Stephen U. Egarievwe

Interferometric studies of coherence in a powder laser

Abstract Longitudinal and transversal coherence in powder laser emission was studied using two different interferometric techniques. It has been shown that longitudinal coherence length in Nd0.5La0.5Al3(BO3)4 powder (ceramics) laser emission corresponds to 0.7 A linewidth. This value is in agreement with the results of direct spectroscopic linewidth measurements published in the literature. The transversal coherence has not been shown to be noticeable when the distance between two points on the emitting surface is equal to ≈85 μm. The possible explanation of the result obtained is discussed.

Study of the pumping regimes in Ti-sapphire and Nd0.5La0.5Al3(BO3)4 powders

Abstract Stimulated emission without a cavity was obtained (at λ=800 nm) in powders of Ti-sapphire laser crystal and compared to that in Nd0.5La0.5Al3(BO3)4. The formation of a narrow channel in a powder sample by a pumping laser beam was found to be advantageous for stimulated emission in Ti-sapphire and disadvantageous in Nd0.5La0.5Al3(BO3)4. The effect of the material volume density on stimulated emission in scattering Nd0.5La0.5Al3(BO3)4 was experimentally studied. The experimental results are explained in terms of absorption of pumping light in scattering materials, penetration depth for pumping, and residence time for emission photon in the pumped volume.

Chemical treatment of CdZnTe radiation detectors using hydrogen bromide and ammonium-based solutions

Surface damages occur in Cadmium zinc telluride (CdZnTe) wafers for radiation detection devices during dicing and polishing. This often results in increased leakage current that limits the performance of the detector. An effective method of removing the surface damage and thus reducing the leakage current is through the use of chemical treatments. The effects discussed in this study include: chemical polishing with a mixture of hydrogen bromide solution followed by passivation with ammonium fluoride in a hydrogen peroxide solution. The effects on the current-voltage measurements and the spectral response were monitored over a 2-week period. X-ray photoelectron spectroscopy (XPS) was also obtained to observe the formation of chemical species on treated surfaces. The resistivity of the treated CdZnTe samples is on the order of 1010 ohm-cm. The current in the I-V measurements increased rapidly immediately following the chemical polishing and surface passivation, and decreased steadily afterwards. The spectral response showed that the 59.5-keV peak of Am-241 was stable in the same position over the test period.

Study of light propagation in scattering powder laser materials

Abstract The penetration and reflection of light in powders of Nd0.5La0.5Al3(BO3)4 and Ti-sapphire laser crystals have been experimentally studied. The experimental results have been satisfactorily described in terms of a two flux model. The absorption, scattering, and reflection parameters determining the light propagation have been determined.

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.

Carbon Coating and Defects in CdZnTe and CdMnTe Nuclear Detectors

CADMIUM zinc telluride (CdZnTe) and cadmium manganese telluride (CdMnTe) are prime materials for detecting X-rays and gamma-rays at room temperature due to their high average atomic numbers that are essential to having high stopping-power for incident high-energy electromagnetic radiations. A major obstacle in developing CdZnTe and CdMnTe detectors lies in growing crystals free from defects, such as Te inclusions, dislocations, sub-grain boundary networks, and precipitates. We present the results of our study of the relationship between carbon coating of the growth ampoule and dislocations in CdZnTe and sub-grain boundary networks in CdMnTe, grown by Bridgman method. For the CdZnTe crystals, a carbon-coating of 2 μm on the ampoule generated fewer dislocations than did a thinner 0.2 - μm carbon-coated one. Furthermore, the ampoules design (normal- or tapered-shape) did not affect the densities of etch pits as much as did the thickness of the carbon-coating. For a CdMnTe ingot with a carbon coating of about 2 μm, created by cracking spectroscopic-grade acetone at ~ 900° C, we observed very few grain boundaries and grain-boundary networks.

Array of virtual Frisch-grid CZT detectors with common cathode readout and pulse-height correction

We present our new results from testing 15-mm-long virtual Frisch-grid CdZnTe detectors with a common-cathode readout for correcting pulse-height distortions. The array employs parallelepiped-shaped CdZnTe (CZT) detectors of a large geometrical aspect ratio, with two planar contacts on the top and bottom surfaces (anode and cathode) and an additional shielding electrode on the crystals sides to create the virtual Frisch-grid effect. We optimized the geometry of the device and improved its spectral response. We found that reducing to 5 mm the length of the shielding electrode placed next to the anode had no adverse effects on the devices performance. At the same time, this allowed corrections for electron loss by reading the cathode signals to obtain depth information.

Effects of Chemical Treatments on CdZnTe X-Ray and Gamma-Ray Detectors

Room-temperature semiconductor detectors, such as cadmium zinc telluride (CdZnTe), often are subjected to surface damage during fabrication, thus reducing their performance in detecting X-rays and gamma-rays. In this study, we compared two surface-passivation chemical solutions: Ammonium fluoride in hydrogen peroxide (NH₄F + H₂O₂ + H₂O) and potassium hydroxide in hydrogen peroxide (0.1 g of KOH + 10 ml of 30% H₂O₂). X-ray photoelectron spectroscopic analysis showed that the NH₄F-based solution is more effective at converting Te species on the CdZnTe surfaces into a more stable TeO₂ layer, attaining values of 4.90 and 5.34 for the Te3d_3/2O₂/Te3d_3/2 and Te3d_5/2O₂/Te3d_5/2 peak-height ratios respectively, compared to the KOH-based solution with 1.25 and 1.19, respectively. The current-voltage measurements showed an increase in the bulk leakage current for freshly passivated samples compared to those of mechanically polished samples. However, within a period of about three to 14 days, their leakage currents reduced to values in the range of the mechanically polished samples. The resistivity of the CdZnTe samples is on the order of 1010 Ω-cm. The NH₄F-based chemical contributed less to the leakage current. Its leakage current at 60 V is 6.3 times that of the mechanically polished sample, compared to 30.5 for the sample passivated with the KOH-based solution. Analysis of the 59.5-keV peak of Am-241 showed that the sample passivated with the NH₄F-based solution has a better energy resolution compared to the one passivated with the KOH-based solution.

Thermal Annealing of CdMnTe Material Being Developed for Nuclear Radiation Detection Applications

Cadmium manganese telluride (CdMnTe) is one of the semiconductor materials with potential applications at room-temperature for nuclear and radiological detection. CdMnTe crystals grown by Bridgman technique are prone to tellurium inclusions and related defects that limit their performance as X-rays and gamma-rays detectors. The major reason for this is that they are grown in a tellurium-rich environment. These defects could trap charges that are generated by X-rays and gamma rays thereby degrading the charge transport properties of the detectors and reducing their carrier lifetime. This in turn leads to poor performance by the detector. One of the solutions to this problem is post-growth thermal annealing. In this paper we present experimental results of annealing a CdMnTe wafer at 720 o C and in cadmium vapor. The CdMnTe wafer and cadmium were sealed in a quartz ampoule at a vacuum of 10 -5 mbar. We used a three-zone furnace that enabled us to adjust the three heating elements to get a flat region of 720 o C in the temperature profile where the wafer was annealed. Infrared transmission microscopy showed changes to the sizes and positions of the tellurium inclusions. There are reductions in the dimensions of the medium-size Te inclusions. Some Te inclusions were completely eliminated while others broke up to form much smaller inclusions. Current-voltage measurements showed that the resistivity of the CdMnTe wafer was reduced by 71 %, from 2.44 x 10 5 -cm to 7.17 x 10 4 -cm

Comparative study on the effects of chemical treatments on CdZnTe nuclear detectors

Room temperature semiconductor detectors such as cadmium zinc telluride (CdZnTe) are often subject to surface damage during fabrication processes, thus affecting detector performance. The surface defects are usually removed through mechanical and chemical polishing, and passivation processes. This paper compares the effects of two surface passivation chemical solutions on CdZnTe detectors. The two chemicals studied are ammonium fluoride in hydrogen peroxide (NH₄F + H₂O₂ + H₂O) and potassium hydroxide in hydrogen peroxide (0.1 g of KOH + 10 ml of 30% H₂O₂) solutions. X-ray photoelectron spectroscopy analysis showed that the NH₄F-based solution is more effective at converting Te species on the CZT wafer surfaces into a more stable TeO₂ layer, with values of 4.90 and 5.34 for the Te₃d_3/2O₂/Te₃d_3/2 and Te₃d_5/2O₂/Te₃d_5/2 peak-height ratios respectively, compared to the KOH-based solution which has 1.25 and 1.19 respectively. Analysis of the 59.5-keV peak of Am-241 showed that the sample passivated with the NH₄F-based solution has a better energy resolution (FWHM = 9.83%) compared to the one passivated with the KOH-based solution (FWHM = 14.60%).