Growth and optical properties of strained (AIP)n (GaP)n short-period superlattices

Biaxial tensile strain is introduced into (AlP)_n(GaP)_n superlattices (SLs) by growing the SLs on slightly lattice-mismatched InGaP intermediate layers on GaP(00l) substrates. A significant enhancement of photoluminescence intensity is observed for the strained (AlP)_n (GaP)_n short-period SLs, especially for those with n≤ 3.     

The effect of AlN/AlGaN superlattices on crystal and optical properties of AlGaN epitaxial layers

We investigate the effect of AlN/AlGaN superlattices (SLs) on crystal and optical properties of AlGaN epitaxial layers. The result indicates that the crystal quality of AlGaN layers is consistent within a wide range of SLs thicknesses, while the optical properties are opposite. With SLs thickness decreasing from 20/44 to 17/36 and 15/29 nm, the full-width at half maximum of X-ray rocking curves for (0002)-and (1012)-plane of n-AlGaN layers grown on SLs are consistent of around 250 arcsec and 700 arcsec, respectively. Meanwhile, the center of the low optical transmittance band decreases from 326 to 279 nm and less than 266 nm as the SLs thickness decreases. 280 nm deep ultraviolet light-emitting diodes (DUV-LEDs) structures are further regrown on the n-AlGaN layers. The electroluminescent intensities of samples are 30% higher than that of the sample whose low optical transmittance band appears around 279 nm. Optical simulations reveal that the SLs acts as distributed Bragg reflectors, thus less photons of the corresponding wavelength escape from the sapphire backside.     

General balance power analysis of HFHPTA with (M,N)-compositesignals

A general balance power analysis of HF high-power tuned amplifiers (HFHPTA) with composite drive and output signal obtained by injection of an N-harmonic signal into an M-harmonic carrier signal, is presented. M and N are positive integers where M相似文献   

基于互质阵列雷达技术的近距离目标探测方法(英文)

一组(M, N)互质阵列由两组结构化排列的子阵列构成:一组包括M个天线单元, 另一组包括N个天线单元。互质阵列稀疏天线仅需要M+N1个收发天线单元就可实现对O(MN)个远距离目标的识别。在相同分辨条件下, 互质阵列雷达技术可利用更少的收发单元来识别更多的雷达目标。因此互质阵列雷达技术能够极大地降低传统雷达收发系统的复杂度。但是, 现有文献中所讨论的互质阵列雷达技术均基于远场近似假设, 当探测近距离目标时, 会由于目标邻近天线单元而产生严重的探测误差。为了解决上述问题, 该文将标准的互质阵列雷达技术的适用范围扩展到用于解决近距离目标探测。该文论证了(M, N)互质阵列雷达技术能够以4k0/MN空域分辨率恢复[2k0, 2k0]空域范围内的目标信息, 其中k0表示自由空间波数, 是场景因子。由此可见(M, N)互质阵列能够以/4的方位向分辨率获得O(MN)个近距离目标方位向位置信息。该文进一步论证了互质阵列雷达技术在穿墙成像中的适用性。最后, 该文提供了一组数值仿真实验结果, 验证了互质阵列雷达技术对于近距离目标探测的有效性。      

Asymptotic average redundancy of Huffman (and other) block codes

We study asymptotically the redundancy of Huffman (and other) codes. It has been known from the inception of the Huffman (1952) code that in the worst case its redundancy-defined as the excess of the code length over the optimal (ideal) code length-is not more than one. However, to the best of our knowledge no precise asymptotic results have been reported in literature thus far. We consider here a memoryless binary source generating a sequence of length n distributed as binomial (n, p) with p being the probability of emitting 0. Based on the results of Stubley (1994), we prove that for p<1/2 the average redundancy R¯_n^H of the Huffman code becomes as n→∞: R¯_n^H={(3/2-(1/ln2+o(1))=0.057304…, α irrational); (3/2-(1/M)(〈βMn〉-½)); (-(1/M(1-2^-1M/))2^-〈nβM〉M/); (+O(ρⁿ), α=N/M rational); where α=log₂ (1-p)/p and β=-log₂(1-p), ρ<1, M, N are integers such that gcd (N, M)=1, and 〈x〉=x-[x] is the fractional part of x. The appearance of the fractal-like function 〈βMn〉 explains the erratic behavior of the Huffman redundancy, and its “resistance” to succumb to a precise analysis. As a side result, we prove that the average redundancy of the Shannon block code is as n→∞: R¯_n^S{(½+o(1), α irrational); (½-1/M (〈Mnβ〉-½)); (+O(ρⁿ), α=N/M rational); where ρ<1. Finally, we derive the redundancy of the Golomb (1966) code (for the geometric distribution) which can be viewed as a special case of the Huffman and Shannon codes, Golomb's code redundancy has only oscillating behavior (i.e., there is not convergent mode). These findings are obtained by analytic methods such as theory of distribution of sequences modulo 1 and Fourier series     

Far-infrared detector based on HgTe/HgCdTe superlattices

HgTe/Hg_0.05Cd_0.95Te superlattices (SLs) were grown on (112)B oriented Cd_0.96Zn_0.04 Te substrates using molecular beam epitaxy (MBE). The SLs, consisting of 100 periods of 80-Å-thick HgTe wells alternating with 77-Å-thick Hg_0.05Cd_0.95Te barriers, were designed to operate as detectors in the far-infrared (FIR) region. Infrared absorption spectroscopy, high-resolution transmission electron microscopy (TEM), Hall effect measurements, and x-ray diffraction were used to characterize the superlattice layers. A series of annealing experiments were initiated to quantify the temperature-dependent interdiffusion of the HgTe wells and Hg_0.05Cd_0.95Te barriers and consequently their degradation, which shifts the absorption edges of the SLs to higher energies, since a high-temperature ex situ anneal is normally required in order to produce the p-type material required for a photovoltaic detector. Results from infrared absorption spectroscopy, TEM, and Hall effect measurements for the annealed samples are presented. A FIR SLs single-element photoconductive (PC) device was designed and fabricated. Both material characterization and device testing have established the applicability of the HgTe/Hg_0.05Cd_0.95Te SLs for the FIR region.     

Strained and Unstrained Layer Superlattices for Infrared Detection

Strained HgTe/CdZnTe or InAs/GaInSb, and essentially unstrained HgTe/CdTe superlattices (SLs), are possible materials systems for implementation in future-generation infrared imaging systems. In addition to cutoff wavelengths spanning the infrared spectrum, they offer degrees of freedom in their design (e.g., layer thicknesses, alloy compositions, and number of layers in one superlattice period) that permit the optimization of an infrared detector’s figures of merit such as detectivity through the tuning of material properties such as recombination lifetimes and optical absorption. This paper provides a brief overview of the anticipated advantages of the SLs over HgCdTe alloy-based infrared photon detector technology and the relative merits of the II–VI and III–V semiconductor-based SLs.     

Loss-imbalance equalization in arrayed waveguide-grating (AWG)multiplexer cascades

How to connect arrayed-waveguide-grating (AWG) multiplexers in cascade is discussed with the goal of equalizing the loss imbalance among frequency-division-multiplexed (FDM) channels. This paper proposes to average the FDM-channel loss over the cascaded multiplexers by shifting port connections between each adjacent multiplexer. A simulation predicts that, in a cascade of M periodic N×N multiplexers, shifting the connections by N/M reduces the loss imbalance from MΔα to Δα/M where Δα [dB] denotes the loss imbalance per multiplexer. This improvement will extend the cascadable node number in all-optical FDM networks. The prediction is confirmed experimentally in an optical add-drop filter (M=2) constructed with a silica-based AWG 16×16 multiplexer; the largest loss difference among 15 FDM channels is reduced from 5.0 dB to 1.5 dB. This paper also reports that imperfect multiplexer periodicity due to waveguide dispersion restricts the equalizable frequency bandwidth to less than several free spectral ranges (FSR's)     

p-type HgTe/CdTe superlattices for very-long wavelength infrared detectors

HgTe/CdTe superlattices (SL) have been studied for applications involving the detection of very-long wavelength infrared (VLWIR) detectors. In this study, p-type HgTe/CdTe SLs were grown by molecular beam epitaxy (MBE). As-grown arsenic δ-doped HgTe/CdTe SLs and undoped HgTe/CdTe SLs were characterized before and after annealing under different conditions using XRD, temperature-dependent Hall-effect measurements, SIMS, and TEM. Incorporated arsenic atoms in a HgTe/CdTe SL were electrically activated without the typical thermal annealing of alloy HgCdTe materials.     

Mid-infrared absorption by short-period InAs/GaSb type II superlattices

We present a theoretical study on optical properties of short-period InAs/GaSb type-II superlattices (SLs) which can serve for mid-infrared (MIR) detection. The miniband structure of such SLs is calculated using the Kronig–Penney model. On the basis of the energy-balance equation derived from the Boltzmann equation we calculate the optical absorption coefficient. The obtained results agree with recent experimental findings. Moreover, the dependence of the MIR absorption in InAs/GaSb type-II SLs on temperature and well-widths are examined.     

Low-temperature growth of nitrogen-doped BeTe and ZnSe/BeTe:N superlattices for delta doping

We propose ZnSe/BeTe:N superlattice (SL) as a new p-type cladding layer with improved p-type dopability, in which nitrogen-doped BeTe (N-doped) layers are inserted into undoped ZnSe layers. We have optimized the growth temperature for ZnSe/BeTe:N SLs. Both equivalent beam pressure (BEP) ratio and growth rate are further optimized in low-temperature growth of BeTe:N layers. The structural properties of BeTe:N layers were analyzed by reflection high-energy electron diffraction and high-resolution X-ray diffraction. Electrical properties were evaluated by the van der Pauw method and capacitance–voltage (C–V) measurements. We achieve the net acceptor concentrations of 1.2×10¹⁹ cm⁻³ at the growth temperature of 350°C, BEP ratio of around 6, and growth rate of 35 nm/h.     

纤锌矿InxGa1-xN/GaN量子阱中的界面声子模

采用赝原胞模型计算讨论纤锌矿InxGa1-xN混晶性质;基于宏观介电连续模型的传递矩阵方法研究任意层纤锌矿量子阱中的界面声子,得出任意层纤锌矿量子阱中的界面声子的本征模解和单量子阱的色散关系,并对InxGa1-xN/GaN单量子阱界面声子的色散关系进行了数值计算和讨论。结果表明,纤锌矿InxGa1-xN混晶中的E1声子和A1声子都表现为单模行为;在对称非应变单量子阱GaN/InxGa1-xN/GaN中,界面声子频率随x的变化呈线性关系。     

GaAs基短周期InAs/GaSb超晶格红外探测器研究

采用分子束外延(MBE)方法,在(001)GaAs衬底上生长了短周期Ⅱ型超晶格(SLs):InAs/GaSb (2ML/8ML)和InAs/GaSb (8ML/8ML).从X射线衍射(HRXRD)中计算出超晶格周期分别为31.2和57.3.室温红外透射光谱表明两种超晶格结构在短波2.1μm和中波5μm处有明显吸收.通过腐蚀、光刻和欧姆接触,制备了短波和中波的单元光导探测器.在室温和低温下进行光谱响应测试和黑体测试,77K下,50%截止波长分别为2.1μm和5.0μm,黑体探测率D·bb均超过2×108cmHz1/2/W.室温下短波探测器D·bb超过108cmHz1/2/W.     

Monolayer heterointerfaces and thin layers (~10 ?) in AlxGa1?xAs-GaAs superlattices grown by metalorganic chemical vapour deposition

Transmission electron microscopy (TEM) and lattice fringe images are used to show that metalorganic chemical vapour deposition (MOCVD) is capable of growing highly uniform AlxGa1?xAs-GaAs quantum-well heterostructures (QWHs) and superlattices (SLs) with monolayer heterointerfaces and layers as thin as ~10 ?.     

Atomic layer epitaxy of CdTe-ZnTe and CdTe-MnTe Superlattices

CdTe-ZnTe superlattices (SLs) with a period ranging from 13 to 38Å have been grown by atomic layer epitaxy (ALE) on (001) GaAs-substrates. In a substrate temperature range between 270 and 290°C, the growth rate for both CdTe and ZnTe regulated itself to exactly 0.5 monolayers per reaction cycle, allowing the growth of very precisely tailored structures. For lower substrate temperatures, the growth rate raised to approximately 0.8 monolayers per cycle, but did not reach one monolayer per cycle before ZnTe started to grow polycrystalline. Using the ALE growth parameters for CdTe, SLs of CdTe and metastable cubic MnTe were prepared. The superlattices were characterized by high resolution x-ray diffraction and photoluminescence. A comparision of x-ray data and computer simulations, based on the dynamical theory of x-ray diffraction, show that the SLs exhibit excellent period constancy and very abrupt interfaces.     

一种基于简单移位的二——十进制相互转换算法

十进制码(BCD)与二进制代码相互转换的问题的研究,主要偏重于软件实现.本文基于数制变换的基本原理,提出了移位为基础的、适合硬件实现的转换算法.并根据该算法,构造了63位二进制与十进制代码的转换器.同时,对该算法又进行了扩充,提出基2^<em>r移位的算法,进一步提高性能.从性能的比较可以看出,该算法速度高,逻辑简单,非常适合实时性要求较强的嵌入式领域应用.     

Terahertz band-gap in InAs/GaSb type-II superlattices

We demonstrate theoretically that it is possible to realize terahertz (THz) fundamental band-gap in InAs/GaSb type-II superlattices (SLs). The presence of such band-gap can result in a strong cut-off of optical absorption at THz bandwidth. This study is pertinent to the application of InAs/GaSb type-II SLs as THz photodetectors.     

On joint transmitter and receiver optimization formultiple-input-multiple-output (MIMO) transmission systems

The problem of joint transmitter and receiver optimization for MIMO systems under the mean-squared error (MSE) criterion is revisited. We address the general problem of N (number of users) ≠M (number of channel inputs) ≠P (number of channel outputs) when the system is Nyquist bandlimited and obtain analytical solutions for the optimal transmit-receive pair. Next, we demonstrate how the above result is directly applicable to the problem where the system has excess bandwidth, thereby generalizing the results of Salz (1985). In conclusion, several numerical examples are included to demonstrate the performance gains obtainable with jointly optimized MIMO systems vis-a-vis systems based only on receiver optimization     

HgTe/HgCdTe superlattices grown on CdTe/Si by molecular beam epitaxy for infrared detection

High-quality HgTe/CdTe superlattices (SLs) and device structures incorporating them were grown by molecular beam epitaxy (MBE) on CdTe/Si substrates. In-situ techniques, such as reflection, high-energy electron diffraction and spectroscopic ellipsometry, were extensively used to rigorously control the growth parameters. The full width at half maximum (FWHM) of x-ray double-crystal rocking curves (DCRCs) were 100–150 arcsec, comparable to those of HgCdTe alloys grown on the same type of substrates. The room-temperature Fourier transform infrared (FTIR) spectrum exhibits two-dimensional features characteristic of SLs. Trial devices in a p⁺-n⁻-n⁺ format were fabricated by diffusing gold in order to further evaluate the HgTe/CdTe SL performance. Gold diffusion was chosen to fabricate photovoltaic junctions in order to preserve the structural integrity of the SLs during the device processing. Though no attempt was made in the current study to optimize the junction properties by Au diffusion, this method has proven to be very useful for rapid preliminary evaluation. The measured spectral-response and detectivity data indicate the possibility to fabricate photovoltaic devices on an HgTe/CdTe SL, although further work is needed to optimize the p-n junction fabrication.     

Interplay between Structural and Thermoelectric Properties in Epitaxial Sb2+xTe3 Alloys

In recent years strain engineering is proposed in chalcogenide superlattices (SLs) to shape in particular the switching functionality for phase change memory applications. This is possible in Sb₂Te₃/GeTe heterostructures leveraging on the peculiar behavior of Sb₂Te₃, in between covalently bonded and weakly bonded materials. In the present study, the structural and thermoelectric (TE) properties of epitaxial Sb_2+xTe₃ films are shown, as they represent an intriguing option to expand the horizon of strain engineering in such SLs. Samples with composition between Sb₂Te₃ and Sb₄Te₃ are prepared by molecular beam epitaxy. A combination of X‐ray diffraction and Raman spectroscopy, together with dedicated simulations, allows unveiling the structural characteristics of the alloys. A consistent evaluation of the structural disorder characterizing the material is drawn as well as the presence of both Sb₂ and Sb₄ slabs is detected. A strong link exists among structural and TE properties, the latter having implications also in phase change SLs. A further improvement of the TE performances may be achieved by accurately engineering the intrinsic disorder. The possibility to tune the strain in designed Sb_2+xTe₃/GeTe SLs by controlling at the nanoscale the 2D character of the Sb_2+xTe₃ alloys is envisioned.