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Structural characterization of SiGe nanoclusters formed by rapid thermal annealing
Applied Surface Science 254 (2008) 6055–6058
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Structural characterization of SiGe nanoclusters formed by rapid thermal annealing Alexandre Miranda P. dos Anjos a, Ioshiaki Doi a,b,*, Jose´ Alexandre Diniz a,b a b
Center for Semiconductor Components, State University of Campinas, Rua Joa˜o Pandia´ Calo´geras 90, 13083-870 Campinas, SP, Brazil School of Electrical and Computer Engineering, State University of Campinas, P.O. Box 6101, 13083-970 Campinas, SP, Brazil
A R T I C L E I N F O
A B S T R A C T
Article history:
This work presents the structural characterization of nanoclusters formed from a-Si:H/Ge heterostructures processed by rapid thermal annealing (RTA) at 1000 8C for annealing times varying between 30 s and 70 s. The a-Si:H layers were grown on electron cyclotron resonance (ECR) using SiH4 and Ar precursor gases. The Ge layer was grown in an e-beam evaporation system. The structural characterizations were performed by high-resolution X-ray diffractometer (HRXRD) on grazing incidence X-ray reflection mode (GIXRR) and micro-Raman measurements. The average grain size, Ge concentration (xGe) and strain were estimated from Lorentzian GIXRR peak fit. The average grain size varied from 3 nm to 7.5 nm and decreased with annealing time. The xGe increase with annealing time and varied from 8% to 19%, approximately. The strain calculated for (1 1 1), (2 2 0) and (3 1 1) peaks at 40 s, 50 s, 60 s and 70 s annealing time suggest the geometrical changes in nanoclusters according to process time. ß 2008 Elsevier B.V. All rights reserved.
Available online 18 March 2008 PACS: 61.46.Hk 61.72.uf 61.82.Rx 68.47.Fg 68.55. a 81.07.Bc 81.16.Rf Keywords: SiGe nanoclusters Rapid thermal annealing Electron cyclotron resonance High-resolution X-ray diffractometer
1. Introduction Amorphous Si1 xGex has drawn interest rather than polysilicon as the basic material in high performance thin-film devices [1,2]. An attractive factor in SiGe film is the compatibility with silicon-based integrated circuits (IC). The fabrication of IC usually involves rapid thermal annealing (RTA) steps. However, structural characterization of SiGe according to the annealing time is important for compatibility with IC process. An important effect from thermal annealing in Si1 xGex alloy is the island formation [3–5]. This property has been explored to growth nanoislands focused in researches in near-IR optoelectronics and microwave devices and detectors [6]. Considerable structural improvement has been obtained from hydrogenated silicon and germanium nanostructures [7]. Both the Ge and Si island stabilization and SiGe nanostructure formation of hydrogenated SiGe films have been reported [3,8,9]. The silyl–germyl (H3Ge)xSiH4 x family of
* Corresponding author at: Center for Semiconductor Components, State University of Campinas, Rua Joa˜o Pandia´ Calo´geras 90, 13083-870 Campinas, SP, Brazil. Tel.: +55 19 3521 5215; fax: +55 19 3521 5177. E-mail address: [email protected] (I. Doi). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.02.119
compounds has been used as precursor gases to form coherent quantum dots [10]. This work presents SiGe nanostructures formation from a-Si:H/ Ge heterostructure by RTA at 1000 8C with times varying from 30 s to 70 s. Structural characterizations of nanoclusters formed at different annealing times were performed by high-resolution Xray diffractometer (HRXRD) X’Pert. From the diffraction spectra the Ge concentration, average grain size and stress were determined. HRXRD results were supported by micro-Raman measurements. 2. Sample growth and processing The a-Si:H/Ge heterostructure was grown on p-type Si (0 0 1). The a-Si:H/Ge structure is composed of 10 nm Ge layer sandwiched between two a-Si:H layers, both with nominal thickness of 40 nm. The a-Si:H layers were grown on electron cyclotron resonance (ECR) using SiH4 and Ar precursor gases. Their flows were set as 200 sccm and 20 sccm, respectively. The chamber pressure was kept at 4.0 mTorr. The frequency of 2.45 GHz and 750 W of ECR microwave power was used. In these conditions, 3 min of growth at 20 8C was used in order to obtain 40 nm of a-Si:H nominal thickness. The Ge layer was grown on e-beam evaporation system. The operational chamber pressure was 8–10 mbar. The Ge layer
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was grown for 5 min in order to obtain a nominal thickness of 10 nm. Then, the sample was carried out on ECR system again for 40 nm top layer growth of a-Si:H reproducing the first a-Si:H layer. The sample was cleaved into six parts, in which five of them were RTA processed at 1000 8C for 30 s, 40 s, 50 s, 60 s and 70 s. The RTA was performed on RTP JIPELEC system model 150 Jetfirst in N2 ambient, with a heating ramp of 50 8C/s. 3. HRXRD characterization The samples processed by rapid thermal annealing and asdeposit one was characterized by HRXRD on grazing incidence Xray reflection mode (GIXRR). The HRXRD measurements were performed on Philips X’Pert MRD with the Cu Ka1 X-ray tube in line focus. Prior to the measurements, the diffractometer was calibrated with a Soller slit with 1/328 divergence and a Cu 0.1mm foil attenuator in the primary optics. In the secondary optics both 0.1 mm anti-scatter slit and a flat crystal graphite monochromator were used. The sample height and the goniometer zero points (2Q = v = 0) were precisely adjusted before each measurement. The GIXRR spectra was measured with a 2Q scan and incidence angle v = 0.38. This angle demonstrated the best spectra resolution compared with angles 1, 0.7 and 0.5. Parameters like peak intensity, Ge concentration (xGe), average grain size and strain were calculated from Lorentzian fit spectra. The Ge concentrations (xGe) were estimated from spectra peaks maximum position using the linear interpolation of the lattice constant known as Vergard’s Law. For diamond cubic Si–Ge the Vergard’s Law can be calculated using xGe = (a0(x) 5.431)/0.226, where a0(x) is the lattice constant of Si1 xGex. The average grain size was estimated using the Scherrer’s formula, D = 1.3865/ (FWHM cos Q), where D is the average grain size for the Q peak on GIXRR spectra. The average strain was calculated using Stokes– Wilson formula, estr = FWHM/(4 tan Q). The FWHM was determined from Lorentzian spectra fit peaks. Instrumental corrections were discarded in consequence of large measured broadening compared with instrumental broadening. Were estimated a variation of only 0.00001 arcsec in FWHM value applying instrumental corrections. GIXRR analysis was supported by micro-Raman (MR) measurements. MR measurements were performed on coupled MR and AFM system NANOFINDER 30 at room temperature and using the 488 nm line with a power of 30 mW measure over the sample. The Laser was focused in a spot with 10 mm of diameter over the sample. The scattered light was analyzed in a backscattering geometry using a monochromator-
Fig. 1. GIXRR spectra for a-Si/Ge heterostructures annealed for 30 s, 40 s, 50 s, 60 s and 70 s. White lines refer to Lorentzian fit.
spectrograph MS5004i and CCD ANDOR DV401-BV detector. Before each measure the focus on the sample was adjusted supported by a microscope OLYMPUS IX-71. The most intense Si Peak determines the best focus. The final spectrum was obtained by integration of 10 successive measurements with 1 s each one. 4. Results and discussion The GIXRR spectra showed characteristics diamond cubic (1 1 1), (2 2 0) and (3 1 1) Si1 xGex peaks for time annealing up to 30 s. Fig. 1 shows the GIXRR spectra measured in samples annealed at 1000 8C for 30 s, 40 s, 50 s, 60 s and 70 s. The sample
Fig. 2. (a) Average grain size (open circles) and Ge concentration (full circles) vs. annealing time. The average grain size was calculated using Scherrer’s formula and the Ge concentration (xGe) was estimated using Vergard’s Law. (b) Raman shift in samples annealed for 40 s, 50 s, 60 s, 70 s and as-deposit.
A.M.P. dos Anjos et al. / Applied Surface Science 254 (2008) 6055–6058
heated during 30 s shows the same characteristics of as-deposited sample. The GIXRR peaks are shifted from the characteristic points of bulk Si and bulk Ge crystals. As-deposit sample has a nominal Ge concentration (xGe) of 11% approximately. Average xGe estimated from GIXRR (1 1 1), (2 2 0) and (3 1 1) spectra peaks varied from 8% to 19% according to time annealing. Fig. 2(a) shows the average xGe increase as function of the time annealing. Average xGe linearly increase from 40 s to 60 s time annealing and abruptly increase at 70 s of RTA. Micro-Raman measurements performed in characteristic Ge–Ge bonds range (250–310 cm 1) showed that xGe increase not formed detectable Ge–Ge bonds, Fig. 2(b). Note that as-deposit sample shows the Ge–Ge peak bond. The average grain size varied from 2.7 nm to 7.5 nm and decreased with annealing time, as shown in Fig. 2. The silicon and germanium hydrides reactions process have been reported on some works [10–13]. Based on it, some hypothesis may be appointed in order to explain the dynamic effects of RTA in both the grain size diminishing and xGe increase with annealing time. We supposed melted Ge and solid Si, at RTA temperature used and rich hydrogen ambient, formed hydrides like digermanium hexahydride (Ge2H6(l)), germane (GeH4(g)), germilene (GeH2(g)), disilane hexahydride (Si2H6(g)) and silane (SiH4(g)). The index (l) and (g) refers to liquid and gaseous states, respectively. The Si–Si and Ge–Ge bonds are more easily fractured than Si–H and Ge–H bonds. Per example, bond strength of Si–Si is 74 kcal/mol while H–Si is 86 kcal/mol. Mass vanish observed during RTA process may be a consequence of gaseous silicon and germanium hydrides desorption. Based on experimental results Cheng and coauthors [11], suggested three pathways for Si2H6(g) dissociative adsorption. As result, were verified the formation of Si2H5(l), SiH3(l), SiH(l) and SiH4(g). Due to the relative similarity between Si and Ge chemistry these results may be extended to hydrogen–germanium compounds. Standard enthalpy values (DH8) reported by Saalfeld and Svec [13] (7.5 kcal/mol), suggest the germylsilane (GeH6Si) trend formation from silyl and germyl radicals (Si/GeH3). SiGe nanoclusters were obtained using germylsilane as precursor gas [10]. The preferential Si–H bond compared to Ge–Si and Si–Si bonds suggest the grain size diminishing by gradual silicon extraction with annealing time leading to xGe increase. Potential energy calculated using density functional theory for clusters of Si15 xGexH16 and Si15 xGexH18 (x = 1, 2) were reported [11]. The xGe between 6.5% and 13%, approximately, discarding H influences on the lattice size calculated from GIXRR, is coherent with xGe observed here. The dynamic of reaction reported by the authors showed both the fracture of silyl radicals and geometrical rearrange in the clusters cells. These facts confirm the xGe increase and agree with strain changes observed here. Probably, the structural characterization changes observed at different annealing times was able only by the abrupt energy input interruption in RTA process. Coherently with grain size diminishing, was observed the exponential (1 1 1) peak intensity declines with time annealing, as shown in Fig. 3. This behavior disagrees with some works [2,14]. Differently, the general crystalline film quality in our work is quite unaffected by RTA process. Furthermore, the X-ray signal decreases with the crystalline clusters diminishing. Classical continuum elasticity, and less frequently, atomistic methods are typically employed to calculate the strains in nanostructures. The strain tensors are directly related to the lattice parameter mismatch between the nanostructure and matrix boundaries. Usually, the nanocluster is considered isotropic or average of the lattice parameters is used in order to simplify the calculations. In practice, crystallites are usually irregular in shape. The X-ray measures are able to detect small variations in lattice parameter of the different crystalline planes and different strain
6057
Fig. 3. Peak intensity calculated from (1 1 1) GIRXX peak for annealing times of 40 s, 50 s, 60 s and 70 s. The intensity decrease was observed in all GIRXX peaks. This behavior agrees with grain size diminishing.
Fig. 4. Average strain estimated for (1 1 1), (2 2 0) and (3 1 1) GIXRR diffraction peaks at different annealing times. The strain was calculated using Stokes–Wilson formula. The different strain values observed for each annealing time may be interpreted like the lattice parameter changes detect by X-ray for different Bragg angles.
tensors may be estimated using Stokes–Wilson formula. Strain estimated for (1 1 1), (2 2 0) and (3 1 1) peaks at 40 s, 50 s, 60 s and 70 s annealing time, respectively, suggest the structural nanocluster changes during thermal process. Fig. 4 shows the almost same strain values after 40 s of thermal annealing at 1000 8C for crystalline planes (1 1 1) and (2 2 0). As the annealing time is increased, the (1 1 1) plane strain increases, while (2 2 0) plane is gradually relaxed to (3 1 1) plane strain values. Another characterization techniques, like the TEM crosssectional micrography, are needed for accurate investigation of the nanoclusters shape changes. 5. Conclusions The used short time annealing, compared with usual process (hours), indicate a potential time processing optimization in Si–Ge nanoclusters-based devices. Annealing times varying between 30 s and 70 s were used to form SiGe nanoclusters with different Ge concentrations, grain sizes and strain. The results showed the average grain size diminishing and Ge concentration increase as function of the annealing time. The strain in (1 1 1), (2 2 0) and (3 1 1) planes suggests re-arrangements of the nanoclusters geometry accordingly to the time annealing. Moreover, the X-ray structural characterizations suggest that RTA process, under
6058
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the used conditions, may be an optimized way to obtain SiGe nanoclusters to device applications and IC process. Acknowledgments The authors are grateful to the financial support from the Brazilian agencies Faepex-Unicamp, CNPq and FAPESP. References [1] L.K. Teh, W.K. Choi, L.K. Bera, W.K. Chim, Solid-State Electron. 45 (2001) 1963. [2] E.V. Jelenkovic, K.Y. Tong, Z. Sun, C.L. Mak, W.Y. Cheung, J. Vac. Sci. Technol. A15 (1997) 2836. [3] L. Xu, P.J. Mcnally, G.D.M. Dilliway, N.E.B. Cowern, C. Jeynes, E. Mendoza, P. Ashburn, D.M. Bagnall, J. Mater. Sci.: Mater. Electron. 16 (2005) 469.
[4] T.K. Nguyen-Duc, V. Le Thanh, J. Derrien, V. Yam, P. Boucaud, D. Bouchier, J. Phys. IV (France) 132 (2006) 163. [5] S. Tripathi, R. Brajpuriya, A. Sharma, T. Shripathi, S.M. Chaudhari, J. Phys. D: Appl. Phys. 39 (2006) 4848. [6] M.Y. Valakh, V.O. Yukhymchuk, V.M. Dzhagan, O.S. Lytvyn, A.G. Milekhim, A.I. Nikiforov, O.P. Pchelyakov, F. Alsina, J. Pascual, Inst. Phys. Publish.: Nanotechnol. 16 (2005) 1464. [7] H. Trinkaus, B. Hollander, St. Rongen, S. Mantl, H.-J. Herzog, J. Kunchenbecker, T. Hackbarth, Appl. Phys. Lett. 76 (2000) 3552. [8] V. Pillard, P. Puech, P.R.I. Cabarrocas, J. Non-Cryst. Sol. 299 (2002) 280. [9] M.A. Ghazala, W. Beyer, H. Wagner, J. Appl. Phys. 70 (1991) 4540. [10] C.J. Ritter, C. Hu, A.V.G. Chimeshya, J. Tolle, D. Klewer, I.S.T. Tsong, J. Kouvetakis, J. Am. Chem. Soc. 127 (2005) 9855. [11] C.-L. Cheng, D.-S. Tsai, J.-C. Jiang, J. Phys. Chem. C 111 (2007) 13466. [12] J. Takahashi, T. Momose, T. Shida, Bull. Chem. Soc. Jpn. 67 (1994) 74. [13] F.E. Saalfeld, H.J. Svec, J. Phys. Chem. 70 (1966) 1753. [14] C. Te`telin, X. Wallart, D. Stie´venard, P. Nys, D.J. Gravesteijn, J. Vac. Sci. Technol. B16 (1998) 1.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Structural characterization of SiGe nanoclusters formed by rapid thermal annealing Alexandre Miranda P. dos Anjos a, Ioshiaki Doi a,b,*, Jose´ Alexandre Diniz a,b a b
Center for Semiconductor Components, State University of Campinas, Rua Joa˜o Pandia´ Calo´geras 90, 13083-870 Campinas, SP, Brazil School of Electrical and Computer Engineering, State University of Campinas, P.O. Box 6101, 13083-970 Campinas, SP, Brazil
A R T I C L E I N F O
A B S T R A C T
Article history:
This work presents the structural characterization of nanoclusters formed from a-Si:H/Ge heterostructures processed by rapid thermal annealing (RTA) at 1000 8C for annealing times varying between 30 s and 70 s. The a-Si:H layers were grown on electron cyclotron resonance (ECR) using SiH4 and Ar precursor gases. The Ge layer was grown in an e-beam evaporation system. The structural characterizations were performed by high-resolution X-ray diffractometer (HRXRD) on grazing incidence X-ray reflection mode (GIXRR) and micro-Raman measurements. The average grain size, Ge concentration (xGe) and strain were estimated from Lorentzian GIXRR peak fit. The average grain size varied from 3 nm to 7.5 nm and decreased with annealing time. The xGe increase with annealing time and varied from 8% to 19%, approximately. The strain calculated for (1 1 1), (2 2 0) and (3 1 1) peaks at 40 s, 50 s, 60 s and 70 s annealing time suggest the geometrical changes in nanoclusters according to process time. ß 2008 Elsevier B.V. All rights reserved.
Available online 18 March 2008 PACS: 61.46.Hk 61.72.uf 61.82.Rx 68.47.Fg 68.55. a 81.07.Bc 81.16.Rf Keywords: SiGe nanoclusters Rapid thermal annealing Electron cyclotron resonance High-resolution X-ray diffractometer
1. Introduction Amorphous Si1 xGex has drawn interest rather than polysilicon as the basic material in high performance thin-film devices [1,2]. An attractive factor in SiGe film is the compatibility with silicon-based integrated circuits (IC). The fabrication of IC usually involves rapid thermal annealing (RTA) steps. However, structural characterization of SiGe according to the annealing time is important for compatibility with IC process. An important effect from thermal annealing in Si1 xGex alloy is the island formation [3–5]. This property has been explored to growth nanoislands focused in researches in near-IR optoelectronics and microwave devices and detectors [6]. Considerable structural improvement has been obtained from hydrogenated silicon and germanium nanostructures [7]. Both the Ge and Si island stabilization and SiGe nanostructure formation of hydrogenated SiGe films have been reported [3,8,9]. The silyl–germyl (H3Ge)xSiH4 x family of
* Corresponding author at: Center for Semiconductor Components, State University of Campinas, Rua Joa˜o Pandia´ Calo´geras 90, 13083-870 Campinas, SP, Brazil. Tel.: +55 19 3521 5215; fax: +55 19 3521 5177. E-mail address: [email protected] (I. Doi). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.02.119
compounds has been used as precursor gases to form coherent quantum dots [10]. This work presents SiGe nanostructures formation from a-Si:H/ Ge heterostructure by RTA at 1000 8C with times varying from 30 s to 70 s. Structural characterizations of nanoclusters formed at different annealing times were performed by high-resolution Xray diffractometer (HRXRD) X’Pert. From the diffraction spectra the Ge concentration, average grain size and stress were determined. HRXRD results were supported by micro-Raman measurements. 2. Sample growth and processing The a-Si:H/Ge heterostructure was grown on p-type Si (0 0 1). The a-Si:H/Ge structure is composed of 10 nm Ge layer sandwiched between two a-Si:H layers, both with nominal thickness of 40 nm. The a-Si:H layers were grown on electron cyclotron resonance (ECR) using SiH4 and Ar precursor gases. Their flows were set as 200 sccm and 20 sccm, respectively. The chamber pressure was kept at 4.0 mTorr. The frequency of 2.45 GHz and 750 W of ECR microwave power was used. In these conditions, 3 min of growth at 20 8C was used in order to obtain 40 nm of a-Si:H nominal thickness. The Ge layer was grown on e-beam evaporation system. The operational chamber pressure was 8–10 mbar. The Ge layer
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was grown for 5 min in order to obtain a nominal thickness of 10 nm. Then, the sample was carried out on ECR system again for 40 nm top layer growth of a-Si:H reproducing the first a-Si:H layer. The sample was cleaved into six parts, in which five of them were RTA processed at 1000 8C for 30 s, 40 s, 50 s, 60 s and 70 s. The RTA was performed on RTP JIPELEC system model 150 Jetfirst in N2 ambient, with a heating ramp of 50 8C/s. 3. HRXRD characterization The samples processed by rapid thermal annealing and asdeposit one was characterized by HRXRD on grazing incidence Xray reflection mode (GIXRR). The HRXRD measurements were performed on Philips X’Pert MRD with the Cu Ka1 X-ray tube in line focus. Prior to the measurements, the diffractometer was calibrated with a Soller slit with 1/328 divergence and a Cu 0.1mm foil attenuator in the primary optics. In the secondary optics both 0.1 mm anti-scatter slit and a flat crystal graphite monochromator were used. The sample height and the goniometer zero points (2Q = v = 0) were precisely adjusted before each measurement. The GIXRR spectra was measured with a 2Q scan and incidence angle v = 0.38. This angle demonstrated the best spectra resolution compared with angles 1, 0.7 and 0.5. Parameters like peak intensity, Ge concentration (xGe), average grain size and strain were calculated from Lorentzian fit spectra. The Ge concentrations (xGe) were estimated from spectra peaks maximum position using the linear interpolation of the lattice constant known as Vergard’s Law. For diamond cubic Si–Ge the Vergard’s Law can be calculated using xGe = (a0(x) 5.431)/0.226, where a0(x) is the lattice constant of Si1 xGex. The average grain size was estimated using the Scherrer’s formula, D = 1.3865/ (FWHM cos Q), where D is the average grain size for the Q peak on GIXRR spectra. The average strain was calculated using Stokes– Wilson formula, estr = FWHM/(4 tan Q). The FWHM was determined from Lorentzian spectra fit peaks. Instrumental corrections were discarded in consequence of large measured broadening compared with instrumental broadening. Were estimated a variation of only 0.00001 arcsec in FWHM value applying instrumental corrections. GIXRR analysis was supported by micro-Raman (MR) measurements. MR measurements were performed on coupled MR and AFM system NANOFINDER 30 at room temperature and using the 488 nm line with a power of 30 mW measure over the sample. The Laser was focused in a spot with 10 mm of diameter over the sample. The scattered light was analyzed in a backscattering geometry using a monochromator-
Fig. 1. GIXRR spectra for a-Si/Ge heterostructures annealed for 30 s, 40 s, 50 s, 60 s and 70 s. White lines refer to Lorentzian fit.
spectrograph MS5004i and CCD ANDOR DV401-BV detector. Before each measure the focus on the sample was adjusted supported by a microscope OLYMPUS IX-71. The most intense Si Peak determines the best focus. The final spectrum was obtained by integration of 10 successive measurements with 1 s each one. 4. Results and discussion The GIXRR spectra showed characteristics diamond cubic (1 1 1), (2 2 0) and (3 1 1) Si1 xGex peaks for time annealing up to 30 s. Fig. 1 shows the GIXRR spectra measured in samples annealed at 1000 8C for 30 s, 40 s, 50 s, 60 s and 70 s. The sample
Fig. 2. (a) Average grain size (open circles) and Ge concentration (full circles) vs. annealing time. The average grain size was calculated using Scherrer’s formula and the Ge concentration (xGe) was estimated using Vergard’s Law. (b) Raman shift in samples annealed for 40 s, 50 s, 60 s, 70 s and as-deposit.
A.M.P. dos Anjos et al. / Applied Surface Science 254 (2008) 6055–6058
heated during 30 s shows the same characteristics of as-deposited sample. The GIXRR peaks are shifted from the characteristic points of bulk Si and bulk Ge crystals. As-deposit sample has a nominal Ge concentration (xGe) of 11% approximately. Average xGe estimated from GIXRR (1 1 1), (2 2 0) and (3 1 1) spectra peaks varied from 8% to 19% according to time annealing. Fig. 2(a) shows the average xGe increase as function of the time annealing. Average xGe linearly increase from 40 s to 60 s time annealing and abruptly increase at 70 s of RTA. Micro-Raman measurements performed in characteristic Ge–Ge bonds range (250–310 cm 1) showed that xGe increase not formed detectable Ge–Ge bonds, Fig. 2(b). Note that as-deposit sample shows the Ge–Ge peak bond. The average grain size varied from 2.7 nm to 7.5 nm and decreased with annealing time, as shown in Fig. 2. The silicon and germanium hydrides reactions process have been reported on some works [10–13]. Based on it, some hypothesis may be appointed in order to explain the dynamic effects of RTA in both the grain size diminishing and xGe increase with annealing time. We supposed melted Ge and solid Si, at RTA temperature used and rich hydrogen ambient, formed hydrides like digermanium hexahydride (Ge2H6(l)), germane (GeH4(g)), germilene (GeH2(g)), disilane hexahydride (Si2H6(g)) and silane (SiH4(g)). The index (l) and (g) refers to liquid and gaseous states, respectively. The Si–Si and Ge–Ge bonds are more easily fractured than Si–H and Ge–H bonds. Per example, bond strength of Si–Si is 74 kcal/mol while H–Si is 86 kcal/mol. Mass vanish observed during RTA process may be a consequence of gaseous silicon and germanium hydrides desorption. Based on experimental results Cheng and coauthors [11], suggested three pathways for Si2H6(g) dissociative adsorption. As result, were verified the formation of Si2H5(l), SiH3(l), SiH(l) and SiH4(g). Due to the relative similarity between Si and Ge chemistry these results may be extended to hydrogen–germanium compounds. Standard enthalpy values (DH8) reported by Saalfeld and Svec [13] (7.5 kcal/mol), suggest the germylsilane (GeH6Si) trend formation from silyl and germyl radicals (Si/GeH3). SiGe nanoclusters were obtained using germylsilane as precursor gas [10]. The preferential Si–H bond compared to Ge–Si and Si–Si bonds suggest the grain size diminishing by gradual silicon extraction with annealing time leading to xGe increase. Potential energy calculated using density functional theory for clusters of Si15 xGexH16 and Si15 xGexH18 (x = 1, 2) were reported [11]. The xGe between 6.5% and 13%, approximately, discarding H influences on the lattice size calculated from GIXRR, is coherent with xGe observed here. The dynamic of reaction reported by the authors showed both the fracture of silyl radicals and geometrical rearrange in the clusters cells. These facts confirm the xGe increase and agree with strain changes observed here. Probably, the structural characterization changes observed at different annealing times was able only by the abrupt energy input interruption in RTA process. Coherently with grain size diminishing, was observed the exponential (1 1 1) peak intensity declines with time annealing, as shown in Fig. 3. This behavior disagrees with some works [2,14]. Differently, the general crystalline film quality in our work is quite unaffected by RTA process. Furthermore, the X-ray signal decreases with the crystalline clusters diminishing. Classical continuum elasticity, and less frequently, atomistic methods are typically employed to calculate the strains in nanostructures. The strain tensors are directly related to the lattice parameter mismatch between the nanostructure and matrix boundaries. Usually, the nanocluster is considered isotropic or average of the lattice parameters is used in order to simplify the calculations. In practice, crystallites are usually irregular in shape. The X-ray measures are able to detect small variations in lattice parameter of the different crystalline planes and different strain
6057
Fig. 3. Peak intensity calculated from (1 1 1) GIRXX peak for annealing times of 40 s, 50 s, 60 s and 70 s. The intensity decrease was observed in all GIRXX peaks. This behavior agrees with grain size diminishing.
Fig. 4. Average strain estimated for (1 1 1), (2 2 0) and (3 1 1) GIXRR diffraction peaks at different annealing times. The strain was calculated using Stokes–Wilson formula. The different strain values observed for each annealing time may be interpreted like the lattice parameter changes detect by X-ray for different Bragg angles.
tensors may be estimated using Stokes–Wilson formula. Strain estimated for (1 1 1), (2 2 0) and (3 1 1) peaks at 40 s, 50 s, 60 s and 70 s annealing time, respectively, suggest the structural nanocluster changes during thermal process. Fig. 4 shows the almost same strain values after 40 s of thermal annealing at 1000 8C for crystalline planes (1 1 1) and (2 2 0). As the annealing time is increased, the (1 1 1) plane strain increases, while (2 2 0) plane is gradually relaxed to (3 1 1) plane strain values. Another characterization techniques, like the TEM crosssectional micrography, are needed for accurate investigation of the nanoclusters shape changes. 5. Conclusions The used short time annealing, compared with usual process (hours), indicate a potential time processing optimization in Si–Ge nanoclusters-based devices. Annealing times varying between 30 s and 70 s were used to form SiGe nanoclusters with different Ge concentrations, grain sizes and strain. The results showed the average grain size diminishing and Ge concentration increase as function of the annealing time. The strain in (1 1 1), (2 2 0) and (3 1 1) planes suggests re-arrangements of the nanoclusters geometry accordingly to the time annealing. Moreover, the X-ray structural characterizations suggest that RTA process, under
6058
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the used conditions, may be an optimized way to obtain SiGe nanoclusters to device applications and IC process. Acknowledgments The authors are grateful to the financial support from the Brazilian agencies Faepex-Unicamp, CNPq and FAPESP. References [1] L.K. Teh, W.K. Choi, L.K. Bera, W.K. Chim, Solid-State Electron. 45 (2001) 1963. [2] E.V. Jelenkovic, K.Y. Tong, Z. Sun, C.L. Mak, W.Y. Cheung, J. Vac. Sci. Technol. A15 (1997) 2836. [3] L. Xu, P.J. Mcnally, G.D.M. Dilliway, N.E.B. Cowern, C. Jeynes, E. Mendoza, P. Ashburn, D.M. Bagnall, J. Mater. Sci.: Mater. Electron. 16 (2005) 469.
[4] T.K. Nguyen-Duc, V. Le Thanh, J. Derrien, V. Yam, P. Boucaud, D. Bouchier, J. Phys. IV (France) 132 (2006) 163. [5] S. Tripathi, R. Brajpuriya, A. Sharma, T. Shripathi, S.M. Chaudhari, J. Phys. D: Appl. Phys. 39 (2006) 4848. [6] M.Y. Valakh, V.O. Yukhymchuk, V.M. Dzhagan, O.S. Lytvyn, A.G. Milekhim, A.I. Nikiforov, O.P. Pchelyakov, F. Alsina, J. Pascual, Inst. Phys. Publish.: Nanotechnol. 16 (2005) 1464. [7] H. Trinkaus, B. Hollander, St. Rongen, S. Mantl, H.-J. Herzog, J. Kunchenbecker, T. Hackbarth, Appl. Phys. Lett. 76 (2000) 3552. [8] V. Pillard, P. Puech, P.R.I. Cabarrocas, J. Non-Cryst. Sol. 299 (2002) 280. [9] M.A. Ghazala, W. Beyer, H. Wagner, J. Appl. Phys. 70 (1991) 4540. [10] C.J. Ritter, C. Hu, A.V.G. Chimeshya, J. Tolle, D. Klewer, I.S.T. Tsong, J. Kouvetakis, J. Am. Chem. Soc. 127 (2005) 9855. [11] C.-L. Cheng, D.-S. Tsai, J.-C. Jiang, J. Phys. Chem. C 111 (2007) 13466. [12] J. Takahashi, T. Momose, T. Shida, Bull. Chem. Soc. Jpn. 67 (1994) 74. [13] F.E. Saalfeld, H.J. Svec, J. Phys. Chem. 70 (1966) 1753. [14] C. Te`telin, X. Wallart, D. Stie´venard, P. Nys, D.J. Gravesteijn, J. Vac. Sci. Technol. B16 (1998) 1.