Low-Cost Approaches to IIIâV Semiconductor Growth for Photovoltaic

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Low-Cost Approaches to III-V Semiconductor Growth for Photovoltaic Applications Ann L Greenaway, Jason W Boucher, Sebastian Z. Oener, Christopher Funch, and Shannon W. Boettcher ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00633 • Publication Date (Web): 31 Aug 2017 Downloaded from http://pubs.acs.org on September 4, 2017

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ACS Energy Letters

1 Low-Cost Approaches to III-V Semiconductor Growth for Photovoltaic Applications

Ann L. Greenaway,a* Jason W. Boucher,b Sebastian Z. Oener,a Christopher J. Funch,a and Shannon W. Boettcher a* a

Department of Chemistry and Biochemistry and bDepartment of Physics, University of Oregon, Eugene, Oregon, 97403, USA.

*Corresponding authors. Email address: [email protected], [email protected] Abstract: III-V semiconductors form the most efficient single- and multi-junction photovoltaics. Metal organic vapor phase epitaxy, which uses toxic and pyrophoric gas-phase precursors, is the primary commercial growth method for these materials. In order for the use of highly efficient III-V-based devices to be expanded as the demand for renewable electricity grows, a lower-cost approach to the growth of these materials is needed. This Review focuses on three deposition techniques compatible with current device architectures: hydride vapor phase epitaxy, closespaced vapor transport, and thin-film vapor-liquid-solid growth. We consider recent advances in each technique, including the available materials space, before providing an in-depth comparison of growth technology advantages and limitations and considering the impact of modifications to the method of production on the cost of the final photovoltaics. Table of Contents Figure:

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2 As the uptake of renewable energy increases, demand for less-expensive photovoltaics (PVs) will continue to increase. While the ultimate installed cost of a PV module depends on many inputs, several of which are external and fixed, the efficiency and cost of the PV devices themselves can improved to help reduce the final price.1 Practical Si- and III-V-based PV cells are highly efficient, making them prime targets for terrestrial PV applications. Si-based PV is a mature technology, with low semiconductor growth costs, so future $/W cost reductions will be primarily driven by improved module efficiencies and decreased balance-of-system costs.1,2 IIIV-based PVs have high semiconductor growth and substrate costs that make fabrication roughly 100-fold more expensive than Si cells,2 limiting these devices to multijunction concentrator and space applications.3 These costs could be directly lowered via a less-expensive growth method, allowing the higher efficiencies of devices from III-Vs to be utilized for single-junction nonconcentrator terrestrial applications.4 The development of growth techniques that can lower the cost of producing III-V semiconductors without compromising materials quality is needed. The binary III-Vs GaAs and InP have appropriate bandgaps for terrestrial one-sun applications and large optical absorption coefficients, enabling efficient, broad-spectrum light collection by thin layers (~ 1 µm).5–7 Ternary and quaternary III-Vs have tunable lattice parameters and bandgaps,8 enabling their use as lattice-matched window layers,9,10 metamorphic buffer layers,10–13 or active layers in multi-junction devices14–17 without compromising crystalline and electronic quality. These characteristics, illustrated in Figure 1, provide design flexibility in the final device. Demonstrated III-V-based PV cells have high efficiencies relative to their theoretical limits as a result of growth and device architecture optimizations that provide excellent carrier collection, surface passivation, and very low dark (recombination) currents.10,17,18 These characteristics make III-Vs an optimal target for use in terrestrial PV, but market penetration is limited by cost.


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3

Figure 1: A) The AM 1.5G solar spectrum with the bandgaps of GaAs, InP, and a typical Ga0.52In0.48P|GaAs|Ge triple-junction solar cell overlaid to illustrate the portion of the solar spectrum absorbed by each material. GaAs (Eg = 1.42) and InP (Eg = 1.35) possess the highest theoretical Shockley-Queisser efficiency limits for a single band gap PV. B) Band gap as a function of lattice constant for common III-V semiconductors, with ternary compositions indicated with lines. Si and Ge are marked for reference. Data adapted from Ref. 8.

The use of single-crystal III-V wafers is a major factor contributing to the expense of IIIV PVs.4 III-V electronic quality is heavily influenced by crystalline quality, generally precluding polycrystalline substrates, and routes to high-quality hetero- or non-epitaxial growth are currently lacking. Heteroepitaxy of III-Vs on Si, which is high quality, low cost, and has a useful bandgap for tandem PV, is difficult due to lattice parameter and thermal expansion coefficient mismatch as well as anti-phase boundaries that form during polar III-V growth on the non-polar Si.19–21 Because homoepitaxy provides the most consistent quality but remains expensive, III-Vs are currently most cost-effective as multi-junction concentrator solar cells, which have small device areas, rather than large-area single- or two-junction devices for one-sun terrestrial applications.3 Techniques allowing for substrate reuse such as spalling22–24 or epitaxial lift-off25– 31 are under development and have been reviewed elsewhere.32 Such approaches are necessary, but cannot be the sole strategy for lowering costs. Semiconductor growth expenses are also drivers of the high cost of III-V PV.4 Virtually all commercial production of these devices uses metal-organic vapor phase epitaxy (MOVPE, also known as metal-organic chemical vapor deposition, MOCVD). MOVPE is the mostdeveloped III-V growth technique, utilizing metal-organic molecules (e.g. trimethylgallium) as group III precursors, typically hydrides (e.g. phosphine and arsine) as group V precursors, and a range of similar precursors for dopants.33 At deposition temperatures used for III-Vs, 550-800 °C, the MOVPE film growth rate is limited by mass transport of the group III precursor to the substrate and the kinetics of precursor decomposition. This has historically provided low growth


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4 rates but afforded excellent control over junction formation, doping profiles, and buffer grading between III-Vs.4,33 As a result, MOVPE semiconductor growth costs are high not only due to the expense and low utilization of its toxic and pyrophoric precursors, but also due to the slow growth leading to large contributions of safety infrastructure,34 reactor depreciation, maintenance, and labor costs per layer.4 The same level of materials control has been demonstrated in molecular beam epitaxy (MBE),35 but production costs for this technique are even higher due to lower growth rates and limits on scalability imposed by high-vacuum growth environments.33,36 Recent economic analysis shows that III-V solar cells produced by MOVPE cannot become cost competitive for non-concentrator applications without a radical change in approach;2,4 techniques that are inherently less expensive while providing the same material quality for high efficiency devices are required.37 Ideally, such systems would be flexible enough to produce multi-junction devices, but demonstrated record efficiencies for single-junction thinfilm GaAs are sufficiently high (η = 28.8% at one sun)38 that even single-junction PV produced by an alternative route could be cost-effective. These growth systems must have the potential for production at scale, and preferably be compatible with techniques that dramatically reduce substrate costs, such as those described above. Some alternative approaches to III-V growth propose to dramatically lower device costs by e.g. reducing materials use (nanowire solar cells grown on-wafer),39–43 utilizing low-purity precursors for growth on non-epitaxial substrates (electrochemical liquid-liquid-solid growth),44–48or eliminating the growth substrate entirely (aerotaxy).49–51 While each of these approaches shows promise, they are not compatible with conventional PV architectures and require substantial demonstration before they can be considered as replacements for MOVPE or for planar device architectures. Here we review three alternative III-V growth techniques that show promise as MOVPE replacements. Two, hydride vapor phase epitaxy (HVPE) and close-spaced vapor transport (CSVT), were initially investigated in the mid-20th century and are undergoing a renaissance as new research demonstrates their flexibility. Another technique, thin-film vapor-liquid-solid growth (TF-VLS), directly addresses substrate cost via non-epitaxial growth while maintaining crystal and electronic quality. This Review discusses recent progress in each of these techniques, compares their strengths, and assesses routes toward commercialization and eventual widespread production of lower-cost, high-quality, one-sun III-V photovoltaics. Hydride vapor phase epitaxy Of the alternative III-V growth techniques discussed here, HVPE is the most similar to MOVPE. However, MOVPE is operated in a kinetically limited growth regime, while HVPE is typically a near-equilibrium process controlled by the supersaturation of reactants at the substrate surface.52 In typical HVPE (shown schematically in Figure 2A), HCl (diluted in H2) at atmospheric pressure reacts with a liquid group III metal to generate a volatile metal chloride. The metal chloride flows downstream to the substrate where it mixes with the flow of a group V hydride, forming a V-III-Cl complex on the substrate, then liberating HCl.53 Gas-phase dopants (e.g. silane, diethylzinc) can be added to the stream (not shown). Because HVPE is operated near equilibrium, addition of HCl at the reaction zone via a second gas flow can slow or reverse deposition, while growth rates can reach > 100 µm hr-1 by increasing precursor supersaturation or mass transport to the substrate.52,54 While there are examples of III-V-based solar cells in the HVPE55,56 and related chloride-vapor phase epitaxy57 literature, III-V PV device growth by HVPE was initially hampered by difficulty producing abrupt junctions;58 controlling metal


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5 chloride chemistry, particularly that of Al-containing species;52 and preventing surface degradation.59 As a result, MOVPE became preferred over HVPE, particularly for depositing low-dimensional structures such as superlattices.36 HVPE remained prominent for growth of LEDs because of its high growth rates and, more recently, its compatibility with III-nitrides, and has been used for anisotropic growth of III-nitride micro- and nanostructures for complex transistor and lasing devices.52

Figure 2: A) Schematic of HVPE growth of GaAs. The background indicates hotter areas (yellow) and cooler (red). B) The two-chamber HVPE reactor designed by NREL, reprinted with permission from Ref. 65. Copyright 2016 IEEE. C) Comparison of IQE for a HVPE GaAs homojunction without a window (black), with a window and growth interrupts (red), and with a window but without growth interrupts (blue), reprinted with permission from Ref. 67. Copyright 2017 IEEE.

HVPE is being re-investigated for III-V growth as the cost barriers for MOVPE-grown III-V solar cells have become more apparent. The Kuech group at the University of Wisconsin – Madison and the Ptak group at the National Renewable Energy Laboratory (NREL) have pushed the field forward substantially, focusing on demonstrating high-quality single-junction GaAs solar cells. The first advance was abrupt GaAs pn junctions grown in a HVPE system with a preheat zone. Interrupting growth by retracting the substrate into a preheat zone that was protected by gas flows counter to the precursor flow allowed the switch between p and n regions to occur over just 16 nm of deposited material.58 Growth interruption afforded a PV device with JSC = 14.2 mA cm-2, VOC = 840 mV, ff = 76.5%, and η = 9.16% (AM 1.5 G illumination, unpassivated PV with no antireflective coating), compared to devices without interruption at η = 5.11%. Providing an arsine overpressure during growth interruption prevented the surface degradation observed in previous studies.58 Another advance came with the introduction of a two-chamber system (Figure 2B) where the substrate could be moved between reaction zones to deposit different layers, rather than interrupting growth. Using this system, the Ptak group demonstrated p- and n-type GaAs with near-ideal majority carrier mobilities deposited at 60-90 µm hr-1. The resulting device characteristics were JSC = 11.80 mA cm-2, VOC = 936 mV, ff = 86%, and η = 9.5%, good performance for an unpassivated device without an AR coating.54


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6 Higher-order III-Vs have also been grown using HVPE, and the challenges associated with depositing such materials have proven similar to MOVPE. HVPE is particularly well-suited to metamorphic buffer layer (MBL) deposition because of its high growth rates; the Kuech group studied In1-xGaxAs for this application. Film composition was found to be a linear function of the InCl partial pressure for indium-poor compositions,60 which is consistent with varying the group III (cation) composition in MOVPE.33 Increasing InCl was found to decrease the growth rate of the buffer layers.60 They later identified factors controlling film tilt over the growth of multiple MBLs61 as well as optimum growth temperatures.62 Higher-order III-Vs also have potential applications as top cells; recently, the NREL HVPE system was used to demonstrate In1-xGaxAs163 yPy for such a purpose. The group V (anion) composition in MOVPE is often more difficult to control because of the high input concentrations of group V precursors and because of differing rates of pyrolysis of those precursors.33 While the same was found to be true in HVPE,63 exploration of the input composition space led to the growth of smooth films lattice matched to GaAs with Eg ≈ 1.7 eV, and devices with reasonable efficiencies were fabricated, illustrating a route to lower-cost devices that could be integrated as tandems on Si. Although MBL and top-cell growth are important applications for higher-order III-Vs, the most common application is as a window layer, as is the case for Ga0.52In0.48P which is lattice-matched to GaAs. Growth of this material by HVPE has been extensively investigated at NREL. Initially, a low growth temperature (560 °C) was required for good In incorporation in the films.64 However, modifying the HVPE system for a higher surface area In boat increased the efficiency of the HCl-to-InCl conversion, which allowed the window layer to be grown at the same temperature as the GaAs homojunction (650 °C).65 VOC in these devices was limited by contamination from a reactor leak, but the addition of a window layer still improved JSC by ~ 6 mA cm-2 over an unpassivated device.65 Recently, the Ptak group demonstrated a η = 12.8% GaxIn1-xP homojunction device as part of an effort to further improve the quality of this material grown by HVPE for tandem solar cell applications.66 Together with an estimated minority carrier diffusion length of 3.5 µm in the homojunction itself, these results showed the feasibility of PV devices similar to MOVPE stateof-the-art grown only using HVPE. This feasibility was further confirmed by the most recent study of single-junction HVPE-grown GaAs PV. By tuning the emitter and window layer growth rates, the Ptak group was able to entirely eliminate growth interruptions, dramatically improving Φint of the devices (shown in Figure 2C).67 They also demonstrated inverted geometry cells where the GaxIn1-xP window layer was grown first. These devices had the same Φext as those grown in an upright geometry, and had improved J-V characteristics: for the upright cell, JSC = 25.90 mA cm-2, VOC = 944 mV, ff = 84%, and η = 20.6%, while the inverted cell had JSC = 27.45 mA cm-2, VOC = 945 mV, ff = 79%, and η = 20.4%.67 Further improvements in device quality have recently been reported,66 but details of the mechanism of improvement have not yet been published. Recent work on HVPE has demonstrated its utility for growing GaAs homojunction devices nearly equivalent to MOVPE-grown cells, as well as the growth of ternary and quaternary III-V systems for further device developments. This work has clarified the advantages of HVPE over MOVPE, particularly the availability of comparable crystalline quality of the thinfilms at much higher growth rates. Because HVPE uses lower flow rates and a smaller V/III ratio, group V precursor costs are reduced in addition to the dramatically lower cost of using a metallic group III precursor compared to organometallics.67 Additionally, substantial modelling work has been performed at NREL to optimize the in situ production of the group III precursor.53


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7 Modelling of HVPE has also helped to identify deficiencies in the current-generation NREL reactor,68,69 enabled by the thermodynamic nature of the growth process. If HVPE were commercialized, these differences could provide a competitive edge. As MOVPE and HVPE are similar in many ways (thermal budget, hydride usage, and dopant sources), adoption of HVPE as a commercial III-V growth technique would require few changes to the existing supply infrastructure. Close-spaced vapor transport While HVPE is closely related to MOVPE, CSVT is distinctly different as all volatile reactants are generated in situ from solid sources. In CSVT, the source material and substrate are in close proximity, typically ≤ 1 mm, which increases growth rates and improves precursor utilization.70 CSVT of III-Vs generally occurs in a H2 ambient at atmospheric pressure. Once growth temperatures are reached, a transport agent (H2O or a halide, particularly chloride) is introduced, liberating a group III oxide or halide and a group V dimer or tetramer at both source and substrate (Figure 3A).71,72 The source is held at a higher temperature in III-V CSVT, causing the vapor-phase reactants to diffuse across the thermal gradient (10 °C < ∆T < 70 °C) from the source to the substrate, where precursor supersaturation causes deposition. Changes in materials systems or dopants are achieved by changing the solid source. As precursors move by diffusion, the need for gas flow management is eliminated, making CSVT reactors relatively simple to engineer. Explorations of CSVT in the literature are more limited than of HVPE; although many semiconductors were demonstrated in the 1960s,70,73,74 the resulting materials were never thoroughly investigated for device applications and the electronic quality remains largely unknown. CSVT did continue to be used for single-crystal III-V growth through the early 2000s. Côté and Dodelet proposed a model for GaAs transport by H2O (broadly applicable to III-Vs) where the growth rate is determined solely by diffusion of As2, As4, and Ga2O.72 Other studies investigated elemental impurities in CSVT-grown material for correlation with photoluminescence75,76 and directly probed defects using deep-level transient spectroscopy.77,78 Despite these investigations, CSVT and its capabilities for producing solar cells was substantially less-well understood than HVPE at the beginning of this decade.

Figure 3: A) Schematic of H2O-CSVT deposition of GaAs with temperature and diffusion gradients indicated. B) The original CSVT reactor (which consists of two 1.5 inch × 3.9 inch graphite heaters contained within a 2 inch


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8 diameter quartz tube) at the University of Oregon, reprinted from Ref. 79. Copyright 2012 American Chemical Society, More information on the reactor can be found in Ref. 79. C) and D) IQE for n- and p-type GaAs films grown by CSVT and measured using non-aqueous PEC, adapted from Ref. 81 with permission from the Royal Society of Chemistry. E) IQE of CSVT-grown homojunction, with inset schematic of the device, adapted with permission from Ref. 85. Copyright 2017 Elsevier. F) Film vs. source P content for GaAs1-xPx films grown using CSVT, reproduced with permission from Ref. 88. Copyright 2016 Royal Society of Chemistry.

Investigation of the electronic properties of H2O-CSVT-grown III-Vs was the starting point for recent work in our laboratory at the University of Oregon. Commercial verticalgradient-freeze (VGF) wafers were used as both source material and substrates in our initial studies, which were aimed at determining material quality possible by CSVT. Our lab-built H2OCSVT reactor is similar to literature descriptions,71 utilizing serpentine graphite heaters with embedded thermocouples to control the source and substrate temperatures (Figure 3B).77,78 We grew n-GaAs at rates up to 420 nm min-1 and characterized its electronic properties using nonaqueous photoelectrochemical (PEC) techniques that allow characterization of PV-relevant properties without fabrication of solid-state junctions or contacts. The GaAs films were on par with similarly-doped MOVPE-grown material and had improved electronic properties over the source wafers.79 The deposited films also showed consistent internal quantum efficiencies for varied transport agent concentrations 200 ppm < [H2O] < 4000 ppm at the same growth temperature.80 Next, we turned our focus to controlling dopant density of both n- and p-type GaAs. CSVT lacks the flexibility of gas-phase doping shared by HVPE and MOVPE. In H2O-CSVT, either the dopant or its predominant oxide must be volatile at the high growth temperature, and slow-diffusing dopants are preferred to limit cross-diffusion when growing multiple layers. Our initial films were grown from Si-doped wafer sources but were found to be doped with S rather than Si, consistent with the rapid conversion of Si to its oxide and subsequent low volatility at high temperatures.81 The S originated from impurities in the graphite heaters, which were removed by high temperature vacuum annealing. Because Si did not transport, Te was chosen as a n-type dopant. Te has near-unity transport efficiency and low diffusivity under H2O-CSVT growth conditions.81 Zn was chosen as the p-type dopant despite its low transport efficiency, ~1%.82 A lack of commercial wafers with high [Zn] led to development of pressed powder sources consisting of ground undoped VGF wafer with added elemental dopants, or a ground wafer of the appropriate ND or NA.81 No significant differences in film crystalline quality or dopant transport efficiency were observed between films grown from powder versus wafer sources. Both n- and p- type films showed controllable dopant densities with improved electronic properties over their source material; by Hall effect, µe and µh approached those of MOVPEgrown films at the same ND or NA, and minority carrier diffusion lengths (measured by PEC) were 2-3 µm for n-type films and 5-8 µm for p-type films (Figure 3C and D).81 This demonstration of controlled doping, and models showing that LD = 1.5 µm could enable η > 20% for a p absorber and n+ emitter with an ideal antireflective coating,83 encouraged us to begin fabricating the first homojunction GaAs devices using H2O-CSVT. Initial devices used a VGF wafer absorber, enabling characterization of CSVT-grown emitters compared to a fixed absorber quality; these devices reached VOC > 900 mV.84 Despite the high growth temperatures, cross-diffusion was limited, and dopant profiles were similarly abrupt to those grown by the Ptak group using HVPE, occurring over ~20 nm.85 Work on solar cells grown entirely by CSVT yielded improved devices, the best having JSC = 13.9 mA cm-2, VOC = 916 mV, ff = 75% and η = 9.5% for an unpassivated device with no antireflective coating (Figure 3E).85 This is comparable to devices with similar structures produced by other methods, including


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9 HVPE,54,67 indicating the promise of CSVT for this application. Further exploration of PV fabrication via H2O-CSVT revealed that repeated use of Zn-doped powder sources resulted in declining NA until films were compensated n-type by background doping. This is consistent with conversion of Zn to ZnO over multiple exposures to the H2O transport agent, which prevents Zn transport as ZnO has limited volatility. Additionally, oxide-related defects were observed on emitter films grown at high [H2O], as well as particulate-related defects consisting of GaAs crystallites originated at the powder source that cross to the substrate during growth.85 Elimination of these defects to improve consistency of device properties is a focus of future work. We have also demonstrated other PV-relevant III-V growth in CSVT. GaAs1-xPx had previously been demonstrated, but not comprehensively studied, in H2O-74,86 and Cl-CSVT.87 Using the method developed for doped powder sources, we made mixed GaAs/GaP sources spanning the composition range and grew GaAs1-xPx films at up to 0.5 µm min-1.88 We found ~10% reduction of [P] from the pellet to film (i.e. for [P] = 33% of the source group V, film [P] = 31%), shown in Figure 3F. This is an improvement over MOVPE, where group V element incorporation rates vary widely by temperature89,90 and a large excess of the P precursor is required.33 Slightly increasing the growth temperature over the H2O-CSVT GaAs growth conditions to a source temperature of 900 °C provided the best JSC for n-GaAs0.7P0.3 measured by non-aqueous PEC, grown at a rate of 0.2 µm min-1.88 This work proved that CSVT is a viable route to higher-order III-V semiconductors as well as binaries, and that composition can be directly controlled by the solid source material. Work on a III-III-V system, GaxIn1-xP, is currently underway. While growth of such a system may be similarly straightforward, H2OCSVT growth rates are determined by diffusion of the group III metal oxide to the substrate,72 and varying the group III composition of the source may have a large influence on growth rate and ultimate film composition. We have used H2O-CSVT to grow n- and p-GaAs on par with MOVPE-grown material and show that integration of these films into homojunction devices produces devices comparable to HVPE. Similar to HVPE, the high growth rates for H2O-CSVT provide a substantial advantage over MOVPE, and growth from solid sources eliminates the majority of the hazards associated with both MOVPE and HVPE while increasing precursor utilization. This could dramatically lower deposition cost. Straightforward composition control for a ternary with a mixed anion composition is also notable, although GaAs1-xPx has no composition that is lattice matched to GaAs, making it less desirable than GaxIn1-xP for PV applications. Selective-area growth of GaAs microstructures has also been shown, demonstrating the applicability of this growth system to non-planar device architectures,91 and our recent study showed that defect concentrations in H2O-CSVT can approach the low levels found in films grown by MOVPE or HVPE.92 While H2O-CSVT does operate at higher temperatures than HVPE and MOVPE (~800 °C compared to ~600 °C), the increase to the thermal budget is slight, and may be counteracted by the higher growth rates of CSVT relative to MOVPE, which could shorten growth cycle times in full production. Although substantial work controlling defects and demonstrating a breadth of III-Vs remains, CSVT is positioned to compete with MOVPE if these barriers can be overcome.

Thin-film vapor-liquid-solid growth While HVPE and CSVT would reduce the cost of epitaxially-grown III-V solar cells by using less-expensive precursors and improving growth rates compared to MOVPE, TF-VLS


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10 growth would make such devices cheaper by growing efficient thin-film devices directly on lowcost substrates. This process, illustrated in Figure 4A, was developed by the Javey research group at the University of California – Berkeley for InP starting in 2013.93 A layer of In metal is deposited on polished Mo foil and capped with SiOx by electron-beam evaporation. The substrate is then heated under H2 ambient and a gaseous P source is introduced. P diffuses through the capping layer to the now-liquid In, eventually becoming supersaturated. It reacts with the In to nucleate InP on the Mo foil, which has a lower surface energy than the capping SiOx; notably, this affords 100% utilization of the initial In film, as measured by the complete disappearance of In peaks by x-ray diffraction.93 After growth, the capping layer is removed for processing and characterization of the InP.

Figure 4: A) Schematic of TF-VLS growth mechanism for InP showing regions of liquid In with dissolved P, liquid In that has been depleted of P, and InP nuclei on Mo foil.. B) Microscope image of quenched InP growth, showing dendritic growth mode, adapted from Ref. 97. Copyright 2014 American Chemical Society. C) Colorized SEM of InP heterojunction solar cell on Mo foil, with inset of surface layers, adapted with permission from Ref. 102. Copyright 2015 John Wiley & Sons.

The Javey group investigated other routes to low-cost InP94,95 before the development of TF-VLS growth. InP was chosen for its low surface recombination velocity (SRV),93 which would help alleviate the requirement for single-crystal films for high performance devices, required for materials like GaAs with higher SRVs.96 Planar and textured InP was grown over a range of temperatures (400-800 °C), with complete conversion of In to InP (measured by x-ray diffraction) and grain sizes of 10-100 µm. While the films were unintentionally doped n-type, steady-state photoluminescence (PL) was on par with that of single-crystal InP. However, there was a strong dependence of minority carrier lifetime (from time-resolved PL) on growth temperature, with the maximum lifetime for the film grown at 750 °C.93 The growth mode was dendritic, as InP nucleation reduces local [P], creating a depletion zone through which more P must diffuse to reach the nuclei, causing dendrites and preventing additional nuclei formation in that zone (Figure 4B). A low flux of P through the SiOx cap was needed compared to its diffusion rate in the liquid In in order to promote large grains.93 Later, they showed that MoOx acted as a nucleation promoter with InP nuclei density two orders of magnitude higher than on


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11 Mo foil. They then directly controlled nucleation by patterning MoOx dots, although random nucleation was not completely suppressed for long growth times.97 Examination of the InP films via PL mapping showed a strong correlation of carrier lifetime to the known dendritic and micron-scale grain structures; from this data, they estimated a low SRV of 3 × 104 cm s-1.98 An ex situ doping method was later developed; Zn3P2 was diffused into the as-grown films to provide both p-type doping and prevent P loss from the film. This provided final NA = 1.9 × 1017 cm-3 and enabled InP photocathodes for PEC water splitting.99 Later work showed that treatment of the film post-doping with hydrogen plasma increased the optoelectronic quality and uniformity of the doped material by forming neutral complexes with Zn.100 With this method for growing high-quality, non-epitaxial InP in hand, the Javey group set out to fabricate solar cells, which required the development of a heterojunction device as this technique cannot be easily expanded to the growth of multiple layers. Such a device would employ p-type doping in the InP layer, as p-type transparent conducting layers compatible with n-InP are lacking. They demonstrated a p-InP|n-TiO2 solar cell based on a commercial InP wafer with η = 19.2%, close to the record homojunction MOVPE-grown InP PV, at the time η = 22.1%.38,101 This proof-of-concept and the previous work on optimizing and doping TF-VLS InP films enabled devices consisting of a Mo|p-InP|n-TiO2|ITO stack (shown in Figure 4C). Ex situ doping of the TF-VLS InP film gave NA = 0.3 – 3 × 1017 cm-3, the n-TiO2 was provided by atomic layer deposition, and ITO deposited by sputtering.102 Zn saturated the front surface of the film and segregated to the back interface, possibly forming a p++ layer and providing the low measured contact resistance. Electron beam-induced current (EBIC) measurements of the fully fabricated PV devices were used to extract LD and the grain boundary recombination velocity (GBRV, for comparison to SRV) for the InP layer. LD was determined to be 1 – 3 µm, on the low end of measured single-crystal p-InP, while GBRV = 0.1 – 4 × 106 cm s-1, which compared favorably to single-crystal SRV of 105 cm-1.102 The best device had JSC = 26.9 mA cm-2, VOC = 692 mV, and ff = 65%, giving η = 12.1%. Examination of device characteristics under varied illumination revealed that parasitic resistances lowered the fill factor, limiting device performance. Device external luminescence efficiency indicated a possible VOC ≈ 795 mV; losses from the luminescence-implied VOC were attributed to optical and contact design of the device.102 While this efficiency fell short of the η = 19.2% previously demonstrated for the wafer-built device, this work confirmed the potential of TF-VLS growth for producing efficient, polycrystalline III-V PV. The mechanism of VOC reduction was considered in follow-up work, which suggested that local Eg narrowing occurred as a result of deviations from stoichiometry, and that local shunting also occurred, both of which caused a reduction in VOC of the solar cells.103 Most recently, the Javey group demonstrated TF-VLS (referred to as templated liquidphase or TLP) growth of patterned InP on glass substrates.104 By encapsulating evaporated MoOx and lithographically patterned In from all sides with SiOx before exposure to the group V precursor flux, they were able to grow a range of patterns of high quality InP at temperatures between 500 – 535 °C. Notably, because of the small pattern size relative to the depletion zone of P caused by nucleation, the patterned InP was effectively single-crystalline.104 They also demonstrated in situ n-type doping using GeH4 up to ND = 1.3 × 1019 cm-3, and investigated the electronic properties of InP MOSFETs. Preliminary growths of GaP and InSb were performed but not extensively characterized. While this paper did not directly investigate PV applications, it was noted that the ability to grow effectively single-crystal III-Vs directly on amorphous surfaces could provide a route to virtual substrates at a very low cost.104


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12 Although TF-VLS growth has a short history compared to HVPE and CSVT, it has significant potential for the growth of III-V-based thin-film polycrystalline PV. TF-VLS relies on an elemental group III precursor, like HVPE, but with 100% utilization of the deposited material and additionally eliminates the cost of a single-crystalline substrate. InP is well-suited to use in polycrystalline devices due to its low SRV, which has enabled devices that can be further improved with modified designs;102 the utility of the heterojunction approach has also been confirmed using a single-crystal substrate.101 However, these thin-film solar cells represent a substantial departure from current III-V PV geometries, and TF-VLS is likely incompatible with current state-of-the-art III-V device architectures, such as use of higher-order III-Vs and complex dopant gradients. Because it is such a departure from the standard set by MOVPE, TF-VLS is perhaps more comparable with other thin-film PV technologies like CIGS and CdTe, whose overall device production is similar.105 However, in order for single-junction InP devices made by TF-VLS to be directly competitive with established thin film technologies like CdTe or CIGS, the InP devices must have significantly higher efficiency than in those technologies, where cells over 20% efficient are possible.38 Prospects for low-cost III-V growth methods Commercialization of a low-cost III-V growth method will be a function of a range of factors. Some of these, such as reactor capital costs, are difficult to predict on the basis of extant lab-scale reactors. However, the fundamental metrics for the three low-cost techniques discussed here are known, and a comparison to MOVPE is presented in Table 1. While MOVPE remains the most versatile technique, the electronic quality and device properties of III-Vs from HVPE, CSVT, and TF-VLS growth are approaching its benchmarks. Table 1: A comparison of growth technique characteristics and capabilities MOVPE

2-469

CSVT Metal oxides/halides generated in situ Dimer or tetramer generated in situ 1

not reported

>95%71

550 – 750 °C33

560 – 750 °C58,67

750 – 900 °C85,88,91

500 – 700 °C100,104

Growth rate

1 100% (group III)93 ~70% (group V)105

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13 a

defined as the temperature of the substrate to facilitate comparisons other materials have historically been demonstrated,108 including Al-containing III-Vs.109 c effort currently underway d for MOVPE, the device characteristics for the current record, GaAs thin-film single crystal cell produced by Alta Devices are used b

The flexibility of MOVPE as a growth method is a result of its use of gas-phase precursors; however, the need for a wide array of specialty gases that are also hazardous adds substantial equipment complexity and can also be considered a drawback of the technique.34 HVPE, CSVT, and TF-VLS growth would all gain cost advantages over MOVPE by eliminating the use of metal-organics as group III precursors. One estimate puts the commonly-used trimethylgallium at ~$9/g Ga, substantially higher than current costs for metallic Ga (for HVPE and TF-VLS, $0.85/g Ga)54 or bulk polycrystalline GaAs (which could be synthesized from elemental sources at low cost). Similar comparisons can be made for In. Further reductions in the cost of the group III element have already been explored for TF-VLS, as electrochemical deposition rather than evaporation could be used to deposit the initial In layer.110 Similarly, it is likely possible to use polycrystalline or powder (and therefore lower cost) stoichiometric solid sources for CSVT.81 Generating the group III precursor from a solid, as in these cases, additionally removes one of the current safety concerns for large-scale III-V growth and thus lowers capital and operational expense. HVPE and TF-VLS growth utilize hydrides as group V precursors as well as gas-phase dopant sources (e.g. SiH4, GeH4, diethyl zinc), although alternative group V and dopant sources have been demonstrated for TF-VLS growth of InP.93,102 CSVT alone generates both group III and V and dopant precursors in situ, and utilizes H2 (in some systems with the addition of a halide gaseous source) as its only hazardous gaseous input. While reducing the number of discrete gaseous hazards is a laudable goal from both a safety and cost perspective, working only from the solid phase may ultimately be a substantial manufacturing disadvantage for CSVT. Without the direct control over dopant concentration afforded by gas-phase systems, the kind of dopant gradients and transitions required by for state-of-the-art PV devices (e.g. for back surface fields) may not be possible. Additionally, problems associated with H2O as a transport agent (i.e. the limited use of Si and Zn as dopants, oxide defects and O incorporation in films)85,92 will likely hinder the use of H2O-CSVT. Work is currently underway on the use of a halide transport agent to overcome these hurdles. Similar difficulties controlling dopant distribution are likely in TF-VLS growth, as dopants percolate to grain edges (where diffusion rates are typically much higher) in the final device.100 Less-expensive precursors are not per se a benefit if dramatically more material is required. Materials utilization is therefore an important factor, but one which is often not directly reported. In those cases, V/III input ratio may serve as a basis for comparison. Utilization in MOVPE is low, at ~30% for group III precursors, ~20% for group V, and as low as ~5% for some dopants.4 Reactants are lost within the reactor itself and also to the various gas lines and purges required for such a system. Utilization is not reported for the most recent HVPE work, but the V/III input is 2-4.65,69 The recent study of GaInP homojunction cells grown by HVPE suggests that lowering the V/III ratio affords higher quality material, suggesting that such low ratios should be utilized commercially.66 Because CSVT uses stoichiometric sources, the input V/III ratio is 1,81 and if the reaction zone is well-confined, precursor utilization approaches 100%.70 For TF-VLS, the group III element is annealed in the presence of the group V precursor,93 so the ultimate V/III ratio is ≥ 1, but is not directly reported. Group III utilization is


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14 100% when the metal film is completely converted to III-V, and ~70% utilization of the group V precursor has been predicted.105 While scale-up will likely influence the final materials utilization of these techniques, these lab-scale results demonstrate that HVPE, CSVT, and TFVLS growth have the potential to outperform MOVPE on this metric. While the efficient use of different precursors has the potential to lower III-V growth cost, process time must also be considered. Growth rates in MOVPE have historically been very low, contributing to long process times and high per-layer costs. Recent work has been successful in improving MOVPE growth rates, although often at the cost of materials quality106,107 and still lagging behind the potential of the techniques described here. As seen in Table 1, HVPE has the highest demonstrated growth rates, with CSVT behind it. Although TFVLS growth also has higher growth rates than MOVPE, these are conversion rates of the group III element in the presence of the group V precursor, linearly dependent on that precursor’s partial pressure.105 Thus, these rates are not directly comparable to other techniques as additional pre-growth deposition steps increase total processing time. Reducing the time to deposit each active layer will lower operating costs per layer, ultimately reducing the production costs of a single device.107 It should be noted that deposition area is another important factor to consider during scale-up which has not been conclusively explored for these low-cost growth systems. Current HVPE work has demonstrated homogeneity over 2” wafers, while CSVT and TF-VLS have much smaller demonstrated areas. As HVPE, CSVT, and TF-VLS growth are all advantaged to various degrees over MOVPE with respect to their growth processes, the factors that will inhibit commercial use of these techniques are the quality and range of III-V semiconductors produced. For quality assessment, J-V characteristics for the best device from each of these techniques can be seen in Figure 5, with schematic representations of the cells in question. As discussed above, HVPEgrown devices are now approaching the efficiency of those grown by MOVPE;67 the application of state-of-the-art device architectures has dramatically improved JSC of these devices and VOC can be further improved with device architecture and light-trapping optimization. Although CSVT-grown devices have not benefited from the same optimization, VOC for the best devices grown by this technique are comparable to HVPE, suggesting that if optimized, these techniques would provide similar characteristics.85 Additionally, recent characterization of H2O-CSVTgrown devices by deep level transient spectroscopy showed defect levels comparable to devices grown by HVPE, further supporting the possibility of achieving >20% efficiencies via CSVT.92 The TF-VLS-grown device shows excellent properties for poly-crystalline III-V; while this device reaches only η = 12.1% (versus 22.1% for the MOVPE-grown InP homojunction), a sufficiently low cost of commercialized cells could make TF-VLS competitive with other III-V solar cells after further improvement in performance.105 Optimization of doping for n-InP (which has since been investigated104 as n-type material has higher lifetimes than the p-type material used in the demonstration of thin-film cells) could afford efficiencies as high as 24% if a suitable window layer could be found.102


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Figure 5: Comparative structures of record cells produced by A) HVPE, B) CSVT, and C) TF-VLS growth. The structures shown are not to scale and omit doping levels; detailed information can be found in Ref. 67, 85 and 102. D) Comparison of JV characteristics of the pictured cells, with data adapted from Ref. 54, 67, 85, and 102. The HVPE cell without AR coating also lacked a window layer and was not removed from its substrate.54

A successful growth method would ideally be compatible with a range of III-Vs, due to the large role that higher-order III-Vs play in state-of-the-art device structures. Homogeneity, compositional flexibility, and materials utilization are most important in this case. Recent HVPE publications do not provide much insight into the balance of precursor inputs required for making ternaries or quaternaries,63 but high quality III-III-V60,64 and III-III-V-V63 systems have been demonstrated. HVPE will likely have similar limitations to MOVPE for III-V-V systems as the same group V precursors are employed. A III-V-V system has been demonstrated in CSVT, where the V/III input ratio was not changed from binary growth and the resulting films were homogeneous with controlled compositions.88 Although target film compositions can be achieved using solid source, the growth of metamorphic buffer layers using CSVT will be difficult, as composition cannot be directly modulated during growth, as in MOVPE and HVPE. Multiple sources would likely be required, increasing the reactor complexity. While higher-order compositions may be possible in TF-VLS growth, they would be expected to be difficult, as nucleation of off-target phases and phase segregation would need to be completely suppressed.111 A further challenge in producing a range of III-V materials comes in the form of Alcontaining compounds, which have found wide use as window or active layers multi-junction IIIV photovoltaics112 as well as sacrificial layers for epitaxial liftoff processes.113 Although there are historical examples of Al-containing compounds being grown by HVPE,109 as yet, no Alcontaining III-Vs have been grown in the recent examples of HVPE and CSVT, and no such compounds have been grown by TF-VLS. The current HVPE system at NREL has component compatibility limitations preventing the growth of Al-based compounds,66 but the development of a new reactor would likely allow for this deposition. Additionally, the recent HVPE work on In1-xGaxAs1-yPy has been with the intent of displacing Al-based III-Vs.63 Our current H2O-CSVT system cannot be used to deposit Al-containing materials due to production of Al2O3; work is currently underway to transition to a halide transport agent, which will eventually enable


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16 deposition of these materials.85 In principle, Al-based III-Vs should be compatible with TF-VLS growth, but this is the least-tested growth method discussed here and unknown roadblocks may yet emerge for the deposition of high-quality materials beyond InP. For the deposition of higherorder and Al-containing III-V semiconductors, HVPE stands out as being the most competitive with MOVPE, but is also the most similar. CSVT and TF-VLS growth, which have a much shorter history, will require extensive development but have, perhaps, a greater opportunity to ultimately reach the very low costs needed for widespread uptake of one-sun III-V PV. Summary and Future Outlook After a long period of dominance by MOVPE, research into HVPE, CSVT, and TF-VLS has widened the field of III-V semiconductor growth for PV. These techniques are at various levels of development, and their ability to compete with MOVPE will depend both on their future development and on potentially cost-saving innovations in MOVPE. Current work in the MOVPE field has focused on improving growth rates and utilization, but those may come at the expense of high electronic and crystal quality, or homogeneity of higher-order systems.107 In all these cases, the transition from lab-scale to mass production will necessitate substantial development, and will depend on factors beyond the scientific aspects of efficiency, materials utilization, etc. discussed here.114 This Review has assessed the current state-of-the-art for HVPE, CSVT, and TF-VLS growth, and aimed to reveal the known benefits and potential challenges for each. Beyond this Review’s scope, but worth considering, are cost reductions that are technique agnostic, such as reducing costs associated substrates via high-quality epitaxial liftoff,115 or with metallization and module processing schemes,1 and the potential long-term impacts of III-V semiconductor development, such as materials scarcity.37,116 As it stands, each of these techniques has significant potential for further development, and may become a dominant growth method in the future to drive the implementation of III-V photovoltaics in the terrestrial, one-sun market. Biographies Ann L. Greenaway is a NSF Graduate Research Fellow and Ph.D. candidate in Chemistry at the University of Oregon. She received her B.A. from Hendrix College in 2012 and was both a Goldwater and Truman Scholar (2011). Her current research focuses on the growth of ternary and microstructure III-Vs for photovoltaic and photoelectrochemical applications. Dr. Jason W. Boucher obtained his Ph.D. in Physics from the University of Oregon. As a Ph.D. candidate with Prof. Shannon Boettcher, his research focused on the fabrication and electrical characterization of III-V solar cells. Dr. Sebastian Z. Oener studied physics at the University of Konstanz, Germany, and at Massachusetts Institute of Technology focusing on photovoltaics. He obtained his Ph.D. from AMOLF in Amsterdam on nanoscale photovoltaics. He is currently a postdoctoral researcher at the University of Oregon, working on catalysis for energy storage. Christopher J. Funch is a Ph.D. candidate in Chemistry at the University of Oregon. He is currently working with the Boettcher laboratory’s chloride CSVT system. With an Applied Physics Master’s degree, and three years as a Process Engineer at IBM, his research focus is the mechanism and optimization of the CSVT growth process. Dr. Shannon W. Boettcher is an Assistant Professor of Chemistry at the University of Oregon. He received his B.A. at the University of Oregon in 2003 and his Ph.D. at UC Santa Barbara in 2008. His postdoctoral work was at the California Institute of Technology. His group


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17 focuses on the fundamental aspects of solar energy conversion and storage. More information can be found at http://boettcher.uoregon.edu. Acknowledgements This work was supported by the Department of Energy SunShot Initiative SIPS program (DE-EE0007361) and by the Research Corporation for Scientific Advancement through a Scialog Collaborative Innovation award (S.W.B.). A.L.G. acknowledges support from the American Association of University Women and P.E.O. International. S.W.B. acknowledges support from a Sloan Fellowship. Work on H2O-CSVT was performed with support from the user program at the Molecular Foundry, Lawrence Berkley National Laboratory under Contract DE-AC02-05CH1123. We thank Dr. Shaul Aloni for useful conversations on this topic.


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Highlighted quotes from the article “As MOVPE and HVPE are similar in many ways… adoption of HVPE as a commercial III-V growth technique would require few changes to the existing supply infrastructure.” “Similar to HVPE, the high growth rates for H2O-CSVT provide a substantial advantage over MOVPE, and growth from solid sources eliminates the majority of the hazards associated with both MOVPE and HVPE while increasing precursor utilization.” “Although TF-VLS growth has a short history compared to HVPE and CSVT, it has significant potential for the growth of III-V based thin-film polycrystalline PV.” “While MOVPE remains the most versatile technique, the electronic quality and device properties of III-Vs from HVPE, CSVT, and TF-VLS growth are approaching its benchmarks.”


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