TRANSPARENT CONDUCTIVE OXIDE THIN FILMS FOR SOLLAR CELLS APLICATION

dehisceforkElectronics - Devices

Nov 2, 2013 (3 years and 10 months ago)

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U.P.B. Sci. Bull., Series B, Vol. 75, Iss. 1, 2013 ISSN 1454
-
2331


TRANSPARENT CONDUCTI
VE OXIDE THIN FILMS
FOR
SOLLAR CELLS APLICAT
ION

Beatrice
-
Gabriela

SB
Â
RCEA
1
,

Lucia Nicoleta
LEONAT
2
,

Ioan

Viorel
BR
Â
NZOI
3

Filme

subţiri

de

oxid

de

zinc

dopate

cu

oxid

de

aluminiu

cu

o

grosime

de
aproximativ

300

nm

au

fost

depuse

pe

subtrat

de

sticlă

prin

tehnica

depunere
laser

pulsată.

Compozite

ceramice

de

ZnO

având

fază

secundară

de

Al
2
O

au fost

folosite

ca

ţ
inte

pentru

ablaţia

laser.

Prin

măsuratori

de

difracţie

de

raze

X se

determină

o

structură

cristalină

a

filmelor

subţiri

ş
i

o

dimensiune

medie

de
cristalit

mai

mică

de

20

nm

pentru

toate

probele.

Spectrele

de

transmisie

pentru
filmele

de

ZnO

dopate

prezintă

o

transmisie

mai

mare

de

80%

î n

domeniul
vizibil.

Aluminum

oxide

doped

zinc

oxide

thin

films

with

thickness

around

300

nm
were

deposited

on

glass

substrate by

pulsed

laser deposition.
Composite ceramics

comprising

ZnO

and

secondary

phase

Al
2
O
3

were

employed

as

targets for laser

ablation. X
-
ray

diffraction

measurements

reveal

a

polycrystalline structure

of

films

and

an

average

crystallite

diameter

of

less

than

20

nm

for

all the

samples.

The

transmission

spectra

of

doped

ZnO

films

on

glass

substrates show optical
transmission larger than 80 % in
the visible range.

Keywords
: ZnO, TCO, thin films, solar cell


1. Introduction


Zinc oxide (ZnO) has been regar
ded as a promising material for
t
ransparent electrodes, solar cells, photo
-
detectors, diodes, sensors, thin film
transistors, and wave resonators
. [1]

Recently, transparent conducting oxides (TCOs) have been widely studied

Among TCOs, zinc oxide (ZnO) is one of the most promising materials for the
fabrication of the next generation of optoelectronic devices in the UV region an
d
optical or display
devices.[2]

Zinc

oxide or impurity (B, Al, Ga, In and Zr) doped zinc oxide films have
been investigated as alternative materials to indium tin oxide (ITO) for organic



1

Ph
ys
.
,

Fa
c
ul
t
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f

App
li
e
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Ch
e
m
istr
y

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d

M
a
teri
a
l
s

S
cie
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ce
.

Un
iversit
y

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LITE
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IC
A

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,

Roman
i
a
,

e
-
ma
il:

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i
_b
e
a
@
y
ahoo
.c
o
m

2

Ph
ys
.
,

Fa
c
ul
t
y

o
f

App
li
e
d

Ch
e
m
istr
y

an
d

M
a
teri
a
l
s

S
cie
n
ce
.

Un
iversit
y

PO
LITE
HN
IC
A

o
f

Bu
c
ha
rest
,

Roman
i
a
,

e
-
ma
il:

l
u
cy
a_
le
o
@
y
ahoo
.c
o
m

3

P
r
o
f
.,

Fa
c
ul
t
y

o
f

App
li
e
d

Ch
e
m
istr
y

an
d

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a
teri
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s

S
cie
n
ce
.

Un
iversit
y

PO
LITE
HN
IC
A

o
f

Bu
c
ha
rest
,

Roman
i
a
,

e
-
mail:

iv_branzoi@yahoo.com

150




Beatrice
-
Gabriela

Sb
â
rcea
,
Lucia Nicoleta Leonat
,
Ioan

Viorel Br
â
nzoi


light emitting diodes (OLEDs) because zinc oxide is nontoxic, inexpensive and
abundant. I
n comparison to ITO, ZnO has the advantages of low cost, nontoxic
and with good thermal stability.

Zinc

oxide

is

a

semiconductor,

which

is

highly

transparent

in

the


visible region

with a wide and direct band gap of about 3.37 eV at room
temperature and a

high exciton binding

energy

of

60

eV.

Generally,

undoped

ZnO

thin

films exhibit

n
-
type conduction

with

a

background

electron

concentration

as

high

as 10
2
1

cm
-
3

[3].

Aluminium,

indium

and

gallium

oxides

have

been

reported

as

effective

n
-

type dopants to
increase the electrical conductivity of pure zinc oxide. [4]

Recently,

aluminium oxide doped zinc oxide (AZO) thin films have been
used as windows and contact layers for thin film solar cells. [5]

Among

the

several

fabrication

techniques,

pulsed

laser

depo
sition

(PLD)

has attracted much

attention

because

the

fabrication

process

is

quite

suitable

for optoelectronic

devices

using

the

ZnO

transparent

electrode.

The

composition

of films

grown

by

PLD

is

quite

close

to

that

of

the

target.

[6]

PLD

films

may

be
crystallized

at

lower

deposition

temperature

in

comparison

with

other

physical vapor

deposition

techniques

due

to

the

high

kinetic

energies

of

the

ionized

and ejected

species in the laser plumes. [3]

In

this

study,

Al2O3

doped

ZnO

thin

films

were

prepared

using

PLD,


On glass

substrate, at different substrate

temperatures, ranging from room
temperature

to

500
o
C.

The

crystallographic

structure

and

optical

properties

of

the
films prepared with different growth parameters will be discussed.


2. Experimental

AZO

thin

films

were

prepared

on

glass

substrates

at

different

temperature

by pulsed

laser

deposition,

using

a

ceramic

target.

The

target

of

AZO

was

fabricated using

high
-
purity

ZnO

(99.99%)

doped

with

3wt%

Al
2
O
3
(99.99%).


The

target was

obtained

by

manually

grinding

the

powder

mixture

for

30

min,

pressing

the powders to pellets at the pressure of 3.5 tons/cm
2
, and sintering
of pellets in air.

A

KrF

excimer

laser



=

248

nm,

pulse

duration

20

ns,

fluence

2

J/cm
2
,

pulse repetition rate 10 Hz) was used

for film growth. An oxygen gas
background

with

pressure

p(O
2
)

=

10
-
3

mbar

is

employed

during

PLD

and

post
-

deposition cool
-
down.

The

AZO

films

were

produced

by

ablating

ZnO

targets
containing

3 wt%
-
3wt% Al
2
O
3
.

Doped

ZnO

thin

films

were

characterized

by

X
-
ray

diffraction

(D8


Discover AXS
-
Bruker diffractometer) to evidence the crystal structure.
The

surface morphology

was

observed

by

atomic

force

microscopy

(AFM

from

Veeco).

The optical transmission measurements were performed using a UV
-
VIS
spectrophotometer

(Jasco

570)

and

the

thin

films

structure

was

investigated

with
scanning electron microscopy (SEM, Auriga from Zeiss).

Transparent conductive oxide thin films for sollar cells aplication



151


3.

Results and discussion


Fig.

1

shows

the

XRD

patterns

of

AZO

films

deposited

at

different
temperature, ranging

from

room

temperature

to

500
o
C.

The

X
-
ray

diffraction
patterns were obtained for 2θ values from 10 deg. to 70 deg.

The

AZO thin films deposited at room temperature were

amorphous while

the

well

crystallized

polycrystalline

phase

appears

at

temperatures

above
30
0
o
C.

All

samples

containing

films

deposited

over

300°C

temperature

presented
strong

c
-
axis texture,

perpendicular

to

the

substrate

with

a

pronounced

[002]
diffraction

peak

at

approximately



=

34.4

degrees.

The

[002]

peak

intensity
increased with the
increase of

the temperature.

In addition, two small peaks [102]
and

[103],

appear

at

approximately

48,8

degrees and

63

degrees,

respectively.
The

peak

intensity

of

[102]

and

[103]

peaks

also

increase

with increasing

the
temperature.

XRD

patterns

show

that

the

deposited

films

were

crystallized

in
hexagonal phase, namely a wurtzite structure.


F
i
g
.
1

X
-
ra
y

d
iffracti
o
n

p
atter
n
s

o
f

AZ
O

t
h
i
n

fil
m
s

d
e
po
site
d

a
t

d
iffere
n
t

s
ub
strat
e

te
mp
erat
u
re

X
-
ray

diffraction parameters are presented in table 1.



152




Beatrice
-
Gabriela

Sb
â
rcea
,
Lucia Nicoleta Leonat
,
Ioan

Viorel Br
â
nzoi


Tab
l
e

1

X
-
r
a
y

d
iffr
a
cti
o
n

da
t
a

o
f

t
h
e

AZ
O

t
h
i
n

fil
m
s

a
t

d
iff
eren
t

t
e
m
pera
t
ure
s

S
a
mp
l
e

n
a
m
e

M
ille
r
i
nd
e
x

(
hk
l
)

2
θ

x
d
e
g
z


l
l

w
i
d
t
h

a
t

h


max
i


(
c
t
e
M
)

a

(
n
m
)


l

3
%

㌰3
o
C


2

3
4
.


1
.
〱0

8
.



l

3
%

㌵3
o
C


2

3
4
.


0
.
㔲5


.



l

3
%

㐰4
o
C


2

3
4
.


0
.
㔲5


.



l

3
%

㐵4
o
C


2

3
4
.


0
.
㐷4


.



l

3
%

㔰5
o
C


2

3
4
.


0
.
㔳5


.



The

grain size of the film from the XRD data was calculated using the

Debye


Scherrer formula:






(1)

where

D
is the grain size of the crystallite, λ (1.54059 Å)

is the wavelength
of the X
-
rays used, B is the broadening of diffraction line measured at the half of
its maximum intensity in radians and θ is the angle of diffraction.

The

transmittance of

the

AZO

thin

films, shown

in
Fig.

2 is an important

factor

for

T
CO

applications,

because

applications

such

as

solar

cells require a
wide bandgap to avoid unwanted absorption of the solar spectra.


F
i
g
.
2

Op
tica
l

tra
n
s
m
issi
o
n

o
f

AZ
O

t
h
i
n

fil
m
s

d
e
po
site
d

a
t

d
iffere
n
t

s
ub
strat
e

te
mp
erat
u
r
e

The

transparency

of

the

films

increased

in

the

visible

range

(>80%)

after aluminum

oxide

was

introduced

in

the

films.

The

widening

of

the

optical

band
-

gap

with

the

substrate

temperature

is

originated

by

the

increase

of

the

electron concentration caused by Al
2
O
3

doping

[7].


Transparent conductive oxide thin films for sollar cells aplication



153












The

surface

topography

and

SEM

micrographs

are

being

presented

in

the next

images,
Fig.

3.



















































154




Beatrice
-
Gabriela

Sb
â
rcea
,
Lucia Nicoleta Leonat
,
Ioan

Viorel Br
â
nzoi





































F
i
g
.

3
.

SE
M

a
n
d

AF
M

i
m
a
g
e
s

o
f

AZ
O

t
h
i
n

fil
m

d
e
po
site
d

a
t

d
iffere
n
t

s
ub
strat
e

te
mp
erat
u
re
s

st
a
rti
n
g

fr
o
m

r
oo
m

t
empe
r
a
t
u
r
e

(top)
,

30
0
o
C
,

350
o
C
,

400
o
C
,

450
o
C
,

500
o
C.
(bottom)

The

surfaces

investigated

by

AFM

look

flat

and

very

few

sharp

peaks
appear

in

the

domain.

The

root
-
mean

square

(RMS)

roughness

value

of

the

AZO
thin

films

fabricated

in

the

range

of

27
-
500
o

C

does

not

vary

linearly

with

the
temperature

but

the

surfaces

become

smoother

with

temperature.

Increasing

the
temperature

also

causes

the

increase

of

the

droplets

sizes,

which

may

be

due

to
the

coalescence

of

the

grains.

However,

the

maximum

value

for

RMS

roughness
does not

exceed

5

nm

for

all

samples

studied.

The

RMS

values

are

shown

in

the
table

2.

Tab
l
e

2

Th
e

R
M
S

va
l
u
e
s

o
f

r
oughn
es
s

f
o
r

t
h
e

AZ
O

t
h
i
n

fil
m
s

a
t

d
iffere
n
t

te
m
p
er
a
t
u
res
:

Sa
m
p
l
e

na
m
e

Roo
t

M
e
a
n

Squa
r
e

(
R
M
S
)

r
oughn
ess
,

n
m

AZ
O

3
%

r
oo
m

te
m
p

2
.
34

A
Z
O

3
%

300
o
C

4
.
48

A
Z
O

3
%

350
o
C

1
.
46

A
Z
O

3
%

400
o
C

2
.
96

A
Z
O

3
%

450
o
C

0
.
82

A
Z
O

3
%

500
o
C

1
.
29

Transparent conductive oxide thin films for sollar cells aplication



155


The

surface

morphologies

of

the

films

are

also

observed

by

FESEM,
Fig.

3.

The

morphology

of

AZO

films

is

found

to

be

compact

and

continuous.
Agglomeration
-
like micrograins arbitrarily dispersed on the surface can be
observed.


4.

Conclusions


3

wt%

Al
2
O
3

doped

ZnO

thin

films

were

prepared

on

glass

substrates

by
Pulsed Laser Deposition (PLD), with different substrate

temperatures. The
structural and optical properties of Al
2
O
3

doped

ZnO thin film

have been
investigated by X
-
ray diffraction, UV
-
Vis

spectroscopy, FESEM and AFM
techniques.

X
-
ray

diffraction

studies

show

a

polycrystalline

wurtzite

structure

and

a
preferential

orientation

along

the

axis

[002].

The

crystallite

size

is

around

18

nm.
From

the

AFM

and

FESEM

images,

the

surfaces

investigated

are

smooth

with
very

few

droplets;

the

root

mean

square

value

of

r
oughness

for

each

sample

does
not

decrease

linearly

with

the

temperature, nevertheless

it

is

not

higher

than

5
nm.

The

optical

transmittance

is

over

80%,

which

makes

these

AZO

thin
films good candidates for TCO application.


Acknowledgement


The

work has
been funded by the Sectoral Operational Programme
Human Resources Development 2007
-
2013 of

the Romanian Ministry of

Labour,
Family and Social Protection through the Financial Agreement
POSDRU/88/1.5/S/60203.

R

E

F

E

R

E

N

C

E

S

[
1
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D
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Cho
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tr
o
l

o
f

a
-

a
n
d

c
-

p
la
n
e
p
refere
n
tia
l

o
rie
n
tati
on
s

o
f

Zn
O

t
h
i
n

fil
m
s
,

App
lie
d

Su
rfac
e

S
cie
n
c
e
,

255
,

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9

p
p

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3484

[
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m
i
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ati
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t
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t
h
ic
kn
es
s

a
n
d

op
tica
l

c
on
sta
n
t
s

o
f

tra
n
s
p
are
n
t

i
nd
i
um
-
dop
e
d

Zn
O

t
h
i
n

fil
m
s

b
y

t
h
e

e
nv
el
op
e

m
et
hod
,

M
aterial
s

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cienc
e
,

Vo
l
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25
,

No
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pp709
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S
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rk
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gam
i
,

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e
n
j
i

Eb
i
ha
r
a
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Pa
ik
-
K
y
u
n

Sh
i
n
,

S
tr
u
ct
u
r
e

a
n
d

p
r
op
ertie
s

o
f

tra
n
s
p
are
n
t
c
ondu
cti
v
e

dop
e
d

Zn
O

fil
m
s

b
y

pu
lse
d

lase
r
d
e
po
siti
on
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App
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d

Su
rfac
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[
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i
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,
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.
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ee
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Y
.

L
ee
,

P
r
op
ertie
s

o
f

mu
ltila
y
e
r

g
alli
u
m

a
n
d

al
um
i
nu
m
dop
e
d
ZnO
(
GZO
/
AZO
)
tra
n
s
p
are
n
t
t
h
i
n
fil
m
s
d
e
po
site
d
b
y
pu
lse
d
lase
r

d
e
po
siti
o
n
p
r
o
cess
,

T
ra
n
sacti
on
s

o
f

Non
ferr
ou
s

M
etal
s

So
ciet
y

o
f


C
h
i
n
a
,

Vo
l

2
1
,

Supp
le
m
e
n
t

1
,

2011
,

p
p

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[
5
]

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.
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,

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y
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g

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o
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i
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.
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.

K
i
m
,

P
r
op
ertie
s

o
f

AZ
O

Th
i
n

F
il
m
s

f
o
r

So
la
r

156




Beatrice
-
Gabriela

Sb
â
rcea
,
Lucia Nicoleta Leonat
,
Ioan

Viorel Br
â
nzoi


C
ells
D
e
po
site
d

o
n

Po
l
y
car
bon
at
e

Sub
strates
,

J
ou
r
n
a
l

o
f

t
h
e

Ko
rea
n

Phy
sica
l

S
o
ciet
y
,

Vo
l
.

5
5
,

No
.

5
,
2009
,

pp
.

1945
-
1949

[
6
]

S
.
M
.

Pa
rk
,

T
.

Ike
gam
i
,

K
.

Eb
i
ha
r
a
,

I
nv
esti
g
ati
o
n

o
f

T
ra
n
s
p
are
n
t

Condu
cti
v
e

Ox
i
d
e

A
l
-
Dop
e
d
Zn
O

F
il
m
s

P
r
odu
ce
d

b
y

Pu
lse
d

L
ase
r

D
e
po
siti
on
,

Ja
p
a
n
es
e

J
ou
r
n
a
l

o
f

A
pp
lie
d

Phy
sics
,

Vo
l
.

44
,

No
.

11
,

2005
,

pp
.

8027

8031

[
7
]

Y
.

L
i
u
,

L
.
Zhao
,

J
.

L
i
an
,

A
l
-
dop
e
d

Zn
O

fil
m
s

b
y

pu
lse
d

lase
r

d
e
po
siti
o
n

a
t

r
oo
m

te
mp
erat
u
re
,

V
ac
uu
m
,

Vo
l
,

8
1
,

200
6

pp
.

18

21

[
8
]

D
.

D
j
ouad
i
,

A
.

Ch
el
ou
c
h
e
,

A
.

A
ks
a
s
,

A
m
p
lificati
o
n

o
f

t
h
e

U
V

E
m
issi
o
n

o
f

ZnO
:
A
l

Th
i
n
F
il
m
s

P
re
p
are
d

b
y

So
l
-
G
e
l

M
et
hod
,

Jou
r
n
a
l

o
f

M
aterial
s

a
n
d

Env
ir
onmen
t
a
l

S
cie
n
ce
s

3
,
vo
l

3
,
2012
,

pp
.

585
-
590

[
9
]

H
.

Kong
,

P
.

Yang
,

J
.

Chu
,

P
r
o
cessi
n
g

P
ara
m
eter
s

a
n
d

P
r
op
ert
y

o
f

AZ
O

Th
i
n

F
il
m

P
re
p
ared

b
y

M
a
gn
etr
o
n

Spu
tteri
ng
,

J
ou
r
n
a
l

o
f

Phy
sics
:

Con
fere
n
c
e

S
erie
s

276
,

2011
,

p
p

012170

[
10
]

C
.

W
eidenthale
r

,

P
itfall
s

i
n

t
h
e

c
h
aracterizati
o
n

o
f

n
a
nopo
r
ou
s

a
n
d

n
a
n
osi
ze
d

ma
t
e
ri
a
ls,

N
a
no
scal
e
,

vo
l

3
,

2011
,

p
p

792
-
810

[
11
]
S
.

V
e
n
k
a
t
a
c
ha
l
am
,

Y
.

Kanno
,

S
.

V
el
uman
i
,

Ch
aracterizati
o
n
o
n
pu
lse
d
lase
r

d
e
po
site
d

n
a
no
cr
y
stalli
n
e

Zn
O

t
h
i
n

fil
m
s
,

V
ac
uum
,

vo
l

84
,

2010
,

p
p

1199

1203

[
12
]

N
.

H
ir
aha
r
a
,

B
.

Onwona
-
Ag
ye
man
,

M
.

Na
k
ao
,

P
re
p
arati
o
n

o
f

A
l
-
dop
e
d

Zn
O

t
h
i
n

fil
m
s

a
s
tra
n
s
p
are
n
t

c
ondu
cti
v
e

s
ub
strat
e

i
n

dye
-
se
n
siti
ze
d

s
o
l
a
r

ce
ll
,

Th
i
n

So
li
d

F
il
m
s
,

Vo
l

520
,

Iss
u
e

6
,
2012
,

p
p

2123

2127

[
13
]
Y
.
K
.

T
se
ng
,

G
.J
.

Gao
,

S
.
C
.

Ch
ie
n
,
Syn
t
h
esi
s
o
f
c
-
a
x
i
s
p
referre
d
o
rie
n
tati
o
n

ZnO
:
A
l
tra
n
s
p
are
n
t

c
ondu
cti
v
e

t
h
i
n

fil
m
s

u
si
n
g

a

nov
e
l

s
o
l
v
e
n
t

me
t
hod
,

Th
i
n

So
li
d

F
il
m
s
,

Vo
l

518
,

Iss
u
e
22
,

2010
,

p
p

6259

6263

[
14
]
J.
N
.

D
i
ng
,
C
.
B
.

Tan
,

N
.
Y
.

Yuan
,

X
.
W
.

F
e
ng
,

X
.
Y
.

Chang
,

F
.

Y
e
,
Th
e
P
re
p
arati
o
n

a
n
d
P
r
op
ertie
s
o
f
A
l
-
Dop
e
d
Zn
O
Th
i
n
F
il
m
s
a
s
T
ra
n
s
p
are
n
t
E
lectr
od
e
s
f
o
r
So
la
r
C
ell
,

Phy
sic
s

P
r
o
ce
d
ia
,

Vo
l

3
2
,

2012
,

P
a
g
e
s

789

794

[
15
]

P
.

Gondon
i
,

M
.

Gh
i
d
elli
,

F
.

D
i

Fon
z
o
,

V
.

Ru
ss
o
,

P
.

B
r
uno
,

J
.

M
a
rtí
-
Ru
j
a
s
,

C
.
E
.

Bo
tt
an
i
,

A.

L
i

Ba
ssi
.

C
.
S
.

Ca
s
a
ri
,

S
tr
u
ct
u
ra
l

a
n
d

f
un
cti
on
a
l

p
r
op
ertie
s

o
f

A
l:
Zn
O

t
h
i
n

fil
m
s

g
r
ow
n

b
y

Pu
lse
d
L
ase
r

D
e
po
siti
o
n

a
t

r
oo
m

te
m
p
erat
u
re
,

Th
i
n

So
li
d

F
il
m
s
,

Vo
l

520
,

Iss
u
e

14
,

2012
,

p
p

4707

4711