# Chromium-Induced Nanocrystallization of a-Si Thin Films  into the Wurtzite Structure

Authors: K. Uma Mahendra Kumar; M. Ghanashyam Krishna
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

Chromium metal-induced nanocrystallization of amorphous silicon (a-Si) thin films is reported.  The nanocrystalline nature of these films is confirmed from X-ray diffraction and Raman spectroscopy. Significantly, the deconvolution of Raman spectra reveals that the thin films were crystallized in a mixed phase of cubic diamond and wurzite structure as evidenced by the lines at 512 and 496cm−1, respectively. The crystallite sizes were between 4 to 8 nm. Optical properties of the crystallized silicon, derived from spectral transmittance curves, revealed high transmission in the region above the band gap. Optical band gap varied between 1.3 to 2.0 eV depending on the nature of crystallinity of these films and remained unaltered with increase in Cr addition from 5 to 30%. This signifies that the electronic structure of the nanocrystalline Silicon films is not affected considerably inspite of the presence of metal silicides and the process of crystallization.


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## Body

## 1. Introduction

In recent years,
nanocrystalline silicon has found substantial applications in a variety of
areas including visible  electroluminescent devices [1–3] field-induced
ballistic electron emitter [4], thermally induced ultrasonic emitter [5], and
biocompatible scaffold [6] to name a few. Nanocrystallization in a-Si has been
achieved by different methods starting from solid phase crystallization (SPC)
[7], excimer laser annealing [8] and metal induced crystallization (MIC) [9].Silicon crystallizes in the
cubic diamond structure under ambient conditions. However, it is also known to
have several polymorphs that are stable only at high pressures.
Hexagonal-wurtzite silicon is one such structure and there have been very few
reports in literature on its stabilization either in bulk or thin form at
ambient pressure and temperature conditions [10]. The Raman spectra of these
stabilized structures are  studied but very little is known about the optical
properties such as transmission, refractive indexand optical band gap of the
wurtzite structured Si films.Chromium-induced
nanocrystallization of silicon has been very sparsely reported previously
although crystallization induced by metals such as Al and Ni has  been extensively reported [11–13]. It is known
that in many silicides the metal-silicon bond length is very similar to that of
Si-Si bond lengths in the wurtzite structure [14]. Furthermore, in the process
of metal-induced crystallization of Si, both the metal and metal silicides that form  as a consequence of the thermal treatment, act as nucleating
agents for the crystallization of Si. The eventual structure of the Si
nanocrystals, then, closely mimics that of the metal silicide. The choice of Cr
was, therefore, based on the premise that its silicides under favourable
thermodynamic conditions could result in the formation of the wurtzite
structure.The objectives of the
present work are therefore to study the possibility of stabilizing the wurtzite
structure of Si in thin film form by the metal-induced nanocrystallization
route. A further objective is to study the optical properties such as spectral
transmission, refractive index, and optical band gap of the wurtzite form of silicon
thin films.


## 2. Experimental

Thin films of Cr/a-Si/Borosilicate
Glass (BSG) substrate stacks are deposited by resistive thermal
evaporation. Silicon thin films were grown by evaporating granular silicon
powder (99.99% pure) from a tantalum boat on to BSG substrates. In order to deposit
Cr films, Chromium (99.99% pure), shots were sublimed from a Tantalum boat.
The vacuum chamber was evacuated using a diffusion-rotary pump combination
equipped with a liquid nitrogen trap to a base pressure of1×10−6 Torr. In all cases, the substrate to source distance was kept constant at 8 cm.
Relative concentration of metal in the a-Si matrix were varied while varying
the a-Si thickness; in the case of Cr/a-Si/BSG system, the Cr concentration was
varied from 5% to 30% by varying the a-Si thin film thickness from 600 nm to
100 nm. After recording the spectral transmission curves, these films were
subjected to thermal annealing. The Cr/a-Si/BSG stacks were annealed at 500°C
for 20 minutes. The thickness was measured in situ by a quartz crystal monitor
and after deposition using a surface profilometer (Model XP-1 Ambios Tech.,USA) and also was
calculated from the spectral transmission curves [15]. Before
and after annealing, the films was  characterized for transmission in the wavelength range 350
to 2500 nm by means of a dual-beam spectrophotometer (UV-VIS-NIR, Model, Jasco
V-570) having a resolution limit of 0.2 nm and a sampling interval of 2 nm. The
transmitted intensities were measured at accuracy better than 0.3%. The
refractive indices were extracted from the measured spectral curves by curve
fitting using the PUMA code [15]. The
Raman spectra were recorded in air using an argon ion laser in the back scattering
geometry in a JY-ISA T64000 spectrometer equipped with a  confocal microscope and 100X objective (1 μm diameter focal
spot size). Care was taken to optimize the parameters of the laser, so that it
does not induce onset of crystallization in the sample. The phase content with
in the samples was investigated in a spectral region 200 to 600 cm−1 with an irradiation time of 50 seconds. The Raman spectra presented in this
paper are the result of a scan with irradiation duration of 50 seconds. X-ray
diffraction patterns were recorded on a powder x-ray diffractometer (CPS120 of
Inel, France) machine equipped with a Co Kα = 0.178896 nm and gas phase position
sensitive detector.


## 3. Results and Discussion

### 3.1. Morphology and Structure

An optical micrograph of
a-Si thin film grown on glass by thermal evaporation is shown in Figure1(a).
The observed microstructures are very familiar features in MBE grown epitaxial silicon
thin films and are known as shallow saucer pits (S-pits) [16]. These structures
have also been interpreted as point defects or clustered crystallographic
imperfections. The silicon thin films shown in these images were X-ray
amorphous and hence the saucer pits seen in our samples cannot be due to
crystallographic imperfection. The possible reasons for this kind of growth in
MBE grown silicon thin films were attributed to poor vacuum conditions (<10−7 Torr) and low substrate temperatures during deposition (<700°C).
According to Pearce and McMahon metal  contamination will also cause
S-pits [17]. In our case, the films were deposited on substrates held at
ambient temperature during deposition and in vacuum of 10−6 Torr and
these can be the causes for the formation of the S-pits.Optical micrograph of (a) a-Si thin film on
BSG substrates showing shallow saucer pits (S-pits). (b) Cr/a-Si/BSG after
annealing at 500°C for 20 minutes. (c) 280 nm thick a-Si film with
30 nm of Cr overlayer annealed at 500°C for 20 minutes.
(a)(b)(c)Optical microscopy reveals
the morphology of the a-Si thin films crystallized by Cr-induced
crystallization after annealing at 500°C for 20 minutes Figure 1(b). Different kinds of morphologies have been
reported earlier in literature for crystalline Si films. Choi et al. reported
disk-like grains, which grow radially upwards on annealing [18], and Yoon et al. observed dendrite-like growth [19]. After annealing the Cr/a-Si/BSG
stack at 500°C in a muffle furnace in ambient atmosphere, the
morphology is as shown in Figure 1(b) for a film
of thickness of 100 nm with a 30 nm of
Cr over the a-Si thin film as a blanket layer and for the film of thickness 280 nm with the same thickness of Cr overlayer in Figure
1(c). In the latter case, the Si thin film seems to be melted. Cr
interaction with a-Si is important since this kind of stack structure is found
in a-Si solar cells [20]. In the present study, after melting, due to the
density gradients in crystalline silicon and a-Si, ripple formation occurred.
These ripple structures are known in laser ablated silicon samples where silicon
melting is evidenced [21, 22].
Levinson reported columnar
structures of CrSi2 in the matrix of Si [23]. However, in our case
the quantity of CrSi2 is very small compared to the a-Si, and
therefore these ripples can be attributed to the crystalline Si.X-ray diffraction patterns
of the annealed samples are shown in Figure2.
The unambiguous identification of the presence of crystalline Si is difficult
because there is an overlap with the reflections from the silicides. There are
two Bragg reflections at 2θ values of 78.06° and 105.33°,
which can be indexed as originating either from Si or CrSi2. To
remove the ambiguity, Raman scattering experiments were carried out in both cases
and results are presented in the next section. However, crystallite size
determination from FWHM shows that the films are nanocrystalline with an
average crystallite size of 6 ± 1 nm.Figure 2
X-ray diffraction pattern of Cr/a-Si/BSG after annealing at 500°C for 20
minutes.


### 3.2. Raman Spectroscopy

The Raman spectrum of a 100 nm thick a-Si film on glass substrate is shown in Figure3. A broad feature centered at 472 cm−1 was observed. After
correcting for the base line, a Voigt-like function was used to curve fit the
Raman spectra. The curve fitting reveals that the Raman signal between 400 and
560 cm−1 is comprised of two resonances, one peak centered at 472 cm−1 which is attributed to the amorphous silicon, while the peak at 504 cm−1 is due to nanocrystalline cubic silicon. Using the Zi model, the derived
crystallite size was estimated ~2 ± 0.5 nm size [24]. However, since these
films were X-ray amorphous, the values may not be entirely accurate.Figure 3
Raman shift of as deposited a-Si thin film on
glass.In our studies, the relative
concentration of Cr metal was varied from 30% to 5% by varying the a-Si film
thickness from 100 nm to 600 nm. After annealing the Cr/a-Si/BSG stack at 500°C
for 20 minutes, crystallization of a-Si started. The Raman shift of the 30%
chromium doped a-Si thin films after annealing is shown in
Figure 4. The spectral components are deconvoluted into four parts that belong
to (a) amorphous silicon (468 cm−1), (b) hexagonal silicon (496 cm−1), (c) cubic silicon (512 cm−1), and (d) Cr2O3 (548 cm−1). Once the concentration of Cr was  decreased to
5%, the silicon  started crystallizing in wurzite structure. The Raman shift of
the wurzite silicon is shown in Figure 5. The
presence of the wurtzite structure, in the current case, is confirmed by the
prominent Γ5′ resonance peak at 496 cm−1. However, the theoretically predicted
resonance peak is at 498 cm−1 [25].
The downward shift of 2 cm−1 can be attributed to the strain or
small crystallite size of wurzite silicon. The peak at 548 cm−1 belongs to Cr2O3 (Oxide formation is not avoidable in a
furnace working in ambient atmosphere). Metal silicide (CrSi2) peaks
at 307 and 350 cm−1 are also observed in the low-frequency part of
the Raman spectra shown in the inset of Figure 4. The presence of Cr2O3,
from the Raman spectra, indicates surface oxidation of the films. This would
also suggest the possibility of some SiOx phase being present in the
films. However, it is difficult to unambiguously establish this since the thin
films are grown on borosilicate glass substrates. Similarly, distinguishing SiOx from films and substrate  would be difficult using FTIR. Hence, while the presence of
SiOx in the films cannot be ruled out, its presence cannot also be
unambiguously established in the current case.Figure 4
The Raman spectral components are deconvoluted
into four parts that belong to (a) amorphous Silicon, (b) hexagonal Silicon,
(c) cubic Silicon, and (d) Cr2O3.Figure 5
The Raman shift of the wurzite silicon
centered at 496 cm−1 due to Γ5′ resonances.Wurzite silicon has three
vibrational peaks corresponding to the two vibrations in the plane of hexagonal
layers and one perpendicular to these layers [25]. Zhang et al. have reported
wurzite silicon by laser ablation [26], and they observed Raman peaks at 516
and 518 cm−1 due to the hexagonal silicon. In contrast, our
investigations indicate that the Si films crystallize in the mixed structure at
high Cr concentrations and in the wurzite structure at low Cr concentrations. A
survey of the structures of Cr2O3 and CrSi2 reveals that Cr2O3 exists in the corundum structure with
a space group R 3¯ c with six formula units in the hexagonal unit
cell. The metal ions are located in special positions along the threefold axis
at ±[0 0 z; 0 0 (1/2) +z ] and the oxygens lie on diads at ±[x 0 (1/4); 0 x (1/4); (1/4) ] [27] and in
the case of CrSi2, they have three formula units per hexagonal unit
cell.  Its lattice parameters are a = 4.431 Å and c = 6.354 Å [28]. It is, therefore, clear
that the silicide and the oxide both exist in the hexagonal form and are
structurally isomorphous to the hexagonal form of Si. As a consequence, they
act as seeds for the formation of the hexagonal-wurtzite structure of silicon
in the crystallization process. To the best of our knowledge, this is the first
experimental evidence of metal-induced wurzite structure crystallization  of
Si.


### 3.3. Optical Properties

The spectral transmittance
of the 100 nm pure a-Si thin film is shown in Figure6(a). The spectral transmittance of this film with a 30 nm Cr thin film
blanket is shown in Figure 6(b) and the stack of these films after annealing at 500°C for 20 minutes
is shown in Figure 6(c). The lower transmittance
prior to annealing is due to the blanket of Cr metal film on top of the a-Si.
After annealing, the increase in the transmittance, to 80%, is a combined
effect of formation of different phases in the Cr/a-Si stack. It may be noted
that Raman spectroscopy revealed the presence of Cr2O3,
after annealing in ambient atmosphere. Apart from these oxide compounds the
formation of CrSi2was also in evidence. CrSi2is
intrinsically a semiconductor with a band gap of 0.35 eV [28], but the
observed absorption edge as estimated from the value at which the spectral
transmittance falls to 20% is much higher. The actual calculated values are
discussed below. From the spectral transmittance curves another important optical
property, refractive index, can also be extracted. The index of refraction is
sensitive to the coordination chemistry and composition around Si. In the
present study, as mentioned earlier, after annealing, different compounds with
varying concentrations are found in the volume of the sample and also on the
surface. This has immense effect on the
thin film refractive index. The refractive index behaviour before (open circles) and after annealing (closed circles), as function of
wavelength, in Figure 7 shows that the values
varied from 2.7 to 2.5 at 1100 nm of wavelength for the film of thickness 100 nm
whereas the bulk silicon value is 3.4.Figure 6
The spectral transmittance of the (a) 100 nm
pure a-Si thin film, (b) a-Si film with a 30 nm Cr thin film blanket, and (c)
after annealing at 500°C for 20 minutes.Figure 7
The refractive index behaviour of a 100 nm Si
thin film as function of wavelength; open circles represent pure a-Si thin film
and closed circles represent the annealed film with a 30 nm Cr thin film
blanket.The optical band gap (Tauc
gap) is determined from the extrapolation of linear part in the (αhυ)1/2 Vs hυ plot assuming an indirect band gap. The band
gap variation with the thickness of the a-Si thin films before and after
annealing is shown in Figure 8. It is observed
that the band gap varied from 2 eV for the 100 nm thick film to 1.3 eV for the
film of thickness 600 nm. It is interesting that the optical absorption edge of
the as-deposited a-Si thin film and the film after annealing with a 30 nm Cr
layer on the top remained unaltered. The edge, in contrast, shifted towards
lower energies for the Cr/a-Si/BSG stack before annealing. This small red shift
in the absorption edge could be due to the formation of metal silicide during
deposition. The absorption edge, which represents the electronic band structure
of the materials, remained almost unaltered after annealing. This signifies the
fact that the different compounds observed during the annealing process do not
disturb or affect the essential optical properties of a-Si. We believe this to
be a very important observation for several optical applications relying on
nanocrystalline-Si.Figure 8
The
band gap variation as function of thickness for the (open triangles) as
deposited pure a-Si films (closed circles) after annealing.


## 3.1. Morphology and Structure

An optical micrograph of
a-Si thin film grown on glass by thermal evaporation is shown in Figure1(a).
The observed microstructures are very familiar features in MBE grown epitaxial silicon
thin films and are known as shallow saucer pits (S-pits) [16]. These structures
have also been interpreted as point defects or clustered crystallographic
imperfections. The silicon thin films shown in these images were X-ray
amorphous and hence the saucer pits seen in our samples cannot be due to
crystallographic imperfection. The possible reasons for this kind of growth in
MBE grown silicon thin films were attributed to poor vacuum conditions (<10−7 Torr) and low substrate temperatures during deposition (<700°C).
According to Pearce and McMahon metal  contamination will also cause
S-pits [17]. In our case, the films were deposited on substrates held at
ambient temperature during deposition and in vacuum of 10−6 Torr and
these can be the causes for the formation of the S-pits.Optical micrograph of (a) a-Si thin film on
BSG substrates showing shallow saucer pits (S-pits). (b) Cr/a-Si/BSG after
annealing at 500°C for 20 minutes. (c) 280 nm thick a-Si film with
30 nm of Cr overlayer annealed at 500°C for 20 minutes.
(a)(b)(c)Optical microscopy reveals
the morphology of the a-Si thin films crystallized by Cr-induced
crystallization after annealing at 500°C for 20 minutes Figure 1(b). Different kinds of morphologies have been
reported earlier in literature for crystalline Si films. Choi et al. reported
disk-like grains, which grow radially upwards on annealing [18], and Yoon et al. observed dendrite-like growth [19]. After annealing the Cr/a-Si/BSG
stack at 500°C in a muffle furnace in ambient atmosphere, the
morphology is as shown in Figure 1(b) for a film
of thickness of 100 nm with a 30 nm of
Cr over the a-Si thin film as a blanket layer and for the film of thickness 280 nm with the same thickness of Cr overlayer in Figure
1(c). In the latter case, the Si thin film seems to be melted. Cr
interaction with a-Si is important since this kind of stack structure is found
in a-Si solar cells [20]. In the present study, after melting, due to the
density gradients in crystalline silicon and a-Si, ripple formation occurred.
These ripple structures are known in laser ablated silicon samples where silicon
melting is evidenced [21, 22].
Levinson reported columnar
structures of CrSi2 in the matrix of Si [23]. However, in our case
the quantity of CrSi2 is very small compared to the a-Si, and
therefore these ripples can be attributed to the crystalline Si.X-ray diffraction patterns
of the annealed samples are shown in Figure2.
The unambiguous identification of the presence of crystalline Si is difficult
because there is an overlap with the reflections from the silicides. There are
two Bragg reflections at 2θ values of 78.06° and 105.33°,
which can be indexed as originating either from Si or CrSi2. To
remove the ambiguity, Raman scattering experiments were carried out in both cases
and results are presented in the next section. However, crystallite size
determination from FWHM shows that the films are nanocrystalline with an
average crystallite size of 6 ± 1 nm.Figure 2
X-ray diffraction pattern of Cr/a-Si/BSG after annealing at 500°C for 20
minutes.


## 3.2. Raman Spectroscopy

The Raman spectrum of a 100 nm thick a-Si film on glass substrate is shown in Figure3. A broad feature centered at 472 cm−1 was observed. After
correcting for the base line, a Voigt-like function was used to curve fit the
Raman spectra. The curve fitting reveals that the Raman signal between 400 and
560 cm−1 is comprised of two resonances, one peak centered at 472 cm−1 which is attributed to the amorphous silicon, while the peak at 504 cm−1 is due to nanocrystalline cubic silicon. Using the Zi model, the derived
crystallite size was estimated ~2 ± 0.5 nm size [24]. However, since these
films were X-ray amorphous, the values may not be entirely accurate.Figure 3
Raman shift of as deposited a-Si thin film on
glass.In our studies, the relative
concentration of Cr metal was varied from 30% to 5% by varying the a-Si film
thickness from 100 nm to 600 nm. After annealing the Cr/a-Si/BSG stack at 500°C
for 20 minutes, crystallization of a-Si started. The Raman shift of the 30%
chromium doped a-Si thin films after annealing is shown in
Figure 4. The spectral components are deconvoluted into four parts that belong
to (a) amorphous silicon (468 cm−1), (b) hexagonal silicon (496 cm−1), (c) cubic silicon (512 cm−1), and (d) Cr2O3 (548 cm−1). Once the concentration of Cr was  decreased to
5%, the silicon  started crystallizing in wurzite structure. The Raman shift of
the wurzite silicon is shown in Figure 5. The
presence of the wurtzite structure, in the current case, is confirmed by the
prominent Γ5′ resonance peak at 496 cm−1. However, the theoretically predicted
resonance peak is at 498 cm−1 [25].
The downward shift of 2 cm−1 can be attributed to the strain or
small crystallite size of wurzite silicon. The peak at 548 cm−1 belongs to Cr2O3 (Oxide formation is not avoidable in a
furnace working in ambient atmosphere). Metal silicide (CrSi2) peaks
at 307 and 350 cm−1 are also observed in the low-frequency part of
the Raman spectra shown in the inset of Figure 4. The presence of Cr2O3,
from the Raman spectra, indicates surface oxidation of the films. This would
also suggest the possibility of some SiOx phase being present in the
films. However, it is difficult to unambiguously establish this since the thin
films are grown on borosilicate glass substrates. Similarly, distinguishing SiOx from films and substrate  would be difficult using FTIR. Hence, while the presence of
SiOx in the films cannot be ruled out, its presence cannot also be
unambiguously established in the current case.Figure 4
The Raman spectral components are deconvoluted
into four parts that belong to (a) amorphous Silicon, (b) hexagonal Silicon,
(c) cubic Silicon, and (d) Cr2O3.Figure 5
The Raman shift of the wurzite silicon
centered at 496 cm−1 due to Γ5′ resonances.Wurzite silicon has three
vibrational peaks corresponding to the two vibrations in the plane of hexagonal
layers and one perpendicular to these layers [25]. Zhang et al. have reported
wurzite silicon by laser ablation [26], and they observed Raman peaks at 516
and 518 cm−1 due to the hexagonal silicon. In contrast, our
investigations indicate that the Si films crystallize in the mixed structure at
high Cr concentrations and in the wurzite structure at low Cr concentrations. A
survey of the structures of Cr2O3 and CrSi2 reveals that Cr2O3 exists in the corundum structure with
a space group R 3¯ c with six formula units in the hexagonal unit
cell. The metal ions are located in special positions along the threefold axis
at ±[0 0 z; 0 0 (1/2) +z ] and the oxygens lie on diads at ±[x 0 (1/4); 0 x (1/4); (1/4) ] [27] and in
the case of CrSi2, they have three formula units per hexagonal unit
cell.  Its lattice parameters are a = 4.431 Å and c = 6.354 Å [28]. It is, therefore, clear
that the silicide and the oxide both exist in the hexagonal form and are
structurally isomorphous to the hexagonal form of Si. As a consequence, they
act as seeds for the formation of the hexagonal-wurtzite structure of silicon
in the crystallization process. To the best of our knowledge, this is the first
experimental evidence of metal-induced wurzite structure crystallization  of
Si.


## 3.3. Optical Properties

The spectral transmittance
of the 100 nm pure a-Si thin film is shown in Figure6(a). The spectral transmittance of this film with a 30 nm Cr thin film
blanket is shown in Figure 6(b) and the stack of these films after annealing at 500°C for 20 minutes
is shown in Figure 6(c). The lower transmittance
prior to annealing is due to the blanket of Cr metal film on top of the a-Si.
After annealing, the increase in the transmittance, to 80%, is a combined
effect of formation of different phases in the Cr/a-Si stack. It may be noted
that Raman spectroscopy revealed the presence of Cr2O3,
after annealing in ambient atmosphere. Apart from these oxide compounds the
formation of CrSi2was also in evidence. CrSi2is
intrinsically a semiconductor with a band gap of 0.35 eV [28], but the
observed absorption edge as estimated from the value at which the spectral
transmittance falls to 20% is much higher. The actual calculated values are
discussed below. From the spectral transmittance curves another important optical
property, refractive index, can also be extracted. The index of refraction is
sensitive to the coordination chemistry and composition around Si. In the
present study, as mentioned earlier, after annealing, different compounds with
varying concentrations are found in the volume of the sample and also on the
surface. This has immense effect on the
thin film refractive index. The refractive index behaviour before (open circles) and after annealing (closed circles), as function of
wavelength, in Figure 7 shows that the values
varied from 2.7 to 2.5 at 1100 nm of wavelength for the film of thickness 100 nm
whereas the bulk silicon value is 3.4.Figure 6
The spectral transmittance of the (a) 100 nm
pure a-Si thin film, (b) a-Si film with a 30 nm Cr thin film blanket, and (c)
after annealing at 500°C for 20 minutes.Figure 7
The refractive index behaviour of a 100 nm Si
thin film as function of wavelength; open circles represent pure a-Si thin film
and closed circles represent the annealed film with a 30 nm Cr thin film
blanket.The optical band gap (Tauc
gap) is determined from the extrapolation of linear part in the (αhυ)1/2 Vs hυ plot assuming an indirect band gap. The band
gap variation with the thickness of the a-Si thin films before and after
annealing is shown in Figure 8. It is observed
that the band gap varied from 2 eV for the 100 nm thick film to 1.3 eV for the
film of thickness 600 nm. It is interesting that the optical absorption edge of
the as-deposited a-Si thin film and the film after annealing with a 30 nm Cr
layer on the top remained unaltered. The edge, in contrast, shifted towards
lower energies for the Cr/a-Si/BSG stack before annealing. This small red shift
in the absorption edge could be due to the formation of metal silicide during
deposition. The absorption edge, which represents the electronic band structure
of the materials, remained almost unaltered after annealing. This signifies the
fact that the different compounds observed during the annealing process do not
disturb or affect the essential optical properties of a-Si. We believe this to
be a very important observation for several optical applications relying on
nanocrystalline-Si.Figure 8
The
band gap variation as function of thickness for the (open triangles) as
deposited pure a-Si films (closed circles) after annealing.


## 4. Conclusions

The stabilization of
wurtzite structure of silicon via metal-induced nanocrystallization is
demonstrated. The identification of the crystalline phase was done using Raman
spectroscopy and x-ray diffraction. The optical properties of the
nanocrystalline silicon thin films are characterized by high transmittance, a
lower refractive index than bulk diamond-like Si but a larger optical band gap.
The presence of CrSi2 and Cr2O3 seems to
trigger the formation of the wurtzite structure.



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*Source: 736534-2008-04-09.xml*