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In this work, two Hg-based chalcogenides were investigated in detail to reveal their potential capability of radiation detection at room temperature (RT). Cs2Hg6S7, with a bandgap of 1.63 eV, which is designed by the dimensional reduction theory proposed by our group, were prepared and characterized. α-HgS, with a bandgap of ~2.10 eV, as a precursor used for the ternary compound synthesis, was also proposed and further investigated. For Cs2Hg6S7, the crystals tended to crystallize into needle form with small grains. Here, the conditions of Bridgman melt growth were optimized to obtain relatively large single crystals. The slight excess of Cs2S as a fluxing agent during growth was found to facilitate better crystallization and large grains. Interestingly, no inclusion or secondary phase was found in the as-grown single crystals. The improvement of bulk resistivity from ~10^6 Ωcm to 10^8 Ωcm was also achieved through the control of stoichiometry during crystal growth. For α-HgS crystals, both physical vapor transport and chemical vapor transport methods have been applied. By modifying the transport temperature and transport agent, single crystal with size about 3x1.5 mm^2 was grown with resistivity higher than 10^11 Ωcm. Photoluminescence (PL) revealed that multiple peaks observed in the 1.6-2.3 eV range and excitonic peak from for α-HgS single crystals were observed indicating good crystalline quality. Finally, the planar detectors for both crystals were tested under Co57 gamma ray source. Both of the crystals showed reasonable gamma ray response, while α-HgS crystals could respond at a relatively higher counting rate.

Thallium based chalcogenide and halide semiconductors such as Tl4HgI6, TlGaSe2, Tl6SeI4 and Tl6SI4 are promising materials for room-temperature hard radiation detection. They feature appropriate band gaps, high mass densities and facile growth technology. However, these materials are being plagued by the Tl oxides impurity from Tl precursor or Tl containing binary precursors, which leads to problems including tube breakage, parasitic nucleation and detector performance deterioration. In this work, we present a facile way to chemically reduce Tl oxidations, and then eliminate oxygen impurity by adding high-purity graphite powder during synthesis and crystal growth. We also further investigated the reactivity between Tl oxides and graphite. The detector performance of Tl6SeI4 crystal was dramatically improved after lowering/removing the oxygen impurities. This result not only indicates the significance of removing oxygen impurity for improving detector performance. Our results suggest that the chemical reduction method we developed by adding carbon powder during synthesis is highly effective in substantially reducing oxygen impurities from Tl containing materials.

The cascading of logic gates is one of the primary challenges for spintronic computing, as there is a need to dynamically create magnetic fields. Spin-diode logic provides this essential cascading, as the current through each spin-diode is modulated by a magnetic field created by the current through other spin-diodes. This logic family can potentially be applied to any device exhibiting strong positive or negative magnetoresistance, and allows for the creation of circuits with exceptionally high performance. These novel circuit structures provide an opportunity for spintronics to replace CMOS in general-purpose computing.

Two recently proposed spintronic logic families, spin-diode logic and emitter-coupled spin-transistor logic, provide an opportunity for high-performance cascaded logic circuits. These logic families make use of magnetoresistive semiconductor heterojunctions, in which the presence of a magnetic field causes a large increase in resistance. Spin-diode logic and emitter-coupled spin-transistor logic produce highly compact circuits with superior speed and power characteristics. In particular, circuits realized in these logic families use three to ten times fewer devices than conventional CMOS. Additionally, there is minimal dynamic power dissipation, presenting a pathway for low power high performance computing beyond 10 GHz.

We demonstrate that mercury and antimony compounds with chalcogens (Q = S, Se, Te) and halogens (X = I, Cl, Br) can be a promising family for radiation detection materials. Chalcogen p-orbitals are usually located near the Fermi level and they are responsible for relative high mobilities but at the same time band gap decreases (from S to Te) due to their extended interactions. Halogens on the other hand have their bands well below the Fermi level and salts between transition metals and halogen are usually insulators. Incorporation of halogen atoms in a mercury or antimony chalcogenide framework can give rise to intermediate properties between the two end members (HgQ and HgX2), i.e. structures composed of heavy elements (Z < 40), wide band gap (1.6 - 2.5 eV), and high carrier mobilities. As a proof of concept, we will present two new chalcohalide families, Hg3Q2X2 and SbQX. Crystal growth of the Hg3Te2Br2 phase (7.8 g/cm3 and 2.5 eV) by a vapor transport method gave mm-sized single crystals with electrical resistivity values more in the GΩ.cm range. Preliminary data for mobility-lifetime products for both electron and hole carriers were around 10-5 cm2/V. SbSeI (5.8 g/cm3 and 1.7 eV) sample grown by relatively fast Bridgman technique showed an MΩ.cm range (2.8 x 106 Ω.cm) resistivity with a similar order of magnitude (10-4 cm2/V) of mobility-lifetime products for both electron and hole carriers.

We report on the characterization of optical and electronic properties of a series of thallium chalcogenide, wide gap ternary semiconductor compounds including Tl6I4Se, Tl6I4S, Tl3SbS3, Tl2SnS3, and Tl7Bi3I16 for ionized radiation detection at the x-ray and γ ray regimes. The semiconductor crystals were grown by the modified Bridgman method. The optical absorption, band gap, mobility-lifetime products for electron and hole carriers were measured at room temperature. For Tl6I4S and Tl6I4Se the mobility-lifetime products are comparable to those of CdZnTe. We measured room temperature detector response to x-ray and γ ray sources. For 137Cs radiation, Tl6I4Se has a well-resolved spectral response comparable to that of CdZnTe.

In this work we investigate carrier dynamics of narrow gap ferromagnetic alloys grown by MOVPE. We determine the intraband and interband relaxation times in these material systems where the samples are excited with photon energies above the band gap of InMnAs and InMnSb films. Our results are important for understanding the electronic states and the relaxation mechanisms in these ferromagnetic materials.

The optical and electronic properties of Tl-chalcogenide, wide gap semiconductors, TlGaSe 2 , Tl 6 I 4 Se, and Tl 2 Au 4 S 3 for x-ray and γ ray detection were characterized. The semiconductor crystals are grown by the modified Bridgman method. The optical absorption and band gap energy of the materials were determined from UV-Vis-near IR transmission and reflection spectra. The mobility-lifetime products were measured. For Tl 6 I 4 Se the values were comparable to those of CdZnTe. We measured room temperature detector response to x-ray and γ ray radiations. Under 57 Co radiation, Tl 6 I 4 Se has a well-resolved spectral response and peak FWHM comparable to those of Cd 0.9 Zn 0.1 Te.

The simulation, fabrication and measurement of nonlinear photonic crystals (PhCs) with hexagonal symmetry in epitaxial BaTiO 3 were investigated. The optical transmission properties of a PhC were simulated by a 2-D finite-difference time domain (FDTD) method. A complete bandgap exists for both the TE and TM optical modes. The fabricated PhC has a well-defined stop band over the spectral region of 1525 to 1575 nm. A microcavity structure was also fabricated by incorporation of a line defect in the PhC. Transmission of the microcavity structure over the spectral region from 1456 to 1584nm shows a well-defined 5 nm wide window at 1495nm. Simulations indicate that the phase velocity matched PhC microcavity device of 0.5 mm long can potentially serve as modulator with a 3 dB bandwidth of 4 THz. Keywords : Hexagonal PhC; Bandgap; EO modulator. 1. INTRODUCTION Photonic crystal (PhC) devices, such as tunable filters, 1 optical switches, 2 optical delay lines 3 as well as low V Πelectro-optic modulators (EOM)

Nonlinear photonic crystal (PhC) waveguides are being developed for ultrafast modulators. To enable phase velocity matching we have investigated one- and two-dimensional structures. Photonic crystal (PhC) waveguides based on epitaxial barium titanate (BTO) thin film in a Si 3 N 4 /BTO/MgO multilayer structure were fabricated by electron beam lithography or focused ion beam (FIB) milling. For both one- and two-dimensional PhCs, simulation shows that sufficient refractive index contrast is achieved to form a stop band. For one-dimensional Bragg reflector, we measured its slow light properties and the group refractive index of optical wave. For a millimeter long waveguide a 27 nm wide stop band was obtained at 1550 nm. A slowing of the light was observed, the group refractive indices at the mid band gap and at the band edges were estimated to be between 8.0 and 12 for the transverse electric (TE) mode, and 6.9 and 13 for the transverse magnetic (TM) mode. For TE optical modes, the enhancement factor of EO coefficient ranges from 7 to 13, and for the TM mode, the factor ranges from 5.9 to 15. Measurements indicate that near velocity phase matching can be realized. Upon realizing the phase velocity matching condition, devices with a small foot print with bandwidths at 490 GHz can be attained. Two-dimensional PhC crystal with a hexagonal lattice was also investigated. The PhCs were fabricated from epitaxial BTO thin film multilayers using focused ion beam milling. The PhCs are based on BTO slab waveguide and air hole arrays defined within Si 3 N 4 and BTO thin films. A refractive index contrast of 0.4 between the barium titanate thin film multilayers and the air holes enables strong light confinement. For the TE optical mode, the hexagonal photonic crystal lattice with a diameter of 155 nm and a lattice constant of 740 nm yields a photonic bandgap over the wavelength range from 1525 to 1575 nm. The transmission spectrum of the PhC waveguide exhibits stronger Fabry Perot resonance compared to that of conventional waveguide. Measured transmission spectra show a bandgap in the ΓM direction in the reciprocal lattice that is in agreement with the simulated results using the finite-difference time-domain (FDTD) method. Compared to polarization intensity EO modulator with a half-wave voltage length product of 4.7 V•mm. The PhC based EO modulator has a factor of 6.6 improvement in the figure of merit performance. The thin film PhC waveguide devices show considerable potential for ultra-wide bandwidth electro-optic modulators as well as tunable optical filters and switches.

In light of the growing interest in spin-related phenomena and devices, there is now renewed interest in the science and engineering of narrow gap semiconductors. In this work, time resolved spectroscopy of InSb-based parabolic multi-quantum wells and narrow gap ferromagnetic alloys grown by MOVPE, have been pursued. In addition, in this study, we report on CR experiments carried out on the ferromagnetic InMnAs film, on which clear resonance signals have been successfully observed in high magnetic fields. Investigation of the electronic structure of III-Mn-V alloys by techniques such as the cyclotron resonance can shed important light on the origin of ferromagnetism and the p-d exchange interaction in III-Mn-V systems. Our results are important for understanding the electronic and magnetic states in these material systems.

In light of the growing interest in spin-related phenomena and devices, there is now a renewed interest in the science and engineering of narrow gap semiconductors. They offer several scientifically unique electronic features such as a small effective mass, a large g-factor, a high intrinsic mobility, and large spin-orbit coupling effects. Our studies have been focused on probing and controlling the coherent and quantum states in InSb quantum wells and InMnAs ferromagnetic semiconductors. Our observations are providing new information regarding the optical control of carriers and spins in these material systems. We demonstrated the generation of spin polarized photo-current in an InSb QW where a non-equilibrium spin population has been achieved by using circularly polarized radiation. In addition, the differential transmission measurements in InSb QWs demonstrated that the initial distribution function strongly influences the carrier relaxation dynamics. We employed the polarization-resolved differential transmission as well as the MOKE measurements to provide information on the spin relaxation dynamics in MOVPE grown InMnAs. Our measured T1 is comparable to the reported measurements in MBE grown InMnAs and several time resolved measurements on InAs.

In this paper we will present innovative design approach for UV Focal Plane Array for Photon Counting Applications. This Focal Plane includes the building a large area silicon micro-channel plate (MCP) using GaN photocathode. In this paper, we will present the design and simulation of silicon micro-channel plate with GaN photocathode with large area array with 2 micron pores and 3 micron pitch. We will also present results for the ICP-RIE process to fabricate 2 micron pores, and Growth of high conductivity GaN photocathodes using MOCVD to produce 40% QE. We will also discuss approaches for development of readout architecture and circuit for Silicon based MCP 4096x4096 UV FPA. The readout architecture scheme uses a series of charge sensitive amplifiers (CSA) to boost the charge received on each anode. The signal from each CSA is then passed into a shaping amplifier (SA) that produces a smooth waveform. The peak of the smoothed waveform is captured by a sample and hold (S/H) circuit that holds the signal until an analog to digital (A/D) converter can sample it. Details of the ROIC circuit will be presented.

We have made InGaN:Mg epitaxial layers and report for the first time the QE versus wavelength for a photo-cathode. The motivation for InGaN is to lower the band gap just enough to enable detection of nitrogen fluorescence 337 nm, 357 nm and 391nm for both Earth observing and for energetic cosmic ray studies. Homogeneous InGaN alloys are difficult to prepare as the indium rich alloy tends to coalesce into quantum dots. The transmission, X-ray, and photo-luminescence measurements of the films indicated a significant concentration of Mg acceptors was incorporated into the film and as such could be converted into a viable photo-cathode upon cessiation. We present our photo-luminescence, X-ray, and near-field scanning microscope (NSOM) and QE measurements of films and compare these with measurements of GaN:Mg. The spectral properties of the photo-cathodes will also be presented.

The nitride-III semiconductors, in particular GaN (band gap energy 3.5 eV), AlN (band gap 6.2 eV) and their alloys AlxGa1-xN are attractive as UV photo-convertors with applications as photocathodes for position sensitive detector systems. These can “fill the gap” in the 150-400nm wavelength regime between alkali halide photocathodes ( 4000A, mutlialkali & GaAs). Currently CsTe photocathodes have fairly low efficiency (Fig. 1) in the 100nm to 300nm regime are sensitive to contamination and have no tolerance to gas exposure. We have prepared and measured a number of GaN photocathodes in opaque and semitransparent modes, achieving >50% quantum efficiency in opaque mode and ~35% in semitransparent mode (Fig. 2). The GaN photocathodes are stable over periods of >1 year and are robust enough to be re-activated many times. The cutoff wavelength is sharp, with a rapid decline in quantum efficiency at ~380-400nm. Application of GaN photocathodes in imaging devices should be feasible in the near future. Further performance improvements are also expected as GaN fabrication and processing techniques are refined.

The basic requirement for an imaging low-light level system (one capable of single photon counting) is that the device has low dark current. Photocathode based devices have the advantage over solid state devices in this regard as the dark current is inherently low. A further requirement for UV detectors is the necessity to suppress the sensitivity in the red, and wide-band gap semi-conductors fill this role well. For nitride based semi-conductors, there is still the issue of making p-type material and making alloys with Al or In to move the red cutoff to the blue (Al) or red (In). Regardless of the material (e.g. another choice is diamond) coupling the resulting photocathode to a device such as a micro-channel plate (MCP) is necessary to produce imaging. Based on advances we have made both in the production of p- type GaN photocathodes, diamond photocathodes, and read-outs of Si MCPs, we are on the verge of making high quality UV imaging systems for astronomy and other low-light level applications. In this paper we will review the progress that has been made over the past few years and provide an update with recent results.

Currently, photo-cathodes hold the highest promise in the near term (next few years) of being able to detect low light level UV signals at high QE while being nearly blind to visible wavelengths. We briefly discuss the requirements for UV detection for astronomical applications, and then we describe our work on producing GaN based photo-cathodes. The p-type GaN films were grown on sapphire at Northwestern University. The films were then converted into opaque photo-cathodes inside photo-tubes at Hamamatsu. Hamamatsu tested detective quantum efficiencies (DQE) of these detectors to be as high as 30% at 200 nm. The ratio of peak DQE at 200 nm to the minimum DQE at 500 nm was measured to be about 6 X 103. We found a dramatic increase in the DQE at 200 nm versus the conductivity, with the break point being near 0.13 1/(Ohm-cm). Based on this dramatic increase, we believe that further improvement in photo-cathode quantum efficiencies can be achieved by increasing the conductivity. We have recently achieved more than an order of magnitude increase in conductivity by co-doping techniques. Improvements in the solar blindness of the devices depend both on characteristics of the film and its surface properties. A detailed discussion of decreasing the visible response and producing a sharper wave-length cutoff is beyond the scope of this work, but we briefly discuss the attributes that most likely affect the wavelength dependence of the photo-cathode response.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Erbium doped BaTiO3 thin films for optical waveguide applications were investigated. Characteristic 4f emission at 1560 nm is observed for Er concentrations ranging from 1018 to 1020 cm-3. Factors which determined the luminescence efficiency were investigated and a model for efficiency was developed. The luminescence intensity depended predominantly upon two factors: the concentration of radiative Er centers and the de-excitation efficiency of the excited 4f electron state. At a growth temperature of 725 degree(s)C, the concentration of radiative Er ions was independent of Er dopant concentration. Annealing the thin films in an oxidizing ambient resulted in increased luminescence efficiency whereas annealing under reducing conditions quenched the luminescence.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

High-Ta superconducting Bi-Sr-Ca--Cu-0 thin films have been prepared by low pressure organometallic chemical vapor deposition (ONCVD). Factors which influence texture and morphology of the OMCVD derived films have been examined, including the effects of precursors, doping and substrates. Under optimal conditions, high quality films with a high degree of preferred orientation are obtained. Initial experiments on in-situ formation of superconducting films are also described.

The chemical vapor deposition of indium phosphide using the In/HCl/PH3/H2 reactant system is described. The deposition kinetics were studied as function of reactant concentration and substrate temperature. Studies of deposition of both polycrystalline and single crystalline material indicated that the growth rate of the thin films was limited by surface kinetics via a Langmuir-type absorption mechanism. Stoichiometry of the gas phase influenced the electrical properties of the as-grown epitaxial layers. For a group V/III ratio of 0.70 in the growth ambient, net donor densities of 1.5 x 1016 cm-3 were observed in the undoped epitaxial layers. Transient capacitance spectroscopy indicated deep level concentrations of the order 1-10 x 1014 cm-3 in the n and p-type epitaxial InP.© (1982) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.