X. M. Bao

Annealing temperature dependence of Raman scattering in Ge+-implanted SiO2 films

We have examined the Raman spectra of violet and infrared emitting Ge+-implanted SiO2 films with special emphasis upon annealing temperature (AT) dependence of Raman scattering. We found that the complete spectrum mainly consists of three bands at 220–280, 300, and 430 cm−1, corresponding to scattering of Ge-related components, Ge nanocrystallites, and localized Si–Si optical phonons in the Ge neighborhoods, respectively. The Ge crystalline band shows an obvious AT dependence. The theoretical result from the phonon confinement model can predict its linewidth change with AT, but cannot explain its constant peak frequency. Based on the experimental result from x-ray diffraction, we attributed the discrepancy mainly to the compressive stress exerted on Ge nanocrystallites, which leads to the upshift of Ge crystallite peak thereby basically compensating the downshift caused by the confinement on phonon frequency.

Strong ultraviolet photoluminescence from silicon oxide films prepared by magnetron sputtering

Intense ultraviolet photoluminescence centered at 370 nm was observed from magnetron-sputtered silicon oxide films after they were annealed at about 1000 °C in N2 atmosphere. This photoluminescence is found to be associated with the formation of nanocrystal silicon particles in the specially structured SiO2, which highly resembles the oxide layer of porous silicon. The luminescence centers at the interface between the nanocrystal silicon particles and the SiO2 matrix are responsible for the strong ultraviolet luminescence.

Anodic alumina template on Au/Si substrate and preparation of CdS nanowires

Abstract A layer of thin gold film was sandwiched between a silicon substrate and an Al film to form the Al/Au/Si structure. Subsequent anodization leads to formation of a Si-based anodic aluminum oxide (AAO) template (AAO/Au/Si structure) with ordered nanopores. This kind of template has unique electrodeposition properties and can bond well with the deposited materials. The anodic process of the Al/Au/Si structure was investigated in detail by in situ monitoring the current–time curve. As an application, CdS nanowires were fabricated on the silicon substrate using this kind of AAO templates. Light-emitting property from the CdS nanowires was observed. This kind of Si-based light-emitting nanowires are expected to have practical applications in optoelectronic integration.

Blue-emitting β-SiC fabricated by annealing C60 coupled on porous silicon

C60 molecules were chemically coupled in the pores of porous Si through a coupling agent and then coated with a layer of Si, and followed by N2 annealing. X-ray diffraction results indicate that the fabricated samples contain β-SiC particles which may exist in the pores, in addition to Si, SiO2, and graphite. The photoluminescence (PL) spectra show an asymmetrical broadband, which can be Gaussian divided into two bands at 380 (3.26 eV) and 454 (2.73 eV) nm. Spectral analyses and the experimental results from infrared spectroscopy and PL excitation measurements suggest that the 380 nm PL band is related to oxygen-vacancy defects in the SiO2 matrix, whereas the blue PL band is closely connected with the β-SiC particles. Our experiments provide a way for fabricating stable blue-emitting β-SiC materials.

Defect-related infrared photoluminescence in Ge+-implanted SiO2 films

SiO2 films with Ge+ implantation at an energy of 60 keV and a dose of 1×1016 cm−2, followed by annealing at different temperature, exhibit a broad infrared photoluminescence (PL) at room temperature under an excitation of the 514.5 nm line of Ar+ laser. With increasing the annealing temperature, the intensity of the infrared PL band decreases, its full width at half maximum increases, and its energy redshifts. Spectral analysis and some experimental results from Raman scattering, electron spin resonance, and infrared spectroscopy strongly suggest that the infrared PL is mainly related to interfacial oxygen-deficient-type defects between the oxide and Ge nanocrystals.

Enhanced ultraviolet photoluminescence from SiO2/Ge:SiO2/SiO2 sandwiched structure

SiO2/Ge:SiO2/SiO2 sandwiched structure was fabricated for exploring efficient light emission. After annealed in N2 (O2<1%), this structure shows three photoluminescence (PL) bands at 293, 395, and 780 nm. The intensity of the 395 nm band is largely enhanced in comparison with that from the monolayered Ge:SiO2 film. Spectral analyses suggest that the three PL bands originate from S1→S0, T∑(T∏)→S0, and T∏′→S0 optical transitions in GeO color centers, respectively. The improvement of the GeO density resulting from the confinement on Ge diffusion is responsible for the enhanced ultraviolet PL. This structure is expected to have important applications in optoelectronics.

Ultraviolet and blue emission from crystalline SiO2 coated with LiNbO3 and LiTaO3

Crystalline SiO2 (α quartz) coated with LiNbO3 (LN) or LiTaO3 (LT) films emits two strengthened luminescence bands at 368 and 468 nm compared with those of pure α-SiO2, when excited with 280 nm light of a Xe lamp. These two bands strongly depend on the coating: the LN-coated α-SiO2 has a much stronger photoluminescence (PL) than the LT-coated α-SiO2. On the other hand, the two bands are different from those of α-SiO2 in that their excitation spectra do not have any noticeable band in the range 200–320 nm. They could be understood based on excitons in the α-SiO2 surface, which are induced by the photorefractive effect of the coated films. The 368 nm PL band is attributed to the optical transitions of the E′ defect pairs and the 468 nm PL band to the radiative recombination of the self-trapped excitons.

Photoluminescence from Ge+-implanted SiO2 films on Si substrate and its mechanism☆

Abstract An intense ultraviolet photoluminescence peaked at 286 nm and 396 nm from Ge + -implanted SiO 2 film thermally grown on crystal silicon was observed. After 1100°C annealing, a weak near-infrared emission peak at 840 nm was measured at low temperature (77 K). Annealing analyses and X-ray photoemission spectroscopy measurement show that the ultraviolet peaks are caused by S 1 → S 0 and T 1 → S 0 transition in GeO color centers formed during implantation and annealing, while the infrared one is due to the quantum confinement effect in Ge clusters formed at high annealing temperatures.

X-RAY DIFFRACTION STUDY OF ALTERNATING NANOCRYSTALLINE SILICON/AMORPHOUS SILICON MULTILAYERS

Structural properties of alternating nanocrystalline silicon/amorphous silicon multilayers with visible light emission at room temperature were examined by means of x-ray diffraction. According to the linewidths and intensities of the diffraction peaks in the low- and high-angle ranges, we have determined the effective interface thickness, the mean crystallite sizes, and the internal strains, which are closely related to the photoluminescence in this material. In addition, the existence of the voids or holes was also observed, indicating that the improved electrical properties of this kind of hydrogenated nanocrystalline materials are due to the inhomogeneous structure of the material.

Self-organized growth and optical emission of silicon-based nanoscale β-SiC quantum dots

Si-based β-SiC quantum dots (QDs) were fabricated for exploring efficient blue emission from β-SiC nanostructures. Microstructural observations and x-ray photoemission spectroscopy reveal that the β-SiC QDs with sizes of 5–7 nm are embedded in the SiO2 and graphite matrices, displaying a locally tetragonal symmetry. Photoluminescence spectral examinations show two narrow blue-emitting bands at 417 and 436 nm, which are determined by both quantum confinement and surface structure of the β-SiC QDs. Electron spin resonance investigation demonstrates that the photoexcited carriers partially come from the β-SiC QD core with a widened band gap, whereas the radiative recombination occurs in Si excess defect centers at the β-SiC QD surface. A theoretical calculation about electronic states caused by the vacancy defects in the gap of balls formed with excess Si atoms at the surfaces of the β-SiC QDs supports our assignment to the two blue-emitting origin.