(551a) Thermal Oxidation of Group III Nitrides

The thermal oxidation of the group three nitride semiconductors (AlN, GaN, and InN) is of fundamental and practical interest for the fabrication of many types of electronic devices. For high power ? high frequency electronics, such as metal-insulator semiconductor hetrojunction field effect transistors (MIS-HFETs), a thin group III oxide layer can form at the interface between the semiconductor and the deposited dielectric layer. For chemical, gas, biological and pressure sensors, a group III oxide is sometime intentionally grown on the gate region of the FETs to make functionalization with chemical or biological components easier than directly to the nitride. Thus, understanding the kinetics of oxide formation, the composition and structure of the oxide and its electrical properties are important for realization of the device function. This work reviews recent studies on group III nitride oxidation.

The thermal oxidation of the group III nitrides is significantly different from the thermal oxidation of silicon, the semiconductor for which oxidation is best understood. In contrast to silicon, the group III nitrides lack inversion symmetry, so they have crystal polarity; the (0001) plane can be either group III or nitrogen polar, each with distinctly different chemical properties. Thus, the dry oxidation rate of AlN is 15% faster on the nitrogen polar (0001) plane than the aluminum polar (0001) plane. The oxide that silicon forms is always amorphous, a consequence of the large volume increase that occurs when it is oxidized. The oxide that forms on the group III nitrides is amorphous at low temperature, and polycrystalline at high temperatures. Silicon forms only SiO2 under either dry or wet (saturated with water) oxidation conditions. There are many oxidation products possible for AlN including Al203, Al00H, and Al(OH)3. Silicon dioxide is strictly an insulator; the products of group III nitride oxidation can be either an insulator (Al2O3) or a semiconductor (Ga2O3).

The thermal oxidation of aluminum nitride single crystals and powders was studied under dry and wet (saturated with water vapor) environments over the temperature range from 800 °C to 1000 °C to gain more insights into this process. The composition and structure of layer grown under wet conditions varied considerably with temperature. At 800 °C and 6 hours, an amorphous layer formed that was rich in hydrogen (7.8 at%), suggesting the layer was predominately an aluminum hydroxide. The layer formed at 950 °C and 6 hours was crystalline Al2O3 (determined by x-ray diffraction) with a much lower hydrogen content (<0.7 at%). A sharp nitride oxide interface formed at high temperatures; in contrast, a more diffuse interface formed at low temperatures. The temperature dependence of the rate constants was determined for both the dry and wet oxidation of AlN, and will be reported.