Dark Current Values Research Articles

Time-varying phenomena in the photoelectric properties of porous silicon

We have undertaken a systematic study of porous silicon with the use of photoconductivity and photoluminescence. During this, we have observed at least three kinds of time-varying photoconductivity, of which two reduce the conductivity and one increases the conductivity during illumination. In addition, we have observed persistent photoconductivity. The time developments of the photoconductivity as well as the persistent photoconductivity depend in intricate ways on parameters such as the wavelength and intensity of the illuminating light, the potential drop across the sample, the surface treatment, and the dark current value. The time scale of these time-varying effects ranges from a few minutes to several hours. The results are discussed in terms of the photoelectric properties of the supporting silicon wafer, diffusion of hydrogen, and photoinduced desorption of hydrogen.

Photocurrents in poly-Si TFTs

Steady-state photocurrents have been measured in poly-Si TFTs fabricated from columnar poly-Si as well as from material crystallised from amorphous LPCVD and PECVD precursors. With top face white light illumination, through the poly-Si gate, the photocurrents from all three technologies were comparable and showed similar trends of increasing photocurrent with film thickness. The photocurrents measured in the thinnest films (600 AA thick) were approximately 7*10-14 A/ µm of channel width/klux. With back face illumination the currents were approximately four times larger. When compared with dark current values of approximately 4*10-14 A/ µm in high quality TFTs, it is apparent that in high light environments, such as LCTV projectors, the photocurrents in unshielded TFTs can be two to three orders of magnitude greater than the dark currents. From the channel length and gate and drain bias dependences, the photocurrent was identified as arising from both optical generation in the drain space charge region and diffusion from the bulk channel. The weak dependence of the photocurrent on drain bias meant that field relieving structures, used for the reduction of dark currents, had a more limited effect upon photocurrents. Spectral photocurrent measurements yielded a value for the recombination lifetime, in a columnar poly-Si TFT, of approximately 2*10-10 s. The detailed spectral response was shown to be due to interference effects in the multiple layer thin film structure.

Hg/sub 1-x/Cd/sub x/Te metal-semiconductor-metal (MSM) photodetectors

Metal-semiconductor-metal (MSM) detectors with active layers of Hg/sub 1-x/Cd/sub x/Te (x=0.62-0.74) and electrode spacings of 2, 4, and 6 mu m have been fabricated and characterized. Direct-current measurements have shown a low dark current and high responsivity from 0.15 to 1.5 A/W at 10-V bias. The lowest values of dark current (0.16 mA cm/sup 2/) were obtained for detectors which incorporated an overlayer of CdTe. For detectors without the overlayer, increasing the Cd mole fraction resulted in a decrease in the dark current and a reduction in the 300-nm responsivity. Measurements of frequency response for these detectors show a maximum loss of 8 dB to 20 GHz. These results compare favorably with high-performance MSM detectors based on In/sub 0.53/Ga/sub 0.47/As with a lattice-matched barrier layer of In/sub 0.52/Al/sub 0.48/As.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Synchrotron-induced surface-photovoltage saturation at intercalated Na/WSe2 interfaces.

The interaction of ultrahigh-vacuum cleaved p-type ${\mathrm{WSe}}_{2}$ (0001) (band gap ${\mathit{E}}_{\mathit{G}}$=1.2 eV) with deposited Na has been investigated by photoelectron spectroscopy using synchrotron radiation and low-energy electron diffraction. At room temperature (RT) the deposited Na diffuses into the bulk of the substrate (intercalation) and creates a compensated degenerate n-doped ${\mathrm{WSe}}_{2}$ surface layer. At low temperatures (LT, 150 K) the deposited Na forms a metallic overlayer. For this condition a maximum surface photovoltage (SPV) of 0.8 eV is induced by synchrotron radiation. Annealing the sample at RT leads to a reduction of SPV and to intercalation of Na. After recooling the sample to LT the synchrotron-induced SPV is saturated (\ensuremath{\approxeq}1 eV). The experimental values of the SPV for the different degrees of intercalation and temperatures are compared with theoretical estimates based on the expected values of the reverse dark current of Schottky diodes and p-n homojunctions. These estimations indicate the detrimental influence of the reverse dark current for the magnitude of the SPV response.

Effects of Plastic Deformation on Charge Transport in Mercuric Iodide Radiation Detectors

AbstractThe effects of bulk plastic deformation of mercuric iodide (HgI2), upon electronic properties which are relevant to the performance of HgI2 as a radiation detector, were examined experimentally. Hole lifetimes, as well as hole and electron mobilities, were measured at various stages of sample deformation. Hole lifetimes decreased by a factor of 2 under strains of several percent; carrier mobilities did not change significantly, except during creep loading where electron and hole mobilities decreased by about 65% and 25%, respectively. Additionally, dark current measurements were made on specimens with varying degrees of accumulated plastic strain; dark current values did not strongly reflect the extent of plastic damage in deformed specimens.

In/sub 0.53/Ga/sub 0.47/As/InP floating guard ring avalanche photodiodes fabricated by double diffusion

In/sub 0.53/Ga/sub 0.47/As/InP separate absorption and multiplication region avalanche photodiodes (SAM-APDs) with doubly diffused floating guard rings have been demonstrated. The planar, front-side illuminated devices are easily fabricated and incorporate strong guarding against edge and surface breakdown. Edge gain is suppressed both by the action of the floating guard rings and by the grading of the p-n junction at the outer edges of the active region that results from the second diffusion. Uniform gains as high as 85 have been measured at multiplied dark currents <100 nA. Multiplied dark currents below 5 nA have been measured at 90% of breakdown, with capacitances below 400 fF for front-side illuminated devices. The low values of dark current and capacitance, as well as the ease of fabrication, make the devices well suited for fiber-optic applications.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Lateral high-speed metal-semiconductor-metal photodiodes on high-resistivity InGaAs

First results are presented on the fabrication and characterization of MSIM photodetectors on high-resistivity Fe-doped InGaAs. The acronym MSIM refers to the fact that the active layer of the photodiodes consists of a semi-insulating (SI) photoabsorbing semiconductor layer. Metal-semiconductor-metal (MSM) photodetectors on high-resistivity InGaAs with a lateral planar structure have been fabricated and characterized. The detectors formed exhibit a low capacitance (<50 fF), a very fast response (>14 GHz), a dark current of about 250 nA at a bias voltage of 10 V, and an external quantum efficiency of 20% at 1.3 mu m. The dark current values of the MSIM devices are only a factor of three larger than the values for MSM photodetectors based on a technologically complex AlInAs/InGaAs superlattice. This result indicates that the Fe doping of the photoactive InGaAs layer yields a significantly increased Schottky barrier. The fabrication of MSM photodetectors on high-resistivity InGaAs is demonstrated. Laterally structured photodiodes are formed with interdigitated contact fingers. The devices show a high response bandwidth and are suitable for high-speed applications.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analytical model for leakage current in InGaAsP avalanche photodiodes

A complete expression for the dark current in InGaAsP/InP avalanche photodetectors is successfully derived by taking into account the diffusion process, the thermal generation process, the tunneling process, and the multiplication process. The expression is computed in order to achieve the I– V reverse characteristics of the device with consideration of the effect of epitaxial layer width, uniform doping concentration, and a doping profile in the active region on the dark current value. Analysis of the computed results indicates that a large tunneling current component appears over the range of 0.75 V B to V B , thereby limiting the effective performance of the device. A trade-off exists among the requirements of a high gain, a low dark current value, a fast response, and a good signal-to-noise power ratio.

Vapor phase epitaxial growth of high purity InGaAs, InP and InGaAs/InP multilayer structures

A trichloride-based vapor phase epitaxial approach for the growth of high-purity InGaAs, InP, and InGaAs/InP multilayer structures is presented. InGaAs lattice-matched to InP is consistently grown with an undoped background level of (1−3)×10 14 cm −3. Undoped background levels in the mid 10 14 cm −3 range are achieved for InP. The difficulties of growing InGaAs/InP structures without contamination of either layer by the reactants of the previously grown layer have been overcome by the introduction of magnetically coupled movable sources and simultaneous trichloride flow controls. This approach alleviates the need for using movable substrate covers or multibarrel reactors. The growth of layers of very uniform composition, doping and thickness over round two-inch diameter substrates is achieved in a single tube reactor with a horizontal rotating substrate holder. InGaAs/InP PIN photodiodes (90 μm diameter) tested at wafer level had mean values of dark current and capacitance of 28 nA and 27 pF, respectively, at − 5V bias.

Light-dependent ion influx into toad photoreceptors.

To measure the influx of Na+ and other ions through the light-dependent permeability of photoreceptors, we superfused the isolated retina of the toad, Bufo marinus, with a low-Ca2+ (10(-8) M), low-Cl- Ringer's solution containing 0.5 mM ouabain. Under these conditions, the membrane potential of the rod is near zero and there is no light-induced potential change either in the rod or in more proximal neurons. The photoreceptors, however, continue to show a light-dependent increase in membrane resistance, which indicates that the light-sensitive channels still close with illumination. Dark-adapted retinas show a larger 22Na+ accumulation than do light-adapted retinas. The extra accumulation of 22Na+ into dark-adapted retinas can be removed if the retinas are washed in darkness with low-Ca2+ Ringer's solutions, or if the ionophore gramicidin D is present in the perfusate. The additional accumulation in dark retinas corresponds to a flux of at least 10(9) Na+ per receptor per second, which is close to the value of the photoreceptor dark current. The light-dependent uptake of 22Na+ can be prevented by exposing the retinas to Ca2+ during the incubation period, but is restored if the phosphodiesterase inhibitor IBMX is added to the perfusate. A significant light-dependent ion accumulation can be observed for the cations K+, Rb+, Cs+, and Tl+, in addition to Na+, but not for methylamine, choline, or tetraethylammonium.

Detection of nighttime atmospheric scattering of lunar UV radiances by the Nimbus 4 BUV instrument

Morphological studies of nighttime dark current data from the Nimbus 4 backscattered UV experiment clearly indicate enhancements during the period of the full moon. This analysis is based on processed and filtered data which have other known perturbations induced by energetic particle events. In order to obtain truly quiescent dark current background values for the 5‐day study intervals centered about several full moon sequences and for the corresponding new moon intervals, the data were further screened so that isolated elements showing spurious or abnormal enhancements could be removed selectively (∼3%). The results demonstrate the presence and uniqueness of a lunar signal, induced by atmospherically backscattered UV radiation. On the Nimbus 4 instrument this signal is not sufficiently strong to provide an accurate map for global nighttime ozone. However, these results do suggest this as a potential application for future, more sensitive instruments.

MIS solar cells: A review

The metal-thin-film insulator-semiconductor (MIS) structure is currently receiving much attention in solar-cell studies. Both theoretical and practical investigations indicate that this structure offers a means of overcoming the principal deficiency of Schottky barrier solar cells, namely low open-circuit photovoltage, while maintaining the attractive features that have led the metal-semiconductor junction to be considered as a possible alternative to the p-n junction for large-area, terrestrial, solar-cell applications. The thin insulating layer allows control over not only the magnitude of the dark current flowing through the diode, but also the dominant type (majority or minority carrier) of this current. Desirably low values of dark current have been postulated for majority carrier devices incorporating suitable charge-trapping centres, located either within the insulator or at the semiconductor-insulator interface, and for minority carrier devices employing suitable insulator thicknesses, metal work functions, and semiconductor resistivities. Theories based on these models are reviewed in this paper and their relevance to explaining photovoltage enhancement in practical Si and GaAs MIS cells is examined. The factors affecting other salient solar-cell properties (photocurrent, fill factor, conversion efficiency) are also considered and suggestions as to the parameters limiting present device performance are given.

Thermionic emission of the GaAs photocathode

The sources of thermionic emission (dark current) from the GaAs/CsO type of photocathode are considered. It is shown that for boat-grown GaAs the dominant emission components are likely to be due to generation of electron-hole pairs, at trapping centers in the band-bending region or via surface states. The expected emission densities are comparable to the dark current of an alkali antimonide photocathode of the S-20 type. For an arbitrary distribution of traps and surface states, the work function should be lowered no more than necessary to give satisfactory photoelectric response. Certain trap distributions should allow simultaneously low values of work function and dark current.