G. F. A. van de Walle

Ge segregation at Si/Si1−xGex interfaces grown by molecular beam epitaxy

The interface quality of Si/Si1−xGex (0.08≤x≤0.33) interfaces grown by molecular beam epitaxy has been studied by means of secondary‐ion mass spectrometry. Ge segregation occurs into the Si capping layers. The segregation is characterized by a 17 nm/dec slope; the total amount of segregated Ge corresponds to a few tenths of a monolayer. The phenomenon is independent of the Ge fraction and does not depend on temperature as long as crystal growth is perfect. A possible explanation is given in terms of a Ge adlayer that is formed during growth as a result of site exchange between subsurface Ge and surface Si atoms. This adlayer is incorporated slowly during further Si growth. The Ge segregation can be suppressed by having an adlayer of Ga on the surface of the growing structure.

Mechanisms of implant damage annealing and transient enhanced diffusion in Si

Interactions between self‐interstitials (I) and {113} interstitial defects during annealing of Si implant damage have been studied. At low damage levels diffusion is ultrafast, driven by I released direct from the ion collision cascade. At higher damage levels, free I are quenched by nucleation of {113} defects. We show that the transient enhanced diffusion seen in most previous studies arises from the subsequent dissolution of the {113} defects.

Heterojunction bipolar transistors with SiGe base grown by molecular beam epitaxy

High-quality SiGe heterojunction bipolar transistors (HBTs) have been fabricated using material grown by molecular beam epitaxy (MBE). The height of parasitic barriers in the conduction band varied over the wafer, and the influence of these barriers on controller current, early voltage, and cutoff frequency were studied by experiments and simulations. Temperature-dependent measurements were performed to study the influence of the barriers on the effective bandgap narrowing in the base and to obtain an expression for the collector-current enhancement. From temperature-dependent measurements, the authors demonstrate that the collector-current enhancement of the HBTs can be described by a single exponential function with a temperature-independent prefactor.<<ETX>>

Germanium diffusion and strain relaxation in Si/Si1−xGex/Si structures

Abstract The thermal stability of strained Si1−xGex layers grown by molecular beam epitaxy on Si(100) was measured using Rutherford backscattering spectrometry, secondary ion mass spectroscopy and high resolution X-ray diffractometry (HRXRD). Diffusion experiments were carried out on Si1−xGex layers 50 nm thick (x = 0.07, 0.16 and 0.33) annealed at temperatures between 775 and 1010 °C for different times. The diffusion of germanium was evaluated from the broadening of the RBS and SIMS germanium profiles, while the strain relaxation was deduced from the angular shift of the (400) reflection in HRXRD. The diffusion coefficient thus measured proved to be strongly dependent on the local germanium concentration in the film. In the tails of the profile, the diffusion coefficient was comparable with the value for germanium in bulk silicon while in the centre of the film an enhanced diffusion was found. Both the initial germanium fraction x in the as-grown film and the presence of misfit dislocations hah only minor influence on the diffusion behaviour. It is concluded that safe thermal processing of these structures is possible up to 850 °C for several hours.

The interaction of Sb overlayers with Si(001)

Abstract Medium-energy ion scattering was used to study the temperature dependence of the saturation coverage, the layer morphology and reordering of the substrate after adsorption of Sb 4 tetramers and Sb 1 monomers on Si(001). Depositions at room temperature result in formation of Sb clusters on top of a dissociatively chemisorbed Sb layer. The clusters desorb at temperatures exceeding 420 K. For depositions at substrate temperatures above 570 K the Sb coverage saturates at 0.7 to 0.9 monolayer. The saturation coverage depends on substrate temperature and absorbing species, i.e., Sb 4 or dissociated Sb 4 . The highest coverages are obtained with a beam of dissociated Sb 4 . These dependencies are explained by the observation of a decreasing binding energy of the chemisorbed Sb with increasing coverage. The Si directly underneath the Sb layer is structurally bulk-like.

Effect of interface quality on the electrical properties of p‐Si/SiGe two‐dimensional hole gas systems

Electrical properties have been examined for single Si/Si0.8Ge0.2 p‐type modulation‐doped heterostructures which have been grown by molecular beam epitaxy. It is shown that the two‐dimensional hole gas in a normal modulation‐doped heterostructure (doped layer on the surface side) has a higher mobility than in an inverted structure (doped layer on the substrate side). Secondary‐ion mass spectrometry analysis indicates that the lower mobility in the inverted structure is due to surface segregation of boron. Hole mobilities as high as 6000 cm2/V s at 2 K and 3800 cm2/V s at 6 K have been obtained which are the highest values reported so far for Si/SiGe heterostructures.

Photoluminescence from Si/Ge superlattices

We have studied the luminescence of short‐period Si/Ge superlattices of varying composition grown on a Si1−xGex alloy buffer layer. X‐ray diffraction and Rutherford backscattering were used to analyze the composition of the samples. Luminescence bands at 1.5 and 1.6 μm originate from the superlattice, as is indicated by etching experiments. A strong change in luminescence intensity is observed as the composition and strain of the superlattice vary.

Positron beam defect profiling of silicon epitaxial layers

Epitaxial layers of silicon grown on a Si(100) substrate by molecular‐beam epitaxy (MBE) and solid‐phase epitaxy (SPE) have been investigated by slow positron beam analysis methods. Results of Doppler broadening measurements revealed that the S parameter of the SPE material is considerably higher than the value measured for the MBE layer, indicative of a higher concentration of open‐volume defects in the former material. This was confirmed by measurements of the positronium fraction at elevated temperatures.

Performance and processing line integration of a silicon molecular beam epitaxy system

Abstract Silicon molecular beam epitaxy (MBE) has been shown to be a promising technology for ultralow temperature epitaxy. Its capability to deposit strained Si-Ge layers and superlattices has received much attention. In this paper, the design, performance tests and processing line integration of a silicon MBE system capable of handling waters of 150 mm diameter is described. Several parameters, such as particle density, deposition uniformity, background doping, etch pit density and metallic contamination, have been carefully monitored during the first year of operation. These parameters are important acceptance criteria for the integration of silicon MBE into a standard processing line. The deposition uniformity across 140 mm was within 2%. Extensive measurements on the particulates identified the major source to be the first handling of wafers outside the cassette. Levels below 100 cm -2 are found. The background doping has been reduced from around 10 16 to 10 15 cm -3 . Strong reductions have been observed for the metallic contaminants tantalum and chromium. The system has been used as part of a standard processing line for the fabrication of thin, low temperature epitaxial layers. Two examples, epitaxial emitters for bipolar transistors, and p-i-n diodes, are given.

Structural characterization of an Sb delta-doping layer in silicon

Delta‐function doped layers in Si have been prepared by deposition of Sb on Si(001) followed by solid phase epitaxy of Si. The morphology and the crystal quality of the grown structures are characterized in situ during all stages of preparation by high‐resolution Rutherford backscattering spectrometry. The obtained doping profile is found to consist of a <0.8‐nm‐wide spike and a 4‐nm‐long tail in front of the spike. A large fraction of about 70% of the Sb atoms is confined to the spike while the remaining 30% is located in the tail. Ion channeling and blocking measurements demonstrate that at least 95% of the Sb atoms is located on substitutional lattice sites. At temperatures exceeding 1000 K, the Sb profile broadens and Sb atoms diffuse towards the surface where they desorb.