Because of its high diffusivity in silicon, aluminum is best suited for deep diffusions often required in high-voltage-power semiconductor devices. The ion implantation technique allows the reproducible low dosage doping necessary, e.g., for the new concepts of junction termination systems. The most important drawback of using aluminum as a p-type dopant in silicon is its low electrical activity after the anneal. In order to obtain a deeper insight into the mechanisms responsible for the loss of the electrical activity, we have studied the states of aluminum implanted into silicon before and after annealing by means of spreading resistance, secondary-ion mass spectroscopy, transmission electron microscopy, and energy-dispersive x-ray techniques. The case study presented here [Czochralski grown (100) silicon, implanted dose 3×1015 cm−2, junction depth 6 μm] reveals that the major source for the loss of the electrical activity is out-diffusion, i.e., segregation into the native silicon oxide layer and/or evaporation into the vacuum. In addition, the activity is reduced by the formation of aluminum oxide precipitates. The results are discussed in the light of optical studies on the same materials performed previously as well as on the basis of a diffusion model which allows for out-diffusion. The large rate constant for out-diffusion indicates that the native oxide layer represents a highly reactive surface for aluminum. From the diffusion model it is possible to calculate an approximate electrical activity Ã(xj) as a function of junction depth xj, which qualitatively reproduces well the observed activity A(xj). This demonstrates that our case study is representative for a large number of samples which were implanted and annealed under widely different conditions. Some technical processes which could possibly enhance the electrical activity are discussed.
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.
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