On the dark currents in germanium Schottky-barrier photodetectors

It is shown that for experimentally observed values of Schottky-barrier height of metal-n-type germanium photodetector structures, the dominant component of dark current can be due to the injection of minority carriers, rather than to the usual majority-carrier component. For barrier heights approaching 0.6 eV, for doping concentrationsN_{d} lsim 10^{15}cm^-3, for short minority-carrier lifetimetau_{p} lsim 1µs, for narrow base widthW_{b} lsim 10µm, or combinations of these conditions, the minority-carrier injection ratio approaches unity and these devices behave in the same way as p⁺-n junctions, with identical dark currents. The low-temperature fabrication requirements and processing simplicity of germanium Schottky barriers makes these devices more attractive under these conditions than germanium p-n junction photodetectors.     

Nitride-Based ultraviolet metal-semiconductor-metal photodetectors with a low-temperature GaN layer

The GaN metal-semiconductor-metal (MSM) ultraviolet (UV) photodetectors with a low-temperature (LT)-GaN layer have been demonstrated. It was found that we could achieve a two orders of magnitude smaller, photodetector-dark current by introducing a LT-GaN layer, which could be attributed to the larger Schottky-barrier height between the Ni/Au metal contact and the LT-GaN layer. It was also found that photodetectors with the LT-GaN layer could provide a larger photocurrent to dark-current contrast ratio and a larger UV-to-visible rejection ratio. The maximum responsivity was found to be 3.3 A/W and 0.13 A/W when the photodetector with a LT-GaN layer was biased at 5 V and 1 V, respectively.     

Source resistance reduction of AlGaN-GaN HFETs with novel superlattice cap layer

We have developed a novel AlGaN-GaN heterojunction field effect transistor (HFET) with an ultralow source resistance by employing the novel superlattice (SL) cap structure. The particular advantage of the SL cap, i.e., the existence of multiple layers of the polarization-induced two-dimensional electron gas (2DEG) with high mobility and high concentration at each AlGaN-GaN interface, is fully exploited for lowering the lateral resistance and the potential barrier at the interface of the SL cap and the HFET barrier layer. By designing the AlGaN-GaN thickness ratio, we have established a method to obtain the optimized SL structure and have achieved an extremely low source resistance of 0.4 /spl Omega//spl middot/mm which is lower not only than HFETs with the conventional structure but also than those with the n-GaN cap structure. The SL cap HFET fabricated on a sapphire substrate exhibited excellent dc and RF performance, i.e., maximum transconductance of over 400 mS/mm, maximum drain current of 1.2 A/mm, a cutoff frequency of 60 GHz, a maximum frequency of oscillation of 140 GHz, and a very low noise figure minimum of 0.7 dB at 12 GHz.     

改进器件工艺降低硅光电探测器暗电流

为降低硅光电探测器ＰＮ结反向暗电流，可在器件制作工艺中采用一系列完美晶体器件工艺（ＰＣＤＴ）。在实验过程中，对吸除工艺，应力补偿工艺等作了改进，进一步降低了反向暗电流。     

Low dark current and internal gain mechanism of GaN MSM photodetectors fabricated on bulk GaN substrate

Metal-semiconductor-metal ultraviolet photodetectors are fabricated on low-defect-density homoepitaxial GaN layer on bulk GaN substrate. The dislocation density of the homoepitaxial layer characterized by cathodoluminescence mapping technique is ∼5 × 10⁶ cm⁻². The photodetector with a high UV-to-visible rejection ratio of up to 1 × 10⁵ exhibits a low dark current of <2 pA at room temperature under 10 V bias. The photo-responsivity of the photodetector gradually increases as a function of applied bias, resulting in a photodetector quantum efficiency exceeding 100% at above medium bias. The photo-responsivity also shows a dependence on the incident optical power density and illumination conditions. The internal gain mechanism of the photodetector is attributed to photo-generated holes trapped at the semiconductor/metal interface as well as high-field-induced image-force lowering effect.     

GaN metal-semiconductor-metal photodetectors with low-temperature-GaN cap layers and ITO metal contacts

Nitride-based metal-semiconductor-metal (MSM) photodetectors (PDs) with low-temperature (LT) gallium nitride (GaN) cap layers and indium-tin-oxide (ITO) metal contacts were successfully fabricated. It was found that we could achieve three orders of magnitude smaller dark current by the introduction of the LT-GaN layer. For the PDs with LT-GaN cap layers, the maximum responsivity at 350 nm was found to be 0.1 and 0.9 A/W when the device was biased at 1 and 5 V, respectively. Operation speed of PDs with LT-GaN cap layers was also found to be faster than that of conventional PDs without LT-GaN cap layers.     

Improvement of dark current using InP/InGaAsP transition layer in large-area InGaAs MSM photodetectors

A large-area InGaAs metal-semiconductor-metal (MSM) photodetector with 1/spl times/1 mm/sup 2/ photoactive area for free-space optical communication applications has been designed, fabricated, and characterized. Interdigitated electrodes of 2-/spl mu/m widths and 15-/spl mu/m spacings are designed to maximize the responsivity, and enable MSM photodetectors to reach a maximum responsivity at 1.53-/spl mu/m wavelength. By employing a two-step InP/InGaAsP transition layer, the dark current density of 45 fA//spl mu/m/sup 2/ was achieved at 10-V bias and at room temperature. Dark current-bias voltage curves were measured as a function of temperature from 40 to 270 K to estimate the activation energy. A 3-dB bandwidth of 210 MHz was obtained at a 10-V bias, and the measured result was compared with the designed bandwidth.     

Inkjet printed polymeric electron blocking and surface energy modifying layer for low dark current organic photodetectors

The reduction of dark current is required to enhance the signal-to-noise ratio and decrease the power consumption in photodetectors. This is typically achieved by introducing additional functional layers to suppress carrier injection, a task that proves to be challenging especially in printed devices. Here we report on the successful reduction of dark current below 100 nA cm⁻² (at −1 V bias) in an inkjet printed photodetector by the insertion of an electron blocking layer based on poly[3-(3,5-di-tert-butyl-4-methoxyphenyl)-thiophene], while preserving a high quantum yield. Furthermore, the electron blocking layer strongly increases the surface energy of the hydrophobic photoactive layer, therefore simplifying the printing of transparent top electrodes from water based formulations without the addition of surfactants.     

AlGaN日盲紫外雪崩光电探测器暗电流研究

为了提高AlGaN日盲紫外雪崩探测器的信噪比,降低暗电流,研制高性能日盲紫外探测器,针对AlGaN日盲紫外雪崩探测器暗电流机制进行了深入研究。首先对传统p-i-n-i-n结构雪崩探测器进行了初步研究,分别设计了GaN和AlGaN的两种雪崩探测器模型,分析了其不同暗电流特性,得到的模拟暗电流特性曲线与实验吻合。在此基础上,针对日盲紫外波段高Al组分AlGaN雪崩探测器,重点分析研究了不同异质界面的负极化电荷、p型有效掺杂以及温度等因素对暗电流的影响。在AlGaN日盲紫外雪崩探测器研究中得到的近零偏工作暗电流为2.510-13 A,在反向138 V左右发生雪崩击穿,雪崩开启电流为18.3 nA左右,击穿电压温度系数约为0.05 V/K,与实验及文献测试结果吻合。     

Transparent photodetectors with ultra-low dark current and high photoresponse for near-infrared detection

A remarkable progress in research works regarding flexibility and transparency of organic optoelectronic devices has been observed in the past decade compared to their inorganic counterparts. However, few studies have been devoted to the advancement of a transparent organic photodetector. In this study, we have used a wavelength-selective bulk-heterojunction of ClAlPc:C₆₀ as active layer and Cu:Ag/WO₃ metal alloy as electrode to realize a see-through organic photodetector (OPD) with an average visible transmission of 76.92%. The optimized transparent OPDs show an average dark current density of 0.36 nA cm⁻² and a rise/fall time of <5 μs under a bias voltage of −2 V, which could be potentially applied in a home security system based on invisible near-infrared detection.     

Normally-Off AlGaN/GaN HEMTs with InGaN cap layer: A simulation study

AlGaN/GaN high electron mobility transistors (HEMTs) are favored for the use in high-power and high-frequency applications. Normally-off operation has been desired for various applications, but proved to be difficult to achieve. Recently, a new approach was proposed by Mizutani et al. [Mizutani T, Ito M, Kishimoto S, Nakamura F. AlGaN/GaN HEMTs with thin InGaN cap layer for normally-off operation. IEEE Elec Dev Lett 2007;28(7):549–51]: a thin InGaN cap layer introduces a polarization field, which raises the conduction band of the AlGaN/GaN interface. As a result, the threshold voltage is shifted in positive direction. Relying on the experimental work of Mizutani et al. we conduct a simulation study of the proposed devices. Our device simulation tool is expanded by material models for InN and InGaN and also an improved high-field mobility model accounting for the specifics of the III-N materials. Using this setup, we further explore the device specific effects and conduct an analysis of the AC characteristics.     

High-breakdown-voltage MESFET with a low-temperature-grown GaAspassivation layer and overlapping gate structure

GaAs MESFETs were fabricated using a low-temperature-grown (LTG) high-resistivity GaAs layer to passivate the doped channel between the gate and source and between the gate and the drain. The gate was fabricated such that the source and drain edges of the metal gate overlapped the LTG GaAs passivation layer. The electric fields at the edges of the gate were reduced by this special combination of LTG GaAs passivation and gate geometry, resulting in a gate-drain breakdown voltage of 42 V. This value is over 60% higher than that of similar MESFETs fabricated without the gate overlap     

KOH溶液处理对AlGaN紫外探测器暗电流的影响

采用KOH溶液表面处理工艺制备得到了128×1线列日盲AlGaN紫外探测器,器件的反偏暗电流为6.88×10-9A(-8 V时),比未采用此项工艺制备得到的器件的暗电流减小近103倍。元素深度分布俄歇电子谱(AES)等测试结果分析表明,采用这种表面处理工艺可以有效地去除干法刻蚀后材料表面的N空位、刻蚀生成物及自然氧化物,减小了界面态密度,改善了电流-电压特性,减小了反偏暗电流。利用传输线模型TLM计算得到了Ti/Al/Ti/Au金属电极与高Al组分n-Al0.65Ga0.35N材料间的接触电阻率为8.35×10-3Ωcm2。     

Effect of a low-temperature-grown GaAs layer on InAs quantum-dot photoluminescence

The photoluminescence of InAs semiconductor quantum dots overgrown by GaAs in the low-temperature mode (LT-GaAs) using various spacer layers or without them is studied. Spacer layers are thin GaAs or AlAs layers grown at temperatures normal for molecular-beam epitaxy (MBE). Direct overgrowth leads to photoluminescence disappearance. When using a thin GaAs spacer layer, the photoluminescence from InAs quantum dots is partially recovered; however, its intensity appears lower by two orders of magnitude than in the reference sample in which the quantum-dot array is overgrown at normal temperature. The use of wider-gap AlAs as a spacer-layer material leads to the enhancement of photoluminescence from InAs quantum dots, but it is still more than ten times lower than that of reference-sample emission. A model taking into account carrier generation by light, diffusion and tunneling from quantum dots to the LT-GaAs layer is constructed.     

A new approach for modeling of dark current characteristics of quantum wire infrared photodetectors

In order to study the dark current characteristics in a quantum wire infrared photodetector(QRIP),the average number of electrons in quantum wires(QRs) must be got,which is mostly too complicated.In this paper we give a simple formula to calculate the average number of carriers in a quantum wire(QR) that can be easily evaluated by mathematical softwares,and then we use this formula to study dark current characteristics of a quantum wire infrared photodetector(QRIP). 更多还原     

高反射性电流阻挡层用于提高InGaN/GaN发光二极管的光输出功率

由SiO₂/TiO₂分布布拉格反射镜(DBR)和Al镜组成的混合式反射电流阻挡层用于提高InGaN/GaN发光二极管的光输出功率。混合式反射电流阻挡层不仅增强了电流扩展效应而且有效的将射向p金属电极的光子反射防止其对p电极焊点附近光子的吸收。实验结果表明,淀积在p-GaN上1.5个周期的SiO₂/TiO₂DBR和Al镜在455nm垂直入射时的反射率高达97.8%。在20mA的工作电流下,与没有电流阻挡层的发光二极管相比,生长1.5对SiO₂/TiO₂ DBR和Al镜作为电流阻挡层的发光二极管的光输出功率提高了12.5%,且光输出功率的分布更加均匀。     

CH3NH3PbI3/C60 heterojunction photodetectors with low dark current and high detectivity

CH₃NH₃PbI₃/C₆₀ heterojunction photodetectors were here fabricated. The peak EQE achieves ∼80% in the visible-light range from 400 to 760 nm. Benefitting from eliminating the leakage current (PEDOT:PSS-free), extremely low dark current density (0.6 nA/cm²) and high specific detectivity (2.7 × 10¹³ Jones) are acquired.     

Influence of quantum capture and tunneling mechanisms on the dark current of bound-to-continuum quantum-well infrared photodetectors

We present a theoretical model for the dark current of bound-to-continuum quantum-well infrared photodetectors (QWIPs), by considering the field-induced mixing effect, tunneling rate and phonon scattering rate between bound and continuum states. Using this model, we can see clearly how these mechanisms significantly influence the Fermi levels of bound and continuum electrons, and thus, the dark current. Nonlinear temperature dependence of the dark current at low temperature is predicted and discussed in detail. The simulated dark currents exhibit good agreement with the experimental results, without use of parameter fitting techniques.     

Optical properties of photodetectors based on single GaN nanowires with a transparent graphene contact

We report the fabrication and optical and electrical characterization of photodetectors for the UV spectral range based on single p–n junction nanowires with a transparent contact of a new type. The contact is based on CVD-grown (chemical-vapor deposition) graphene. The active region of the nitride nanowires contains a set of 30 radial In_0.18Ga_0.82N/GaN quantum wells. The structure is grown by metal-organic vaporphase epitaxy. The photodetectors are fabricated using electron-beam lithography. The current–voltage characteristics exhibit a rectifying behavior. The spectral sensitivity of the photodetector is recorded starting from 3 eV and extending far in the UV range. The maximal photoresponse is observed at a wavelength of 367 nm (sensitivity 1.9 mA/W). The response switching time of the photodetector is less than 0.1 s.     

具有电流阻挡层的垂直结构氮化镓发光二级管的光电特性研究

本文对具有电流阻挡层（CBL）的氮化镓垂直结构发光二极管（LED）进行了研究。不带有CBL、带有非欧姆接触CBL和带有二氧化硅CBL的垂直LED芯片样品被制作出来,并对它们进行了光电性能的测试。结果表明这种带有非欧姆接触CBL和带有二氧化硅CBL的氮化镓垂直结构LED的光输出效率相比不带有CBL的分别高出40.6%和60.7%。不带有CBL的、带有非欧姆接触CBL和带有二氧化硅CBL的氮化镓垂直结构LED在350毫安下的效率分别下降到各自最高效率的72%、78%和85.5%。另外,带有非欧姆接触CBL的氮化镓垂直结构LED具有更优越的抗静电性能。