R. A. van den Heuvel

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.

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.

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.

Diffusion and strain relaxation in silicon/silicon-germanium/silicon structures studied with Rutherford backscattering spectrometry

Abstract The thermal stability of strained Si/Si1−xGex/Si structures grown by molecular beam epitaxy was investigated by resistive heating and in situ Rutherford backscattering spectrometry. Ge profiles obtained from a 50 nm Si1−xGex layer on a Si(100) substrate capped with 50 nm Si were evaluated for different Ge concentrations after sequential heating periods at a particular temperature between 850 and 1010° C. The diffusion coefficients, calculated from the increase in signal in the tail of the Ge profile, proved to be comparable to the value for Ge in bulk Si. A more pronounced decrease of the signal at the center of the Ge profile indicated a faster diffusion within the SiGe layer which was confirmed by analysis of the FWHM of the Ge profile. Ion channeling measurements were used to characterize tetragonal strain in the buried SiGe layers. Angular scans through the 〈111〉 direction were interpreted with Monte Carlo channeling calculations and used to study strain relaxation in dislocation-free and partially relaxed layers.

Thermal stability of strained Si/Si1-xGex/Si structures

The thermal stability of strained Si1-xGex layers grown epitaxially by molecular beam epitaxy on Si(100) and capped by a Si top layer was studied using Rutherford backscattering spectrometry. Diffusion coefficients were evaluated from the Ge scattering profiles of a 50 nm SiGe film, capped with 50 nm Si, for anneal temperatures between 850 degrees C and 1010 degrees C. The diffusion coefficient, calculated from the increase in signal in the tail of the Ge profile, proved to be comparable with the value for Ge in bulk Si. An enhanced decrease in signal at the centre of the Ge profile indicated a faster diffusion within the SiGe layer. This was confirmed by analysis of the FWHM of the Ge profile, from which a diffusion coefficient was derived which was up to 20 times higher.

Co diffusion and precipitation in Si/SiGe heterostructures

Several Si/Si1−xGex multilayer structures, grown by molecular‐beam epitaxy (MBE) or synthesized by high‐dose Ge implantation into Si substrates, have been implanted with Co atoms at doses of typically 1.0×1015 and 1.0×1016 ions/cm2. These samples are annealed at elevated temperatures (typically 850 °C) to study the diffusion and precipitation of the Co atoms. Depth distributions of Co atoms are measured with secondary‐ion mass spectrometry. X‐ray diffraction is used to measure the strain evolution of the Si/Si1−xGex MBE multilayers during processing. Channeling Rutherford backscattering spectrometry is used to study the lattice position of the Co atoms after heat treatment. Upon annealing, Co atoms are observed to diffuse out of the Si1−xGex layers. The efficiency of this outdiffusion process strongly depends on the implantation dose. The released Co atoms are gettered by the Si layers adjacent to the Si1−xGex layers and, in case of the MBE samples, at the interface between the substrate and MBE‐grown buf...

Transport properties of p-Si/SiGe single-modulation-doped heterostructures grown by MBE

Abstract Electrical properties have been examined for single Si/Si 1- x Ge x p-channel modulation-doped heterostructures grown by MBE. Heterostructures with the doped layer on top have a higher hole mobility than structures with the doped layer on the substrate side. This difference is caused by B segregation during growth. The effects of B concentration and Ge content on the electrical properties are studied. Hole mobilities as high as 6000 cm 2 /V⋯ at 2 K have been obtained, which are the highest values reported so far.

Mono- and polycrystalline silicon emitters for bipolar transistors grown by molecular beam epitaxy

Abstract Molecular beam epitaxial silicon was used to grow epitaxial and polysilicon emitters in self-aligned bipolar transistors with the “double poly” structure. The emitter characteristics of the transistors were compared with various types of polysilicon emitters grown by low pressure chemical vapour deposition. It is shown that the molecular beam epitaxial silicon emitter provides the lowest emitter series resistance and has an adequate emitter efficiency.