Dark Current Values Research Articles

Ge/Si (100) heterojunction photodiodes fabricated from material grown by low-energy plasma-enhanced chemical vapour deposition

We have fabricated a series of p–i–n Ge/Si heterojunction photodetectors with different thicknesses of the intrinsic Ge layer, different doping levels of the p and n layers and different diode diameters. Epitaxial Ge was deposited on Si(100) using low-energy plasma-enhanced CVD (LEPECVD) followed by cyclic annealing. Dark current values as low as 0.04 mA/cm 2 were achieved for 1 μm thick p–i–n photodiodes on lightly doped substrates at − 1 V bias, and external quantum efficiencies of 56% at 1.30 μm and 44% at 1.55 μm for 3 μm thick p +–i–n + photodiodes on highly doped substrates under 0.5 V reverse bias. For a 30 µm diameter diode a RC frequency of 21 GHz is obtained at a reverse bias of 1 V. With such characteristics, these diodes are attractive for telecommunication and optoelectronic applications.

Feasibility of sequential multiphoton absorption for terahertz radiation detection

AbstractMultilevel architectures based on semiconductor nanostructures appear as promising designs for unipolar midinfrared detectors. In this paper we theoretically investigate their feasibility in the terahertz frequency range. Here, the performances of common quantum‐well photodetectors, based on a single bound subband, are severely hindered by high dark current and low Tblip values. We will demonstrate that, in this respect, the multi‐subband scheme represents a better alternative to the conventional design. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A photon detector with very high gain at low bias and at room temperature

We report on a photon detector aimed at low light detection, which is based on the combination of small sensing volumes and large absorbing regions. Fabricated devices show stable gain values in the range of 1000–10 000 at bias voltages of ∼1V at 1.55μm at room temperature. Submicron devices show dark current less than 90nA and unity gain dark current density values less than 900nA∕cm2. The noise equivalent power (NEP) is measured to be 4fW∕Hz0.5 at room temperature without any gating, which is similar to NEP of current InGaAs∕InP avalanche photodetectors in gated operation.

Transient Current Behavior of Vertically Integrated Amorphous Silicon Diodes

AbstractMonolithic image sensors based on Thin Film on CMOS (TFC) Technology are becoming more and more attractive as an alternative solution to conventional active pixel sensors (APS). Imager with high sensitivity, high dynamic coupled with low dark current values (10-100 pA/cm2 @ 104 V/cm) have been developed. However, issues such as light-induced degradation and image lag hinder the commercial development of a-Si:H based image sensors. The problem of image lag is caused by residual current due to the release of trapped charges after the switch off of the illumination.In this paper, we present a comprehensive study of the transient behavior of the photocurrent in a-Si:H photodiodes deposited on glass, as well as in corresponding diodes implemented in a TFC image sensor when illumination is switched off or periodically varied. The influence of the pixel architecture for two different cases is also analyzed: One setup reproduces the typical 3 transisor APS pixel architecture behavior, in which the bias voltage of the diode varies with the photogenerated charge while the second setup keeps a constant bias voltage applied to the diode by using a charge integrator.The influence of the light-induced defect creation on the performance of the sensors is also presented and discussed.

Theoretical comparison between quantum well and dot infrared photodetectors

The aim of the authors is a performance comparison of quantum dot infrared photodetectors (QDIPs) with quantum well infrared photodetectors (QWIPs) under various operating conditions. This type of photodetector is interesting from the point-of-view that QWIPs have numerous advantages over photodetectors based on HgCdTe. From a quantum structure point-of-view, QDIPs have several advantages over QWIPs, especially their wide spectrum range that can be covered, as well as the availability of absorption normal incident light. However, they still have some problems that include high values of dark current and lower values of their responsivity. For these reasons, there is a need to make a theoretical comparison between the two fundamental types of quantum photodetectors. The more interesting parameters of these detectors to achieve this comparison are calculated. These parameters include dark current, photocurrent, responsivity and detectivity. Numerical results show that QDIPs have a lower value for their responsivity and detectivity for the same operating conditions. The aim is to reduce the dark current by adjusting the doping level of these detectors. Moreover, the effect of lateral size and its associated adaptive value can improve the behaviour of the QDIPs that are processed to some extent.

Optical and Electrical Characterization of GaAs-Based High-Speed and High-Sensitivity Delta-Doped Resonant Cavity-Enhanced HMSM Photodetector

We have previously reported an AlGaAs-GaAs-based delta modulation doped resonant cavity-enhanced, heterostructure metal-semiconductor-metal (HMSM) photodetector design with wavelength selectivity, high sensitivity, and high-speed properties. In this paper, the influences of the resonant cavity and the delta modulation-doped layer on the optical and electrical performances of HMSM photodetectors have been studied by using the transmission line model and Ramo's theorem, respectively. The microcavity offers narrow spectral bandwidth detection accompanied by a drastic increase of the optical field at the resonant wavelength, solving the tradeoff between high efficiency and high speed with the thinner absorption regions. The top AlGaAs delta modulation-doped layer presents an enhanced barrier height associated with the vertically oriented two-dimensional potential and field profiles rather than the horizontally oriented ones, providing very low dark current values and an increase in device responsivity and speed of the response. Optimizing the optical and electric field simultaneously results in high-performance photodetection without further scaling down the devices. Two separate groups of photodetectors with various geometries have been fabricated, characterized, and simulated: One with a delta modulation-doped structure, the other without this doped layer. Delta-doped photodetectors show wavelength selectivity at 850 nm with about a 30-nm full width at half-maximum (FWHM), 9.2 fA//spl mu/m/sup 2/ dark current, less than 30 fF capacitance, 10.6-ps FWHM, and 18.4-ps fall time. Photocurrent spectral response, dark current, time response, and capacitance-voltage measurements consistently show that the delta-doped detectors have the better optical and electrical performances over the undoped ones.

Effects of Silicon Nitride Passivation Layer on Mean Dark Current and Quantum Efficiency of CMOS Active Pixel Sensors

AbstractIn the present study, we have analyzed the influence of the silicon nitride (SiN) passivation layer properties on the mean dark current and the quantum efficiency of CMOS APS (Active Pixel Sensor) through electrical characterization of lot wafers processed with three different SiN passivation films. The SiN layers were characterized by Spectroscopic Ellipsometry (SE) and Fourier Transform Infra Red (FTIR) spectroscopy to get the optical indices and the hydrogen content of the films, respectively. Hydrogen desorption was also studied by Thermal Desorption Spectrometry (TDS) experiments.The different chemical bond concentrations enable to explain the device performance. It is shown that high [Si-H] and low [Si-N] bonds concentrations lead to high hydrogen desorption from the SiN films. Thus, the lowest dark current values have also been obtained with such silicon nitride passivation layers. Consequently, results are in agreement with the hydrogen passivation of defects being responsible of thermally generated carriers. Concerning the quantum efficiency (QE), it is highly influenced by the optical indices of the SiN passivation layer.Actually, the thickness and complex refractive index of the SiN layer affect the light reflection, while the light absorption, in the visible range, is controlled by the [Si-Si] bonds concentration. There are also competing requirements to minimize dark current and to maximize quantum efficiency. In fact, high [Si-H] and low [Si-N] bonds concentrations (for good passivation) are generally observed in silicon-rich SiN films, which also contain high amount of [Si-Si] bonds (inducing high absorption, thus low QE). Increasing simultaneously the [Si-H] and [N-H] bonds concentrations in the passivation layer can be a way to have high hydrogen desorption during passivation anneal just preserving a transparent layer.

High bandwidth-efficiency solar-blind AlGaN Schottky photodiodes with low dark current

Al 0.38Ga 0.62N/GaN heterojunction solar-blind Schottky photodetectors with low dark current, high responsivity, and fast pulse response were demonstrated. A five-step microwave compatible fabrication process was utilized to fabricate the devices. The solar-blind detectors displayed extremely low dark current values: 30 μm diameter devices exhibited leakage current below 3 fA under reverse bias up to 12 V. True solar-blind operation was ensured with a sharp cut-off around 266 nm. Peak responsivity of 147 mA/W was measured at 256 nm under 20 V reverse bias. A visible rejection more than 4 orders of magnitude was achieved. The thermally-limited detectivity of the devices was calculated as 1.8 × 10 13 cm Hz 1/2 W −1. Temporal pulse response measurements of the solar-blind detectors resulted in fast pulses with high 3-dB bandwidths. The best devices had 53 ps pulse-width and 4.1 GHz bandwidth. A bandwidth-efficiency product of 2.9 GHz was achieved with the AlGaN Schottky photodiodes.

Solar-Blind AlGaN-Based p-i-n Photodiodes With Low Dark Current and High Detectivity

We report solar-blind Al/sub x/Ga/sub 1-x/N-based heterojunction p-i-n photodiodes with low dark current and high detectivity. After the p+ GaN cap layer was recess etched, measured dark current was below 3 fA for reverse bias values up to 6 V. The device responsivity increased with reverse bias and reached 0.11 A/W at 261 nm under 10-V reverse bias. The detectors exhibited a cutoff around 283 nm, and a visible rejection of four orders of magnitude at zero bias. Low dark current values led to a high differential resistance of 9.52/spl times/10/sup 15/ /spl Omega/. The thermally limited detectivity of the devices was calculated as 4.9/spl times/10/sup 14/ cm/spl middot/Hz/sup 1/2/W/sup -1/.

Performance analysis of a-Si:H detectors deposited on CMOS chips

ABSTRACTImage and particle sensors based on thin-film on CMOS technology are currently being developed at our laboratory. In this technology, amorphous silicon detectors are vertically integrated on top of dedicated CMOS chips. For both, vision and particle detection, this approach is expected to enhance the performances. In fact very high fill factors, increased sensitivity, and integration level, coupled with extremely low dark current density values can potentially be attained.A first optimization of the a-Si:H diodes (>1mm2) on glass substrates, with the primary focus on reducing dark current densities, gave Jdark values as low as 1 pA/cm2 (at -1 V for 1 m thick detectors). These detectors were then deposited on CMOS readout chips, but so far this step was unfortunately accompanied by an increase in Jdark to values over 10 nA/cm2.Here, the possible cause for such an increase in Jdark as well as possible “remedies” against this effect will be discussed; the principle cause is supposed to be the influence of chip topology. Possible solutions include surface treatments as well as the use of metal-i-p diode configuration. Results obtained so far with these methods are given.

Soft-input decoder for decoding of internally channel coded fiber-optic CDMA communication systems

We propose using a soft-input decoder for the decoding of internally convolutional coded Poisson noise-dominated fiber-optic code-division multiple-access (CDMA) communication systems using optical orthogonal codes. We first compute the coded symbol log-likelihoods at the output of the Poisson noise-dominated channel, which is then used by a soft-input maximum-likelihood decoder, for a fiber-optic CDMA system assuming both on-off keying and binary pulse position modulation schemes. Furthermore, we develop a discrete soft-output channel model for a Poisson noise-dominated channel, with which we evaluate the upper bound on the bit error probability of the internally coded Poisson noise-dominated fiber-optic CDMA system using a soft-input decoder. It is shown that the soft-input decoder significantly outperforms the hard-input decoder. Furthermore, the performance of the soft-input decoder is also evaluated in the presence of different values of dark current.

Developments in the fabrication and performance of high-quality HgCdTe detectors grown on 4-in. Si substrates

We are continuing to develop our growth and processing capabilities for HgCdTe grown on 4-in. Si substrates by molecular beam epitaxy (MBE). Both short-wave and mid-wave infrared (SWIR and MWIR) double-layer hetero-junctions (DLHJs) have been fabricated. In order to improve the producibility of the material, we have implemented an in-situ growth composition-control system. We have explored dry etching the HgCdTe/Si wafers and seen promising results. No induced damage was observed in these samples. Detector results show that the HgCdTe/Si devices are state-of-the-art, following the diffusion-limited trend line established by other HgCdTe technologies. Focal-plane array (FPA) testing has been performed in order to assess the material over large areas. The FPA configurations range from 128 × 128 to 1,024 × 1,024, with unit cells as small as 20 µm. The MWIR responsivity and NEDT values are comparable to those of existing InSb FPAs. Pixel operabilities well in excess of 99% have been measured. We have also explored the role of growth macrodefects on diode performance and related their impact to FPA operability. The SWIR HgCdTe/Si shows similar results to the MWIR material. Short-wave IR FPA, median dark-current values of less than 0.1 e-/sec have been achieved.

Resonant-cavity-enhanced heterostructure metal–semiconductor–metal photodetector

We report a GaAs-based high-speed, resonant-cavity-enhanced, heterostructure metal–semiconductor–metal photodetector with Al0.24Ga0.76As/Al0.9Ga0.1As distributed Bragg reflector operating around 850 nm. The photocurrent spectrum shows a clear peak at this wavelength with full width at half maximum (FWHM) of around 30 nm. At resonance wavelength, a seven-fold increase can be achieved in quantum efficiency compared to a detector of the same absorption depth. The top reflector is a delta modulation doped Al0.24Ga0.76As that also acts as the barrier enhancement layer thus providing very low dark current values. The breakdown voltage is above 20 V. Time response measurements show rise time, fall time, and FWHM of 8.8 ps, 9 ps, and 8.1 ps, respectively, giving a 3-dB bandwidth of about 33 GHz. Combination of low dark current, fast response, wavelength selectivity, and compatibility with high electron mobility transistors makes this device especially suitable for short haul communications purposes.

Dark Current Reduction of Avalanche Photodiode Using Optimized InGaAsP/InAlAs Superlattice Structure

We studied reduction in dark current of avalanche photodiodes (APDs) using an optimized InGaAsP/InAlAs superlattice (SL) structure, which was fabricated by gas-source molecular beam epitaxy. For the sample which has the thickest barrier and the thinnest well, the dark current value of 0.4 µA at a multiplication factor of 10 was achieved. This is the lowest dark current for InGaAsP/InAlAs SL-APDs to our knowledge. This result shows that the band-to-band tunneling current, which affects dark current, was suppressed by increasing the effective band gap energy of the SL multiplication layer. Moreover, we discussed the mechanism of avalanche multiplication and confirmed the reason why sufficient avalanche multiplication occurred in optimized InGaAsP/InAlAs SL-APD even at low electric field. It was concluded that an optimized InGaAsP/InAlAs SL structure effectively enhances electron impact ionization and also improves dark current.

Low-rate super-orthogonal channel coding for fiber-optic CDMA communication systems

In this paper, we consider using practical low-rate error correcting codes in fiber-optic code division multiple-access (CDMA) communication systems. To this end, a different method of low-rate channel coding is proposed. As opposed to the conventional coding schemes, this method does not require any further bandwidth expansion for error correction in fiber-optic CDMA communication systems. The low-rate channel codes that are used for demonstrating the capabilities of the proposed method are super-orthogonal codes. These codes are near optimal and have a relatively low complexity. We evaluate the upper bounds on the bit-error probability of the proposed coded fiber-optic CDMA system assuming both on-off keying and binary pulse position modulation schemes. It is shown that the proposed method significantly outperforms the uncoded systems for various receiver structures such as a correlator with and without hard-limiter and chip-level detector. Furthermore, the performance of the proposed coded fiber-optic CDMA system is also evaluated in the presence of different values of dark current.

Effect of dye aggregation on the photosensitization effect of conducting polypyrrole sensitized with Prussian Blue

Abstract The effect of dye aggregation at the small concentration (0–4 mM/monomer) of dispersed Prussian Blue (PB) on the photocurrents were studied in solid state cells fabricated using conducting polypyrrole (Ppy) coated electrodes sandwiched with solid polymer electrolytes viz. polyvinyl alcohol (PVA) with phosphoric acid. These exhibit good photoresponse to visible light: photocurrents being 25 times more than that of the dark current values depending on the concentration of the sensitizer and the applied field. The sensitization increases with concentration of the dye at low concentrations but at high concentration formation of dye aggregates (H-type) has a negative impact on the photosensitization process. The current–voltage (I–V) characteristics in such cells reveal that the charge transport is mainly governed by the space charge effects and the sensitization effects are due to the lowering of the potential barrier formed at the Ppy/PVA+PB interface under photoexcitation.

Displacement damage effects in InGaAs detectors: experimental results and semi-empirical model prediction

Proton irradiation results are presented for InGaAs multiplexed detector arrays used on the SPOT 4 satellite. Mean and extreme dark current values and random telegraph signal are analyzed and extreme dark current values are fitted with semiempirical models. Prediction of orbital behavior is in good agreement with flight data.

Effect of moisture (in solid polymer electrolyte) on the photosensitivity of conducting polypyrrole sensitized by prussian blue in solid-state photocells

The effect of small concentration of dispersed prussian blue (PB) [in solid polymer electrolyte viz polyvinyl alcohol (PVA) with phosphoric acid] on the photocurrents were studied (in wet state and in dry state of solid polymer electrolyte) in solid state photocells fabricated using conducting polypyrrole (PPy). These exhibit good photoresponse to visible light: photocurrents being 20 to 25 times greater than the dark currents depending upon the concentration of the sensitizer. The current–voltage ( I– V) characteristics in such cells reveals that the charge transport is mainly governed by the space charge effects and the sensitization effects are due to the lowering of the potential barrier formed at the polypyrrole (PPy)/polyvinyl alcohol (PVA) interface under photoexcitation. Also, sensitization effects were studied in the wet state and in dry state of solid polymer electrolyte (PVA). It has been observed that the moisture content in the solid polymer electrolyte reduces the photosensitivity factor ( S= I l/ I d, where I l is the light current and I d is the dark current), since it increases the ionic conductivity in the solid polymer electrolyte thereby increasing the dark current values which results in overall decrease in the photosensitivity factor ( S). The importance of the other factors such as dye concentration, dye aggregation etc. have been explained on the basis of the experimental observations. The sensitization effects are explained on the basis of the energy band diagram of the materials forming the photocells.

Voronoi network modelling of multicrystalline silicon solar cells

A model of a multicrystalline silicon solar cell with columnar grains having different carrier recombination properties is constructed from a two-dimensional Voronoi network. The prismatic grains are approximated by cylinders embedded in an effective medium with diffusion length Le, and the charge collection probability (r) within each grain is calculated by solving a three-dimensional diffusion equation. The value of Le is calculated self-consistently through the integral of over the cell volume. The function also allows the calculation of the contribution of the base to the reverse saturation current of the cell. Numerical examples illustrate the influence of areal inhomogeneities and grain boundary recombination on the value of Le, dark current and open-circuit voltage. It is found that small-area grains with low lifetime mainly reduce the open-circuit voltage without significantly lowering Le and the photocurrent. Histograms obtained for multicrystalline cells by the light beam induced current technique are simulated and compared with experiment.

Role of intermediate temperature molecular beam epitaxy grown GaAs defects in tunneling and diffusion

Current transport in GaAs grown by molecular beam epitaxy in the intermediate temperature (IT-GaAs) range of about 400 °C is investigated. A model is proposed which explains the direct role of deep defects in assisting tunneling from Schottky contacts to semiconductor, where, due to rapidly changing potential, the Wentzel–Kramers–Brillouin approximation cannot be used. The indirect role of defects in diffusion process in the IT-GaAs semiconductor is also investigated. The model is used to explain the dark I–V behavior of metal–semiconductor–metal photodetectors made on unannealed, i.e., as-grown, IT-GaAs. dc responses of annealed and unannealed IT-GaAs are compared showing much smaller values of dark current in low biases in the unannealed device but sharp increase due to defect assisted tunneling at medium bias levels. Since the defect that best fits the model has an activation energy of about 0.5 eV, the annealing behavior suggests that the As interstitial has all the requisite properties of the defect which determines conduction behavior in this material. The observed reduction of dark response by light at high biases, the negative photoresponse, can then be explained based on occupancy of these defects. Device applications include ohmic contacts based on tunneling through IT-GaAs, and, conversely, reduction of tunneling through growth of a barrier layer between Schottky metal and semiconductor.