Wafer bonding technology and its applications in optoelectronicdevices and materials

The direct wafer bonding process has found broad applications in many critical areas including both commercial and state-of-the-art photonic devices and more recently, formation of semiconductor compliant substrates. Using the wafer bonding technology, we have demonstrated 1.3-μm vertical-cavity surface-emitting lasers (VCSELs) with a 1-mA continuous-wave (CW) threshold current and 0.83-mA pulsed threshold current. Superior device performance has also been achieved with photodetectors and micromachined tunable devices. Applying the wafer bonding process in a novel way, we have fabricated compliant universal substrates on which largely mismatched (e.g., 15% mismatch) heteroepitaxial layers can be grown defect free     

Hybrid integration of VCSEL's to CMOS integrated circuits

Three hybrid integration techniques for bonding vertical-cavity surface-emitting lasers (VCSELs) to CMOS integrated circuit chips have been developed and compared in order to determine the optimum method of fabricating VCSEL based smart pixels for optical interconnects and free-space optical processing. Each of the three bonding techniques used different ways of attaching the VCSEL to the integrated circuit and making electrical contacts to the n- and p-mirrors. All three techniques remove the substrate from the VCSEL wafer leaving an array of individual VCSELs bonded to individual pixels. The 4×4 and/or 8×8 arrays of bonded VCSELs produced electrical and optical characteristics typical of unbonded VCSELs. Threshold voltages down to 1.5 V and dynamic resistance as low as 30 Ω were measured, indicating good electrical contact was obtained. Optical power as high as ~10 mW for a VCSEL with a 20-μm aperture and 0.7 mW with a 6-μm aperture were observed. The VCSELs were operated at 200 Mb/s (our equipment limit) with the rise and fall times of the optical output <1 nS     

Preparation and properties of Pb(Zr, Ti)O3 ferroelectric thin films and compositionally graded thin films on LaNiO3/Si substrates

LaNiO₃ (LNO) thin films were prepared on Si (100) wafer by MOD method. Pb(Zr, Ti)O₃ ferroelectric thin films and their compositionally graded thin films were prepared on LNO/Si (100) substrates by a modified sol–gel method. The composition depth profile of a graded film was determined by using a combination of Auger electron spectroscopy and Ar ion etching. The results confirmed that the processing method produces graded composition change. XRD analysis showed that the graded thin film possessed a composite structure of tetragonal and rhombohedral. The dielectric constant of the graded thin films was higher than that of each thin film unit, but the loss tangent was near to each other at 10 kHz. The temperature characteristics of the dielectric constant of the graded thin films at different frequencies showed three peaks and ferroelectric relaxor feature to some extent. Hysteresis loops showed that graded thin film had higher remanent polarization, smaller coercive field than each thin film unit. The pyroelectric coefficient of the graded thin films increased gradually with temperature, and was higher than that of each thin film unit.     

EFFECTS OF PZT BUFFER LAYERS ON PZFNT FERROELECTRIC THIN FILMS

ABSTRACT

PZFNT thin films were fabricated on 5-inch Pt/Ti/SiO₂/Si and PZT/Pt/Ti/SiO₂/Si substrates by RF magnetron sputtering method and rapid thermal annealing process. By investigating the two thin films at various annealing temperatures, the results show that the annealing temperature of PZFNT thin films without PZT buffer layers is about 730°C, which is higher than that of PZFNT films with PZT buffer layers. By use of PZT buffer layers, the annealing temperature of PZFNT films is decreased greatly, and the dielectric and ferroelectric properties are improved. In the optimum process, the thin films with PZT buffer layers have a dielectric constant of 1199 and dielectric loss of 3.0% at 1 KHz. The remanent polarization and coercive field of the thin films are 21.1 μC/cm² and 53.5 KV/cm respectively. The films have the significant potential for FRAM and pyroelectric infrared detectors.     

Structural dependence of 1.3-μm narrow-beam lasers fabricated byselective MOCVD growth

The effect of structural parameters on the lasing characteristics of 1.3-μm narrow beam lasers has been investigated. Monolithically integrated vertically tapered multiquantum-well (MQW) waveguide, fabricated by use of selective metal-organic chemical vapor deposition (MOCVD), is used for the expansion of the optical spot size. It is experimentally shown that the energy separation between the gain and waveguide regions that is formed simultaneously by selective MOCVD is shown to be an important parameter in order to achieve low-threshold current density and good temperature characteristics. The lengths of gain and waveguide regions have been investigated in terms of temperature characteristics of threshold current and far-field angle. A lower threshold current density and a higher characteristic temperature were obtained for longer gain region, We also have estimated the waveguide loss of the mode-field converter lasers diodes (MFC-LD's). High performance of 1.3-μm integrated vertically tapered waveguide lasers were achieved in an optimized device     

Monolithic integration of spot-size converters with 1.3-μmlasers and 1.55-μm polarization insensitive semiconductor opticalamplifiers

To reduce packaging costs, it is necessary to use passive alignment between the laser diodes and optical fiber. Such an alignment requires low-coupling loss and large positional alignment tolerances. This is achievable with integrated spot-size converters, which permit to match the near field of a laser to that of a flat-end single-mode fiber (SMF). In this paper, we first review briefly the different technological approaches to realize spot-size converters. Then, we focus on the double-core structure developed both for 1.3-μm Fabry-Perot lasers and 1.55-μm semiconductor optical amplifiers (SOAs). The spot-size expansion is simulated using a two-dimensional (2-D) beam propagation method analysis. Short spot-size converters (100 μm) integrated with 1.3-μm lasers and 1.55-μm SOAs exhibit beam divergences as low as 12°×12° and 12°×15°, respectively. The performances of devices with integrated spot-size converters are reported and discussed. A 2-in wafer process is used thanks to the versatility of the double-core structure and its compatibility with buried ridge stripe technology     

High-power 1.55-μm mass-transport-grating DFB lasers forexternally modulated systems

High-output-power operation of 1.55-μm-wavelength distributed-feedback (DFB) lasers with a novel mass-transport grating (MTG) structure which is composed of InAsP buried with InP are reported. To improve high output power characteristics, we have investigated the influence of the width of the active layer on the light output power and the spectral linewidth at high injection current. It is confirmed that the increase of the active layer width is effective to realize high output power and to reduce the linewidth power product. The fabricated lasers show high single-longitudinal-mode output power of 180 mW, which is the highest value reported for 1.55-μm DFB lasers. They also exhibit narrow spectral linewidths less than 0.3 MHz and low noise characteristics of -159 dB/Hz. Moreover, we have obtained the mean time to failure of longer than 10⁵ h with a lifetime test over 200 h at 50°C     

1.55-μm vertical-cavity laser arrays for wavelength-divisionmultiplexing

We report on the fabrication and operation of the first electrically pumped 1.55-μm vertical-cavity laser array for wavelength-division-multiplexing applications. The array consisted of four channels operating between 1509 and 1524 nm. Wafer bonding was used to integrate GaAs-AlGaAs distributed Bragg reflectors with an InP-InGaAsP active region     

1.3-μm InAsP n-type modulation-doped MQW lasers grown bygas-source molecular beam epitaxy

The effect of n-type modulation doping as well as growth temperature on the threshold current density of 1.3-μm InAsP strained multiple-quantum-well (MQW) lasers grown by gas-source molecular beam epitaxy (GSMBE) was investigated for the first time. We have obtained threshold current density as low as 250 A/cm² for 1200-μm long devices. The threshold current density per well for infinite cavity length J_th/N_w∞ of 57 A/cm² was obtained for the optimum n-doping density (N_D=1×10¹⁸ cm^-3) and the optimum growth temperature (515°C for InP and 455°C for the SCH-MQW region), which is about 30% reduction as compared with that of undoped MQW lasers. A very low continuous-wave threshold current of 0.9 mA have been obtained at room temperature, which is the lowest ever reported for long-wavelength lasers using n-type modulation doping, and the lowest results grown by all kinds of MBE in the long-wavelength region. The differential gain was estimated by the measurement of relative intensity noise. No significant reduction of differential gain was observed for n-type MD-MQW lasers as compared with undoped MQW lasers. The carrier lifetime was also reduced by about 33% by using n-type MD-MQW lasers. Both reduction of the threshold current and the carrier lifetime lead to the reduction of the turn-on delay time by about 30%. The 1.3-μm InAsP strained MQW lasers using n-type modulation doping with very low power consumption and small turn-on delay is very attractive for laser array application in high-density parallel optical interconnection systems     

Large area deposition of Pb(Zr,Ti)O<Subscript>3</Subscript> thin films for piezoelectric MEMS devices

Pb(Zr,Ti)O₃ (PZT) thin films deposited on insulating ZrO₂ buffered silicon wafer are intended to be employed for in-plane polarized piezoelectric MEMS devices. Multi-target reactive sputtering system for large area deposition of in-situ crystallized PZT thin films and the ZrO₂ buffer layer has been employed. The interface analysis of multilayer structures by high resolution transmission microscope, X-ray diffraction, optical refraction, and absorption spectra studies has been presented. At a substrate temperature of 520°C and excess lead deposition condition, the formation of a PZT superstructure has been revealed. The substrate temperature of 580°C leads to the crystallization of PZT directly into a single phase perovskite crystal structure. A pronounced Urbach behavior in our PZT thin films has been observed by optical absorption studies. The surface roughness of PZT films deposited on a ZrO₂ buffer layer is much higher than that on conducting platinized silicon wafer.     

Structure and properties of sol-gel processed (Pb,La)TiO3 ferroelectric thin films

Abstract

The influence of heat treatment and substrate materials on PLT thin films by sol-gel processing has been researched. Epitaxial growth PLT thin films with perovskite-type structure on sapphire, SrTiO₃ and MgO single crystal substrates have been prepared. The epitaxial relations are (100)PLT28/(100)SrTiO₃, (111)PLT28//(0001)sapphire and (100)PLT14//(100)MgO. The PLT polycrystal thin films with perovskite-type structure on Si single crystal and quartz glass substrates have been prepared. The remanent polarization Pr and the coercive field Ec of PLT15 ceramic thin films are 7.7 μc/cm² and 34 kv/cm at 4 KHz respectively. The optical transmittance of PLT28 ceramic thin films within the wavelength range of λ = 500–1000 nm is approximately 80%.     

Growth of MgTiO3 thin films by pulsed laser deposition and their electrical properties

Abstract

MgTiO₃ thin films have been grown on various substrates by pulsed laser deposition (PLD) to investigate the application for microwave dielectrics and optical devices. Highly oriented MgTiO₃ thin films were obtained on sapphire (c-plane Al₂O₃). MgTiO₃ thin films deposited on SiO₂/Si and platinized silicon (Pt/Ti/SiO₂/Si) substrates were polycrystalline nature. MgTiO₃thin films grown on sapphire were transparent in the visible and had a sharp absorption edge at 280 nm. These MgTiO₃ thin films had extremely fine feature of surface morphology, i.e., rms roughness of 0.87 nm. Dielectric constant and loss of MgTiO₃ thin films deposited by PLD were about 24 and 1.5% at 1MHz, respectively. These MgTiO₃ thin films also exhibited little dielectric dispersion.     

Polymeric optical waveguide films for short-distance opticalinterconnects

We describe three different applications of polymeric waveguide films as short-distance optical interconnects. We fabricated the waveguide films, which were 6.5 cm long and mounted in MT-compatible (MTC) connectors by passive alignment, for MM fiber systems with a 50-μm diameter graded index (GI) core. The average insertion loss of these devices was approximately 0.6 dB at 0.85-μm wavelength. We also fabricated waveguide films with a 350 mirror and an MTC connector for use as 90° out-of-plane optical deflectors, and they exhibited an insertion loss of 1 dB. Two silica planar waveguides for single-mode (SM) fiber systems were also connected by a polymeric waveguide film. Low insertion losses were obtained in both MM and SM films designed to be employed as bending waveguides. This reveals their good potential for use as practical short-distance optical interconnects     

Fabrication and characterization of Au/SBT/LZO/Si MFIS structure

Ferroelectric SrBi₂Ta₂O₉ (SBT) films on a p-type Si (100) wafer with a LaZrO _x (LZO) buffer layer have been fabricated to form a metal-ferroelectric-insulator–semiconductor (MFIS) structure. The LZO thin film and SBT films were deposited by using a sol–gel method. The equivalent oxide thickness (EOT) value of the LZO thin film was about 8.83 nm. Also, the leakage current density of the LZO thin film is about 3.3?×?10^?5 A/cm² at bias sweeping voltage of ±5 V. SBT films were crystallized in polycrystalline phase with highly preferred (115) orientation. Also, the intensity of each pick slightly increased as thickness of SBT films increased. The C–V characteristics of Au/SBT/LZO/Si structure showed clockwise hysteresis loop. The memory window width increased as the thickness of SBT films increased. The leakage current density of Au/SBT/LZO/Si structure decreased as thickness of SBT films increased.     

Effects of temperature on structural and optical properties of ZnMgO films fabricated by pulsed laser deposition

ZnMgO thin films have been deposited on Si(111) substrates by pulsed laser deposition (PLD) technique at growth temperature from 300 to 700°C in nitrogen ambient of 1.0 Pa. The effects of growth temperature on structural and optical properties of deposited ZnMgO thin films have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared absorption (IR) spectra and photoluminescence (PL) spectra. The results of XRD and SEM analyses show that the film fabricated at 400°C possesses good crystallinity with hexagonal wurtzite structure and surface morphology. The Mg has been incorporated into ZnO in the form of substitutional Zn. The IR spectra reveal the typical absorption peaks of ZnMgO. The band-gap values have been obtained from 2.96 to 4.23 eV with increasing growth temperature. The PL spectra show that the highest UV emission is obtained at growth temperature of 600°C, and the obvious blue-shift is observed. This may be assigned as the change of the band-gap due to the increasing incorporation of Mg²⁺ ions with the increasing growth temperature.     

Structure and dielectric properties of Ba(Sn0.2Ti0.8)O3 thin films grown on different substrates by a sol-gel process

Ba(Sn_0.2Ti_0.8)O₃ (BTS) thin films were deposited on Pt/Ti/SiO₂/Si and LaNiO₃/Pt/Ti/SiO₂/Si substrates by a sol-gel processing technique, respectively. The influences of substrates on the phase and microstructure of the thin films were examined. Dielectric properties of the thin films were investigated as a function of frequency and direct current electric field. The results showed that the substrates strongly influenced the microstructure and the dielectric properties of the films. The properties of BTS thin films on LaNiO₃/Pt/Ti/SiO₂/Si substrates were superior to that of the films grown on Pt/Ti/SiO₂/Si substrates.     

Effects of Heating Process on Crystalline Orientation and Electrical Properties of (Bi,La)4Ti3O12 Thin Films Derived by Chemical Solution Deposition Method

(Bi,La)₄Ti₃O₁₂ (BLT) thin films were prepared on Pt-coated Si (Pt/Ti/SiO₂/Si(100)) substrates by chemical solution deposition method. The effect of heating process on structural and electrical properties of BLT thin films was investigated. The c-axis preferentially oriented films were obtained by lower drying temperature with heating from bottom by a hot plate or IR lamp heater. On the other hand, randomly oriented films were obtained by higher drying temperature with heating from top of films by IR lamp heater or with heating substrate all around by tube type furnace. The orientation of BLT thin films deposited on Pt-coated substrates can be controlled by heating process.     

Blueshift of absorption edge and photoluminescence in Al doped ZnO thin films

ABSTRACT

Pure and Al doped ZnO thin films are fabricated on quartz substrates by sol–gel method, and then analyzed by X-ray diffraction (XRD), transmittance spectra, and photoluminescence (PL) measurements respectively. XRD results reveal that all the thin films have a preferential c-axis orientation. With the increase of Al doping, however, the peak position of the (002) plane is shifted to a low 2θ value. On the other hand, the data of spectrometer transmittance are obtained, with which the band gap energies of Al doped ZnO films are calculated by a linear fitting method. The band gap is found to be broadening, and the absorption edge has an obvious blueshift to the shorter wavelength with increasing dopant concentration. PL measurement is also conducted, and deep-level (DL) emission and near band edge (NBE) emission are observed in pure ZnO thin films. But DL emissions are depressed when Al is doped into thin films. And the peak of NBE emission has a blueshift to the region of higher photon energy as the Al concentration increases, a performance which tallies with observations through the optical transmittance data. The study demonstrates that the blueshift of optical properties in the ZnO:Al films, can nevertheless be easily manipulated and managed by controlling dopant concentration, a big plus to the said films in their applications in broadband UV photodetectors with highly tunable wavelength resolution.     

Structural and dielectric properties of epitaxial Ba0.6Sr0.4TiO3 thin films grown on Si substrates with thin SrO buffer layers

(100) epitaxial Ba_0.6Sr_0.4TiO₃ (BST) thin films were grown on Si substrates using a 9 nm thick SrO buffer layer. The phase shifter fabricated on BST films grown on a SrO buffered Si substrate showed a larger figure of merit (FOM) of 24.7°/dB as a result of improving the phase tuning while retaining an appropriate insertion loss compared to that (15.3°/dB) for the BST/MgO structure. This work demonstrates that a thin SrO buffer layer plays an important role in the successful integration of BST-based microwave tunable devices onto Si wafers.     

Characteristics of Bi4Ti3O12 thin films on ITO/glass and Pt/Si substrates prepared by R.F. sputtering and rapid thermal annealing

Bi₄Ti₃O₁₂ thin films are deposited on ITO/glass and Pt/Ti/Si(100) substrates by R.F. magnetron sputtering at room temperature. The films are then heated by a rapid thermal annealing (RTA) process conducted in oxygen atmosphere at temperatures ranging from 550–700^∘C. X-ray diffraction examination reveals that the crystalinity of the films grown on Pt/Ti/Si is better than that of the films grown on ITO/glass under the same fabrication conditions. SEM observation shows that the films grown on Pt/Ti/Si are denser than those grown on ITO/glass substrates. Interactive diffusion between the Bi₄Ti₃O₁₂ film and the ITO film increases with the increase of annealing temperature. The optical transmittance of the thin film annealed at 650^∘C is found to be almost 100% when the effect of the ITO film is excluded. The relative dielectric constants, leakage currents and polarization characteristics of the two films are compared and discussed.