10. Appendix

Ultra-Pure Germanium 555
Ultra-Pure Germanium:
From Applied to Basic Research or
an Old Semiconductor Offering New Opportunities
Eugene E. Haller
University of California, Berkeley, USA
Imagine arriving one morning at the laboratory and somebody comes to ask
you if single crystals of germanium with a doping impurity concentration in
the 10 10 –10 11 cm 3 range can be grown! You quickly compare this concentration
with the number of Ge atoms per cm 3 , which is close to 4 × 10 22 . Well,
you pause and wonder how anybody can ask if a 99.999999999% pure substance
can be made. The purest chemicals available are typically 6 or 7 nines
pure. Robert N. Hall of the General Electric Company proposed in 1968 [1]
that such crystals could be grown and that they would be most useful in fabricating
very large volume (up to 400 cm 3 ) p-i-n junctions working as gammaray
detectors [2].
When I arrived at Berkeley as a postdoc I joined the group of F.S. (Fred)
Goulding, who headed one of the leading groups of semiconductor detector
and electronics experts at the Lawrence Berkeley Laboratory (LBL), then
called the Radiation Laboratory. There I met W.L. (Bill) Hansen, who had
started the race towards the ultra-pure Ge single-crystal goal believed to be attainable
by Hall. Bill was extremely knowledgeable in chemistry, physics, and
general laboratory techniques. In addition, he was the fastest-working experimentalist
I had ever encountered. Somewhat overwhelmed, I started to work
with Bill and Fred on these Ge crystals. When Bill tried out various Czochralski
crystal growth configurations [3], he rigorously pursued ultra-purity by using
the simplest crystal growth design, the purest synthetic silica (SiO2) container
for the Ge melt, and hydrogen gas purified in a Pd diffusion system. I,
on the other hand, tried to build up an arsenal of characterization techniques
which would allow us to find out within hours the purity and crystalline perfection
we had achieved. The IEEE meetings on nuclear science, which were
held every fall, provided the forum where we “crossed swords” with Hall [4–
7]. It was a close race. Hall had the advantage of enormous experience, which
started way back when Ge was first purified and single crystals were grown
for transistors. We had the advantage of blissful ignorance but also excellent
and helpful colleagues. Furthermore, nobody could match Bill’s agility in trying
out new purification and crystal growth methods. One major development
for us was learning, through Hall, about a super-sensitive photoconductivity
technique which was capable of identifying extremely small numbers of impurities
in Ge single crystals. The technique had been discovered by Russian
scientists at the Institute of Radio-engineering and Electronics in Moscow [8,
6.85]; see Figs. 6.39 and 6.40. They found that a two-step ionization process of