Materials in smaller scales exhibit promising properties that are useful for wide variety of applications. Semiconductor quantum wells and quantum dots are two main examples of low-dimensional systems, where the quantum wells act as two-dimensional systems and the quantum dots act as zero-dimensional systems. Alternatively, semiconductor nanowires act as one-dimensional materials, and they exhibit promising and device applicable properties. These nanowires are relatively new class of materials compared to the quantum wells and the quantum dots. The semiconductor nanowires are expected to be the building blocks for future nanoelectronic and nano-optoelectronic device technology. Vapor-liquid-solid (VLS) mechanism is a widely used mechanism for the growth of semiconductor nanowires, where metal nanoparticles are used as the catalysts for the nanowires growth. This growth mechanism offers a flexibility to control the size, morphology and location of the semiconductor nanowires. In the VLS growth, changing the chemical composition of vapor constituents produce consequent compositional modulation in each nanowire. The compositional modulation along the nanowire axis produces axial nanowire heterostructures and in radial direction produces radial nanowire heterostructures. Such compositional modulation within an individual nanowire enables the designing of band structure of a nanowire and thereby allows the fabrication of single nanowire devices. These nanowire heterostructures show many potential properties and consequent applications. Although the semiconductor nanowire heterostructures are promising semiconductor nanostructures, the fundamental growth mechanisms of axial and radial nanowire heterostructures have not been explored sufficiently due to their complex nature of the growth. In this regard, this PhD thesis addresses the fundamental issues associated with axial and radial nanowire heterostructures. For such fundamental investigations, this PhD work chooses InAs/GaAs nanowire heterostructure system due to its potential applications. In fact, InAs/GaAs nanowire heterostructures are the first reported axial nanowire heterostructure system. However, no detailed investigations were reported on this system so far. The detailed nucleation and growth mechanisms associated with InAs/GaAs nanowire heterostructure system are explored in this thesis using electron microscopy investigations. This objective is achieved in the following steps. • InAs/GaAs nanowire heterostructures are grown using Au nanoparticles and metal-organic chemical vapor deposition (MOCVD) method. To determine the axial and radial growth evolution of InAs on GaAs nanowires, different InAs/GaAs nanowire heterostructures are produced by depositing InAs for different durations on GaAs nanowires. The GaAs nanowires are initially grown for 30 min and then the InAs is deposited on these nanowires for 1, 3, 5 and 30 min. • These InAs/GaAs nanowire heterostructures are subjected to scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigations. These investigations determine that, in the initial stages of the InAs axial growth (1 min), the Au particles move sidewards and subsequently downwards by maintaining an interface with the GaAs nanowire. Such a movement of Au catalysts is attributed to lower Au/GaAs interfacial energy than Au/InAs. The detailed TEM investigations show that this Au movement depends upon the crystallographic nature of the GaAs nanowire. The Au particle is always tend to move towards {112}B sidewall of the GaAs nanowire rather than its {112}A sidewalls. Increase in InAs growth duration shows that InAs branches evolve from GaAs-InAs core-shell structures. Such evolution is observed in following steps: (1) the movement of Au particle terminates when it encounters the radially grown InAs on GaAs nanowires; (2) further growth of InAs leads to the InAs nanowire growth from those terminated Au nanoparticles in the form of branches. • The TEM observations of InAs/GaAs nanowire heterostructures show that, in the initial stages of InAs radial growth on GaAs nanowires, InAs nucleates preferentially in the concave regions of the non-planar sidewalls of the GaAs nanowire. The further growth of InAs leads to the preferential formation of InAs shell structure at the regions of concave regions. Such heterogeneous formation of shell structure resembles InAs nanoring structures around GaAs nanowire cores. InAs growth on the planar {112} sidewalls of GaAs nanowires with hexagonal cross sections shows different growth phenomena to the above described InAs nanorings formation. In this case, InAs preferentially nucleates on {112}A sidewalls of the GaAs nanowires and with further deposition of InAs, the complete shell structure of InAs form with {110} sidewalls on the GaAs nanowire cores. • In addition to the above mentioned investigations, to observe the growth evolution of GaAs on InAs nanowires, GaAs is grown for 3 and 30 min on InAs nanowires. The TEM investigations of these nanostructures show that the axial GaAs/InAs hetero-interface contains an InGaAs transition segment in contrast to the sharp InAs/GaAs (InAs on GaAs) hetero-interface. The different nature of hetero-interfaces is attributed to the different affinities between Au catalysts and Ga or In. The radial growth of GaAs on InAs nanowires show that the GaAs shell has grown in wurtzite structure around the wurtzite structured InAs nanowire cores. Overall, through the extensive SEM and TEM investigations, this PhD thesis addresses the fundamental issues related to the growth of axial and radial nanowire heterostructures. Such fundamental investigations are expected to advance the processing and application prospective of the semiconductor nanowires and their associated heterostructures.
| Identifer | oai:union.ndltd.org:ADTP/282829 |
| Creators | Mohanchand Paladugu |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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