The decay of carbon, luminescence in liquid-encapsulated Czochralski-grown semi-insulating GaAs
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I. INTRODUCTION
Carbon has been identified1"4 as the dominant shallow acceptor in undoped semi-insulating (SI) GaAs grown by the liquid-encapsulated Czochralski (LEC) technique from pyrolytic BN crucibles. The presence of an appropriate amount of carbon, together with deep donor levels, commonly known as EL2,2 is in fact essential to obtain reliable semi-insulating material. Semi-insulating GaAs is useful as a substrate for GaAs devices, where device isolation and low parasitic capacitance are important for full integration on a single chip. Carbon in epitaxial GaAs has been studied extensively using photoluminescence spectroscopy and the shallow acceptor level CAs has an ionization energy of —26 meV.5'6 The luminescence lines of carbon are well known to be at 1.493 and 1.490 eV, which have been attributed to the conduction band-to-acceptor (B-A) transition, and the donor-to-acceptor (D-A) transition, respectively.6"9 In a previous paper10 we have reported that the photoluminescence decay of the —1.49 eV emission obeys a double exponential and a power-law behavior with the exponent p = 1.4 at 4.2 K. We have seen that the decay time constant of the slower component, b, of the double exponential decay follows the Arrhenius relation rb = Tbo exp(AEb/kT), and gives an activation a)Current
address: Department of Chemistry, Tulane University, New Orleans, Louisiana 70118. J. Mater. Res., Vol. 5, No. 2, Feb 1990
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energy of AEb = 15 ± 1 meV (where k is the Boltzmann constant and T is the temperature). Jonscher and de Polignac11 have given a review on the decay of luminescence in solids where the p values of the power laws are between 0.5 and 2. From our studies of the temperature dependence of the decay for several samples of LEC SI GaAs (with carbon concentration ranging from =1014 to =1016 cm"3) in the temperature range of 4.2 to 34 K, we have found that in the temperature range of 18 ^ T < 30 K, the exponent p of the power-law decay is linearly dependent on temperature according to the expression p{T) = /3T - 1, where /3 in turn is a linear function of the concentration of the CAs acceptors. The/? value increases up to =3.5 at —28 K. This seems to contradict the more familiar expression of p{T) = akT + I,12 where akT = 1 for an ideal bimolecular decay and a = 0 for a solid with uniform trap distribution. We suggest, instead, that the observed large values of p at higher temperatures are due to two parallel processes involving detrapping and quenching of the luminescence signal. We will also present a theory that can explain some aspects of the temperature dependence of p. II. EXPERIMENTAL PROCEDURE
Prior to measurements, all the LEC SI GaAs samples (courtesy of Cominco Ltd., Electronic Materials Div., Trail, British Columbia) were chemo-mechanically polished on lint-free tissues (LABX 170) using freshly © 1990 Materials Research Society
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C. K. Teh, J. Tuszyriski, and F. L. Weichman: Decay of carbon luminescence
prepared 2% Br2 in
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