Defect States in Hydrogenated- nanocrystalline Si thin films

Hydrogenated nanocrystalline silicon films (nc-Si:H) is a very promising material for photovoltaic applications. Notwithstanding its interesting properties, many issues regarding its electronic and optical properties and the influence of structural defects on them are not completely understood yet. This is due to the fact that nc-Si:H is a very complex material due to the coexistence of different phases: crystalline nano-grains, nanocrystalline columns, hydrogenated amorphous Si (a-Si:H), defects and voids. Surface Photovoltage Spectroscopy (SPS) [1] is a valuable method for the non-contact and non-destructive investigation of several semiconductor properties, such as optical band-gap, defect-related optical transitions, Van Hove singularities or phase inhomogeneities. Hydrogenated nanocrystalline Si films for photovoltaic applications were analysed by SPS. The films have been deposited by Low Energy Plasma Enhanced Chemical Vapor Deposition on different substrates (crystalline Si, oxidized crystalline Si, glass, glass covered by transparent conductive oxide), at deposition temperature around 200 °C and SiH4 dilution ratio d= [SiH4]/([SiH4]+[H2]) ranging from 1% to 50%. The crystal fraction, measured by Raman spectroscopy, ranges from 25 to 75%. Surface Photovoltage spectra 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. One example of a surface photovoltage spectrum of highly crystalline nc-SI:H film is reported in Fig.1, while a sketch of the band structure of the film, as obtained by SPS, is shown in fig.2. Intra-gap transitions in nc-Si:H are often attributed to dangling bonds. In the present study, a tentative identification of the defects responsible of such transitions has been carried out: as SPV spectra measured before and after H etching showed a very similar behavior, thus intra gap transitions should be not related only with dangling bonds, that should reduce their concentration after H-induced passivation. 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. The comparison between films grown at different dilution ratios and with different cristallinity has allowed us to localize the defects responsible for intra-gap optical transitions, i.e. to attribute them to a certain phase in the material. The role played by the different phases on the optical properties of the film is also clarified. One of the most interesting physical issues regarding photovoltaic applications concerns the optical properties, in this respect SPS is a valuable tool as it allows the determination of optical properties in a non-destructive and non-contact mode.