The availability of high-current implanters, now makes it possible to perform high-dose ion-implantations.
By modifying the implant species, dose, energy and substrate temperature and by choosing the suitable thermal annealing conditions buried layers below a monocrystalline Si overlayer can be formed.
By implanting into Si oxygen, nitrogen or a combination of both Silicon-On-Insulator (SO!) structures are obtained. Another application consists in the implantation of transition metals to obtain buried silicide layers. To achieve device grade material a low defect density in the top Si layer is required. In the present paper the formation of the buried layer and the defects in the Si overlayer are discussed as studied by means of high-voltage (HVEM) and highresolution (HREM) transmission electron microscopy in the case of oxygen [1], nitrogen, oxygen/nitrogen combinations [2,3] and cobalt [4J ion-implantations. For all the high dose implantations, in the as-implanted samples [113) defects are found below the buried layer.
Strong evidence exists to interprete these rod-like defects as hexagonal phase silicon precipitates. Their presence is then an indication of silicon selfinterstitial supersaturation. During the implantation the substrate temperature is kept above 500ยฐC, in order to avoid amorphisation of the surface silicon. As such, on top of the buried layer a Si layer with the same orientation as the substrate is preserved.
For the oxygen and nitrogen implantation this buried layer is amorphous. During annealing the silicon dioxide layer remains amorphous, whereas in the nitrogen case the layer crystallises in the a-Si 3N4