Free carrier absorption and lattice vibrational modes in bulk ZnO

The electronic properties of semiconductors are highly dependent on carrier scattering mechanisms determined by crystalline structure, band structure, and defects in the material. Experimental characteristics of lattice vibrational modes and free carrier absorption in single-crystal ZnO samples obtained from different sources are presented in this work to provide a further understanding of carrier scattering processes pertaining to electronic properties. Infrared absorption measurements indicate strong absorption peaks due to a combination of optical and nonpolar phonon modes in the 9–13 μm spectral region. The Raman spectra obtained for these samples similarly reveal the presence of these phonon modes. Infrared absorption measurements also demonstrate free carrier absorption in the 3–9 μm spectral region for higher conductivity samples, where a λ^m dependence is observed with m=2.7–3, indicating both longitudinal optical phonon scattering and ionized impurity scattering. From these results, we show that infrared absorption can be used as a routine nondestructive technique to determine the material characteristics and quality of bulk ZnO.     

Infrared studies of be-doped GaAs grown by molecular beam epitaxy at low temperatures

Samples of molecular beam epitaxial GaAs grown at low temperatures doped with Be defects are studied as a function of growth temperature (T_G)-by measuring their localized vibrational modes at 77K using BOMEM Fourier transform infrared spectrometer. Localized vibrational modes of⁹Be_Ga in samples grown at T_G>350°C have been identified at 482 cm⁻¹. Secondary ion mass spectroscopy measurements show that the densities of Be defects remain approximately constant as T_G is lowered, however, additional structure in the⁹Be_Ga localized vibrational mode is observed. Calculations based on Green's function theory suggest that the additional structure in Be-doped LT GaAs can best be explained in terms of a complex center [⁹Be_Ga-As_Ga] involving an intrinsic defect.     

Layer-number dependent high-frequency vibration modes in few-layer transition metal dichalcogenides induced by interlayer couplings

Two-dimensional transition metal dichalcogenides (TMDs) have attracted extensive attention due to their many novel properties. The atoms within each layer in two-dimensional TMDs are joined together by covalent bonds, while van der Waals interactions combine the layers together. This makes its lattice dynamics layer-number dependent. The evolutions of ultralow frequency (<50 cm^-1) modes, such as shear and layer-breathing modes have been well-established. Here, we review the layer-number dependent high-frequency (>50 cm^-1) vibration modes in few-layer TMDs and demonstrate how the interlayer coupling leads to the splitting of high-frequency vibration modes, known as Davydov splitting. Such Davydov splitting can be well described by a van der Waals model, which directly links the splitting with the interlayer coupling. Our review expands the understanding on the effect of interlayer coupling on the high-frequency vibration modes in TMDs and other two-dimensional materials.     

Metallic mesh properties and design of submillimeter filters

By using a Michelson interferometer in the asymmetric mode with helium cooled bolometer, we have measured with precision the complex transmittance and reflectance of metallic meshes in the wavelength region 0.7<λ/g<5, where λ is the wavelength, and g is the period of the mesh. For λ?g we observed a minimum power reflectance smaller than 10^?4 at normal incidence. The important variations of the transmittance with the angle of incidence have been thoroughly investigated. Changes around the maximum transmittance are explained by the propagation of diffracted modes. Phase measurements show that a sharp dip appearing at λ_d with 1<λ_d/g<1.8 is related to the finite thickness of the mesh. From our measurements as well as other data precedently published it appears that there is a linear dependance between λ_d/g and the relative width of the slots of the mesh. All these deviations from classical models must absolutely be taken in account when designing high performances far infrared filters.     

Raman study of defects in a GaAs buffer layer grown by low-temperature molecular beam epitaxy

Raman scattering measurements on high-resistivity layers of GaAs grown by molecular beam epitaxy at low temperature are presented. Several defect-related features are ob-served, including two peaks attributed to quasi-localized vibrational modes of point de-fects, one with a frequency of 223 cm⁻¹ similar to a mode previously observed in elec-tron-irradiated GaAs, and the other with a frequency of 47 cm⁻¹ similar to a mode observed in ion-implanted GaAs. We suggest that these are due to arsenic interstitials and gallium vacancies, respectively. We also observe peaks at 200 and 258 cm⁻¹, which we believe may be due to vibrational modes in small clusters of arsenic. The 223 cm⁻¹ mode is the only defect-related mode still observed after a 10-min annealing treatment at 600° C, although it is significantly broader and has different symmetry from the 223 cm⁻¹ mode in the unannealed material. This indicates that the 223 cm⁻¹ mode in the annealed material is due, at least in part, to a defect other than the arsenic interstitial.     

Cryptanalysis of Triple Modes of Operation

Multiple modes of operation and, in particular, triple modes of operation were proposed as a simple method to improve the strength of blockciphers, and in particular of DES. Developments in the cryptanalysis of DES in recent years have popularized the triple modes of DES, and such modes are now considered for ANSI standards. In a previous paper we analyzed multiple modes of operation and showed that the security of many multiple modes is significantly smaller than expected. In this paper we extend these results, with new cryptanalytic techniques, and show that all the (cascaded) triple modes of operation are not much more secure than a single encryption—in the case of DES they can be attacked with up to an order of 2 ⁵⁶ —2 ⁶⁶ chosen plaintexts or ciphertexts and complexity of analysis. We then propose several candidates for more secure modes. Received 19 August 1996 and revised 29 September 1997     

Effect of Metal Substitution in BSCCO Ceramic Superconductors

The fabrication, electrical, and optical properties of ceramic Bi_2-xM_xSr₂Ca₂Cu₃O_y (where M=Al or Ni and x=0.3) superconductors are described. Resistivity measurements reveal that when Al is partially substituted at the bismuth site in the BiSrCaCuO compound, T _c decreases to 77 K and the nickel-substituted compound shows a T _c of 70 K, compared to the undoped ceramic BiSrCaCuO samples with T _c of 85 K. Infrared reflectance measurements, which cover the 50-4000 cm^?1 range, find that the reflectance from the ceramic samples decreases in the metal-doped samples. The observed phonon modes in the infrared conductivity spectra (obtained by Kramers-Kronig analysis) decrease in strength and some features are completely smeared out. Furthermore some of the modes are observed to shift slightly in the frequency on doping, the change in the T _c and vibrational modes is attributed to destabilization of 2223 phase.     

Nearly resonant third-order optical non-linearity in c60 cast films

We have carried out measurements of the third-order optical non-linearityX⁽³⁾ of C₆₀ cast films using the z-scan technique. The measurements have shown that two-photon absorption is the dominant non-linear process around the 3.76 eV absorption region. The real and imaginary parts of X⁽³⁾ of C₆₀ films have been measured at 665 nm are found to be 2.8 × 10^?8 and ?3.2 × 10^?8 esu respectively.     

Effect of low-temperature annealing on characteristics of SiC detectors with radiation-induced defects

p ⁺-n-n ⁺ detector structures based on CVD films with an uncompensated donor concentration of 2 × 10¹⁴ cm^?3 have been studied. The p ⁺-region was created by implantation of Al ions. The detectors were preliminarily irradiated with 8-MeV protons at a fluence of 3 × 10¹⁴ cm^?2 and then annealed at 600°C for 1 h. In measurements performed in the temperature range 20–150°C, the forward-and reverse-bias modes were compared. It is shown that the annealing leads to a higher collection efficiency of carriers generated by nuclear radiation and to a decrease in the amount of charge accumulated by traps in the course of testing. Despite the positive effect of the annealing, a considerable amount of radiation defects remain, which is manifested, in particular, in the kinetics of the forward current.     

Analytic theory of a tapered gyrotron resonator

An analytic theory has been derived for determining the eigenfrequencies, RF-field distribution and Q of the TE_mpq modes of a gyrotron resonator consisting of a circular cylinder joined to a slowly tapered section. Explicit results are obtained for a linear taper. The cavity modes are found to have an RF-field distribution which is useful for prebunching the electron beam and enhancing efficiency. For high Q cavities, the cavity Q depends on axial mode number q as q^?2, which is important for mode discrimination. Proper selection of taper length is found to reduce the Q of high q modes, also aiding in mode discrimination. The present approach may be applied to other forms of weakly irregular cavities, such as cavities with nonlinear tapers.     

A Complete Frequency-Field Chart for the Antiferromagnetic Resonance in MnF2

Antiferromagnetic resonance (AFMR) measurements on a well characterized single crystal sample of MnF₂ have been performed using various kinds of millimeter and submillimeter sources up to 570 GHz and a superconducting magnet up to 20 T. All the AFMR modes predicted from the theory have been observed. We report a complete frequency-field chart for these AFMR modes.     

Boron and nitrogen im gallium phosphide grown by the liquid encapsulated czochralski process

Boron and nitrogen concentrations have been determined, using charged particle activation analysis, in a number of GaP ingots grown by the Liquid Encapsulated Czochralski process in order to determine the extent of the chemical interaction between the Gap melt and the liquid B₂O₃ encapsulant, and the nitrogen ambient used to pressurize the crystal puller. The effects of deliberate oxygen, nitrogen and boron doping have also been investigated. Boron concentrations have been found in the range 3 x 10¹⁸ to3 x 10¹⁸ cm^-3, and boron incorporation is seen to be suppressed by the presence of oxygen and nitrogen. Channeled nuclear reaction and local mode absorption measurements have shown that the boron is predomin-antly located on substitutional gallium sites. The nitrogen gas used to pressurize the crystal puller results in nitrogen incorporation in the range 1–2 × 10¹⁷ cm⁻³. Comparison with optical absorption measurements indicates that the formulas used to determine nitrogen from optical measurements overestimate the nitrogen by a factor of 4. Leave of absence from the Physics Department, King's College, London     

Characterization of oxide traps in 0.15 μm MOSFETs using random telegraph signals

Random telegraph signals (RTS) have been used to characterize oxide traps of W×L=0.97×0.15 μm² medium-doped drain n-MOSFETs. RTS have been measured in the linear and saturation regions of operation, both in forward and reverse modes where the drain and source are reversed. The contribution of mobility fluctuations as well as number fluctuations to the amplitude of RTS has been investigated. The scattering coefficient due to screened Coulomb scattering effect is computed from the measured data as a function of channel carrier density. The depth of the position of the trap in the oxide from Si–SiO₂ interface is calculated utilizing the dependence of the emission and capture times on the gate voltage. In addition, the position of the trap along the channel with respect to the source is obtained using the difference in the drain voltage dependence of the capture and emission times between the forward and reverse modes. Knowing the location of the trap in the oxide and along the channel, the energy associated with the trap can be extracted accurately from the data. This technique allows one to evaluate the trap energy at the point where the trap is located without any assumptions about the location of the trap or the need for variable temperature measurements. The probed trap was found to be an acceptor type center (repulsive for an n-MOSFET) located at about 27 Å deep the oxide, half-way between drain and source with an energy of E_Cox−E_T=3.04 eV, slightly above the conduction band edge.     

Effect of extreme radiation fluences on parameters of SiC nuclear particle detectors

Detectors based on modern CVD-grown films were irradiated with 8 MeV protons at a fluence of 3 × 10¹⁴ cm^?2. The concentration of primary radiation defects was ～10¹⁷ cm^?3, which is three orders of magnitude higher than the concentration of the initially present uncompensated donors. The resulting deep compensation of SiC enabled measurements of detector parameters in two modes: under reverse and forward bias. The basic parameters of the detectors degraded by no more than a factor of 1.7, compared with the fluence of 1 × 10¹⁴ cm^?2. However, there appeared a polarization voltage, which indicates that a space charge is accumulated by radiation defects.     

Analysis of mode characteristics for hexagonal resonator lasers

Mode characteristics of hexagonal resonators are numerically simulated by the two-dimensional (2-D) finite-difference time-domain (FDTD) technique. For the hexagonal resonator with the side length of 3 μm and the refractive index of 3.2, the mode quality (Q) factors decrease from 10⁴ to 10² as the resonant wavelength increases from 1.1 to 3.2 μm. The modes have a relatively high Q factor as their even and odd states have different mode wavelengths and different Q factors, i.e., they are accidentally degenerate modes. The azimuthal mode numbers obtained from mode field distribution for the accidentally degenerate modes satisfy the mode number relation derived from the symmetry characteristics for the split of double-degenerate modes. Furthermore, the numerical results indicate that an output waveguide parallel to one of the sides is the better scheme for directional emission. For the hexagonal resonator with 3 m side length and 0.3 μm wide output waveguide, the Q factor of 5.05×10³ and output efficiency of 33% are obtained at the mode wavelength of 1.29 μm.     

The analysis of wideband spiral antennas using modal decomposition

This paper discusses modal decomposition and its application to wideband antenna measurements. Applying network analysis, a simple finite Fourier series decomposes measured far-field patterns into characteristic modes to determine the power in each mode. The technique is useful for identifying measurement inaccuracies and errors due to antenna design or manufacturing problems. The analysis can be applied to a general class of antennas that radiate a set of characteristic modes; however, this paper demonstrates the concept with wideband, multi-mode spiral antenna measurements.     

An accurate and compact large signal model for III–V HBT devices

An accurate and compact large signal model is proposed for modeling heterojunction bipolar transistors (HBTs) based on III–V materials. In DC mode, the model includes self-heating, Kirk and Early effects, as well as the temperature dependence of the model parameters. In small signal mode, the model captures the variation of various AC parameters with bias. The procedure of extracting the model parameters uses DC and multiple bias S-parameter measurements. The model is compiled in the HP–ADS circuit simulator as user-compiled model and is verified by comparing its simulations to measurements in all modes of operation for an AlGaAs/GaAs transistor with an emitter area of 2 × 25 μm².     

Input impedance of annular ring microstrip antenna using circuit theory approach

Input impedance and bandwidth of an annular ring microstrip antenna have been determined by modeling the antenna as a section of radial line loaded with wall admittances. The effect of mutual coupling between the radiating apertures has been taken into account. The theoretically calculated values of input impedance for TM12-mode are compared with measurements. The agreement is good. Higher order modes have been found to be present in the vicinity of TM12-mode. The reactance due to these modes can be utilized to increase the voltage standing-wave ratio (VSWR) bandwidth of the antenna.     

Tsc measurements of coupled levels in ion implanted Si

Thermally Stimulated Current (TSC) measurements have been made on deep states in ion implanted silicon. The nature of the resulting curves and the relationship of the peak to valley transition currents clearly preclude the conventional analysis based on independent emission to the valence band. The results have been analyzed with a model based on coupled levels with interlevel transitions and the fit yields reasonable energy levels and exponential pre-factors. The measurements were made by using shallow n⁺ guard ring diodes implanted with a low dose (3×10¹³ ions/cm²) of 1 MeV B⁺ ions and annealed at 350‡C for one hour. The energy was selected so that the end of the ion track would be in the depletion region of the diodes.     

Coupling constant of triple-clad long-period fiber grating

Because of its high sensitivity to external refractiveindex,the long-period fiber grating(LPFG) has beenused as chemical sensor1-3.Now the multi-clad LPFGwiththe fil ms coated on cladding area of the fiber grat-ing has drawn much attention[4].It cani mpro…