Thickness Dependent Structural, Electronic, and Optical Properties of Ge Nanostructures

Abstract

In the present paper, we have investigated structural, optical as well as electronic properties of electron beam evaporated Ge thin films having layer thicknesses ranging from ultra-thin (5 nm) to thick (200 nm). The Raman spectra show that all peaks are shifted towards lower wave number as compared to their bulk counterparts and are considered as a signature of nanostructure formation and quantum confinement effect. The Raman line exhibits transformation from nanocrystalline to microcrystalline phase with a reduction in blue shift of peak position with increase in Ge film thickness (>5 nm). Similarly, the optical absorption spectra corresponding to these films also show reduction in blue shift effect, although Ge 5 nm film shows the absorption behaviour quite different from higher thickness films. The corresponding band gap values obtained from absorption measurements are much larger than bulk Ge and are mainly attributed to the effect of quantum confinement as expected for small size particles calculated from GIXRD patterns. AFM data in each case are correlated and discussed with structural as well as optical results to support the effect of growth morphology on the above-mentioned observations. The results are further supported by photoelectron spectroscopy (PES), photoluminescence (PL) and resistivity measurements and are interpreted in terms of crystallinity and quantum confinement effect.

Nanostructured multilayer TiO2–Ge films with quantum confinement effects for photovoltaic applications

“…This pronounced effect may be due to TiO 2 in the multilayer films that tend to isolate Ge crystallites/islands and layers from each other for enhanced quantum confinement effects. It is well-known that threedimensional quantum confinement in nanoparticles is much more effective that one-dimensional quantum confinement in 2-D layers [14]. The pronounced increase with annealing in our multilayer films seems to arise from gradual conversion of 2-D layers into 3-D islands, crystallites, or quantum dots partly isolated from each other by TiO 2 .…”

“…Vapor-deposited Ge films exhibit higher band gap energy [14,15,19], in comparison with bulk Ge ($0.7 eV). Gorokhov et al [15] have found that higher band gap energy is due to small particle size of Ge crystals isolated by a minute amount of oxide.…”

“…It has been shown earlier that nanostructured Ge films exhibit quan- tum confinement effects [14,15,19], and annealing of Ge films causes blue shift in photo-absorption (from 1.1 eV to about 1.35 eV) due to enhanced quantum confinement effects. On the other hand, a band gap of Ge in the present TiO 2 -Ge multilayer films exhibits a more pronounced shift with annealing with a very significant increase to about 1.72 and 1.77 eV (direct band gap) after crystallization at 450 and 500°C, respectively.…”

“…1b). This may be related to the fact that the structure of thin films (or layers in the multilayer films) may depend on thickness [14]. Furthermore, the amorphous structure of underlying Ge layers may also be responsible for the formation of the amorphous phase in thin TiO 2 layers, being deposited alternatively.…”

“…It is well-known that band gap energy of thin films changes with film thickness [14], deposition techniques [18], and annealing [19], which is because of the formation of nanoparticles [14], nanoporosity [20], and an oxide network [15,19] causing quantum confinement effects associated with nanostructures [14,15,19]. It has been shown earlier that nanostructured Ge films exhibit quan- tum confinement effects [14,15,19], and annealing of Ge films causes blue shift in photo-absorption (from 1.1 eV to about 1.35 eV) due to enhanced quantum confinement effects.…”