Work function of doped zinc oxide films deposited by ALD

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FOCUS ISSUE

ATOMIC LAYER DEPOSITION FOR EMERGING THIN-FILM MATERIALS AND APPLICATIONS

Work function of doped zinc oxide films deposited by ALD Peter G. Gordon1,a)

, Goran Bacic1, Gregory P. Lopinski2, Seán T. Barry1

1

Department of Chemistry, Carleton University, Ottawa, Ontario K1S 5B6, Canada Measurement Science and Standards, National Research Council of Canada, Ottawa, Ontario K1A 0R6, Canada a) Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. 2

Received: 30 August 2019; accepted: 15 October 2019

Al-doped ZnO (AZO) is a promising earth-abundant alternative to Sn-doped In2O3 (ITO) as an n-type transparent conductor for electronic and photovoltaic devices. We have deposited AZO films with resistivities as low as 1.1 × 10−3 Vcm by atomic layer deposition (ALD) using trimethylaluminum (TMA), diethylzinc (DEZ), and water at 200 °C. The work functions of the films were measured using a scanning Kelvin probe (sKP) to investigate the role of aluminum concentration. The work function of AZO films prepared by two different ALD recipes were compared: a “Al-terminated” recipe and a “ZnO-terminated” recipe. As aluminum doping increases, the Alterminated recipe produces films with a consistently higher work function than the ZnO-terminated recipe. The resistivity of the Al-terminated recipe films shows a minimum at a 1:16 Al:Zn atomic ratio and using a ZnOterminated recipe, minimum resistivity was seen at 1:19. The film thicknesses were characterized by ellipsometry, chemical composition by EDX, and resistivity by a four-point probe.

Introduction Most modern devices contain components that leverage an interaction between light and electrons: displays, cameras, many sensors, photovoltaics and so on. A material that is transparent to visible light and also electrically conductive is often essential to these devices. Of the materials that fit these requirements, the most commonly used are transparent conducting oxides (TCOs), typically integrated into devices as thin films [1, 2]. Because of its high transparency and conductivity, the most widely used TCO is indium tin oxide (ITO), where tin is the dopant [3]. The relatively low natural abundance of indium drives up the financial and environmental cost of devices using this conductor [4]. Al-doped ZnO (AZO) is emerging as an earth-abundant replacement for ITO for applications that require transparent conductive n-doped thin films. Resistivity values as low as 103 X cm have been achieved for ZnO films, and doping can yield even lower resistivities [5]. Thin film deposition methods such as magnetron sputtering at moderate temperatures have been reported that match the relevant properties of ITO films [4]. AZO films have been deposited by many methods including chemical vapor deposition, sol–gel spin coating, aqueous synthesis, and magnetron sputtering, yielding a wide range of resistivities (102–104 X cm) [6, 7, 8, 9].

ª Materials Research Society 2019

Atomic layer deposition (ALD) is a well-kno