Optical study of localized and delocalized states in GaAsN/GaAs

Taking advantages of short pulse excitation and time-resolved photoluminescence (PL), we have studied the exciton localization effect in a number of GaAsN alloys and GaAsN/GaAs quantum wells (QWs). In the PL spectra, an extra transition located at the higher energy side of the commonly reported N-related emissions is observed. By measuring PL dependence on temperature and excitation power along with PL dynamics study, the new PL peak has been identified as a transition of the band edge-related recombination in dilute GaAsN alloy and delocalized transition in QWs. Using selective excitation PL we further attribute the localized emission in QWs to the excitons localized at the GaAsN/GaAs interfaces. This interface-related exciton localization could be greatly reduced by a rapid thermal annealing.

Influence of In0.2Ga0.8As strain-reducing layer on the active region of quantum dot superluminescent diodes

We have investigated the optical properties of asymmetric multiple layer stacked self-assembled InAs quantum dot with different interlayer. We found that asymmetric multiple stacked QD samples with In0.2Ga0.8As + GaAs interlayer can afford a 180nm flat spectral width with strong PL intensity compared to other samples at room temperature. We think this result is due to the introduction of In0.2Ga0.8As strain-reducing layer. Additionally, for the broad spectral width and the strong PL intensity, this structure can be a promising candidate for quantum-dot superluminescent diodes.

Polymorphous silicon nanowires synthesized by plasma-enhanced chemical vapor deposition

Polymorphous Si nanowires (SiNWS) have been successfully synthesized on Si wafer by plasma enhanced chemical vapor deposition (PECVD) at 440degreesC,using silane as the Si source and Au as the catalyst. To grow the polymorphous SiNWS preannealing the Si substrate with Au film at 1100 degreesC is needed. The diameters of Si nanowires range from 15 to 100 urn. The structure morphology and chemical composition of the SiNWS have been characterized by high resolution x-ray diffraction, scanning electron microscopy, transmission electron microscopy, as well as energy dispersive x-ray spectroscopy. A few interesting nanowires with Au nanoclusters uniformly distributed in the body of the wire were also produced by this technique.

Microstructure of the silicon film prepared near the phase transition regime from amorphous to nanocrystalline

A kind of hydrogenated diphasic silicon films has been prepared by a new regime of plasma enhanced chemical vapor deposition (PECVD) near the phase transition regime from amorphous to nanocrystalline. The microstructural properties of the films have been investigated by the micro-Raman and Fourier transformed Infrared (FT-IR) spectra and atom force microscopy (AFM). The obtained Raman spectra show not only the existence of nanoscaled crystallites, but also a notable improvement in the medium-range order of the diphasic films. For the FT-IR spectra of this kind of films, it notes that there is a blueshift in the Si-H stretching mode and a redshift in the Si-H wagging mode in respect to that of typical amorphous silicon film. We discussed the reasons responsible for these phenomena by means of the phase transition, which lead to the formation of a diatomic hydrogen complex, H-2* and their congeries.

High-mobility Ga-polarity GaN achieved by NH3-MBE

GaN epilayers were grown on (0001) sapphire substrates by NH3-MBE and RF-MBE (radio frequency plasma). The polarities of the epilayers were investigated by in-situ RHEED, chemical solution etching and AFM surface examination. By using a RF-MBE grown GaN layer as template to deposit GaN epilayer by NH3-MBE method, we found that not only Ga-polarity GaN films were repeatedly obtained, but also the electron mobility of these Ga-polarity films was significantly improved with a best value of 290 cm(2)/V.s at room temperature. Experimental results show it is an easy and stable way for growth of high quality Ga-polarity GaN films.

The influence of oxygen content on photoluminescence from Er-doped SiOx

SiOx films with oxygen concentrations ranging 13-46 at.% were deposited by plasma enhanced chemical vapor deposition (PECVD) technique using: pure SiH4 and N2O mixture. Erbium was then implanted at an energy of 500 KeV with dose of 2x10(15) ions/cm(2). The samples were subsequently annealed in N-2 for 20 sec at temperatures of (300-950 degrees C). Room temperature (RT) photo-luminescence (PL) data were collected by Fourier Transform Infrared Spectroscopy (FTIS) with an argon laser at a wavelength of 514.5 nm and an output power from 5 to 2500 mw. The intense room-temperature luminescence was observed around 1.54 mu m. The luminescence intensity increases by 2 orders of magnitude as compared with that of Er-doped Czochralski (CZ) Si. We found that the Er3+ luminescence depends strongly on the SiOx microstructure. Our experiment also showed that the silicon grain radius decreased with increasing oxygen content and finally formed micro-crystalline silicon or nano-crystalline silicon. As a result, these silicon small particles could facilitate the energy transfer to Er3+ and thus enhanced the photoluminescence intensity.

High temperature annealing behaviors of luminescent SIOx : H films

The effects of high temperature annealing on the microstructure and optical properties of luminescent SiOx:H films have been investigated. Micro-Raman scattering and IR absorption, in combination with atomic force microscopy (AFM), provide evidence for the existence of both a-Si clusters in the as-grown a-SiOx:H and Si nanocrystals in the 1170 degrees C annealed films. The dependence of optical coefficients (alpha) on photon energy (h nu) near the absorption edge (E-g) is found to follow the square root law: (alpha h nu)(1/2) proportional to (E-g - h nu), indicating that nano-Si embedded in SiO2 is still an indirect material. A comparison of the deduced absorption edge with the PL spectra shows an obvious Stokes shift, suggesting that phonons should be involved in the optical transition process.

Light-excited structural instability of a-Si : H.

With the accumulation of experimental data, it has been recognized by many that the light-induced metastable change of a-Si:H, Staebler-Wronski effect (SWE), may be related to a structural instability of the whole a-Si:H network. However, direct evidence of such a structural change is still lacking. In the present paper, the efforts of our laboratory in this direction will be reviewed, including the light-induced changes of Si-H bond absorption, low frequency dielectric response, and an apparent photo-dilation effect.

High quality hydrogenated amorphous silicon films with significantly improved stability

High quality hydrogenated amorphous silicon (a-Si:H) films have been prepared by a simple "uninterrupted growth/annealing" plasma enhanced chemical vapor deposition (PECVD) technique, combined with a subtle boron-compensated doping. These a-Si:H films possess a high photosensitivity over 10(6), and exhibit no degradation in photoconductivity and a low light-induced defect density after prolonged illumination. The central idea is to control the growth conditions adjacent to the critical point of phase transition from amorphous to crystalline state, and yet to locate the Fermi level close to the midgap. Our results show that the improved stability and photosensitivity of a-Si:H films prepared by this method can be mainly attributed to the formation of a more robust network structure and reduction in the precursors density of light-induced metastable defects.

Structural properties of polycrystalline silicon films formed by pulsed rapid thermal processing

A novel pulsed rapid thermal processing (PRTP) method has been used for realizing the solid-phase crystallization of amorphous silicon films prepared by PECVD. The microstructure and surface morphology of the crystallized films are investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM). The results indicate that this PRTP is a suitable post-crystallization technique for fabricating large-area polycrystalline silicon films with good structural qualities such as large grain size, small lattice microstain and smooth surface morphology on low-cost substrate.

Effect of carrier relaxation and emission on photoluminescence of InAs quantum dots

　

The role of hydrogen in semi-insulating INP

Complexes of vacancy at indium site with one to four hydrogen atoms and isolated hydrogen or hydrogen dimer and other infrared absorption lines, tentatively be assigned to hydrogen related defects were investigated by FTIR. Hydrogen cam passivate imperfections, thereby eliminating detrimental electronic states from the energy bandgap. Incorporated hydrogen can introduce extended defects and generate electrically-active defects. Hydrogen also can acts as an actuator for creating of antistructure defects. Isolated hydrogen related defects(e.. H-2*) may play an important role in the conversion of the annealed wafers from semiconducting to the semi-insulating behavior. H-2* may be a deep donor, whose energy level is very near the iron deep acceptor level in the energy gap.

The effect of low temperature GaAs nucleation on the growth of GaN on Silicon (001) during MOVPE process

High quality cubic GaN was grown on Silicon (001) by metalorganic vapor phase epitaxy (MOVPE) using a GaAs nucleation layer grown at low temperature. The influence of various nucleation conditions on the GaN epilayers' quality was investigated. We found that the GaAs nucleation layer grown by atomic layer epitaxy (ALE) could improve the quality of GaN films by depressing the formation of mixed phase. Photoluminescence (PL) and X-ray diffraction were used to characterize the properties of GaN epilayers. High quality GaN epilayers with PL full width at half maximum (FWHM) of 130meV at room temperature and X-ray FWHM of 70 arc-min were obtained by using 10-20nm GaAs nucleation layer grown by ALE.