Reduction Of Dark Current Research Articles

フェムト秒電子シングルバンチの生成・計測・利用

A new S-band laser photocathode RF gun and chicane-type magnetic pulse compressor were installed in the twin linac system of Nuclear Engineering Research Laboratory, University of Tokyo. Low emittance (6πmm. mrad) femtosecond (240-290fs at FWHM) electron single bunch of 16MeV, 2% energy spread and 350pC/bunch was obtained. Femtosecond streak camera (200fs resolution) with dispersionless reflective optics, coherent transition radiation interferometer and far-infrared polychromator were used. We conclude that the streak camera and polychromator are the best for longer than 200fs (FWHM) bunch and for shorter, respectively. Detailed performances of the RF gun including the linac-laser synchronization and the reduction of dark currents were precisely evaluated. Further, we tried a new femtosecond quantum beam based pump-and-probe analysis, which is the time-resolved X-ray diffraction to visualize atomic motions. X-ray diffraction patterns of CuKα 1.2 from several monocrystal such as GaAs, KCl, CaF 2 were obtained using 10ps X-ray pulses. The visualization is under way. Finally, we introduce the new femtosecond ultrafast quantum phenomena research facility.

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.

Effect of GaSb growth temperature on p-GaSb/n-GaAs diodes grown by MOVPE

Despite the 7% lattice mismatch, the reverse bias dark current of p-GaSb/n-GaAs junctions has been found to be unexpectedly low. MOVPE-grown diodes exhibited dark currents up to two orders of magnitude lower than comparable GaSb homojunctions. The initial nucleation of GaSb on GaAs was found to play an important role in determining the properties of the bulk layer. A larger number of small nucleation sites produced diodes with lower reverse bias leakage currents and growth temperature was in turn critical in determining the size of these nucleation islands. The authors present a study correlating growth temperature with nucleation site density, dark current and open circuit voltage under illumination. Diodes were grown at temperatures ranging from 490-550/spl deg/C using TMGa and TMSb. A reduction in reverse bias dark current of several orders of magnitude was observed across this narrow temperature range. The 'turn-on' voltage also increased with reduction in growth temperature and there was a corresponding increase in the average open voltage, V/sub oc/, under illumination. However, V/sub oc/ did exceed /spl sim/0.25 V. This is low compared to GaSb homojunctions (typically 0.45 V) and it may be limited by a low shunt resistance under illumination as well as low efficiency from the thin GaSb layers. Other alkyls [TIPGa and t-DMASb], which allow a further reduction in growth temperature, are being investigated.

Radiation response and injection annealing of p +n InGaP solar cells

The effects of 1 MeV electron and 3 MeV proton irradiation on single-junction p +n InGaP solar cells are studied. The cell radiation response is monitored through measurements of current vs voltage (both in the dark and under simulated solar light (1 sun, AM0)), quantum efficiency, and capacitance vs voltage. Analysis of the data is used to determine the basic mechanisms for the cell response. High current injection causes the irradiated cells to partially recover due to a reduction in the junction dark current, which induces recovery in the cell open circuit voltage. The degradation data measured under electron and proton irradiation are correlated in terms of displacement damage dose, from which a characteristic degradation curve for this cell technology is determined.

Wet-chemistry surface treatment for dark-current reduction that preserves lateral dimensions of reactive ion etched Ga/sub 0.47/In/sub 0.53/As p-i-n diode photodetectors

A wet-chemistry treatment, consisting of a fresh mixture of one-to-one 2-propanol plus concentrated sulfuric acid added to photoresist developer, which is tetramethyl ammonium hydroxide, was found to reduce the dark-current density at the -5-V dc reverse-bias to 2 pA/μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (2×10/sup -4/ A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ), and simultaneously to maintain the lateral geometrical dimensions, of reactive ion etched, 3 micrometer tall, right-cylinder mesa, Ga/sub 0.47/In/sub 0.53/As p-i-n diode photodetectors with diameters of 100, 50, and 25 μm.

Cryogenic performance of a high-speed GaInAs/InP p-i-n photodiode

The cryogenic performance of a high-speed GaInAs/InP p-i-n photodiode, with graded bandgap layers at the heterostructure interfaces, was investigated for the first time. DC measurements show that the dark current of the diode decreases sharply as the temperature decreases from 300 to 200 K. A factor of 1000 in dark current reduction was found for this photodiode, when it was cooled from room temperature to about 150 K. Similar modulation bandwidths were found for this device for temperatures between 9 and 300 K, with a bandwidth greater than 20 GHz. No degradation was found in performance at cryogenic temperature compared to room temperature. This enables direct integration of high-speed photodiodes with superconductive and other cryogenic electronics.

D.C. I–V characteristics and steady-state photoconductivity of Au/Pb2CrO5/SnO2 sandwich-structure films under illumination in the visible region

The room temperature d.c. current–voltage (I–V) characteristics of an Au/Pb2CrO5/SnO2 sandwich-structure 1.39 μm thick film have been measured for d.c. voltages, Vd.c., in the range 0.25 V≤Vd.c.≤5.0 V. These measurements were carried out under both dark and visible-light illumination conditions. For Vd.c.<2.5 V, the I–V curves of the sample in both dark and light environments were found to be non-linear and conform to space-charge-limited (SCL) current governed by traps uniformly distributed in energy. At higher d.c. voltages, a nearly Mott–Gurney V2 behaviour of the dark current has been observed, whereas the I–V behaviour of the illuminated specimen was a combination of an ohmic conduction and a V2 dependence at low illumination levels and became highly ohmic at large light intensities. This behaviour can be understood in terms of a reduction in the SCL dark current in favour of a larger ohmic d.c. photocurrent as a result of neutralization of the majority-carrier space charge by the photogenerated minority carriers of the electron–hole pairs produced under the illumination with visible light of energy ℏω≅EG(∼2.1–2.3 eV for the Pb2CrO5 material). The d.c. photocurrent, Iphot, at a fixed d.c. voltage, was found to follow a power-law dependence on light intensity, F, of the form Iphot∝Fγ, with the exponent γ being dependent on the applied d.c. voltage. At the low-voltage side (Vd.c.<1.5 V), γ∼0.5, a value usually obtained when the photoconductivity behaviour is governed by bimolecular recombination mechanisms. As the d.c. voltage is increased further, γ increases monotonically until it saturates at a value of about 0.9 for d.c. voltages beyond 3.5 V, where monomolecular recombination processes seem to be more operative with increasing d.c. voltage.

Synchrotron X-Ray photoconductor detector arrays made on MBE grown CdTe

We have been fabricating x-ray photoconductor linear array detectors using molecular beam epitaxially (MBE) grown (lll)B undoped CdTe layers on (100) Si substrates. A novel technique was developed to remove the Si and to mount the fragile MBE grown CdTe layers onto insulating ceramic substrates. 256 channel linear photoconductor array devices were fabricated on the resulting CdTe layers. The resistivity of MBE (lll)B CdTe was high (>108 \cm) enough to utilize the material for low energy (8 ~ 25 keV) x-ray detectors. The stability of the detectors are satisfactory, and they were tested at room temperature routinely for over a year. The performance of the photoconductor was greatly improved when the detector was cooled to 230K. Due to its reduced dark current at low temperatures, the dynamic range of the detector response increased to nearly four decades at 230K.

Enhanced quantum well infrared photodetector with novel multiple quantum well grating structure

An enhanced quantum well infrared photodetector (EQWIP) with lower dark current and improved performance relative to a conventional QWIP is described. Dark current reduction and external quantum efficiency improvements are achieved by novel structural enhancements that involve patterning the GaAs/AlGaAs multiple quantum well into a diffraction grating and reducing the number of wells. A 64×64 long wave infrared EQWIP array with 60 μm pixel pitch and peak D*∼8×1010 cm Hz1/2/W was demonstrated at 77 K. The low bias current permits hybridization to conventional readout circuits. Test results for pixel pitches down to 30 μm show that high EQWIP performance is achievable in the small pixels required for large focal plane array formats.

Switch opening time reduction in high power photoconducting semiconductor switches

It is shown experimentally and theoretically that the decay time of the current generated in a photoconductive switch can be controlled and significantly reduced, by the application of a strong magnetic field ( μ 0 B>1), without affecting its conductance during the illumination state. The application of the magnetic field has the added benefit of leading to a substantial reduction of the switch dark current.

Effect of asymmetric barriers on performances of GaAs/AlGaAs quantum well detectors

Recently, the efficient tuning of the carrier capture efficiency of quantum well by introducing a barrier asymmetry was demonstrated [J. M. Gérard, Appl. Phys. Lett. 63, 240 (1993)]. The electron launching effect was proposed to improve the gain of multiquantum well detectors. In this article, we give experimental results on the dark current, noise, optical gain, responsivity, and detectivity of barrier asymmetric quantum well detectors. We confirm that the electron launching reduces the capture efficiency and thus improves the gain. In addition to this effect, the modification of the barrier potential profile induces a dark current reduction and a blue shift of the intersubband transition. All these effects are then taken into account for analyzing detector performances. As a result of the comparison between symmetric and asymmetric detectors, we do not see any net advantage of using asymmetric structures for improving detector performances.

Generalized relationship between gain, noise, and capture probability of quantum well infrared photodetectors

We show that due to the discrete nature of multiple quantum well structures, the decay function of an energetic electron depends on the spatial extent of the electron wave function. As a result, the current gain of a quantum well infrared photodetector (QWIP) cannot be uniquely specified by the single well capture probability pc. Specifically, for a given pc the current gain of an extended state electron is found to be larger than that of a localized electron. Consequently, for a typical QWIP, it is possible that the photocurrent gain is larger than the dark current gain, resulting in a reduction of dark current induced generation-recombination noise from its expected value.

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.

Mapping of Defects in Metal-Semiconductor-Metal (MSM) Detectors in Hg1−xCdxTe by Nuclear Microprobe

AbstractThe incidence of compositional and structural inhomogenieties in MSM detectors based on Hg1−xCdxTe/GaAs and Hg1−xCdxTe/GaAs/Si has been examined by nuclear microprobe. With a 2 MeV He+ beam focussed to ≤5 μm, the microprobe has demonstrated the capability for RBS channelling in the active region of a Hg1−xCdxTe device and the imaging of defects by Channelling Contrast Microscopy (CCM). A series of linear growth defects in some Hg1−xCdxTe devices were identified using CCM. The channelling RBS spectra from these regions have shown an increase in χmin compared with the surrounding high quality crystal. The occurence of these defects was associated with a degradation in the performance of affected devices in an array. RBS spectra have also revealed the presence of an anomalous CdTe layer, correlating with a significant reduction in dark current and increase in breakdown voltage of these devices. RBS channelling of individual devices has identified differences in χmin between arrays which were prepared under equivalent conditions of growth and processing.

Visual transduction in dialysed detached rod outer segments from lizard retina.

1. Properties of a new preparation for studying the physiology and biochemistry of phototransduction in retinal rods are described. Whole-cell voltage clamp was used to record the generation, maintenance and light-sensitivity of dark current in rod outer segments that had been isolated from the rest of the receptor cell by detachment at the connecting cilium. 2. Detached outer segments dialysed with standard internal solution supplemented with physiological amounts of ATP (5 mM) and GTP (1 mM) developed a standing inward dark current that was the sum of three components: approximately 91% light-sensitive current, approximately 6% Na(+)-Ca2+,K+ exchange current and approximately 3% leakage current. Light-sensitive dark current (mean amplitude approximately -63 pA) was suppressed transiently by brief flashes in an intensity-dependent manner. Light responses had the same kinetics, sensitivity and intensity-response relationship as those recorded from intact rods. 3. Dialysed outer segments differed from intact rods in that intense flashes evoked saturating responses that recovered incompletely to a plateau of reduced dark current caused by incomplete inactivation of the transduction cascade. Light sensitivity was reduced for a short time following an intense flash and then recovered despite persistent reduction of dark current. This suggests that there is no fixed relationship between dark current amplitude and light sensitivity. 4. Light-sensitive dark current faded rapidly when outer segments were not supplied with nucleotides. Outer segments dialysed with solution that contained cyclic GMP, but no ATP or GTP, supported dark current at a level that increased with [cyclic GMP]. When basal phosphodiesterase (PDE) activity is inhibited, 8 microM cyclic GMP supports a dark current of approximately 70 pA. 5. Light sensitivity decreased during recordings made with solution that contained only cyclic GMP, consistent with the inhibition of G protein activation by loss of GTP. After thorough nucleoside triphosphate depletion, however, intense illumination evoked a transient increase rather than a decrease in dark current, i.e. an inverted light response. This result suggests that isomerized rhodopsin may generate a signal that causes either inhibition of basal PDE activity or release of bound cyclic GMP. 6. Sustained Na(+)-Ca2+,K+ exchange current was recorded during steady illumination when Ca2+, but not when Mg2+, was added to the dialysis solution. Exchange current increased with the amount of added Ca2+ and saturated at approximately 18 pA when the dialysis solution contained > or = 10 mM Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)

Reduction of dark current in photodiodes by the use of a resonant cavity

The advantages of using a resonant cavity enhancement scheme to decrease the dark current of heterojunction photodiodes are considered. It is shown that the employment of very thin absorption layers, allowed by this scheme, provides a significant reduction of the generation component of the dark current. Experimental results for InGaAs/InAlAs pin photodiodes suggest that the tunnelling component of the dark current can be also suppressed in resonant cavity enhanced photodetectors.

Modeling of a quantum well infrared detector with a dark current discriminator

A model is developed to provide estimates of the performance of quantum well intersubband infrared photodetectors. By introducing an energy filter to reduce the dark current, the quantum well device performance is improved. The amount of reduction in dark current depends upon the height of the energy filter barrier which can be varied by bias voltage. The energy distributions of the dark current and photocurrent electrons are discussed. Calculated results are presented for detectors with different quantum well widths, including the cases of bound-to-bound and bound-to-continuum transitions. The reduction in dark current results in a higher detectivity, and a substantial improvement over traditional designs can be obtained for some well widths.

Mapping of Defects in Metal-Semiconductor-Metal (MSM) Detectors in Hg1−xCdxTe by Nuclear Microprobe

ABSTRACTThe incidence of compositional and structural inhomogenieties in MSM detectors based on Hg1−xCdxTe/GaAs and Hg1−xCdxTe/GaAs/Si has been examined by nuclear microprobe. With a 2 MeV He+ beam focussed to ≥5 μm, the microprobe has demonstrated the capability for RBS channelling in the active region of a Hg1−xCdxTe device and the imaging of defects by Channelling Contrast Microscopy (CCM). A series of linear growth defects in some Hg1−xCdxTe devices were identified using CCM. The channelling RBS spectra from these regions have shown an increase in χmin compared with the surrounding high quality crystal. The occurence of these defects was associated with a degradation in the performance of affected devices in an array. RBS spectra have also revealed the presence of an anomalous CdTe layer, correlating with a significant reduction in dark current and increase in breakdown voltage of these devices. RBS channelling of individual devices has identified differences in χmin between arrays which were prepared under equivalent conditions of growth and processing.

Current conduction in bound-to-miniband transition III-V multiquantum well/superlattice infrared photodetectors

The use of the optical matrix method for computing the energy states and transmission probability in the bound-to-miniband (BTM) and step-bound-to-miniband (SBTM) transition III-V quantum well infrared photodetectors (QWIPs) is described. The effect of exchange energy due to electron-electron interaction was taken into account in the calculation of the Fermi level in the quantum well. Numerical simulations of the dark current as a function of temperature and bias voltage have been performed for both BTM and SBTM QWIPs. It is shown that dark current in these miniband transport QWIPs is dominated by the thermionic-assisted tunnelling conduction via the miniband for T>or=60 K, whereas resonant tunnelling conduction prevails for T<or=40 K. The results show that a significant reduction in dark current can be achieved for the BTM QWIP as compared with that of a conventional GaAs QWIP.

Magneto-optic and magnetotransport study of InAs/Ga1-xInxSb superlattices

InAs/Ga1-xInxSb superlattices have been investigated by magneto-optical and magnetotransport techniques. Bandgaps, determined from interband magneto-optical measurements and from the temperature dependence of the intrinsic carrier concentration, are in the long-wavelength infrared region (8.3 to 12.4 mu m) and are in good agreement with gaps calculated from a two-band model. Both electron and hole effective masses were measured by cyclotron resonance and the electron effective mass is found to be a factor of 4-5 times larger than in HgCdTe (the industry-standard IR material). This is necessary for reduced dark currents and good optical absorption coefficients in this material. Finally, the first observation of higher-order electron and hole subbands (by interband magneto-optics) in this material system is reported.