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Unabated, worldwide trends in CO2 production project growth to > 43-BMT per year over the next two decades. Efficient power electronics are crucial to fully realizing the CO2 mitigating benefits of a worldwide smart grid (~ 18% reduction for the United States alone). Even state-of-the-art SiC high voltage junction devices are inefficient because of slow transition times (~ 0.5-μs) and limited switching rates at high voltage (~ 20-kHz at ≥ 15-kV) resulting from the intrinsically limited charge carrier drift speed (7-cm-s−1). Slow transition times and limited switch rates waste energy through transition loss and hysteresis loss in external magnetic components. Bulk conduction devices, where carriers are generated and controlled nearly simultaneously throughout the device volume, minimize this loss. Such devices are possible using below bandgap excitation of semi-insulating (SI) SiC single crystals. We explored carrier dynamics with a 75-fs single wavelength pump/supercontinuum probe and a modified transient spectroscopy technique and also demonstrated a new class of efficient, high-speed, high-gain, bi-directional, optically-controlled transistor-like power device. At a performance level six times that of existing devices, for the first time we demonstrated prototype operation at multi-10s of kW and 20-kV, 125-kHz in a bulk conduction transistor-like device using direct photon-carrier excitation with below bandgap light.

We demonstrate a novel illumination technique for extrinsic photoconductive switches based on the creation of a total internal reflection trap to increase the optical path length. The optimal geometry is a square substrate illuminated from one of the corners along the diagonal, ensuring that the total internal reflection condition is maintained after any reflection off the device lateral sides. The optical absorption uniformity throughout the device bulk was improved by designing the entry window to have a cylindrical shape and by using a square core optical fiber as an illumination source. The concept is experimentally validated on a vanadium-doped silicon carbide device that shows approximately four-fold improvement in responsivity compared to normal illumination.

This paper presents the design of a SiC-based photoconductive semiconductor switch (PCSS) in which a picosecond laser pulse generates excess free electrons and holes that are rapidly separated by applying a lateral electric field across the switch. Due to the high velocity and low recombination rate of carriers in high-quality, semi-insulating SiC, the PCSS can potentially operate at terahertz frequency at 10's of watts of output power, thereby improving the frequency-power-size tradeoff of high-electron mobility transistors and traveling wave tubes. The key contributions of this paper are as follows. First, we quantify the impact of material properties, doping, traps, laser spot size, and electric field on the transient response of the switch. Second, we develop a new compact model that can describe the performance of the PCSS over broad operating conditions. Excellent agreement of the model against numerical data is demonstrated. Finally, we identify an upper bound on the frequency of operation of the switch and obtain “frequency versus length” and “frequency versus laser spot size” scaling under low optical generation conditions.

This work demonstrates the potential of silicon pillars filled with boron-10 as a sensor technology for a compact and portable neutron multiplicity system. Solid-state, semiconductor based neutron detectors may enable completely new detector form factors, offer an alternate approach to helium-3 based systems, and reduce detector weight and volume requirements. Thirty-two pillar-structured neutron detectors were assembled into a system with an active area of over 20 cm 2 and were used in this work to demonstrate the feasibility of this sensor technology as a potential replacement for helium-3 based gas detectors. Multiplicity measurements were successfully carried out using a californium-252 neutron source , in which the source mass, system efficiency, and die-away time were determined. This demonstration shows that these solid-state detectors could allow for a more compact and portable system that could be used for special nuclear material identification in the field.

Evaluation of the photoresponse in wurtzite GaN photoconductive switches is presented based on kinetic Monte Carlo simulations. The focus is on electron transport physics and assessment of high frequency operation. The roles of GaN band structure, Pauli exclusion, and treatment of internal fields based on the fast multipole method are all comprehensively included. The implementation was validated through comparisons of velocity-field characteristics for GaN with computational results in the literature. Photocurrent widths of less than ∼7 ps for the 1 μm device can be expected, which translates into a 100 GHz upper bound. Photocurrent pulse compression below the laser full width at half maxima at high applied fields are predicted based on the interplay of space-charge effects and the negative differential velocity characteristics of GaN.

Plasma-etched micropillars in 4H-SiC were etched in forming gas (4% H2, 96% N2) at 1500, 1550, and 1600 °C at 2.4 and 9.4 standard liters per min (slm). At 2.4 slm, oxygen from the aluminum oxide components of the tube furnace oxidized the SiC surface, and the pillars etched isotropically. At 9.4 slm, the pillars etched crystallographically at 1500 and 1550 °C, and sharp 4H-SiC needless with tips as narrow as 15 nm were produced. The lateral etch rates of both 4H- and 6H-SiC were measured with the a-plane etching faster than the m-plane at 1500 and 1550 °C. At 1600 °C, the m-plane and a-plane etch at comparable rates. Due to the difference in etch rates between the m-plane and a-plane, convex surfaces tend to produce facets parallel to the a-plane, while concave surfaces produce facets parallel to the m-plane.

Carrier-transport mechanisms are studied in high-purity diamond irradiated with 6 MeV electrons in the dose range of 1012–1016 cm−2 and annealed at different temperatures up to 1450 °C. Lifetimes and diffusion coefficients are extracted using two pump–probe techniques based on free-carrier absorption and transient-grating principles and then correlated with the corresponding defect evolution from spectroscopic measurements. The neutral monovacancy is revealed as the main carrier recombination center in the as-irradiated diamond, providing bipolar carrier lifetimes of a few nanoseconds at the highest irradiation dose. Carrier-capture cross sections are reduced during annealing as vacancies aggregate into divacancies at ≤1000 °C and extended vacancy clusters at 1450 °C.

In this paper, we demonstrate a detector that has a high neutron-to-gamma discrimination of 8.5×105 with a high thermal neutron detection efficiency of 39% when exposed to a high gamma-ray field of 109 photons/cm2s. The detector is based on a silicon pillar structure filled with a neutron converter material (10B) designed to have high thermal neutron detection efficiency. The pillar dimensions are 50 µm pillar height, 2 µm pillar diameter and 2 µm spacing between adjacent pillars.

An optical pump–probe technique was used to detect spatial distribution of carrier lifetimes across the thickness of a high-quality diamond device structure. Two samples with as-received and boron implanted surfaces were compared to assess the role of implantation and related processing on carrier recombination mechanisms. It was found that the two implanted surfaces show very different behaviors despite undergoing the same treatment. At one of the surfaces, carrier lifetimes remained relatively unchanged, indicating surface recombination rates in the 102–103 cm/s range. At the other surface, carrier lifetimes were almost a magnitude lower and correlated with the locally detected nitrogen vacancy defect that was attributed to the elevated concentration of residual nitrogen.

To mymargin evaluate nitrogen-doped diamond as a candidate for Photoconductive Semiconductor Switches (PCSS) triggered in the sub-bandgap visible range, we have fabricated and tested diamond PCSS from a set of diamond grades of varying nitrogen concentration. The nitrogen-doped diamond PCSS have higher responsivity than other diamond optoelectronic devices in the visible range. We present a novel demonstration of sub-bandgap triggered diamond PCSS, which when scaled to higher voltage, can compete with and even surpass performance of existing ultraviolet-triggered diamond PCSS. This work suggests that nitrogen-doped diamond is a promising candidate for high-power, fast optoelectronic devices.

High gain photoconductive switching using Si and GaAs was studied previously for pulsed high voltage switching. A laser is used to generate charge carriers within the material to render the bulk conductive. We have begun the study of photoconductive switching using wide bandgap materials. These materials appear to operate in a non-high gain mode and the on resistance can be directly controlled with the laser intensity over many decades. It is presently believed that the conduction mechanism may be due to (a) excitation of deep states or (b) multi-photon pumping of carriers from the valance band. We present the study of the physics processes and development of a device operating at >20-kV.

Thallium bromide (TlBr) crystals subjected to hydrochloric acid (HCl) chemical treatments have been shown to advantageously affect device performance and longevity in TlBr-based room temperature radiation detectors, yet the exact mechanisms of the improvements remain poorly understood. Here, we investigate the influence of several HCl chemical treatments on device-grade TlBr and describe the changes in the composition and electronic structure of the surface. Composition analysis and depth profiles obtained from secondary ion mass spectrometry (SIMS) identify the extent to which each HCl etch condition affects the detector surface region and forms of a graded TlBr/TlBrCl surface heterojunction. Using a combination of X-ray photoemission spectroscopy (XPS) and hybrid density functional calculations, we are able to determine the valence band offsets, band gaps, and conduction band offsets as a function of Cl content over the entire composition range of . This study establishes a strong correlation between device process conditions, surface chemistry, and electronic structure with the goal of further optimizing the long-term stability and radiation response of TlBr-based detectors.

The Symposium on Radiation Measurements and Applications (SORMA) convened for the third time on the West Coast, May 22–26, 2016, at the Clark Kerr Campus of the University of California, Berkeley, CA, USA. With radiation detectors increasing in number, variety, and societal importance, we are alternating between SORMA (in Ann Arbor, MI, USA) and SORMA West so that the forum will be available every two years.

Random nanotexturing of optics and semiconductors has proven to be a highly effective way to produce high transmittivity, low reflective surfaces. Here, the authors report a one-step technique using SF6 reactive ion etching to produce nanotexturing of SiC. The effect of etching process power and time is optimized to demonstrate sub-1% specular reflectance and below 5% total reflectance over the 400–2000 nm spectral range. The technique is applied to compare measurements of a photoconductive switch with untreated and treated surfaces. Using a white light source emitting in the range 400–750 nm, the authors show that the decrease in specular reflectivity results in a 20% increase in the photocurrent response.

Free carrier absorption spectra are measured along the different polarization directions with respect to the c-axis of 4H-SiC using ultrafast differential transmission spectroscopy. Probing of excited carrier spectra in an undoped material reveals intrinsic resonances within the conduction band. Widths of the detected resonance peaks are shown to be wider than their theoretical estimates and more comparable to the ones observed in a low doped material. Relative strength of the peaks, on the other hand, is shown to be nearly excitation independent in contrast to the doping induced absorption weakening for the same transitions in n-type samples. Free carrier cross sections are extracted from the excitation dependency of the detected spectra and linked to the individual electron and hole contributions in the near infrared range.

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.

CdZnTe-based gamma detectors require a reduction of electronic noise contributed by apparent device and surface leakage current, especially for advanced readout schemes such as the Co-planar Grid or Pixelated Grid. In this work, we describe a combination of surface treatments and amorphous semiconductor layers that result in a reduction of both apparent device and surface leakage current compared to metal contacts on Br:MeOH etched devices. Characterization by scanning electron microscopy (SEM), spectroscopic ellipsometry (SE), x-ray photoelectron spectroscopy (XPS), current-voltage (IV), and pulse height spectra is performed.

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.

Isotopically enriched 10boron for use in pillar-structured neutron detectors was successfully etched in an electron cyclotron resonance (ECR) plasma using SF6-based plasmas. The effects of radio frequency (RF) power, ECR power, gas flow rate, H2 and O2 incorporation into the plasma, and gas mixture ratios were examined. Etch rates up to approximately 1.35 μm/min were realized. In addition, etch morphology was examined, and the final shape of 10boron-coated pillars could be controlled through the etch gas mixture utilized. Selectivity to the underlying Si structure was apparent from scanning electron microscopy (SEM) micrographs of completed etches.

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.

InGaN quantum well structures on GaN epilayers were exposed to 500 keV alpha particles to fluences above 1014 cm2 to probe the relative radiation tolerance of the epilayer and wells. Performance was estimated by the intensity of ion-beam induced luminescence. Two separate types of quantum well structures emitted at 470 and 510 nm prior to irradiation, and only small wavelength shifts were observed even with the highest alpha fluences. Complementary cathodoluminescence experiments showed that luminescence in the quantum wells is strongly influenced by charges injected deep into the GaN epilayer. The 500 keV alpha penetration depth was ~ 1 mum, so that defects were created at a faster rate in GaN compared to InGaN as alpha particles slowed and stopped within a minority carrier diffusion length of the quantum wells. However, the rate of luminescent decay was similar for both materials. Taken together with the cathodoluminescence data, this ion-beam induced luminescence comparison indicates that the quantum well luminescence decay rate is dominated by radiation-induced defects in the GaN epilayer. InGaN quantum wells are then demonstrated to be not more than a factor of ten more radiation sensitive than GaN, and may be substantially less sensitive than this upper bound.

Due to its wide band gap (2.68 eV) and high stopping power, thallium bromide (TlBr) is being investigated as a room-temperature semiconductor gamma-ray spectrometer. When cooled to −20°C, performance of better than 1% FWHM at 662 keV has been observed on 5×5×5 mm3 pixelated TlBr detectors. The room-temperature lifetime (under continuous bias) of thick TlBr detectors operated at high fields (∼2000 V/cm) has been limited to weeks or months due to polarization caused by ionic conduction. Previous work has shown that the degradation process is limited to the surface and that surface preparation techniques can extend the lifetime of thin planar TlBr devices at room temperature. In this work, the lifetime and stability of two large 5×5×5 mm3 pixelated TlBr arrays are presented. Detector performance is compared for different surface preparation techniques and improved stability is observed. Additionally, we use depth-sensing techniques to track the depth-dependent photopeak centroids over time and conclude that once surface degradation effects are mitigated, TlBr performance can improve over time at room temperature. The improvement is likely similar to conditioning at −20°C in which the electric field stabilizes and becomes more uniform.

Degradation of room temperature operation of TlBr radiation detectors with time is thought to be due to electromigration of Tl and Br vacancies within the crystal as well as the metal contacts migrating into the TlBr crystal itself due to electrochemical reactions at the metal/TlBr interface. Scanning Auger electron spectroscopy (AES) in combination with sputter depth profiling was used to investigate the metal contact surface/interfacial structure on TlBr devices. Device-grade TlBr was polished and subjected to a 32% HCl etch to remove surface damage and create a TlBr1-xClx surface layer prior to metal contact deposition. Auger compositional depth profiling results reveal non-equilibrium interfacial diffusion after device operation in both air and N2 at ambient temperature. These results improve our understanding of contact/device degradation versus operating environment for further enhancing radiation detector performance.

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.

TlBr radiation detector operation degrades with time at room temperature and is thought to be due to electromigration of Tl and Br vacancies within the crystal as well as the metal contacts migrating into the TlBr crystal itself due to electrochemical reactions at the metal/TlBr interface. X-ray photoemission spectroscopy (XPS) was used to investigate the metal contact surface/interfacial structure on TlBr devices. Device-grade TlBr was polished and subjected to a 32% HCl etch to remove surface damage prior to Mo or Pt contact deposition. High-resolution photoemission measurements on the Tl 4f, Br 3d, Cl 2p, Mo 3d and Pt 4f core lines were used to evaluate surface chemistry and non-equilibrium interfacial diffusion. Results indicate that anion substitution at the TlBr surface due to the HCl etch forms TlBr 1-x Cl x with consequent formation of a shallow heterojunction. In addition, a reduction of Tl 1+ to Tl 0 is observed at the metal contacts after device operation in both air and N2 at ambient temperature. Understanding contact/device degradation versus operating environment is useful for improving radiation detector performance.

The surface of the layered III–VI chalcogenide semiconductor GaSeTe was treated with (NH 4 ) 2 S at 60 °C to modify the surface chemistry and determine the effect on transport properties. X-ray photoelectron spectroscopy and room temperature photoluminescence were used to examine the surface reactions and effect on surface defect states of the (NH 4 ) 2 S treatment. Metal overlayers were deposited on the (NH 4 ) 2 S treated surfaces and the I–V characteristics were measured. The measurements were correlated to understand the effect of (NH 4 ) 2 S modification of the interfacial electronic structure with the goal of optimizing the metal/GaSeTe interface for radiation detector devices.

Isotopically enriched 10boron films have been successfully etched in an Electron Cyclotron Resonance (ECR) etching tool using CF4 and SF6 based plasmas. Comparisons between the two are made with regard to etch rate, selectivity to the underlying Si device structure, and morphology of the 10boron post-etching. Our present film etching development is expected to be critical for the fabrication of next generation thermal neutron solid state detectors based on 10boron.

Device-grade TlBr was subjected to various chemical treatments used in room temperature radiation detector fabrication to determine the resulting surface composition and electronic structure. Samples of as polished TlBr were treated separately with 2%Br:MeOH, 10%HF, 10%HCl and 96%SOCl 2 solutions. High-resolution photoemission measurements on the valence band electronic structure and Tl 4f, Br 3d, Cl 2p and S 2p core lines were used to evaluate surface chemistry. Results suggest anion substitution at the surface with subsequent shallow heterojunction formation. Surface chemistry and valence band electronic structure were further correlated with the goal of optimizing the long-term stability and radiation response. Keywords: Thallium bromide, photoelectron spectroscopy, radiation detection 1. INTRODUCTION The development of room temperature radiation detectors requires new materials with large band gaps and high atomic number species. Thallium bromide (TlBr) meets these requirements having a wide band gap (2.68 eV), high Z (

In this paper the fabrication and characterization of lead gallium selenide (PbGa2Se4) semiconductor devices are reported. PbGa2Se4 semiconductor crystals were grown by vertical Bridgman method and planar devices were fabricated from cleaved crystals. The resistivity of the samples with chromium gold contacts was measured to be in the 1013 Ω cm range. Pulse height spectra were collected using 241Am, 109Cd, 57CO, and 137Cs gamma-ray sources. A full energy peak was also observed from the Ag x-rays of 109Cd gamma ray source with 250 μm thick PbGa2Se4 planar device.

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.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Development of thermal neutron detectors is critical for a number of homeland security and physics applications. In this work, we describe our efforts towards developing boron-10 oxide (10B2O3) as a thermal neutron conversion material for textured PIN silicon pillar diode platforms. The filling of the textured diode was achieved by first infiltrating the structures with aqueous boric acid, followed by thermal annealing at above the melting point of boron oxide to achieve a high fill and conversion of the materials into boron oxide. The present solution phase processing strategy has the potential to yield amenable and economical manufacturing processes for coating neutron converter materials on detectors. Potential performance of a boron oxide filled detector is also discussed.

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.

This report summarizes key research, development, and technology advancements in Lawrence Livermore National Laboratory's Engineering Directorate for FY2010. These efforts exemplify Engineering's nearly 60-year history of developing and applying the technology innovations needed for the Laboratory's national security missions, and embody Engineering's mission to ''Enable program success today and ensure the Laboratory's vitality tomorrow.'' Leading off the report is a section featuring compelling engineering innovations. These innovations range from advanced hydrogen storage that enables clean vehicles, to new nuclear material detection technologies, to a landmine detection system using ultra-wideband ground-penetrating radar. Many have been recognized with RD all are examples of the forward-looking application of innovative engineering to pressing national problems and challenging customer requirements. Engineering's capability development strategy includes both fundamental research and technology development. Engineering research creates the competencies of the future where discovery-class groundwork is required. Our technology development (or reduction to practice) efforts enable many of the research breakthroughs across the Laboratory to translate from the world of basic research to the national security missions of the Laboratory. This portfolio approach produces new and advanced technological capabilities, and is a unique component of the value proposition of the Lawrence Livermore Laboratory.more » The balance of the report highlights this work in research and technology, organized into thematic technical areas: Computational Engineering; Micro/Nano-Devices and Structures; Measurement Technologies; Engineering Systems for Knowledge Discovery; and Energy Manipulation. Our investments in these areas serve not only known programmatic requirements of today and tomorrow, but also anticipate the breakthrough engineering innovations that will be needed in the future.« less

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 H 2 O 2 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 H 2 O 2 to remove this layer have been shown to display superior performance as compared to unetched detectors followed a period of “field annealing”.

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.

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.

High energy resolution gamma ray detectors based on CdZnTe are needed for a variety of homeland security, nuclear nonproliferation and astronomy applications. The energy resolution of the detectors is dependent on background noise levels. This noise is often the result of leakage current through the devices, which generally operate as resistors. Because of this, increasing the resistivity of the CdZnTe has been one of the primary goals of research in these materials. However, an alternate approach to achieving similar results is to increase the effective resistivity through the use of metal Schottky contacts.[1] Researchers have also demonstrated that amorphous — crystalline semiconductor heterojunctions can improve the blocking behavior contacts for radiation detectors. [2,3] In this work, we investigate the effect of hydrogenated amorphous Si (a-Si:H) layers on oriented CdZnTe on leakage current and the gamma detection response.

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.

This work reports characterization and modeling of a novel three dimensionally integrated silicon p+, intrinsic, n+ (PIN) diode micropillar array for thermal neutron detection.

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.

The layered III-VI semiconductor Gallium Telluride has potential for room temperature gamma ray spectroscopy applications due to its band gap of 1.67 eV, and average atomic number of 45 : 31 (Ga) and 52 (Te). The physical properties of GaTe are highly anisotropic due to covalent bonding within the layer and van der Waals bonding between layers. This work reports the results of surface and bulk processing, surface characterization, and electrical characterization of diodes formed on both the laminar and non-laminar GaTe surfaces. Alpha detection measurements were also performed.

Surface passivation of device-grade radiation detector materials was investigated using x-ray photoelectron spectroscopy in combination with transport property measurements before and after various chemical treatments. Specifically Br-MeOH (2% Br), KOH with NH4F/H2O2 and NH4OH solutions were used to etch, reduce and oxidize the surface of Cd(1-x)ZnxTe semiconductor crystals. Scanning electron microscopy was used to evaluate the resultant microscopic surface morphology. Angle-resolved high-resolution photoemission measurements on the valence band electronic structure and core lines were used to evaluate the surface chemistry of the chemically treated surfaces. Metal overlayers were then deposited on these chemically treated surfaces and the I-V characteristics measured. The measurements were correlated to understand the effect of interface chemistry on the electronic structure at these interfaces with the goal of optimizing the Schottky barrier height for improved radiation detector devices.

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.