Layered doped GaN photoelectric cathode

Abstract

The invention discloses a layered doped GaN photoelectric cathode which is structurally characterized in that the bottommost layer is a substrate, a buffer layer, a GaN light absorption layer and a GaN electron migration layer are sequentially grown on the substrate, a Cs / O activation layer is adsorbed on the GaN electron migration layer, the GaN light absorption layer is unintentionally doped or p-type doped, the GaN electron migration layer is p-type doped, and the doping concentration is higher than that of the GaN light absorption layer. By cooperatively controlling the doping concentration and thickness of p-GaN, the layered doped GaN photoelectric cathode structure provided by the invention effectively avoids the problem that high crystal quality and high-concentration p-type doping of a GaN photoelectric cathode material of a traditional structure are difficult to be compatible in epitaxial growth, and is improved in three aspects of photoelectron generation, migration and emission; the effect of improving the quantum efficiency of the photocathode is achieved.

Description

Technical Field

[0001] The invention relates to a layered doped GaN photocathode, belonging to the technical field of semiconductor materials. Background Art

[0002] Gallium nitride (GaN) material is a semiconductor material with excellent properties such as wide bandgap, corrosion resistance, high temperature resistance, and radiation resistance. The photocathode made of GaN material has the advantages of high quantum efficiency, small dark current, and concentrated distribution of emitted electron energy. Therefore, it has broad application prospects in ultraviolet photoelectric detection fields such as vacuum electron source, ultraviolet vacuum detection, flame detection, and ultraviolet lidar detection.

[0003] The working principle of the GaN photocathode is based on the photoelectric effect, which mainly includes three processes: (1) GaN absorbs the energy of the incident light, causing the electrons in the valence band to jump to the conduction band; (2) The electrons in the conduction band diffuse and move to the cathode surface; (3) The electrons on the cathode surface overcome the electron affinity of GaN and escape from the surface and are emitted into the vacuum.

[0004] At present, the quantum efficiency of GaN photocathode is limited by many aspects such as the performance of the material itself and the preparation technology. There are problems such as low p-type doping activation efficiency and high defect density of epitaxial film. For this reason, some researchers have proposed to improve the quantum efficiency of GaN photocathode by gradient doping and exponential doping. The so-called gradient doping or exponential doping requires that the p-type doping concentration of GaN gradually increases from the surface to the inside, that is, the cathode surface doping concentration is low and the internal doping concentration is high. However, this method still has some problems:

[0005] (1) In order to achieve the negative electron affinity of GaN, a higher p-type doping concentration is required on the cathode surface. If the internal p-type doping concentration is further increased, on the one hand, the lattice distortion will be aggravated, which will lead to an increase in the epitaxial defect density. On the other hand, the excessively high doping concentration will lead to a compensation effect, and the p-type activation efficiency will be greatly reduced. Therefore, in fact, the p-type doping concentration inside GaN cannot be much higher than the surface, which makes the role of gradient doping or exponential doping in improving the cathode quantum efficiency limited;

[0006] (2) Gradient doping or exponential doping structures require precise control of the GaN p-type doping concentration and thickness in each small area, which places more stringent requirements on the epitaxial growth process and increases the difficulty and complexity of manufacturing. Summary of the invention

[0007] The object of the present invention is to provide a layered doped GaN photocathode.

[0008] The purpose of the present invention is achieved through the following technical solutions:

[0009] A layered doped GaN photocathode, the bottom layer of which is a substrate, on which a buffer layer, a GaN light absorption layer, and a GaN electron transport layer are sequentially grown, a Cs / O activation layer is adsorbed on the GaN electron transport layer, the GaN light absorption layer is unintentionally doped or p-type doped, and the GaN electron transport layer is p-type doped, and the doping concentration is higher than that of the GaN light absorption layer. The GaN light absorption layer is unintentionally doped or p-type doped, and its function is to increase the electron diffusion length, increase the concentration of photogenerated electrons, and make the electrons move to the electron transport layer under the action of built-in electric field and diffusion.

[0010] Preferably, the thickness T of the GaN electron transport layer is related to the doping concentration N A The relationship is:

[0011]

[0012] Where ε0 is the dielectric constant of vacuum, ε r is the relative dielectric constant of GaN, and Vs is the p-GaN surface potential.

[0013] Preferably, the buffer layer is made of AlN or AlGaN material, which has a bandgap width greater than that of GaN. The heterojunction formed by the light absorption layer and the buffer layer has a high potential barrier, which on the one hand blocks the diffusion of electrons to the substrate, and on the other hand, the negatively polarized charges at the heterojunction interface generate a built-in electric field pointing to the substrate, promoting the movement of electrons inside the absorption layer to the cathode surface.

[0014] Preferably, the substrate is made of sapphire, silicon carbide or silicon.

[0015] Preferably, when the GaN light absorbing layer is p-type doped, the doping concentration is 1×10 16 -5×10 18 / cm 3 .

[0016] Preferably, the doping concentration of the GaN electron transport layer is 5×10 18 -5×10 19 / cm 3 .

[0017] Preferably, the total thickness of the GaN light absorption layer and the GaN electron transport layer is above 100 nm.

[0018] Theoretical simulation results show that by purposefully designing the thickness and doping concentration of the electron transport layer and light absorption layer and regulating the band structure of the GaN photocathode, it is possible to use the built-in electric field to improve the transport efficiency of photogenerated electrons, reduce the defect recombination centers in the GaN body, and increase the electron diffusion length, ultimately achieving the goal of improving the quantum efficiency of the GaN photocathode.

[0019] The working principle of the present invention is explained as follows:

[0020] (1) The principle of low-concentration p-type doping in GaN absorption layer: Since III-V nitride materials such as GaN, AlN, and AlGaN have strong spontaneous polarization and piezoelectric polarization characteristics, there is a high density of negative polarization charges at the GaN / AlN or GaN / AlGaN heterojunction interface, which induces the generation of high concentration of two-dimensional hole gas (2DHG) and raises the heterojunction interface energy band, such as Figure 1 As shown. The present invention utilizes the polarization characteristics of GaN / AlN or GaN / AlGaN heterojunctions. The GaN light absorption layer still has a high hole concentration in the body under low concentration doping or unintentional doping conditions, so that the energy band maintains a high energy level position. In order to facilitate the description of the influence of the polarization effect on the GaN photocathode energy band, the photocathode energy band is simulated with or without polarization conditions. Figure 2 As shown in the figure, the solid line and dotted line correspond to the conduction band and valence band diagram of the GaN photocathode under non-polarized and polarized conditions, respectively. Under non-polarized conditions, the energy band near the GaN / AlN interface is concave, forming an electron trap. Electrons have to overcome certain potential barriers to move to the surface. Under polarized conditions, the two-dimensional hole gas at the GaN / AlN interface will raise the energy band, making the energy band in the absorption layer higher than the electron diffusion layer, avoiding the formation of electron traps. In addition, low-concentration p-type doping of the absorption layer is also conducive to the extension of the depletion region of the electron migration layer to the inside of the photocathode, forming a relatively uniform intensity and consistent direction of the built-in electric field inside the photocathode, promoting the movement of electrons to the surface.

[0021] (2) The principle of controlling the p-type doping concentration and thickness of the GaN electron migration layer: After being fully activated by Cs / O, p-GaN with a high doping concentration forms a negative electron affinity, that is, the p-GaN surface work function is lower than the work function in the body. The difference between the two is the surface potential Vs. At the same time, a width t is formed on the GaN surface. d Based on this premise, the present invention proposes that the surface layer with high doping concentration p-GaN is used as the electron migration layer, and its thickness T is controlled to be less than or equal to the depletion region width t d , so that the light absorption layer also forms a built-in electric field, promoting the rapid migration of photogenerated electrons to the cathode surface. The thickness of the electron migration layer T and the width of the depletion region t d and doping concentration N A The relationship is:

[0022]

[0023] Where ε0 is the dielectric constant of vacuum, ε r is the relative dielectric constant of GaN, Vs is the height of the p-GaN surface barrier, N A is the p-type doping concentration.

[0024] By synergistically controlling the p-GaN doping concentration and thickness, the layered doped GaN photocathode structure proposed in the present invention effectively avoids the compatibility problem of high crystal quality and high concentration p-type doping in epitaxial growth of traditional structure GaN photocathode materials, and improves the three aspects of photoelectron generation, migration and emission, thereby achieving the effect of improving the quantum efficiency of the photocathode.

[0025] The present invention has the following beneficial effects:

[0026] (1) The doping concentration of the GaN light absorption layer is low, which reduces impurity scattering and recombination centers and increases the lifetime and diffusion length of electrons;

[0027] (2) The thickness of the electron transport layer is smaller than the width of the depletion region, forming a built-in electric field pointing to the substrate inside the GaN, improving the transport efficiency of photogenerated electrons inside the cathode and increasing the probability of electron escape;

[0028] (3) The electron transfer layer is thin, and the electrons are less affected by impurity scattering, which is conducive to improving electron mobility;

[0029] (4) Thanks to the larger electron diffusion length and built-in electric field, even if the thickness of the light absorption layer is increased, the internal electrons can be effectively transported to the surface. Based on this, increasing the thickness of the GaN light absorption layer can not only increase the number of absorbed photons, but also reduce the defect density of the epitaxial film. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Schematic diagram of the layered doped GaN photocathode structure and the corresponding energy band diagram.

[0031] Figure 2 This is the simulation diagram of the GaN photocathode energy band with and without polarization conditions.

[0032] Figure 3 Schematic diagram of the layered doped GaN photocathode structure and energy band diagram: when the electron transport layer is 5nm, the doping concentration of the light absorption layer is changed.

[0033] Figure 4 The electric field strength and energy band diagram of the layered doped GaN photocathode: when the electron transfer layer is 5nm, the inner layer doping concentration is changed.

[0034] Figure 5 Energy band diagram for layered doped GaN photocathode: changing the thickness of the light absorbing layer. DETAILED DESCRIPTION

[0035] The following is a clear and complete description of the technical solutions in the embodiments of the present invention in combination with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians without creative work are within the scope of protection of the present invention.

[0036] Example 1

[0037] A layered doped GaN photocathode structure, such as Figure 1 As shown, it includes a substrate, an AlN buffer layer, a GaN light absorption layer, a GaN electron transport layer and a Cs / O activation layer in sequence.

[0038] In order to obtain the best structural parameters, the energy band simulation modeling is first performed. Figure 3 As shown, the doping concentration of the electron transport layer is set to 1×10 19 cm -3 , the surface potential Vs is 1.2ev, and the depletion region width t corresponding to this concentration can be calculated. d =10.9nm, set the thickness of the electron transport layer to 10nm, and the buffer layer is unintentionally doped AlN with a thickness of 2μm. The thickness of the GaN light absorption layer is fixed to 140nm, and simulate the effect of different doping concentrations on the photocathode energy band. Figure 3 As shown in the figure, as the p-type doping concentration of the light absorption layer decreases, the GaN conduction band gradually moves down, and the band tilt slope increases, which indicates that the holes in the light absorption layer are gradually depleted, the depletion region extends to the inside of the light absorption layer, and the built-in electric field strength increases. Under the premise of ensuring the flatness of the band, the p-type doping concentration of the light absorption layer is 1×10 18 -5×10 18 cm -3 It has a large adjustment range, which is beneficial to actual growth control. In order to minimize the impurity scattering caused by doping and improve the electron mobility, the optimal p-type doping concentration of the light absorption layer under this condition is 1×10 18 cm -3 .

[0039] Example 2

[0040] A layered doped GaN photocathode structure, in Figure 3 (a) Based on the photocathode structure, the thickness of the electron transport layer is further reduced, while the total thickness of GaN is kept unchanged at 150nm. In order to optimize the doping concentration parameters of the light absorption layer, its energy band and electric field are simulated. Figure 4 As shown in Figure 1, when the electron migration layer is much smaller than the depletion region width, the holes in the light absorption layer will be depleted in large quantities, resulting in a high-intensity built-in electric field inside the entire GaN layer. Changing the p-type doping concentration of the absorption layer can adjust the built-in electric field strength and distribution, such as Figure 4 As shown in (a), the p-type doping concentration of the light absorption layer is increased from 1×10 17 cm -3 Reduced to 1×10 16 cm -3 When the electrons are transported out of the GaN light absorption layer, the electric field strength near the electron migration layer decreases, but the internal electric field strength increases significantly, and the overall distribution of the built-in electric field becomes more uniform, which is conducive to transporting electrons far away from the surface of the GaN light absorption layer through the action of the electric field. Figure 4 (b) is the energy band diagram corresponding to the electric field intensity. Reducing the p-type doping concentration makes the holes in the GaN layer begin to be exhausted. However, thanks to the two-dimensional hole gas induced by the polarization charge at the GaN / AlN interface, the GaN energy band only tilts more sharply without any depression, which hinders the electron overflow. On the contrary, it increases the built-in electric field near the heterogeneous interface and promotes the movement of electrons to the surface. Therefore, in order to improve the electron mobility and diffusion length, the optimal p-type doping concentration of the light absorption layer under this condition is 1×10 16 cm -3 , which is actually close to the level of unintentionally doped GaN.

[0041] Example 3

[0042] A layered doped GaN photocathode structure, the thickness of the electron transport layer is 5nm, which is much smaller than the width of the depletion region, and the p-type doping concentration of the light absorption layer is only 1×10 16 cm -3 Therefore, the probability of GaN internal electrons being scattered by impurities is small, while maintaining a high built-in electric field strength. The combined effect of the two makes the GaN light absorption layer have a large electron diffusion length and mobility. Based on this, further increasing the thickness of the light absorption layer can increase the number of photons absorbed by the photocathode on the one hand, and reduce the defect density of GaN epitaxy on the other hand, improve the quality of thin film crystals, which are all helpful to improve the quantum efficiency of the photocathode. Figure 5 As shown in the figure, due to the action of the Cs / O activation layer and the polarization charge, the energy bands of the GaN surface and the GaN / AlN interface are "pinned" at fixed positions. Therefore, as the thickness of the light absorption layer increases, the slope of the energy band decreases, but the built-in electric field always exists, but the intensity decreases. This layered doped GaN photocathode structure has a high electron diffusion length and a built-in electric field pointing to the substrate, so even if the thickness of the light absorption layer is increased, it can ensure the effective transport of internal electrons to the surface.

[0043] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A layered doped GaN photocathode, wherein the bottom layer of the structure is a substrate, on which a buffer layer, a GaN light absorption layer, and a GaN electron transport layer are sequentially grown, and a Cs / O activation layer is adsorbed on the GaN electron transport layer, characterized in that: The GaN light absorption layer is unintentionally doped or p-type doped, and the GaN electron migration layer is p-type doped, and the doping concentration is higher than that of the GaN light absorption layer.

2. The GaN photocathode according to claim 1, characterized in that: The thickness T of the GaN electron transport layer and the doping concentration N A The relationship is: Where ε0 is the dielectric constant of vacuum, ε r is the relative dielectric constant of GaN, and Vs is the p-GaN surface potential.

3. The GaN photocathode according to claim 2, characterized in that: The buffer layer is AlN or AlGaN.

4. The GaN photocathode according to claim 3, characterized in that: The substrate is made of sapphire, silicon carbide or silicon.

5. The GaN photocathode according to any one of claims 1 to 4, characterized in that: When the GaN light absorption layer is p-type doped, the doping concentration is 1×10 16 -5×10 18 / cm 3 .

6. The GaN photocathode according to claim 5, characterized in that: The doping concentration of the GaN electron transport layer is 5×10 18 -5×10 19 / cm 3 .

7. The GaN photocathode according to any one of claims 1 to 4, characterized in that: The total thickness of the GaN light absorption layer and the GaN electron migration layer is above 100 nm.

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