Selection, Growth, and Characterization of Gate Insulators on MOCVD Gallium Nitride for the Use in High Power Field Effe
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Mat. Res. Soc. Symp. Proc. Vol. 482 0 1998 Materials Research Society
Table I. Properties of the selected insulators Bandgap (eV @ 300 K) Dielectric Strength (10"V/cm) Dielectric Constant Thermal Conductivity W/cm.K Density (g/cm3 )
144
201
258
315
372 429
486
Electron Energy (eV)
543
GaO.•
AIN
? ? 3-6 ? 6.44
6.3 0.6 - 1.5 8.5 2.0 3.2
201
600
258
315 372
SiO0 9 1 3.9 .014 2.2
429
4
Electron Energy (eV)
Figure 2. AES of GaN surface following 30W N 2 plasma clean at 300'C
Figure 1.AES of as-loaded GaN surface after ex-situ clean described in text
oxygen remained on the surface, as shown by the Auger electron spectroscopy (AES) results in Fig. 1. The GaN was then placed on a quartz boat, inserted into a quartz tube furnace which was evacuated and backfilled with oxygen flowing through the system at 2500 sccm. The oxidation of the GaN surface was investigated for times ranging from three - six hours at 700°C, 800'C, 825°C, 900°C, 1000°C. The films were analyzed using electron dispersion spectroscopy (EDS) for composition and scanning electron microscopy (SEM) to observe the morphology. The AIN films were deposited on the GaN layers by molecular beam epitaxy. The surface of each GaN sample was cleaned prior to deposition ex-situ in the same manner as prior to the oxide growth, and in-situ at 800'C in a 5 sccm ammonia flux for 30 minutes. All traces of oxide were removed using the latter procedure, as determined by x-ray photoelectron spectroscopy (XPS). Details for cleaning and surface preparation have been reported in Reference 6. The AMN was then deposited at a temperature of 800*C for 20 minutes with an ammonia flux of 10 sccm. The Si0 2 deposition was performed in a RPECVD system. Due to the inability to reach high temperatures with ammonia in this system, cleaning steps involving a nitrogen plasma were implemented. The GaN samples were cleaned with the aforementioned solvents and the HCI:DI water dip as before, placed upon a Si wafer in the RPECVD system, heated between 250° and 300°C and immersed in a nitrogen plasma. Varying plasma powers and system pressures were used in an attempt to remove all of the surface contaminates. A plasma power of 30W at a pressure of 0.1 Torr for 5 minutes removed all of the surface contaminates except oxygen, as shown in Figure 2. The oxide was deposited on the GaN, following a 15 second exposure to an oxygen plasma using silane and molecular oxygen as the precursor gases. Details of the growth and characterization of the resulting films have been reported in Reference 7. Following deposition the samples were annealed in a rapid thermal annealing furnace under an argon atmosphere at 900°C for 1 minute to densify the oxide. Metal insulator semiconductor capacitors were fabricated using all three insulators. These devices were fabricated laterally, i.e., both the Al gate contact and the Al ohmic contact were deposited via thermal evaporation using a shadow mask on the surface of the structure, because of the insulating AIN buffer layer on which of the GaN epila
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