Nikhil Pokharel - Academia.edu (original) (raw)
Uploads
Papers by Nikhil Pokharel
2017 IEEE 44th Photovoltaic Specialist Conference (PVSC), 2017
What is the best method to determine long-term PV system performance and degradation rates? Ideal... more What is the best method to determine long-term PV system performance and degradation rates? Ideally, one universally applicable methodology would be desirable so that a single number could be derived. However, data sets vary in their attributes and evidence is presented that defining two methodologies may be preferable. Monte Carlo simulations of artificial performance data allowed investigation of different methodologies and their respective confidence intervals. Tradeoffs between different approaches were delineated, elucidating as to why two separate approaches may need to be included in a standard. Regression approaches tend to be preferable when data sets are less contaminated by seasonality, noise and occurrence of outliers although robust regression can significantly improve the accuracy when outliers are present. In the presence of outliers, marked seasonality, or strong soiling events, year-on-year approaches tend to outperform regression approaches.
2017 IEEE 44th Photovoltaic Specialist Conference (PVSC), 2017
Aluminum could be used as a structural buffer and sacrificial layer that is lattice-matched on th... more Aluminum could be used as a structural buffer and sacrificial layer that is lattice-matched on the (001) surface to GaInAs having a 45° in-plane rotation. Conventional MOCVD from trimethylaluminum results in nanocrystalline aluminum carbide. By using hydrogen-plasma assistance in the reactor inlet, we avoid carbide formation, allowing deposition of elemental aluminum. The microstructures of these films resulting from initial applications of this method are discussed.
Journal of Physics D: Applied Physics, 2021
Direct bandgap Al x In1−x P alloys offer an advantage for red and amber light-emitting diode (LED... more Direct bandgap Al x In1−x P alloys offer an advantage for red and amber light-emitting diode (LED) operation over conventional (Al x Ga1−x )0.5In0.5P alloys due to their higher direct bandgap energies. However, the coupled variation of its bandgap energy and lattice constant present challenges for fabricating quantum well (QW)-based LED devices on GaAs substrates. Here, we present the design and demonstration of Al x In1−x P red and amber LEDs incorporating multiple QW structures. Strain balancing the QW layers and manipulating the Al x In1−x P conduction band energy through control of spontaneous atomic ordering produce structures with higher energetic barriers to electron leakage compared to (Al x Ga1−x )0.5In0.5P LEDs. We also discuss future improvements that must be made to realize high efficiency red and amber LEDs.
Journal of Microscopy, 2021
Precession electron diffraction (PED) was used to measure the long‐range order parameter in latti... more Precession electron diffraction (PED) was used to measure the long‐range order parameter in lattice‐mismatched AlInP epitaxial films under investigation for solid‐state‐lighting applications. Both double‐ and single‐variant films grown at 620, 650 and 680 °C were analysed in TEM cross‐section. PED patterns were acquired in selected‐area‐diffraction mode through external microscope control using serial acquisition, which allows inline image processing. The integrated peak intensities from experimental patterns were fit using dynamical simulations of diffraction from the ordered domain structures. Included in the structure‐factor calculations were mean atomic displacements of the anions (P) due to ordering, which were found by valence‐force‐field calculations to have a nearly linear dependence on order parameter. A maximum order parameter of S = 0.36 was measured for a double‐variant specimen grown at 650 °C.
Journal of Crystal Growth, 2020
The use of plasma enhancement for growth of III-V compound semiconductor materials by metalorgani... more The use of plasma enhancement for growth of III-V compound semiconductor materials by metalorganic chemical vapor deposition (MOCVD) is examined, to improve control of microstructure, develop understanding of the underlying growth mechanisms, and expand the range of materials combinations for photovoltaic and solidstate lighting applications. Whereas plasma is commonly used with group-IV materials, such as silicon, few studies have examined the impact of plasma-enhanced MOCVD (PE-MOCVD) for III-V materials growth. PE-MOCVD provides improved decomposition of metalorganic precursors, which is driven by hydrogen plasma to augment hydride reactions and thermal pyrolysis, but plasma generation requires low reactor pressure. We have demonstrated elemental Al films grown by PE-MOCVD, which show distinct crystallographic texturing, and assume epitaxial microstructure upon post-growth annealing. We also demonstrate PE-MOCVD of GaAs at temperatures as low as 300°C. With increasing radio-frequency power, the GaAs growth rate shows a transition from power limited to mass-transport limited. PE-MOCVD grown Ga 0.5 In 0.5 P shows no detectable spontaneous atomic ordering, which offers a mechanism to form order/disorder unicompositional heterostructures, without temperature change or impurity incorporation. In its current implementation, PE-MOCVD grown films have shown microstructural degradation, that we attribute to direct plasma exposure during growth. Improvements in the system design and growth sequences are at the focus of ongoing efforts.
Materials Science in Semiconductor Processing, 2019
A method for growth of aluminum epilayers on III-V substrates using metalorganic chemic vapor dep... more A method for growth of aluminum epilayers on III-V substrates using metalorganic chemic vapor deposition (MOCVD) processes could be beneficial for optoelectronic and photovoltaic device fabrication. However, deposition from common precursors under standard MOCVD conditions results in polycrystalline Al 4 C 3. Both Al 4 C 3 films grown on GaAs (100) substrates, and those grown on GaAs (111) B substrates, show strong alignment of the rhombohedral [110] for the Al 4 C 3 crystallites with the in-plane GaAs < 110 > directions. However, by using plasma-enhanced MOCVD at low growth temperatures, elemental Al can be deposited. Al growth by this method on (001), (111) B and (110) GaAs substrates gave textured, polycrystalline films. In all cases, the Al crystallites are primarily oriented with a < 110 > aligned with an in-plane < 110 > of the substrate at initial growth stages. These insights may lead towards growth of large-grained or epitaxial Al films.
2017 IEEE 44th Photovoltaic Specialist Conference (PVSC), 2017
What is the best method to determine long-term PV system performance and degradation rates? Ideal... more What is the best method to determine long-term PV system performance and degradation rates? Ideally, one universally applicable methodology would be desirable so that a single number could be derived. However, data sets vary in their attributes and evidence is presented that defining two methodologies may be preferable. Monte Carlo simulations of artificial performance data allowed investigation of different methodologies and their respective confidence intervals. Tradeoffs between different approaches were delineated, elucidating as to why two separate approaches may need to be included in a standard. Regression approaches tend to be preferable when data sets are less contaminated by seasonality, noise and occurrence of outliers although robust regression can significantly improve the accuracy when outliers are present. In the presence of outliers, marked seasonality, or strong soiling events, year-on-year approaches tend to outperform regression approaches.
2017 IEEE 44th Photovoltaic Specialist Conference (PVSC), 2017
Aluminum could be used as a structural buffer and sacrificial layer that is lattice-matched on th... more Aluminum could be used as a structural buffer and sacrificial layer that is lattice-matched on the (001) surface to GaInAs having a 45° in-plane rotation. Conventional MOCVD from trimethylaluminum results in nanocrystalline aluminum carbide. By using hydrogen-plasma assistance in the reactor inlet, we avoid carbide formation, allowing deposition of elemental aluminum. The microstructures of these films resulting from initial applications of this method are discussed.
Journal of Physics D: Applied Physics, 2021
Direct bandgap Al x In1−x P alloys offer an advantage for red and amber light-emitting diode (LED... more Direct bandgap Al x In1−x P alloys offer an advantage for red and amber light-emitting diode (LED) operation over conventional (Al x Ga1−x )0.5In0.5P alloys due to their higher direct bandgap energies. However, the coupled variation of its bandgap energy and lattice constant present challenges for fabricating quantum well (QW)-based LED devices on GaAs substrates. Here, we present the design and demonstration of Al x In1−x P red and amber LEDs incorporating multiple QW structures. Strain balancing the QW layers and manipulating the Al x In1−x P conduction band energy through control of spontaneous atomic ordering produce structures with higher energetic barriers to electron leakage compared to (Al x Ga1−x )0.5In0.5P LEDs. We also discuss future improvements that must be made to realize high efficiency red and amber LEDs.
Journal of Microscopy, 2021
Precession electron diffraction (PED) was used to measure the long‐range order parameter in latti... more Precession electron diffraction (PED) was used to measure the long‐range order parameter in lattice‐mismatched AlInP epitaxial films under investigation for solid‐state‐lighting applications. Both double‐ and single‐variant films grown at 620, 650 and 680 °C were analysed in TEM cross‐section. PED patterns were acquired in selected‐area‐diffraction mode through external microscope control using serial acquisition, which allows inline image processing. The integrated peak intensities from experimental patterns were fit using dynamical simulations of diffraction from the ordered domain structures. Included in the structure‐factor calculations were mean atomic displacements of the anions (P) due to ordering, which were found by valence‐force‐field calculations to have a nearly linear dependence on order parameter. A maximum order parameter of S = 0.36 was measured for a double‐variant specimen grown at 650 °C.
Journal of Crystal Growth, 2020
The use of plasma enhancement for growth of III-V compound semiconductor materials by metalorgani... more The use of plasma enhancement for growth of III-V compound semiconductor materials by metalorganic chemical vapor deposition (MOCVD) is examined, to improve control of microstructure, develop understanding of the underlying growth mechanisms, and expand the range of materials combinations for photovoltaic and solidstate lighting applications. Whereas plasma is commonly used with group-IV materials, such as silicon, few studies have examined the impact of plasma-enhanced MOCVD (PE-MOCVD) for III-V materials growth. PE-MOCVD provides improved decomposition of metalorganic precursors, which is driven by hydrogen plasma to augment hydride reactions and thermal pyrolysis, but plasma generation requires low reactor pressure. We have demonstrated elemental Al films grown by PE-MOCVD, which show distinct crystallographic texturing, and assume epitaxial microstructure upon post-growth annealing. We also demonstrate PE-MOCVD of GaAs at temperatures as low as 300°C. With increasing radio-frequency power, the GaAs growth rate shows a transition from power limited to mass-transport limited. PE-MOCVD grown Ga 0.5 In 0.5 P shows no detectable spontaneous atomic ordering, which offers a mechanism to form order/disorder unicompositional heterostructures, without temperature change or impurity incorporation. In its current implementation, PE-MOCVD grown films have shown microstructural degradation, that we attribute to direct plasma exposure during growth. Improvements in the system design and growth sequences are at the focus of ongoing efforts.
Materials Science in Semiconductor Processing, 2019
A method for growth of aluminum epilayers on III-V substrates using metalorganic chemic vapor dep... more A method for growth of aluminum epilayers on III-V substrates using metalorganic chemic vapor deposition (MOCVD) processes could be beneficial for optoelectronic and photovoltaic device fabrication. However, deposition from common precursors under standard MOCVD conditions results in polycrystalline Al 4 C 3. Both Al 4 C 3 films grown on GaAs (100) substrates, and those grown on GaAs (111) B substrates, show strong alignment of the rhombohedral [110] for the Al 4 C 3 crystallites with the in-plane GaAs < 110 > directions. However, by using plasma-enhanced MOCVD at low growth temperatures, elemental Al can be deposited. Al growth by this method on (001), (111) B and (110) GaAs substrates gave textured, polycrystalline films. In all cases, the Al crystallites are primarily oriented with a < 110 > aligned with an in-plane < 110 > of the substrate at initial growth stages. These insights may lead towards growth of large-grained or epitaxial Al films.