Soumyendu Guha

VISIBLE LIGHT EMISSION FROM SI NANOCRYSTALS GROWN BY ION IMPLANTATION AND SUBSEQUENT ANNEALING

Photoluminescence (PL), electron spin resonance (ESR), and high resolution transmission electron microscopy (HRTEM) were used to investigate the luminescence mechanism in Si nanocrystals. Si ions were implanted in SiO2 films at 190 keV to a dose of 3×1017/cm2.An intense photoluminescence (PL) band at 755 nm (1.65 eV) was observed when the implanted films were annealed above 800 °C in air or in nitrogen. HRTEM images showed Si nanocrystals of sizes between 1 and 6 nm from these annealed samples. ESR indicated Si dangling bonds. Upon annealing at 900 °C in air a few times, the particle sizes were reduced to less than 2 nm due to oxidation. The red PL band is attributed to emission from Si nanocrystals.

Characterization of Si nanocrystals grown by annealing SiO2 films with uniform concentrations of implanted Si

We have performed physical and optical characterization of Si nanocrystals grown by ion implantation of Si+ ions at multiple energies with varying doses into thermally grown SiO2 films. The purpose of multiple implants was to achieve uniform composition of the added Si profile throughout the SiO2 film to produce Si particles with a narrow size distribution upon annealing at 1000 °C in a nitrogen atmosphere. The depth distribution of the composition and sizes of the Si particles in SiO2 films before and after the anneal were determined using Rutherford backscattering (RBS), forward recoil spectroscopy, small-angle x-ray diffraction (SXRD), and high-resolution transmission electron microscopy (HRTEM). From RBS we concluded that the amount of free silicon was reduced by annealing, presumably due to oxidation in the annealing process. The mean cluster sizes of the annealed samples were determined by SXRD. HRTEM was also employed to determine the average size of Si particles. Photoluminescence spectra (PL) fro...

Characterization of Si+ ion-implanted SiO2 films and silica glasses

We report here electron spin resonance (ESR), Raman scattering, photoluminescence (PL), and absorption studies of Si+ ions implanted into silica glasses and thermally grown SiO2 films on Si wafers. The aim is to understand the defect formation and luminescence mechanism as the films were annealed at several temperatures. In as-grown Si-implanted films, paramagnetic defects in the form of Si–E′ and nonbridging oxygen hole centers (NBOHC) were detected by ESR. A luminescence band, centered at 650 nm, was observed in these samples. The E′ and NBOHC defects were annealed out in samples annealed above 500 °C, but Si dangling bonds in the form of Pb centers were observed. In films annealed above 800 °C, Si nanocrystals of varying size between 1 and 5 nm were formed, as detected by transmission electron microscopy. A red PL band (>700 nm) and a Si-phonon band at 510 cm−1, gradually evolved as a function of anneal temperature. The observation of the Si-phonon mode with a 15 cm−1 shift in the peak position from th...

Resonant Raman scattering and photoluminescence studies of porous silicon membranes

Raman scattering measurements as a function of excitation wavelength are conducted on a 55‐μm‐thick porous silicon (PS) membrane of an approximate porosity of 70%. A narrowing of the Si–Si phonon peak with increasing excitation wavelength is observed. We attribute this to a resonance of the excitation energy with the electronic states of nanocrystallites in PS. We estimate the size of silicon particles from the shift and broadening of the phonon band. The absorption band edge is next blue shifted by annealing the sample at 300 and 500 °C. A collapse of the nanostructure with a decrease in particle size is observed. The photoluminescence (PL) spectra exhibit a red shift of the PL band upon annealing. We also estimate the size of silicon particles from the PL peak.

Growth and characterization of Ge nanocrystals

Abstract We have synthesized Ge nanocrystals of mean sizes 4, 8, and 12 nm by ion-implanting Ge+ ions into thermally grown SiO2 films and subsequent annealing of the films at 830°C for 30 min in nitrogen. These films were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and Raman spectroscopy. A distribution of particle size was identified by TEM in a 100 nm band below the surface. Particle sizes were estimated by these three techniques.

Photoluminescence and Raman studies of porous silicon in polymethyl methacrylate

We report the observation of strong luminescence from porous Si grains containing Si nanocrystallites embedded in polymethyl methacrylate disks. In concentrated samples, forward and backscattered photoluminescence spectra were just as strong as in porous Si on Si wafers. This new result indicates that porous Si grains could be embedded in nonconducting polymers and would retain all its inherent characteristics as in porous Si layers on Si wafers.

Observation of excitonic states in PbSe nanocrystals

We report the observation of excitonic states in PbSe nanocrystals with a size distribution varying between 2 and 6 nm. Size selective photoluminescence (PL) and PL excitation spectroscopy (PLE) are used to probe the discrete excitonic states associated with PbSe nanocrystals. By using various excitation wavelengths, we observed two broad PL bands between 300 and 900 nm. These PL bands were identified as the first- and second-exciton recombination in PbSe nanocrystals. A reasonable agreement is obtained between the observed strong bands and calculated oscillator strengths for exciton recombination in PbSe nanocrystals with an average particle size of 4 (±2) nm.

Raman and infrared studies of cupric oxide

Polarized Raman and fourier-transform infrared (FTIR) measurements have been made on a single crystal of CuO. Group theory predicts nine vibrations of which three (Ag, 2Bg) are Raman-active and six (3Au, 3Bu) are infrared-active. We have observed three Raman modes at 296 (Ag), 346 (Bg1) and 636 (Bg2) cm−1. We have also observed six infrared modes at 146 (Bu3), 164 (Au2), 355 (Au3), 480 (Bu1), 542 (?) and 603 (Bu2) cm−1. The normal frequencies and eigenvectors have been calculated using Wilson’s FG method; a good fit between theory and experiment has been obtained.

Optical Characterization of Free-Standing Porous Silicon Films

We report here the optical characterization of two free-standing porous silicon (PS) films of porosity 70% and 85%, grown on a p- and a n-type Si. We determine the optical band gap in these films from the excitation wavelength dependence of the photoluminescence band. As the excitation wavelength is changed from red (800 nm) to UV (355 nm), a blue shift of the photoluminescence (PL) band is observed. We attribute the observed blue shift of the PL band to the emission due to the distribution of bandgap in PS. Both samples are oxidized in air and we believe that the observed bandgap in these films arises from the inhomogeneous distribution of Si particle sizes. However, we find that intrinsic defects play a dominant role in the process of luminescence. Electron spin resonance measurement indicates the presence of defects leading to the saturation of the optical absorption spectra and a decrease in intensity of the PL band. The lineshape of the PL band is modeled using a weighted asymmetric Gaussian that selects a gap distribution function at each excitation wavelength. From Raman measurements in these two films, the quantum confinement effect in Si nanostructures is clearly observed in both films.

Excitation wavelength dependence of Raman and photoluminescence spectra of porous Si membranes

A frequency shift and a band narrowing of the Raman and photoluminescence (PL) spectra from porous Si free-standing films are observed with increasing excitation wavelength. We attribute this to a distribution of particle size in Si nanostructures. From the PL and Raman band shapes we determine the energy gap distribution and particle size in Si nanostructures.