SunUng Kim

Gamma-Ray Response of Semi-Insulating CdMnTe Crystals

Semi-insulating Cd0.9Mn0.1Te:In crystals are grown by vertical Bridgman method. The segregation coefficient of Mn in CdTe is nearly 1 so that all the CdMnTe samples obtained from one ingot have nearly a same Mn composition. Also sulfur-based passivation effectively prevent the formation of Te-oxide but large amount of Mn exist as a MnO on the CdMnTe surface. The resistivity of CdMnTe samples are low 1010 Omegacm and well resolved 241Am gamma peaks are seen for all detectors. The difference in spectral response can be attributed to the effect of excess Te and conductivity change due to over-compensation induced by indium segregation in CdMnTe. The mobility-lifetime products evaluated from the dependence of peak location on the bias voltage are 1 times 10-3 cm2/V. The higher mobility-lifetime products in our CdMnTe crystals than other previous reports are assumed due to minimization of impurity contents in MnTe by several zone refining process.

Photoluminescence study on the effects of the surface of CdTe by surface passivation

The etching effects on the CdTe surface treated by the different chemical etchants, such as 2% Br-methanol (BM) and 2% Br-20% latic acid in ethylene glycol (BLE) solutions are studied by both photoluminescence (PL) and atomic force microscopy. After etching, the dielectric materials ZnS and CdZnTe are used for the passivation of the CdTe surface. The PL studies of the CdTe surfaces passivated by the dielectrics are carried out, and the results are correlated with the value of the lifetime of the CdTe. The results show that the etching of CdTe surface by BLE solution yields the better surface condition for CdTe. The surface trap states of CdTe are reduced by the CdZnTe and ZnS passivant which results in the increase in the surface lifetime of CdTe.

Improved Carrier-Transport Properties of Passivated Cdmnte Crystals

By analyzing photoconductive decay curves, we compared the surface recombination velocities of semi-insulating CdMnTe:In crystals grown by the vertical Bridgman method with or without surface passivation. Sulfur passivation effectively prevents the formation of a conductive Te oxide layer on the CdMnTe surface and reduces the surface recombination velocities by about one third. We demonstrated, from IR observations of the distribution maps of Te precipitates, that their configuration affects the anomalous photoconductive decay curves and the gamma-ray spectrum in some areas of the CdMnTe crystal. Notably, not only the size but also the spatial configuration of the Te precipitates modulates the carrier-transport properties.

Effects of material growth technique and Mg doping on Er3+ photoluminescence in Er-implanted GaN

Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies have been carried out at 6 K on the ∼1540 nm 4I13/2–4I15/2 emissions of Er3+ in Er-implanted and annealed GaN. These studies revealed the existence of multiple Er3+ centers and associated PL spectra in Er-implanted GaN films grown by metalorganic chemical vapor deposition, hydride vapor phase epitaxy, and molecular beam epitaxy. The results demonstrate that the multiple Er3+PL centers and below-gap defect-related absorption bands by which they are selectively excited are universal features of Er-implanted GaN grown by different techniques. It is suggested that implantation-induced defects common to all the GaN samples are responsible for the Er site distortions that give rise to the distinctive, selectively excited Er3+PL spectra. The investigations of selectively excited Er3+PL and PLE spectra have also been extended to Er-implanted samples of Mg-doped GaN grown by various techniques. In each of these samples, the so-called viol...

Performance of a Digital Gamma-Imaging System Based Upon CdTe-CMOS Sensor and

As a continuation of our digital radiographic sensor R&D, we have developed a digital gamma-imaging system based upon the commercially-available CdTe-CMOS sensor (AJAT, SCAN1000) and the 75Se gamma source (MDS, Gamma Mat@ SE) for our ongoing application of nondestructive testing. Here the sensor has a 750-mum-thick CdTe photoconductor as an efficient radiation converter and a CMOS pixel array having 100times100 mum2 pixel size and 5.41times51.0 mm2 active area, bump-bonded to the photoconductor for signal readout. The source has about 62.8 Ci activity and a physical size of 3.0 mm in diameter. For the first time in this project, we have succeeded in obtaining useful gamma images from the imaging system and evaluated the imaging performance in terms of the resolving power, the line spread function (LSF), the modulation transfer function (MTF), the noise power spectrum (NPS), and the detective quantum efficiency (DQE). For comparison, we also evaluated the image quality by using a microfocus X-ray source (Hamamatsu, L9121-01) having a focal spot size of about 5 mum.

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As a continuation of our digital X-ray imaging sensor R&D, we have developed a cost-effective, portable, digital radiographic system based on the CMOS image sensor coupled with a fiber optic plate (FOP) and a conventional scintillator. The imaging system mainly consists of a commercially available CMOS image sensor of 48times48 mum2 pixel size and 49.2times49.3 mm2 active area (RadEyetrade2 from Rad-icon Imaging Corp.), a FOP bundled with several millions of glass fibers of about 6 mum in diameter, a phosphor screen such as Min-R or Lanex series, a readout IC board and a GUI software we developed, and a battery-operated X-ray generator (20-60 kVp; up to 1 mA). Here the FOP was incorporated into the imaging system to reduce the performance degradation of the CMOS sensor and the readout IC board caused by irradiation, and also to improve image qualities. In this paper, we describe each imaging component of the fully-integrated portable digital radiographic system in detail, and also present its performance analysis with experimental measurements and acquired X-ray images in terms of system response with exposure, contrast-to-noise ratio (CNR), modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE).

Se Source for Nondestructive Testing

In this paper, we report the capacitance-voltage (C-V) properties of metal-insulator-semiconductor (MIS) devices on CdTe/HgCdTe by the metalorganic chemical vapor deposition (MOCVD) and CdZnTe/HgCdTe by thermal evaporation. In MOCVD, CdTe layers are directly grown on HgCdTe using the metal organic sources of DMCd and DiPTe. HgCdTe layers are converted to n-type and the carrier concentration, ND is low 1015 cm-3 after Hg-vacancy annealing at 260 degrees Celsius. In thermal evaporation, CdZnTe passivation layers were deposited on HgCdTe surfaces after the surfaces were etched with 0.5 - 2.0% bromine in methanol solution. To investigate the electrical properties of the MIS devices, the C-V measurement is conducted at 80 K and 1 MHz. C-V curve of MIS devices on CdTe/HgCdTe by MOCVD has shown nearly flat band condition and large hysteresis, which is inferred to result from many defects in CdTe layer induced during Hg-vacancy annealing process. A negative flat band voltage (VFB approximately equals -2 V) and a small hysteresis have been observed for MIS devices on CdZnTe/HgCdTe by thermal evaporation. It is inferred that the negative flat band voltage results from residual Te4+ on the surface after etching with bromine in methanol solution.

Development of a portable digital radiographic system based on FOP-coupled CMOS image sensor and its performance evaluation

We propose a fabrication process using dielectrophoretic (DEP) force for plasmonic devices as a light source. The 100nm wide Au nanowires fabricated by e-beam lithography and lift-off were used to trap 25nm diameter cadmium selenide (CdSe) QDs on its end-facet with DEP force. DEP force was induced around the nanowire using 8 Vpp, 3MHz sine wave. An Electric field of 108 V/m order and electric field gradient of 1015 V/m2 order intensity were calculated with COMSOL multiphysics simulation tool. And the values are enough to induce DEP force for QD trapping. Before the QD manipulations, polystyrene bead was used which is more rigid and influenced by DEP force than QD. Concentration of 10-5% order and approximately 120sec reaction time are considered with polystyrene bead and QD manipulations are accomplished with the conditions. Finally, the QDs were manipulated to the nanowires array and ‘QD on nanowire’ nanostructure was formed as a practical plasmonic device using DEP force.

Electrical properties of MIS devices on CdZnTe/HgCdTe

Abstract A scheme for mixing simulated and real events in a system is proposed. The purpose of the scheme is to build a computer controlled system with none or a part of all the necessary hardware devices. The rest of the devices are simulated. This study first identifies real activities in computer controlled system, performed by either physical or logical processes in the system. Then it devises a way of synchronizing real and simulated events. The usefulness and advantages of the scheme are also discussed.

Nano plasmon device fabrications using dielectrophoretic (DEP) force

We report in-situ growth of MWIR P-on-n HgCdTe on GaAs by Metal Organic Vapor Phase Epitaxy. HgCdTe epi layers were grown by interdiffused multilayer process (IMP). Tris- dimethylarminoarsenic (DMAAs) was used as a precursor for arsenic doping (p-type layer) and isoprophyliodide (IPI) was used for iodine doping (n-type layer). Standard bubbler configuration was used for both precursors. Doping concentration could be controlled accurately in the range of 2 X 1015 to 7 X 1016 cm-3. After growth, HgCdTe layers were annealed in Hg-atmosphere at 415 degrees Celsius and 260 degrees Celsius consecutively. This Hg- annealing is for activating dopants and then reducing Hg- vacancy concentration. The layers doped with iodine in low 1015 cm-3 concentration showed higher Hall mobility than undoped layers. The Hall mobility of iodine doped layers decreased with increasing doping concentration. Secondary ion mass spectroscopy (SIMS) analyses for the iodine-doped layer showed sharp decrease of iodine concentration after IPI precursor being turned off, indicating negligible memory effect and very slow diffusion of iodine during growth and Hg-annealing. SIMS analyses for the arsenic doped layer showed that arsenic diffused by about 1 micrometer during growth and Hg-annealing. These results show that IPI and DMAAs could be used as stable precursors for in-situ growth of HgCdTe heterojunction. A P-on-n structure was grown. The P-layer has x composition of 0.32 and acceptor concentration of 6 X 1016 cm-3. The n-layer has x composition of 0.30 and donor concentration of 2 X 1015 cm-3. SIMS depth profile for the structure shows well-defined regions of doping concentration and alloy composition. After Hg-annealed, P-on-n structures were fabricated into MESA structure diodes. Electron beam evaporated CdZnTe was used as a passivation layer. This MWIR diode had RoA value of about 3 X 104 (Omega) .cm2.