Why wide bandgap semiconductor (SiC or GaN)?
Wide bandgap semiconductors are semiconductors that have a bandgap energy greater than the 1.1 eV bandgap of silicon. The larger bandgap allows these materials to be electrically active at higher temperatures than silicon, allows greater band gap tuning for photonic devices, and these materials typically have higher electrical breakdown fields.  Silicon carbide (SiC) and gallium nitride (GaN), are two desireable wide bandgap semiconductors for robust, high-temperature, high-power, and high-frequency applications.  Next-generation device will require next-generation semiconductor substrates like SiC and GaN.

Hydrogen cleaning of 6H-SiC
One disadvantage of wide bandgap 6H-SiC, is that the commercially available substrates are not as perfect or defect-free as silicon wafers.  See Figure 1a. We use a custom built hydrogen furnace (designed after Ramachandran et. al) to improve the surface of the wafer as seen in figure 1b.  The hydrogen-flow furnace removes oxygen contamination and surface scratches due to polishing from the 6H-SiC (0001) substrates.  Figure 2 shows the RHEED patterns of a cleaned surface and formation of atomic steps with a final root-mean-square (rms) roughness of 0.44±0.03 nm, including the atomic steps. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) confirm a clean, reproducible starting surface, as is necessary for effective functional oxide integration.
1a
1b

Fig. 2. Morphological and crystallographic characterizations of 6H-SiC(0 001) by AFM and RHEED after ex-situ hydrogen cleaning; (a) 1umx1um AFM scan showing atomic steps; RHEED pattern showing a sharp root3xroot3 surface reconstruction pattern with electron incidence along (b) (112¯0) azimuth and (c) (11¯00) azimuth.

Fig. 3. Hydrogen furnace for cleaning 6H-SiC substrate to get atomic flat starting surface

Integration of magnesium oxide (MgO) on SiC as a heteroepitaxial template for complex oxide integration with SiC
Magnesium oxide (MgO) has a rock salt structure, and thus the {001} presents a cubic surface structure and the {111} presents a hexagonal surface structure. Based on our previous work, MgO has the potential to be a simple oxide that can be integrated with SiC and used to effectively create an O-O bridge to more complex oxides. MgO thin films have been deposited using a Mg effusion cell and an oxygen plasma source by a Mg adsorption controlled mechanism. We have demonstrated smooth, aligned films 10 to 380 Å thick deposited at a low substrate temperature of 140 deg. C. Figure 4 shows the RHEED and AFM of the 20 Å MgO film. The rms roughness along the steps is within the noise of the AFM, and this smooth surface is important in further integration of functional oxides. In addition, these films have been shown to be stable in vacuum and air at high temperatures, and stable in active oxygen plasma environments below 400 deg.C (publication in progress). This stability is important in order to ensure that the interlayer will survive the MBE or PLD processing of the functional oxide layer.

Fig. 4. Morphological and crystallographic characterizations of MgO (111) by AFM and RHEED; (a) 1umx1um AFM scan showing smooth surface with atomic steps; RHEED pattern with electron incidence along (b) (11¯0) azimuth and (c) (11¯2) azimuth.

  

References:
T. L. Goodrich, J. Parisi, Z. Cai, K. S. Ziemer, “Low temperature growth of crystalline magnesium oxide on hexagonal silicon carbide (0001) by molecular beam epitaxy“, Applied Physics Letters, Vol. 90, 042910/1-042910/3, 2007.

T. L. Goodrich, Z. Cai, M. D. Losego, J-P. Maria, and K. S. Ziemer, “Thin, Crystalline MgO on Hexagonal 6H-SiC (0001) by Molecular Beam Epitaxy for Functional Oxide Integration “Journal of Vacuum Science and Technology, B, 25(3) pages 1033-1038, 2007.

T. L. Goodrich, Z. Cai, K. S. Ziemer, “Exploring stability of MgO(111) films grown on 6H-SiC(0001) by molecular beam epitaxy for two-step integration of functional oxides” submitted to Applied Surface Science, 7/25/07

Ramachandran, V., Brady, M.F., Smith, A.R., Feenstra, R.M., Greve,
D.W., Preparation of atomically flat surfaces on silicon carbide using hydrogen etching, J. Elec. Mat. 27 (4), 308-312, 1996.