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  • 1.
    Bertilsson, Kent
    et al.
    KTH, Superseded Departments, Electrum Laboratory.
    Harris, C
    Nilsson, H E
    Calculation of lattice heating in SiC RF power devices2004In: Solid-State Electronics, ISSN 0038-1101, E-ISSN 1879-2405, Vol. 48, no 12, p. 2103-2107Article in journal (Refereed)
    Abstract [en]

    Silicon carbide MESFET devices are suitable for high-speed and high-power applications. In this paper we are studying thermal effects in 4H-SiC RF power devices. The simulations are based on a combination of 2D device simulations for the electrical transport, and 3D thermal simulations for the lattice heating. We show that the method gives good accuracy, efficiency, flexibility and capacity dealing with tasks, where a 2D coupled electrical-thermal simulation is not sufficient. We also present an improvement of Roschke and Schwierz mobility model, based on Monte Carlo simulations for the temperature dependencies of the mobility parameters beta and v(sat).

  • 2. Lindberg, H
    et al.
    Strassner, Martin
    KTH, Superseded Departments, Electrum Laboratory.
    Bengtsson, J
    Larsson, A
    InP-based optically pumped VECSEL operating CW at 1550 nm2004In: IEEE Photonics Technology Letters, ISSN 1041-1135, E-ISSN 1941-0174, Vol. 16, no 2, p. 362-364Article in journal (Refereed)
    Abstract [en]

    We present a vertical external-cavity surface-emitting laser operating at 1550 run. The laser comprises an InGaAsP-based gain element, with a resonant periodic gain structure on top of a distributed Bragg reflector, and a high reflectivity spherical mirror as the external reflector. Optical pumping is achieved using a 1250-nm fiber Raman laser. A maximum continuous output power of 70 mW was obtained under multitransverse mode operation at 233 K. Under single-mode operation, we obtained a maximum power of 60 mW with a beam quality factor M-2 less than 1.1.

  • 3. Lindberg, H.
    et al.
    Strassner, Martin
    KTH, Superseded Departments, Electrum Laboratory. Ctr. Natl. de la Rech. Scientifique, Lab. de Photon. et de Nanstructures, France .
    Bengtsson, J.
    Larsson, A.
    Optically pumped VECSEL operating at 1550 nm2004In: Proceedings of SPIE - The International Society for Optical Engineering, 2004, p. 25-33Conference paper (Refereed)
    Abstract [en]

    We present the design and characteristics of an optically pumped vertical external cavity surface emitting laser emitting near 1550 nm. The InP-based laser was grown by Metalorganic vapor-phase epitaxy including an InGaAsP gain element and an InP/InGaAsP mirror. The gain element comprises 20 strain compensated quantum wells on top of a distributed Bragg reflector. As an external mirror we used a concave spherical mirror, which also provides the outcoupling of light. Gain is achieved by optical pumping with a high power, 1250 nm fiber Raman laser focused on the gain chip. Essential for achieving high output power is to reduce the temperature of the gain material and this is accomplished by bonding an intra-cavity silicon heat spreader to the surface of the gain element. The maximum output power is 260 mW at multi transverse mode operation and 230 mW at single transverse mode operation with a near Gaussian beam profile (M2=1.22) at 240 K. At room temperature the output power was limited to 12 mW. The maximum output power greatly depends on the operating temperature and studies of pump induced thermal effects show that thermal lensing imposes limitations on the attainable power.

  • 4.
    Westerberg, Staffan
    et al.
    KTH, Superseded Departments, Electrum Laboratory.
    Wang, C
    Chou, K
    Somorjai, G A
    High-pressure ammonia adsorption and dissociation on clean Fe(111) and oxygen-precovered Fe(111) studied by sum frequency generation vibrational spectroscopy2004In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 108, no 20, p. 6374-6380Article in journal (Refereed)
    Abstract [en]

    The adsorption of gases N-2, H-2, O-2, and NH3 that play a role in ammonia synthesis have been studied on the Fe(111) crystal surface by Sum Frequency Generation (SFG) vibrational spectroscopy using an integrated ultrahigh vacuum/higla-pressure system. SFG spectra are presented for the dissociation intermediates, NH2 (similar to3325 cm(-1)) and NH (similar to3235 cm(-1)) under high pressure of ammonia (200 Torr) on the clean Fe(111) surface. Addition of 0.5 Torr of oxygen to 200 Torr of ammonia does not significantly change the bonding of dissociation intermediates to the surface. However, it leads to a phase change of nearly 180degrees between the resonant and nonresonant second-order nonlinear susceptibility of the surface, demonstrated as a reversal of the SFG spectral features. Heating the surface in the presence of 200 Torr of ammonia and 0.5 Torr of oxygen reduces the oxygen coverage, which can be seen from the SFG spectra as another relative phase change of 180degrees. The reduction of the oxide is also supported by Auger electron spectroscopy. The result suggests that the phase change of the spectral features could serve as a sensitive indicator of the chemical environment of the adsorbates. Clean Fe(111) is found to have a large SFG nonresonant signal. The magnitude of the nonresonant signal was dependent on the adsorption species; O-2 and N-2 decrease, while H-2 and NH3 increase the SFG nonresonant signal. The change in nonresonant signal is correlated to the change in work function for Fe(111) upon adsorption. Adsorption-induced changes in the SFG nonresonant signal was used as an indicator of surface conditions and to monitor surface reactions.

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