A broad spectrum of accessible functionalities and material properties based on slight changes in crystal structure and composition makes perovskites a unique class of materials. Understanding and leveraging of its properties have resulted in its use in a myriad of applications. Today, performance and efficiency demands require these materials in miniaturized devices; hence, a precise control of perovskite synthesis in terms of thickness, crystallinity, and stoichiometry is indispensable. Atomic layer deposition (ALD) makes these requirements potentially conceivable due to the technology's unique self-limiting deposition process. Yet, not all properties of perovskites have been leveraged in the thin film regime due to limited understanding of their synthesis. In part one of a two-part review, we discuss the ALD growth of perovskite-based thin films. After explaining the specific growth characteristics of ALD perovskites, effects of process parameters, and thin film treatments on properties, we discuss an important functional perovskite strontium titanate (STO). In part two, we discuss ALD-deposited perovskites for next generation electronic applications. K E Y W O R D Satomic layer deposition, high-k, interfacial growth, nucleation and growth, perovskites, ultrathin films, piezo-electric

From part one, we learned that perovskites are interesting materials with tunable properties. Here, four current applications are elaborated on; high-κ dielectrics, piezoelectrics, optoelectronics, and solar to energy conversion devices. To start with, we discuss perovskite based dynamic random-access memory (DRAM) capacitors, where ALD strontium titanate (STO) of thickness 10 nm can achieve dielectric constants (k) of up to 146. Next, we discuss ALD perovskite piezoelectric-based device design of nanoelectromechanical systems (NEMS) and microelectromechanical systems (MEMS). There is a renewed interest in barium-based ternary compounds which have piezoelectric coefficients up to 500 pC/ N. We further explore ALD perovskitebased solar photovoltaics (PVs), where conformal and uniform layers of lead sulfide (PbS) absorbing layers allow deposition on large surfaces, facilitating perovskite architectures with conversion efficiency reaching 20%. Finally, we learn how lanthanumbased perovskites can replace cerium oxide, which is currently utilized for thermochemical processes for solar to energy conversion. Subsequently, we discuss different characterization techniques allowing us to deepen our understanding of process property relationships eventually leading to further performance enhancements. K E Y W O R D Satomic layer deposition, high-κ, interfacial growth, nucleation and growth, perovskites, piezoelectric, ultra-thin films | PEROVSKITE-BASED APPLICATIONSIn the first part of this two-part review, we developed a fundamental understanding of ALD deposition of perovskites, based on growth mechanisms, interfacial interactions, precursor chemistries, and energetics. Now, we review applications where the ALD deposition of perovskites have been leveraged.

Nanoscale free‐standing membranes are used for a variety of sensors and other micro/nano‐electro‐mechanical systems devices. To tune performance, it is indispensable to understand the limits of aspect ratios achievable. Herein, vapor hydrofluoric (VHF) processes are employed to release 3D shell structures made of atomic‐layer‐deposited Al2O3 etch‐stop layers. Structure heights of 100–600 nm and widths of 1–200 nm are fabricated for membranes with 20 and 50 nm thickness. Undercut depths of and aspect ratios of 475:1 etch depth to structure width (50 nm films) and etch depth to membrane thicknesses of 495:0.02 (20 nm films) are achieved. The etch‐rate stagnates above a ratio of 31% hydrofluoric (HF), where decreasing EtOH shares reduce reproducibility. Etch rates reach 0.75 mm min−1 and are generally constant over vapor etch depth. For 100 nm heights and widths of , etch rates however stagnate for deeper depths. All explored structures remained stable with widths up to 5 μm independent of the height. Above width, top membranes deflect, likely from stress accumulated during deposition. Herein, exploring and understanding the limits of aspect ratio in future free‐standing membrane devices are helped.

No abstract