Weiguo Yao

Structure and optical properties of SiO2 films containing Ge nanocrystallites

Abstract SiO 2 thin films embedded with Ge nanocrystallines (nc-Ge) were prepared on p-type Si and optical quartz glasses substrates by RF-magnetron co-sputtering method from a composite target of Ge and SiO 2 . The as-deposited films were annealed in the temperature range of 300–1000 °C under nitrogen ambience. The structure of the films was evaluated by X-ray diffraction and X-ray photoemission spectroscopy. Results show that the average size of nc-Ge in the SiO 2 matrix is in the range of 2–10 nm which can be controlled by the annealing temperature. Quantum confinement effect was observed in the films by the measurements of absorption spectrum. Photoluminescence (PL) spectra were observed in visible range at room temperature. Possible origins of the visible PL are discussed.

Origin of photoluminescence peaks in Ge–SiO2 thin films

Abstract Ge–SiO2 thin films were prepared by the RF magnetron sputtering technique on p–Si substrates from a Ge–SiO2 composite target. The asdeposited films were annealed in the temperature range of 300–10000C under nitrogen ambience. The structure of films was evaluated by X–ray diffraction, X-ray photoemission spectroscopy and Fourier transform infrared absorption spectroscopy. Results show that the content of Ge and its oxides in thefilms change with increasing annealing temperature (Ta), the photoluminescence (PL) characteristics are closely dependent on the contents of Ge and its oxides in SiO2 matrix. The dependence observed strongly suggests that the PL peak at 394 nm is related to the existenceof GeO and 580 nm to that of Ge nanocrystal (nc-Ge) in the films.

Room-temperature visible electroluminescence of Al-doped silicon oxide films

A series of Al-doped amorphous silicon oxide films have been grown on p-type silicon (100) substrates by a dual ion beam cosputtering method. Visible electroluminescence (EL) from the devices, made by films with different contents of Al, can be seen with the naked eye under forward bias and reverse bias for films containing sufficient amounts of Al. The EL spectra are found to have a luminescence band peaked at 510 nm (2.4 eV), which is the same result as that obtained from silicon oxide films. With the increase in the amounts of Al, the peak position does not shift, the onset of the bias decreases, and the intensity of EL peak increases. Experiment results show that the doping of Al is beneficial to improving the conduction condition of films while the structure of the films associated with luminescence centers is affected hardly at all.

Structural and magnetic effects of Ni addition in FeN films

Fe–Ni–N films were prepared on glass substrates by dual ion beam sputtering. The effect of nickel (Ni) addition on the structure, magnetic properties, and thermal stability of Fe–N films was investigated. In the γ′-(Fe,Ni)4N phase, the addition of Ni influences the growth, nitrogen content, and the preferred orientation of the film. The (200) plane of the γ′-phase in the Fe–Ni–N films grows dominantly parallel to the substrate plane with increasing Ni content, indicating the possibility of controlling the crystal orientation. A single γ′-(Fe,Ni)4N phase film with all grains oriented in the (200) direction was actually grown for a Ni content of 21.2 at. %. The coercivity HC of Ni-containing films was lower than that of the Ni-free films. The film with Ni content of 6.7 at. % showed good soft magnetic properties, with HC=0.68 kA/m and MS=2.04 T. The thermal stability, however, was weakened due to the existence of Ni in the Fe–N films.

Strong green electroluminescence from silicon based oxide films

Abstract Electroluminescence (EL) devices have been fabricated on three types of silicon based oxide films (Ge–SiO 2 films, Si–SiO 2 films, and Al–SiO 2 films). Visible EL from the devices can be seen with the naked eye when the bias voltage greater than a critical value is applied. Each EL spectrum is found to have only one luminescence band peaked at 510 nm and the peak position does not shift with the bias voltage, annealing temperature, and the type of doping atoms in Si oxide matrixes. Spectra analyses suggest that the origin of EL can be attributed to the luminescence centers (LCs) in the silicon oxide films.

Growth of InAs nanocrystals embedded in SiO2 films by radio-frequency magnetron cosputtering

Abstract In situ radio-frequency (RF) magnetron cosputtering was used to prepare InAs nanocrystals embedded in SiO 2 matrices. The growth behavior of InAs in the composite films has been studied systematically for the first time by transmission electron microscopy. It is found that with increasing substrate temperature, InAs in the matrix undergoes a series of transitions from an initial dispersed phase to a fractal structure, then to nucleation, and finally to grain growth. The average size and the size distribution of the InAs nanocrystals as a function of the RF sputtering parameters were presented and discussed with respect to the crystal growth dynamics. Optical absorption spectra analysis shows that size-controllable InAs nanocrystals embedded in SiO 2 films with a narrow size distribution have been obtained. The blue shift of optical absorption edge versus the average size has been explained by the effective-mass approximation method.

Quantum confinement effect in SiO2 films containing Ge microcrystallites

SiO2 thin films embedded with Ge microcrystallites (Ge-SiO2 films) were prepared by RF-magnetron co-sputtering method from a composite target of Ge and SiO2. The average size of Ge crystallites can be modulated by the experiment parameters. The optical absorption and non-linear optical properties of Ge-SiO2 films were measured. The blue shift of the optical absorption edge, the saturated absorption and two-photon absorption under the condition of resonant absorption have been observed, and are discussed according to the quantum confinement effect.

Photoluminescence from Ge-SiO2 thin films and its mechanism

Ge-SiO2 thin films were deposited on p-type Si substrates using the radio frequency (rf) magnetron sputtering technique with a Ge-SiO2 composite target. Films were annealed in N2 ambience for 30 min at 300°C–1000°C with an interval of 100°C. Through the X-ray diffraction, the average size of Ge nanocrystals (nc-Ge) was determined. They increased from 3.9 to 6.1 nm with increasing annealing temperature in the range of 600°C–1000°C. Under ultraviolet excitation, all samples emit a strong violet band centered at 396 nm. With the formation of nc-Ge, the samples exhibit another emission of orange band with the peak at 580 nm and its intensity increases with the increasing size of nc-Ge. The peak positions of two bands do not shift obviously. Experimental data indicate that the violet band comes from GeO defect and the orange band originates mainly from the luminescence centers at the interface between the nc-Ge and SiO2 matrix.

Preparation and optical absorption of InAs nanocrystal-embedded thin films

InAs-SiO2 nanocrystal-embedded films were prepared by using radio-frequency cosputtering. The growth behavior of InAs in the composite film has been studied by a transmission electron microscope. It has been found that with the increasing substrate temperature, InAs in the matrix undergoes transitions from an initial dispersed phase to a fractal structure of the lnAs phase, then to nucleation, and finally to grain growth. Large blueshift of the optical absorption edges of the films was observed from the optical absorption spectra. The relationship between the blueshift of optical absorption edge and the average size of the nanocrystals has been explained by the effective-mass approximation.