Impedance Spectroscopy Study of Core-shell GaAs Nanowire p-n Junction Structures

M. TIRADO; JORGE CARAM; CLAUDIA SANDOVAL; D. COMEDI; JOSEF CZABAN; R. R. LAPIERRE

Turku

Conferencia; 9th International Electrokinetics Conference; 2010

Electrical impedance measurements performed over broad frequency and bias ranges provide information of the whole measured system: bulk material and interfaces. Recently, photovoltaic (PV) p-n junction devices employing semiconductor nanowires (NWs) have received attention due to their enormous potential for lower cost and greater energy conversion efficiency compared to conventional thin film devices. However, the current status of PV devices is far from satisfactory due to laboratory efficiencies that are much lower than the theoretical limits. Some of the problems may include charge carrier trapping at the NW surface or core-shell interface, insufficient control of dopant distribution across the NWs, and high contact resistance. In this work, we present an impedance spectroscopy study of core-shell GaAs nanowire PV devices aimed at determining charge conduction and trapping mechanisms. Samples having two distinct p and n spatial distributions and whit Te as the n-dopant, were studied over the 103?107 Hz frequency and the −1.5 to 1.5 V bias ranges. For a large n-core/p-shell overlap region within the NWs in a coaxial geometry, the p?n junction properties (DC rectification and p?n depletion capacitance) are found to prevail. When the excitation frequency is increased above around 104 Hz, the device responses are found to decay abruptly, an effect that can be attributed to carrier trapping at and release from deep bandgap levels. By comparing the estimated trap energies with those reported in the literature for planar GaAs surfaces, the deep levels are attributed to NW surface states. Electrostatic calculations in the NW core-shell geometry show that NW surfaces lie within or very close to the depletion region, backing the above results. An increasing conductance with increasing frequency for low bias is observed, suggesting hopping transport through localized states. For large bias values, the conductance increases exponentially with bias and is frequency independent, indicating conduction through extended states in this regime.