The Experts below are selected from a list of 10437 Experts worldwide ranked by ideXlab platform
Yong Zhang - One of the best experts on this subject based on the ideXlab platform.
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New strategy for Highly Effective P-Type Doping of Nitrides: Energy Band Tailoring
Light Energy and the Environment, 2017Co-Authors: Xiaoyan Yi, Junxi Wang, Jinmin Li, Na Lu, Ian T. Ferguson, Yong ZhangAbstract:A new strategy for achieving efficient P-Type Doping has been proposed and investigated. Furthermore, a diagnostic technique for examining the impurity level was proposed based on photoluminescence thermal quenching
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impurity resonant states p type Doping in wide band gap nitrides
Scientific Reports, 2016Co-Authors: Zhiqiang Liu, Xiaoyan Yi, Junxi Wang, Jinmin Li, Na Lu, Ian T. Ferguson, Zhiguo Yu, Gongdong Yuan, Yang Liu, Yong ZhangAbstract:Group III-nitride semiconductors possess a number of excellent properties including a tunable, direct band gap, high drift velocity, high mobility, and strong light absorption1,2,3,4. Such properties make them viable for a broad range of electronic and optoelectronic devices and applications. Despite the tremendous progress which has been made in the growth and fabrication of such Group III semiconductors, achieving a high P-Type conductivity in nitrides has been shown to be extremely difficult, which hinders further improvement in the performance of nitride-based devices. It is well known that, similar to most wide-band-gap semiconductors such as diamond and ZnO, nitrides have a “unipolar” or “asymmetric” Doping problem. This can be attributed to low dopant solubility, hydrogen passivation, relatively low valence-band maximum (VBM) and high defect ionization energies5,6,7. Considerable effort has been expended to address this P-Type Doping issue in Group III-nitrides8,9,10. Recent advances in crystal growth technology have shown that the issues of low solubility and hydrogen passivation can, at least to some extent, be overcome by using non-equilibrium growth techniques and high-temperature annealing. However, alleviating the more fundamental problem of high activation energies has, to date, not yet been satisfactorily achieved. The underlying physical mechanism in this problem is attributed to the electronic structure of the host material. Nitrogen is strongly electronegative and has a deep 2p atomic orbital. Thus, the valance band maximum (VBM) of nitrides, which contain mostly N 2p orbitals, is at relatively low energies. This leads to a relatively deep acceptor energy level which makes it very inefficient for thermal activation. To date, the most promising acceptor for III-nitrides continues to be Mg. Unfortunately, even with Mg dopant ions, the activation energy Ea of the Mg dopant in GaN is still in the range of 160 and 200 meV. For AlN, the activation energy can be as high as 630 meV. Consequently, only a small fraction of Mg can be activated at room temperature11,12. Various approaches have been sought to lower the acceptor levels and reduce the acceptor ionization energy in nitrides. Recently, B. Gunning et al. proposed a strategy for lowering the acceptor impurity states by extremely high Doping4. They argue that, as the electrically active acceptor concentration increases, the isolated deep acceptor levels begin to interact and split into an impurity band, which is closer to the valence band thus lowering the effective activation energy. Peter and Schubert13,14 demonstrated another strategy and found that by polarization induced modulation of the valence band edge in a superlattice, the low Doping efficiency could be partially overcome. Simon and Jena15 also suggested that a 3D hole gas could be produced using the built-in electronic polarization in nitrides. However, in these previous works more direct evidence is required to further delineate and discriminate the characteristics of the 3D and 2D hole gases, which usually coexist in superlattice-like Doping systems, for instance multiple–quantum-well structures, compositionally graded layer structures, or heterojunction interfaces13,14,15,16,17. Elevating the VBM of the host material by co-Doping has been regarded as another strategy to address this issue8,18, for example by Si-Mg co-Doping and mutually passivated defect pair co-Doping. However, intensive theoretical analyses show that this type of energy level coupling is too small to significantly reduce the acceptor ionization energy due to different symmetries and wave-function characteristics10. Therefore, although partial successes have been achieved, the mechanisms of those methods are still controversial and poorly understood. Better approaches or alternative strategies to create more stable and shallower acceptors in nitrides are highly desired. As discussed above, the behavior of Mg as an acceptor in nitride semiconductors is strongly linked to the position of the Mg impurity states related to the VBM of the host materials. Besides co-Doping, a periodic oscillation of the valance band edge produced by a superlattice structure, such as AlxGa1−xN/GaN, can also modify the characteristics and energy position of the VBM13,14. Based on this consideration, a novel strategy for efficient P-Type Doping is proposed to overcome the fundamental problem of high activation energy by inducing impurity resonant states in an Mg doped AlxGa1−xN/GaN superlattice structure. As schematically shown in Fig. 1, in the structure developed using our proposed strategy, the discrete wave-functions of Mg impurity states are able to overlap to form continuous miniband-like impurity states19,20. Therefore, the initially localized impurity states in AlxGa1−xN barrier layers form resonant states in the GaN layer (i.e. with energy levels below or close to the GaN VBM, it strongly depends on the Al percentage in AlxGa1−xN). To see the exact energy position of Mg impurity state, one would need to use a pretty large cell. Alternatively, in this work we offer the above qualitative band-diagram to explain the idea of resonant state P-Type Doping. In the case of considerable acceptor density, these impurity states are broadened4,21,22,23, which can further enhance the coupling between them. In this new scenario, electrons are able to drop from the VBM of GaN into the impurity states or band without any energy barrier, which means the acceptors are self-ionized. Hence, high concentration of the acceptors can be expected. In addition, as proposed by previous reports, the polarization effect also enhances the ionization of the deep acceptors and leads to the accumulation of carriers as a hole sheet, which further increase the effective hole concentration in the host materials13,14,15. In this work, to test these proposed concepts, the impact of impurity resonant states on the ionization energy of Mg acceptors is analyzed through both theoretical and experimental methods. Figure 1 Schematic model showing the mechanism of impurity resonant states P-Type Doping.
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impurity resonant states p type Doping in wide band gap nitrides
Scientific Reports, 2016Co-Authors: Xiaoyan Yi, Junxi Wang, Jinmin Li, Na Lu, Ian T. Ferguson, Zhiguo Yu, Gongdong Yuan, Yong ZhangAbstract:In this work, a new strategy for achieving efficient P-Type Doping in high bandgap nitride semiconductors to overcome the fundamental issue of high activation energy has been proposed and investigated theoretically, and demonstrated experimentally. Specifically, in an AlxGa(1-x)N/GaN superlattice structure, by modulation Doping of Mg in the AlxGa(1-x)N barriers, high concentration of holes are generated throughout the material. A hole concentration as high as 1.1 × 10(18) cm(-3) has been achieved, which is about one order of magnitude higher than that typically achievable by direct Doping GaN. Results from first-principle calculations indicate that the coupling and hybridization between Mg 2p impurity and the host N 2p orbitals are main reasons for the generation of resonant states in the GaN wells, which further results in the high hole concentration. We expect this approach to be equally applicable for other high bandgap materials where efficient P-Type doing is difficult. Furthermore, a two-carrier-species Hall-effect model is proposed to delineate and discriminate the characteristics of the bulk and 2D hole, which usually coexist in superlattice-like Doping systems. The model reported here can also be used to explain the abnormal freeze-in effect observed in many previous reports.
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Remote P-Type Doping in GaSb/InAs Core-shell Nanowires.
Scientific Reports, 2015Co-Authors: Feng Ning, Li-ming Tang, Yong Zhang, Ke-qiu ChenAbstract:: By performing first-principles calculation, we investigated the electronic properties of remotely P-Type Doping GaSb nanowire by a Zn-doped InAs shell. The results show that for bare zinc-blende (ZB) [111] GaSb/InAs core-shell nanowire the Zn P-Type doped InAs shell donates free holes to the non-doped GaSb core nanowire without activation energy, significantly increasing the hole density and mobility of nanowire. For Zn Doping in bare ZB [110] GaSb/InAs core-shell nanowire the hole states are compensated by surface states. We also studied the behaviors of remote P-Type doing in two-dimensional (2D) GaSb/InAs heterogeneous slabs, and confirmed that the orientation of nanowire side facet is a key factor for achieving high efficient remote P-Type Doping.
Xiaoyan Yi - One of the best experts on this subject based on the ideXlab platform.
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New strategy for Highly Effective P-Type Doping of Nitrides: Energy Band Tailoring
Light Energy and the Environment, 2017Co-Authors: Xiaoyan Yi, Junxi Wang, Jinmin Li, Na Lu, Ian T. Ferguson, Yong ZhangAbstract:A new strategy for achieving efficient P-Type Doping has been proposed and investigated. Furthermore, a diagnostic technique for examining the impurity level was proposed based on photoluminescence thermal quenching
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impurity resonant states p type Doping in wide band gap nitrides
Scientific Reports, 2016Co-Authors: Zhiqiang Liu, Xiaoyan Yi, Junxi Wang, Jinmin Li, Na Lu, Ian T. Ferguson, Zhiguo Yu, Gongdong Yuan, Yang Liu, Yong ZhangAbstract:Group III-nitride semiconductors possess a number of excellent properties including a tunable, direct band gap, high drift velocity, high mobility, and strong light absorption1,2,3,4. Such properties make them viable for a broad range of electronic and optoelectronic devices and applications. Despite the tremendous progress which has been made in the growth and fabrication of such Group III semiconductors, achieving a high P-Type conductivity in nitrides has been shown to be extremely difficult, which hinders further improvement in the performance of nitride-based devices. It is well known that, similar to most wide-band-gap semiconductors such as diamond and ZnO, nitrides have a “unipolar” or “asymmetric” Doping problem. This can be attributed to low dopant solubility, hydrogen passivation, relatively low valence-band maximum (VBM) and high defect ionization energies5,6,7. Considerable effort has been expended to address this P-Type Doping issue in Group III-nitrides8,9,10. Recent advances in crystal growth technology have shown that the issues of low solubility and hydrogen passivation can, at least to some extent, be overcome by using non-equilibrium growth techniques and high-temperature annealing. However, alleviating the more fundamental problem of high activation energies has, to date, not yet been satisfactorily achieved. The underlying physical mechanism in this problem is attributed to the electronic structure of the host material. Nitrogen is strongly electronegative and has a deep 2p atomic orbital. Thus, the valance band maximum (VBM) of nitrides, which contain mostly N 2p orbitals, is at relatively low energies. This leads to a relatively deep acceptor energy level which makes it very inefficient for thermal activation. To date, the most promising acceptor for III-nitrides continues to be Mg. Unfortunately, even with Mg dopant ions, the activation energy Ea of the Mg dopant in GaN is still in the range of 160 and 200 meV. For AlN, the activation energy can be as high as 630 meV. Consequently, only a small fraction of Mg can be activated at room temperature11,12. Various approaches have been sought to lower the acceptor levels and reduce the acceptor ionization energy in nitrides. Recently, B. Gunning et al. proposed a strategy for lowering the acceptor impurity states by extremely high Doping4. They argue that, as the electrically active acceptor concentration increases, the isolated deep acceptor levels begin to interact and split into an impurity band, which is closer to the valence band thus lowering the effective activation energy. Peter and Schubert13,14 demonstrated another strategy and found that by polarization induced modulation of the valence band edge in a superlattice, the low Doping efficiency could be partially overcome. Simon and Jena15 also suggested that a 3D hole gas could be produced using the built-in electronic polarization in nitrides. However, in these previous works more direct evidence is required to further delineate and discriminate the characteristics of the 3D and 2D hole gases, which usually coexist in superlattice-like Doping systems, for instance multiple–quantum-well structures, compositionally graded layer structures, or heterojunction interfaces13,14,15,16,17. Elevating the VBM of the host material by co-Doping has been regarded as another strategy to address this issue8,18, for example by Si-Mg co-Doping and mutually passivated defect pair co-Doping. However, intensive theoretical analyses show that this type of energy level coupling is too small to significantly reduce the acceptor ionization energy due to different symmetries and wave-function characteristics10. Therefore, although partial successes have been achieved, the mechanisms of those methods are still controversial and poorly understood. Better approaches or alternative strategies to create more stable and shallower acceptors in nitrides are highly desired. As discussed above, the behavior of Mg as an acceptor in nitride semiconductors is strongly linked to the position of the Mg impurity states related to the VBM of the host materials. Besides co-Doping, a periodic oscillation of the valance band edge produced by a superlattice structure, such as AlxGa1−xN/GaN, can also modify the characteristics and energy position of the VBM13,14. Based on this consideration, a novel strategy for efficient P-Type Doping is proposed to overcome the fundamental problem of high activation energy by inducing impurity resonant states in an Mg doped AlxGa1−xN/GaN superlattice structure. As schematically shown in Fig. 1, in the structure developed using our proposed strategy, the discrete wave-functions of Mg impurity states are able to overlap to form continuous miniband-like impurity states19,20. Therefore, the initially localized impurity states in AlxGa1−xN barrier layers form resonant states in the GaN layer (i.e. with energy levels below or close to the GaN VBM, it strongly depends on the Al percentage in AlxGa1−xN). To see the exact energy position of Mg impurity state, one would need to use a pretty large cell. Alternatively, in this work we offer the above qualitative band-diagram to explain the idea of resonant state P-Type Doping. In the case of considerable acceptor density, these impurity states are broadened4,21,22,23, which can further enhance the coupling between them. In this new scenario, electrons are able to drop from the VBM of GaN into the impurity states or band without any energy barrier, which means the acceptors are self-ionized. Hence, high concentration of the acceptors can be expected. In addition, as proposed by previous reports, the polarization effect also enhances the ionization of the deep acceptors and leads to the accumulation of carriers as a hole sheet, which further increase the effective hole concentration in the host materials13,14,15. In this work, to test these proposed concepts, the impact of impurity resonant states on the ionization energy of Mg acceptors is analyzed through both theoretical and experimental methods. Figure 1 Schematic model showing the mechanism of impurity resonant states P-Type Doping.
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impurity resonant states p type Doping in wide band gap nitrides
Scientific Reports, 2016Co-Authors: Xiaoyan Yi, Junxi Wang, Jinmin Li, Na Lu, Ian T. Ferguson, Zhiguo Yu, Gongdong Yuan, Yong ZhangAbstract:In this work, a new strategy for achieving efficient P-Type Doping in high bandgap nitride semiconductors to overcome the fundamental issue of high activation energy has been proposed and investigated theoretically, and demonstrated experimentally. Specifically, in an AlxGa(1-x)N/GaN superlattice structure, by modulation Doping of Mg in the AlxGa(1-x)N barriers, high concentration of holes are generated throughout the material. A hole concentration as high as 1.1 × 10(18) cm(-3) has been achieved, which is about one order of magnitude higher than that typically achievable by direct Doping GaN. Results from first-principle calculations indicate that the coupling and hybridization between Mg 2p impurity and the host N 2p orbitals are main reasons for the generation of resonant states in the GaN wells, which further results in the high hole concentration. We expect this approach to be equally applicable for other high bandgap materials where efficient P-Type doing is difficult. Furthermore, a two-carrier-species Hall-effect model is proposed to delineate and discriminate the characteristics of the bulk and 2D hole, which usually coexist in superlattice-like Doping systems. The model reported here can also be used to explain the abnormal freeze-in effect observed in many previous reports.
Debdeep Jena - One of the best experts on this subject based on the ideXlab platform.
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polarization induced hole Doping in wide band gap uniaxial semiconductor heterostructures
Science, 2010Co-Authors: John D. Simon, Chuanxin Lian, Huili Xing, Vladimir Protasenko, Debdeep JenaAbstract:Impurity-based P-Type Doping in wide–band-gap semiconductors is inefficient at room temperature for applications such as lasers because the positive-charge carriers (holes) have a large thermal activation energy. We demonstrate high-efficiency P-Type Doping by ionizing acceptor dopants using the built-in electronic polarization in bulk uniaxial semiconductor crystals. Because the mobile hole gases are field-ionized, they are robust to thermal freezeout effects and lead to major improvements in P-Type electrical conductivity. The new Doping technique results in improved optical emission efficiency in prototype ultraviolet light-emitting–diode structures. Polarization-induced Doping provides an attractive solution to both p- and n-type Doping problems in wide–band-gap semiconductors and offers an unconventional path for the development of solid-state deep-ultraviolet optoelectronic devices and wide–band-gap bipolar electronic devices of the future.
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Polarization-Induced Hole Doping in Wide–Band-Gap Uniaxial Semiconductor Heterostructures
Science, 2010Co-Authors: John D. Simon, Chuanxin Lian, Huili Xing, Vladimir Protasenko, Debdeep JenaAbstract:Impurity-based P-Type Doping in wide–band-gap semiconductors is inefficient at room temperature for applications such as lasers because the positive-charge carriers (holes) have a large thermal activation energy. We demonstrate high-efficiency P-Type Doping by ionizing acceptor dopants using the built-in electronic polarization in bulk uniaxial semiconductor crystals. Because the mobile hole gases are field-ionized, they are robust to thermal freezeout effects and lead to major improvements in P-Type electrical conductivity. The new Doping technique results in improved optical emission efficiency in prototype ultraviolet light-emitting–diode structures. Polarization-induced Doping provides an attractive solution to both p- and n-type Doping problems in wide–band-gap semiconductors and offers an unconventional path for the development of solid-state deep-ultraviolet optoelectronic devices and wide–band-gap bipolar electronic devices of the future.
Jinmin Li - One of the best experts on this subject based on the ideXlab platform.
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New strategy for Highly Effective P-Type Doping of Nitrides: Energy Band Tailoring
Light Energy and the Environment, 2017Co-Authors: Xiaoyan Yi, Junxi Wang, Jinmin Li, Na Lu, Ian T. Ferguson, Yong ZhangAbstract:A new strategy for achieving efficient P-Type Doping has been proposed and investigated. Furthermore, a diagnostic technique for examining the impurity level was proposed based on photoluminescence thermal quenching
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impurity resonant states p type Doping in wide band gap nitrides
Scientific Reports, 2016Co-Authors: Zhiqiang Liu, Xiaoyan Yi, Junxi Wang, Jinmin Li, Na Lu, Ian T. Ferguson, Zhiguo Yu, Gongdong Yuan, Yang Liu, Yong ZhangAbstract:Group III-nitride semiconductors possess a number of excellent properties including a tunable, direct band gap, high drift velocity, high mobility, and strong light absorption1,2,3,4. Such properties make them viable for a broad range of electronic and optoelectronic devices and applications. Despite the tremendous progress which has been made in the growth and fabrication of such Group III semiconductors, achieving a high P-Type conductivity in nitrides has been shown to be extremely difficult, which hinders further improvement in the performance of nitride-based devices. It is well known that, similar to most wide-band-gap semiconductors such as diamond and ZnO, nitrides have a “unipolar” or “asymmetric” Doping problem. This can be attributed to low dopant solubility, hydrogen passivation, relatively low valence-band maximum (VBM) and high defect ionization energies5,6,7. Considerable effort has been expended to address this P-Type Doping issue in Group III-nitrides8,9,10. Recent advances in crystal growth technology have shown that the issues of low solubility and hydrogen passivation can, at least to some extent, be overcome by using non-equilibrium growth techniques and high-temperature annealing. However, alleviating the more fundamental problem of high activation energies has, to date, not yet been satisfactorily achieved. The underlying physical mechanism in this problem is attributed to the electronic structure of the host material. Nitrogen is strongly electronegative and has a deep 2p atomic orbital. Thus, the valance band maximum (VBM) of nitrides, which contain mostly N 2p orbitals, is at relatively low energies. This leads to a relatively deep acceptor energy level which makes it very inefficient for thermal activation. To date, the most promising acceptor for III-nitrides continues to be Mg. Unfortunately, even with Mg dopant ions, the activation energy Ea of the Mg dopant in GaN is still in the range of 160 and 200 meV. For AlN, the activation energy can be as high as 630 meV. Consequently, only a small fraction of Mg can be activated at room temperature11,12. Various approaches have been sought to lower the acceptor levels and reduce the acceptor ionization energy in nitrides. Recently, B. Gunning et al. proposed a strategy for lowering the acceptor impurity states by extremely high Doping4. They argue that, as the electrically active acceptor concentration increases, the isolated deep acceptor levels begin to interact and split into an impurity band, which is closer to the valence band thus lowering the effective activation energy. Peter and Schubert13,14 demonstrated another strategy and found that by polarization induced modulation of the valence band edge in a superlattice, the low Doping efficiency could be partially overcome. Simon and Jena15 also suggested that a 3D hole gas could be produced using the built-in electronic polarization in nitrides. However, in these previous works more direct evidence is required to further delineate and discriminate the characteristics of the 3D and 2D hole gases, which usually coexist in superlattice-like Doping systems, for instance multiple–quantum-well structures, compositionally graded layer structures, or heterojunction interfaces13,14,15,16,17. Elevating the VBM of the host material by co-Doping has been regarded as another strategy to address this issue8,18, for example by Si-Mg co-Doping and mutually passivated defect pair co-Doping. However, intensive theoretical analyses show that this type of energy level coupling is too small to significantly reduce the acceptor ionization energy due to different symmetries and wave-function characteristics10. Therefore, although partial successes have been achieved, the mechanisms of those methods are still controversial and poorly understood. Better approaches or alternative strategies to create more stable and shallower acceptors in nitrides are highly desired. As discussed above, the behavior of Mg as an acceptor in nitride semiconductors is strongly linked to the position of the Mg impurity states related to the VBM of the host materials. Besides co-Doping, a periodic oscillation of the valance band edge produced by a superlattice structure, such as AlxGa1−xN/GaN, can also modify the characteristics and energy position of the VBM13,14. Based on this consideration, a novel strategy for efficient P-Type Doping is proposed to overcome the fundamental problem of high activation energy by inducing impurity resonant states in an Mg doped AlxGa1−xN/GaN superlattice structure. As schematically shown in Fig. 1, in the structure developed using our proposed strategy, the discrete wave-functions of Mg impurity states are able to overlap to form continuous miniband-like impurity states19,20. Therefore, the initially localized impurity states in AlxGa1−xN barrier layers form resonant states in the GaN layer (i.e. with energy levels below or close to the GaN VBM, it strongly depends on the Al percentage in AlxGa1−xN). To see the exact energy position of Mg impurity state, one would need to use a pretty large cell. Alternatively, in this work we offer the above qualitative band-diagram to explain the idea of resonant state P-Type Doping. In the case of considerable acceptor density, these impurity states are broadened4,21,22,23, which can further enhance the coupling between them. In this new scenario, electrons are able to drop from the VBM of GaN into the impurity states or band without any energy barrier, which means the acceptors are self-ionized. Hence, high concentration of the acceptors can be expected. In addition, as proposed by previous reports, the polarization effect also enhances the ionization of the deep acceptors and leads to the accumulation of carriers as a hole sheet, which further increase the effective hole concentration in the host materials13,14,15. In this work, to test these proposed concepts, the impact of impurity resonant states on the ionization energy of Mg acceptors is analyzed through both theoretical and experimental methods. Figure 1 Schematic model showing the mechanism of impurity resonant states P-Type Doping.
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impurity resonant states p type Doping in wide band gap nitrides
Scientific Reports, 2016Co-Authors: Xiaoyan Yi, Junxi Wang, Jinmin Li, Na Lu, Ian T. Ferguson, Zhiguo Yu, Gongdong Yuan, Yong ZhangAbstract:In this work, a new strategy for achieving efficient P-Type Doping in high bandgap nitride semiconductors to overcome the fundamental issue of high activation energy has been proposed and investigated theoretically, and demonstrated experimentally. Specifically, in an AlxGa(1-x)N/GaN superlattice structure, by modulation Doping of Mg in the AlxGa(1-x)N barriers, high concentration of holes are generated throughout the material. A hole concentration as high as 1.1 × 10(18) cm(-3) has been achieved, which is about one order of magnitude higher than that typically achievable by direct Doping GaN. Results from first-principle calculations indicate that the coupling and hybridization between Mg 2p impurity and the host N 2p orbitals are main reasons for the generation of resonant states in the GaN wells, which further results in the high hole concentration. We expect this approach to be equally applicable for other high bandgap materials where efficient P-Type doing is difficult. Furthermore, a two-carrier-species Hall-effect model is proposed to delineate and discriminate the characteristics of the bulk and 2D hole, which usually coexist in superlattice-like Doping systems. The model reported here can also be used to explain the abnormal freeze-in effect observed in many previous reports.
Na Lu - One of the best experts on this subject based on the ideXlab platform.
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New strategy for Highly Effective P-Type Doping of Nitrides: Energy Band Tailoring
Light Energy and the Environment, 2017Co-Authors: Xiaoyan Yi, Junxi Wang, Jinmin Li, Na Lu, Ian T. Ferguson, Yong ZhangAbstract:A new strategy for achieving efficient P-Type Doping has been proposed and investigated. Furthermore, a diagnostic technique for examining the impurity level was proposed based on photoluminescence thermal quenching
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impurity resonant states p type Doping in wide band gap nitrides
Scientific Reports, 2016Co-Authors: Zhiqiang Liu, Xiaoyan Yi, Junxi Wang, Jinmin Li, Na Lu, Ian T. Ferguson, Zhiguo Yu, Gongdong Yuan, Yang Liu, Yong ZhangAbstract:Group III-nitride semiconductors possess a number of excellent properties including a tunable, direct band gap, high drift velocity, high mobility, and strong light absorption1,2,3,4. Such properties make them viable for a broad range of electronic and optoelectronic devices and applications. Despite the tremendous progress which has been made in the growth and fabrication of such Group III semiconductors, achieving a high P-Type conductivity in nitrides has been shown to be extremely difficult, which hinders further improvement in the performance of nitride-based devices. It is well known that, similar to most wide-band-gap semiconductors such as diamond and ZnO, nitrides have a “unipolar” or “asymmetric” Doping problem. This can be attributed to low dopant solubility, hydrogen passivation, relatively low valence-band maximum (VBM) and high defect ionization energies5,6,7. Considerable effort has been expended to address this P-Type Doping issue in Group III-nitrides8,9,10. Recent advances in crystal growth technology have shown that the issues of low solubility and hydrogen passivation can, at least to some extent, be overcome by using non-equilibrium growth techniques and high-temperature annealing. However, alleviating the more fundamental problem of high activation energies has, to date, not yet been satisfactorily achieved. The underlying physical mechanism in this problem is attributed to the electronic structure of the host material. Nitrogen is strongly electronegative and has a deep 2p atomic orbital. Thus, the valance band maximum (VBM) of nitrides, which contain mostly N 2p orbitals, is at relatively low energies. This leads to a relatively deep acceptor energy level which makes it very inefficient for thermal activation. To date, the most promising acceptor for III-nitrides continues to be Mg. Unfortunately, even with Mg dopant ions, the activation energy Ea of the Mg dopant in GaN is still in the range of 160 and 200 meV. For AlN, the activation energy can be as high as 630 meV. Consequently, only a small fraction of Mg can be activated at room temperature11,12. Various approaches have been sought to lower the acceptor levels and reduce the acceptor ionization energy in nitrides. Recently, B. Gunning et al. proposed a strategy for lowering the acceptor impurity states by extremely high Doping4. They argue that, as the electrically active acceptor concentration increases, the isolated deep acceptor levels begin to interact and split into an impurity band, which is closer to the valence band thus lowering the effective activation energy. Peter and Schubert13,14 demonstrated another strategy and found that by polarization induced modulation of the valence band edge in a superlattice, the low Doping efficiency could be partially overcome. Simon and Jena15 also suggested that a 3D hole gas could be produced using the built-in electronic polarization in nitrides. However, in these previous works more direct evidence is required to further delineate and discriminate the characteristics of the 3D and 2D hole gases, which usually coexist in superlattice-like Doping systems, for instance multiple–quantum-well structures, compositionally graded layer structures, or heterojunction interfaces13,14,15,16,17. Elevating the VBM of the host material by co-Doping has been regarded as another strategy to address this issue8,18, for example by Si-Mg co-Doping and mutually passivated defect pair co-Doping. However, intensive theoretical analyses show that this type of energy level coupling is too small to significantly reduce the acceptor ionization energy due to different symmetries and wave-function characteristics10. Therefore, although partial successes have been achieved, the mechanisms of those methods are still controversial and poorly understood. Better approaches or alternative strategies to create more stable and shallower acceptors in nitrides are highly desired. As discussed above, the behavior of Mg as an acceptor in nitride semiconductors is strongly linked to the position of the Mg impurity states related to the VBM of the host materials. Besides co-Doping, a periodic oscillation of the valance band edge produced by a superlattice structure, such as AlxGa1−xN/GaN, can also modify the characteristics and energy position of the VBM13,14. Based on this consideration, a novel strategy for efficient P-Type Doping is proposed to overcome the fundamental problem of high activation energy by inducing impurity resonant states in an Mg doped AlxGa1−xN/GaN superlattice structure. As schematically shown in Fig. 1, in the structure developed using our proposed strategy, the discrete wave-functions of Mg impurity states are able to overlap to form continuous miniband-like impurity states19,20. Therefore, the initially localized impurity states in AlxGa1−xN barrier layers form resonant states in the GaN layer (i.e. with energy levels below or close to the GaN VBM, it strongly depends on the Al percentage in AlxGa1−xN). To see the exact energy position of Mg impurity state, one would need to use a pretty large cell. Alternatively, in this work we offer the above qualitative band-diagram to explain the idea of resonant state P-Type Doping. In the case of considerable acceptor density, these impurity states are broadened4,21,22,23, which can further enhance the coupling between them. In this new scenario, electrons are able to drop from the VBM of GaN into the impurity states or band without any energy barrier, which means the acceptors are self-ionized. Hence, high concentration of the acceptors can be expected. In addition, as proposed by previous reports, the polarization effect also enhances the ionization of the deep acceptors and leads to the accumulation of carriers as a hole sheet, which further increase the effective hole concentration in the host materials13,14,15. In this work, to test these proposed concepts, the impact of impurity resonant states on the ionization energy of Mg acceptors is analyzed through both theoretical and experimental methods. Figure 1 Schematic model showing the mechanism of impurity resonant states P-Type Doping.
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impurity resonant states p type Doping in wide band gap nitrides
Scientific Reports, 2016Co-Authors: Xiaoyan Yi, Junxi Wang, Jinmin Li, Na Lu, Ian T. Ferguson, Zhiguo Yu, Gongdong Yuan, Yong ZhangAbstract:In this work, a new strategy for achieving efficient P-Type Doping in high bandgap nitride semiconductors to overcome the fundamental issue of high activation energy has been proposed and investigated theoretically, and demonstrated experimentally. Specifically, in an AlxGa(1-x)N/GaN superlattice structure, by modulation Doping of Mg in the AlxGa(1-x)N barriers, high concentration of holes are generated throughout the material. A hole concentration as high as 1.1 × 10(18) cm(-3) has been achieved, which is about one order of magnitude higher than that typically achievable by direct Doping GaN. Results from first-principle calculations indicate that the coupling and hybridization between Mg 2p impurity and the host N 2p orbitals are main reasons for the generation of resonant states in the GaN wells, which further results in the high hole concentration. We expect this approach to be equally applicable for other high bandgap materials where efficient P-Type doing is difficult. Furthermore, a two-carrier-species Hall-effect model is proposed to delineate and discriminate the characteristics of the bulk and 2D hole, which usually coexist in superlattice-like Doping systems. The model reported here can also be used to explain the abnormal freeze-in effect observed in many previous reports.
