Nirmalendu Deo

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.

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.

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...

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.

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.

Pillar structured thermal neutron detector

This work describes an innovative solid state device structure that leverages advanced semiconductor fabrication technology to produce an efficient device for thermal neutron detection which we have coined the ¿Pillar Detector¿. State-of-the-art thermal neutron detectors have shortcomings in simultaneously achieving high efficiency, low operating voltage while maintaining adequate fieldability performance. By using a three dimensional silicon PIN diode pillar array filled with isotopic 10boron (10B), a high efficiency device is theoretically possible. Here we review the design considerations for going from a 2-D to 3-D device and discuss the materials trade-offs. The relationship between the geometrical features and efficiency within our 3-D device is investigated by Monte Carlo radiation transport method coupled with finite element drift-diffusion carrier transport simulations. To benchmark our simulations and validate the predicted efficiency scaling, experimental results of a prototype device are illustrated. The fabricated pillar structures reported in this work are composed of 2 ¿m diameter silicon pillars with a 2 ¿m spacing and pillar height of 12 ¿m. The pillar detector with a 12 ¿m height achieved a thermal neutron detection efficiency of 7.3% at a reverse bias of - 2 V.

Analysis of design parameters of holographic solar concentrators for large acceptance angle

This paper presents a detailed analysis of design parameters of holographic solar concentrators for large acceptance angle. It has been shown that by optimizing the design parameters like depth of the film (d), refractive index modulation (n<inf>1</inf>) and fringe spacing (∧) the acceptance angle of holographic-concentrators can be increased appreciably with reasonably good diffraction efficiency over the desired wavelength range. Theoretical analysis revel that angular acceptance of ± 2 degree with 10 percent fall in diffraction efficiency from maximum can be achieved for for n<inf>1</inf> = 0.0954, ∧ = 0.8696 ¼m and d = 4¼m having appreciable diffraction efficiency over the wavelength range of about 0.6 ¼m to 1.5 ¼m. Also, for n<inf>1</inf> = 0.0954, ∧ = 0.8696 ¼m and d = 12¼m we may achieve an angular acceptance of ± 2 degree at 90 percent diffraction efficiency that allows mainly wavelengths of around 0.7 to 0.9 ¼m to diffract through it with high diffraction efficiency.