Session G01: Lunchtime Physics

Electrical Activation of Indium Dopants in Germanium and Silicon-Germanium Alloys

Electrical deactivation of indium dopants at high doping levels in germanium and silicon-germanium alloys is a major obstacle to using germanium to create high-performance semiconductor devices. We used X-ray absorption spectroscopy (EXAFS and XANES) to determine the local structure changes responsible for the decrease in the electrical activation of indium dopants observed in these semiconductors as doping levels increase. We then performed ab initio theoretical calculations (DFT and MD) to further study indium-vacancy complexes and indium clusters at a range of doping levels. With carbon co-doping, we were able to triple the electrically active fraction of indium at concentrations of up to ~0.7 at%.  Further experimental and theoretical analyses show that this significant improvement results from carbon-indium pair formation, relieving indium-induced strain in the germanium lattice and reducing the precipitation of indium.   

The Youngest Protostars in the Orion Molecular Cloud: Observations Revealing the Physics of Protostar Formation

We present a study five of the most deeply embedded protostars (stars still accreting mass) known in the Orion Molecular Clouds, identified with the Herschel Space Observatory by their bright emission at 70 microns as part of the Herschel Orion Protostar Survey. Using the Atacama Large Millimeter Array (ALMA) and Very Large Array (VLA) radio interferometers, we map the thermal dust emission at 0.87 mm and 8 mm.  Using the Planck function, we analyze this emission, which arises from the ~50 K dust grains in the collapsing gas clouds forming the stars.  We show images of dense structures in the inner regions of the collapsing gas clouds, and argue that these are images are within 10,000 years of onset of star formation that has previously been limited to theoretical work.

Resolving structures down to 40 AU with high optical depth shows  that radiation cooling is inefficient in these regions. We estimate dust masses of up to ~1.5 solar masses concentrated  in regions 160 AU in extent from the 8 mm data. These data gives us the first observational insights into the adiabatic contraction phases of the gas clouds that initiates the formation of hydrostatically supported protostars (see Larson 1969 and Bhandare et al. 2018).   

Wide Angle Compton Coincidence technique for determining scintillator nonlinearity

The energy response of most scintillator materials is nonlinear at some level. The linearity of a scintillation detector can be measured using Compton scattering into a known-to-be-linear detector. In the Wide Angle Compton Coincidence (WACC) technique, Compton scattering events (from the scintillation detector under test) are sampled over many angles at once, enabling the energy response to be efficiently mapped out over a range of energies. We report on the implementation of this technique with a portable WACC setup based on a HPGe detector and a multi-channel digital data acquisition system.