Compact solid-state lamps based on light-emitting diodes (LEDs) are of current technological interest as an alternative to conventional light bulbs. The brightest LEDs available so far emit red light and exhibit higher luminous efficiency than fluorescent lamps. If this luminous efficiency could be transferred to white LEDs, power consumption would be dramatically reduced, with great economic and ecological consequences. But the luminous efficiency of existing white LEDs is still very low, owing to the presence of electrostatic fields within the active layers. These fields are generated by the spontaneous and piezoelectric polarization along the [0001] axis of hexagonal group-III nitrides--the commonly used materials for light generation. Unfortunately, as this crystallographic orientation corresponds to the natural growth direction of these materials deposited on currently available substrates. Here we demonstrate that the epitaxial growth of GaN/(Al,Ga)N on tetragonal LiAlO2 in a non-polar direction allows the fabrication of structures free of electrostatic fields, resulting in an improved quantum efficiency. We expect that this approach will pave the way towards highly efficient white LEDs.

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We investigate the growth of M-plane GaN(1 100) on g-LiAlO 2 (100) by molecular beam epitaxy. The crystal orientation and structural properties of buffer layers are examined by means of reflection high-energy electron diffraction, high-resolution transmission electron microscopy, X-ray diffraction, Raman scattering, and atomic force microscopy. The layers are shown to be single-phase GaN(1 100) within the measurements' sensitivity. In contrast to the ubiquitous C-plane GaN[0001] orientation, the M-plane of wurtzite nitrides is free of electrical polarization. This is experimentally verified for (Al,Ga)N/GaN heterostructures by continuous-wave and time-resolved luminescence.

Electronic excitations in nominally undoped GaN have been investigated by Raman scattering. Peaks in the range between 18.5 and 30 meV have been assigned to internal shallow donor transitions in cubic and hexagonal GaN, respectively. The photon energy dependence of the scattering efficiencies in the cubic phase is explained by Raman scattering in resonance with the so-called ‘‘yellow’’ luminescence transitions. This interpretation supports models for the notorious yellow luminescence in which shallow donors are involved. These shallow donors most likely do not originate from native point defects.

We report on the selectivity of growth on patterned GaAs (311)A substrates by solid-source molecular beam epitaxy. For mesa stripes oriented along the [01-1] direction, the selectivity of growth is qualitatively different from that on patterned GaAs (100) substrates with a higher growth rate on one of the side facets of the stripes. This growth mode develops a convex curved surface profile enclosing thicker wirelike regions of GaAs due to preferential migration of Ga atoms from both sides toward the sidewall leaving behind thinner regions on the adjacent mesa top and bottom areas. A mechanism for the formation of the surface profile is proposed.