Crystal Surface Roughness Research Articles

Preparation and characterization of HfO2/VO2/HfO2 sandwich structures with low phase transition temperature, excellent thermochromic properties, and superior durability

In the last few decades, smart windows made from VO2-based thermochromic films have attracted extensive attention, but their actual commercial applications are limited by low luminous transmittance (Tlum), low solar modulation ability (ΔTsol), high phase transition temperature (Tc), and poor durability. In this study, glass/HfO2/VO2/HfO2 tri-layer films were designed and deposited on glass substrates by pulse laser deposition. Crystal structures, surface morphology, surface roughness, electrical properties, and optical properties of as-prepared sandwich structure films were analyzed. Results showed that both HfO2 buffer layer and antireflection layer (ARL) were monoclinic phase and grew along the (020) and (−111) crystal planes, respectively. HfO2 buffer layer not only reduced Tc of VO2 film by about 20 °C, but also played an important role in regulating crystal quality and surface morphology of VO2 films. More importantly, by covering films with HfO2 ARL, Tlum and ΔTsol of VO2 film were greatly improved. In particular, when the thicknesses of HfO2 buffer layer and ARL were 80 nm and 120 nm, the obtained HfO2/VO2/HfO2 tri-layer film reached a balance between high Tlum (∼47.2%), high ΔTsol (∼9.1%) and low Tc (∼49.1 °C). In addition, after 216 h of boiling water treatment, Tlum and ΔTsol of HfO2/VO2/HfO2 film covered with 120 nm thick ARL still remained at 49.3% and 7.0%, showing excellent durability. This research provides a new strategy for designing VO2-based smart windows with high performance and good durability.

The retrieval of snow properties from SLSTR Sentinel-3 – Part 1: Method description and sensitivity study

Abstract. The eXtensible Bremen Aerosol/cloud and surfacE parameters Retrieval (XBAER) algorithm has been designed for the top-of-atmosphere reflectance measured by the Sea and Land Surface Temperature Radiometer (SLSTR) instrument on board Sentinel-3 to derive snow properties: snow grain size (SGS), snow particle shape (SPS) and specific surface area (SSA) under cloud-free conditions. This is the first part of the paper, to describe the retrieval method and the sensitivity study. Nine pre-defined SPSs (aggregate of 8 columns, droxtal, hollow bullet rosette, hollow column, plate, aggregate of 5 plates, aggregate of 10 plates, solid bullet rosette, column) are used to describe the snow optical properties. The optimal SGS and SPS are estimated iteratively utilizing a look-up-table (LUT) approach. The SSA is then calculated using another pre-calculated LUT for the retrieved SGS and SPS. The optical properties (e.g., phase function) of the ice crystals can reproduce the wavelength-dependent and angular-dependent snow reflectance features, compared to laboratory measurements. A comprehensive study to understand the impact of aerosols, SPS, ice crystal surface roughness, cloud contamination, instrument spectral response function, the snow habit mixture model and snow vertical inhomogeneity in the retrieval accuracy of snow properties has been performed based on SCIATRAN radiative transfer simulations. The main findings are (1) snow angular and spectral reflectance features can be described by the predefined ice crystal properties only when both SGS and SPS can be optimally and iteratively obtained; (2) the impact of ice crystal surface roughness on the retrieval results is minor; (3) SGS and SSA show an inverse linear relationship; (4) the retrieval of SSA assuming a non-convex particle shape, compared to a convex particle shape (e.g., sphere), shows larger retrieval results; (5) aerosol/cloud contamination due to unperfected atmospheric correction and cloud screening introduces underestimation of SGS, “inaccurate” SPS and overestimation of SSA; (6) the impact of the instrument spectral response function introduces an overestimation into retrieved SGS, introduces an underestimation into retrieved SSA and has no impact on retrieved SPS; and (7) the investigation, by taking an ice crystal particle size distribution and habit mixture into account, reveals that XBAER-retrieved SGS agrees better with the mean size, rather than with the mode size, for a given particle size distribution.

Evaluation of dual-ended readout GAGG-based DOI-PET detectors with different surface treatments.

Parallax error is a major issue in small animal positron emission tomography (PET) scanners which are used in preclinical studies or detailed scanning of human organs. Several methods have been proposed and investigated to reduce this radial artifact in PET images by estimating the depth of interaction (DOI) of 511-keV photons in the crystal. Among all, the dual-ended readout method seems to be very simple and effective as it does not have any fabrication and readout complications. In the past, some studies suggested that increasing the roughness of crystal lateral surfaces improves DOI resolution. In this paper, this was experimentally examined for four Ce:GAGG crystals with different surface structures. Four 1.2×1.2×20mm3 GAGG crystals with following surface treatment were examined: polished with optical finishing, fine grinding (using a fine surface grinding machine), fine cutting (no treatment), and coarse cutting (no treatment). These crystals were coupled individually to two SiPMs for dual-ended readout and placed in a coincidence detection circuit for electronic collimation of 511keV incidents. The crystals were compared in terms of energy response and DOI estimation capability. DOI function for each crystal was extracted and FWHM DOI resolution was calculated. DOI resolution for the polished crystal varied in the range of 0.54-4.14mm throughout the length of the crystal due to its nonlinear DOI function. The fine grinding crystal showed a linear DOI function within the dynamic range of (-0.75, 0.75), and its DOI resolution varied in the range of 1.24-1.50mm (1.37±0.13mm DOI resolution). The fine-cut crystal had almost a linear DOI function and a wider dynamic range of (-0.85, 0.85) and therefore the best performance with 1.2±0.08mm DOI resolution. However, for the crystal with the roughest surface (coarse-cut crystal), even though the dynamic range expanded to (-0.95, 0.95), its DOI function became nonlinear resulting in 1.24±0.28mm DOI resolution. This means there is an optimum surface roughness to provide the crystal with the best DOI capability. The pulse-height spectrum measured at each depth was used as a measure to compare the energy performance of the four crystals. The photopeak of 511keV was observed for all depths, all crystals. The photopeak position for the coarse-cut crystal had extensive depth dependency which results in poorer energy resolution unless the energy window is calibrated for each depth. This variation of photopeak for the fine-cut and fine grinding crystals was comparable with that of polished crystal. This paper reports 1.2±0.08mm FWHM DOI resolution for a fine-cut unpolished crystal. This resolution is as narrow as the crystal width, resulting in the complete elimination of parallax error in PET images. Results suggest that there is an optimum roughness for the best performance of the dual-ended method and further increase in the roughness, degrades DOI resolution. Thanks to the high light yield of GAGG, the energy performance of the fine-cut crystal is acceptable, and the depth dependency of the spectrum is negligible.

Faceted-rough surface with disassembling of macrosteps in nucleation-limited crystal growth

To clarify whether a surface can be rough with faceted macrosteps that maintain their shape on the surface, crystal surface roughness is studied by a Monte Carlo method for a nucleation-limited crystal-growth process. As a surface model, the restricted solid-on-solid (RSOS) model with point-contact-type step–step attraction (p-RSOS model) is adopted. At equilibrium and at sufficiently low temperatures, the vicinal surface of the p-RSOS model consists of faceted macrosteps with (111) side surfaces and smooth terraces with (001) surfaces (the step-faceting zone). We found that a surface with faceted macrosteps has an approximately self-affine-rough structure on a ‘faceted-rough surface’; the surface width is strongly divergent at the step-disassembling point, which is a characteristic driving force for crystal growth. A ‘faceted-rough surface’ is realized in the region between the step-disassembling point and a crossover point where the single nucleation growth changes to poly-nucleation growth.

Investigation of the trajectory uniformity in water dissolution ultraprecision continuous polishing of large-sized KDP crystal

Large-sized potassium dihydrogen phosphate (KDP) crystals are an irreplaceable nonlinear optical component in an inertial confinement fusion project. Restricted by the size, previous studies have been aimed mainly at the removal principle and surface roughness of small-sized KDP crystals, with less research on flatness. Due to its low surface damage and high machining efficiency, water dissolution ultraprecision continuous polishing (WDUCP) has become a good technique for processing large-sized KDP crystals. In this technique, the trajectory uniformity of water droplets can directly affect the surface quality, such as flatness and roughness. Specifically, uneven trajectory distribution of water droplets on the surface of KDP crystals derived from the mode of motion obviously affects the surface quality. In this study, the material removal mechanism of WDUCP was introduced. A simulation of the trajectory of water droplets on KDP crystals under different eccentricity modes of motion was then performed. Meanwhile, the coefficient of variation (CV) was utilized to evaluate the trajectory uniformity. Furthermore, to verify the reliability of the simulation, some experimental tests were also conducted by employing a large continuous polisher. The results showed that the CV varied from 0.67 to 2.02 under the certain eccentricity mode of motion and varied from 0.48 to 0.65 under the uncertain eccentricity mode of motion. The CV of uncertain eccentricity is always smaller than that of certain eccentricity. Hence, the uniformity of trajectory was better under uncertain eccentricity. Under the mode of motion of uncertain eccentricity, the initial surface texture of the 100 mm × 100 mm × 10 mm KDP crystal did achieve uniform planarization. The surface root mean square roughness was reduced to 2.182 nm, and the flatness was reduced to 22.013 μm. Therefore, the feasibility and validity of WDUCP for large-sized KDP crystal were verified.

Optimization of synthesis conditions of thin Te-doped InSb films and first principles studies of their physicochemical properties

The main objective of our study was to develop an electrochemical synthesis method for obtaining Te-doped InSb thin films. The electrodeposition was performed using a pulse mode in a citrate bath containing 0.06 M InCl3, 0.045 M SbCl3 and 0–0.008 M TeO2. Firstly, the optimization of following parameters was carried out: the potential of pulse “off” (Eoff), duration of pulse “off” (toff), and the ratio of time “on” to time “off” (ton/toff). We have confirmed that appropriate selection of electrodeposition conditions allows obtaining a material with a controllable morphology and chemical composition. Based on performed studies, optimal parameters for electrodeposition of thin Te-doped InSb films were selected. In the next stage, the pulse electrodeposition of Te-doped InSb films at previously optimized conditions was conducted from a solution containing different concentrations of the TeO2 precursor. It was demonstrated that adjustment of the TeO2 content in the citrate bath allows controlling the dopant concentration in thin Te-doped InSb films. Such a controllable synthesis creates the opportunity for tuning physicochemical properties of the material. The effect of doping on the crystal structure, phase composition, surface roughness, and optical band gap of thin Te-doped InSb films was discussed in detail.

Ice crystal characterization in cirrus clouds II: radiometric characterization of HaloCam for the quantitative analysis of halo displays

Abstract. We present a procedure for geometric, spectral, and absolute radiometric characterization of the weather-proof RGB camera HaloCamRAW and demonstrate its application in a case study. This characterization procedure can be generalized to other RGB camera systems with similar field of view. HaloCamRAW is part of the automated halo observation system HaloCam and designed for the quantitative analysis of halo displays. The geometric calibration was performed using a chessboard pattern to estimate camera matrix and distortion coefficients. For the radiometric characterization of HaloCamRAW, the dark signal and vignetting effect were determined to correct the measured signal. Furthermore, the spectral response of the RGB sensor and the linearity of its radiometric response were characterized. The absolute radiometric response was estimated by cross calibrating HaloCamRAW against the completely characterized spectrometer of the Munich Aerosol Cloud Scanner (specMACS). For a typical measurement signal the relative (absolute) radiometric uncertainty amounts to 2.8 % (5.0 %), 2.4 % (5.8 %), and 3.3 % (11.8 %) for the red, green, and blue channel, respectively. The absolute radiometric uncertainty estimate is larger mainly due to the inhomogeneity of the scene used for cross calibration and the absolute radiometric uncertainty of specMACS. Geometric and radiometric characterization of HaloCamRAW were applied to a scene with a 22∘ halo observed on 21 April 2016. The observed radiance distribution and 22∘ halo ratio compared well with radiative transfer simulations assuming a range of ice crystal habits and surface roughness values. This application demonstrates the potential of developing a retrieval method for ice crystal properties, such as ice crystal size, shape, and surface roughness using calibrated HaloCamRAW observations together with radiative transfer simulations.

On the nucleation of polylactide by melt-soluble oxalamide based organic compounds

In this work, a series of oxalamide based organic compounds (OBOC) are synthesized and their capability to enhance the nucleation of polylactide is reported. The OBOCs are soluble in the polylactide melt and crystallize upon cooling, providing surface for heterogeneous nucleation to the polylactide matrix. One interesting observation is that the nucleation efficiency of the OBOCs increases at high cooling rates, making the use of OBOCs as a nucleating agent attractive for industrial processing conditions. The paper addresses the mechanism involved in the cooling rate dependence on the nucleation efficiency of the OBOC: The nucleation efficiency of polylactide is significantly enhanced in the presence of OBOC crystals, as a transcrystalline PLA morphology grows from the OBOC crystal surface (at relatively low supercooling from the equilibrium melting temperature of PLA). However, such crystallization of PLA occurs only when the OBOC crystals are formed at or below 145 °C while cooling at 10 °C/min. In contrast, when the OBOC crystals are formed above 145 °C (i.e. from a lower supersaturated state), the OBOC crystals display a significantly reduced capability for PLA nucleation. These findings override the possibility of both epitaxy and soft-epitaxy as a plausible nucleation mechanism. Supported by polarized optical microscopy, differential scanning calorimetry, plate-plate rheology and molecular modelling we evaluate the possibility of nucleation resulting from local stresses imposed on the polylactide melt invoking stress-enhanced nucleation. The imposed local shear rates, facilitated by the rapid growth of the OBOC crystals, are found high enough to facilitate contour orientation of the high molecular weight PLA chains next to the growing OBOC crystals, confirming the possibility for stress-enhanced nucleation. In addition, we identify surface roughness of OBOC crystals as a second parameter that influences the PLA nucleation process; the OBOC crystallization at high supersaturation is expected to yield smaller/defected OBOC crystals, which are presumed to provide high surface area and surface roughness. In contrast, when the OBOC crystals are grown at lowered supersaturation (>150 °C), they are bound to anneal during crystallization, providing a smoother surface and lower surface area – retrospectively exhibiting a decreased capability to promote the PLA nucleation, irrespective of the PLA supercooling. Interestingly, both the surface roughness of OBOC crystals and the local stresses they impose on the PLA melt increase when the OBOC crystal growth proceeds from a highly supersaturated state, providing an explanation to the cause of the favored crystallization of PLA at the high cooling rates in the presence of the chosen OBOCs.

Experimental and theoretical modeling study on the infrared properties of ZrB2 thin film

Surface oxidation and deterioration of mechanical properties in high-temperature environment have hamperd application of low infrared emissivity materials. Zirconium diboride (ZrB2) exhibits excellent thermal stability and electrical conductivity due to the combination of properties of ceramics and metallic materials, which is promising for application as low infrared emissivity material in high-temperature environment. In this work, we demonstrate the deposition of ZrB2 thin films with different thicknesses, varying from 20 nm to 600 nm, on Si (111) and ZnS substrates by direct current magnetron sputtering method. Infrared emissivity characterization is carried out in the range of 3 to 14 μm, whereas thickness dependence is elaborated in terms of crystal structure, surface roughness and electrical conductivity. Experimental results reveal that normal infrared emissivity of ZrB2 films decreases with an increase in film thickness, from 100 nm to 400 nm, due to influence of substrate, surface roughness and electrical conductivity. The relationship among surface roughness, electrical conductivity and infrared emissivity is theoretically illustrated in detail.

Improved Field Electron Emission Properties of Phosphorus and Nitrogen Co-Doped Nanocrystalline Diamond Films.

Nanocrystalline diamond (NCD) field emitters have attracted significant interest for vacuum microelectronics applications. This work presents an approach to enhance the field electron emission (FEE) properties of NCD films by co-doping phosphorus (P) and nitrogen (N) using microwave plasma-enhanced chemical vapor deposition. While the methane (CH4) and P concentrations are kept constant, the N2 concentration is varied from 0.2% to 2% and supplemented by H2. The composition of the gas mixture is tracked in situ by optical emission spectroscopy. Scanning electron microscopy, atomic force microscopy (AFM), transmission electron microscopy, and Raman spectroscopy are used to provide evidence of the changes in crystal morphology, surface roughness, microstructure, and crystalline quality of the different NCD samples. The FEE results display that the 2% N2 concentration sample had the best FEE properties, viz. the lowest turn-on field value of 14.3 V/µm and the highest current value of 2.7 µA at an applied field of 73.0 V/µm. Conductive AFM studies reveal that the 2% N2 concentration NCD sample showed more emission sites, both from the diamond grains and the grain boundaries surrounding them. While phosphorus doping increased the electrical conductivity of the diamond grains, the incorporation of N2 during growth facilitated the formation of nano-graphitic grain boundary phases that provide conducting pathways for the electrons, thereby improving the FEE properties for the 2% N2 concentrated NCD films.

(Invited) Gap Nanowires with Twinning Superlattices: Structure, Optical Properties and Applications

Semiconductor nanowires (NWs) are a good candidate for future optoelectronic devices. However, the control of the essential parameters that determine the electronic and optical quality of NWs, such as crystal structure and incorporation of impurity dopants, are still challenging problems. Most III-V NWs exhibit crystal defects, which are typically randomly distributed zincblende twinning segments and stacking faults that can affect the optical and electrical properties of NW devices. The incorporation of intentional impurity dopants in NWs is important for the fabrication of p-n junctions and control of the electrical conductivity of NWs. The effect of Te and Be impurity dopant concentration on the crystal structure, surface roughness and optical properties of GaAs NWs will be presented. Four identical GaAs NW arrays were grown: an undoped sample (as a reference) and 6 samples with different Te and Be doping concentration. High resolution transmission electron microscopy (HRTEM) revealed an unusual superlattice twinning, with periodicity that became wider and more regular as the doping level increased. Twin boundaries in GaP are shown to act as an atomically narrow plane of wurtzite phase with a type-I homostructure band alignment. Twin boundaries and stacking faults (wider regions of the wurtzite phase) lead to the introduction of shallow trap states observed in photoluminescence studies. Controlling the surface roughness, the periodicity, and the width of twinning planes with doping concentration might open new possibilities for high efficiency NW-based thermoelectric devices, but also has wide implications for all NW devices.

(Invited) Doping Considerations for Finfet, Gate-All-Around, and Nanosheet Based Devices

The semiconductor industry has been based on Si device scaling for many decades. The motivation for scaling Si is well known, as the reduced dimensions can lead to increased field-effect transistor (FET) device (logic) performance with lower power consumption, thereby enabling a vast array of consumer electronic products. While other materials such as Ge, III-Vs, and 2D materials have threatened to replace Si in FET device channels, it remains at the core of modern digital logic transistors and circuits.Si has been scaled below 10 nm in multi-gate and silicon-on-insulator (SOI) device technologies, but its thickness cannot be reduced indefinitely and larger surface-to-bulk ratios will impact the behaviour of the Si. In general, surface physics cannot be discounted as thickness approaches 5 nm. It is known that effects must be considered at these scales such as carrier surface scattering, dielectric screening, interface states, as well as decreased doping efficiency, increased dopant trapping, and an increase in Si bandgap. If not properly controlled, or at least understood, these factors may inhibit proper semiconductor operation.Damage-free doping processes are desirable as crystal defects or surface roughness can, not only, negatively impact individual transistor device performance, but can also introduce an uncontrolled variability among devices. A conformal process implies access to all surfaces equally in order to, in this case, uniformly and reliably modify the material. Uniformity is necessary as it reduces local resistance variation which again can lead to poor electrical behaviour. As we transition to Gate-All-Around (GAA) device architectures, where all sides of the semiconductor are accessible in theory, having processes that can actually demonstrate access to the bottom surface are becoming increasingly sought after. When dealing with 3D structures the impact of surface effects becomes crucial in determining the active dopant levels. These surface effects lead to problems for dopant incorporation/activation or as gettering points where dopants pile up. Another problem with the greater exposure to the perimeter of the 3D devices is when the doping process etches the available surface, severely damaging or destroying these devices.In this presentation we will consider doping techniques based on ion implant, solid-source in-diffusion, liquid-source in-diffusion, and gas-source in-diffusion. As Si devices scale, the effect of beamline induced crystal damage compounds as a greater percentage of the device area will consist of damaged crystal. Beamline induced surface damage will become a more prominent issue with increasing surface to volume ratios of Si devices. Sputtering of the Si surface by the ion beam is likely to become a more limiting factor once device pitch becomes scaled limiting the acceptable implantation angles. Limitation of the beamline angles will lead to shadowing of the Si devices, and therefore asymmetrical dopant levels. With the advent of GAA devices, line-of-sight implantation methodologies may be unsuitable for the doping of 3D stacked devices.On the other hand in-diffusion techniques suffer due to a surface barrier that must be overcome in order to enter the Si, while ions can always be implanted beyond the surface into the target. Pioneering solutions may exist in the fields of proximity doping and novel surface passivation techniques. Gas-phase doping has a greater potential than liquid or solid phase processes to dope the increasingly complex 3D geometries of Si devices as liquid coverage can be limited by wetting, and solid sources can be limited when fin and nanowire pitches are scaled.

Tuning the mechanical performance efficiently of various LLM-105 based PBXs via bioinspired interfacial reinforcement of polydopamine modification

Inspired by mussel adhesion, the self-polymerization of dopamine was adopted to coat 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) crystals with different crystal characteristics. It has been demonstrated that the crystal quality and surface roughness played a crucial role in mechanical sensitivity and bioinspired interfacial reinforcement of LLM-105 based energetic composites. The high crystal quality brought benefits for the safety performances of polydopamine (PDA) modified LLM-105 crystals, with the drop height of 50% explosion probability (H50) higher than 112.2 cm. After PDA coating for 6 h, the compressive and tensile strength of LLM-105 based polymer bonded explosives (PBXs) could be improved by 27%–52% and 47%–100%, respectively. The enhancing effect in mechanical properties was more pronounced for LLM-105 crystals with smooth surface, due to the physical “interlocking block” at the interface induced by the increase of surface roughness after PDA modification. The overall results indicated that PDA coating had great potential in improving the mechanical properties of different LLM-105 based PBXs while maintaining safety performance.

Effect of annealing atmosphere (CO and N2 gas flow) on surface morphology and crystal quality of AlN buffer layer

In this study, the effect of the annealing atmosphere (N2–CO and pure N2 atmosphere) on the crystal quality, surface morphology, and surface O content of the annealed AlN buffer layers was studied. The FWHM of the (0002) plane of an XRD rocking curve showed that the screw dislocation density of the as-grown AlN film can be reduced by annealing in both N2–CO and pure N2 atmosphere. The AlN film showed an improvement in crystal quality under pure N2 annealing atmosphere. As annealing the AlN buffer layer improves the crystal quality, surface roughness, and surface O content of the AlN film, it should be well controlled for the subsequent growth of III-nitride layers. The surface oxidation that occurred on the AlN films during annealing in N2–CO atmosphere resulted in an increase in surface O content and surface roughness of the AlN films, which is undesirable for the subsequent growth of III-nitride layers. We observed that the surface O content (in the depth of 2-nm) of all annealed AlN films was reduced in the N2 annealing atmosphere, which could be due to the O outgassing from the AlN surface to the pure N2 annealing atmosphere. The O outgassing of the surface O content in the AlN surface layer was related to the volume of space in the annealing setups. However, a large volume of annealing space under pure N2 gas flow resulted in higher thermal decomposition on the annealing AlN films, leading to pits forming on the annealed AlN surface and a smearing of the original step-terrace structure. Therefore, we found that there is an optimal annealing atmosphere space in maintaining the original surface morphology and achieving low surface O content in the annealed AlN film.

Influence of Al Content on Micro-Properties of TiAlSiN Coatings Deposited by Plasma Enhanced Magnetron Sputtering

TiN coating has attracted substantial attention due to its high hardness and good wear resistance. To meet the current more demanding service environment, Al and Si were added to TiN coating to further improve its comprehensive performance in this work. TiAlSiN coatings with different Al content governed by Al target power were prepared on GH169 superalloy by plasma enhanced magnetron sputtering, and the Al content effects on microstructure and mechanical properties of the coatings were explored in detail. The crystal structure, chemical composition, micromorphology and surface roughness of the coatings were analyzed by XRD, SEM, EDS and AFM, respectively. The mechanical properties of the coatings at room temperature including micro-hardness, elastic modulus and bonding strength were evaluated by nano-indentation and nano-scratch, respectively. With the increase of Al target power, Al content in coatings increased, and the preferred orientation of TiN(111) plane was also more and more obvious. At the same time, the surface morphology of the coatings changed from the round cell shape to the hexagonal star shape. The hardness, elastic modulus and adhesion of the coatings increased first and then decreased with the Al content. The TiAlSiN coating with Al content of 19.25% possesses the best comprehensive properties.

A High Count-Rate and Depth-of-Interaction Resolving Single-Layered One-Side Readout Pixelated Scintillator Crystal Array for PET Applications

Organ-specific, targeted Field-of-View (FoV) Positron Emission Tomography (PET)/Magnetic Resonance Imaging (MRI) inserts are viable solutions for a number of imaging tasks where whole-body PET/MRI systems lack the necessary sensitivity and resolution. To meet the required PET detector performance of these systems, high count-rates and effective spatial resolutions on the order of a few mm, a novel two-axis patterned reflector foil pixelated scintillator crystal array design is developed and its proof-of-concept illustrated in-silico with the Monte Carlo radiation transport modelling toolkit Geant4. It is shown that the crystal surface roughness and phased open reflector cross-section patterns could be optimised to maximise either the PET radiation detector's effective spatial resolution, or count rate before event pile up. In addition, it was illustrated that these two parameters had minimal impact on the energy and time resolution of the proposed PET radiation detector design. Finally, it is shown that a PET radiation detector with balance performance could be constructed using ground crystals and phased open reflector cross-section pattern corresponding to the middle of the tested range.

Epitaxial characteristics of MBE-grown ZnTe thin films on GaAs (211)B substrates

Highly crystalline ZnTe thin films were grown on GaAs (211)B substrates by molecular beam epitaxy (MBE) for potential applications such as MCT detectors and optoelectronic devices. We investigated the effects of Te to Zn (VI/II) flux ratio on the quality of ZnTe films in terms of crystal orientation, elemental composition, surface roughness, and dislocation density. Atomic concentrations of Zn, Te, and oxygen complexes due to oxygen contamination on the film surfaces were analyzed by X-ray photoelectron spectroscopy. X-ray double crystal rocking curve full width half maximum (FWHM) of ZnTe (422) peak was observed as 233 arcseconds for a 1.66 μm thick film, which indicates high crystallinity. Wet chemical etching was applied to the films to quantify the crystal quality by calculating etch pit densities (EPD) from scanning electron microscope images. A very low EPD value of 1.7 × 107 cm−2 was measured. Additionally, the root mean square roughness values, obtained from atomic force microscopy topography images were in the range of 10–25 nm. These values were supported by FWHM values of red green blue color intensity histograms obtained from Nomarski Microscope images. The results of our analyses indicate that the VI/II flux ratios of 4 and 4.5 produce the best quality ZnTe films on GaAs (211)B substrates.

Advancements in High Indium Content Alinn Grown Via Metal Modulated Epitaxy and Application Towards Polar/Non-Polar Optical Devices

AlInN has been a topic of recent study due to its unique property among III-nitrides of lattice matching to GaN at a composition of 18% indium, making it a strong candidate for power electronic and optoelectronic applications. A widely unexplored area is the application of AlInN towards visible/Near-IR optical devices. AlInN has a tunable bandgap, spanning the infrared to ultraviolet range, of 0.7 to 6.1eV. At a composition of about 70% indium AlInN has a perfect bandgap, 1.7eV, for tandem solar cells with silicon and for various optical communications applications. Challenges exist with the growth of AlInN as a result of the large lattice parameter mismatch and differences in growth regimes between the two binary components. At the low temperatures required for the growth of AlInN the aluminum adatoms have a low mobility, often leading to lateral phase separation in the film. Metal Modulated Epitaxy (MME) offers a good solution to the growth issues of AlInN. This flux modulated technique allows for growth under metal rich conditions, increasing surface diffusion lengths of the Aluminum while limiting droplet formation and terminating in a dry surface suitable for devices. For comparison ~100 nm thick high indium content AlInN samples were grown using nitrogen rich (0.8 III/V ratio) MBE and MME using a III/V ratio of 1.3 with a dose designed to prevent surface segregation (details supplied at the conference). Both films were grown cold at 375 degrees C to limit phase separation The MME sample showed improvement over the MBE sample in crystal quality and surface roughness, with the XRD (0002) reflection FWHM improving from 783 arcsec to 184 arcsec, the XRD (10-15) reflection FWHM improving from 2456 arcsec to 1421 arcsec, and the AFM surface roughness improving from 1.52nm rms to 0.882nm rms between the MBE and MME samples respectively. The carrier concentrations of both samples were found to be about 1E19 cm-3 via hall measurement implying that like InN, In-rich AlInN may suffer from excessive residual electron concentrations and/or surface fermi-level pinning. Optical measurements will be presented at the conference. The same MME growth conditions were used to grow 100nm of indium rich AlInN directly on a (111) p-silicon substrate. Simple indium contacts were used to contact the Si and AlInN. Vertical conduction and rectification between the silicon and AlInN layers were observed in this large area (~0.25x0.25 cm2) device. Understanding of the limitations of and physics governing this promising vertical polar/non-polar heterojunction will be given at the conference. Additionally, results from various growth conditions will be detailed. Figure 1

Experimental evaluation of depth-encoding absorber designs for prompt-gamma Compton imaging in proton therapy

Prompt-gamma Compton imaging (PGCI) has been presented as a promising in-vivo method for proton range verification. An accurate estimation of the Bragg-peak position can potentially be achieved by imaging high-energy prompt-gamma rays (in the range of several MeV). Therefore, scintillation detectors with thick and high Z crystals are mostly used as Compton absorbers for high-energy gamma-ray detection. However, an absorber using thick crystals degrades the angular resolution unless the absorber can provide continuous depth-of-interaction (DOI) measurement. The study investigated various detector configurations using thick crystals for developing a Compton absorber with DOI resolving capability without compromising energy performance. Two groups of lutetium yttrium oxyorthosilicate (LYSO) arrays with dimensions of 1.8 × 1.8 × 50 mm3 and 1.8 × 1.8 × 20 mm3, and each with four different surface treatments (combining crystal surface finishing and type of reflector coverage), were constructed for the study. The DOI detector utilized the dual-ended readout of pixelated scintillator arrays for depth encoding. The results revealed that the influences of type of the reflector coverage and crystal surface roughness on the performance of the DOI detectors for the 50- and 20-mm-thick LYSO differed greatly. Moreover, the combined use of the proposed partial inter-crystal reflector coverage and the unpolished surface finishing on the 50-mm-thick crystals were shown to improve both the DOI resolution and the energy performance but degraded the flood map quality. These results provide useful guidance for selecting an applicable Compton DOI absorber design for developing a high-performance PGCI system.

Carbon monoxide gas sensing performance of ZnO:Al thin films prepared using different solvent–stabilizer combinations

Carbon monoxide (CO) sensing ability of Al-doped zinc oxide thin films deposited using the sol–gel method was investigated for the first time employing different solvent–stabilizer combinations. The coating solutions for the films were prepared using different solvent–stabilizer combinations of two solvents, ethanol and 2-methoxyethanol and three stabilizers, monoethanolamine, diethanolamine and triethylamine (TEA). Structural, morphological and optical characterizations of the films were performed by X-ray diffraction, atomic force microscopy and UV–Visible spectrophotometer. Varying solvent–stabilizer combinations caused a considerable change in crystal size, surface roughness and transparency of the films. Furthermore, the results certainly unveiled that the combination of solvent and stabilizer significantly affected CO sensing properties of the films. A considerable high responsivity made the film consisting of 2-methoxyethanol-TEA combination highly suitable for CO gas detection.