Layer-by-Layer Growth of GaN on Sapphire by Low Temperature Cyclic Pulsed Laser Deposition / Nitrogen RF Plasma
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Layer-by-Layer Growth of GaN on Sapphire by Low Temperature Cyclic Pulsed Laser Deposition / Nitrogen RF Plasma P. Sanguino1, M. Niehus1, S. Koynov1, R. Schwarz1 H. Alves2, B. Meyer2 1 2
Departamento de Física, Instituto Superior Técnico, Lisboa, Portugal Justus-Liebig University, Giessen, Germany
ABSTRACT Recently we have proposed a new layer-by-layer method for deposition of group-III nitrides from elemental precursors (Ga, N2) [1,2]. This technique is based on a two-step cyclic process, which alternates Pulsed Laser Deposition (PLD), of a liquid gallium target and nitrogen plasma treatment. In this work, we proceed on the development of this flexible cyclic deposition technique and study the influence of the power and time duration of the 1 mbar nitrogen RF plasma on the GaN thin films. The layers are deposited on pre-nitridated sapphire (0001) substrates at low deposition temperature (600º C) to minimise reevaporation. The cyclic GaN thin films thus obtained are compared in terms of crystal alignment and nitrogen incorporation. X-ray diffraction and optical transmission spectra are the selected tools used to characterise and compare the deposited films.
INTRODUCTION Pulsed Laser Deposition (PLD) is not a totally new technique for the growth of a broad set of materials [3-7]. However, for the deposition of group III nitrides it is still not has developed as the conventional metalorganic chemical vapour deposition (MO-CVD) or molecular beam epitaxy (MBE). Specially, since the fabrication of the first super bright blue GaN LEDs, MO-CVD is having a lot of success in the industry. As an example, the consumer market is now being invaded by all kinds of super bright led torches deposited by this wellestablished method. Nevertheless, PLD can present some advantages over those techniques. One of the most important advantages is that the ablated species are available at high kinetic energies (typically 10-100eV). This permits the use of a greater variety of substrates without thermal degradation or diffusion of impurities from substrate to film. Also, the possibility to use high purity elemental sources (like Ga and N2 for GaN deposition) can circumvent the unwanted incorporation of hydrogen that is a problem in techniques like MO-CVD and MBE. The main problem still remains to be the production of laser droplets by subsurface boiling. Expectations are set on the future commercial femtosecond lasers to solve this problem [3]. Presently, this is solved by the reduction of laser intensity, which has the undesirable effect of decreasing the deposition rate [6]. Laser exfoliation droplets are usually avoided by target rotation or using a liquid target [3, 9-12]. In order to achieve the ideal balance between advantages and disadvantages various research groups develop different variations of GaN PLD techniques. Most of them use UV lasers on liquid Ga target [9-13] and reactive nitrogen rf plasma as the elemental sources [1,2,8,12,13]. K7.6.1 Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 23
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