HgTe and PbS colloidal quantum dots (CQD) with first excitonic absorption peak of about 2 μm
and shorter (down to about 1 μm) have been synthesized and characterized. The synthesized
CQDs were characterized using FTIR spectroscopy and TEM technique. The nanomaterials were
tested for photo-electrical properties with photoconductive (PC) devices. The devices were
fabricated by drop-casting a suspension of the CQDs on the fanout, followed by solid state ligand
exchange (SSLE) process, and then spectral and electrical photoresponse of the device were
measured. The SSLE process was evaluated thru absorption spectra of test samples. The device
fabrication parameters were the number of deposited layers, the thickness of individual layers,
the type of the substituting ligand, and the ligand exchange duration. For selected devices
external quantum efficiency (EQE) was also determined.
The three most important desirable features in the search for room temperature semiconductor detector (RTSD) candidate as an alternative material to current commercially off-the-shelf (COTS) material for gamma and/or thermal neutron detection are: low cost, high performance and long term stability. This is especially important for pager form application in homeland security. Despite years of research, no RTSD candidate so far can satisfy the above 3 features simultaneously. In this work, we show that mercurous halide materials Hg2X2 (X= I, Cl, Br) is a new class of innovative compound semiconductors that is capable of delivering breakthrough advances to COTS radiation detector materials. These materials are much easier to grow thicker and larger volume crystals. They can detect gamma and potentially neutron radiation making it possible to detect two types of radiation with just one crystal material. The materials have wider bandgaps (compared to COTS) meaning higher resistivity and lower leakage current, making this new technology more compatible with available microelectronics. The materials also have higher atomic number and density leading to higher stopping power and better detector sensitivity/efficiency. They are not hazardous so there are no environmental and health concerns during manufacturing and are more stable making them more practical for commercial deployment. Focus will be on Hg2I2. Material characterization and detector performance will be presented and discussed. Initial results show that an energy resolution better than 2% @ 59.6 keV gamma from Am-241 and near 1% @ 662 keV from Cs-137 source can be achieved at room temperature.
In this paper, development of single crystalline n- and p- type PbTe crystals and PbTe bulk nanocomposites using PbTe
nano powders and emerging field assisted sintering technology (FAST) are discussed. Materials requirements for efficient
thermoelectric power generation using waste heat at intermediate temperature range (6500 to 8500 K) will be discussed.
Recent results on production of n- and p- type PbTe crystals and their thermoelectric characterization will be presented.
Relative characteristics and performance of PbTe bulk single crystals and nano composites for thermoelectric power
generation will be discussed.
High performance HgCdTe IR detector fabrication on silicon substrates first requires low defect density CdTe buffer
layers to be grown on silicon. The objective of this paper is to demonstrate dislocation reduction in CdTe epitaxial layers
grown on silicon substrate by using intermediate nanocrystalline CdTe buffer layers. Colloidal synthesis of high quality
CdTe nanocrystals was accomplished and spin coating of these CdTe nanocrystals as buffer layers on silicon substrates
was carried out. CdTe layers were grown on these buffered substrates by metalorganic chemical vapor deposition
(MOCVD). However, the incomplete removal of SiO2 on silicon substrate (by chemical treatment) prevented the exact
orientation of the nanocrystals with the silicon substrate and over layer growth of continuous single crystal CdTe
epitaxial film. Two new approaches were further investigated: (i) First a thin film of Ge was grown on Si, followed by
the deposition of thin SiO2 followed by nanopatterning using block co-polymer (BCP) lithography. Transmission
electron microscopy (TEM) showed defect reduction in the CdTe layers grown on these substrates, but the x-ray rocking
curves over a larger area gave wider full width half maximum values compared to that of layers grown on blanket
surfaces. This was attributed to non uniform nanopatterning in these initial studies; (ii) SiO2 coated silicon substrates
were nanopatterned using interference lithography with a honeycomb array of holes. These substrates will be used for
the selective growth of germanium and CdTe by MOCVD.
The objective of this study is to determine feasibility of ternary Fe:CdMnTe crystals for room temperature lasing in the
mid-infrared spectral range. Fe:CdMnTe samples were grown with a modified Bridgman technique. At room temperature, Fe:CdMnTe features wide (2800-6000nm) absorption and emission (3700-6500nm) bands. The kinetics of the photoluminescence were measured over 14K-300K temperature range under 2920nm and 532nm excitation. The low temperature (14K) kinetic of luminescence under 2920nm excitation was a single exponential with a decay time of 75μs. The room temperature emission cross-section was estimated to be 2x10-18cm2.
Large crystals of Hg2Cl2 and Hg2Br2 (48 mm in diameter, up to 600g in weight) have been grown by self-seeded
contactless Physical Vapor Transport (PVT) technique in closed ampoules in vertical configuration. Seed selection
was accomplished in the small diameter tubing at the ampoule tip. Material purification was done by resublimation
processing. The crystals show high transparency, with good transmittance and good crystallographic quality. An
acousto-optic modulator built from the Hg2Cl2 crystal showed good performance consistent with predicted device
High quality CdTe crystals with resistivities higher than 108 Ω cm were grown by the 'contactless' PVT technique. Group III elements In and Al, and the transition metal Sc were introduced at the nominal level of about 6 ppm to the source material. Low-temperature photoluminescence (PL) has been employed to identify the origins of PL emissions of the crystals. It was found that the emission peaks at 1.584 eV and 1.581 eV exist only in the In-doped crystal. The result suggests that the luminescence line at 1.584 eV is associated with Cd-vacancy/indium complex. The intensity of the broadband centered at 1.43 eV decreases dramatically with introduction of Sc.
A numerical model of the thermal field in the furnace-ampoule system developed for the 'contactless' physical vapor transport growth geometry is presented. The model is used to assess the effect of the system geometry and growth parameters on the conditions of 'contactless' growth of cadmium telluride crystals.
The effect of desorption from, and diffusion through the wall on inert gas pressure in sealed fused silica ampoules was investigated. It is shown, that desorption from the surface and the bulk of silica may lead to an accumulation of residual gas on the order of a few Torr or more upon annealing. A prior outgassing of the ampoules under vacuum at high temperature reduces the amount of gas released from the glass by at least one order of magnitude. Presence of oxide and other impurities in the source material was found to increase the residual gas pressure, affect its composition, and reduce the vapor transport rate in PVT systems. It is shown, that light gases (hydrogen, helium, and neon) diffuse through silica wall and may change the pressure inside the sealed ampoule considerably even at moderate temperatures.