A. Tukiainen

An electric current spike linked to nanoscale plasticity

The increase in semiconductor conductivity that occurs when a hard indenter is pressed into its surface has been recognized for years, and nanoindentation experiments have provided numerous insights into the mechanical properties of materials. In particular, such experiments have revealed so called pop-in events, where the indenter suddenly enters deeper into the material without any additional force being applied; these mark the onset of the elastic-plastic transition. Here, we report the observation of a current spike--a sharp increase in electrical current followed by immediate decay to zero at the end of the elastic deformation--during the nanoscale deformation of gallium arsenide. Such a spike has not been seen in previous nanoindentation experiments on semiconductors, and our results, supported by ab initio calculations, suggest a common origin for the electrical and mechanical responses of nanodeformed gallium arsenide. This leads us to the conclusion that a phase transition is the fundamental cause of nanoscale plasticity in gallium arsenide, and the discovery calls for a revision of the current dislocation-based understanding of nanoscale plasticity.

Annealing of the deep recombination center in GaInP/AlGaInP quantum wells grown by solid-source molecular beam epitaxy

Time resolved photoluminescence and deep level transient spectroscopy have been used to monitor the effect of rapid thermal annealing on bulk GaInP and GaInP/AlGaInP quantum wells grown by solid source molecular beam epitaxy similar to those used in 650 nm range lasers. Following rapid thermal annealing at temperatures up to 875 °C, reductions in the concentration of several deep level traps are observed. Correlation of these data with photoluminescent intensity and lifetime measurements indicate that the defect labeled N3, 0.83 eV below the conduction band, is the dominant recombination center. The combination of these two transient spectroscopy measurement techniques is therefore not only able to measure the change in deep level concentration, but also to correlate this change with improved carrier lifetimes and, ultimately, reduced threshold current densities in quantum well lasers. There is also evidence to suggest that this same defect, possibly a phosphorous vacancy or a related complex, plays an im...

Performance assessment of multijunction solar cells incorporating GaInNAsSb

We have measured the characteristics of molecular beam epitaxy grown GaInNAsSb solar cells with different bandgaps using AM1.5G real sun illumination. Based on the solar cell diode characteristics and known parameters for state-of-the-art GaInP/GaAs and GaInP/GaAs/Ge cells, we have calculated the realistic potential efficiency increase for GaInP/GaAs/GaInNAsSb and GaInP/GaAs/GaInNAsSb/Ge multijunction solar cells for different current matching conditions. The analyses reveal that realistic GaInNAsSb solar cell parameters, render possible an extraction efficiency of over 36% at 1-sun AM1.5D illumination.PACS88.40.hj; 88.40.jm; 88.40.jp; 81.15.Hi

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.

Postgrowth annealing of GaInAs∕GaAs and GaInAsN∕GaAs quantum well samples placed in a proximity GaAs box: A simple method to improve the crystalline quality

The effects of thermal annealing on GaInAs∕GaAs and GaInAsN∕GaAs quantum wells, grown by molecular beam epitaxy, were investigated. Optical and structural properties were examined upon annealing when the samples had a 200nm thick SiO2 cap layer, or were placed in a so-called GaAs box or were left uncapped. The GaAs box gave rise to the strongest photoluminescence without significant blueshift or structural changes at moderate annealing temperature. Capping with SiO2 impaired the samples and caused a more pronounced blueshift for the GaInAs quantum wells than for the GaInAsN ones. These results consolidate our understanding of the blueshift mechanisms.

SS-MBE-grown short red wavelength range AlGaInP laser structures

Short-wavelength GaInP/AlGaInP quantum-well (QW) laser diodes emitting in the 618-650 nm range at room temperature have been fabricated and characterized. Several variations in laser structures have been tested, including changes in QW composition, thickness, strain and number; changes in the barrier/waveguide composition and thickness; changes in cladding structure; use of multi-quantum-barriers and changes in the doping profile. The experiments showed that the threshold current characteristic temperature (T0) increases with the number of QWs and is higher for compressive strain. The use of graded-index (GRIN) waveguides and higher p-cladding doping induced both a reduction in threshold current density and an increase in T0, mostly at shorter wavelengths. Waveguide thickness optimization can be carried out, for both constant composition and GRIN waveguides, using the QW optical confinement as a first-approximation optimization criterion. Modified cladding structures reduced the vertical far-field full-width-at-half-maximum below 20° without significantly affecting the threshold current. Devices designed using some of the guidelines resulted from our study achieved, with different structures and under different operating conditions, performances like emitting more than 2W at 650 nm in continuous wave operation or lasing down to 618 nm at room temperature, which is among the shortest wavelengths from lasers grown by solid-source molecular-beam-epitaxy.

Influence of deep level impurities on modulation response of InGaP light emitting diodes

The effect of deep level impurities on static and dynamic properties of InGaP-based light emitters grown by all-solid-source molecular-beam epitaxy is analyzed. The improvement of the output power and the decrease in modulation bandwidth induced by the burn-in process are explained by the recombination enhanced annealing of one deep level trap. This assumption is experimentally proven through comparison of small-signal analysis for resonant cavity light-emitting diodes operating at 650 nm and deep level transient spectroscopy results. Finally, the concentration of the midgap recombination center N3 in the active region is shown to play an important role in the performance of the InGaP devices.

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.

Study of nitrogen incorporation into GaInNAs: The role of growth temperature in molecular beam epitaxy

GaInNAs has an important impact on developing GaAs-based optoelectronics and multijunction solar cells, but the complex nature of the nitrogen incorporation into GaInAs is still not fully understood. By combining x-ray diffraction, photoluminescence, reflection high-energy electron diffraction, and photoelectron spectroscopy measurements, we show that nitrogen incorporation is enhanced with increasing growth temperature in the range of 300–450 °C. We study the growth front and show that the surface reconstruction is (1 × 3) regardless of growth temperature in this range. The enhanced nitrogen incorporation can be modeled as a thermally activated process with activation energy of about 0.1 eV.

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.