Large Dark Current Research Articles

Photoconductive gain in solar-blind ultraviolet photodetector based on Mg0.52Zn0.48O thin film

An ultraviolet photodetector was fabricated on MgZnO thin film grown by metal-organic chemical vapor deposition. The peak response of the device centers at 238 nm and cutoff wavelength is 253 nm. The peak responsivity is 129 mA/W at 15 V bias, and the UV/visible reject ratio is 4 orders of magnitude. Internal gain is due to the hole trapping at interface that brings low response speed. Native defects at the Au/MgZnO interface degrade the barrier effect, which caused large dark current and high visible response.

Solar blind metal-semiconductor-metal ultraviolet photodetectors using quasi-alloy of BGaN/GaN superlattices

Large internal gains that can be obtained in wide band gap semiconductors-based (GaN and ZnO types) Schottky and/or metal-semiconductor-metal photodetectors are generally accompanied by large dark current and time response. We show that, using quasi-alloy of BGaN/GaN superlattices as the active layer, the dark current can be lowered while maintaining high internal gain (up to 3 × 104) for optical power in the nW range and low time response (few tens of ns) for optical power in the W range. Furthermore, the boron incorporation allows the tuning of the cutoff wavelength.

Electron self-injection into an evolving plasma bubble: Quasi-monoenergetic laser-plasma acceleration in the blowout regime

An electron density bubble driven in a rarefied uniform plasma by a slowly evolving laser pulse goes through periods of adiabatically slow expansions and contractions. Bubble expansion causes robust self-injection of initially quiescent plasma electrons, whereas stabilization and contraction terminate self-injection thus limiting injected charge; concomitant phase space rotation reduces the bunch energy spread. In regimes relevant to experiments with hundred terawatt- to petawatt-class lasers, bubble dynamics and, hence, the self-injection process are governed primarily by the driver evolution. Collective transverse fields of the trapped electron bunch reduce the accelerating gradient and slow down phase space rotation. Bubble expansion followed by stabilization and contraction suppresses the low-energy background and creates a collimated quasi-monoenergetic electron bunch long before dephasing. Nonlinear evolution of the laser pulse (spot size oscillations, self-compression, and front steepening) can also cause continuous self-injection, resulting in a large dark current, degrading the electron beam quality.

Performance of novel materials for radiation detection: Tl3AsSe3, TlGaSe2, and Tl4HgI6

Abstract In this paper we report on the electrical characteristics of three novel ternary compounds, Tl 3 AsSe 3 (TAS), TlGaSe 2 (TGS), and Tl 4 HgI 6 (THI), pertaining to their use as radiation detectors. The details for growth and material characterization are not presented. A semiconductor based gamma ray detector requires a material with high Z , high density, high resistivity, appropriate bandgap (1.5–2 eV), low energy/electron–hole pair, and a high μτ product. CZT is currently the best semiconductor material for room temperature gamma ray spectroscopy; however, it is extremely difficult to produce large volumes of detector grade material, making it expensive and in limited supply. DNDO/DHS began searching for other materials that might perform as well as CZT but be easier to grow and in the end lower the cost. For this purpose, we investigated the above three materials as possible replacements for CZT as gamma ray detectors. The bulk resistivity, I – V curves, X-ray response, and gamma ray response measurements for doped and undoped crystals are presented and discussed. TAS shows good X-ray and gamma ray response, but has poor resistivity, which results in large dark current and poor spectral response. TGS has good resistivity, but shows poor X-ray and gamma ray response. THI has excellent resistivity, shows some X-ray and gamma ray response, and has great potential as a gamma ray detector.

15 μm Quantum Well Infrared Photodetector for thermometric imagery in cryogenic windtunnel

Quantum Well Infrared Photodetector (QWIP) usually suffer from a too moderate quantum efficiency and too large dark current which is often announced as crippling for low flux applications. Despite this reputation we demonstrate the ability of QWIP for the low infrared photon flux detection. We present the characterization of a state of the art 14.5 μm QWIP from Alcatel-Thales III–V Lab. We developed a predictive model of the performance of an infrared instrument for a given application. The considered scene is a cryogenic wind tunnel (ETW), where a specific Si:Ga camera is currently used. Using this simulation tool we demonstrate the QWIP ability to image a low temperature scene in this scenario. QWIP detector is able to operate at 30 K with a NETD as low as 130 mK. In comparison to the current detector, the operating temperature is 20 K higher. The use of a QWIP based camera would allow a huge simplification of the optical part.

A novel photon detector made of silicon and carbon nanotubes

The use of silicon and the development of micro-technology made possible the transition from macro- to micro-detection systems and of very large-scale integration systems for microelectronics. The most powerful micro-detector recently developed is the Silicon PhotoMultiplier (SiPM) that allows a pixel dimension of microns in a hundred-pixel array. The use of nanotechnology opens now a new field of applications in which materials are “built” chemically up to the desired dimensions. This process allows the realization of complex structures that have finely pixelled sub-micron dimensions and that are cheap, light, and easy to produce. Between the materials suitable for nanotechnology, carbon nanotubes (CNT) play a main role because of their facility of being produced and their unique mechanical and electrical properties. They can be grown chemically in a very easy and cheap way, assembled in the desired geometry and directly connected to readout electronics devices. In addition, they can be coupled to silicon substrates to obtain mixed micro–nano structures with intermediate electronic properties. The first radiation detector using CNTs grown through a chemical vapour deposition (CVD) process has been realized using a sapphire substrate. This device, sensitive to the radiation in the range from 220 to more than 850 nm, exhibits a relevant increase in the photocurrent toward UV wavelengths both with continuous light and with pulsed radiation. This opens the door to the realization of a new kind of low-cost radiation detector with sub-micron spatial resolution and high sensitivity in the UV radiation region. In order to avoid the large dark current observed, a new detector has been realized using an p-doped silicon substrate. Electrical and optical properties of this novel detector have been intensively studied as well as the coupling between nanotubes and silicon. The strong matching found between the two different materials suggests the possibility of realizing a single photon detector with high quantum efficiency in the UV wavelength region.

Enhanced Characteristics of In0.5Ga0.5As Quantum Dot Infrared Photo Detector with Hydrogen Plasma Treatment

Device characteristics of In0.5Ga0.5As/GaAs quantum dot infrared detector (QDIP) have been enhanced with hydrogen plasma treatment. After the hydrogen (H) plasma treatment, the dark currents were noticeably decreased and photoluminescence (PL) intensity was increased by H-passivation of interfacial traps between quantum dots and GaAs and of non-radiative defect centers caused during QD growths. Photo response, which could not be observed in as-grown QDIP due to large dark currents which obscured the photocurrent signal, was measured successfully after H-treatment due to H-passivation.

High sensitivity organic photodiodes with low dark currents and increased lifetimes

Organic semiconductors hold the promise for large-area, low-cost image sensors and monolithically integrated photonic microsystems. This requires the availability of photodiodes offering at the same time high quantum efficiency, low noise and long lifetimes. Although published structures of organic photodiodes offer high external quantum efficiencies (EQE) of up to 76% [F. Padinger, R.S. Rittberber, N.S. Sariciftci, Effects of postproduction treatment on plastic solar cells, Advanced Functional Materials 13 (2003) 1, P. Schilinsky, C. Waldauf, C.J. Brabec, Recombination loss analysis in polythiophene based bulk heterojunction photodetectors, Applied Physics Letters 81 (20) (2002) 3885], [1,2] they normally suffer from short lifetimes of only a few hundred hours as well as large dark currents. In our work the lifetime of a poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) heterojunction photodiode structure was increased to several thousand hours by omitting the widely used poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) anode layer. In addition, a simple model of optical interference and absorption effects was used to find the optimum thickness that combines high quantum efficiency with low dark current. As a result, we report on organic photodiodes with state-of-the-art EQE of 70% at 0V bias, an on/off current ratio of 106 at −1V and 40mW/cm2 illumination, dark current densities below 10nA/cm2 at −1V, and a lifetime of at least 3000h.

ZnSe homoepitaxial MSM photodetectors with transparent ITO contact electrodes

We report the homoepitaxial growth of ZnSe layers on ZnSe substrates by molecular beam epitaxy (MBE). It was found that we can only observe an extremely strong ZnSe (004) x-ray peak with a full-width-at-half-maximum of 21.5 arcsec, which is much smaller than that observed from ZnSe grown on GaAs substrates. Photoluminescence and Hall measurement also indicate that the quality of our homoepitaxial ZnSe layers is good. ZnSe-based homoepitaxial metal-semiconductor-metal photodetectors with transparent indium-tin-oxide (ITO) contact electrodes were also fabricated. It was found that although ITO transparent contact electrodes can result in large photon absorption and large photocurrents, the low Schottky barrier height between ITO and homoepitaxial ZnSe would also result in relatively large dark currents. With an incident wavelength of 450 nm and a 1-V applied bias, it was found that the maximum responsivity is about 0.13 A/W, which corresponds to a quantum efficiency of 35%. Furthermore, it was found that the detector responsivity drops by more than two orders of magnitude across the cutoff region.

Influence of rapid thermal annealing on a 30 stack InAs/GaAs quantum dot infrared photodetector

In this article the effect of rapid thermal annealing (RTA) on a 30 stacked InAs/GaAs, molecular beam epitaxially grown quantum dot infrared photodetector (QDIP) device is studied. Temperatures in the range of 600–800 °C for 60 s, typical of atomic interdiffusion methods are used. After rapid thermal annealing the devices exhibited large dark currents and no photoresponse could be measured. Double crystal x-ray diffraction and cross sectional transmission electron microscopy studies indicate that this could be the result of strain relaxation. V-shaped dislocations which extended across many quantum dot (QD) layers formed in the RTA samples. Smaller defect centers were observed throughout the as-grown sample and are also likely a strain relaxation mechanism. This supports the idea that strained structures containing dislocations are more likely to relax via the formation of dislocations and/or the propagation of existing dislocations, instead of creating atomic interdiffusion during RTA. Photoluminescence (PL) studies also found that Si related complexes developed in the Si doped GaAs contact layers with RTA. The PL from these Si related complexes overlaps and dominates the PL from our QD ground state.

Conductive liquid crystalline compounds having a piperazine structure

New conductive thermotropic liquid crystalline materials having a piperazine ring in the central core: 1-[4-(9-decenyloxy)phenyl]-4-alkylpiperazines (6) were synthesized. The mesomorphic behaviours of these compounds and their conductivity in the liquid crystal phase were measured. The principal features of these compounds are to exhibit a smectic B phase around room temperature (for example 6: Cr 50 SmB 81 I, °C) and to exhibit a large dark current (6d: 430 μA cm−2, applied voltage 20 V, at 70°C) in the smectic B phase.

An mK×nK bouwblok CCD image sensor family. I. Design

This paper presents a bouwblok family of CCD image sensors, a set of devices of varying size built with a single pixel design from one mask set. It describes the original design goals and how they led to dividing the layout into smaller blocks that could be stitched together to form a complete sensor. The architecture is traditional for large-area scientific devices. The pixel design is optimized for large charge packets, antiblooming and low dark current, the horizontal register for binning, and the output amplifier for fast pixel rates. The paper describes how the design is split and modified to cover all possible configurations and how to minimize the effects of stitching.

The Poisson multiple-access channel

The Poisson multiple-access channel (MAC) models many-to-one optical communication through an optical fiber or in free space. For this model we compute the capacity region for the two-user case as a function of the allowed peak power. Focusing on the maximum throughput we generalize our results to the case where the users are subjected to an additional average-power constraint and to the many-users case. We show that contrary to the Gaussian MAC, in the Poisson MAC the maximum throughput is bounded in the number of users. We quantify the loss that is incurred when time-division multiple access (TDMA) is employed and show that while in the two-user case and in the absence of dark current the penalty is rather mild, the penalty can be quite severe in the many-users case in the presence of large dark current. We introduce a generalized TDMA technique that mitigates this loss to a large extent.

Long-wavelength λc=18 μm infrared hot-electron transistor

A long-wavelength λc=18 μm infrared hot-electron transistor (IHET) with low dark current is demonstrated. In order to achieve long-wavelength absorption, a low barrier height is required, which in turn results in a large dark current. Therefore, operation of a normal long-wavelength quantum-well infrared photodetector (QWIP) structure is limited to very low temperatures and biases due to the thermally activated dark current. In the IHET, a high-energy pass filter placed after 30 periods of GaAs/AlGaAs quantum wells blocks the temperature-activated dark current while allowing high-energy photoexcited electrons to pass and be collected as photocurrent. A comparison of the dark current to the 300 K background photocurrent shows that the QWIP structure without the high-energy pass filter demonstrates background-limited infrared photodetection (BLIP) only at T≤35 K. Furthermore, in order to avoid saturating a typical readout circuit, detector operation of the QWIP is restricted to biases less than 0.08 V at 35 K. In contrast, the filtered dark current in the IHET is reduced by two to four orders of magnitude such that BLIP performance can be achieved for temperatures up to T=55 K without saturating the readout circuit. Because of the preferential current filtering effect, the noise equivalent temperature difference of the IHET can be improved by a factor of 100 at T=55 K. The dark-current-limited detectivity was found to be D*=1×1010 cm Hz1/2/W at λp=15 μm, Ve=−0.2 V, and T=55 K.

Thin-film photosensor design for liquid crystal spatial light modulators

Liquid crystal (LC) spatial light modulators (SLMs) are addressed optically with semiconductor thin-film photosensors incorporated into the devices. Nematic LCs, which are insensitive to the polarity of the applied voltage, are addressed by optically modifying the effective resistance of the photosensors to be much smaller than or much larger than a threshold value. Much faster ferroelectric LCs, which are polarity sensitive, are addressed by supplying sufficient photogenerated charge. Because the spatial resolution of the devices decreases rapidly with increasing mobility of carriers at the photosensor/LC interface, very low mobilities, less than 1 cm2 V-1 s-1, are required. Photodiodes of hydrogenated amorphous silicon in p-i-n, Schottky, and metal insulator semiconductor configurations form practical photosensors for optically addressed SLMs. Thin-film photoconductors, having nonblocking contacts, cannot be used in many cases because of their large dark currents.

Evaluation of low dark current InSb photovoltaic detectors

This paper reports on the fabrication and evaluation of high-quality InSb photovoltaic detectors. Etchpits on the <111 > face of the InSb substrates were studied. This distribution usually shows a “T” shape. Detectors fabricated in an etchpit rich region show a large dark current while those from a pit-free region show low dark current. This is because the tunneling current becomes dominant at different reverse bias levels for devices fabricated in the different regions. We also present what are the first reported effects of a reverse injection current on the performance of these detectors. The breakdown voltage of the detector increases after the injection process. However, the dark current at small reverse bias (−200 mV) increases and then decreases with an increase of the reverse injection current. This result may be due to hole trapping in the passivation layer during the injection process.

Electrochemical Behavior of p-Type Indium Selenide Single Crystal Electrodes in Dark and under Illumination

Abstract The electrochemical behaviors of van der Waals face of p-InSe single crystal electrodes in 0.5M H2SO4 and 1M NaOH (1M=1 mol dm−3) with and without illumination have been thoroughly investigated. In 0.5M H2SO4, relatively large cathodic dark current was observed and the onset potentials for the current in dark and under illumination were ca. −0.8 V (vs. Ag/AgCl) and ca. −0.5 V, respectively, at as-prepared electrode. The current seemed to be mainly due to hydrogen evolution reaction. The dark current decreased significantly and the photo-current increased by removing surface steps. These results and impedance analyses show that the surface steps create surface states which act as recombination centers and mediators for charge transfer. Both the dark- and photocurrent onset potentials in 1M NaOH were more negative than those in 0.5M H2SO4 by ca. 0.5 V. The dark current was quite large and the photocurrent was small in 1M NaOH even after removing surface steps. By depositing Pt on p-InSe, the photocurrent onset potential shifted towards positive by several hundred mV. The reaction mechanism of photoelectrochemical hydrogen evolution was discussed by considering the above results.

II-VI infrared superlattices

Superlattice materials systems have recently been proposed to overcome the inherent problems of large tunnel and dark currents that limit the operation of infrared detectors at longer wavelengths (≥10 μm). We have utilized the envelope function approximation and a simple two-band Kane model to calculate the dependence of cutoff wavelength and effective mass on specific layer thicknesses for the HgTe-CdTe materials system. Infrared absorption coefficient calculations have been carried out on this system (with Eg =0.1 eV) and compared with both HgTe and 0.1-eV HgCdTe alloy. Infrared device performance for various proposed superlattice systems is predicted from a consideration of effective mass and absorption coefficient values. Superlattice systems involving materials with the inverted gray tin band structure are seen to offer the greatest potential for future infrared devices.

Photoelectrochemical properties of polycrystalline TiO 2 electrodes: Anomalous photoeffects

We combined optical and impedance measurements with conventional electrochemical techniques to investigate the photoprocesses that occur on polycrystalline TiO 2 electrodes in the presence of reducible adsorbates in solution. The semiconductor-electrolyte junction is treated as a simple Schottky barrier. The photopotential, photocurrent and capacitance are described using this model. The observed anomalous large cathodic photoeffects on n-type TiO 2 result from either anodic photoproduction or the diffusion of the dissolved oxygen adsorbed on the electrode surface. Anomalous photocurrents occur together with relatively large dark currents, which are caused in n-type semiconductors by electron transfer from the conduction band to the interface via tunnelling through the space charge layer. The influence of the surface states on the determination of the flat-band potential by measurements of either the photocurrent or the impedance was also examined. It was found that reliable results could be obtained by scanning from cathodic to anodic potentials after prepolarization at -1.5 V. The value determined in this way for the flat-band potential was -0.5±0.05 V.

Sputtered Schottky barrier solar cells on p-type GaAs

Schottky barrier solar cells have been produced on single crystal p-type GaAs by sputtering gold to form the barrier. The rectifying action for the gold sputtered p-type device is unique to the method since normal thermal evaporated gold onto p-type produced Ohmic contacts. The opposite behavior was observed for n-type GaAs. Barrier heights have been measured for both p-type (sputtered) and n-type (thermal) diodes using current-voltage and capacitance-voltage methods and are 0.90 and 0.95 eV, respectively. The power conversion efficiencies without AR coatings have values of 6.56% (p) and 5.58% (n). Deep level transient spectroscopy has been used to identify the trap center concentrations and energy levels for both diodes to account for the relatively large dark currents in the p-type (sputtered) diodes.