The atmospheric pressure plasma-assisted chemical vapor deposition technique has successfully demonstrated unintentionally doped (UID) Ga2O3 growth at 350 °C. This technique allows independent and ...homogeneous multiple nuclei growth of Ga2O3, leading to three-dimensional grain growth at a rate of ⁓0.17 μm/h. In the study of Schottky barrier diodes, the Schottky-like current (I)–voltage (V) response shows typical behavior on Ga2O3. This is a good sign at an early stage of device development on the grown sample. The extracted barrier height of ⁓2.20 eV was higher, which may be due to unintentional PtOx formation on the Ga2O3 surface. Furthermore, the extracted capacitance (C)–voltage (V) depth profiling of the effective impurity concentration was nearly flat, ⁓1.5 × 1017 cm−3, in the unintentionally doped grown film. The effective impurity concentration is comparable to the UID carrier concentration of epitaxial films fabricated using the high temperature growth technique. Therefore, low-temperature-grown homoepitaxial Ga2O3 thin films grown by atmospheric pressure-plasma-assisted chemical vapor deposition can be used in future Ga2O3-based power device applications.
Low-temperature homoepitaxial growth of β-Ga2O3(-201) has been successfully demonstrated by the atmospheric pressure plasma-enhanced chemical vapor deposition technique. To search for low-temperature ...growth, temperature-dependent studies were carried out between 350 and 600 °C. A high N2 gas flow rate, low gallium source concentration, and high oxygen flow rate ratio played key roles in growing independent and homogeneous multiple nuclei of Ga2O3, leading to three-dimensional grain growth mode, single crystallinity, and the highest growth rate of ⁓0.17 µm/h at 350 °C. The highly reactive atmospheric pressure oxygen plasma actively led to epitaxial growth. The low thermal budget homoepitaxial growth is a record reduction reported thus far.
Polarisation domain structure is a microstructure specific to ferroelectrics and plays a role in their various fascinating characteristics. The piezoelectric properties of ferroelectrics are ...influenced by the domain wall contribution. This study provides a direct observation of the contribution of domain walls to the direct piezoelectric response of bismuth ferrite (BiFeO
) films, which have been widely studied as lead-free piezoelectrics. To achieve this purpose, we developed a scanning probe microscopy-based measurement technique, termed direct piezoelectric response microscopy (DPRM), to observe the domain structure of BiFeO
films via the direct piezoelectric response. Quantitative analysis of the direct piezoelectric response obtained by DPRM, detailed analysis of the domain structure by conventional piezoelectric force microscopy, and microscopic characterisation of the direct piezoelectric properties of BiFeO
films with different domain structures revealed that their direct piezoelectric response is enhanced by the walls between the domains of spontaneous polarisation in the same out-of-plane direction.
Monolayer molybdenum disulfide (MoS2) is an atomically thin semiconducting material with a direct band gap. This physical property is attributable to atomically thin optical devices such as sensors, ...light-emitting devices, and photovoltaic cells. Recently, a near-unity photoluminescence (PL) quantum yield of a monolayer MoS2 was demonstrated via a treatment with a molecular acid, bis(trifluoromethane)sulfonimide (TFSI); however, the mechanism still remains a mystery. Here, we work on PL enhancement of monolayer MoS2 by treatment of Brønsted acids (TFSI and sulfuric acid (H2SO4)) to identify the importance of the protonated environment. In TFSI as an acid, different solvents1,2-dichloroethane (DCE), acetonitrile, and waterwere studied, as they show quite different acidity in solution. All of the solvents showed PL enhancement, and the highest was observed in DCE. This behavior in DCE would be due to the higher acidity than others have. Acids from different anions can also be studied in water as a common solvent. Both TFSI and H2SO4 showed similar PL enhancement (∼4–8 enhancement) at the same proton concentration, indicating that the proton is a key factor to enhance the PL intensity. Finally, we considered another cation, Li+ from Li2SO4, instead of H2SO4, in water. Although Li and H atoms showed similar binding energy on MoS2 from theoretical calculations, Li2SO4 treatment showed little PL enhancement; only coexisting H2SO4 reproduced the enhancement. This study demonstrated the importance of a protonated environment to increase the PL intensity of monolayer MoS2. The study will lead to a solution to achieve high optical quality and to implementation for atomically thin optical devices.
The discovery of the HfO2‐based ferroelectric films has opened new opportunities for using this silicon‐compatible ferroelectric material to realize low‐power logic circuits and high‐density ...non‐volatile memories. The functional performances of ferroelectrics are intimately related to their dynamic response to external stimuli, such as electric fields at finite temperatures. In the case of HfO2‐based films, the time‐dependent imprint and wake‐up effect, which distinguish them from conventional ferroelectrics, play important roles in understanding the remaining reliability issues, such as insufficient endurance. In this study, the time‐dependent imprint process is carefully investigated using Hf0.5Zr0.5O2 (HZO) films with different ferroelectric properties and defect density. The amount of redistributed charge, which causes imprint during polarization retention, is affected by the remanent polarization of the ferroelectric layer, suggesting that the depolarization field corresponding to the remanent polarization generates and works as a driving force of charge redistribution. The time‐dependent measurement of the imprint distinguishes the origins of charge redistribution processes, which have different time constants. In addition, the correlation between the amount of redistributed charge and the dielectric relaxation of the HZO films is discussed. Correlations are identified between the redistributed charge and the dielectric relaxation, indicating that the mobile charge contributes to the time‐dependent imprint.
This work investigated the time‐dependent imprint of Hf0.5Zr0.5O2 ferroelectric films. Redistributed charge, which causes imprint, is affected by the remanent polarization, suggesting that the depolarization field corresponding to the remanent polarization works as a driving force of charge redistribution. Additionally, correlations are identified between the redistributed charge and the dielectric relaxation, indicating that the mobile charge contributes to the imprint.
To advance the development of atomically thin optoelectronics using two-dimensional (2D) materials, engineering strong luminescence with a physicochemical basis is crucial. Semiconducting monolayer ...transition-metal dichalcogenides (TMDCs) are candidates for this, but their quantum yield (QY) is known to be poor. Recently, a molecular superacid treatment of bis(trifluoromethane)sulfonimide (TFSI) generated unambiguously bright monolayer TMDCs and a high QY. However, this method is highly dependent on the processing conditions and therefore has not been generalized. Here, we shed light on environmental factors to activate the photoluminescence (PL) intensity of the TFSI-treated monolayer MoS2, with a factor of more than 2 orders of magnitude greater than the original by photoactivation. The method is useful for both mechanically exfoliated and chemically deposited samples. The existence of photoirradiation larger than the band gap demonstrates enhancement of the PL of MoS2; on the other hand, activation by thermal annealing, as demonstrated in the previous report, is less effective for enhancing the PL intensity. The photoactivated monolayer MoS2 shows a long lifetime of ∼1.35 ns, more than a 30-fold improvement over the original as exfoliated. The consistent realization of the bright monolayer MoS2 reveals that air exposure is an essential factor in the process. TFSI treatment in a N2 environment was not effective for achieving a strong PL, even after the photoactivation. These findings can serve as a basis for engineering the bright atomically thin materials for 2D optoelectronics.
Due to the direct band gap nature, extensive studies have been conducted to improve the optical behavior in monolayer transition metal dichalcogenides (TMDCs) with a formula of MX2 (M = Mo, W; X = S, ...Se, Te). One of the strongest modulating agents of optical behavior is a molecular superacid treatment; however, the chemical event has not been unveiled. Also, the engineering protocol for keeping the treatment is immature. In this work, we systematically study the superacid treatment procedures on monolayer molybdenum disulfide (MoS2) and propose that the interaction, a hydrophilic interaction, between the superacid molecule and MoS2 surface would be critical. As a result of the interaction, the superacid molecules spontaneously form an acidic layer with the thickness of several nanometers on the surface. The power-dependent photoluminescence (PL) measurement indicates the edge of MoS2 flake is more effective and electronically modulated by the treatment. By understanding the superacid nanolayer formation by the treatment, we succeeded in maintaining the ultrastrong PL in the superacid-treated MoS2 for more than 30 days in the ambient air by encapsulation with transparent organic polymers. This study advances the understanding and designing applications of strong luminescent properties in the superacid-treated TMDCs and paves the way toward engineering exciton dynamics and an experimental platform for treating multibody states.
Carrier modulation in transition-metal dichalcogenides (TMDCs) is of importance for applying electronic devices to tune their transport properties and controlling phases, including metallic to ...superconductivity. Although the surface charge transfer doping method has shown a strong modulation ability of the electronic structures in TMDCs and a degenerately doped state has been proposed, the details of the electronic states have not been elucidated, and this transport behavior should show a considerable thickness dependence in TMDCs. In this study, we characterize the metallic transport behavior in the monolayer and multilayer MoS2 under surface charge transfer doping with a strong electron dopant, benzyl viologen (BV) molecules. The metallic behavior transforms to an insulative state under a negative gate voltage. Consequently, metal–insulator transition (MIT) was observed in both monolayer and multilayer MoS2 correlating with the critical conductivity of order e 2/h. In the multilayer case, the BV molecules strongly modulated the topmost surface layer in the bulk MoS2; the transfer characteristics suggested a crossover from a heterogeneously doped state with a doped topmost layer to doping in the deep layers caused by the variation in the gate voltage. The findings of this work will be useful for understanding the device characteristics of thin-layered materials and for applying them to the controlling phases via carrier modulation.
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