Martin Elborg

Fabrication of GaNAs/AlGaAs Heterostructures with Large Band Offset Using Periodic Growth Interruption

We studied the growth of GaNAs/AlGaAs heterostructures on GaAs (100) substrates by plasma-assisted molecular beam epitaxy. By introducing periodic growth interruption and nitrogen (N) supply to the interrupted surfaces during the growth of GaNAs, we achieved high controllability of the average N concentration in GaNAs layers. We observed three-dimensional island growth of GaNAs on the N-rich surfaces. The GaNAs island structures exhibit narrow photoluminescence emission at around 1 µm at low temperature, indicating the formation of a large band offset between GaNAs and AlGaAs.

Open-Circuit Voltage in AlGaAs Solar Cells With Embedded GaNAs Quantum Wells of Varying Confinement Depth

We investigate the photovoltaic properties of AlGaAs solar cells with embedded GaNAs quantum wells (QWs) with N concentrations in the range of 0-3.1%, for which the QW confinement energy can be tuned by adjusting the N concentration. We systematically study the dependence of open-circuit voltage VOC in relation to the lowest band-to-band transition energy. In samples with low N concentrations (shallow QW confinement), VOC degrades and is limited by the lowest transition energy in the solar cell, i.e., the QW transition. With increasing N concentration, N > 0.5% (deep QW confinement), VOC does not degrade further and is no longer limited by the QW transition energy. The highest N sample exhibits a remarkably small offset between the lowest transition energy and the achieved VOC of 0.23 V, which is beyond the detailed balance limit of standard solar cells. VOC dependence is explained by analyzing the current-voltage (I-V) characteristics under different illumination conditions, from which information about the balance of escape and recombination rates of carriers from the QWs is extracted. In the deeply confined QWs, tunneling and thermal carrier escape is completely suppressed, allowing the recovery of VOC.

Extension of Absorption Wavelength in GaAs/AlGaAs Quantum Dots with Underlying Quantum Well for Solar Cell Application

We fabricated a GaAs/AlGaAs quantum dot solar cell (QD-SC) in which GaAs QDs with an underlying quantum well (QW) layer were grown to enable the absorption of long-wavelength photons. Photoluminescence emission of these QDs with an underlying QW, referred to as QDW structure, is redshifted and significantly narrower than that of plain QDs. The extension of absorption wavelength is clearly observed in the spectral response of the samples while keeping zero-dimensional confinement as confirmed by calculation. Improvement in fill factor in the QDW sample compared with plain QDs is demonstrated.

Two-Color Photoexcitation in a GaNAs/AlGaAs Quantum Well Solar Cell

We demonstrate an efficient two-color photoexcitation process in a GaNAs/AlGaAs multiple quantum well (MQW) solar cell. The introduction of N into the GaAs MQW induces a marked reduction in bandgap energy, forming a large conduction band offset, and the formation of localized states. Owning to this deep confinement, the thermal escape of photogenerated carriers from the QWs is greatly suppressed even at room temperature, resulting in a reduction in photocurrent. An additional photocurrent is generated by a two-color absorption process of sub-bandgap photons.

Optical transitions in GaNAs quantum wells with variable nitrogen content embedded in AlGaAs

We investigate the optical transitions of GaNxAs1−x quantum wells (QWs) embedded in wider band gap AlGaAs. A combination of absorption and emission spectroscopic techniques is employed to systematically investigate the properties of GaNAs QWs with N concentrations ranging from 0 – 3%. From measurement of the photocurrent spectra, we find that besides QW ground state and first excited transition, distinct increases in photocurrent generation are observed. Their origin can be explained by N-induced modifications in the density of states at higher energies above the QW ground state. Photoluminescence experiments reveal that peak position dependence with temperature changes with N concentration. The characteristic S-shaped dependence for low N concentrations of 0.5% changes with increasing N concentration where the low temperature red-shift of the S-shape gradually disappears. This change indicates a gradual transition from impurity picture, where localized N induced energy states are present, to alloying pic...

Ga metal nanoparticle-GaAs quantum molecule complexes for terahertz generation

A hybrid metal-semiconductor nanosystem for the generation of THz radiation, based on the fabrication of GaAs quantum molecules-Ga metal nanoparticles complexes through a self assembly approach, is proposed. The role of the growth parameters, the substrate temperature, the Ga and As flux during the quantum dot molecule (QDM) fabrication and the metal nanoparticle alignment are discussed. The tuning of the relative positioning of QDMs and metal nanoparticles is obtained through the careful control of Ga droplet nucleation sites via Ga surface diffusion. The electronic structure of a typical QDM was evaluated on the base of the morphological characterizations performed by atomic force microscopy and cross sectional scanning electron microscopy, and the predicted results confirmed by micro-photoluminescence experiments, showing that the Ga metal nanoparticle-GaAs quantum molecule complexes are suitable for terahertz generation from intraband transition.

Excitonic Aharonov-Bohm effect in QD-on-Ring nanostructures

We show by the first-order perturbation theory and the configuration interaction method that the Coulomb interaction in quantum rings mixes electron-hole pair states with the same total angular momentum, which makes it difficult to observe a clear excitonic Aharonov-Bohm (A-B) effect. To avoid this situation, we propose the use of a combined structure of a quantum dot on the top of a quantum ring with an applied static electric field. Under moderate experimental conditions with respect to the applied electric and magnetic fields, we show that we can observe the excitonic A-B effect due to the reduction of the Coulomb interaction and an increase in the difference between the average radii of the electron and hole trajectories.

Nitrogen-concentration control in GaNAs/AlGaAs quantum wells using nitrogen δ-doping technique

GaNAs/Al0.35Ga0.65As multiple quantum wells (MQWs) with nitrogen δ-doping were fabricated on GaAs (100) substrates by plasma-assisted molecular beam epitaxy. High controllability of nitrogen-concentrations in the MQWs was achieved by tuning nitrogen δ-doping time. The maximum nitrogen concentration in the MQWs was 2.8%. The MQWs exhibit intense, narrow photoluminescence emission.

Nitrogen Concentration Dependence on Two-Step Photocurrent Generation in GaNAs/AlGaAs Solar Cells

The photocurrent characteristics of GaNAs/AlGaAs solar Intermediate Band Solar Cells are investigated. The photocurrent is increased by a two-step photon absorption process and distinct differences in the bias dependence for different nitrogen concentrations are presented.