Growth and Analysis of Semiconductor Heterojunction Nanowires
Date Issued
2016
Date
2016
Author(s)
Shen, Tzu-Hsien
Abstract
Formation of abrupt and defect-free Si/Ge heterostructures is of great importance for device applications. One of the advantages of this structure is that the lattice strain due to the 4.18% lattice mismatch between Si and Ge may be helpful to improve the optoelectronic properties of the two materials. However, when such heterojunctions are fabricated in thin-film structures, misfit dislocations are inevitably formed at the heterointerfaces, losing the desired strain state. One approach to get rid of this obstacle is by growing the heterojunction in nanowires, in which the strain can be relaxed elastically. The aim of this study is to grow nearly perfect Si/Ge heterojuncitons in nanowires with sufficient understanding of the controls of their morphology, composition, interfacial abruptness, and strain field near the interface. An ultra-high vacuum chemical vapor deposition (UHV-CVD) reactor is used for the nanowire growth, and it is equipped with in-situ metal evaporation systems for preparing the catalysts. In order to create compositionally abrupt Si/Ge interfaces, AgAu solid catalysts are used to fabricate Si/Ge/Si heterojunction nanowires via the vapor-solid-solid (VSS) mechanism. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) analysis shows that the Si/Ge heterointerface can be as sharp as merely 1 nm and no misfit dislocations are observed at the heterointerface; therefore, coherent strains are produced in the lattice near the interface and the strain distribution is quantitatively measured using the geometric phase analysis (GPA) method. The strain effect on the electronic property of Si is revealed in electron energy-loss spectroscopy (EELS) analysis – a slight shift of the loss energy is observed in the strained silicon lattice. For applications, the heterojunction nanowires should be long enough for device fabrication and we therefore use a two-step growth method using AgAu alloy catalysts: a long Si nanowires is grown via the vapor-liquid-solid (VLS) method and Si/Ge/Si heterojunctions are subsequently grown via the VSS method on the Si nanowires. In order to create a larger strain in Si lattice, we propose to grow Ge/Si/Ge heterojunction nanowires. Yet, the Ge nanowires can be grown using Au as the catalysts and it is found that the individual Ge nanowires are fabricated with the use of disperse AuSi eutectic droplets prepared by a pre-annealing treatment.
Subjects
Si/Ge heterojunction nanowires
vapor-liquid-solid (VLS)
vapor-solid-solid (VSS)
transmission electron microscopy (TEM)
geometric phase analysis (GPA)
electron energy-loss spectroscopy (EELS)
Type
thesis
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