Horst Kibbel

The n-channel SiGe/Si modulation-doped field-effect transistor

At the heterointerface of Si1-xGex/Si the existence of two-dimensional carrier gas has recently been demonstrated. The electrons are confined inside the large-gap material Si. We report the first fabrication of n-channel modulation-doped SiGe/Si hetero field-effect transistors by use of molecular-beam epitaxial growth. Though neither layer sequence nor parasitic resistances were optimized, these first transistors exhibit an extrinsic transconductance of 40 mS/mm for a gate length of 1.6 µm. This value is higher than that of conventional Si MESFETs of comparable carrier concentration. Technological processing steps and device evaluation are described.

Ultrathin SimGen strained layer superlattices-a step towards Si optoelectronics

Ultrathin SimGen (m monolayers (ML) Si, n ML Ge) strained layer superlattices (SLS) have been grown by molecular beam epitaxy. The optical properties of these structures depend on the concept of band-structure engineering by Brillouin zone folding and strain adjustment of the SLS by a Si1-ybGeyb alloy buffer layer. The energies and the oscillator strengths of the bandgap and intersubband transitions have been studied theoretically for SimGen SLS with a variety of period lengths, particularly those of m+n=10. Various characterization tools such as X-ray diffraction, transmission electron microscopy, Raman spectroscopy, photoluminescence (PL) and photocapacitance measurements have been used to analyse growth quality, interface sharpness, morphology, strain distribution and optical properties of the superlattice experimentally. The PL data indicative of the quasidirect energy gap of the 10 ML strain-symmetrized SLS in the near-infrared spectral regime (h nu approximately 0.8 eV) are presented and discussed as well as complementary photocapacitance measurements on a p-n doped Si4Ge4 SLS diode. The fabrication of test mesa diodes from Si/Ge SLS structures is described. Finally, device applications offering the possibility of monolithic integration of superlattice devices with complex silicon-based electronic circuits are outlined.

Growth of 100 GHz SiGe-Heterobipolar Transistor (HBT) Structures

The transit frequency f T of SiGe-heterobipolar transistors (HBTs) was increased from 20 GHz to 100 GHz. This was mainly achieved by thickness reduction of the double heterojunction SiGe-base from 65 nm to 25 nm. The complete vertical structure of the SiGe-HBTs (collector, base, emitter, emitter contact) was grown in one run by Si molecular beam epitaxy (Si-MBE). The growth temperature was varied from 650° C at the collector side to 325° C at the emitter contact side. The different n-type doping levels (1017/ cm3, 1018/ cm3, 1020/ cm3) were obtained by applying three different Sb-doping techniques (secondary implantation, adatom pre build-up, low temperature doping). The p-type base was doped with boron. The doping level in the base (6×1019/ cm3) exceeded the emitter doping level by a factor of 30 (doping level inversion).

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 550 °C (325–450 °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 600 °C.

Boron delta doping in Si and Si0.8Ge0.2 layers

By using an elemental boron effusion cell, B delta doping structures (5×1013 B atoms/cm2) were grown on Si (100) by molecular beam epitaxy at different substrate temperatures and cap layer compositions (Si and Si0.8Ge0.2). Close to the delta interface the B profiles are characterized by an exponential decay in growth direction. For the Si cap the results suggest the existence of a transition from equilibrium segregation (exponential decay length ≂20 nm) to kinetically limited segregation (transition temperature ≂600 °C at 0.1 nm/s). The doping profiles also give evidence of a temporal change of the segregation coefficient which is probably caused by clustering of segregating B atoms.

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=500 °C growth temperature surface recombination dominates the low bias range. At Li=35 nm and Tg=325 °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.

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.