| Summary: | 碩士 === 國立交通大學 === 電子工程學系 電子研究所 === 102 === Silicon carbide (SiC) is an ideal semiconductor material for the higher power and high temperature applications due to its wide bandgap, high critical electric field, and good thermal conductivity. For high breakdown voltage device, the quality of the n- epi-layers is very important, because it influences the on-resistance, junction leakage current, and breakdown voltage of device. Deep level transient spectroscopy (DLTS) is capable of characterizing the defect energy, defect capture cross section, and density of defect quantitatively. Besides, DLTS has high sensitivity and is relatively easier than the other defect detection systems to be setup. Hence, we choose DLTS to analysis the defects in the epi-layer. The widest applications of SiC are its power diodes like Schottky barrier diode, pn diode, and junction barrier Schottky (JBS) diode. The JBS diodes have many different variations of structure for the improvement in performance or the ease of process. The trench junction barrier Schottky (TJBS) diode is one of the latest innovations, so we are going to figure out the design guidelines of the TJBS diodes by the Sentaurus TCAD tool.
The setup of DLTS system consists of a semiconductor parameter analyzer with a capacitance measurement unit and a probe station with a temperature heating controller. Our DLTS system can measure the same signal but weaker signal intensity in comparison with the commercial DLTS system due to the sampling rate limitation of the capacitance measurement unit.
To measure the defects in epi-layer, Ni/SiC Schottky barrier diode and SiC MOS capacitor are fabricated. For the Schottky barrier diode, there is no defect measured by our DLTS system. In order to create defects in the epi-layers, low dose nitrogen ion implantation is introduced. After ion implantation, strong signal is measured in the DLTS, but severe overlap of signals from different defects makes it hard to extract the defect energy. Different processes are applied to eliminate the defects induced by ion implantation. The most effective one is annealing at 1600 oC.Almost all the defects induced by ion implantation are eliminated. However, high temperature annealing could generate Z1/2 defect.
The interface state of the SiC MOS capacitor is also investigated by our DLTS system. The gate dielectric of MOS capacitor is thermal oxide. The DLTS of MOS capacitor is dominated by oxide trap in SiO2. The signals from oxide traps overlap the signals from interface states. If the DLTS is not only composed of interface states, the direct transformation of DLTS into interface state density would be incorrect. For MOS capacitor fabricated on the ion-implanted epi-layer, poor capacitance-voltage characteristic is exhibited, but the DLTS result is the same as that measured on the Schottky barrier diode.
The design guidelines of the TJBS diode have been investigated by considering different geometric parameters. As the trench depth becomes deeper, the cut-in voltage becomes slight lower due to the extra sidewall current. We define the transition voltage as the voltage at which the current transport changes from unipolar conduction (Schottky junction dominant) to bipolar conduction (pn junction dominant). TJBS diodes have lower transition voltage, and thus lower specific on-resistance and higher forward conduction current after bipolar conduction. The transition voltage increases as wider Schottky contact width, but decreases as wider pn junction width or deeper trench depth. Because of trench corner and thinner epi-layer under the pn junction due to the trench structure, relatively low breakdown voltage is observed on TJBS diodes than JBS diodes. For TJBS diodes, the breakdown voltage depends on structural design, which can be compensated by edge termination design.
In conclusion, the setup and verification of DLTS system have been accomplished. Different defects after nitrogen ion implantation has been detected in Schottky barrier diode. These defects can be annihilated by a 1600 oC annealing, but meanwhile Z1/2 defect will be generated. The DLTS of MOS capacitor are dominated by oxide trap. The Dit from DLTS would be overestimated if other traps are mixed in the signal. Under the same process condition, Schottky barrier diode has identical DLTS result to MOS capacitor. The design guidelines of TJBS diodes are proposed. The trench affects all the characteristics of device. It is suggested that TJBS diodes are suitable for the higher current application, while JBS didoes are favored for high voltage application.
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