Estiak Ahmad – Seminar/Ph.D. Thesis Defense – Thursday, June 29, 2017 at 2:00 P.M.

JSNN – Estiak Ahmad – Ph.D. Thesis Defense/Thursday Seminar

Candidate: Estiak Ahmad

Advisor and Committee Chair: Shanti Iyer, Ph.D.

Department: Nanoengineering

Date: Thursday, June 29, 2017

Time: 2:00 P.M. – 4:00 P.M.

Location: JSNN 209 Open Room

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.”


Sb-based III-V nanowire (NW) 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 photodetectors, 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, and GaAs1-xSbx NWs are promising due to the higher sensitivity for photon detection offered by one dimensional architecture of the NW.

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, and maximum red shift in the PL emission wavelength that can be achieved before the onset of degradation in the optical quality. Optical emission tuning of 1.12 eV at room temperature on Si(111) substrates has been achieved for the first time, 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.

Controlled doping is a primary requirement for reliable nanowire-based opto-electronic device fabrication. Group VI element, Te, was investigated as an 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. Finally, axial p-i-n junction GaAs1-xSbx/AlxGa1-xAs core-shell nanowire ensemble photodetector was fabricated. Photodetector performance characteristics, namely responsivity of 523 A/W, external quantum efficiency of 1.2×105 % and detectivity of 2.6×1012 jones were achieved at room temperature. Based on these results, this work demonstrates great potential for GaAs1-xSbx based p-i-n photodetector in the near-infrared region, and the insight provided by this study pave the way to fabricate reliable Sb based novel opto-electronic devices integrated on Si platform.