Materials Characterization and Growth Mechanisms of ZnO, ZrO2, and HfO2 Deposited by Atomic Layer Deposition

Gallium Nitride (GaN) is recognized as one of the best candidates for high-power high-frequency metal-oxide-semiconductor field-effect-transistors (MOSFETs). The critical component to enable this technology is the development of a robust oxide with low density of defects and preferential mobility properties that can produce an enhancement mode transistor rather than a depletion mode transistor. Zirconium oxide (ZrO2) and hafnium oxide (HfO2) are considered as two promising oxides for the gate oxide of the GaN MOSFETs. On the other hand, zinc oxide (ZnO) is an alternative wide bandgap semiconductor for GaN. ZnO has some advantages over GaN in optoelectronics due to its large exciton binding energy (~60 meV), and is widely used as the active channel in thin film transistors (TFTs). To control the electrical properties of the deposited thin films, a fundamental understanding of the nucleation and growth mechanisms is essential. In this dissertation, the material characterization and growth mechanisms of atomic layer deposition (ALD) of the three important oxides in semiconductor industry, ZnO, ZrO2, and HfO2, were investigated. The oxides were deposited using thermal and plasma-enhanced ALD on Si(100) substrate at various deposition temperatures. Different analytical techniques, including spectroscopic ellipsometry (SE), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM) were utilized to analyze the optical, chemical, and morphological characteristics of the oxide thin films. Based on the results, nucleation and growth mechanisms were proposed for thermal and plasma enhanced ALD of ZnO, ZrO2, and HfO2. The role of ALD parameters, as well as –OH reaction sites on the nucleation and growth mechanisms were described. Atomistic growth mechanisms of thermal ALD ZnO, ZrO2, and HfO2 were studied using a density functional theory (DFT) approach. The important role of formation of intermediate structures between surface reaction sites and the precursor molecules were emphasized. The results were found to be consistent with the variation of growth rate of the ALD oxides with the deposition temperature. Finally, it was found that PEALD ZrO2 offered the best properties for the gate oxide of the GaN MOSFETs with the lowest value of density of interface traps.