10. Appendix

A4.1 A Prototypical Deep Center in N-Type Zincblende-Type Semiconductors 679
iments have firmly established that the deep centers induced by pressure in
GaAs have the same properties as the DX center found in AlGaAs and, furthermore,
this conversion of shallow donors to DX centers occurred also for
other group IV donors, such as Ge and Sn, and group VI donors like S, Se
and Te. In fact, it has been shown that the effect of a hydrostatic pressure of
0.1 GPa on the DX in GaAs is almost equivalent to adding 1% of Al [Wei89].
It has been known that both pressure and alloying with Al have the similar
effect of lowering the conduction band minima at the X point of the Brillouin
zone in GaAs relative to the zone-center conduction band valley. With
sufficient pressure or Al concentration GaAs can be converted from a direct
band gap semiconductor into an indirect one. At ambient pressure the DX
level associated with donor atoms in GaAs is actually a resonant state above
the conduction band. As a result of the change in the conduction band structure
caused by alloying or by pressure, the DX level emerges from the conduction
band into the band gap and becomes the stable ground state of the
donor. Thus, one can conclude that the DX center is the result of a so-called
“shallow-to-deep transformation” of substitutional donors in GaAs induced by
changes in the conduction band structure. In this transformation the chemical
nature of the impurity plays only a secondary role.
E (eV)
2.5
2.0
1.5
Al Ga As
x 1-x
1.4
0 0.2 0.4 0.6 0.8 1.0
GaAs AlAs Mole Fraction x AlAs
Γ
L
X
E(eV)=0.54x+1.57
Fig. A4.4 The DX center energy
level (open squares) determined by
Chand et al. from temperature-dependent
Hall effect measurement on
Si doped AlGaAs as a function of
the AlAs mole fraction. For comparison
the solid curves labeled as °,
L and X show, respectively, the corresponding
dependence of the direct
band gap energy at the zone center,
the indirect band gap energies between
the conduction valleys at the
L and X points and the zone-center
valence band maximum. Reproduced
from Chand et al. [Chand84].