Widening Of Optical Gap Research Articles

Effect of dehydrogenation on optical constants of silicon nitride thin films

This study presents annealing experiments conducted on near-stoichiometric hydrogenated amorphous silicon nitride (a-SiNx:H) thin films deposited via Plasma Enhanced Chemical Vapor Deposition. The heat treatment was carried out under an inert atmosphere within a temperature range of 400–1000 °C, with increments of 100 °C. After each annealing step, a-SiNx:H films were subjected to characterization using visible and FTIR spectroscopies, evaluating their optical and structural properties, as well as hydrogen concentration. The infrared analysis reveals a significant reduction in Si–H bond concentration within the temperature range of 400–700 °C, becoming virtually undetectable at higher annealing temperatures. Conversely, the N–H bond concentration primarily decreases at elevated temperatures, persisting at 23% of its initial value at 1000 °C. Optical constants were determined from transmittance spectra using a dispersion model that combines two unbounded Tauc-Lorentz oscillators. The refractive index experiences an increase up to 600 °C due to material densification, followed by a decrease at higher temperatures attributed to optical gap widening. Dispersion curves of the extinction coefficient reveal a wide sub-gap absorption band, which vanishes after a 900 °C annealing step, rendering the material suitable for waveguide applications.

Study of Infra-red Spectroscopy on Bonding Environment and Structural Properties of Nanocrystalline Silicon Thin Films Grown by VHF-PECVD Process

Infra-red spectroscopy as an effective tool used to establish platelet like configuration in nanocrystalline silicon thin films (nc-Si:H). These films were deposited using 60 MHz assisted very high frequency plasma enhanced chemical vapor deposition process with varying pressure from 4 to 40 Pa. The deposited films were characterized by X-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM), Atomic force microscopy (AFM), Raman spectroscopy (RS), Fourier transform infra-red spectroscopy (FTIR), Dark conductivity, UV-Vis Spectra and photosensitivity. Infra-red studies of these films reveals that the hydrogen bonding is a platelet like configuration (Si-H groups at 2033 cm− 1) located at grain boundaries resulting in crystalline grains wrapped with hydrogen rich amorphous tissues that provides good passivation resulting in less dangling bonds on grain boundary surface. Structural transformation from amorphous silicon (a-Si:H) to nanocrystalline (nc-Si:H ) phase has been identified at pressure of 24 Pa. The increase in deposition pressure provides clear evidence of improved crystallinity (∼ 37%) which were depicted by increased grain size (∼ 12 nm), reduced hydrogen bonded content (∼ 5.0%), widening of optical gap (∼ 1.9 eV) with enhanced polymerization in the network.

Melting point of Sn as the optimal growth temperature in realizing the favored transparent conducting properties of In2O3:Sn films

At substrate temperature (TS) close to melting point of Sn (TSn) rapid incorporation of metallic dopants in significant amount introduces sharp rise in mobility (μ) and concentration (ne) of charge carriers, leading to substantial reduction in resistivity (ρ); simultaneous sharp widening in optical gap (Eg) results in optimum Figure-of-Merit (Φ). The Eg vs. ne2/3 plot demonstrates two distinct regimes of TS across TSn, leading to higher reduced effective mass of charge carriers, mvc∗ by 0.072m0 owing to rapid incorporation of Sn4+ at substitutional site of In3+ in In2O3 matrix. Consequently, the self-energy due to electron-impurity scattering rises and/or the material self-converts to an alloy-like ensemble. On further increase in TS, Φ reduces due to enhanced optical absorption by metallic dopants and dopant induced defects, as noted by enhanced Urbach-energy (EU). Rather than any arbitrary TS, TSn has been demonstrated as an optimal growth temperature for ITO films grown by RF magnetron sputtering.

Evolution of size distribution, optical properties, and structure of Si nanoparticles obtained by laser-assisted fragmentation

We investigate the physical properties of Si-based nanoparticles produced by an environment-friendly three-step method relying on: (1) laser ablation of a solid target immersed in water, (2) centrifugation and separation, and (3) laser-assisted fragmentation. The evolution of size distribution is followed after each step by means of dynamic light scattering (DLS) measurements and crosschecked by transmission electron microscopy (TEM). The as-ablated colloidal suspension of Si nanoparticles presents a large size distribution, ranging from a few to hundreds of nanometers. Centrifugation drives the very large particles to the bottom eliminating them from the remaining suspension. Subsequent irradiation of height-separated suspensions with a second high-fluence (40 mJ/pulse) Nd:YAG laser operating at the fourth harmonic ( $$\lambda =266$$ nm) leads to size reduction and ultra-small nanoparticles are obtainable depending on the starting size. Si nanoparticles as small as 1.5 nm with low dispersion ( $$\pm 0.7$$ nm) are observed for the uppermost part after irradiation. These nanoparticles present a strong blue photoluminescence that remains stable for at least 8 weeks. Optical absorption (UV–Vis) measurements demonstrate an optical gap widening as a consequence of size decrease. Raman spectra present features related to pure silicon and silicon oxides for the irradiated sample. Interestingly, a defect band associated with silicon oxide is also identified, indicating the possible formation of defect states, which, in turn, supports the idea that the blue photoluminescence has its origin in defects.

Nanocrystalline silicon prepared at high growth rate using helium dilution

Growth and optimization of the nanocrystalline silicon (nc-Si: H) films have been studied by varying the electrical power applied to the helium diluted silane plasma in RF glow discharge. Wide optical gap and conducting intrinsic nanocrystalline silicon network of controlled crystalline volume fraction and oriented crystallographic lattice planes have been obtained at a reasonably high growth rate from helium diluted silane plasma, without using hydrogen. Improving crystallinity in the network comprising ∼ 10 nm Si-nanocrystallites and contributing optical gap widening, conductivity ascending and that obtained during simultaneous escalation of the deposition rate, promises significant technological impact.

Properties of a-SiO:H films prepared by RF glow discharge

Hydrogenated amorphous silicon oxygen alloy (a-SiO:H) films have been prepared by RF plasma enhanced chemical vapour deposition from (SiH 4+CO 2+H 2) gas mixture. Films have been characterized, in detail, by electrical, optical as well as structural studies. The effect of the oxygen incorporation into the Si-network was studied by controlling various deposition parameters e.g., CO 2 to SiH 4 flow ratio, H 2 dilution of the plasma, total flow rate of the reacting gases, RF power applied to the electrodes, working gas pressure in the plasma chamber and the substrate temperature. Optical gap of the films increased due to the incorporation of O, and a lowering in photoconductivity with optical gap widening was monitored. Increasing polyhydrogenation at higher O-content resulted in a rise in defect density. O-incorporation into the Si-network increased the light-induced degradation in photoconductivity.

Nanocrystallization in Si:H and quantum size effect on optical gap

Interruption of growth and H-plasma exposure on stacking layers of Si:H film resulted in a remarkable change in material properties. Widening of optical gap and increase in dark conductivity were simultaneous with the reduction in photoconductivity, bonded hydrogen content and optical absorption. An associated change in the network structure from amorphous towards crystalline was observed. Enhanced dose of plasma exposure resulted in the gradual lowering in the size of nanograins and increase in their number density. Systematic widening in optical gap during dehydrogenation of the network appears to be a unique feature related to amorphous semiconductors, which suggests nanocrystallization and quantum size effect in hydrogenated binary alloy.

Physical and electronic structure of amorphous silicon carbon hydrogen alloy

The physical and electronic structure of hydrogenated amorphous silicon carbide (a-SiC:H) are identified by measuring its photoluminescence and infrared spectra at various stages of thermal annealing. It is found that Brodsky’s [Solid State Commun. 36, 55 (1980)] quantum well model can be successfully applied to explain the observed results, such as the double peak structure in photoluminescence spectrum and the physical origin of the optical gap widening due to carbon incorporation.