Experimental investigation on nonlinearities of PIN photodiodes

In this report, we experimentally investigate nonlinearity of PIN photodiode as a function of incident light power with different bias. It is found that the nonlinearity can be related with effective resistance of this device itself. According to ambipolar diffusion model, the resistance is divided into two parts, i.e. intrinsic region resistance and series resistance originating from non-Ohmic contact. Forward Current (I_f)-Voltage(V_f) plots indicate that fabrication of high-quality Ohmic contact is necessary to improve linear performance for PIN photodiodes.     

Surface passivation technology for III-V semiconductor nanoelectronics

The present status and key issues of surface passivation technology for III-V surfaces are discussed in view of applications to emerging novel III-V nanoelectronics. First, necessities of passivation and currently available surface passivation technologies for GaAs, InGaAs and AlGaAs are reviewed. Then, the principle of the Si interface control layer (ICL)-based passivation scheme by the authors’ group is introduced and its basic characterization is presented. Ths Si ICL is a molecular beam epitaxy (MBE)-grown ultrathin Si layer inserted between III-V semiconductor and passivation dielectric. Finally, applications of the Si ICL method to passivation of GaAs nanowires and GaAs nanowire transistors and to realization of pinning-free high-k dielectric/GaAs MOS gate stacks are presented.     

A renormalization scheme for calculating the spectrum of a vibronic system occurring in molecules or impurities in insulators

An iteration scheme which makes use of a numerical renormalization group approach is used to calculate the spectrum of vibronic levels. This spectrum resulted from dynamic effects occurring in certain molecules or impurities in insulators.The Hamiltonian of these systems is expressed in the matrix form, using products of suitable electron-phonon states as a basis. In applying this method to multimode electron-phonon systems, phonon modes are coupled in a chain-like fashion. Then a finite chain calculation in terms of Hubbard X-operators is explored by setting up the vibronic Hamiltonian.Calculations are based on Lanczos algorithm, in which only the nearest neighbor matrix elements along the chain need to be taken into account. The iterative scheme is then applied to a two-level electronic system coupled to phonons. A single-particle Green's function corresponding to a two-level system is applied to calculate the spectral density of states, which, coupled to single mode is carried out. The strength of lines in density of states is affected by the coupling constant as well as the temperature dependence of some measurable quantities.     

Improvement of interface properties of AlGaN/GaN heterostructures under gamma-radiation

The cathodoluminescence (CL) spectra of AlGaN/GaN heterostructures grown on sapphire substrate were studied before and after gamma irradiation treatment. The CL spectroscopy results reveal strong yellow and blue luminescence transformation under gamma radiation treatment. The changes in CL spectra are compared with changes in the electrical characteristics of two-dimensional gas in AlGaN/GaN heterostructures. The origins of the observed improvement in properties of AlGaN/GaN heterostructures after gamma radiation treatment with 1 × 10⁶ rad are discussed on the basis of compensation and structural ordering of native defects.     

Nanosecond pulse laser melting investigation by IR radiometry and reflection-based methods

Experimental system for nanosecond laser melting investigation was developed containing three independent noncontact methods: infrared radiometry, time-resolved reflectivity of He-Ne laser and sample surface reflected KrF heating laser pulse. The system was applied to the investigation of laser melting of Cu, Mo, Ni, Si, Sn, Ti, steel ?SN 15330 and stainless steel ?SN 17246 samples. For metallic samples the IR radiometry signal was transformed to temperature. Obtained surface temperature and reflectivity spectra in nanosecond time scale (10-1000 ns) for wide range of energy densities (100-5500 mJ cm⁻²) are presented. Interesting evolutions were found. Melting thresholds and melting durations were determined from the measured curves. The applicability of the methods is evaluated.     

Effect of focus position on informational properties of acoustic emission generated by laser-material interactions

To achieve desired accuracy, precision and surface roughness during laser-material removal process, monitoring and control of the process parameters related to laser, optics, workpiece material and its motion are required. Focus position, defined as a gap between the focusing lens and the surface of the sample workpiece, is one of the most critical process parameters, which determines the projection of the intensity of the laser beam on the surface to be ablated and therefore directly affects volume and geometry of the material removed and there by machining quality. In this paper, acoustic emission (AE) generated by laser-material interactions was statistically analyzed with respect to the variations in the focus position. The study involved on-line measurements of the AE signal from the laser-material interaction zone as a function of the focus position and the width of the machined trenches. Several basic statistical parameters, e.g. average amplitude, variance and power spectrum density were analyzed to select distinct informational parameters. Pattern recognition analysis of three informational parameters based on variances within frequency diapasons of 20-180, 180-300, and 300-500 kHz was used for reliable classification of the focus position and width of the machined trenches. The results provide important information for future development of on-line monitoring and control systems for laser-material removal process.     

视频播放设备中的K系数指标的测量及应用

K系数指标是评价电视信号质量优劣的重要指标之一,是在把各种波形失真按人眼视觉特性给以不同评价的基础上来度量图像损伤的一套系统的方法,它由K50,Kb,Kpb,Kp等指标组成,在图像上分别反映了垂直方向上的亮度不均匀,水平方向上的亮度变化,以及图像的细节、边缘、轮廓等的失真.     

Influence of strain on the tunneling magnetoresistance in diluted magnetic semiconductor trilayer and double barrier structures

The influence of strain on hole tunneling in trilayer and double barrier structures made of two diluted magnetic semiconductors (DMS) (Ga, Mn)As, separated by a thin layer of non-magnetic AlAs is investigated theoretically. The strain is caused by lattice mismatch as the whole structure is grown on a (In_0.15Ga_0.85)As buffer layer. The tensile strain makes the easy axis of magnetization orient along the growth direction. We found that biaxial strain has a strong influence on the tunneling current because the spin splitting at is comparable to the Fermi energy E_F. Tensile strain decreases the tunneling magnetoresistance ratio.