Mebougna Drabo

Study of surface passivation and contact deposition techniques in CdZnTe X-Ray and gamma-ray detectors

Passivation and improved contact deposition techniques are known to reduce the surface leakage current and decrease noise levels of CdZnTe X-Ray and gamrna-ray detectors by improving their spectral energy resolutions. This paper presents a comparative study of surface passivation process in CdZnTe X-Ray and gamma-ray detectors. The experimental study compares three surface passivation processes: mechanically polished with a O.9μm Alumina Powder (AI₂O₃) finishing; mechanically polished with a O.9μm Alumina Powder (AI₂O₃) and etched with HBr + H₂O₂ solution for 2 minutes; mechanically polished with a O.9,.m Alumina Powder (AI₂O₃) and chemomechanically polished with bromine-methanol-ethylene glycol solution. The results show that chemo-mechanical polishing with bromine-methanol-ethylene glycol solution proved to be the best method out of the three for reducing surface leakage current. The preliminary results of aging studies on H₂O₂ (Hydrogen Peroxide), NH4F (Ammonium Fluoride) + H₂O₂ + H₂O and (NH4hS (Ammonium Sulfide) passivation agents are also presented.

Study of the diffusion of Te inclusions in CdZnTe nuclear detectors in post-growth annealing

Despite immense endeavor invested in optimizing the crystal growth parameters and the post growth improvement methodologies proposed by numerous studies, there are still unresolved shortcomings of CdZnTe crystals to produce commercial-grade CdZnTe detectors. Post-growth thermal annealing under Zn, Te, or Cd vapor overpressure at various temperature have been the approach attempted to improve the crystallinity of CdZnTe crystals. This paper presents results of post growth annealing of CdZnTe detectors that shows both reduction in the sizes of Te inclusions and the migration of the inclusions towards the high-temperature side of the crystal. Two set of annealing experiments were made. The first is annealing under Cd vapor overpressure in vacuum at 600 °C for 45 minutes at a temperature gradient of 10 °C/cm. The second set of CdZnTe post-growth annealing experiments was carried out at 700 °C CdZnTe annealing temperature with the Cadmium temperature at 650 °C, 30 minutes annealing time, and temperature gradient of 10 °C/cm. The reduction in the sizes of Te inclusions ranges from 8% to 38%.

Measurement and Analysis of Unsteady Flows in IC Engines

Unsteady velocity measurements made near the spark-plug location of an IC engine using a single-component LDV fiber-optic spark-plug LDV probe are reported. Measurements were made within a motored Briggs and Stratton single-cylinder air-cooled engine at ~780 RPM. Data obtained was analyzed using Ensemble averaging, High-pass filtering, Wavelet Decomposition (WD) techniques to determine the unsteady mean velocity and unsteady turbulence variations in such an engine. The results of the analysis are compared to each other. Cycle-to-cycle variations of the mean velocity and fluctuating velocity are investigated.

Surface passivation and contacts in CdZnTe X-rays and gamma-rays detectors

Surface passivation and contact deposition are important processes in the fabrication of cadmium zinc telluride (CdZnTe) X-rays and gamma-rays detectors. Surface passivation is used to reduce the effects of fabrication-induced surface defects. It is also used to reduce defects that could result from aging of the detector. Another important factor in charge collection is the interface between the electrical contacts and the detector wafer. In this paper, we present the results of the effects of chemo-mechanically polishing CdZnTe wafers with a hydrogen bromide in hydrogen peroxide solution followed by passivation in a mixture of potassium hydroxide in hydrogen peroxide solution by monitoring their energy resolutions over a 4-day period. The results showed that the 59.5-keV gamma peak of Am-241 remains under the same channel over the 4 days.

Batch annealing of CdMnTe wafers for nuclear detector applications

Post-growth annealing is used to eliminate performance-limiting Te inclusions and related defects from cadmium manganese telluride (CdMnTe) wafers for nuclear detector applications. These defects could cause problems in applications that require large-volume detectors, where they trap charges that are generated by nuclear radiation. The trapping of charges degrades the charge transport properties and reduce their carrier lifetime, thereby limiting the overall performance for the detector. We have carried out batch annealing of three CdMnTe wafers in different ampoules at the same temperature of 720 °C using a three-zone furnace. The experiment compared annealing in tellurium (Te) vapor, cadmium (Cd) vapor, and a mixture of cadmium and tellurium (Cd-Te) vapors. For the samples annealed in Te and Cd-Te vapors, it appears that Te deposited on their surfaces. This limited the imaging of the inside of the wafer since Te is opaque to infrared light. The infrared imaging of the matrix of the wafer annealed in Cd showed that the sizes of tellurium inclusions were reduced.

Unsteady Flow Analysis in a Fired Briggs-Stratton Internal Combustion Engine

Unsteady velocity measurements made near the spark-plug location of a fired Briggs and Stratton IC engine using a single-component LDV fiber-optic spark-plug LDV probe are reported. An off-the shelf engine was first equipped with an IMPCO carburetor adapter to allow the engine run using methane gas. Velocity data obtained during the engine start and the engine run were analyzed using high-pass filtering, wavelet decomposition, proper orthogonal decomposition, and wavelet decomposition together with proper orthogonal decomposition techniques to separate the mean and fluctuating velocity components. Data was further analyzed to determine the standard deviation of the fluctuating velocity signal and the cross correlation between the pressure and velocity signals. Results show that mean velocity delayed by 0.034 seconds show a high correlation with the pressure signal.