On the Growth of High-Temperature Epitaxial AlN (AlGaN) Layers on Sapphire Substrates by Ammonia Molecular Beam Epitaxy

The growth of high-temperature AlN and AlGaN layers on (0001) sapphire substrates by ammonia based molecular-beam epitaxy is studied. Factors affecting the formation of inverted domains in high-temperature AlN films are examined. The density of inverted domains is found to correlate with the density of nucleation islands during the initial stages of growth. The denser coverage of a surface by nucleation islands suppresses the formation of inverted domains. It is possible to increase the density of surface coating at the nucleation growth stage by increasing the degree of substrate nitriding, reducing the deposition temperature, and using intense ammonia fluxes during deposition of the initial layers. The kinetic model in the mean field approximation is developed to explain the observed effects of growth parameters on the density of nucleation islands. The growth features of AlN and its structure are taken into account. The obtained results are used to grow AlN/AlGaN layers with improved structural quality. The grown films have a root-mean-square surface roughness of 2 Å and 120 arc s FWHM of X-ray diffraction peaks for the AlN 0002 reflection. The density of inverted domains is decreased to below 10⁵ cm^-2. Improvement in the quality of the AlN films is achieved by using two-step growth and by the application of gallium as a surfactant.     

Mechanism of silicon epitaxy on porous silicon layers

The mechanism of silicon epitaxy on porous Si(111) layers is investigated by the Monte Carlo method. The Gilmer model of adatom diffusion extended to the case of arbitrary surface morphology is used. Vacancies and pendants of atoms are allowed in the generalized model, the activation energy of a diffusion hop depends on the state of the neighboring positions in the first and second coordination spheres, and neighbors located outside the growing elementary layer are also taken into account. It is shown that in this model epitaxy occurs by the formation of metastable nucleation centers at the edges of pores, followed by growth of the nucleation centers along the perimeter and the formation of a thin, continuous pendant layer. Three-dimensional images of surface layers at different stages of epitaxy were obtained. The dependence of the kinetics of the epitaxy process on the amount of deposited silicon is determined for different substrate porosities. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 7, 512–517 (10 April 1998)     

The activation energy of crystallization of amorphous Fe40Ni40P14B6

The isothermal crystallization of Fe₄₀Ni₄₀P₁₄B₆ (Metglas 2826) has been studied by transmission electron microscopy, using static observations of partially crystallized ribbons at room temperature and in situ dynamic registration of the crystallization process at elevated temperatures. At all temperatures crystallization takes place by the nucleation and growth of individual crystals. Analysis of the transformation kinetics allowed to determine the nucleation rates and the activation energy for crystal growth. The growth velocity of the crystal phase was found to be controlled by the diffusion coefficient of phosphorus in this alloy withD ₀=2.5×10^10±1cm{swu2}/s andQ=(3.4±0.15)eV.     

负偏压热灯丝CVD金刚石膜核化和早期生长的研究

利用扫描电子显微镜、Raman谱和X射线光电子能谱，研究了Si衬底上热灯丝CVD金刚石膜的核化和早期生长.在-300V和100mA条件下预处理15min，镜面抛光的Si(100)表面上金刚石核密度超过了10⁹cm^-2，但是核的分布极不均匀且可分为三个区域：A区，边缘处以锥体为主；B区，位于边和中心之间过渡区是纳米金刚石；C区，中心处有SiC层.无偏压下生长4h后，A区形成许多大而弧立的金刚石颗粒，B区成为织构金刚石膜，而C区变为含有大量缺陷的连续金刚石膜.衬底负 关键词：     

Nucleation, growth and properties of Co nanostructures electrodeposited on n-Si(1 1 1)

In the present work, cobalt thin films deposited directly on n-Si(1 1 1) surfaces by electrodeposition in Watts bath have been investigated. The electrochemical deposition and properties of deposits were studied using cyclic voltammetry (CV), chronoamperometry (CA), ex situ atomic force microscopy (AFM), X-ray diffraction (XRD) and alternating gradient field magnetometer (AGFM) techniques. The nucleation and growth kinetics at the initial stages of Co studied by current transients indicate a 3D island growth (Volmer-Weber); it is characterized by an instantaneous nucleation mechanism followed by diffusion limited growth. According to this model, the estimated nucleus density and diffusion coefficient are on the order of magnitude of 10⁶ cm⁻² and 10⁻⁵ cm² s⁻¹, respectively. AFM characterization of the deposits shows a granular structure of the electrodeposited layers. XRD measurements indicate a small grain size with the presence of a mixture of hcp and fcc Co structures. The hysteresis loops with a magnetic field in the parallel and perpendicular direction and showed that the easy magnetization axis of Co thin film is in the film plane.     

Effect of growth pressure on the morphology evolution and doping characteristics in nonpolar a-plane GaN

Nonpolar a-plane GaN layers grown on r-plane sapphire substrates were examined by using a two-step growth process. The higher initial growth pressure for the nucleation layer resulted in the improved crystalline quality with lower density of both threading dislocations and basal stacking faults. This was attributed to the higher degree of initial roughening and recovery time via a growth mode transition from three-dimensional (3D) to quasi two-dimensional (2D) lateral growth. Using Hall-effect measurements, the overgrown Si doped GaN layers grown with higher initial growth pressure were found to have higher mobility. The scattering mechanism due to the dislocations was dominant especially at low temperature (<200 K) for the lower initial growth pressure, which was insignificant for the higher initial growth pressure. The temperature-dependent Hall-effect measurements for the Mg doped GaN with a higher initial growth pressure yielded the activation energy and the acceptor concentration to be 128 meV and 1.2 × 10¹⁹ cm⁻³, respectively, corresponding to about 3.6% of activation at room temperature. Two-step growth scheme with a higher initial growth pressure is suggested as a potential method to improve the performance of nonpolar a-plane GaN based devices.     

Thermal decomposition of nickel carbide thin films

Thin films of nickel carbide are produced by evaporating fourteen atomic layers (1.8 × 10¹⁶ atoms cm^?2) of nickel onto the (0001) surface of graphite at room temperature. The presence of nickel carbide is indicated by the characteristic carbide Auger electron signal. LEED shows that no ordered structural change takes place on the graphite (0001) surface when nickel carbide is produced in this manner. Isothermal heating of the sample leads to an irreversible change of the carbon Auger signal. The times required for this change range from 150 min at 150°C to 45 min at 185°C. The times required for decomposition yield an activation energy for decomposition equal to 50 kJ mole^?1. The decomposition of nickel carbide thin films obeys zero order kinetics. Depth profiling of the film after decomposition reveals that the observed decomposition is limited only to the top two or three atomic layers.     

Solutions for heteroepitaxial growth of GaN and their impact on devices

GaN technology relies on highly mismatched heteroepitaxial growth, mainly on sapphire or SiC substrates, and therefore suffers from 10⁹ to 10¹⁰ threading dislocations per cm². The origin and the deteriorating influence of the extremely high dislocation densities are analyzed with regard to the specific circumstances of GaN technology. Various attempts to cope with heteroepitaxial growth are discussed, from the use of nucleation layers to the growth on GaN single bulk crystals. Special focus is put on the impact of the approaches on the device performance.     

Effects of damage rate on Cu precipitation in Fe–Cu model alloys under neutron irradiation

The formation of Cu precipitates and point defect clusters was investigated in two Fe–Cu binary model alloys, Fe–0.3Cu and Fe–0.6Cu, irradiated at 573?K at three different damage rates, namely 3.8?×?10^?10, 1.5?×?10^?8 and 5?×?10^?8?dpa (displacements per atom)/s, up to about 1.6?×?10^?2?dpa. Results of positron annihilation experiments indicated that Cu precipitates were formed in these irradiations with different damage rates. The growth of Cu precipitates does not increase monotonously with increasing irradiation dose, but it rather depends on the nucleation and growth of microvoids. It is also clear that the nucleation and growth of microvoids are influenced by the irradiation dose rate.     

Oxidation of iron within the scanning electron microscope: Oxide crystallite growth kinetics during the oxidation of iron in CO2

The growth of magnetite (Fe₃O₄) on a polycrystalline iron substrate has been continuously followed in situ in the S.E.M. at 500°C in 13.3 N m^?2 CO₂. Medium resolution (250 Å) micrographs of the same area have been taken at time intervals over periods of oxidation in excess of 200 hr. Successive layers of oxide crystallites grow according to the same logarithmic rate law, but the overall thickening rate of the oxide film conforms to the parabolic rate law. A theoretical analysis of this situation suggests that the dominant factor determining the overall growth kinetics is the frequency of oxide nucleation steps. It can be shown that the basic logarithmic growth of individual oxide crystallites could lead to a variety of overall growth processes, depending upon the conditions required for nucleation.     

40Ar+诱导无定形碳到金刚石纳米晶相变的研究

通过60 keV的⁴⁰Ar⁺辐照无定形碳靶合成了大量尺寸不同的金刚石纳米颗粒.高分辨透射电子显微镜配合能量色散X射线谱和电子衍射以及Raman谱分析的结果表明，这些嵌于具有扰动石墨结构薄膜中的纳米金刚石颗粒，其成核率很高(约为10¹³/cm²)，而且可以生长到较大的尺寸，有的甚至可以达到微米量级.对其相转变过程也进行了初步探讨. 关键词： 离子束 金刚石纳米晶 相变 透射电子显微镜     

Kinetics and thermodynamics of sucrose crystal growth in the presence of a non-ionic surfactant

Batch experiments were carried out to study the effect of Hodag CB6, a non-ionic surfactant, on the growth kinetics of sucrose crystals as a function of supersaturation, impurity concentration and temperature. The growth promoting effect of the added impurity, studied using a recently introduced spiral nucleation model (SNM), was due to the decrease in the surface free energy induced by the added surfactant. The growth process was influenced by both kinetic and thermodynamic effect, the latter being predominant. The coverage of impurity molecules on the sucrose surface followed a Henry type expression according to Langmuir isotherm at studied temperatures. In the case of a pure system, the total active kink density was found to be around 10¹⁶ kinks/m². The active growth sites on the crystal surface were found to be two orders of magnitude lower than the total number of sucrose molecules.     

The dehydration kinetics of gypsum at high pressure and high temperature

An in situ dehydration kinetics study of gypsum under water-saturated condition was performed in the temperature and pressure ranges of 383–423?K and 343–1085?MPa by using a hydrothermal diamond anvil cell and Raman spectroscopy. Kinetic analysis shows that the dehydration rate k increases with pressure, suggesting a negative pressure dependence on dehydration rate. The elevation of temperature can contribute to the dehydration. The n values increase with pressure, indicating that the nucleation process becomes slower relative to the growth process. According to the n values of ～1.0, the dehydration of gypsum is dominated by an instantaneous nucleation and diffusion-controlled growth mechanism. The obtained average activation volume ?V is equal to 5.69?cm³/mol and the calculated activation energy E_a and the pre-exponential factor A are 66.9?kJ/mol and 4.66?×?10⁵?s^?1. The activation energy may be dependent upon grain size, shape, temperature and pressure, and surrounding water.     

The kinetics of solidification of Al-Si eutectic alloys under high pressure

Abstract

The kinetics of crystallization of eutectic alloys Al_100-xSi_x (χ=12, 18, 26 at.%) was investigated under pressures p=0.5, 2.5, 4.5 GPa. The values of supercooling and average grain size of silicon crystals were determined for alloys quenched from melt under different pressures with the cooling rate 10³ K/s. The data obtained were used to evaluate quantitatively the pressure dependences of surface tension (between melt and crystal) and activation energy of crystal growth which, in turn, have made it possible to determine the relative change of nucleation frequency and of the rate of crystal growth with pressure.

The possibility is shown, based on the investigation of mechanical properties of the samples obtained under high pressure, for improving the strength and the plasticity of A1-Si alloys by means of high pressure-high temperature treatment.     

Direct Observation of Crystallization of Polybutene-1 Under Low Shear Rate Flow

The isothermal crystallization process of polybutene-1 melt under shear flow was investigated with an optical microscope and a device (shear flow direct observation system, SF-DOS) newly developed by our group. The nucleation rate and growth rate of polybutene-1 were studied under slow shear rates (0–0.1 s^?1) at high crystallization temperature (102–108°C) with the SF-DOS. The nucleation remains heterogeneous. The number of nuclei after long times increased and induction time decreased by increasing the shear rate. Anisotropic and distorted spherulites were observed under shear flow, while the spherulites in the static condition were isotropic. It was clearly observed that the spherulites were rotating under shear. The average growth rates were enhanced by increasing shear rates, which acts as the main factor affecting the overall crystallization kinetics. Finally, the crystallization kinetics were analyzed on the basis of the secondary nucleation theory of Hoffman and Lauritzen. Even under very low shear rates, the product of lateral‐surface free energy σ _s and fold-surface free energy σ _e was found to be reduced as shear rate increased.     

Influence of nucleation on the kinetics of boron precipitation in silicon

A study of the precipitation of boron implanted into pre-amorphized silicon has been carried out following several thermal processes in the temperature range between 550 and 900 °C. It will be shown that the formation of inactive boron takes place even during a low-temperature solid-phase epitaxy of the regrowing layer, when the starting concentration level exceeds about 3.5×10²⁰ atoms/cm³. The presence, at the beginning of the annealing process, of inactive atoms in the form of small aggregates which behave as nucleation centers, markedly affects the precipitation kinetics during the thermal treatments. In these conditions the experimental data follow the Ham's theory of precipitation. On the contrary, if all the dopant is incorporated in the lattice side, nucleation is the limiting factor for boron deactivation.     

New method and experimental setup for visual positioning of nanocontacts

A new method and original experimental setup for visualizing nanoobjects are presented. The setup developed makes it possible to obtain a shadow nanoobject image on a luminescence screen with a magnification of 10⁶, ensure real visualization of carbon nanotube nucleation and growth, and provide controlled positioning of nanocontacts within 0.2–0.4 nm. An example of positioning of a cantilever tip with respect to a carbon nanotube spiral end is considered.     

Shear‐Enhanced Nucleation of Isotactic Polypropylene in Limited Space

The nucleation rate was measured by directly counting the number of nuclei, which were developed while an isotactic polypropylene melt was flowing under shear in a thin film. The nucleation rate was enhanced with an increased rate of shear, e.g., by a factor of 10 larger at the rate of shear of 14 s^?1 compared with the quiescent state, at 134°C. The ratio of the shear‐enhanced nucleation rate to the nucleation rate in the quiescent state was larger at a higher temperature of crystallization, i.e., about 10 times at 134°C to 590 times at 140°C. The increase of the nucleation rate under shear flow was explained by a reduction of the lateral and end (fold) surface free energies; the product σ _s ² σ _e decreased to 3.2×10^?7 for the sheared melt, from 6.0×10^?7 (J m^?2)³ for the isotropic state. The free energy reduction was caused by transition of the nucleus formation mode from three‐dimensional folded chain nuclei to two‐dimensional bundle nuclei, in which chains lie down on the glass substrate, aligning parallel to the flow direction.