Relevance of Threading Dislocations for the Thermal Oxidation of GaN (0001)
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Relevance of Threading Dislocations for the Thermal Oxidation of GaN (0001) Maria Reiner1,2, Christian Koller1, Kurt Pekoll1, Rudolf Pietschnig2 and Clemens Ostermaier1 1
Infineon Technologies Austria AG, Siemensstr. 2, 9500 Villach, Austria 2 Institute of Chemistry and CINSaT, University of Kassel, Heinrich-Plett-Str. 40, 34132 Kassel, Germany
ABSTRACT The influence of threading dislocations (TDs) on the dry thermal oxidation of c-plane gallium nitride (GaN) is investigated for oxidation temperatures above 800°C. The transformation of GaN to gallium oxide (Ga2O3) is preferably found at TDs and grain boundaries, showing enhanced vertical oxidation, compared to defect free surface sites. Therefore, the increase in surface roughness commonly obtained upon oxidation is explained by an inhomogeneous chemical reactivity associated with those crystal defects. Additionally, annealing in an N2 atmosphere showed that also decomposition is favored at such chemically reactive spots. Comparison between decomposition and oxidation suggests that at temperatures above 950°C, the Ga2O3 formation is supported by the decomposition of GaN and subsequent oxidation of the metallic gallium. INTRODUCTION Even though GaN metal insulator semiconductor - high electron mobility transistors (MIS-HEMTs) exhibit several benefits in device performance over Schottky-contact based HEMTs, the topic of finding a suitable gate dielectric with a reliable interface remains an open challenge [1]. So far no suitable gate dielectric for normally-off operation has been found for deposited layers on c-plane GaN [2-4]. Devices exhibit large threshold voltage drifts caused by a very high density of interface states above 1013 cm-² even for bias stress conditions below lifetime requirements [5]. In contrast, silicon and silicon carbide devices have shown strong improvements in achieving high quality interfaces using thermal oxidation [6]. Hence native oxidation could be the key for comparable interface qualities also in III-N based devices. Natively grown Ga2O3, either by chemical processing [7] or thermal oxidation is predominantly found as β-Ga2O3 [8-10]. After the growth an increase in surface and interface roughness [10-12] as well as certain hillocks have been observed, that are presumably located on threading dislocations (TDs) [13]. As epitaxial grown GaN (0001) on Si (111) occurs in high densities of threading dislocations (TDs) of up to 1010 cm-2, they are likely to participate in the oxidation reaction from GaN to Ga2O3. In contrast to the inertness of defect free GaN, TDs represent chemically more reactive areas, as observed from defect etching [14]. It is further suggested by Zhou et al. that GaN surface decomposition might be responsible for the formation of pits found at the GaN surface after oxide removal. Nevertheless, detailed understanding on the origin of interface roughening as well as the influence of TDs and the role of decomposition in the mechanism of oxidation would be desirable.
In this work we propose a new perspective on the dry
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