Stimuli

Tributes are paid to Zhores Alferov by presenting personal anecdotes from the fields, where Alferov performed his pioneering research: masers, lasers, solar cells and heterojunctions.

What a pleasure and honor to pay tribute to Zhores Alferov in this Festschrift. Member of a remarkable laboratory and originator of imaginative and useful ideas for semiconductor physics and technology; a happy birthday! I would like to use this opportunity to ramble a little about the physics of masers, lasers, heterojunctions, solar cells— all themes of such vital importance in Alferov's career—and also tangible in my own endeavors. I start out with an anecdote of a colloquium presentation in my youthful days at Göttingen.

The Physics Colloquium at Göttingen University presented a serious weekly meeting. Werner Heisenberg and Carl Friedrich von Weizsäcker attended, often Wolfgang Pauli visited from Zurich; Otto Hahn always sat in the first row, on the left corner— and he smoked his cigar. I had just obtained my doctorate [1]— it was 1958, and my boss Rudolf Hilsch ordered me to contribute a colloquium talk. He hoped that I would report on color centers in alkali halides or review experiments on quenched amorphous bismuth, a surprising superconductor [2], or on my own dissertation [1], all recent results of our team. I, however, being an avid reader of the latest American physics literature, begged to differ. The English language gave me no problems because I had in 1951/52 spent a year at the University of Kansas. This experience in the friendly American Midwest provided me with a definite linguistic advantage over most of my German fellow students. I was fascinated by those very first reports on the maser, this molecular amplifier using ammonia for stimulated emission, and therefore decided, quite to the chagrin of my boss Hilsch, to choose this particular topic for a report at the Colloquium.

So I went to the rostrum in the small auditorium 'Hörsaal II' and delivered a well-rehearsed talk. The audience was intrigued by this new principle of stimulated coherent microwave radiation [3]. Friedrich Hund, famous for his 'rule' was then our theory professor, he sat in the second row. He was very surprised, and asked me in the discussion if he had understood correctly. If it were true what I had just suggested, then the maser coherence length would go from the Earth to the Moon. I paused a little, pondered and observed my microwave-conscious friends in the audience nodding encouragingly. 'Yes, sir; I think so!' 'I don't believe it', Hund retorted. How could a youngster react? I remained silent and obediently, quite imperceptibly shrugged my shoulders. After the talk, Professor Lamla, an editor of a science journal came to congratulate me and asked for a manuscript. I delivered [4]. This item on my early publication list may have contributed to the fact that I was hired in 1959 by William Shockley to join his fledgling company Shockley Transistor in this old apricot barn on 391 South San Antonio Road in Mountain View, California [5].

I knew that it would be extremely difficult to extend the frequency into the optical regime, you have to fight against the square of the frequency. Nevertheless, I refrained from making the statement in my paper that reaching an optical maser might be hopeless [4]. 'Never say never' is an appropriate adage, not only for seniors. A young colleague, who had also written a review paper, dared to support a more pessimistic view [6]. He anticipated in his very last sentence that stimulated emission would probably prevail merely in the microwave regime.

This defeatist attitude seemed to have ruled throughout Germany, as already preached in the famous textbooks by Pohl [7], and also assumed by physics Professor Hellwege at Darmstadt, who was the leading expert regarding luminescence of materials such as ruby crystals; yet Maiman and others surpassed him [8]. Silicon came next for me, working, for example, with Shockley on the theory of maximal efficiency for solar cells, not really a topic regarding coherent radiation [9]. Once, however, a discussion evolved during one of those nearly dreaded hamburger lunches with Shockley at Kirk's charcoal restaurant on El Camino Real in Mountain View. Those frugal lunches ended with a demanding one-on-one interrogation, stricter and tougher than any doctoral oral examination. 'What, you do not know of Einstein's A and B coefficients?' Next afternoon I dutifully looked them up in the Stanford physics library.

My first, rather indirect contacts with semiconductor heterojunctions occurred in this former apricot barn of Shockley's. Improving junction transistors required a maximum of the emitter efficiency. The emitter-to-base junction should carry only a forward current, no particles should flow from base to emitter [10]. This requirement can be met with a heterojunction: some other semiconductor material covering the silicon. Shockley had already contemplated this possibility while still at Bell Laboratories [11]. One day, a physicist by the name of Herbert Krömer visited us. This young man had also studied at Göttingen, especially with the memorable theoretician Richard Becker, whom we all admired. Krömer had in Princeton contributed to the theoretical understanding [12] of such wide-gap emitter/base junctions, and Shockley urgently wanted to hire him. But Herb preferred to join Varian Associates, just up the road in Palo Alto. Later, it was my great pleasure to attend the Nobel Festivities for Herb and Zhores Alferov in Stockholm.

In the early sixties, I became a Member of Technical Staff at the Bell Laboratories in Murray Hill, New Jersey. Now, compound semiconductors, such as gallium arsenide, had to attract my interest. By the time of the mid-sixties, helium/neon-lasers were quite the vogue; Bell Labs actually established a little workshop with a production line to fabricate them and spread them throughout the departments. 'The solution in search of a problem', as sceptics joked about this new light source, was of vital interest to us because of the high frequencies to carry plenty of information channels. Transmission of laser light straight through the air, from Building 1 to Building 2 at Murray Hill, however, showed that the atmosphere was by far too unstable. We discussed silver-plated tubes and glass fibers, which eventually became so unbelievably pure that nowadays they provide a wealth of inexpensive communication channels.

A gas laser did not appear to emerge into a viable, convenient engineering solution, nor did the ruby. A diode laser source had to be developed. I used laser-induced photoluminescence to search for more efficient GaAs materials, which resulted in detecting crystals with amphoteric silicon doping of very high output in the near-infrared [13]. This invention was patented in 37 countries and provided millions of diodes, such as for TV remote control devices. I had to sign off my inventor's reward for one US dollar, which I actually did not even receive. (In earlier years, patentors obtained one silver dollar; but not anymore!) Yet my little diodes, however efficient, could not be stimulated to emit coherent light, alas!

Together with my colleagues and friends Morton Panish and Craig Casey, later famous textbook authors on diode lasers [14], we searched for solutions, although colleagues at the famed RCA Laboratories in Princeton had predicted that a laser diode was impossible [15]. I remember one morning when Mort told us of a talk he had just heard at a meeting in New York City, where our friendly competitors at the IBM Labs in Yorktown Heights, NY had suggested that heterojunctions could nicely confine and concentrate carriers, maybe also photons. Such heterojunctions were then tried in Panish's lab to be grown via liquid-phase epitaxy, Stan Sumski being the expert technician. At that time, the Leningraders, under leadership of Zhores Alferov were working hard and highly successfully with this crystal growth technique. We were very much impressed by the success in Leningrad. Liquid-phase epitaxy yields, in principle, exceedingly pure crystals, but we were unhappy about the principal lack of direct monitoring during this growth process, which we deemed absolutely necessary for obtaining reproducible heterojunctions with tightly controlled small dimensions.

Ultrahigh-vacuum epitaxy seemed to be the inescapable solution. Delicate molecular beams had to be gently used and monitored! What a costly proposition! I clearly remember the day when Mort and I went to the Laboratory director John Galt. A little bit fearful and subdued, we explained our project. No, not expensive, rather a very expensive idea! We anxiously watched John with his usual stern demeanour; he paused and contemplated: 'All right, we do it—go ahead!' Construction for equipment needed for the Molecular Beam Epitaxy (MBE) began, and in Al Cho, an excellent new employee was hired for this task. A little later I left Bell Labs, this fabulous 'Mecca of Solid State' for a physics professorship at the Goethe University in Frankfurt-on-the-Main in Germany. Meanwhile, successful work on semiconductor lasers bore ample fruit worldwide.

In Frankfurt, I used gas laser sources for photoluminescence diagnostics of elemental and compound semiconductors. With my astute doctoral student 'Teddy' Güttler, for example, we observed impurity photoluminescence in Au-doped silicon and concluded that doping of solar cells with deep impurities would not be beneficial for cell efficiency; just the opposite would happen because of increased carrier recombination [16]. In 1968, Western Germany experienced an ultra-left-wing student rebellion. Frankfurt students violently attacked me and accused me of war research since I used lasers, obviously a deadly weapon of mass destruction. Dieter Bimberg, our co-editor of this Festschrift, will undoubtedly remember those happenings when he was a doctoral candidate. In 1968, we all assembled in Moscow for the International Conference on the Physics of Semiconductors; what a unique opportunity to meet so many Russian colleagues, including this intellectual elite from the most remarkable Joffe Institute, with Zhores Alferov a major player.

In 1970, I became a founding director of the Max-Planck-Institute for Solid State Research at Stuttgart, in the Southwest of Germany. There I eventually succeeded—against massive opposition—to establish a group for MBE, which became truly successful under the very capable leadership of Klaus Ploog [17], to whom was bestowed a prize of the Seibold-Foundation for Japan-Germany Science Cooperation. Klaus von Klitzing's group in our Max-Planck-Institute in Stuttgart relies on MBE to the present day for research on the quantum Hall effect [18]. Equally, my former doctoral student Horst Stormer had to utilize excellent MBE for his Nobel-Prize winning research on the fractional quantum Hall effect [18].

We fondly remember one congenial dinner party at our Stuttgart house, with Zhores Alferov and Helmut Lotsch as our valued guests; it must have been in the mid-seventies. My wife Inge had prepared a dessert in the shape of the title page of the Springer journal Applied Physics, with chocolate and orange cream. Herr Lotsch had won Alferov to become part of our board of editors, a most valuable connection to the excellence of Soviet semiconductor research!

Many Japanese colleagues, especially from industrial electronics labs came to learn the tricks of MBE from us in Stuttgart; the German electronics industry, however, was reluctant and remained completely disinterested—but the French equipment maker RIBER was our staunch ally, and this company grew with the international acceptance of MBE for small, high-frequency devices. One diligent young visitor at my Stuttgart laboratories, Ozamu Kumagai from the SONY Corporation, did especially well. Back at home, he most cleverly devised novel technologies for efficient and low-cost production of laser diodes and thus earned a promotion to Vice Presidency.

One of the most recent, gratifying encounters with Zhores Alferov happened to me in a cozy retreat in the forests near Madrid, with Antonio Luque being our gracious host for a solar cell symposium. We Stuttgarters had hoped to use multi-pair generation in perfected silicon solar cells [19], but a better chance to capture more photons from the solar spectrum exists most likely in multi-junction cells [20], with fancy tunnel-contacts interconnecting between heterojunctions. We shall see if this approach might eventually lead to more efficient, yet still economical solar energy conversion.

Semiconductor heterojunctions for communications and consumers! Many of Alferov's present activities in St Petersburg and Berlin are governed by this magic modern prefix nano, which might one day also provide some applications in solar cells; but we have yet to carefully investigate [21]!

References

[1] Queisser H J 1958 Z.Physik 152 507 and 495

[2] Buckel W and Hilsch R 1956 Z. Physik 146 27

[3] Wittke J P 1957 Proc. IRE 45 291 with references to earlier work

[4] Queisser H J 1959 Naturwiss. 46 394

[5] Queisser H J 1988 The Conquest of the Microchip (Cambridge, MA: Harvard University Press)

[6] Wolf H C and Agnew Z 1958 Physik 10 480

[7] Pohl R W Optik (Heidelberg: Springer)

[8] Yariv A 1968 Quantum Electronics (New York: Wiley)

[9] Shockley W and Queisser H J 1961 J. Appl. Phys. 32 510

[10] For details, see Sze S M and Ng K K 2007 Physics of Semiconductor Devices 3rd edn (Hoboken, NJ: Wiley)

[11] Shockley W 1951 US Patent Specification 2.569.347

[12] Krömer H 1957 Proc. IRE 45 1535

[13] Queisser H J 1966 J. Appl. Phys. 37 2909 (this paper was withheld internally for some time due to the patent application: US Pat.3.387.163)

[14] Panish M B and Casey C H 1978 Heterostructure Lasers (New York: Academic)

[15] Kressel H Private communications

[16] Güttler G and Queisser H J 1996 J. Appl. Phys. 40 4994

[17] Ploog K and Graf K 1984 MBE of III-V Compounds (Berlin: Springer)

[18] For recent coverage, see Chakraborty T and Pietiläinen P 1995 The Quantum Hall Effect (Berlin: Springer)

[19] Werner J H, Kolodinski S and Queisser H J 1993 Phys. Rev. Lett. 72 3851

[20] Yamaguchi M 2002 Physica E 14 84

[21] Queisser H J 2002 Physica E 14 1 and many other contributions in this issue