This FRG (Focused Research Group) project aims for greater understanding of materials science and device physics aspects of dipole layers at heterojunction interfaces, and to modify them for improved device performance across a variety of common compound semiconductor structures: lasers, HBTs, MOSFETs, HEMTs, and Schottky barriers. The project involves theory, growth, characterization, and prototype device studies. First principle calculations will be performed by collaborators at EPFL (Switzerland), and coupled with material processing for improved device performance. Recently dual purpose molecular beam epitaxy (MBE) facilities devoted to the exploration of interface properties and the fabrication of working devices have produced tunability of band offsets and Schottky barriers through local modifications of the atomic termination of critical interfaces. In parallel with experiment, a convergence of different theoretical models, including first principles calculations, the theoretical alchemy approach and linear response theory results, are establishing a common theoretical framework. Most theoretical and experimental studies have identified heterovalent semiconductor junctions with polar orientation as those that exhibit the strongest dependence of interface properties on local interface termination. Therefore IV/III-V, II-VI/III-V, III-V/IV/III-V and III--V/II-VI/III-V interfaces may represent a class of highly tunable interface systems, as compared to the more conventional isovalent systems (e.g., III-V/III-V) where the valence difference across each interface allows fabrication of large electrostatic dipoles with orientation and magnitude largely controlled by the growth conditions. For heterojunction interfaces, parameters such as the valence and conduction band discontinuities, and the built-in potentials affect carrier confinement on both sides of the active region where radiative recombination occurs in heterojunction lasers, emitter efficiencies in HBT's, as well as the gate voltage swing and the gate leakage current in MOSFET and HEMT structures. For metal/semiconductor interfaces, present in all solid state devices, the possibility of reducing or increasing the Schottky barrier height without changing the doping of the semiconductor constituent would enhance ability to fabricate low resistivity contacts to new wide bandgap materials for which doping technology is still limited (e.g., III-V nitrides, silicon carbide, diamond), simplify the exploitation of ballistic transport in practical devices, decrease the leakage current in MESFET's, and potentially yield Schottky barrier photon detectors with tunable long -wavelength cut-off and/or lower dark currents. Thus, the project seeks to exploit heterovalency-induced, extrinsic local interface dipoles in a variety of III-V materials systems of current practical interest, including AlGaAs/GaAs, GaInAs/InP, GaInP/GaAs, and AlGaN/GaN. Different ultrathin heterovalent interlayers (e.g., Si, Ge, Si-C, Zn-O) will be fabricated by MBE to tune the band alignments as well as in related metal/semiconductor junctions. Interlayer type and growth conditions which minimize out-diffusion, and assess the range of offset tuning that can be achieved in high-quality, device grade structures will be determined. The ultimate goal is to clarify the microscopic mechanisms that determine the offset tuning while developing a series of new interfacial engineering principles for device optimization. This FRG project is co-supported by two NSF programs, and the MPS OMA(Office of Multidisciplinary Activities). %%% The project addresses basic research issues in a topical area of materials science and engineering having high potential technological relevance. The research will contribute new knowledge at a fundamental level to important fabrication aspects of electronic/photonic devices. The basic knowledge and understanding gained from the research is expected to contribute to improving the perform-ance and stability of advanced devices and circuits. An important feature of the program is the integration of research and education through the training of students in a fundamentally and technologically significant area. ***
