Decrease In Dark Current Research Articles

Compositional inhomogeneities in AlGaN thin films grown by molecular beam epitaxy: Effect on MSM UV photodetectors

Ultraviolet (UV) MSM photodetectors (PD) based on AlGaN alloys find many applications, including flame sensing. In this work we investigate the dependence of AlGaN based photodetectors grown by MBE on the kinetics of growth. MSM photodetectors were fabricated in the interdigitated configuration with Ni/Au contacts having 400 μm finger length and 10 μm finger spacing. Bulk Al0.4Ga0.6N films were grown on to sapphire substrates using an AlN buffer layer. A series of PDs were developed using the Al0.4Ga0.6N films grown under different group III/V flux ratios ranging from stoichiometric conditions to much higher than unity. Upon testing, it was observed that the otherwise identical photodetectors show significant decrease in dark current as AlGaN deposition conditions change from stoichiometric to excess group III, due to reduction of unintentional incorporation of oxygen-related point defects. In addition, the intensity and spectral dependence of the photocurrent also change, showing an extended low energy tail for the former and a sharp and prominent excitonic peak for the latter. The optical transmission measurements indicate a variation in Urbach energy with deposition conditions of the AlGaN films, although they have the same absorption edge. While all samples show a single red-shifted photoluminescence peak at room temperature, upon cooling, multiple higher energy peaks appear in the photoluminescence (PL) spectra, indicating that the alloys contain complex compositional inhomogeneities. Two types of alloy fluctuations, determined by the growth conditions, have been identified that modulate the optoelectronic properties of AlGaN by changing the spatial localization of excitons, thereby altering their stability. We identified that growth under stoichiometric conditions leads to compositional inhomogeneities that play a detrimental role in the operation of MSM photodetectors, which reduces the sharpness of the sensitivity edge, while growth under excess metal conditions enhances it.

A Theoretical Analysis of the Dark Current in Quantum Dot Infrared Photodetector using Non- Equilibrium Green’s Function Model

Objectives: This paper entails the theoretical computation of dark current in asgrown and annealed Quantum Dot Infrared Photodetectors (QDIP). Methods/Statistical analysis: Here, non-equillibrium Green’s function was used to model the dark current characteristics in asgrown and annealed QDIPs. Post-growth thermal annealing, which reduces the traps states in the QD is also simulated using Fick’s second law of diffusion. We have developed a selfconsistent Poisson’s equation solver to compute the potential profile and quasi Fermi level at QD and at the contacts. Findings: The theoretically computed dark current obtained from our model is in good agreement with the experimental data, which validates the efficacy of our model. Our computation also predicts decrease in dark current in the QD with increase in annealing temperature at low bias voltage from0.25-1.5 volts. Application/ Improvements: Here, we have optimized the dark current for different annealing temperatures for improving the performance of QD device for infrared sensing applications

The injected dark current of a p+n and a p+n n+ silicon solar cell taking into account the narrowing of band gap due to heavy doping

In addition to the photocurrent, the dark current of solar cell plays a vital role in determining the overall efficiency of a solar cell. In this paper, the injected dark current of a p + n junction solar cell and that of a p + n n + back-surface-field cell have been studied analytically, taking into account the band gap narrowing effect that exists at high doping concentrations. It is observed that there is a significant difference in the magnitude of the injected dark current obtained when the band gap narrowing effect is taken into consideration as compared to the case when it is not considered. Also, it is observed that the magnitude of the injected dark current decreases for smaller values of back surface recombination velocity, which corresponds to BSF structure. This is one of the reasons why the p + n n + BSF solar cell has better output than the conventional p + n cell. The results obtained here are similar to the case of an n + p p + solar cell reported earlier by researchers.

On/off-ratio dependence of bulk hetero junction photodiodes and its impact on electro-optical properties

Optical sensing from the near ultra-violet to the visible range of light is very important for industrial process monitoring, health care, and life science. Here, we present organic photodiodes (OPD), operating between 300 and 750nm wavelength fabricated in bottom absorption geometry with the semiconducting polymer blend PTB7:PC71BM. A maximal responsivity of ~100mA/W at 630nm with 54mW/cm2 light intensity has been achieved at room temperature. An approach to improve the on/off-ratio (light to dark) by modifying the absorption layer thickness has been investigated. The dark current decreases with the thickness of polymer blend PTB7:PC71BM. The transient response for OPD rise- and fall-time is in the μs regime, with an on/off-ratio of four orders of magnitude at −2V bias. The rise of responsivity is tunable by increasing the absorption layer thickness and shows a dependency in relation to dark current and transient response.

Annealing of the superlong CdS nanotubes for enhanced performance in fully nanostructured photodetector

The superlong single-crystalline CdS nanotubes have been prepared via a physical evaporation process. The effect of annealing process on the structural and photoluminescence (PL) characterizations are investigated, which plays important roles in the improvement of the crystalline quality and the optical property. A fully nanostructured photodetector with CdS NTs as active layer and Ag nanowires as electrodes have been fabricated and characterized. Compared with the performance of as-grown NTs-based photodetector, the annealed NTs-based device provides about 2–3 orders of magnitude decrease of dark current in air and about 30 times increase of photocurrent and decrease of the response time in vacuum, which are dependent on the high crystalline quality and defect-free property in annealed sample.

新型复合盖层延伸波长InGaAs红外探测器结构优化设计

The near-infrared detector of In0. 82Ga0. 18 As with PIN structure was designed and simulated by APSYS. The long-wave cutoff wavelength was extended to 2. 6 μm. Three doped layers were deposited on the absorption layer,which contained P type N type,N type and by growth order. The thickness and concentration of each doped layer in cap layer were analyzed. The results show that In0. 82Ga0. 18 As,In As0. 6P0. 4and In0. 82Al0. 18 As layers by growth order are determined to obtain the excellent performance. The relative spectral responsivity of the detector keeps almost the same,and dark current decreases by one magnitude compared with the PIN structure. The defect tunneling current predominates when the detector works at 120- 250 K,the inter-band tunneling current dominates the dark current when the detector works at 250- 300 K,and the G-R current and diffusion current dominates the dark current when the detector works above 300 K.

Atomic layer deposited HfO2 based metal insulator semiconductor GaN ultraviolet photodetectors

A report on GaN based metal insulator semiconductor (MIS) ultraviolet (UV) photodetectors (PDs) with atomic layer deposited (ALD) 5-nm-thick HfO2 insulating layer is presented. Very low dark current of 2.24 × 10−11 A and increased photo to dark current contrast ratio was achieved at 10 V. It was found that the dark current was drastically reduced by seven orders of magnitude at 10 V compared to samples without HfO2 insulating layer. The observed decrease in dark current is attributed to the large barrier height which is due to introduction of HfO2 insulating layer and the calculated barrier height was obtained as 0.95 eV. The peak responsivity of HfO2 inserted device was 0.44 mA/W at bias voltage of 15 V.

Amorphous Ge Passivation Effects on Ge Solar Cells

Amorphous Ge (α-Ge) deposited from a high purity Ge source was adopted to study the passivation applications for Ge solar cells. The measured results indicate that postannealing treatment onto the solar cell with α-Ge plays an important role in determining the solar cell performance. For samples with α-Ge and a GaAs contact layer outside the front metal grid region, more than one-order decrease of dark current can be obtained after conducting a postannealing step at 300 °C for 2-10 min. In addition, solar cell performance characteristics are dramatically improved with 1-sun Voc from 0.035 to 0.16 V, Jsc from 16.1 mA/cm 2 to 27 mA/cm 2 , fill factor from 0.17 to 0.40, and efficiency η from 0.1% to 1.7% after completing the postannealing process. Furthermore, after removing the GaAs contact layer outside the front metal-grid region, solar cells with α-Ge show 1-sun efficiency 1.19% (6.54% versus 5.35%) and fill factor 0.14 (0.68 versus 0.54) higher than those samples without α-Ge, which correspond to 22.2% and 25% relative increase in the efficiency and the fill factor, respectively.

Temperature dependence of performance of ZnO-based metal-semiconductor-metal ultraviolet photodetectors

ZnO as a wide bandgap semiconductor material can be used in ultraviolet photodetection for its direct bandgap (3.37eV) corresponding to the energy of an ultraviolet photon. Recently, ZnO based ultraviolet photodetectors have been deeply investigated, people have successfully fabricated high-performance MSM (Metal-semiconductor-metal) photodetectors and vertical-structured ultraviolet photodetectors, moreover the sensitive spectral region for detection has been extended into solar-blind spectral range. However, the temperature dependence of photodetector's performance has seldom been studied, so we fabricated high performance ZnO based MSM ultraviolet photodetectors and then the temperature dependence of performance has been investigated. We compared the photo-response measurement results at room temperature with the results measured at other higher temperatures. We found that photocurrent and responsivity decrease with temperature rising in the temperature range from room temperature to 200°C due to bandgap shrinkage and lattice scattering. Dark current decreases with temperature for mobility decrease below 100°C, but it increases with temperature for thermal generation above 100°C. These are not good for the stability of devices. Decreasing the working voltage may be an option to lessen the influence of temperature, but a low signal-to-noise ratio is the cost.

Understanding the positive effects of (Co–Pi) co-catalyst modification in inverse-opal structured α-Fe2O3-based photoelectrochemical cells

Recent studies found that Co–Pi as a co-catalyst is of great significance for water oxidation in photoelectrochemical (PEC) cell because it efficiently suppresses surface electron/hole recombination. Besides, it leads to a cathodic shift for the onset potential obviously. In this study, the synergic effect between inverse-opal structured α-Fe2O3 and cobalt–phosphate (Co–Pi) component is investigated. As expected, the photocurrent density of the novel Co–Pi/α-Fe2O3 photoanode shows a significant enhancement compared to that of bare inverse-opal structured α-Fe2O3 especially at low potential, which can be ascribed to the efficient charge separation by the introduction of Co–Pi and 3D nanostructure. Not only that, a dramatic decrease in dark current is also noticed after being Co–Pi modification. To explain this phenomenon, a mechanism study has been carried out by using a series of joint characterizations, including I/V curve feature, Mott–Schottky plot and impedance analysis. The results reveal that the Co–Pi modification in 3D α-Fe2O3 structure can also facilitate electron transport in skeleton structure. These findings make 3D nanostructure-based PEC with Co–Pi co-catalyst modification a more promising approach to generate hydrogen/oxygen from water splitting.

Characterization of Si hybrid CMOS detectors for use in the soft X-ray band

We report on the characterization of four Teledyne Imaging Systems HAWAII Hybrid Si CMOS detectors designed for X-ray detection. Three H1RG detectors were studied along with a specially configured H2RG. Read noise measurements were performed, with the lowest result being 7.1e− RMS. Interpixel capacitive crosstalk (IPC) was measured for the three H1RGs and for the H2RG. The H1RGs had IPC upper limits of 4.0–5.5% (up & down pixels) and 8.7–9.7% (left & right pixels), indicating a clear asymmetry. Energy resolution is reported for two X-ray lines, 1.5 & 5.9keV, at multiple temperatures between 150 and 210K. The best resolution measured at 5.9keV was 250eV (4.2%) at 150K, with IPC contributing significantly to this measured energy distribution. The H2RG, with a unique configuration designed to decrease the capacitive coupling between ROIC pixels, had an IPC of 1.8±1.0% indicating a dramatic improvement in IPC with no measurable asymmetry. We also measured dark current as a function of temperature for each detector. For the detector with the lowest dark current, at 150K, we measured a dark current of 0.020±0.001 (e−s−1pixel−1). There is also a consistent break in the fit to the dark current data for each detector. Above 180K, all the data can be fit by the product of a power law in temperature and an exponential. Below 180K the dark current decreases more slowly; a shallow power law or constant must be added to each fit, indicating a different form of dark current is dominant in this temperature regime. Dark current figures of merit at 293K are estimated from the fit for each detector.

Improved Performance of Photosensitive Field-Effect Transistors Based on Palladium Phthalocyanine by Utilizing Al as Source and Drain Electrodes

In conventional photosensitive organic field-effect transistors (FETs) (OFETs) (photo-OFETs) based on p-type organic semiconductors, high work function metals such as Au are generally used as source/drain electrodes, whose photosensitivity is generally low. We report on the performance improvements of photo-OFETs based on palladium phthalocyanine (PdPc) by using Al as source and drain electrodes. It is concluded that the dark currents of the photo-OFET based on PdPc with Al as source and drain electrodes (denoted as Al-PdPc device) are about only one-thousandth of that of the photo-OFET based on PdPc with Au as source and drain electrodes (denoted as Au-PdPc device). This tremendous decrease of dark current results in about a three-order-of-magnitude increase for photosensitivity at the gate and drain voltages of -50 V. The enormous decrease of dark current is ascribed to the Schottky contacts between the Al source/drain electrodes and PdPc. In addition, the Al-PdPc devices show also larger photoresponsivity compared with the Au-PdPc devices.

Effect of Various Structures on the Efficiency of Organic Solar Cells

Small molecule organic solar cell with an optimized structure of indium tin oxide (ITO)/copper phthalocyanine (CuPc) (10nm)/CuPc: C60mixed (20nm)/fullerene (C60) (20nm)/bathocuproine (BCP) (10nm)/Al) was fabricated. With optimizing the hybrid planar-mixed molecular heterojunction (PM-HJ) from the double layer heterojunction (HJ) and the bulk heterojunction (BHJ), the short-circuit current density (Jsc) increased from 3.09 to 5.11mA/cm2, the open-circuit voltage (Voc) increased from 0.40 to 0.47V, and the power conversion efficiency (ηp) increased from 0.66 to 1.28% under 100mW/cm2 AM1.5G illumination. These improvements were attributed to reach the optimal balance among the light absorption efficiency, the exciton dissociation efficiency and the carrier collection efficiency of the device, resulting in enhancement of Jscwithout affecting the value of fill factor (FF) and the reduction of the dark current. Furthermore a decrease of dark current is caused to the higher Voc.

NBn dark current reduction by UV hydrogenation

This study reports on hydrogenation of InAs nBn photodetectors grown on a lattice mismatched GaAs substrate. The mismatched growth causes increases in dark current by factors of 8–16 (voltage- and temperature-dependent), with respect to similar lattice matched growth. UV hydrogenation of the mismatched nBn’s produced significant decrease in dark current without decreasing the photocurrent.

Effect of indium doping and annealing on photoconducting property of wurtzite type CdS

In this paper we observed the effect of doping and annealing on the dark current and anomalous photoconducting behavior of hexagonal wurtzite CdS, synthesized by solid state reaction method. Undoped CdS sample shows higher anomalous behavior in photoconductivity as well as contains larger dark current of 19 nA. With the doping of Indium in CdS, dark current decreases from 19 nA to 1 nA but the anomalous behavior is not completely removed. While, after annealing at 150°C for four hour, indium doped CdS sample shows good switching property with rise and decay time of 360 ± 10 & 322 ± 6 seconds respectively. The anomalous photoconducting behavior is completely removed from annealed sample. X-ray diffraction patterns confirm the existence of hexagonal wurtzite phase of indium doped and undoped CdS samples while energy dispersion X-ray spectrum exhibits the elemental presence of cadmium, indium & sulfur in the indium doped sample. UV-Visible absorption spectra show the blue shift in absorption edge on indium doping from 475 nm to 425 nm in comparison to undoped sample. Photoluminescence spectra confirm the indium doping and reveal that annealed CdS sample has lesser defects among other samples due to which annealed sample has best switching performance.

Photoelectric characteristics of diodes in prototype photosensitive pixels for a monolithic array infrared photodetector

Test photodiodes in the form of mesa structures with different areas from 30 × 30 to 100 × 100 μm in size are fabricated based on a CdxHg1 − xTe/Si structure at x = 0.235, grown by molecular-beam epitaxy (MBE). The current-voltage characteristics of the diodes are measured in the dark and under background light conditions. The experimental results are compared with theoretical calculations. It is found that the dependence of the photodiode photocurrent and dark current on the mesa structure size appears in the mesa size ranges from 30 × 30 to 80 × 80 μm. The dark current decreases and the photocurrent increases with decreasing mesa size. The mechanisms affecting the behavior of current-voltage characteristics are discussed.

AlAsSb Avalanche Photodiodes With a Sub-mV/K Temperature Coefficient of Breakdown Voltage

The temperature dependence of dark current and avalanche gain were measured on AlAsSb p-i-n diodes with avalanche region widths of 80 and 230 nm. Measurements at temperatures ranging from 77 to 295 K showed that the dark current decreases rapidly with reducing temperature while avalanche gain exhibits a weak temperature dependence. No measurable band to band tunneling current was observed in the thinner diodes at an electric field of 1.07 MV/cm, corresponding to a bias of 95% of the breakdown voltage. Temperature coefficients of breakdown voltage of 0.95 and 1.47 mV/K were obtained from 80 and 230 nm diodes, respectively. These are significantly lower than a range of semiconductor materials with similar avalanche region widths. Our results demonstrated the potential of using thin AlAsSb avalanche regions to achieve low temperature coefficient of breakdown voltage without suffering from high band to band tunneling current.

Improvement in photoconductor film properties by changing dielectric layer structures

In recent times, digital X-ray detectors have been actively applied to the medical field; for example, digital radiography offers the potential of improved image quality and provides opportunities for advances in medical image management, computer-aided diagnosis and teleradiology. In this study, two candidate materials (HgI2 and PbI2) have been employed to study the influence of the dielectric structure on the performance of fabricated X-ray photoconducting films. Parylene C with high permittivity was deposited as a dielectric layer using a parylene deposition system (PDS 2060). The structural and morphological properties of the samples were evaluated field emission scanning electron microscopy and X-ray diffraction. Further, to investigate improvements in the electrical characteristics, a dark current in the dark room and sensitivity to X-ray exposure in the energy range of general radiography diagnosis were measured across the range of the operating voltage. The electric signals varied with the dielectric layer structure of the X-ray films. The PbI2 film with a bottom dielectric layer showed optimized electric properties. On the other hand, in the case of HgI2, the film with a top dielectric layer showed superior electric characteristics. Further, although the sensitivity of the film decreased, the total electrical efficiency of the film improved as a result of the decrease in dark current. When a dielectric layer is deposited on a photoconductor, the properties of the photoconductor, such as hole-electron mobility, should be considered to improve the image quality in digital medical imaging application. In this study, we have thus demonstrated that the use of dielectric layer structures improves the performance of photoconductors.

Giant Improvement of the Performance of ZnO Nanowire Photodetectors by Au Nanoparticles

ZnO nanowire photodetectors with and without Au nanoparticles have been fabricated and investigated. The dark current decreases by 2 orders of magnitude and the ratio of photo current to dark current increases after covering with Au nanoparticles. Meanwhile, the decorated Au nanoparticles can drastically improve the response speed of ZnO nanowire photodetectors. A physical model based on band energy theory was developed to illustrate the origin of the improvement of performance for ZnO nanowire photodetector attached with Au. Our work suggests that rational integration of ZnO wire and metal nanoparticles is a viable approach to improving the performance of ZnO nanowire photodetectors, which may help to advance optoelectronic devices.

Long-Wave InAs/GaSb Superlattice Detectors Based on nBn and Pin Designs

The development of type-II InAs/GaSb superlattice (SL) detectors with nBn and pin designs for the long-wave infrared (LWIR) spectral region is discussed. The dependence of dark current density and responsitivity of the pin photodetectors on doping type and level is investigated, and it is shown that dark current density decreases while responsivity and detectivity increase by p-doping the absorbing region of pin detectors. Comparison of optical and electrical properties of SL photodetectors based on the nBn and pin designs is reported. nBn devices have higher specific detectivity (D*), responsivity, and lower dark current density as compared to the pin detectors. The decrease in dark current in nBn devices is due to suppression of Shockley-Reed-Hall and surface leakage currents. A specific detectivity (D*) of 7.15 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> cm Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> W <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> at 0.1 V, a responsivity of 1.28 A/W and a quantum efficiency of 21.3% under 0.2 V biasing at 77 K and 7 ¿m, assuming unity gain, was obtained in the nBn device.