D. V. Mashovets

Transient-current measurement of the trap charge density at interfaces of a thin-film metal∕ferroelectric∕metal structure

A method providing estimation of the trap density at metal∕ferroelectric interfaces of a depleted ferroelectric film located between back-to-back Schottky barriers has been developed. The method is based on the recharge of interface traps induced by external bias pulse applied to the metal∕ferroelectric∕metal structure. It is shown that the transient current under bias pulse can be controlled by the trap recharge on the reverse-biased interface. Using the method, the trap charge density on interfaces of the Pt∕PZT∕Ir(Ti∕SiO2∕Si) capacitor was found from transient-current measurements to be two orders of magnitude less than the remnant polarization of PbZrxTi1−xO3 film.

White electroluminescence from ZnO/GaN structures

White electroluminescence (EL) from ZnO/GaN structures fabricated by pulsed laser deposition of ZnO:In onto GaN:Mg/GaN structures MOCVD-grown on Al2O3 substrates has been observed. The white light is produced by superposition of the two strongest emission lines, narrow blue and broad yellow, peaked at 440 and 550 nm, respectively. The intensity ratio of different EL lines from ZnO/GaN/Al2O3 structures depends on the ZnO film quality and drive current. The white EL is due to the high density of structural defects at the n-ZnO/p-GaN interface. A band diagram of the n-ZnO/p-GaN/n-GaN structure is constructed and a qualitative explanation of the EL is suggested.

Ageing of Thin-Film Capacitor Structures Based on PZT

Thin-film Pt/PZT/Ir(Ti/SiO/Si) and Ir/PZT/PTO/Ir(Ti/SiO2/Si) capacitors, as-grown and after ageing, are investigated. A comprehensive study based on the Auger spectroscopy and measurement of electrical and ferroelectrical characteristics is performed. Ageing results in increasing of the oxygen content in PZT, broadening of interfaces owing to formation of metal oxides, diffusion of Pb and Ti towards the top electrode along the grain boundaries, and diffusion of Ti into PZT grain. These processes induce decreasing of the structure capacitance, healing of the interface traps, enhance leakage and dielectric loss.

Auger spectroscopy and properties of Ir(Pt)/PZT(PZT/PT)/Ir nanosized thin-film structures

Studies of the elemental distribution profiles through the thickness of thin-film ferroelectric capacitor structures using Auger spectroscopy made it possible to establish the relation between the elemental and phase composition of the structures, on the one hand, and their electrophysical properties, on the other. Special features were observed in the behavior of the lead titanate underlayer both in the as-fabricated and aged structures. It is shown that the variation in the characteristics of the capacitor structures during aging is caused by diffusion of the elements at the interfaces and in the PZT film against a background of a significant increase in the oxygen concentration. As a result, interface-modifying oxide layers form and the trap density on the upper and lower interfaces decreases in all samples.

Strongly Modulated Conductance in Ag/PLZT/LSCO Ferroelectric Field-Effect Transistor

A possibility of fabricating an all-perovskite field effect transistor with a deep conductance modulation is shown. A 5-10 nm thick LSCO film was used as a channel, and a ferroelectric gate insulator was 100 nm thick PLZT. The channel conductance modulation in the studied transistors was found to be ∼70%, much higher than that reported up to now and could be sufficient for reliable readout of data stored. Relatively low carrier density, small thickness, and rather high surface quality of the channel have provided the penetration of the electric field into the major part of the channel and deep conductance modulation.

Strongly Modulated Conductivity in Ag/PLZT/LSCO Field-Effect Transistor

A possibility of deep (>70%) modulation of La 1.94 Sr 0.06 CuO 4 and La 1.85 Sr 0.15 CuO 4 channel conductance has been shown in all-perovskite field effect transistor with a (Pb 0.95 La 0.05 )(Zr 0.2 Ti 0.8 )O 3 ferroelectric as a gate insulator. Relatively low carrier density (∼10 18 m 10 19 cm m 3 ), small thickness, and rather high surface quality of the channel have provided the penetration of the electric field into the major part of the channel and deep conductance modulation, sufficient, in principle, for operation of a nonvolatile memory cell with a nondestructive readout of information.

PHOTOVOLTAIC EFFECT BASED ON POLARIZATION CHARGE IN POLYCRYSTALLINE Pb(ZrTi)O3 FILM

ABSTRACT A steady-state short-circuit photocurrent across polycrystalline Pb(ZrTi)O3(PZT) film is found to be directed opposite to ferroelectric polarization, defined by the magnitude of polarization, but it is not the depolarization current of the ferroelectric. To explain this, the authors assume that Pt/PZT/Ir capacitor consists of PZT grains and semiconductor PbO phase segregated on PZT grain boundaries and forming conducting channels. In such medium, uncompensated polarization charge can arise at the PZT/PbO interface and generate the electric field acting on carriers in PbO channels, thus producing the photocurrent across the film. The relation between the polarization charge and electric field is considered.

A ferroelectric field effect transistor based on a Pb(ZrxTi1−x )O3/SnO2 heterostructure

The possibility of fabricating a ferroelectric FET based on a Pb(ZrxTi1−x)O3/SnO2 (PZT/SnO2) heterostructure is investigated. Sb-doped epitaxial SnO2/Al2O3 thin film deposited by YAG laser ablation from a metal target is used as the FET channel. The highest obtained electron mobility in the channel is 25 cm2/(V s) at an electron density of 8 × 1019 cm−3. The possibility of growing PZT film directly on SnO2 film using two different techniques, laser ablation and magnetron sputtering, is demonstrated. Both methods have been used in the fabrication of Au/PZT/SnO2 capacitor heterostructures, whose top Au electrode is 250×250 μm2 in size. These cells demonstrate a capacitance of 1000 pF at a 10-V bias and remnant polarization up to 16 μC/cm2. A Au/PZT/SnO2/Al2O3 transistor structure with 94% modulation of the channel current is fabricated. The difference in the channel current under the effect of positive and negative remnant polarization of the undergate ferroelectric is 37%.

Trap Charge Density at Interfaces of MOCVD Pt(Ir)/PZT/Ir(Ti/SiO 2 /Si) Structures

A method providing estimation of the trap density at metal/ferroelectric interfaces of a depleted ferroelectric film located between back-to-back Schottky barriers has been developed. The method is based on the recharge of interface traps induced by external bias pulse applied to the metal/ferroelectric/metal structure. It is shown that the transient current under bias pulse can be controlled by the traps recharge on the reverse-biased interface. Using the method, the trap charge density on interfaces of MOCVD Pt/PZT/Ir(Ti/SiO 2 /Si) and Ir/PZT/Ir(Ti/SiO 2 /Si) capacitors were found from transient current measurements.

Effect of the Deep Center Recharge on the Process of Ferroelectric Switching in Au/PLZT/LSCO Structures

The analysis of transient currents in Au/PLZT/LSCO gate-capacitor structure has been used to determine the distribution of surface traps at the F/S interface. The total charge associated with the surface states recharge under the bias variation was found to be Qt ∼3 μC/cm2, which is far less than the remnant polarization at the operating bias values.