Enhanced charge storage characteristics of silicon nanocrystals fabricated by electron-beam coevaporation of Si and SiOx(x=1 or 2)

In this article, a simple and flexible electron-beam coevaporation (EBCE) technique has been reported of fabrication of the silicon nanocrystals (Si NCs) and their application to the nonvolatile memory. For EBCE, the Si and SiOx(x=1 or 2) were used as source materials. Transmission electron microscopy images and Raman spectra measurement verified the formation of the Si NCs. The average size and area density of the Si NCs can be adjusted by increasing the Si:O weight ratio in source material, which has a great impact on the crystalline volume fraction of the deposited film and on the charge storage characteristics of the Si NCs. A memory window as large as 6.6 V under +/- 8 V sweep voltage was observed for the metal-oxide-semiconductor capacitor structure with the embedded Si NCs.

Photoluminescence and Raman scattering of silicon nanocrystals prepared by silicon ion implantion into SiO2 films

Photoluminescence (PL) and Raman spectra of silicon nanocrystals prepared by Si ion implantion into SiO2 layers on Si substrate have been measured at room temperature. Their dependence on annealing temperature was investigated in detail. The PL peaks observed in the as-implanted sample originate from the defects in SiO2 layers caused by ion implantation. They actually disappear after thermal annealing at 800 degrees C. The PL peak from silicon nanocrystals was observed when thermal annealing temperatures are higher than 900 degrees C. The PL peak is redshifted to 1.7 eV and the intensity reaches maximum at the thermal annealing temperature of 1100 degrees C. The characterized Raman scattering peak of silicon nanocrystals was observed by using a right angle scattering configuration. The Raman signal related to the silicon nanocrystals appears only in the samples annealed at temperature above 900 degrees C. It further proves the formation of silicon nanocrystals in these samples. (C) 2000 American Institute of Physics. [S0021-8979(00)00215-2].

360-nm Photoluminescence from Silicon Oxide Films Embedded with Silicon Nanocrystals

Si-rich silicon oxide films were deposited by RF magnetron sputtering onto composite Si/SiO2 targets. After annealed at different temperature, the silicon oxide films embedded with silicon nanocrystals were obtained. The photoluminescenee（PL） from the silicon oxide films embedded with silicon nanocrystals was observed at room temperature. The strong peak is at 360 nm, its position is independent of the annealing temperature. The origin of the 360-nm PL in the silicon oxide films embedded with silicon nanoerystals was discussed.

Strong red light emission from silicon nanocrystals embedded in SIO2 matrix

In this study, silicon nanocrystals embedded in SiO2 matrix were formed by conventional plasma enhanced chemical vapor deposition (PECVD) followed by high temperature annealing. The formation of silicon nanocrystals (nc-Si), their optical and micro-structural properties were studied using various experimental techniques, including Fourier transform infrared spectroscopy, micro-Raman spectra, high resolution transmission electron microscopy and x-ray photoelectron spectroscopy. Very strong red light emission from silicon nanocrystals at room temperature (RT) was observed. It was found that there is a strong correlation between the PL intensity and the substrate temperature, the oxygen content and the annealing temperature. When the substrate temperature decreases from 250degreesC to RT, the PL intensity increases by two orders of magnitude.

Raman scattering and infrared absorption of silicon nanocrystals in silicon oxide matrix

Structural dependence on annealing of a-SiOx:H was studied by using infrared absorption and Raman scattering. The appearance of Raman peaks in the range of 513-519cm(-1) after 1170 degreesC annealing was interpreted as the formation nanocrystalline silicon with the sizes from 3-10nm. The Raman spectra also show the existence of amorphous-like silicon phase, which is associated with Si-Si bond re-construction at boundaries of silicon nanocrystallites. The presence of the shoulder at 980cm(-1) of Si-O-Si stretching vibration at 1085cm(-1) in infrared spectra imply that except that SiO2 phase, there is silicon sub-oxide phase in the films annealed at 1170 degreesC. This sub-oxide phase is located at the interface between Si crystallites and SiO2, and thus support the shell model for the mixed structures of Si grains and SiO2 matrix.

Origin and evolution of photoluminescence from Si nanocrystals embedded in a SiO2 matrix

A detailed analysis of the photoluminescence (PL) from Si nanocrystals (NCs) embedded in a silicon-rich SiO2 matrix is reported. The PL spectra consist of three Gaussian bands (peaks A,B, and C), originated from the quantum confinement effect of Si NCs, the interface state effect between a Si NC and a SiO2 matrix, and the localized state transitions of amorphous Si clusters, respectively. The size and the surface chemistry of Si NCs are two major factors affecting the transition of the dominant PL origin from the quantum confinement effect to the interface state recombination. The larger the size of Si NCs and the higher the interface state density (in particular, Si = O bonds), the more beneficial for the interface state recombination process to surpass the quantum confinement process, in good agreement with Qin's prediction in Qin and Li [Phys. Rev. B 68, 85309 (2003)]. The realistic model of Si NCs embedded in a SiO2 matrix provides a firm theoretical support to explain the transition trend.

Study on Er3+ emission from the erbium-doped hydrogenated amorphous silicon suboxide film

The erbium-doped hydrogenated amorphous silicon suboxide films containing amorphous silicon clusters were prepared. The samples exhibited photoluminescence peaks at around 750 nm and 1.54 mum, which could be assigned to the electron-hole recombination in amorphous silicon clusters and the intra-4f transition in Er3+, respectively. Correlations between the intensities of these two photoluminescence peaks and oxidation and dehydrogenation of the films during annealing were studied. It was found that the oxidation is triggered by dehydrogenation of the films even at low annealing temperatures, which decisively changes the intensities of the two photoluminescence peaks. On the other hand, the increase of Er content in the erbium-doped hydrogenated amorphous silicon suboxide film will enhance Er3+ emission at 1.54 mum, while quench amorphous silicon cluster emission at 750 nm, such a competitive relationship, was also observed in the erbium-doped silicon nanocrystals embedded in SiO2 matrix. Moreover, we found that Er3+ emission is not sensitive to whether silicon clusters are crystalline or amorphous. The amorphous silicon clusters can be as sensitizer on Er3+ emission as that of silicon nanocrystals. (C) 2003 American Institute of Physics.

Photoluminesfcence of Er-doped SiO2 films containing Si nanocrystals and Er

Correlations between Si nanocrystal (nc-Si) related photoluminescence (PL), Er3+ emission and nonradiative defects in the Er-doped SiO2 films containing nc-Si (SRSO) are studied. Upon 514.5 nm laser excitation the erbium-doped SRSO samples exhibit PL peaks at around 0.8 and 1.54 mum, which can be assigned to the electron-hole recombination in nc-Si and the intra-4f transition in Er3+, respectively. With increasing Er3+ content in the films, Er3+ emission becomes intense while the PL at 0.8 mum decreases, suggesting a strong coupling of nc-Si and Er 31 ions. Hydrogen plasma treatment for the samples improve the PL intensities of the 0.8 and 1.54 mum bands, indicating H passivation for the nonradiative defects existing in the samples. Further-more, from the effect of hydrogen treatment for the samples, we observe variation of the number of nonradiative defects with annealing temperatures. (C) 2003 Elsevier Science B.V. All rights reserved.

Getting high-efficiency photoluminescence from Si nanocrystals in SiO2 matrix

Silicon nanocrystals in SiO2 matrix are fabricated by plasma enhanced chemical vapor deposition followed by thermal annealing. The structure and photoluminescence (PL) of the resulting films is investigated as a function of deposition temperature. Drastic improvement of PL efficiency up to 12% is achieved when the deposition temperature is reduced from 250 degreesC to room temperature. Low-temperature deposition is found to result in a high quality final structure of the films in which the silicon nanocrystals are nearly strain-free, and the Si/SiO2 interface sharp. The demonstration of the superior structural and optical properties of the films represents an important step towards the development of silicon-based light emitters. (C) 2002 American Institute of Physics.

Optical properties of nanometer silicon prepared by silicon ion implanted into SiO2 layers

The samples of silicon nanocrystals (nc-Si) were prepared by Si ion implanted into SiO2 layers. Photoluminescence spectra were measured at room temperature and their dependence on thermal annealing was investigated. The experimental results show that PL peaks originate from the defects in SiO2 layers caused by ion implantation when the thermal annealing temperature is lower than 800 C. The PL peak from nc-Si was observed when the thermal annealing temperature was higher than 900 C, and PL intensity reached its maximum at the thermal annealing temperature of 1100 C. As the annealing temperature increases the red shift of PL peak from nc-Si shows the quantum size effect. The characterized Raman scattering peak of nc-Si was observed at the right angle scattering configuration for the first time. It provides further support for the PL measurements.

Photoluminescence spectroscopy of sputtering Er-doped silicon-rich silicon nitride films

Er-doped silicon-rich silicon nitride (SRN) films were deposited on silicon substrate by an RF magnetron reaction sputtering system. After high temperature annealing, the films show intense photoluminescence in both the visible and infrared regions. Besides broad-band luminescence centered at 780 nm which originates from silicon nanocrystals, resolved peaks due to transitions from all high energy levels up to ~2H_(11/2) to the ground state of Er~(3+) are observed. Raman spectra and HRTEM measurements have been performed to investigate the structure of the films, and possible excitation processes are discussed.

Enhancement of electroluminescence in p-i-n structures with nano-crystalline Si/SiO2 multilayers

Nano-crystalline Si/SiO2 multilayers were prepared by alternately changing the ultra-thin amorphous Si film deposition and the in situ plasma oxidation process followed by the post-annealing treatments. Well-defined periodic structures can be achieved with 2.5 nm thick SiO2 sublayers. It is shown that the size of formed nano-crystalline Si is about 3 nm. Room temperature electroluminescence can be observed and the spectrum contains two luminescence bands located at 650 nm and 520 nm. In order to improve the hole injection probability, p-i-n structures containing a nanocrystalline Si/SiO2 luminescent layer were designed and fabricated on different p-type substrates. It is found that the turn-on voltage of p-i-n structures is obviously reduced and the luminescence intensity increases by 50 times. It is demonstrated that the use of a heavy-doped p-type substrate can increase the luminescence intensity more efficiently compared with the light-doped p-type substrate due to the enhanced hole injection.

Anomalous temperature dependence of photoluminescence from stoichiometric GD(2)O(3-x) film

A stoichiometric Gd2O3-x thin film has been grown on a silicon (10 0) substrate with a low-energy dual ion-beam epitaxial technique. Gd2O3-x shares Gd2O3 structures although there are many oxygen deficiencies in the film. The photoluminescence (PL) measurements have been performed in a temperature range 5-300 K. The detailed characters of the peak position, the full-width at half-maximum (FWHM) and the peak intensity at different temperature were reported. An anomalous intensity behavior of the PL spectra has been observed, which is similar to that of some other materials such as porous silicon and silicon nanocrystals in silicon dioxide. Therefore, we suggest that the nanoclusters with the oxygen deficiencies contribute to the PL emission and employ the model of singlet-triplet exchange splitting of exciton to discuss the four peaks observed in the experiment. (C) 2003 Elsevier B.V. All rights reserved.

Nonlinear optical response of nc-Si-SiO2 films studied with femtosecond four-wave mixing technique

Nonlinear optical properties of silicon nanocrystals (nc-Si) embedded in SiO2 films are investigated using time-resolved four-wave mixing technique with a femtosecond laser. the off-resonant third-order nonlinear susceptibility chi((3)) is observed to be 1.3 x 10(-10) esu at 800 nm. The relaxation time of the film is fast as short as 50 fs. The off-resonant nonlinearity is predominantly electronic in origin and enhanced due to quantum confinement.

Electroluminescence afterglow from indium tin oxide/Si-rich SiO2/p-Si structure

Indium tin oxide/Si-rich SiO2/p-Si structured devices are fabricated to study the electroluminescence (EL) of the Si-rich SiO2 (SRO) material. The obvious peaks at similar to 1050nm and similar to 1260nm in the EL are ascribed to localized state transitions of amorphous Si (alpha-Si) clusters. The EL afterglow associated with alpha-Si clusters is observed from this structure at room temperature, while the afterglow is absent in the case of optical pumping. It is believed that carrier-induced defects act as trap centres in the alpha-Si clusters, resulting in the EL afterglow. The phenomenon of the EL afterglow indicates the limits of EL performance and electrical modulation of the SRO material with a larger fraction of alpha-Si clusters.