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 decay mechanism of NEA GaN photocathode: A first-principles research

Using the first-principles method based on the density functional theory (DFT), the work function of seven different GaN (0001) (1×1) surface models is calculated. The calculation results show that the optimal ratio of Cs to O for activation is between 3:1 and 4:1. Then, Cs/O activation and stability testing experiments on reflection-mode negative electron affinity GaN photocathodes are performed. The surface model [GaN (Mg): Cs] Cs-O after being activated with cesium and oxygen is used. The experiment results illustrate that the adsorption of O contained in the residual gas increases the surface potential barrier and the reduction of the effective dipole quantity is the basic cause of the quantum efficiency decay.

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.

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.

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.

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.

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.