J. F. Jia

Controlled growth of Zn-polar ZnO epitaxial film by nitridation of sapphire substrate

Surface nitridation is used to eliminate O-polar inversion domains and control the growth of single-domain Zn-polar ZnO film on sapphire (0001) substrate by rf-plasma-assisted molecular-beam epitaxy. It is found that the nitridation temperature is crucial for achieving quality AlN buffer layers and ZnO films with cation polarity, as demonstrated by ex situ transmission electron microscopy. Under optimal growth conditions, a 4×4 surface reconstruction was observed, which is confirmed to be a characteristic surface structure of the Zn-polar films, and can be used as a fingerprint to optimize the ZnO growth.

Low-temperature interface engineering for high-quality ZnO epitaxy on Si(111) substrate

ZnO(0001)∕Si(111) interface is engineered by using a three-step technique, involving low-temperature Mg deposition, oxidation, and MgO homoepitaxy. The double heterostructure of MgO(111)∕Mg(0001)∕Si(111) formed at −10°C prevents the Si surface from oxidation and serves as an excellent template for single-domain ZnO epitaxy, which is confirmed with in situ reflection high-energy electron diffraction observation and ex situ characterization by transmission electron microscopy, x-ray diffraction, and photoluminescence. The low-temperature interface engineering method can also be applied to control other reactive metal/Si interfaces and obtain high-quality oxide templates accordingly.

Cubic nitridation layers on sapphire substrate and their role in polarity selection of ZnO films

Well-defined cubic AlN ultrathin layers formed by nitridation of Al2O3 (0001) substrate at various temperatures were observed by high-resolution transmission electron microscopy. The polarity of the AlN layers strongly depends on the substrate pretreatment and nitridation temperature. The structure of the AlN layers plays a key role in polarity selection of subsequent ZnO films, and both Zn-polar and O-polar ZnO films could be steadily obtained by control of the cubic AlN layers.

Interface engineering for lattice-matched epitaxy of ZnO on (La,Sr)(Al,Ta)O3(111) substrate

ZnO∕(La,Sr)(Al,Ta)O3(LSAT) heterointerface is engineered to control the crystallographic orientation of ZnO films grown by plasmas-assisted molecular beam epitaxy. Lattice-matched in-plane alignment of [112¯0]ZnO‖[112¯]LSAT has been realized using Mg modification of the substrate surface, which is confirmed with in situ reflection high-energy electron diffraction observation, and ex situ characterization of x-ray diffraction and transmission electron microscopy. The low-temperature deposition and high-temperature treatment of the Mg layer on the oxygen-terminated LSAT(111) surface results in selective nucleation of a MgO interface layer which serves as a template for single-domain epitaxy of ZnO. Oxygen-polar ZnO film with an atomically smooth surface has been obtained, which is favorable for metal-ZnO Schottky contact with high barrier height.

Epitaxial orientation of Mg2Si(110) thin film on Si(111) substrate

Epitaxial Mg(2)Si(110) thin film has been obtained on Si(111) substrate by thermally enhanced solid-phase reaction of epitaxial Mg film with underlying Si substrate. An epitaxial orientation relationship of Si(111)parallel to Mg(2)Si(110) and Si parallel to Mg(2)Si has been revealed by transmission electron microscopy. The formation of the unusual epitaxial orientation relationship is attributed to the strain relaxation of Mg(2)Si film in a MgO/Mg(2)Si/Si double heterostructure. (c) 2007 American Institute of Physics.

Two-dimensional growth of Fe thin films with perpendicular magnetic anisotropy on GaN(0001)

The growth and magnetism of Fe thin films on the GaN(0001) surface are studied by scanning tunneling microscopy and surface magneto-optic Kerr effect. It is found that Fe grows in a layer-by-layer mode on the pseudo-1×1 surface at room temperature, and the film develops magnetism at 1.2 ML and shows perpendicular magnetic anisotropy below 6 ML. On the bulk-terminated 1×1 surface, Fe grows in a three-dimensional mode, and ferromagnetization with in-plane anisotropy is observed only above 4.3 ML. Fe-induced √7×√7 reconstruction on the pseudo-1×1 surface plays the key role in reducing the interface reaction and promoting the two-dimensional growth.

Structure and magnetism of ultrathin Co film grown on Pt(100)

Ultrathin Co films were deposited on Pt(100) at room temperature in ultrahigh vacuum, and investigated in situ by low-energy electron diffraction, scanning tunneling microscopy (STM), and surface magneto-optic Kerr effect. The Co film was grown into a wedge shape to provide a continuous change of the film thickness. We find that the Co film forms single-crystal ultrathin films at least up to 5 monolayers (ML). For as-grown films, we observe only in-plane magnetization. After annealing the film, the Co film develops a perpendicular magnetic anisotropy, leading to a spin reorientation transition at 2.7 ML Co thickness. STM measurements were performed at room temperature both before and after annealing the film. We found very different surface morphology and alloy formation after the film annealing, and attribute the perpendicular magnetic anisotropy to the formation of the Co-Pt alloy layer at the Co∕Pt(100) interface.Ultrathin Co films were deposited on Pt(100) at room temperature in ultrahigh vacuum, and investigated in situ by low-energy electron diffraction, scanning tunneling microscopy (STM), and surface magneto-optic Kerr effect. The Co film was grown into a wedge shape to provide a continuous change of the film thickness. We find that the Co film forms single-crystal ultrathin films at least up to 5 monolayers (ML). For as-grown films, we observe only in-plane magnetization. After annealing the film, the Co film develops a perpendicular magnetic anisotropy, leading to a spin reorientation transition at 2.7 ML Co thickness. STM measurements were performed at room temperature both before and after annealing the film. We found very different surface morphology and alloy formation after the film annealing, and attribute the perpendicular magnetic anisotropy to the formation of the Co-Pt alloy layer at the Co∕Pt(100) interface.