Low Resistance Non-Transparent ohmic Pt-contacts on p-GaN
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Low Resistance Non-Transparent ohmic Pt-contacts on p-GaN Andreas Weimar, Stefan Bader, Georg Brüderl, Volker Kümmler, Alfred Lell and Volker Härle OSRAM-Opto Semiconductors, 93049 Regensburg, Germany ABSTRACT The metal – p-GaN junction for low resistance ohmic contacts is still a challenge to be applied in GaN-based opto electronics as well as in power and high frequency devices. Currently, we try to improve the performance of our blue laser diodes. In order to decrease heat generation during device operation it is necessary to ensure as small contact resistances as possible. In this work, we achieved a specific contact resistance value of RC = 1.8 ± 1.7 ⋅ 10-5 Ωcm2 for Ptcontacts on MOVPE-grown p-GaN. The Pt-layers were deposited by e-beam and thermally assisted vacuum evaporation after a standard cleaning process. For evaluation of Rc we used optimised circular TLM test patterns defined by photolithography. Best contacts were formed by annealing in Nitrogen athmosphere at 500°C. We also investigated the dependence of the contact resistance on the Mg doping concentration. Therefore p-GaN layers with different Mg-concentrations were grown on SiC-substrates and Ptcontacts were processed. For those samples, we investigated the Mg-concentrations, verified by secondary ion mass spectroscopy (SIMS), the hole concentrations and mobilities in dependence of C(Mg), which we obtained from HALL-measurements, and the contact and sheet resistances, measured by circular TLM measurements. The experiments showed that the optimum Mg-concentration for low contact resistances is higher than 2*1019 cm-3 which was found to provide a maximum hole concentration near 7*1017 cm-3. The influence of self-compensation in p-GaN in bulk and near interfaces will be discussed. INTRODUCTION The semiconductor material system Ga(In,Al)N can be used in many applications including opto electronic, power and high frequency devices. Because of its large band gap it is possible to realise light emitting devices in the ultra violet, blue and green colour range with potential to red. At OSRAM – Opto semiconductors in Regensburg, Germany, blue LEDs are produced in large scale since 1998. Meanwhile the product spectrum has extended by white LEDs using converter technology. For all devices, the n- and p-doped Ga(In,Al)N-layers are grown by MOVPE (Metal Organic Vapour Phase Epitaxy) on 2‘‘-SiC-substrates. We are also working on the performance improvement of blue laser diodes for applications like data storage, printing, projection displays and illumination. The current status of our laser diode lifetimes in continuous wave operation is several minutes which is mainly limited by the high operation temperature due to the threshold current density of about 7,5 kA/cm2. To lower the heat generation, low series, interface and contact resistances inside the device are necessary. In this work we concentrate on the metal-pGaN contact which is still one of the major heat sources in our laser diodes. After giving a short survey of the theoretical background, the C-TLM (
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