HVPE and MOVPE GaN Growth on Slightly Misoriented Sapphire Substrates
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Olivier Parillaud, Volker Wagner, Hans-Jörg Bühlmann, François Lelarge, and Marc Ilegems Institut de Micro- et Optoélectronique, Ecole Polytechnique Fédérale de Lausanne, CH1015 Lausanne, Switzerland ABSTRACT We present preliminary results on gallium nitride growth by HVPE on C-plane sapphire with 2, 4 and 6 degrees misorientation towards M and A directions. A nucleation GaN buffer layer is deposited prior the growth by MOVPE. Surface morphology and growth rates are compared with those obtained on exact C-plane oriented sapphire, for various growth conditions. As expected, the steps already present on the substrate surface help to initiate a directed step-flow growth mode. The large hillocks, which are typical for HVPE GaN layers on (0001) sapphire planes, are replaced by more or less parallel macro-steps. The width and height of these steps, due to step bunching effect, depend directly on the angle of misorientation and on the growth conditions, and are clearly visible by optical or scanning electron microscopy. Atomic force microscopy and X-ray diffraction measurements have been carried out to quantify the surface roughness and crystal quality.
INTRODUCTION (0001)-oriented sapphire is the main substrate used for the realization of GaNbased devices [1]. To our knowledge, very few reports deal with MOVPE of GaN on slightly misoriented sapphire [2-4], and only one concerns HVPE on such substrates [5]. However, for other III/V compounds GaAs or InP, it has been shown that the use of substrates misoriented by a few degrees can be very efficient to improve both crystal quality and surface morphology of the layers grown by HVPE. This is related to the fact that the supersaturation of the gas phase is quite small in the hydride technique as opposed to the conditions during MOVPE growth and growth occurs near the thermodynamic equilibrium as predicted by the Burton, Cabrera, and Frank theory [6].
EXPERIMENTAL DETAILS Growth experiments have been carried out in two separate reactors. An horizontal home made reactor was used to grow low temperature GaN buffer layers prior to the growth by HVPE. Nitrogen and gallium precursors were ammonia (NH3)
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Figure 1 AFM picture of the GaN buffer layer surface before annealing. Small GaN grain structure is clearly visible and a 10 nm high macro-step can be observed in the center of the image. and trimethylgallium (TMG) respectively and carrier gas was nitrogen. Deposition temperature was 600°C after a nitridation step of the Al2O3 surface at 1100°C during 30 minutes. All the experiments were carried out at atmospheric pressure. HVPE layers were subsequently grown in an horizontal Aixtron HVPE reactor. Details on the reactor geometry have been published in a previous paper [7]. GaCl resulting from the reaction of HCl with metallic gallium and NH3 are brought separately to the deposition zone. Nitrogen and mixed nitrogen / hydrogen were used as carrier gas. Source temperature and deposition temperature were kept constant at 900°C and 1050°C respectively for all the experiment
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