A highly sensitive UV-detector is devised for the first time from ion beam irradiated nanocrystalline CdS (nc-CdS) thin films. The UV-sensors are fabricated using pulsed laser deposited nc-CdS thin films on Si wafer and subsequent irradiation treatment. Swift heavy ion irradiation (SHII) of the nc-CdS thin films is carried out using 70 MeV 58Ni6+ ions. The sensors used in the present study are easy to fabricate and require inexpensive materials; they feature characteristics similar to those of UV sensors designed with complex structures and expensive procedures. Current–voltage (I–V) measurements reveal an enrichment in carrier concentration and improvement in conductivity after exposure to SHII. This giant conductivity may be attributed to an enhancement in sulfur vacancies as a consequence of SHII. The sensor exhibits improvements in the responsivity, photosensitivity, and efficiency as a function of the ion fluence and attains maximum values of ∼53 W A−1, 576.4% and 15.6 × 103% respectively for a film irradiated at a fluence of 1 × 1013 ions per cm2. The response time of the sensor reduces with the increase in ion fluence and reaches a minimum of a rise time of 165 ms and fall time of 65 ms at 3 V for these sensors. A possible mechanism involved in the SHII induced moderation of conductivity and consequently photosensitivity is explained on the basis of variation in the defect densities.

Giant UV-sensitivity of ion beam irradiated nanocrystalline CdS thin films

The influence of swift heavy ion irradiation (SHII) on surface phonon mode (SPM) and green emission in nanocrystalline CdS thin films grown by chemical bath deposition is studied. The SHII of nanocrystalline CdS thin films is carried out using 70 MeV Ni ions. The micro Raman analysis shows that asymmetry and broadening in fundamental longitudinal optical (LO) phonon mode increases systematically with increasing ion fluence. To analyze the role of phonon confinement, spatial correlation model (SCM) is fitted to the experimental data. The observed deviation of SCM to the experimental data is further investigated by fitting the micro Raman spectra using two Lorentzian line shapes. It is found that two Lorentzian functions (LFs) provide better fitting than SCM fitting and facilitate to identify the contribution of SPM in the observed distortion of LO mode. The behavior of SPM as a function of ion fluence is studied to correlate the observed asymmetry (Γa/Γb) and full width at half maximum of LO phonon mode and to understand the SHII induced enhancement of SPM. The ion beam induced interstitial and surface state defects in thin films, as observed by photoluminescence (PL) spectroscopy studies, may be the underlying reason for enhancement in SPM. PL studies also show enhancement in green luminescence with increase in ion fluence. PL analysis reveals that the variation in population density of surface state defects after SHII is similar to that of SPM. The correlation between SPM and luminescence and their dependence on ion irradiation fluence is explained with the help of thermal spike model.

Correlation between surface phonon mode and luminescence in nanocrystalline CdS thin films: An effect of ion beam irradiation

Lattice deformation is observed in nanocrystalline silicon synthesized by ion beam induced phase separation of SiOx films. The separation of silicon and silicon dioxide phases is achieved in the suboxide films after irradiation with a 100-MeV Ag beam. The interaction of a high-energy heavy ion beam with the material is dominated by an electronic energy loss process. A large amount of energy is deposited uniformly in the material through this process. Glancing angle X-ray diffraction (GAXRD), micro-Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), and energy-dispersive X-ray spectroscopy (EDX) measurements were carried out. The results confirm the occurrence of phase separation in non-stoichiometric silicon oxide by means of ion-beam irradiation leading to the formation of silicon nanocrystals in the films. The lattice of silicon nanocrystals is found to be deformed (contracted). The X-ray diffraction studies and HRTEM studies reveal the occurrence of two different phases of silicon, viz. cubic (fcc) phase and high-pressure (contracted lattice) phase, simultaneously. The observations are explained on the basis of ion beam induced pressure and thermal spike in the ion track.

Lattice distortion in ion beam synthesized silicon nanocrystals in SiOx thin films

Synthesis and characterization of nano-crystalline silicon grown by atom beam sputtering technique are reported. Rapid thermal annealing of the deposited films is carried out in Ar + 5% H2 atmosphere for 5 min at different temperatures for precipitation of silicon nano-crystals. The samples are characterized for their optical and structural properties using various techniques. Structural studies are carried out by micro-Raman spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy (TEM), high resolution transmission electron microscopy, and selected area electron diffraction. The optical properties are studied by photoluminescence and UV-vis absorption spectroscopy, and bandgaps are evaluated. The bandgaps are found to decrease after rapid thermal treatment. The micro-Raman studies show the formation of nano-crystalline silicon in as-deposited as well as annealed films. The shifting and broadening in Raman peak suggest formation of nano-phase in the samples. Results of micro-Raman, photoluminescence, and TEM studies suggest the presence of a bimodal crystallite size distribution for the films annealed at higher temperatures. The results show that atom beam sputtering is a suitable technique to synthesize nearly mono-dispersed silicon nano-crystals. The size of the nano-crystals may be controlled by varying annealing parameters.

https://link.springer.com/content/pdf/10.1186%2F1556-276X-7-547.pdf

Opto-structural studies of well-dispersed silicon nano-crystals grown by atom beam sputtering

In the present work, non-stoichiometric silicon oxide films (SiOx) deposited using a hot filament chemical vapor deposition technique at short time and simple parameters of depositions are reported. This is motivated by the numerous potential applications of SiOx films in areas such as optoelectronics. SiOx films were characterized with different spectroscopic techniques. The deposited films have interesting characteristics such as the presence of silicon nanoclusters without applying thermal annealing, in addition to a strong photoluminescence after applying thermal annealing in the vicinity of 1.5 eV, which may be attributed to the presence of small, oxidized silicon grains (less than 2 nm) or silicon nanocrystals (Si-nc). An interesting correlation was found between oxygen content, the presence of hydrogen, and the formation of defects in the material, with parameters such as the band gap and the Urbach energies. This correlation is interesting in the development of band gap engineering for this material for applications in photonic devices.

Spectroscopic Properties of Si-nc in SiOx Films Using HFCVD.

The formation and size tuning of silicon nanocrystals embedded in silicon oxide films grown by reactive pulsed laser ablation followed by ion beam irradiation is reported. Nanocrystalline silicon is prepared in silicon oxide matrix by ablating silicon target in oxygen environment at definite oxygen partial pressure. Irradiation of these films with 120 MeV Ni leads to the uniformity, size reduction, surface smoothness and shaping of these nanostructures. The structural, optical and morphological studies show that the nanocrystalline silicon undergoes further reduction due to irradiation. The results are explained by combining spinodal decomposition phenomena and thermal spike model.

Ram Janey Choudhary

Papers

Controlled formation of silicon nanocrystals by dense electronic excitation in PLD grown SiOX films