Photosensitive Area Research Articles

A DOI Detector With Crystal Scatter Identification Capability for High Sensitivity and High Spatial Resolution PET Imaging.

A new phoswich detector is being developed at the Crump Institute, aiming to provide improvements in sensitivity, and spatial resolution for PET. The detector configuration is comprised of two layers of pixelated scintillator crystal arrays, a glass light guide and a light detector. The annihilation photon entrance (top) layer is a 48 × 48 array of 1.01 × 1.01 × 7 mm3 LYSO crystals. The bottom layer is a 32 × 32 array of 1.55 × 1.55 × 9 mm3 BGO crystals. A tapered, multiple-element glass lightguide is used to couple the exit end of the BGO crystal array (52 × 52 mm2) to the photosensitive area of the Position Sensitive Photomultiplier Tube (46 × 46 mm2), allowing the creation of flat panel detectors without gaps between the detector modules. Both simulations and measurements were performed to evaluate the characteristics and benefits of the proposed design. The GATE Monte Carlo simulation indicated that the total fraction of the cross layer crystal scatter (CLCS) events in singles detection mode for this detector geometry is 13.2%. The large majority of these CLCS events (10.1% out of 13.2%) deposit most of their energy in a scintillator layer other than the layer of first interaction. Identification of those CLCS events for rejection or correction may lead to improvements in data quality and imaging performance. Physical measurements with the prototype detector showed that the LYSO, BGO and CLCS events were successfully identified using the delayed charge integration (DCI) technique, with more than 95% of the LYSO and BGO crystal elements clearly resolved. The measured peak-to-valley ratios (PVR) in the flood histograms were 3.5 for LYSO and 2.0 for BGO. For LYSO, the energy resolution ranged from 9.7% to 37.0% full width at half maximum (FWHM), with a mean of 13.4 ± 4.8%. For BGO the energy resolution ranged from 16.0% to 33.9% FWHM, with a mean of 18.6 ± 3.2%. In conclusion, these results demonstrate that the proposed detector is feasible and can potentially lead to a high spatial resolution, high sensitivity and DOI PET system.

Implementation Study of Single Photon Avalanche Diodes (SPAD) in <formula formulatype="inline"><tex Notation="TeX">$0.8~\mu\hbox{m}$</tex> </formula> HV CMOS Technology

Single Photon Avalanche Diodes (SPAD) are known for their excellent timing performance which enables Time of Flight capabilities in positron emission tomography (PET). However, current array architectures juxtapose the SPAD with its ancillary electronics at the expense of a poor fill factor of the SPAD array. The 3D vertical integration of SPADs and readout electronics represents a solution to the aforementioned problem. Compared to systems with external electronics readout, 3D vertical integration reduces the SPAD interconnect parasitic capacitance while greatly increasing the photosensitive area and improving overall performances. This paper presents the implementation of two SPAD structures designed for PET. The SPAD structures are designed using Teledyne DALSA high voltage (HV) CMOS technology targeted for a 3-dimensional single photon counting module (3DSPCM). SPAD with two types of guard ring (diffusion-based and virtual guard ring) are designed, fabricated and characterized. All structures are based on a $p + $ anode in an $n$ -well cathode and are implemented along with active quenching circuits for proper characterization. The results show that the contact distribution and the anode-cathode spacing impact the dark count rate (DCR). The design of SPADs with a diffusion guard ring have a DCR down to $3~\hbox{s}^{- 1}\mu \hbox{m}^{-2}$ at room temperature, afterpulsing probability of $ , timing resolution of 27 ps FWHM and PDE of 49% at 480 nm.

AlGaAs/GaAs photodetectors for detection of luminescent light from scintillators

The AlxGa1-xAs / GaAs photosensitive heterostructures were grown by molecular beam epitaxy. On their basis the discrete photodiodes (PD) and phototransistors (PT) operating in the visible (0.4–0.7 μm) wavelength range and having photosensitive area diameters of 1.5 mm and 180 pm correspondingly were created. The current-voltage and spectral characteristics of discrete PD and PT were measured and analyzed. Distinctive features of the proposed photodetectors are a high monochromasy with the maximal sensitivity of 0.13 A/W at λ = 530-570 nm and a low dark current of 4.7 nA and 530 pA at 5 V reverse bias for PD and PT correspondingly.

Design and optimization of backside illuminated image sensor for epiretinal implants

Backside illuminated pixel structure is proposed and evaluated as the building block for the image sensor being used as epiretinal prosthesis implant. The image sensor pixel is designed with the parameters of 90nm technology node of standard CMOS (Complementary Metal Oxide Semiconductor) process. The image sensor is consisted of a p-sub/n-well structure as the photosensitive area with the pixel pitch of 20μm. The maximum fill factor is observed due to separation of photosensitive area with the readout transistor surface in backside illumination technology. 90% quantum efficiency at 600nm wavelength and the dark current of 74.6nA/cm2 at room temperature is achieved for the optimized pixel. The application of deep backside Deep Trench Isolation (DTI), with high depth n-well doping profiles, results in a significant reduction of crosstalk (5.6% total crosstalk).

Temperature-sensitive junction transformations for mid-wavelength HgCdTe photovoltaic infrared detector arrays by laser beam induced current microscope

In this paper, we report on the disappearance of the photosensitive area extension effect and the unusual temperature dependence of junction transformation for mid-wavelength, n-on-p HgCdTe photovoltaic infrared detector arrays. The n-type region is formed by B+ ion implantation on Hg-vacancy-doped p-type HgCdTe. Junction transformations under different temperatures are visually captured by a laser beam induced current microscope. A physical model of temperature dependence on junction transformation is proposed and demonstrated by using numerical simulations. It is shown that Hg-interstitial diffusion and temperature activated defects jointly lead to the p-n junction transformation dependence on temperature, and the weaker mixed conduction compared with long-wavelength HgCdTe photodiode contributes to the disappearance of the photosensitive area extension effect in mid-wavelength HgCdTe infrared detector arrays.

Photoelectric characteristics of an inverse U-shape buried doping design for crosstalk suppression in pinned photodiodes

A design of an inverse U-shape buried doping in a pinned photodiode (PPD) of CMOS image sensors is proposed for electrical crosstalk suppression between adjacent pixels. The architecture achieves no extra fill factor consumption, and proper built-in electric fields can be established according to the doping gradient created by the injections of the extremely low P-type doping buried regions in the epitaxial layer, causing the excess electrons to easily drift back to the photosensitive area rarely with a diffusion probability; the overall junction capacitance and photosensitive area extensions for a full well capacity (FWC) and internal quantum efficiency (IQE) improving are achieved by the injection of a buried N-type doping. By considering the image lag issue, the process parameters of all the injections have been precisely optimized. Optical simulation results based on the finite difference time domain method show that compared to the conventional PPD, the electrical crosstalk rate of the proposed architecture can be decreased by 60%–80% at an incident wavelength beyond 450 nm, IQE can be clearly improved at an incident wavelength between 400 and 600 nm, and the FWC can be enhanced by 107.5%. Furthermore, the image lag performance is sustained to a perfect low level. The present study provides important guidance on the design of ultra high resolution image sensors.

The Design and Experiment of a Two-Dimensional Position Sensitive Detecting (PSD) Sensor Based on Avalanche Breakdown

A two-dimensional position sensitive detecting sensor (PSD) based on avalanche breakdown is introduced in this paper. The structure of the sensor is designed under the assumption that the breakdown of the PN junction in the sensor occurs at the bottom of the PN junction. The breakdown structure and characteristics of the sensor are simulated by Medici software and the doping structure and process conditions are calculated by Tsuprem4 software. By using COMSOL Multiphysics, we obtained current allocation of the straight and right angle type electrodes, which is corresponding to the optimal structure. In simulation, the root mean square error of the rectangular-shaped electrode and the straight line-shaped electrode are 0.198, 0.145 respectively. Experiment results show that in the 50% photosensitive area with the center as the origin, the rectangular-shaped electrode error is much smaller than a straight line-shaped electrode and fits in to linear relationship better. But the error of the angle the boundary of the electrode is significantly worse than the line-shaped electrode.

Silicon photodetectors integrated with vertical silicon nitride waveguides as image sensor pixels: Fabrication and characterization

The current trend toward image sensors with ever-increasing pixel counts is prompting continual reductions in pixel area, leading to significant cross-talk and efficiency challenges. The realization of image sensor pixels containing waveguides presents a means for addressing these issues. The fabrication of such pixels is however not straightforward. Conventional waveguides employed in integrated optics are horizontal, but waveguides needed for the proposed sensor must be vertical and integrated with photodetectors. Here, the authors describe a fabrication process for vertical silicon nitride waveguides integrated with silicon photodetectors. The authors describe the etching, deposition, and planarization techniques that enable the formation of silicon nitride waveguides embedded in silicon dioxide. They also describe a fabrication process for silicon photodetectors, including a means for ensuring that their photosensitive areas have sizes consistent with those of photodetectors employed in conventional image sensors. In addition, the authors perform optical and electrical characterization of the fabricated devices. The results demonstrate the ability of the fabricated waveguides to guide light onto the photodetectors with high efficiency.

Point spread function and lateral resolution analysis of digital holographic microscopy system

A major issue so far for digital holography is the low lateral resolution. In order to improve the resolution, we deduce the point spread function of the most common and practical digital holographic microscopy system. Based on the derived formulas, the determinant factor of lateral resolution of the system is analyzed. The results show that the optimal resolution of this system is limited by the numerical aperture of microscope objective, and the resolution can be further deteriorated by the recording media parameters and recording distance. By decreasing the recording distance the resolution can be improved dramatically. In addition, by choosing large photosensitive area and small pixel size camera the resolution of the holographic microscopy system can be improved a little. Moreover, the highest resolution and the best image quality can be achieved by image-plane digital holography. Experiment results demonstrate the correctness of the theoretical analysis.

Performance Optimization of InSb Infrared Focal-Plane Arrays with Diffractive Microlenses

In this paper, diffractive microlens arrays are studied to concentrate incident light onto the effective photosensitive area of InSb infrared focal-plane arrays and thus enhance the quantum efficiency and reduce the crosstalk. Four designs of diffractive microlenses are investigated by a phase-matched Fresnel-elements approach. The quantum efficiency and crosstalk of the devices are calculated by using a two-dimensional device simulation with unit cell of 50 μm. Light propagation through the diffractive microlenses is simulated by the finite-difference time-domain method based on a rigorous vector solution of Maxwell’s equations. The results show that the highest quantum efficiency of the device with a diffractive microlens array is about 51.6% and the corresponding crosstalk is 5.06%. The quantum efficiency is 2.1% higher than that of the device with a spherical refractive microlens array.

Design and analysis of a multi-element dual band infrared detector for counter-countermeasure

A dual band multi-element infrared (IR) detector which enables effective target tracking in a complex tactical environment is presented. The photosensitive elements are designed like the stripes of the Lovell optical reticle and arranged close to each other. The thin films of the dual band filter are precisely coated onto an IR transparent substrate, which is then bonded directly onto the surface of photosensitive area with an IR transparent adhesive. The optical system of the detector adopts a spin-scan mode. The filter-detector combination rotates with a mechanical rotor during scanning. The dual band filters temporally encode target location into the incoming radiation signals, and the photosensitive elements then convert the modulated radiation into an electrical signal. The ratio of radiation intensity from two bands can be calculated to distinguish target from IR target-flare mixed signals. Because the pattern of the detector is similar to the Lovell reticle, the detector can also perform spatial filtering to eliminate background noise. In addition, relatively high stability, together with simplicity of the hardware and low cost, indicates that the presented method has a potential application at the field in which precision is not strictly requested.

Implementation of an AlGaN-based solar-blind UV four-quadrant detector

An AlGaN-based front illuminated intrinsically solar-blind ultraviolet four-quadrant Schottky detector was fabricated and characterized. A layered ohmic structure was deposited followed by a multi-step annealing method. Ultraviolet transmissive iridium oxide was used as the Schottky barrier material and formed by a two-step annealing method. Au contacts were deposited on the Schottky contacts and annealed. The detector was mounted onto a commercial chip carrier and wires were epoxy bonded from the ohmic and Au contacts to the carrier strips. The detector had an average ideality factor of 1.97±0.08, a Schottky barrier height of (1.22±0.07)eV, a reverse leakage current density of (2.1±4)nA/cm2, a series resistance of (120±30)Ω and a free carrier concentration of (1.6±0.3)×1018cm−3. Spectral characterization on the photosensitive area of 7.3×10−3cm2 yielded a cut-off wavelength at (275±5)nm (4.59eV to 4.23eV) for each quadrant, corresponding to the absorption edge of a (46±3)% Al content AlGaN-based material. The detector had an average responsivity of (28±2)mA/W and a quantum efficiency of (14±1)% at 250nm. The ultraviolet-to-visible and near-infrared rejection ratio was between 103 and 105 for most of the quadrants. Characterization showed uniformity across the quadrants, proving the detector feasible for implementation in future ultraviolet-sensitive electro-optic devices.

Small Prototype Gamma Spectrometer Using CsI(Tl) Scintillator Coupled to a Solid-State Photomultiplier

Small solid-state photomultipliers (SSPMs) are an alternative scintillator light-detection technology to traditional photomultiplier tubes that offer advantages such as lower bias voltages and insensitivity to magnetic fields. A digital spectrometer using a commercially available SSPM was constructed and characterized at Oregon State University as a prototype for small, highly-mobile, low-power, robust spectroscopy devices. The SSPM has over 19,000 microcells in a photo-sensitive area of 6.32 × 6.32 mm and was coupled to 6 × 6 × 10 mm reflectively-coated CsI(Tl) crystals. The rest of the spectrometer consists of a fast preamplifier and 200 MHz, 12-bit digital pulse processor based around a field-programmable gate array (FPGA). The efficiency, resolution, linearity, and peak-to-Compton ratio of the system were characterized.

Analysis of spatial resolution and pinhole size for single-shot point-diffraction interferometer using in closed-loop adaptive optics

A closed-loop adaptive optics system using a single-shot point-diffraction interferometer (PDI) and a liquid crystal spatial light modulator is presented. Only one CCD camera is required to achieve simultaneous acquisition of four phase-stepped interference patterns, and then the aberrated wavefront is directly reconstructed with a simple algorithm. To reasonably choose or utilize the photosensitive area of a single camera using in this class of single-shot interferometers, the relationship of the spatial resolution and the reconstructed accuracy is analyzed. Additionally, to increase the intensity transmission to the utmost extent and thus improve the visibility of interference patterns especially in the beginning of closed-loop correction, the maximal pinhole size allowed in a closed-loop adaptive optics is studied. Numerical simulation and closed-loop correction experiments are performed.

Detection of electrically formed photosensitive area in Ca-doped BiFeO3 thin films

We report on the visualization of n-p junctions formed by oxygen vacancy movement under the application of an electric field in a Ca-doped BiFeO3 thin film through spatially resolved scanning photocurrent mapping. The photocurrent mapping, in conjunction with the spectroscopic approach, provides clues to local electronic structures and defect levels associated with oxygen vacancies. These observations provide insights into the spatial redistribution of oxygen vacancies in an electric field.

Long wavelength infrared detector based on Type-II InAs / GaSb superlattice

报道了50％截止波长为12．5μm的InAs／GaSbII类超晶格长波红外探测器材料及单元器件．实验采用分子束外延技术在GaSb衬底上生长超品格材料．吸收区结构为15ML（InAs）／7ML（GaSb），器件采用PBIN的多层异质结构以抑制长波器件暗电流．在77K温度下测试了单元器件的电流-电压（I—V）特性，响应光谱和黑体响应．在该温度下，光敏元大小为100μm×100μm的单元探测器RmaxA为2．5Ωcm2，器件的电流响应率为1．29A／W，黑体响应率为2．1×109cmHz1/2／W，11μm处量子效率为14．3％．采用四种暗电流机制对器件反向偏压下的暗电流密度曲线进行了拟合分析，结果表明起主导作用的暗电流机制为产生复合电流．

Synthesis and Properties of Novel Aromatic Azo Compounds from Hydroxyazobenzene and Cyanuric Chloride

AbstractA new series of aromatic azo compounds with one, two, three and five azobenzene units were facilely constructed by dehydrochlorination reaction between p‐hydroxyazobenzene, 4,4′‐dihydroxyazobenzene and cyanuric chloride under very mild conditions. Their solubility, crystallinity, thermostability, UV‐vis absorption properties and photoisomerization behaviors were carefully examined. When the number of azobenzene unit grew, the solubility of the synthesized compounds basically decreased, the crystallinity of the synthesized compounds firstly rose then dropped, while the thermostability and UV‐vis absorption ability of the synthesized compounds greatly improved. All the aromatic azo compounds exhibited typical reversible photoisomerization behaviors and were expected to be applied in photosensitive material areas.

Photodiodes based on InAs/InAsSb/InAsSbP heterostructures with quantum efficiency increased by changing directions of reflected light fluxes

It is shown, using the example of InAs/InAsSb/InAsSbP heterostructures, that the formation of a curvilinear reflecting surface consisting of hemispherical etch pits on the rear side of a photodiode chip leads to an increase in the quantum efficiency of photodiodes by a factor of 1.5–1.7 in the entire mid-IR wave-length interval studied (λ = 3–5 μm). For the obtained photodiodes with a cutoff wavelength of 4.8 μm, a photosensitive area of 0.1 mm2, and a chip area of 0.9 mm2, a monochromatic responsivity at λ = 4.0 μm reached 0.6 A/W, while a dark current at a reverse bias voltage of 0.2 V was within 4–6 A/cm2.

A novel gamma-ray detector with submillimeter resolutions using a monolithic MPPC array with pixelized Ce:LYSO and Ce:GGAG crystals

We have developed a large-area monolithic Multi-Pixel Photon Counter (MPPC) array consisting of 4×4 channels with a three-side buttable package. Each channel has a photosensitive area of 3×3mm2 and 3600 Geiger mode avalanche photodiodes (APDs). For typical operational gain of 7.5×105 at +20°C, gain fluctuation over the entire MPPC device is only ±5.6%, and dark count rates (as measured at the 1p.e. level) amount to ≤400kcps per channel. We first fabricated a gamma-ray camera consisting of the MPPC array with one-to-one coupling to a Ce-doped (Lu,Y)2(SiO4)O (Ce:LYSO) crystal array (4×4 array of 3×3×10mm3 crystals). Energy and time resolutions of 11.5±0.5% (FWHM at 662keV) and 493±22ps were obtained, respectively. When using the charge division resistor network, which compiles signals into four position-encoded analog outputs, the ultimate positional resolution is estimated as 0.19mm in both X and Y directions, while energy resolution of 10.2±0.4% (FWHM) was obtained. Finally, we fabricated submillimeter Ce:LYSO and Ce-doped Gd3Ga3Al2O12 (Ce:GGAG) scintillator matrices each consisting of 1.0×1.0, 0.7×0.7 and 0.5×0.5mm2 pixels, to further improve the spatial resolution. In all types of Ce:LYSO and Ce:GGAG matrices, each crystal was clearly resolved in the position histograms when irradiated by a 137Cs source. The energy resolutions for 662keV gamma-rays for each Ce:LYSO and Ce:GGAG scintillator matrix were ≤14.3%. These results suggest excellent potential for its use as a high spatial medical imaging device, particularly in positron emission tomography (PET).

Temperature dependence on photosensitive area extension in mercury cadmium telluride photodiodes using laser beam induced current

We report on the temperature-dependent extension of n-type inversion regions in mercury cadmium telluride (HgCdTe) photodiodes at low temperatures (87 K) compared to inversion regions at room temperature (300 K). Laser-beam-induced-current (LBIC) measurement techniques are used to obtain the photosensitive area extensions of n-type inversion in HgCdTe photodiodes for typical n+-on-p HgCdTe photovoltaic IR detectors. The effect of temperature on the extension of n-type conversion region is investigated by considering the sign of the LBIC signal. Theoretical results show that the hole concentration decreases in multidoped HgCdTe due to the freeze-out effect as the temperature decreases. Consequently, hole concentration is much lower than electron concentration at 87 K. The n-type inversion region extension is shown to be caused with the p-to-n type conversion.