Growth and Characterization of III-V Heterostructure Nanowires on Si (111) Substrate by MOCVD for Low Power Applications

Autor:	Ramesh Kumar Kakkerla, 康克拉
Rok vydání:	2018
Druh dokumentu:	學位論文 ; thesis
Popis:	107 In this thesis, the growth and characterization of vertically aligned Au-free InAs and InAs/GaSb heterostructure nanowires (NWs) on Si (111) substrate by Metal Organic Chemical Vapor Deposition (MOCVD) are studied. Field-emission scanning electron microscopy (FEG-SEM), high resolution transmission electron microscopy (HR-TEM), fast fourier transform (FFT) and energy dispersive x-ray spectroscopy (EDX) analysis measuremen¬ts have been used to investigate the morphology, GaSb shell thickness, crystal structures, and chemical compositions of the NWs. The effect of growth temperature on the morphology and growth rate of the InAs NWs is investigated. Control over diameter and length of the InAs NWs is achieved by varying the growth temperature. All the InAs NWs were grown layer by layer in 〈111〉direction. Due to a very small energy difference between the two Wurtzite (WZ) and Zinc-blende (ZB) phases for InAs along 〈111〉growth direction, a small change in the sequence of the layers in 〈111〉growth direction can easily lead to the change in the WZ and ZB crystal structures, resulting in mixture of WZ and ZB phases i.e, polytype crystal structure. A combination of optimized gas flow rates and growth temperature has given good crystal quality of InAs nanowires. Pure crystalline WZ InAs NWs were grown using the properly controlled growth parameters. Pure ZB InAs NWs are grown by adding antimony (Sb) during the growth. The effect of growth temperature on the morphology and growth rate of the GaSb on InAs NWs is investigated. Control over the GaSb shell thickness is achieved by using growth temperature. As the GaSb growth temperature increase, GaSb radial growth rate increases due to the increase in alkyl decomposition at the substrate surface. Diffusivity of the adatoms increases as the GaSb growth temperature increase which results in tapered GaSb shell growth. SEM and TEM measuremen¬ts revealed that the morphology and shell thickness can be tuned by the growth temperature. Electron microscopy also shows the formation of GaSb both in radial and axial directions outside the InAs NW core can be controlled by the growth temperature. GaSb shell thickness is also controlled by the growth time. The effect of V/III ratio variation on the morphology and growth rate of the GaSb on InAs NWs is also investigated. GaSb is grown under three different V/III ratios, while keeping the growth temperature and TMSb flux constant. Ga adatoms are observed to have no influence on GaSb shell growth rate. But, Ga droplets form on the nanowire tips and GaSb axial growth is observed when a relatively higher TMGa flow rate is used. Whereas, the droplets are missing and only radial GaSb shell is observed for low TMGa flow rate. The growth of GaSb on various InAs nanowire crystal structures is investigated. The GaSb is grown radially and axially on WZ, ZB and Polytype (mixture of WZ and ZB) InAs nanowires. The effect of the various InAs crystal structures on GaSb is studied. InAs NWs with Polytype, WZ and ZB crystal structures were grown using the properly controlled growth parameters. The GaSb was grown on polytype InAs core resulted in the same polytype GaSb shell. Whereas, axially grown GaSb possesses ZB structure. In case of InAs NW with WZ structure, a very good crystalline WZ GaSb shell was obtained. Surprisingly, axially grown GaSb also has the InAs core crystal structure (WZ). Axially grown GaSb possessing WZ crystal structure has never been observed before even using the external catalyst. For ZB InAs NW, both axially and radially grown GaSb have the ZB structure. Simulation of InAs/GaSb heterostructure nanowire based Tunnel Field Effect Transistor (TFET) is performed using Synopsys’s Sentaurus TCAD. The effects of device intrinsic parameters such as shell thickness, spacer length on the performance of the InAs/GaSb nanowire Tunneling FETs are investigated. Device on-current (ION) was chosen as the key figure of merit in this study. ON-current initially increases with radius up to 2 nm shell radius thickness and then starts to decrease. This increase in ON-current for a thin shell around core is attributed to the injection of carriers and the increase in the intrinsic mobility of core than shell. The length of spacer was varied from 1 nm to 5 nm with a step size of 1nm and its effect on Id-Vg was studied. The addition of spacer between source/gate electrodes results in the increase in ON-current and gate capacitance (Cgg) starts to decrease as the length of the spacer increases for core-shell nanowire TFET. It is found that ION can be improves by adding spacer up to a critical limit. To characterize the basic electrical properties of the grown InAs/GaSb heterostructure nanowires, single-nanowire with metal contacts were fabricated. The I – V characteristics show a linear Ohmic behavior so that the device resistivity can be obtained from the inverse slope ( Vsd / Id) of the curve. The calculated resistivity is around 3.4 -cm which is roughly one order of magnitude larger than the previously reported by others for undoped GaSb thin films.
Databáze:	Networked Digital Library of Theses & Dissertations
Externí odkaz:	http://ndltd.ncl.edu.tw/handle/n8npyc Zobrazit plný text záznamu