Synthesis and stress analysis of germanium nanocrystals embedded in dielectric matrices 5

Chapter Results & Discussions V Chapter Results & Discussions V: Effect of Substrate Geometry on the Germanium Diffusion and the Formation of Nanocrystals 8.1 Introduction In Chapter 4, the influence of ambient and Ge concentration on the formation of Ge nanocrystals have been systematically studied over a range of annealing temperature It was concluded that H2 plays a very important role in assisting the formation of the nanocrystals In addition, there are further explorations on the stress state of these samples in Chapter The large compressive stresses in the as-grown nanocrystals were found to be linked to annealing time, annealing temperature, Ge concentration and annealing ambient as well as the size, shape, density and quality of the nanocrystals It should be pointed out that, all the SiO2 + Ge films mentioned in the pervious chapters were grown on top of flat silicon surface of (100) orientation However, there is a lack of comprehensive study on the Ge diffusion and nanocrystal growth on different geometries In this chapter, the different substrate - 165 - Chapter Results & Discussions V geometries will be created by the laser interference lithography and anisotropic chemical etching The diffusion of Ge atoms on such a substrate and the formation of the Ge nanocrystals will be further discussed 8.2 Fabrication of V-shape and U-shape groove by Laser Interference Lithography V-shape and U-shape grooves were fabricated on a p-type silicon wafer with a (100) orientation and the processing steps are shown in Figure 8.1 Firstly, a silicon oxide layer of 10 nm in thickness was thermally grown in O2 ambient at 900°C (see Figure 8.1 (a)) This was followed by spin coating of a negative photoresist layer (TSMR-iN032) of 100 nm (see Figure 8.1 (b)) Then, Laser interference lithography (LIL) system with the Lloyd’s mirror setup and a 325 nm helium-cadmium (He-Cd) continuous wave laser light source were used to pattern the photoresist A anisotropic etching of Si was carried out to selectively remove the area of silicon after the developing and silicon oxide etching processes (see Figure 8.1 (c)) The U-shape groove structures were fabricated by the anisotropic etching of Si using a 30 % Potassium Hydroxide (KOH) solution at room temperature with silicon oxide as masking layer (see Figure 8.1 (d)) It should be noted that, as KOH is known to etch (100) plane much faster than (111) plane [1], therefore, with sufficient etching time, a V-shape groove structure would be created as shown in Figure 8.1 (e) - 166 - Chapter Figure 8.1: 8.3 Results & Discussions V Cross section view of the process flow chart for the fabrication of U-shape groove arrays on (100) silicon substrate by laser interference lithography Synthesis of Ge nanocrystals in V-shape groove The SiO2 + Ge thin film was firstly deposited in the V-shape groove by the co-sputtering a SiO2 + Ge target in argon at room temperature The samples were subsequently subjected to rapid thermal annealing (RTA) at 1000°C for 60 seconds in order to synthesize the nanocrystals - 167 - Chapter Results & Discussions V Figure 8.2 shows the cross section transmission electron microscopy (XTEM) image of the sample after the RTA process It can be seen from this figure that, the film deposited on the bottom of the V-shape groove is relatively thinner as comparing to the side wall of the groove and the mesa area It has been suggested that the income flux is the function of the arriving angle The anisotropic KOH etching of silicon has resulted in a relatively smaller arriving angle at the bottom of the groove (i.e 70.6°) as comparing to the one of the corner of the mesa (i.e 144.7°) This difference in the arriving angle will inevitably cause the shadowing effect and results in the thicker film on the mesa and thinner film inside the groove It is well established that thermal annealing of Ge and silicon oxide system will lead to Ge diffusion out from the matrix [2, 3], due to the low solubility of Ge in the silicon oxide matrix Therefore, during the subsequent thermal annealing process, it was easier for the Ge atom to diffuse out from the thin silicon oxide matrix to the ambient and to the silicon substrate and results in no nanocrystal formation, as shown in the Figure 8.2 - 168 - Chapter Figure 8.2: Results & Discussions V Cross section transmission electron microscopy image of V-shape groove sample annealed at 1000°C for 60 seconds In addition, although there is a relatively thicker film at the corner of the mesa, it is interesting to observe that there are very few nanocrystals formed in such an area This phenomenon will be further discussed in the following sections 8.4 Synthesis of Ge nanocrystals in U-shape groove In order to minimize the shadowing effect and make a relatively conformal film deposition, the KOH etching process is carefully calibrated and the time is limited to fabricate the U-shape groove sample as shown in Figure 8.1 (d) In this section, two sets of samples were prepared Sample A was co-sputtered film with the U-shape groove substrate Sample B, which is a control sample, was co-sputtered film on Si (100) substrate - 169 - Chapter Results & Discussions V Figure 8.3 shows the Raman spectra and the Raman peak position of both Sample A and Sample B after the rapid thermal annealing We have shown previously that the formation of Ge nanocrystal is kinetically limited at 700°C However, at 800°C, the diffusivity of the Ge atoms and the collision frequency between the Ge atoms will increase and this should lead to a higher probability for nucleation and hence the nanocrystal formation This is confirmed by the Raman band at around 300cm-1 in Figure 8.3 (a) for both Sample A and Sample B In addition, the Raman peak positions of both set of samples exhibit a blue shift as the annealing temperature increases from 800°C to 1000°C This is probably due to the growth of the nanocrystal at higher temperature that causes an increase in the compressive stress experienced by the nanocrystals - 170 - Chapter Figure 8.3: Results & Discussions V (a) Raman spectra of Sample A and Sample B after the RTA; (b) summary of Raman peak position of annealed Sample A and Sample B It is interesting to note that, by applying the patterned substrate, the peak position of the Raman band is located at relatively smaller wavenumber as compared to the blanket substrate, as shown in Figure 8.3 (b) As the complete - 171 - Chapter Results & Discussions V miscibility of Ge and Si at high annealing temperate [3], and the extremely low solubility of Ge in silicon oxide matrix, there will be always a driving force for Ge to diffuse away from the matrix and towards the silicon substrate This diffusion will be significantly enhanced when the annealing temperature exceeds the melting point of Ge (i.e 937°C) By creating the U-shape groove substrate and hence increasing the Si/silicon oxide interfaces, one enables more Ge outdiffusion during annealing process This outdiffusion will effectively lower the Ge supersaturation near the interface and results in increase of formation of the smaller nanocrystal and hence the red shift of the Raman band [4] This effect of geometry (i.e U-shape groove) on the diffusion of Ge atoms to the Si can be further examined by the Raman results shown in Figure 8.4, whereby there exists a weak peak between 410-440 cm-1 for only the sample A annealed at 1000°C for 60 seconds The peak detected in the U-shape groove sample has been attributed to the Si-Si phonon mode (from the substrate) in the near vicinity of Ge atoms and this is caused by a significant diffusion of Ge atoms to the Si substrate when annealed at 1000°C [5] The absence of such peak for the sample B (i.e the blanket substrate sample) indicates the clearly the effect of substrate geometry on the diffusion of Ge atoms when annealed at elevated temperatures - 172 - Chapter Figure 8.4: Results & Discussions V Raman spectra of Sample A and Sample B after the RTA at 1000°C for 60 seconds In order to confirm the existence and the distribution of the nanocrystal, cross section transmission electron microscopy (XTEM) was done for both Sample A and Sample B At 800°C anneal (See Figure 8.5 and the inset), numerous small Ge nanocrystals with the diameter of around 4nm can be seen in the entire bulk of the film When the annealing temperature increases to 900°C one can observe from Figure 8.6 that, the nanocrystals grow in size and generally adopt a spherical shape This is in agreement with the observed blue shift of the Raman band The size variation of the nanocrystals is also greater as compared to the one annealed at 800°C, indicating coarsening has taken place The significant increase in the diffusivity of Ge is most likely due to the fact that the annealing temperature was near the melting point of Ge such that it enables the Ge atoms to - 173 - Chapter Results & Discussions V overcome kinetic limitations and enhance the nucleation and growth of the nanocrystal Figure 8.5: Cross section transmission electron microscopy image of U-shape groove sample annealed at 800°C for 60 seconds The inset is high resolution transmission electron microscopy image of the Ge nanocrystals Figure 8.6: Cross section transmission electron microscopy image of U-shape groove sample annealed at 900°C for 60 seconds - 174 - Chapter Results & Discussions V Figures 8.7 (a) and (b) show the XTEM picture of Sample A and Sample B annealed at 1000°C for 60 seconds One can observe the relatively denser and larger Ge nanocrystals from Sample B as compare to the one from Sample A As there are larger Si/silicon oxide interface created by the U-sharp groove, it is reasonable to expect that, for Sample A, more Ge will diffuse into the Si substrate from the matrix which will become most severe when the temperature reached 1000°C The depletion of Ge will inevitably reduce Ge supersaturation, especially near the interface, and hence results in smaller and less nanocrystal formation, as shown in Figure 8.7 (a) Note that, there is always a region around the corner of the mesa where no Ge nanocrystal forms The energy dispersive x-ray (EDX) in Figure 8.7 (a) reveals that there is a significant Ge outdiffusion from the region This could be explained by the observation that the film quality of the silicon oxide film is relatively poorer near the corner of the mesa, which is probably caused by higher income flux due to the larger arriving angles This low film quality will make the Ge atom easier to diffuse out from such region On the other hand, it has also been suggested that the strain field will greatly affect the diffusion of the atoms [6] Couple with the fact that the thermal expansion coefficient is very different between Si and silicon oxide, it is reasonable to suggest that the protruded mesa will generate a high strain field during the thermal process This strain field as well as the poor film quality may assist the Ge diffusion away from such area and results in depletion of Ge and no formation of the nanocrystal - 175 - Chapter Figure 8.7: Results & Discussions V Cross section transmission electron microscopy image of (a) sample A and (b) sample B annealed at 1000°C for 60 seconds - 176 - Chapter 8.5 Results & Discussions V Summary Artificial V-shape and U-shape grooves were fabricated on (100) Si substrate via laser interference lithography It was found that a threshold of 800°C is necessary for the synthesis of Ge nanocrystals in the silicon oxide matrix The pile up of Ge nanocrystal near the Si/silicon oxide interface and the void region of Ge nanocrystal around the corner of the mesa are suggested to be linked to the Ge outdiffusion and strain-assisted Ge diffusion, respectively - 177 - Chapter Results & Discussions V References [1] G T A Kovacs, N I Maluf, and K E Petersen, “Bulk Micromachining of Silicon”, Proceedings of the IEEE, vol 86, 1998 [2] Y Maeda, “Visible photoluminescence from nanocrystallite Ge embedded in a glassy SiO2 matrix: Evidence in support of the quantum-confinement mechanism”, Phys Rev B, vol 51, pp 1658-1670, 1995 [3] K H Heinig, B Schmidt, A Markwitz, R Grӧtzschel, M Strobel, and S Oswald, “Precipitation, ripening and chemical effects during annealing of Ge+ implanted SiO2 layers”, Nucl Instrum Meth B, vol 148, pp 969974, 1999 [4] J L Liu, G Jin, Y S Tang, Y H Luo, K L Wang, and D P Yu, “Optical and acoustic phonon modes in self-organized Ge quantum dot superlattices”, Appl Phys Lett., vol 76, pp 586-588, 2000 [5] M I Alonso and K Winer, “Raman spectra of c-Si1-xGex alloys”, Phys Rev B, vol 39, pp 10056-10062, 1989 [6] E Bassous, H N Yu, and V Maniscalco, “Topology of Silicon Structures with Recessed SiO2”, J Electrochem Soc., vol 123, pp pp 1729-1737, 1976 - 178 - ... discussed in the following sections 8.4 Synthesis of Ge nanocrystals in U-shape groove In order to minimize the shadowing effect and make a relatively conformal film deposition, the KOH etching process... bottom of the groove (i.e 70.6°) as comparing to the one of the corner of the mesa (i.e 144.7°) This difference in the arriving angle will inevitably cause the shadowing effect and results in the... enhanced when the annealing temperature exceeds the melting point of Ge (i.e 937°C) By creating the U-shape groove substrate and hence increasing the Si/silicon oxide interfaces, one enables