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The development of integrated optical isolators is critical to the functional integration of optical devices and systems. This work will primarily elucidate a methodology to grow, by a semiconductor compatible process, the critical active material in monolithically integrated magneto-optical isolators; yttrium iron garnet (YIG: Y3Fe5O12). Reactive radio frequency (RF) sputtering was used to grow YIG on MgO, which is a promising buffer layer material for optical devices. The chemical, structural, optical, magneto-optical and magnetic properties of the resulting films have been studied by various techniques including energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Faraday rotation measurements and vibrating sample magnetometry (VSM). Low forward powers (lower limit of 12.3 W/cm2) grew YIG nuclei in an amorphous matrix and the number of these nuclei increased with increasing forward power. At powers exceeding 19W/cm2 film cracking occurred. The films with YIG had strong in-plane magnetizations with small coercive fields. Optical cladding layers compatible with YIG films have been grown through plasma enhanced chemical vapor deposition (PECVD) and thin film permanent magnets for biasing have been grown and optimized.
Ordered arrays of pores in Si provided the first (two dimensional) photonic crystals with bandgaps in the μm region. The paper explores the potential of pore etching for two- and threedimensional photonic crystals in GaAs, InP, and GaP. A striking feature of pore etching in III-V semiconductors is the strong tendency to self-organization and pattern formation. As an example, self-organized well-defined pore lattices (a = 100 nm – 1 μm) can be made in InP. All materials show self organized diameter oscillations, often synchronized over large distances between pores. Extremely strong diameter oscillations are observed in GaAs. Pores in all materials tend to grow in <111> directions, but can be induced to grow in the direction of current flow, too. These features can be used to produce two- and three dimensional photonic crystals. The latter goal might be achieved by switching periodically between different pore morphologies with depth, or by modulating the diameter with depth - always helped by the tendency to self organization. Self organization, however, will not lead to perfect crystal structures; lithographically defined nucleation is needed and has been tried. First results show that there are pronounced differences to what is known from Si. While the production of externally defined photonic crystals in the sub μm region appears to be feasible, the strong tendency to self organization must be taken into account by matching internal time and length scales to the desired external ones.
Bulk-quantity single crystalline wurtzite gallium nitride nanowires with a mean diameter of 25 nm were synthesized on silicon substrate using a catalyst-assisted reaction of gallium and gallium nitride mixture with ammonia. They exhibit a strong and broad photoluminescence in the energy range of 2.9-3.6 eV with no yellow band. X-ray diffraction and Raman scattering data suggest that the nanowires would experience biaxial compressive stresses in the inward radial direction and the induced tensile uniaxial stresses in the wire axis. The blue photoluminescence would originate from the recombination of the bound excitons under the compressive and tensile stresses.
We report on the growth of quaternary AlInGaN layers and MQWs by two different metalorganic chemical vapor deposition (MOCVD) techniques such as pulsed atomic layer epitaxy (PALE) and pulsed MOCVD (PMOCVD). For both growth processes, emission wavelength of quaternary MQWs can be tuned from 350 nm to 300 nm by simply changing the unit growth cell configurations. The PALE grown AlInGaN MQWs have a very smooth surface, few band tail states and exhibit a band-to-band emission. The PMOCVD grown AlInGaN MQWs exhibit a high density of band tail states, which strongly enhance spontaneous emission. Based on the characterization by photoluminescence, X-ray diffraction and AFM, both MOCVD techniques grown quaternary samples are shown to be promising for fabricating the active region of deep UV LEDs.
Photoluminescence (PL) spectra and time resolved PL from self-assembled InAs/GaAs quantum dots (QDs) grown by metal organic chemical vapor deposition are studied. A reduction in the emission linewidth with increasing temperature was observed at low temperature range and an increase in the linewidth at higher temperature. It was also observed that the variation of PL peak energy with temperature does not follow Varshni's equation. These anomalous behaviors of PL can be explained in term of thermal redistribution of carriers. It was also found that the PL decay time increases with photon wavelength, which is due to the carrier transfer between laterally coupled QDs.
While physical properties of ideal antimonide superlattices (ASL) indicate that they should significantly outperform mercury cadmium telluride (MCT) based infrared photodiodes for low dark current applications in the long and very long wave-infrared (LWIR and VLWIR), this potential has not yet been fully realized. Even though measured Auger and tunneling rates in ASL's are reduced as predicted, overall carrier lifetimes remain much shorter, and dark currents much higher than expected. The large carrier losses are the result of defects in the ASL structure, with contributions measured from large bulk defects and surface channels along mesa sidewalls, and the remaining component believed to be due to midgap states.
In this presentation we report on several studies of epitaxial growth parameters and their influence on defect formation. X-ray photoelectron spectroscopy analysis of oxide desorption from GaSb substrates shows the presence of both antimony and gallium oxides, along with their decomposition and desorption behavior with anneal temperature. A study of buffer growth shows that defect density and size are critically dependent on growth temperature, with an optimal growth window between 480 and 500 °C.. Side-by-side GaSb buffer growths on vicinal ((100) + 1 ° (111)) and flat (100) substrates show that while growing on vicinal material can suppress mound formation, it does not yield epilayers as flat as can be obtained on (100) substrates grown under optimal conditons. Finally, the ratio of As to In flux during superlattice growth can be used to tune the lattice parameter both above and below that of GaSb, with strain-related defects appearing when the mismatch reaches roughly 0.1%.
The polarization state of visible light is found to be altered upon reflection from artificial two-dimensional chiral media. Arrays of metallic planar chiral structures were fabricated by electron beam lithography and ion beam milling. The characteristic dimensions on the chiral elements correspond to wavelengths in the near-IR. Our chiral media are found to induce strong polarization effects, with the handedness of individual elements having a direct effect on the sense and magnitude of rotation of the diffracted light.
We have studied the effects of Al0.1Ga0.9N(150 nm)/AlN Composite Nucleation Layers (CNLs) having different thicknesses of AlN ranging from 20 to 41 nm on the growth characteristics of GaN/Si(111) epitaxy. The surface morphologies of the GaN epitaxial layers which were grown on Al0.1Ga0.9N(150nm)/AlN CNLs showed that the number of thermal etch pits and cracks was abruptly decreased with the increase of AlN thickness from 20 to 35 nm. However, the morphology of GaN epitaxy which was grown on Al0.1Ga0.9N(150 nm)/AlN CNL having AlN of 41 nm thick above 35 nm showed that the number of them was increased again. So, the GaN/Si(111) epitaxy which was grown using Al0.1Ga0.9N(150 nm)/AlN(35 nm) CNL showed the highest crystallinity having the FWHM of 1157 arcsec for the (0002) diffraction. Photoluminescence spectrum at room temperature for GaN/Si(111) epitaxy grown using Al0.1Ga0.9N(150 nm)/AlN(35 nm) CNL showed a sharp band edge emission at 364 nm, which especially doesn't have yellow luminescence related to various defects such as vacancy and dislocation. Meanwhile, the spectra at room temperature for the others showed yellow luminescence at around 580 nm except each band edge emission. Moreover, the FWHM of main exitonic peak at 10 K for the GaN/Si(111) epitaxy which was grown using Al0.1Ga0.9N(150 nm)/AlN(35 nm) CNL is the lowest value of 12.81 meV among them. It is obvious that the Al0.1Ga0.9N(150 nm)/AlN CNL having suitable thickness of AlN plays an important role in improving the crystallinity and optical properties of GaN/Si(111) heteroepitaxy without any defects such as pits and cracks over the surface by reducing the mismatch of thermal expansion coefficient and lattice constant between GaN and Si(111) comparing with AlxGa1-xN or AlN nucleation layer alone.
We present the results on investigation and analysis of photoluminescence (PL) dynamics of quaternary AlInGaN epilayers and AlInGaN/AlInGaN multiple quantum wells (MQWs) grown by a novel pulsed metalorganic chemical vapor deposition (PMOCVD). The emission peaks in both AlInGaN epilayers and MQWs show a blueshift with increasing excitation power density. The PL emission of quaternary samples is attributed to recombination of carriers/excitons localized at band-tail states. The PL decay time increases with decreasing emission photon energy, which is a characteristic of localized carrier/exciton recombination due to alloy disorder. The obtained properties of AlInGaN materials grown by a PMOCVD are similar to those of InGaN. This indicates that the AlInGaN system is promising for ultraviolet applications such as the InGaN system for blue light emitting diode and laser diode applications.
The electrical properties of heterojunctions composed of polycrystalline films of beta-irondisilicide and n-type germanium substrate are investigated. The heterojunctions have been prepared by co-sputtering of iron and silicon on germanium substrate followed by thermal annealing. The samples were prepared over various annealing temperature and chemical compositions. Most of the samples showed rectifying characteristics in current-voltage characteristics measurement. However, large backward leakage current was observed. The result is consistent with that in the case of beta-irondisilicide/silicon heterojunctions. In addition, the leak current showed significant dependence on annealing condition and chemical composition. It was suggested that the high density of trap levels existing at the interface caused by diffusion of Fe into substrate induce the inadequate electrical properties of the samples.
We demonstrate two ways in which the optical band-gap of a 2-D macroporous silicon photonic crystal can be tuned. In the first method the temperature dependence of the refractive index of an infiltrated nematic liquid crystal is used to tune the high frequency edge of the photonic band gap by up to 70 nm as the temperature is increased from 35 to 59°C. In a second technique we have optically pumped the silicon backbone using 150 fs, 800 nm pulses, injecting high density electron hole pairs. Through the induced changes to the dielectric constant via the Drude contribution we have observed shifts up to 30 nm of the high frequency edge of a band-gap.
Synthesis and characterization of transparent Co-doped ZnO and TiO2 diluted magnetic semiconductor (DMS) films are described. The films are prepared by single sputtering deposition. They are ferromagnetic at temperatures as high as 350 K. The films were characterized by X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), X-ray fluorescence (XRF). Optical transparency was measured on UV/VIS spectrometer. The Codoped ZnO films had wurtzite structure similar to ZnO with the (002) preferential texture. Neither XRD nor XPS showed any presence of pure Co or CoO in the samples. The Co-doped TiO2 samples were amorphous, and some unoxidized Co was found in the films.
We investigate the localized coupled-cavity modes in two-dimensional dielectric photonic crystals. The transmission, phase, and delay time characteristics of the various coupled-cavity structures are measured and calculated. We observed waveguiding through the coupled cavities, splitting of electromagnetic waves in waveguide ports, and switching effect in such structures. The corresponding field patterns and the transmission spectra are obtained from the finite-difference-time-domain (FDTD) simulations. We also develop a theory based on the classical wave analog of the tight-binding (TB) approximation in solid state physics. Experimental results are in good agreement with the FDTD simulations and predictions of the TB approximation.
In this work we describe a novel system for photovoltaic applications which combines InGaAs based strain-balanced multiple quantum wells (MQWs) with a “virtual substrate”, designed to extend the absorption edge of the photovoltaic devices to about 1 eV. The virtual substrate is designed by properly choosing a sequence of InGaAs layers having different In content, in order to obtain the desired lattice parameter at the topmost layer and to confine at the deepest interfaces the misfit dislocations, well away from the QW active region.
A series of InGaAs p-i-n junctions, containing a strain balanced MQW in the intrinsic region, were deposited by metallorganic chemical vapor deposition on different virtual substrates. In all the samples the virtual substrates were proved to be successful to grow zero net strain MQW and to confine defects at the buffer/substrate interface. Transmission electron microscopy observation shows that no defects propagate from the strain accommodating layers to the active region. The total density of threading dislocations reaching the surface was found to be less than 1*E5/cm2.
The confined misfit dislocation network, however, results in marked cross-hatched morphology that was found to affect the lateral strain distribution in the whole structure. By optimizing the growth condition of the structures, the influence of the surface roughness induced by CH pattern is partially suppressed.
Antimony sulfide (Sb2S3) and bismuth sulfide (Bi2S3) thin films of 200 nm thickness each were deposited from aqueous baths on glass substrates. Subsequently, thin films of thallium sulfide (Tl2S) with thickness around 120 nm were deposited on to these films from a bath containing thallium nitrate, sodium citrate, sodium hydroxide and thiourea. The multilayer films of Sb2S3-Tl2S, Bi2S3-Tl2S and Bi2S3-Sb2S3-Tl2S, thus produced, were heated in a nitrogen atmosphere around 300°C. XRD studies confirmed the formation of TlSbS2, TlBiS2, Tl4Bi2S5 and TlSb3S5 compounds. Optical band gaps of these materials are 1.85 eV (TlSbS2), 0.15 eV (TlBiS2), and about 1 eV for the composite film (Tl4Bi2S5 + TlSb3S5). In the visible spectral region, the optical absorption coefficients of these materials are about 105 cm-1. Values of dark conductivity are 10-7 Ωcm-1 (TlSbS2), 10-4 Ωcm-1 for TlBiS2 and 10-6 Ωcm-1 for the composite film. All the films are photoconductive.
We present a systematic study on the nonlinear optical properties of silicon nanocrystals (Si-nc) grown by plasma enhanced chemical vapour deposition (PECVD). The sign and magnitude of both real and imaginary parts of third-order nonlinear susceptibility χ(3) of Si-nc are measured by Z-scan method. While the closed aperture Z-scan reveals a sign of positive nonlinearity, the open aperture measurements suggests a nonlinear absorption coefficients. Absolute values of χ(3) are in the order of 10-9 esu and show systematic correlation with the Si-nc size, due to quantum confinement related effects.
A one-dimensional Si/SiO2 photonic crystal with a large, tunable air defect cavity is fabricated. Multiple resonant modes are observed within the photonic band gap. The free spectral range (FSR) is large compared to other resonant structures, with more than 100nm bandwidth. Simultaneous low voltage tuning around two telecom wavelengths, 1.55μm and 1.3μm, is realized using electrostatic force. The whole process is at low temperature and can be CMOS compatible. Potential applications include switching, modulation, and wavelength conversion devices, particularly WDM devices.
Charge distributions N(z) and electroluminescence spectra of blue and green light-emitting diodes (LEDs) based on InGaN/AlGaN/GaN heterostructures with multiple quantum wells. MQWs were modulated doped by Si donors in GaN barriers, electrons from donors being in InGaN wells. N(z) were determined using dynamical capacitance (C-V) method. Acceptor and donor concentrations near the p-n- junction were approximately NA ≥1.1019 cm-3 >> ND ≥ 1.1018 cm-3. Functions N(z) have periodic maxima and minima; their number was 4 and a period of 10 %15 nm, according to the details of growth. The extrema reflect charge distributions in MQWs on the n-side of the junctions with accuracy in z of the order of the Debye length (2-3 nm). An energy diagram of the structures is calculated according these measurements. Shifts of spectral maxima with current (J = 10-6 – 3.10-2 A) for these LEDs are comparatively low (3-12 meV for blue LEDs and 20-50 meV for green ones), much less than for previously studied green LEDs (up to 150 meV). This behavior is explained by screening of piezoelectric fields by electrons in the wells. Quantum efficiency versus current is correlated with N(z) distributions and currentvoltage characteristics of the LEDs.
Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) studies of GaAs/GaN interfaces, obtained by direct wafer bonding, are presented. TEM observations show that most of the interface area was well bonded. A thin oxide layer, confirmed by EDX, was present at the interface in the well-bonded regions. Plan-view TEM studies showed the presence of two dislocation networks in such regions. They formed to accommodate: (1) tilt between bonded crystals and (2) strain, which appeared during sample cooling due to mismatch in thermal expansion coefficients. Asymmetrical, often elongated, cavities, formed on the GaAs side, were present at the interface between the well-bonded regions. It was shown by EDX that the walls of these cavities are covered with native oxide.
Large area hydrogenated amorphous silicon p-i-n structures with low conductivity doped layers were proposed as single element image sensors. This work is focused on the analysis of the dynamic behavior of the sensor. Additionally some sensor parameters like maximum scanning speed, from which depends the maximum achievable frame rate are presented and discussed.
In order to evaluate the sensor response to a time varying light excitation the sensor was locally illuminated with a focused chopped light source and the generated photocurrent was measured under different load conditions. Results show that the sensor is mainly capacitive and a signal rise time of approximately 100 νs was measured under a 1 kΩ load. A model for the sensor was created from the experimental data and was used to simulate the dynamic behavior of the sensor. The simulation results obtained are in good agreement with the experimental ones.
As conclusion one can expect a trade off between the frame rate and the number of pixels. A frame rate higher than 10 fps was achieved for 100×100 pixels readout without a significant degradation in the image quality.