Modeling GaN Growth by Plasma Assisted MBE in the Presence of Low Mg Flux
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Internet Journal Nitride Semiconductor Research
Modeling GaN Growth by Plasma Assisted MBE in the Presence of Low Mg Flux Nathan Sipe1 and Rama Venkat1 1University
of Nevada, Las Vegas,
(Received Wednesday, August 15, 2001; accepted Wednesday, January 16, 2002)
A rate equation model is developed to investigate the plasma assisted MBE growth of GaN in the presence of a fractional monolayer of Mg. Four distinct cases were identified and modeled – (i) Galimited regime (ii) Low N-limited regime (iii) Medium N-limited regime and (iv) High N-limited regime. In the model, it is assumed that Ga arriving on a Mg site undergoes faster incorporation into the epilayer through an exchange reaction compared to Ga arriving directly on a N surface. Additionally the incorporation rate of Ga was assumed to depend on the size of the Ga cluster. The results of the model are in good agreement with that of experiments. The non-monotonic behavior of growth rate with Ga flux for moderate Mg coverage is explained based on the incorporation rate dependence of Ga on the cluster size.
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Introduction
Group III nitrides are important for many opto-electronic devices. The future technologies based on microelectronic devices such as blue light emitting diodes, ultra-violet lasers for high capacity storage devices, and high temperature, high field heterojunction bipolar devices depend on these materials [1]. One method of producing III-nitride based device structures is molecular beam epitaxy. The understanding of the surface kinetics of the growth processes in molecular beam epitaxy is limited despite active experimental research. Thorough understanding of the growth process will help realize reliable production of high quality material in large quantities, which in turn will help in commercializing III-nitride based devices. In order to make useful solid-state devices, a p-type dopant must be identified. Mg with its thermal activation energy of ~200 meV, is the most suitable candidate at the current level of technology [2]. It is, therefore, important to understand the influence of Mg on the surface kinetics of the GaN MBE growth process. In addition to acting as a p-type dopant, Mg has the benefit of inhibiting Ga droplet formation under N-limited conditions in the MBE growth of GaN [3]. Ga droplets get incorporated into the growing epilayer as pockets of Ga, thus resulting in poorer quality materials [4]. For high Mg fluxes resulting in 1/4 to 1/2 monolayer (ML) surface
coverage, the Mg incorporates into the epilayer [2]. However, increasing Mg above 1 ML results in reversal from Ga-polarity surface to N-polarity surface and in lower Mg incorporation [5]. Mg has a much different behavior below 1/4 ML surface coverage. At a low Mg flux, the growth rate increases up to 30% for addition of Mg up to 1/4 ML [3]. Improved growth rates are necessary for producing high yield of material in a short amount of time. Daudin et al. [3] found three different types of growth behaviors for three different coverages of Mg on the surface (CMg). When CMg
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