Sejoon Lee

Robust bi-stable memory operation in single-layer graphene ferroelectric memory

With the motivation of realizing an all graphene-based circuit for low power, we present a reliable nonvolatile graphene memory device, single-layer graphene (SLG) ferroelectric field-effect transistor (FFET). We demonstrate that exfoliated single-layer graphene can be optically visible on a ferroelectric lead-zirconate-titanate (PZT) substrate and observe a large memory window that is nearly equivalent to the hysteresis of the PZT at low operating voltages in a graphene FFET. In comparison to exfoliated graphene, FFETs fabricated with chemical vapor deposited (CVD) graphene exhibit enhanced stability through a bi-stable current state operation with long retention time. In addition, we suggest that the trapping/de-trapping of charge carriers in the interface states is responsible for the anti-hysteresis behavior in graphene FFET on PZT. V C 2011 American Institute of Physics. [doi:10.1063/1.3619816] Graphene is considered to be an exceptional material with high potential for future electronics, owing to its excellent electronic properties; 1 linear electron energy dispersion, and high room temperature mobility. If feasible, an all graphene-based circuit, including logic, analog, and memory devices, would be of great interest to further extend the performance of current Si-based electronics. Among various device applications, graphene based memory structures are still in their infancy in comparison to its logic and analog applications. To date, graphene memory has been demonstrated through chemical modification, 2 filament-type memristor, 3 nanomechanical switch, 4 and graphene FFETs. 5‐7 In graphene FFETs, however, the ambipolar conduction leads to undesirable on/off states for memory applications. Moreover, the absence of an electronic bandgap and controlled doping makes it difficult to resolve such issues. Therefore, a systematic study of graphene FFET is beneficial to realize graphene-based memory structures. In this Letter, we investigate graphene/PZT FFET structures using exfoliated- and CVD-SLG and their mechanism of operation. We show that exfoliated SLG can be optically identified on a PZT substrate and exhibit a hysteresis of the Vshaped conductance with a large memory window at low operating gate voltages. We compare exfoliated- with CVDSLG FFETs and show that devices made of CVD-SLG exhibit a robust bi-stable current state with a long retention time. In order to construct the SLG FFET, we first engineered a ferroelectric substrate to identify SLG. Previously, we have demonstrated that SLG is invisible under the optical micro

Vertical graphene-base hot-electron transistor.

We demonstrate vertical graphene-base hot-electron transistors (GB-HETs) with a variety of structures and material parameters. Our GB-HETs exhibit a current saturation with a high current on-off ratio (>10(5)), which results from both the vertical transport of hot electrons across the ultrathin graphene base and the filtering of hot electrons through a built-in energy barrier. The influences of the materials and their thicknesses used for the tunneling and filtering barriers on the common-base current gain α are studied. The optimization of the SiO2 thickness and using HfO2 as the filtering barrier significantly improves the common-base current gain α by more than 2 orders of magnitude. The results demonstrate that GB-HETs have a great potential for high-frequency, high-speed, and high-density integrated circuits.

Suspended few-layer graphene beam electromechanical switch with abrupt on-off characteristics and minimal leakage current

Suspended few-layer graphene beam electro-mechanical switches (SGSs) with 0.15 μm air-gap are fabricated and electrically characterized. The SGS shows an abrupt on/off current characteristics with minimal off current. In conjunction with the narrow air-gap, the outstanding mechanical properties of graphene enable the mechanical switch to operate at a very low pull-in voltage (VPI) of 1.85 V, which is compatible with conventional complimentary metal-oxide-semiconductor (CMOS) circuit requirements. In addition, we show that the pull-in voltage exhibits an inverse dependence on the beam length.

Observation of Single Electron Transport via Multiple Quantum States of a Silicon Quantum Dot at Room Temperature

Single electron transport through multiple quantum levels is realized in a Si quantum-dot device at room-temperature conditions. The energy spacing of more than triple the omnipresent thermal energy is obtained from an extremely small ellipsoidal Si quantum dot, and high charge stability is attained through a construction of the gate-all-around structure. These properties may move us a step closer to practical applications of quantum devices at elevated temperatures. An in-depth analysis on the transport behavior and quantum structure is presented.

Impact of gate work-function on memory characteristics in Al2O3/HfOx/Al2O3/graphene charge-trap memory devices

Graphene-based non-volatile memory devices composed of a single-layer graphene channel and an Al2O3/HfOx/Al2O3 charge-storage layer exhibit memory functionality. The impact of the gate material’s work-function (Φ) on the memory characteristics is investigated using different types of metals [Ti (ΦTi = 4.3 eV) and Ni (ΦNi = 5.2 eV)]. The ambipolar carrier conduction of graphene results in an enlargement of memory window (ΔVM), which is ∼4.5 V for the Ti-gate device and ∼9.1 V for the Ni-gate device. The increase in ΔVM is attributed to the change in the flat-band condition and the suppression of electron back-injection within the gate stack.

Improved ferromagnetism of (Zn0.93Mn0.07)O through rapid thermal annealing

After annealing at 900°C, the ferromagnetic properties of (Zn0.93Mn0.07)O thin films were dramatically improved. The resultant remanent magnetization (Mr) and Curie temperature (TC) were 1.17μB∕Mn and 83K. The improvement of ferromagnetism was confirmed to as resulting from the enhancement of magnetic anisotropy. This result is attributed to the improvement of crystallinity and the stabilization of unstably bonded Mn2+ ions by thermal treatments. These results suggest that ferromagnetism of (Zn1−xMnx)O thin films can be improved by modifying the crystal magnetic anisotropy through postgrowth thermal treatments.

Structural, optical, and magnetic properties of As-doped (Zn0.93Mn0.07)O thin films

The As-doped (Zn0.93Mn0.07)O thin film prepared by As+ ion implantation showed a clear peak of (A0,X) having acceptor binding energy of 181meV. The sample showed high TC ferromagnetism persisting up to 285K. The contribution of magnetization from Mn ion at 280K was determined to be 0.13μB∕Mn. The improved ferromagnetism is expected to be originated from hole-induced ferromagnetism and enhanced magnetic anisotropy because crystallographically improved sample showed p-type conductivity with hole concentration of 4.8×1018cm−3 and hole mobility of 11.8cm2V−1s−1. These results suggest that high TC ferromagnetism can be realized by codoping the acceptor dopant and improving the magnetic anisotropy.

Enhanced ferromagnetism in H2O2-treated p-(Zn0.93Mn0.07)O layer

Enhanced ferromagnetism was observed from the H2O2-treated p-type (Zn0.93Mn0.07)O:As layer. Compared with the untreated sample, the H2O2-treated sample showed the enlarged ferromagnetic hysteresis loop with approximately two-times-increased spontaneous magnetization. And also, in comparison with the untreated sample (TC∼280 K), the H2O2-treated sample exhibited to have the increased TC persisting up to above 350 K. These results were confirmed to originate from the enhanced p-d hybridization due to the decrease in negatively charged residual background carriers. This is because the increased effective g-factor resulting from the decrease in oxygen-related defects acting as native deep donors was observed from the H2O2-treated sample.

Impact of defect distribution on transport properties for Au/ZnO Schottky contacts formed with H2O2-treated unintentionally doped n-type ZnO epilayers

The Au/ZnO Schottky contacts fabricated using H2O2-treated unintentionally doped ZnO epilayers showed an abnormal behavior in their transport properties; i.e., the background carrier density-dependent trade-off relation between the barrier height and the ideality factor was observed. This result is attributed to the difference in carrier transport mechanisms for each sample fabricated using ZnO epilayers with different background carrier concentrations; namely, the observed trade-off relation originates from a result that the difference in the distribution of oxygen vacancies near the surface and depletion regions, which depends on the initial background carrier concentration of each sample, causes the different carrier transport mechanism.

Extremely high flexibilities of Coulomb blockade and negative differential conductance oscillations in room-temperature-operating silicon single hole transistor

A unique feature of the extremely long-range-extended blockade regime with its shape of a long stick, where the Coulomb blockade oscillation and negative differential conductance peak-positions can be systematically and precisely modulated for both extremely-wide VG and VD ranges, was clearly observed in a room-temperature-operating silicon single hole transistor. These results originate from the large quantum level spacing, large tunnel-barrier height, small tunnel-barrier curvature, small bias-induced barrier modulation, and large voltage gain, attributing to the formation of an ultrasmall dot in the gently sloped tunnel barriers along the [100] Si nanowire channel having the large subband modulation.