Searching for the Influence of the Sapphire Nitridation Conditions on GaN Films Grown by Cyclic PLD
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L3.3.1
Searching for the Influence of the Sapphire Nitridation Conditions on GaN Films Grown by Cyclic PLD P. Sanguino1, M. Niehus1, S. Koynov2, L. Melo1, R. Schwarz1, M. J. Soares3, C. Boemare3, T. Monteiro3 1
Physics Department, Instituto Superior Técnico, Lisbon, Portugal CL-SENES, Bulgarian Academy of Sciences, Sofia, Bulgaria 3 Physics Department, Aveiro University, Aveiro, Portugal 2
Abstract We have deposited highly c-axis oriented GaN films on sapphire by the Cyclic Pulsed Laser Deposition Technique. Nitridation of the sapphire substrates for these samples was performed at 200 ºC, 400 ºC and 600 ºC. For that purposed, we used a radio frequency nitrogen plasma during four hours. The films were compared in terms of crystal structure, surface morphology and optical quality. Although small, the biggest differences were detected in the surface morphology of the films. Additionally, a typical GaN sample nitridated at 200 ºC was analysed by photoluminescence and showed the typical donor bound excitonic luminescence (D0X ) transition at 3.47 eV and a line near 3.42 eV. These lines show a FWHM of 20 meV and 30 meV at 13K, respectively.
Introduction The preparation of high quality semiconductor films of group-III nitrides (GaN, AlN, InN and their alloys) is a forefront task of the present technological research because of the attractive applications of such films in “blue and UV” optoelectronic devices such as blue LEDs and Blue laser diodes [1,2]. The Pulsed Laser Deposition (PLD) is a promising alternative of the common MOCVD and MBE techniques for preparation of these materials [3-8]. The heteroepitaxial growth of GaN is usually done on sapphire substrates due to their wide availability. The most usual solution to circumvent the large lattice mismatch between sapphire and GaN is the nitridation treatment of the substrate prior to the low temperature buffer layer growth. It has been reported by various groups that this nitridation step is crucial to the structural and optical properties of the GaN epilayer [9-11]. Depending on the deposition method, the nitridation is usually done with ammonia (NH3) or nitrogen plasma. High temperature (850 ºC to 1050 ºC) is usually necessary to thermally dissociate the NH3 in active nitrogen species. This is not the case if nitrogen plasma is used. Here, the plasma generates the active species and a high temperature is not necessary. In fact it has been reported that oxygen diffusion from the Al2O3 occurs over distances of about 1.5 µm in samples nitridated at high temperatures [12]. Recent studies have pointed to the 200 ºC as the best temperature for nitridation with nitrogen plasma [13]. In this work, we investigate the influence of the temperature of the sapphire nitridation process on the growth of GaN films grown by cyclic PLD. The GaN samples thus obtained are compared in terms of optical and structural quality. X-ray diffraction, UV-visible transmission
L3.3.2
spectroscopy, atomic force microscopy (AFM) and photoluminescence are the techniques used to characterise an
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