Electrical characterization of very-narrow-gap bulk HgCdTe single crystals by variable magnetic field hall measurements

Variable-magnetic-field Hall measurements (0 to 1.5 T) are performed on very-narrow-gap bulk-grown Hg_1−xCd_xTe single crystals (0.165 ≤ x ≤ 0.2) at various temperatures (10 to 300K). The electron densities and mobilities are obtained within the one-carrier (electrons) approximation of the reduced-con-ductivity-tensor scheme. The present data together with the selected data set reported by other workers exhibit a pronounced peak when the electron mobility is plotted against the alloy composition x-value which has been predicted to be due to the effective-mass minimum at the bandgap-crossing (E_g ≈ 0). The observed position (x ≈ 0.165), height (≈4 x 10² m²Vs), and width (≈0.01 in x) of the mobility-peak can be explained by a simple simulation involving only ionized-impurity scattering. A lower bound of the effective mass is introduced as a fitting parameter to be consistent with the finiteness of the observed electron mobility and is found to be of the order of 10⁻⁴ of the mass of a free electron.     

Improved arsenic doping in metalorganic chemical vapor deposition of HgCdTe andin situ growth of high performance long wavelength infrared photodiodes

Controlled and effective p-type doping is a key ingredient forin situ growth of high performance HgCdTe photodiode detectors. In this paper, we present a detailed study of p-type doping with two arsenic precursors in metalorganic chemical vapor deposition (MOCVD) of HgCdTe. Doping results from a new precursortris-dimethylaminoarsenic (DMAAs), are reported and compared to those obtained from tertiarybutylarsine (TBAs). Excellent doping control has been achieved using both precursors in the concentration range of 3 × 1015-5 × 10¹⁷ cm⁻³ which is sufficient for a wide variety of devices. Arsenic incorporation efficiency for the same growth temperature and partial pressure is found to be higher with DMAAs than with TBAs. For doping levels up to 1 × 10₁₇ cm⁻³, the alloy composition is not significantly affected by DMAAs. However, at higher doping levels, an increase in the x-value is observed, possibly as a result of surface adduct formation of DMAAs dissociative products with dimethylcadmium. The activation of the arsenic as acceptors is found to be in the 152–50% range for films grown with DMAAs following a stoichiometric anneal. However, a site transfer anneal increases the acceptor activation to near 100%. Detailed temperature dependent Hall measurements and modeling calculations show that two shallow acceptor levels are involved with ionization energies of 11.9 and 3.2 meV. Overall, the data indicate that DMAAs results in more classically behaved acceptor doping. This is most likely because DMAAs has a more favorable surface dissociation chemistry than TBAs. Long wavelength infrared photodiode arrays were fabricated on P-on-n heterojunctions, grownin situ with iodine doping from ethyl iodide and arsenic from DMAAs on near lattice matched CdZnTe (100) substrates. At 77K, for photodiodes with 10.1 and 11.1 (im cutoff wavelengths, the average (for 100 elements 60 × 60 μm² in size) zero-bias resistance-area product, R₀A are 434 and 130 ohm-cm², respectively. Quantum efficiencies are ≥50% at 77K. These are the highest R₀A data reported for MOCVDin situ grown photodiodes and are comparable to state-of-the-art LPE grown photodiodes processed and tested under identical conditions.     

Synthesis of branched 'nanotrees' by controlled seeding of multiple branching events

The formation of nanostructures with controlled size and morphology has been the focus of intensive research in recent years. Such nanostructures are important in the development of nanoscale devices and in the exploitation of the properties of nanomaterials. Here we show how tree-like nanostructures ('nanotrees') can be formed in a highly controlled way. The process involves the self-assembled growth of semiconductor nanowires via the vapour-liquid-solid growth mode. This bottom-up method uses initial seeding by catalytic nanoparticles to form the trunk, followed by the sequential seeding of branching structures. Each level of branching is controlled in terms of branch length, diameter and number, as well as chemical composition. We show, by high-resolution transmission electron microscopy, that the branching mechanism gives continuous crystalline (monolithic) structures throughout the extended and complex tree-like structures. The controlled seeding method that we report here has potential as a generic means of forming complex branching structures, and may also offer opportunities for applications, such as the mimicking of photosynthesis in nanotrees.     

Generation of size-selected gold nanoparticles by spark discharge — for growth of epitaxial nanowires

One-dimensional semiconductor nanowires are a promising candidate for future electronic devices. The epitaxial growth of nanowires is often mediated by metal seed particles, usually gold particles. In this paper the setup of a simple and robust technique to generate nanometer-sized aerosol gold particles by spark discharge is described. Furthermore we demonstrate for the first time that particles generated by spark discharge can be used to design advanced nanoelectronic structures, namely nanowires. In order to obtain compact, spherical particles suitable for nanowire growth, the sparkgenerated agglomerate particles were reshaped in a special compaction furnace. The reshaped particles were used to seed the growth of epitaxial GaP and InP nanowires, by metal organic vapor phase epitaxy, which was shown to be a reliable and reproducible method. This work indicates the possibility of using spark-discharge generated gold particles for the creation of new electronic devices even at large scale processing.     

Calgary Depression Scale for Schizophrenia: a study of the validity of a French-language version in a population of schizophrenic patients

A case report of outpatient maintenance electroconvulsive therapy (ECT) is presented in a patient with bipolar disorder type I refractory to conventional medication treatment but responsive to ECT. A cost comparison is made showing substantial savings when outpatient maintenance ECT is used in lieu of inpatient hospitalization with ECT. A detailed life chart illustrating multiple medication trials that failed to stabilize the patient accompanies the financial summary. This case highlights the advantages of outpatient maintenance ECT for bipolar depression particularly with regard to safety, efficacy, and significant health care cost reduction.     

Graft copolymers of cellulose and vinylic ketones. I. Preparation of poly(methyl vinyl ketone)–cotton cellulose copolymers

The graft copolymerization reaction of methyl vinyl ketone from several solvents with fibrous cotton cellulose, preirradiated to a dosage of 5.2 × 10¹⁹ eV/g with γ-radiation from ⁶⁰Co, was investigated. Solvents included water, methanol, N,N-dimethyl formamide, and several combinations of these solvents. From water a maximum yield of copolymer product was obtained after 2 hr at 25°C. The addition of methanol to aqueous solutions of methyl vinyl ketone, in all concentrations, inhibited the graft copolymerization reaction. The addition of a small amount of N,N-dimethylformamide to aqueous solutions of the monomer increased the rate of the copolymerization reaction; however, the addition of large amounts of N,N-dimethylformamide to these solutions also inhibited the reaction. From solutions of methanol or N,N-dimethylformamide and monomer, little or no copolymerization of monomer with irradiated cellulose occurred. The copolymer products exhibited a strong infrared absorption band at 5.85 μ which is characteristic of the ? C?O group of the grafted poly(methyl vinyl ketone). Fibrous copolymer yarns exhibited increased yarn number and decreased breaking strength and average stiffness, as compared with unmodified cotton yarns.     

Measurement of space charge generation-recombination current in Hg1−xCdxTe photodiodes by deep level transient spectroscopy

Deep Level Transient Spectroscopy (DLTS) measurements have been used to characterize n⁺ ? pHg_1?xCd_xTe junction photodiode performance. Deep level results obtained on a x = 0.320 liquid phase epitaxial grown photodiode and a x = 0.219 bulk quench anneal-grown photodiode have identified deep Shockley-Read recombination centers. Detailed characterization of trap energy, trap density, and capture cross sections for these traps located within the diode depletion region have been used to predict a space charge generation-recombination current and dynamic resistance-area product at zero bias voltage. This paper presents for the first time a direct correlation of DLTS parameters with photodiode device performance.     

A Novel Stress Characterization Technique for the Development of Low-Stress Ohmic Contacts to HgCdTe

HgCdTe material intended for long-wavelength infrared detection is particularly susceptible to damage from stress. As a result, an ideal ohmic contact needs to have good adhesion and low specific contact resistance. The contact should act as a diffusion barrier and induce the least amount of stress in the underlying material. In this paper we present a set of stress measurements from different ohmic contact materials deposited on short- and long-wavelength HgCdTe films grown by liquid-phase epitaxy (LPE). Using a new experimental technique we remove the substrate and measure the stress induced on single- and multilayered HgCdTe cantilevers. To interpret our results, we develop a theoretical model that describes the physics of elastic deformation in HgCdTe layers. Our model is based on classical thin-plate bending theory and explicitly takes into account the realistic boundary conditions that are present in the experimental setup by using a variational approach.     

Long‐Term Electrical and Mechanical Function Monitoring of a Human‐on‐a‐Chip System

The goal of human‐on‐a‐chip systems is to capture multiorgan complexity and predict the human response to compounds within physiologically relevant platforms. The generation and characterization of such systems is currently a focal point of research given the long‐standing inadequacies of conventional techniques for predicting human outcome. Functional systems can measure and quantify key cellular mechanisms that correlate with the physiological status of a tissue, and can be used to evaluate therapeutic challenges utilizing many of the same endpoints used in animal experiments or clinical trials. Culturing multiple organ compartments in a platform creates a more physiologic environment (organ–organ communication). Here is reported a human 4‐organ system composed of heart, liver, skeletal muscle, and nervous system modules that maintains cellular viability and function over 28 days in serum‐free conditions using a pumpless system. The integration of noninvasive electrical evaluation of neurons and cardiac cells and mechanical determination of cardiac and skeletal muscle contraction allows the monitoring of cellular function, especially for chronic toxicity studies in vitro. The 28‐day period is the minimum timeframe for animal studies to evaluate repeat dose toxicity. This technology can be a relevant alternative to animal testing by monitoring multiorgan function upon long‐term chemical exposure.