Bandgap and Interface Engineering for Advanced Electronic and Photonic Devices
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LLETIN/JUNE1991
Figure 1. High-resolution OU TEM cross section showing a lattice image of an InCaAs quantum wcll three molecular layers thick sandwiched between two InP lattice-matched barriers, grown by gas source MBE by Morton B. Panish at AT&T Bell Laboratories. The normal black-and-white contrast has been coded according to the color spectrum, the red end corresponding to higher intensifies. The red spots represent tunnels between pairs of atoms. The minimum séparation between the tunnels in InP is 3.4 A. Thèse abrupt heterointerfaces can be used as ballistic électron launchers in several transistor and detector applications (see Figure 2). (Courtesy of J.M. Gibson and S.N.C. Chu.)
motion is quantized by the présence of two heterojunction potential barriers, are among the early achievements of MBE. 3 However, it was not until the 1980s that MBE became a technology capable of producing "device quality materials" ready for commercial exploitation and semiconductor s t r u c t u r e s with arbitrary spatial control of doping and composition. 4 Thèse new material structures hâve allowed an enormous breadth of new devices. Near infrared GaAs lasers for compact dise players, IMPATT diodes, and modulation doped field-effect transistors are among the many devices manufactured today by MBE. The purpose of this article is to acquaint the reader with bandgap engin e e r i n g and to illustrate how thèse structures can be both useful devices and tools to study the physics of transport in small dimensions. Abrupt Heterointerfaces as Launching Ramps: Ballistic D e v i c e s Heterojunction interfaces grown by MBE are atomically abrupt, as shown by the électron micrograph of Figure 1. This represents a TEM cross section of two InP/GalnAs lattice-matched heterojunctions forming a quantum well, grown by gas source MBE. This material combination is currently used in lasers and detectors for lightwave communications Systems operating in the 1.3-1.55 /xm wavelength région and is also important, as we shall see, for ultrahigh speed bipolar transistors. The atomically abrupt heterointerfaces g r o w n by MBE give rise to potential steps (band-discontinuities) in the conduction and valence bands, denoted by A£ c a n d A£ v , r e s p e c t i v e l y , as illustrated in Figure 2. Thèse energy band diagrams show the conduction and valence band edges as a function of distance inside the materials. They are a powerful aid for the device physicist and engineer to visualize the motion of électrons and holes in thèse structures and to visualize device opération. The structures of Figure 2 use abrupt heterojunctions as ballistic launching r a m p s for électrons to i m p r o v e device performance. An électron crossing thèse potential steps gains a kinetic energy equal to the conduction band discontinuity. This process is a ballistic accélération over a distance equal to the interface w i d t h (=1 monolayer). Such a launching ramp has been used to i m p r o v e t h e h i g h - s p e e d performance of heterojunction bipolar tran-
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Bandga
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