Praloy Mondal

Photoluminescence phenomena prevailing in c-axis oriented intrinsic ZnO thin films prepared by RF magnetron sputtering

Substantial c-axis orientation of the hexagonal ZnO crystals with wurtzite structure demonstrates only those two preferred peaks in the first-order spectra which are permitted by the Raman scattering selection rule viz., the Ehigh2 and A1 (LO) modes, that identify the improved structural quality of the undoped ZnO film grown by magnetron sputtering in Ar ambient at an RF power of P = 200 W. The presence of a substantial amount of hydroxyl groups attached to the Zn lattice has been correlated to the dominant c-axis orientation of the ZnO crystals which exhibited a distinct UV luminescence band that arises due to the typical exciton emission or near-band-edge emission. At higher applied powers, disorder-activated Raman scattering introduces a well resolved Bhigh1 mode and gradually growing second order Raman peaks, (Ehigh2 − Elow2) and (Bhigh1 − Blow1), which are caused by the breakdown of translational symmetry of the lattice by defects or impurities and lead to deviation from preferred c-axis orientation with I002/I103 < 1. Out diffusion of oxygen from the network creates increasing oxygen vacancy states and in addition, various other defects e.g., Zn interstitial , doubly ionized Zn vacancy and oxygen antisite (OZn) as the dynamic acceptor defects, act as the origins of different visible photoluminescence components classified in the UV-violet, violet, violet-blue, blue and green regions.

Low temperature grown ZnO:Ga films with predominant c-axis orientation in wurtzite structure demonstrating high conductance, transmittance and photoluminescence

The Ga doped ZnO films grown at a low substrate temperature ∼50 °C and a low RF power ∼50 W in RF magnetron sputtering possess a predominant c-axis orientation in wurtzite structure with I〈002〉/I〈101〉 ∼ 40 in the XRD pattern; which has been further supported by the most prominent presence of the allowed Raman active A1 (LO) mode. The reduced oxygen vacancies in the ZnO network has been correlated to the enhanced optical transparency while the sharp increase in the presence of metallic Ga in the network identified from the XPS data has been attributed to the increasing electrical conductivity of the films prepared at lower temperature. Associated UV luminescence arises as a result of the typical exciton emission or near-band-edge emission, i.e., due to recombination of photo-generated electrons with holes in the valence band or in traps near the valence band. Highly conducting and optically transparent ZnO:Ga films with a dominant c-axis orientation, demonstrating intense UV-luminescence, are extremely useful for many optoelectronic devices including in solar cells, particularly when prepared at such a low substrate temperature which is most compatible for their fabrication.

Preferential 〈220〉 crystalline growth in nanocrystalline silicon films from 27.12 MHz SiH4 plasma for applications in solar cells

It has been experimentally demonstrated that silicon nanocrystallites (Si-ncs) are generally of 〈111〉 crystallographic orientation from random nucleation, which are associated to highly defective polyhydride networks at the grain-boundary; however, ultra-nanocrystallites preferably harvest a 〈220〉 alignment due to the thermodynamically preferred grain growth with concomitant monohydride bonding at the boundary. Using an excitation frequency (27.12 MHz) higher than the conventional frequency of 13.56 MHz, and its stimulus impact in terms of larger ion flux densities with reduced peak ion-energy in the plasma and its associated ability to efficiently generate atomic hydrogen, nanocrystalline silicon (nc-Si) films are produced. The nc-Si:H films grown at elevated pressures demonstrate enhanced growth rates, lower hydrogen contents, lower microstructure factors, preferred 〈220〉 crystallographic orientation and possess a significant fraction of ultra-nanocrystalline component in the Si-network, along with a higher intensity of monohydride bonding at the grain boundary by bond-centered Si–H–Si modes in a platelet-like configuration. The material prepared at a low power and low temperature is extremely suitable, in every aspect, for efficient application in the fabrication of nc-Si p–i–n solar cells.

Preferred C-axis oriented photoluminescent ZnO thin films prepared by RF magnetron sputtering

Undoped ZnO thin films were prepared by RF magnetron sputtering at 300° C. High value of optical transparency, distinct (002) preferential crystallographic orientation, high growth rate, good electrical conductivity and surface roughness are obtained at 200 W of RF power. XRD and Raman studies reveal good crystallinity with c-axis orientation. The Photoluminescence spectrum shows UV, violet, blue and green emission from the material.

Highly conducting and preferred oriented boron doped nc–Si films for window layers in nc–Si solar cells

Growth and optimization of the boron dopednanocrystalline silicon (nc-Si) films have been studied by varyingthe gaspressure applied to the hydrogendiluted silane plasma in RF (13.56 MHz) plasma-enhanced chemical vapor deposition (PECVD) system, using diborane (B2H6) as the dopant gas. High magnitudeof electrical conductivity (~102 S cm−1) and orientedcrystallographic lattice planes have been obtained with high crystalline volume fraction (~86 %) at an optimum pressure of 2.5 Torr. XRD and Raman studies reveal good crystallinity with preferred orientation, suitable for applications in stacked layer devices, particularly in nc–Si solar cells.Growth and optimization of the boron dopednanocrystalline silicon (nc-Si) films have been studied by varyingthe gaspressure applied to the hydrogendiluted silane plasma in RF (13.56 MHz) plasma-enhanced chemical vapor deposition (PECVD) system, using diborane (B2H6) as the dopant gas. High magnitudeof electrical conductivity (~102 S cm−1) and orientedcrystallographic lattice planes have been obtained with high crystalline volume fraction (~86 %) at an optimum pressure of 2.5 Torr. XRD and Raman studies reveal good crystallinity with preferred orientation, suitable for applications in stacked layer devices, particularly in nc–Si solar cells.

Transparent c-axis oriented photoluminescent ZnO:Ga films grown by low temperature magnetron sputtering

ZnO:Ga thin films have been prepared by rf magnetron sputtering, varying the substrate temperature, at a very low rf power (50 W). Films have been characterized by optical, electrical, structural and chemical analysis. The ZnO:Ga film prepared at close to ambient temperature (∼50 °C) demonstrates the widest optical gap, highest transparency over the entire visible region and good electrical conductivity, along with most favored c-axis orientation with I(002)/I(101) ∼40. Presence of metallic Ga, as identified from XPS studies, enhances electrical conductivity. Dominant c-axis orientation identifies improved structural configuration and exhibited intense UV-luminescence band arises due to the typical exciton emission or near-band-edge emission.

Conducting intrinsic nanocrystalline silicon films with high growth rate prepared at 27.12 MHz frequency

Growth and characterization of intrinsic nanocrystalline silicon (nc-Si:H) thin films have been studied using 27.12 MHz H2−diluted SiH4 plasma in PECVD, by optimizing the gas pressure during deposition. High electrical conductivity (∼10−3 S cm−1) and narrow optical gap (1.762 eV) of the mostly crystalline (∼80%) network with optimum hydrogenation and dominance towards thermodynamically preferred (220) crystallographic orientation, obtained at a high growth rate (39 nm/min), signify its promising use in the fabrication of devices e.g., solar cells in particular.