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aPhys. Status Solidi C 6, No. 2, 596–598 (2009) / DOI 10.1002/pssc.200880406

Fabrication of MgZnO films by molecular precursor method and their application to UV-transparent electrodes

Yoshihiro Mashiyama1, Kaori Yoshioka1, Shigetoshi Komiyama1, Hirohisa Nomura2, Shunsuke Adachi2, Mitsunobu Sato2, and Tohru Honda*,1

1 Department of Electronic Engineering, Kogakuin University, 2665-1 Nakano-machi, Hachiohji, Tokyo 192-0015, Japan 2 Coordination Engineering Laboratory, Kogakuin University, 2665-1 Nakano-machi, Hachiohji, Tokyo 192-0015, Japan

Received 8 August 2008, revised 24 October 2008, accepted 24 October 2008

Published online 21 November 2008

PACS 61.05.cp, 72.80.Ey, 73.61.Ga, 78.60.Fi, 81.05.Dz, 81.15.Lm

* Corresponding author: e-mail ct11761@ns.kogakuin.ac.jp, Phone: +81 426 22 9291, Fax: +81 425 8982

© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Recently, transparent electrodes such as those of indium tin oxide (ITO), which is one of the transparent conductive oxides (TCO), are attracting much attention for the application to flat-panel displays (FPDs). UV transparent electrodes are also promising for the realiz-ing high light-extraction efficiency in GaN-based UV light-emitting diodes (LEDs). However, the conventional indium tin oxide (ITO) is not suitable for the UV transpar-ent use. The ZnO-based transparent electrode is one such candidate for a near UV spectral region [1]. The molecular precursor method (MPM) is a spin-coating technique [2] that is suitable for the cost-effective fabrication of those electrodes [3]. Metal-organic complexes are used for the starting materials for MPM. After the spin coating, metal nta and edta release metals at RT and those are oxidized during the annealing. This mechanism is different from the spray pyrolysis [4]. The organic parts after the metal re-leasing are oxidized and evaporated. The merit of MPM is the flexibility of the mixture of metal-organic materials if their common solvent is available. Although the classical absorption edge of ZnO is 3.37 eV [5], this electrode is in-

sufficient for the GaN-based LEDs operating in UV spec-tral regions. However, there are few reports concerning the fabrica-tion of MgZnO films by MPM. In this paper, the fabrica-tion of MgZnO films by MPM is reported. Their transpar-ent spectra and the solid phase composition of Mg in the film are also discussed in view of transparent electrodes. The possibility of their application to UV GaN-based LEDs is also discussed 2 Experiments The base solution was prepared by the reaction of Zn-nitrilotriacetic acid (nta) complex, which was obtained from the reacted NTA aqueous solu-tion with zinc acetate, with butylamine in ammonia aque-ous. The dosed solution was prepared by the reaction of Mg-ethylenediamine-N,N,N’,N’-tetraacetic acid (edta) complex. Then, the dosed solution was mixed into the base precursor solution. Here, the amount of the dosed solution (Mg-precursor) was controlled. Thus, a coating solution for MgZnO was obtained. The clear solution was spin-coated onto quartz glass (1st 3000 rpm · 3 s /2nd 4000 rpm · 5 s).

MgZnO thin films were fabricated by the molecular precursor

method (MPM) for the realization of cost-effective near-

ultraviolet (UV) transparent electrodes for GaN-based UV

light-emitting diodes (LEDs). The fabrication by MPM re-

quires a common solution. It was clarified that ammonia

aqueous is a common solvent for Mg, Zn and Ga precursors.

MgZnO thin film was successfully fabricated on a quartz

glass substrate using MgZnO precursor solution. The solid

phase composition of Mg in the film is 40-50% of its molar

fraction in the liquid phase. The X-ray diffraction patterns in-

dicate the films have a hexagonal single phase, the same as in

the case of ZnO. Ga doping of the MgZnO films enables their

resistivity control. The possibility of applying MgZnO films

for UV transparent electrodes on GaN-based UV LEDs is

discussed.

Phys. Status Solidi (c) 6, No. 2 (2009) 597

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The precursor films were dried at 70 °C for 10 min and then fired at 600 °C for 90 min. The 3rd annealing atmos-phere was N2 (3rd 600 °C · 3 h). These procedures are summarized in Fig. 1. The films were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transparent spectra. The resistivity was measured using a conventional four-probe method. Tungsten needles were used as the probes. 3 Results 3.1 Fabrication of MgZnO films A common sol-vent for Zn-nta and Mg-edta was required for the fabrica-tion of MgZnO films by MPM. Some solvents such as ethanol, methanol and ammonia aqueous were tested and we found that ammonia solution is suitable. The transpar-ent spectra of MgZnO films fabricated by MPM are shown in Fig. 2. With increasing molar fraction of Mg sources, the excitonic absorption peak shifted towards the higher energy side. This indicates that the deposited films were MgZnO alloys. The XRD spectra of MgZnO films fabri-cated by MPM are shown in Fig. 3. The XRD patterns show that the structure of the deposited films is hexagonal and polycrystalline. No diffraction peaks from MgO and other materials are observed. The Mg concentration and Mg composition of MgZnO films observed by XPS spectra are shown in Fig. 4. The solid phase composition of Mg in the film is 40-50% of its molar fraction in the liquid phase. Thus, these results indicate that the MPM is suitable for the cost-effective fabrication of MgZnO alloy films. 3.2 Ga doping to MgZnO films The realization of low resistivity was required for the application of transpar-ent electrodes. It is reported that Ga or Al doping in ZnO is effective for the realization of low resistivity [6-8] in the case of pulsed laser deposition (PLD) and sputtering fabri-cations. Thus, Ga doping in the MgZnO films was per-formed. Ga-nitrilotriacetic acid (nta) complex was used for

doping. It was mixed into the Mg-EDTA and Zn-NTA so-lutions and spin-coated onto the glass substrates. The an-nealing conditions were the same as a case of the fabrica-tion of undoped MgZnO films. The resistivity of the fabricated Ga-doped ZnO and MgZnO films is shown in Fig. 5. Here, their thickness is approximately 0.1 µm. With an increasing amount of the Ga source, the resistivity of ZnO decreased. The MgZnO films also showed low resistivity. The I-V plotting with the outside probes of the MgZnO (Mg 20 mol%, Ga 5 mol%) film is also shown in the inset of Fig. 5. Ohmic-like contact between the tungsten probe and the MgZnO film was real-ized. These indicate that the doping technique is effective for the fabrication of ZnO-based UV transparent electrodes by MPM. 4 Discussion It is found that the ammonia aqueous can be used as a common solution. The fabrications of MgZnO films and Ga-doping to them were achieved using it. Although carbon contaminations were not investigated, no disadvantages originated from carbon contaminations were observed at present. Generally, the composition of transparent electrodes based on multinary compound semiconductors is deter-mined on the basis of their fundamental absorption edge. However, the transparent spectra, as shown in Fig. 2, indi-cate that the composition of MgxZn1–xO should be deter-mined in accordance with its excitonic absorption shape. For example, the Mg composition will be over 10% for the fabrication of MgZnO-based transparent electrodes on GaN-based LED operating in the spectral range of ap-proximately 370 nm [9]. The MgZnO films whose Mg compositions over 10% could be fabricated by MPM. The resistivity control of MgZnO films is possible by the Ga-doping technique in the case of MPM fabrication. This indicates the future possibility of the fabrication of cost-effective UV transparent electrodes, although their re-

Figure 1 Schematic drawing of the fabrication process of MgZnO films by MPM.

Figure 2 Transparent spectra of MgZnO films observed at RT. An electroluminescent spectrum of the GaN-based light-emitting diode [8] is also shown in the figure.

598 Y. Mashiyama et al.: Fabrication of MgZnO films by molecular precursor method and their application

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sistivities are still higher than those of reported ZnO films fabricated by sputtering [8] at present. Because the MgZnO films were polycrystalline, the control of their grain size is one of the future technical issues for the realization of the low resistivity. We consider that the annealing conditions are an important factor to control the grain size. We con-sider that the annealing conditions are important factor to control the grain size. The Ga-doping enables to the resis-tivity control from 104 to 100 Ω·cm, as shown in Fig. 5.The contact property, as shown in the inset of the figure, indi-cates that the crystallites in the films have a low resistivity. We consider that the control of grain size will reduce the resistivity. Nevertheless the roles of generated defects and of the impurities still need to be addressed, aiming to reach highly transparent and conductive films. 5 Summary MgZnO fabrication by MPM requires a common solvent for both Zn and Mg precursor solutions. We found that ammonia solution is a common solvent. MgZnO thin film was fabricated on a quartz glass substrate using the MgZnO precursor solution. The Mg composition in the films was estimated by X-ray photoelectron spectroscopy (XPS). The solid phase composition of Mg in the film was 40-50% of its molar fraction in the liquid phase. The resistivity con-trol of MgZnO films was possible by the Ga-doping tech-nique. The results indicated that the MPM is suitable for the cost-effective fabrication of MgZnO alloy films.

Acknowledgments The authors thank Professors Emeriti

Y. Suematsu and K. Iga of Tokyo Institute of Technology for en-

couragement and Professor H. Kawanishi of Kogakuin University

for support. This work was supported by a Grant-in-Aid (No.

18560344) from the Ministry of Education, Culture, Sports, Sci-

ence and Technology.

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Figure 3 X-ray diffraction patterns of MgZnO films fabricated on glass substrates.

Figure 4 Solid phase composition of Mg in the film as a func-

tion of molar fraction in the liquid phase.

Figure 5 Resistivity of ZnO and MgZnO films (Mg: 20

mol%) as a function of Ga molar fraction in the liquid phase.

Inset of the figure, I-V characteristics of Ga-doped (5 mol%)

MgZnO films are shown.