Harish Kumar Yadav

Influence of postdeposition annealing on the structural and optical properties of cosputtered Mn doped ZnO thin films

The influence of postgrowth annealing on the structural and optical properties of rf cosputtered Mn doped ZnO thin films deposited on glass substrate at room temperature has been investigated. All as deposited Zn1−xMnxO films are highly textured, with the c axis of the wurtzite structure along the growth direction. The as grown films are in a state of compressive stress and a reduction in stress with postgrowth annealing treatment are observed. The band gap of Mn doped ZnO films (3.34eV) is slightly larger than the pure ZnO film (3.30eV) and is found to decrease with an increase in annealing temperature for all the samples. The optical dispersion of refractive index with photon energy in Zn1−xMnxO films with varying x and different annealing temperature is studied in the light of single oscillator and Pikhtin-Yas’kov [Sov. Phys. Semicond. 15, 81 (1981)] model, respectively.

Defect induced activation of Raman silent modes in rf co-sputtered Mn doped ZnO thin films

We study the influence of Mn doping on the vibrational properties of rf sputtered ZnO thin films. Raman spectra of the Mn doped ZnO samples reveal two additional vibrational modes, in addition to the host phonon modes, at 252 and 524?cm?1. The intensity of the additional modes increases continuously with Mn concentration in ZnO and can be used as an indication of Mn incorporation in ZnO. The modes are assigned to the activation of ZnO silent modes due to relaxation of Raman selection rules produced by the breakdown of the translational symmetry of the crystal lattice with the incorporation of Mn at the Zn site. Furthermore, the A1 (LO) mode is observed with very high intensity in the Raman spectra of undoped ZnO thin film and is attributed to the built-in electric field at the grain boundaries.

Enhanced response from metal/ZnO bilayer ultraviolet photodetector

Enhanced ultraviolet (UV) photoresponse is observed in rf magnetron sputtered ZnO thin films interfaced with ultrathin overlayers of different metals (Al, Cu, Sn, Pb, Au, and Te). Electrons transferred from a metal layer to the semiconductor at the interface compensate the surface states and increase the dark conductivity. Depending on the number of injected electrons, surface states are compensated to different extents and correspondingly increase the photoconductivity. The photoresponse of the Sn∕ZnO bilayer sample exhibits the highest responsivity (∼6kV∕W) at low UV intensity of 140μW∕cm2 (λ=365nm) with a relatively fast response speed of 65ms.

Study of metal/ZnO based thin film ultraviolet photodetectors: The effect of induced charges on the dynamics of photoconductivity relaxation

Ultraviolet photoconductivity relaxation in ZnO thin films deposited by rf magnetron sputtering are investigated. Effect of oxygen partial pressure in the reactive gas mixture and film thickness on the photoconductivity transients is studied. A different photodetector configuration comprising ZnO thin film with an ultrathin overlayer of metals like Cu, Al, Sn, Au, Cr, and Te was designed and tested. Photoresponse signal were found to be stronger (four to seven times) in these configurations than the pure ZnO thin films. Sn(30 nm)/ZnO sample exhibits highest responsivity of ∼8.57 kV/W whereas Te(20 nm)/ZnO structure presents highest sensitivity of ∼31.3×103 compared to unloaded ZnO thin film. Enhancement in the photoresponse of ZnO thin films is attributed to the change in surface conductivity due to induced charge carriers at the interface because of the difference in work function and oxygen affinity values of metal overlayer with the underlying semiconducting layer. Charge carrier transfer from the meta...

Persistent photoconductivity due to trapping of induced charges in Sn/ZnO thin film based UV photodetector

Photoconductivity relaxation in rf magnetron sputtered ZnO thin films integrated with ultrathin tin metal overlayer is investigated. Charge carriers induced at the ZnO-metal interface by the tin metal overlayer compensates the surface lying trap centers and leads to the enhanced photoresponse. On termination of ultraviolet radiation, recombination of the photoexcited electrons with the valence band holes leaves the excess carriers deeply trapped at the recombination center and holds the dark conductivity level at a higher value. Equilibrium between the recombination centers and valence band, due to trapped charges, eventually stimulates the persistent photoconductivity in the Sn/ZnO photodetectors.

A comparative study of ultraviolet photoconductivity relaxation in zinc oxide (ZnO) thin films deposited by different techniques

Photoresponse characteristics of ZnO thin films deposited by three different techniques namely rf diode sputtering, rf magnetron sputtering, and electrophoretic deposition has been investigated in the metal-semiconductor-metal (MSM) configuration. A significant variation in the crystallinity, surface morphology, and photoresponse characteristics of ZnO thin film with change in growth kinetics suggest that the presence of defect centers and their density govern the photodetector relaxation properties. A relatively low density of traps compared to the true quantum yield is found very crucial for the realization of practical ZnO thin film based ultraviolet (UV) photodetector.

Low-frequency zone boundary phonons in Li doped ZnO ceramics

Room temperature Raman spectra of Li doped ZnO (Zn1−xLixO) ceramics with varying Li concentrations (x=0.0, 0.05, 0.10, and 0.15) are investigated in this study. Four peaks were identified at 96.6, 127, 157, and 194 cm−1 in the Li doped samples. The peaks at 127, 157, and 194 cm−1 are assigned to zone boundary phonons in ZnO [J. M. Calleja and M. Cardona, Phys. Rev. B 16, 3753 (1977)], and appear due to disorder in ZnO lattice with Li incorporation. Lithium, owing to its smaller radius, adjusts itself anywhere in the ZnO lattice and breaks the crystal translational symmetry to a large extent, compared to other dopants. Disorder in the lattice is seen to be finely modulated with varying Li content. The peak at 96.6 cm−1 is hypothesized to be a projection of the vibrational motion of Li atoms at lower frequencies, which contributes in a major fashion at higher frequencies, due to its lighter mass than Zn or O atoms.

Raman spectroscopy and dielectric Studies of multiple phase transitions in ZnO:Ni

We present Raman and dielectric data on Ni-doped ZnO (Zn1−xNixO) ceramics as a function of Ni concentration (x=0.03, 0.06, and 0.10) and temperature. A mode (around 130cm−1) is identified as TA(M) [J. M. Calleja and M. Cardona, Phys. Rev. B 16, 3753 (1977)] and appears due to an antiferromagnetic phase transition at low temperatures (100K) via the spin-orbit mechanism [P. Moch and C. Dugautier, Phys. Lett. A 43, 169 (1973)]. A strong dielectric anomaly occurs at around 430–460K, depending on Ni concentration, and is due to extrinsic electret effects (Ni ionic conduction) and not to a ferroelectric phase transition.

Plasmonic assisted enhanced photoresponse of metal nanoparticle loaded ZnO thin film ultraviolet photodetectors

An ultraviolet (UV) photodetector exhibiting enhanced response characteristics has been realized successfully after integrating various metal nanoparticles (NPs) such as silver (Ag), gold (Au) and platinum (Pt) with sol–gel derived ZnO thin film (NPs–ZnO). The metal NP based photodetector (Ag, Au, Pt-NPs–ZnO) exhibits a relatively high photoresponse in comparison to the bare ZnO based UV photodetector and gives a maximum value of about 4.27 × 103. The combined effect of the lowering of dark current due to the formation of a Schottky barrier at the interface of the metal NPs with the ZnO thin film and the photocurrent upon UV illumination due to the plasmonic effect of loaded NPs results in an enhanced photoresponse of the prepared metal NP–ZnO photodetector. The trapping of incident UV radiation mainly through the enhanced optical absorption by loaded metal NPs due to the plasmonic effect and subsequent coupling of harvesting photons into underlying optical modes of the surface of photoconducting ZnO thin films lead to a significant increase in photocurrent. The observed results provide an indication that the plasmonic assisted UV response of the novel metal NP–ZnO photodetector might provide a breakthrough for the development of next generation photodetectors.

Temperature dependent dynamics of ZnO nanoparticles probed by Raman scattering: A big divergence in the functional areas of nanoparticles and bulk materials

Temperature dependent Raman scattering of the E2(low) and E2(high) modes of ZnO nanoparticle powder samples has been investigated. The dynamics exhibited by the ZnO nanoparticles, the E2 modes, follow single crystal like behavior but with strong cubic or/and quadratic anharmonicity by supporting additional channels for immediate lattice relaxation. The strong anharmonic character associated with the E2 modes reflects enhanced thermal conductivity of the ZnO nanoparticles. An anomalous variation in linewidth of the E2(low) mode is observed and is attributed to the superposition of the large displacement field, associated with the surface atoms of the nanoparticles and the lattice mode.