Surface properties of Al_xGa_1‐xN/GaN heterostructures treated by fluorine plasma: an XPS study

Abstract

The variations in surface potential and the Schottky barrier height ΦB in fluorine‐plasma‐treated AlxGa1‐xN/GaN heterotructures are systematically studied by x‐ray photoelectron spectroscopy (XPS), giving insights into the mechanisms underlying the strong threshold voltage shift in AlxGa1‐xN/GaN HEMTs by the F plasma treatment technology. It is found that the treatment resulted in a fluorinated surface containing masses of AlF3, with the surface potential of Al0.25Ga0.75N/GaN heterostructure increased by ∼0.38 eV during the first 60 seconds of the treatment. Annealing at 400 °C in N2 ambient for 10 minutes does not affect the surface potential, but results in quick reduction of AlF3, consistent with the relative poor thermal stability of AlF3 reported in literature. ΦB between Ni and F‐plasma‐treated Al0.25Ga0.75N surface was extrapolated from the shift in Ga 2p3/2 core‐level and exhibits a small increase of 0.20 eV. The enhanced ΦB is much smaller than the positive shift in Vth observed from HEMTs fabricated with the same treatment conditions, suggesting that the primary factor responsible for the conversion from depletion‐mode to enhancement‐mode AlxGa1‐xN/GaN by F plasma treatment is not the surface modifications, but rather the negative fixed charges carried by F ions in AlxGa1‐xN/GaN heterostructures. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Impact of Surface Treatment on Interface States of ALD Al2O3/GaN Interfaces

“…41 XPS studies of fluorine treated GaN have shown that Ga-F bonds can result in significant chemical shifts in the Ga 3d peak. 42,43 However, in this study, no significant shifts were observed, likely due to the much lower fluorine content. Here, the presence of fluorine is accompanied by evidence of a Ga-rich surface, which could result in fluorine binding at the nitrogen site, as et al that the substitutional fluorine level is near the GaN conduction band and would act as an interface state.…”

Comparison between AlGaN surfaces etched by carbon tetrafluoride and argon plasmas: Effect of the fluorine impurities incorporated in the surface

“…The presently obtained results of the XPS Al 2p and Ga 2p spectra differ from those reported in the earlier work [34]. The earlier work shows that the AleF and GaeF peaks clearly appear in the XPS Al 2p and Ga 2p spectra of the CF 4 plasma-etched AlGaN surface, respectively [34].…”

“…The earlier work shows that the AleF and GaeF peaks clearly appear in the XPS Al 2p and Ga 2p spectra of the CF 4 plasma-etched AlGaN surface, respectively [34]. The clear appearance of the peaks suggests that a large number of the fluoride impurities incorporated in the surface contribute to the formations of AlF 3 and GaF x .…”

“…The black curves in Fig. 2 (b) correspond to those fitted with a linear combination of two Gaussian functions associated with the GaeN peak at 1118.3 eV [33] (blue curves) and with the GaeF peak at 1119.35 eV [34] (green curves). As shown in Fig.…”

“…The possible cause for these formations is the fact that the RF power used in the earlier work is high. The RF power used for the formations of AlF 3 and GaF x is 90 W [34], whereas the maximum value of the RF power used in the present study is the above-described value of 32 W. The high RF power promotes the dissociative ionization between electrons and CF 4 gas particles in the plasma, and thereby increases the number of fluorine ions impinging on the surface [28e30]. Since the increase in the number of the impinging fluorine ions causes the preferential removal of a large number of nitrogen atoms, the fluorine impurities incorporated in the surface under the high RF power condition are readily substituted for the nitrogen atoms in the surface; namely, the incorporated fluorine impurities readily bond with the aluminum and gallium atoms in the case where the high RF power is high.…”