Incomplete elimination of precursor ligands during atomic layer deposition of zinc-oxide, tin-oxide, and zinc-tin-oxide

Abstract

For atomic layer deposition (ALD) of doped, ternary, and quaternary materials achieved by combining multiple binary ALD processes, it is often difficult to correlate the material properties and growth characteristics with the process parameters due to a limited understanding of the underlying surface chemistry. In this work, in situ Fourier transform infrared (FTIR) spectroscopy was employed during ALD of zinc-oxide, tin-oxide, and zinc-tin-oxide (ZTO) with the precursors diethylzinc (DEZ), tetrakis(dimethylamino)tin (TDMASn), and H2O. The main aim was to investigate the molecular basis for the nucleation delay during ALD of ZTO, observed when ZnO ALD is carried out after SnO2 ALD. Gas-phase FTIR spectroscopy showed that dimethylamine, the main reaction product of the SnO2 ALD process, is released not only during SnO2 ALD but also when depositing ZnO after SnO2, indicating incomplete removal of the ligands of the TDMASn precursor from the surface. Transmission FTIR spectroscopy performed during ALD on SiO2 powder revealed that a significant fraction of the ligands persist during both SnO2 and ZnO ALD. These observations provide experimental evidence for a recently proposed mechanism, based on theoretical calculations, suggesting that the elimination of precursor ligands is often not complete. In addition, it was found that the removal of precursor ligands by H2O exposure is even less effective when ZnO ALD is carried out after SnO2 ALD, which likely causes the nucleation delay in ZnO ALD during the deposition of ZTO. The underlying mechanisms and the consequences of the incomplete elimination of precursor ligands are discussed.

Atomic Layer Deposition of Zinc Oxide: Study on the Water Pulse Reactions from First-Principles

“…The commonly made assumption that all the surface ethyl ligands are removed from the surface after the water pulse has been shown to be incorrect by Mackus et al 11 The authors conducted an in situ gas-phase and surface FTIR spectroscopy measurements on the ALD of zinc tin oxide (ZTO) thin films and note that after a typical water exposure 30–50% of the surface ethyl ligands remain on the surface during the growth of a ZnO thin film. Even after extended periods of water exposure, approximately 16% of surface ethyl ligands remain on the surface at 150 °C temperature, and only at elevated temperatures around 200 °C have virtually all the ligands been removed from the surface.…”

Efficient Planar Perovskite Solar Cells Using Passivated Tin Oxide as an Electron Transport Layer

“…It is seen that the weight loss occurs rapidly in a range between 70 and 150 °C, with a total weight loss of ≈92%. The DSC result shows one endothermic peak at 230 °C, indicating that the decomposition of the precursor and the residual precursor can coexist with SnO 2 below the temperature 23, 24. From our observation, we can draw a probable structure of the SnO 2 ETL as depicted in Figure 2g.…”

Band gap engineering of atomic layer deposited ZnxSn1‐xO buffer for efficient Cu(In,Ga)Se2 solar cell

“…The slow reaction kinetics at low temperatures suggests the possibility of unreacted ligands after the SnO 2 cycles, which reduce the available reactive sites for subsequent ZnO growth. A similar contention was made for ZTO films prepared by ALD using DEZ, tetrakis(dimethylamino)tin, and H 2 O where dimethylamine ligands were still detected on the reaction surface even after extremely long H 2 O pulses (900 s) 24. The report however argued that unremoved ligands only exist on the immediate growing surface and therefore are not incorporated in the films, as a consequence resulting in impurity free SnO 2 films.…”