Engineering ZnO/GaN Interfaces for Tunneling Ohmic Contacts to GaN
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V6.7.1
ENGINEERING ZnO/GaN INTERFACES FOR TUNNELING OHMIC CONTACTS TO GaN E. Kaminska1, A. Piotrowska1, K. Golaszewska1, R. Kruszka1, A. Kuchuk1, J. Szade2, A. Winiarski2, A. Barcz1,3, J. Jasinski4, Z. Liliental-Weber4 1
Institute of Electron Technology, Warsaw, Poland, Institute of Physics, University of Silesia, Katowice, Poland, 3 Institute of Physics, PAS, Warsaw, Poland, 4 Lawrence Berkeley National Laboratory, Berkeley, CA, USA. 2
ABSTRACT We have investigated two approaches for the fabrication of thin ZnO films: sputter deposition from the ZnO target and thermal oxidation of vacuum deposited Zn. The microstructure and electronic properties after consecutive steps of the formation of n-ZnO/pGaN contacts have been studied using electron transmission microscopy and x-ray photoelectron spectrometry. We have achieved ohmic contacts by Zn oxidation and explain their ohmic behaviour in terms of a tunnel n+-ZnO - p-GaN junction. INTRODUCTION Transparent conducting oxides (TCOs) are becoming increasingly important in the modern semiconductor optoelectronics. Their exceptional combination of properties like low resistivity, high thermal stability, transparency to visible light and high work function might be essential for the development of transparent ohmic contacts to light emitting and light detecting GaN-based devices. Prior attempts to form transparent ohmic contacts to GaN had focused on ultra thin metal films. They were partly satisfying: higher transparency was achieved, but at the cost of increased contact resistivity and poor thermal stability. As for TCOs, indium tin oxide (ITO) has drown much attention for its application in GaN optoelectronic devices and also as current spreading layers in LEDs [1]. However, ITO in contact with p-type and n-type GaN exhibited non-ohmic behaviour [1, 2]. In this work we consider the use of a different TCO material, namely ZnO as a contact to ptype GaN. It has been chosen basing on the following premises. First, ZnO is very similar to GaN in terms of band gap, crystalline structure, optical properties and thermal stability. Second, we presume that the technology of a two-component compound could be easier to tailor as compared to the three-component one. We have used two approaches for the fabrication of thin ZnO films: sputter deposition from the ZnO target and thermal oxidation of vacuum deposited Zn. The electrical properties of ZnO/GaN contacts were assessed from conductivity, I-V-T and contact resistivity measurements. The modification of p-GaN surface by ZnO deposition processes and the structure of ZnO/GaN interface were investigated by X-ray photoemission spectroscopy (XPS) and transmission electron microscopy. For that reason, a particular care has been paid to the preparation of damage- and contamination-free GaN surface prior to ZnO deposition [3]. Basing on the obtained results, we demonstrate the feasibility of a tunnelling n+ZnO/p-GaN contact, suitable for application as a transparent ohmic contact to p-type GaN.
V6.7.2
EXPERIMENTAL PROCEDURE GaN (0001) s
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