Phil Ahrenkiel - Academia.edu (original) (raw)

Papers by Phil Ahrenkiel

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.

Materials and Energy, 2018

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.

Biophysical Journal, 2020

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.

CrystEngComm, 2018

Device-quality germanium is achieved using liquid phase epitaxy on single-crystalline-like german... more Device-quality germanium is achieved using liquid phase epitaxy on single-crystalline-like germanium templates.

Microscopy and Microanalysis, 2015

Accurate measurements of composition provide critical information in understanding and optimizing... more Accurate measurements of composition provide critical information in understanding and optimizing epitaxial growth of compound semiconductors and alloys particularly used in optoelectronic devices. Energy-dispersive X-ray spectrometry (EDX) is widely used technique in transmission electron microscopy (TEM) to rapidly identify compositions of multilayer structures. It is easy to derive qualitative conclusions instantly with this method, quantitative analysis of the generated data remains a challenge that requires specialized software tools.

Applied Physics Letters, 2014

We demonstrate heteroepitaxial growth of single-crystalline-like n and p-type doped GaAs thin fil... more We demonstrate heteroepitaxial growth of single-crystalline-like n and p-type doped GaAs thin films on inexpensive, flexible, and light-weight metal foils by metal-organic chemical vapor deposition. Single-crystalline-like Ge thin film on biaxially textured templates made by ion beam assisted deposition on metal foil served as the epitaxy enabling substrate for GaAs growth. The GaAs films exhibited strong (004) preferred orientation, sharp in-plane texture, low grain misorientation, strong photoluminescence, and a defect density of ∼107 cm−2. Furthermore, the GaAs films exhibited hole and electron mobilities as high as 66 and 300 cm2/V-s, respectively. High mobility single-crystalline-like GaAs thin films on inexpensive metal substrates can pave the path for roll-to-roll manufacturing of flexible III-V solar cells for the mainstream photovoltaics market.

2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 2016

This paper describes the demonstration of the flexible single-junction III-V solar cells based on... more This paper describes the demonstration of the flexible single-junction III-V solar cells based on high-quality epitaxial GaAs thin films on a low-cost flexible metal substrate. The single-crystal-like semiconductor material structure is fabricated to photovoltaic devices with front illumination geometry. We fabricate a proof-of-concept epitaxial GaAs thin film solar cell with an open-circuit voltage of 0.3 V and short-circuit current of 6 mA/cm2, resulting in conversion efficiency of ~1% in AM1.5G condition. Relatively low efficiency can be further increased by material crystalline quality improvement and device optimization. This development has the potential to open a new avenue for next-generation low-cost and high efficiency flexible PV devices.

2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 2016

We demonstrate heteroepitaxial growth of single-crystalline-like InP thin films by metal organic ... more We demonstrate heteroepitaxial growth of single-crystalline-like InP thin films by metal organic chemical vapor deposition (MOCVD) on low-cost flexible metal foils. The epitaxy was enabled by a multilayer oxide buffer made using ion beam assisted deposition (IBAD). The InP films were biaxially textured with sharp in-plane texture and exhibited strong (002) preferential out-of-plane orientation. Strong room-temperature photoluminescence was also observed with band gap of ~ 1.27 eV. Electron mobility of > 700 cm2/V-s at a carrier concentration of 5 × 1017 cm-3 was obtained. High quality single crystalline-like InP films on low-cost metal substrates may potentially be used in the fabrication of inexpensive flexible InP solar cells.

2014 IEEE 40th Photovoltaic Specialist Conference (PVSC), 2014

We discuss metamorphic, epitaxial nanostructures, using the III-V system GaAs/GaPAs as a particul... more We discuss metamorphic, epitaxial nanostructures, using the III-V system GaAs/GaPAs as a particular example. The materials were synthesized by low-pressure metalorganic chemical vapor deposition, incorporating strain-engineered compositional grades. Vertically aligned, quantum-dot-array like structures were observed by cross-sectional transmission electron microscopy of short-period superlattice regions. Z-contrast images and geometric phase analysis of lattice images both confirm the presence of coherent GaAs dots within GaPAs barriers.

Materials Science Forum, 2005

InP and InAs quantum rods were synthesized via the reactions of monodispersed indium droplets wit... more InP and InAs quantum rods were synthesized via the reactions of monodispersed indium droplets with phosphide or arsenide ions, respectively. In these reactions indium droplets, which do not act as a catalyst but rather as a reactant, are completely consumed. For the synthesis of InP and InAs quantum rods with a narrow size distribution, a narrow size distribution of indium particles is required because each indium droplet serves as a template to strictly limit the lateral growth of individual InP or InAs nanocrystals. Free-standing InP (130 Å diameter and 870 Å length) and InAs (180 Å diameter and 745 Å length) quantum rods without residual metallic catalyst at the rod tip were synthesized from the diluted transparent solutions of metallic indium. Both kinds of synthesized nanorods are in the strong confinement regime since the Bohr diameters of InP and InAs are 200 and 700 Å, respectively.

Epitaxial Growth Processes, 1994

ABSTRACT Spontaneous CuPt-type ordering of Group III atoms on the (111) subplanes of the GaInAs2 ... more ABSTRACT Spontaneous CuPt-type ordering of Group III atoms on the (111) subplanes of the GaInAs2 and AlInAs2 epitaxially deposited by atmospheric pressure organometallic vapor phase epitaxy is observed by transmission electron microscopy. We find positive correlation between the observation of CuPt-like (111) superlattice diffraction spots in transmission electron diffraction (TED) patterns and reduced band gap energies, with a reduction of more than 75 meV for GaInAs2 and 25 meV for AlInAs2. For these materials, ordering depends strongly on growth temperature, but only moderately on substrate misorientation. Room temperature time-resolved photoconductivity of ordered GaInAs2 exhibit 50 microsecond(s) ec decay and behavior indicative of carrier localization.

Surface Engineering, 2013

ABSTRACT

Microscopy and Microanalysis, 2012

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, ... more Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Journal of Nanoscience and Nanotechnology, 2013

The mechanism of electrical charge transport in hydrogenated nanocrystalline silicon (nc-Si:H) an... more The mechanism of electrical charge transport in hydrogenated nanocrystalline silicon (nc-Si:H) and the enhancement in electrical conductivity by hydrogen plasma exposure has been studied. Nanoscale electrical conduction measurements (laterally on the surface) suggested that the dominant charge transport in nc-Si:H occurs through the crystalline grain interiors while grain boundaries are highly resistive. Room temperature low-power/short-duration (10 W, 10 s) surface hydrogen plasma treatment enhanced the local surface and bulk electrical conductivity of nc-Si:H films which was attributed to improved passivation of surface and bulk dangling bonds, increase in crystalline fraction and decrease in grain boundary (GB) fraction. However, the improvement in electrical conductivity due to high-power/long-duration (50 W, 10 min) hydrogen plasma exposure was not as pronounced as low-power/short-duration exposure. Temperature-dependent dark conductivity measurements showed dual activation-energy behavior; increase in activation energy in the high-temperature regime (400-585 K) was attributed to the temperature dependence of tunneling probability of carriers and explained using a heteroquantum dots model. A decrease in activation energy with plasma exposure was observed which was explained using the framework of a three-phase model of nc-Si:H where GB width and barrier potential played a critical role in determining the relative contribution of tunneling and thermally activated carrier transport.

Chemical Communications, 2013

0.47 and InAs_yP_1-y (InAsP) epitaxial alloy layers on InP substrates with lattice mismatch [LMM=... more 0.47 and InAs_yP_1-y (InAsP) epitaxial alloy layers on InP substrates with lattice mismatch [LMM=a(film)/a(InP)-1] as large as 2.6%. InAsP step grades (0.3 mum/step, 0.1% LMM/step) provide adequate surfaces for the subsequent deposition of both GaInAs and InAsP layers with reasonably low dislocation densities. The heterointerfaces contain the expected cross-hatched misfit dislocation networks that promote relaxation. Variations with alloy composition of the mechanical properties of these alloys may partly account for the vertical confinement of dislocations. We use high-resolution transmission electron diffraction to examine the strain state and crystallographic tilting of the constituent step-grade layers. Residual strain persists in the films, leading to surface facets with corresponding threading dislocations at the strained facet troughs in films thicker than 2 mum.

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.

Materials and Energy, 2018

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.

Biophysical Journal, 2020

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.

CrystEngComm, 2018

Device-quality germanium is achieved using liquid phase epitaxy on single-crystalline-like german... more Device-quality germanium is achieved using liquid phase epitaxy on single-crystalline-like germanium templates.

Microscopy and Microanalysis, 2015

Accurate measurements of composition provide critical information in understanding and optimizing... more Accurate measurements of composition provide critical information in understanding and optimizing epitaxial growth of compound semiconductors and alloys particularly used in optoelectronic devices. Energy-dispersive X-ray spectrometry (EDX) is widely used technique in transmission electron microscopy (TEM) to rapidly identify compositions of multilayer structures. It is easy to derive qualitative conclusions instantly with this method, quantitative analysis of the generated data remains a challenge that requires specialized software tools.

Applied Physics Letters, 2014

We demonstrate heteroepitaxial growth of single-crystalline-like n and p-type doped GaAs thin fil... more We demonstrate heteroepitaxial growth of single-crystalline-like n and p-type doped GaAs thin films on inexpensive, flexible, and light-weight metal foils by metal-organic chemical vapor deposition. Single-crystalline-like Ge thin film on biaxially textured templates made by ion beam assisted deposition on metal foil served as the epitaxy enabling substrate for GaAs growth. The GaAs films exhibited strong (004) preferred orientation, sharp in-plane texture, low grain misorientation, strong photoluminescence, and a defect density of ∼107 cm−2. Furthermore, the GaAs films exhibited hole and electron mobilities as high as 66 and 300 cm2/V-s, respectively. High mobility single-crystalline-like GaAs thin films on inexpensive metal substrates can pave the path for roll-to-roll manufacturing of flexible III-V solar cells for the mainstream photovoltaics market.

2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 2016

This paper describes the demonstration of the flexible single-junction III-V solar cells based on... more This paper describes the demonstration of the flexible single-junction III-V solar cells based on high-quality epitaxial GaAs thin films on a low-cost flexible metal substrate. The single-crystal-like semiconductor material structure is fabricated to photovoltaic devices with front illumination geometry. We fabricate a proof-of-concept epitaxial GaAs thin film solar cell with an open-circuit voltage of 0.3 V and short-circuit current of 6 mA/cm2, resulting in conversion efficiency of ~1% in AM1.5G condition. Relatively low efficiency can be further increased by material crystalline quality improvement and device optimization. This development has the potential to open a new avenue for next-generation low-cost and high efficiency flexible PV devices.

2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 2016

We demonstrate heteroepitaxial growth of single-crystalline-like InP thin films by metal organic ... more We demonstrate heteroepitaxial growth of single-crystalline-like InP thin films by metal organic chemical vapor deposition (MOCVD) on low-cost flexible metal foils. The epitaxy was enabled by a multilayer oxide buffer made using ion beam assisted deposition (IBAD). The InP films were biaxially textured with sharp in-plane texture and exhibited strong (002) preferential out-of-plane orientation. Strong room-temperature photoluminescence was also observed with band gap of ~ 1.27 eV. Electron mobility of > 700 cm2/V-s at a carrier concentration of 5 × 1017 cm-3 was obtained. High quality single crystalline-like InP films on low-cost metal substrates may potentially be used in the fabrication of inexpensive flexible InP solar cells.

2014 IEEE 40th Photovoltaic Specialist Conference (PVSC), 2014

We discuss metamorphic, epitaxial nanostructures, using the III-V system GaAs/GaPAs as a particul... more We discuss metamorphic, epitaxial nanostructures, using the III-V system GaAs/GaPAs as a particular example. The materials were synthesized by low-pressure metalorganic chemical vapor deposition, incorporating strain-engineered compositional grades. Vertically aligned, quantum-dot-array like structures were observed by cross-sectional transmission electron microscopy of short-period superlattice regions. Z-contrast images and geometric phase analysis of lattice images both confirm the presence of coherent GaAs dots within GaPAs barriers.

Materials Science Forum, 2005

InP and InAs quantum rods were synthesized via the reactions of monodispersed indium droplets wit... more InP and InAs quantum rods were synthesized via the reactions of monodispersed indium droplets with phosphide or arsenide ions, respectively. In these reactions indium droplets, which do not act as a catalyst but rather as a reactant, are completely consumed. For the synthesis of InP and InAs quantum rods with a narrow size distribution, a narrow size distribution of indium particles is required because each indium droplet serves as a template to strictly limit the lateral growth of individual InP or InAs nanocrystals. Free-standing InP (130 Å diameter and 870 Å length) and InAs (180 Å diameter and 745 Å length) quantum rods without residual metallic catalyst at the rod tip were synthesized from the diluted transparent solutions of metallic indium. Both kinds of synthesized nanorods are in the strong confinement regime since the Bohr diameters of InP and InAs are 200 and 700 Å, respectively.

Epitaxial Growth Processes, 1994

ABSTRACT Spontaneous CuPt-type ordering of Group III atoms on the (111) subplanes of the GaInAs2 ... more ABSTRACT Spontaneous CuPt-type ordering of Group III atoms on the (111) subplanes of the GaInAs2 and AlInAs2 epitaxially deposited by atmospheric pressure organometallic vapor phase epitaxy is observed by transmission electron microscopy. We find positive correlation between the observation of CuPt-like (111) superlattice diffraction spots in transmission electron diffraction (TED) patterns and reduced band gap energies, with a reduction of more than 75 meV for GaInAs2 and 25 meV for AlInAs2. For these materials, ordering depends strongly on growth temperature, but only moderately on substrate misorientation. Room temperature time-resolved photoconductivity of ordered GaInAs2 exhibit 50 microsecond(s) ec decay and behavior indicative of carrier localization.

Surface Engineering, 2013

ABSTRACT

Microscopy and Microanalysis, 2012

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, ... more Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Journal of Nanoscience and Nanotechnology, 2013

The mechanism of electrical charge transport in hydrogenated nanocrystalline silicon (nc-Si:H) an... more The mechanism of electrical charge transport in hydrogenated nanocrystalline silicon (nc-Si:H) and the enhancement in electrical conductivity by hydrogen plasma exposure has been studied. Nanoscale electrical conduction measurements (laterally on the surface) suggested that the dominant charge transport in nc-Si:H occurs through the crystalline grain interiors while grain boundaries are highly resistive. Room temperature low-power/short-duration (10 W, 10 s) surface hydrogen plasma treatment enhanced the local surface and bulk electrical conductivity of nc-Si:H films which was attributed to improved passivation of surface and bulk dangling bonds, increase in crystalline fraction and decrease in grain boundary (GB) fraction. However, the improvement in electrical conductivity due to high-power/long-duration (50 W, 10 min) hydrogen plasma exposure was not as pronounced as low-power/short-duration exposure. Temperature-dependent dark conductivity measurements showed dual activation-energy behavior; increase in activation energy in the high-temperature regime (400-585 K) was attributed to the temperature dependence of tunneling probability of carriers and explained using a heteroquantum dots model. A decrease in activation energy with plasma exposure was observed which was explained using the framework of a three-phase model of nc-Si:H where GB width and barrier potential played a critical role in determining the relative contribution of tunneling and thermally activated carrier transport.

Chemical Communications, 2013

0.47 and InAs_yP_1-y (InAsP) epitaxial alloy layers on InP substrates with lattice mismatch [LMM=... more 0.47 and InAs_yP_1-y (InAsP) epitaxial alloy layers on InP substrates with lattice mismatch [LMM=a(film)/a(InP)-1] as large as 2.6%. InAsP step grades (0.3 mum/step, 0.1% LMM/step) provide adequate surfaces for the subsequent deposition of both GaInAs and InAsP layers with reasonably low dislocation densities. The heterointerfaces contain the expected cross-hatched misfit dislocation networks that promote relaxation. Variations with alloy composition of the mechanical properties of these alloys may partly account for the vertical confinement of dislocations. We use high-resolution transmission electron diffraction to examine the strain state and crystallographic tilting of the constituent step-grade layers. Residual strain persists in the films, leading to surface facets with corresponding threading dislocations at the strained facet troughs in films thicker than 2 mum.