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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.

The strong atomistic spin-orbit coupling of holes makes single-shot spin readout measurements difficult because it reduces the spin lifetimes. By integrating the charge sensor into a high bandwidth radio frequency reflectometry setup, we were able to demonstrate single-shot readout of a germanium quantum dot hole spin and measure the spin lifetime. Hole spin relaxation times of about 90 μs at 500 mT are reported, with a total readout visibility of about 70%. By analyzing separately the spin-to-charge conversion and charge readout fidelities, we have obtained insight into the processes limiting the visibilities of hole spins. The analyses suggest that high hole visibilities are feasible at realistic experimental conditions, underlying the potential of hole spins for the realization of viable qubit devices.

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.

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 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.

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.

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.

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.

MBE grown pseudomorphic Si 1−x Ge x Si (001) quantum wells (QW's) with germanium contents from 10% to 36% and various thicknesses were studied by photoluminescence (PL) under external stress applied parallel or perpendicular to the growth direction. Stress perpendicular to the growth direction reduces the symmetry and leads to additional splittings in both barrier and layer material. Bending the whole wafer, we create compressive or tensile in-plane strains in the sample. In both cases we could not observe the splittings in the SiGe layers, but the SiGe PL shifts like the SiGe band edge, as expected from deformation potential theory. Parallel stress conserves the tetragonal symmetry and reduces the strain in the SiGe-layer, and at the same time leads to a splitting of valence and conduction band states in the Si barrier. For stress values up to 600 MPa we find an upshift of the SiGe PL, whereas the lowest branch of the bound exciton emission of the Si barriers shifts down. The observed shift rates depend on both Ge concentration and layer thickness. For thick layers, the shift rates are as expected from deformation potential theory for the SiGe band edge. For thin layers significant deviations occur. To interpret these deviations, calculations have been carried out which take into account quantization energies in the QW's, stress dependent exciton binding energies, band bending effects, and stress dependence of band offsets.

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.

Antidot superlattices with a period of a=500 nm and different etch depths were fabricated in two-dimensional hole gases confined in surface-side as well as in substrate-side modulation-doped Ge/SiGe heterostructures by holographic lithography and reactive ion etching and were then studied by magnetotransport at T=4.2 K. Even though the elastic mean free paths are smaller than or comparable to the superlattice period, typical features of locally ballistic transport around groups of antidots, such as commensurability oscillations and additional, non-quantized Hall plateaus, are observed in both types of samples at sufficient etch depth. These geometry-related effects are found to be rather robust against inhomogeneities and defects brought about by imperfect fabrication.

Two-dimensional (2D) periodic arrays of Ge islands were realized on prepatterned Si (001) substrates by solid-source molecular-beam epitaxy. Atomic-force microscopy images demonstrate that the Ge islands are formed in the 2D laterally ordered pits of patterned substrates. The 2D periodicity of the substrate pattern is replicated throughout a stack of Ge island layers by strain-driven vertical ordering. Photoluminescence spectra of the ordered Ge islands show well-resolved peaks of the no-phonon signal and the transverse-optical phonon replica. These peaks are observed at nearly the same energy as those of random Ge islands deposited under the same conditions on unpatterned Si substrates.

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.

In this work we observed strain relaxation at 550 °C following the interaction of palladium with strained Si 1− x Ge x epilayers. The germanium concentrations x in the Si 1− x Ge x films were 0.09 and 0.21 with thicknesses of 3500 A and 2300 A respectively, which are below the critical thickness h c . An overlayer of palladium, 1300 A thick, was deposited onto the Si 1− x Ge x epilayers, and the samples were annealed at 250 and 550 °C in a vacuum furnace for compound formation. Low temperature anneals (250 °C) were characterized by the formation of a uniform, highly textured ternary compound (Pd 2 Si 1− y Ge y with y x ) on top of the unreacted Si 1− x Ge x layer, and no substantial change in the state of strain was observed in the underlying unreacted Si 1− x Ge x layer. On the contrary, a higher temperature anneal (550 °C) of metallized samples was characterized by the formation of a double-layer structure, with a highly defected germanium-rich Si 1− z Ge z layer (0.37 z 1− x Ge x . In these samples a considerable decrease in the vertical lattice parameter and correspondingly strain relaxation in the unreacted Si 1− x Ge x layer was observed.

We studied the nucleation and ordering mechanism of Ge islands on hill-patterned Si (001) templates. Like in the case of pit-patterned substrates, the initial Ge wetting layer decorates the inclined surfaces of the hill pattern with corrugations consisting entirely of {105} faceted prisms and {001} terraces. Upon further Ge deposition, islands nucleate as pairs in the V-shaped troughs between neighboring hills, and subsequently merge into a single island near the center of each trough. Finite element calculations show that island nucleation and the subsequent movement toward the center of the trough are governed by elastic energy minimization. © 2008 American Institute of Physics.

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

A variety of SiGe quantum well (QW) samples were grown by solid‐source molecular beam epitaxy (MBE) to study the influence of growth temperature TG and QW width Lz on the photoluminescence (PL) properties. For all growth temperatures investigated (350 °C≤TG≤750 °C) we found intense, well‐resolved PL signals from the SiGe QWs. The PL intensity increases with TG, and the stability against measurement temperature becomes better. A formerly reported PL band below the SiGe band edge is either completely absent, or very weak in the 4.2 K spectra of our samples. Thus, the defects or complexes responsible for this signal are obviously not inherent properties of MBE growth.

Modulation-doped FET (MODFET) structures with hole channels consisting of pure Ge were grown by molecular beam epitaxy (MBE) on Si substrates. To overcome the relatively large lattice mismatch, between Si and Ge, a relaxed Si/sub 1-x/Ge/sub x /buffer layer with linearly graded Ge concentration and a final x value of around 70% was grown first. Hall mobilities of up to 1300 cm/sup 2//V-s at room temperature and 14000 cm/sup 2//V-s at 77 K were measured. Devices with and without gate recess were fabricated, which result in enhancement- and depletion-type FETs. Maximum extrinsic transconductances of 125 and 290 mS/mm at room temperature and 77 K, respectively, were found for gate lengths L/sub G/ around 1.2 mu m. >

Electrical properties of Sb implanted strained Si1−xGex alloy layers are reported. Two sets of Si1−xGex epilayers with compositions of x=0.08 and 0.18, MBE grown on (100)Si substrates, were implanted at room temperature with Sb+ ions at energies of 200 and 100 keV, respectively, and doses of 1013 and 1015 cm−2. Secondary‐ion mass spectrometry and spreading resistance profiling measurements show that Sb implantation, with a dose below the critical value for amorphization (1013 cm−2), formed a p‐type region upon annealing at 500 °C. Only higher temperature anneals transformed the implanted layer into the expected n‐type doping. Maximal values of electrical activity (45±10%) and mobility were obtained in this case only at temperatures around 800–900 °C. For the high dose implantation (1015 cm−2), it was found that the highest activation efficiency at the implantation profile peak was obtained at 500–600 °C, while at the end‐of‐range region the activation efficiency was very low.

We have investigated the resonant tunneling feature variations with temperature in molecular beam epitaxy grown strained Si0.78Ge0.22/Si double‐barrier structures with different well width w and spacer thickness LS. Both w and LS have strong effects on the temperature characteristics of the peak current JP, the valley current JV, and the current peak‐to‐valley ratio (PVR). When w is reduced, both JP and JV are greatly increased. The resonant tunneling devices (RTDs) also have better temperature stability and PVR decreases slower. The RTD with larger LS has higher peak current. Beside w and LS, the quasi‐Fermi level position has a very important influence on the resonant tunneling feature variations, which is stressed in the present work.

The photoluminescence spectra of type-I alignment Si/Si 1 − x Ge x /Si heterostructures contained thin Si 1 − x Ge x layers ( d = 25–70 nm) are studied under various temperatures and excitation intensities. It was shown, excitation intensity increase at low temperatures leads to the exciton condensation resulting electron–hole liquid (EHL) formation in Si 1 − x Ge x layer. Electron–hole pair density n 0 and binding energy of the EHL relative to exciton gas φ decrease noticeably while х increases. The decrease in the binding energy and density of the electron–hole liquid is attributed to splitting of conduction and valance bands due to internal strains in the Si 1 − x Ge x layer. The Mott transition (from the exciton gas to electron–hole plasma) occurs above the critical temperatures for high excitation intensities.

We show that both the morphology and the optoelectronic properties of SiGe islands growing in the pits of periodically pre-patterned Si(001) substrates are determined by the amount of Ge deposited per unit cell of the pattern. Pit-periods (p) ranging from 300 to 900 nm were investigated, and Ge growth was performed by molecular beam epitaxy (MBE) at temperatures of 690 and 760 °C. The ordered SiGe islands show photoluminescence (PL) emission, which becomes almost completely quenched, once a critical island volume is exceeded. By atomic force and transmission electron microscope images we identify the transition from pyramid-shaped to dome-shaped islands with increasing p. Eventually, the nucleation of dislocations in the islands leads to PL quenching. Below a critical Ge coverage a narrowing and a blue shift of the PL emission is observed, as compared to islands grown on a planar reference area of the same sample.

This work demonstrates a rib waveguide photodetector based on a vertical Si p-i-n junction with Ge islands operating in the spectral region around lambda=1.55 microm at room temperature. A vertical stack of four layers of Ge islands is grown by molecular beam epitaxy on a silicon-on-insulator. Each layer is organized in a two-dimensional square grid with a period of 460 nm. The spectral response of the detector extends well beyond 1.6 mum at 300 K. The absorption length of approximately 135 microm (at 1/e decrease of intensity) at lambda=1.55 microm along the waveguide allows for relatively small-size devices for all-on-one-platform integration.

The peak positions in photoluminescence spectra of Ge wetting layers (WL) deposited at 700 °C were measured versus the Ge coverage with an extremely high relative resolution of 0.025 monolayers. A nearly linear redshift of the peaks with increasing Ge coverage is observed. We derived quantitative WL composition profiles by fitting this shift, and its dependence on the deposition temperature of the capping layer (Tc), to results of band structure calculations. Despite the high growth temperature, the Ge content in the WL exceeds 80%. It is shown that the composition profile is dominated by surface segregation of Ge on Si.

Stranski-Krastanow island growth is demonstrated for tensile strained silicon epilayers on Ge (001) substrates over a wide range of growth temperatures. Small, Si-rich islands show sidewall faces near {1,1,10}, whereas larger islands are {113}-terminated truncated pyramids with an aspect ratio near 0.1. In contrast to compressively strained Ge on Si, we find for Si on Ge a significantly thicker wetting layer of >8 ML and coexistence of islands and dislocations.

Multilayer Si/Ge nanostructures with germanium layers of different thicknesses are grown by molecular-beam epitaxy at low temperatures (

Si 1− x Ge x layers with overcritical thicknesses grown on (100 Si by molecular beam epitaxy (MBE) were characterized by Rutherford backscattering/channeling (RBS), transmission electron microscopu (TEM) and Nomarski differential interference contrast (NDIC) microscopy. The influence of growth temperature and subsequent furnace annealing or rapid thermal annealing (RTA) on crystal quality and surface morphology was investigated. Microstructural investigations by TEM of plane-view and cross-section specimens show an array of misfit dislocations at the interface with the substrate as well as threading dislocations which extend from the interface to the surface. RTA up to 1150°C led to a significant reduction of the threading dislocation density. The minimum yield ξ min of a sample grown at 450°C decreased from 6.7 to 3.9% after RTA at 1100°C for 30s. NDIC microscopy shows that the surface remains smooth after RTA in contrast to in-situ thermal annealing and ex situ furnace annealing. Similar experiments were carried out with pure germanium layers on (100) Si. RTA up to 600°C leads to an improvement of crystal quality. Higher temperatures result in the formation of square-shaped holes.

We present our work on the spin relaxation of electrons confined in SiGe islands. Ge islands are grown on unstructured and structured Si(100) substrates by MBE, and lead to strain in the Si layer that is deposited on top. These quantum dot structures are investigated by photoluminescence and electron spin resonance (ESR) experiments, the latter both in continuous wave and pulsed mode. We observe a g-factor and an ESR line width that correspond to Si conduction band electrons with additional inhomogeneous broadening.