Fabricating Si[111] Nanostructures on Graphene by Aluminum-Induced Crystallization for High Yield Vertical III-V Semiconductor Nanowire Growth

III-V semiconductor nanowire-graphene photovoltaics is an emerging technology that has the potential of highly efficient, flexible and ultra-thin solar cells. This thesis has explored aluminum-induced crystallization(AIC) of amorphous silicon on graphene and graphite surfaces, with the aim of increasing the nucleation yield of nanowires. It is demonstrated that thin films of aluminum and amorphous silicon, separated by an oxide membrane, produces [111]-oriented crystalline silicon after annealing at 470-500°C, well below the crystallization temperature of silicon, for glass, kish graphite and graphene substrates. The crystal orientation and elemental composition of the samples are characterized by X-ray diffraction(XRD), electron backscatter diffraction(EBSD) and energy-dispersive X-ray spectroscopy(EDX). The crystallized silicon forms semi-continuous films on glass and graphene, while it forms dendrite structures on kish graphite substrates. Silicon crystallization is achieved with both sputtering and electron beam evaporation of aluminum, and it is suggested that the aluminum microstructure is the determining factor for whether silicon crystallization occurs or not. The developed process for AIC of amorphous silicon is combined with the one-shot exposure electron beam lithography(EBL) technique to pattern silicon nanodot arrays on few-layer graphene substrates, showing high yields and controllable dot diameter sizes of 80-160 nm. The processes developed in this thesis would have a high potential for the high-density nucleation of vertically aligned, self-catalyzed gallium arsenide(GaAs) nanowires on graphene, enabling the fabrication of an ultra-thin solar cell with GaAs nanowires as the photoactive component and graphene as the bottom electrode

Semiconductor-graphene hybrid structures and optoelectronic devices

In order to utilise the intriguing properties discovered in nanomaterials during the last decades, structures and devices that do exactly that have to be developed. Graphene, the carbon wondermaterial discovered in 2004, has baffled the scientific community with its endless possibilites for quantum physics experiments, as well as extreme properties for current conduction, transparency and heat dissipation. Semiconductor nanowires with their one-dimensionality have been able to solve some fundamental challenges in thin films, as they enable the integration of optically active semiconductors onto both conventional and new substrates. Together with a precise control of crystal design, this enables the integration of lasers, solar cells, photodetectors and light emitting diodes with different substrates not suitable for bulk growth. To take advantage of the possibilities offered by combining these two very different nanostructures, complex fabrication strategies, diligent and multifaceted characterisation work and extensive collaborations have to be employed to reach the end-goal of actual functional devices. We have studied several approaches to enable the fabrication of graphenesemiconductor hybrid devices. The transfer of graphene with a metal protection layer has been studied in detail to try and realise molecular beam epitaxycompatible chemical vapour deposited graphene for the direct growth of semiconductor nanowires. The growth of GaAs, GaAsSb and GaN nanowires on kish graphite has been investigated both with and without an oxide mask for position-controlled growth of the nanowires. A two-layer mask structure consisting of alumina and silicon oxide was found to give the best selectivity, while the oxygen plasma and subsequent hydrogen annealing treatment of graphite was found to increase the nucleation probability of nanowires. To further increase the nucleation probability of GaAs and GaAsSb nanowires on chemical vapour deposited graphene, a thin film consisting of aluminium-induced crystallised Si was deposited. By varying the silicon surface oxide formation and optimising the nanowire growth, high-density vertical nanowires could be grown. Not only did the thin film of polycrystalline Si strongly increase the density of vertical nanowires, it was also discovered that the presence of graphene directly affected the crystallinity of the aluminium-induced crystallised Si, when compared with the same substrate without the graphene layer. The deposited Si had an increase in (111)-oriented grains with respect to the surface plane, which is beneficial for the growth of vertical GaAs nanowires, which also grow in the [111]-direction. The graphene is not damaged as aluminium-induced crystallisation of Si takes place at relatively low temperatures (< 577 C), and apparently adopts the strain present in the polycrystalline Si. An epitaxial model is suggested to explain the effect of graphene on Si crystallisation. A photodetector consisting of the graphene and aluminium-induced silicon was fabricated, but as of yet reliable operation could not be achieved. The growth of GaN/AlGaN nanowires on bi- and single-layer graphene was studied and used for the fabrication of ultraviolet light emitting diodes using graphene as a transparent conductive electrode. The nitrogen plasma extensively damages graphene, but through the use of a special nucleation scheme using an AlN buffer layer the damage can be decreased and graphene can inject current through the photoactive nanowires. Emission in the UVA region (365 and 352 nm) was achieved, and the simultaneous use of graphene as growth substrate and transparent conductive electrode was demonstrated for the first time for any device. Together, this work demonstrates the possibilities and potential platforms of certain semiconductor-graphene optoelectronic devices, possibly enabling the development of commercial technology in the future

Fabricating Si[111] Nanostructures on Graphene by Aluminum-Induced Crystallization for High Yield Vertical III-V Semiconductor Nanowire Growth

III-V semiconductor nanowire-graphene photovoltaics is an emerging technology that has the potential of highly efficient, flexible and ultra-thin solar cells. This thesis has explored aluminum-induced crystallization(AIC) of amorphous silicon on graphene and graphite surfaces, with the aim of increasing the nucleation yield of nanowires. It is demonstrated that thin films of aluminum and amorphous silicon, separated by an oxide membrane, produces [111]-oriented crystalline silicon after annealing at 470-500°C, well below the crystallization temperature of silicon, for glass, kish graphite and graphene substrates. The crystal orientation and elemental composition of the samples are characterized by X-ray diffraction(XRD), electron backscatter diffraction(EBSD) and energy-dispersive X-ray spectroscopy(EDX). The crystallized silicon forms semi-continuous films on glass and graphene, while it forms dendrite structures on kish graphite substrates. Silicon crystallization is achieved with both sputtering and electron beam evaporation of aluminum, and it is suggested that the aluminum microstructure is the determining factor for whether silicon crystallization occurs or not. The developed process for AIC of amorphous silicon is combined with the one-shot exposure electron beam lithography(EBL) technique to pattern silicon nanodot arrays on few-layer graphene substrates, showing high yields and controllable dot diameter sizes of 80-160 nm. The processes developed in this thesis would have a high potential for the high-density nucleation of vertically aligned, self-catalyzed gallium arsenide(GaAs) nanowires on graphene, enabling the fabrication of an ultra-thin solar cell with GaAs nanowires as the photoactive component and graphene as the bottom electrode

Fabrication of Si(111) crystalline thin film on graphene by aluminum-induced crystallization

We report the fabrication of a Si(111) crystalline thin film on graphene by the aluminum-induced crystallization (AIC) process. The AIC process of Si(111) on graphene is shown to be enhanced compared to that on an amorphous SiO2 substrate, resulting in a more homogeneous Si(111) thin film structure as revealed by X-ray diffraction and atomic force microscopy measurements. Raman measurements confirm that the graphene is intact throughout the process, retaining its characteristic phonon spectrum without any appearance of the D peak. A red-shift of Raman peaks, which is more pronounced for the 2D peak, is observed in graphene after the crystallization process. It is found to correlate with the red-shift of the Si Raman peak, suggesting an epitaxial relationship between graphene and the adsorbed AIC Si(111) film with both the graphene and Si under tensile strain

GaN/AlGaN nanocolumn ultraviolet light-emitting diode using double-Layer graphene as substrate and transparent electrode

The many outstanding properties of graphene have impressed and intrigued scientists for the last few decades. Its transparency to light of all wavelengths combined with a low sheet resistance makes it a promising electrode material for novel optoelectronics. So far, no one has utilized graphene as both the substrate and transparent electrode of a functional optoelectronic device. Here, we demonstrate the use of double-layer graphene as a growth substrate and transparent conductive electrode for an ultraviolet light-emitting diode in a flip-chip configuration, where GaN/AlGaN nanocolumns are grown as the light-emitting structure using plasma-assisted molecular beam epitaxy. Although the sheet resistance is increased after nanocolumn growth compared with pristine double-layer graphene, our experiments show that the double-layer graphene functions adequately as an electrode. The GaN/AlGaN nanocolumns are found to exhibit a high crystal quality with no observable defects or stacking faults. Room-temperature electroluminescence measurements show a GaN related near bandgap emission peak at 365 nm and no defect-related yellow emission

Graphene-Based Transparent Conducting Substrates for GaN/AlGaN Nanocolumn Flip-Chip Ultraviolet Light-Emitting Diodes

Flip-chip ultraviolet light-emitting diodes based on self-assembled GaN/AlGaN nanocolumns have been fabricated, exploiting single-layer graphene not only as a growth substrate but also as a transparent conducting electrode. High crystalline quality of the nanocolumns is confirmed by detailed electron microscopy characterization, also showing the intrinsic GaN quantum disk in the active region of the nanocolumns. These features are further confirmed in the optical emission, where the absence of defect-related yellow emission and the presence of blue-shifted (from the usual 365 nm band gap emission of bulk wurtzite GaN) emission at ∼350 nm, ascribed to quantum confinement and strain effects, are observed. Despite a noticeable graphene damage after the nanocolumn growth that causes high sheet resistance of graphene and high turn-on voltage, the proof of concept of single-layer graphene used as the transparent conducting substrate for a nanocolumn device is demonstrated. This study offers an alternative platform for the fabrication of next-generation nano-optoelectronic and electronic devices