Effects of dimensional nanoscaling on the optical and electrical properties of crystalline Si thin films

Abstract

Thin film Si structures between 10 and 200nm in thickness and configured into two terminal metal-semiconductor-metal structures have been characterized for optical and electrical properties. Dark currents, spectral response, dc quantum efficiency, and ultrafast time response up to 400nm femtosecond laser illuminations at low fields have been studied. Dark currents and dc photocurrent measurements showed an increase in the film conductivity between 75 and 35nm, suggesting an increase in the carrier effective velocities due to confinement. An increase in the carrier effective velocity below 75nm was also confirmed through the transient response analysis. The measured spectral responses are in good agreement with Fresnel’s theoretical model for thin film coupling. The electron-limited transient signal has a full width at half maximum (FWHM) approximately 40ps for the 10nm Si film as compared to 490ps for a 200nm structure. For a hole-limited transit time signal the FWHM was about 82ps for the 10nm film as compared to 2.5ns for the 200nm film reduction in FWHM for both electrons and holes for the 10nm film, signifying that carriers travel much faster as the film thickness is reduced.