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A microwave photoconductivity decay (MPCD) technique, which probes conductivity change in wafers in response to either an above-band-gap or below-band-gap laser pulse, has been used to characterize recombination lifetime in high-purity 4H-SiC substrates produced with three different anneal processes. The above-band-gap (266 nm) decay times vary from ∼10 ns to tens of microseconds in the 4H-SiC substrates depending on the wafer growth parameters. Wafers produced using the three processes A (as-grown), B (annealed at 2000 °C), and C (annealed at 2600 °C) have decay times of 10–20 ns, 50–500 ns, and tens of microseconds, respectively. The differences in decay times are attributed to low, medium, and high densities of recombination centers in process C, B, and A wafers, respectively. The MPCD results correlate with other characterization results such as deep level transient spectroscopy, which also showed that the 2600 °C anneal process significantly reduces defect densities, resulting in the enhanced recombination lifetimes. Modeling and one-dimensional simulations indicate a trapping center closer to the conduction band results in a longer MPCD decay transient, but such a trapping based model for the enhanced lifetimes is not compatible with the wide range of experimental characterization results described in this work, which indicate an annealing out of recombination centers at 2600 °C. [ABSTRACT FROM AUTHOR]

We present results of a thermal anneal process that increases the minority carrier lifetime in SiC substrates to in excess of 3 μs, compared to the starting as-grown substrates with lifetimes typically in the <10 ns range. Measurement of lifetimes was conducted using microwave-photoconductive decay. Electron beam induced current measurements exhibited minority carrier diffusion lengths of up to 65 μm, confirming the enhanced carrier lifetime of the annealed substrate material. Additionally, positron annihilation spectroscopy and deep level transient spectroscopic (DLTS) analysis of samples subjected to this anneal process indicated that a significant reduction of deep level defects, particularly Z1/Z2, may account for the significantly enhanced lifetimes. The enhanced lifetime is coincident with a transformation of the original as-grown crystal into a strained or disordered lattice configuration as a result of the high temperature anneal process. The operational performance of p-i-n diodes employing drift layers fabricated from the annealed high-lifetime substrates confirmed conductivity modulation in the diodes consistent with ambipolar carrier lifetimes in the microsecond range. [ABSTRACT FROM AUTHOR]

The magnetocaloric effect of the FeCrMoCuGaPCB alloy series, suitable for being prepared as bulk amorphous alloys, has been studied. Optimal refrigeration cycles have a cold reservoir close to room temperature. The refrigerant capacity of these alloys is comparable to that of a Mo-containing Finemet-type alloy and is ∼40% bigger than that of other bulk amorphous alloys with comparable working temperatures. Analysis of the field dependence of the magnetic entropy change evidences a power law for all the magnetic regimes. [ABSTRACT FROM AUTHOR]

Multicomponent Fe65.5Cr4Mo4Ga4P12C5B5.5 amorphous alloy is known to have good soft magnetic properties combined with large glass-forming ability. Searching for a nanocrystalline structure that could improve its magnetic properties, Cr and Mo were partially substituted by Cu. The devitrification process of Cu-containing alloys shows an additional broad crystallization event that can be described by an Avrami law, with an exponent close to unity, typical for nanocrystallization phenomena. The microstructure of the nanocrystalline alloys consists of Fe(Ga) nanocrystals with a mean grain size of 5–10 nm embedded in an amorphous matrix. The maximum crystalline volume fraction achieved at the end of the nanocrystallization process was found to be of around 20% and for alloys with high Cu content a number of crystals of larger size (∼40 nm) were observed. The combined use of x-ray diffraction and Mössbauer spectrometry allowed to estimate the compositions of both the nanocrystalline grains and the amorphous residual phase of nanocrystalline alloys. The complex hyperfine structure of the studied alloys at the end of the nanocrystallization process was characterized. [ABSTRACT FROM AUTHOR]

A radio-frequency photoreflectance technique, which senses changes in sample conductivity as carriers recombine following excitation by a laser pulse, has been used to measure the recombination parameters in 0.55-eV InGaAsSb lattice matched to GaSb. Doubly capped lifetime structures with variable active layer thicknesses are used to extract the surface recombination velocity, while an analysis of the samples with different doping concentrations is used to obtain Auger (C) and radiative (B) recombination parameters. Parameter extraction for the samples evaluated gives C=(1±0.4)×10-28 cm6/s and B=(3±1.5)×10-11 cm3/s for 0.55-eV InGaAsSb lattice matched to GaSb. The Auger and radiative recombination coefficients obtained from high-level injection decay times in low doping concentration samples show very good agreement with the values obtained from low-level injection conditions. [ABSTRACT FROM AUTHOR]

A Mössbauer spectrometry and magnetic study have been performed on FeCoSiAlGaPCB amorphous alloys in order to get information about the variations in the atomic short range order with the substitution of Fe by Co. The decrease of the Curie temperature with decreasing Fe content was analyzed in the framework of the molecular field theory. The average hyperfine magnetic field at 77 K decreases linearly with increasing Co content as does the average magnetic moment per transition metal atom and a linear dependence between both quantities is found. The standard deviation of the hyperfine magnetic field distribution at 77 K does not change with Fe substitution and the average isomer shift increases linearly with the Co content of the alloy. The hyperfine magnetic field distributions show a bimodal character that evidences the presence of two types of Fe environments in the as-cast alloys. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The kinetics of the glass transition and the crystallization process of Co[sub 62 - x]Fe[sub x]Nb[sub 6]Zr[sub 2]B[sub 30] (x = 0,16) amorphous alloys with large supercooled liquid region was investigated by differential scanning calorimetry. The dependence of the glass transition on the heating rate was analyzed in terms of the Vogel-Fulcher-Tamman equation. The glass-forming ability of these alloys is discussed with respect to the width of the supercooled liquid region, the reduced glass transition temperature, and the fragility parameter. The studied alloys are found to be rather strong glass formers. The crystalline phases formed after the devitrification are identified by means of x-ray diffraction and thermomagnetic measurements. The complicated crystallization process requiring long-range atomic rearrangements seems to contribute to the high stability of the supercooled liquid against crystallization. [ABSTRACT FROM AUTHOR]

The glassf-forming ability of (Fe[sub x]Co[sub y]B[sub z]C[sub u])[sub 80]Si[sub 3]Al[sub 5]Ga[sub 2]P[sub 10] with x = 5 - 70, y = 0-63, z = 5-12, and u = 0- 5 amorphous alloys has been analyzed in terms of the width of the supercooled liquid region, the reduced glass transition temperature, and the Vogel-Fulcher-Tammann parameters. Substitution of Fe by Co slightly decreases the glass-forming ability of the studied alloys. The value of the fragility parameter m is discussed in the frame of the general classification scheme of glass-forming liquids. The crystalline phases formed during the first crystallization step are identified. Magnetic moment at low and room temperature, Curie temperature, room temperature magnetostriction, and coercivity decrease with increasing Co content. [ABSTRACT FROM AUTHOR]

This report presents an assessment of the efficiency and power density limitations of thermophotovoltaic (TPV) energy conversion systems for both ideal (radiative limited) and practical (defect-limited) systems. Thermodynamics is integrated into the unique physics of TPV conversion, and used to define the intrinsic tradeoff between power density and efficiency. The results of the analysis reveal that the selection of diode band gap sets a limit on achievable efficiency well below the traditional Carnot level. In addition it is shown that filter performance dominates diode performance in any practical TPV system and determines the optimum band gap for a given radiator temperature. It is demonstrated that for a given radiator temperature, lower band-gap diodes enable both higher efficiency and power density when spectral control limitations are included. The goal of this work is to provide a better understanding of the basic system limitations that will enable successful long-term development of TPV energy conversion technology. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents a study wherein an electron-beam-induced-current technique was extended to the case where the grain size is less than the diffusion length. Effect of grain boundary on carrier transport; Comparison of the measured grain size and the calculated grain-boundary-recombination velocity; Parameters that were used to characterize the epitaxial layer/substrate structure.

Presents a study which evaluated the effect of processing conditions on the interface state density from capacitance-voltage measurements on metal oxide semiconductor capacitors, particularly on gallium arsenide field effect transistors. Examination of the optimum annealing condition; Experiment; Results and discussion.

Presents a study which proposed a verification method to determine the defect parameters for gallium arsenide (GaAs) and silicon (Si). Assumption on the deep level transient spectroscopy; Problems with comparable minority-carrier emission rate; Derivation of accurate defect parameters; Details on deep levels in GaAs and Si.

Evaluates the electrical characterization of plasma-grown gallium arsenide oxides on gallium arsenide. Details on the experiment; Properties of gallium arsenide oxides; Information on the damage effects.

Examines heat treatment of bulk gallium arsenide using a phosphosilicate glass cap. Experimental procedure; Results of the study. Conclusions.

The kinetics of the glass transition and the crystallization process of Co(sub 62-x)Fe(sub x)Nb(sub 6)Zr(sub 2)B(sub 30)(x=0,16) amorphous alloys with large supercooled liquid region is investigated by differential scanning calorimetry. The glass-forming ability of these alloys is discussed with respect to the width of the supercooled liquid region, the reduced glass transition temperature, and the fragility parameter.

Amorphous samples of nominal composition Co62Nb6Zr2B30 have been prepared using mechanical alloying (MA) and rapid quenching (RQ) techniques. Differences appear in Curie temperature and the phases developed after crystallization. Refrigerant capacity is enhanced 20% in the MA-sample with respect to that of RQ-sample. Neglecting the demagnetizing factor of powder samples significantly affects the exponent n characterizing the field dependence of the maximum magnetic entropy change. [ABSTRACT FROM AUTHOR]

The influence of different compositional modifications on the magnetic entropy change and refrigerant capacity of Finemet, Nanoperm, HiTperm, and bulk amorphous alloys is presented. For all the studied alloys, the field dependence of the magnetic entropy change exhibits a quadratic dependence in the paramagnetic regime, a linear dependence in the ferromagnetic temperature range, and a potential law with a field exponent ∼0.75 at the Curie temperature. This exponent can be explained using the critical exponents of the Curie transition. It is shown that for alloys of similar compositional series, the magnetic entropy change follows a master curve behavior. [ABSTRACT FROM AUTHOR]

Presents information on a study which analyzed possible defects generated by low-energy ions during partially ionized beam (PIB) depositions. Illustration of several deep level transient spectroscopy spectra measured on different samples; Performance of a series of thermal anneal in a nitrogen ambient; Advantages of the PIB technique.