Effect of atomic hydrogen in highly lattice-mismatched molecular beam epitaxy

Effects of atomic hydrogen on the growth of lattice-mismatched InAs/GaAs and GaAs/InP systems have been investigated. The irradiation of atomic H has resulted in a delay of the onset of formation of three-dimensional islands maintaining flat surface morphology and increase of the critical layer thickness (CLT) from 4 to 10 å in the case of the InAs/GaAs system. The effect of atomic H was shown to be dependent on the growth conditions such as the growth temperature and V/III flux ratio. The increase of CLT with atomic H irradiation may be explained by the uniform distribution of the total misfit stress in the plane of the surface as a result of enhanced two-dimensional growth by atomic H acting as a surfactant.     

Ionization enhanced crystallization in amorphous germanium

The effect of high-energy electron irradiation upon the kinetics of crystallization of amorphous germanium layers has been studied, using conductivity measurements, during isochronal and isothermal warm-up. An increase in the growth rate of crystallization, induced at the surface or in the bulk, has been observed. It is suggested that this increase is the consequence of the recombination of electron-hole pairs created by the irradiation.     

Micromorphology and defects of non-doped gallium arsenide layers

Electron microscopy was used to study non-doped GaAs layers obtained in a chloride system Ga-AsCl₃-H₂. Electron concentrations is 10¹³–10¹⁵ cm⁻³, mobility at liquid nitrogen temprature reaches 200000 cm²/v. sec. It is shown that a fraction of the growth surface taken with (110) faces is less for films with less electron concentration. The main type of defects in pure GaAs layers is precipitates at pinning centres of growth steps. Estimations based on microdiffraction patterns and irradiation effects show that the impurity concentration in them is 10¹⁶–10¹⁹ cm⁻³. The impurity in precipitates is assumed to be electrically non-active, mainly, in interstitial positions.     

Dynamic process of crystallization of Sb2Se3 from Sb50Se50 amorphous film

Dynamics of crystallization of amorphous antimony-selenium film deposited on carbon substrate have been studied by the high-resolution transmission electron microscopy. The amorphous film was suddenly crystallized at 200°C by heating in vacuum. By the electron beam irradiation crystallization occurred at the focused electron beam region in the amorphous film. The growth process of crystallization by electron beam irradiation was recorded on a video image at the atomic resolution mode. The growth front of crystallization showed nano-concave and -convex shapes. The recrystallization with the different orientation at the first grown crystal have been found, and discussed as the influence of remaining antimony crystallites at the first crystallized film region.     

稀土Sm掺杂对纳米TiO2结构和可见光催化性能的影响

采用溶胶-凝胶法制备了纯TiO2和稀土Sm掺杂TiO2纳米粉体( Sm-TiO2),通过XRD、XPS、FT-IR、UV-Vis-DRS、PL和Nano-sizer纳米粒度仪等对样品进行表征,以亚甲基蓝( MB)的光催化降解为探针反应,探讨稀土Sm掺杂对纳米TiO2的结构和可见光催化性能的影响。结果表明,Sm掺入TiO2后在表面存在Sm3+和Sm2+两种价态, Sm掺杂抑制了TiO2从锐钛矿向金红石的相转变,阻碍纳米晶粒生长,增加了纳米粉体表面羟基含量;适量的Sm掺杂能使TiO2吸收光谱的阈值波长红移,有效降低光生e-/h+的复合率,提高TiO2光催化活性。热处理温度500℃时,掺杂1.0wt;Sm的纳米TiO2样品在普通日光灯下对MB在6 h内的光催化降解效率达97;,明显高于同等条件下Degussa公司产品P25的降解率56;。     

Surface modification and crystallization of the BaO–B2O3–SiO2 glassy system using CO2 laser irradiation

The surface modification and crystallization process of BaO–B₂O₃–SiO₂ glass compositions when exposed to CO₂ laser irradiation was evaluated as a function of the laser power, irradiation time and surface condition. The glass surface was modified by the application of laser power exceeding 0.40 W and an irradiation time of more than 300 s. Micro-Raman and X-ray diffraction measurements revealed at high laser power the formation of β-BaB₂O₄ (β-BBO) crystalline phase. The crystallization of the irradiated region was enhanced when β-BBO micrometer sized particles were dispersed on the surface of the glass sample. The intensity of the second harmonic generation observed in the crystallized region was found to depend mainly on the condition of the glassy surface prior to glass irradiation.     

Laser induced modification on 40BaO–45B2O3–15TiO2 glass composition

This paper describes a study concerning the permanent modification of 40BaO–45B₂O₃–15TiO₂ (mol%) glass composition when irradiated by a continuous CO₂ laser beam. Three parameters of the permanent modification and crystallization processes induced by irradiation were studied: laser power, irradiation time and the presence of micrograins of β-BaB₂O₄ (β-BBO) crystalline phase on the glass sample surface prior to irradiation. The geometry of permanent deformation induced by irradiation depended mainly on the laser power and sample surface condition. Crystallization of the irradiated region was observed when laser power at around 0.72 W and an irradiation time of around 300 s were used. The structural characterization performed by micro-Raman and X-ray diffraction techniques showed the formation of only the BaTi(BO₃)₂ crystalline phase on the as-polished sample. On the other hand, when β-BBO microcrystals were homogeneously dispersed onto the glass surface, the formation of the β-BBO phase was induced. In this condition, the β-BBO becomes the main crystalline phase and presents a certain degree of texture.     

Experimental and numerical studies of Ar-laser-assisted Si doping of GaAs with SiBr4 by chemical beam epitaxy

Doping results from Ar⁺-laser-assisted chemical beam epitaxy with triethylgallium, tris(dimethylamino) arsenic and silicon tetrabromide as group-III, group-V and dopant precursors, respectively, are reported. Enhancements in the n-type doping concentration are observed with laser irradiation in the investigated substrate-temperature range 390 – 500°C. With a 300 W/cm² irradiation power density, an increase in the carrier concentration by 70 times is obtained at 390°C substrate temperature. A numerical model developed by us for epitaxial growth and doping with the above precursors is used to assess the contribution of laser-induced thermal heating to the observed doping increase. A reaction scheme for photo-induced decomposition of physisorbed silicon tetrabromide is proposed. A kinetic rate equation for the photolysis is derived and used to estimate the absorption cross section required to reproduce the observed concentration enhancements resulted from laser irradiation. The possibility is established that dramatic increases in carrier concentration at low growth temperatures are due to photolysis of physisorbed silicon tetrabromide.     

Structure investigations of the Ge-GeO2 system by high-resolution electron microscopy

The crystal structure of thermally oxidized Ge was investigated by high-resolution electron microscopy (HREM), mainly the interface Ge/oxide. Under special conditions the reaction Ge + O₂ → GeO₂ which takes place at (111) surface planes leads to suitable thin crystal regions. The GeO₂ occurs normally as amorphous films on the crystal surface. Furthermore, hexagonal GeO₂ can grow at the interface Ge/oxide by a topotaxial reaction; the orientation relation between these two lattices was ascertained. Intensive electron irradiation was used to initiate and to observe structure changes in boundary regions.     

The growth of ZnSe by photo-assisted metalorganic chemical vapor deposition (MOCVD)

ZnSe epitaxy layers were grown on (1 0 0)GaAs substrates by photo-assisted MOCVD using DMZn and DMSe as group II and VI sources, respectively. Irradiation can improve the growth rate efficiently, but the irradiation intensity influences the growth rate and the crystalline quality negligibly in a large range. Due to an oxidation reaction on the surface of ZnSe, the growth rate and the flow ratio of group II and VI sources influence the crystalline quality.     

CFD modelling of single crystal growth of potassium dihydrogen phosphate (KDP) from binary water solution at 30 °C

Crystal growth rate depends on both diffusion and surface reaction. In industrial crystallizers, there exist conditions for diffusion-controlled growth and surface reaction-controlled growth. Using mathematical modelling and experimental information obtained from growth studies of single crystals, it is possible to separate these phenomena and study how they are affected by concentration, slip velocities of particles, temperature and finally estimate the parameters for crystal growth models.In this study, a power-law growth model using activity-based driving force is created. Computational fluid dynamics (CFD) was used to evaluate the thickness of a diffusion layer around the crystal. Parameters of the crystal growth model were estimated using a non-linear optimization package KINFIT. Experimental data on growth rate of the (1 0 1) face of a potassium dihydrogen phosphate (KDP) single crystal and simulated data on the thickness of a diffusion layer at the same crystal face were used in parameter estimation. The new surface reaction model was implemented into the CFD code. The model was used to study the effect of flow direction on growth rate of the whole crystal with various slip velocities and solute concentrations.The developed method itself is valid in general but the parameters of crystal growth model are dependent on the system. In this study, the model parameters were estimated and verified for KDP crystal growth from binary water solution.     

基于光谱测试的纯净金刚石品质与缺陷研究

本文利用低温拉曼与光致发光光谱对纯净金刚石晶片的结晶质量、晶体应力分布进行表征分析,并结合电子辐照和快速退火对晶片的杂质缺陷结构开展研究。通过拉曼光谱对金刚石特征峰的表征分析发现,由于金刚石生长机制以及晶片的切割、抛光等因素影响,晶片的边缘与表面应力分布较高。光致发光光谱中的零声子线具有明显的温度依赖性,根据Jahn-Teller效应与电子-声子耦合理论阐明了测试温度变化引起中性单空位缺陷零声子线分裂与红移的机理。对晶片做电子辐照与退火调控处理后,晶片中氮-空位(NV)缺陷显著增多,表明在纯净晶片中氮主要以替代位氮杂质的形式存在。     

Low energy and low dose electron irradiation of potassium–lime–silicate glass investigated by XPS. I. Surface composition

X-ray induced photoelectron spectroscopy has been used to study the influence of low-energy electron beam on the pristine potassium–lime–silicate glass surface prepared by fracturing in situ under ultrahigh vacuum. Relatively low-energy electron beam of 1600 eV with low-electron beam current density of 0.02–0.22 A/m² and low-electron dose of 29–5200 C/m² was used. The expected modified near-surface region thickness is in this case comparable with the mean sampling depth of the analytical tool. Therefore, possible changes and modifications due to electron irradiation could be recorded with high sensitivity. The freshly fractured glass surface was found to be significantly enriched with potassium, and slightly with calcium. As a consequence of the lowest electron dose irradiation used, the potassium signal substantially increased by a factor 1.24 relative to the value found for the fresh surface. For higher doses used, the potassium signal continuously increased with the dose to a maximum and decreased thereafter. This variation was accompanied with the qualitative opposite behaviour of calcium signal. The concentrations of the other elements present in the glass, oxygen and silicon, varied only slightly with the electron dose. They can be considered to be constant within experimental uncertainty. In agreement with experimental results, a model assuming mobility of only two most mobile cations, potassium and calcium, was suggested. The models assuming one layer and two layers on the bulk were developed. Their results reproduce well experimental findings: (i) the formation of a potassium-rich surface layer, and (ii) the opposite-like signal variation of calcium in comparison with potassium.     

Si(111)外延GaN的表面形貌及形成机理研究

用高温AlN作缓冲层在Si(111)上外延生长出GaN薄膜.通过对薄膜表面扫描电子显微镜(SEM)和高分辨率双晶X射线衍射(DCXRD)的分析,确定缓冲层对外延层形貌的影响,分析解释了表面形貌中凹坑的形成及缓冲层生长温度对凹坑的影响.结果表明:温度的高低通过影响缓冲层初始成核密度和成核尺寸来影响外延层表面形貌.     

The effect of crystal surface roughness on impurity adsorption

The effect of crystal surface roughness on impurity adsorption was investigated in a fluidized bed crystallizer and in a batch crystallizer. The crystallisation of sucrose in pure and impure systems was the study subject. Calcium chloride was utilized as impurity in this work. The results show that the impurity adsorption is growth rate dependent and is strongly influenced by the crystal surface properties. Crystals with high surface roughness have lower impurity adsorption. Based on experimental evidences, a new theoretical model is proposed to quantify the surface roughness influence on the impurity adsorption, allowing, by operating at the more adequate supersaturation, to control the impurity transfer into crystals. The used impurity does not have a significant influence on the growth rates at the studied temperatures. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

RHEED studies of the growth of Si(001) by gas source MBE from disilane

The growth of Si(001) from a gas source molecular beam epitaxy system (Si-GSMBE) using disilane (Si₂H₆) was investigated using reflection high-energy electron diffraction (RHEED). The surface reconstructions occurring between 100 and 775°C were studied as a function of both substrate temperature and surface coverage. We report the first observation of (2x2) and c(4x4) reconstructions during growth at substrate temperatures near 645°C using Si₂H₆. All growth was found to be initiated by the formation of three-dimensional (3D) islands which coalesced at substrate temperatures above 600°C. The surface reconstruction was found to change from a disordered to an ordered (2x1)+(1x2) structure at 775°C via intermediate (2x2) and c(4x4) phases. Thereafter, growth was found to proceed in a 2D layer-by-layer fashion, as evidenced by the observation of RHEED intensity oscillations. This technique has been used, for the first time, to calibrate growth rates during Si-GSMBE. The intensity oscillations were measured as a function of both substrate temperature and incident beam flux. Strong and damped oscillations were observed between 610 and 680°C, in the two-dimensional growth regime. At higher temperatures, growth by step propagation dominated while at lower temperatures, growth became increasingly three-dimensional and consequently oscillations were weak or absent. Similarly, there was a minimum flux limit ( <0.16 SCCM), below which no oscillations were recorded.     

In-situ RHEED study on the effect of light irradiation on Ge/Si heteroepitaxial growth by GeH4 source MBE

Effects of light irradiation on growth processes in the initial stage of growth of Ge films on (100)Si substrates by gas source molecular beam epitaxy using GeH₄ have been investigated by in-situ RHEED observations. It has been found that the growth rate is enhanced by an Ar⁺ ion laser and a D₂ lamp irradiation, and, moreover, that the growth process sequence of the Ge films irradiated by the Ar⁺ laser and D₂ lamp is the same as that without irradiation. The irradiation effect on the photolysis of GeH₄ gas has been examined by mass spectroscopy. The increase of the growth rate is due to the photolysis of GeH₄ and the excitation of substrate surfaces by vacuum-ultraviolet light in the D₂ lamp irradiation, and is due to only the excitation of substrate surfaces in the Ar⁺ laser irradiation.     

Investigation of GaN heteroepitaxial layers by means of the kossel effect technique

GaN heteroepitaxial layers grown on spinel substrates have been investigated by the Kossel effect technique. The excitation of GaK_α X-rays inside the crystal lattice was carried out either by means of 1.25 MeV protons or 40 KeV electrons. Both methods give similar results concerning the values of the lattice parameters and the sign of polarity. Following the proton irradiation a lattice expansion normal to the surface occurs. A comparison of our results with those of other authors shows that the growth direction is the same for {111} spinel and {0001} sapphire substrates.     

In-situ HVEM Investigations under the Influence of Ion Beams

A 200 kV ion accelerator was coupled to a 500 kV TEM for in-situ studies of the 80 keV He⁺ irradiation of 316 stainless steel. Dynamic and static observations of bubble growth, exfoliation and blistering show that slip bands, incoherent twin boundaries and grain boundaries are preferred sites for helium bubble formation. Bubble growth is enhanced by plastic deformation during irradiation. Exfoliation and blistering occur in samples thinner than the projected range of 80 keV He⁺ ions. Crack propagation during tensile tests proceeds through the bubbles enlarging them in diameter by a factor of two. Bubble enlargement also accompanies the formation of a blister cap. Brittle intergranular failure, even in samples with a large bubble population on the grain boundaries, occurred only when tensile tests were performed at elevated temperatures.     

Growth kinetics on the (100) and (001) faces of TGS crystals

The growth kinetics and mechanisms on the (001) and (100) faces of TGS crystals were investigated. A phase contrast microscope with a CCD camera was used to observe the growth of the crystal. We found the growth on the (001) and (100) faces at high supersaturation was mainly controlled by a BCF surface diffusion mechanism. The kinetic data for the (100) face were also fitted by the nucleation and layer growth model of two-dimension nucleation at high supersaturation. Some important growth parameters for TGS crystals, such as edge energy, activation energy, and so on, were estimated.