Appendix: Pioneers of Semiconductor Physics Remember... Semiconductor physics has a long and distinguished history. The early devel­ opments culminated in the invention of the transistor by Bardeen, Shockley, and Brattain in 1948. More recent work led to the discovery of the laser diode by three groups independently in 1962. Many prominent physicists have con­ tributed to this fertile and exciting field. In the following short contributions some of the pioneers have recaptured the historic moments that have helped to shape semiconductor physics as we know it today. They are (in alphabetical order): Elias Burstein Emeritus Mary Amanda Wood Professor of Physics, University of Pennsylvania, Philadelphia, PA, USA. Editor-in-chief of Solid State Communications 1969-1992; John Price Wetherill Medal, Franklin Institute 1979; Frank Isakson Prize, American Physical Society, 1986. Marvin Cohen Professor of Physics, University of California, Berkeley, CA, USA. Oliver Buckley Prize, American Physical Society, 1979; Julius Edgar Lilienfeld Prize, American Physical Society, 1994. Leo Esaki President, Tsukuba University, Tsukuba, Japan. Nobel Prize in Physics, 1973. Eugene Haller Professor of Materials Science and Mineral Engineering, University of California, Berkeley, CA, USA. Alexander von Humboldt Senior Scientist Award, 1986. Max Planck Research Award, 1994. Conyers Herring Professor of Applied Physics, Stanford University, Stanford, CA, USA. Oliver Buckley Prize, American Physical Society, 1959; Wolf Prize in Physics, 1985. 538 Appendix Charles Kittel Emeritus Professor of Physics, University of California, Berkeley, CA, USA. Oliver Buckley Prize, American Physical Society, 1957; Oersted Medal, American Association of Physics Teachers, 1978. Neville Smith Scientific Program Head, Advanced Light Source, Lawrence Berkeley Laboratory, Berkeley, CA, USA. c.J Davisson and L.H. Germer Prize, American Physical Society, 1991. Jan Taue Emeritus Professor of Physics and Engineering, Brown University, Providence, RI, USA. Alexander von Humboldt Senior Scientist Award, 1981; Frank Isakson Prize, American Physical Society, 1982. Klaus von Klitzing Director, Max-Planck-Institut fUr Festkorperforschung, Stuttgart, Germany. Nobel Prize in Physics, 1985. Ultra-Pure Germanium 539 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 1010_1011 cm3 range can be grown! You quickly compare this concentra­ tion with the number of Ge atoms per cm - 3, which is close to 4 x 1022 . Well, you pause and wonder how anybody can ask if a 99.999999999% pure sub­ stance 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 fab­ ricating very large volume (up to 400 cm3) p-i-n junctions working as gamma­ ray detectors [2]. When I arrived at Berkeley as a postdoc I joined the group of ES. (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 at­ tainable by Hall. Bill was extremely knowledgeable in chemistry, physics, and general laboratory techniques. In addition, he was the fastest-working experi­ mentalist I had ever encountered. Somewhat overwhelmed, I started to work with Bill and Fred on these Ge crystals. When Bill tried out various Czochral­ ski crystal growth configurations [3], he rigorously pursued ultra-purity by us­ ing the simplest crystal growth design, the purest synthetic silica (Si02) con­ tainer 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 per­ fection 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 try­ ing 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 im­ purities 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 540 Appendix shallow hydrogenic donors or acceptors in a very cold crystal would lead to photoconductivity peaks which were very sharp and unique for each dopant species. Paul Richards, of the Physics Department at the University of Califor­ nia at Berkeley, had a home-built Fourier-transform far-infrared spectrometer and the necessary liquid helium temperature dewar. By the end of the first day of experimenting we had a spectrum of a p-type high-purity Ge crystal with only 1010 cm-3 net amount of acceptors and we knew also that phosphorus and aluminum were the major residual impurities. In parallel with a number of novel and interesting physics studies we fabri­ cated gamma-ray detectors at LBL. We broke records in the resolution of the gamma-ray photopeaks with our ultra-pure crystals [2]. Soon the commercial detector manufacturers became interested and started their own ultra-pure Ge crystal-pulling programs. In a few years several companies in the US and in Europe succeeded in developing large-diameter (~ 8 cm) single crystals with incredibly good yield, excellent purity « 2 X 1010 cm - 3) and very small con­ centrations (108 cm-3) of deep-level defects which would detrimentally affect the charge collection in large-size coaxial p-i-n diodes. In order to achieve the best spectral resolution, electrons and holes had to have mean-free-paths of up to several meters. Most semiconductor physicists simply shook their heads and could not comprehend these numbers. How pure is ultra-pure Ge? The person who cares only about electrically active impurities would say that crystals with a few 1010 cm-3 of impurities are routinely grown. But are there other inactive impurities? Yes, of course there are. Hydrogen, oxygen, silicon and carbon are usually present at con­ centrations of up to 1014 cm-3, depending on the crystal growth conditions. These impurities do not interfere with Ge's operation as radiation detectors provided certain rules are followed: no heating to temperatures above 350°C and no rapid temperature changes. Can we reduce the concentration of these four electrically inactive impurities? Yes, we can, but we pay a price. Elimi­ nating hydrogen by growing in vacuum leads to the introduction of impurities which can no longer be "flushed" out of the crystal puller. Furthermore, hy­ drogen will passivate the very small concentrations of deep-level defects and impurities which are always present. Free oxygen and silicon are generated by the reduction of the ultra-pure silica crucible by the liquid Ge. We do not know of any substance which can replace silica with, perhaps, the exception of graphite. Numerous attempts to grow ultra-pure Ge in graphite crucibles have failed so far because the resultant crystals contain too many Al acceptors. Most recently, the interest in Ge has sharply increased because isotopically pure Ge can be obtained from Russia. Isotopically pure Ge bulk crystals [9- 12] and isotope superlattices [13] have been grown. New phonon physics and electronic transport studies are currently being pursued by several groups with these isotopically controlled crystals and multilayers. Have we arrived at the ultimately ideal material: isotopically and chemi­ cally pure and crystallographically perfect Ge single crystals? Perhaps the an­ swer is no, but I certainly do not know of another parameter that can be con­ trolled. Ultra-Pure Germanium 541 References 1 R.N. Hall: in Proc. of the 12th Int. Conf on Physics of Semiconductors, ed. by M.H. Pilkuhn (Teubner, Stuttgart 1974), p. 363 2 E.E. Haller, ES. Goulding: Handbook on Semiconductors, Vol. 4, ed. by C. Hilsum (Elsevier, New York 1993), Chap. 11, p. 937-963 3 w.L. Hansen, E.E. Haller: Mater. Res. Soc. Proc. 16, 1 (1983) 4 R.N. Hall, T.1. Soltys: IEEE Trans. Nucl. Sci. NS-18, 160 (1971) 5 E.E. Haller, w.L. Hansen, ES. Goulding: IEEE Trans. Nucl. Sci. NS-20, 481 (1973) 6 E.E. Haller, w.L. Hansen, G.S. Hubbard, ES. Goulding: IEEE Trans. Nucl. Sci. NS- 23, 81 (1976) 7 E.E. Haller, w.L. Hansen, ES. Goulding: Adv. Phys. 30, 93 (1981) 8 E.E. Haller: Physics 146B, 201 (1987) 9 E.E. Haller: Semicond. Sci. Techno!. 5, 319 (1990) 10 E.E. Haller: Solid State Phenom. 32-33, 11 (1993) 11 G. Davies, 1. Hartung, V. Ozhogin, K. Itoh, w.L. Hansen, E.E. Haller: Semicond. Sci. Techno!. 8, 127 (1993) 12 H.D. Fuchs, P. Etchegoin, M. Cardona, K. Itoh, E.E. Haller: Phys. Rev. Lett. 70, 1715 (1993) 13 1. Spitzer, T. Ruf, M. Cardona, W. Dondl, R. Schorer, G. Abstreiter, E.E. Haller: Phys. Rev. Lett. 72, 1565 (1994) 542 Appendix Two Pseudopotential Methods: Empirical and Ab Initio Marvin L. Cohen University of California, Berkeley, USA It took a relatively long time to develop methods capable of determining the detailed electronic structure of solids.