Low-temperature fabrication of layered self-organized Ge clusters by RF-sputtering
© Pinto et al; licensee Springer. 2011
Received: 3 November 2010
Accepted: 14 April 2011
Published: 14 April 2011
In this article, we present an investigation of (Ge + SiO2)/SiO2 multilayers deposited by magnetron sputtering and subsequently annealed at different temperatures. The structural properties were investigated by transmission electron microscopy, grazing incidence small angles X-ray scattering, Rutherford backscattering spectrometry, Raman, and X-ray photoelectron spectroscopies. We show a formation of self-assembled Ge clusters during the deposition at 250°C. The clusters are ordered in a three-dimensional lattice, and they have very small sizes (about 3 nm) and narrow size distribution. The crystallization of the clusters was achieved at annealing temperature of 700°C.
Semiconductor nanocrystals (NCs) have shown a big potential for application in flash memory devices . Most quantum dot (QD) flash memory research studies have used Si NCs in floating gate. However, several groups have proposed systems using Ge dots  instead of Si dots. The band gap of Ge provides both a higher confinement barrier for retention mode and a smaller barrier for program and erase mode. This makes Ge dots a strong candidate for floating gates.
However, the fabrication of Ge dots on insulators is much more difficult to obtain than Si dots because of the low evaporation temperature of Ge and the difference in surface energy with respect to the oxide. Si1- x Ge x can offer an intermediate solution to this issue. In fact, embedding silicon or silicon germanium (SiGe) dots in an insulator structure has been proposed for non-volatile memory devices [3–6]. Magnetron sputtering has been proven to be a useful, cheap, and easy technique with less energy consuming, for the fabrication of Si, Ge, and Si1- x Ge x NCs embedded in SiO2 films [7, 8].
The most challenging part in the production of nanoclusters for potential applications is the control over their size and arrangement properties. Earlier studies have reported layered Ge NCs produced at temperatures of 500°C and higher [9, 10]. However, the nanoclusters formed were not regularly ordered. Recently, it has been reported of a possibility to grow self-assembled NCs in amorphous silica matrix [11, 12]. However, the ordering was only found for a single deposition temperature, and it was performed only for Ge nanoclusters. The control of ordering of the particles is important because the spatial regularity implies narrowing of the QDs size distribution, which is very important for the collective behavior effects and consequently for potential applications of the system.
The complete crystallization of the NCs was achieved at temperatures of 800°C and higher [8, 13, 14]. In this article, we report the formation of self-assembled Ge nanoclusters by the magnetron sputtering technique at quite a low deposition temperature of 250°C. The nanoclusters formed are very small in size (about 3 nm), and well ordered in a three-dimensional FCC-like nanocluster lattice. The parameters of the nanocluster lattice formed are precisely determined using grazing incidence small angle X-ray scattering (GISAXS) and high-resolution transmission electron microscopy (HRTEM) techniques, while their crystalline quality and chemical composition are examined using Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The mutual distances of the nanoclusters are found to be very small (distance of about 3 nm between the nanocluster edges), while their size distribution is found to be very narrow. These properties make this material very suitable for different nano-based applications.
SiO2/Si1- x Ge x + SiO2/SiO2 multilayers films containing 20 bi-layers were prepared on Si (100) substrates using RF magnetron co-sputtering machine Alcatel SCM650. The structures were grown using a composite target, a SiO2 (99.99%) plate partially covered by polycrystalline chips of Si and Ge, and a second target of pure SiO2. The surface ratio of the Si and Ge pieces in the SiO2 target was 2:1. Before sputtering, a pressure of at least 1 × 10-6 mbar was reached inside the chamber. Substrate and targets were subjected to in situ argon plasma treatment to clean the surfaces and remove any impurities. The layers were grown at 250°C, and the argon pressures were 1 × 10-2 and 1 × 10-3 mbar, for the pure target and the composite target, respectively. The thickness of both types of layers was controlled by the deposition time. The deposition rates were found to be 7.4 and 7.8 nm/min, for SiO2 and SiGe + SiO2 layers, respectively. The thicknesses of SiGe + SiO2 and SiO2 layers are 2 and 5 nm, respectively. A top SiO2 layer was deposited to prevent the diffusion of Ge atoms out of the surface. The samples were subsequently thermally annealed at temperatures between 700 and 1000°C, in N2 atmosphere for 1 h.
Rutherford backscattering spectrometry (RBS) measurements were performed with a 2-MeV 4He+ ion beam impinging on the target at grazing angles of 78°, 80°, and 82° to obtain sufficiently high depth resolution to separate the signals arising from the different layers, and to detect and investigate possible compositional changes.
Conventional TEM and high-resolution TEM images were acquired with a Tecnai F30 FEG-TEM microscope operating at 300 kV. TEM cross-sectional samples were produced by mechanical polishing followed by ion beam milling to have sufficiently large electron transparent areas. GISAXS measurements were performed at the SAXS beamline of the Elettra synchrotron, using monochromatic radiation with wavelength 0.154 nm, and several grazing incidence angles slightly above the critical angle of total external reflection. The incidence direction of the X-ray radiation was along the x axis, perpendicular to the detector (y-z) plane. Data were measured by a two-dimensional (1024 × 1024 pixel) CCD detector, with a sample-detector distance of approx. 1.72 m. A thin Al-stripe (beam stopper) was inserted in front of the 2D detector to attenuate the very intense specular beam (reflected beam, Yoneda peak, etc.) and thus avoid the overflow of the detector, and increase the sensitivity for scattered signal outside the specular plane. Raman scattering spectra were recorded using a Jobin-Yvon T64000 system with an optical microanalysis system and a CCD detector, in the backscattering geometry. These measurements were performed at room temperature using the 488 nm line of an argon ion laser. The laser beam was focused on the sample surface with a beam spot size of 1 μm and a power of 0.2 mW to avoid the heating of the sample. XPS were measured using a Thermo Scientific K-Alpha ESCA instrument equipped with aluminum Ka1.2 monochromatized radiation at 1486.6 eV X-ray source.
Results and discussion
In the GISAXS map of the film annealed at 700°C, a rearrangement of the Bragg spots' positions is visible. From the new arrangement, it follows that the clusters are not any more correlated in the vertical direction, while the correlation of lateral clusters still exists. The results of the numerical analysis show formation of NCs which are larger than in as-deposited multilayer (R = 2.5 ± 0.3 nm), with larger mutual distance (L = 17.8 ± 0.3 nm) and significantly larger vertical disorder parameter (σV = 1.6 ± 0.1 nm). The in-layer disorder is also larger than for the as-deposited case (σL = 9.1 ± 0.1 nm), but the separation L is also larger. Growth of QDs during the annealing treatment causes the destruction of the vertical dot correlation. Initially regularly ordered QDs coalesce, thereby changing their lateral positions. The size distribution is still relatively narrow, but broader than in the as-deposited film case. Annealing at 800°C causes a further growth of QDs (R = 3.8 ± 0.5 nm), and a further decrease of the regularity in the QD positions. For this film (Figure 3c), no Bragg spots are visible in the GISAXS intensity distribution. The size distribution, shown in Figure 4, is found to be very broad in this film.
Contrary to the general tendency observed in the literature concerning the growth of NCs, we have shown the possibility to grow the self-assembled nanoclusters at low temperature (250°C). Low-cost process will be explored further to obtain well-separated crystalline NCs.
In this study, we have shown formation of self-organized Ge nanoclusters at low temperature (250°C) in amorphous silica matrix by the magnetron sputtering technique. The size distribution of the clusters formed is found to be very narrow because of the self-ordering growth. The annealing of those films caused the formation of crystalline Ge clusters with larger sizes. Furthermore, the regular spatial arrangement of clusters has undergone changes by the annealing treatment. RBS results show that annealing at 800 and 1000°C promote the out-diffusion from the surface of Ge atoms.
grazing incidence small angle X-ray scattering
high-resolution transmission electron microscopy
Rutherford backscattering spectrometry
X-ray photoelectron spectroscopy.
This study has been partially funded by: (i) FEDER funds through the COMPETE program "Programa Operacional Factores de Competitividade and by Portuguese funds through Portuguese Foundation for Science and Technology (FCT) in the frame of the Project PTDC/FIS/70194/2006; (ii) Bilateral Cooperation Program BC/CRUP - B 26/08 financed by the British Council and the Council of the Portuguese Rectores,; (iii) ELETTRA Synchrotron Radiation Center through the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement no 226716; (iv) European COST MP0901-NanoTP Action; (v) Scientific and Technological Cooperation Program between Portugal (FCT) and Morocco (CNRST)-2010/2011. S.R.C.P. is grateful for financial support through the FCT grant SFRH/BD/29657/2006. M.B. acknowledges the support from the Croatian Ministry of Science Higher Education and Sport (project number 098-0982886-2866).
The authors thank Dra Carmen Serra from C.A.C.T.I. of University of Vigo in Spain for the assistance of XPS measurementsDr. Rosário Correia from Physics Department of University of Aveiro in Portugal and Dr. M. Ivanda from Rudjer Boskovic Institute, Zagreb in Croatia for Raman discussions.
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