Synthesis and stress analysis of germanium nanocrystals embedded in dielectric matrices 2

Chapter Results & Discussions II Chapter Results & Discussions II: Synthesis and Growth of Ge Nanocrystals in High-κ Dielectric Matrix 5.1 Introduction Due to the aggressive scaling requirement, Electrically Erasable Programmable Read-Only Memory (EEPROM) is facing the problem of the 10 years charge retention requirement for non-volatility The consequence of this restriction causes the programming and erasing speed of the EEPROM to reach a plateau and prevents the applied gate voltage from being scaled down further To overcome these constraints, the first nanocrystal based flash memory structures were proposed by Tiwari et al [1], using discrete nanocrystals instead of a continuous floating gate as charge storage nodes to reduce the local defect related leakage and improve data retention [2] Although the first nanocrystal based flash memory was demonstrated using Si nanocrystals embedded in silicon oxide matrix [1], several groups have demonstrated a better performance of Ge nanocrystal based memory devices over - 101 - Chapter Results & Discussions II Si nanocrystal based devices [3-6] For example, Lu et al have pointed out that Ge has a narrower bandgap and a similar electron affinity as Si which provides a higher confinement barrier for the retention mode and a smaller barrier for the program and erase modes for the memory devices [6] Meanwhile, various high dielectric constant (high-κ) materials have also been suggested to replace the gate oxide due to the relentless scaling down of the gate oxide thickness Among which, hafnium oxide (HfO2) based material has emerged as one of the potential candidates because of its reasonably high dielectric constant, low leakage current, comparatively higher crystallization temperature and bandgap [7] There has been a demonstration of very good flash memory characteristics recently based on Ge nanocrystals embedded in hafnium aluminium oxide (HfAlO) matrix [8,9] However, to date, there is a lack of a comprehensive study on the formation and growth of Ge nanocrystals in such a material In this chapter, the results of an investigation on the effects of annealing temperature, annealing time and Ge concentration on the formation of Ge nanocrystals embedded in HfAlO matrix will be presented Three series of samples with increasing Ge content, namely, Samples A (4 at.%), B (10 at.%) and C (22 at.%), were prepared for this work We will also present a discussion on the comparison of the growth of Ge nanocrystals in silicon oxide and HfAlO matrices - 102 - Chapter 5.2 Results & Discussions II Ge nanocrystals in HfAlO matrix In order to synthesize the Ge nanocrystals in the HfAlO matrix, Samples A were annealed at 700 to 1000°C for durations varying from 15 to 60 minutes in N2 ambient The Raman spectra of these annealed samples were featureless indicating the absence of Ge nanocrystals (see Figure 5.1) This implies that there are no Ge nanocrystals in the HfAlO matrix even when the temperature reaches as high as 1000°C Note that, from the RBS results, it was estimated that Samples A contained a relatively low amount of Ge of around at.% in the HfAlO matrix It is therefore reasonable to expect that the low Ge concentration will increase the barriers to nucleation, thus making it difficult for Ge atoms to nucleate and form nanocrystals As a result, no Ge peak can be observed in the Raman spectra of these samples Figure 5.1: Raman spectra for Sample A annealed for 15 minutes to 60 minutes from 700 to 1000°C in N2 - 103 - Chapter Results & Discussions II In order to lower the barriers to nucleation, a series of Sample B was prepared with the higher Ge concentration of about 10 at.% These samples were also annealed at the same conditions as Samples A Figure 5.2 shows the Raman spectra of annealed Samples B It can be seen from this figure that at 700°C, after annealing for 15 minutes, a broad peak can be detected at ~289.4 cm-1 For the same annealing duration of 15 minutes, when the annealing temperature was increased to 800°C, a peak at ~ 299.6 cm-1 with a full width at half maximum (FWHM) value of cm-1 can be observed indicating the presence of Ge nanocrystals in the HfAlO matrix However, at higher annealing temperatures (i.e 900 and 1000°C), no Ge peak can be detected Note that increasing the annealing duration to 60 minutes made no observable changes to all the spectra in Figure 5.2 as compared to those annealed for 15 minutes Figure 5.2: Raman spectrum for Sample B annealed for 15 minutes to 60 minutes from 700 to 1000°C in N2 - 104 - Chapter Results & Discussions II Figures 5.3 (a) and (b) are the cross section TEM (XTEM) images of sample B annealed at 800oC for 15 and 60 minutes, respectively Figure 5.3 (a) shows that the nanocrystals are uniformly distributed throughout the matrix and they are approximately ~4 nm in diameter All these nanocrystals show clear lattice fringes (see inset of Figure 5.3 (a)) which indicates good crystallinity of the nanocrystals It is found that a longer annealing duration of 60 minutes enabled the nanocrystal to increase its diameter to ~6.5 nm (see inset of Figure 5.3 (b)) These nanocrystals also exhibited clear lattice fringes and were evenly distributed in the HfAlO matrix Meanwhile, the diffraction pattern obtained from sample B annealed at 800°C for 15 minutes shown in Figure 5.4 indicates that the HfAlO matrix remain amorphous Note that, a similar pattern was also observed for the sample annealed for 60 minutes - 105 - Chapter Results & Discussions II Figure 5.3: TEM micrographs of sample B annealed at 800°C for (a) 15 minutes and (b) 60 minutes The insets are the HRTEM micrographs of the nanocrystals from the corresponding samples Figure 5.4: The diffraction pattern for sample B annealed at 800°C for 15 minutes - 106 - Chapter Results & Discussions II According to simulation results of She et al [10], for FLASH applications, the optimum size of Ge nanocrystals embedded in SiO2 should be nm in diameter for the most efficient write/erase speed and retention time For the case of Ge nanocrystals embedded in HfAlO, the optimum size should also be similar or slightly larger as the barrier height is lower [11] Thus the present way to synthesize Ge nanocrystals provides a promising way of achieving the ideal size for Ge nanocrystal FLASH memory devices The size required could be obtained by controlling the annealing duration, i.e if a larger size is desired, the annealing time could be increased The XTEM image of sample B annealed at 900°C for 15 minutes (see Fig 5.5 (a)) reveals that there is an absence of Ge nanocrystals in the HfAlO film This is in good agreement with the featureless Raman spectrum of this sample shown in Figure 5.2 Figure 5.5 (b) shows the diffraction pattern of this sample annealed at 900°C for 15 minutes It shows that the HfAlO film had crystallized when annealed under this condition The crystallization of HfAlO at 900°C has also been reported by Teresawa et al [7] The crystallized HfAlO structure could enhance the diffusion of Ge atoms through grain boundaries [12] and lead to a reduction in Ge concentration in the HfAlO film This would raise the barriers to nucleation Furthermore, a higher annealing temperature will lead to an increase in the critical nucleus size and would also raise the barriers to nucleation - 107 - Chapter Figure 5.5: Results & Discussions II (a) Cross section transmission electron microscopy (TEM) image of sample B annealed at 900°C 15 minutes anneal, (b) the corresponding diffraction pattern of the annealed sample Figure 5.6 (b) shows the SIMS results of the as-sputtered sample B and that annealed at 900°C for 15 minutes It was found that there is indeed significant diffusion of the Ge atoms away from the bulk of the HfAlO film either into the ambient or into the Si substrate As a result, it is reasonable to expect no Ge nanocrystals in samples annealed at 900°C One would also expect the reduction of Ge in the HfAlO matrix to increase further for a longer annealing duration This will definitely prevent the formation of Ge nanocrystals in the HfAlO matrix, as confirmed by the featureless Raman spectrum of such sample in Figure 5.2 At a higher annealing temperature of 1000°C, firstly, the diffusivity of the Ge atoms would be even higher than that at 900°C as the Ge atoms would be in its liquid state and, secondly, the HfAlO matrix would crystallize even more These - 108 - Chapter Results & Discussions II two factors will cause a significant depletion of Ge in the HfAlO matrix, thus make it impossible for the nucleation and growth of the nanocrystals Again, this is confirmed by the Raman spectra of samples annealed at 1000°C for 15 and 60 minutes shown in Figure 5.2 Figure 5.6: (a) Secondary ion mass spectrometry (SIMS) profiles of assputtered and annealed (800°C for 15 and 60 minutes) Samples B, (b) SIMS profiles of as-sputtered and annealed (900°C for 15 minutes) Samples B In comparison, Figure 5.6 (a) shows the SIMS results of the as-sputtered sample B and those annealed at 800°C for 15 and 60 minutes The Ge concentration profiles show that there is virtually no change in the Ge concentration in the bulk of the HfAlO matrix for the as-prepared and the annealed samples There is a slight reduction in the Ge concentration at the surface of the annealed samples probably due to outdiffusion of Ge when annealed at elevated temperatures There is also some indication of Ge diffusion to the Si substrate when annealed as there is an increase in the Ge concentration near the HfAlO-Si interface for the annealed samples On the whole, Fig 5.6 (a) - 109 - Chapter Results & Discussions II indicates a good retention of Ge in the bulk of the HfAlO film when annealed at 800°C This provides the Ge supersaturation and driving force for the Ge nanocrystal formation and growth 5.3 Ge nanocrystals in crystallized HfAlO matrix It has been reported that the existence of Ge and Ge diffusion may decrease the crystallization temperature of HfAlO [13] In order to examine the effects of matrix crystallization and Ge concentration on the formation of Ge nanocrystals, a series Sample C (Ge content 23 at.%) were prepared Figure 5.7 shows the Raman spectra of Samples C annealed at 700°C and 800°C for 15 minutes and 60 minutes Similar to samples B, samples C also show a broad amorphous Ge band when annealed at 700°C for 15 and 60 minutes However, a sharp Ge peak located at ~300 cm-1 with a FWHM of cm-1 is observed for Sample C annealed at 800°C for 15 minutes Further increase in the annealing duration to 60 minutes resulted in the disappearance of such peak It should be pointed out here that annealing Sample C at 900°C and beyond resulted in severe bubbling and evaporation of the HfAlO film This is probably due to the fact that HfAlO film with such a high Ge content will significantly deteriorate the film quality and thus reduce its thermal stability - 110 - Chapter Figure 5.7: Results & Discussions II Raman spectrum for Sample C annealed at 700 and 800°C in N2 for 15 and 60 minutes Figures 5.8 (a) and (b) show the XTEM and HRTEM images of a Sample C annealed at 800°C for 15 minutes, respectively Numerous Ge nanocrystals can be observed in Figure 5.8 (a) and this agrees with the clear Raman peak observed in Figure 5.7 Note that from the energy dispersive X-ray (EDX) analysis, the Ge content at the bulk of the films and at the HfAlO/Si interface was estimated to be 11.8 and 17.6 %, respectively However, unlike Sample B, it is found that the HfAlO film started to crystallize at 800°C for Sample C, as can be clearly seen from the lattice fringes found in the HfAlO film in Figure 5.8 (b) This is further proven by the diffraction pattern of the sample (see the inset of Figure 5.8 (a)) In addition, X-ray diffraction (XRD) spectrum of the sample shown in Figure 5.9 clearly confirms that there is indeed Ge reflection peak and that the matrix has - 111 - Chapter Results & Discussions II crystallized with the occurrence of the HfO2 (211), (400), (402) and (611) reflection peaks [13] It should be noted that, for sample B annealed at the same condition, the HfAlO matrix remained amorphous with a weak HfO2 halo at ~35° and the Ge (111) and (220) reflections [14] Liu et al have also reported a decrease in the crystallization temperature for HfAlO films with the incorporation of Ge They observed localized crystallization of the HfAlO plus Ge films at 800°C [15] This is in good agreement with our results shown in Figure 5.8 (b) In fact, by comparing the results of Figure 5.3 (a) and 5.8 (a), we show that the incorporation of Ge into HfAlO matrix will lower the crystallization temperature of the HfAlO film from 900 to 800°C when the Ge concentration in the matrix reaches a value of ~ 23.3 at.% Figure 5.8: (a) XTEM and (b) high magnification XTEM of Sample C annealed for 15 minutes at 800°C in N2 The inset is the diffraction pattern of the sample in (a) showing that it has crystallized - 112 - Chapter Figure 5.9: Results & Discussions II θ-2θ x-ray diffraction patterns of the samples annealed at different conditions The SIMS profiles of the as-sputtered and annealed (at 800°C for 15 minutes) Samples C are shown in Figure 5.10 Note that annealing at 800°C for 15 minutes causes a reduction of Ge concentration from 23.3 to ~11.5 at.% in the bulk of the HfAlO film and an accumulation of Ge atoms near the HfAlO/Si interface This is in good agreement with the EDX results mentioned above In contrast to Sample B which showed minimum Ge outdiffusion (see Figure 5.6 (a)) upon the same annealing conditions, this ~50% reduction in Ge content in the bulk of the sample is due to the outdiffusion of Ge atoms caused by (i) the crystallization of the HfAlO matrix and (ii) the higher Ge content in the original as-prepared sample C that creates a relatively steeper concentration gradient - 113 - Chapter Results & Discussions II between the film and the surroundings (c.f Sample B); that provides a larger driving force for Ge to diffuse away from the matrix [16,17] Figure 5.10: SIMS depth profile of Ge concentration in the HfAlO matrix for as-sputtered sample and sample annealed for 15 minutes at 800°C in Sample C It should be pointed out that, although the crystallization of the HfAlO matrix of Sample C makes it difficult for the nucleation of the nanocrystals as more Ge atoms would be able to diffuse out of the matrix via the grain boundaries, there is still ~11.5 at.% of Ge in the HfAlO matrix From the results of Sample B, a Ge concentration of ~10.5 at.% would be sufficient for the formation of nanocrystals when annealed at 800°C for 15 minutes Therefore, a - 114 - Chapter Results & Discussions II critical Ge concentration (at a particular annealing temperature) is the most important factor that determines the formation of the nanocrystals It has been also observed from the TEM image of a Sample C annealed at 800°C for 60 minutes (see Figure 5.11) that there was a reduction in the number of Ge nanocrystals in the bulk of the HfAlO film when the annealing time was increased from 15 to 60 minutes The EDX analysis showed that there was a significant decrease in the Ge concentration in the bulk of the matrix to 4.5 at.% The Ge concentration near the HfAlO/Si interface was estimated to be 20.3 at.% Figure 5.11: Cross section transmission electron microscopy (TEM) image of sample C annealed at 800oC for 60 minutes It appears that with an increase in annealing time, the Ge atoms in the matrix had out-diffused to the ambient instead of aiding in the growth of the existing nucleus, which is probably due to the crystallization of the matrix The - 115 - Chapter Results & Discussions II Ge content near the HfAlO/Si interface was kept at a relatively high level of ~20.3 at.% because this interface was a very efficient sink for the Ge atoms in the HfAlO matrix Note that such a phenomenon has also been observed in the SiO2 + Ge on Si system [18,19] The higher Ge concentration at the HfAlO/Si interface will lead to the growth of large Ge nanoclusters Such large nanoclusters have a much lower equilibrium concentration of dissolved Ge at their interface than the smaller nanocrystals in the bulk of the matrix Due to the different Ge gradient between the smaller nanocrystals and the bigger nanoclusters, it is possible for the nanocrystals to dissolve and provides the Ge atoms for the growth of the nanoclusters 5.4 Comparison of nanocrystals growth in HfAlO and Si oxide matrix It has been shown in Figure 4.6 of the pervious chapter that a SiO2 + Ge system with medium Ge concentration of 9.9 at.% (from RBS experiments) shows the presence of a clear crystalline Ge Raman peak at annealing temperatures ranging from 700 to 1000°C after annealing for 15 minutes Figure 4.8 reveals that a 15 minutes annealing at 800°C resulted in the formation of nanocrystals of around nm in diameter These nanocrystals are also uniformly distributed throughout the matrix but they are relatively much denser and more numerous as comparing to the HfAlO + Ge system with similar concentration and the sample annealing condition (see Figure 5.3(a)) The faster nucleation and growth rate of these nanocrystals in SiO2 matrix as compared to those synthesized in HfAlO under similar conditions implies that - 116 - Chapter Results & Discussions II the enthalpy of mixing between the HfAlO and Ge phase is more negative than the silicon oxide and Ge phase From thermodynamic calculations and study on the SiO2 + Ge system, it has been concluded that Ge is almost insoluble in silicon oxide [20,21], whereas it has been found that thermal processing of HfO + Ge systems can lead to the formation of hafnium germinate (HfGeOx) [22,23] The formation of the hafnium germanate phase during annealing of our HfAlO + Ge samples could reduce the Ge atoms available for nucleation and growth by binding them to the matrix atoms It should be also noted that silicon oxide has a high resistance towards crystallization as compared to many other materials, including HfAlO [24-26] Thus at an annealing temperature of 800°C the HfAlO matrix will be closer to its crystallization temperature (i.e 900°C) [7] relative to the silicon oxide matrix It would therefore be reasonable to expect a reduction in defects in the HfAlO matrix at this temperature as the matrix atoms start to develop order The presence of these defects can act as heterogeneous nucleation sites with low activation energy barriers to seed the nucleation of the Ge nanocrystals These defective regions would have high interfacial energy due to the dangling bonds present By nucleating at such sites, the Ge atoms would be able to help in minimizing the total interfacial free energy of the system Although nanocrystals cannot form at 900°C in the HfAlO matrix, a comparison with the case of the nanocrystals grown in silicon oxide under similar conditions (see Figure 4.9) reveals that the nucleation of the nanocrystals was possible The difference between the two systems is that the silicon oxide matrix - 117 - Chapter Results & Discussions II was still amorphous whereas the HfAlO matrix has already crystallized The crystallized HfAlO structure would have grain boundaries which can enhance the diffusion of the Ge atoms and lead to a depletion of Ge within the matrix, thus increasing the barriers to nucleation The situation would be even worse for the higher annealing temperature, i.e 1000°C, as the size of the critical nucleus required would be larger and the outdiffusion of Ge is expected to be more significant 5.5 Summary In this work, the formation of Ge nanocrystals in HfAlO matrix was examined with different Ge concentrations, annealing temperature and annealing time It was found that at moderate Ge concentration, good quality Ge nanocrystals can be obtained when annealed at 800°C while the matrix remains amorphous The slow growth rate of the nanocrystals enables them to develop good crystallinity By controlling the annealing duration, it would be possible to control the size of these nanocrystals The higher annealing temperature or a very high Ge content in HfAlO film will cause a significant Ge outdiffusion at the film surface or diffusion into the Si substrate In addition, the high Ge concentration would also result in lowering the crystallization temperature of HfAlO film Moreover, it was also found that the nucleation and growth processes of Ge nanocrystals in an HfAlO matrix is more difficult as compared to those in a Si oxide matrix The difference in the Ge nanocrystal formation characteristics in the HfAlO and silicon oxide matrices has been attributed mainly due to (i) the - 118 - Chapter Results & Discussions II difference in the crystallization temperatures of HfAlO and silicon oxide films, which influences the defects level in the films and the nucleation process; (ii) the formation of the HfGeOx phase, thus reducing the supply of Ge atoms available for nucleation and growth - 119 - Chapter Results & Discussions II References [1] S Tiwari, F Rana, K Chan, L Shi and H Hanafi, “Single charge and confinement effects in nano-crystal memories”, Appl Phys Lett., vol 69, no 9, pp 1232-1234, 1996 [2] V Y Thean and J P Leburton, “Flash memory: towards singleelectronics”, IEEE Potentials, vol 21, pp 35-41, 2002 [3] Y C King, T J King, and C Hu, “Charge-trap memory device fabricated by oxidation of Si1-x Gex”, IEEE Trans Electron Devices, vol 48, pp 696700, 2001 [4] J K Kim, H J Cheong, Y Kim, J Y Yi, and H J Bark, “Rapidthermal-annealing effect on lateral charge loss in metal–oxide– semiconductor capacitors with Ge nanocrystals”, Appl Phys Lett., vol 82, pp 2527-2529, 2003 [5] M Kanoun, A Souifi, T Baron, and F Mazen, “Electrical study of Genanocrystal-based metal-oxide-semiconductor structures for p-type nonvolatile memory applications”, Appl Phys Lett., vol 84, pp 50795081, 2004 [6] X B Lu, P F Lee, and J Y Dai, “Synthesis and memory effect study of Ge nanocrystals embedded in LaAlO3 high-k dielectrics”, Appl Phys Lett., vol 86, pp 203111-203113, 2005 [7] N Terasawa, K Akimoto, Y Mizuno, A Icyimiya, K Sumitani, T Takahashi, X W Zhang, H Sugiyama, H Kawata, T Nabatame, A - 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123 - ... comparison of the growth of Ge nanocrystals in silicon oxide and HfAlO matrices - 1 02 - Chapter 5 .2 Results & Discussions II Ge nanocrystals in HfAlO matrix In order to synthesize the Ge nanocrystals in. .. almost insoluble in silicon oxide [20 ,21 ], whereas it has been found that thermal processing of HfO + Ge systems can lead to the formation of hafnium germinate (HfGeOx) [22 ,23 ] The formation of the... 20 04 [ 12] 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, 20 04