Columnar Dots Research Articles

Single photon emission up to liquid nitrogen temperature from charged excitons confined in GaAs-based epitaxial nanostructures

We demonstrate a non-classical photon emitter at near infrared wavelength based on a single (In,Ga)As/GaAs epitaxially grown columnar quantum dot. Charged exciton complexes have been identified in magneto-photoluminescence. Photon auto-correlation histograms from the recombination of a trion confined in a columnar dot exhibit sub-Poissonian statistics with an antibunching dip yielding g(2)(0) values of 0.28 and 0.46 at temperature of 10 and 80 K, respectively. Our experimental findings allow considering the GaAs-based columnar quantum dot structure as an efficient single photon source operating at above liquid nitrogen temperatures, which in some characteristics can outperform the existing solutions of any material system.

Improving the characteristics of intermediate-band solar cell devices using a vertically aligned InAs/GaAsSb quantum dot structure

This study demonstrates the feasibility of improving the optical properties of a vertically aligned quantum dot (QD) structure and the performance of a quantum dot intermediate band solar cell (QD-IBSC) by capping a GaAsSb layer on the InAs QDs. Experimental results indicate that dot-size uniformity is significantly improved due to the strain modification in the evolution of the successive vertically aligned dot layer growth. A solar cell device with an InAs/GaAsSb columnar dot structure increases the short-circuit current density (Jsc) by 8.8%, compared to a GaAs reference cell. This dot structure also increases quantum efficiency by up to 1200nm through the absorption of lower-energy photons. The InAs/GaAsSb QD-IBSC also improves the open-circuit voltage (Voc), indicating a reduction in misfit defect density and recombination current density. The results of this study confirm the ability of a columnar InAs/GaAsSb QD structure to enhance the device performance.

High optical property vertically aligned InAs quantum dot structures with GaAsSb overgrown layers

This study investigates the feasibility of growing high quality columnar InAs/GaAsSb quantum dots (QDs) on a GaAs (100) substrate using a molecular beam epitaxial system. Structural and photoluminescence (PL) studies are conducted on vertically aligned, ten-period InAs quantum dot (QD) stacks with two different overgrown layer designs. Experimental results indicate an increased dot density of 5×1010cm−2 with completely suppressed coalescences in vertically aligned InAs quantum dots capped by a GaAsSb layer. This finding demonstrates a columnar dot structure with an enhanced luminescent intensity, activation energy, and narrow spectral line width of 22meV.

Controlling the Aspect Ratio of Quantum Dots: From Columnar Dots to Quantum Rods

We demonstrate the feasibility and flexibility of artificial shape engineering of epitaxial semiconductor nanostructures. Novel nanostructures including InGaAs quantum rods (QRs), nanocandles, and quantum dots (QDs)-in-rods were realized on a GaAs substrate. They were formed by depositing a short-period GaAs/InAs superlattice (SL) on a seed QD layer by molecular beam epitaxy growth. The InAs layer thickness in the SL plays an important role in obtaining the QRs. The growth of the QRs is very sensitive to growth interruption and growth temperature. By properly choosing both growth parameters, QRs with length of 41 nm corresponding to an extremely large aspect ratio of 4.1 were obtained. The evolution from a 0-D to 1-D confinement type is evidenced in the optical properties. The origin of the optical transitions from the QRs was understood by calculations of the electronic states within a fully 3-D approach in the eight-band k <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex>$\cdot$</tex> </formula> p approximation. The QRs are embedded in a GaAs matrix and are therefore free from surface traps. This feature enables high material quality and consequently their application in real devices. At room temperature, laser diodes based on QR active regions lasing around 1120 (or 1130) nm are demonstrated.

Optical and electronic properties of GaAs-based structures with columnar quantum dots

The electronic properties of a structure with columnar quantum dots obtained by close stacking of InAs submonolayers have been investigated by contactless electroreflectance (CER) and photoluminescence. These dots have an almost ideally rectangular cross section and uniform composition, which is promising for polarization independent gain. After energy level calculations in the effective mass approximation using composition profiles obtained from cross-sectional transmission electron microscopy the part of the CER spectrum related to the two-dimensional surrounding layer has been explained and single heavy-hole-like and light-hole-like transitions related to the columnar dots identified, due to a single electron state confined in a shallow in-plane potential.

Lattice distortion in dry-etched Si/SiGe quantum dot array studied by 2D reciprocal space mapping using synchrotron X-ray diffraction

Using synchrotron X-ray two-dimensional reciprocal space mapping around the (004) and (224) reciprocal lattice points, we have studied the changes of the lattice strain in a 30-period 3 nm Si/3 nm Si 0.7Ge 0.3 superlattice (SL) that has been processed into a φ≈50 nm columnar dot array by e-beam patterning combined with reactive ion etching. It has been found that the superlattice within the columns was partially relaxed after nano-fabrication. The observation of high-order SL satellites from columnar dots indicates a good long-range ordering of SL layers. An analysis based on elastic theory showed that the strain relief in the superlattice occurs by a partial relaxation in SiGe layers together with a biaxial lattice expansion in the thin Si layers, i.e. a strain symmetrisation effect. A staggered band line-up between Si and SiGe is then expected. We propose that an enhanced recombination between electrons and holes confined in adjacent quantum wells at the hetero-interfaces may give rise to the observed increase in the luminescence efficiency in this type of structures.