Materials Science Forum Vols. 778-780

Abstract: Cost of silicon carbide (SiC) wafer has been improved owing to the development of larger and higher quality wafer technologies, while the process stays long and complicated. In this paper, we propose a novel short process of ion implantation and provide the fabrication model SiC schottky barrier diodes (SiC-SBDs) devices. Currently common mask layer of ion implantation employs high heat resistant materials such as metal oxides. Because the ion is implanted to SiC wafer at high temperature between 300 °C and 800 °C due to avoid the damage of SiC crystal structure. The process using oxide layer tends to became long and complicated. On the other hand, our proposal process uses a heat resistant photoresist material as the mask instead of the oxide layer. The heat resistant photoresist is applied to newly developed SP-D1000 produced by Toray Industries, Inc.. We demonstrated to fabricate model SiC-SBDs devices based on our proposal process with SP-D1000 and confirmed the device working as same as a current process.

Abstract: Extreme temperature measurements of Ni/Ti/Al contacts to p-type SiC (N_a = 1∙10¹⁸cm^-3), with a specific contact resistivity ρ_c = 6.75∙10^-4 Ωcm² at 25 °C, showed a five time increase of the specific contact resistivity at -40 °C (ρ_c = 3.16∙10^-3 Ωcm²), and a reduction by almost a factor 10 at 500 °C (ρ_c = 7.49∙10^-5 Ωcm²). The same response of ρ_c to temperature was seen for contacts on lower doped epitaxial layer. Also N-type nickel contacts improved with higher operational temperature but with a considerably smaller variation over the same temperature interval. No degradation of the performance was seen to either the Ni/Ti/Al or the Ni contacts due to the high temperature measurements.

Abstract: Because Al and B (elements of III group) in SiC are deep-level acceptors and these acceptors cannot reduce the resistivity of p-type SiC very much, Mg (element of II group) that may emit two holes into the valence band is investigated. A p-type 4H-SiC layer is obtained by 1800 °C annealing of the Mg-implanted layer, not by 1600 and 1700 °C annealing. It is found that a Mg acceptor level in 4H-SiC is too deep to determine the reliable density and energy level of the Mg acceptor using the frequently-used occupation probability, i.e., the Fermi-Dirac distribution function. Using the distribution function including the influence of the excited states of the Mg acceptor, therefore, its density and energy level can be determined to be approximately 1×10¹⁹ cm^-3 and 0.6 eV, respectively. Judging from the Mg implantation condition, the obtained values are considered to be reliable.

Abstract: In this work, a partial amorphization is introduced to form a Nickel silicide ohmic contact for 4H-SiC bottom electrode. In a conventional Nickel silicide electrode, a carbon agglomeration at the silicide/SiC interface has been occured, and contant resistance between Ni silicide and SiC substrate became larger. For the reduction of the contact resistance, the partial amorphization of surface of SiC substrate was introduced. By this partial amorphization, the space position of the carbon agglomeration is controlled, and contact resistance can be reduced. As a result, with an amorphous 100 nm line pattern, a reliable contact resistance of 1.9×10^-3Ωcm² was realized.

Abstract: This paper presents a detailed physical and electrical analysis of 4H-SiC ohmic contacts to p-type material, the main aim being to examine their ruggedness under high temperature conditions. XRD, FIB-TEM and SEM are techniques that have been utilized to examine the microstructure and interface properties respectively. A detailed physical study revealed the presence of a crystalline hexagonal Ti layer orientated in the same direction as the 4H-SiC epitaxial layer. This factor seems to be important in terms of electrical performance, having the lowest measured specific contact resistivity of 1x10^-6 Ωcm². We attribute this to the optimized formation of Ti₃SiC₂ at the metal/SiC interface. An initial high temperature study shows thermionic emission occurring across the metal/semiconductor junction.

Abstract: Distribution of chemical composition in nickel-based ohmic contacts to n-type 4H-SiC was investigated with XEDS mapping performed on plan-view and cross-sectional TEM samples. Obtained results indicate that local deviations in stoichiometry from that of Ni₂Si phase significantly contribute to degradation of surface morphology.

Abstract: A high inversion channel mobility is a key parameter of normally off Silicon-Carbide MOS field effect power transistors. The mobility is limited by scattering centers at the interface between the semiconductor and the gate-oxide. In this work we investigate the mobility of lateral normally-off MOSFETs with different p-doping concentrations in the channel. Additionally the effect of a shallow counter n-doping at the interface on the mobility was determined and, finally, the properties of interface traps with the charge pumping method were examined. A lower p-doping in the cannel reduces the threshold voltage and increases the mobility simultaneously. A shallow counter n-doping shows a similar effect, but differences in the behavior of the charge pumping current can be observed, indicating that the nitrogen has a significant effect on the electrical properties of the interface, too.

Abstract: We report on the electrical characteristics of Ni/4H-SiC Schottky contacts fabricated on a Ge-doped 4H-SiC epilayer. The morphology and the current mapping carried out by conductive atomic force microscopy on the epilayer allowed observing nanoscale preferential conductive paths on the sample surface. The electrical characteristics of Ni contacts have been studied before and after a rapid thermal annealing process. A highly inhomogeneous Schottky barrier was observed in as-deposited diodes, probably related to the surface electrical inhomogeneities of the 4H-SiC epilayer. A significant improvement of the Schottky diodes characteristics was achieved after annealing at 700°C, leading to the consumption of the near surface epilayer region by Ni/4H-SiC reaction. After this treatment, the temperature behavior of the ideality factor and Schottky barrier height was comparable to that observed on commercial 4H-SiC material.

Abstract: This paper presents a physical model based on interface traps to explain both the larger barrier heights of practical Schottky diodes in comparison to the theoretically expected values and the appearance of a knee in the log I–V characteristics. According to this model, acceptor-type interface traps near the valance band increase the Schottky barrier height, which shifts the log I–V characteristic to higher forward-bias voltages. In addition to the acceptor traps, donor-type interface traps can appear near the conduction band, and when they do, they cause the knee in the log I–V characteristics as their energy level falls below the Fermi level and the charge associated with these traps changes from positive to neutral.

Abstract: Si wafers were directly bonded to 6H-SiC wafers without the formation of an intermediate layer. Heterojunctions of n-Si/n-SiC and p-Si/n-SiC exhibited ohmic and rectifying characteristics, respectively, as expected based on their band lineups. Band bending of Si at the bonded interface was observed for Si/semi-insulating 6H-SiC heterointerfaces. This band bending can be explained by either heterojunction formation based on Anderson’s model or the existence of negative charge with a density of ~2 × 10¹⁰ cm⁻² at the Si/SiC interface.