Infoscience

Thesis

Physical Properties of Al1-xInxN/(AIN)/GaN (0.07<=x<=0.21) Heterostructures and their Application for High Power Electronics

AlInN is a material which is known to be difficult to be grown among the III-nitride ternary compounds. It attracted much attention only recently and starts to be studied intensively mainly for electronic applications. The aim of this work is manifold. Various structural, electrical and optical measurements are brought together to underline the outstanding quality of AlInN epi-layers grown by metalorganic vapor phase epitaxy (MOVPE). Especially the electrons confined at the heterointerface of coherently grown AlInN on GaN buffer layers determine crucially the electronic properties. An appropriate model based on charge balance is proposed allowing the extraction of all significant electrical properties. An AlN interlayer has been introduced at the AlInN/GaN interface and was found to tremendously affect transport properties. This issue is studied intensively and results indicate that even slightest deviations from atomically perfect interfaces leads to the creation of huge piezoelectric fields disturbing carrier transport in the case of strongly mismatched epilayers. A similar debate was conducted concerning the localization of carriers in InGaN quantum wells. High electron mobility transistors (HEMT) processed from these heterostructures exhibits excellent device performance exceeding 2 A/mm even on sapphire substrates. This are the highest currents ever reported for nitride heterostructures. The same structures grown on Si(111) substrates allow reaching cut-off frequency in excess of 100 GHz . However, results are puzzling and on at first sight striking since the device saturation currents of structures grown on SiC are similar to results on sapphire despite its much larger thermal diffusivity. Therefore we studied the temperature arising in the device under applied voltages using a micro-photoluminescence (µPL) setup. To our knowledge this is the first study of this type. The main advantage is the use of the short wavelength for excitation in comparison with well-established Raman methods. However we find from this analysis a significant heat development up to 1200 K for devices with 5 µm drain-source distance. The heating can be understood in terms of excited optical phonons due to field accelerated electrons under applied voltages. The comparison of the available amount of phonons in the finite volume covered by the 2D electrons and the number of created phonons in the acceleration process allows formulating a simple relation for the saturation currents as a function of the transport properties. However, this mechanism also explains why the saturation currents are far below expected values from simple velocity saturated currents models and independent of the substrate. However, heat, i.e. the decomposition of local optical phonons into acoustical phonons with large group velocity, is dissipated differently depending on substrate influencing therefore mainly the reliability of the devices. Beside the electrical breakdown due to avalanche carriers a second thermally induced breakdown mechanisms is proposed. Finally, the capability of those heterostructures for sensing is discussed. Especially the electrically response on external stimulus such as UV radiation and the exposition of the surface to polar liquids is investigated.

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