Strained Gaasp Photocathode With GaAs
Quantum Well.
Yu.Yashin1, Yu.Mamaev1, A.Rochansky1 and D.Vinokurov2
1- State Poly technical University, Polytekhnicheskaya 29,
195251, St, Petersburg, Russia
2 - loffe Physico-Technical Institute, RAS, St, Petersburg, 194021, Russia
Abract. By varying of the phosphorous contents "x" and "y" at the GaAsi.xPx/GaAsi.yPy
cathodes they can be tuned to the wavelength, corresponding to maximum light power of the
certain accelerator laser system. The parameters of strained GaAsP sample have been modified
to enhance the quantum yield value at polarization maximum. The modification consisted of the
incorporation of heavily doped thin GaAs quantum well layer at the top part of the structure. At
the polarization maximum the yield enhancement of up to ten times has been achieved.
INTRODUCTION
Strained GaAsi.xPx/GaAsi.yPy photocathodes, which we have developed [1,2], have
been already applied in the source of polarized electrons attached to the MAMI
accelerator in Mainz [3]. The measurement of polarization transfer from the electron
to the neutron in the quasielastic reaction at 855 MeV has been performed
successfully with such cathodes installed. In more than 1000 hours of beamtime the
source produced a 20 jiA electron beam with a polarization of 75%.
It turned out that the sources at different accelerators are equipped with various
lasers. By varying of the phosphorous contents "x" at the strained overlayer GaAsi.xPx
and "y" at the GaAsi.yPy buffer, such cathodes can be tuned to the wavelength,
corresponding to maximum light power of the certain accelerator laser system (see
fig.l). The values of the phosphorus fraction x and y were designed with help of
computer "Band - Edge" program, developed by A. Subashiev, to have welcome
energy gap Eg of the strained overlayer GaAsi_xPx , high coherent strain and, thus,
sufficient energy splitting 8def (about 60 meV) of the Heavy holes and Light holes
bands. The samples were grown in the horizontal MOCVD reactor at the top of
commercial GaAs (001) wafer. The growth of the epitaxial structures was carried out
at the reactor pressure 50 Torr. Trimethylgallium (TMG) and trimethylindium
(TMIn) were used as sources of III group elements; 30% PH3 -H2 and 20% AsH3-H2
mixtures were used as the sources of V group elements. Growth temperature was
varied from 600 to 750° C and carrier gas flow rate was varied from 3 to 11 1/min.
TMGa and TMIn bubblers temperatures were kept constant at -10° C and +18° C,
respectively. TMGa bubbler pressure was 1050 Torr. Low pressure in the growth
reactor made it possible to use reduced pressure in TMIn bubbler and, hence, to obtain
increased vapor concentration of TMIn, which has the low pressure of the saturation
vapor (approx. 1.8 Torr at room temperature). We kept the pressure in TMIn bubbler
at 2000 Torr level. The ratio of the III and V group elements in the vapor phase was
varied from 50 to 200. Growth rate was typically 5-10 A/sec.
CP675, Spin 2002:15th Int'l. Spin Physics Symposium and Workshop on Polarized Electron
Sources and Polarimeters, edited by Y. L Makdisi, A. U. Luccio, and W. W. MacKay
© 2003 American Institute of Physics 0-7354-0136-5/03/$20.00
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10-4
- 20
_
LU
- 10
10'
650
700
750
800
850
Light wavelength, nm
FIGURE 1. Electron spin polarization (solid symbols) and quantum yield (open symbols) as a function
of excitation energy for the GaAsPx/GaAsPy strained samples with various phosphorous fractions "x"
and "y". Room temperature. Sample 1 (x=0.08, y=0.38) - circles, sample 2 (x=0.12, y=0.36) - squares,
sample 3 (x=0.18, y=0.37) - up triangles.
Layer
Thickness
Doping
GaAs quantum well
20 nm
gradient doping of up to
5«1019 cm 3 Mg
GaAso.9iPo.09
strained overlayer
140 nm
5«1018cm3Mg
GaAso.6sPo.32 buffer
1.0 jam
SL 10 pairs
GaAso.5sPo.45
GaAso.85Po.i5
GaAso.6sPo.32
GaAso.sPo.2
GaAso.9Po.i
7nm
7nm
500 nm
500 nm
500 nm
GaAs(lOO) - substrate
0.5 mm
Uniform
Mg
Doping
MO18 cm3
Intrinsic
Table I. Structure of MOCVD grown GaAsP photocathode with GaAs quantum well.
1007
550
600
650
700
750
800
850
900
Excitation light wavelength, nm
FIG. 2. Electron spin polarization (solid symbols) and quantum yield (open symbols) as a function of
excitation energy for the GaAsP strained sample with (circles) and without (triangles) thin GaAs
quantum well at room temperature.
= 827 nm
Room temperature
LU
10'J
Quantum Yield, %
FIGURE 3. Polarization evolution upon the degradation of the GaAs/GaAs0.9iPo.o9/GaAs0.68Po.32
sample at room temperature. Excitation light wavelength 827 nm.
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Strained GaAsP photocathodes allowed to achieve the polarization a little less than
90%. Nevertheless, rather low value of quantum yield at polarization maximum was
still a disadvantage of such cathodes. To make the photocathodes features better the
parameters of the samples once more have been improved on the base of X-ray,
Raman and polarized photoluminescence studies. The modification consisted of the
incorporation of heavily doped thin GaAs quantum well layer at the top part of the
structure, based upon GaAsP strained overlayer, as it was discussed in [4]. The goal
was to enhance electron emission from strained wide-gap low doped overlayer
through heavily doped thin GaAs quantum well layer (one can call it as "field
assistant emission"). Table I shows the composition of a photocathode under
consideration. In the sample the intermediate layers with graded phosphorus fraction
"z" serve to adjust the lattice parameters of the wafer and the buffer layer, and to
withdraw unwelcome strain in the last one. The existence of the intermediate
GaAsPo.45/GaAsP0.i5 Superlattice is of crucial importance for the growing of the
photocathodes with rather thick high quality strained overlayers. Thorough
characterisation of MOCVD grown heterostructures with AlGalnAs strained
overlayers by the Transmission Electron Microscopy and X-ray diffraction techniques
has shown [5] that the intermediate SL significantly diminishes the initial density of
structural defects, and, hence, influences on the process of elastic strain relaxation in
lattice mismatched epitaxial structure. As a result the main part of thick lattice
mismatched overlayer remains crystal lattice perfection.
The set of spectral and temperature experiments has been realised with the
modified GaAs/GaAso.9iPo.o9/GaAso.6sPo.32 and GaAso.9iPo.o9/GaAso.6sPo.32 strained
MOCVD grown heterostructures both at room and 130K temperatures. All
measurements have been performed at the experimental set-up [6], which includes the
polarized electron source and the spin detector and is based upon the Russian
commercial UHV system USU-4. It has an additional cryogenic pump and two
chambers separated by a valve. The smaller one has an activation system, Auger
analyser and the load-lock system and equipped with a manipulator with cooling and
temperature control systems. The bigger chamber serves as a chamber for mini-Mott
polarimetry. All experiments are performed under computer control. Fig. 2 shows
P(A,) and quantum yield Y(A,) curves both for GaAs/GaAs0.9iPo.o9/GaAs0.6sPo.32 and
GaAso.9iPo.o9/GaAso.6sPo.32 cathodes at room temperature.
One can see that the polarization maximum value for GaAso.9iPo.o9/GaAso.6sPo.32
cathode is higher, than for GaAs/GaAso.9iPo.o9/GaAso.6sPo.32 one. But the value of
quantum yield at the polarization maximum for GaAs/GaAso.9iPo.o9/GaAso.6sPo.32
cathode is about ten times higher, than in the case of GaAs0.9iPo.o9/GaAs0.6sPo.32
cathode, which means that thin GaAs quantum well is in fact very effectively
improves the electrons escape conditions. The reason of it is the absence of a potential
barrier, even at extremely high doping of the GaAs quantum well. At the same time
highly doped GaAs quantum well helps to achieve really Negative Electron Affinity
surface and, hence, high quantum yield. Fig 3 illustrates the polarization and quantum
yield evolution upon the degradation of the GaAs/GaAso.9iPo.o9/GaAso.6sPo.32 sample
at room temperature, excitation light wavelength being equal to 827 nm. A
considerable increase of a polarization can be explained by high polarization of the
"hot" electrons, which have high probability to be escaped prior the spin relaxation.
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The depolarization of such electrons in GaAs QW is less, than for slow electrons. It is
seen that upon the quantum yield decreasing of about ten times, the polarization value
increases of up to 86%, which means that the initial electron polarization is really very
high. Further perfection and optimisation of the GaAsP strained structures is
underway.
CONCLUSIONS
The tuning of the GaAsP photocathodes has been experimentally demonstrated.
The value of quantum yield of strained GaAsP sample has been improved by
incorporating of a thin heavily doped GaAs layer. At the polarization maximum the
yield enhancement of up to ten times has been achieved.
ACKNOWLEDGMENTS
This work was supported by INTAS under grant 99-00125, CRDF under grant
RP1-2345-ST-02 and Russian Fond for Basic Research under grant 00-02-16775.
REFERENCES
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3. Drescher, P. et al., Appl. Phys. A 63, 203 (1996).
4. Maruyama T. et. al, "Investigation of the charge limit phenomenon in GaAs photocathodes", SPIN 2000
proceedings, AIP, Melville, New York, 976 (2001).
5. Mamaev, Yu. et al., "Photocathodes for Spin - Polarized Electron Source with Strained AlGalnAs Layers ",
Proc. of Int. Workshop on Polarized beams and Polarized Gas Targets, June 1995, Cologne, Germany, World
Scientific, 303 (1996).
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(2000).
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