Using La0.5Sr0.5CoO3 bottom electrode layers, Bi4Ti3O12 thin films were grown on LaAlO3(001), Al2O3(0001), and Si(001) substrates. Crystalline orientation of the Bi4Ti3O12 thin films was examined by x-ray diffraction techniques. The cross-sectional microstructures of Bi4Ti3O12/La0.5Sr0.5CoO3 heterostructures are investigated. It is found that the crystalline orientation and the microstructure affect leakage current behavior of the Bi4Ti3O12 layers.

We analyze the computational complexity of sphere decoding (SD) for maximum likelihood detection (MLD) according to initial radius selection schemes, and also propose an efficient initial radius reduction scheme that reduces further the initial radius. As the initial radius for SD, we use the Euclidean distance between the received signal vector and the lattice vector corresponding to a suboptimum initial estimate. The proposed initial radius reduction scheme selects a new lattice vector closer to the received signal vector than the initial lattice vector in order to reduce the initial radius further. From our analyses, the reduction in the overall complexity due to further reduction of initial radius gets more significant as the SNR decreases. The ZF-DFE scheme in a combination with the proposed radius reduction scheme has the fewest computations over practical SNR range for communications, and its computations are less than that of the vertical Bell-labs layered space-time (V-BLAST) detection scheme with optimal ordering, even at low SNR values achieving an uncoded bit error rate (BER) of 0.1.

In this paper, we propose a generalized structure for constructing space-time codes (STCs), which achieves generalized optimum diversity (GOD). A GOD class of STCs can achieve the full diversity and an optimized coding gain with minimum delay for any number of transmit antennas (Nt) and any rate (R), if the rate is less than or equal to the rank of the channel matrix. The structure combines mutually exclusive R data symbols with complex weights at each transmit antenna and the same set of data symbols is repeated at every time slot in a code block, but is transmitted through different transmit antennas with different sets of complex weights. In addition, it does not need any restrictions to the complex weights, thus the code can be designed with the maximum degrees of freedom. Optimized codes based on the proposed structure are obtained under the well-known rank and determinant criteria by applying two design constraints in order to reduce the design complexity. Even using two constraints, however, the proposed structure has an unaffordable design complexity when the number of complex weights gets larger. Hence, we provide some guidelines for code design, which can further reduce the design complexity by examining the characteristics of the determinant of codeword difference matrices

We propose a generalized code structure for constructing space-time codes (STCs) that can achieve generalized optimum diversity (GOD) with minimum delay. As a GOD code, it can achieve the full diversity and a maximum coding gain for any number of transmit antennas ( t N ) and any rate ( R ), if the rate is less than or equal to the rank of the channel matrix. As a minimum delay code, the structure requires only t N time slots in order to achieve the full diversity. The proposed structure combines mutu- ally exclusive R data symbols with complex weights at each transmit antenna and the same set of data symbols is repeated at every time slot in a code block, but is transmitted through different transmit antennas with different sets of complex weights. In addi- tion, the structure does not need any restrictions to the complex weights, thus the code can be designed with the maximum degrees of freedom. The design of an optimum code would be done by finding the set of the complex weights so that the minimum abso- lute determinant of code word difference matrices could be maxi- mized. The optimum code could be easily designed so that two types of constraints, that is, the power constraints and the orthogo- nality constraint could be satisfied. The two constraints make the channel resource fully utilized when the channel state information (CSI) is not available at the transmitter and also reduce to the uni- tary constraint for capacity lossless code design in the case of t N R = . We give two STC design examples: (1) an STC with rate 2 for two transmit antennas and (2) an STC with rate 3 for three transmit antennas. We also provide some guidelines for code de- sign. Computer simulations are performed to demonstrate the per- formance of the proposed designs. From simulation results, we see that the proposed code structure and code designs can achieve GOD with minimum delay.

The inherent background coefficients that exactly describe the background amplitudes in the scattered field have been presented for the scattering of plane acoustic waves by a system of concentrically multilayered solid and/or fluid shells submerged in a fluid. The coefficients have been obtained by replacing the mechanical surface admittance function with the zero-frequency limit of the admittance function for the analogous fluid system, where the shear wave speeds in the solid layers are set to zero. By taking advantage of the concept of incoming and outgoing waves, we find the surface admittance function for the fluid system in such a form that the analytical generalization for any number of layers and the physical interpretation are very easy. The background coefficients obtained are independent of the bulk wave speeds in the system: they depend on the mass densities and the thicknesses of the shells. With increasing frequency, the inherent background undergoes a transition from the soft to the rigid background. The transition frequency fbr the spherical system is lower than that for the cylindrical system, but the difference becomes negligible in high order partial waves. The resonance coefficients that directly describe the resonance amplitudes in the scattered field are also found, and the validity of the analysis is numerically demonstrated for the case of empty spherical shells of rubber-coated steel and aluminum submerged in water.

The effect of multiple quantum well (MQW) parameters on the free carrier distribution (FCD) and the apparent carrier distribution (ACD) obtained from capacitance–voltage (C–V) profiling has been investigated using a self-consistent simulation technique and the C–V profiling technique. The FCD in MQW structures reveals large outer peaks and small inner peaks even when all the parameters of each quantum wells (QWs) are the same. Such a feature becomes more apparent as the barrier layer becomes thinner or the barrier doping level becomes lower. These characteristics are found to originate from the fact that the density of carriers confined in each well is mostly determined by the depletion region formed alongside the well via the charge neutrality condition. The ACD is found to vary drastically as the thickness or the doping level of barrier changes. When the Debye averaging process is prominent, the ACD peaks are broader and smaller than the FCD peaks and are displaced toward the bottom layer side. The ACD inner peaks even disappear completely when the Debye screening length is comparable to or larger than the QW period, while real free carriers are well confined in each well. The effect of temperature on the ACD through the Debye averaging process is also investigated. When the Debye length is much smaller than the QW period, the full width at half maximum of the ACD is determined by the change of the position expectation value of the two-dimensional differential carriers. This change of position expectation value is found to be much smaller than the well width for relatively narrow QWs. The accuracy of our simulation results is confirmed by the excellent agreements between the simulated ACDs and the measured ACDs of In0.2Ga0.8As/GaAs MQWs. As an example of extracting the qualitative informations from the measured C–V profile, the C–V profiles of partially strain relaxed InxGa1−xAs/GaAs MQWs with x=0.15 and x=0.25, grown by metal organic chemical vapor deposition, are discussed. These results show systematically how the QW parameters affect the FCD and the ACD.

The effects of partial dopant ionization on the capacitance–voltage (C–V) characteristics of δ-doped structures have been investigated using self-consistent simulations. The simulation results show that partially ionized δ-doped dopants should produce a much sharper C–V profile compared with the case of fully ionized dopants. The results reveal also that the main factor which determines the spatial resolution and the full width at half maximum of the C–V peak is the spatial extent of the dopant profile rather than the spatial extent of the ground-state wave function. From this, it is suggested that the δ-doped C–V data should be interpreted always with the effect of partial ionization of δ-doped dopants.

The temperature dependence of the spatial resolution of capacitance-voltage (C−V) profiles in the compositional quantum well (CQW) is investigated. The apparent carrier distribution (ACD) peak in the In0.2Ga0.8As/GaAs single QW is observed to show a strong temperature dependence, compared to that in Si δ-doped GaAs. The ACD peak in CQW is wider (narrower) than the spatial extent of ground-state wave function at high (low) temperatures. The self-consistent numerical simulations on the carrier distribution show that the full width at half maximum of ACD peak in CQW is mainly affected by the debye averaging process at high temperatures and the change in the position expectation value of the two-dimensional electrons at low temperatures. This change in the position expectation value is found to be much smaller than the spatial extent of ground-state electron wave function.

Deep donor levels in group VI impurity-doped In0.18Ga0.82As0.28P0.72 grown on GaAs0.61P0.39 substrates by liquid phase epitaxy are studied by deep level transient spectroscopy and thermally stimulated capacitance measurements. The activation energies of deep donor levels are found to be 0.26, 0.23, and 0.14 eV for S-, Se-, and Te-doped In0.18Ga0.82As0.28P0.72, respectively, and all of the samples show persistent photoconductivity. It is suggested that group VI impurities in In0.18Ga0.82As0.28P0.72 form DX centers. Two deep levels with the activation energies of 0.15 and 0.48 eV are detected in undoped sample.

Carrier profiles of multiple quantum wells are studied using self-consistent simulations. The free carrier density of the well is found to be distributed nonuniformly and symmetrically, although the doping level in barriers is uniform. The calculated apparent carrier density obtainable from the capacitance-voltage profile is found to be distributed asymmetrically. Simulation results show that, even if electrons are confined in quantum wells, the apparent electron distribution can be flattened if barrier thickness or doping level in barriers are reduced to such an extent that the Debye length is comparable to the barrier thickness.

Electronic properties of partially relaxed InxGa1−xAs/GaAs multiple quantum well (MQW) structures are investigated using capacitance–voltage (C–V) profiling and deep level transient spectroscopy (DLTS). As the In composition becomes large, the depletion of carriers confined in QWs and the concentration of dislocation-related deep traps are increased. The carrier depletion is observed to occur predominantly in the QWs adjacent to the bottom layer. This depletion is believed to be due to electron capture at the acceptor-like misfit dislocation-related traps. Our results thus show that the C–V and DLTS measurements, combined with the numerical simulation of C–V profiles, can be used to study the influence of nonuniformly distributed misfit dislocations on the carrier distribution in MQW structures.

A self-consistent simulation technique for the analysis of capacitance-voltage profiles of quantum-well (QW) structures, which iteratively solves the one-dimensional Schrodinger and Poisson equations, is develolped. Using this simulation technique, the carrier profiles in QW structures with one, three, and five QWs are investigated. The carrier-density distributions of the wells are found to be not identical even when the parameters of the wells and the barriers are the same. It is found that the wells adjacent to a wide top or bottom layer confine more carriers than the other wells. The apparent carrier distribution peaks are smaller and broader compared with the free carrier distribution peaks. Also, the former peaks are found to be shifted from the well positions while the latter peaks remain at the well positions. These results are attributed to the Debye averaging process.

Anomalous increase of band gap energy was observed in the temperature-dependent photoluminescence spectra of S-, Se- and Te-doped In0.32Ga0.68P. This blue shift could be explained by the change of many body effect of the conduction electrons due to the electron capture at the DX centers. The ratio of the electron concentration captured at the DX centers to the free electron concentration could be determined empirically from the values of the observed band gap energy shift. The ratios were 0.40, 0.12 and 0.15 for S-, Se- and Te-doped In0.32Ga0.68P samples with dopant concentration of mild 1017 cm−3, respectively.

The effects of (NH4)2Sx treatments on the interface traps in Au/n-GaAs and Au/n-In0.5Ga0.5P Schottky contacts are investigated by deep level transient spectroscopy measurements. The interface trap concentration in Au/GaAs increases when the residual S overlayer is sublimated after (NH4)2Sx treatments. But the trap concentration decreases when the S overlayer is rinsed by de-ionized water. In Au/InGaP, the phosphorus vacancy-related interface traps are passivated effectively by the (NH4)2Sx treatment. These results are attributed to the fact that S bonds with As but not with P.

Deep level properties of S-, Se-, and Te-doped In1-xGaxP layers have been studied by DLTS and capacitance-temperature measurements. In In0.32Ga0.68P layers, S, Se, and Te each form deep states whose activation energies are 0.26, 0.23, and 0.14 eV, respectively. In In0.49Ga0.51P layer, only S forms a deep state. From the investigation of defect formation by impurity species, the compositional dependence of binding energy, and the persistent photoconductivity, these deep donors are attributed to the DX centers. The binding energy of S DX center is observed to be the largest in the investigated DX centers.

Properties of deep levels in S-, Se-, and Te-doped In0.18Ga0.82As0.2P0.72 grown on GaAs0.61P0.39 substrates by liquid phase epitaxy are studied by deep level transient spectroscopy and thermally stimulated capacitance measurements. The donor-related deep levels are observed and their activation energies are found to be 0.26, 0.23, and 0.14 eV for S-, Se-, and Te-doped In0.18Ga0.82As0.28P0.72, respectively. Persistent photoconductivity is observed in all the samples doped with these impurities. It is clear from these results that the S, Se, and Te donors form DX centers in In0.18Ga0.82As0.28P0.72.

We suggest a model which can explain the shifting of carrier concentration peaks in the temperature-dependent capacitance-voltage carrier profiles of heterojunction (HJ) structures. The shift of concentration peaks, which was frequently observed in the inverted isotype HJs was previously attributed to the traps at the heterointerface. The main feature of our model is the role of band offset as a limiter to the test signal current. The model can explain the difference of the peak shift in the carrier profiles of the normal and inverted type HJs. According to this model, the peak shifts at low temperatures occur naturally for the inverted type HJs.

The effects of (NH4)2Sx solution treatment on the interface properties of metal‐In0.5Ga0.5P Schottky contacts have been investigated by capacitance‐voltage measurements and deep‐level transient spectroscopy measurements. The (NH4)2Sx‐treated samples show Schottky barrier heights that are more sensitive to the metal work functions. It is also found that (NH4)2Sx treatment of In0.5Ga0.5P can passivate the phosphorus‐vacancy‐related interface deep traps of Schottky contacts as well as suppress the generation of interface deep traps due to heat treatment.