Showing papers on "Responsivity published in 1997"

Low noise p-π-n GaN ultraviolet photodetectors

Abstract: We report on the fabrication and characterization of p-π-n GaN ultraviolet detectors. The peak responsivity at ∼363 nm is measured to be 0.1 A/W in the photovoltaic mode, and 0.14 A/W with a bias of −15 V. Speed measurements have shown the photoresponse to be RC-limited with the response time decreasing from 17.4 ns at zero bias to 10.3 ns at −6 V bias. For a 200×200 μ m2 device, we measure the dark current to be 2.7 pA at −3 V bias, and a noise density of less than 10−25 A2/Hz, the noise floor of the measurement. Extrapolating the noise data taken at higher reverse biases, we estimate the noise equivalent power to be 6.6×10−15 W/Hz1/2.

High speed, low noise ultraviolet photodetectors based on GaN p-i-n and AlGaN(p)-GaN(i)-GaN(n)structures

Abstract: We have investigated the spectral response of front-surface-illuminated GaN and AlGaN/GaN p-i-n ultraviolet photodetectors prepared by reactive molecular beam epitaxy on sapphire substrates. GaN homojunction p-i-n photodiodes exhibited a peaked response near the band edge. This enhanced response was absent in the AlGaN/GaN heterojunction p-i-n detectors. We analyzed the effect of p-layer thickness of the GaN p-i-n diodes on the magnitude of the peak photoresponse. The AlGaN/GaN photodiodes had a maximum zero-bias responsivity of 0.12 A/W at 364 nm, which decreased by more than 3 orders of magnitude for wavelengths longer than 390 nm. A reverse bias of −10 V raised the responsivity to 0.15 A/W without any significant increase in noise. The root-mean-square noise current in a 1 Hz bandwidth is ∼1.0 pA, corresponding to a noise-equivalent-power of ∼8.3 pW. We measured extremely fast decay times of 12 ns for the AlGaN/GaN and 29 ns for the GaN photodiodes.

High performance InAs/Ga1-xInxSb superlattice infrared photodiodes

Abstract: The optical and electrical properties of infrared photodiodes diodes based on InAs/(GaIn)Sb superlattices grown by molecular beam epitaxy were investigated. The diodes, with a cut-off wavelength around 8 μm show a current responsivity of 2 A/W. By proper adjustment of the p-doping level above the n-background concentration the depletion width exceeds a critical size of about 60 nm, leading to the suppression of band-to-band tunneling currents. Above that critical width the dynamic impedance R0A at 77 K reaches values above 1 kΩ cm2 leading to a Johnson-noise-limited detectivity in excess of 1×1012 cm√Hz/W.

Ultraviolet-sensitive, visible-blind GaN photodiodes fabricated by molecular beam epitaxy

Abstract: GaN p–i–n photovoltaic diode arrays were fabricated from epitaxial films deposited on sapphire by molecular beam epitaxy. Peak UV responsivity was 0.11 A/W at 360 nm, corresponding to 48% internal quantum efficiency. Visible rejection over 400–800 nm was 3–4 orders of magnitude. Typical pulsed time response was measured at 8.2 μs. Spectral response modeling was performed to analyze the photocurrent contributions from photogenerated carrier drift in the depletion region and from minority carrier diffusion in the p and n layers. With the model, a maximum internal quantum efficiency of 55% at 360 nm was calculated for the photovoltaic diode structure.

Very low dark current metal-semiconductor-metal ultraviolet photodetectors fabricated on single-crystal GaN epitaxial layers

Abstract: We report a very low dark current (∼57 pA at 10 V reverse bias) metal–semiconductor–metal photodetectors fabricated on GaN epitaxial layers grown by low-pressure metalorganic chemical vapor deposition. The photodetectors exhibit the typical sharp band-edge cutoff, with good responsivity. There is indication of a photoconductive gain mechanism. We also performed a Medici simulation to establish an effective area for current density calculations.

Ultrafast Photoconductive Detectors Based on Semi-Insulating GaAs and InP

Abstract: Photoconductive dipole antennas fabricated on semi-insulating (SI) GaAs and SI-InP were used to detect terahertz (THz) pulses. The responses of these long-carrier-lifetime photoconductive detectors were compared to that of the photoconductive antenna fabricated on a low-temperature grown GaAs (LT-GaAs) with a subpicosecond carrier lifetime. The SI-InP-based photoconductive detector showed a higher responsivity and a better signal-to-noise ratio (SNR) than the LT-GaAs-based photoconductive detector at low gating laser powers. The SI-GaAs-based detector, however, showed a responsivity comparable to that of the LT-GaAs photoconductive detector only at very weak gating laser power, and the SNR of the SI-GaAs-based detector was poor for overall gating laser powers due to the high background noise originating from a large amount of stray photocurrent.

A high strain two-stack two-color quantum well infrared photodetector

Abstract: A high strain two-stack, two-color, InGaAs/AlGaAs and AlGaAs/GaAs quantum well infrared photodetector for midwavelength infrared (MWIR) and long wavelength infrared (LWIR) detection has been demonstrated. Each stack is designed to have detection in one of the two atmospheric windows, 3–5 μm and 8–12 μm, respectively. The MWIR stack has employed 35% of indium in the InGaAs well, which not only achieved peak wavelength at 4.3 μm, but also obtained very high peak responsivity of Rp=0.65 A/W, using 45° light coupling. Normal incidence without grating coupling also has high responsivity with 40%–50% in the MWIR stack and 35%–45% in the LWIR stack, respectively, compared with the 45° incidence. Despite the large in-plane compressive strain induced by the high indium concentration, the device is highly uniform and has very low dark current in the MWIR stack. The background limited temperature is 125 K for the MWIR stack with a cutoff wavelength λc=4.6 μm, and is 70 K for the LWIR stack with λc=10 μm.

Growth and characterization of InAs/GaSb photoconductors for long wavelength infrared range

Abstract: In this letter we report the molecular beam epitaxial growth and characterization of InAs/GaSb superlattices grown on semi-insulating GaAs substrates for long wavelength infrared detectors. Photoconductive detectors fabricated from the superlattices showed photoresponse up to 12 μm and peak responsivity of 5.5 V/W with Johnson noise limited detectivity of 1.33×109 cm Hz1/2/W at 10.3 μm at 78 K.

Characteristics of quantum well infrared photodetectors

Abstract: A model is presented for the performance of quantum well infrared photodetectors (QWIPs) utilizing intersubband electron transitions and tunneling injection electrons. The dark current and the responsivity are derived as functions of the QWIP parameters, including the number of the QWs, in an analytical form. Nonlinear effects in the QWIP operation at high infrared power are considered and the threshold value of power density is estimated.

Photovoltaic quantum well infrared photodetectors: The four-zone scheme

Abstract: We have investigated a particular class of photovoltaic quantum well intersubband photodetectors. Each period of the active region in these structures consists of four zones, namely an excitation zone, a drift zone, a capture zone, and a tunneling zone. The devices show pronounced photovoltaic behavior and high detectivities. In particular, the responsivity without external bias is substantially enhanced if resonant carrier capture is achieved due to an appropriate design of the capture zone.

Diluted- and distributed-absorption microwave waveguide photodiodes for high efficiency and high power

Abstract: We propose a new microwave photodetector with high responsivity, which can handle both high optical power and deliver high microwave-output power. The distributed-absorption waveguide photodiode (DWGPD) is specially designed to equally distribute the photocarriers over the maximum surface area available in order to reduce the nonlinearities of electrical response due to electric-field screening effects. The expected calculated performances of this DWGPD is compared to other types of broad-band photodetectors in terms of linear microwave-output power, quantum efficiency, and thermal behavior. The fabricated DWGPDs have a responsivity of 1 A/W, cutoff frequency of 29 GHz, and linear response up to 8 mA limited in this experiment by contacting electrodes.

Optimization of noise and responsivity in CMOS active pixel sensors for detection of ultralow-light levels

Abstract: In this paper, we present results of the investigation of the design and operation of CMOS active pixel sensors for detection of ultra-low light levels. We present a detailed noise model of APS pixel and signal chain. Utilizing the noise model, we have developed APS pixel designs that can achieve ultra-low noise and high responsivity. We present results from two test chips, that indicate (1) that less than 5 electrons of read noise is possible with CMOS APS by reducing the size of the pixel transistors, and (2) that high responsivity can be achieved when the fill-factor of the photodiode is reduced.

Photoconductivity nonlinearity at high excitation power in quantum well infrared photodetectors

Abstract: Nonlinear photoconductivity effects at high excitation power in quantum well infrared photodetectors (QWIPs) are studied both experimentally and theoretically. The photoconductivity nonlinearity is mainly caused by a redistribution of the electric potential at high power, which leads to a decrease of electric field in the bulk of the QWIP. As a result of the decreased field, the photoexcited electron escape probability and drift velocity decrease resulting in a decrease of responsivity.

Bias effects in high performance GaAs homojunction far-infrared detectors

Abstract: A high performance, bias tunable, p-GaAs homojunction interfacial workfunction internal photoemission far-infrared detector is demonstrated. A responsivity of 3.10±0.05 A/W, a quantum efficiency of 12.5%, and a detectivity D* of 5.9×1010 cmHz/W were obtained at 4.2 K for cutoff wavelengths from 80 to 100 μm. The bias dependences of the quantum efficiency, detectivity, and cutoff wavelength were measured and are well explained by the theoretical model. The effect of the layer number on detector performance and the uniformity of the detectors are discussed. A comparison with Ge:Ga photoconductive detectors suggests that similar or even better performance may be obtainable with a far-infrared detector.

Properties of a photovoltaic detector based on an n-type GaN Schottky barrier

Abstract: In this article, we report on the characterization of a photovoltaic detector based on an n-type GaN Schottky barrier. We first present the photovoltaic responsivity above the gap. Its spectrum is explained by the combined effects of absorption and diffusion. The hole diffusion length is estimated to be in the 0.1 μm range with a numerical model. The photoresponse below the gap is also investigated and it is shown that the current generated by the internal photoemission is the major contribution to the photocurrent at reverse biases at 80 K. At room temperature, an additional component to the photocurrent is clearly demonstrated and identified. This extra current stems from the existence of traps. Several spectroscopy techniques are used to characterize those traps. The supplementary current emitted from the traps in the depletion region accounts for the spectral and the temporal behavior of the Schottky photodetector at room temperature.

High-speed polysilicon resonant-cavity photodiode with SiO/sub 2/-Si Bragg reflectors

Abstract: Previously, it has been shown that the bandwidth of Si photodiodes can be increased by more than an order of magnitude, without sacrificing responsivity, by a resonant-cavity structure that utilized GeSi-Si asymmetric Bragg reflectors. In this letter, we report an interdigitated p-i-n polysilicon resonant-cavity photodiode, which employs a Si-SiO/sub 2/ Bragg reflector, that is more compatible with standard Si processing technology. For an absorbing region thickness of only 0.5 /spl mu/m, a peak quantum efficiency of 40% was achieved and the dark current was <60 nA at 10 V. For 2 /spl mu/m/spl times/2-/spl mu/m finger width and spacing the bandwidth was 10 GHz.

GaAs multilayer p+-i homojunction far-infrared detectors

Abstract: A molecular beam epitaxy grown wavelength tunable GaAs p+-i homojunction interfacial work-function internal photoemission far-infrared detector is developed The multilayer (p+-i-p+-i-…) detector structures consist of 2, 5, and 10 emitter layers Experimental results are explained in terms of the number of emitter layers and the doping concentrations of the emitter layer A detector with 10 multilayers and an emitter layer doping concentration (Ne) of 3×1018 cm−3 shows a current responsivity of 2 A/W, an effective quantum efficiency of 92% (at 263 μm) with a cutoff wavelength of 85 μ and the noise equivalent power of 218×10−12 W/Hz at 42 K

Fabrication of semiconducting YBaCuO surface-micromachined bolometer arrays

Abstract: Thermal infrared detectors require thermal isolation to permit the infrared-sensitive material to integrate the incident photon energy and thereby obtain high responsivity and detectivity. This paper describes the fabrication of semiconducting YBaCuO microbolometer arrays into thermal isolation structures by employing Si surface-micromachining techniques. An isotropic HF:HNO/sub 3/ etch was used to remove the underlying Si substrate from the front-side of the wafer and suspend SiO/sub 2/ membranes into 1/spl times/10 pixel-array structures. The infrared-sensitive material, YBaCuO, was subsequently deposited onto the thermal isolation structures and patterned to form the detector arrays. The high-temperature coefficient of resistance and low noise of semiconducting YBaCuO at room temperature is attractive for uncooled infrared detection. The fabrication process was conducted entirely at room temperature. In this manner, infrared detectors are fabricated in a process that is compatible with CMOS technology to allow for the integration with on-chip signal processing circuitry. The end result is low-cost infrared-detector arrays for night vision in a variety of applications including transportation and security. Preliminary results show a temperature coefficient of resistance above 3%, voltage responsivity close to 10/sup 4/ V/W, and detectivity over 10/sup 7/ cm/spl middot/Hz/sup 1/2//W at a bias current of 0.79 /spl mu/A.

High quantum efficiency metal-semiconductor-metal ultraviolet photodetectors fabricated on single-crystal GaN epitaxial layers

Abstract: The authors report a GaN metal-semiconductor-metal photodetector with high quantum efficiency (~50%) in the absence of internal gain. These photodetectors have a flat responsivity above the bandgap (measured at ~0.15 A/W) with a sharp, solar-blind cutoff at the band edge. There is no discernible responsivity for photons below the bandgap energy. In addition, a very low dark current was obtained, measured at ~800 fA at –10 V reverse bias.

Antenna-coupled polycrystalline silicon air-bridge thermal detector for mid-infrared radiation

Abstract: Polycrystalline silicon (polysilicon) airbridges coupled to log-periodic and bow-tie planar antennas have been fabricated for mid-infrared radiation detection. The detectors in thermocouple and bolometer modes exhibit a higher electrical responsivity than previously reported metal based antenna-coupled thermal detectors. Measurements made at 10.2 μm wavelength indicate that the linearly polarized antennas have an on-axis cross-polarization ratio of about −4.5 dB and the pattern in the free-space side has a directivity of approximately 5–9 dB. The coupling efficiency is about 15%. Frequency responses for modulated optical signals agree with thermal transport models.

A high-speed ITO-InAlAs-InGaAs Schottky-barrier photodetector

Abstract: A high-speed and high-sensitivity vertical indium-tin-oxide-InAlAs-InGaAs Schottky barrier photodetector has been designed, fabricated, and characterized. The devices had dark current densities as low as 8.87/spl times/10/sup -5/ A/cm/sup -2/ at an applied bias of 5 V. The responsivity for all the devices tested ranged from 0.55-0.60 A/W at a wavelength of 1.31 /spl mu/m, and 0.563-0.583 A/W at 1.55 /spl mu/m. The 15-/spl mu/m diameter devices exhibited a -3 dB bandwidth of 19 and 25 GHz at a wavelength of 1.55 /spl mu/m and an applied bias of 5 and 10 V, respectively. These are the best values of responsivity and bandwidth for a vertical InGaAs-based Schottky-barrier photodetector reported to date.

Optical quenching of photoconductivity in GaN photoconductors

Abstract: The observation of optical quenching of photoconductivity in GaN photoconductors at room temperature is reported on. Three prominent quenching bands were found at Ev+1.44, 1.58, and 2.20 eV, respectively. These levels are related to three hole traps in GaN materials based on a hole trap model to interpret the quenching mechanism. The responsivity was reduced about 12% with an additional He–Ne laser shining on the photoconductor.

Normal incident long-wavelength quantum well infrared photodetectors using electron intersubband transitions

Abstract: Grating-free quantum well infrared photodetectors (QWIPs) for 8–10 μm applications have been fabricated. The QWIPs fabricated with InGaAs/GaAs quantum wells show excellent performance with normal-incident infrared radiation. A detectivity of 2×1010 cm Hz1/2/W and a responsivity of 0.23 A/W have been obtained for these devices. The devices were compared with QWIPs with 1D and 2D surface gratings. No significant improvement in responsivity is observed for devices with 1D gratings. Devices with 2D gratings have significantly higher peak responsivity but only in a very limited spectral range. The GaAs/AlGaAs QWIPs in the similar spectral range have also been fabricated and, however, shown much weaker TE absorption.

High-speed infrared detection by uncooled photovoltaic quantum well infrared photodetectors

Abstract: We have investigated the temporal response of uncooled photovoltaic quantum well infrared photodetectors (PV QWIP) using 2 ps pulses from a free electron laser. We measured rise and fall times of 8 ps and 63 ps, respectively. We have also determined the temperature dependence of the voltage responsivity and resistance of the sample from 70 K to 300 K. Based on these measurements, we discuss the figures of merit of PV QWIPs in a high-bandwidth 50 Ω environment.

Characterization of a polarization-independent transmission trap detector.

Abstract: A six-element polarization-independent transmission trap detector with coaxial input and output beams has been constructed and full characterized. The measured optical parameters are compared with their values, predicted by Fresnel equations. Measured transmittances are in agreement with the predicted values within 2 × 10-5 in the wavelength region from 450 to 650nm. The spectral responsivity of the transmission trap detector is in agreement with the predicted values within 0.035% at 543.5- and 633.0-nm vacuum wavelengths. The spatial uniformity of the responsivity is ±0.03% across the active area of approximately 5 × 6 mm2, measured with a laser beam of 1-mm diameter. The angular uniformity of the transmission trap detector is better than ±0.01% for ±3° rotation around two perpendicular axes.

GaN and AlGaN metal–semiconductor–metal photodetectors

Abstract: GaN based interdigital metal–semiconductor–metal (MSM) photodetectors have been successfully fabricated. The MSM structures were patterned on highly resistive GaN and the ternary compound, AlGaN. For the highly resistive GaN detector, the lowest dark current is ∼0.1 nA and the UV responsivity of the device was about 460 A W−1 at a DC bias of 5 V. The AlGaN with 24% Al exhibited larger gains of up to 106 A W−1 at 20 V, but at a very high dark current, 1 mA, and very long detector responses, greater than 60 s. The high gain in this device is not well understood. The preliminary measurements indicate that tunneling occurs at high electric fields since a negative temperature coefficient for the breakdown voltage was observed.

Development of radiation-hard optical links for the CMS tracker at CERN

Abstract: A radiation-hard optical link is under development for readout and control of the tracking detector in the future CMS experiment at the CERN Large hadron Collider. We present the optical system architecture based on edge-emitting InGaAsP laser-diode transmitters operating at a wavelength of 1.3 /spl mu/m, single mode fiber ribbons, multi-way connectors and InGaAs Pin photodiode receivers. We report on radiation hardness tests of lasers, photodiodes, fibers and connectors. Increases of laser threshold and pin leakage currents with hadron fluence have been observed together with decreases in laser slope-efficiency and photodiode responsivity. Short lengths of single-mode optical fiber and multi-way connectors have been found to be little affected by radiation damage. We analyze the analog and digital performance of prototype optical links transmitting data generated at a 40 MSamples/s rate. Distortion, settling time, bandwidth, noise, dynamic range and bit-error-rate results are discussed.