Kyoung Su Lee

Toward negligible charge loss in charge injection memories based on vertically integrated 2D heterostructures

Two-dimensional (2D) crystals have a multitude of forms, including semi-metals, semiconductors, and insulators, which are ideal for assembling isolated 2D atomic materials to create van der Waals (vdW) heterostructures. Recently, artificially-stacked materials have been considered promising candidates for nanoelectronic and optoelectronic applications. In this study, we report the vertical integration of layered structures for the fabrication of prototype non-volatile memory devices. A semiconducting-tungsten-disulfide-channel-based memory device is created by sandwiching high-density-of-states multi-layered graphene as a carrier-confining layer between tunnel barriers of hexagonal boron nitride (hBN) and silicon dioxide. The results reveal that a memory window of up to 20 V is opened, leading to a high current ratio (>103) between programming and erasing states. The proposed design combination produced layered materials that allow devices to attain perfect retention at 13% charge loss after 10 years, offering new possibilities for the integration of transparent, flexible electronic systems.

Band gap modulation of ZnTe1-xOx alloy film by control of oxygen gas flow rate during reactive magnetron sputtering

The band gap energy of ZnTe1-xOx alloy films grown on c-plane sapphire substrates was modulated by controlling the argon-oxygen ratio during radio frequency magnetron sputtering. The ZnTe1-xOx samples were deposited at a substrate temperature of 200 °C and with gas mixtures of 2%–8% oxygen in argon. The optical transparency of the ZnTe1-xOx samples was measured in the 1.5–6.0 eV energy range by optical transmission spectra. The optical band gap, obtained from plots of (αhν)2 as a function of hν, increased from 2.2 to 4.9 eV with increasing oxygen ratio, believed to be a result of a change in bonding structure through composition exchange during film deposition by reactive magnetron sputtering. These results show that the band gap energy of ZnTe1-xOx, ZnOTeO, and (ZnO)1-x(TeO2)x alloy films can be modulated, making them more suited for applications as windows and as active layers for ZnTe-based intermediate band solar cells.

Effect of space layer doping on photoelectric conversion efficiency of InAs/GaAs quantum dot solar cells

We report an effect of photoelectric conversion efficiency (PCE) by space layer doping in InAs/GaAs quantum dot solar cells (QDSC) and δ-doped QDSC grown by molecular beam epitaxy. The PCEs of QDSC and δ-doped QDSC without anti-reflection coating were 10.8% and 4.3%, respectively. The QDSC had about four electrons per QD, and its ideality factor was temperature-independent, which implies that recombination of electron-hole pairs is suppressed by strong potential barriers around charged dots. From the deep level transient spectroscopy measurements, four defect levels, including QD with the activation energy ranges from 0.08 eV to 0.50 eV below GaAs conduction band edge, appeared. Especially, the M1 defect (Ec-0.14 eV) was newly formed in δ-doped QDSC and its density was higher than those of M3 (Ec-0.35 eV) and M4 (Ec-0.50 eV) levels in QDSC. These results suggest that the photo-carriers recombining at M1 defect might be responsible for the reduction of PCE in δ-doped QDSC.

Optimization of the p+-ZnTe layer for back contacts of ZnTe thin-film solar cells

We have studied the performance of ZnTe-based heterojunction diodes grown on p-GaAs substrates by using pulsed laser deposition (PLD). In the X-ray diffraction measurements, ZnTe and CdS thin films grown on p-GaAs (100) substrates appeared to have a cubic crystalline structure while the ZnO film showed a hexagonal structure. The structure of n+-ZnO/i-ZnTe/p-GaAs with insertion of a CdS buffer layer between n+-ZnO and i-ZnTe showed strong rectifying diode characteristics, and its open-circuit voltage (Voc) and short-circuit current density (Jsc) under 1,000 Wm−2 air mass 1.5 global (AM 1.5G) illumination were measured about 0.32 V and 0.61 mAcm−2, respectively. When an inserted another layer, a ZnTe:Cu layer, as a p+-layer for the contacts on p-GaAs substrate, the value of Voc and Jsc of the n+-ZnO/CdS/i-ZnTe/ZnTe:Cu/p-GaAs structure increased to 0.39 V and 0.81 mA/cm2, respectively. As the thickness of i-ZnTe were increased from 210 to 420 nm, the photoelectric conversion efficiency of the n+-ZnO/CdS/i-ZnTe/ZnTe:Cu/p-GaAs structure increased to 0.24%.

Electronic states of deep trap levels in a-plane GaN templates grown on r-plane sapphire by HVPE

We report on the defect states incorporated in a-plane GaN crystals grown on r-plane sapphire substrates by hydride vapor phase epitaxy (HVPE), using deep level transient spectroscopy (DLTS). Two defect states were observed at 0.2 eV and 0.55 eV below the conduction band minimum with defect densities of 5 × 1012/cm3 and 4.7 × 1013/cm3, respectively. The size of capture cross section, non-linear relation of trap densities from the depth profile, filling pulse width, and PL measurements indicated that the electronic deep trap levels in a-plane GaN on r-plane sapphire by HVPE originated from non-interacting point defects such as NGa, complex defects involving Si, O, or C, and VGa-related centres. Even though the a-plane GaN templates were grown by HVPE with high growth rates, the electronic deep trap characteristics are comparable to those of a-plane GaN layers of high crystal quality grown by MOCVD. This study prove that the growth of a-plane GaN templates on r-plane sapphire by HVPE is a promising method to obtain a-plane GaN layers efficiently and economically without the degradation of electrical characteristics.

Defect States in InP/InGaAs/InP Heterostructures by Current–Voltage Characteristics and Deep Level Transient Spectroscopy

We studied defect states in In0.53Ga0.47As/InP heterojunctions with interface control by group V atoms during metalorganic chemical vapor (MOCVD) deposition. From deep level transient spectroscopy (DLTS) measurements, two defects with activation energies of 0.28 eV (E1) and 0.15 eV (E2) below the conduction band edge, were observed. The defect density of E1 for In0.53Ga0.47As/InP heterojunctions with an addition of As and P atoms was about 1.5 times higher than that of the heterojunction added P atom only. From the temperature dependence of current- voltage characteristics, the thermal activation energies of In0.53Ga0.47As/InP of heterojunctions were estimated to be 0.27 and 0.25 eV, respectively. It appeared that the reverse light current for In0.53Ga0.47As/InP heterojunction added P atom increased only by illumination of a 940 nm-LED light source. These results imply that only the P addition at the interface can enhance the quality of InGaAs/InP heterojunction.

Memory effect by carrier trapping into V3Si nanocrystals among SiO2 layers on multi-layered graphene layer.

We report the electrical characteristics and conduction mechanism of a resistive switching memory device consisting of V3Si nanocrystals embedded in the SiO2 layer on multi-layered graphene. The V3Si nanocrystals with average size of 5 nm were formed between the SiO2 layers by thin film deposition and post-annealing process at 800 degrees C for 5 s. The current values of high (HRS) and low resistance states (LRS) at 1 V were measured to be about 3.26 x 10(-9) A and 3.11 x 10(-8) A, respectively. The ratio of the HRS and LRS after applying sweeping bias of ± 6 V appeared to be about 9.54 at 1 V. The resistance switching could originate from the effect of carrier trap and emission into the V3Si nanocrystals via the tunneling, space charge limited current, and thermionic emission mechanisms controlled by the modulation of the Fermi level of the graphene layer. The V3Si nanocrystals memory device has a strong possibility for the application of nonvolatile memory devices.

Emission enhancement of InGaN/GaN light emitting diode by using Ag nanoparticles.

We have studied the effect of surface plasmon (SP) coupling to enhance emission efficiency of light emitting diode (LED) with multiple quantum wells (MQWs) structure by positioning Ag nanoparticles on the line-arrayed patterns. The line-arrayed patterns were fabricated by photolithography and inductively coupled plasma reactive ion etching process. The Ag nanoparticles were formed by thermal annealing at 300 degrees C and 400 degrees C for 30 min for Ag films with thickness of 10 nm and 15 nm deposited on the patterned LED structures, respectively. The photoluminescence (PL) emission intensity of line-patterned LED with Ag nanoparticles was overall enhanced. According to the spectra of time resolved PL, carrier life times of line-patterned LED with and without Ag nanoparticles appeared about 0.47 and 5.47 ns, respectively.