Electronics used for space applications (e.g. communication satellites) are susceptible to space weather, primarily consisting of electrons and protons. As more critical equipment is used in space, a comprehensive monitoring network is needed to mitigate risks associated with radiation damage. Compact detectors suited for this requirement have been too complicated or do not provide sufficient information. As the damage from electrons (e.g. total ionizing dose effects) is significantly different compared to protons (e.g. displacement damage effects), monitors that can provide unique measurements of the dose and/or spectral information for electrons and protons separately are necessary for mission assessment to determine strategies for maintaining function. Previously, we demonstrated that the Proton-Electron Discrimination Detector (PEDD) is space-compatible and can discriminate fast electrons from protons using a diphenylanthrecene (DPA) scintillator coupled to a CMOS silicon photomultiplier (SiPM). The SiPM has a temperature dependence, and a circuit has been developed to provide a stable response as a function of temperature. The PEDD detector is scheduled to participate on the RHEME experiment to be flown on the ISS, scheduled for launch in 2016.
Avalanche photodiodes (APD) manufactured at RMD are fabricated using deep diffusion processes, resulting in a thick reach-through APD with excellent performance characteristics. These include a high quantum efficiency (<50% for visible photons) and low excess noise (F ~ 2). Due to the structure of the APD, the devices have very low junction capacitance (~0.7pF/mm2). These devices have been made as squares or hexagons on the order of 2-4” dimensionally and require <1000 V for operation. Due to the high operating bias, studies on the Geiger behavior were dismissed. The low capacitance is conducive to developing large-area devices, and the large drift region allows for charge steering toward the high breakdown field region. These results provide initial data on the performance characteristics of RMD’s APDs when operated in Geiger mode. Due to the thickness of these devices, they provide a high gain-bandwidth product for near IR single photon counting. A small area (~4 mm2) APD was biased beyond the reverse bias breakdown voltage (~1700 V at -50 C), where the device showed typical Geigermode behavior with a low dark count rate (<54 kHz at 1700 V at an excess bias of 3 V). The data indicates a uniform response over the diode region, yet due to the large dark currents, the device was only operated to 5 V in excess bias beyond the breakdown voltage. The Geiger probability at 5V excess bias was measured as 3%, which is consistent with simulations that suggest an excess bias of ~300 V is required for 100% Geiger probability.
The development of high-performance scintillation materials that emit light below 400 nm has prompted the development of improved solid-state UV photodetectors. While silicon provides a mature context for UV photodetectors, the high dark current due to its low band-gap (1.1 eV) limits the signal-to-noise performance when scaling the detector to large areas. Photodetectors fabricated in materials with a larger band-gap have the potential to surmount the performance limitations experienced by silicon. AlxGa1-xAs, is a material that provides a band gap from 1.55 eV to 2.13 eV, depending on the Al concentration. Using high Al concentration (0.7 < x < 1), AlxGa1-xAs to engineer a wider bandgap > 2eV is very desirable in terms of reducing dark noise. Due to its strong absorption of UV-light at the material surface, however, surface effects limit the quantum efficiency below 400 nm. Introducing surface layers that have a longer penetration depth for UV photons promises to boost the quantum efficiency in the UV while maintaining low dark current. This work describes the development of a photodiode fabricated in AlxGa1-xAs, x > 0.7, compared to an AlxGa1-xAs, x > 0.7 photodiode with an AlAs surface (x = 1). It presents the design of the photodiodes, simulations of their performance, the fabrication process, along with characterization data of fabricated photodiodes. We report on the surface effects of high aluminum concentration AlxGa1-xAs, x > 0.7, to provide a high quantum efficiency for photons below 400 nm, by examining the charge collection.
Solid-state photomultipliers (SSPM) are high gain photodetectors composed of Geiger photodiodes (GPD) operating above device breakdown voltage. In scintillation based radiation detection applications, SSPMs fabricated using silicon (SiPMs, MPPCs, etc) provide a compact, low cost alternative to photomultiplier tubes (PMTs), however, the high dark count rate due to its low band-gap (1.1eV) limits the signal-to-noise performance as the silicon SSPM is scaled to large areas. SSPMs fabricated in materials with a larger band-gap have the potential to surmount the performance limitations experienced by silicon. AlGaAs is a material that provides a bandgap from 1.55eV to 2.13 eV, depending on Al concentration. Using high Al concentration AlGaAs to engineer a wideband- gap (>2eV) SSPM is very desirable in terms of reducing dark noise, which promises better signal-to-noise performances when large detector areas is needed. This work describes the development of Geiger photodiodes (GPDs), the individual elements of a SSPM, fabricated in AlGaAs with 80% Al concentration. We present the design of the GPDs, the fabrication process, along with characterization data of fabricated GPD samples. To the best of our knowledge, we have demonstrated for the first time, a passively quenched Geiger photodiode in Al0.8Ga0.2As.
Early CMOS SSPM pixel designs utilize a highly doped layer near the surface as a component for the Geiger junction, which limits the collection of charge from the surface and the UV response of the high gain solid state photodetector. To address these limitations, we are developing a new generation of CMOS SSPMs using pixel elements with a buried layer as a component of the Geiger junction in a process with smaller feature sizes. The new SSPM, an array of newly designed Geiger photodiode elements, is designed and fabricated to provide improvements in blue light response and dark noise performance. This work compares the performance of the early and new CMOS SSPM designs. Results showed ~2-4× improvement of detection efficiency in the blue/shallow UV region (350nm to 450nm), and a 10× reduction in detector dark count rate. Due to higher operating bias, the after pulse multiplier is no larger than a factor of 1.5 larger than the previous design. Inter-pixel cross-talk is similar to previous SSPM designs at comparable Geiger probabilities.
Exploration in nuclear physics may require extreme conditions, such as temperatures down to a few Kelvin, high
magnetic fields of several Tesla, or the small physical dimensions of a few centimeters. As a standard technique for
radiation detection using scintillation materials, it is desirable to develop photodetectors that can operate under these
harsh conditions. Though photomultiplier tubes (PMTs) have been used for most applications for readout of scintillation
materials, they are bulky, highly susceptible to magnetic fields, and present a large heat load in cryogenic environments.
Avalanche photodiodes are a reasonable alternative to PMTs in that they are extremely compact and less susceptible to
magnetic fields. Avalanche photodiodes have been developed in a commercial CMOS process for operation at
temperatures below 100 Kelvin. Here we present the overall operation of the photodiodes at 5 Kelvin. The diodes show a
quantum efficiency of at least 30% at 532 nm at 5 Kelvin. At about 30 Kelvin, the diodes exhibit an internal resistive
term, which generates a second breakdown point. The prototype diode shows a proportional response to the intensity of
light pulses down to 150 detected photons with a hole to electron ionization ratio, k, of 2.3x10-13 at 5 Kelvin. The
properties of the photodiodes and the readout electronics will be discussed for general photon detection below 100 K.
High-energy, gamma-ray calorimetry typically employs large scintillation crystals coupled to photomultiplier tubes.
These calorimeters are segmented to the limits associated with the costs of the crystals, photomultiplier tubes, and
support electronics. A cost-effective means for construction of a calorimeter system is to use solid-state photomultipliers
(SSPM) with front-end electronics, which is at least half the cost, but the SSPM must provide the necessary energy
resolution defined by the physics goals. One experiment with plans to exploit this advantage is an upgrade to the
PRIMEX experiment at Jefferson Laboratories. We have developed a large-area SSPM (1 cm × 1 cm) for readout of
large scintillation crystals. As PbWO4 has excellent properties (small Molière radius and radiation hard) for high-energy
gamma-rays (>1 GeV) but low light yields (~150 photons/MeV at 0 °C), evaluation of the SSPM and support readout
electronics with LaBr3 provides a measure of the device performance. Using the known detection efficiency and dark
current of the SSPM, an excess noise factor associated with after pulsing and cross talk is determined. The contribution
to the energy resolution from the detector module is calculated as <1% for gamma rays greater than ~2.5 GeV (0.7% at
Detection of single photons is crucial for a number of applications. Geiger photodiodes (GPD) provide large gains with
an insignificant amount of multiplication noise exclusively from the diode. When the GPD is operated above the reverse
bias breakdown voltage, the diode can avalanche due to charged pairs generated from random noise (typically thermal)
or incident photons. The GPD is a binary device, as only one photon is needed to trigger an avalanche, regardless of the
number of incident photons. A solid-state photomultiplier (SSPM) is an array of GPDs, and the output of the SSPM is
proportional to the incident light intensity, providing a replacement for photomultiplier tubes.
We have developed CMOS SSPMs using a commercial fabrication process for a myriad of applications. We present
results on the operation of these devices for low intensity light pulses. The data analysis provides a measured of the
junction capacitance (~150 fF), which affects the rise time (~2 ns), the fall time (~32 ns), and gain (>106). Multipliers
for the cross talk and after pulsing are given, and a consistent picture within the theory of operation of the expected dark
current and photodetection efficiency is demonstrate. Enhancement of the detection efficiency with respect to the
quantum efficiency at unity gain for shallow UV photons is measured, indicating an effect due to fringe fields within the
diode structure. The signal and noise terms have been deconvolved from each other, providing the fundamental model
for characterizing the behavior at low-light intensities.
Solid-state photomultipliers (SSPM) are photodetectors composed of avalanche photodiode pixel arrays operating in
Geiger mode (biased above diode breakdown voltage). They are built using CMOS technology and can be used in a
variety of applications in high energy and nuclear physics, medical imaging and homeland security related areas. The
high gain and low cost associated with the SSPM makes it an attractive alternative to existing photodetectors such as the
photomultiplier tube (PMT). The capability of integrating CMOS on-chip readout circuitry on the same substrate as the
SSPM also provides a compact and low-power-consumption solution to photodetector applications with stringent area
and power requirements. The optical performance of the SSPM, specifically the detection and quantum efficiencies, can
depend on the geometry and the doping profile associated with each photodiode pixel. The noise associated with the
SSPM not only includes dark noise from each pixel, but also consists of excess noise terms due to after pulsing and
inter-pixel cross talk. The magnitude of the excess noise terms can depend on biasing conditions, temperature, as well as
pixel and inter-pixel dimensions. We present the optical and noise performance of SSPMs fabricated in a conventional
CMOS process, and demonstrate the dependence of the SSPM performance on pixel/inter-pixel geometry, doping
profile, temperature, as well as bias conditions. The continuing development of CMOS SSPM technology demonstrated
here shows that low cost and high performance solid state photodetectors are viable solutions for many existing and
future optical detection applications.