Electrical characterization of process- and radiation-induced defects in 4H-SiC

Abstract

Devices for operation in aerospace, manufacturing industries, defence and radiation-harsh environments need to be manufactured from materials that are resistant to the frequent damage caused by irradiation and high-temperature environments. Silicon carbide (SiC) is a wide-bandgap semiconductor material that promises to provide solutions to these problems based on its capability to operate under extreme conditions of temperature and radiation. These conditions introduce defects in the materials. Such defects play an important role in determining the properties of devices, albeit beneficial or detrimental. Therefore it is very important to characterize the defects present in as-grown material as well as defects introduced during processing and irradiation. In this research, resistive evaporation (RE) as well as electron-beam deposition was employed for the fabrication of ohmic and Schottky barrier contacts on nitrogen-doped, n-type 4H-SiC substrate. The quality of the Schottky barrier diodes (SBDs) deposited was confirmed by current-voltage (I-V) and capacitance-voltage (C-V) measurements. Deep level transient spectroscopy (DLTS) and high-resolution Laplace DLTS were successfully used to characterize the electrically active defects present in the 4H-SiC SBDs before and after bombarding them with high-energy electrons and alpha-particles as well as after exposing the sample to electron beam deposition conditions. I-V and C-V measurements showed that the SBDs deposited by RE were of good quality with an ideality factor close to unity, a low series resistance and low reverse leakage current. After irradiation, the electrical properties deviated significantly based on the irradiation types and fluences. Thermionic emission dominated at high temperatures close to room temperature, while other current transport mechanisms became dominant at lower temperatures. The ideality factor increased and Schottky barrier heights decreased with decreasing temperature, which has been attributed to barrier inhomogeneities at the metal 4H-SiC interface. Irradiation by high-energy particles had no effect on mean barrier height, but influenced the modified Richardson constant of the devices. Results obtained from the DLTS and LDLTS measurements revealed the presence of four electrically active deep levels in the as-grown 4H-SiC, and the presence of two and three extra defects after bombardment with alpha-particle and high-energy electron (HEE) irradiation, respectively. The irradiation by both alpha-particle and HEE caused an increase in concentration of electrically active defects attributed to nitrogen impurities as well as the Z1/Z2 intrinsic defect attributed to the carbon vacancy. However, the structure of two of the defects observed with energies EC 0.22 and EC 0.76 eV were unknown. Electron-beam deposition as well as exposure to electron-beam deposition conditions introduced two additional electrically active defects in 4H-SiC These two defects were close to the metal-semiconductor junction, but the process did not cause any noticeable increase in the concentration of defects previously observed in the as-grown 4H-SiC SBDs deposited by resistive evaporation technique. These electrically active defects with energies EC 0.42 and EC ~0.70 eV were probably caused by the product of elastic collisions between 10 keV electrons and residual vacuum gases which were ionized around the filament and accelerated by the electric field towards the substrate.