Microstructure, growth mechanisms and electro-optical properties of heteroepitaxial GaN layers on sapphire (0001) substr
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Internet Journal o f
Nitride S emiconductor Research
Volume 1, Article 19
Microstructure, growth mechanisms and electro-optical properties of heteroepitaxial GaN layers on sapphire (0001) substrates S. Christiansen, M. Albrecht, W. Dorsch, H. P. Strunk Institut für Werkstoffwissenschaften, Lehrstuhl VII, Universität Erlangen-Nürnberg C. Zanotti-Fregonara, G. Salviati CNR-MASPEC Institute A. Pelzmann, M. Mayer, Markus Kamp , KJ Ebeling Abteilung Optoelektronik, Universität Ulm This article was received on June 11, 1996 and accepted on October 22, 1996.
Abstract We investigate the structure, growth morphology and the related electro-optical properties of gallium nitride (GaN) films deposited on (0001) sapphire substrates by gas source molecular beam epitaxy (GSMBE) and use transmismission electron microscopy, atomic force microscopy and scanning tunneling microscopy, photoluminescence (PL) and cathodoluminescence (CL). We find two types of specimens: one type which shows a strong UV luminescence (band-to-band transition at 358nm/3.46eV) in CL and PL and only faint yellow luminescence (Gaussian shaped CL/PL peaks at around 528nm/2.35eV), specimen ‘B‘, and another type, which shows a strong UV and a comparably strong yellow luminescence, specimen ‘Y‘. These two types of specimens have a rough layer surface, specimen ‘Y‘ even an islanded one with, facetted hexagonal islands with a width of 1-2µm at a height of 50nm. A correlation of spectrally resolved CL images to the observed defect structure shows: (i) the yellow luminescence is homogeneously distributed over the complete specimen for ‘B‘ and ‘Y‘ specimens. Our investigations strongly suggest the yellow luminescence to be related to screw dislocations with , which are found randomly distributed in ‘B‘ and ‘Y‘ specimens with a high density of 1.3·109cm-2; (ii) the strong UV luminescence in ‘Y‘ specimens is located in the troughs between adjacent surface islands, where dislocations essentially in small angle grain boundaries of edge type, i.e. with or are located; (iii) in the case of the ‘B‘ specimens these dislocations are randomly distributed and so is the luminescence.
1. Introduction Today α-GaN (hexagonal polytype with wurzite structure) can be deposited successfully by metalorganic chemical vapor deposition (MOVPE) [1] [2] [3] as well as molecular beam epitaxial (MBE) methods [4] [5] [6]. It is now the timely task to correlate differences in electro-optical properties (i.e. photoluminescence (PL), cathodoluminescence (CL)) of those films to differences in the micro-and defect structure. While in high quality α-GaN layers only band-to-band luminescence at 358nm/3.46eV, which is responsible for the UV light emission [7] [8] [9] is observed, in lower quality material in addition a so-called yellow luminescence at about 528nm/2.35eV [9] [10] [11] [12] occurs. It is generally accepted (cf. [12]), that the undefected volume of α-GaN is responsible for the UV luminescence, while the origin of the yellow luminescence which acts as a lifetime killer and suppresses t
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