Mott Transition Field Effect Transistor: Experimental Results
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make switching devices. However, these superconducting switches operate only at temperatures below that of the super conducting transition of the channel materia, and the results reported so far exhibit a low on-off ratio [4, 6-8]. A main feature of our devices is the fact that the mobility in the channel depends on the carrier concentration. These carriers, are drawn into the channel electrostatically from the source and drain electrodes by application of a voltage to the gate electrode, forming a thin conductive sheet at the interface between the channel material and the gate insulator. In order to fabricate devices such as those just described, we have selected perovskite-structure cuprate compounds as a channel material. Cuprates can be grown epitaxially on single crystal substrates of strontium titanate (STO). These materials exhibit layer conductivity and have been largely characterized. They have the advantage of being compatible with STO, which is an appropriate choice of gate oxide material in view of its high dielectric constant and dielectric strength. EXPERIMENT Device Fabrication The devices have been fabricated by epitaxially growing the oxide layers on a single crystal STO surface by means of pulsed laser deposition PI.1). The deposition chamber and experimental 243 Mat. Res. Soc. Symp. Proc. Vol. 574 ©1999 Materials Research Society
setup has been described elsewhere [9]. Prior to deposition, the substrates were ultrasonically cleaned, sequentially, in acetone, isopropanol and ethanol. We have investigated two architectures. The simplest one is shown in Fig. 1.This device is constructed by depositing an STO film approximately 100 nm thick directly onto the substrate, followed by the deposition of a channel layer approximately 10 nm thick. Typical materials used for the later were La 2CuO 4 (LCO) and Y•Pri.,Ba 2Cu 3OT 7 s (YPBCO). In this architecture, the substrate is a 0.5 % Nb doped (100) oriented STO single crystal, so that the substrate is a conductor and constitutes the common gate. After deposition of the oxide films, the Pt source and drain electrodes are evaporated through a stencil mask. Finally, isolation trenches are opened with an excimer laser around each device. Because of the coupling produced by the common gate electrode, these devices are not expected to offer a good performance at high frequencies, and hereafter they will be referred to as DC devices. Results arising from these devices have been already shown elsewhere [21 in more detail, and some of them are also shown for comparison in the next section. The DC devices, although simpler to make, present problems related to epitaxial growth and to deterioration of the channel layer because of its contact to ambient atmospheres.
source
channel
gate insulator
Isolation trench gate isolation trench
gate oxide
drain
chanl aye
Nb doped STO
substrate insulating (STO)
Molt channel layer
Fig. 1.Schematic representation of the structure with the top channel layer.
drain
source
Fig. 2. Schematic representation of the structur
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