Contents lists available at SciVerse ScienceDirect
Applied Surface Science
jou rn al h om epa g e: www.elsev ier .com/ locate /apsusc
ffect of sputtering power on the properties of ZnO:Ga transparent conductivexide films deposited by pulsed DC magnetron sputtering with a rotatingylindrical target
Department of Advanced Materials Science and Engineering, Sungkyunkwan University, 300 Cheoncheon-Dong, Suwon, Gyeonggi 440-746, Republic of KoreaDepartment of Materials Science and Engineering, Yonsei University, 134 Shinchon-Dong, Seoul 120-749, Republic of KoreaSchool of Nano and Advanced Materials Engineering, Changwon National University, Changwon National University, 9 Sarim-Dong, Changwon, Gyeongnam 641-773, Republic oforea
r t i c l e i n f o
rticle history:eceived 8 June 2012eceived in revised form 23 January 2013ccepted 23 January 2013vailable online 31 January 2013
a b s t r a c t
Effect of sputtering power on the properties of ZnO:Ga (GZO) transparent conductive oxide (TCO) filmswas investigated on the films deposited by pulsed DC magnetron sputtering with a rotating cylindricaltarget. At lower sputtering power up to 2.0 kW, the films showed flat surfaces with some pit-like struc-tures. However, films morphology deteriorated with higher sputtering power up to 3.5 kW as reflected
eywords:ransparent conductive oxidea-doped ZnOulsed DC magnetron sputtering
in the rough porous surfaces. Microstructures of the films evolved into the uniformly shaped colum-nar grains well aligned with the c-axis with the increasing sputtering power up to 2.0 kW, but furtherincreasing the sputtering power caused the irregularity of the grain shapes and their orientations. Accord-ingly, the film deposited at 2.0 kW showed the lowest electrical resistivity of 4.89 × 10−4 � cm achievedthrough the highest Hall mobility of 25.9 cm2 V−1 s−1. All the GZO films in this study showed the opticaltransmittances higher than 80%.
. Introduction
Implementation of today’s information technology (IT) requireshe use of interactive user interface for their functionalities [1].nteractive user interfaces are in many cases integrated into flatanel displays in which transparent electrodes are used as essentialomponents [2]. Transparent electrodes must have substantiallyow electrical resistivity and be transparent to visible light [3]. Suchroperties can be achieved by degenerately doping wide bandgapoxide) semiconductors [4]. Currently, indium tin oxide (ITO) isidely used as transparent conductive oxide (TCO) due to the
ow electrical resistance in the range of 10−4 � cm and ease ofide-area deposition by simple techniques [5,6]. However, with
ver-expanding realms of ubiquitous computing such as mobileomputing, scarcity of the precious metal indium could raise price
oncern in the IT industry [6]. Therefore, efforts have been made toeek suitable alternatives, in which a wide bandgap ZnO has beenntensively studied [7,8]. ZnO, of which the electrical resistivity can
be made comparable to that of ITO by doping with Al and Ga, isreadily available and chemically stable ensuring lower price andenvironmental safety [9].
So far, Al-doped ZnO has been the focus of research, but Al isreactive in oxidizing atmosphere including air [10]. Instead of Al,chemically less reactive Ga can be used as dopant [11]. Anotheradvantage of Ga is that its ionic radius is smaller than Al so thatit induces smaller lattice distortion on doping as compared to Al,which may allow improved electrical properties [11,12]. For elec-trode applications in IT devices, TCOs must be deposited in formsof thin films using simple techniques [13]. Most of the thin filmsdeposition techniques can be used to deposit ZnO:Ga (GZO) filmson various substrates and magnetron sputtering has been exclu-sively considered due to its simplicity and scalability to large-areadeposition with uniformity [14]. While DC (reactive) magnetronsputtering is regarded as most suitable method, recent studiessuggest that pulsed DC magnetron sputtering (p-DCMS) can bea promising candidate [15,16]. This relatively novel technique isadvantageous in that it allows high sputtering power reaching fewkW ranges with high plasma density, long-term process stability
with arc prevention, and enhanced dynamic deposition rates lead-ing to high-speed deposition of high-quality films [15,17]. Furtherimprovement in the deposition process is possible by incorporat-ing cylindrical target as an anode, by which target efficiencies can
Fig. 1. SEM micrographs showing the changes in the mo
e enhanced over 70% reducing the cost [15,18,19]. By comparison,lanar targets have the efficiency only of 20–30% due to partialputtering [19]. Being a relatively novel process technique, worksn GZO film deposition by p-DCMS with rotating cylindrical targets relatively scarce and therefore more work is needed to estab-ish the optimal condition for film deposition. In this study, effectf sputtering powers on the properties of GZO films was investi-ate to understand relations between the process variables and theesulting film properties.
. Experimental
GZO films were deposited on soda-lime glass substrates by p-CMS at 230 ◦C with varying sputtering powers. Glass substratesere cleaned using trichloroethylene, acetone, and methanol in
rder and then rinsed in deionized water. For the film deposi-ion, glass substrates were first placed in the load-lock chamberhich was subsequently evacuated to 1.0 × 10−6 Torr. At this pres-
ure, the gate valve between the load-lock chamber and the mainputtering chamber was open to transfer the substrate in the sput-ering position which is located 160 mm away from the target. Theorking pressure was maintained at 3.0 × 10−3 Torr for sputter-
ng by feeding Ar (5 N) gas during the film deposition. A cylindricalnO target (250 mm in length and 45 mm in diameter) containing.0 wt% Ga2O3 was used as the source. Pulse frequency was fixedt 70 kHz while the sputtering powers were varied from 1.0 kW to.5 kW. The GZO films were deposited to the thickness of about00–620 nm.
Morphology of the GZO films was observed using a scanning
lectron microscope (SEM, Hitachi S-4200). Crystallinity of thelms was assessed using an X-ray diffractometer (XRD, Rigaku/MAX-2500H) and a transmission electron microscope (TEM, JEOL
SM-2100F). Electrical properties were estimated based on carrier
logy of the GZO films with increasing sputtering power.
concentration, Hall mobility and resistivity obtained from van derPauw Hall measurements at room temperature. Optical transmit-tance and reflectivity of the films were measured using a UV-VISspectrometer (JASCO, V-570).
3. Results and discussion
The SEM micrographs in Fig. 1 show the morphological charac-teristics of the GZO films deposited with varying sputtering powers.All the films are characterized by more or less densely packedand vertically grown columnar grains (details about the columnarstructure will be discussed with TEM later). However, the surfacemorphologies of the films exhibit different details with varyingsputtering powers. Films deposited at 1.0–2.0 kW have almost flatsurfaces on which pit-like structures can be seen. These pit-likestructures decrease in population slightly with increasing sput-tering power up to 2.0 kW. With further increasing the sputteringpower, the pit-like structures evolves into small crater-like struc-tures that make the surfaces appear severely eroded.
Higher sputtering power increases the plasma density whichin turn increases the flux of sputtered species delivered to thesubstrate surface [20]. At the same time, higher sputtering powerprovides more energy to the particles impinging on the substratesurface [20,21]. The former may result in the higher deposition ratewhile the latter allows the sputtered species to diffuse within thegrowing film to find their crystallographic sites more readily [22].When the deposition rate is high, sputtered species impinged onthe surfaces have short time to diffuse within the growing film,which could be compensated by high energy of the species that
helps adjusting their suitable positions in the lattice structure [23].It is believed that these two conflicting contributions to the growthkinetics were balanced during the films growth so that the mor-phological characteristics appeared similar when the sputtering
2 face Sc
pdsstptmt
oa(1wpfpt(1tsZ(iste
(vidfvtwFitt
d
wl[nbs
m1ttpc
TC
18 K.-J. Ahn et al. / Applied Sur
owers were moderate (1.0–2.0 kW in the present case). Slightlyecreasing population of the pit-like structures with increasingputtering power up to 2.0 kW seems to have resulted from highurface mobility associated with higher power [23]. However, whenhe sputtering power is further increased, energy of the impingingarticles can be excessive such that some of the particles bombardhe surfaces of growing film to yield the highly rugged (eroded)
orphology as observed in Fig. 1 for the sputtering power higherhan 2.5 kW [12,23].
Fig. 2 shows the XRD spectra obtained from the GZO films. In allf these spectra, strong diffraction peaks can be seen at 2� = ∼34.3◦
nd secondary peaks at 2� = ∼72.6◦, which are originated from ZnO0 0 0 2) planes and ZnO (0 0 0 4) planes, respectively (JCPDS No. 36-451). These diffraction peaks indicate that the GZO films haveurtzite structures and are c-axis oriented. Intensities of theseeaks initially increase slightly as the sputtering power is increasedrom 1.0 kW to 2.0 kW and then decrease with the higher sputteringowers. Also in Fig. 2, peaks at 2� = ∼43.4◦ appear, which is assignedo spinel ZnGa2O4 (4 0 0). While only the traces of this ZnGa2O44 0 0) peak can be seen in the spectra for the samples deposited at.0–3.0 kW, the peak becomes noticeably intense when the sput-ering power is 3.5 kW. Further, in the XRD spectra from the 3.5 kWample, additional peak appears at 2� = ∼62.5◦, which is assigned tonO (1 0 1 3). The decreasing intensity of the ZnO (0 0 0 2) and ZnO0 0 0 4) peaks accompanied by the appearance of ZnO (1 0 1 3) andntensification of ZnGa2O4 (4 0 0) peaks with high sputtering poweruggests the deterioration of the crystalline quality possibly due tohe excessively energetic sputtered particles that have detrimentalffects on the microstructural evolution of the GZO films [11].
When closely examined, the diffraction angles for the ZnO0 0 0 2) peaks in Fig. 2 are deviated slightly from the standardalue of 34.44◦ [7]. The diffraction angles (2�) for various sputter-ng powers are listed in Table 1 in which it can be seen that the 2�’secrease from 34.29◦ to 34.21◦ with increasing sputtering powerrom 1.0 kW to 3.5 kW. Diffraction angles smaller than the standardalue imply that the lattice spacings along the c-axis are larger thanhe lattice constant of 5.207 A [24]. Also listed in Table 1 are the fullidth at half maximum (FWHM) of the ZnO (0 0 0 2) peaks. These
WHM values remain almost the same when the sputtering powers between 1.0 and 2.0 kW, but increase rapidly with higher sput-ering powers. Crystallite sizes (d’s) are related to these FWHMs viahe Scherrer formula [25]:
= K�
cos �(1)
here is the FWHM, � is the diffraction angle, � is the X-ray wave-ength and K is the shape factor which is normally taken to be 0.925,26]. Consequently, the estimated crystallite sizes in Table 1 areearly the same for the films grown with the sputtering powersetween 1.0 and 2.0 kW while they decrease gradually at higherputtering powers, i.e. 2.5–3.5 kW.
Crystallinity of the GZO films estimated by XRD is comple-ented by the TEM images shown for the sputtering powers of
.0, 2.0 and 3.5 kW, respectively, in Fig. 3. The bright field con-
rast images showing the cross sections of the films indicate thathe microstructure of the GZO films deposited with the sputteringower of 2.0 kW evolved with relatively regular and well-alignedolumnar grains. In the case of the film deposited at 1.0 kW, while
able 1hanges in the position and FWHM of the ZnO (0 0 0 2) peaks with varying sputtering pow
Sputtering power (kW) 1.0 1.5
2� (◦) 34.29 34.26
FWHM (radian) 0.2523 0.2606
Crystallite size (nm) 32.93 31.89
ience 271 (2013) 216– 222
the vertically aligned columnar structures are observable, they lookless ordered as compared to the case of the 2.0 kW sample. Incomparison, the grain structure of the 3.5 kW sample is somewhatirregular in shape while their alignments are loosely vertical.
Concerning the crystalline qualities of these films, the diffractionspots in the selected area electron diffraction (SAED) pattern forthe 2.0 kW sample form regular and ordered array, which suggeststhat the columnar grains are oriented parallel to each other. In thisSAED pattern, some rows of diffraction spots offset small distancefrom the original positions are also noticed. This suggests that somecolumnar grains are slightly rotated about their columnar axes withrespect to other columnar grains. Meanwhile, SAED pattern for the1.0 kW sample appears as the overlap of two diffraction patternswith their own zone axes suggesting that the columnar structureshave been evolved with two main different crystallographic groupshaving c-axis orientations. In any case, this SAED pattern indicatesthat the 1.0 kW sample also grew with some structural ordering. Onthe other hand, the SAED pattern for the 3.5 kW sample exhibits agroup of uniform ordered array of diffraction spots that is superim-posed on somewhat irregular diffraction pattern corresponding topolycrystalline structures which seem to be related to the appear-ance of the (1 0 1 3) and ZnGa2O4 (4 0 0) peaks in the XRD spectra(Fig. 2). TEM observations, together with the XRD results, con-firm that increasing the sputtering power assists improvementsof the crystalline quality of the GZO films at moderate levels [27].On the contrary, too high sputtering power results in the deteri-oration of the crystalline quality due to excessive energy of thesputtered particles impinging on the film surface and increasedflux of the sputtered materials that facilitates random nucleationand fast growth of the grains reducing time for structural ordering[15].
The morphological and crystallographic characteristicsreflected in Figs. 1–3 can be associated with the electricalproperties shown in Fig. 4. In general, electron concentrationincreases slightly with higher sputtering power, varying between4.98 × 1020 and 5.71 × 1020 cm−3. Since the target compositionwas fixed, such moderate increase in carrier concentration withincreasing sputtering power would indicate some improvementin doping efficiency. It was previously reported that increasingsputtering power resulted in higher carrier concentration when thecomposition is fixed [23,28]. This was ascribed to the higher energyof the particles associated with higher sputtering power allowingthem to form intended boding with the neighboring atoms moreeasily [22]. However, unlike the case of existing work, sputteringpower dependence of the carrier concentration (or doping effi-ciency) is marginal in this study. Rather, the electrical propertywas mainly governed by the Hall mobility. In Fig. 4, the mobilityinitially increases from 22.9 cm2 V−1 s−1 to 25.9 cm2 V−1 s−1 andthen decreases slightly to 25.1 cm2 V−1 s−1as the sputtering powerincreases from 1.0 kW to 2.5 kW via 2.0 kW, but decrease substan-tially to 14.3 cm2 V−1 s−1 with further increasing the sputteringpower above 3.0 kW. This is ascribed to the deterioration ofthe crystalline quality reflected in the decreasing intensity andincreasing FWHM of the ZnO (0 0 0 2) peaks (decreasing crystallite
size), larger lattice distortion or increasing structural disorder, andappearance of the ZnO (1 0 1 3) and ZnGa2O4 (4 0 0) peaks withhigher sputtering power [29]. In addition, damaged surface shownin Fig. 1 suggests that bombardments by higher energy particles
ers and the crystallite sizes estimated from the FWHMs.
Fig. 2. XRD spectra showing the changes in crystal
ay have caused structural damages which also have disadvan-ageous effect on the mobility. As a result, electrical resistivity is
elatively lower at lower sputtering power regime where it reacheshe lowest value of 4.89 × 10−4 � cm at the sputtering power of.0 kW. On the other hand, it increases to 7.70 × 10−4 � cm ashe sputtering power increases to 3.0 kW. In any case, electrical
of the GZO films with increasing sputtering power.
resistivities of the GZO films considered in this study all fulfill therequirements for the TCO materials for electrode applications in
terms of the electrical properties.
Finally, optical properties of the GZO films are evaluated basedon their optical transmittances and absorption characteristicsshown in Fig. 5. In the transmittance spectra shown in Fig. 5(a),
ig. 3. TEM contrast images (left side column) and accompanying SAED patternsputtering powers.
t can be seen that all the GZO films are transparent to visible light� = 380–750 nm) and that their transmittances at � = 550 nm areigher than 80% satisfying the requirements for transparent elec-rodes. Optical bandgaps of these films estimated from the Tauc
side column) showing the evolution of crystalline qualities of film with varying
plot in Fig. 5(b) are more or less insensitive to the sputtering power,varying between 3.72 and 3.73 eV. Since the carrier concentrationsof the films did not show noticeable dependence on the sputter-ing power, it is expected that the Fermi levels of the films would
K.-J. Ahn et al. / Applied Surface Sc
Fig. 4. Changes in the electrical properties of the GZO films in terms of the electricalresistivity, carrier concentration, and Hall mobility with varying sputtering powers.
Fa
br
4
awssio
[
[
[
[
[
[
[
[
ig. 5. (a) Transmittances of the GZO films deposited with various sputtering powersnd (b) Tauc plots showing the almost identical optical bandgaps of these films.
e almost identical and therefore the optical bandgap would alsoemain constant [30,31].
. Conclusions
GZO films were deposited by p-DCMS with rotating targetpplying various sputtering powers. Morphology of the GZO filmsas initially characterized by flat surface and columnar grains
howing good crystallographic alignments along the c-axis as theputtering power increased from 1.0 kW to 2.0 kW. Accompany-ng such microstructural evolution, lowest electrical resistancef 4.89 × 10−4 � cm was reached mainly due to the increased
[
ience 271 (2013) 216– 222 221
Hall mobility of 25.9 cm2 V−1 s−1, while the carrier concentra-tion remain almost the same regardless of the sputtering power.When the sputtering power was increased, deteriorations in thefilm morphology and the crystallinity were resulted which causeddegradation of electrical properties represented by increasingelectrical resistivity and decreasing Hall mobility. The electricalresistivity, and also the carrier concentration and Hall mobility,remained almost similar over wide range of sputtering powerfrom 1.0 kW to 2.5 kW while the optical properties fulfilled therequirements for TCO films regardless of the sputtering power. Thisdemonstrates the technical advantage of p-DCMS with a rotatingcylindrical target in terms of simplicity in process control in addi-tion to the possible scalability of this technique.
Acknowledgements
This work was supported by the Technology Innovation Program(Industrial Strategic technology development program, 10040741,Development of the Low Damage Sputtering Source for TCO Layers)funded by the Ministry of Knowledge Economy (MKE, Korea).
References
[1] S. Narushima, M. Orita, M. Hirano, H. Hosono, Electronic structure and transportproperties in the transparent amorphous oxide semiconductor 2 CdO GeO2,Physical Review B 66 (2002) 352031–352038.
[2] K. Nomura, H. Ohta, K. Ueda, T. Kamiya, M. Hirano, H. Hosono, Thin-film tran-sistor fabricated in single-crystalline transparent oxide semiconductor, Science300 (2003) 1269–1272.
[3] X. Nie, S.-H. Wei, S. Zhang, Bipolar doping and band-gap anomalies indelafossite transparent conductive oxides, Physical Review Letters 88 (2002)0664051–0664054.
[4] H. Hosono, H. Ohta, M. Orita, K. Ueda, M. Hirano, Frontier of transparent con-ductive oxide thin films, Vacuum 66 (2002) 419–425.
[5] K. Suzuki, N. Hashimoto, T. Oyama, J. Shimizu, Y. Akao, H. Kojima, Large scaleand low resistance ITO films formed at high deposition rates, Thin Solid Films226 (1993) 104–109.
[6] T. Minami, Present status of transparent conducting oxide thin-film devel-opment for indium-tin-oxide (ITO) substitutes, Thin Solid Films 516 (2008)5822–5828.
[7] A. Mondal, N. Mukherjee, S.K. Bhar, Galvanic deposition of hexagonal ZnO thinfilms on TCO glass substrate, Materials Letters 60 (2006) 1748–1752.
[8] M. Purica, E. Budianu, E. Rusu, M. Danila, R. Gavrila, Optical and structural inves-tigation of ZnO thin films prepared by chemical vapor deposition (CVD), ThinSolid Films 403 (2002) 485–488.
[9] J. Sans, J. Sánchez-Royo, A. Segura, G. Tobias, E. Canadell, Chemical effectson the optical band-gap of heavily doped ZnO:MIII (M = Al,Ga,In): an inves-tigation by means of photoelectron spectroscopy, optical measurementsunder pressure, and band structure calculations, Physical Review B 79 (2009)1951051–1951059.
11] J.-H. Park, B.-K. Shin, H.-M. Moon, M.-J. Lee, K.-I. Park, K.-J. Ahn, W. Lee, J.-M.Myoung, Effect of the substrate temperature on the properties of Ga-doped ZnOfilms for photovoltaic cell applications deposited by a pulsed DC magnetronsputtering with a rotating cylindrical target, Vacuum 86 (2012) 1423–1427.
12] X. Yu, J. Ma, F. Ji, Y. Wang, X. Zhang, C. Cheng, H. Ma, Effects of sputtering poweron the properties of ZnO:Ga films deposited by r.f. magnetron-sputtering atlow temperature, Journal of Crystal Growth 274 (2005) 474–479.
13] K. Chopra, S. Major, D. Pandya, Transparent conductors – a status review, ThinSolid Films 102 (1983) 1–46.
14] K. Ellmer, Magnetron sputtering of transparent conductive zinc oxide: relationbetween the sputtering parameters and the electronic properties, Journal ofPhysics D: Applied Physics 33 (2000) R17–R32.
15] B.-K. Shin, T.-I. Lee, J. Prakash Kar, M.-J. Lee, K.-I. Park, K.-J. Ahn, K.-Y. Yeom,J.-H. Cho, J.-M. Myoung, Effect of deposition power on structural and electri-cal properties of Al-doped ZnO films using pulsed direct-current magnetronsputtering with single cylindrical target, Materials Science in SemiconductorProcessing 14 (2011) 23–27.
16] J. Musil, P. Baroch, J. Vlcek, K.H. Nam, J.G. Han, Reactive magnetron sputteringof thin films: present status and trends, Thin Solid Films 475 (2005) 208–218.
17] I. Brodie, L. Lamont, R. Jepsen, Production of high-energy neutral atoms by scat-
tering of ions at solid surfaces and its relation to sputtering, Physical ReviewLetters 21 (1968) 1224–1226.
18] X.Y. Li, D. Depla, W.P. Leroy, J. Haemers, R. De Gryse, Influence of deposition onthe reactive sputter behaviour during rotating cylindrical magnetron sputter-ing, Journal of Physics D: Applied Physics 41 (2008) 0352031–0352036.
2 face Sc
[
[
[
[
[
[
[
[
[
[
[
[
22 K.-J. Ahn et al. / Applied Sur
19] J. Musschoot, D. Depla, G. Buyle, J. Haemers, R.D. Gryse, Influence of the geomet-rical configuration on the plasma ionization distribution and erosion profile ofa rotating cylindrical magnetron: a Monte Carlo simulation, Journal of PhysicsD: Applied Physics 39 (2006) 3989–3993.
20] D.G. Lim, D.H. Kim, J.K. Kim, O. Kwon, K.J. Yang, K.I. Park, B.S. Kim, S.W. Lee, M.W.Park, D.J. Kwak, Improved electrical properties of ZnO:Al transparent conduct-ing oxide films using a substrate bias, Superlattices and Microstructures 39(2006) 107–114.
21] R. Mientus, K. Ellmer, Reactive magnetron sputtering of tin-doped indium oxide(ITO): influence of argon pressure and plasma excitation mode, Surface andCoatings Technology 142 (2001) 748–754.
22] S. Kim, J. Seo, H.W. Jang, J. Bang, W. Lee, T. Lee, J.M. Myoung, Effects of H2 ambientannealing in fully 0 0 2-textured ZnO:Ga thin films grown on glass substratesusing RF magnetron co-sputter deposition, Applied Surface Science 255 (2009)4616–4622.
23] Q.-B. Ma, Z.-Z. Ye, H.-P. He, J.-R. Wang, L.-P. Zhu, B.-H. Zhao, Preparation and
characterization of transparent conductive ZnO:Ga films by DC reactive mag-netron sputtering, Materials Characterization 59 (2008) 124–128.
24] X. Sun, H. Kwok, Optical properties of epitaxially grown zinc oxide films onsapphire by pulsed laser deposition, Journal of Applied Physics 86 (1999)408–411.
[
ience 271 (2013) 216– 222
25] A. Patterson, The Scherrer formula for X-ray particle size determination, Phys-ical Review 56 (1939) 978–982.
26] R. Viswanatha, S. Sapra, S.S. Gupta, B. Satpati, P. Satyam, B. Dev, D. Sarma, Syn-thesis and characterization of Mn-doped ZnO nanocrystals, Journal of PhysicalChemistry B 108 (2004) 6303–6310.
27] H.Y. Park, S.R. Lee, Y.J. Lee, B.W. Cho, W.I. Cho, Bias sputtering and charac-terization of LiCoO2 thin film cathodes for thin film microbattery, MaterialsChemistry and Physics 93 (2005) 70–78.
28] S. Lee, D. Cheon, W.-J. Kim, M.-H. Ham, W. Lee, Ga-doped ZnO films depositedwith varying sputtering powers and substrate temperatures by pulsed DC mag-netron sputtering and their property improvement potentials, Applied SurfaceScience 258 (2012) 6537–6544.
29] S.-S. Lin, J.-L. Huang, P. Sajgalik, The properties of Ti-doped ZnO films depositedby simultaneous RF and DC magnetron sputtering, Surface and Coatings Tech-nology 191 (2005) 286–292.
30] J.-P. Lin, S.-W. Chen, J.-M. Wu, Influence of nanocrystalline grains on electronic
conduction in sol–gel derived Al-doped ZnO films, Electrochemical and SolidState Letters 12 (2009) J1–J4.
31] S. Das, S. Bhattacharyya, R. Gayen, A. Pal, Optical properties of Si-doped GaNnanocrystals in SiO2/GaN/SiO2 thin film structure, Journal of Physics D: AppliedPhysics 42 (2009) 1354021–1354029.
