Microwave measurement of temperature and current dependences ofsurface impedance for high-Tc superconductors

Perturbation formulas for TE₀₁₁-mode dielectric rod resonator and for a TE₀₁₁-mode circular cavity resonator are derived to determine the surface impedance Z_s of superconductors from measured values of resonant frequencies and unloaded Q. The relation between the maximum surface current density of a superconductor, J_s, and output power from a signal generator is derived. On the basis of these analytical results, a measurement technique is proposed to evaluate the temperature and J_s dependencies of Z_s for superconductors. The measured results of the temperature dependence of Z_s for YBCO and copper plates are presented. From these results, it is verified that the dielectric resonator is suitable for measuring the surface reactance for YBCO. From these Z_s values the temperature dependences of the skin depth and the penetration depth and those of the complex conductivity are obtained on the basis of the two-fluid model. These measured values agree well with the theoretical curves     

AC losses in HTSC conductor elements

AC self-field losses and DC dynamic resistivity in HTSC conductor elements have been measured. The data are discussed with reference to the relevant theory. For two very different sample types, the J _c calculated on the basis of the AC self-field loss is about an order of magnitude lower than the J_c based on the 1-μV/cm DC voltage drop criterion often used for HTSC materials. This is attributed to the onset of resistive behavior at currents much lower than the reported J_c based on the 1-μV/cm criterion. The J_c determined from measurements of the dynamic resistivity of a melt-textured YBCO fiber is within 15% of that determined by magnetic measurements     

A novel and effective PECVD SiO2/SiN antireflectioncoating for Si solar cells

It is shown that sequential plasma-enhanced chemical vapor deposition (PECVD) of SiN and SiO₂ can produce a very effective double-layer antireflection (AR) coating. This AR coating is compared with the frequently used and highly efficient MgF₂/ZnS double layer coating. The SiO₂/SiN coating improves the short-circuit current (J_SC) by 47%, open-circuit voltage (V_OC) by 3.7%, and efficiency (Eff) by 55% for silicon cells with oxide surface passivation. The counterpart MgF₂/ZnS coating gives similar but slightly smaller improvement in V_OC and Eff. However, if silicon cells do not have the oxide passivation, the PECVD SiO₂/SiN gives much greater improvement in the cell parameters, 57% in J_SC, 8% in V_OC, and 66% in efficiency, compared to the MgF₂/ZnS coating which improves J_SC by 50%, V_OC by 2%, and cell efficiency by 54%. This significant additional improvement results from the PECVD deposition-induced surface/defect passivation. The internal quantum efficiency (IQE) measurements showed that the PECVD SiO₂/SiN coating a absorbs fair amount of photons in the short-wavelength range (<500 nm); however, the improved surface/defect passivation more than compensates for the loss in J_SC and gives higher improvement in the cell efficiency compared to the MgF₂/ZnS coating     

Evaluation of the quantization error in denominator-separable 2-Drecursive filters

The evaluation of the quantization error in two-dimensional (2-D) digital filters involves the computation of the infinite square sum J=Σ^∞_m=φΣ ^∞_n=φy² (m, n). A simple method is presented for evaluating J based on partial fraction expansion and using the residue method provided the Z-transform Y(Z₁, Z₂) of the sequence y(m, n) having quadrant support is a causal bounded input, bounded output (BIBO) stable denominator-separable rational function. The value of J is expressed as a sum of simple integrals which can easily be evaluated. The simple integrals are tabulated for ready reference. The proposed method is suitable for analytical as well as numerical computation and can easily be programmed     

On the semiconductor laser logarithmic gain-current densityrelation

The simplified relation, α=G₀ In (η _iJ/J₀), between material gain α and current density J is shown to be a very good shape approximation, for quantum wells and bulk materials, essentially independent of the type of recombination processes present. Simulations show that for a given material system, G₀ decreases by only about 30% from pure electron-hole-recombination-dominated to pure Auger-recombination-dominated. A generic quantum-well situation is explored to reveal the density of states and recombination coefficient dependence of G₀ and to formulate simple estimates for G₀. The results were tested against published data for eight quantum-well diode lasers. The predicted values of G ₀ were generally found to be in agreement with experiments only for the wider gap diodes. The discrepancies were attributed in part to carrier induced absorption, and it is shown that the formalism can be modified in selected cases to incorporate this without changing the basic form of the gain. A new expression which relates the temperature dependence of the measured parameters to the characteristic temperature, T₀, is provided     

Low-threshold-current-density vertical-cavity surface-emittingAlGaAs/GaAs diode lasers

The optimization of electrically pumped vertical-cavity surface emitters (VCSEs) with conductive semiconductor-stack rear reflectors and mirror-reflectivity products of 0.93 for minimum threshold current density, J_th, is presented. Devices of two different active layer thicknesses are fabricated: 3 μm, as for conventional devices, and 0.6 μm, which, according to theoretical calculations, provides J_th for lasers with a mirror reflectivity product in the 0.90-0.95 range. For structures of 0.16-μm thick active layers, J_th values as low as 10 kA/cm² are obtained, in good agreement with theory. Analysis shows that for active layer thicknesses ⩽1 μm, J_th reaches similar minimum values for double-heterostructure and multiquantum-well structure devices of the same active medium thickness. A brief discussion of thin active medium structures is presented     

AlInGaAs-AlGaAs strained single-quantum-well diode lasers

Diode lasers with a strained AlInGaAs active layer and AlGaAs confining and cladding layers are reported. Broad-stripe devices were fabricated in graded-index separate-confinement heterostructures grown by organometallic vapor-phase epitaxy on GaAs substrates and containing a single Al_yIn_xGa_1-x-yAs quantum well with 0.12⩽x⩽0.14 and five values of y between 0.05 and 0.17. With increasing Al content the emission wavelength decreases from 890 to 785 nm. The threshold current density J _th is less than 200 A-cm^-2, with one exception, and the differential quantum efficiency ranges from 78 to 81%. A preliminary reliability test was made on a saw-cut, uncoated broad-area device, with an Al_0.17In_0.12Ga_0.71As active layer, that was fabricated without using special precautions to minimize damage and was mounted junction side up. After 20 h of CW operations at a constant current of 1.125 times threshold, J _th has increased by only 3.5%     

Studies of diffused phosphorus emitters: saturation current,surface recombination velocity, and quantum efficiency

The surface recombination velocity s for silicon surfaces passivated with thermal oxide was experimentally determined as a function of surface phosphorus concentration for a variety of oxidation, anneal, and surface conditions. This was accomplished by measuring the emitter saturation current density J₀ of transparent diffusions for which the J₀ is strongly dependent on s. At the lowest doping levels, the value of s was confirmed by measurements of s on substrates with uniform phosphorus doping. The impact of these measurements on solar cell design is discussed     

InGaAs/InAlAs/InP collector-up microwave heterojunction bipolartransistors

Collector-up InGaAs/InAlAs/InP heterojunction bipolar transistors (HBTs) were successfully fabricated, and their DC and microwave characteristics measured. High collector current density operation (J_c>30 kA/cm²) and high base-emitter junction saturation current density (J₀>10^-7 A/cm²) were achieved. A cutoff frequency of f _t=24 GHz and a maximum frequency of oscillation f _max=20 GHz at a collector current density of J₀ =23 kA/cm² were achieved on a nominal 5-μm×10-μm device     

Quantum-mechanical constraints on electron-beam brightness

Bounds on the brightness and quality of fermion beams are derived from Pauli's exclusion principle and the finite density of quantum eigenstates for free fermions. These bounds are applied to electron-beam sources to derive limitations on the quality (γJ/Δγ_z) of the electron beams they generate. In the absence of electromagnetic fields, the quality of a relativistic electron beam of current density J and kinetic energy (γ-1)mc² cannot exceed (Jγπec)^1/2λ_c ^-3/2, where λ_c is the Compton wavelength. The normalized brightness cannot exceed 2Δγeccλ_c^-3, where Δγmc² is the electron energy spread, and will be 1/π times the beam quality for a beam of maximal quality. It is concluded that the qualities of electron beams produced by existing linear accelerators for free-electron lasers could be increased by six to eight orders of magnitude before reaching the quality limit     

Theoretical and experimental studies of the effects of compressiveand tensile strain on the performance of InP-InGaAs multiquantum-welllasers

The authors have studied, both theoretically and experimentally, the effects of biaxial strain upon the performance characteristics of broad-area InP-InGaAsP-In_xGa_1-xAs (0.33⩽x ⩽0.73) separate confinement heterostructure multiquantum-well lasers. The theoretical calculations include the effects of strain on the bandstructure and the Auger recombination rates. A pronounced dependence of the threshold current density J_th upon x is observed. The lowest measured J_th is 589 A/cm² in an 800-μm laser with x=0.68. Also, internal quantum efficiencies as high as unity and loss coefficients as low as 5.6 cm^-1 have been measured for x=0.58     

Pulse dispersion distortion in open and shielded microstrips usingthe spectral-domain method

The spectral-domain method is used to compute the effective dielectric constant [ϵ_r/_eff(f)] of open and shielded microstrip lines to analyze the dispersion distortion of short electrical pulses. Precise expressions for the longitudinal and transverse current distributions allow a high level of accuracy for ϵ_r/_eff(f). It is determined that computation time can be minimized for the open microstrip calculations by using the shielded microstrip formulation provided large dimensions for the conducting walls are assumed     

Optimized silicon-rich oxide (SRO) deposition process for 5 V onlyflash EEPROM applications

A process for depositing in-situ very-thin (<10 nm) SiO₂ films on top of a silicon-rich oxide (SRO) layer in a standard low-pressure chemical vapor deposition (LPCVD) reactor has been optimized. Polysilicon-gate MOS capacitors using this stacked dielectric have shown high tunneling current at low voltages and an extraordinary endurance to electrical stress. Capacitors with 7 nm LPCVD SiO₂ on top of 10 nm SRO did not show any relevant shift on either the low or high portion of the I-V characteristic, after a fluence of more than 500 C/cm² at J=0.1 A/cm² . The results add further support to the usefulness of implementing these stacked dielectric structures in a variety of nonvolatile memory devices     

Trellis coding with multidimensional QAM signal sets

Trellis coding using multidimensional quadrature amplitude modulation (QAM) signal sets is investigated. Finite-size 2D signal sets are presented that have minimum average energy, are 90° rotationally symmetric, and have from 16 to 1024 points. The best trellis codes using the finite 16-QAM signal set with two, four, six, and eight dimensions are found by computer search (the multidimensional (multi-D) signal set is constructed from the 2-D signal set). The best moderate complexity trellis codes for infinite lattices with two, four six, and eight dimensions are also found. The minimum free squared Euclidean distance and number of nearest neighbors for these codes were used as the selection criteria. Many of the multi-D codes are fully rotationally invariant and give asymptotic coding gains up to 6.0 dB. From the infinite lattice codes, the best codes for transmitting J, J+1/4, J+1/3, J+1/2, J+2/3, and J+3/4 b/sym (J an integer) are presented     

Bounds and constructions of disjoint sets of distinct differencesets

An (I,J)-DDD is a set of I disjoint sets of distinct difference sets each having J elements. A number of constructions are given. Upper and lower bounds on the maximal element in a DDD (disjoint distinct difference) set are given. It is shown that regular DDD sets exist for I≳4J     

Potential methods for the fabrication ofhigh-Tc superconductors for wires and cables

Studies of practical properties, such as upper critical field (H_C2) and critical-current density, of high-T _c oxide superconductors of Y-Ba-Cu-O Bi-Sr-Ca-Cu-O, and Tl-Ba-Ca-Cu-O systems are reviewed. The H_c2 of these materials is much higher than that of conventional metallic superconductors, indicating a high potential for practical applications, even when they are used in liquid nitrogen. However, the H_C2 of these materials is also highly anisotropic, as can be expected from an examination of their crystal structure. In addition to this anisotropy, the presence of weak links and a weak pinning force in this material limit the transport current density J₃ to much lower levels than that required for practical applications. Recent fabrication processes that have good potential for producing wire or tape conductors or high-T_c oxide are reviewed. Some details are presented of the powder method and other fabrication processes using diffusion, solidification, and deposition techniques. For the Ag-sheathed oxide tapes, J_c values exceeding 10000 A/cm² at 77 K and 0 T have been reported for both Bi and Tl oxide materials     

Monte Carlo studies of the effect of emitter junction grading onthe electron transport in InAlAs/InGaAs heterojunction bipolartransistors

InAlAs/InGaAs HBTs with various emitter junction gradings are simulated using a self-consistent Monte Carlo simulator. The effects of the emitter junction grading and the shift of the emitter-base p-n junction into the emitter depletion region due to diffusion of the base dopant are investigated. A minimum transit time of 1.18 ps is predicted for an In(Ga_1-xAl_x)As grading with x=0.6 at the E-B interface and J_C=0.7×10⁵ A/cm². Graded-base designs do not offer any transit time performance improvement compared with the graded E-B approach. For transient performance, the device switching time is found to remain constant at about 2.2 ps up to x₀~0.7 but increases for larger values. A cutoff frequency as high as 270 GHz was observed for x₀=0.7, indicating that the best transport can be achieved from intermediately graded rather than abrupt E-B junction designs     

Microwave shielding effectiveness of EC-coated dielectric slabs

Correct formulas for the microwave shielding effectiveness (SE ) of a thin metallic layer deposited on top of a dielectric slab are derived. For coatings much thinner than the skin depth, the following holds: (a) in a half-wave geometry, SE is a function of a sheet resistance only, SE (in dB)=20×log(1+188.5/R_s) if R_s is in ohms per square; (b) in a quarter-wave geometry, SE (in dB)=20×log[(1+ε_r)/(2√ε _r)+188.5/(√ε_rR_s)], where ε_r refers to the dielectric constant of the substrate. These formulas provide upper and lower limits for the effective shielding performance of an electroconductive coated dielectric slab     

The new two-dimensional electron gas base HBT (2DEG-HBT):two-dimensional numerical simulation

The bipolar/FET characteristics of the 2DEG-HBT are analyzed extensively by a two-dimensional numerical simulator based on a drift-diffusion model. For bipolar operations at high collector current densities, it is confirmed that the cutoff frequency f_T is determined mainly by the collector transit time of holes and by the charging time of the extrinsic base-collector capacitance C _bc^EXT. The charging times of the emitter and base regions and the base transit time are shown to be negligible. A high cutoff frequency F_T (88 GHz) and current gain h_FE (760) are obtained for an emitter size of 1×10 μm², and undoped collector thickness of 150 nm, and a collector current density J_c of 10⁵ A/cm². The FET operation of the same 2DEG-HBT structure shows a threshold voltage V_th of 0.74 V, the transconductance G_m^max of 80 mS/mm, and maximum cutoff frequency F_T^max of 15 GHz. The dependence of the device performance on material parameters is analyzed extensively from a device design point of view     

New low capacitance transverse junction stripe AlGaAs/GaAs laserfor planar laser-MESFET integration

A transverse junction stripe laser structure with a very low parasitic capacitance, compatible with an entirely planar laser-MESFET integration, is presented. The laser, obtained by two successive n (S) and p(Zn) diffusions across undoped AlGaAs/GaAs double heterostructure layers, has a threshold current of I_th =55 mA. A very low parasitic capacitance C_s=0.6 pF is measured, with a corresponding rolloff time constant of t_r=9 ps