Effect of deposition temperature on electron-beam evaporated polycrystalline siliconthin-film and crystallized by diode laserJ. Yun, S. Varalmov, J. Huang, K. Kim, and M. A. Green

Citation: Applied Physics Letters 104, 242102 (2014); doi: 10.1063/1.4883863 View online: http://dx.doi.org/10.1063/1.4883863 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/104/24?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Origin of preferential grain orientation in excimer laser-induced crystallization of silicon thin films Appl. Phys. Lett. 100, 161906 (2012); 10.1063/1.4704559 Characterization and control of crystal nucleation in amorphous electron beam evaporated silicon for thin filmsolar cells J. Appl. Phys. 110, 063530 (2011); 10.1063/1.3627373 Texture development and grain boundary faceting in an excimer laser-crystallized silicon thin film J. Vac. Sci. Technol. B 24, 2322 (2006); 10.1116/1.2353845 Aluminum-induced crystallization of amorphous silicon–germanium thin films Appl. Phys. Lett. 85, 2134 (2004); 10.1063/1.1789245 Formation of porous grain boundaries in polycrystalline silicon thin films J. Appl. Phys. 91, 9408 (2002); 10.1063/1.1476088

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Effect of deposition temperature on electron-beam evaporatedpolycrystalline silicon thin-film and crystallized by diode laser

J. Yun,1,a) S. Varalmov,1 J. Huang,1 K. Kim,1,2 and M. A. Green1

1School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney,New South Wales 2052, Australia2Suntech R&D Australia, Botany, New South Wales 2019, Australia

(Received 24 April 2014; accepted 2 June 2014; published online 16 June 2014)

The effects of the deposition temperature on the microstructure, crystallographic orientation, and

electrical properties of a 10-lm thick evaporated Si thin-film deposited on glass and crystallized

using a diode laser, are investigated. The crystallization of the Si thin-film is initiated at a

deposition temperature between 450 and 550 �C, and the predominant (110) orientation in the

normal direction is found. Pole figure maps confirm that all films have a fiber texture and that it

becomes stronger with increasing deposition temperature. Diode laser crystallization is

performed, resulting in the formation of lateral grains along the laser scan direction. The laser

power required to form lateral grains is higher in case of films deposited below 450 �C for all scan

speeds. Pole figure maps show 75% occupancies of the (110) orientation in the normal direction

when the laser crystallized film is deposited above 550 �C. A higher density of grain boundaries is

obtained when the laser crystallized film is deposited below 450 �C, which limits the solar cell

performance by n¼ 2 recombination, and a performance degradation is expected due to severe

shunting. VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4883863]

Polycrystalline silicon (poly-Si)-based thin-film solar

cells on glass, which combine the advantages of Si wafer

and thin-film solar cell technologies, are candidates for the

next-generation Photovoltaics. Poly-Si thin-film solar mod-

ules fabricated by CSG Solar have achieved a photovoltaic

conversion efficiency of 10.4%.1 The record minimodule

consisted of 2-lm thick Si thin-film on borosilicate glass

with grains of 1.5–2 lm in size that was processed by solid-

phase crystallization (SPC). However, the SPC process

resulted in a high density of intragrain defects that led to life-

time limiting recombination, thereby greatly reducing the

open-circuit voltage.2 As an alternative to SPC, continuous

wave (CW) diode laser crystallization of Si thin-films on

glass is reported to lead to high-quality grains with sizes up

to a few tenths of a millimeter in length and up to a few

millimeters in width.3,4 Recently, efficiencies up to 11.7%

have been achieved using this process.5,6 For the diode laser

crystallization of sputter deposited a-Si, it was shown that

the microstructure and crystallographic orientation were

strongly dependent on the optical properties of the as-

deposited film, which, in turn, were dependent on the

deposition conditions.7 In the present work, we report on the

effects of the deposition temperature on the microstructure,

crystallographic orientation, and electrical properties of

10-lm thick evaporated Si films on glass, crystallized by a

diode laser.

On a 3.3 mm thick planar borosilicate glass (Schott

Borofloat33) of 50� 50 mm2 size, 100 nm thick SiOx barrier

layer was deposited by plasma-enhanced chemical vapor

deposition (PECVD). Then, the SiOx layer was subjected to

a dehydrogenation annealing step at 500 �C for 2 h under N2

flow. 10 lm thick Si films were deposited by electron beam

(e-beam) evaporation at 650 �C. During the deposition,

in-situ boron doping was performed on to realize a concen-

tration of �2� 1016 cm�3. Then, the thin-film was crystal-

lized by a CW line-focus diode laser (808 nm wavelength

and 12 mm� 170 lm FWHM). The samples were placed on

a stage pre-heated to 650 �C to prevent cracks in the glass

and films during laser scanning. After removal of native ox-

ide formed during the crystallization by HF dip, phosphorus

dopant source (P508, Filmtronics, Inc.) was spin-coated.

Subsequently, diffusion was preceded in a belt furnace at

870 �C for <10 min. Junction depth around 300 nm was

achieved and emitter sheet resistance of 300–500 X/� was

obtained. Hydrogen passivation was performed in a

cold-wall vacuum system with an inductively coupled

remote plasma source at a temperature of 650 �C for 20 min.

The crystallographic orientation is analyzed by X-ray dif-

fraction (XRD) using Cu-Ka radiation with k¼ 1.5405 A at

an operating voltage of 35 kV and current of 40 mA.

Receiving slit of 3–8 mm2 was used to measure the (111),

(400), and (220) pole figures with background subtraction.

The defocused errors and pole figures were corrected and

calculated, respectively, using the texture analysis software

X’pert Texture from PANalytical B.V version 1.1a.

Fig. 1 depicts the Raman spectra and XRD pole figure

maps of e-beam evaporated Si thin-films for different

deposition temperatures. The Raman peak shifts from

501.2 cm�1 towards the position of the crystalline Si peak

(519–521 cm�1) as the deposition temperature increases

from 350 �C to 550 �C. Between 550 �C and 650 �C, the peak

position remains unchanged. For the qualitative analysis of

the crystallinity, the spectrum was deconvoluted into two

Gaussian and one mixed Lorentzian-Gaussian function near

480, 510, and 520 cm�1, corresponding to the transverse op-

tical (TO) modes of the amorphous phase (Ia), intermediate

phase (Im), and crystalline phase (Ic), respectively. The

a)Author to whom correspondence should be addressed. Electronic mail:

j.yun@unsw.edu.au. Tel.: þ61 2 93855000

0003-6951/2014/104(24)/242102/5/$30.00 VC 2014 AIP Publishing LLC104, 242102-1

APPLIED PHYSICS LETTERS 104, 242102 (2014)

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degree of crystallinity is estimated by the crystalline fraction

X, which is defined as

X ¼ ðIc þ ImÞ=ðIc þ Im þ IaÞ: (1)

The crystalline fraction increases from 74% to 96% as the

temperature increases from 350 �C to 550 �C and remains

unchanged up to 650 �C. These results are consistent with

the Raman spectra and XRD pole figure maps. The occupan-

cies of each crystal orientation in the normal direction of the

as-deposited Si thin-film, as analyzed by the XRD pole fig-

ures, are summarized in Table I. At the temperature of

350 �C, the (111) orientation in the normal direction has an

occupancy of 20% while the occupancy of the (110) and

(100) orientation amounts to 35% and 45%, respectively.

The (111) orientation switches to (110) when the deposition

temperature is increased to 450 �C. A further temperature

raise to 550 �C and 650 �C results in a predominant (110) ori-

entation in the normal direction. The (110) orientation has

occupancies of 98% and 93% at 550 �C and 650 �C, respec-

tively. A fiber texture in which one crystallographic axis of

the film is parallel to the substrate normal while there is a

rotational degree of freedom around the fiber axis, is evident

for each pole map at all deposition temperatures. There are

rings around 35� and around 90�, 45�, and 60� in the (111),

(110), and (100) pole figure maps, respectively, which

become stronger as the temperature increases. The formation

of a predominant (110) orientation when the temperature is

raised from 450 to 550 �C confirms an abrupt increase in

crystallinity, as likewise shown by the Raman spectra.

Ouyang8 reported on the formation of a preferential (110)

orientation in the normal direction for identical e-beam

evaporated Si thin-films deposited on a SiNx-coated

borosilicate-glass substrate at a substrate temperature of

500 �C. Also, evaporated Si films on a quartz substrate

showed preferential (110) orientation in the normal direction

for temperatures between 765 and 900 �C, as reported by

Mountvala and Abowitz.9

The absorption (%) at the wavelength of 808 nm was

measured for each sample prepared at a different deposition

temperature and the results are shown in Fig. 2. The samples

deposited at 350 �C and 450 �C have similar absorption val-

ues �74% at 808 nm, while an abrupt decrease in absorption

is observed for the samples deposited at 650 �C and 550 �C.

These results confirm the conclusion derived from the

Raman and XRD measurements that the crystallinity of the

thin-films abruptly increases between 450 and 550 �C.

The samples deposited at different temperatures were

crystallized by a diode laser. For a given scan speed, a cer-

tain laser power is required to fully melt the film and form

lateral grains in the plane normal and along the scan direc-

tion,4,5 as depicted in the upper inset of Fig. 3. The measured

results of the required power density in dependence of the

scan speed are depicted in Fig. 3. The samples deposited at

350 and 450 �C, and at 550 and 650 �C have identical

required power densities at each scan speed. The required

power density was higher for the films deposited at 350 and

450 �C. In our previous works, specific laser parameters to

achieve lateral grains were dependent on the optical and

dewetting properties. This is confirmed by the introduction

of different intermediate layers10 and a capping layer,4

respectively, which are not intrinsically related to the film

properties. There are various factors that are intrinsic to the

film quality and result in different laser parameters to form

lateral grains, such as the density7 and crystallinity of the

film. Since the thermal properties of a-Si and c-Si are not

identical,11 the impurity content12 and the roughness of the

film surface13 are also important factors. Therefore, further

data and more detailed investigations are required to

adequately interpret the above results.

FIG. 1. (Left) Raman spectra and (right) XRD pole figure maps of Si films

deposited by e-beam evaporation at different deposition temperatures. X rep-

resents the crystallinity fraction.

TABLE I. Occupancies of the different crystal orientations of the as-

deposited Si thin-films in the normal direction, analyzed by XRD pole figures

at different deposition temperatures. These occupancies took into account the

angular tolerance of the crystal orientation within the range of 5�.

Deposition temperature (�C) (100) (110) (111)

350 20% 35% 45%

450 17% 50% 33%

550 1% 98% 1%

650 3% 93% 4% FIG. 2. Measured absorption (%) at 808 nm for the Si films deposited at dif-

ferent temperatures.

242102-2 Yun et al. Appl. Phys. Lett. 104, 242102 (2014)

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The films deposited at 350 and 450 �C contain large vol-

umes of a high density of parallel grain boundaries, as shown

in Figs. 4(a) and 4(b), which can be up to 8 mm in width and

usually extend up to several cm along the scan direction as

indicated by an arrow. It was previously reported that a high

density of parallel grain boundaries, mostly of R3 coincident

site lattice (CSL) boundaries, are always formed as indicated

in Fig. 4(c).14 The films deposited at 550 and 650 �C do not

have such large defective regions and form relatively larger

grains, as shown in the inset of Fig. 3. It has been reported

that the lateral growth of grains is expected to occur in the

normal direction to the film surface since the heat conduction

through the glass substrate is extremely high.14,15 Therefore,

high densities of grain boundaries are likely to be formed

during the lateral growth in the normal direction to the

surface. To investigate nucleation and grain growth behav-

ior, transient conductance and surface optical reflectance

measurements are required under diode laser irradiation onto

the Si film, as described by Stiffler et al.16

XRD pole figure maps of the laser crystallized samples

at a speed of 40 cm/min and a power of 200 kW (350 and

450 �C) and 170 kW (550 and 650 �C) are shown in Fig. 5.

The crystal orientation is also found to be strongly dependent

on the orientation of the as-deposited film. The orientation

occupancies of the laser crystallized film in the normal direc-

tion analyzed by the XRD pole figure method for different

deposition temperatures are summarized in Table II. For the

temperatures of 550 and 650 �C, the predominant grain ori-

entation in the normal direction is (110), which occupies

65%. There is a fiber texture reflected in rings around 45�,60�, and 90� in the (100), (110), and (111) pole figure map,

respectively, as previously revealed earlier in as-deposited

films at deposition temperatures above 550 �C. For the films

deposited at 350 and 450 �C, the (111) orientation in the nor-

mal direction has the highest occupancy, followed by the

(110) and (100) orientation. At these deposition tempera-

tures, no fiber texture can be found. It has been previously

reported that the crystal orientation of the as-deposited Si

film has great influence on the final orientation of the laser

crystallized Si films under a melting condition such that the

initial film structure is partially maintained.17 In the vicinity

of the melting point, unmelted islands of Si at the underlying

SiO2 interface act as seed crystals.18 In our case, a part of the

FIG. 4. (a) Optical images of highly dense parallel grain boundaries after

defect etching. (b) Close-look of the parallel boundaries. (c) Electron back-

scatter diffraction mapping of area with the parallel boundaries with respect

to the layer surface.

FIG. 5. XRD pole figure maps of laser crystallized Si films deposited by

e-beam evaporation at 350 and 450 �C (upper row), and at 550 and 650 �C(lower row).FIG. 3. Required laser power density to form lateral grains in Si films vs.

laser scan speed for different deposition temperatures. Insets show the

optical-microscopy images of lateral and columnar grains.

TABLE II. Crystal-orientation occupancies of the laser crystallized poly-

crystalline Si thin-film in the normal direction, analyzed by the XRD pole

figure method for different deposition temperatures. These occupancies took

into account the angular tolerance of the crystal orientation within the range

of 5�.

Deposition temperature ( �C) (100) (110) (111)

350 and 450 10% 55% 35%

550 and 650 20% 75% 5%

242102-3 Yun et al. Appl. Phys. Lett. 104, 242102 (2014)

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predominant (110)-orientated crystals in the as-deposited

film (deposition temperature above 550 �C) could have sur-

vived the near-melting conditions under laser irradiation and

could act as seed crystals during solidification.

During the growth, the grain orientation is extremely

important as it determines the growth speed and the forma-

tion of grain boundaries.19,20 Each orientation of the grains

has its unique growth velocity,20 surface energy,20 and melt-

ing temperature,21 and these properties have great impact on

the overall grain growth and microstructure.22

The electrical properties of the laser crystallized films

are assessed by Suns-Voc, a quasi-steady-state open-circuit

voltage measurement method. Suns-Voc curves are useful to

identify diode properties, such as the open-circuit voltage,

pseudo fill factor, and diode ideality factors, and to deter-

mine recombination effects of the bulk and depletion regions

of non-metallized mesa-type diodes.23 In Suns-Voc measure-

ments, the sample and a calibrated reference solar cell are

simultaneously illuminated with a slowly decaying (�10 ms)

flashlight and the open-circuit voltage Voc as a function of

the illuminated light intensity is recorded. A pseudo-

current–voltage curve is then produced using an assumed

short-circuit current density of 20 mA/cm2 and the corre-

sponding shunt resistance is calculated. The obtained data

are fitted according to the two-diode model (n¼ 1 and

n¼ 2), and V1 and V2 are fitted in compliance with the 1-Sun

Voc of the n¼ 1 and n¼ 2 diodes, respectively. The results

are shown in Fig. 5. Generally, the n¼ 2 diode (V1>V2)

accounts for Shockley-Read-Hall recombination in the

space-charge region of the junction and at grain boundaries,

whereas the n¼ 1 diode (V1<V2) accounts for bulk and sur-

face recombination.

To determine the effect of the deposition temperature,

the results of the Suns-Voc measurements are compared in

Fig. 6 for the groups of films deposited at lower (350 and

450 �C) and at higher temperatures (550 and 650 �C). For this

experiment, a set of 12 samples (1.2 cm� 5 cm) was scanned

by a diode laser at a speed of 40 cm/min and a power of

200 kW (350 and 450 �C) and 170 kW (550 and 650 �C).

Before hydrogen passivation, 0.1-Sun Voc for the low-

temperature group is lower by 63.8 mV while 1-Sun Voc is

lower by 9 mV, in comparison to the results after passivation.

A logarithmic plot of Suns-Voc in dependence of the light in-

tensity showed that the n¼ 2 diode is greatly influencing Sun

Voc at low illumination levels.24 Therefore, a reduced value

of 0.1-Sun Voc means that Voc is dominated by n¼ 2 recombi-

nation which, in turn, results in a higher V1 than V2. A high

density of parallel grain boundaries is formed when the depo-

sition temperatures were 350–450 �C, and this is likely

responsible for the n¼ 2 recombination. However, the

high-temperature group revealed fewer grain boundaries and

is dominated by n¼ 1 recombination. The shunt resistance

Rsh of the low-temperature group is 352 X cm2 whereas the

high-temperature group has infinite Rsh. Rsh below 500 X cm2

can significantly degrade the performance of solar cells.

Again, the high density of grain boundaries can be considered

responsible for such a low Rsh as phosphor can diffuse selec-

tively from the emitter along the grain boundaries and make

contacts for both emitter and base.

After hydrogenation passivation, 0.1 and 1-Sun Voc

increased by 49 mV and 16 mV, respectively, for the low-

temperature group. Moreover, V2 increased by 48.6 mV

whereas V1 remained almost unchanged compared to the val-

ues before passivation. The strong increase in V2 implies that

the grain boundaries were effectively passivated. However,

this increased V2 is still lower than V2 of the non-passivated

samples of the high-temperature group. Now, the dominant

recombination is changed from the n¼ 2 to the n¼ 1 limit

caused by the increase in V2. The shunt resistance increased

by 249 X cm2, which is not a large improvement and, there-

fore, deleterious effects onto the cell performance are still

expected. For the high-temperature group, 0.1 and 1-Sun Voc

increased by 19.8 mV and 12.4 mV, respectively. The n¼ 1

recombination behavior remained unchanged, as well as Rsh.

Enhancements in 0.1 and 1-Sun Voc are relatively small com-

pared to the low-temperature group and V2 increased by

36 mV, which is a smaller enhancement than observed in

case of the low-temperature group. Since the films deposited

at higher temperatures have a lower grain-boundary density,

the passivation could not be as effective as for the

low-temperature group. Thus, no significant improvement is

achieved.

In conclusion, 10-lm thick Si films on glass have been

deposited using e-beam evaporation at different temperatures

and their structural, optical, and electrical properties are

characterized. The XRD pole figure maps agree well with

Raman spectra and imply that crystallization of the Si thin-

films is initiated at deposition temperatures between 450 and

550 �C. The crystals are predominantly (110)-orientated in

the normal direction. When the films are crystallized using a

CW diode laser, large lateral grains can be grown at opti-

mum laser conditions. Films deposited above 550 �C require

a lower power density at a given scan speed. The microstruc-

ture of laser crystallized films show a large region with a

FIG. 6. 0.1-Suns-Voc, 1-Suns-Voc, V1,

and V2 of the laser crystallized samples

deposited at different substrate

temperatures of 350 and 450 �C (low-

temperature group), and 550 and

650 �C (high-temperature group) (a)

before and (b) after hydrogenation

passivation.

242102-4 Yun et al. Appl. Phys. Lett. 104, 242102 (2014)

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high density of parallel grain boundaries in case of films de-

posited below 450 �C. XRD pole figure maps confirm that

films deposited above 550 �C have higher occupancies of the

(110) orientation, which could be attributed to the predomi-

nant (110) orientation of the as-deposited film. The electrical

properties of the laser crystallized films are characterized by

Suns-Voc measurements after formation of diffused emitter

junction. Values of both 0.1 and 1-Sun Voc were lower for

films deposited below 450 �C, compared to films deposited

above 550 �C. A high density of grain boundaries is assumed

to limit the solar-cell performance by n¼ 2 recombination.

This program has been supported by the Australian

Government through the Australian Renewable Energy

Agency (ARENA). The Australian Government, through

ARENA, is supporting Australian research and development

in solar photovoltaic and solar thermal technologies to help

solar power become cost competitive with other energy sour-

ces. The author acknowledges support from the Australian

Government through the ARENA Ph.D. scholarship. They

thank Dr. Q. Zakaia for assistance with the EBSD imaging,

and Dr. Y. Wang for assistance with the XRD measurement.

Special thanks to Dr. F. Falk for helpful discussions.

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