Negative Electron Affinity GaN Photocathode Research Articles

Negative electron affinity GaN photocathode with Mg delta-doping

Mg δ-doping GaN material is grown by MOCVD and activated by Cs/O in UHV subsequently, then NEA GaN photocathode can be obtained. Compared with the p-type uniform doping, the δ-doping NEA GaN photocathode shows better photoemission performance, and the photocurrent is twice as the uniform. The QE of δ-doping and uniform doping are 24% and 10% at 5.16 eV, respectively. The hole concentration of δ-doping and uniform doping are 8.7 × 1017 cm−3 and 3.3 × 1017 cm−3, while better crystal quality for δ-doping is proved by the XRD results simultaneously. Profited from the higher hole concentration and better crystal quality, the P and L of δ-doping NEA GaN photocathodes are raised to 0.51 and 258 nm, respectively. After introduction of O during the activation, the photocurrent of uniform doping GaN photocathode has obvious improvement, however, it is even worse than before for the δ-doping. On the basis of [GaN(Mg)- Cs]:[O-Cs] model, it is found that the surface roughness is beneficial for improving photocurrent during the Cs/O alternating activation stage.

Quantum efficiency for reflection-mode varied doping negative-electron-affinity GaN photocathode

As a new kind of ultraviolet photocathode material, the negative-electron-affinity (NEA) GaN photocathode needs to further improve its photoemission performance and the stable performance in practical applications. Under the limit of GaN photocathode material growth level, how to further improve the quantum efficiency of cathode is an important problem. The varied doping technology can help to solve the problem under such circumstances. According to the photoemission mechanism of varying doping NEA GaN photocathode material, the built-in electric field formulas and the quantum efficiency formulas for reflection-mode varied doping NEA GaN photocathode are given. The preliminary structure of varied doping NEA GaN photocathode is designed. The varied doping material sample is divided into four layers according to the doping concentration. Using the self-developed experimental equipment, the varied doping GaN photocathode sample is activated with Cs/O. The activation process and the change characteristics of photocurrent for varied doping NEA GaN photocathode are discussed. At the beginning, the photocurrent is increased steady with the introduction of Cs, then the Cs kill phenomenon appears in the presence of excessive Cs. After the introduction of O, the photocurrent value starts to rise again. The spectral response of varied doping GaN photocathode is tested in situ after activation, and the quantum efficiency values ranging from 240 nm to 354 nm are obtained. On the basis of the obtained experimental results of quantum efficiency, combining to the typical quantum efficiency curve from University of California, the characteristics of quantum efficiency curves are analyzed. The results show that the quantum efficiency value for reflection-mode varied doping NEA GaN photocathode can reach 56% at 240 nm because of the built-in electric field, yet the quantum efficiency maximum value for uniform doping GaN photocathode is only 37% at 230 nm. The tested quantum efficiency maximum value of varied doping NEA GaN photocathode is improved much more than that of the uniform doping GaN photocathode. In a wider range of the incident light wavelength, the quantum efficiency of varied doping NEA GaN photocathode is relatively stable, and the excellent properties of varied doping GaN photocathode are confirmed. The reason why the value of quantum efficiency decreases with the increase of incident light wavelength is given. First, the photon energy decreases with the increase of incident light wavelength. Second, the incident light is absorbed from the front surface of cathode for reflection mode. In addition, the quantum efficiency curves of varied doping GaN photocathode show obvious sharp cut-off characteristics near the threshold, and the sharp cut-off characteristic is necessary for high detection sensitivity. The property of negative electron affinity for varied doping GaN cathode material after successful activation is also proved by the sharp cut-off feature.

Theoretical study for heterojunction surface of NEA GaN photocathode dispensed with Cs activation

For the disadvantages of conventional negative electron affinity (NEA) GaN photocathodes activated by Cs or Cs/O, new-type NEA GaN photocathodes with heterojunction surface dispensed with Cs activation are investigated based on first-principle study with density functional theory. Through the growth of an ultrathin [Formula: see text]-type GaN cap layer on [Formula: see text]-type GaN emission layer, a [Formula: see text]–[Formula: see text] heterojunction is formed on the surface. According to the calculation results, it is found that Si atoms tend to replace Ga atoms to result in an [Formula: see text]-type doped cap layer which contributes to the decreasing of work function. After the growth of [Formula: see text]-type GaN cap layer, the atom structure near the [Formula: see text]-type emission layer is changed while that away from the surface has no obvious variations. By analyzing the E-Mulliken charge distribution of emission surface with and without cap layer, it is found that the positive charge of Ga and Mg atoms in the emission layer decrease caused by the cap layer, while the negative charge of N atom increases. The conduction band moves downwards after the growth of cap layer. Si atom produces donor levels around the valence band maximum. The absorption coefficient of GaN emission layer decreases and the reflectivity increases caused by [Formula: see text]-type GaN cap layer.

A DFT study on NEA GaN photocathode with an ultrathin n-type Si-doped GaN cap layer

Due to the drawbacks of conventional negative electron affinity (NEA) GaN photocathodes activated by Cs or Cs/O, a new-type NEA GaN photocathodes with heterojunction surface dispense with Cs activation are proposed. This structure can be obtained through the coverage of an ultrathin n-type Si-doped GaN cap layer on the p-type Mg-doped GaN emission layer. The influences of the cap layer on the photocathode are calculated using DFT. This study indicates that the n-type cap layer can promote the photoemission characteristics of GaN photocathode and demonstrates the probability of the preparation of a NEA GaN photocathode with an n-type cap layer.

GaN(Mg)-Cs]:[O-Cs] model for the Negative Electron Affinity GaN (0001) surface

Because of the limitations of the existing Negative Electron Affinity (NEA) photocathode surface emission models, [GaN(Mg)-Cs]:[O-Cs] photoemission model based on dual-dipole model is established, which can explain the photoemission mechanism of the NEA GaN photocathode well. Cs, O adsorption process on the GaN (0001) surface during the activation is discussed, and the GaN(Mg)-Cs dipole layer is found having a unified direction which is conducive for photoelectron escaping, so photocurrent has a significant increase when Cs is introduced. There is not a unified direction for the O-Cs dipole layer. Because of the surface defects, part of the O-Cs dipoles have the direction which is conducive for photoelectron escaping, and the photocurrent has a modest growth after introducing O. Finally, the performance of GaN and GaAs photocathode are compared based on the photoemission model, and the reason of better stability of NEA GaN photocathode is interpreted.

Theoretical study of cesium and oxygen activation processes on GaN (0001) surface

The structure properties and work function of reflection-mode GaN photocathodes are investigated by using first-principles calculation within density-function theory (DFT). Seven different GaN (0001) (1×1) surface models are used in this paper to simulate cesium and oxygen activation process. Before starting the surface models calculations, clean bulk GaN surface is first optimized, when compared with the models before optimization, the correctness of the calculations method is verified. Next, change of work function of different surface models caused by adsorption is calculated and analyzed, the results show that over-cesiuminizd atmosphere is not only benefit for work function declining but also conductive to the formation of negative electron affinity (NEA), the optimal ratio of Cs to O for activation is between 3:1 and 4:1, excess Cs or O modules increase the work function and undermine the photocathode. Then a series of experiments are performed to verify the calculation results. First of all, NEA GaN photocathode activation and evaluation system is established, “yo-yo” activation method is verified. Then, an activation experiment is performed on high quality p-type Mg-doped reflection-mode GaN substrate grown by metal organic chemical vapor deposition (MOCVD), the photocurrent is controlled and monitored by multi-information on-line monitoring system, the results are consistent with our calculation models. Finally, combined with the data provided by Stanford University, schematic energy band variation of the GaN photocathode after only-Cs and Cs/O activations is given. Our result opens the possibility to engineer the activation properties of GaN photocathode.

Effect of surface potential barrier on electron escape probability of GaN photocathode

Aiming to inverstigate the effect of surface potential barrier on electron escape probability of negative electron affinity GaN photocathode, the electron escape probability is calculated by using the Boltzmann distribution and the method of transfer matrix based on Airy function. It is found that barrier I is a key influencing factor of electron escape probability, while barrier II has a limited influence. The activation photocurrent curve of the transmission-mode GaN photocathode is measured by using our built activation and evaluation experimental system of NEA GaN photocathode. The obvious increase of electron escape probability can be achieved mainly by activating Cs only. The increase of electron escape probability is not large in Cs/O activation process with only Cs activated sufficiently. The theoretical calculations are in good agreement with the photocurrent curves from experimental test. The reason is that the contribution of activating only Cs to the reducing of vacuum level for obtaining NEA state is much larger than that of activating Cs/O.

Effects of NEA GaN photocathode performance parameters on quantum efficiency

Using research on the negative electron affinity GaN photocathode photoemission mechanism, we obtained the reflective-type and transmission-type GaN photocathode quantum efficiency formulas. The influence on quantum efficiency and sensitivity of integral of cathode performance parameters such as electron surface escape probability P, electron diffusion length LD, absorption coefficient α, back-interface recombination rate Sv and cathode thickness Te, were analyzed using these formulas. It was found that to obtain negative electron affinity GaN optoelectronic cathodes with high quantum efficiencies, we must constantly improve cathode activation technologies and the surface escaping probability of cathode. Also, we must increase the electronic diffusion length, reduce the rate of compounding, and find the optimal thickness of the cathode transmit layer based for the specific electronic diffusion length.

Comparison between gradient-doping and uniform-doping GaN photocathodes

In order to enhance the quantum efficiencies of negative electron affinity (NEA) GaN photocathodes, gradient-doping reflection-mode GaN photocathodes are grown by metal organic chemical vapor deposition (MOCVD)at doping concentrations of 1×1018cm-3, 4×1017cm-3, 2×1017cm-3 and 6×1016cm-3 from the body to the surface, with the thickness of each doping region being about 45nm and the total thickness of GaN 180 nm. The gradient-doping GaN photocathodes are activated in an ultra-high vacuum system and are compared with two kinds of uniform-doping GaN photocathodes whose thicknesses are both 150 nm and doping concentrations are 1.6×1017cm-3 and 3×1018cm-3 separately. The results show that both the photocurrent growth rate and the maximum photocurrent of the gradient-doping GaN photocathodes are greater than those of the uniform-doping GaN in the Cs/O activation process, and the multi-test system measured maximum quantum efficiency of the gradient-doping NEA GaN photocathode is about 56% which is as high as the double of the uniform-doping. Calculations show that the energy band bendings of the gradient-doping GaN photocathodes are 0.024eV, 0.018eV and 0.031eV from the body to the surface, a larger electron drift and diffusion length are gained due to the built-in electric field formed by the energy band bending, because of the 0.073eV total energy band bending the photoelectrons reaching the surface have higher energies and pass through the surface barrier more easily. Therefore the gradient-doping NEA GaN photocathodes have greater quantum efficiencies.

Research and development of GaN photocathode

Negative electron affinity GaN photocathode with greatly advanced photoelectricity performance is described. The research of GaN photocathode focuses on the three points, i. e. , quantum yield, electron energy distribution and surface model, in the last decade. The domestic research of GaN photocathode is still in its infancy, the basic theory is not established, and preparation technology is not mature. In this paper we review emission mechanism, material growth, surface cleaning, activation process optimization, varied-doping structure design and stability of GaN photocathode. The latest experimental results confirm that the fabrication technology of GaN photocathode is feasible.

Study of spectral response characteristics of negative electron affinity GaN photocathode

Aiming at determining the quantum efficiency of fully activated negative electron affinity(NEA) GaN photocathodes, we have investigated the spectral response of NEA GaN photocathodes by using a dedicated ultraviolet spectral response measurement instrument. The quantum efficiency curve of reflection-mode NEA GaN photocathode has been obtained in the band region from 230nm to 400nm. The experimental results show that the quantum efficiency of reflection-mode NEA GaN photocathode reaches up to 37.4% at 230 nm, and at 365 nm, which is the threshold of GaN photocathode, the quantum efficiency remains 3.75%. A sharp cutoff characteristic with over two orders of magnitude degradation from 230 nm to 400 nm has been observed. Based on the former research results, the factors influencing quantum efficiency were also comprehensively analyzed in the paper.

Quantum efficiency decay mechanism for reflection-mode negative electron affinity GaN photocathode

Aimming at the decay tendency of reflection-mode negative electron affinity (NEA) GaN photocathode and the different decay speeds of quantum efficiency corresponding to the different wave bands, and referring to the decay tendency of quantum efficiency curve provided by foreign authors for reflection-mode NEA GaN photocathode, the quantum efficiency decay mechanism for reflection-mode NEA GaN photocathode was studied. The surface model ［GaN (Mg) : Cs］: O-Cs for GaN photocathode after being activated with cesium and oxygen was used. And the change of surface barrier in the decay course of quantum efficiency was considered. The reduction of the effective dipole quantity is the basic reason causing quantum efficiency reduction. And it is the change of surface I, II barrier shape that causes the difference of dropping speeds of quantum efficiencies corresponding to different wave bands.

Activation mechanism of negative electron affinity GaN photocathode

The photocurrent curves during either Cs or Cs/O activation of GaN photocathode were tested by using dedicated experimental system for activation and evaluation of negative electron affinity （NEA） photocathode. Aiming at explaining the formation of NEA property of GaN photocathode and according to the rule of photocurrent change during activation period and the surface model of a fully activated photocathode，the activation mechanism for NEA GaN photocathode was studied. The experiment results show that：the obvious NEA property is be induced in GaN photocathode mainly due to the activation by Cs. The increase extent of photocurrent is not large after introducing O during Cs/O activation process for GaN photocathode. The NEA property formation reasons of GaN photocathode after being activated successfully can be well explained using the double dipole layer model ［GaN（Mg）：Cs］：O—Cs.