Reduction of grown-in dislocation density in Ge Czochralski-grown from the B2O3-partially-covered melt

Germanium (Ge) single crystals with an extremely low density of grown-in dislocations were grown by the Czochralski (CZ) technique with boron oxide (B₂O₃) liquid. Because attachment of particles floating on the melt surface to a growing Ge crystal leads to generation of dislocations during the growth, partial covering of the Ge melt surface with B₂O₃ liquid was attempted. Such attachment of particles was drastically suppressed or the particles were caught by the introduction of B₂O₃ liquid, and a particle-free Ge melt was realized in the central region of the melt surface. Ge single crystals were successfully grown from such melt, the grown-in dislocation density being 0–1×10³ cm⁻², which was remarkably lower than that in Ge crystals grown by a conventional CZ technique. The contaminations by B and O atoms of the grown crystal detected by SIMS analysis were very low. These Ge crystals have the potential for application to be applied as high-quality, dislocation-free substrates of GaAs solar cells for various usages including in space.     

Study of microstructural evolutions in phosphorus-doped ZnO films grown by pulsed laser deposition

The microstructure of P-doped ZnO films grown on the c-plane sapphire substrate by pulsed laser deposition (PLD) was investigated. ZnO films were highly textured along c-axis with two different in-plane orientations. The textured domain was surrounded by the threading dislocations, resulting in the formation of low-angle grain boundary. It was found that the degree of texture and crystalline quality of P-doped ZnO films decreased with increasing the phosphorus atomic percent. For the microstrain study, X-ray diffraction line profile analysis (LPA) was performed. The 0.5 at% P-doped ZnO film showed much higher microstrain than the 1.0 at% P-doped ZnO film as well as as-grown film, which indicated that the phosphorus in former film was effectively incorporated into ZnO film. X-ray photoelectron spectroscopy (XPS) results showed that the phosphorus in 1.0 at% P-doped ZnO film tended towards segregation, which was well consistent with XRD results.     

Fabrication of freestanding m-plane GaN wafer by using the HVPE technique on an aluminum carbide buffer layer on an m-plane sapphire substrate

A freestanding m-plane GaN wafer is fabricated by using the hydride vapor-phase epitaxy (HVPE) technique on an aluminum carbide buffer layer on an m-plane sapphire substrate. X-ray pole-figure measurements show a clear m-plane orientation of the GaN surface. The full-width at half-maximum (FWHM) of GaN (1 1¯ 0 0) X-ray rocking curve (XRC) with the scattering vector along the [1 1 2¯ 0] direction is approximately 800 arcsec; this indicates good crystallinity. On the other hand, the FWHM for the case in which the scattering vector is oriented along the [0 0 0 1] direction is broad; this suggests the influence of structural defects along this direction. In fact, basal plane stacking faults (BSF) with a density of approximately 3×10⁵ cm⁻¹ is observed by transmission electron microscopy (TEM). The preparation of a 45-mm-diameter m-plane GaN wafer due to spontaneous separation of the GaN layer from the sapphire substrate is demonstrated.     

Measurement and analysis of the dextran partition coefficient in sucrose crystallization

The effect of crystallization conditions on the dextran partition coefficient between impure syrup and sugar crystal has been investigated in a batch crystallizer. The crystallizer is operated isothermally at temperatures of 30, 40, and 50 °C, at constant relative supersaturations of 0.05, 0.07, and 0.09, and with mother liquor dextran concentrations of 1000 and 2000 ppm/Brix. The dextran content has been determined by the CSR method. A 1:1 mass ratio of high-fraction dextran (approximately 250,000 Da) and low-fraction dextran (60,000-90,000 Da) is used to represent a wide range of dextran contamination. It is seen that the dextran partition coefficient in sucrose crystallization increases with both increasing supersaturation and increasing crystallization temperature. However it appears that these are secondary effects, with the partition coefficient strongly correlating with crystal growth rate alone, despite the regressed data having large variations in temperature, mother liquor dextran content, and supersaturation. Dextran incorporation into the sugar crystal results from both dextran adsorption onto the crystal surface and mother liquor inclusions. The explanation for the variation in the dextran content in sugar crystal with respect to the growth rate is due to increased adsorption due to the higher surface roughness of crystals grown at high growth rates. Although the dextran concentration in the solution affects the dextran content in the crystal, it does not strongly affect the dextran partition coefficient.     

Defect reduction of laterally regrown GaN on GaN/patterned sapphire substrates

Structural properties of GaN epilayers on wet-etched protruding and recess-patterned sapphire substrates (PSSs) have been investigated in detail using high-resolution double-crystal X-ray diffraction (DCXRD) and etch-pit density methods. The DCXRD results reveal various dislocation configurations on both types of PSSs. The etch pits of GaN on the recess PSS exhibit a regular distribution, i.e. less etch pits or threading dislocation density (TDD) onto the recess area than those onto the sapphire mesas. On the contrary, an irregular distribution is observed for the etch pits of GaN on the protruding PSS. A higher crystal quality of the GaN epilayer grown onto the recess PSS can be achieved as compared with that onto the protruding PSS. These data reflect that the GaN epilayer on the recess PSS could be a better template for the second epitaxial lateral overgrowth (ELOG) of GaN. As a result, the GaN epilayers after the ELOG process display the TDDs of around ∼10⁶ cm⁻².     

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.     

Growth of GaN single crystals with extremely low dislocation density by two-step dislocation reduction

We have discovered a mechanism which can significantly reduce the dislocation density during the growth of GaN single crystals in the Na flux method. The significant reduction of the dislocation density occurs in the later stage of LPE growth, rather than solely at the seed-LPE interface for which we have already reported evidence indicating the presence of bundling dislocations. The two-step dislocation reduction is the key in achieving extremely low dislocation density using this method.     

Effect of NH3 flow rate on m-plane GaN growth on m-plane SiC by metalorganic chemical vapor deposition

This paper reports a study of the effect of NH₃ flow rate on m-plane GaN growth on m-plane SiC with an AlN buffer layer. It is found that a reduced NH₃ flow rate during m-plane GaN growth can greatly improve the recovery of in situ optical reflectance and the surface morphology, and narrow down the on-axis (1 0 1¯ 0) X-ray rocking curve (XRC) measured along the in-plane a-axis. The surface striation along the in-plane a-axis, a result of GaN island coalescence along the in-plane c-axis, strongly depends on the NH₃ flow rate, an observation consistent with our recent study of kinetic Wulff plots. The pronounced broadening of the (1 0 1¯ 0) XRC measured along the c-axis is attributed to the limited lateral coherence length of GaN domains along the c-axis, due to the presence of a high density of basal-plane stacking faults, most of which are formed at the GaN/AlN interface, according to transmission electron microscopy.     

A new in-situ surface treatment during MBE-grown PbSe on CaF2/Si(1 1 1) heterostructure

The continual improvement of IV-VI materials grown by molecular beam epitaxy (MBE) is a key step in the development of IV-VI infrared semiconductor devices on silicon substrates. This study presents a novel surface-treatment method which is carried out during MBE growth of monocrystalline PbSe on Si(1 1 1)-oriented substrates. Details of the experimental procedures are described and supported by reflection high-energy electron diffraction (RHEED) patterns. The effect of the in-situ surface-treatment method is exhibited in the forms of improved electrical and morphological properties of PbSe thin films. Specially, the carrier mobility increases almost three-fold at 77 K and nearly two-fold at 300 K. The density of the growth pits undergoes almost three-fold reduction, whereas the density of the threading dislocations decreases around four-fold.     

Misoriented domain formation in 6H-SiC single crystal

6H-SiC single crystals were grown by the physical vapor transport (PVT) technique. Misoriented domains (MDs) were observed in as-grown crystals. Raman spectra and X-ray diffraction indicated that the MDs are 4H polytype with either (1 0 1¯ 2) or (1 0 1¯ 6) growth plane. Formation probability of MDs increased continuously as the thermal insulator had been repeatedly used. Simulations based on heat transfer demonstrated that the changes of the temperature and the temperature axial gradient at the center of the growth front were responsible for the phenomenon. The formation mechanism was put forward in terms of atomic structure of various crystal planes.     

HRTEM investigation of high-reflectance AlN/GaN distributed Bragg-reflectors by inserting AlN/GaN superlattice

A high-quality AlN/GaN distributed Bragg-reflectors (DBR) was successfully grown on sapphire substrate by low-pressure metal-organic chemical vapor deposition using ultra-thin AlN/GaN superlattice insertion layers (SLILs). The reflectivity of AlN/GaN DBR with ultra-thin AlN/GaN SLIL was measured and achieved blue peak reflectivity of 99.4% at 462 nm. The effect of ultra-thin AlN/GaN superlattice insertion layer was examined in detail by transmission electron microscopy, and indicated that the crack of AlN/GaN DBR can be suppress by inserting AlN/GaN SLIL. For electronic properties, the turn on voltage is about 4.1 V and CW laser action of vertical-cavity surface-emitting laser (VCSEL) was achieved at a threshold injection current of 1.4 mA at 77 K, with an emission wavelength of 462 nm.     

Characterizations of GaN film growth by ECR plasma chemical vapor deposition

The electron cyclotron resonance plasma-enhanced metalorganic chemical vapor deposition technology (ECR–MOPECVD) is adopted to grow GaN films on (0 0 0 1) α-Al₂O₃ substrate. The gas sources are pure N₂ and trimethylgallium (TMG). Optical emission spectroscopy (OES) and thermodynamic analysis of GaN growth are applied to understand the GaN growth process. The OES of ECR plasma shows that TMG is significantly dissociated in ECR plasma. Reactants N and Ga in the plasma, obtained easily under the self-heating condition, are essential for the GaN growth. They contribute to the realization of GaN film growth at a relatively low temperature. The thermodynamic study shows that the driving force for the GaN growth is high when N₂:TMG>1. Furthermore, higher N₂:TMG flow ratio makes the GaN growth easier. Finally, X-ray diffraction, photoluminescence, and atomic force microscope are applied to investigate crystal quality, morphology, and roughness of the GaN films. The results demonstrate that the ECR–MOPECVD technology is favorable for depositing GaN films at low temperatures.     

AlAs/GaAs分布布拉格反射镜(DBR)的反射谱拟合与优化生长

对分布布拉格反射镜(DBR)的原理和特征进行了分析,使用传输矩阵方法计算了不同对数GaAs/AlAs反射镜的反射率曲线.利用分子束外延(MBE)设备生长了波长为920nm和980nm的半导体多层膜DBR反射镜,分析了实验测得的反射谱与理论拟合曲线之间的差异及其产生原因,实现了材料的优化生长,获得了反射率大于99;、中心波长和带宽接近理论计算值的DBR材料.该DBR的反射谱拟合与优化生长研究可应用于VCSEL和VECSEL激光器.     

Structural and optical properties of quaternary AlInGaN epilayers grown by MOCVD with various TMGa flows

AlInGaN quaternary epilayers have been grown with various TMGa flows by metalorganic chemical vapor deposition to investigate the influence of growth rate on the structural and optical properties. Triple-axis X-ray diffraction measurements show AlInGaN epilayers have good crystalline quality. Photoluminescence (PL) measurements show that the emission intensity of AlInGaN epilayers is twenty times stronger than that of AlGaN epilayer with comparable Al content. V-shaped pits are observed at the surface of AlInGaN epilayers by atomic force microscopy (AFM) and transmission electron microscopy (TEM). High growth rate leads to increased density and size of V-shaped pits, but crystalline quality is not degraded.     

Phase-field simulations of Te-precipitate morphology and evolution kinetics in Te-rich CdTe crystals

Te precipitates are one of principal defects that form during cooling of melt-grown CdTe or CZT crystals when grown Te-rich. Many factors such as the kinetic properties of intrinsic point defects (vacancy, interstitial, and antisite defects); stresses associated with the lattice mismatch between precipitate and matrix; temperature gradients and extended defects (dislocations, twin and grain boundaries); non-stoichiometric composition; thermal treatment history all affect the formation and growth/dissolution of Te precipitates in CdTe. A good understanding of these effects on Te precipitate evolution kinetics is technically important in order to optimize material processing and obtain high-quality crystals. This research develops a phase-field model capable of investigating the evolution of coherent Te precipitates in a Te-rich CdTe crystal undergoing cooling from the melt. Cd vacancies and Te interstitials are assumed to be the dominant diffusing species in the system, which is in two-phase equilibrium (matrix CdTe and liquid Te inclusion) at high temperatures and three-phase equilibrium (matrix CdTe, Te precipitate, and void) at low temperatures. Using available thermodynamic and kinetic data from experimental phase diagrams and thermodynamic calculations, the effects of Te interstitial and Cd vacancy mobility, cooling rates and stresses on Te precipitate, and void evolution kinetics are investigated.     

Systematic studies of Si and Ge hemispherical concave wafers prepared by plastic deformation

Si and Ge hemispherical concave wafers can be prepared by plastic deformation using Si and Ge single- and polycrystal wafers. Deformation regions in which such Si and Ge hemispherical wafers can be obtained by high-temperature plastic deformation were systematically studied. The deformation regions in which well-shaped Si concave wafers can be obtained were studied for (1 0 0), (1 1 1), and polycrystal Si. It was found that Si (1 1 1) crystal wafers can be more easily deformed to a perfect hemispherical shape than (1 0 0) wafers because of the crystallographic symmetry. Si hemispherical wafers with a small radius of 25 mm can be perfectly deformed when 0.5-mm-thick Si (1 1 1) crystal wafers are used. Ge hemispherical wafers with a radius of 100 mm can be perfectly deformed when 0.5-mm-thick Ge crystal wafers are used. Ge hemispherical concave wafers with a perfect shape can be more easily obtained using Ge (1 1 1) wafers than (1 0 0) wafers. According to these results, the deformation behavior of Ge wafers is very similar to that of Si wafers at a normalized pressing temperature. As both the radius and the load on Si and Ge hemispherical wafers increase, thicker Si and Ge wafers can be used to obtain hemispherical wafers with a perfect shape. The dislocation density in the shaped wafers markedly decreases as the pressing temperature and hemisphere radius increase. Thus, it is suggested that Si and Ge shaped wafers are of sufficient quality and have the high potential for use as several types of lens such as those in Si and Ge X-ray monochromators.     

Unsteady-state transfer of impurities during crystal growth of sucrose in sugarcane solutions

In this work, we present growth rate data of sucrose crystals in the presence of impurities that can be used by both sugar technologists and crystal growth scientists. Growth rate curves measured in a pilot-scale evaporative crystallizer suggest a period of slow growth that follows the seeding of crystals into supersaturated technical solutions. The observed trend was enhanced by adding typical sugarcane impurities such as starch, fructose or dextran to the industrial syrups. Maximum growth rates of sucrose resulted at intermediate rather than high supersaturation levels in the presence of the additives. The effects of the additives on the sucrose solubility and sucrose mass transfer in solution were taken into account to explain the observed crystal growth kinetics. A novel mechanism was identified of unsteady-state adsorption of impurities at the crystal surface and their gradual replacement by the crystallizing solute towards the equilibrium occupation of the active sites for growth. Specifically designed crystallization experiments at controlled supersaturation confirmed this mechanism by showing increasing crystal growth rates with time until reaching a steady-state value for a given supersaturation level and impurity content.     

Transmission electron microscopy observations of twin boundaries and sub-boundary networks in bulk CdZnTe crystals

The microstructure characteristics of the twin boundaries and sub-boundary networks in bulk cadmium zinc telluride (CdZnTe) crystals have been studied by transmission electron microscopy (TEM). Three types of twin boundaries were identified and characterized, which are (i) single straight twin boundary, (ii) tilt twin boundary, and (iii) twin boundary with steps. Boundary dislocations at tilt twin boundary and high dislocation density around steps were observed. The origin of the boundary dislocations is ascribed to the lattice misfit between two tilt grains. The formation of the twin boundary with steps is suggested to be the interaction between the twin boundary and dislocations. Honeycomb-like sub-boundary defects were also observed. The probable reason for the formation of the sub-boundary networks is discussed.     

Estimation of growth rate in unidirectionally solidified multicrystalline silicon by the growth-induced striation method

Multicrystalline silicon was grown by unidirectional solidification method using the accelerated crucible rotation technique. The application of the accelerated crucible rotation technique in unidirectional solidification method induced growth striations across the axial direction of the grown crystal. This striation pattern was observed from carbon concentration distribution, obtained by using Fourier transform infrared spectroscopy. The generated striation pattern was found to be weak and discontinuous. Some striations were absent, probably due to back melting, caused during each crucible rotation. From the growth striations and applied time period in crucible rotation, the growth rate was estimated by using Fourier transformation analysis.     

N-face GaN growth on c-plane sapphire by metalorganic chemical vapor deposition

In this work, we report the growth of smooth, high-quality N-face GaN on c-plane sapphire by metalorganic chemical vapor deposition. It is found that the nitridation temperature of sapphire has a critical effect on the surface morphology of N-face GaN. Sapphire after a severe nitridation gives rise to a high density of hexagonal hillocks during N-face GaN growth. Smooth N-face GaN has been grown on appropriately nitridized sapphire. The N-polarity of the GaN film has been confirmed with no inversion domain by convergent beam electron diffraction. Controlled growth interruption is carried out to study the nucleation evolution during N-face GaN growth, which is found distinctly different from the two-step growth of Ga-face GaN. Atomically smooth N-face GaN has been achieved with comparable structural quality to Ga-face GaN.