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ABSTRACT Nitrogen is the common n-type dopant of the various polytypes of silicon carbide. The nitrogen levels in 4H-SiC (at EC-53 meV and EC-100 meV) and in 6H-SiC (at Ec-89 meV, Ec100 meV, and Ec-125 meV) have been studied in detail by temperature dependent Hall effect measurements, electron spin resonance (ESR), and thermal admittance spectroscopy. Until now, such detailed studies of the nitrogen levels in 15R-SiC have not been carried out. Lely-grown 15R samples were used in these studies. The net carrier concentrations (ND-NA), determined by room temperature CV measurements, ranged from I x 1018 to 3 x 1018 cm- 3 . The nitrogen levels in 15R-SiC were studied using thermal admittance spectroscopy. Optical admittance spectroscopy (OAdS) was used to study the deeper defects in this polytype. It was found that optical transitions to the conduction band were inhibited in the heavily doped material. INTRODUCTION Because of its many superior properties, including a wide bandgap and high breakdown field, silicon carbide is being investigated for a variety of electronic applications including high temperature, high power, microwave, and radiation hard, devices. Significant advances have been made in recent years in both materials and devices[l]. Now that improved material is becoming available, both in bulk and epitaxial form, researchers are investigating the deep levels present in the material. Deep levels can limit carrier lifetime and adversely affect device performance. In addition, there is a need, particularly in the area of high power microwave devices, for insulating substrates. Compensation of residual impurities by intentionally doping with deep level impurities is required to produce this material. Deep levels in SiC have been studied for some time by a variety of techniques but perhaps the most powerful are those based on capacitance spectroscopy. Deep level transient spectroscopy (DLTS) is the most widely used technique and a number of levels have been detected in 6H-SiC. In particular, a level near 0.70 eV has been reported by Uddin and coworkers[2,3], and Jang et al.[4], and by us[5]. However, no one has been able to identify the defect responsible. Unless special high-temperature facilities are used, 0.7 eV is about as deep a level as can be observed by conventional DLTS. Other techniques have been used to investigate deeper levels, and photoluminescence[6] and electron paramagnetic resonance[7] have proven useful, particularly for the study of transition metal impurities such as vanadium and titanium. The samples studied were 15R-SiC grown by the Lely method. The crystals were all ntype due to the predominance of the nitrogen donor levels. Wafer preparation consisted of oxidation and etching to remove polishing damage[9]. To create the capacitance specimens, nickel was annealed on the Si face to create the ohmic contact and unannealed sputtered aluminum formed the Schottky contacts, which ranged in size from 100 to 600 gim in diameter.

705 Mat. Res. Soc. Symp. Proc. Vol. 423 0 1996 Materials Research