ZnO: A matter of properties

Richard O'Donoghue, Ruhr-University-Bochum, Bochum, Germany
Daniel Peeters, Ruhr-University-Bochum, Bochum, Germany
Anjana Devi, Ruhr-University-Bochum, Bochum, Germany
Detlef Rogalla, DTL of RUBION, RUB, Bochum, Germany
Hans Werner Becker, DTL of RUBION, RUB, Bochum, Germany

Zinc oxide (ZnO) has garnered a lot of attention in recent times. This is in part due to a unique set of intrinsic properties which have been recognized for quite some time. One of the main highlights of those is the fact that ZnO is a semiconductor with a wide and direct band gap of 3.37eV which in principle enables optoelectronic applications in the near UV spectral range including light emitting diodes and photodetectors. If you couple with this a high transparency to visible light in combination with its tunable electric conductivity, which enables use in thin film transistors as well as buffer layers in solar cells, the reason for the invested attention becomes quite apparent.[1] However, the material is not without its drawbacks. Controlling the conductivity of ZnO, where it gets its n-type character and the ability to obtain p-type doped ZnO, have been issues in the past and require an understanding of the native point defects and impurity incorporation in order to find solutions.[2]

ZnO can be grown in both bulk-crystal and thin films by a whole host of methods including gas or vapour transport, hydrothermal and pressurized melt growth for bulk and pulsed laser deposition (PLD), chemical vapour deposition (CVD), metal-organic CVD, sputtering and MBE. Atomic layer deposition (ALD) is a subset of CVD which is based on self-limiting and saturated surface reactions which allows low temperature and conformal growth with control at the nanoscale. Previously ALD saw minimal use for the growth of ZnO thin films in comparison to other deposition techniques. However, the requirement for reduced dimensions and the increasing control in microelectronic applications has caused a massive surge in its research within the last 10 years.

Herein we present an overview of ZnO thin films in terms of broad properties, general challenges and applications with respect to ALD. A brief history of the known implemented precursors and an analysis of the corresponding growth parameters will be divulged. Finally, recent results upon this topic from our group will be discussed.

[1] T. Tynell and M. Karppinen, Semicond. Sci. Technol. 29 2014 043001
[2] A. Janotti and C. G. Van de Walle, Rep. Prog. Phys. 72 2009 126501

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