Formation and Characterization of Oxides on GaN surfaces
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Formation and Characterization of Oxides on GaN surfaces D. Mistele, T. Rotter, F. Fedler, H. Klausing, O.K. Semchinova, J. Stemmer, J. Aderhold, and J. Graul Laboratory for Information Technology, University of Hanover, Schneiderberg 32, D-30167 Hannover, Germany ABSTRACT We characterized oxides formed directly on n-GaN surfaces. The methods used for oxide layer formation were both photoanodic oxidation and thermal oxidation. The photoanodic oxidation took place in aqueous solutions of potassium hydroxide with pH values lower than 13. Homogenous oxide films were obtained in the voltage range from -0.6 V to 0.4 V vs the saturated calomel electrode (SCE). The characterization of the oxide layers was performed primarily by Auger electron spectroscopy (AES). First the surface chemistry was determined, proving that Ga-oxide is formed with an attributed stoichiometry of Ga2O3. Secondly, depth profiling shows the oxide thickness to be dependent on the photoanodic voltage and oxidation time. Complementary X-ray diffraction (XRD) studies suggest an amorphous state of the formed layers. Annealing GaN in O2-atmospheres above 900°C also lead to surfaces fully covered with gallium oxide. We found that N-polar surfaces oxidize faster than Ga-polar surfaces, which is in agreement to the theoretical work of Zywietz et al [1]. Furthermore, we report on the electrical properties of the anodized oxide layers by analyzing MOS structures. INTRODUCTION A lot of progress has been achieved in the development of GaN-based devices over the last couple of years. Applications range from optoelectronic devices (LEDs, LDs, photodetectors) to different kinds of electronic applications. From the chemical and physical point of view GaN and its ternary alloys are especially suited for high power and high temperature electronics and should also be useful as high frequency devices. Ambitious efforts in this field led to a lot of different types of transistors such as HBTs [2], HFETs [3], MODFETs [4], and MOSFETs [5]. For both optical and electronic devices the controlled formation of an oxide layer on GaN surfaces could induce great progress. On one hand it could serve as a passivation layer improving optical characteristics [6] and on the other hand its use as a dielectric opens the wide field to many FET devices. For comparison one may consider the Si/SiO2 system which is one key for the successful MOS technology used in millions of integrated circuits e.g. in computer chip fabrication. Main obstacles in order to reach oxide qualities useful for devices are the homogeneity of the oxide layers, their reproducibility, the insulating and dielectric properties, and last but not least the density of interface states. In comparison to the elementary semiconductor Si and its oxide SiO2 the oxide formation differs in the case of III/V compound semiconductors [7]. A high quality oxide formation on GaAs for example is hindered by the competing formation of Ga2O3 and As2O3 or by elemental As which remains in the interface region resulting in bad high frequenc
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