Stress and morphology of a nonpolar a-plane GaN layer on r-plane sapphire substrate

The anisotropic strain of a nonpolar (1120) a-plane GaN epilayer on an r-plane (1102) sapphire substrate, grown by low-pressure metal-organic vapour deposition is investigated by Raman spectroscopy. The room-temperature Raman scattering spectra of nonpolar a-plane GaN are measured in surface and edge backscattering geometries. The lattice is contracted in both the c- and the m-axis directions, and the stress in the m-axis direction is larger than that in the c-axis direction. On the surface of this sample, a number of cracks appear only along the m-axis, which is confirmed by the scanning electron micrograph. Atomic force microscopy images reveal a significant decrease in the root-mean-square roughness and the density of submicron pits after the stress relief.     

Optical and structural investigation of a-plane GaN layers on r-plane sapphire with nucleation layer optimization

Nonpolar a-plane GaN epilayers are grown on several r-plane sapphire substrates by metal organic chemical vapour deposition using different nucleation layers:(A) a GaN nucleation layer deposited at low temperature(LT);(B) an AlN nucleation layer deposited at high temperature;or(C) an LT thin AlN nucleation layer with an AlN layer and an AlN/AlGaN superlattice both subsequently deposited at high temperature.The samples have been characterized by Xray diffraction(XRD),atomic force microscopy and photoluminescence.The GaN layers grown using nucleation layers B and C show narrower XRD rocking curves than that using nucleation layer A,indicating a reduction in crystal defect density.Furthermore,the GaN layer grown using nucleation layer C exhibits a surface morphology with triangular defect pits eliminated completely.The improved optical property,corresponding to the enhanced crystal quality,is also confirmed by temperature-dependent and excitation power-dependent photoluminescence measurements.     

InGaN/GaN多量子阱的组分确定和晶格常数计算

采用金属有机化学气相沉积(MOCVD)技术以蓝宝石为衬底在n型GaN单晶层上生长了InGaN/GaN多量子阱结构外延薄膜，利用高分辨X射线衍射(HRXRD)，卢瑟福背散射/沟道(RBS/channeling)，以及光致发光(PL)技术对InGaN/GaN多量子阱结构薄膜分别进行了平均晶格常数计算、In原子替位率计算和In组分的定量分析.研究表明：InGaN/GaN多量子阱的水平和垂直方向平均晶格常数分别为a_epi=0.3195nm，c_epi=0.5198nm，In原子的替位率为99.3%，利用HRXRD和RBS/channeling两种分析技术计算In的组分分别是0.023和0.026，并与样品生长时设定的预期目标相符合，验证了两种实验方法的准确性；而用室温条件下的光致发光谱(PL)来计算InGaN/GaN多量子阱中In的组分是与HRXRD和RBS/channeling的实验结果相差很大，说明用PL测试In组分的方法是不适宜的. 关键词： InGaN/GaN多量子阱 高分辨X射线衍射 卢瑟福背散射/沟道 光致发光     

A new method to preserve the c-axis growth of thick YBa2Cu3O7–δ films grown by pulsed laser deposition

YBa₂Cu₃O_7–δ (YBCO) films were prepared on (1 0 0) MgO substrates by pulsed laser deposition (PLD) method. In order to eliminate the a-axis growth, which is commonly observed in the YBCO film thicker than a critical value, we developed a new PLD target that was sintered at a temperature far below YBCO 123 phase formation. The surface analysis made by AFM technique confirmed that very fine particles of around 20 nm size could be ejected from the new target to the substrate. The fine oxide clusters could be easily moved and incorporated into the YBCO phase thus benefited the c-axis growth even in the thick films. For instance, only the c-axis growth in the new film with a thickness of about 650 nm was larger than a critical thickness of the a-axis growth. However, in the standard film of the same thickness, there is 24.5% of the a-axis growth accompanying the main c-axis growth. Therefore, the c-axis growth could be preserved in the very thick YBCO film by a non-superconducting target.     

Magnetic properties of V5S8 single crystals

The magnetic properties of V₅S₈ single crystals have been investigated by susceptibility and torque measurements. The susceptibilities parallel and perpendicular to the magnetic easy axis show a remarkable anisotropy below the Neel temperature of 32 K and are nearly Isotropie above that temperature. The easy axis is found to be nearly along the c-axis of the monoclinic lattice, being inclined at an angle of 9.6 ± 1.0° from the c-axis toward the a-axis. The torque curves, measured up to 24.4 kOe at liquid helium temperature, deviate from the usual form of a sine function with increasing magnetic field. The analysis of these torque curves suggests that spin flopping may occur at 43 kOe, a comparatively low critical field. Using these experimental results, a localized d-electron model for a particular site of the vanadium-ion, proposed by previous investigators, is examined.     

Characterization of a-plane orientation ZnO film grown on GaN/Sapphire template by pulsed laser deposition

In this study, the authors have investigated the structural and optical properties of ZnO layer grown by pulsed laser deposition on GaN/r-plane sapphire. X-ray diffraction results demonstrate the ZnO film to be highly preferentially deposited at a-axis orientation; the different rocking curve values along the two orthogonal directions indicate the low C_2v symmetry in the growth a-plane ZnO. From free stress to large tensile stress (about 1.34 × 10⁹ Pa) distribution along the growth direction of ZnO is revealed by visible Raman mapping spectra. The enhanced significantly high-order longitudinal-optical (LO) phonon modes up to 4th and no TO phonons have been observed in Raman spectrum under UV 325 nm by resonance conditions; an intense and broad disorder activated surface phonon mode is also observed, resulting from the increased disorder on the film surface with stripe-like growth features. Low-temperature photoluminescence measurements reveal that the band-edge emission of ZnO is dominated by neutral donor-bound exciton and free electrons to neutral acceptor emissions. Interfacial microstructure of ZnO/GaN has been examined by transmission electron microscopy, with the epitaxial relationship () ZnO//() GaN. All these results indicated that GaN template played an important role in the growth of ZnO film, with full advantage of small lattice mismatch.     

Anomalous shear ultrasonic attenuation peak at 95.5°K in holmium2

An anomalous attenuation peak at 95.5°K has been observed for shear ultrasonic waves transmitted along the c-axis of two holmium single crystals. The velocity change across this temperature is on the order of 10^?4. In the presence of an external magnetic field, the peak moves toward high temperatures for both H∥a and H∥c. A rotation of the field in the basal plane produces a two-fold symmetry pattern in the attenuation. A double peak structure centered at the c-plane has also been observed when the field is rotated away from the a-axis. A considerable fraction of the attenuation peak appears to go as sin² θ where θ is the angle between the shear wave polarization and the external field.     

The etching of a-plane GaN epilayers grown by metal--organic chemical vapour deposition

Morphology of nonpolar (1120) a-plane GaN epilayers on r-plane (1102) sapphire substrate grown by low-pressure metal-organic vapour deposition was investigated after KOH solution etching. Many micron-and nano-meter columns on the a-plane GaN surface were observed by scanning electron microscopy. An etching mechanism model is proposed to interpret the origin of the peculiar etching morphology. The basal stacking fault in the a-plane GaN plays a very important role in the etching process.     

金属有机物化学气相沉积生长的$lt;i$gt;a$lt;/i$gt;(1120)面GaN三角坑缺陷的消除研究

用金属有机物化学气相沉积方法在r面蓝宝石上生长了非极性a面GaN薄膜,通过采用AlGaN多量子阱插入层,得到了高质量的非极性GaN材料. 用原子力显微镜和高分辨X射线衍射仪研究了a面GaN的表面形貌和结晶质量,发现非极性材料上典型的三角坑缺陷被消除,（1120）面X射线双晶摇摆曲线的半峰宽为680″. 关键词： GaN 原子力显微镜 高分辨X射线衍射仪 非极性     

Low-temperature heat transport in α-HgI2 single crystals

The thermal conductivity of several samples from α-HgI₂ crystals grown by two different methods has been measured from 50 mK to 200 K. The thermal conductivity is found to be intrinsic but anisotropic above 15 K: it is smaller along c-axis than along a-axis, the anisotropy ratio being about 5 between 15 and 200 K. Below 15 K, the thermal conductivity is sample dependent and the calculated Casimir limit is not reached at the lowest temperatures. The results have been interpreted considering phonon scattering by structural defects. A simple quantitative analysis of the curves suggests that phonons are scattered mainly by large clusters of interstitial defects due to the lack of stoichiometry of the crystals; the typical dimensions of these clusters are not smaller than 1 μm perpendicular to c-axis and 0.3 /gmm along c-axis. The presence of plane defects is also detected. Point defect scattering is relatively small and explained by residual metallic impurities and carbon at interstitial sites. The intrinsic anisotropy is briefly discussed.     

Phase diagram of Fe(1−x)CoxCl2; a random mixture of two antiferromagnets with competing spin anisotropies

Concentration vs transition temperature phase diagram of a random mixture of two anisotropic antiferromagnets, FeCl₂ and CoCl₂, is obtained from the measurement of the susceptibility on the single crystals. Two distinct critical points, one along the c-axis and the other in the c-plane, are observed for the respective concentrations between x = 0.264 and x = 0.481. Three kinds of ordered phases, namely, Fe-rich and Co-rich antiferromagnetic phases and a new phase are found. The phase diagram shows a tetracritical point as well.A hump is observed in the temperature dependence of the susceptibility along the c-axis (c-plane for x ? 0.3) near the ordering temperature occuring in the c-plane (c-axis for x ? 0.3).     

NMR study of KCuF3 at low temperatures

The NMR of F¹⁹ nuclei in KCuF₃ has been measured in the a-type single crystal at 1.7 K. Two types of magnetic domains exist; one occupies most part of the crystal and has easy axis along <110>, and the other occupies the rest of the crystal and has easy axis along <100>. In both domains the moment directions are distributed around the easy axes over a considerable angular range. The spin-flop begins with nearly zero applied field in the <110>;-domain. When the magnetic field is rotated in the c-plane, an angular dependence has been observed for those F¹⁹ nuclei which lie on the c-axis. This dependence arises from the alternate stacking of the ground state wave functions of Cu²⁺ ions.     

Vortex states in mesoscopic single crystals Bi2Sr2CaCu2O8+δ in high magnetic fields

The c-axis resistivity measurements were performed in the vicinity of the ab-plane in order to investigate the interaction between Josephson vortices and pancake vortices in Bi₂Sr₂CaCu₂O_8+δ mesoscopic single crystals. It was found that the angular dependence of the c-axis resistivity drastically changes in high magnetic field regime. The vortex lock-in transition becomes considerably broad in high magnetic fields, while the angular dependence of resistance exhibits the sharp lock-in features in low magnetic field region.     

Electrical conductivity anisotropy in alkali feldspar at high temperature and pressure

The electrical conductivity of alkali feldspar along different orientations was determined at 1.0 GPa and at temperatures of 823–1286 K in a cubic anvil apparatus using alternating current impedance spectroscopy. Impedance arcs representing crystal conductivity occur in the frequency range of ～10³–10⁶ Hz. The electrical conductivity of alkali feldspar increases with increasing temperature. The highest electrical conductivities in alkali feldspars were measured along the a-axis, with somewhat lower conductivities along the b-axis, and the lowest conductivities along the c-axis, suggesting minor anisotropy. The activation enthalpies ranged from 100 to 110 kJ/mol. The anisotropic results were combined to yield an isotropic model with an activation enthalpy of 102 kJ/mol. By comparing these results with previous results, we suggest that the dominating charge carriers for alkali feldspars are alkali ions. The minor anisotropy in conductivity for alkali feldspar may not account for the anisotropy of the crust.     

Measurement of iron self-diffusion in hematite single crystals by secondary ion-mass spectrometry (SIMS) and comparison with cation self-diffusion in corundum-structure oxides

Iron bulk self-diffusion coefficients were measured in Fe₂O₃ single crystals using original methodology based on the utilization of ⁵⁷Fe stable isotope as iron tracer and depth profiling by secondary ion-mass spectrometry (SIMS). The iron self-diffusion coefficients were measured along and perpendicular to the c-axis, between 900 and 1100°C, in an oxygen atmosphere. Along the c-axis, the coefficients can be described by D _{∥ c} (cm²/s)?=?5.2?×?10⁶?exp[?510 (kJ/mol)/RT], and are close to reliable data, available in the literature, obtained by means of radioactive techniques. Perpendicular to the c-axis, D _⊥c (cm²/s)?= 83?exp[?430 (kJ/mol)/RT], and the coefficients are smaller than coefficients along the c-axis. The data are compared with previously results of cation bulk self-diffusion in Cr₂O₃ and Al₂O₃ single crystals.     

Growth of nonpolar a-plane GaN on nano-patterned r-plane sapphire substrates

Sapphire substrates were nano-patterned by inductive coupled plasma etching process. Nonpolar a-plane GaN films were grown on planar and nano-patterned r-plane sapphire substrates by metal organic chemical vapor deposition. The anisotropic characteristic and the crystalline quality of the a-plane GaN films were studied through XRD rocking curves. The cross section and surface morphologies of the a-plane GaN films were studied using SEM and AFM measurements, respectively. The crystal quality and surface flatness of the nonpolar a-plane GaN were greatly improved through the usage of the nano-patterned r-plane sapphire substrates.     

Study on the relationships between Raman shifts and temperature range for a-plane GaN using temperature-dependent Raman scattering

In this paper, Raman shifts of a-plane GaN layers grown on r-plane sapphire substrates by low-pressure metal-organic chemical vapor deposition (LPMOCVD) are investigated. We compare the crystal qualities and study the relationships between Raman shift and temperature for conventional a-plane GaN epilayer and insertion AlN/AlGaN superlattice layers for a-plane GaN epilayer using temperature-dependent Raman scattering in a temperature range from 83 K to 503 K. The temperature-dependences of GaN phonon modes (A₁ (TO), E₂ (high), and E₁ (TO)) and the linewidths of E₂ (high) phonon peak are studied. The results indicate that there exist two mechanisms between phonon peaks in the whole temperature range, and the relationship can be fitted to the pseudo-Voigt function. From analytic results we find a critical temperature existing in the relationship, which can characterize the anharmonic effects of a-plane GaN in different temperature ranges. In the range of higher temperature, the relationship exhibits an approximately linear behavior, which is consistent with the analyzed results theoretically.     

Investigation of threshold switching mechanism based on Mn-doped ZnO/GaN heterojunction by photogenerated carrier injection

Highly c-axis oriented ZnO thin films have been prepared on n-type GaN-coated sapphire substrates by radio frequency reactive magnetron sputtering at 100 °C followed by thermal annealing at 740 °C for 2 h. The ZnO/GaN heterojunction devices show a steady threshold switching (TS) characteristic. In addition, Mn-doped ZnO (MZO)/GaN heterojunction has been prepared and it shows lower threshold voltage and higher on/off ratio than those of ZnO/GaN heterojunction. The formation of TS characteristic may be attributed to trapping and detrapping of electrons in heterojunction device. Furthermore, the devices are investigated by the injection of photogenerated carriers, it is illustrated that the external carrier injection may affect the rate of the trapping of electrons and results in the decrease of the threshold voltage.     

Anisotropic thermoelectric power of TTF-TCNQ

We have measured the thermoelectric power along the a-axis as well as the highly conducting b-axis in TTF-TCNQ crystals. Measurements were taken both parallel and perpendicular to the long axis on two sets of crystals. One set grew along the a-axis and the other along the b-axis. The thermopowers are of opposite sign except near 60°K where both cross zero at slightly different temperatures. The a-axis thermopower is consistent with non-metallic diffusive transport in the a direction.     

Optical properties of nonpolar -plane GaN layers

We have studied optical properties of nonpolar a-plane GaN layers grown on r-plane sapphire by metalorganic chemical vapor deposition and hydride vapor phase epitaxy using different nucleation schemes. Several emission bands, which are not typical for c-plane GaN, are observed in the photoluminescence spectra and their excitation-intensity, temperature, and polarization dependencies are examined. In addition, the spatial distribution of the emissions was examined by cathodoluminescence imaging and relations of the different emissions with particular structural features in the layers are revealed. The results are discussed with emphasis on the origin of the emission line and particular recombination mechanisms.