Thermal annealing and surface pre-treatment methods
Annealing of InGaN layers is also used to initiate quantum dot formation. After deposition of a thin Ino.20Gao.80N layer annealing at the quantum well growth temperature in pure nitrogen atmosphere leads to In droplets on the surface. These In droplets may transform into InGaN quantum dots upon capping with GaN. The density of these droplets increases from 5-109 to 3-1010 cm~2 with decreasing growth temperature of the cap layer between 900° C and 700° C. Using an annealing atmosphere of H2/NH3 a meander-like InGaN layer surface is created. These structures are attributed to additional etching effects which hydrogen has on InGaN layers. Similar structures are obtained for layers with very high Incontent of xIn >90% when annealed at 700°C and 750°C, respectively. Spinodal decomposition is predicted to take place at such high In concentrations [39, 50]. By TEM analysis of uncapped InGaN layers thermally treated at 700° C and 750°C, large truncated pyramids and spherical islands with 20 nm height and 40-50 nm base length are found besides flat structures of 1-1.5 nm height. High In concentrations of 80-90% are measured for the large pyramidal and spherical objects, whereas only 20% In is remaining in the flat structures. Upon overgrowth, the large objects dissolve and the excess In may form a quantum well layer with 6-7% In above the layer containing the flat meandering structures. Quantum dots are identified by sharp emission lines emerging in low-temperature luminescence spectra and by an increased radiative efficiency at room temperature.
Opposite to InGaAs/GaAs-based QDs which are very sensitive to surface recombination, ex situ surface patterning methods may be effective to fabricate luminescent InGaN quantum dot structures. This can be inferred from the relatively strong luminescence intensities of InGaN surfaces which point to low non-radiative surface recombination rates.
A very effective way of producing InGaN quantum dots is to pre-treat the growth surface prior to the InGaN growth. By supplying Si to the growth surface the growth mode for subsequent GaN and InGaN layers changes from 2D to 3D growth, thus leading to high-density arrays of quantum dots (see Fig. 5.17) [16, 48]. Originally, an anti-surfactant effect by silicon atoms suppressing In diffusion on the surface has been assumed. More probable is the formation of SiNK nano-masks on the surface, which lead to selected area growth within the openings of the nano-masks [49, 64]. This is further supported by growth experiments in which Si and ammonia are simultaneously fed into the growth chamber to intentionally allow for SiNK formation on the growth surface. Similar three-dimensional growth modes are obtained for appropriate exposure times, and thin SiN^-layers are in such cases identified beneath the InGaN quantum dots. For extended Si and ammonia treatment times InGaN growth can be completely suppressed . Very high quantum dot densities approaching 1011 cm~2 are realized with Si pre-treatment. A disadvantage of the approach is related to the Si donor properties in the nitride material system. Due to the spatial and temporal proximity of the Si treatment to the quantum dot growth it is very
Fig. 5.17. AFM image of InGaN quantum dots grown by MOVPE on Si pre-treated GaN surfaces. (Reprinted with permission from , © 1996 American Institute of Physics.)
likely that Si atoms will incorporate into quantum dots where Si would act as a charged impurity state. Conclusive evidence that Si atoms are not incorporated into QDs are not reported so far.
Another approach of surface pre-treatment is wet-chemical removal of SiO2 masks completely covering a GaN surface. These masks are deposited ex situ without any subsequent patterning. Direct growth of InGaN layers on such surfaces results in 3D growth and QD ensembles with a density of 9-1010 cm~2 result. As Fig. 5.18 shows, average lateral QD size is increasing, while density decreases gradually with increasing deposition times, indicating coalescence of the islands.