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Wurtzite AlN is widely used for deep ultraviolet optoelectronic devices (DUV), which are generally grown along the [0001]-direction of the wurtzite structure on currently available substrates. However, huge internal electrostatic fields are presented within the material along [0001] axis induced by piezoelectric and spontaneous polarization, which has limited the internal quantum efficiency of AlN based DUV LEDs dramatically. The internal fields can be strongly reduced by changing the epitaxial growth direction from the conventional polar c-direction into less polar crystal directions. Twinned crystal is a crystal consisting of two or more domains with the same crystal lattice and composition but different crystal orientations. In other words, twins can be induced to change crystal directions. In this work we demonstrated that the epitaxial growth of () semi-polar AlN on (0001) AlN by constructing () and () twin structures. This new method is relative feasible than conventional methods and it has huge prospect to develop high-quality semi-polar AlN.
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(a) X-ray 2θ/ω scan of AlN thick film; (b) Cross-sectional TEM image of AlN thick film, which contains the sawtooth layer and the columnar layer; (c) SAED pattern taken from the sawtooth layer of (b); (d,e) are the SAED patterns taken from the neighboring column A and B in (b), respectively.
Just by () twin growth mode, AlN thick film grown on (0001) sapphire substrate changed growth direction from (0001) polar to () semi-polar. Therefore, the formation of () twin boundary is crucial for the change of growth direction. As () twin structures are formed on the () facets, the formation of the first sawtooth layer is the precondition. This process could be explained by the equilibrium crystal shape theory (ECS)18,19. ECS theory indicates that the surface energies of crystal plane vary with the growth temperature. The surface free energy of a certain crystal plane ω is20
(a) XRD 2θ/ω scan of AlN film grown on patterned 6H-SiC by ELOG, which indicates that 0002 and 101 diffraction peaks coexist in the film. (b) TEM cross sectional image of AlN film. (c,d) are SAED pattern and HRTEM image taken from the left interface. (e,f) are SAED pattern and HRTEM image taken from the right interface. Both interfaces are () incoherent twin boundaries.
(a) TEM cross sectional image of AlN film. (b) XRD 2θ/ω scan of AlN film indicates that 0002 and peaks coexist in the film. (c,d) are SAED patterns correspond to the left interface and the right interface in (b), respectively, which indicate both interfaces are () twin boundaries preliminarily. (e) HRTEM image taken from the interfaces between triangle and the column, which demonstrates that the two interfaces are () coherent twin boundaries further.
Therefore, []-directed AlN thick films can be grown on (0001) sapphire or 6H-SiC substrates by twin growth mode (both () and () twin structures). The geometrical features of twin structure can change the growth orientation of crystals, which shows schematically in Fig. 6. The black and blue dash lines represent cross section of (0001) and () planes, while red and yellow solid lines are those of () and () facets on surface, respectively. Considering the interfacial angle between (0001) and () is 61 and the value between (0001) and () is 32. If the () facet on surface is formed firstly (Fig. 6a), it will develop () twin structure. If the facet on surface is () plane (Fig. 6b), then () twin structure will be formed. It needs to be pointed out that the real situations may be much more complicated than the simple picture presented here. However, further research work towards this promising method of fabricating other kind of semi-polar AlN thick films may be worthwhile.
In summary, () semi-polar AlN were grown on conventional c-plane sapphire by HVPE. It was found that twin structures were formed at the interface of (0001)-polar and () semi-polar layers, which changed the growth direction and resulted in the growth of semi-polar AlN materials. TEM measurements showed that tilt facets occurred on the surface during the growth of the (0001)-polar layer, which may be crucial for the following twin growth mode. Based on the idea, (0001) polar AlN with different tilt facets on the surface were grown on trench-patterned c-plane 6H-SiC by ELOG technique. Then () semi-polar AlN was successfully fabricated on the polar layer by forming () or () twin structures, which depended on the tilt angle of facets on the surface. This work presented a method for growing semi-polar AlN on conventional c-plane sapphire or 6H-SiC. Further research work towards this promising fabrication method of semipolar or nonpolar AlN may be worthwhile.
XRDand SEM Analysis of AlN films: The characterization of AlN template was measured by x-ray diffraction (XRD) on a Bruker D8 Discover. The cross sectional morphology of the AlN film was characterized by scanning electron microcopy (SEM) on a Quant 400.
How to cite this article: Liu, T. et al. Nucleation and growth of () semi-polar AlN on (0001) AlN by Hydride Vapor Phase Epitaxy. Sci. Rep. 6, 26040; doi: 10.1038/srep26040 (2016).
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