1 Optoelectronics, Department of Photonics Engineering, Technical University of Denmark2 Department of Photonics Engineering, Technical University of Denmark3 Nanophotonics, Department of Photonics Engineering, Technical University of Denmark
The fabrication, characterization and exploitation of self-assembled quantum dot (QD) heterostructures have attracted much attention not only in basic research, but also by the promising device applications such as QD lasers. The Stranski-Krastanow (SK) growth and the submonolayer (SML) deposition are two different methods of growing self-assembled QDs. In the case of SK growth, which has been widely used for lattice mis-matched materials, such as In(Ga)As/Ga(Al)As, coherent three dimensional (3D) islands form on top of a wetting layer to relax the strain energy. However,in the case of SML deposition, the deposition of a short-period InAs/GaAs superlattice on GaAs (100) surface with an InAs effective thickness of less than 1 monolayer (ML), results in the formatioin of nanometer scale (In,Ga)As QDs of a non-SK class.In this thesis, the SML InGaAs/GaAs QDs are formed by 10 cycles of alternate deposition of 0.5 ML InAs and 2.5 MLGaAs. The growth, structure, and optical properties of SML InGaAs/GaAs QD heterostructures are investigated in detail. SML InGaAs/GaAs QD lasers lasing even at room temperature have been successfully realized. The gain properties of SML InGaAs QD lasers are studied.The growth temperature of SML InGaAs/GaAs QDs in one sample was as low as 480 0C. Plan-view transmission electron microscopy observation shows that SML QDs are slightly elongated along the [1 -1 0] crystal direction, and the QD density is extremely high (> 1011 cm-2). By using lower temperature micro-photoluminescence (PL), selective PL with excitation energy below the GaAs band gap, the temperature-dependent PL, for the first time, the SML InGaAs/GaAs QD heterostructure is verified to be quantum-dot-quantum-well structure, i.e., the Indium righ QDs are embedded in a lateral quantum well (QW) with lower Indium content. This conclusion was further confirmed in the high-resolution x-ray diffraction analysis, which shows that the vertical lattice mismatch of the InAs monolayer with respect to GaAs is around 1.4%, while the lattice mismatch in the QW is negligible. As the temperature increases, a sigmoidal behavior of the PL peak energy and narrowing of the PL linewidth of the SML QD ensemble are observed, and explained by carrier transfer from smaller dots to larger dots via QW states. At room temperature, the PL signal of QDs quenches due to the thermal escape of carriers from QD to QW states. In the edge geometry, strong contribution of the TM mode to PL signal has been observed, indicating the vertical coupling of the SML InAs islands in the GaAs matrix. The PL rise time (~ 35 ps) and decay time (~ 700 ps) of the ground states of SML QDs, at 5 K, are found to be comparable to those of typical SK QDs.The SML InGaAs/GaAs QDs in another sample was grown at 500 0C. For this sample, the PL signal of QD ground states is still observable at room temperature. Furthermore, strong contribution of the TM mode to PL signal is also observed in the edge geometry, at room temperature. In our SML InGaAs/GaAs QD lasers, the growth conditions of QDs are the same as those of this sample.The lasing wavelength, the threshold current density, and the characteristic temperature of a SML InGaAs/GaAs QD broad area laser with a 628 ìm-long cavity and a 100 ìm-wide stripe, are 965 nm, 373 A/cm2 and 81 K, respectively, at 30 0C. The gain spectra at 30 0C were measured, by using the Hakki-Paoli method. It is found that the maximum modal gain of QD ground states is 43.9 cm-1, which is the highest value obtained for a single sheet of self-assembled In(Ga)As/GaAs QDs, to the best of our knowledge. Furthermore, no gain saturation takes place below the threshold at the lasing wavelength, and the gain spectrum becomes symmetric with respect to the lasing wavelength when the injection current is about 0.98 Ith. The zero linewidth-enhancement-factor at the lasing wavelength has been observed, when the injection current is about 0.98 Ith. This is the first time for the zero linewidth-enhancement-factor to be observed for a QD laser. These properties are attributed to the high density and the high uniformity of SML QDs in our laser diodes, which are very useful for the application of SML QDs in high power lasers or vertical cavity surface emitting lasers.