Redox‐Based Resistive Switching Memories – Nanoionic Mechanisms, Prospects, and Challenges

An overview of the state of art in ferroelectric thin films is presented. First, we review applications: microsystems' applications, applications in high frequency electronics, and memories based on ferroelectric materials. The second section deals with materials, structure (domains, in particular), and size effects. Properties of thin films that are important for applications are then addressed: polarization reversal and properties related to the reliability of ferroelectric memories, piezoelectric nonlinearity of ferroelectric films which is relevant to microsystems' applications, and permittivity and loss in ferroelectric films-important in all applications and essential in high frequency devices. In the context of properties we also discuss nanoscale probing of ferroelectrics. Finally, we comment on two important emerging topics: multiferroic materials and ferroelectric one-dimensional nanostructures. (c) 2006 American Institute of Physics.

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Ferroelectric thin films: Review of materials, properties, and applications

An object is to provide a semiconductor device of which a manufacturing process is not complicated and by which cost can be suppressed, by forming a thin film transistor using an oxide semiconductor film typified by zinc oxide, and a manufacturing method thereof. For the semiconductor device, a gate electrode is formed over a substrate; a gate insulating film is formed covering the gate electrode; an oxide semiconductor film is formed over the gate insulating film; and a first conductive film and a second conductive film are formed over the oxide semiconductor film. The oxide semiconductor film has at least a crystallized region in a channel region.

Semiconductor device, and manufacturing method thereof

The present invention relates to an amorphous oxide and a thin film transistor using the amorphous oxide. In particular, the present invention provides an amorphous oxide having an electron carrier concentration less than 10 18 /cm 3 , and a thin film transistor using such an amorphous oxide. In a thin film transistor having a source electrode 6 , a drain electrode 5 , a gate electrode 4 , a gate insulating film 3 , and a channel layer 2 , an amorphous oxide having an electron carrier concentration less than 10 18 /cm 3 is used in the channel layer 2.

Amorphous oxide and thin film transistor

In a thin film transistor, a gate insulating film having a first insulating film and a second insulating film is formed on a gate electrode, and a semiconductor layer including ZnO etc. is formed on the second insulating film. The first insulating film is formed by using SiN x having a high insulating characteristic, and the second insulating film is formed by using an oxide (for example, SiO 2 ). This structure improves a crystalline characteristic of the semiconductor layer that constitutes an interface in combination with the second insulating film, and decreases a defective level of the interface between the semiconductor layer and the second insulating film. Further, the second insulating film is constituted of the oxide, so that it is possible to restrain a material for the second insulating film from depriving oxygen of the semiconductor layer. This keeps a crystalline characteristic of the semiconductor layer under a preferable condition in the vicinity of the interface between the second insulating film and the semiconductor layer. As a result, it is possible to realize a thin film transistor such that: a leak current level at an OFF area is low, and the mobility is high, and a switching characteristic is preferable. Thus, in the thin film transistor having a transparent semiconductor film, a TFT characteristic is improved.

Thin film transistor and matrix display device

A semiconductor device includes a transparent switching element (1) with two connection electrodes (2, 3) of a transparent material and an interposed transparent channel region (4) of a semiconductor material provided with a transparent gate electrode (5) of a conductive material, separated from the channel region (4) by a transparent insulating layer (6). The semiconductor material is a degenerate semiconductor material with a basic material having a bandgap (10) between conduction band (11) and valence band (12) of electrons greater than 2.5 eV and a mobility of charge carriers greater than 10 cm2 /Vs provided with dopant atoms which form a fixed impurity energy level (13) adjacent or in the valence band (12) or conduction band (11) of the basic material. The degenerate semiconductor material is transparent because the absorption of visible light is not possible owing to the great bandgap (10), while also no absorption of visible light takes place through the impurity energy levels (13). The device is capable of comparatively fast switching.

Semiconductor device having a transparent switching element

A Schottky contact consisting of a semiconducting ferroelectric material and a high work function metal shows a bistable conduction characteristic. An on/off ratio of about 2 orders of magnitude was obtained in a structure consisting of a 0.2 \ensuremath{\mu}m ferroelectric PbTi${\mathrm{O}}_{3}$ film, a Au Schottky contact, and a ${\mathrm{La}}_{0.5}$${\mathrm{Sr}}_{0.5}$Co${\mathrm{O}}_{3}$ Ohmic bottom electrode. The observations are explained by a model in which the depletion width of the ferroelectric Schottky diode is determined by the polarization dependence of the internal electric field at the metal-ferroelectric interface.

Ferroelectric Schottky diode.

A switching element is provided with two electrodes (1, 2) with a semiconducting dielectric (3) therebetween, one electrode (2) having a material which forms a Schottky contact with the semiconducting dielectric (3), while a space charge region (3') of the Schottky contact forms a tunnelling barrier for electrons during operation. It is desirable in many applications for the switching element to hold a certain switching state, such as open or closed, during a longer period. The switching element may then be used, for example, as a memory element. The dielectric (3) includes a ferroelectric material with a remanent polarization which influences a dimension of the tunnelling barrier. In this manner the switching element has various switching states depending on the remanent polarization of the dielectric (3). These switching states are held until the polarization of the dielectric (3) changes.

Switching element with memory provided with Schottky tunnelling barrier

The field dependence of the dielectric constant of SrTiO3 is investigated using the capacitance versus bias voltage characteristics of various metal‐SrTiO3:Nb Schottky diodes. The relative dielectric constant is shown to decrease one order of magnitude for electric fields ranging from 0.1 to 10 MV/cm. At low fields the permittivity follows the Curie–Weiss law, whereas at fields larger than 500 kV/cm the permittivity is nearly temperature independent. At high doping densities the field dependent permittivity gives rise to a reduction of the depletion width of the Schottky diode.

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Paulus Wilhelmus Maria Blom

Papers

Semiconductor device having a transparent switching element

Ferroelectric Schottky diode.

Switching element with memory provided with Schottky tunnelling barrier

Field-dependent permittivity in metal-semiconducting SrTiO3 Schottky diodes

Growth, structuring and characterisation of all-oxide thin film devices prepared by pulsed laser deposition