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While light-emitting nanostructures composed of group-IV materials fulfil the mandatory compatibility with CMOS-fabrication methods, factors such as the structural stability of the nanostructures upon thermal annealing, and the ensuing photoluminescence (PL) emission properties, are of key relevance. In addition, the possibility of improving the PL efficiency by suitable post-growth treatments, such as hydrogen irradiation, is important too. We address these issues for self-assembled Ge quantum dots (QDs) that are co-implanted with Ge ions during their epitaxial growth. The presence of defects introduced by the impinging Ge ions results in pronounced PL-emission at telecom wavelengths up to room temperature (RT) and above. This approach allows us to overcome the severe limitations of light generation in the indirect-band-gap group-IV materials. By performing in-situ annealing, we demonstrate a high PL-stability of the defect-enhanced QD (DEQD) system against thermal treatment up to 600 &deg, C for at least 2 h, even though the Ge QDs are structurally affected by Si/Ge intermixing via bulk diffusion. The latter, in turn, allows for emission tuning of the DEQDs over the entire telecom wavelength range from 1.3 &micro, m to 1.55 &micro, m. Two quenching mechanisms for light-emission are discussed, first, luminescence quenching at high PL recording temperatures, associated with the thermal escape of holes to the surrounding wetting layer, and second, annealing-induced PL-quenching at annealing temperatures &gt, 650 &deg, C, which is associated with a migration of the defect complex out of the QD. We show that low-energy ex-situ proton irradiation into the Si matrix further improves the light emission properties of the DEQDs, whereas proton irradiation-related optically active G-centers do not affect the room temperature luminescence properties of DEQDs.

Si growth on pit-patterned Si (001) substrates fabricated by electron beam lithography and reactive ion etching was studied by in situ scanning tunnelling microscopy. After reactive ion etching, the pits have cylindrical shape with vertical side walls, which begins to change from the bottom already after annealing at 740 °C. During Si overgrowth, the pit shape evolves into inverted pyramid-like or into multi-facetted inverted dome-like geometries depending on the growth conditions as well as thickness of the deposited Si layer. High-resolution STM images clearly show that the walls of the pits after overgrowth are composed predominantly of DB-type of steps.

The electron-hole liquid has been found in strained SiGe thin films of Si/Si1−x Gex/Si heterostructures. The density and binding energy of the electron-hole liquid have been determined. Owing to the presence of internal strains in the SiGe layer, the density and binding energy are significantly smaller than the respective quantities for the electron-hole liquid in a bulk single crystal of the solid solution of the same composition. The critical temperature of the transition from the exciton gas to the electron-hole liquid is estimated using the experimental data. The Mott transition (from the exciton gas to electron-hole plasma) occurs above the critical temperatures for high excitation intensities.

Self-organization during strained-layer heteroepitaxy has been a field of intense research in the last few years. Especially the Si/SiGe heterosystem is considered a model system for strain-driven Stranski–Krastanow growth. In this review article, we concentrate on ordered Stranski–Krastanow growth of Ge and SiGe islands on prepatterned Si(001) substrates, which provide preferential nucleation sites. Perfectly ordered SiGe dot arrays in two and three dimensions are demonstrated, and the basic mechanisms for preferential nucleation in geometrically well-defined etch pits are discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A split gate technique is used to form a lateral quantum dot in a two-dimensional electron gas of a modulation-doped silicon/silicon-germanium heterostructure. e-beam lithography was employed to produce split gates. By applying negative voltages to these gates the underlying electron gas is depleted and a lateral quantum dot is formed, the size of which can be adjusted by the gate voltage. We observe single-electron operation with Coulomb blockade behavior below 1K. Gate leakage currents are well controlled, indicating that the recently encountered problems with Schottky gates for this type of application are not an inherent limitation of modulation-doped Si/SiGe heterostructures, as had been speculated., 3 pages

SiGe islands grown on pit-patterned Si(001) substrates show significant dependence on the surface geometry of the substrate, especially when the period of the patterning is reduced to below 300 nm. With different geometry of pit-patterned substrates, SiGe islands are observed to preferentially nucleate at the bottom of shallow pits after a ripple formation of the SiGe wetting layer, or at the top terrace when the pits are deep and steep.

Two dimensionally (2D) periodic pit structures are obtained by a lithograph technique on Si (001) substrate. Under proper growth conditions, the pit-pattern can be reserved after a thick Si buffer growth although the depth and the geometrical profiles of the pits are changed. During Ge deposition, GeSi islands preferentially grow at the pit bottoms, resulting in 2D ordered islands. In addition, the size homogeneity of these ordered islands is significantly improved. A kinetic model is proposed to qualitatively explain these results. By growing multilayer GeSi islands separated by thin Si spacers, three-dimensionally ordered islands or island crystal can be realized on pre-patterned substrates. These ordered islands helps to analyze the properties of a single island and island ensemble.

We report on the fabrication and characterization of silicon-on-insulator (SOI) photonic crystal slabs (PCS) with commensurately embedded germanium quantum dot (QD) emitters for near-infrared light emission. Substrate pre-patterning defines preferential nucleation sites for the self-assembly of Ge QDs during epitaxial growth. Aligned two-dimensional photonic crystal slabs are then etched into the SOI layer. QD ordering enhances the photoluminescence output as compared to PCSs with randomly embedded QDs. Rigorously coupled wave analysis shows that coupling of the QD emitters to leaky modes of the PCS can be tuned via their location within the unit cell of the PCS.

Using ESR, we investigate g-factor and spin coherence time τ of electrons confined in 2D Si1−xGex {channels} (x 0.2 and in SiGe quantum dots grown on prepatterned Si substrates. The quantum wells exhibit 2D-anisotropy of both g and τ which can be explained in terms of the Bychkov–Rashba field. The latter increases with increasing Ge content in the well indicating that the increasing spin-orbit coupling is more important than interface properties. The narrow ESR permits selective spin manipulation already for x > 0.02. Large, regular arrays of Ge quantum dots (about 109) were grown on prepatterned substrates. Strain in the Si capping layer lowers the conduction band relative to that of Ge causing confinement. The g-shift observed implies the possibility of g-tuning by confinement. The line width shows substantial inhomogeneous broadening whereas the longitudinal spin lifetime is hardly changed with respect to 2D structures.

The morphology of annealed patterned Si(0 0 1) wire templates was studied by several techniques. We found an enormous Si-mass transport on the Si surface at usual oxide desorption temperatures around 900°C under UHV conditions. Heat treatment of 5 min transforms the initially rectangular wire profiles with a height of 300 nm to flat ( nm ) and faceted triangular ridges exhibiting thermodynamically preferred {1 1 1} - and {3 1 1} -facets. It was found that the natural SiO2 on the predefined wire pattern must be responsible for the degradation of the wire structure. Removing the SiO2 layer from the Si wires ex situ with an HF dip preserves the rectangular structures during high-temperature annealing. The Si–SiO2 interface was investigated with high-resolution transmission electron microscopy to image the Si wire surface and the natural oxide layer in detail.

The vicinal Si(0 0 1) surfaces exhibit a bunching instability of the atomic height steps. Here we present a study to investigate the effect of germanium on the kinetic instability which occurs in silicon homoepitaxy. For the growth parameters employed we find no evidence for strain-induced step-bunching neither in single Si1−xGex layers nor in Si/Si1−xGex superlattices. The effect of germanium is to kinetically suppress the formation of step-bunches. No significant influence of the Ge-related strain on the surface morphology can be found. The effects of growth temperature and the influence of the amount of deposited pure Si are very pronounced.

The structural characteristics of silicon samples containing one and two Si1 − xGex/Si quantum wells 1.8 to 15 nm thick were determined by high-resolution X-ray diffraction. A detailed analysis of X-ray rocking curves made it possible to reproduce the Ge-concentration profiles in the quantum wells. The diffusion of germanium (up to 20%) into interface layers was observed, with a consequent broadening of the quantum well interfaces.

A comparative study is reported of kinetically and thermodynamically driven growth instabilities in the Si/SiGe heterosystem on vicinal Si (0 0 1) . Investigating single Si 1− x Ge x -layers and Si 1− x Ge x /Si superlattices, a wide range of the relevant growth parameter space is mapped out. In contrast to earlier reports no evidence for strain-induced step-bunching is found. Single Si 1− x Ge x layers replicate the morphology of the underlying buffer layers and tend to form hut clusters near thermal equilibrium. In Si 1− x Ge x /Si superlattices, no significant influence of the Ge-related strain on the step-bunching morphology can be seen. On the other hand, the effect of growth temperature is very pronounced, again indicating the dominant role of kinetics in the formation of step-bunches.

We report on the realization of high-Q/V photonic crystal cavities in thin silicon membranes, with resonances around 1.55 μm wavelength. The cavity designs are based on a recently proposed photonic crystal implementation of the Aubry-André-Harper bichromatic potential, defined from the superposition of two one-dimensional lattices with a non-integer ratio between their periodicity constants. In photonic crystal nanocavities, this confinement mechanism is such that optimized figures of merit can be straightforwardly achieved, in particular an ultra-high-Q factor and diffraction-limited mode volume. Several silicon membrane photonic crystal nanocavities have been realized with measured Q-factors in the 1 × 106 range, as evidenced by resonant scattering. The generality of the proposed designs and their easy implementation and scalability make these results particularly interesting for realizing highly performing photonic nanocavities on different material platforms and operational wavelengths.

Dislocated Si, SiGe, SiC, and SiGeC n-type heterostructures, grown by molecular beam epitaxy, were characterized by capacitance–voltage profiling and deep-level transient spectroscopy. Exclusively dislocation-related defects were found in the different layers, which correspond to the well-known defects found in plastically deformed silicon.

The vicinal Si(001) surfaces are known to exhibit both kinetically and thermodynamically driven instabilities during overgrowth with Si or SiGe. Here, we present a comparative study that allows the discrimination of kinetic effects and strain-induced equilibrium effects. Under kinetic growth conditions, layers with low Ge content become smoother than their homoepitaxial Si counterparts. In contrast, for layers with high Ge content, hut cluster formation is identified as the dominant mechanism of the epilayer to relax strain. We find no evidence for one-dimensional strain-induced step bunching on small angle miscut substrates. Our results strongly suggest that kinetic or kinetically limited processes rather than thermodynamic effects determine the morphology of Si/SiGe interfaces at 550°C.

We have conducted an atomic force microscopy (AFM) study of homoepitaxial silicon layers deposited on vicinal Si(001) substrates under frequently employed molecular beam epitaxy (MBE) growth conditions. Samples with different miscut angles and miscut directions were overgrown with epilayers of thicknesses up to 7500 A and subsequently examined. A previously unknown kinetic growth instability results in distinct, highly regular surface morphologies with a considerable height of up to 40 A and periods of up to 0.45 μm. The surface morphological features can be associated with the intrinsic properties of the Si surface and show a strong sensitivity to the growth temperature. With increasing layer thickness the morphology undergoes a simultaneous steepening and coarsening process. A qualitative model is discussed to explain our experimental results.

In this work we present some recent results concerning the α-particles irradiation of Si1−yCy alloy epitaxially grown on silicon. The study of the damage process is interesting because of the extensive use of backscattering technique as a tool of characterisation of this kind of materials and because of the possibility of adding information about the transformations that this metastable material undergoes. We point out that the irradiation damage process causes a change in the material structure different from that due to the thermal treatments. The irradiation damage occurs at a rate much higher than in Si, however it involves only a silicon atom fraction that appears to be proportional to the substitutional carbon content.

We report new types of kinetic growth instabilities during silicon homoepitaxy on vicinal Si(001) surfaces with miscut angles $\ensuremath{\le}{2}^{\ensuremath{\circ}}$. They occur under frequently employed growth conditions in an extensively studied material system. The characteristic features are (i) kinetic step bunching at low growth temperatures, (ii) a macroscopic zigzag morphology of $〈110〉$ segments at intermediate temperatures, and (iii) a smooth, noncorrelated morphology at high growth temperatures and after annealing. A qualitative model is discussed that relates the morphological features with the kinetics of step incorporation and diffusion in connection with a symmetry-breaking miscut.

Elastic relaxation in dry-etched periodic wires fabricated from molecular beam epitaxy grown Si/SiGe multilayers was studied by coplanar and grazing incidence (GID) high-resolution x-ray diffraction. The inhomogeneous strain distribution in the wires was calculated by the finite element method, which provided the input data for simulations of the scattered intensities using kinematical diffraction theory used for comparison with measured reciprocal space maps. A fabrication-induced layer covering the wire surfaces, modifies the strain distribution. Using GID, the geometrical shape of the wires and their in-plane strain can be determined independently of each other.

The epitaxial growth conditions for silicon on germanium substrates were investigated as a function of growth temperature and epilayer thickness. 3D Stranski–Krastanow growth was observed in a temperature range from 400 to 750 °C, although strong alloying with the substrate occurred for the highest temperatures. At a lower growth temperature alloying is reduced. For the growth at 550 °C island formation takes place only after a wetting layer thickness of about 14 monolayers. With decreasing temperature the wetting layer thickness shifts to lower Si coverages. Nevertheless, the equilibrium critical thickness for dislocation formation in the wetting layer is reached for temperatures down to 450 °C. Partial ordering of the islands along oriented lines could be seen at higher Si coverages, which is most likely caused by ordering along misfit dislocation segments.

Silicon-based heterostructures have come a long way from the discovery of strain as a new and essential parameter for band structure engineering to the present state of electron and hole mobilities, which surpass those achieved in the traditional Si/SiO 2 material combination by more than an order of magnitude and rapidly close up to the best III–V heteromaterials. Besides the inherent performance enhancement, undoubtedly the main attraction of high-mobility Si/SiGe and Si/SiC heterostructures is their basic compatibility with standard Si technologies. This unique combination of properties makes these materials the natural choice for future high performance VLSI circuits based on the contemporary MOSFET. It is the purpose of this contribution to critically review the most recent developments and the performance levels achieved to date in these material systems, to address problems related with device fabrication and process integration, and to discuss alternative routes that could circumvent the use of strain-adjusting epilayers, which are presently the bottleneck for an introduction of these promising materials into a production environment.

Our experiments demonstrate that Schottky-barrier reducing mechanisms can be overcome by adequately designed Si/SiGe heterostructures and that single electron transistor (SET) functionality can be achieved in modulation-doped Si/SiGe heterostructures with a standard split-gate approach that can easily be integrated into an array of coupled SETs

Silicon-based heterostructures have come a long way from the discovery of strain as a new and essential parameter for band structure engineering to the present state of electron and hole mobilities, which surpass those achieved in the traditional material combination by more than an order of magnitude and are rapidly approaching the best III - V heteromaterials. It is the purpose of this article to report on the most recent developments, and the performance level achieved to date in this material system, in a concise and critical manner. The first part of this review is concerned with the structural and electronic properties of the lattice-mismatched Si/SiGe heterostructure. Emphases are put on the effects of strain both on the band structure and on the band offsets, as well as on means to actually control the strain in a stack of heteroepitaxial layers. The second part is dedicated to the transport properties of low-dimensional carrier systems in Si/SiGe and Ge/SiGe heterostructures. The prospects and limitations of the different layer concepts are discussed in terms of scattering mechanisms and experimental results. This part also reviews the most recent magneto-transport experiments on quantum wires and quantum point contacts, which became possible by the enhanced mean free paths in these materials. The third part covers the device aspects of these high-mobility materials, which is of special interest, because silicon-based heterostructures can significantly enhance the performance level of contemporary Si devices without sacrificing the essential compatibility with standard Si technologies. The recent achievements in this application-driven research field, but also the foreseeable problems and limitations, are discussed, and an assessment of the possible role of such heterodevices in future microelectronic circuits is given.

We have studied the strain relaxation in Si1−xGex/Si (001) structures with high electron mobility grown by molecular beam epitaxy. The structures contain a Si1−xGex layer with linearly graded composition, followed subsequently by a uniform composition buffer Si1−yGey, a thin Si layer serving as two-dimensional electron gas channel, and a modulation n-doped Si1−xGex layer. We found that a major part of the graded layer is basically completely strain relaxed, whereas a very thin layer close to the graded-uniform layer interface, as well as the uniform alloy buffer, are just partly relaxed. We performed also model calculations of the strain status of a graded-uniform two-layer system using an equilibrium approach. It is found that for our Si0.7Ge0.3 systems, the residual strains of the samples with different composition, grading rate, and a uniform buffer thickness of 0.6 μm is almost the same at equilibrium. However, experiments show a clear dependence of the residual strain on the grading rate of the grade...

We report on the MBE growth and X-ray characterization of Si 1-y C y /Si 1-x Ge x superlattices (SLs). The concentrations and thicknesses of the layers were chosen such as to get strain-symmetrized superlattices, lattice-matched to the Si (0 0 1) substrates. In-situ RHEED investigations showed increasing roughness during the growth of the Si 1-y C y layers and a smoothing effect of the subsequent Si 1-x Ge x layers. Further characterizations comprised double and triple axis X-ray diffraction, X-ray refraction (XRR), and atomic force microscopy (AFM). Dynamical simulations of the various X-ray configurations yielded the structural parameters of the SLs as well as information on the morphological and replication properties of the interfaces. With increasing carbon content we found an overall interface roughening concomitant with a significant decrease of the replication length. Lateral correlation length fits were compared with AFM-measurements, which give a more detailed picture of the interface morphology as long as the replication lengths are large.

Electroreflectance spectroscopy was used to measure the direct transitions in strained Si 1− x Ge x layers in the energy range from 3–6 eV. We were able to detect the transitions E 1 , E 1 + Δ 1 , E ′ 0 , E 0 , E 0 + Δ 0 , E 2 ( X ), E 2 ( Σ ) and E ′ 1 for multiple samples with germanium concentrations from 12.5–28.1% for temperatures from 10–300 K. The transitions E 1 + Δ 1 , E 0 , E 0 + Δ 0 , E 2 ( Σ ) and E ′ 1 were detected for the first time in strained Si 1− x Ge x layers and it was also the first investigation of the temperature dependence of direct transitions in strained Si 1− x Ge x . Good agreement with theoretical calculations of strain shifts was found for the E 0 transition, while deviations occur for the E 1 transition.

Self-assembled Ge islands were grown by solid-source molecular-beam epitaxy on the submicron stripe-patterned Si(001) substrates at 650 °C. Atomic-force microscopy shows that the Ge islands grow preferentially at the sidewall of the Si stripes, oriented along the [−110] direction. The migration of the Ge adatoms from the top terrace down to the sidewall accounts for the island formation at the sidewall of the stripes. However, most of the Ge islands are formed on the top terraces when the patterned stripes are covered by a strained GeSi multilayer buffer prior to Ge island growth. Apparently, the strained buffer layer acts as a stressor and contributes to the preferential growth of islands on the top terrace.

High-mobility two-dimensional electron gases in Si/Si 1- x Ge x single heterostructures are studied by Shubnikov-De Haas (SdH), Hall, and cyclotron resonance (CR) measurements. These samples were grown by MBE on relaxed SiGe buffer layers with linearly increasing Ge contents and therefore only the Si layers experience tensile strain. This modern buffer concept leads to an enormous increase of the electron mobilities compared to samples grown on conventional single step buffers. By evaluating the SdH data at low magnetic fields it is shown that long-range scattering is the dominant scattering mechanism in these samples. CR spectra yield narrow line widths and strong absorption. The mobility derived from the CR scattering rate is somewhat lower than the magnetotransport result.

The electroluminescence (EL) and photoluminescence (PL) of ultrametastable Si 1− x Ge x layers grown by molecular beam epitaxy (MBE) on Si(100) substrates has been studied. Two layers 150 and 175 nm thick (samples B2005 and C0997 respectively) were pseudomorphically grown on silicon in two different MBE chambers at 450 °C with x ≈ 0.25 and x = ≈ 0.23 respectively. At liquid-helium temperature both samples show luminescence spectra from the Si 1− x Ge x layers consisting of resolved band edge recombination and a broad low energy band peaking at about 810 meV. In one sample (B2005) the broad band is dominant; in the other sample (C0997) the band edge PL and EL signals are dominant. Variation of the exciting current density or laser power over a wide range shows a different power dependence for the low energy band, the Si 1− x Ge x band edge luminescence and the silicon substrate luminescence. In addition, we have studied the EL of fully strained Si 1− x Ge x single quantum wells. At constant excitation current density the EL signal increases with decreasing layer thickness, while the peak energy shifts to higher energy owing to the quantum size effect. For thin quantum wells, two lines at 969 and 897 meV are observed, known as the G line and the E line respectively, which are associated with C s SiC s defect.

Monolithic integrated IMPATT diodes are manufactured as millimeter wave power generators in slot line resonator-antennas. Silicon molecular beam epitaxy (Si-MBE) is used for the growth of the active layers. A fabrication process with a self stopping etchant, self aligned contacts, silicon nitride passivation and air-bridge technology is used. The emitted radiation of the millimeter wave transmitter chip is 2.6 mW at 90.6 GHz in cw-operation and 50 mW at 92 GHz in pulsed operation.

Negative differential resistance, caused by hole resonant tunneling (RT) in Si 1− x Ge x /Si triple-barrier/double-quantum-well structures (TBSs), are studied in an attempt to explore new possibilities for device design optimization and new means to study RT mechanism. The current-voltage characteristics show mainly two resonances, and some small features are observed in the first order differential curve. The current-voltage curves also show strong asymmetry between two opposite bias polarities. The temperature dependences of the RT characteristics are investigated. Calculations on the tunneling transmission probability T p of the TBSs are carried out to assist the analyses. The experimental results are compared with the measurements on the double barrier structures with similar device parameters.

The optical and structural properties of sub-micrometer scale Si/SiGe wires grown by solid source molecular beam epitaxy on Si (001) substrates are presented. The structures were grown selectively using SiO 2 and Si 3 N 4 /SiO 2 masks, and have been studied by photoluminescence and high-resolution X-ray diffraction. All samples exhibited a photoluminescence signal from SiGe, but the most intense and narrowest photoluminescence peak (comparable to the Si substrate) was observed from Si/SiGe wires grown in deep-etched Si trenches.

On high resistive silicon substrates Schottky barrier diodes have been monolithically integrated with planar antenna structures. The Schottky barrier diodes were fabricated on n+ diffusion regions with thin epitaxial layers grown by molecular beam epitaxy. Low series resistance (≪ 6 ?), ideality factor of less than 1.1 and cut-off frequencies up to 1 THz have been achieved. The maximum sensitivity of the detector-was 90 mV/(mW cm?2) at 94 GHz.

Individually addressed Si-bipolar diodes stacked monolithically in two planes were fabricated, applying a variety of novel technologies. A two-step differential Si-MBE process allows us to grow monocrystalline devices embedded in a poly-Si surrounding which provides lateral or vertical isolation and buried interconnections. The doping levels of the active devices ( n , p ≲ 10 17 cm −3 ) are concomitant with a high resistivity poly surrounding (10 4 -10 5 Ω cm), while highly doped contact layers (> 10 19 cm −3 ), grown between the two device levels and above the second level are of low resistivity in the monocrystalline (10 −3 -10 −1 Ω cm) and in the poly area (10 −2 −5 × 10 −1 Ω cm). MBE-growth was interrupted after deposition of the intermediate contact layer to define interconnections by photo-lithography, which were subsequently overgrown by a second MBE process. Patterning of the highly conductive cap layer and etching of contact holes to the by now buried leads permits electrical access to both device levels. The measured device characteristics demonstrate the feasibility of the concept, which also appears very attractive for three-terminal devices. Routes towards optimized growth and doping sequences are discussed.

The differential MBE technique is suggested as a tool for separation or interconnection of advanced devices on a common chip. Bipolar diodes with various diameters of 23 to 800 mu m were fabricated and their electrical behaviour measured. Implantation of etching methods were used to eliminate the influence of a highly doped contact layer on top of the epitaxial layer sequence. Leakage currents from 10-7 to 10-9 A are slightly higher than those of reference mesa diodes. It is estimated that the differential technique is applicable to many advanced devices for planar integration.

Patterned multilayers are deposited by means of differential silicon MBE. Simultaneously monocrystalline islands are formed in a polycrystalline embedment. The structure is investigated with respect to the poly/mono-boundary and the polysilicon. The poly-embedment can work as an isolation or as an interconnection for a mono-island. Several devices have been realized, namely differential diodes, a differential three-dimensional IMPATT, and a differential resonant tunnel diode and these demonstrate the feasibility of the differential multilayer silicon MBE.

n-type modulation-doped Si/Si 0.5 Ge 0.5 strained layer multiple quantum well structures have been grown on (100)Si substrates by molecular beam epitaxy. The concept of strain symmetrization has been applied by inserting a strain-relaxed SiGe buffer layer. The samples have been investigated by Hall measurements supplemented by Shubnikov-de Haas and Rutherfored backscattering analysis. We present mobility results on the two-dimensional electron gas in the silicon wells as a function of electron concentration, quantum well width, and spacer thickness. The influence of the sample parameters on the electron mobility is discussed. With an electron sheet concentration of 2.4×10 12 cm −2 per well, a well width of 10 nm, and a spacer thickness of 10 nm a Hall mobility at room temperature of 1280 cm 2 (V s) -1 has been achieved.

Gallium doping of silicon molecular beam epitaxial layers is investigated in the temperature range between 550 and 300°C. Down to about 450°C, the incorporation behaviour can be described by a simple kinetic model. Deviations from an activated behaviour are indicative of kinetic limitations. At lower temperatures a complicated incorporation dynamics with two distinct regimes is observed. Doping by secondary implantation is investigated as a function of ion energy. A significant implantation yield is observed at low energies, while at higher energies sputter desorption of the gallium adlayer becomes dominant. Implications of the results for more efficient gallium doping techniques are discussed.

We investigate the initial stage of the two‐dimensional to three‐dimensional transition of strained Ge layers deposited on pit‐patterned Si(001) templates. Within the pits, which assume the shape of inverted, truncated pyramids after optimized growth of a Si buffer layer, the Ge wetting layer develops a complex morphology consisting exclusively of {105} and (001) facets. These results are attributed to a strain‐driven step‐meandering instability on the faceted sidewalls of the pits, and a step‐bunching instability at the sharp concave intersections of these facets. Although both instabilities are strain driven, their coexistence becomes mainly possible by the geometrical restrictions in the pits. It is shown that the morphological transformation of the pit surface into low‐energy facets has strong influence on the preferential nucleation of Ge islands at the flat bottom of the pits.

The epitaxial growth conditions for silicon on germanium substrates were investigated as a function of growth temperature and monolayer coverage. Island formation was observed for the hole studied temperature range, although strong alloying with the substrate occurred for the highest temperatures. Carbon pre-deposition offers suitable nucleation centers for the Si island and reduction of alloying. pre-structured Ge substrates were prepared to enhance islanding and to achieve ordering., Comment: presented 26.July 2006 at ICPS 2006 Vienna

We report the observation of the apparent metal‐insulator transition (MIT) of two‐dimensional electrons in a high quality modulation doped Si/Si1−xGex heterostructure. The critical density nc, at which the thermal coefficient of its low temperature resistivity changes sign, is 0.32×1011 cm−2, much smaller than the nc of ∼ 0.8×1011cm−2 seen in clean Si‐MOSFET’s. This is consistent with previous observations in 2D holes in GaAs that nc decreases with decreasing disorder.

To study the conditions for substitutional incorporation of carbon into Si layers, we grew pseudomorphic Si1−yCy/Si superlattices with absolute carbon concentrations between 0.3% and 2% and growth temperatures varying between 400 and 650 °C. We employed a novel technique to derive the amount of substitutional carbon, which is based on comparative x-ray rocking analyses of Si1−yCy/Si superlattices grown with constant and step-graded temperature profiles. At growth rates around 1 A/s, we find complete substitutional incorporation of carbon up to growth temperatures of 550 °C and carbon concentrations of about 1%. At higher growth temperatures and/or higher carbon concentrations, the percentage of substitutional carbon drops significantly, concomitant with a thus far unnoticed morphological transition to island growth.

We report here a systematic study of the energy gaps at the odd-integer quantum Hall states nu = 3 and 5 under tilted magnetic (B) fields in a high quality Si two-dimensional electron system. Out of the coincidence region, the valley splitting is independent of the in-plane fields. However, the nu = 3 valley gap differs by about a factor of 3 (Deltav approximately 0.4 vs 1.2 K) on different sides of the coincidence. More surprisingly, instead of reducing to zero, the energy gaps at nu = 3 and 5 rise rapidly when approaching the coincidence angles. We believe that such an anomaly is related to strong couplings of the nearly degenerate Landau levels.

The apparent metal-insulator transition is observed in a high-quality two-dimensional electron system (2DES) in the strained Si quantum well of a Si/Si{sub 1-x}Ge{sub x} heterostructure with mobility {mu} = 1.9 x 10{sup 5} cm{sup 2}/V s at density n = 1.45 x 10{sup 11} cm{sup -2}. The critical density, at which the thermal coefficient of low T resistivity changes sign, is -0.32 x 10{sup 11} cm{sup -2}, a very low value obtained in Si-based 2D systems. The in-plane magnetoresistivity {rho}(B{sub ip}) was measured in the density range, 0.35 x 10{sup 11} B{sub c} , with B{sub c} the full spin-polarization field. Surprisingly, {rho}(B{sub c})/{rho}(0)-1.8 for all the densities, even down to n = 0.35 x 10{sup 11} cm{sup -2}, only 10% higher than n{sub c}. This is different from that in clean Si metal-oxide-semiconductor field-effect transistors, where the enhancement is strongly density dependent and {rho}(B{sub c})/{rho}(0) appears to diverge as n {yields} n{sub c}. Finally, we show that in the fully spin-polarized regime, dependent on the 2DES density, the temperaturemore » dependence of {rho}(B{sub ip}) can be either metallic-like or insulating.« less

Silicon K a band BARITT ( Bar rier I njection T ransit T ime) diodes in the frequency regime from 26 to 31 GHz have been grown by MBE with a p - n - p doping sequence. CW powers of 4 mW at 29 GHz have been realized from fabricated mesa diodes with diameters of ⊘ = 55 to 65 μm , the diodes exhibited a signal to noise ratio of S /( NΔf ) = 93 dBc/Hz ( Δf = 100 kHz).