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REVIEW the History and Future of Semiconductor Heterostructures REVIEW The history and future of semiconductor heterostructures Zh. I. Alferov A. F. Ioffe Physicotechnical Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia ~Submitted July 22, 1997; accepted for publication July 27, 1997! Fiz. Tekh. Poluprovodn. 32, 1–18 ~January 1998! The history of the development of semiconductor heterostructures and their applications in various electron devices is presented, along with a brief historical survey of the physics, production technology, and applications of quantum wells and superlattices. Advances in recent years in the fabrication of structures utilizing quantum wires and especially quantum dots are discussed. An outline of future trends and prospects for the development and application of these latest types of heterostructures is presented. © 1998 American Institute of Physics. @S1063-7826~98!00101-X# 1. INTRODUCTION heterostructures.1 In the same year A. F. Ioffe and Ya. I. It would be very difficult today to imagine solid state Frenkel’ formulated the theory of current rectification at a physics without semiconductor heterostructures. Semicon- metal-semiconductor interface, based on the tunneling 2 In 1931 and in 1936 Frenkel’ published his cel- ductor heterostructures and especially double heterostruc- effect. ebrated papers,3 tures, including quantum wells, quantum wires, and quantum in which he predicted excitonic phenomena dots, currently comprise the object of investigation of two and, naming them as such, developed the theory of excitons thirds of all research groups in the physics of semiconduc- in semiconductor heterostructures. Excitons were eventually 4 The first diffu- tors. detected experimentally by Gross in 1951. sion theory of a rectifying p – n heterojunction, laying the While the feasibility of controlling the type of conduc- foundation for W. Shockley’s theory of the p – n junction, tivity of a semiconductor by doping it with various impuri- was published by B. I. Davydov in 1939.5 Research on in- ties and the concept of nonequilibrium carrier injection are termetallic compounds commenced at the Physicotechnical the seeds from which semiconductor electronics has sprung, Institute at the end of the forties on the initiative of Ioffe. heterostructures provide the potential means for solving the The theoretical prediction and experimental discovery of the far more general problem of controlling fundamental param- properties of III–V semiconductor compounds were eters in semiconductor crystals and devices, such as the achieved independently by N. H. Welker6 and by N. A. width of the bandgap, the effective masses and mobilities of Goryunova and A. R. Regel’ at the Physicotechnical charge carriers, the refractive index, and the electron energy Institute.7 We have drawn very heavily from the high theo- spectrum. retical, technological, and experimental level of the research The development of the physics and technology of semi- conducted at the Physicotechnical Institute in that era. conductor heterostructures has brought about tremendous changes in our everyday lives. Heterostructure-based elec- 2. CLASSICAL HETEROSTRUCTURES tron devices are widely used in many areas of human activ- ity. Life without telecommunication systems utilizing The idea of using heterostructures in semiconductor double-heterostructure ~DH! lasers, without heterostructure electronics emerged at the very dawn of electronics. Already light-emitting diodes ~LEDs! and bipolar transistors, or with- in the first patent associated with p – n junction transistors out the low-noise, high-electron-mobility transistors W. Shockley8 proposed the application of a wide-gap emitter ~HEMTs! used in high-frequency devices, including satellite to achieve one-way injection. At our Institute A. I. Gubanov television systems, is scarcely conceivable. The DH laser is first analyzed theoretically the current-voltage (I – V) curves now found in virtually every home as part of the compact- of isotypic and anisotypic heterojunctions9; however, some disk ~CD! player. Solar cells incorporating heterostructures of the most important theoretical explorations in this early are used extensively in both space and terrestrial programs; stage of heterostructure research were carried out by H. Kro- for almost a decade now the Mir space station has been uti- emer, who introduced the concept of quasielectric and quasi- lizing solar cells based on AlGaAs heterostructures. magnetic fields in a smooth heterojunction and hypothesized Our interest in semiconductor heterostructures did that heterojunctions could possess extremely high injection not come about by accident. A systematic investigation of efficiencies in comparison with homojunctions.10 Various semiconductor heterostructures was launched in the very notions concerning the application of semiconductor hetero- early thirties at the Physicotechnical Institute ~FTI! under structures in solar cells evolved in that same time period. the direct supervision of its founder, Abram Fedorovich The next important step was taken several years later, Ioffe. In 1932, V. P. Zhuze and B. V. Kurchatov studied the when we and Kroemer11 independently formulated the con- intrinsic and impurity conductivities of semiconductor cept of DH–based lasers. In our patent we noted the feasi- 1 Semiconductors 32 (1), January 1998 1063-7826/98/010001-14$15.00 © 1998 American Institute of Physics 1 bility of attaining a high density of injected-carriers and It was mainly on account of this general skepticism that population inversion by ‘‘double’’ injection. We specifically only a few research groups were attempting to find the mentioned that homojunction lasers ‘‘do not provide con- ‘‘ideal’’ pair, and indeed the task was a formidable one. tinuous lasing at elevated temperatures,’’ and to demonstrate Many conditions had to be met to find the right compatibility an added benefit of DH lasers, we explored the possibility of between the thermal, electrical, and crystal-chemical proper- ‘‘increasing the emitting surface and utilizing new materials ties of the interfaced materials, not to mention their crystal to achieve emission in different regions of the spectrum.’’ and band structures. In his article Kroemer proposed that DHs be used to At that time an auspicious combination of several confine carriers to the active zone. He postulated that a pair properties—specifically low effective masses and a wide of heterojunction injectors could be used to achieve lasing in bandgap, effective radiative recombination and a sharp opti- many indirect-gap semiconductors and to improve it in the cal absorption edge due to the ‘‘direct’’ band structure, high direct-gap kind. electron mobility at the absolute minimum of the conduction In the beginning theoretical research significantly out- band and a drastic reduction in mobility at the nearest mini- paced its experimental implementation. In 1966, we mum at the ~100! point—had already garnered gallium ars- predicted12 that the injected-carrier density could well be enide a reputable place in semiconductor physics and elec- several orders of magnitude greater than the carrier density tronics. Since the maximum effect is attainable by in a wide-gap emitter ~the ‘‘superinjection’’ phenomenon!. interposing a heterojunction between a semiconductor func- That same year, in a paper13 submitted to the new Soviet tioning as the active zone of a device and a material having a journal Fizika i Tekhnika Poluprovodnikov @in translation: wider bandgap, the most promising systems studied at the Soviet Physics Semiconductors#, I generalized our concep- time were GaP–GaAs and AlAs–GaAs. For ‘‘compatibil- tion of the principal advantages of DHs for various devices, ity’’ the materials of the pair had to satisfy the first and most particularly for lasers and high-power rectifiers: important condition: closest proximity of the lattice con- ‘‘The regions of recombination, light emission, and stants. Heterojunctions in the system AlAs–GaAs were the population inversion coincide and are concentrated entirely preferred choice for this reason. However, a certain psycho- in the middle layer. Owing to potential barriers at the bound- logical barrier had to be overcome before work could begin ary of semiconductors with different bandgap widths, even on the preparation and investigation of the properties of these for large displacements in the direction of transmission, there heterojunctions. By that time AlAs had already long been in 15 is absolutely no indirect passage of electron and hole cur- production, but many properties of this compound had yet rents, and the emitters have zero recombination ~in contrast to be studied, because AlAs was known to be chemically with p – i – n, p – n – n1, n – p – p1, where recombination unstable and to decompose in a humid atmosphere. The pos- plays a decisive role!. sibility of obtaining a stable heterojunction suitable for prac- ‘‘Population inversion to generate stimulated emission tical applications held little promise in this system. can be achieved by pure injection means ~double injection! Our attempts to construct a double heterostructure ini- and does not require a high doping level of the middle region tially focused on the incompatible-lattice system GaAsP. We and especially does not require degeneracy. Because of the successfully fabricated the first lasers using DHs in this sys- appreciable difference in the dielectric constants, light is tem by vapor-phase epitaxy ~VPE!. However, owing to the concentrated entirely in the middle layer, which functions
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