C. Dieker

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.

Formation techniques for porous silicon superlattices

Abstract Porosity superlattices (SLs) are a new type of Si-based heterostructures which exhibit a periodical variation of the porosity in depth. These structures have been investigated by transmission electron microscopy. Different formation techniques for porous Si SLs will be presented: SLs on p-type doped Si were formed by periodic variation of the formation current density or by using periodically doped Si substrate layers. An influence of the substrate quality on the interface roughness has been found. On n-type Si the illumination intensity has been periodically changed during the etching process which leads to a periodical variation in the macropore radii. An explanation for this dependence is suggested.

Lateral confinement by low pressure chemical vapor deposition-based selective epitaxial growth of Si1−xGex/Si nanostructures

Among the growth approaches being considered currently to realize quantum dots and quantum wires is the selective epitaxial growth on patterned substrates. With this technique the feature size and geometry are mainly limited by the lithographic process. With optical lithography we achieved a lateral dimension of ⩾0.4 μm. Therefore, to further reduce the lateral dimension, but still using optical lithography, the tendency toward facet formation during selective epitaxial growth was investigated. Si0.70Ge0.30 multiple quantum well structures with Si0.935Ge0.065 spacers and buffers were deposited on (001) Si. The buffer thickness was varied so as to achieve facet junction. While on large areas the Si0.935Ge0.065 buffer was relaxed, for dots ⩽300 μm or narrower the structures remained strained even for buffer thicknesses exceeding by a factor of two–three the critical thickness of large area. In dots and wires where facet junctioning has taken place a rounded region between facets (approximately 50 nm broad) ...

Growth kinetics of iron silicides fabricated by solid phase epitaxy or ion beam synthesis

Abstract The growth kinetics of the sequential formation of FeSi and β-FeSi 2 layers on (111)Si during solid phase epitaxy were investigated over a wide range of temperatures. Surface layers of β-FeSi 2 were produced by evaporation of Fe onto (111)Si substrates and subsequent rapid thermal annealing. The composition and the thickness of the growing layers were determined by Rutherford backscattering spectroscopy. The phases were identified by X-ray diffraction. First, the FeSi phase forms in the temperature range 500–625 °C. The thickness of the FeSi layer increases with the square root of time, which is indicative of diffusion-limited growth of the silicide layer. An activation energy of 1.36 ± 0.25 eV was deduced. The formation of β-FeSi 2 layers was found to be nucleation controlled with an approximate activation energy of 2.6 ± 0.5 eV. In addition, the coarsening of FeSi 2 precipitates in (111)Si during the formation of buried FeSi 2 layers was studied using ion beam synthesis and thermal annealing. The estimated activation energy for the coarsening process was 3.63 ± 0.33 eV, which is consistent with Ostwald ripening.

Evidence for strain relaxation via composition fluctuations in strained quaternary / quaternary and quaternary / ternary multiple quantum well structures

Abstract The occurence of large-scale contrast modulations is discussed, which have been observed in the transmission electron microscope in cross-sectional samples containing Ga x In 1− x As y P 1− y with a number of different x and y values. 90°-wedge samples were used because of their known geometry, in which thin-foil contrast effects can be excluded. The samples exhibiting large-scale modulations of a period greater than 100 nm all contained quaternary / quaternary or quaternary / ternary multiple quantum well stacks. Indications for a correlation between the onset, strength and frequency of the modulations with the well/barrier composition and its proximity to the centre of the miscibility gap are presented. The contrast is also found to be influenced by the stack thickness and strain. In large stacks, the modulations are related to waviness in epi-layer growth. The contrast is discussed in terms of composition modulation patterns connected with strain fluctuations, which are set up in interfacial regions and which evolve into wavy layer growth as means of misfit strain relief.

Ion beam synthesis of buried epitaxial FeSi2

Abstract We fabricated continuous buried layers of the metallic α-FeSi 2 phase by high dose implantation of Fe + into (111) silicon wafers and subsequent rapid thermal annealing at 1150 °C for 10 s. As determined from Rutherford backscattering spectrometry, these α-FeSi 2 layers have a stoichiometry of Fe 0.83 Si 2 , corresponding to approximately 17% iron vacancies. Schottky diodes were fabricated on n-type silicon with ideality factors of n = 1.4 ± 0.1 and Schottky barrier heights of Φ B = 0.84 ± 0.03 eV . Deep level transient spectroscopy measurements of these diodes showed a low concentration of iron in silicon of about 1 × 10 13 cm −3 . Semiconducting stoichiometric β-FeSi 2 layers were produced by transforming buried α-FeSi 2 layers into β-FeSi 2 by furnace annealing at 800 °C foor 17 h.

Formation of buried CoSi2 layers with ion beam synthesis at low implantation energies

The formation of Si/CoSi2/Si heterostructures by high dose Co implantation has been studied for the implantation energy range of 30–200 keV. Various implantation and post-implantation annealing procedures were investigated in order to determine the conditions for producing the thinnest possible CoSi2 layers. Layer thicknesses ranging from 70 nm at 200 keV to as thin as 18 nm at 30 keV were produced in both (100) and (111) oriented Si. For implantation energies < 70 keV, it was necessary to keep the annealing temperatures below 950°C in (100) Si — a restriction not encountered in (111) Si — to produce a continuous layer. The mechanism for this temperature dependence is discussed. Channeled implantation was also investigated as a technique to produce a thin buried layer at low energies, but comparison of such layers with typical random implants showed no improvement in heterostructure quality nor any apparent processing advantages. Finally, the suitability of the top Si layer of the heterostructures as a seed for epitaxial Si has been investigated with low pressure vapor phase epitaxy. The heterostructures were analyzed by Rutherford backscattering spectroscopy, ion channeling, cross section transmission electron microscopy and resistivity measurements.

Investigation of dislocations in Si1−xGex/Si heterostructures grown by LPCVD

Abstract The morphology and distribution of dislocations in both single-layer and multiple-quantum-well Si 1− x Ge x /Si structures epitaxially grown by LPCVD between 700 and 750°C have been investigated by techniques of chemical etch, transmission electron microscopy and photoluminescence. A special morphology of initial-stage misfit dislocations and precipitate colonies formed along the misfit dislocations after annealing relaxation are presented and discussed. Dislocation-related photoluminescence has been distinguished and well correlated to the corresponding misfit and threading dislocations in the heterostructures. The combination of Schimmel etching and photoluminescence technique has allowed the experimental determination of the critical thickness of as-grown strained single layers grown at 750°C. A step-by-step etch in depth followed by taking photoluminescence spectra made possible the localization of the misfit dislocations in the multiple-quantum-well structures.

SUPPRESSION OF WAVY GROWTH IN METALORGANIC VAPOR PHASE EPITAXY GROWN GAINAS/INP SUPERLATTICES

The effects of growth temperature, V–III partial pressure ratio in the gas phase, and the nature of the carrier gas on the morphology of lattice matched InGaAs/InP multiple quantum well stacks are investigated. Preliminary results of an extension of this study to strained InGaAs/InP systems are presented.

Electron energy loss spectroscopy on FeSi2/Si(111) heterostructures grown by gas source molecular‐beam epitaxy

The growth of FeSi2 on Si(111) by gas source molecular‐beam epitaxy is reported. Fe(CO)5 and SiH4 are used as source gases for the silicide growth on the heated substrate. A wide range of growth temperatures, 400–800 °C, has been examined and the thickness of the grown layers ranges between 50 and 200 nm. High resolution electron energy loss spectroscopy, with its sensitivity to the electronic properties of heterostructures in a range of several hundreds angstroms below the surface, turns out to be very effective for the in situ characterization of the growth. Photoelectron spectroscopy and ex situ high‐resolution transmission electron microscopy complete this study. Three main growth modes are identified. For Tgrowth≤500 °C, different phases are growing, with an early stage of β‐FeSi2 nucleation. Above 550 °C only the semiconducting β phase is formed with increasing tendency towards three dimensional growth as the temperature increases. In the range 500–550 °C the β‐FeSi2 layer grows uniformly and the pr...