Quantitative in situ synchrotron X-ray analysis of the ALD/MLD growth of transition metal dichalcogenide TiS2 ultrathin films.

We present the results of a full quantitative analysis of X-ray absorption spectroscopy (XAS) performed in situ during the growth of ultrathin titanium disulfide (TiS2) films via an innovative two-step process, i.e. atomic layer deposition/molecular layer deposition (ALD/MLD) followed by annealing. This growth strategy aims at separating the growth process from the crystallization process by first creating an amorphous Ti-thiolate that is converted later to crystalline TiS2via thermal annealing. The simultaneous analysis of Ti and S K-edge XAS spectra, exploiting the insights from density functional theory calculations, allows us to shed light on the chemical and structural mechanisms underlying the main steps of growth. The nature of the bonding at the base of the interface creation with the SiO2 substrate is disclosed in this study. Evidence of a progressive incorporation of S in the amorphous Ti-thiolate is given. Finally, it is shown that the annealing step plays a critical role since the transformation of the Ti-thiolate into nanocrystalline TiS2 and the loss of S are simultaneously induced, validating the two-step synthesis approach, which entails distinct growth and crystallization steps. These observations contribute to a deeper understanding of the bonding mechanism at the interface and provide insights for future research in this field and the generation of ultra-thin layered materials.

The initial stages of ZnO atomic layer deposition on atomically flat In0.53Ga0.47As substrates.

InGaAs is one of the III-V active semiconductors used in modern high-electron-mobility transistors or high-speed electronics. ZnO is a good candidate material to be inserted as a tunneling insulator layer at the metal-semiconductor junction. A key consideration in many modern devices is the atomic structure of the hetero-interface, which often ultimately governs the electronic or chemical process of interest. Here, a complementary suite of in situ synchrotron X-ray techniques (fluorescence, reflectivity and absorption) as well as modeling is used to investigate both structural and chemical evolution during the initial growth of ZnO by atomic layer deposition (ALD) on In0.53Ga0.47As substrates. Prior to steady-state growth behavior, we discover a transient regime characterized by two stages. First, substrate-inhibited ZnO growth takes place on InGaAs terraces. This leads eventually to the formation of a 1 nm-thick, two-dimensional (2D) amorphous layer. Second, the growth behavior and its modeling suggest the occurrence of dense island formation, with an aspect ratio and surface roughness that depends sensitively on the growth condition. Finally, ZnO ALD on In0.53Ga0.47As is characterized by 2D steady-state growth with a linear growth rate of 0.21 nm cy-1, as expected for layer-by-layer ZnO ALD.