Michael Choi

III–VI Chalcogenide Semiconductor Crystals for Broadband Tunable THz Sources and Sensors

The layered chalcogenide semiconductor GaSe has been grown under various crystal growth conditions for optimum performance for tunable terahertz (THz) wave generation and broadband THz detection. Low-temperature photoluminescence (PL), Raman spectroscopy, optical absorption/transmission, electrical charge transport property measurements, and THz time-domain spectroscopy (TDS) have been used to characterize the grown crystals. It is observed that indium doping enhances hardness of the grown GaSe crystals, which is very useful for processing and fabricating large-area devices. GaSe crystals have demonstrated promising characteristics with good optical quality (absorption coefficient les0.1 cm-1 in the spectral range of 0.62-18 mum), high dark resistivity (ges109 Omega cm), wide bandgap (2.01 eV at 300 K), good anisotropic (parand perp) electrical transport properties (mue/h, taue/h, and mutaue/h) and long-term stability. The THz emission measurements have shown that the GaSe crystals are highly efficient for broadband tunable THz sources (up to 40 THz), and sensors (up to 100 THz). Additionally, new THz frequencies (0.1-3 THz) have been observed for the first time from an anisotropic binary and a ternary semiconductor crystal. Details of characterizations as well as optimum crystal growth conditions including simulation and computer modeling are described in this paper.

Crystal Growth, Characterization and Anisotropic Electrical Properties of GaSe Single Crystals for THz Source and Radiation Detector Applications

The single crystal growth of large semi‐insulating GaSe by the vertical Bridgman technique using zone‐refined selenium (Se) and HP gallium (Ga) is described. The grown crystals (up to 10 cm long and 2.5 cm diameter) have been characterized thoroughly by X‐ray diffraction (XRD), energy dispersive analysis by x‐rays (EDAX), optical absorption/transmission, X‐ray photoelectron spectroscopy (XPS), charge carrier electrical property measurements, second harmonic test, and radiation detection measurements.

Rare-Earth Doped Potassium Lead Bromide Mid-IR Laser Sources for Standoff Detection

The single crystal growth of KPb2Br5 by vertical Bridgman technique using in-house processed zone refined PbBr2 and KBr with rare-earth terbium doping has been studied. The grown moisture resistant crystals (1.5 cm diameter and 10 cm length) have shown high promise for low phonon energy room temperature solid-state laser applications in the longer side of mid-IR (4-15 µm) due to their high storage lifetimes, wide tunability, and excellent optical quality. The processed crystals are highly transparent (T= ≥80%) in the 0.4-25 µm spectral region. Repeated melting-freezing cycles during differential scanning calorimetry (DSC) experiments did not reveal any appreciable variation in the melting point or phase transitions, which is indicative of their excellent thermal stability. The emission spectra pumped with a 2 µm source show broadband emissions with peak wavelength of 3 µm (7F4→ 7F6), 5µm (7F5→ 7F6) and 7.9µm (7F4→ 7F5). The KPb2Br5:Tb laser crystals will be highly useful for standoff detection of incoming chemical and biological threats using unique infrared absorption signatures.

GaSe and GaTe anisotropic layered semiconductors for radiation detectors

High quality detector grade GaSe and GaTe single crystals have been grown by a modified vertical Bridgman technique using high purity Ga (7N) and in-house zone refined (ZR) precursor materials (Se and Te). A state-of-the-art computer model, MASTRAPP, is used to model heat and mass transfer in the Bridgman growth system and to predict the stress distribution in the as-grown crystals. The model accounts for heat transfer in the multiphase system, convection in the melt, and interface dynamics. The crystals harvested from ingots of 8-10 cm length and 2.5 cm diameter, have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, low temperature photoluminescence (PL), atomic force microscopy (AFM), and optical absorption/transmission measurements. Single element devices up to 1 cm2 in area have been fabricated from the crystals and tested as radiation detectors by measuring current-voltage (I-V) characteristics and pulse height spectra using 241Am source. The crystals have shown high promise as nuclear detectors with their high dark resistivity (≥109 Ω.cm), good charge transport properties (μτe ~ 1.4x10-5 cm2/V and μτh ~ 1.5x10-5 cm2/V), and relatively good energy resolution (~4% energy resolution at 60 keV). Details of numerical modeling and simulation, detector fabrication, and testing using a 241Am energy source (60 keV) is presented in this paper.

Crystal Growth and Characterization of CdTe and Cd 0.9 Zn 0.1 Te for Nuclear Radiation Detectors

CdTe and Cd 0.9 Zn 0.1 Te (CZT) crystals have been studied extensively at EIC Laboratories, Inc. for various applications including x- and γ-ray imaging and high energy radiation detectors. The crystals were grown from in-house zone refined ultra pure precursor materials using a vertical Bridgman furnace. The growth process has been monitored, controlled and optimized by a computer simulation and modeling program (MASTRAPP). The grown crystals were thoroughly characterized after sequential surface passivations and post-growth annealing treatments with and without component overpressures. The infrared (IR) transmission images of the post-treated CdTe and CZT crystals showed average Te inclusion size of ∼10 μm for CdTe crystal and ∼8 μm for CZT crystal. The etch pit density was ≤ 5×10 4 cm −2 for CdTe and ≤ 3×10 4 cm −2 for CZT. Various planar and Frisch collar detectors were fabricated and evaluated. From the current-voltage measurements, the electrical resistivity was estimated to be ∼1.5×10 10 Ω·cm for CdTe and 2-5×10 11 Ω·cm for CZT. The Hecht analysis of electron and hole mobility-lifetime products (μτ e and μτ h ) showed μτ e =2×10 −3 cm 2 /V (μτ h =8×10 −5 cm 2 /V) and μτ3-6×10 −3 cm 2 /V (μτ h =4-6×10 −5 cm 2 /V) for CdTe and CZT, respectively. Final assessments of the detector performance have been carried out using 241 Am (60 keV) and 137 Cs (662 keV) energy sources and the results are presented in this paper.

Crystal growth, characterization, and testing of Cd0.9Zn0.1Te single crystals for radiation detectors

This paper describes our recent research in growing large single crystals of Cd0.9Zn0.1Te (CZT) by the vertical Bridgman technique using in-house processed zone refined precursor materials (Cd, Zn, and Te). The grown semi-insulating CZT crystals have shown high promise for high-resolution room-temperature radiation detectors due to their high dark resistivity (~1010 Ωcm), reasonably good charge transport properties [(μτ)e = (2-5) x 10-3 cm2/V] and low cost. The grown CZT single crystals (~2.5 cm diameter and up to 10 cm long) have demonstrated a very low radial Zn concentration deviation, low dislocation densities and Te precipitate/inclusions, and high infrared transmission. Details of the CZT single crystal growth, their physical and chemical analysis, surface processing, nuclear radiation detector fabrication, and testing of these devices are also presented.

Amorphous selenium based detectors for medical imaging applications

We have developed and characterized large volume amorphous (a-) selenium (Se) stabilized alloys for room temperature medical imaging devices and high-energy physics detectors. The synthesis and preparation of well-defined and high quality a-Se (B, As, Cl) alloy materials have been conducted using a specially designed alloying reactor at EIC and installed in an argon atmosphere glove box. The alloy composition has been precisely controlled and optimized to ensure good device performance. The synthesis of large volume boron (B) doped (natural and isotopic 10B) a-Se (As, Cl) alloys has been carried out by thoroughly mixing vacuum distilled and zone-refined (ZR) Se with previously synthesized Se-As master alloys, Se-Cl master alloys and B. The synthesized a-Se (B, As, Cl) alloys have been characterized by x-ray diffraction (XRD), differential scanning calorimetry (DSC), Fourier transform infra-red spectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS), inductively coupled plasma mass spectroscopy (ICP-MS), and detector testing. The a- Se alloys have shown high promise for x-ray detectors with its high dark resistivity (1010-1013 Ωcm), good charge transport properties, and cost-effective large area scalability. Details of various steps about detector fabrication and testing of these imaging devices are also presented.

Progress in Producing Large Area Flexible Dye Sensitized Solar Cells

Dye sensitized nanocrystalline TiO 2 solar cells have been reported with over 11% efficiency and are extremely promising as very low cost and lightweight photovoltaic sources. However, most reports are for cells of low area fabricated on glass, which withstands processing temperatures of ∼450°C. In this paper, we describe the fabrication and performance of cells made on flexible ITO-coated polyethylene terephthalate (PET) substrates with 6” × 3” dimensions. To improve the efficiency in the cells, we enhanced the ITO current collection efficiency with metallization fingers. The fingers resulted in a >10 fold increase in short-circuit current under normal solar illumination compared to cells without metallization. Further improvements were realized by passivating the metallization fingers at the metal/polymer electrolyte interface.

CdTe and Cd0.9Zn0.1Te crystal growth and characterization for nuclear spectrometers

Large volume single crystals of CdTe and Cd0.9Zn0.1Te (CZT) have been grown by a controlled vertical Bridgman technique using in-house zone refined precursors and characterized through structural, electrical, optical, and spectroscopic methods. The grown crystals (diameter greater than or equal to 2.5 cm and length >10 cm) have shown promising characteristics for high-resolution room temperature solid-state radiation detectors due to their high resistivity (~1010 Ω-cm for CdTe, and >1011 Ω-cm for CZT) and good charge transport properties [μτe ~ (2-5)x10-3 cm2/V]. The fabricated detectors in planar single element and Frisch collar configurations have shown very low leakage currents and high count rates for various sources, including Am-241, and Cs-137. The grown crystals have been further characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), and transmission two-modulator generalized ellipsometry (2-MGE). Details of the CdTe and CZT characterization results, detector fabrication steps, and testing with radiation sources are presented. The CdTe and CZT crystals have shown high prospects for low power rating solid-state nuclear spectrometers and medical imaging devices.