AlN/AlGaN heterostructures for selective ultraviolet MSM detectors

Technical Physics - MSM photodetectors based on AlN/AlGaN heterostructures offer a low dark current, and their spectral characteristics render them promising for selective solar-blind detectors...     

Photoconductive ultraviolet detectors based on ZnO films

Properties of photoconductive ultraviolet detectors fabricated on ZnO films were presented. Highly c-axis oriented ZnO films were grown on glass substrates by pulsed laser deposition. Ultraviolet photodetectors were fabricated based on metal-semiconductor-metal planar structures. The photoresponsivity and the quantum efficiency are much higher in the ultraviolet range than in the visible range, and the peak values are around 360 nm. Photocurrent transients show that the detector has a large photocurrent with the peak value of 2.8 mA, and a slow photoresponse with a rise time of 5 min and a decay time of 7 min. The response curve of the detector is fitted well with exponential curve. The large photocurrent should result from the both effects of the accumulation of conduction electrons and the decrease of the barrier height between crystallites. The relaxation time constant τ obtained from the curve fitting represents the time accumulation during the process. The neutralization of photogenerated holes by negatively charged oxygen ions plays a key role in the photoconductive characteristics of ZnO polycrystalline films.     

Performance improvements of ultraviolet/infrared dual-band detectors

Results are reported on dual-band detectors based on a GaN/AlGaN structure operating in both the ultraviolet–midinfrared (UV–MIR) and ultraviolet–farinfrared (UV–FIR) regions. The UV detection is due to an interband process, while the MIR/FIR detection is from free carrier absorption in the emitter/contact followed by internal photoemission over the barrier at the GaN/AlGaN interface. The UV detection, which was observed from 300 K to 4.2 K, has a threshold of 360 nm with a peak responsivity of 0.6 mA/W at 300 K. The detector shows a free carrier IR response in the 3–7 μm range up to 120 K, and an impurity response around 54 μm up to 30 K. A response in the range 7–13 μm, which is tentatively assigned to transitions from C impurities and N vacancies in the barrier region, was also observed. It should also be possible to develop a detector operating in the UV–visible–IR regions by choosing the appropriate material system. A dual-band detector design, which allows not only to measure the two components of the photocurrent generated by UV and IR radiation simultaneously but also to optimize the UV and IR responses independently, is proposed.     

Assessment of integrated solar ultraviolet radiation by PM-355 detectors

The increase in environmental solar UV radiation due to depletion of ozone layer is a recent challenge to human health (skin cancer and eye effects) in countries having clear skies. Therefore, applying integrated, passive and inexpensive techniques to assess solar UV radiation is very much essential. Measurements of environmental solar UV radiation in Dhahran, Saudi Arabia area were carried out for a period of two months in the summer period in 1996 using two techniques in parallel namely: passive nuclear track detectors and active solar UV radiometers. Some of the nuclear track detectors were mounted in different conditions such as: under shadow band, on solar tracking mechanism following the solar rays. Others were mounted on perpendicular, tilted and horizontal surfaces in sunlight. All detectors were attached to a wooden background of the same thickness (0.5 cm) to eliminate interference of the heat effect of various support materials and have uniformity of the support materials. The assessment was carried out for different periods extending from two to nine weeks continuously. The investigated period covered the hottest months in Saudi Arabia (July and August) when the sky was clear of clouds. The results indicate linear correlation between alpha track diameters and the integrated exposure to solar UV as measured by the solar UV radiometer for all nuclear track detector positions and orientations. The highest slope has been observed for the detectors placed on solar tracking mechanism following the solar rays and the lowest from detectors oriented under the shadow band on horizontal position (measuring the diffused UV radiation only). The results show that most of the measured UV radiation (60%) were from the diffused UV radiation. The characteristics of the upper layer of the detectors are changed after chemical etching very quickly, with increase in the exposure time to UV solar radiation at certain orientation. The results encourage the use of nuclear track detectors for environmental and personal solar UV dosimetry on a large scale in Saudi Arabia and similar hot and clear-sky countries.     

The comparison of ZnO nanowire detectors working under two wavelengths of ultraviolet

Schottky-barrier ultraviolet (UV) detectors based on ZnO-nanowires (NWs) were fabricated with Pt as electrodes in this investigation. The ZnO NWs synthesized by the hydrothermal method were characterized by field-emission scanning electron microscopy (FE-SEM), Raman and PL spectroscopy. Photoelectric properties under 254 and 365 nm UV light were investigated. It is found that the photo-response properties of the devices under 365 nm UV light are better than those under 254 nm UV light, which is further illustrated by light transmission theory, energy-band diagram and absorption spectra. The results demonstrate that ZnO NWs detectors with selectivity to near-UV (NUV) light are promising candidates in photoelectric devices.     

一维纳米氧化锌自驱动紫外探测器的构建与性能研究

本文通过化学气相沉积法制备了ZnO纳米材料, 利用扫描电镜、光致发光谱、X衍射光谱及拉曼光谱等方法对制备的材料进行了表征. 基于制备的单根ZnO线分别构建了三种不同结构的紫外探测器件: Ag-ZnO-Ag肖特基型、PEDOT:PSS/n-ZnO结型和p-Si/n-ZnO结型紫外探测器, 并对器件的性能进行了研究. 结果表明: 三种不同结构的器件都表现出良好的整流特性, 对紫外线均有明显的光响应; 在零偏压下, 都有明显的自驱动特性. 三种器件中, p-Si/n-ZnO型紫外探测器性能最为优异: 在零偏压下, 暗电流约在1.2×10^-3 nA, 光电流在5.4 nA左右, 光暗电流比为4.5×10³, 上升和下降时间分别为0.7 s和1 s. 通过三类器件性能比较, 表明无机p-Si更适合与ZnO构建pn结型自驱动紫外探测器.     

The use of nuclear track detectors as an environmental solar ultraviolet radiation dosimeter

Investigation of using CR-39 nuclear track detectors as solar ultraviolet radiation (SUVR) dosimeter was studied. The nuclear track detectors were irradiated with Cf-252 (alpha and fission fragments) before exposure to sun light. The exposure of nuclear track detectors to SUVR were carried out under two different conditions (i) the detector was mounted on a solar tracker mechanism which always rotate towards the sun, and (ii) fixed on a horizontal surface. The measurements were performed for different periods extended from one to eight weeks continuously at Dhahran - Saudi Arabia. The period extended from the middle of July to middle of September: the hottest months in Saudi Arabia. The weekly integrated SUVR measured by Eppley sensor was ranged from 2400 W. h. m⁻² for one week up to 21000 W.h.m⁻² for eight weeks. The results indicate linear correlation between both the track diameters of alphas and fission fragments and the total exposure to SUVR. The results clearly demonstrate that it is possible to use the nuclear track detectors as an environmental and personal SUVR dosimeters.     

Silver microgrid transparent conductive electrode based on bulk plasmon effect for ultraviolet wavelength application

Silver microgrid/TiO₂/ITO (indium tin oxide glass) sandwich‐structure photodetectors with various pore sizes are fabricated by a microsphere lithography strategy. This ultraviolet (UV) detector possesses ohmic contact characteristics. The high UV transmittance results from an enhanced bulk plasmon effect of the silver microgrid electrode. Photoelectric properties of the sandwich structure with various pore sizes are discussed. The short transport distance of carriers and the low blocking of incoming UV light are the main reasons of the high activity for silver microgrid/TiO₂/ITO sandwich‐structure photodetectors. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the passive electrode signal in X-ray detectors based on superconducting tunnel junctions

X-ray detectors based on superconducting tunnel junctions with the multilayer electrode structure described by the formula Ti/Nb/Al, AlO _x /Al/Nb/NbN were studied. The main signal arose during X-ray absorption in the top electrode and had an energy resolution of ∼90 eV at the 5.9-keV line. The bottom passive Ti/Nb electrode provided rapid absorption of excess quasiparticles. The residual signal of the passive electrode was from 7 to 17% of the main signal amplitude. The dependences of the amplitude of this signal on the voltage and the absorbed X-ray energy were measured for detectors with different thicknesses of the top and bottom electrodes. The rate of quasiparticle trapping by the energy trap in the Ti/Nb bilayer was estimated. The main mechanisms of the formation of the passive electrode signal formation were considered and methods for its suppression were proposed.     

Fast response ultraviolet photoconductive detectors based on Ga-doped ZnO films grown by radio-frequency magnetron sputtering

A metal-semiconductor-metal photoconductive detector was fabricated on c-axis preferred oriented Ga-doped ZnO (ZnO:Ga) thin film prepared on quartz by radio-frequency magnetron sputtering. With a 10 V bias, a responsivity of about 2.6 A/W at 370 nm was obtained in the ultraviolet region. The photocurrent increases linearly with incident power density for more than two orders of magnitude. The transient response measurement revealed photoresponse with a rise time of 10 ns and a fall time of 960 ns, respectively. The results are much faster than those reported in photoconductive detectors based on unintentionally doped n-type ZnO films.     

Radiative heating of thin Al foils by intense extreme ultraviolet radiation

The effect of induced transparency of thin Al foils radiatively heated by intense extreme ultraviolet (EVU) radiation has been observed. The radiation of the plasma of Z-pinches appearing under the compression of tungsten liners at the Angara-5-1 facility has been used as the radiation that heats the Al foil (peak illumination on the foil ~0.55 TW/cm²) and is transmitted through it. The photoabsorption has been studied in the formed aluminum plasma at temperatures of ~10–30 eV in the density range of ~1–20 mg/cm³ in the wavelength range of ~5–24 nm. Absorption lines of Al^4+...7+ ions have been identified in the experimental spectrum. In addition, radiative gas-dynamic simulations of the foil heating and expansion have been performed taking into account radiation transfer processes.     

CuI/Al双层电极的有机场效应晶体管

制备了CuI/Al为源极和漏电极的并五苯基场效应晶体管.相对于纯金属(Al, Au)电极的晶体管,所研制的晶体管的迁移率、阈值电压VT、开关电流比I_on/I_off等参数都有明显改善.研究发现,在Al电极与并五苯半导体之间引入CuI作为空穴注入层,能够明显降低Al电极与并五苯之间的空穴注入势垒.紫外-可见光谱和X射线光电子能谱数据表明,这种空穴注入势垒的降低源自并五苯和Al向CuI的电子转移. 关键词： 有机场效应晶体管 CuI/Al双层源漏电极 电子转移     

退火温度对声表面波检测器电极 表面粗糙度的影响*

该文实验研究了退火温度对声表面波检测器电极表面粗糙度的影响。电极表面的粗糙度随着退火温度不同而变化,实验中分别选择常温(25?C)、200?C和300?C作为退火温度对两种镀膜方式制备的声表面波器件进行退火,最后得到退火温度和电极表面粗糙度的对应关系。从实验结果来看,退火温度为200?C时,得到的电极表面粗糙度最大。该研究为声表面波检测器表面粗糙度优化及灵敏度提升提供了基础。     

应用于气体传感器的具有铝/金电极的单模式两端对声表面波谐振器

为改善气体传感器性能,通过器件优化设计获得了一种应用于气体传感器的具有低损耗、高品质因子(Q)的单模式两端对声表面波(SAW)谐振器。该谐振器由两个换能器、分置于换能器两边的短路栅反射器以及在换能器之间分布的用于敏感膜镀膜的约2.5mm金属薄层构成。谐振器采用铝/金双层电极以降低测试气体环境的腐蚀影响。利用经典耦合模(COM)理论对器件性能进行了仿真以提取优化的结构设计参数。基于仿真结果,实验研制了基于300MHz频率的新型铝/金电极SAW两端对谐振器,测试结果显示所研制器件具有较低损耗(〈7dB),较高Q值(-3000)以及单一谐振模式的特点,并且,以所研制的新型谐振器为频率控制单元的谐振器型振荡器表现出良好的频率稳定度(t15Hz/h),这对于改善气体传感器的检测下限及稳定性等性能指标具有重要意义。     

Optimized preamplifiers for pyroelectric detectors

The performance of pyroelectric detectors largely depends on the design of the preamplifier. In this paper the amplifier parameters affecting detector overall performance will be discussed and some design details improving rise time without sacrificing noise figure will be described. Results achieved with two prototype amplifiers will be presented.     

Photoconducting tellurium for submillimeterwave detectors

Undoped tellurium single crystals show at low temperatures (2.2 K) strong photoeffect under submillimeter irradiation. Therefore, tellurium was used as a FIR-detector. For a comparison with other detectors signal-to-noise ratio, response time, and noise equivalent power were determined. Tests were carried out with HCN-laser radiation ( = 337 m, = 30 cm- t). The sensitivity of the Tc-photoconductor is comparable to that of a TGS-bolometer but lower than that of a Ge-bolometer. The Te-detector is much faster than the bolometers.     

Thin-film photoelectromagnetic detectors for infrared radiation

Using the theory of the PEM effect for thin-film samples, in which the interference of radiation internally reflected from the sample surfaces is possible, the performance of PEM far-infrared detectors is discussed. The results of numerical computation done for CdHgTe at 300K are presented. The voltage responsivity, spectral responsivity, total responsivity, detectivity and time constant are reported as functions of sample thickness, magnetic field, and the semiconductor parameters: carrier lifetime, recombination velocities at illuminated and unilluminated sample surfaces, absorption coefficient and doping level. The thickness of thin-film PEM detectors is one of the more important parameters which can affect their performance.