Duan Feng

Phonon mode study of Si nanocrystals using micro‐Raman spectroscopy

First‐order Raman spectra of hydrogenated nanocrystalline silicon (nc:Si:H) films show unexpected features in their optical vibrational modes for crystallites with sizes ranging from 2 to 6 nm. Two size‐dependent spectral regions, one with the stronger intensity peaking at 505–509 cm−1 and another a shoulder‐like band between 512 and 517 cm−1, are clearly identified using a detailed line‐shape analysis and the strong phonon confinement model. The strong size dependence of the relative integrated intensities of the two bands suggests that the modification of the vibrational spectra can be attributed to an effect induced by the atomic vibrations from the near‐surface region of the nanocrystals.

Visible photoluminescence in crystallized amorphous Si:H/SiNx:H multiquantum‐well structures

Visible photoluminescence has been observed in crystallized a‐Si:H/a‐SiNx:H multiquantum‐well structures at room temperature. The MQW heterostructures consisting of 72 layers were formed by computer controlled plasma‐enhanced chemical‐vapor deposition method and then crystallized by Ar+ laser annealing technique. The crystallinity and average grain size of the silicon microcrystals were determined by means of Raman and x‐ray diffraction spectroscopy. The crystallized samples with well‐layer thickness Ls=40 A showed an intense photoluminescence which is peaked at 2.1 eV with a full width at half‐maximum of 0.25 eV. This is consistent with calculations based on the quantum confinement model.

Raman scattering of alternating nanocrystalline silicon/amorphous silicon multilayers

Nanocrystallite size distribution and structural properties in alternating hydrogenated nanocrystalline silicon/amorphous silicon multilayers were investigated by means of Raman scattering. The obtained Raman spectra show a broad peak at ∼480 cm−1 from amorphous Si and some small peaks superposed on the broad peak. According to the positions of the crystallite peak, the mean crystallite size and volume fraction of the crystalline were calculated. Since these small peaks have strong size dependence of their relative intensities, an effect induced by the atomic vibrations from the near‐surface region of nanocrystals is considered to be responsible for the modification of the vibrational properties and the stable photoluminescence from our samples.

Influence of growth striations on para‐ferroelectric phase transitions: Mechanism of the formation of periodic laminar domains in LiNbO3 and LiTaO3

The concentration distributions of Y dopings in LiTaO3 across the growth striations in as‐grown crystals are obtained by means of x‐ray energy dispersive spectrum analysis in a scanning transmission electron microscope at room temperature. The results show that it is the concentration gradient of the dopings which determines the configurations of ferroelectric domains. The mechanism for the formation of the periodic laminar ferroelectric domains (PLFDs) is explained theoretically by considering the influence of the distributions of the dopings on the para‐ferroelectric phase transitions in LiTaO3 and LiNbO3 crystals. It is shown that the coupling between the order parameter and the conjugate external field, which results from the ambipolar diffusion or from the strain‐induced polarization mechanism associated with the dopings and their compensated point defects, is just the physical reason for the formation of such PLFDs in these two materials.

OPTICAL PROPERTIES OF PULSED LASER DEPOSITED ZnO THIN FILMS

Abstract With the help of the pulsed laser deposition technique, completely (001) textured ZnO thin films were achieved on (001) α-SiO2 single crystalline substrates at moderate substrate temperatures of 225–500°C. The films exhibited oscillated optical transmittance, from which the refractive indices vs the wavelength were calculated and fitted in Cauchys equation. The index dispersion showed the same tendency as that of the bulk ZnO crystal. By the prism-film coupling method a wave of 0.633 μm was well guided in the films. The m-lines for both TE and TM multimodes were sharp and clear. Light propagation lines were observed within the films and the propagation loss was determined to 2.8 dB/cm. Waveguide equations suiting the anisotropic waveguide structures were derived to determine the ordinary and extraordinary refractive indices. Values of no = 1.96 and ne = 1.98 were much closer to that of the bulk crystal. The as-deposited films were of high quality in crystallinity and optical properties.

Branchy alumina nanotubes

Branchy alumina nanotubes (bANTs) have been shown to exist in aluminum oxide. Electron-beam evaporated 400 nm Al film on Si substrate is stepwise anodized in dilute sulfuric acid under the constant dc voltage 40 V at 10.0 °C. This electrochemical-anodizing route resulted in the formation of individual bANTs. Transmission electron microscopy showed that the length of the bANTs was around 450 nm, and the inner diameter was around 10–20 nm. We deduced that the bANTs, the completely detached multibranchy cells of anodic porous alumina (APA) film, should be evolved from the stagnant cells of the APA mother film. The bANTs may be used as templates in fabrication of individual branchy nanoscale cables, jacks, and heterojunctions. The proposed formation mechanisms of the bANTs and the stagnant cells should give some insights into the long-standing problem of APA film, i.e., the self-ordering mechanism of the cells arrangement in porous anodization of aluminum.

Low electric field induced (001) oriented growth of LiNbO3 films by pulsed laser ablation

Abstract It has been demonstrated that a low electric field of a very few V/cm can induce textured growth of LiNbO 3 thin films fabricated by the pulsed laser ablation technique. By applying an electric field of 7 V/cm perpendicular to the substrate, completely (001) oriented LiNbO 3 films have been grown on fused silica by this method. The films are stoichiometric and very smooth. Optical transmittance measurements showed that the films are of high transparency and have a refractive index close to that of bulk crystals. The effects of electric field in the growth of ferroelectric films were analysed in terms of Volmer-Weber nucleation theory taking into consideration the electrostatic energy. This method should open a new way to produce LiNbO 3 multilayers composed of periodic domain layers with alternatively reversed poling direction.

The role of an electric field applied during pulsed laser deposition of LiNbO3 and LiTaO3 on the film orientation

A low electric field applied during film deposition has a significant influence on the orientation of ferroelectric thin films prepared by pulsed laser deposition. C‐axis oriented growth of LiNbO3 films was demonstrated on fused silica with the aid of a low bias voltage Vb=0–120 V, and complete c‐axis orientation was achieved at Vb=110 V. In contrast, the electric field cannot induce c‐axis orientation of LiTaO3 films. Theoretical analysis suggests that the electrostatic energy provides an extra driving force for the c‐axis oriented growth of the ferroelectric films if the permittivity components satisfy e33/e11≤2, as in LiNbO3 film, but not if e33/e11>2 at the deposition temperature, as in LiTaO3 film.

Comparative study of laser ablation techniques for fabricating nanocrystalline SnO2 thin films for sensors

Abstract The microstructure and sensing properties of SnO 2 thin films prepared by using SnO 2 and Sn targets at different substrate temperatures with and without post annealing were investigated systematically. It is found that nanocrystalline SnO 2 films with grain size of 4.0–5.2 nm can be achieved by two methods: (1) by crystallization of amorphous SnO 2 films at 400 °C; and (2) by oxidation of Sn films at 400 °C. The nanocrystalline SnO 2 films prepared with these methods exhibit much higher C 2 H 5 OH gas sensitivity in comparison with SnO 2 films prepared using SnO 2 and Sn targets at higher substrate temperatures with grain sizes of several tens of nm upon exposure to air containing 2000 ppm C 2 H 5 OH.

Role of hydrogen surface coverage during anodic plasma deposition of hydrogenated nanocrystalline germanium

Nanocrystalline Ge:H films were deposited on the anode of a conventional plasma-enhanced chemical vapor deposition system using highly H2-diluted GeH4 as source gas. The structural changes with substrate temperature, hydrogen dilution, and rf power were investigated by x-ray diffraction and Raman scattering. The hydrogen content and H bonding configuration were determined by infrared absorption spectroscopy. It was found that increased rf power mainly favors nucleation of grains while increased H2 dilution mainly favors the subsequent growth of grains. Transitions from an amorphous to a crystalline and then back to an amorphous phase are observed as the substrate temperature is increased. The amorphous phase at high temperatures and variations in the preferred crystallite orientations are explained by changes in the diffusion length of adsorbed precursors due to changing hydrogen coverage on the growing surface.