K. P. Adhi

Role of Al doping in structural, microstructural, electrical and optical characteristics of as-deposited and annealed ZnO thin films

The role of the Al dopant (0.0 to 3.0 wt%) in modifying the structural, microstructural, electrical and optical properties of pulsed laser-deposited ZnO thin films is reported in both as-deposited (AD) and annealed (AN) films. Incorporation of Al3+ ions in the ZnO matrix (AZO) created localized lattice distortions in AD thin films. Breakdown in the local translational symmetry resulted in the observation of otherwise Raman inactive modes in the phonon spectra of the AZO thin films. Furthermore, Al doping enhanced the n-type character of the films, with the charge carrier density exceeding the Mott critical density of ZnO. Charge transport study at low temperatures revealed the metal–semiconductor transition. The increase in charge carrier density with Al doping concentration resulted in a blue shift in the absorption band-edge of these films. These well-characterized thin films were annealed at 800 °C in ambient air for 4 hours. Subsequent characterization revealed drastic modifications in the properties of the AZO thin films. Annealing resulted in reduced lattice distortions, thus improving the crystalline quality of the thin films. This is also supported by the enhanced intensity of E2High phonon mode and the disappearance of Raman inactive modes in the phonon spectra of AN thin films. AN AZO thin films showed reduced charge carrier density and increased resistivity. These radical changes in characteristics suggest that the segregation of Al3+ ions to grain boundaries is a consequence of annealing.

Benzophenone doped polydimethylsiloxane: self developable composite resist system for its use in a direct write laser lithography application

This paper reports a benzophenone doped polydimethylsiloxane (PDMS) composite resist system, for micro patterning using direct write laser lithography for its use in lab-on-chip (LOC) applications. A 248 nm excimer laser with a 20 ns pulse width is used for microfabrication of doped-PDMS. The effect of two major aspects viz. resist composition and laser processing parameters on the quality of fabricated microstructures is studied and optimized. The lithographic analysis reveals that the doped-PDMS shows self developable sensitivity at lower threshold fluence, 250 mJ cm−2. The optimized composition ratio 10: 1: 0.3 (wt%) of the PDMS monomer: curing agent: Benzophenone (P:C:B) is used for processing and analysis. Comprehensive analysis on the effect of laser ablation parameters (fluence, frequency and number of laser pulses) on etching performance (etch rate, geometry of micropattern and quality of surface) is presented. Increase in etch rate with fluence (250–650 mJ cm−2) is observed and considered to be in a working range. Simultaneously, increase in surface roughness as a function of fluence >650 mJ cm−2 is observed which can be associated with rapid rise in the photothermal decomposition of the composite resist. However pulse repetition rate (PRR) at 1, 5 and 10 Hz does not offer any significant effect on etch rate. The surface quality at a higher PRR is deprived due to redeposition of ablated material which concludes 1 Hz as a suitable working frequency. The deterioration of surface quality with increasing PRR is associated with the formation of a heat affected zone, due to cumulative heating, as the increase in temperature is 362 °C at 5 Hz and 624 °C at 10 Hz, above Ts ~ 1099 °C for 1 Hz. However, the number of pulses and etch rate are inversely related due to the plume effect. The overall study provides a guideline for precise control on fast prototyping direct write laser lithography processes used in LOC applications.

Highly conducting phosphorous doped n-type nc-Si:H films by HW-CVD for c-Si heterojunction solar cells

Phosphorous doped hydrogenated nanocrystalline silicon (nc-Si:H) films were prepared using the hot wire chemical vapor deposition (HW-CVD) method at a low substrate temperature of 200 °C. The microstructure and opto-electrical properties of these films were systematically studied using Raman spectroscopy, low angle XRD, high resolution transmission electron microscopy (HR-TEM), UV-Visible spectroscopy, Fourier transform infrared (FTIR) spectroscopy, dark conductivity and its activation energy measurements and Hall measurement as a function of PH3 gas-phase ratio. It has been found that with an increase in the PH3 gas-phase ratio, both the volume fraction of the crystallites and its size decrease, signifying that the phosphorous atom favors the growth of amorphization in the nanocrystalline Si network. At the optimized PH3 gas-phase ratio we have obtained n-type nc-Si:H films with a band gap of ∼1.84 eV, high dark conductivity (∼6.78 S cm−1) with low hydrogen content (∼1.72 at. %), at a reasonably high deposition rate (∼10 A s−1). Finally, Al/ZnO:Al/n-nc-Si:H/buffer a-Si:H/p-c-Si/Al heterojunction solar cells were fabricated using the optimized n-layer, showing excellent photovoltaic performance with Voc = 719 mV, Jsc = 9.94 mA cm−2, FF = 53.8%, and an energy conversion efficiency of 5.2%. These are very encouraging results for the future fabrication of high efficiency silicon heterojunction solar cells and thin film tandem solar cells.

Effect Of Eu Doping On The Structural, Optical And Electrical Properties Of ZnO Thin Films Deposited By PLD

Effect of Eu doping on the structural, optical and electrical properties of pulsed laser deposited ZnO thin film is reported. Thin films of Eu doped ZnO (Zn(1−x)EuxO, where, x = 0, 0.01 and 0.03) were deposited on the c−Al2O3 substrate. X‐ray diffraction (XRD) study reveals the growth of highly c‐axis oriented, single phase Zn(1−x)EuxO thin films which are strained, when compared with ZnO thin film. Eu doping activates the silent Raman modes of ZnO viz. B1L and 2B1L, due to breakdown of local translation symmetry. Optical investigations show a blue shift in the band gap of Zn(1−x)EuxO thin films. Hall measurements indicate that the charge carrier concentration has increased from 1017 cm−3 (for ZnO) to 1019 cm−3 (for Zn0.97Eu0.03O) thin films.

Effect of Al or Bi on The Properties of PLD Grown Zn0.95Mn0.05OThin Films

Thin films of Zn0.95Mn0.05O (ZMO) and Al or Bi (1 weight %) incorporated ZMO thin films (viz. Al: Zn0.95Mn0.05O (AZMO) and Bi: Zn0.95Mn0.05O (BZMO)) were grown on Al2O3 substrate by pulsed laser deposition (PLD). X‐ray diffraction studies indicate that all the deposited thin films were highly c‐axis oriented and showed single phase formation with wurtzite structure. Incorporation of 1 wt. % Al leads to reduction of the extensive strain (in AZMO) when compared to the ZMO thin film. However in case of BZMO, compressive strain is observed. Raman studies indicate that the wurtzite structure of the host material is preserved albeit incorporation of 1 wt. % highly mismatched Al or Bi ions. The optical transmission spectra (in the range 350 to 700 nm) of the AZMO and BZMO thin films show significant changes compared to ZMO thin film. The otherwise spin forbidden d‐d transitions of Mn2+ in the tetrahedral symmetry give rise to a strong absorption in the visible region for the ZMO thin film. This absorption band, ...

Effect of Al or Bi on The Properties of PLD Grown Zn[sub 0.95]Mn[sub 0.05]O Thin Films

Thin films of Zn0.95Mn0.05O (ZMO) and Al or Bi (1 weight %) incorporated ZMO thin films (viz. Al: Zn0.95Mn0.05O (AZMO) and Bi: Zn0.95Mn0.05O (BZMO)) were grown on Al2O3 substrate by pulsed laser deposition (PLD). X‐ray diffraction studies indicate that all the deposited thin films were highly c‐axis oriented and showed single phase formation with wurtzite structure. Incorporation of 1 wt. % Al leads to reduction of the extensive strain (in AZMO) when compared to the ZMO thin film. However in case of BZMO, compressive strain is observed. Raman studies indicate that the wurtzite structure of the host material is preserved albeit incorporation of 1 wt. % highly mismatched Al or Bi ions. The optical transmission spectra (in the range 350 to 700 nm) of the AZMO and BZMO thin films show significant changes compared to ZMO thin film. The otherwise spin forbidden d‐d transitions of Mn2+ in the tetrahedral symmetry give rise to a strong absorption in the visible region for the ZMO thin film. This absorption band, ...

Effect of Al or Bi on The Properties of PLD Grown Zn0.95Mn0.05O Thin Films

Thin films of Zn0.95Mn0.05O (ZMO) and Al or Bi (1 weight %) incorporated ZMO thin films (viz. Al: Zn0.95Mn0.05O (AZMO) and Bi: Zn0.95Mn0.05O (BZMO)) were grown on Al2O3 substrate by pulsed laser deposition (PLD). X‐ray diffraction studies indicate that all the deposited thin films were highly c‐axis oriented and showed single phase formation with wurtzite structure. Incorporation of 1 wt. % Al leads to reduction of the extensive strain (in AZMO) when compared to the ZMO thin film. However in case of BZMO, compressive strain is observed. Raman studies indicate that the wurtzite structure of the host material is preserved albeit incorporation of 1 wt. % highly mismatched Al or Bi ions. The optical transmission spectra (in the range 350 to 700 nm) of the AZMO and BZMO thin films show significant changes compared to ZMO thin film. The otherwise spin forbidden d‐d transitions of Mn2+ in the tetrahedral symmetry give rise to a strong absorption in the visible region for the ZMO thin film. This absorption band, ...