An official website of the United States government
Abstract Enhanced and controlled light absorption, as well as field confinement in optically thin materials, are pivotal for energy‐efficient optoelectronics and nonlinear optical devices. Highly doped transparent conducting oxide (TCO) thin films can support the so‐called epsilon near zero (ENZ) modes in a frequency region of near‐zero permittivity, which can lead to the perfect light absorption and ultrastrong electric field intensity enhancement (FIE) within the films. To achieve full control over absorption and FIE, one must be able to tune the ENZ material properties as well as the film thickness. Here, engineered absorption and FIE are experimentally demonstrated in aluminum‐doped zinc oxide (AZO) thin films via control of their ENZ wavelengths, optical losses, and film thicknesses, tuned by adjusting the atomic layer deposition (ALD) parameters such as dopant ratio, deposition temperature, and the number of macrocycles. It is also demonstrated that under ENZ mode excitation, though the absorption and FIE are inherently related, the film thickness required for observing maximum absorption differs significantly from that for maximum FIE. This study on engineering ENZ material properties by optimizing the ALD process will be beneficial for the design and development of next‐generation tailorable photonic devices based on flat, zero‐index optics.
more »
« less
Review Article: Atomic layer deposition of doped ZnO films
This article reviews the process-structure-property relationship in doped ZnO thin films via atomic layer deposition (ALD). ALD is an important manufacturing-scalable, layer-by-layer, thin film deposition process that precisely controls dopant type and concentration at the nanoscale. ZnO is an important technological material, which can be doped to modulate structure and composition to tailor a wide variety of optical and electronic properties. ALD doped ZnO is viewed as a transparent conducting oxide for application in solar cells, flexible transparent electronics, and light-emitting diodes. To date, there are 22 elements that have been reported as dopants in ZnO via ALD. This article studies the underlying trends across dopants and establishes generalized relationships for (1) the role of ALD process parameters, (2) the impact of these parameters on the structure of the ZnO matrix, and (3) the impact of dopants on the optical and electrical properties. The article ends with a brief discussion on the limitations of the ALD-based doping scheme, knowledge gaps in the compositional maps, and a perspective on the future of ALD doped ZnO films.
more »
« less
- Award ID(s):
- 1908167
- PAR ID:
- 10584136
- Publisher / Repository:
- American Vacuum Society
- Date Published:
- Journal Name:
- Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films
- Volume:
- 37
- Issue:
- 5
- ISSN:
- 0734-2101
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Enhanced and controlled light absorption as well as field confinement in an optically thin material are pivotal for energy-efficient optoelectronics and nonlinear optical devices. Highly doped transparent conducting oxide (TCO) thin films with near-zero permittivity can support ENZ modes in the so-called epsilon near zero (ENZ) frequency region, which can lead to perfect light absorption and ultra-strong electric field intensity enhancement (FIE) within the films. To achieve full control over absorption and FIE, one must be able to tune the ENZ material properties as well as the film thickness. Here, we experimentally demonstrate engineered absorption and FIE in aluminum doped zinc oxide (AZO) thin films via control of their ENZ wavelengths, optical losses, and film thicknesses, tuned by adjusting the atomic layer deposition (ALD) parameters such as dopant ratio, deposition temperature, and number of macro-cycles. We also demonstrate that under ENZ mode excitation, though the absorption and FIE are inherently related, the film thickness required for observing maximum absorption differs significantly from that for maximum FIE. This study on engineering ENZ material properties by optimizing the ALD process will be beneficial for the design and development of next- generation tunable photonic devices based on flat, zero-index optics.more » « less
-
Transparent conducting oxides, such as Ga-doped ZnO (GZO) and Al-doped ZnO (AZO) are attractive materials for high-performance plasmonic devices operating at telecommunication wavelengths. In this contribution, we compare the growth of epsilon-near-zero GZO and AZO films on sapphire by two different deposition techniques: molecular beam epitaxy (MBE) and atomic layer deposition (ALD). For MBE of GZO, a multiple buffer consisted of a high-temperature MgO layer, a low-temperature ZnO, followed by a high-temperature ZnO layer is employed to assure the crystalline quality of the GZO film. By controlling the growth parameters, including Ga doping level, VI/II ratio, substrate temperature, we are able to produce GZO films at 350 °C with electron mobility between 30 and 50 cm2/V.s, electron concentration up to 7×1020 cm-3, and resistivity down to 2.5×10-4 Ω.cm. For ALD of AZO, without using any buffer, by reducing the Al pulse duration, we are able to grow the AZO films under a large ratio of Al to Zn pulses of 1:6, which improves the activation of Al as an effective dopant. Hence AZO films with electron concentration above 7×1020 cm-3, electron mobility between 10 and 20 cm2/V.s, and resistivity below 6×10-4 Ω.cm have been obtained at 250 °C. The corresponding epsilon-near-zero point in the ALD-grown material was tuned down to 1470 nm. Our data indicate that the ALD method provides a low-temperature route to plasmonic TCOs for telecommunication wavelength range. Effect of electron mobility on optical loss and, therefore, plasmonic figure of merit is discussed.more » « less
-
Typical titanium oxide (TiO2) films are transparent in the visible range, allowing for their index of refraction and thickness to be extracted by single-angle spectroscopic ellipsometry (SE) using a Cauchy model. However, TiO2 films grown by atomic layer deposition (ALD) from tetrakis(dimethylamino)titanium (IV) (TDMAT) and H2O at 350 °C absorb in the visible range due to the formation of Ti-O-N/Ti-N bonds. Single-angle SE is inadequate for extracting the optical constants of these films, as there are more unknowns (n, k, d) than measurable parameters (ψ, Δ). To overcome this limitation, we combined SE with transmission (T) measurements, a method known as SE + T. In the process, we developed an approach to prevent backside deposition on quartz substrates during ALD deposition. When applying a B-spline model to SE + T data, the film thicknesses on the quartz substrates closely matched those on companion Si samples measured via standard lithography. The resulting optical constants indicate a reduced refractive index, n, and increased extinction coefficient, k, when compared to purer TiO2 thin films deposited via a physical vapor deposition (PVD) method, reflecting the influence of nitrogen incorporation on the optical properties.more » « less
-
Major technological breakthroughs are often driven by advancements in materials research, and optics is no different. Over the last few years, near-zero-index (NZI) materials have triggered significant interest owing to their exceptional tunability of optical properties and enhanced light-matter interaction, leading to several demonstrations of compact, energy-efficient, and dynamic nanophotonic devices. Many of these devices have relied on transparent conducting oxides (TCOs) as a dynamic layer, as these materials exhibit a near-zero-index at telecommunication wavelengths. Among a wide range of techniques employed for the deposition of TCOs, atomic layer deposition (ALD) offers advantages such as conformality, scalability, and low substrate temperature. However, the ALD process often results in films with poor optical quality, due to low doping efficiencies at high (>1020cm−3) doping levels. In this work, we demonstrate a modified ALD process to deposit TCOs, taking Al:ZnO as an example, which results in an increase in doping efficiency from 13% to 54%. Moving away from surface saturation for the dopant (aluminum) precursor, the modified ALD process results in a more uniform distribution of dopants (Al) throughout the film, yielding highly conductive (2.8×10−4Ω-cm) AZO films with crossover wavelengths as low as 1320nm and 1370nm on sapphire and silicon substrates, respectively.more » « less
