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Second-generation high-temperature superconductor (2G HTS) tapes have demonstrated the ability to generate high magnetic fields and critical currents at a wide operating temperature range. In this paper, we study the mechanical properties of 2G HTS tape measured simultaneously with its critical current. The lattice deformations in the tape's substrate caused by applied mechanical stress were measured by neutron diffraction. In our experiments, the 2G HTS tape was exposed to uniaxial tensile force ranging from 250 to 1100 N at temperature 77 K. The current through the tape was scanned in the range from 22 to 42 A. The experimental results have been obtained in a cryogenic testing chamber for neutron scattering measurements of internal stresses under load with the incorporated HTS current leads. Critical stress is a vital parameter required for the modeling and designing of advanced superconducting magnets and also a variety of different superconducting applications based on 2G HTS tapes. [ABSTRACT FROM AUTHOR]

Compressively strained Ge quantum well devices have a spin-splitting in applied magnetic field that is entirely consistent with a Zeeman effect in the heavy hole valence band. The spin orientation is determined by the biaxial strain in the quantum well with the relaxed SiGe buffer layers and is quantized in the growth direction perpendicular to the conducting channel. The measured spin-splitting in the resistivity ρxx agrees with the predictions of the Zeeman Hamiltonian where the Shubnikov-deHaas effect exhibits a loss of even filling factor minima in the resistivity ρxx with hole depletion from a gate field, increasing disorder or increasing temperature. There is no measurable Rashba spin-orbit coupling irrespective of the structural inversion asymmetry of the confining potential in low p-doped or undoped Ge quantum wells from a density of 6 × 1010 cm-2 in depletion mode to 1.7 × 1011 cm-2 in enhancement. [ABSTRACT FROM AUTHOR]

A thin, flat, and single crystal germanium membrane would be an ideal platform on which to mount sensors or integrate photonic and electronic devices, using standard silicon processing technology. We present a fabrication technique compatible with integrated-circuit wafer scale processing to produce membranes of thickness between 60 nm and 800 nm, with large areas of up to 3.5 mm2. We show how the optical properties change with thickness, including appearance of Fabry-Pérot type interference in thin membranes. The membranes have low Q-factors, which allow the platforms to counteract distortion during agitation and movement. Finally, we report on the physical characteristics showing sub-nm roughness and a homogenous strain profile throughout the freestanding layer, making the single crystal Ge membrane an excellent platform for further epitaxial growth or deposition of materials. [ABSTRACT FROM AUTHOR]

This study investigates the onset of islanding (Stranski-Krastanow transition) in strained pure germanium (Ge) and dilute silicon-germanium (SiGe) alloy layers grown by chemical vapour deposition on Si(001) substrates. Integration of compositional profiles is compared to a novel method for quantification of X-ray maps acquired in cross-sectional scanning transmission electron microscopy, together with simulations of surface segregation of Ge. We show that Si1-xGex alloys for germanium concentrations x≤0.27 grow two-dimensionally and stay flat up to considerable layer thicknesses, while layers with concentrations in the range 0.28

For the first time, the I-V-T dataset of a Schottky diode has been accurately modelled, parameterised, and fully fit, incorporating the effects of interface inhomogeneity, patch pinch-off and resistance, and ideality factors that are both heavily temperature and voltage dependent. A Ni/SiC Schottky diode is characterised at 2 K intervals from 20 to 320 K, which, at room temperature, displays low ideality factors (n < 1.01) that suggest that these diodes may be homogeneous. However, at cryogenic temperatures, excessively high (n > 8), voltage dependent ideality factors and evidence of the so-called 'thermionic field emission effect' within a T0-plot, suggest significant inhomogeneity. Two models are used, each derived from Tung's original interactive parallel conduction treatment of barrier height inhomogeneity that can reproduce these commonly seen effects in single temperature I-V traces. The first model incorporates patch pinch-off effects and produces accurate and reliable fits above around 150 K, and at current densities lower than 10-5 A cm-2. Outside this region, we show that resistive effects within a given patch are responsible for the excessive ideality factors, and a second simplified model incorporating these resistive effects as well as pinch-off accurately reproduces the entire temperature range. Analysis of these fitting parameters reduces confidence in those fits above 230 K, and questions are raised about the physical interpretation of the fitting parameters. Despite this, both methods used are shown to be useful tools for accurately reproducing I-V-T data over a large temperature range. [ABSTRACT FROM AUTHOR]

Epitaxial growth of Ge on Si has been investigated in order to produce high quality Ge layers on (110)- and (111)-orientated Si substrates, which are of considerable interest for their predicted superior electronic properties compared to (100) orientation. Using the low temperature/high temperature growth technique in reduced pressure chemical vapour deposition, high quality (111) Ge layers have been demonstrated almost entirely suppressing the formation of stacking faults (< 107 cm-2) with a very low rms roughness of less than 2 nm and a reduction in threading dislocation density (TDD) (∼ 3 × 108 cm-2). The leading factor in improving the buffer quality was use of a thin, partially relaxed Ge seed layer, where the residual compressive strain promotes an intermediate islanding step between the low temperature and high temperature growth phases. (110)-oriented layers were also examined and found to have similar low rms roughness (1.6 nm) and TDD below 108 cm-2, although use of a thin seed layer did not offer the same relative improvement seen for (111). [ABSTRACT FROM AUTHOR]

We report the observation of the quantum-confined Stark effect in Ge/SiGe multiple quantum well heterostructures grown on Si0.22Ge0.78 virtual substrates. The large compressive strain in the Ge quantum well layers caused by the lattice mismatch with the virtual substrate results in a blue shift of the direct absorption edge, as well as a reduction in the Γ-valley scattering lifetime because of strain-induced splittings of the conduction band valleys. We investigate theoretically the Γ-valley carrier lifetimes by evaluating the Γ→L and Γ→Δ scattering rates in strained Ge/SiGe semiconductor heterostructures. These scattering rates are used to determine the lifetime broadening of excitonic peaks and the indirect absorption in simulated absorption spectra, which are compared with measured absorption spectra for quantum well structures with systematically varied dimensions. We find that Γ→Δ scattering is significant in compressively strained Ge quantum wells and that the Γ-valley electron lifetime is less than 50 fs in the highly strained structures reported here, where Γ→Δ scattering accounted for approximately half of the total scattering rate. [ABSTRACT FROM AUTHOR]

This work presents a detailed characterization, using high-resolution x-ray diffraction, of multilayered Si1-xGex heterostructures grown on (001), (011), and (111) Si substrates by reduced pressure chemical vapor deposition. Reciprocal space mapping has been used to determine both the strain and Ge concentration depth profiles within each layer of the heterostructures after initially determining the crystallographic tilt of all the layers. Both symmetric and asymmetric reciprocal space maps were measured on each sample, and the evaluation was performed simultaneously for the whole data set. The ratio of misfit to threading dislocation densities has been estimated for each individual layer based on an analysis of diffuse x-ray scattering from the defects. [ABSTRACT FROM AUTHOR]

The effect of compositional grading rate on reverse linear graded silicon germanium virtual substrates, grown by reduced pressure chemical vapor deposition, is investigated. For a Si(001)/Ge/RLG/Si0.22Ge0.78 buffer of 2.4 μm total thickness the threading dislocation density (TDD) within the top, fully relaxed, Si0.22Ge0.78 layer is 4×106 cm-2, with a surface roughness of 3 nm. For a thicker buffer, where the grading rate is reduced, a lower TDD of 3×106 cm-2 and a surface roughness of 2 nm can be achieved. The characteristics of reverse graded Si0.22Ge0.78 virtual substrates are shown to be comparable to, or exceed, conventional buffer techniques, leading to thinner high-quality high Ge composition SiGe virtual substrates. [ABSTRACT FROM AUTHOR]

The transport properties of a two-dimensional hole gas (2DHG), with very high room-temperature drift mobilities, formed in 7.5- (2540 cm2 V-1 s-1) and 20-nm (2940 cm2 V-1 s-1) strained Ge quantum wells of multilayered Si/Si0.33Ge0.67/Ge/Si0.33Ge0.67/Si0.73Ge0.27/Si(001) p-type modulation-doped heterostructures were investigated experimentally in the temperature range of 10–295 K. For both samples the drift mobility of the 2DHG behaves in the same way as the Hall mobility, increases as the temperature decreases. The sheet carrier density of the 2DHG increases with decreasing temperature, which is opposite to the behavior of Hall-effect sheet carrier density. We found that the 2DHG formed in the thicker Ge quantum well has a higher drift mobility at 295 K but a lower one at low temperatures. For both samples the Hall factor increases with decreasing temperature, reaching unity at 10 K, when carriers in parallel conducting layers completely freeze-out. The interpretation of these results is carried out and the scattering mechanisms limiting the 2DHG mobility are discussed. [ABSTRACT FROM AUTHOR]

Large scale fabrication using Complementary Metal Oxide Semiconductor compatible technology of semiconductor nanostructures that operate on the principles of quantum transport is an exciting possibility now due to the recent development of ultra-high mobility hole gases in epitaxial germanium grown on standard silicon substrates. We present here a ballistic transport study of patterned surface gates on strained Ge quantum wells with SiGe barriers, which confirms the quantum characteristics of the Ge heavy hole valence band structure in 1-dimension. Quantised conductance at multiples of 2e²/h is a universal feature of hole transport in Ge up to 10 × (2e²/h). The behaviour of ballistic plateaus with finite source-drain bias and applied magnetic field is elucidated. In addition, a reordering of the ground state is observed. [ABSTRACT FROM AUTHOR]

The anisotropic nature of carrier mobility in simple cubic crystalline semiconductors, such as technologically important silicon and germanium, is well understood as a consequence of effective mass anisotropy arising from a change in band structure along non-identical surface crystal directions. In contrast to this, we show experimentally that this type of anisotropy is not the dominant contribution. Recent advances in epitaxial growth of high quality germanium enabled the appearance of high mobility 2D carriers suitable for such an experiment. A strong anisotropy of 2D carrier mobility, effective mass, quantum, and transport lifetime has been observed, through measurements of quantum phenomena at low temperatures, between the <110> and <100> in-plane crystallographic directions. These results have important consequences for electronic devices and sensor designs and suggest similar effects could be observed in technologically relevant and emerging materials such as SiGe, SiC, GeSn, GeSnSi, and C (Diamond). [ABSTRACT FROM AUTHOR]

We report a magneto-transport study of a two-dimensional hole gas confined to a strained Ge quantum well grown on a relaxed Si0.2Ge0.8 virtual substrate. The conductivity of the hole gas measured as a function of a perpendicular magnetic field exhibits a zero-field peak resulting from weak anti-localization. The peak develops and becomes stronger upon increasing the hole density by means of a top gate electrode. This behavior is consistent with a Rashba-type spin-orbit coupling whose strength is proportional to the perpendicular electric field and hence to the carrier density. In the low-density, the single-subband regime, by fitting the weak anti-localization peak to an analytic model, we extract the characteristic transport time scales and a spin splitting energy ΔSO ~ 1 meV. Tight-binding calculations show that ΔSO is dominated by a cubic term in the in-plane wave vector. Finally, we observe a weak anti-localization peak also for magnetic fields parallel to the quantum well and associate this finding to an effect of intersubband scattering induced by interface defects. [ABSTRACT FROM AUTHOR]

Characterisation of membranes produced for use as micro-electro-mechanical systems using vibrational techniques can give a measure of their behaviour and suitability for operation in different environments. Two membranes are studied here: germanium (Ge) and cubic silicon carbide (3C-SiC) on a silicon (Si) substrate. When driven at higher displacements, the membranes exhibit self-protecting behaviour. The resonant vibration amplitude is limited to a maximum value of around 10 nm, through dissipation of energy via higher harmonic vibrations. This is observed for both materials, despite their different Young's moduli and defect densities. [ABSTRACT FROM AUTHOR]

The magnetoresistance of 50 nm thick epilayers of doped germanium is measured at a range of temperatures down to 1.6 K. Both n- and p-type devices show quantum corrections to the conductivity in an applied magnetic field, with n-type devices displaying weak localization and p-type devices showing weak antilocalization. From fits to these data using the Hikami-Larkin-Nagaoka model, the phase coherence length of each device is extracted, as well as the spin diffusion length of the p-type device. We obtain phase coherence lengths as large as 325 nm in the highly doped n-type device, presenting possible applications in quantum technologies. The decay of the phase coherence length with temperature is found to obey the same power law of 1ϕ ∝ Tc, where c = -0.68 ± 0.03, for each device, in spite of the clear differences in the nature of the conduction. In the p-type device, the measured spin diffusion length does not change over the range of temperatures for which weak antilocalization can be observed. The presence of a spin-orbit interaction manifested as weak antilocalization in the p-type epilayer suggests that these structures could be developed for use in spintronic devices such as the spin-FET, where significant spin lifetimes would be important for efficient device operation. [ABSTRACT FROM AUTHOR]

The complex quantum transport of a strained Ge quantum well (QW) modulation doped heterostructure with two types of mobile carriers has been observed. The two dimensional hole gas (2DHG) in the Ge QW exhibits an exceptionally high mobility of 780 000 cm2/Vs at temperatures below 10 K. Through analysis of Shubnikov de-Haas oscillations in the magnetoresistance of this 2DHG below 2K, the hole effective mass is found to be 0.065 m0. Anomalous conductance peaks are observed at higher fields which deviate from standard Shubnikov de-Haas and quantum Hall effect behaviour due to conduction via multiple carrier types. Despite this complex behaviour, analysis using a transport model with two conductive channels explains this behaviour and allows key physical parameters such as the carrier effective mass, transport, and quantum lifetimes and conductivity of the electrically active layers to be extracted. This finding is important for electronic device applications, since inclusion of highly doped interlayers which are electrically active, for enhancement of, for example, room temperature carrier mobility, does not prevent analysis of quantum transport in a QW. [ABSTRACT FROM AUTHOR]

We report magnetotransport measurements of a SiGe heterostructure containing a 20 nm p-Ge quantum well with a mobility of 800 000 cm² V−1 s−1. By dry etching arrays of wires with widths between 1.0 μm and 3.0 μm, we were able to measure the lateral depletion thickness, built-in potential, and the phase coherence length of the quantum well. Fourier analysis does not show any Rashba related spin-splitting despite clearly defined Shubnikov-de Haas oscillations being observed up to a filling factor of ν = 22. Exchange-enhanced spin-splitting is observed for filling factors below ν = 9. An analysis of boundary scattering effects indicates lateral depletion of the hole gas by 0.5 ± 0.1 μm from the etched germanium surface. The built-in potential is found to be 0.25 ± 0.04 V, presenting an energy barrier for lateral transport greater than the hole confinement energy. A large phase coherence length of 3.5 ± 0.5 μm is obtained in these wires at 1.7 K. [ABSTRACT FROM AUTHOR]

We report the observation, through Shubnikov-de Haas oscillations in the magnetoresistance, of spin splitting caused by the Rashba spin-orbit interaction in a strained Ge quantum well epitaxially grown on a standard Si(001) substrate. The Shubnikov-de Haas oscillations display a beating pattern due to the spin split Landau levels. The spin-orbit parameter and Rashba spin-splitting energy are found to be 1.0 x 10-28 eVm³ and 1.4meV, respectively. This energy is comparable to 2D electron gases in III-V semiconductors, but substantially larger than in Si, and illustrates the suitability of Ge for modulated hole spin transport devices. [ABSTRACT FROM AUTHOR]

We describe optical characterisation of a strained silicon cold electron bolometer (CEB), operating on a 350 mK stage, designed for absorption of millimetre-wave radiation. The silicon cold electron bolometer utilises Schottky contacts between a superconductor and an n++ doped silicon island to detect changes in the temperature of the charge carriers in the silicon, due to variations in absorbed radiation. By using strained silicon as the absorber, we decrease the electron-phonon coupling in the device and increase the responsivity to incoming power. The strained silicon absorber is coupled to a planar aluminium twin-slot antenna designed to couple to 160GHz and that serves as the superconducting contacts. From the measured optical responsivity and spectral response, we calculate a maximum optical efficiency of 50% for radiation coupled into the device by the planar antenna and an overall noise equivalent power, referred to absorbed optical power, of 1:1× 10-16WHz-1/2 when the detector is observing a 300K source through a 4K throughput limiting aperture. Even though this optical system is not optimized, we measure a system noise equivalent temperature difference of 6mKHz-1/2. We measure the noise of the device using a cross-correlation of time stream data, measured simultaneously with two junction field-effect transistor amplifiers, with a base correlated noise level of 300 pV Hz-1/2 and find that the total noise is consistent with a combination of photon noise, current shot noise, and electron-phonon thermal noise. [ABSTRACT FROM AUTHOR]

Scanning X-ray micro-diffraction has been used as a non-destructive probe of the local crystalline quality of a thin suspended germanium (Ge) membrane. A series of reciprocal space maps were obtained with ~4 μm spatial resolution, from which detailed information on the strain distribution, thickness, and crystalline tilt of the membrane was obtained. We are able to detect a systematic strain variation across the membranes, but show that this is negligible in the context of using the membranes as platforms for further growth. In addition, we show evidence that the interface and surface quality is improved by suspending the Ge. [ABSTRACT FROM AUTHOR]

A self doping contact consisting of a silver/antimony alloy that produces an Ohmic contact to moderately doped n-type germanium (doped to a factor of four above the metal-insulator transition) has been investigated. An evaporation of a mixed alloy of Ag/Sb (99%/1%) onto n-Ge (ND = 1 × 1018 cm-3) annealed at 400°C produces an Ohmic contact with a measured specific contact resistivity of (1:1±0.2) × 10-5 Ω-cm². It is proposed that the Ohmic behaviour arises from an increased doping concentration at the Ge surface due to the preferential evaporation of Sb confirmed by transmission electron microscope analysis. It is suggested that the doping concentration has increased to a level where field emission will be the dominate conduction mechanism. This was deduced from the low temperature electrical characterisation of the contact, which exhibits Ohmic behaviour down to a temperature of 6.5 K. [ABSTRACT FROM AUTHOR]

In this paper, we report on anisotropic transport properties of strained germanium (sGe) quantum wells grown on Si (001) substrates with p-type doping beneath the sGe channel. Mobility measurements were made along orthogonal [110] directions. The level of measured resistivity anisotropy in the [110] and [110] orientations was found to vary between 2 and 9 for different samples. This corresponds to an actual mobility anisotropy ratio of between 1.3 and 2, values that are significantly higher than previously found for sGe. From modeling of the low temperature (12 K) mobility, using the relaxation time approach, the anisotropy in mobility was accounted for by a difference in interface roughness scattering between the two orientations. For the [110] orientation, a step height of δ=28nm and interface roughness periodicity of l=7 nm were found while for the [110] orientation, l= reduced to 4nm and l increased to 0.42nm. High-resolution X-ray diffraction and transmission electron microscopy confirmed a 1° off-cut in the wafer towards the [110] direction. [ABSTRACT FROM AUTHOR]

In this letter, we propose a heterostructure design for tunnel field effect transistors with two low direct bandgap group IV compounds, GeSn and highly tensely strained Ge in combination with ternary SiGeSn alloy. Electronic band calculations show that strained Ge, used as channel, grown on Ge1-xSnx (x > 9%) buffer, as source, becomes a direct bandgap which significantly increases the tunneling probability. The SiGeSn ternaries are well suitable as drain since they offer a large indirect bandgap. The growth of such heterostructures with the desired band alignment is presented. The crystalline quality of the (Si)Ge(Sn) layers is similar to state-of-the-art SiGe layers. [ABSTRACT FROM AUTHOR]

In this paper, we report a Hall mobility of one million in a germanium two-dimensional hole gas. The extremely high hole mobility of 1.1?×?106 cm² V-1 s-1 at a carrier sheet density of 3?×?1011?cm-2 was observed at 12?K. This mobility is nearly an order of magnitude higher than any previously reported. From the structural analysis of the material and mobility modeling based on the relaxation time approximation, we attribute this result to the combination of a high purity Ge channel and a very low background impurity level that is achieved from the reduced-pressure chemical vapor deposition growth method. [ABSTRACT FROM AUTHOR]

Enhanced electron cooling is demonstrated in a strained-silicon/superconductor tunnel junction refrigerator of volume 40 μm3. The electron temperature is reduced from 300 mK to 174 mK, with the enhancement over an unstrained silicon control (300 mK-258 mK) being attributed to the smaller electron-phonon coupling in the strained case. Modeling and the resulting predictions of silicon-based cooler performance are presented. Further reductions in the minimum temperature are expected if the junction sub-gap leakage and tunnel resistance can be reduced. However, if only tunnel resistance is reduced, Joule heating is predicted to dominate. [ABSTRACT FROM AUTHOR]

High-resolution x-ray diffraction rocking curve (RC), x-ray reflectivity (XRR), and transmission electron microscopy (TEM) were used to characterize strained Ge epilayers grown on relaxed SiGe/Ge/Si(100) virtual substrates by reduced pressure chemical vapor deposition. The investigation focused on the reliability and accuracy of thickness measurement by these different techniques. The authors found that both XRR and RC could give reliable values that agree well with TEM results over a wide range of Ge epilayer thicknesses. The best-fit thickness from both XRR and RC is within ±5% of the TEM measurement for a thickness in the range of 10-120 nm, with XRR producing more accurate values than RC. However, neither RC nor XRR could give reliable results for very thin Ge epilayers (<7 nm) because of the complicated heterostructure and interface/surface quality. Agreement is also not as good for the thickest Ge epilayers (>122 nm) due to surface roughening caused by strain relaxation. [ABSTRACT FROM AUTHOR]

The effect of charge-carrier overheating in a two-dimensional (2D) hole gas is realized in a Si1-xGex quantum well, where x=0.13, 0.36, 0.8, or 0.95. The Shubnikov–de Haas (SdH) oscillation amplitude is used as a “thermometer” to measure the temperature of overheated holes. The temperature dependence of the hole-phonon relaxation time is found from an analysis of the change of the dependence of the amplitude of the SdH oscillations on temperature and applied electrical field. Analysis of the temperature dependence of the hole-phonon relaxation time reveals a transition of the 2D system from the regime of “partial inelasticity” to conditions of small-angle scattering. [ABSTRACT FROM AUTHOR]

The weak localization and interaction effects for charge carriers in a two-dimensional hole gas in a pure germanium quantum well in a SiGe/Ge/SiGe heterostructure with a hole density of 5.68×1011 cm-2 and mobility of 4.68×104 cm2 V-1 s-1 are investigated. The resistance measurements were made at temperatures from 46 mK to 10 K and magnetic fields up to 15 T. The magnetic-field dependence of the resistivity exhibits Shubnikov-de Haas oscillations and quantum Hall effect steps. At very low magnetic fields (B<0.1 T) a weak localization effect for holes is revealed, which makes for a negative magnetoresistance and growth of the resistance with decreasing temperature (at T<2 K). The manifestation of the interaction effect is observed and analyzed over a wide range of temperatures and magnetic fields. With increasing temperature the manifestation of the interaction-induced quantum correction passes from the diffusive regime to an intermediate and then to the ballistic regime. In all regions the behavior of the interaction quantum correction is in good agreement with the modern theoretical predictions. [ABSTRACT FROM AUTHOR]

The Shubnikov–de Haas oscillations in a two-dimensional hole gas in a quantum well of pure germanium in a SiGe/Ge/SiGe heterostructure with a hole concentration pH=5.68×1011 cm2 and mobility μ=4.68×104 cm2V-1s-1 are investigated in magnetic fields up to 15 T at temperatures from 40 mK to 4 K. The observed deviation from the known relation describing the conductivity oscillations in the Shubnikov–de Haas effect are explained by additional broadening of the Landau levels due to the existence of a nonuniform distribution of the concentration of charge carriers, and, accordingly, of their energy, in the plane of the two-dimensional gas. It is assumed that the latter is due to natural atomic-step variations of the well width. The effective hole mass (m*=0.112m0) is determined from the temperature dependence of the oscillation amplitude, and its dependence on magnetic field is used to determine the quantum scattering time and the value of the carrier concentration fluctuations. [ABSTRACT FROM AUTHOR]

The magnetic-field dependence (up to 110 kOe) of the resistance of Si[sub0.7]Ge[sub0.3] /Si[sub0.2]Ge[sub0.8]/Si[sub0.7]Ge[sub0.3] with a 2D hole gas in a Si[sub0.2]Ge[sub0.8] quantum well is measured in the temperature range 0.335-10 K and in a range of variation of the currents from 100 nA to 50 μA. Shubnikov-de Haas oscillations are observed in the region of high magnetic fields, and in the low-field region H &les; 1 kOe a positive magnetoresistance is observed which gives way to a negative magnetoresistance as the field is increased. This peculiarity is explained by effects of weak localization of the 2D charge carriers under conditions when the spin-orbit scattering time τ[SUBso] is close to the inelastic scattering time τ[SUBϕ], and it is evidence of a splitting of the spin states under the influence of a perturbing potential due to the formation of a two-dimensional potential well (the Rashba mechanism). Analysis of the weak localization effects gave the values of the characteristic relaxation times as τ[SUBϕ] = 7.2T[SUP-1] × 10[SUP-12] s and τ[SUBso] = 1.36 × 10[SUP-12] s. From these characteristics of the heterostructure studied, a value of Δ = 2.97 meV was obtained for the spin splitting. [ABSTRACT FROM AUTHOR]

A low temperature nickel process has been developed that produces Ohmic contacts to n-type germanium with specific contact resistivities down to (2.3 ± 1.8) × 10-7 Ω-cm2 for anneal temperatures of 340 °C. The low contact resistivity is attributed to the low resistivity NiGe phase which was identified using electron diffraction in a transmission electron microscope. Electrical results indicate that the linear Ohmic behaviour of the contact is attributed to quantum mechanical tunnelling through the Schottky barrier formed between the NiGe alloy and the heavily doped n-Ge. [ABSTRACT FROM AUTHOR]

We demonstrate significant modification of the electron-phonon energy loss rate in a many-valley semiconductor system due to lattice mismatch induced strain. We show that the thermal conductance from the electron system to the phonon bath in strained n+Si, at phonon temperatures between 200 and 480 mK, is more than an order of magnitude lower than that for a similar unstrained sample. [ABSTRACT FROM AUTHOR]

Epitaxial growth of a compressively strained Ge quantum well (QW) on an ultrathin, 345 nm thick, Si0.4Ge0.6/LT-Si0.4Ge0.6/Si(001) virtual substrate (VS) has been demonstrated. The VS, grown with a low temperature Si0.4Ge0.6 seed layer on a Si(001) substrate, is found to be fully relaxed and the Ge QW is fully strained. The temperature dependence of Hall mobility and carrier density clearly indicates a two-dimensional hole gas in the Ge QW. At room temperature, which is more relevant for electronic devices applications, the samples show a very high Hall mobility of 1235 cm2 V-1 s-1 at a carrier density of 2.36×1012 cm-2. [ABSTRACT FROM AUTHOR]

An innovative approach is proposed for epitaxial growth of high Ge content, relaxed Si1-xGex buffer layers on a Si(001) substrate. The advantages of the technique are demonstrated by growing such structures via chemical vapor deposition and their characterization. Relaxed Ge is first grown on the substrate followed by the reverse grading approach to reach a final buffer composition of 0.78. The optimized buffer structure is only 2.8 μm thick and demonstrates a low surface threading dislocation density of 4×106 cm-2, with a surface roughness of 2.6 nm. The buffers demonstrate a relaxation of up to 107%. [ABSTRACT FROM AUTHOR]

Very high two-dimensional hole gas (2DHG) drift mobility of 3100 cm2/V s is obtained at extremely high density of 41×1011 cm-2 in the modulation doped, 20 nm thick, strained Ge quantum well (QW) of SiGe heterostructure at room temperature. Very high 2DHG density is achieved by increasing the boron modulation doping, reducing the spacer layer thickness located between it and Ge QW, and increasing the valence-band offset of Ge QW, which also results in the enhancement of mobility. The obtained 2DHG mobility and carrier density exceed those reported for two-dimensional electron gas in the strained Si QW of SiGe heterostructures. [ABSTRACT FROM AUTHOR]

The room-temperature two-dimensional hole gas (2DHG) conductivity as high as 649.3 μS is obtained by implementation of double-side modulation doping (DS-MOD) of an 8 nm thick strained Ge quantum well in a SiGe heterostructure. This conductivity is about three times higher than that of the conventional SiGe heterostructure with single-side modulation doping (SS-MOD). While the low-temperature (T=3 K) mobility with DS-MOD is two times higher than that with SS-MOD, the room-temperature mobility of the two is practically the same, suggesting that phonon scattering is the dominant limiting mechanism at the device operating temperatures. [ABSTRACT FROM AUTHOR]

A significant improvement of transport properties of two-dimensional hole gas (2DHG) in the strained Ge quantum well (QW) of SiGe heterostructure was obtained via implementation of double-side modulation doping from bottom and top sides of Ge QW. Around two times enhancement of 2DHG Hall mobility up to 30 000 cm2 V-1 s-1 and sheet carrier density up to 1.9×1012 cm-2 were obtained at 3 K. By employing this way of modulation doping hole’s wave function was moved away from the interface towards the center of Ge QW that allowed enhancement of hole mobility due to reduction of interface roughness scattering. [ABSTRACT FROM AUTHOR]

We report an ultrahigh room-temperature hole drift mobility obtained in a Si0.2Ge0.8 quantum well with a parabolic-like Ge profile, which has at its core a p-type modulation-doped (MOD) Si/Si0.2Ge0.8/Si0.65Ge0.35/Si(001) heterostructure. High-conductivity holes at 293 K with a drift mobility of 3600 cm2 V-1 s-1 at a sheet carrier density of 4.94×1012 cm-2 were obtained in the Si0.2Ge0.8 quantum well after optimum annealing at 750 °C for 30 min. Hall mobility and sheet carrier density of this heterostructure are 1776 cm2 V-1 s-1 and 2.37×1013 cm-2, respectively. Structural characterization of the as-grown and the annealed samples revealed that the annealing had caused Si0.2Ge0.8 channel broadening, smearing of interfaces, and formation of a parabolic-like Ge profile that significantly improved room-temperature hole transport properties. The reported values of hole mobility are much higher than in the bulk Ge. [ABSTRACT FROM AUTHOR]

We have demonstrated reduced 1/f low-frequency noise in sub-μm metamorphic high Ge content p-Si[sub 0.3]Ge[sub 0.7] metal–oxide–semiconductor field-effect transistors (MOSFETs) at 293 K. Three times lower normalized power spectral density (NPSD) S[sub I[sub D]]/I[sub D][sup 2] of drain current fluctuations over the 1–100 Hz range at V[sub DS]=-50 mV and V[sub G]-V[sub th]=-1.5 V was measured for a 0.55 μm effective gate length p-Si[sub 0.3]Ge[sub 0.7] MOSFET compared with a p-Si MOSFET. Performed quantitative analysis clearly demonstrates the importance of carrier number fluctuations and correlated mobility fluctuations (CMFs) components of 1/f noise for p-Si surface channel MOSFETs, and the absence of CMFs for p-Si[sub 0.3]Ge[sub 0.7] buried channel MOSFETs. This explains the reduced NPSD for p-Si[sub 0.3]Ge[sub 0.7] MOSFETs in strong inversion. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We performed systematic low-temperature (T = 350 mK-15 K) magnetotransport measurements on the two-dimensional hole gas with various sheet carrier densities P[sub s] = (0.57-2.1) × 10[sup 12] cm[sup -2] formed in the strained Ge channel modulation-doped (MOD) SiGe heterostructures grown on Si substrates. It was found that the effective hole mass deduced by temperature dependent Shubnikovde Hass oscillations increased monotonically from (0.087 ± 0.05)m[sub 0] to (0.19 ± 0.01)m[sub 0] with the increase of P[sub s], showing large band nonparabolicity in strained Ge. In contrast to this result, the increase of the mobility with increasing P[sub s] (up to 29000 cm²/V s) was observed, suggesting that Coulomb scattering played a dominant role in the transport of the Ge channel at low temperatures. In addition, the Dingle ratio of the transport time to the quantum lifetime was found to increase with increasing P[sub s], which was attributed to the increase of remote impurity scattering with the increase of the doping concentration in MOD SiGe layers. [ABSTRACT FROM AUTHOR]

The effect of post-growth furnace thermal annealing (FTA) on the Hall mobility and sheet carrier density measured at 9–300 K in the Si[sub 0.2]Ge[sub 0.8]/Si[sub 0.7]Ge[sub 0.3]/Si(001) p-type modulation-doped heterostructures was studied. FTA treatments in the temperature range of 600–900 °C for 30 min were performed on similar heterostructures but with two Si[sub 0.2]Ge[sub 0.8] channel thicknesses. The annealing at 600 °C is seen to have a negligible effect on the Hall mobility as well as on the sheet carrier density. Increases in the annealing temperature resulted in pronounced successive increases of the mobility. For both samples the maximum Hall mobility was observed after FTA at 750 °C. Further increases of the annealing temperature resulted in a decrease in mobility. The sheet carrier density showed the opposite behavior with an increase in annealing temperature. The mechanism causing this behavior is discussed. Structural characterization of as-grown and annealed samples was done by cross-sectional transmission electron microscopy. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

To extract the room-temperature drift mobility and sheet carrier density of two-dimensional hole gas (2DHG) that form in Ge strained channels of various thicknesses in Ge/Si[sub 0.33]Ge[sub 0.67]/Si(001) p-type modulation-doped heterostructures, the magnetic field dependences of the magnetoresistance and Hall resistance at temperature of 295 K were measured and the technique of maximum entropy mobility spectrum analysis was applied. This technique allows a unique determination of mobility and sheet carrier density of each group of carriers present in parallel conducting multilayers semiconductor heterostructures. Extremely high room-temperature drift mobility (at sheet carrier density) of 2DHG 2940 cm[sup 2] V[sup -1] s[sup -1] (5.11×10[sup 11] cm[sup -2]) was obtained in a sample with a 20 nm thick Ge strained channel. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The behavior of the conductance upon changes in temperature (in the interval 1.5-40 K) and magnetic field (up to 20 kOe) is investigated for a series of samples with a δ layer in Si, with hole concentrations in the conducting δ layer of 2.5x10[sup 13]-2.2x10[sup 14] cm[sup -2]. It is shown that the temperature and field dependences obtained can be explained successfully as a manifestation of the weak localization effect and the interaction of mobile charge carriers (holes) in a two-dimensional electron system under conditions of strong spin-orbit interaction. An analysis of the behavior of the quantum corrections yields the temperature dependence of the phase relaxation time of the carriers, τ[sub φ]=AT[sup -1], with Aapprox. (1.4±0.3)x10[sup -12] K·s, where this temperature dependence is treated as a manifestation of hole-hole scattering processes, and the values of the interaction constants are also obtained (λ[sub T]approx. 0.64-0.73). © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]