Response of semi-insulating GaAs detectors to low energy protons

The performance of semi-insulating GaAs detectors, grown with the LEC technique, has been studied by irradiating the Schottky diodes with 2 and 2.4 MeV monoenergetic protons in a pencil beam with sub-millimeter width (70 or 600 um). The beam was moved across the surface of the front (Schottky) contact, in order to investigate the uniformity of the detection characteristics over the sensitive area ofthe diodes, and to study the electric field behavior around the Schottky contact. For each scanning position, a pulse-height spectrum was measured. Then, the charge collected and the energy resolution were obtained as a function of the irradiation position both on the contact and outside it. The data show that - the best spectroscopic response occurs for the beam with 70 um width, - when the beam is incident onto the contact, the energy resolution is between 1% and 5%, and the variation ofthe charge collected, for different irradiation position, is less than 30%, - when the beam is incident onto the border of the contact (substrate is irradiated), the spectrum is degraded and no clear peak is present, - collection of charge still occurs at distances up to about 500 um from the border of the Schottky contact (for a pixel size of 200 um), or up to about 200 um (for a pixel diameter of 3 mm), - saturation ofthe curve collected charge vs. reverse bias voltage occurs at about 100 V for both 2 MeV (range=32 um) and 2.4 MeV protons (range=41 um).