A.M. Barnett

SiC X-ray detectors for harsh environments

We have characterised a number of SiC Schottky Diodes as soft X-ray photon counting detectors over the temperature range -30°C to +80°C. We present the spectroscopic performance, as measured over the energy range ~ 6 keV–30 keV and correlate the data with measurements of the temperature dependence of the device leakage current. The results show that these detectors can be used for X-ray photon counting spectroscopy over a wide temperature range. Measurement of the radiation tolerance of Semi Transparent SiC Schottky Diodes (STSSD) has shown that these devices can still operate as photon counting X-ray spectrometers after proton irradiation (total dose of 1013 cm−2, 50 MeV). We present measurements on proton irradiated STSSDs that indicate that radiation induced traps, located in the upper half of the bandgap, have reduced the mobility and concentration of charge carriers. X-ray spectra predicted using a Monte Carlo model for SiC diodes are compared with measured spectra.

Temperature dependent characterization of gallium arsenide X-ray mesa p-i-n photodiodes

Electrical characterization of two GaAs p+-i-n+ mesa X-ray photodiodes over the temperature range 0 °C to 120 °C together with characterization of one of the diodes as an X-ray detector over the temperature range 0 °C to 60 °C is reported as part of the development of photon counting X-rayspectroscopic systems for harsh environments. The randomly selected diodes were fully etched and unpassivated. The diodes were 200 μm in diameter and had 7 μm thick i layers. The leakage current density was found to increase from (3 ± 1) nA/cm−2 at 0 °C to (24.36 ± 0.05) μA/cm−2 at 120 °C for D1 and from a current density smaller than the uncertainty (0.2 ± 1.2) nA/cm−2 at 0 °C to (9.39 ± 0.02) μA/cm−2 at 120 °C for D2 at the maximum investigated reverse bias (15 V). The best energy resolution (FWHM at 5.9 keV) was achieved at 5 V reverse bias, at each temperature; 730 eV at 0 °C, 750 eV at 20 °C, 770 eV at 40 °C, and 840 eV at 60 °C. It was found that the parallel white noise was the main source of the photopeak broadening only when the detector operated at 60 °C, at 5 V, 10 V, and 15 V reverse bias and at long shaping times (>5 μs), whereas the sum of the dielectricnoise and charge trapping noise was the dominant source of noise for all the other spectra.

Avalanche Gain and Energy Resolution of Semiconductor X-ray Detectors

Realistic Monte Carlo simulations for the avalanche gain of absorbed X-ray photons were carried out in a study of the relationship between avalanche gain and energy resolution for semiconductor X-ray avalanche photodiodes (APDs). The work explored how the distribution of gains, which directly affects the energy resolution, depends on the number of injected electron-hole pairs (and, hence, the photon energy), the relationship between ionization coefficients, and the mean gain itself. We showed that the conventional notion of APD gains significantly degrading energy resolution is incomplete. If the X-ray photons are absorbed outside the avalanche region, then high avalanche gains with little energy resolution penalty can be achieved using dissimilar ionization coefficients. However, absorption of X-ray photons within the avalanche region will always result in broad gain distribution (degrading energy resolution), unless electrons and holes have similar ionization coefficients.

Temperature dependence of the average electron-hole pair creation energy in Al0.8Ga0.2As

The temperature dependence of the average energy consumed in the creation of an electron-hole pair in the wide bandgap compound semiconductor Al 0.8Ga0.2As is reported following X-ray measurements made using an Al0.8Ga0.2As photodiode diode coupled to a low-noise charge-sensitive preamplifier operating in spectroscopic photon counting mode. The temperature dependence is reported over the range of 261 K-342 K and is found to be best represented by the equation e AlGaAs 7.327-0.0077 T, where eAlGaAs is the average electron-hole pair creation energy in eV and T is the temperature in K.

Electrical and ultraviolet characterization of 4H-SiC Schottky photodiodes

Fabrication and electrical and optical characterization of 4H-SiC Schottky UV photodetectors with nickel silicide interdigitated contacts is reported. Dark capacitance and current measurements as a function of applied voltage over the temperature range 20 °C - 120 °C are presented. The results show consistent performance among devices. Their leakage current density, at the highest investigated temperature (120 °C), is in the range of nA/cm(2) at high internal electric field. Properties such as barrier height and ideality factor are also computed as a function of temperature. The responsivities of the diodes as functions of applied voltage were measured using a UV spectrophotometer in the wavelength range 200 nm - 380 nm and compared with theoretically calculated values. The devices had a mean peak responsivity of 0.093 A/W at 270 nm and -15 V reverse bias.

Al0.52In0.48P 55Fe x-ray-photovoltaic battery

An Al0.52In0.48P 55Fe radioisotope microbattery is demonstrated over the temperature range −20 °C to 160 °C. Al0.52In0.48P p+-i-n+ mesa structures were used to collect the photons from a 238 MBq 55Fe radioisotope x-ray source. The effects of temperature on the key microbattery parameters were studied. Increasing the temperature, the saturation current increased; whilst the open circuit voltage, the maximum power and the conversion efficiency decreased. An open circuit voltage of 0.97V and a conversion efficiency of 22% were measured in a single p+-i-n+ mesa structure at −20 °C. The highest total microbattery maximum output power using two mesa structures was 1.2 pW at −20 °C.

Gallium arsenide 55Fe X-ray-photovoltaic battery

The effects of temperature on the key parameters of a prototype GaAs 55Fe radioisotope X-ray microbattery were studied over the temperature range -20 °C to 70 °C. A p-i-n GaAs structure was used to collect the photons from a 254 Bq 55Fe radioisotope X-ray source. Experimental results showed that the open circuit voltage and the short circuit current decreased with increased temperature. The maximum output power and the conversion efficiency of the device decreased at higher temperatures. For the reported microbattery, the highest maximum output power (1 pW, corresponding to 0.4 μW/Ci) was observed at -20 °C. A conversion efficiency of 9% was measured at -20 °C.

Characterisation of Al0.52In0.48P mesa p-i-n photodiodes for X-ray photon counting spectroscopy

Results characterising the performance of thin (2 μm i-layer) Al0.52In0.48P p+-i-n+ mesa photodiodes for X-ray photon counting spectroscopy are reported at room temperature. Two 200 μm diameter and two 400 μm diameter Al0.52In0.48P p+-i-n+ mesa photodiodes were studied. Dark current results as a function of applied reverse bias are shown; dark current densities <3 nA/cm2 were observed at 30 V (150 kV/cm) for all the devices analysed. Capacitance measurements as a function of applied reverse bias are also reported. X-ray spectra were collected using 10 μs shaping time, with the device illuminated by an 55Fe radioisotope X-ray source. Experimental results showed that the best energy resolution (FWHM) achieved at 5.9 keV was 930 eV for the 200 μm Al0.52In0.48P diameter devices, when reverse biased at 15 V. System noise analysis was also carried out, and the different noise contributions were computed.

Determination of the electron-hole pair creation energy in Al0:8Ga0:2As

The average energy consumed in the creation of an electron-hole pair (commonly called the electron-hole pair creation energy) in the compound semiconductor Al0.8Ga0.2As has been experimentally measured for the first time at X-ray energies using 55Fe and 109Cd radioisotope sources and a GaAs X-ray photodiode as a reference detector. The value measured (5.1 eV ± 0.08 eV at 294 K) is compared to values previously estimated in the literature.

First spectroscopic X-ray and beta results from a 400 μm diameter Al0.8Ga0.2As photodiode

Results characterising the performance of thin (1 μm active layer) 400 μm diameter Al0.8Ga0.2As mesa photodiodes operated at room temperature are presented showing the spectral response under illumination with moderate energy X-rays (5.9 keV and 22.16 keV) from radioisotope X-ray sources (55Fe and 109Cd) and beta particles from a 14C source (endpoint energy = 156.48 keV). The X-ray FWHM is established as 1.95 keV at 5.9 keV and 22.16 keV, primarily limited by electronics noise from the preamplifier. The diodes show a spectral response to 14C beta particles, but the endpoint energy is not reached due to the detectors thinness. The X-ray results are compared with 200 μm diameter Al0.8Ga0.2As photodiodes previously characterised using X-ray radioisotope sources at our laboratory.