Title: III-V Semiconductor Materials Grown by Molecular Beam Epitaxy for Infrared and High-Speed Transistor Applications
Description: Semiconductor devices based on III-V materials have been the focus of intense research due to their superior electron mobility and favorable energy direct bandgap which are applicable in infrared wavelength range optoelectronics and high speed electronic systems. The thesis presented here consists of two thrusts; the first focusing on infrared applications, and the second focusing on InP-based heterojunction bipolar transistors (HBTs). In the first thrust, we investigate type-II InAs/GaSb superlattice IR detector devices and the effect of substrate orientation on InSb and InAs nanostructure morphology. In the second thrust, we study InP-based high frequency HBTs. A low resistance InAs ohmic contact is demonstrated, and we presented along with a study of the crystalline qualities in GaAs0.5Sb0.5 films grown on tilted- axis InP substrates. Chapter 2 presents fabrication and characterization of two type-II superlattice structures with 15 monolayer (ML) InAs/12ML GaSb and 17ML InAs/7ML GaSb grown on GaSb (100) substrates by solid-source molecular beam epitaxy (MBE).

The X-ray diffraction (XRD) measurements of both the 15ML InAs/12ML GaSb and 17MLInAs/7ML GaSb superlattices indicated excellent material and interface qualities. The cutoff wavelengths of 15ML InAs/12ML GaSb and 17ML InAs/7ML GaSb superlattices photodetectors were measured to be 6.6μm and 10.2μm, respectively. These different spectral ranges were achieved by growing alternating layers of varying thicknesses which allowed for bandgap engineering of the superlattices of InAs and GaSb. Lastly, a mid-IR type-II superlattice photodiode was demonstrated at 80K with a cutoff wavelength at 6.6µm. The device exhibited a near background limited performance (BLIP) detectivity at 80K and higher temperature operation up to 280K. In Chapter 3, we show that the (411) orientation, though not a naturally occurring surface, is a favorable orientation to develop a buffer layer into a super flat surface at a certain high growth temperature. The (411) surface is a combination of localized (311) and (511) surfaces but at a high growth temperature, adatoms can obtain enough energy to overcome the energy barrier between these localized (311) and (511) surfaces and form a uniform (411) surface with potential minima.

This results in a super flat surface which is promising for high-density nanostructure growth. In this work, this is the first time that the highest InSb and InAs nanostructures density can be achieved on the (411) surface which is in comparison with the (100), (311), and (511) surfaces. Chapter 4 of this thesis addresses the use of an InAs layer as a low-resistance ohmic contact to InP-based heterostructure devices. Selective area crystal growth of InAs on a dielectric (Benzocyclobutene, BCB polymer) covered InP (100) substrate and direct growth of InAs on InP substrate were performed by MBE. Heavy doping of InAs using Te was carried out to determine the lowest sheet resistance. Based on scanning electron microscope (SEM) and XRD measurements, increasing substrate temperature from 210 ℃ to 350 ℃, led to an improvement in crystallinity from a polycrystalline layer to a single crystal layer with a corresponding improvement of surface morphology. Moreover, a narrow X-ray diffraction peak indicated full-relaxation of the inherent 3.3% lattice-mismatch in InAs/InP layers.

Furthermore, around 290 ℃ a tradeoff was reached between crystallinity and optimized dopant incorporation of Te into InAs for the lowest sheet resistance. Lastly, Chapter 5 discusses the effect of substrate tilting on the material properties of MBE grown GaAsSb alloys closely lattice-matched to an InP substrate. InP(100) substrates tilted 0°off-(on-axis), 2°off-, 3°off-, and 4°off-axis were used for MBE growth; then the material qualities of GaAsSb epitaxial layers were compared using various techniques, including high resolution XRD, photoluminescence (PL) and transmission-line measurements (TLM). Substrate tilting improved the crystalline quality of the GaAsSb alloys, as shown by a narrower XRD linewidth and enhanced optical quality as evidenced by a strong PL peak. The results of TLM show that the lowest sheet resistance was achieved at a 2° off-axis tilt.