Article pubs.acs.org/JPCC
Phase Transition and Microstructural Changes of Sol−Gel Derived ZrO2/Si Films by Thermal Annealing: Possible Stability of Tetragonal Phase without Transition to Monoclinic Phase Soo Min Hwang, Jun Hyuk Choi, Seung Muk Lee, Jun Hyung Lim,* and Jinho Joo* School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea S Supporting Information *
ABSTRACT: Stabilization of high-temperature phases such as tetragonal (t-) or cubic phases has been a pivotal issue for technological applications of polymorphic ZrO2. In this work, we fabricated ZrO2/Si films using a sol−gel deposition route and investigated the phase transformation, microstructural evolution, surface morphological changes, and interfacial chemical structures by thermal annealing. The ZrO2 precursor solution was prepared using a zirconium acetylacetonate, coated, dried on Si substrates, and finally annealed at 300−950 °C in ambient air. The sol−gel-derived ZrO2 layer crystallized into the t-phase as the annealing temperature increased. Despite high-temperature annealing, the t-phase was stabilized without a noticeable transition to the monoclinic phase, probably because of the relatively low film thickness (∼15 nm), enlarged surface/interface area due to thermal grooving, and strain effects. The probable t(112) orientation was developed after annealing at ≥800 °C, which could be related to minimization of the sum of the surface, interface, and strain energies. High-temperature thermal annealing resulted in the contraction of the ZrO2 layer as a result of the pyrolysis of the remnant organics, surface roughening by thermal grooving, and thickening of the amorphous interface layer (predominantly SiOx) between the ZrO2 and Si. derived amorphous (a) gel state and the t-phase.20,21 The metastable t-phase eventually transforms into the m-phase generally as a result of high-temperature processing.12−17,19 In a previous work, we fabricated ZrO2/Si thin films using a sol−gel method for use as high-k dielectrics as alternatives to conventional SiO2 and observed microstructural evolution from the a- to the t-phase upon increasing the film thickness and increasing the annealing temperature to 700 °C.22 However, the transition from the t- to the m-phase was not detected until 700 °C, a finding that is inconsistent with those of other studies in which m-phases were observed in ZrO2 films deposited by conventional vacuum-based processes at similar thicknesses.23−26 Above all, transmission electron microscopy (TEM) observations of the phase transformation of ZrO2/Si films together with grain morphology have been rarely reported.27,28 In addition, we could not clearly elucidate the chemical composition of the amorphous interfacial layer (IL) between the ZrO2 and Si formed during deposition and annealing. It was assumed that the IL composition was either ZrxSi1−xO or SiOx (1 < x < 2) at annealing temperatures of
