10. Appendix

674 Appendix C
A4.1.2 Historical Background
The DX center was discovered in 1979 by Lang and coworkers [Lang79a] in
the ternary alloys n-AlxGa1xAs with Al concentration x 0.22. As shown in
4.2.2, Group IV dopants like Te substituting atoms on the Group VI sublattice
of III-V semiconductors like GaAs form hydrogenic donors. Lang et al. found
that the substitutional Te atoms suddenly behave more like deep centers in
AlxGa1xAs when x is 0.22. For example, the energy required to thermally
ionize the Te impurities (known as the thermal ionization energy) increases
from the hydrogenic donor binding energy of ∼5 meV by more than one order
of magnitude to ∼0.1 eV. The center exhibits a higher-energy, metastable
(or long-lived) and conducting state which can be excited optically. As a result,
samples containing these centers exhibit persistent photoconductivity at low
temperatures (such as T 100 K), i.e. their conductivity is greatly increased
by light irradiation but, unlike ordinary photoconductivity, the sample remains
in this conducting state for a very long time even after the light is turned
off. These unusual properties of the Te donors in AlGaAs have been demonstrated
by Lang et al. with two different kinds of capacitance transient techniques.
These techniques are known as Deep Level Transient Spectroscopy
(or DLTS) and Thermally Stimulated Capacitance (TSCAP). Their results are
shown in Fig. A4.1.
DLTS Signal (arb. units)
TOTAL DIODE CAPACITANCE (pF)
+
0
-
80
60
40
20
0
iii
h
i
ii
(a) DLTS
Al xGa 1-xAs(Te)
x=0.36
(b) TSCAP
0 100 200 300
TEMPERATURE (K)
Fig. A4.1 The DLTS(a) and thermally
stimulated capacitance (TSCAP) results
(b) in AlGaAs:Te obtained
by Lang et al. Reproduced from
[Lang79a].