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  • 1
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: This article presents a study of the quasi-longitudinal optical and quasi-transverse optical modes in wurtzite AlN which originate from the interaction of phonons belonging to the A1 and E1 symmetry groups. In order to analyze the allowed quasi as well as pure Raman modes, the modes were observed in a rotating crystallographic coordinate system, and the Raman tensors of the wurtzite crystal structure were calculated as a function of the crystallographic rotation. The frequencies of the quasimodes of wurtzite AlN were also analyzed in terms of the interaction of the polar phonons with the long range electrostatic field model. The experimental values of the Raman frequencies of the quasiphonons concur with these expected from the model, implying that the long range electrostatic field dominates the short range forces for polar phonons in AlN. © 1999 American Institute of Physics.
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  • 2
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The incorporation of silicon into boron nitride films (BN:Si) has been achieved during ion beam assisted deposition growth. A gradual change from cubic boron nitride (c-BN) to hexagonal boron nitride (h-BN) was observed with increasing silicon concentration. Ultraviolet photoelectron spectroscopy, field emission, and field emission electron energy distribution experiments indicated that the observed electron transport and emission were due to hopping conduction between localized states in a band at the Fermi level for the undoped c-BN films and at the band tails of the valence band maximum for the BN:Si films. A negative electron affinity was observed for undoped c-BN films; this phenomenon disappeared upon silicon doping due to the transformation to h-BN. No shift of the Fermi level was observed in any BN:Si film; thus, n-type doping can be excluded. © 1998 American Institute of Physics.
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  • 3
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Selective growth of arrays of silicon-doped GaN (Si:GaN) pyramids for field emitter applications has been achieved. The electron emission characteristics of these arrays has been measured using techniques such as field emission, field emission energy distribution analysis (FEED), photoemission electron microscopy (PEEM), and field emission electron microscopy (FEEM). The field emission current–voltage (I–V) results indicate an average threshold field as low as 7 V/μm for an emission current of 10 nA. It is suggested that the low threshold field value is a consequence of both the low work function of Si:GaN and the field enhancement of the pyramids. The results of the FEEM and FEED measurements indicate agreement with the field emission I–V characteristics. The FEED results indicate that the Si:GaN pyramids are conducting, and that no significant ohmic losses are present between the top contact to the array and the field emitting pyramids. The PEEM and FEEM images show that the emission from the arrays is uniform over a 150 μm field of view. © 1998 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 84 (1998), S. 5248-5260 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Successful ex situ and in situ cleaning procedures for AlN and GaN surfaces have been investigated and achieved. Exposure to HF and HCl solutions produced the lowest coverages of oxygen on AlN and GaN surfaces, respectively. However, significant amounts of residual F and Cl were detected. These halogens tie up dangling bonds at the nitride surfaces hindering reoxidation. The desorption of F required temperatures 〉850 °C. Remote H plasma exposure was effective for removing halogens and hydrocarbons from the surfaces of both nitrides at 450 °C, but was not efficient for oxide removal. Annealing GaN in NH3 at 700–800 °C produced atomically clean as well as stoichiometric GaN surfaces. © 1998 American Institute of Physics.
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  • 5
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A method of measuring molecular diffusion rates in microscopic sample volumes is described. This method utilizes the standing wave interference created by colliding two counterpropagating laser beams at the focus of two opposing microscope objectives, creating a periodic light distribution in a volume on the order of 1 fl. By using a Pockels cell to vary the laser intensity with a time resolution of milliseconds, we show how this experimental geometry can be used to perform ultrahigh resolution fluorescence recovery after patterned photobleaching (FRAPP) experiments. A mathematical treatment of the experiment shows that the laser excitation profile has two characteristic length scales, the width of the focal spot and the period of the standing wave, which permits the simultaneous measurement of dynamics on two separate length scales. This feature may be used to determine whether the measured diffusion is anomalous. We present experimental results using a femtosecond Ti:sapphire laser to create a two-photon excitation profile with a fringe visibility on the order of 100. This standing wave is used to demonstrate FRAPP in both model dye/polymer systems and in more complex systems like living cells stained with a fluorescent dye. By combining the advantages of standing wave microscopy and two-photon fluorescence recovery after photobleaching, this technique permits the measurement of very short length motions in localized sample volumes, which should be useful in both biology and the study of diffusion in microscopically heterogeneous systems. © 2002 American Institute of Physics.
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  • 6
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The continuous high average-power microsecond pulser (CHAMP) ion accelerator is being constructed at Los Alamos National Laboratory. Progress on the testing of the CHAMP diode is discussed. A direct simulation Monte Carlo computer code is used to investigate the puffed gas fill of the CHAMP anode. High plenum pressures and low plenum volumes are found to be desirable for effective gas puffs. The typical gas fill time is 150–180 μs from initiation of valve operation to end of fill. Results of anode plasma production at three stages of development are discussed. Plasma properties are monitored with electric and magnetic field probes. From this data, the near coil plasma density under nominal conditions is found to be on the order of 1×1016 cm−3. Large error is associated with this calculation due to inconsistencies between tests and the limitations of the instrumentation used. The diode insulating magnetic field is observed to result in lower density plasma with a more diffuse structure than for the cases when the insulating field is not applied. The importance of these differences in plasma quality on the beam production is yet to be determined. © 2000 American Institute of Physics.
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  • 7
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A newly constructed universal crossed molecular beams apparatus for studies of bimolecular chemical reaction dynamics is described. The apparatus employs two rotatable molecular beam sources and a fixed ultrahigh vacuum (UHV) quadrupole mass spectrometer with electron impact or pulsed photoionization of reaction products. Electronically cold neutral supersonic transition metal atomic beams are produced in one of the rotatable sources using laser vaporization. The beams are characterized by laser induced fluorescence spectroscopy, photodepletion spectroscopy, and time-of-flight analysis. Photoionization of the ZrC2H2 products from the crossed beam reaction Zr+C2H4→ZrC2H2+H2 is carried out using the pulsed 157 nm radiation from a F2 excimer laser in the UHV region of the mass spectrometer. Compared to conventional electron impact ionization, 157 nm photoionization improves signal-to-noise ratios by more than two orders of magnitude for experiments using pulsed beam sources where cross correlation methods cannot be used. We also demonstrate the use of 1+1 resonance enhanced multiphoton ionization for state selective detection of nonreactively scattered transition metal atoms from decay of long-lived collision complexes. Due to the small reaction cross sections for Y+C2H6→YC2H4+H2 and Y+CD3CDO→DYCD3+CO, these reactions could not be studied using electron impact ionization. However, photoionization detection permitted direct studies of the reactions with excellent signal-to-noise ratios. The greatly improved sensitivity of the photoionization technique facilitates studies of transition metal systems not previously amenable to the crossed beams method. © 1999 American Institute of Physics.
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  • 8
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 70 (1999), S. 1459-1463 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We describe the design and development of a scanning tunneling microscope (STM) which can operate at temperatures down to 240 mK and in magnetic fields up to 7 T with high spatial and energy resolution. The compact and rigid STM head is mounted directly on a low vibration, single shot, 3He refrigerator. This refrigerator can be operated at its base temperature continuously for several days before the 3He needs to be recondensed. The system is equipped with a sample transport manipulator from room temperature, and a cleavage device at low temperature, so that the cryogenic ultrahigh vacuum condition inside the cryostat can be utilized. A superconducting magnet provides a magnetic field of up to 7 T at the sample along the STM tip direction. Test results have shown that, at the base temperature, this instrument has better than 0.5 pm z-direction resolution in imaging mode, and better than 20 μV energy resolution in spectroscopy mode. © 1999 American Institute of Physics.
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  • 9
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A new reflectron time-of-flight mass spectrometer for surface analysis has been developed that incorporates a Schwarzschild all-reflecting microscope. The instrument is configured for secondary ion mass spectrometry and secondary neutral mass spectrometry using either ion beam bombardment or laser ablation for sample atomization. The sample viewing and imaging system of this instrument enables in situ laser microanalysis with a lateral resolution below 1 μm. The major advantages of using a Schwarzschild objective include good lateral resolution, easy design, low cost, complete achromatism, and both viewing the sample and extracting secondary or photoions normal to its surface. The instrument has a mass resolution of m/Δm≥2000 and is capable of measuring elemental and isotopic compositions at trace levels using resonance ionization. The isotopic ratios of trace concentrations of Ti in μm size SiC grains separated from meteorites were measured. The extremely low ablation laser power used in the above experiment points to the possibility of using low-cost laser systems for laser microprobe applications. © 1995 American Institute of Physics.
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  • 10
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A nonresonant laser absorption technique is described for studying the density and temperature profiles in a high density z-pinch discharge. A long pulse (700-ns) dye laser backlights the pinch and a streak camera records the transmitted laser intensity. Spatially and temporally resolved absorption coefficients are inferred from the fractional transmission through the plasma. The measured values are related to plasma density and temperature profiles using theoretical calculated absorption coefficients and equations of state. The results demonstrate the fine spatial resolution that can be obtained with this technique, reveal a complex radial structure in z-pinch discharges not observable in emission light streak photographs, and establish the practicality of using the technique to evaluate and benchmark theoretical model predictions. © 1995 American Institute of Physics.
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