V. Venkataraman

High room-temperature hole mobility in Ge0.7Si0.3/Ge/Ge0.7Si0.3 modulation-doped heterostructures

Modulation-doped two-dimensional hole gas structures consisting of a strained germanium channel on relaxed Ge0.7Si0.3 buffer layers were grown by molecular-beam epitaxy. Sample processing was optimized to substantially reduce the contribution from the parasitic conducting layers. Very high hall mobilities of 1700 cm2/V s for holes were observed at 295 K which are the highest reported to date for any kind of p-type silicon-based heterostructures. Hall measurements were carried out from 13 to 300 K to determine the temperature dependence of the mobility and carrier concentration. The carrier concentration at room temperature was 7.9×1011 cm−2 and decreased by only 26% at 13 K, indicating very little parallel conduction. The high-temperature mobility obeys a T−α behavior with α∼2, which can be attributed to intraband optical phonon scattering.

Alloy scattering limited transport of two‐dimensional carriers in strained Si1−xGex quantum wells

The mobility of two‐dimensional electron systems in strained Si1−xGex quantum wells was studied at low temperatures as a function of the germanium content in the well. While mobilities exceeding 30 000 cm2/V s have been achieved for pure Si layers, they dropped to less than 3000 cm2/V s for Si0.88Ge0.12 wells, demonstrating the effect of strong alloy disorder scattering. This was confirmed by the relatively weaker dependence of mobility on carrier concentration in gating experiments. By accounting for valley degeneracy effects in the standard two‐dimensional alloy scattering model, an effective scattering potential Valloy=0.8±0.1 eV has been derived for electrons. Using a weaker potential, it is shown that the maximum low temperature mobilities for holes reported in literature are also consistent with the alloy scattering model.

Temperature dependence of spin lifetime of conduction electrons in bulk germanium

Optically generated spin polarized electrons in bulk n-type Ge samples have been detected by using a radio-frequency modulation technique. Using the Hanle effect in an external magnetic field, the spin lifetime was measured as a function of temperature in the range 90 K to 180 K. The lifetime decreases with increasing temperature from similar to 5 ns at 100 K to similar to 2 ns at 180 K. We show that the temperature dependence is consistent with the Elliott-Yafet spin relaxation mechanism R. J. Elliot, Phys. Rev. 96, 266 (1954)]

All Inorganic Spin-Coated Nanoparticle-Based Capacitive Memory Devices

We demonstrate all inorganic, robust, cost-effective, spin-coated, two-terminal capacitive memory metal-oxide nanoparticle-oxide-semiconductor devices with cadmium telluride nanoparticles sandwiched between aluminum oxide phosphate layers to form the dielectric memory stack. Using a novel high-speed circuit to decouple reading and writing, experimentally measured memory windows, programming voltages, retention times, and endurance are comparable with or better than the two-terminal memory devices realized using other fabrication techniques.

Symmetric Si/Si1-xGex two-dimensional hole gases grown by rapid thermal chemical vapor deposition

Single and symmetric double p‐type modulation‐doped structures have been fabricated in Si/SiGe for the first time by rapid‐thermal chemical‐vapor deposition. Temperature‐dependent electrical measurements and high‐field magnetotransport measurements demonstrate the presence of a well‐confined two‐dimensional hole gas in these samples. Nominally‐symmetric normal and inverted structures differ in carrier density and mobility at most by 20%, indicating that there is little asymmetry due to surface segregation or autodoping effects. Measurements on double heterostructures confirm that the interfaces are symmetric to within 10 A. Peak mobilities reached 2500 cm2/V s at 10 K, comparable to those obtained in similar samples grown by ultrahigh vacuum techniques.

Alloying induced degradation of the absorption edge of InAsxSb1−x

InAsxSb1−x alloys show a strong bowing in the energy gap, the energy gap of the alloy can be less than the gap of the two parent compounds. The authors demonstrate that a consequence of this alloying is a systematic degradation in the sharpness of the absorption edge. The alloy disorder induced band-tail (Urbach tail) characteristics are quantitatively studied for InAs0.05Sb0.95.

Enhanced sensitivity in detection of Kerr rotation by double modulation and time averaging based on Allan variance

Experiments in spintronics necessarily involve the detection of spin polarization. The sensitivity of this detection becomes an important factor to consider when extending the low temperature studies on semiconductor spintronic devices to room temperature, where the spin signal is weaker. In pump-probe experiments, which optically inject and detect spins, the sensitivity is often improved by using a photoelastic modulator (PEM) for lock-in detection. However, spurious signals can arise if diode lasers are used as optical sources in such experiments, along with a PEM. In this work, we eliminated the spurious electromagnetic coupling of the PEM onto the probe diode laser, by the double modulation technique. We also developed a test for spurious modulated interference in the pump-probe signal, due to the PEM. Besides, an order of magnitude enhancement in the sensitivity of detection of spin polarization by Kerr rotation, to 3x10(-8) rad was obtained by using the concept of Allan variance to optimally average the time series data over a period of 416 s. With these improvements, we are able to experimentally demonstrate at room temperature, photoinduced steady-state spin polarization in bulk GaAs. Thus, the advances reported here facilitate the use of diode lasers with a PEM for sensitive pump-probe experiments. They also constitute a step toward detection of spin-injection in Si at room temperature.

Simplified theory of optical nonlinearities in spin-polarized semiconductors

The spin degree of freedom is largely disregarded in existing theories of the density-dependent optical properties of an interacting electron-hole plasma in quasiequilibrium. Here, we extended the pair equation, which is applicable to a bulk semiconductor at elevated temperatures, to calculate optical nonlinearities due to a spin-polarized plasma. We obtained agreement with recent circular dichroism data in laser-excited GaAs by using the plasma density alone as the fitting parameter. The simplicity of our theory, based on the analytical pair-equation formula, makes it ideal for conveniently modelling absorption in a carrier spin-polarized semiconductor.

Enhanced room temperature mobilities and reduced parallel conduction in hydrogen passivated Si/SiGe heterostructures

The effect of hydrogen passivation on the room temperature mobility and density is studied on n-type

A power-efficient thermocycler based on induction heating for DNA amplification by polymerase chain reaction

Si_{0.62}Ge_{0.38}