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