MBE Growth of GaN using NH 3 and Plasma Sources
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MBE Growth of GaN using NH3 and Plasma Sources A.V. Sampath, A. Bhattacharyya, I. Sandeep, H.M. Ng*, E. Iliopoulos, T.D. Moustakas Electrical and Computer Engineering, Photonics Center, Boston University 8. St. Mary’s St., Boston, MA 02215
Abstract We report on a comparative study of the growth of GaN in an arsenic free MBE system using either the method of plasma activation of molecular nitrogen or catalytic decomposition of ammonia on a heated substrate. We find that while growth with a plasma source leads to smooth films only under Ga- rich conditions, growth with ammonia leads to smooth films under ammonia-rich conditions. In both cases we find a 2x2 surface reconstruction when using an AlN buffer, which is evidence that material grown with this buffer layer has the Ga-polarity. In the case of plasma growth we also investigated the use of a GaNbuffer and found that at the growth temperature the surface is unreconstructed, however it undergoes 3x3 reconstruction upon cooling to 300 ºC. This observation is evidence that material grown on a GaN buffer has the N-polarity.
Introduction The III- Nitrides is an important family of direct band gap semiconductors with potential applications in optoelectronic, electronic and electromechanical devices [1,2]. The deposition of GaN and other III-Nitrides by MBE requires the development of appropriate nitrogen sources, since molecular nitrogen (N2) does not chemisorb on Ga due to its large binding energy of 9.5 eV. Two approaches have led to the deposition of high quality films at relatively high growth rates (~1 µm/hr). The first employs plasmas (microwave or RF) to activate molecular nitrogen [3], while the second uses the catalytic dissociation of ammonia (NH3) on a heated GaN surface [4,5]. The kinetics of growth using plasma sources was found to be different than what has been observed for other III/V compounds. Specifically, it was found that two dimensional (2D) growth occurs under group-III rich conditions, while group-V rich conditions lead to three dimensional (3D) growth [6]. In the case of GaN growth using ammonia, both Grandjean and coworkers [7] and Held and coworkers [8] reported observing RHEED oscillations at growth temperatures below 700 ºC which are characteristic of 2D nucleation and growth. Grandjean and coworkers also reported that during the growth of GaN at 830 ºC, films are smoother under group-V rich conditions. In particular, they found that the film roughness increased from 3.6 nm to 22.3 nm as the V/III ratio decreased from 4 to 1. Simultaneously, RHEED studies on the same samples showed a transition from streaky-like to spotty-like diffraction. This result is in contrast to that reported for plasma-assisted growth, where Ga-rich conditions are necessary for the deposition of smooth films. In this paper we report on a comparative study of the growth of GaN in the same MBE system using either plasma-activated molecular nitrogen or catalytically decomposed ammonia.
Experimental Method All the films reported in this paper were grown in a Varian Ge
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