The Role of EL2 for the Mobility-Lifetime Product of Photoexcited Electrons in GaAs
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The role of EL2 for the mobility-lifetime product of photoexcited electrons in GaAs G. C. Valley Hughes Research Laboratories Malibu, Ca 90265 - USA H.J. von Bardeleben Groupe de Physique des Solides de I' Ecole Normale Sup~rieure Centre national de la Recherche Scientifique Tour 23, 2 place Jussieu 75251 Paris Cedex 05, France. H. Rajbenbach Laboratoire Central de Recherches Thomson-CSF Domaine de Corbeville 91404 Orsay Cedex, France
abstract The mobility
lifetime
products for
photo-electrons
in semi-insulating
GaAs,
which
fit
successfully the results of photorefractive studies undertaken in the presence of electric fields
are
three
orders
of
magnitude
smaller
than
those
inferred
from
transport
measurements or from the photorefractive effect with no applied electrical field. Consideration of enhanced recombination via EL2 effective-mass states linked to the Lconduction band minimum allows us to fit the dependence of the photorefractive beam coupling gain coefficient on the grating period for both AC fields and moving gratings. A cascade-capture process, which is three orders of magnitude faster than recombination by multiphonon emission from the products for field influence
of
the
strengths EL2
r
band to EL2, leads to greatly reduced mobility-lifetime
greater than 1 kV/cm. Our results establish
defect properties
on the
recombination
processes
the dominant essential
for
modelling and optimizing the photorefractive effect in semi-insulating GaAs.
Undoped semi-insulating GaAs is an important electronic material and since 1984 has been shown to have very promising properties for beam coupling, phase conjugation and optical data
processing
[1-7].
EL2
is the dominant native defect in this material with a 16 concentration of typically 2.10 cm -3. It dominates the extrinsic absorption spectrum by the photoionisation from the valence band and to the conduction band. Without an external applied field, in GaAs
however, coupling coefficients in photorefractive experiment are rather small
because of its small electro-optic coefficient. To increase the coupling coefficient
many researchers have applied external DC and AC fields to enhance charge transport and Mat. Res. Soc. Symp. Proc. Vol. 163. @1990 Materials Research Society
838
obtain large internal space-charge field grating [3,4,6,7]. This has led to larger coupling coefficients, but the results are in disagreement with theoretical predictions based on photorefractive theories and intrinsic properties of semi-insulating GaAs [7-9]. Here we explicitly demonstrate this discrepancy between theoretical prediction and experimental result in measurements made on undoped GaAs in which the defect responsible for the photorefractive effects in EL2. The results can be consistently interpreted in terms of enhanced recombination in the presence of large electric fields.
Nd:YA6 laser X,= 1.06pro
I
IS.
C,aAs
's
Fig.l: Photorefractive two-beam coupling experiment (ref.15). The optical beams photoinduce a phase volume grating in GaAs a
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