L. Vescan

The single-mode condition for semiconductor rib waveguides with large cross section

A comparison of the single-mode condition for semiconductor rib waveguides with large cross section obtained by the effective index method and the corrected formula proposed by Soref et al. [see IEEE Journal of Quantum Electronics, vol. 27, p. 1971, 1991] with experimental data is carried out. It is found that the effective index method is in a better agreement with Rickmans experimental data than the modified version. Moreover a stronger condition is proposed for single-mode design purposes.

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.

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.

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.

Selective epitaxial growth of SiGe alloys—influence of growth parameters on film properties

Abstract In this paper the selective growth of SiGe by low-pressure chemical vapor deposition is discussed on the basis of the present understanding of the selective growth process, with emphasis on the Si-Ge-Cl-H system and SiO 2 masks. The selectivity of growth is a function of total pressure, temperature, Cl/H ratio and partial pressures of the reactive species. These factors, among others, influence strongly the chemistry of the deposition reaction. Their role must therefore be clearly understood, interpreted and predicted by thermochemical and kinetic analysis. An attempt will be made to determine the most significant parameters and to find selection criteria. The growth rate and composition dependence is affected by the pad size; this dependence is weaker at low-pressure than at atmospheric-pressure conditions. The evolution of relaxation the pad size is demonstrated for 16%Ge. Finally, intense photoluminescence observed in dots 500 nm wide with multiple quantum wells is presented.

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.

Misfit dislocations in finite lateral size Si1-xGex films grown by selective epitaxy

Abstract In this paper, the reduction of misfit dislocation density on small pads of selectively grown Si 1- x Ge x films is studied experimentally and theoretically. The experiments were performed mainly on Si 1- x Ge x layers with x =0.12 grown on patterned substrates with pad sizes of 10×10 to 10 4 ×10 4 μm 2 . The misfit dislocations were detected by optical and transmission electron microscopy. It was observed that on small pads, misfit dislocations are generated at a significantly higher critical thickness than on extended areas, while pads of size 10×10 μm 2 or smaller showed no evidence of misfit dislocations at all. The theoretical analysis was performed in two steps. First, an elastic strain model was used to calculate the pad size dependence of the critical thickness. The main hypothesis of the model is that the density of misfit dislocations is solely affected by the elastic relaxation at the edges of small epitaxial areas. This equilibrium model can explain only the experimentally observed absence of misfit dislocations on small pads; however, it predicts a critical thickness for finite sizes much lower than the observed one. Second, a kinetic approach was further performed, in which the relaxation is supposed to be due to nucleation of misfit dislocations at defects and self-multiplication. The best fit with the experimental results was obtained for time constants for generation from defects of 600 min and for self-multiplication of 7 min and a gliding velocity of 12 μm/min. While the onset of relaxation seems to be due generation at defects, the self-multiplication process determines to a great extend the density of misfit dislocations.

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) ...

Size distribution and electroluminescence of self-assembled Ge dots

In this article we study the electroluminescence of p-i-n diode structures with Ge dots consisting of coherent three-dimensional small (pyramids) and larger (dome) islands. The Ge dots are formed through strain-induced islanding. The diode structures, including one layer with Ge dots, were deposited on Si mesas with variable areas in order to study the influence of limited area deposition on self-assembling. It was observed that the reduction of deposited area improves island uniformity. The combined analysis of island distribution and electroluminescence spectra has lead to the conclusion that domes in small diodes have a smaller Si content or are less relaxed than domes in larger diodes. The diodes are found to emit up to room temperature near the optical communication wavelength of 1.3 microns.