Spectroscopic Ellipsometry on GaN: Comparison Between Hetero-epitaxial Layers and Bulk Crystals
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Internet Journal Nitride Semiconductor Research
Spectroscopic Ellipsometry on GaN: Comparison Between Hetero-epitaxial Layers and Bulk Crystals A. R. A. Zauner1, M. A. C. Devillers1, P. R. Hageman1, P. K. Larsen1 and S. Porowski2 1Research 2High
Institute of Materials, University of Nijmegen, Pressure Research Center,
(Received Tuesday, June 23, 1998; accepted Thursday, September 10, 1998)
In the present study spectroscopic ellipsometry was used for characterising GaN bulk crystals obtained at high pressure and thin films grown on sapphire. The undoped GaN films grown by MOCVD show interference fringes below the fundamental gap. The ellipsometric data (∆ and Ψ), measured in the wavelength range between 500 nm and 680 nm, were analysed using a multilayer description in the transfer matrix formalism. For the thin films grown on sapphire the model includes a buffer layer, a GaN epilayer, and a hypothetical overlayer. Furthermore, both, the real and imaginary part of the complex refractive index were taken into account. The real part of the refractive . For bulk index n1 was found to follow a Cauchy-type of dispersion n1 = 2.290 + 0.06 crystals n1 was found to be 2.337 ± 0.010 at the wavelength of 632.8 nm. This value compares well with MOCVD grown GaN where n1(λ = 632.8 nm) = 2.328 ± 0.003.
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Introduction
Due to its material properties gallium nitride (GaN) has attracted enormous attention in recent years, certainly after the success of GaN-based blue-light emitting diodes. The lack of lattice-matched substrates for epitaxial growth of GaN films has led to the application of a variety of substrates of which sapphire is most abundantly used [1]. Despite the large mismatch in the lattice constants between GaN and sapphire, a two-step growth process, proposed by Amano et al. [2], makes it possible to grow device quality hetero-epitaxial GaN layers. However, these layers are still highly defective thereby degrading device performance. GaN single crystals would provide perfect lattice matched substrates which can be easily cleaved. Although considerable progress has been made in increasing the size of the GaN single crystals [3], at present, their relatively small crystal-size is still a bottleneck for the application as lattice matched substrate. Ellipsometry has been used by several authors [4] [5] [6] [7] to study GaN. It is a non-destructive technique which can be performed on optically flat surfaces of almost any material in order to study surface as well as bulk properties. If polarised light is reflected from the sample at an angle of reflection ϕ, the reflected beam will be elliptically polarised. Dividing the p-polarised
component by the s-polarised component one can define two ellipsometric quantities ∆ and Ψ, given by, (1)
The two parameters, ∆ and Ψ, contain information on the sample as thickness and refractive index of the film. They are very sensitive to the presence of overlayers on the surface of the material such as a natural oxide layer or a rough surface layer [8] [9]. In this paper ellipsometry
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