Vol 18 No 10, October 2009
c
° 2009 Chin. Phys. Soc.
1674-1056/2009/18(10)/4541–06
Chinese Physics B
and IOP Publishing Ltd
Distribution of carriers in gradient-doping
transmission-mode GaAs photocathodes grown
by molecular beam epitaxy
∗
Zhang Yi-Jun(张益军), Chang Ben-Kang(常本康)
†
, Yang Zhi(杨 智),
Niu Jun(牛 军), and Zou Ji-Jun(邹继军)
Institute of Electronic Engineering and Optoelectronic Technology, Nanjing University of Science and Technology,
Nanjing 210094, China
(Received 11 January 2009; revised manuscript received 7 April 2009)
The gradient-doping structure is first applied to prepare the transmission-mode GaAs photo cathode and the
integral sensitivity of the sealed image tube achieves 1420 µA/lm. This paper studies the inner carrier concentration
distribution of the gradient-doping transmission-mode GaAs photo cathode after molecular beam epitaxy (MBE) growth
using the electrochemical capacitance-voltage profiling. The results show that an ideal gradient-doping structure can be
obtained by using MBE growth. The total band-bending energy in the gradient-doping GaAs active-layer with doping
concentration ranging from 1×10
19
cm
−3
to 1×10
18
cm
−3
is calculated to be 46.3 meV, which helps to improve the
photoexcited electrons movement toward surface for the thin epilayer. In addition, by analysis of the band offsets, it is
found that the worse carrier concentration discrepancy between GaAs and GaAlAs causes a lower back interface electron
potential barrier which decreases the amount of high-energy photoelectrons and affects the short-wave response.
Keywords: GaAs photocathode, gradient doping, molecular beam epitaxy, carrier concentration
distribution
PACC: 7960, 7280E, 7320A
1. Introduction
Negative electron affinity (NEA) GaAs photo-
cathodes activated by the coadsorption of cesium and
oxygen have already found widespread application
in modern night vision image intensifiers.
[1−3]
The
GaAs photocathodes have been typically grown by
metal-organic chemical vapor deposition (MOCVD)
and molecular beam epitaxy (MBE).
[4,5]
The MOCVD
technique is widely used due to its fast growth veloc-
ity and high throughput, whereas the MBE technique
offers a precise control over the epilayer thickness and
doping concentration level, which provides superior
epitaxial quality. In the MBE growth of p-type GaAs
photocathodes, the element beryllium (Be) is expected
to be a suitable acceptor impurity for heavy doping
and steep impurity profiles, because of its unique char-
acteristics, such as near-unity sticking co efficient, low
vap or pressure and high solubility in GaAs.
[6]
Photoemission from GaAs NEA photocathodes
is described as a three-step process of optical ab-
sorption, electron transport to the surface, and es-
cape across the surface into vacuum.
[7]
In order to
improve the performance of GaAs NEA photocath-
odes, such as electron diffusion length and surface
electron escape probability, a gradient-doping struc-
ture on reflection-mode GaAs photocathodes was
put forward,
[8,9]
which can obtain higher quantum
efficiency than the uniform-doping one. However,
there are few reports concerning the gradient-doping
transmission-mode photocathodes. In this paper, we
first apply this gradient-doping structure to prepare
the transmission-mode GaAs photocathodes. In order
to know the inner actual gradient-doping structure,
we study the carrier concentration distribution in such
structural epilayers after the MBE growth with elec-
trochemical capacitance-voltage (ECV) profiling. Be-
sides, the inherent mechanism of electron transport in
gradient-doping transmission-mode GaAs photocath-
odes is analyzed in detail.
2. Experiment
The gradient-doping transmission-mode GaAs
sample was grown on the high quality p-type GaAs
(100) oriented substrate with Be doping by using an
∗
Project supported by the National Natural Science Foundation of China (Grant Nos 60678043 and 60801036).
†
Corresponding author. E-mail: bkchang@mail.njust.edu.cn
http://www.iop.org/journals/cpb
http://cpb.iphy.ac.cn