Surface Photovoltage Spectroscopy of Semiconductor Nanostructures

Surface Photovoltage Spectroscopy (SPS) [1] is a valuable method for the non-contact and non-destructive investigation of several semiconductor bulk properties, such as optical band-gap, defect-related optical transitions, minority carrier diffusion length and lifetime, Van Hove singularities or phase inhomogeneities. Moreover, SPS allows for obtaining a detailed picture of the electronic structure of surfaces and interfaces, of quantum structures such as quantum wells and superlattices. In SPS, changes in band bending (both at the free semiconductor surface and at buried interfaces) are monitored as a function of external illumination. In the present contribution the physical principles of the method and the experimental details will be discussed, together with several applications concerning a wide variety of materials, alloys and low dimensional semiconductor structures. SPS has been applied to study hydrogenated nanocrystalline silicon films (nc-Si:H) [2]. nc-Si:H is a very complex material made by Si nano-crystals dispersed in hydrogenated amorphous Si (a-Si:H) matrix that is very promising for photovoltaic applications. Notwithstanding its interesting applications, many issues regarding its electronic and optical properties and the influence of structural defects on them are not completely understood yet. The SPS analyses of nc-Si:H films have allowed for the determination of defect states, energy gap and Urbach tails in the films. Several characteristics of the films were obtained: slight n-type conductivity, optical gap around 1.5 eV, Urbach tails around 50meV and the presence of intra-gap transitions relevant to defective states. Optical transitions (around 1.86 eV and 1.14 eV) were also detected and attributed to the optical gap of the amorphous phase and crystalline phases, respectively. Another application will be shown, relevant to Cd1–xZnxTe radiation detectors studied by SPS [3]. SPV spectra have been obtained on semi-insulating Cd1-xZnxTe samples with varying Zn concentration. A reliable and very accurate determination of the energy gap in Cd1-xZnxTe (with x ranging from 0 to 6.76%) has been achieved, needed to assess the exact Zn concentration which significantly affects the transport and lattice parameters of the alloys. It was also found that the electron-hole recombination at the surface has a strong effect on the determination of band gap energy through SPV spectra. Moreover a band structure feature relevant the Γ8 splitting of the valence band in CdTe has been found. A final application will concern Surface photovoltage spectroscopy of AlInN based semiconductor heterostructures. The SPS has allowed for the determination of energy gap in buried semiconductor layers and of intra-gap defect states. References: [1] L. Kronik Y. Shapira Surf. Interface Anal. 31, 954 (2001) [3] A. Cavallini, D. Cavalcoli, M. Rossi, A. Tomasi, S. Pizzini, D. Chrastina, G. Isella. PHYSICA. B, Cond. Matt 401-402, 519 - 522. (2007). [2] D. Cavalcoli, B. Fraboni, and A. Cavallini J Appl Phys 103, 043713 (2008).