Application of ZnO to Passivate the GaN-based Device Structures
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1035-L08-08
Application of ZnO to Passivate the GaN-based Device Structures Eliana Kaminska1, Anna Piotrowska1, Marie-Antoinette di Forte Poisson2, Sylvain Delage2, Hacene Lahreche3, Norbert Kwietniewski1, Iwona Pasternak1, Renata Kruszka1, Marek Guziewicz1, Piotr Boguslawski4, Elzbieta Dynowska4, and Michal Borysiewicz1 1 Semiconductor Processing for Photonics, Institute of Electron Technology, Al. Lotnikow 32/46, Warsaw, 00-743, Poland 2 Thales-Alcatel III-V Lab, Route de Nozay, Marcoussis, 91461, France 3 Picogiga, Place M. Rebuffat, Courtaboeuf, 91971, France 4 Institute of Physics PAS, Al. Lotnikow 32/46, Warsaw, 02-668, Poland
ABSTRACT The fabrication of high-resistivity ZnO-based thin films lattice-matched to AlGaN/GaN structures has been developed. It relies on low-temperature reactive sputter deposition of ZnO:Sb from ZnSb target. Taking into account the hygroscopic nature of ZnO surface, an additional coating by Si3N4 films is applied to ensure the humidity protecition. The developped passivation suppresses leakage currents in Schottky diods, and substantially improves output characteristics of AlGaN/GaN HEMT. INTRODUCTION The same crystalline structure with a close lattice match, similar band gaps, and thermal stability of ZnO and GaN or other III-nitrides have previously been used in fabrication of ZnO/GaN hybrid substrates and heterojunctions. Here, we propose to farther exploit these features, and use highly resistive ZnO as passivating coating for GaN-based heterostructures. In fact, GaN-based heterostructures are intensively studied owing to their excellent properties for applications in high power/high frequency electronics. While high output power devices have been demonstrated, one of the key unsettled issues is that of large leakage currents in Schottky gates fabricated on GaN and AlGaN surfaces, seriously limiting their performance. It is believed that the dominant leakage mechanism is the tunnelling via donor states (nitrogen vacancy or oxygen impurity) introduced near the semiconductor surface during surface processing of devices [1]. Various surface treatments, such as N2 plasma exposure and/or passivation with dielectric films have been investigated [2]. A novel approach to solve this problem presented in this work relies on application of semi-insulating ZnO as passivating lattice-matched coating. Since nominally undoped ZnO exhibits n-type conductivity we apply additional acceptor doping to achieve semi-insulating material. The close lattice match at GaN/ZnO interface should reduce the strain effects. Finally, thin Si3N4 film was deposited to protect ZnO from humidity. We present the results of comparative study of unpassivated and passivated using polycrystalline HfO2, amorphous Si3N4 and lattice-matched ZnO AlGaN/GaN high electron mobility transistors (HEMT) structures.
EXPERIMENTAL To study passivation effects on device characteristics, Schottky barriers and HEMTs were fabricated. AlGaN/GaN structures were grown by both MOCVD and MBE on sapphire or composite substrates [3]. The proc
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