Apr 6, 2010
Impedance spectroscopy profiles GaAs nanowire core-shell p-n junctions
Photovoltaic (PV) p-n junction devices employing semiconductor nanowires (NWs) have an enormous potential for lower cost and greater energy-conversion efficiency compared with conventional thin-film devices. However, charge carrier trapping at the NW surfaces, insufficient control of dopant distribution across the NWs and high contact resistances have been major obstacles to achieving good rectifying characteristics and high solar-energy-conversion efficiencies.
Using impedance spectroscopy in the 103–107 Hz range, researchers at the National University of Tucumán, Argentina, have shown that the degree of spatial overlap of the n-core and the p-shell along the NW axis is critical to obtain good p-n junction electrical characteristics in GaAs NW-based core-shell PV devices.
The tested NW devices were fabricated by the LaPierre research group at McMaster University, Canada, and consisted of n-cores and coaxial p-shells aligned perpendicular to a GaAs substrate. Tellurium was used as the n-type dopant instead of the more common silicon due to the amphoteric behaviour that silicon may have in GaAs NWs. The two kinds of studied devices had very different degrees of core-shell spatial overlap along the NW axes.
When the DC voltage was changed from forward to reverse bias at low frequencies in devices having large core-shell overlap ("sample D" in the figure), the measured capacitance qualitatively followed the behaviour expected for a cylindrical space-charge region characterizing the core-shell p-n interface. In contrast, samples with little overlap ("sample A" in the figure) did not follow this behaviour.
Furthermore, when the excitation frequency was increased above 104 Hz, the device responses decayed abruptly, an effect that the team 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 could be attributed to NW surface states. This was supported by electrostatic calculations of the NW core-shell geometry, which showed that NW surfaces lay within or very close to the depletion region.
The scientists believe that these findings will help improve future NW-based PV devices for efficient solar-energy conversion.
The group presented its work in the journal Nanotechnology.
About the author
The work was performed at the National University of Tucumán (UNT), Argentina, and McMaster University, Canada, and was supported by the Inter-American Collaboration in Materials Research (CIAM), a multi-agency initiative that supports opportunities for new and ongoing interactions between materials researchers that leverage the strengths of each country's scientific community. In Argentina, funding was provided by Argentina's National Research Council (CONICET), the National University of Tucumán Research Council (CIUNT) and the Fund for Scientific and Technological Research (FONCyT) of the National Agency for the Promotion of Science and Technology (ANPCyT). In Canada, the work was funded by Natural Sciences and Engineering Research Council of Canada (NSERC). Jorge Caram and Claudia Sandoval are bachelors in physics studying and working towards their Licenciado degree in physics at the Physics Department, Faculty of Science and Technology (FACET) of the UNT. Dr Mónica Tirado is assistant professor at the Departments of Physics and Bioengineering at the FACET, UNT and member of the Dielectric Properties of Matter Laboratory (LPDM-UNT). Dr David Comedi is an independent investigator with CONICET, graduate school lecturer at the FACET-UNT and member of the Solid State Physics Laboratory (LAFISO-UNT). Josef Czaban is a postdoctoral researcher at the Centre for Emerging Device Technologies (CEDT), McMaster University. Prof. David Thompson is with the CEDT and Emeritus professor at McMaster University. Prof. Ray LaPierre is associate professor at the CEDT and the Engineering Physics Department, McMaster University.