In this paper we report on results obtained for different AlGaN/GaN heterostructure field effect transistors (HFET) applying a recently developed photooxidation technique. We have processed and characterized MOS-HFETs in which a photoanodic oxide, a PECVD deposited SiO 2 layer, or a combined stack of the photoanodic oxide followed by SiO 2 (tandem-approach) is used as gate dielectric. All tandem-MOS-HFET devices have in common a large gate voltage swing and a broad transconductance peak. Additionally, the leakage and gate currents are several orders of magnitude smaller than for the Schottky gate HFETs and are in the range of pA/mm. Tandem MOSH-FET devices achieved values of 465 mA/mm in enhancement mode and 46 mS/mm for the drain current I Dmax and transconductance g m , respectively. These values are about 50% (I Dmax ) and 15% (g m ) higher than for the HFETs processed on the same wafer. Thermal oxidation at 900 C for 15 min before SiO 2 deposition leads to a normally off transistor type.
AlGaN/GaN based hetero field effect transistors (HFETs) were capped with different dielectrics, characterized, and tested for DC performance. As dielectrics we use SiO2 and photoelectrochemical (PEC) grown AlxGa2-xO3. Combination of this two dielectrics show best performance with respect to gate leakage current and controllability of the drain current ID. TheMOSHEFTs work also at positive gate voltages in accumulation, which is also demonstrated in a broad transconductance peak. The PEC oxidation shows low density of interface states Dit and the insulating properties depend strongly on the PEC conditions. Pre-treatments before the SiO2 deposition result in varying threshold voltages |Vth| and it seems that (NH4)Sx pre-treatment leaves the surface in best conditions. Comparison of Ti/Al and Ti/Al/Ti/Au as source/drain contacts for AlGaN/GaN HFETs are done and the annealing behavior of Ti/Al/Ti/Au is displayed resulting in contact resistance as low as RC = 2 &mm after annealing at 850°C in N2.
Up to now an in-line method for parameter determination of deposited thin polysilicon films is not available. In this paper a method for monitoring the polysilicon deposition process in device manufacturing by variable angle spectroscopic ellipsometry (VASE) is demonstrated. Therefore several polysilicon films are deposited on thermally oxidized [100] silicon wafers. These samples are characterized by VASE in the optical range of 450 - 850 nm. Parameters are determined by simulation using a multilayer model consisting of air, interface layer (surface roughness), polysilicon, SiO2 and silicon substrate. Different optical models representing the properties of polysilicon are tested. The free parameters are the oxide thickness, the composition and the thickness of the interface layer (air, polysilicon) as well as the thickness and the complex refractive index of the polysilicon layer. Results of the spectroscopic analysis are verified by AFM and SEM measurements. It can be shown that parameters of the deposited polysilicon films, which often could only be determined by complex and destructive off-line analysis methods are also accessible by non-destructive in-line VASE measurements.
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