Thin Solid Films 450 (2004) 75–78 0040-6090/04/$ - see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.10.146 UV photoreflectance for wide band gap nitride characterization C. Bru-Chevallier *, S. Fanget , G. Guillot , S. Ruffenach , O. Briot a, a a b b Laboratoire de Physique de la Matiere, UMRCNRS 5511 – LPM INSA de Lyon – Bat Blaise Pascal, 7, Avenue Jean Capelle 69621, a ` ˆ Villeurbanne, Cedex, France Groupe d’Etude des Semiconducteurs, UMRCNRS 5650 – Universite Montpellier II, France b ´ Abstract The optical properties of hetero-polarization GaNyAlGaN (12% Al and 16.5% Al) are studied by photoluminescence (PL), photoreflectance (PR) and photoluminescence excitation (PLE) at low temperature. This material system is known to exhibit very strong internal electric fields and we show that PR spectroscopy is one of the best techniques to get a quantitative measurement of such fields, as it is a direct all-optical measurement. Electric field strength derived from FKO is approximately 120 kVycm in AlGaN (12% Al) and 850 kVycm in a 4.2 nm GaN QW. These values are in good agreement with other indirect determinations realized on the same samples by PL and PLE spectroscopy, but which involve computer simulations that need an accurate knowledge of geometrical and composition parameters of the samples.  2003 Elsevier B.V. All rights reserved. Keywords: Photoreflectance; Piezoelectric fields; GaNyAlGaN 1. Introduction Group III nitride semiconductor compounds have demonstrated their capacity for light emission w1x and have recently gained increasing interest in high-power, high-frequency electronics w2x. Most of the work usually addresses the hexagonal phase of GaN, which is ther- modynamically stable, but in which very high sponta- neous and piezoelectric fields are present w3x. Such internal fields strongly influence optical as well as electronic properties of nitride heterostructures and then device performance, either light emitting devices or high electron mobility transistors. Therefore, the experimental determination of such internal electric fields is unavoid- able for the device design. In this paper, UV range photoreflectance (PR) spec- troscopy in the Franz Keldysh regime is used to optically determine the internal electric field in hexagonal phase GaNyAlGaN quantum wells (QW). Complementary photoluminescence (PL) and PL excitation (PLE) meas- urements are also performed on the same samples. The electric field value in the QW is deduced from the fitting of QW energy transition vs. the well width using *Corresponding author. Tel.: q33-4-72-43-89-06; fax: q33-4-72- 43-85-31. E-mail address: bru@insa-lyon.fr (C. Bru-Chevallier). electric field as a parameter. Both experimental and theoretical determinations will be compared. 2. Experiment The GaNyAlGaN samples were grown by low pres- sure metal-organic chemical vapor deposition (MOCVD) using an AIXTRON 200y4 RF-S system. Based on (0001) sapphire, a 25 nm GaN nucleation layer is grown at low temperature (540 8C), followed by a 1 mm GaN buffer layer grown at 1140 8C. A single GaN quantum well is then grown between 30 nm AlGaN barriers. Two different aluminium compositions 12 or 16.5% are used for the barriers. The whole structure is unintentionally doped. The aluminium composition and the strain state in the barriers were determined by X-ray reciprocal space mapping w4x. Different QW thicknesses were grown between 1.3 and 6.5 nm. Low temperature PL (8K) was performed using a frequency-doubled Argon laser emitting at 244 nm (5.08 eV). The same laser beam was used in PR experiments to photo modulate the internal electric field near the surface of the sample. A Xe white light source dispersed through a HR 640 monochromator was used as the probe beam. PLE measurements were performed at 8 K using the same Xe lamp as excitation source, dispersed through a monochromator.