Electrical characterization of the near-surface region of semiconductor materials by photoconductive decay.

摘要

As the trend in mainstream IC manufacturing continues to move towards using very thin layers of silicon and very shallow junctions, the near-surface electrical properties become more important. Likewise, in the photovoltaic industry, thin layers of amorphous silicon and shallow junction solar cells using single crystal and multi-crystalline silicon are of significant interest. For all of these applications, an effective method of in-line monitoring of near-surface electrical properties is essential. The near-surface properties of solar cell materials are of particular interest, where there is a delicate balance between having the surface textured sufficiently to minimize reflectivity, but not too excessively as to dramatically reduce carrier lifetime. As solar cell wafers are becoming thinner and bulk recombination lifetimes improve, carrier diffusion lengths will begin to exceed the thickness of the wafer. Hence, the back surface recombination velocity, which directly affects cell efficiency, becomes a critical factor.;Difficulties in determining the impact of near-surface effects on carrier transport properties of thin layer semiconductors have been encountered with traditional methods of electrical characterization. The goal of this research was to investigate the near-surface electrical properties of semiconductor materials, including multi-crystalline silicon used in the photovoltaic industry, by a modified method of electrical characterization based on the photoconductive decay (PCD) effect.;The project was completed in two phases. The first phase involved verification of a photoconductive decay method with a newly developed tool, in both a non-contact version and a physical contact version, with respect to capability for characterizing a shallow subsurface region of selected semiconductor materials. To evaluate the capability of the tool, the longer recombination lifetimes of single crystal wafers of indirect bandgap semiconductors Si and Ge (10--100 micros), and the relatively shorter recombination lifetimes of single crystal direct bandgap semiconductors GaAs and InP (50 ns--1 micros) were verified. In an early stage of the study, the sensitivity of the non-contact version of the tool in its current configuration was found to be too low for characterizing low mobility materials. Subsequently, a high sensitivity temporary contact version of the tool was developed and utilized for a major part of the project.;The near-surface PCD measured response to surface treatments (native oxide removal and growth of a passivation layer) performed on prime single crystal Si wafers correlated with the electrical property changes that are known to occur in response to change of the chemical condition of the surface. The reduction in measured minority carrier lifetime and mobility in response to minor increase in surface roughness (∼1 nm RMS) of Ge wafers, and the increase in measured minority carrier lifetime of Si wafers in response to increasing SiO2 thickness (Deltatox ≅ 43 nm) demonstrates that the near-surface PCD method measures carrier transport properties in a very shallow sub-surface region of semiconductor dominated by surface effects.;The second phase was to utilize the adapted photoconductive decay characterization method to investigate surface carrier transport properties of solar cell grade multi-crystalline silicon material. Near-surface mobility measured by the PCD tool shows a clear distinction between saw damaged and chemically treated wafer surfaces, with the mobility of the latter being approximately 4 times higher than that of the former, while bulk (Hall) mobility measurements did not distinguish between the two surface conditions. Similarly, the near-surface recombination lifetime measurements show a distinct difference between the two wafer surface conditions (Deltatau = 12micros), while no conclusion could be drawn from the bulk recombination lifetime difference.;A method of monitoring surface passivation of mc-Si wafers, a critical step for maximizing solar cell efficiency, utilizing the near-surface PCD tool has been developed. Experimental results obtained show that the measured recombination lifetime improves incrementally with SiO2 thickness (Deltat ox ≅ 43 nm) for both sawed and chemically polished wafers, with the most significant improvement occurring in going from a bare wafer surface to tox ≅ 91 nm.;Studies of optimizing the surface texture etch of sawed mc-Si wafers in an acidic solution were performed by monitoring the recombination lifetime with the near-surface PCD method. It was found that wafers etched in HF/HNO 3/H2O (14:1:5) solution for 2 minutes provided the best compromise between uniform texturization of the surface and low surface recombination velocity.