M. M. Saad

Exploring grain size as a cause for “dead-layer” effects in thin film capacitors

Pulsed laser deposition was used to make a series of Au/Ba0.5Sr0.5TiO3 (BST)/SrRuO3/MgO thin film capacitors with dielectric thickness ranging from ∼15 nm to ∼1 μm. Surface grain size of the dielectric was monitored as a function of thickness using both atomic force microscopy and transmission electron microscopy. Grain size data were considered in conjunction with low field dielectric constant measurements. It was observed that the grain size decreased with decreasing thickness in a manner similar to the dielectric constant. Simple models were developed in which a functionally inferior layer at the grain boundary was considered as responsible for the observed dielectric behavior. If a purely columnar microstructure was assumed, then constant thickness grain-boundary dead layers could indeed reproduce the series capacitor dielectric response observed, even though such layers would contribute electrically in parallel with unaffected bulk-like BST. Best fits indicated that the dead layers would have a relat...

High-field conduction in barium titanate

We present current–voltage studies of very thin (∼77nm) barium titanate single crystals up to 1.3GV∕m applied field. These show that the mechanism of leakage current at high fields is that of space charge limited conduction (SCLC) in a regime with a continuous distribution of traps, according to the original model of Rose [Phys. Rev. 97, 1538 (1955)]. This study represents a factor of ×5 in field compared with the early studies of BaTiO3 conduction [A. Branwood et al., Proc. Phys. Soc. London 79, 1161 (1962)]. Comparison is also given with ceramic multilayer barium titanate capacitors, and with variable range hopping [B. I. Shklovskii, Sov. Phys. Semicond. 6, 1964 (1973)], reported in SrTiO3 films [D. Fuchs, M. Adam, and R. Schneider, J. Phys. IV France 11, 71 (2001)].

Conformal oxide coating of carbon nanotubes

The International Roadmap for Ferroelectric Memories requires three-dimensional integration of high-dielectric materials onto metal interconnects or bottom electrodes by 2010. Here, we demonstrate the possibility of conformally coating carbon nanotubes with high-dielectric oxide as a first step toward ultrahigh integration density of three-dimensional ferroelectric random access memories.

Characteristics of single crystal "thin film" capacitor structures made using a focused ion beam microscope

The focused ion beam microscope was used to isolate thin parallel-sided lamellae from single crystals of BaTiO3 and SrTiO3. Damage and ion-implantation on the faces of the lamellae were observed, but could be removed by thermal annealing. A series of lamellae ranging in thickness from ∼660 to ∼300 nm were made from a SrTiO3 single crystal, and after thermal annealing, gold electrodes were deposited on either side to form parallel-plate capacitor structures. The room temperature functional properties of these single crystal “thin film” capacitors were investigated. Although space-charge artifacts dominated the low frequency response, above 10 kHz, examination of the change in capacitance as a function of lamellar thickness allowed the room temperature relative dielectric constant to be meaningfully extracted. This was found to be ∼300, as is the case in bulk. For these single crystal lamellae there was therefore no evidence of a collapse in dielectric constant associated with thin film dimensions.

Investigating the effects of reduced size on the properties of ferroelectrics

A series of experiments has been undertaken to understand more about the fundamental origin of the thickness-induced permittivity collapse often observed in conventional thin film ferroelectric heterostructures. The various experiments are discussed, highlighting the eventual need to examine permittivity collapse in thin film single crystal material. It has been seen that dielectric collapse is not a direct consequence of reduced size, and neither is it a consequence of unavoidable physics associated with the ferroelectric-electrode boundary. Research on three-dimensional shape-constrained ferroelectrics, emphasizing self-assembled structures based on nanoporous alumina templates and on FIB-milled single crystals, is also presented, and appears to represent an exciting area for ongoing research

LIQUID SOURCE MISTED CHEMICAL DEPOSITION PROCESS OF THREE-DIMENSIONAL NANO-FERROELECTRICS WITH SUBSTRATE HEATING

ABSTRACT A modification of liquid source misted chemical deposition process (LSMCD) with heating mist and substrate has developed, and this enabled to control mist penetrability and fluidity on sidewalls of three-dimensional structures and ensure step coverage. A modified LSMCD process allowed a combinatorial approach of Pb(Zr,Ti)O3 (PZT) thin films and carbon nanotubes (CNTs) toward ultrahigh integration density of ferroelectric random access memories (FeRAMs). The CNTs templates were survived during the crystallization process of deposited PZT film onto CNTs annealed at 650°C in oxygen ambient due to a matter of minute process, so that the thermal budget is quite small. The modified LSMCD process opens up the possibility to realize the nanoscale capacitor structure of ferroelectric PZT film with CNTs electrodes toward ultrahigh integration density FeRAMs.

Thin Film Capacitor Cut from Single Crystals Using Focused Ion Beam Milling

The focused ion beam microscope (FIB) has been used to fabricate thin parallel-sided ferroelectric capacitors from single crystals of BaTiO3 and SrTiO3. A series of nano-sized capacitors ranging in thickness from ∼660 nm to ∼300 nm were made. Cross-sectional high resolution transmission electron microscopy (HRTEM) revealed that during capacitor fabrication, the FIB rendered around 20 nm of dielectric at the electrode-dielectric interface amorphous, associated with local gallium impregnation. Such a region would act electrically in series with the single crystal and would presumably have a considerable negative influence on the dielectric properties. However, thermal annealing prior to gold electrodes deposition was found to fully recover the single crystal capacitors and homogenise the gallium profile. The dielectric testing of the STO ultra-thin single crystal capacitors was performed yielding a room temperature dielectric constant of ∼300, as is the case in bulk. Therefore, there was no evidence of a collapse in dielectric constant associated with thin film dimensions.

Nanoscale ferroelectrics machined from single crystals

ABSTRACT This paper summarises some of the most recent work that has been done on nanoscale ferroelectrics as a result of a joint collaborative research effort involving groups in Queens University Belfast, the University of Cambridge and the University of St. Andrews. Attempts have been made to observe fundamental effects of reduced size, and increasing morphological complexity, on ferroelectric behaviour by studying the functional response and domain characteristics in nanoscale single crystal material, whose size and morphology have been defined by Focused Ion Beam (FIB) patterning. This approach to nanoshape fabrication has allowed the following broad statements to be made: (i) in single crystal BaTiO3 sheets, permittivity and phase transition behaviour is not altered from that of bulk material down to a thickness of ∼ 75 nm; (ii) in single crystal BaTiO3 sheets and nanowires changes in observed domain morphologies are consistent with large scale continuum modeling.

CH022: Behaviour of ferroelectrics influenced by nanoscale morphology

In an attempt to explore fundamental aspects of the behaviour of ferroelectrics at reduced dimensions, a focused ion beam microscope has been used to cut thin sheets, or lamellae, and nanowires directly from single crystal BaTiO3. Permittivity characteristics and the nature of domain configurations adopted have been investigated. While the permittivity of nanoscale samples appears to remain unaltered from bulk, the domain periodicities and polar orientations observed show a dramatic sensitivity to both size and shape. Indeed, once the physics of the domain response of the nanoscale ferroelectrics is understood, it has been shown that nanoscale morphology alone can be used to control polar orientation along the lengths of single nanowires.