Optical properties of AlN determined by vacuum ultraviolet spectroscopy and spectroscopic ellipsometry data
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Optical properties of AlN determined by vacuum ultraviolet spectroscopy and spectroscopic ellipsometry data D.J. Jones and R.H. Frencha) DuPont Central Research, E356-384 Experimental Station, Wilmington, Delaware 19880
H. Mu¨llejansb) Max-Planck-Institut fu¨r Metallforschung, Seestraße 92, D-70174 Stuttgart, Germany
S. Loughin DuPont Central Research, E356-384 Experimental Station, Wilmington, Delaware 19880
A.D. Dorneich Max-Planck-Institut fu¨r Metallforschung, Seestraße 92, D-70174 Stuttgart, Germany
P.F. Carcia DuPont Central Research, E356-384 Experimental Station, Wilmington, Delaware 19880 (Received 2 November 1998; accepted 3 August 1999)
Precise and accurate knowledge of the optical properties of aluminum nitride (AlN) in the ultraviolet (UV) and visible (VIS) regions is important because of the increasing application of AlN in optical and electro-optical devices, including compact disks, phase shift lithography masks, and AlN/GaN multilayer devices. The interband optical properties in the vacuum ultraviolet (VUV) region of 6–44 eV have been investigated previously because they convey detailed information on the electronic structure and interatomic bonding of the material. In this work, we have combined spectroscopic ellipsometry with UV/VIS and VUV spectroscopy to directly determine the optical constants of AlN in this range, thereby reducing the uncertainty in the preparation of the low-energy data extrapolation essential for Kramers–Kronig analysis of VUV reflectance. We report the complex optical properties of AlN, over the range of 1.5–42 eV, showing improved agreement with theory when contrasted with earlier results. I. INTRODUCTION
Interest in aluminum nitride (AlN) has increased ever since Slack1 and McNelly2,3 synthesized high-purity, high thermal conductivity, single crystals. This material has found many technological applications4 motivating considerable study in theoretical5–13 and experimental14 investigations. Due to its high intrinsic thermal conductivity, AlN is attractive as an electronic packaging substrate. AlN is also used as a phase contrast layer in optical disks and has potential as a phase shift material in lithographic photo masks.15,16 AlN/GaN alloys and multilayers find roles in optical devices such as blue lasers. These applications generate an interest in the optical properties and electronic structure of AlN. The literature on AlN spans vibrational spectroscopy,17 ultraviolet
a)
Address all correspondence to this author. e-mail: [email protected] b) Present address: Institute for Advanced Materials, Joint Research Centre, European Commission, PO Box 2, NL-1755 ZG Petten, The Netherlands. J. Mater. Res., Vol. 14, No. 11, Nov 1999
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(UV) spectroscopy in the band gap region18–27 including temperature-dependent studies28 and polarization dependent studies,20,29 a number of vacuum ultraviolet (VUV) studies,30,37 electron energy loss spectroscopy (EELS) studies,3
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