Pseudomorphic InAs/GaAs quantum dots on low index planes

Pyramidal nm-size InAs dots in a GaAs matrix arranged on a two-dimensional primitive square lattice are formed self-organized by coherent island Stranski-Krastanov growth mode during molecular beam epitaxy on exactly oriented or vicinal GaAs (001) substrate. Details of their geometry, size distribution and arrangement are revealed using plan view and high resolution cross section transmission electron microscopy. Strain is found to present the driving force of dot formation. Occurence of a particular dot size is energetically favorable. The strain distribution in and around a covered QD is simulated numerically. The hydrostatic part of the strain is mostly confined to the dot, while the anisotropic part of the strain is shared between the dot and the surrounding GaAs barriers and has a minimum in the dot center. The strain induced modification of the confinement potential is used as input for a numerical solution of the three-dimensional effective mass Schrodinger equation for the experimentally observed geometry. For a typical dot size (base length 12 nm) only one electron level is confined, but several bound hole levels exist. Large local charge non-neutrality is found. Coulomb interaction of carriers can be treated as a perturbation; exciton binding energy around 20 meV is found. Due to a finite size distribution of the dots the ensemble luminescence, as probed by broad area excitation photoluminescence (PL), has a typical halfwidth of 50 meV. The PL peak coincides energetically with the absorption peak. Excited state energies, revealed at high excitation intensity, agree with the theoretical predictions. High spatial resolution, low acceleration voltage cathodoluminescence (Ck) excites only a small number of dots, such that the spectrum consists of a series of sharp lines, each due to recombination in a single QD. The luminescence halfwidth of such single QD luminescence has a FWHM < 0.15 meV, which remains that small even for elevated temperatures, thus unambiguously proving the S-function density of states in the QDs. Monochromatic CL images visualize the distribution of QDs having the same ground state energy. Energy relaxation via multi-phonon processes is evidenced using excitation and Raman spectroscopy and found to circumvent the phonon bottleneck effect. The observed phonon energy of 32.2 +/- 0.5 meV agrees with the value predicted from strain theory. A broad area laser with QDs as active gain medium is demonstrated to have a low threshold current density and a large T-0.