New information on the electron-hole wave functions in InAs-GaAs self-assembled quantum dots is deduced from Stark effect spectroscopy. Most unexpectedly it is shown that the hole is localized towards the top of the dot, above the electron, an alignment that is inverted relative to the predictions of all recent calculations. We are able to obtain new information on the structure and composition of buried quantum dots from modeling of the data. We also demonstrate that the excited state transitions arise from lateral quantization and that tuning through the inhomogeneous distribution of dot energies can be achieved by variation of electric field.

The use of a high-growth-temperature $\mathrm{GaAs}$ spacer layer is demonstrated to significantly improve the performance of $1.3\mu \mathrm{m}$ multilayer self-assembled $\mathrm{InAs}∕\mathrm{InGaAs}$ dot-in-a-well lasers. The high-growth-temperature spacer layer inhibits threading dislocation formation, resulting in enhanced electrical and optical characteristics. Incorporation of these spacer layers allows the fabrication of multilayer quantum-dot devices emitting above $1.3\mu \mathrm{m}$ , with extremely low room-temperature threshold current densities and with operation up to $105°\mathrm{C}$ .

We report first quantitative measurements by energy-selected imaging in a transmission electron microscope of In segregation within an uncapped islanded ${\mathrm{In}}_{0.25}{\mathrm{Ga}}_{0.75}\mathrm{As}$ layer grown epitaxially on GaAs. This layer has the lowest In concentration at which islanding occurs and, then, only after a flat $\sim 3\mathrm{nm}$ alloy layer has been formed. In buildup by segregation at the surface of this initial flat layer is considered the driving force for islanding and, importantly, the segregation process introduces the characteristic delay seen before the Stranski-Krastanow transition. We observe strong inhomogeneous In enrichment within the islands (up to $x\left(\mathrm{In}\right)\approx 0.6$ at the apex) and a simultaneous In depletion in the remaining flat layer.

We present a cross-sectional scanning-tunneling microscopy investigation of the shape, size, and composition of InAs quantum dots in a GaAs matrix, grown by molecular beam epitaxy at low growth rate. From the dimensional analysis we conclude that the investigated quantum dots have an average height of 5 nm, a square base of 18 nm oriented along [010] and [100] and the shape of a truncated pyramid. From outward relaxation and lattice constant profiles we conclude that the dots consist of an InGaAs alloy and that the indium concentration increases linearly in the growth direction. Our results justify the predictions obtained from previous photocurrent measurements on similar structures and the used theoretical model.

The structural and optical properties of GaAs-based 1.3 μm InAs/InGaAs dots-in-a-well (DWELL) structures have been optimized in terms of different InGaAs and GaAs growth rates, the amount of InAs deposited, and In composition of the InGaAs quantum well (QW). An improvement in the optical efficiency is obtained by increasing the growth rate of the InGaAs and GaAs layers. A transition from small quantum dots (QDs), with a high density ${\text{(∼5.3×10}}^{10} {\mathrm{cm}}^{\text{−2}})$ and broad size distribution, to larger quantum dots with a low dot density ${\text{(∼3.6×10}}^{10} {\mathrm{cm}}^{\text{−2}})$ and narrow size distribution, occurs as the InAs coverage is increased from 2.6 to 2.9 monolayers. The room-temperature optical properties also improve with increased InAs coverage. A strong dependence of the QD density and the QD emission wavelength on the In composition of InGaAs well has been observed. By investigating the dependence of the dot density and the high-to-width ratio of InAs islands on the matrix of InGaAs strained buffer layer (SBL), we show that the increasing additional material from wetting layer and InGaAs layer into dots and the decreasing repulsive strain field between neighboring islands within substrate are responsible for improving QD density with increasing In composition in InGaAs SBL. The optical efficiency is sharply degraded when the InGaAs QW In composition is increased from 0.15 to 0.2. These results suggest that the optimum QW composition for 1.3 μm applications is $\text{∼15\%.}$ Our optimum structure exhibits a room temperature emission of 1.32 μm with a linewidth of 27 meV.

We report a spectroscopic investigation of the electronic energy levels and carrier-relaxation mechanisms in self-organized InAs/GaAs quantum dots. Power-dependent photoluminescence (PL) and photoluminescence excitation (PLE) are used to study the energy-level structure. Two excited states, 74 and 120 meV above the luminescent ground state, are identified. As expected for a zero-dimensional system, it is not possible to observe PL from the ground state of the dots when exciting between the energies of the ground and first excited state due to the discrete, atomiclike nature of the electronic states. Selectively excited PL and PLE reveal two mechanisms for the relaxation of carriers from the excited states to the ground state: a nonresonant mechanism dominant in the upper state, and a resonant mechanism, involving the emission of one or more LO phonons of well-defined energy, which is dominant in the lower excited state. The resonant mechanism is shown to be a consequence of the distribution of energy-level spacings in the inhomogeneous ensemble of dots; preferentially selecting dots with an energy-level spacing close to an integer multiple of the LO phonon energy.