We have studied the polarization of surface and edge-emitted photoluminescence ͑PL͒ from structures with vertically coupled In 0.5 Ga 0.5 As/GaAs quantum dots ͑QD's͒ grown by molecular beam epitaxy. The PL polarization is found to be strongly dependent on the number of stacked layers. While single-layer and 3-layer structures show only a weak TE polarization, it is enhanced for 10-layer stacks. The 20-layer stacks additionally show a low-energy side-band of high TE polarization, which is attributed to laterally coupled QD's forming after the growth of many layers by lateral coalescence of QD's in the upper layers. While in the single, 3-and 10-layer stacks, both TE polarized PL components are stronger than the TM component, the ͓110͔ TE component is weaker than the TM component in the 20-layer stack. This polarization reversal is attributed to an increasing vertical coupling with increasing layer number due to increasing dot size.

We report on resonant photoluminescence ͑PL͒ of InGaN inclusions in a GaN matrix. The structures were grown on sapphire substrates using metal-organic chemical vapor deposition. Nonresonant pulsed excitation results in a broad PL peak, while resonant excitation into the nonresonant PL intensity maximum results in an evolution of a sharp resonant PL peak, having a spectral shape defined by the excitation laser pulse and a radiative decay time close to that revealed for PL under nonresonant excitation. Observation of a resonantly excited narrow PL line gives clear proof of the quantum dot nature of luminescence in InGaN-GaN samples. PL decay demonstrates strongly nonexponential behavior evidencing coexistence of quantum dots having similar ground-state transition energy, but very different electron-hole wave-function overlap.

We report photopumped room-temperature surface-mode lasing at 401 nm in a InGaAlN vertical-cavity surface-emitting laser grown on a sapphire substrate using metal–organic vapor-phase epitaxy. A 2λ cavity was formed by a quarter-wave Al0.15Ga0.85N/GaN distributed Bragg reflector on the one side of the active layer and a GaN–air interface on the other. A multilayer structure composed of 12-fold-stacked ultrathin InGaN insertions in a GaN matrix served as an active layer providing ultrahigh material gain and making possible vertical lasing without use of the upper Bragg reflector.

A comparative analysis is made of laser diodes based on Stranski-Krastanow (SK) and sub-monolayer (SML) InAs/GaAs quantum dots, emitting at about 940 nm. Owing to the better uniformity of sub-monolayer quantum dots, the SML QD laser surpasses the SK QD one in power characteristics. A maximum output power of 3.9 W and a peak power conversion efficiency of 59% have been achieved for SML QD 100 µm wide lasers at 10 • C.

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Mechanism for improvements of optical properties of 1.3-μ m InAs ∕ GaAs quantum dots by a combined InAlAs -InGaAs cap layer J. Appl. Phys. 98, 083516 (2005); 10.1063/1.2113408 Tuning InAs/GaAs quantum dot properties under Stranski-Krastanov growth mode for 1.3 μm applicationsSuppression of temperature sensitivity of interband emission energy in 1.3-μm-region by an InGaAs overgrowth on self-assembled InGaAs/GaAs quantum dots Quantum dots ͑QDs͒ formed on GaAs͑100͒ substrates by InAs deposition followed by ͑Al,Ga͒As or ͑In,Ga,Al͒As overgrowth demonstrate a photoluminescence ͑PL͒ peak that is redshifted ͑up to 1.3 m͒ compared to PL emission of GaAs-covered QDs. The result is attributed to redistribution of InAs molecules in the system in favor of the QDs, stimulated by Al atoms in the cap layer. The deposition of a 1 nm thick AlAs cover layer on top of the InAs-GaAs QDs results in replacement of InAs molecules of the wetting layer by AlAs molecules, leading to a significant increase in the heights of the InAs QDs, as follows from transmission electron microscopy. This effect is directly confirmed by transmission electron microscopy indicating a transition to a Volmer-Weber-like QD arrangement. We demonstrate an injection laser based on this kind of QDs.

(In)GaAsN bulk layers and quantum wells usually demonstrate lower photoluminescence intensity than the nitrogen-free compositions. In the present work we have carefully optimized both conductance and operation of a nitrogen plasma source as well as growth parameters of GaAsN layers. We found conditions when incorporation of nitrogen did not lead to formation of additional nonradiative recombination. There is some minimum growth rate to obtain good crystal and optical quality of GaAsN. At growth rates below this value the pattern of reflection high energy electron diffraction turns spotty and the growth proceeds in a three-dimensional mode. This leads to a steep decrease in luminescence efficiency of the grown layer. The minimum value of growth rate depends on nitrogen content and growth temperature. Defects caused by low temperature growth are removed by post-growth annealing. We achieved the same radiative efficiency of GaAsN samples with nitrogen content up to about 1.5% grown at 520 °C as that of a reference layer of GaAs grown at 600 °C. Compositional fluctuation in the GaAsN layers leads to the S-shape temperature dependence of photoluminescence peak position. Post-growth annealing reduces compositional fluctuation.

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