Ab initio DFT 계산을 통해서 Si/SiO2 계면의 Band offset을 계산 했다. Si과 SiO2 각각의 물질을 계산한 결과로 얻은 로컬 퍼텐셜을 기준으로 Valence band와 Conduction band의 band edge의 위치를 결정할 수 있다. 그리고 계면 계산으로 얻은 로컬포텐셜을 이용하여 두물질의 로컬 퍼텐셜의 상대적인 위치를 결정할 수 있고 이를 이용하여 Band offset을 결정 할 수 있었다.

The atomic and electronic properties of molybdenum disurfide (MoS2) nanostructures are investigated through density functional theory (DFT) calculations. We find that the band gap is indirect (about 1.79 eV) and direct (about 1.84 eV) in GGA for 2-dimensional MoS2 in our calculations. On the other hand, 1-dimensional armchair nanoribbons have semiconductor properties (band gap is about 0.11 ~0.28 eV), while 1-dimensional zigzag nanoribbons are metallic.

In this study, we investigate the electronic and atomic structure of graphene on O-terminated MgO(111) using density functional theory (DFT) calculations. To suggest a possible direction for future band gap engineering of graphene on MgO(111), adsorption of carbon atoms on graphene/MgO(111) is studied by considering the several adsorption sites. Details in adsorption properties of carbon atoms on graphene/MgO(111) are analyzed in terms of energy band structure.

In this paper, we study the channel doping concentration characteristics of junctionless nanowire transistors (JLT) using Edison nanowire FET device simulation. JLT has no junctions by very simple fabrication process. And this device has less variability and better electrical properties than classical inversion-mode transistors with PN junctions at the source and drain. In this simulation we use tri-gate structure. Source and drain doping concentration is 1020 atoms/cm3. The simulation results show that I-V characteristics of JLT change due to the variation of channel doping concentration.

We investigate co-doping effects of conjugated P-N B-N with increasing of N concentration in the graphene sheets using a first principles based on the density functional theory. N doping sites of the graphene consider two possible sites (pyridinic and porphyrin-like). Energy calculation shows that additional doping of B atom in the porphyrin-like N doped graphene (V+B-Nx) is hard to form. At the low chemical potential of N, one N atom with additional doping in the graphene (V+P-N1, P/B-N1) has low formation energy on the other hand at high chemical potential of N, high concentration of N (V+P-N4, P/B-N3) in the graphene is governing conformation. From the results of electronic band structure calculation, it is found that V+P-N4 and P/B-N3 cases change the Fermi energy therefore type change is occurred. On the other hand, the cases of V+P-N1 and N+B recover the electronic structure of pristine graphene.

본 실험에서는 탄소 cluster 중에서 fullerene 구조를 가질 수 있는 가장 작은 cluster인 C20 cluster에 대해, 기존 연구에서 가장 안정한 것으로 제시된 cage(fullerene), bowl, ring의 3가지 구조와 Si20 cluster를 모방한 구조 하나의 안정성을 확인하였다. ab-initio calculation을 지원하는 Edison nanophysics의 LCAODFLab을 이용하여 LDA-CA, GGA-PBE 두 가지 방법으로 계산하였다. 계산 값을 바탕으로 각 구조의 원자화에너지를 비교한 결과 LDA와 GGA 모두 cage, bowl, ring의 순서로 안정하였다. 최적화한 구조에 대하여 구조분석을 진행하였다. 최적화 결과 Bowl은 C5v, ring은 D10h, cage는 C2h 대칭성을 가지는 구조였으며, LDA, GGA 계산 모두 C20 구조의 spin polarization에는 영향을 받지 않았다.

In this paper, novel Carbon Nanotube Gate-Elongated Tunneling Field Transistor(CNT G-E TFET) is proposed. This proposed device is designed to decrease off-current around 2~6 orders of magnitude compared to the gate-channel size matched TFET. Mechanism of CNT G-E TFET creates additional steps in energy band structure so that off-current can be reduced. Since CNT TFETs show a great probability for tunneling processes and they are beneficial for the overall device performance in terms of switching speed and power consumption, CNT G-E TFET looks pretty much promising.

A semiconducting single-walled carbon nanotube (SWCNT) field-effect transistor (FET) in a top-gate model was constructed. The effect of different high-κ dielectric materials (Al2O3, HfO2 and HfSiON) and various temperatures with a wide range from 50K to 500K on the performance of such nominal device were investigated. Several key device parameters including the on/off ratio of the current, transconductance (gm), subthreshold swing, and carrier mobility were used to evaluate the device performance. The simulated results fit well with the experiment results previously published.

본 연구에서는 S/D이 n+/n+로 구성된 CNTMOSFETs, 금속으로 이루어진 SB-CNTFETs와 p+/n+로 구성된 CNT-TFETs에 대한 각각의 I d-Vg 특성과 포텐셜 프로파일을 확인하였다. 그리고 각 소자의 특성 및 특징을 연구하고, 이 중에서 BTB에 가장 큰 영향을 받는 CNT-TFETs의 특성을 VDS, 분자 비대칭성과 Tox에 따른 특성 변화를 연구하였다. 그 결과 예상과 다르게 오히려 작은 VDS와 큰 Eg을 가질 때, 향상된 SS를 가진다는 것을 확인 할 수 있었다. 특히, (7,0) CNT-TFETs에서 비록 I d는 작지만, SS를 57mV/dec까지 개선할 수 있었다. 또한, Tox를 얇게 하면, 비록 60 mV/dec 이하의 결과는 보여주지 못했지만, SS와 Ion 모두 개선할 수 있음을 확인할 수 있었다.

Topological insulator (TI) has non-trivial metallic surface states and has provoked many studies of property of this metarial. One of TI, Bi2Se3 is the promising metarial due to application of quantum devices. We investigate the effect of Ag adatom in the Bi2Se3 (111) surface. The silver atom prefers to locate within the vdW gap between the QLs rather than on the top surface. The effect of Ag adsorption is the rise of the Fermi level implying that the adsorbed Ag atoms behave like electron donors.

실리콘 나노선 전계 효과 트랜지스터(Field Effect Transisor: FET)의 특성을 시뮬레이션을 통해 연구하였다. 일반적인 트랜스컨덕턴스(transconductance) 값을 이용하여 소자의 전계 효과 이동도(field effect mobility)를 추출했고, Y-function 방법을 이용하여 저전계 이동도(low field mobility)와 문턱전압(threshold voltage)를 구했다. 채널길이가 10nm로 매우 짧을 때와 100nm의 일반적인 길이 일때의 전하 이동도 특성을 비교하여 Si 나노선 FET의 쇼트 채널 효과(short channel effect)를 보았다.

탄화규소(SiC)는 물리적 성질이 뛰어나 여러 전자기기에 다양한 활용 가능성이 제시되고있다. 이 논문에서는 2D 구조의 탄화규소 벌집 구조에 대하여 전자 구조 계산을 수행하고 탄소와 규소의 2D 구조체와 전자구조적 차이점을 논의했다. 단원자 2D 구조와는 달리 탄화규소의 합성 구조는 반도체의 성질을 띄는 것으로 나타났고, 이는 두 원자 간의 전기음성도 차이로 인한 전자의 국소화 현상 때문인 것으로 분석되었다.

How the different thickness of thin films of black-P has an effect on its electronic band structure and structure has been studied by using SIESTA code. Although the interaction between the thin films has something to do with band reduction, it does not affect the inter-atomic distance between two nearest neighbour puckered layers.

이번 연구에서는 제일원리 계산을 통해 실리콘의 전자구조를 분석하였다. 특히 strain이 걸렸을 때에 실리콘의 전자이동도는 전자구조의 변화와 밀접하게 관련이 있음을 밝혔다. Strain 이 걸린 경우와 그렇지 않은 경우에 대한 conduction band의 effective한 유효질량 계산을 하였고 이를 통해 tensile strain이 걸린 경우 전자의 이동도가 증가하는 것을 보였다.

The size effects in ultrathin ZnO nanowires have been investigated based on first-principles total energy calculations. We observed that the wurtzite phase is most stable than the hexagonal and rocksalt structures. Thus, we have considered three different diameters of wurtzite ZnO nanowires with [0001] growth direction. We found that most of the atomic relaxations take place on surface atoms, and surface atoms cause change of the electronic structure of ZnO nanowires. The thinner nanowire has more surface atoms and dangling bonds, The bandgap depend on the diameter of nanowire, while single walled nanotubes nearly independent of tubular structure and diameters.

We studied the thermal and electron properties of covalent one-dimensional (1D) monatomic linear chains of carbon, particularly carbyne. We found the α- carbyne (Polyyne, alternating single and triple C-C bond co-existing) is more stable than β-carbyne(Equally-spaced based on C-C double bond) energetically. As investigation of electron density of states (EDOS), polyyne and cumulene had different electronic characteristic, which corresponding metallic and semiconducting respectively. We also calculate the phonon dispersion, phonon density of states (PDOS) and phonon transmission of carbynes.

We investigate special arrangements of lithium atoms adsorbed on graphene. By changing adsorption sites and increasing number of atoms, adsorption of lithium atoms on (3x3) graphene is investigated using the density functional theory (DFT) calculations. In this study, three kinds of adsorption sites are considered, such as top, bridge and hollow sites.

A FeRh alloy is a well-known efficient magnetocaloric material and some experimental and theoretical studies of bulk FeRh have been reported already by several groups. In this study we report first-principles calculations on magnetic properties of different thickness FeRh thin films in order to investigate the possibility to enhance further the magnetocaloric efficiency. We used two methods of a Vienna Ab-initio Simulation Package (VASP) code and SIESTA package. We found that the FeRh thin films have quite different magnetic properties from the bulk when the thickness is thinner than 6-atomic-layers. While bulk FeRh has a Gtype antiferromagnetic(AFM) state, thin films which are thinner than 6-atomic-layers have an A-type AFM state or a ferromagnetic (FM) state. We will discuss possibility of magnetic phase transitions of the FeRh thin films in the view point of a magnetocaloric effect. And we found 4-, 5-, 6-layers films with Fe surface and 7-layers film with Rh surface are FM and they have relatively small magnetocrystalline anisotropy (MCA) energy about less than 70 meV. The small MCA energy leads to reduction of the strength of magnetic field in operating a magnetic refrigerator.

Sub 10-nm thick of Si plate is simulated with the software for Nanowire Field Effect Transistor (FET) device simulation. With usual single crystal Si technology, it is difficult to realize flexible electronic devices. Here, we suggest a FET device based on thinned Si layer. The simulation implied a practical limitation of the Si plate thickness for flexible devices as 2 nm. With around this thickness, Si plate may have much flexibility than existing bulk MOSFETs.

Nano MOS with Er2O3/SiO2 double-layer gate dielectrics was demonstrated by EDISON Nanophysics software. Double-layer structure decreases the gate leakage current in two orders compared with SiO2 single-layer gate dielectric.