Two GaAs mesa
p+-i-n+ photodiodes intended
for photon counting X-ray spectroscopy, having an i layer thickness of
7 μm and diameter of 200 μm, have been characterized electrically, for their
responsivity at the wavelength range 580 nm to 980 nm and one of them for its
performance at detection of soft X-rays, at room temperature. Dark current and
capacitance measurements as a function of applied forward and reverse bias are
presented. The results show low leakage current densities, in the range of
nA/cm2 at the maximum internal electric field (22 kV/cm). The
unintentional doping concentration of the i layer, calculated from
capacitance measurements, was found to be <1014 cm−3.
Photocurrent measurements were performed under visible and near infrared light
illumination for both diodes. The analysis of these measurements suggests the
presence of a non-active (dead) layer (0.16 μm thickness) at the
p+ side top contact interface, where the photogenerated
carriers do not contribute to the photocurrent, possibly due to recombination.
One of the diodes, D1, was also characterized as detector for room temperature
photon counting X-ray spectroscopy; the best energy resolution achieved
(FWHM) at 5.9 keV was 745 eV. The noise analysis of the system, based
on spectra obtained at different shaping times and applied reverse biases,
showed that the dominant source of noise is the dielectric noise. It was also
calculated that there was at least (165±24) eV charge trapping noise at
0 V.
Keywords: Gallium Arsenide; p-i-n
photodiodes; X-ray spectroscopy; Visible and near infrared
responsivity
Source: Sciencedirect
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