The Morphology and Cathodoluminescence of GaN Thin Films
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Internet Journal o f
Nitride S emiconductor Research
Volume 1, Article 6
The Morphology and Cathodoluminescence of GaN Thin Films Carol Trager-Cowan , P. G. Middleton, K. P. O'Donnell Department of Physics and Applied Physics, University of Strathclyde This article was received on May 31, 1996 and accepted on September 5, 1996.
Abstract In this paper we compare gallium nitride (GaN) films grown by molecular beam epitaxy on sapphire (Al2O 3), gallium arsenide (GaAs (111)B) and lithium gallate (LiGaO2) substrates. Atomic force microscopy, scanning electron microscopy, cathodoluminescence imaging and cathodoluminescence spectroscopy are used to characterise the films. From growth runs carried out to date, GaN films on GaAs substrates exhibit the best surface uniformity and the cleanest luminescence.
1. Introduction The commercialisation of light emitting diodes based upon InGaN [1] has resulted in an explosion of interest in wide bandgap III-V materials. The ability to produce light emitting diodes spanning the UV to orange spectral region makes these materials highly attractive for the development of full colour displays. Recent announcements by Nagoya Institute of Technology [2], of stimulated emission at 380 nm from a InGaN/AlGaN diode at room temperature, and by Nichia Chemicals, of a laser diode operating at 410 nm [3], has generated further interest in developing coherent light emitters for high density optical data storage, reprographics, underwater communications and so on. In spite of recent successes, much fundamental physics remains to be done on these materials. Working devices exhibit huge defect densities [4]. Unidentified deep centres contribute a strong green/yellow band in competition with band edge emission. We use atomic force microscopy, scanning electron microscopy, cathodoluminescence imaging and cathodoluminescence spectroscopy, to study both the morphological and optical properties of GaN films and to investigate the correlation between them. In this paper we compare results obtained from MBE grown GaN on three different substrates: (0001) orientated sapphire (Al2O3), gallium arsenide (GaAs (111)B) and the novel lattice-matched oxide substrate lithium gallate (LiGaO2).
2. Sample Growth All the films discussed in this paper were grown by molecular beam epitaxy using a solid source for gallium and an RF plasma source for atomic nitrogen [5]. Prior to growth, the sapphire substrates were degreased using standard solvents. They were then thermally heated at ~800oC for 20-30mins and finally lowered to the growth temperature of 750oC. The epi-ready GaAs(111)B substrates were thermally cleaned at ~600oC to remove the surface oxide and were nitrided for a few minutes as the temperature was raised to the growth temperature of 700 oC. As for the LiGaO o 2 substrates, these were etched in a solution of phosphoric and suphuric acids HPO4:H2SO4 (1:1) at 80 C for about 10-15 mins before introducing them into the MBE system. The gallate was then nitrided as the substrate temperature was increased to 630
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