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.

半导体激光器抽运新型Yb：GYSO混晶实现连续锁模激光输出

掺杂Yb^2＋离子的激光材料具有能级结构简单、抽运波长与振荡波长相近、量子效率高等优点，十分适合作为半导体激光器（LD）直接抽运的高功率激光光源。近年来，随着高性能InGaAs激光二极管的发展和成本的降低，掺Yb抖激光介质的研究受到人们的极大关注，并已研制出了许多新型激光晶体，如Yb：YAG，Yb：KYW，Yb：KGW，Yb：YAB，Yb：GGG和Yb：CaF2等。但是这些晶体还有很多不足之处，譬如生长比较困难、发射谱带相对窄和晶伙执导忡能相对姜等．

激光二极管抽运的高效率低阈值Yb：GSO激光器

近年来，掺Yb离子的晶体备受关注：掺Yb离子晶体能级结构简单，可以避免激发态再吸收、频率上转换、弛豫振荡和浓度猝灭等效应。此外，掺Yb离子晶体的吸收光谱位于900～1000nm，无需严格的温度控制即可与InGaAs激光二极管有效耦合，并且具有很宽范围的荧光发射谱，因此这种晶体很有潜力成为1gm波段的宽调谐及超快激光光源。

半导体可饱和吸收镜调Q的Yb：LSO激光器

报道了一个激光二极管（LD）抽运多波长连续输出的激光器和一个被动调Q的固体激光器。该激光器的增益材料是一种新型掺Yb^3＋的晶体Yb^3＋：Lu2SiO5（Yb^1LSO）。当吸收的抽运功率为2．57W时，连续输出的最大功率为490mW，斜率效率为22．2％，光-光转换效率为14．2％，激光阈值为299mW，输出激光波长为1084nm。多波长输出时，波长调谐范围为1034～1085nm。利用InGaAs可饱和吸收镜实现调Q输出时，斜率效率为3．0％，激光波长为1058nm。脉冲重复频率为25～39kHz，

Yb：FAP晶体的生长及光谱性能

应用中频感应提拉法生长出不同掺杂浓度的Yb：FAP激光晶体，运用电感耦合等离子体原子发射光谱仪（ICP-AES）测定了Yb^3＋离子存Yb：FAP晶体中的分凝系数约为0．03。随着晶体的生长，晶体中Yb^3＋离子的轴向浓度逐渐增大。研究Yb：FAP晶体在77K和300K温度下的吸收光谱发现，振动谱的变化主要是由电子-声子近共振耦合作用引起的。系统地研究了不同Yb^3＋离子掺杂浓度Yb：FAP晶体的吸收光谱和荧光光谱。通过吸收光谱的测量计算了晶体的吸收截面。Yb：FAP晶体在904nm和982nm处存在Yb

激光二极管抽运Na，Yb：CaF2晶体实现低阈值激光输出

与Nd^3+相比，Yb^3+具有能级结构简单，本征量子缺陷低(<0．1)，辐射量子效率高，而且吸收和发射光谱非常宽，适合激光二极管(LD)抽运宽带调谐激光运转和超短脉冲的产生。特别是掺Yb晶体适合高亮度的InGaAs激光二极管抽运，从而成为近年来激光二极管抽运全固态激光器中备受关注的增益介质。但是掺Yb激光晶体属三能级系统，抽运阈值普遍较高。因此，寻找低阈值、实用化掺Yb晶体介质是近年来激光晶体的重要发展方向．

LD抽运新型Yb：LYSO晶体实现1085nm激光输出

Yb^3＋激光材料在900～980nm范围具有较强的吸收，能与高效的InGaAs激光二极管（波长为9001100nm）有效地耦合，且能级简单，抽运波长与振荡波长相近，量子效率高。这些优点十分有利于在1000nm附近实现超快高功率激光输出。而随着高性能InGaAs激光二极管的发展和成本的降低，近年来，掺Yb^3＋激光介质的研究受到人们的极大关注，并研制出了许多新型激光晶体，如Yb：YAG，Yb：KYW，Yb：GdVO4，Yb：SYS，Yb：YAB，

Investigation of the spectroscopic properties of Yb3+-doped yttrium lanthanum oxide transparent ceramic

Spectroscopic properties of (Y0.9-xLa0.1Ybx)(2)O-3 transparent ceramic were studied. Two main absorption peaks of the specimen are centered at 940 and 970 nm, which are suitable for InGaAs laser diode pumping. The main emission peaks were located at 1032 and 1075 nm with larger emission cross-section and longer fluorescence lifetime than those of Yb:Y2O3. These properties of (Y0.9-xLa0.1Ybx)(2)O-3 transparent ceramic are favorable to achieve high efficiency and high power laser output. (c) 2007 Elsevier B.V. All rights reserved.

Growth and spectral properties of Yb : FAP single crystal

In this paper, single crystal of ytterbium (Yb) doped Ca-5(PO4)(3)F (FAP) has been grown along the c-axis by using the Czochralski method. The segregation coefficients of Yb3+ in the Yb:FAP crystal has been determined by ICP-AES method. The absorption spectrum, fluorescence spectrum and fluorescence lifetime of the Yb:FAP crystal has been also measured at room temperature. In the absorption spectra, there are two absorption bands at 904 and 982 nm, respectively, which are suitable for InGaAs diode laser pumping. The absorption cross-section (sigma(abs)) is 5.117 x 10(-20) cm(2) with an FWHM of 4 nm at 982 nm. The emission cross-section is (sigma(em)) 3.678 x 10(-20) cm(2) at 1042 nm. Favorable values of the absorption cross-section at about 982 nm are promising candidates for laser diode (LD) pumping. (c) 2005 Elsevier B.V. All rights reserved.

Demonstration of multi wavelength picosecond pulse generation with resonant RF drive using an monolithically integrated multi-grating cavity (MGC) laser

Multiwavelength pulses were generated using a monolithically integrated device. The device used is an integrated InGaAs/InGaAsP/InP multi-wavelength laser fabricated by selective area regrowth. The device self pulsated on all of the four wavelength channels. 48 ps pulses were obtained which were measured by a 50GHz oscilloscope and 32GHz photodiode which was not bandwidth limited. Simultaneous multi-wavelength pulse generation was also achieved.

1 × 4 space switching and simultaneous all-optical wavelength conversion at 2.488Gbit/s using a single monolithically integrated multiwavelength laser

A novel InGaAs/InGaAsP/InP integrated multiwavelength grating cavity laser is presented, which has been used to demonstrate space switching and simultaneous all-optical wavelength conversion at bit rates of 2.488 Gbit/s. This has been achieved using a single monolithically integrated device without the need for post-filtering to separate the converted signal from the input.

External-cavity mode-locked quantum-dot laser diodes for low repetition rate, sub-picosecond pulse generation

This paper presents an investigation of the mode-locking performance of a two-section external-cavity mode-locked InGaAs quantum-dot laser diode, focusing on repetition rate, pulse duration and pulse energy. The lowest repetition rate to-date of any passively mode-locked semiconductor laser diode is demonstrated (310 MHz) and a restriction on the pulse energy (at 0.4 pJ) for the shortest pulse durations is identified. Fundamental mode-locking from 310 MHz to 1.1 GHz was investigated, and harmonic mode-locking was achieved up to a repetition rate of 4.4 GHz. Fourier transform limited subpicosecond pulse generation was realized through implementation of an intra-cavity glass etalon, and pulse durations from 930fs to 8.3ps were demonstrated for a repetition rate of 1 GHz. For all investigations, mode-locking with the shortest pulse durations yielded constant pulse energies of ∼0.4 pJ, revealing an independence of the pulse energy on all the mode-locking parameters investigated (cavity configuration, driving conditions, pulse duration, repetition rate, and output power). © 2011 IEEE.

Modelling 2DEG charges in AlGaN/GaN heterostructures

In this paper we compare different approaches to calculating the charge density in the 2DEG layer of AlGaN/GaN HEMTs. The methods used are (i) analytical theory implemented in MATLAB, (ii) finite-element analysis using semiconductor TCAD software that implements only the Poisson and continuity equations, and (iii) 1D software that solves the Poisson and Schrödinger equations self-consistently. By using the 1D Poisson-Schrödinger solver, we highlight the consequences of neglecting the Schrödinger equation. We conclude that the TCAD simulator predicts with a reasonable level of accuracy the electron density in the 2DEG layer for both a conventional HEMT structure and one featuring an extra GaN cap layer. In addition, while the sheet charge density is not significantly affected by including Schrödinger, its confinement in the channel is found to be modified. © 2012 IEEE.

Nature of heterointerfaces in GaAs/InAs and InAs/GaAs axial nanowire heterostructures

The structural and compositional characteristics of heterointerfaces of Au-catalyzed GaAs/InAs and InAs/GaAs axial nanowire heterostructures were comprehensively investigated by transmission electron microscopy. It has been found that the GaAs/InAs interface is not sharp and contains an InGaAs transition segment, and in contrast, the InAs/GaAs interface is atomically sharp. This difference in the nature of heterointerfaces can be attributed to the difference in the affinity of the group III elements with the catalyst material. © 2008 American Institute of Physics.

Structural and optical properties of III-V nanowires and nanowire heterostructures grown by metalorganic chemical vapour deposition

We have investigated the structural and optical properties of III-V nanowires grown by metalorganic chemical vapour deposition. Binary GaAs, InAs and InP nanowires, and ternary InGaAs and AlGaAs nanowires, have been fabricated and characterised. A variety of axial and radial heterostructures have also been fabricated, including GaAs/AlGaAs core-multishell and GaAs/InGaAs superlattice nanowires. GaAs/AlGaAs core-shell nanowires exhibit strong photoluminescence as the AlGaAs shell passivates the GaAs nanowire surface reducing the surface nonradiative recombination. © 2007 IEEE.