Scanning tunneling microscopy of hydrogenated amorphous silicon: high-resolution topography and local apparent barrier heights

As-grown films of phosphorous-doped hydrogenated amorphous silicon a-Si:H.were investigated by scanning tunneling microscopy STM.using a combination of high-resolution constant-current imaging and apparent barrier height FA. imaging. We observed a distribution of atomic scale surface structures which is characterized by a local root-mean-square roughness, d l, ranging from 0.2 A°to 1.8 A°and which is superimposed on the array of hills described previously. The structures can be ascribed to surface phases consisting of silicon monohydride species andror adsorbed hydrogen as well as to higher hydrides, SiH xG2.. Correlations were found between F and cos2w, where w is the angle between the local x A surface normal and the direction of tip movement perpendicular to the surface, and between FA and dl. The frequency distributions of FA, cos2w and dldepend significantly on the tunneling voltage, Ut. The frequency distribution of FA, obtained for atomically smooth areas dlf0.3 A°.and for Uts2.3 V, is peaked at 3.1 eV which is in reasonable agreement with a simple model of vacuum tunneling. The dependence of FA on Utcan be explained by intermediate state tunneling. q1999 Elsevier Science B.V. All rights reserved.