This study presents the crystalline and luminescence properties of silicon-rich oxide (SRO)/SiO2 superlattices in which the SRO layers were prepared with a low-energy (<60 eV) argon ion-beam treatment. Experimental results evidenced that density of the Si nanocrystals (NCs) in the SRO layer was increased by ion-beam treatment after annealing, increasing the surface roughness. The stoichiometry of the as-prepared SRO layer was unchanged but the phase separation of the annealed SRO layer was enhanced by the ion-beam treatment, yielding visible white photoluminescence from the E’ centers and Si NCs.
© 2013 OSA
Silicon-rich oxide (SRO) or nitride (SRN) films in which are embedded silicon nanocrystals (Si NCs) have been demonstrated to exhibit visible luminescence and to be integratable with Si technology, with great potential as next-generation solid-state lighting sources with large area scalability and low production cost . Light-emitting diodes (LEDs) that are based on SRN films have a low operating voltage and can cover all visible wavelengths, owing to their low potential barrier and the low density of interfacial states between the SRN and Si NCs [2,3]. White-light LEDs have been obtained by stacking several SRN or SRN/SRO layers with differently sized Si-NCs . However, the quantum efficiency of such SRN-based LEDs is low, owing to insufficient quantum confinement and low carrier retention . Therefore, recent research into Si NCs focused on SRO-based devices for lighting purposes.
Although the free-standing Si NCs that were made by electrochemical methods showed high quantum efficiency and tunable band gap , they are difficult to integrate with the thin-film technologies. The application of them was focused on passive light-converting materials. As for the case of thin films, SRO films with embedded Si NCs have been fabricated by the oxidation of Si nanostructures, the annealing of Si-ion-implanted SiO2, and the annealing of vacuum-deposited SRO films [7–9]. Implanting Si ions into SiO2 generates separate blue-green and red-orange photoluminescence (PL) bands, whose intensity depends on the amount of excess Si and the annealing time. White PL has been observed by co-implanting high doses Si or C ions into SiO2 and post-annealing . Emission from both oxygen-related defects (blue-green) and quantum-confined Si NCs (red-infrared) suggests a way to achieve white-light emission. However, implantation induces many defects and the corresponding spectra typically include up to three bands. The high color rendering reduces luminous efficiency. SRO films that are fabricated by common thin-film deposition technique like plasma-enhanced chemical vapor deposition generally exhibit sharp red-orange luminescence. However, the blue-green luminescence of such films is weak because of a lack of radiative recombination centers and the inhibition of size-dependent band-gap widening by highly localized interfacial states . Earlier, sputtering has been used to synthesis the SRO films. Broad spectrum was obtained. Besides, ion-beam assisted deposition has been attempted to synthesize the SRO films. Unfortunately, the phase separation of the SRO films was reduced because of the preferring bombardment of Si .
In this investigation, an ion-beam was used to irradiate simultaneously on the sputtered SRO films that were inserted in SiO2/SRO superlattices. Contrary to previous report, the use of low-energy (<60eV) ion-beam was found to enhance the phase separation of amorphous silicon suboixde films. Density and crystallinity of Si NCs were increased. Blue emission from E’ centers and red-yellow emission from the Si NCs of the ion-beam treated SRO films contributed to strong white PL that was observable by the naked eye at room-temperature, making this process potential to fabricate Si NCs-based lighting sources.
Figure 1 illustrates the simplified deposition system. An end-hole gridless ion source with a plasma bridge neutralizer and two sputtering guns were used to prepare SRO/SiO2 supperlattices. The anode voltage of the Ar ion source was varied from 0 to 60 V, yielding an ion energy of 0-60 eV. Metal plates were used to shield the interaction between the ion beam and other plasma sources (Si, SiO2). The Ar ion-beam irradiated on the SRO layer as the substrate rotated. The ion beam affected mainly the surface reactions of the SRO films rather than the plasma chemistry. Twenty-period SRO/SiO2 superlattices were deposited on p-type (100) Si wafer by repeating the following procedure: a 2 nm-thick SiO2 layer was firstly deposited by the rf-sputtering of an SiO2 target; then a 4 nm-thick SRO layer was deposited by the dc-sputtering of a pure Si target, and the Ar ion-beam was then irradiated on the SRO surface by rotating the substrate. The Ar (20 sccm) gas was introduced into the chamber during deposition. Post-annealing was performed at 1000 °C for 180 min in a furnace with flowing forming gas (95%N2 + 5% H2). X-ray photoelectron spectroscopy (XPS), cross-sectional high-resolution transmission-electron microscopy (HR-TEM, JEOL/JEM-2100) and grazing-angle X-ray diffraction (GIXRD, Siemens/D5000) were used to investigate the films. Atomic-force microscopy was used to observe the surface morphology. An He-Cd laser (325 nm) with a power of ~40 mW was used for PL measurement.
3. Results and discussion
Control of the phase separation of thin-film SiOx (x<2) into nanocrystalline Si and SiO2 is known to strongly depend on thermal annealing . Figure 2 displays the HR-TEM images of the annealed SRO films. The bright-field images [Figs. 2(a) and 2(b)] demonstrate that interface of the SRO/SiO2 superlattices was abrupt and all SRO layers had the same thickness. It indicated that the sputtering and densification effects of the ion-beam on the growing films could be ignored. Figures 2(c) and 2(d) show the dark field HRTEM images. Amount of white spots was increased by the ion-beam treatment, revealing an increase in the density of Si NCs (white spots) in the SRO film.
TEM images usually present very local information in nano-meter scale. Therefore, XRD was performed to further determine the crystalline behaviors of Si NCs. Figure 3 presents the GIXRD patterns of the SRO/SiO2 multilayers in which the SRO layers were treated using various ion energies. Ion-beam treatment of the SRO layer markedly increased the XRD intensity, indicating an increase in Si content. Moreover, the full-width at half maximum of XRD peaks increased with ion-beam energy, showing a decrease in grain size according to Sherrer’s law. Figure 4(a)-4(d) show the AFM images. It was found that the ion-beam did not change the surface roughness of the as-prepared sample, but it did increase that of the annealed samples, and this effect was evidently associated with the formation of Si NCs . Accordingly, the nanostructural observations implied that the ion-beam enhanced the phase separation of SRO films after annealing.
Figure 5 displays the Si 2p XPS spectra of the annealed and as-prepared SRO layers that were treated using various ion energies. Quantitative analysis revealed that the stoichiometry of the as-prepared SRO films(x in SiOx) was unchanged by the ion-beam because the sputtering yields (Y(E)) of the oxygen and silicon atoms from the Ar ion sources were similar, according to the equation:15]. The spectra of annealed SRO films that were treated by ion-beam yielded a shoulder emerged in the high binding energy side (~104 eV). The appearance of the high binding energy feature was associated with the increases in both the Si0+ and Si4+ contents, which were determined by fitting the XPS profiles by five Gaussian functions (Si0+, Si1+, Si2+, Si3+, Si4+) (Table 1). The phase separation of SRO layer was greatly enhanced by the low-energy ion-beam treatment. This finding contradicted the studies of Kim , in which the ion-beam caused a preferential sputtering of Si, which inhibited the phase separation. Similarly, Lee used ion-beam with 200-600 eV to assist depositing SiOx films . Only improvements in the optical properties and film density were obtained. It was believed that the re-sputtering effect occurred when a higher ion-beam energy was used (>1000 eV) . Therefore, the ion energy was responsible for the different observations of how the ion beam influenced the SRO films. From a thermodynamic perspective, the formation of the Si NCs within the SRO films was a result of a demixing process of Si and O atoms. Because the bonding energy of Si-Si was smaller than that of Si-O or Si = O , the ion-beam possibly broke the Si-Si bonds that reduced the size of the Si clusters or introduced dangling bonds, increasing the total free energy that drove the decomposition of the SRO layer. Based on the XPS results, models of atomic arrangement of Si NCs were represented in Figs. 6(a) and 6(b). The Si NCs that were prepared by ion-beam treatment were surrounded by SiO2; while that deposited by sputtering were surrounded by Si1+, Si2+, and Si 3+ atoms.
Figure 7(a) shows the PL spectra. The PL peak in the vicinity of ~750 nm was slightly blue-shifted by ion-beam treatment, also suggesting a reduction of the Si cluster size by the ion-beam. Additionally, this peak was further blue-shifted when the sample was annealed in an oxygen ambient, confirming that this emission was associated with the radiative recombination of Si NCs. The PL spectrum of the as-prepared SRO film that was treated using ion-beam was resolved by three Gaussian functions that were centered at 410, 450, and 510 nm, representing the weak oxygen bond (WOB), the neutral oxygen vacancy (NOV), and the E’ center, respectively . The diamagnetic NOV center at 2.7 eV (~450 nm) dominated the luminescence of the SRO film that was not treated by ion-beam and annealing. On the contrary, strong luminescence from E’ centers dominated the PL of the ion-beam-treated SRO sample. In both cases, after annealing most of the NOV defects had been transferred to the E’ centers. This phenomenon was associated with hole trapping . The presence of the E’ centers that were enhanced by ion-beam treatment was further confirmed by the electron paramagnetic resonance (EPR) signals around 3498 Gauss (Fig. 8). Very weak EPR signal was obtained for SRO film that was simply prepared by sputtering. Figure 7(b) points the position of the spectra in the CIE chromaticity diagram. The ion-beam treatment increased the color temperature of the SRO from 3021 to 5204 K. Additionally, visible electroluminescence with little Stark shift was obtained, and the PL quantum yield was roughly around 2%, suggesting that the low-energy ion-beam-assisted sputtering process has the potential to make Si-based lighting devices. Table 1 summarizes the changes in binding energies and roughness of the SRO films to which were applied annealing and ion-beams.
In summary, a low-energy ion-beam was used to assist the sputtering of SRO films. The density of Si NCs in SRO films was thus increased, increasing their surface roughness. Quantitative analysis showed that the ion-beams did not change the stoichiometry but did enhance the phase separation of the SRO films. Finally, low-energy ion-beam treatment increased the PL intensity and color temperature of SRO/SiO2 superlattices by promoting luminescence from E’ centers. White photoluminescence was thus obtained. This finding will be helpful for development of Si-based light emitters.
The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC100-2221-E-006-130-MY2.
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