H. Lafontaine

Transimpedance amplifier-based full low-frequency noise characterization setup for Si/SiGe HBTs

An experimental setup, based on current/voltage conversion through transimpedance amplifiers (TAs), has been implemented for the direct full low-frequency noise (LFN) characterization of Si/SiGe heterojunction bipolar transistors (HBTs) in terms of base and collector short-circuit current noise sources. This setup performs a full characterization, as it measures simultaneously the two noise current sources and their correlation, thanks to an original technique based on the specific properties of a specially designed buffer amplifier using a low-noise common-base bipolar transistor (CB BJT). By means of translation formulae, the obtained measurements are compared with those carried out with a multi-impedance technique. They show a good agreement both for the noise sources spectral densities and for their correlation. The TA-based setup provides enhanced capabilities in terms of measurement speed and remote control potentialities.

Kinetic critical thickness for surface wave instability vs. misfit dislocation formation in GexSi1−x/Si (100) heterostructures

Abstract The kinetic critical thicknesses for surface wave formation and misfit dislocation generation in UHVCVD-grown GeSi/Si have been quantitatively determined using microscopical techniques. A refined morphological instability theory has been developed using a coupled continuum/atomistic treatment that incorporates a nucleation barrier to the onset of surface wave formation. The theory accurately predicts the onset of surface wave formation as a function of thickness, composition, temperature and deposition rate. The interplay between misfit dislocation generation and surface wave formation can be elucidated from two-dimensional strain relaxation instability diagrams obtained from a 4-D parameter space.

Electronic contribution to secondary electron compositional contrast in the scanning electron microscope

Abstract Scanning electron microscopy of cleavage surfaces through a variable thickness SiGe 0.25 Si 0.75 heterostructure is shown to reveal the high sensitivity of the secondary electron signal to small changes in band structure. Ge 0.25 Si 0.75 layers that are coherently strained appear brighter in secondary electron micrographs than equal thickness layers of the unstrained Ge 0.25 Si 0.75 alloy. This effect has been studied quantitatively and is explained in terms of the 0.1 eV strain-induced raising of the Ge 0.25 Si 0.75 valence band edge resulting in an increased secondary electron escape probability.

Ultra low phase noise SiGe HBT. Application to a C band sapphire resonator oscillator

In this paper, the electrical and noise performance of a 0.8 /spl mu/m Silicon Germanium (SiGe) transistor optimized for the design of low phase noise circuits are described. The nonlinear model developed for the transistor and its use for the design of low phase noise C band Sapphire resonator oscillator are reported. The best measured phase noise (at ambient temperature) is -133 dBc/Hz at 1 kHz offset from a 4.85 GHz carrier frequency for a loaded Q/sub L/ factor of 60,000.

Photoluminescence in UHV-CVD-grown Si1−xGex quantum wells on Si(100): band alignment variation with excitation density and applied uniaxial stress

Abstract New photoluminescence data are presented which show an intrinsic conduction band alignment in Si 1− x Ge x quantum wells grown by ultra-high vacuum chemical vapor deposition (UHV-CVD) on Si(001) such that the Si 1− x Ge x layers form electron barriers (type 11 band alignment). The variation of the photoluminescence peak energies with applied (110) tensile stress for various excitation intensities for a number of quantum well samples of different germanium concentration shows an intrinsic conduction band offset which increases with germanium concentration. At higher excitation densities a photo-induced type I band alignment was observed with electrons confined to the Si 1− x Ge x layers through the electrostatic interaction with the excitation-created holes. Supporting temperature dependence data are also presented.

Ion beam intermixing of semiconductor heterostructures for optoelectronic applications

The ability of radiation enhanced quantum well (QW) intermixing to produce active integrated photonic devices has been demonstrated by the manufacture of a set of wavelength tuned lasers from a single semiconductor wafer. Defects, created in the InP-based structure by a high energy (1 MeV) P implant, enhance the diffusion of atomic species across the as-grown heterojunctions during subsequent rapid thermal annealing (90 s at 700°C). As a result, the QW band gap energy is blue shifted with respect to unirradiated regions. It is shown that by implanting through a SiO2 mask of varying thickness, the bandgap of the QW can be selectively tailored across the wafer. Additional results from GaAs- and SiGe-based QW systems are presented to illustrate how bandgap engineering techniques may be improved through a better understanding of the defect interactions involved. In the GaAs-based structure, defect trapping at structural interfaces has been identified as a possible hindrance to ion assisted intermixing. In contrast, data from the group IV QWs highlights the benefits of a low temperature (24 h at 630°C) anneal prior to irradiation. By removing defects from the as-grown material with pre-annealing, the relative bandgap shift induced by ion bombardment is doubled.

Phonon-resolved photoluminescence at λ=1.55 μm from undulating Si0.5Ge0.5 epitaxial layers

Si0.5Ge0.5/Si multiquantum well structures are grown using a production-compatible ultrahigh vacuum chemical vapor deposition system. The structures are designed in order to obtain dislocation-free undulating strained layers. A photoluminescence emission corresponding to the direct “no phonon” transition is measured at energies systematically smaller than calculated for planar layers, implying that any increase in band gap due to elastic relaxation of the lattice strain at the undulation crests is compensated for by a confinement energy decrease together with a Ge accumulation at the undulation crests. The photoluminescence “no phonon” emission peaks at a wavelength that increases with nominal well thickness up to 1.55 μm. This opens the possibility of using dislocation-free silicon–germanium undulating layers as an absorber for photodetector applications at the telecommunication wavelengths of λ=1.3–1.55 μm.

Photoluminescence study of initial interdiffusion of SiGe/Si quantum wells grown by ultrahigh vacuum‐chemical vapor deposition

SiGe quantum wells were grown at 525 °C using a commercially available, ultrahigh vacuum–chemical vapor deposition system, in which the purity of the material and quality of interfaces have already been demonstrated. Changes in photoluminescence line energies are monitored and the extent of interdiffusion in the wells during annealing is calculated. A strong initial enhancement of the diffusivity is observed in as‐grown material. Material annealed using a two‐step process in which strain and Ge peak concentrations are unchanged after the first (low temperature) step, shows a much lower interdiffusion during the second step. It is argued that strain alone cannot explain the enhanced interdiffusion, which is, thus, attributed to grown‐in, nonequilibrium point defects.

Laser-induced selective area band-gap tuning in Si/Si1−xGex microstructures

A one-step process is reported for selective area band-gap tuning of as-grown quantum well (QW) material consisting of Si/Si1−xGex microstructures. The process takes advantage of the ability of increasing the local temperature of the wafer, in excess of 900 °C, by applying the beam of a high-power cw Nd:YAG laser, which leads to controlled intermixing between the quantum well and barrier material. A microstructure with the band-gap blueshifted by 142 meV has been fabricated from as-grown 980 meV band-gap material. The results indicate that this approach has the potential for “writing” of Si/Si1−xGex QW microstructures with the selectively tuned band gap required in the fabrication of optoelectronic integrated circuits.

X-Ray scattering investigation of the interfaces in Si/Si1−xGex superlattices on Si(001) grown by MBE and UHV-CVD

Abstract We have used X-ray scattering techniques to compare the interface quality in similar Si/Si 1− x Ge x superlattices on Si(001) grown by either molecular beam epitaxy (MBE) or ultra-high vacuum chemical vapor deposition (UHV-CVD). High-resolution X-ray diffraction revealed that both structures were fully strained. The superlattices grown by chemical vapor deposition showed less thickness fluctuations from period to period, but the interfaces were slightly less chemically abrupt than that of the corresponding molecular beam epitaxy-grown structure. Grazing incidence X-ray specular and diffuse scattering revealed significant differences in the microstructural properties of the interface roughness. Transverse rocking scans of satellite peaks and detector scans showed a very different diffuse component in MBE and UHV-CVD. The structure grown by MBE is characterized by a long auto-correlation length (0.5 μ m) while the UHV-CVD sample possess a shorter correlation length (20–50 nm). In both cases, offset reflectivity scans showed that the diffuse scattering peaks at values of perpendicular wave vector transfer corresponding to the superlattice satellite peaks, indicating a strong cross-correlation of the interface roughness.