effects of the pre-annealing temperature on structural and optical properties of sol–gel deposited...

CERAMICSINTERNATIONAL

Available online at www.sciencedirect.com

http://dx.doi.org/0272-8842/& 20

Please cite thizinc oxide, Ce

Ceramics International ] (]]]]) ]]]–]]]www.elsevier.com/locate/ceramint

Effects of the pre-annealing temperature on structural and optical propertiesof sol–gel deposited aluminium doped zinc oxide

Ian Y.Y. Bu

Department of Microelectronics Engineering, National Kaohsiung Marine University, 81157 Nanzih District, Kaohsiung City, Taiwan, Republic of China

Received 25 January 2014; received in revised form 5 April 2014; accepted 5 April 2014

Abstract

Highly transparent aluminium doped zinc oxide (AZO) thin films were deposited by the sol–gel deposition technique. The effects of variouspre-sintering temperatures on the film's structural, optical and electrical properties were investigated using scanning electron microscopy, X-raydiffraction spectroscopy, photoluminescence emission measurements, UV–vis spectroscopy and electrical characterization. It was found that thepre-sintering temperature greatly affected the film's structural properties and resulted in an almost 100% increase in grain size within theinvestigated experimental window. Photoluminescence emission and photovoltaic measurements confirmed that the best optoelectronic propertiesof AZO thin films can be achieved with a pre-sintering temperature of 400 1C.& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: A. Sintering; Annealing; Aluminium doped zinc oxide; Optical; Sol–gel

1. Introduction

Zinc oxide (ZnO) is a semiconductor material, with ahexagonal wurtzite structure, that possesses a direct wide bandgap (Eg¼3.37 eV) and large exciton binding energy (60 meV)[1–4]. Nanostructured ZnO has been proposed as a potentialmaterial for use in various optoelectronic device applications,such as solar cells[5], light emitting diodes [6], waveguides[7,8] and lasers [9]. In addition, ZnO is piezoelectric andmoisture sensitive, and its use has also been explored innanogenerators [10] and humidity sensors [11]. In addition toits unique properties, ZnO possesses a distinct processingadvantage over other nanomaterials, as it can adapt manydifferent geometries (nanorods [2], nanowires [12,13], nano-flowers [14], and so on).

As-deposited ZnO tends to exhibit an n-type conductionbehaviour due to its structural defects [15]. This pronouncedn-type behaviour can be further enhanced through the additionof dopants; such as Al [16], In [17], and B [18]. Owing to itslow production-cost, non-toxicity and high optical transpar-ency, Al doped ZnO (AZO) is one of the materials touted toreplace the indium tin oxide (ITO) commonly used in solar

10.1016/j.ceramint.2014.04.03014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

s article as: I.Y.Y. Bu, Effects of the pre-annealing temperatureramics International (2014), http://dx.doi.org/10.1016/j.ceramint.20

cells [19,20]. Although it is possible to deposit high qualityAZO through thermal oxidation [21], such a method suffersfrom the drawbacks of low material yield and expensivevacuum set-up. On the other hand, the sol–gel based deposi-tion method is of interest, as it offers precise compositioncontrol, simple set-up and high material utilization [15,22,23].A typical sol–gel preparation procedure for AZO utilizes zinccontaining compounds in solvents, with Al as the dopant [24].During the sol–gel synthesis, the AZO films undergo twoheating processes at different temperatures. First, the films arepre-annealed at around 250–300 1C to remove the solvents[25]. This process is followed by the second thermal annealingprocedure at 500–600 1C to crystallize the film [15]. Indeed, aswill be shown in this paper, pre-annealing is important notonly in removing organic compounds, but also in governingthe subsequent orientation and grain size of the crystallites.The temperature of the pre-annealing step is generally set atabove the boiling point of the solvent and chelating agents inorder to ensure complete removal of the organic compounds[26,27]. Previous studies have shown that properties of thesol–gel deposited ZnO films are highly influenced by solconcentration [28], choice of stabilizers [27,29], pH [23], aging

on structural and optical properties of sol–gel deposited aluminium doped14.04.030

www.sciencedirect.com/science/journal/02728842

http://dx.doi.org/10.1016/j.ceramint.2014.04.030

www.elsevier.com/locate/ceramint







I.Y.Y. Bu / Ceramics International ] (]]]]) ]]]–]]]2

period [30] and post annealing temperature [31]. However,there have been a limited number of studies on the effects ofpre-annealing temperature on the subsequent properties ofAZO thin films.

In this study, AZO thin films were deposited on Corningglass substrates by a sol–gel technique. Samples were preparedby pre-heating at different temperatures (300–550 1C). Theeffects of the pre-annealing temperature on the structural andoptical properties of the AZO films are investigated byscanning electron microscopy X-ray-diffraction, photolumines-cence emission and electrical measurements.

2. Experimental

AZO thin films were prepared by sol–gel process. Al dopingwas achieved by adding 2 at% of Al into the ZnO precursorsolution using AlCl3 (hexahydrate). The Al dopant concentra-tion was selected from the previously reported optimizedconditions [32,33]. The films were coated on pre-cleanedCorning glass substrates using a spin coater set at 3000 rpm.The films were then pre-annealed at 300, 400, 500 and 550 1Cand post annealed at 550 1C. This coating/drying/post anneal-ing cycle was repeated five times to obtain the desiredthickness (around 250 nm).

The surface morphology and structural properties of the sol–gelsynthesized AZO thin films were investigated by using an FEI

Fig. 1. SEM image of sol–gel derived AZO with pre-sintering temperatures

Please cite this article as: I.Y.Y. Bu, Effects of the pre-annealing temperaturezinc oxide, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.20

Quanta 400 F environmental scanning electron microscope (SEM).The chemical composition of the deposited thin films wasdetermined by an energy dispersive spectroscope (EDS) withinthe SEM chamber. A Siemens D5000 X-ray Diffractometer withCu Kα radiation was used to determine the crystalline orientationand grain size of the thin films. Hall effect measurements in theVan der Pauw configuration were used to determine the electricalconductivity type, resistivity and mobility. Optical transmittancewas measured by an UV–vis NIR (Hitachi U-4100). The quality ofthe deposited AZO thin films was evaluated by the photolumines-cence (PL) emission using a 325 nm He/Cd laser (Jasco ModelFP-6000). Dark and illuminated current–voltage (i–v) character-istics were measured using a Keithley 2400 source-measure unit,with illumination (100 mW/cm2) provided by a solar simulator(Science-tech.).

3. Results and discussions

Fig. 1(a)–(d) shows the SEM images of AZO thin films, pre-sintered at various temperatures and post annealed at 550 1C.It can be seen that the surface morphology of the films isstrongly influenced by the pre-sintering temperature. The sur-face morphology of films deposited at pre-sintering temperatureof 300 1C consists of small crystallites with an average diameterof around 50 nm. As the pre-sintering temperature increases, thegrain size becomes larger and the diameter rises up to around

of (a) 300 1C, (b) 400 1C, (C) 500 1C and (d) 550 1C (scale bar 1 μm).





Fig. 3. (a) PL emission of the AZO thin films deposited at different pre-sintering temperatures and (b) the extracted I380/I436 as a function of variouspre-sintering temperature. (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of this article.)

I.Y.Y. Bu / Ceramics International ] (]]]]) ]]]–]]] 3

100 nm. A close examination reveals that cracks and voids havealso been formed. The cracks probably originate from thecombination of larger crystallite nucleation and differences inthermal expansion between AZO and the substrate. Previousstudies have shown that the post annealing temperature andheating rate have significant effects on the structural propertiesof sol–gel derived AZO [34,35]. The grain size generallyincreases with a higher post annealing temperature, due to themerging process that occurs during thermal treatment. However,an increase in the heating rate usually results in a decrease ingrain size. Our data supplements these studies and suggests thatthe pre-sintering temperature can strongly affect the grain size(�100%), with a greater impact than either the heating rate(�70%) [35]and post-sintering temperature (�50%) [34].

Fig. 2 shows the XRD patterns of AZO thin films pre-sintered at temperatures between 300 and 550 1C and post-sintered at 550 1C. All of the films exhibit a dominant peak at(002), orientation accompanied by a small peak at (100),without any secondary phases, such as Al2O3. It is observedthat the intensity of the (002) diffraction peak rises as the pre-sintering temperature is increased. This tendency correlateswith the SEM results presented in Fig. 1. It is recognized thatfilms tend to grow along the plane with the lowest surfaceenergy [36]. Fujimura et al. [37] indicated that the surfaceenergy density of the (002) orientation is the lowest in ZnOcrystals. The number of grains with the lowest surface energywill increase as the film grows, thus resulting in a preferentialgrowth orientation. The preferential c-axis orientation in AZOfilms is further supported based on the model proposed by Vander Drift [38]. According to this model, crystallite growth is acompetitive process that begins with nucleation of variousorientations, but eventually only those crystallites with thefastest growth rates will remain on the substrate. It can also beobserved from Fig. 2 that as the pre-sintering temperatureincreases, all the peaks shift toward higher angles due tooxygen out-diffusion during the pre-sintering stage [39].

Photoluminescence (PL) measurements are a powerful toolthat is often employed to determine the structural defectswithin deposited films. The PL spectra of the films sintered

Fig. 2. XRD patterns of sol–gel derived AZO with pre-sintering temperature.


between 300 and 550 1C are presented in Fig. 3(a). Regardlessof the deposition condition, all the PL emission spectra possesstwo distinct peaks: a narrow peak centred at 380 nm (UV) anda broad peak centred at 436 nm (blue/purple). The UVemission has been attributed to the presence of ZnO nanopar-ticles [8], in good agreement with the SEM images. On theother hand, the defect-related green band is believed to berelated to zinc/oxygen vacancies and zinc/oxygen interstitials[8]. The defective PL emission peak from AZO thin film ismuch higher than those from intrinsic ZnO, due to theintroduction of dopant (alumina) [15].The quality of AZO thin films has been quantitatively evaluated

by the integrated values of the PL emission spectra at 380 nm (I380)and 436 nm (I436), respectively, and plotted in Fig. 3(b). It isevident that the AZO thin films synthesised at pre-sinteringtemperature of 400 1C possess the highest I380/I436 ratio, whichis a useful indication of quality. In another study, differentialscanning colorimetry measurements of sol–gel derived AZO thinfilms have confirmed that solvents and organics are completelyevaporated at around 317 1C [40]. The same study also concludedthat ZnO crystallization occurs at temperatures greater than 490 1C.





Fig. 4. (a) UV–vis spectroscopy transmittance measurements of sol–gelderived AZO with different pre-sintering temperatures and (b) the re-plottedgraph used to extract optical band gap.

Fig. 5. Carrier concentration, Hall mobility and sheet resistance of sol–gelderived AZO as a function of pre-sintering temperature.


The poor quality of AZO crystals produced with a pre-sinteringtemperature of 300 1C is thus likely to be due to the incompleteremoval of organics. However, the experimental results of thisstudy also reveal that films deposited at a pre-sintering temperatureat 500 1C are of inferior quality. It is believed that films pre-sintered at around 500 1C have undergone a rapid crystallizationprocess that may have incorporated unwanted organic residue. Thisexplanation is supported by the appearance of dark (carbon) spotsunder visual inspection. Other scholars [34,41] have reported thatthe highest UV band edge emissions occur in AZO films postsintered at 450 1C, due to the slow formation of defects and thepresence of oxygen related defects. Furthermore, more of thedefects responsible for the non-radiative transition will be intro-duced into AZO thin films sintered at temperatures greater than450 1C [41]. Although one of these earlier studies [41] refers toAZO thin films sintered at 450 1C (without pre-sintering tempera-ture), its results can still be used to aid our understanding of therole of pre-sintering temperature. In fact, this previous studyshowed a similar reversal in trend to that shown in Fig. 3(b), withthe lowest UV intensity occurring at 550 1C [41], as compared tothe 500 1C observed in the present study.

The effects of pre-sintering temperatures on the optical trans-mittance spectra of AZO thin films in the visible range arepresented in Fig. 4. The optical transmittance increases significantlywith a higher pre-sintering temperature. All of the AZO thin filmshave an average transmittance greater than 89% at a wavelengtharound 550 nm. The weak fluctuation of the plotted data is due tointerference between the thin layers. In general, the optical bandgap can be extracted from Fig. 4(a) using the following equation:

ðαhvÞ ¼ Aðhv�EgÞ1=2

where α is the absorption coefficient; hv is the photon energy; A isa constant; and Eg is the band gap. The optical band gap of AZOthin films has been determined by extrapolating the slope, and theresults are summarized in Fig. 4(b). The extracted band gap is inthe range between 3.22 and 3.24 eV, which correlates well with thevalues obtained from other studies [41–43]. The optical band gapof the AZO films exhibits a slightly higher value than those ofundoped ZnO film (3.2 eV) [43]. In general, films that containmore dopants possess larger band gaps. Since the dopantconcentration is fixed during this study, no significant changes inthe band gap can be observed. According to previous study,the refractive index of the deposited sample should be around1.505 [7].

Apart from the optical properties, the electrical properties arealso an important parameter of the deposited AZO thin films.Fig. 5 shows the effects of pre-annealing temperature on the carrierconcentration, mobility and sheet resistance of the AZO thin films.The carrier concentration is related to the number of electroncreated by the ionization of interstitial zinc atoms and oxygenvacancies [44]. It can be observed that as the pre-sinteringtemperature rises the carrier concentration increases, with acorresponding decrease in sheet resistance. Such trends in carrierconcentration and sheet resistance can be attributed to enhancedcrystallization, as seen in the increased XRD pattern at the (002)orientation. The mobility reaches a maximum in the film depositedat a pre-annealing temperature of around 400 1C, and decreases


with further increases. This reversal in trend is believed to originatefrom the increased grain boundaries due to crack formation (asobserved in the SEM images in Fig. 1).The electrical characteristics of the deposited AZO thin films

were evaluated by integrating the AZO films onto p-type siliconin order to form heterojunction devices. The dark and solar light





I.Y.Y. Bu / Ceramics International ] (]]]]) ]]]–]]] 5

illuminated current–voltage characteristics are measured and pre-sented in Fig. 6. The rectifying behaviour indicates that apn-junction is formed in our p-type silicon/AZO heterojunctionstructure. Under forward bias, it is found that there is a sharpincrease in the turn on voltage at around 0.26 V for the AZO thinfilms pre-sintered at 300 1C. The AZO/Si devices, pre-sintered at atemperature of 550 1C, exhibit an increased turn on voltage of0.3 V. Fig. 5 also shows that the forward current (at 1 V) decreasesfrom 3.74� 10�4 to 3.86� 10�5 A, and there is a significantdrop in ON/OFF ratio as the pre-sintering temperature increases.Since all of the AZO thin films have been doped with the sameamount of Al, the reduction in rectifying behaviour is clearly due todifferences in structural properties. The electrical data can also berelated to the SEM images in Fig. 1, which shows the amount ofgrain boundaries and cracks increases as the pre-sintering tem-perature rises. As a result, it is reasonable to assume that thisincrease in cracks and grain boundaries is the main cause for thereduction in forward current and rectifying behaviour.

The illuminated i–v characteristics were measured andplotted in Fig. 5.The illuminated i–v devices exhibit a muchlarger current than that of the devices measured in the dark.For example, for a device fabricated using AZO pre-sintered at300 1C, at the reverse bias of �1 V, the dark current is only1.66� 10�5 A, but the photocurrent reaches 2.30� 10�4 A.

Fig. 7. Illustration of the proposed mechanism of the

Fig. 6. Dark and illuminated i–v characteristics of the AZO/Si devices with theAZO deposited using different pre-sintering temperatures.


An understanding of the photovoltaic behaviours underlyingthe i–v characteristics of a p-type silicon/AZO heterojunctioncan be obtained with the aid of the proposed model presentedin Fig. 7. Upon deposition, a pn junction is formed through thecombination of n-type AZO and p-type silicon. Since the AZOis highly doped, a built-in potential and depletion region isformed. The UV–vis spectroscopy measurements reveal thatthe as-deposited AZO thin films are highly transparent(transmittance greater than 88%) in the visible region. Uponillumination by simulated solar light, visible light passingthrough the AZO thin films will be either reflected or absorbedby the underlying p-type silicon. Photons absorbed by thesilicon will generate electron–hole pairs, and hence thephotocurrent. In terms of photovoltaic behaviour, the bestperformance was obtained in AZO thin films deposited at apre-sintering temperature of 400 1C (open circuit voltagearound 0.1 V, and short circuit current around 5μA), and thusfilms deposited under this condition will have the bestoptoelectronic characteristics. Although this photovoltaic per-formance is not high enough for power generation, it should beuseful in light sensor applications.

4. Conclusion

In summary, highly transparent AZO thin films with lowresistivity have been prepared by a cost effective sol–geltechnique. The effects of the pre-annealing temperature on thefilm's structural, optical and electrical properties are investigated.SEM images reveal that the pre-sintering temperature cansignificantly affect the grain size. The same tendency is alsoobserved from the XRD data, which shows the diffraction peakintensity increases along with the pre-sintering temperature. ThePL measurements and illuminated i–v characteristics suggestthat the best quality film can be obtained by using a pre-sintering temperature of around 400 1C. Electrical measure-ments confirm that all of the deposited films are n-type, with acarrier concentration in the range of 1019 cm2. The dark i–vcharacteristics also confirm the n-type conduction behaviour,due to the formation of rectifying junctions between p-typesilicon and n-type AZO.

n-type AZO/p-type silicon heterojunction device.






References

[1] L.-H. Cheng, L.-Y. Zheng, L. Meng, G.-R. Li, Y. Gu, F.-P. Zhang,R.-Q. Chu, Z.-J. Xu, Electrical properties of Al2O3-doped ZnO varistorsprepared by sol–gel process for device miniaturization, Ceram. Int. 38(2012) S457–S461.

[2] I.Y. Bu, Rapid synthesis of ZnO nanostructures through microwaveheating process, Ceram. Int. 39 (2012) 1189–1194.

[3] L. Cui, G.-G. Wang, H.-Y. Zhang, R. Sun, X.-P. Kuang, J.-C. Han, Effectof film thickness and annealing temperature on the structural and opticalproperties of ZnO thin films deposited on sapphire (0001) substrates bysol–gel, Ceram. Int. 39 (2012) 3261–3268.

[4] D. Fang, P. Yao, H. Li, Influence of annealing temperature on thestructural and optical properties of Mg–Al co-doped ZnO thin filmsprepared via sol–gel method, Ceram. Int. 40 (2013) 5873–5880.

[5] I.Y. Bu, Novel fabrication process for flexible dye sensitized solar cellusing aluminum doped zinc oxide, Mater. Sci. Semicond. Process. 16 (6)(2013) 1730–1735.

[6] U. Ozgur, D. Hofstetter, H. Morkoç, ZnO devices and applications: a review ofcurrent status and future prospects, Proc. IEEE 98 (7) (2010) 1255–1268.

[7] A. Chiappini, C. Armellini, A. Chiasera, M. Ferrari, R. Guider, Y. Jestin,L. Minati, E. Moser, G. Nunzi Conti, S. Pelli, Preparation and characterizationof ZnO particles embedded in organic–inorganic planar waveguide by sol–gelroute, J. Non-Cryst. Solids 355 (18) (2009) 1132–1135.

[8] A. Chiappini, C. Armellini, A. Chiasera, M. Ferrari, R. Guider, Y. Jestin,L. Minati, E. Moser, G.N. Conti, S. Pelli, Preparation and characterizationof ZnO particles embedded in organic–inorganic planar waveguide bysol–gel route, in: Proceeedings of the Photonics Europe, InternationalSociety for Optics and Photonics, 2008, pp. 699607–699608.

[9] C.-L. Kuo, C.-L. Wang, H.-H. Ko, W.-S. Hwang, K.-M. Chang,W.-L. Li, H.-H. Huang, Y.-H. Chang, M.-C. Wang, Synthesis of zincoxide nanocrystalline powders for cosmetic applications, Ceram. Int. 36(2) (2010) 693–698.

[10] Z.L. Wang, J. Song, Piezoelectric nanogenerators based on zinc oxidenanowire arrays, Science 312 (5771) (2006) 242–246.

[11] I.Y. Bu, C.-C. Yang, High-performance ZnO nanoflake moisture sensor,Superlattices Microstruct. 51 (6) (2012) 745–753.

[12] I.Y. Bu, Y.-M. Yeh, Effects of sulfidation on the optoelectronic propertiesof hydrothermally synthesized ZnO nanowires, Ceram. Int. 38 (5) (2012)3869–3873.

[13] I.Y. Bu, Novel all solution processed heterojunction using p-type cupricoxide and n-type zinc oxide nanowires for solar cell applications, Ceram.Int. (2013).

[14] B. Fang, C. Zhang, W. Zhang, G. Wang, A novel hydrazine electro-chemical sensor based on a carbon nanotube-wired ZnO nanoflower-modified electrode, Electrochim. Acta 55 (1) (2009) 178–182.

[15] I.Y. Bu, Facile synthesis of highly oriented p-type aluminum co-dopedzinc oxide film with aqua ammonia, J. Alloys Compd. 509 (6) (2011)2874–2878.

[16] C. Muiva, T. Sathiaraj, K. Maabong, Effect of doping concentration onthe properties of aluminium doped zinc oxide thin films prepared byspray pyrolysis for transparent electrode applications, Ceram. Int. 37 (2)(2011) 555–560.

[17] S. Major, A. Banerjee, K. Chopra, Highly transparent and conductingindium-doped zinc oxide films by spray pyrolysis, Thin Solid Films 108(3) (1983) 333–340.

[18] B. Joseph, P. Manoj, V. Vaidyan, Studies on the structural, electrical andoptical properties of Al-doped ZnO thin films prepared by chemical spraydeposition, Ceram. Int. 32 (5) (2006) 487–493.

[19] M. Mazilu, N. Tigau, V. Musat, Optical properties of undoped andAl-doped ZnO nanostructures grown from aqueous solution on glasssubstrate, Opt. Mater. 34 (11) (2012) 1833–1838.

[20] I.Y. Bu, M.T. Cole, A highly conductive and transparent solutionprocessed AZO/MWCNT nanocomposite, Ceram. Int. (2013).

[21] B. Chen, X. Sun, C. Xu, Fabrication of zinc oxide nanostructures ongold-coated silicon substrate by thermal chemical reactions vaportransport deposition in air, Ceram. Int. 30 (7) (2004) 1725–1729.


[22] I.Y. Bu, Sol–gel synthesis of ZnS (O, OH) thin films: Influence of precursorand process temperature on its optoelectronic properties, J. Lumin. (2012).

[23] I.Y. Bu, Effect of NH4OH concentration on p-type doped ZnO film bysolution based process, Appl. Surf. Sci. 257 (14) (2011) 6107–6111.

[24] M. Ohyama, H. Kouzuka, T. Yoko, Sol–gel preparation of ZnO filmswith extremely preferred orientation along (002) plane from zinc acetatesolution, Thin Solid Films 306 (1) (1997) 78–85.

[25] D. Raoufi, T. Raoufi, The effect of heat treatment on the physical properties ofsol–gel derived ZnO thin films, Appl. Surf. Sci. 255 (11) (2009) 5812–5817.

[26] Y. Ohya, H. Saiki, Y. Takahashi, Preparation of transparent, electricallyconducting ZnO film from zinc acetate and alkoxide, J. Mater. Sci. 29(15) (1994) 4099–4103.

[27] S.H. Yoon, D. Liu, D. Shen, M. Park, D.-J. Kim, Effect of chelatingagents on the preferred orientation of ZnO films by sol–gel process, J.Mater. Sci. 43 (18) (2008) 6177–6181.

[28] C.-Y. Tsay, K.-S. Fan, S.-H. Chen, C.-H. Tsai, Preparation andcharacterization of ZnO transparent semiconductor thin films by sol–gelmethod, J. Alloys Compd. 495 (1) (2010) 126–130.

[29] I. Winer, G.E. Shter, M. Mann-Lahav, G.S. Grader, Effect of solvents andstabilizers on sol–gel deposition of Ga-doped zinc oxide TCO films, J.Mater. Res. 26 (10) (2011) 1309–1315.

[30] Y. Li, L. Xu, X. Li, X. Shen, A. Wang, Effect of aging time of ZnO solon the structural and optical properties of ZnO thin films prepared by sol–gel method, Appl. Surf. Sci. 256 (14) (2010) 4543–4547.

[31] W. Tang, D. Cameron, Aluminum-doped zinc oxide transparent conductorsdeposited by the sol–gel process, Thin Solid Films 238 (1) (1994) 83–87.

[32] T. Minami, H. Nanto, S. Takata, Highly conductive and transparentaluminum doped zinc oxide thin films prepared by RF magnetronsputtering, Jpn. J. Appl. Phys. 23 (5) (1984) L280–L282.

[33] P. Nunes, E. Fortunato, P. Tonello, F. Braz Fernandes, P. Vilarinho,R. Martins, Effect of different dopant elements on the properties of ZnOthin films, Vacuum 64 (3) (2002) 281–285.

[34] S.-Y. Kuo, W.-C. Chen, F.-I. Lai, C.-P. Cheng, H.-C. Kuo, S.-C. Wang,W.-F. Hsieh, Effects of doping concentration and annealing temperatureon properties of highly-oriented Al-doped ZnO films, J. Cryst. Growth287 (1) (2006) 78–84.

[35] M. Gao, X. Wu, J. Liu, W. Liu, The effect of heating rate on the structuraland electrical properties of sol–gel derived Al-doped ZnO films, Appl.Surf. Sci. 257 (15) (2011) 6919–6922.

[36] S. Goto, N. Fujimura, T. Nishihara, T. Ito, Heteroepitaxy of zinc oxidethin films, considering non-epitaxial preferential orientation, J. Cryst.Growth 115 (1) (1991) 816–820.

[37] N. Fujimura, T. Nishihara, S. Goto, J. Xu, T. Ito, Control of preferredorientation for ZnOx films: control of self-texture, J. Cryst. Growth 130(1) (1993) 269–279.

[38] A. Van der Drift, Evolutionary selection, a principle governing growthorientation in vapour-deposited layers, Philips Res. Rep. 22 (3) (1967)267–288.

[39] J. Kim, J. Han, K. Park, The characteristics of Al-doped ZnO films depositedwith RF magnetron sputtering system in various H2/(ArþH2) gas ratios, 2012.

[40] K.-J. Chen, T.-H. Fang, F.-Y. Hung, L.-W. Ji, S.-J. Chang, S.-J. Young,Y. Hsiao, The crystallization and physical properties of Al-doped ZnOnanoparticles, Appl. Surf. Sci. 254 (18) (2008) 5791–5795.

[41] A. Verma, F. Khan, D. Kumar, M. Kar, B. Chakravarty, S. Singh,M. Husain, Sol–gel derived aluminum doped zinc oxide for application asanti-reflection coating in terrestrial silicon solar cells, Thin Solid Films518 (10) (2010) 2649–2653.

[42] P. Banerjee, W.-J. Lee, K.-R. Bae, S.B. Lee, G.W. Rubloff, Structural,electrical, and optical properties of atomic layer deposition Al-doped ZnOfilms, J. Appl. Phys. 108 (4) (2010) 043504–043507.

[43] Y. Li, G. Meng, L. Zhang, F. Phillipp, Ordered semiconductor ZnOnanowire arrays and their photoluminescence properties, Appl. Phys.Lett. 76 (15) (2000) 2011–2013.

[44] H.-M. Zhou, D.-Q. Yi, Z.-M. Yu, L.-R. Xiao, J. Li, Preparation ofaluminum doped zinc oxide films and the study of their microstructure,electrical and optical properties, Thin Solid Films 515 (17) (2007)6909–6914.


http://refhub.elsevier.com/S0272-8842(14)00558-6/sbref1




































































































































effects of the pre-annealing temperature on structural and optical properties of sol–gel deposited...

Documents