Journal of Crystal Growth
210 (2000) 516-520
Effect of gallium incorporation on the
physical properties of ZnO films grown by
spray pyrolysis
K.T. Ramakrishna Reddya, H. Gopalaswamya, P.J. Reddya, R.W. Milesb,*
aDepartment
of Physics, Sri Venkateswara University, Tirupati 517 502, India
bNewcastle Photovoltaics Application Centre, School of Engineering, University of
Northumbria, Ellison Place, Newcastle upon Tyne NE1 8ST, UK
指導教授：林克默 博士
報告學生：郭俊廷
報告日期：99/11/29
1
Outline




Introduction
Experimental procedure
Results and discussion
Conclusions
2
Introduction



Although gallium doped zinc oxide films grown by sputtering have
shown very low resistivities of 10-3~10-4 Ωcm, the technique is
neither economically viable nor suitable for coating large area
substrates.
In this study, we have chosen spray pyrolysis for the deposition of
ZnO:Ga films in view of its simplicity and economics.
In the present paper, we report on the influence of gallium doping
concentration on the physical properties of ZnO : Ga films prepared
by spray pyrolysis.
3
Experimental procedure





The spraying liquid consisted of zinc acetate dissolved in isopropyl
alcohol and deionised water with a solution concentration of 0.1 M.
The solution was sprayed at the rate of 6 ml min-1 onto Corning
7059 glass substrates kept at a substrate temperature, (TS), of 350 ℃.
Purified compressed air with a flow rate of 8 l min-1 was used as the
carrier gas.
The nozzle to substrate distance was about 25 cm.
For doping the films with gallium, gallium trichloride solution was
added to the starting solution and the doping concentration was
varied from 0.0 to 8.0 at.%.
4




The structural studies were carried out using X-ray diffraction and
scanning electron microscopy.
The grain size of the films was calculated using the Scherrer
formula [21] and the extent of preferred orientation of the films was
evaluated using the texture coefficient of the films[22].
The electrical properties of the films at room temperature were
measured using the van der Pauw method.
The optical transmission of the films was recorded using a Hichachi,
U3400 UV-VIS-NIR, double beam spectrophotometer in the
wavelength range 300~2500 nm.
5
Results and discussion


It can be seen from the XRD spectra
and the scanning electron
micrographs that all the deposited
films are polycrystalline and from the
XRD spectra that the films have the
wurtzite crystal structure with a (002)
preferred orientation.
The other weak orientations observed
were (100), (101), (102) and (110).
Fig. 1. XRD spectra of sprayed
ZnO:Ga thin films formed at
Ts=350 ℃.
6
Fig. 2. Scanning electron micrographs of ZnO:Ga films grown at Ts=350℃.
7


The decrease of texture
coefficient at higher doping
levels has also been observed by
Goyal et al.[24] in indium-doped
zinc oxide films grown by spray
pyrolysis.
They have attributed this
behaviour to the interstitial
inclusion of dopant atoms
resulting in the poor structure of
the films and therefore the
texture coefficient decreases.
Fig. 3. Variation of texture coefficient
and grain size with gallium content in
ZnO films.
8


It varied from 3.6×10-1 to 1.0×10-3
Ωcm and the films formed with
5.0 at% of gallium had a
resistivity of 1.5×10-3 Ωcm.
The decrease in the resistivity of
the films by gallium doping can
be explained by considering the
atomic radius and
electronegativities of zinc and
gallium.
Fig. 4. Change of electrical resistivity
and transmittance with gallium
content in ZnO films.
9



In addition, the electronegativity of gallium (1.13) is close to that of
zinc (0.99) when compared to other dopants so that the localization
effect of conduction electrons by gallium becomes very small.
The mobility of the charge carriers followed the same behaviour of
the texture coefficient with a maximum value of 17 cm2 V-1 s-1 at a
gallium concentration of 5.0 at%.
As the doping level is increased, more dopant atoms occupy the
zinc lattice sites which results in more charge carriers. However,
after a certain level of doping, the Ga atoms can not occupy zinc
lattice sites and they have a tendency to occupy interstitial sites
where they form neutral defects and become ineffective as dopant
impurities.
10
Conclusions




隨著不同濃度Ga的摻雜，GZO薄膜在物理特性上有明顯的改變。
透過XRD的結構分析得知，GZO薄膜為多晶纖鋅礦結構，在
(002)方面有優選取向。
在摻雜5 at.%的GZO薄膜，有最低的電阻率1.5×10-3 Ωcm，其穿
透率也大於85%，已達可應用在太陽能電池之透明電極上。
在這研究中，摻雜鎵得到了高載子密度濃度，在電阻率方面也
比摻雜鋁和銦來得低。
11
Thanks for your attention
12