Aluminum nitride (AlN) has a direct band gap exceeding 6 eV, so it is a promising material for light-emitting devices (LEDs) with ultra-short wavelengths[1].  Conventional AlN are grown on polar c -plane, especially commercial products. In this case, due to spontaneous/piezoelectric polarization of wurtzite structure, large internal electric field is induced along the [0001] direction, which spatially separates electrons and holes to degrade the performances of devices. Furthermore, due to the strong polarization, the a-plane AlN LED showed strong emission along the surface normal, while the conventional c-plane AlN LED showed weak emission along the surface normal[2]. To obtain higher efficiencies, research on nonpolar AlN is necessary due to the suppression of built-in electrostatic fields in the polar direction.

 This study reports the growth of nonpolar AlN films on r-plane sapphire substrate by plasma-assisted molecular beam epitaxy (PAMBE) with different III-V ratio. Sapphire substrates of r-plane orientation were cleaned by organic solvents and thermally cleaned in UHV. The growths of main AlN films were started after a substrate-nitridation step at 860 °C.  The effects of III-V ratio on the films were characterized by reflection high energy diffraction (RHEED), atomic force microscopy (AFM) and high-resolution x-ray diffraction rocking curves (XRCs). RHEED in-situ observation showed that these films had two crystal orientations or only one crystal orientation depended on the III-V ratio. The higher III-V ratio growth condition prefers a-plane AlN films growth direction. More interesting, in only one growth process, there could be a change from two crystal orientations to one crystal orientation of AlN film and vice versa. From the AFM results, the AlN film which was grown with higher III-V ratio provides smoother surface and smaller grains. The crystal qualities of these films were compared based on the full width at half maximum values measured by XRCs. The a-plane AlN film grown in higher III-V ratio has better crystal quality. It is opposite to the case of c-plane AlN grown on c-plane sapphire in the same condition of our survey.

References

[1]  J. Li et al., Appl. Phys. Lett. 83, 5163-5165 (2003).
[2]  Y. Taniyasu et al., Appl. Phys. Lett. 96, 221110 (2010).

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