Investigation of metastability and instability effects on the minority carrier transport properties of microcrystalline silicon thin films by using the steady-state photocarrier grating technique

Metastability effects in hydrogenated microcrystalline silicon thin films due to air, high purity nitrogen, helium, argon, and oxygen were investigated using temperature-dependent dark conductivity, photoconductivity, and steady-state photocarrier grating methods. It was found that short-term air, nitrogen, and inert gases caused a small reversible increase of sigma(Dark) and sigma(photo) within a factor of two, but they did not affect the minority carrier mu tau-products significantly. These changes are partially reduced by vacuum treatment and completely reduced after heat treatment at 430 K. However, oxygen gas treatment at 80 degrees C resulted in more than an order of magnitude increase in both sigma(Dark) and sigma(photo) and an increase in the diffusion length, L-D, by 50% from that of the annealed-state value in highly crystalline samples, while no significant metastability is detected in amorphous and low crystalline silicon thin films. A following heat treatment partially recovers both sigma(Dark) and sigma(photo) to their annealed-state values, while L-D decreases only slightly. Such increase in the L-D values could be due to a decrease in the density of recombination centers for holes below the Fermi level, which may be related to passivation of defects by oxygen on the surface of crystalline grains.