Comparative Analysis of Strain and Stress in MBE and MOCVD grown GaN thin films on sapphire
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In recent years, GaN and related compounds have attracted a lot of academic as well as commercial interest. This is due to the potential applications for UV-based opto-electronic applications as well as high-temperature electronics [1]. Very bright blue and green InGaN single quantum well diodes light-emitting diodes have been developed and commercialized [2], and a laser diode consisting of 4 InGaN multi quantum wells has been reported to have a room temperature cw-operational lifetime of more than 10.000 hrs [3]. For the most part, epitaxial layers of GaN are deposited on substrate materials like sapphire, SiC or GaAs. The lattice mismatch between layer and substrate as well as the difference in thermal expansion coefficient results in stress which can reach values of up to 1.2 GPa [4], either compressive or tensile. Though methodologies have been empirically developed to control stress to some extent, very little fundamental understanding of the causes and impact of stress on material performance has been gained. It was not until very recently that the influence of stress on major GaN film properties has been systematically explored. It has been found that stress in the film during growth can alter the surface mobility of Ga ad-atoms and consequently the growth mode, thereby vastly influencing the surface morphology [5]. Unlike most other III-V semiconductors, the covalent radii of the two matrix elements gallium and nitrogen differ by a factor of almost two. Conse-
"Lawrence Berkeley Lab, m/s 2-200, 1 Cyclotron Road, (510) 486-5083(ph.) / -5530 (Fax), [email protected] 447 Mat. Res. Soc. Symp. Proc. Vol. 482 0 1998 Materials Research Society
quently, deviations from stoichiometry are expected to result changes in the lattice consant of the thin film that can be considered as a hydrostatic strain. In fact, this prediction has been experimentally verified for the MBE growth process [6]. Furthermore, it has been shown that the incorporation ofdoping atoms, such as Si [7,8] and Mg [9], changes the amount of stress in the film as well. Consequently, by proper tailoring of the lattice constant via engineering the main layer's strain and stress components, the efficiency of dopant incorporation should be influenced. In the case of Mg, it has been proven that by variation of the c lattice constant the incorporated amount of Mg can be changed by more than two orders of magnitude [9]. Also, the optical properties of epitaxially grown GaN films are effected by stress. For the bandedge related luminescing transitions, stress was found to be the main broadening mechanism at helium temperatures [10]. By appropriate design of the growth conditions, a line width of the donor bound exciton as narrow as 1.2 meV was accomplished for a completely relaxed MBEgrown film being only 0.5 [m thick. For heteroepitaxially grown thin films, this benchmark is unprecedented and has only been surpassed by homo-epitaxially MOCVD grown GaN (linewidth of 0.7 meV) [11].
It is the purpose of this study to explore the causes of biaxial and hy
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