Materials Science Forum Vol. 590

Abstract: Nitride-based optoelectronic devices prepared in the c orientation have been successfully introduced to the global marketplace and are changing the way we think about lighting. A part of the research interest has shifted toward nonpolar and semipolar orientations, which has the potential to broaden the scope and impact of this technology. This is because quantum-well structures prepared in nonpolar and semipolar orientations are able to suppress the quantum-confinement Stark effect, which has a negative impact on optoelectronic device performance. The lower crystal symmetry of such orientations provides spontaneously polarized light emission. Despite these attractive properties of nonpolar and semipolar orientations, the corresponding materials growth is not trivial. The present chapter discusses our efforts on growth of III-nitride materials in nonpolar and semipolar orientations and the related material properties.

Abstract: Threading dislocations (TDs) in (Al,In,Ga)N semiconductors are known to affect the luminescence efficiency of near-band-edge (NBE) emissions in bulk films and quantum structures. However, the principal role of point defects such as vacancies on the luminescent properties has not been fully understood. In this article, impacts of point defects on the luminescence quantum efficiency of NBE emissions and on the intensity of deep emission bands will be described, based on the results of steady-state and time-resolved photoluminescence (TRPL) and positron annihilation measurements. The room temperature nonradiative lifetime (τNR) of the NBE excitonic photoluminescence (PL) peak in polar (0001) and (000-1) , nonpolar (11-20) and (10-10), and zincblende (001) GaN layers prepared by various growth techniques was shown to increase with the decrease in concentration or size of Ga vacancies (VGa) and with the decrease in gross concentration of point defects including complexes, leading to an increase in the NBE PL intensity. As the edge TD density decreased, the concentration or size of VGa tended to decrease and τNR tended to increase. However, there existed remarkable exceptions. The results indicate that the nonradiative recombination process is governed not by single point defects, but by certain defects introduced with the incorporation of VGa, such as VGa-defect complexes. Similar relations were found in AlxGa1-xN alloy films grown by metalorganic vapor phase epitaxy: i. e. τNR at room temperature increased with the decrease in the concentration of cation vacancies (VIII) and with the decrease in gross concentration of point defects. In addition to nonradiative processes, the VIII concentration was found to correlate with the intensity ratio of characteristic deep emission band to the NBE emission (Ideep/INBE). For example, Ideep/INBE at low temperature for the deep emission bands at 4.6, 3.8, and 3.1 eV of AlN epilayers grown by NH3-source molecular beam epitaxy had a linear correlation with the concentration or size of Al vacancies (VAl). Since the relative intensities of 3.1 eV and 3.8 eV bands increased remarkably with lowering the supply ratio of NH3 to Al (V/III ratio) and growth temperature (Tg), they were assigned to originate from VAl-O as well as VAl-shallow donor complexes. The VAl concentration could be decreased by adjusting the V/III ratio and Tg. In the case of AlxGa1-xN alloys, the concentration or size of VIII and Ideep/INBE at 300 K increased simultaneously with the increase in x up to approximately 0.7. Similar to the case for GaN and AlN, the deep emission band was assigned as being due to the emission involving VIII-O complexes.

Abstract: Nanoscopic optical characterization using scanning near-field optical microscopy was performed on both InxGa1-xN/GaN single quantum wells (SQWs) grown on polar (0001) orientation and a semipolar (1122) microfacet SQW fabricated by a re-growth technique. The photoluminescence intensity of a conventional (0001) SQW emitting in the blue was completely independent of the threading dislocations (TDs) due to the small diffusion length less than 100 nm. In contrast, the photoluminescence intensity was well correlated with the TDs in the sample emitting in the green due to the association of In-rich clusters with dislocations, and the effect was enhanced by the larger diffusion length contribution from the longer radiative recombination lifetime. It was found that in a (1122) SQW, the suppression of the piezoelectric field leads to orders-of-magnitude faster radiative lifetime and consequently, a shorter diffusion length. In addition, the highest internal quantum efficiency was approximately 50% at 520 nm, which is about 50 nm longer than in (0001) QWs, suggesting that (1122) QWs are suitable for green emitters.