10. Appendix

Optical Spectroscopy of Shallow Impurity Centers 571
the split-off valence band, which is not unexpected since the spin–orbit interaction
is small in Si and the optical spectra are broadened appreciably.
Efforts to detect photoconductivity in n- and p-type Si at 77 K were unsuccessful,
due to the presence of large numbers of thermally excited carriers.
However, a photoconductive response was observed at liquid helium temperature.
The spectral response of relatively pure n-Si is shown in Fig. 2 [5]. The
dips in the photoconductive response between 8 and 24 Ìm correspond to lattice
vibration absorption peaks. The data yielded a donor optical ionization
energy of 0.04 eV. Photoconductivity studies were later carried out at liquid
helium temperature on Ge doped with group III and V impurities. The photoconductive
response was found to extend out to 38 Ìm, the limit of measurement
at that time [6].
Fairly complete optical studies were carried out for the group III acceptors
(B, Al, Ga and In) and for the group V donors [7,8]. Absorption spectra
for the group III centers are shown in Fig. 1. The variations in the ionization
energy (Fig. 3) are accompanied by changes in the character of the excitation
and photoionization absorption spectra. The positions of the excitation bands
for Al, Ga and In centers, unlike B, do not correspond to a hydrogenic model,
their oscillator strengths also differ appreciably from those of a hydrogenic
model (Fig. 3). These deviations, which become more pronounced on going
from B to In, are due to central cell corrections. The states with s character
have their energies and wavefunctions rather strongly modified, since their
wavefunctions are relatively large at the impurity atoms. The states with p
characters, whose wavefunctions are small at the center of the impurity atom,
are affected to a lesser degree. The agreement between the experimental ionization
energy for B and the predictions of the hydrogenic model is probably
due to a cancellation of different effects.
Relative photoconductivity
per incident photon
10
5
1
0.5
0.1
0.05
0.01
0.6 1.0 5.0 10 20 30 40
Wavelength [µm]
10
α [cm –1 ]
Fig. 2. The relative photoconductive response per incident photon of a relatively pure
n-Si sample [4]. The dips in the photoconductive response between 8 and 24 Ìm correspond
to the peaks in the optical absorption due to lattice vibrations