The Effect of Gold on the Properties of the Si–SiO2 System

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Abstract
The admittance and surface‐state density of metal‐oxide‐silicon structures are studied as a function of fabrication processing, emphasizing the effects of gold doping. Experimental investigations show that the process used in fabricating an MOS structure has as much effect on its surface‐state density as the introduction of gold. The theoretical effect of gold in the bulk semiconductor on the C‐V curves is derived. This theory must be used to evaluate the surface‐state density in order to separate the bulk effects from the surface effects. A high‐frequency theory for gold‐doped structures is used to show that the gold which causes the experimentally observed positive shift in the MOS C‐V curves can cause an increase in the surface‐state density of 3×1011 states/eV·cm2. This gold is believed to be located at the silicon‐silicon dioxide interface and has a continuous rather than a discrete energy distribution within the energy gap. The state of the gold is shown to be best described in terms of a bonding model rather than a band structure viewpoint. A low‐frequency method of evaluating surface‐state density gives excellent agreement with the high‐frequency analysis for non‐gold‐doped structures. However, this low‐frequency method does not give agreement in the gold‐doped case nor do the experimental low‐frequency gold‐doped C‐V curves agree well with the low‐frequency capacitance theory, which is extended here to include the effect of gold in the bulk silicon. It is shown that this discrepancy may be caused by the gold introducing appreciable loss into the low‐frequency admittance of the bulk silicon. This loss illustrates a limitation in the applicability of previous low‐frequency capacitance theories.