Surface Photovoltage studies of surface and defect states in semiconductors

The Surface photovoltage spectroscopy (SPS) is a valuable and flexible method for the non-contact and non-destructive investigation of electronic states in semiconductors [1,2]. In this contribution, a throughout description and discussion of the SPS method will be presented. The basic physical principles, the details of the experimental set-up and the relevant results, the capability of the method to extract material properties like optical band gap and defect related states will be discussed. The method presents several advantages, as it allows for the identification of conduction vs. valence band nature of the defect-related transitions and the defect level positions within the band gap. Moreover it allows for the detection of relatively low densities of surface defects as well as their cross sections. Some examples of the application of the method to different materials and structures are discussed, ranging from bulk semiconductors, to low-dimensional systems, to nano- structures. In more detail, SPS has been used to determine the energy gap of bulk crystalline CdTe and Cd1-xZnxTe, and its variation with Zn concentration, to determine surface recombination effects and below-band gap defect states. SPV spectroscopy has also allowed for optical characterization of buried layers in III-N based heterostructures, where the effect that the 2 dimensional electron gas forming in InAlGaN/GaN has on the energy gap value has been demonstrated. Impurity related levels, gap values and bowing parameters have been obtained in InxGa1-xN as a function of In content. The potentialities of the method have further been shown on Si based nanostructures, in this case the method helped obtaining evidence of quantum confinement in Si nanocrystals and doping-induced phase changes in Si nanowires. Finally, the application of the method to nanoporous Ge has contributed to the identification of phase transitions induced by thermal treatment, quantum confinement effects and enhanced light trapping effects due to Au nanoparticles embedded in the np-Ge matrix. In conclusion, the here reported examples evidence the potential, the flexibility and the ease of application of the SPV method, that can be used to investigate a wide range of different systems allowing to achieve relevant physical information on material properties that can hardly be otherwise obtained. References 1. Kronik L. Shapira Y. 2001. Surface photovoltage spectroscopy of semiconductor structures: at the crossroads of physics, chemistry and electrical engineering. Surf. Interface Anal.; 31, 954–965. 2. D Cavalcoli, B Fraboni, A. Cavallini; Surface and Defect States in Semiconductors Investigated by Surface Photovoltage, Chapt 7, Semiconductors and Semimetals, Elsevier, 91, 251-278, 2015