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Now showing 1 - 20 of 70
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    Deep Ultraviolet Light Emitting Diodes Based on Short Period Superlattices of AlN/AlGa(In)N
    (The Japan Society of Applied Physics, 2003-11-15) Temkin, H.; Holtz, M.; Ahmad, I.; Kipshidze, G.; Borisov, B.; Chandolu, A.; Kuryatkov, V.; Nikishin, S.
    We report a systematic study of the optical properties of superlattices of AlN/Al0:08Ga0:92(In)N with periods in the range of 1.25–2.25 nm. The superlattices were grown on sapphire substrates using gas source molecular beam epitaxy with ammonia. Effective bandgaps between 4.5 eV (276 nm) and 5.3 eV (234 nm), as determined by optical reflectivity measurements, were obtained by adjusting the barrier and well thickness. These superlattices can be doped n- and p-type. We demonstrate double heterostructure light emitting diodes operating at wavelengths as short as 262 2 nm.
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    Structural properties of fluorinated amorphous carbon films
    (American Institute of Physics, 2001-01-01) West, M.; Startham, M.; Gangopadhyay, S.; Temkin, H.; Bouldin, K.; Harris, H.; Wang, X.
    Fluorinated amorphous carbon films have been deposited in a plasma-enhanced chemical vapor deposition system, and the optical properties examined by Fourier transform infrared and ultraviolet-visible absorption spectroscopy. The infrared absorption spectra in the region from 1000 to 1800 cm21 were resolved into ten peaks, which were assigned to various carbon–fluorine and carbon–carbon vibration modes. A relationship between the optical band gap and the aromatic carbon (sp2) concentration is demonstrated.
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    Electrical properties of fluorinated amorphous carbon films
    (American Institute of Physics, 2001-04-15) Gangopadhyay, S.; Temkin, H.; Celebi, G.; Wang, X.; Harris, H.; Biswas, N.
    We have studied the capacitance–voltage (C–V), conductance–voltage (G–V), and current– voltage characteristics of fluorinated amorphous carbon (a-C:Fx) films using metal/a-C:Fx /Si and metal/a-C:Fx /metal structures, respectively. Samples annealed in a vacuum were also studied. The C–V curves of the as-deposited sample are stretched about the voltage axis. Interface state density of 4.131011 cm22 eV21 at the midgap was calculated. Annealing the sample deposited on Si in a vacuum caused more frequency dispersion in the C–V and G–V curves, probably due to the diffusion of carbon into silicon. The bulk density of states for samples deposited on metal, measured by space-charge-limited current technique, decreased from 431018 eV21 cm23 for the as-deposited sample, to 731017 eV21 cm23 for the annealed sample.
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    Determination of oscillator strength of C–F vibrations in fluorinated
    (American Institute of Physics, 2001-05-01) West, M.; Strathman, M.; Gangopadhyey, S.; Temkin, H.; Harris, H.; Wang, X.
    Fluorinated amorphous-carbon (a-CFx) films deposited by plasma-enhanced chemical-vapor deposition were investigated by Fourier transform infrared transmission spectroscopy and Rutherford backscattering. The proportionality constant between the fluorine concentration and the integrated absorption of C–F vibration modes is 3.5260.331019 cm22, and is constant within experimental uncertainty over a wide range of processing conditions. It is shown that the fluorine content can be accurately determined from the infrared absorption spectrum of a-CFx films.
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    Plasma enhanced metalorganic chemical vapor deposition of amorphous
    (American Institute of Physics, 2001-12-01) Strathman, M.; Gangopadhyay, S.; Temkin, H.; Biswas, N.; Harris, H.
    Plasma enhanced deposition of amorphous aluminum nitride ~AlN! using trimethylaluminum, hydrogen, and nitrogen was performed in a capacitively coupled plasma system. Temperature was varied from 350 to 550 °C, and pressure dependence of the film structure was investigated. Films were characterized by Fourier transform infrared, Rutherford backscattering ~RBS!, ellipsometry, and x-ray diffraction ~XRD!. The films are amorphous in nature, as indicated by XRD. Variations in the refractive index were observed in ellipsometric measurements, which is explained by the incorporation of carbon in the films, and confirmed by RBS. Capacitance–voltage, conductance– voltage (G–V), and current–voltage measurements were performed to reveal bulk and interface electrical properties. The electrical properties showed marked dependence on processing conditions of the AlN films. Clear peaks as observed in the G–V characteristics indicated that the losses are predominantly due to interface states. The interface state density ranged between 1010 and 1011 eV21 cm22. Annealing in hydrogen resulted in lowering of interface state density values.
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    (Materials Research Society, 2000) Temkin, H.; Nikishin, S.; Wilson, S.; Gregory, R.; Konkar, A.; Zollner, Stefan
    We measured the ellipsometric response from 0.7-5.4 eV of c-axis oriented AlN on Si (111) grown by molecular beam epitaxy. We determine the film thicknesses and find that for our AlN the refractive index is about 5-10% lower than in bulk AlN single crystals. Most likely, this discrepancy is due to a low film density (compared to bulk AlN), based on measurements using Rutherford backscattering. The films were also characterized using atomic force microscopy and x-ray diffraction to study the growth morphology. We find that AlN can be grown on Si (111) without buffer layers resulting in truely two-dimensional growth, low surface roughness, and relatively narrow x-ray peak widths.
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    Visible and ultraviolet Raman scattering studies of Si1ÀxGex alloys
    (American Institute of Phsyics, 2000-09) Liu, R.; Zollner, S.; Duncan, W.; Holtz, M.
    We report Raman studies of the Si–Si phonon band in Si12xGex alloys, where the excitation is by visible and ultraviolet ~351 nm! light. At a wavelength 351 nm, the optical penetration depth is extremely shallow ~'5 nm!. By varying the excitation from 351 to 514 nm, the optical penetration depth spans from 5 to 300 nm. Two sets of samples were examined. Thin layers grown using molecular beam epitaxy were coherently strained to match the lattice constant of the silicon substrate. Thick layers grown using organo–metallic chemical vapor deposition were strain relaxed. For the thin, strained layers, visible excitation produces a spectrum, which is a superposition of the substrate and the epilayer phonon bands. Reducing the wavelength ~and, consequently, penetration depth! allows us to isolate the epilayer spectrum. Phonon energies obtained using all excitation wavelengths agree. We conclude that Raman scattering from these alloys using 351 nm laser light gives us bulk alloy properties pertinent to the near-surface composition and strain. The epilayers show no evidence of compositional variance or strain relaxation near the surface.
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    Selective growth of high quality GaN on Si 111 substrates
    (American Institute of Physics, 2000-04-03) Temkin, H.; Faleev, N.; Nikishin, S.; Holtz, M.; Prokofyeva, T.; Seon, M.
    We demonstrate selective growth of high-quality GaN by gas-source molecular beam epitaxy on Si~111! wafers patterned with SiO2. GaN was grown on wafers having two different buffer layers. The first buffer layer contains two AlGaN/GaN superlattices, separated by GaN spacer, grown on AlN, with a total thickness of 400 nm. The second is a thin AlN ~1.5 nm! buffer layer. X-ray diffraction confirms ~0001! growth orientation, smooth interfaces, and coherence lengths comparable to the layer thickness in both samples. In the case of the thin AlN buffer layer, the tensile stress measured by the E2 Raman line shift is attributed to the mismatch in the thermal expansion coefficients of GaN and Si. However, when the AlGaN/GaN superlattice buffer layer is grown first, a reduced stress is measured. High carrier concentrations ~'1018 cm23! are seen in the GaN grown on the thin AlN buffer layer, which we attribute to the incorporation of silicon from the substrate during the growth process. The superlattice buffer layer is seen to inhibit this diffusion, resulting in a carrier concentration of ,1017 cm23 in the GaN
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    HfO2 gate dielectric with 0.5 nm equivalent oxide thickness
    (American Institute of Physics, 2002-08-05) Temkin, H.; Gangopadhyay, S.; Nikishin, S.; Kipshidze, G.; Biswas, N.; Chandolu, A.; Mehta, N.; Choi, K.; Harris, H.
    Hafnium dioxide films have been deposited using reactive electron beam evaporation in oxygen on hydrogenated Si(100) surfaces. The capacitance–voltage curves of as-deposited metal(Ti)–insulator–semiconductor structures exhibited large hysteresis and frequency dispersion. With post-deposition annealing in hydrogen at 300 °C, the frequency dispersion decreased to less than 1%/decade, while the hysteresis was reduced to 20 mV at flatband. An equivalent oxide thickness of 0.5 nm was achieved for HfO2 thickness of 3.0 nm. We attribute this result to a combination of pristine hydrogen saturated silicon surfaces, room temperature dielectric deposition, and low temperature hydrogen annealing
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    Vibrational properties of AlN grown on 111 -oriented siliconPHYSICAL REVIEW B, VOLUME 63, 125313
    (American Physical Society, 2001) Zollner, S.; Temkin, H.; Faleev, N.; Nikishin, S.; Holtz, Mark; Vanbuskirk, J.; Seon, M.; Prokofyeva, T.
    We study the vibrational spectrum of AlN grown on Si~111!. The AlN was deposited using gas-source molecular beam epitaxy. Raman backscattering along the growth c axis and from a cleaved surface perpendicular to the wurtzite c direction allows us to determine the E2 1 , E2 2 , A1~TO!, A1~LO!, and E1~TO! phonon energies. For a 0.8-mm-thick AlN layer under a biaxial tensile stress of 0.6 GPa, these are 249.0, 653.6, 607.3, 884.5, and 666.5 cm21, respectively. By combining the Raman and x-ray diffraction studies, the Raman stress factor of AlN is found to be 26.361.4 cm21/GPa for the E2 2 phonon. This factor depends on published values of the elastic constants of AlN, as discussed in the text. The zero-stress E2 2 energy is determined to be 657.4 60.2 cm21. Fourier-transform infrared reflectance and absorption techniques allow us to measure the E1~TO! and A1~LO! phonon energies. The film thickness ~from 0.06 to 1.0 mm! results in great differences in the reflectance spectra, which are well described by a model using damped Lorentzian oscillators taking into account the crystal anisotropy and the film thickness
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    AlGaInN-based ultraviolet light-emitting diodes grown on Si 111
    (American Institute of Physics, 2002-05-20) Temkin, H.; Nikishin, S.; Holtz, M.; Borisov, B.; Kuryatkov, V.; Kipshidze, G.
    Ultraviolet light-emitting diodes grown on Si(111) by gas-source molecular-beam epitaxy with ammonia are described. The layers are composed of superlattices of AlGaN/GaN and AlN/AlGaInN. The layers are doped n and p type with Si and Mg, respectively. Hole concentration of 4×1017 cm−3, with a mobility of 8 cm2/Vs, is measured in Al0.4Ga0.6N/GaN. We demonstrate effective n- and p-type doping of structures based on AlN/AlGaInN. Light-emitting diodes based on these structures show light emission between 290 and 334
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    Plasma etching of AlNÕAlGaInN superlattices for device fabrication
    (American Institute of Physics, 2002) Holtz, Mark; Temkin, H.; Nikishin, S.; Kipshidze, G.; Borisov, B.; Kuryatkov, V.; Zhu, K.
    We report a study of plasma etching of GaN, AlN, and AlN/AlGaN superlattices for the processing of deep ultraviolet light emitting diodes. Etching was carried out using inductively coupled plasma of chlorine diluted with argon under reactive ion etching conditions. Using parameters selected for etch rate, anisotropy, and surface smoothness, we study etching of n- and p-type superlattices. The former etches at a rate of 250 nm/min, which is intermediate to that of AlN and GaN, while the latter exhibits a slower etch rate of 60 nm/min. Based on these studies, we prepare low-leakage p–n junctions and mesa light emitting diodes with peak emission at 280 nm
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    Deep Ultraviolet AlGaInN-Based Light-Emitting Diodes
    (Wiley, 2002) Temkin, H.; Chu, S.; Holtz, M.; Nikishin, S.; Borisov, B.; Kuryatkov, V.; Kipshidze, G.
    Ultraviolet light-emitting diodes (LEDs) with emission wavelength as short as 280 nm, grown by gas source molecular beam epitaxy with ammonia, are described. The typical multi-quantum well (MQW) structure LED consists of an AlN buffer layer deposited on Si(111) or sapphire, followed by a (Al)GaN buffer layer and two superlattice structures, n- and p-type, with the MQW active region placed between them. Room temperature Hall measurements of n- and p-type AlN/AlGaInN superlattice structures show average hole concentrations of 1 × 1018 cm—3, with mobility of 3–4 cm2/Vs, and electron concentrations of 3 × 1019 cm—3, with mobility of 10–20 cm2/Vs. Room temperature electroluminescence spectra of mesa-etched devices show predominant emission at 280 nm.
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    Optical properties of GaN grown on Si 111 by gas source molecular beam
    (American Institute of Physics, 2002-02-01) Holtz, M.; Prokofyeva, T.; Temkin, H.; Kuryatkov, V.; Kipshidze, G.; Nikishin, S.; Zubrilov, A.
    We report a study of the optical properties of GaN grown on Si~111! by gas source molecular beam epitaxy with ammonia. Temperature dependence of edge luminescence was studied in the range of 77–495 K for samples with low background carrier concentrations, as determined by capacitance voltage profiling and Raman spectroscopy, and the results were fitted using Passler’s and Varshni’s models. We also demonstrate strong correlation between electron concentration in GaN and relative Raman intensity of A1 ~longitudinal optical! and E2 2 modes. The binding energy of free excitons is estimated to be 2962 meV. The contributions of different mechanisms to free exciton line broadening are discussed.
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    Effects of power truncation on the insertion loss and crosstalk
    (American Institute of Physics, 2003-09) Temkin, H.; Frisbie, S.; Grave de Peralta, Luis; Bernussi, A.
    A measurement of the insertion loss and crosstalk in folded, 100 GHz, arrayed waveguide multiplexers as a function of the number of grating waveguides is described. The number of grating waveguides can be varied in a single device to evaluate power truncation effects with high accuracy. We show that the central peak insertion loss decreases exponentially with the number of grating waveguides. The crosstalk decreases with increased number of waveguides and the measured dependence shows valleys and peaks associated with the passband spectrum of individual channels. These observations are in good agreement with simulation results. With the arrayed waveguide design used in this work the crosstalk performance becomes phase error limited for the number of waveguides greater than 250.
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    Reflective Arrayed Waveguide Grating Multiplexer
    (IEEE, 2003-10) Temkin, H.; Gale, R.; Frisbie, S.; Bernussi, A.; Grave de Peralta, Luis
    High-performance reflective arrayed waveguide grating multiplexer/demultiplexer designed for compatibility with silicon integrated circuit processing is described. The grating, folded by a flat reflecting surface, can be printed in a single-exposure field of a photolithographic stepper. Advanced plasma assisted deposition is used to prepare waveguides with very low loss and minimum birefringence. Multiplexers with 40 channels separated by 100 GHz show intrinsic insertion losses of –2.4 dB, channel uniformity less than 0.6 dB, and very low polarization dependent wavelength shift of 0.012 nm.
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    Silicon-Dioxide Waveguides With Low Birefringence
    (IEEE, 2003-07) Doucette, David; Borhani, Marcus; Temkin, H.; Bernussi, Ayerton; Grave de Peralta, L.
    We describe the use of highly boron-doped silicon dioxide for the preparation of optical waveguides with very low birefringence. Plasma-enhanced chemical vapor deposition was used to vary the boron content from 5 wt% to 10 wt%, at a constant phosphorus content of 4.8%. A transition from compressive to tensile stress was observed at a boron concentration of 9.1%. Pedestal-type waveguides formed with the high-boron top cladding layer show low loss of 0.02 dB/cm. Arrayed waveguide grating devices with a polarization-dependent wavelength shift of 0.01 nm and excellent stability have been demonstrated.
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    Influence of nanocrystal growth kinetics on interface roughness
    (American Institute of Physics, 2003-12-29) Temkin, H.; Yun, J.; Chandolu, A.; Berg, J.; Daugherty, M.; Holtz, M.; Aurongzeb, D.
    We study the layer morphology of Ni/Al multilayer structures, with 50 nm period, as deposited and following 10 min anneals up through the melting temperature of Al. X-ray reflectivity measurement of the as-deposited film shows interference fringes, characteristic of a well-defined multilayer stack, with ∼1 nm interface roughness. Over a narrow anneal range of 360–500 °C these fringes diminish in amplitude and disappear, indicating elevated interface roughening. However, fringes are observed for anneal temperatures both below and above this range, indicating the presence of well-defined layers with smooth interfaces. A model, in which nanocrystal domains of intermetallic nickel aluminides form at the interfaces, is developed to quantify the annealing induced interface roughness. This model agrees well with the experimental results
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    Microfabrication and Characterization of Teflon AF-Coated Liquid Core Waveguide Channels in Silicon
    (Institute of Elecrtonics and Electrical Engineers., 2003-12-06) Dasgupta, P.; Temkin, H.; Holtz, M.; Dallas, T.; Gangopadhyay, S.; Ahmad, I.; Manor, R.; Kuban, P.; Dhar, A.; Eom, I.; Datta, A.
    The fabrication and testing of Teflon AF-coated channels on silicon and bonding of the same to a similarly coated glass wafer are described.With water or aqueous solutions in such channels, the channels exhibit much better light conduction ability than similar uncoated channels. Although the loss is greater than extruded Teflon AF tubes, light throughput is far superior to channels described in the literature consisting of [110] planes in silicon with 45 sidewalls. Absorbance noise levels under actual flow conditions using an LED source, an inexpensive photodiode and a simple operational amplifier circuitry was 1 10 4 absorbance units over a 10-mm path length (channel 0.17-mm deep 0.49-mm wide), comparable to many commercially available macroscale flow-through absorbance detectors. Adherence to Beer’s law was tested over a 50-fold concentration range of an injected dye, with the linear 2 relating the concentration to the observed absorbance being 0.9993. Fluorescence detection was tested with fluorescein as the test solute, a high brightness blue LED as the excitation source and an inexpensive miniature PMT. The concentration detection limit was 3 10 9 M and the corresponding mass detection limit was estimated to be 5 10 16 mol.
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    AlNÕAlGaInN superlattice light-emitting diodes at 280 nm
    (American Institute of Physics, 2003-02-01) Temkin, H.; Nikishin, S.; Holtz, M.; Borisov, B.; Zhu, K.; Kuryatkov, V.; Kipshidze, G.
    Ultraviolet light-emitting diodes operating at 280 nm, grown by gas source molecular-beam epitaxy with ammonia, are described. The device is composed of n- and p-type superlattices of AlN~1.2 nm thick!/AlGaInN~0.5 nm thick! doped with Si and Mg, respectively. With these superlattices, and despite the high average Al content, we obtain hole concentrations of (0.7– 1.1)31018 cm23, with the mobility of 3–4 cm2/V s and electron concentrations of 331019 cm23, with the mobility of 10–20 cm2/V s, at room temperature. These carrier concentrations are sufficient to form effective p – n junctions needed in UV light sources