Effects of N_2 Pressure on the Structural and Electrical Properties of TiN Films Deposited by Laser Molecular Beam Epitaxy

Titanium nitride （TiN） films were deposited on Si（100） substrates by laser molecular beam epitaxy（LMBE）,and their properties of structure and resistivity with varying N2 pressure were investigated.The results showed that atomically flat TiN films with layer-by-layer growth mode were successfully grown on Si（100） substrates,and （200） was the preferred orientation.With the increasing of N2 pressure,the N/Ti ratio gradually increased and the diffraction peak progressively shifted towards lower diffraction angle.At pressure of 0.1 Pa,stoichiometric TiN film was formed which exhibited the characteristic diffraction angle of （200） plane.All films showed high reflectance to infrared spectrum and the films with overstoichiometry and understoichiometry had a higher resistivity owing to the surface particles and lattice distortion,while the stoichiometric TiN film depicted the minimum resistivity,around 19 μΩ·cm.

Effect of thickness on the microstructure of GaN films on Al_2 O_3 (0001) by laser molecular beam epitaxy

Heteroepitaxial GaN films are grown on sapphire （0001） substrates using laser molecular beam epitaxy. The growth processes are in-situ monitored by reflection high energy electron diffraction. It is revealed that the growth mode of GaN transformed from three-dimensional （3D） island mode to two-dimensional （2D） layer-by-layer mode with the increase of thickness. This paper investigates the interfacial strain relaxation of GaN films by analysing their diffraction patterns. Calculation shows that the strain is completely relaxed when the thickness reaches 15 nm. The surface morphology evolution indicates that island merging and reduction of the island-edge barrier provide an effective way to make GaN films follow a 2D layer-by-layer growth mode. The ll0-nm GaN films with a 2D growth mode have smooth regular hexagonal shapes. The X-ray diffraction indicates that thickness has a significant effect on the crystallized quality of GaN thin films.

Current transport in ZnO/Si heterostructure grown by laser molecular beam epitaxy

The n-ZnO/p-Si heterojunction was fabricated by depositing high quality single crystalline aluminium-doped n-type ZnO film on p-type Si using the laser molecular beam epitaxy technique. The heterojunction exhibited a good rectifying behavior. The electrical properties of the heterojunction were investigated by means of temperature dependence current density-voltage measurements. The mechanism of the current transport was proposed based on the band structure of the heterojunction. When the applied bias V is lower than 0.15 V, the current follows the Ohmic behavior. When 0.15 V ~ V 〈 0.6 V, the transport property is dominated by diffusion or recombination in the junction space charge region, while at higher voltages （V 〉 0.6 V）, the space charge limited effect becomes the main transport mechanism. The current-voltage characteristic under illumination was also investigated. The photovoltage and the short circuit current density of the heterojunction aproached 270 mV and 2.10 mA/cm^2, respectively.

High quality YBCO superconductive thin films fabricated by laser molecular beam epitaxy

High quality YBa2Cu3O6+x (YBCO) superconductive thin films have been fabricated on the SrTiO3(100) substrate using laser molecular beam epitaxy (laser-MBE).The active oxygen source was used,which made the necessary ambient oxygen pressure be 2-3 orders lower than that in pulsed laser deposition (PLD).Tc0 is 85-87 K,and Jc～1.0×106 A/cm2.Atomic force microscopy (AFM) measurements show that no obvious particulates can be observed and the root mean square roughness is 7.8 nm.High stability DC superconducting quantum interference devices (DC-SQUID) was fabricated using this YBCO thin film.

Structure and chemical states of highly eptiaxial CeO_2(001) films grown on SrTiO_3 substrate by laser molecular beam epitaxy

Highly epitaxial and pure（001）-oriented CeO2 films were grown on SrTiO3（001） substrates by laser molecular beam epitaxy method without any gas ambient.Layer-by-layer epitaxial growth mode of CeO2 was confirmed by in situ reflection high-energy electron diffraction（RHEED） observations.High-resolution X-ray diffraction（HRXRD） and high-resolution transmission electron microscopy（HRTEM） results indicated the STO（100）//CeO2（100）,STO[100]//CeO2 [110] epitaxial relationship for out-of-plane and in-plane,respectively.The formation mechanism of the epitaxial film was also discussed in the light of a theoretical model.Chemical states of the LMBE ceria films were evaluated and evidences for the existence of Ce3＋and oxygen vacancies were presented.

Detection of Fe 3d electronic states in the valence band and magnetic properties of Fe-doped ZnO film

Fe-doped ZnO film has been grown by laser molecular beam epitaxy（L-MBE） and structurally characterized by X-ray diffraction（XRD） and scanning electron microscopy（SEM）,all of which reveal the high quality of the film.No secondary phase was detected.Resonant photoemission spectroscopy（RPES） with photon energies around the Fe 2p-3d absorption edge is performed to detect the electronic structure in the valence band.A strong resonant effect at a photon energy of 710 eV is observed.Fe3＋ is the only valence state of Fe ions in the film and the Fe 3d electronic states are concentrated at binding energies of about 3.8 eV and 7 eV～8 eV.There are no electronic states related to Fe near the Fermi level.Magnetic measurements reveal a typical superparamagnetic property at room temperature.The absence of electronic states related to Fe near the Fermi level and the high quality of the film,with few defects,provide little support to ferromagnetism.

The Growth and Microstructure of GaAs Embedded with Al Nanocrystals

The preparation and the microstructure of GaAs embedded with Al nanocrystals prepared by Laser molecular beam epitaxy were investigated.The microstructure of the sample was observed by transmission electron microscope.The reflection high-energy electron diffraction(RHEED)pattern varied from the stripe pattern to the spot pattern at the beginning of the Al nanocrystals growth,and then the spot pattern tended to change to the stripe pattern.There was a large lattice mismatch between Al and GaAs substrate,and Al formed three-dimensional islands on the GaAs substrate,which led to the RHEED transformation into the spot pattern.Otherwise,the dislocations would be formed between the GaAs layer and Al islands due to the large lattice mismatch.Meanwhile,there was some polycrystal of GaAs around the Al islands.

Temperature dependence of surface and structure properties of ZnCdO film

Zn1-xCdx O films are grown on c-sapphire substrates by laser molecular beam epitaxy（LMBE） at different temperatures. Their crystallographic structures, compositions, surface electronic structures are investigated. The a-axis lattice constant of Zn0.95Cd0.05 O is 3.20. Moreover, the epitaxial relationship shows a 30°-in-plane rotation of the film with respect to the c-sapphire substrate. When the substrate temperatures arrives at 500℃, the in situ reflection high-energy electron diffraction（RHEED） pattern of Zn Cd O film shows sharp streaky pattern. The maximum Cd content of Zn Cd O film grown at low substrate temperatures increases up to about 29.6 at.%, which is close to that of the ceramic target. In situ ultraviolet photoelectron spectroscopy（UPS） measurements demonstrate that Zn Cd O film exhibits intense peaks at 4.7 e V and 10.7 e V below the Fermi level, which are assigned to the O 2p and Zn 3p states. Energetic distance between Zn 3d and Cd 4d is 0.60 e V. Above 470 nm, the thin film shows excellent optical transmission.

The Micro-structural Studies of Ni-BaTiO_3 Nanocomposite Films by TEM and EELS

Epitaxial BaTiO3 films with embedded metallic Ni nanocrystal （Ni-BaTiO3） were successfully fabricated on SrTiO3 （001） single-crystalline substrate through the laser molecular beam epitaxial （L-MBE） technique.High resolution transmission electron microscopy （HRTEM） and electron energy loss spectrum （EELS） with Kramers-Kronig analysis methods were employed to characterize the microstructures,elementary distribution and the electron structure of these films.HRTEM results suggested that the structure of BaTiO3 was tetragonal with lattice parameters of a=0.399 nm and c=0.403 nm.Energy dispersive X-Ray spectroscopy （EDX） confirmed metallic Ni nanocrystal embedded successfully in BaTiO3 epitaxial films.The Ni-BaTiO3 composite films were compound of the epitaxial BaTiO3 （110） layers alternating with Ni NCs array （111） layers.Furthermore,the existence of the misfit dislocations induced by the embedding of Ni nanoparticles was also clearly demonstrated by the HRTEM images.The Ni L2,3 edges of EELS revealed that Ni NCs in their metallic state were embedded uniformly in the BaTiO3 matrix.A chemical shift of about 7 eV regarding L3 edges in the Ni EELS was also observed.The optical band gap of BaTiO3 in these films was about 3.84 eV,higher than 3.55 eV for pure BaTiO3 films at room temperature.