In low-temperature epitaxial Si deposition methods such as molecular beam epitaxy (MBE), pre-epitaxial substrate preparation usually incorporates a high temperature (≳800 °C) step. Elimination of this step is essential to wider applicability of these epitaxial methods. We show that Si(100) wafers exposed to HF vapors in a laboratory ambience are bulk terminated and that such termination is stable in air for several tens of minutes, and in vacuum for several hours. It is possible to obtain good epitaxy, as determined by surface diffraction and transistor characteristics, provided epitaxy is commenced on these bulk-terminated surfaces. We also give evidence that under certain conditions, bulk-terminated surfaces are maintained in low-temperature epitaxy using the method of ultrahigh vacuum chemical vapor deposition.
We report the first use of a (silicon)/(heavily doped polysilicon)/(metal) structure to replace the conventional high-low junction or back-surface-field (BSF) structure, of silicon solar cells. Compared with BSF and back-ohmic-contact (BOC) control simples, the polysilicon-back solar cells show improvements in red spectral response (RSR) and open-circuit voltage. Measurement reveals that a decrease in effective surface recombination velocity S is responsible for this improvement. Decreased S results for n-type (Si:As) polysilicon, consistent with past findings for bipolar transistors, and for p-type (Si:B) polysilicon, reported here for the first time. Though the present polysilicon-back solar cells are far from optimal, the results suggest a new class of designs for high efficiency silicon solar cells. Detailed technical reasons pre advanced to support this view.
Ultrathin silicon oxide films 5–6 nm thick have been grown in a double-walled furnace and annealed in N2 and Ar at temperatures varying between 850 and 1100 °C. The breakdown field distribution obtained is very tight and centered above 11 MV/cm for as-grown oxides at 850 °C. The oxides that received a post-oxidation anneal (POA) at 1000 °C show a consistent improvement in breakdown field distribution and breakdown charge density as compared to the oxides annealed at lower temperatures. Furthermore, under high field current stress, oxides with a POA at 1000 °C show a positive voltage flatband Vfb shift, while oxides with POA at a temperature T<1000 °C show a negative Vfb shift. These results point out the efficacy of a high-temperature POA of 5–6 nm oxides on breakdown strength and on the reduction of some defects responsible for the positive charge trapping.
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