Date of Degree
PhD (Doctor of Philosophy)
John P. Prineas
First Committee Member
Thomas F Boggess
Second Committee Member
David R Andersen
Third Committee Member
Fourth Committee Member
Michael E Flatté
GaInAsSb is a promising material for mid-infrared devices such as lasers and detectors because it is a direct band gap material with large radiative coefficient and a cut-off wavelength that can be varied across the mid-infrared (from 1.7 to 4.9 μm) while remaining lattice matched to GaSb. On the other hand, the potential of the alloy is hampered by predicted ranges of concentration where the constituents of the alloy become immiscible when the crystal is grown near thermodynamic equilibrium at typical growth temperatures. There have been efforts to extend the wavelength of GaInAsSb alloys through such techniques as digital alloy growth and non-equilibrium growth, but most of the compositional range has for a long time been inaccessible due to immiscibility challenges. Theoretical studies also supported the existence of thermodynamic immiscibility gaps for non-equilibrium growth conditions.
Lower growth temperatures lead to shorther adatom diffusion length. While a shorter adatom diffusion length suppresses phase separation, too short an adatom length is associated with increased defect formation and eventually loss of crystallinity. On the other hand, hotter growth temperatures move epitaxial growth closer to thermodynamic equilib- rium conditions, and will eventually cause phase separation to occur. In this study thick 2 μm; bulk GaInAsSb layers lattice-matched to GaSb substrates were grown across the entire (lattice-matched) compositional range at low growth temperatures (450°C), including the immiscibility region, when grown under non-equilibrium conditions with MBE. High quality epitaxial layers were grown for all compositions, as evidenced by smooth morphology (atomic force microscopy), high structural quality (X-ray diffraction), low alloy fluctuactions (electron dispersive spectroscopy in cross sectioned samples), and bright room temperature photoluminescence.
Because initial theoretical efforts have suggessted that lattice strain can influence layer stability, we have studied effects of strain on alloy stability. Unstable and metastable alloys were grown hot enough for the onset of phase separation, then progressively strained and characterized. We show that strain is effective in suppressing phase separation.
Finally, we performed time-resolved carrier lifetime measurements for InAsSb alloy with low concentrations of Ga to investigate the role of Ga in influencing nonradiative carrier recombination. There have been studies on non-Ga containing antimonide structures (InAsSb, InAs/InAsSb) that show long carrier lifetimes, which suggest that Ga plays a role in reducing carrier lifetime, because Ga-containing structures such as InAs/GaSb superlattices have much shorter carrier lifetimes. Ga may reduce carrier lifetime through native defects that increase background carrier concentration, or that create mid-gap electronic states. Here, a series of GaInAsSb alloys were grown with low to zero Ga concentration. No difference in carrier lifetime was observed between Ga and Ga-free structures, and minority carrier lifetimes > 600 ns were observed. Additional work remains to be done to obtain background carrier densities in the samples with Hall measurements.
GaInAsSb semiconductor alloys grown on GaSb are a potentially important material for mid-infrared devices (2 − 5 μm). However, growth of this alloy has been plagued by an immiscibility gap (elements do not mix properly and the alloy may segregate into multiple binaries and alloys). In this dissertation, it is shown that using strain and nonequilibrium epitaxial growth techniques (such as molecular beam epitaxy growth), phase separation of the alloy into binary, ternary, or other quaternary parts can be suppressed, and an optically high quality, mixed alloy can be grown. Additionally, Ga containing alloys have been implicated in degrading the carrier lifetime, an important metric for many types of optoelectronic devices. Ga is induced to InAsSb alloys and the role of Ga defects in carrier lifetime is investigated.
publicabstract, GaInAsSb, MBE growth, phase separation, semiconductor
xiii, 102 pages
Includes bibliographical references (pages 96-102).
Copyright 2014 Asli Yildirim
Yildirim, Asli. "Phase separation and defect formation in stable, metastable, and unstable GaInSaSb alloys for infrared applications." PhD (Doctor of Philosophy) thesis, University of Iowa, 2014.