Theoretical and experimental investigations of the temperature dependent continuous wave lasing characteristics and the switch-on dynamics of an InAs/InGaAs quantum-dot semiconductor laser

We investigate spectrally resolved continuous wave (CW) and spectrally and time-resolved switch-on emission properties of an InAs/InGaAs quantum-dot laser. The temperature and injection current dependence of the excited-state and ground-state emission dynamics is studied in the range between 20 °C and 50 °C under CW and gain-switching operation. All the experimental results are found to be in good agreement with simulation results based on a multi-population rate equation model incorporating all of the peculiar properties of the quantum-dot material as homogeneous and inhomogeneous broadening of the emission linewidth, different dynamics for electrons and holes, cascaded and direct capture paths of carriers from the wetting layer into the dot and Auger non-radiative recombination. This coincidence between simulations and experiments allows explaining the complicated behavior of the CW characteristics and the switch-on dynamics in the investigated temperature range.     

Field effect and charge injection in hybrid nanorod heterostructure

Recent studies on organic/inorganic heterostructures have indicated that interface morphology plays an important role in determining the charge transport properties. Hybrid heterostructure light-emitting diodes mixing donor and acceptor semiconductors appear to offer the best opportunity in achieving superior performance and there are indications that a network of percolated heterojunctions can be very effective in promoting light absorption/emission. Charge transport however can be more complex in a nanorod heterostructure as the charge flow at the interface will depend on the injection mechanism(s) as well as the interface field strength. In this work, we examined the current density–voltage characteristics of the hybrid NPB (N, N′-di(napth-2-yl)-N-N′-diphenylbenzidine)–ZnO nanorod heterostructure and attempted to identify the transport mechanism(s) close to the tips of the nanorods. Our study indicated that charge flow essentially followed the conventional pattern changing from a linear regime (emission-limited) to a quadratic regime (space-charge limited) and possibly to a rapid rise in current (trap-free injection). Detailed evaluation of the changes in the reported conductivity data further suggested the conduction mechanism (up to a p-layer thickness of 400 nm) was dominated by space-charge limited current in the NPB layer, which also resulted in substantial charge pile-up near the tips of the nanorods. An interface charge layer responsible for the barrier height modification effect could be used to explain the observed “blue-shift” in the emission spectra of the nanorod heterostructure light-emitting diode as reported by Sun et al. [2].     

Interband tunneling depopulation in type-II InAs/GaSb cascade laser heterostructure

An exact analytical representation has been obtained for electron eigenstates in the full isotropic 8-band Kane model and applied to calculate the depopulation rate of the lower lasing state in the active region of a type-II intersubband cascade laser. We show that interband tunneling rate takes its maximum value when the depopulated states belong to the upper of the coupled electron- and hole-like subbands in the “leaky window” of the broken-gap InAs/GaSb heterostructure.     

InAs和InAs/GaSb异质结的MOCVD生长和表征

以三甲基铟(TMIn)、砷烷(AsH3)、三甲基镓(TMGa)和三甲基锑(TMSb)为源,用水平常压MOCVD技术,在较低的Ⅴ/Ⅲ比的条件下(1.5～4)于GaAs和GaSb衬底上成功地生长了InAs合金和InAs/GaSb异质结。实验表明,生长温度在500℃～620℃范围内,InAs外延生长是扩散控制的。在Ⅴ/Ⅲ比为2.5时,生长效率(相对Ⅲ族源)为3×103μm/mol.不掺杂InAs外延层为n型的,室温迁移率为2000cm2/V.s.InAs/GaSb异质结的12KPL谱为一个在375meV处较宽的与杂质相关的跃迁峰,和一个在417meV附近的几乎被杂质峰湮没的带边峰.     

Electrical and deep levels characteristics of ZnO/Si heterostructure by MOCVD deposition

ZnO films have been prepared on p-type Si substrates by metal-organic chemical vapour deposition （MOCVD） at different total gas flow rates. The current versus voltage and temperature （I - V - T） characteristics, the deep-level transient spectroscopy （DLTS） and the photoluminescence （PL） spectra of the samples were measured. DLTS shows two deep-level centres of E1 （Ec-0.13±0.02eV） and E2 （Ec-0.43±0.05eV） in sample 1202a, which has a ZnO/p-Si heterostructure. A deep level at Ec-0.13±0.01 eV was also obtained from the I -T characteristics. It was considered to be the same as E1 obtained from DLTS measurement. The emission related to this deep level center was detected by PL spectra. In addition, the energy location and the relative trap density of E1 was varied when the total gas flow rate was changed.     

Current transport in ZnO/Si heterostructure grown by laser molecular beam epitaxy

The n-ZnO/p-Si heterojunction was fabricated by depositing high quality single crystalline aluminium-doped n-type ZnO film on p-type Si using the laser molecular beam epitaxy technique. The heterojunction exhibited a good rectifying behavior. The electrical properties of the heterojunction were investigated by means of temperature dependence current density-voltage measurements. The mechanism of the current transport was proposed based on the band structure of the heterojunction. When the applied bias V is lower than 0.15 V, the current follows the Ohmic behavior. When 0.15V 0.6 V), the space charge limited effect becomes the main transport mechanism. The current-voltage characteristic under illumination was also investigated. The photovoltage and the short circuit current density of the heterojunction aproached 270 mV and 2.10 mA/cm 2 , respectively.     

High-sensitive phototransistor based on vertical HfSe2/MoS2 heterostructure with broad-spectral response

Van der Waals heterostructures based on the two-dimensional (2D) semiconductor materials have attracted increasing attention due to their attractive properties. In this work, we demonstrate a high-sensitive back-gated phototransistor based on the vertical HfSe₂/MoS₂ heterostructure with a broad-spectral response from near-ultraviolet to near-infrared and an efficient gate tunability for photoresponse. Under bias, the phototransistor exhibits high responsivity of up to 1.42×10³ A/W, and ultrahigh specific detectivity of up to 1.39×10¹⁵ cm·Hz^1/2·W^-1. Moreover, it can also operate under zero bias with remarkable responsivity of 10.2 A/W, relatively high specific detectivity of 1.43×10¹⁴ cm·Hz^1/2·W^-1, ultralow dark current of 1.22 fA, and high on/off ratio of above 10⁵. These results should be attributed to the fact that the vertical HfSe₂/MoS₂ heterostructure not only improves the broadband photoresponse of the phototransistor but also greatly enhances its sensitivity. Therefore, the heterostructure provides a promising candidate for next generation high performance phototransistors.     

基于异质结界面优化的光子晶体二极管单向传输特性研究

基于光子晶体异质结结构的全光二极管是目前的一个研究热点,使其具有更好的单向传输特性是研究的一个目标.本文中通过改变异质结界面处的光子晶体结构,提出了不同的优化设计方案,在宽频带内实现了高对比度全光二极管的高效率单向传输,并且该结构还具有分束特性.     

ZnO纳米棒的低温溶液法制备、光电特性研究及其在有机/无机复合电致发光中的应用

利用水热法制备了垂直于衬底的定向生长的ZnO纳米棒,利用扫描电子显微镜及光致发光的方法对其形貌及光学特性进行了表征,利用场发射性能测试装置对ZnO纳米棒的场发射性能进行了测试.结果表明:利用水热法在较低的温度(95 ℃) 下生长了具有较好形貌和结构的ZnO纳米棒,并表现出了较好的场发射特性,当电流密度为1 μA/cm²时,开启电场是2.8 V/μm,当电场为6.4 V/μm时,电流密度可以达到0.67 mA/cm²,场增强因子为3360.稳定性测试表明,在5 h内,4.5 V/μm的电场下,其波动不超过25%.将制备的ZnO纳米棒应用到有机/无机电致发光中,其中ZnO纳米棒为电子传输层,m-MTDATA(4,4',4″-tris{N,(3-methylphenyl)-N-phenylamino}-triphenylamine) 为空穴传输层,得到了ZnO的342 nm的紫外电致发光,此发光较ZnO纳米棒光致发光的紫外发射有约40 nm的蓝移. 关键词： ZnO纳米棒 场发射 水热法 有机/无机复合电致发光     

Rectifying and photovoltaic properties of ZnCo2O4/Si heterostructure grown by pulsed laser deposition

ZnCo2O4/Si heterostructures have been fabricated by a pulsed laser deposition method, and their transport behaviors and photovoltaic properties have been characterized. The ZnCo2O4/Si heterostructures show a good rectifying behavior at five different temperatures ranging from 50 K to 290 K. The measurements of the photovoltaic response reveals that a photovoltage of 33 mV is generated when the heterostructures are illuminated by a 532 nm laser of 250 mW/cm2and mechanically chopped at 2500 Hz. Both the photocurrent and the photovoltage clearly increase with the increase of the laser intensity at room temperature. However, the heterostructures' photovoltage peak decreases with the increase of the temperature. This work may open new perspectives for ZnCo2O4/Si heterostructure-based devices.     

A hybrid OPCPA/Nd:phosphate glass multi-terawatt laser system for seeding of a petawatt laser

To enhance radiographic abilities on its Z-Accelerator, Sandia National Laboratories is incorporating a petawatt laser system into the existing Z-Backlighter laser facility. As part of this work, a short-pulse laser has been constructed to seed the larger Beamlet type Nd:phosphate glass slab amplifiers. This seed laser consists of an optical parametric chirped pulse amplification (OPCPA) system joined to a Nd:phosphate glass rod amplifier system in order to achieve multi-Joule sub-picosecond operation. The rod amplifier system has been modeled using the Miró code which shows good agreement with the experimental results. This system can achieve focal intensities up to 10¹⁸ W/cm² at a repetition rate of once every 20 min and has been applied to produce k-alpha X-rays in copper.     

The effects of carrier degeneracy on transport properties of the double heterostructure injection laser

It is shown that carrier degeneracy plays an important role in determining the electrical properties of double heterostructure injection lasers. Correct statistical treatment of the charge cariers in the device leads to a generalised Einstein relation between the carrier transport coefficients. Such a relation (for both electrons and holes) is used in numerical solutions of semiconductor transport equations applicable to the laser. Experimentally observed I–V characteristics of the laser are explained in terms of carrier degeneracy effects.     

In-situ thin film growth/etch measurement and control by laser light reflectance analysis

In this article, I report in-situ thin film growth and etching measurement and control with ultra high precision. A stack of thin film layers of AlAs/Al_0.1Ga_0.9As to form Bragg reflector for the surface emitting semi-conductor laser was grown by gas source molecular beam epitaxy on a rotating wafer and the growth process was monitored with inclined incident polarized He–Ne laser light reflectance measurement. The multiple thin film layers were preferred to increase the visibility of the oscillating intensity variation signal for feasibility experiments. The inferred growth rate from the reflectance analysis was found to be 0.15 nm/s which remained in agreement with set growth parameters. The laser light reflectance analysis method was also employed for in-situ control and measurement of thin film etching process. The inferred etching rate was found to be 0.75 nm/s by the chemical etchant H₃PO₄:H₂O₂:H₂O::3:1:100. Calibrated optical reflectance signal for growth and etching process is proposed for measuring and control their respective thickness and depth with very high accuracy and precision.     

Dependence of the experimental stability diagram of an optically injected semiconductor laser on the laser current

An experimental study of the dynamical properties of a semiconductor laser subjected to external optical injection is presented. The effect of the laser current on the dynamical regions as found in an experiment is reported on for the first time. The nonlinear dynamical regions are mapped in the parameter plane consisting of the detuning between lasers and the injection strength, by utilizing a method of condensing the information in output spectra to two-dimensional images. Corresponding maps for different values of the slave laser current are recorded. The recordings present conclusive experimental evidence that the overall locations of the dynamical regions scale with respect to the relaxation–oscillation frequency and the injection strength relative to the free running laser power. The results further support the theoretical prediction that the linewidth-enhancement factor is the parameter which most strongly affects the dynamics. Locally, specific chaotic regions are found to grow for higher operating points of the slave laser. A complete characterization of the parameters that enter the rate-equations for the visible output AlGaInP laser used in this experiment is performed.     

基于二维六方氮化硼材料的光子晶体非对称传输异质结构设计

二维六方氮化硼(hexagonal boron nitride,hBN)材料在产生光学稳定的超亮量子单光子光源领域有着潜在应用,有望用于量子计算和信息处理平台,已成为研究热点.而光学非对称传输设备是集成量子计算芯片中的关键器件之一.本文从理论上提出了一种基于hBN材料光子晶体异质结构的纳米光子学非对称光传输器件.运用平...     

真空热蒸发制备CdS/ZnS核/壳异质结纳米线

将传统的真空热蒸发镀膜实验加以改进,先以催化剂辅助蒸发制备出CdS纳米线,再将其作为模板,以ZnS为蒸发源物质,二次蒸发包覆ZnS层,成功制备出大量的CdS/ZnS核/壳异质结纳米线.经X射线衍射、X射线能量色散谱、透射电镜分析表明,所得CdS/ZnS异质结纳米线的核心部分为CdS单晶纳米线,外层为ZnS多晶层.本文的实验方法简便易行,所得纳米结构在光电纳米器件领域有一定应用前景.     

酞菁铜与MoS_2(0001)范德瓦耳斯异质结研究

利用光电子能谱、原子力显微镜以及低能电子衍射等表面研究手段系统研究了真空沉积生长的酞菁铜薄膜与衬底MoS2(0001)之间的范德瓦耳斯异质结界面电子结构和几何结构.角分辨光电子能谱清楚地再现了MoS2(0001)衬底在Γ点附近的能带结构.低能电子衍射结果表明,CuPc薄膜在MoS2(0001)表面沿着衬底表面[11ˉ20],[1ˉ210]和[ˉ2110]三个晶向有序生长,反映了衬底对CuPc的影响.原子力显微镜结果表明,CuPc在MoS2衬底上遵循层状-岛状生长模式:在低生长厚度下(单层薄膜厚度约为0.3 nm),CuPc分子平面平行于MoS2表面上形成均匀连续的薄膜;在较高的沉积厚度下,CuPc沿衬底晶向形成棒状晶粒,表现出明显的各向异性.光电子能谱显示界面偶极层为0.07 eV,而且能谱在膜厚1.2 nm饱和,揭示了酞菁铜与MoS2(0001)范德瓦耳斯异质结的能级结构.     

两种调制方式下主从激光器同步性能的理论研究

 以单周期信号为例,数值模拟并讨论了注入频率和注入光场波动强度对主从式半导体激光器同步性能的影响。分别通过直接调制和外部调制两种方式获得单周期信号,并采用改进的参数——相似指数评价系统的同步性能。研究结果表明：当注入强度足够大时,系统实现注入锁定同步,外部调制的相似指数大于内部调制;注入强度恒定时,同步品质几乎不受调制指数的影响,而受调制频率的影响;由于内部、外部调制引起主激光器的输出反相,两种调制方式下同步品质随调制频率的变化趋势也相反。     

Online monitoring of thermo-cycles and its correlation with microstructure in laser cladding of nickel based super alloy

Laser cladding, basically a weld deposition technique, is finding applications in many areas including surface coatings, refurbishment of worn out components and generation of functionally graded components owing to its various advantages over conventional methods like TIG, PTA etc. One of the essential requirements to adopt this technique in industrial manufacturing is to fulfil the increasing demand on product quality which could be controlled through online process monitoring and correlating the signals with the mechanical and metallurgical properties. Rapid thermo-cycle i.e. the fast heating and cooling rates involved in this process affect above properties of the deposited layer to a great extent. Therefore, the current study aims to monitor the thermo-cycles online, understand its variation with process parameters and its effect on different quality aspects of the clad layer, like microstructure, elemental segregations and mechanical properties. The effect of process parameters on clad track geometry is also studied which helps in their judicious selection to deposit a predefined thickness of coating. In this study Inconel 718, a nickel based super alloy is used as a clad material and AISI 304 austenitic steel as a substrate material. The thermo-cycles during the cladding process were recorded using a single spot monochromatic pyrometer. The heating and cooling rates were estimated from the recorded thermo-cycles and its effects on microstructures were characterised using SEM and XRD analyses. Slow thermo-cycles resulted in severe elemental segregations favouring Laves phase formation and increased γ matrix size which is found to be detrimental to the mechanical properties. Slow cooling also resulted in termination of epitaxial growth, forming equiaxed grains near the surface, which is not preferred for single crystal growth. Heat treatment is carried out and the effect of slow cooling and the increased γ matrix size on dissolution of segregated elements in metal matrix is studied.     

Carrier hopping in InAs/AlyGa1−yAs quantum dot heterostructures: effects on optical and laser properties

The optical properties of InAs/Al_yGa_1−yAs self-assembled quantum dots are studied as a function of temperature from 10 K to room temperature. The temperature dependence of carrier hopping between dots is discussed in terms of the depth of the dot confinement potential and the dispersion in dot size and composition. We show that carrier hopping between dots influences both the electrical and optical properties of laser devices having dots as active medium.