Quantum efficiency decay mechanism of NEA GaN
photocathode: A first-principles research
Yang Shen (沈 洋)
1,
*, Liang Chen (陈 亮)
1,2,
**, Shuqin Zhang (张淑琴)
1
,
and Yunsheng Qian (钱芸生)
2
1
Institute of Optoelectronics Technology, China Jiliang University, Hangzhou 3100 18, China
2
School of Electronic and Optical Engineering, Nanjing University of Science and Technology,
Nanjing 210094, China
*Corresponding author: 920778028@qq.com; **corresponding author: LChen@cjlu.edu.cn
Received June 12, 2015; accepted July 31, 2015; posted online September 8, 2015
Using the first-principles method based on the density functional theory (DFT), the work function of seven
different GaN (0001) ð1×1Þ 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.
OCIS codes: 040.7190, 160.1890, 230.0040, 260.7210.
doi: 10.3788/COL201513.100401.
With the rapid development of lithographic manufactur-
ing and ultraviolet (UV) detection technology, a high
performance UV photocathode is urgently needed
[1–5]
.
A GaN-based photocathode is a new type of semiconduc-
tor material with the excellent characteristics of wide
bandgap, low dielectric constant, corrosion resistance,
high temperature resistance, and radiation resistance, so
it has recently gained much interest among those who wish
to use it in high power, optoelectronic, and microelectronic
devices
[6–8]
. It has become an attractive candidate due to its
stable physical and chemical properties, high quantum ef-
ficiency (QE), low dark current, and concentrative emit-
ting electron energy distribution
[9–11]
. Because of the wide
bandgap, GaN photocathodes are naturally “solar blind”
and do not respond to visible light, so they have plenty of
uses in the fields of UV detection, fire alarm technology,
atmospheric monitoring, lithographic manufacture, and
others. These areas require not only higher sensitivity
and good emission properties but also the stability of
the QE of the cathode. But the practical application has
encountered a number of obstacles; the most important is
a stable GaN photocathode in a vacuum system in nature,
that is, the QE of the GaN photocathode attenuation
problem in a vacuum system. To take full advantage of
the excellent properties of a negative electron affinity
(NEA) GaN photocathode, it is essential for researchers
to study the reason for the QE decay, and thus improve
its stability.
A GaN photocathode activated by Cs/O can reach a
higher QE because of the NEA
[12–16]
, but after a while
the adsorption of residual gases such as oxygen in a
demountable vacuum system can act on the activated
NEA surface and influence photocathode stability. How
to well analyze the QE decay mechanism of the NEA
GaN photocathode further becomes the important part
of our study, and only knowing the reason why it declines
can we better improve the QE of the photocathodes. From
the studies of the NEA GaAs photocathode it can be found
that the QE mainly depends on the performance of the
material and the technique of preparation
[17]
. Wang et al.
have studied the influence of the p-type doping concentra-
tion on the GaN photocathode, and the optimal concen-
tration has been found
[18]
. In this Letter, we investigated
and discussed the influence of residual gas on the QE after
activation and optimized the preparation technology of
the NEA GaN photocathode.
Calculations in this Letter are performed with the
quantum mechanics program Cambridge Serial Total
Energy Package (CASTEP) code
[19]
. The generalized-
gradient approximation (GGA) parameterized by
Perdew–Burke–Ernzerhof (PBE) is adopted to calculate
the exchange-corre lation energy. The Broyden–Fletcher–
Goldfarb–Shanno (BFGS) algorithm
[20–22]
is used to relax
the structure of the crystal model
[23]
. All calculations are
carried out in reciprocal space and the atomic pseudopo-
tentials, described by the ultra-soft pseudopotential, are
generated from the following electronic configurations:
Ga:3d
10
4s
2
4p
1
, N:2s
2
2p
3
, Cs:5s
2
5p
6
6s
1
, O:2s
2
2p
4
, and
H:1s. The convergence parameters are set as follows:
energy change below 2 × 10
6
eV∕atom, force less than
0.005 eV/nm, stress less than 0.05 GPa, and change
in displacement less than 1 × 10
5
nm in an iterative
process. The Brillouin zone integral is sampled with the
Monkhorst–Pack mesh scheme and special k points
of high symmetry. After a series of tests, the energy
cutoff of the final sets of energies is set as 400 eV and
the number of k points
[24]
for a GaN (0001) surface is
set as 6 × 6 × 1.
COL 13(10), 100401(2015) CHINESE OPTICS LETTERS October 10, 2015
1671-7694/2015/100401(5) 100401-1 © 2015 Chinese Optics Letters
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