Changes of electronic properties of AlGaN/GaN HEMTs by surface treatment
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Changes of electronic properties of AlGaN/GaN HEMTs by surface treatment W. Pletschen1, St. Linkohr1, L. Kirste1, V. Cimalla1, S. Müller1, M. Himmerlich2, S. Krischok2 and O. Ambacher1 1 Fraunhofer-Institute of Applied Solid State Physics, D-79108 Freiburg, Germany, 2 University of Technology, D-98693 Ilmenau, Germany, ABSTRACT The impact of device processing and plasma treatments at different plasma conditions on the electronic transport properties of GaN/AlGaN/GaN heterostructures was investigated as well as annealing in nitrogen atmosphere at 425°C. The electrical properties are characterized by Halleffect measurements while electron spectroscopy and X-ray measurements are used to investigate changes in the surface chemical composition and in the layer structure, respectively. It is demonstrated that these layer structures are quite sensitive even to non-plasma based processing. Furthermore, treatments in SF6 and N2 based plasmas strongly affect the 2DEG properties of the heterostructure due to altering of the surface barrier accompanied by thinning of the layer structure. Depending on the layer structure and the plasma conditions used the electronic properties may be recovered by annealing. INTRODUCTION AlGaN/GaN High Electron Mobility Transistors (HEMTs) are well suited for high power applications in the frequency range from 2 to 100 GHz due to a high breakdown voltage and large charge densities in the channel even without doping the AlGaN barrier. The electric fields induced by the polarization sheet charges present in these layer structures provide the channel for the two dimensional electron gas (2DEG) [1] and it is assumed that the 2DEG carriers originate from surface donors [2]. Therefore, it can be expected that the 2DEG may be changed by altering the surface of the HEMT structure. In particular plasma based techniques like dielectric film deposition and its dry etching as well as annealing used in device processing have been shown to affect the 2DEG properties strongly [3, 4], including a change of transistor operation from depletion to enhancement [5]. In a previous work [3, 4] we have analyzed the degradation of AlGaN/GaN heterostructures with 22 nm AlGaN barriers by typical SF6, CF4 and N2 plasma processing steps. Furthermore, the possibilities and limitations to recover the performance by annealing was demonstrated. The present study extends this work by the investigation of the impact of different fluorine and nitrogen based plasmas on the electronic properties of AlGaN/GaN HEMT structures having variable AlGaN barrier and GaN cap thicknesses. It will be shown for which of the heterostructures the plasma induced damage might be eliminated by annealing. EXPERIMENTAL The undoped GaN/AlGaN HEMT structures of this study were grown on 3 inch sapphire substrates by metal-organic chemical vapor deposition using an AIXTRON 12x3” multi wafer reactor. The layer stack consists of a 1.9 µm-thick GaN buffer layer followed by an AlGaN
barrier layer having different thicknesses with Al contents of 14 - 30% and a
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