Superlattices and Microstructures 60 (2013) 407–413

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Superlattices and Microstructures

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o ca t e / s u p e r l a t t i c es

Spray pyrolysis derived ZnMgO:In thin films:Investigation of the structural, optical andelectrical properties

0749-6036/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.spmi.2013.05.021

⇑ Corresponding author at: Faculty of Materials Science and Chemistry, China University of Geosciences, WuhanChina. Tel.: +86 18986126026.

E-mail address: jintan_cug@163.com (J. Tan).

Wei Yan a, Jin Tan a,b,⇑, Wei Zhang a, Yujun Liang a,b, Shan Feng a,Xinrong Lei a,b, Hongquan Wang b

a Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, Chinab Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan 430074, China

a r t i c l e i n f o

Article history:Received 29 April 2013Received in revised form 15 May 2013Accepted 18 May 2013Available online 29 May 2013

Keywords:ZnMgO:InEnergy band gapElectrical propertiesSpray pyrolysis

a b s t r a c t

Zn1�xMgxO:In films were deposited on quartz substrates via ultra-sonic spray pyrolysis (USP). By tuning the molar ratio of Zn and Mgsources, the tunable band gap in ZnMgO:In films were realized. Theobtained films exhibited wurtzite crystal structure with a preferen-tial c-axis orientation. Larger grain size was obtained with increas-ing the Mg introduction. Photoluminescence (PL) indicated that thepeaks of near-band-edge (NBE) emission appeared as a blue-shiftfrom 378 nm to 370 nm, labeled as x value increase from 0 to0.15. Optical band gap (Eg) calculated from the absorption edge fur-ther confirmed the blue-shift phenomenon due to the Mg substitu-tion for Zn lattice sites. The average transmittance was about 90%in the visible wavelength region (400–800 nm). The increasingresistivity from 6.70 � 10�3 X cm to 2.14 � 10�2 X cm anddecreasing mobility from 24.7 cm2 V�1 S�1 to 6.46 cm2 V�1 S�1

were observed.� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Nowadays, ZnO thin films, including ZnO:Al (AZO), ZnO:Ga (GZO) and ZnO:In (IZO), emerge as thepromising materials due to their exceptional properties, such as non-toxicity, environmental-friend-liness, cheapness as well as the excellent optical and electrical properties [1–3]. Based on the above

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advantages, ZnO films show potential application in the field of solar cells [4], chemical sensors [5],laser diodes and optoelectronic ultraviolet devices [6,7]. With the direct band gap (3.37 eV) and largeexciton binding energy (60 meV), ZnO presents the favorable candidates for the optoelectronic devices[8,9]. However, the achievement of the practical application for ZnO is challenged by the band gapengineering. Many approaches have been adopted to modulate the band gap of ZnO, among whichalloying with Mg, Be or Cd elements is the common one [10–13]. For the addition of Mg, Be or Cd ele-ments to substitute for Zn lattice sites in ZnO bulk, band gap modulation of ZnO has been successfullyachieved. Recently, Zn1�xMgxO films have aroused scientists’ great interests for the sake of the closeionic radius of Mg2+ (0.57 Å) and Zn2+ (0.60 Å) as well as the wide band gap of MgO (7.80 eV)[14,15]. By Mg doping, Zn1�xMgxO films with the tunable band gap from 3.37 eV to 7.80 eV were real-ized [10]. Though the investigation of the large conduction band offset on ZnO/ZnMgO films has beenconducted, the transmittance of the films still remains controversial for ultraviolet devices [16,17].Moreover, band gap modulation with high concentration of Mg doping (x > 0.3) for Zn1�xMgxO filmshas been successfully achieved, but the electrical properties decreased (q � 1 X cm) greatly [18].Therefore, it is worthwhile to further investigate the tunable band gap with desirable conductivityby doping.

Current researches on the fabrication of ZnMgO films are reported by pulsed-laser-deposition (PLD)[19], molecular-beam epitaxy (MBE) [20], metal organic chemical vapor deposition (MOCVD) [21],magnetron sputtering (MS) [22] and ultrasonic spray pyrolysis (USP). Among them, USP techniquepresents the advantage of cost-efficiency; process-simplicity and easy-scalability in the fabricationof ZnO based thin films [23]. Within this study, the USP technology has been employed to depositZn1�xMgxO:In films. To combine the transparent conductive properties with well-tunable band gap,the adjustment of Mg doping content in the starting sources is desirable. In addition, electrical prop-erties correlated to optical and crystal structure properties of the Zn1�xMgxO:In films are investigatedsystematically.

2. Experimental details

Indium doped ZnMgO films were deposited by USP technology on the quartz substrates, whichwere prepared 10 nm ZnO film as the buffer by MS. During the USP deposition, the growth tempera-ture was maintained at 420 �C. The Zn(Ac)2�2H2O, Mg(Ac)2�4H2O and In(NO3)3 solutions were used asthe precursors of Zn, Mg and In sources, respectively. The optimized In/(Zn + Mg) ratio of 2.25 at.% wasemployed to control the In doping for obtaining good electrical performance. In addition, different Mgconcentration of Zn1�xMgxO:In films (x = 0, 0.05, 0.10 and 0.15) were used in order to achieve the Mgdoping. The aerosol of precursor solution was generated by the commercial ultrasonic nebulizer, andtransported to the substrates heated at 420 �C according to optimized results. Subsequently, theobtained products were annealed in an oven with mixed Ar2 + H2 (flux ration = 90:10) atmosphereat 400 �C for 20 min.

Crystal structure were measured by X-ray diffraction (XRD, D8-FOCUS) using Cu Ka radiation(k = 0.15406 nm). The morphologies of films were characterized by field-emission scanning electronmicroscopy (FESEM, Hitachi S-8010) equipped with EDS. The PL spectra were taken by aFluoroMax-4P spectrofluorometer system with Xe light source operating at 325 nm. The optical trans-mittance was evaluated using UV–VIS-2450 spectrophotometer in the wavelength range from 350 to1000 nm. The electrical properties were analyzed by Hall-effect measurements (Bio-Rad HL 5500) inthe Van der Pauw configuration.

3. Results and discussion

Fig. 1a presents the XRD patterns of In-doped Zn1�xMgxO films (x = 0, 0.05, 0.10 and 0.15) preparedat 420 �C. As shown in Fig. 1a, polycrystalline and hexagonal wurtzite structure of Zn1�xMgxO films areformed with a highly preferential (002) orientation. All diffraction peaks including (002) and (103)planes coincide well with hexagonal-phase (JCPDS 36-1451). No peaks related to Mg or In oxidesare detected. It is confirmed that the doping Mg and In elements replace the Zn sites rather than

Fig. 1. a XRD patterns of the Zn1�xMgxO:In films (x = 0, 0.05, 0.10, 0.15) prepared at 420 �C. (b) The variation of (002) peakposition and FWHM of Zn1�xMgxO thin films as a function of Mg concentration.

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act as the grain-boundary segregation. Fig. 1b shows the variation of Full-Width Half-Maximum(FWHM) and (002) peak position of the Zn1�xMgxO films as a function of Mg concentration in thesource solutions. It is clearly observed that the FWHM values of the (002) peak decrease with a con-tinued increment of Mg content from 0 to 0.15, indicating the crystalline has been improved by the Mgintroduction. However, the diffraction angle of the corresponding films switches to large region withincreasing Mg content, which is consistent with the equation: k = 2d sinh. The XRD results of theZn1�xMgxO:In films indicate that Zn lattice sites are successfully substituted by both Mg and In ele-ments. We infer that the tensile stress of films have been introduced by the lattice constant expansiondue to the In and Mg incorporation [24].

Fig. 2 shows the FESEM images of the prepared samples grown in different molar ratio of Zn/Mgstarting solutions. It is observed that the Mg incorporation plays a crucial role in influencing the filmsurface morphology. In Fig. 2a, regular grains with homogeneous distribution of 100 nm in size arefound in the ZnO:In films. However, the larger crystalline size are achieved by increasing Mg molarratio, and the size can be up to 300 nm as the Mg molar ratio from 0.05 to 0.15 (Fig. 2b–d). The trendcan be explained that the Mg introduction enhances the surface reaction and the sticking process

Fig. 2. The FESEM images of the Zn1�xMgxO:In films grown in different Mg concentration solutions. (a) x = 0, (b) x = 0.05, (c)x = 0.10, (d) x = 0.15, (e) cross-sectional morphology of the samples, distribution of In and Mg element of the sample withx = 0.15, corresponding to (f and g).

Fig. 3. Actual In element and Mg element content of Zn1�xMgxO:In films as a function of Mg concentration in the precursorsolutions.

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during the USP process. The increased crystalline size after the Mg introduction is in agreement withthe FWHM results, which are shown in Fig. 1b. The cross-sectional morphology of the ZnO:In filmshown in Fig. 2e reveals that the thickness of the prepared films is approximately 300 nm. Obviously,the Zn1�xMgxO:In films are closely attached to the quartz substrates. Fig. 2f and g depicts the EDSmappings of In and Mg element at x = 0.15. From the EDS mappings, the distribution of In and Mg ele-ments is homogeneous. However, there are deviations between the actual doping content and thenominal source ratio. The actual molar ratios of Mg and In elements are 1.70 at.% and 0.95 at.%,respectively.

Fig. 3 summarizes the actual In and Mg element incorporation of the prepared films, derived by EDSmeasurement. It is apparent that as increasing x value of the Zn1�xMgiO films, the actual Mg and Incomposition in the films are far below the expectation. This can be deduced that the efficient surfacereaction and sticking process are influenced by Mg introduction. With increasing the molar ratio from0.05 to 0.15, the actual Mg doping content changes from 0.41 at.% to 1.7 at.%. However, high Mg sourceconcentration (x P 0.10) will lead to the reduction of In element corporation, which is limited by theUSP technology.

Fig. 4 plots the photoluminescence (PL) spectra of the Zn1�xMgxO:In films as a function of Mg con-tent in the sources. It is acknowledged that there exists two main emission bands in ZnO PL spectra,NBE and deep-level (DL) emission, respectively [25]. However, in our PL spectra, the films exhibit

Fig. 4. Photoluminescence (PL) spectra of the Zn1�xMgxO:In films as a function of Mg content in the starting solutions.

Fig. 5. (a) The transmittance spectra of the thin films deposited with different Mg contents and (b) the correlation between thevalue of a and photo energy. The inset shows the optical band gap (Eg) as determined by the onset of absorption in UV–Vistransmission measurements.

Fig. 6. The resistivity, carrier concentration and mobility of Zn1�xMgxO:In films (x = 0, 0.05, 0.10, 0.15).

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strong peaks located in the UV region, which originate from the NBE emission. Note that the NBE emis-sion peaks appear as a blue-shift from 378 nm to 370 nm with increasing the Mg content to 0.15 in thesource solutions. Compared the NBE emission, the DL emission is hardly observed due to less nativedefects in Zn1�xMgxO films. It is deduced that the film lattice is not disturbed by the introduction ofMg and In atoms, especially in the formation of interstitials, vacancies or antisites. Meanwhile, theintroduction of Mg content leads to the general broadening of Zn1�xMgxO spectra lines, which arethe consequence of the element alloying [26].

Fig. 5a shows the transmittance spectra of the thin films deposited with different Mg contents inthe solutions. From Fig. 5a, the average transmittance of all the films in the visible wavelength region(400–800 nm) is about 90%. This high transmission is attributed to the surface with large crystal sizeand pore-like morphology. Fig. 5b shows the correlation between the value of a and photo energy (eV),where a is the absorption coefficient. There is an obvious shift of the absorption edge to shorterwavelength with increasing Mg content. The inset of Fig. 5b exhibits the optical band gap (Eg) whichis determined by the onset of absorption in UV–Vis transmission measurements. As can be clearly seenthat Eg turns to be larger as increasing the Mg content to 0.15. From the inset, Eg with the band gap of3.27, 3.29, 3.32 and 3.37 eV are evaluated for x = 0, 0.05, 0.10 and 0.15 in Zn1�xMgxO films, which indi-cates the introduction of Mg can effectively realize band gap broadening. This effect of widening the

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band-gap is attributed primarily to the Burstein-Moss shift in semiconductor [27,28]. Besides, theblue-shift of transmittance spectra is consistent well with that of PL spectra.

The Hall measurement of the Zn1�xMgxO:In films grown at different Mg introduction is presentedin Fig. 6. The resistivity of 6.70 � 10�3 X cm, carrier concentration of 2.36 � 1019 cm�3 and mobility of24.7 cm2 V�1 S�1 are found for pure ZnO:In film. Subsequently, resistivity increases to 2.14 �10�2 X cm and mobility drops to 6.46 cm2 V�1 S�1 when increasing the Mg content to 0.15 in theprecursor solutions. This is the result of the higher grain-boundary scattering introduced by Mg ion.Meantime, the incorporation of Mg elements in turn deteriorates the In ion actual doping contentin the crystal structure evidenced by the lines in Fig. 3. Furthermore, the carrier concentration doesnot significantly increase and even fluctuate with increasing nominal source ratio, indicating thatthe main reason of electrical properties is attribute to the intrinsic doping and morphology.

4. Conclusion

Zn1�xMgxO:In transparent conductive oxide films with tunable energy band gap were prepared byUSP technology. With increasing Mg content, the grain size becomes larger due to the improvedcrystalline. The actual Mg doping enhances to 1.7 at.% at the x value fixed at 0.15 in Zn1�xMgxO:Infilms. All the films show high transmittance above 90% in visible wavelength region (400–800 nm).PL NBE blue-shift from 378 nm to 370 nm with the band gap (Eg) 3.27 eV to 3.37 eV is observeddue to the Mg introduction. When increasing the Mg content from 0 to 0.15 in the starting solutions,the resistivity raise from 6.70 � 10�3 X cm to 2.14 � 10�2 X cm and the mobility decrease from24.7 cm2 V�1 S�1 to 6.46 cm2 V�1 S�1. These Zn1�xMgxO:In films are expected as a good candidatefor the UV photoelectrical devices.

Acknowledgment

This work was supported by National Natural Science Foundation of China (Grant No. 40643018).The authors thanked Jiang Qingjun for technical support.

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