Performance and modeling of proton exchanged LiTaO3branching modulators

Proton-exchanged LiTaO₃ branching modulators are demonstrated experimentally and modeled using a normal mode coupled-mode theory. The normal modes are approximated by ordinary coupled-mode theory parameters which are experimentally obtained. Agreement between the model and the experimental results is demonstrated. Crosstalk in these linear branch devices is found to be linearly proportional to the voltage-length product, a result which leads to an analytic expression for the output power     

Investigation of structure and properties of nanocrystalline silicon on various buffer layers

A comparative study of deposition nanocrystalline silicon (nc-Si) on various buffer layers is investigated. The nc-Si films were deposited in a hot-wire chemical vapor deposition (CVD) system. Through Hall measurement, scanning electron microscopy (SEM), atomic force microscope (AFM), Raman, and x-ray diffraction (XRD) analyses, it was found that the columnar grain (CG) size, mobility, and volume fraction of crystalline-deposited nc-Si films increase with an increase of the buffer layers’ surface roughness. The nc-Si film deposited on the nc-Si buffer layer possesses the highest Xc (volume fraction of crystalline) of 84.32%, Hall mobility of 45.9 (cm²/V s), and CG size of 200–220 nm, and it shows the strongest intensity of the XRD diffraction peak in (111).     

Characterization of annealed proton exchanged LiNbO3waveguides for nonlinear frequency conversion

A thorough and detailed characterization of annealed proton-exchanged (APE) waveguides in Z-cut LiNbO₃ is described. The mode index measurements in planar waveguides as a function of wavelength and annealing time are reported, including useful analytical relations for the refractive index change, its dispersion, and the depth profile as a function of annealing parameters. Analytical expressions for the mode propagation characteristics are presented and experimentally verified with reasonable accuracy. It is shown that the planar waveguide characterization results can be used to model the channel waveguide characteristics accurately. The model provides closed-form expressions for the mode index and the mode field profile, and the theoretical results are in excellent agreement with the measured data. The technique is used to accurately predict the phase mismatch between the fundamental and second harmonic modes in frequency-doubling experiments using APE channel waveguides. An optimum waveguide geometry for which the phase mismatch is relatively insensitive to the waveguide nonuniformity was predicted and verified experimentally     

Numerical simulation of ion-exchange in glass for integratedoptical components

A numerical model based on the finite-element method for solving the nonlinear diffusion equation of ion-exchange in glasses is presented. The model is applied to the calculation of the concentration profile for a Ag⁺-Na⁺ ion-exchanged channel waveguide. The calculated concentration profile is in good agreement with a measured refractive-index profile of a waveguide fabricated with the same parameters as used in the calculations     

Crystalline and optical quality of proton exchanged waveguides

In this paper we demonstrate the possibility of obtaining high-quality proton-exchanged (PE) lithium niobate guides by performing the exchange at high temperatures (300°C) in benzoic acid diluted with lithium benzoate. Rutherford backscattering studies suggest that the higher exchange temperatures avoid the production of niobium dislocations, and, IR absorption measurements suggest that the use of lithium benzoate diluted melts prevents the creation of interstitial hydrogen.     

Metal-semiconductor-metal photodetectors with InAlGaP capping and buffer layers

To improve the Schottky contact performance and carrier confinement of GaAs metal-semiconductor-metal photodetectors (MSM-PDs), we employed the wide bandgap material, In/sub 0.5/(Al/sub 0.66/Ga/sub 0.34/)/sub 0.5/P, for the capping and buffer layers. We directly evaluated the Schottky contact parameters on the MSM-PD structure. The reverse characteristics of the Schottky contacts were examined by taking into account the Schottky barrier height depended on the electric field in the depletion region, and hence on the applied bias. The ideality factor and Schottky barrier height of Ti-Pt-Au contacts to In/sub 0.5/(Al/sub 0.66/Ga/sub 0.34/)/sub 0.5/P are 1.02 and 1.05 eV, respectively. Extremely low dark currents of 70 and 620 pA were obtained for these MSM-PDs when they were operated at a reverse bias of -10 V at room temperature and at 70/spl deg/C, respectively.     

Biaxial characteristics of planar proton exchanged lithium niobatewaveguides

Results of a theoretical and experimental study on proton exchanged lithium niobate (LiNbO₃) planar waveguides are presented. Experimental results are successfully explained by the theoretical model only if the guiding layer is assumed to be anisotropic with biaxial characteristics, this biaxiality being induced by the proton exchange process in the uniaxial lithium niobate substrate     

Characterization of deep levels in Pt-GaN Schottky diodes depositedon intermediate-temperature buffer layers

Gallium nitride (GaN)-based Schottky junctions were fabricated by RF-plasma-assisted molecular beam epitaxy (MBE). The GaN epitaxial layers were deposited on novel double buffer layers that consist of a conventional low-temperature buffer layer (LTBL) grown at 500°C and an intermediate-temperature buffer layer (ITBL) deposited at 690°C. Low-frequency excess noise and deep level transient Fourier spectroscopy (DLTFS) were measured from the devices. The results demonstrate a significant reduction in the density of deep levels in the devices fabricated with the GaN films grown with an ITBL. Compared to the control sample, which was grown with just a conventional LTBL, a three-order-of-magnitude reduction in the deep levels 0.4 eV below the conduction band minimum (E_c) is observed in the bulk of the thin films using DLTFS measurements     

InAs flip-chip LEDs with InGaAsSb buffer layers

Under study are the electrical and optical properties of n-InGaAsSb epitaxial layers with composition close to InAs and lattice-matched with it, fabricated on InAs substrates by LPE from Te-containing melts. The layers are transparent in the 3-μm range owing to the Moss-Burstein effect. The light-current characteristics and near-field emission pattern of InAs-based flip-chip LEDs with the emission extracted through n ⁺-InGaAsSb layers are presented and discussed.     

Microwave performance of GaAs-on-Si MESFETs with Si buffer layers

The incorporation of silicon-buffer layers is shown to be critical in attaining optimized microwave performance for GaAs on silicon MESFETs. A current gain cutoff frequency (f_t) of 18 GHz and maximum power cutoff frequency (f_max) of 30 GHz is reported for relaxed geometry devices. The low parasitic capacitance and excellent device isolation make this structure suitable for monolithic integration     

The effects of finite melt volume on proton exchanged lithiumniobate

The effects of using a benzoic acid melt of limited volume on proton-exchanged LiNbO₃ crystals are discussed. It was found, by observing the strength of the O-H peak as a function of exchange time, that the amount of hydrogen entering the crystal saturates if the benzoic acid melt is of limited volume. This behavior has been modeled using the condition that the concentration of free protons in the melt decreases as the exchange proceeds. A good fit of the data based on this model can be obtained by considering the total amount of solute in the system to a constant. Diffusion coefficients deduced from the model agree well with the ones reported in the literature     

AlGaN/GaN Schottky barrier diodes on silicon substrates with various Fe doping concentrations in the buffer layers

This study demonstrated AlGaN/GaN Schottky barrier diodes (SBDs) for use in high-frequency, high-power, and high-temperature electronics applications. Four structures with various Fe doping concentrations in the buffer layers were investigated to suppress the leakage current and improve the breakdown voltage. The fabricated SBD with an Fe-doped AlGaN buffer layer of 8 × 10¹⁷ cm^− 3 realized the highest on-resistance (R_ON) and turn-on voltage (V_ON) because of the memory effect of Fe diffusion. The optimal device was the SBD with an Fe-doped buffer layer of 7 × 10¹⁷ cm^− 3, which exhibited a R_ON of 31.6 mΩ-cm², a V_ON of 1.2 V, a breakdown voltage of 803 V, and a buffer breakdown voltage of 758 V. Additionally, the low-frequency noise decreased when the Fe doping concentration in the buffer layer was increased. This was because the electron density in the channel exhibited the same trend as that of the Fe doping concentration in the buffer layer.     

Epitaxial growth of GaN layers on metallic TiN buffer layers

To improve GaN light-emitting diode light emission, we produced metal organic chemical vapor deposition (MOCVD)-grown, continuous, flat GaN layers on metallic TiN buffer layers deposited on sapphire substrates. Three important conditions were found: (a) the sapphire substrate surface plane should be (1120), (b) the TiN layer surface plane should be (111), and (c) the TiN buffer layer nitrogen content should be higher than that of stoichiometric TiN. Reduction of TiN layer thickness reduced TiN buffer layer surface roughness. Threading dislocation density in GaN layers grown on TiN buffer layers was much lower than that in GaN layers grown on AlN.     

Characterization of SiO2 dielectric breakdown forreliability simulation

The authors analyze breakdown distributions of thin oxide MOS capacitors to reveal the limitations of accelerated procedures for reliability simulation at circuit level. The detailed analysis of the breakdown distributions corresponding to different stress voltages shows the presence of three modes of breakdown, associated with different kinds of defects. These breakdown nodes control the shape of the distributions at different oxide field ranges. The mean values of time-to-breakdown, charge-to-breakdown, and energy-to-breakdown and their dependence on oxide field are found to be directly related to the mode dominating the distribution. Since the shape of the breakdown distribution changes with stress voltage, an accelerated testing procedure must provide a way to extrapolate to operation conditions not only the mean value, but also the shape of the distribution. The results indicate that different physical mechanisms cause the breakdown at low and high fields     

Comparison of organic photovoltaic with graphene oxide cathode and anode buffer layers

In this report, we present inverted organic solar cells integrating solution-processed aluminum doped zinc oxide (AZO) and trilayer graphene oxide (GO) as an electron selective and anode buffer layers, respectively. The polymers in this inverted architecture are PCDTBT, PBDTTPD and PCBM as an electron donor and acceptor, respectively and the photovoltaic performance were recorded at 300 K under 100 mW/cm² light intensity. The characteristics of PCDTBT and PBDTTPD-based inverted solar cells were: open-circuit voltages (V_oc’s) 0.74 and 0.70 V, short-circuit current densities (J_sc’s) −12.09 and −12.06 mA/cm², fill factors (FFs) of 60.73% and 60.03%, with an overall power conversion efficiencies (PCEs) of about 5.46% and 5.07%. The fabricated inverted cells show better performances compared to conventional structure reference cells.     

Full-wave analysis of coplanar waveguides for LiNbO3optical modulators by the mode-matching method considering nonidealconductors on etched buffer layers

Rigorous analysis of traveling-wave coplanar waveguide electrodes for LiNbO₃ optical modulator applications is presented by using an extended full-wave mode-matching method. The microwave propagation characteristics under the composite influence of substrate anisotropy, uniform or etched buffer layers, finite electrode thickness and conductivity, and metallization undercutting are accurately assessed by employing a network equivalent formulation. Variations of the coplanar waveguide microwave effective index and the characteristic impedance at low frequencies due to finite electrode conductivity are illustrated, and are important even though the mode is quasi-TEM in nature. The effect of etching the SiO₂ buffer layer is shown to be one possible method for lowering the microwave effective index while keeping the conductor loss at a fixed level     

Impact of low-temperature buffer layers on nitride-based optoelectronics

A historical overview and recent trends in the research and development of nitride-based light emitters are presented. The growth of GaN using a low-temperature-deposited buffer layer conductivity control, and the use of GaInN alloys, by which nonradiative recombination centers in nitrides are screened, have been employed to fabricate high-efficiency blue and green light-emitting diodes. Today, luminous flux efficiency of up to 50 lm/W at a specific wavelength is available. Electrical pumping has realized commercial laser diodes in the violet and blue regions. Several milestones in the realization of these achievements are reviewed. Future prospects of the nitride-based light emitters are also discussed.     

High-speed-response InGaAs/InP heterostructure avalanche photodiode with InGaAsP buffer layers

The optical response time of an InGaAs/InP heterostructure avalanche photodiode (HAPD) with InGaAsP buffer layers is reported. It is shown that the buffer layers play an important role in reduction of the pile-up effect and are considered to be effective in achieving high-speed InGaAs/InP HAPDs.     

Planarization of dielectric layers for multilevel metallization

A process to planarize low-pressure chemical-vapor deposition (LPCVD) SiO₂ films formed over the abrupt topography of fine-line (2.0-μm pitch) integrated circuits with two levels of metallization and pillar interconnections has been developed with sacrificial photoresist and plasma etching using response-surface methodology. To produce flat dielectric surfaces with this topography, the ratio of the measured etch rate of photoresist to that of phosphorus-doped SiO₂ must be maintained at ~0.4 (3800 and 9100 Å/min, respectively) with an Ar/CF₄/O₂ high pressure plasma generated in a low radio-frequency etching system     

Near-field microwave tomography of subsurface dielectric layers

There is represented a method of 3D visualization of a structure of dielectric objects with small loss on a basis of data, obtained by scanning with near-field microwave microscope. Theoretical model was proven by means of experimental research of amount of test objects. We researched possibility of probes with complex geometry application for defectoscopy of dielectric materials analysis of volumetric non-uniformities.