Photon emission by hot carrier injection across a lateral p-n or n-p junction

Abstract

Apparatus for generating singular light photons, comprises a source of electrons (100), a p-n junction stage comprising a terminal coupled to the source of electrons (100), the terminal being located at an n-type region (104) of the p-n junction stage, a p-type region (106) adjacent the n-type region (104) and remote from the source of electrons (100), a boundary (112) between the n-type and p- type regions (104, 106), wherein the boundary presents an energy step from the n- type region (104) to the p-type region (106), a source (170) of holes (122) coupled to the p-type region (106), an energy-relaxation suppression device (160) disposed across the n-type region (104), wherein the energy-relaxation suppression device (160) is actuatable to keep excitation energy of an electron (100) disposed in the n-type region (104) and travelling from the source of electrons (100) to the p-type region (106); wherein the energy-relaxation suppression device is operable to maintain sufficient excitation energy of an electron (100) to enable the electron to pass across the boundary (112) and into the p-type region (106). The apparatus also comprises an electron guiding unit, preferably an electromagnet, configured to guide an electron from the source to the p-type region. A method of generating singular light photons is also disclosed.

Description

[0001] PHOTON EMISSION BY HOT CARRIER INJECTION ACROSS A LATERAL P-N OR N-P JUNCTION

[0002] Field of the Invention

[0003] The present invention relates to apparatus and a method for generating single photons. Preferred embodiments disclosed herein provide for a reliable source of single photons, for use for example in quantum optics, quantum communication and quantum imaging. This is typically carried out by hot carrier, that is electron or hole injection across a lateral p-n or n-p junction, or across p-i-n junctions.

[0004] Background of the Invention

[0005] On-demand sources of single photons have been sought after in the field of quantum optics towards applications such as quantum communication and quantum imaging.

[0006] Such sources require the controlled preparation of an excited state in order to avoid accidental emission of secondary photons, either simultaneously or within a delay shorter than the detector jitter duration. There is usually a trade-off between the photon emission rate (or brightness) and the coincidence counts characterised by g(2)(0). This issue can be mitigated, for example, using an optical cavity, where a brightness of 65% and g(2)(0) less than 1 % has been observed (see N. Somaschi et al., Nature Photonics 10, 340 (2016)]. N. Tomm et al. demonstrated up to 57% on-demand generation of photons at the output of the final optical fibre [N. Tomm et al., Nature Nanotechnology 16, 399 (2021 )). However, as long as the emitter is pumped by continuous excitation (optically or electrically) albeit for a short duration in pulse, it is difficult to eliminate the probability of secondary photon emission completely. The latter reference mentioned above states that the purity of single-photon generation [g(2)(0) = 2.1%] is limited by a small amount of laser light leaking into the detection channel (0.3% of total signal) and double-excitation events. A different approach was proposed by C.L. Foden, V. Talyanskii, G.J.

[0007] Milburn, M. Leadbeater, and M. Pepper, in High-frequency acoustoelectric singlephoton source, Physical Review A 62, 011803 (2000)). In their proposed scheme, electrons are carried one by one by surface acoustic waves across a lateral n-i-p junction that is biased below the threshold. As long as the injection of electrons is separated long enough compared to the radiative recombination time, and as long as only one electron is injected at a time, the probability of second photon emission is theoretically zero. A recent experiment demonstrated the possibility of single-photon emission using this scheme, although g(2)(0) was as high as 39%. Details can be found in: T.-K. Hsiao, A. Rubino, Y. Chung, S.-K. Son, H. Hou, J. Pedros, A. Nasir, G. Ethier-Majcher, M.J. Stanley, R.T. Phillips, T.A. Mitchell, J.P. Griffiths, I. Farrer, D.A. Ritchie, and C.J.B. Ford, Single photon emission from single-electron transport in a saw-driven lateral light-emitting diode, Nature Communications 11 , 1 (2020). However, this method can only provide injection of electrons in periodic manner, that is at a given frequency determined by the periodic wave and cannot provide on demand injection. Furthermore, the system cannot guarantee single injection of electrons as the system can enable for multiple electrons in a given period, and also for the missed electron injection in a period. The smallest experimental error rate of single-electron transfer by surface acoustic waves demonstrated so far was 10’4, which will ultimately limit the achievable g(2)(0).

[0008] An idea to use a tunable-barrier single-electron pump for a variablefrequency photon emitter was initially suggested by Blumenthal et al. [M. D. Blumenthal, et al., Gigahertz quantized charge pumping, Nature Physics 3, 343 (2007)]. Following their idea, Buonacorsi et a / (B. Buonacorsi, F. Sfigakis, A. Shetty, M.C. Tam, H.S. Kim, S.R. Harrigan, F. Hohls, M.E. Reimer, Z.R. Wasilewski, and J. Baugh, Non-adiabatic single-electron pumps in a dopant-free GaAs / AIGaAs 2DEG, Applied Physics Letters 119, 114001 (2021 ), discusses quantized charge pumping using non-adiabatic single-electron pumps in dopant-free GaAs two-dimensional electron gases. It is suggested that the dopant free 111— V platform allows for ambipolar devices, such as p-i-n junctions, which could be combined with such pumps to form electrically driven single photon sources.

[0009] Summary of the Present Invention

[0010] The present invention seeks to provide an improved source of individual photons and an improved method of generating individual photons.

[0011] According to an aspect of the present invention, there is provided apparatus for generating singular light photons, comprising: an electron pump operable to provide a source of conduction band electrons; a p-n junction stage comprising a terminal coupled to the electron pump, the terminal being located at an n-type region of the p-n junction stage; a p-type region adjacent the n-type region and remote from the electron pump; a boundary between the n-type and p-type regions, wherein the boundary presents an energy step from the n-type region to the p-type region; a source of valence band holes coupled to the p-type region; an energy-relaxation suppression device disposed across the n-type region, wherein the energy-relaxation suppression device is actuatable to suppress the energy relaxation of an electron disposed in the n-type region and travelling from the electron pump to the p-type region; wherein the energy-relaxation suppression device is operable to keep sufficient excitation energy of an electron to enable the electron to pass across the boundary and into the p-type region.

[0012] The teachings herein provide a mechanism, system or method, which is configured in one embodiment to enable an electron to be carried from a single-electron source to the lateral p-n junction (or a hole to be carried from a single-hole source to the lateral n-p junction). In order to suppress undesirable photon emission by the recombination process of electrons (or holes) other than those injected by the single-electron (or single-hole) source, the lateral p-n (on n- p) junction is set under-biased (that is below the threshold of forward bias current). The energy step created at the junction is significantly larger than the thermal energy, in order to avoid the electrons (or holes) in the Fermi sea to cross the p-n junction by thermal excitation. This means that the energy step needs to be set around 10 meV or larger, even when the device is cooled down to liquid helium temperature or below, therefore, the electrons (holes) emitted from the singleelectron (hole) source are in a high energy state in order to overcome the energy step to enter the p-type (n-type) region. However, such high-energy (hot) carriers (electrons or holes) are short lived and they do not travel long distance easily. The teachings herein solve this issue by a verified method to carry hot electrons (or holes) from a source to a lateral p-n (or n-p) junction emitting photons by recombination process.

[0013] The p-n junction stage is preferably a lateral p-n junction. In the described embodiments, the lateral p-n junction consists of or comprises two-dimensional electron gas and two-dimensional hole gas in the same plane.

[0014] According to another aspect of the present invention, there is provided apparatus for generating singular light photons, comprising: a hole pump operable to provide a source of valence band holes; a n-p junction stage comprising a terminal coupled to the hole pump, the terminal being located at a p-type region of the n-p junction stage; an n-type region adjacent the p-type region and remote from the hole pump; a boundary between the p-type and n-type regions, wherein the boundary presents an energy step from the p-type region to the n-type region; a source of electrodes coupled to the n-type region; and an energy-relaxation suppression device disposed across the p-type region, wherein the energy-relaxation suppression device is actuatable to keep the excitation energy of a hole disposed in the p-type region and travelling from the hole pump to the n-type region; wherein the energy-relaxation suppression device is operable to keep sufficient excitation energy of a hole to enable the hole to pass across the boundary and into the n-type region.

[0015] The p-n junction stage is preferably a lateral p-n junction. In the described embodiments, the lateral p-n junction consists of or comprises two-dimensional electron gas and two-dimensional hole gas in the same plane. Preferably, the energy-relaxation suppression device comprises a magnet configured to generate a magnetic field having at least a magnetic component extending in a direction perpendicular to a direction between the hole pump and the n-p junction stage (preferably the plane of two-dimensional holes and electrons) , wherein the magnetic filed is operable to guide holes from the hole source across the n-p junction stage.

[0016] Advantageously, the n-p junction stage provides a lateral n-p junction.

[0017] In either of these aspects, the magnet preferably is configured to guide an electron from the electron pump to the p-type region of the p-n junction stage or a hole from the hole pump to the n-type region of the n-p junction stage.

[0018] The magnet may be an electromagnet.

[0019] In the preferred embodiments:

[0020] (i) the n-type region of the p-n junction stage has an energy (conduction band) below an energy (conduction band) of the p-type region of the p-n junction stage, wherein the n-type region of the p-n junction stage provides an electron bath, or

[0021] (ii) the p-type region of the n-p junction stage has an energy (valence band) above an energy (valence band) of the n-type region of the n-p junction stage, wherein the p-type region of the n-p junction stage provides a hole bath.

[0022] The electron / hole pump may be a controllable source of singular electrons / holes.

[0023] Advantageously, the electron pump or the hole pump is a tunable-barrier quantum dot pump.

[0024] In some embodiments, the energy-relaxation suppression device comprises a depletion gate disposed across the n-type region of the p-n junction stage or disposed across the p-type region of the n-p junction stage.

[0025] Advantageously, the depletion gate is operable to be maintained at a controllable voltage.

[0026] The apparatus may comprise an inducing gate as the source of holes or electrons. The holes or electrons are induced by voltage operation of the inducing gate.

[0027] Preferably: (i) the p-n junction stage is part of a two-dimensional electron gas and two- dimensional hole gas, or

[0028] (ii) the n-p junction stage is part of a two-dimensional hole gas and two- dimensional electron gas.

[0029] In the preferred embodiments, the two-dimensional electron gas system comprises a GaAs / AIGaAs hetero-structure.

[0030] The apparatus according to any preceding claim may comprise a pump control unit connected: (i) to the electron pump and operable to tune an exit barrier height to fix an electron emission energy, or (ii) to the hole pump and operable to tune an exit barrier height to fix a hole emission energy.

[0031] The pump control unit in practical embodiments may comprise an AC voltage source and a DC voltage source.

[0032] In the case of a continuous hot-electron emission, a voltage is applied to the source to raise the source potential to emit hot electrons over the barrier (depicted in Figure 3, described below). In the case of electron pump (depicted in Figure 1 , described below), such a source-drain voltage is not applied, but a radiofrequency signal on the entrance gate barrier lifts up the energy of an electron captured in a pump, and emits the electron as "hot electron" over the exit-barrier potential. The electron emission energy is controlled by the height of exit barrier.

[0033] The electron guiding unit could be chiral one-dimensional edge channels such as quantum-Hall edge states formed by an application of magnetic field or chiral transport channels in topological insulator, in which case an external magnetic field may not be needed.

[0034] Advantageously, the apparatus is configured to maintain the Fermi energy of the n-type region adjacent to the p-n junction below the energy of the conduction band in the p-type region by an amount much larger than the thermal energy BT, where the AB is the Boltzmann constant and T is the device temperature, such that the electrons in the n-type Fermi sea cannot travel across the junction.

[0035] According to another aspect of the present invention, there is provided a method of generating singular light photons, comprising the steps of: supplying from an electron pump a source of conductance band electrons to an n-type region of a p-n junction stage, wherein the p-n junction stage comprises a p-type region adjacent the n-type region and remote from the supply of electrons; generating a boundary between the n-type and p-type regions, wherein the boundary presents an energy step from the n-type region to the p-type region; providing a source of valence band holes to the p-type region; suppressing energy relaxation of an electron disposed in the n-type region and travelling from the electron pump to the p-type region; wherein the energyrelaxation suppression is operable to keep sufficient excitation energy of an electron to enable the electron to pass across the boundary and into the p-type region; the energy-relaxation suppression device comprising a magnet, the method including the step of configuring the magnet to generate a magnetic field having at least a magnetic component extending in a direction perpendicular to a plane of two-dimensional electron gas, wherein the magnetic filed is operable to guide electrons from the electron source across the p-n junction stage.

[0036] Advantageously, the p-n junction stage is a lateral p-n junction.

[0037] According to another aspect of the present invention, there is provided a method of generating singular light photons, comprising the steps of: supplying from a hole pump a source of valence band holes to a p-type region of an n-p junction stage, wherein the n-p junction stage comprises an n- type region adjacent the p-type region and remote from the supply of holes; generating a boundary between the p-type and n-type regions, wherein the boundary presents an energy step from the p-type region to the n-type region; providing a source of conduction band electrons to the n-type region; suppressing energy relaxation of a hole disposed in the p-type region and travelling from the hole pump to the n-type region; wherein the energy-relaxation suppression is operable to keep sufficient excitation energy of a hole to enable the hole to pass across the boundary and into the n-type region; the energy-relaxation suppression device comprising a magnet, the method including the step of configuring the magnet to generate a magnetic field having at least a magnetic component extending in a direction perpendicular to a plane of two-dimensional hole gas, wherein the magnetic filed is operable to guide holes from the hole source across the n-p junction stage.

[0038] Advantageously, the n-p junction stage is a lateral n-p junction.

[0039] In these aspects, the method preferably comprises the step of:

[0040] (i) guiding an electron from the electron pump to the p-type region of the p-n junction stage; or

[0041] (ii) guiding a hole from the hole pump to the n-type region of the n-p junction stage.

[0042] The electron or hole may be guided by means of an electromagnet.

[0043] The method advantageously includes the step of:

[0044] (i) maintaining the n-type region of the p-n junction stage at an energy (conduction band) below an energy (conduction band) of the p-type region of the p-n junction stage, wherein the n-type region of the p-n junction stage provides an electron bath, or

[0045] (ii) maintaining the p-type region of the n-p junction stage at an energy (valence band) above an energy (valence band) of the n-type region of the n-p junction stage, wherein the p-type region of the n-p junction stage provides a hole bath. For holes, the energy in the p-type region needs to be "below" that in the n-type region. However, the energy-band diagram is usually plotted for electron energy, the sign of which is opposite for holes. Therefore, the (electron) energy in the p-type region needs to be above that in the n-type region.

[0046] The method may include the step of controlling the source of electrons or holes.

[0047] The step of providing excitation energy may utilise a depletion gate disposed across the n-type region of the p-n junction stage or across p-type region of the n-p junction stage.

[0048] Preferably, the method includes controlling the voltage of the depletion gate.

[0049] There may be provided the step of:

[0050] (i) inducing holes in the p-type region of the p-n junction stage, or

[0051] (ii) inducing electrons in the n-type region of the n-p junction stage. Holes or electrons are preferably induced by voltage operation of an inducing gate.

[0052] The method preferably includes:

[0053] (i) providing at the p-n junction stage a two-dimensional electron gas; or

[0054] (ii) providing at the n-p junction stage a two-dimensional hole gas.

[0055] In some embodiments, the two-dimensional electron gas comprises a GaAs / AIGaAs hetero-structure.

[0056] Preferably, the method includes tuning an exit barrier height to fix an electron or hole emission energy.

[0057] According to another aspect of the present invention, there is provided apparatus for generating singular light photons, comprising:

[0058] (a) an electron pump operable to provide a source of conduction band electrons; an n-i junction stage comprising a terminal coupled to the electron pump, the terminal being located at an n-type region of the n-i junction stage, an intrinsic region adjacent the n-type region and remote from the electron pump, a boundary between the n-type and intrinsic regions, wherein the boundary presents an energy step from the n-type region to the intrinsic region; and

[0059] (b) a hole pump operable to provide a source of valence band holes, a p-i junction stage comprising a terminal coupled to the hole pump, the terminal being located at a p-type region of the p-i junction stage, an intrinsic region adjacent the p-type region and remote from the hole pump, a boundary between the p-type and intrinsic regions, wherein the boundary presents an energy step from the p-type region to the n-type region; and an energy-relaxation suppression device disposed across the n-type region of the n-i junction stage and the p-type region of the p-i junction stage, wherein the energy-relaxation suppression device is actuatable to keep the excitation energy of an electron disposed in the n-type region of the n-i junction stage and travelling from the electron pump to the intrinsic region and of a hole disposed in the p-type region and travelling from the hole pump to the intrinsic region of the p-i junction stage; wherein the energy-relaxation suppression device is operable to keep sufficient excitation energy of an electron to enable the electron to pass across the boundary and into the intrinsic region, and to keep sufficient excitation energy of a hole to enable the hole to pass across the boundary and into the intrinsic region; the energy-relaxation suppression device comprising a magnet configured to generate a magnetic field having at least a magnetic component extending in a direction perpendicular to a plane of the two-dimensional electron gas and two- dimensional hole gas, wherein the magnetic filed is operable to guide electrons from the electron pump across the n-i junction stage and to guide holes from the hole pump across the p-i junction stage.

[0060] Preferably, the n-i junction stage provides a lateral n-i junction, and the p-i junction stage provides a lateral p-i junction.

[0061] The magnet is preferably configured to guide an electron from the electron pump to the intrinsic region and a hole from the hole pump to the intrinsic region.

[0062] In the preferred embodiments, the magnet is an electromagnet.

[0063] Advantageously: (i) the n-type region has an energy (conduction band) below an energy (conduction band) of the intrinsic region, wherein the n-type region provides an electron bath, and (ii) the p-type region has an energy (valence band) above an energy (valence band) of the intrinsic region, wherein the p-type region provides a hole bath.

[0064] Preferably, the electron pump is a controllable source of singular electrons, and the hole pump is a controllable source of singular holes.

[0065] One or both of the electron pump and the hole pump may be a tunable- barrier quantum dot pump.

[0066] The energy-relaxation suppression device may comprise a depletion gate disposed across at least one of the n-type region of the n-i junction stage or the p-type region of the p-i junction stage.

[0067] Advantageously, the depletion gate is operable to be maintained at a controllable voltage.

[0068] The apparatus may comprise at least one inducing gate as the source of holes or electrons, in which holes or electrons are preferably induced by voltage operation of the inducing gate. Advantageously, (i) the n-i junction stage is part of a two-dimensional electron gas and an intrinsic quantum well, and (ii) the p-i junction stage is part of a two-dimensional hole gas and an intrinsic quantum well.

[0069] In some embodiments, the two-dimensional electron gas comprises a GaAs / AIGaAs hetero-structure.

[0070] The apparatus may comprise a pump control unit connected: (i) to the electron pump and operable to tune an exit barrier height to fix an electron emission energy, and (ii) to the hole pump and operable to tune an exit barrier to fix a hole emission energy.

[0071] According to another aspect of the present invention, there is provided a method of generating singular light photons, comprising the steps of:

[0072] (a) supplying from an electron pump a source of conductance band electrons to an n-type region of a n-i junction stage, wherein the n-i junction stage comprises an intrinsic region adjacent the n-type region and remote from the supply of electrons, generating a boundary between the n-type and intrinsic regions, wherein the boundary presents an energy step from the n-type region to the intrinsic region,

[0073] (b) supplying from a hole pump a source of valence band holes to a p-type region of a p-i junction stage, wherein the p-i junction stage comprises an intrinsic region adjacent the p-type region and remote from the supply of holes, generating a boundary between the p-type and intrinsic regions, wherein the boundary presents an energy step from the p-type region to the intrinsic region; and suppressing energy relaxation of an electron disposed in the n-type region of the n-i junction stage and travelling from the electron pump to the intrinsic region of the n-i junction stage, and of a hole disposed in the p-type region of the p-i junction stage and travelling from the hole pump to the intrinsic region of the p-i junction stage; wherein the energy-relaxation suppression is operable to keep sufficient excitation energy of an electron to enable the electron to pass across the boundary and into the intrinsic region and travelling from the electron pump to the intrinsic region of the n-i junction stage, and to keep sufficient excitation energy of a hole to enable the hole to pass across the boundary and into the intrinsic region and travelling from the hole pump to the intrinsic region of the p-i junction stage; the energy-relaxation suppression device comprising a magnet, the method including the step of configuring the magnet to generate a magnetic field having at least a magnetic component extending in a direction perpendicular to a plane of two-dimensional electron gas and two-dimensional hole gas, wherein the magnetic filed is operable to guide electrons from the electron source across the n-i junction stage and to guide holes from the hole source across the p-i junction stage; whereby an electron and a hole recombine in the intrinsic region to emit a photon of light.

[0074] In practice, the intrinsic region provides what could be described as a pocket to which the two electron and hole are trapped and recombine to emit a photon.

[0075] The method may comprise the steps of:

[0076] (i) guiding an electron from the electron pump to the intrinsic region; and

[0077] (ii) guiding a hole from the hole pump to the intrinsic region.

[0078] Advantageously, the electron and hole are guided by means of an electromagnet.

[0079] The method may include the steps of:

[0080] (i) maintaining the n-type region at an energy (conduction band) below an energy (conduction band) of the intrinsic region, wherein the n-type region provides an electron bath and

[0081] (ii) maintaining the p-type region at an energy (valence band) above an energy (valence band) of the intrinsic region, wherein the p-type region provides a hole bath.

[0082] It preferably includes the step of controlling the sources of electrons and holes.

[0083] Advantageously, the step of providing excitation energy utilises depletion gates disposed across the n-type region of the n-i junction stage and the p-type region of the p-i junction stage.

[0084] The method preferably includes the step of controlling the voltages of the depletion gates. The method may include:

[0085] (i) providing at the n-i junction stage a two-dimensional electron gas; and

[0086] (ii) providing at the p-i junction stage a two-dimensional hole gas.

[0087] In the preferred embodiments, the includes tuning exit barrier height to fix electron and hole emission energies.

[0088] Brief Description of the Drawings

[0089] Embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings, in which:

[0090] Figure 1 is a schematic energy and circuit diagram showing the preferred electrical set-up of the p-n junction stage;

[0091] Figure 2 is scanning electron microscope image of the device in false colour with circuit diagram components superimposed on the image, providing a top view of the p-n junction stage matching the schematic view of Figure 1 ;

[0092] Figure 3 is a schematic diagram similar to Figure 1 , showing the effect of controlled operation of an energy-relaxation suppression device of the apparatus of Figure 2;

[0093] Figures 4(a) and (b) show the injection of hot electrons into p-type region against depletion voltage and a bias applied across the p-n junction;

[0094] Figure 5 shows magnetic field dependence of spectrum for electroluminescence from the p-n junction caused by hot electrons and electrons at ground level;

[0095] Figure 6 is a cross-sectional view of an example of device structure that realises the energy structure shown in Figure 1 and Figure 3 and the device geometry shown in Figure 2;

[0096] Figure 7 is a cross-sectional view of another example of device structure formed as GaAsZAIo.33Gao.67As p-type and configured to pump holes into an n-region; Figure 8 is a cross-sectional view of another example of device structure formed of Si / SiojGeo.s as an n-type embodiment and using different materials compared to the embodiment of Figure 6;

[0097] Figure 9 shows an embodiment of wafer structure for the GaAsZAIo.33Gao.67As lateral p-n junction;

[0098] Figures 10A to 10G shows an example of device fabrication stages, with Figure 10A showing an n-type dopant removal stage;

[0099] Figure 10B shows the isolation etch stage;

[0100] Figure 10C shows the creation of the p-type ohmic contact;

[0101] Figure 10D shows the creation of the n-type ohmic contact;

[0102] Figure 10E shows the creation of the gate electrodes for an n-type single electron pump;

[0103] Figure 10F shows the insulation formation;

[0104] Figure 10G shows the creation of the inducing gate electrode;

[0105] Figure 11 shows another embodiment of the present invention, which provides for pumping holes rather than electrons as in the embodiment of the preceding Figures, and specifically is a schematic energy and circuit diagram showing the preferred electrical set-up of the n-p junction stage;

[0106] Figure 12 shows another embodiment of the present invention, and specifically is a schematic energy and circuit diagram showing the preferred electrical set-up of an electron and hole pumping device; and

[0107] Figure 13 is a schematic illustration of the device with circuit diagram components superimposed on the image, providing a top view of the diode junction stages matching the schematic view of Figure 12.

[0108] Description of the Preferred Embodiments

[0109] The preferred embodiments disclosed herein provide for a reliable source of single photons, preferably generated by use of an electron pump which is coupled to feed electrons one by one to a p-n junction stage. The apparatus includes a mechanism to cause an electron ejected from the electron pump into the p-n junction stage to maintain its energy, in effect keeping the electron as a hot electron, as it travels across the n-type region and until it enters the p-type region. The electron, being the only electron in the conduction band in the p-type region, can radiatively recombine with a hole in the valence band, so as to cause a single photon matching the band-gap energy to be emitted. The p-n junction is preferably biased below threshold so that any other electrons / holes cannot travel across the junction. Advantageously, the apparatus includes a mechanism to direct the singular electron across the p-n junction, which can optimise the efficacy of the generation of the desired single photon.

[0110] The preferred embodiments, as well as providing very reliable sources of single photons, are also able to generate single photons on demand.

[0111] An example of an electron pump suitable for the embodiments disclosed herein is a tunable-barrier quantum dot pump, for instance of a type described by: M.D. Blumenthal, B. Kaestner, L. Li, S. Giblin, T.J.B.M. Janssen, M. Pepper, D. Anderson, G. Jones, and D.A. Ritchie, in Gigahertz quantized charge pumping, Nature Physics 3, 343 (2007), the disclosure of which is incorporated herein by reference.

[0112] The disclosed tunable-barrier quantum dot pump comprises two modulated barriers and operates in a nonadiabatic pump mode. Transmission modulation of the two barriers is achieved by monochromatic sinusoidal signals, the narrow frequency spectrum of which allows effective impedance matching of the various transmission lines between a radio-frequency source and the device gates, generating a clean and controllable signal. The device comprises two Ti / AI finger gates, all of which are over an etched channel and provide a time-dependent potential, the remaining three being earthed. A static quantum dot over the wire is induced by applying negative de voltages to both gates. These gates provide the entrance and exit barriers. The sinusoidal signal from the output of the radio frequency source is fed to the entrance-barrier gate such that the ac sinusoidal signals ride the static voltages. This controls the ac-dc potentials at an individual electron. At any point during the cycle one of the outer barriers is always sufficiently high to prevent tunnelling and such that any pumped current is independent of the applied source-drain voltage. It should be understood that the pumping does not have to be by sinusoidal wave as it can be done by arbitrary shape. Furthermore, the timing of emission can be tuned with a picosecond resolution. Therefore, injection of electron can be performed on demand with picosecond resolution in timescale.

[0113] It should also be understood that the described ac sinusoidal signal is not necessary for the teachings herein. In embodiments, an AC signal could be applied to one gate (for instance the entrance barrier).

[0114] Another example of electron pumps have been described by: B. Buonacorsi, F. Sfigakis, A. Shetty, M. C. Tam, H. S. Kim, S. R. Harrigan, F. Hohls, M. E. Reimer, Z. R. Wasilewski, and J. Baugh, in Non-adiabatic single-electron pumps in a dopant-free GaAs / AIGaAs 2DEG, Applied Physics Letters 119, 114001 (2021 ), https: / / doi.Org / 10.1063 / 5.0062486, the disclosure of which is incorporated herein by reference.

[0115] Other examples of electron pumps and other electron sources are known in the art.

[0116] The teachings herein provide a source of single photons by coupling the output of the quantum dot pump, or other source of electrons, to a p-n junction stage. In the preferred embodiment, the p-n junction is biased below threshold, such that electrons / holes in the Fermi sea (including those thermally excited) cannot normally travel across the junction. This has the advantage of eliminating the background photon emission. An electron emitted by the quantum dot pump or other source is made to travel as a “hot electron”, that is controlled so as not to lose its excitation energy, until it reaches and enters the p-type region. This is the only electron in the conduction band, so when it radiatively recombines with a hole in the valence band, only one photon matching the band-gap energy is emitted.

[0117] The p-n junction stage preferably comprises a lateral p-n junction with an n-type region and a p-type region, in which holes are induced by gate voltage operation. In the preferred embodiments, the p-n junction stage is part of a two dimensional electron gas (2DEG) system of GaAs / GaAIAs hetero-structure. Other embodiments can use instead a two dimensional electron gas structure of, for example, indium arsenide, or indium aluminium arsenide, or silicon or silicon germanium. Other two dimensional electron gas systems will be apparent to the person skilled in the art on the basis of the teachings herein.

[0118] Combining a single-electron pump device with a lateral p-n junction (lateral connection of two-dimensional electron gas and two-dimensional hole gas in semiconductor materials) is not straightforward in terms of device fabrication. Furthermore, there is also a technical question as to how to transport electrons emitted by single-electron pumps into a p-type region, which must be placed away from the pump at a short distance (typically a few pm). The electrons emitted by single-electron pumps will have a high energy (typically ~ 100 meV) compared to the other electrons (the Fermi sea) in the region. In order to avoid background photon emission by the electrons in the Fermi sea entering into the p-type region, the solution disclosed herein biases the p-n junction below the threshold, meaning that there is a barrier or step in the electrostatic potential that the incoming electrons (from the single-electron pump) must overcome. This potential step is set below the energy of pumped electrons (~ 100 meV) and is higher than the thermal excitation of the Fermi sea (~ 0.1 meV at 1 K). Setting the potential step to a few tens of meV has been found to meet this requirement efficiently. As a consequence, the incoming electrons must travel as “hot electrons” maintaining a threshold amount of energy without completely relaxing onto the Fermi energy before reaching the p-type region.

[0119] Referring to Figure 1 , this is an energy and circuit diagram showing the preferred electrical set-up of the p-n junction stage of the system according to the preferred embodiments disclosed herein.

[0120] Individual electrons 100 are emitted by the electron source, preferably an electron pump as previously described, at a relatively high excitation energy. The p-n junction stage 102 comprises an n-type region 104 and a p-type region 106. The n-type region is held at a lower energy level (conduction band) 108 compared to the energy level (conduction band) 110 at the p-type region. This energy differential creates an energy step 112 from the n-type region up to the p-type region. The electron sea, or Fermi sea, in the n-type region acts as a sink and traps any electrons that fail to have the required energy to reach and enter the p- type region. This assists in ensuring that only the desired electrons reach the p- type region at any time.

[0121] Figure 1 also depicts the passage of electron 100 through the n-type region. In the preferred embodiments the electron is made to maintain its excitation energy across the gap formed by the n-type region, in a manner as described below. The electron 100 in this state is hereinafter referred to as a hot electron. Its energy state across the n-type region is depicted by the arrow 120 in Figure 1 . As the hot electron 100 is maintained at a high energy state, it is able to cross the energy barrier or step 112 between the n-type and p-type regions and thereby enter the p-type region 106. At the same time, any other electrons in the n-type region are kept there by being at a lower excitation energy 108.

[0122] The individual electron 100, once in the p-type region, can couple to a hole 122, at which it releases a photon 124 of energy. Given the individual electron 100 passing into the p-type junction 106, only a single photon 124 is generated.

[0123] Further photons 124 can be generated by feeding further electrons 100 from the source across the n-type region of the p-n junction stage. However, since the generation of the electrons 100 is, in the preferred embodiment, an individual electron at a time, and since the maintenance of the excitation energy 120 can be controlled, the generation of further photons can be precisely controlled, a single photon at a time.

[0124] The preferred embodiments also provide a mechanism to direct or guide the electron 100 across the n-type region. In the preferred embodiment this achieved by the provision of a magnetic field, generated by an external coil disposed around the p-n junction stage and which in practice powered to generate a magnetic field a direction pointing into the paper in the view of Figure 2. This is described in further detail below.

[0125] A practical embodiment of the schematic arrangement shown in Figure 1 is shown in Figure 2. For ease of understanding, Figure 2 is scanning electron microscope image of the device in false colour with circuit diagram components superimposed on the image, to present a top view of the p-n junction stage 102 matching the schematic view of Figure 1 . Ohmic contacts are indicated with cross mark in a box and magnetic field B is applied perpendicular to the substrate. The p-n junction stage comprises a two-dimensional electron gas 150 (depicted in blue in Figure 2). An entrance gate 152 (depicted in pink in Figure 2) is positioned at one end of n-type region 154 and is coupled to an electron source such as a pump as described above. The entrance gate 152 controls the input energy of the (hot) electron. When pumping, the exit gate 153 (the green gate on the right) controls the energy of hot-electrons. When using as continuous hot- electron source, then the combination of the entrance gate and bias applied from the current source 180 controls the hot-electron energy.

[0126] The p-n junction stage also includes a depletion gate 160 (depicted in green in Figure 2), which preferably extends across the entire extent of the edge of the n- type region 104. However, as shown in this embodiment, it may cover the edge of a part of the n-type region 104 of the p-n junction, as long as the gate 160 remains sufficient to maintain the excitation energy of the hot electron 100 across the entire extent of the n-type region 104. The depletion gate 160 in practice controls the rate of energy relaxation due to phonon emission and electron-electron interactions for hot electrons 100, as described below.

[0127] The edge-depletion gate 160 is not an essential component of the apparatus. It is not necessary when a strong magnetic field is used. It becomes useful in the cases of weaker magnetic fields, where the provision of edgedepletion gate 160 assists in maintaining the trajectory of the electron.

[0128] The edge depletion gate typically works in combination with the magnetic field. The system and method can work without applying a gate voltage if the magnetic field is large enough. In such cases, the large magnetic field acts to suppress energy relaxation, so the magnet itself is an effective energy-relaxation suppression device. When the magnetic field is not sufficiently large, the edge depletion gate acts also as an energy-relaxation suppression device.

[0129] The p-type region 106 is located adjacent the n-type region 104 and adjacent the depletion gate 160. Disposed within the zone of the p-type region is an inducing gate 170 (depicted in yellow in Figure 2), which acts to induce holes in the p-type region 106 from a p-type ohmic contact 184.

[0130] The white arrow 172 depicts the hot electron trajectory making across the p-n junction while guided by a negative high magnetic field generated by a surrounding magnet. The white arrow 174 depicts the trajectory of an electron that has lost energy and cannot enter the p-type region.

[0131] The n-type region 104 may be defined by conventional lithography techniques, etching with acids, metal deposition with vacuum evaporator and alloying of Ge / Au / Ni to get electric contacts to the two dimensional electron gas system. The p-type region 106 is in the preferred embodiment a p-type metal- oxide semiconductor field effect transistor (p-MOSFET). First, the dopant layer of n-AIGaAs in p-type region is etched away to eliminate the two-dimensional electron gas (2DEG). Second, a layer of AuBe is deposited and annealed to establish ohmic contacts to the induced holes. Then, a layer of AI2O3 is deposited by atomic layer deposition to insulate the ohmic contacts and the Inducing gate 170, which allows induced holes to intrude into the p-type contacts. Gate electrodes of Ti / Au on n-type region are deposited after removing the insulating layer. The insulator need not be a layer of AI2O3, as other insulators could be used, as the skilled person will appreciate.

[0132] With reference also to Figure 1 , applying a negative voltage Vdep to the depletion gate 160 modifies the optical phonon emission rate and electron-electron scattering rate, which enables hot electrons to travel long distances while maintaining their energy, at least above the energy level required to enter into the p-type region 106. A hot electron 100 which overcomes the p-n junction step passes into the p-type region 106 and relaxes into the valence band with the emission of a photon.

[0133] Holes in the p-type region 106 are induced by applying a gate voltage Vind to the inducing gate 170.

[0134] When the p-n junction stage is subjected to a strong magnetic field, a hot electron passes across the p-n junction stage along with the bottom edge in Figure 2, along the line of the arrows 172. Initially, a hot electron is injected through the potential barrier created at the entrance gate 152, its gate voltage being noted Vent. In this particular experiment that demonstrated hot-electron injection into a lateral p-n junction using continuous source of hot electrons, a source measure and / or control unit 180 connected to the entrance contact 152 tunes Vinj to fix the injection current nj. A hot electron travels along with the bottom edge 172 losing energy, in dependence upon the voltage applied to the depletion gate, as described below. At the boundary 112 between the n-type and p-type regions 104, 106, an electron retaining sufficient energy will overcome the p-n junction 112 potential barrier step and intrude into p-type region 106. Any hot electron intruding into p-type region combines with a hole and emits a photon of light. If the hot- electron injection source is replaced by a single-electron pump, the emission of photon can be controlled one by one. The sequence can be repeated with multiple electrons, each supplied on demand from the electron source, and are able to generate a photon of light at the rate at which photons are induced. Therefore, the emission of one or more photons of light in sequence can be controlled: by the rate of supply of electrons using the entrance gate 152, by the generation of holes by means of the inducer gate 170, by the energy retained in an electron passing through the n-type region 104.

[0135] In the preferred embodiments, the device is operated in a large magnetic field (~ 10 T) having at least a component that is perpendicular to the plane of the lateral p-n junction stage, so that the electrons 100 from the single-electron pump travel along the edge of the two-dimensional electron gas in the n-type region 104. It will be appreciated that the most efficient arrangement is by a magnet disposed or operable to generate a magnetic field that is perpendicular to the plane of the lateral p-n junction stage. However, the apparatus and system will function with a magnet that does not generate its field directly perpendicular to the plane of the lateral junction, as long as there is a component of that magnetic field that is along the perpendicular line. A magnetic field of around 10 Tesla provides optimum results, although the field need only be strong enough to guide the electrons along the edge of the junction. Magnetic fields of 3-4 Tesla have been found to work, though preferably the magnetic fields are between 5 to 10 Tesla. At lower magnetic fields there is a risk of electron scattering. In practice, the minimum magnetic field that can provide reliable operation of the system can be determined by routine experimentation.

[0136] The edge-depletion gate 160 that covers over this electron path controls the electrostatic potential, which in turn controls the rate of energy relaxation by phonon emission. The depletion of the two-dimensional electron gas under the gate, in the path of hot electron, also reduced the rate of electron-electron scattering. Advantageously, the semiconductor device design and device fabrication topology disclosed herein allows the integration of a single-electron pump, an edge-depletion gate 160, and a lateral p-n junction stage.

[0137] The generated light can be detected by a spectrometer through a lens assembly and optical fibre. A source measure unit 182 connected to p-type contact 184 can detect the number of electrons that contribute to photon emission, as current lp. Any other electrons that did not enter the p-type region will follow the path 174 and will flow into the top contact 186 and their current lncan be detected by a digital multimeter 190 via a current-voltage converter). The magnitude of the current lndepends on the voltage Vdep applied to the depletion gate 160 and on the voltage Vpapplied to the p-type contact 184. Depletion gate voltage VdePmodifies the device edge potential and optical phonon emission rate, which allows hot electrons 100 to maintain their energy for longer distances. As the band gap is intrinsically defined based on the material, the p-type contact voltage Vpmodifies the step size of the p-n junction potential barrier 112. Were the step height 112 lowered significantly, electrons even at ground level can intrude into the p-type region 106. The measured voltage thresholds for this embodiment are:

[0138] Vp= 1 ,525V and 1 ,535V at B = 0T and -10 T, respectively.

[0139] Figure 4 shows excess current lp0.05 seconds after triggering nd to induce holes, as functions of nj and Vpat Vdep = (a) 0V and (b) -1 ,2V. In Figure 5 all data measurements were taken at hnj = 300 nA and B = -10 T.

[0140] The images of Figures 4(a) and 4(b) are computed with interpolation based on excess current lpand applied bias voltage nj on the hot electron reservoir 100 as functions of Vent and Vpat nj = 300 nA and B = -10 T. The blue regions indicate a very low lpand therefore low or no photon generation. The red regions depict a high lpand photon generation.

[0141] As the p-n junction works 112 acts as a potential barrier and its height is modified by Vp, the figures give hot electron energy distribution at Vdep = (a) 0V and (b) -1 ,2V, respectively. It has been established that electrons 100 intrude into the p-type region 106 when nj is larger than -0.03V (i.e. electrons are injected at least 30 meV above the Fermi energy in the n-type region 104). This is consistent with the excess energy (the size of energy step formed at the p-n junction) plotted on the right axis in Figure 4. Electrons must have at least this excess energy to enter the p-type region. As can be seen in Figure 4, the application of voltage Vdep at the depletion gate 160, of around -1 .2 V in this example, while Vinj is larger than (below) about -0.03V ensures passage of an electron 100 into the p-type region 106 for the generation of a photon of light.

[0142] Referring now to Figure 3, this shows in schematic form similar to Figure 1 the effect of applying a differing depletion voltage Vdep to the depletion gate 160. The section of the graph in red shows the energy loss of an electron 100 as it passes across the n-type region 104 of the p-n junction stage. In the case where Vdep is close to 0 V, specifically below the required threshold, the electron 100 will lose excitation energy and will relax towards the electron pool 108. They will not maintain enough energy to overcome the barrier 112 between the n-type and p-type regions.

[0143] On the other hand, when Vdep is sufficiently negative, this will contribute to maintaining the energy of the electron 100 and while this may drop off shortly before the p-n junction 112, depending on the physical arrangement and extent of the depletion gate 160 as well as the voltage Vdep at the gate 160, the electron 100 will still maintain sufficient energy to overcome the barrier 112 and reach the p-type region for emission of a photon of light.

[0144] In practice, the amount of current lpis enhanced by applying optimised negative voltage on Vdep. Applying negative voltage on the depletion gate electrode 160 separates the orbital of electron wavefunction before and after optical phonon emission, which suppresses phonon emission rate and assists hot electrons 100 to maintain their energy. A majority of hot electrons 100 would lose their energy along the trajectory between the deletion gate 160 and inducing gate 170 in cases where phonon emission is not suppressed. In addition, the depletion of the background two-dimensional electron gas under the gate also suppresses the energy relaxation by electron-electron interactions. When the maximum energy is smaller than the p-n junction barrier step height 112, no electrons can intrude into the p-type region. Figure 5 shows magnetic field dependence of spectrum for electroluminescence from the p-n junction caused by (a) hot electrons 100 and (b) electrons at ground level, which shows that injected hot electrons 100 travelling along a quantum Hall edge channel are converted into photons. A spectrum at each magnetic field consists of averaged spectrum offset to make the background signal level equal to zero. Raw spectra are filtered to eliminate strong peak signals generated by cosmic rays and are averaged 30 times to improve noisesignal ratio.

[0145] For Figure 5(a), the amount of 500 nA hot current is injected under the condition Vp= 1 .510V that is too small for the electrons in the Fermi sea 109 to enter the p-type region (or for holes in the p-type region to enter the n-type region) to get photo emissions without hot electrons to acquire photon emission. In Figure 5(a) electroluminescence is observed only at high negative magnetic field where injected hot electrons can intrude into the p-type region thanks to the chiral transport through quantum Hall edge channel following the path 172. When the direction of the magnetic field is reversed, the chirality is reversed, and electrons emitted from the source 152 travel upwards in Fig. 2 and sink into the contact 186. Therefore, they do not reach the p-n junction, and no photo emission is observed. On the other hand, in Figure 5(b), a large enough Vpis applied to achieve electroluminescence without hot electron injection to acquire photon emission. In this case, the chirality is not important, and a symmetric pattern with respect to the magnetic field is observed.

[0146] As a first step to realise an on-demand photon source, success was achieved in getting photo emissions generated by hot electrons travelling on quantum Hall edge channels. Single hot electron injection with a tunable barrier tunnelling electron source can generate a single photon by means of a p-n junction stage as herein described and that is free from any secondary photon emission. Two perspectives are considered here.

[0147] One is the measurement of the second-order correlation function with Hanbury-Brown-Twiss effect to show that a device generates a single photon. This can be achieved well by optimisation of the position of the lens unit, using a larger diameter fibre to capture larger amounts of photons, and implementing setup for second-order correlation function measurement in a dilution refrigerator.

[0148] The perspective provides circular polarisation measurement. Photons generated by transitions between Landau levels are expected to have circular polarisation depending on the spin states of the electron and hole due to the conservation law on angular momentum. Circular polarisation can be detected by inserting a quarter-wave plate into the device in order to convert them into linear polarisation and by using a half wave plate to resolve the linear polarisation. Conversion is advantageously carried out in a refrigerated environment in which photons converted into linear polarisation a dilution refrigerator can be guided to the spectrometer through linear polarisation maintaining fibres.

[0149] Figure 6 shows an example of an integrated circuit lay-out or topography for the circuit arrangement of the p-n junction stage of Figure 2. The n-type contact 104 and the p-type contact 106 are as previously described and disposed at opposing sides of the chip. The two-dimensional electron gas is in this embodiment is created by a GaAs / AIGaAs heterostructure 150. The layer 150’ is Si-doped Alo.33Gao.67As, and the layer 150” is intnnsic-Alo.33Gao.67As. The upper layer 150’ is covered by a layer 200 which is a GaAs cap layer. A GaAs quantum well 201 contains two-dimensional electron gas and two-dimensional hole gas 122.

[0150] An insulating layer 190, of AI2O3 in this embodiment, insulates the p-type contact 106 and the inducing gate 170 from one another.

[0151] As will be apparent from Figure 6, an electron 100 will travel along a channel from the n-type region to the p-type region, assuming it has sufficient excitation energy, to hole 122, through the GaAs quantum well 201 .

[0152] A complementary embodiment to the embodiment using a lateral p-n junction is shown in Figure 7, which comprises a p-type region 302 with a two dimensional hole gas (2DHG) and n-type region 304 in which electrons (instead of holes) are induced by gate 306 voltage operation. The structure also includes gate electrodes 307, similar to the gate electrodes 160 of the first described embodiment. In principle, the teachings herein can be implemented on any semiconductor material system and structure with a 2DEG (or 2DHG) and also induce holes (or electrons) by gate voltage operation.

[0153] Figure 8 shows another embodiment of structure, having a lateral p-n junction, implemented in a different semiconductor material system such as Si / SiGe. Other examples include InAs / lnP, GaN / lnGaN, and so on.

[0154] Figure 9 illustrates a wafer in accordance with an embodiment of the present invention. The skilled person will appreciate that variables including layer thickness, number of matrix repeat, doping concentration, and so on, are readily determinable by the skilled person on the basis of their common knowledge in the art and are typically tailored for a particular designed emission wavelength or energy band structure. The values set out below in the description of an example structure are therefore to be understood as being examples only and not determinative of essential characteristics of the structure.

[0155] The semiconductor material illustrated in this embodiment comprises a AIGaAs / GaAs heterostructure 310 on a GaAs substrate 312.

[0156] Semiconductor material for the device can be produced by molecular beam epitaxy (MBE) on a semi-insulating GaAs substrate. Other common crystal growth techniques on lattice-matched substrates may also be employed, such as metal-organo chemical vapour deposition or vapour phase epitaxy. Epitaxial growth commences with a 500 nm intrinsic GaAs buffer on the substrate.

[0157] Next, a graded 250 nm AlxGai-xAs (x increased from 0.05 to 0.33), interspaced with 0.6 nm GaAs and repeated several times to form the buffer. Alternatively, a distributed Bragg reflector stack of multiple GaAs / AlyGai-yAs (typically y > 0.5) or GaAs / AIAs layers can be included to enhance the output photon emission efficiency.

[0158] A 15 nm intrinsic i-GaAs quantum well is sandwiched between a lower 50 nm and upper 40 nm intrinsic i-Alo.33Gao.67As barrier. The 2DEG will form preferably in this narrow 15 nm i-GaAs quantum well. Next, a 40 nm n-type Alo.33Gao.67As doped with Si concentration of 1x1018cm-3followed by an intrinsic 10 nm i-GaAs is grown on the underlying structure. (A 2DHG version can be produced in the 15 nm i-GaAs quantum well by using a p-type dopant such as Be or C in the 40 nm Alo.33Gao.67As layer.) Fabrication

[0159] Figure 6, as described above, depicts a schematic cross-sectional view of the final device structure in accordance with an embodiment of the present invention. It represents the principal elements along the central red axis 185 of Figure 2, although Figure 6 does not show the n and p-type ohmic contacts that provide electrical connections located away from the main active region shown in Figure 2.

[0160] Figures 10A to 10G illustrate the preferred embodiment of device fabrication sequence.

[0161] The first step is usually to restrict the bulk 2DEG across the whole wafer substrate into a smaller mesa region. This step can be done with a photoresist mask defined using photo or electron beam or any other compatible lithographic techniques. Areas left unprotected by photoresist masked areas will be removed by chemical etching with a combination of diluted acid and oxidising solvents or dry plasma based processes using the appropriate gases. Etching is stopped partway in the buffer region.

[0162] Next, with reference to Figure 10A, the n-type dopants in the n-type Alo.33Gao.67As are removed from the areas where the induced p-regions will be formed, for example by employing a similar resist defined mask and etching technique discussed above. The narrower single electron pump channel on the large mesa is also formed concurrently with this step (or separately if a different etch depth is required or preferred).

[0163] With reference to Figure 10B, another, optional, deeper isolation etch stage can then be performed to reduce potential electrical and optical leakage paths.

[0164] As depicted in Figure 10C, AuBe (or AuZn) p-type ohmic contacts are then deposited, preferably by the standard resist masked lift off process, in the areas where the n-type dopants were removed and then annealed into the substrate.

[0165] Figure 10D shows the AuGeNi n-type ohmic contacts being formed and alloyed, preferably using a similar technique. With reference to Figure 10E, thin (< 50nm) sub-micrometre scale metal gate electrodes are defined and then evaporated onto the 2DEG channel in the n- type region, typically using electron beam lithography and a resist lift off process. Examples of typical gate electrode metals can be a combination of Ti / Au or Cr / Au or Pt / Au. Assuming similar metal thickness, this gate electrode on the n-side can be delayed and done concurrently with the final stage with the p-side gate electrodes contacts.

[0166] As shown in Figure 10F, an insulator in then formed on the p-type region. This layer can be laid down using atomic layer deposition, for example with AI2O3 or HfO2, or sputtered or chemical vapour deposition, for example with SiO2 or SisN4, or cross linked PMMA or any other compatible material and associated method, such as by etching or lift off or multiple electron beam exposures.

[0167] As shown in Figure 10G, the inducing gate electrode for the p-region is then defined preferably using the standard resist lift off process. Ideally, this gate electrode is transparent, such as indium tin oxide (or similar metal oxide based), or semi-transparent, such as very thin Ti (< 5nm).

[0168] Finally, all the electrodes in the n and p-regions are connected to the large area bond pads for electrical wiring using metal interconnects (e.g. Ti / Au or Cr / Au or Pt / Au) defined by lithography and the standard lift off process.

[0169] As described above, other embodiments provide in place of electron pumping, a hole pumping system, in which the particle that is moved is a hole rather than an electron. This can generate a photon of light, in an analogous manner to the electron pumping embodiment.

[0170] Referring to Figure 11 , this is an energy and circuit diagram showing the preferred electrical set-up of the n-p junction stage of the system according to this embodiment.

[0171] Individual holes 400 are emitted by the hole source, preferably a hole pump, at a relatively high excitation energy. The n-p junction stage 402 comprises an p-type region 404 and an n-type region 406. The p-type region is held at a higher energy level (valence band) 408 compared to the energy level (conduction band) 410 at the n-type region. This energy differential creates an energy step 412 from the p-type region up to the n-type region. The hole sea, or hole Fermi sea, in the p-type region acts as a sink and traps any holes that fail to have the required energy to reach and enter the n-type region. This assists in ensuring that only the desired holes reach the n-type region at any time.

[0172] Figure 11 also depicts the passage of hole 400 through the p-type region 404. In the preferred embodiments the hole is made to maintain its excitation energy across the gap formed by the p-type region, in a manner as described below. The hole 400 in this state is hereinafter referred to as a hot hole. Its energy state across the p-type region is depicted by the arrow 420 in Figure 11 . As the hot hole 400 is maintained at a high energy state, it is able to cross the energy barrier or step 412 between the p-type and n-type regions and thereby enter the n-type region 406. At the same time, any other holes in the p-type region are kept there by being at a lower excitation energy 408.

[0173] The individual hole 400, once in the n-type region, can couple to an electron 422, at which it releases a photon 124 of energy. Given the individual hole 400 passing into the n-type junction 406, only a single photon 124 is generated.

[0174] Further photons 124 can be generated by feeding further holes 400 from the source across the p-type region of the n-p junction stage. However, since the generation of the holes 400 is, in the preferred embodiment, an individual hole at a time, and since the maintenance of the excitation energy 420 can be controlled, the generation of further photons can be precisely controlled, a single photon at a time.

[0175] The preferred embodiments also provide a mechanism to direct or guide the hole 400 across the p-type region. In the preferred embodiment this achieved by the provision of a magnetic field, generated by an external coil disposed around the n-p junction stage and which in practice powered to generate a magnetic field a direction perpendicular to the plane of the two-dimensional hole gas and two- dimensional electron gas.

[0176] The invention also provides for a system which pumps both electrons and holes, so that a single electron can combine with a single hole to generate a single photon. Such an embodiment can be precisely controlled, thereby to control the generation of single photons. As with the other embodiments, a series of electrons and holes can be pumped, with each electron / hole pair in the sequence generating a photon.

[0177] Figure 12 depicts an energy and circuit diagram showing the preferred electrical set-up of the n-i and p-i junction stages of the system according to this embodiment.

[0178] These stages are equivalent to the electron pump and hole pump embodiments described above and shown in Figures 1 and 11 , save for the fact that they are arranged such that the electrons and holes travel towards one another so as to combine in overlapping intrinsic regions 502, 504.

[0179] As with the embodiments of Figures 1 and 11 , the excitation energy of the electrons and the holes can be maintained sufficiently high by a magnetic field and as appropriate by an edge depletion gate 160 as taught above.

[0180] It is expected that full quantum state transfer from electron / hole to a photon is possible this way, as the states of both particles (electrons and holes) can be controlled.

[0181] Figure 13 is a view similar to Figure 2, that is a schematic illustration of the device of Figure 12 with circuit diagram components superimposed on the image, illustrating how such a device can be formed to pump both electrons and holes. Electrons and holes have different chirality when a perpendicular magnetic field is applied, and hence they can move towards each other. In the embodiment shown in Figure 13, a pocket 500 is created in the middle (in the intrinsic region) of the device, where both particles 100, 400 are trapped, find each other to form an exciton, and then emit a photon.

[0182] The structure of device shown in Figure 13 will be appreciated by the skilled person having regard to Figure 12 and its accompanying description.

[0183] Figure 13 represents a top view of the n-i junction and p-i junctions stages matching the schematic view of Figure 12. Ohmic contacts are indicated with cross mark in a box and magnetic field B is applied perpendicular to the substrate.

[0184] The n-i junction stage comprises a two-dimensional electron gas 154 (depicted in blue in Figure 13). An entrance gate 152 is positioned at one end of n-type region 154 and is coupled to an electron source such as a pump as described above. The entrance gate 152 controls the input energy of the (hot) electron. When pumping, the exit gate 156 controls the energy of hot-electrons. When using as continuous hot-electron source, then the combination of the entrance gate and bias applied from the current source 180 controls the hot- electron energy.

[0185] The n-i junction stage may also include a depletion gate 160, which preferably extends across the entire extent of the edge of the n-type region 104. However, as shown in this embodiment, it may cover the edge of a part of the n- type region 104 of the n-i junction stage, as long as the gate 160 remains sufficient to maintain the excitation energy of the hot electron 100 across the entire extent of the n-type region 104. The depletion gate 160 in practice controls the rate of energy relaxation due to phonon emission and electron-electron interactions for hot electrons 100, as described above.

[0186] On the other side of the structure, the p-i junction stage 402 comprises a two-dimensional hole electron gas 454. An entrance gate 452 is positioned at one end of p-type region 454 and is coupled to a hole source such as a hole pump as described above. The entrance gate 452 controls the input energy of the (hot) hole. When pumping, the exit gate 460 controls the energy of hot-holes. When using as continuous hot-hole source, then the combination of the entrance gate and bias applied from the current source 480 controls the hot-electron energy.

[0187] The stage 402 may also include a depletion gate 460, which preferably extends across the entire extent of the edge of the p-type region 404. However, as shown in this embodiment, it may cover the edge of a part of the p-type region 404 of the p-i junction 402, as long as the gate 460 remains sufficient to maintain the excitation energy of the hot hole 400 across the entire extent of the p-type region 404. The depletion gate 460 in practice controls the rate of energy relaxation due to phonon emission and hole-hole interactions for hot holes 400, as described above.

[0188] The edge-depletion gates 160, 460 are not essential components of the apparatus. They are not necessary when a strong magnetic field is used. They become useful in the cases of weaker magnetic fields, where the provision of edge-depletion gates 160, 460 assist in maintaining the trajectory of the electron and hole.

[0189] The regions of the device depicted in Figure 13 may be formed in the same way and with the same materials as described above in relation to the other embodiments.

[0190] In some implementations, there is a chance that inducing a hole layer can be unreliable due to a charging effect (potentially caused by the holes leaking out from the quantum well confinement and piling up at the interface between the insulator and semiconductor layers). In order to refresh the accumulated charges, a large positive bias can be applied, which then induces trapped negative charges, most likely at the same insulator / semiconductor interface. The trapped negative charge can be used to induce holes, rather than using a negative voltage on the inducing gate. So, a sequence can be: 1 ) initially apply a large positive voltage on the inducing gate, 2) suddenly drop the voltage to zero. This induces holes, which decay over (roughly) one second. Once the holes have been lost, steps 1 ) and 2) can be repeated. This process was published in “Formation of a lateral p-n junction light-emitting diode on an n-type high-mobility GaAs / AI0.33Ga0.67As heterostructure - lOPscience”.

[0191] It will be appreciated that in practice the apparatus and method disclosed herein can be used to generate multiple photons in order to create, for example, brighter emissions. In all cases, a single photon can be generated at any point in time, with multiple photons being generated in sequence. The apparatus and method can operate at up to very high frequencies, of 1 GHz or more.

[0192] The benefit of this single-photon source technology is the ability to generate photons on demand. It is expected that the timing of photon emission can be controlled with a precision of ~ 100 picoseconds (limited by the radiative recombination time). Since the error of electron emission from the single-electron pumps can be less than 1 part in 107, we also expect that the chance of two-photon emission will be very low.

[0193] The skilled person will appreciate that above-described are the preferred embodiments of the device and method, and that modifications will be apparent to the skilled person having regard to the teachings herein. The disclosures in British patent application number GB 2304452.2, from which this application claims priority, and in the abstract accompanying this application are incorporated herein by reference.

Claims

CLAIMS1 . Apparatus for generating singular light photons, comprising: an electron pump operable to provide a source of conduction band electrons; a p-n junction stage comprising a terminal coupled to the electron pump, the terminal being located at an n-type region of the p-n junction stage; a p-type region adjacent the n-type region and remote from the electron pump; a boundary between the n-type and p-type regions, wherein the boundary presents an energy step from the n-type region to the p-type region; a source of holes coupled to the p-type region; an energy-relaxation suppression device disposed across the n-type region, wherein the energy-relaxation suppression device is actuatable to keep the excitation energy of an electron disposed in the n-type region and travelling from the electron pump to the p-type region; wherein the energy-relaxation suppression device is operable to keep sufficient excitation energy of an electron to enable the electron to pass across the boundary and into the p-type region; the energy-relaxation suppression device comprising a magnet configured to generate a magnetic field having at least a magnetic component extending in a direction perpendicular to a plane of two-dimensional electron gas and two- dimensional hole gas, wherein the magnetic filed is operable to guide electrons from the electron source across the p-n junction stage.

2. Apparatus according to claim 1 , wherein the p-n junction stage provides a lateral p-n junction.

3. Apparatus for generating singular light photons, comprising: a hole pump operable to provide a source of valence band holes; a n-p junction stage comprising a terminal coupled to the hole pump, the terminal being located at a p-type region of the n-p junction stage; an n-type region adjacent the p-type region and remote from the hole pump;a boundary between the p-type and n-type regions, wherein the boundary presents an energy step from the p-type region to the n-type region; a source of electrodes coupled to the n-type region; an energy-relaxation suppression device disposed across the p-type region, wherein the energy-relaxation suppression device is actuatable to keep the excitation energy of a hole disposed in the p-type region and travelling from the hole pump to the n-type region; wherein the energy-relaxation suppression device is operable to keep sufficient excitation energy of a hole to enable the hole to pass across the boundary and into the n-type region; the energy-relaxation suppression device comprising a magnet configured to generate a magnetic field having at least a magnetic component extending in a direction perpendicular to a plane of two-dimensional electron gas and two- dimensional hole gas, wherein the magnetic filed is operable to guide holes from the hole source across the n-p junction stage.

4. Apparatus according to claim 3, wherein the n-p junction stage provides a lateral n-p junction.

5. Apparatus according to any preceding claim, wherein the magnet is configured to guide an electron from the electron pump to the p-type region of the p-n junction stage or a hole from the hole pump to the n-type region of the n-p junction stage.

6. Apparatus according to any preceding claim, wherein the magnet is an electromagnet.

7. Apparatus according to any preceding claim, wherein:(i) the n-type region of the p-n junction stage has an energy (conduction band) below an energy (conduction band) of the p-type region of the p-n junction stage, wherein the n-type region of the p-n junction stage provides an electron bath, or(ii) the p-type region of the n-p junction stage has an energy (valence band) above an energy (valence band) of the n-type region of the n-p junction stage, wherein the p-type region of the n-p junction stage provides a hole bath.

8. Apparatus according to any preceding claim, wherein the electron pump is a controllable source of singular electrons, or the hole pump is a controllable source of singular holes.

9. Apparatus according to claim 8, wherein the electron pump or the hole pump is a tunable-barrier quantum dot pump.

10. Apparatus according to any preceding claim, wherein the energy-relaxation suppression device comprises a depletion gate disposed across the n-type region of the p-n junction stage or disposed across the p-type region of the n-p junction stage.11 . Apparatus according to claim 10, wherein the depletion gate is operable to be maintained at a controllable voltage.

12. Apparatus according to any preceding claim, comprising an inducing gate as the source of holes or electrons.

13. Apparatus according to claim 12, wherein holes or electrons are induced by voltage operation of the inducing gate.

14. Apparatus according to any preceding claim, wherein:(i) the p-n junction stage is part of a two-dimensional electron gas and two- dimensional hole gas, or(ii) the n-p junction stage is part of a two-dimensional hole gas and two- dimensional electron gas.

15. Apparatus according to claim 14, wherein the two-dimensional electron gas comprises a GaAs / AIGaAs hetero-structure.

16. Apparatus according to any preceding claim, comprising a pump control unit connected: (i) to the electron pump and operable to tune an exit barrier height to fix an electron emission energy, or (ii) to the hole pump and operable to tune an exit barrier height to fix a hole emission energy.

17. A method of generating singular light photons, comprising the steps of: supplying from an electron pump a source of conductance band electrons to an n-type region of a p-n junction stage, wherein the p-n junction stage comprises a p-type region adjacent the n-type region and remote from the supply of electrons; generating a boundary between the n-type and p-type regions, wherein the boundary presents an energy step from the n-type region to the p-type region; providing a source of valence band holes to the p-type region; suppressing energy relaxation of an electron disposed in the n-type region and travelling from the electron pump to the p-type region; wherein the energyrelaxation suppression is operable to keep sufficient excitation energy of an electron to enable the electron to pass across the boundary and into the p-type region; the energy-relaxation suppression device comprising a magnet, the method including the step of configuring the magnet to generate a magnetic field having at least a magnetic component extending in a direction perpendicular to a plane of two-dimensional electron gas and two-dimensional hole gas, wherein the magnetic filed is operable to guide electrons from the electron source across the p-n junction stage.

18. A method according to claim 17, wherein the p-n junction stage is a lateral p-n junction.

19. A method of generating singular light photons, comprising the steps of:supplying from a hole pump a source of valence band holes to a p-type region of an n-p junction stage, wherein the n-p junction stage comprises an n- type region adjacent the p-type region and remote from the supply of holes; generating a boundary between the p-type and n-type regions, wherein the boundary presents an energy step from the p-type region to the n-type region; providing a source of conduction band electrons to the n-type region; suppressing energy relaxation of a hole disposed in the p-type region and travelling from the hole pump to the n-type region; wherein the energy-relaxation suppression is operable to keep sufficient excitation energy of a hole to enable the hole to pass across the boundary and into the n-type region; the energy-relaxation suppression device comprising a magnet, the method including the step of configuring the magnet to generate a magnetic field having at least a magnetic component extending in a direction perpendicular to a plane of two-dimensional electron gas and two-dimensional hole gas, wherein the magnetic filed is operable to guide holes from the hole source across the n-p junction stage.

20. A method according to claim 19, wherein the n-p junction stage is a lateral n-p junction.21 . A method according to any one of claims 17 to 20, comprising the step of:(i) guiding an electron from the electron pump to the p-type region of the p-n junction stage; or(ii) guiding a hole from the hole pump to the n-type region of the n-p junction stage.

22. A method according to any one of claims 17 to 21 , wherein the electron or hole is guided by means of an electromagnet.

23. A method according to any one of claims 17 to 22, including the step of:(i) maintaining the n-type region of the p-n junction stage at an energy (conduction band) below an energy (conduction band) of the p-type region of thep-n junction stage, wherein the n-type region of the p-n junction stage provides an electron bath, or(ii) maintaining the p-type region of the n-p junction stage at an energy (valence band) above an energy (valence band) of the n-type region of the n-p junction stage, wherein the p-type region of the n-p junction stage provides a hole bath.

24. A method according to any one of claims 17 to 23, including the step of controlling the source of electrons or holes.

25. A method according to any one of claims 17 to 24, wherein the step of providing excitation energy utilises a depletion gate disposed across the n-type region of the p-n junction stage or across p-type region of the n-p junction stage.

26. A method according to claim 25, including controlling the voltage of the depletion gate.

27. A method according to any one of claims 17 to 26, including the step of:(i) inducing holes in the p-type region of the p-n junction stage, or(ii) inducing electrons in the n-type region of the n-p junction stage.

28. A method according to claim 27, wherein holes or electrons are induced by voltage operation of an inducing gate.

29. A method according to any one of claims 17 to 28, including:(i) providing at the p-n junction stage a two-dimensional electron gas; or(ii) providing at the n-p junction stage a two-dimensional hole gas.

30. A method according to claim 29, wherein the two-dimensional electron gas comprises a GaAs / AIGaAs hetero-structure.31 . A method according to any one of claims 17 to 30, including tuning an exit barrier height to fix an electron or hole emission energy.

32. Apparatus for generating singular light photons, comprising:(a) an electron pump operable to provide a source of conduction band electrons; an n-i junction stage comprising a terminal coupled to the electron pump, the terminal being located at an n-type region of the n-i junction stage, an intrinsic region adjacent the n-type region and remote from the electron pump, a boundary between the n-type and intrinsic regions, wherein the boundary presents an energy step from the n-type region to the intrinsic region; and(b) a hole pump operable to provide a source of valence band holes, a p-i junction stage comprising a terminal coupled to the hole pump, the terminal being located at a p-type region of the p-i junction stage, an intrinsic region adjacent the p-type region and remote from the hole pump, a boundary between the p-type and intrinsic regions, wherein the boundary presents an energy step from the p-type region to the n-type region; and an energy-relaxation suppression device disposed across the n-type region of the n-i junction stage and the p-type region of the p-i junction stage, wherein the energy-relaxation suppression device is actuatable to keep the excitation energy of an electron disposed in the n-type region of the n-i junction stage and travelling from the electron pump to the intrinsic region and of a hole disposed in the p-type region and travelling from the hole pump to the intrinsic region of the p-i junction stage; wherein the energy-relaxation suppression device is operable to keep sufficient excitation energy of an electron to enable the electron to pass across the boundary and into the intrinsic region, and to keep sufficient excitation energy of a hole to enable the hole to pass across the boundary and into the intrinsic region; the energy-relaxation suppression device comprising a magnet configured to generate a magnetic field having at least a magnetic component extending in a direction perpendicular to a plane of the two-dimensional electron gas and two-dimensional hole gas, wherein the magnetic filed is operable to guide electrons from the electron pump across the n-i junction stage and to guide holes from the hole pump across the p-i junction stage.

33. Apparatus according to claim 32, wherein the n-i junction stage provides a lateral n-i junction, and the p-i junction stage provides a lateral p-i junction.

34. Apparatus according to claim 32 or 33, wherein the magnet is configured to guide an electron from the electron pump to the intrinsic region and a hole from the hole pump to the intrinsic region.

35. Apparatus according to any one of claims 32 to 34, wherein the magnet is an electromagnet.

36. Apparatus according to any one of claims 32 to 35, wherein:(i) the n-type region has an energy (conduction band) below an energy (conduction band) of the intrinsic region, wherein the n-type region provides an electron bath, and(ii) the p-type region has an energy (valence band) above an energy (valence band) of the intrinsic region, wherein the p-type region provides a hole bath.

37. Apparatus according to any one of claims 32 to 36 wherein the electron pump is a controllable source of singular electrons, and the hole pump is a controllable source of singular holes.

38. Apparatus according to claim 37, wherein one or both of the electron pump and the hole pump is a tunable-barrier quantum dot pump.

39. Apparatus according to any one of claims 32 to 38, wherein the energyrelaxation suppression device comprises a depletion gate disposed across at leastone of the n-type region of the n-i junction stage or the p-type region of the p-i junction stage.

40. Apparatus according to claim 39, wherein the depletion gate is operable to be maintained at a controllable voltage.41 . Apparatus according to any one of claims 32 to 40, comprising at least one inducing gate as the source of holes or electrons.

42. Apparatus according to claim 41 , wherein holes or electrons are induced by voltage operation of the inducing gate.

43. Apparatus according to any one of claims 32 to 42, wherein:(i) the n-i junction stage is part of a two-dimensional electron gas and an intrinsic quantum well, and(ii) the p-i junction stage is part of a two-dimensional hole gas and an intrinsic quantum well.

44. Apparatus according to claim 43, wherein the two-dimensional electron gas system comprises a GaAs / AIGaAs hetero-structure.

45. Apparatus according to any one of claims 32 to 44, comprising a pump control unit connected: (i) to the electron pump and operable to tune an exit barrier height to fix an electron emission energy, and (ii) to the hole pump and operable to tune an exit barrier to fix a hole emission energy.

46. A method of generating singular light photons, comprising the steps of:(a) supplying from an electron pump a source of conductance band electrons to an n-type region of an n-i junction stage, wherein the n-i junction stage comprises an intrinsic region adjacent the n-type region and remote from the supply of electrons,generating a boundary between the n-type and intrinsic regions, wherein the boundary presents an energy step from the n-type region to the intrinsic region;(b) supplying from a hole pump a source of valence band holes to a p-type region of a p-i junction stage, wherein the p-i junction stage comprises an intrinsic region adjacent the p-type region and remote from the supply of holes, generating a boundary between the p-type and intrinsic type regions, wherein the boundary presents an energy step from the p-type region to the intrinsic; and suppressing energy relaxation of an electron disposed in the n-type region of the n-i junction stage and travelling from the electron pump to the intrinsic region of the n-i junction stage, and of a hole disposed in the p-type region of the p-i junction stage and travelling from the hole pump to the intrinsic region of the p-i junction stage; wherein the energy-relaxation suppression is operable to keep sufficient excitation energy of an electron to enable the electron to pass across the boundary and into the intrinsic region and travelling from the electron pump to the intrinsic region of the n-i junction stage, and to keep sufficient excitation energy of a hole to enable the hole to pass across the boundary and into the intrinsic region and travelling from the hole pump to the intrinsic region of the p-i junction stage; the energy-relaxation suppression device comprising a magnet, the method including the step of configuring the magnet to generate a magnetic field having at least a magnetic component extending in a direction perpendicular to a plane of two-dimensional electron gas and two-dimensional hole gas, wherein the magnetic filed is operable to guide electrons from the electron source across the n-i junction stage and to guide holes from the hole source across the p-i junction stage.

47. A method according to claim 46, wherein the n-i junction stage is a lateral n- i junction, and the p-i junction stage is a lateral p-i junction.

48. A method according to claim 46 or 47, comprising the steps of:(i) guiding an electron from the electron pump to the intrinsic region; and(ii) guiding a hole from the hole pump to the intrinsic region.

49. A method according to any one of claims 46 to 48, wherein the electron and hole are guided by means of an electromagnet.

50. A method according to any one of claims 46 to 49, including the steps of:(i) maintaining the n-type region at an energy (conduction band) above an energy (conduction band) of the intrinsic region, wherein the n-type region provides an electron bath and(ii) maintaining the p-type region at an energy (valence band) below an energy (valence band) of the intrinsic region, wherein the p-type region provides a hole bath.51 . A method according to any one of claims 46 to 50, including the step of controlling the sources of electrons and holes.

52. A method according to any one of claims 46 to 51 , wherein the step of providing excitation energy utilises depletion gates disposed across the n-type region of the n-i junction stage and the p-type region of the p-i junction stage.

53. A method according to claim 52, including controlling the voltages of the depletion gates.

54. A method according to any one of claims 46 to 53, including:(i) providing at the n-i junction stage a two-dimensional electron gas; and(ii) providing at the p-i junction stage a two-dimensional hole gas.

55. A method according to claim 54, wherein the two-dimensional electron gas comprises GaAs / AIGaAs hetero-structures.

56. A method according to any one of claims 46 to 55, including tuning exit barrier heights to fix electron and hole emission energies.

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