In this work comparison of the MBE growth of GaN and AlGaN on dislocation free bulk substrates and GaN/sapphire MOCVD templates is presented. The RF plasma source is used as a source of active Nitrogen. In order to achieve 2D growth, the optimalisation of the Ga to N ratio was performed. The step flow growth mode was obtained for Ga rich conditions (high Ga/N ratio) [1]. We found that for good quality GaN layers, the total Ga flux consists of two components: a) proportional to the growth rate (equal to the Nitrogen flux), b) constant flux which is desorbed from the surface during growth. The latter one strongly depends on the growth temperature. We will discuss the growth diagram and the conditions where smooth step flow growth mode without formation of the Ga droplets on the surface is achieved.
The AFM and photoluminescence data on MBE grown GaN samples confirms high layer quality. In 1 μm GaN layers grown on bulk substrates we observe very narrow donor bound exciton lines (D⁰X) with the half-width of 0.3-0.5 meV. Our results are comparable to the best PL data from GaN layers grown on bulk crystals by Grandjean et al [2] using ammonia MBE. However, in contrast to their work we do not observe acceptor bound exciton (A⁰X) in our spectra. This result indicates considerably lower acceptor concentrations in our layers.
We succeed in the growth of high quality of GaN/AlGaN heterojunctions. The growth on the semi-insulating bulk substrates results in high mobilities of 2D electron gas: μ=25000 cm²/Vs for high electron density n_2d=1x10¹³ cm^-2 at T=4.2K. Moreover at room temperature we found that electron mobility is higher than 2300 cm2/Vs for n=3x10¹³ cm^-2 which is very promising for device performance based on the dislocation free substrates. This suggest also that mobility limits at room temperature is not only govern by optical phonon scattering but also depends on scattering by dislocations. We will discuss the dependence of AlGaN spacer width and Al content on the transport properties of 2D gas. The role of polarization effects on the formation of 2D gas will be pointed out.

References.
[1] B. Heying, I. Smorchkova, C. Poblenz, C. Elsass, P. Fini, and S. Den Baars, U. Mishra, J. S. Speck, Appl. Phys. Lett 77, 2885 (2000)
[2] N. Grandjean, B. Damilano, J. Massies in "Low -dimensional nitride semiconductors" edited by B. Gil, p.121, (2002), Clarendon Press, Oxford

Corresponding author: Czesław Skierbiszewski, High Pressure Research Center, Polish Academy of Sciences,Sokołowska 29/37, 01-142 Warsaw, Poland
Phone: +48 22 6325010, Fax: +48 22 6324218, e-mail: [email protected]

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