Quantum Pyramid Research Articles

External catalyst-free InGaN photoelectrode for highly efficient energy conversion and H2 generation

GaN is a well-known material whose energy band edges can straddle the redox potentials deep in the visible and infrared wavelengths, thereby promising a drastically improved photon-to-current efficiency under applied bias. However, the material is still limited by the half-reactions of water splitting due to its high defect density, low light absorption, small reaction area, and large energy band bending. Here, our study provides a turn-key solution to all these issues. The synergistic effect of InGaN/GaN quantum pyramids on nanowires (QPs-NWs) directly addresses the performance degradation of the photocathodes (PCs). New InGaN QP structures on non-polar GaN nanowire show a unique tunable energy band (Eg: ∼2 eV to ∼1.36 eV) by quantum-sliding interface recombination effect. Without the use of external catalysts, the photoelectrochemical water splitting (PEC-WS) of QPs-NWs PC demonstrated enhanced performance with a current density of 34.36 mA cm−2 and a photon-to-current efficiency of 13.75 % under the −0.8 to 0 V applied biasing condition, which is much higher than in previous reports. The current density and the H2 production were measured to be ∼61.81 mA cm−2 and 11.5 mmol cm−2 for 10 h. The external catalyst-free electrode and the metal organic chemical vapor deposition (MOCVD) process will open a new platform for the commercialization of III-nitride based water splitting hydrogen technology.

Calculation of Confined Electron and Hole States in a Strained InAs-GaAs Pyramidal Quantum Dot System Based on Effective Mass Approximation

Computational studies on zero dimensional semiconductor structure have been centred on typically produced quantum dot of various geometries namely pyramidal and lens with lateral sizes ranging from 10 nm to 24 nm. In the case of an epitaxially grown quantum dot, strain plays another essential role apart from its size and shape in determining its electronic properties [. Among the most studied strained structures is the self-assembled InAs quantum dot capped by a GaAs matrix. A study by [ on InAs pyramidal quantum dot predicted no confined electron states for quantum dot with base lengths 6 nm and below. Nevertheless, a calculation by [3] based on atomistic psedudopotential predicted at most two confined states for both electron and hole in a self-assembled InAs-GaAs quantum pyramid system of base length 6.06 nm.

How well can you know the edge of a quantum pyramid?

We consider a symmetric quantum communication scenario in which the signal states are edges of a quantum pyramid of arbitrary dimension and arbitrary shape, and all edge states are transmitted with the same probability. The receiver could employ different decoding strategies: he could minimize the error probability, or discriminate without ambiguity, or extract the accessible information. We state the optimal measurement scheme for each strategy. For large parameter ranges, the standard square-root measurement does not extract the information optimally. †We dedicate this work to János Bergou–friend, colleague, and grandmaster of unambiguous discrimination–on the occasion of his 60th birthday

Linewidth enhancement factor and chirp in quantum dot lasers

We have made a comparative study of the linewidth enhancement factor (LEF) and chirp in quantum dot (QDL’s) and quantum well lasers (QWL’s). The simulations are based on the quasiequilibrium approximation and on semiempirical transition energies and amplitudes of InGaAs quantum pyramid structures. We have accounted for the carriers confined in the active material as well as for the carriers in all the other material layers. It is found that in the quasiequilibrium approximation inhomogeneous broadening leads to asymmetric population of the quantum dot ground state. If the QDL is operated at the gain maximum, the asymmetry leads to nonzero chirp even for a single bound resonance state located at a large distance from other resonances. Our calculations show that, by detuning the laser emission to ∼15 nm shorter wavelengths with a frequency selective cavity and by tailoring the resonance energies and inhomogeneous broadening, the LEF and chirp of a QDL can be made very small. This detuning does not add a substantial penalty to the efficiency of the laser. For QWL’s, a similar reduction of chirp is generally not feasible due to the fundamentally different density of states. Therefore QDL’s have an important advantage over QWL’s as directly modulated light sources in applications where the stability of the emission wavelength is critical.

Self-Organized Growth of II-VI Wide Bandgap Quantum Dot Structures

We present our recent investigations on the self-organized growth and optical properties of quantum structures including CdSe quantum dots (QDs) embedded in ZnSe, ZnSe QDs in ZnS, and ZnO quantum pyramids. The self-organized mechanism is shown to play an important role in the formation of CdSe QDs on ZnSe(111). The ZnSe QDs are found to form on a 1 ML thick ZnSe wetting layer embedded in ZnS(100). The self-organized ZnO quantum pyramids form on ZnO(0001) buffer layers through preferred facet nucleation rather than strain-induced islanding.

InAs‐GaAs quantum dots: From growth to lasers

AbstractInjection lasers based on InAsGaAs and InGaAsGaAs quantum pyramids (QPs) with a lateral size ranging from 80 to 140 Å, are realized. The structures with relatively small dots (≈80 Å) exhibit properties predicted earlier for quantum dot (QD) lasers such as low threshold current densities (below 100 A cm−2) and ultrahigh characteristic temperatures (T0 = 350 to 425 K). For temperatures of operation above 100 to 130 K T0 decreases and the threshold current density increases (up to 0.95 to 3.3 kA cm−2 at room temperature) due to carrier evaporation from QPs. Larger InAs QPs (≈140 Å) providing better carrier localization exhibit saturation of the ground state emission and enhanced nonradiative recombination rate at high excitation densities. The radiative lifetime shows a weak dependence on the dot size (80 to 140 Å) being close to ≈1.8 to 2 ns, respectively. A significant decrease in radiative lifetime is realized in vertically‐coupled quantum dots formed by a QP shape‐transformation effect. The final arrangement represents a three‐dimensional tetragonal array of InAs islands inserted in a GaAs matrix each composed of several vertically merging InAs parts. The first injection lasing in such an array is achieved.

Electronic structure and energy relaxation in strained InAs/GaAs quantum pyramids

We perform experimental and theoretical studies of the electronic structure and relaxation processes in pyramid shaped InAs/GaAs quantum dots (QDs), grown by molecular beam epitaxy in the Stranski-Krastanow growth mode. Structural properties are characterized with plan view and cross section transmission electron microscopy. Finite difference calculations of the strain and the 3D Schrödinger equation, taking into account piezoelectric and excitonic effects, agree with experimental results on transition energies of ground and excited states, revealed in luminescence and absorption spectra. We find as relative standard deviation of the size fluctuation ξ=0.04; the pyramid shape fluctuates between {101} and {203} side facets. Carrier capture into the QD ground state after carrier excitation above barrier is a very efficient process. No luminescence from excited states is observed at low excitation density. Energy relaxation processes in the zero-dimensional energy states are found to be dominated by phonon energy selection rules. However, multi-phonon emission (involving GaAs barrier, InAs wetting layer, InAs QD and interface modes) allows for a large variety of relaxation channels and thus a phonon bottleneck effect does not exist here.

InAs–GaAs Quantum Pyramid Lasers: In Situ Growth, Radiative Lifetimes and Polarization Properties

We have realized injection lasers based on InAs–GaAs and InGaAs–GaAs quantum pyramids (QPs) with a lateral size ranging from 80 to 140 Å. The structures with relatively small dots (∼80 Å) exhibit properties predicted earlier for quantum dot (QD) lasers such as low threshold current densities (below 100 Acm-2) and ultrahigh characteristic temperatures (T0=350–425 K). For operation temperatures above 100–130 K, T0 decreases and the threshold current density increases (up to 0.95–3.3 kAcm-2 at room temperature) due to carrier evaporation from QPs. Larger InAs QPs (∼140 Å) providing better carrier localization exhibit saturation of the ground-state emission and enhanced nonradiative recombination rate at high excitation densities. The radiative lifetime shows a weak dependence on the dot size in the range 80–140 Å being close to ∼1.8–2 ns, respectively. A significant decrease in radiative lifetime is realized in vertically coupled quantum dots formed by a QP shape-transformation effect. The final arrangement corresponds to a three-dimensional tetragonal array of InAs islands inserted in a GaAs matrix each composed of several vertically merging InAs parts. We achieved injection lasing in such an array for the first time.

Fabrication and characterization of MBE grown InAs/GaAs strained-layer superlattices on variously oriented substrates

InAs/GaAs strained-layer superlattices (SLSs), grown by molecular-beam epitaxy on variously oriented GaAs substrates, have been investigated. X-ray diffraction and photoluminescence (PL) studies reveal the formation of coherent SLS and microstructures due to the three-dimensional nucleation is strongly dependent on the substrate orientation and the growth parameters. The relationship between PL spectra and growth modes and potential applications of self-aligned microstructures, including quantum pyramids, are described.