Tuning Morphology, Optics, and Conductivity of Al₂O₃/ZnO Nanolaminates Through Bilayer Thickness Control

Background

Nanolaminates are composite structures composed of alternating nanometer‑thick layers of distinct materials, granting properties that often surpass those of the individual constituents [1–4]. Recent work has harnessed these structures for energy storage, advanced optical components, and temperature‑sensitive biosensor substrates [5–7]. Notably, PAN/ZnO and graphene/ZnO nanolaminates have shown enhanced electronic and optical performance suitable for sensor applications [8–9].

Al‑doped ZnO (AZO) is a leading transparent conductive oxide (TCO) owing to its abundance, low cost, and excellent plasma stability. Conventional AZO tuning relies on adjusting Al‑doping levels [10–12], but structural modulation via Al₂O₃/ZnO nanolaminates offers a simpler, more scalable alternative.

Atomic‑layer deposition (ALD) delivers precise control over individual layer thicknesses through self‑limiting surface reactions, enabling the fabrication of high‑quality, smooth nanolaminates with well‑defined interfaces [13–15]. In this work, we employ ALD to systematically vary bilayer thicknesses and investigate the resulting morphological, optical, and electrical properties.

Methods

Synthesis of Nanolaminates by ALD

Al₂O₃/ZnO bilayers were deposited on SiO₂/Si and quartz substrates at 150 °C in a PICOSUN reactor. Zn, Al, and O precursors were diethylzinc (DEZ), trimethylaluminum (TMA), and deionized water, respectively, with high‑purity nitrogen as carrier/purge gas (50 sccm). Al₂O₃ layers were grown via TMA/H₂O cycles (0.03 s/3 s/15 s/0.03 s/5 s/15 s), while ZnO layers used DEZ/H₂O cycles with identical timing. Five bilayer thicknesses were prepared: 25/25 nm, 10/10 nm, 5/5 nm, 2/2 nm, and 1/1 nm, ensuring total stack thicknesses remained comparable by adjusting the number of bilayers (Table 1).

Characterization

Cross‑sectional TEM (FEI Tecnai G2 F20) and AFM (Bruker Dimension Icon) assessed morphology and surface roughness. Spectroscopic ellipsometry (SE; J.A. Woollam) measured optical constants and thicknesses across 200–1000 nm at 65° incidence. UV–vis spectroscopy (Shimadzu UV‑3600) recorded transmittance. Electrical properties were extracted from Hall‑effect measurements (Ecopia HMS3000) using a four‑point probe on quartz samples.

Results and Discussion

Morphological Characteristics

TEM cross‑sections reveal well‑defined Al₂O₃/ZnO interfaces for all bilayer thicknesses (Fig. 2). X‑ray diffraction shows amorphous behavior, confirmed by high‑resolution TEM images. Even the thickest 25/25 nm bilayers remain amorphous, underscoring the strong blocking effect of Al₂O₃ on ZnO crystallization.

AFM scans (Fig. 3) demonstrate that surface roughness (R_q) decreases from ~1.3 nm (25/25 nm) to ~0.8 nm (1/1 nm), following a linear trend that saturates at the smallest bilayers. The smoothness originates from the Al₂O₃ layers preventing ZnO nanocrystal growth, a phenomenon independent of individual layer thickness but dependent on the number of interleaved Al₂O₃ layers.

Optical Properties

SE modeling (Fig. 5) yielded accurate thicknesses matching TEM data and low RMSE values (<0.05). The refractive index n decreases progressively with thinner bilayers, while the extinction coefficient k approaches zero in the visible–near‑IR, confirming high transparency (>90 %) (Fig. 6). A pronounced blue shift of the absorption edge occurs as bilayer thickness decreases, attributable to the Burstein–Moss effect and quantum‑confinement in sub‑Bohr‑radius ZnO layers. Bandgap energies extracted via Tauc plots rise from 3.20 eV (25/25 nm) to 3.60 eV (1/1 nm) (Fig. 7), aligning with the theoretical BM expression E_g = E_g⁰ + (ħ²/8m_e*) (3/π)²⁄³ n_e²⁄³.

Transmission spectra (Fig. 8) corroborate the SE results, displaying sharp absorption edges in the 200–400 nm UV range that shift to shorter wavelengths with decreasing bilayer thickness. The high transmittance across visible and near‑IR confirms suitability as TCO material.

Electrical Properties

Hall‑effect measurements (Fig. 9) reveal carrier concentrations of ~5 × 10¹⁹ cm⁻³ for the thicker bilayers (25/25 nm, 10/10 nm, 5/5 nm), while thinner bilayers (2/2 nm, 1/1 nm) exhibit markedly reduced carriers (~1 × 10¹⁹ cm⁻³) and increased resistivity, indicating a transition toward insulating behavior. The tunable conductivity—high for thicker bilayers and low for thinner ones—offers a straightforward route to engineer both transparent conductors and high‑resistivity layers.

Conclusions

By systematically varying bilayer thickness in Al₂O₃/ZnO nanolaminates, we achieved precise control over morphology, optical bandgap, and electrical conductivity. Thinner bilayers produce smoother surfaces, a blue‑shifted bandgap driven by Burstein–Moss and quantum‑confinement effects, and a tunable resistivity spanning two orders of magnitude. These results position Al₂O₃/ZnO nanolaminates as versatile candidates for high‑transparency conductive layers and high‑resistivity barriers in next‑generation optoelectronic devices.

Abbreviations

AFM

Atomic force microscopy

ALD

Atomic layer deposition

AZO

Al‑doped ZnO

BM

Burstein–Moss

DEZ

Diethylzinc

FB

Forouhi–Bloomer

RMSE

Root‑mean‑square error

SE

Spectroscopic ellipsometry

TCO

Transparent conductive oxide

TEM

Transmission electron microscope

TMA

Trimethylaluminum