H. Lüth

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.

Effect of the heterointerface on the spin splitting in modulation doped InxGa1−xAs/InP quantum wells for B→0

Spin splitting of conduction band electrons in In0.53Ga0.47As/In0.77Ga0.23As/InP heterostructures due to spin-orbit coupling is studied by performing Shubnikov–de Haas measurements on nongated and gated Hall bars. From an analysis of the beating pattern in the Shubnikov–de Haas oscillations, the spin-orbit coupling constant is determined. For a symmetric sample no beating pattern and thus no spin splitting is observed. This demonstrates that the k3 contribution to the spin-orbit coupling constant can be neglected. By applying an envelope function theory it is shown that the major contribution to the Rashba spin-orbit coupling originates from the band offset at the interface of the quantum well. Using gated Hall bar structures it is possible to alter the spin-orbit coupling by application of an appropriate gate voltage. A more negative gate voltage leads to a more pronounced asymmetry of the quantum well, which gives rise to a stronger spin-orbit coupling.

Mechanism of molecular beam epitaxy growth of GaN nanowires on Si(111)

GaN nanowires have been grown without external catalyst on Si(111) substrates by plasma-assisted molecular beam epitaxy. Nanowire aspect ratios (length/diameter) of about 250 have been achieved. During the initial stage of the growth, there is a nucleation process in which the number of wires increases and the most probable nucleation diameter of about 10nm has been observed, which slowly increases with deposition time. For deposition time longer than the nucleation stage, the nanowire length as a function of diameter monotonically decreases. This phenomenon can be explained by adatom diffusion on the nanowire lateral surface towards the tip.

Porosity superlattices: a new class of Si heterostructures

Porosity superlattices have been investigated by transmission electron microscopy, photoluminescence and reflectance spectroscopy. The superlattices were formed on p-type doped Si using two different techniques. Firstly, for homogeneously doped substrates we have periodically varied the formation current density and thereby the porosity. Secondly, the current density was kept constant while etching was performed on periodically doped Si layers. For the first type of superlattices the layer thicknesses were determined by transmission electron microscopy. The results are in good agreement with the values calculated from the etching rate and time. For both types of superlattices, reflectance and photoluminescence spectra show strong modulation due to the periodicity of the superlattice.

Dielectric filters made of PS: advanced performance by oxidation and new layer structures

Copyright (c) 1997 Elsevier Science S.A. All rights reserved. For the formation of PS dielectric filters a detailed calibration of the etch rates and refractive indices is required. The effective dielectric function of PS was determined for different substrate doping levels as a function of the anodization current density by fitting reflectance spectra. Based on these results a number of different dielectric filters were realized. For device applications a thermal oxidation step is necessary to reduce aging effects which occur as a result of the native oxidation of PS. In addition, thermal oxidation results in a qualitatively improve filter performance due to a reduced absorption in the PS layers. Therefore the dielectric functions of PS oxidized in dry O 2 at temperatures up to 950 °C were determined. A continuous variation of the porosity and hence the refractive index with depth was used to realize so-called rugate filters. This type of interference filter allows the design of structures with more complex reflectance or transmittance characteristics than structures consisting of discrete single layers.

Porous silicon as a substrate material for potentiometric biosensors

For the first time porous silicon has been investigated for the purpose of application as a substrate material for potentiometric biosensors operating in aqueous solutions. Porous silicon was prepared from differently doped silicon substrates by a standard anodic etching process. After oxidation, penicillinase, an enzyme sensitive to penicillin, was bound to the porous structure by physical adsorption. To characterize the electrochemical properties of the so build up penicillin biosensor, capacitance - voltage (C - V) measurements were performed on these field-effect structures.

Size distribution of Ge islands grown on Si(001)

Atomic force microscopy analyses were performed on Ge islands on Si(001) grown by low pressure chemical vapor deposition in the temperature range from 525 to 700 °C. A comparison with theoretical models describing the growth of coherently strained and plastically relaxed islands is given to describe the observations. The mean diameter of coherently strained islands is found to be 170 nm over a wide range of temperature, whereas plastically relaxed islands grow up to diameters >500 nm. The aspect ratio turns out to be independent of the presence of dislocations. For the sample grown at 700 °C three size regimes could be observed, whereas the sample grown at the lowest temperature exhibits no island formation. At 550 °C islands with an average diameter of 33 nm and a low aspect ratio were observed; these could prove to be interesting in applications using quantum confinement effects.

A detailed Raman study of porous silicon

Abstract Porous silicon layers formed on p-doped substrates with different doping levels were studied by Raman spectroscopy. The porosity of the samples varied between 36% and 65%. One set of samples had been preoxidized. The presence of nanocrystals in the porous film was clearly observed by an asymmetric broadening of the optical silicon phonon in the Raman spectra. The diameters of the nanocrystals were obtained by a detailed line shape analysis of the phonon Raman peak. With increasing porosity the amount of small nanocrystals increased compared with the amount of those with diameters larger than 35 A. Furthermore, changes are observed in the multiphonon regime which are due to surface-assisted multiphonon processes which are enhanced in porous films.

Investigation and design of optical properties of porosity superlattices

Abstract We have investigated the optical properties of porosity superlattices and complex multilayer systems. Type II superlattices reveal a more complex layer structure than expected from the substrate doping levels. Type I layer systems have been used to form highly reflective layer systems and Fabry-Perot filters.

Ion beam synthesis of buried α-FeSi2 and β-FeSi2 layers

Using high dose implantation of Fe+ into (111)Si, followed by rapid thermal annealing (RTA) at 1150 °C for 10 s, we fabricated continuous buried layers of the metallic α‐FeSi2 phase. Rutherford backscattering experiments indicate that these layers contain a large number of Fe vacancies, up to 18%. By implanting through a SiO2 mask, we produced Schottky diodes with idealty factors of 1.4±0.1 and a Schottky barrier height of ΦB=0.84±0.03 eV on (111) n‐Si. In this letter we report for the first time the formation of the semiconducting stoichiometric FeSi2 (β‐FeSi2) phase by annealing the buried α‐FeSi2 layers below the phase transition temperature of 937 °C; specifically at 750 °C for 20 h.