Ga Ordering and Electrical Conductivity in Nanotwin and

Article pubs.acs.org/crystal

Ga Ordering and Electrical Conductivity in Nanotwin and Superlattice-Structured Ga-Doped ZnO Sang-Won Yoon,†,‡,⊥ Jong-Hyun Seo,†,‡ Tae-Yeon Seong,‡ Tae Hwan Yu,§ Yil Hwan You,§ Kon Bae Lee,*,∥ Hoon Kwon,∥ and Jae-Pyoung Ahn*,† †

Nano Analysis Center, Korea Institute of Science and Technology, Hawolkok-dong, Sungbuk-ku, Seoul 130-650, Korea Department of Materials Science and Engineering, Korea University, Seoul 136-713, Korea § Electronic Materials Lab, Samsung Corning Precision Materials, 644-1 Jinpyeong-dong, Gumi, Korea ∥ School of Advanced Materials Engineering, Kookmin University, Seoul 136-702, Korea ‡

ABSTRACT: We have investigated the Ga-ordering controlled by structural changes from nanotwin to superlattice in Ga-doped ZnO (GZO) targets for transparent conductive oxides (TCOs) and discussed the distribution effect of Ga atoms on electrical conductivities of GZOs. The nanotwin and superlattice structures were preferentially formed by Ga-doping and sintering at high temperature. The relative fraction of nanotwin increased above transition concentration (TC ≈ 5.6 wt % Ga). Here, we found that Ga atoms at nanotwin are distributed as clustered and disordered states, while they are completely ordered in superlattice. Ultimately, the superlattice leads to high electrical conductivity in GZOs rather than the nanotwin. last two decades and now have values comparable with ITO.1−9 In addition, a notable fact is that the minimum resistivity of impurity-doped ZnO films is still decreasing, whereas those of impurity-doped SnO2 and In2O3 films have essentially remained stagnant for more than the past twenty years.5 In order to breakthrough this situation, it is very important to understand the electrical and transport properties of doped ZnO films, which is critical for further improvement of TCO characteristics. It is well-known that the conductivity of TCOs can be improved by increasing the carrier concentration and Hall mobility. Although impurity addition (such as Ga and Al atoms) in ZnO increases the carrier concentration and conductivity, many researches reported that there is an optimum dopant concentration for the improvement of conductivity in the impurity-doped ZnO, which was dependent on the film deposition method as well as the dopant used.4,8−13 In the case of highly conducting TCOs, both extrinsic and intrinsic donors are known to affect the carrier concentration and the conductivity, but the exact nature and interdependence

1. INTRODUCTION Transparent conducting oxides (TCOs) have found applications in several optoelectronic devices such as light emitting diodes (LEDs), organic light-emitting-diodes (OLEDs), solar cells, touch panels, and flat panels as well as flexible displays.1−6 Currently, indium tin oxide (ITO) is used for most of the TCO applications because of its high transmittance in the visible region and resistivity close to 1.0 × 10−4 Ω cm.4−9 As a result, because market demand of indium for transparent electrode applications has been dramatically expanding in recent years, this situation brought about serious concerns on the high cost and scarcity of indium. In addition to the depletion of indium resources, the toxicity of indium compound powders has been recently reported.2,5 Therefore, many researchers have attracted much attention on research to develop substitute materials for ITO transparent electrodes and/or to develop technology for reducing indium usage in transparent electrodes.1−9 Among possible substitute materials, impurity-doped ZnO materials have recently attracted much interest as a promising alternative to ITO because of their several advantages including (a) scarcity of indium, (b) low cost (resource availability), (c) improved thermal and chemical stability, (d) lack of toxicity, and (e) process integrability.1 As a result, the electrical and optical properties of ZnO as a TCO have improved over the © 2012 American Chemical Society

Received: August 19, 2011 Revised: November 30, 2011 Published: January 3, 2012 1167

dx.doi.org/10.1021/cg2010908 | Cryst. Growth Des. 2012, 12, 1167−1172

Crystal Growth & Design

Article

Figure 1. SEM images of GZOs with the Ga-doping concentration of (a) 0, (b) 2.7, (c) 5.6, and (d) 6.6 wt %.

of these entities is far from understood.4 For further improvement of TCO characteristics, correct understanding on the electrical transport properties of doped ZnO films is necessary and critical. As we know, however, most research for the resistivity improvement with dopant addition in TCO materials has been focused on the fabrication method and condition. In addition, almost all studies have been mainly discussed in terms of carrier concentration and hall mobility to explain the mechanism responsible for resistivity variation. However, the resistivity of the impurity-doped ZnO materials is related to the doping concentration, intrinsic defects (e.g., Zn interstitials and O vacancies), respective concentrations of impurity atoms at substitutional and interstitial sites, and various scattering mechanisms.8 In recent, several researches reported on the relationship between electronic transport property as well as optical property and structural defects (twins, stacking faults, etc.) in various types of materials, such as bulk, thin film, and nanostructure.10−32 Nevertheless, in the case of Ga-doped ZnO materials, the internal structural parameters have not been well-known compared to other materials for TCOs applications. Furthermore, details concerning crystal defects such as point defects and twin boundaries that dominate carrier scattering as well as the relationship between defects and scattering mechanisms have to be investigated sufficiently. Therefore, we have tried to elucidate the effect of Ga concentration on resistivity of the GZO using a new approach different from previous studies, namely, the structural point of view. We fabricated sintered GZO targets as a standard sample via the conventional sintering process, and a systematic study of structural and electrical properties of GZO materials as a function of Ga content has been discussed in detail.

mixture of ZnO and Ga2O3 powders at the sintering temperature of 1200−1400 °C for 7 days. For the microstructural observations as well as electrical measurements, the sintered GZO targets were mechanically polished using 0.25 μm Al2O3 slurry and were chemically etched by the etchant of 36% HCl and distilled water (7:3). The overall microstructures were observed using SEM (Environmental SEM, FEI Co.). The GZO structures were characterized by high resolution X-ray diffraction (HRXRD, 8C2 beamline at PAL) and TEM (Titan 80-300, FEI Co.). Especially, the crystal structure of the nanotwin and superlattice was identified using the rotation holder (650 single tilt rotation analytical holder, Gatan Co.), which makes various tilting and rotation at a GZO grain possible. We applied Titan Cs-corrected probe to the precise element analysis at the atomic scale, with a spatial resolution of 0.07 nm in STEM mode. In addition, the nanotwin and superlattice were investigated from various zone axes using the Kikuchi map drawn by Electron Diffraction (ED) program. The electrical properties of GZO targets were measured using the van der Pauw technique on the sample surface of a size of 5 × 5 × 1 mm3 at room temperature.

3. RESULTS AND DISCUSSION 3.1. Structural Characterization of GZOs. In GZO targets sintered by the mixed powder of ZnO and Ga2O3, the addition of Ga atoms led to the dramatic change of grain structure as shown in Figure 1. While the size and shape of grains in GZOs is significantly changed from a polygon of 20− 50 μm to a rectangle of 2−7 μm up to 2 wt % Ga addition, the microstructural change is negligible above 5.6 wt % Ga. The reduction in the grain size with increasing Ga concentration might be due to the interstitial inclusion of the Ga atoms as suggested by the previous reports.9 Figure 2 shows the XRD spectra of the GZO targets with different concentrations of Ga. All diffracted peaks of undoped ZnO were completely matched with wurtzite ZnO structure of JCPDS (80-0075). The peak width of GZOs is slightly broader than that of undoped ZnO, indicating a lattice distortion due to the introduction of Ga atoms. In addition, the peak shift in

2. EXPERIMENTAL SECTION The GZO containing up to 6.6 wt % Ga were prepared by Samsung Corning Precision Materials Co., Ltd., which were sintered from the 1168



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Figure 2. XRD patterns of GZOs with the Ga concentration of 0, 5.6, 6.1, and 6.6 wt %. The diffracted positions of ZnO in JCPDS data for the reference are present at the bottom.

Figure 4. Simulated Kikuchi map and SAD patterns, which were acquired at various zone axes that are aligned after tilting a superlattice-structured grain of 5.6 wt % Ga doped ZnO. The superlattice structure was found at different zone axes of [001], [011], [111], [211], and [101]. A representative HRTEM image showing superlattice is also observed at the zone axis of [211].

GZO targets occurred within 0.15%, meaning the evolution of extremely small strain. Interestingly, small extra peaks shown as arrows in XRD spectra are observed in GZO different from undoped ZnO, which those peaks were not reported in similar GZO materials so far. By TEM characterization in Figures 3

microstructural observation in detail via HRTEM and selected area diffraction (SAD) pattern analyses. At first, the twin system generated in GZOs was investigated from SAD patterns acquired after tilting the zone axis of a grain, which were assisted by a Kikuchi map drawn from the Electron Diffraction (ED) program as shown in Figure 3. By tilting a grain based on the Kikuchi map, we can change a specific zone axis showing nanotwins to other zone axes, which can effectively identify a twin system. Crystallographically, in order to image twins by TEM observation, incident electron beam should be aligned parallel to the twin plane (TP). Therefore, twins can be observed by tilting parallel to specific TP, while they cannot be observed when the incident beam direction is not parallel to the TP. Figure 3 displays a Kikuchi map showing a (−113) TP of ZnO. When a grain was tilted to the various zone axes of [2-11], [411], and [932] parallel to this TP (as marked by A direction), twin structure was always observed. It is clearly seen that bright field images of twin observed at the above zone axes show alternating boundaries parallel to the TP, revealing that twin boundaries (TBs) appear atomically sharp and as separate twinned regions (twin lamellar width of 4−6 nm). However, when same grain was tilted in the B direction, the twin was not observed as shown at the zone axis of [211] any more. It means that GZOs have a TP of {−113}. In addition, a shearing direction of nanotwins was [21-35], which is calculated at the zone axis of [411] showing the largest twin angle. Finally, we would set a twin system of {113}⟨2 13 5⟩ in GZOs. The TP observed in this work is totally different from those reported in wurtzite structures so far, which are known as a plane of {011}, {012}, and {013}.14−18 While theoretical calculations revealed that the {013} twin has a relatively low energy, this plane was not observed in this study.14,15 Ding et al. reported the formation of (−212) TP in ZnO nanobelt as well as three types of TP above referred.16 As previously mentioned, although there have been a few reports about the internal structure of GZO, it has not been well-known so far. Juarez et al. reported natural formation of TBs in GZO6 using first-principles methods.19 They suggested that the location of the Ga agglomerates play a very important role in

Figure 3. Simulated Kikuchi map and HRTEM images, which were acquired at various zone axes that are aligned after tilting a twinstructured grain of 5.6 wt % Ga doped ZnO. Each inset are the SAD patterns corresponding to the HRTEM images.

and 4, it revealed that extra peaks are related to the structural change due to the formation of twin and superlattice with the addition of Ga atoms. It is obviously attributed to the diffraction of superlattice because the twin cannot be distinguished from the matrix by X-ray diffractions, when twin and superlattice coexist in a sample. Therefore, the existence of extra peaks can imply the formation of superlattice in Ga-doped GZOs. In order to identify the structural evolution with Ga addition, we have performed a systematic study of 1169



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the formation of the polarity inversion and hence in the twinboundary formation. In addition, Chunfei et al.20 and Barf et al.18 reported experimental results on the formation of twin structure in sintered body of the GZO, similar to our study. Especially, twin planes existing in nanostructural materials can significantly affect mechanical properties as well as their effects on electrical and optical properties.17,21−26 Although a few studies reported the effect of twin structure on the resistivity in the materials containing twins, its effect has not been wellknown so far. We will discuss the correlation between twin structure and resistivity in GZO later. Figure 4 shows a HRTEM image and SAD patterns obtained at various zone axes, tilted in a same grain using the tilting method similar to Figure 3. Here, extra spots between two adjacent main spots are evidently observed in each SAD pattern. It means that a superlattice structure was formed by Ga doping in GZOs as shown in a HRTEM image and XRD results in Figure 2. Such extra spots are also observed at the various zone axes of [001], [011], [111], [211], and [101] irrespective of tilting direction, which are different from the case of nanotwins. The difference in the number of extra spots indicates that Ga atoms are positioned with the different period along each crystal direction in order to minimize shear stress induced by Ga doping. The formation of superlattices means that Ga atoms are completely distributed as the atomically ordered state in GZOs. Although most of the superlattice structures are created through an artificial process, the natural superlattice structures are also observed in some impuritydoped ZnO (such as IZO, AZO, and IAZO) compounds as well as pure ZnO.27−32 Recently, thin films and onedimensional materials with a periodic superlattice structure have attracted considerable interest because these types of compounds have demonstrated novel optical and electrical properties. In addition, according to the authors’ knowledge, this is the first time that superlattice structures have been observed in GZO materials. We will discuss the correlation between resistivity and superlattice as well as twin structure in GZO later. A natural question conceivable from Figure 3 and 4 is whether the nanotwin and superlattice can coexist within the same grain. Although we tried to find the twin by tilting a zone axis in a grain showing the superlattice, we could not observe twin structure and vice versa. This means that both structures independently exist in each grain. 3.2. Ga Distribution and Electrical Characterization in Nanotwin and Superlattice. The formation mechanism of nanotwins caused by Ga addition in pure ZnO was investigated in detail by TEM observation. Figure 5 shows a low magnified TEM microstructure of 2.2 wt % GZO specimen, showing twins in a grain (as marked arrows). By energy dispersive spectroscopy (EDS) measurements, Ga was not detected in the GZO matrix, where twins are not observed, but the twin region contained the Ga concentration of 2−3%. Ga atoms are physically doped in thin film fabricated by the sputtering process but are thermally doped by the diffusion process during sintering in this work. Ga atoms are concentrated at certain regions diffused from Ga rich regions in the mixture powders of ZnO and Ga2O3 at about 1200 °C. Here, the nanotwins may be formed due to the lattice distortion and shear stress induced by Ga diffusion into ZnO lattice. However, if the sintering temperature increases up to 1400 °C, nanotwins are dissolved, and Ga atoms are redistributed in the matrix by a thermally activated process, finally resulting in the formation of superlattice. These results showed that it is energetically

Figure 5. Low magnification TEM image observed at 2.2 wt % GZO.

more favorable to dissolve nanotwins because of the high solubility of Ga atoms at high temperature and to form the Gaordering in the superlattice-structured GZOs. As abovementioned, although superlattice structures were observed in impurity-doped ZnO materials, their formation mechanism is unclear so far. Recently, Gao et al. reported a superlattice structured nanohelix of ZnO and suggested that reducing the polar surfaces could be the driving force for forming the superlattice structure.30 In order to elucidate the effects of nanotwin and superlattice on the electrical conductivity of GZOs, we measured the electrical conductivity of sintered ZnOs as a function of Ga concentration as shown in Figure 6. Electrical properties were

Figure 6. (a) Variations of resistivity measured by van der Pauw method and the fraction of twin and superlattice-structured grains, which is counted by SAD characterizations and by HRTEM observations from about a hundred grains, as a function of Ga concentration. (b) HRTEM image shows a nanotwin structure observed at a zone axis of [411] and a Ga elemental line profile measured at twin matrix and twin boundary by EDS in 2.2 wt % GZO.

measured from the samples with the sintered density above 99% in order to exclude the effect of sintered density. In addition, relative fractions of twin-to-superlattice in each sintered GZO were also superimposed in Figure 6. Both results are summarized in Table 1. The resistivity begins to decrease dramatically from 1.26 Ω cm to 4.574 × 10−3 Ω cm by the addition of 2.2 wt % Ga in pure ZnO and then, is gradually reduced up to 5.6 wt % Ga, at which the resistivity shows the lowest value of 3.259 × 10−3 Ω cm. The Ga concentration above 5.6 wt % resulted in an opposite electrical trend, increasing again up to 1.246 × 10−2 Ω cm in resistivity. It means that there is a transition concentration (TC), leading to the change of electrical behavior as a function of Ga concentration in nanotwin and superlattice-structured GZOs. Although TC 1170



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properties because of the reduced mobility of charge carriers.17,19−25 However, the distribution of Ga atoms in superlattice-structured GZO grains is perfectly uniform. Because Ga atoms are mainly distributed at TBs compared to superlattice structure, TBs may prevent electron transport by high electron localization, resulting in the reduction of the resistivity of the GZO. In contrast to the twin structure, the superlattice structure played an important role to enhance the electrical conductivity of GZO because crystallographically perfect Ga ordering makes homogeneous electron delocalization possible. So far, although a few researchers separately reported the effect of twin and superlattice on the resistivity in various materials, their combined contribution to the resistivity is not disclosed. Therefore, our research is the first meaningful report that specific structural features can affect differently the electrical properties of the GZO. Furthermore, because these structural features may significantly have an impact on the mechanical, optical, and electrical properties of nanosized materials, further information on these issues will be required to develop the future nanostructure-based devices. We anticipate that the results of this research will help to understand the electrical behavior depending on the specific structure generated by impurity doping.

Table 1. Resistivity of GZOs As a Function of Ga Concentration and the Normalized Fraction of Twin and Superlattice-Structured GZO Grainsa resistivity (Ω cm) pure ZnO 2.2 wt % GZO 5.6 wt % GZO 6.1 wt % GZO 6.6 wt % GZO

1.26 4.57 3.26 5.60 1.25

× × × ×

10−3 10−3 10−3 10−2

pure (%)

twin (%)

superlattice (%)

100 33.6 3.7 2.9 2.0

0 3.4 22.8 26.2 49.5

0 63.0 73.5 70.9 48.5

a

The pure ZnO means a Wurtzite structure of original undoped ZnO, which does not contain the twin and superlattice structure.

depends on the deposition method and dopant type in impurity-doped ZnO, many researchers reported the existence of TC for the minimum resistivity was obtainable with dopant concentration.4,8−13 Both the carrier concentration and mobility increased continuously as the dopant concentration was increased up to TC. However, when the dopant concentration was increased beyond TC, the carrier concentration saturated, but the carrier mobility decreased. Namely, beyond TC, the dopant atoms form into neutral defects by entering in the interstitial sites and become ineffective as dopant atoms.4,8−13 At high doping concentrations, ionised impurity scattering starts to dominate, and this leads to a reduction in the carrier mobility with a consequent reduction in the electrical conductivity of the films. However, there were no reports on experimental evidence elucidating these entities. From structural observation in this study, we could identify the formation of nanotwin and superlattice in GZO, which were not formed in undoped ZnO. In order to find the relationship between the variation of resistivity and structural change (i.e., nanotwin and superlattice), we investigated relative twin-to-superlattice ratios from SAD patterns analysis of more than one hundred at each sample, which was plotted together on the right y axis in Figure 6a. It is clearly seen that the relative fraction of nanotwin and superlattice is oppositely changed to each other with the Ga concentration. As the Ga concentration increases up to 5.6 wt % Ga (TC) showing the minimum resistivity, the fraction of superlattice increases, while the fraction of nanotwin decreases. However, the fraction trend is changed by the Ga addition beyond TC. Interestingly, it reveals that the change of resistivity is strongly related to the relative fraction of nanotwin and superlattice-structured grains as shown in Figure 6a. In order to examine the negative effect of twins on the resistivity of the GZO, we investigated Ga distribution in a nanotwin using EDS. Figure 6b shows a HRTEM image and a line profile of Ga element measured by EDS at the same area. It is clearly seen that the Ga concentration periodically changes between twin boundaries (TBs). The Ga concentration of the TBs is much higher than that of twin matrixes. Until now, there are contradictory reports on Ga concentration of the twin region in GZO.18−20 Barf et al. reported that the Ga contents of the twin matrix were approximately twice as high as that of the TBs using energy-filtering transmission electron microscopy (EFTEM).18 However, Chunfei et al.20 and Juarez et al.19 reported opposite results, which were measured by EDS and simulation, respectively. These results indicate that the distribution of Ga atoms in twin-structured GZO grains is nonuniform, which was also identified in this study. It was suggested that TBs can significantly affect the electronic

4. CONCLUSIONS We revealed that significant nanotwin and superlattice structures were formed by Ga addition in ZnO through XRD and TEM investigations of GZO targets sintered at 1400 °C. Nanotwins in GZOs had a twin system of {113}⟨2 13 5⟩ and Ga distribution in the superlattice had various periods along crystal directions. Ga atoms doped below TC were fully dissolved in ZnO matrix and preferentially formed a superlattice. As the superlattice structure makes the enhancement of electrical properties possible, the highest conductivity was accomplished at 5.6 wt % Ga of which the GZO contains the maximum fraction of superlattice. Above TC, however, the fraction of nanotwin increased due to the undissolved excess Ga atoms. The nanotwins led to the relatively low conductivity by forming the Ga cluster and disordering. These results suggest that the Ga-ordered superlattice in GZOs is more preferred for the higher electrical conductivity, which can be evaluated as new criteria to interpret interrelations between structural change and electrical properties in GZOs.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.-P.A.); [email protected] (K.B.L.). Present Address ⊥

Electronic Materials Lab, Samsung Corning Precision Materials, 644-1 Jinpyeong-dong, Gumi, Korea.

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ACKNOWLEDGMENTS This work was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea, by the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology, by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and 1171



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Technology (2009-0093814), and also by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea Government (MEST) (R11-2005-048-00000-0, ERC Program, Center for Materials and Processes of Self-Assembly), and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0075739).

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REFERENCES

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Ga Ordering and Electrical Conductivity in Nanotwin and

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