The effect of pair-induced energy transfer on the performance ofsilica waveguide amplifiers with high Er3+/Yb3+concentrations

High-concentration Er³⁺/Yb³⁺ co-doped silica waveguide amplifiers are numerically analyzed. With optimized rare-earth concentrations the effect of Er³⁺/Er³⁺ ion-pairs can be neglected and each Er³⁺ ion can be assumed to be paired only to the surrounding Yb³⁺ ions. The rate-equations model includes uniform upconversion mechanisms from ⁴I_13/2 and ⁴I_11/2 erbium levels and an Yb³⁺ to Er³⁺ pair-induced energy transfer process. Numerical results demonstrate the possibility of fabricating short- and high-gain integrated optical amplifiers; it is shown that net gain as high as 3 dB/cm can be obtained     

Reliable Low-Cost Fabrication of Low-Loss $hbox{Al}_{2}hbox{O} _{3}{:}hbox{Er}^{3+}$ Waveguides With 5.4-dB Optical Gain

A reliable and reproducible deposition process for the fabrication of Al₂O₃ waveguides with losses as low as 0.1 dB/cm has been developed. The thin films are grown at ~ 5 nm/min deposition rate and exhibit excellent thickness uniformity within 1% over 50times50 mm² area and no detectable OH^- incorporation. For applications of the Al₂O₃ films in compact, integrated optical devices, a high-quality channel waveguide fabrication process is utilized. Planar and channel propagation losses as low as 0.1 and 0.2 dB/cm, respectively, are demonstrated. For the development of active integrated optical functions, the implementation of rare-earth-ion doping is investigated by cosputtering of erbium during the Al₂O₃ layer growth. Dopant levels between 0.2-5times10²⁰ cm^-3 are studied. At Er³⁺ concentrations of interest for optical amplification, a lifetime of the ⁴I_13/2 level as long as 7 ms is measured. Gain measurements over 6.4-cm propagation length in a 700-nm-thick Al₂O₃:Er³⁺ channel waveguide result in net optical gain over a 41-nm-wide wavelength range between 1526-1567 nm with a maximum of 5.4 dB at 1533 nm.     

Novel methods to incorporate deuterium in the MOS structures

The deuterium concentration as high as 2×10²⁰ cm ^-3 can be incorporated in rapid thermal oxide layers by a process of deuterium prebake and deuterium post oxidation anneal. The deuterium distributed not only at Si/oxide interface but also in the bulk oxide. The deuterium incorporation shows the improvement on soft breakdown characteristics and the interface state density at SiO₂ /Si after stress. The addition of very high vacuum prebake process yields a deuterium concentration of 9× 10²⁰ cm^-3 , but also leads to the formation of rough oxide     

Submicrometer Inversion-Type Enhancement-Mode InGaAs MOSFET With Atomic-Layer-Deposited Al2O3 as Gate Dielectric

High-performance inversion-type enhancement-mode n-channel In_0.53Ga_0.47As MOSFETs with atomic-layer-deposited (ALD) Al₂O₃ as gate dielectric are demonstrated. The ALD process on III-V compound semiconductors enables the formation of high-quality gate oxides and unpinning of Fermi level on compound semiconductors in general. A 0.5-mum gate-length MOSFET with an Al₂O₃ gate oxide thickness of 8 nm shows a gate leakage current less than 10^-4 A/cm² at 3-V gate bias, a threshold voltage of 0.25 V, a maximum drain current of 367 mA/mm, and a transconductance of 130 mS/mm at drain voltage of 2 V. The midgap interface trap density of regrown Al₂O₃ on In_0.53Ga_0.47As is ~1.4 x 10¹²/cm² ldr eV which is determined by low-and high-frequency capacitance-voltage method. The peak effective mobility is ~1100 cm² / V ldr s from dc measurement, ~2200 cm²/ V ldr s after interface trap correction, and with about a factor of two to three higher than Si universal mobility in the range of 0.5-1.0-MV/cm effective electric field.     

Some aspects affecting transmittance spectra of composite smartfilm WO3

The transmittance spectra (TS) of sol-gel electrochromic smart film single-doped Mo⁶⁺ and double-doped Mo⁶⁺, Cr ⁶⁺ have been studied in this paper. For the film α-WO ₃ single-doped with Mo⁶⁺, it is evident that the colloid stability increases and that the absorption peak moves to shorter wavelength side nearly 770 nm. For the film double doped with Mo ⁶⁺ and Cr⁶⁺, the peak will move to longer wavelength side nearly 440 nm and infrared band and the threshold voltage will increase from 0.2 to 0.8 V, simultaneously. The optical absorption effect is interrelated to transition process of small polar     

Measurement of low densities of surface states at the Si---SiO2-interface

By a careful process Si---SiO₂-interfaces can be made with a low oxide charge Q_ox and with a low surface states density N_ss. For dry oxides on (100) N_ss-values as low as a few 10⁹ cm⁻² eV⁻¹ are found on samples with an oxide charge density of 3·0 × 10¹⁰ cm⁻². Only the Nicollian-Goetzberger conductance method is proved to give reasonable results on these structures. The quasi-static low frequency C-V-technique is in good agreement with the conductance technique for samples with N_ss-values higher than 1·0 × 10¹⁰ cm⁻² eV⁻¹. The spatial fluctuation of surface potential, mainly caused by oxide charge fluctuations, is an important parameter when studying the high or low frequency C-V-characteristics. Some irregularities in the experimental N_ss-φ_s-curves are explained.     

Hot phonons and Auger related carrier heating in semiconductoroptical amplifiers

We have directly measured the carrier temperature in semiconductor optical amplifiers (SOAs) via spontaneous emission and we demonstrate an unexpectedly high carrier temperature. The direct correlation of the temperature increase with the carrier density suggests Auger recombination as the main heating mechanism. We have developed a model based on rate equations for the total energy density of electrons, holes, and longitudinal-optical phonons. This model allows us to explain the thermal behavior of carrier and phonon populations. The strong heating observed is shown to be due to the combined effects of hot phonon and Auger recombination in the valence band. We also observe an evolution of the Auger process, as the density is increased, from cubic to square dependence with coefficients C₃ = 0.9 10^-28 cm⁶ s^-1 and C₂ = 2.4 10^-10 cm³ s^-1. This change is explained by the hole quasi-Fermi level entering the valence band     

Red, Green, and Blue Light Through Cooperative Up-Conversion in Sol-Gel Thin Films Made With Yb0.80La0.15Tb0.05F3 and Yb0.80La0.15Eu0.05F3 Nanoparticles

Silica and zirconium dioxide sol-gel thin films made with Yb_0.80La_0.15Tb_0.05F₃ or Yb _0.80La_0.15Eu_0.05F₃ nanoparticles are reported. Bright blue (413 and 435 nm), green (545 nm), and red (585 and 625 nm) emissions are produced from Tb³⁺ ions through cooperative up-conversion of 980 nm light. Similarly, red (591 and 612 nm) emission is generated from Eu³⁺ ions. These up-convertors may find use in white light sources. The cooperative up-conversion of Yb³⁺-Tb³⁺ ions is more efficient than of Yb³⁺-Eu³⁺ ions because the efficiency of energy transfer from excited Yb³⁺ ions to a Tb ³⁺ ion (0.37) is more than two-times higher than of excited Yb³⁺ ions to a Eu³⁺ ion (0.15), as estimated from the lifetime of excited Yb³⁺ ion. The estimated quantum yields of both Tb³⁺ ion and Eu³⁺ ion emissions are on the order of 40%, and hence are not the cause of the difference in efficiency. This approach does not work for Sm³⁺, Pr³⁺ , and Dy³⁺. Incorporation of the respective Ln³⁺ ions in nanoparticles is crucial, as controls, in which the various Ln³⁺ ions are incorporated directly into the sol-gel, that do not show cooperative up-conversion     

4H-SiC MOSFETs utilizing the H2 surface cleaningtechnique

The H₂ cleaning technique was examined as the precleaning of the gate oxidation for 4H-SiC MOSFETs. The device had a channel width and length of 150 and 100 μm, fabricated on the p-type epitaxial layer of 3×10¹⁶ cm^-3. The gate oxidation was performed after the conventional RCA cleaning, and H₂ annealing at 1000°C. The obtained channel mobility depends on the pre-cleaning process strongly, and was achieved 20 cm²/N s in the H₂ annealed sample. The effective interface-state density was also measured by the MOS capacitors fabricated on the same chips, resulting 1.8×10¹² cm^-2 from the photo-induced C-V method     

Fabrication and characterization of Yb3+:Er3+phosphosilicate fibers for lasers

Fabrication process of efficient Yb³⁺,Er³⁺ codoped phosphosilicate fibers by modified chemical vapor deposition (MCVD) combined with the solution doping technique is studied in detail. We show that the process can be adapted to incorporate low viscosity phosphate glass and some important issues in the fabrication process are discussed. These include the sensitive presintering pass. We also report on the fabrication of a low loss all-glass double clad Yb³⁺:Er³⁺ codoped fiber. We explain how we evaluate the fibers and discuss the effect of the ytterbium to erbium concentration ratio on the laser characteristics. Finally, we present results of some investigations into the mechanisms which can affect the efficiency of the lasers, and show that the detrimental up-conversion from the metastable level of the erbium ions is dramatically reduced by the presence of the ytterbium ions     

Planar AlGaAs/GaAs HBT fabricated by MOCVD overgrowth with a grownbase

A planar heterojunction bipolar transistor (HBT) with an AlGaAs emitter layer epitaxially grown onto a selectively defined grown base layer, where the base is grown with the collector as part of the original epi, is discussed. The transistors fabricated with this process exhibit good gain and output characteristics. Transistors with 7×7 μm² emitters have exhibited a DC current gain of 10 to 1000 for base doping from 1×10¹⁹ to 8×10¹⁷ cm³, respectively, and Early voltages ⩾100 V. The propagation delay of 19-stage ring oscillators was 87 ps/gate. The transistor-fabrication process was designed to be manufacturable, and the planar nature of the transistor surface should permit large-scale integration with good yields     

Finite-element analysis of single-frequency silica-based Er3+-Yb3+ co-doped waveguide lasers

Continuous wave laser operations of silica-based Er³⁺-Yb³⁺ co-doped waveguides have been numerically analyzed by means of a finite-element method. The theoretical model, based on propagation-rate equations, describes uniform upconversion by a dipole-dipole interaction between Er³⁺ ions, and includes a pair-induced energy transfer process from Yb ³⁺-Er³⁺ Numerical results show that single-frequency operation with slope efficiency higher than 50% and threshold pump powers of few mW can be achieved in short and heavily doped waveguides equipped with input dielectric mirrors and output distributed Bragg reflectors     

Thin oxide grown on heavily channel-implanted substrate by using a low temperature wafer loading and N2 pre-annealing process

In this paper, we present high integrity thin oxides grown on the channel implanted substrate (3 × 10¹⁷ cm⁻³) and heavily doped substrate (1 × 10²⁰ cm⁻³) by using a low-temperature wafer loading and N₂ pre-annealing process. The presented thin oxide grown on the channel implanted substrate exhibits a very low interface state density (1 × 10¹⁰ cm⁻² eV⁻¹) and a very high intrinsic dielectric breakdown field (15 MV/cm). It also shows a lower charge trapping rate and interface state generation rate than the conventional thermal oxide. For the thin oxide grown on the heavily-doped substrate by using the proposed recipe, the implantation-induced damage close to the silicon surface can be almost annealed out. The presented heavily-doped oxide shows much better dielectric characteristics, such as the dielectric breakdown field and the charge-to-breakdown, as compared to the conventional heavily-doped oxide.     

A comparison of optoelectronic properties of lattice-matched andstrained quantum-well and quantum-wire structures

The k·p formalism is used to study the absorption spectra, material and differential gain in quantum wires as a function of orientation, built-in strain, and wire dimensions. The results for material and differential gain are compared with those for an optimized quantum-well structure. We find that for quantum wires at 300 K, the gain becomes positive at a carrier density of 1.27·10¹⁸ cm^-3, while in quantum wells this density is calculated to be 1.82·10¹⁸ cm^-3. Incorporating tensile strain in the wires reduces the transparency carrier concentration to 0.96·10¹⁸ cm^-3 while compressive strain allows one to obtain positive gain for densities greater than 1.08·10¹⁸ cm^-3. Orienting the wire along the [111] direction reduces the transparency carrier density to 0.60·10¹⁸ cm^-3. The differential gain in quantum-well structures for injections near the threshold is on the order of 10^-14 cm^-4, while for 50 Å·100-Å quantum wires the differential gain near the threshold is found to be on the order of 10^-13 cm^-4 . The differential gain in wires whose wire axis is parallel to the [111] direction has also been found to be on the order of 10^-13 cm^-4 for carrier injections close to the threshold     

Design study of AlGaAs/GaAs HBTs

The frequency performance of AlGaAs/GaAs heterojunction bipolar transistors (HBTs) having different layouts, doping profiles, and layer thicknesses was assessed using the BIPOLE computer program. The optimized design of HBTs was studied, and the high current performances of HBTs and polysilicon emitter transistors were compared. It is shown that no current crowding effect occurs at current densities less than 1×10⁵ A/cm² for the HBT with emitter stripe width S_E<3 μm, and the HBT current-handling capability determined by the peak current-gain cutoff frequency is more than twice as large as that of the polysilicon emitter transistor. An optimized maximum oscillation frequency formula has been obtained for a typical process n-p-n AlGaAs/GaAs HBT having base doping of 1×10 ¹⁹ cm^-3     

Triple correlations

The (auto)triple correlation l⁽³⁾(t1, t2) is defined as the triple function integral, applied to the signal l(t) l⁽³⁾(t1, t2) = ∫ l(t)l(t + t1)l(t + t2) dt. The triple correlation l⁽³⁾(t1, t2) is less popular than the standard (double) correlation l⁽²⁾(t1) for several reasons: l⁽²⁾is sometimes easier to observe and to process, l⁽³⁾is small for many bipolar or complex signals, the mathematics associated with l⁽²⁾is better known. On the other hand, the triple correlation l⁽³⁾knows more about the signal l than does the ordinary autocorrelation l⁽²⁾. Also l⁽³⁾is in some ways more sensitive, in other ways less sensitive to noise, to bias drifts, etc. Hence, there are situations, where it is quite favorable to evaluate one-dimensional signals or two-dimensional pictures by means of their triple correlations. We will review the underlying mathematical tools and report on our projects where triple correlations were employed for studying laser pulse shapes, sound quality, halftone print statistics, mobility of bacteria, and astronomical speckle interferometry. We will mention also how others have used the triple correlation for ocean waves, engine noises, intensity interferometry, and other optical signal processing tasks.     

Saturable absorbers based on impurity and defect centers incrystals

Saturation of near-infrared absorption and transmission dynamics are investigated in tetravalent-chromium-doped Gd₃Sc₂ Ga₃O₁₂, Gd₃Sc₂Al₃ O₁₂, and Mg₂SiO₄ crystals, as well as in reduced SrTiO₃ using 20 ps 1.08 μm laser pulses. An absorption cross section of (5±0.5)×10^-18 cm² in garnets and (2.3±0.3)×10^-18 cm² in forsterite is estimated for the ³A ₂-³T₂ transition of tetrahedral Cr⁴⁺. Q-switched and ultra-short pulses are realized in neodymium lasers using chromium-doped crystals as the saturable absorbers. Saturation of free-carrier absorption with ultra-short relaxation time is observed in SrTiO₃ at 10⁸-10 ¹⁰ W/cm² pump intensities, while at 10¹⁰-10¹¹ W/cm² three-photon interband transitions predominate. The free-carrier absorption cross section is estimated to be (2.7±0.3)×10^-18 cm²     

Test structures to measure the heat capacity of CMOS layersandwiches

We report novel thermal characterization microstructures to measure the heat capacity of CMOS thin film sandwiches. This parameter is relevant, e.g., for the dynamic response of thermal CMOS microtransducers and for the thermal management of integrated circuits. The test structures were fabricated using a commercial 2-μm CMOS process, followed by maskless micromachining. The propagation of heat waves in the structures is monitored, which provides the thermal conductivity and heat capacity of CMOS thin film sandwiches. At 300 K, volumetric heat capacities of (1.71±0.12)×10⁶ Jm ^-3K^-1 and (2.41±1.88)×10⁶ Jm^-3K^-1 were obtained for the sandwich of CMOS dielectrics and for the lower CMOS metal, respectively. These values do not deviate significantly from available bulk data of such materials     

Effects of F- ions and F2 molecules on theoscillation process of a discharge-pumped ArF excimer laser

Time-resolved number densities of the fluorine negative ion in a discharge-pumped ArF excimer laser are measured by a dye laser absorption method. The peak density of F^- is 0.93 ×10 ¹⁵ cm^-3 at a total gas pressure of 2.5 atm, a gas mixture ratio of F₂-Ar-He=0.2-10.0-89.8, and a charging voltage of 28 kV for a 68-nF storage capacitor bank. The dependences of the peak F^- density and the ArF laser output power on the F ₂ gas fraction in F₂-Ar-He mixture are investigated. The effects of F^- ions and F₂ molecules on the ArF laser oscillation process are discussed by considering the F₂ mixture-ratio dependences of particle densities, laser output power, mean electron energy, and laser power extraction efficiency. With increasing F₂ mixing ratio, the ArF* excimer formation first increases as F^- increases, but in F₂-rich conditions the laser power decreases because of the laser photon absorption due to F^- ions and quenching of ArF* with F₂ molecules