OPTICAL-PROPERTIES OF ALUMINUM-IMPLANTED AND ANNEALED SILICON

The main problem associated with the use of aluminum as a p-type dopant in silicon power devices is its low electrical activity in silicon after the anneal process. In order to obtain a deeper insight into the possible mechanisms responsible for the loss of electrical activity, it is necessary to study three different states of the p-n junction fabrication: (1) the unimplanted starting or reference material; (2) the aluminum-implanted material; and (3) the implanted and annealed material. In this paper we present a detailed analysis of reflectivity and transmission measurements of the three different states extending from the far-infrared to the UV region, as well as depth profiles of the reflectivity from as-implanted and bevelled samples. From these investigations we have obtained information about two important aspects, namely lattice damage and free-carrier properties. The refractive index across the implanted layer is essentially constant and considerably larger than that of the crystalline state; together with recent transmission electron microscopy studies it is suggested that this change in refractive index is due to the formation of broken or weakened bonds. In the annealed state those defects induced by implantation which produce a change of the optical properties are healed out to a high degree. From the free-carrier absorption observed in the far-infrared direct information is obtained about the electrical properties, i.e., the mean concentration and mobility of the holes associated with the electrically active aluminum atoms in the thin p-type layer produced by annealing. We obtain an electrical activity (percentage of electrically active Al atoms) of 17%. This result is discussed and compared with recent sheet resistance-, spreading resistance-, and secondary-ion mass spectrometry data obtained from the same sample.