PEMBE-Growth of Gallium Nitride on (0001) Sapphire: A comparison to MOCVD grown GaN
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Internet Journal o f
Nitride S emiconductor Research
Volume 1, Article 15
PEMBE-Growth of Gallium Nitride on (0001) Sapphire: A comparison to MOCVD grown GaN H. Angerer, O. Ambacher, R. Dimitrov, Th. Metzger, W. Rieger, M. Stutzmann Walter Schottky Institut, Technische Universität München This article was received on June 2, 1996 and accepted on October 8, 1996.
Abstract Thin films of GaN on c-plane sapphire were grown by plasma-enhanced molecular beam epitaxy (PEMBE). The influence of different growth conditions on the quality of the epitaxial layers was studied by x-ray diffraction (XRD), atomic force microscopy (AFM) and Hall measurements. For low deposition temperatures, the growth of a thin buffer layer of AlN results in a decrease of the XRD rocking curve full width at half maximum (FWHM) but also in poorer quality in electronic and optical properties. Samples of 3µm thickness with 570 arcsec FWHM in the XRD rocking curve, a near band gap PL-emission FWHM at 5 K of 7 meV, charge carrier densities of ne = 2 x 1017 cm-3, and Hall mobilities of 270 cm2/Vs at 300 K were grown without a buffer layer. A comparison of the morphology and XRD rocking curves with those of GaN films deposited by metalorganic chemical vapour deposition (MOCVD) shows that the two methods have different growth mechanisms.
1. Introduction Metalorganic chemical vapour deposition (MOCVD) and molecular beam epitaxy (MBE) are the most common methods for epitaxial growth of GaN films. Both techniques have their advantages. The MOCVD is a comparatively inexpensive method, because ultrahigh vacuum is not necessary. The MBE benefits from the clean environment and therefore low concentration of unwanted extrinsic defects, excellent control of the deposition parameters, and lower deposition temperatures which should lead to fewer thermally induced intrinsic defects. The plasma-enhanced molecular beam epitaxy (PEMBE) uses nitrogen radicals, produced in a nitrogen rf-plasma. Hydrogen, probably responsible for the passivation of dopants such as magnesium [1], is thus avoidable during PEMBE growth in contrast to MBE using other nitrogen sources [2]. Despite these advantages, the best film quality has so far been achieved by the MOCVD method [3]. In this paper we discuss the PEMBE growth of GaN and emphasize some differences in MOCVD and PEMBE-grown GaN.
2. Experimental The PEMBE GaN films are grown in a Tectra Mini MBE® chamber with a background pressure of 1 x 10-10 mbar . The gallium and aluminum sources are conventional hot lip and cold lip effusion cells, respectively. As nitrogen source, an Oxford Applied Research CARS25 radical beam source is used. In this source the molecular nitrogen is decomposed in a radio frequency (rf) plasma at 13.56 MHz. The activated nitogen is predominantly atomic nitrogen and has only a negligble fraction of ions [4]. This is the main difference to the electron cyclotron resonance (ECR) plasma source, [4] the other commonly used source for PEMBE growth of III-V nitrides. During the growth, the samples were rotated
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