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Combined with the research achievements of silicon material characteristics, the asymmetric depletion layer with mesa structure is used in the design of engineering parameters for the 225GHz pulsed-mode IMPATT device to improve the output power. Based on the operating frequency, output power, and other constrained conditions, the epitaxial layer parameters of IMPATT device are designed by theoretical calculation and equivalent derivation. The tolerance of the epitaxial layer parameters is simulated, analyzed, and optimized by Silvaco TCAD device process simulation software. A test system, built based on standard instruments and frequency multipliers and mixers, is employed to evaluate the electrical property of the IMPATT device samples. The test results are consistent with the design values. The operating frequency of the samples is (225±1)GHz and the output power is about 170mW.

We present modeling and analysis of carrier dynamics in a single-photon avalanche diode (SPAD) operated with a capacitive quenching (CQ) method. The CQ method is regarded as a conventional resistive quenching (RQ) with its quenching resistance infinite or an open circuit. The SPAD is modelled as a lumped circuit consisting of a voltage dependent charge generator representing an avalanching depletion region and a capacitance of the depletion region and parasitic components. The carrier dynamics inside the device is described by time-dependent bipolar continuity equations (BCE) derived from the carrier continuity equations. We solve the BCE numerically with a 0.1 ps time resolution and investigate numbers of carriers in each circuit element as functions of time and of excess bias voltage (|𝑉ex|). We find two important characteristics of the CQ method; (1) a single-photon triggered Geiger-mode pulse is guaranteed to be quenched in a stable state (2) a voltage drop of the internal bias of SPAD due to the charges stored on the capacitance is proportional to |𝑉ex| with the proportionality factor of two. The results, in turn, enables one to design a SPAD free from after-pulse and from overflow. Such a SPAD pixel is shown to be compatible with a conventional complementary metal-oxide semiconductor (CMOS) image sensor (CIS) with a four transistors configuration pixel circuit. Finally, effectiveness of the present methodology is demonstrated by the subrange synthesis (SRS) time-of-flight (ToF) ranging experiments using a 6 μm size 400 × 400 pixels SPAD-based CIS.

The InAs/InSb/GaSb/AlSb family of III-V alloys and superlattice materials offer unique possibilities for band structure engineering, because they can be grown on GaSb or InSb substrates with high quality and satisfactory control of strain, doping and composition. The band profiles and oscillator strengths are also quite predictable, enabling full simulation of detector performance from a basic knowledge of layer and stack thicknesses. In conventional III-V p-n devices, Shockley-Read-Hall (SRH) traps generate a significant flow of thermal carriers in the device depletion region. At SCD, we have overcome this problem by developing XBn and XBp barrier device architectures that suppress these depletion currents, leading to higher operating temperatures or lower dark currents. Our first barrier detector product was launched in 2013 and operates at 150K. It uses a mid-wave infrared (MWIR) XBn device with an InAsSb absorber well matched to the most transparent of the atmospheric windows, at wavelengths between 3 and 4.2μm. However to span the full MWIR and to sense the long-wave infrared (LWIR) spectrum, we have investigated InAs/GaSb type II superlattices (T2SLs), because they offer full tunability. In this work we show that minority carriers in n-type T2SLs are localized and diffuse by variable range hopping, even when the period is short and the valence miniband has a width of 30-40 meV. Unfortunately, this leads to sub-micron diffusion lengths and a low quantum efficiency (QE) of ~20% in a full MWIR XBn device. On the other hand, p-type layers exhibit “metallic” minority carrier transport with much longer diffusion lengths, typically ~7 μm in our LWIR device layers. The successful development of p-type devices has led to our second barrier detector product, which uses an XBp LWIR T2SL and operates at 77K with a cut-off wavelength of 9.5 μm, a focal plane array (FPA) QE of ~50% and background limited performance up to ~90K at F/3. Moreover, the FPA operability is typically above 99.5%, based on stringent production-line criteria. Together with high spatial uniformity and good temporal stability, these barrier detectors are already a realistic alternative to MCT photodiode arrays, and further products operating at other wavelengths will be launched in due course.

We present a design principle for photonically-active double-heterojunction (DHJ) diodes that utilizes spatial control over the distribution of dopant impurities in the growth direction to suppress non-radiative Shockley-Read-Hall (SRH) carrier generation and recombination processes. These non-radiative processes constitute major parasitics in numerous devices, including light-emitting diodes (LEDs), photovoltaics (PVs), and photo-diode detectors (PDs). The design principle is general to all diodes with carrier-confining heterojunctions and is agnostic to material system. As a result, devices based on semiconductors in Group IV (e.g. SiGe), III-V (e.g. GaN, AlGaAs, GaInAsP, GaInAsSb), II-VI (e.g. MgZnCdTe, ZnCdSeS), and others can theoretically benefit from it. To illustrate the principle, here we will model LEDs, PVs, and PDs in the InP-lattice-matched GaInAsP material system. We show that LEDs’ Internal Quantum Efficiency (IQE) is raised, with a major impact at low forward bias voltages. The modeling presented here shows that, because thermophotonic refrigerator LEDs operate at these voltages, the design principle could prove to be a major step toward realizing the Kelvinscale and larger temperature reductions that have remained experimentally elusive to date. Next, we show that redesigned PVs exhibit higher open-circuit voltage and efficiency, with significant improvement in cells with the high defect densities typical of lattice mismatched III-V cells grown on inexpensive Silicon substrates. Finally, we show that PDs exhibit improvements in dark current during reverse bias operation and shunt resistance during photovoltaic operation, quantities that can impact the noise floor of receivers in optical communication systems and thus the overall power consumption of photonic links.

The InAs/InAsSb superlattices are attractive materials for the replacement of both RoHS restricted bulk HgCdTe and strongly Shockley-Read (SR) generation limited InAs/GaSb superlattices. Two main factors limit the performance of InAs/InAsSb photodiodes: the rate of the SR processes, especially in the depletion region, which is the source of the large dark current and a short vertical diffusion length of charge carriers in superlattice absorbers which results in poor responsivity. In this paper, we report on the status of HOT LWIR detectors based on InAs/InAsSb superlattices at VIGO System S.A. The uncooled and Peltier cooled LWIR photoconductors are the most successful devices developed so far. The practical InAs/InAsSb SL-based photoconductors have been fabricated by MBE heteroepitaxial growth on buffered 3” wafers. The design of the devices, material composition and doping, has been optimized for operation at temperatures from 200 to 300 K at a spectral range up to 18 μm. Some of the detectors were supplied with immersion microlenses formed in the GaAs substrates. The devices were characterized by measurements of the spectral responsivity and frequency-dependent noise density. The measured spectral detectivities of the best SL devices were found to be close or better compared to the HgCdTe counterparts operating at the same conditions. The devices are now offered as commercial products. Vigo present efforts are focused on the development of HOT LWIR photodiodes including monolithic cascade devices and thin absorber devices with the plasmonic enhancement of absorption. The development roadmap of advanced HOT devices is also sketched.

The depletion region around each p-n junction in HgCdTe HAWAII-2RG detector arrays decreases in volume as charge is collected, causing the pixel capacitance to change continuously throughout an integration period. This changing capacitance manifests as a steadily decreasing measured signal rate while observing a constant flux. Ignoring this nonlinear response to signal accumulation can lead to underestimating the number of detected pho- tons by as much as 10%. Presented here are two methods, one simple and one complex, of measuring this signal nonlinearity and a theoretical framework behind a nonlinearity correction method. Additionally, experimental data are compared with simulations to explain methods to reduce noise in the nonlinearity measurement and identify deviations from the expected behavior that merit further study.

The HgCdTe avalanche photodiode (APD) with built-in gain mechanism has great application prospects in the field of weak light signal detection. Any dark current will be converted into noise affecting the work efficiency of photodetectors. Therefore, the study of dark current mechanism is an important way to obtain high performance HgCdTe APD. In this paper, the photoelectric detection mechanism of planar junction electron injection HgCdTe APD is systematically studied, focusing on two aspects of structural optimization design and performance improvement. The dependence of device performance parameters on structure parameters is obtained by comparing the simulation results and experimental results. The lower the trap concentration, the smaller the dark current under small reverse bias voltage is, and the higher the operation voltage is. The dark current density of HgCdTe APD array is about 10-8 A/cm2 under small bias voltage. In addition to the material parameters, the dark current is mainly related to the internal electric field, which is affected by the width and doping concentration of the depletion region. The results show that device structure is the major determinant for the performance difference of HgCdTe APD. The structural optimization direction of high-performance HgCdTe APD is to further suppress the local electric field and reduce the dark current by adjusting the process.

We report on the fabrication and characterization of a high-resolution Schottky barrier radiation detector for high temperature (HT) applications using high quality 150 𝜇m thick n-4H-SiC epitaxial layers with an ultra-low micropipe density of ≤0.1 cm-2. To evaluate the depletion region parameters of the detector, it was first characterized temperature- dependent capacitance voltage (C-V-T) measurements which showed barrier heights Φ𝐵 ranging from 2.09 to 2.24 eV and a carrier concentration of ~2 x 1014 cm-3 at room temperature (RT) which linearly increased at a rate of 9.65 x 1010 cm-3/K. From the depletion region simulation studies, it was shown that the change in doping concentration with temperature (T) can increase the needed bias to fully capture charged particles by up to 9V for 241Am and over 30V for a higher energy particles from 213Po. To examine the barrier properties and leakage currents of the detectors, we have systematically characterized temperature-dependent current - voltage measurements (I-V-T). The forward bias characteristics showed two linear regions - a low voltage region corresponding to a Shockley-Read-Hall (SRH) recombination center at 𝐸𝐶 − 0.93 eV and an upper region which corresponded to the barrier height of 2.02 eV. The reverse bias leakage currents were measured to be ~6 pA at ~65V at 300K implying a high signal to noise ratio (SNR) at RT and exhibited a current of less than 1 nA until 500K suggesting that detector should be operable with high SNR at ~200 oC. The Arrhenius plot of the reverse I-V-T showed an activation energy of 0.11 eV up to 400K and then 0.73 eV for 450K to 600K suggesting that most of the excess current at HT is derived from deep level state Z1/2 recombination center. Deep level transient spectroscopy (DLTS) results showed low defect concentrations of ~1011 cm-3 confirming that charge loss from trapping is negligible. Density functional theory (DFT) calculations suggested that the measured trap levels corresponded primarily to carbon (C) vacancies while the level at 0.93 eV corresponded to Si vacancies near the surface. The pulse height spectra (PHS) of the detector showed an excellent RT energy resolution of 0.55% FWHM at 5486 keV alpha particles after gaussian correction owed gamma response at 59.54 keV.

The electrical and detection properties of the X/γ-ray detectors, fabricated as Cr/CdTe/Au diode structures using semiinsulating p-CdTe single crystals with different concentrations of uncompensated impurities (N ~ 4 × 1010 cm-3, 1 × 1011 cm-3, 2 × 1011 cm-3 and 2 × 1012 cm-3), which determines the width of the space-charge region (SCR), were studied. All the diode-structured detectors were fabricated by the same elaborated technique with vacuum deposition of a Cr electrode (rectifying Schottky contact) and Au electrode (near Ohmic contact) onto the B- and A-face of CdTe(111) crystals pre-treated with Ar-ion bombardment. The dark I-V characteristics of the Cr/CdTe/Au Schottky diodes evidenced steep rectification properties that allowed applying high reverse bias voltage (V = 1-1.5 kV) under moderately low dark currents (I = 3-10 nA at rectifying contact area of 10 mm2 and T = 300 K). The charge transport mechanisms in the Schottky diodes were analyzed and attributed to generation currents in the SCR at V < 100 V, charge transport in the conditions when the SCR width exceeded the crystal thickness at elevated V, and currents limited by space charge at higher V. The as Cr/CdTe/Au Schottky-diode detectors, fabricated using CdTe with N ≤ 5×1010 cm-3, demonstrated the highest detection efficiency and energy resolution (0.6-1.0 %@662 keV). With increasing N, these characteristics deteriorated. Dependences of detection efficiency on N were calculated for different applied bias voltages V.

An integrated high-speed novel electro-optic power splitter based on interleaved PN junction in a silicon microring resonator has been proposed, which can be used as an ultra-fast, dynamically tunable optical power splitter to efficiently distribute power between various optical stations in Network-on-chip (NoC). The main working principle behind such a device is that we can change the refractive index of the PN junction waveguide under an application of an external reverse bias voltage, enabling the resonator to work as a power splitter. The interleaved PN junction waveguides utilizing the features of greater overlap of the optical waveguide mode with the depletion layer compared to normal PN junction, provide the opportunity to maximize the change in refractive index with minimum applied reverse bias voltage. The carriers (electrons and holes) are depleted near to the p region and n region interface on applying a reverse bias voltage. The depletion region increases with the applied reverse bias voltage, which enhances more interaction with the propagating light. To evaluate the performance of the splitter, we have calculated the change in the effective refractive index and the optical loss due to the free carrier concentrations. The proposed optical power splitter is capable of tuning the power splitting ratio from 0.15 to 7.82, which is a very wide range of tunability with low power consumption (35 mW) and low external voltage (0-3.5 V). We have proposed the use of a high-speed Digital to Analog converter (DAC) to apply an external reverse bias voltage to the interleaved PN junction waveguide.

In this work we review the reported results as well as the analytical and 3D numerical modeling tools we used to analyze dark current and quantum efficiency data from lattice matched InP/In0.53Ga0.47/InP double layer planar 15 μm pixel pitch focal plane arrays (FPAs). These imaging sensors are designed to operate in the near infrared under overcast “Night-Glow” illumination conditions. A notable finding is that the diffusion dark current component is the dominant current component near and above 300 K and is limited by band-to-band radiative recombination processes. The Shockley-Read-Hall (SRH) recombination through band gap states situated at the intrinsic Fermi level is the dominant component for temperatures below 300 K. 3D numerical simulations consisting of both bulk area and perimeter dependent components explains the dark current component of origin in the space charge region (SCR) with an SRH lifetime of τSRH = 107 μs. Image sensors with extended cut-off wavelength of 2.5 μm at room temperature built on InP/InGaAs are not lattice matched and needed are reductions in the mismatch throughout the InP/InGaAs multilayer epitaxial structure. We analyzed devices with a buffer layer introduced between the InP and the In0.81Ga0.10As absorber and showed that the measured dark current consisted of a diffusion current limited by a back surface interface recombination velocity and a shunt current component expected to be dominant at T ≤ 200 K. At 293 K we calculated a factor of 10 increase in the diffusion current caused by lattice mismatch. Recommended are hole minority carrier lifetime measurements and variable junction diode geometries to assess suspected perimeter/area dependency.

The electrical and detection properties of the X/gamma-ray detectors, fabricated using commercial detector-grade p-like CdTe single crystals grown by Acrorad Co., were studied. The detectors were developed as Cr/CdTe/Au Schottky diodes using CdTe wafers with an area of 5 × 5 mm2 and different thicknesses (0.25, 0.5, 0.75, 1.0 and 2.0 mm). Both the Schottky and Ohmic contacts were formed on the opposite sides of semi-insulating CdTe(111) crystals after preliminary chemical and Ar-ion etchings using different parameters. The Cr/CdTe/Au Schottky diodes demonstrated steep rectification that made it possible to apply high reverse bias voltage up to V = 1500-2000 V at moderately low dark currents. Having the rectifying contact area of 10 mm2, the diodes showed dark currents ~ 2-3 nA at V = 1000 V (T = 300 K). The dominant charge carrier transport mechanisms were analyzed using the I-V characteristics and determined as: generation-recombination in the space-charge region (SCR) at V = 1-100 V, the charge transport in the conditions when the SCR width exceeded the thickness of a semiconductor crystal at elevated voltages and, finally currents limited by space charge at even higher bias voltages. For radiation from a 137Cs radioisotope, the Cr/CdTe/Au detectors fabricated on thin (0.5 mm) semiconductor crystals showed the highest energy resolution (FWHM = 0.5%@662 keV). The detection efficiency increased in 4 times with increasing the crystal thickness (up to 2 mm), although the energy resolution deteriorated from 0.5 % to 3 % that, however, was still acceptable for spectroscopic operation of the detectors.

InAsSb ternary alloy is considered to be an alternative to HgCdTe (MCT) in long-wavelength infrared (LWIR) spectral region. The high operation temperature conditions are successfully reached with AIIIBV bariodes, where InAsSb/AlAsSb system is playing dominant role. Since theoretically there is no depletion region in the active layer, the generation-recombination and trap-assisted tunneling mechanisms are suppressed leading to lower dark currents in comparison with standard photodiodes. As a consequence, the bariodes operate at a higher temperature than standard photodiodes which could be used in wide range of system applications, especially where the size, weight, and power consumption are crucial, to include - Free Space Optics (FSO) system. In terms of FSO the LWIR provides limited scattering and atmosphere turbulence in comparison to the commonly used ~ 1550 nm range. Currently, as reported HgCdTe LWIR with peak wavelength ~ 10 μm exhibiting ~ 9×109 cmHz1/2/W (unbiased) and ~ 3×1010 cmHz1/2/W (200 mV) and response time 0.3-4 ns are used. The paper presents detailed analysis of the bariode’s performances (such as dark current, detectivity and response time) versus applied voltage, operating wavelength, temperatures (4-, 2- stage thermoelectrical cooling) and structural parameters (active layer Sb composition, xSb). The optimal working conditions are simulated (non-equilibrium). Theoretical predictions of bariode’s performances are compared with experimental data published for commonly used FSO HgCdTe detectors.

This article presents the fabrication of the front-illuminated planar type InGaAs infrared detector based on the lateral collection structure. The detector with the cutoff wavelength 1.7μm was fabricated on the NIN-type InP/InGaAs/InP hetero-structure materials with sealed-ampoule method using Zn3P4 as the diffusion source. And the detector with the dimension of 460μm×1000μm consists of four lateral collection regions and the width of each region is 15μm. Furthermore, the electrical properties and photo response characteristics were investigated between detectors with the lateral collection structure and normal structure. The Laser beam induced current (LBIC) map shows that the photoresponse of lateral collection InGaAs detector at 296 K is quite uniform and the photoresponse signals generated in the lateral collection regions are the same as them in PN junction regions. The lateral collection regions disappear from view since the electron/hole pairs generated in the regions are all collected by the electrical field of depletion region. It turns out that the average peak detectivity and the density of dark current of the detectors with lateral collection structure and normal structure is 3.22×1012 cm·Hz1/2/W and 3.00×1012 cm·Hz1/2/W, 4.85 nA/cm2 and 22 nA/cm2 at -100 mV respectively. Therefore, the lateral collection structure could substantially reduce the dark current by 70% compared with the normal structure.

X-ray Imaging Spectrometers (XIS) are the X-ray CCD cameras onboard Suzaku. They were operated in orbit from 2005 to 2015 and produced lots of findings with their good energy resolution and low non X-rat background. Precise calibration including the 10 eV accuracy in the energy scale reinforced them. Nevertheless, there has been a unresolved calibration issue in the spectral response around the Si-K edge (1.839 keV) appearing as systematic residuals up to 10%. The residual is negative peaking at 1.85 keV in the front illuminated (FI) sensors and positive peaking at 1.8 keV in the back illuminated (BI) sensor for X-ray sources dominated by continuum X-ray emission. Various attempts to eliminate these residuals by changing response parameters or quantum efficiency models have been insufficient. In this paper, we revisit this problem by focusing on the relation between incident X-ray energy and pulse height. We introduce a jump in that relation at the Si-K edge by modifying the , and optimize its value so as to minimize the residuals in the fit of the X-ray spectra for the black hole binary LMC X-3, a source dominated by continuum emission. We find the introduction of a jump significantly reduces the residuals. The optimized jump values are +4:2 channel, +4:0 channel, and -3:1 channel, corresponding to 15.3 eV, -11:3 eV, and 14.6 eV, for XIS0, XIS3 (FI), and XIS1 (BI), respectively. The direction of the jump is opposite for the FI and for the BI. We revise the response matrices generator so as to include the jump for each XIS sensor, and apply it to the X-ray spectra of the Perseus cluster of galaxies which has various emission lines in the spectra, and the blazar PKS2155-304 which was observed various epoch in the Suzakuoperation. We confirm the residuals are significantly reduced for these sources, too. We finally suggest the jump at Si-K edge in the energy and pulse height relation is qualitatively explained, if some of charges are lost in course of charger collection to the electrode of the CCD in the depletion later, and its amount is large for larger travel length in the depletion layer. If this explanation is correct, the Si-K edge problem and its solution presented in this paper is not specific only for the SuzakuXIS but also for other X-ray CCDs.

In this work we use analytical and 3D numerical modeling tools to analyze data from InP/In0.53Ga0.47/InP double layer planar 15 pixel pitch focal plane arrays (FPAs) designed to image in the near infrared to determine array suitability for operation under low-level illumination, including overcast, “Night Glow”- only conditions. Notable is that the diffusion dark current component is the dominant current component near and above 300K and is limited by band-to-band radiative recombination processes. The Shockley- Read- Hall (SRH) minority carrier lifetime is τSRH =107μs. Recombination through band gap states in the space charge region (SCR) situated at the intrinsic Fermi level is the dominant component for temperatures below 300K as previously demonstrated using 3-D numerical simulations consisting of both bulk area and perimeter dependent components. 3-D numerical simulations in combination with scanning capacitance microscopy are paramount to characterizing the Zn-diffused p+n shallow step homojunction and to identifying technology limitations in small pitch high density (FPAs). Photon recycling effects, i.e., effects caused by repeated trapping of photons, are not observed in the measurements of the minority carrier lifetime and the diffusion dark current component. As a result, the diffusion current component Jdiffusion α to the radiative recombination rate Gr(α) and the radiative minority carrier lifetime τradiative = 1/BNd, where B is the radiative recombination coefficient and Nd the majority carrier concentration.

InP/InGaAs DHBTs and frequency dividers are irradiated by low energy proton, and displacement damage effect of the devices are analyzed. InP/InGaAs DHBTs has been made DC characteristics measurements, and the function measurement for frequency dividers has been done both before and after proton irradiation. The breakdown voltage of InP DHBTs drop to 3.7V When the fluence up to 5x10 13 protons/cm 2 . Meanwhile, the function of frequency dividers get out of order. Degradation of DC characteristics of DHBTs are due to the radiation-induced defects in the quasi neutral base and the space charge region of base-collector and base-emitter junctions. The performance deterioration of DHBTs induce the fault of frequency dividers, and prescaler may be the most sensitive circuit.

Silicon modulators based on the carrier depletion mechanism are extensively used in recent years for high-speed data transmission. Lateral PN junctions are the most common electro-optical phase shifters for silicon Mach-Zehnder modulators (MZMs) due to its ease of fabrication. They have a relatively high DC VπLπ of around 2.5 V.cm in the Oband. An alternative approach is to design and optimize vertical PN junctions for lower DC VπLπ, which is currently lacking in the literature for silicon MZMs that operates using carrier depletion mechanism in the O-band. In this work, we look into the design and optimization of silicon phase shifters based on vertical PN junctions for high-modulationefficiency with VπLπ ≤ 1 V.cm, while meeting the stringent low loss budget of ≤ ∼1 dB/mm for data communication in the O-band. This is achieved by varying the offsets of the vertical PN junction with respect to different doping concentrations (2e17/cm3 – 3e18/cm3 ) near the depletion region. Different types of doping schemes are explored and optimized. Our optimized vertical PN junction designs are predicted to give low DC VπLπ of 0.26–0.5 V.cm for low DC reverse bias of ≥ –2V and low propagation loss of ≤ ∼1dB/mm, resulting in α.VπLπ = 1.7 for the best designs, which to the best of our knowledge, is the lowest α.VπLπ at the O-band to date. Electrical and optical modeling are based on our in-house proprietary software that is able to perform both optical and electrical simulations without loss of data fidelity.

Thermal poling, a technique to create permanently effective second-order susceptibility in silica optical fibers, has a wide range of applications, such as frequency conversion, electro-optic modulation, switching and polarization-entangled photon pairs generation. After many works where a conventional configuration anode-cathode was used, in 2009 a new electrode configuration (double-anode) was adopted, which allows for a more temperature-stable depletion region formation and a higher value of effective Chi2 1, 2 . In this work we demonstrate, via numerical simulations realized in COMSOL® Multiphysics, that in a double-anode configuration the effective value of Chi2 strongly depends on the relative position of the core with respect to the electrodes, requiring an accurate and precisely tailored geometry of the fiber, while in a single-anode configuration this value, for standard poling conditions 1 is almost independent of the position of the core, offering the possibility of relaxing the manufacturing constraint of the fiber fabrication. We also demonstrate that, in the same experimental conditions, the maximum value of effective Chi2 induced by thermal poling in double-anode configuration is smaller than the one obtained in single-anode configuration for any position of the core in the range between the anodic surface and the geometric center of the fiber. Finally, we report the experimental observation of depletion region formation in a twin-hole fused silica fiber poled in single-anode configuration. Using a QPM SHG experimental set-up, we demonstrate that the value of Chi2 obtained in the single anode configuration is at least as large as for the double anode one.

This manuscript presents a survey of the extended short wavelength infrared (SWIR) FPA technologies with the emphasis on InxGa1-xAs imagers. We discuss the limitations of 2.5 μm cut-off extended InxGa1-xAs photodetector technology arising from the lattice mismatch between the absorber material and the InP substrate, as well as presenting the characteristics of molecular beam epitaxy grown In0.83Ga0.17As photodetectors on InP substrates with different buffer layers. It is possible to achieve desirable pixel and large format focal plane array (FPA) characteristics with In0.83Ga0.17As epilayers on linearly graded AlxIn1-xAs buffer layer. No considerable difference was detected in the G-R components of the dark currents of the detectors with graded AlInAs and InGaAs buffers. Therefore, the utilization of these materials as the buffer layer seems to generate similar trapping centers in the In0.83Ga0.17As absorber layer. While no considerable 1/f noise was observed in the extended SWIR In0.83Ga0.17As detectors operating under small reverse bias at 200 K, the signal to noise ratio of the detectors was limited by 1/f noise for higher reverse bias voltages. The 1/f noise was found to be related with the shunt leakage and the generation-recombination processes in the depletion region with a relatively larger shunt leakage noise coefficient in the order of 10-3. We also present a comparison of extended SWIR InGaAs and HgCdTe detector technologies and discuss the potential of these technologies for future developments. While 2.5 μm cut-off mesa type In0.83Ga0.17As FPA pixels exhibit dark current densities in the same order with those of the best planar-type HgCdTe sensors on CdZnTe substrates near room temperature, HgCdTe on CdZnTe sensors provide more than an order of magnitude smaller dark current density at the typical operation temperature of these detectors. On the other hand, In0.83Ga0.17As sensors on InP substrates and HgCdTe detectors on Si substrates display dark current densities in the same order with different dominancy levels of shunt and G-R leakage currents. High noise coefficient of the shunt leakage is an important limitation for the lattice mismatched detectors narrowing the operation range of these sensors.

Progress on the modeling, fabrication, and characterization of the transistor-injected quantum-cascade laser (TI-QCL) is presented. As a novel variant of the quantum cascade laser, the TI-QCL has been projected to have advantages over conventional QCLs in certain applications because of its 3-terminal nature. The separation of field and current is expected to allow separate amplitude and frequency modulation, and the location of the cascade structure in a p-n junction depletion region is expected to reduce free carrier absorption and improve efficiency. At the same time, the added complexity of the structure creates challenges in the realization of working devices. An overview of the basic operating principles of the TI-QCL is first given, and projected advantages discussed. Next, work on modeling GaAsbased TI-QCLs is presented, and a design for devices in this system is presented. Finally, work on fabrication and characterization of devices is examined and ongoing challenges are discussed. The role of quantum state alignment in the QCL region on electron-hole recombination in the base is also examined, showing the capability of using basecollector voltage to modulate the optical output from the direct-bandgap transistor base.

The present paper analyzes the сharge transport mechanisms and spectrometric properties of In/CdTe/MoOx heterojunctions prepared by magnetron sputtering of indium and molybdenum oxide thin films onto semi-insulating p-type single-crystal CdTe semiconductor, produced by Acrorad Co. Ltd. Current-voltage characteristics of the detectors at different temperatures were investigated. The charge transport mechanisms in the heterostructures under investigation were determined: the generation-recombination in the space charge region (SCR) at relatively low voltages and the space charge limited currents at high voltages. The spectra of 137Cs and 241Am isotopes taken at different applied bias voltages are presented. It is shown that the In/CdTe/MoOx structures can be used as X/γ-ray detectors in the spectrometric mode.

The modeling and optimization of several photodetectors by semiconductor simulation tool Silvaco Atlas are reported. First is the simulations of p-i-n InP/In 0.53 Ga 0.47 As/InP photodetector at low bias. How the dark current, photoresponse and the transient response are influenced by the doping concentration and thickness of the absorption layer are reported. Second is a two-terminal p-n-p heterojunction phototransistors (2T-HPTs) based on In 0.53 Ga 0.47 As/InP. To optimize the device performance, the adjustment of the doping level, width, and compositional grading of base, the effects of high-low doping in collector region and the insertion of a thin undoped InGaAs layer in the base region have been investigated. The last is a simulation for InGaAs/InAlAs separate absorption, grading, charge, and multiplication avalanche photodetectors (SAGCM APDs), study the effect of multiplication layer parameters on the operating voltage ranges of APD.

Experiment shows that thin film bulk acoustic resonator (FBAR) is feasible to detect gamma irradiation, but the sensing mechanism is not studied deeply. For this problem, different sensing mechanisms are proposed to explain the resonance frequency shift after gamma irradiation according to two different FBAR structures. One FBAR structure is four - layers stacked (metal layer - piezoelectric layer - oxide layer - metal layer). After gamma irradiation, a voltage will be formed in the radiation sensitive layer (oxide layer), which is equivalent to impose a DC voltage to the piezoelectric layer that makes resonant frequency shift. There is a semiconductor layer between oxide layer and piezoelectric layer in the other FBAR structure, which is the difference between the two structures. A voltage formed in the oxide layer after irradiation will change the surface potential of the semiconductor and then change the space charge layer capacitor in semiconductor that makes the resonant frequency shift. The results of two mechanisms are obtained by simulation and compared with those in related literature, it is found that the trends and magnitudes of frequency shift are the same, so the two mechanisms are feasible.

Colloidal quantum dots (QDs) based on lead sulfide (PbS) have acquired scientific interest for infrared optoelectronic devices with potential bandgap tunability and ease of fabrication on arbitrary substrates. In this work, we show how device analysis data feed back into process optimization, towards the realization of high performance QD NIR photodetectors. Using the combination of transient PL, carrier transport and CV measurements we obtain the carrier density, lifetime and diffusion length in the layers. From the measured short diffusion length of the minority carriers, we deduce the need to achieve a wide depletion region to minimize recombination and thus enhance the carrier harvesting. Process optimization lead to a depletion region of more than 150 nm, resulting in high photon to carrier conversion. Furthermore, the complex index of refraction of all layers is characterized using ellipsometry and reflection/transmission, and these values are used as input for a transfer matrix method. Using the first interference peak, we show that a maximum EQE of 25% can be expected from optical modeling, a value that we almost reach experimentally (20%). Combining all of the above, we demonstrate 1450-nm photodetectors with dark current in the range of μA and specific detectivity (D*) of 10^11 Jones.

Isolating spectral signatures and/or the carrier dynamics that are specific to semiconductor junctions and not just the interface or bulk is challenging. Junctions that form between a semiconductor surface and a contacting layer are the key to their function. Equilibration of chemical potential at such junctions creates an internal electric field and establishes a region where mobile charges are driven away (depletion region). Absorption of light produces electrons and holes within the depletion region where the charges are separated. We developed transient photoreflectance (TPR) as an innovative time-resolved spectroscopic probe that can directly monitor carrier dynamics within and across such junctions. In the TPR method, the change in reflectance (ΔR) of a broadband probe from a specific interface is monitored as a function of pump-probe delay. The spectral nature of the reflected beam provides quantitative information about the built-in field; thus, TPR is a non-contact probe of the electric field at that interface. We applied TPR to study charge transfer at p-type gallium-indium phosphide (p-GaInP2) and n-type gallium-arsenide (n-GaAs) interfaces. We monitored the formation and decay of transient electric fields that form upon photoexcitation within bare p-GaInP2, p-GaInP2/platinum (Pt), and p-GaInP2/amorphous titania (TiO2) interfaces. A field at both the p-GaInP2/Pt and p-GaInP2/TiO2 interfaces forms that drives charge separation, however, recombination at the p-GaInP2/TiO2 interface is significantly reduced compared the p-GaInP2/Pt interface. On the other hand, n-GaAs forms an ohmic contact with TiO2 while only a small field forms at the n- GaAs/NiO interface that promotes hole transfer to nickel oxide (NiO).

Diffusion doped p-i-n/p-n diodes in SOI substrate is proposed for the fabrication of active silicon photonics devices with scalable waveguide cross-sections. The p-type and n-type diffusion doping parameters are optimized for the fabrication of tunable single-mode waveguide phase-shifters with microns to submicron cross-sectional dimensions. The simulations results show that the shape of depletion layer can be effectively engineered by suitably positioning the rib waveguide with respect to the gap between doping windows. We could thus introduce an additional control parameter to optimize over-all figure of merits of the phase-shifter for various applications. For an optimized set of diffusion parameters, the VπLπ of single-mode waveguides designed with 1μm, 0.5μm, and 0.25μm device layers (under reverse bias operating in TE-polarization at λ ~ 1550 nm) are found as 2.7 V-cm, 2.1 V-cm, and 1.6 V-cm, respectively. The typical p-n junction capacitance of an optimized 0.25μm single-mode waveguide is estimated to be < 0.5 fF/μm, which is comparable to that of ion-implanted p-n waveguide junctions.

III-V materials, which exhibit high absorption coefficients and charge carrier mobility, are ideal templates for solar energy conversion applications. This work describes the photoelectrochemistry research in several IIIV/electrolyte junctions as an enabler for device design for solar chemical reactions. By designing lattice-matched epitaxial growth of InGaP and GaP on GaAs and Si, respectively, extended depletion region electrodes achieve photovoltages which provide an additional boost to the underlying substrate photovoltage. The InGaP/GaAs and GaP/Si electrodes drive hydrogen evolution currents under aqueous conditions. By using nanowires of InN and InP under carefully controlled growth conditions, current and capacitance measurements are obtained to reveal the nature of the nanowire-electrolyte interface and how light is translated into photocurrent for InP and a photovoltage in InN. The materials system is expanded into the III-V nitride semiconductors, in which it is shown that varying the morphology of GaN on silicon yields insights to how the interface and light conversion is modulated as a basis for future designs. Current extensions of this work address growth and tuning of the III-V nitride electrodes with doping and polarization engineering for efficient coupling to solar-driven chemical reactions, and rapid-throughput methods for III-V nanomaterials synthesis in this materials space.

Theoretical and experimental investigations on the response time improvement of biased and unbiased long-wave infrared (LWIR) HgCdTe detectors operating at temperatures T = 230K were presented in this paper. MOCVD technology is an excellent tool in fabrication of different HgCdTe detector structures with a wide range of composition, donor/acceptor doping and without post grown ex-situ annealing. Donor doping efficiency in (111) and (100) oriented HgCdTe layers has been discussed. The time constant is lower in biased detectors due to Auger suppression phenomena and reduction of diffusion capacitance related to wider depletion region. The relatively high bias currents requirements and excessive low frequency noise which reduces the detectivity of biased detectors inspire researches on the time constant improvement of unbiased detectors. The response time of high-operating temperature (HOT) LWIR HgCdTe detectors revealed complex behavior being dependent on the applied the reverse bias, the operating temperature, the absorber thickness and doping, the series resistance and the electrical area of the devices.

In this paper, we utilize the fin-shaped channel to form the AlGaN/GaN HEMTs which can be considered as “Fin- HEMTs” to adjust the threshold voltage (VTH) toward positive value. The gate metal here is deposited directly on the AlGaN/GaN semiconductor to form the Schottky contact. Although the fin-widths are in the level of micron-scale, the shifts of VTH are still observable and the VTH becomes more positive with the smaller fin width. This is attributed to the assistant of the side-gate control which can be regarded as the depletion layer formed by Schottky contact at side-gate will deplete the 2DEG in the channel. Therefore, with the smaller fin width, the channel can be pinched off faster which is similar to the double gate MESFETs. The VTH of planar device is shifted from -3.81 V to -3.37 V with 2-μm-fin-width. On the other hand, unlike to carrier transportation in the conventional FinFET with nano-scale fin width which is dominated by the surface scattering, our Fin-HEMTs with micron-scale fin width exhibit higher drain current than planar device and this is because of the smaller thermal resistance (RTH) for the fin-HEMT. We extract the RTH by varying the measured temperature and the RTH of the device with 4-μm-fin-width and planar device is 58.5 K/W and 249.5 K/W, respectively.

In this paper, the two-dimensional (2-D) simulations of p-i-n type InP/In0.53Ga0.47As/InP photodetector at low bias are presented. The modeling results fit the experimental results well, verifying the validity of our model and the desirability of the simulated results. In order to further optimize the detector structure, the effects of thickness and doping concentration of the absorption layer on the dark current are both simulated and discussed, the results can be explained by the depletion region width in the junction area, and the details are shown in the energy band diagrams and the electric field maps of the p-i-n InP/In0.53Ga0.47As/InP photodetector.

The Soft X-ray Imager (SXI) is an X-ray CCD camera onboard the ASTRO-H X-ray observatory. The CCD chip used is a P-channel back-illuminated type, and has a 200-µm thick depletion layer, with which the SXI covers the energy range between 0.4 keV and 12 keV. Its imaging area has a size of 31 mm x 31 mm. We arrange four of the CCD chips in a 2 by 2 grid so that we can cover a large field-of-view of 38’ x 38’. We cool the CCDs to -120 °C with a single-stage Stirling cooler. As was done for the CCD camera of the Suzaku satellite, XIS, artificial charges are injected to selected rows in order to recover charge transfer inefficiency due to radiation damage caused by in-orbit cosmic rays. We completed fabrication of flight models of the SXI and installed them into the satellite. We verified the performance of the SXI in a series of satellite tests. On-ground calibrations were also carried out and detailed studies are ongoing.

We demonstrate visible-light electroluminescence due to d-d transitions in GaAs:Mn based light emitting diodes (LEDs) [1][2]. We prepared p+n junctions with a p+GaAs:Mn layer. At a reverse bias voltage (-3 to -6V), holes are injected from the n-type layer to the depletion layer and accelerated by the intense electric field, and excite the d electrons of Mn in the p+GaAs:Mn layer by impact excitations. We observe visible-light emission E1 = 1.89eV and E2 = 2.16eV, which are exactly the same as the 4T1 -> 6A1 and 4A2 -> 4 T1 transition energy of Mn. Furthermore, by utilizing optical transitions between the p-d hybridized orbitals of Mn atoms doped in Si, we demonstrate Si-based LEDs that continuously emit reddish-yellow visible light at room temperature. The Mn p-d hybrid states are excited by hot holes that are accelerated in the depletion layers of reverse biased Si pn junctions. Above a threshold reverse bias voltage of about -4V, our LEDs show strong visible light emission with two peaks at E1 = 1.75eV and E2 = 2.30eV, corresponding to optical transitions from the t-a (spin-down anti-bonding) states to the e- (spin-down non-bonding) states, and from the e- to the t+a (spin-up anti-bonding) states. The internal quantum efficiency of the E1 and E2 transitions is 3-4 orders of magnitude higher than that of the indirect band-gap transition [3]. [1] P. N. Hai, et al., APL 104, 122409 (2014). [2] P. N. Hai, et al., JAP 116, 113905 (2014). [3] P. N. Hai, et al., submitted.

This paper presents recent developments done at CEA-LETI Infrared Laboratory on processing and characterization of p-on-n HgCdTe (MCT) planar infrared focal plane arrays (FPAs) in LWIR and VLWIR spectral bands. These FPAs have been grown using liquid phase epitaxy (LPE) on a lattice matched CdZnTe substrate. This technology presents lower dark current and lower serial resistance in comparison with n-on-p vacancy doped architecture and is well adapted for low flux detection or high operating temperature. This architecture has been evaluated for space applications in LWIR and VLWIR spectral bands with cutoff wavelengths from 10μμm up to 17μm at 78K. Innovations have been introduced to the technological process to form a heterojunction with a LPE growth technique. The aim was to lower dark current at low temperature, by decreasing currents from the depletion region. Electro-optical characterizations on p-on-n photodiodes have been performed on QVGA format FPAs with 30μm pixel pitches. Results show excellent operabilities in current and responsivity, with low dispersion and noise limited by current shot-noise. Studies performed on dark current show that dark current densities are consistent with the heuristic prediction law "Rule07" at 78K. Below this temperature, dark current varies as a pure diffusion current.

A detailed understanding of limiting dark current mechanisms in InAs/GaSb type-II superlattice (T2SL) infrared detectors is key to improve the electrooptical performance of these devices. We present a six-component dark current analysis which, for the first time, takes account of sidewall-related dark current contributions in mesa-etched T2SL photodiodes. In a wide temperature range from 30K to 130K, the paper compares limiting mechanisms in two homojunction T2SL photodiode wafers for the long-wavelength infrared regime. While the two epi wafers were fabricated with nominally the same frontside process they were grown on different molecular beam epitaxy systems. In the available literature a limitation by S hockley-Read-Hall processes in the space charge region giving rise to generation-recombination (GR) dark current is the prevailing verdict on the bulk dark current mechanism in T2SL homojunction photodiodes around 77K. In contrast, we find that investigated photodiode wafers are instead limited by the diffusion mechanism and the ohmic shunt component, respectively. Furthermore, our in-depth analysis of the various dark current components has led to an interesting observation on the temperature dependence of the shunt resistance in T2SL homojunction photodiodes. Our results indicate that the GR and the shunt mechanism share the same dependence on bandgap and temperature, i.e. , a proportionality to exp(- E

Multi-Quantum well solar cells (MQWSC) have been shown to present several advantages, among which are low dark currents and tunable bandgaps. They are especially suited for implementation in multi-junction cells, and are highly promising for absorbers in Hot Carrier Solar Cells (HCSC). Such applications require high concentration ratio, which arises the issue of collection efficiency. Whereas it is usually considered that collection in MQW is very close to unity at one sun, it has been shown to not be the case under high concentration at the maximum power point. We propose in this work to take advantage of the luminescence spectral variation to investigate the depth collection efficiency. In order to validate the model, a series of strain compensated InGaAs/GaAsP MQW solar cells with intentional variation of the MQW doping concentration are grown. This has the effect of switching the space charge region position and width as well as the electric field intensity. Recording the luminescence spectra at various illumination intensities and applied voltages, we show that the in-depth quasi-Fermi level splitting and thus collection properties can be probed. Other measurements (EQE, luminescence intensity variation) are shown to be consistent with these results. Regarding their use as HCSC, the luminescence of MQW solar cells has been mainly used so far for investigating the quasi-Fermi level splitting and the temperature. Our results improve our understanding by adding information on carrier transport.

We show here that n-type InAs/InGaSb superlattices can be electrically isolated from lightly doped n-type GaSb substrates at much higher temperatures than from the more common undoped p-type GaSb substrates without the use of a large band gap insulating buffer layer. Temperature dependent Hall effect measurements show superlattice conduction up to near room temperature, which is significantly higher than the 20 K observed for p-type substrates. Multi-carrier analysis of magnetic field dependent transport data demonstrate the absence of a substrate related conduction channel. We argue that the isolation is due to the depletion layer at the p-n junction between the p-type buffer layer and the n-type substrate.

Under elevated defect concentrations, MWIR, III-V nBn detectors exhibit diffusion limited performance with elevated dark current densities. The resulting diffusion current is limited by the generation of carriers through defect states in the neutral n-type absorber and a dark current dependence on the defect density described by one of two limits, a short absorber or long absorber limit. This characteristic contrasts that exhibited by defect limited, conventional pn junction based photodiodes which exhibit performance limited by Shockley-Read-Hall generation in the depletion layer rather than diffusion based processes.

We have been developing monolithic active pixel sensors, known as Kyoto's X-ray SOIPIXs, based on the CMOS SOI (silicon-on-insulator) technology for next-generation X-ray astronomy satellites. The event trigger output function implemented in each pixel offers microsecond time resolution and enables reduction of the non-X-ray background that dominates the high X-ray energy band above 5--10 keV. A fully depleted SOI with a thick depletion layer and back illumination offers wide band coverage of 0.3--40 keV. Here, we report recent progress in the X-ray SOIPIX development. In this study, we achieved an energy resolution of 300~eV (FWHM) at 6~keV and a read-out noise of 33~e- (rms) in the frame readout mode, which allows us to clearly resolve Mn-K$\alpha$ and K$\beta$. Moreover, we produced a fully depleted layer with a thickness of $500~{\rm \mu m}$. The event-driven readout mode has already been successfully demonstrated., Comment: 7pages, 12figures, SPIE Astronomical Telescopes and Instrumentation 2014, Montreal, Quebec, Canada. appears as Proc. SPIE 9147, Space Telescopes and Instrumentation 2014: Ultraviolet to Gamma Ray

X-ray CCDs are widely used as the focal plane detectors of the X-ray telescopes. Among them, backside illuminated CCDs with a deep depletion layer are preferred because of their high quantum efficiency in both soft and hard X-ray bands. However, they tend to have poorer energy resolution and higher background due to the relatively large charge diffusion. We carried out simple experiments to apply a magnetic field of 0.25 T or 0.4 T to the CCD, which is expected to suppress the charge diffusion very slightly and to bring subtle improvement in the performance of the CCD. We found unexpectedly that grade branching ratios of Grade 3 and Grade 4, both are horizontal split events, symmetrically changed depending on the direction of the applied magnetic field. Although the cause of the change is not understand yet, it clearly demonstrate that the charge cloud in the CCD is affected by the externally applied magnetic field. We also found a decrease of Grade 7 only in the experiment 2. We consider this may be caused by the supress of the charge diffusion by the magnetic field, although other possibilities can not be excluded. No significant improvement was detected in the energy resolution. We could show with these experiments that the charge cloud in the CCD may be controlled by the externally applied magnetic field. Magnetic field may become useful tool in future to improve the performance of CCDs.

Recently, a new strategy used to achieve high operation temperature (HOT) infrared photodetectors including III-V compound materials (bulk materials and type-II superlattices) and cascade devices has been observed. Another method to reduce detector’s dark current is reducing volume of detector material via a concept of photon trapping detector. The barrier detectors are designed to reduce dark current associated with Shockley-Read (SR) processes and to decrease influence of surface leakage current without impeding photocurrent (signal). In consequence, absence of a depletion region in barrier detectors offers a way to overcome the disadvantage of large depletion dark currents. So, they are typically implemented in materials with relatively poor SR lifetimes, such as all III-V compounds. From considerations presented in the paper results that despite numerous advantages of III-V barrier detectors over present-day detection technologies, including reduced tunneling and surface leakage currents, normal-incidence absorption, and suppressed Auger recombination, the promise of a superior performance of these detectors in comparison to HgCdTe photodiodes, has not been yet realized. The dark current density is higher than that of bulk HgCdTe photodiodes, especially in MWIR range. To attain their full potential, the following essential technological limitations such as short carrier lifetime, passivation, and heterostructure engineering, need to be overcome.

As the technologies in focal plane array (FPA) progresses, the industry is pushing for smaller pixel size and spacing between the pixels. The reduction in pixel size and spacing will increase both the resolution and fill factor which reduces the cost and increases the performance. However, as the density of the array elements increases, the crosstalk between the nearest neighboring pixels become a significant issue. Here we examine the case for a planer FPA with epitaxially grown NIN+ structure and the planer junctions are formed by diffusing P-type dopant into the N doped layer. We first examine the possible spacing by considering the lateral depletion region width to set the upper boundary for the spacing. The depletion region width is calculated by solving Poisson’s equation for Gaussian doping profile and the isolation of adjacent pixel is dependent on the formation of the back to back diodes to block the current flowing towards the device. Therefore overlap of the depletion regions indicates shorting and sets the minimum possible spacing for this structure. The electrical and optical crosstalks are modeled by using a DC resistive model to gauge the effect of current flow as the spacing reduces. Series of device arrays with various device pitches and device sizes ranging from 5 μm to 10 μm with device pitch from 5.5 μm to 15 μm are fabricated and tested under both dark and illumination conditions for their electrical performances including the crosstalk. The simulated and measured results will be presented.

The short wavelength infrared (SWIR) band near 1.0-3.0μm plays an important role in many applications such as weather forecast, earth environmental or resource observation, low light level systems and astronomical observation. It is well known that InGaAs detectors can shift the cutoff wavelength from 1.7μm to 2.5μm with the higher fraction of indium in the ternary In X Ga 1-X As material grown on InP, which results to material defects and poorer device characteristics due to the lattice mismatch. Dark current characteristics of extended wavelength InGaAs detectors were investigated in this paper. Dark current mechanisms for extended InGaAs detectors with different absorption layer parameters and device fabrication process were analyzed according to current-voltage curves at different temperatures and bias voltages. Activation energy of devices was extracted from current-voltage curves. Activation energy is related with absorption layer concentration and test temperature. Activation energy is the higher for the devices with the higher absorption layer concentration at lower bias voltage at the same temperature range, which shows that the narrower width of the depletion layer in the devices results to the lower generation-recombination current. The devices with the optimized etching and passivation parameters show higher thermal activation energy and the lower dark current. Dark current mechanisms of the ones are dominated by diffusion current at the higher temperature and lower bias voltage, whereas dominated by internal generation-recombination current and ohmic leakage current at the lower temperature.

CuInSe2 films for photovoltaic applications grown under Cu-excess have been rarely investigated up to now. In general CuInSe2 solar cells use an overall Cu-poor absorber. In this work we argue that it is valuable to investigate Cu-rich solar cells, since all the basic material properties are better in Cu-rich absorbers. With less defects in the bulk and better transport properties it is somehow intriguing why devices with Cu-rich absorber perform less. We demonstrate that this can be attributed to the too high doping of these films. Such a high native doping leads to tunneling enhanced recombination and interface recombination, strongly affecting the devices performances. We demonstrate different attempts to overcome the problem of doping: at first a Cu-poor surface was grown on the Cu-rich absorbers which enables to decrease the doping in the space charge region, then to directly decrease the doping in the bulk, the influence of sodium content was investigated. Finally, here we show that different selenium activity during the absorber growth enables to decrease the doping of these films and to open thus a way to fully exploit the favorable properties of the Curich CuInSe2 films.

Recently excellent infrared detectors have been demonstrated using InAs/GaSb superlattice materials sensitive at wavelength from 3um to greater than 32um. Using empirical tight binding method (ETBM), different structures as InAs(xML)/GaSb(8ML), (x=2, 4, 6, 8) and InAs(14ML)/GaSb(7ML) were designed for various cut-off wavelengths from short to long IR wavelength. These materials were grown by MBE with valved cracker cells for arsenic and antimony on p-type GaSb(001) substrates. The microstructure and the bandgap Eg were verified by high resolution X-ray diffraction and photoresponse spectra. The temperature dependence of Eg and photoresponse responsivity Rv were studied. The differential resistance under zero bias R0 in MWIR photodiode was measured up to 106 ohms. The ideality factor in the range of 1.5 to 2.1 indicates the generation-recombination current and surface leakage current are the dominant leakage in the depletion region. These results will promote InAs/GaSb superlattices infrared detectors research in multi-color from short to long wave IR application.

InAsSb ternary alloy is potentially capable of operating at the longest cut-off wavelength (about 9 μm at 77 K) in the entire III-V family. Recently, there has been a considerable progress in development of the InAsSb focal plane arrays. The high operation temperature conditions were successfully achieved with A III B V unipolar barrier structures including InAsSb/AlAsSb material system. In this paper, the performance of medium wavelength infrared (MWIR) InAsSb-based nBnnn+ detectors, called also "bariodes", is examined theoretically taking into account thermal generation governed by the Auger and radiative mechanisms. In our model, the heterojunction barrier-active region (absorber) is assumed to be decisive as the contributing dark current mechanism limiting detector's performance. Since there is no depletion layer in the active layer of such devices, generation-recombination and trap assisted tunneling mechanisms are suppressed leading to lower dark currents in bariode detectors in comparison to standard diodes. Detailed analysis of the detector’s performance (such as dark current, RA product, and current responsivity) versus bias voltage and operating temperatures are performed pointing out optimal working conditions. The theoretical predictions of bariode parameters are compared with experimental data published in the literature. Finally, the bariode performance is compared with standard p + -on-n InAsSb photodiodes operated at room temperature with the same bandgap wavelength.

This paper presents one-dimensional numerical simulations and analytical modeling of ideal (only diffusion current and only Auger-1 and radiative recombination) InAs nBn detectors having n-type barrier layers, with donor concentrations ranging from 1.8×1015 to 2.5×1016 cm-3. We examine quantitatively the three space charge regions in the nBn detector with an n-type barrier layer (BL), and determine criteria for combinations of bias voltage and BL donor concentration that allow operation of the nBn with no depletion region in the narrow-gap absorber layer (AL) or contact layer (CL). We determine the quantitative characteristics of the valence band barrier that is present for an n-type BL. From solution of Poisson’s equation in the uniformly doped BL, we derive analytical expressions for the valence band barrier heights versus bias voltage for holes in both the AL and the CL. These expressions show that the VB barrier height varies linearly with the BL donor concentration and as the square of the BL width. Using these expressions, we constructed a phenomenological equation for the dark current density versus bias voltage which agrees reasonably well with the shape of the J(V) curves from numerical simulations. Our simulations suggest that the nBn detector should be able to be operated at or near zero-bias voltage.

The noise behavior of InAs/GaSb superlattice photodiodes for high-performance thermal imaging in the mid- and longwavelength infrared atmospheric windows at 3-5 μm and 8-12 μm is complex and up to now not very well understood. In order to characterize these devices we have developed a noise measurement setup with a noise current resolution in the femtoampere range. First, we show that, when sidewall leakage is absent, InAs/GaSb superlattice photodiodes with a low dark current very close to the generation-recombination limited dark current level of the bulk behave according to the well-known shot noise expression. Next, we investigate a set of 18 large-area diodes with a bandgap in the midwavelength infrared regime, which show an increased dark current depending linearly on the applied reverse bias. For these diodes the common shot noise model generally fails to describe the noise experimentally observed in the white part of the noise spectrum. Instead, we find that McIntyre’s excess noise model for electron-initiated avalanche multiplication processes fits our data remarkably well for the entire set of diodes, which covers about three orders of magnitude in dark current and a wide range of reverse bias voltage. Thus, to explain the mechanism leading to the increased reverse dark current and observed excess noise we tentatively suggest that primary electrons originating from Shockley-Read-Hall states within the space charge region might initiate avalanche multiplication processes within high electric field domains localized around sites of macroscopic crystallographic defects.

The radiation detectors based on the third era semiconductor material silicon carbide (SiC) with wide energy band gap are the most promising ionizing radiation detectors in high temperature and harsh radiation environment. This paper illustrated several important advantages of the SiC neutron detector and described its fabrication and detection principle briefly. Furthermore, we evaluated the neutron sensor’s measurement performance when detecting the 2.5MeV and 14MeV neutrons under different conditions of sensor’s active layer thickness and polyethylene converter film thickness based on MCNP simulation. According to the results of simulation, the sensor’s optimal configuration was designed. For the sensor whose radius and depletion layer thickness are 3mm and 30μm respectively, the detection efficiency can reach 3.16×10 -18 coulomb per neutron (c/n) and 1.80×10 -17 c/n for 2.5MeV and 14MeV neutrons respectively. When adding a polyethylene converter film of 90μm thickness to the above sensor, the detection efficiency to 2.5MeV neutron will be 3.7 times that without neutron converter film; and with the converter film of 2mm thickness, the detection efficiency to 14MeV neutron will be improved by 246%.

In this paper we investigate the performance of a transparent photoconductive sensor based on a double layer of organic materials (m-MTDAB / PTCBI) which are deposited on two interdigitated transparent ITO electrodes. The performance of the sensor is demonstrated with electro-optical measurements: the I(V) curves consist of two linear sections meeting at a knee voltage Vt. Linear regression performed on the I(V) curves below Vt show that the conductance is a power law of the luminance incident on the device. We present a model to describe the behaviour of the sensor below Vt. We present measurements of I(t) for a transient illumination of the sensor. Plotting the inverse of the current as a function of time we find that the transient is consistent with the model for voltages below Vt. For voltages above Vt we find that the sensor behaves like a resistor in series with a space charge (SC) region. We present a local illumination experiment that confirms the existence of a SC region between the electrodes of the photoconductive sensor for V