Matthew O. Reese

Antenna-Coupled Niobium Bolometers for Terahertz Spectroscopy

We report characterizations of antenna-coupled hot electron bolometers designed for laboratory-based terahertz spectroscopy. These direct detectors combine sub-nanosecond response, high sensitivity, and the ability to operate below saturation when viewing a room temperature background. The optimum small-signal responsivity is 4.4 times 104 V/W, measured at a bath temperature Tb ap 0.9Tc. The corresponding saturation power is 7 nW. The saturation power increases and the small-signal responsivity decreases as the bath temperature is lowered. The measured noise equivalent power is 2.0 times 10-14 W/(Hz)frac12, near the predicted thermal fluctuation limit. The noise is white from approximately 100 Hz to 100 MHz.

A far-infrared Fourier transform spectrometer with an antenna-coupled niobium bolometer

We have designed and constructed a custom far-infrared Fourier transform spectrometer using an antenna-coupled bolometer as a detector. The active element of the detector is a superconducting niobium microbridge, and the far-infrared signal is coupled to the microbridge via a planar antenna mounted on a hyperhemispherical silicon lens. The spectrometer uses a broadband blackbody source with frequency-independent optical components, and thus the system bandwidth is set by the detector antenna. We have fabricated devices with two different antenna types, the double dipole and the log spiral, and have characterized the spectral response of each. This spectrometer can utilize the fast response of the niobium bolometer to perform time-resolved far-infrared spectroscopy on nanosecond to millisecond timescales. These timescales are too fast for standard commercial bolometers and too long for a typical optical delay line.

Niobium Hot Electron Bolometer Development for a Submillimeter Heterodyne Array Camera

We are developing a proof-of-concept for a diffusion cooled hot electron bolometer (HEB) array submillimeter camera. The ultimate objective is to create a working 64 pixel array with the University of Arizona for use on the Heinrich Hertz Telescope. We have fabricated Nb HEBs using a novel angle deposition process. We have characterized these devices using heterodyne mixing at 20 GHz. We also report on optimizations in the fabrication process that improve device performance and a dc screening test for device quality. Using this process, HEBs with a sharp resistive transition (<300 mK) can consistently be produced. These devices have good suppression of superconductivity in the contact pads, with a pad transition ~1 K below the main bridge transition. Furthermore, the bandwidth was investigated above and below the pad transition and found to be ~15% larger for 400 nm long bridges with non- superconducting contact pads.

Niobium direct detectors for fast and sensitive terahertz spectroscopy

We report the performance of a niobium hot-electron bolometer designed for laboratory terahertz spectroscopy. The antenna-coupled detector can operate above 4.2 K and has fast (subnanosecond) response. Detailed microwave measurements of performance over a wide range of operating conditions were correlated with quantitative terahertz measurements. The maximum responsivity is 4 x 10(4) VW with a noise equivalent power at the detector of 2 x 10(-14) W/Hz(12), approaching the intrinsic thermal fluctuation limit for the device. This detector enables a variety of novel laboratory spectroscopy measurements.

Superconducting microbolometers for time-resolved terahertz spectroscopy

We report the characterization of superconducting niobium microbolometers designed for time-resolved terahertz spectroscopy on nanosecond to millisecond timescales. Coupling of the incident signal is achieved via a planar antenna mounted on a hyperhemispherical silicon lens. We have integrated these detectors into a custom Fourier-transform spectrometer. The spectrometer optics are frequency independent over the spectral range 0.1-3 terahertz and thus the system bandwidth is set by the detector antenna. We have fabricated devices with two different antenna geometries, the double-dipole and the log spiral, and have characterized the spectral response of each. This detector will enable a variety of novel spectroscopy applications.

Two-Dimensional Cadmium Chloride Nanosheets in Cadmium Telluride Solar Cells

In this study we make use of a liquid nitrogen-based thermomechanical cleavage technique and a surface analysis cluster tool to probe in detail the tin oxide/emitter interface at the front of completed CdTe solar cells. We show that this thermomechanical cleavage occurs within a few angstroms of the SnO2/emitter interface. An unexpectedly high concentration of chlorine at this interface, ∼20%, was determined from a calculation that assumed a uniform chlorine distribution. Angle-resolved X-ray photoelectron spectroscopy was used to further probe the structure of the chlorine-containing layer, revealing that both sides of the cleave location are covered by one-third of a unit cell of pure CdCl2, a thickness corresponding to about one Cl-Cd-Cl molecular layer. We interpret this result in the context of CdCl2 being a true layered material similar to transition-metal dichalcogenides. Exposing cleaved surfaces to water shows that this Cl-Cd-Cl trilayer is soluble, raising questions pertinent to cell reliability. Our work provides new and unanticipated details about the structure and chemistry of front surface interfaces and should prove important to improving materials, processes, and reliability of next-generation CdTe-based solar cells.

Semi-insulating Sn-Zr-O: Tunable resistance buffer layers

Highly resistive and transparent (HRT) buffer layers are critical components of solar cells and other opto-electronic devices. HRT layers are often undoped transparent conducting oxides. However, these oxides can be too conductive to form an optimal HRT. Here, we present a method to produce HRT layers with tunable electrical resistivity, despite the presence of high concentrations of unintentionally or intentionally added dopants in the film. This method relies on alloying wide-bandgap, high-k dielectric materials (e.g., ZrO2) into the host oxide to tune the resistivity. We demonstrate SnxZr1−xO2:F films with tunable resistivities varying from 0.001 to 10 Ω cm, which are controlled by the Zr mole fraction in the films. Increasing Zr suppresses carriers by expanding the bandgap almost entirely by shifting the valence-band position, which allows the HRT layers to maintain good conduction-band alignment for a low-resistance front contact.