Raman spectra of p-type transparent semiconducting Cr 2 O 3 :Mg Karsten Fleischer ⁎, David Caffrey, Leo Farrell, Emma Norton, Daragh Mullarkey, Elisabetta Arca, Igor V. Shvets School of Physics and Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, Dublin 2, Ireland abstract article info Available online xxxx Keywords: p-Type transparent conducting oxide Raman spectroscopy Dopant clustering Cr2O3 Chromium sesquioxide We present an analysis of the Raman spectra of p-type transparent conducting Cr 2 O 3 :Mg grown by various tech- niques including spray pyrolysis, pulsed laser deposition, molecular beam epitaxy and reactive magnetron sputtering. The best performing films show a distinct broad range Raman signature related to defect-induced vi- brational modes not seen in stoichiometric, undoped material. Our comparative study demonstrates that Raman spectroscopy can quantify unwanted dopant clustering in the material at high Mg concentrations, while also being sensitive to the Mg incorporation site. By correlating the Raman signature to the electrical properties of the films, growth processes can be optimised to give the best conducting films and the local defect structure for effective p-type doping can be studied. © 2015 Elsevier B.V. All rights reserved. 1. Introduction p-Type transparent conductors are not only much sought after for the realisation of fully transparent CMOS type electronics [1], but also can play an important role in hole injecting or extracting layers for organic light emitting diodes or photovoltaic solar cells respectively [2–5]. In particular, oxide materials have been studied intensively since the first report of p-type transparent CuAlO 2 [6]. One of the better performing p-type transparent conducting oxides (TCOs) is CuCrO 2 :Mg [7]. Despite its promising properties, the processing of this doped terna- ry oxide remains challenging and large variations in electrical properties depending on growth conditions have been reported [8]. Part of these problems is related to the dopant distribution and the other part is due to de-phasing of the ternary oxide itself [9–11]. Recently we have reported on a related doped binary oxide (Cr 2 O 3 :Mg and Cr 2 O 3 :Mg,N) with similar overall properties to CuCrO 2 :Mg [12–14]. In contrast to CuCrO 2 , Cr 2 O 3 is the thermodynamically most stable chromium oxide, and hence the impact of the dopant distribution can be much easier studied. In this paper we investigate the magnesium inclusion in this simpler binary system and will correlate how growth conditions influence the local micro-structure of the dopant measured by Raman spectroscopy. This is made possible by the rather simple Raman spectra of the binary oxide, allowing for a qualitative and quantitative analysis of defect in- duced vibrational modes introduced by the Mg dopant. We will show that electrical properties of Cr 2 O 3 :Mg are comparable between different growth techniques once the conditions are optimised to favour dissoci- ation of dopant clusters during growth or post-annealing. 2. Experimental details Thin film samples with a thickness between 100 and 300 nm have been grown on glass substrates or epitaxially on Al 2 O 3 (0001) by a vari- ety of different techniques such as radio frequency (RF) magnetron sputtering, molecular beam epitaxy (MBE), spray pyrolysis (SP) and pulsed laser deposition (PLD). RF sputtered samples have been prepared by co-sputtering from both a Cr metal target and an MgO ceramic target in a reactive plasma with Argon flow of 10 sccm with a substrate to target distance of 11 cm. The O 2 flow rate was raised to be the highest possible value (1.6 sccm) without system instability resulting from the formation of oxide on the metallic target surface (the oxide point). Alternatively, films have been prepared from sputtering in Ar only using a Cr 2 O 3 / MgO mixed target with nominal MgO weight content of 5%. Samples are typically grown at a pressure of 0.45 Pa and deposited onto a sub- strate heated to 950 K. MBE grown samples have been prepared in a DCA M600 MBE with base pressure of 1.3 × 10 -7 Pa at a substrate temperature of 875 K. Cr metal or ceramic Cr 2 O 3 pieces were evaporated by e-beam heating with an oxygen partial pressure ranging from 1 × 10 -4 to 5 ×10 -3 Pa. The doping was achieved by controlling the supplied power to a second e-beam evaporator loaded with MgO. Details of the crystallographic and electrical properties of MBE grown films have been published elsewhere [14]. Details of the in-house built spray pyrolysis system and dependence of the Cr 2 O 3 :Mg properties on the precursor solution have been de- scribed in detail elsewhere [13]. For this study, the best performing films were grown using water based solutions of 0.1 M Cr(NO 3 ) 3 as a Cr precursor, 0.01 M MgCl 2 ·6(H 2 O) as a cation dopant and 0.5 M NH 4 CH 3 CO 2 as an additive to improve film quality. Samples were sprayed using an air blast type nozzle operated with 120 kPa O 2 , where substrates have been heated to 780 K. Thin Solid Films xxx (2015) xxx–xxx ⁎ Corresponding author. E-mail address: fleisck@tcd.ie (K. Fleischer). TSF-34234; No of Pages 5 http://dx.doi.org/10.1016/j.tsf.2015.03.076 0040-6090/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf Please cite this article as: K. Fleischer, et al., Raman spectra of p-type transparent semiconducting Cr 2 O 3 :Mg, Thin Solid Films (2015), http:// dx.doi.org/10.1016/j.tsf.2015.03.076