Gallium Arsenide

InAs/GaAs quantum dot heterostructures grown by molecular-beam epitaxy are studied using cross-sectional scanning tunneling microscopy and spectroscopy. The images reveal individual InAs quantum dots (QDs) having a lens shape with maximum base diameter of 10.5 nm and height of 2.9 nm. Analysis of strain relaxation of the QDs reveals an indium composition varying from 65% at the base of the QD, to 95% at its center, and back to 65% at its apex. Room-temperature tunneling spectra acquired 3-4 nm from the center of a dot show a peak located in the upper part of the GaAs bandgap originating from the lowest electron confined state of the QD, along with a tail in the conductance extending out from the valence band and originating from QD hole states. A computational method is developed for simulating the tunneling spectra using effectivemass bands treated in an envelope-function approximation. By comparison of the computations to low-current spectra, the energy of the lowest electron and highest hole QD states are determined. These energies are found to be in reasonably good agreement both with optical measurements and prior theoretical predictions of Wang et al. [Phys. Rev. B 59, 5678 (1999)].

We show that by measuring the separation between the Stokes and anti-Stokes peaks excited by two different laser lines we obtain a very precise determination of absolute phonon energies. The method is useful for measuring small changes of these energies with strain, temperature, laser power, etc. It doubles the changes and avoids the necessity of using the reference lines in the Raman spectra. The method can be applied for the determination of phonon deformation potentials, for the characterization of strained heteroepitaxial layers, and for micro-Raman analysis of strain in silicon integrated circuits. We give examples of phonon shifts in Si, Ge, GaAs, InAs, and GaP as a function of applied biaxial strain, laser power, and temperature.

In selective-area metalorganic vapor-phase epitaxy, polycrystals are generated on masks of large area. The generation of such crystals should be avoided because they act as uncontrollable sink of film precursors. Although generation of polycrystals is sometimes associated with surface migration of a precursor on masks, our observation suggested that polycrystals are generated when the gas-phase concentration of a film precursor just above masks exceeds a critical value. Reducing either total pressure or partial pressure of the source gas suppressed generation of polycrystals, which is consistent with the gas-phase mechanism. The threshold of gas-phase precursor concentration for generation of polycrystals was investigated as a function of growth temperature and V/III ratio for both GaAs and InP. The threshold increased with temperature, that is to say, fewer polycrystals were observed at higher temperature. The threshold for InP was significantly larger than that for GaAs. This information is useful for designing mask patterns which do not generate polycrystals, and it helps to provide a detailed understanding of nucleation phenomena. r

A surface emitting laser is very attractive for lightwave communication as well as optoelectronics when taking advantage of the possible two-dimensional arrayed configuration. In this paper, we describe the research progress of developing a vertical cavity surface emitting (SE) injection laser based on GaAIAs/GaAs and GaInAsP/InP systems. Ultimate laser characteristics, device design, state-of-the-art performances, possible device improvement, and future prospects will also be discussed. The authors proposed a vertical cavity surface emitting semiconductor laser and currently much work is done in our laboratory to realize it. In order to reduce the threshold current, we have improved the laser reflector and introduced a circular buried heterostructure. The microcavity structure, which is 7 pm long and 6 pm in diameter, was realized with a threshold of 6 mA. Thus, possibilities of an extremely low threshold current SE laser device and a densely packed two-dimensional array are suggested.

The effect of postgrowth annealing on shape and ordering of a single layer of InGaAs/GaAs͑001͒ quantum dots is investigated by three dimensional grazing incidence small angle x-ray scattering. A transition from disordered dots to two-dimensional lateral ordering is found. This transition is accompanying a quantum dot shape transformation. Grazing incidence diffraction measurements relate the observed ordering type to strain driven self organization. The role of different growth conditions leading to lateral correlation is discussed by comparing the results to recent experimental achievements in the field.

Introduction: Low level laser therapy (LLLT) has been shown to enhance collagen production and wound healing but its effect on cartilage repair from biomechanical point of view is not known yet. The aim of present study was to evaluate the biomechanical behaviour of repairing osteochondral defect in rabbits which received a pulsed low-level gallium-arsenide (Ga-As) laser irradiation. Materials and methods: Osteochondral defects with 5 mm diameter and 4 mm in depth induced by drilling in right femoral patellar grooves of 41 adolescent male rabbits. They were divided into experimental and control groups. Experimental group received pulsed Ga-As (890 nm) laser irradiation with energy density of 4.8 J/cm 2 . The rabbits in control group received placebo LLLT with shut-down equipment. The control defects were allowed to heal spontaneously. Each group were divided into three subgroups: A, B and C. Subgroups A, B and C were sacrificed on 4, 8, and 16 weeks after surgery. The knee joint were removed, and the defects were examined biomechanically by in situ-indentation method. The thickness, instantaneous and equilibrium indentation stiffness was measured during the test. Data were analysed using ANOVA and independent sample t-test. Result: While no difference was observed in the repaired cartilage biomechanical properties among 4th, 8th, 16th weeks in study groups. The equilibrium indentation stiffness of experimental group was significantly higher in 8th week in comparison with control group. Conclusion: LLLT significantly enhances the stiffness of repairing tissue in the 8th week post injury in osteochondral defects in rabbits.

This study investigated the relationship between the self-efficacy of basic school headteachers in Effutu municipality and their leadership effectiveness. The purpose of the study was to determine whether the selfefficacy of Basic School Heads in Effutu Municipality affects their leadership effectiveness. The quantitative method was used for the study. A population of 85 headteachers and 145 teachers were drawn from the 85 basic schools in Effutu municipality via total/whole enumeration and random sampling techniques respectively. The structured questionnaires were used to collect the data. The quantitative survey data was analysed with SPSS, specifically, frequencies and correlation. One research question and two hypotheses were formulated to guide the study. The major finding of the study was that the basic school headteachers self-efficacy affects their leadership effectiveness and that there is a positive relationship between self-efficacy and leadership effectiveness. G.E.S. should consider the self-efficacy of teachers before selecting them as heads for Basic Schools for effective academic performance or should invest in stimulating the self-efficacy of heads so as to work effectively. Other organizations should consider building the self-efficacy of their leadership so as to improve their effectiveness in performing their duties and also, rate selfefficacy high on recruitment requirements of applicants at recruitment interviews.

The VGF-method was applied for the growth of Si-doped GaAs crystals of 3 and 4 inch diameter (n410 18 cm À3 ). When the average dislocation density in these crystals falls below 200 cm À2 the stress-induced 601-dislocations disappear and other dislocation types dominate the crystal. These remaining dislocations are classified and the conditions for their occurence are analyzed.

Optical reflection, transmission, and absorption in arrays of GaAs and GaAs/AlGaAs core-shell nanowires are studied using transfer matrix and photonic bandgap formalisms, analyzing the effects of size, geometry, height, packing density, and polarization. Energy dependence of the spectra demonstrates optical modes in the dielectric, similar to guided resonant modes, and also the air bands. Simulation of polarization dependence verifies higher absorption with the electric field along the wire axis. Higher absorption at much lower volume compared to thin film, combined with excellent charge transport, make core-shell nanowire arrays excellent candidates for optoelectronics applications.

A GaAs solar cell without prismatic covers, with the highest efficiency known to the authors in the range of 1000-2000 suns for a single junction, is presented. Low temperature liquid phase epitaxy is used for its growth. In addition to improvements such as the achievement of a good quality material or a low contact resistance, this solar cell exhibits specific enhanced aspects. Among the most noticeable are: 1) an innovative design; 2) a double and gradual emitter layer; 3) a small size: 1 mm 2 , 4) a finger width of the front metal grid of 3 m; and 5) a tailored ARC deposition based on a nondestructive and accurate AlGaAs window layer characterization. As a consequence, an efficiency of 26.2% at 1000 suns and 25.0% at 2000 suns AM1.5D (standard conditions) is achieved thanks mainly to a short-circuit current density at 1000 suns of 26.8 A/cm 2 (and 53.6 A/cm 2 at 2000 suns) with a simultaneous series resistance of 3 m cm 2 .

Electron distribution function, drift velocity, mean energy, valley population fractions and diffusion coefficient were calculated at high, up to 100 kV/cm, electric fields. Calculations were carried out by the Monte Carlo method. The three-valley model of GaAs conduction band was used. The obtained results were compared with the experimental data.

There are over 100 conducting polymers which have been synthesised by chemists with a wide range of specific electrical conductivities. Many of these polymers are suitable for elec tronic device fabrication. Semi-conducting and conducting polymers have potential for application in several areas. In this paper electronic and microwave properties are explored. Fabrication of elec tronic and microwave devices can be

Edge emitting diode lasers with highly strained In-GaAs quantum wells and GaAs waveguide layers emitting at 1150 nm were investigated focusing on the impact of the waveguide design on the laser performance. Using a thick GaAs waveguide layer broad area devices with low vertical divergence of 20 • FWHM and reliable operation at a power level of 80-mW/µm stripe width were demonstrated.

In the present study, we determine exact analytical expression of the current flow through a Schottky barrier diode as a function of the input voltage. The Schottky diode is modeled by an electronic circuit containing four physical parameters: a series resistance Rs, a shunt resistance Rsh, a Schottky diode reverse saturation current Is and a Schottky diode ideality factor ¿. Firstly, we solve the characteristic equation and determine the analytical expression of the input current I as a function of the input voltage of the Schottky diode V using the LambertW Function. Secondly, We present two different methods to extract the four physical parameters appearing in the electronic circuit. These methods are applied for two junctions: Iridium-Silicon Carbide Schottky barrier diode at 200 K and Gold-Gallium Arsenide at 300 K. Finally, we compare the results obtained via the two methods presented.

This paper present the design of 2-stage 15 GHz power amplifier (PA) using 0.15 µm GaAs p-HEMT technology. At operating frequency of 15 GHz (Ku-band), each single PA stage was designed for optimum power and efficiency of the transistor, with 50 Ω input and output impedance matching. In this design, the active devices were selected from the depletion p-HEMT type with voltage supply of 4.5 V and DC bias of -0.2 V. The PA delivers maximum linear output power of 22.21 dBm while achieving maximum power-added-efficiency (PAE) of 37.02 %. The PA has an input and output return loss at 20.76 dB and 25.19 dB respectively. Having a current consumption of 132 mA, this PA achieves a small signal gain of 22.34 dB. The proposed PA is designed within a die size of about 2.08 x 1.03 mm 2 on GaAs substrate. Index Terms -Cascade, GaAs p-HEMT, Ku-band, MMIC, power added efficiency, power amplifiers.

This paper describes the design and measured performance of Monolithic Microwave Integrated Circuit (MMIC) power amplifier for wireless LA applications in the 2.4GHz band. The power amplifier (PA) is designed using 0.15µ µ µ µm GaAs power PHEMT technology. The die size of this PA is 1.2mm x 0.7mm and this PA is also offered in 16-pin QF package. With only 3.0 V of drain voltage (V DS ), the packaged PA exhibits an output power at 1dB gain compression (P1dB) of 14.01 dBm, Power-Added Efficiency (PAE) of 31.70% and gain of 7.51 dB, respectively. The maximum current, Imax of this packaged amplifier is 77.90mA and the power consumption for the device is 233.70mW.

The coplanar waveguide (CPW) structure is subdivided into five classes based on substrate thickness, backside metallization, and ground plane width. Radiation and guided modes are studied in each class, and their effects on loss and dispersion are described. For coplanar waveguides, dispersion due to interaction with surface wave parallel-plate modes in the substrate strongly depends on the ground-to-ground spacing (W+2G). If this spacing is small compared to dielectric wavelength and substrate thickness, both dispersion and radiation losses are minimized. In finite ground plane cases, small ground-to-ground spacing compared to substrate thickness is necessary to avoid the excitation of the microstrip mode and to minimize the deviation of the CPW mode from the odd mode of the ideal line. Limitations on the reduction of (W+2G) come from conductor loss, which imposes a minimum width requirement on the center conductor. If substrate thickness is to be kept large compared with this dimension, it is inevitable that, at high frequencies, the operation of the CPW will not be below the cutoff frequency of surface wave modes. However, the interaction of the CPW mode with surface wave modes is negligible for integrated circuit applications when ground-to-ground spacing is small compared with dielectric wavelength

The Thirteenth Space Photovoltaic Research and Technology (SPRAT) Conference gathered representatives from 26 commercial corporations, 8 universities and 7 governmental agencies, including Europe, for two and a half days of presentations and discussions regarding the status and future of space photovoltaics.

Front-side bulk micromachining based on 0.2 mm GaAs HEMT MMIC technology is presented. Several chemical solutions have been used to perform the etching procedure characterization in respect to the obtained vertical profiles. It has been verified that citric acid solution is the most appropriate selective etchant to build suspended GaAs/AlGaAs mesa-shaped structures, while both H 3 PO 4 and NH 4 OH based anisotropic systems seem to be the most suitable for the free-standing triangular prism-shaped structures. Moreover, all these three solutions could be applied to suspend only metal and intermetallic materials. Etch rates as well as cross-section profiles were obtained. Furthermore, the compatibility of the etching procedure with the integrated electronics and the pad metallization has been successfully tested. The features and applications linked to the obtained structures are also discussed.

This paper analytically describes the concept of enhancing the bandwidth of second-harmonic generation in the mid-infrared region in an isotropic tapered semiconductor slab configuration. In this slab geometry, the lengths between successive reflection points due to total internal reflection of the incident laser radiation increase when it propagates through the slab. A computer aided simulation has been carried out to determine the possibility of generating broadband second harmonic intensity for broadband fundamental laser radiations as they are allowed to undergo total internal reflection inside the tapered isotropic semiconductor crystal slab made of either gallium arsenide (GaAs) or zinc selenide (ZnSe). The simulated results indicate wide 3 dB bandwidths of 187 nm and 196 nm in a 30 mm long tapered slab of GaAs and ZnSe respectively. The conversion efficiency, after considering the absorption and reflection losses, is quite satisfactory (≈ 1%). The effects of variations in temperature, incident angle of the fundamental laser radiations at the air-slab interface, length and tapering angle of the semiconductor slab have been studied for generated second harmonic radiations in both GaAs and ZnSe crystals. Optimising these parameters a wider broadband frequency converter with appreciable conversion efficiency can be designed.

For use in millimeter-wave integrated circuits, theoretical and experimental design data for various useful capacitive transmission lines in coplanar waveguide on gallium arsenide are presented. Multifinger and metal-insulator-metal (MIM) lines enabling low characteristic impedance in the order of 10 as well as broad-band capacitive coupled transmission lines are investigated. Simple design rules and accurate models validated with measurements up to 120 GHz demonstrate that such planar transmission lines can be used in monolithic microwave integrated circuits (MMIC's) to extend the impedance range and the design flexibility with respect to the conventional CPW.

Heat generated by silicon-based transistors due to high energy photo irradiation interferes with the electronic conductivity of transistors in the ICs. There is need to search for an alternative semiconductor material for making diodes and transistors with little or no heat dissipation. Dilute magnetic semiconductor such as Gallium Arsenide (GaAs) has demonstrated to be a better candidate to substitute silicon in electronic technology. UV-VIS-IR light was illuminated on the Ga 1-x Mn x As samples of thickness 500nm−1000nm of varied doping levels during the study.

Abstrmt-A novel experimental technique to perform in-siru Hall effect mobility profiling of short-channel devices is described in this paper. This is based on the measurement of the Hall current in the conduction channel of a transistor with a split source or drain contact.

A3. Hydride vapor phase epitaxy B1. Gallium phosphide and gallium arsenide B2. Nonlinear optic materials a b s t r a c t This paper describes advances in the development of quasi-phase-matched (QPM) gallium phosphide (GaP) crystals for agile laser sources in the mid-infrared regions between 3-5 and 8-12 mm. In the quest for a nonlinear optical material with the potential to efficiently convert near infrared energy (wavelength $ 1 mm) to a powerful mid-infrared source, we have investigated the growth of GaP by hydride vapor phase epitaxy (HVPE). The process is shown to produce high quality thick layers at rapid growth rates in a low-pressure horizontal reactor. This process was used to grow thick layers on orientation-patterned (OP) templates. The OP-GaP templates were fabricated by lithographic patterning of inverted wafer-fused GaP. HVPE growth on both OP-GaP and OP-GaAs templates was performed, showing that HVPE can successfully replicate the initial template pattern. However, for longer growth duration at given conditions the patterned structure can be lost, with annihilation of every other domain.

Avalanche breakdown behavior at the collector junction of the GaAs/AIGaAs heterojunction bipolar transistor (HBT) has been studied. Junction breakdown characteristics displaying hard breakdown, soft breakdown, and negative resistance breakdown behavior were observed and are interpreted by analysis of localized microplasma effects, uniform microplasma-free behavior, and associated current gain measurements. Light emission from the collector-base junction of the GaAs/AIGaAs HBT was observed and was used to investigate breakdown uniformity. Using a simple punchthrough breakdown model, the theoretical breakdown curves at different collector doping concentrations and thicknesses were computed and found to be in agreement with maximum breakdown voltages measured from devices displaying the most uniform junction breakdown.

Secondary electron imaging combined with immersion lens and through-the-lens detection has been used to analyze various semiconductor junctions. Dopant contrast imaging was applied for multi-doped 4H-SiC, growth-interrupted n þ /p and n/n þ homoepitaxial interfaces, and an AlGaAs/ GaAs p-n junction light-emitting diode structure. Dopant contrast was explained by the local variation in secondary electron escape energies resulting from the built-in potential difference. The effect of varying electron affinity on contrast for the heterostructures is also discussed. The contrast profile of the n-doped AlGaAs compared reasonably well to the simulated valence bandedge energy using a previously determined efficiency of dopant ionization.

A comparison between compositionally stepped and alternating step-graded structures used in the production of a relaxation buffer layer is carried out by means of transmission electron microscopy. The latter shows higher efficiency in relieving the strain. A simple balance force model permits us to understand the reason for a higher generation of threading dislocations observed in the alternating step-graded structures. The presented results can be applied as new design rules for buffer fabrication that contrast in some key points with previous published rules as, for example, the ''zero-net-strain'' precept ͓D.

The time evolution of a GaAs(100) 'epi-ready' wafer exposed to air at normal conditions of pressure and temperature has been studied by means of Auger electron spectroscopy and argon-ion depth profiling. Our results show that the oxygen signal near the surface evolves during times as long as 5 months. The evolution of the oxygen signal is accompanied by the penetration of an oxidation front as the air exposure increases. This behavior is confirmed by means of different indicators obtained through the study of the low-energy Auger line-shape spectra.

H eterojunction fi eld effect transistors (HFET) based on gallium nitride (AlGaN/GaN) and metal semiconductor fi eld effect transistors (MESFETs) based on silicon carbide (SiC) are the preferred transistors for high-power amplifi er circuit designs rather than MESFETs, high electron mobility transistors (HEMTs) and pseudomorphic HEMTs based on gallium arsenide (GaAs) or indium phosphide (InP) semiconductor technology. While AlGaN/GaN and SiC are good candidates for high-power applications, GaAs and InP semiconductor technologies are the preferred transistors in low-power, low-voltage, and low-noise applications .

We have investigated structural properties of GaAs grown on top of a pseudomorphic Si interlayer deposited on a GaAs substrate by x-ray reflectivity and Bragg scattering. The surface and interface roughness of the 500-Å thick GaAs overlayer depends critically on the thickness of the Si interlayer. The surface roughness increases from roughly 10 to 40 Å when the Si thicknesses increases from 3 to 5 Å, whereas the interface roughness remains roughly constant for Si thicknesses larger than 9 Å. The Laue oscillation of the GaAs͑004͒ Bragg reflection together with the asymmetry of the intensity allow a precise determination of the pseudomorphic Si interlayer.

This paper gives an overview of the materials and methods used for fabricating photovoltaic solar cell devices. The technologies discussed include those based on the use of silicon (in the crystalline, multicrystalline, amorphous and micro-crystalline forms), the IIIeV compounds (e.g. gallium arsenide, indium phosphide and gallium antimonide), the polycrystalline compounds (e.g. cadmium telluride, copper gallium indium diselenide and copper indium disulphide), and organic materials (e.g. dyes, polymers and fullerenes). The paper also discusses the important environmental and energy issues with regard to the manufacture, use and disposal of the solar cells and modules.

A detailed analysis of the band alignment between atomic-layer deposition (ALD)grown HfO 2 thin films and n-type Ge(1 0 0) substrate is presented. The valence band offset (VBO) is determined by X-ray photoelectron spectroscopy (XPS) after careful evaluation of the experimental data and accurate removal of the artefacts induced by differential charging phenomena occurring during XPS measurement. VBO values are 2.770.1 eV for all samples independent of the different growth conditions. A conduction band offset (CBO) of 2.070.1 eV and a band gap of 5.670.1 eV have been obtained by internal photoemission (IPE) and photoconductivity measurements, respectively. VBO and CBO values obtained by the different techniques are in excellent agreement within the experimental error.

Impurity-free vacancy disordering (IFVD) using SiO 2 and SrF 2 dielectric caps to induce selective quantum-well (QW) intermixing in the GaAs-AlGaAs system is studied. The intermixing rate of IFVD was found to be higher in n-i-p and intrinsic than in p-i-n structures, which suggests that the diffusion of the Group III vacancy is not supported in p-type material. Single-mode waveguides have been fabricated from both as-grown and bandgap-tuned double-quantum-well (DQW) laser samples. Propagation losses as low as 8.5 dB1cm 01

The characteristics of an electrically injected spin polarized single photon source have been investigated. The GaAs-based microcavity diode consists of a single InAs/GaAs self-organized quantum dot as the single photon source and a MnAs/ Al 0.1 Ga 0.9 As Schottky tunnel barrier for the ferromagnetic contact to inject spin polarized electrons. The measured output circular polarization of the biexciton emission at ϳ 1130 nm exhibits a switching behavior as a function of magnetic field, in the Faraday geometry, the value remaining near-zero for B Ͻ 1 T and ϳ6% -8% for B Ն 1 T. The linear polarization shows a complementary trend. The results are explained in the framework of the exciton fine structure in the quantum dot and the effects of electron-hole exchange splitting, due to in-plane quantum dot anisotropy, and Zeeman splitting on the spin eigenstates and their coupling to the photon field.

A figure of merit based on material parameters has been used for a comparison of various Heterojunction Power Bipolar Transistors (HBT's). The general tendency is for use of narrow-bandgap materials such as Ge or InGaAs as the base and wide-bandgap materials such as AIGaAs, InP, Sic, or GaN as the collector, technology permitting.

We explore the potential for high efficiency multijunction solar cells that are not lattice matched to currently available single crystal substrates, but are still formed of component subcells that can be lattice matched to one another in the III-V GaInAsP compound semiconductor material system. Using detailed balance calculations, the theoretical efficiency of three and four junction cells with lattice constants in the range from 5.65325 to 5.8687 Å are explored. In this range, a lattice-matched three junction cell with an InxAl1-xAs top subcell has a 56.2% detailed balance efficiency (500 suns). A latticematched four junction cell with 54.5% (500 suns) detailed balance efficiency is also identified.

Photonic integration is a key area of information and communications technology. It allows the development of not only  photonics systems with better performance, but also devices that would not be possible without integration. Challenges will be encountered in the development of integrated photonics; mostly due to thermal and electrical crosstalk between the composing devices.

This project requires an integrated device to be developed as part of heat assisted magnetic recording, an incorporated laser heated system designed to increase data densities on hard drive disks. It is a precise process requiring a high level of control over the device output power. Space is also limited, meaning a well integrated photodetector is required. As well as strict requirements in terms of the device integration, the laser  device itself must also must also meet specific performance targets. Heat assisted magnetic recording requires significant optical power. Given the laser will be mounted in a read/write head of a hard drive, thermal dissipation paths will be limited. Also given the fact it is situated in an operating computer; ambient temperature will be high. The laser performance requirements are for a high performance laser which is highly thermally stable. A laser has been designed, fabricated and characterised for high output power with significantly reduced temperature sensitivity. An integrated photodetector system has also been designed, fabricated and tested.