Nanocrystal and interface defects related photoluminescence in silicon-rich Al2O3 films
Introduction
Silicon nanostructures such as porous silicon, silicon nanocrystal-rich SiO2 have attracted many researchers’ attention for their prominent photoluminescence properties and possible applications in optoelectronics [1], [2], [3], [4], [5]. Apart from the widely studied Si-rich SiO2 (SRSO) system, some researchers focused on finding new energy barriers for silicon nanocrystals such as Al2O3 [6], [9], ZnO [7] and MgO [8] in order to explore new properties and clarify the mechanisms of light emitting. Both nanocrystal and defect related light emissions have been reported in silicon-rich ZnO and MgO films. However, in nanocrystal silicon-rich Al2O3 films, little or only very weak nanocrystal related light emission is observed as far as we know. The interface of the nanocrystalline silicon and Al2O3 matrix has been observed to be abrupt and devoid of recombination centers. This is considered to impede the light emitting from the nanostructure according to the QCE model. In fact, as a new gate dielectric material replacing SiO2, Al2O3 film has an atomically abrupt interface with silicon substrate when growing at UHV [10], [11], but non-negligible SiOx grows at the interface after annealing the films in O2 or N2 atmosphere [12]. One could also expect such SiOx layer at the interface between the silicon nanocrystal and the Al2O3 matrix to form after proper annealing process and improve the photoluminescence property of the film. Moreover, using Al2O3 as the energy barrier for silicon nanocrystals has more advantage. The energy gap of Al2O3 is 9.2 eV, comparable to SiO2 (8.7 eV), and the microconstruction at the silicon nanocrystal interface can be tuned by simply controlling the annealing process. These features may allow us to discover more details about the nanocrystal interface defects and clarify the light-emitting mechanism.
In this study, we synthesized nanocrystal silicon-rich Al2O3 film (SRA) by co-sputtering a silicon and alumina composite target and annealing in N2 atmosphere. Post-oxygen-annealing processes were carried out to modify the interface defect centers. Infrared (IR), Raman and UV–visible spectroscopy were used to investigate the microstructure of the film. Typical nanocrystal and interface defects related PL spectra were observed and the emitting mechanism is discussed according to the experiment results.
Section snippets
Experiments
The samples investigated were fabricated by co-sputtering a silicon and alumina composite target through an RF sputtering process. Three pieces of sector-shaped silicon plates (vertex 30° and radius 3 cm) were symmetrically placed on Al2O3 plate () to form the composite target. The substrates used in this study were n-type Si (1 1 1) wafers with a resistance of 1.5 Ω cm and fused quartz plates. All substrates were cut into 1×1 cm2 wafers. Before being loaded into the vacuum chamber, the wafers
Results and discussion
The as-prepared film has silicon content of 58.9 at% as analyzed by EDS. Fig. 1 shows the Raman spectrum of the sample after annealing in N2. For a comparison, the spectrum of the as-deposited sample is also presented. The inset shows the detail of the spectrum from the annealed sample. For the as-deposited sample, two broad Raman emission bands can be seen with the center located at 150 and 470 cm−1, respectively, which are originated from amorphous silicon. After annealing, a narrower and
Conclusions
Silicon nanocrystal and interface defects related photoluminescence in silicon-rich Al2O3 films have been reported in this study by annealing the samples in N2 ambient at a high temperature. Raman, IR and UV-absorption spectra have confirmed the formation of silicon nanocrystals. Two typical PL bands of silicon nanocrystals have been observed at 1.54 (IR band) and 1.69 eV (R band). Post-oxidation process has been carried out to clarify the emission mechanism of the sample. Despite the redshift
Acknowledgment
Project 50571050 supported by the National Natural Science Foundation of China.
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