Modulation in current density of metal/n-SiC contact by inserting Al2O3 interfacial layer
© Zheng et al.; licensee Springer. 2013
Received: 4 November 2012
Accepted: 31 January 2013
Published: 2 March 2013
Metal contact to SiC is not easy to modulate since the contact can be influenced by the metal, the termination of the SiC, the doping, and the fabrication process. In this work, we introduce a method by inserting a thin Al2O3 layer between metal and SiC to solve this problem simply but effectively. The Al2O3/n-SiC interface composition was obtained with X-ray photoemission spectroscopy, and the electrical properties of subsequently deposited metal contacts were characterized by current–voltage method. We can clearly demonstrate that the insertion of Al2O3 interfacial layer can modulate the current density effectively and realize the transfer between the Schottky contact and ohmic contact.
KeywordsContact resistance Schottky barrier height SiC Atomic layer deposition
Silicon carbide is a promising material for numerous electronic applications due to its wide bandgap, high breakdown electric field, high thermal conductivity, and high saturation velocity . These excellent properties make SiC suitable for high-temperature, high-power, and high-frequency applications. For high-performance and high-frequency devices in these applications, metal/SiC contact plays very important roles. However, the traditional method for fabricating Schottky contact and ohmic contact are so different, and it will unavoidably add to the processing difficulty and cost .
The Schottky barrier height (SBH) is the key factor that determines whether the electrical behavior is an ohmic contact or Schottky contact: a low SBH is necessary to create a good ohmic contact, while a large SBH is required to form a good Schottky contact. According to the thermionic emission model , the direct reflection of the SBH is the reverse current density, and therefore, by controlling the Schottky barrier height, we can modulate the current density and acquire the needed contact type without modifying the fabrication process.
In this work, we demonstrate the modulation of the current density in the metal/n-SiC contact by inserting a thin Al2O3 layer into a metal-insulator-semiconductor (MIS) structure. Al2O3 is chosen as the interfacial insulator for its large areal oxygen density (σ) which means that the formation of dipole is much stronger and shifts the SBH more effectively than that induced by other insulators based on the bond polarization theory  and Kita’s model . As for the choice of metal, aluminum (Al) is suitable due to its low work function (4.06 to 4.26 eV) for the investigations of the Fermi level shift toward the conduction band of SiC (electron affinity = 3.3 eV).
The analysis of the Al2O3/SiC interface during the formation of Al2O3 was obtained with X-ray photoemission spectroscopy (XPS), and the electrical properties of Al/ Al2O3/SiC with different thicknesses of the inserted Al2O3 were characterized by current–voltage (I-V) method. Since the current density as well as the contact resistance was found to be sensitive to the Al2O3 thickness, we carefully varied the Al2O3 thickness from 0.97 to 6.3 nm and finally have acquired the experiment results that can describe the modulation of current density by changing the thickness of the insulator.
In order to determine the generation of SiO2 and the content ratio of SiO2 and SiC, the XPS method is used. XPS experiments were carried out on a RBD-upgraded PHI-5000C ESCA system (PerkinElmer, Waltham, MA, USA) with Mg Kα radiation (hν = 1,253.6 eV), and the base pressure of the analyzer chamber was about 5 × 10−8 Pa. Ar ion sputtering was performed to clean the sample in order to alleviate the influence of carbon element in the air. Samples were directly pressed to a self-supported disk (10 × 10 mm) and mounted on a sample holder, then transferred into the analyzer chamber. The whole spectra (0 to 1,100 eV) and the narrow spectra of Si 2p, O 1s, C 1s, and Al 2p with much high resolution were both recorded, and binding energies were calibrated using the containment carbon (C 1s = 284.6 eV). Since the XPS spectra obtained consist of numerous overlapping peaks, curve fitting is necessary to separate the peaks from each other. The binding energies for the species were all correlated to the binding energies determined from standards in the handbook of XPS  and earlier studies [18, 19]. These standards were also used to determine the full width at half-maximum (FWHM) and band type for curve fitting of multicomponent spectra, and it was found that the Gaussian distribution was the best model. Background removal was adopted according to the Shirley model and performed prior to curve fitting.
Results and discussion
Figure 5b shows the I-V characteristics of an Al/ Al2O3/SiC diode with different thicknesses of Al2O3. Reverse bias current first decreases due to the increase of Al2O3 thickness which can block off the current and then has its minimum at the thickness of 1.98 nm which is suitable for the Schottky contact. When keeping on increasing the thickness, the reverse current rises since the formation of positive dipole between Al2O3 and SiO2 pulls down the SBH, and then, the reverse current reaches its maximum at the thickness of 3.59 nm which is suitable for ohmic contact. Next, the reverse current decreases as Al2O3 thickness increases owing to the large tunnel barrier induced by the thick Al2O3 film. The experimental I-V characteristics clearly indicate that current density is effectively modulated with the insulator’s thickness.
In this work, we successfully realize the modulation of current density at the metal/SiC contact by inserting a thin Al2O3 layer between the metal and semiconductor. By varying the thickness of Al2O3, we can acquire the ideal current density and contact resistance based on our demands and achieve a transfer between Schottky contact and ohmic contact. The mechanism appears to be the coaction of a positive dielectric dipole decreasing the barrier and the tunneling resistance increasing the barrier. Consequently, this is a promising method to increase the performance of SiC electronic applications.
This work was supported by the NSFC (61076114, 61106108, and 51172046), the Shanghai Educational Develop Foundation (10CG04), SRFDP (20100071120027), the Fundamental Research Funds for the Central Universities, and the S&T Committee of Shanghai (1052070420).
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