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Formation of Ge-nanocrystals in SiO2 matrix by magnetron sputtering and post-deposition thermal treatment
Superlattices and Microstructures 44 (2008) 323–330 www.elsevier.com/locate/superlattices
Formation of Ge-nanocrystals in SiO2 matrix by magnetron sputtering and post-deposition thermal treatment U.V. Desnica a,∗ , K. Salamon b , M. Buljan a , P. Dubcek a , N. Radic a , I.D. Desnica-Frankovic a , Z. Siketic a , I. Bogdanovic-Radovic a , M. Ivanda a , S. Bernstorff c a R. Boskovic Institute, Physics Department, Bijenicka 54, HR-10000 Zagreb, Croatia b Institute of Physics, Bijenicka 56, HR-10000 Zagreb, Croatia c Sincrotrone Trieste, SS 14 km 163.5, 34012 Basovizza (TS), Italy
Available online 4 March 2008
Abstract Germanium Quantum Dots (Ge QDs) were formed in SiO2 by RT magnetron sputtering co-deposition of Ge and SiO2 and subsequent annealing. Films were deposited in the form of alternating (Ge + SiO2 ) layers (40:60 molar ratio) and pure SiO2 layers, serving as spacers. Grazing incidence small angle x-ray scattering (GISAXS) was applied for structural characterization of the QDs synthesized in the SiO2 amorphous matrix. The chemical composition and phase of the QDs were determined by Raman spectroscopy, and the spatial distribution and concentration of the Ge atoms by Rutherford Backscattering. The 2D GISAXS patterns, besides giving information on the layered structure, were used to reveal the onset of the synthesis of Ge QDs in SiO2 and to determine the average size and shape of QDs. It has been shown that the insertion of spacer SiO2 layers between (Ge + SiO2 ) layers transforms the 3D growth of Ge QDs into a preferentially 2D growth, within each 7 nm thick (Ge + SiO2 ) layer. This resulted in a considerably smaller average size of Ge QDs in the layered films. The synthesis of well crystallized, moderately sized, spherical Ge QDs was achieved by post-deposition annealing in the 700–800 ◦ C range. c 2008 Elsevier Ltd. All rights reserved.
Keywords: Quantum dots; Nanocrystals; GISAXS; RBS; Raman; Magnetron sputtering
∗ Corresponding address: R. Boˇskovi´c Institute, P.O. Box 180, HR-10000 Zagreb, Croatia. Tel.: +385 1 4561173; fax: +385 1 4680114. E-mail address: [email protected] (U.V. Desnica).
c 2008 Elsevier Ltd. All rights reserved. 0749-6036/$ - see front matter doi:10.1016/j.spmi.2008.01.021
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1. Introduction Semiconductor materials in the form of quantum dots (QDs) display a significant dependence of electronic, optical and other properties on the size of the nanoparticles [1–5]. Specifically, Ge quantum dots (Ge QDs) embedded in a transparent matrix exhibit intense photo- and electro-luminescence, strong third-order optical nonlinearities and tunable absorption, which are particularly strongly dependent on the nanoparticle size [6]. This enables the tunability of the nano-Ge band-gap over a considerable range of the visible light wavelengths. These physical properties of nano-Ge material makes it suitable for many electronic, optoelectronic and photonic applications, including integrated opto-couplers in micro-systems in biotechnology, etc [1]. Furthermore, Ge QDs are prime candidates for electronic nonvolatile memories, overcoming the limitations of current technologies by enabling higher endurance and lower operating voltages. Ge nanocrystals embedded in SiO2 can serve as light emitters exhibiting bright blue–violet electro-luminescence, favoring their application in Si-integrated sensor technology. Such materials are also ideal for nonvolatile high-speed memory applications, due to their excellent charging and retention time [5]. In thick films, including those obtained by ion implantation [6,7] the diffusion of Ge atoms at elevated annealing temperatures Ta occurs in all 3 dimensions, leading to relatively large QDs through Oswald ripening. In this paper this problem is addressed through the deposition of multilayered thin films containing alternating (Ge + SiO2 ) and SiO2 layers. The intention was that the ‘spacer’ SiO2 layer, if sufficiently thick, prevents Ge atoms from other (Ge+SiO2 ) layers from participating in the growth of GeQDs in a given (Ge + SiO2 ) layer, thus resulting in smaller QDs. 2. Experimental details Ge QDs were formed in SiO2 by magnetron sputtering co-deposition, either in the form of thick films (400 nm thick) or as multilayered films containing 20 bi-layers. The substrate was either SiO2 or h111i Si. Each bi-layer consisted of a layer of co-sputtered mixture of 40% mol Ge and 60% mol SiO2 (‘active layer’), and a layer of pure SiO2 , serving as a spacer between ‘active’ layers. The deposition temperature, Td , ranged from room temperature to 700 ◦ C. A multi-source magnetron sputtering KJLC CMS-18 system was used for the depositions. Pure Ge and pure SiO2 (99.995%) were used as targets in DC (14 W) and RF (250 W) operated magnetrons, respectively. The as-deposited samples were subsequently thermally annealed for one hour in vacuum up to Ta = 1000 ◦ C. Rutherford Backscattering (RBS) measurements were done with a 6 MV EN Tandem Van de Graaff accelerator in Zagreb, using 1.5 MeV 4 He ions, which were extracted from the Alphatross ion source. The accelerated alphas were directed normally to the target surface. Spectra were collected using the charge particle detector positioned at 165◦ relative to the beam direction. GISAXS experiments were carried out using X-ray photons of energy E = 8 keV (wavelength, λ = 0.154 nm) at the Austrian SAXS beamline of the synchrotron radiation facility ELETTRA, Trieste, Italy [9]. Two-dimensional GISAXS patterns were recorded with a 2D CCD detector having 1024 × 1024 pixels, placed in the y–z plane, perpendicularly to the specular x–z plane [8]. The patterns were first corrected for background intensity and detector response, and then for refraction and absorption effects [10]. In Raman spectroscopy the first order, dipoleallowed Raman spectra were obtained at room temperature (RT) by excitation with the 2.57 eV (514.5 nm) line from an Ar-ion laser, with low laser power to avoid heating. The scattered
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Fig. 1. RBS spectra of samples with (Ge+SiO2 ) magnetron sputtering deposited films on Si substrate. The temperatures of deposition (Td ) and of post-deposition annealing (Ta ) are indicated in the figure in ◦ C. (a): Whole RBS spectra for as-deposited samples at RT and at 700 ◦ C, with the other deposition parameters kept constant. (b): Ge-related part of the RBS spectra after post-deposition annealing at different Ta .
light was filtered with a triple spectrometer (DILOR Z-24) and Raman spectra were taken in the 190–550 cm−1 range, with 2 cm−1 steps. 3. Results and discussion 3.1. RBS most important results To determine the success of co-deposition of (Ge + SiO2 ) films, as well as to determine the stability limits of Ge atoms in SiO2 matrix to post-deposition thermal treatments, RBS spectroscopy was applied. Examples of the results for thick (Ge + SiO2 ) films on h111i Si substrate, for two deposition temperatures and for post/deposition treatments at several annealing temperatures, Ta , are shown on Fig. 1. Fig. 1a depicts the whole spectra for the as-deposited samples, while Fig. 1b shows in more detail the higher energy part of the RBS spectra belonging to Ge, for as-deposited and for several annealed samples. All spectra were normalized to the same number of incident particles for comparison. The quantitative assessment of the concentrations of the incorporated Ge atoms in different samples was calculated using the SIMNRA program [11]. The error in the Ge-concentration determined in this way was estimated to be +5%.
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The main findings from the RBS measurements can be summarized as following: (a) For RT deposition almost all the magnetron sputtered Ge and SiO2 material was successfully deposited on the Si substrate: The quantity of actually deposited Ge atoms corresponds to the calculated quantity of Ge in the films (9.3 × 1014 /cm2 in each nm of 40%Ge + 60%SiO2 film) within 10%. (b) After post-deposition thermal processing up to Ta = 700 ◦ C no visible change of the position of the Ge atoms in the deposited film occurs and apparently there is no loss of Ge atoms from the sample (i.e. there is no perceivable loss of Ge, which would exceed the inaccuracy of the measurement). (c) For Ta = 800 ◦ C, a loss of about 10% of the Ge atoms is observed. It seems that those lost Ge atoms out-diffused from the sample — there is no in-diffusion of Ge atoms into the substrate. It appears that the Ge out-diffusion is partly counterbalanced by trapping of Ge atoms at/close to the surface: there is a dip in the Ge distribution close to the surface, with some pile-up of Ge atoms at/close to the sample surface, which decelerates the escape of Ge atoms. (d) The diffusion and loss of Ge becomes strong for Ta = 1000 ◦ C. Now more than 30% of the deposited Ge atoms are lost from the layer. Part of the Ge out-diffused from the surface, but also a fraction of the Ge atoms in-diffused into the Si substrate, creating an apparently thicker deposited layer. Also a very small fraction of the Ge atoms in-diffused much deeper into the substrate, probably diffusing faster along some linear defect lines in the Si substrate, like dislocations, etc. All of these effects indicate that at about Ta = 800 ◦ C the diffusion of Ge in SiO2 starts to become significant. For Ta = 1000 ◦ C both in-diffusion into the Si substrate and outdiffusion from the surface are very strong, up to a level which significantly changes the initial composition of the (Ge + SiO2 ) film, both regarding the fraction of Ge atoms and their depth distribution. (e) Films deposited at Td = 700 ◦ C, with the other parameters nominally the same as for the Td = RT films, had dramatically lower deposition rates: (Ge + SiO2 ) films are less than half as thick as RT deposited films. When comparing the integral Ge signal intensities, only about 10% of the Ge atoms ended up in the deposited layer in comparison with RT deposition. Hence these films were not studied further. (f) Films deposited as (Ge + SiO2 )/SiO2 bi-layers, each (7 + 7) nm thick, have RBS spectra (not shown) very similar to the ones of thick films. Since the depth resolution of a RBS measurement (≈20 nm) is larger than the thickness of bi-layers the depth structure regarding the Ge concentration cannot be resolved. RBS spectra of the bi-layered films as a whole follow generally the annealing dynamics of the thick films: small changes of the Ge atom position and a weak out-diffusion become visible above Ta = 800 ◦ C, and are becoming significant above Ta = 1000 ◦ C. The dynamics of in- and out-diffusion of the Ge atoms from bi-layered films, either for Td = RT or Td = 700 ◦ C, is very similar to the one from thick films (including a specific depth distributions of Ge atoms within the deposited films, with a characteristic, Ta -dependent, ‘dip’ in depth distributions). The most important practical implication of the RBS results is that the Ge atoms remain within the deposited films up to 700 ◦ C, and that out-diffusion becomes considerable above Ta = 800 ◦ C. RBS gives depth-distributions and concentrations of Ge atoms. However, to find out when/whether they are agglomerated into nanoparticles and of which size other methods are needed.
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Fig. 2. 2D GISAXS patterns from magnetron deposited (Ge + SiO2 ) layers at RT on h111i Si substrate, prior to and after annealing at various temperatures, Ta , indicated in each figure. (A) (first column): Thick (Ge + SiO2 ) film. (B) (2. and 3. pattern in 1. row): Bi − layeredfilms; thicknesses of each of the (Ge + SiO2 ) and SiO2 layers being 7 nm and 7 nm respectively. (C) (2. and 3. pattern in 2. row): Same as (B), but thicknesses of each of the (Ge + SiO2 ) and SiO2 layers being 7 nm and 3.5 nm, respectively. (D) 1D GISAXS profiles for samples annealed at 800 ◦ C for different thickness of the SiO2 layer serving as a spacer between (Ge + SiO2 ) layers: (a) dspacer = 0, (b) dspacer = 3.5 nm, (c) dspacer = 7 nm.
3.2. GISAXS most relevant results Fig. 2 shows 2D GISAXS patterns from (Ge + SiO2 ) films deposited on Si substrate. The grazing incidence angle of the X-ray beam, α, was α = αc + 0.05◦ (which corresponds to a penetration depth of about 200 nm), where αc denotes the critical angle for the total external reflection. The 2D patterns exemplify maps of scattering intensities in reciprocal q space (q = q y + qz is the wave vector, where q y = (2π/λ) sin(2ϕ) and qz = (2π/λ) sin(2β); 2β and 2ϕ being the scattering angles in plane and out of plane of incidence, respectively). In order to block strong surface signals (reflected beam, Yoneda peak, etc.) in/close to the specular plane, a
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Table 1 The role of spacer layer on the size of Ge quantum dots formed in SiO2 matrix Thickness of the spacer layer
0 (thick film) 3.5 nm 7 nm
Substrate SiO2 Radius (nm)
Si
5.9 ± 0.2 4.1 ± 0.3 3.9 ± 0.4
6.1 ± 0.2 4.1 ± 0.3 3.8 ± 0.4
Average radius of Ge Quantum Dots (in nm) as a function of the thickness of the spacer layers of pure SiO2 deposited between the 7 nm thick (Ge + SiO2 ) layers. All samples were deposited at RT on either Si or SiO2 substrate and subsequently annealed at Ta = 800 ◦ C. Except of the thickness of spacer layers all other parameters of the deposition and annealing were the same for all samples.
partly transparent beam stopper was inserted, allowing a better sensitivity for the weak diffuse scattering at larger q. Group A of the GISAXS pattern in Fig. 2A refers to thick films (no spacer layer) deposited at RT, and subsequently annealed at the temperature Ta indicated in each pattern. For the as-deposited sample (Ta = RT ) the GISAXS signal comprises practically the entire intense region close to the center, decaying very fast for larger q y . This signal comes primarily from the surface, which is not of particular interest in this paper. After post-deposition annealing at Ta = 500 ◦ C and above, an additional scattering signal at larger q becomes visible, indicating the formation of QDs. The regions in the 2D maps having the same scattering intensities have apparently circular form, indicating that the QDs are approximately spherical [8,12]. With further annealing (higher Ta ) the intensity of the particle-related signal decreases faster with q, indicating an increase of the size of the QDs. Fig. 2. also shows the 2D profiles of two types of (Ge+SiO2 )/SiO2 bi-layered films deposited at RT. In both films the (Ge + SiO2 ) layers were 7 nm thick, while the SiO2 spacer layers were either 7 nm (group B pattern) or 3.5 nm (group C pattern) thick. The pattern of RT deposited bi-layered films (group B and C) are characterized by “Bragg sheets” [13]. Their off-specular intensity indicates a vertical correlation of the layers/interface roughness, indicating a highly conformal layered structure of the film, while QDs are not yet formed. Hence, the shape of the 2D GISAXS pattern is dominated by the bi-layered structure, with the distance between Bragg sheets being determined by the thickness of the bi-layers (14 nm and 10.5 nm, respectively). Annealing of as-deposited films at Ta = 800 ◦ C, (right panels in B and C) resulted in the appearance of an additional signal, belonging to the formed nanoparticles, but the remnants of the signal belonging to the layered structure is still clearly resolvable, which are better visible in Fig. 2D (at 0.4 and 0.9 qz2 , for 7 nm and 3.5 nm spacer thicknesses, respectively). Fig. 2D shows one-dimensional (1D) GISAXS profiles of samples annealed at 800 ◦ C, whose 2D patterns are shown in Fig. 2A, B and C. The 1D GISAXS plots were obtained by cross-sectioning the 2D pattern parallel to the z-axis close to the beam-stopper. When the traditional particle scattering model (Guinier approximation) is applied in the analysis of such 1D GISAXS profiles, the average cluster radius, R, is obtained from the radius of gyration, Rg , (R = (5/3)0.5 ∗ Rg ) computed from the slope of the linear region of the lnI(q) versus q 2 dependence [8,12]. There is no doubt that the insertion of the SiO2 spacer indeed resulted in a dramatic reduction of the QDs size (steeper slope of lnI (q) versus q 2 dependence), as intended. Numerical results for the average QD radii are given in Table 1, obtained in films deposited either on Si or SiO2 substrate. Already spacers as thin as 3.5 nm, reduce the average radii by ca 50%; from 6.1 ± 0.2 nm to 4.1 ± 0.3 nm. Doubling the thickness of the spacer layer from 3.5 to 7 nm results in just a small further decrease of R by less
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Fig. 3. Raman spectra of (Ge + SiO2 ) films deposited at Td = RT on SiO2 substrate, after different post- deposition annealings at the temperatures indicated in the figure.
than 10%. Hence, one can conclude that already a 3.5 nm spacer prevents a large majority of the Ge atoms from neighboring (Ge + SiO2 ) layers from participating in the formation of GeQDs in any given (Ge + SiO2 ) layer. For a thicker spacer layer (7 nm) the diameter of the Ge QDs is very close to the thickness of the (Ge + SiO2 ) layer. It seems safe to conclude that Ge QDs in these (Ge + SiO2 ) layers are practically exclusively formed from Ge atoms deposited in the same layer. As expected, the choice of the substrate (indicated as Si or SiO2 in Table 1) played only a minimal or no role. The growth of spherical QDs in parallel layers separated by SiO2 is confirmed for these samples also by cross-sectional Transmission Electron Microscopy (not shown). For a very narrow range of deposition parameters [14] the formation of Ge QDs in a 3D superlattice can be obtained. 3.3. Raman spectroscopy main results Representative Raman spectra are presented in Fig. 3 for as deposited thick films as well as for the same films after annealing at several different Ta . Already in the as-deposited samples the Ge atoms are distributed into the amorphous network, as revealed by a broad band centered at 275 cm−1 . These clusters have to be very small since they are not observed in GISAXS pattern. Up to at least Ta = 500 ◦ C the Ge QDs remain fully amorphous. After annealing at Ta = 700 ◦ C or higher the a-Ge agglomerates crystallize into crystalline Ge QDs (TO c-Ge line at 300 cm−1 ). This line is somewhat broadened due to confinement effects, related to the nanometric size of the QDs. Ge-related Raman features are superimposed on the substrate signal, a broad band related to the amorphous SiO2 matrix. The Raman spectra also indicate that the quality of the deposited SiO2 improves with thermal annealing. Upon annealing at Ta = 700 ◦ C, the SiO2 part of the spectra became very similar to the spectrum of standard SiO2 substrate. 4. Conclusions Ge nanocrystals were formed in SiO2 after magnetron co-sputtering of Ge and SiO2 . The deposited films were either thick films containing 40%mol Ge and 60%mol SiO2 , or in the form of bi-layers, each bi-layer consisting of a 7 nm thick layer of the same (Ge + SiO2 ) mixture and a layer of pure SiO2 (either 3.5 or 7 nm-thick), serving as a spacer. Sputtering was performed
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either at room temperature (RT) or at 700 ◦ C. As-deposited samples were subsequently thermally annealed up to Ta = 1000 ◦ C. RBS revealed successful (Ge + SiO2 ) deposition at RT . In contrast, the deposition temperature Td = 700 ◦ C yielded only about 10% of Ge and about 40% of SiO2 deposited, in comparison with RT deposition. The 2D GISAXS patterns, besides detecting the bi-layered structure, revealed the formation of nanoparticles (QDs) in the deposited films after post-deposition annealing at 500 ◦ C. Raman spectroscopy found that these QDs are amorphous Ge aggregates, which crystallize into Ge-QDs after annealing at Ta = 700 ◦ C or higher. The insertion of 7 nm of pure SiO2 spacers between each (Ge + SiO2 ) layer during deposition hinders the interaction of Ge atoms from different (Ge + SiO2 ) layers, transforming 3D growth of Ge QDs from all directions occurring in thick films into preferentially 2D, in-layer, growth, which results in much smaller Ge QDs. Even if spacers are only 3.5 nm thick, the Ge QDs in each particular (Ge + SiO2 ) layer are formed mostly (cca 90%) from the Ge atoms that originated from the same (Ge + SiO2 ) layer. The best Ta ’s for the formation of moderately sized, spherical, well crystallized Ge QDs were found to be in the Ta = 700–800 ◦ C range. Acknowledgment This work was supported by the Ministry of Science, Croatia, by the following projects: 119-0982886-1009, 098-1191005-2876, 098-0982886-2897, 098-0982886-2895, 098-09828862866, 098-0982904-2898. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
A.P. Alivisatos, Science 271 (1996) 933. R.F. Service, Science 271 (1996) 920. W. Scorupa, L. Rebohle, T. Gebel, Appl. Phys. A 76 (2003) 1049. A.L. Stepanov, et al., Appl. Phys. A 74 (2002) 441. T.C. Chang, Electrochem. Solid-State Lett. 7 (2004) G17. C. Boested, T. van Buuren, T.M. Willey, N. Franco, L.J. Terminello, C. Heske, T. Moeller, Appl. Phys. Lett 84 (2004) 4056. U. Serincan, G. Kartopu, A. Guennes, T.G. Finstad, R. Turan, Y. Ekinci, S.C. Bayliss, Semicon. Sci. Technol. 19 (2004) 1. U.V. Desnica, P. Dubcek, K. Salamon, I.D. Desnica-Frankovic, M. Buljan, S. Bernstorff, R. Turan, Nuclear Instrum. Methods Phys. Res. B 238 (2005) 272. H. Amenitsch, S. Bernstorff, M. Kriechbaum, D. Lombardo, H. Mio, M. Rappolt, P. M,Laggner, J. Appl. Cryst. 30 (1997) 872. M. Buljan, K. Salamon, P. Dubcek, S. Bernstorff, I.D. Desnica-Frankovic, O. Milat, U.V. Desnica, Vacuum 71 (2003) 65. M. Mayer, Technical Report IPP 9/113, Max-Planck Institut fur Plasmaphysik, Garching, Germany, 1997. U.V. Desnica, P. Dubcek, I.D. Desnica-Frankovic, M. Buljan, S. Bernstorff, S.W. White, J. Appl. Cryst. 36 (2003) 443. V. Holy, T. Baumbach, Phys. Rev. B 49 (1994) 10668. M. Buljan, U.V. Desnica, G. Drazic, M. Ivanda, N. Radic, P. Dubcek, K. Salamon, S. Bernstorff, V. Holy, Nature Mater. (2008) (submitted for publication).
Formation of Ge-nanocrystals in SiO2 matrix by magnetron sputtering and post-deposition thermal treatment U.V. Desnica a,∗ , K. Salamon b , M. Buljan a , P. Dubcek a , N. Radic a , I.D. Desnica-Frankovic a , Z. Siketic a , I. Bogdanovic-Radovic a , M. Ivanda a , S. Bernstorff c a R. Boskovic Institute, Physics Department, Bijenicka 54, HR-10000 Zagreb, Croatia b Institute of Physics, Bijenicka 56, HR-10000 Zagreb, Croatia c Sincrotrone Trieste, SS 14 km 163.5, 34012 Basovizza (TS), Italy
Available online 4 March 2008
Abstract Germanium Quantum Dots (Ge QDs) were formed in SiO2 by RT magnetron sputtering co-deposition of Ge and SiO2 and subsequent annealing. Films were deposited in the form of alternating (Ge + SiO2 ) layers (40:60 molar ratio) and pure SiO2 layers, serving as spacers. Grazing incidence small angle x-ray scattering (GISAXS) was applied for structural characterization of the QDs synthesized in the SiO2 amorphous matrix. The chemical composition and phase of the QDs were determined by Raman spectroscopy, and the spatial distribution and concentration of the Ge atoms by Rutherford Backscattering. The 2D GISAXS patterns, besides giving information on the layered structure, were used to reveal the onset of the synthesis of Ge QDs in SiO2 and to determine the average size and shape of QDs. It has been shown that the insertion of spacer SiO2 layers between (Ge + SiO2 ) layers transforms the 3D growth of Ge QDs into a preferentially 2D growth, within each 7 nm thick (Ge + SiO2 ) layer. This resulted in a considerably smaller average size of Ge QDs in the layered films. The synthesis of well crystallized, moderately sized, spherical Ge QDs was achieved by post-deposition annealing in the 700–800 ◦ C range. c 2008 Elsevier Ltd. All rights reserved.
Keywords: Quantum dots; Nanocrystals; GISAXS; RBS; Raman; Magnetron sputtering
∗ Corresponding address: R. Boˇskovi´c Institute, P.O. Box 180, HR-10000 Zagreb, Croatia. Tel.: +385 1 4561173; fax: +385 1 4680114. E-mail address: [email protected] (U.V. Desnica).
c 2008 Elsevier Ltd. All rights reserved. 0749-6036/$ - see front matter doi:10.1016/j.spmi.2008.01.021
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1. Introduction Semiconductor materials in the form of quantum dots (QDs) display a significant dependence of electronic, optical and other properties on the size of the nanoparticles [1–5]. Specifically, Ge quantum dots (Ge QDs) embedded in a transparent matrix exhibit intense photo- and electro-luminescence, strong third-order optical nonlinearities and tunable absorption, which are particularly strongly dependent on the nanoparticle size [6]. This enables the tunability of the nano-Ge band-gap over a considerable range of the visible light wavelengths. These physical properties of nano-Ge material makes it suitable for many electronic, optoelectronic and photonic applications, including integrated opto-couplers in micro-systems in biotechnology, etc [1]. Furthermore, Ge QDs are prime candidates for electronic nonvolatile memories, overcoming the limitations of current technologies by enabling higher endurance and lower operating voltages. Ge nanocrystals embedded in SiO2 can serve as light emitters exhibiting bright blue–violet electro-luminescence, favoring their application in Si-integrated sensor technology. Such materials are also ideal for nonvolatile high-speed memory applications, due to their excellent charging and retention time [5]. In thick films, including those obtained by ion implantation [6,7] the diffusion of Ge atoms at elevated annealing temperatures Ta occurs in all 3 dimensions, leading to relatively large QDs through Oswald ripening. In this paper this problem is addressed through the deposition of multilayered thin films containing alternating (Ge + SiO2 ) and SiO2 layers. The intention was that the ‘spacer’ SiO2 layer, if sufficiently thick, prevents Ge atoms from other (Ge+SiO2 ) layers from participating in the growth of GeQDs in a given (Ge + SiO2 ) layer, thus resulting in smaller QDs. 2. Experimental details Ge QDs were formed in SiO2 by magnetron sputtering co-deposition, either in the form of thick films (400 nm thick) or as multilayered films containing 20 bi-layers. The substrate was either SiO2 or h111i Si. Each bi-layer consisted of a layer of co-sputtered mixture of 40% mol Ge and 60% mol SiO2 (‘active layer’), and a layer of pure SiO2 , serving as a spacer between ‘active’ layers. The deposition temperature, Td , ranged from room temperature to 700 ◦ C. A multi-source magnetron sputtering KJLC CMS-18 system was used for the depositions. Pure Ge and pure SiO2 (99.995%) were used as targets in DC (14 W) and RF (250 W) operated magnetrons, respectively. The as-deposited samples were subsequently thermally annealed for one hour in vacuum up to Ta = 1000 ◦ C. Rutherford Backscattering (RBS) measurements were done with a 6 MV EN Tandem Van de Graaff accelerator in Zagreb, using 1.5 MeV 4 He ions, which were extracted from the Alphatross ion source. The accelerated alphas were directed normally to the target surface. Spectra were collected using the charge particle detector positioned at 165◦ relative to the beam direction. GISAXS experiments were carried out using X-ray photons of energy E = 8 keV (wavelength, λ = 0.154 nm) at the Austrian SAXS beamline of the synchrotron radiation facility ELETTRA, Trieste, Italy [9]. Two-dimensional GISAXS patterns were recorded with a 2D CCD detector having 1024 × 1024 pixels, placed in the y–z plane, perpendicularly to the specular x–z plane [8]. The patterns were first corrected for background intensity and detector response, and then for refraction and absorption effects [10]. In Raman spectroscopy the first order, dipoleallowed Raman spectra were obtained at room temperature (RT) by excitation with the 2.57 eV (514.5 nm) line from an Ar-ion laser, with low laser power to avoid heating. The scattered
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Fig. 1. RBS spectra of samples with (Ge+SiO2 ) magnetron sputtering deposited films on Si substrate. The temperatures of deposition (Td ) and of post-deposition annealing (Ta ) are indicated in the figure in ◦ C. (a): Whole RBS spectra for as-deposited samples at RT and at 700 ◦ C, with the other deposition parameters kept constant. (b): Ge-related part of the RBS spectra after post-deposition annealing at different Ta .
light was filtered with a triple spectrometer (DILOR Z-24) and Raman spectra were taken in the 190–550 cm−1 range, with 2 cm−1 steps. 3. Results and discussion 3.1. RBS most important results To determine the success of co-deposition of (Ge + SiO2 ) films, as well as to determine the stability limits of Ge atoms in SiO2 matrix to post-deposition thermal treatments, RBS spectroscopy was applied. Examples of the results for thick (Ge + SiO2 ) films on h111i Si substrate, for two deposition temperatures and for post/deposition treatments at several annealing temperatures, Ta , are shown on Fig. 1. Fig. 1a depicts the whole spectra for the as-deposited samples, while Fig. 1b shows in more detail the higher energy part of the RBS spectra belonging to Ge, for as-deposited and for several annealed samples. All spectra were normalized to the same number of incident particles for comparison. The quantitative assessment of the concentrations of the incorporated Ge atoms in different samples was calculated using the SIMNRA program [11]. The error in the Ge-concentration determined in this way was estimated to be +5%.
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The main findings from the RBS measurements can be summarized as following: (a) For RT deposition almost all the magnetron sputtered Ge and SiO2 material was successfully deposited on the Si substrate: The quantity of actually deposited Ge atoms corresponds to the calculated quantity of Ge in the films (9.3 × 1014 /cm2 in each nm of 40%Ge + 60%SiO2 film) within 10%. (b) After post-deposition thermal processing up to Ta = 700 ◦ C no visible change of the position of the Ge atoms in the deposited film occurs and apparently there is no loss of Ge atoms from the sample (i.e. there is no perceivable loss of Ge, which would exceed the inaccuracy of the measurement). (c) For Ta = 800 ◦ C, a loss of about 10% of the Ge atoms is observed. It seems that those lost Ge atoms out-diffused from the sample — there is no in-diffusion of Ge atoms into the substrate. It appears that the Ge out-diffusion is partly counterbalanced by trapping of Ge atoms at/close to the surface: there is a dip in the Ge distribution close to the surface, with some pile-up of Ge atoms at/close to the sample surface, which decelerates the escape of Ge atoms. (d) The diffusion and loss of Ge becomes strong for Ta = 1000 ◦ C. Now more than 30% of the deposited Ge atoms are lost from the layer. Part of the Ge out-diffused from the surface, but also a fraction of the Ge atoms in-diffused into the Si substrate, creating an apparently thicker deposited layer. Also a very small fraction of the Ge atoms in-diffused much deeper into the substrate, probably diffusing faster along some linear defect lines in the Si substrate, like dislocations, etc. All of these effects indicate that at about Ta = 800 ◦ C the diffusion of Ge in SiO2 starts to become significant. For Ta = 1000 ◦ C both in-diffusion into the Si substrate and outdiffusion from the surface are very strong, up to a level which significantly changes the initial composition of the (Ge + SiO2 ) film, both regarding the fraction of Ge atoms and their depth distribution. (e) Films deposited at Td = 700 ◦ C, with the other parameters nominally the same as for the Td = RT films, had dramatically lower deposition rates: (Ge + SiO2 ) films are less than half as thick as RT deposited films. When comparing the integral Ge signal intensities, only about 10% of the Ge atoms ended up in the deposited layer in comparison with RT deposition. Hence these films were not studied further. (f) Films deposited as (Ge + SiO2 )/SiO2 bi-layers, each (7 + 7) nm thick, have RBS spectra (not shown) very similar to the ones of thick films. Since the depth resolution of a RBS measurement (≈20 nm) is larger than the thickness of bi-layers the depth structure regarding the Ge concentration cannot be resolved. RBS spectra of the bi-layered films as a whole follow generally the annealing dynamics of the thick films: small changes of the Ge atom position and a weak out-diffusion become visible above Ta = 800 ◦ C, and are becoming significant above Ta = 1000 ◦ C. The dynamics of in- and out-diffusion of the Ge atoms from bi-layered films, either for Td = RT or Td = 700 ◦ C, is very similar to the one from thick films (including a specific depth distributions of Ge atoms within the deposited films, with a characteristic, Ta -dependent, ‘dip’ in depth distributions). The most important practical implication of the RBS results is that the Ge atoms remain within the deposited films up to 700 ◦ C, and that out-diffusion becomes considerable above Ta = 800 ◦ C. RBS gives depth-distributions and concentrations of Ge atoms. However, to find out when/whether they are agglomerated into nanoparticles and of which size other methods are needed.
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Fig. 2. 2D GISAXS patterns from magnetron deposited (Ge + SiO2 ) layers at RT on h111i Si substrate, prior to and after annealing at various temperatures, Ta , indicated in each figure. (A) (first column): Thick (Ge + SiO2 ) film. (B) (2. and 3. pattern in 1. row): Bi − layeredfilms; thicknesses of each of the (Ge + SiO2 ) and SiO2 layers being 7 nm and 7 nm respectively. (C) (2. and 3. pattern in 2. row): Same as (B), but thicknesses of each of the (Ge + SiO2 ) and SiO2 layers being 7 nm and 3.5 nm, respectively. (D) 1D GISAXS profiles for samples annealed at 800 ◦ C for different thickness of the SiO2 layer serving as a spacer between (Ge + SiO2 ) layers: (a) dspacer = 0, (b) dspacer = 3.5 nm, (c) dspacer = 7 nm.
3.2. GISAXS most relevant results Fig. 2 shows 2D GISAXS patterns from (Ge + SiO2 ) films deposited on Si substrate. The grazing incidence angle of the X-ray beam, α, was α = αc + 0.05◦ (which corresponds to a penetration depth of about 200 nm), where αc denotes the critical angle for the total external reflection. The 2D patterns exemplify maps of scattering intensities in reciprocal q space (q = q y + qz is the wave vector, where q y = (2π/λ) sin(2ϕ) and qz = (2π/λ) sin(2β); 2β and 2ϕ being the scattering angles in plane and out of plane of incidence, respectively). In order to block strong surface signals (reflected beam, Yoneda peak, etc.) in/close to the specular plane, a
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Table 1 The role of spacer layer on the size of Ge quantum dots formed in SiO2 matrix Thickness of the spacer layer
0 (thick film) 3.5 nm 7 nm
Substrate SiO2 Radius (nm)
Si
5.9 ± 0.2 4.1 ± 0.3 3.9 ± 0.4
6.1 ± 0.2 4.1 ± 0.3 3.8 ± 0.4
Average radius of Ge Quantum Dots (in nm) as a function of the thickness of the spacer layers of pure SiO2 deposited between the 7 nm thick (Ge + SiO2 ) layers. All samples were deposited at RT on either Si or SiO2 substrate and subsequently annealed at Ta = 800 ◦ C. Except of the thickness of spacer layers all other parameters of the deposition and annealing were the same for all samples.
partly transparent beam stopper was inserted, allowing a better sensitivity for the weak diffuse scattering at larger q. Group A of the GISAXS pattern in Fig. 2A refers to thick films (no spacer layer) deposited at RT, and subsequently annealed at the temperature Ta indicated in each pattern. For the as-deposited sample (Ta = RT ) the GISAXS signal comprises practically the entire intense region close to the center, decaying very fast for larger q y . This signal comes primarily from the surface, which is not of particular interest in this paper. After post-deposition annealing at Ta = 500 ◦ C and above, an additional scattering signal at larger q becomes visible, indicating the formation of QDs. The regions in the 2D maps having the same scattering intensities have apparently circular form, indicating that the QDs are approximately spherical [8,12]. With further annealing (higher Ta ) the intensity of the particle-related signal decreases faster with q, indicating an increase of the size of the QDs. Fig. 2. also shows the 2D profiles of two types of (Ge+SiO2 )/SiO2 bi-layered films deposited at RT. In both films the (Ge + SiO2 ) layers were 7 nm thick, while the SiO2 spacer layers were either 7 nm (group B pattern) or 3.5 nm (group C pattern) thick. The pattern of RT deposited bi-layered films (group B and C) are characterized by “Bragg sheets” [13]. Their off-specular intensity indicates a vertical correlation of the layers/interface roughness, indicating a highly conformal layered structure of the film, while QDs are not yet formed. Hence, the shape of the 2D GISAXS pattern is dominated by the bi-layered structure, with the distance between Bragg sheets being determined by the thickness of the bi-layers (14 nm and 10.5 nm, respectively). Annealing of as-deposited films at Ta = 800 ◦ C, (right panels in B and C) resulted in the appearance of an additional signal, belonging to the formed nanoparticles, but the remnants of the signal belonging to the layered structure is still clearly resolvable, which are better visible in Fig. 2D (at 0.4 and 0.9 qz2 , for 7 nm and 3.5 nm spacer thicknesses, respectively). Fig. 2D shows one-dimensional (1D) GISAXS profiles of samples annealed at 800 ◦ C, whose 2D patterns are shown in Fig. 2A, B and C. The 1D GISAXS plots were obtained by cross-sectioning the 2D pattern parallel to the z-axis close to the beam-stopper. When the traditional particle scattering model (Guinier approximation) is applied in the analysis of such 1D GISAXS profiles, the average cluster radius, R, is obtained from the radius of gyration, Rg , (R = (5/3)0.5 ∗ Rg ) computed from the slope of the linear region of the lnI(q) versus q 2 dependence [8,12]. There is no doubt that the insertion of the SiO2 spacer indeed resulted in a dramatic reduction of the QDs size (steeper slope of lnI (q) versus q 2 dependence), as intended. Numerical results for the average QD radii are given in Table 1, obtained in films deposited either on Si or SiO2 substrate. Already spacers as thin as 3.5 nm, reduce the average radii by ca 50%; from 6.1 ± 0.2 nm to 4.1 ± 0.3 nm. Doubling the thickness of the spacer layer from 3.5 to 7 nm results in just a small further decrease of R by less
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Fig. 3. Raman spectra of (Ge + SiO2 ) films deposited at Td = RT on SiO2 substrate, after different post- deposition annealings at the temperatures indicated in the figure.
than 10%. Hence, one can conclude that already a 3.5 nm spacer prevents a large majority of the Ge atoms from neighboring (Ge + SiO2 ) layers from participating in the formation of GeQDs in any given (Ge + SiO2 ) layer. For a thicker spacer layer (7 nm) the diameter of the Ge QDs is very close to the thickness of the (Ge + SiO2 ) layer. It seems safe to conclude that Ge QDs in these (Ge + SiO2 ) layers are practically exclusively formed from Ge atoms deposited in the same layer. As expected, the choice of the substrate (indicated as Si or SiO2 in Table 1) played only a minimal or no role. The growth of spherical QDs in parallel layers separated by SiO2 is confirmed for these samples also by cross-sectional Transmission Electron Microscopy (not shown). For a very narrow range of deposition parameters [14] the formation of Ge QDs in a 3D superlattice can be obtained. 3.3. Raman spectroscopy main results Representative Raman spectra are presented in Fig. 3 for as deposited thick films as well as for the same films after annealing at several different Ta . Already in the as-deposited samples the Ge atoms are distributed into the amorphous network, as revealed by a broad band centered at 275 cm−1 . These clusters have to be very small since they are not observed in GISAXS pattern. Up to at least Ta = 500 ◦ C the Ge QDs remain fully amorphous. After annealing at Ta = 700 ◦ C or higher the a-Ge agglomerates crystallize into crystalline Ge QDs (TO c-Ge line at 300 cm−1 ). This line is somewhat broadened due to confinement effects, related to the nanometric size of the QDs. Ge-related Raman features are superimposed on the substrate signal, a broad band related to the amorphous SiO2 matrix. The Raman spectra also indicate that the quality of the deposited SiO2 improves with thermal annealing. Upon annealing at Ta = 700 ◦ C, the SiO2 part of the spectra became very similar to the spectrum of standard SiO2 substrate. 4. Conclusions Ge nanocrystals were formed in SiO2 after magnetron co-sputtering of Ge and SiO2 . The deposited films were either thick films containing 40%mol Ge and 60%mol SiO2 , or in the form of bi-layers, each bi-layer consisting of a 7 nm thick layer of the same (Ge + SiO2 ) mixture and a layer of pure SiO2 (either 3.5 or 7 nm-thick), serving as a spacer. Sputtering was performed
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either at room temperature (RT) or at 700 ◦ C. As-deposited samples were subsequently thermally annealed up to Ta = 1000 ◦ C. RBS revealed successful (Ge + SiO2 ) deposition at RT . In contrast, the deposition temperature Td = 700 ◦ C yielded only about 10% of Ge and about 40% of SiO2 deposited, in comparison with RT deposition. The 2D GISAXS patterns, besides detecting the bi-layered structure, revealed the formation of nanoparticles (QDs) in the deposited films after post-deposition annealing at 500 ◦ C. Raman spectroscopy found that these QDs are amorphous Ge aggregates, which crystallize into Ge-QDs after annealing at Ta = 700 ◦ C or higher. The insertion of 7 nm of pure SiO2 spacers between each (Ge + SiO2 ) layer during deposition hinders the interaction of Ge atoms from different (Ge + SiO2 ) layers, transforming 3D growth of Ge QDs from all directions occurring in thick films into preferentially 2D, in-layer, growth, which results in much smaller Ge QDs. Even if spacers are only 3.5 nm thick, the Ge QDs in each particular (Ge + SiO2 ) layer are formed mostly (cca 90%) from the Ge atoms that originated from the same (Ge + SiO2 ) layer. The best Ta ’s for the formation of moderately sized, spherical, well crystallized Ge QDs were found to be in the Ta = 700–800 ◦ C range. Acknowledgment This work was supported by the Ministry of Science, Croatia, by the following projects: 119-0982886-1009, 098-1191005-2876, 098-0982886-2897, 098-0982886-2895, 098-09828862866, 098-0982904-2898. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
A.P. Alivisatos, Science 271 (1996) 933. R.F. Service, Science 271 (1996) 920. W. Scorupa, L. Rebohle, T. Gebel, Appl. Phys. A 76 (2003) 1049. A.L. Stepanov, et al., Appl. Phys. A 74 (2002) 441. T.C. Chang, Electrochem. Solid-State Lett. 7 (2004) G17. C. Boested, T. van Buuren, T.M. Willey, N. Franco, L.J. Terminello, C. Heske, T. Moeller, Appl. Phys. Lett 84 (2004) 4056. U. Serincan, G. Kartopu, A. Guennes, T.G. Finstad, R. Turan, Y. Ekinci, S.C. Bayliss, Semicon. Sci. Technol. 19 (2004) 1. U.V. Desnica, P. Dubcek, K. Salamon, I.D. Desnica-Frankovic, M. Buljan, S. Bernstorff, R. Turan, Nuclear Instrum. Methods Phys. Res. B 238 (2005) 272. H. Amenitsch, S. Bernstorff, M. Kriechbaum, D. Lombardo, H. Mio, M. Rappolt, P. M,Laggner, J. Appl. Cryst. 30 (1997) 872. M. Buljan, K. Salamon, P. Dubcek, S. Bernstorff, I.D. Desnica-Frankovic, O. Milat, U.V. Desnica, Vacuum 71 (2003) 65. M. Mayer, Technical Report IPP 9/113, Max-Planck Institut fur Plasmaphysik, Garching, Germany, 1997. U.V. Desnica, P. Dubcek, I.D. Desnica-Frankovic, M. Buljan, S. Bernstorff, S.W. White, J. Appl. Cryst. 36 (2003) 443. V. Holy, T. Baumbach, Phys. Rev. B 49 (1994) 10668. M. Buljan, U.V. Desnica, G. Drazic, M. Ivanda, N. Radic, P. Dubcek, K. Salamon, S. Bernstorff, V. Holy, Nature Mater. (2008) (submitted for publication).