10-um-thick non-hydrogenated amorphous-silicon (a-Si) films were deposited at relatively high rates (_>10 Å/s) by radio-frequency magnetron sputtering (RFMS) on different large-area buffer-layer-coated glass substrates at deposition temperatures ranging from room temperature (RT) to 300oC. These amorphous samples were subsequently crystallized by means of a continuous-wave diode laser, looking for conditions to reach liquid-phase crystallization. The influence of deposition conditions on the quality of the final micro-crystalline silicon films has been studied

Canteli, D.

Cárabe, J.

Fernández, S.

Gandía, J. J.

García-Perez, F.

González, J.P.

Gómez-Mancebo, M.B.

Molpeceres Álvarez, Carlos Luis

Morales Furió, Miguel

Saugar, E.

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid

LASER-INDUCED CRYSTALLIZATION OF SPUTTERED UNHYDROGENATED SILICON AT LOW 
TEMPERATURES 
 
E. Saugar1, J.P. González1, S. Fernández1, J.J. Gandía1, J. Cárabe1, D. Canteli2, M. Morales2, C. Molpeceres2, F. García-
Pérez3, M.B. Gómez-Mancebo3. 
1Energy Department, CIEMAT, Madrid, 28040, Spain 
2Centro Láser, Universidad Politécnica de Madrid, Madrid, 28038, Spain 
3Chemistry Division, CIEMAT, Madrid, 28040, Spain 
 
ABSTRACT: 10-µm-thick non-hydrogenated amorphous-silicon (a-Si) films were deposited at relatively high rates 
(≥ 10 Å/s) by radio-frequency magnetron sputtering (RFMS) on different large-area buffer-layer-coated glass 
substrates at deposition temperatures ranging from room temperature (RT) to 300ºC. These amorphous samples were 
subsequently crystallized by means of a continuous-wave diode laser, looking for conditions to reach liquid-phase 
crystallization. The influence of deposition conditions on the quality of the final micro-crystalline silicon films has 
been studied. 
Keywords: amorphous silicon, liquid-phase crystallization, multicrystalline silicon 
 
 
1 INTRODUCTION 
 
 The current photovoltaic scenario is dominated by 
wafer-based silicon technology, which has led the 
worldwide market for a long time owing to its good 
performance-to-cost ratio [1]. Nevertheless, and in spite 
of the dramatic cost reduction undergone by mono- and 
multicrystalline silicon wafers in the last decade, the 
further gain in competitiveness of photovoltaic devices 
requires intensive search for new approaches in what 
concerns semiconductor devices [2]. The road to find 
new solutions and new forms of silicon go through 
thickness reduction, kerf avoidance and manufacturing-
cost lowering. From this point of view, liquid-phase 
crystallization (LPC) has arisen as a promising and novel 
method [3] above solid-phase crystallization (SPC), for 
which a higher thermal budget is needed. Furthermore, 
cells made from SPC silicon exhibit lower open-circuit 
voltages [4].  
 Previous works have demonstrated satisfactory LPC-
silicon results in terms of material grain sizes and cell 
open-circuit voltages well above 600 mV [5]. While it is 
true that those results are promising, the substrate 
temperatures involved exceed 600ºC, thus limiting the 
attractiveness of the process[6]. 
 In this work the quality of polycrystalline silicon 
(poly-Si) obtained from a-Si deposited at relatively low 
temperatures (300ºC) is analyzed in what concerns 
crystalline fraction and grain size. The effects of 
deposition conditions, buffer layer, grain size and crystal 
preferential orientation are discussed. 
 
2 EXPERIMENTAL 
 
 10-µm-thick non-hydrogenated a-Si films were 
deposited onto Corning Eagle XG® glass substrates 
coated with a 200-nm SiO2 layer by using an RFMS 
system. Prior to the deposition, the substrates were 
cleaned in an ultrasonic bath, subsequently rinsed in 
ethanol and dried with nitrogen. Two series were 
deposited respectively at room temperature and 300ºC, 
under a working pressure of 1.07 Pa. The measurement of 
substrate temperature was performed by means of a 
type-K reference thermocouple.   
 The a-Si was treated with a continuous-wave diode 
laser emitting at 940 nm and forming a spot which is 
linear along the X axis and Gaussian in the Y axis, and 
whose dimensions in focus are 16.5 mm × 2.16 mm. 
During the crystallization process, in order to reduce 
thermal stress and prevent cracks, the samples were 
subjected to high temperatures (650ºC) for a short time. 
The samples were moved in a straight line using a 
constant scanning speed ranging from 400 mm/min to 
1000 mm/min and irradiance from 900 J/cm2·s to 1650 
J/cm2·s. Each sample was crystallized in distinct zones at 
different laser conditions. 
 The quality of the micro-crystalline silicon obtained 
has been evaluated by X-Ray diffraction (XRD) using a 
PANalytical X’pert Pro diffractometer operating in θ – θ 
configuration, with CuKα radiation (45kV – 40 mA), in 
the angular range 20º < 2θ < 80º. Phase identification was 
obtained by comparison to the Inorganic Crystal 
Structure Database (ICDS). In order to study the material 
modifications caused during the laser process, Raman 
spectra were measured by using a commercial InVia 
Renishaw Raman spectrometer employing the 514.5-nm 
line of an Ar laser. 
 
3 RESULTS AND DISCUSSION 
 
3.1 Buffer layer and stability 
 As known from literature [7], diffusion barriers play 
an important role in the laser crystallization process. On 
the one hand, they are necessary in order to avoid 
contamination of the silicon due to diffusion of species 
from the glass substrate [8]. On the other hand, they 
prevent the formation of droplets due to surface tension 
during liquid-phase crystallization [3][8]. With this in 
mind, we have mutually compared the behavior of 
different samples on glass substrates with and without 
a 200-nm SiO2 coating during the laser process. 
 
Table 1: Process conditions and results obtained for two 
samples deposited at room temperature. A1 on bare glass 
and B1 on 200-nm SiO2-coated glass. 
ID Zone 
Irradiance 
(J/cm2·s) 
Scan 
speed 
(mm/min) 
Status 
A1 1 1164 ± 49 600 Damaged 
B1 1 1164 ± 49 600 Crystallized 
 
 Table I shows the effect of the diffusion barrier on 
the laser crystallization process. The diffusion barrier not 
only prevents the formation of droplets [8] and 
contamination but also favors crystallization, as it is 
shown on Table 1. 
 
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3.2 Structural properties 
 Once the need for diffusion barriers had been 
established, the influence of substrate temperature on the 
crystallization process was investigated. Although very 
high deposition temperatures, such as those previously 
cited [6] (about 600 ºC) were avoided, samples C1 and D1 
were respectively deposited by sputtering at 300 ºC and 
room temperature (RT). Different regions of these samples 
were subsequently laser-processed in different conditions 
(see please Table 2). 
 Figure 1a and 1b show the corresponding Raman 
spectra of the samples, including a <100> monocrystalline-
silicon wafer for reference. The Raman spectra were 
deconvoluted into three Gaussian peaks respectively at 480 
cm-1 (corresponding to the amorphous-silicon phase), 510 
cm-1 (corresponding to a defective crystalline phase) [9] 
and at 520 cm-1 (corresponding to silicon crystallites). 
From the integration of the deconvoluted Gaussian peaks it 
is possible to calculate the crystalline fraction (ΧC) through 
the following equation: 
𝑋𝐶 =
(𝐼520 + 𝐼510)
(𝐼520 + 𝐼510 + 𝐼480)
 
 
 
(a) 
 
(b) 
Figure 1: Raman spectra of 10-µm-thick magnetron-
sputtered silicon films deposited at different 
temperatures: sample C1 at 300ºC (a) and D1 at room 
temperature (RT) (b). The spectrum of a c-Si reference 
has been added. The graph inset shows a detail of the 
peak region. 
 As it can be observed on Figure 1, the spectrum of 
the c-Si reference (green curves) has its peak at 520 cm-1 
with a full-width half maximum (FWHM) of 4.5 cm-1 
approximately. However, the peaks of samples deposited 
at respectively 300ºC and room temperature differ both 
mutually and with respect to the c-Si reference peak both 
in position and in FWHM. Furthermore, for sample D1, a 
slight contribution due to the amorphous phase present in 
the material can be observed at 490 cm-1. 
 The sample deposited at relatively high temperature 
(C1) allows higher irradiances and this means higher 
crystalline fraction values of up to 20% more than the 
sample deposited at room temperature. 
 These samples have also been evaluated concerning 
their crystal orientation and grain size from X-Ray 
diffraction (XRD). Figure 2 shows the respective XRD 
patterns of samples C1 (a) and D1 (b). 
 
 
(a) 
 
(b) 
Figure 2: X-Ray diffraction patterns (XRD) of 
respectively sample C1, deposited at 300ºC (Figure 2a) 
and sample D1, deposited at RT (Figure 2b). The 
different patterns in each plot correspond to different 
zones over the samples. 
 The XRD results evidence that laser-induced 
crystallization generates different crystal orientations in 
different zones, i.e. for different laser irradiances. 
However, by comparing the relative intensities of the 
XRD patterns it becomes easy to notice that <111> is the 
main orientation in both samples. Table 2 shows the 
crystalline fraction of each zone and the respective grain 
size as obtained from the Scherrer equation [10].  
Table 2 also reveals that grain size increases abruptly, 
up to 2450 nm, when crystalline fraction reaches high 
values, beyond XC = 95%. Furthermore, as suggested by 
Figure 3, not only irradiance but also sample temperature 
favors the jump to really large grain sizes. Sample C1 is 
believed to have undergone LPC.  
 
 
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Table 2: Crystalline fraction derived from the integration 
of deconvoluted Raman peaks, and grain size obtained 
from XRD pattern by using Scherrer equation, of two 
samples deposited respectively at 300ºC (C1) and RT 
(D1). 
ID Zone 
Irradiance / 
Scan speed  
XC 
Grain Size 
(nm) 
C1 
1 
1384 ± 51 W cm-2 
1000 mm min-1 91 ± 3 70 ± 2 
2 
1538 ± 52 W cm-2 
1000 mm min-1 93 ± 3 134 ± 5 
3 
1630 ± 53 W cm-2 
1000 mm min-1 96 ± 3 2450 ± 400 
D1 
1 
914 ± 47 W cm-2 
400 mm min-1 84 ± 3  50 ± 1 
2 
963 ± 48 W cm-2 
400 mm min-1 81 ± 2 57 ± 1 
3 
1164 ± 49 W cm-2 
600 mm min-1 75 ± 1 57 ± 1 
4 
1222 ± 50 W cm-2 
600 mm min-1 85 ± 3 71 ± 2 
 
 
(a) 
Figure 3: Grain size in logarithmic scale versus 
irradiance. 
 In order to further investigate how substrate 
temperature affects the samples and their compactness, 
the refractive index of different samples deposited at 
different substrate temperatures has been studied. It is 
well known that refractive index is an important optical 
parameter related to mass- and void density [11]. From 
the Clausius-Mossoti equation, mass density can be 
drawn [12]. 
 
𝜌𝑎−𝑆𝑖 = (
𝑛∞
2 − 1
𝑛∞2 + 2
)
3𝑚𝑆𝑖
8𝜋𝛼𝑆𝑖−𝑆𝑖
 
 
where 𝑚𝑆𝑖 is the mass of the silicon atom, n∞ is the 
refractive index at long wavelengths and 𝛼𝑆𝑖−𝑆𝑖 is the 
bond polarizability in the amorphous phase. The value 
𝛼𝑆𝑖−𝑆𝑖 = 1.96 × 10
−24 cm3 is taken from [12].  
 Four a-Si samples have been deposited in the same 
conditions as for C1 and D1 except for substrate 
temperature, which has been varied between RT and 
300ºC. 
 Figure 4 shows the refractive indexes of four samples 
deposited at 1.07 Pa at different temperatures ranging 
from RT to 300ºC. In the whole range covered an abrupt 
refractive-index growth with increasing substrate 
temperature is appreciated. This result is attributed to a 
reduction of void density [13]. 
 From the analysis of refractive-index dispersion by 
means of the Wemple-DiDomenico dispersion model 
[14], below the interband absorption edge it is possible to 
obtain important information about atomic structure and 
mass density. The refractive index values were fitted to 
the WDD dispersion model according the following 
equation: 
 
𝑛(𝐸)2 − 1 =
𝐸𝑑𝐸0
(𝐸𝑑
2 − 𝐸2)
 
 
 
 
Figure 4: Refractive indexes of four a-Si samples 
deposited at different substrate temperatures. The circle 
inset illustrates the refractive-index rise with 
temperature. 
Ed is an important parameter, which is closely related to 
the structural characteristics of the material and obeys the 
following relationship: 
 
𝐸𝑑 = 𝛽𝑁𝑐𝑍𝑎𝑁𝑒, 
 
where NC is the coordination number of the cation nearest 
neighbor to the anion, and Za, Ne, β are constants [14]. 
The voids and the low packing of the amorphous lattice 
result in low density and coordination-number values 
[12]. In this sense, the estimate of the coordination 
number can provide information about the evolution of 
void density with substrate temperature. According to 
Wemple [15], the following expression allows us 
calculate the coordination number: 
 
𝐸𝑑
𝑎
𝐸𝑑
𝑥 = (
𝜌𝑎
𝜌𝑥
)(
𝑁𝑐
𝑎
𝑁𝑐
𝑥) 
 
where ρ is the density, x and a refer to crystalline- and 
amorphous phases respectively.  
 Figure 5 shows the mass density drawn from the 
Clausius-Mossoti equation and the coordination number 
derived from the fitting of the Wemple-DiDomenico 
dispersion model. As it can be observed, mass density 
and coordination number increase with substrate 
temperature as a consequence of silicon-atom 
rearrangement due to substrate-temperature enhancement. 
At higher temperatures, the diffusion processes are more 
likely, so greater compactness is induced. 
 
33rd European Photovoltaic Solar Energy Conference and Exhibition
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Figure 5: Mass density (black dots with short dash error 
bars) and coordination number Nc (blue triangles with 
solid error bars) versus substrate temperature. 
 The mass-density values obtained increase from 
2.1 g/cm3 for sample deposited at RT to 2.3 g/cm3 for 
sample deposited at 300ºC, whereas the coordination 
number stretches from 2.1 to 3.7 in the same substrate-
temperature range. The typical values, according to 
bibliography [12][16], are ρ = 2.33 g/cm3 and Nc = 4 for 
c-Si, ρ = 2.29 g/cm3 and Nc = 3.7 for a-Si. 
 In view of the results obtained, the samples deposited 
at moderately high deposition temperatures show higher 
compactness than those deposited at RT. Such a result is 
consistent with the finding that high crystalline fractions 
and grain sizes of the order of microns are only obtained 
from amorphous-silicon samples deposited at moderately 
high temperatures (of the order of 300 ºC, yet well below 
the high temperatures largely found in the literature). The 
emergence of this microcrystalline material is attributed 
to the achievement of liquid-phase crystallisation in the 
conditions indicated. 
 
4 CONCLUSIONS 
 
 In this work several samples have been deposited 
onto different substrates, either bare or coated with 
a 200-nm SiO2 layer, with different substrate 
temperatures. Firstly, it has been noted that the buffer 
layer plays a fundamental role to obtain a successful 
crystallization process by improving wettability and 
enhancing thermal stability under high laser irradiances. 
 Secondly, the relevance of substrate deposition 
temperature has been assessed. It has been observed that 
the samples deposited at moderately high (although much 
lower than those typically found in the literature) 
substrate temperatures show higher compactness and 
mass density than those deposited at RT. The effect is 
attributed to the activation of different diffusion 
processes during deposition, leading to the rearrangement 
of silicon atoms. Furthermore, these samples offer better 
performance under high irradiances, resulting in 
increased crystalline fractions and grain sizes. The 
results, in summary, suggest that the samples having 
micron-size crystallites are those for which process 
conditions trigger liquid-phase crystallization. Such an 
achievement in spite of the moderate substrate 
temperatures (not higher than 300ºC) is considered quite 
relevant. 
 
5 ACKNOWLEDGEMENTS 
 
This research has been partially supported by the Spanish 
Research Agency (AEI) and Ministry of Economy, 
Industry and competitiveness through project CHENOC 
(ENE2016-78933-C4). 
 
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Laser-Induced Crystallization of Sputtered Unhydrogenated Silicon at Low Temperatures

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Laser-Induced Crystallization of Sputtered Unhydrogenated Silicon at Low Temperatures

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