Homo- and Hetero-Epitaxial Gallium Nitride Grown by Molecular Beam Epitaxy
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Cite this article as: MRS Internet J. Nitride Semicond. Res. 4S1, G4.11 (1999) Abstract Various methods have been used to initiate growth by Molecular Beam Epitaxy (MBE) of GaN on sapphire, or other substrates, but there is always a problem with morphology and with a high defect density which results in the formation of a sub-grain boundary structure. We show that by using, homo-epitaxial growth on properly prepared bulk GaN substrates, combined with high temperature growth, we obtain a significant improvement in surface morphology. Growth at sufficiently high temperature leads to a rapid smoothing of the surface and to almost atomically flat surfaces over relatively large areas. Multi-Quantum Well structures grown on such GaN epitaxial films are dislocation free with abrupt interfaces.
Introduction The Group Il-Nitrides are an important class of semiconductors now being used for light emitting diodes (LEDs), short wavelength blue/ultra-violet (UV) laser diodes (LDs) and high temperature electronic devices. LEDs are already commercially available from a number of suppliers [1,2]. Recently, blue/UV laser diodes have been demonstrated with room temperature cw operational lifetimes in excess of 3000 hours [3]. Projected lifetimes of >10000 hours have been announced at a number of recent international conferences. Other device structures based on the group III-Nitride system, grown both by Metal Organic Vapour Phase Epitaxy (MOVPE) and Molecular Beam Epitaxy (MBE), including high-power high-frequency FETs [4] and solar blind UV photo-detectors [5] have also been reported. During the growth of GaN layers, either by MOVPE or MBE, one would ideally grow under stoichiometric conditions. However, for films grown by MBE, it is difficult to achieve growth under exact stoichiometric conditions and films are usually grown slightly Ga or N-rich. Films grown under N-rich conditions show a columnar structure [61 and they are not suitable for device purposes. Films grown under Ga-rich conditions at low temperatures show large hexagonal features of wurtzite GaN, in a polycrystalline background and with additional Ga droplets. It is common practise, therefore, to use a slightly Ga-rich growth mode at high temperature, the excess Ga being desorbed.
G 4.11
Mat. Res. Soc. Symp. Proc. Vol. 537 @1999 Materials Research Society
Amongst the various growth methods for group III-nitrides, MBE has provided most insight into the growth kinetics, because of the in-situ analytical measurements such as reflection high energy electron diffraction (RHEED). A variety of GaN RHEED patterns have been seen and reported in the literature, during or after MBE growth, these include (1 x 1), (2 x 1), (2 x 2), (2 x 3), (3 x 2), (3 x 3), (4 x 6) and (5 x 5). The un-reconstructed (I x 1) pattern has been shown to correspond to a monolayer of Ga, which is tightly bound to the GaN [7,8]. On top of this relatively stable Ga adlayer, there are mobile Ga adatoms which give rise to the surface reconstruction. One of the most severe problems hindering both MBE a
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