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GaN epitaxy on off-axis Ge(111) substrates by MBE Host Publication: Finds and Results from the Swedish Cyprus Expedition: A Gender Perspective at the Medelhavsmuseet Authors: R. Lieten, Y. Zhang, S. Degroote, M. Kuijk, C. Humphreys and G. Borghs Publication Date: Oct. 2007
Abstract: III nitrides show interesting physical properties allowing many electronic and optoelectronic devices. Nitride growth is hampered by the lack of suitable substrates. Therefore nitride materials are grown on foreign substrates. Sapphire, SiC, and Si are most commonly used. These substrates have large lattice mismatches with respect to GaN of, respectively, +15%, +3.5%, and minus 17% [1]. SiC and Si do not allow direct growth of high quality GaN without Al(Ga)N interlayers whereas Sapphire is an insulator. A semiconductor substrate which allows high quality GaN growth without intermediate layers would allow the fabrication of heterostructure devices such as heterojunction bipolar transistors (HBTs). Recently we have found that direct growth of GaN on Ge(111) is relatively straightforward with Plasma Assisted Molecular Beam Epitaxy (PAMBE) [2]. A streaky reflection high energy electron diffraction (RHEED) pattern was observed immediately at the onset of growth and is maintained during growth. It was shown that a FWHM of the XRD rocking curve of only 371 arc seconds for a 38 nm GaN layer is possible. Fringes in the XRD omega/2theta scan indicated an abrupt interface between the GaN and Ge, see Figure 1. An advantage of GaN growth on Ge is the small thermal mismatch (minus 5.5%). The thermal mismatches between GaN and sapphire, SiC and Si are much larger: minus 34%, +25% and +54%, respectively [1]. Thermal mismatch leads to high thermally induced stress when cooling the sample down after growth and can lead to cracking. Structural investigations by TEM and XRD showed that it is energetically favorable to match the GaN and Ge crystals with a 4 ° twist with respect to each other. Two phases of GaN are created on a Ge(111) on axis substrate, each with a 4 ° twist. To obtain only one GaN phase, we investigated the use of off axis orientated Ge(111) substrates. From these investigations it is seen that off axis orientated Ge(111) substrates are succesful in obtaining only one GaN phase and reduce the number of defects in the GaN layer.
[1] L. Liu and J. H. Edgar, Mater. Sci. Eng., R. 37, 61 (2002).
[2] R.R. Lieten, S. Degroote, K. Cheng, M. Leys, M. Kuijk and G. Borghs, Appl. Phys. Lett. 89, 2118 (2006)
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