Microstructural and electrical characterization of Si/4H-SiC heterojunction diodes

Abstract

With the growing demand for high power, highly efficient, and fast switching power electronics, silicon carbide (SiC) is besides gallium nitride (GaN) the substrate material of choice due to its outstanding properties. For this stongly growing field of application SiC offers great potential, as this compound semiconductor has a wide band gap, a high thermal conductivity, a high mechanical strength as well as a high chemical resistance. In the last years, SiC wafers entered a price regime suited even for mass production and became available in up to 6-inch diameter with very low defect densities. Almost all conventional silicon (Si) based devices could be replicated in SiC technology. One of the most important and quite simple structures is the metal-semiconductor, or Schottky contact. This thesis investigates the potential when combining the well-established semiconductor Si with the wide band gap semiconductor 4H-SiC. The use of Si as contact material on 4H-SiC allows an adjustment of the Schottky barrier height in a wide range, by changing the Si doping concentration. In addition, the temperature stability and the overall temperature budget during device fabrication are enhanced, since most metal contacts alloy with SiC forming silicides, whereas Si is stable on SiC up to 1000 °C and even above. Heterojunction diodes based on Si/4H-SiC, which act as Schottky junctions, are fabricated using different growth and interface preparation techniques and are characterized electrically in a wide temperature range. The first step was to investigate the growth of Si on monocrystalline 4H-SiC. By applying sputter-deposition techniques at temperatures below 600 °C only amorphous Si is grown. Investigations on the influence of different post deposition annealing steps at temperatures up to 1100 °C were performed to achieve recrystallization of the Si thin films on 4H-SiC. A different approach to enable the realization of low-temperature crystalline Si on 4H-SiC is metal-induced crystallization. By applying aluminum serving as crystallization promotor, the recrystallization temperature of Si on 4H-SiC could be reduced to about 200 °C. Disadvantageous is, however, that the homogeneity of the achieved films still needs further improvement. The direct growth of crystalline Si was studied using low-pressure chemical vapor deposition. A strong influence of the deposition temperature on the grain size and the preferred growth orientation is observed. Microstructural investigations of the samples were mainly performed with state-of-the-art scanning electron microscopy, transmission electron microscopy, and X-ray diffractometry. Apart from the different growth techniques, the influence of interface pre-conditioning is investigated. Argon ion bombardment of the 4H-SiC surface prior to Si deposition shows promising results of Schottky barrier height tuning. Also the influence of amorphous a-SiC:H interface layers, with different thickness values below 4 nm, on the performance of conventional Ti/4H-SiC Schottky diodes was investigated. The impact of Schottky barrier inhomogeneities was found to be strong at p-Si/4H-SiC heterojunctions due to their large barrier height. A new fitting procedure based on Tung’s model was applied to extract the density of the interface inhomogeneities in a wide temperature range. All in all, the findings of this thesis proved the well-rectifying properties of the Si/4H-SiC heterojunction and the manufacturability using standard silicon micro technologies. Additionally, many theoretical and practical results of the growth and the electrical behavior significantly strengthened the knowledge about the Si/4H-SiC heterojunction interface.