Electrical properties of boron diffused emitters

Abstract

The effective passivation of boron emitters is a critical precursor to the migration of the silicon solar cell industry to n-type substrates. With this requirement in mind, this thesis focuses upon the surface passivation quality of boron diffused emitters in crystalline silicon. The chief analytical approach applied involves the investigation of the dependence of surface recombination on the charge density. The investigation is undertaken on two structures, namely LPCVD Si{u2083}N{u2084}/SiO{u2082}/Si and PECVD SiNx/Si. Measurement of emitter saturation current (J{u2080}e) under strong accumulation provides a meaningful comparison of the interface quality of different samples, since the surface recombination is independent of surface doping and only weakly dependent on surface charge density. In all cases, the J{u2080}e measured following negative corona charge deposition in boron diffused samples is significantly reduced and saturated, even for samples with relatively high surface doping. Correlation of boron surface concentration and the Si/SiO{u2082} interface properties of oxides grown using low temperature wet-dry oxidation as an intermediate layer of an LPCVD Si{u2083}N{u2084}/SiO{u2082} stack is reported. The proportion of J{u2080}e due to the defects at the surface is estimated after the Auger recombination in the emitter bulk has been taken into account based on the emitter profiles. The Si/SiO{u2082} interface of boron diffused (111) samples with LPCVD Si{u2083}N{u2084}/SiO{u2082} is substantially inferior to the corresponding (100) surface, as indicated by higher effective surface recombination velocity (Sejf) under accumulation. For the range of boron surface concentrations investigated in this thesis (-"4x10{u00B9}{u2078} to 8x10{u00B9}{u2078} cm-{u00B3}), the interface properties of LPCVD Si{u2083}N{u2084}/SiO{u2082} on (100) and (111) surfaces are found to be insensitive to the changes in boron surface concentration. This conclusion is supported by electron paramagnetic resonance results, which illustrate that a comparable Pb defect densities exist at the various surfaces. A long term stability investigation on the thermal oxide and LPCVD Si{u2083}N{u2084}/SiO{u2082} passivated samples shows that the degradation observed after long term storage is due to moisture. The detrimental impacts of the moisture can mostly be avoided with the addition of an effective barrier such as a capping layer of LPCVD nitride. The application of negative charge on undiffused, PECVD SiNx passivated surfaces leads to a significant disagreement between the Sef! determined by two separate techniques. In particular, for samples not subjected to a high temperature anneal, significant discrepancies exist between Sef! extracted from effective lifetime (-ref!) and J{u2080}e data. The reason for this is not well understood, however, several possible mechanisms are discussed. On PECVD SiNx passivated samples featuring a boron diffused emitter, the higher J{u2080}e values under accumulation for boron diffused samples compared to that of undiffused samples implies that the boron diffusion introduces defects at the PECVD SiNx/Si interface of planar (100) and (111) surfaces. Further, the results of J{u2080}e under accumulation for samples within the range of surface concentrations and sheet resistances studied in this thesis suggest that the interface degradation is not a strong function of the boron surface concentration, the total boron dose, or the surface orientation.