The high costs of substrates and epitaxial growth present significant barriers to the scaling of III–V photovoltaics, despite their remarkable efficiencies. Combining methods for device exfoliation, substrate recovery and reuse, and a rapid epitaxial growth technique may significantly lower the manufacturing cost of III–V photovoltaics. In this work, we investigate controlled spalling of the (211)B gallium arsenide (GaAs) substrate orientation and epitaxial growth of GaAs devices on spalled (211)B wafers with high-growth-rate halide vapor phase epitaxy (HVPE). The selection of substrate orientation is crucial, as it must result in a flat surface after spalling, with minimal faceting, while also being suitable for high-performance epitaxial regrowth. The (211)B orientation demonstrates significant potential in meeting both requirements. Initial spalling results from 50 mm diameter (211)B substrates reveal large, specular, and facet-free areas with root mean square (RMS) roughness of 0.98 nm across a 75 μm × 75 μm atomic force microscopy (AFM) scan that are promising for device regrowth. We studied the effect of HVPE growth parameters, including GaCl partial pressure, AsH3 carrier flow and deposition temperature on roughness and growth rate of GaAs on (211)B-oriented substrates. Finally, we fabricated an upright GaAs photovoltaic device on a spalled (211)B wafer that showed performance equivalent to a reference device deposited on an unspalled, epi-ready (211)B wafer with an open-circuit voltage (VOC) of 1.01 V measured under the AM1.5G spectrum. These results suggest that (211)B-oriented GaAs is a promising platform for low-cost III–V photovoltaics manufacturing.

We report simple and potentially low-cost techniques for creating high-quality n-type gallium arsenide (GaAs) and GaAs p/n junctions and fabricate GaAs p/n junction solar cells. Detailed-balance modeling suggests that 20% AM1.5G efficiency p/n homojunction devices may be possible if the surface doping concentration can be limited to values less than similar to 5 & times; 10(19) cm(-3). Our process exploits an open-tube, vapor-phase, deposition-free, zinc diffusion technique for forming p-type layers in melt-grown n-GaAs substrates that results in sheet resistances less than 1 k Omega/square. In addition, we have improved the minority carrier diffusion lengths of melt-grown GaAs from less than one micron to over five microns using an open-tube, vacuum-free, annealing process which reduces the density of EL2 midgap defects. Finally, we have combined these advances to fabricate epitaxy-free, GaAs solar cells with a validated AM1.5G efficiency of 15.3%.

We investigate the (211) GaAs substrate orientation as a pathway to enable lower cost III-V photovoltaics. Substrates and epitaxial growth are two major costs that limit the market for III-V photovoltaics despite their high efficiencies. The combination of a rapid epitaxial growth technique along with a method like controlled spalling for substrate recovery and reuse would enable dramatic reduction in the production cost of III V photovoltaics. The choice of substrate orientation must consider both aspects of the process flow, because a substrate orientation amenable to the spalling process may not enable high performance device epitaxy, and vice versa. Here, we present initial spalling results from a 50 mm diameter (211)B substrate, revealing a large, specular, and facet-free area that is promising for device regrowth. We measure a 0.98 nm root mean square surface roughness from 75 µm x 75 µm atomic force microscopy scan. We also use halide vapor phase epitaxy to grow a GaAs device on a polished, epitaxy-ready (211)B GaAs substrate and measure an open-circuit voltage of 1.020 V under the AM1.5G spectrum. Taken together, the spalling and device results demonstrate that (211) is a promising substrate orientation for low-cost manufacturing of III-V photovoltaics.

Halide vapor phase epitaxy shows promise for low-cost photovoltaic device manufacturing because of its high growth rates and lower cost elemental precursors but previously has not been used to deposit epitaxial Ge. Here, we demonstrate Ge deposition by generating GeCl2 in situ from solid Ge and HCl in a N2 ambient. To achieve Ge growth, we inject AsH3 and PH3 as sources of active hydrogen to the growth surface to create a driving force for growth. We do not observe Ge growth unless a supply of hydrogen is added, consistent with thermodynamic calculations. Furthermore, we show the hydrogen source must crack readily on the substrate surface to enable growth; relatively stable sources such as H2 do not cause growth. Unintentional group V doping is one drawback of using AsH3 and PH3 to drive the Ge reaction. We observed As or P concentrations in the Ge films ranging from 4 x 1017 to 1 x 1018 atoms/cm3, concentrations that can drastically influence device characteristics. However, we note there are numerous other "helper molecule" options that can provide active hydrogen without doping or etching the material. This work provides a path forward for Ge deposition for optoelectronic devices from an elemental source.

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. Here, 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.

III-V devices are used in countless applications due to their excellent physical properties. They could become more prevalent, especially in area-intensive applications such as solar power, if they can achieve significant cost decreases through increasing scale. The development of high-throughput growth systems can help to achieve this scale, leading to the use of III-V devices in areas where they are not currently economically feasible. Here, we describe a pilot-production, pseudo inline HVPE reactor with the potential to greatly increase the throughput of III-V devices. We show computational modeling results that both informed system design and the understanding of the impact of different process parameters on the deposition. We show the throughput possibilities of this reactor with an example solar cell device design but note that this system is agnostic to the device structure and can be used to increase the throughput of lasers, LEDs, transistors, and more III-V devices.

III-V rear heterojunctions offer enhanced efficiency over traditional homojunction designs, but there is a large parameter space available and a number of tradeoffs that must be considered. Efficiency is maximized by pushing the junction depletion region into the wider bandgap side of the junction, limiting the impact of non-radiative recombination at point defects. However, too low of an emitter doping creates barriers to current flow if heteroband offsets are too large, reducing fill factor and efficiency. Thus, careful choice of emitter bandgap, emitter electron affinity, and/or emitter doping are required. We present an combined experimental and computational study of III-V heterojunction solar cells and show how the emitter doping, emitter bandgap, and heteroband offsets impact device efficiency. We use modeling to create a roadmap for heterojunction design using commonly available alloy systems and use this guidance to optimize of the performance of real solar cells. In one example, we increase the efficiency of heterojunctions grown by hydride vapor phase epitaxy by switching from a GaAs/GaInP design to a more optimal GaAs/GaInPAs design, achieving a 27% efficient single junction cell under the AM1.5G spectrum. We also show that heterojunctions yield proportionally larger efficiency improvements in lower quality materials. These results are theoretically applicable to materials systems outside III-Vs, if non-radiative recombination at defects is the dominant loss mechanism.

We analyze the effect of surface morphology created by planarizing spalled GaAs wafers on GaAs solar cells grown by HVPE. Controlled spalling of (100)-oriented GaAs has potential to reduce substrate costs for III-V photovoltaics (PV); however, it creates regularly faceted surfaces that complicate the growth of high quality III-V PV devices. We leverage the crystallographic-direction-dependent growth capability of hydride vapor phase epitaxy (HVPE) to planarize these faceted GaAs substrates, reducing the surface roughness and degree of faceting. We then demonstrate that GaAs solar cells grown on planarized surfaces are nominally identical to those grown on a planar, epi-ready GaAs surface. We observe slightly degraded device performance in cases where facets are not completely removed. We use device-scale imaging techniques combined with characterization of device cross sections to analyze this performance degradation. Lastly, we discuss the growth mechanisms that contribute to the planarization of faceted surfaces. These investigations into the mechanisms of both device degradation and planarizing growth will ultimately enable high-performing III- V PV with the cost reduction potential of controlled spalling. This advancement, when combined with low-cost epitaxy by HVPE, provides one of the most promising routes to low-cost III- V PV to date.

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.

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.

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 similar to 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.

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 PH3 to etch and redeposit scattering morphologies on Ga0.5In0.5P in a dynamic hydride vapor phase epitaxy (D-HVPE) reactor. We show that the addition of PH3 leads to redeposition during the vapor phase HCl etching of Ga0.5In0.5P and that HCl flow rate and time exposed to HCl-PH3 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.

On page 3, the second paragraph, the first sentence has been corrected as "Textured morphologies were imaged with a JEOL JSM-7000F7 field emission SEM."On page 6, the last paragraph, the second sentence has been corrected as "Fig.3 shows that the average feature size and thickness trend toward zero at zero time and have a constant growth rate, indicating island growth starting from nucleation."

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 mu m 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 mu m x 215 mu m 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.

We couple halide vapor phase epitaxy (HVPE) growth of III-V materials with liftoff from an ultrathin carbon release layer to address two significant cost components in III-V device - epitaxial growth and substrate reusability. We investigate nucleation and growth of GaAs layers by HVPE on a thin amorphous carbon layer that can be mechanically exfoliated, leaving the substrate available for reuse. We study nucleation as a function of carbon layer thickness and growth rate and find island-like nucleation. We then study various GaAs growth conditions, including V/III ratio, growth temperature, and growth rate in an effort to minimize film roughness. High growth rates and thicker films lead to drastically smoother surfaces with reduced threading dislocation density. Finally, we grow an initial photovoltaic device on a carbon release layer that has an efficiency of 7.2%. The findings of this work show that HVPE growth is compatible with a carbon release layer and presents a path toward lowering the cost of photovoltaics with high throughput growth and substrate reuse.

We present the first AlInP-passivated solar cells grown by dynamic hydride vapor phase epitaxy (D-HVPE). D-HVPE has potential to reduce the costs of III-V solar cell production, but historical challenges with the growth of high-quality Al-containing compounds placed a ceiling on photoconversion efficiencies of D-HVPE-grown solar cells. Our single junction (1J) GaAs and GaInP/GaAs (2J) tandems with AlInP passivation achieve AM1.5G efficiencies of 26.0% and 28.5%, respectively, which are the highest reported efficiencies for HVPE-grown devices of each type. 1J devices passivated with either AlInP or GaInP have an open circuit voltage of 1.06 V and similar long wavelength current collection, indicating that both windows provide a similar degree of passivation. Adding AlInP passivation to the 2J solar cell improves the VOC by ~50 mV relative to the device passivated by a GaInP emitter. AlInP windows enable short circuit current densities of >29 mA/cm2 and >14 mA/cm2 for the 1J cell and 2J GaInP top cell, respectively. These achievements remove one of the last barriers limiting parity of HVPE device efficiencies with state-of-the-art.

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°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.

We study the effect of doping density on the performance of inverted metamorphic ~1 eV GaInAs solar cells grown by dynamic hydride vapor phase epitaxy. We find that the doping efficiency of Se in this material is extremely high, with 5x10 17 cm -3 being the lowest controlled value we could achieve with our present Se source. We compared rear heterojunction devices grown with intentional n-doping of 5x10 17 cm -3 and unintentional n-doping of 5x10 15 cm -3 . The intentionally doped sample exhibited an open circuit voltage ~80 mV higher than the unintentionally doped sample due to reduced dark current, and a bandgap voltage offset, or W OC , of 0.407 V. We believe that future doping optimization will improve carrier collection and W OC in these devices.