Effect of Al Doping on Properties of SiC Films

Undoped and Al-doped 3C-SiC films are deposited on Si（100） substrates by low-pressure chemical vapour deposition. Effects of aluminium incorporation on crystallinity, strain stress, surface morphology and growth rate of SiC films have been investigated. The x-ray diffraction patterns and rocking curves indicate that the crystallinity is improved with aluminium doping. Raman scatting patterns also demonstrate that the strain stress in SiC films is released due to the incorporation of Al ions and the increase of film thickness. Furthermore, due to the catalysis of surface reaction which is induced by trimethylaluminium, the growth rate is increased greatly and the growth process varies from three-dimensional island-growth mode to step-flow growth mode.     

一个改进的聚合物结晶生长的相场模型

The irrationality of existing phase field model is analyzed and a modified phase-field model is proposed for polymer crystal growth, in which the parameters are obtained from real materials and very simple to use, and most importantly, no paradoxical parameters appeared in the model. Moreover, it can simulate different microstructure patterns owing to the use of a new different free energy function for the simulation of morphologies of polymer. The new free energy function considers both the cases of T<T_m and T≥T_m, which is more reasonable than that in published literatures that all ignored the T≥T_m case. In order to show the validity of the modified model, the finite difference method is used to solve the model and different crystallization morphologies during the solidification process of isotactic polystyrene are obtained under different conditions. Numerical results show that the growth rate of the initial secondary arms is obviously increased as the anisotropy strength increases. But the anisotropy strength seems to have no apparent effect on the global growth rate. The whole growth process of the dendrite depends mainly upon the latent heat and the latent heat has a direct effect on the tip radius and tip velocity of side branches.     

Hot deformation in nanocrystalline Ndben Feben B backward extruded rings

Radially oriented Nd-Fe-B rings are prepared by backward extrusion of fine grained melt-spun powder. Melt-spun powder with the nominal composition of Nd_30.5Fe_bal.Co_6.0Ga_0.6Al_0.2B_0.9 (wt%) is used as starting material. The effects of process variables, such as deformation temperature (T_d), strain rate (ε) and height reduction (Δh%), on the magnetic properties of the rings are investigated. A scanning electron microscope (SEM) equipped with an energy spectrum device is used to study the metallograph and microfracture of the extruded rings. The B_r and (BH)_max reach the optimum values at T_d=800℃, ε =0.01 mm/s, and Δh% =70%. It is found by SEM observations that the particle boundaries, which seemingly correspond to the interfaces of the starting melt-spun powders, emerge after the corrosion of metallography specimens. This is helpful for studying the effects of powder-powder interface on the local deformation and deformation homogeneity in the rings. For different spatial positions of the extruded rings, there are characteristic metallographies and microfractures. The upper end of the rings has the least deformation and worst texture, and therefore the worst magnetic properties. The magnetic properties in the radial direction increase slightly along the axis from the bottom to the middle, then steeply decrease at the upper end of the ring. The deformation and the formation-of-texturing processes are discussed. The deformation and the texturing formation of melt-spun Nd-Fe-B alloys probably involve grain boundary sliding and grain rotation, the solution-precipitation process and preferential growth of Nd₂Fe₁₄B nanograins along the easy growth a-axis.     

Weak ferromagnetism in copper metaborate CuB2O4

CuB₂O₄ single crystals have been grown and their magnetic and resonance properties have been investigated for the first time. The temperature dependence of the susceptibility was found to contain features at T=21 and 10 K. The CuB₂O₄ single crystal transformed at T=21 K to a weakly ferromagnetic state. The sharp drop in susceptibility at T<10 K is caused by a transition of the magnetic system of CuB₂O₄ to an antiferromagnetic state. The effective magnetic moment of the Cu₂₊ ion, determined from the high-temperature part of the magnetic susceptibility, is 1.77 μ _B. The room-temperature g factors are, respectively, 2.170 and 2.133 for magnetic field parallel and perpendicular to the c axis of the crystal. The antiferromagnetic resonance parameters in the weakly ferromagnetic and antiferromagnetic phases were measured. Fiz. Tverd. Tela (St. Petersburg) 41, 1267–1271 (July 1999)     

Nonpolar a-plane light-emitting diode with an in-situ SiNx interlayer on r-plane sapphire grown by metal-organic chemical vapour deposition

We report on the growth and fabrication of nonpolar a-plane light emitting diodes with an in-situ SiN_x interlayer grown between the undoped a-plane GaN buffer and Si-doped GaN layer. X-ray diffraction shows that the crystalline quality of the GaN buffer layer is greatly improved with the introduction of the SiN_x interlayer. The electrical properties are also improved. For example, electron mobility and sheet resistance are reduced from high resistance to 31.6 cm²/(V·s) and 460 Ω/口 respectively. Owing to the significant effect of the SiN_x interlayer, a-plane LEDs are realized. Electroluminescence of a nonpolar a-plane light-emitting diode with a wavelength of 488nm is demonstrated. The emission peak remains constant when the injection current increases to over 20 mA.     

THE ANNEALING FEATURES OF Ta/Si MULTILAYER FILMS

The Ta/Si multilayers on (100) Si substrate have been studied over the annealing temper-ature range from 500 to 900℃ by X-ray diffraction and cross-section transmission electron microscopy. The periodicity of the multilayers becomes worse with increasing annealing tem-perature and disappears at 750℃. At 600℃, two kinds of modulation wavelength coexist because the size of several TaSi₂ grains is larger than the contracted original modulation wavelength. The films are contracted after annealing. The largest contraction, at least 40nm decreasing in thickness, occurs at 600℃. When the annealing temperature is lower than 600℃, h-TaSi₂ grains grow randomly and the growth is not affected by the substrate. At temperatures higher than 750℃, h-TaSi₂ grows preferentially in [001] direction parallel to [100] axis of Si substrate. The appearance of texture depends on whether the atomic diffusion is short range or long range at the corresponding annealing temperature.     

Electroluminescence from Si／SiO2 films deposited on p—Si substrates

The structure of Au/Si/SiO₂/p-Si has been fabricated using the magnetron sputtering technique. It has a very good rectifying behaviour. Visible electroluminescence (EL) has been observed from the Au/Si/SiO₂/p-Si structure at a forward bias of 5V or larger. A broad band with one peak around 650－660 nm appears in all the EL spectra of the structure. The effects of the thickness of the Si layer in the Si/SiO₂ films and of the input electrical power on EL spectra are studied systematically.     

Growth and properties of wide spectral white light emitting diodes

Wide spectral white light emitting diodes have been designed and grown on a sapphire substrate by using a metal-organic chemical vapor deposition system. Three quantum wells with blue-light-emitting, green-light-emitting and red-light-emitting structures were grown according to the design. The surface morphology of the film was observed by using atomic force microscopy. The films were characterized by their photoluminescence measurements. X-ray diffraction θ/2θ scan spectroscopy was carried out on the multi-quantum wells. The secondary fringes of the symmetric ω/2θ X-ray diffraction scan peaks indicate that the thicknesses and the alloy compositions of the individual quantum wells are repeatable throughout the active region. The room temperature photoluminescence spectra of the structures indicate that the white light emission of the multi-quantum wells is obtained. The light spectrum covers 400-700 nm, which is almost the whole visible light spectrum.     

Effects of V/III ratio on the growth of a-plane GaN films

The non-polar a-plane GaN is grown on an r-plane sapphire substrate directly without a buffer layer by metal-organic chemical vapour deposition and the effects of V/III ratio growth conditions are investigated. Atomic force microscopy results show that triangular pits are formed at a relatively high V/III ratio, while a relatively low V/III ratio can enhance the lateral growth rate along the c-axis direction. The higher V/III ratio leads to a high density of pits in comparison with the lower V/III ratio. The surface morphology is improved greatly by using a low V/III ratio of 500 and the roughness mean square of the surface is only 3.9 nm. The high resolution X-ray diffraction characterized crystal structural results show that the rocking curve full width at half maximum along the m axis decreases from 0.757° to 0.720°, while along the c axis increases from 0.220° to 0.251° with the V/III increasing from 500 μmol/min to 2000 μmol/min, which indicates that a relatively low V/III ratio is conducible to the c-axis growth of a-plane GaN.     

A Study on the Components of MgB2 Thick Film Preparedvia HPCVD

Superconducting MgB₂ thick film has been prepared via hybrid physical-chemical vapor deposition method on Al₂O₃ (0001) substrate by using B₂H₆ and magnesium ingot as raw materials reacted from 730 to 830°C for 40 min under 20 to 30 kPa. Its thickness is about 40 μm. The MgB₂ thick film shows T _c (onset) = 39.0 K and T _c (0) = 37.2 K. X-ray diffraction pattern shows that the film grown along (101) direction has small amount of impurities of Mg and MgO. Scanning electron microscopy and energy dispersive X-ray spectroscopy indicated that these impurities existed indeed and were Mg rich. The MgO film was formed on the surface of the MgB₂ thick film to further protect the sample from oxidation. We presented a new mechanism for the formation of the thick film. Translated from Chinese Journal of Low Temperature Physics, 2005, 27(1) (in Chinese)     

Characteristics of ZnO epilayer on the post-annealed buffer layer on GaN/sapphire substrate by pulsed laser deposition

We employed epi-GaN substrates for ZnO film growth, and studied the deposition and post-annealing effects. ZnO films were grown by pulsed laser deposition (PLD) method. The as-grown films were annealed for one hour under atmospheric pressure air. ZnO morphologies after annealing were investigated and the post-annealed ZnO films grown at T _g =700^oC have very smooth surfaces and the rms with roughness is about 0.5 nm. Finally, ZnO post-annealed buffer layer was inserted between ZnO epilayer and GaN/sapphire substrates. It is confirmed by AFM that growth temperature of 700^oC helps the films grow in step-flow growth mode. It is observed by cathode luminescence spectrum that the ZnO film grown at 700^oC has very low visible luminescence, indicating the decrease of the deep level defects. It is also revealed by Hall measurements that carrier concentration is decreased by increasing the growth temperatures. It is suggested that low temperature buffer layer growth and post-annealing technique can be used to fabricate ZnO hetero-epitaxy.     

SrTiO3 homoepitaxy by the pulsed laser deposition method: island, layer-by-layer, and step-flow growth

SrTiO₃ homoepitaxy was investigated under various conditions using the pulsed laser deposition method. The growth mode was determined by in-situ reflection high-energy electron diffraction, and the surface of the films was characterized by ex-situ atomic force microscopy. At the laser fluence of 0.68 J/cm², island growth was observed below 500 °C substrate temperature, while the growth mode turned into layer-by-layer growth above 500 °C. On further raising the substrate temperature, the step-flow growth mode prevailed above 800 °C. We thus demonstrated that step-flow growth in SrTiO₃ homoepitaxy is possible at a temperature as low as 800 °C.     

气相外延生长的ZnSe单晶薄膜及电致发光

用窄空间外延方法,在GaAs(100)衬底上外延生长了ZnSe(100)单晶薄膜.实验条件是,T衬=550℃,T源=650℃,H2-HCl气流速率为0.4-0.45l/min,生长速率为0.25-0.3μ/h.外延片在700℃的Zn和MnCl2蒸气中处理40-60分钟,以降低ZnSe的电阻率及掺入杂质Mn.利用这一外延层制作了MS结发光二极管,在反向偏压下获得黄色电致发光.     

Silicide formation by furnace annealing of thin Si films on large-grained Ni substrates

Si films with a thickness of approximately 250 nm have been electron-beam evaporated on thick, large-grained Ni substrates (grain size a few mm to 1 cm in diameter). An in situ sputter cleaning procedure has been used to clean the Ni surface before the Si deposition. Thermal annealings have been performed in a vacuum furnace. Ni₂Si is the first phase that grows at temperatures between 240 °C and 300 °C as a laterally uniform interfacial layer with a diffusion-controlled kinetics. The layer thicknessx follows the growth lawx ²=kt, withk=k ₀ exp(-E _a k _B T), wherek ₀=6.3 × 10^–4cm 2/s andE _a=(1-1±0.1) eV. Because of the virtually infinite supply of Ni, annealing at 800 °C for 130min yields a Ni-based solid solution as the final phase. The results are compared with those reported in the literature on suicide formation by the reaction of a thin Ni film on Si substrates, as well as with those for interfacial phase formation in Ni/Zr bilayers.     

Linear expansion coefficient of the SiC/Si biomorphic composite

In the temperature range 100–650 K, the linear expansion coefficient β was measured for the SiC/Si biomorphic composite, a new cellular ecoceramic fabricated from a porous cellular carbon matrix prepared through pyrolysis of wood (white eucalyptus) in an argon ambient with subsequent infiltration of molten Si into the channels of the matrix and the formation of 3C-SiC. The SiC/Si samples studied had an “excess” ～30% volume concentration of Si and a porosity of ～13–15%. The measurements were conducted on samples cut along (β_∥) and across (β_⊥) the tree growth direction. The measured values of β(T) of SiC/Si are compared with literature data available for the linear expansion coefficients of Si and 3C-SiC.     

The application of submetallic phosphorus-doped Si as ESR marker

With the aim of applying it in high-accuracy electron spin resonance spectral analysis, some “marker” qualities of P-doped Si have been investigated. K-band data of submetallic Si:P powders doped to (1.7 ± 0.1) × 10¹⁸ CM⁻³ in the temperature range T = 1.6−300 K are presented. If this material is used in accurate ESR calibration work, full attention should be paid to the reported g versus T variations over the whole T range covered. Comparison to other data at T less than 4.2 K indicates an essentially different behavior of g and ΔB_pp versus T when studied in either low or high magnetic field regimes. Hence, when calibrating spectra, the ESR marker data obtained in the same observational frequency band as the one of interest should be used.     

Si-W/Si/SiO2/Si(100)的横截面电子显微镜研究

本文用横截面电子显微镜法分析了Si-W/Si/SiO₂/Si(100)在440—1000℃退火后的晶化过程,以及各个界面的变化情况.发现Si-W合金膜中,WSi₂并未优先在表面、界面处形成晶核.当退火温度不高于700℃时,反应在合金膜内发生,表面、界面起伏和缓.退火温度高达800—1000℃时,界面、表面出现原子扩散,造成剧烈的界面起伏;表面则出现小的热沟槽,Si/SiO₂界面也出现高分辨电子显微镜才能观察到的起伏.表面、界面的原子迁移的动力来源于晶界与表面、界面张力.由于SiO₂中Si—O键很稳定,不易发生Si和O在界面处的互扩散,所以Si/SiO₂界面起伏很小. 关键词：     

On new two-dimensional structures produced on the Si (111) and Si (100) surface upon molecular-beam epitaxy of cobalt and silicon

Highly perfect epitaxial heterostructure CoSi₂ films have been grown on the surface of Si (111) and Si (100) single crystals by the method of molecular-beam epitaxy. The optimal regimes of the film growth with different thicknesses have been determined. It has been shown that short-term annealing of epitaxial films at T = 900–950 K leads to the formation of new CoSi₂/Si(111)-2 × 2 and CoSi₂/Si(100)−2 × 4 superstructures.     

Substrate-step-induced effects on the growth of CaF2 on Si (111)

The growth of CaF₂ on vicinal Si (111) surfaces was studied by scanning tunneling microscopy (STM) and atomic force microscopy (AFM) for the temperature range from 300 to 750 °C. In the submonolayer range a transition from terrace to step nucleation is observed for increasing temperature. The first monolayer grows in the step-flow growth mode since second layer islands are not nucleated before completion of the wetting layer. For the subsequent growth of CaF₂ on the CaF interface layer, substrate-induced steps do not act as preferential nucleation sites, but rather as growth barriers. Thus CaF₂ films grow very inhomogeneously at high temperatures. At lower deposition temperatures, the film homogeneity increases due to the increased (homogeneous) nucleation rate on terraces. The lattice mismatch leads to (lateral) relaxation of thicker CaF₂ film close to substrate steps. In addition, CaF₂ self-decoration effects are caused by the relaxed regions close to the film steps at temperatures above 500 °C. Received: 7 August 2001 / Accepted: 23 October 2001 / Published online: 3 April 2002     

Study of the features of BaF<Subscript>2</Subscript> heteroepitaxy on CaF<Subscript>2</Subscript>/Si(100) layers obtained in the high-temperature growth mode

The surface morphology of BaF₂ epitaxial films grown by MBE (molecular beam epitaxy) in various modes on the surface of CaF₂/Si(100) is investigated by AFM. The CaF₂ layers on Si(100) are obtained in the high-temperature growth mode (Т _S = 750°C). It is shown that the epitaxy of BaF2 at a temperature of 600°C at the initial stage of growth leads to the formation of defects such as perforations in the epitaxial film, while epitaxy at a temperature of 750°C provides a defect-free film with a surface morphology suitable for the subsequent growth of semiconductors of IV–VI type and solid solutions based on them.