H. Kibbel

MBE-grown Si/SiGe HBTs with high beta , f/sub T/, and f/sub max/

Si/SiGe heterojunction bipolar transistors (HBTs) were fabricated by growing the complete layer structure with molecular beam epitaxy (MBE). The typical base doping of 2*10/sup 19/ cm/sup -3/ largely exceeded the emitter impurity level and led to sheet resistances of about 1 k Omega / Square Operator . The devices exhibited a 500-V Early voltage and a maximum room-temperature current gain of 550, rising to 13000 at 77 K. Devices built on buried-layer substrates had an f/sub max/ of 40 GHz. The transit frequency reached 42 GHz.<<ETX>>

Secondary implantation of Sb into Si molecular beam epitaxy layers

We report on the influence of low‐energy Si+ ions on the incorporation of Sb adatoms existing on growing (100) Si molecular beam epitaxy layers. At a growth temperature of 650 °C employed for these experiments an increase of incorporation of about three orders of magnitude compared to the spontaneous incorporation is obtained at ion flux densities of typically 1012 cm−2 s−1. Dopant activation coefficients of almost unity are established up to 1019 cm−3. The number of incorporated adatoms is found to increase proportionally with preadjusted adatom density as well as with Si+ ion dose. At an ion energy of 500 eV the constant of proportionality is estimated to be σI =(5±2)×10−16 cm2.

Strain relaxation of epitaxial SiGe layers on Si(1 0 0) improved by hydrogen implantation

Abstract We propose a new method to fabricate strain relaxed high quality Si1−xGex layers on Si by hydrogen implantation and thermal annealing. Hydrogen implantation is used to form a narrow defect band slightly below the SiGe/Si interface. During subsequent annealing hydrogen platelets and cavities form, giving rise to strongly enhanced strain relaxation in the SiGe epilayer. As compared to thermally induced strain relaxed Si–Ge epilayers, the hydrogen implanted and annealed samples show a greatly reduced threading dislocation density and a much higher degree of strain relaxation (90%). We assume that the hydrogen induced defect band promotes strain relaxation via preferred nucleation of dislocation loops in the defect band which extend to the interface to form misfit segments. The samples have been investigated by X-ray diffraction, Rutherford backscattering spectrometry and transmission electron microscopy.

Test of Vegard's law in thin epitaxial SiGe layers

Abstract Ultrametastable pseudomorphic Si 1− x Ge x layers (on (100) Si) with Ge contents x up to 32% and partly relaxed SiGe layers with Ge contents between 35% and 75% were grown by molecular beam epitaxy at growth temperatures between 310 and 575°C. The relation between lattice constant of the alloy layers and chemical composition (Ge content) was measured by X-ray methods (reciprocal space mapping) and by Rutherford backscattering. Slight negative deviations from the linear relationship (Vegards law) were found which confirm very early bulk data gained by Dismukes.

Investigation of strain‐symmetrized and pseudomorphic SimGen superlattices by x‐ray reciprocal space mapping

Double‐crystal and triple‐axis x‐ray diffractometry was used to characterize in detail the strain and composition of short period Si6Ge4, Si8Ge8, Si9Ge6, and Si17Ge2 strained‐layer superlattices (SLSs), grown by molecular‐beam epitaxy. Nominally strain‐symmetrized superlattices, intended to be free standing from underlying buffer layers and the substrate, grown on rather thin (20 nm thick) SiGe alloy buffers with constant Ge content (Si6Ge4 and Si8Ge8) are compared to those grown on 1.3‐μm‐thick step‐graded SiGe alloy buffers (Si6Ge4 and Si9Ge6). Due to the much higher instrumental resolution offered by triple‐axis diffractometry (Δ2Θ=12 arcsec) buffer and SLS peaks are clearly separated from each other, which overlap in corresponding double‐crystal‐diffractometry measurements (Δ2Θ in the range of 180 arcsec to 2°). The lattice constants parallel and perpendicular to the [001] growth direction are determined independently from each other and thus precise strain data of the buffers and the SLS constituting...

Self-assembled Ge-islands for photovoltaic applications

Abstract We have fabricated silicon solar cells with embedded germanium layers to form three-dimensional islands in the Stranski–Krastanov growth mode. The additional Ge-layers increase the infrared absorption in the base of the cell to achieve higher overall photocurrent and overcome the loss in open circuit voltage of the heterostructure. In an UHV-MBE chamber up to 75 layers of germanium, each about 8 monolayers thick, separated by Si-spacer layers (9– 16 nm ) were grown on each other using standard 10 Ω cm p-type Si-substrates. The density of islands in the layers was increased by the use of antimony as surfactant, thus densities >10 11 cm −2 were realized. The islands were covered by a 200 nm thick Si-layer (n-type) on top which is used as emitter of the cell. Photoluminescence measurements, AFM and TEM-microscopy were used to characterize the growth of Ge-islands under various growth conditions and post-thermal treatment. Photocurrent measurements exhibit a higher response of the fabricated solar cells in the infrared regime compared to standard Si-cells.

Annealing effects in short period Si-Ge strained layer superlattices

Annealing effects in Si/Ge strained layer superlattices (SLS) are studied by Raman scattering. Spectra from optical and folded acoustic phonons are recorded after each temperature treatment. Above 600 degrees C the Ge LO phonon shifts to lower frequencies and the Si-Ge alloy mode gains in intensity due to diffusion of Si into the Ge layers. The Si mode remains unchanged up to annealing temperatures of about 800 degrees C. The intensity ratios of folded LA phonons are also sensitive to the annealing process. Their frequencies shift only slightly. Raman spectroscopy allows the authors to separate a different interdiffusion process of species in adjacent layers.

Room‐temperature electroluminescence from Si/Ge/Si1−xGex quantum‐well diodes grown by molecular‐beam epitaxy

Tunable room‐temperature electroluminescence, photocurrent, and photoluminescence in the near infrared (λ∼1.3 μm) has been observed from Ge/Si/Ge/Si1−xGex quantum‐well (QW) diodes grown by molecular‐beam epitaxy. The QWs are grown on a p+‐doped 〈100〉‐Si substrate and consist of two thin Ge wells separated by a thicker Si middle layer, and the whole structure is embedded by two Si0.85Ge0.15 alloy layers. Our theoretical analysis of the data suggests that the strength of the spectra is linked to states localized at the interface.

Properties of Si layers grown by molecular beam epitaxy at very low temperatures

(100) silicon molecular beam epitaxy films with etch pit densities below 103 cm−2 and χmin values of 3.3–3.9% were grown at very low temperatures (Ts =250–350 °C). Although dopant activation is significantly below unity at n=1018 Sb atoms/cm3 Hall mobilities of homogeneously Sb‐doped samples (Ts =250 °C, 300 °C) are found to match reasonably bulk values. δ doping at a monolayer Sb deposition shows a dopant activation of 0.45–0.81 with no detectable broadening at Ts =200 °C.

Mesa and planar SiGe-HBTs on MBE-wafers

SiGe-HBTs have the potential for outstanding analog and digital or mixed-signal high frequency circuits widely based on standard Si technology. Here we review on MBE grown transistors and circuits. Processes and results of a research-like SiGe HBT and two possible production relevant HBT versions are presented. The high frequency results with fmax and fT up to 120 GHz and a minimum noise figure of 0.9 dB at 10 GHz demonstrate the advantage of using MBE samples with steep and high base doping and high germanium contents. A comparison to the concept of reported low doped, low germanium and triangular profiled SiGe base layers, realized by UHV-CVD, is given. In addition, some circuit demonstrators of SiGe-ICs will be presented.