Abstract
This paper describes our recent progress on InN, In‐rich In~x~ Ga~1–x~ N, and InN/In~x~ Ga~1–x~ N quantum wells (QWs) grown by radio‐frequency plasma‐assisted molecular‐beam epitaxy. Among the essential growth sequences to obtain high‐quality InN, the nitridation process of (0001) sapphire substrates was reexamined. It was found that the lower‐temperature and longer–period nitridation‐process was very effective in improving crystalline quality of InN films. We succeeded in dramatically improving c ‐axis orientation of InN films without deteriorating their a ‐axis orientation by nitridating the substrates at a relatively low‐temperature of 300 °C for a relatively long period of 2 h. The full widths at half maximum (FWHMs) of (0002) X‐ray rocking curves as narrow as 1 arcmin were obtained from only 400 nm thick InN films. These FWHMs are the narrowest value ever reported for InN and moreover approximately a twentieth part of the values obtained from our conventional InN films with a similar thickness, which were grown via the conventional nitridation process carried out at 550 °C for 1 h. Furthermore interference fringes in X‐ray diffraction (ω –2__θ__ scan) were observed from these improved InN films. These high crystalline quality InN layers have been employed as a template for the growth of In‐rich In~x~ Ga~1–x~ N layers (0.70 ≤ x ≤ 0.94). The resultant In~x~ Ga~1–x~ N layers have shown dramatic improvements in not only the surface morphology but also both the a ‐ and c ‐axis orientations. By employing In~0.8~Ga~0.2~N layers of improved quality as the bottom barrier layer, InN/In~0.8~Ga~0.2~N multiple quantum well (MQW) and single quantum well (SQW) structures with different well widths were fabricated. Clear satellite peaks of X‐ray diffraction were observed from these MQW structures. Both of these SQW and MQW structures have exhibited a blue shift of the photoluminescence peak energy with decreasing well width. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)