Improvement of Electrical Properties of Metal/SiC Contact by Plasma Pretreatments
|School||Dalian University of Technology|
|Course||Microelectronics and Solid State Electronics|
|Keywords||Silicon Carbide Ohmic contact Schottky contact Surface state density Barrer height|
Silicon carbide (SiC) is a promising candidate for applications in high temperature, high voltage, high power electronic devices because of its outstanding properties such as wide band gap, high critical electric field and high thermal conductivity. Metal/SiC contacts, serving as Ohmic contacts or rectifying contacts form an intrinsic and vital part of SiC devices. The efficiency, gain and switching speed of these devices are strongly depended on the quality of Metal/SiC contacts. Therefore, the formation of good Ohmic contacts and Schottky contacts on SiC is a key factor in improving the electrical performance and reliability of SiC devices. The electrical properties of metal-semiconductor contact not only depend on the work function of metal and the doping concentration of semiconductor, but also on the surface states of semiconductor. Compared to other semiconductors like Si and GaN, the surface states on SiC surfaces are much more complex. For example, SiC surfaces are still contaminated by C and O impurity ions after the traditional RCA cleaning. Consequently, the electrical properties of metal/SiC contacts are strongly influenced by the surface properties of SiC. Therefore, it is of great importance to improve the properties of SiC Ohmic and Schottky contacts through developing new surface treatment technology to improve the surface properties of SiC.In this paper, low-temperature low-damage electronic cyclotron resonance (ECR) microwave hydrogen plasma treatment (HPT) was employed to improve the properties of SiC surface. With the HPT, the improvements in electrical characteristics of metal/SiC contacts were studied. Also, the corresponding mechanisms and related theories were further investigated. The main research contents and results are as follows:1. The improvements in the properties of4H-SiC surfaces with the HPT have been studied. The effects of the HPT on the structure, morphology, chemical and electronic properties of surfaces were characterized by in-situ reflection high energy electron diffraction (RHEED), Atomic Force Microscope (AFM) and X-ray photoelectron spectroscopy (XPS). With the HPT, RHEED results indicate that smooth, atomically ordered, unreconstructed SiC surfaces are achieved. AFM results display that RMS is reduced to be as low as0.268nm. The XPS results show that the surface oxygen is greatly reduced and the carbon contaminations are completely removed from the4H-SiC surfaces. The hydrogenated SiC surfaces exhibit an unprecedented stability against oxidation in the air with the density of surfaces states as low as1010cm-2eV-1scale. The removal of surface contaminations and the reduction of surface state density would be significant for improving the properties and reliabilities of SiC devices.2. The improvements in the Ohmic properties of Ti/n-type4H-SiC contact with the HPT and the corresponding mechanism have been studied. It is found that Ti Ohmic contacts to relatively highly doped (1×1018cm-3) n-type4H-SiC with low resistivity have been produced without high temperature annealing. To elucidate the Ohmic formation mechanism of Ti contact to4H-SiC with the HPT, the electrical properties of metals with different work functions contact to SiC with different doping concentrations before and after the HPT are systematically investigated. The experimental results show that the Ohmic behavior of Ti contact is mainly attributed to low barrier height at Ti/4H-SiC contact interface. The HPT releases the Fermi level from pinning by reducing surface state density. Consequently, barrier height is significantly decreased to as low as0.41eV by the low surface state density together with the effects of band-gap narrowing, image-force and thermionic-field emission at relatively high doping. The formation of SiC Ohmic contact without high temperature annealing and high doping concentration would be helpful for improving the stability of SiC electronic devices and reduce the production cost.3. The improvements in the rectifying properties of Ni, Pt/n-type4H-SiC contacts with the HPT and the corresponding mechanism have been studied. It is found the rectifying behaviors of Ni, Pt/4H-SiC contacts have been remarkably enhanced, and are optimized after annealing at400℃. In order to reveal the mechanism involved, XPS was employed to investigate the changes in the position of surface Fermi level and surface state density. XPS measurements show that the surface Fermi level moves toward the conduction band edge by the HPT. It almost attains the bulk Femi level position after annealing at400℃with the lowest density of surface states. Combining the observation of Ti/4H-SiC contact, the the correlation between the barrier height of metal/4H-SiC contact, the metal work function and the surface state density is discussed. It is found the experimental results obey the barrier height theory proposed by Cowley and Sze. The improvement of rectifying properties of metal/4H-SiC contact may help promote the development of Schottky contact related SiC electronic devices.4. The barrier inhomigeneities at metal/n-type4H-SiC contact has been studied. Through controlling the surface properties of4H-SiC by the HPT for different periods and annealing process, the Schottky barrier properties of Pt/4H-SiC contacts are investigated by current-voltage-temperature (I-V-T) and capacitance-voltage (C-V-T) measurements. Using the experimental results, the dependence of the barrier height and barrier inhomogeneities on the SiC surface properties is discussed and analyzed. It is found that the barrier height and barrier inhomogeneities of Pt contacts to4H-SiC appear to be strongly influenced by the surface properties of4H-SiC. The effective barrier height increases with decreasing the degree of Fermi level pinning. Electrically homogeneous contacts with barrier heights close to Bardeen limit and ideal Schottky limit are observed for the samples with "pinning" and "pinning free" of Fermi level, respectively. However, when the Fermi level is partially pinned, Gaussian distribution of inhomogeneous barrier height is found and the inhomogeneity decreases with reducing the degree of Fermi level pinning. An analysis model, by considering changes in the magnitude and spatial distribution of surface state density after different pretreatments, is proposed to clarify the correlation between the barrier height, the barrier inhomogeneties of metal/SiC, and the surface state density and the position of surface Fermi level of SiC. This study provides a clear insight into physical information on the barrier inhomogeneties of metal/SiC, which is of significance in controling the Schottky barrier of metal/SiC contact precisely and further improving the performance and reliability of SiC electronic devices.