D.O. Henderson

ENCAPSULATED SEMICONDUCTOR NANOCRYSTALS FORMED IN INSULATORS BY ION BEAM SYNTHESIS

Abstract Both elemental and compound semiconductor nanocrystals have been formed in insulators by ion beam synthesis. Si nanocrystals in SiO2 give rise to strong optical absorption and intense photoluminescence (PL). The dose dependence of optical absorption provides evidence for size dependent changes in the Si nanocrystal bandgap due to quantum confinement, but the PL results suggest that surface or defect states play an important role in PL. CdS and CdSe nanocrystals have been formed in SiO2 and in Al2O3. Their structure, size, and optical properties are discussed.

Optical properties and electrical characterization of p-type ZnO thin films prepared by thermally oxiding Zn3N2 thin films

In this paper, we report a simple method for preparing p-type ZnO thin films by thermal oxidization of Zn 3 N 2 thin films. The Zn 3 N 2 films were grown on fused silica substrates by using plasma-enhanced chemical vapor deposition from a Zn(C 2 H 5 ) 2 and NH 3 gas mixture. The Zn 3 N 2 film with a cubic antibixbyite structure transformed to ZnO:N with a hexagonal structure as the annealing temperature reached 500 °C. When the annealing temperature reached 700 °C, a high-quality p-type ZnO film with a carrier density of 4.16 x 10 1 7 cm - 3 was obtained, for which the film showed a strong near-band-edge emission at 3.30 eV without deep-level emission, and the full width at half-maximum of the photoluminescence spectrum was 120 meV at room temperature. The origin of the ultraviolet band was the overlap of free exciton and the bound exciton. The N concentration was as high as 10 2 1 cm - 3 , which could be controlled by adjusting the parameters of the annealing processes.

Vibrational spectra of bismuth silicate glasses and hydrogen-induced reduction effects

Abstract Raman (10–1200 cm −1 ) and infrared reflectance (100–2000 cm −1 ) spectra are reported for bismuth silicate glasses of the formula x Bi 2 O 3 · (10− x )SiO 2 with x = 9, 8, 7, 6, 5, 4. The basic vibrational characteristics and the possible glass structure are discussed. The results provide evidence that bismuth oxide behaves as the network former while silicon oxide is present as isolated SiO 4 tetrahedra. Hydrogen treatment was performed at 300°C on a bismuth silicate glass with x = 5. The intensity of most Raman bands decreased with increasing exposure time. The infrared spectrum after reduction showed vibrational bands at 1124 and 1220 cm −1 which are characteristic of SiOSi stretching vibrational modes in a fused-silica network. The spectral changes observed indicate the formation of bismuth metallic colloids as well as the formation of the bridged SiOSi structure in the glass surface layer during reduction.

GaAs nanocrystals formed by sequential ion implantation

Sequential ion implantation of As and Ga into SiO2 and α‐Al2O3 followed by thermal annealing has been used to form zinc‐blende GaAs nanocrystals in these two matrices. In SiO2, the nanocrystals are nearly spherical and randomly oriented, with diameters less than 15 nm. In Al2O3, the nanocrystals are three dimensionally aligned with respect to the crystal lattice. Infrared reflectance measurements show evidence for surface phonon modes in the GaAs nanocrystals in these matrices.

Microstructure of sulfide nanocrystals formed by ion-implantation

Abstract Ion implantation is a versatile tool for the formation of compound semiconductor nanocrystal precipitates in a host medium with the ultimate goal of producing a uniform size distribution of quantum dots for use in optical devices. Ion implantation was used to form ZnS, CdS, and PbS nanocrystals in both glass and crystalline hosts, and the precipitate microstructures and size distributions were investigated by TEM techniques. The size distributions were found to be significantly different for the three compounds, and several unusual microstructures were observed.

Production, structure, and optical properties of ZnO nanocrystals embedded in CaF2 matrix

High-quality ZnO nanocrystals have been fabricated by zinc ion implantation (160 keV, 1×1017 ions/cm2) into a CaF2(111) single-crystal substrate followed by thermal annealing from 300 to 700 °C. X-ray diffraction results show that ZnO nanocrystals in CaF2(111) substrate have a (002) preferred orientation. The average grain size is ranging from 14 to 19 nm corresponding to the annealing temperatures from 500 to 700 °C. A very strong ultraviolet near-band edge emission is observed from 372 to 379 nm. The emission intensity is enhanced and linewidth is narrowed as the annealing temperature increases. The commonly observed visible green emission associated with deep-level defects in ZnO is suppressed.

A transmission electron microscopy investigation of sulfide nanocrystals formed by ion implantation

Ion implantation was used to form compound semiconductor nanocrystal precipitates of ZnS, CdS, and PbS in both glass and crystalline matrices. The precipitate microstructures and size distributions were investigated by cross-sectional transmission electron microscopy techniques. Several unusual features were observed, including strongly depth-dependent size variations of the ZnS precipitates and central void features in the CdS nanocrystals. The morphology and crystal structure of the nanocrystal precipitates could be controlled by selection of the host material. The size distribution and microstructural complexity were significantly reduced by implanting a low concentration of ions into a noncrystalline host, and by using multi-energy implants to give a flat concentration profile of the implanted elements. (c) 1999 Materials Research Society.

Study of tellurium precipitates in CdTe crystals

The effect of tellurium precipitates was studied in medium resistivity (103–106 Ω cm) undoped and Cl‐doped CdTe using differential scanning calorimetry (DSC) and mid‐infrared spectroscopy and the results were correlated with near‐infrared microscopy photographs. When present in a significant quantity (∼0.25 wt %), we show that Te precipitates are detectable using DSC measurements. In the mid‐infrared, the contribution of the absorption by free‐carriers is negligible, and therefore, the effect of the Te precipitates in these crystals can be considered uncoupled from the effects of Cd vacancies.

Optical spectra of Pb implanted fused silica

Abstract The extinction coefficient over the spectral range of 1.8–6.2 eV and the infrared reflectance over the 5000 cm −1 to 75 cm −1 range were measured for a series of Pb ion-implanted high-purity silica substrates. The samples were implanted at room temperature with an energy of 320 keV for nominal doses of 1, 3 and 6 × 10 16 ions/cm 2 . A sample implanted with a nominal dose of 6 × 10 16 ions/cm 2 was annealed in air at 800°C for 1 h. The Pb ions form both elemental Pb and PbO particles in the as-implanted state. The extent of formation for each chemical state is dose-dependent. Upon annealing, the elemental Pb and PbO particles form a lead silicate phase in the implanted layer that is similar to conventionally prepared bulk lead silicate glasses.

Formation of CdS and CdSe nanocrystals by sequential implantation

Abstract Nanocrystals of CdS and CdSe have been formed in SiO2, Al2O3, and Si by sequential ion implantation and annealing. In SiO2, the nanocrystals have the hexagonal wurtzite structure and are randomly oriented. In Al2O3 and Si, nanocrystals are three dimensionally oriented. They have the cubic zincblende structure in Si and can be produced with either structure in Al2O3 by controlling the implantation conditions. In SiO2 and Al2O3, nanocrystals exhibit strong optical absorption and photoluminescence (PL). Evidence for quantum confinement is observed, and the PL results for CdSe are strongly temperature dependent.