Microstructure-functionality relationships of pulsed laser deposited CuI thin films

Abstract text (incl. figure legends and references)

In the field of transparent p-type semiconductors, copper iodide (CuI) holds an outstanding position. It combines high transparency in the visible spectral range with excellent hole transport properties [1]. To make CuI thin films suitable for complementary electronics, i. e. to develop CuI into an equivalent partner for well investigated transparent n-type semiconductors, control of electrical and morphological properties is required.

Pulsed laser deposition (PLD) showed recently successful outcomes regarding film quality and systematic functionality tuning [2]. The use of powder-based targets allows a high flexibility regarding composition and can provide lateral and vertical composition gradients as well as controlled doping. With the possibility to change source targets in-situ, template layers can be grown to improve lattice match and surface smoothness due to the decrease of grain boundaries.

This works focuses on the microstructural investigation of (doped) CuI films employing EBSD, SEM, FIB, TEM and STEM-EDX. The obtained results are used to explore microstructure-functionality relationships by linking XRD findings and electrical properties with the results gained by the aforementioned electron microscopy techniques.

The first part of this work examines the impact of intentional p-doping with selenium (Se) on the film morphology. CuI with a lateral Se gradient was PLD grown on a two-inch diameter sapphire wafer. Site-specific TEM samples were prepared by focused ion beam (FIB) machining corresponding to a certain Se content. Whereas for each Se value, growth starts with the ´default´ zinc-blende γ-phase, high resolution TEM showed that a relatively high Se content seems to favor the CuI beta phase (trigonal crystal system) within the ongoing growth process. For a lower Se content, a new CuI phase, presumably with an up to now unpublished wurtzite 4H structure, was discovered with high-resolution TEM and related diffraction analysis (Fig. 1). This crystalline phase change also matches with results regarding XRD and electrical properties, where considerable changes occur for a selenium content above x(Se) = 1 at% [3].

The second part investigates the possibility to suppress rotational domains by employing PLD grown template layers such as sodium bromide (NaBr). By growing NaBr on cubic SrF₂ (111) substrates before depositing the CuI thin film, a heterostructure was constructed where the moisture sensitivity of NaBr could be employed to obtain free standing CuI thin films. XRD phi-scans showed single-crystallinity and no occurrence of rotational domains up to 1-2 µm film thickness. Nevertheless, films need to be sufficiently thicker to be utilized as, for example, front-back device processing. Samples out of reasonable thick films could be prepared for EBSD to visualize domain orientation, SEM to depict surface roughness and TEM to analyze grain boundaries to examine the cross-section as well as the front- and backside of the CuI film. It was shown that for thicknesses above 1-2 µm, the single crystallinity gets lost, probably due to accumulation of defects during the PLD growth [4].

[1] M. Grundmann et al., phys. stat. sol. (a) 210, 1671-1703 (2013).

[2] P. Storm et al., APL Mater. 8, 091115 (2020).

[3] P. Storm et al., Phys. Status Solidi RRL 15(8), 202100214:1-6 (2021).

[4] P. Storm et al., J. Mater. Chem. C 10, 4124-4127 (2022).

Fig. 1: CuI:Se film with lateral Se composition gradient on 2-inch diameter c-sapphire wafer with marked position for FIB sample extraction (left). High-resolution TEM images 1,2, and 3 close to the interface substrate-film (right).