# Metal-to-Insulator Transition in Ultrathin Manganite Heterostructures
## 3 The Dead Layer in LSMO  
The bulk LSMO is suggested to be half-metal, but the epitaxial LSMO films are found to gradually become insulating with reducing thickness. A thickness-driven MIT can be found, where above and below a critical thickness, the film is metallic and insulating, respectively, as shown by an example of LSMO films grown on STO substrates (Figure 4a) [39]. The insulating LSMO film is defined as the electrical dead layer. Although there are a lot of reports of the dead layer thickness from different groups, the exact thickness of the electric dead layer is not conclusive. The thicknesses of electric dead layer on different substrates reported by different group are listed in Figure 4b. On the same substrate, for example, STO, quite different thicknesses of dead layer, e.g., 4.0 nm [33], 3.1 nm [18], 2.7 nm [35], and 2.3 nm [39], have been reported. The reported thicknesses of the dead layer on the LaAlO\({}_{3}\) (LAO) substrate even has a big gap (5 nm [31] vs 12 nm [34; 39]). There must be some extrinsic elements causing the dead layer. For example, with the presence of oxygen deficiency during growth, due to an oxidizing background that is not strong enough, gas is found to increase the dead layer thickness [39]. The growth condition, e.g., oxygen partial pressure-dependent Sr-segregation near the top surface is  
Figure 3: Surface morphology of (**a**) the SrTiO\({}_{3}\) substrate and (**b**) 10 u.c. La\({}_{2/3}\)Sr\({}_{1/3}\)MnO\({}_{3}\) (LSMO) thin film imaged by scanning tunneling microscopy. Inset of (**a**) and (**b**) show the corresponding Reflection High-Energy Electron Diffraction (RHEED) pattern. (**c**) RHEED intensity monitored during growth at different oxygen partial pressures. (**d**) Cross-section of a scanning transmission electron microscopy (STEM) image of the LSMO/STO (TiO\({}_{2}\)-terminated SrTiO\({}_{3}\)). Figure is adapted from Ref. [39]. also expected to vary the actual dead layer thickness [46; 47; 48; 49]. Interdiffusion would also play a role in deteriorating the properties, and contribute to the origin of the dead layer [44; 50]. Although there is big discrepancy about the dead layer thickness reported from different research groups, a statistical tendency that dead layer thickness is smaller with smaller strain can be observed if plotting the dead layer thickness as a function of the lattice mismatches, as shown in Figure 4b. This fact strongly suggests an important role of lattice strain in inducing the dead layer, and a minimum dead layer thickness will be expected for a non-strained LSMO films. Unfortunately, ideal freestanding LSMO films cannot be experimentally synthesized, leaving an open question on what is the intrinsic dead layer that is driven by solely dimensionality effect. Since the existence of the dead layer can degrade the device performance, and hinder the miniaturization of the device, it is an important research goal for material science communities to remove the dead layer. Though the underlying physics for the dead layer is still not fully understood, research in the past few decades has already revealed the important role of stoichiometry (oxygen deficiency, cation off-stoichiometry, Sr-segration, interdiffusion), surface broken symmetry-induced structure and electronic reconstruction, charge transfer, and strain in affecting the dead layer, laying a frame for engineering, and eliminating the dead layer. A thinner dead layer is expected when those factors can be mitigated or eliminated. Hereto, an ultrathin dead layer of 3 u.c. was achieved in STO-sandwiched LSMO grown on NdGaO\({}_{3}\) (NGO) (110) substrate by combining the growth condition optimization, strain reduction, interface engineering, and surface capping [39]. It was found that the thickness of the dead layer decreased with increasing oxygen pressure [39]. By using high ozone partial pressure the as background gas during the growth, oxygen deficiency was minimized. The NGO substrate had a smaller lattice mismatch (\(-0.5\%\)) with LSMO.