Interaction Features of Stepped-Impedance Stripline Resonators in Comb Filters

Mixed coefficients of coupling between the closely spaced stepped-impedance resonators in comb filters of stripline design have been investigated. Transmission zeros at frequencies f _zi correspond to mixed coupling coefficients k _i . These zeros can be moved with respect to the filter passband central frequency f₀ by modifying the shape of resonators. It was proved that the reduction of gap between resonators made it possible to locate frequencies f _z and f₀ closer to one another. The existing restrictions on the minimal value of gap between resonators limit the degree of proximity between f _z and f₀. The N-resonator stripline comb filters with mixed coupling can have N?1 transmission zeros. The absence of cross-coupling links in stripline filters simplifies their construction. It has been established that the thickness of central conductors of stripline resonators affects the positive and negative mixed coupling coefficients. The paper presents measurement data of miniature stripline three-resonator comb filter having an enhanced selectivity at the expense of two transmission zeros. The central frequency of filter is f₀ = 1850 MHz, the bandwidth BW = 100 MHz. The filter having dimensions 5.8×4.2×2 mm was implemented by connecting two ceramic substrates having relative dielectric permittivity ε _r = 92 and the metallized patterns deposited on them.     

Electron transport and detection of terahertz radiation in a GaN/AlGaN submicrometer field-effect transistor

Electron transport and photoresponse in the terahertz range in a GaN/AlGaN field-effect transistor with the submicrometer gate (0.25 μm) and two-dimensional electron gas in the channel (the electron concentration n _s = 5 × 10¹² cm^?2) were studied at 4.2 K. The charge-carrier mobility in the transistor’s channel μ = 3500 cm²/(V s) was determined from the dependence of the conductance on magnetic field. It is found that the dependence of photovoltage at the radiation frequency f = 574 GHz on the gate voltage (i.e., on the concentration of two-dimensional electrons) features a characteristic maximum, which is related to a resonance response of the subgate plasma in the transistor channel.     

Low-frequency noise in gallium nitride epitaxial layers with different degrees of order of mosaic structure

The correlation between the noise level 1/f and the degree of mosaic-structure order in gallium nitride epitaxial layers was studied for the first time. Samples with a doping level of N _d ?N _a ≈8×10¹⁶ cm^?3 and a relatively high degree of order were characterized by the Hooge parameter α≈1.5×10^?3. This value is unprecedently low for thin GaN epitaxial films. The Hooge parameter was significantly higher for samples with N _d ?N _a ≈1.1×10¹⁸ cm^?3 and a low degree of order despite the fact that α generally decreases with increasing doping level at the same degree of order. Thus, the degree of mosaic-structure order affects not only the optical and electrical characteristics but also the fluctuation parameters of GaN epitaxial layers.     

Graphene-Based\ Waveguide Terahertz Wave Attenuator

We design an electrically controllable terahertz wave attenuator by using graphene. We show that terahertz wave can be confined and propagate on S-shaped graphene waveguide with little radiation losses, and the confined terahertz wave is further manipulated and controlled via external applied voltage bias. The simulated results show that, when chemical potential changes from 0.03 into 0.05 eV, the extinction ratio of the terahertz wave attenuator can be tuned from 1.28 to 39.42 dB. Besides the simplicity, this novel terahertz wave attenuator has advantages of small size (24?×?30 μm²), a low insertion loss, and good controllability. It has a potential application for forthcoming planar terahertz wave integrated circuit fields.     

Microwave Dielectric Properties of BiCu<Subscript>2</Subscript>PO<Subscript>6</Subscript> Ceramics with Low Sintering Temperature

A BiCu₂PO₆ microwave dielectric ceramic was prepared using a solid-state reaction method. As the sintering temperature increased from 800°C to 880°C, the bulk density of BiCu₂PO₆ ceramic increased from 6.299 g/cm³ to 6.366 g/cm³; the optimal temperature was 860°C. The best microwave dielectric properties [permittivity (? _r ) = ～16, a quality factor (Q × f) = ～39,110 GHz and a temperature coefficient of resonant frequency (τ _f ) = ～?59 ppm/°C] were obtained in the ceramic sintered at 860°C for 2 h. Then, TiO₂ with a positive τ _f (～+400 ppm/°C) was added to compensate the τ _f value. The composite material was found to have a near-zero τ _f (+2.7 ppm/°C) and desirable microwave properties (? _r  = 19.9, Q × f = 24,885 GHz) when synthesized at a sintering temperature of 880°C. This system could potentially be used for low-temperature co-fired ceramics technology applications.     

A Modified 8<Emphasis Type="Italic">f</Emphasis> Geometry with Reduced Optical Aberrations for Improved Time Domain Terahertz Spectroscopy

We present a modified 8f geometry for time domain terahertz (THz) spectroscopy (TDTS) experiments. We show, through simulations and data, that a simple rearranging of the off-axis parabolic mirrors, which are typically used to focus and direct THz radiation in TDTS experiments, results in a nearly 40 % reduction in the THz focal spot diameter. This effect stems from significant reduction of the principle optical aberrations which are enhanced in the conventional 8f geometry but partially compensated in the modified 8f experimental setup. We compare data from our home-built TDTS spectrometer in the modified 8f geometry to that of previous iterations that were designed in the conventional 8f geometry to demonstrate the effect.     

Functionalized Multiwalled Carbon Nanotube Electrochemical Sensor for Determination of Anticancer Drug Flutamide

An electrochemical sensor based on functionalized multiwalled carbon nanotubes (MWCNT_f) has been developed and applied for determination of anticancer drug flutamide in pharmaceutical and artificial urine samples. The electrode was prepared by modifying a glassy carbon electrode with MWCNT_f, denoted herein as MWCNT_f/GCE. The MWCNT_f/GCE electrode exhibited high catalytic activity, high sensitivity, and high stability and was applicable over a wide concentration range for flutamide. The effects of the scan rate, pH, and nature of the electrolyte on the electrochemical behavior of flutamide on the MWCNT_f/GCE were investigated. The results showed that this electrode presented the best square-wave voltammetric response to flutamide in Britton–Robinson buffer solution at pH 5.0 at frequency of 50 Hz and amplitude of 0.06 V. The proposed sensor presents a wide linear response range from concentration of 0.1 μmol L^?1 up to 1000 μmol L^?1 (or 27.6 μg L^?1 up to 0.27 g L^?1), with limit of detection, limit of quantification, and sensitivity of 0.03 μmol L^?1, 0.1 μmol L^?1, and 0.30 μA μmol^?1 L, respectively. The MWCNT_f/GCE electrode was successfully applied for determination of flutamide in pharmaceutical formulations and artificial urine samples, giving results in agreement with those obtained by a comparative method described in literature. A paired Student’s t-test revealed no statistical difference between the reference and proposed method at 95% confidence level. The average recovery for fortified samples was 101 ± 1%.     

Structural and Electrical Properties of Ag/<Emphasis Type="Italic">n</Emphasis>-TiO<Subscript>2</Subscript>/<Emphasis Type="Italic">p</Emphasis>-Si/Al Heterostructure Fabricated by Pulsed Laser Deposition Technique

We have investigated the structural and electrical characteristics of the Ag/n-TiO₂/p-Si/Al heterostructure. Thin films of pure TiO₂ were deposited on p-type silicon (100) by optimized pulsed laser ablation with a KrF-excimer laser in an oxygen-controlled environment. X-ray diffraction analysis showed the formation of crystalline TiO₂ film having a tetragonal texture with a strong (210) plane as the preferred direction. High purity aluminium and silver metals were deposited to obtain ohmic contacts on p-Si and n-TiO₂, respectively. The current–voltage (I–V) characteristics of the fabricated heterostructure were studied by using thermionic emission diffusion mechanism over the temperature range of 80–300 K. Parameters such as barrier height and ideality factor were derived from the measured I–V data of the heterostructure. The detailed analysis of I–V measurements revealed good rectifying behavior in the inhomogeneous Ag/n-TiO₂/p-Si(100)/Al heterostructure. The variations of barrier height and ideality factor with temperature and the non-linearity of the activation energy plot confirmed that barrier heights at the interface follow Gaussian distributions. The value of Richardson’s constant was found to be 6.73 × 10⁵ Am^?2 K^?2, which is of the order of the theoretical value 3.2 × 10⁵ Am^?2 K^?2. The capacitance–voltage (C–V) measurements of the heterostructure were investigated as a function of temperature. The frequency dependence (Mott–Schottky plot) of the C–V characteristics was also studied. These measurements indicate the occurrence of a built-in barrier and impurity concentration in TiO₂ film. The optical studies were also performed using a UV–Vis spectrophotometer. The optical band gap energy of TiO₂ films was found to be 3.60 eV.     

Low-temperature (77 K) impurity breakdown in <Emphasis Type="Italic">p</Emphasis>-type 4<Emphasis Type="Italic">H</Emphasis>-SiC

The impact ionization of acceptors in aluminum-doped 4H-SiC epitaxial films (Al concentration 2 × 10¹⁵ cm^?3) at a temperature of 77 K is studied. It is found that the impact-ionization coefficient exponentially depends on the reverse electric field: α _p = α*_pexp(?F*/F). The largest ionization coefficient is α* _p = 7.1 × 10⁶ cm^?3 s^?1, and the threshold field is F* = 2.9 × 10⁴ V/cm.     

Detection of terahertz radiation by resonant tunneling nanoheterostructures

The phenomenon of terahertz radiation detection by resonant tunneling structures (RTSs) has been studied. The calculations of the changes ΔI₀ in the direct current (DC) component I₀ under the action of an alternating electric field were carried out by the solution of a nonstationary Schrodinger equation with a time-periodic electric field based on the Floquet mode expansion of the wave functions. The dependences of the DC component I₀ in resonant tunneling structures on the frequency ν and AC signal amplitude V_ac have been built. It is shown that the ΔI₀ value in triple-barrier RTSs at resonance frequency hv ≈ E_r2–E_r1 (E_r1 and E_r2 are the energies of the size-quantized levels) can exceed a low-frequency value by more than an order of magnitude. The parameters of the structures have been optmized, in order to use them in the terahertz radiation detectors in the anbsence of an external bias. The possibility of tuning the resonance frequency in the terahertz range by changing the DC bias has been shown.     

Features of the infrared reflection spectra of SmS semiconductor in the homogeneity range

Infrared reflection spectra in the range of 5200–380 cm^?1 were measured for polycrystalline samarium monosulfide samples in the semiconductor phase with compositions Sm_{1 + x} S lying within the homogeneity range (0 ≤ x ≤ 0.17) in the temperature range of 300–600 K. Five peaks with energies in the range of 1150–880 cm^?1, whose positions depend weakly on the composition and temperature. It was shown that the reflection peaks are associated with ⁷ F ₀–⁷ F ₂ transitions of 4f electrons of Sm²⁺ ions.     

Observation of Ultrafast Magnon Dynamics in Antiferromagnetic Nickel Oxide by Optical Pump-Probe and Terahertz Time-Domain Spectroscopies

We have studied the ultrafast magnon dynamics in an antiferromagnetic 3d-transition-metal monoxide, nickel oxide (NiO), using optical pump-probe spectroscopy and terahertz time-domain spectroscopy (THz-TDS). THz damped magnon oscillations were observed in the Faraday rotation signal and in the transmitted THz electric field via optical pump-probe spectroscopy and THz-TDS, respectively. The magnon signals were observed in both the optical pump-probe spectroscopy and THz-TDS experiments, which shows that both Raman- and infrared-active modes are included in the NiO magnon modes. The magnon relaxation rate observed using THz-TDS was found to be almost constant up to the Néel temperature T _N (= 523 K) and to increase abruptly near that temperature. This shows that temperature-independent spin-spin relaxation dominates up to T _N . In our experiment, softening of the magnon frequency near T _N was clearly observed. This result shows that the optical pump-probe spectroscopy and THz-TDS have high frequency resolution and a high signal to noise ratio in the THz region. We discuss the observed temperature dependence of the magnon frequencies using three different molecular field theories. The experimental results suggest that the biquadratic contribution of the exchange interaction plays an important role in the temperature dependence of the sublattice magnetization and the magnon frequency in cubic antiferromagnetic oxides.     

Special features of radiation-defect annealing in silicon <Emphasis Type="Italic">p-n</Emphasis> structures: The role of Fe impurity atoms

Deep-level transient spectroscopy is used to study the formation of complexes that consist of a radiation defect and a residual impurity atom in silicon. It is established that heat treatment of the diffused Si p⁺-n junctions irradiated with fast electrons lead to the activation of a residual Fe impurity and the formation of the FeVO (E_0.36 trap) and FeV₂ (H_0.18 trap) complexes. The formation of these traps is accompanied by the early (100–175°C) stage of annealing of the main vacancy-related radiation defects: the A centers (VO) and divacancies (V₂). The observed complexes are electrically active and introduce new electron (E_0.36: E _t ^e =E _c -0.365 eV, σ _n =6.8×10^?15 cm²) and hole (H_0.18: E _t ^h =E _v +0.184 eV, σ _p =3.0×10^?15 cm²) levels into the silicon band gap and have a high thermal stability. It is believed that the complex FeVO corresponds to the previously observed and unidentified defects that have an ionization energy of E _t ^e =E _c ?(0.34–0.37) eV and appear as a result of heat treatment of irradiated diffused Si p⁺-n junctions.     

Analysis of the magnetic properties of (La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>0.93</Subscript>MnO<Subscript>3</Subscript> manganites at high temperatures

For the first time, the Faraday method is used to measure the temperature dependence of paramagnetic susceptibility χ(T) of (La_{1 ? x} Sr _x )_0.93MnO₃ (x = 0.2, 0.25, or 0.3) manganites in the temperature interval 60–850°C. It is demonstrated that the dependences have two kinks and three linear sections. The kink of curve χ^?1(T) is related to polymorphic transformations (Q′ → Q* and Q* → R) that take place in the crystal lattices of the samples. The main magnetic characteristics of the samples are determined with the least-squares processing of curve χ^?1. Is is demonstrated that dependence χ^?1(T) obeys the Curie-Weiss law. The energy state of the magnetoactive manganese atom in the Q′-and Q*-phase samples is close to the energy state of a free Mn²⁺ ion. In the R phase, this state is close to the state of a free Mn³⁺ ion.     

Investigation of the ZnS-CdHgTe interface

The ZnS-Cd_xHg_1?xTe interface was investigated using the capacitance-voltage characteristics of MIS structures in experimental samples. During fabrication of the n⁺-p junctions based on p-Cd_xHg_1?xTe, the density of states within the range N _ss =(1–6)×10¹¹ cm^?2 eV^?1 at T=78 K was obtained. The experiments showed that the conditions in which n⁺-p junctions are fabricated only slightly affect the state of the ZnS-CdHgTe interface. The negative voltages of the at bands V _FB , even if immediately after deposition of the ZnS films V _FB >0, point to the enrichment of the ZnS-p-CdHgTe near-surface layer with majority carriers, specifically, holes. This led to a decrease in the leakage current over the surface. During long-term storage (as long as ～15 years) in air at room temperature, no degradation of differential resistance R _d , current sensitivity S _i , and detectivity D* of such n⁺-p junctions with a ZnS protection film was observed.     

Terahertz absorption and emission upon the photoionization of acceptors in uniaxially stressed silicon

Experimental data on the spontaneous emission and absorption modulation in boron-doped silicon under CO₂ laser excitation depending on the uniaxial stress applied along the [001] and [011] crystallographic directions are presented. Room-temperature radiation is used as the probe radiation. Low stress (less than 0.5 kbar) is shown to reduce losses in the terahertz region by 20%. The main contribution to absorption modulation at zero and low stress is made by A⁺ centers. Intersubband free hole transitions additionally contribute to terahertz absorption at higher stress. These contributions can be minimized by compensation.     

Electrical properties and luminescence spectra of light-emitting diodes based on InGaN/GaN heterostructures with modulation-doped quantum wells

The distribution of charged centers N(w), quantum efficiency, and electroluminescence spectra of blue and green light-emitting diodes (LED) based on InGaN/AlGaN/GaN p-n heterostructures were investigated. Multiple InGaN/GaN quantum wells (QW) were modulation-doped with Si donors in GaN barriers. Acceptor and donor concentrations near the p-n junction were determined by the heterodyne method of dynamic capacitance to be about N _A ≥ 1 × 10¹⁹ cm^?3 ? N _D ≥ 1 × 10¹⁸ cm^?3. The N(w) functions exhibited maxima and minima with a period of 11–18 (±2–3 nm) nm. The energy diagram of the structures has been constructed. The shifts of spectral peaks with variation of current (J=10^?6–3×10^?2 A) are smaller (13–12 meV for blue and 20–50 meV for green LEDs) than the corresponding values for the diodes with undoped barriers (up to 150 meV). This effect is due to the screening of piezoelectric fields in QWs by electrons. The dependence of quantum efficiency on current correlates with the charge distribution and specific features in the current-voltage characteristics.     

Iodine Doping of CdTe and CdMgTe for Photovoltaic Applications

Iodine-doped CdTe and Cd_1?x Mg _x Te layers were grown by molecular beam epitaxy. Secondary ion mass spectrometry characterization was used to measure dopant concentration, while Hall measurement was used for determining carrier concentration. Photoluminescence intensity and time-resolved photoluminescence techniques were used for optical characterization. Maximum n-type carrier concentrations of 7.4 × 10¹⁸ cm^?3 for CdTe and 3 × 10¹⁷ cm^?3 for Cd_0.65Mg_0.35Te were achieved. Studies suggest that electrically active doping with iodine is limited with dopant concentration much above these values. Dopant activation of about 80% was observed in most of the CdTe samples. The estimated activation energy is about 6 meV for CdTe and the value for Cd_0.65Mg_0.35Te is about 58 meV. Iodine-doped samples exhibit long lifetimes with no evidence of photoluminescence degradation with doping as high as 2 × 10¹⁸ cm^?3, while indium shows substantial non-radiative recombination at carrier concentrations above 5 × 10¹⁶ cm^?3. Iodine was shown to be thermally stable in CdTe at temperatures up to 600°C. Results suggest iodine may be a preferred n-type dopant compared to indium in achieving heavily doped n-type CdTe.     

Electron traps in thin layers of low-temperature-grown gallium arsenide with As-Sb nanoclusters

Electron traps in low-temperature-grown ～40-nm-thick GaAs layers containing nanometer As-Sb clusters have been studied using deep-level transient spectroscopy. Measurements at various bias voltages and small-amplitude filling pulses have allowed the identification of two groups (T1 and T2) of traps with substantially different thermal electron emission rates. It is shown that the density of traps T2 (with an activation energy of 0.56 ± 0.04 eV and electron capture cross section of 2 × 10^?13?10^?12cm²) is ～2 × 10¹²cm_?2, while the density of traps T1 (0.44 ± 0.02 eV and 2 × 10^?14?10^?13 cm², respectively) is ten times lower. It is assumed that, according to the existence of the two cluster groups observed in the layers under study, traps T2 are associated with clusters 4–7 nm in diameter and traps T1, with clusters up to ～20 nm in diameter.     

Terahertz luminescence of GaAs-based heterostructures with quantum wells under the optical excitation of donors

Spontaneous emission from selectively doped GaAs/InGaAs:Si and GaAs/InGaAsP:Si heterostructures is studied in the frequency range of ～3–3.5 THz for transitions between the states of the two-dimensional subband and donor center (Si) under the condition of excitation with a CO₂ laser at liquid-helium temperature. It is shown that the population inversion and amplification in an active layer of 100–300 cm^?1 in multilayered structures with quantum wells (50 periods) and a concentration of doping centers N _D ≈ 10¹¹ cm^?2 can be attained under the excitation-flux density 10²³ photons/(cm² s).