Thomas J. Gramila

Observation of strain in pseudomorphic Si1−xGex by tracking phonon participation in Si∕SiGe resonant interband tunnel diodes via electron tunneling spectroscopy

High-sensitivity and low-noise electron tunneling spectroscopy was used to measure the phonon spectra via band-to-band tunneling in Si∕SiGe resonant interband tunneling diodes (RITD), tracking the effects of the weighted average Ge percentage in the central tunneling spacer. With a composite RITD tunneling barrier consisting of 4nm of intrinsic Si0.60Ge0.40 and n nm of intrinsic Si (n=4,6,8,10) all grown on Si substrates, the transverse acoustic (TA) phonon of Si0.60Ge0.40 was identified and the energy was measured to be 16±1meV. This is higher than the ∼14meV energy of the TA phonon in Si0.60Ge0.40 reported from measurements of Esaki tunnel diodes fabricated from bulk single crystals. The increase is attributed to the compressive strain in the Si0.60Ge0.40 layer grown on Si substrates. The observation of the upshift of phonon energy with strain by electron tunneling spectroscopy demonstrates the capability of electron tunneling spectroscopy to characterize residual strain.

Implementation of a Baffle Cooled Approach for a Cryostat Magnet Lead System

We discuss magnet leads for a small cryostat suitable for use in storage dewars, intended to be a testbed for a new, baffle cooled approach to the design of magnet leads. The essential elements of the lead design are discussed. A comparison of measurements of the current dependent boiloff rates with numerical modeling of the system is presented, providing validation of the overall design approach and its capabilities.