Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Low-Temperature Plasma-Enhanced Atomic Layer Deposition of Tin(IV) Oxide from a Functionalized Alkyl Precursor: Fabrication and Evaluation of SnO2‑Based Thin-Film Transistor Devices Lukas Mai,†,∇ David Zanders,†,∇ Ersoy Subaşı,‡ Engin Ciftyurek,⊥ Christian Hoppe,# Detlef Rogalla,§ Wolfram Gilbert,⊥ Teresa de los Arcos,# Klaus Schierbaum,⊥ Guido Grundmeier,# Claudia Bock,∥ and Anjana Devi*,† Downloaded via TULANE UNIV on January 10, 2019 at 11:56:22 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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Inorganic Materials Chemistry, ‡Werkstoffe und Nanoelektronik, §RUBION, and ∥Mikrosystemtechnik, Ruhr University Bochum, 44801 Bochum, Germany ⊥ Abteilung für Materialwissenschaft, Institut für Experimentelle Physik der kondensierten Materie, Heinrich-Heine-Universität Düsseldorf, Universitätsstraße 1, 40225 Düsseldorf, Germany # Technical and Macromolecular Chemistry, University of Paderborn, 33098 Paderborn, Germany S Supporting Information *
ABSTRACT: A bottom-up process from precursor development for tin to plasma-enhanced atomic layer deposition (PEALD) for tin(IV) oxide and its successful implementation in a working thin-film transistor device is reported. PEALD of tin(IV) oxide thin films at low temperatures down to 60 °C employing tetrakis-(dimethylamino)propyl tin(IV) [Sn(DMP)4] and oxygen plasma is demonstrated. The liquid precursor has been synthesized and thoroughly characterized with thermogravimetric analyses, revealing sufficient volatility and long-term thermal stability. [Sn(DMP)4] demonstrates typical saturation behavior and constant growth rates of 0.27 or 0.42 Å cycle−1 at 150 and 60 °C, respectively, in PEALD experiments. Within the ALD regime, the films are smooth, uniform, and of high purity. On the basis of these promising features, the PEALD process was optimized wherein a 6 nm thick tin oxide channel material layer deposited at 60 °C was applied in bottom-contact bottom-gate thin-film transistors, showing a remarkable on/off ratio of 107 and field-effect mobility of μFE ≈ 12 cm2 V−1 s−1 for the as-deposited thin films deposited at such low temperatures. KEYWORDS: tin(IV) oxide, thin films, atomic layer deposition, precursors, thin-film transistors
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the Sn4+ cation.16,17 Thus, amorphous SnO2 thin films have been proven to be applicable in TFTs while facilitating transistor features such as threshold voltages as low as 0.1 V.14 As Chin et al. showed, the resulting TFT characteristics are highly dependent on the thickness of the channel layer as they observed an optimal performance of the device at a thickness of only 4.5 nm.15 To be able to tune the thickness of the layers in the nanometer range with a high precision, the atomic layer deposition (ALD) technique is of high relevance as it enables the growth of pinhole-free, highly uniform, and conformal films onto three-dimensional surface geometries with a defined thinfilm stoichiometry over large surface areas at low deposition temperatures due to the self-limiting nature of the growth process.18−21 Even though several ALD processes for the fabrication of high-quality SnO2 thin films are known,1,22−27
INTRODUCTION Tin(IV) oxide (SnO2) is an n-type oxide semiconductor material with excellent electrical, optical, and electrochemical properties.1−3 These characteristics qualify SnO2 thin films to be employed in a broad range of devices. As the oxygendeficient nature of SnO2 surfaces allows the conductivity to be highly dependent on the amount of preadsorbed oxygen ions, SnO2 thin films facilitate highly sensitive sensing of reducing gases that consume the adsorbate ions.4−6 Besides, various approaches were made to employ SnO2 thin films in photovoltaics as the front contact layer coated on glass, because the transparent conductive oxide provides high transmission of visible light accompanied by low absorbance.7−9 In thin-film transistors (TFTs), e.g., in transparent organic light emitting diode displays,1,10 amorphous SnO2 thin films are considered to be a favorable candidate as a channel material11 that offers outstanding electron mobilities of μe = 6−40 cm2 V−1 s−1 12−15 and carrier densities of N = 1.6 × 1017−1.2 × 1021 cm−3 due to the high spatial s-state overlap of © XXXX American Chemical Society
Received: September 20, 2018 Accepted: December 18, 2018
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DOI: 10.1021/acsami.8b16443 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
nitric acid, yielding high-quality thin films at temperatures as low as 50 and 120 °C, respectively.24,37 However, this compound has not yet been used as precursor in PEALD. To summarize, so far very few precursors have been tested in low-temperature PEALD (
