NASA has considerable interest in the growth of strategically important materials in microgravity. Bridgman-Stockbarger directional solidification within a sealed ampoule are planned for CdZnTe, HgCdTe, HgZnTe and GaAs. In order to optimize the thermal parameters for the growth conditions, it is necessary to characterize the furnace and understand its behavior. This entails the use of appropriate thermal modelling based on the known physical properties of the material, the ampoule material and the furnace itself. A suitable test is the determination of the shape and location of the solid-liquid interface relative to the furnace. Electrical and chemical etching techniques have been used to locate this interface for certain materials. One of these, germanium doped with gallium, a material with well-known properties, has been used to characterize the furnace model. Demarcation has been extended to cadmium telluride by post-growth examination of the precipitation of tellurium inclusions at positions of abrupt thermal changes in the furnace regime. Deliberately imposed mechanical vibration of the melt can also produce evidence of interface location. This work was done with infrared microscopy. Other techniques, available in termary solid solutions, involve the effect of change of lower zone furnace temperatures on the compositional profile. Examples of demarcation are shown in gallium-doped germanium, cadmium telluride, and mercury zinc telluride.
The successful MBE growth of CdMnTe-CdTe heterostructures and superlattices has demonstrated the feasibility of growing layered structures incorporating dilute magnetic semiconductor materials (DMS). These materials exhibit new and interesting properties. These properties allow the band-gap engineering to continue after the structure has been grown through the application of an external magnetic field. During the growth process the engineering can be accomplished through traditional means, i.e., through the choice of layer thickness and/or the choice of the strain state of the structure.
A noncontact, noninvasive lifetime characterization technique for measuring the excess carrier lifetime in long wavelength infrared (LWIR) Hg1-xCdxTe (x approximately equals 0.20-0.225) using transient millimeter-wave (90 GHz) reflectance is presented. Excess carrier lifetime results for both p-type (vacancy-doped >= 1 X 1016 cm-3) and n-type (annealed <= 1x1015 cm-3 epilayer material are given. The lifetimes in vacancy-doped p-type HgCdTe are short, i.e., approximately 25 ns at 80 K, thereby requiring a short-pulsed laser source and high-bandwidth electronics in the lifetime test system. The lifetime test system employs either YAG (1.06 micrometers ) or CO2 (9-11 micrometers ) pulsed laser excitation. Lifetime results for both frontside and backside laser illumination of the epilayer HgCdTe are presented. The effect of surface recombination of lifetime and reflected millimeter-wave signal amplitude is discussed.
A theoretical study of the photoconductivity decay method is presented for determining excess carrier lifetime of p-type HgCdTe when recombinations are due to trapping centers. The solutions to the equations describing the transient decay are numerically evaluated and compared with steady-state lifetimes determined using the Shockley-Read equations. It is found for p-type HgCdTe that for short pulse width photoexcitation, the minority carrier lifetime can be obtained from photoconductivity decay measurements.
In ternary tellurides two reststrahlen bands are associated with vibration of the two types of near neighbor atoms. A very simple model allows for evaluation of the corresponding effective charges and force constants for each type of dipole. The results show clearly the destabilization of the HgTe bond when Cd is substituted for Hg and the strengthening of this bond when Zn is substituted for Hg. The force constants behavior in CdZnTe appears of the same type as in CdHgTe but is more complex.
The formation of anodic oxides on HgZnTe has been studied and characterized by XPS technique. The physical properties of anodized p-type layers have been investigated by the differential Hall effect, photoconductivity, and photoluminescence measurements. It was found that the tendency to form surface inversion layers on HgZnTe by anodization is considerably lower than that for HgCdTe. There is a considerable increase in effective lifetime values and in photoluminescence intensities. In addition, significant differences between the voltametric analysis curves of HgZnTe and HgCdTe were observed. The results are discussed in view of the bonding characteristics of the two materials.
Experimental results concerning the properties of anodic fluoride films grown on HgCdTe (O.2 xO.3) are presented and discussed. The analysis of the growth rate and XPS measurements indicate that the films are composed of a mixture of Cd, Hg and Te fluorides, together with unreacted HgTe and elemental Te. The films are characterized by some of their optical, electrical and chemical properties. Capacitance—voltage measurements of MIS devices are employed to determine the electronic properties of the anodic fluoridelllgCdTe interface. The results show that a positive and unstable charge density is present at the interface.
Ion implantation in HgCdTe is a well-established approach for fabricating IR- sensitive photovoltaic devices with n-on-p type junctions. The technique typically uses ion implantation of light species (usually B) to form the n region by the diffusion of irradiation-induced defects, including Hg atoms, in the material doped by Hg-vacancy acceptors. Also, the approach to form electrical junctions by the classical technique of ion implantation for chemical doping followed by the diffusion and activation of the implanted species has been demonstrated. This paper focuses mainly on p-type extrinsic doping with As diffusion from an ion implanted source.
It has been shown that thin films of the high temperature superconductors (HTS) exhibit a change in their electrical properties when exposed to optical radiation. The motivation for this research is the promise of a fast detector operating at elevated temperatures that is sensitive to low-level optical signals and that operates out to the far IR. In order to make a practical detector out of HTS materials, the mechanisms of this response must be fully understood. The purpose of this research is to investigate the spectral, temporal, and thermal characteristics of this response in an effort to better understand the mechanisms involved.
Responsivity measurements of silicide/silicon IR detectors, i.e., PtSi/Si and IrSi/Si, are interpreted by physical models for photoemission. The quantum yield for photoemission is composed of the optical absorption and the electrical emission yield across the Schottky barrier. For thin (2-6 nm) silicide films, the IR responsivity is enhanced by diffuse wall scattering of charge carriers at the interfaces. The enhancement is high near the long wavelength cut-off of the detector and decreases to shorter wavelengths due to inelastic scattering of mobile carriers for increasing photoexcitation energy. The electronic properties of the PtSi/Si interface are also characterized by scanning tunneling micrographs and tunneling spectroscopy. The results indicate a smooth and continuous film with a disordered interface which is created by interdiffusion of the components at elevated temperatures for silicide formation. Interdiffusion increases the emission barrier and reduces the responsivity.
Responsive pyroelectric linear arrays are described. After a short representation of the principal detector function, the pyroelectric materials L-alanine doped triglycine sulfate (DTGS:L-A) and lithium niobate (LiNbO3) are characterized, and the system parts pyroelectric chip, CCD-multiplexer, and hybrid arrangement are described in detail. Finally, the measured properties responsivity, noise equivalent power, and modulation transfer function are summarized.
Neutron transmutation doping (NTD) of silicon (Si) in nuclear reactors is known as a successful way of obtaining high-quality material for power devices. Using NTD of Si in order to obtain high-quality material for detectors requires an accurate neutron fluence control. The paper presents results obtained in this field using silicon slices as neutron calibration material. Irradiation was done in a vertical channel of the VVR-S nuclear reactor in Bucharest, Romania. Final resistivity of slices used for calibration was between 20 and 50 ohm X cm. Special attention was paid to the accuracy of initial and final characterization of the slices concerning resistivity, lifetime, and defects, as well as to the neutron fluence calibration and control used during exposure. High-performance detectors were obtained using this material.
The photoemission from quantum wells, quantum wires, and quantum dots of nonlinear optical materials is studied, taking n-CdGeAs2 as an example. The authors have formulated the photoemission from the aforementioned materials by deducing the new carrier energy spectra in all cases, taking into account various types of anisotropies of the energy band parameters. It is found that the photoemission increases with incident photon energy in a ladder-like manner and also exhibits oscillatory dependences with changing film thickness and the carrier density, respectively, for all cases. The numerical values of the photoemission is greatest in quantum dots and least in quantum wells. The well-known results have been shown as special cases under certain limiting conditions of our generalized expressions. The theoretical formulations are in agreement with the experimental observations as reported elsewhere.
In this paper we have studied the effective electron mass in infrared materials under various quantiations of band states (e.g. under magnetic quantization, cross—field configuration, size quanti— zation, quantum wells under cross—field configuration, quantum wires electric field aided quantum wires, magnetic field aided QWs, cross— field configuration of QWs, QWVvs and electric field aided QWWs 4Lüv eleetron energy spe ctra in the re spe ctive ease S. We have also forniu— 1 a te d the ele c tro n s ta ti s ti Cs fo r the Pu rpo se o f s tu dying the do ping dependences of effective electron mass under the said conditions. We have plotted the effective mass with various physical variables, t aki rig n—Hg1CdTe a s an e x arnpl e whi c1 al so fi n d s e x ten si ye appl i Ca— tions as IR compounds and photovoltaic detector arrays. It is found that the effective masses increase with increasing electron concen— tration, oscillate with magnetic field and film thickness in various manners. The effective masses become juantui number dependent under crossed electric and magnetic field, quantuni wells under cross—field configurations and electric field aided quantum wires. The theoreti— cal results are in agreement with the experimental observations as given elsewhere. In addition, the eorsponding results of two—band Kane model and that of parabolic energy bands have been obtained from our generalized expressions as special cases under certain limiting condi tions.
Structural, electrical, and infrared optical properties of screen-printed vanadium oxide thick films have been studied. It is seen that the original starting material, in the form of V2O3, undergoes a global transformation to its next higher oxide V2O5 during firing. This has been confirmed through differential thermal analysis (DTA) and x-ray diffraction analysis. The crystalline morphology of the transformed V2O5 seems to improve as a function of firing temperature in the range of 400- 600 degree(s)C. The associated screen-printed resistors have temperature coefficient of resistance in the range of -37,000 to -17,000 ppm per K over a temperature range of -65 to +155 degree(s)C and thermistor constant equal to 2000 K, which are independent of firing temperature. It is observed that both electrical resistivity and infrared emissivity decreases with increase of firing temperature, attaining values 1.12 X 102 ohm cm for electrical resistivity and 82% for infrared emissivity at a firing temperature of 550 degree(s)C.
This paper compares melt processed calcium aluminate fibers from two technologies in terms of their optical and structural properties. Strong amorphous fibers with a modulus 15-16 Mpsi (vs. 9-12 Mpsi for silica fibers) can be drawn from quaternary, low-silica calcium aluminate melts with 42-44%/wt. Al2O3, <6% SiO2, and <5% MgO, and from quaternary, non-silica Ca aluminate melts with 46% Al2O3, 4% MgO, and 14% BaO. These fibers have excellent structural properties. Amorphous Ca aluminate melts with 51.5-80.2% Al2O3 (both with and without silica) have much lower viscosities. They cannot be drawn from supercooled melts, but can be spun, by inviscid melt spinning, whereby a low viscosity jet is ejected into propane, affording chemically induced jet stabilization. These fibers were weaker, but were found to have sapphire-like infrared transmission spectra. A carbon sheath from the pyrolytic decomposition of propane was formed on the surface of most spun fibers. It acted like a hermetic coating and was found to significantly enhance the hydrolytic stability of the fibers.