Schottky junctions on semi-insulating LEG gallium arsenide for X- and 7-ray spectrometers operated at and below room temperature

This work deals with the study of a Schottky junction used as an X- and 7-ray detector in a spectrometer operated in the temperature range from -30 °C to +22 C. The device (7 mm2 active area and 100 /urn thickness), fabricated on liquid encapsulated Czochralski (LEG) semi-insulating Gallium Arsenide, is designed with a noninjecting ohmic contact which allows biasing voltages up to 550 V. At room temperature (22 °C) the energy resolution is found to be relatively poor (15.5-keV full-width at half-maximum (FWHM) at 59.5 keV) due to the large junction reverse current, whose density (7-37 nA/mm2 at Vbias -100-500 V) is within the typical values for Schottky junctions on SI LEG GaAs. By cooling of the detector to -30 °C, the noise of the reverse current is drastically lowered, thus achieving electronic noise levels around 160-180 rms electrons (1.6-1.8 keV FWHM). At 500-V bias, the 241Am spectrum has been resolved down to an energy of 4 keV with charge collection efficiency of cce 97% and a resolution of about 2-keV FWHM for the Np L lines and 2.4-keV FWHM for the 59.5-keV 7 photons. The linearity of the detector has been measured to be better than ±0.6% within the explored energy range (14-59 keV). From the experimental spectra, it has been analyzed how either the electronic noise or the trapping of the signal charge contribute to the energy resolution of the spectrometer. The result is that despite the high measured cce, the trapping gives a contribution higher than 1.5keV FWHM for the 59.5-keV spectral line. A comparison between the experimental results and Monte Carlo simulations, based on the Hecht model of charge trapping in detectors, is shown to give a satisfactory justification of the observed phenomena. A total mean drift length of carriers has been experimentally derived, finding an exponential dependence upon the bias voltage applied to the detector.