Heteroepitaxy of Group IV and Group III-V semiconductor alloys for photovoltaic applications

Photovoltaic energy conversion represents a economically viable technology for realizing collection of the largest energy resource known to the Earth -- the sun. Energy conversion efficiency is the most leveraging factor in the price of energy derived from this process. This thesis focuses on two routes for high efficiency, low cost devices: first, to use Group IV semiconductor alloy wire array bottom cells and epitaxially grown Group III-V compound semiconductor alloy top cells in a tandem configuration, and second, GaP growth on planar Si for heterojunction and tandem cell applications.

Metal catalyzed vapor-liquid-solid grown microwire arrays are an intriguing alternative for wafer-free Si and SiGe materials which can be removed as flexible membranes. Selected area Cu-catalyzed vapor-liquid solid growth of SiGe microwires is achieved using chlorosilane and chlorogermane precursors. The composition can be tuned up to 12% Ge with a simultaneous decrease in the growth rate from 7 to 1 μm/min^-1. Significant changes to the morphology were observed, including tapering and faceting on the sidewalls and along the lengths of the wires. Characterization of axial and radial cross sections with transmission electron microscopy revealed no evidence of defects at facet corners and edges, and the tapering is shown to be due to in-situ removal of catalyst material during growth. X-ray diffraction and transmission electron microscopy reveal a Ge-rich crystal at the tip of the wires, strongly suggesting that the Ge incorporation is limited by the crystallization rate.

Tandem Ga_1-xIn_xP/Si microwire array solar cells are a route towards a high efficiency, low cost, flexible, wafer-free solar technology. Realizing tandem Group III-V compound semiconductor/Si wire array devices requires optimization of materials growth and device performance. GaP and Ga_1-xIn_xP layers were grown heteroepitaxially with metalorganic chemical vapor deposition on Si microwire array substrates. The layer morphology and crystalline quality have been studied with scanning electron microscopy and transmission electron microscopy, and they provide a baseline for the growth and characterization of a full device stack. Ultimately, the complexity of the substrates and the prevalence of defects resulted in material without detectable photoluminescence, unsuitable for optoelectronic applications.

Coupled full-field optical and device physics simulations of a Ga_0.51In_0.49P/Si wire array tandem are used to predict device performance. A 500 nm thick, highly doped "buffer" layer between the bottom cell and tunnel junction is assumed to harbor a high density of lattice mismatch and heteroepitaxial defects. Under simulated AM1.5G illumination, the device structure explored in this work has a simulated efficiency of 23.84% with realistic top cell SRH lifetimes and surface recombination velocities. The relative insensitivity to surface recombination is likely due to optical generation further away from the free surfaces and interfaces of the device structure.

Finally, GaP has been grown free of antiphase domains on Si (112) oriented substrates using metalorganic chemical vapor deposition. Low temperature pulsed nucleation is followed by high temperature continuous growth, yielding smooth, specular thin films. Atomic force microscopy topography mapping showed very smooth surfaces (4-6 Å RMS roughness) with small depressions in the surface. Thin films (~ 50 nm) were pseudomorphic, as confirmed by high resolution x-ray diffraction reciprocal space mapping, and 200 nm thick films showed full relaxation. Transmission electron microscopy showed no evidence of antiphase domain formation, but there is a population of microtwin and stacking fault defects.

A mathematical and experimental investigation of microcycle spectral estimates of semiconductor flicker noise

The experimental portion of this thesis tries to estimate the density of the power spectrum of very low frequency semiconductor noise, from 10^-6.3 cps to 1. cps with a greater accuracy than that achieved in previous similar attempts: it is concluded that the spectrum is 1/f^α with α approximately 1.3 over most of the frequency range, but appearing to have a value of about 1 in the lowest decade. The noise sources are, among others, the first stage circuits of a grounded input silicon epitaxial operational amplifier. This thesis also investigates a peculiar form of stationarity which seems to distinguish flicker noise from other semiconductor noise.

In order to decrease by an order of magnitude the pernicious effects of temperature drifts, semiconductor "aging", and possible mechanical failures associated with prolonged periods of data taking, 10 independent noise sources were time-multiplexed and their spectral estimates were subsequently averaged. If the sources have similar spectra, it is demonstrated that this reduces the necessary data-taking time by a factor of 10 for a given accuracy.

In view of the measured high temperature sensitivity of the noise sources, it was necessary to combine the passive attenuation of a special-material container with active control. The noise sources were placed in a copper-epoxy container of high heat capacity and medium heat conductivity, and that container was immersed in a temperature controlled circulating ethylene-glycol bath.

Other spectra of interest, estimated from data taken concurrently with the semiconductor noise data were the spectra of the bath's controlled temperature, the semiconductor surface temperature, and the power supply voltage amplitude fluctuations. A brief description of the equipment constructed to obtain the aforementioned data is included.

The analytical portion of this work is concerned with the following questions: what is the best final spectral density estimate given 10 statistically independent ones of varying quality and magnitude? How can the Blackman and Tukey algorithm which is used for spectral estimation in this work be improved upon? How can non-equidistant sampling reduce data processing cost? Should one try to remove common trands shared by supposedly statistically independent noise sources and, if so, what are the mathematical difficulties involved? What is a physically plausible mathematical model that can account for flicker noise and what are the mathematical implications on its statistical properties? Finally, the variance of the spectral estimate obtained through the Blackman/Tukey algorithm is analyzed in greater detail; the variance is shown to diverge for α ≥ 1 in an assumed power spectrum of k/|f|^α, unless the assumed spectrum is "truncated".

High-performance 4H-SiC based metal-semiconductor-metal ultraviolet photodetectors with Al2O3 SiO2 films

4H-silicon carbide (SiC) metal-semiconductor-metal (MSM) ultraviolet (UV) photodetectors with Al2O3/SiO2 (A/S) films employed as antireflection/passivation layers have been demonstrated. The devices showed a peak responsivity of 0.12 A/W at 290 nm and maximum external quantum efficiency of 50% at 280 nm under 20 V electrical bias, which were much larger than conventional MSM detectors. The redshift of peak responsivity and response restriction effect were found and analyzed. The A/S/4H-SiC MSM photodetectors were also shown to possess outstanding features including high UV to visible rejection ratio, large photocurrent, etc. These results demonstrate A/S/4H-SiC photodetectors as a promising candidate for OEIC applications. (C) 2008 American Institute of Physics.

Application of electron beams in thermal processing of semiconductor materials and devices

Rapid and effective thermal processing methods using electron beams are described in this paper. Heating times ranging from a fraction of a second to several seconds and temperatures up to 1400°C are attainable. Applications such as the annealing of ion implanted material, both without significant dopant diffusion and with highly controlled diffusion of impurities, are described. The technique has been used successfully to activate source/drain regions for fine geometry NMOS transistors. It is shown that electron beams can produce localised heating of semiconductor substrates and a resolution of approximately 1 μm has been achieved. Electron beam heating has been applied to improving the crystalline quality of silicon-on sapphire used in CMOS device fabrication. Silicon layers with defect levels approaching bulk material have been obtained. Finally, the combination of isothermal and selective annealing is shown to have application in recrystallisation of polysilicon films on an insulating layer. The approach provides the opportunity of producing a silicon-on-insulator substrate with improved crystalline quality compared to silicon-on-sapphire at a potentially lower cost. It is suggested that rapid heating methods are expected to provide a real alternative to conventional furnace processing of semiconductor devices in the development of fabrication technology. © 1984 Benn electronics Publications Ltd, Luton.

Thickness control of Molecular Beam Epitaxy grown layers at the 0.01-0.1 monolayer level

Electron tunnelling through semiconductor tunnel barriers is exponentially sensitive to the thickness of the barrier layer, and in the most common system, the AlAs tunnel barrier in GaAs, a one monolayer variation in thickness results in a 300% variation in the tunnelling current for a fixed bias voltage. We use this degree of sensitivity to demonstrate that the level of control at 0.06 monolayer can be achieved in the growth by molecular beam epitaxy, and the geometrical variation of layer thickness across a wafer at the 0.01 monolayer level can be detected.

Highly efficient spin transport in epitaxial graphene on SiC

Spin information processing is a possible new paradigm for post-CMOS (complementary metal-oxide semiconductor) electronics and efficient spin propagation over long distances is fundamental to this vision. However, despite several decades of intense research, a suitable platform is still wanting. We report here on highly efficient spin transport in two-terminal polarizer/analyser devices based on high-mobility epitaxial graphene grown on silicon carbide. Taking advantage of high-impedance injecting/detecting tunnel junctions, we show spin transport efficiencies up to 75%, spin signals in the mega-ohm range and spin diffusion lengths exceeding 100μm. This enables spintronics in complex structures: devices and network architectures relying on spin information processing, well beyond present spintronics applications, can now be foreseen. © 2012 Macmillan Publishers Limited. All rights reserved.

Rapid Electron Beam Reacted Tantalum/Titanium Bilayers on Silicon : Semiconductors and Semiconductor Devices

The electrical and structural characteristics of tantalum-titanium bilayers on silicon reacted by electron beam heating have been investigated over a wide range of temperature and time conditions. The reacted layers exhibit low sheet resistance and stable electrical characteristics up to at least 1100℃. Titanium starts reacting from 750℃ onwards for 100 milliseconds reaction times whereas tantalum starts reacting only above 900℃ for such short reaction times. RBS results confirm that silicon is the major diffusing species and there is no evidence for the formation of ternary silicides. Reactions have also been explored on millisecond time scales by non-isothermal heating.

Measuring the composition of AlGaN layers in GaN based structures grown on 150 mm Si substrates using (2 0 5) reciprocal space maps

A critical element for the successful growth of GaN device layers on Si is accurate control of the AlGaN buffer layers used to manage strain. Here we present a method for measuring the composition of the AlGaN buffer layers in device structures which makes use of a one-dimensional x-ray detector to provide efficient measurement of a reciprocal space map which covers the full compositional range from AlN to GaN. Combining this with a suitable x-ray reflection with low strain sensitivity it is possible to accurately determine the Al fraction of the buffer layers independent of their relaxation state. © 2013 IOP Publishing Ltd.

Microstructure and strain analysis of GaN epitaxial films using in-plane grazing incidence x-ray diffraction

This paper investigates the major structural parameters, such as crystal quality and strain state of (001)-oriented GaN thin films grown on sapphire substrates by metalorganic chemical vapour deposition, using an in-plane grazing incidence x-ray diffraction technique. The results are analysed and compared with a complementary out-of-plane x-ray diffraction technique. The twist of the GaN mosaic structure is determined through the direct grazing incidence measurement of (100) reflection which agrees well with the result obtained by extrapolation method. The method for directly determining the in-plane lattice parameters of the GaN layers is also presented. Combined with the biaxial strain model, it derives the lattice parameters corresponding to fully relaxed GaN films. The GaN epilayers show an increasing residual compressive stress with increasing layer thickness when the two dimensional growth stage is established, reaching to a maximum level of -0.89 GPa.

Cathodoluminescence study of GaN-based film structures

GaN films grown on sapphire substrate with an emphasis on epitaxial lateral overgrown (ELOG) layers with an array of rhombic shaped mask area as well as InGaN/GaN MQW laser diode layer structures were investigated by cathodoluminescence (CL) spectroscopy and CL imaging at room and low temperatures. The microscopic imaging with a high-spatial resolution clearly reveals the distribution of threading dislocations and point defects in ELOG GaN films. The secondary electron and CL data measured on cleaved faces of laser diodes are analyzed in consideration with luminescence mechanisms in semiconductor heterostructures and around the p - n junction, providing important information on the defects and carrier dynamics in laser diode devices.

Spin states in InAs/AlSb/GaSb semiconductor quantum wells

We investigate theoretically the spin states in InAs/AlSb/GaSb broken-gap quantum wells by solving the Kane model and the Poisson equation self-consistently. The spin states in InAs/AlSb/GaSb quantum wells are quite different from those obtained by the single-band Rashba model due to the electron-hole hybridization. The Rashba spin splitting of the lowest conduction subband shows an oscillating behavior. The D'yakonov-Perel' spin-relaxation time shows several peaks with increasing the Fermi wave vector. By inserting an AlSb barrier between the InAs and GaSb layers, the hybridization can be greatly reduced. Consequently, the spin orientation, the spin splitting, and the D'yakonov-Perel' spin-relaxation time can be tuned significantly by changing the thickness of the AlSb barrier.

Curvature Correction of FWHM in the X-Ray Rocking Curve of Bent Heteroepitaxial Films

A method for accurate determination of the curvature radius of semiconductor thin films is proposed. The curvature-induced broadening of the x-ray rocking curve (XRC) of a heteroepitaxially grown layer can be determined if the dependence of the full width at half maximum (FWHM) of XRC is measured as a function of the width of incident x-ray beam. It is found that the curvature radii of two GaN films grown on a sapphire wafer are different when they are grown under similar MOCVD conditions but have different values of layer thickness. At the same time, the dislocation-induced broadening of XRC and thus the dislocation density of the epitaxial film can be well calculated after the curvature correction.

EPITAXIAL LATERAL OVERGROWTH OF GaN ON SILICON-ON-INSULATOR

From a single process, GaN layers were laterally overgrown on maskless stripe-patterned (111) silicon-on-insulator (SOI) substrates by metalorganic chemical vapor deposition. The influence of stress on the behavior of dislocations at the coalescence during growth was observed using transmission electron microscopy (TEM). Improvement of the crystallin equality of the GaN layer was demonstrated by TEM and micro-Raman spectroscopy. Furthermore, the benefits of SOI substrates for GaN growth are also discussed.

Growth Parameter Dependence of Structural Characterizations of Diluted Magnetic Semiconductor (Ga, Cr)As

We report the influence of growth parameters and post-growth annealing on the structural characterizations and magnetic properties of (Ga, Cr)As films. The crystalline quality and magnetic properties are sensitive to the growth conditions. The single-phase (Ga, Cr)As film with the Curie temperature of 10 K is synthesized at growth temperature T-s = 250 degrees C and with nominal Cr content x = 0.016. However, for the films with x > 0.02, the aggregation of Cr atoms is strongly enhanced as both T. and x increase, which not only brings strong compressive strain in the epilayer, but also roughens the surface. The origin of room-temperature ferromagnetism in (Ga, Cr)As films with nanoclusters is also discussed.