Fabrication & Characterization of Titania-Germanium (TiO2-Ge) Based Nanocomposites.

Abstract

Composite nano-films of TiO2-Ge were grown by ‘pulsed laser deposition’ (PLD) technique on Si wafers while deposition conditions were changed. Firstly single run deposition for a longer time (30 min) with varying Ge concentration in N and p-type polished Si wafers. Secondly films were deposited as single, bi- and tri-layers on n-type polished and unpolished Si wafers such that each layer was deposited for 5 mins. In the first batch of samples target-substrate distance was varied to find its effect on optoelectronic properties of film. It was observed that Ge concentration decreased as target-substrate separation was increased. Also substrate type and separation between target and substrate effected the crystallinity, optical & electrical response of film. On p-type (111) Si wafer thin films showed dominant amorphous behaviour with decreasing Ge concentration. Thin films deposited on Si wafer showed an improvement in crystallinity as target-substrate distance was increased resulting in decrease in crystallite size, increase in defects and strains. Raman spectroscopic results and EDX analysis confirmed the Ge presence in all the samples. Ge identification in Raman and its non-identification through XRD may possibly be due to non-crystalline nature of Ge. Composite nature was identified by the Ge peak related to cubic structure rather than tetragonal (i.e. did not follow crystalline structure of TiO2) i.e. no doping occurred. Films grown at a distance of 6 cm from the target showed better optoelectronic properties which exhibited minimum reflectance but maximum direct and indirect absorption transitions it is also confirmed by its photoluminescence (PL) response. It has a constant refractive index with a stable extinction coefficient. This film demonstrated a positive dielectric constant and a negligible dielectric loss confirming its stable optoelectronic behaviour which was confirmed by its I-V response. Better optoelectronic response for the above mentioned film can be positively due to strains and size reduction of crystallite size. In the second batch of experiment single, di- and tri-layered composite films were grown on polished and n-type unpolished Si (100) wafer. Amorphous nature was observed dominantly in all the samples. Di-layered film deposited on polished n-type Si wafer showed a better optical response which is due to minimum variation in its n. Maximum direct and indirect transitions are also observed in it and a broad PL peak is observed around 2-3.5 eV. Although it does not show better electrical properties as compared to tri-layered thin films but overall optoelectronic response of this film is better as compared to other samples. The reason may be the outcome of crystalline nature of film. Thin films (for 5 min) shows reduced crystallinity and hence optoelectronic response in comparison to that for 30 min, this variance can be attributed to the reduced thickness of film. Single layer film has variable n responsible for lower dielectric constant and hence lowest electrical response.