GaN nanocolumns grown on Si(111) by plasma-assisted MBE: Correlation of structural and optical properties with growth parameters.
Résumé
The morphology and low-temperature photoluminescence spectra of GaN samples grown by plasma-assisted molecular beam epitaxy on Si(111) were systematically studied as a function of impinging Ga/N flux ratio and growth temperature (730-850ºC). Two different growth regimes were identified: compact and nanocolumnar. A growth diagram was established as a function of growth parameters, exhibiting the transition between growth regimes, and showing under which growth conditions GaN cannot be grown due to thermal decomposition and Ga desorption. The results indicate that adatoms diffusion length and the actual Ga/N ratio on the growing surface are key factors to achieve nanocolumnar growth. A correlation can be established between growth parameters such Ga flux and growth temperature, on one hand, and structural characteristics such as nanocolumn (NC) densities and diameters, on the other hand. Clear correlation is obtained, too, between sample morphology and the relative intensities of emission lines related to structural defects. All lines comprised between 3.10 and 3.42 eV rapidly lose intensity as growth conditions tend to reduce the NC density, thus relating the associated defects to the NC coalescence. On the other hand, the doublet line around 3.45 eV, well-known characteristic of GaN NC samples, shows an opposite behaviour and therefore its different nature.