Germanium tin nanowires: synergy at the nanoscale

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Doherty, Jessica
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University College Cork
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The race to create alternative, Si compatible, scalable, tuneable device materials over the past number of years has led to a focus on group IV elements. Alloying group IV semiconductors, such as Ge or Si with group IV metals such as Sn and Pb, can lead to direct bandgap semiconductors, as in III-V materials, but with the distinct advantage over III-Vs of being Si compatible. A direct bandgap group IV semiconductor would be beneficial for the development of mid-IR optoelectronic devices such as photodetectors. However, due to the lattice mismatch between Ge and Sn, Ge1-xSnx thin films often experience large amounts of strain. Compressive strain shifts the energy gap to lower wavelengths, therefore, in order to achieve a direct bandgap, more Sn incorporation is necessary. A promising solution to overcome strain induced in GeSn thin films is the move towards 1-D GeSn nanostructures; nanowire morphology allows for increased strain relaxation compared to thin films due to free sidewall facets. This thesis aims to demonstrate the advancements in Ge1-xSnx nanowires, and branched nanostructures, and their application in various fields and devices. Chapter 1 presents a review of the recent advances in Ge1-xSnx materials, with a focus on recent advances in the field, in particular concerning nanostructures i.e. nanowires and nanoparticles. I aim to summarise the recent developments in the growth and characterisation of Ge1-xSnx films and nanostructures, and their application in electronic, optoelectronic and other devices. I will briefly discuss the theoretical insights on Ge1-xSnx material to provide an essential historical starting point for what has become an increasingly popular material. The growth and properties of Ge1-xSnx thin films will also be considered, as this material has been reported on sporadically over the last 30 years prior to the popularisation of Ge1-xSnx. Chapter 2 describes the growth of Ge1-xSnx nanowires with x = 0.09. The addition of an annealing step close to the Ge-Sn eutectic temperature (230 ºC) during cool-down further facilitated the excessive dissolution of Sn in the nanowires. Sn was distributed throughout the Ge nanowire lattice with no metallic Sn segregation or precipitation at the surface or within the bulk of the nanowires. The non-equilibrium incorporation of Sn into the Ge nanowires was attributed to a kinetic trapping model for impurity incorporation at the triple-phase boundary during growth. Chapter 3 details the incorporation of this same high Sn content (x = 0.09) without the use of a eutectic anneal, thereby increasing the relative atomic ordering. This Sn incorporation was achieved by altering the growth parameters of the system to increase the nanowire growth rate, thereby confirming solute trapping as the mechanism of Sn inclusion. The profound impact of growth kinetics on the incorporation of Sn; from 7 to 9 at. %; in Ge1-xSnx nanowires was clearly apparent, with the fastest growing nanowires (of comparable diameter) containing a higher amount of Sn. The participation of a kinetic dependent, continuous Sn incorporation process in the single-step VLS nanowire growth resulted in improved ordering of the Ge1-xSnx alloy lattice; as opposed to a randomly ordered alloy. The amount of Sn inclusion and the Sn impurity ordering in Ge1-xSnx nanowires has a profound effect on the quality of the light emission (narrowing of the photoluminescence spectra) and on the directness of the band gap as confirmed by temperature dependent photoluminescence study and electron energy loss spectroscopy. Chapter 4 reports the functionality of these Ge1-xSnxnanowires in optoelectronics as photodetectors (x = 0.105). The structural and optical quality of these high Sn content GexSn1-x nanowires was investigated to determine their applicability and functionality in photodetector devices. The as-grown Ge1-xSnx nanowires were single crystalline with a direct bandgap of 0.59 eV, as determined from photoluminescence spectroscopy. These highly crystalline direct bandgap Ge1-xSnx nanowires, with narrow emission widths, uniform morphologies and chemical homogeneity were found to be to be ideal candidates for photodetectors due to their high responsivity and broad range photoresponse. Chapter 5 also explores the functionality of Ge1-xSnx nanowires, this time with lower Sn incorporation (x = 0.048), in energy storage as Li-ion anode materials. Ge1-xSnx nanowires were predominantly seeded from the Au0.80Ag0.20 catalysts with negligible amount of growth also catalysed from stainless steel substrate. The electrochemical performance of the the Ge1-xSnx nanowires as an anode material for Li-ion batteries was investigated via galvanostatic cycling and detailed analysis of differential capacity plots. Chapter 6 informs on the development of Ge1-xSnx branched nanostructures. A growth mechanism is proposed for these novel nanostructures; with trunk components comprised of 4.4 at. % Sn and branches containing 8.0 at. % Sn; fabricated in a one step growth. The trunks are seeded from Au0.80Ag0.20 nanoparticles followed by the epitaxial growth of secondary branches (diameter ~ 50 nm) from the excess of Sn on the sidewalls of the trunks, as determined by scanning transmission electron microscopy (STEM) and energy dispersive X-ray (EDX) analysis. The nanowires, with directed GeSn branches oriented at ~ 70 ° to the trunks, have no apparent defects or change in crystal structure at the trunk-branch interface; structural quality is retained at the interface with epitaxial crystallographic relation. These Ge1-xSnx nanostructures are also explored as anode materials for Li-ion batteries, as their increased charge carrier pathways, mechanical strength and surface area result in increased capacities over conventional nanowires. Chapter 7 depicts the influence of pressure on the growth Ge1-xSnx nanowires. A move to a supercritical fluid growth regime results in the incorporation of colossal amounts of Sn in the Ge nanowire lattice, with 0.1 ≤ x ≤ 0.35. Sn incorporation in the Ge1-xSnx nanowires was found to be strongly diameter dependent, with small diameter nanowires containing higher amounts of Sn relative to nanowires with larger diameters. A colossal Sn content of 35 at. % was achieved in Ge1-xSnx nanowires with diameters of ~ 20 nm. EDX analysis of the Ge1-xSnx nanowires verified the homogeneous distribution of Sn throughout the nanowires, even for the high Sn content nanowires, without apparent clustering or segregation of Sn. Finally, Chapter 8 details the conclusions of this thesis, and a future outlook for Ge1-xSnx nanowires is provided.
Nanowires , Electron microscopy , Germanium tin
Doherty, J. A. 2018. Germanium tin nanowires: synergy at the nanoscale. PhD Thesis, University College Cork.