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Materials Science in Semiconductor Processing

Materials Science in Semiconductor Processing 27 (2014) 19–25

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Sol–gel production of aluminium doped zinc oxide usingaluminium nitrate

Ian Y.Y. Bu n

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

a r t i c l e i n f o

Keywords:NanostructuresSemiconductorsSol–gel chemistryElectrical properties

x.doi.org/10.1016/j.mssp.2014.06.01101/& 2014 Elsevier Ltd. All rights reserved.

þ886 972506900.ail address: ianbu@hotmail.com

a b s t r a c t

Sol–gel synthesis of aluminium doped zinc oxide was performed via a spin coating processthrough a precursor solution that consisted of zinc acetate, aluminium nitrate andammonia. The effects of sintering temperature on the optoelectronic properties of thederived films were investigated through scanning electron microscopy, X-ray diffraction,photoluminescence, UV–vis spectroscopy and Hall effect measurements. It was found thatas the process temperature increases the film changes from a grain-like morphologytowards a nanowire structure. This trend is also evident in the presented XRD data.Optical measurements revealed the derived films to be highly transparent with an opticaltransparency �92%.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Recently, metal oxide semiconductor materials haveattracted considerable attention due to their potential applica-tions in next generation optoelectronic devices [1]. Amongthesematerials, zinc oxide (ZnO) is one of themost intensivelystudied, due to its high electron mobility, piezoelectric proper-ties, large band gap (3.37 eV) and large exciton binding energy(60 meV) [2]. Consequently, applications of ZnO have beenproposed with regard to transparent conductive oxide [3],light emitting diodes [4], humidity sensors [5]and piezo-electric nanogenerators [6].

In terms of deposition, ZnO can be synthesized throughboth solution [7] and vacuum-based processes [8]. Vacuum-based processes include chemical vapour deposition (CVD)[9], sputtering, laser ablation [10] and spray pyrolysis [11].Although high-quality ZnO can be deposited via vacuum-based deposition methods, these techniques often require

complicated and expensive equipment. Alternatively, one canuse solution-based deposition methods, such as chemical bathdeposition [12], sol–gel process [13], electrodeposition [14]and a hydrothermal process [15], in order to deposit ZnO.These solution-based processes offer superior control ofgeometry, and involve only simple equipment. Among thedifferent chemical based techniques sol–gel possesses areparticularly attractive due to the advantages of precise controlof chemical composition and efficient material utilization [16].

Generally, as-deposited ZnO is n-type, due to theincorporation of impurities during the synthesis process[17]. The conductivity of ZnO can be further increased bythe addition of Al [18] and Ga [19]. Due to the materialabundance and high optical transparency, Al doped ZnO(AZO) has been proposed to replace indium tin oxide astransparent conductive film. Previous studies have shownthat physical properties of the sol–gel deposited AZO thinfilms are highly influenced by dopant concentration [13],selection of stabilizers [20] and post-sintering temperature[21]. However, there have been a limited number ofstudies on the effects of dopant source on the subsequentproperties of AZO thin films. In this study, the effects of

I.Y.Y. Bu / Materials Science in Semiconductor Processing 27 (2014) 19–2520

sol–gel synthesis temperature on the production of AZOusing aluminium nitrate are investigated. The resultantZnO films were thoroughly investigated through scanningelectron microscopy, energy dispersive spectroscopy, X-raydiffraction, photoluminescence spectroscopy, Raman spec-troscopy and electrical measurements.

2. Experimental

Corning glass (Eagle 2000) substrates were cleanedusing baths of acetone, isopropanol and D.I. water withinan ultrasonicator. All the chemicals used in this study wereof analytical grade and used without further purification.AZO thin films were deposited on pre-cleaned glass sub-strates by spin coating the derived sol. Firstly, zinc acetatedehydrate was mixed into isopropanol to form 0.7 M ofprecursor solution. Then 2 ml of monoethanolamine(MEA) was then added as a chelating agent to preventprecipitation during the mixing stage of the process.Doping was achieved through addition of 1 wt% of alumi-nium nitrate nonhydrate. The solution was mixed for 1 h at60 1C and left to age for 48 h. Sol–gel deposition wasperformed through a spin coating at 3000 rpm and atwo-step annealing process at 250 1C and 400–550 1C.Each spin-sintering cycle yielded around 50 nm of filmthickness and was repeated five times to form 250 nmof AZO.

The crystalline orientation of the films was studiedusing an X-ray diffractometer (Philips, Model: PW1830)with Cu Kα radiation with an angular step of 0.011. TheXRD scan was taken between 2 theta between 201 and 801.The structural properties of sol–gel derived AZO thin filmswere characterized using an FEI Quanta 400 F environ-mental scanning electron microscope (SEM) equippedwith energy dispersive spectroscopy (EDS). Photolumines-cence emission and Raman spectroscopy measurementswere obtained through the use of a Dongwoo MacroRaman spectrometer. The optical transmittance of thederived samples was obtained using UV–vis–NIR spectro-scopy (JASCO). Hall effects measurements were taken withan Ecopia HMS-3000 Hall effect measurement system. As ademonstration, pn junctions were formed through spincoating of AZO onto p-type silicon substrates. In order toensure Ohmic contacts to the fabricated device Zn/Al wereevaporated onto the surface through a pre-fabricated maskto form electrodes. Current density–voltage (J–V) charac-teristics were measured using a Keithley 2400 sourcemeasure unit.

3. Discussions

The surface morphology of the deposited AZO thinfilms was evaluated using SEM. Fig. 1a–d shows the SEMimages of the sol–gel synthesized AZO thin films sinteredat 400 1C, 450 1C, 500 1C and 550 1C. It can be observedfrom the SEM images that the sol–gel derived AZO thinfilm was composed of uniform particles with diametersvarying from 20 to 60 nm. The microstructures of the thinfilms were found to be highly dependent on the post-sintering temperature and transformed from small particlesinto larger interconnected grains as the temperature was

increased from 400 1C to 550 1C. Fig. 1a shows that AZOthin films annealed at 400 and 450 1C (Fig. 1b) consistedof nano-particles with average grain sizes of around 10–20 nm. For the AZO thin films annealed at 500 1C (Fig. 1c),the grain size increased to around 40 nm in diameter, withclearly observed pores between the nano-particles. In thecase of AZO thin films annealed at 550 1C the grain sizeincreased up to 60 nm with the pores closed. During thesol–gel synthesis of AZO the thin films underwent twostages of heating: 1) a pre-sintering process at around250 1C and 2) a post-sintering process at 400–550 1C.At the pre-sintering stage, most of the organic compoundswere eliminated from the precursor coated substrates. Thepost-sintering process supplied the required energy forcrystallite nucleation. The formation of pores between thecrystallites is due to the elimination of organic mattersfrom the precursor solution. Fig. 1e) shows the represen-tative EDS compositional analysis of the AZO thin filmdeposited at 550 1C. As expected Zn, O, Al and N weredetected, with a near equal at% of Zn:O. The present studyinvolves precursors prepared under an excessive ammoniacondition that results in the formation of ammonia com-plexes and aluminium hydroxide, as shown in the follow-ing equation:

Al(NO3)3(aq)þNH4OH(aq)(aqueous NH3)———4Al(OH)3 (s)þNH4NO3(aq) (1)

The orientations of the deposited AZO thin films as afunction of sintering temperature were determined usingXRD. Fig. 2a shows the XRD peaks as a function ofannealing temperature. All the deposited AZO thin filmswere polycrystalline with a hexagonal wurtzite structureand a c-axis (002) orientation. Such a c-axis orientation isa common feature of ZnO derivative thin films depositedusing the sol–gel deposition technique. Distinct XRD peaksat 31.991, 34.491, 36.381 and 56.791 correspond to the(100), (002), (101) and (110) orientations. As can beobserved from Fig. 2a, AZO thin films post-sintered at500 1C exhibited the highest diffraction intensity andhence crystallinity. Furthermore, it can be deduced fromFig. 2a that as post-sintering temperature increased up to500 1C the intensity of the (002), (100) and (101) diffrac-tion peaks also increased. A further increase in the post-sintering temperature to 550 1C resulted in a decrease inthe intensity of the (002) peak. The XRD data correspondswell with the SEM image shown in Fig. 1, where grains sizeis shown to increase as a function of temperature. Speci-fically, Fig. 1c and d shows that as the sintering tempera-ture was raised to 500 1C, this resulted in a grain-likemorphology due to growth in the (100) and (101) orienta-tions. The full-width half-maximum (FWHM) is a usefulvalue to evaluate film properties and can be extracted fromthe XRD pattern. It can be observed from Fig. 2a that thereis a simultaneous decrease in the peak intensity at the(002) orientation with an increase in the (110) orientation.This suggests that films grown at 550 1C increased growthat the (110) orientation and thus leading to decreasedgrowth in the (002) orientation. It is known that films tendto grow with the plane with the lowest surface energy.Fujimura et al. [22] observed that the surface energy

Fig. 1. SEM image of the sol–gel derived AZO thin films post-annealed at a) 400 1C, b) 450 1C, c) 500 1C and d) 550 1C. e) The representative EDScomposition of the film deposited at 550 1C.

I.Y.Y. Bu / Materials Science in Semiconductor Processing 27 (2014) 19–25 21

density in the (002) orientation is the lowest in ZnO and thusthe film grew in this orientation until the sintering tempera-ture reached 500 1C. The preferential c-axis orientation in ZnOthin films can be explained using the theoretical modelproposed by Van der Drift [23]. According to the Van derDrift model, crystallite growth consists of a competitiveprocess that begins with nucleation of different orientationsbut only crystallites with the fastest growth rate will even-tually remain on the substrate, which are those in the (002)orientation in the present study. However, when the films

sintered at a higher temperature other ZnO phases nucleateand disrupt the growth at the (002) orientation.

Fig. 2b shows the extracted FWHM of AZO deposited atvarious temperatures. As expected, the FWHM decreasedwith increasing annealing temperature up to 500 1C,which indicates enhancement in crystallinity. Filmsannealed at 550 1C have a higher FWHM value, whichsuggests a degradation in film properties. This reversal inthe FWHM value is due to the growth of other ZnO phasesand correlates well with the SEM image.

Fig. 2. a) XRD patterns of AZO post-annealed at different temperaturesand b) the extracted FWHM from the XRD pattern.

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Photoluminescence (PL) measurements are a usefultool for the evaluation of optically induced defects withinZnO related materials. Fig. 3a shows the PL spectra of theAZO thin films deposited at different post-sintering tem-peratures. In general, two signature PL peaks emissions arefound from ZnO at 385 nm and 500–550 nm, respectively.The ultraviolet near-band centred at around 385 nm(3.22 eV) originates from the exciton recombination pro-cess (donor–acceptor pair recombination), whereas thegreen PL emission at around 500–550 nm (2.25–2.47 eVis related to Zn interstitials (donor) and single ionized Ovacancy (donor) [24,25], respectively. In the present study,two PL peaks were observed in all the deposited AZO thinfilms, at 385 and 440 nm (2.82 eV), respectively. The newemission peak at around 440 nm differed from the com-monly observed green band. Furthermore, the PL emissionat 440 nm was rather broad, which indicates that theemission originated from deep level defects. It is interest-ing to note that PL emission at 440 nm is absent from sol–gel derivations of AZO when using AlCl as the co-dopant[26], which suggests that incorporation of aluminiumnitrate have contributed to the emission peak. Previousstudies have revealed that PL emission at 440 nm is onlypresent in AlN thin films with oxygen impurities, whichimplies incorporation of AlN into ZnO [27]. Therefore, the

PL data suggests that Al is incorporated into ZnO throughformation of AlN. The decrease in PL emission at 440 nmfrom films sintered at higher temperature suggests adecrease in AlN content probably due to evaporationduring the sintering process. From the PL measurements,the absence of 500–550 nm suggests that intrinsic defectsare suppressed.

The quality of deposited ZnO thin films can be evalu-ated using the integrated PL intensity at 385 nm and550 nm, respectively. In order to evaluate the quality ofour AZO thin films, we modified the integrated ratio usingthe integrated PL peak at I385/I440. Fig. 3b shows the ratioof I385/I440 as a function of sintering temperature. It can beseen that as the sintering temperature increases the ratioof I385/I440 increases, which indicates improvement in thestructure of the AZO thin film.

The Raman spectra of AZO thin films sintered atdifferent temperatures are shown in Fig. 4. Three distinctpeaks appear in these Raman spectra centred at 100, 432,573, 852 and 1125 cm�1, respectively. The Raman spectraat 100 cm�1 is assigned to the E2 (low) peak which hasbeen associated with oxygen displacement and the sub-lattice and have been used to determine the crystal qualityof the film [28]. The 432 cm�1 Raman peak is attributed tothe high frequency E2 mode of ZnO. The 573 cm�1 Ramanpeaks have been identified in nitrogen doped ZnO thinfilms and are assigned as an electric field induced A1 (Lo)mode [29] and are considered as nitrogen-induced defects.Fig. 4 shows that the 573 cm�1 peak significantly reducesas sintering temperature increases, which suggests areduction in defects and is in good agreement with thePL and XRD data presented earlier. Possible mechanism forreduction in defects includes AZO lattice restructuring andout diffusion of interstitial N from the ammonia.

The peak at 1125 cm�1 is related to a carbon relateddefect complex, whereas the 852 cm�1 peaks have beentermed local vibrational modes [30].

Fig. 5 shows the optical transmittance spectra of thesynthesized AZO thin films determined by UV–vis spectro-scopy measured in a wavelength range between 200 and1200 nm. Clearly, the optical transmittance of the films ishighly dependent on post-annealing temperature. AZOthin films deposited at 400 1C are shown to exhibit thelowest film optical transparency of around 80% in thevisible region. Further increases in post-sintering tempera-ture resulted in highly transparent thin film with anaverage transparency of around 92% in the visible region.The absorption coefficient of direct band gap semiconduc-tors can be determined by using the following formula:

α¼ 1t

� �ðlnðT%ÞÞ

ðahvÞ2 ¼ hv�Eg

where α is the absorption coefficient, t is the film thick-ness, T% is the transmittance, hv is the photon energy andEg is the band gap. The band gap can be determined byextrapolating the linear region of the graph (αhv)2 vs. hv,as shown in Fig. 5b. The inset of Fig. 5b shows theextracted band gap plotted as a function of temperature.It can be observed from the inset that as the sintering

Fig. 3. a) Photoluminescence emission of the AZO film deposited at different temperatures and b) the extracted I385/I440 intensity as a function oftemperature.

I.Y.Y. Bu / Materials Science in Semiconductor Processing 27 (2014) 19–25 23

temperature of AZO thin films increases, the band gapincreases, due to a reduction in defect density [31]. Thisimprovement in optoelectronic properties correlates wellwith the XRD and PL measurements, which suggests thatthere are improvements in the film properties as the post-sintering temperature increases.

Hall-effect measurements were performed to identifythe carrier concentration and resistivity of the deposited

AZO thin films, which are presented in Table 1. All thedeposited films exhibited high carrier concentration withthe films deposited at 500 1C having the highest at2.214�10þ19 cm�3. A previous study has reported thatthere is an experimental window which exists for sol–geldeposition of Al/N doped ZnO at a sintering temperature of500–550 1C [31]. It is believed that the absence of such anexperimental window in this study maybe due to the

Fig. 4. Raman spectroscopy spectra of the deposited AZO thin films.

Fig. 5. a) Optical transmittance of the deposited AZO thin film depositedat various temperatures and b) the variation of ðαhvÞ2 against hv used toextract the optical bandgap (the inset shows the extracted band gap).

Table 1Carrier concentration, mobility, resistivity and conduction type for thederived AZO thin films.

Post-sinteringtemperature (1C)

Carrierconcentration(cm�3)

Mobility(cm2 V s�1)

Resistivity(Ω cm)

400 1.211Eþ18 5.340E�01 9.650Eþ00450 1.698Eþ19 3.818E�02 3.818E�02500 2.214Eþ19 3.022E�02 9.329Eþ00550 7.898Eþ18 8.137E�02 9.712Eþ00

Fig. 6. a) Illustration of the fabricated pn junction device constructedwith p-type silicon/AZO and b) the measured I–V characteristics

I.Y.Y. Bu / Materials Science in Semiconductor Processing 27 (2014) 19–2524

excessive N concentration (N/Al �100) that was employedin the previous report. In the past, our group has shownthat excessive ammonia yields thin films with precipitatesthat degrade the carrier concentration [32]. All the AZOsamples exhibit high carrier concentration and low

resistivity, which indicate high crystalline structure andoxygen displacement and correlates well with the E2 (low)Raman spectra obtained in Fig. 4.

In order to evaluate the electronic properties of thederived AZO thin films, the optimized AZO thin filmsintered at 500 1C was spun onto p-type substrate to forma heterojunctional device. Fig. 6a shows the schematicdiagram of the fabricated device, which consists of p-typesilicon/AZO and Al/Zn Ohmic contacts. Fig. 6b shows theI–V characteristics of the fabricated heterojunction device.The fabricated heterojunction exhibits good rectifyingbehaviour with the threshold voltage in the region of0.2 V and leakage current of around 1.366�10�5 A.

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4. Conclusion

In summary, this paper investigated the effects of post-sintering temperature on sol–gel derived aluminiumdoped ZnO thin films using aluminium nitrate and ammo-nia as co-dopants. The synthesized thin films were exten-sively characterized through SEM, PL, EDS, XRD andUV–vis. From the SEM and PL emission measurements, itcan be concluded that there exists an experimental win-dow of post-sintering temperature (500 1C) that is bene-ficial towards high quality AZO thin film. Further increasein post-sintering temperature degrades the thin film andgrowth in other crystal phases. UV–vis spectroscopy mea-surements indicate that the optimized AZO thin film ishighly transparent (�92%) and the band gap increaseswith higher sintering temperature. Electrical measure-ments have confirmed that aluminium nitrate is an effec-tive dopant for producing highly conductive AZO thin filmby the sol–gel deposition method.

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