Metalorganic vapor phase epitaxy
In 1996, Tanaka et al. reported the very first result on GaN QD growth employing silicon surface pre-treatment in MOVPE growth . The method allows growth of GaN quantum dots on Al^Ga^^N (0.07 < x < 0.20) buffer layers by MOVPE. In this case a SiNK mask may form and the growth of GaN resembles a mode which may be best described by selective area growth (SAG). Self-assembled QD formation via a 2D/3D transition was observed for GaN growth on AlN in a temperature range of 960-970° C using a horizontal reactor geometry. The apparently large difference to MBE growth temperatures is to a great extent caused by the indirect measurement with a thermocouple, reading an inner susceptor temperature which may be offset to the surface temperature by up to 150°C. As shown in Fig. 5.4, the QD density can be effectively controlled between 2-108 and 5-1010 cm~2 by tuning the GaN coverage from 3.4 and 9 MLs. Besides, control of the dot density is achieved by growth interruptions of 3-30 s and by tuning the V/III ratio during GaN growth. High QD densities of >1010 cm~2 are obtained at very low V/III ratios of 20-30, whereas for V/III ratios of >10000 one finds densities between 107 and 109 cm~2 [17, 18]. This agrees well with the conclusion
Fig. 5.4. Surfaces for different coverages of GaN (AFM images) grown by MOVPE on AlN(0001)/sapphire. (Reprinted with permission from , © 2002 American Institute of Physics.)
from MBE results that the surface diffusion of Ga atoms is regulated by the nitrogen supply. Interestingly, for high V/III ratios the critical layer thickness for QD formation is well below 2.5 MLs and non-exponential growth of the QD density is observed. These discrepancies to MBE results has been ascribed to tolerances on the growth-rate estimations as well as to possibly non-homogeneous wetting layer thicknesses which might be locally exceeding the critical value of about 2 MLs. There is also a noticeable reduction in lateral size of the QD pyramids from 35 nm to 20 nm when the V/III ratio is reduced, and hence the density is increased [17, 18]. Vertically ordered stacks of up to ten QD planes separated by 20 nm AlN spacer layers are also fabricated using MOCVD. The relatively large vertical distance of 20 nm as compared to the upper limit of 8 nm seen for MBE-grown, vertically correlated QD samples is explained by the larger lateral QD size (20 nm to 10 nm) . Similar to the results obtained by MBE, photoluminescence spectra also show reduced size broadening and red-shift of the peak wavelength upon stacking, indicating more homogeneous and larger dot sizes.