JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS Vol. 19, No. 7 - 8, July – August 2017, p. 522 - 527
Resolution characteristics of transmission-mode
exponential-doping GaN photocathodes
HONGGANG WANG
a,b,c,d*
, JUNJU ZHANG
b,d
, GANG WANG
a
, QINFENG XU
a
, JIAN LIU
b,d
, XUGUANG ZHAO
a
a
School of Information and Electrical Engineering, Ludong University, 264025, Yantai, CHN
b
Ministerial Key Laboratory of JGMT, Nanjing University of Science and Technology, 210094, Nanjing, CHN
c
Jiangsu Key Laboratory of Spectral Imaging & Intelligent Sense, 210094, Nanjing, CHN
d
School of Electronic and Optical Engineering, Nanjing University of Science and Technology, 210094, Nanjing, CHN
According to the electron transport equation for the transmission-mode uniform-doping GaN photocathode, we have obtained
the electron transport equation for the transmission-mode exponential-doping GaN photocathode. And then through solving
this equation, the expression of modulation transfer function for an exponential-doping GaN photocathode is determined.
Subsequently, the resolution characteristics of transmission-mode exponential-doping and uniform-doping GaN
photocathodes are calculated and comparatively analyzed. Simultaneously, the quantum efficiencies of both GaN
photocathodes are given. These calculated results show that the exponential-doping structure can upgrade remarkably not
only the resolution but also the quantum efficiency of a negative electron affinity GaN photocathode, compared with the
uniform-doping structure. This upgradation differs from the approach for high resolution by shortening the thickness of
emission layer
and the diffusion length of electron
or by increasing the recombination velocity of back-interface
,
which leads to a low quantum efficiency. Furthermore, the upgradation of resolution and quantum efficiency for a
transmission-mode exponential-doping GaN photocathode result mainly from the facilitation of the electron transport and
constraint of the lateral diffusion by the built-in electric field.
(Received August 3, 2016; accepted August 9, 2017)
Keywords: GaN photocathode, Resolution, Exponential-doping, Modulation transfer function
1. Introduction
Negative electron affinity (NEA) GaN photocathodes
find wide usage in ultraviolet (UV) detection for its high
quantum efficiency, and become an ideal photocathode
used in ultraviolet image intensifiers [1]. Concretely, the
UV image intensifiers employing transmission-mode or
reflection-mode GaN photocathode have been developed
[2]. However, most research into a NEA GaN photocathode
has focused on its quantum efficiency, preparation process
and energy distribution [3-7], and relatively little attention
has been paid to the resolution characteristics.
In image intensification applications, the role of the
resolution characteristics of a photocathode is very
important [8].This is mainly because the photocathode must
not only detect the incident light but also faithfully convert
the light image to a photoelectron image. Precisely during
this conversion, the resolution is degraded within the
photocathode mainly due to the lateral diffusion of
photoelectrons. Some meaningful work [9-11] indicates
that the degradation of resolution may be offset to some
degree, if there is an electric field which is contrary to the
direction of photoelectrons transport towards the surface of
a NEA photocathode. As can be seen from Figs. 1 and 2,
the exponential -doping structure shapes the bent-band
region that linearly slopes downwards and then generates a
constant built-in electric field [12]. Therefore, an
exponential-doping GaN photocathode just meet the
requirement of offsetting the degradation of resolution.
Additionally, this photocathode can achieve higher
quantum efficiency, which has been experimentally
verified [13, 14]. Fig. 3 shows that the diameter of
dispersion circle formed by the lateral diffusion of
photoelectrons at an exponential-doping GaN photocathode
surface is smaller than that at a uniform-doping GaN
photocathode surface, since the latter cannot generate a
built-in electric field. Naturally, the resolution of GaN
photocathode would be upgraded by the electron drift
motion which results from this electric field. It will be
important in further discussions to determine the
dependence of resolution on the parameters of a GaN
photocathode. Besides, the quantum efficiency of a GaN
photocathode, which is an another important indicator for
an UV image intensifier, should also be considered. In other
words, the relationship between the quantum efficiency and
the resolution must be researched. To this end, using the
modulation transfer function (MTF), we have obtained a
family of curves that describe the above relationship among
variables. Accordingly, comparative analysis of the
resolution characteristics of transmission-mode
exponential-doping and uniform-doping GaN
photocathodes is presented, and the corresponding values
of quantum efficiencies are given in this paper.
2. Derivation of an MTF Expression
MTF is a standard measure of the resolution
characteristics of an imaging system, in this case, a
photocathode [8]. In this section, by establishing and then