Optical characteristics of hexagonal and cubic GaN self-assembled quantum dots

Abstract

Studies on the optical properties related to built-in internal electric field and carrier localization present in the GaN self-assembled quantum dots (QDs) are essential for the physical interest in atomic-like confined system and the visible and ultraviolet light emitting applications. We have systematically studied the optical properties of hexagonal GaN (<i>h</i>-GaN) and cubic GaN (<i>c</i>-GaN) self-assembled QDs by means of photoluminescence (PL), PL excitation (PLE), cathodoluminescence (CL), and time-resolved PL experiments. The GaN self-assembled QD samples were grown in Stranski-Krastanov mode by plasma-assisted molecular beam epitaxy. The substrates for the growth of <i>h</i>-GaN and <i>c</i>-GaN were 6H-SiC and 3C-SiC, respectively. With increasing temperature, the PL intensity of GaN quantum wells was dramatically decreased while that of GaN QDs was not changed much. From the wavelength-resolved CL images, strong carrier localization in the QD confinement was clearly observed. An apparent Stokes-like shift between PLE absorption edge and PL emission from the <i>h</i>-GaN QDs increases with increasing detection wavelength (so, with QD size), which is attributed to the separation of wavefunction overlap due to the built-in internal field present in the QDs. From the time-resolved PL experiments, we found that the measured lifetime of the <i>h</i>-GaN QDs emission increased with emission wavelength (i.e., with QD size), while that of the <i>c</i>-GaN QDs kept almost constant. It is concluded that the <i>h</i>-GaN QD emissions are strongly influenced by built-in internal electric field as well as carrier localization in the QDs.