Charge transport properties of as-grown CZT by traveling heater method

In this present work we have grown Cd_0.9Zn_0.1Te doped with indium by the traveling heater method (THM) technique. Large 2 in diameter CZT ingots of more than 1 kg each were successfully grown by the THM technique in vertical configuration. In order to evaluate our as-grown CZT samples, charge transport characteristics were studied at and below room temperature. The key parameter investigated for as-grown CZT samples was the mobility-trapping time product and its temperature variation. Mobility-trapping time values as high as 9×10⁻³ cm²/V at 30 °C were measured for samples exhibiting resistivities in the 1-2×10¹⁰ Ω cm range. The as-grown samples showed moderately good resolution of 1.5-3.5% at 662 keV when fabricated. The variation of the internal electric field along the depth of the detector was studied for as-grown material to evaluate deformations inside the crystal due to the presence of residual stress or other defects.     

Extended deep level defects in Ge-condensed SiGe-on-Insulator structures fabricated using proton and helium implantations

SiGe-on-Insulator (SGOI) structures were created using the Ge condensation method, where an oxidation process is performed on the SiGe/Si structure. This method involves rapid thermal chemical vapor deposition and H⁺/He⁺ ion-implantations. Deep level defects in these structures were investigated using deep level transient spectroscopy (DLTS) by varying the pulse injection time. According to the DLTS measurements, a deep level defect induced during the Ge condensation process was found at 0.28 eV above the valence band with a capture cross section of 2.67 × 10^− 17 cm², two extended deep levels were also found at 0.54 eV and 0.42 eV above the valence band with capture cross sections of 3.17 × 10^− 14 cm² and 0.96 × 10^− 15 cm², respectively. In the SGOI samples with ion-implantation, the densities of the newly generated defects as well as the existing defects were decreased effectively. Furthermore, the Coulomb barrier heights of the extended deep level defects were drastically reduced. Thus, we suggest that the Ge condensation method with H⁺ ion implantation could reduce deep level defects generated from the condensation and control the electrical properties of the condensed SiGe layers.     

Influence of the diamond grain size on the electrical properties of nano-crystalline diamond film detectors

In this work, we developed X-ray radiation detectors with sandwich structure fabricated from nano-crystalline diamond (NCD) films. These NCD films with different grain size ranging from 15 nm to 160 nm were grown on silicon substrates using a hot-filament chemical vapor deposition technique. I-V measurement results indicate that with reducing of the grain size, the resistivity of diamond films decreases from 9.5 × 10⁸ to 6.20 × 10⁷ Ω cm and the ratio of the photocurrent to the dark-current (I_ph/I_d) of the detectors decreases rapidly from 0.45 to 0.09 at an electric field of 50 kV/cm. Typical spectral response to 5.9 keV ⁵⁵Fe X-rays shows that counting efficiency and energy resolution of NCD detectors with large grains are better than those of detectors with small grains, due to the less defects and grain-boundaries contained in the film.     

Development of an energy-binned photon-counting detector for X-ray and gamma-ray imaging

The aim of this study is to develop an energy-binned photon-counting (EBPC) detector that enables us to provide energy information of x-rays with a reasonable count statistics. We used Al-pixel/CdTe/Pt semiconductor detectors, which had an active area of 8 mm×144 mm and consisted of 18 modules aligned linearly. The size of a CdTe detector module was 8 mm×8 mm and the thickness of the CdTe crystal was 1 mm. Each module consisted of 40×40 pixels and the pixel size was 200 μm×200 μm. We applied the bias voltage of −500 V to the Pt common electrode. The detector counted the number of x-ray photons with four different energy windows, and output four energy-binned images with pixel depths of 12, 12, 11 and 10 bits at a frame rate of 1200 Hz (300 Hz×4 energy bins). The basic performance of the detector was evaluated in several experiments. The results showed that the detector realized the photon counting rate of 0.4×10⁶ counts/sec/pixel (10⁷ counts/sec/mm²), energy resolution 4.4% FWHM at 122 keV. The integral uniformity of the detector was about 1% and the differential uniformity was about 1%. In addition, the image quality was examined with a resolution chart and step-wedge phantoms made of aluminum and polymethyl methacrylate. And we compared the quality of an acquired image with that acquired with an energy integration detector. The results of these experiments showed that the developed detector had desirable intrinsic characteristics for x-ray photon counting imaging.     

Polarized spectroscopic properties of a potential self-frequency doubling crystal, Nd:BaCaBO3F

In this paper, the polarized absorption and fluorescence spectra of Nd³⁺ ion in the BaCaBO₃F non-linear optical crystal have been investigated at room temperature. The Judd-Ofelt theory, extended to anisotropic system, has been applied to calculate the phenomenological intensity parameters, spontaneous transition probabilities, branching ratios and radiative lifetimes. The maximum emission cross-sections have been calculated to be 7.22 × 10⁻²⁰ cm² and 15.21 × 10⁻²⁰ cm² at 1068 nm for π- and σ-polarization using the Füchtbauer-Ladenburg (F-L) equation. The fluorescence lifetime of ⁴F_3/2 manifold is equal to 57.7 μs, and the quantum efficiency is 26.8%, which is higher than those of most of other borates. The potential of Nd³⁺:BaCaBO₃F as a new self-frequency doubling laser crystal has also been evaluated preliminarily. Appreciable self-absorption at the second-harmonic wavelength of Nd³⁺:BaCaBO₃F crystal should be considered for further laser operations.     

Growth and electrical properties of mercury indium telluride single crystals

A novel photoelectronic single crystal, mercury indium telluride (MIT), has been successfully grown by using vertical Bridgman method (VB). The crystallinity, thermal and electrical properties of the MIT crystal were investigated. The results of X-ray rocking curve show that the as-grown MIT crystal has good crystal quality with the FWHM on (3 1 1) face of about 173 in. DSC measurement reveals that the Hg element is easy to solely evaporate from the compound when the temperature is higher than 387.9 °C in the open system. Hall measurements at room temperature show that the resistivity, carrier density and mobility of the MIT crystal were 4.79 × 10² Ω cm, 2.83 × 10¹³ cm⁻³ and 4.60 × 10² cm² V⁻¹ s⁻¹, respectively. The reduction of carrier mobility and the increase of the resistivity are related to the adding of In₂Te₃ into HgTe, which changes the energy band structure of the crystal.     

Observations on dual-ended readout of 100 mm long LYSO crystals

We are investigating using dual-ended readout of axially oriented long thin scintillator crystals in detectors for a compact geometry, small ring diameter animal PET system. The axial position of interaction is determined from the light sharing between two photodetectors at opposite ends of the crystal. We examine the light output, energy resolution and axial spatial resolution of 1.5-5×2×100 mm³ polished LYSO crystals by irradiating with an electronically collimated beam of 511 keV photons oriented perpendicular to the long axis and read out at either end by position sensitive photomultiplier tubes (PSPMTs). Three reflector materials, namely Teflon, 3 M enhanced specular reflector (ESR) and black paint are examined for the 2×2×100 mm³ crystal size. The light output increases and energy resolution improves with the crystal cross-section. Generally, the spatial resolution worsens with increase in crystal cross-section. For the 2×2×100 mm³ crystal size, the mean energy resolutions of the photopeak over the nine irradiation positions were 14.4±0.4%, 16.0±1.2% and 28.3±2.1% with mean spatial resolutions of 7.0±1.0, 9.4±3.3 and 26.0±5.0 mm using ESR, Teflon and black paint, respectively. ESR reflector gave the best light output, energy and axial spatial resolutions. These characterization results of PSPMT-based dual-ended long LYSO crystals will be useful in the design of detector modules for a highly compact geometry preclinical PET system using this detector technology.     

Growth and properties of UV nonlinear optical crystal ZnCd(SCN)4

A second-order nonlinear optical coordination crystal, zinc cadmium thiocyanate, ZnCd(SCN)₄ (ZCTC) was grown as a frequency doubler, emitting UV light. A large typical single crystal with dimensions up to 15×7×7 mm³ has been obtained by slow solvent-evaporation method for the first time. The infrared (IR) spectroscopy and X-ray powder diffraction (XRPD) of single crystals were performed at room temperature. The specific heat of the crystal has been measured to be 367.9 J/mol K at 300 K. The thermal expansion coefficients a- and c-oriented, have been measured to be −1.69×10⁻⁵ and 1.95×10⁻⁴ K⁻¹, respectively. The second harmonic generation (SHG) efficiency of ZCTC crystal is 51.6 times as high as that of urea reference, and the measured transmittance spectra from 190 to 3200 nm showed that the UV transparency cutoff occurs at 290 nm and the transmission is 73.22% at 380 nm. UV laser light of wavelength 380 nm has been achieved by the frequency doubling of a 760 nm laser diode at room temperature.     

Activation behavior of boron implanted poly-Si on glass substrate

The activation behavior of boron (B) implanted poly-Si films on glass substrates has been investigated. The effect of B dose and annealing temperature on crystal defects and electrical properties of the films were evaluated by Raman spectroscopy and Hall measurement. It was found that the maximum activation ratio of the film with B dose of 1 × 10¹⁵ cm^− 2 was obtained when Raman peak associated with disordered amorphous silicon disappeared. However, reverse anneal was observed in the film when the annealing temperature further increased. The results from secondary ion mass spectrometry and Hall measurement revealed that B segregation at the top and bottom interface and deactivation of B substitutional occurred simultaneously in the high-dose specimens when the annealing temperature increased from 600 to 750 °C.     

Thermal and optical properties of polycrystalline CdS thin films deposited by the gradient recrystallization and growth (GREG) technique using photoacoustic methods

In this work we report the study of the thermal and optical properties of polycrystalline CdS thin films deposited by the gradient recrystallization and growth technique. CdS films were grown on pyrex glass substrates. These studies were carried out using an open photoacoustic cell made out of an electret microphone. From X-ray diffraction, atomic force microscope and photoluminescence measurements we observed polycrystalline CdS films with good morphology and crystalline quality. We obtained a thermal diffusivity coefficient of our samples with values ranging from 3.15 to 3.89 × 10^− 2 cm²/s. For comparison, we measured a value of 1.0 × 10^− 2 cm²/s for the thermal diffusivity coefficient of a CdS single crystal. We measured an energy gap value of 2.42 eV for our samples by using a photoacoustic spectroscopy system.     

Crystal growth and spectral properties of Pr:La2(WO4)3

Pr³⁺-doped La₂(WO₄)₃ single crystal with dimensions up to Ø 20 mm × 35 mm has been grown by the Czochralski method. The structure of the Pr³⁺:La₂(WO₄)₃ crystal was determined by the X-ray powder diffraction and the Pr³⁺ concentration in this crystal was determined. The absorption and fluorescence spectra of Pr³⁺:La₂(WO₄)₃ crystal were measured at room temperature, and the fluorescence lifetime of main emission multiplets were estimated from the recorded decay curves. The spectral properties related to laser performance of the crystal were evaluated.     

Continuous measurement of radiation damage of standard CMOS imagers

In this work we have irradiated a standard CMOS VGA imager with a 24 MeV proton beam at INFN Laboratori Nazionali del Sud, up to a nominal fluence of 10¹⁴ protons/cm². The device under test was fabricated with a 130 nm technology without radiation hardening. During the damaging the detector was fully operational to monitor the progressive damaging of the sensor and the associated on-pixel electronics in terms of detection efficiency, charge collection and noise. We found that the detector is still working at 10¹³ protons/cm², with a moderate increase of the noise (20%).     

Visible photoluminescence from Er-doped TiO2 nanofibres by electrospinning

Er³⁺-doped TiO₂ nanofibres were fabricated with electrospinning method followed by annealing in air at 420, 600, 800 and 1000 °C, respectively. The obtained nanofibres are relatively straight and have an average diameter of ∼ 75 nm. X-ray diffraction measurements showed that the crystal structure transforms from anatase to rutile phase with the increase of annealing temperature. Visible photoluminescence peaking at 528.1, 566.6 and 669.3 nm is detected which is ascribed to ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ions and the PL intensities increase with the increase of annealing temperature. Meanwhile at high annealing temperatures, near-infrared photoluminescence peaking at 815 nm due to the defect states associated with Ti³⁺ ions is also found. The strong green photoluminescence of Er³⁺ ions may have potential applications in one-dimensional luminescent nanodevices.     

Structural and electrical properties of HfO2/Dy2O3 gate stacks on Ge substrates

In the present work we report on the structural and electrical properties of metal-oxide-semiconductor (MOS) devices with HfO₂/Dy₂O₃ gate stack dielectrics, deposited by molecular beam deposition on p-type germanium (Ge) substrates. Structural characterization by means of high-resolution Transmission Electron Microscopy (TEM) and X-ray diffraction measurements demonstrate the nanocrystalline nature of the films. Moreover, the interpretation of the X-ray reflectivity measurements reveals the spontaneous growth of an ultrathin germanium oxide interfacial layer which was also confirmed by TEM. Subsequent electrical characterization measurements on Pt/HfO₂/Dy₂O₃/p-Ge MOS diodes show that a combination of a thin Dy₂O₃ buffer layer with a thicker HfO₂ on top can give very good results, such as equivalent oxide thickness values as low as 1.9 nm, low density of interfacial defects (2-5 × 10¹² eV^− 1 cm^− 2) and leakage currents with typical current density values around 15 nA/cm² at V_g = V_FB − 1V.     

Growth, spectroscopic properties and laser performance of Nd-doped potassium ytterbium tungstate laser crystal

The 5 at.% Nd³⁺-doped potassium ytterbium tungstate (Nd³⁺:KYb(WO₄)₂, hereafter Nd:KYbW) laser crystal with the dimension up to 28 mm × 15 mm × 12 mm was grown by the top seeded solution growth (TSSG) method. The infrared spectrum of crystal sample was measured, and the vibrational peaks were assigned. According to the absorption and emission spectra of crystal sample, the absorption and emission cross-sections are 16.03 × 10⁻²⁰ cm² at 808 nm and 10.72 × 10⁻²⁰ cm² at 1067 nm, respectively. The fluorescence life of ⁴F_3/2 energy level is 196.33 μs, and the fluorescence branching ratio for the ⁴F_3/2-⁴I_11/2 transition at 1067 nm is 55.74%. The energy transfer between Nd³⁺ and Yb³⁺ ions was observed from the fluorescence spectra pumped by 808 and 980 nm LD sources and the Stark levels of Yb³⁺ in Nd:KYbW crystal were determined. Highly efficient laser output up to 305 mW of Nd:KYbW crystal at 1067 nm has been achieved under pumping by a CW 808 nm laser diode at room temperature. The optical-optical conversion efficiency is 33.9% and the slope efficiency is 46.8%.     

Charge collection efficiency characterization of a HgI2 Frisch collar spectrometer with collimated high energy gamma rays

In this study, a 2.1×2.1×4.1 mm³ HgI₂ Frisch collar device was characterized through probing the device with a highly collimated 662 keV gamma rays (¹³⁷Cs check source) along the length and width of the device. In a systematic series of experiments, the detector was probed along its central line under different operating voltages of 1600, 1300, 1000, 800, 600 and 500 V. The experimental results were confirmed through a simulation of the charge collection for a device with the same size and operating conditions. It is shown that the HgI₂ Frisch collar device has a uniform response to gamma rays over two-thirds of the detector volume. The HgI₂ crystals and the Frisch collar detectors were grown and fabricated within the S.M.A.R.T Laboratory at Kansas State University.     

Effects of low-energy hydrogen ion implantation on optical properties of ZnO nanowires

The low-energy hydrogen ions (2 keV; 1 × 10¹⁵ to 1 × 10¹⁸ cm⁻² per dose) implantation was used to study the passivation effect of defects and photoluminescence properties of ZnO nanowires. The implanted H⁺ effectively passivated deep level native defects, making the visible emission at 500 nm disappear completely and the UV emission (380 nm) of nanowires enhance for seven times. H⁺ implantation at higher dose induced a strongly new violet emission broad peak (around 410 nm) which may originate from the hydrogen related complex of defects. However, this violet emission disappeared after annealing in argon atmosphere at 300 °C, confirming that the new violet emission is related to hydrogen. All emission peaks vanished due to the formation of a large quantity of nonradiative recombination centers at high dose implantation. This controllable method of hydrogen doping may find potential application in UV/violet optoelectronic and especially in nano-optoelectronic devices.     

Growth and characterization of a new semi-organic nonlinear optical sodium paranitrophenolate paranitrophenol dihydrate single crystal

A new semi-organic nonlinear optical sodium paranitrophenolate paranitrophenol dihydrate single crystal is grown successfully using methanol as solvent by slow evaporation technique to dimensions of 14 × 5 × 4 mm³ in a period of 7 days. The grown crystal is characterized by X-ray diffractometry and UV-Visible spectral analysis. X-ray diffraction data reveals that the crystal belongs to monoclinic system with space group C2. Optical absorption studies illustrate low absorption in the entire UV and Visible region. The second harmonic generation (SHG) efficiency of the crystal measured by Kurtz's powder technique infers that the crystal has NLO coefficient 5 times greater than that of KDP crystal. Remarkable mechanical strength with the work hardening coefficient less than 2 and thermal stability up to 120 °C of the grown crystal is reported.     

Test beam results of Current Injected Detectors (CID) irradiated up to 5×10 1 MeV neq/cm

Two full size strip detectors were investigated in this study: one with p⁺ strips (p⁺/n⁻/n⁺) and another with n⁺ strips (n⁺/p⁻/p⁺). Both detectors, are made of magnetic Czochralski silicon (MCz-Si) and irradiated to S-LHC fluencies, were tested with 225 GeV muon beam in the CERN H2 area. The Current Injected Detector (CID) sensors were operated in a cooling box capable of providing a −53 °C temperature. Results indicate a relative charge collection efficiency (CCE) at 5×10¹⁵ n_eq/cm² above 30% in irradiated p⁺/n⁻/n⁺ CID detector at 600 V bias voltage. The signal to noise ratio of this CID module was about eight and a forward current of 30 μA was needed for detector biasing. In standard reverse bias, the same detector could not provide a sufficiently large signal for particle tracking purposes. A p-type (n⁺/p⁻/p⁺) sensor was irradiated to a fluence of 2×10¹⁵ n_eq/cm² and measured under the same test beam conditions. According to the theory of CIDs developed by the CERN RD39 Collaboration, this detector module could be biased up to only 230 V due to the low irradiation fluence. The CCE at 230 V was 35% in CID operation and 20% when reverse biased.     

Laser annealing of Pb(Zr0.52Ti0.48)O3 thin films for the pyroelectric detectors

Lead zirconate titanate (Pb(Zr_0.52Ti_0.48)O₃, PZT) thin films fabricated by magnetron sputtering technique on the Pt/Ti/SiO₂/Si substrates at room temperature, were annealed by means of CO₂ laser with resulting average substrate temperature below 500 °C. The crystal structure, surface morphology and pyroelectric properties of the PZT films before and after annealing were investigated by X-ray diffraction, atomic force microscopy, and pyroelectric measurements. The results show that the annealed PZT thin film with a laser energy density of 490 W/cm² for 25 s has a typical perovskite phase, uniform crystalline particles with a size of about 90 nm, and a high pyroelectric coefficient with 1.15 × 10^− 8 Ccm^− 2 K^− 1.