J. Phys. Chem. C 2008, 112, 9223–9228
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Stress Tuning of Ge Nanocrystals Embedded in Dielectrics F. Zheng,† W. K. Choi,*,† F. Lin,‡ S. Tripathy,‡ and J. X. Zhang§ Department of Electrical and Computer Engineering, National UniVersity of Singapore, 4 Engineering DriVe 3, Singapore 117576, Singapore, Institute of Material Research and Engineering, 3 Research Link, Singapore 117602, Singapore, and Department of Material Science and Engineering, National UniVersity of Singapore, 9 Engineering DriVe 1, Singapore 117576, Singapore ReceiVed: February 20, 2008; ReVised Manuscript ReceiVed: April 3, 2008
Germanium (Ge) nanocrystals have been synthesized by annealing cosputtered SiO2 + Ge or HfAlO + Ge samples in N2 ambient at temperatures ranging from 800 to 1000 °C. We conclude that the annealing technique, the dielectric matrix, and the capping stressor have a significant influence on the formation and stress evolution of the nanocrystals. We show that a careful selective etching of the annealed samples in hydrofluoric acid solution enables the embedded Ge nanocrystals to be liberated from the matrix. From the Raman results of the as-prepared and the etched samples, we establish that the nanocrystals generally experience compressive stress in the Si oxide matrix and that the stress state of the nanocrystal can be tuned by the different matrix material as well as by the capping stressor. 1. Introduction Metal or semiconductor nanostructures are currently under intensive study for both practical and fundamental reasons.1–4 There are substantial reports on the synthesis and characterization of Ge nanocrystals with the majority focused on the luminescence properties of the nanocrystals.1,4–6 There are two commonly encountered methods of fabrication of Ge nanocrystals in silicon oxide matrix, namely, by cosputtering of Ge and silicon oxide targets1,4 or by implanting Ge into silicon oxide films.5 All cosputtered or ion-implanted samples required hightemperature anneal for the synthesis of nanocrystals. The main reason for the keen interest on nanocrystals embedded in dielectrics is due to its possible applications in optoelectronic and memory devices.1,7 Liu et al. have recently shown that, in the nanocrystal-matrix systems, the distribution of the stress and strain field plays an important role in deciding the physical and thermodynamic properties of the nanocrystals.8 Moreover, Ishikawa et al. have also suggested a way of fabricating the potential high performance photodetector by varying Ge band gap via tensile strain.9 In our previous work, the stress state of Ge nanocrystals embedded in Si oxide matrix was studied. It was found that the compressive stress exerted on Ge nanocrystals by the silicon oxide matrix has a significant effect on its shape and distribution.10 In this study, the effects of the annealing conditions, of the capping stressor, and of the dielectric matrix on the stress development of Ge nanocrystals were investigated. 2. Experimental Section The samples were prepared by cosputtering a SiO2 + Ge or HfAlO + Ge target in argon at room temperature. The target was a 3 in. SiO2 disk or HfAlO disk with pieces of Ge (99.999% * Author to whom correspondence should be addressed. E-mail: elechoi@ nus.edu.sg. † Department of Electrical and Computer Engineering, National University of Singapore. ‡ Institute of Material Research and Engineering. § Department of Material Science and Engineering, National University of Singapore.
pure, 5 mm × 10 mm × 0.3 mm) attached. The sputtering pressure and power were fixed at 3 × 10-3 mbar and 100 W, respectively. The thickness of the sputtered film was approximately 300 nm. The silicon nitride (SiN) capping stressor with a thickness of around 200 nm was grown by the plasmaenhanced chemical vapor deposition (PECVD) technique at 280 °C with SiH4 and NH3 as source gases. The synthesis of Ge nanocrystals was carried out either by conventional furnace annealing (CFA) in N2 ambient from 800 to 1000 °C for 15 min or by rapid thermal annealing (RTA) in N2 ambient from 800 to 1000 °C for 60 s. The Ge nanocrystals were studied by using transmission electron microscopy (TEM) using a JEOL 2010F system with an operating voltage of 200 kV. The distribution profiles of Ge of the as-prepared and annealed samples were obtained by secondary-ion mass spectrometry (SIMS) experiments using 25 kV Ga for analysis and 3 kV Cs for sputtering. In the estimation of stress experienced by the nanocrystals, the Raman spectroscopy was first performed on the annealed cosputtered samples. We followed the procedure outlined by Sharp et al.11 and dipped the samples in 1:1 HF:H2O solution to selectively etch away the oxide to obtain free-standing nanocrystals (see Figure 1). After the selective etching, the piling up of the free-standing nanocrystals on Si substrate was observed by cross section transmission electron microscopy (XTEM) as shown in Figure 1d and Figure 1e. This could be explained by the attractive van der Waals forces. These free-standing nanocrystals were stable as the surface oxidation of the nanocrystals was self-limiting (see Figure 1e). This has also been reported by Sharp et al.11 Another round of Raman experiments was performed on these samples to obtain the Raman peak position of the free-standing nanocrystals. The magnitude of the stress was then calculated10,11 using the following equation for hydrostatic pressure P:
P )
ωembedded - ωetched 3 γω0 (S11 + 2S12)
(1)
where ωembedded and ωetched are the Raman peak positions of nanocrystals embedded in the dielectric matrix and of free-
10.1021/jp801529j CCC: $40.75 2008 American Chemical Society Published on Web 05/29/2008
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Figure 1. (a-c) The schematic of the chemical etching process to obtain free-standing Ge nanocrystals. (d) The XTEM micrograph of freestanding Ge nanocrystals. (e) The high-resolution transmission electron microscopy (HRTEM) micrograph of the nanocrystals.
standing Ge nanocrystals, respectively, ω0 is the Raman peak position of bulk Ge, γ ) 0.89 cm-2 is the mode-Gru¨neisen parameter,12 and S11 and S12 are components of the elastic compliance tensor with S11 + 2S12 ) 0.44 × 10-12 dyn-1 cm2. 3. Results and Discussion 3.1. Influence of the Annealing Technique. To examine the effects of the annealing technique on the growth and the stress states of the Ge nanocrystals, two sets of samples were annealed by RTA and CFA, respectively. Figure 2a-c is the XTEM micrographs of the RTA samples with annealing temperatures of 800, 900, and 1000 °C, respectively. It can be seen from the figure that when RTA is at 800 °C numerous small Ge nanocrystals are uniformly distributed in the entire bulk of the film. The voided region at the surface of the film can be explained by the out-diffusion of Ge to the ambient.13 When the annealing temperature was further increased to 900 and 1000 °C, one could observe that the nanocrystals grew in size and generally adopted a spherical shape. The size variation of the nanocrystals is also greater as compared to the one annealed at 800 °C indicating that coarsening has taken place or is in the process of occurring. This is most likely because when annealed at 900 and 1000 °C, Ge atoms are able to overcome kinetic limitations and enhance the nucleation and growth of the nanocrystals. The XTEM micrographs of samples furnace-annealed at 800, 900, and 1000 °C for 15 min are shown in Figure 2d-f, respectively. In comparison to the samples annealed by RTA, the furnace-annealed samples generally exhibit larger nanocrystals at the same annealing temperatures. The size variation of the nanocrystals is attributed to the much longer annealing duration of furnace annealing which assists the diffusion of Ge atoms and therefore the growth of the nanocrystals. In addition, it is interesting to note from the inset of Figure 2e that the nanocrystals synthesized using furnace annealing at 900 °C were well formed showing facets that are bounded by crystal planes. This implies that it is possible to attain the equilibrium interface energy minimizing configuration at this condition. Moreover, one can observe the lineup of the Ge nanocrystals near the Si oxide/Si interface in Figure 2f. This is probably due to the huge
increase of Ge diffusivity at 1000 °C, which allows the diffusion of Ge atoms toward the Si substrate. Figure 3 summarizes the compressive stress experienced by the Ge nanocrystals which was calculated from eq 1. It can be seen from the figure that, for the RTA samples, the compressive stress increases gradually from 0.3 GPa to 1.2 GPa as the annealing temperature increases from 800 to 1000 °C. For the furnace-annealed samples, the compressive stress decreases from 0.4 GPa to 0.16 GPa when the annealing temperature increases from 800 to 900 °C followed by a sharp increase to 1.3 GPa at 1000 °C. It is reasonable to expect that, for the RTA samples, the low compressive stress experienced by the nanocrystals when annealed at 800 °C is because of the small size of the nanocrystals as observed in Figure 2a. The size of the nanocrystal increases when the annealing temperature reaches 900 and 1000 °C; therefore, it becomes more difficult for the silicon oxide matrix to accommodate those nanocrystals resulting in a buildup and increase in stress. On the other hand, the furnace annealing at 800 °C allows the growth of the nanocrystals because of its longer annealing time. This accounts for a relatively higher value of P obtained at 800 °C. A few groups have suggested that CFA generally leads to higher activation energy for nucleation or a slower crystallization process as compared to RTA.14,15 This, coupled with the right diffusivity of Ge at 900 °C, makes the nanocrystals able to form facets so as to minimize the interfacial energy. In the process of faceting, it would be energetically favorable for the nanocrystals to grow along planes that exerted the least pressure on the SiO2 matrix as it enables them to minimize their strain energy and thus to minimize stress for the nanocrystals. At 1000 °C, the Ge atoms would become molten and would lose their atomic ordering. This resulted in a significant increase in the diffusivity of the Ge atoms. Consequently, the nanocrystals will form very rapidly giving rise to a large compressive stress exerted on the nanocrystals. This large compressive stress will then cause the nanocrystals to adopt a spherical shape to minimize the surface-to-volume ratio of the nanocrystals and thus to minimize the strain energy of the nanocrystals. Under such a large compressive stress, nanocrystals are observed to be defective with multiple twinning shown in Figure 2f.
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Figure 3. Comparison of stress experienced by Ge nanocrystals between RTA and CFA samples.
Figure 2. Ge plus Si oxide sample rapid thermal annealed at (a) 800 °C, (b) 900 °C, and (c) 1000 °C for 60 s and conventional furnace annealed at (d) 800 °C, (e) 900 °C, and (f) 1000 °C for 15minutes. The inset is the HRTEM micrograph of Ge nanocrystal from the corresponding sample.
3.2. Influence of the Capping Stressor. In this section, we present results of stress development of the Ge nanocrystals by applying the SiN capping stressor. It has been reported that the annealed PECVD SiN films exhibit considerable tensile strain. This tensile strain increases with annealing temperature from 750 °C and saturates at a value of 1.2 GPa at around 1100 °C.16–18 This is suggested to be linked to the release of hydrogen and reformation of Si-N bond network after the annealing.16 The possible reaction is shown as follows.
Si2-N-H + 2N-H f 2Si-N + NH3
(2)
Figure 4a and b shows the XTEM micrographs of the 1000 °C rapid thermal annealed samples without and with the SiN cap, respectively. As can been seen, the sample without the SiN cap exhibited much less Ge nanocrystals as compared to the SiN capped sample. SiN is commonly used as a barrier material to prevent interdiffusion of metal and semiconductor. Therefore,
it is reasonable to expect that, in the sample with the SiN cap, the relatively higher Ge supersaturation would lead to a reduction of barrier to nucleation and hence more nanocrystal formation. This is further proven by the SIMS results shown in Figure 5, whereby comparing to the as-prepared sample, Ge content of the capped sample is still well preserved inside the silicon oxide matrix even after annealing. For the SiN capped sample, there are voided regions of Ge nanocrystal near the SiN/Si oxide interface and Si/Si oxide interface (see Figure 4c). From the energy-dispersive X-ray (EDX) analysis shown in Figure 4d, the Ge content near the Si/Si oxide interface is estimated to be less than 2%. This is in good agreement with the SIMS result, whereby a very significant Ge depletion near the interface was observed. However, such a phenomenon was not found from both the sample without the SiN cap and the control sample with the Si oxide cap. This probably suggests that the enhanced Ge diffusion may be linked to the large intrinsic tensile strain of the SiN film. Figure 6 shows the comparison of the stress experienced by Ge nanocrystal in the sample with and without the SiN cap. There are two competing factors which will influence the stress state of the nanocrystals in dielectrics: one is the capping effect and the other is temperature-dependent intrinsic tensile strain of SiN film. At a low temperature of 800 °C, the breakage of N-H bond and the reformation of Si-N bond network is very minimal and results in the insignificant intrinsic stress. Coupled with the fact that the SiN cap will effectively prevent the outdiffusion of Ge and hence will lead to denser nanocrystals, the stress experienced by the Ge nanocrystal from the SiN cap sample is therefore expected to be slightly higher as compared to the uncapped sample. However, when the annealing temperature is increased to 900 and 1000 °C, although the nanocrystals are denser, the observed compressive stress of the nanocrystals is very much reduced owing to the reformation of the Si-N network shown in eq 2. This suggests that, at such high temperature, the large intrinsic tensile strain of the SiN film starts to dominate over the capping effect and therefore relaxes the nanocrystals. 3.3. Influence of the Dielectric Matrix. In this section, we evaluate the stress or strain experienced by the Ge nanocrystals in different dielectric matrices (i.e., Si oxide and HfAlO). It has been reported that, at an annealing temperature of 900 °C, the HfAlO matrix is likely to recrystallize and aids the outdiffusion of Ge atom leading to no formation of Ge nanocrystal.3,19,20 Therefore, we will limit the discussion here regarding the stress development and only focus on the samples subjected to the RTA at 800 °C.
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Figure 4. (a) Uncapped Ge plus Si oxide sample rapid thermal annealed at 1000 °C for 60 s. (b) SiN capped Ge plus Si oxide sample rapid thermal annealed at 1000 °C for 60 s; the inset is the HRTEM micrograph of Ge nanocrystal from the corresponding sample. (c) The enlarged region I of b. (d) The EDX spectrum from region II of c.
sample with free-standing nanocrystals) observed from the inset of Figure 8. This suggests that the nanocrystals embedded inside the HfAlO matrix experience tensile strain. It can be seen from Figure 8 that the Ge nanocrystals in HfAlO are under a tensile strain of about 0.62 GPa. However, for Si oxide matrix sample, the Ge nanocrystals experience a compressive stress of about 0.3 Gpa. The tensile strain experienced by the HfAlO film upon cooling because of the thermal mismatch between the Si substrate and the HfAlO film can be approximated as Figure 5. SIMS profiles of as-sputtered and RTA annealed SiN capped sample.
Figure 7a and b shows the XTEM micrographs of samples employing the Si oxide and HfAlO as the host materials with RTA at 800 °C. The Ge nanocrystals in HfAlO matrix were much less than those in Si oxide matrix. The slower nucleation and growth rate of the nanocrystals in the HfAlO matrix implies that the enthalpy of mixing between the HfAlO and Ge phase is more negative than the Si oxide and Ge phase. From thermodynamic calculations and study on the Si oxide plus Ge system, it has been concluded that Ge is almost insoluble in Si oxide.21,22 However, it has been also found that thermal processing of HfO plus Ge systems can lead to the formation of hafnium germinate (HfGeOx).23,24 Therefore, the formation of the hafnium germinate phase during annealing of the HfAlO plus Ge samples could reduce the Ge atoms available for nucleation and growth by binding them to the matrix atoms. There is a typical Raman shift from 297 cm-1 (for the annealed HfAlO plus Ge sample) to 300 cm-1 (corresponding
σtherm )
( 1 -Y ν )
(Rsub - Rfilm)(T - Td)
film
(3)
where Y and ν are the Young’s modules and the Poisson’s ratio of the film, respectively, which were estimated from the mechanical data of HfO2 and Al2O3 to be ∼357 GPa and ∼0.25;25 Rsub ) 3.1 × 10-6 K-1 and Rfilm ) 7.7 × 10-6 K-1 and are the thermal expansion coefficients of the Si substrate and the HfAlO film.26 T and Td are the process temperature and the environmental temperature, respectively. The pressure acting on the nanocrystals should be different from the pressure applied from the outside7 because of the difference in stiffness between the matrix and the nanocrystal material. Therefore, the pressure exerted on the nanocrystal, PNC, could be estimated by
PNC )
9B(1 - ν) P 2Y + 3(1 + ν)B film
(4)
where B is the bulk modulus of the nanocrystal material (i.e., B ) 75 GPa for Ge) and Pfilm is the hydrostatic pressure
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Figure 6. (a) Comparison of stress experienced by Ge nanocrystals between uncapped and SiN capped samples. (b) Typical Raman spectra of as-grown and etched samples with RTA at 1000 °C for 60 s.
Figure 7. XTEM micrographs of (a) Ge plus Si oxide and (b) Ge plus HfAlO samples with RTA at 800 °C for 60 s. The inset is the HRTEM micrograph of Ge nanocrystal from the corresponding sample.
4. Conclusions
Figure 8. Comparison of stress experienced by Ge nanocrystals between Si oxide matrix and HfAlO matrix samples with RTA at 800 °C for 60 s; the inset is the typical Raman spectra of as-grown and etched Ge plus HfAlO samples with RTA at 800 °C for 60 s.
There are several reports on the compressive stress experienced by Ge nanocrystals in silicon oxide matrix.4,10,11 In the present study, we aim to tune the stress state of the nanocrystals from its intrinsic compressive state to tensile stress by changing the annealing conditions, the capping stressor, or the dielectric matrix material. By comparing the Raman results of as-prepared and free-standing nanocrystal samples, we are able to quantitatively study the hydrostatic pressure experienced by the Ge nanocrystals in different dielectric matrices. Among the three tuning methods, we are only able to change the stress state of the nanocrystals from compressive to tensile by introducing the dielectric matrix to that of HfAlO. With the other two methods, one can engineer the amount of compressive stress experienced by the nanocrystals in silicon oxide matrix. This is very important as the stress state of Ge is intimately connected to the band gap of the material.
experienced by the matrix.27 From eqs 3 and 4, we estimated that the Ge nanocrystals in HfAlO matrix were under tensile strain of around 0.86 GPa when annealed at 800 °C which is fairly close to the experimental value of 0.62 GPa.
Acknowledgment. F. Zheng would like to acknowledge the provision of a research scholarship from National University of Singapore and the provision of research stipends from the Chartered Semiconductors Manufacturing Ltd. We would also
9228 J. Phys. Chem. C, Vol. 112, No. 25, 2008 like to acknowledge the Singapore-MIT Alliance for providing financial assistance for this work. References and Notes (1) (a) Choi, W. K.; Chim, W. K.; Heng, C. L.; Teo, L. W.; Ho, V.; Ng, V.; Antoniadis, D. A.; Fitzgerald, E. A. Appl. Phys. Lett. 2002, 80, 2014. (b) Choi, W. K.; Ho, V.; Ng, V.; Ho, Y. W.; Ng, S. P.; Chim, W. K. Appl. Phys. Lett. 2005, 86, 143114. (2) (a) Fang, X. S.; Bando, Y.; Ye, C. H.; Shen, G.; Gautam, U. K.; Tang, C.; Golberg, D. Chem. Commun. 2007, 29, 3048. (b) Fang, X. S.; Bando, Y.; Shen, G. Z.; Ye, C. H.; Gautam, U. K.; Costa, P. M. F. J.; Zhi, C. Y.; Tang, C. C.; Golberg, D. AdV. Mater. 2007, 19, 2593. (3) Zheng, F.; Chew, H. G.; Choi, W. K.; Zhang, J. X.; Seng, H. L. J. Appl. Phys. 2007, 101, 114310. (4) (a) Kolobov, A. V.; Wei, S. Q.; Yan, W. S.; Oyanagi, H.; Maeda, Y.; Tanaka, K. Phys. ReV. B 2003, 67, 195314. (b) Yue, L.; He, Y. J. Appl. Phys. 1997, 81, 2910. (5) (a) Heinig, K. H.; Schmidt, B.; Markwitz, A.; Gro¨tzschel, R.; Strobel, M.; Oswald, S. Nucl. Instrum. Methods Phys. Res., Sect. B 1999, 148, 969. (b) Teo, K. L.; Kwok, S. H.; Yu, P. Y.; Guha, S. Phys. ReV. B 2000, 62, 1584. (c) Tyschenko, I. E.; Talochkin, A. B.; Cherkov, A. G.; Zhuravlev, K. S.; Yankov, R. A. Solid State Commun. 2004, 129, 63. (6) Talochkin, A. B.; Teys, S. A.; Suprun, S. P. Phys. ReV. B 2005, 72, 115416. (7) (a) Takeoka, S.; Fujii, M.; Hayashi, S.; Yamamoto, K. Phys. ReV. B 1998, 58, 7921. (b) Min, K. S.; Shcheglov, K. V.; Yang, C. M.; Atwater, H. A.; Brongersma, M. L.; Polman, A. Appl. Phys. Lett. 1996, 68, 2511. (c) Kim, J. K.; Cheong, H. J.; Kim, Y.; Yi, J. Y.; Bark, H. J. Appl. Phys. Lett. 2003, 82, 2527. (8) Liu, L.; Teo, K. L.; Shen, Z. X.; Sun, J. S.; Ong, E. H.; Kolobov, A. V.; Maeda, Y. Phys. ReV. B 2004, 69, 125333. (9) (a) Ishikawa, Y.; Wada, K.; Cannon, D. D.; Liu, J. F.; Luan, H. C.; Kimmerling, L. C. Appl. Phys. Lett. 2003, 82, 2044. (b) Liu, J.; Cannon, D. D.; Wada, K.; Ishikawa, Y.; Danielson, D. T.; Jongthammanurak, S.; Michel, J.; Kimmerling, L. C. Phys. ReV. B 2004, 70, 155309. (10) (a) Choi, W. K.; Chew, H. G.; Zheng, F.; Chim, W. K.; Foo, Y. L.; Fitzgerald, E. A. Appl. Phys. Lett. 2006, 89, 113126. (b) Chew, H. G.; Zheng, F.; Choi, W. K.; Chim, W. K.; Foo, Y. L.; Fitzgerald, E. A. Nanotechnology 2007, 18, 065302.
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