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We report Al0.8Ga0.2As recessed-window single-photon avalanche photodiodes with high internal single-photon detection efficiency and low dark count probability. External quantum efficiency was increased by a factor of 2 at \lambda = 405 nm. Annealing in arsine with hydrogen carrier gas reduced the dark count probability by a factor of 100, to \sim 10^{-6} /gate with a \sim 5$ ns gate, at room temperature. The activation energies of primary carrier traps, which give rise to afterpulsing, are extracted in a temperature range from 150 to 200 K. [ABSTRACT FROM PUBLISHER]

We report Al0.8Ga0.2As avalanche photodiodes with low dark current and low excess noise. In linear mode when biased to a photocurrent gain of 200, a 150-μm diameter device exhibits dark current of 32 pA (180 nA/cm2). The external quantum efficiency is 31% at 490 nm and the excess noise factor corresponds to k ~ 0.15. In Geiger mode operation, low dark count probability of 10-5 at room temperature is reported. [ABSTRACT FROM PUBLISHER]

This paper examines the performance of CMOS avalanche photodiode pixels operated in a Geiger mode. The pixels, called Geiger photodiode (GPD) pixels, convert an incident analog photon flux into a digital count rate. The maximum detection efficiency of the characterized GPD pixel for 632-nm light is 22%. The passively quenched GPD pixel exhibits an after pulsing at excess bias voltages above 2 V, and a minimum in the after-pulsing correction factor, of 0.53, occurs at an excess bias of 5.8 V. The after pulsing increases the fluctuations, or noise, in the count-rate signal. The following expression accurately describes the noise in the characterized GPD pixel, which exhibits a relatively low after-pulsing probability: σ² = ¯ndp + ¯nap + 2 · [¯ndp · ¯nap]1/2, where σ represents the count-rate fluctuations, the ‘dp’ subscript stands for ‘detected photons,’ the ‘ap’ subscript stands for ‘after pulses,’ and the ¯n represents the ‘average count rate of’ dp, or ap. The noise-equivalent illumination exhibits a minimum of 300 Hz at an operating voltage of 28 V. The best operating voltage for the GPD pixel increases from 28 V with increasing signal intensity. [ABSTRACT FROM AUTHOR]

Abstract: The CMOS solid-state photomultiplier (SSPM) is an array of Geiger avalanche photodiodes (GPD) read out in parallel. These devices are not susceptible to magnetic fields, less expensive to fabricate than many other photodetector technologies, compact, and allow for on-chip integration of signal processing circuits. A number of nuclear detection applications require detector sizes on the order of 1cm. Standard silicon fabrication technology limits the size of the SSPM die, and tiling of the silicon die can result in large-area devices but results in dead space between die for bonding purposes. Radiation Monitoring Devices (RMD) has fabricated 1×1cm SSPM arrays on a single die. The size of these devices is large enough to provide an alternative detector for scintillation detector applications compared to photomultiplier tubes. Although the size increases the dark noise, we will demonstrate that the large-area SSPM can provide a PMT-like response for 22Na gamma rays using an LYSO crystal. Each of the noise terms associated with the large-area SSPM is discussed, quantifying the cross talk and after pulse multipliers, which are scaling factors to the gain to account for the additional output charge from the SSPM. The excess noise factor associated with cross talk and after pulsing has a linear dependence on the multiplier term. The signal and noise terms have been compiled to provide the best operating voltage of roughly 6V above breakdown for a 1×1cm CMOS SSPM to be operated with a short integration time (<10ns) and at 0°C. [Copyright &y& Elsevier]

Abstract: Experiments and simulations using various solid-state photomultiplier (SSPM) designs have been performed to evaluate pixel layouts and explore design choices. SPICE simulations of a design for position-sensing SSPMs showed charge division in the resistor network, and anticipated timing performance of the device. The simulation results predict good position information for resistances in the range of 1–5kΩ and 150-Ω preamplifier input impedance. Back-thinning of CMOS devices can possibly increase the fill factor to 100%, improve spectral sensitivity, and allow for the deposition of anti-reflective coatings after fabrication. We report initial results from back illuminating a CMOS SSPM, and single Geiger-mode avalanche photodiode (GPD) pixels, thinned to 50μm. [Copyright &y& Elsevier]

Abstract: Active Pixel Sensors (APSs) in complementary metal-oxide-semiconductor (CMOS) technology are augmenting Charge-Coupled Devices (CCDs) as imaging devices and cameras in some demanding optical imaging applications. Radiation Monitoring Devices are investigating the APS concept for nuclear detection applications and has successfully migrated avalanche photodiode (APD) pixel fabrication to a CMOS environment, creating pixel detectors that can be operated with internal gain as proportional detectors. Amplification of the signal within the diode allows identification of events previously hidden within the readout noise of the electronics. Such devices can be used to read out a scintillation crystal, as in SPECT or PET, and as direct-conversion particle detectors. The charge produced by an ionizing particle in the epitaxial layer is collected by an electric field within the diode in each pixel. The monolithic integration of the readout circuitry with the pixel sensors represents an improved design compared to the current hybrid-detector technology that requires wire or bump bonding. In this work, we investigate designs for CMOS APD detector elements and compare these to typical values for large area devices. We characterize the achievable detector gain and the gain uniformity over the active area. The excess noise in two different pixel structures is compared. The CMOS APD performance is demonstrated by measuring the energy spectra of X-rays from 55Fe. [Copyright &y& Elsevier]

Abstract: Gamma-ray detector technologies, capable of providing adequate energy information, use photomultiplier tubes (PMTs) or silicon avalanche photodiodes to detect the light pulse from a scintillation crystal. A new approach to detect the light from scintillation materials is to use an array of small photon counting detectors, or a “detector-on-a-chip” based on a novel “Solid-state Photomultiplier” (SSPM) concept. A CMOS SSPM coupled to a scintillation crystal uses an array of CMOS Geiger photodiode (GPD) pixels to collect light and produce a signal proportional to the energy of the radiation. Each pixel acts as a binary photon detector, but the summed output is an analog representation of the total photon intensity. We have successfully fabricated arrays of GPD pixels in a CMOS environment, which makes possible the production of miniaturized arrays integrated with the detector electronics in a small silicon chip. This detector technology allows for a substantial cost reduction while preserving the energy resolution needed for radiological measurements. In this work, we compare designs for the SSPM detector. One pixel design achieves maximum detection efficiency (DE) for 632-nm photons approaching 30% with a room temperature dark count rate (DCR) of less than 1kHz for a 30-μm-diameter pixel. We characterize after pulsing and optical cross talk and discuss their effects on the performance of the SSPM. For 30-μm diameter, passively quenched CMOS GPD pixels, modeling suggests that a pixel spacing of approximately 90μm optimizes the SSPM performance with respect to DE and cross talk. [Copyright &y& Elsevier]