Z.G. Liu

Investigation of high-quality ultra-thin LaAlO3 films as high-k gate dielectrics

We have studied the formation of a high-quality LaAlO3 (LAO) film directly on silicon substrates by the pulsed laser deposition method as a novel high-k gate dielectric. The LAO films can remain amorphous at temperatures up to 850°C. An atomic force microscopy study indicated a very smooth surface of the deposited films with a rms of 0.14 nm for an 8 nm LAO film. The structures and electrical properties of metal–dielectric–semiconductor (Pt/LAO/Si) capacitors were investigated with LAO films deposited under different ambient conditions. High-resolution transmission electron microscopy indicated that interfacial reactions often occur for films of LAO deposited under oxygen ambient. A small effective oxide thickness of 1.2 nm was obtained for those films deposited under 20 Pa nitrogen ambient, with the corresponding leakage current density 17.1 mA cm−2 at +1 V gate voltage. It is proposed that amorphous LAO films are a novel promising alternative high-k gate dielectric material in future ultra-large scale integrated devices.

PULSED EXCIMER (KRF) LASER INDUCED CRYSTALLIZATION OF PBZR0.44TI0.56O3 AMORPHOUS FILMS

Amorphous PZT thin films 600 nm thick were rf sputtered from a PbZr0.44Ti0.56O3 ceramic target with excess PbO onto glass substrates maintained at room temperature. After irradiation with a 248 nm KrF pulsed excimer laser with a power density of 2.3×107 W/m2 at a frequency of 50 Hz and a pulsed width of 30 ns for 2 min, the films crystallized into the PZT perovskite structure to a depth of about 120 nm. Comparisons of this work with PZT crystallization obtained from a traditional oven and 488 nm Ar+ laser postdeposition treatments are also given.

Enhanced fatigue and retention properties of Pb(Ta0.05Zr0.48Ti0.47)O3 films using La0.25Sr0.75CoO3 top and bottom electrodes

Ferroelectric perovskite Pb(Ta0.05Zr0.48Ti0.47)O3 thin-film capacitors having LaxSr1−xCoO3 bottom and top electrodes have been prepared on Pt/TiO2/SiO2/Si(001) substrates by pulsed-laser deposition. It is found that La0.25Sr0.75CoO3 bottom electrodes with cubic structure strongly promote the formation of (001) texture of PTZT films and improve the fatigue and retention properties of the capacitors. The polarization of the La0.25Sr0.75CoO3/Pb(Ta0.05Zr0.48Ti0.47)O3/La0.25Sr0.75CoO3 capacitors with a Pb(Ta0.05Zr0.48Ti0.47)O3 thickness of 400 nm were subjected to no degradation after 1×1010 switching cycles at an applied voltage 5 V with a frequency of 1 MHz. The capacitor retains more than 92.6% of its polarization after a retention time up to 105u200as. The possible microstructural background responsible for the excellent fatigue and retention properties was discussed.

Low electric field induced (001) oriented growth of LiNbO3 films by pulsed laser ablation

Abstract It has been demonstrated that a low electric field of a very few V/cm can induce textured growth of LiNbO 3 thin films fabricated by the pulsed laser ablation technique. By applying an electric field of 7 V/cm perpendicular to the substrate, completely (001) oriented LiNbO 3 films have been grown on fused silica by this method. The films are stoichiometric and very smooth. Optical transmittance measurements showed that the films are of high transparency and have a refractive index close to that of bulk crystals. The effects of electric field in the growth of ferroelectric films were analysed in terms of Volmer-Weber nucleation theory taking into consideration the electrostatic energy. This method should open a new way to produce LiNbO 3 multilayers composed of periodic domain layers with alternatively reversed poling direction.

The epitaxial growth of wurtzite ZnO films on LiNbO3 (0001) substrates

Abstract ZnO epitaxial films were deposited on LiNbO 3 (0xa00xa00xa01) substrates by pulsed laser deposition. The smaller lattice misfit (−8.5%) between ZnO along 〈1 0 1 0〉− direction and LiNbO 3 (0xa00xa00xa01) along 〈1 1 2 0〉− direction, as compared with that in the case of normally used sapphire (0xa00xa00xa01) substrates, favored the epitaxial growth of ZnO films. The transmittance spectra of ZnO films deposited in vacuum after annealed in pure oxygen show a sharp absorption edge at 375.6xa0nm (E g =3.31 eV ) .

The interaction of ambient background gas with a plume formed in pulsed laser deposition

Abstract The propagation of KrF excimer laser-produced plasmas from Pb(Zr 0.52 Ti 0.48 )O 3 has been studied with emphasis on topics relevant to the interaction of a plume with ambient gas in oxide film growth by pulsed laser deposition. A gated CCD was employed to investigate the overall shape and propagation of the laser plasma in different background gases. It is found that the plume image in both 20 Pa O 2 and Ar is spherical. With increasing O 2 pressure to 60 Pa, the plumes are prolonged along the target normal direction and suppressed in the direction parallel to the target surface. Above 60 Pa, the plumes are suppressed along both directions. When the ambient gas is Ar, the changes of the plumes are contradictious. The similarity of plume shape in 20 Pa O 2 and Ar has been interpreted as that the interaction of ablated species with ambient gas is mainly elastic. The differences are thought to be due to the interaction of ablated species with ambient gas, which is reactive in O 2 and elastic in Ar. It was proposed that the elastic scattering of ablated species by background gas leads to the nonstoichiometry of the deposited film, while the reactive collision of ablated species with ambient gas is favorable to reduce the nonstoichiometry and to enhance the oxygen incorporation in the film. It was proposed that the propagation of the ablated species can be divided into three regimes.

Fabrication and characterization of pulsed laser deposited HfO2 films for high-k gate dielectric applications

We investigated the structural and electrical properties of HfO2 films fabricated by the pulsed laser deposition technique. HfO2 films were deposited directly on n-Si (100) substrates and Pt coated silicon substrates, respectively, at 300°C in a 20 Pa N2 ambient, and in situ post-annealed in a 20 Pa N2 ambient. X-ray diffraction indicates that films post-annealed at temperatures more than 500°C exhibit a polycrystalline monoclinic structure. High-resolution transmission electron microscopy images clearly show that an interfacial layer (IL) between an amorphous HfO2 layer and the Si substrate exists. An x-ray photoelectron spectroscopy measurement was performed to identify this IL as nonstoichiometric Hf-silicate. The dielectric constant of amorphous HfO2 was determined to be about 26 by measuring the Pt/HfO2/Pt capacitor structures. Capacitance–voltage measurements show that a small equivalent oxide thickness of 1.26 nm for the 5 nm HfO2 film on the n-Si substrate, with a leakage current of 2.2 mA cm−2 at 1 V gate voltage was obtained.

The effect of the deposition temperature on the low-field magnetoresistance of polycrystalline La0.5Sr0.5MnO3 thin films produced by pulsed laser deposition

Pulsed laser deposition of La0.5Sr0.5MnO3 (LSMO) thin films on quartz wafers at different deposition temperatures has been carried out. The microstructural, electrical and low-field magnetotransport properties of these films are evaluated as functions of the deposition temperature. The film crystallinity depends substantially on the deposition temperature. Significantly enhanced low-field magnetoresistance for the samples deposited from 570xa0°C to 600xa0°C, in which amorphous phase and polycrystalline phase coexist, is observed. The electrical and low-field magnetotransport properties of the thin films are explained by the two-channel model where insulating channels of variable-range-hopping conduction and metallic ones coexist.

Comparative study of laser ablation techniques for fabricating nanocrystalline SnO2 thin films for sensors

Abstract The microstructure and sensing properties of SnO 2 thin films prepared by using SnO 2 and Sn targets at different substrate temperatures with and without post annealing were investigated systematically. It is found that nanocrystalline SnO 2 films with grain size of 4.0–5.2 nm can be achieved by two methods: (1) by crystallization of amorphous SnO 2 films at 400 °C; and (2) by oxidation of Sn films at 400 °C. The nanocrystalline SnO 2 films prepared with these methods exhibit much higher C 2 H 5 OH gas sensitivity in comparison with SnO 2 films prepared using SnO 2 and Sn targets at higher substrate temperatures with grain sizes of several tens of nm upon exposure to air containing 2000 ppm C 2 H 5 OH.

Pulsed laser deposition and characterization of optical waveguiding (Pb,La)(Zr,Ti)O3 thin films

The ferroelectric (Pb,La)(Zr,Ti)O3 optical waveguiding thin films have been prepared on MgO coated (100)LiF substrates by pulsed laser deposition. X‐ray θ‐2θ scans revealed that the films are single‐phase pseudocubic perovskite and highly 〈100〉 textured. The surface chemical composition of the as‐grown films was determined by x‐ray photoelectron spectroscopy. The ferroelectric properties of the films as grown on Pt/Ti coated silicon were demonstrated by using a modified Sawyer–Tower circuit, and the optical waveguiding properties of the films were characterized by using a rutile prism coupling method. The as‐grown films have an average transmittance of 75% in the wavelength range of 400–2000 nm and a refractive index of 2.2 at 632.8 nm close to the bulk PLZT. The distinct m lines of the guided TM and TE modes of the films as grown on MgO coated LiF substrates have been observed.