Colloidal CdS nanocrystals show strongly enhanced two-photon absorption (TPA) cross sections δ(2) of 104 to 105 GM over a wide NIR spectral range, making them ideal markers for confocal two-photon microscopy. We present a systematic study of the size and shape dependence of the linear and TPA cross sections for colloidal CdS dots and rods of diameters d between 2 and 5 nm. On the basis of z-scan measurements, we observe that the TPA cross section of colloidal CdS dots at 800 nm in and near the absorption continuum is manly shape and volume dependent, whereas it is less influenced by the electronic confinement. In the case of resonance to the lowest excitonic transition, a significant confinement-induced TPA enhancement with respect to the off-resonant case is observed. Colloidal CdS rods can exhibit a factor on the order of 10 larger δ(2) compared to CdS dots of the same diameter. In very small CdS rods a non-negligible three-photon absorption is found and assigned to a change in valence band symmetry.

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We investigate the spectral dependence of the linear and two-photon absorption of wurtzite CdS nanoparticles (dots and rods) by means of quantitative one-and two-photon photoluminescence excitation spectroscopy and effective mass theory modeling. Absolute two-photon absorption cross sections free from spectrally varying beam related uncertainties are obtained by means of a new reference dyebased method. The two-photon spectrum features of rods strongly differ from those of dots, due to the distinct energy structure of quasi-one-dimensional systems. The transversal confinement is found to dominate the energy of the absorption maxima while the longitudinal one dominates their absorption intensity. This suggests two-photon transition energy and intensity can be controlled independently in nanorods. For both geometries we observe a sizable spectral shift between the first one-and two-photon absorption maxima, which we conclude is inherent to the small rates of near-bandgap two-photon transitions rather than to the particular geometry of the absorber. The provided understanding of the spectral dependence of the two-photon absorption of CdS dots and rods is of strong interest for the design of nanocrystals with optimized two-photon absorption properties for bioimaging and phototherapy applications.

Carbon‐doping in the concentration range from [C] = 5 × 1017 to 1.2 × 1019 cm−3 is employed to achieve semi‐insulating properties of GaN layers as required for electronic power devices. Using propane as a carbon precursor, an independent analysis of the carbon incorporation during growth and its impact on electrical properties of the layers was obtained as growth parameters for optimum GaN quality could be applied. We observe that C is within precision of measurements fully incorporated in GaN as compensating deep acceptor. In a series of Si + C co‐doped samples, semi‐insulating properties were obtained for [C] > [Si] and the compensation efficiency for electrons is around unity. Through the extrinsic C‐doping technique previous ambiguous results on electrical and optical properties of GaN:C layers are clarified.

This paper studies the key parameters affecting on-resistance and current crowding in quasi-vertical GaN power devices by experiment and simulation. The current distribution in the drift region, n À-GaN, was found to be mainly determined by the sheet resistance of the current spreading layer, n þ-GaN. The actual on-resistance of the drift region significantly depends on this current distribution rather than the intrinsic resistivity of the drift layer. As a result, the total specific on-resistance of quasi-vertical diodes shows a strong correlation with the device area and sheet resistance of the current spreading layer. By reducing the sheet resistance of the current spreading layer, the specific on-resistance of quasi-vertical GaN-on-Si power diodes has been reduced from $10 mXÁcm 2 to below 1 mXÁcm 2. Design space of the specific on-resistance at different breakdown voltage levels has also been revealed in optimized quasi-vertical GaN power diodes.