We have investigated a novel approach for improving GaN crystal quality by utilizing a stack of quantum dots (QDs) in GaN grown on sapphire substrates by molecular beam epitaxy. The GaN films were grown on GaN/AlN buffer layers containing multiple QDs and characterized using x-ray diffraction, photoluminescence, atomic force microscopy, and transmission electron microscopy. The density of the dislocations in the films was determined by defect delineation wet chemical etching and atomic force microscopy. It was found that the insertion of a set of multiple GaN QD layers in the buffer layer effectively reduced the density of the dislocations in the epitaxial layers. As compared to a density of ∼1010 cm-2 in typical GaN films grown on AlN buffer layers, a density of ∼3×107 cm-2 was demonstrated in GaN films grown with the QD layers. Transmission electron microscopy observations confirmed termination of threading dislocations by the QD layers.

We report for the first time the use of the relatively low index material SiON (n=1.56) for photonic crystal applications. With this system it is possible to enlarge the complete TE-band gap of a 2D-photonic crystal by properly designing the unit cell filling. This may be practically realised because of the larger dimensions of photonic crystals in low index materials, which allows less stringent etch conditions when compared to traditional InP based materials. The optimal distribution of the refractive index is found by placing the displacement field in high or in low dielectric material. The structure is defined by two figures of merit, the width of the band gap and the smallest feature size, which is the key factor in etching the structure. In addition, we suggest the use of a graded-index profile to reduce out of plane scattering. This graded index profile can be realised by controlling the nitrogen flow during deposition of the SiON layer.

Photoluminescence (PL) and Time-resolved PL (TR-PL) are used to measure the luminescence energy and carrier lifetime of InGaN/GaN quantum well (QW) structures as a function of biaxial strain and excitation density. A blueshift of the transition energy and a decrease in the carrier lifetime reveal a field-dependent spatial electron-hole (e-h) wavefunction separation. This behavior is observed both under the application of tensile, biaxial strain, which directly affects the piezo-related field, and under increased excitation density, which effectively screens the electric field. Our results show an increased carrier separation with increasing QW thickness.

We have explored the growth of GaN on porous SiC substrates by plasma-assisted molecular beam epitaxy. The porous 4H- and 6H-SiC(0001) substrates used in this study contain 10 to 100-nm sized pores and a thin skin layer at the surface. This skin layer was partially removed prior to the growth by H-etching. Transmission electron microscopy (TEM) observations indicate that the epitaxial GaN growth initiates from the surface areas between pores, and the exposed surface pores tend to extend into GaN as open tubes and trap Ga droplets. Plan-view TEM observations indicate that the GaN layers grown on porous substrates contain fewer dislocations than layers grown on non-porous substrates by roughly a factor of two. The GaN layers grown on a porous SiC substrate were also found to be mechanically more relaxed than those grown on non-porous substrates; electron diffraction patterns indicate that the former are free of misfit strain or are even in tension after cooling to room temperature.

We summarize structural properties of thick HVPE GaN templates from the point of view of their application as substrates for growth of nitride layers. This is followed by the results of optical and structural studies, mostly transmission electron microscopy, of nitride layers grown by MOCVD on top of the HVPE substrates. The results indicate high structural quality of these layers with a low density of threading dislocations (in the range of 106 cm-2). Convergent beam electron diffraction studies showed that the MOCVD GaN films have Ga-polarity, the same polarity as the HVPE GaN substrates. Structural studies of an InGaN layer grown on top of the MOCVD GaN film showed the presence of two layers, which differed in lattice parameter and composition. The upper layer, on the top of the structure had a clattice parameter about 2 % larger than that of GaN and contained 10.3 ± 0.8 % of In. Values measured for the thinner, intermediate layer adjacent to the GaN layer were about 2.5 times lower.

We present an efficient approach for computing the optical properties of defect structures embedded in Photonic Crystals. This approach is based on an expansion of the electromagnetic field into optimally localized photonic Wannier functions and maximally utilizes the information of the underlying Photonic Crystals available from photonic bandstructure computations. While we demonstrate the efficiency of this approach by considering several defect structures for TM-polarized radiation in two-dimensional Photonic Crystals, the approach itself can easily be extended to the case of TE-polarization and three-dimensional Photonic Crystals.

Usage of air holes in optical fibers has become a hot subject in fiber optics because of the possibilities for novel transmission properties. Although photonic crystal fibers based on photonic bandgap guidance are the most drastic innovation in this subject, optical fibers containing air holes but not having photonic crystal structures are also being intensively studied. Such air-silica microstructured fibers are more practical than the photonic bandgap fibers because the lack of photonic crystal structure makes the fabrication far easier. Even without the photonic bandgap, the microstructured fibers can exhibit valuable properties in terms of group velocity dispersion and nonlinearity, because the index contrast between air and silica is 10 or more times as large as that of the conventional optical fibers based on doped silica glasses. However, one of the major challenges for practical applications of the air-silica microstructured fibers has been their high transmission losses, which have been several tens to hundreds times higher than those of the conventional fibers. As a solution to this problem, we have proposed a more practical structure called hole-assisted lightguide fiber (HALF). In addition to the air holes for realizing novel optical properties, this structure has a material index profile for waveguiding, and hence is closer to the conventional fibers than the other microstructured fibers are. As a result, novel optical properties can be realized without severe degradation in transmission loss. In experiments, an anomalous group velocity dispersion as large as +35 ps/nm/km at 1550 nm wavelength, which would be unattainable in the conventional fibers, has been realized with a loss of 0.41 dB/km, which is comparable to those of the conventional fibers. Analyses of the losses of the fabricated HALFs suggest that the loss should be lowered by mitigating the effect of the drawing tension and minimizing the power fraction in the holes. It is also shown that the full-vector finite element method realizes accurate modeling of the properties such as dispersion and macrobend loss.

Based on the Laser Scanned Photodiode (LSP) image sensor we present an optical fingerprint reader for biometric authentication. The device configuration and the scanning system are optimized for this specific purpose.

The scanning technique for fingerprint acquisition is improved and the effects of the probe beam size, wavelength and flux, the scan time and modulation frequency on image contrast and resolution will be analyzed under different electrical bias. An optical model of the image acquisition process is presented and supported by a two dimensional simulation.

Results show that a trade-off between read-out parameters (fingerprint scanner) and the biometric sensing element structure (p-i-n structure) are needed to minimize the cross talk between the fingerprint ridges and the fingerprint valleys. In the heterostructures with wide band gap/low conductivity doped layers the user-specific information is detected with a good contrast while the resolution of the sensor is around 20 νm. A further increase in the contrast is achieved by slightly reverse biasing the sensor with a sensitivity of 6.5 νWcm-2 and a flux range of two orders of magnitude.

A Low Thermal Stress (LTS) process, involving the removal of most of the InGaAs(P) material, was used to allow for wafer-fusion of InGaAs(P) samples to Si. The formation and behavior of bubbles of trapped gas between the thinned layer of InGaAs(P) material and the thick Si substrate were studied due to the yield problems such bubbles represent. Observations revealed a low temperature regime <300°C, during which significant gas generation occurred but only weak, reversible bonds were formed. The higher temperature regime, 300°-650°C, was characterized by no significant increase in bubble density, but resulted in a reduction or complete blockage of gas escape during storage. Further investigation of the higher temperature regime indicated continued production of gas, but with a tendency of the gases produced above 300°C to escape from the interface without the formation of bubbles under normal conditions. This information led to the development of a scalable LTS process requiring no modification of the wafers prior to bonding and utilizing sample pre-heating to 300°C before InGaAs(P) removal and subsequent high temperature treatment to 650°C for permanent bond formation.

InGaN device quality films and their related heterostructures play a critical role in the development of nitride devices. InGaN growth needs to be performed at much lower temperatures than GaN growth, due to the lower dissociation temperature of InN. Furthermore, decomposition of ammonia becomes less efficient with decreasing temperature due to high kinetic barrier for breaking N-H bonds and InGaN growth needs high NH3/TMIn ratio. We investigated the optical and structural properties of InGaN bulk layers and GaN/InGaN MQWs using thermally precracked ion supplied metalorganic chemical vapor deposition (TPISMOCVD) system. The temperature range for this study was 670 -770°C. In a low NH3 flow condition, In metal droplet appeared on the surface on InGaN layer in conventional MOCVD system, but it disappeared in TPIS-MOCVD system. An increasement of In mole fraction in InGaN could be achieved even in low NH3 flow. As the NH3 flow rate and the InGaN growth temperature decreased, In metal droplet was more effectively reduced by ammonia precracking. The quality of InGaN/GaN MQWs was evaluated with high resolution XRD and TEM.

Novel organic, polymeric materials and processes of depositing thin films on electronics substrates by chemical vapor deposition (CVD) have been developed and the lithographic behavior of photoresist coated over these CVD films at deep ultraviolet (DUV) wavelength has been evaluated. The specific monomers synthesized for DUV applications include [2.2](1,4)- naphthalenophane, [2.2](9,10)-anthracenophane and their derivatives which showed remarkable film uniformity on flat wafers and conformality over structured topography wafers, upon polymerization by CVD. The chemical, physical and optical properties of the deposited films have been characterized by measuring parameters such as thickness uniformity, solubility, conformality, adhesion to semiconductor substrates, ultraviolet-visible spectra, optical density, optical constants, defectivity, and resist compatibility. Scanning electron microscope (SEM) photos of cross-sectioned patterned wafers showed verticle profiles with no footing, standing waves or undercut. Resist profiles down to 0.10 νm dense lines and 0.09 νm isolated lines were achieved in initial tests. CVD coatings generated 96-100% conformal films, which is a substantial improvement over commercial spin-on polymeric systems. The light absorbing layers have high optical density at 248 nm and are therefore capable materials for DUV lithography applications. CVD is a potentially useful technology to extend lithography for sub-0.15 νm devices. These films have potential applications in microelectronics, optoelectronics and photonics.

In this paper, we are investigating the growth of a ZnSe based vertical cavity surface emitting laser (VCSEL). Undoped and p-type doped distributed Bragg reflectors (DBRs) with reflectivities exceeding 99% have been grown by molecular beam epitaxy (MBE) using Zn(S)Se layers for the high refractive index material and ZnSe/MgS superlattices (SLs) for the low index material. An undoped monolithic VCSEL structure containing a ZnCdSSe quantum well (QW) emitting in the blue-green reaches a quality factor (Q-factor) of 100, which is the best value reported so far. Temperature dependent photoluminescence (PL) measurements show that the emission of the QW is effectively guided by the cavity resonance.

We have studied the electrical and optical characteristics GaN/InGaN based light-emitting diodes (LEDs) grown on sapphire using metalorganic chemical vapor deposition (MOCVD). Strong correlation has been found between material quality and the mechanism of current transport through the wide-bandgap p-n junction. Tunneling behavior dominates throughout all injection regimes in the devices with high-density defects in the space-charge region, which act as deep-level carrier traps. The approximately current-squared dependence of light output at low currents indicates dominant nonradiative recombination in the active region. However, in a high quality LED diode, tunneling current is only a major contributor at low forward biases. At moderate biases, temperature dependent diffusion-recombination current has been identified as I0 exp(qV/1.6kT). In these devices, nonradiative recombination centers are saturated at current densities as low as 1.4×10-2 A/cm2.

Highly-oriented ZnO thin films have been successfully deposited on (100)Si by metal organic chemical vapor deposition (MOCVD) at 250°C∼400°C. We report on the structural properties of ZnO thin film at various temperatures. The crystallinity of thin films improved and the surface smoothness decreased with increasing growth temperature. In x-ray reflection analysis with respect to ZnO (0002) peak, the full width at half maximum (FWHM) of 0.4°was achieved at 400°.

We have developed a thermally stimulated narrow-band infrared source for sensing, spectroscopy and thermophotovoltaic applications by combining the unique advantages of two different structures: a photonic crystal that consists of an array of holes etched into a dielectric substrate and a periodically perforated metallic thin film. The dielectric photonic crystal structure is passive and exhibits a strong absorption at resonance. This acts as a radiation reservoir for the conductive array, which plays an active role through plasmon interactions and is opaque at all wavelengths except those at which coupling occurs. We have fabricated the arrays on silicon, silicon dioxide and silicon nitride substrates using MEMS-based processing methods. Infrared spectroscopic studies were used to characterize reflection, absorption and emission in the 2 to 14 micron range showing narrow band resonance. Spectral tuning was accomplished by controlling symmetry and lattice spacing of the arrays. The effects of the etch depth, metal and dielectric properties have been studied experimentally and theoretically. These structures have been used as an emitter/detector sensor chip to selectively detect industrial pollutants like carbon dioxide.

The use of dry hydrogen plasma etching is evaluated for determination of GaN polarity and critically compared to wet etching in NaOH. It is shown that hydrogen plasma etching is effective in revealing inversion domains (IDs) and some types of dislocations. This is because the surface morphology is unchanged by the hydrogen treatment, and, hence, the surface reactivity is not masked.

The effects of proton irradiation on carrier dynamics were measured by time-resolved photoluminescence on InGaAs/GaAs quantum dot structures with different dot density and substrate orientation, as well as on InAlAs/AlGaAs quantum dots. Results were compared to irradiation effects on carrier dynamics in thin InGaAs quantum wells. We find that carrier lifetimes in QDs are much less affected by proton irradiation than in quantum wells, which can be attributed to the three-dimensional carrier confinement in quantum dots.

Luminescence on Si-epitaxial layers grown on Si-implated Al2O3 ( 1102) with 30 keV Si+ to a dose of 5×1015/cm2 has been investigated. To active the implanted Si+ ions in Al2O3, the post annealing was performed at 1100°C in Ar ambiant. Also, cathodoluminescence (CL) and photoluminescence (PL) have been used to study struc Al2O3tural and optical properties of nc-Si in the Si+-implanted (1102) Al2O3 substrates. In the PL and CL spectra for Si+-implanted samples, peaks to be responsible for nanocrystalline-Si (nc-Si) appear at 316 nm (3.92 eV) and 574 nm (2.16 eV), respectively. The crystallinity of nc-Si imbeded in Al2O3 has been about 5nm in size and the dislocation aligned parallel to the (0001) planes of (1102) Al2O3, confirmed by transmission electron microscopy (TEM).

A perfect 2D porous alumina photonic crystal with 500 nm interpore distance was fabricated on an area of 4 cm2 via imprint methods and subsequent electrochemical anodization. A 4” imprint stamp consisting of a convex pyramid array was obtained by modern VLSI processing using DUV-lithography, anisotropic etching, LPCVD Si3N4 deposition and wafer bonding. The optical properties of the porous alumina photonic crystal were measured with an infrared microscope in Г-M direction. For both polarizations, a bandgap is observed at around 1 μm for r/a = 0.42. A reflectivity of almost unity for E-polarization in the region of the bandgap is a sign of the high quality of the structure, indicating almost no scattering losses. These experimental results could be correlated very well to the bandstructure as well as reflectivity calculations assuming a dielectric constant of å = 2.0 for the anodized alumina.

This paper describes the results of extensive performance and reliability characterization of a silicon-based surface micro-machined tunable optical filter. The device comprises a high-finesse Fabry-Perot etalon with one flat and one curved dielectric mirror. The curved mirror is mounted on an electrostatically actuated silicon nitride membrane tethered to the substrate using silicon nitride posts. A voltage applied to the membrane allows the device to be tuned by adjusting the length of the cavity. The device is coupled optically to an input and an output single mode fiber inside a hermetic package. Extensive performance characterization (over operating temperature range) was performed on the packaged device. Parameters characterized included tuning characteristics, insertion loss, filter line-width and side mode suppression ratio. Reliability testing was performed by subjecting the MEMS structure to a very large number of actuations at an elevated temperature both inside the package and on a test board. The MEMS structure was found to be extremely robust, running trillions of actuations without failures. Package level reliability testing conforming to Telcordia standards indicated that key device parameters including insertion loss, filter line-width and tuning characteristics did not change measurably over the duration of the test.