10. Appendix

676 Appendix C
from the slopes of these plots are 0.33 eV and 0.26 eV, respectively. Notice
that the emission activation energy Eem is not the same as the capture activation
energy Ecap. Their difference Eem Ecap is equal to the thermal ionization
energy which is defined as the energy required to thermally ionize carriers out
of the deep center. The relation between the three energies will become clear
when we discuss the large lattice relaxation model.
ELECTRON EMISSION OR CAPTURE RATE (sec -1)
10 7
10
10
10
10
10
5
3
10
-1
-3
-5
0.33 eV
0.26 eV
Al Ga As(Te)
0.36 0.64
ELECTRON EMISSION
RATE
ELECTRON CAPTURE
RATE
THERMAL
QUENCHING OF
PHOTOCONDUCTIVITY
0.18 eV
5 7 9 11 13 15 17
1000/T (1/K)
Fig. A4.2 The Arrhenius plots
for the emission and capture
rates of the deep centers in Al-
GaAs:Te obtained by Lang et al.
Reproduced from [Lang79a].
In the TSCAP measurement shown in Fig. A4.1(b) the total capacitance (as
distinct from the capacitance difference measured in DLTS) is measured as a
function of temperature. An increase in capacitance indicates that a smaller
number of electrons are trapped at the deep centers. The curve labeled as (1)
is the steady-state state zero-bias capacitance curve. This curve is reversible for
increasing and decreasing temperatures. Curves labeled as (2) and (3) are irreversible
with respect to temperature cycling. Curve (2) is obtained by first
cooling the sample in the dark from about 200 K to 50 K with a bias of
1 V. This positive bias causes electrons to be trapped on the deep centers
thus returning them to the neutral charged state. At the lowest temperature
the bias voltage is set to zero so that the electrons want to escape from the
deep centers. However, the emission rate at low temperature is very slow and,
therefore, the deep centers remain in a non-equilibrium state. As the sample
is warmed above 100 K the emission process is thermally activated and be-