Qinghui Shao

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.

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.

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.

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.

Leakage current quenching and lifetime enhancement in 3D pillar structured silicon PIN diodes

Structures containing three dimensional (3D) pillars, wires and tubes have applications in sensors, photodetectors as well as solar cells. We have been developing a thermal neutron detector by utilizing a 3D pillar architecture where the pillars are constructed out of a PIN diode material, and filling this structure with a neutron sensitive material,10B [1, 2]. By optimizing the pillar diameter, spacing and height, neutron detection efficiency > 50% is possible [3, 4]. Fig. 1 shows the schematic of a pillar structured detector. However, the large surface-to-volume ratio resulting from the 3D design can result in a high reverse leakage current, which results in a high noise floor for the device, causing a decrease in detector efficiency. Here we report passivation methods to reduce the leakage current by passivating the sidewall surface by wet chemical treatements and oxidation.

Smooth Bosch Etch for Improved Si Diodes

A modified Bosch process is used to reduce leakage current resulting from surface damage and roughness for high aspect ratio pillars fabricated from Si p-i-n structures. C4F8 is used during both the etch and passivation steps to achieve a scallop-free and vertical structure. A 5× decrease in both the reverse bias leakage current and corresponding improvement in effective carrier density, charge density, depletion width, and minority carrier lifetime are observed using this process, indicating that surface charge states are decreased using this process. This can impact a number of 3-D next-generation devices.

Blue shift of GaAs micropillars strained with silicon nitride

Strain engineering has been shown to induce shifts in the band structure of semiconductors. In this work, we demonstrate a blue shift in the band gap of GaAs micropillars of greater than 50 meV using SiNx. GaAs micropillars were fabricated and conformally coated with highly strained SiNx. The band gap and strain state of the micropillars were measured using room temperature photoluminescence and Raman spectroscopy. The GaAs was shown to be in uniaxial compression, leading to a linear increase in the band gap. Removal of the strained layer resulted in relaxation back to the unstrained state.

Ultradeep electron cyclotron resonance plasma etching of GaN

Ultradeep (≥5 μm) electron cyclotron resonance plasma etching of GaN micropillars was investigated. Parametric studies on the influence of the applied radio-frequency power, chlorine content in a Cl2/Ar etch plasma, and operating pressure on the etch depth, GaN-to-SiO2 selectivity, and surface morphology were performed. Etch depths of >10 μm were achieved over a wide range of parameters. Etch rates and sidewall roughness were found to be most sensitive to variations in RF power and % Cl2 in the etch plasma. Selectivities of >20:1 GaN:SiO2 were achieved under several chemically driven etch conditions where a maximum selectivity of ∼39:1 was obtained using a 100% Cl2 plasma. The etch profile and (0001) surface morphology were significantly influenced by operating pressure and the chlorine content in the plasma. Optimized etch conditions yielded >10 μm tall micropillars with nanometer-scale sidewall roughness, high GaN:SiO2 selectivity, and nearly vertical etch profiles. These results provide a promising rou...

Sintered Cr/Pt and Ni/Au ohmic contacts to B12P2

Icosahedral boron phosphide (B12P2) is a wide-bandgap semiconductor possessing interesting properties such as high hardness, chemical inertness, and the reported ability to self-heal from irradiation by high energy electrons. Here, the authors developed Cr/Pt and Ni/Au ohmic contacts to epitaxially grown B12P2 for materials characterization and electronic device development. Cr/Pt contacts became ohmic after annealing at 700 °C for 30 s with a specific contact resistance of 2 × 10−4 Ω cm2, as measured by the linear transfer length method. Ni/Au contacts were ohmic prior to any annealing, and their minimum specific contact resistance was ∼l–4 × 10−4 Ω cm2 after annealing over the temperature range of 500–800 °C. Rutherford backscattering spectrometry revealed a strong reaction and intermixing between Cr/Pt and B12P2 at 700 °C and a reaction layer between Ni and B12P2 thinner than ∼25 nm at 500 °C.