Wavelength-selective photodetectors responding to deep-ultraviolet (DUV) wavelengths (λ = 200−300 nm) are drawing significant interest in diverse sensing applications, ranging from micrometer biological molecules to massive military missiles. However, most DUV photodetectors developed thus far have suffered from long response times, low sensitivity, and high processing temperatures, impeding their practical use. Here, we report fast, high-responsivity, and general-substrate-compatible DUV photodetectors based on ultrathin (3−50 nm) amorphous gallium oxide (GaO X ) films grown by low-temperature (∼<250 °C) atomic layer deposition (ALD) for the first time. ALD-grown GaO X films on glass substrates display a typical amorphous nature, which is identified by electron beam diffraction and X-ray diffraction measurements, while their band gap is sharply featured at ∼4.8 eV. Metal−semiconductor−metal photodetectors (active area of 30 × 30 μm 2 ) using the 30-nm-thick GaO X films work reliably only for DUV wavelengths; the responsivity is maximized to 45.11 A/W at λ = 253 nm, which dropped off at λ ≈ 300 nm (i.e., a cutoff wavelength). The dark current measured at 10 V is as low as 200 pA and the signal-to-noise ratio reaches up to ∼10 4 , underpinning the pristine material quality of the ALD-grown GaO X films. In addition, the rise time (i.e., the time interval for photocurrent to increase from 10% to 90%) is as quick as 2.97 μs at λ = 266 nm. Such a reliable and fast photoresponse is achieved for even atomically thin (∼3 nm) devices. The substrate-compatible and low-temperature ALD growth permits the demonstration of flexible DUV photodetectors using amorphous GaO X films grown on polyimide substrates, suggesting their facile integration into other curved optoelectronic systems. We believe that photodetectors developed herein will provide an economically viable solution for high-performance DUV detection and create a variety of sensing applications.

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.

Graphene has emerged as a promising 2dimensional (2D) material composed of a monolayer of carbon atoms, which is expected to be utilized for nano-and optoelectronic device applications. In order to fabricate high speed graphene transistors with low power consumption, the growth of insulating thin films with high dielectric constant (high-k) on graphene is essential. Atomic layer deposition (ALD) is one of the best deposition techniques to grow functional thin films, however, it is extremely challenging to grow high-k thin films on graphene by ALD because of the lack of surface functional groups (such as hydroxyl groups) on graphene. Here, we demonstrate that the graphene surface is fully covered by Al 2 O 3 thin films (10−30 nm), with significantly reduced leakage current (decreased by a factor of ∼10 7 ), through simple surface treatment of the graphene in the ALD chamber prior to the deposition of the Al 2 O 3 layer by ALD to provide surface nucleation sites on the graphene, without breaking vacuum and changing entire process temperature (100 °C). Physisorbed nuclei were created on the graphene as a form of Al 2 O 3 with the surface treatment using trimethylaluminum (TMA) and H 2 O that are typical ALD precursors for Al 2 O 3 growth. Negligible defects were generated during the graphene surface treatment, which provides promising opportunities in graphene electronics.

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.