P. Fisher

Stoichiometric, nonstoichiometric, and locally nonstoichiometric SrTiO3 films grown by molecular beam epitaxy

SrTiO3 films were grown by reactive molecular beam epitaxy to have varying degrees of both global and local cationic nonstoichiometries (with stoichiometry defined as a 1:1 ratio of Sr:Ti). Slight global excesses of Sr and Ti resulted in two-fold reconstructions in the reflection high-energy electron diffraction patterns along the [110] and [100] azimuths, respectively. Larger global nonstoichiometries (2:1 and 1:2 ratios) were also accommodated into the film’s crystalline structure and affected the long-range crystalline order as observed in the x-ray diffraction patterns, both of which were related to the parent perovskite pattern. Local nonstoichiometries were introduced by depositing multiple monolayers (MLs) (from 2 to 33) of SrO and TiO2 in an alternating fashion, while maintaining the global SrTiO3 stoichiometry. These layered structures of SrO and TiO2 blocks inter-reacted during growth to form highly crystalline epitaxial SrTiO3. Films grown in this manner with blocks thicker than 8 MLs were full...

A series of layered intergrowth phases grown by molecular beam epitaxy: SrmTiO2+m(m=1–5)

SrmTiO2+m phases having one TiO2 layer sandwiched between m SrO layers were grown using molecular beam epitaxy. The out-of-plane (in-plane) lattice parameters determined by x-ray diffraction were c(a)=9.14A (3.78A), 23.55A (3.75A), and 14.60A (3.75A) for Sr3TiO5, Sr4TiO6, and Sr5TiO7, respectively. Both lattice parameters change abruptly on going from the m=2 Ruddlesden-Popper phase to m=3 phase, indicating a significant change in the bond lengths (or strain states) on transitioning from the known members to the higher order members of this structural family. Electron microscopy confirmed the artificially layered structures.

Structural properties of SrO thin films grown by molecular beam epitaxy on LaAlO3 substrates

SrO films were grown on LaAlO3 substrates by molecular beam epitaxy and characterized using reflection high-energy electron diffraction (RHEED) and x-ray diffraction (XRD). The evolution of the RHEED pattern is discussed as a function of film thickness. 500A thick SrO films were relaxed and exhibited RHEED patterns indicative of an atomically smooth surface having uniform terrace heights. Films had the epitaxial relationship (001)SrO‖(001)LaAlO3; [010]SrO‖[110]LaAlO3. This 45° in-plane rotation minimizes mismatch and leads to films of high crystalline quality, as verified by Kikuchi lines in the RHEED patterns and narrow rocking curves of the (002) XRD peak.

Growth of GaN films on GaAs substrates in an As-free environment

We investigated growth of GaN films on [001] GaAs substrates by plasma-assisted molecular beam epitaxy in an As-free chamber. The crystalline quality and the surface morphology of the films were studied with x-ray diffraction and transmission electron, scanning electron, and atomic force microscopes. We determined that direct GaN deposition on the thermally desorbed substrate resulted in the growth of a polycrystalline film containing misoriented grains and inclusions. We achieved a significant improvement of the film quality by adopting a procedure consisting of a substrate nitridation, deposition of a low-temperature buffer layer, and a high-temperature overgrowth.