Thermal oxidation is the key process to achieve high quality interfaces on Si and SiC devices. On c-plane GaN no suitable gate dielectric for normally-off operation has been found for deposited layers, showing large drifts caused by very high density of interface states lying above 1E13 cm-2 ; hence native oxidation could be the key for comparable interface qualities as on other materials. Epitaxially grown GaN (0001) on Si (111) occur in high densities of threading dislocations (TDs) up to 1E10 cm-2. Though GaN represents a very inert material, these TDs serve as preferred sites for chemical reactions, as is observed after defect etching. Their influence on the oxidation mechanism of GaN has not been understood yet, despite their importance especially in the presence of high dislocation density. In this work we propose a new perspective on the dry thermal oxidation mechanism of GaN studied with AFM, TEM and EDX. Our results reveal (1) enhanced vertical oxidation at threading dislocations (TDs) and (2) enhanced decomposition at these TDs. Both observations will explain the commonly observed increase of interface roughness during thermal oxidation of GaN [3, 4]. The transformation of GaN to Ga2O3 is significantly slower at defect free surface sites compared to TD areas. Our investigations suggest that at higher temperatures (T>900ï¿½C), the Ga2O3 formation is solely controlled by the decomposition of GaN and subsequent oxidation of the metallic gallium. Owing to the exothermic formation enthalpy of Ga2O3 from GaN and O2 above 750ï¿½C  the actual local temperature of the sample is higher than the apparent one. This means that adding oxygen to the system supports the decomposition; hence the oxidation mechanism is promoted by the amount of oxygen added due to both, the excess offer of oxygen and the availability of metallic gallium. This mechanism is preferred at TDs and grain boundaries and is true for oxidation temperatures above 850ï¿½C. Oxidation promoted decomposition can additionally be seen to play a role at defect free interfaces at T=1100ï¿½C as it will be demonstrated by TEM investigations. The resulting surface roughness after oxidation is therefore explained by an inhomogeneous Ga supply at the defect free interface compared to grain boundaries and TDs. The results are relevant for process improvements of thermal oxidation and explain why interface roughness is an intrinsic feature of such processes.  P. Lagger et al., Appl. Phys. Lett. 105, 033512 (2014).  T. Hossain et al., Phys. Status Solidi C 3-4, 565 (2014).  H.S. Oon and K.Y. Cheong, J. Mater. Eng. Perform. 22, 1341 (2013).  M.R. Ranade et al. J. Phys. Chem. B 104, 4060 (2000).
Infineon Technologies Austria AG, University of Kassel
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