Growth and characterization of high quality gaAs nanowires

Semiconductor nanowires have received tremendous attention in the recent years due to their interesting properties, but their applications have been hindered by some difficulties, including the high presence of defects in their structure. In this study, motivations for growth of high quality, defect free GaAs nanowires have been discussed briefly. To do this, the advantages and benefits of utilization of nanowires in solar cell applications have been discussed. Theoretical background on the difference between different polarities of GaAs wires, A and B, is presented, and the reason for appearance of defects in polarity B is also shortly mentioned. Furthermore, thermodynamical favorability of defected B polar wires over defect free A polar ones is reviewed. The cleanroom process required for sample preparation, including wafer etching, spin coating and annealing is described. Silicon oxide have been used on the GaAs wafers as mask for GaAS nanowire growth. Molecular Beam Epitaxy (MBE) is employed for self- catalyzed Vapor-Liquid-Solid (VLS) growth of GaAs nanowires on (100) GaAs substrate. Scanning Electron Microscopy (SEM) images are used to guess the polarities, considering the geometric constrains of A and B polar wires with respect to the underlying substrate. Optimization of MBE process for getting a low occurrence of parasitic growth, in combination with high yield of defectless A polar wires and high reproducibility is discussed in length. Parametric study on the effect of Ga flux, V/III ratio and temperature is done to uncover the best condition for high yield of A polar wires. In addition, the effect of using (111)A substrate instead of (100) and also using Atomic Layer Deposition (ALD) oxide instead of spin-coating based oxide are presented. Next, Ccathodoluminescnece (CL) technique is used to characterize the optical properties of A and B polar GaAs wires. This have been done on four layered GaAs-AlAS-AlGaAs-GaAs core-shell hetrostructure nanowires, in which the AlGaAs capping also had been done using the MBE. Extensive CL analysis of A polar GaAS nanowires has been reported for the first time. Two samples with different shell thicknesses have been used for the measurements. For each sample, room temperature and 20 K measurements have been performed. Detailed analysis of CL data reveals the difference between the optical responses of A and B polar wires and between the signals for room and cold temperature measurements. In addition, the effect of shell thickness on the optical response of the core-shell structure is discussed by direct comparison of the signals for the two samples. Finally, nanowires have been deposited on a marked TEM grid, and a combined Cathodoluminescence/ High -Angle Annular Dark-Field Imaging Scanning Transmission Electron Microscopy (HAADF-STEM) method is used to assign the definite polarities to wires. This is done by direct mapping of element in crystal lattice. The combined approach gives the opportunity to directly correlate optical properties with structural features of the wires. Complex distribution of defects at the root of some A polar wires, which includes anti-phase boundaries and in-plane inversion of polarities are shown by ultra-high resolution STEM images. This study addresses the problems of high density of defects in GaAa nanowires, and hence, paves the way for improved performance in nanowire based devices, such as solar cells and transistors.