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Water-vapor-mediated close-spaced vapor transport growth of epitaxial gallium indium phosphide films on gallium arsenide substrates Ann L Greenaway, Benjamin F. Bachman, Jason W Boucher, Christopher Funch, Shaul Aloni, and Shannon W. Boettcher ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00199 • Publication Date (Web): 12 Jan 2018 Downloaded from http://pubs.acs.org on January 14, 2018

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ACS Applied Energy Materials

Water-Vapor-Mediated Close-Spaced Vapor Transport Growth of Epitaxial Gallium Indium Phosphide Films on Gallium Arsenide Substrates

Ann L. Greenaway,*a Benjamin F. Bachman,a Jason W. Boucher,b Christopher J. Funch,a Shaul Aloni,c and Shannon W. Boettcher*a a

Department of Chemistry and Biochemistry, University of Oregon, Eugene, OR, 97403, USA. b Department of Physics, University of Oregon, Eugene, OR, 97403, USA. c The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA. *Corresponding authors. Email address: [email protected], [email protected]

Abstract: Ga1-xInxP is a technologically important III-V ternary semiconductor widely utilized in commercial and record-efficiency solar cells. We report the growth of Ga1-xInxP by water-vapormediated close-spaced vapor transport. Because growth of III-V semiconductors in this system is controlled by diffusion of metal-oxide species, we find that congruent transport from the mixed powder source requires complete annealing to form a single alloy phase. Growth from a fullyalloyed source at water vapor concentrations of ~7000 ppm in H2 at 850 °C affords smooth films with electron mobility of 1070 cm2 V-1 s-1, and peak internal quantum efficiency of ~90% for carrier collection in a non-aqueous photoelectrochemical test cell. Keywords: low-cost, photovoltaics, III-V semiconductor, epitaxy, Hall effect, photoelectrochemistry

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2 Ga1-xInxP has been critical to the success of III-V semiconductor-based solar cells. Ga0.52In0.48P, which is lattice-matched to GaAs but less sensitive to oxygen incorporation than Al1-xGaxAs,1 has been particularly important. It is often employed in high-efficiency GaAs homojunctions as a window layer2 and as the top cell of the most efficient tandem solar cells,3 as well as in multijunction cells,4 including those integrated with Si.5 Ga1-xInxP can also be used for graded metamorphic buffer layers, enabling other device architectures.6,7 Demonstrating the growth of Ga1-xInxP is desirable for alternative III-V growth methods to compete with state-ofthe-art processes. Water-vapor-mediated close-spaced vapor transport (H2O-CSVT) is a nonconventional growth method for III-V semiconductors.8–10 CSVT is an alternative to metal-organic and hydride vapor phase epitaxy (MOVPE and HVPE), both of which utilize gas-phase reactants, generally in many-fold excess. In CSVT, group III and group V precursors are generated in situ by reaction of stoichiometric solid source material (a wafer or powder) with a transport agent (in this case H2O) at high temperature. The hallmark of this technique is the narrow gap (~1 mm) between source and substrate; precursors diffuse from hot source (at a temperature Tsrc) to cooler substrate (with the temperature difference, ∆T, between 10 and 100 °C). Close-spacing affords, in principle, high growth rates (>500 nm min-1)11 and precursor utilization approaching 100%.12 CSVT was developed in the 1960s,13 but has only recently been investigated for device applications. The first entirely H2O-CSVT-grown GaAs pn homojunction solar cells were 9.5% efficient, with maximum open circuit potential VOC = 916 mV,10 on par with recent HVPE-grown devices of similar geometry.14,15 The short circuit current, JSC, was limited in part by lack of a window layer, the addition of which would improve internal quantum efficiency, Φint, and thereby JSC. The whole composition range of a III-V ternary, GaAs1-xPx, has been grown by H2O-


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3 CSVT, demonstrating that higher-order III-Vs are accessible. While reasonable quality material was produced,16 GaAs1-xPx has no composition lattice-matched to GaAs, making it a poor alternative to Ga1-xInxP for these device applications. In addition to improving CSVT-grown solar cells, Ga1-xInxP growth provides an opportunity to explore the chemistry of CSVT. Controlled growth of higher-order III-V alloys is a challenge across techniques,11 and, although GaAs1-xPx has previously been grown by CSVT,17– 19

there have been no reports of CSVT-grown Ga1-xInxP. In most III-V growth methods,

deposition of mixed group V alloys is more challenging due to differing rates of pyrolysis and sticking coefficients between hydrides,20,21 but previous work on GaAs1-xPx showed that congruent transport was readily achieved in H2O-CSVT.16 Models of III-V growth by H2OCSVT indicate that the process is controlled by diffusion of volatile group III oxide species.22 Therefore, Ga1-xInxP is expected to transport predominantly by the reaction, 2Ga1-xInxP(s) + H2O(g) ⇌ (Ga2O) 1-x(g) + (In2O)x(g) + P2(g) + H2(g) and have more complex growth dynamics than GaAs1-xPx due to the generation of two diffusioncontrolling oxide species. Here we report the crystalline and electronic quality of Ga1-xInxP grown epitaxially on GaAs substrates from powder sources. We show that film composition is controlled by the alloy phase of the source, and that complete alloying of the source material is required for growth of the targeted compositions. Evidence is presented showing that the water vapor concentration [H2O] influences both growth rate and crystalline quality. We further demonstrate promising electronic quality for H2O-CSVT-grown Ga1-xInxP, with maximum electron mobility of 1130 cm2 V-1 s-1. These initial results indicate that further development of Ga1-xInxP growth by CSVT is warranted, and suggests that high-quality material can be grown with optimization.



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4 Growth of Ga1-xInxP. Reproducible transport and film growth is paramount to the development of H2O-CSVT as a viable growth method for III-Vs, and was a characteristic of GaAs1-xPx growth in this system.16 However, there is a reported difference in the CSVT literature of growth temperatures for GaP and InP (Tsrc = ~950 °C and 725 °C, respectively);12 GaAs and GaP transport at more similar temperatures (for GaAs, 800 °C ≤ Tsrc ≤

900 °C).9,10,23

Additionally, the group V dimers are more diffusive than the group III precursors (metal oxides). For GaAs1-xPx, this means that diffusion of gallium oxide controls the film growth rate;22 for Ga1xInxP,

the two oxides would likely transport at different rates. Given this, we were initially

concerned that In and Ga would not transport congruently from the powder source. Powder source material was made by grinding, mixing, and pressing InP and GaP wafers into a pellet (see Supporting Information, Table S1 for details). To study the transport of individual phases and formation of the Ga1-xInxP alloy, the first source was only sintered for 10 min under H2 at 850 °C. The initial composition was calculated based on the input GaP/InP mass ratio; after sintering (and following subsequent growths) the elemental composition was measured by X-ray fluorescence spectroscopy (XRF). The targeted In concentration, [] , was 40%; post-sinter, the concentration measured by XRF, [] , was very similar, 41.8%.

Because XRF only provides elemental composition, X-ray diffraction (XRD) was used to probe the crystalline phases of the powder source. The In concentration of the majority alloy phase is referred to as [] . The post-sinter θ/2θ scan (Figure 1A) shows peaks consistent with a

mixed Ga1-xInxP phase (between the reference values for InP and GaP); using Vegard’s law24 and [111] peak position, [] = 76%. The discrepancy between [] and [] is explained

by the presence of GaP reference peaks in the powder source XRD scans, indicating some GaP


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5 was not incorporated into the alloy. No InP peaks are present, suggesting that InP was entirely incorporated, leaving the Ga1-xInxP alloy In-rich.

Figure 1: A) Evolution of powder source θ/2θ XRD scans over the six growths from it. Grey areas highlight peaks indicative of a merged Ga1-xInxP phase and distinct GaP phase. InP and GaP reference powder patterns are adapted from the International Crystal Structure Database. B) Evolution of [ ] (open squares), [ ] (open circles), and [ ] (closed circles) from the unannealed Ga1-xInxP pellet. Varied growth parameters are indicated by labels and colors.

Six Ga1-xInxP films were grown epitaxially on GaAs substrates from the sintered powder source. First, Tsrc was varied while [H2O] was kept constant; then Tsrc was held constant while [H2O] was varied (see Tables S2 and S3 for details). The position of the [004] Ga1-xInxP peak

was used to calculate the In content of the films, [] , via Vegard’s law. Compositions were based on the position of the largest peak, but all films had minority peaks indicating some off

stoichiometric growth (film θ/2θ scans are shown in Figure S1A). For each growth, [] was



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6 compared to the post-growth [] and [] (Figure 1B). For the first two growths,

[] is ~70%, similar to [] but much higher than [] . This difference is eliminated

after the third growth, at Tsrc = 950 °C, after which [] = 34% and [] = 30%, nearly in agreement with [] = 38%. For the last three growths at Tsrc = 850 °C, increasing [H2O]

increased [] , producing films with the closest composition to [] .

The changing relationship between [] and [] can be explained by the evolving phases of the source pellet, monitored as [] (Figure 1A). After the initial sintering

process, GaP peaks were present, which persisted following the first two growths. Following the third growth, at Tsrc = 950 °C, the Ga1-xInxP peaks in the powder source XRD pattern shift to higher angles and the intensity of the GaP peaks is reduced. Both changes indicate increased GaP

incorporation into the alloy. At this point the values of [] , [] , and [] become

similar (Figure 1B). The high temperature growth nearly completely diffused the GaP phase into the Ga1-xInxP alloy (Figure 2A). The increased incorporation of GaP into the alloy means that [] better reflects the total elemental composition of the pellet, [] .

Figure 2: Schematics of A) evolution of powder source, from mixed GaP/InP material to (nearly) fully alloyed Ga1reflecting the reduction in GaP peak intensity measured by XRD. B) A proposed microscopic schematic view

xInxP,


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7 of Ga1-xInxP alloy transfer via H2O-CSVT mechanism. First, In leaves the source (step 1), as does any surface P (step 2). This leads to a Ga-P rich surface, and Ga is transported (step 3). The process continues with the rate limited by the slower Ga-P etching and transport, effectively leading to near-congruent transport of Ga1-xInxP.

While the growth of films where [] is close to [] indicates near-congruent transport is possible, counter to our initial hypothesis, the strong correlation between [] and

[] across all growths suggests that Ga1-xInxP only transports congruently from the alloy phase of the powder source. After initial sintering, when both In-rich Ga1-xInxP and GaP are present, it is likely that only indium oxide and P2 initially transport, with gallium oxide transporting from the mixed phase once it has been sufficiently depleted of In (Figure 2B). Low Tsrc do not provide enough energy to break Ga—P bonds in the lattice of pure GaP crystallites. However, the growth with Tsrc = 950 °C provided enough energy to break those Ga—P bonds and reconfigure the pellet (based on XRD peak shifts, Figure 1A), forming a more-complete alloy. Subsequently, a lower temperature (Tsrc = 850 °C) is sufficient to achieve near-congruent transport. Following this discovery, we predicted that pre-formation of the alloy would provide

consistency between [] and [] , and therefore control over [] . A second powder

source was made from the same GaP and InP wafers and annealed in an evacuated quartz ampoule for 12 h at 850 °C (see Table S1). Five films were grown from this source using various

[H2O] and fixed Tsrc (Table S2). Although there are slight disparities between [] , [] , and [] , prolonged vacuum annealing did have the desired effect: only trace GaP is apparent

by XRD (Figure 3A) and [] was consistent across all five growths (Figure 3B; see Figure S1B for film θ/2θ scans).



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8

Figure 3: A) Evolution of vacuum-annealed source θ/2θ XRD scans; the grey highlighted areas show mainly Ga1xInxP peaks, with only a trace amount of GaP in the (220) region. B) Composition evolution of a pre-annealed pellet as a function of growth number showing [ ] (open squares), [ ] (open circles), and [ ] (closed circles). Although there is some spread in the values of [ ] , they are much more consistent with [ ] and [ ] , suggesting near-congruent transport of the pre-annealed alloy. Two [ ] values are given for the third film in the series due to the presence of two peaks on the XRD (Figure S1B).

Ga1-xInxP morphology and growth rate. Having achieved growth that appears to be nearly congruent, we considered the influence of Tsrc and [H2O] on film composition and quality.

While annealing the source afforded control over [] , and varying Tsrc changed [] by modifying the alloy phase in the source material as discussed above, [H2O] does not clearly

influence [] . Although increasing [H2O] for growths from the sintered pellet increased

[] from 34% to ~40% (Figure 1B), varying [H2O] for the vacuum-annealed pellet did not

substantially change [] (Figure 3B). Scanning electron microscopy (SEM) was then used to assess film quality and determine the effects of Tsrc and [H2O] on film morphologies. XRD


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9 rocking curves were also measured, and full-width at half-maximum data are reported with film thicknesses, measured using optical profilometry, in Table S4. Plan-view and cross-section (inset) SEM images of the Ga1-xInxP films grown from both sintered and vacuum-annealed powder sources are shown in Figure 4, along with the growth rates for different conditions. For the initial growths from the sintered source, with [H2O] ~3000 ppm (Figure 4A), increasing Tsrc increased film thickness as well as roughness. Higher growth rates are expected at higher Tsrc because the driving force for deposition is higher. The increase in film roughness is attributed to the high growth rate as well as lattice mismatch, as the films are far from the composition lattice-matched to GaAs, Ga0.52In0.48P. For films with varied [H2O] grown from the sintered source (Figure 4B), increasing [H2O] mainly afforded smoother films, with a much lower effect on growth rate than varying Tsrc (Figure 4D); however, the true effect of varying [H2O] is difficult to discern on the samples grown from the sintered powder source given the changing composition of the films over the series. The films grown from the vacuum-annealed source were grown with varied [H2O] (Figure 4C). The highest [H2O], ~7000 ppm, afforded a thick, smooth film, while a growth with no water input ([H2O] ~300 ppm due to water adsorbed to the porous graphite heaters) resulted in an extremely thin, rough film. The three growths from this pellet at ~3000 ppm had similar, smooth appearances by SEM, and intermediate thicknesses. The growth rates for these films are shown in Figure 4E. Based on the increase in growth rate with increasing [H2O] for the vacuumannealed source (where consistent, congruent transport is obtained), Ga1-xInxP transports by a H2O-mediated reaction. For both the sintered and vacuum-annealed sources, the observed growth rates are lower than those generally reported for growth of GaAs by H2O-CSVT.10 However,



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10 they are in the range of growth rates observed for GaAs1-xPx under some conditions in this system.16

Figure 4: Plan-view and cross-sectional (inset) SEM images of the Ga1-xInxP films grown for this study. A) films 13 and B) films 4-6 from the sintered source; C) films 1, 4 and 5 from the vacuum-annealed source. The regular, crystallographic defects observed in several of the images (white arrows) are attributed to particulate-related defects which have also been observed in H2O-CSVT-grown GaAs.10 D) Growth rate vs. temperature for films grown from the sintered pellet. The high [H2O] films in this series are indicated; the lower growth rate for these films is attributed to the difference in [ ] and the fact that the films grown at ~3000 ppm are rough. E) Growth rate vs. [H2O] for the films grown from the vacuum-annealed pellet with Tsrc = 850 °C. Film thicknesses, from which growth rates were calculated, are given in Table S4.

Electronic properties and photoelectrochemistry. The electronic properties of the Ga1xInxP

films were also characterized. Films from the sintered and vacuum-annealed sources were

grown on semi-insulating GaAs substrates, allowing for Hall effect measurements to determine the electron mobility, µe, of only the Ga1-xInxP films (all results given in Table S4). For the films

grown from the sintered pellet, µe varied widely but was generally low. The range of [] for those samples makes it difficult to compare µe given the known variation in mobility with [In],25 compounded by the corollary lattice mismatch with the GaAs substrate. Dopant densities ND ranged from 0.3 – 3.0 × 1017 cm-3. The films grown from the vacuum-annealed pellet had µe of


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11 530 – 1100 cm2 V-1 s-1, and the dopant density, ND, ranged from 1.2 – 5.0 × 1017 cm-3. The film

grown at ~7000 ppm [H2O], with [] = 56%, had µe = 1070 cm2 V-1 s-1, slightly lower than the maximum of 1130 cm2 V-1 s-1 obtained for one of the films grown with ~3000 ppm [H2O]

([] = 60%). At these carrier concentrations and [] , the observed µe are indicative of good electronic quality material.26 For example, Macksey et al. measured µe ~ 1100 cm2 V-1 s-1 for melt-grown []

= 64% in the same range of ND.25 The low µe of films grown from the

sintered source compared to the vacuum-annealed source, even though the former has a lower carrier concentration, suggests that growth from an inhomogeneous source may also negatively influence µe. This is likely due to variability in the composition of the deposited films and poor crystallinity, evident from the multiple phases apparent from XRD (Figure S1) and large XRD rocking curve full-width at half-maximum values for films from the sintered source (Table S4). Because time-of-flight secondary-ion mass spectrometry of the fourth film from the vacuum annealed source did not reveal any obvious dopants from possible impurities present in the reactor (e.g. S or Si)9 and nominally undoped source powders were used, the doping is thus attributed to native defects, as discussed by Krynicki et al.27 As the final component of electronic characterization, a Ga0.41In0.59P film was grown on n+-GaAs from another vacuum-annealed pellet produced in the same manner as previously described (growth conditions given in Table S2). This film was fabricated into electrodes for non-aqueous photoelectrochemical characterization using well-established techniques to assess photovoltaic-relevant properties (VOC, JSC, ND, and external and internal quantum efficiencies, Φext and Φint), without fabrication of a solid-state junction.8,9,28 Figure 5 shows Φext and Φint calculated from the current response of one electrode under monochromatic illumination, with the J-E response of that electrode under simulated AM 1.5G (100 mW cm-2) illumination inset



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12 (details on both measurements are given in the SI). The average JSC = 6.8 ± 0.3 mA cm-2

-

compares favorably with that measured by Olson et al. for MOVPE-grown material of the same composition;1 they used aqueous photoelectrochemistry to obtain JSC ~7.8 mA cm-2.29 The use of a ferrocene redox couple here complicates a direct comparison to the literature, as the light absorbed by the colored redox electrolyte suggests that a lower JSC would be expected for a nonaqueous measurement. Φint provides a better basis for comparison; Olson et al. measured peak Φint ~90%, comparable to that measured here. We note also that the results reported here represent a marked improvement over the electronic quality (particularly Φint) of GaAs1-xPx also produced using CSVT.16 The small difference between the electronic quality produced in this study and literature values from MOVPE suggests that optimization of Ga1-xInxP growth by H2OCSVT can yield high-quality material.

Figure 5: Quantum efficiency, Φ, characteristics of a Ga0.41In0.59P electrode measured using non-aqueous photoelectrochemistry, with J-E inset and average characteristics at the bottom left.

Ga1-xInxP films were grown epitaxially on GaAs substrates using H2O-CSVT. Pre-growth

annealing of the mixed powder sources was necessary to control [] , as only the alloyed phases of the mixed material transported from the pellet to the film. Growth rates were low, but increased with both increasing Tsrc and [H2O], and increasing [H2O] was found to afford


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13 smoother, higher-quality crystalline films. Initial investigations of electronic quality indicate that Ga1-xInxP grown by this method is promising, but will require optimization to compete with MOVPE-grown material. In comparison to our previous work on GaAs1-xPx, the Ga1-xInxP system is more sensitive to source composition (requiring additional pre-treatment of the source to achieve congruent transport) and [H2O]. However, this work does indicate that, like III-Vs with mixed group V compositions, III-V ternaries with mixed group III compositions can be grown controllably by H2O-CSVT. Further investigations of Ga1-xInxP growth will enable fundamental studies of III-V growth phenomena such as CuPt ordering, which is known to occur in all VPE growth systems30 but has not been documented in CSVT, as well as studies of intentional doping parallel to previous work in this system.9,10 Additional work on Ga1-xInxP will focus on the optimization of growth parameters to increase electronic and crystalline quality, and on analysis of long-term compositional stability from a single source, before this material is implemented in a CSVT-grown solar cell.

Supporting Information Additional details on powder source fabrication; details of all Ga1-xInxP film growths and characterization; details of electronic and photoelectrochemical characterization.

Acknowledgments This study was funded by the Research Corporation for Scientific Advancement through a Scialog Award. The authors also acknowledge support from the Department of Energy SunShot Initiative SIPS program (DE-EE0007361). A.L.G. acknowledges support from the American Association of University Women and P.E.O. International. S.W.B. acknowledges



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14 support from a Sloan Fellowship. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. We thank Dr. Stephen Golledge, Dr. Matthew Kast, Dr. Michaela Burke, Lisa Enman, and Elizabeth Cochran for their support on this project.

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