Aluminum‑Doped Zinc Oxide Thin Films: Infrared Optical Properties and Near‑Perfect Absorption Design

Background

Plasmonics and metamaterials have propelled advances in negative‑index materials, sub‑diffraction imaging, and cloaking. Traditionally, noble metals have dominated these fields, but heavily doped semiconductors—such as aluminum‑doped zinc oxide (AZO) and titanium nitride (TiN)—offer tunable carrier concentrations and lower loss in the infrared. AZO’s wide band gap and high dopant solubility make it a versatile plasmonic platform, especially when combined with ZnO in epitaxial superlattice structures that reduce interfacial loss.

While the visible and near‑infrared behavior of AZO has been extensively explored, its infrared properties remain less understood. Prior work using recombination‑model simulations provided a general view but lacked speed and convenience. Here, we extract AZO permittivity from 210 to 5000 nm via point‑by‑point SE analysis, a method that depends only on primary SE simulations and delivers both speed and precision. We dissect the thickness dependence in visible and infrared bands, examine band‑gap shifts, and uncover that the effective plasma frequency vanishes for ultrathin (<25 nm) films in the infrared, precluding metamaterial applications at these scales. Finally, we employ finite‑difference time‑domain (FDTD) simulations to design void‑cylinder arrays on AZO/ZnO multilayers, achieving broadband near‑perfect absorption between 2 and 5 µm.

Methods

Atomic layer deposition (ALD) offers sub‑nanometer thickness control and excellent conformity, making it ideal for plasmonic thin‑film growth. We deposited AZO on p‑type Si(100) substrates by alternating diethylzinc (DEZ) and trimethylaluminum (TMA) with deionized water (H₂O) in a Picosun ALD reactor at 190 °C. A single AZO cycle comprises 14 ZnO sub‑cycles followed by one Al‑O sub‑cycle, each sub‑cycle consisting of a 0.1 s precursor pulse, a 5 s N₂ purge, a 0.1 s H₂O pulse, and a 5 s N₂ purge. The ZnO ALD reaction is:

with surface reactions:

and

Al doping follows a similar cycle (Zn:Al = 14:1) with:

and

We varied the film thickness by adjusting ALD cycle counts (150, 300, and 450 cycles). Structural and optical characterization employed a J.A. Woollam spectroscopic ellipsometer (incident angle 65°, wavelength ranges 210–1000, 1000–2000, and 2000–5000 nm) and Fourier transform infrared spectroscopy (FTIR) for reflection and transmission. X‑ray diffraction (XRD) assessed crystallinity.

Results and Discussions

Optical Properties of AZO Films in Visible and Infrared Broadband

Leveraging ALD’s low interfacial roughness, we modeled AZO as a monolayer. Ellipsometric parameters (Ψ, Δ) were extracted from SE, and the refractive index (n), extinction coefficient (k), and thickness (d) were fitted by minimizing the root‑mean‑square error (RMSE):

with RMSE defined as:

For ZnO we used a Forouhi‑Bloomer (F‑B) fit in the 300–800 nm range, but AZO’s metallic character necessitated a Cauchy model (400–800 nm) and a Drude‑Lorentz model (1500–5000 nm). Point‑by‑point analysis yielded the full spectral n and k (see Fig. 1). In the visible (210–800 nm), AZO behaves as a semiconductor: k ≈ 0, and n shows thickness dependence driven by interfacial effects—thinner films exhibit reduced permittivity due to substrate coupling. In the infrared (800–5000 nm), k rises sharply, indicating increased absorption and metallic behavior; n remains thickness‑dependent but now reflects crystallinity effects.

Refractive index (n) and extinction coefficient (k) simulated via point‑by‑point analysis.

We determined the band gap (E_g) by extrapolating (αE)² → 0, where α = 4πk/λ:

Linear extrapolation (Fig. 2) shows a blue‑shift of E_g from 3.62 to 3.72 eV with increasing thickness, attributable to free‑electron screening that suppresses excitonic absorption.

Band‑gap determination via (αE)² extrapolation.

XRD analysis (Fig. 3) indicates that AZO crystallinity improves with thickness: the 450‑cycle film shows a pronounced (100) wurtzite peak, while thinner films are less crystalline. Reduced crystallinity correlates with higher lattice defects, lower carrier concentration, and a blue‑shifted band gap.

XRD patterns for AZO films of varying thickness.

Converting n and k to complex permittivity (ε_r = n² − k² + i·2nk) (Fig. 4) reveals that the real part of ε_r becomes negative at the plasma frequency. As thickness increases, the plasma frequency blue‑shifts, and for 150‑cycle films the zero‑crossing disappears in the infrared, leaving ε_r strictly positive. Consequently, ultrathin AZO cannot function as a negative‑index metamaterial in the infrared.

Real and imaginary parts of ε_r for AZO films of different thicknesses.

Reflection, absorption, and transmission measurements (Fig. 5) corroborate the SE analysis: reflection is higher for thicker films on SiO₂, while absorption peaks shift with thickness in the infrared. FTIR data (Fig. 5d) confirm that thicker AZO films suppress transmission between 2500 and 5000 nm.

Reflection, absorption, and transmittance of AZO films on Si and SiO₂ substrates.

Near‑Perfect Absorption via Void‑Cylinder Arrays on AZO/ZnO Multilayers

AZO’s low extinction coefficient (k ≈ 0.2–5 µm) compared to noble metals (Au, Ag) makes it an attractive absorber in the infrared (Fig. 6).

Extinction coefficient k of AZO, Au, and Ag across 0.2–5 µm.

Building on previous 32‑layer AZO/ZnO stacks (≈1.92 µm, each 60 nm) that exhibited near‑perfect absorption at ~1.9 µm, we used SE‑derived optical constants and FDTD simulations to design void‑cylinder arrays that broaden absorption to 2–5 µm. The structure (Fig. 7) comprises a periodic array of cylindrical voids (radius R, period P) etched into the multilayer.

Void‑cylinder array geometry on AZO/ZnO multilayers.

Two configurations were optimized: Array A (R = 0.6 µm, P = 1.8 µm) and Array B (R = 0.8 µm, P = 2.0 µm). Array B delivers >90 % absorption across 2.04–5 µm, while Array A excels in the near‑infrared. The negative real part of AZO’s permittivity enables phase matching of reflected waves, and the low‑k multilayer stack enhances broadband absorption.

Reflection and absorption spectra for Arrays A and B.

We tested Array A on three substrates (void, Si, quartz). Despite varying refractive indices (n = 1–3.56), absorption remained largely unchanged (Fig. 9), demonstrating the absorber’s substrate independence.

Absorption of Array A on different substrates; inset shows substrate n and k.

Conclusions

Our comprehensive analysis demonstrates that AZO film thickness modulates permittivity in both visible and infrared regimes via interfacial effects and thickness‑dependent crystallinity. The resulting blue‑shift of the effective plasma frequency—and its disappearance below 25 nm—highlights the limits of ultrathin AZO for metamaterial use. Harnessing these properties, we fabricated a 32‑layer AZO/ZnO absorber with void‑cylinder arrays that achieves near‑perfect absorption from 2 to 5 µm, independent of substrate. These findings advance the understanding of AZO’s optical behavior and open pathways for infrared plasmonic devices.