Der Einfluß des Kontaktes auf die durchα-Teilchen induzierten Stromimpulse in Cadmiumsulfid-Kristallen

Abstract

The effect of the electrode contact on pulses induced byα-particles in single crystals of CdS grown by theFrerichs method was investigated to ascertain whether such crystals are suitable as particle counters. Thin, transparent metal electrodes were prepared on both sides of the crystals at room temperature, or above, by vacuum evaporation after surface cleaning by glow discharge. Crystals with Ohmic contacts (In, Ce) showed space charge limited pulse currents; the rise time of these pulses was longer than the dielectric relaxation time. When a thin sheet of an insulating material was interposed between crystal and electrode, polarization caused pulse amplitude to decrease very rapidly. It follows from the results that the polarizing charge is located at the electrodes. Saturation pulses were obtained with Cr, Mn and Au electrodes deposited on crystals at higher temperatures. Measured photovoltaic effects confirmed the existence of a surface-barrier potential at the electrodes. An energy resolution of about 20% forα-particles of 4·8 MeV was obtained for a crystal with Au electrodes. In regard to the charge carried by holes, the amount of energy required to produce an electron-hole pair was determined to 7·25±1·2 eV. The mobility range of the holes was estimated to be 2≦μ≦8 cm²/ Vsec. These results show that a CdS crystal can only be used as anα-particle detector with good time and energy resolution if there is a potential barrier immediately under the electrode.