Semiconductor Nano-Air-Channel Diode Terahertz Optical Mixer

Abstract

The present invention discloses a semiconductor nano-air-channel diode optical mixer, belonging to the technical field of semiconductor photodetectors and millimeter-wave / terahertz devices. The optical mixer includes a substrate, a semiconductor thin film, a nano-insulating layer, an upper electrode, and a lower electrode; it is a vertical structure or a horizontal structure. By using the nano-air channel as the channel for high-speed ballistic transport of photo-electrons, an optical mixer with a bandwidth above THz can be realized; moreover, applying a small bias voltage of several volts between the cathode and anode on both sides of the nano-air channel can generate carrier ionization multiplication in the semiconductor, doubling the photocurrent, responsivity, and optical-to-terahertz conversion efficiency; the semiconductor nano-air-channel optical mixer combines the advantages of near-light-speed ballistic transport of electrons in vacuum devices and high-integration batch processing of semiconductor devices, and is an effective solution for realizing a large-bandwidth and high-efficiency optical mixer.

Description

Technical Field

[0001] The present invention belongs to the technical field of semiconductor optoelectronic detectors and millimeter-wave / terahertz devices, and particularly relates to a semiconductor nano-air-channel diode terahertz optical mixer. Background Art

[0002] Optical mixing is an important technical approach for generating millimeter waves and terahertz waves in the field of photonics. By heterodyning two laser beams with different frequencies to down-convert them into a continuous coherent electromagnetic wave with the frequency difference between the two, a millimeter-wave and terahertz source with a wide continuous tunable frequency range, narrow linewidth, high coherence, room-temperature operation, and on-chip integration can be achieved, without being restricted by phase matching and the Manley-Row relationship in nonlinear medium optical mixing. More importantly, optical mixing technology can break through the modulation bandwidth limitation of traditional electronic millimeter-wave devices and achieve lower phase noise and higher operating frequencies than electronic signal sources, thereby obtaining higher communication rates and radar resolutions. Therefore, the optoelectronic integrated millimeter-wave and terahertz technology based on optical mixing is considered to be one of the most promising solutions for realizing 6G communication and high-resolution millimeter-wave / terahertz radar. An optical mixer is the core optoelectronic conversion device in optical mixing technology. In terms of working principle, it is a super-fast optoelectronic detector that converts signals such as the amplitude and phase of the difference frequency of two laser beams into a modulated high-frequency photocurrent based on an ultra-fast carrier transport process, and then converts it into a radio-frequency electromagnetic wave through a load. In addition to being used to generate millimeter waves and terahertz waves, an optical mixer can also be used as a room-temperature heterodyne coherent detector to down-convert and receive millimeter-wave / terahertz signals, achieving near-quantum-limited detection of weak terahertz signals at room temperature.

[0003] Semiconductor photoconductive antennas (PCAs), PIN photodiodes, and uni-traveling-carrier photodiodes (UTC-PDs) are the most commonly used optical mixers at present. However, their responsivities and the conversion efficiency from light to terahertz waves are both relatively low, generally less than one percent. In addition to the DC component lost during the optical mixing process, mainly because only one electron-hole pair can be generated by one photon in such devices, and in order to achieve a large bandwidth, the full absorption of incident light or the full collection of photo-generated carriers is often sacrificed. At the same time, affected by lattice scattering, the carrier transport speed in the semiconductor is limited, restricting the bandwidth and output power. Therefore, traditional all-solid-state semiconductor optical mixers face the bottleneck that it is difficult to simultaneously improve the bandwidth, responsivity, and efficiency.

[0004] In an ideal optical mixer, carriers should undergo ballistic transport at nearly the speed of light without being affected by scattering. However, in traditional semiconductor optical mixers, carriers are affected by the lattice scattering of the solid medium, and ballistic transport at nearly the speed of light cannot be achieved because the high electric field used to increase the carrier velocity also increases scattering. Vacuum is an ideal medium for the ballistic transport of electrons without scattering, where the saturation velocity of electrons is close to the speed of light. Compared with traditional semiconductor solid-state optoelectronic devices, vacuum optoelectronic devices have unique advantages in terms of frequency, power, radiation resistance, high temperature resistance, sensitivity, etc. due to the absence of lattice collisions and energy dissipation and low dark current. However, traditional vacuum optoelectronic devices use the external photoelectric effect to achieve field-assisted photoemission of electrons on the photocathode surface, and generally suffer from problems such as small emission current, high working voltage, low responsivity and quantum efficiency, and the need for a vacuum working environment, resulting in a large volume and difficulty in miniaturization and integration. Summary of the Invention

[0005] In view of the deficiencies in the prior art, the present invention proposes a semiconductor nano-air channel diode optical mixer, aiming to provide a new type of optical mixer that can operate without a vacuum environment, and at the same time has large bandwidth, high responsivity, high output power, high conversion efficiency, and can be batch-produced on a large-area wafer.

[0006] The present invention is achieved through the following technical solutions:

[0007] A semiconductor nano-air channel diode optical mixer, characterized in that: the optical mixer includes a substrate, a semiconductor thin film, a nano-insulating layer, a first electrode, and a second electrode;

[0008] The optical mixer is of a vertical structure or a horizontal structure;

[0009] When the optical mixer is of a vertical structure, a semiconductor thin film is disposed above the substrate; the semiconductor thin film is a stepped thin film with a height difference, wherein the region with a lower height is the first mesa, and the region with a higher height is the second mesa; a first electrode is disposed above the first mesa; a nano-insulating layer is disposed above the second mesa; a second electrode is disposed above the nano-insulating layer; the region occupied by the nano-insulating layer is smaller than the second mesa and the second electrode, and the outside of the nano-insulating layer is a nano-air channel.

[0010] When the optical mixer is of a horizontal structure, an insulating layer is disposed above the substrate; a second electrode is disposed in the right region above the insulating layer, a semiconductor thin film is disposed in the left region, and a first electrode is disposed above the semiconductor thin film; the nano-air channel is between the semiconductor thin film and the second electrode.

[0011] Furthermore, the function of the semiconductor thin film is to absorb light and convert it into electron-hole pairs. When the semiconductor thin film serves as the cathode, p-type doping is used and a negative bias voltage is applied to the first electrode. When the semiconductor thin film serves as the anode, n-type doping is used and a positive bias voltage is applied to the first electrode.

[0012] Furthermore, the first electrode and the second electrode are made of highly conductive materials, including but not limited to metals, metalloids, transparent conductive films, and graphene. The first electrode and the second electrode also serve as radiation antennas for millimeter waves and terahertz waves.

[0013] Furthermore, the length of the nano air channel is less than the average free path of electron scattering in air (the average free path of scattering is about 70 nm under 1 standard atmosphere). Electrons can perform high-speed ballistic transport in the nano air channel without a vacuum environment.

[0014] Furthermore, when the semiconductor nano air channel diode adopts a vertical structure, the electric field intensity generated by the bias voltages applied to the first electrode and the second electrode in the semiconductor is greater than its ionization threshold electric field, and optical carrier ionization multiplication occurs in the semiconductor, doubling the photocurrent, responsivity, terahertz output power, and the optical-to-terahertz conversion efficiency.

[0015] Furthermore, when the semiconductor nano air channel diode adopts a vertical structure, when the reverse bias voltage applied to the semiconductor is greater than its strong inversion bias voltage, a strong inversion layer is generated on the semiconductor surface, forming a two-dimensional electron gas or a two-dimensional hole gas, and zero-barrier electron emission is achieved by the edge electrons of the two-dimensional electron gas under the action of the Coulomb repulsive force of the internal electrons.

[0016] The basic working principle and process of the semiconductor nano-air-channel diode optical mixer of the present invention are as follows: Under the excitation of two mixing light beams and the action of an externally applied electric field, the semiconductor emits photoelectrons, or a high concentration of holes is generated on the semiconductor surface to induce the metal opposite the nano-air channel to emit photoelectrons. The photoelectrons enter the nano-air channel and undergo high-speed ballistic transport before being received by the anode, thereby generating a high-frequency photocurrent. The high-frequency photocurrent is transmitted through the electrodes to loads such as dipole antennas or coplanar waveguides to radiate millimeter waves or terahertz waves. The transport distance of electrons between the cathode and anode (i.e., the length of the nano-air channel) is on the nanoscale, and its transport time is on the femtosecond scale. The corresponding bandwidth and cut-off frequency can be as high as hundreds of THz. The semiconductor generates a depletion region under reverse bias and generates an inversion layer two-dimensional electron gas (2DEG) or hole gas (2DHG) near the surface. Under illumination, the photo-generated carriers in the depletion region drift to the 2DEG or 2DHG region under the action of an electric field to increase their concentration. When the electric field strength generated by the externally applied voltage between the cathode and anode in the semiconductor is greater than its ionization threshold electric field strength, carrier ionization multiplication occurs in the semiconductor, doubling the photocurrent, responsivity, terahertz output power, and the optical-to-terahertz conversion efficiency. The nano-air-channel optical mixer combines the advantages of near-light-speed ballistic transport of electrons in vacuum devices and high-integration batch processing of semiconductor solid-state devices, and is an effective solution to break through the technical bottlenecks of traditional semiconductor optical mixers and vacuum optical mixers and achieve large-bandwidth and high-efficiency optical mixers.

[0017] The advantages of the present invention are as follows:

[0018] 1. There is no lattice scattering in the nano-air channel, and electrons can undergo ballistic transport at near-light speed. Therefore, the bandwidth of the semiconductor nano-air-channel optical mixer can reach above THz, featuring an ultra-large bandwidth.

[0019] 2. The nano-air channel has an ultra-high intrinsic breakdown voltage close to vacuum and lower heat generation. Therefore, the semiconductor nano-air-channel optical mixer has the advantage of achieving a large output power.

[0020] 2. Benefiting from the fact that the electron transport process in the nano-air channel is not affected by solid lattice scattering, this device also has the advantages of radiation resistance, high and low temperature tolerance, and can work in harsh environments.

[0021] 3. The distances between the cathode and anode on both sides of the nano-air channel are very close. Only a small working voltage of a few volts is required to achieve a large electric field, thereby obtaining efficient electron field emission, greatly reducing the requirement for a working voltage of hundreds of volts for traditional vacuum optoelectronic devices. At the same time, the electrons at the edge of the 2DEG layer can greatly compress the surface barrier under the Coulomb repulsion force, achieving zero-barrier electron emission and overcoming the problem of low electron emission efficiency of traditional vacuum optoelectronic devices.

[0022] 4. When the electric field strength generated by the applied voltage between the cathode and anode of the vertical - structure semiconductor nano - air - channel diode in the semiconductor is greater than its ionization threshold electric field, photo - generated carrier ionization multiplication occurs in the semiconductor, doubling the photocurrent, responsivity, terahertz output power, and the optical - to - terahertz conversion efficiency;

[0023] 5. The semiconductor nano - air - channel diode photomixer can be fabricated on a large - area wafer using micro - nano technology. Compared with traditional vacuum optoelectronic devices, it has the advantages of miniaturization, high integration, and large - scale batch production. Compared with UTC - PD, it has a simple structure, low requirements for material growth and processes, and is suitable for low - cost and high - yield production. Description of the Drawings

[0024] Figure 1 It is a schematic diagram of a terahertz photomixer of a vertical - structure nano - air - channel diode with a semiconductor as the cathode; wherein: 11. Semi - insulating InP substrate; 12. p - type InGaAs thin film; 13. Nano - air channel; 14. SiO2 nano - insulating layer; 15. Ni / Au anode; 16. Ti / Pt / Au ohmic contact electrode.

[0025] Figure 2 It is a schematic diagram of a millimeter - wave photomixer of a vertical - structure nano - air - channel diode with a semiconductor as the anode; wherein: 21. SiO2 substrate; 22. n - type Si thin film; 23. Nano - air channel; 24. HfO2 nano - insulating layer; 25. Hf / Au cathode; 26. MXene thin - film contact electrode.

[0026] Figure 3 It is a schematic diagram of a terahertz photomixer of a horizontal - structure nano - air - channel diode with a semiconductor as the cathode; wherein: 31. Semi - insulating Si substrate; 32. p - type Ge thin film; 33. Nano - air channel; 34. SiO2 insulating layer; 35. ITO transparent anode; 36. Graphene ohmic contact electrode.

[0027] Figure 4 It is a schematic diagram of a terahertz photomixer of a horizontal - structure nano - air - channel diode with a semiconductor as the anode. Wherein: 41. Substrate; 42. Semiconductor thin film; 43. Nano - air channel; 44. Insulating layer; 45. Cathode; 46. Contact electrode. Detailed Embodiments

[0028] The technical solutions in the present invention will be further described below with reference to the drawings and embodiments.

[0029] Embodiment 1

[0030] A terahertz photomixer of a vertical - structure nano - air - channel diode with a semiconductor as the cathode, as Figure 1As shown, the substrate in this embodiment is a semi-insulating InP substrate. Above the substrate is a p-type InGaAs thin film. The thickness of the lower region, which serves as the first mesa, is 50 nm, and the thickness of the higher region, which serves as the second mesa, is 100 nm. An ohmic contact electrode composed of a Ti / Pt / Au thin film is provided on the first mesa. A 50-nm-thick SiO2 nano-insulating layer is provided on the second mesa, and above the SiO2 nano-insulating layer is a Ni / Au anode. A nano-air channel is formed between the edge of the SiO2 nano-insulating layer, the InGaAs thin film, and the NiAu anode. The channel length is determined by the thickness of the SiO2 nano-insulating layer.

[0031] The working process of this device is as follows: A negative bias voltage is applied to the ohmic contact electrode of the p-type InGaAs thin film. A depletion region is generated in the p-type InGaAs thin film under the reverse bias voltage, and an inversion layer is formed near its surface to form a 2DEG. Under the illumination of two near-infrared lights in the vicinity of the 1550-nm band with a wavelength difference in the terahertz frequency band, the photo-generated electrons in the depletion region drift to the 2DEG layer under the action of a large electric field, increasing its concentration. During this process, ionization collisions and multiplication occur, and at the same time, the Coulomb repulsion force between electrons also increases. The electrons at the edge of the 2DEG cross the surface barrier and are emitted into the nano-air channel under the action of the internal electron Coulomb repulsion force and the large electric field between the anode and cathode. The surface electrons at other positions are directly field-emitted into the nano-air channel under the large electric field and quickly transit to the anode to be collected, forming a photocurrent. This photocurrent is modulated by the superposition optical field of the two near-infrared lights. Its oscillation frequency is equal to the frequency difference of the two incident near-infrared lights, and the changes in amplitude and phase are also synchronized with the amplitude and phase of the superposition optical field of the two near-infrared lights. The high-frequency photocurrent is transmitted through the electrode to loads such as dipole antennas or coplanar waveguides to radiate terahertz waves, thus realizing the generation of terahertz signals by optical mixing. In the device of this embodiment, the 3dB bandwidth limited by the carrier transit time can reach 2 THz.

[0032] Embodiment 2

[0033] A vertical structure nano-air channel diode millimeter-wave optical mixer with a semiconductor as the anode, as Figure 2 shown, the substrate in this embodiment is an insulating SiO2 substrate. Above the substrate is an n-type Si thin film. The thickness of the lower region, which serves as the first mesa, is 100 nm, and the thickness of the higher region, which serves as the second mesa, is 200 nm. An ohmic contact electrode composed of MXene thin film is provided on the first mesa. A 70-nm-thick HfO2 nano-insulating layer is provided on the second mesa, and above the HfO2 nano-insulating layer is an Hf / Au cathode. A nano-air channel is formed between the edge of the HfO2 nano-insulating layer, the Si thin film, and the Hf / Au cathode. The channel length is determined by the thickness of the HfO2 nano-insulating layer.

[0034] The working process of the device is as follows: A positive bias voltage is applied to the ohmic contact electrode of the n-type Si thin film. The n-type Si generates a depletion region under a reverse bias voltage, and an inversion layer is generated near its upper surface to form 2DHG. Under the illumination of two visible lights with wavelengths near 633 nm and a wavelength difference in the millimeter wave band, the photo-generated holes in the depletion region drift to the 2DHG layer under the action of a large electric field to increase its concentration. During this process, ionization collisions and multiplication occur. At the same time, 2DEG is induced on the lower surface of the Hf / Au cathode on the other side of the nano-insulating layer and the air channel, and the Coulomb repulsion force between the electrons therein also increases. The electrons at the edge of the 2DEG cross the surface barrier under the action of the internal electron Coulomb repulsion force and the large electric field between the anode and the cathode and are emitted into the nano-air channel. The surface electrons at other positions are directly field-emitted into the nano-air channel under the large electric field and quickly transit to reach the Si anode and are collected to form a photocurrent. This photocurrent is modulated by the superimposed optical field of the two near-visible lights. Its oscillation frequency is equal to the frequency difference of the two incident visible lights, and the changes in amplitude and phase are also synchronized with the amplitude and phase of the superimposed optical field of the two visible lights. The high-frequency photocurrent is transmitted through the electrode to loads such as dipole antennas or coplanar waveguides to radiate millimeter waves or terahertz waves, thereby realizing the generation of optically mixed millimeter wave signals. The 3dB bandwidth limited by the carrier transit time in the device of this embodiment can reach 225 GHz.

[0035] Embodiment 3

[0036] A horizontal structure nano-air channel diode terahertz optical mixer with a semiconductor cathode, as Figure 3 shown. In this embodiment, the substrate is a semi-insulating Si substrate. Above the substrate is a 1μm-thick SiO2 insulating layer. Above the SiO2 insulating layer are a cathode composed of a 100nm-thick p-type Ge thin film and an anode composed of 100nm-thick Ti / Au respectively. The nano-air channel with a spacing of 20nm is between the p-type Ge thin film cathode and the Ti / Au anode. Above the p-type Ge thin film is a graphene transparent ohmic contact electrode.

[0037] The working process of the device is as follows: A negative bias voltage is applied to the ohmic contact electrode of the p-type Ge thin film. Under the illumination of two near-infrared lights with wavelengths near 1310 nm and a wavelength difference in the terahertz band, the photo-generated electrons on the surface of the Ge cathode overcome the surface barrier under the action of a large electric field, are field-emitted into the nano-air channel, and quickly transit laterally to reach the anode and are collected to form a photocurrent. This photocurrent is modulated by the superimposed optical field of the two near-infrared lights. Its oscillation frequency is equal to the frequency difference of the two incident near-infrared lights, and the changes in amplitude and phase are also synchronized with the amplitude and phase of the superimposed optical field of the two near-infrared lights. The high-frequency photocurrent is transmitted through the electrode to loads such as dipole antennas or coplanar waveguides to radiate millimeter waves or terahertz waves, thereby realizing the generation of optically mixed millimeter wave and terahertz signals. The 3dB bandwidth limited by the carrier transit time in the device of this embodiment can reach 10 THz.

[0038] Example 4

[0039] A horizontal-structured nano-air-channel diode terahertz optical mixer with a semiconductor anode, as Figure 4 shown. In this embodiment, the substrate is a semi-insulating GaAs substrate. Above the substrate is a 1-μm-thick Si3N4 insulating layer. Above the Si3N4 insulating layer are an anode composed of a 100-nm-thick n-type GaAs thin film and a cathode composed of 100-nm-thick Hf / Au respectively. The nano-air channel with a spacing of 10 nm is between the n-type GaAs thin film anode and the Hf / Au anode. Above the n-type GaAs thin film is an ITO transparent ohmic contact electrode.

[0040] The working process of this device is as follows: A positive bias voltage is applied to the ohmic contact electrode of the n-type GaAs thin film. Under the illumination of two near-infrared lights near the 850-nm band with a wavelength difference in the terahertz frequency band, a large number of photo-generated holes are generated and accumulated on the surface of the GaAs anode side, and a large number of electrons are induced on the surface of the Hf / Au cathode side opposite to the nano-air channel. The electrons on the surface of the Hf / Au cathode side overcome the surface barrier under the action of a large electric field, field emit into the nano-air channel, and quickly cross laterally to reach the GaAs anode and are collected, forming a photocurrent. This photocurrent is modulated by the superposition optical field of the two near-infrared lights. Its oscillation frequency is equal to the frequency difference of the two incident near-infrared lights, and the changes in amplitude and phase are also synchronized with the amplitude and phase of the superposition optical field of the two near-infrared lights. The high-frequency photocurrent is transmitted through the electrode to loads such as dipole antennas or coplanar waveguides to radiate millimeter waves or terahertz waves, thus realizing the generation of optical mixing millimeter wave and terahertz signals. The 3-dB bandwidth limited by the carrier transit time in the device of this embodiment can reach 20 THz.

[0041] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not restrictive. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the purpose and scope of the technical solutions of the present invention, and they should all be covered by the scope of the claims of the present invention.

Claims

1. A semiconductor nano-air-channel diode optical mixer, characterized in that: The optical mixer includes a substrate, a semiconductor thin film, a nano-insulating layer, a first electrode, and a second electrode; The optical mixer is of a vertical structure or a horizontal structure; When the optical mixer is of a vertical structure, a semiconductor thin film is disposed above the substrate; the semiconductor thin film is a stepped thin film with a height difference, wherein the region with a lower height is the first mesa, and the region with a higher height is the second mesa; a first electrode is disposed above the first mesa; a nano-insulating layer is disposed above the second mesa; a second electrode is disposed above the nano-insulating layer; the region occupied by the nano-insulating layer is smaller than the second mesa and the second electrode, and a nano-air channel is formed outside the nano-insulating layer; When the optical mixer is of a horizontal structure, an insulating layer is disposed above the substrate; a second electrode is disposed in the right region above the insulating layer, a semiconductor thin film is disposed in the left region, and a first electrode is disposed above the semiconductor thin film; a nano-air channel is formed between the semiconductor thin film and the second electrode; The function of the semiconductor thin film is to absorb light and convert it into electron-hole pairs; when the semiconductor thin film serves as the cathode, p-type doping is used, and a negative bias voltage is applied to the first electrode; when the semiconductor thin film serves as the anode, n-type doping is used, and a positive bias voltage is applied to the first electrode.

2. The semiconductor nano air-channel diode optical mixer according to claim 1, wherein: The first electrode and the second electrode are made of a highly conductive material.

3. The semiconductor nano-air channel diode optical mixer according to claim 2, wherein: The materials of the first electrode and the second electrode are metal, metalloid, transparent conductive film or graphene.

4. The semiconductor nano air-channel diode optical mixer according to claim 2, characterized in that: The length of the nano-air channel is less than the average free path of electron scattering in air.

5. The semiconductor nano-air-channel diode optical mixer according to claim 4, characterized in that: When the optical mixer is of a vertical structure, the electric field intensity generated in the semiconductor by the bias voltages applied to the first electrode and the second electrode is greater than its ionization threshold electric field, and optical carrier ionization multiplication occurs in the semiconductor, doubling the photocurrent, responsivity, terahertz output power, and optical-to-terahertz conversion efficiency.

6. The semiconductor nano-air-channel diode optical mixer according to claim 4, characterized in that: When the optical mixer is of a vertical structure, when the reverse bias voltage applied to the semiconductor is greater than its strong inversion bias voltage, a strong inversion layer is generated on the semiconductor surface, forming a two-dimensional electron gas or a two-dimensional hole gas, and zero-barrier electron emission is achieved by the edge electrons of the two-dimensional electron gas under the action of the Coulomb repulsion force of the internal electrons.

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