Synthesis and stress analysis of germanium nanocrystals embedded in dielectric matrices 1

They concluded that for samples with high Ge concentration Ge concentration = 60 mol.%, annealing results in the formation of nanocrystals, whereas for samples with low Ge concentration

Chapter 4

Results & Discussions I:

Synthesis and Growth of Ge

nanocrystals in Silicon Oxide Matrix

4.1 Introduction

With the observation of visible photoluminescence and charge storage properties from Si and Ge nanocrystals embedded in silicon oxide matrix, there has been tremendous research interest in this area As mentioned in Chapter 2, there are currently several techniques being used to synthesize Ge nanocrystals Among these techniques, there is a keen interest in the synthesis of Ge nanocrystals in SiO2 via furnace annealing of a co-sputtered SiO2 + Ge film [1,2] However, it was found that an as-prepared RF co-sputtered system of Ge and SiO2contains not only elemental Ge and oxides of Si but also oxides of Ge [3] Due to its large free energy, these GeO2 will tend to decompose and form Ge in the presence of a reducing agent with O2 transferred to the reducing agent

Taraschi et al had provided a detailed analysis of the crystalline

nanostructure of Ge nanocrystals formed by annealing oxidized SiGe films [4] They varied the H2 partial pressure and processing temperature in their

experiments and were able to conclude that the presence of H2 in the annealing ambient can directly impact the structural evolution of the nanocrystal by influencing processes of nucleation, growth and coarsening However, they were unable to examine the influence of Ge concentration on these processes in the presence of H2 as they used samples with a fixed Ge concentration in their work

On the other hand, Kobolov et al had performed a study on the local

structure of Ge nanocrystals embedded in a SiO2 matrix by annealing co-sputtered samples with different Ge concentration via the X-ray absorption fine structure technique They concluded that for samples with high Ge concentration (Ge concentration = 60 mol.%), annealing results in the formation of nanocrystals, whereas for samples with low Ge concentration (Ge concentration = 25-40 mol.%), little or no indication of Ge cluster formation was observed after the annealing [5] Unfortunately, the effect of annealing ambient on the formation of

Ge nanocrystals was not examined as their experiments were done only in inert argon ambient

In this chapter, a systematic study on the influence of annealing ambient, temperature and Ge concentration on the growth of nanocrystals in a silicon oxide matrix is carried out with three series of samples (i.e Samples A, B and C) with different Ge concentrations The Ge content in Samples A, B and C were estimated to be 3, 10 and 15 at.%, respectively, by the Rutherford backscattering spectroscopy (RBS) technique

4.2 Effect of reductant

In order to study the effect of reductants on the formation of Ge nanocrystals in the silicon oxide matrix, Sample A (i.e low Ge concentration sample) have been annealed in both N2 and forming gas (10% H2 + 90% N2) at different temperatures Figure 4.1 shows the Raman spectra of those samples annealed at different conditions It can be seen from the figure that annealing in

N2 ambient up to 1000°C resulted in relatively featureless spectra, indicating that

no Ge nanocrystals has been formed However, when the sample was annealed in forming gas, a significant Ge peak can be observed when Sample A was annealed

at 900°C

Figure 4.1: Raman spectra of Sample A annealed between 700°C to 1000°C

for 15 minutes The top four curves represent samples annealed in forming gas (10% H2 + 90% N2) while the bottom two curves are from samples annealed in N2

Figure 4.2 shows the cross-sectional TEM (XTEM) images of Sample A annealed in forming gas at 800°C for 15 minutes Numerous small Ge nanocrystals can be seen to be distributed throughout the entire bulk of the film The presence of these small nanocrystals accounts for the weak Ge peak in Figure 4.1 For 900°C anneal in forming gas (see Figure 4.3), the nanocrystals become larger in the bulk of the film The inset of Figure 4.3 is the high resolution TEM (HRTEM) image of a well-formed single crystalline Ge nanocrystal There also exists a region that is void of nanocrystals between the substrate and a band of nanocrystals in the bulk of the film The large nanocrystals gave rise to the significant Ge peak in Figure 4.1 For 1000°C anneal in forming gas (Figure 4.4), the distribution of the nanocrystals follows the same trend as the 900°C case but the density of nanocrystals has decreased significantly This may explain the reduction in intensity of the Ge peak in Figure 4.1 for Sample A annealed at 1000°C

Figure 4.2: XTEM image of Sample A annealed at 800°C in forming gas (10%

H2 + 90% N2) for 15 minutes

Figure 4.3: XTEM image of Sample A annealed at 900°C in forming gas (10%

H2 + 90% N2) for 15 minutes The inset is a HRTEM image of a nanocrystal

Figure 4.4: XTEM image of Sample A annealed at 1000°C in forming gas

(ii) Diffusion of liberated Ge in the oxide matrix,

(iii) Nucleation due to Ge–Ge collisions,

(iv) Growth, whereby diffusing Ge atoms bond to existing Ge nuclei, and (v) Coarsening of nanocrystals due to Ostwald ripening

The direct decomposition of GeO2 is the simplest reaction for the reduction of GeO2 to Ge as given by: GeO2 → Ge + O2 However, Maeda has shown that the

direct decomposition of GeO2 at 800°C at 1 atmospheric pressure is not possible

without the participation of reductants [6]

It has been established that the Ge oxides and suboxides in a Si–O–Ge

system could be reduced to elemental Ge by Si at an elevated temperature above

800°C [6] As mentioned in the pervious chapter, these reduction reactions of

GeOx and GeO2 to Ge by Si are as follows:

In addition, It has shown that the main source of Si for the reduction of

GeO2 is not from the excess Si atoms originally present in the oxide matrix but

from Si atoms diffused from the Si substrate due to the abundance of Si in the

substrate However, although Si can diffuse from the substrate to the film to

reduce the Ge oxides in the samples to increase the supply of Ge atoms,

apparently this effect alone is not sufficient to trigger nucleation during the

annealing process as no Ge–Ge Raman mode can be detected in the sample

annealed in N2 alone

On the other hand, the presence of H2 in the annealing ambient could also

act as a reducing agent for GeO2 in a reduction reaction given by [1,3]

As a result, when H2 is present in the annealing ambient, there is evidence of

nucleation and growth of the Ge nanocrystals from the Raman and TEM results It

has been suggested [7] that annealing the co-sputtered silicon oxide plus Ge films

in a H2 containing ambient can cause the incorporation of hydroxyl groups (–OH)

into the oxide matrix The –OH acts as a network modifier in the system as their presence opens up the oxide structure, consequently enhancing the diffusivity of

Ge In addition, the presence of H2 in the annealing ambient makes it possible for

Ge to form volatile, fast diffusing GeHx species which will also enhance the diffusivity of Ge [8] H2 is also important in assisting the nucleation of the Ge

nanocrystals due to its high values of diffusivity in silica (~5.6×10−5 cm2s−1) for the temperature range concerned [9] whereas for Si, even at 1000°C, the diffusivity of Si in SiO2 had only been estimated to be in the range of 4.2×10−13

cm2s−1 [10] By diffusing through the SiO2 matrix rapidly, H2 can hasten the nucleation and growth processes by reducing germanium oxide to increase the supply of Ge in the matrix All these factors will assist in the formation of Ge nanocrystals

The voided region at the surface of the film for an annealing temperature

of 800°C (i.e Figure 4.2) can be explained by the outdiffusion of Ge due to the low solubility of Ge in SiO2 [11], or by the re-oxidation of Ge by the small concentration of oxidants present in the annealing gas to form GeO2 [12] The voided region between the substrate and the band of nanocrystals observed for samples annealed at 900 and 1000°C can be attributed to the diffusion of Ge into the Si substrate due to the complete miscibility between Ge and Si The significant increase in the diffusivity of Ge at temperatures of 900°C or higher is most likely due to the fact that such temperatures are very close to the melting point of bulk Ge such that it enables the Ge atoms to overcome kinetic limitations and diffuse into the Si substrate

The diffusion of Ge towards the Si substrate will result in Si-Ge bonds being formed at the Si surface This accounts for the very clear Raman peaks at ~ 410-440 cm-1 shown in Figure 4.5 The intensity of these peaks becomes more prominent as the annealing temperature increases

Figure 4.5: Raman spectrum showing the growth of the low frequency Si peak,

between 300 to 500 cm-1, with increase in annealing temperature due to the localized Si-Si optic mode in near vicinity of Ge atoms The random introduction of Ge atoms into an initially pure Si crystal reduces the local symmetry, which leads to the localization of the Si-Si optical phonons (phonon confinement) in the Ge neighborhoods The frequencies of these modes are reduced through the effect of the larger mass Ge, which pulls modes out of the main Si-Si optic-phonon band to lower frequencies [13]

However, this process is highly temperature driven as it requires long range diffusion activities of Ge to take place At 800°C and below, due to the kinetic limitations encountered by the Ge atoms at such temperature, this process will not be prevalent, and thus accounting for the absence of these peaks at ~410-

440 cm-1 for this temperature anneal range When the temperature increases to 900°C and 1000°C, which is near to or above the melting point of Ge, the values for Ge diffusivity would be very high; thus the effects of this diffusion is more significant and can be observed

The diffusion of Ge towards the Si substrate leads to a net reduction in the

Ge content within the film This will result in a net reduction of the collision frequencies of the Ge atoms within the silicon oxide matrix as the average distance the Ge atoms need to travel before they collide with each other increases

as there are now less of them Consequently the probability of nucleation events drops due to an increase in activation energy for nucleation brought on by the reduction in Ge supersaturation Within the voided region of Figures 4.3 and 4.4, the rate of diffusion of Ge into the Si substrate apparently dominates over the nucleation rate, and thus no nanocrystals can form As this phenomenon is dependent on the diffusivity of Ge, the voided region is larger for 1000°C anneal

as compared to 900°C anneal For 800°C anneal, the relatively lower temperature would mean a lower Ge diffusivity, thus this phenomenon is less obvious as the

Ge atoms are unable to overcome the kinetic limitations

4.3 Effect of Ge concentration

It has been mentioned in the Chapter 2 that the supply of Ge for the formation of the nanocrystals can also come from the excess elemental Ge atoms originally existing in the matrix [10,14] In such cases, when the Ge concentration

is high, it becomes possible for the nanocrystal formation process to bypass the reduction steps, to supply the Ge atoms, and nucleation and growth can occur at a earlier time and a faster rate In order to examine the influence of Ge concentration on the formation of nanocrystals, a comparison of the Raman and TEM results of Samples B and C (i.e of medium and high Ge concentration) with Sample A (i.e of low Ge concentration) will be made in this section

The Raman spectra of Samples B and C annealed in forming gas are shown in Figure 4.6 and Figure 4.7 In contrast to the Raman spectra of the forming gas annealed Sample A, a relatively significant Raman peak can already

be observed for Sample B annealed at 800°C, as shown clearly in the figures For sample C annealed at 800°C, the relative stronger the intensity and the reduced Full Width at Half Maximum (FWHM) suggested that the formation of Ge nanocrystal are even denser and with larger size and better crystalline In addition, unlike sample A whereby there is no noteworthy Raman band for the sample annealed in N2 ambient (see Figure 4.1), the Raman spectra of Sample B and C annealed in N2 at same temperature are identical as comparing to the one annealed

in the forming gas

Figure 4.6: Raman spectra of Sample B annealed between 800°C to 1000°C

for 15 minutes in forming gas (10% H2 + 90% N2)

Figure 4.7: Raman spectra of Sample C annealed between 800°C to 1000°C

for 15 minutes in forming gas (10% H2 + 90% N2)

Figure 4.8 shows the XTEM image of Sample B annealed in forming gas

at 800°C for 15 minutes Numerous small Ge nanocrystals can be seen in the entire bulk of the film This is a much higher density of nanocrystals as compared

to Sample A in Figure 4.2 (i.e., of lower Ge concentration) at the same annealing temperature The higher density of nanocrystals resulted in the appearance of a more significant Ge peak in the Raman spectrum shown in Figure 4.6 This is expected because when the Ge concentration is high, the critical nucleus size is smaller and nucleation barriers are lowered due to the higher Ge supersaturation Consequently, Ge nanocrystals will be able to nucleate and form earlier and faster

in Sample B After 900°C anneal of Sample B (Figure 4.9), the nanocrystals are

well-formed, showing facets that are bounded by crystal planes as can be seen in the HRTEM image shown in the inset of Figure This implies that it is possible to attain the equilibrium interface energy minimizing configuration at 900°C When the temperature was further increased to 1000°C (Figure 4.10), the nanocrystals become large and spherical with a very dense band of nanocrystals close to the Si substrate/SiO2 interface due to the diffusion of Ge towards the Si substrate Figures 4.11 to 4.13 are the XTEM images of Sample C annealed in forming gas

at 800°C, 900°C and 1000°C, respectively, for 15 minutes The characteristics and the distribution of the Ge nanocrystals are very similar as comparing to the Sample B (see Figures 4.8 to 4.10) However, it should be noted that, with the highest Ge concentration (i.e Sample C), the nanocrystal are general larger and denser as comparing to the other set of the samples This is in good agreement of the sharp Raman spectra shown in Figure 4.7

Figure 4.8: XTEM image of Sample B annealed at 800°C in forming gas (10%

H2 + 90% N2) for 15 minutes

Figure 4.9: XTEM image of Sample B annealed at 900°C in forming gas (10%

H2 + 90% N2) for 15 minutes The inset is a HRTEM image of a

Figure 4.10: XTEM image of Sample B annealed at 1000°C in forming gas

(10% H2 + 90% N2) for 15 minutes

Figure 4.11: XTEM image of Sample C annealed at 800°C in forming gas (10%

H2 + 90% N2) for 15 minutes

Figure 4.12: XTEM image of Sample C annealed at 900°C in forming gas (10%

H2 + 90% N2) for 15 minutes The inset is a HRTEM image of a nanocrystal

Figure 4.13: XTEM image of Sample C annealed at 1000°C in forming gas