利用梯度掺杂获得高量子效率的GaAs光电阴极

获得高量子效率且稳定性良好的阴极一直是近年来发展GaAs光电阴极的重要方向。对晶面为(100),掺杂Be,厚度为1μm分子束外延生长的反射式GaAs发射层,设计了一种从体内到表面掺杂浓度由高到低分布的新型梯度掺杂结构。掺杂浓度的范围从1×1019cm-3到1×1018cm-3,并利用(Cs,O)激活技术制备了GaAs光电阴极。光谱响应测试曲线显示,与传统均匀掺杂的GaAs光电阴极相比,梯度掺杂的GaAs光电阴极的量子效率在整个波段都有提高,积分灵敏度可达1580μA/lm,且具有更好的稳定性。讨论了这种新型GaAs光电阴极获得更高量子效率的内在机理。该设计结构是现实可行的,且具有很大发展潜力,它为国内发展高性能GaAs光电阴极提供了一条重要途径。     

Narrow line-width resonant cavity enhanced photodetectors operating at 1.55 μm

A method for fabrication of long-wavelength narrow line-width InGaAs resonant cavity enhanced (RCE) photodetectors in a silicon substrate operating at the wavelength range of 1.3-1.6 μm has been developed. A full width at half maximum (FWHM) of 0.7 nm and a peak responsivity of 0.16 A/W at the resonance wavelength of 1.55 μm have been accomplished by using a thick InP layer as part of the resonant cavity. The effects of roughness and tilt of the InP layer surface, and its free carrier absorption, as well as the thickness deviation of the mirror pair on the resonance wavelength shift and the peak quantum efficiency of the RCE photodetectors are analyzed in detail, and approaches for minimizing them toward superior performance are suggested.     

近紫外波段NEA GaN阴极响应特性的研究

为了深入理解近紫外波段NEA GaN阴极的光谱响应特性, 在超高真空系统中对MOCVD生长的不同发射层厚度和掺杂浓度的三个样品进行激活实验, 并在线测试样品光谱响应. 利用反射式GaN阴极量子效率公式和最小二乘法对入射光波长为0.25—0.35 μ之间的 阴极响应量子效率实验数据进行拟合, 分别得到后界面复合速率和拟合直线L的斜率, 并使用量子效率公式对入射光波长为0.35 μ时的反射式GaN阴极光谱响应量子效率进行仿真. 结果表明, 后界面复合速率和直线v的斜率都能很好地反映GaN阴极的响应性能, 当GaN阴极后界面复合速率小于10⁵ cm/s, 发射层的厚度取0.174—0.212 μ时, 阴极光谱响应性能最好. 关键词： 反射式GaN 势垒 最小二乘法 后界面缺陷     

微光像增强器光阴极灵敏度理论极限问题研究

光阴极灵敏度(量子效率)是微光像增强器最重要和最基本的性能参数之一,它决定着微光成像系统在低照度下的视距和图像清晰度。根据半导体光电发射物理模型及普朗克黑体辐射理论,简介了光电发射5个环节（光子不完全吸收、GaAlAs/GaAs后界面、GaAs光阴极激活层体特性缺陷、GaAs光阴极表面位垒和GaAs光阴极-MCP之间近贴电场电子隧道效应）对光阴极量子效率的影响,给出了相关数学表达式。在假定5个环节子量子效率均为100%的前提下,估算出蓝延伸GaAs光阴极在（0.41～0.93）μm波段内的极限积分灵敏度,其值为6569μA/lm。文末,对此结果的意义给予评价。     

Optical performance simulation and optimization for CMOS image sensor pixels

Finite-difference time-domain (FDTD) method is used to perform three dimension simulations for the optical performance of 1.75 μm pitch pixels of CMOS image sensor. A three dimension pixel model for CMOS image sensor pixels is set up. Micro-lens optimization, dielectric stack height reduction can decrease the optical power loss. A SiN layer with proper thickness can reduce the reflection at the Si–SiO₂ interface. A high refractive index lightpipe is proposed to confine the light within the pixel. The simulation results show that the optical efficiency of the optimized 1.75 μm pixel compare to that before optimized is promoted by more than 10% and the cross-talk is reduced by 50%.     

高性能透射式GaAs光电阴极量子效率拟合与结构研究

为了探索高性能透射式GaAs光电阴极的特征结构,对光电阴极量子效率公式进行了光谱反射率与短波截止限的修正,并利用修正后的公式对ITT透射式GaAs光电阴极量子效率(≈43%)曲线进行了拟合,得到拟合相对误差小于5%时的结构参数为:窗口层Ga_1-xAl_xAs的厚度介于0.3-0.5 μm,Al组分x值为0.7,发射层GaAs的厚度介于1.1-1.4 μm.另外,根据拟合结果讨论了均匀掺杂透射式GaAs光电阴极的优化结构参数,如果光电阴极具有0.4 μm厚的Ga_1-xAl_xAs(x=0.7)窗口层和1.1-1.5 μm厚的GaAs发射层,则积分灵敏度可以达到2350 μA/lm以上. 关键词： 透射式GaAs光电阴极 量子效率 积分灵敏度 光学性能     

Design of fiber Bragg grating-based Fabry–Perot sensor for simultaneous measurement of humidity and temperature

A new sensor for simultaneous measurement of humidity and temperature is proposed. The sensor consists of Fabry–Perot cavity formed by two identical uniform fiber Bragg gratings. To make the cavity serves as humidity sensor, moisture sensitive polymer, which is polyimide, is coated on the FBG and on the cavity with different thickness. When the sensor is exposed to the relative humidity change, the polyimide will expand and stretch the fiber and induces strain on the FBG and on the cavity. The induced strain alters the grating period, cavity length and effective refractive index of fiber. The simulation results show that the humidity sensitivity and thermal sensitivity are 1.92 pm/%RH and 8.87 pm/°C, respectively, for polyimide coating thickness of 10 μm on the FBG and 15 μm on the cavity.     

Optimization of the CIGS based thin film solar cells: Numerical simulation and analysis

Chalcopyrite Cu(In,Ga)Se (CIGS) is a very promising material for thin film photovoltaics and offers a number of interesting advantages compared to the bulk silicon devices. CIGS absorbers today have a typical thickness of about 1–2 μm. However, on the way toward mass production, it will be necessary to reduce the thickness even further. This paper indicates a numerical study to optimization of CIGS based thin film solar cells. An optimum value of the thickness of this structure has been calculated and it is shown that by optimizing the thickness of the cell efficiency has been increases and cost of production can be reduces. Numerical optimizations have been done by adjusting parameters such as the combination of band gap and mismatch as well as the specific structure of the cell. It is shown that by optimization of the considered structure, open circuit voltage increases and an improvement of conversion efficiency has been observed in comparison to the conventional CIGS system. Capacitance–voltage characteristics and depletion region width versus applied voltage for optimized cell and typical cell has been calculated which simulation results predict that by reducing cell layers in the optimized cell structure, there is no drastically changes in depletion layer profile versus applied voltage. From the simulation results it was found that by optimization of the considered structure, optimized value of CIGS and transparent conductive oxide thickness are 0.3 μm and 20 nm and also an improvement of conversion efficiency has been observed in comparison to the conventional CIGS which cell efficiency increases from 17.65 % to 20.34%, respectively.     

向短波红外延伸的微光夜视技术及其应用

对传统的微光像增强器与向短波红外延伸的InGaAs光阴极像增强器进行了比较,分析了通过调节组分而使响应波段覆盖夜天光辐射主要波段的InGaAs材料特性,揭示了以InGaAs半导体材料为光阴极的像增强器将在夜间具有更高的量子效率和响应度。介绍了InGaAs器件技术的国内外研究现状,InGaAs光阴极微光器件在短波红外波段的辐射响应是传统像增强器的100~1 000倍,InGaAs全固态探测器可在0.4 m~1.7 m宽光谱成像,在0.9 m~1.7 m范围的量子效率大于80%,表明该器件在激光探测、远距离定位与跟踪、情报侦察、夜间辅助驾驶等方面可以获得广泛应用。     

Optimizing GaN photocathode structure for higher quantum efficiency

To improve the quantum efficiency of GaN photocathode, we optimized the photocathode's structure in three aspects. We use AlN replacing GaN as the buffer layer, which can act as potential barrier to reflect electrons back to surface. The optimal thickness of emission layer is calculated as 162.5 nm, and considering the graded doping profile, we optimized the thickness as 180 nm. Three built-in electric fields are introduced by Mg graded dope, and the intensities of the high fields are calculated to give the quantitive results of their influence. After surface cleaning and activation, quantum efficiency of the optimized sample was increased and the highest value of 56% was achieved at 240 nm. More quantum efficiency enhancement is possible by further optimizing the photocathode structure.     

Theoretical research on optical properties and quantum efficiency of Ga1−xAlxN photocathodes

In order to design the optimal component structure of transmission-mode (t-mode) Ga_1−xAl_xN photocathode, the optical properties and quantum efficiency of Ga_1−xAl_xN photocathodes are simulated. Based on thin film principle, optical model of t-mode Ga_1−xAl_xN photocathodes is built. And the quantum efficiency formula is put forward. Results show that Ga_1−xAl_xN photocathodes can satisfy the need of detectors with “solar blind” property when the Al component is bigger than 0.375. There is an optimal thickness of Ga_1−xAl_xN layer to get highest quantum efficiency, and the optimal thickness is 0.3 μm. There is close relation between absorptivity and quantum efficiency, which is in good agreement with the “three-step” model. This work gives a reference for the experimental research on the Ga_1−xAl_xN photocathodes.     

Numerical simulation of a coupling effect on electronic states in quantum dots

Lasers operating at 1.3 μm have attracted considerable attention owing to their potential to provide efficient light sources for next-generation high-speed communication systems. InAs/GaAs quantum dots (QDs) were pointed out as a reliable low-cost way to attain this goal. However, due to the lattice mismatch, the accumulation of strain by stacking the QDs can cause dislocations that significantly degrade the performance of the lasers. In order to reduce this strain, a promising method is the use of InAs QDs embedded in InGaAs layers. The capping of the QD layer with InGaAs is able to tune the emission toward longer and controllable wave-lengths between 1.1 and 1.5 μm. In this work, using the effective-mass envelope-function theory, we investigated theoretically the optical properties of coupled InAs/GaAs strained QDs based structures emitting around 1.33 μm. The calculation was performed by the resolution of the 3D Schrödinger equation. The energy levels of confined carriers and the optical transition energy have been investigated. The oscillator strengths of this transition have been studied with and without taking into account the strain effect in the calculations. The information derived from the present study shows that the InGaAs capping layer may have profound consequences as regards the performance of an InAs/GaAs QD based laser. Based on the present results, we hope that the present work make a contribution to experimental studies of InAs/GaAs QD based structures, namely the optoelectronic applications concerning infrared and mid-infrared spectral regions as well as the solar cells.     

Effect of thickness and porous structure of SiC layers on the spectral response of the Pd/SiC-pSi Schottky photodiodes

In this work, we study the effect of the thickness and porous structure of silicon carbide (PSC) layers on the electrical properties of Schottky photodiodes by using a palladium (Pd) layer deposited on non-porous silicon carbide (SiC) and porous-SiC (PSC) layers. The non-porous and porous-SiC layers were realized on a p-type silicon (Si(1 0 0)) substrate by pulsed laser deposition using a KrF laser (248 nm) and thermal deposition of a thin Pd layer. The porous structure of the SiC layer deposited was developed by an electrochemical (anodization) method. The electrical measurements were made at room temperature (295 K) in an air ambience. The effect of the porous surface structure and the thickness of the SiC layer were investigated by evaluating electrical parameters such as the ideality factor (n) and barrier height (?_Bp). The thickness of the porous layer significantly affects the electrical properties of the Schottky photodiodes. Analysis of current-voltage (I-V) characteristics showed that the forward current might be described by a classical thermal emission theory. The ideality factor determined by the I-V characteristics was found to be dependent on the SiC thickness a value For a thin SiC layer (0.16 μm) n was around 1.325 with a barrier height 0.798 eV, while for a thick layer (1.6 μm), n and ?_Bp were 1.026 and 0.890 eV, respectively for Pd/SiC-pSi. These results indicate Schottky photodiodes with high performance are obtained for thicker SiC layer and for thin layer of PSC. This effect showed the uniformity of the SiC layer. In the same case the ideality factor (n) decreases for Pd/PSC-pSi(1 0 0) for low SiC thickness by report of Pd/PSC-pSi(1 0 0) Schottky photodiodes, but for Pd/PSC-pSi(1 0 0) n increase for large SiC thickness layer. We notice that the barrier height (?_Bp) was reversely depend by report of ideality factor. A spectral response value of (SR) of 34 mA/W at λ = 400 nm was measured for Pd/0.16 μm SiC-pSi Schottky photodiode with low SiC thickness. On the other hand, a value of SR = 0.14 mA/W at λ = 900 nm was obtained when we used PSC layer (Pd/PSC-pSi(1 0 0)). A reverse behaviour occurs for thicker SiC layer. Finally, it was found that the thickness and surface porous structure have strong effect on sensitivity.     

Near-infrared emissions and quantum efficiencies in Tm-doped heavy metal gallate glasses for S- and U-band amplifiers and 1.8 μm infrared laser

Intense 1.8 μm and efficient 1.48 μm infrared emissions have been recorded in Tm³⁺-doped alkali-barium-bismuth-gallate (LKBBG) glasses with low phonon energies under the excitation of 792 nm diode laser. The maximum emission cross-sections for 1.8 and 1.48 μm emission bands are derived to be 6.26×10⁻²¹ and 3.34×10⁻²¹ cm², respectively, and the peak values are much higher than those in Tm³⁺-doped ZBLAN glass. In low-concentration doping, the full-widths at half-maximum (FWHMs) of the two emission bands are 223 and 122 nm, and the quantum efficiencies of the ³F₄ and ³H₄ levels are proved to be ∼100% and 86%, respectively. When the doping concentration increases to 1 wt%, the quantum efficiency of the ³H₄ level is reduced to 60% due to the cross-relaxation processes in high-concentration doping. Efficient 1.8 μm infrared emission in Er³⁺/Tm³⁺-codoped LKBBG glass has also been achieved under the excitation of 970 nm diode laser, and the probability and the efficiency of non-radiative energy transfer from Er³⁺ to Tm³⁺ are as high as 354 s⁻¹ and 58.4%, respectively. Efficient and broad 1.8 and 1.48 μm infrared emission bands indicate that Tm³⁺-doped LKBBG glasses are suitable materials in developing S- and U-band amplifiers and 1.8 μm infrared laser.     

透射式蓝延伸GaAs光电阴极光学结构对比

用金属有机物化学气相沉积法外延制备了一个透射式蓝延伸GaAs光电阴极,积分灵敏度达到1980 μA/lm,同时与美国ITT公司的一条蓝延伸阴极光谱响应曲线对比,分别对两者进行了光学结构拟合. 结果表明,国内阴极在Ga_1-xAl_xAs层厚度、Al组分、电子扩散长度和后界面复合速率上与国外 存在差距,这导致国内阴极的蓝延伸性能不及国外.国内蓝延伸阴极的表面电子逸出几率、发射层厚度与 国外阴极拟合结果一致,这使得两者长波响应性能差别远小于短波部分的差别.另外响应波段全谱的吸收率 小于国外阴极,导致国内透射式蓝延伸GaAs光电阴极光谱响应、积分灵敏度尚不及国外.     

透射式蓝延伸GaAs光电阴极的光电发射特性研究

利用计算光学性能、量子效率和积分灵敏度的理论模型,分别研究比较了我国和ITT典型透射式蓝延伸GaAs光阴极的光电发射特性,包括阴极的光学性质和性能参数。结果表明我国的透射式蓝延伸光阴极积分灵敏度已经达到2 100μA.lm-1,但与ITT的2 750μA.lm-1相比还存在一定的差距。分析的主要原因是一方面是GaAlAs窗口层的厚度和Al组分大小对于短波响应,特别是对蓝延伸起着决定的作用;另一方面阴极性能参数电子扩散长度和后界面复合速率的大小对长波响应和短波响应也有着重要的影响,这些因素都受制于基础工业制造水平的落后。     

Effects of NEA GaN photocathode performance parameters on quantum efficiency

Using research on the negative electron affinity GaN photocathode photoemission mechanism, we obtained the reflective-type and transmission-type GaN photocathode quantum efficiency formulas. The influence on quantum efficiency and sensitivity of integral of cathode performance parameters such as electron surface escape probability P, electron diffusion length L_D, absorption coefficient α, back-interface recombination rate S_v and cathode thickness T_e, were analyzed using these formulas. It was found that to obtain negative electron affinity GaN optoelectronic cathodes with high quantum efficiencies, we must constantly improve cathode activation technologies and the surface escaping probability of cathode. Also, we must increase the electronic diffusion length, reduce the rate of compounding, and find the optimal thickness of the cathode transmit layer based for the specific electronic diffusion length.     

Design of a silicon RCE Schottky photodetector working at 1.55 μm

In this paper, the design of a resonant cavity-enhanced (RCE) Schottky photodetector, based on internal photoemission effect and working at 1.55 μm, is presented. In order to estimate the theoretical quantum efficiency we take the advantage of analytical formulation of the internal photoemission effect (Fowler theory), and its extension for thin films, while for the optical analysis of device a numerical method, based on the transfer matrix method, has been implemented. Finally, we complete our design calculating bandwidth and bandwidth-efficiency product.Our numerical results prove that a quantum efficiency of 0.1% is obtained at resonant wavelength (1.55 μm) with a very thin absorbing metal layer (30 nm). Theoretical values of 100 GHz and 100 MHz were obtained, respectively, for the carrier-transit time limited 3-dB bandwidth and bandwidth-efficiency. The proposed photodetector can work at room temperature and its fabrication is completely compatible with standard silicon technology.     

Enhanced field emission and patterned emitter device fabrication of metal-tetracyanoquinodimethane nanowires array

Ag(TCNQ) and Cu(TCNQ) nanowires were synthesized via vapor-transport reaction method at a low temperature of 100 °C. Field emission properties of the as-obtained nanowires on ITO glass substrates were studied. The turn-on electric fields of Ag(TCNQ) and Cu(TCNQ) nanowires were 9.7 and 7.6 V/μm (with emission current of 10 μA/cm²), respectively. The turn-on electric fields of Ag(TCNQ) and Cu(TCNQ) nanowires decreased to 6 and 2.2 V/μm, and the emission current densities increased by two orders at a field of 8 V/μm with a homogeneous-like metal (e.g. Cu for Cu(TCNQ)) buffer layer to the substrate. The improved field emission is due to the better conduct in the nanowires/substrate interface and higher internal conductance of the nanowires. The patterned field emission cathode was then fabricated by localized growing M-TCNQ nanowires onto mask-deposited metal film buffer layer. The emission luminance was measured to be 810 cd/m² at a field of 8.5 V/μm.     

Structural and optical analysis of size-controlled InAs quantum dashes

Self-organized InAs and InGaAs quantum dashes have been investigated by chemically sensitive scanning transmission electron microscopy and photoluminescence spectroscopy. The quantum dashes exhibit a triangular cross section. Using electron energy loss spectroscopy, we show that no intermixing between quantum dashes and embedding barrier occurs during growth. By adjusting the nominal thickness of the InAs layer, the emission wavelength of the InAs quantum dashes can be tuned between 1.37 and 1.9 μm in a straightforward way.