Materials Science Forum Vol. 806

Abstract: This work reports on the improvement of ohmic contacts made on heavily p-type doped 4H-SiC epitaxial layer selectively grown by Vapor-Liquid-Solid (VLS) transport. Even before any annealing process, the contact is ohmic. This behavior can be explained by the high doping level of the VLS layer (Al concentration > 10²⁰ cm^-3) as characterized by SIMS profiling. Upon variation of annealing temperatures, a minimum value of the Specific Contact Resistance (SCR) down to 1.3x10^-6 Ω.cm² has been obtained for both 500 °C and 800 °C annealing temperature. However, a large variation of the SCR was observed for a same process condition. This variation is mainly attributed to a variation of the Schottky Barrier Height.

Abstract: We report on the stabilization of 15R Silicon Carbide (15R-SiC) grown by seeded sublimation method. It was found that polytype transitions are directly related to the occurrence of facets on the grown crystals. Once a foreign polytype is formed, its propagation is governed mainly by the interface shape of the crystal and its evolution during growth. A concave crystal shape enhances the expansion of foreign polytypes, usually formed at the periphery of the crystal. Then, foreign polytypes can either overlap the original polytypes (constant concave crystal shape) or form inclusions (change to convex crystal shape). On the contrary, an initially slightly convex interface repels foreign polytypes towards the edge of the crystal. The optimization of the growth interface shape can be a key issue towards the stabilization of bulk 15R-SiC crystals.

Abstract: Investigation of excess carrier dynamics in a 15R-SiC bulk layer grown by physical vapour transport (PVT) on 15R-SiC substrate has been carried out using pump-probe techniques: an interband carrier injection by a picosecond laser pulse and measuring the induced absorption and diffraction of a probe beam. For this task, differential transmittivity (DT) and light induced transient grating (LITG) techniques were used. Room temperature carrier lifetime varied in the 3 ns 8 ns range at excess carrier densities above ΔN₀ = 7×10¹⁷ cm^-3 and was ascribed to the recovery time of optically recharged carrier traps, and their activation energy of E_a = 75 meV was determined. The presence of recharged traps caused the injection-dependence of the diffusion coefficient D, whereby its value dropped below 0.1 cm²/s at ΔN₀ < 1×10¹⁸ cm^-3 and gradually increased up to 0.7 cm²/s at higher injections. At elevated temperatures (300 K < T < 700 K), when the traps are thermally activated, the diffusivity increased up to ~ 1.5 cm²/s and was independent on ΔN₀. The overgrown layer parameters were comparable to those of the used 15R PVT seed.

Abstract: So far, most of the SiC homoepitaxy has been realized on 8-deg-off and 4-deg-off substrates, whereas GaN heteroepitaxy is done on SiC on-axis (up to 0.3 deg off). As 6-inch SiC wafers are being introduced into the market, a decrease of the substrate off-cut to 2 deg for SiC homoepitaxy is desirable to reduce the manufacturing costs. If both, GaN heteroepitaxy and SiC homoepitaxy are successful on 2-deg-off SiC substrates, this would pave the way to monolithic integration of both kinds of devices, as well as to obtain cheap insulating SiC substrates for AlGaN/GaN microwave and THz devices. In this work, we present our present status of AlGaN/GaN growth on SiC 2-deg off. Comparing to the on-axis situation, we obtained similar structural (XRD and TEM data) and electrical characteristics, but not morphological ones. Therefore, we propose two ways of a decrease of AlGaN surface roughness when grown on SiC 2 deg-off: i) by planarization, ii) by lateral patterning.

Abstract: The aim of this paper is to optimize the epitaxial layer structure of an AlGaN/GaN high electron mobility transistor (HEMT) for high power density at high frequency. The idea is to play on the polarization engineering with the different layers of the epitaxial stack. The influence of the cap and barrier layer thicknesses, the aluminum content in the barrier and the insertion of an AlGaN buffer layer are studied through the electron gas density, electron mobility and sheet resistance. This permits to find out the best trade-off in order to satisfy the requirements for high performances.

Abstract: The growth of graphene on 3C-SiC/Si heterostructure is a promising approach, which provides low production cost, high scalability and easiness of nanoelectromechanical system fabrication. However, the quality of graphene is still insufficient for device applications due to mediocre morphological and structural quality of the 3C-SiC epilayers compared to bulk SiC crystals and to excessive Si out-diffusion from the Si substrate. Here, we propose a solution of inserting a 4H-AlN layer between 3C-SiC and Si, which allows us to polish the 3C-SiC film without worrying about enhancement of the Si out-diffusion despite the thinning after the polishing. With this insertion, a considerable quality improvement is achieved in our graphene on silicon.

Abstract: Electrochemical immunosensor devices comprise of an antibody immobilised onto a semiconducting or conducting substrate. The use of epitaxial graphene in immunosensors allows for the detection of an antigen specifically bound to the immobilised antibody by monitoring the current modulation of lithographically fabricated graphene channel devices. Multilayer epitaxial graphene (MEG) was produced on semi-insulating 4H-SiC(0001) substrates by annealing at 1700°C at 1x 10^-5 mbar using a graphite cap. Thickness and morphology of the graphene was studied using Raman spectroscopy, XPS, AFM, and SEM. Selective areas of graphene were targeted for modification by adding a protective window of PMMA. In order to immobilise the antibody to the graphene substrate, an amine-terminated surface is required. (3-aminopropyl) triethoxysilane (APTES), is used to achieve amine termination, which is itself bound to a hydroxyliated graphene surface. Hydroxylation was achieved using Fenton chemistry and changes in surface hydrophobicity are confirmed using contact angle measurements. Attachment of APTES to the hydroxyl terminated graphene channel was confirmed using cyclic voltammetry (CV), XPS, and Raman spectroscopy. This functionalization method can be used to attach any antibody to the graphene substrate that can bind to an amine group. This platform is therefore easily adaptable for the fabrication of a range of immunosensor devices for the detection of different biomarkers.

Abstract: In this paper, the electronic transport in epitaxial graphene (EG) grown on the Si face of 8° off-axis 4H-SiC has been investigated, using both electrical characterization of macroscopic devices and conductive atomic force microscopy (CAFM). In particular, current measurements on linear transmission line model (TLM) structures with different orientations showed a current transport anisotropy related to steps orientation, with the resistance of EG in the direction orthogonal to the steps ~2× higher than in the parallel direction. Two dimensional morphology and current maps in EG over the stepped SiC surface were obtained by CAFM and revealed a local resistance increase of EG over the (11-2n) facets with respect to the (0001) basal planes. This effect allows to account for the observed macroscopic current transport anisotropy and can be explained in terms of a different interface nature between EG and SiC on the two faces, leading to a locally different substrate induced doping of EG.

Abstract: Silicon photovoltaic cells require anti-reflection treatments in order to minimise optical losses and improve cell efficiencies. Commercially, the silicon surface is textured using a chemical etchant followed by the addition of an anti-reflective coating to further suppress reflectivity. We present a process using metal assisted etching to create porous silicon features capable of reducing reflectivity to less than 5%. A method for producing porous silicon using Substrate Conformal Imprint Lithography (SCIL) has been developed in order to pattern the nanoscale anti-reflective structures onto silicon wafers.

Abstract: This work reports on the fabrication and electrical characterization of 3 different diodes. The first one is a Schottky diode with a single 50 mm P⁺ ring between the edge termination and the active area. The two other diodes are JBS with a 3 mm P⁺ strips separated by 4 mm and 8 mm respectively. The breakdown voltage ranges from 2.7kV up to 3.7kV depending on the P⁺/N area. The 3 different diodes exhibit a similar on-resistance versus the temperature behavior. Moreover, no contribution of the bipolar conduction is observed and no degradation has been observed when a forward stress is performed in forward mode and also in reverse.