Site-selective growth

Deterministically addressable individual quantum dots are of major interest for devices operating under the constraints of purely quantum-mechanically defined electronic states like in atoms. Nitride-based quantum dots are promising sources for single photon emission due to large conduction and valence band offsets between AlN-GaN-InN, which may yield large confinement potentials and energy level splittings larger than the kinetic energy at room temperature [73, 74]. Nucleation of single quantum dots at pre-defined sites during growth is possible only if the surface free energy is locally altered, whereby nucleation of atomic species such as In or Ga atoms is attractive at those sites. As the base size of a quantum dot takes on only a few nanometers, the surface free energy has to be changed on about the same length scale. Many technologies exist nowadays to create nanometer-sized patterns on a surface for ordering of nanostructures [75]. For example, electron-beam lithography, nano-imprinting, and focused ion beam implantation were successfully applied for site-controlled growth of InGaAs/GaAs quantum dots [4]. However, one problem of such surface patterning methods is their limited reach of influence being restricted to a few nanometers. In case of nitride growth the short-range impact raises concerns, as growth on lattice-mismatched substrates yields huge densities of threading dislocations which act also as nucleation sites by locally widening the lattice [11]. Also, emission-line broadening by spectral diffusion arising from defects in the vicinity of QDs can be reduced by burying the usually contaminated initial growth surface by a few hundred nm thick buffer layer.

Most approaches for site-controlled quantum dot growth in the nitrides are utilizing, therefore, selective area epitaxy under growth conditions which result in pyramidal cones with very sharp, nanometer-sized apexes and {1011} smooth side facets [76, 78-82]. MOVPE is preferred over MBE because of the larger diffusion lengths attainable on the masked areas. After growing nominally 30 nm of GaN onto a SiO₂ masked template with sub-pm openings pyramids, as shown in Fig. 5.20, can be obtained [76]. Suitable growth conditions for the

Fig. 5.20. SEM image of hexagonal pyramids grown on an SiO₂-masked GaN(0001)/sapphire template showing well-defined side facets and sharp apexes. Quantum dots are formed on top of the apexes. (Reprinted with permission from [76], © 2005 American Institute of Physics.)

Fig. 5.21. Excitation-power-dependent single QD photoluminescence spectra. The inset reveals a linear relationship on a log-log scale indicating excitonic emission. (Reprinted with permission from [77], © 2011 American Chemical Society.)

formation of GaN hexagonal pyramids favor growth in the [0001] direction and suppress lateral adatom diffusion on the (0001) plane. Growth temperatures of around 900°C yield smooth GaN(0001) surfaces [83, 84]. On top of hexagonal GaN pyramids, quantum dots can be formed by depositing thin layers of InGaN. The nanometer-scaled size of the apexes naturally leads to three-dimensionally confined quantum-dot structures without being forced to fulfill the conditions of the SK regime. Excitonic emission of single quantum dots located at the apexes of the pyramids is demonstrated as shown in Fig. 5.21 [77]. Linewidths as small as 350 peV at temperatures of 5 K are measured, confirming comparable optical quality to QDs on planar substrates.