10. Appendix

Optical Spectroscopy of Shallow Impurity Centers 569
Optical Spectroscopy of Shallow Impurity Centers
Elias Burstein
University of Pennsylvania, Philadelphia, USA
In the fall of 1948, Frank Isakson, head of the Physics Section of the Office
of Naval Research, was a frequent visitor at the Naval Research Laboratory,
where I was a member of the Crystal Branch. During one of our frequent discussions
of projects of mutual interest, he informed me about the Navy’s interest
in developing an infrared (IR) photoconductor with a response beyond
7 Ìm, the long wavelength limit of PbS films, an intrinsic photoconductor developed
in Germany during World War II. The properties of the III–V semiconductors
were still unknown at that time. In the summer of 1949 I had the
good fortune of being able to attend the annual Modern Physics Symposium
at the University of Michigan, one of a series of symposia that started in 1928.
The lecturers that summer were Luis Alvarez (High Energy Physics), Richard
Feynman (Path Integral Method), Frederick Seitz (Solid State Physics) and
Gordon B.B. Sutherland (Infrared Spectroscopy of Solids).
In his lectures on semiconductors, Seitz discussed the nature of the impurity
levels in Si and Ge and summarized the thermal ionization energies of
group III acceptors and group V donors that had been obtained by Pearson
and Bardeen at Bell Telephone Laboratories [1] from data on the temperature
dependence of the carrier densities derived from resistivity and Hall measurements.
He also discussed their conclusions that the ionization energies of the
group III acceptors (0.048 eV) and group V donors (0.045 eV) were in reasonable
agreement with a simple effective-mass hydrogen model. It was at that
point in the lecture that the idea came to me to make use of the photoionization
of un-ionized hydrogenic impurity centers in Si and Ge as the basis for
IR detectors.
Shortly after returning to Washington, DC, I went to see John Bardeen,
who provided me with several Si samples. Together with John J. Oberly, James
W. Davisson and Bertha Henvis, I started measurements of the low temperature
IR absorption spectra of the Si samples. I wanted to study the absorption
spectra associated with photoionization of un-ionized impurity centers before
making an effort to observe the photoconductive response. Our first measurements,
using a Perkin-Elmer model 12C spectrometer with interchangeable
NaCl, KBr, KSR-5(TlBr+I) prisms and mirror optics, were carried out at 77 K,
since a simple calculation based on the thermal ionization energy of impurities
indicated that over 90% of the impurity centers would remain un-ionized
at this temperature.