Investigation of single crystal growth of GaPO4 by the flux method

GaPO₄ crystals have been grown by a spontaneous nucleation method and the top-seeded solution growth method using three different solvents, 3MoO₃-Li₂O, 3MoO₃-K₂O and 2KPO₃-5MoO₃-3LiF. All of the as-grown crystals were characterized by means of X-ray powder diffraction. The results show that all the crystals were well crystallized and belong to the point group 32, and 2KPO₃-5MoO₃-3LiF flux is the best for nucleation and growth of transparent GaPO₄ crystals. The infrared spectrum of GaPO₄ single crystal obtained by the flux method shows that there is no incorporation of OH groups during the crystallization, which is beneficial for high temperature piezoelectric applications.     

Interface of GaN grown on Ge(1 1 1) by plasma assisted molecular beam epitaxy

Early efforts to grow GaN layers on germanium substrates by plasma assisted molecular beam epitaxy led to GaN domains, rotated by 8° relative to each other. Increased insight in the growth of GaN on germanium resulted in the suppression of these domain and consequently high quality layers. In this study the interface of these improved layers is investigated with transmission electron microscopy. The GaN layers show high crystal quality and an atomically abrupt interface with the Ge substrate. A thin, single crystalline Ge₃N₄ layer is observed in between the GaN layer and Ge substrate. This Ge₃N₄ layer remains present even at growth temperatures (850 °C) far above the decomposition temperature of Ge₃N₄ in vacuum (600 °C). Triangular voids in the Ge substrate are observed after growth. Reducing the Ga flux at the onset of GaN growth helps to reduce the triangular defect size. This indicates that the formation of voids in the Ge substrate strongly depends on the presence of Ga atoms at the onset of growth. However complete elimination was not achieved. The formation of voids in the germanium substrate leads to diffusion of Ge into the GaN layer. Therefore we examined the diffusion of Ge atoms into the GaN layer and Ga atoms into the Ge substrate. It was found that the diffusion of Ge into the GaN layer and Ga into the Ge substrate can be influenced by the growth temperature but cannot be completely suppressed. Our results suggest that Ga atoms diffuse through small imperfections in the Ge₃N₄ interlayer and locally etch the Ge substrate, leading to the diffusion of Ga and Ge atoms.     

Reducing Inversion Twinning in Single Crystal Growth of GaPO4

Gallium orthophosphate (GaPO₄) single crystals were grown by the reverse temperature gradient method from phosphoric acid solutions under hydrothermal conditions. Twins after (110) were studied by etching faces having been cut perpendicular to one of the twofold axes. Based on the determination of the twin boundary position as well as on the knowledge of the growth rates of different crystallographic forms, a few faces have been chosen to be quite promising for growing high‐quality GaPO₄ single crystals if they are offered at the referring seed crystal. From the characterization of the grown crystals conditions have been found, which may lead to the reduction of the inversion twin number during the growth process.     

Growth kinetics,structure and surface morphology of AlN/α-Al2O3 epitaxial layers

The gas-phase epitaxy of AlN on saphire substrates in the system Al HCl NH₃–Ar was investigated. The uniformity and structural perfection of the layers were shown to be determined by gas dynamics in the growth zone. The growth rate of AlN-layer is proportional to AlCl concentration in the growth zone and does not depend on NH₃ partial pressure. This is in accordance with the law of diffusional stoichiometry. The morphology of AlN-layers was shown to be dependent on the growth rate and the substrate orientation. Smooth mirror-like layers were prepared on the sapphire planes (0001) and (1120) at low growth rates. X-ray microanalysis showed the oxygen impurity content at the level 1 — 5 · 10²¹ cm⁻³. Other impurities were not available.     

Doping Properties of Pr2O3 Associate InP Liquid Phase Epitaxy

InP epilayers were grown on semi-insulating InP substrates by liquid phase epitaxy with Pr₂O₃-doping. Most grown layers yield mirror-like surfaces and good crystal quality. Hall measurements indicate that n-type background concentration of those grown InP layers will decrease from a value of 2.8 × 10¹⁷ to 3.0 × 10¹⁶ cm⁻³. Their correspondent 77 K mobility also varied from a value of 1326 to 3775 cm²/V s. The photoluminescence (PL) spectra of Pr₂O₃-doped InP epilayers display narrower FWHM and stronger intensity ratios (for band peak to the impurity peak). These PL spectra also demonstrate that the grown layers exhibit a pure crystal quality.     

Ga and In incorporation rates in Ga1−xInxAs growth by chemical beam epitaxy

GaAs, InAs and Ga_1?xIn_xAs layers were grown by chemical beam epitaxy (CBE) using triethylgallium, trimethylindium and tertiarybutylarsine as precursors for Ga, In and As, respectively. The growth rate during the homoepitaxial growth of GaAs and InAs, deduced from the frequency of reflection high-energy electron diffraction intensity oscillations, was used to calibrate the incorporation rates for the III elements. The In content of the Ga_1?xIn_xAs layers was measured by Rutherford backscattering spectrometry and compared with the value predicted from the above calibration data; while the measured In mole fraction is close to the predicted value for the samples grown for low In to Ga flux ratios (x<0.2), the In incorporation is enhanced for larger values of this ratio. The results obtained on layers grown at different substrate temperatures show that In mole fraction is almost constant at growth temperatures in the range 400–500 °C, but a strong dependence on the substrate temperature has been found outside this range. The above results, not observed for samples grown by solid source molecular beam epitaxy, indicate that some interaction between Ga and In precursors at the sample surface could take place during the growth by CBE.     

Growth Mechanism in Atomic Layer Epitaxy (II). A Model of the Growth Process of CdTe on CdTe (111) Substrates

A realistic model of CdTe growth by atomic layer epitaxy (ALE) has been proposed. This model is based on experimental studies concerning the isothermal re-evaporation rates of elemental Cd and Te deposits on the (lll)A and (lll)B surfaces of CdTe substrates, on a study of surface morphology and crystal structure of CdTe single crystal overlayers grown by ALE on CdTe(lll)B substrates under various crystallization conditions as well as on the existing theories related to the interaction of thermally activated atoms or molecules with hot solid surfaces. This model includes: (i) an existence of transition layers of both Cd and Te₂ species, intermediate between a chemisorbed and a bulk-like film, which create reaction zones 3–4 monolayers thick near the substrate surface, and (ii) partial re-evaporation of the first, chemisorbed monolayer of the deposited constituent elements.     

Preparation and Characterization of MBE-grown Si/CaF2 Heterostructures

Growth and fabrication of silicon-on-insulator structures, based on heteroepitaxial growth of insulating films on Si, is an area of research that has rapidly developed in recent years. Thin CaF₂ films and Si/CaF₂ multilayers were prepared on {111}-oriented Si substrates by molecular beam epitaxy (MBE) and were investigated by RHEED and RBS. For epitaxial growth the Si substrates were cleaned using the ultraviolet/ozone surface cleaning method which is an effective tool to remove contaminants from the surface by low-temperature in-vacuo preheating. The growth of CaF₂ on such Si{111} substrates provides epitaxial layers with a high structural perfection. When this layer system, however, is used as a substrate for epitaxial Si growth the Si layers show always twin formation. Si layers without twins could be obtained only after deposition of thin amorphous Si buffer layers at room temperature, immediately followed by Si growth at 700 °C.     

Effect of an applied electric current in epitaxial growth of GaAs layer on patterned GaAs substrate

Selective epitaxial growth of a GaAs layer on SiN_x masked Si-doped semi-insulating (1 0 0) GaAs substrate was performed by current-controlled liquid-phase epitaxy (CCLPE) in the conventional liquid-phase epitaxy. Experiments were carried out with and without the application of electric current. Surface morphology of (1 0 0) facet of the grown layer and the vertical and lateral growth rates were significantly improved under applied electric current. A thick layer of about 330 μm was achieved at relatively low growth time of 6 h with a current density of 20 Acm⁻². The epitaxial growth is realized by both electromigration of the solute and supercooling under a constant rate of furnace cooling. The dislocation density of the grown layer was significantly reduced, compared with that of the substrate (4×10⁴ cm²).     

Structural and electrical characteristics of InGaAsN layers grown by LPE

Crystallographic and transport properties of nominally undoped and Sn-doped InGaAsN layers grown by low-temperature LPE have been studied and related to the growth conditions.In the case of lattice matching, flat and uniform mirror-like layers of 8–10 μm in thickness are obtained. The compositions of the layers under study have been determined by combination of X-ray microanalysis and X-ray diffraction methods to be In_0.035Ga_0.065As_0.086N_0.014. The lattice mismatch between layer and substrate Δa_l/a_s calculated from X-ray diffraction curves is less than?7×10^?4 for all samples. The layers grown at lower epitaxy temperatures exhibit the highest crystalline quality, better lattice match and better homogeneity. This is in good agreement with the results of morphological study by atomic force microscopy which show root mean-square surface roughness of 0.18 nm for the best layers.CV and Hall measurements reveal that intentionally undoped InGaAsN layers are n-type with free carrier concentration about one order of magnitude higher in comparison to layers not containing nitrogen and high electron mobility values over 2000 cm²/Vs. A dramatic reduction in the free carrier concentration and slightly increase in mobility are observed for Sn-doped InGaAsN layers.     

LPE growth of DH laser structures with the double source method

For the reproducible processing of double heterojunction injection laser structures to stripe geometry laser diodes, e.g. by proton bombardment, the thickness of the p-Ga_0.7Al_0.3As confining layer and of the p-GaAs top layer are of great importance. It is shown that the thickness control of these layers grown by liquid phase epitaxy can be improved considerably by introducing two source crystals into the growth system. Both source crystals are brought into contact with the solutions consecutively and prior to the seed. If this method is combined with the use of relatively thin melts (about 2 mm), a growth scheme may be chosen such that the growth rates of the p-GaAlAs and of the p-GaAs layers have reached a practically constant value on the seed crystal, independent of the initial degree of supersaturation. This behaviour is found to be in accordance with the diffusion-limited growth model applied for thin solutions, including a temperature dependence for both the diffusion constant and the slope of the liquidus curve. The results indicate that a second solid phase may appear in the p-GaAlAs solutions at a supersaturation as small as 4°C, which reduces the growth rate on the substrates by a factor of about 2. This situation is different from that of GaAs solutions, where a second phase appears only at a much higher degree of supersaturation. For the p-GaAlAs and the p-GaAs layers a thickness control of ⁺_-0.2 μm is thus achieved with this growth method, without the necessity of very precise temperature control and weighing so as to keep the total supersaturation less than 0.2°C.     

Raman spectroscopy study of the structure of gallium nitride epitaxial layers of different orientations

The composition nonstoichiometry and structural quality of undoped gallium nitride layers grown by the hydride vapor phase epitaxy on sapphire substrates of different orientations have been estimated using Raman spectroscopy. It is found for the first time that the peak position of the phonon mode E₂(high) in the Raman spectra of gallium nitride films at a wave vector of 572 cm^–1 depends on the initial orientation of sapphire substrate and is low-frequency shifted when passing from Ga-polar to partially N-polar orientation. Additional modes are found in the spectra of GaN layers grown on substrates with m and r orientations. It is shown that a decrease in the composition deviation from stoichiometry, caused by reducing the HCl flow through the gallium source during the growth of GaN layers, leads to an increase in the phonon-mode intensity in the Raman spectrum.     

Structural Characteristics of Er Doped LiNbO3 Thin Films Grown by the Liquid Phase Epitaxy Method

Erbium (Er³⁺) doped LiNbO₃ single crystal thin films have been grown LiNbO₃ (001) substrate by the liquid phase epitaxy method. The crystallinity was determined by high‐resolution X‐ray diffraction. The lattice mismatch between Er³⁺ doped LiNbO₃ films and LiNbO₃ (001) substrate was investigated by X‐ray rocking curve analysis. Also we studied the structural characteristics of Er³⁺ doped LiNbO₃ films and surface morphology dependent on the film thickness.     

Epitaxial growth of α-SiC layers by chemical vapor deposition technique

Single crystals of α-SiC were grown on α-SiC substrates at a temperature between 1570 and 1630°C with the standard gas flow rate: H₂ ～ 1 liter/min, SiCl₄ ～ 1.7 ml/min and C₃H₈ ～ 0.1 ml/min. The grown layers were transparent greenish-blue, and surfaces were mirror-like. By an X-ray back-reflection Laue pattern and a reflection electron diffraction method, the grown layer was identified as 6H-SiC, one polytype of α-SiC. Crystal growth was influenced by substrate temperature, flow rates of reaction gases and the surface polarity of the substrate. The growth rate decreased with increase of the substrate temperature in the above temperature region. A lamellar structure was observed on the (0001) Si surface and a mosaic structure was observed on the (000$\text{1}$)C surface. The mole ratios of both SiCl₄ and C₃H₈ to H₂ and that of Si to C had some influence on crystal growth. Undoped layers were n-type due to nitrogen. P-type SiC was grown by doping Al during crystal growth. Doping effects were studied by photoluminescence and electrical measurements.     

Large area lateral overgrowth of mismatched InGaP on GaAs(111)B substrates

Application of InGaAs/InGaP double‐heterostructure (DH) lasers increases the band offset between the cladding layer and the active layer more than the use of conventional 1.3 µm InGaAsP/InP lasers. As a first step in realizing 1.3 µm InGaP/InGaAs/InGaP DH lasers, we proposed InGaP lattice‐mismatched epitaxial lateral overgrowth (ELO) technique and successfully carried out the InGaP growth on both GaAs (100), (111)B and InP (100) substrates by liquid phase epitaxy. In this work, we grew the InGaP crystal on GaAs (111)B substrate by adjusting Ga and P composition in In solution, to obtain In_0.79Ga_0.21P (λ = 820 nm) virtual substrate for 1.3 µm InGaAs/InGaP DH lasers. To grow the InGaP all over the lateral surface of the substrate, the growth time was extended to 6 hours. The amount of InGaP lateral growth up to 2 hours was gradually increased, but the lateral growth was saturated. The InGaP lateral width was about 250 µm at the growth time of 6 hours. We report the result that optical microscope observation, CL and X‐ray rocking curve measurements and reciprocal lattice space mapping were carried out to evaluate the crystal quality of the grown InGaP layers. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Highly reflective AlGaAsSb/InP Bragg reflector at 1.55 μm grown by MOVPE

The metalorganic vapor-phase epitaxy growth of a highly reflective 24-pair AlGaAsSb/InP-distributed Bragg reflector (DBR) is reported for the first time. The influence of the growth parameters such as the V/III input ratio, the growth temperature and the pressure, the total H₂ flow, the gas velocity and the switching sequence of the source gases at the interfaces has been deeply investigated and optimized to achieve stable growth conditions. The DBR achieves a reflectivity as high as 99.5% around 1.55 μm, a uniform stable composition, and an excellent crystal quality over the 2 inch wafer, with a surface free of crosshatch and a defect density below 1/cm². For the optical characterizations, measurements of linear and nonlinear reflectivity, transmission, pump-probe and photoluminescence were done. The interfaces and bulk layers of InP/AlGaAsSb/InP heterostructures were analyzed by transmission electron microscopy. High resolution X-ray diffraction measurements were used to determine the composition shift in the growth plane of the DBR. The measurements show the high quality of the growth and demonstrate that thick AlGaAsSb/InP heterostructures can be grown by metalorganic vapor-phase epitaxy (MOVPE), and in particular DBRs above 1.31 μm.     

Epitaxial growth of tin oxide film on TiO2(1 1 0) using molecular beam epitaxy

Growth of tin oxide thin films using molecular beam epitaxy in a pyrolyzed nitrogen dioxide atmosphere on a titanium dioxide (1 1 0) substrate was investigated using X-ray photoelectron spectroscopy (XPS), electron diffraction, and atomic force microscopy (AFM). Properties of deposited films were studied for their dependence on substrate temperature and oxidation gas pressure. Analyses using XPS data revealed that tin atoms were fully oxidized to Sn⁴⁺ and SnO₂ films were grown epitaxially in deposition conditions of substrate temperatures of 627 K or higher and NO₂ pressure greater than 3×10⁻³ Pa. At a substrate temperature of 773 K, a smooth surface with atomic steps was visible in the SnO₂ films, but above or below this temperature, fine grains with crystal facets or porous structures appeared. At pressures of 8×10⁻⁴ to 3×10⁻⁴ Pa, the randomly oriented SnO phase was dominantly grown. Further decreasing the pressure, the Sn metal phase, which was epitaxially crystallized at less than 500 K, was also grown.     

The dependence of cobalt concentration on the growth temperature in synthetic blue quartz

Co-doped blue quartz crystals were grown hydrothermally in an Na₂CO₃ solution. The concentration of cobalt in as-grown quartz crystal was observed to be independent of the concentration of cobalt in the nutrient but to be strongly affected by the growth temperature. It is concluded that transparent dark-blue quartz could be grown in the narrow temperature range, that is, 340–345°C. From TEM observations, the origin of the blue color in synthetic blue quartz is thought to be an inclusion effect resulting from the entrapped clusters of Co–Si–OH colloids, which is different from other reports [Wood, Ballman, Am. Mineral. 51 (1966) 216; Lehmann, J. Phys. Chem. Solids 30 (1969) 395]. The as-grown synthetic blue quartz crystal was jewel grade.     

Defects and stresses in MBE-grown GaN and Al0.3Ga0.7N layers doped by silicon using silane

The electric and structural characteristics of silicon-doped GaN and Al_0.3Ga_0.7N layers grown by molecular beam epitaxy (MBE) using silane have been analyzed by the Hall effect, Raman spectroscopy, and high-resolution X-ray diffractometry. It is established that the electron concentration linearly increases up to n = 4 × 10²⁰ cm^?3 with an increase in the silane flow rate for GaN:Si, whereas the corresponding dependence for Al_0.3Ga_0.7N:Si is sublinear and the maximum electron concentration is found to be n = 4 × 10¹⁹ cm^?3. X-ray measurements of sample macrobending indicate a decrease in biaxial compressive stress with an increase in the electron concentration in both GaN:Si and Al_0.3Ga_0.7N:Si layers. The parameters of the dislocation structure, estimated from the measured broadenings of X-ray reflections, are analyzed.     

Gettering Properties of Praseodymium in GaAs,In0.53Ga0.47As,and InP Grown by Liquid Phase Epitaxy

The incorporation of praseodymium (Pr) into GaAs, In_0.53Ga_0.47As, and InP during liquid-phase epitaxy were investigated by double crystal x-ray diffraction, Hall effect, low temperature photoluminescence (PL) measurements. The lattice mismatch slightly vary with Pr concentration in the growth melts. Examinations of the electrical property illustrate that the lower carriers concentrations and a higher mobilities are obtain from Pr-doped epilayers than undoped sample (In_0.53Ga_0.47As and InP). The PL spectra (15-K) show that the intensity of the impurity related peaks decreases and the near-band-to-band luminescence intensity increase. They also reveal that the impurities are gettered by Pr ions during LPE growth. Thus, for the purpose of purification, proper amount of Pr in the growth melts is suggested. No intra-4f-shell transition line is observed from the Pr-doped GaAs, In_0.53Ga_0.47As, and InP layers.