Optical properties of InAs quantum dots (QDs) embedded in GaAsSb barriers with delta-doping levels equivalent to 0, 2, 4, and 6 electrons per dot (e/dot) are studied using time-integrated photoluminescence (PL). When the PL excitation power is increased the full width at half maximum (FWHM) of the 4 and 6 e/dot samples is found to increase at a much greater rate than the FWHMs for the 0 and 2 e/dot samples. PL spectra of the 4 e/dot sample show a high energy peak attributed to emission from the first excited states of the QDs, a result deduced to be due to preoccupation of states by electrons supplied by the delta-doping plane. When temperature dependent PL results are fitted using an Arrhenius function, the thermal activation energies for the 4 and 6 e/dot samples are similar and greater than the thermal activation energies for the 0 and 2 e/dot samples (which are similar to each other). This increased thermal activation energy is attributed to the enhanced Coulombic interaction in the InAs QD area by the delta-doping plane for higher doping levels. It is concluded that delta-doping of the barrier in QD systems is a feasible method for controlling the level of carrier occupation in a QD mediated intermediate band.

InAs quantum dots grown on GaAsSb buffer layers with varying Sb content have been studied. Atomic force microscopy results show that the dot size is reduced as the Sb content increases with a concomitant increase in number density. Analysis of the size distribution indicates that the spread of dot sizes narrows with increasing Sb content. This is confirmed by photoluminescence measurements showing a significant narrowing of the dot emission peak for a GaAs0.77Sb0.23 buffer compared to a GaAs buffer. The results are attributed to the strained buffer reducing interactions between dots and the Sb acting as a surfactant.

We describe InAs quantum dot creation in InAs/GaAsSb barrier structures grown on GaAs (001) wafers by molecular beam epitaxy. The structures consist of 20-nm-thick GaAsSb barrier layers with Sb content of 8%, 13%, 15%, 16%, and 37% enclosing 2 monolayers of self-assembled InAs quantum dots. Transmission electron microscopy and X-ray diffraction results indicate the onset of relaxation of the GaAsSb layers at around 15% Sb content with intersected 60° dislocation semi-loops, and edge segments created within the volume of the epitaxial structures. 38% relaxation of initial elastic stress is seen for 37% Sb content, accompanied by the creation of a dense net of dislocations. The degradation of In surface migration by these dislocation trenches is so severe that quantum dot formation is completely suppressed. The results highlight the importance of understanding defect formation during stress relaxation for quantum dot structures particularly those with larger numbers of InAs quantum-dot layers, such as those proposed for realizing an intermediate band material.

We have studied the material properties and device performance of InAs/GaAs quantum dot solar cells (QDSCs) made using three different QD growth modes: Stranski-Krastanov (S-K), quasi-monolayer (QML), and sub-monolayer (SML) growth modes. All QDSCs show an extended external quantum efficiency (EQE) at near infrared wavelengths of 950–1070 nm from the QD absorption. Compared to the S-K and SML QDSCs, the QML QDSC with a higher strain exhibits a poor EQE response in the wavelength region of 300–880 nm due to increased non-radiative recombination. The conversion efficiency of the S-K and SML QDSCs exceeds that of the reference cell (13.4%) without QDs due to an enhanced photocurrent (>16% increase) produced by the silicon doped QD stacks. However, as expected from the EQE of the QML QDSC, the increase of strain-induced crystalline defects greatly degrades the photocurrent and open-circuit voltage, leading to the lowest conversion efficiency (8.9%).

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