Structural properties of MOVPE GaN layers grown by a new multi-buffer aproach
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Internet Journal Nitride Semiconductor Research
Structural properties of MOVPE GaN layers grown by a new multi-buffer aproach J. Kozlowski1, R. Paszkiewicz1, R. Korbutowicz1, M. Panek1, B. Paszkiewicz1 and M. Tlaczala1 1Institute
of Microsystem Technology, Wroclaw University of Technology,
(Received Tuesday, June 23, 1998; accepted Friday, September 25, 1998)
GaN undoped layers of good morphology, good crystallinity and electrical properties were grown on c-plane sapphire substrates by the atmospheric pressure MOVPE technique using a new multibuffer growth approach. A suitable buffer layer growth technique was worked out which enabled growth of GaN layers with properties superior to those grown in a conventional process scheme. Additional buffer layers, deposited with increasing temperature and increasing V/III molar ratio, were inserted between the low temperature buffer layer and the high temperature GaN layer grown on it. The c and a lattice constants of the high temperature GaN overgrown layer were evaluated from X-ray data. The layer mosaicity and c-lattice parameter variation were determined. The relationship between c and a lattice parameters and the second buffer layer growth scheme has been studied. The effect of second buffer layer growth conditions, buffer layer annealing time as well as the influence of V/III molar ratio during the high temperature GaN deposition on the crystalline and electrical properties of overgrown GaN epitaxial layers are presented. Characterization includes surface morphology examination by SEM and Nomarski optical microscope, X-ray diffraction and C-V measurements.
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Introduction
Wurtzite GaN is an attractive material for realization of high temperature electronic, optoelectronic devices and cold cathodes due to its direct band gap of 3.4 eV at room temperature, high thermal conductivity, high thermal stability and excellent physical properties. But its epitaxial growth presents several serious problems caused mainly by the lack of commercially available, large size, lattice matched substrates. Despite the fact that device quality GaN has been achieved, many fundamental aspects of the growth are still unclear and need further investigation. The most common substrate utilized for GaN deposition is sapphire because of its low cost and availability while MOVPE seems to be the most successful and the most promising epitaxial technique, especially for large scale production. Typically GaN crystals nucleate and grow on sapphire by island formation. Iinitial sapphire substrate nitridation [1] and low temperature (450-600°C) AlN [2], GaN [3] or even double GaN/AlN [4] nucleation layers have been tried to promote oriented lateral growth. Although the exact role of the low temperature buffer layer is not fully understood yet, it is believed that it provides a large number of nucleation sites and is
responsible for converting the growth mechanism from three dimensional to pseudo-two dimensional. After low temperature buffer layer deposition, annealing is performed before the high t
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