Nitric acid oxidation of Si (NAOS) method for low temperature fabrication of SiO2/Si and SiO2/SiC structures

We have developed low temperature formation methods of SiO2/Si and SiO2/SiC structures by use of nitric acid, i.e., nitric acid oxidation of Si (or SiC) (NAOS) methods. By use of the azeotropic NAOS method (i.e., immersion in 68 wt% HNO3 aqueous solutions at 120 °C), an ultrathin (i.e., 1.3–1.4 nm) SiO2 layer with a low leakage current density can be formed on Si. The leakage current density can be further decreased by post-metallization anneal (PMA) at 200 °C in hydrogen atmosphere, and consequently the leakage current density at the gate bias voltage of 1 V becomes 1/4–1/20 of that of an ultrathin (i.e., 1.5 nm) thermal oxide layer usually formed at temperatures between 800 and 900 °C. The low leakage current density is attributable to (i) low interface state density, (ii) low SiO2 gap-state density, and (iii) high band discontinuity energy at the SiO2/Si interface arising from the high atomic density of the NAOS SiO2 layer. For the formation of a relatively thick (i.e., ≥10 nm) SiO2 layer, we have developed the two-step NAOS method in which the initial and subsequent oxidation is performed by immersion in ∼40 wt% HNO3 and azeotropic HNO3 aqueous solutions, respectively. In this case, the SiO2 formation rate does not depend on the Si surface orientation. Using the two-step NAOS method, a uniform thickness SiO2 layer can be formed even on the rough surface of poly-crystalline Si thin films. The atomic density of the two-step NAOS SiO2 layer is slightly higher than that for thermal oxide. When PMA at 250 °C in hydrogen is performed on the two-step NAOS SiO2 layer, the current–voltage and capacitance–voltage characteristics become as good as those for thermal oxide formed at 900 °C. A relatively thick (i.e., ≥10 nm) SiO2 layer can also be formed on SiC at 120 °C by use of the two-step NAOS method. With no treatment before the NAOS method, the leakage current density is very high, but by heat treatment at 400 °C in pure hydrogen, the leakage current density is decreased by approximately seven orders of magnitude. The hydrogen treatment greatly smoothens the SiC surface, and the subsequent NAOS method results in the formation of an atomically smooth SiO2/SiC interface and a uniform thickness SiO2.