GaAsP Nanowires Grown by Aerotaxy

We have grown GaAsP nanowires with high optical and structural quality by Aerotaxy, a new continuous gas phase mass production process to grow III-V semiconductor based nanowires. By varying the PH3/AsH3 ratio and growth temperature, size selected GaAs1-xPx nanowires (80 nm diameter) with pure zinc-blende structure and with direct band gap energies ranging from 1.42 to 1.90 eV (at 300 K), (i.e., 0 ≤ x ≤ 0.43) were grown, which is the energy range needed for creating tandem III-V solar cells on silicon. The phosphorus content in the NWs is shown to be controlled by both growth temperature and input gas phase ratio. The distribution of P in the wires is uniform over the length of the wires and among the wires. This proves the feasibility of growing GaAsP nanowires by Aerotaxy and results indicate that it is a generic process that can be applied to the growth of other III-V semiconductor based ternary nanowires

New Methods in the growth of InP on Si and Regrowth of Semi-insulating InP for Photonic Devices

This thesis addresses new methods in the growth of indium phosphide on silicon for enabling silicon photonics and nano photonics as well as efficient and cost-effective solar cells. It also addresses the renewal of regrowth of semi-insulating indium phosphide for realizing buried heterostructure quantum cascade lasers with high power and wall plug efficiency for sensing applications. As regards indium phosphide on silicon, both crystalline and polycrystalline growth methods are investigated. The crystalline growth methods are: (i) epitaxial lateral overgrowth to realize large area InP on Si, for silicon photonics (ii) a modified epitaxial lateral overgrowth method, called corrugated epitaxial lateral overgrowth, to obtain indium phosphide/silicon heterointerface for efficient and cost effective solar cells and (iii) selective growth of nanopyramidal frusta on silicon for nanophotonics. The polycrystalline growth method on silicon for low cost solar cell fabrication has been realized via (i) phosphidisation of indium oxide coating synthesized from solution chemistry and (ii) phosphidisation cum growth on indium metal on silicon. All our studies involve growth, growth analysis and characterization of all the above crystalline and polycrystalline layers and structures. After taking into account the identified defect filtering mechanisms, we have implemented means of obtaining good optical quality crystalline layers and structures in our epitaxial growth methods. We have also identified feasible causes for the persistence of certain defects such as stacking faults. The novel methods of realizing indium phosphide/silicon heterointerface and nanopyramidal frusta of indium phosphide on silicon are particularly attractive for several applications other than the ones mentioned here. Both the polycrystalline indium phosphide growth methods result in good optical quality material on silicon. The indium assisted phosphidisation cum growth method normally results in larger grain size indium phosphide than the one involving phosphidisation of indium oxide. These two methods are generic and can be optimized for low cost solar cells of InP on any flexible substrate. The method of regrowth of semi-insulating indium phosphide that is routinely practiced in the fabrication of buried heterostructure telecom laser has been implemented for quantum cascade lasers. The etched ridges of the latter can be 6-15 µm deep, which is more than 2-3 times as those of the former. Although this is a difficult task, through our quick and flexible regrowth method we have demonstrated buried heterostructure quantum cascade lasers with an output power up to 2. 5 W and wall plug efficiency up to 9% under continuous operation.QC 20140523</p

New Methods in the growth of InP on Si and Regrowth of Semi-insulating InP for Photonic Devices

This thesis addresses new methods in the growth of indium phosphide on silicon for enabling silicon photonics and nano photonics as well as efficient and cost-effective solar cells. It also addresses the renewal of regrowth of semi-insulating indium phosphide for realizing buried heterostructure quantum cascade lasers with high power and wall plug efficiency for sensing applications. As regards indium phosphide on silicon, both crystalline and polycrystalline growth methods are investigated. The crystalline growth methods are: (i) epitaxial lateral overgrowth to realize large area InP on Si, for silicon photonics (ii) a modified epitaxial lateral overgrowth method, called corrugated epitaxial lateral overgrowth, to obtain indium phosphide/silicon heterointerface for efficient and cost effective solar cells and (iii) selective growth of nanopyramidal frusta on silicon for nanophotonics. The polycrystalline growth method on silicon for low cost solar cell fabrication has been realized via (i) phosphidisation of indium oxide coating synthesized from solution chemistry and (ii) phosphidisation cum growth on indium metal on silicon. All our studies involve growth, growth analysis and characterization of all the above crystalline and polycrystalline layers and structures. After taking into account the identified defect filtering mechanisms, we have implemented means of obtaining good optical quality crystalline layers and structures in our epitaxial growth methods. We have also identified feasible causes for the persistence of certain defects such as stacking faults. The novel methods of realizing indium phosphide/silicon heterointerface and nanopyramidal frusta of indium phosphide on silicon are particularly attractive for several applications other than the ones mentioned here. Both the polycrystalline indium phosphide growth methods result in good optical quality material on silicon. The indium assisted phosphidisation cum growth method normally results in larger grain size indium phosphide than the one involving phosphidisation of indium oxide. These two methods are generic and can be optimized for low cost solar cells of InP on any flexible substrate. The method of regrowth of semi-insulating indium phosphide that is routinely practiced in the fabrication of buried heterostructure telecom laser has been implemented for quantum cascade lasers. The etched ridges of the latter can be 6-15 µm deep, which is more than 2-3 times as those of the former. Although this is a difficult task, through our quick and flexible regrowth method we have demonstrated buried heterostructure quantum cascade lasers with an output power up to 2. 5 W and wall plug efficiency up to 9% under continuous operation.QC 20140523</p

Electron Tomography Reveals the Droplet Covered Surface Structure of Nanowires Grown by Aerotaxy

For the purpose of functionalizing III-V semiconductor nanowires using n-doping, Sn-doped GaAs zincblende nanowires are produced, using the growth method of Aerotaxy. The growth conditions used are such that Ga droplets, formed on the nanowire surface, increase in number and concentrations when the Sn-precursor concentration is increased. Droplet-covered wires grown with varying Sn concentrations are analyzed by transmission electron microscopy and electron tomography, which together establish the positioning of the droplets to be preferentially on {−111}B facets. These facets have the same polarity as the main wire growth direction, [−1−1−1]B. This means that the generated Ga particles can form nucleation sites for possible nanowire branch growth. The concept of azimuthal mapping is introduced as a useful tool for nanowire surface visualization and evaluation. It is demonstrated here that electron tomography is useful in revealing both the surface and internal morphologies of the nanowires, opening up for applications in the analysis of more structurally complicated systems like radially asymmetrical nanowires. The analysis also gives a further understanding of the limits of the dopants which can be used for Aerotaxy nanowires

Calculation of hole concentrations in Zn doped GaAs nanowires

We have previously demonstrated that we can grow p-type GaAs nanowires using Zn doping during gold catalyzed growth with aerotaxy. In this investigation, we show how to calculate the hole concentrations in such nanowires. We base the calculations on the Zhang–Northrup defect formation energy. Using density functional theory, we calculate the energy of the defect, a Zn atom on a Ga site, using a supercell approach. The chemical potentials of Zn and Ga in the liquid catalyst particle are calculated from a thermodynamically assessed database including Au, Zn, Ga, and As. These quantities together with the chemical potential of the carriers enable us to calculate the hole concentration in the nanowires self-consistently. We validate our theoretical results against aerotaxy grown GaAs nanowires where we have varied the hole concentration by varying the Zn/Ga ratio in the aerotaxy growth

Trends in heteroepitaxy of III-Vs on silicon for photonic and photovoltaic applications

We present and compare the existing methods of heteroepitaxy of III-Vs on silicon and their trends. We focus on the epitaxial lateral overgrowth (ELOG) method as a means of achieving good quality III-Vs on silicon. Initially conducted primarily by near-equilibrium epitaxial methods such as liquid phase epitaxy and hydride vapour phase epitaxy, nowadays ELOG is being carried out even by non-equilibrium methods such as metal organic vapour phase epitaxy. In the ELOG method, the intermediate defective seed and the mask layers still exist between the laterally grown purer III-V layer and silicon. In a modified ELOG method called corrugated epitaxial lateral overgrowth (CELOG) method, it is possible to obtain direct interface between the III-V layer and silicon. In this presentation we exemplify some recent results obtained by these techniques. We assess the potentials of these methods along with the other existing methods for realizing truly monolithic photonic integration on silicon and III-V/Si heterojunction solar cells

Hot-Carrier Extraction in Nanowire-Nanoantenna Photovoltaic Devices

Nanowires bring new possibilities to the field of hot-carrier photovoltaics by providing flexibility in combining materials for band engineering and using nanophotonic effects to control light absorption. Previously, an open-circuit voltage beyond the Shockley-Queisser limit was demonstrated in hot-carrier devices based on InAs-InP-InAs nanowire heterostructures. However, in these first experiments, the location of light absorption, and therefore the precise mechanism of hot-carrier extraction, was uncontrolled. In this Letter, we combine plasmonic nanoantennas with InAs-InP-InAs nanowire devices to enhance light absorption within a subwavelength region near an InP energy barrier that serves as an energy filter. From photon-energy- and irradiance-dependent photocurrent and photovoltage measurements, we find that photocurrent generation is dominated by internal photoemission of nonthermalized hot electrons when the photoexcited electron energy is above the barrier and by photothermionic emission when the energy is below the barrier. We estimate that an internal quantum efficiency up to 0.5-1.2% is achieved. Insights from this study provide guidelines to improve internal quantum efficiencies based on nanowire heterostructures

Aerotaxy: High throughput gas-phase epitaxy of nanostructures

Aerotaxy is an aerosol-based growth method for semiconductors and we present in detail how aerotaxy can be used to grow nanowires continuously with controlled nanoscale dimensions, with a high degree of crystallinity and remarkable throughput, including process details and our current understading of the growth processes. Catalytic size-selected Au aerosol particles travel through a heated flow-through reactor and mix with III–V precursor flux, which nucleates the growth of nanowires. We demonstrate that the method allows sensitive and reproducible control of the nanowire dimensions and shape. The reported continuous and potentially high-throughput method is expected to substantially reduce the cost of producing high-quality nanowires and may enable the low-cost fabrication of nanowire-based devices on an industrial scale

N-type doping and morphology of GaAs nanowires in Aerotaxy

Controlled doping in semiconductor nanowires modifies their electrical and optical properties, which are important for high efficiency optoelectronic devices. We have grown n-type (Sn) doped GaAs nanowires in Aerotaxy, a new continuous gas phase mass production technique. The morphology of Sn doped nanowires is found to be a strong function of dopant, tetraethyltin to trimethylgallium flow ratio, Au-Ga-Sn alloying, and nanowire growth temperatures. High temperature and high flow ratios result in low morphological quality nanowires and in parasitic growth on the wire base and surface. Alloying and growth temperatures of 400 °C and 530 °C, respectively, resulted in good morphological quality nanowires for a flow ratio of TESn to TMGa up to 2.25 ×10-3. The wires are pure zinc-blende for all investigated growth conditions, whereas nanowires grown by metal-organic vapor phase epitaxy with the same growth conditions are usually mainly Wurtzite. The growth rate of the doped wires is found to be dependent more on the TESn flow fraction than on alloying and nanowire growth temperatures. Our photoluminescence measurements, supported by four-point probe resistivity measurements, reveal that the carrier concentration in the doped wires varies only slightly (1-3) ×1019 cm-3 with TESn flow fraction and both alloying and growth temperatures, indicating that good morphological quality wires with high carrier density can be grown with low TESn flow. Carrier concentrations lower than 1019 cm-3 can be grown by further reducing the flow fraction of TESn, which may give better morphology wires