Jeff Beeman

Dopant and self-diffusion in extrinsic n-type silicon isotopically controlled heterostructures

We present experimental results of dopant- and self-diffusion in extrinsic silicon doped with As. Multilayers of isotopically controlled {sup 28}Si and natural silicon enable simultaneous analysis of {sup 30}Si diffusion into the {sup 28}Si enriched layers and dopant diffusion throughout the multilayer structure. In order to suppress transient enhanced self- and dopant diffusion caused by ion implantation, we adopted a special approach to dopant introduction. First, an amorphous 250-nm thick Si layer was deposited on top of the Si isotope heterostructure. Then the dopant ions were implanted to a depth such that all the radiation damage resided inside this amorphous cap layer. These samples were annealed for various times and temperatures to study the impact of As diffusion and doping on Si self-diffusion. The Si self-diffusion coefficient and the dopant diffusivity for various extrinsic n-type conditions were determined over a wide temperature range. We observed increased diffusivities that we attribute to the increase in the concentration of the native defect promoting the diffusion.

NTD-GE-based microcalorimeter performance

Abstract Our group has been developing X-ray microcalorimeters consisting of neutron transmutation-doped (NTD) germanium thermistors attached to superconducting tin absorbers. We discuss the performance of single pixel X-ray detectors, and describe an array technology.

Preliminary test results of a low-noise 2x16 Ge:Sb array with a CTIA readout

Germanium detectors are extensively used in astronomical instruments for far infrared observations. To meet the science objectives of future space projects, large-format far IR detectors are needed. As a first step toward this goal, we have fabricated a 2x16 Ge:Sb array with the 1x32, SB-190 CTIA cryogenic readout. The detector design as well as the preliminary results of our parametric tests are presented here. The array exhibits very good noise performance with an NEP as low as 4.0E-18 W/√Hz, and confirms the viability of the design for large format arrays.

The first look at SB349, a 32x32, CTIA readout multiplexer for far IR focal-plane arrays

Development of two-dimensional cryogenic readouts suitable for far infrared and submillimeter detectors is a key step in fabrication of large format far infrared focal-plane arrays. In collaboration with Raytheon Vision Systems, we have designed and fabricated the first 32x32 readout multiplexer, SB349, capable of operating at cryogenic temperatures as low as 1.7K. This readout is a capacitive-transimpedance amplifier multiplexed to eight outputs and is buttable on two sides to form a 64x64 mosaic array. It features eight selectable gain settings, AC coupling (auto zero) for better input uniformity, sample-and-hold circuitry, and provisions to block the readout glow. A special, 2-micron cryo-CMOS process has been adopted to prevent freeze out and ensure low noise and proper operation at deep cryogenic temperatures. An overview of the design and the results of the initial functionality tests on this device are reported in this paper.

EBIT diagnostics using X-ray spectra of highly ionized Ne

Abstract We have carried out a detailed analysis of highly ionized neon spectra collected at the NIST EBIT using an NTD germanium X-ray microcalorimeter developed at the Harvard–Smithsonian Center for Astrophysics [Nucl. Instr. and Meth. A 444 (2000) 156]. Our attention was focused especially on the Ne IX He-like triplet to check electron density diagnostics through the intercombination/forbidden line ratio. We have investigated possible effects of the ion dynamics on the plasma emission line intensities, looking at the dependence of the count-rate and the charge state distribution on the electron beam energy and current. The temperature and spatial distribution of the neon ions, and hence the overlap between the electron beam and the ion cloud, depend on the electron beam operating parameters. The overlap affects the average electron density seen by the ions, and in turn the measured line ratio. These results underscore the value of future improved studies of the trapped ion dynamics, both for understanding the EBIT performance and for allowing experimenters to take full advantage of its potential for astrophysical plasma diagnostics.