A. Hartmann

Photoluminescence and electroluminescence of SiGe dots fabricated by island growth

We present a study of photo‐ and electroluminescence of SiGe dots buried in Si and compare them with structures containing smooth SiGe layers. The SiGe dot structures were fabricated by low‐pressure chemical vapor deposition using the Stranski–Krastanov growth mode (island growth). We show that the localization of excitons in the dots leads to an increase of the luminescence efficiency at low excitation compared to smooth SiGe layers (e.g., quantum wells). At higher excitation the efficiency decreases which is attributed to nonradiative Auger recombination processes in the dots.

Optical and structural investigation of SiGe/Si quantum wells

In this letter we report photoluminescence and structural results obtained on asymmetrically strained Si0.7Ge0.3/Si single and multiple quantum wells epitaxially grown by low pressure chemical vapor deposition. Well‐resolved peaks were obtained which can be attributed to quantum well excitons and their transversal optical phonon replica. A good correlation between peak properties and structure results was found. From the photoluminescence peak energies a valence band offset of 0.27 eV and an effective hole mass of 0.25 were estimated.

Photoluminescence and microstructure of self‐ordered grown SiGe/Si quantum wires

Employing self‐ordered growth in convex corners of nonplanar Si substrates, SiGe quantum wires of approximately 30 nm lateral dimension were fabricated. Photoluminescence spectra of these structures are dominated by transitions originating from the quantum wires at measurement temperatures above 20 K. The energetic positions of the quantum wire transitions are in good agreement with Ge concentrations measured by spatially resolved energy dispersive x‐ray spectroscopy using a scanning transmission electron microscope. We find that the Ge concentration inside the wire is considerably lower than the nominal value for growth on planar parts of the substrate. In addition we find a pronounced gradient in the Ge concentration of the wire.

Growth of modulation-doped GaAsAlGaAs quantum wires on V-groove patterned substrates

Abstract In order to achieve modulation-doped V-groove quantum wires for transport measurements, two different combinations of precursor groups have been tested: TMGa, TMAl and AsH 3 (set 1) and TEGa, dimethylethylaminealane (DMEAAl) and AsH 3 (set 2). Perfect selectivity of growth on SiO 2 masked substrates was obtained for set 1 and quantum wires were formed with lateral dimensions of 20 nm. However, selective epitaxy results in large local growth rate variations in areas of different SiO 2 surface coverage, complicating the design of modulation-doped structures. Using precursor set 2, growth is not selective. Crystalline regions on unmasked areas join smoothly the polycrystalline film growing with a similar growth rate on SiO 2 masked regions. The result is a growth rate independent of the local substrate patterns. Since the semi-insulating polycrystalline film does not degrade the quality of crystalline regions, these samples exhibit the first magnetotransport measurements that clearly indicate the existence of a 2DEG in our V- and U-groove structures. As a result we are able to define a layer structure that combines the specific advantages of precursor set 1 and 2 for future work.

Electrical and structural studies of AlGaAs/GaAs wires grown on patterned substrates

Abstract The electrical and structural properties of U- and V-shaped Al 0.3 Ga 0.7 As/GaAs wire structures grown by low-pressure metal-organic vapor phase epitaxy were investigated. By using dimethylethylamine alane (DMEAA) and triethylgallium (TEGa) for the growth of the bottom Al 0.3 Ga 0.7 As buffer layer, a non-conductive, polycrystalline layer is formed on top of the SiO 2 masked areas, leading to an effective electrical isolation between adjacent wire structures. The characterization of the morphology by TEM confirmed the formation of a wire structure at the bottom of the V-groove. Additionally, it was observed that if DMEAA/TEGa is used as a source for the growth of the lower Al 0.3 Ga 0.7 As barrier layer, the radius of curvature of the crescent-shaped quantum wires at bottom of the V-shaped groove is larger than in structures where this barrier layer is based on trimethylaluminum and trimethylgallium sources. From magneto-transport measurements under different orientations of the magnetic field it could be concluded that in U-shaped structures, the main conductive channel is located at the (100) bottom layer, while for the V-shaped structures the side-wall quantum well and the quantum wire contribute to the conductance.

Photoluminescence and magnetotransport of 2-D hole gases in Si/SiGe/Si heterostructures

Abstract p -Type modulation-doped Si/Si 1− x Ge x /Si strained layer double heterostructures with x ∼ 0.2 have been epitaxially grown on (100) Si substrates using low pressure chemical vapor deposition (LPCVD). The modulation doping effect has been obtained by two remote boron-doped Si layers ∼ 10 nm thick. Hole mobilities as high as 5900 cm 2 /Vs have been obtained at 4.2 K comparable to the best reported values, so far. The temperature dependence of the 2-D hole concentration and mobility shows characteristic remote ion-dominated scattering. Clear quantum Hall plateaus and Shubnikov-de Haas oscillations were observed. The photoluminescence (PL) from the SiGe quantum well (QW) shows excitonic behaviour in its variation with excitation power. A general trend of increasing PL intensity with the mobility of the 2-D hole gas was observed. The high hole mobility and strong PL reflect the good interfacial quality of the LPCVD Si/SiGe/Si heterointerfaces.

Intense photoluminescence from strained SiGe sub-100 nm wires selectively grown on Si by LPCVD

Abstract Selective epitaxial growth by low pressure chemical vapor deposition has been used to produce SiGe-heterostructures in oxide-windows parallel to 〈110〉 or 〈100〉 directions on (001) silicon substrates. For 〈110〉 oxide-wall directions, {111} and {311} facets are formed at the edge of the epitaxial areas, while only {110} facets develop for 〈100〉 directions. Transmission electron microscopy characterizations have clearly shown the formation of quantum wires near the intersecting (001) and {110} planes. The wires have a regular width of 100 nm and a thickness of 6 nm and show a very strong photoluminescence.

Spontaneous formation of a tilted AlGaAs/GaAs superlattice during AlGaAs growth

Abstract In transmission electron microscopical (TEM) cross-sections of AlGaAs layers grown on vicinal 111✓A facets of patterned substrates, we observe a periodic structure of Ga-rich and Al-rich layers forming angles of up to 25° with the growth front. This spontaneous tilted superlattice (TSL) formation is found in layers grown by metal-organic vapor phase epitaxy using the group III precursors dimethylethylaminealane (DMEAAl) and triethylgallium (TEGa) and in samples grown by solid source MBE. We present a growth model explaining the TSL formation in terms of step bunching and adatom diffusion. The step bunches required by the model are experimentally found by atomic force microscopy as well as high resolution TEM. Finally, we introduce a Monte Carlo simulation, which is able to reproduce the main features of the spontaneously formed TSL.