Lars F. Voss

High aspect ratio composite structures with 48.5% thermal neutron detection efficiency

The pillar structured thermal neutron detector is based on the combination of high aspect ratio silicon p-i-n pillars surrounded by the neutron converter material 10B. By etching high aspect ratio pillar structures into silicon, the result is a device that efficiently absorbs the thermal neutron flux by accommodating a large volume fraction of 10B within the silicon pillar array. Here, we report a thermal neutron detection efficiency of 48.5% using a 50 μm pillar array with an aspect ratio of 25:1.

Fabrication Methodology of Enhanced Stability Room Temperature TlBr Gamma Detectors

Thallium bromide (TlBr) is a material of interest for use in room temperature gamma ray detector applications due to is wide bandgap 2.7 eV and high average atomic number (Tl 81, Br 35). Researchers have achieved energy resolutions of 1.3% at 662 keV, demonstrating the potential of this material system. However, these detectors are known to polarize using conventional configurations, limiting their use. While high quality material is a critical starting point for excellent detector performance, we show that the room temperature stability of planar TlBr gamma spectrometers can be significantly enhanced by treatment with both hydrofluoric and hydrochloric acid. By incorporating F or Cl into the surface of TlBr, current instabilities are eliminated and the longer term current of the detectors remains unchanged. In addition the choice of electrode metal is shown to have a dramatic effect on the long term stability of TlBr detector performance 241Am spectra are also shown to be more stable for extended periods; detectors have been held at 4000 V/cm for 50 days with less than 10% degradation in peak centroid position.

Si pillar structured thermal neutron detectors: fabrication challenges and performance expectations

Solid-state thermal neutron detectors are desired to replace 3He tube tube-based technology for the detection of special nuclear materials. 3He tubes have some issues with stability, sensitivity to microphonics and very recently, a shortage of 3He. There are numerous solid-state approaches being investigated that utilize various architectures and material combinations. Our approach is based on the combination of high-aspect-ratio silicon PIN pillars, which are 2 μm wide with a 2 μm separation, arranged in a square matrix, and surrounded by 10B, the neutron converter material. To date, our highest efficiency is ~ 20 % for a pillar height of 26 μm. An efficiency of greater than 50 % is predicted for our device, while maintaining high gamma rejection and low power operation once adequate device scaling is carried out. Estimated required pillar height to meet this goal is ~ 50 μm. The fabrication challenges related to 10B deposition and etching as well as planarization of the three-dimensional structure is discussed.

Nine Element Si-based Pillar Structured Thermal Neutron Detector

Solid state thermal neutron detectors are desirable for replacing the current 3He based technology, which has some limitations arising from stability, sensitivity to microphonics and the recent shortage of 3He. Our approach to designing such solid state detectors is based on the combined use of high aspect ratio silicon PIN pillars surrounded by 10B, the neutron converter material. To date, our highest measured detection efficiency is 20%. An efficiency of greater than 50% is expected while maintaining high gamma rejection, low power operation and fast timing for multiplicity counting for our engineered device architecture. The design of our device structure, progress towards a nine channel system and detector scaling challenges are presented.

Long-term room temperature stability of TlBr gamma detectors

TlBr is a material of interest for use in room temperature gamma ray detector applications due to is wide bandgap 2.7 eV and high average atomic number (Tl 81, Br 35). Researchers have achieved energy resolutions of 1.3 % at 662 keV, demonstrating the potential of this material system. However, these detectors are known to polarize using conventional configurations, limiting their use. Continued improvement of room temperature, high-resolution gamma ray detectors based on TlBr requires further understanding of the degradation mechanisms. While high quality material is a critical starting point for excellent detector performance, we show that the room temperature stability of planar TlBr gamma spectrometers can be significantly enhanced by treatment with both hydrofluoric and hydrochloric acid. By incorporating F or Cl into the surface of TlBr, current instabilities are eliminated and the longer term current of the detectors remains unchanged. 241Am spectra are also shown to be more stable for extended periods; detectors have been held at 2000 V/cm for 52 days with less than 10% degradation in peak centroid position. In addition, evidence for the long term degradation mechanism being related to the contact metal is presented.

Surface processing of TlBr for improved gamma spectroscopy

Planar detectors have been fabricated on 0.5 mm thick TlBr crystals grown by Radiation Monitoring Devices (RMD). The crystals have been characterized by microhardness measurements. A surface damage layer resulting from mechanical polishing has been measured to be approximately 3.7 μm thick. We have removed this layer with H2O2 chemical etching and compared device performance with and without the presence of the surface damage layer and found significant differences in the initial and long term current-voltage behavior and radiation response. Detectors treated with H2O2 to remove this layer have been shown to display superior performance as compared to unetched detectors followed a period of “field annealing”.

Gamma discrimination in pillar structured thermal neutron detectors

Solid-state thermal neutron detectors are desired to replace 3He tube based technology for the detection of special nuclear materials. 3He tubes have some issues with stability, sensitivity to microphonics and very recently, a shortage of 3He. There are numerous solid-state approaches being investigated that utilize various architectures and material combinations. By using the combination of high-aspect-ratio silicon PIN pillars, which are 2 μm wide with a 2 μm separation, arranged in a square matrix, and surrounded by 10B, the neutron converter material, a high efficiency thermal neutron detector is possible. Besides intrinsic neutron detection efficiency, neutron to gamma discrimination is an important figure of merit for unambiguous signal identification. In this work, theoretical calculations and experimental measurements are conducted to determine the effect of structure design of pillar structured thermal neutron detectors including: intrinsic layer thickness, pillar height, substrate doping and incident gamma energy on neutron to gamma discrimination.

Planarization of high aspect ratio p-i-n diode pillar arrays for blanket electrical contacts

Two planarization techniques for high aspect ratio three dimensional pillar structured p-i-n diodes have been developed in order to enable a continuous coating of metal on the top of the structures. The first technique allows for coating of structures with topography through the use of a planarizing photoresist followed by reactive ion etch-back to expose the tops of the pillar structure. The second technique also utilizes photoresist but instead allows for planarization of a structure in which the pillars are filled and coated with a conformal coating by matching the etch rate of the photoresist to the underlying layers. These techniques enable deposition using either sputtering or electron beam evaporation of metal films to allow for electrical contact to the tops of the underlying pillar structure. These processes have potential applications for many devices comprised of three dimensional high aspect ratio structures.

Analysis of strain in dielectric coated three dimensional Si micropillar arrays

Stress induced in [100] oriented Si circular micropillars by coatings of low pressure chemical vapor deposited 10B, SiyNx, and plasma enhanced chemical vapor deposited SiO2 were measured using micro-Raman spectroscopy. Both tensile and compressive strains in the Si micropillars were observed. Exceptionally large stresses were found to exist in some of the measured Si micropillars. The cross-sectional shapes of these structures were shown to be an important factor in correlating their strain concentrations which could fracture the micropillar.

Surface current reduction in (211) oriented Cd0.46Zn0.04Te.50 crystals by Ar bombardment

Cd{sub 0.46}Zn{sub 0.04}Te{sub .50} crystals have been exposed to high density Ar plasmas in order to modify the surface chemistry and control the surface conductivity. X-ray photoelectron spectroscopy (XPS) reveals that this bombardment results in a modified surface atomic ratio, with Cd being preferentially removed compared to Te. In addition, the native oxide is removed and suppressed for an extended period of time. Current-voltage data is analyzed in order to determine the effect on surface leakage current after exposure. It is found that surface leakage current can be decreased by approximately 2.5 orders of magnitude following Ar{sup +} bombardment.