Underlying most of our modern semiconductor devices, and hence optical and electronic systems, are semiconductor heterostructures composed of thin lattice-matched or pseudomorphic epitaxial layers. While some measure of lattice-mismatched derived strain can be accommodated within an epitaxial structure, the constraints of lattice-matching and thermodynamic stability of the alloy material have limited the choices of semiconductor alloy materials that can be used for structure design and device development. In order to increase the degrees of freedom within the design of devices, such as independent control over lattice parameter, band gap, band gap alignments, and electronic carrier transport characteristics, the chemical complexity of the epitaxial layer must increase through the incorporation of additional alloying elements. Unfortunately, most semiconductor alloys of arbitrary composition lie within a miscibility gap present in their respective phase diagrams. Epitaxial strain can be used to modify the phase relationships in epitaxial and pseudomorphic alloys. Alternatively, a variety of growth techniques have been used in attempts to form these thermodynamically immiscible materials as metastable phases. The formation of these potentially desirable alloys through epitaxial growth techniques that are characterized by being near thermodynamic equilibrium at the growth surface is generally impossible They can be formed, however, by suppressing those surface processes, such as surface mass transport, which lead to local phase separation during growth. Low growth temperature and chemical modification of the growth front can both be used to inhibit or suspend such surface transport processes and lead the formation of homogeneous alloys deep within a miscibility gap. Once formed, these alloys can be used in device applications since any subsequent phase separation would require some bulk diffusion, which is kinetically “frozen-out” under normal device operating conditions. This talk will discuss the thermodynamic relationships and kinetic processes that can be realized in a variety of alloy systems using established epitaxial growth techniques. Recent work on the formation of alloys which contain multiple anion substitutions in compound semiconductors and their extension to pentanary alloys will be presented as examples of the development of new materials that can open new areas of device design and technology.

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