Occurrence of 'accidental' InN quantum dots in indium gallium nitride/gallium nitride heterostructures

Nakamura and co-workers were first to report the observation of spontaneously formed In-rich clusters in InGaN quantum wells (QW), using transmission electron microscopy (TEM) [1,2]. Some of the present authors have argued that the exceptional optical efficiency of nitride devices is due to the presence of nearly pure InN QD which form accidentally during crystal growth [3]. Pure InN QD with very small radius would emit light in the required spectral region, under the opposing effects of intrinsic electric fields and quantum confinement [4]. We have recently used extended X-ray absorption fine stucture to examine nanostructure [5] of molecular beam epitaxial (MBE) quantum boxes (QB), and photoluminescence excitation spectroscopy to examine their optical properties [6]. As revealed by AFM, QB are densely packed mesoscopic structures, 10-40 nm wide and 2-3 nm high. Our analysis of the QB EXAFS shows that the filling factor is of order 1%: each QB therefore contains a single QD. The optical evidence suggests that InN dots may be cubic. In contrast, TEM shows that QD in thick epilayers are sparsely distributed [7]. We re-examine recent TEM data on QW in the light of these results.