InGaO3(ZnO)m Thin Films with Layered Structure

We fabricated the crystallized InGaZnO thin films by sol-gel process and high-temperature annealing at 900C. Prior to the deposition of the InGaZnO, ZnO buffer layers were also coated by sol-gel process, which was followed by thermal annealing. After the synthesis and annealing of the InGaZnO, the InGaZnO thin film on the ZnO buffer layer with preferred orientation showed periodic diffraction patterns in the X-ray diffraction, resulting in a superlattice structure. This film consisted of nanosized grains with two phases of InGaO


Introduction
The complex materials based on PbTe and SiGe have been considered for thermoelectric application due to high ZT values at mid-(500-900 K) and high-(>900 K) temperature range.Recently, it was reported that nanosized inclusions (in bulk system) or nanocomposites (in low-dimensional system) in host material enhance dimensionless thermoelectric figure of merit, ZT =  2 / (S: Seebeck coefficient, : electrical conductivity, T: temperature, and : thermal conductivity).There are two principle reasons for ZT increase.First, Seebeck coefficient is remarkably increased by energy filtering for electrons or holes with low energy.Second, thermal conductivity is decreased due to more frequent phonon scattering by new generated interfaces, which is the main reason for the dramatic ZT increase [1].In PbTe based materials, it was proven that nanoinclusions related to Pb, Sb, and Bi decrease thermal conductivity by forming numerous interfaces in PbTe matrix, theoretically [2] and experimentally [3].In addition, p-type SiGe nanocomposite synthesized by sintering nanopowder also showed a half of thermal conductivity ( nanocomposite = ∼2.3Wm −1 K −1 at 1000 K) compared to that ( bulk = ∼4.5 Wm −1 K −1 at 1000 K) of SiGe bulk due to new generated grain boundaries [4].The oxide thermoelectric materials such as Na 0.75 CoO 2 [5], Ca 3 Co 4 O 9 [6], and SrTiO 3 [7] have been researched for thermoelectric application at high temperature due to high chemical and thermal stability as well as nontoxic elements.
Over the past ten years, there has been a rising amount of interest in fabricating RAO 3 (MO)  (R = In and rare earth elements; A = Ga, In, Al, and Fe; M = Mg, Co, Cu, and Zn; m = integer) superlattice structures due to their unexpected physical properties by quantum confinement effects [8,9].Thus, a multilayered oxide material with long-range periodic atomic arrangement shows exceptional properties, such as high field-effect mobility in thin-film transistors (TFTs) [10], giant magnetoresistance in ferromagnetic materials [11], and extremely high Seebeck coefficient as well as low thermal conductivity in thermoelectric materials [7].Hosono group first suggested the InGaO 3 (ZnO)  film with multilayered structure on sapphire substrate by reactive solid-phase epitaxy method based on vacuum process [12].In addition, the InGaZnO TFT using the InGaO 3 (ZnO) 5 superlattice grown by reactive solid-phase epitaxy as a channel layer exhibited high field-effect mobility (∼80 cm 2 V −1 s −1 ) [10].As a similar approach, our group produced the InGaO 3 (ZnO)  superlattice film using advanced process based on solution process for thermoelectric application [13], because Dresselhaus et al. proposed that multilayered structure (superlattice) can remarkably improve thermoelectric properties [1].
In this study, an all-solution process for fabricating InGaZnO superlattice structure was developed based on the chemical reaction of InGaZnO solution and reactive solid phase epitaxy growth proposed by the Hosono group [12].ZnO films were first deposited on sapphire substrates by solgel coating and then annealed at 900 ∘ C for 2 hours.Repeated coating processes led to the formation of ZnO thin film preferentially oriented along the c-axis.Then, amorphous InGaZnO was deposited on ZnO buffer by sol-gel coating and annealed at 900 ∘ C for 9 hours.This study is the first report on the synthesis of oxide thin films with superlattice structure based on all-solution method.

Experiments
The deposition of the ZnO buffer layer and amorphous InGaZnO layer was performed by sol-gel method.Indium nitrate hydrate [In(NO 3 ) 3 ⋅H 2 O, Aldrich], gallium nitrate hydrate [Ga(NO 3 ) 3 ⋅H 2 O, Aldrich], and zinc acetate dehydrate [Zn(CH 3 COO) 2 , Aldrich] were used as the metal precursors, and 2-methoxyethanol (2ME, C 3 H 8 O 2 ) and monoethanolamine (MEA, C 2 H 7 NO) were employed as the solvent and stabilizer, respectively.First, ZnO buffer layers were prepared on sapphire substrates by repeated spin coating processes.The solution mixture consisted of zinc acetate dehydrate, 0.2 M 2ME and 0.3 M MEA, and was processed in a glove box under a nitrogen atmosphere (O 2 and H 2 O < 10 ppm).After constant magnetic stirring at 400 rpm on a 70 ∘ C hot plate for 1 hour, stable ZnO sol was spin-coated on sapphire substrates at 3000 rpm for 30 s and dried at 250 ∘ C for 10 min.The coating process was repeated several times to achieve proper thickness, and postannealing was performed at 900 ∘ C for 2 hours for the crystallization of the ZnO film.Next, the zinc acetate, indium nitrate, and gallium nitrate were blended at a ratio of 1 : 1 : 1 and spin-coated on the ZnO buffer layer for the formation of the InGaZnO thin films, which was followed by postannealing at 900 ∘ C for 9 hours.The overall process for fabricating the InGaZnO thin films is schematically presented in Figure 1.This approach is the same as the one from our previous report [13] except for the synthesis of ZnO seed layer.
Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were performed at temperatures ranging from room temperature to 400 ∘ C with a heating rate of 8 ∘ C/min in air for regulating the chemical reaction temperature.The crystal phase of the crystallized InGaZnO thin films was investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM).The surface morphology of the buffer layers was observed by field-emission scanning electron microscopy (FE-SEM).

Results and Discussion
The TG-DTA results of InGaZnO sol are shown in Figure 2.There are two stages that can be seen at 55-110 ∘ C and 230-250 ∘ C. The first weight loss at 55-110 ∘ C indicates that Zn acetate, In nitrate, and Ga nitrate were decomposed and hydrolyzed to Zn-OH, In-OH, and Ga-OH.The second weight loss at 230-250 ∘ C indicates dehydroxylation reaction of M-OH to M-O (M = Zn, In, Ga), and most organic products such as 2ME, CH 3 COOH, and HNO 3 , as well as H 2 O, were evaporated.The overall chemical reaction of the metal precursors for the formation of Zn-O, In-O, and Ga-O was explained in previous research [14].The coating process was established with TG/DTA results, and hightemperature postannealing was performed for crystallization of these films.For the coating of the ZnO buffer layers, different solute concentrations and repetitions were used to induce different surface morphologies and to achieve proper buffer thicknesses, respectively.As shown in Figure 3(a), the solute concentration of 0.5 M with 2 coatings induced a rough surface with island shape, due to inhomogeneous Zn-O distribution coming from excessive solvent concentration.On the other hand, the solute concentration of 0.2 M with 5 coatings led to a homogeneous and flat surface (Figure 3(b)), after post annealing at 900 ∘ C for 2 hours similar to previous research [15].In addition, the flat surface of the ZnO buffer layer was closely related to the strong c-axis preferred orientation.A more intense ZnO (0002) diffraction peak was observed in the InGaZnO thin film/flat ZnO buffer structure than in the InGaZnO thin film/rough ZnO buffer structure, as shown in Figures 4(b) and 4(c).
Through the solution-based synthesis of the ZnO and InGaZnO and thermal annealing, the crystallized InGaZnO thin films were grown on sapphire substrates with the help of the ZnO buffer layer with high c-axis preferred orientation.Figure 4 shows XRD patterns of InGaZnO thin films before and after annealing at 900 ∘ C for 9 hours on different ZnO buffer layers, where the InGaZnO thin film without ZnO buffer was used for comparison.In our study, the ZnO layer oriented along the c-axis acts as a seed layer for the enhanced crystallization of the InGaZnO thin films and finally disappears due to the complete intermixing of the ZnO and InGaZnO layers.The intermixing can be confirmed by no sign of the ZnO diffraction peaks after thermal annealing, as shown in Figures 4(e) and 4(f).As confirmed in Figures 4(a   and 4(d), the sample without ZnO buffer shows faint crystallized diffraction patterns that are indexed as GaO, GaZnO, and so forth and are not related to any InGaZnO crystals.This indicates that higher annealing temperature is required for the crystallization of the InGaZnO thin film.Also, a similar result was observed in the sample with rough ZnO buffer layer (or less c-axis preferred orientation), as shown in Figure 4(e).However, it is noticeable that many InGaZnOphases-related peaks were observed from the InGaZnO thin film on the ZnO buffer layer with strong c-axis preferred orientation, as shown in Figure 4(f).From the XRD data and JCPS cards (nos.38-1104 and 40-0252), the 2Θ peaks at 10.2, 20.4, and 30.9 ∘ correspond to the (0003), (0006), and (0009) planes of the InGaO 3 (ZnO) 1 crystal phase, respectively, and the 2Θ peaks at 15.8, 23.8, and 31.9 ∘ are in accordance with the (0004), (0006), and (0008) planes of the InGaO 3 (ZnO) 2 crystal phase, respectively.This result indicates that the ZnO buffer layer with c-axis preferred orientation promotes the crystallization of the InGaZnO thin films, and significantly reduces the crystallization temperature [16].
Figures 5(a)-5(c) show cross-sectional bright-field (BF) TEM image, selective area diffraction pattern (SADP), and high-resolution transmission electron microscopy image obtained from InGaZnO thin film using ZnO buffer layer with strong c-axis preferred orientation, respectively.The cross-sectional BF TEM image revealed that the InGaZnO film consists of nano-size grains with the dimension of 100∼ 150 nm, and the film is polycrystal.The nano-size grains have the width of ∼150 nm and the height of ∼100 nm.As shown in Figures 5(b We had already proven the effectiveness of the InGaO 3 (ZnO)  superlattice structure for improved thermoelectric properties [17], which was attributed to frequent phonon scattering at the interface of InO (0010) frequent phonon scattering at the surface of nanograins as well as many interfaces in each InGaO 3 (ZnO)  superlattice nanograin, as shown in Figure 5(d).In previous report by Seo et al. [17], the polycrystalline InGaZnO thin film fabricated by sputtering exhibited high thermal conductivity (7.53 Wm −1 K −1 at 300 K).In addition, thermal conductivities of ZnO-based materials were also quite high (2∼10 Wm −1 K −1 ) [18][19][20].On the contrary, the polycrystalline InGaZnO thin film with superlattice nanograins fabricated by all-solution process showed considerably low thermal conductivity (1.14 Wm −1 K −1 at 325 K and 0.99 Wm −1 K −1 at 355 K).

Conclusions
Semiconducting ZnO-based oxide thin films had been developed for various electronic and optoelectronic applications such as transparent TFTs, optical and chemical sensors, and thermoelectrics.These applications can be improved by using layered structures as an active layer.In this study, InGaZnO thin films were fabricated by an all-solution process.Unlike the single phase InGaO 3 (ZnO)  observed in InGaZnO thin film using a epitaxial ZnO buffer grown by vacuum deposition process, the solution-processed ZnO buffer layer with preferred orientation along the c-axis resulted in polycrystalline InGaZnO film formed with the crystal grains of two phases, such as InGaO 3 (ZnO) 1 and InGaO 3 (ZnO) 2 .We also obtained extremely low thermal conductivity (1.14 Wm −1 K −1 at 325 K).This was due to frequent phonon scattering at the surface of nanograins as well as the interfaces of the InGaO 3 (ZnO)  superlattice. )

Figure 1 :
Figure 1: The overall fabrication process of the crystallized InGaZnO thin films with nanosize InGaO 3 (ZnO)  superlattice grains, which consisted of sol preparation, spin coating, and thermal annealing.

Figure 2 :
Figure 2: Thermogravimetric analysis and differential thermal analysis curves of the InGaZnO sol.
) and 5(c), SADP and HRTEM image show the existence of InGaO 3 (ZnO)  superlattice along the vertical direction, which is equivalent with XRD data.One nanograin in the bottom region of cross-sectional HRTEM image certainly shows InGaO 3 (ZnO)  superlattice in polycrystalline InGaZnO film.

Figure 4 :
Figure 4: XRD patterns of InGaZnO thin films (a)-(c) before and (d)-(f) after annealing on different buffer layers.(a) and (d) without ZnO buffer, (b) and (e) randomly oriented ZnO buffer layer, and (c) and (f) preferentially oriented ZnO buffer layer.

Figure 5 :
Figure 5: (a) Cross-sectional bright-field TEM image, (b) selective area diffraction pattern (SADP) and (c) HRTEM image obtained from InGaO 3 (ZnO)  thin film on ZnO buffer layer with strong c-axis preferred orientation.(d) Frequent phonon scattering at the surface of nano-grains as well as the interfaces of the InGaO 3 (ZnO)  superlattice.