Sigen Wang

Effect of dislocations on charge carrier mobility–lifetime product in synthetic single crystal diamond

The authors report correlations between variations in charge transport of electrons and holes in synthetic single crystal diamond and the presence of nitrogen impurities and dislocations. The spatial distribution of these defects was imaged using their characteristic luminescence emission and compared with maps of carrier drift length measured by ion beam induced charge imaging. The images indicate a reduction of electron and hole mobility–lifetime product due to nitrogen impurities and dislocations. Very good charge transport is achieved in selected regions where the dislocation density is minimal.

Mapping of polarization and detrapping effects in synthetic single crystal chemical vapor deposited diamond by ion beam induced charge imaging

Diamond has been regarded as a promising radiation detector material for use as a solid state ionizing chamber for decades. The parameters degrading the charge transport from what is expected from an ideal crystal are still not completely understood. Recently, synthetic chemical vapor deposited (CVD) single crystal diamond has become available, offering the opportunity to study the properties of synthesized material independent of grain boundaries. We have studied the charge transport of a synthetic single crystal diamond with α-particle induced charge transients as a function of temperature and established the presence of a shallow hole trap with an activation energy of 0.29±0.02eV in some parts of the detector. Ion beam induced charge imaging has been used to study the spatial variations of the charge transport in a synthetic single crystal diamond. Pulses influenced by the shallow hole trap had their origin close to the substrate∕CVD interface of the sample. They could be clearly distinguished from pul...

Pronounced hysteresis and high charge storage stability of single-walled carbon nanotube-based field-effect transistors

In this letter, pronounced hysteresis loops were observed in single-walled carbon nanotube-based field-effect transistors (CNTFETs). The shift in threshold voltage was found to increase with increasing gate voltage sweep ranges. A significant enhancement in the charge storage stability over 14 days was obtained at room temperature after a two-stage hydrogen and air annealing process was applied to the CNTFETs. The passivation of interface traps by annealing in hydrogen and the removal of physisorption solvent molecules by annealing in air are suggested to be responsible for the improvement of the charge storage stability.

Performance improvement of polycrystalline diamond ultraviolet photodetectors by room-temperature plasma treatment

Enhancement of ultraviolet (UV) photoresponsivity in chemical-vapor-deposited (CVD) diamond photodetectors was observed by posttreatment in a plasma of oxygen and carbon tetrafluoride at room temperature. This room-temperature plasma posttreatment was found to be an efficient process in suppressing the extrinsic photoresponse of CVD diamond UV photodetectors in the visible region. Nearly four orders of magnitude difference in the photoresponsivity between the UV and visible light regions were obtained. The results of photoluminescence mapping and Raman spectra indicate that this performance improvement may have resulted from the effective removal of the sp2-bonded carbon impurities and passivation of the silicon-vacancy defects in the diamond.

Temperature-dependent hole detrapping for unprimed polycrystalline chemical vapor deposited diamond

Rise-time distribution spectra of a polycrystalline chemical vapor deposited diamond detector were directly measured from alpha-particle induced pulse shapes over a temperature range of 240–280K. Pulses due to hole-dominated charge transport showed a strong delayed component due to thermal detrapping of charge from a shallow level, with a mean rise time that decreased strongly with increasing temperature. The activation energy of this shallow hole trap was directly measured using an Arrhenius plot, with a value of 0.31±0.03eV. No priming or pre-irradiation of the device was required in order to observe thermal detrapping, indicating that the concentration of shallow hole traps in this sample is relatively high. In contrast, no delayed component was observed from electron transport, indicating that only deep electron-trapping levels are active.

Improvement of Electron Field Emission in Patterned Carbon Nanotubes by High Temperature Hydrogen Plasma Treatment.

In this paper, we report a significant improvement of electron field emission property in patterned carbon nanotubes films by using a high temperature (650 °C) hydrogen plasma treatment. This treatment was found to greatly increase the emission current, emission uniformity and stability. The mechanism study showed that these enhanced properties are attributed to the lowering of the potential barrier and the creation of geometrical features through the removal of amorphous carbon, catalyst particles and the saturation of dangling bonds after such a hydrogen plasma treatment.

SU-E-T-188: An Effective Quality Assurance Method for TomoTherapy Craniospinal Irradiation Patients with A 3D Semiconductor Phantom

PURPOSE This study aims to develop an effective and efficient approach to perform delivery quality assurance (DQA) to any large length spine in Tomo craniospinal irradiation (CSI) with a 3D semiconductor phantom. METHODS It is challenging to use conventional method to perform DQA to an entire large length of spine because the Tomo machine limits the maximum longitudinal distance between red and green laser (=18 cm). In order to overcome this limit, three DQA plans for each CSI patient were generated for covering the entire treatment site, in which we intentionally overlapped the red and green laser in DQA calculation, then we measured the distance (d) between the centers of the phantom and lasers. When using SunNucelar QA software to compute the plannar dose, we set the center of the cylindrical surface by applying d to the IEC Z coordinate. In phantom setup, we first align the center of phantom with green laser then manually shift the phantom superiorly or inferiorly d. With this new approach, we measured five CSI patients using a commercial 3D semiconductor phantom, the ArcCHECK (SunNuclear). The measured and computed dose is compared with gamma analysis and (3%, 3 mm) agreement criteria. RESULTS For all CSI patients and sections measured, the acquired dose distribution covered the complete treatment length and matched the computed data very well. All the DQA passed the gamma criteria with an average passing rate of 99.1%. The total delivery time of DQA using the new method was 66.7% of the time using the conventional method. CONCLUSION The measured DQA results from the five CSI patients demonstrate that this new method overcomes the distance limitation of 18 cm between the red and green laser in the conventional DQA method. The DQA delivery becomes more efficient with 33.3% reduction of beam-on time.

MO‐FF‐A3‐02: In‐Vivo Correspondence Validation of a Deformable Image Registration Method

Purpose: During a course of radiation therapy for the prostate, organs such as the bladder and rectum may change shape significantly from day to day, causing the delivereddose to differ from that planned. Deformable image registration has been used to overlap structures with different shape. In this project, we propose a method to validate the correspondence accuracy of a deformable registration algorithm using images of previously treated prostate patients. Method and Materials: The planning and one daily treatmentCTimages for each of five patients with three Calypso transponders implanted in their prostate were retrospectively selected for the study. Two CT volumes of each patient were pre‐processed to mask the fiducials. A fluid‐based image registration method was used to register images. The displacement fields obtained were applied on the original images with fiducials. The overlap of prostate volumes between the registered images was evaluated by the coincidence index (CI). The centroid positions of each transponder were calculated. The distance between the corresponding transponders was used to quantify the accuracy of correspondence of image registration.Results: The fluid deformable image registration produced satisfactory results of the overlap of the prostate volumes. The mean CI is 95.3%, with 91.5% as minimum and 98.7% as maximum. The average distance between the centroid of all pairs of transponders is 0.35 mm. The displacement is related to the quality of image registration. For image registration with CI 98.7%, all displacement of three pairs of transponders is within 0.22 mm. When less overlap is achieved, CI 91.5%, one pair of transponder is 0.78 mm apart. Conclusion: We have used real patient data to validate the correspondence of the fluid deformable image registration method. For the prostate, it has maintained sub‐millimeter mapping accuracy, which is dependent on image registration quality.

The Fabrication and Performance of CVD Diamond-Based X-Ray Detectors

In this study, X-ray detectors with coplanar metal-semiconductor-metal structure, were fabricated employing high quality chemical vapour deposited (CVD) diamond film grown by a direct current arc jet plasma system. In which the electrical contacts with dimension of 25 µm in width with a 25 µm inter-electrode spacing, were patterned on the growth side of the diamond film using lift-off technology. The performance of the fabricated detectors was evaluated by steady-state X-ray illumination. The photoconductivity of the diamond detectors was found to linearly increase with increase in the X-ray photon flux. This demonstrates that high quality CVD diamond can be used for X-ray detectors.