Materials Science Forum Vols. 645-648

Abstract: We performed electrochemical deposition of ZnO on the surfaces of 4H-SiC epilayers and characterized Ni Schottky diodes fabricated on the same epilayers. We found correlation between positions where ZnO was deposited and positions where Schottky barrier height of Ni contacts is lower than of the rest of the contacts. Parts of the surface where ZnO was deposited were observed by AFM after removal of the ZnO layer, and we discussed the origin of the low Schottky barrier height from the AFM images.

Abstract: In this paper we describe how a merged pn Schottky diode (MPS diode) is capable to drive surge current levels far beyond the normal current of the diode and how to improve the device in order to achieve even higher surge current levels. For a sine half wave of 10 µs an 8A MPS diode (size: 2.52mm2) with conventional Al pad metallization shows surge current levels of greater than 500A, using Cu it can be increased to ~900A. For 10ms pulse length a different behaviour was observed, here diodes with Al pad metallization show a higher surge current level (80A) compared to Cu pad metallization (~ 40A). The root cause for this negative result at longer pulse time is based in a chemical interaction between Cu and the Schottky metal (Ti). Additionally, an outlook is given how Cu can contribute to improved surge current capability also at longer pulse lengths.

Abstract: The electrical characteristics of Au/3C-SiC Schottky diodes were studied and related to crystal defects. A structural analysis performed by transmission electron microscopy (TEM), combined with a current mapping of the surface by conductive atomic force microscopy (C-AFM), indicated that stacking faults (SFs) are the conductive defects having the biggest influence on the electrical properties of the Schottky barrier on 3C-SiC. Further, C-AFM current mapping of the semiconductor surface also showed that an ultraviolet (UV) irradiation process enables the electrical passivation of the SFs, due to their preferential oxidation. From current-voltage (I-V) measurements in diodes of different area (different amount of defects) it was observed that, for the non-irradiated surface, no significant dependence of the Schottky barrier height (ΦB) on the contact area could be observed. On contrast, after the UV-irradiation, ΦB gradually increases with decreasing contact area, ultimately leading to a nearly ideal value of the barrier height for the smallest diodes. The results indicate that even after the passivation of SFs there are still some electrically active defects contributing to deleterious conduction, responsible for a worsening of the electrical properties of the diodes.

Abstract: The electrical characteristics and the reliability of different oxides on the 4H-SiC Si-face for gate oxide application in MOS devices are compared under MOSFET operation conditions at room temperature, at 100°C and at 130°C. The oxides are either an 80nm thick deposited oxide annealed in NO or an 80nm thick grown oxide in diluted N2O. The deposited oxide shows significant higher QBD- and lower Dit-values as well as a stronger decrease of drain current under stress than the grown oxide. Although for the deposited oxide, the leakage current below subthreshold increases more than one order of magnitude during constant circuit stress at room temperature, for the thermal oxide it is quite constant, but at higher level for higher temperatures.

Abstract: The effect of ammonia (NH3) post-oxidation annealing (POA) technique on the reliability of thermal oxides grown on a n-type 4H-SiC (0001) face by dry oxidation has been investigated. Comparing other POA techniques using hydrogen and nitrous oxide gases, it was indicated that the NH3 POA after dry oxidation remarkably improves the insulating properties of thermal oxides. The mode value of field-to-breakdown for thermal oxides prepared by NH3 POA was 12.1 MV/cm. The charge-to-breakdown (QBD) in the NH3 POA sample was the highest in all samples, and the QBD value at 63% cumulative failure rate was 19.1 C/cm2. In addition, the NH3 POA maintained excellent electron trapping characteristics of thermal oxides against the electron injection.

Abstract: The consequences of thermal treatments in nitric oxide atmospheres on the characteristics of dielectric films / SiC structures was investigated by high-frequency capacitance-voltage measurements, X-ray photoelectron spectroscopy, and X-ray reflectometry techniques. It was observed that nitrogen incorporation in dielectric films / SiC structures leads to the formation of a thinner interfacial layer that contains carbon. This fact was related to the improvement of electrical properties of those structures.

Abstract: We report on the benefits and the shortcomings of the NO annealing process following observations made on capacitors and transistors with various nitrogen densities at the SiO2/SiC interface. While NO annealing leads to a progressively lower interface state density and higher inversion mobility, consistent with Coulomb-limited transport, MOSFET properties are still limited by the relatively poor interface quality. Moreover, NO induces a large amount of hole traps in the oxide. We establish that these properties are not related to the oxidation rate and we discuss them in terms of the nitrogen content.

Abstract: Boron (B) ions were implanted into 4H-SiC. In order to avoid the out-diffusion of B ions during the subsequent annealing process, two processing techniques were applied. Either a box-shaped B-profile was implanted, which was followed by a two-step annealing (900°C for 120 min + annealing temperature TA for 30 min), or a box-shaped B-profile was implanted together with two carbon (C) Gaussian profiles located on both edges of the B box-profile followed by a one-step annealing (TA for 30 min). The annealing temperature TA ranged from 1500°C to 1750°C. The electrically activated B acceptor concentration was measured by temperature-dependent Hall effect and the energy for the formation of the B acceptor was determined assuming a first order process.

Abstract: Manganese diffusion in 4H-SiC for possible spintronic applications is investigated. Ion implantation is used to introduce manganese in n-type and p-type 4H-SiC and subsequent heat treatment is performed in the temperature range of 1400 to 1800 °C. The depth distribution of manganese is recorded by secondary ion mass spectrometry and Rutherford backscattering spectrometry in the channeling direction is employed for characterization of crystal disorder. After the heat treatment, the crystal order is improved and a substantial rearrangement of manganese is revealed in the implanted region. However, no pronounced manganese diffusion deeper into the sample is recorded.

Abstract: Effects of implantation temperature on electrical properties of heavily-Al-doped 4H-SiC layer formed with Al implantation have been investigated. To form the p++ 4H-SiC with the original 4H-stacking structure, the implantation temperature above 175 °C is needed. A decrease in the implantation temperature below 250 °C leads to an increase in the NA-ND. It is suggested that an increase in the density of vacancies with a decrease in the implantation temperature promotes the Al substitution to lattice sites during activation annealing. The lower-temperature implantation also causes a decrease in activation energy for the p-type electrical conduction and a decrease in p-type ohmic contact resistivity. We presume that the increase in the Al acceptors at low-implantation temperatures gives expansion of the impurity bands and formation of valence band tail-states, causing the decrease in the impurity binding energy. The properties obtained with the lower-temperature implantation are desirable for practical applications especially at low temperatures.