Ramakanta Naik

Role of rigidity and temperature in the kinetics of photodarkening in Ge x As (45-x) Se 55 thin films

We present insightful results on the kinetics of photodarkening (PD) in Ge(x)As(45-x)Se55 glasses at the ambient and liquid helium temperatures when the network rigidity is increased by varying x from 0 to 16. We observe a many fold change in PD and its kinetics with decreasing network flexibility and temperature. Moreover, temporal evolution of PD shows a dramatic change with increasing x.

Photoinduced transparency of effective three-photon absorption coefficient for femtosecond laser pulses in Ge16As29Se55 thin films

We report a dramatic change in effective three-photon absorption coefficient of amorphous Ge16As29Se55 thin films, when its optical band gap decreases by 10 meV with 532 nm light illumination. This large change provides valuable information on the higher excited states, which are otherwise inaccessible via normal optical absorption. The results also indicate that photodarkening in chalcogenide glasses can serve as an effective tool to tune the multiphoton absorption in a rather simple way

Kinetics and chemical analysis of photoinduced interdiffusion in nanolayered Se/As2S3 films

We have studied the kinetics of photoinduced effects in nanolayered Se/As2S3 film by in situ optical absorption measurements, which reveal that photodarkening in these films is followed by photoinduced diffusion. An increase in disorder during photodarkening and its subsequent decrease during photoinduced diffusion were also observed. The observation of photodarkening of Se at room temperature when confined between As2S3 layers suggests that the glass transition temperature of Se shifts to higher energy. The analysis shows that the atoms which take part in photodarkening play a vital role in photoinduced diffusion. The x-ray photoelectron spectroscopy measurements show the atomic movements during photoinduced diffusion. It also shows that some of the As–S bonds are converted into As–Se bonds. Since it is energetically difficult to break an As–S bond to form an As–Se bond, we assume that the new bond formations are taking place by the bond rearrangement mechanism.

Optical band gap tuning by laser induced Bi diffusion into As2Se3 film probed by spectroscopic techniques

Amorphous chalcogenide semiconducting materials are very sensitive to electromagnetic radiation and are useful for infrared optics and play a pivotal role in modern technology. In the present article, As2Se3 and bilayer Bi/As2Se3 thin films were prepared by thermal evaporation method. The 532 nm laser induced diffusion of active Bi top layer into barrier As2Se3 film was probed through spectroscopic techniques. An X-ray diffraction study reveals no structural change due to laser irradiation while the optical parameters are affected by both Bi addition and laser irradiation which brings a change in the transmittivity and absorption coefficient. The indirect optical band gap is found to be decreased by 0.11 eV due to Bi addition to As2Se3 which is explained on the basis of density of defect states with an increase in disorder. The laser irradiated Bi diffusion increases the optical band gap by 0.05 eV (photobleaching) with a decrease in disorder. The Tauc parameter and Urbach energy which measures the degree of disorder change with Bi doping and irradiation. The refractive index is modified by the illumination process which is useful for optical applications. The optical property change is well supported by X-ray photoelectron core level spectra and Raman spectra.

Photo-induced optical bleaching in Ge12Sb25S63 amorphous chalcogenide thin films: effect of 532 nm laser illumination

The photo-induced effects of Ge12Sb25S63 films illuminated with 532 nm laser light are investigated from transmission spectra measured by FTIR spectroscopy. The material exhibits photo-bleaching (PB) when exposed to band gap light for a prolonged time in a vacuum. The PB is ascribed to structural changes inside the film as well as surface photooxidation. The amorphous nature of thin films was detected by x-ray diffraction. The chemical composition of the deposited thin films was examined by energy dispersive x-ray analysis (EDAX). The refractive indices of the films were obtained from the transmission spectra based on an inverse synthesis method and the optical band gaps were derived from optical absorption spectra using the Tauc plot. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple-DiDomenico model. It was found that the mechanism of the optical absorption follows the rule of the allowed non-direct transition. Raman and x-ray photoelectron spectra (XPS) were measured and decomposed into several peaks that correspond to the different structural units which support the optical changes.

Photo induced effects on the optical properties of As20Sb20S60 thin films

The thermally evaporated As20Sb20S60 amorphous film of 800 nm thickness was subjected to light exposure for photo induced studies. The as-prepared and illuminated thin films were studied by X-ray diffraction, Fourier Transform Infrared Spectroscopy and X-ray Photoelectron Spectroscopy and Raman spectroscopy. The optical band gap was reduced due to photo induced effects along with the increase in disorder. These optical properties changes are due to the change of homopolar bond densities. The core level peak shifting in XPS spectra and Raman shift supports the optical changes happening in the film due to light exposure.

Influence of Ge addition on the optical properties of As40Se50Ge10 thin film probed by spectroscopy techniques

ABSTRACT The thin films of As40Se60 and As40Se50Ge10 were prepared on glass substrates by thermal evaporation method with thickness 1000 nm. The prepared films were amorphous in nature which was confirmed through X-ray diffraction. The chemical composition and the surface picture were obtained from energy dispersive X-ray analysis and Scanning Electron Microscopy analysis. The transmission data of the two films were collected in the wavelength range 400–1000 nm. The transmission percentage is found to be decreased whereas the absorption coefficient is increased with the Ge addition. The addition of Ge into As40Se60 is found to increase the refractive index and the extinction coefficient of As40Se50Ge10 thin film. The decrease in optical band gap is explained on the basis of increase in density of states and disorderness due to Ge addition. The optical absorption in the film is due to allowed indirect transition, and the homopolar bond density is increased with Ge addition. The Raman shift observed in the two films clearly supports the optical changes due to Ge addition.

Influence of thermal annealing on optical and structural properties change in Bi-doped Ge30Se70 thin films

ABSTRACT The effect of annealing on the structural and optical properties of thermally evaporated Ge30Se70 and Ge30Se60Bi10 thin films is reported in this paper. The prepared films were thermally annealed at 250°C to optimize the optical properties which can be used for the optical device fabrication. X-ray diffraction study revealed no structural transformation whereas the surface morphology changed as observed from scanning electron microscopy. The optical properties of the deposited and annealed films have been investigated by using a UV–VIS–NIR spectrophotometer in the wavelength range of 400–1100 nm. The optical band gap of the Ge30Se70 annealed film is found to be increased while the energy gap of the Bi-doped annealed Ge30Se60Bi10 thin film decreased which is explained by the chemical disorderness, defect states and density of localized states in the mobility gap. The Tauc parameter and Urbach energy which measure the degree of disorder changed with the annealing process. The transmittivity increases and the absorption power decreases in the Ge30Se70 annealed film, whereas the reverse effect is noticed for the annealed Ge30Se60Bi10 thin film. The irreversible nature of this change can be useful for optical recording purposes.

Low-temperature growth of γ phase in thermally deposited In2Se3 thin films

ABSTRACT In the present report, we have studied the structural and optical change in the In2Se3 thin films prepared by the thermal evaporation method, deposited on a glass substrate and annealed at 250°C. Both the structural and optical studies revealed the formation of γ-In2Se3 phase on annealing at such low temperature as 250°C which is not observed before. Raman analysis indicates that the as-prepared film consists of both α and γ-In2Se3 phases and the annealed film contains only γ-In2Se3 phase. The absorption mechanism in the studied film follows the direct allowed transition. The optical band gap is found to be decreased with annealing due to the increase in the width of localized states near to the band edges. Transmittance is found to be decreased and the absorption is increased with annealing due to the change in the film density which enhances its suitability for solar cell applications.

Effect of bismuth on structural and optical properties of Ge30Se70 amorphous thin film

The present paper emphasizes on the structural and optical properties change in thermally evaporated Bi doped Ge30Se70 thin film. Thin films of Ge30Se70 and Ge30Se60Bi10 were prepared by thermal evaporation technique from the bulk material under high vacuum condition. X-ray diffraction study (XRD) revealed the amorphous nature of both the films as no sharp peak was found. Scanning electron microscopy (SEM) study indicated the smooth surface for both the films. The elemental presence was confirmed from Energy Dispersive X-ray spectroscopy (EDXS). The optical transmission was recorded by Fourier Transform Infra Red (FTIR) spectroscopy in the range 500-1100 nm. The optical band gap calculated by Taucs relation shows a large decrease in band gap from 1.67 eV (Ge30Se70) to 1.05 eV (Ge30Se60Bi10) with Bi incorporation. This decrease in band gap is discussed in terms of defect states and degree of disorderness in the film. After incorporation of Bi, the increase in width of localized states in the band gap region reduces the optical band gap.