early demonstrations (1952–64) of the usefulness of high pressure in semiconductor research

phys. stat. sol. (b) 241, No. 14, 3095–3098 (2004) / DOI 10.1002/pssb.200405201

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Early demonstrations (1952–64) of the usefulness of high pressure in semiconductor research

William Paul*

Department of Physics and Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachustetts, 02138, USA

Received 12 August 2004, revised 16 August 2004, accepted 17 August 2004 Published online 22 October 2004

PACS 62.50.+p, 71.20.–b

The usefulness of experiments at high pressure done in the period 1952–64, on the establishment of semi-conductor band structures and the explanation of semiconductor properties at atmospheric pressure is re-viewed.


Introduction

The organizers of this conference have asked me to give a brief retrospective on the origins of research on semiconductors at high pressure, albeit seen from my personal, and thus restricted, point of view. This will not be an all-inclusive review of the early work from all sources [1]. If I seem to concentrate unduly on the Harvard contribution, please recognize that these accomplishments are properly shared with my many thesis students and research fellows, and that the overall progress in the field is attributable to the subset of the large community of semiconductor physicists represented here who use high pressures as a fundamental weapon of investigation. There are now about 50 years of our special contribution to semi-conductor research, and I trust that this contribution will continue as we apply ourselves to the new sys-tems and new phenomena being addressed this week.

Work at Aberdeen

Part of my thesis work at Aberdeen, completed in 1951, involved the anomalous increase with tempera-ture of the minimum band gap of lead sulfide, a result not understood in terms of broadening of band edges by lattice vibrations. I sought to follow the problem by separating the explicit effect of temperature on the band edges from the volume dependence, an experimental approach that required learning the techniques of high pressure. Thus from the very beginning my high pressure studies were driven by the desire to understand the properties at atmospheric pressure.

Harvard

At the time, the principal practitioner of high pressures was P. W. Bridgman, who had been awarded the Nobel Prize for his high-pressure work in 1946. And so, using a Carnegie Fellowship from the Universi-ties of Scotland, I set out for a year at Harvard in 1952. I expected the laboratory of the great Bridgman to be modern, well-equipped, clean, full of shining equipment. What a surprise awaited me! What I found was three rooms of old equipment, no frills,

* e-mail: [email protected]

3096 W. Paul: Early demonstrations of the usefulness of high pressure in research


smelling of oil and grease like a not-too-clean garage, and modernity represented by a single vacuum tube incorporated by a student in a temperature-control apparatus. Bridgman had one laboratory assistant and one mechanic, no students, in fact he had had few students and no postdoctoral fellows essentially throughout his career. I was ignorant of all of that, which might have influenced my decision to go there. However, I was to find excellence in what the three members of that team did, unfailing helpfulness to me, sharing of all apparatus to give me a fast start. Bridgman and I had a relationship of respect on my part, and generosity on his, to the end of his life. And I had the privilege, once, of actually directing a semiconductor experiment at high pressures with him manning the pressure apparatus pumps.

An experiment with P. W. Bridgman

An aside on that experience. My interpretation of Bridgman’s early quasi-hydrostatic experiments to 10 GPa on extrinsic Ge involved interband scattering and eventual transfer of the electrons from the (lll) minima to the (100). I proposed at a later date to demonstrate that, in the fully hydrostatic pressure range of his apparatus to 3 GPa, that transfer would take place at lower pressure in SiGe alloys, where the (100) minima were already lowered toward the (lll) set by the Si alloying. But, Bridgman was using the apparatus I required. He kindly agreed to inserting my experiment, provided he participated in the pres-sure generation. So, over 70 by that time, he manned the two sets of pumps, doing the heavy lifting, while I sat, comfortably on a stool measuring the resistance changes. Unfortunately near the (lll) and (100) band crossover, the temporary temperature effects were great, so that each pressure point took ages as pressure and temperature settled. The experiment was entirely successful, but took all day, much longer than Bridgman usually assigned to a run. At the end, he remarked that now I knew why he had not continued the study of semiconductors: he did not have the patience for such long experiments, so differ-ent from his own studies on metallic alloys. Of course, I was happy with his decision. But, to return to 1952: It was convenient for me to begin by extending experiments Bridgman had done on Ge, rather than plunging into the PbS problem. His experiments on Ge had been on quite good material from Bell Labs, but done at room temperature; it apparently had not concerned him that he was measuring the changes in extrinsic or impurity conductivity. Thus I began, with his apparently contented concurrence, by arranging to dump his 1/2 ton pressure apparatus into a vat of oil whose temperature I raised to get the Ge into its intrinsic range. I think I can summarize that first year or two at Harvard, where I worked alone, as having three prin-cipal results and conclusions.

1 Germanium

My analysis of the totality of my results on Ge led to the conclusion that, although the conduction band structure of Ge was as yet unknown, it was plausible that at high pressures the electrons changed their position in the Brillouin Zone as a second extremum with different basic characteristics came into com-petition with the first. As far as I know, this publication by Harvey Brooks and me in 1954 was the first time a detail of electron behavior, such as transfer between conduction band extrema, and concurrent phonon-aided scattering between extrema, had been suggested in the literature. That sounds a trivially obvious phenomenon today. Then, it was not. Note well, however: a high pressure investigation was contributing to the knowledge of the atmospheric pressure band structure.

2 Silicon

I succeeded also in measuring the intrinsic resistivity of Si, thanks to progress in Si refinement at Bell. At high pressures, the intrinsic resistivity decreased, which implied that the band gap decreased with pressure, the opposite of Ge. The band structures of Si and Ge at atmospheric pressure were there-fore different! This was a major new unexpected conclusion. I recall being told by a prominent theo-

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retician, later to become a member of both US national academies, that if I stated such a conclusion at a meeting of the Physical Society, I would be laughed off the stage. Perhaps that will tell you of the relative lack of sophistication of our interpretation of band structures at that time. And I still recall the delicious thrill of realizing, from my measurement of the Si resistivity at the first high pressure point taken, that this pressure technique was establishing new knowledge not yet available by other means. It did not take long to establish that the energy gaps between the valence band maxima at (000) and the (100) conduction band minima had the same negative coefficient in Si and Ge. Furthermore, subse-quent research on the III–V compounds soon after permitted the acceptance of an Empirical Rule: the energy separations of extrema of a specified symmetry in all the semiconductors of the Si family had nearly equal pressure coefficients.

3 A new approach to pressure measurements on semiconductors

The third conclusion I reached, over a period of time I guess, was that a new approach to the traditional practice of high pressure measurements on solids was appropriate, and was necessary for full understand-ing. The traditional method employed by Bridgman, and wholly appropriate to his era, was to build an apparatus dedicated to the measurement of a particular property, and then to apply it to explore all manner of substances. What I realized was that a different approach contained a different challenge, and had a different objective. The different approach was to choose one substance, or one group of closely selected substances, like Si and Ge, and the challenge was to develop techniques to measure under pressure all the properties that were informative at atmospheric pressure. Thus, for example, in order to fully understand the change in conductivity with pressure, one needed separate measurements of the change with pressure of elastic constants, dielectric constant, effective mass and so on. And the new objective? The new objectives – first dimly glimpsed but increasingly recognized – were to contribute to the estab-lishment of the energy band structure at atmospheric pressure and to the understanding of phenomena taking place in these atmospheric pressure band structures. For example, the symmetry of the dominant conduction band extrema could be identified exclusively from the pressure coefficient of the minimum gap. Indeed, the entire conduction band structure of GaP was first established by optical experiments at high pressures, and pressure results were interpreted to yield the new zero-gap semiconductors such as α-Sn and HgTe. Poorly understood atmospheric pressure phenomena could be clarified by their re-measurement at high pressures using knowledge of the shifts in the energy of the extrema with pressure. An outstanding example is the explanation at Bell Labs of the properties of hot electrons resulting in the so-called Gunn Effect, an electron instability resulting in very high frequency electronic oscillations. The cause is a transfer of electrons from a high mobility (000) conduction band minimum to low mobility minima lying higher in energy, under the influence of an electric field. The transfer is identified from our knowledge of the shifts under pressure of the extrema. Time restraints prevent me continuing to report to you the later investigation in many laboratories of the impurity states associated with the several conduction band extrema, and of the DX and similar cen-ters. Enough to repeat that atmospheric pressure phenomena were being investigated, and that high pres-sures were essential to the examinations. It is worthwhile to point out that the circumstances at the time were propitious for this outcome: first, the immense interest in materials of the Si transistor family re-sulted in easily obtained financial support. Second, innovative new types of experimentation on the structural, electrical and especially optical properties of semiconductors were ingeniously replicated at high hydrostatic pressures, primarily as a result of the development of flexible 1/8” diameter stainless steel tubing capable of retaining pressures up to about 1.5 GPa. This permitted pressure to be conveyed from massive pressure generators via tubing to vessels inside cryostats, between magnet poles, at the exit slits of spectrometers, and so on. Third, the results of experimentally varying the lattice constant were compared to the parallel results of band structure theoreticians, pioneered by Frank Herman, then fol-lowed by the very productive work based on pseudopotential theory.

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The present situation

What then, can one say of the present state of high pressure work on semiconductors, by examining this week’s program? First, we must recognize the exciting progress in the accurate identification of new phases at high pressure. Second, the case I have described for the usefulness of pressure measurements in contributing to further clarification of the atmospheric pressure properties of the diamond-like semicon-ductors, whether in bulk form, or multi-quantum-well form, is indisputable. However, we should recognize that the techniques used in the early period (1952–64) are not in as much use today. Much of today’s research uses diamond anvil cells, which reach for very high pressures, but are not easily adaptable to many types of experiment, and especially transport. Will high pressure be able to contribute in the future to the understanding of new bulk systems and importantly to clarification of the atmospheric pressure properties of such as the nanoscale arrangements of atoms? I feel confident that bulk semiconductors – such as the IV–VI series, the group V’s, the transition metal oxides, the or-ganics, and the amorphous materials, will yield useful information, but only if there is a renewal of effort on the gas and liquid techniques. The nanostructures present an exciting challenge, and I look forward to papers such as “High pressure as tool to tune electronic coupling in self-assembled quantum dot nanostructures”. When I think myself back to the days of 1952, this is a mind-boggling title. The jury is still out on our contributions to such materials. Let us see what we can do in helping understand them.

References

[1] A much fuller account is given in an article titled “High Pressure in Semiconductor Physics: A Historical Over-view”, by W. Paul in: Semiconductors and Semimetals, Vol. 54, Treatise editors Robert K. Willardson and Eicke R. Weber, Volume editors Tadeusz Suski and William Paul (Academic Press, New York, 1998).

early demonstrations (1952–64) of the usefulness of high pressure in semiconductor research

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