Estiak Ahmad – Seminar/Ph.D. Preliminary Defense, Friday, August 26, 2016 at 11:00 A.M.

JSNN – Estiak Ahmad – Ph.D. Preliminary Thesis Defense/Friday Seminar

Candidate: Estiak Ahmad

Advisor: Shanthi Iyer, Ph.D.

Department: Nanoengineering

Date: Friday, August 26, 2016

Time: 11:00 A.M. – 1:00 P.M.

Location: JSNN Auditorium

2907 E. Gate City Blvd., Greensboro, NC 27401

Title: “A Study of GaAs1-xSbx Axial Nanowires and PIN Junctions by Self-Catalyzed Molecular Beam Epitaxy.”


Semiconductor nanowires have stimulated great interest due to its one-dimensional architecture, which exhibits unique properties and enables elastic relaxation of the lattice mismatched-induced strain at the edges. These features offer the potential of innovative opportunities for low-cost, high-performance nanophotonic devices and a viable pathway for integration with electronic devices on Si platform, as well as other nanoscopic and microscopic systems. Generally, nanowires are grown via the vapor-liquid-solid mechanism using gold (Au) as a catalyst. However, the commonly used seed, Au, induces mid-gap levels in Si-compatible technological processes. Self-assisted (Au-free) growth overcomes these issues and yields nanowires of high structural and optical qualities that are also scalable for large scale production.

Among NWs, Sb-based III-V nanowire material systems have attracted great interest due to its tunable bandgap and high intrinsic mobility. The bandgap of GaAs1-xSbx covers an important wavelength range from 870 nm (GaAs) to 1700 nm (GaSb), which lends itself to applications in the next generation optoelectronic devices, namely solar cells, optical telecommunication applications, photonic integrated circuits and quantum information science. In particular, the wavelength near 1300 nm represents one of the telecommunication wavelength windows of interest.

The goal of this work is to successfully demonstrate the p-i-n junction in axial configured GaAs1-xSbx NWs and its spectral responsivity close to telecommunication wavelength range for nanoscale photodetector applications. This requires growth of GaAs1-xSbx nanowires of high quality and doping of these NWs, in addition to determining the maximum red shift in wavelength that can be achieved as close as possible to 1.3 µm by Sb incorporation.

Self-catalyzed molecular beam epitaxy technique has been used for the growth of GaAs1-xSbx NWs in the axial configuration. A systematic investigation has been carried out using a variety of characterization techniques to ascertain the influence of Sb incorporation on the compositional homogeneity, microstructural quality, nature of the photoluminescence (PL) transitions, and maximum red shift in the PL emission wavelength that can be achieved before the onset of degradation in the optical quality. Sb incorporation up to 16 at.% has been achieved for the first time corresponding to an optical emission tuning of 1.12 eV at room temperature on Si(111) substrates, with no noticeable planar defects.

It has been challenging to further reduce the bandgap corresponding to the wavelength of 1.3 µm, due to the narrow growth window as the growth mechanism for axial configuration is primarily a thermodynamically driven vapor-liquid-solid growth process. Hence, a systematic study has been carried out to ascertain the impact of different growth parameters on Sb incorporation and correlated to NW morphology, density and growth rate. The results of this study successfully enabled a two-phased growth temperature technique to achieve maximum Sb incorporation of 34 at.% corresponding to a red shift of the wavelength to the desired 1.3 µm. Although the NWs are not of high quality, a plausible pathway for incorporating higher Sb content has been demonstrated.

The other primary requirement towards reliable nanowire-based electronic and optoelectronic device fabrication is the controlled doping. Thus far, the group IV element Si has been commonly used as the n-type dopant for III-V thin films and nanowires. However, due to its strong amphoteric behavior, group VI element, Te, is examined for n-type dopant. As direct determination of the Te concentration is not possible, or at least not possible with the usual characterization facility that is available for normal researchers, indirect evidence of the incorporation of the dopant in the NWs has been shown via comprehensive study of doped NWs using variety of characterization techniques. Using the above studies on the growth of axial GaAs1-xSbx NWs and Te as the n-type dopant, first reports on the experimental realization of a GaAs1-xSbx axial p-type/intrinsic/n-type (p-i-n) junction NW ensemble device is demonstrated on a Si substrate utilizing Te as the n-type dopant. Preliminary data on the transport parameters and dopant concentration extracted for both Te-doped and p-i-n GaAs1-xSbx nanowires by best fit of the simulated data to the experimental PL spectra and I-V characteristics, respectively, will also be presented. Based on these results, this work demonstrates great potential of GaAs1-xSbx based p-i-n photodetector. ”