The presence of Laue oscillations in a film grown on a solid surface is broadly taken as indicating a high quality, crystallographically aligned film of the targeted compound. In this paper we briefly review the origins of both Laue oscillations and Kiessig fringes and show how they can be used together to determine if extra thickness exists above or below the coherently diffracting domains. The differences between experimental and “ideal” films are discussed and the effect of structural features (roughness, different thickness coherently diffracting domains and thickness in addition to the coherently diffracting domains) are illustrated with experimental and simulated data for metal and mixed-metal chalcogenide films of titanium, bismuth, vanadium/iron, and bismuth/molybdenum. Examples are given showing how quantitative information can be extracted from experimental diffraction patterns.

Recently, it was shown that the metastable heterostructure (Bi-Se) 0.97 MoSe 2 , consisting of alternating bilayers of BiSe and MoSe 2 trilayers, contained both the semiconducting 2H-and the metallic 1T-polytype of MoSe 2 . In this study, we investigated five other structures containing different stackings of BiSe, Bi 2 Se 3 , and MoSe 2 layers in the repeat unit to understand what facilitates the stabilization of metastable constituent layers. X-ray diffraction was used to characterize the stacking sequences and the structure of the different layers, and X-ray photoelectron spectroscopy and Raman spectroscopy were carried out to understand the occurring interlayer interactions. A sufficient transfer of electrons from the BiSe layers into MoSe 2 is required to form 1T-MoSe 2 upon crystallization. The yield of 1T-MoSe 2 is reduced if charges are localized at antiphase boundaries in BiSe or if filling of conduction band states in Bi 2 Se 3 or 2H-MoSe 2 occurs. If the repeat unit contains more than one layer of MoSe 2 , band structure changes allow charges from BiSe to be distributed over all available dichalcogenide layers, suppressing the formation of both 1T-MoSe 2 and antiphase boundaries. Understanding the interplay between the different layers may enable the design of heterostructures with the desired constituent structure and properties.

Lead selenide (PbSe) has extensively been investigated due to its thermoelectric and photoconductive properties. More recently, predictions of emergent properties have focused synthetic efforts on preparing ultra-smooth layers of crystallographically aligned PbSe layers with a thickness equal to an integer number of monolayers. While rough films are typically formed through Volmer-Weber growth on heated substrates, the data presented herein indicates that a crystallographically aligned, ultrathin layer with a defined number of PbSe layers forms on mostly amorphous MoÀ Se and VÀ Se coated Si substrates during nominally room temperature deposition. Since the films are uniform in thickness, the number of PbSe bilayers formed depends on the amount of Pb (or Se) deposited. If the amount of Pb deposited is close to that required for an integral number of unit cells and there is sufficient Se, Laue oscillations are present in the as-deposited samples. The Laue oscillations reflect the quality and uniformity of the crystalline PbSe domains. This study demonstrates that the substrate does not need to be crystalline to form uniform, crystallographically aligned, integer monolayer thick PbSe films. These results suggest that tuning the strength of interactions between substrate and growing film is necessary to form smooth, crystallographically aligned layers. The substrate does not need to be crystalline for these interactions to be strong enough to enable growth of crystallographically aligned films.

There have been a number of surprising reports of unexpected products when preparing heterostructures of Bi 2 Se 3 with other 2D layers. These reports prompted us to explore the formation of metastable heterostructures containing Bi 2 Se 3 using X-ray diffraction techniques to follow the reaction pathway. We discovered that the products formed depend on the electronic properties of the second constituent. Bi|Se layers deposited in a 2:3 ratio with enough atoms to make a single five-plane layer evolved to form thermodynamically stable Bi 2 Se 3 as expected from the phase diagram. When the same Bi|Se layers were sequentially deposited with M|Se layers that form semiconductor layers (PbSe and 2H-MoSe 2 ), Bi 2 Se 3 -containing heterostructures formed. When the same Bi|Se layers were deposited with M|Se layers that form metallic layers (TiSe 2, VSe 2 , and 1T-MoSe 2 ), BiSe-containing heterostructures formed. The amount of excess Se in the precursor controls whether [(Bi 2 Se 3 ) 1+δ ] 1 [(MoSe 2 )] 1 or [(BiSe) 1+γ ] 1 [(MoSe 2 )] 1 forms. XPS data indicates that a mixture of both metallic 1T and semiconducting 2H-MoSe 2 is present in [(BiSe) 1+γ ] 1 [(MoSe 2 )] 1 , while only semiconducting 2H-MoSe 2 is present when layered with Bi 2 Se 3 . The electronic structure of adjacent layers impacts the formation of different structures from layers with similar local compositions. This provides an important additional parameter to consider when designing the synthesis of heterostructures, similar to substituent effects in molecular chemistry.

While a linear growth behavior is one of the fingerprints of textbook atomic layer deposition processes, the growth often deviates from that behavior in the initial regime, i.e., the first few cycles of a process. To properly understand the growth behavior in the initial regime is particularly important for applications that rely on the exact thickness of very thin films. The determination of the thicknesses of the initial regime, however, often requires special equipment and techniques that are not always available. We propose a thickness determination method that is based on X-ray reflectivity (XRR) measurements on double layer structures, i.e., substrate/base layer/top layer. XRR is a standard thin film characterization method. Utilizing the inherent properties of fast Fourier transformation in combination with a multi-Gaussian fitting routine permits the determination of thicknesses down to t≈2 nm. We evaluate the boundaries of our model, which are given by the separation and full width at half maximum of the individual Gaussians. Finally, we compare our results with data from x-ray fluorescence spectroscopy, which is a standard method for measuring ultra-thin films.