Rebecca J. Nikolic

6:1 aspect ratio silicon pillar based thermal neutron detector filled with B10

Current helium-3 tube based thermal neutron detectors have shortcomings in achieving simultaneously high efficiency and low voltage while maintaining adequate fieldability performance. By using a three-dimensional silicon p-i-n diode pillar array filled with boron-10 these constraints can be overcome. The fabricated pillar structured detector reported here is composed of 2μm diameter silicon pillars with a 4μm pitch and height of 12μm. A thermal neutron detection efficiency of 7.3+∕−0.6% and a neutron-to-gamma discrimination of 105 at 2V reverse bias were measured for this detector. When scaled to larger aspect ratio, a high efficiency device is possible.

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 of Pillar-structured thermal neutron detectors

Pillar detector is an innovative solid state device structure that leverages advanced semiconductor fabrication technology to produce a device for thermal neutron detection. State-of-the-art thermal neutron detectors have shortcomings in achieving simultaneously high efficiency, low operating voltage while maintaining adequate fieldability performance. By using a 3-dimensional silicon PIN diode pillar array filled with isotopic boron 10, (10B) a high efficiency device is theoretically possible. The fabricated pillar structures reported in this work are composed of 2 mum diameter silicon pillars with a 4 mum pitch and pillar heights of 6 and 12 mum. The pillar detector with a 12 mum height achieved a thermal neutron detection efficiency of 7.3% at 2 V.

Numerical Simulations of Pillar Structured Solid State Thermal Neutron Detector: Efficiency and Gamma Discrimination

This paper reports numerical simulations of a three-dimensionally integrated, Boron-10 (10B) and Silicon p+, intrinsic, n+ (PIN) diode micropillar array for thermal neutron detection. The inter-digitated device structure has a high probability of interaction between the Si PIN pillars and the charged particles (alpha and 7Li) created from the neutron-10B reaction. In this paper, the effect of both the 3-D geometry (including pillar width, separation and height) and energy loss mechanisms are investigated via simulations to predict the neutron detection efficiency and gamma discrimination of this structure. The simulation results are demonstrated to compare well with the experimental data available at this time, for 7- and 12-mum tall micropillar arrays. This indicates that upon scaling the pillar height, a high efficiency thermal neutron detector is possible.

Conformal filling of silicon micropillar platform with b10oron

A recently proposed micropillar semiconductor platform filled with a high volume of isotopic b10oron (B10) has great potential to yield efficient thermal neutron detectors because B10 has a high thermal neutron cross section. Here, the authors report the development of conformal filling of high aspect ratio silicon micropillar platforms with B10 by low pressure chemical vapor deposition (LPCVD) using B10-enriched decaborane (B10H14). The relationships between the pillar structure and the key process parameters including reaction temperature, process pressure, and buffer gas flow rates were investigated to optimize the conformal filling on these structures. Reaction temperature of 420–530 °C, process pressure of 50–450 mTorr, 0.3 SCCM (SCCM denotes cubic centimeter per minute at STP) B10H14 flow rate, and argon buffer gas flow rate of 0–200 SCCM were used to deposit B10 materials into the micropillar structures with aspect ratios of 3:1, 6:1, and 10:1. All three mentioned pillar structures were found to be...

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.

X-ray photoemission analysis of chemically treated GaTe semiconductor surfaces for radiation detector applications

The surface of the layered III-VI chalcogenide semiconductor GaTe was subjected to various chemical treatments commonly used in device fabrication to determine the effect of the resulting microscopic surface composition on transport properties. Various mixtures of H3PO4:H2O2:H2O were accessed and the treated surfaces were allowed to oxidize in air at ambient temperature. High-resolution core-level photoemission measurements were used to evaluate the subsequent chemistry of the chemically treated surfaces. Metal electrodes were created on laminar (cleaved) and nonlaminar (cut and polished) GaTe surfaces followed by chemical surface treatment and the current versus voltage characteristics were measured. The measurements were correlated to understand the effect of surface chemistry on the electronic structure at these surfaces with the goal of minimizing the surface leakage currents for radiation detector devices.

Electrical and optical gain lever effects in InGaAs double quantum-well diode lasers

In multisection laser diodes, the amplitude or frequency modulation (AM or FM) efficiency can be improved using the gain lever effect. To study gain lever, InGaAs double quantum-well (DQW) edge-emitting lasers have been fabricated with integrated passive waveguides and dual sections providing a range of split ratios from 1:1 to 9:1. Both the electrical and the optical gain lever have been examined. An electrical gain lever with greater than 7-dB enhancement of AM efficiency was achieved within the range of appropriate dc biasing currents, but this gain dropped rapidly outside this range. We observed a 4-dB gain in the optical AM efficiency under nonideal biasing conditions. This value agreed with the measured gain for the electrical AM efficiency under similar conditions. We also examined the gain lever effect under large signal modulation for digital logic switching applications. To get a useful gain lever for optical gain quenched logic, a long control section is needed to preserve the gain lever strength and a long interaction length between the input optical signal and the lasing field of the diode must be provided. The gain lever parameter space has been fully characterized and validated against numerical simulations of a semi-3-D hybrid beam propagation method (BPM) model for the coupled electron-photon rate equation. We find that the optical gain lever can be treated using the electrical injection model, once the absorption in the sample is known.

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.