J. Ahopelto

Reduction of the thermal conductivity in free-standing silicon nano-membranes investigated by non-invasive Raman thermometry

We report on the reduction of the thermal conductivity in ultra-thin suspended Si membranes with high crystalline quality. A series of membranes with thicknesses ranging from 9 nm to 1.5 μm was investigated using Raman thermometry, a novel contactless technique for thermal conductivity determination. A systematic decrease in the thermal conductivity was observed as reducing the thickness, which is explained using the Fuchs-Sondheimer model through the influence of phonon boundary scattering at the surfaces. The thermal conductivity of the thinnest membrane with d = 9 nm resulted in (9 ± 2) W/mK, thus approaching the amorphous limit but still maintaining a high crystalline quality.

Enhanced optical properties of in situ passivated near‐surface AlxGa1−xAs/GaAs quantum wells

An epitaxial method for in situ passivation of epitaxial AlxGa1−xAs/GaAs surfaces is reported. The deposition of an ultrathin InP layer (about one monolayer) on the surface of AlxGa1−xAs/GaAs structures by metalorganic vapor phase epitaxy results in drastically reduced surface recombination. The effect is studied by low‐temperature photoluminescence of near‐surface Al0.22Ga0.78As/GaAs quantum wells where the top barrier thickness is varied from 0 to 50 nm. At the thicknesses of ≤5 nm, the intensity from passivated samples is more than four orders of magnitude larger than that obtained from unpassivated structures. For a passivated surface quantum well where InP is deposited directly onto the GaAs quantum well, we observe a blueshift of 15 meV and an intensity reduction of only a factor of 10 as compared to the luminescence from a quantum well placed at a depth of 50 nm from the surface.

Pauli-blocking imaging of single strain-induced semiconductor quantum dots

The photoluminescence (PL) of InP strained-induced quantum dots in a GaInAs/GaAs quantum well is measured at low temperature (4.2 K) using near-field scanning optical microscopy. The PL originating from the first three confined levels of eight individual dots is mapped out over an area of 1.4×1.4 μm. The spatial resolution of the PL of the lowest energy level is found to be limited to about 0.5 μm. In contrast, the mapping of the PL of the higher excited state shows a much improved spatial resolution of the order of 150 nm which is the instrument resolution. This effect is understood in terms of Pauli-blocking of the dot level filling.

Electron–phonon heat transport and electronic thermal conductivity in heavily doped silicon-on-insulator film

Electron–phonon interaction and electronic thermal conductivity have been investigated in heavily doped silicon at subKelvin temperatures. The heat flow between electron and phonon systems is found to be proportional to T6. Utilization of a superconductor–semiconductor–superconductor thermometer enables a precise measurement of electron and substrate temperatures. The electronic thermal conductivity is consistent with the Wiedemann–Franz law.

Mechanical nanomanipulation of single strain-induced semiconductor quantum dots

We report on low temperatures (4 K) in situ nanomanipulation of the confining potential of single strain-induced Ga0.9In0.1As quantum dots. This was achieved by scanning a metal coated tapered optical fiber tip over the self organized InP stressor islands that are responsible for the localized strain field in the GaInAs/GaAs quantum well. By scanning the tip with a shear force contact of the order of 1 nN, we thinned down the InP stressor islands in an unexpectedly reproducible and controlled way. The modification of the confining potential was directly monitored by measuring in situ the photoluminescence of each manipulated dot using a near-field scanning optical microscope.

Red luminescence from strain‐induced GaInP quantum dots

The strain of self‐organized InP islands is used to induced quantum dots in near‐surface GaInP/AlGaInP quantum wells. To obtain quantum dot luminescence in a widely tunable wavelength range of 630–700 nm, the composition and thickness of the GaInP quantum well is varied. The effect of different cap layer materials, i.e., GaAs, AlGaAs, GaInP, and AlGaInP on the InP island formation and quantum dot luminescence properties is investigated. The luminescence intensity ratio of the quantum dot peak to the quantum well peak is found to be highest when a GaAs cap is used.

Maskless selective growth of InGaAs/InP quantum wires on (100) GaAs

A new fabrication process to create InGaAs/InP quantum wires on (100) GaAs substrates is demonstrated. The process is based on the selectivity of the growth of InP on lines created by focused ion beam bombardment, together with the selectivity of the growth of InGaAs on the InP wires. Intense photoluminescene is observed from the wires and the emission shows clear polarization parallel and perpendicular to the wires. Cathodoluminescene images confirm that the luminescence originates from the wires.

Potential of amorphous Mo-Si-N films for nanoelectronic applications

The properties of amorphous metallic molybdenum-silicon-nitrogen (Mo-Si-N) films were characterised for use in nanoelectronic applications. The films were deposited by co-sputtering of molybdenum and silicon targets in a gas mixture of argon and nitrogen. The atomic composition, microstructure and surface roughness were studied by RBS, TEM and AFM analyses, respectively. The electrical properties were investigated in the temperature range 80 mK to 300 K. No transition into a superconductive state was observed. Nanoscale wires were fabricated using electron beam lithography with their properties measured as a function of temperature.

Fabrication of GaInAs quantum disks using self-organized InP islands as a mask in wet chemical etching

GaInAs quantum disks are fabricated by wet chemical etching from a GaInAs/GaAs near‐surface quantum well using self‐organized InP islands as an etch mask. InP islands are formed in coherent Stranski–Krastanow growth mode by metalorganic vapor phase epitaxy. The free‐standing GaInAs/GaAs columns, produced by a three‐step etching process, are overgrown at 550 °C. The luminescence efficiency per emitting area from the regrown quantum disks is one order of magnitude larger than that from a regrown reference quantum well.