Hirohiko Niu

SIZE EFFECTS OF EPITAXIAL AND POLYCRYSTALLINE PB(ZR, TI)O3 THIN FILMS GROWN BY METALORGANIC CHEMICAL VAPOR DEPOSITION

In order to distinguish the dependence of electrical properties on thickness from that on grain size, the crystalline and electrical properties of epitaxial and polycrystalline Pb(Zr, Ti)O3 (PZT) thin films grown on SrRuO3 (SRO)/SrTiO3(100) and SRO/SiO2/Si by metalorganic chemical vapor deposition (MOCVD) were investigated. The tetragonality of our epitaxial PZT films slightly increased with decreasing thickness. Relative dielectric constant(er) of PZT films decreased with decreasing thickness. The thickness dependence of er was stronger for polycrystalline PZT thin films than for epitaxial films. The remanent polarization of both epitaxial and polycrystalline films was not dependent on thickness. The coercive field of both epitaxial and polycrystalline films increased markedly as the thickness decreased. In our experiment, the minimum thickness of epitaxial PZT films which showed D-E hysteresis was 40 nm.

Low-Temperature Fabrication of Ir/Pb(Zr,Ti)O3/Ir Capacitors Solely by Metalorganic Chemical Vapor Deposition

Ir/Pb(Zr, Ti)O3(PZT)/Ir capacitors were fabricated solely by metalorganic chemical vapor deposition (MOCVD). Both top and bottom Ir electrodes with mirror like surfaces were obtained at 300°C by MOCVD. Ferroelectric PZT thin films were successfully prepared at 395-540°C by MOCVD using (C2H5)3PbOCH2C(CH3)3 as a Pb precursor and PbTiO3 as a seed. Both Ir electrodes and PZT thin films showed good step coverage of 70–80%. PZT films prepared at 445–540°C exhibited well-saturated hysteresis loops with remanent polarization of 19–25 µC/cm2.

Observations of Island Structures at the Initial Growth Stage of PbZrxTi1-xO3 Thin Films Prepared by Metalorganic Chemical Vapor Deposition

Island structures of PbZrxTi1-xO3(PZT) (x=0.26 and 0.74) were observed prior to formation of a continuous film at the initial growth stage on a Pt(111)/SiO2/Si substrate by metalorganic chemical vapor deposition (MOCVD). Triangular-shaped (111)-oriented PZT islands were observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Transmission electron microscopy (TEM) observations revealed that PZT islands 80 to 100 nm wide and 20 to 30 nm high had a twin structure due to the presence of tetragonal or rhombohedral structures. Hence, small PZT islands could have spontaneous polarization. In piezoresponse measurements using scanning probe microscopy (SPM), the phase difference between the vibrational signal of the SPM cantilever and the applied ac voltage changed from 0 to 180°, resulting in a hysteresis loop due to polarization reversal by a dc bias voltage. From this piezoresponse result, it was found that PZT islands had ferroelectricity.

Structural control of self-assembled PbTiO3 nanoislands fabricated by metalorganic chemical vapor deposition

Self-assembled PbTiO3 nanoislands of three different shapes with orderly in-plane directions were fabricated on Pt∕SrTiO3 substrates by metalorganic chemical vapor deposition. The shapes of the nanoislands were triangular-shaped (width 50–110nm, height 20–30nm), triangular-prism-shaped (width 50–110nm, length 100–550nm, height 10–30nm) and square-shaped (width 40–130nm, height 4–10nm) on Pt∕SrTiO3(111), (101), and (001), respectively. The PbTiO3 nanoislands were epitaxially grown on the Pt∕SrTiO3 substrates and consisted of {100} and {001} facets irrespective of the orientation of the substrates indicating that structural control of shape and in-plane direction of self-assembled PbTiO3 nanoislands can be achieved through epitaxial relations. The self-assembled PbTiO3 nanoislands with three different shapes were found to be ferroelectric by piezoresponse force microscopy.

Application of Lateral Photovoltaic Effect to the Measurement of the Physical Quantities of P-N Junctions?Sheet Resistivity and Junction Conductance of N2+ Implanted Si

A new method of measuring the physical quantities of a thin film p-n junction such as the one formed by ion implantation has been developed; by means of this method difficulties of making reliable electrical contacts with thin layers and leakage currents which are often encountered when applying the bias voltage can be avoided. Lucovskys theory for the lateral photovoltaic effect has been extended such that nonzero values for sheet resistivities of both p and n layers and a finite value for the length of the junction are taken into consideration, and the modified theory has been experimentally verified using p-Si samples implanted with N2+ at 8 keV. With the aid of the theory, sheet resistivities, junction conductances and some other physical quantities are obtained by analysing the measured lateral photovoltage (LPV). The use of the LPV method together with the ordinary one increases the reliability of the measurement.

Step Coverage Characteristics of Pb(Zr, Ti)O3 Thin Films on Various Electrode Materials by Metalorganic Chemical Vapor Deposition

The step coverage characteristics of Pb(Zr, Ti)O3 (PZT) thin films on steps with various electrode materials, such as Pt/SiO2, Ir/SiO2 and IrO2/SiO2, by metalorganic chemical vapor deposition (MOCVD) were investigated. The step coverage of PZT films deposited on Pt/SiO2 at 500 and 550° C was 80–87%. Deformation of Pt/SiO2 steps was observed when PZT films were deposited at 600° C. The cause of this deformation of steps was mainly the diffusion of Pb into the steps. When PZT thin films were deposited on Ir/SiO2 at 500–600° C and IrO2/SiO2 at 500 and 550° C, good step coverage of 76–93% and 70–85% was obtained, respectively.

Self-Assembled PbTiO3 Nano-Islands Prepared on SrTiO3 by Metalorganic Chemical Vapor Deposition

PbTiO3 nano-islands were prepared on Pt(111)/SiO2/Si(100), Pt(111)/SrTiO3(111) and Pt(110)/SrTiO3(110) substrates by metalorganic chemical vapor deposition (MOCVD). When PbTiO3 was deposited on Pt(111)/SiO2/Si, PbTiO3 nano-islands on different Pt grains had different in-plane orientations. On the other hand, all PbTiO3 nano-islands on Pt(111)/SrTiO3(111) showed the same in-plane orientation. The size of PbTiO3 nano-islands on the Pt(111)/SrTiO3(111) substrate was uniform compared with that on the Pt(111)/SiO2/Si(100) substrate. When PbTiO3 islands were deposited on a Pt(110)/SrTiO3(110) substrate, triangular-prism-shaped PbTiO3 islands, which preferentially grew along the direction, were obtained. These PbTiO3 nano-islands had also uniform in-plane orientation. These results showed that the size, shape and in-plane orientation of self-assembled ferroelectric nano-islands by MOCVD were fairly controllable using the epitaxial relation.

Epitaxial Growth and Ferroelectric Properties of the 20-nm-Thick Pb(Zr, Ti)O3 Film on SrTiO3(100) with an Atomically Flat Surface by Metalorganic Chemical Vapor Deposition

We investigated the influences of the surface flatness of a substrate on the surface morphology and ferroelectric properties of Pb(Zr, Ti)O3 (PZT) ultrathin films by metalorganic chemical vapor deposition (MOCVD). When SrTiO3 (STO) (100) was etched in a buffered NH4F–HF solution and annealed, an atomically flat surface with steps corresponding to one unit cell height was obtained. SrRuO3 (SRO) deposited on the treated STO also showed a flat surface with steps. Epitaxial PZT films with thickness of 20–60 nm on SRO/treated STO were successfully obtained. The 20-nm-thick PZT ultrathin film showed remanent polarization of 30 µC/cm2 and coercive electric field of 730 kV/cm. On the other hand, when PZT films were grown on SRO/nontreated STO, PZT films with thicknesses of 20–32 nm showed no hysteresis loops due to high leakage current. These results suggest that surface flatness of STO and SRO plays a very important role in the growth of PZT ultrathin films.

Crystalline and Ferroelectric Properties of Pb(Zr, Ti)O3 Thin Films Grown by Low-Temperature Metalorganic Chemical Vapor Deposition

Low-temperature growth of Pb(Zr, Ti)O3 (PZT) thin films, as low as 390°C, by metalorganic chemical vapor deposition (MOCVD) using PbTiO3 seeds was successfully achieved. Influence of the crystalline nature of PbTiO3 seeds on crystalline and ferroelectric properties of the PZT thin films were investigated. The orientation of PZT films and its degree were strongly influenced by those of PbTiO3 seeds. PZT thin films grown at 390°C showed higher orientation as the deposition temperatures of the seeds increased from 380 to 450°C. PZT thin films grown with the 380–450°C-deposited seeds showed remanent polarization (2Pr) of 6.9–15.9 µC/cm2 and coercive field (2Ec) of 93–162 kV/cm. Crystalline and ferroelectric properties of PZT films with and without seeds were also investigated.

Effects of Pt/SrRuO3 Top Electrodes on Ferroelectric Properties of Epitaxial (Pb, La)(Zr, Ti)O3 Thin Films

Epitaxially c-axis oriented PLZT films with the composition (Pb0.925La0.075)(Zr0.4Ti0.6)O3 were deposited on Pt/MgO(100) substrate by rf-magnetron sputtering using a compacted powder target with the composition (0.8PLZT+0.2PbO). Pt/SrRuO3(SRO)/PLZT/Pt capacitors were successfully fabricated by forming Pt/SRO top electrodes onto PLZT films. The 206-nm-thick PLZT films with layered Pt/SRO top electrodes exhibited a slightly high leakage current at a low electric field, compared with PLZT films with the same thickness and a single Pt top electrode. The values of switchable polarization after 1010 cycles for Pt/PLZT/Pt capacitors decreased up to around 17% of their initial values, whereas the switchable polarization for Pt/SRO/PLZT/Pt capacitors hardly exhibited any fatigue degradation due to polarization reversal. The layered Pt/SRO is useful as a top-electrode material for fabricating the low leakage and high endurance ferroelectric capacitors.