COL 10(11), 110401(2012) CHINESE OPTICS LETTERS November 10, 2012
Research on surface photovoltage spectroscopy for GaAs
photocathodes with Al
x
Ga
1−x
As buffer layer
Shuqin Zhang (ÜÜÜÔÔÔ)
1,2
, Liang Chen ( )
2∗
, and Songlin Zhuang (BBBttt)
1
1
Institute of Optical Electronic Information and Computer Engineering,
University of Shanghai for Science and Technology, Shanghai 200093, China
2
Institute of Optoelectronics Technology, China Jiliang University, Hangzhou 310018, China
∗
Corresponding author: 52571497@qq.com
Received March 26, 2012; accepted May 17, 2012; posted online September 14, 2012
Surface photovoltage spectroscopy equations for cathode materials with an Al
x
Ga
1−x
As buffer layer are
determined in order to effectively measure the body parameters for transmission-mode (t-mode) photo-
cathode materials before Cs-O activation. Body parameters of cathode materials are well fitted through
experiments and fitting calculations for the designed Al
x
Ga
1−x
As/GaAs structure material. This inves-
tigation examines photo-excited performance and measurements of body parameters for t-mode cathode
materials of different doping structures. It also helps study various doping structures and optimize struc-
ture designs in the future.
OCIS codes: 040.7190, 160.1890, 230.0040, 260.7210.
doi: 10.3788/COL201210.110401.
After Cs-O activation, the surface barriers of GaAs pho-
tocathodes can become negative electron affinity (NEA),
driving photo-excited electro ns to tunnel through surface
barriers into a vacuum. Thus, NEA GaAs photocathodes
have been widely used in night-vision image intensifiers,
photomultiplier tube s, and polarized electron sources due
to their excellent wavelength response, high quantum
efficiency, and good spin polarization
[1]
. NEA photo-
cathodes are of two types: a reflection-mode (r-mode)
cathode, in which the incident light and electron emission
surface are on the same side, and a transmission-mode (t-
mode) cathode, in which the incident light and electron
emission surface are on different sides. In practical ap-
plications, NEA photocathodes often operate in t-mode,
but the measurement techniques for t-mode cathode ma-
terials before activation are inadequate
[2]
.
As the buffer layer matching the lattice between the
Si
3
N
4
antireflective film a nd GaAs active layer, the
Al
x
Ga
1−x
As layer helps reflect the photo-excited elec-
trons to the active layer and finally promotes quantum
efficiency for NEA photocathodes. In previous studies,
the mea surements for GaAs photocathodes mainly de-
pend on the spectral response curve (SRC) after activa-
tion. However, the parameters of GaAs photocathodes
cannot be effectively measur ed by SRC alone, because
the performances of cathodes after activation are influ-
enced not only by material parameters mainly co nnected
with the doping structure and epitax ial growth tech-
nique, but als o by surface barriers mainly related to Cs-O
activation techniques. Surface photovoltage spectroscopy
(SPS) is connected only to the performance of c athode
materials. Thus, in recent res e arch, SPS has been widely
used to measure material par ameters, such as electron
diffusion length and interface recombination velocity, for
cathode materials with an r-mode structure. A compar-
ative research between SPS before activa tio n and SRC
after activatio n has also exa c tly fitted the surface escape
probability
[3]
. The SPS fitting theory is inadequate for
the t- mode material, which has a more complex struc-
ture than the r-mode cathode. Thus, the SPS research
for cathode materials with the Al
x
Ga
1−x
As buffer layer
can help in further building the SPS theory fo r t-mode
photocathodes
[4]
.
The main difference between the t-mo de and r-mode
structures is the Al
x
Ga
1−x
As buffer layer. Figure
1 shows the complete t- mode ca thode module in the
glass/Si
3
N
4
/Al
x
Ga
1−x
As/GaAs/pole structure. Thus,
we focused on the Al
x
Ga
1−x
As/GaAs structure material,
which simplified the SPS research by removing other lay-
ers o f the t-mode cathode module. The cathode material
was designed by molec ular beam epitaxial (MBE) with
p-type beryllium Be doping. Figure 2 shows the mate-
rial structure, where the GaAs substrate has been grown
with high-quality p-type doping concentration of about
1.0×10
18
cm
−3
. T he Al
x
Ga
1−x
As buffer layer was de-
signed with a Be doping concentration of 2.0×10
18
cm
−3
and Al mole fraction of 0.63.
The Al mole fraction is 0.63, because the lattice con-
stant of Al
x
Ga
1−x
As material is calculated by
[5]
a
Al
x
Ga
1−x
As
= 5.6 533 + 0.078x. (1)
When the Al mole fraction x=0.63, the lattice constant
of the Al
x
Ga
1−x
As ma terial is 5.7024×10
−10
m in accor-
dance with that of GaAs materia l at 5.6533×10
−10
m.
First, the cathode material was allowed to pass succes-
sively through acetone, hydrofluoric acid, and absolute
Fig. 1. Diagram of the t-mode photocathode module.
1671-7694/2012/110401(4) 110401-1
c
2012 Chinese Optics Letters