All-pass filters based on CCII− and CCCII−

This paper proposes a first-order all-pass filter using one negative type second-generation current conveyor (CCII?), two resistors and a capacitor. From the proposed CCII? based filter, by replacing the CCII? and a resistor by a negative second-generation current-controlled conveyor (CCCII?) a new filter is developed which requires only one each of CCCII?, capacitor and resistor. The experimental and simulated results are found in good agreement with theoretical analysis. The circuits are also analyzed for non-ideal current conveyors to show that they still function as all-pass filters but with slight altered values of gain and phase.     

Modeling of Ga1−xInxAs1−y−zNySbz/GaAs quantum well properties for near-infrared lasers

The aim of this work is to model the properties of GaInAsNSb/GaAs compressively strained structures. Indeed, Ga_1?xIn_xAs_1?y?zN_ySb_z has been found to be a potentially superior material to GaInAsN for long wavelength laser dedicated to optical fiber communications. Furthermore, this material can be grown on GaAs substrate while having a bandgap smaller than that of GaInNAs. The influence of nitrogen and antimony on the bandgap and the transition energy is explored. Also, the effect of these two elements on the optical gain and threshold current density is investigated. For example, a structure composed of one 7.5 nm thick quantum well of material with In=30%, N=3.5%, Sb=1% composition exhibits a threshold current density of 339.8 A/cm² and an emission wavelength of 1.5365 μm (at T=300 K). It can be shown that increasing the concentration of indium to 35% with a concentration of nitrogen and antimony, of 2.5% and 1%, respectively, results in a decrease of the threshold current density down to 253.7 A/cm² for a two well structure. Same structure incorporating five wells shows a threshold current density as low as 221.4 A/cm² for T=300 K, which agrees well with the reported experimental results.     

Au−Ni−Sn intermetallic phase relationships in eutectic Pb−Sn solder formed on Ni/Au metallization

Recent work has shown that a Au−Ni−Sn ternary compound with a nominal composition of Au_0.5Ni_0.5Sn₄ redeposits and grows at the interface between eutectic Pb−Sn solder and Ni/Au metallization during aging at 150°C. The present work verifies the existence of the Au_0.5Ni_0.5Sn₄ phase by examining the Sn-rich corner of the Au−Ni−Sn ternary phase diagram. The reconfiguration mechanism of the AuSn₄ from the bulk solder is also discussed, with detailed observations of the Au_0.5Ni_0.5Sn₄ microstructure. The results show that the Ni solubility limit in the AuSn₄ phase is approximately 12 at.% at 150°C and thus, the Au_0.5Ni_0.5Sn₄ phase is a ternary AuSn₄-based compound with high Ni solubility. Due to the slight solubility and the fast diffusion of Au in the eutectic Pb−Sn at 150°C, the AuSn₄ intermetallics in the bulk solder can reconfigure to form a Au_xNi_1−xSn₄ compound at the interface where Ni is available. The Au_xNi_1−xSn₄ compound layer consists of nanocrystals arranged in a larger grainlike morphology. It appears that the inherent lattice strain of the Au_xNi_1−xSn₄ compound and the volume change due to its formation results in a nanocrystalline microstructure.     

Fundamentals of Ge1−xSnx and SiyGe1−x-ySnx RPCVD epitaxy

We have studied epitaxial growth of Ge_1−xSn_x and Si_yGe_1−x-ySn_x materials in 200 mm and 300 mm industrial CVD reactors using industry standard precursors. The growth kinetics of undoped GeSn were firstly studied via varying growth parameters including growth temperatures, GeH₄and SnCl₄precursor flows, which indicated that the material growth is highly dependent on surface kinetic limitations involving the SnCl₄reaction pathway. Secondly, the growth kinetics of doped layer growth by varying the growth temperatures and the PH₃and B₂H₆dopants flows were investigated. It was shown that B₂H₆had the effect of increasing the growth rate and decreasing the Sn incorporation whereas PH₃had no effect on the growth rate but increased the Sn incorporation. Thirdly, the SiGeSn growth kinetics using SiH₄, GeH₄, and SnCl₄as precursors were discussed, which revealed that the careful control of the growth rate was required to produce compositionally homogenous SiGeSn alloy. Moreover, the material and optical characterizations have been conducted to examine the material quality. Finally, the GeSn quantum well structure was grown to exhibit the precise control of the growth parameters.     

Reinvestigation of Thermoelectric Properties of n- and p-Type Ba8−d Au x Si46−x−y Clathrate

We have synthesized n- and p-type clathrates Ba_8?d Au _x Si_46?x?y with various Au contents (4.6 < x < 6.0) by arc-melting, annealing at 1173 K, and spark plasma sintering at 1073 K. The Au compositions found by wavelength-dispersive x-ray spectrometry for the synthesized samples were slightly lower than the nominal compositions. Ba_7.8Au_4.6Si_41.4 and Ba_7.7Au_4.9Si_41.1 samples showed n- and p-type conduction, respectively. According to the electron count (Ba²⁺)₈Au(^3?)_5.33Si_40.67, the clathrate composition with x = 5.33 is expected to be an intrinsic semiconductor. Our experimental results show that increase of the Au composition causes a transition from n-type to p-type conduction between x = 4.6 and 4.9. We have also calculated the band structures of the Ba₈Au _x Si_46?x clathrate including a vacancy by ab initio calculation based on density functional theory with structure optimization. It was found that the vacancy behaves like an electron acceptor and the numbers of vacancies at 24k sites for the synthesized Ba₈Au _x Si_46?x?y clathrates can be estimated as ～0.4 in a unit cell.     

Thermoelectric Performance of Multiple-Doped Co4Sb12−x−y−z Ge x Te y S z Skutterudite Compounds

CoSb₃ skutterudites multiply doped with Ge, Te, and S were synthesized by solid-state reaction and spark plasma sintering. x-Ray diffraction studies revealed that Ge, Te, and S entered the lattice of the CoSb₃ compounds, and while Te increased the lattice volume, Ge and S decreased it. Compared with the undoped and single-doped CoSb₃ compounds, the thermal conductivity and lattice thermal conductivity are significantly suppressed due to greatly increased point defect scattering. It is found that S is more effective for decreasing the lattice thermal conductivity than Te and Ge. The highest thermoelectric figure of merit, ZT, exceeds 1.1 for the Co₄Sb_11.25Ge_0.05Te_0.63S_0.07 compound at 800 K.     

Vacancies in Hg1−xCdxTe

Measurements have been performed of the carrier concentrations in vacancy-doped Hg_1−xCd_xTe with x=0.22, 0.29, 0.45, and 0.5. Anneals to establish the carrier concentrations were performed on both the mercury- and tellurium-rich sides of the phase field. When these results were added to earlier data for x=0.2 and 0.4, and assuming that all vacancies are doubly ionized, then vacancy concentrations for all values of x and anneal temperature can be represented by simple equations. On the mercury side of the phase field, the vacancy concentrations varied as 2.50×10²³(1−x) exp[−1.00/kT] for low concentrations, and as 3.97×10⁷(1−x)^1/3n _i ^2/3 exp[−0.33/kT] for high concentrations, where n_i is the intrinsic carrier concentration. On the tellurium rich side, the vacancy concentrations varied as 2.81 × 10²²(1−x) exp[−0.65/kT] for low concentrations and as 1.92×10⁷(1−x)^1/3n _i ^2/3 exp[−0.22/kT] for high concentrations.     

Optical band gap of Cd1−x MnxTe and Zn1−x MnxTe semiconductors

The dependence of the optical band gap for Zn_1?x Mn_xTe and Cd_1?x Mn_xTe semiconductor compounds was investigated by the methods of cathodoluminescence and optical reflection. It was found that, for Zn_1?x Mn_xTe compounds in the region x?0.2, the band gap is additionally broadened by a magnitude of about 0.08 eV, which is related to the high density of interstitial-type defects in single crystals. For x?0.3, the probability of the existence of these defects decreases substantially, which is related to the distortion of tetrahedra of the crystal lattice of Zn_1?x Mn_xTe by Mn atoms, which are incorporated into each tetrahedron.     

Pure-Mode Gm−C Biquad Filters*

A continuous time differential voltage mode G_m–C biquad and its adjoint current-mode version are described, employing FCS (floating current source) circuits as building blocks. The biquad operates in the pure mode, i.e. no resistors are used to convert internally voltage into current or vice versa. Both f_p and Q_p are tunable by current sources applied externally. The current mode version is capable of providing values of Q_p 160. Due to the increasing Loop transmission, THD improves significantly towards low frequencies. The input stage of the voltage mode version employs two OTAs, whose outputs are connected in parallel in order to add their differential inputs. This results in the hitherto unnoticed property of subtracting their common mode input, providing an open loop gain which is high for differential, but low for common mode signals.     

Defects in Zn1−x−yCoxMgyO nanoparticles: Probed by XRD,RAMAN and PAS techniques

Wurtzite Zn_1?x?yMg_xCo_yO nanoparticles of size 14–20 nm are synthesized by the conventional coprecipitation route and are analyzed using XRD, FESEM, UV–visible, Raman, and Positron annihilation spectroscopic techniques. XRD patterns reveal formation of a single wurtzite phase of ZnO on adding Mg, Co or both. In addition to six Raman active modes corresponding to the wurtzite structure of space group C_6ν⁴, we also observe additional Raman modes at 519, 544 and 673 cm^?1 irrespective of the dopant type and concentration. These modes exactly match with the silent vibrational modes of ZnO lattice as calculated by the ab initio calculations. From positron life time measurements, we observe that while the shortest lifetime τ₁, the lifetime of positrons that annihilate in the grain boundary regions match well with the lifetime of positrons in a defect free ZnO (τ₁～158 ps), the intermediate lifetime, τ₂ of all three samples match with the life time of positron annihilating at the cluster of (Zn+O) di-vacancies. We conclude that the origin of additional Raman modes is not due to impurities as reported in the literature rather is due to host lattice defects.     

Long-wavelength photodiodes based on Ga1−x InxAsySb1−y wit composition near the miscibility boundary

The possibility of using liquid-phase epitaxy to obtain Ga_1−x In_xAs_ySb_1−y solid solutions isoperiodic with GaSb near the miscibility boundary is investigated. The effect of crystallographic orientation of the substrate on the composition of the solid solutions grown in this way is examined, and the indium concentration is observed to grow from 0.215 to 0.238 in the Ga_1−x In_xAs_ySb_1−y solid phase in the series of substrate orientations (100), (111)A, (111)B. A change in the composition of the solid solution leads to a shift of the long-wavelength edge of the spectral distribution of the photosensitivity. The use of a GaSb (111)B substrate made it possible, without lowering the epitaxy temperature, to increase the indium content in the solid phase to 23.8% and to create long-wavelength photodiodes with spectral photosensitivity threshold λ _th=2.55 μm. The primary characteristics of such photodiodes are described, along with aspects of their fabrication. The proposed fabrication technique shows potential for building optoelectronic devices (lasers, LED’s, photodiodes) based on Ga_1−x In_xAs_ySb_1−y solid solutions with red boundary as high as 2.7 μm. Fiz. Tekh. Poluprovodn. 33, 249–253 (February 1999)     

Photocatalytic Coupling of CH4 and CO2 to Ethanol on Asymmetric Ce−O−Zn Sites

Partial oxidation of methane (CH₄) to value-added products is significantly challenging due to the highly inert chemical property of CH₄ at ambient conditions and easy over-oxidation into carbon dioxide (CO₂) or carbon monoxide (CO) at elevated temperatures and pressures. Targeting this challenge, the efficient photocatalytic coupling of CO₂ and CH₄ into ethanol is demonstrated, using a cerium (Ce)-doped zinc oxide (ZnO) photocatalyst with abundant Ce─O─Zn units. Under light illumination, CO₂ is adsorbed on the Ce atoms and photo-reduced to CO, and CH₄ is captured by the Zn atoms and photo-oxidized to hydroperoxymethane (CH₃OOH). The close proximity of Ce and Zn atoms on the Ce─O─Zn units allowed to further efficiently couple the as-formed CO and CH₃OOH into ethanol. Without additional Oxygen (O₂) oxidant or sacrificial regent, the ethanol production rate reached 580 µmol g⁻¹ h⁻¹, substantially exceeding previously reports on photocatalytic CH₄ oxidation. This work features to convert two greenhouse gases into value-added chemicals with adjacent and asymmetric reaction sites, suggesting attractive potentials for CH₄ and CO₂ utilization.     

Structure and optical properties of heterostructures based on MOCVD (Al x Ga1 − x As1 − y P y )1 − z Si z alloys

Epitaxial heterostructures produced by MOCVD on the basis of Al _x Ga_{1 ? x} As ternary alloys with the composition parameter x ≈ 0.20–0.50 and doped to a high Si and P atomic content are studied. Using the high-resolution X-ray diffraction technique, scanning electron microscopy, X-ray microanalysis, Raman spectroscopy, and photoluminescence spectroscopy, it is shown that the epitaxial films grown by MOCVD are formed of five-component (Al _x Ga_{1 ? x} As_{1 ? y} P _y )_{1 ? z} Si _z alloys.     

Superstructured ordering in Al x Ga1 − x As and Ga x In1 − x P alloys

Epitaxial heterostructures produced on the basis of Al _x Ga_{1 ? x} As and Ga _x In_{1 ? x} P ternary alloys by metal-organic chemical vapor deposition are studied. The composition parameter x of the alloys was ～0.50. By X-ray diffraction studies, scanning electron microscopy, atomic force microscopy, and photoluminescence spectroscopy, it is shown that superstructured ordered phases with the stoichiometry composition III_{1 ? η}III_{1 + η}V₂ can be formed. As a consequence of this effect, not only does the cubic crystal symmetry change to the tetragonal type in the new compound, but also the optical properties become different from those of disordered alloy with the same composition.     

Hole scattering mechanisms in Hg1−xCdxTe

In this paper, we analyze and discuss the roles of nine different scattering mechanisms—ionized impurity, polar and nonpolar optical, acoustic, dislocation, strain field, alloy disorder, neutral impurity, and piezoelectric—in limiting the hole mobilities in p-type Hg_1−xCd_xTe crystals. The analysis is based on obtaining a good fit between theory and experiment for the light and heavy hole drift mobilities by optimizing certain unknown (or at the most vaguely known) material parameters such as the heavy hole mobility effective mass, degree of compensation, and the dislocation and strain field scattering strengths. For theoretical calculations, we have adopted the relaxation time approach, keeping in view its inadequacy for the polar scattering. The energy dispersive hole relaxation times have been drawn from the published literature that take into account the p-symmetry of valence band wave functions. The temperature dependencies of multiple charge states of impurities and of Debye screening length have been taken into account through a numerical calculation for the Fermi energy. Mobility data for the present analysis have been selected from the HgCdTe literature to represent a wide range of material characteristics (x=0.2–0.4, p=3×10¹⁵–1×10¹⁷ cm⁻³ at 77K, μ_peak≅200-1000cm²V⁻¹s⁻¹). While analyzing the light hole mobility, the acoustic deformation and neutral impurity potentials were also treated as adjustable. We conclude that

Large VLWIR Hg1−xCdxTe photovoltaic detectors

Very long wavelength infrared (VLWIR; 15 to 17 μm) detectors are required for remote sensing sounding applications. Infrared sounders provide temperature, pressure and moisture profiles of the atmosphere used in weather prediction models that track storms, predict levels of precipitation etc. Traditionally, photoconductive VLWIR (λ_c >15 μm) detectors have been used for sounding applications. However, photoconductive detectors suffer from performance issues, such as non-linearity that is 10X – 100X that of photovoltaic detectors. Radiometric calibration for remote sensing interferometry requires detectors with low non-linearity. Photoconductive detectors also suffer from non-uniform spatial optical response. Advances in molecular beam epitaxy (MBE) growth of mercury cadmium telluride (HgCdTe) and detector architectures have resulted in high performance detectors fabricated in the 15 μm to 17 μmm spectral range. Recently, VLWIR (λ_c ∼ 17 μm at 78 K) photovoltaic large (1000 μm diameter) detectors have been fabricated and measured at flux values targeting remote sensing interferometry applications. The operating temperature is near 78 K, permitting the use of passive radiators in spacecraft to cool the detectors. Detector non-AR coated quantum efficiency >60% was measured in these large detectors. A linear response was measured, while varying the spot size incident on the 1000 μm detectors. This excellent response uniformity, measured as a function of spot size, implies that low frequency spatial response variations are absent. The 1000 μm diameter, λ_c ∼ 17 μm at 78 K detectors have dark currents ∼160 μA at a −100 mV bias and at 78 K. Interfacing with the low (comparable to the contact and series resistance) junction impedance detectors is not feasible. Therefore a custom pre-amplifier was designed to interface with the large VLWIR detectors operating in reverse bias. A breadboard was fabricated incorporating the custom designed preamplifier interfacing with the 1000 μm diameter VLWIR detectors. Response versus flux measurements were made on the large VLWIR detectors and non-linearity <0.15% was measured at high flux values in the 2.5×10¹⁷ to 3.5×10¹⁷ ph-cm⁻²sec⁻¹ range. This non-linearity is an order of magnitude better than for photoconductive detectors.     

Organometallic VPE growth of AlxGa1−xAs

The " hybrid" organometallic VPE process for the growth of Al_xGa_l-xAs has been explored. Six growth parameters have been considered; substrate orientation, substrate temperature during growth, total flow rate and ratios of Al, HC1 and As to the total group III flow rate. The effects of these six growth parameters on the growth process (growth rate, composition and surface morphology) and the materials properties (carrier concentration, photoluminescence intensity and spectrum) have been systematically studied. A technique has been developed for the growth of heterostructures by changing only the H₂ diluent flow rate. This results in high quality heterostructures with x changing from 0.05 to 0.30 in > 100 å.     

Improved operability in Hg1−xCdxTe detector arrays

As liquid phase epitaxial (LPE) growth and array fabrication processes have matured to give excellent wafer average performance, the yield limiter for infrared focal plane arrays (IRFPAs), especially large ones, have become outages. In this work, significant progress has been made in identifying the source and eliminating outages from LPE grown Hg_1−xCd_xTe P-on-n structures. Historically, studies of the sources of outages have employed defect etches to look for dislocations and other crystalline defects, and secondary ion mass spectroscopy (SIMS), imaging SIMS, and sputter initiated resonance ion spectrometry (SIRIS) to look for impurities at critical interfaces. Using these techniques, trends were established, but direct correlation with outages have been observed. In LPE grown materials, where the dislocation densities are always below 5×10⁵ cm⁻², and often below 1×10⁵ cm⁻² on CdZnTe substrates, dislocations only account for a few outages. In order to understand the source(s) of outages, a failure analysis was performed on several long wavelength IRFPAs. Using a dilute etchant, the metals and then cap layers of some 64×64 pixel IRFPAs which had excellent average performance, but suffered from a high density of pixels with excessive leakage current, were removed. Using a scanning electron microscope with energy dispersive spectroscopy capability, the presence of carbon particles was correlated with excessive leakage current on a 1:1 pixel basis. A series of experiments was then conducted which isolated the source of the particles to the cap layer growth process, which was consequently changed to eliminate them. The process improvements have reduced the particle density to below the measurement limit of the optical measurement technique implemented to monitor the density of particles on witness wafers. These improvements are resulting in IRFPAs with significantly improved operability.     

Band-edge photoluminescence of heavily doped InxGa1−x As1−y Py(λ=1.2 µm)

Band-edge photoluminescence spectra of heavily donor-doped samples of In_xGa_1−x As_1−y P_y (x=0.77, y=0.53) were investigated in the temperature range (77–300) K. A theory of luminescence that takes into account fluctuations in the band-edge potentials due to nonuniform distribution of impurities is used to calculate temperature dependences of the positions and half-widths of peaks in these spectra. Good agreement is obtained between experimental and calculated curves. For heavily doped In_xGa_1−x As_1−y P_y samples with either p-or n-type conductivity, the peak energy of the band-edge PL is observed to shift towards lower frequencies at low temperatures. This shift is accompanied by broadening of the spectra and a decrease in the photoluminescence intensity compared to the analogous parameters for the spectra of undoped material. Possible mechanisms for radiative recombination are analyzed. Fiz. Tekh. Poluprovodn. 33, 907–912 (August 1999)