Photoinduced electromotive force on the surface of InN epitaxial layers

TL;DR

This paper investigates the generation of photo-induced electromotive force on InN epitaxial layers, revealing unique surface effects and developing a theoretical model based on surface electron gas and inhomogeneous surface potential distribution.

Contribution

It introduces a novel understanding of surface photo-EMF and negative photoconductivity in InN layers, supported by experimental evidence and a new theoretical model.

Findings

Surface photo-EMF varies in magnitude and polarity under illumination.

Negative photoconductivity observed on InN surfaces.

Inhomogeneous surface potential distribution confirmed experimentally.

Abstract

We report the generation of photo-induced electromotive force (EMF) on the surface of c-axis oriented InN epitaxial films grown on sapphire substrates. It has been found that under the illumination of above band gap light, EMFs of different magnitudes and polarities are developed on different parts of the surface of these layers. The effect is not the same as the surface photovoltaic or Dember potential effects, both of which result in the development of EMF across the layer thickness, not between different contacts on the surface. These layers are also found to show negative photoconductivity effect. Interplay between surface photo-EMF and negative photoconductivity result in a unique scenario, where the magnitude as well as the sign of the photo-induced change in conductivity become bias dependent. A theoretical model is developed, where both the effects are attributed to the 2D electron gas (2DEG) channel formed just below the film surface as a result of the transfer of electrons from certain donor-like-surfacestates, which are likely to be resulting due to the adsorption of certain groups/adatoms on the film surface. In the model, the photo-EMF effect is explained in terms of a spatially inhomogeneous distribution of these groups/adatoms over the surface resulting in a lateral non-uniformity in the depth distribution of the potential profile confining the 2DEG. Existence of such an inhomogeneity in the distribution of surface potential has indeed been experimentally found for these layers.