Brajesh K. Rai

Photoluminescence study of deep levels in Cr-doped ZnSe

Single crystals of intrinsic ZnSe were grown by the seeded physical vapor transport method and the diffusion doping was utilized to incorporate Cr in these crystals. The radiative recombinations in these samples with Cr concentration in the range 1.0–10.2×1019 cm−3 were studied by the steady state photoluminescence technique. It was found that the Cr deep centers inhibit the band-to-band emission in Cr-doped ZnSe. Except in undoped single crystals, no emission corresponding to the band-to-band transition was observed from any of the doped samples. Instead, the higher wavelength emissions associated with Cr deep levels were obtained. This capture of photoexcited carriers by deep centers was verified using different excitation wavelengths. The role of chromium impurities in nonradiative recombination processes was also evidenced from the sharp decreases in the deep level emission intensity with increasing Cr concentration.

SIMULTANEOUS OBSERVATION OF STRONG AND WEAK QUANTUM CONFINEMENT EFFECT IN CHEMICALLY DEPOSITED CDSE THIN FILMS : A SPECTRO-STRUCTURAL STUDY

CdSe thin films deposited chemically on glass substrates for 4, 8, and 16 h, and subsequently annealed at 400 °C for 1 h, have been studied by a combination of spectroscopic (photoluminescence and Raman scattering) and structure-determining (x-ray diffraction and atomic force microscopy) techniques. Due to a size distribution of constituent grains, photoluminescence spectra of the as-deposited films show weak but broad bands at ∼2.2 eV (strongly confined band) and ∼1.73 eV (weakly confined band). On annealing, intensity of the weakly confined band, at ∼1.7 eV increases as a result of an improvement in the crystalline quality of CdSe nanoclusters. A surface-optic Raman mode at ∼250 cm−1 in as-deposited samples has been observed for the first time. The x-ray diffraction studies of annealed samples show a diffraction peak at 2θ=13° from the (001) plane. The improvement in crystallinity of the films as observed by atomic force microscopy and photoluminescence techniques, the appearance of (001) reflection in ...

Controlled micro oxidation of CdTe surface by laser irradiation: A micro‐spectroscopic study

A small area of CdTe single crystal within a spot of ≤2 μm diameter has been oxygenated under ambient conditions using 514.5 nm radiation from Ar+ laser. The oxygenation could be monitored using laser Raman spectroscopy, and it has been established that the asymmetric TeO2−3 ion is formed on the surface. The significance of the process for basic material science and technological applications in electronic and optoelectronic devices has been emphasized.

RAMAN AND PHOTOLUMINESCENCE STUDIES ON INTRINSIC AND CR-DOPED ZNSE SINGLE CRYSTALS

Intrinsic and Cr-doped single crystals of ZnSe, grown by the seeded physical vapor transport technique, are studied using Raman and photoluminescence spectroscopic techniques. The coupling between the longitudinal–optical (LO) phonon and the hole plasmons, in Cr-doped ZnSe, results in a downward shift of the LO peak with increasing dopant concentration. The ratio of integrated intensities of LO and transverse–optical (TO) modes in ZnSe:Cr shows a systematic increase, with increasing temperature and decreasing excitation wavelength. This occurs due to the interaction of the LO phonon field with the surface electric field in the depletion layer as an enhancement of LO mode intensity. The localized Raman modes of CrZn–Se are seen in Cr-doped samples, which corroborate the photoluminescence findings of the presence of Cr2+ and Cr1+ deep levels. The LO and TO modes of intrinsic ZnSe show upward renormalization in their peak positions with decreasing excitation wavelength. The interaction of discrete phonons wi...

Study of visible emission and phase transition in nanocrystalline A1−xA′xTiO3 systems

Abstract Room temperature photoluminescence spectra on Ba 1− x Pb x TiO 3 and Sr 1− x Pb x TiO 3 , with grain sizes of ∼ 60 and ∼ 30 nm, respectively, have been measured. The Pb content dependence of the visible emission can be used to provide evidence for the origin of photoluminescence occuring in these nanocrystalline ABO 3 oxides. The phase transitions for Ba 1− x Pb x TiO 3 and Sr 1− x Pb x TiO 3 are found to have an obvious effect on the visible emission band.

Raman investigation on (Ta2O5)1−x(TiO2)x system at different temperatures and pressures

Abstract Micro Raman spectra for (Ta2O5)1−x(TiO2)x reveal an induced Raman enhancement of the low frequency phonon modes for a critical TiO2 content, x = 0.08. The temperature and pressure effects on these modes have also been studied. The Raman enhancement is shown to be associated with effects of the superstructure which is composed of TiO6 octahedra and Ta-O clusters. Pressure Raman scattering on (Ta2O5)0.92(TiO2)0.08 indicates that a phase transition occurs at ~6.22 Gpa. Based on our Raman scattering studies, the origin responsible for the enhanced dielectric constant in samples with TiO2, content, x = 0.08, has been provided and discussed.

Photoluminescence in PbMg1/3Nb2/3O3–PbIn1/2Nb1/2O3 systems

Photoluminescence for PbMg{sub 1/3}Nb{sub 2/3}O{sub 3} (PMN){endash}PbIn{sub 1/2}Nb{sub 1/2}O{sub 3} (PIN) solid solutions in the temperature ranging from 35 to 295 K have been obtained for the first time. An abrupt photoluminescence enhancement of the system has been observed to occur at {approximately}210 K, which can be attributed to the growing and merging of dynamic polar microregions or to the phase transformation of the material at this temperature. The PIN content has been found to affect the photoluminescence of the PMN-PIN system significantly. The photoluminescence mechanism for PMN-PIN has been studied. {copyright} {ital 1998 Materials Research Society.}

Visible emission study in nanocrystalline Ba1-xPbxTiO3 and Sr1-xPbxTiO3 systems

Abstract Photoluminescence spectroscopy measurements., on Ba1−xPbxTiO3 and Sr1−xPbxTiO3, and with grain sizes of 60 and 30 nm respectively, have been carried out. A single-emission band at ~ 2.0 eV for pure BaTiO3 and at ~ 2.14 eV in the case of pure SrTiO3, had been observed at room temperature. The dependences of the band-position, -intensity, and -width of the visible emission on Pb concentration using the excitation line of 514.5, has been studied and offered a model responsible for the origin of photoluminescence in nanocrystalline BaTiO3 and SrTiO3. The thermal quenching of photoluminescence for pure bulk materials has been discussed.