Inga A. Fischer

Electrical spin injection and transport in germanium

We report the first experimental demonstration of electrical spin injection, transport, and detection in bulk germanium (Ge). The nonlocal magnetoresistance (MR) in n-type Ge is observable up to 225 K. Our results indicate that the spin relaxation rate in the n-type Ge is closely related to the momentum scattering rate, which is consistent with the predicted Elliot-Yafet spin relaxation mechanism for Ge. The bias dependence of the nonlocal MR and the spin lifetime in n-type Ge is also investigated.

Comparison of spin lifetimes in n-Ge characterized between three-terminal and four-terminal nonlocal Hanle measurements

We compared the temperature dependence of spin lifetime in n-Ge characterized from three-terminal (3T) and four-terminal (4T) Hanle measurements using single-crystalline Fe/MgO/n-Ge tunnel junctions. The bias conditions of the two schemes were chosen to be about the same in order to compare the spin lifetimes (?3T and ?4T). The temperature dependences of ?3T and ?4T behave in a very similar way at the low temperature region (T ? 10?K), and both ?3T and ?4T decrease as the temperature increases, which is consistent with the dominating Elliot?Yafet spin relaxation mechanism in bulk Ge. However, when the temperature is higher than 10?K, ?4T is longer than ?3T, which may be explained by the fact that 3T Hanle measurements are more easily affected by additional scattering effects caused by the accompanied charge current and electric field in the 3T geometry.

Silicon tunneling field-effect transistors with tunneling in line with the gate field

We present experimental results on the fabrication and characterization of vertical Si tunneling field-effect transistors (TFETs) in a device geometry with tunneling in line with the gate field. Compared to vertical Si TFETs without this geometry modification, on-currents are increased by more than one order of magnitude with <i>I</i>_ON = 1.1 μA/μm at <i>V</i>_DS = 0.5 V and an <i>I</i>_ON/<i>I</i>_OFF ratio of 3.4 ·10⁴ in n-channel operation. We present further suggestions for device improvements.

Crystalline phases in chiral ferromagnets: Destabilization of helical order

In chiral ferromagnets, weak spin-orbit interactions twist the ferromagnetic order into spirals, leading to helical order. We investigate an extended Ginzburg-Landau theory of such systems where the helical order is destabilized in favor of crystalline phases. These crystalline phases are based on periodic arrangements of double-twist cylinders and are strongly reminiscent of blue phases in liquid crystals. We discuss the relevance of such blue phases for the phase diagram of the chiral ferromagnet MnSi.

Tuning the Ge(Sn) Tunneling FET: Influence of Drain Doping, Short Channel, and Sn Content

We present experimental results on the realization of p-channel mode Ge(Sn) heterojunction band-to-band tunneling field effect transistors. We investigate the influence of three device parameters (drain doping, channel length, and tunnel barrier height at source side) of the semiconductor body of the devices on the device performance. We achieve a complete suppression of the n-channel mode in p-type operating conditions by systematically reducing the p-type drain doping from 1·1020 to 2 · 1017 cm-3, examined in sample series A. In the second sample series B, we investigate the influence of a reduction of the channel length down to 15 nm on transistor performance. To improve the ON current ION without degrading the OFF current IOFF, we introduce a 10-nm delta layer of a Ge1-xSnx alloy at the source/channel junction in the third sample series C. We demonstrate an improved ON current ION compared with the reference sample without a GeSn delta layer.

Growth and characterization of SiGeSn quantum well photodiodes.

We report on the fabrication and electro-optical characterization of SiGeSn multi-quantum well PIN diodes. Two types of PIN diodes, in which two and four quantum wells with well and barrier thicknesses of 10 nm each are sandwiched between B- and Sb-doped Ge-regions, were fabricated as single-mesa devices, using a low-temperature fabrication process. We discuss measurements of the diode characteristics, optical responsivity and room-temperature electroluminescence and compare with theoretical predictions from band structure calculations.

Field-tuned quantum critical point of antiferromagnetic metals

A magnetic field applied to a three-dimensional antiferromagnetic metal can destroy the long-range order and thereby induce a quantum critical point. Such field-induced quantum critical behavior has been the focus of many recent experiments. We theoretically investigate the quantum critical behavior of clean antiferromagnetic metals subject to a static, spatially uniform, external magnetic field. The external field not only suppresses (or induces in some systems) antiferromagnetism, but also influences the dynamics of the order parameter by inducing spin precession. This leads to an exactly marginal correction to spin-fluctuation theory. We investigate how the interplay of precession and damping determines the specific heat, magnetization, magnetocaloric effect, susceptibility, and scattering rates. We point out that precession can change the sign of the leading

Compositional dependence of the band-gap of Ge1−x−ySixSny alloys

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Hanle-effect measurements of spin injection from Mn5Ge3C0.8/Al2O3-contacts into degenerately doped Ge channels on Si

correction to the specific-heat coefficient

Ge-on-Si PIN-photodetectors with Al nanoantennas: The effect of nanoantenna size on light scattering into waveguide modes

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