Resolution properties of reflection-mode exponential-doping
GaAs photocathodes
Honggang Wang
a,b
, Yunsheng Qian
a,
n
, Yujie Du
a
, Xinlong Chen
a
,
Jian Liu
a
, Benkang Chang
a
a
School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, China
b
School of Information and Electrical Engineering, Ludong University, Yantai, Shandong 264025, China
article info
Available online 11 April 2014
Keywords:
Resolution
Exponential doping
Modulation transfer function
Reflection-mode GaAs
abstract
Resolution properties of reflection-mode exponential-doping and uniform-doping GaAs
photocathodes were calculated and comparatively analyzed by using the modulation
transfer function obtained by establishing and solving the two-dimensional continuity
equation. The calculated results show that compared with the uniform-doping GaAs
photocathode, the exponential-doping structure can improve significantly not only the
resolution, but also the quantum efficiency of the photocathode. This improvement differs
from the approach for achieving high resolution by reducing the emitting layer thickness
T
e
and the electron diffusion length L
D
or by increasing the recombination velocity of
back-interface S
V
, which results in low quantum efficiency. Furthermore, the improve-
ment of resolution and quantum efficiency of the reflection-mode exponential-doping
GaAs photocathode is due the effect of the built-in electric field on electron transport and
on lateral diffusion.
& 2014 Els evier Ltd. All rights reserved.
1. Introduction
Most imaging devices require operation of the photo-
cathode in transmission mode, which conveniently sepa-
rates the optical and electro-optical functions [1–10].
Reflection-mode photocathodes have specific advantages
over their transmission-mode counterparts, namely, sim-
pler fabrication of the photocathode due to the non-
criticality of thickness, amenability to the use of reflecting
substrates for substantially enhancing the sensitivity, and
reduced internal reflections inside the tube [11]. There is
also sufficient incentive for evaluating more methods for
using reflection-mode photocathodes in imaging devices.
Electro-optical devices employing reflection-mode
photocathodes have been reported by Johnson et al. [11],
Palmer et al. [12], and Tremsin et al. [13].
The resolution of a photocathode is an important
parameter in its imaging applications [14]. Not only must
the photocathode detect incident light, but also it must
faithfully convert the optical image into a photoelectron
image. During conversion of the light signal into an
electronic signal, the resolution is degraded within the
photocathode mainly because of lateral diffusion of photo-
electrons. Previous works have found that [15,16] degra-
dation of the resolution may be partially offset if there is
an electric field along the opposite direction of the photo-
electron motion. Figs. 1 and 2 show that the exponential-
doping structure forms the band-bending region, which
linearly slopes downward, generating a constant built-in
electric field [17]. Notably, an exponential-doping photo-
cathode with constant built-in electric field satisfies the
requirement well and achieves higher quantum efficiency,
which has been experimentally verified [17–19]. Because
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Materials Science in Semiconductor Processing
http://dx.doi.org/10.1016/j.mssp.2014.03.006
1369-8001/& 2014 Elsevier Ltd. All rights reserved.
n
Corresponding author. Nanjing university of science and technology,
School of Electronic and Optical Engineering, 200, Xiaolingwei, Nanjing,
Jiangsu 210094, China. Tel.: þ86 02584315437.
E-mail address: yshqian@mail.njust.edu.cn (Y. Qian).
Materials Science in Semiconductor Processing 24 (2014) 215–219