A broadband microwave Corbino spectrometer at 3^3He temperatures and high magnetic fields

We present the technical details of a broadband microwave spectrometer for measuring the complex conductance of thin films covering the range from 50 MHz up to 16 GHz in the temperature range 300 mK to 6 K and at applied magnetic fields up to 8 Tesla. We measure the complex reflection from a sample terminating a coaxial transmission line and calibrate the signals with three standards with known reflection coefficients. Thermal isolation of the heat load from the inner conductor is accomplished by including a section of NbTi superconducting cable (transition temperature around 8 $-$ 9 K) and hermetic seal glass bead adapters. This enables us to stabilize the base temperature of the sample stage at 300 mK. However, the inclusion of this superconducting cable complicates the calibration procedure. We document the effects of the superconducting cable on our calibration procedure and the effects of applied magnetic fields and how we control the temperature with great repeatability for each measurement. We have successfully extracted reliable data in this frequency, temperature and field range for thin superconducting films and highly resistive graphene samples

Reduction of Effective Terahertz Focal Spot Size By Means Of Nested Concentric Parabolic Reflectors

An ongoing limitation of terahertz spectroscopy is that the technique is generally limited to the study of relatively large samples of order 4 mm across due to the generally large size of the focal beam spot. We present a nested concentric parabolic reflector design which can reduce the terahertz focal spot size. This parabolic reflector design takes advantage of the feature that reflected rays experience a relative time delay which is the same for all paths. The increase in effective optical path for reflected light is equivalent to the aperture diameter itself. We have shown that the light throughput of an aperture of 2 mm can be increased by a factor 15 as compared to a regular aperture of the same size at low frequencies. This technique can potentially be used to reduce the focal spot size in terahertz spectroscopy and enable the study of smaller samples

Ledge Design of InGaP Emitter GaAs Based HBTs

A wide range of emitter composition, thickness, and doping is studied via dc current gain measurements on large area GaAs based heterojunction bipolar transistors (HBTs) at both room and elevated temperatures. InGaP emitters offer the widest thickness and doping design window in terms of dc peak current gain, as compared with AlGaAs emitters. Remarkably, a 50 Å InGaP emitter HBT retains 50% gain of a more standard 500 Å emitter device. For state-of-the-art HBTs, a degraded peak gain is argued to be caused by an increased reverse hole injection current (IRHI). In light of previously published results which implicate IRHI as a mechanism for materials limited HBT reliability, we suggest dc current gain measurements on large-area HBTs give meaningful insights into the long term reliability of the structure. Specifically, the wider emitter thickness and doping design window offered by an InGaP emitter HBT could apply to reliability as well as to the demonstrated gain stability