Chapter Results & Discussions IV Chapter Results & Discussions IV: Stress Tuning of Ge Nanocrystals Embedded in Dielectrics 7.1 Introduction In Chapter 6, it has been shown that synthesis of Ge nanocrystals from conventional furnace annealing of co-sputtered SiO2 + Ge films will induce large compressive stresses for the nanocrystals The magnitude of the stress exhibits an intimate link to the annealing temperature, annealing time and the Ge concentration in the co-sputtered films, which in turn determine the size, shape, density and quality of the nanocrystals The origin of the stress is attributed to the fact that the SiO2 matrix is unable to accommodate the growing Ge nucleus However, it should be noted that, due to the volumetric difference between the nanocrystals and the matrix cavity, the intrinsic stress is found always to be compressive in nature In a recent theoretical study, Liu et al have shown that, in the nanocrystalmatrix system, the distribution of the stress and strain field plays an important role in deciding the physical and thermodynamic properties of the nanocrystals [1] - 142 - Chapter Results & Discussions IV Moreover, Wada et al have also presented a potential high performance photodetector by varying the bandgap of Ge film via tensile strain [2] Therefore, it is important to develop a method to alter the stress state of Ge nanocrystals embedded in the dielectrics such that one may be able to tune the bandgap of the nanocrystals In this chapter, three different stress tuning methods will be proposed, namely the annealing techniques, the capping stressor and the dielectric matrix The stress state of Ge nanocrystals will be analyzed using Equation 6.1 highlighted in Section 6.3 7.2 Influence of the Annealing Technique In order to examine the effects of annealing technique on the growth and the stress states of the Ge nanocrystals, two sets of samples were prepared by cosputtering a SiO2 + Ge target in argon at room temperature and annealed by rapid thermal annealing (RTA) for 60 seconds or conventional furnace annealing (CFA) for 15 minutes, respectively Figures 7.1 (a)-(c) are the cross section TEM (XTEM) images of the RTA samples annealed at 800, 900 and 1000°C, respectively It can be seen from these figures that, when RTA is preformed 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 When the annealing temperature was increased to 900 and 1000°C, one can observe that, the nanocrystals grew in size and generally adopted a spherical - 143 - Chapter Results & Discussions IV 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 due to the fact that 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 images of samples furnace annealed at 800, 900 and 1000°C for 15 minutes are shown in Figures 7.1 (d)-(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 7.1 (e), unlike the nanocrystals of the RTA sample shown in the inset of Figure 7.1 (b), 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 7.1 (f) This is probably due to the huge increase of Ge diffusivity at 1000°C, which allow the diffusion of Ge atoms towards the Si substrate - 144 - Chapter Figure 7.1: Results & Discussions IV Ge plus Si oxide sample rapid thermal annealed at (a) 800°C, (b) 900°C, (c) 1000°C for 60 seconds and conventional furnace annealed at (d) 800°C, (e) 900°C, (f) 1000°C for 15 minutes The inset is the HRTEM micrograph of Ge nanocrystal from the corresponding sample - 145 - Chapter Results & Discussions IV Figure 7.2 summarizes the compressive stress experienced by the Ge nanocrystals which was calculated from Equation 6.1 in Section 6.3 It can be seen from the figure that, for the RTA samples, the compressive stress increases gradually from 0.3GPa to 1.2GPa as the annealing temperature increases from 800°C to 1000°C Whereas for the furnace annealed samples, the compressive stress decreases from 0.4GPa to 0.16GPa when the annealing temperature increases from 800°C to 900°C, follow by a sharp increase to 1.3GPa at 1000°C Figure 7.2: Comparison of stress experienced by Ge nanocrystals between RTA and CFA samples 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 7.1 (a) The size of the nanocrystal increases when the annealing temperature reaches 900°C and 1000°C, - 146 - Chapter Results & Discussions IV therefore, it becomes more difficult for the silicon oxide matrix to accommodate those nanocrystals, and hence resulting in a build-up and increase in stress On the other hand, a furnace annealing at 800°C for 15 minutes allows the growth of the nanocrystals due to its longer annealing time This accounts for a relatively higher value of P obtained at 800°C A few groups have suggested that conventional furnace annealing generally leads to higher activation energy for nucleation or a slower crystallization process, as compared to RTA [3,4] This, coupled with the right diffusivity of Ge at 900°C, makes it possible for the nanocrystals to form facets so as to minimize the interfacial energy In the process of faceting, it would be energetically favourable for the nanocrystals to grow along planes that exerted the least pressure on the silicon oxide matrix as it enables them to minimize their strain energy and thus minimize stress for the nanocrystals At 1000°C, the Ge atoms would become molten and 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 in order to minimize the surface-to-volume ratio of the nanocrystals and thus minimize the strain energy of the nanocrystals Note that, under such a large compressive stress, nanocrystals are observed to be defective with multiple twinning, as shown in Figure 7.1 (f) - 147 - Chapter 7.3 Results & Discussions IV Influence of the capping stressor In this section, we present results of stress development of the Ge nanocrystals by applying the silicon nitride (SiN) capping stressor It has been reported that the annealed plasma enhanced chemical vapour deposition (PECVD) SiN films exhibit considerable tensile strain This tensile strain increases with annealing temperature from 750°C and saturates at a value of 1.2GPa at around 1100°C [5,6] This has been suggested to be linked to the release of hydrogen and reformation of Si-N bond network after the annealing [5] A possible reaction is shown as follows Si2-N-H + 2N-H 2Si-N + NH3 (7.1) In order to examine the effect of the SiN capping stressor on the formation and stress state of the nanocrystals, three sets of samples were prepared by cosputtering a SiO2 + Ge target in argon at room temperature One set of samples was free of capping layer to serve as a control sample, while the other two sets of samples were subject to a capping layer growth of either SiN or SiO2 The SiN capping stressor with a thickness of around 200nm was grown by the PECVD technique at 280°C with SiH4 and NH3 as source gases The SiO2 capping layer with a thickness of around 200nm was grown also by the PECVD technique at 280°C with SiH4 and N2O as source gases In addition, the nanocrystals were then synthesized by rapid thermal annealing of the samples from 800°C to 1000°C for 60 seconds Figures 7.3 and 7.4 are the XTEM images of the 1000°C rapid thermal annealed samples without and with the SiO2 cap, respectively As can be seen, for - 148 - Chapter Results & Discussions IV both samples, the nanocrystals are quite uniformly distributed throughout the film with a lineup of the Ge nanocrystals near the Si oxide/Si interface This is probably due to the diffusion of the Ge atoms as mentioned before Though both samples exhibit the very similar distribution pattern of the nanocrystals, it should be noted that the size of nanocrystals from the sample with SiO2 cap is relatively larger as compared to the one without the capping layer This could be explained by the fact that the SiO2 capping layer will limit the out-diffusion of the Ge atoms and therefore enhance the nucleation and growth of the nanocrystals Figure 7.3: Ge plus Si oxide sample rapid thermal annealed at 1000°C for 60 seconds - 149 - Chapter Figure 7.4: Results & Discussions IV Ge plus Si oxide sample with SiO2 capping layer rapid thermal annealed at 1000°C for 60 seconds Figure 7.5 (a) shows the XTEM image of the 1000°C rapid thermal annealed samples with the SiN cap and the inset are the HRTEM image of the Ge nanocrystals As compared to the one without the SiN cap (see Figure 7.3), the sample with the SiN cap exhibited much denser Ge nanocrystals Note that, SiN is commonly used as a barrier material to prevent inter-diffusion of metal and semiconductor Therefore, it is reasonable to expect that, in the sample with the SiN cap, the higher Ge supersaturation would lead to a reduction of barrier to nucleation and hence more nanocrystal formation This is further proven by the secondary ions mass spectrometry (SIMS) results shown in Figure 7.6, whereby - 150 - Chapter Results & Discussions IV comparing to the as-prepared sample, Ge content of the capped sample is still well preserved inside the silicon oxide matrix even after annealing It is also interesting to note that, 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 7.5 (a)) From the energy dispersive x-ray (EDX) analysis shown in the inset of Figure 7.5 (b), 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 silicon oxide cap This probably suggests that the enhanced Ge diffusion may be linked to the large intrinsic tensile strain of the SiN film - 151 - Chapter Figure 7.5: Results & Discussions IV (a) Ge plus Si oxide sample with SiN capping layer rapid thermal annealed at 1000°C for 60 seconds (b) HRTEM image of Si/Si oxide interface (i.e region (I) in (a)) The inset is the EDX spectrum from the voided region of Figure 7.5 (b) - 152 - Chapter Figure 7.6: Results & Discussions IV SIMS profiles of as-sputtered and RTA annealed SiN capped sample Figure 7.7 shows the comparison of the stress experienced by Ge nanocrystal in the sample with and without the SiN cap Note that, 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 out-diffusion of Ge and hence 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 described by Equation 7.1 This suggests that, at such high temperature, the large intrinsic - 153 - Chapter Results & Discussions IV tensile strain of the SiN film starts to dominate over the capping effect, and therefore relax the nanocrystals Figure 7.7: (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 and without SiN cap with RTA at 1000°C for 60seconds - 154 - Chapter 7.4 Results & Discussions IV Influence of the Dielectric Matrix In this section, we evaluate the stress or strain experienced by the Ge nanocrystals in a different dielectric matrix (i.e HfAlO) There are three series of samples with increasing Ge content, namely, Samples A, B and C, prepared for this work According to Rutherford backscattering spectroscopy (RBS) experiment, the Ge content in Samples A, B and C were estimated to be 10, 18 and 22 at.%, respectively All the samples were subject to the rapid thermal annealing for 60 seconds in order to synthesize the Ge nanocrystals It has been reported that, at an annealing temperature of 900°C, the HfAlO matrix is likely to re-crystallize and aid the out-diffusion of Ge atom leading to no formation of Ge nanocrystal [7,8] We have confirmed this by annealing Sample A via RTA at 900°C for 60 seconds Figure 7.8 (a) shows the XTEM image of Sample A annealed at 900°C for 60 seconds It reveals an absence of Ge nanocrystal which is in agreement with the featureless Raman results Note that the diffraction pattern shown in Figure 7.8 (b) indicates that the HfAlO matrix had crystallized The grain boundaries of the HfAlO matrix can act as fast diffusion pathways for the Ge atoms [9] This can lead to a reduction in Ge supersaturation within the matrix and raise the barriers to nucleation The increase in Ge atom diffusion is clearly shown by the SIMS data in Figure 7.9 It can be seen that Sample A upon RTA at 800°C for 60 seconds showed a minimum reduction of Ge content as compared to the as-prepared sample The slight dip of Ge concentration at the film surface is probably due to the out diffusion of Ge to ambient during RTA However, for the sample annealed at 900°C for 60 seconds, - 155 - Chapter Results & Discussions IV the depletion of Ge in the film is very significant As a result, it is reasonable to expect that it is harder to form nanocrystals in samples annealed at 900°C due to the lower Ge concentration This is in agreement with our TEM results of Figure 7.8 Figure 7.8: (a) Cross section transmission electron microscopy (XTEM) image of sample A annealed at 900°C for 60 seconds; (b) The corresponding diffraction pattern of the annealed sample A - 156 - Chapter Figure 7.9: Results & Discussions IV Secondary ion mass spectrometry (SIMS) profiles of as-prepared and annealed (800°C and 900°C for 60 seconds) Samples A Figure 7.10 shows the hydrostatic pressure experienced by the Ge nanocrystals embedded in HfAlO matrix There is a Raman shift from 297 cm-1 (from the annealed sample A) to 300 cm-1 (corresponding sample with free standing nanocrystals) as can be observed from the inset of Figure 7.10 This suggests the nanocrystals embedded inside the HfAlO matrix experiences tensile stress Moreover, Figure 7.10 suggests that the Ge nanocrystals in Sample A are under a tensile stress of about 0.62 GPa However, for Sample C, with same annealing process, the Ge nanocrystals experience a compressive stress of about 0.29 GPa The tensile stress experienced by the HfAlO film upon cooling due to the thermal mismatch between the Si substrate and HfAlO film can be approximated as - 157 - Chapter Results & Discussions IV σ therm = ( Y ) film (α sub − α film )(T − Td ) 1− v (7.2) where Y and ν are the Young’s modules and Poisson’s ratio of the film, respectively, which had been estimated from the mechanical data of HfO2 and Al2O3 to be ~357 GPa and ~0.25 [10-13], αsub = 3.1×10-6 K-1 and αfilm = 7.7×10-6 K-1 are the coefficients of thermal expansion of the Si substrate and the HfAlO film [14], 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 outside due to the difference in stiffness between the matrix and nanocrystals material [15] The pressure exerted on the nanocrystal, PNC can be estimated by PNC = B (1 − v) Pfilm 2Y + 3(1 + v) B (7.3) where B is the bulk modulus of the nanocrystal material (i.e B = 75 GPa for Ge) and Pfilm is the hydrostatic pressure experienced by the matrix [16] From Equations 7.2 and 7.3, we estimated that the Ge nanocrystal was under tensile stress of ~0.86 GPa when annealed at 800°C However, the experimental value for Sample A (i.e 0.62 GPa) is lower than this value This suggests that the nanocrystals might also experience another source of stress that is compressive in nature - 158 - Chapter Results & Discussions IV Figure 7.10: Hydrostatic Pressure (P) experienced by Ge nanocrystals embedded in hafnium aluminium oxide matrix as a function of Ge concentration; the inset shows a set of typical Raman spectra of sample A annealed at 800°C for 60 seconds before and after selective matrix removal Figure 7.10 shows that at higher the Ge concentration, the stress experienced by the Ge nanocrystals becomes more compressive To further investigate the evolution of the stress, two sets of the TEM experiments were done for Samples A and C annealed at 800°C for 60 seconds and Figure 7.11 (a) and (b) are the corresponding XTEM images Figure 7.11 (a) shows that, numerous nanocrystals are uniformly distributed throughout the matrix The inset of the Figure 7.11 (a) reveals that the nanocrystals are approximately ~5 nm in diameter Compare to Sample A, Figure 7.11 (b) shows that much bigger nanocrystals are observed in the annealed Sample C with a diameter of ~10 nm The higher Ge content in Sample C resulted in a higher Ge supersaturation that - 159 - Chapter Results & Discussions IV lowers the barriers to nucleation and enhances the nucleation and growth rate of the nanocrystals With bigger nanocrystals, this can cause a compressive stress to be exerted on the Ge nanocrystals, as had been observed for nanocrystals embedded in a silicon oxide and sapphire matrices [17,18] Therefore, for samples with a higher Ge concentration (Sample C), it is possible for the larger Ge nanocrystals to experience a higher amount of compressive stress When this compressive stress is larger than the tensile stress (due to the thermal mismatch between the substrate and the film), it will result in a net compressive stress (i.e 0.29 GPa) on the sample Figure 7.11: (a) XTEM image of sample A annealed at 800°C for 60 seconds, the inset shows a typical high resolution transmission electron microscopy (HRTEM) image of Ge nanocrystals embedded inside the annealed sample A; (b) XTEM image of sample C annealed at 800°C for 60 seconds, the inset shows a typical HRTEM image of Ge nanocrystals embedded inside the annealed sample C - 160 - Chapter 7.5 Results & Discussions IV Summary It should be pointed out here that there are several reports on the compressive stress experienced by Ge nanocrystals in silicon oxide matrix In the present work, we aim to tune the stress state of the nanocrystals from its intrinsic compressive state to tensile stress by changing the annealing conditions, capping stressor, or the dielectric matrix material By comparing the Raman results of asprepared and free standing nanocrystal samples, we were 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 - 161 - Chapter Results & Discussions IV References [1] L Liu, K L Teo, Z X Shen, J S Sun, E H Ong, A V Kolobov and Y Maeda, “Raman scattering investigation of a Ge/SiO2/Si nanocrystal system under hydrostatic pressure”, Phys Rev B, vol 69, pp 125333, 2004 [2] Y Ishikawa, K Wada, D D Cannon, J F Liu, H C Luan and L C Kimerling, “Strain-induced band gap shrinkage in Ge grown on Si substrate”, Appl Phys Lett., vol 82, pp 2044-2046, 2003 [3] L K Lam, S Chen and D G Ast, “Kinetics of nickel-induced lateral crystallization of amorphous silicon thin-film transistors by rapid thermal and furnace anneals”, Appl Phys Lett., vol 74, pp 1866-1868, 1999 [4] W O Adekoya, J C Muller, and P Siffert, “Annealing kinetics during rapid and classical thermal processing of a laser induced defect in n-type silicon”, Appl Phys Lett., vol 49, pp 1429-1431, 1986 [5] P Morin, E Martinez, F Wacquant and J L Regolini, “Thermal Stress Relaxation of Plasma Enhanced Chemical Vapour Deposition Silicon Nitride”, Mater Res Soc Symp Proc., vol 875, pp O14.6.1, 2005 [6] M P Hughey, and R F Cook, “Massive stress changes in plasmaenhanced chemical vapor deposited silicon nitride films on thermal cycling”, Thin Solid Films, vol 460, pp 7-16, 2004 [7] N Terasawa, K Akimoto, Y Mizuno, A Ichimiya, K Sumitani, T Takahashi, X W Zhang, H Sugiyama, H Kawata, T Nabatame, and A - 162 - Chapter Results & Discussions IV Toriumi, “Crystallization process of high-k gate dielectrics studied by surface X-ray diffraction”, Appl Surf Sci., vol 244, pp.16-20, 2005 [8] P F Lee, X B Lu, J Y Dai, H L W Chan, E Jelenkovic and K Y Tong, “Memory effect and retention property of Ge nanocrystal embedded Hf-aluminate high-k gate dielectric”, Nanotechnology, vol 17, pp 12021206, 2006 [9] Q C Zhang, N Wu, L K Bera, and C X Zhu, “Study of Germanium Out-Diffusion in HfO2 Gate Dielectric of MOS Device on Germanium Substrate”, Mater Res Soc Symp Proc., vol 829, pp 449, 2004 [10] S L Dole, O Hunter, and C J Wooge, “Elastic Properties of Monoclinic Hafnium Oxide at Room Temperature”, J Am Ceram Soc., vol 60, pp 488-490, 1977 [11] D P H Hasselman, “On the Porosity Dependence of the Elastic Moduli of Polycrystalline Refractory Materials”, J Am Ceram Soc., vol 45, pp 452-453, 1962 [12] R M Spriggs, “Effect of Open and Closed Pores on Elastic Moduli of Polycrystalline Ahmina”, J Am Ceram Soc., vol 45, pp 454, 1962 [13] J B Wachtman and D G Lam JR “Young's Modulus of Various Refractory Materials as a Function of Temperature”, J Am Ceram Soc., vol 47, pp 254-260, 1959 [14] R Thielsch, A Gatto, and N Kaiser, “Mechanical Stress and ThermalElastic Properties of Oxide Coatings for Use in the Deep-Ultraviolet Spectral Region”, Appl Opt., vol 41, pp 3211-3217, 2002 - 163 - Chapter [15] Results & Discussions IV S Takeoka, M Fujii, S Hayashi, and K Yamamoto, “Size-dependent near-infrared photoluminescence from Ge nanocrystals embedded in SiO2 matrices”, Phys Rev B, vol 58, pp 7921-7925, 1998 [16] M Haselhoff, K Reimann, and H J Weber, “CuCl nanocrystals in alkalihalide matrices under hydrostatic pressure”, Eur Phys J B, vol 12, pp 147-156, 1999 [17] A Wellner, V Paillard, C Bonafos, H Coffin, A Claverie, B Schmidt, and K H Heinig, “Stress measurements of germanium nanocrystals embedded in silicon oxide”, J Appl Phys., vol 94, pp 5639-5642, 2003 [18] I D Sharp, Q Xu, D O Yi, C W Yuan, J W Beeman, K M Yu, J W Ager III, D C Chrzan and E E Haller, “Structural properties of Ge nanocrystals embedded in sapphire”, J Appl Phys., vol.100, pp 114317114324, 2006 - 164 - ... 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... atoms and therefore the growth of the nanocrystals In addition, it is interesting to note from the inset of Figure 7.1 (e), unlike the nanocrystals of the RTA sample shown in the inset of Figure... hence resulting in a build-up and increase in stress On the other hand, a furnace annealing at 800°C for 15 minutes allows the growth of the nanocrystals due to its longer annealing time This