Dependence of the growth rate of an AlN layer on nitrogen pressure in a reactor for sublimation growth of AlN crystals

The dependence of the layer growth rate on nitrogen pressure in a reactor has been examined in order to analyze the conditions of growth of AlN thick layers and bulk crystals by the sublimation sandwich method. It is shown that the layer growth rate steadily increases as the pressure in the reactor is lowered within the range 1–0.02 bar. This suggests that a key role in the layer growth kinetics is played by the source-to-substrate transfer of the components (Al, N), rather than by their adsorption (desorption) on the substrate surface.     

Effect of the duration of the growth process on the properties of GaN grown by the sublimation method

Variation in the structural and morphological features and luminescent characteristics of thick epitaxial GaN layers grown by the sublimation sandwich method with the duration of the crystallization process has been studied. This was, in particular, done by means of scanning electron microscopy in the secondary-electron and color-cathodoluminescence modes. It was found that rather high-quality GaN layers with a thickness of up to 0.5 mm can be grown in a time of about 1.5 h, with their surface hardly exhibiting any luminescence in the visible spectral range. However, making the growth process longer in order to obtain thicker layers impairs the quality of a crystal being grown, which is accompanied by an increase in the intensity of cathodoluminescence from its surface layer in the visible (predominantly yellow) region of the spectrum. Reasons for the poorer quality of GaN layers in this case are discussed. It is suggested that, as the evaporation rate from the source decreases, the amount of active nitrogen near the growth surface becomes lower.

     

Methods for the doping of silicon layers in growth by sublimation MBE

Epitaxial layers doped with various impurities were grown by sublimation MBE on Si (100) substrates. Doping with phosphorus was controlled at electron densities ranging from 2×10¹³ to 10¹⁹ cm^?3. A high dopant concentration of ～10²⁰ cm^?3 was obtained from the evaporation of partly molten Si sources. It shown that the type and concentration of an impurity in the sublimation MBE process can be controlled by the fabrication of multilayer p ⁺?n ⁺ structures.     

Effect of thermal etching on silicon epitaxial growth by vacuum sublimation

Silicon epitaxial growth by vacuum sublimation was studied with emphasis on the effect of thermal etch of the substrate on the surface concentration of carbon.A calculation of surface concentration of carbon on a Si substrate during thermal etch in the vacuum system shows that prolonged thermal etch at a higher temperature progressively enhances a pile-up of involatile impurity on the substrate surface and formation of precipitates above solid solubility limits, which would cause generation of interfacial defects. This effect was experimentally confirmed with various temperatures and periods of thermal etch and with the use of LOPEX material.Experimentally optimized temperature and period of thermal etch with the use of LOPEX Si yielded a good quality epitaxial P/N junction; low defect density 0–100 over the area of 10^?4 cm^?2, low reverse saturation current of less than 1 × 10^?4A/cm² at half of breakdown voltage of 90 V. Uniformity of multiplication factor M over the reverse biased P/N junction was measured with a laser probe. The variation around the average value over the area of a mesa junction structure 600 μm diam. was less than 10 per cent. Static I–V characteristics of PN junction, Schottky barrier NN⁺ diodes and P⁺NN⁺ diodes fabricated from grown layers by vacuum sublimation were also demonstrated.     

Dependence of the concentration of ionized donors on epitaxy temperature for Si:Er/Si layers grown by sublimation molecular-beam epitaxy

The dependence of the concentrations of the Er impurity and ionized donors on the epitaxy temperature has been studied before and after annealing of Si:Er/Si layers grown by sublimation molecular-beam epitaxy. n-Si:Er layers have been grown in the temperature range 400–800°C and annealed in hydrogen atmosphere at a temperature of 800°C for 30 min. The possible nature of the donor centers is discussed.     

Dependence of intersymbol interference upon the signalling rate

When the channel characteristics of a digital communications system are known, it is possible in theory to modify the system to meet any one of Nyquiat's criteria to eliminate intersymbol interference. This paper examines the performance of such a system when signalling rate changes by about ± 40% about the Nyquist optimum value. The results are presented in a graphical form in terms of the eye parameters. The cases considered are Nyquist I (with cosine roll-off factor of 0, 0·25, 0·50, 0·75 and 1·0 respectively) and Nyquist II (with cosine roll-off factor of 0 and 1·0 respectively).     

用升华法释放表面硅工艺中的悬浮结构

介绍了在谐振器结构释放后的干燥过程中,采用的一种有效的、防粘附的方法———升华法。在升华法的基础上,开发了充分置换升华法,对若干种释放方法进行实验、比较,确定了充分置换法的更优性;并找出了影响分离长度的因素,为提高结构释放产品率奠定了基础。     

Dependence of error rate degradation on graded-index-fiber excitation state

Degradation of error-rate characteristics has been observed in practical high-bit-rate digital systems using graded-index multimode fibers with center dips in the refractive index profiles. This degradation is caused by substantial intersymbol interference. To understand this phenomenon better, the relationship between the excitation state of a fiber and its pulse response has been studied both in theory and through experiments. It has been found that this phenomenon results from a center dip in the refractive index profile of a fiber, and that core center excitation brings about a fatal degradation in the error rate characteristics. Solutions to this problem are also discussed.     

Dependence of structural and optical properties of QD arrays in an InAs/GaAs system on surface temperature and growth rate

Properties of InAs QD arrays on a <100> GaAs surface in relation to the surface temperature and InAs growth rate are studied experimentally and theoretically. A kinetic model of QD formation in heteroepitaxial growth is developed, which allows the calculation of the mean lateral size and surface density of islands as functions of the growth conditions and duration. Experimental study of optical and structural properties is performed for QDs with an effective thickness of 2 ML, grown at different substrate temperatures and growth rates. The calculated results correlate well with the experimental data. The raising of the surface temperature and slowing of the growth rate result in a considerable increase in the QD mean lateral size and a decrease in their surface density.     

低码率应用中的一种视频码率控制方法

针对低码率视频应用,该文根据信源的线性模型,在满足码率约束的条件下,提出一种使宏块量化参数集中的优化策略。通过该方法,可以使宏块失真度分布更加集中,图像质量更加均匀,更符合人眼视觉特性;同时减少了编码码流中用于表征宏块间量化参数变化的句法比特数。仿真表明,在低码率及甚低码率的视频应用中,该方法能改善主客观视觉质量。     

LTE网络用户感知速率提升策略研究

用户感知速率是LTE网络带给用户的最直观体验,本文引入Throughput速率作为评价用户感知速率的指标。通过对低速率小区的指标分析发现,导致用户感知速率低的主要原因为覆盖不合理、小区高负荷以及高干扰。通过对以上网络问题的优化,可以明显提升用户感知速率,改善网络质量。     

沉积、退火条件以及刻蚀液对PECVD氮化硅薄膜湿法刻蚀速率的影响

The influence of deposition, annealing conditions, and etchants on the wet etch rate of plasma enhanced chemical vapor deposition （PECVD） silicon nitride thin film is studied. The deposition source gas flow rate and annealing temperature were varied to decrease the etch rate of SiNx：H by HF solution. A low etch rate was achieved by increasing the SiH4 gas flow rate or annealing temperature, or decreasing the NH3 and N2 gas flow rate. Concentrated, buffered, and dilute hydrofluoric acid were utilized as etchants for Sit2 and SiNx：H. A high etching selectivity of Sit2 over SiNx：H was obtained using highly concentrated buffered HE     

Dependence of wet etch rate on deposition, annealing conditions and etchants for PECVD silicon nitride film

tchants for SiO2 and SiNx:H. A high etching selectivity of SiO2 over SiNx:H was obtained using highly concentrated buffered HE     

Stoichiometry control of some II–VI ternary solid solutions during sublimation growth

A useful method is presented to control the stoichiometric composition y in Cd_1+y(S_xSe_1−x) and Zn_1+y(S_xSe_1−x) systems. The stoichiometry y is controlled through vapor growth process by regulating the sum of chalcogen partial pressures P_ps (=P_S2+P_Se2), and by keeping the pressure ratio P_r (=P_S2/P_Se2) constant in each growth experiment. The electrical properties of the grown crystal change obviously depending on P_ps and it is confirmed that the deviation y from stoichiometric composition y is effectively controlled without changing the composition x. The applicability of this method to other II–VI solid solutions is presented.     

The effect of a continuous etch on the growth rate and morphology of InP prepared by the vapor phase epitaxial-hydride method

A study of the effect of a continuousin situ etch of HC1 on growth rate and the properties of epitaxial InP layers prepared by the vapor phase epitaxial-hydride technique is reported. Growth rates were determined as a function of the following variables: HC1 flow rates in the mixing and source zones, PH₃ flow rates, and mixing zone temperatures. Epitaxial InP structures with good morphology were obtained when the continuous HC1 etch was varied between 0.8 and 1.5 cc/min. The average values (77K) of the carrier concentrations and mobilities were 1.3 × 10¹⁵ cm^s−3 and 23,000 cm²V^−l Sec⁻¹, respectively. The study indicates that the continuousin situ HCl etch improves the quality of epitaxial InP layers.     

Slew rate enhancement method for folded-cascode amplifiers

A new circuit is proposed to enhance the slew rate (SR) of the folded-cascode amplifier (FCA). The proposed circuit is automatically activated during the slewing phase. Simulation results show a fourtimes improvement in the SR and close to 40% reduction in the settling time, compared to a conventional FCA     

Transient phenomena in the sublimation of cadmium sulfide

New results on transient effects in basal-surface sublimation of cadmium sulfide and on sublimation morphologies of its basal surfaces are presented and discussed in terms of the terrace-ledge-kink theory of sublimation. In this context, various mechanisms for the stepwise sublimation of II-VI compounds are reviewed. The results are shown to be consistent with activation control of sublimation at the kink sites as influenced by surface charge.     

一些染料分子中电子单重激发态的无辐射内转换效率与溶剂粘度的关系

本文应用倍频锁模钕玻璃激光器产生脉冲时间为6微微秒、波长530毫微米的激光脉冲,激发3,3′-diethyl-9-methyl thiacarbocyanino Bromide(DMTCB)等染料分子,然后用时间分辨率为微微秒级的条纹相机测定它们的荧光衰变动力学光谱,以研究这些染料分子的荧光寿命与它们的溶剂粘度之间的关系。     

STC单片机波特率自适应方法

为了缩小电路体积、降低硬件成本,同时又保证串口稳定可靠通信,采用的一种波特率自适应方法,该方法充分利用STC单片机运行速度快、拥有片内振荡器、片内资源丰富的特点,在串行通信程序中,利用单片机I／O口和定时器,对主机发送的固定字符进行测量、计算,得到合适的波特率常数,从而实现波特率自适应。给出了设计原理、实现方法、误差分析、流程图和部分程序,并通过应用实例验证该方法切实可行。     

Periodic variation method for blind symbol rate estimation

In order to obtain unknown symbol rate of incoming signal at a receiver, in this paper, cyclostationary features of linear digitally modulated signals are exploited by proposed periodic variation method. A low complexity but highly accurate symbol rate estimation technique is obtained. The proposed method is based on a superposed epoch analysis over autocorrelations obtained blindly in different sampling frequencies. The obtained autocorrelations are analyzed in the frequency domain, and it is seen that there are large oscillations when the autocorrelation is obtained around the symbol rate. Then, a superposed epoch analysis is developed in order to estimate symbol rate based of the periodic variations on the frequency responses of autocorrelations. The proposed algorithm is quite accurate in the noisy environment because the noise is having no frequency component after taking Fourier transform of autocorrelations in all sampling rates, and this feature is also valid for the offset frequency that the purposed estimation is not affected by offset frequency. Thus, a successful blind symbol rate estimation algorithm is obtained, and it performs much better error performance than those using the well‐known cyclic correlation based symbol rate estimations, as it is proven by the obtained performances presented in the paper. Copyright © 2013 John Wiley & Sons, Ltd.