Characterization of GaN MOS Structures Using Photoanodically Grown Oxides with Respect to FET Devices
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Characterization of GaN MOS Structures Using Photoanodically Grown Oxides with Respect to FET Devices D. Mistelea, T. Rotter, R. Ferretti*, F. Fedler, H. Klausing, O.K. Semchinova, J. Stemmer, J. Aderhold, and J. Graul Laboratory for Information Technology, University of Hannover, Schneiderberg 32, D-30167 Hannover, Germany; *Institute for Semiconductor Technology, University of Hannover, Appelstr.4A, D-30167 Hannover, Germany a E-mail: [email protected]
ABSTRACT Photoanodically grown Ga2O3 layers were characterized with respect to their suitability as gate dielectrics for GaN based MOSFET Device applications. The Ga2O3 layers were produced in a photoelectrochemical cell using aqueous solutions of KOH. IV characterization of MOS structures show insulating behavior of the oxide layers and CV measurements indicate a small density of states at the oxide/GaN interface. Integrating the wet chemical oxide growth in a MOSFET device fabricating process includes tungsten as gate metal together with H2O2 as etching solution for the gate metal. Source/drain areas were made free of oxide by the alkaline developer of a conventional lithographic step and metallization was done by using the liftoff technique. MOS structures show no inversion mode but strong depletion in reverse biasing mode.
INTRODUCTION AND MOTIVATION Gallium nitride is still of increasing interest for high power and high frequency electronic devices. Material qualities such as the wide, direct bandgap, the good thermal conductivity, the high breakdown field, and last but not least the piezoelectric properties make GaN very attractive for electronic power applications. So far many transistor devices have been realized featuring a lot of excellent device characteristics: HBTs [1,2], MODFETs [3], HFETs [4], MESFETs [5], and MOSFETs [6] with current densities as high as 2.55 kA/cm2 [2], operation at elevated temperatures up to 750°C [7], and cutoff frequencies ft of 50 GHz [4]. One advantage of MOSFET devices is the low power consumption due to the insulated gate. Also a large ft can be realized but there is a need for good gate dielectrics with a low density of interface states Dit. In previous reports of our research we showed the ability of photoelectrochemical (PEC) smooth etching of n-GaN surfaces [8,9] and the formation of oxide layers [10,11]. In these recent publications we emphasized on the formation procedure together with phototransient considerations. We also compared there the photoandic oxide growth with thermal oxide growth and tested the dependence on the polarity of GaN. In these ongoing studies now we are motivated to combine these processes to create a gate recessed MOSFET (figure 1). An advantage of the photoelectrochemical technology is the lower surface damage produced by PEC G11.42.1
ohmic contact
gate metal
ohmic contact
highly doped GaN
PEC oxide
highly doped GaN
(high mobility) undoped channel
substrate (sapphire/SiC/GaN?)
Figure 1: Schematic gate recess MOSFET (normally on/depletion mode). The channel might be u
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