R. Hiskes

Formation and observation of 50 nm polarized domains in PbZr1−xTixO3 thin film using scanning probe microscope

Nanometer size polarized domains were written in a PbZr1−xTixO3 (PZT) thin film using an atomic force microscope (AFM) and the relationship between the polarized domain and the grain of the film was investigated. The polarized domain was formed by applying a pulse voltage to the ferroelectric PZT thin film through a conductive AFM tip. The polarized domain structure was observed by imaging the piezoelectric‐induced surface vibration by an AFM with an ac voltage applied between the tip and the bottom electrode of a sample. The polarized domains with a diameter of 50 nm were written within a single grain.

Anneal‐tunable Curie temperature and transport of La0.67Ca0.33MnO3

Resistivity measurements on a La0.67Ca0.33MnO3 film are reported for a series of argon anneals at successively higher temperatures. Tc, the ferromagnetic ordering temperature, increases uniformly with increasing annealing temperature and annealing time. Hence, Tc can be tuned by appropriate annealing. In order to fully anneal these samples, i.e., achieve bulk properties, it proves sufficient to anneal them in argon. Further annealing in oxygen produces only minor changes in the resistivity. Data from Tc up to 1200 K show activated conduction with ρ=BTeEa/kT, the temperature dependence predicted by the Emin–Holstein theory of adiabatic polaron hopping. Their model fits both data from the partially annealed and fully annealed samples better than the variable range hopping or semiconductor models which have been used by previous workers. The activation energy Ea and resistivity coefficient B decrease with increasing maximum anneal temperature. These changes, together with the increase in Tc, are consistent w...

Single source metalorganic chemical vapor deposition of low microwave surface resistance YBa2Cu3O7

YBa2Cu3O7 films 2000–7500 A thick were deposited onto [100] LaAlO3 substrates using a novel single source metalorganic chemical vapor deposition technique. We have so far observed critical currents as high as 4×106 A/cm2 at 77 K, transition temperatures (Tc at R=0) as high as 91 K, and the 10 GHz microwave surface resistance Rs as low as 40 μΩ at 4.2 K and 400 μΩ at 77 K.

Influence of strain and microstructure on magnetotransport in La0.7Ca0.3MnO3 thin films

A La0.7Ca0.3MnO3 thin film made by pulsed laser deposition (PLD) and another film of the same composition made by metal organic chemical vapor deposition (MOCVD), both on single crystal LaAlO3, were subject to a series of six, short, controlled anneals. The oxygen content was purposely not changed in the films from the first anneal to subsequent anneals. After each anneal, the film microstructures were characterized to determine average grain size, lattice constants, nonuniform strain, and crystalline mosaic spread, and these parameters were correlated with the magnetotransport properties. For both sets of films, the influence of annealing was to both increase the temperature at which the maximum in the magnetoresistance occurs (Tm) and the maximum magnetoresistance (MR) value. The improvement in film properties occurred in conjunction with stress relaxation and improved crystallinity, as a result of grain growth. The MOCVD films showed poorer grain coupling and poorer epitaxy compared to the PLD films. T...

Low dislocation density, large diameter, liquid encapsulated Czochralski growth of GaAs

Abstract We have grown 70 nm diameter, 1200 g, 〈100〉 oriented GaAs single crystals in a low thermal gradient environment. Without intentional doping we have obtained average KOH etch pit densities (EPD) of 5000 cm -2 over 70% of the wafer area, and throughout 75% of the ingot length. This material is semi-insulating with a resistivity of 2 × 10 7 Ω cm. By doping with Si to give free carrier concentrations in the range of 1 to 5 × 10 18 cm -3 , we reproducibly obtain average EPDs of -2 . Near the bottom of the ingot, wafers are virtually dislocation free except very near the wafer edge. The distribution of etch pits across the wafer is in general agreement with the thermoelastic stress model: high at the edges and in the center, but with an annular region essentially dislocation free (Si-doped). The axial distribution, however, does not fit this model: generally high at the seed and tail ends of the crystal and low through the bulk of the ingot. Transmission X-ray topographs confirm the nature and extent of the dislocation distribution.

Characteristics of PZT thin films as ultra-high density recording media

Abstract Ferroelectric thin films as the recording media of scanning probe microscope-based storage devices were investigated using an atomic force microscope(AFM) technique. Polarization domains were formed in the PbZrxTi1-xO3(PZT) thin films epitaxialy grown on the epitaxial SrRuO3(SRO) thin films on SrTiO3 substrate by applying a pulse voltage between the conductive tip of AFM and SRO as a bottom electrode. The polarized domains were observed by detecting the inverse-piezoelectricity-induced surface vibration of the PZT thin film caused by applying an ac modulation voltage to the conductive tip. The recording density of polarized domains, domain switching speed and preliminary retention characteristics of polarized domains were studied. The polarized domains as small as 30 nm are formed in the PZT thin film with the thickness of 45 nm. The small domains can be formed by applying a 100 ns pulse of 10 V to the conductive tip. As for the retention characteristics of polarized domains with a size of 90–110...

Solid source MOCVD for the epitaxial growth of thin oxide films

A novel solid source metalorganic chemical vapor deposition (MOCVD) process has been used to grow highly textured thin films of CeO2 and MgO, and highly oriented thin films of Sr1-xBaxNb206 (SBN). Both (100)-oriented and (lll)-oriented CeO2 films were reproducibly grown on (1102)-oriented Al203 (r-plane sapphire) substrates. Higher substrate temperatures and higher growth rates were found to favor the (111) orientation. The epitaxial (100) CeO2 films, 150 A thick, had very smooth surfaces, with an RMS surface roughness of 7 A, as measured by atomic force microscopy. MgO films with (100) orientation were readily grown on both r-plane sapphire and (100) SrTiO3 substrates at temperatures below 600°C. SBN films, 1.6 μm thick, were also grown on r-plane sapphire substrates at 700-800°C using a CeO2 buffer layer and all tetramethylheptanedionate (Sr, Ba, and Nb) metalorganic sources.

Magnetoconductivity and Hall effects in La0.67Ca0.33MnO3

The observed H2 and |H| dependence of the magnetoresistance above and below TC, respectively, may be explained by general time‐reversal symmetry considerations. We further find empirically that the saturation observed in the magnetoresistance is best fit by a simple resistor in series with a magnetoconductor: ρ(H)=ρ∞+1/(σ0+γ|H|) for T TC. This provides a functional form to analyze and predict the magnetoresistance over a wide range of fields. This suggests that the underlying mechanism of ‘‘colossal magnetoresistance ’’ may be magnetoconductive, not magnetoresistive. The magnetoresistance and Hall effects on an annealed epitaxial thin films of La0.67Ca0.33MnO3 were measured at 0.9 TC and 1.1 TC. At low fields, anisotropic magnetoresistance plays a dominant role. The high field Hall effect shows holelike carriers above and below TC. The apparent change in sign at low fields is likely due to the anomalous Hall effect.

Single source MOCVD of epitaxial oxide thin films

A novel and inexpensive single source metalorganic chemical vapor deposition (MOCVD) reactor has been developed at Hewlett Packard Laboratories, which permits the controlled and reproducible growth of device quality thin film heterostructures on a variety of substrates, using marginally volatile β-diketonate powdered solid sources. This reactor has been used to deposit state-of-the-art dielectrics, ferroelectrics and yttrium barium copper oxide (YBa 2 Cu 3 0 7 or YBCO) high temperature superconducting multi layers. The technique is readily applicable to many other oxide and non-oxide material systems. We describe herein the reactor and present the results of deposition experiments for single layers and heterostructures of YBCO on substrates up to 4 inches in diameter. To demonstrate the versatility of the reactor, we also present initial results for the deposition of other materials, including MgO, CeO 2 , Pr60 11 , barium cerate, hematite, and strontium barium niobate.

Crystalline quality and surface morphology of (100) CeO2 thin films grown on sapphire substrates by solid source metalorganic chemical vapor deposition

(100)-oriented, cerium oxide epitaxial thin films, approximately 250 A thick, were grown on r-plane sapphire substrates using a solid source MOCVD technique. Rocking curve FWHM values as low as 0.63° and AFM surface roughness on the order of 5 A RMS were obtained with substrate temperatures near 680°C and growth rates of 0.13 A/s. Lower substrate temperatures and higher metalorganic source feed rates resulted in rougher layers, which had broader XRD rocking curve widths, indicating poorer out-of-plane alignment. RBS channeling spectra showed that the best films had high crystalline quality: only at the film-substrate interface were slight distortions due to lattice or thermal expansion mismatch detected.