Growth of narrow band gap semiconductor nanowires on silicon and graphitic substrates by droplet epitaxy

Anyebe, Ezekiel and Zhuang, Qiandong (2015) Growth of narrow band gap semiconductor nanowires on silicon and graphitic substrates by droplet epitaxy. PhD thesis, Lancaster University.

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This thesis is focused on the growth of narrow band gap semiconductor nanowires (NWs) on silicon and graphite by droplet epitaxy. First, the growth conditions of In droplets suitable for the nucleation of NWs was identified. Vertically-aligned and non-tapered InAs NWs were then realized on bare Si. It is shown that the diameter and areal density of NWs are defined by the geometry of pre-deposited In droplets. The NWs exhibit a dominant PL peak associated with the band to band (BtB) emission in addition to a distinct BtB temperature dependent red-shift, strong emission efficiency (up to 2500C) and record narrow spectral linewidth of ~20 meV (at 10K) which is relatively smaller than previously reported values. This demonstrates the high optical properties of the droplet epitaxy grown InAs NWs. Vertically-aligned and non-tapered InAs1-xSbx NWs have been demonstrated on Si without the commonly used NWs stems. In addition, the effect of Sb addition to the morphology of self-catalyzed InAsSb NWs grown directly on Si is systematically investigated for the first time. It is shown that trace Sb flux significantly promotes lateral NWs growth while at the same time suppressing axial growth. Furthermore, Sb-induced crystal phase evolution is elucidated as a function of Sb content. Although, pure InAs NWs show a mixture of Wurtzite (WZ) and Zinc-Blende (ZB) phases, a crystal phase evolution from a highly polytypic InAs to a quasi-pure WZ InAsSb NWs (2-4% Sb content) and a quasi-pure ZB InAsSb NWs crystals (~10% Sb content) is demonstrated in addition to a significant reduction in the stacking fault density in as-grown NWs with increasing Sb content. The recent discovery of flexible graphene has triggered a new wave of optoelectronic revolution. In order to fully exploit the enormous potential of functional monolithic NWs/graphene hybrid structures, the optimal growth conditions for realizing morphologically and structurally superior InAs NWs on graphitic substrates has been identified. Vertically well-aligned and thin InAs NWs were obtained in a narrow growth window of 420-450oC while a high yield of NWs was realized within a restricted domain of growth rate and V/III flux ratio. Compared to the growths on Si, the graphitic substrate is shown to enhance adatom mobility and enable growth at high growth rate which is highly promising for cost-effective devices. In addition, the NWs on graphite show a significantly reduced density of defect in comparison to the growth on conventional Si substrates owing to van der Waals epitaxy growth technique resulting from the absence of dangling bonds. Moreover, high aspect ratio NWs are essential for functional device applications however, the growth of thin InAs1-xSbx NWs is extremely challenging owing to Sb-induced lateral growth. The growth of ultra-high aspect ratio InAs1-xSbx NWs (0 ≤ x ≤ 0.12) on graphite is demonstrated for the first time at highly As-rich conditions with potential for applications in ultra-sensitive, eco-friendly, flexible and cost-effective infrared photodetectors. It is shown that the graphitic thin film promotes Sb incorporation and is more favourable for InAsSb NWs growth in comparison to Si substrates. Finally, a morphological evolution from InN NCs to three dimensional (3D) InN islands is demonstrated with increasing growth temperature attributable to lowered surface free energy of the growing crystals with disproportionate growth velocities along different growth fronts.

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Thesis (PhD)
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08 Jul 2015 13:28
Last Modified:
12 Mar 2024 00:01