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<p> InGaAs/InP heterostructure nanowires (NWs) grown by gas source molecular beam epitaxy (GS-MBE) have been analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive x-ray spectroscopy (EDS). The morphology and interfacial properties of these structures have been compared to pure InP NWs and lattice-matched InGaAs!InP superlattice (SL) structures, respectively. Based on high-resolution x-ray diffraction (HRXRD) and photoluminescence (PL) measurements of the SLs a detailed structural model is proposed, consisting of strained InAsP and InGaAsP mono layers due to group-V gas switching and atomic exchange at the SL interfaces. The interfaces of the heterostructure NW s were an order of magnitude larger than those of the SLs and showed a distinct bulging morphology. Both of these characteristics are explained based on the slow purging of group-III material from the Au catalyst. Growth of lnGaAs on the sidewalls of the InP base of these wires was also observed, and occurs due to the shorter diffusion length of Ga adatoms as compared to In. </p> / Thesis / Master of Science (MSc)

<p> In the current research, use of p-n junction GaAs nanowires (NWs) grown by gas source molecular beam epitaxy on GaAs (111) B substrates for the fabrication of flexible solar cells are reported. The solar cells were fabricated by embedding the NWs in a polymer matrix (SU8 2), followed by ohmic contact formation to the tops of the NWs as well as the rear side of the substrate. I-V characteristic curves were obtained by illuminating the solar cells using a solar simulator, indicating a photovoltaic effect. NWs were also detached from the substrate by different methods and successfully transferred onto a flexible substrate for potential use as solar cells. Scanning electron microscopy was used throughout the research for characterization and optimization of the fabrication processes including NW embedment, removal from the substrate, and contact formation.</p> / Thesis / Master of Applied Science (MASc)

InAs/GaAs quantum dots (QDs) embedded within InP/GaP nanowires (NWs) were grown on Si substrates by Au-assisted and self-assisted vapor-liquid-solid (VLS) growth using molecular beam epitaxy (MBE). The morphology and structure of the NWs was characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The samples were analysed using photoluminescence (PL) and photocurrent measurements to study the properties of NW-based QDs. The composition of InAs x P 1-x QDs embedded within InP NWs was varied from x = 0.25 to x = 1, demonstrating the tuning of quantum confined energy levels. PL measurements demonstrated an emission peak that shifted towards lower energy levels as the As composition was increased. This result was also observed for QD absorption peaks through wavelength-dependent room temperature photocurrent measurements. InP NWs were successfully passivated with an AlInP shell, which was demonstrated through PL analysis. The growth mechanism of patterned self-assisted GaP NWs on Si was studied through SEM and TEM analysis. It was found that for large V/III flux ratios the Ga seed particle reduced in volume throughout growth, which led to a smaller NW diameter. Conversely, for small V/III flux ratios the Ga seed particle increased in volume throughout growth, resulting in larger NW diameters. The dependence of V/III flux ratio on NW growth was characterized, allowing the tuning of NW diameter. iv GaP NWs with p-i-n junctions were fabricated on a Si substrate with GaAs QDs embedded within the intrinsic region. To the author’s knowledge, this is the first time such a device was demonstrated. The device demonstrated diode characteristics as expected for a p-n junction. Wavelength-dependent photocurrent measurements demonstrated the absorption of light within GaAs QDs, which was collected through electric field dependent tunneling and thermionic emission. The absorption of light extended beyond the bandgap of GaP due to the GaAs QDs. / Thesis / Doctor of Philosophy (PhD)

Multispectral capabilities of nanowires (NWs) were explored for InAs and InAsSb NWs on Si(111) substrates. NWs were grown with the vapour-solid (VS) growth mode in a molecular beam epitaxy (MBE) system using an oxide template to control positions and diameters. Early attempts to integrate InSb NWs and silicon substrates proved unsuccessful. Instead studies of InAs NWs on silicon, and eventually InAsSb/InAs NWs on silicon were completed to achieve large-diameter, infrared (IR) sensitive photodetectors. InAs NWs were grown on silicon substrates to study their morphology characteristics and vertical NW yield. The five different growth modes explored were (1) Au-assisted vapour-liquid-solid (VLS), (2) positioned Au-assisted, (3) vapour solid, (4) positioned Au-assisted VLS using a patterned oxide mask (VLS-SAE), and (5) selective area epitaxy (SAE) using a patterned oxide mask. Optimal temperature and V/III flux ratios for achieving a high vertical yield were found for the SAE growth mode. Further understanding of the InAs SAE growth mode was gained through modeling of various oxide hole filling scenarios. Each scenario was defined by the arrival rates of the group III and group V materials to the holes. A parameter space is discussed for the growth of high yield InAs NWs, dependent on the V/III flux ratio and temperature of growth. Large diameter InAsSb NWs for IR absorptance were grown on silicon using a high yield InAs stem. Several NW array diameters were grown simultaneously on the same substrate to measure multispectral photodetection. Diameters were controlled by NW spacing. Fourier transform IR (FTIR) spectroscopy was used to measure absorptance in the NWs. NW diameters spanned 440 – 520 nm which resulted in enhanced absorptance in the short-wave IR region. Simulations of the HE11 resonances of the NW arrays were performed and compared with the FTIR measurements. Initial electrical measurements demonstrated a diameter-dependent photocurrent. / Thesis / Doctor of Philosophy (PhD)

Since strain changes the interatomic spacing of matter and alters electron and phonon dispersion, an applied strain ϵ can modify the thermal conductivity κ of a material. This thesis shows how the strain induced by heteroepitaxy is a passive mechanism to change κ in a thin film and how the film thickness is key to the functional form of κ(ϵ). Molecular Dynamics simulations of the physical vapor deposition and epitaxial growth of ZnTe thin films provide insights into the role of interfacial strain on the thermal conductivity of a deposited film. ZnTe films grown on a lattice mismatched CdTe substrate exhibit ~6% in-plane biaxial tension and ~7% out-of-plane uniaxial compression. In the T=700 K to 1100 K temperature range, the conductivities of strained ZnTe layers that are 5 unit cells thick decrease by ~ 35%, a result that is relevant to thermoelectric devices since strain can also enhance charge mobility and increase their overall efficiency. The resulting understanding of dκ/dT shows that strain engineering can also be used to create a thermal rectifier in a material that is partly strained and partly relaxed, like at the junction of an axial nanowire heterostructure. To better isolate the role of strain, the study is extended to free-standing ZnTe films with thicknesses between 116 Å to 1149 Å under the application of both uniform and biaxial strain between -3% to 3% at 300 K. Since the boundaries of the film are diffuse, κ becomes size dependent when the film thickness approaches the order of the mean free path of the phonons. As this thickness is decreased, the magnitude of κ decreases until boundary scattering dominates so that κ(ϵ) depends on v_g (ϵ). This conclusion is important as it can be generalized to other materials and potential functions; it suggests that if a film is thin enough for boundary scattering to dominate, then the behavior of κ(ϵ) can be predicted based on the bulk dispersion curve alone, which should greatly simplify strain-based device design. / Thesis / Doctor of Philosophy (PhD) / Since strain changes the interatomic spacing of matter and alters electron and phonon dispersion, an applied strain ϵ can modify the thermal conductivity κ of a material. This thesis shows how the strain induced by heteroepitaxy is a passive mechanism to change κ in a thin film and how the film thickness is key to the functional form of κ(ϵ). Molecular Dynamics simulations of the physical vapor deposition and epitaxial growth of ZnTe thin films provide insights into the role of interfacial strain on the thermal conductivity of a deposited film. The result is relevant to thermoelectric devices since strain can also enhance charge mobility and increase their overall efficiency. The resulting understanding of dκ/dT shows that strain engineering can also be used to create a thermal rectifier in a material that is partly strained and partly relaxed, like at the junction of an axial nanowire heterostructure.

The high degree of control over the morphology and optoelectronic properties of semiconductor nanowires (NWs) makes them attractive for applications such as thermoelectrics, quantum emitters, and photodetectors. However, NW growth is still not fully understood as many parameters play a role in the determination of NW morphology and crystal structure, which in turn governs resulting optoelectronic properties. We report tellurium-doped GaP NWs with positive tapering and radii measuring as low as 5 nm grown by the self-assisted vapor–liquid–solid mechanism using selective-area molecular beam epitaxy. The occurrence of ultrathin nanoantenna showed a dependence on pattern pitch (separation between NWs) with a predominance at 600 nm pitch, and exhibited radius oscillations that correlate with polytypic zincblende (ZB)/wurtzite (WZ) segments. A growth model explains the positive tapering of the NW leading to an ultrathin tip from the suppression of surface diffusion of Ga adatoms on the NW sidewalls by Te dopant flux. The model also provides a relationship between the radius modulations and the oscillations of the droplet contact angle, predicting the quasi-periodic radius oscillations and corresponding crystal phase transitions. Photoluminescence and cathodoluminescence at 10 K reveal distinct spectra corresponding to either the ZB or WZ phase. Emission above and below ~2.15 eV are associated with ZB and WZ, respectively. The characteristic WZ spectrum arises from a bound exciton and its phonon replicas, consistent with published results. The origin of emission in the ZB regime is less conclusive, but may originate from the splitting of a bound exciton by the field of an axial defect. The results presented in this thesis establish a link between NW growth, morphology, and optoelectronic properties to inform future work involving ultrathin NWs. / Thesis / Master of Applied Science (MASc) / A nanowire (NW) is a tiny rod with a length on the order of one millionth of a meter and diameter on the order of one billionth of a meter. We made gallium phosphide (GaP) NWs by stacking gallium and phosphorus atoms in a column. The NWs were separated by a constant distance. In some cases, we also added beryllium and tellurium atoms to our NWs. The addition of tellurium caused our NWs to grow into extremely sharp points, which we measured with a microscope that uses electrons instead of light. The microscope images also revealed that the arrangement of the atoms in the NW changes along its length. By detecting the light emission from the NWs, it was possible to distinguish between two unique arrangements. Overall, the small dimensions of our GaP NWs make them interesting for applications that require the emission or detection of single particles of light.

Thesis (PhD)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: This study developed and optimised zinc oxide (ZnO) nanowire-based nanogenerator. The nanogenerator works on the piezoelectric effect that is, a mechanical force is converted to an electrical voltage. The ZnO nanowires are piezoelectric and when any force is applied to the nanowires an output voltage is generated. This ZnO nanowire-based nanogenerator can be used to power small electronic devices, such as pacemakers. The nanogenerator can also be incorporated into clothes and shoes to generate electricity to charge a cell phone for example. The problem experienced currently is that the nanogenerator does not generate enough electricity to be of practical use and needs to be further optimised. Simulations and mathematical models were used to identify areas where the nanogenerator could be optimised in order to increase the output voltage. It is shown that the morphology of the nanowires can have a considerable effect on the output voltage. For this reason the growth of the nanowires was investigated first. Different methods were used to propagate the nanowires in order to select the method that, on average, has the highest output voltage. Accordingly, one parameter at a time and design of experiments were used to optimise the nanowire growth. Consequently, these two methods were used to optimise the growth parameters with the respect to the output voltage. The aqueous solution method was found to yield nanowires that give the highest generated output voltage. After growing over 600 nanowire samples, optimal growth parameters for this method were found. These optimal growth parameters were subsequently used to grow nanowires that were used to manufacture the nanogenerator. The nanowires were grown on a solid substrate and hence the nanogenerator was also manufactured on the solid substrate. Through various optimisations of the manufacturing process the maximum output voltage achieved was about 500 mV. However, this output voltage is too low to be of practical use, even though the output has been raised considerably. The main problem was found to be the fact that the contact between the nanowires and the electrode was weak due to contamination. A new method was therefore required where the electrode and the nanowires would be in proper contact to ensure that higher output voltages were achieved. Subsequently, a flexible nanogenerator was manufactured in order to solve this problem. Accordingly, the nanowires were grown on the flexible polyimide film and a buffer layer was then spun onto the flexible substrate, leaving only the nanowire tips exposed. The electrode was then sputtered on top of this buffer layer, covering the nanowire tips. This ensured proper contact between the nanowires and the electrode. The nanogenerator, which was manufactured with non-optimal growth parameters, gives a maximum voltage output of 1 V, double the maximum achieved with the solid nanogenerator. When the optimal growth parameters were used the output voltage was raised to 2 V. Various optimisation techniques were performed on the nanogenerator, including plasma treatment and annealing and the use of various materials in the buffer layer. Combining these optimisation methods subsequently led to an optimised nanogenerator that can generate an output voltage of over 5 V. This was achieved after over 1200 nanogenerators had been manufactured. However, the output voltage was not in a usable form. Circuitry was therefore developed to transform the voltage generated by the nanogenerator to a useable form. The best circuit, the LTC3588, was used to power an LED for 10 seconds. The completed device was found to achieve a power output of 0.3 mW, enough for small electronic devices. / AFRIKAANSE OPSOMMING: ‘n Sink-oksied (ZnO) nanodraad gebaseerde nanogenerator is ontwikkeld en geöptimeer. Die nanogenerator werk met behulp van die piezoelektriese effek - meganiese krag work omgesit in ‘n elektriese spanning. Die ZnO nanodrade is piezoelektries en wanneer ‘n krag op die drade aangewend word, word ‘n uittree spanning gegenereer. Die nanogenerator kan gebruik word om klein elektroniese toestelle, soos ‘n pasaangeër, van krag te voorsien. Die nanogenerator kan in klere en skoene geïnkorporeer word om elektrisiteit op te wek vir die laai van ‘n selfoon. Die probleem is egter dat die nanogenerator tans nie genoeg krag opwek om prakties van nut te wees nie en verdere optimasie word benodig. Simulasies en wikundige modelle work gebruik om areas te identifiseer waar die nanogenerator geöptimeer kan word, met die doel om die uittreespanning te verhoog. Dit word bewys dat die morfologie van die nanodrade ‘n groot effek het op die uittreespanning. Dus word die groei van die nanodrade eerste ondersoek. Verskillende metodes word gebruik om die nanodrade te groei en die beste metode, wat die hoogste uittreespanning op gemiddeld verskaf, word gekies. Een parameter op ‘n slag en ontwerp van eksperimente word gebruik om die nanodraad groei te optimeer. Die groei parameters word geöptimeer deur van die twee metodes gebruik te maak, en die optimeering word gedoen in terme van die uittreespanning. Die oplossing groei metode lei tot nanodrade wat die hoogste uittreespanning verskaf. Na oor die 600 nanodraad monsters gegroei is, is die optimale parameters gevind. Hierdie optimale parameters word uitsluitlik gebruik om die nanogenerator te vervaardig. Die nanodrade word op ‘n soliede substraat gegroei en dus word die nanogenerator op dieselfde soliede substraat vervaardig. Verskeie metodes is gebruik om die vervaardiging te optimeer en die hoogste uittreespanning wat bereik is, is 500 mV. Die uittreespanning is te laag om van praktiese nut te wees alhoewel dit heelwat verhoog is. Die grootste probleem is die swak kontak tussen die nanodrade en die elektrode, wat veroorsaak word deur kontaminasie. ‘n Nuwe metode word verlang wat beter kontak tussen die nanodrade en elektrode sal verseker. ‘n Buigbare nanogenerator is vervaardig om die probleem op te los. Die nanodrade word nou op ‘n buigbare film gegroei. ‘n Bufferlaag word tussen die nanodrade in gedraai, tot net die punte van die nanodrade nog sigbaar is. Die elektrode word bo-op die bufferlaag gedeponeer, wat behoorlike kontak tussen die nanodrade en elektrode verseker. Die nanogenerator wat met nie-optimale groei parameters vervaardig is, bereik ‘n uittreespanning van 1 V, dubbel die soliede nanogenerator. Met optimale groei parameters word die uittreespanning tot 2 V verhoog. Verskeie optimasie tegnieke word op die nanogenerator toegepas. Die metodes sluit in suurstof plasma behandeling, verhitting en die inkorporasie van verskillende materiale in die bufferlaag. ‘n Kombinasie van die metodes geïnkorporeer in een nanogenerator lei tot ‘n uittreespanning van 5 V. Die uittreespanning is bereik na oor die 1200 nanogenerators vervaardig is. The uittreespanning is nog nie in ‘n bruikbare vorm nie. Spesiale stroombane is ontwikkel wat die nanogenerator spanning omskakel na ‘n bruikbare vorm. Die beste stroombaan, die LTC3588, kan ‘n LED aanskakel vir 10 sekondes. The toestel kan ook 0.3mWuittreekrag voorsien, genoeg vir klein elektroniese toestelle om te werk.