Modulation in current density of metal/n-SiC contact by inserting Al2O3 interfacial layer

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.


Background
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 [1]. 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 [2].
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 [3], 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 a previous study, Connelly et al. [4] have raised a method to reduce the SBH of the metal/Si contact by using a thin Si 3 N 4 through the creation of a dielectric dipole [5]. Similar researches have been dedicated to the study of the SBH modulation on Ge [6][7][8][9], GaAs [10], InGaAs [10,11], GaSb [12], ZnO [13], and organic material [14] by inserting different dielectrics or bilayer dielectrics. According to the bond polarization theory [15], an electronic dielectric dipole is formed between the inserted insulator and semiconductor native oxide which results in a shift of the SBH, as Figure 1 depicts. The origin of the dipole formation at the dielectric/SiO 2 interface is described in Kita's model [16], and in this model, the areal density difference of oxygen atoms at the dielectric/SiO 2 interface is the driving force to form the dipole. Since the areal density of oxygen atoms (σ) of Al 2 O 3 is larger than that of SiO 2 , the σ difference at the interface will be compensated by oxygen transfer from the higher-σ to the lower-σ oxide which creates oxygen vacancies in the higher-σ oxide (Al 2 O 3 ) and negatively charged centers in the lower-σ oxide (SiO 2 ), and the corresponding direction of the dipole moment is from SiO 2 to Al 2 O 3 . As a result, this dipole is a positive dipole which can reduce the SBH and therefore increases the current density. As the thickness of the inserted insulator increases, it becomes more difficult for the current to tunnel through the insulator, and the tunneling barrier is the dominant factor of the total barrier height, which decreases the current density in the end.
In this work, we demonstrate the modulation of the current density in the metal/n-SiC contact by inserting a thin Al 2 O 3 layer into a metal-insulator-semiconductor (MIS) structure. Al 2 O 3 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 [15] and Kita's model [16]. 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 Al 2 O 3 /SiC interface during the formation of Al 2 O 3 was obtained with X-ray photoemission spectroscopy (XPS), and the electrical properties of Al/ Al 2 O 3 /SiC with different thicknesses of the inserted Al 2 O 3 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 Al 2 O 3 thickness, we carefully varied the Al 2 O 3 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.

Methods
We prepared an Al/Al 2 O 3 /SiC MIS structure on n-type C-terminated 6H-SiC with a carrier concentration of 1 × 10 16  surface as the top contact through shadow masks, and back side contact was also formed through the evaporation of Al. The MIS structure is depicted in Figure 2a. Figure 2b is a cross-sectional transmission electron microscope (TEM) image of Al/Al 2 O 3 /SiC which presents that Al 2 O 3 was uniformly deposited as a fully amorphous film.
In order to determine the generation of SiO 2 and the content ratio of SiO 2 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 [17] 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. and Si-O bonds (102.8 eV, FWHM = 2.27 eV). As illustrated in Figure 3a,b,c,d, all the Si 2p3 spectrum samples have a Si-C peak which associates with SiC from the substrate. Si-O species indicates that SiO 2 exists at the Al 2 O 3 /SiC interface. This SiO 2 is probably generated from SiC-heated substrate oxidized by Al 2 O 3 since all the samples have been completely cleaned before the ALD process. Figure 4 demonstrates the evolution in the content ratio of SiO 2 and SiC which is calculated by using the area of Gaussian fitting curve of the Si-O bond divided by the area of Gaussian fitting curve of the Si-C bond. It clearly and deliberately shows that the content of SiO 2 oxidized by Al 2 O 3 reaches an increase at the Al 2 O 3 thickness of 1.98 nm. The content ratio of SiO 2 / SiC stays nearly at 17% in the Al 2 O 3 film with the thickness beyond 1.98 nm. However, the content ratio of SiO 2 /SiC increases to 21.58% at the Al 2 O 3 thickness of 2.32 nm and almost remains around 21.89% at the Al 2 O 3 thickness of 3.59 nm and thicker samples. The content ratio of SiO 2 /SiC rises by about 24% from the 1.98-nm sample to the 2.32-nm sample, which is possibly due to the fact that the well-oxidized SiO 2 begins to generate when the Al 2 O 3 thickness is thicker than 1.98 nm.

Results and discussion
The I-V characteristics of the Al/Al 2 O 3 /SiC MIS structure were measured by the circuit connections of the back-to-back Schottky diode as illustrated in Figure 5a. One advantage of the back-to-back diode measurement is that the large resistance contributed from the series resistance and the large resistance caused by the substrate can be eliminated. Another advantage is that both in positive and negative biasing, only the reverse current is measured, and fortunately, the change in reverse saturation current reflects the characteristic of the contact where maximum reverse saturation current is desired for ohmic contacts. Figure 5b shows the I-V characteristics of an Al/ Al 2 O 3 /SiC diode with different thicknesses of Al 2 O 3 . Reverse bias current first decreases due to the increase of Al 2 O 3 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 Al 2 O 3 and SiO 2 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 Al 2 O 3 thickness increases owing to the large tunnel barrier induced by the thick Al 2 O 3 film. The experimental I-V characteristics clearly indicate that current density is effectively modulated with the insulator's thickness.
Contact resistance (R C ) of the Al/Al 2 O 3 /SiC MIS structure was further evaluated through contact end resistance method [20]. R C involves two resistances in a series: a tunneling resistance (R T ) due to the insulator and a resistance (R SB ) caused by the Schottky barrier. When the thickness of Al 2 O 3 is thinner than 1.98 nm, the dipole was not completely formed, and as a result, the inserted insulator blocks the current. In this range, along with the increase of the insulator, the contact resistance increases. According to the XPS result discussed above, the electronic dielectric dipole begins to create at the thickness of 1.98 nm. The formation of the dipole at the interface reduces the tunneling barrier and then raises the current across the contact in a reasonable region. Figure 6 shows the R C versus the thickness of Al 2 O 3 , which provided that the contact resistance is modulated by the thickness of the insulator. It is interesting to find that there exists a trough because of the trade-off between a reduced barrier by the electronic dielectric dipole and an increased tunneling resistance by the accretion of the insulator's thickness.

Conclusions
In this work, we successfully realize the modulation of current density at the metal/SiC contact by inserting a thin Al 2 O 3 layer between the metal and semiconductor. By varying the thickness of Al 2 O 3 , 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.