T. Hakkarainen

Tilting of lattice planes in InP epilayers grown on miscut GaAs substrates: the effect of initial growth conditions

Abstract The relative misorientation (tilt) between the epilayer and substrate (400) lattice planes of InP epilayers grown on (100) GaAs substrates misoriented towards the (100) plane was studied by high resolution X-ray diffraction. The epilayers were grown by gas-source molecular beam epitaxy. For the growth temperature of 490–500°C, the direction of the relative tilt was nearly coincident with the direction of the lattice plane tilting of the substrate, according to previous models. In contrary, when a buffer layer was deposited at a lower temperature of 400–450°C prior to growth, an azimuthal rotation of about 45° was found between the directions of the relative tilt and the substrate lattice plane tilting. In order to explain the results, a temperature-dependent anisotropic nucleation model is proposed. The effect of misfit dislocations is also considered.

Large array of single, site-controlled InAs quantum dots fabricated by UV-nanoimprint lithography and molecular beam epitaxy

We present the growth of single, site-controlled InAs quantum dots on GaAs templates using UV-nanoimprint lithography and molecular beam epitaxy. A large quantum dot array with a period of 1.5 µm was achieved. Single quantum dots were studied by steady-state and time-resolved micro-photoluminescence experiments. We obtained single exciton emission with a linewidth of 45 µeV. In time-resolved experiments, we observed decay times of about 670 ps. Our results underline the potential of nanoimprint lithography and molecular beam epitaxy to create large-scale, single quantum dot arrays.

Structural and optical properties of InAs quantum dot chains grown on nanoimprint lithography structured GaAs with different pattern orientations

We use large-scale UV nanoimprint lithography prepatterned GaAs substrates for site-controlled growth of InAs quantum dot chains by molecular beam epitaxy. We demonstrate simultaneous fabrication of quantum dot chains with high optical quality along four different crystal orientations, [011], [011¯], [010], and [001]. We show that the [011¯], [010], and [001]-oriented quantum dot chains not only have similar morphology but also experience similar in-plane optical anisotropy, which tends to align along the axis of the quantum dot chain. Our optical and structural results show that InAs quantum dot chains could be a potential platform for nanophotonic waveguiding and integrated circuits.

Lithography-free oxide patterns as templates for self-catalyzed growth of highly uniform GaAs nanowires on Si(111).

We report self-catalyzed growth of GaAs nanowires (NWs) on Si/SiOx patterns fabricated by a lithography-free method. The patterns are defined using droplet epitaxy of GaAs nanocrystals, spontaneous oxidation, and thermal annealing. We investigate the influence of the size and density of the nucleation sites on the NW growth process and show that this approach enables the fabrication of highly uniform GaAs NWs with controllable density. The pattern fabrication and NW growth process are studied and discussed in relation to the surface morphology and chemical properties of the Si/SiOx patterns. Furthermore, the optical quality of the NWs is investigated by photoluminescence experiments performed for GaAs–AlGaAs core–shell NWs.

Electron-irradiation enhanced photoluminescence from GaInNAs∕GaAs quantum wells subject to thermal annealing

Electron irradiation of a 1.3‐μm‐GaInNAs∕GaAs multi-quantum-well heterostructure, grown by molecular beam epitaxy and subsequently rapid-thermal annealed, is found to induce much stronger photoluminescence than what is observed for an identical as-grown sample upon annealing. Annealing of the irradiated sample also causes a small additional spectral blueshift and reduces alloy potential energy fluctuations at the conduction band minimum. These irradiation-related phenomena are accompanied by small but discernable changes in x-ray diffraction features upon annealing, which indicate compositional and∕or structural changes in the quantum wells.

Structural characterization of InAs quantum dot chains grown by molecular beam epitaxy on nanoimprint lithography patterned GaAs(100)

We combine nanoimprint lithography and molecular beam epitaxy for the site-controlled growth of InAs quantum dot chains on GaAs(100) substrates. We study the influence of quantum dot growth temperature and regrowth buffer thickness on the formation of the quantum dot chains. In particular, we show that by carefully tuning the growth conditions we can achieve equal quantum dot densities and photoluminescence ground state peak wavelengths for quantum dot chains grown on patterns oriented along the [011], [01 ̄1], [011] and [001] directions. Furthermore, we identify the crystal facets that form the sidewalls of the grooves in the differently oriented patterns after capping and show that the existence of (411)A sidewalls causes reduction of the QD density as well as sidewall roughening.

Interface effects on electrical properties of high purity InP grown by gas-source molecular beam epitaxy

Abstract The effect of the substrate-epilayer interface of InP on Hall mobilities and carrier concentrations at 300 and 77K is studied. It is shown that photoluminescence (PL) and Hall results are contradictory due to the highly conductive interface of InP, which falsifies the Hall results. After correcting mathematically for the interface effect in the Hall results, PL and Hall data show that our InP layers are of high purity and only slightly compensated. The SIMS profiles reveal an accumulation of carbon and silicon at the interface.

Optical properties of ion irradiated and annealed InGaAs/GaAs quantum wells and semiconductor saturable absorber mirrors

Optical properties of Ni+ irradiated and thermally annealed InGaAs/GaAs multiple quantum wells and semiconductor saturable absorber mirrors (SESAMs) have been studied using photoluminescence (PL) and non-linear reflectivity measurements. Rapid decrease of PL intensity and lifetime with increasing irradiation dose was accompanied by undesirable degradation of the non-linear optical properties of SESAMs. However, some of the irradiation-created defects could be removed and the non-linear optical properties improved by rapid thermal annealing. The combination of ion irradiation and annealing provided a selective method for controlling the absorption recovery time of SESAMs while preserving the non-linear properties. Irradiation with 1012 cm−2 of 6 MeV Ni+ ions and 1 s annealing at 400°C led to an absorption recovery time of ~1 ps while the modulation depth, the non-saturable losses and the saturation fluence were all close to their as-grown values.

The influence of growth conditions on the composition of GaInAsP grown by gas-source molecular beam epitaxy

Abstract We have studied the effect of growth temperature and growth rate on the composition of GaInAsP grown by gas-source molecular beam epitaxy on InP substrates. The composition of GaInAsP was found to be independent of substrate temperature below a transition temperature, approximately 530°C, at which appeared an abrupt change in the group V composition of GaInAsP for the wavelengths 1.05

Te-doping of self-catalyzed GaAs nanowires

Tellurium (Te)-doping of self-catalyzed GaAs nanowires (NWs) grown by molecular beam epitaxy is reported. The effect of Te-doping on the morphological and crystal structure of the NWs is investigated by scanning electron microscopy and high-resolution transmission electron microscopy. The study reveals that the lateral growth rate increases and axial growth rate decreases with increasing Te doping level. The changes in the NW morphology can be reverted to some extent by changing the growth temperature. At high doping levels, formation of twinning superlattice is observed alongside with the {111}-facetted sidewalls. Finally, the incorporation of Te is confirmed by Raman spectroscopy.