Abstract
A circular mesa (400 μm diameter) GaAs p+-i-n+ photodiode with a 30 μm thick i layer was characterized for its performance as a detector in photon counting x-ray spectroscopy at 20 °C. The detector was fabricated from material grown by molecular beam epitaxy (MBE). An earlier MBE-grown detector fabricated using a different fabrication process and material from a different area of the same epiwafer was shown to suffer from: relatively high leakage current at high temperatures; a high effective carrier concentration that limited its depletion layer width; and material imperfections (butterfly defects) [Lioliou et al 2019 Nucl. Instrum. Methods Phys. Res. A 946 162670]. However, the new detector has better performance (lower leakage current and effective carrier concentration within the i layer). Using the new detector and low noise readout electronics, an energy resolution of 750 eV ± 20 eV Full Width at Half Maximum (FWHM) at 5.9 keV was achieved at 20 °C, equal to that reported for high quality GaAs detectors made from high quality material grown by metalorganic vapour phase epitaxy [Lioliou et al 2017 J. Appl. Phys. 122 244506]. The results highlight the substantially different performances of detectors made from the same epiwafer when the wafer qualities are not uniform and the effects of different fabrication processes.
Highlights
Applications both in space [1, 2] and on Earth [3] benefit from x-ray spectroscopy
The detector suffered from: the presence of butterfly defects which arose during the epitaxial growth possibly from the creation of unwanted non-integer step heights on the surface and were non-uniformly distributed [11]; a limited depletion of the intrinsic layer due to high carrier concentrations in what was intended to be the unintentionally doped i layer; and possibly surface leakage current over the mesa side walls which contributed to the measured leakage current [11]
The new detector had a leakage current of 38.3 pA ± 0.7 pA at −100 V reverse bias, which was lower than that of the old detector (111.2 pA ± 0.8 pA) at the same applied reverse bias. This was partly attributed to the use of the H2SO4:H2O2:H2O finishing etch, which is thought to have supressed the surface leakage current; similar leakage current reductions attributable to the use of H2SO4:H2O2:H2O finishing etches have been reported previously for other GaAs [14] and InAs [15] mesa photodiodes
Summary
Among the different semiconductor detectors employed in photon counting x-ray spectroscopy, GaAs (1.42 eV bandgap [4]) photodiodes are advantageous due to their radiation hardness [5, 6], high temperature tolerance [7], and good stopping power for x-ray photons of up to moderate energy [8]. Investigation of molecular beam epitaxy (MBE) for growing high-quality thick GaAs structures, such as x-ray detectors, is interesting due to the relative simplicity of its epitaxial growth concept, which makes MBE a powerful technique with unparalleled control and reproducibility. The detector suffered from: the presence of butterfly defects (material imperfections having a characteristic butterfly shape) which arose during the epitaxial growth possibly from the creation of unwanted non-integer step heights on the surface and were non-uniformly distributed [11]; a limited depletion of the intrinsic layer due to high carrier concentrations in what was intended to be the unintentionally doped i layer; and possibly surface leakage current over the mesa side walls which contributed to the measured leakage current [11]
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