Types of Defects and Impurities Passivated

Gold has been used for many years as a minority carrier lifeline controller in Si. As such, it is introduced in a controlled manner, usually by diffusion into transistor structures to decrease the carrier lifetime in the base region in order to increase the switching speed (Ravi, 1981). Conversely, the uncontrolled presence of Au is clearly deleterious to the performance of devices, both because of the increased recombination within the structure and the increase of pipe defects, which can cause shorting of the device. These pipe defects consist of clusters of metallic impurities at dislocations bounding epitaxial stacking faults. 

Most of the other metal-related deep levels in Si are also passivated by reaction with hydrogen (Pearton, 1985). Silver, for example, gives rise in general to a donor level at Ee + 0.54 eV and an acceptor level at Ec - 0.54 e V (Chen and Milnes, 1980 Milnes, 1973). These levels are very similar to those shown by Au, Co and Rh and raise the question of whether Au might actually be introduced into all of the reported samples or a contaminant, or whether as discussed by several authors there is a similar core to these impurity centers giving rise to similar electronic properties (Mesli et al., 1987 Lang et al., 1980). This problem has not been adequately decided at this time. It has been 

Palladium and platinum are also used as carrier lifetime controllers in Si. Pd creates an electron trap at Ec - 0.22 eV and a hole trap at Ev + 0.32 eV in Si (Chen and Milnes, 1980). Pt induces a single electron trap at Ec + 0.28 eV (Chen and Milnes, 1980). All of these levels are passivated by atomic hydrogen (Pearton and Haller, 1983) suggesting that hydrogen might be profitably used during silicide formation to passivate electrically active levels near the silicon-silicide interface. 

Copper and nickel are also common contaminants in Si and can often be introduced during annealing treatments. Both of these impurities are extremely rapid diffusers and cannot be retained in electrically active form even by rapid quenching of diffused samples (Weber, 1983). Quite often, complexes involving Cu or Fe impurities are observed by DLTS in heat-treated Si. All of these centers are hole traps, with Cu giving rise to levels at Ev + 0.20 eV, Ev + 0.35 eV and Ev + 0.53 eV, whereas Ni is related to levels at Ev + 0.18 eV, Ev + 0.21 eV and Ev + 0.33 eV. All of these levels are passivated by reaction with atomic hydrogen (Pearton and Tavendale, 1983), and are restored by annealing at 400°C. 

The effect of low energy (0.4 eV) H+ ion implantation into Si diffused with Ti, V or Cr has also been examined (Singh et al., 1986). The electrically active concentration of a Cr-related level at Ev + 0.30 eV was reduced after hydrogenation, although substantial loss of this level was also 

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The difference between the results of the saturated storage tests and the lifetests can thus be explained as follows. Failures during the saturated storage tests only occur after delamination between the plastic and the die, and are related to the impurity concentration of the plastic, whereas failures during lifetests will usually be attributed to passivation defects and in this case delamination is not a necessary precondition for failure. Thus saturated storage tests only assess the plastic, while lifetests assess both the plastic and the passivation. With some combinations of plastic and passivation, the two types of test will... 

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