Control and understanding growth of III-V nanowires structures

Yunlong Zi, Purdue University

Abstract

Scaling down conventional field effect transistors (FET) has shown the limitation of the inverse subthreshold slope S>=60 mV/dec, since the operation of the conventional FET is based on the gate modulation of the thermal emission of charge carriers. To overcome this limitation, the tunneling FET based on the modulation of carriers tunneling through bandgap barriers has been demonstrated to have S smaller than 60 mV/dec. With the low effective mass and the high mobility, III-V nanowires have been proven to be the perfect material to enhance the performance of the tunneling FET. Especially, the InAs/GaSb core/shell nanowire heterostructure with the broken bandgap alignment will yield the minimum tunneling barrier and high tunneling probability. To achieve that, synthesis of InAs, GaSb nanowires and core/shell nanowires have to be understood and well controlled. To do that, controlled nanowire growth in a simple vapor deposition system was demonstrated in several nanowire systems. Metal-organic chemical vapor deposition (MOCVD) was also utilized to perform the GaSb film growth. ^ Au-catalyzed InAs nanowires were synthesized in our home-built simple vapor deposition system. Free-standing and planar InAs nanowires were grown with the same catalytic interface of (111)B even though the growth directions are different. The different growth behaviors in the termination time were observed in the growth of these two nanowires. A model based on the revised Gibbs-Thomson equations for the growth of the both nanowires was developed, showing the difference in supersaturations. By using this model, we predicted and successfully confirmed through experiments that the free-standing InAs nanowire growth was expected to be promoted by higher arsenic vapor partial pressure. The difference in critical diameters of both the planar and free-standing nanowire growths was also predicted and demonstrated experimentally. ^ Self-catalyzed InAs nanowire growth on III-V substrates was also performed in our simple vapor deposition system. The growth direction was always along <111>B directions. The pits thermally created on the substrates' surface were hypothesized to initiate the self-catalyzed nanowire growth. The selected area growth with pits created by thermal annealing on GaAs substrate was performed to demonstrate the hypothesis, with pits identified in prepared cross-sectional TEM sample. The pre-growth annealing step was demonstrated to promote the density of nanowire growth. ^ GaSb nanowires were grown in our simple vapor deposition system with Au particles as catalysts. The growth direction was found to be <220>. The grown GaSb nanowires were found to be highly p-type, which shows very few gate modulations in fabricated nanowire FET. In-situ doping method with Te as the doping element was developed to counter-dope GaSb nanowires. Results from TEM-EDX and electrical characterization of Te-doped GaSb nanowires show successful doping of Te in nanowires. ^ Au-catalyst-free InAs nanowires were preferred as core material for core/shell structure synthesis. In the simple vapor deposition system with solid GaSb powder as precursors, serrated GaSb shell with the thickness of about 6 nm was grown, as the core/shell structure identified by TEM and EDX. In the MOCVD system, GaSb thin film growth was performed on GaSb and InAs substrates. Further optimized recipe is still required to be developed to enhance the shell thickness and to achieve more uniform surface. ^ The outlook includes fabrication and characterization of the tunneling FET based on the core/shell heterostructure, the transfer-free self-aligned InAs nanowire FET fabrication based on planar nanowire growth, position-controlled self-catalyzed InAs nanowire arrays, and III-V nanowires with differently doped segments.^

Degree

Ph.D.

Advisors

Chen Yang, Purdue University.

Subject Area

Nanoscience|Physics, Astronomy and Astrophysics|Physics, Theory|Engineering, Materials Science

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