Materials Science Forum Vol. 924

Abstract: In the last decade, silicon carbide (SiC) has gained a remarkable position among wide bandgap semiconductors due to its high temperature, high frequency, and high power electronics applications. SiC heterostructures, based on the most prominent polytypes like 3C-SiC, 4H-SiC and 6H-SiC, exhibit distinctive electrical and physical properties that make them promising candidates for high performance optoelectronic applications. The results of simulations of nn-junction 3C-4H/SiC and 6H-4H/SiC heterostructures, at the nanoscale and microscale, are presented in this paper. Nanoscale devices are simulated with QuantumWise Atomistix Toolkit (ATK) software, and microscale devices are simulated with Silvaco TCAD software. Current-voltage (IV) characteristics of nanoscale and microscale simulated devices are compared and discussed. The effects of non-ideal bonding at the heterojunction interface due to lattice misplacements (axial displacement of bonded wafers) are studied using the ATK simulator. These simulations lay the groundwork for the experiments, which are targeted to produce either a photovoltaic device or a light-emitting diode (working in the ultraviolet or terahertz spectra), by direct bonding of SiC polytypes.

Abstract: First objective of present contribution is to supply a compact description of two thickness-dependent characteristics of state of the art as-grown 3C-SiC/Si templates: structural quality and surface morphology. Second objective is to point out the benefits of surface planarization and indicate the temperature limits of thermal treatment that can be applied to polished 3C-SiC/Si templates without deterioration of planarization results. We believe that this kind of overview, based on our long term experience in fabrication of 3C-SiC/Si templates, may be useful for the scientific community working on future applications of cubic silicon carbide polytype.

Abstract: Recently the intense terahertz electroluminescence from monopolar n⁺⁺–n^– –n⁺ structures of 6H- and 8H-SiC of natural superlattices at helium temperatures due to Bloch oscillations was discovered. In the present work we present the THz emission spectra of bipolar n⁺⁺–π–n⁺ structures (π is a high-resistance layer of p-type conductivity) of natural superlattices 4H-, 8H- and 15R-SiC at 7 K. The bipolar n⁺⁺–π–n⁺ structures of 4H- and 8H-SiC were analogous to those of structures for which the negative differential conductivity effect was observed earlier for three polytypes (4H, 6H and 8H) at T=300 K. We demonstrate resemblance and differences of the spontaneous THz emission spectra for the monopolar and bipolar 4H-, 6H- 8H- and 15R-SiC natural superlattices caused by Bloch oscillations of electrons in the SiC natural superlattice.

Abstract: The optical response of graphene on 6H-SiC was investigated by means of IR-reflectance measurements. Thereby, the anisotropy of the substrate is considered and its influence was studied by performing measurements with s- and p-polarized light. The anisotropy causes a splitting of the reststrahlen band in p-polarization, but does not affect spectra recorded with s-polarization. In both cases a thin film approximation was used to simulate the reflectance spectra. A model consisting of SiC, graphene and air enables the extraction of the graphene layer count.

Abstract: The silicon carbide cubic polytype (3C-SiC) is perfectly appropriate to fabricate microelectromechanical systems. However, for such applications, the stress can largely influence both the fabrication of 3C‑SiC‑based microsystems and their related mechanical properties. Accordingly, in this study, we investigated the influence of strong aluminum incorporation towards the mechanical properties of 3C-SiC epilayers grown on silicon substrates.

Abstract: Band structure calculations for radiofrequency-sputtered AlN-films doped with various 3d-transition-metals (TM: V, Cr, and Mn) were conducted to investigate the origin of the characteristic optical absorption structures. Experimentally evaluated crystal structures and lattice constants of the synthesized films were adopted for supercells. The model calculations showed that additional energy bands mainly consisting of 3d e and t states of TMs are formed in the band gap of AlN (6.2 eV), and that their potentials depend on the TM species. It was also shown that the Fermi levels of Cr- and Mn-doped AlN lie within the spin-up t band, while the Fermi level of V-doped AlN lies between the spin-up e and t bands. These findings imply that the materials have TM species-dependent, multiple absorption paths with lower energy than the band gap energy of AlN, resulting in optical absorption in the near-ultraviolet, visible, and infrared regions.

Abstract: The electrical activation of 1×10²⁰ cm^-3 implanted Al in 4H-SiC has been studied in the temperature range 1500 - 1950 °C by the analysis of the sheet resistance of the Al implanted layers, as measured at room temperature. The minimum annealing time for reaching stationary electrical at fixed annealing temperature has been found. The samples with stationary electrical activation have been used to estimate the thermal activation energy for the electrical activation of the implanted Al.

Abstract: The physics and technology of metal/semiconductor interfaces are key-points in the development of silicon carbide (SiC) based devices. Although in the last decade, the metal to 4H-SiC contacts, either Ohmic or Schottky type, have been extensively investigated with important achievements, these remain even now an intriguing topic since metal contacts are fundamental bricks of all electronic devices. Hence, their comprehension is at the base of the improvement of the performances of simple devices and complex systems. In this context, this paper aims to highlight some relevant aspects related to metal/semiconductor contacts to SiC, both on n-type and p-type, with an emphasis on the role of the barrier and on the carrier transport mechanisms at the interfaces.

Abstract: We propose the Si-vapor ambient anneal as a cap-free activation annealing (A/A) method for Silicon Carbide (SiC) using Tantalum Carbide / Tantalum composite materials (TaC/Ta). This method prevents the roughening of SiC surface by controlling the process function without conventional Carbon (C)-cap [1,2]. In this report, we evaluated the warping behavior of SiC wafer to confirm the effect of ion implantation (I/I) temperature (T_I/I) and epi-ready treatment using Si-vapor ambient anneal. Wafer warp suppressing effect of high temperature I/I was confirmed and large wafer warpage occurred due to thinning of the wafer thickness. Furthermore we also observed the simultaneous improvement of the sharp edge shape and sidewall roughness of the trench under the appropriate conditions of the Si-vapor ambient anneal. It is possible to shape the round shape of the trench edge and to improve the roughness of trench sidewall by Si-vapor ambient anneal simultaneously with activation annealing process.

Abstract: We investigate electrical characteristics of the pn-diode fabricated using the epitaxial films on the surface damage-free 4H-SiC (0001) Si-face 4° off-cut wafers prepared by the completely thermal-chemical etching process; Si-vapor etching (Si-VE) technology. The forward and reverse current-voltage (I-V) characteristics of pn-diodes correlated to the epitaxial defects are discussed. The device at the defect-free area includes 11 % failed diodes on the chemo-mechanical polishing (CMP) wafer while 0 % on the Si-VE wafer. The latent scratches and mechanical damages, which increase the forward and reverse leakage current of the pn-diodes, are completely removed by the Si-VE. The Si-VE exposes the carbon inclusions in the wafer to form the small bump which ends up with the larger bump defect on the epitaxial surface. These bumps cause leak current of the forward characteristics while all of the reverse characteristics are normal. The epitaxial film on the Si-VE surface has less density of the basal plane dislocations (BPDs) than the conventional CMP. It is hard to recognize the safe device on the CMP wafer without additional reliability test. The Si-VE wafer shows the apparent breakdown voltage fail on every small-number diode including BPDs under the simple test. It is considered that the Si-VE is possible to reduce ambiguity of the device characteristics under the relationship with the defects in comparison with the CMP.