Sang Yeon Lee

Uniform ZnO nanorod/Cu2O core–shell structured solar cells by bottom-up RF magnetron sputtering

Cu2O is a good candidate material for use as a p-type absorber in solar cells. Here, uniform ZnO nanorod (NR)–Cu2O core–shell structures are fabricated and their diode performances are studied. ZnO NRs are grown on fluorine-doped tin oxide (FTO) glass using a hydrothermal method. Cu2O is then deposited on the ZnO NRs using bottom-up RF magnetron sputtering. The crystal structures of the deposited ZnO NRs and Cu2O are characterized using X-ray diffraction. From secondary electron microscopy analysis, the uniform core–shell structure and its size are identified. UV-vis spectroscopy measurements show that the optical bandgap of the Cu2O in this structure is 2.3 eV. The diode characteristics of the fabricated nanostructures depend on the thickness of Cu2O; 2.7 μm-thick Cu2O on ZnO NRs shows diode properties. Lastly, we propose a band alignment model based on X-ray photoelectron spectroscopy analysis and demonstrate a possible approach for fabricating CuxO–ZnO nanohybrids for further improvements to device efficiency, highlighting a need for interfacial band offset medication for oxide heterojunction solar cells.

Depth resolved band alignments of ultrathin TiN/ZrO2 and TiN/ZrO2-Al2O3-ZrO2 dynamic random access memory capacitors

In this paper, we investigated the interface band alignment of TiN/ZrO2 and TiN/ZrO2-Al2O3-ZrO2 (TiN/ZAZ) structures by analyzing the conduction band offset (CBO) and valence band offset at the electrode/dielectric interface using depth-resolved spectroscopy techniques. At the center of the interface, which is defined by the chemical composition depth profile, CBO values of 2.03 eV and 2.57 eV for ZrO2 and ZAZ were found, respectively. Subcutaneous TiON, which is induced by the process, was identified at this interface, and it played an important role in creating sub-band states. Based on combined analyses on both intrinsic and sub-band structures, a band alignment model is proposed. It was confirmed that the Al2O3 layer in ZAZ leads to a lowering of the Fermi energy or a p-doping effect, thereby increasing both the CBO and the tunneling barrier height in metal-insulator-metal capacitors.

Forming-less and Non-Volatile Resistive Switching in WO X by Oxygen Vacancy Control at Interfaces

Resistive switching devices are recognized as candidates for next-generation memory devices in that they can replace conventional memory devices. In these devices, a WOX film deposited by RF magnetron sputtering with a significant number of oxygen vacancies exhibits a resistive switching property and does not involve the use of a forming process. The resistive switching mechanism involves the hopping of electrons through the sub-band states of the oxygen vacancies in E-field-driven electromigration. X-ray photoemission spectroscopy, ultra-violet photoemission spectroscopy, and transmission electron microscopy-electron energy loss spectroscopy were performed to analyze local variations in the O-vacancies and in the electronic band structure of a WOX thin film. The band structure is responsible for the correlation between the motion of the electrons under the interface effect at the electrodes with the change in the resistance and the bias-polarity dependence of the I-V property of the device. The optimized metal-insulator-metal structure (Pt/WOX/Au), which has an asymmetric electrode and many oxygen vacancies, gives rise to excellent resistive-switching properties with a high on/off ratio on the order of 105 times, a low set voltage of <0.34 V, and a uniform DC cyclic performance in the order of 1500 cycles at room temperature. These specifications can be further adopted for application to non-volatile memory-device applications.

Photochemical tuning of ultrathin TiO2/p-Si p-n junction properties via UV-induced H doping

We report a modified TiO2/p-Si electronic structure that uses ultraviolet exposure for the incorporation of H. This structure was characterized using various photoelectron spectroscopic techniques. The ultraviolet (UV) exposure of the TiO2 surface allowed the Fermi energy level to be tuned by the insertion of H radicals, which induced changes in the heterojunction TiO2/p-Si diode properties. The UV exposure of the TiO2 surface was performed in air. On UVexposure, a photochemical reaction involving the incorporation of UV-induced H radicals led to the creation of a surface Ti-O-OH group and caused interstitial H doping (Ti-H-O) in the bulk, which modified the electronic structures in different ways, depending on the location of the H. On the basis of the band alignment determined using a combined spectroscopic analysis, it is suggested that the UV-induced H incorporation into the TiO2 could be utilized for the systematic tuning of the heterojunction property for solar cells, photocatalytic applications, and capacitors.

Creation of a Short-Range Ordered Two-Dimensional Electron Gas Channel in Al2O3/In2O3 Interfaces

The tuning of electrical properties in oxides via surface and interfacial two-dimensional electron gas (2DEG) channels is of great interest, as they reveal the extraordinary transition from insulating or semiconducting characteristics to metallic conduction or superconductivity enabled by the ballistic transport of spatially confined electrons. However, realizing the practical aspects of this exotic phenomenon toward short-range ordered and air-stable 2DEG channels remains a great challenge. At the heterointerface formed after deposition of an Al2O3 layer on a nanocrystalline In2O3 layer, a dramatic improvement in carrier conduction equivalent to metallic conduction is obtained. A conductivity increase by a factor of 1013 times that in raw In2O3, a sheet resistance of 850 Ω/cm2, and a room temperature Hall mobility of 20.5 cm2 V-1 s-1 are obtained, which are impossible to achieve by tuning each layer individually. The physicochemical origin of metallic conduction is mainly ascribed to the 2D interfacially confined O-vacancies and semimetallic nanocrystalline InOx (x < 2) phases by the clustered self-doping effect caused by O-extraction from In2O3 to the Al2O3 phase during ALD. Unlike other submetallic oxides, this 2D channel is air-stable by complete Al2O3 passivation and thereby promises applicability for implementation in devices.

Electric field-triggered metal-insulator transition resistive switching of bilayered multiphasic VO x

Electric field-triggered Mott transition of VO2 for next-generation memory devices with sharp and fast resistance-switching response is considered to be ideal but the formation of single-phase VO2 by common deposition techniques is very challenging. Here, VOx films with a VO2-dominant phase for a Mott transition-based metal-insulator transition (MIT) switching device were successfully fabricated by the combined process of RF magnetron sputtering of V metal and subsequent O2 annealing to form. By performing various material characterizations, including scanning transmission electron microscopy-electron energy loss spectroscopy, the film is determined to have a bilayer structure consisting of a VO2-rich bottom layer acting as the Mott transition switching layer and a V2O5/V2O3 mixed top layer acting as a control layer that suppresses any stray leakage current and improves cyclic performance. This bilayer structure enables excellent electric field-triggered Mott transition-based resistive switching of Pt-VOx-Pt metal-insulator-metal devices with a set/reset current ratio reaching ~200, set/reset voltage of less than 2.5 V, and very stable DC cyclic switching upto ~120 cycles with a great set/reset current and voltage distribution less than 5% of standard deviation at room temperature, which are specifications applicable for neuromorphic or memory device applications.