Noise Characteristics of ZnO-Nanowire Photodetectors Prepared on ZnO:Ga/Glass Templates

In this paper, we report the fabrication of vertically well-aligned ZnO nanowire ultraviolet (UV) photodetectors on ZnO:Ga/glass templates. With 1 V applied bias, it was found that dark current density of the device was only 1.37times10^-7 A/cm². It was also found that UV-to-visible rejection ratio of the fabricated photodetector was around 1000 with a maximum quantum efficiency of 12.6%. It was also found that noise equivalent power and normalized detectivity of the ZnO nanowire photodetector were 5.73times10^-11 W and 6.17times10⁹ cmHz^0.5W^-1, respectively.     

GaN metal-semiconductor-metal ultraviolet sensors with various contact electrodes

Indium-tin-oxide (ITO), Au, Ni, and Pt layers were deposited onto n-GaN films and/or glass substrates by electron-beam evaporation. With proper annealing, it was found that we could improve the optical properties of the ITO layers and achieve a maximum transmittance of 98% at 360 nm. GaN-based MSM UV sensors with ITO, Au, Ni, and Pt as contact electrodes were also fabricated. It was found that we could achieve a maximum 0.12 A photocurrent and a photocurrent to dark current contrast higher than five orders of magnitude for the 600/spl deg/C-annealed ITO/n-GaN MSM UV sensor at a 5-V bias voltage. We also found that the maximum responsivity at 345 nm was 7.2 A/W and 0.9 A/W when the 600/spl deg/C-annealed ITO/n-GaN MSM UV sensor was biased at 5 V and 0.5 V, respectively. These values were much larger than those observed from other metal/n-GaN MSM UV sensors. However, the existence of photoconductive gain in the 600/spl deg/C-annealed ITO/n-GaN MSM UV sensor also results in a slower operation speed and a smaller 3-dB bandwidth as compared with the metal/n-GaN MSM UV sensors.     

Influence of two-tier structuring on the performance of black silicon-based MSM photodetectors

Black silicon, fabricated by alkaline anisotropic etching along with metal-assisted etching, consists of regular distributed micro-scale pyramids and irregular distributed nano-scale pores, in which the pore size plays an important role in the performance of the black silicon-based metal–semiconductor–metal photodetectors (BS MSM PDs). It is found that the dark current characteristic of BS MSM PD is different from that of traditional silicon MSM PD, and the former has negative differential resistance in part of its operating range because of the quantum tunneling effect exists in black silicon. Moreover, it is interesting to note that BS MSM PD with longer metal-assisted etching time (larger nanopore size) has higher responsivity at high bias voltage but lower responsivity at low bias voltage.     

Characterization of AlGaN/GaN Metal- Semiconductor-Metal Photodetectors With a Low-Temperature AlGaN Interlayer

AlGaN/GaN metal-semiconductor-metal photodetectors (MSM PDs) with a low-temperature (LT) AlGaN interlayer (IL) were fabricated. Compared with the conventional AlGaN/GaN MSM PD, it was found that leakage current can be suppressed by insertion of a LT AlGaN IL due to the reduction of surface pits and improvement of crystalline quality. It was also found that larger photoresponsivity can be achieved due to the enhanced electric field strength as a result of inserting a LT AlGaN IL. Furthermore, suppressed photoconductive gain, lower noise level, and larger detectivity of MSM PD can also be achieved by using a LT AlGaN IL.      

GaN-UV photodetector integrated with asymmetric metal semiconductor metal structure for enhanced responsivity

Fabrication of very thin GaN ultraviolet photodetectors on Si (111) substrate integrated with asymmetric (Pt–Ag, Pt–Cr) metal–semiconductor–metal (MSM) structure have been illustrated. Designed GaN photodetection device displays significant enhancement in responsivity for asymmetric (Pt–Ag) MSM structure (280 mA/W) in comparison to symmetric (Pt–Pt) MSM structure (126 mA/W) at 10 V bias. The fabricated asymmetric and symmetric devices also exhibit fast response time in the range of 30–59 ms. The enhancement in responsivity using asymmetric MSM structure ascribed to large difference in work function which lead to change in Schottky barrier height of the metal semiconductor junction. Additionally, power dependent photoresponse analysis of GaN asymmetric (Pt–Ag) ultraviolet photodetector was showing a responsivity of 116 mA/W at low optical power of 1 mW. Such GaN asymmetric MSM ultraviolet photodetectors having high responsivity can extensively be used for low power, high speed ultraviolet photo detection applications.     

A ZnO-nanowire phototransistor prepared on glass substrates

The fabrication of a phototransistor via the bridging of two prefabricated electrodes with a laterally grown ZnO nanowire is reported. It was found that the fabricated device is an n-channel enhancement-mode phototransistor with a dark carrier concentration of 6.34 × 10(17) cm(-3) when the gate voltage is biased at 5 V. With an incident-light wavelength of 360 nm and a zero gate bias, it was found that the noise equivalent power and normalized detectivity (D*) of the fabricated ZnO phototransistor were 6.67 × 10(-17) W and 1.27 × 10(13) cm Hz(0.5) W(-1), respectively. It was also found that the current in the device can be modulated efficiently by tuning the wavelength of the excitation source.     

Gallium arsenide metal-semiconductor-metal photodiodes as optoelectronic mixers for microwave single-sideband modulation

Gallium arsenide (GaAs) metal-semiconductor-metal (MSM) photodetectors have unique properties including high-bandwidth, linearity, and biphase response that make them suitable as mixers and programmable weights for microwave and communications applications. An optical technique for microwave single-sideband modulation that uses GaAs MSM photodiodes as mixers is reported. It uses MSM Schottky photodiodes formed in a GaAs/Al(0.3)Ga(0.7)As materials system to detect microwave in-phase and quadrature signals on optical carriers. Modulation of the photodetector bias voltages results in a single-sideband modulation of the microwave signal. Radio frequency and undesired-sideband suppression of 36 and 27 dB, respectively, were achieved. The optical wavelength was 850 nm, and the bandwidth of the photodetectors was >/=29 GHz.     

GaN and AlGaN metal–semiconductor–metal photodetectors

GaN based interdigital metal–semiconductor–metal (MSM) photodetectors have been successfully fabricated. The MSM structures were patterned on highly resistive GaN and the ternary compound, AlGaN. For the highly resistive GaN detector, the lowest dark current is 0.1 nA and the UV responsivity of the device was about 460 A W⁻¹ at a DC bias of 5 V. The AlGaN with 24% Al exhibited larger gains of up to 10⁶ A W⁻¹ at 20 V, but at a very high dark current, 1 mA, and very long detector responses, greater than 60 s. The high gain in this device is not well understood. The preliminary measurements indicate that tunneling occurs at high electric fields since a negative temperature coefficient for the breakdown voltage was observed.     

肖特基型氮铝镓紫外光电探测器

通过采用插入缓冲层的办法,利用金属有机气相外延(MOCVD)得到高质量的AlGaN薄膜,克服了AlGaN薄膜容易产生裂纹的缺点.在此基础上,我们通过采用传统的紫外光刻和湿法刻蚀的方法,制备得到了金属-半导体-金属(MSM)结构的Al0.25Ga0.75N紫外光电探测器.结果表明,在1 V偏压下,器件的暗电流仅为20 pA,如此低的暗电流主要是由于器件中存在一定量的缺陷而导致电阻过大的原因造成的.器件的最高峰值出现在308 nm,大小为0.07 A/W,器件的上升时间为10 ns,下降时间为190 ns.     

Crabwise ZnO Nanowire UV Photodetector Prepared on ZnO : Ga/Glass Template

Vertical well-aligned and crabwise ZnO nanowires were prepared on patterned ZnO:Ga/glass substrates by reactive evaporation method under different growth conditions. The average length and diameter of vertical well-aligned ZnO nanowires were around 1 mum and 50-100 nm, respectively. In contrast, the average length and diameter of crabwise ZnO nanowires were around 5 mum and 30 nm, respectively. Upon illumination with UV light (lambda = 362 nm), it was found that measured responsivities were 0.015 and 0.03 A/W for the crabwise ZnO nanowire photodetector biased at 10 and 15 V, respectively. Furthermore, a rejection ratio of approximately 10 was obtained for the crabwise ZnO nanowire photodetector with an applied bias of 10 V.     

Surface HCl treatment in ZnO photoconductive sensors

In this paper, we report the effect of surface HCl treatment on ZnO photoconductive sensors with Ni/Au electrodes after the etching process. Epitaxial ZnO photoconductive sensor film on sapphire substrates was fabricated and then treated with different HCl concentrations. With an incident light wavelength of 370 nm and an applied bias of 10 V, the responsivity of the sensor measured is around 141 mA/W after being treated with 0.7% HCl solution. The result indicates that the a larger surface available for photodetection could be realized by increasing the surface roughness of the ZnO photoconductive sensor. It was also found that the low-frequency and high-frequency noises of the fabricated sensors were dominated by 1/f-type and shot noises, respectively.     

A high photocurrent gain in UV photodetector based on Cu doped ZnO nanorods on PEN substrate

Vertical well-aligned Cu-doped ZnO nanorods were successfully synthesized by chemical bath deposition (CBD) method on low cost and flexible polyethylene naphthalate (PEN) substrate. The structural and optical investigations exhibited the high quality of the Cu-doped ZnO nanorods on a flexible PEN substrate. The metal-semiconductor-metal (MSM) configuration was used to fabricate UV photodetector based on the Cu-doped ZnO nanorods grown on PEN substrate. Under a 5 V applied bias, the values of dark current and photocurrent of the Cu-doped ZnO nanorods photodetector were 14.9 µA and 3.27 mA, respectively. Meanwhile, calculated photocurrent gain of the UV photodetector was 219 at 5 V bias voltage. Upon exposure to 365 nm UV light, the UV device exhibited fast response time and recovery time of 0.317 and 0.212 s, respectively, at a bias voltage of 5 V.     

A ZnO nanowire vacuum pressure sensor

In this study, we report the growth and characterization of lateral ZnO nanowires (NWs) on ZnO:Ga/glass templates. Using x-ray diffraction and micro-Raman spectroscopy, it was found that crystal quality of the as-grown ZnO NWs is good. It was also found that the average length and average diameter of the laterally grown ZnO NWs were 5?μm and 30?nm, respectively. A vacuum pressure sensor was then fabricated using a single NW bridging across two electrodes. By measuring the current-voltage characteristics of the samples at low pressure, we found that the currents were of 17, 34.28, 57.37 and 96.06?nA for the ZnO NW measured at 1 × 10(-3)?Torr, 1 × 10(-4)?Torr, 3 × 10(-5)?Torr and 5 × 10(-6)?Torr, respectively. These values suggest that the laterally grown ZnO NWs prepared in this study are potentially useful for vacuum pressure sensing.     

Front and backside-illuminated GaN/Si based metal-semiconductor-metal ultraviolet photodetectors manufactured using micromachining and nano-lithographic technologies

This paper presents the fabrication and characterization of GaN/Si based Ultraviolet (UV) Metal/Semiconductor/Metal (MSM) photodetectors. The thin GaN membranes have been obtained by semiconductor micromachining techniques. The two MSM interdigitated structures are contrived of fingers and interdigit spacings 100 and 200 nm wide respectively, obtained by nanolithographic techniques on GaN. Responsivity measurements were performed using both front side as well as backside-illumination. For front side illumination and for a wavelength of 365 nm and 2.5 V bias the structure with 100 nm wide fingers/interdigit spacing, exhibited the high value of 1.45 A/W. Backside-illumination responsivity of the same structure was ~ 0.37 A/W at the same wavelength and bias. Backside-illuminated photodetctors are interesting in two dimensional UV CCD imaging array manufacturing.     

Positive temperature coefficient of resistance of single ZnO nanorods

ZnO nanorods were synthesized by a simple aqueous solution method. Crystal structures and morphology studies show that the ZnO nanorods are single crystalline with the growth direction aligned with the c axis of ZnO. An Au-ZnO nanorod-Au (metal-semiconductor-metal, MSM) device using the synthesized nanorod was fabricated. An electronic model with two back-to-back Schottky diodes in series with a nanorod was used to describe the electrical transport of the MSM device. A positive temperature coefficient of resistance is observed on a single ZnO nanorod from 383 to 473 K. A simple model has been proposed to explain such an abnormal behavior including the effect of the interface states and the adsorption-desorption of the water/oxygen molecules on the surface of the nanorod.     

Vertically aligned and ordered ZnO/CdS nanowire arrays for self-powered UV–visible photosensing

Photodetectors based on photoconductivity effect are usually driven by an external power source. A self-powered photodetector can be powered by incident light using the photovoltaic effect. Here, photoelectrochemical cells with periodically aligned ZnO/CdS nanowire arrays as photoanodes were fabricated and investigated for detecting UV and visible light. At zero bias, this self-powered UV–visible photodetector showed high responsivities of 35.4 and 23.2 mA/W for UV and visible light, a fast rise time of 0.18 s, and a decay time of 0.32 s. The spectral responses of the self-powered photodetectors based on ZnO/CdS nanowire arrays exhibited superior photoresponse in both UV and visible regions in comparison with ZnO nanowire film and ZnO nanowire arrays. The high photosensing performance originates from the excellent light trapping ability at broadband wavelengths and the high charge collection efficiency of the highly ordered ZnO/CdS nanowire arrays. The results indicate that the ZnO/CdS heterojunctions with periodic nanostructures provide a facile frame for UV–visible detecting applications.     

Vertical single-crystal ZnO nanowires grown on ZnO:Ga/glass templates

Vertical single-crystal ZnO nanowires with uniform diameter and uniform length were selectively grown on ZnO:Ga/glass templates at 600/spl deg/C by a self-catalyzed vapor-liquid-solid process without any metal catalyst. It was found that the ZnO nanowires are grown preferred oriented in the [002] direction with a small X-ray diffraction full-width half-maximum. Photoluminescence, field-emission scanning electron microscopy, and high-resolution transmission electron microscopy measurements also confirmed good crystal quality of our ZnO nanowires. Field emitters using these ZnO nanowires were also fabricated. It was found that threshold field of the fabricated field emitters was 14 V//spl mu/m. With an applied electric field of 24 V//spl mu/m, it was found that the emission current density was around 0.1 mA/cm/sup 2/.     

Enhanced sensitivity and response speed of graphene oxide/ZnO nanorods photodetector fabricated by introducing graphene oxide in seed layer

Graphene/ZnO nanocomposites photodetectors hold great potential for UV detection because of the combination of photosensitive ZnO and high electron-mobility graphene. In this paper, graphene oxide (GO)/ZnO nanorods photoconductive photodetector with seed layer of GO and ZnO nanocrystals (NCs) hybrids is fabricated via a low-cost solution process. Uniform and oriented GO/ZnO nanorods have been obtained due to the positive role of GO in the growth process of ZnO nanorods, which gives rise to less light scattering and thereby stronger absorption and enhanced photocurrent. When the growth time is 1 h, the optimum photocurrent of GO/ZnO nanorods is about 9.4 times than pure ZnO nanorods, meanwhile, the corresponding detectivity reaches 7.17?×?10¹¹ cm Hz^1/2 W^?1. In addition, owing to the high carrier mobility of graphene, the response time t ₉₀ of GO/ZnO photodetector beneficially decreases to ~1 s, which is much faster than many other GO/ZnO hybrid photodetectors.     

ZnO纳米棒/ZnCo2O4纳米片异质结的制备及其光电探测性能

ZnO纳米材料异质结是构筑高性能紫外光电探测器的有力候选之一。本工作中, 设计并制备了一种新型ZnO纳米棒/ZnCo₂O₄纳米片异质结, 研究了其电学性能及光电探测性能。使用油水界面自组装, 将ZnCo₂O₄纳米片在ITO玻璃上组装为均匀的薄膜; 通过调控ZnO种子层厚度, 在ZnCo₂O₄纳米片薄膜上水热生长了取向一致、密度适中的ZnO纳米棒阵列, 获得了高质量的ZnO纳米棒/ZnCo₂O₄纳米片异质结。该异质结具有优良的整流特性, 整流比达到673.7; 其工作在反偏状态时, 光暗电流比超过2个量级, 紫外-可见判别比为29.4, 在光电探测中有良好的波长选择特性。研究表明, 该异质结有潜力应用于构筑高性能紫外光电探测器。     

Optical sensor with transparent conductive oxides electrodes for microposition detection applications

This paper presents an optical sensor structure for microposition detection application using transparent electrodes of indium doped ZnO (IZO). The optical microsensor consists of two linear arrays of metal - semiconductor - metal (MSM) silicon photodetectors with IZO transparent electrodes integrated with a polymer optical waveguide.IZO layers with a thickness of 460-580 nm have been deposited by dc magnetron sputtering technique on silicon epitaxial wafers of 30-50 Ω cm resistivity and a thickness of 23 µm. Due to their high optical transmittance (> 90%) over the 0.4-0.9 µm spectral range, these layers contributed to an increased responsivity of the MSM photodiode structure of about 0.34 A/W, thus improving the optical position microsensor sensitivity.