Narrow line-width resonant cavity enhanced photodetectors operating at 1.55 μm

A method for fabrication of long-wavelength narrow line-width InGaAs resonant cavity enhanced (RCE) photodetectors in a silicon substrate operating at the wavelength range of 1.3-1.6 μm has been developed. A full width at half maximum (FWHM) of 0.7 nm and a peak responsivity of 0.16 A/W at the resonance wavelength of 1.55 μm have been accomplished by using a thick InP layer as part of the resonant cavity. The effects of roughness and tilt of the InP layer surface, and its free carrier absorption, as well as the thickness deviation of the mirror pair on the resonance wavelength shift and the peak quantum efficiency of the RCE photodetectors are analyzed in detail, and approaches for minimizing them toward superior performance are suggested.     

Fluorination of perovskite-related phases of composition La1−xSrxFe1−yCoyO3−δ

We report here on the fluorination of the perovskite-related phases La_1−xSr_xFe_1−yCo_yO_3−δ. The introduction of fluorine in place of oxygen is achieved through a low-temperature (400 °C) reaction with poly(vinylidene fluoride). X-ray powder diffraction data show that in all cases the fluorination leads to an expansion in the unit cell, which is consistent with partial replacement of oxygen by fluorine and consequent reduction in the oxidation state of iron and/or cobalt. This reduction in oxidation state is confirmed by X-ray absorption- and Mössbauer-spectroscopy. The Mössbauer spectra show complex magnetically split hyperfine patterns for the fluorinated samples, reflecting the interactions between Fe³⁺ ions, which are not possible in oxides containing Fe⁴⁺.     

Low-temperature synthesis of nanocrystalline Ca1−xHoxMnO3−δ (0?x?0.3) powders

Nanocrystalline Ca_1−xHo_xMnO_3−δ (0?x?0.3) manganites were synthesized as phase-pure by a simple and instantaneous solution autogel combustion method, which is a low temperature initiated synthetic route to obtain fine grain size. All the samples, heated at 800 °C for 18 h, can be produced as phase-pure; the polycrystalline powders are homogeneous and possess ultrafine particle size. The holmium-doped calcium manganites retain the orthorhombic phase of the undoped sample. The scanning electron microscope (SEM) images revealed that the combustion-derived compounds exhibit particle size that decreases with holmium content from 300 to 80 nm. All manganites show two active IR vibrational modes near 400 and 600 cm⁻¹. The high temperature dependence of resistivity was measured using a standard four-probe method in the range 25-600 °C. All the samples exhibit semiconductor behaviour and holmium induces a marked decrease in the electrical resistivity when compared with the parent CaMnO₃. The results can be well attributed to the Mn⁴⁺/Mn³⁺ ratio and to the particle grain size.     

Spin transistor and quantum spin Hall-effects in CdBxF2−x–p-CdF2–CdBxF2−x sandwich nanostructures

Planar CdB_xF_2−x–p-CdF₂–CdB_xF_2−x sandwich nanostructures prepared on the surface of the n-type CdF₂ bulk crystal are studied to register the spin transistor and quantum spin Hall-effects. The current–voltage characteristics of the ultra-shallow p⁺–n junctions verify the CdF₂ gap, 7.8 eV, and the quantum subbands of the 2D holes in the p-type CdF₂ quantum well confined by the CdB_xF_2−xδ-barriers. The temperature and magnetic field dependencies of the resistance, specific heat and magnetic susceptibility demonstrate the high temperature superconductor properties for the CdB_xF_2−xδ-barriers. The value of the superconductor energy gap, 2Δ = 102.06 meV, determined by the tunneling spectroscopy method appears to be in a good agreement with the relationship between the zero-resistance supercurrent in superconductor state and the conductance in normal state, πΔ/e, at the energies of the 2D hole subbands. The results obtained are evidence of the important role of the multiple Andreev reflections in the creation of the high spin polarization of the 2D holes in the edged channels of the sandwich device. The high spin hole polarization in the edged channels is shown to identify the mechanism of the spin transistor and quantum spin Hall-effects induced by varying the top gate voltage, which is revealed by the first observation of the Hall quantum conductance staircase.     

Simulation of the Metal-Semiconductor-Metal photodetector based on InGaAs for the photodetection at the wavelength 1.55 μm

In this paper we present the simulation of Metal-Semiconductor-Metal photodetector (MSM-PD) of interdigitated planar structure based on InAlAs/InGaAs adapted for photodetection at the wavelength 1.55 μm. We use the theoretical models to plot the variations of the dark current, the photocurrent, the capacity, and the cut-off frequency of the photodetector as a function of bias voltage and the interelectrode distance. The obtained results show a very low dark current, mainly due to the introduction of a thin layer to increase the Schottky barrier based on In_0.52Al_0.48As in the epitaxial structure of component. The obtained photocurrent and cut-off frequencies are very appreciable, these latter are mainly limited by the transit time of the photo-generated carriers given the low component capacity obtained by simulation.     

Elastic properties of mono- and poly-crystalline PbO-type FeSe1−xTex (x = 0–1.0): A first-principles study

The structural parameters of the alloys are obtained as non-magnetic cases for which justification is provided. The elastic coefficients and various moduli of the monocrystalline FeSe_1−xTe_x system as a function of doping are predicted for the first time using density functional method. The bulk moduli, shear moduli, Young’s moduli, Poisson’s ratios, velocities of sound and Debye temperature of the corresponding poly-crystalline aggregates have been calculated and the results discussed.     

La2−xSrxCuO4−δ superconducting samples prepared by the wet-chemical method

In this work, we report on the physical properties of good-quality polycrystalline superconducting samples of La_2−xSr_xCu_1−yZn_yO_4−δ (y=0, 0.02) prepared by a wet-chemical method, focusing on the temperature dependence of the critical current. Using the wet-chemical method, we were able to produce samples with improved homogeneity compared to the solid-state method. A complete set of samples with several carrier concentrations, ranging from the underdoped (strontium concentration x≈0.05) to the highly overdoped (x≈0.25) region, were prepared and investigated. The X-ray diffraction analysis, zero-field cooling magnetization and electrical resistivity measurements were reported on earlier. The structural parameters of the prepared samples seem to be slightly modified by the preparation method and their critical temperatures were lower than reported in the literature. The temperature dependence of the critical current was explained by a theoretical model which took the granular structure of the samples into account.     

Structural and magnetic properties of Zn1−xCoxO (0<x⩽0.30) nanoparticles

We present a systematic investigation on the structural and magnetic properties of Zn_1−xCo_xO nanoparticles synthesized by an auto-combustion method. The single-phase Zn_1−xCo_xO crystallize in the wurtzite-type structure with a homogeneity range as large as x≈0.30, which enables the observation of some anomalies. The lattice parameter a and the unit cell volume V increase with the Co content, and anomalies are discernable around x=0.15 on the a−x and V−x curves. The magnetization data show no evidence of ferromagnetic (FM) ordering in our samples down to T=5 K, and the magnetization at 5 K and 5 T exhibits a maximum around x=0.125. Based on the detailed analysis of the magnetization data and the donor impurity band exchange model, the anomalies on composition dependence of both the lattice parameters and magnetization can be associated with an occurrence of cation percolation around the threshold x_p (≈1.5/Z=0.125 for three-dimensional lattice with coordination number Z=12). Within the framework of the donor impurity band exchange model, the absence of FM in the well-characterized Zn_1−xCo_xO can be attributed to insufficient donor electron concentration.     

Anomalous Hall effect in sputter-deposited CoxTi1−xO2–δ films

Hall effect measurements were performed on epitaxial Co_xTi_1−xO_2–δ thin films grown on (0 0 1) LaAlO₃ by reactive RF magnetron co-sputter deposition. Magnetization measurements reveal ferromagnetic behavior in M–H loop at room temperature for Co_xTi_1−xO_2–δ thin films for which x?0.02. An anomalous Hall effect was observed for Co_0.10Ti_0.90O_2−δ films grown with the partial pressure of water P(H₂O)=4×10⁻⁴ Torr or less. These films exhibit a positive ordinary Hall coefficient and a positive magnetoresistance. X-ray diffraction on films grown under these conditions shows evidence for Ti_nO_2n−1 phase due to the deficiency of oxygen. In contrast, Hall measurements taken for undoped and Co-doped TiO₂ thin films grown under more oxidizing conditions show only the ordinary Hall effect with a negative Hall coefficient consistent with n-type conduction. For these films, the magnetoresistance was positive and increased monotonically with increasing magnetic field. The results suggest that Co-doped Ti_nO_2n−1 may be a dilute magnetic semiconducting oxide for which the carriers couple to the spin polarization.     

Valence changes of manganese and praseodymium in Pr1−xSrxMn1−yInyO3−δ perovskites upon cation substitution as determined with XANES and ELNES

Systematic valence changes in Pr_1−xSr_xMn_1−yIn_yO_3−δ upon cation substitution with Sr²⁺ and In³⁺ have been found using Mn K-edge and Pr L-edge X-ray absorption, and Mn L_II,III and Pr M_IV,V electron energy-loss spectroscopy. The average valence of the praseodymium ions is close to +3.0 and virtually constant over the sample set when the samples also contained manganese ions. Pr_0.5Sr_0.5InO_3−δ showed a distinct increase in the praseodymium valence state. In contrast, the average valence of the manganese ions changed from the trivalent state to intermediate values between +3.0 and +4.0 and approached the tetravalent state depending on the level of substitution. The knowledge of the valence is required to understand the conduction mechanisms in the material due to the small polaron hopping (electronic conductivity) and motion of oxygen ions along the vacancies (ionic conductivity). Addition of strontium and indium led to the formation of oxygen vacancies. A previously assumed intermediate valence of praseodymium as causal factor for the higher oxygen catalytic activity cannot be confirmed with room temperature measurements.     

Amplification characteristics measurement of 1.55 μm multi-mode LD

The amplification characteristics of 1.55 μm multi-mode LD are presented experimentally. It demonstrates that nine wavelengths across 1547–1557 nm have a good amplification, the maximum gain of ∼43 dB at 1552.14 nm is obtained with pump power of 130 mW @980 nm, and noise figure of ∼5.6 dB at 1554.3 nm is achieved.     

Structural,magnetic, and transport properties in La(2+4x)/3Sr(1−4x)/3Mn1−xCuxO3 (0⩽x⩽0.20) system

A series of the double-doping samples La_(2+4x)/3Sr_(1−4x)/3Mn_1−xCu_xO₃(0?x?0.2)$(0?x?0.2)$ with the Mn³⁺/Mn⁴⁺ ratio fixed at 2:1 have been prepared. The structural, magnetic, transport properties and magnetoresistance of the series samples have been investigated. It is found that no apparent crystal structure change is introduced by Cu doping up to x=0.20$x=0.20$. But the Curie temperature _TC$T_{\mathrm{C}}$ and magnetization M   are strongly affected by Cu substitution. A remarkable magnetotransport behavior, characterized by double bumps, is observed, and an obvious low-temperature upturn is found in the range of 0.07?x?0.12$0.07?x?0.12$. As a result, the temperature range of colossal magnetoresistance (CMR) is greatly broadened. Moreover, it is found that the room temperature magnetoresistance (MR) of double-doping samples is obviously larger that the undoped La_2/3Sr_1/3Mn_1−xCu_xO₃ at 300 K, which can give a guide for the adequate selection of the room temperature CMR materials.     

Heterogeneous magnetic and electronic states in Ca1−xLaxMnO3−δ (x⩽0.12) single crystals with electron doping

The magnetic, transport, and optical properties of electron-doped Ca_1−xLa_xMnO_3−δ single crystals with x  ?0.12 were studied. The magnetic measurements show that in single crystals with x=0$x=0$ and 0.05 the G-type AFM phase with weak FM component is realized and in crystals with x=0.10$x=0.10$ and 0.12 the G- and C-type AFM phases coexist. The C-type magnetic structure arises at less concentration of La than in polycrystalline samples as a result of oxygen vacancies being additional source of electrons. Under magnetic transitions in the G- and C-type phases, resistivity and magnetoresistance of the doped single crystals have anomalies. Optical absorption in IR range indicates formation of a charge gap in crystals with x=0.10$x=0.10$ and 0.12 at appearance of the C-AFM and monoclinic phase with orbital/charge ordering. By comparing optical and transport properties, heterogeneous electronic state and its relation with heterogeneous magnetic state are shown.     

1.55 μm distributed feedback (DFB) lasers subject to strong 1.4 μm optical injection

An experimental analysis of the influence of optical injection at 1.4 μm wavelength into two different commercial 1.55 μm DFB lasers is reported. The results demonstrate the strong dependence of the DFB behaviour on the injection parameters. Complete mode suppression or signal amplification can be obtained by varying the excitation wavelength and/or intensity, suggesting that these devices could be operated as logic ports or signal amplifiers, according to the injected signal.     

Surface and redox chemistry of Sm0.95Ce0.05Fe1 − xNixO3 − δ perovskites

A new series of perovskite materials with formula Sm_0.95Ce_0.05Fe_1 − xNi_xO_3 − δ (0 ≤ x ≤ 0.10) has been prepared by sol-gel combustion via a citrate precursor route. X-ray diffraction data showed that materials prepared by this method had a single orthorhombic phase belonging to the Pnma (62) space group. The study of powders sintered in air and in reducing atmospheres reveals that these materials do not show phase separation in air (up to 1350 °C) nor under 5% v/v H₂/N₂ (up to 700 °C), but a phase separation of Sm₂O₃ does occur at and above 800 °C under 5% v/v H₂/N₂ without deterioration of the perovskite phase. The surfaces of all the powders (fresh, in-situ reduced and ex-situ reduced) were Sm rich, and multiple oxidation states for Fe were observed. XP analysis of in-situ reduced samples (800 °C and above) shows that metallic Fe forms in all nickel doped materials except x = 0.07. The surface oxygen vacancies and percentages of lattice and adsorbed oxygen for this series of Ni doped materials were determined and the oxygen recapturing ability is explained in terms of the multiple oxidation states of Fe.     

Glass transition behavior of binary GaxSe100−x (0?x?10) glass systems

The glass transition behavior of glassy Ga_xSe_100−x (x=0, 2.5, 5, 7.5 and 10) systems were investigated using differential scanning calorimetry (DSC). The variation of glass transition temperature, T_g, with Ga concentration has been studied. The value of activation energy of glass transition, E_g, has been found to increase with increase in Ga content. This increase in E_g has been explained in terms of the average heat of atomization for these glasses.     

Long range surface plasmon polariton waveguides at 1.31 and 1.55 μm wavelengths

We investigate characteristics of gold metal strip waveguides based on long range surface plasmon polaritons (LRSPPs) along thin metal strips embedded in a polymer for practical applications at the telecommunication wavelengths of 1.31 and 1.55 μm. Guiding properties of the gold strip waveguides are theoretically and experimentally evaluated with the limited thickness and width up to ∼20 nm and ∼10 μm, respectively. The lowest propagation loss of ∼1.4 dB/cm is obtained with a 14.5-nm-thick and 2-μm-wide gold strip at 1.55 μm. With a single-mode fiber, the lowest coupling loss of ∼0.4 dB/facet is achieved with a 14.5-nm-thick and 5-μm-wide gold strip at 1.55 μm. The lowest insertion losses are obtained 8-9 dB with 1.5 cm-long gold strips of a limited thickness and width at both the wavelengths. We demonstrate a 10 Gbps optical signal transmission via the LRSPP waveguide with a 14 nm-thick, 2.5 μm-wide, and 4 cm-long gold strip. These LRSPP waveguides have potential applications for optical interconnects and communications.     

Influence of the concentration in Fe-doped BaTiO3 on magnetoelectric couping of layered composites BaTi1−xFexO3–Tb1−yDyyFe2−z

Perovskites BaTi_1−xFe_xO₃ has been synthesized with the concentration x ranging from 0.01 to 0.02. Their transformation point of ferroelectric to paraelectric and the corresponding latent heat of the phase transformation were observed to decrease with increasing the doping level of Fe³⁺. Bonded layered composites BaTi_1−xFe_xO₃–Tb_1−yDy_yFe_2−z have been fabricated and their magnetoelectric effect has been investigated. The sample containing a layer of perovskite BaTi_0.985Fe_0.015O₃ was found to show the maximum transverse ME voltage coefficient, which is about 1422 mV Oe⁻¹ cm⁻¹ under a magnetic field of 1580 Oe, in these bilayers. Analysis shows that the Fe-doped BaTiO₃ with doping level at about 1.5% should have largest piezoelectric coefficient in these ceramics BaTi_1−xFe_xO₃.     

Non-adiabatic polaron hopping conduction in CaMn1−xCrxO3 (0?x?0.3)

DC electrical conductivity (σ_dc) of electron-doped antiferromagnetic CaMn_1−xCr_xO₃ (0?x?0.3) has been discussed elaborately in the light of polaron hopping conduction. The increase in Cr doping concentration increases the conductivity and decreases the activation energy. Non-adiabatic polaron hopping conduction is observed in all the manganites at high temperatures. The analysis of σ_dc data shows that small polarons are formed at lower concentrations (?5%) of Cr doping and undoped samples. However, large polarons are materialized at higher doping (?10%) concentrations. This is consistent with the fact that doped Cr³⁺ has larger ionic size compared to that of Mn⁴⁺. Again, strong electron-phonon (e-ph) interaction is perceived in undoped and 5% Cr-doped samples but not in manganites with larger doping concentration. This also confirms the formation of larger polarons with the increase of x. Mott's variable range hopping (VRH) model can elucidate the dc conductivity at very low temperatures. It has been detected that single phonon-assisted hopping is responsible for the dc conduction in the Cr-doped CaMnO₃ manganites.     

Electrical and switching properties of the Se90Te10−xAgx (0?x?6) films

Amorphous Se₉₀Te_10−xAg_x (0?x?6) films are obtained by thermal evaporation technique under vacuum from the synthesized bulk materials on pyrographite and glass substrates. X-ray analysis shows the amorphous nature of the obtained films. The dc electrical conductivity was studied for different thicknesses (165-711 nm) as a function of temperature in the range (298-323 K) below the corresponding T_g for the studied films. The obtained results show that the conduction activation energy has a single value through the investigated range of temperature which can be explained in accordance with Mott and Davis model. The I-V characteristic curves for the film compositions are found to be typical for a memory switch. The mean value of the threshold voltage increases linearly with increasing film thickness (165-711 nm), while it decreases exponentially with increasing temperature in the investigated range for the studied compositions. The results are explained in accordance with the electrothermal model for the switching process. The effect of Ag on the studied parameters is also investigated.