Conference-ISSS-7-Evaluation of ZnO-MgO Mixed Thin Films Grown by Metal-Organic Decomposition

The influence of ZnO-MgO mixed metal-organic decomposition (MOD) coating materials have been investigated based on crystal growth and film properties of ZnO-MgO mixed thin films. These mixed thin films were grown on quartz substrates by dip coating and then sintered at 550◦C. The film properties were evaluated using an atomic force microscope and a UV-VIS-NIR spectrophotometer, and by performing X-ray diffraction measurements. The growth mechanism is changed by MgO in ZnO-MgO mixed MOD coating materials. Mg segregated to the surface side in the ZnO-MgO mixed thin film, and some of the segregated Mg formed ZnMgO. In addition, the grain size of ZnO-MgO mixed thin films increased with an increase in the amount of MgO additive. Therefore, it can be concluded that grain boundary segregation of Mg enhances crystal growth. This enhancement of crystal growth by the intentional addition of an impurity can be applied to improving the characteristics of other semiconductor materials. [DOI: 10.1380/ejssnt.2015.185]


INTRODUCTION
ZnO is an attractive material because its raw materials are cheap, the exciton binding energy is large, and fabrication of n-type ZnO is easy.As a result of these properties, ZnO thin films have been extensively researched for application in optical and electronic devices such as light emitting diodes (LEDs) and transparent conductive oxides (TCOs) [1,2].In addition, ZnO nanoparticles have been investigated for medical applications [3].Thus, ZnO is a material expected to have applications in various fields.Therefore, it is important to lower the cost of manufacturing equipments and of the overall fabrication process.ZnO nanoparticles have been grown by inexpensive techniques such as gas evaporation and chemical methods [4,5].Although the fabrication of p-type ZnO is difficult due to the self-compensation effect by intrinsic defects, fabrication of p-type ZnO nanoparticles by the gas evaporation method has been reported [6].On the other hand, ZnO thin films with good properties have been grown by pulsed laser deposition (PLD), metalorganic chemical vapor deposition (MOCVD), and sputtering [7][8][9].Although MOCVD and sputtering are good mass-production techniques, the metal-organic decomposition (MOD) method is expected to further decrease production costs.Generally, using the MOD method, it is difficult to realize good material properties equivalent to those obtained with vacuum processes.Therefore, it is important to improve and control the properties of films grown by the MOD method.Control of film properties such as p-type doping, n-type doping, and band gap engineering are necessary for device applications of ZnO thin films.However, there are few reports on ZnO thin films grown by the MOD method [10,11].In this study, ZnO-MgO mixed thin films were grown by the MOD method using mixed source materials of ZnO and MgO.In addition, ZnMgO is a semiconductor alloy that is promising material for band gap engineering [12].The aim of the present study is to reveal the influence of ZnO-MgO mixed MOD coating materials on crystal growth and film properties of ZnO-MgO mixed thin films.

II. EXPERIMENTAL
ZnO-MgO mixed thin films were grown on quartz substrates by dip-coating in a DC4300 instrument (Aiden Co., Ltd.) at a withdrawal rate of 2.0 mm s −1 .The MOD ZnO and MgO coating materials were Zn-05 and Mg-03, respectively (Kojundo Chemical Laboratory Co., Ltd.).The composition of the MOD coating materials are shown in Table I.MgO contents x 1 = MgO/(ZnO + MgO) were 0, 0.020, 0.062, 0.107, and 0.157 in the mixed MOD coating materials.After dip-coating, drying was carried out in vacuum for 10 min at 180 • C. Sintering was performed in air and nitrogen for 60 min (MOTOYAMA MS-2940) at 550 • C at a heating rate of 5 Surface morphologies of the ZnO-MgO mixed thin films were observed using an atomic force microscope (AFM) (JEOL JSPM-5200).The optical properties were evaluated using a UV-VIS-NIR spectrophotometer (JASCO V-670), and the Mg contents x 2 in the films were measured by X-ray fluorescence (XRF) analysis (SHIMADZU XRF-1800).X-ray diffraction (XRD) measurements were performed using CuKα 1 radiation (RIGAKU RINT-2000).

A. Effect of growth condition on surface morphology
Effects on surface morphology of the MgO content x 1 and the sintering atmosphere were investigated.In the samples sintered in air (Figure 1), large crystal grains about 150 nm in size were observed on the film surface when x 1 = 0.020.The observed grain size decreased with further increase in MgO content x 1 .However, the grain size of these films was larger at x 1 = 0.062, 0.107 and 0.157 than at x 1 = 0.In the samples sintered in nitrogen (Figure 2), large crystal grains were observed at x 1 = 0.020 as well as in the sample sintered in air.Although grain size decreased at x 1 = 0.062 and 0.107, large crystal grains appeared again at x 1 = 0.157.Therefore, these variations observed in crystal grain size suggest that the MgO content in the ZnO-MgO mixed MOD coating materials influences the growth mechanism.
The grain size was increased by adding MgO in the ZnO MOD coating material in both air and nitrogen atmospheres, although the observed trend of crystal grain variation is different.Figure 3 shows the magnified AFM images of Figures 1b and 2b.A facet appeared at the surface of a large crystal grain on the sample sintered in air (Figure 3a).On the other hand, the surface of the large crystal grain of the sample sintered in nitrogen reflects the surrounding surface morphology (Figure 3b).The facet may not appear only when the Mg substitutes the Zn site.These results suggest that the rock-salt MgO (RS-MgO) was formed in air atmosphere since Mg reacts with oxygen in air across the entire film surface.http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) e-Journal of Surface Science and Nanotechnology

B. Sintering atmosphere and MgO content dependence of optical properties
The observed dependence of the transmittance spectra on MgO content x 1 is shown in Figure 4.The thicknesses of ZnO-MgO mixed thin films were calculated from the transmittance spectra.The film thicknesses were the almost same regardless of the sintering atmosphere and the MgO content x 1 (about 280 nm).The table inserted in the figure indicates the average transmittance of the thin films in visible (from 400 to 800 nm) and NIR (from 800 to 2500 nm) regions.Higher average transmittance in the visible region was observed on the sample sintered in nitrogen compared to that sintered in air.It is suggested that the light scattering of the sample sintered in nitrogen is small because the grain size is large as compared to that of the sample sintered in air.On the other hand, the sample sintered in air has a higher transmittance in the NIR region compared to the sample sintered in nitrogen.Reflections due to free carriers appear in the NIR region as a threshold value, defined as the plasma frequency ω p .The variation of carrier concentration can be explained by the change in the transmittance in the NIR region.The ω p value can be calculated by solving the motion equation of free carriers based on the Drude theory, which is expressed as follows [13]: where e, N f , m * , and ϵ are the electron charge, free carrier concentration, effective mass, and permittivity, respectively.Hence, the transmittance in the NIR region decreases with increasing carrier concentration.The oxygen vacancy acts as a donor in ZnO [14].In addition, we previously demonstrated that the electron concentration was decreased by the adsorbed atmospheric component at grain boundaries in other oxide thin film [15].Therefore, the comparatively high transmittance of the NIR region for the sample sintered in air suggests that oxidation of oxygen vacancies and the electron traps at grain boundaries decreased the electron concentration.The exciton absorption peak and a blue shift with increasing MgO content x 1 were observed by absorption measurements (data not shown).As shown in Figure 5, the Tauc plot was used to calculate the band gap energy E g using the following equation [16]: where α, hν, and A are the light absorption coefficient, photon energy, and proportionality constant, respectively.The band gap energy (E g ) was calculated as the point where the linear region intersects with the x-axis.Calculated E g is indicated in the table insert in Figure 5.A red shift appeared on the E g of the ZnO film (x 1 = 0), although the E g of ZnO is 3.37 eV.It is considered that this shift is caused by the low film density [17] and the exciton binding energy E X .The absorption peak position of the exciton is expressed as follows: The absorption edge shifted to the high-energy side with http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) increasing x 1 .This shift suggests that ZnMgO can be formed by MOD using ZnO-MgO mixed MOD coating materials.It has been reported that the E g of ZnMgO follows Vegard's law [12].Therefore, the E g of the ZnO-MgO mixed thin film was compared using Vegard's law after measuring precise values of Mg content x 2 in the films by XRF.The Mg contents x 2 were 0.014, 0.046, 0.073, and 0.098 relative to the MgO contents x 1 = 0.020, 0.062, 0.107, and 0.157 in the mixed MOD coating materials, respectively.The present research as well as previous study [11] demonstrated that Mg content x 2 of ZnO-MgO mixed thin film depend on MgO content x 1 in the mixed MOD coating material.The difference between MgO contents x 1 and Mg contents x 2 indicates that the partial MgO is precipitated in the ZnO-MgO mixed MOD ing material.As shown in Figure 6, the E g of ZnO-MgO mixed thin films did not follow Vegard's law.This misfit suggests that Mg rarely substitutes the Zn site in the ZnO-MgO mixed thin film.

C. Film structure of ZnO-MgO mixed thin films
Figures 7a and 7b show the MgO content x 1 dependence on XRD patterns of samples sintered in air and in nitrogen, respectively.Phase separation was not observed in ZnO-MgO mixed thin films since the MgO peak was not observed when the MgO content x 1 increased.The XRD patterns of samples sintered in air exhibit a strong (002) orientation as shown in Figure 7a.On the other hand, random crystalline orientation appeared for the sample sintered in nitrogen.The preferential orientation of ZnO is the (002) orientation.These results indicate that the driving force for growth is insufficient in nitrogen.However, the grain size of the sample sintered in nitrogen is larger when compared to that of the sample sintered in air.
Generally, the full width at half maximum (FWHM) of the XRD peaks narrow with increasing grain size.How-ever, the FWHM of each peak in the XRD patterns for these samples did not change.In addition, the peak positions in the XRD pattern move to a larger angle due to the change of atomic distance because of the substitution of Mg in the Zn site.Nevertheless, this peak position shift was not observed, although ZnMgO was clearly formed due to the variation of E g .These results suggest that the MgO affected the properties of the film surface, which is not thick enough for XRD measurement.The ZnO-MgO mixed thin films were considered to have crystallized from the quartz substrate side out of the thermal environment during the drying and sintering process.Therefore, Mg in the ZnO-MgO mixed MOD coating material segregates by the following equation [18]: where C S , C 0 , k, and l are incorporated impurity concentration in the crystal, initial impurity concentration in solution, equilibrium segregation coefficient and solidification rate, respectively.
From the surface morphology dependence on sintering atmosphere (Figures 1 and 2), the unchanged E g (Figure 6) and the unchanged XRD spectra (Figure 7), the behavior of Mg in the ZnO-MgO mixed thin films is explained as follows.Most Mg segregated to the surface side of the thin film on the sample sintered in both air and nitrogen.RS-MgO was formed by the reaction with oxygen in air atmosphere.In contrast, RS-MgO on the film surface and the MgO peak in the XRD pattern were not observed in the sample sintered in nitrogen.However, Mg substituted few Zn sites since the variation of E g was significantly small when compared with Vegard's law.Therefore, it is speculated that Mg segregates to grain boundaries in the ZnO-MgO mixed thin films sintered in nitrogen.

D. Enhancement of crystal growth
The grain size of ZnO-MgO mixed thin films increased by adding MgO to ZnO MOD coating material.A significant increase of grain size was observed in the sample sintered in nitrogen despite a lack of growth driving force.When the sample was sintered in air, MgO may be formed by reaction with oxygen on the film surface and at grain boundaries.On the other hand, it can be assumed that Mg segregates to grain boundaries when the sample was sintered in nitrogen.It may be concluded that segregated Mg at grain boundaries contributed to the enhancement of crystal growth.
This enhancement can be explained by a previous study wherein Kitahara et al. reported that the lateral growth of Si is enhanced by grain boundary segregation of Ge during the growth of Si 0.7 Ge 0.3 [19].This finding leads to a conclusion that the segregated Ge lowers the grain boundary potential or reduces the Gibbs free energy.Mg in ZnO is considered to behave in a way similar to Ge in Si.Thus, we show that the MOD coating material MgO acts as a growth enhancer for ZnO grown by MOD.

IV. CONCLUSION
In the present study, ZnO-MgO mixed thin films were grown by dip-coating using MOD.Mg segregated to the surface side in the ZnO-MgO mixed thin film, and partially segregated Mg formed ZnMgO.We showed that a wet process could easily form the ZnMgO/ZnO structure.In addition, the grain size of the ZnO-MgO mixed thin films became larger with an increasing amount of the MgO additive.The change in grain size by the intentional introduction of an impurity can be applied to improve the characteristics of other semiconductor materials.

FIG. 4 .
FIG. 4. MgO content x1 dependence of transmittance spectra on samples sintered in (a) air and (b) nitrogen.The inserted table shows the average transmittance in the visible and NIR regions.

FIG. 5 .
FIG. 5. Dependence of photon energy (αhν) 2 for samples with different MgO content x1, sintered in (a) air and (b) nitrogen.The inserted table shows the band gap energy.

TABLE I .
The composition of MOD coating materials.