A layered doped gaN photocathode

Abstract

The application discloses a layered doped GaN photocathode, which comprises a substrate, a buffer layer, a GaN light absorption layer and a GaN electron transfer layer which are sequentially arranged on the substrate, and a Cs / O activation layer is adsorbed on the GaN electron transfer layer; the GaN light absorption layer is non-intentionally doped or p-doped, and the GaN electron transfer layer is p-doped and has a doping concentration higher than that of the GaN light absorption layer. By synergistically controlling the p-GaN doping concentration and thickness, the layered doped GaN photocathode structure can effectively avoid the problem that high crystal quality and high concentration p-doped GaN photocathode materials are difficult to be compatible in epitaxial growth, and can improve photoelectron generation, transfer and emission, thereby improving the quantum efficiency of the photocathode.

Description

TECHNICAL FIELD

[0001] The application relates to a layered doped GaN photocathode and belongs to the technical field of semiconductor materials. BACKGROUND

[0002] GaN material is a semiconductor material with excellent properties such as wide band gap, corrosion resistance, high temperature resistance and radiation resistance. A photocathode made of GaN material has advantages such as high quantum efficiency, small dark current and concentrated electron energy distribution, and therefore has wide application prospects in the fields of ultraviolet photoelectric detection such as vacuum electron source, ultraviolet vacuum detection, flame detection and ultraviolet laser radar detection.

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

[0004] At present, the quantum efficiency of the GaN photocathode is limited by the performance of the material itself and the preparation technology, and there are problems such as low p-type doping activation efficiency and high epitaxial film defect density. Therefore, some researchers propose to improve the quantum efficiency of the GaN photocathode by gradient doping and exponential doping. Gradient doping or exponential doping requires the p-type doping concentration of GaN to gradually increase from the surface to the inside, that is, the doping concentration on the surface of the cathode is low and the doping concentration in the inside is high. However, this method still has some problems:

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

[0006] (2) The gradient doping or exponential doping structure requires accurate control of the p-type doping concentration and thickness of GaN in each small area, which puts higher requirements on the epitaxial growth process, increases the manufacturing difficulty and complexity. SUMMARY

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

[0008] The purpose of the present application is achieved by the following technical solutions:

[0009] The layered doped GaN photocathode has a substrate as the bottom layer, a buffer layer, a GaN light absorption layer and a GaN electron migration layer sequentially grown on the substrate, and a Cs / O activation layer adsorbed on the GaN electron migration layer, wherein the GaN light absorption layer is non-intentionally doped or p-doped, and the GaN electron migration layer is p-doped and has a doping concentration higher than that of the GaN light absorption layer. The GaN light absorption layer is non-intentionally doped or p-doped to increase the electron diffusion length, increase the photoelectron concentration, and make the electrons move to the electron migration layer under the built-in electric field and diffusion effect.

[0010] Preferably, the thickness T of the GaN electron migration layer and the doping concentration N of the GaN electron migration layer satisfy the following relationship: A

[0011]

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

[0013] Preferably, the buffer layer is made of AlN or AlGaN material, and has a band gap greater than that of GaN. The heterojunction formed by the light absorption layer and the buffer layer has a high potential barrier, which blocks the diffusion of electrons to the substrate and promotes the movement of electrons in the absorption layer to the surface of the photocathode under the built-in electric field generated by the negative polarization charge at the heterojunction interface.

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

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

[0016] Preferably, the doping concentration of the GaN electron migration 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 migration layer is greater than 100 nm.

[0018] Theoretical simulation results show that by purposefully designing the thickness and doping concentration of the electron migration layer and the light absorption layer, adjusting the band structure of the GaN photocathode, the photoelectron transport efficiency can be improved by the built-in electric field, the electron diffusion length can be increased by reducing the defect recombination centers in the GaN body, and the quantum efficiency of the GaN photocathode can be ultimately improved.

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

[0020] (1) Principle of low-concentration p-type doping in GaN absorber layer: Due to the strong spontaneous polarization and piezoelectric polarization characteristics of III-V nitride materials such as GaN, AlN, and AlGaN, a high density of negative polarization charge exists at the interface of GaN / AlN or GaN / AlGaN heterojunctions, inducing the generation of a high concentration of two-dimensional hole gas (2DHG), raising the energy band of the heterojunction interface, such as... Figure 1 As shown. This invention utilizes the polarization characteristics of GaN / AlN or GaN / AlGaN heterojunctions. Even under low-concentration doping or unintentional doping conditions, the GaN light-absorbing layer still maintains a high hole concentration, thus preserving a high energy level in the band structure. To facilitate the illustration of the polarization effect on the GaN photocathode band structure, simulations were performed under both polarized and non-polarized conditions. For example... Figure 2 As shown in the figure, the solid and dashed lines correspond to the conduction and valence band diagrams of the GaN photocathode under unpolarized and polarized conditions, respectively. Under unpolarized conditions, the band structure near the GaN / AlN interface is recessed, forming an electron trap. Electrons must overcome a certain potential barrier to reach the surface. Under polarized conditions, the two-dimensional hole gas at the GaN / AlN interface raises the band structure, making the band structure within the absorption layer higher than that of the electron diffusion layer, thus preventing the formation of electron traps. In addition, the low concentration of p-type doping in the absorption layer also facilitates the extension of the depletion region of the electron migration layer into the photocathode, forming a relatively uniform and directional built-in electric field inside the photocathode, promoting electron movement to the surface.

[0021] (2) Principle of controlling p-type doping concentration and thickness in GaN electron migration layer: After sufficient Cs / O activation, p-GaN with higher doping concentration forms a negative electron affinity potential, that is, the work function of p-GaN surface is lower than the work function in bulk, and the difference between the two is the surface potential Vs. At the same time, a width t is formed on the GaN surface. d The depletion region is characterized by downward bending of the energy bands. Based on this premise, this invention proposes a highly doped p-GaN surface layer as an electron migration layer, controlling its thickness T to be less than or equal to the width t of the depletion region. d This creates a built-in electric field in the light-absorbing layer, promoting the rapid migration of photogenerated electrons to the cathode surface. Electron migration layer thickness T, depletion region width t d and doping concentration N A The relationship is:

[0022]

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

[0024] By synergistically controlling the p-GaN doping concentration and thickness, the layered doped GaN photocathode structure provided by the application effectively avoids the problem that high crystal quality and high concentration p-type doping of a traditional structure GaN photocathode material are difficult to be compatible in epitaxial growth, and improves the photoelectron generation, migration and emission in three aspects, and realizes the effect of improving the quantum efficiency of the photocathode.

[0025] The application has the following beneficial effects:

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

[0027] (2) The thickness of the electron migration layer is less than the width of the depletion region, a built-in electric field pointing to the substrate is formed in the GaN, the transport efficiency of photo-generated electrons in the cathode is improved, and the probability of electron escape is increased;

[0028] (3) The thickness of the electron migration layer is thin, and the influence of impurity scattering on the electron is small, which is beneficial to improve the electron mobility;

[0029] (4) Benefiting from the greater electron diffusion length and built-in electric field, even if the thickness of the light absorption layer is increased, the effective transport of internal electrons to the surface can be ensured. 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 DRAWINGS

[0030] Figure 1 It is a schematic diagram of a layered doped GaN photocathode structure and a corresponding energy band diagram.

[0031] Figure 2 It is a GaN photocathode energy band simulation diagram under polarization and non-polarization conditions.

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

[0033] Figure 4 It is a layered doped GaN photocathode electric field intensity and energy band diagram: when the electron migration layer is 5nm, the inner layer doping concentration is changed.

[0034] Figure 5 It is a layered doped GaN photocathode energy band diagram: the thickness of the light absorption layer is changed. DETAILED DESCRIPTION

[0035] The technical solutions in the embodiments of the present application will be described clearly and completely in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of the present application.

[0036] Embodiment 1

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

[0038] In order to obtain the best structure parameters, firstly, a band simulation model is established. As shown in Figure 3 , the electron transport layer doping concentration is set to 1×10 19 cm -3 , the surface potential Vs is 1.2ev, and the calculated corresponding depletion region width t d =10.9nm, the electron transport layer thickness is set to 10nm, the buffer layer is unintentionally doped AlN, and the thickness is 2μm. The GaN light absorption layer thickness is fixed to 140nm, and the influence of different doping concentrations on the band of the photocathode is simulated. As shown in Figure 3 , with the decrease of the p-type doping concentration of the light absorption layer, 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 expands into the light absorption layer, and the built-in electric field strength increases. Under the premise of ensuring the flat band, the p-type doping concentration of the light absorption layer is 1×10 18 -5×10 18 cm -3 , which has a larger adjustment range and is conducive to actual growth control. In order to minimize the impurity scattering caused by doping and improve the electron mobility, the optimal value of the p-type doping concentration of the light absorption layer under this condition is 1×10 18 cm -3 .

[0039] Embodiment 2

[0040] A layered doped GaN photocathode structure, on the basis of Figure 3 (a) the photocathode structure, the electron transport layer thickness is further reduced while keeping the total GaN thickness of 150nm unchanged, and the band and electric field are simulated to optimize the doping concentration parameters of the light absorption layer. As shown in Figure 4 , when the electron transport layer is much smaller than the depletion region width, the holes in the light absorption layer will be largely depleted, resulting in a high-intensity built-in electric field in the entire GaN layer. Changing the p-type doping concentration of the absorption layer can adjust the built-in electric field strength and distribution, as shown inFigure 4 As shown in (a), the p-type doping concentration of the light-absorbing layer ranges from 1×10⁻⁶. 17 cm -3 Reduced to 1×10 16 cm -3 At that time, the electric field strength near the electron migration layer region is reduced, but the internal electric field strength is greatly increased, and the overall distribution of the built-in electric field is more uniform. This is beneficial for transporting electrons that are far from the surface of the GaN light absorption layer out through the action of the electric field. Figure 4 (b) shows the band structure corresponding to the electric field strength. Decreasing the p-type doping concentration causes the holes in the GaN layer to be depleted as well. However, thanks to the two-dimensional hole gas induced by the polarization charge at the GaN / AlN interface, the GaN band structure only experiences a steepening of the tilt, without any dip that would hinder electron overflow. Conversely, it increases the built-in electric field near the heterojunction, promoting electron movement towards the surface. Therefore, to improve electron mobility and diffusion length, the optimal p-type doping concentration for the light-absorbing layer under this condition is 1 × 10⁻⁶. 16 cm -3 In fact, it is close to the level of unintentionally doped GaN.

[0041] Example 3

[0042] A layered doped GaN photocathode structure has an electron migration layer thickness of 5 nm, much smaller than the depletion region width, and a p-type doping concentration of only 1 × 10⁻⁶ in the light absorption layer. 16 cm -3 Therefore, the probability of electrons inside GaN being scattered by impurities is low, while maintaining a high built-in electric field strength. The combined effect of these two factors results in a large electron diffusion length and mobility in the GaN photoabsorbing layer. Based on this, further increasing the thickness of the photoabsorbing layer can, on the one hand, increase the number of photons absorbed by the photocathode, and on the other hand, reduce the defect density during GaN epitaxial growth, improving the quality of the thin film crystal, both of which contribute to improving the quantum efficiency of the photocathode. Figure 5 As shown, due to the effects of the Cs / O activation layer and polarization charge, the energy bands of the GaN surface and GaN / AlN interface are "pinned" to fixed positions. Therefore, increasing the thickness of the light-absorbing layer reduces the band tilt slope, but the built-in electric field always exists, albeit with a reduced intensity. This layered doped GaN photocathode structure, due to its high electron diffusion length and built-in electric field pointing towards the substrate, ensures effective electron transport to the surface even with increased light-absorbing layer thickness.

[0043] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A layered doped GaN photocathode, wherein the bottom layer is a substrate, on which a buffer layer, a GaN light-absorbing layer, and a GaN electron migration layer are sequentially grown, and a Cs / O activation layer is adsorbed on the GaN electron migration layer, characterized in that, The GaN light-absorbing layer is unintentionally doped or p-type doped, and the GaN electron migration layer is p-type doped with a higher doping concentration than the GaN light-absorbing layer. After sufficient Cs / O activation, p-GaN forms a negative electron affinity potential, meaning the work function at the p-GaN surface is lower than the work function in the bulk. The difference between the two is the surface potential Vs. Simultaneously, a width t is formed on the GaN surface. d The depletion region, with a doping concentration of 5 × 10⁻⁶. 18 -5×10 19 / cm 3 p-GaN is used as an electron migration layer, and its thickness T is controlled to be less than or equal to the depletion region width t. d This allows the GaN light-absorbing layer to also form a built-in electric field.

2. The GaN photocathode according to claim 1, characterized in that, The thickness T and doping concentration N of the GaN electron migration layer A The relationship is: in The vacuum permittivity, is the relative permittivity 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 1-4, characterized in that: When the GaN optical absorption layer is p-type doped, the doping concentration is 1×10⁻⁶. 16 -5×10 18 / cm 3 .

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

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