We report the field-effect transistors using quasi-two-dimensional electron gas generated at an ultrathin (∼10 nm) AlO/TiO heterostructure interface grown via atomic layer deposition (ALD) on a SiO/Si substrate without using a single crystal substrate. The 2DEG at the AlO/TiO interface originates from oxygen vacancies generated at the surface of the TiO bottom layer during ALD of the AlO overlayer. High-density electrons (∼10 cm) are confined within a ∼2.2 nm distance from the AlO/TiO interface, resulting in a high on-current of ∼12 μA/μm. The ultrathin TiO bottom layer is easy to fully deplete, allowing an extremely low off-current, a high on/off current ratio over 10, and a low subthreshold swing of ∼100 mV/decade. Via the implementation of ALD, a mature thin-film process can facilitate mass production as well as three-dimensional integration of the devices.
A high-performance, transparent, and extremely thin (<15 nm) hydrogen (H 2 ) gas sensor is developed using 2D electron gas (2DEG) at the interface of an Al 2 O 3 /TiO 2 thin film heterostructure grown by atomic layer deposition (ALD), without using an epitaxial layer or a single crystalline substrate. Palladium nanoparticles (≈2 nm in thickness) are used on the surface of the Al 2 O 3 /TiO 2 thin film heterostructure to detect H 2 . This extremely thin gas sensor can be fabricated on general substrates such as a quartz, enabling its practical application. Interestingly, the electron density of the Al 2 O 3 /TiO 2 thin film heterostructure can be tailored using ALD process temperature in contrast to 2DEG at the epitaxial interfaces of the oxide heterostructures such as LaAlO 3 / SrTiO 3 . This tunability provides the optimal electron density for H 2 detection. The Pd/Al 2 O 3 /TiO 2 sensor detects H 2 gas quickly with a short response time of <30 s at 300 K which outperforms conventional H 2 gas sensors, indicating that heating modules are not required for the rapid detection of H 2 . A wide bandgap (>3.2 eV) with the extremely thin film thickness allows for a transparent sensor (transmittance of 83% in the visible spectrum) and this fabrication scheme enables the development of flexible gas sensors.
A two-dimensional electron gas (2DEG) was formed at the interface of an ultrathin Al 2 O 3 /TiO 2 heterostructure that was fabricated using atomic layer deposition (ALD) at a low temperature (<300 °C) on a thermally oxidized SiO 2 /Si substrate. A high electron density (∼10 14 cm −2 ) and mobility (∼4 cm 2 V −1 s −1 ) were achieved, which are comparable to those of the epitaxial LaAlO 3 /SrTiO 3 heterostructure. An in situ resistance measurement directly demonstrated that the resistance of the heterostructure interface dropped significantly with the injection of trimethylaluminum (TMA), indicating that oxygen vacancies were formed on the TiO 2 surface during the TMA pulse in the ALD of Al 2 O 3 films, such that they provide electron donor states to generate free electrons at the interface of the ultrathin Al 2 O 3 /TiO 2 heterostructure. The activation energy of the electron donor states to move to the Ti 3d conduction band plays an essential role in the electrical characteristics of the 2DEG. Interestingly, the donor state level can be tailored by the control of TiO 2 crystallinity, which eventually adjusts the electron density. The activation energy was decreased to less than 20 meV to generate ultrashallow donor states while improving the TiO 2 crystallinity, such that the 2D electrons become readily delocalized, even at room temperature, to create a 2DEG.
This
research demonstrates, for the first time, the development
of highly uniform resistive switching devices with self-compliance
current for conductive bridge random access memory using two-dimensional
electron gas (2DEG) at the interface of an Al2O3/TiO2 thin-film heterostructure via atomic layer deposition
(ALD). The cell is composed of Cu/Ti/Al2O3/TiO2, where Cu/Ti and Al2O3 overlayers are
used as the active/buffer metals and solid electrolyte, respectively,
and the 2DEG at the interface of Al2O3/TiO2 heterostructure, grown by the ALD process, is adopted as
a bottom electrode. The Cu/Ti/Al2O3/TiO2 device shows reliable resistive switching characteristics
with excellent uniformity under a repetitive voltage sweep (direct
current sweep). Furthermore, it exhibits a cycle endurance over 107 cycles under short pulse switching. Remarkably, a reliable
operation of multilevel data writing is realized up to 107 cycles. The data retention time is longer than 106 s
at 85 °C. The uniform resistance switching characteristics are
achieved via the formation of small (∼a few nm width) Cu filament
with a short tunnel gap (<0.5 nm) owing to the 2DEG at the Al2O3/TiO2 interface. The performance and
operation scheme of this device may be appropriate in neuromorphic
applications.
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