10. Appendix

680 Appendix C
Fig. A4.5 (a)–(d)The Deep Level Transient Spectroscopy (DLTS) results in GaAs:Si measured
at different pressures. From the temperature of the dip in the spectra and experimental
conditions it is possible to deduce the thermal ionization energy (EI) of the deep
center. The absence of a dip in the spectra (a)–(c) indicates the absence of any center
with EI larger than 0.2 eV. Spectrum (e) was obtained from a AlGaAs:Si sample under
similar experimental conditions except that no high pressure was applied to that sample.
From the size of the dip in the spectrum (d) an approximate concentration of the deep
center more than 10 17 cm 3 could be deduced. The inset shows the appearance of a persistent
effect in the photoconductivity in GaAs:Si under pressure exceeding 2.5 GPa. The
solid curves were measured with the sample in the dark before any light illumination
while the broken curves were measured after the sample was illuminated momentarily by
light. Reproduced from Mizuta et al. [Mizuta85].
A.4.1.4 Theoretical Models of the DX Centers
Many theoretical models of the DX centers have been proposed. At one time
there were controversies surrounding the atomic and electronic configurations
of the DX center. One such controversy involves the question of whether
there are large [Lang79a, Chadi88, Khachaturyan89] or small lattice relaxations
[Hjalmarson86, Henning87, Bourgoin89, Yamaguchi90] associated with
the formation of the DX center. Another issue is whether the DX center has