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G. Kipshidze,
S. Nikishin,
V. Kuryatkov,
K. Choi,
Ìu. Gherasoiu,
T. Prokofyeva,
M. Holtz, H. Temkin,
K. D. Hobart,
F. J. Kub,
M. Fatemi
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ABSTRACT: Epitaxial layers of AlN and GaN were grown by gas source molecular-beam epitaxy on a composite substrate consisting of a thin
(250 nm) layer of silicon (111) bonded to a polycrystalline SiC substrate. Two dimensional growth modes of AlN and GaN were
observed. We show that the plastic deformation of the thin Si layer results in initial relaxation of the AlN buffer layer
and thus eliminates cracking of the epitaxial layer of GaN. Raman, x-ray diffraction, and cathodoluminescence measurements
confirm the wurtzite structure of the GaN epilayer and the c-axis crystal growth orientation. The average stress in the GaN
layer is estimated at 320 MPa. This is a factor of two less than the stress reported for HVPE growth on 6H-SiC (0001).
Journal of Electronic Materials 04/2012; 30(7):825-828. · 1.47 Impact Factor
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ABSTRACT: We demonstrate a universal reconfigurable add-drop optical filter based on arrays of digital micromirrors. Our approach allows for reconfiguration of the device parameters for operation in different sets of channel wavelengths, even or uneven channel-to-channel separation, adjustable channel passband, and dynamic add-drop of optical signals. The proposed device shows insertion loss at the center wavelength in the all-pass configuration of 7.6 dB, extinction ratio of 35 dB, channel isolation better than 24dB, negligible loss ripple, and possibility of reconfiguration time in the order of microseconds. Although designed for demonstration on optical telecommunication frequencies, the high-flexibility features of the proposed add-drop filter allow it to be used for other optical wavelength-based filtering/switching applications.
Journal of Lightwave Technology 02/2008; 26(2):237-242. · 2.78 Impact Factor
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ABSTRACT: We present a new method for measuring thermal conductivities of films with nanoscale thickness. The method combines a micro electrothermal test structure with a finite-element- based data analysis procedure. The test device consists of two serpentine nickel structures, which serve as resistive heaters and resistance temperature detectors, on top of the sample. The sample is supported by a silicon nitride membrane. Analytical solution of the heat flow is infeasible, making interpretation of the data difficult. To address this, we use a finite-element model of the test structure and apply nonlinear least-squares estimation to extract the desired material parameter values. The approach permits simultaneous extraction of multiple parameters. We demonstrate our technique by simultaneously obtaining the thermal conductivity of a 280 mum x 80 mum x 140 nm thick aluminum sample and the 360 mum x 160 mum x 180 nm thick silicon nitride support membrane. The thermal conductivity measured for the silicon nitride thin film is 2.1 W/mK, which is in agreement with reported values for films of this thickness. The thermal conductivity of the Al thin film is found to be 94 W/mK, which is significantly lower than reported bulk values and consistent both with reported trends for thin metallic films and with values that were obtained using electrical resistivity measurements and the Wiedemann-Franz law.
Journal of Microelectromechanical Systems 11/2007; · 2.10 Impact Factor
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ABSTRACT: We present a general quantitative description of the temporal and spectral responses of arrayed waveguide gratings pulse shapers to a single ultrashort optical pulse. Operation of the pulse shaper was described in terms of superposition of pulse replicas that are overlapping in the slab waveguide region of the device. We demonstrate that the analytical formulation requires only a combination of fast Fourier transform algorithm with waveguide-mode profile simulation to accurately describe the spectral and temporal responses of the pulse shaper. The proposed model was applied to pulse shapers based on digital amplitude and phase modulations. A supporting evidence of the effectiveness of the model described in this paper was obtained from the good agreement between simulations and experiments.
Journal of Lightwave Technology 10/2007; 25(9):2410-2416. · 2.78 Impact Factor
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ABSTRACT: The ultrafast limit of the response of arrayed waveguide gratings is studied both theoretically and experimentally. We present new experimental results that show that interference occurs inside of an arrayed waveguide pulse shaper even when pulses that travel through different paths do not overlap. A comprehensive discussion of this extreme case is presented
IEEE Journal of Quantum Electronics 07/2007; · 1.88 Impact Factor
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ABSTRACT: We report a new approach to the generation of arbitrary sequences of ultrafast pulses using an integrated optics space-to-time optical processor. The processor combines a reflective arrayed waveguide grating multiplexer with a phase-only mask containing sets of pixels of constant phase separated by sets of pixels with phases alternating between 0 and pi rad
IEEE Photonics Technology Letters 03/2007; · 2.19 Impact Factor
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ABSTRACT: We demonstrate a novel approach to generation of sequences of ultrafast pulses in which external stress is applied to arrayed waveguide grating multiplexers using a corrugated pattern. Stress applied to the grating region modifies the free spectral range resulting in modulation of the response spectrum with the corresponding sequence of ultrafast pulses at each output channel of the device. The repetition rate, number, and width of the pulses within the sequence are determined by the corrugation length and the period of the pattern
IEEE Photonics Technology Letters 06/2006; · 2.19 Impact Factor
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ABSTRACT: We demonstrate for what we believe to be the first time the generation of sequences of ultrafast optical pulses by phase modulation in a direct space-to-time pulse shaper. The pulse shaper is based on the combination of a reflective arrayed waveguide grating multiplexer and an external reflector. The spatial modulation of the phase was obtained by fabricating corrugated patterns on the external reflector. We demonstrate that pulse sequences with different repetition rates can be obtained by changing the period in the patterned mask.
Optics Letters 04/2006; 31(5):640-2. · 3.40 Impact Factor
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ABSTRACT: Low contact resistance to digital alloys of n-type AlGaN/AlN with high average Al concentration is described. Low-energy electron diffraction was used to evaluate surface precleaning with HCl and buffered HF. The contact metallization consisting of a stack of Ti/Al/Ti/Au, 20/100/45/60 nm in thickness, was e-beam deposited and etch-patterned. The lowest specific contact resistance of 5.6×10<sup>-5</sup> Ω·cm<sup>2</sup> was obtained after annealing in N<sub>2</sub> ambient at 700°C.
IEEE Electron Device Letters 02/2006; · 2.85 Impact Factor
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ABSTRACT: We describe an optical Fourier transform pulse shaper based on a combination of a diffraction grating and a reconfigurable array of digital micromirrors. The pulse shaper allows for adjustment of separation between consecutive pulses within a pulse burst and the length of the burst. Pulse separation, burst length, and number of pulses can be dynamically reconfigured by changing the periodic pattern imposed to the array of digital micromirrors. Experimental results on binary intensity modulation are in excellent agreement with basic design relations describing the spectral and temporal characteristics of the pulse shaper.
IEEE Photonics Technology Letters 11/2005; · 2.19 Impact Factor
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ABSTRACT: We report a significant improvement in the room temperature cathodoluminescence efficiency of AlGaN quantum wells when the three-dimensional growth mode is induced by reduced flux of ammonia. We interpret this observation in terms of formation of quantum dots of AlGaN in Al0.45Ga0.55N wells. Reflection high electron diffraction images and detailed measurements of the cathodoluminescence intensity, linewidth, and wavelength as a function of growth conditions are consistent with the presence of quantum dots.
Applied Physics Letters 10/2005; 87(19):191902-191902-3. · 3.84 Impact Factor
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ABSTRACT: We describe a dynamic direct space-to-time pulse shaper based on the combination of a reflective arrayed waveguide grating multiplexer and an array of digital micromirrors used as an external reflector. Spatial modulation was obtained by imposing a reconfigurable periodic structure on the digital micromirror array. The period of this structure and the number of reflecting micromirrors within each period was varied to produce pulse sequences with variable repetition rates within the burst and different pulsewidths. Experimental results are in excellent agreement with simulated temporal output response.
IEEE Photonics Technology Letters 10/2005; · 2.19 Impact Factor
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ABSTRACT: We describe experiments on Si doping in AlxGa1−xN grown by gas source molecular beam epitaxy with ammonia and silane. Growth conditions that minimize self-compensation were used to assure Si incorporation at a level of 2×1020 cm−3 for the entire range of compositions investigated, from x = 0.56 to 1.0. These conditions resulted in donor concentrations of ∼ 1×1019 cm−3 up to x = 0.85. Layers of AlxGa1−xN up to x = 0.85 show good mobility and low resistivity. In these layers, the activation energy, Ea, of Si stays below ∼ 25 meV and Si can be considered a shallow donor. For AlN content above x = 0.85 the donor activation energy increases to Ea ∼ 250 meV in AlN. The change in donor activation energy correlates with increased incorporation of oxygen and carbon.
Applied Physics Letters 09/2005; 87(13):132106-132106-3. · 3.84 Impact Factor
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ABSTRACT: We present a reconfigurable optical filter with multiple passband channels based on the combination of a diffraction grating and an array of digital micromirrors. The filter allows for adjustment of the central frequency of its individual passband channels to match standard wavelength grids with precision better than 0.033 nm. The number of channels, the channel passband width, and the channel peak profile can be dynamically reconfigured by changing the periodic pattern imposed to the digital micromirror array. Experimental results are in excellent agreement with basic design relations describing the spectral characteristics of the filter.
IEEE Photonics Technology Letters 09/2005; · 2.19 Impact Factor
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ABSTRACT: We report the nonselective plasma etching of epitaxial GaN:Mg, Al0.63Ga0.37N, and AlN/Al0.08Ga0.92N short-period superlattices with various doping properties. Etching is performed using mixed CF4/Ar feed gases in a combined inductively coupled plasma and reactive-ion etching chamber. A uniform etch rate of ∼ 23 nm/min is obtained for each of the compositions studied under identical conditions. This nonselective etching is also found to preserve the surface uniformity studied by atomic force microscopy and quantified using surface roughness and lateral correlation length. By adding Cl2 gas, etch rates are increased to 230–250 nm/min without degrading the surface properties.
Journal of Applied Physics 03/2005; 97(7):073302-073302-5. · 2.17 Impact Factor
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ABSTRACT: We report the formation of Ni nanodots on Si(111). Island density is varied by annealing temperature and time and is studied using atomic force microscopy (AFM) and magnetic force microscopy. Activation energies of 0.09±0.02 and 0.31±0.05 eV are observed for the formation of these islands. These are associated with Ni surface self-diffusion across the (111) and (110) Ni facets, respectively. For brief 500 °C anneals, regular nanodots are observed with self-limiting sizes of height ∼ 16 nm and area 180 nm×260 nm, while density exhibits a power-law time dependence with exponent 1.13±0.12. AFM analysis reveals a “truncated hut” shape consistent with (110) top and (111) sidewall surfaces.
Applied Physics Letters 03/2005; 86(10):103107-103107-3. · 3.84 Impact Factor
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ABSTRACT: Controlled and reproducible growth of GaN nanowires is demonstrated by pulsed low-pressure metalorganic chemical vapor deposition. Using self-assembled Ni nanodots as nucleation sites on (0001) sapphire substrates we obtain nanowires of wurtzite-phase GaN with hexagonal cross sections, diameters of about 100 nm, and well-controlled length. The nanowires are highly oriented and perpendicular to the growth surface. The wires have excellent structural and optical properties, as determined by x-ray diffraction, cathodoluminescence, and Raman scattering. The x-ray measurements show that the nanowires are under a complex strain state consistent with a superposition of hydrostatic and biaxial components.
Applied Physics Letters 01/2005; 86(3):033104-033104-3. · 3.84 Impact Factor
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ABSTRACT: A new approach to compensate for the channel-wavelength shift due to fabrication errors and thermal effects in arrayed waveguide-grating multiplexers is described. The method combines a silica-based reflective multiplexer with a composite mirror made of materials with different coefficients of thermal expansion. Differential thermal expansion of the mirror assembly rotates its reflecting surface at a constant rate with temperature, compensating for the temperature-induced changes in the effective index of refraction of the waveguide material. The use of external mirror also allows for wavelength trimming that centers the channel wavelength at the standard grid. The channel wavelength can be tuned by up to 2 nm without increased insertion loss or changes in channel-to-channel separation. The channel wavelength shifts linearly with the external mirror angle at a rate of ∼66 nm/deg in excellent agreement with simulation. A finite element analysis of the composite mirror shows negligible deformation of the reflecting surface over a wide range of temperatures, in good agreement with experimental results.
IEEE Journal of Quantum Electronics 01/2005; · 1.88 Impact Factor
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ABSTRACT: The phenomena that accompany the growth of aluminum nitride (AlN) by metal-organic molecular beam epitaxy with trimethylaluminum and ammonia as sources of aluminum and nitrogen, respectively, have been systematically investigated. Optimizing the growth temperature, flux ratios, and the ammonia injector temperature, we obtained an efficient growth with a rate of 500 nm∕h and a low consumption of ammonia. Layers of AlN with x-ray diffraction linewidth as low as 141 arcsec for the (0002) reflection, and 800 arcsec for the (11-24) reflection are demonstrated on Si(111) substrates. High temperatures of ammonia injector result in lower growth rates but facilitate transition to the two-dimensional growth. These phenomena are discussed in terms of surface hydrogen, manifested through surfactant effect, and passivation of nitrogen bonds.
Journal of Applied Physics 11/2004; 96(11):6272-6276. · 2.17 Impact Factor
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ABSTRACT: High-quality short-period superlattices of AlN∕Al0.08Ga0.92N have been grown by gas-source molecular-beam epitaxy with ammonia on Al face of AlN (0001) substrates. A significant reduction was achieved in the dislocation density, down to 3×108 cm−2. Complete removal of residual Al2O3 surface oxide is needed in order to obtain low dislocation density in homoepitaxy on AlN. We show that the presence of Al2O3 islands with the surface coverage as low as 0.2% results in increased dislocation density.
Applied Physics Letters 11/2004; 85(19):4355-4357. · 3.84 Impact Factor
