We apply low temperature confocal optical microscopy to spatially resolve, and spectroscopically study, a single self-assembled quantum dot. By comparing the emission spectra obtained at various excitation levels to a theoretical many body model, we show that (a) single exciton radiative recombination is very weak, and (b) sharp spectral lines are due to optical transitions between confined multiexcitonic states among which excitons thermalize within their lifetimes. Once these few states are fully occupied, broadbands appear due to transitions between states which contain electrons in the continuum.

/pdf/multiexciton-spectroscopy-of-a-single-self-assembled-quantum-141meav1vn.pdf

Multiexciton Spectroscopy of a Single Self-Assembled Quantum Dot

This paper presents recent progress in the field of semiconductor lasers and optical amplifiers with InAs-based self-assembled quantum dots in the active region for optical telecommunication. Based on our design in terms of the maximum bandwidth for high-speed modulation and p-type doping in quantum dots for high temperature stability, we realized temperature-insensitive 10 Gb s−1 laser diodes on a GaAs substrate at 1.3 µm. The output waveform at 10 Gb s−1 maintained a clear eye opening, average output power and extinction ratio without current adjustments from 20°C to 70°C. We developed ultrawide-band high-power amplifiers in the 1.5 µm wavelength region on an InP substrate. The amplifier showed ultrafast gain response under gain saturation, and enabled signal regeneration at 40 Gb s−1 by suppressing the '1'-level noise due to the beating between the signal and amplified spontaneous emission. We present our amplifier module with polarization diversity to enable a stable polarization-insensitive performance, and also, discuss prospects for polarization-insensitive quantum dots by the close stacking technique.

https://iopscience.iop.org/article/10.1088/0022-3727/38/13/008/pdf

Recent progress in self-assembled quantum-dot optical devices for optical telecommunication: temperature-insensitive 10 Gb s−1 directly modulated lasers and 40 Gb s−1 signal-regenerative amplifiers

We investigate exciton states theoretically in strained GaN/AlN quantum dots with wurtzite (WZ) and zinc-blende (ZB) crystal structures, as well as strained WZ GaN/AlGaN quantum dots. We show that the strain field significantly modifies the conduction- and valence-band edges of GaN quantum dots. The piezoelectric field is found to govern excitonic properties of WZ GaN/AlN quantum dots, while it has a smaller effect on WZ GaN/AlGaN, and very little effect on ZB GaN/AlN quantum dots. As a result, the exciton ground state energy in WZ GaN/AlN quantum dots, with heights larger than 3 nm, exhibits a redshift with respect to the bulk WZ GaN energy gap. The radiative decay time of the redshifted transitions is large and increases almost exponentially from 6.6 ns for quantum dots with height 3 nm to 1100 ns for the quantum dots with height 4.5 nm. In WZ GaN/AlGaN quantum dots, both the radiative decay time and its increase with quantum-dot height are smaller than those in WZ GaN/AlN quantum dots. On the other hand, the radiative decay time in ZB GaN/AlN quantum dots is of the order of 0.3 ns, and is almost independent of the quantum-dot height. Our results are in good agreement with available experimental data and can be used to optimize GaN quantum-dot parameters for proposed optoelectronic applications.

Excitonic properties of strained wurtzite and zinc-blende GaN/AlxGa1−xN quantum dots

Nanometre-scale semiconductor devices have been envisioned as next-generation technologies with high integration and functionality. Quantum dots, or the so-called ‘artificial atoms’, exhibit unique properties due to their quantum confinement in all 3D. These unique properties have brought to light the great potential of quantum dots in optoelectronic applications. Numerous efforts worldwide have been devoted to these promising nanomaterials for next-generation optoelectronic devices, such as lasers, photodetectors, amplifiers, and solar cells, with the emphasis on improving performance and functionality. Through the development in optoelectronic devices based on quantum dots over the last two decades, quantum dot devices with exceptional performance surpassing previous devices are evidenced. This review describes recent developments in quantum dot optoelectronic devices over the last few years. The paper will highlight the major progress made in 1.3 μm quantum dot lasers, quantum dot infrared photodetectors, and quantum dot solar cells.

https://iopscience.iop.org/article/10.1088/0022-3727/48/36/363001/pdf

Quantum dot optoelectronic devices: lasers, photodetectors and solar cells

A brief review on the present knowledge of the electronic properties of the Ga(In)NAs ternary and quaternary alloys is given mainly from an experimental perspective. The discussion is focused on Ga(In)NAs with low N composition (< 10 %), where a large amount of experimental work has been done. Important fundamental electronic properties of the material system are analyzed with the emphasis on the nature of the giant band gap bowing in the alloy and nitrogen-induced modifications of the electronic structure of the conduction band. The current knowledge of the key material parameters, relevant for the device applications, such as electron effective mass, recombination processes and band alignment in Ga(In)NAs/GaAs heterostructures, is also reviewed.

/pdf/electronic-properties-of-ga-in-nas-alloys-4u4hxqn7tn.pdf

Electronic Properties of Ga(In)NAs Alloys

Growth of self-assembled InAs and InAsxP1−x dots on InP by metalorganic vapour phase epitaxy

In the recombination spectra of indirect-band-gap alloys of ${\mathrm{GaAs}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{P}}_{\mathrm{x}}$, a peculiar and asymmetric luminescence band, ${M}_{0}^{x}$, has previously been reported to be due to the fluctuating alloy potential. Detailed experiments are reported here on ${\mathrm{GaAs}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{P}}_{\mathrm{x}}$ samples which exhibit exceptionally strong ${M}_{0}^{x}$ luminescence. The properties and line shape of ${M}_{0}^{x}$ have been studied as a function of the sample composition, excitation power densities, and decay times. It is concluded that recombining electron-hole pairs are trapped by the fluctuations of the band edges which form exponential tails in the gap and that the charge carriers are subject to localization below a mobility edge. Calculations performed using this model are able to explain most of the observed behavior of the ${M}_{0}^{x}$ emission band.

Photoluminescence study of localization effects induced by the fluctuating random alloy potential in indirect band-gap GaAs1-xPx.

We report the detection of quantum confinement in single InAs-InP core-shell nanowires. The wires having an InAs core with ~25 nm diameter are characterized by emission spectra in which two peaks are identified under high excitation intensity conditions. The peaks are caused by emission from the ground and excited quantized levels, due to the quantum confinement in the plane perpendicular to the nanowire axis. We have identified in the emission spectra different energy contributions related to the wurtzite structure of the wires, the strain between the wurtzite core and shell, and the confinement energy of the InAs core. Calculations based on 6-band strain-dependent k.p theory allow the theoretical estimation of the confined energy states in such materials and we found these results to be in good agreement with those from the photoluminescence studies.

http://export.arxiv.org/pdf/cond-mat/0701082

Quantum confinement effects in InAs-InP core-shell nanowires

Quantum wire structures of GaAs/AlGaAs have been grown by metalorganic vapor phase epitaxy in V grooves using pre‐etched corrugated substrates and have been characterized by high‐resolution transmission electron microscopy. Low‐temperature cathodoluminescence (CL) identifies luminescence peaks with the spatial distributions of the different recombinations, achieving a top view spatial resolution of ≊0.2 μm in the CL images. Principally we report how a scanning tunneling microscope (STM) induces local luminescence in the sample structure, and we spectrally resolve STM‐induced luminescence for the tip in different positions relative to the wires. We have recorded the luminescence from a single wire and observed band‐filling effects resulting from varying levels of excitation into a wire. We have demonstrated the difference between recombination of electron‐hole pairs generated in CL and the recombination of injected holes from the STM tip with a thermalized distribution of accumulated electrons in scanning tunneling luminescence.

Scanning tunneling microscope and electron beam induced luminescence in quantum wires

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Papers

Growth of self-assembled InAs and InAsxP1−x dots on InP by metalorganic vapour phase epitaxy

Photoluminescence study of localization effects induced by the fluctuating random alloy potential in indirect band-gap GaAs1-xPx.

Quantum confinement effects in InAs-InP core-shell nanowires

Scanning tunneling microscope and electron beam induced luminescence in quantum wires

Scanning tunneling microscope and electron beam induced luminescence in quantum wires