Surface Passivation of AlGaN terminated and GaN Terminated HEMT Structures Studied by XPS
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Surface Passivation of AlGaN terminated and GaN Terminated HEMT Structures Studied by XPS B.P. Gila1, E. Lambers1, B. Luo2, A.H. Onstine1, K.K. Allums1, C.R. Abernathy1, F. Ren2 and S.J. Pearton1 1
Dept. of Materials Science and Engineering, University of Florida, Gainesville, FL 32611 2 Dept. of Chemical Engineering, University of Florida, Gainesville, FL 32611 ABSTRACT Samples of both AlGaN and GaN terminated HEMT structures were studied using x-ray photo spectroscopy (XPS). It was found that the XPS spectra of both AlGaN and native oxide surfaces were shifted by a surface charge of 0.5 to 1.0 eV. The samples were then oxidized using an UV-ozone treatment for 25 minutes at room temperature. The ozone oxide XPS spectra of the AlGaN terminated surface was found to have the same 0.5 to 1.0 eV shift while the ozone oxide XPS spectra of the GaN terminated surface was found to have a 5.0 eV shift, indicating that the native GaN oxide is more insulating. Processed HEMT devices using both surface terminations were given the same UV-ozone treatment followed by a 10nm MBE grown Sc2O3 film for passivation. The GaN terminated HEMT structures showed on average a 10% increase in channel current (from gate-lag measurements, pulse mode) over the AlGaN terminated HEMT structures. INTRODUCTION Remarkable progress has been made in the field of high performance and high power AlGaN/GaN high-electron-mobility transistors (HEMTs); namely, in understanding the device physics and demonstrating excellent microwave power performance.[1] One problem with the implementation of these devices is that the RF power obtained is much lower than that expected from the dc characteristics. This RF power decrease is manifested by a drain current collapse that is believed to be caused by the presence of surface states between the gate and drain, which further depletes the channel.[2-12] This phenomenon can also be observed by a current dispersion between dc and pulsed test conditions or a degraded rf output power. The carriers in the 2-dimensional electron gas can be lost either to the surface or buffer traps [2,3, 13-16]. The surface traps may be mitigated by the use of appropriate surface passivation, most often SiNX deposited by plasma-enhanced chemical vapor deposition (PECVD). The bulk traps are a function of the epitaxial growth conditions and can only be reduced by improving material growth techniques. In the situation in which surface states dominate the current collapse, the use of SiNX passivation typically restores 70-80% of the dc current. Other surface passivation has been achieved by employing MgO or Sc2O3 and the recovery of current to is shown to approach 100%.[17-19] After performing identical treatments and passivating oxide growths on AlGaN terminated and GaN terminated HEMT structures, tendencies are that GaN terminated HEMTs show a higher degree of current recovery than the
L11.40.2
AlGaN terminated HEMTs, Table 1. In this paper, XPS spectra of as received AlGaN and GaN terminated HEMT structures are compared to spect
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