Increase In Dark Current Research Articles

Particle and X-ray damage in pn-CCDs

The fully depleted pn-junction charge coupled device (pn-CCD) has been developed as a detector for X-ray imaging and high-resolution spectroscopy for the X-ray satellite missions XMM and ABRIXAS. If the detector is exposed to a particle radiation environment, the energy resolution is degraded due to charge transfer losses and a dark current increase. In a first experiment, prototype devices were irradiated with 10 MeV protons. After completion of the detector development, the proton irradiation was repeated for a quantitative study of the radiation damage, relevant for the satellite missions. The irradiation test was extended by a 5.5 MeV α-particle and a 6 keV X-ray exposure of the pn-CCD, including the CAMEX preamplifier chip.

Temperature investigation of dark current and its electrical noise in GaAs/AlGaAs multiquantum well photodiodes

Dark current fluctuations in three types of multiquantum well (MQW) GaAs/AlGaAs avalanche photodiodes have been investigated. This work investigated both forward and reverse biases and furthered the understanding of the physics of noise in MQW structures used in photodetection (avalanche photodiodes) and emitters (semiconductor lasers). Under reverse bias, the increase of dark current and flicker noise compared with a classical PIN junction or Schottky junction is caused by thermionic field effect (TFE) conduction at the MQWs barriers. A method is developed as a function of temperature to detect the TFE at each barrier of the MQW before the onset of multiplication. Decreasing temperature is known to produce an increased resistance limit across an MQW emitter photodiode. Under forward bias, white and flicker noises are governed by the increase of resistance limit.

Design, fabrication, and analysis of crystalline Si-SiGe heterostructure thin-film solar cells

One possible method to improve the efficiency of crystalline silicon (Si) solar cells is by alloying with germanium (Ge). Although the improved absorption of the alloy leads to a gain in the current, the reduction of the alloy bandgap causes a loss in voltage, which overrides the increased current of the SiGe-alloy solar cell. There has been a number of theoretical studies to circumvent this behavior. However, to date there has been no detailed study, which discusses the technological implementation of these concepts in solar cells. In this paper, the design issues of crystalline Si-SiGe heterostructure will be dealt with in an attempt to reduce the effect of the increased dark current of the alloyed cells, while at the same time sustaining the enhancement in the current. The enhanced back surface field at the back p/sup +/-Si/p-SiGe interface reduces the base component of the recombination current of the heterostructure cell if recombination caused by dislocations is neglected. A higher infrared (IR) response which results in a higher short-circuit current (2 mA/cm/sup 2/ higher than a reference Si cell) has been recorded for the Si-Si/sub 0.9/Ge/sub 0.1/-thin-film structure of 15 /spl mu/m thickness. The reduction in dark saturation current which has been predicted based on the theoretical calculations could not be realized in the heterostructure SiGe/Si cell due to the degradation effect of the misfit dislocations that decreases the bulk lifetime, and increases the interface recombination velocity. In a structure which contains a p/sup +/-SiGe buffer layer, an efficiency of 12.5% is achieved for a SiGe cell with 15 /spl mu/m thickness without texturing or optical confinement, which is about the same as the Si reference cell with equal active thickness, but with a higher short-circuit current. These results, for the first time, experimentally prove that alloying with Ge offers a higher current and might have a room for improving the efficiency of the multijunction solar cells or dual bandgap cells when SiGe is used to convert the IR-part of the spectrum.

Radiation damage to charge coupled devices in the space environment

We have investigated the characteristics of radiation damage to charge coupled devices (CCDs) in the space environment. The X-ray astronomy satellite ASCA launched on February 20, 1993 in low Earth orbit carries CCDs specially developed for soft X-ray detection. We have traced the performance of the CCDs for 3 years. We have observed both the gradual decrease of charge transfer efficiency (CTE) and the increase of dark current. These are phenomenologically explained by the increase of charge traps due to irradiation by high energy charged particles. However, some of the effects of the radiation damage in the CCD are quite non-uniform over the chip on various scales. We discuss characteristics of the charge traps and possible origins of the non-uniformity.

Junction characteristics of electron-irradiated Ga0.5In0.5P n+-p diodes and solar cells

The effects of 1 MeV electron irradiation on n+-p mesa diodes and solar cells fabricated from metalorganic chemical vapor deposition grown Ga0.5In0.5P on GaAs have been studied. Upon electron irradiation, Ga0.5In0.5P n+-p junction dark current increased by more than one order of magnitude, but it could be reduced by about 75% after annealing at 250 °C for 10 min. The increase in junction dark current was mainly attributed to reductions in minority carrier lifetimes. It was also found that Ga0.5In0.5P solar cell open-circuit voltage, short-circuit current, fill factor, and air-mass zero conversion efficiency were all reduced on the average by 11%, 18%, 8%, and 33%, respectively, following exposure to 1016 electrons/cm2. However, a significant improvement in the electron-irradiated Ga0.5In0.5P solar cell efficiency was observed via thermal annealing at 250 °C, demonstrating good potential for a longer solar cell lifetime in space through on-orbit thermal annealing. The degradation in solar cell conversion efficiency following 1 MeV electron irradiation was smaller than that observed in Si and GaAs, but greater than that in InP solar cells.

Sulphur passivation of InAs(Sb)

The sulphur passivation for the InAs surface was studied by photoluminescence (PL) and spectroscopic ellipsometry (SE). Effects of the treatment are compared with I- V characteristics and photoresponse (PR) of MESA structure devices fabricated with LPE multilayer of InAs 1− y (Sb y ). It is expected that the dangling bonds at the surface are terminated in the forms of SIn, SSb bonds by the passivation, which results in a remarkable decrease of surface recombination center density. This phenomenon was confirmed by observing increases of PL intensity and of the PR of photodiodes. Four order reductions of dark current and remarkable increase of PR have been observed on sulphur passivated photodetectors. Stability of the effect of the sulphur passivation was also examined by SE as well as PR measurements. SE measurements showed that dielectric function of the surface layer for the InAs wafer has no absorption in the measured spectral range between 1.5 and 5.5 eV. After sulphur passivation, optical absorption appeared due to submonolayer sulphur atoms in good agreement with the results obtained from the InAs(Sb) photodetectors.

Radiation Damage in Si Avalanche Photodiodes by 1-Mev Fast Neutrons and 220-Mev Carbon Particles

AbstractResults are presented of a study on the degradation of the electrical and optical performance of n+p Si avalanche photodiodes, subjected to 1-MeV fast neutrons and to a 220-MeV carbon irradiation. The dark current increases after irradiation, while the photo current decreases. Two dominant hole capture levels, which are responsible for the degradation of performance, are after irradiation observed by DLTS (Deep Level Transient Spectroscopy). The degradation caused by neutron irradiation is smaller than that for carbon irradiation. The differences in the radiation damage are explained by the differences in the number of knock-on atoms and the nonionizing energy loss (NIEL). The recovery behavior of the device performance by isochronal annealing is also reported.

Influence of Front Contact Material on Silicon Heterojunction Solar Cell Performance

ABSTRACTThe emitter of amorphous/crystalline silicon heterojunction (HJ) solar cells is normally very thin. Consequently, the metal used as a front contact can produce a partial or even total depletion of this layer. As a result, the diffusion potential of the p-n junction deviates from its maximum value. In this paper, we report the results concerning HJ in which either metal dots (Au, Al), semitransparent metal layers, or indium tin oxide (ITO) dots or layers were used as front contact on the same HJ structure, namely (p)a-Si:H / (i)a-Si:H / (n)c-Si / Al. We show that, for thin p-layers, the dark and light J-V characteristics of HJ solar cells depend on the material used as front contact. In particular, we found that the dark saturation current increases if a low work function material is used. This increase is interpreted in terms of p-layer depletion, and is shown to directly influence the J-V characteristics under illumination, producing a reduction of the open circuit voltage of solar cells.

Recrystallization of Amorphous Silicon Deposited on Ultra Thin Microcrystalline Silicon Layers

ABSTRACTThis study reports on a method to reduce the thermal crystallization time and temperature of amorphous silicon films by initially depositing an ultra thin μc-Si:H seed layer. After rapid thermal annealing (RTA), films were characterized by means of Raman spectroscopy, x-ray diffraction, reflection high energy electron diffraction, atomic force microscopy, and dark and photocurrent. The results show that the microcrystalline particles in the seed layer act as nucleation centers, promoting crystallization of a-Si:H at lower temperatures and at shorter times, compared to a-Si:H films deposited without any seed layer. Additionally, it was found that the seed layer affects the orientation of the crystallized films. The dark current increases abruptly over 4 orders of magnitude in the first 15 second anneal, then decreases as the time increases, and tends to saturate. The photocurrent has an opposite behavior. These transport results can be understood in terms of a change in defect density and band gap shrinkage.

Conductivity effects in hydrogenated amorphous silicon induced by gamma-ray irradiation

Hydrogenated amorphous silicon films 0.45 and 1.12 μm thick, grown by RF glow discharge plasma PECVD on Corning-7059 glass, were irradiated by a 60Co source (activity 2.8 × 10 15 Bq) in the dose range 10–450 kGy at different dose rates. The dark current of the samples, under 100 V applied voltage, was measured before, during and after gamma irradiation. When the irradiation starts, the dark current increases up to a level linearly dependent on the dose rate. When the irradiation is switched off, the dark current returns close to the initial value in a few days. Such behaviour suggests a possible application of hydrogenated amorphous silicon films in dosimetry and as gamma ray sensors. Moreover, the film optical constants were computed from the transmittance spectra measured in the range 500–2500 nm, before and after irradiation. After irradiation, the film optical absorption is slightly increased and the energy gap is substantially unchanged.

Temperature-humidity-bias-behavior and acceleration model for InP planar PIN photodiodes

The reliability of InP planar PIN photodiodes in humid ambients has been studied. The dependence of the degradation rate in temperature, humidity and bias was determined by aging devices at temperatures between 50 and 150/spl deg/C, humidities between 50 and 85% RH, and biases between 2 and 25 V. Failure occurred as a result of a sudden increased dark current. This increase in dark current had little affect on the device responsivity. At all aging conditions the failure distributions were well represented by a lognormal distribution in time with a dispersion of less than 0.3. From these data, an acceleration factor (AF) was developed and is given by AF=exp(E/kT)exp[A(RH)/sup 2/]exp[B(V)] where E=-0.42 eV, A=-4.6/spl times/10/sup -4/, and B=-6.7/spl times/10/sup -2//V. Extensive failure mode analysis was done on more then 500 failed devices. Based on this, a failure mechanism was proposed. The model requires ingress of moisture to the InP surface. The moisture is reduced at the p-contact region of the device region, giving off hydrogen. Under negative bias In and P react with the hydrogen to form gaseous IH and PH/sub 3/. This lead to the semiconductor erosion. The erosion continues until device failure occurs. The worst case concentrate of PH/sub 3/ which if created would produce less than 0.1 ppb PH/sub 3/ per cm/sup 3/ of air. Thus it is not a fire or health hazard. Finally, we present reliability prediction for nonhermetic PIN's encapsulated in an optically clear silicone based polymer. We estimate a 20 year hazard rate of less than 100 FIT's for devices operating at an ambient of 45/spl deg/C/50% RH. For comparison, the hermetic counterparts of similar design have a 20 year hazard rate of less than 10 FIT's.

2-D Simulation and New Structures of Heterojunction Phototransistor for Higher Optical Amplification in Optoelectronic Integrated Devices

The improvement of optical gain is an important priority for performance design in most of optoelectronic integrated devices (OEIDS). A two-dimensional simulator is developed for the rigorous analysis and optimum design of devices. With it, we analyze the characteristics of heterojunction phototransistor (HPT) which acts as a light-detector in OEID, and we propose new structures of HPT to increase the conversion gain without much increase in dark current. Then, the simulation of an OEID composed of an HPT integrated over a light-emitting diode (LED) is implemented, and it is shown that the new structures significantly improve the optical amplification for low input light power over the conventional structure.

The Dark Current at the TiO2 Electrode of a Dye-Sensitized TiO2 Photovoltaic Cell

One of the major factors limiting the performance of the dye-sensitized TiO2 photovoltaic cell is the rapid increase in dark current at potentials above 0.5 V. This current is due to the reduction of the oxidant within the electrolyte at the dye-sensitized TiO2 electrode. Owing to the porous nature of the TiO2 film, the dark reaction can occur at either the TiO2 surface or the surface of the conducting glass. Dark current measurements were made on Libbey -Owens-Ford TEC 10 conducting glass and on the same glass coated with electrophoretically deposited Degussa P-25 TiO2 covered with a monolayer of the sensitizing dye cis-di( thiocyanato ) bis (2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) in an electrolyte consisting of the I3-/I- couple in 1:1 propylene carbonate/ethylene carbonate. The contribution to the dark current resulting from the reaction at the surface of the conducting glass was relatively small at ambient temperatures. The dark reaction at the surface of the dye-sensitized TiO2 was found to make the major contribution to the dark current under these conditions. Tentative results for the Tafel parameters which indicate that the dark reaction occurs via the reduction of both I3- and I2, which are in equilibrium, are also presented.

Improved thermal stability of In/sub 0.53/Ga/sub 0.47/As metal-semiconductor-metal photodetectors with Al/sub 2/O/sub 3/ interfacial layer

The thermal stability of In/sub 0.53/Ga/sub 0.47/As metal-semiconductor-metal photodetectors has been improved by using thin electron-beam evaporated Al/sub 2/O/sub 3/ interfacial layers of different thickness. The addition of the interfacial layer allows for an increase in anneal temperature from 250/spl deg/ to 350/spl deg/C with a four-fold dark current increase. Measurements of impulse responses at 1.3 /spl mu/m showed, that the frequency behaviour is not adversely affected by the increased capacitance caused by oxide charges, since metal-semiconductor-metal photodetectors are carrier transit-time limited. >

Fundamental investigation of vacuum PD tube [photodiode

As a fundamental study of photodiodes (PDs) for electron bombardment, two types of PDs have been experimentally investigated to be applied in electron tubes. A PD bombarded from the front surface (FB-PD), where a pn junction of planar structure existed, was evaluated to measure the fast response characteristics such as a 2.1 ns rise time. However, more than an order of magnitude increase of dark current was also confirmed after a long term stability test of 1000 hours. On the other hand, a PD bombarded by electrons from the rear surface (RB-PD) showed no increase of dark current after the stability test and a fast rise time of 2.7 ns. However, it was clarified that the rise time of RB-PD depended on the applied voltage to the PD, and an applied voltage of 200 V was necessary to achieve such a fast response. Since it was a much higher voltage than expected, some modifications may be necessary to achieve a fast response with a lower applied voltage, considering the yield of the PDs. Comparisons of the two types of PDs for some other characteristics are discussed. Preliminary test results of an electron bombardment APD are also discussed. >

Defects Induced in p‐Type Silicon by Photocathodic Charging of Hydrogen

Capacitance‐voltage (C‐V) and current‐voltage (I‐V) measurements were used to study the effects of photocathodic charging of hydrogen in boron doped crystalline silicon. The electrolyte was a 5% hydrofluoric solution. Within the first minutes of hydrogenation, a negative charge variation occurs at the surface, due to the formation of a thin hydride‐like layer. The hydrogen charging over an extended period of time (≃30 h) results in a drastic increase in the reverse dark current . This effect can be totally suppressed when the electrodes are stepped to rest‐potential (0 bias) for about 20 min. The resulting I–V characteristics are similar to those obtained for a nonhydrogenated sample. However, the surfaces exhibit a rough aspect, and ellipsometry measurements show that an amorphized layer is present. Secondary ion mass spectrometry measurements reveal that hydrogen (deuterium) is also injected up to 100 nm depth. It is concluded that an amorphous hydrogenated layer forms during an electrolytical charging, due to the incorporation of a high density of hydrogen atoms within the surface region. The raise of is ascribed to hydrogen‐induced defects, operating as electron‐hole generation centers. This layer partly dissolves when the cathodic polarization is removed (0 bias condition) and consequently vanishes.

Electrical switching in AuPbPcAu thin film sandwich devices

Electrical switching has been observed in Au-PbPc-Au thin film sandwich devices. The d.c. dark current increases over a period of 10 s at a critical applied bias prior to switching to a higher impedance state. This behaviour is repeated at successively higher critical voltages. Arrhenius plot measurements indicate an activation energy which suggests that oxygen acceptor levels are not involved. However, annealing leads to a higher activation energy suggesting that the switching process may be associated with a thermally activated localised structure change in the film.

In0.53Ga0.47As p-i-n photodiodes with transparent cadmium tin oxide contacts

A new type of p-i-n In0.53Ga0.47As photodiode having an optically transparent composite top electrode consisting of a thin semitransparent metal layer and a transparent cadmium tin oxide (CTO) layer was investigated. The composite functions as the n or p contact, an optical window, and an antireflection coating. The transparent contact also prevents shadowing of the active layer by the top electrode, thus allowing greater collection of incident light. Since the CTO contact is nonalloyed, interdiffusion into the i-region is not relevant avoiding an increased dark current. The photodiodes exhibited leakage currents of ≤8 nA and some as low as 23 pA, with reverse breakdown voltages of ≥15–17 V. Responsivity was measured using a 1.55 μm InGaAsP diode laser focused onto an unpassivated 60 μm diam p-i-n photodiode and was ≥0.41 A/W. Photoresponse of the diodes to 3 ps pulses from a Nd:YLF laser (λ=1.047 μm) was 169 and 86 ps for the 60 and 9 μm diodes, respectively. The maximum frequency response of the 9 μm diode is packaging limited, and is expected to have an intrinsic response time of 20–30 ps.

The role of calcium in light adaptation in vertebrate rods

The mechanisms by which sulfhydryl (SH-) binding reagents modulate the light-sensitive conductance of retinal rods were investigated by current recording from single rods, by patch clamp recording from the plasma membrane of the rod outer segment (ROS), and by biochemical study of their effects on the light-induced hydrolysis of cyclic GMP. The electrophysiology, as well as measurements of the reagents' ability to traverse the ROS plasma membrane, was done on amphibian (Rona and Ambystoma) rods, and the biochemistry on bovine rods. The main SH-reagents used were N-ethyl-maleimide (NEM) and iodoacetamide (IAA). Both transiently increased rod current, but part of the large current could not be turned off by light. After a few minutes' exposure, NEM, but not IAA, caused a continuous decay of the rod's light sensitivity. In patch-clamp recordings from the ROS plasma membrane, the reagents increased conductivity both in the presence and absence of cGMP, consistent with the observation that the drug-induced current increase in intact rods involved both light-sensitive and light-insensitive components. In vitro, NEM was found to be a powerful inhibitor of cGMP hydrolysis, which can explain the gradual loss of light sensitivity in the rod and could initially contribute to the increased dark current via elevated cGMP levels. Thus, SH-reagents act both by modifying the light-sensitive channel and by inhibiting phototransduction inside the rod.

Degradation mechanisms of gamma irradiated LWIR HgCdTe photovoltaic detectors

Planar n/sup +/p Hg/sub 1-x/ Cd/sub x/Te (x=0.23) photodiodes passivated with ZnS were irradiated by a Co/sup 60/ gamma source. A strong increase in the reverse dark current was observed for doses above 0.3 Mrad(air). A similar effect is found by exposing the photodiodes to UV illumination from a high-pressure mercury lamp. By filtering the UV light it is shown that the degradation in the performance of the photodiodes is caused by the light or radiation absorbed in the ZnS layer above the implanted n-type region. C-V measurements of irradiated MIS devices showed a significant increase in the fast surface state density. Galvanomagnetic and lifetime measurements made on irradiated p-type HgCdTe layer showed no significant changes in the bulk transport parameters. Based on these findings, a model for the degradation mechanism is proposed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>