Elsevier

Journal of Alloys and Compounds

Volume 603, 5 August 2014, Pages 268-273
Journal of Alloys and Compounds

Study of the functional properties of ITO grown by metalorganic chemical vapor deposition from different indium and tin precursors

https://doi.org/10.1016/j.jallcom.2014.03.088Get rights and content

Highlights

  • Study of ITO layers obtained from three indium and two tin precursors by pulsed MOCVD.

  • Optimized tin doping depends on the tin chemical precursor.

  • Combination including acetyl acetonate ligand present the same characteristics.

  • Films elaborated between 350 and 700 °C with InMe2OtBu present a constant resistivity.

Abstract

Functional properties of tin doped indium oxide (ITO) layers grown by MOCVD from different indium and tin precursors are investigated. Selected indium precursors are In(acac)3, In(tmhd)3 and InMe2OtBu, and tin precursors are DBTDA and Sn(acac)2. ITO layers are optically and electrically characterized to determine the better doping conditions. Differences in electrical properties of ITO layers are found when using InMe2OtBu, as compared to In(acac)3 and to In(tmhd)3. The best films present a resistivity of 2.5 × 10−4 Ω cm and a transmittance higher than 84% for high deposition temperatures (T  600 °C). The nature of tin precursors modifies the optimal doping at which these characteristics are achieved. When doped by DBTDA optimal doping is 8 at.%, therefore close to the solubility limit of tin in In2O3 matrix; but when using Sn(acac)2, or In(acac)3/DBTDA combination, best functional characteristics are obtained for the maximal doping content obtained, i.e. 2.5 at.%. For optimized conditions, the resistivity decreases when deposition temperature increases except when using the couple InMe2OtBu/DBTDA without oxygen addition during deposition. For this combination of precursors a resistivity of 1 × 10−3 Ω cm is obtained at a deposition temperature of 350 °C and remains constant up to 600 °C. Only the films obtained from InMe2OtBu/DBTDA are crystalline state at a deposition temperature of 350 °C.

Introduction

Tin doped indium oxide (ITO) is a transparent conductive oxide widely used as transparent front electrode in solar cell or in touch screen display applications [1], [2], [3], [4]. Commercial requests for the two major functional properties area sheet resistance lower than 10 Ω/□ and a transmittance higher than 83 %. Chemical Vapor Deposition (CVD) technique allows the fabrication of films with such characteristics but a high energetical cost [5], [6], [7]. Besides, published results are highlighting a minimal resistivity of 2 × 10−4 Ω cm obtained for different tin doping comprised between 3 and 10 at.%. Different strategies are currently followed in order to improve ITO’s functional properties such as stacking of different layers [8], [9], co-doping of ITO with Zn and/or Mg [10], [11], mixing of carbon nanotubes or metallic nanowires with ITO matrix [12], [13], [14]. In these multiple studies, the choice of the indium precursor is not often supported by physical explanations. As motivations, the non toxicity of the raw product is pointed out by Kane et al. [5] or the advantage of directly combine indium and tin in a single molecule is underlined by Veith et al. [15].

To understand the influence of tin doping, it should be reminded that in the In2O3 crystallographic structure (cubic bixbyite), two non equivalent cation site (b and d) exist due to a characteristic indium bonding with oxygen: b-site, when the two vacancies are along the cubic diagonal, or d-sites when they are along the face diagonal [16]. When incorporating in the In2O3 matrix, tin atoms accomodate in one of these two sites and form Sn–O or O–Sn–O bonding. From an electrical point of view, Sn–O acts as an electron acceptor while the second one has an electron donor character. As specified by Franck and Köstlin [17], the incorporation of tin (<4 at.%) in the In2O3 matrix forms a loosely bounded complex which therefore neutralizes the oxygen, and, for tin concentration higher than 4 at.%, a strongly bounded Sn2O4 complex is formed in a second interstitial position. Atomistic simulations reported by Warschkow et al. [18] point out a d-site preference intrinsic to dopant while b-site preference occurs in defect cluster.

In this paper we investigate several indium and tin precursors for metalorganic CVD to asses their influence on the functional properties of resulting ITO layers. The six possible combinations of precursors are: In(acac)3/DBTDA, In(tmhd)3/DBTDA, InMe2OtBu/DBTDA and In(acac)3/Sn(acac)2, In(tmhd)3/Sn(acac)2, InMe2OtBu/Sn(acac)2. We have previously established the correlation of tin concentration in the layer and the structural characterization for each possible combination of precursors [19]. Here, ITO layers are electrically and optically characterized to determine the better doping condition. Differences between tin precursors are pointed out and are explained in term of different incorporation sites. On the other hand, crystalline state of the layer is a key parameter to reduce the resistivity. The observed variation of properties resulting from the choice of precursor emphasizes the importance of good precursor selection and opens a route to grow an optimized ITO film at low deposition temperature by CVD technique.

Section snippets

Experimental

Depositions took place in a CVD reactor using pulsed injection and evaporation of the metalorganic precursor solution [20]. The experimental conditions were detailed in our previous work [19]. To summarize, depositions were performed in a hot-wall reactor MOCVD on alkaline earth boro-aluminosilicate glass (corning 1737). The deposition temperature was varied between 250 °C and 700 °C. The total flux of gas (Ar + O2) was kept constant at 1500 sccm and the total pressure of the system was 10 Torr. The

Results and discussion

This study evaluated the functional properties of ITO films deposited by MOCVD combining three indium and two tin precursors. The kinetic and structural properties of the films obtained by the combinations of these precursors have been previously published [19]. A major result previously found is that ITO films can be deposited using InMe2OtBu precursor even without adding oxygen gas during the reaction. As a consequence two other combinations of precursors are included in this study, InMe2OtBu0

Conclusion

Three indium precursors In(acac)3, In(tmhd)3 and InMe2OtBu, were combined with two tin precursors, namely, DBTDA and Sn(acac)2 to synthesize tin doped indium oxide ITO layers by MOCVD. The functional properties of the resulting ITO layers were studied as a function of the combination of precursors used. ITO layers synthesized with InMe2OtBu presented some specificities with respect to those elaborated with the two other indium precursors: oxygen gas addition is not necessary for the growth of

Acknowledgements

This study has been financially supported by the European funded (FEDER) and by Oseo through the PrecInov project.

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