Fabrication of Ultra-Small Zirconia Nano-Islands Using Thermal Atomic Layer Deposition
Authors: El-Atab Nazek , Suh Junkyo , Islam Raisul , Saraswat Krishna C. , Nayfeh Ammar 
Published: February 2020

Zirconia (ZrO2) nanostructures have been used in various applications such as protective barriers in optics, interference filters, photo-catalysts, oxygen sensors, in addition to buffer layers or gate oxides in microelectronic devices, etc. All of these applications of ZrO2 are based on the outstanding mechanical, physical, electronic, optical, and chemical properties exhibited by this oxide. ZrO2 can be synthesized and deposited by various methods such as thermal decomposition, sol-gel synthesis, hydrothermal synthesis, and precipitation [1]. It is reported that the synthesis method affects the crystallinity and morphological structure of ZrO2 which would affect the electronic and optical properties of the material such as the bandgap, the light harvesting capability, the lifetime of electron-hole pairs, etc. and therefore affect the performance of the devices where ZrO2 is used [2]. Therefore, a good manipulation of the ZrO2 structure and properties is needed in order to reach the application goals. ZrO2 is well known to have three polymorphs with three different bandgap values [3]: monoclinic (3.25 eV), tetragonal (3.58 eV), and cubic (4.33 eV) [4]. Moreover, the dielectric constant of the cubic and tetragonal polymorphs is reported to be higher than the monoclinic and amorphous ZrO2. In this work, we study the deposition of ZrO2 nanoislands using thermal Atomic Layer Deposition (ALD) and their structural and optical and electronic properties are investigated using transmission electron microscopy (TEM), atomic force microscopy (AFM), UV-Vis-NIR spectrophotometer, and Raman spectroscopy.
As a matter of fact, it is known that the growth rate and growth mode in ALD are affected by the chemistry of the substrate surface in addition to the deposition process parameters such as temperature, pressure, precursors, and precursor’s dose times. In this work, three different samples are used as the substrate, each having a different substrate termination: (1) Si with native SiO2, (2) Si after etching the native oxide with HF, and (3) Si with 4-nm Al2O3 deposited by ALD. Moreover, the effect of different deposition temperatures (150°C, 200°C, and 250°C), Zr precursor dose times (30 ms, 165 ms, and 300 ms) and number of cycles (20, 30 and 40 cycles) on the deposited film is analyzed. AFM AC-in-Air mode is conducted for the different samples, and it is found that nano-islands are obtained on the three substrates under the following deposition conditions: temperature of 250°C, Zr dose time of 300 ms, and a pressure of 300 mtorr. On the substrates (1) and (3), nanoislands with different sizes were obtained with 20 and 30 cycles, while 40 cycles leads to a continuous layer. On substrate (2) which is pre-treated with HF and therefore has a hydrophobic –H terminated surface, only 20 cycles showed the growth of nanoislands while 30 and 40 cycles showed continuous layers. The reason for the nanoislands growth is investigated and found to be based on the agglomeration of the adsorbed atoms (ad-atoms), in fact, at 250°C, the ad-atoms have enough energy and mobility which allows them to migrate and form nanoislands [5]. The non-availability of nanoislands at lower deposition temperatures further confirms this analysis. The Raman measurements and TEM imaging on the islands show that they are polycrystalline. In addition, the transmittance and reflectance spectra are measured using the UV-Vis-NIR spectrophotometer and the well-established method “Kubelka-Munk” is used to extract the bandgap of the islands and it is found to be around 4.2 eV which is in between the bandgap values of the cubic and tetragonal-ZrO2 reported in literature. Finally, the obtained results in this work are important for fabricating nanoislands and understanding the behavior of different devices with ZrO2 nano-islands such as non-volatile memory devices with Zirconia nano-islands as the charge trapping layer or solar cells with Zirconia nanoislands as a light trapping scheme.
The authors gratefully acknowledge financial support for this work provided by the Masdar Institute of Science and Technology and the Office of Naval Research global grant N62909-16-1-2031. N.E.A. acknowledges L’Oréal-UNESCO For Women in Science Middle East Fellowship.
[1] Sohn JR et al., Surface Characterization of chromium oxide zirconia catalyst. Langmuir, vol. 126, p. 31, 1993.
[2] Sreethawong T. et al, Synthesis of crystalline mesoporous-assembled ZrO2 nanoparticles via a facile surfactant-aided sol-gel process and their photocatalytic dye degradation activity. Chem. Eng. Journal, 228256-62, 2013.
[3] Gao PT et al., Study of ZrO2-Y2O3 films prepared by RF magnetron reactive sputtering. Thin Solid Films pp. 37732-6, 2000.
[4] Basahel et al., Nanoscale Research Letters, vol. 10, p. 73, 2015.
[5] Ritala, M.; Leskela, M. Handbook of Thin Film Materials; Academic Press: San Diego, CA, 2002.