Window Thickness and Rectifying Process in Surface Barrier Detectors

The thickness of the entrance window of surface barrier detectors is experimentally studied by determining the pulse height defect obtained from a comparison of the pulse heights measured for protons and for Y rays of 80 keV. The used detectors have resistivities ranging from 150 to 50,000 ¿-cm. It is shown that for low resistivity materials (p ¿ 1000 ¿-cm) the window is formed by the gold layer only. At higher resistivities, the existence of an additional silicon dead layer is demonstrated ; the thickness of this region varies in terms of the resistivity ¿ and the applied bias voltage V following a function of the form A ln(B¿/V), where A and B are constants. The experimental results are interpreted by taking into account the role of oxygen in the formation of the rectifying contact. Two possible mechanisms are discussed ; in the first, oxygen is assumed to diffuse within the silicon crystal, where it acts as an acceptor ; in the second, oxygen is supposed to remain adsorbed at the surface and to form an inversion layer. The first mechanism is shown to lead to the best description of the present experimental data, but the validity of the second cannot be considered as absolutely disproved.