Effect of Annealing on Microstructure in (Doped and Undoped) Hydrogenated Amorphous Silicon Films

Abstract

Laser heating and annealing of hydrogenated amorphous silicon (a-Si:H) films is of interest for improved material properties. Due to the variety of possible laser treatments with regard to wavelength, pulse duration, scan time etc., the definition of laser impact on the material is a challenge which we try to approach by comparing properties of laser and oven treated materials. Here we report on the effect of oven heat treatment (up to T A = 575°C) on microstructure and hydrogen content of hydrogenated amorphous silicon films, as detected by measurements of infrared absorption and of effusion of hydrogen as well as of implanted helium. The latter technique has been found to measure isolated voids (cavities) of the size of silicon divacancies and larger. Undoped as well as phosphorus and boron doped plasma-deposited aSi:H films of various hydrogen content (< 15 at.%) were investigated, including undoped device grade a-Si:H. The results show little indication for void-related microstructure in the asdeposited and annealed state for material with a concentration of silicon bonded hydrogen below 5 at. %. At higher hydrogen concentration, evidence is found that hydrogen out-diffusion due to annealing causes isolated voids in concentrations up to about 10 20 cm -3 . A possible mechanism for the annealing induced (micro-)void generation is discussed.

Temperature and hydrogen diffusion length in hydrogenated amorphous silicon films on glass while scanning with a continuous wave laser at 532 nm wavelength

“…Several studies of structural and optoelectronic changes of (dense) a-Si:H by H outdiffusion exist, based on furnace annealing experiments performed at T ≤ 500°C. 37,56,[72][73][74] Conspicuous effects caused by H release are the reduction of the optical bandgap of a-Si:H, 73 the generation of microvoids 73,74 and of Si dangling bond related defects. 75 For H outdiffusion by laser scanning, similar annealing effects as for furnace annealing are expected to occur for the same H diffusion lengths, but due to the much shorter annealing time periods much higher annealing temperatures are required.…”

Investigation of Thermal Stability Effects of Thick Hydrogenated Amorphous Silicon Precursor Layers for Liquid‐Phase Crystallized Silicon

“…Apparently, this H maximum marks the plane where most part of the film peeled off. As an explanation for the peeling at this particular depth of 400–500 nm, we propose void generation caused by inhomogeneous material shrinking [ 52 ] paired with stress and the filling of these voids or bubbles by H 2 during annealing. The stress likely arises next to the substrate by the different thermal expansion coefficients of a‐Si:H and the glass substrate.…”