We apply He implantation and subsequent annealing to MBE gown pseudomorphic Si1-xGex /Si(100) hetero structures to induce strain relief. The He implant is located slightly below the Si1-xGex/Si(100) interface. Through structural characterization by transmission electron microscopy, X-ray diffraction and Rutherford-backscattering we investigate the strain relaxation mechanism of the Si1-xGex buffer. The variation of the implantation dose and the annealing conditions change the characteristics of the dislocation configuration and of the He bubble structure. At a dose of 2x10(16) cm(-2) a high degree of relaxation is accompanied by a low density of threading dislocations of about 10(7) cm(-2) for a Ge content of 30%.

Depending on the detailed geometry, gate voltage, and circuitry, nanoscale Si/SiGe cross junctions at low temperatures exhibit full-wave rectification arising from different mechanisms like change in the number of current-carrying modes, stationary ballistic charging of a current-free voltage lead, and hot-electron thermopower. We study the rectifier structures on high-mobility Si/SiGe heterostructures consisting of a straight voltage stem and oblique current-injecting leads. Local gate electrodes are used to control the electron density in the voltage or current channel. Compared to three-terminal Y-branch junctions, the four-terminal cross junction eliminates the mode effect. A gradual increase of output voltage as gate-voltage is reduced until threshold voltage is identified as contribution of hot-electron thermopower. Heating the initially cold reservoir from a second orthogonal cross junction eliminates the electron temperature gradient and suppresses the thermopower. Even if the operation as six-terminal device re-induces a mode-controlled contribution, we demonstrate that it is negligible. As expected, the ballistic signal can be reliably separated from other mechanisms by measurements under positive gate voltage. The ballistic voltage can be described by a parabolic function of the injected current and is proportional to the cosine of the injection angle.

Quantum point contacts (QPCs) are fabricated on modulation-doped Si/SiGe heterostructures and ballistic transport is studied at low temperatures. We observe quantized conductance with subband separations up to 4 meV and anomalies in the first conductance plateau at 4e2/h. At a temperature of T = 22 mK in the linear transport regime, a weak anomalous kink structure arises close to 0.5(4e2/h), which develops into a distinct plateau-like structure as temperature is raised up to T = 4 K. Under magnetic field parallel to the wire up to B = 14 T, the anomaly evolves into the Zeeman spin-split level at 0.5(4e2/h), resembling the "0.7 anomaly" in GaAs/AlGaAs QPCs. Additionally, a zero-bias anomaly (ZBA) is observed in nonlinear transport spectroscopy. At T = 22 mK, a parallel magnetic field splits the ZBA peak up into two peaks. At B = 0, elevated temperatures lead to similar splitting, which differs from the behavior of ZBAs in GaAs/AlGaAs QPCs. Under finite dc bias, the differential resistance exhibits additional plateaus approximately at 0.8(4e2/h) and 0.2(4e2/h) known as "0.85 anomaly" and "0.25 anomaly" in GaAs/AlGaAs QPCs. Unlike the first regular plateau at 4e2/h, the 0.2(4e2/h) plateau is insensitive to dc bias voltage up to at least VDS = 80 mV, in-plane magnetic fields up to B = 15 T, and to elevated temperatures up to T = 25 K. We interpret this effect as due to pinching off one of the reservoirs close to the QPC. We do not see any indication of lifting of the valley degeneracy in our samples.

The magnetoresistance (MR) of a two-dimensional hole gas in a quantum well of compressively strained Si0.05Ge0.95 has been investigated as a function of temperature. The MR shows a maximum at intermediate magnetic fields between the regions of weak localization and the Shubnikov-de Haas oscillations, which is discussed in terms of a recent theoretical study of the electron-electron interaction effect by Sedrakyan and Raikh (SR). The magnetic field MR dependence is clearly observed to cross over from quadratic to linear at T=7.8 K and B≈0.3 T. It is shown that the SR theory provides a good description of both the measured quadratic and positive linear MR, but over estimates the field position of the MR maximum and does not account for the shift in position with temperature that is observed. Earlier theories of electron-electron interaction (by Altshuler and Aronov, Gornyi and Mirlin) show a better agreement with the experimentally observed behavior of the MR maximum, but fit the low field MR less accurately.

In a ballistic rectifier consisting of an asymmetric cross junction the injection of electrons induce a nonlocal voltage in the current‐free voltage stem, whose polarity is determined by the geometry and not by the direction of the input current. This inertial‐ballistic signal is imposed by a hot‐electron thermopower arising from a gradient in the electron temperature over a gate‐confined region of the stem channel. We succeeded to separate both signal components by adding an orthogonal cross junction to the stem at the other side of the confined region. Current‐induced heating of the electrons in the orthogonal leads purely causes hot‐electron thermopower of opposite polarity. Appropriately designed geometry yields a difference signal which exclusively represents the inertial‐ballistic component.

Injection-type ballistic rectification is achieved in an asymmetric Si/SiGe cross junction made from narrow channels, where the injectors are inclined with respect to the straight voltage stem. In this geometry an inertial-ballistic signal establishes due to the momentum direction of the injected electrons towards the lower part of the central voltage stem. Additionally, a diffusion hot-electron thermopower signal is superimposed which arises from an electron temperature gradient over a gate-confined region of the stem channel. We investigate the influence of the stem width on the rectifier performance. The inertial-ballistic signal disappears at a stem width larger than the 2D electron mean free path in contrast to the diffusion thermopower, which is independent of the stem width.

Quantum interference effects, such as weak localization and electron-electron interaction (EEI), have been investigated in magnetic fields up to 11 T for hole gases in a set of Si 1− x Ge x quantum wells with 0.13< x <0.95. The temperature dependence of the hole phase relaxation time has been extracted from the magneto-resistance between 35 mK and 10 K. The spin-orbit effects that can be described within the Rashba model were observed in low magnetic fields. A quadratic negative magneto-resistance was observed in strong magnetic fields, due to the EEI effect. The hole-phonon scattering time was determined from hole overheating in a strong magnetic field.

Tunable inertial-ballistic rectification is studied in a nanoscale injection-type Si/SiGe rectifier in the hot-electron regime. The rectifier consists of a cascade of two nanoscale cross junctions in series. Two pairs of opposing current injectors merge under 30∘ into a straight central voltage stem. The electron densities in the injectors and the stem can be adjusted separately by two local top-gates. The measurements reveal a substantial efficiency increase for a nearly depleted stem. The efficiency of ballistic rectifiers can be expressed by the transfer resistance RT (output voltage divided by input current), the best value we achieve is 800Ω.

Injection-type ballistic rectifiers on Si/SiGe are studied with respect to the influence of gate voltage on the transfer resistance RT (output voltage divided by input current) for different positions of a local gate electrode. The rectifiers are trifurcated quantum wires with straight voltage stem and oblique current-injecting leads. Depending on the gate configuration, thermopower contributions arise from nearly-pinched stem regions which either cancel each other or impose upon the ballistic signal with same or opposite polarity. At best, this enhances RT to a maximum value of 470 Ohm close to threshold voltage.

Strained-Si modulation doped field effect transistors have been studied as detectors of 0.2 THz and 1.6 THz electromagnetic radiation at room temperature. The difference in the gate voltage dependences for 0.2 THz and 1.6 THz radiation and spatial pattern of the transistor response to focused 1.6 THz radiation confirms that the mechanism of detection is linked to the excitations of the two-dimensional electrons in the device channel.

MOS-gated strained-Si modulation doped Field Effect Transistors (MOSMODFETs) traditionally suffer from parallel conduction causing degradation of the device performance below that of the Si control fabricated in the same batch. We present a MOSMODFET in which parallel conduction is avoided through the use of ultra-thin modulation doped layers and TMAH etching to remove the top SI parasitic layer. A low thermal budget and deposited oxides are used to conserve material integrity. This approach has lead to MOSMODFETs that show RF performance improvement over the Si control MOSFET and improved DC operation over a temperature range from 10K to 300K. The influence of the low temperature processing on the characteristics is an increase from 0.3 to 1.2 Omega mm of the contact resistance, and the deposited oxide increases the interface state density.

The influence of parasitic capacitance on high frequency performances of SiGe n-HFET is reported. These capacitances arising from the fringe and electrostatic contributions of the gate are extracted from HF measurements and estimated with a 2D hydrodynamic modelling and with a 2D electrostatic modelling of the device. The main electrostatic contribution arises from the strong direct coupling between the mushroom-shaped gate and the 2D electron gas in the quantum well increased by the thin oxide layer above the undoped Si cap layer. The height of the gate foot, the shape of the top of the gate are investigated, showing that if the mushroom shape increase the capacitance by 22% compared to a simple-shaped gate, the increase of its size has less impact than the oxide thickness and the aspect ratio. The extrinsic capacitances can degrade by 27% the intrinsic properties of the transistors reported here.

Ballistic electron transport, namely quantized conductance, bend resistance, and its polarity-dependent breakdown, is studied in an asymmetric nanoscale cross junction prepared from a high-mobility Si/SiGe heterostructure. The cross junction is composed of four orthogonal leads which merge into two wide and two narrow quantum point contacts (QPCs). For a wide–narrow combination of leads the two-terminal differential conductance as a function of finite DC bias voltage reveals oscillations and half-plateaus characteristic for quantized conductance. In four-terminal bend resistance configuration with current injection into orthogonal leads negative bend voltage develops between the probing leads. Above a given current threshold the negative bend resistance breaks down just in that bias current polarity where electrons enter the cross region from the narrow QPC. This breakdown is attributed to phonon emission by hot electrons.

SiGe HFETs have reached in recent years good HF noise performance. We report here an investigation about the contribution of electrostatic parasitics on the noise performance using a PRC electrical model for the intrinsic device. A strong reduction of the capacitance due to gate shape improvement and technological optimization should strongly enhance all the HF performance.

We present a method to produce thin SiGe virtual substrates suitable for electronic applications. This method is based on the gas phase process of low-energy plasma-enhanced chemical vapor deposition. The strain-relaxed buffers are characterized by X-ray diffractometry, transmission electron microscopy and atomic force microscopy. We find threading dislocation densities lower than 3×108cm−2 and a surface rms roughness of 1.8nm, for a buffer thickness of 500nm. Room temperature electrical results are also presented, which are competitive with those obtained on SiGe buffers produced by other methods.

N-type Schottky-gated Si:SiGe heterostructure field-effect transistors with physical gate lengths between 70 and 450nm are characterized over a wide temperature range (T=10 K...300 K) for low electric fields. The room-temperature maximum low-field transconductance increases 61% to 440 mS/mm at T=10 K for the 70-nm device. The minimum subthreshold slope is 14...19 mV/dec at T=10 K. The off-state currents I/sub OFF/ are limited by parallel conduction at high temperatures and by the gate leakage current at low temperatures. Substrate leakage currents are found to be due to generation of carriers within the drain/substrate depletion layer and only make a minor contribution to I/sub OFF/. Operation of the devices at the lowest temperature is found to result in the occurrence of the floating-body kink effect, as a consequence of substrate freeze-out and subsequent self-biasing by impact ionization currents. Low temperature characteristics exhibit a nonlinear low-field drain current dependence on the drain voltage, due to the presence of parasitic Schottky source/drain contacts. An extraction method for access resistance consistent with this phenomenon is presented.

In order to investigate impact ionization we have performed electrical DC measurements and electroluminescence (EL) spectroscopy on strained Ge on Si0.4Ge0.6 p-MODFETs. These measurements are discussed in comparison with energy band structure calculation and high electric field transport simulation. The energy band diagram is calculated using an original 30 band k·p Hamiltonian taking into account strain. The transport simulation based on a matrix resolution of the Boltzmann transport equation allows us to calculate impact ionization coefficients. The interpretation of EL results is based on hole energy subbands calculated in the confined Ge quantum well.

An asymmetric nanoscale cross junction is fabricated from a high-mobility Si∕SiGe heterostructure. At T=4.2K, the four-terminal current-voltage characteristics reveal a polarity-dependent breakdown of the negative bend resistance. The breakdown is accompanied by negative differential conductance found in the two-terminal current-voltage characteristics of the orthogonal current leads. We attribute this behavior to phonon emission by hot electrons. From gate-voltage-dependent measurements, we determine a phonon threshold of 19meV.