自组织InAs/GaAs量子点的表面氮化研究

用低温光荧光(PL)和透射电子显微镜(TEM)研究了表面氮化自组织InAs/GaAs量子点的光学性能和微观结构。结果表明氮化后形成薄层的InAsN薄膜作为应变缓和层覆盖在量子点的表面,使得随着氮化时间的增加,InAs量子点的位错密度提高、尺寸变大、纵横比提高、发光波长变长、强度变低。     

Graph‐based resource allocation algorithms for multiuser downlink MIMO‐OFDMA networks

Multiuser multiple‐input multiple‐output orthogonal frequency division multiple access (MIMO‐OFDMA) is considered as the practical method to attain the capacity promised by multiple antennas in the downlink direction. However, the joint calculation of precoding/beamforming and resource allocation required by the optimal algorithms is computationally prohibitive. This paper proposes computationally efficient resource allocation algorithms that can be invoked after the precoding and beamforming operations. To support stringent and diverse quality of service requirements, previous works have shown that the resource allocation algorithm must be able to guarantee a specific data rate to each user. The constraint matrix defined by the resource allocation problem with these data rate constraints provides a special structure that lends to efficient solution of the problem. On the basis of the standard graph theory and the Lagrangian relaxation, we develop an optimal resource allocation algorithm that exploits this structure to reduce the required execution time. Moreover, a lower‐complexity suboptimal algorithm is introduced. Extensive simulations are conducted to evaluate the computational and system‐level performance. It is shown that the proposed resource allocation algorithms attain the optimal solution at a much lower computational overhead compared with general‐purpose optimization algorithms used by previous MIMO‐OFDMA resource allocation approaches. Copyright © 2012 John Wiley & Sons, Ltd.     

强流金属离子注入

金属等离子体浸没注入（MEPIII）技术与金属蒸气真空弧（MEVVA）源注入技术有相同之处,也有各自不同的特点。采用两种方法进行钛离子注入硅,RBS方法分析注入层,并对两种注入技术予以比较。     

Influence of bismuth doping of InAs quantum-dot layer on the morphology and photoelectronic properties of Gas/InAs heterostructures grown by metal-organic chemical vapor deposition

GaAs/InAs quantum dot (QD) heterostructures prepared by metalloorganic chemical vapor deposition (MOCVD) are investigated. It is established that the introduction of isovalent bismuth doping during the growth of InAs QD layer results in the suppression of the nanocluster coalescence and favors the formation of more uniform QDs. Bismuth itself is virtually not incorporated into the dots, its role being mainly in limiting the migration mobility of atoms at the surface of the growing layer. A method for investigating the morphology of buried layers of InAs QDs in GaAs matrix by atomic-force microscopy is developed; it relies on the removal of the cap layer by selective chemical etching. The photoluminescence (PL) and photoelectric sensitivity spectra of the fabricated heterostructures and their relation to the morphology of the QD layer are studied. In doped structures, PL and selective photosensitivity owing to the QDs are observed at a wavelength of 1.41 µm with the linewidth of 43 meV at room temperature. Some of the morphological features and photoelectronic properties of the MOCVD-grown heterostructures are related to the formation of a transitional layer at the GaAs/InAs QD interface due to the diffusion-induced mixing of the components.     

Optimization of the specific on-resistance of the COOLMOSTM

The optimized values for the physical and geometrical parameters of the p- and n-regions used in the voltage-sustaining layer of the COOLMOS^TM are presented. Design of the parameters is aimed to produce the lowest specific on-resistance, R_on for a given breakdown voltage, V_B. A new relationship between the R_on and V_B for the COOLMOS^TM is developed as R_on=C·V_B^1.32, where the constant C is dependent on the cell dimension and pattern geometry. It is also found that by putting a thin layer of insulator between the p-region and its neighboring n-regions, the value of R_on can be further reduced. The possibility of incorporating the insulating layer may open up opportunities for practical implementation of the COOLMOS^TM for volume production     

An SOI LDMOS/CMOS/BJT technology for integrated power amplifiersused in wireless transceiver applications

This paper describes a SOI LDMOS/CMOS/BJT technology that can be used in portable wireless communication applications. This technology allows the complete integration of the front-end circuits with the baseband circuits for low-cost/low-power/high-volume single-chip transceiver implementation. The LDMOS transistors (0.35 μm channel length, 3.8 μm drift length, 4.5 GHz f_T and 21 V breakdown voltage), CMOS transistors (1.5 μm channel length, 0.8/-1.2 V threshold voltage), lateral NPN transistor (18 V BV_CBO and h _FE of 20), and high Q-factor (up to 6.1 at 900 MHz and 7.2 at 1.8 GHz) on-chip inductors are fabricated. A fully-functional high performance integrated power amplifier for 900 MHz wireless transceiver application is also demonstrated     

Analytical model for the first and second moments of an adaptiveinterpolated FIR filter using the constrained filtered-X LMS algorithm

The authors present an analytical model for the mean weight behaviour and weight covariance matrix of an adaptive interpolated FIR filter using the LMS algorithm to adapt the filter weights. The particular structure of this adaptive filter determines that special analytical considerations must be used. First, the introduction of an interpolating block cascaded with the adaptive sparse filter requires that the input signal correlations must be considered. It is well known that such correlations are disregarded by the independence theory, which is the basis for the analysis of the LMS algorithm adapting FIR structures. Secondly a constrained analysis is used to deal mathematically with the sparse nature of the adaptive section. Experimental results demonstrate the effectiveness of the proposed analytical models as compared with the results obtained by classical analysis     

Characteristics of planar n-p junction diodes made bydouble-implantations into 4H-SiC

Double implantation technology consisting of deep-range acceptor followed by shallow-range donor implantation was used to fabricate planar n⁺-p junction diodes in 4H-SiC. Either Al or B was used as the acceptor species and N as the donor species with all implants performed at 700°C and annealed at 1650°C with an AlN encapsulant. The diodes were characterized for their current-voltage (I-V) and capacitance-voltage (C-V) behavior over the temperature range 25°C-400°C, and reverse recovery transient behavior over the temperature range 25°C-200°C. At room temperature, the B-implanted diodes exhibited a reverse leakage current of 5×10^-8 A/cm² at a reverse bias of -20 V and a carrier lifetime of 7.4 ns     

Magnetooptical waveguides with polarization-independentnonreciprocal phase shift

Magnetooptical waveguides having nonreciprocal phase shift for both TE and TM modes can be prepared by properly adjusting the spatial variation of the Faraday rotation. Such waveguides are attractive to realize optical isolators. We investigate four concepts of magnetooptical waveguides which yield equal nonreciprocal phase shifts for the fundamental TE and TM modes. A polarization-independent Mach-Zehnder-type integrated optical isolator is presented. All the calculations are performed using material parameters typical for garnet films     

Iterative decoding of convolutionally encoded signals overmultipath Rayleigh fading channels

We analyze and compare several strategies for iteratively decoding trellis-encoded signals over channels with memory. Soft-in/soft-out extensions of reduced-complexity trellis search algorithms such as delayed decision-feedback sequence estimating (DDFSE) or parallel decision-feedback decoding (PDFD) algorithms are used instead of conventional BCJR and min-log-BCJR algorithms. It has been shown that for long channel impulse responses and/or high modulation orders where the BCJR algorithm becomes prohibitively complex, the proposed algorithms offer very good performance with low complexity. The problem of channel estimation in practical implementation of turbo detection schemes is studied in the second part. Two methods of channel reestimation are proposed: one based on the expectation-maximization (EM) algorithm and the second on a simple Bootstrap technique. Both algorithms are shown to dramatically improve the performance of the classical pseudo inverse channel estimation performed initially on a training sequence