Persistent Questions About The Oxygen Donor In Silicon
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PERSISTENT QUESTIONS ABOUT THE OXYGEN DONOR IN SILICON MICHAEL STAVOLA AT&T Bell Laboratories, Murray Hill, NJ 07974 In spite of 30 years of study the most basic questions about the oxygen donor in silicon remain unanswered. There are not accepted models for the structure or formation kinetics. There is not even agreement on what the donor's constituents are. Nonetheless, substantial progress has been made in this field in recent years [11 that narrows model ideas and helps to focus continuing research. Prolonged heat treatment of oxygen rich silicon gives rise to up to 3x 1016 shallow donors [21. Resistivity measurements showed that the initial formation rate was proportional to [0]4 and that the maximum concentration was proportional to [0]3. Kaiser, Frisch, and Reiss [3] proposed an unspecified SinO 4 structure that was built up in successive reaction steps. Subsequent work has shown that the oxygen donor is more complicated than was suspected. The oxygen diffusion constant [4] is too small for the Kaiser et al. kinetic model. Infrared measurements [51 show that the oxygen donor is a double donor and that it is a family of complexes. There are 9 distinct series of helium-like absorption features that appear sequentially with continued heat treatment. The ground state binding energy decreases monotonically for successive complexes. The growth kinetics of the individual complexes has also been studied. In EPR measurements [6,7] the spectrum NL8 has been assigned to the oxygen donor and shows a C2v g tensor anisotropy. Uniaxial stress has been used in conjunction with DLTS [11, EPR [7] and IR absorption measurements [81 to provide structural information. These measurements show ground state splittings that can be understood in terms of an effective masslike ground state that is constructed from wave functions associated with a single pair of conduction band valleys. A further hint to the donor structure comes from the high resolution TEM observations [9] of the ribbon-like defects formed during long anneals at temperatures between about 450 and 650°C. ENDOR measurements [61 are in agreement with the effective mass theory picture of the donor that has been developed and have the potential to provide detailed structural information about the individual donor species. With so much data measured by a variety of techniques it is reasonable to ask what prevents structural and kinetic models from being constructed. Most models involve small complexes constructed entirely from oxygen. Based on the TEM observation of ribbon-like structures formed for low temperature anneals, several groups have suggested that the family of oxygen donor complexes are chain-like structures extended along [110] directions [lI. The small oxygen cluster was supposed to incorporate an electrically active portion that was formed to relieve strain. Oxygen atoms are added to the defect to give rise to the family of donor species. Ourmazd, Bourret, and Schroter [10] (OBS) have developed a detailed kinetic model that was used to fit the growth kinetic
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