Transcript 下載/瀏覽

Highly stable transparent and
conducting gallium-doped
zinc oxide thin films for
photovoltaic applications
E. Fortunato
, L.Raniero,L.Silva,A.Gonc-alves,A.Pimentel,P.Barquinha,H.A´guas, L.Pereira, G. Goncalves,I.Ferreira,E.Elangovan,R.Martins
CENIMAT/I3N,MaterialsScienceDepartment,FCT-UNLandCEMOP-UNINOVA,CampusdeCaparica,2829-516Caparica,Portugal
教授:林克默博士
學生:董祐成
日期:99/09/13
Outline
1. Introduction
2. Experimental
3. Results and discussion
4. Conclusions
Introduction
 Transparent conducting oxide (TCO)
with an optical
transmission exceeding 80% in the visible region (500–650nm)
and a resistivity less than 10-3 Ωcm have been widely used
inavariety of applications for more than a half-century.
 However , most of these techniques require moderate substrate
temperatures to obtain low resistivity . Among the available
techniques, RF magnetron sputtering presents several
advantages including the production of highly transparent and
conducting gallium zinc oxide (GZO) without heating the
substrate.
Experimental(1)
 The GZO films were deposited initially on soda lime glass
substrates by RF (13.56MHz) magnetron sputtering using a
5cm planar ceramic oxide target consisting ZnO
(98wt%):Ga2O3 (2 wt%) from Super Conductor Materials, Inc.
with a purity of 99.99%.
 The sputtering was carried out at RT with an argon deposition
pressure of 0.15Pa. The substrate–target distance (10cm) and
the RF power (175W) were maintained constant for all
depositions. The typical growth rate for these deposition
conditions is 30 nm/min.
Experimental(2)
 The deposition conditions optimized from the GZO films
deposited on glass substrates were later used to deposit the
GZO films on polyethylene naphthalate (PEN) substrates. The
PEN substrates possess high stiffness, low thermal shrinkage,
and high chemical resistance.
 The film thickness was measured using a surface profilometer
(Dektak 3D from Sloan Tech.). The surface morphology was
analyzed using a field effect scanning electron microscope
(FE-SEM,S-1400 Hitachi) . The electrical resistivity (ρ), free
carrierconcentration (N) and Hall mobility (μ) were inferred
by the four point probe method and Hall effect measurements
in van der Pauw geometry (Biorad HL5500) at a constant
magnetic field of 0.5T.
Experimental(3)

X-ray diffraction measurements were performed using Cu-Ka
radiation (Rigaku DMAX III-Cdiffractometer) in Bragg–
Brentanogeometry(θ/2θcoupled).
 The optical transmittance measurements were per-formed
with a Shimadzu UV/VIS 3100 PC double beam spectrophotometer in the wavelength range from 300 to 2500 nm.
Results and discussion
Dependence of the electrical resistivity (ρ), carrierconcentration (N)
and Hall mobility (μ) as a function of film thickness of the GZO
films deposited at room temperature (inthisstudy).
 A maximum bulk resistivity (ρ) of ~ 5.7×10-4Ωcm obtained for
the 100 nm thick films was decreased with the increasing film
thickness to reach a minimum of ~ 2.8×10-4Ωcm at 1100nm.
 We have observed a continuous increase on the mobility (μ) as
the thickness increases (from 11 to 18 cm2/Vs) .This behavior
can be attributed to a reduction in the ionized impurity
scattering and or an increase in the crystallite size.
 However , we notice that ρ tends to saturate for thicknesses
above 500 nm . Similar results have been obtained for Aldoped ZnO.
XRD patterns obtained from the GZO films deposited with different
thicknesses (inset shows the pattern from the thickest sample).
 For all the films , only the ZnO (002) peak at 2θ = 34.31° is
observed revealing that the films are polycrystalline with a
hexagonal structure and a preferred orientation along the caxis perpendicular to the substrate.
 As the thickness increases the peak intensity corresponding to
the plane (002) increases significantly , whereas the peak
width decreases.
Dependence of Hall mobility of the GZO films with the crystallite size (data
obtained through Scherrer’s formula).
 A linear dependence between the mobility and the crystallite
size was obtained through the equation , μ =k+0.5983dc, where
k is a constant and dc is crystallitesize.
 This is consistent with the previous results since the mobility
is mainly dependent on the grain boundary scattering and
lattice defects, which decrease with the increase of the
crystallite size.
 This also suggests that the only way to decrease the resistivity
is by increasing the mobility .
Surface SEM micrographs (with 40° tilt angle) of GZO films deposited at room
temperature, with different thicknesses
 Shows two typical SEM micrographs of GZO films with a low
(110nm) and high (1110nm) film thicknesses respectively,with
an apparent viewing angle of 40 °
 The surface roughness is increased with the increasing film
thickness, suggesting an enhancement of the grain size as
already confirmed by the electrical and X-ray diffraction
measurements. .
Optical transmittance obtained from the GZO films as a function of
film thickness.
 shows the optical transmittance vs. wavelength in the visible
and near-infrared region obtained from the films with thicknesses ranging from 110 to 1100 nm.
 The near-infrared transmittance decreases as the film thickness
increases, whereas the average transmittance at the visible
range is obtained as 80% and 90% from the low and high
thickness films, respectively. These changes in the optical
properties are consistent with the changes observed in the
electrical, structural and morphological properties.
Conclusions
 The set of data achieved shows that highly conducting and
transparent GZO films can be deposited by RF magnetron
sputtering at room temperature.
 The data show that the crystalline structure , surface
morphology and the electro-optical properties are dependent
on the film thickness.Overall,the produced films present a
resistivity close to 2.8×10-4Ωcm, Hall mobility 18 cm2/Vs and
transmittance ＞80%.
 Further work is under way to increase the Hall
mobility without heating the substrate , either during
deposition or after deposition , to be compatible with
the emergent plastic electronic industry.
THANKS FOR YOUR ATTENTION