A. G. U. Perera and M. H. Francombe
Department of Physics and Astronomy, Georgia State University, Atlanta, Ga 30303J, -W Choe
Department of Physics, Kyung Hee University, Suwon 449-701, KoreaIntroduction and Background
For almost four decades there has been active and growing interest in the crystal growth and properties of narrow-energy-gap semiconductor materials for use in infrared applications. The primary application has been infrared detection and imaging; however, over the past 30 years an increasing segment of research in this area has addressed the use of interband transitions in such materials for the development of solid-state infrared radiation sources. Compact and integratable sources, especially covering the mid-infrared (2 - 5 µm), are required for a wide range of uses, such as laser radar, optical communications, remote sensing, pollution monitoring, molecular spectroscopy, medical care and infrared countermeasures. In some of these applications the successful development of longer-wavelength sources (e.g. 8 - 12 µm) would also be extremely valuable. For most situations, room-temperature (or modestly cooled) operation and high-power output are important for attaining desired system performance at reasonable cost.
Diode-type IR sources operating via interband transitions have been fabricated from solid-solution materials based on direct energy gap III-V, IV-VI or II-VI compounds. The work in this field prior to 1985 has been reviewed comprehensively in Volume 22 (Lightwave Communication Technology) of the Academic Press Series, "Semiconductors and Semimetals". In particular, some of the earliest studies involved attempts to develop both detectors and laser sources in lead chalcogenide alloys such as Pb1-xSnxTe and Pb1-xSnxSe, and continuing efforts over the years have led to the successful development and application of modestly cooled lasers for use at high powers. Both lead salt alloys and the II-VI mercury telluride (HgxCd1-xTe) alloys are capable of being compositionally tuned to provide very long wavelength IR response. However, at the short wavelength end, the lead salts are limited (at 77K) to about 4 µm (for PbS), while the HgxCd1-xTe alloys can be extended down to about 1 µm. Consequently, the III-V and II-VI alloys also provide better wavelength matching to the midwave operating needs of optical communications.
Research on epitaxially grown III-V alloys for optical emitters has been strongly motivated by the potential for effective large-scale application in high-bandwidth optical communication systems. Earlier activity in this field exploited the availability of already established GaAs/GaAlAs commercial processing, and focused initially on the development of integrated optic structures operating at shorter wavelengths (0.82 - 0.88 µm) , utilizing GaAlAs/GaAs lasers as light sources together with silica fiber link technology. However, longer-wavelength sources were soon developed, based on the lattice-matched InGaAsP/InP materials system, and these made it possible to exploit the lower attenuation of silica fibers at wavelengths in the range 1.3 - 1.55 µm. As a result, significant increases in repeater spacing (twelve-fold) for commercial communication systems became feasible. Much lower optical attenuation is available by operating with fibers made, for example, from thallium bromide and chalcogenide materials, at wavelengths within the mid-infrared range (viz. 4 - 5.5 µm), and this has stimulated greatly increased interest in integratable sources operating at longer wavelengths.
Most diode-type IR emitter devices presently employ the double-heterostructure (DH) configuration, in which the active light-generating region (of bandgap corresponding to the desired emission wavelength), is sandwiched between wider-bandgap p- and n-type layers of similar composition. Under forward bias, injected electrons recombine with injected holes in the active region via both radiative and non-radiative processes.The desired radiative carrier recombination within the active region is critically affected by several important materials and device design factors. Efficient carrier injection from the p- and n-type wider bandgap regions is favored by developing large band offsets at the active region interfaces, and by achieving low structural defects (and hence low recombination velocity) at these interfaces during epitaxial growth. The latter condition requires low lattice mismatch, with deltaa/a values well below 10-3. Similarly, high crystal perfection of the active layer is important in order to reduce non-radiative recombination processes due to Shockley-Read centers. Also, it is critical to limit Auger recombination, which becomes more of a problem at narrower bandgap values and higher carrier densities. Power output and efficiency also are determined by parameters such as active layer thickness (relative to minority carrier diffusion length), doping concentration and interface barrier height. This last factor influences carrier and optical containment within the active region.
The performance limitations of compound semiconductor junction devices, designed either for the detection or generation of infrared radiation, have prompted more detailed studies of interface chemistry and physics, and of layer growth techniques such as MOCVD and MBE suitable for the precision fabrication of such structures. The unique design flexibility afforded by controlled, low-temperature epitaxial growth of a wide range of III-V binary, ternary and quaternary solid solutions, has led to numerous novel, multi-layer device configurations of great significance to both IR detector and emitter technology. Of particular concern to this review is the contribution made to the evolution of IR source technology through the special properties of quantum wells and superlattices.
In recent years, a rapidly growing emphasis has been placed upon intersubband transition phenomena in quantum wells as a route towards developing tailored infrared properties of interest in optical systems. The ease with which arbitrary intersubband energy values can be engineered in epitaxial superlattice systems such as GaAs-AlGaAs offers an attractive means of replacing direct interband devices in HgCdTe and PbSnTe, with much more stable and reproducible intraband components in chemically stable materials. The large electric dipole moment between energy levels in superlattices makes the direct optical transitions between the levels even more promising. This concept has already been demonstrated convincingly in the case of infrared detectors, to the point where LWIR focal plane array cameras of MQW devices are in the market.
In the present chapter, we discuss two main types of applications for quantum wells and superlattices in infrared emitter devices. First, we review briefly the use of such structural features to improve the operation of diode emitters and lasers of the conventional interband type. Here, the key objectives are to improve layer and interface quality, and to modify band offsets and strain conditions, so as to lower non-radiative defect- and Auger-recombination and enhance carrier and optical containment. In turn, these modifications should lead to the capability of higher power and efficiency, and to the achievement of lower operating threshold currents for optical generation at higher temperatures. Second, and more importantly, we address very recent and ongoing investigations in the new field of infrared emission via intersubband carrier transitions in quantum well structures. The demonstration of optically excited intersubband quantum-well transitions, both in a variety of wider-bandgap III-V alloy systems and in the Si-Ge system, has led to a voluminous research literature in this area, and to the successful development of a new class of uniform,integratable, high-performance, and chemically stable long-wavelength IR detector arrays. In the present article, rather than IR detection properties, we discuss the electrical stimulation of infrared radiation in such intersubband structures.
Our treatment of intersubband sources begins with a brief historical survey of earlier theoretical and experimental studies, and next proceeds to a more detailed discussion of the evolution and modeling of candidate emitter structures particularly for shorter wavelength (MWIR and LWIR) emission. Recent successful development of cascade laser devices incorporating novel and effective carrier injection features is reviewed, together with properties such as power capability, efficiency, spectral characteristics, temperature behavior, etc. We conclude with a summary of current research and technology directions, anticipated further developments and potential applications.
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