전이금속이 첨가된 Wide Band-gap 반도체의 제일원리계산을 이용한 이론적 재료설계 Theoretical Materials Design for Transition Metal Doped Wide Band-gap Semiconductor Systems by the First-principles Calculation원문보기
전이금속이 치환된 wide band-gap 반도체는 묽은자성반도체로도 잘 알려져있는 재료로써 전자의 스핀특성을 이용하여 물리적 한계에 다다른 오늘날 반도체 기술을 극복하기 위한 소재로 최근 각광을 받고 있다. 전자의 스핀특성을 소자로서 이용하기 위해 재료가 가져야 하는 물성은 크게 3가지이다. 첫째로 강자성을 가져야 한다. 전자의 스핀을 이용하기 위해서는 스핀분극율이 높은 재료가 유리하기 때문이다. 둘째로 반금속 특성을 가져야 한다. 반금속 특성이란 ...
전이금속이 치환된 wide band-gap 반도체는 묽은자성반도체로도 잘 알려져있는 재료로써 전자의 스핀특성을 이용하여 물리적 한계에 다다른 오늘날 반도체 기술을 극복하기 위한 소재로 최근 각광을 받고 있다. 전자의 스핀특성을 소자로서 이용하기 위해 재료가 가져야 하는 물성은 크게 3가지이다. 첫째로 강자성을 가져야 한다. 전자의 스핀을 이용하기 위해서는 스핀분극율이 높은 재료가 유리하기 때문이다. 둘째로 반금속 특성을 가져야 한다. 반금속 특성이란 전도전자, 즉 페르미 준위에 분포하는 전자의 스핀이 한 방향만을 가진 특성으로 한 쪽 스핀방향으로는 도체이고 다른 쪽 스핀방향으로는 부도체인 특성을 말한다. 끝으로 상온에서 강자성을 유지할 수 있어야 한다. 즉 큐리온도가 상온 이상인 재료가 소자로서의 응용가치가 있기 때문이다. 본 논문에서는 wide band-gap 재료로 탄화규소(SiC), 산화아연(ZnO), 이산화티타늄(TiO₂)에 어떠한 전이금속이 치환될 경우 강자성과 반금속 특성이 나타나는지에 대해 제일원리를 이용하여 알아보았다. 먼저 전이금속이 치환되었을 때 자화에너지와 자기모멘트를 계산하여 에너지적으로 강자성이 안정한 치환위치와 치환금속을 알아보았고, 반금속 특성이 나타나는지 확인하기 위하여 상태밀도를 계산하여 보았다.
전이금속이 치환된 wide band-gap 반도체는 묽은자성반도체로도 잘 알려져있는 재료로써 전자의 스핀특성을 이용하여 물리적 한계에 다다른 오늘날 반도체 기술을 극복하기 위한 소재로 최근 각광을 받고 있다. 전자의 스핀특성을 소자로서 이용하기 위해 재료가 가져야 하는 물성은 크게 3가지이다. 첫째로 강자성을 가져야 한다. 전자의 스핀을 이용하기 위해서는 스핀분극율이 높은 재료가 유리하기 때문이다. 둘째로 반금속 특성을 가져야 한다. 반금속 특성이란 전도전자, 즉 페르미 준위에 분포하는 전자의 스핀이 한 방향만을 가진 특성으로 한 쪽 스핀방향으로는 도체이고 다른 쪽 스핀방향으로는 부도체인 특성을 말한다. 끝으로 상온에서 강자성을 유지할 수 있어야 한다. 즉 큐리온도가 상온 이상인 재료가 소자로서의 응용가치가 있기 때문이다. 본 논문에서는 wide band-gap 재료로 탄화규소(SiC), 산화아연(ZnO), 이산화티타늄(TiO₂)에 어떠한 전이금속이 치환될 경우 강자성과 반금속 특성이 나타나는지에 대해 제일원리를 이용하여 알아보았다. 먼저 전이금속이 치환되었을 때 자화에너지와 자기모멘트를 계산하여 에너지적으로 강자성이 안정한 치환위치와 치환금속을 알아보았고, 반금속 특성이 나타나는지 확인하기 위하여 상태밀도를 계산하여 보았다.
This thesis presents a theoretical study of bulk and surface materials using ab initio methods based on the density functional theory (DFT). The quantum-mechanical electronic and spin properties of the transition metal doped wide band-gap semiconductor systems (SiC, ZnO, and TiO₂) were intensively i...
This thesis presents a theoretical study of bulk and surface materials using ab initio methods based on the density functional theory (DFT). The quantum-mechanical electronic and spin properties of the transition metal doped wide band-gap semiconductor systems (SiC, ZnO, and TiO₂) were intensively investigated. These systems are well known as a diluted magnetic semiconductors (DMS), in which magnetic elements substitute for a portion of host elements in semiconductor lattices, and their hetero-structures have attracted a great deal of interest because of their favorable physical (or spin-electronic) properties for applications in spin-related electronic device. The main part of this thesis focuses on the electronic structure of the DMS. Different approaches such as magnetization energy calculations for ferromagnetic stability, density of states calculation, and band structure calculations have been used to determine the electronic structures of the bulk DMS system. Surface energy calculations have also been performed for the (001) and (0001) surfaces of the SiC and ZnO system, respectively. The band structure calculation was employed to investigate the defect states in the band-gap of TiO₂ DMS system. The general relations between the electronic structure and materials properties of the DMS have been elaborated in the thesis. For the SiC-based system, the ferromagnetic phase was found to be energetically favorable for the case of SiC:TMSi (TM=V, Cr, Mn). These diluted ferromagnetic semiconductors exhibit a half-metallic property with metallic majority-spin electrons and insulating minority-spin electrons. It is found that the SiC:Cr reveals stable ferromagnetism with permanent magnetic moments as large as 2 μB regardless of substitution site, and the total magnetic moment of SiC:Cr is independent of the substitution site and amount of doping concentration. Using the density of states calculation, it is shown that SiC:Cr has half-metallic properties for selected doping concentrations of 1.56%, 3.13%, and 6.25%, irrespective of substitution site. The conduction electron mobility of SiC:CrC was expected to be higher than that of SiC:Cr_(Si). On the contrary, SiC:Cr_(Si) has a wider spin band gap compared to SiC:CrC. It is predicted that SiC with 6.25% Cr doping represents desirable characteristics for realizing spintronic devices. The SiC:Mn shows stable ferromagnetism, and the total magnetic moment of SiC:Mn depends on the substitution site of Mn in the SiC lattice. Using the density of states calculation, it is shown that SiC:Mn has half-metallic properties for selected doping concentrations of 1.56, 3.13 and 6.25%, irrespective of substitution site. The conduction electron mobility of SiC:MnC was expected to be higher than that of SiC:MnSi. On the contrary, SiC:MnSi has a wider spin band gap compared to SiC:MnC. It is predicted that SiC with 12.5% Mn doping represents desirable characteristics for realizing spintronic devices, which include stable ferromagnetism, half-metallic properties, faster electron mobility and a wide spin band gap. The magnetic and electronic properties of the polar-ZnO surface system with adsorbed manganese and cobalt were investigated using the ab initio projector-augmented-wave method. It is found that the Mn and Co adsorbed polar-ZnO surfaces show stable ferromagnetism. The O-terminated ZnO:Mn surface system is predicted to have a half-metallic property with a wide spin band gap and stable ferromagnetism of 4.98 μB magnetic moment, which is a desirable property for application to spintronic devices. And also, the electronic structure and magnetic properties of a Cu adsorbed polar-ZnO surface system were investigated. It is found that the H3 site is the most stable adsorption site of Cu adsorbed polar-ZnO surfaces with stable ferromagnetism. The Zn-terminated ZnO:Cu surface system H3 site adsorption is predicted to have a half-metallic property and stable ferromagnetism of 0.72 μB magnetic moments. The electronic structure of 6.25% Ni doped rutile titanium dioxide (Ni/TiO₂) was investigated by ab initio ultrasoft pseudopotential plane wave method based on density functional theory. When a Ni atom was substituted for a Ti site of TiO₂ lattice, two additional unoccupied 3d states appeared as defect states in band gap, and occupied 3d states were hybridized with oxygen 2p states. From the calculation of band decomposed charge density and density of states, it was found that the lower defect state (D1), a Ni 3d-orbital, was hybridized with unoccupied oxygen 2p states and the higher one (D2), localized 3d-orbital of Ni, was located at the middle of band gap. The electronic structure and defect states of 6.25% transition metal (TM = Fe and Co) doped rutile titanium dioxide were investigated. When a Fe or Co atom was substituted for a Ti site of TiO₂ lattice, additional unoccupied 3d-states of TM appeared as defect states in band-gap and occupied 3d-states represented metallic behavior (half-filled states). It was found that two occupied defect states and one half-filled defect state were the 3d-states of TM from the calculation of band decomposed charge density. The higher unoccupied states were hybridized with unoccupied oxygen 2p-states and located at the middle of band-gap.
This thesis presents a theoretical study of bulk and surface materials using ab initio methods based on the density functional theory (DFT). The quantum-mechanical electronic and spin properties of the transition metal doped wide band-gap semiconductor systems (SiC, ZnO, and TiO₂) were intensively investigated. These systems are well known as a diluted magnetic semiconductors (DMS), in which magnetic elements substitute for a portion of host elements in semiconductor lattices, and their hetero-structures have attracted a great deal of interest because of their favorable physical (or spin-electronic) properties for applications in spin-related electronic device. The main part of this thesis focuses on the electronic structure of the DMS. Different approaches such as magnetization energy calculations for ferromagnetic stability, density of states calculation, and band structure calculations have been used to determine the electronic structures of the bulk DMS system. Surface energy calculations have also been performed for the (001) and (0001) surfaces of the SiC and ZnO system, respectively. The band structure calculation was employed to investigate the defect states in the band-gap of TiO₂ DMS system. The general relations between the electronic structure and materials properties of the DMS have been elaborated in the thesis. For the SiC-based system, the ferromagnetic phase was found to be energetically favorable for the case of SiC:TMSi (TM=V, Cr, Mn). These diluted ferromagnetic semiconductors exhibit a half-metallic property with metallic majority-spin electrons and insulating minority-spin electrons. It is found that the SiC:Cr reveals stable ferromagnetism with permanent magnetic moments as large as 2 μB regardless of substitution site, and the total magnetic moment of SiC:Cr is independent of the substitution site and amount of doping concentration. Using the density of states calculation, it is shown that SiC:Cr has half-metallic properties for selected doping concentrations of 1.56%, 3.13%, and 6.25%, irrespective of substitution site. The conduction electron mobility of SiC:CrC was expected to be higher than that of SiC:Cr_(Si). On the contrary, SiC:Cr_(Si) has a wider spin band gap compared to SiC:CrC. It is predicted that SiC with 6.25% Cr doping represents desirable characteristics for realizing spintronic devices. The SiC:Mn shows stable ferromagnetism, and the total magnetic moment of SiC:Mn depends on the substitution site of Mn in the SiC lattice. Using the density of states calculation, it is shown that SiC:Mn has half-metallic properties for selected doping concentrations of 1.56, 3.13 and 6.25%, irrespective of substitution site. The conduction electron mobility of SiC:MnC was expected to be higher than that of SiC:MnSi. On the contrary, SiC:MnSi has a wider spin band gap compared to SiC:MnC. It is predicted that SiC with 12.5% Mn doping represents desirable characteristics for realizing spintronic devices, which include stable ferromagnetism, half-metallic properties, faster electron mobility and a wide spin band gap. The magnetic and electronic properties of the polar-ZnO surface system with adsorbed manganese and cobalt were investigated using the ab initio projector-augmented-wave method. It is found that the Mn and Co adsorbed polar-ZnO surfaces show stable ferromagnetism. The O-terminated ZnO:Mn surface system is predicted to have a half-metallic property with a wide spin band gap and stable ferromagnetism of 4.98 μB magnetic moment, which is a desirable property for application to spintronic devices. And also, the electronic structure and magnetic properties of a Cu adsorbed polar-ZnO surface system were investigated. It is found that the H3 site is the most stable adsorption site of Cu adsorbed polar-ZnO surfaces with stable ferromagnetism. The Zn-terminated ZnO:Cu surface system H3 site adsorption is predicted to have a half-metallic property and stable ferromagnetism of 0.72 μB magnetic moments. The electronic structure of 6.25% Ni doped rutile titanium dioxide (Ni/TiO₂) was investigated by ab initio ultrasoft pseudopotential plane wave method based on density functional theory. When a Ni atom was substituted for a Ti site of TiO₂ lattice, two additional unoccupied 3d states appeared as defect states in band gap, and occupied 3d states were hybridized with oxygen 2p states. From the calculation of band decomposed charge density and density of states, it was found that the lower defect state (D1), a Ni 3d-orbital, was hybridized with unoccupied oxygen 2p states and the higher one (D2), localized 3d-orbital of Ni, was located at the middle of band gap. The electronic structure and defect states of 6.25% transition metal (TM = Fe and Co) doped rutile titanium dioxide were investigated. When a Fe or Co atom was substituted for a Ti site of TiO₂ lattice, additional unoccupied 3d-states of TM appeared as defect states in band-gap and occupied 3d-states represented metallic behavior (half-filled states). It was found that two occupied defect states and one half-filled defect state were the 3d-states of TM from the calculation of band decomposed charge density. The higher unoccupied states were hybridized with unoccupied oxygen 2p-states and located at the middle of band-gap.
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