Atomic layer deposition (ALD) is a technique capable of producing ultrathin conformal films with atomic level control over thickness. A major drawback of ALD is its low deposition rate, making ALD less attractive for applications that require high throughput processing. An approach to overcome this drawback is spatial ALD, i.e., an ALD mode where the half-reactions are separated spatially instead of through the use of purge steps. This allows for high deposition rate and high throughput ALD without compromising the typical ALD assets. This paper gives a perspective of past and current developments in spatial ALD. The technology is discussed and the main players are identified. Furthermore, this overview highlights current as well as new applications for spatial ALD, with a focus on photovoltaics and flexible electronics.

The deposition of cubic fluorite structure group II fluorides onto several well known semiconductors has been studied under clean, controlled, monitored conditions in an MBE system. The fluoride beams were generated from a single Knudsen effusion oven for each fluoride. Modulated beam mass spectrometry (MBMS) confirmed that the fluorides sublimed as group II molecules. Fluoride films therefore grow by a simple process of molecular condensation from the fluoride beam. BaF2 films deposited onto semiconductor surfaces held at room temperature were polycrystalline. However, growth at temperatures≳200 °C onto ion-bombarded, annealed InP and CdTe surfaces resulted in twin-free epitaxial films with the parallel epitaxial relation: (001)BaF2∥(001)InP, CdTe, [100]BaF2∥[100]InP, CdTe. Growth onto InP(001) surfaces subjected to no ion-bombardment annealing treatment also resulted in twin free epitaxial films with the same epitaxial relation. In situ electron diffraction studies during fluoride film growth combined with ex situ studies suggest that film growth occurs by a 2D mechanism rather than a 3D island mechanism. Electrical measurements of BaF2 and CaF2 films show typical film resistivities of ≳1013 Ω cm at 77 and 300 K. Typical breakdown fields are ∠5×105 V cm−1 and promising interface properties for MIS (BaF2 insulator) structures on p-type Si and n-type Hg0.79Cd0.21Te have been achieved. Cubic fluorite structure group II fluorides form a promising new class of epitaxial dielectrics which can be grown at low (∠200 °C) substrate temperatures.