Antaryami Mohanta

Shallow acceptor complexes in p-type ZnO

We show that N-doped ZnO films grown on sapphire can exhibit significant (∼1018 cm−3) room-temperature p-type behavior when sufficient nitrogen (N) is incorporated and the material is annealed appropriately. Substitutional N on the oxygen (O) sublattice is a deep acceptor; however, shallow acceptor complexes involve N, H, and zinc vacancies (VZn). Combining secondary ion mass spectrometry, Raman-scattering, photoluminescence, and Hall-effect data, we establish the evolution of N from its initial incorporation on a Zn site to a final shallow acceptor complex VZn_NO_H+ with an ionization energy of ca. 130 meV. This complex is responsible for the observed p-type behavior.

Photoluminescence study of ZnCdO alloy

The photoluminescence (PL) properties of ZnCdO alloy are investigated at various temperatures. Each PL profile contains four distinct peaks: P1, P2, P3, and P4. Peak P4 is due to free-excitonic recombination. The temperature mapping of the P3 peak position shows an S-shaped shift similar to the InGaN system due to the inhomogeneous distribution of Cd in ZnCdO alloy. The peak P2 is attributed to the phonon replica of the localized excitonic emission (P3). The linewidth of the photoluminescence profiles of the ZnCdO alloy is shown to depend on the concentration of Cd.

Photoluminescence study of ZnO nanowires grown by thermal evaporation on pulsed laser deposited ZnO buffer layer

The ZnO nanowires are grown by thermal evaporation on pulsed laser deposited ZnO buffer layer on silicon. Photoluminescence of ZnO nanowires is studied at various temperatures in the range from 6 K to room temperature and varying excitation intensities. At 6 K, the photoluminescence profile of ZnO nanowires shows bound exciton, free excitons (FXAn=1,FXAn=2), longitudinal optical phonon replicas (FXAn=1-mLO, m=1, 2, 3) of free exciton (FXAn=1), and donor acceptor pairs. The first longitudinal optical phonon replica (FXAn=1-1LO) dominates at room temperature. Photoluminescence peak shifts by 47 meV from free exciton energy with the increase in temperature to room temperature.

Temperature-dependent S-shaped photoluminescence in ZnCdO alloy

Photoluminescence (PL) of ZnCdO alloy with medium Cd concentration (50 wt %) is studied at different temperatures and at varying excitation intensity. The PL peak position shows red-blue-red (S-shaped) shift with increase in temperature due to potential fluctuations on alloying. The line width of PL profile shows increasing-decreasing-increasing behavior with increase in temperature. The dependence of S-shape behavior on Cd concentration is analyzed.

Rayleigh scattering from gaseous phase nanoparticles synthesized by pulsed laser ablation of ZnO

Rayleigh scattering and photoluminescence techniques are used to show the evidence of formation of ZnO nanoparticles in gaseous phase due to condensation of laser ablated ZnO plasma species in air. The intensity of Rayleigh scattered signal increases with the increase in time delays between the probe and the ablating pulse. Plasma emission dominates close to the target surface whereas Rayleigh scattered signal dominates at larger distances where nanoparticles are abundant. Photoluminescence spectra are observed in laser ablated ZnO plasma using fourth harmonic of a Nd:YAG (yttrium aluminum garnet) laser as an excitation source, which confirms the formation of ZnO nanoparticles in gaseous phase. The photoluminescence profiles observed from gaseous phase ZnO nanoparticles show blueshift with respect to that observed from bulk ZnO under similar excitation condition implying quantum confinement.

Photoluminescence from ZnO nanoparticles in vapor phase

Photoluminescence (PL) is observed from ZnO nanoclusters formed in ZnO plasma due to cooling of plasma species by passing a collimated beam of fourth harmonic (266 nm) of Nd:YAG (yttrium aluminum garnet) laser through ZnO plasma formed by third harmonic (355 nm) of Nd:YAG laser. A shift of 42 meV in peak position of PL profile of ZnO clusters from that observed in bulk ZnO is seen, which shows the effect of quantum confinement. The behaviors of PL profiles are studied at varying ablating intensities with fixed probe intensity. A superlinear increase in PL peak intensity with narrowing of emission linewidth above a particular ablating intensity is observed. The defect related emission band, usually occurring due to oxygen vacancy, is not observed from the ZnO clusters formed due to cooling of laser induced ZnO plasma. The observed redshift of PL peak positions with increasing ablating intensities could be due to temperature-induced band gap shrinkage arising due to enhancement of electron temperature.

Carrier recombination dynamics in Si doped InN thin films

Time-integrated and time-resolved photoluminescence (PL) of InN thin films of different background carrier concentrations are investigated. The PL formation mechanism is attributed to the “free-to-bound” transition by analyzing the time-integrated PL spectra at different pump fluences. The dependence of the PL decay time with emission energy is investigated using a theoretical model which speculates upon the carrier localization in InN thin films. The radiative lifetime, mobility edge, and carrier localization energy are obtained from the dependence of the PL decay time on emission energy and are studied at different background carrier concentrations. The effect of intervalley scattering between the Γ1 and Γ3 valley on the radiative lifetime, mobility edge, and carrier localization energy is discussed. The longer radiative lifetime and smaller values of the mobility edge and localization energy for 3.06 eV excitation are observed than that for the 1.53 eV excitation due to the intervalley scattering process.

Observation of weak carrier localization in green emitting InGaN/GaN multi-quantum well structure

Green emitting InGaN/GaN multi-quantum well samples were investigated using transmission electron microscopy, photoluminescence (PL), and time-resolved photoluminescence (TRPL) spectroscopy. Weak carrier localization with characteristic energy of ∼12 meV due to an inhomogeneous distribution of In in the InGaN quantum (QW) layer is observed. The temperature dependence of the PL peak energy exhibits S-shape phenomenon and is comparatively discussed within the framework of the Varshnis empirical formula. The full width at half maximum of the PL emission band shows an increasing-decreasing-increasing behavior with increasing temperature arising from the localized states caused by potential fluctuations. The radiative life time, τr, extracted from the TRPL profile shows ∼T3/2 dependence on temperature above 200 K, which confirms the absence of the effect of carrier localization at room temperature.

Time-integrated photoluminescence and pump-probe reflection spectroscopy of Si doped InN thin films

Temperature and excitation power dependent time-integrated photoluminescence of Si doped InN thin films are investigated. Photoluminescence (PL) spectra at low temperatures are described by single emission peak ensued due to “free-to-bound” recombination; whereas PL spectra at higher temperatures above 150 K are characterized by both “band-to-band” and “free-to-bound” transition. Carrier dynamics of Si doped InN thin films is studied using pump-probe reflection spectroscopy at room temperature. The hot electron cooling process is well described by electron-electron scattering. The dependence of the hot electron cooling rate on total electron density shows sublinear to linear behavior with increase of background electron density. The variation of the carrier recombination lifetime with total electron density implicates the dominance of the defect-related nonradiative recombination channel over other recombination processes.

Exciton-exciton scattering in vapor phase ZnO nanoparticles

Photoluminescence (PL) properties of suspended ZnO nanoparticles formed in vapor phase due to the condensation of the Nd:YAG laser ablated ZnO plasma species are investigated by varying both ablating and excitation intensity at different time delays with respect to the ablating pulse and at different axial distances from the target surface. Emission due to inelastic exciton-exciton (X-X) scattering is observed and is found to be dependent on the size of the vapor phase ZnO nanoparticles. The PL intensity shows nonlinear behavior with increasing ablating intensity, indicating generation and participation of more excitons in X-X scattering process in lager size ZnO nanoparticles.