Effect of Fe doping on optical properties of freestanding semi-insulating HVPE GaN:Fe P. Gladkov a,b,n , J. Humlı ´c ˇek c , E. Hulicius a , T. ˇ Simec ˇek a , T. Paskova d , K. Evans d a Institute of Physics of Academy of Sciences of the Czech Republic, v. v. i., Cukrovarnicka ´ 10, CZ-162 00, Prague 6, Czech Republic b Institute of Photonics and Electronics Academy of Sciences of the Czech Republic, v. v. i., Chaberska 57, CZ-182 51, Prague 8, Czech Republic c Institute of Condensed Matter Physics, Faculty of Science, Masaryk University, Kotla ´ˇ rska ´ 2, CZ-611 37, Brno, Czech Republic d Kyma Technologies Inc., 8829 Midway West Road, Raleigh. NC, USA article info Available online 24 November 2009 Keywords: A1. Fe-doping A1. Optical characterization A3. Hydride vapor phase epitaxy B1. Nitrides abstract Systematic study of optical properties of undoped and Fe-doped substrates grown by hydride vapor phase epitaxy has revealed a strong dependence of the photoluminescence, transmission, reflectivity and ellipsometric spectra on the Fe-doping level. The changes of the near-band-gap transmission, reflectivity and photoluminescence has been observed and ascribed to the absorption introduced by the density of states tails, and the Fe 3+ -ions incorporated in the GaN-lattice. Several approaches towards quantifying the Fe-doping level are suggested. & 2009 Elsevier B.V. All rights reserved. 1. Introduction Semi-insulating GaN:Fe is the preferred substrate material for the AlGaN(InAlN)/GaN based high power, high frequencies electro- nics. Bulk freestanding GaN substrates (fsGaN), both conducting and semi-insulating, became available recently. For utilization of these GaN substrates in the epitaxial processes, extended knowledge of their relevant physical parameters is needed. Apart from the electrical parameters of fsGaN:Fe there is relatively scarce systema- tic information on their optical properties. We report here on the optical properties of both undoped and Fe-doped fsGaN materials produced by Kyma Technologies with their proprietary hydride vapor phase epitaxy (HVPE) process [1]. A semi-insulating GaN doped with Fe to the concentration of 10 18 cm 3 has been investigated by several techniques in Ref. [2]. We focus here on the effect of doping, aiming at several pronounced signatures in transmission, reflectance, ellipsometric and photoluminescence spectra, covering a range of the doping concentrations. 2. Experimental We have studied fsGaN samples covering the doping range of (3  10 17 –3  10 18 ) cm 3 as determined by SIMS measurements. The samples were grown in a vertical HVPE hot-wall reactor, at sub-atmospheric pressure. Bis(cyclopentadienyl) iron (Cp 2 Fe) was used as the iron source, which was introduced into the reactor by the N 2 carrier gas. The GaN films were deposited on c-plane (0 0 0 1) sapphire substrates provided with AlN buffer layer, in the downstream part of the reactor where the GaCl 3 and the NH 3 , transported by N 2 gas, react. GaN growth was carried out at substrate temperatures of approximately 1000 1C. The growth rate of the films was 150 mm/h. layers with thickness of 1–2 mm were grown and removed from the underlying sapphire to create freestanding, bulk material. Electrical measurements of the samples indicate their resistivity to be in the range of 10 9 O cm [1]. Within the studied range of Fe-doping levels the dislocation density is typically 10 5 cm 2 . The oxygen content in the samples with the highest Fe-concentration of 3  10 18 cm 3 determined by SIMS measurements is 2  10 16 cm 3 . The photoluminescence (PL) measurements were performed in the temperature range 4–300 K with He closed cycle optical cryostat. The near infrared PL was excited with the 457 nm line of Ar + ion laser and analyzed with Thermo 5700 FTIR spectrometer equipped with cooled InGaAs photodetector. Optical transmission and near-normal (151) reflectance spectra were measured at room temperature with the UV–vis spectrophotometer Specord 210. Double-sided polishing was used to measure the transmittance of  0.7 mm thick plan-parallel plates; in order to suppress spurious reflection in the reflectance measurements the backside of the samples was subsequently grinded by 10 mm BC powder. Far- infrared (FIR) reflectance and transmittance were measured using vacuum spectrometer Bruker IFS66, equipped with a near-normal (101) reflectance accessory. The excitonic structure at the band- gap was studied in detail with a multichannel fiber spectrometer, with the spatial resolution of  200 mm, and with a home-built ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth 0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.11.032 n Corresponding author at: Institute of Photonics and Electronics Academy of Sciences of the Czech Republic, v. v. I, Chaberska 57, CZ-182 51, Prague 8, Czech Republic. E-mail address: gladkov@ufe.cz (P. Gladkov). Journal of Crystal Growth 312 (2010) 1205–1209