Materials Science Forum Vols. 615-617

Abstract: The kinetics of 4H-SiC thermal oxidation by RTP technique and the properties of thin thermal oxide was reported. The thickness of the thermal oxide was determined by spectroscopic ellipsometry and confirmed by electrical measurements. The conductance method was applied to analyse the surface states parameters. The lifetime, density and cross-section of the surface traps were extracted for as-fabricated MOS capacitors and after thermal annealing processes.

Abstract: This work is focusing on the effect of a high concentration of nitrogen (N) introduced by ion implantation at the SiO2/4H-SiC interface in MOS capacitors. The N implanted sample (Ninterface ~1x1019cm-3) is compared with a non-implanted one (Ninterface ~1x1016cm-3) by means of the electron interface trap density (Dit). The Dit is determined via High-Low frequency C-V method and Thermal Dielectric Relaxation Current (TDRC) technique. It is shown that the TDRC method, mainly used so far for determination of near interface oxide charges, can be exploited to gain information about the Dit too. The determined value of Dit in the N-implanted sample is nearly one order of magnitude lower than that in the sample without N implantation. Good agreement between the TDRC results and those obtained from High-Low frequency C-V measurements is obtained. Furthermore, the TDRC method shows a high accuracy and resolution of Dit evaluation in the region close to the majority carrier band edge and gives information about the traps located into the oxide.

Abstract: In this work the effect of oxidation temperature of 4H-SiC on the density of near-interface traps is studied. It is seen that the portion of traps with slower emission times decreases with increasing oxidation temperature. Despite this reduction, high temperature oxidation alone is not useful to achieve low density of interface traps at the SiO2/4H-SiC interface.

Abstract: We propose the use of an aluminum oxynitride (AlON) gate insulator for 4H-SiC MIS devices. Since direct deposition of AlON on 4H-SiC substrate generates a large amount of interface charge due to an interfacial reaction, a thick AlON layer was deposited on underlying thin SiO2 thermally grown in N2O ambient. To reduce the negative fixed charge density in the aluminum oxide (Al2O3) film, we used reactive sputtering of Al in an N2/O2 gas mixture. The fabricated MIS capacitor with AlON/SiO2 stacked gate dielectric shows no flat band voltage shift and negligible capacitance-voltage hysteresis (30 mV), indicating the dielectric is almost free from both fixed charges and electrical defects. Owing to the high dielectric constant of AlON (k=6.9), as compared to single N2O-SiO2 gate insulator, significant gate leakage reduction was achieved by AlON/SiO2 stacked gate dielectrics even at high-temperature, especially in a high electric field condition (>5 MV/cm).

Abstract: The effects of post deposition annealing in forming gas (5 % H2 in 95 % N2) ambient at different temperatures (850, 950, and 1050 oC) on metal-oxide-semiconductor characteristics of sol-gel derived HfO2 gate on n-type 4H-SiC have been investigated. After 30 min of the annealing, an accumulation of positive effective oxide charge (Qeff) has been observed in all samples. The total interface trap density and Qeff of the oxides annealed at 850 and 950 oC are comparable but an increment and reduction of the respective densities have been recorded when the oxide was annealed at 1050 oC. A reduction of near interface trap density has been revealed as the annealing temperature has been increased. These observation was closely related to the increment of leakage current density as the annealing temperature increased.

Abstract: Reliability of the gate oxide is influenced by the device structure and the processes. In the SiC MOSFET, the surface morphology is degraded by the high temperature activation RTA, and the degradation is remarkable on the n+ source region. This study develops the method to suppress the degradation of the reliability of the gate oxide on the carbon face. By utilizing the carbon cap for the RTA and the high density O2 plasma etching to remove the carbon cap, the reliability is drastically improved both on the un-implanted and the implanted surfaces. Especially, the degradation of the reliability is perfectly suppressed on the un-implanted surface.

Abstract: This paper discusses the issues regarding reliability of large-area (up to 9mm2) gate oxide on the C-face of 4H-SiC. We first show that the initial failure in TDDB characteristics of large area gate oxide is strongly correlated with the surface-defect density. Using wafers with low surface-defect density wafers, scaling analysis of the area-dependence of TDDB characteristics has been performed. It has shown that the reliability of a large area gate oxide is dominated by initial and random failures. Further, we have shown that, by optimizing the temperatures of post-oxidation anneal in hydrogen atmosphere, the random failures of TDDB characteristics are substantially reduced.

Abstract: The oxide reliability of metal-oxide-semiconductor (MOS) capacitors on 4H-SiC(000-1) carbon face was investigated. The gate oxide was fabricated by using N2O nitridation. The effective conduction band offset (Ec) of MOS structure fabricated by N2O nitridation was increased to 2.2 eV compared with Ec = 1.7 eV for pyrogenic oxidation sample of. Furthermore, significant improvements in the oxide reliability were observed by time-dependent dielectric breakdown (TDDB) measurement. It is suggested that the N2O nitridation as a method of gate oxide fabrication satisfies oxide reliability on 4H-SiC(000-1) carbon face MOSFETs.

Abstract: One major problem when operating SiC power devices at a junction temperature of more than 200°C is the pronounced degradation of the Ni2Si-based ohmic contacts caused by interaction with the Al interconnect. In this paper, measures against such trouble and their effectiveness are discussed. Two measures highly compatible with Si device technology have been devised and experimentally implemented: (1) insertion of a Ta/TaN barrier metal between the Al interconnect and the Ni2Si contacts; (2) use of 1 wt% Si-doped Al as the interconnect. A failure lifetime of more than 12000 hours has been attained in a temperature range up to 385°C.

Abstract: We studied the heat resistance of AuTiBx (ZrBx) barrier contacts to n-SiC 6H and n-GaN. The Schottky barrier diode (SBD) parameters, the concentration depth profiles for contact structure components and the phase composition of contact metallization were measured both before and after rapid thermal annealing (RTA) at temperatures up to 900 °С (1000 °С) for contacts to GaN (SiC 6H). It is shown that the layered structure of metallization and electrophysical properties of Schottky barriers (SBs) remain stable after RTA, thus indicating their heat resistance. The ideality factor n of the I-V characteristic of SBDs after RTA was 1.2, while the SB height φВ was ~0.9 eV (~0.8 eV) for the gallium nitride (silicon carbide) barrier structures.