Growth of epitaxial TmFeCuO4 thin films by pulsed laser deposition

Epitaxial thin films of TmFeCuO₄ with a two-dimensional triangular lattice structure were successfully grown on yttria-stabilized-zirconia substrates by pulsed laser deposition and ex situ annealing in air. The films as-deposited below 500 °C showed no TmFeCuO₄ phase and the subsequent annealing resulted in the decomposition of film components. On the other hand, as-grown films deposited at 800 °C showed an amorphous nature. Thermal annealing converted the amorphous films into highly (0 0 1)-oriented epitaxial films. The results of scanning electron microscopic analysis suggest that the crystal growth process during thermal annealing is dominated by the regrowth of non-uniformly shaped islands to the distinct uniform islands of hexagonal base.     

Intrinsic gettering in germanium-doped Czochralski crystal silicon crystals

The intrinsic gettering (IG) of germanium-doped Czochralski (GCZ) silicon with different concentrations of germanium has been investigated in this paper. The conventional Czochralski (CZ) and the GCZ silicon samples were annealed using a one-step high temperature process followed by a sequence of low–high temperature annealing cycles. It was found that the good defect-free denude zones in the near surface of the GCZ silicon could be achieved using simply a one-step high temperature annealing process. Furthermore, the density of bulk microdefects as IG sites was higher than that in the CZ silicon, as a result of germanium enhancing oxygen precipitation during three-step annealing. Meanwhile, the experimental results showed that germanium also enhanced the out-diffusion of oxygen. Furthermore, it is believed that germanium doping can increase the ability of IG in CZ silicon wafers.     

Crystal growth and spectroscopic properties of erbium doped Lu2SiO5

A high optical quality erbium doped Lu₂SiO₅ single crystal has been grown by the Czochralski method. The distribution coefficient of Er³⁺ was measured to be ∼0.926. The absorption and emission spectra as well as the fluorescence decay curve of the excited state ⁴I_13/2 were measured at room temperature. The spectroscopic parameters were calculated using the Judd–Ofelt theory, and the J–O parameters Ω₂, Ω₄ and Ω₆ were found to be 4.451×10^-20, 1.614×10^-20 and 1.158×10^-20 cm², respectively. The room-temperature fluorescence lifetime of the Er³⁺⁴I_13/2→⁴I_15/2 transition was measured to be 7.74 ms. The absorption and emission cross-section as well as the gain cross-section in the eye-safe regime of 1400–1700 nm were also determined and discussed.     

Synthesis of AlN from Li3N and Al: Application to vapor phase epitaxy

We performed synthesis of AlN using Al and Li₃N. In this method, there are two problems that must be solved for obtaining single-phase AlN. One of them is suppression of Li₃AlN₂ formation and the other is precipitation of LiAl from the residual source materials during the cooling process. In the present work, we analyzed phase stability of products and found that AlN was stable at high temperatures and low Li–N/Al molar ratios. However, the products still contained LiAl and Al. Therefore, we examined the effectiveness of vapor phase epitaxy for separating AlN from the extra phase (LiAl and Al formed from residual source materials). From the experimental results, feasibility of vapor phase epitaxy was confirmed. That is, we can deposit only an AlN layer on a sapphire substrate by optimizing the growth conditions, i.e., temperature range above 1150 °C and Li–N/Al molar ratio less than 1.     

Crystal structure, carrier concentration and IR-sensitivity of PbTe thin films doped with Ga by two different methods

The comparison of the results of chemical composition, crystal structure, electronic properties and infrared photoconductivity investigations of PbTe/Si and PbTe/SiO₂/Si heterostructures doped with Ga atoms by two different techniques is presented in this work. One of these techniques is principally based on the vapour-phase doping procedure of PbTe/Si and PbTe/SiO₂/Si heterostructures, which were previously formed by the modified “hot wall” technique. The second method of PbTe(Ga)/Si and PbTe(Ga)/SiO₂/Si heterostructure preparation is based upon the fabrication of lead telluride films, which have been doped with Ga atoms in the layer condensation process directly. The lattice parameter and charge carrier density evolutions with the Ga impurity concentration show principally the different character of PbTe(Ga)/Si films prepared by these techniques. It has been proposed that complicated amphoteric (donor or acceptor) behaviour of Ga atoms may be explained by different mechanisms of substitution or implantation of impurity atoms in the crystal structure of lead telluride.     

Growth control of stoichiometry in LaMnO3 epitaxial thin films by pulsed laser deposition

We have studied structural, magnetic, and optical transport properties of LaMnO₃ (LMO) thin films grown on SrTiO₃. While the stoichiometric LMO is an insulating antiferromagnet, it tends to be a ferromagnetic insulator when grown as thin films. By exploring the majority of growth parameters, we have found that the bulk-like electronic and magnetic phases can be stabilized by growing thin films under reducing atmospheres and by using more energetic laser processes. These conditions are found to reduce the La deficiency in the film resulting in the greatly improved cation stoichiometry. Since oxides are prone to reduce the oxygen content and to alter the cation ratio under such growth conditions, it suggests that the cation and oxygen stoichiometries in complex oxide thin films can be improved by properly optimizing the growth parameters.     

Study of Ge epitaxial growth on Si substrates by cluster beam deposition

Ge epitaxial layers with reasonable quality were grown on Si (1 1 1) substrates by cluster beam deposition (CBD) process. Molecular dynamics study of the low energy Ge clusters deposition process utilizing the Stillinger–Weber two- and three-body interaction potentials was carried out to compare the experimental results. Both experimental and simulation results prove that the substrate temperature plays a dominant role in the epitaxial growth of Ge films in CBD process. The influence mechanisms of temperature are discussed.     

High-temperature conductivity in chemical bath deposited copper selenide thin films

This paper reports high-temperature (305–523 K) electrical studies of chemical bath deposited copper (I) selenide (Cu_2−xSe) and copper (II) selenide (Cu₃Se₂) thin films. Cu_2−xSe and Cu₃Se₂ have been prepared on glass substrates from the same chemical bath at room temperature by controlling the pH. From X-ray diffraction (XRD) profiles, it has been found that Cu_2−xSe and Cu₃Se₂ have cubic and tetragonal structures, respectively. The composition of the chemical constituent in the films has been confirmed from XRD data and energy-dispersive X-ray analysis (EDAX). It has been found that both phases of copper selenide thin films have thermally activated conduction in the high-temperature range. In this paper we also report the variation of electrical parameters with film thickness and the applied voltage.     

Influence of trivalent ions codoping on the Mo:PbWO4 luminescence properties

Trivalent ions codoping Mo:PWO boules were grown along the c-axis using the Czochralski technique. Trivalent ions codoping improved the fast components of luminescence for Mo:PWO due to suppression/compensation of hole-trapping centers. Correspondingly, trivalent ions codoping increased the fast components of the light yield. The La³⁺ and Gd³⁺ concentration in the crystal gradually decreased along the growth direction because they increased the melting point. The Y³⁺ concentration increased along this direction due to the different influence on the melting point. Compared with La:Mo:PWO, Y:Mo:PWO shows a more uniform transmittance along the growth direction.     

Properties of Ge-doped,high-quality bulk GaN crystals fabricated by hydride vapor phase epitaxy

We investigated the properties of Ge-doped, high-quality bulk GaN crystals with Ge concentrations up to 2.4×10¹⁹ cm⁻³. The Ge-doped crystals were fabricated by hydride vapor phase epitaxy with GeCl₄ as the dopant source. Cathodoluminescence imaging revealed no increase in the dislocation density at even the highest Ge concentration, with values as low as 3.4×10⁶ cm⁻². The carrier concentration, as determined by Hall measurement, was almost identical to the combined concentration of Ge and unintentionally incorporated Si. The electron mobilities were 260 and 146 cm² V⁻¹ s⁻¹ for n=3.3×10¹⁸ and 3.35×10¹⁹ cm⁻³, respectively; these values are markedly larger than those reported in the past for Ge-doped GaN thin films. The optical absorption coefficient was quite small below the band gap energy; it slightly increased with increase in Ge concentration. Thermal conductivity, estimated by the laser-flash method, was virtually independent of Ge concentration, maintaining an excellent value around 2.0 W cm⁻¹ K⁻¹. Thermal expansion coefficients along the a- and m-axes were approximately constant at 5.0×10⁻⁶ K⁻¹ in the measured doping concentration range.     

Tetragonal tungsten oxide nanobelts synthesized by chemical vapor deposition

Tungsten trioxide (WO₃) nanobelts in tetragonal structure were grown on Si substrates by a hot-wall chemical vapor deposition (CVD) method without using catalysts. The products were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, and photoluminescence (PL) spectrum. The width of the nanobelts is in the range of 50–100 nm with width-to-thickness ratios of 5–10 and lengths of up to tens of micrometers. These nanobelts grew along the [0 0 1] direction and can be identified as the tetragonal WO₃ structures. Raman and PL measurements indicate the high quality of the nanobelts. The vapor–solid growth mechanism could be applicable in our experiment.     

Influence of Be on N composition in Be-doped InGaAsN grown by RF plasma-assisted molecular beam epitaxy

Undoped and Be-doped InGaAsN layers were grown on GaAs substrates under the same growth conditions by radio frequency plasma-assisted molecular beam epitaxy. Increased tensile strain (Δa/a=3×10⁻³) was observed for Be-doped InGaAsN layers, compared to undoped InGaAsN layers. The strain is shown to originate from the increase in N composition related to Be incorporation, rather than solely from Be atoms substituting Ga atom sites (Be_Ga). A possible reason is the high Be–N bond strength, which inhibits the loss of N from the growth surface during epitaxial growth, thereby increasing the N composition in the Be-doped InGaAsN layer.     

A new method for predicting thermal flow stability: application to a heated rotating disk

In this paper, an approximate analytical formulation is used to develop criteria for predicting thermal flow stability of a heated rotating disk reactor. For fluid flow directed vertically onto a heated horizontal rotating disk, gravitationally induced thermal recirculation results if the density difference between the disk and the incoming gas stream overcomes the stabilizing influence of the viscous forces. From both a fundamental and a practical standpoint, one would like a simple rule that predicts the flow regimes that are free of thermal recirculations. The new stability criteria are based on a variable property similarity transform that makes it possible to obtain approximate analytical solutions to the flow-governing equations. Stability criteria are developed by integrating and combining various terms of the resulting approximate equations. The new stability criteria developed herein are compared to existing criteria as well as to experimental results.     

Growth and spectroscopic properties of Er single doped and Er–Yb co-doped YAl3(BO3)4 crystals

Er³⁺-doped and Er³⁺–Yb³⁺ co-doped yttrium aluminum borate (YAB) single crystals have been grown by the top-seeded solution growth method using a new flux system, namely NaF–MoO₃–B₂O₃. The Er³⁺ concentrations were 1.3 mol% for both single doped and co-doped crystals and the Yb³⁺ concentration in the Er³⁺–Yb³⁺ co-doped crystal was 20.0 mol% in the raw materials. The distribution coefficients of Er³⁺ single doped and Er³⁺–Yb³⁺ co-doped crystals were measured. The polarized absorption and fluorescence spectra of Er³⁺–Yb³⁺ co-doped crystal were recorded and compared with those of Er³⁺ single doped crystal. The results demonstrate that Er³⁺–Yb³⁺ co-doped YAB crystal is a potential candidate for 1.55 μm laser materials.     

Synthesis of AlGaAs-based strained separately confined heterostructure laser diodes by low temperature liquid-phase epitaxy

AlGaAs-based strained separately confined heterostructure laser diodes operating in the 800–900 nm wavelength range have been fabricated from the structures grown using low temperature liquid-phase epitaxy (LPE). Addition of P/Sb to the active region introduces tensile/compressive strain in the structure and is found useful in tuning the emission wavelength. Details of the growth experiments, strain measurements, fabrication and characterization of the laser diodes are described.     

Microstructure and composition analysis of group III nitrides by X-ray scattering

The importance of Group III-nitride structures for both light-emitting devices and high-power field effect transistors is well known (J.W. Orton, C.T. Foxon, Rep. Prog. Phys. 61 (1998) 1). In both cases, different alloy composition and doping levels or type are utilised and the device performance also depends critically on the interface quality and defect density. We have used high resolution X-ray scattering to measure the state of strain in the individual layers on an absolute scale to derive the alloy composition, i.e. we have avoided the conventional method of using the substrate as an internal reference since it could be strained. The composition and individual layer thickness are derived through simulation of the profile with this additional strain information and the best-fit profile is obtained with an automatic procedure. These structures are laterally inhomogeneous arising from defects breaking up the structure into narrow vertical columns of nearly perfect material and this produces significant broadening of the diffraction pattern. This broadening in the diffraction pattern has been modelled using an extended dynamical scattering model (P.F. Fewster, X-Ray Scattering from Semiconductors, Imperial College Press, World Scientific, Singapore, 2000) to yield the size distribution of perfect crystal regions. The measurement of the rotation about an axis defined by the growth direction of the GaN with respect to the sapphire is determined and is found to be small. However, a poor quality sample indicates that a large range of rotations is possible in these structures.     

Growth of strain-compensated GaInNAs/GaAsP quantum wells for 1.3 μm lasers

Strain-compensated GaInNAs/GaAsP quantum well structures and lasers were grown by gas source molecular beam epitaxy using a RF-plasma nitrogen radical beam source. The optimal growth condition for the quantum well structure was determined based on room-temperature photoluminescence measurements. Effects of rapid thermal annealing (RTA) on the optical properties of GaInNAs/GaAsP quantum well structures as well as laser diodes are examined. It was found to significantly increase the photoluminescence from the quantum wells and reduce the threshold current density of the lasers, due to a removal of N induced nonradiative centers from GaInNAs wells.     

In situ optical monitoring of AlGaN thickness and composition during MOVPE growth of AlGaN/GaN microwave HFETs

Real-time spectral reflectometry has been implemented to monitor the MOVPE growth of AlGaN/GaN microwave HFET structures. The aim is to monitor and control the thickness and composition of the thin AlGaN layer during growth. In order to extract useful information from the in situ spectra the optical constants of AlGaN as a function of alloy composition are required at the growth temperature (1050°C). As the first step to obtaining the high temperature optical constants, a room temperature spectroscopic ellipsometry study (energy range 1.65–4.95 eV) has been carried out on thin AlGaN films of various thickness (30 and 100 nm) and aluminium content (0.15 and 0.25). The multilayer model of each sample from the ellipsometry study is used to generate a reflectance spectrum which is compared with the in situ spectral reflectometry spectrum of the same sample acquired at room temperature to verify the technique. Further work is in progress to model the bandgap and optical constants of GaN and AlGaN at growth temperature.