Ohmic contacts to Mg-doped p-GaN grown by MOCVD [1] are studied using a circular transmission line model (TLM) to avoid the need for isolation. For samples which use a p-dopant activation anneal before metallization, no appreciable difference in the specific contact resistance, r,, as a function of different capping options is observed. However, a lower r, is obtained when no pre-metallization anneal is employed, and the post-metallization anneal simultaneously activates the p-dopant and anneals the contact. This trend is shown for Pt/Au, Pt, Pd/Pt/Au, and Ni/Au contacts to p-GaN. The r 's for these metal contacts are in the range of 1.4-7.6 x 10.-ohm-cm 2 at room temperature at a bias of 1OmA. No particular metallization formula clearly yields a consistently superior contact. Instead, the temperature of the contact has the strongest influence.Detailed studies of the electrical properties of the Pt/Au contacts reveal that the I-V linearity improves significantly with increasing temperature. At room temperature, a slightly rectified I-V characteristic curve is obtained, while at 200'C and above, the I-V curve is linear. For all the p-GaN samples, it is also found that the sheet resistance decreases by an order of magnitude with increasing temperature from 25'C to 350'C. The specific contact resistance is also found to decrease by nearly an order of magnitude for a temperature increase of the same range. A minimum r, of 4.2 x 10-4 ohm-cm 2 was obtained at a temperature of 350'C for a Pt/Au contact. This result is the lowest reported r, for ohmic contacts to p-GaN.
We demonstrate autocorrelation measurements of 85-fs Ti:sapphire laser pulses, using a 32-pixel ZnSe detector array in a single-shot geometry. The two-photon photoconductor is fabricated by deposition of an array of interdigitated gold fingers on a single-crystal ZnSe substrate.
We present a ultrasensitive method for measuring the ultrafast optical response of a sample with respect to its amplitude and phase. Combining interferometric crosscorrelation, a wavemeter and a numerical lock-in technique, the method is distinguished by its dynamic range (7 orders of magnitude in the intensity) and phase resolution ( A @ /~T = 1/50). Time resolution depends on residual dispersion ofthe set-up but is limited to -40 fs by the width of the input pulses. We demonstrate, as an example, high precision measurements of nonlinear Is-exciton-polariton propagation through GaAs.Most experiments in ultrafast spectroscopy involve two pulses, an input and an output pulse. If the input pulse can be characterized in a separate set-up, linear correlation techniques can be used to determine the optical response function &(t,t'). The advantages of linear techniques are their potential high sensitivity regarding minimum average power and dynamic range. Our method consist of two parts, the optical set-up and a numerical evaluation algorithm. Three signals are measured in the same Mach-Zehnder interferometer simultaneously ( Fig. 1): i) a linear crosscorrelation signal using both exit interference patterns that is proportional to the optical response function of the sample, ii) an autocorrelation of the pulses to determine the power spectrum of the pulses used, and ii) the autocorrelation of a HeNe laser that represents a reference with which mechanical or thermal drifts of the interferometer can be removed. The amplitude and phase of all signals are determined by a numerical lock-in technique.To demonstrate the performance of the I &h".
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