Engineering topics
Ptak, Aaron J.
Publications and source records attributed to Ptak, Aaron J..
24% Single-Junction GaAs Solar Cell Grown Directly on Growth-Planarized Facets Using Hydride Vapor Phase Epitaxy
A 24%-efficient single-junction GaAs solar cell grown directly on a faceted, spalled (100) GaAs substrate after in situ planarization growth by hydride vapor phase epitaxy (HVPE) is achieved. Controlled spalling, a promising low-cost substrate reuse technique, produces large facets in (100)-oriented GaAs substrates due to the orientation of the fracture planes used for lift-off. Planarization by HVPE offers a path toward direct use of these spalled substrates without costly polishing steps. In this report the growth rate anisotropy enabling planarization arising from diffusion and differences in the adsorption of growth species on {n11}B-type facets relative to (100) is determined. Consecutive planarization and device growth that results in a solar cell with a minimal performance difference relative to a control cell grown on an epitaxy-ready substrate are demonstrated. These results show that controlled spalling coupled with HVPE planarization is a viable pathway for lowering the cost of III-V photovoltaics.
Nucleation and Growth of GaAs on a Carbon Release Layer by Halide Vapor Phase Epitaxy
Not Available
Light-trapping structures fabricated in situ for ultrathin III-V solar cells
Here, we describe a fully in situ method of fabricating light-scattering structures on III-V materials that generates a rough morphology via vapor phase etching and redeposition. Fully in situ methods support higher industrial throughput by utilizing the growth reactor to generate the light-trapping structures after device growth without removal from the reactor. We use HCl and PH 3 to etch and redeposit scattering morphologies on Ga 0.5 In 0.5 P in a dynamic hydride vapor phase epitaxy (D-HVPE) reactor. We show that the addition of PH 3 leads to redeposition during the vapor phase HCl etching of Ga 0.5 In 0.5 P and that HCl flow rate and time exposed to HCl-PH 3 each independently cause a linear increase in the redeposited feature size, indicating that redeposition proceeds by island growth in a III-Cl-limited, hydride-enhanced HVPE regime. Auger electron spectroscopy and scanning transmission electron microscopy with energy dispersive spectroscopy (STEM-EDS) reveal redeposition to be highly Ga-rich GaInP, i.e., Ga(In)P. The Ga-rich nature of the redeposition results from the higher thermodynamic driving force for Ga incorporation than for In during HVPE growth and the difference in the volatility of the III-Cl etch products. The resulting morphologies have high broadband scattering, as determined by normal specular reflectance and integrating sphere measurements, indicating effectiveness as light-scattering structures. In a 270-nm-thick GaAs photovoltaic device with a textured back surface, we achieve a 4.9% increase in short circuit current density (J SC ) without any loss in open-circuit voltage (V OC ) relative to a planar control using only a 60 s in situ texturing treatment.
Modeling and design of III-V heterojunction solar cells for enhanced performance
Heterojunctions can increase the efficiency of solar cell devices relative to homojunctions, but there is a large parameter space with significant tradeoffs that must be considered. Here, we present an experimental and computational study of III-V heterojunction solar cells and show how the emitter doping, emitter band gap, and heteroband offsets impact device efficiency. Efficiency is maximized by pushing the junction depletion region into the wider band gap material while minimizing the effects of heteroband offsets through optimized choice of emitter band gap, emitter electron affinity, and/or emitter doping density. We use these results to guide optimization of devices grown by halide vapor phase epitaxy, achieving 27% efficiency in a GaAs/GaInPAs heterojunction device. We also show that heterojunctions yield proportionally larger efficiency improvements in lower-quality materials. Although the modeling was developed and validated using III-V materials, the results are theoretically applicable to materials systems outside III-Vs.
Analysis of Crystalline Defects Caused by Growth on Partially Planarized Spalled (100) GaAs Substrates
We analyze the effect of growth on non-(100) surfaces resulting from incomplete planarization of spalled GaAs wafers on the defect structure of GaAs solar cell layers grown by hydride vapor phase epitaxy (HVPE). Controlled spalling of (100)-oriented GaAs has the potential to reduce substrate costs for III-V epitaxy; however, it creates regularly faceted surfaces that may complicate the growth of high-quality III-V optoelectronic devices. We leverage the anisotropic growth rate of HVPE to planarize these faceted GaAs substrates, reducing the surface roughness and degree of faceting. We observe degraded solar cell performance and material quality in sample areas where facets are not completely removed. We used dark lock-in thermography and photoluminescence to identify recombination in areas that were not fully planarized. We used cathodoluminescence to identify the presence of extended defects in these regions, which are correlated with bandgap fluctuations in the material. We hypothesize that these defects were created by strain from compositional fluctuations in ternary alloys grown on the faceted surfaces. This work elucidates the potential issues of solar cells grown on faceted surfaces and builds understanding toward realizing high performance III-V photovoltaics with the cost-reduction potential of controlled spalling.
Molecular Beam Epitaxy of Monocrystalline GaAs on Water-Soluble NaCl Thin Films
The goal of this project was to demonstrate the feasibility of employing an epitaxial NaCl thin film as a water-soluble release layer for III-V photovoltaic devices and GaAs substrate reuse. Over the course of this project efforts were focused on: 1) achieving crystalline NaCl thin films on GaAs (100) substrate, 2) exploring the growth parameters for subsequent GaAs on NaCl thin films in effort to improve crystallinity of the semiconductor layer, 3) deposition and removal of single crystalline solar cell devices from the parent substrate, and 4) improving morphology and reducing large scale defects in the removed layers to fabricate a working a solar cell device. There was no previous work on direct integration of GaAs/NaCl/GaAs heterostructures at the time of this study. We used molecular beam epitaxy (MBE) to deposit both the alkali halide salt and subsequent III-V material in the same chamber, with no vacuum break. Single crystalline GaAs films were achieved on NaCl layers through careful tuning of the growth parameters and exposure to the reflection high energy electron diffraction (RHEED) beam. Dissolution of the NaCl layer in water provided rapid release of the semiconductor layer from the substrate. Monocrystalline solar cells were grown on the III-V templates using MBE and dynamic hydride vapor phase epitaxy (HVPE). However, the extended time at elevated temperatures required for the cell growth resulted in large area defects from fusion of the semiconductor overlayer to the substrate, causing shorts in fabricated devices. By changing the way that the template layer was grown, the density of these defects could be reduced. Unfortunately, the defects were not reduced to a level allowing for fabrication of a working device.
Design of Planarizing Growth Conditions on Unpolished and Faceted (100)-Oriented GaAs Substrates Using Hydride Vapor Phase Epitaxy
Here we performed a design of experiments (DoE) analysis to determine the effect of various growth parameters on in situ planarizing overgrowth of rough substrates using hydride vapor phase epitaxy (HVPE). We used two types of rough (100)-oriented GaAs substrates to compare the effect of the initial morphology on the epitaxial growth behavior: an irregular, as-cut surface resultant from cutting the wafer from a GaAs boule and a regularly faceted surface produced by controlled spalling. The DoE analysis identified trends in the overgrowth behavior with changing growth conditions, and we used these trends to design favorable planarization growth conditions for each surface type. These favorable conditions enabled full planarization of a spalled surface with >2.5 um facet height within 10.8 min of growth. The root mean squared (RMS) surface roughness of the resulting (100) surface was 24.5 nm over a 286 um x 215 um area. Our results show that planarization growth by HVPE is a promising technique to enable device growth directly on rough substrates, and the trends revealed through DoE analysis indicate a path toward further optimization of planarization growth conditions for as-cut and spalled surfaces.
AlInP-passivated III–V solar cells grown by dynamic hydride vapor-phase epitaxy
We report the development of AlInP-passivated solar cells grown by dynamic hydride vapor-phase epitaxy (D-HVPE) with AM1.5G efficiencies of 26.0% for single-junction (1J) GaAs cells and 28.0% for GaInP/GaAs (2J) tandems. We compare the device performance of solar cells passivated with AlInP versus control cells passivated with GaInP, which has already enabled near-unity carrier collection in GaAs solar cells. 1J devices passivated with either AlInP or GaInP have an identical open-circuit voltage (V OC ) of 1.06 V and long-wavelength current collection near 95%, indicating that both window materials provide a similar degree of passivation. Adding AlInP passivation to each solar cell structure improves the current collection by 1.3 and 1 mA/cm 2 for the 1J and 2J, respectively. The AlInP also results in a top cell V OC boost of ~40 mV relative to a tandem device passivated only by a thin, highly doped GaInP emitter. Secondary-ion mass spectrometry measurements indicate that although O and Si both incorporate in the AlInP window, they do not appear in the subsequently grown absorber layers and do not impact its ability to passivate the front surface. We expect that these achievements, along with continued optimization, will enable parity of hydride vapor-phase epitaxy (HVPE)-grown device efficiencies with state-of-the-art devices grown by other epitaxial methods in the near future.
Consideration of the Intricacies Inherent in Molecular Beam Epitaxy of the NaCl/GaAs System
Not Available
High‐Efficiency Solar Cells Grown on Spalled Germanium for Substrate Reuse without Polishing
Abstract Radical reduction of III–V device costs requires a multifaceted approach attacking both growth and substrate costs. Implementing device removal and substrate reuse provides an opportunity for substrate cost reduction. Controlled spalling allows removal of thin devices from the expensive substrate; however, the fracture‐based process currently generates surfaces with significant morphological changes compared to polished wafers. 49 single junction devices are fabricated across the spalled surface of full 50 mm germanium wafers without chemo‐mechanical polishing before epitaxial growth. Device defects are identified and related to morphological spalling defects—arrest lines, gull wings, and river lines—and their impact on cell performance using physical and functional characterization techniques. River line defects have the most consistent and detrimental effect on cell performance. Devices achieve a single junction efficiency above 23% and open‐circuit voltage of 1.01 V, demonstrating that spalled germanium does not need to be returned to a pristine, polished state to achieve high‐quality device performance.
(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.
Molecular beam epitaxy of GaAs templates on water soluble NaCl thin films
Expensive III-V substrates are cost limiting for the adoption of many technologies. Consequently, being able to reuse the original substrate is highly desirable. Existing substrate reuse techniques have significant drawbacks, but this work discusses a new method using molecular beam epitaxy deposition of water soluble NaCl thin films on commonly employed (0 0 1) GaAs substrates. Single-crystal GaAs templates are grown on continuous NaCl layers utilizing careful exposure of the NaCl to an in-situ electron beam and a low temperature nucleation layer. The template layers can be quickly removed from the substrate via dissolution of the NaCl. After liftoff, the original wafer shows an increase in rms surface roughness of only 0.2 nm.
Performance of III–V Solar Cells Grown on Reformed Mesoporous Ge Templates
We demonstrate a solar cell on reformed porous Ge with an efficiency of 7.7%. We generate mesopores in (100) Ge by bipolar electrochemical etching and anneal them at high temperature. The pores coalesce deep in the structure rather than at the surface as desired, although resulting in coarse superficial morphology unsuitable for device growth. To combat this issue, we developed a surface treatment involving an HBr dip, annealing at 415 degrees C, and a postannealing ultrasonic De-ionized water dip to improve the surface structure, resulting in a smoother reformed surface on which we grow a GaInAs solar cell. The structure retains embedded pores after growth and the transitions between Ge and III-V layers are distinct. The solar cell fabricated using the improved coalescence has an efficiency of 4.5%. The efficiency improves to 7.7% by isolating the rest of the device from three limiting localized shunt areas. Protruding defects in the porous Ge and III-V layers still limit the performance, but this work establishes a step toward the technical viability of this exfoliation approach, showing decent efficiency if protruding defects can be removed or reduced.
Radiation Effects in III-V Solar Cells Grown by Dynamic Hydride Vapor Phase Epitaxy
The recent development of hydride vapor phase epitaxy (HVPE) is potentially promising as a route to lower the cost of high-efficiency III-V solar cells for space applications. HVPE produces the same materials and device structures as the industry-standard organometallic vapor phase epitaxy (OMVPE) process, and although HVPE has shown promising device efficiencies, it lags the OMVPE growth technique in technical maturity. For example, there are no reports of the performance of HVPE-grown devices in radiation environments. There is an expectation that high-quality (that is, single crystal and low-defect) III-V materials will behave similarly regardless of the growth method. However, it is important to verify assumptions as facts. In this project, we produced materials and devices using both HVPE and OMVPE at NREL and irradiated them using 1 MeV electrons to ascertain the effect of radiation on the materials. We also performed initial work on producing more radiation-hard structures, including devices with the pn junction at the front of the device, and devices with a graded doping profile, both of which help with radiation tolerance. Measurements of the devices post radiation exposure showed that the open-circuit voltage (V OC ) of the HVPE-grown solar cells degraded least on a percent basis, although they also started from a lower baseline than the OMVPE-grown devices. All solar cells reached approximately the same V OC after irradiation, indicating that the dose used was sufficient to degrade all device equally. The short-circuit current in the HVPE devices did degrade more than the other solar cells, and this was attributed to a higher-than-expected doping density in the base layer of that cell. The results of these experiments, while by no means comprehensive, do not show any material difference in the radiation effects in OMVPE- and HVPE-grown materials and devices.
Surface chemistry models for GaAs epitaxial growth and hydride cracking using reacting flow simulations
Hydride vapor phase epitaxy (HVPE) is a promising technology that can aid in the cost reduction of III-V materials and devices manufacturing, particularly high-efficiency solar cells for space and terrestrial applications. However, recent demonstrations of ultrafast growth rates (~500 µm/h) via uncracked hydrides are not well described by present models for the growth. Therefore, it is necessary to understand the kinetics of the growth process and its coupling with transport phenomena, so as to enable fast and uniform epitaxial growth. In this work, we derive a kinetic model using experimental data and integrate it into a computational fluid dynamics simulation of an HVPE growth reactor. We also modify an existing hydride cracking model that we validate against numerical simulations and experimental data. Here, we show that the developed growth model and the improved cracking model are able to reproduce experimental growth measurements of GaAs in an existing HVPE system.
Inverted metamorphic GaInAs solar cell grown by dynamic hydride vapor phase epitaxy
We present an inverted metamorphic rear heterojunction ~1.0 eV GaInAs solar cell deposited by dynamic hydride vapor phase epitaxy (D-HVPE) with high growth rate. This device uses a Ga 1-x In x P compositionally graded buffer (CGB) to bridge the lattice constant gap between the GaAs substrate and the Ga0.71In0.29As emitter layer. High-resolution x-ray diffraction and transmission electron microscopy confirm that the Ga 0.71 In 0.29 As emitter is grown lattice-matched to the in-plane lattice constant of the CGB with minimal generation of defects at the GaInAs/GaInP interface. The device contains a threading dislocation density of 2.3 × 10 6 cm -2 , a level that enables high-performance minority carrier devices and is comparable to previously demonstrated GaInP CGBs grown by D-HVPE. The device exhibits an open-circuit voltage of 0.589 V under a one-sun AM1.5G illumination condition and a bandgap-voltage offset of 0.407 V, indicating metamorphic epitaxial performance nearly equal to state-of-the-art devices. We analyze the dark current of the device and determine that reducing recombination in the depletion region, which can be achieved by reducing the threading dislocation density and optimizing the device doping density, will improve the device performance. Furthermore, the CGB and device layers, comprising ~8 µm of thickness, are grown in under 10 min, highlighting the ability of D-HVPE to produce high-quality metamorphic devices of all types with the potential for dramatically higher throughput compared to present technology.