Strain-Dependent Effects on Confinement of Folded Acoustic and Optical Phonons in Short-Period (XC)m/(YC)n with X,Y (≡Si, Ge, Sn) Superlattices.

Carbon-based novel low-dimensional XC/YC (with X, Y ≡ Si, Ge, and Sn) heterostructures have recently gained considerable scientific and technological interest in the design of electronic devices for energy transport use in extreme environments. Despite many efforts made to understand the structural, electronic, and vibrational properties of XC and XxY1-xC alloys, no measurements exist for identifying the phonon characteristics of superlattices (SLs) by employing either an infrared and/or Raman scattering spectroscopy. In this work, we report the results of a systematic study to investigate the lattice dynamics of the ideal (XC)m/(YC)n as well as graded (XC)10-∆/(X0.5Y0.5C)∆/(YC)10-∆/(X0.5Y0.5C)∆ SLs by meticulously including the interfacial layer thickness ∆ (≡1-3 monolayers). While the folded acoustic phonons (FAPs) are calculated using a Rytov model, the confined optical modes (COMs) and FAPs are described by adopting a modified linear-chain model. Although the simulations of low-energy dispersions for the FAPs indicated no significant changes by increasing ∆, the results revealed, however, considerable "downward" shifts of high frequency COMs and "upward" shifts for the low energy optical modes. In the framework of a bond polarizability model, the calculated results of Raman scattering spectra for graded SLs are presented as a function of ∆. Special attention is paid to those modes in the middle of the frequency region, which offer strong contributions for enhancing the Raman intensity profiles. These simulated changes are linked to the localization of atomic displacements constrained either by the XC/YC or YC/XC unabrupt interfaces. We strongly feel that this study will encourage spectroscopists to perform Raman scattering measurements to check our theoretical conjectures.

Novel Electrodes and Engineered Interfaces for Halide-Semiconductor Radiation Detectors.

A new class of inorganic halide semiconductors are emerging as high-efficiency low-cost candidates for spectroscopic radiation detection. We report on solving one of the major challenges of these halide radiation detectors. At room temperature halide semiconductor detectors polarize under applied electric field, which not only degrades the charge collection efficiency of the detectors, but also promotes chemical reaction of the metal electrodes with the halide ions. This increases the metal-semiconductor interface noise and early failure of the spectroscopic detection capabilities of the device. We report on a solution to this challenge by application of novel electrodes on Thallium Bromide (TlBr) radiation detectors with virtually defect-free electrode-semiconductor interfaces, showing low noise and high detection stability for an extended period of time under accelerated ageing conditions. A number of TlBr detectors fabricated by this technique have demonstrated continuous stable detection performance (e.g. ±1% change in 662 keV gamma channel) for more than 4000 hours at room temperature. This report also shows continuously recorded 137Cs gamma radiation response of a unidirectionally-biased pixelated TlBr detector over more than 2 months (a total of 2880 data sets), which exhibit excellent stability. The developed approach has resulted in unprecedented low-noise stable performance of halide semiconductor detectors at room temperature, overcoming one of the major obstacles to the full consideration of TlBr (and other halide semiconductors) as a potentially low-cost replacement for Cadmium Zinc Telluride (CZT).

Resonant-cavity-enhanced mid-infrared photodetector on a silicon platform.

In this paper, we demonstrate high optical quantum efficiency (90%) resonant-cavity-enhanced mid-infrared photodetectors fabricated monolithically on a silicon platform. High quality photoconductive polycrystalline PbTe film is thermally evaporated, oxygen-sensitized at room temperature and acts as the infrared absorber. The cavity-enhanced detector operates in the critical coupling regime and shows a peak responsivity of 100 V/W at the resonant wavelength of 3.5 microm, 13.4 times higher compared to blanket PbTe film of the same thickness. Detectivity as high as 0.72 x 10(9) cmHz(1/2)W(-1) has been measured, comparable with commercial polycrystalline mid-infrared photodetectors. As low temperature processing (< 160 degrees C) is implemented in the entire fabrication process, our detector is promising for monolithic integration with Si readout integrated circuits.