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Metaferia, Wondwosen

Publications and source records attributed to Metaferia, Wondwosen.

11 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

(110)-Oriented GaAs Devices and Spalling as a Platform for Low-Cost III-V Photovoltaics

Here, we demonstrate the growth of GaAs solar cells by hydride vapor phase epitaxy (HVPE) on epi-ready and previously spalled (110) GaAs wafers as an advance towards a potentially low-cost (110)-based device platform. Controlled spalling offers a fracture-based path to substrate cost amortization, enabling device exfoliation and substrate reuse, but the faceted surface generated during the spalling of (100)-oriented GaAs presents hurdles to direct regrowth of subsequent devices. Spalling of (110) substrates instead eliminates faceting by aligning the substrate surface with the predominant crystal cleavage plane. III-V epitaxy of solar cells is significantly less developed on (110)-oriented substrates, however. Here, we develop (110)-based GaAs solar cells grown by HVPE on epi-ready substrates, demonstrating equal performance to devices grown on the more standard (100) orientation. We also characterize the surface of spalled, (110)-oriented substrates, revealing flat terraces separated by steps with sub-micron-scale height on average. Finally, we present an initial device grown on a previously spalled surface without additional surface re-preparation with nearly 16% efficiency under a simulated AM1.5G spectrum. Together, these results provide preliminary evidence of a potentially low-cost path to enable terrestrial III-V photovoltaics via the (110) substrate orientation.

14 SOLAR ENERGY↗
DOE OSTI2021

(110)-Oriented GaAs Devices and Spalling as a Platform for Low-Cost III-V Photovoltaics

We demonstrate the growth of (110) GaAs solar cells by hydride vapor phase epitaxy (HVPE) as an advance towards a (110)-oriented device platform with substrate reuse via spalling. Controlled spalling offers a fracture-based path to substrate amortization, allowing device removal and substrate reuse, but the faceted surface generated in spalling of (100)-GaAs presents hurdles to direct regrowth of subsequent devices. Spalling of (110)-oriented substrates takes advantage of the natural (110) cleavage plane in zinc-blende III-V materials, eliminating this faceting. The vast majority of III-V epitaxy development is on (100)-oriented substrates, but epitaxy is not limited to this orientation. Here, we demonstrate (110)-based GaAs solar cells grown by HVPE with equivalent performance to devices grown on the more standard (100) orientation. We also demonstrate spalling of (110) oriented substrates, revealing surfaces with flat terraces separated by steps. These results provide preliminary evidence of a potentially low-cost path for terrestrial III-V photovoltaics via the (110) substrate orientation.

(110) orientation↗
DOE OSTI2021

Control of Surface Morphology during the Growth of (110)-Oriented GaAs by Hydride Vapor Phase Epitaxy

We present a study of the effect of hydride vapor phase epitaxy (HVPE) growth conditions on the morphology of GaAs grown on vicinal and nominally exact (110) GaAs substrates. We evaluate epilayer surfaces using a combination of Nomarski microscopy and atomic force microscopy. The surface morphology strongly depends on the growth conditions employed, as well as the substrate orientation. On substrates offcut 3° toward (111)A, faceting generally develops under conditions in which the growth is Ga-limited, although Ga-limited growth is not the only requirement for faceting. Low growth temperatures lead to a faceted morphology, while higher growth temperatures favor smooth growth and facet-free morphology. We show that the surface morphology is kinetically controlled and that the tendency to facet correlates with the growth rate. 3D growth occurs on nominally exact substrates, resulting in hillock formation, but reduction of the group V precursor partial pressure results in smooth morphology. Overall, our results are consistent with models for step-bunching-induced surface faceting that invoke a negative Ehrlich–Schwoebel step-edge barrier.

(110) oriented GaAs↗
DOE OSTI2021

Compositionally graded Ga 1-x In x P buffers grown by static and dynamic hydride vapor phase epitaxy at rates up to 1 μm/min

Here, we demonstrate Ga 1-x In x P compositionally graded buffers (CGBs) grown on GaAs with lattice constants between GaAs and InP by hydride vapor phase epitaxy (HVPE). Growth rates were up to ~1 μm/min and threading dislocation density (TDD) was as low as 1.0 x 10 6 cm -2 . We studied the effect of substrate offcut direction, growth rate, and strain grading rate on CGB defect structure. We compared the effect of a “dynamic grading” style, which creates compositional interfaces via mechanical transfer of a substrate between two growth chambers, vs. “static grading” where the CGB grows in a single chamber. Dynamic grading yielded smoother grades with higher relaxation, but TDD was not significantly different between the two styles. Substrate offcut direction was the most important factor for obtaining CGBs with low defect density. (001) substrates offcut towards (111)B yielded smoother CGBs with lower TDD compared to CGBs grown on substrates offcut towards (111)A. Transmission electron microscopy of static and dynamic CGBs grown on A and B-offcuts only found evidence of phase separation in a static A-offcut CGB, indicating that the B offcut limits phase separation, which in turn keeps TDD low. Reduced growth rate led to the appearance CuPt-type atomic ordering, which affected the distribution of dislocations on the active glide planes but did not alter TDD. Higher growth rates led to smoother CGBs and did not appreciably increase TDD as otherwise predicted by steady-state models of plastic relaxation. These results show HVPE’s promise for lattice-mismatched applications and low-c ost InP virtual substrates on GaAs

36 MATERIALS SCIENCE↗
DOE OSTI2020

Aerotaxy: gas-phase epitaxy of quasi 1D nanostructures

Cost- and resource-efficient growth is necessary for many applications of semiconductor nanowires. We here present the design, operational details and theory behind Aerotaxy, a scalable alternative technology for producing quality crystalline nanowires at a remarkably high growth rate and throughput. Using size-controlled Au seed particles and organometallic precursors, Aerotaxy can produce nanowires with perfect crystallinity and controllable dimensions, and the method is suitable to meet industrial production requirements. Here, we explain why Aerotaxy is an efficient method for fabricating semiconductor nanowires and explain the technical aspects of our custom-built Aerotaxy system. Investigations using SEM (Scanning Electron Microscope), TEM (Transmission Electron Microscope)and other characterization methods are used to support the claim that Aerotaxy is indeed a scalable method capable of producing nanowires with reproducible properties. We have investigated both binary and ternary III–V semiconductor material systems like GaAs and GaAsP. In addition, common aspects of Aerotaxy nanowires deduced from experimental observations are used to validate the Aerotaxy growth model, based on a computational flow dynamics (CFD) approach. We compare the experimental results with the model behaviour to better understand Aerotaxy growth.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2020

GaAs growth rates of 528 μm/h using dynamic-hydride vapor phase epitaxy with a nitrogen carrier gas

Herein we demonstrate record GaAs growth rates approaching 530 μm/h using nitrogen carrier gas and 400 μm/h using hydrogen carrier gas in a dynamic-hydride vapor phase epitaxy reactor. We measured root mean square surface roughness below 1 nm using a 1 μm × 1 μm atomic force microscopy scan for GaAs growth rates up to 483 μm/h using a nitrogen carrier gas and 400 μm/h using a hydrogen carrier gas. We performed computational fluid dynamics modeling to study the effect of the carrier gas choice on the thermal profile within the reactor and how that influences the degree of AsH 3 decomposition. The modeling suggests that the lower thermal conductivity of the nitrogen carrier gas minimizes the amount of AsH 3 that thermally decomposes before reaching the wafer surface and the heavier atomic mass decreases the likelihood that AsH 3 will reach the heated reactor walls, leading to a growth rate enhancement relative to the hydrogen carrier case in a hydride-enhanced growth regime.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Analysis of GaAs Solar Cells Grown on 50 mm Wafers at 700 degrees C by Dynamic Hydride Vapor Phase Epitaxy

We grew and analyzed GaAs photovoltaics devices grown by dynamic hydride vapor phase epitaxy (D-HVPE) on 50 mm GaAs substrates. These were grown at 700 degrees C in a kinetically-limited growth regime. These devices exhibited nearly identical performance to our previous single-junction GaAs devices (>25% efficiency) that were grown at 650 degrees C in a transport-limited growth regime. We determined that the relatively low non-uniformity in device efficiency was not related to variations in structural characteristics, such as GaAs base thickness or Ga x In 1-x P composition or thickness, but rather uniformity of the absorber doping. These results show that growth of high quality material is possible in multiple growth regimes by D-HVPE.

14 SOLAR ENERGY↗
DOE OSTI2020

High Performance GaAs Solar Cells Grown at High Growth Rates by Atmospheric-Pressure Dynamic Hydride Vapor Phase Epitaxy

We report GaAs solar cells grown at rates exceeding 300 um/h by atmospheric-pressure dynamic hydride vapor phase epitaxy (D-HVPE). The surface morphology of the GaAs layers grown at those rates is smooth, with root mean square surface roughness of < 1.4 nm. We grew GaAs solar cells at rates from 35 - 309 um/h with negligible degradation in open circuit voltages (VOC), which ranged from 1.04 - 1.07 V. We showed the slight decrease in VOC is due to lack of doping optimization at higher growth rates. Deep level transient spectroscopy measurements showed low point defect densities that increases slightly with growth rate from 35 - 309 um/h. The low point defect concentrations and the high VOCs indicate that high material quality can be maintained at these extremely high growth rates, enabling the deposition of high efficiency cells in mere seconds instead of hours.

14 SOLAR ENERGY↗
DOE OSTI2019

Growth of AlGaAs, AlInP, and AlGaInP by Hydride Vapor Phase Epitaxy

In this work, we demonstrate hydride vapor phase epitaxy (HVPE) of Al x Ga 1-x As, Al x In 1-x P, and Al x Ga y In 1-x-y P using an AlCl 3 precursor. We study the growth of the Al x Ga 1-x As alloy system to elucidate the effects of deposition temperature, V/III ratio, and group V precursor species on Al solid incorporation via AlCl 3 . Crucially, the presence of group V hydride at the growth front kinetically promotes the solid incorporation of Al. We use these insights to demonstrate controlled deposition of Al x Ga 1-x As, and for the first time by HVPE, Al x In 1-x P and Al x Ga y In 1-x-y P. Finally, these results create exciting implications for HVPE-grown high-efficiency III-V solar cells and devices with reduced cost.

14 SOLAR ENERGY↗