High electron mobility in AlGaN/GaN HEMT grown on sapphire: strain modification by means of AlN interlayers.
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Y10.22.1
High electron mobility in AlGaN/GaN HEMT grown on sapphire: strain modification by means of AlN interlayers. Marianne Germain1, Maarten Leys1, Steven Boeykens1, Stefan Degroote1, Wenfei Wang1, Dominique Schreurs1, Wouter Ruythooren1, Kang-Hoon Choi1, Benny Van Daele2, Gustaaf Van Tendeloo2, and Gustaaf Borghs1 1
IMEC, Microsystems, Components and Packaging, Kapeldreef 75, B-3001 Leuven, Belgium. 2 Electron Microscopy for Materials Research (EMAT), University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium.
ABSTRACT The performance of AlGaN/GaN High Electron Mobility (HEMT) transistors is directly related to the electrical characteristics of the two-dimensional electron gas formed at the interface thanks to the piezoelectric field. Modification of the Al content or thickness of the AlGaN layer can within a certain limit modify the carrier density and mobility in the 2DEG. However, further reduction of the sheet resistance requires strain engineering of the heterostructure. An effective way to reduce the sheet resistance, as well as to lower the threading dislocation (TD) density, is to perform strain engineering through the use of low temperature AlN interlayers inserted in the GaN buffer layer. From correlation of AFM, TEM and HRXRD mapping of the HEMT layers, the strain modification, as well as the mechanism reducing the TD density, can be explained by the highly defected nature of the AlN interlayer grown at low temperature, as well as its very small thickness. The LT AlN acts as a second nucleation layer for the GaN grown on top. Contrarily, when the AlN interlayer is grown at 1050°C, its high crystalline quality and the possibility to grow pseudomorphic and abrupt interfaces, allows its use at the AlGaN/GaN interface. Optimal combination of the AlGaN and AlN layer thickness leads to record values of the mobility at room temperature of 2050 cm2/Vs, for heterostructures grown on sapphire, which is approaching state-of-the-art for HEMT grown on SiC.
INTRODUCTION Very promising for high power microwave systems, AlGaN/GaN HEMT devices offer always more interesting perspectives because of the very high power densities they can afford. An important figure of merit, from the material point of view, is the sheet resistivity of the twodimensional electron gas (2DEG) that is formed spontaneously at the AlGaN/GaN heterojunction itself. In other words, one aims at maximizing the product of the carrier density (ns) and of the electron mobility in the 2DEG (µ). It is now known that both the piezoelectric and the spontaneous polarization fields are responsible for the apparition of the 2DEG [1]. By increasing the Al molar fraction in the top AlGaN layer, one can increase the carrier concentration in the 2DEG, and thus increase the ns µ product. For too high Al content or too thick AlGaN layers, the AlGaN top layer starts to relax, thereby reducing the carrier concentration in the 2DEG [2]. Cracking of the top AlGaN layer and alloy disorder scattering degrades the mobility values.
Y10.22.2
In order to
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