Effects of ion irradiation on 4H- SiC p-n junction

The 4H-SiC is a wide bang gap semiconductor that for its properties such as the high hardness, the temperature stability, the chemical inertness and the high thermal conductivity is a promising material for devices operating in hostile environment such as radiation rich and high temperature. Ion irradiation affects the electrical performance of different kind of 4H-SiC based devices with a subsequent increase in the electrical resistance, leakage current and with a reduction in doping concentration, carrier mobility and lifetime. All these effects can be mainly related to the radiation induced point defects due to the displacement damage. The defects induce the formation of deep levels within the semiconductor band gap which act like recombination and trapping centers for carriers. The 4H- SiC p-n junction is the base component of the bipolar junction devices such as BJT and IGBT which have applications in the power device field as well as in microelectronics and solid- state devices. They find wide applications in hostile environments such as in space and high energy physics experiments involving charged particles and photons. In addition, light ion irradiation as proton and helium, can be used to control the carrier lifetime. In this contest a full knowledge of defect structure, of its introduction rate and of its impact on electrical device parameters in a wide range of irradiation fluence is necessary. In the present thesis, 4H-SiC p-n junctions were irradiated with 700 keV He+ ions in the fluence range 10^12 - 10^15 cm^-2. The effects of irradiation on devices electric properties were investigated by current-voltage (I-V) and capacitance-voltage (C-V) measurements, while Deep Level Transient Spectroscopy (DLTS) was used to study the traps introduced by irradiation defects. Modifications of the device's electrical performances are observed after irradiation and two fluence regimes are identified. In the low fluence range (≤ 10^13 cm^-2), the I-V characteristics, in the high voltage region, evidence an increase of series resistance associated with the decrease of the dopant concentration as also denoted by C-V measurements; instead, in the low voltage region, the increase of irradiation fluence produces an increase in both ideality factor and saturation current mainly due to the increase of point defects concentration. The former parameter evidences that the recombination mechanism dominates the current flow; instead, the second parameter is inversely proportional to the carrier lifetime which decreases by increasing the fluence. The main produced defect states, detected by DLTS measurements, are the Z1/2, RD1/2, and EH6/7 centers whose concentration increases with fluence. At high fluence (>10^13 cm^-2), the I-V curves show a saturation of ideality factor and a strong decrease of the saturation current, while DLTS evidences a rearrangement of defects. The detailed electrical characterization of the p-n junction performed at different temperatures highlights that high fluence irradiation induces the formation of conduction paths with peculiar electrical properties. In particular, the results suggest the formation of localized highly resistive regions related with point defect agglomeration.