NSUF 20-4205: Evolution of Ga2O3 Native Point Defects, Donors and Acceptors with Neutron Irradiation
We propose to use neutron irradiation to study the evolution of Ga2O3 native point defects and the resultant changes in electrical transport properties as a function of neutron fluence. The physical and electronic properties of native point defects are key to understanding the nature of Ga2O3 device instability and breakdown. These defects can compensate free carriers, degrade mobility[1,2], form gap states [3] that “pin” Fermi levels [4], and form trapping states that initiate electrical breakdown. Of high importance for devices required to function in high neutron radiation fields, the physical and electrical nature of defects in Ga2O3 and their interaction to complex or recombine under high neutron irradiation is still relatively unexplored. We will use nanometer-scale depth-resolved cathodoluminescence spectroscopy (DRCLS) and surface photovoltage spectroscopy (SPS) to identify the physical nature and energetics of specific native point defects and temperature-dependent Hall (TDH) effect measurements [5] to establish their correlation with the resulting changes in mobility µ, donor density ND and acceptor density NA on neutron irradiation dose as well as with subsequent forming gas (FG) anneals. Our previous study has revealed that neutron irradiation creates both donors and acceptors that exhibit non-monotonic variations with increasing neutron dose that reflect the interplay of increasing defect densities with defect recombination and healing. [6,7] We used our combination of techniques and processing to explore the effect of neutron irradiation on the creation of native point defects such as gallium vacancies VGa, oxygen vacancies VO, Ga interstitials GaI, and related complexes. Isolated VGa point defects act as acceptors, while VO, and GaI act as donors. Theoretical studies [3,8] indicate that many of native point defects in Ga2O3 create deep level defects within the band gap that can compensate free carriers, lower mobilities, and reduce the sensitivity of free carrier transport to applied bias. We are now able to measure changes in oxygen vacancies, gallium vacancies, and hydrogen-passivated gallium vacancies using DRCLS, SPS, and Hall effect measurements in tandem.[6] We have already shown that neutron irradiation creates VGa defects with optical and electrostatic signatures in agreement with theoretically derived energy levels within the Ga2O3 band gap. To produce these gallium vacancies, a β- Ga2O3 crystal was neutron-irradiated at the Ohio State University Research Reactor (Columbus, OH). Ga2O3 crystals grown by pulsed laser deposition (PLD) and low pressure chemical vapor deposition (LPCVD) were held for 41 minutes at 2 kW power in the central irradiation facility (CIF) reactor with an equivalent total neutron dose of 2.27 x1014 neutrons cm –2 termed “1xn”. Sequential irradiations with systematically increasing equivalent total doses of 2, 4, 10, 100, 500, and 2000xn were performed with subsequent DRCLS and Hall measurements after each irradiation. This sequence of irradiations with optical and electronic measurements has revealed pronounced, non-monotonic variations in n, µ, ND and NA that indicate a combination of various defect creation, complex formation, and recombination with neutron fluence. In addition, forming gas (FG) annealing of these irradiated samples causes both increases and decreases in spectral features that can be correlated with isolated and hydrogen passivated VGa. These forming gas anneals at elevated temperatures caused interstitial Ga to recombine with VGa as well as hydrogen to passivate VGa. Both act to increase n and µ.We now propose to study the optical, electrostatic, and electrical properties of neutron irradiated Ga2O3 over a wider range of irradiation doses, starting at the highest previous dosage, 4.54 x 1017 neutrons cm-2. We propose 2 neutron irradiations: 1x1018 and 2 x1018 neutrons cm–2 which will require 2 and 4 days, respectively, of reactor operations. We will irradiate both PLD and LPCVD types of β-Ga2O3 sample together per irradiation. At each stage of irradiation dosage, we will use DRCLS and SPS to measure optical luminescence transitions involving defect states with energy level positions in the Ga2O3 band gap. Likewise, our TDH facilities permit us to measure not only carrier density n and mobility µ but also ND and NA of the irradiated samples at each dosage level and with FG anneals. Annealing can promote the recombination of VGa with GaI which reduces the total number ND+ NA of charged scattering centers, which in turn increases µ. (The elimination of VGa with GaI removes both a donor and an acceptor, which affects n depending on their charge states.) This removal increases with increasing anneal temperature. The diffusion of H into the Ga2O3 crystal results in passivation of VGa, reducing the number of acceptors and charged scattering centers, thereby increasing both n and µ. Based on the neutron dose dependence, the results of this study would be used to relate the physical nature of specific defects and complexes to donors and acceptors that will dominate the electronic properties of Ga2O3 devices in high radiation fields. 1. A. J. Green, K.D. Chabak, M. Baldini, N. Moser, R. Gilbert, R.C. Fitch, Jr., G. Wagner, Z. Galazka, J. McCandless, A. Crespon, K. Leedy, and G.H. Jessen, IEEE Electron Dev. Lett. 38, 790 (2017). 2. T. Oishi, Y. Koga, K. Harada, and M. Kasu, Appl. Phys. Express 8, 031101 (2015). 3. J. B. Varley, H. Peelaers, A. Janotti, and C. G. Van de Walle, J. Phys.: Condens. Matter 23, 334212 (2011). 4. K. Irmscher, Z. Galazka, M. Pietsch, R. Uecker, and R. Fornari, J. Appl. Phys. 110, 063720 (2011). 5. D.C. Look, “Two-layer Hall-effect model with arbitrary surface-donor profiles: application to ZnO,” J. Appl. Phys. 104, 063718 (2008). 6. H. Gao, S. Muralidharan, N. Pronin, Md. R. Karim, S. M. White, T. Asel, G. Foster, S. Krishnamoorthy, S. Rajan, L. R. Cao, M. Higashiwaki, H. von Wenckstern, M. Grundmann, H. Zhao, D. C. Look, and L J. Brillson, “Optical Signatures of Deep Level Defects in Ga2O3,” Appl. Phys. Lett. 112, 242102 (2018). 7. H. Gao, S.Muralidharan, R. Karim, S.M. White, R.L. Cao, K. Leedy, H. Zhao, D. Look, and L.J. Brillson, “Neutron Irradiation and Forming Gas Anneal Impact on Ga2O3 Deep Level Defects, unpublished. 8. J.B. Varley, J.R. Weber, A. Janotti, and C.G. Van de Walle, “Oxygen Vacancies and Donor Impurities in β-Ga2O3” Appl. Phys. Lett. 97, 142106 (2010).
Additional Info
| Field | Value |
|---|---|
| Abstract | We propose to use neutron irradiation to study the evolution of Ga2O3 native point defects and the resultant changes in electrical transport properties as a function of neutron fluence. The physical and electronic properties of native point defects are key to understanding the nature of Ga2O3 device instability and breakdown. These defects can compensate free carriers, degrade mobility[1,2], form gap states [3] that “pin” Fermi levels [4], and form trapping states that initiate electrical breakdown. Of high importance for devices required to function in high neutron radiation fields, the physical and electrical nature of defects in Ga2O3 and their interaction to complex or recombine under high neutron irradiation is still relatively unexplored. We will use nanometer-scale depth-resolved cathodoluminescence spectroscopy (DRCLS) and surface photovoltage spectroscopy (SPS) to identify the physical nature and energetics of specific native point defects and temperature-dependent Hall (TDH) effect measurements [5] to establish their correlation with the resulting changes in mobility µ, donor density ND and acceptor density NA on neutron irradiation dose as well as with subsequent forming gas (FG) anneals. Our previous study has revealed that neutron irradiation creates both donors and acceptors that exhibit non-monotonic variations with increasing neutron dose that reflect the interplay of increasing defect densities with defect recombination and healing. [6,7] We used our combination of techniques and processing to explore the effect of neutron irradiation on the creation of native point defects such as gallium vacancies VGa, oxygen vacancies VO, Ga interstitials GaI, and related complexes. Isolated VGa point defects act as acceptors, while VO, and GaI act as donors. Theoretical studies [3,8] indicate that many of native point defects in Ga2O3 create deep level defects within the band gap that can compensate free carriers, lower mobilities, and reduce the sensitivity of free carrier transport to applied bias. We are now able to measure changes in oxygen vacancies, gallium vacancies, and hydrogen-passivated gallium vacancies using DRCLS, SPS, and Hall effect measurements in tandem.[6] We have already shown that neutron irradiation creates VGa defects with optical and electrostatic signatures in agreement with theoretically derived energy levels within the Ga2O3 band gap. To produce these gallium vacancies, a β- Ga2O3 crystal was neutron-irradiated at the Ohio State University Research Reactor (Columbus, OH). Ga2O3 crystals grown by pulsed laser deposition (PLD) and low pressure chemical vapor deposition (LPCVD) were held for 41 minutes at 2 kW power in the central irradiation facility (CIF) reactor with an equivalent total neutron dose of 2.27 x1014 neutrons cm –2 termed “1xn”. Sequential irradiations with systematically increasing equivalent total doses of 2, 4, 10, 100, 500, and 2000xn were performed with subsequent DRCLS and Hall measurements after each irradiation. This sequence of irradiations with optical and electronic measurements has revealed pronounced, non-monotonic variations in n, µ, ND and NA that indicate a combination of various defect creation, complex formation, and recombination with neutron fluence. In addition, forming gas (FG) annealing of these irradiated samples causes both increases and decreases in spectral features that can be correlated with isolated and hydrogen passivated VGa. These forming gas anneals at elevated temperatures caused interstitial Ga to recombine with VGa as well as hydrogen to passivate VGa. Both act to increase n and µ.We now propose to study the optical, electrostatic, and electrical properties of neutron irradiated Ga2O3 over a wider range of irradiation doses, starting at the highest previous dosage, 4.54 x 1017 neutrons cm-2. We propose 2 neutron irradiations: 1x1018 and 2 x1018 neutrons cm–2 which will require 2 and 4 days, respectively, of reactor operations. We will irradiate both PLD and LPCVD types of β-Ga2O3 sample together per irradiation. At each stage of irradiation dosage, we will use DRCLS and SPS to measure optical luminescence transitions involving defect states with energy level positions in the Ga2O3 band gap. Likewise, our TDH facilities permit us to measure not only carrier density n and mobility µ but also ND and NA of the irradiated samples at each dosage level and with FG anneals. Annealing can promote the recombination of VGa with GaI which reduces the total number ND+ NA of charged scattering centers, which in turn increases µ. (The elimination of VGa with GaI removes both a donor and an acceptor, which affects n depending on their charge states.) This removal increases with increasing anneal temperature. The diffusion of H into the Ga2O3 crystal results in passivation of VGa, reducing the number of acceptors and charged scattering centers, thereby increasing both n and µ. Based on the neutron dose dependence, the results of this study would be used to relate the physical nature of specific defects and complexes to donors and acceptors that will dominate the electronic properties of Ga2O3 devices in high radiation fields. 1. A. J. Green, K.D. Chabak, M. Baldini, N. Moser, R. Gilbert, R.C. Fitch, Jr., G. Wagner, Z. Galazka, J. McCandless, A. Crespon, K. Leedy, and G.H. Jessen, IEEE Electron Dev. Lett. 38, 790 (2017). 2. T. Oishi, Y. Koga, K. Harada, and M. Kasu, Appl. Phys. Express 8, 031101 (2015). 3. J. B. Varley, H. Peelaers, A. Janotti, and C. G. Van de Walle, J. Phys.: Condens. Matter 23, 334212 (2011). 4. K. Irmscher, Z. Galazka, M. Pietsch, R. Uecker, and R. Fornari, J. Appl. Phys. 110, 063720 (2011). 5. D.C. Look, “Two-layer Hall-effect model with arbitrary surface-donor profiles: application to ZnO,” J. Appl. Phys. 104, 063718 (2008). 6. H. Gao, S. Muralidharan, N. Pronin, Md. R. Karim, S. M. White, T. Asel, G. Foster, S. Krishnamoorthy, S. Rajan, L. R. Cao, M. Higashiwaki, H. von Wenckstern, M. Grundmann, H. Zhao, D. C. Look, and L J. Brillson, “Optical Signatures of Deep Level Defects in Ga2O3,” Appl. Phys. Lett. 112, 242102 (2018). 7. H. Gao, S.Muralidharan, R. Karim, S.M. White, R.L. Cao, K. Leedy, H. Zhao, D. Look, and L.J. Brillson, “Neutron Irradiation and Forming Gas Anneal Impact on Ga2O3 Deep Level Defects, unpublished. 8. J.B. Varley, J.R. Weber, A. Janotti, and C.G. Van de Walle, “Oxygen Vacancies and Donor Impurities in β-Ga2O3” Appl. Phys. Lett. 97, 142106 (2010). |
| Award Announced Date | 2020-07-14T14:16:00.793 |
| Awarded Institution | None |
| Facility | None |
| Facility Tech Lead | Raymond Cao |
| Irradiation Facility | Ohio State University Research Reactor |
| PI | Leonard Brillson |
| PI Email | [email protected] |
| Project Type | RTE |
| RTE Number | 4205 |