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

Chapter 5 Results & Discussions II
- 101 -

Chapter 5
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
Chapter 5 Results & Discussions II
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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 (HfO

2
) 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.


Chapter 5 Results & Discussions II
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5.2 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
N
2
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 4 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 N
2
.
Chapter 5 Results & Discussions II
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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 8 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 N
2
.
Chapter 5 Results & Discussions II
- 105 -
Figures 5.3 (a) and (b) are the cross section TEM (XTEM) images of
sample B annealed at 800
o
C 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.


Chapter 5 Results & Discussions II
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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.

Chapter 5 Results & Discussions II
- 107 -
According to simulation results of She et al. [10], for FLASH applications,
the optimum size of Ge nanocrystals embedded in SiO
2
should be 5 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.
Chapter 5 Results & Discussions II

- 108 -

Figure 5.5: (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
Chapter 5 Results & Discussions II
- 109 -
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 as-
sputtered 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)
Chapter 5 Results & Discussions II
- 110 -
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 8 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.
Chapter 5 Results & Discussions II
- 111 -

Figure 5.7: Raman spectrum for Sample C annealed at 700 and 800°C in N
2

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
Chapter 5 Results & Discussions II
- 112 -
crystallized with the occurrence of the HfO
2
(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 HfO
2

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 N
2
. The inset is the diffraction
pattern of the sample in (a) showing that it has crystallized.
Chapter 5 Results & Discussions II
- 113 -

Figure 5.9: 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
Chapter 5 Results & Discussions II
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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
Chapter 5 Results & Discussions II
- 115 -
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 800
o

C 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
Chapter 5 Results & Discussions II
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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 SiO
2
+
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 SiO
2
+ 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 8 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 SiO
2
matrix
as compared to those synthesized in HfAlO under similar conditions implies that
Chapter 5 Results & Discussions II
- 117 -
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 SiO
2
+ 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 (HfGeO
x
) [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
Chapter 5 Results & Discussions II
- 118 -
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
Chapter 5 Results & Discussions II
- 119 -
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 HfGeO
x
phase, thus reducing the supply of Ge atoms available
for nucleation and growth.
Chapter 5 Results & Discussions II
- 120 -
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