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247 nm solar-blind ultraviolet p-i-n photodetector
(American Institute of Physics, 2006-11-06) Holtz, Mark; Song, D.Y.; Sokolovskii, G.; Kuchinskii, V.; Asomoza, R.; Kudryavtsev, Yu.; Nikishin, S.; Borisov, B.A.; Kuryatkov, V.
We describe solar-blind p-i-n photodetectors based on short period superlattices of AlN/Al0.08Ga0.92N. The devices are grown using gas source molecular beam epitaxy with ammonia on transparent sapphire substrates. The cutoff wavelength for the device is 247 nm. For diodes with 150 m diameter mesas and sidewall surfaces passivated in oxygen plasma, we obtain extremely low dark leakage current of 3 pA/cm2 and high zero-bias resistance of 6 1014 . At 10 V reverse bias the observed responsivity is 62 mA/W.
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
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
Analysis of nonselective plasma etching of AlGaN by CF4/Ar/Cl2
(American Institute of Physics, 2005-03-21) Holtz, Mark; Menon, L.; Patibandla, S.; Temkin, H.; Nikishin, A.; Saxema, J.; Borisov, B.; Kuryatkov, V.
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.
Control of Center Wavelength in Reflective-ArrayedWaveguide-Grating Multiplexers
(Institute of Electronics and Electrical Engineers, 2004-12) Temkin, H.; Berg, J.; Gorbounov, V.; Bernussi, Ayrton; Grave de Peralta, Luis
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.
Controlled growth of GaN nanowires by pulsed metalorganic chemical
(American Institute of Physics, 2005-01-11) Temkin, H.; Holtz, Mark; Aurongzeb, D.; Ahmad, I.; Kuryatkov, V.; Yon, J.; Chandolu, A.; Yavich, B.; Kipshidze, G.
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 s0001d 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
Decay of zone-center phonons in AlN with A1, E1, and E2 symmetries
(American Institute of Physics, 2007-12-06) Holtz, Mark; Song, D.Y.; Pandit, P.
Raman studies are reported for the A1 TO , E1 TO , E2 2, and A1 LO symmetry phonons of AlN from 20 to 375 K. By applying anharmonic decay theory to the observed temperature dependences of the phonon energies and linewidths, we determine the phonon decay mechanisms of these zone-center vibrations. Thermal expansion is taken into account using published temperature-dependent coefficients. The A1 TO , E1 TO , and E2 2 vibrations are described by symmetric two-phonon decay. The A1 LO band is interpreted by an asymmetric two-phonon decay. Phonon lifetimes are obtained based on the observed linewidths and the dependence allows us to estimate the impurity-related phonon lifetime for each vibration. The latter ranges from 2.9 to 9.1 ps.
Decay of zone-center phonons in GaN with A1, E1, and E2 symmetries
(American Institute of Physics, 2007-03-15) Dmitriev, V.; Usikov, A.; Soukhoveev, V.; Holtz, M.; Nikishin, S.A.; Song, D.Y.
We report Raman studies of the A1 TO , E1 TO , E2 2, A1 LO , and E1 LO symmetry phonons of GaN from 20 to 325 K. By applying anharmonic decay theory to the observed temperature dependences of the phonon energies and linewidths, we determine the phonon decay mechanisms of these zone-center vibrations. Thermal expansion is taken into account using published temperature-dependent coefficients. The A1 TO and E1 TO vibrations are described by symmetric two-phonon decay. The E2 2 decays via the creation of three phonons. Both A1 LO and E1 LO bands are interpreted by an asymmetric two-phonon decay, with a minor contribution to the decay of the former from the three-phonon creation. Phonon lifetimes are obtained based on the observed linewidths
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.
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.
Deep ultraviolet photodetectors grown by gas source molecular beam
(SPIE, 2006-10-13) Asomoza, R.; Kudryavtsev, Yu.; Dmitriev, V.; Vsikov, A.; Nikishin, S.; Borisov, B.; Song, D.Y.; Kuryatkov, V.; Holtz, M.
Optically-based chemical and biological sensors require optoelectronic devices with specific emission and detection wavelength ranges. Semiconductor optoelectronic devices applicable to this sensing are of particular interest due to their low power consumption, compact size, long lifetime, and low cost. We report the electrical and optical properties of deep UV p-i-n photodiodes (PDs) based on short period superlattices (SPSLs) of AlN/AlGaN. All device and test structures are grown by gas source molecular beam epitaxy with ammonia on sapphire and AlGaN/sapphire substrates. AlGaN/sapphire substrates were grown by stress controlled hydride vapor phase epitaxy (HVPE). The cutoff wavelength of PDs based on these SPSLs can be varied from 250 to 280 nm by changing the SPSL barrier/well thickness ratio. For mesa diodes with 150 μm diameter we obtain extremely low dark leakage current of ~ 3 pA/cm2, and high zero-bias resistance of ~ 6 × 1014 Ω. A cutoff wavelength of 247 nm is obtained for these devices with four orders of magnitude rejection by 315 nm. We obtain a maximum responsivity of 60 mA/W.
Dependence of the stress–temperature coefficient on dislocation density
(American Institute of Physics, 2004-02-15) Temkin, H.; Faleev, N.; Holtz, Mark; Ahmad, I.
We report measurements of stress in GaN epitaxial layers grown on 6H–SiC and a-Al2O3 substrates. Biaxial stresses span 11.0 GPa ~tensile! to 21.2 GPa ~compressive!. Stress determined from curvature measurements, obtained using phase-shift interferometry ~PSI! microscopy, compare well with measurements using accepted techniques of x-ray diffraction ~XRD! and Raman spectroscopy. Correlation between XRD and Raman measurements of the E2 2 phonon gives a Raman-stress factor of 23.460.3 cm21/GPa. We apply PSI microscopy for temperature dependent stress measurements of the GaN films. Variations found in the stress–temperature coefficient correlate well with threading dislocation densities. We develop a phenomenological model which describes the thermal stress of the epitaxial GaN as a superposition of that for ideal GaN and the free volume existing in the layers due to the threading dislocations. The model describes well the observed dependence.
Depth dependence of defect density and stress in GaN grown on SiC
(American Institute of Physics, 2005-12-21) Melnik, Yu.; Holtz, Mark; Ahmad, I.; Temkin, H.; Faleev, N.
We report high resolution x-ray diffraction studies of the relaxation of elastic strain in GaN grown on SiC 0001 . The GaN layers were grown with thickness ranging from 0.29 to 30 m. High level of residual elastic strain was found in thin 0.29 to 0.73 m thick GaN layers. This correlates with low density of threading screw dislocations of 1-2 107 cm−2, observed in a surface layer formed over a defective nucleation layer. Stress was found to be very close to what is expected from thermal expansion mismatch between the GaN and SiC. A model based on generation and diffusion of point defects accounts for these observations.
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.
DIELECTRIC FUNCTION OF AlN GROWN ON Si (111) BY MBE
(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.
Diffusion process and formation of super-spin-glass state in soft magnetic Fe/Pt system.
(American Institute of Physics, 2006-08-28) Moon, Latika; Holtz, M.; Aurongzeb, D.
We report results on surface and micromagnetic structures of Fe thin films consisting of a Pt underlayer. We use atomic force microscopy to study the surface structure evolution of the Fe films as a function of annealing time at an annealing temperature of 800 °C. Power spectral density analysis shows saturation in roughness exponent after 15 min of annealing. However, lateral correlation length and roughness continue to increase for up to 45 min. At high annealing temperature, the authors find two separate phase correlation lengths and a single surface correlation length indicating super-spin-glass state in the system.
Digital Alloys of AlN/AlGaN for Deep UV Light Emitting Diodes
(The Japan Society of Applied Physics, 2005-10-11) Temkin, H.; Holtz, Mark; Nikishin, S.
We report a systematic study of the optical and electrical properties of deep ultraviolet light emitting diodes based on digital alloy structures of AlN/Al0:08Ga0:92N grown by gas source molecular beam epitaxy with ammonia. Digital alloys are formed by short period superlattices consisting of Al0:08Ga0:92N wells, 0.50 or 0.75 nm thick, and AlN barriers, 0.75 to 1.5 nm thick. For digital alloys with effective bandgap of 5.1 eV, average AlN composition 72%, we obtain room temperature electron concentrations up to 1 1019 cm 3 and resistivity of 0.005 cm and hole concentrations of 1 1018 cm 3 with resistivity of 6 cm. Light emitting diodes based on digital alloys are demonstrated operating in the range of 250 to 290 nm
Direct space-to-time pulse shaper with reflective
(Optical Society of America, 2006-03-01) Temkin, H.; Bernussi, A.A.; Kuryatkov, V.; Grave de Peralta, L.; Krishnan, A.
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.
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.
Electric Field Distribution in a Grating of a Folded
(Institute of Electronics and Electrical Engineers, 2004-02) Temkin, H.; Grave de Peralta, L.; Bernussi, A.
We present direct measurements of the electric field distribution at the reflecting surface folding a grating of an arrayed-waveguide multiplexer. The field distribution obtained using different input channels of a device can be described by a single-Gaussian function. Grating and coupler-slab waveguide mode-field radii were determined directly. The field profile was used to model the transmission spectrum of the device. Excellent agreement was found between the measured and calculated spectra.

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