Method for electrical characterization of nanowires

Abstract

The ability to control conductivity in semiconductor nanostructures is often challenged bysurface states trapping the majority of the charge carriers. Addressing this challengerequires a reliable method for assessing electrical properties such as carrier concentrationand mobility. Unfortunately, here we are facing another challenge, as the Hall effect isgeometrically inapplicable to nanowires while the field effect model is also challenged bythe geometry of the common nanowire field effect transistor, and can only yield channelmobility which is very different from Hall mobility. In this paper, we propose a method thatcombines resistivity and photovoltage measurements with a chemical perturbation tothe surface to measure carrier concentration and mobility, as a function of wirediameter, and also to measure the surface state density and the surface band bendingbefore and after the chemical treatment. We apply this method to CVD grownGaN nanowires, before and after a mild HCl etch. Using transmission electronmicroscope and x-ray photoelectron spectrometry we find that HCl removes thenative gallium oxide. The etch is found to reduce the surface state density from1 × 1012 to5.3 × 1011 cm2, which is calculated from a reduction of the critical radius for full depletion from 7.6 to4 nm, and a reduction of the surface band bending from 0.53 to 0.29 eV, observed usingsurface photovoltage. On the average, the values of carrier concentration we obtain areabout ten times smaller, and the mobility about ten times greater, than values obtainedusing field effect transistors. Interestingly, the weak size dependence of the mobilitydisappears after etching, suggesting a causal linkage between the as-grown size dependenceof the mobility and the density of surface states. The proposed method provides anexperimental handle to the study of surface states and their effects on the electricalproperties of nanowires.