E. Kasper

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>>

Direct observation of band‐edge luminescence and alloy luminescence from ultrametastable silicon‐germanium alloy layers

Ultrametastable silicon‐germanium (Si1−xGex) layers with a Ge content x in the range from about 20% to 27% were grown by Si‐MBE at temperatures far below 550u2009°C (325–450u2009°C). The thicknesses of the layers (up to 500 nm) exceed the equilibrium thickness by a factor of up to 50. We observe in the as‐grown samples without any annealing both the excitonic Si1−xGex band‐edge luminescence and a broad alloy luminescence of unknown origin. The two peaks have an energy difference of ≊144 meV and shift linearly with the Ge content. The alloy band luminescence disappears when strain relaxation sets on upon annealing at around 600u2009°C.

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...

Electroluminescence at room temperature of a SinGem strained-layer superlattice

We report for the first time on room temperature electroluminescence in the region 1.3–1.7 μm from a strain‐adjusted Si6Ge4 superlattice. These results, together with photoluminescence, short‐circuit photocurrent spectroscopy, and voltage‐intensity and current‐intensity measurements indicate that the observed electroluminescence consists of two emission bands which are believed to be caused by defect and interband recombination processes.

Forward‐bias characteristics of Si bipolar junctions grown by molecular beam epitaxy at low temperatures

Forward‐bias current‐voltage characteristics of molecular beam epitaxy grown Si p+‐i‐n+ junctions have been determined at room temperature. At small widths of the i zone (Li=5 and 10 nm) band‐to‐band tunneling with a maximum peak‐to‐valley ratio of two is observed. Up to Li=30 nm (trap assisted) forward‐bias tunneling is apparent with saturation tunneling current densities somewhat lower than in p‐n junctions at comparable widths of the space‐charge region WSCR(0). For Li≳30 nm and Tg=500u2009°C growth temperature surface recombination dominates the low bias range. At Li=35 nm and Tg=325u2009°C, both surface and bulk recombination is observed. We found evidence that Si molecular beam epitaxy layers grown at this low temperature get an increasing density of crystalline defects with growing thickness.

Photoluminescence of confined excitons in MBE-grown Si1-xGex/Si(100) single quantum wells

Abstract MBE-grown single quantum wells of pseudomorphic Si 1 − x Ge x /Si(100), with x = 21%, 24% and 36% and well widths L z from 1.2 nm to 250 nm were studied by photoluminescence using Fourier transform spectroscopy. Observations showed very low thermal deactivation energies of the luminescence, power-dependent luminescence intensities which vary with L z in a characteristic way, and power-dependent luminescence energy shifts, again showing characteristic L z dependence. Moreover, the TO-phonon sidebands were decomposed into their parent Siue5f8Si, Siue5f8Ge and Geue5f8Ge vibrations. These components exhibit relative strengths depending strongly on the optical power. All these data conspire to suggest Type II staggered band line-up in the Si 1 − x Ge x /Si heterojunction system for x ⩽ 36%, whereas there is no easy explanation of the data by invoking a Type I band alignment.

Si/SiGe heterojunction bipolar transistor made by molecular-beam epitaxy

Si/SiGe heterostructure bipolar transistors (HBTs) were fabricated and compared to Si homojunction transistors with similar doping levels. Low-temperature Si-MBE (molecular-beam epitaxy) was used to form the heterojunction and the homojunction layer sequences. A wet chemical selective etching technique was used to contact the thin (80 nm) base layer of the heterojunction transistor. A peak current gain of 200 to 400 was measured for the heterostructure devices, compared to a gain of two for the homojunction structure. The current gain collector current dependence of the heterostructure device could be due to surface recombination effects. >

Photoluminescence studies of Si/Si1 − xGex quantum wells and SimGen superlattices

Abstract We report on photoluminescence studies of Si/Si1 − xGex quantum wells with systematically varied growth temperatures and well thicknesses. Well resolved band gap luminescence could be observed in quantum well structures grown at temperatures above 600 °C while for structures grown at lower temperatures defect-related lines dominate the luminescence spectra. We also present photoluminescence and electroluminescence studies for a strain-symmetrized Si5Ge5 superlattice. The photoluminescence observed below the band gap of the corresponding alloy is shown to be enhanced by growing the superlattice on a thick, single-step alloy buffer layer. Absorption measurements on the superlattice show an onset of the absorbance at an energetic position close to the observed photoluminescence. These findings provide strong evidence for band gap related photoluminescence and electroluminescence in a strain-symmetrized Si5Ge5 superlattice.

Characterization of short-period Sim Gen superlattices by high-resolution transmission electron microscopy and X-ray diffraction

Abstract High-resolution and analytical transmission electron microscopy as well as X-ray diffraction were used to characterize the structure of short-period strained-layer (Si m Ge n ) N superlattices ( m monolayers Si, n monolayers Ge, total number of periods N T = 300–500 °C) on different SiGe alloy buffer layers on Si(100) substrates. By a combination of these methods, detailed information can be obtained about periodicity, interface roughness on an atomic scale, strain and average composition of the superlattices. Superlattices of good morphology were grown, although defects were still present. Superlattices on thin buffers contained rather high defect-densities in general, whereas the defect-densities were much lower for superlattices grown on thick buffers, especially for those with composition gradients.

Analysis of strain and mosaicity in a short‐period Si9Ge6 superlattice by x‐ray diffraction

Triple axis x‐ray diffractometry was employed for the structural characterization of a 100 period Si9Ge6 superlattice grown by molecular beam epitaxy on a thick step‐graded SiGe alloy buffer. From the distribution of diffusely scattered intensity around reciprocal lattice points the correlation function of the deformation field due to structural defects has been calculated using kinematical theory of x‐ray diffraction. From the extension of the correlation function it turns out that on the average the entire superlattice (0.2 μm thick) scatters coherently along growth direction, whereas laterally the coherently scattering regions are extended only over about 40 nm.