AN INVESTIGATION OF THE PHOSPHORUS DOPING MECHANISM IN A-SI BY SWEEP-OUT EXPERIMENTS

The paper is concerned with a study of the metastable state produced in P-doped hydrogenated amorphous silicon (a-Si) by thermal or bias annealing. Sweep-out experiments were used to determine the electron density, n(BT), in the electron bandtail states. Improvements to the experimental technique and specimen design are described. In particular a thin a-SiN(x) blocking layer has been incorporated close to the doped reservoir layer which makes it possible to extend measurements to 500 K and also provides a steep depletion layer for highly effective bias annealing. Direct measurements of the temperature dependence of n(BT) lead to results in substantial agreement with the Xerox work. Reverse bias annealing on P-doped specimens, deposited at gaseous doping levels between 0.2 and 20 vppm, produce a metastable state with a greatly increased active donor density. In all cases saturation of n(BT) with bias potential has been observed and it is suggested that under these conditions all P-sites have been converted to the P4+ doping configuration. From the saturation level the density of incorporated P can be obtained. It is demonstrated in subsidiary experiments that for quantitative meaningful sweep-out experiments after bias annealing it is essential to re-establish the flat-band situation at the a-SiN(x)/a-Si interface. This is readily done by means of a laser probing technique. The time dependence of the dopant activation during bias annealing has been studied and can be described by a pure exponential law with time constant tau-act. In contrast, the relaxation of the metastable state follows a stretched exponential, in agreement with the Xerox work. Tau-rel congruent-to tau-act and their temperature dependence has an activation energy of 1.0 eV. It is pointed out that the pure exponential activation law provides strong support for the single-step process involved in removing the H from a H-passivated donor site. It is suggested that, in the doping range investigated, this configuration represents the predominant non-doping site.