Transition

Letter pubs.acs.org/JPCL

Detecting the Photoexcited Carrier Distribution Across GaAs/ Transition Metal Oxide Interfaces by Coherent Longitudinal Acoustic Phonons Kevin L. Pollock,† Hoang Q. Doan,† Avinash Rustagi,‡ Christopher J. Stanton,‡ and Tanja Cuk*,†,¶ †

Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, United States Department of Physics, University of Florida, Gainesville, Florida 32611, United States ¶ Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ‡

S Supporting Information *

ABSTRACT: A prominent architecture for solar energy conversion layers diverse materials, such as traditional semiconductors (Si, III−V) and transition metal oxides (TMOs), into a monolithic device. The efficiency with which photoexcited carriers cross each layer is critical to device performance and dependent on the electronic properties of a heterojunction. Here, by time-resolved changes in the reflectivity after excitation of an nGaAs/p-GaAs/TMO (Co3O4, IrO2) device, we detect a photoexcited carrier distribution specific to the p-GaAs/TMO interface through its coupling to phonons in both materials. The photoexcited carriers generate two coherent longitudinal acoustic phonons (CLAPs) traveling in opposite directions, one into the TMO and the other into the p-GaAs. This is the first time a CLAP is reported to originate at a semiconductor/TMO heterojunction. Therefore, these experiments seed future modeling of the built-in electric fields, the internal Fermi level, and the photoexcited carrier density of semiconductor/ TMO interfaces within multilayered heterostructures.

S

the electronic properties of the buried interface between the Si or III/V photovoltaic cell and the TMO catalyst, where terminating the Si or III/V semiconductor by the appropriate doping1 and sensitively tuning the material properties of the oxide, such as a defect-induced conductivity in TiO2,5 led to significantly enhanced performance. Recently, by current− voltage measurements, a high equilibrium hole concentration at the Si/SiO2/TMO interface was associated with the lack of a barrier for extracting photoexcited holes, which led to increased anodic activity at higher photovoltages independent of the TMO thickness.9 In this Letter, we report on a direct, optical method of detecting the photoexcited carrier distribution specific to the buried semiconductor/TMO interface within the monolithic device. By ultrafast optical spectroscopy of an n-GaAs/p-GaAs/ TMO (Co3O4, IrO2) device, in which the GaAs bandgap is excited by 800 nm light and the front (TMO) surface is probed in reflectance, we detect coherent longitudinal acoustic phonons (CLAP) traveling in opposite directions: one into the TMO and the other into the p-GaAs. They result from a coupling of the photoexcited carrier distribution to phonons in both materials. While commonly observed to originate at

olar energy conversion processes rely on disparate physical steps to convert solar energy into a usable form. In solar to electrical energy conversion, absorbed sunlight creates electron−hole pairs that are separated into a charge flow. In solarto-fuel conversion, absorbed sunlight is first separated into electrons and holes, and then the charge is sent to a catalyst to convert chemical bonds into a storable fuel. A prominent strategy to achieve high conversion efficiencies in scalable systems is to augment each of these stepsabsorption, chargeseparation, and in fuel generation, catalysisby designing diverse materials and material interfaces into a monolithic device. For solar-to-fuel conversion especially, heterostructures couple a light-absorbing and charge-separating photovoltaic cell to an electrocatalytically active catalyst, allowing the decoupling of the light absorbing and catalytic duties in the device.1−4 For demonstrations of water splitting, the materials predominantly involve thin-film transition metal oxides (TMOs) for the catalytic overlayer with III/V or Si semiconductors for the underlying photovoltaic cell;5−8 the transition metal oxide promotes O2 evolution and to some extent, protects the III/V or Si semiconductor from degradation in aqueous solution. In these monolithic devices, the carrier transport between each of the layers, which essentially connects one step in the energy conversion process to another, is critical to the performance. For water splitting, the photon-to-O2 evolving current efficiency from the anode ranges widely depending on © XXXX American Chemical Society

Received: December 3, 2016 Accepted: February 2, 2017 Published: February 2, 2017 922

DOI: 10.1021/acs.jpclett.6b02835 J. Phys. Chem. Lett. 2017, 8, 922−928

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The Journal of Physical Chemistry Letters homogeneous semiconductor junctions and at semiconductor surfaces,10−18 this is the first time a CLAP is reported to originate from a semiconductor/TMO heterojunction. Figure 1 shows the design of the monolithic device. With a heavily doped 50 nm n-GaAs layer and lightly doped 860 nm p-

Figure 1. Schematic diagram of the sample with band diagram. The pump pulse (800 nm) excites from the back (n-GaAs) or the front (TMO or p-GaAs) side while the probe pulse records the change in reflectivity at the TMO and/or p-GaAs side. Inset: zoom on the pGaAs TMO interface. The probe beam reflects from the TMO/air, TMO/CLAP wave, and TMO/p-GaAs interfaces. Δn refers to the CLAP-induced refractive index change in the TMO and p-GaAs.

GaAs layer, the electric field of the GaAs photovoltaic cell is concentrated in the back of the device, where the bandgapexcited electron−hole pairs are preferentially separated and holes are shuttled to the front surface (see Figures S3−S4 in the Supporting Information). The 100 nm InGaP layer, by serving as an etch stop for a window in the GaAs substrate, allows for excitation from the back of the GaAs cell. The device was designed to isolate the critical p-GaAs/TMO interface from the n-p GaAs junction and not to optimize photovoltaic performance. Figure 2 shows the change in reflectivity induced by photoexcitation, both with and without the TMO layer. In all cases, the reflectivity is probed from the front and the device is excited from the back. Without a TMO layer, probing with deeply penetrating 800 nm light (aborption depth 890 nm) results in an exponentially decaying signal that is attributable to recombination of photogenerated electron−hole pairs within the bulk of the GaAs cell (Figure 2a). Switching the probe wavelength to 400 nm light (Figure 2b), where the absorption depth in GaAs is much reduced (15 nm), results in a signal that is much more surface sensitive.19,20 The surface sensitivity is in part evidenced by the significantly lower signal of a smaller probed cross-section. Further, a 7 ps exponential rise, which is also the transit time of holes through the GaAs cell (see calculation with Figure S5 in the Supporting Information), suggests carrier accumulation from the full length of the device to the probed region at the front surface. With the TMO layer, probing with 800 nm light results in the exponential decay characteristic of bulk carriers within GaAs (Figure 2c). However, with the TMO layer (here, IrO2) and when probing with 400 nm light, large oscillations (typical magnitude ∼ δR/R = 10−3) appear (Figure 2d). The oscillatory component of the

Figure 2. Transient reflectivity for n-p GaAs device pumped in the back at 800 nm with an absorbed fluence of 1 mJ cm−2 (a) without TMO layer and probed at 800 nm, (b) without TMO layer and probed at 400 nm, (c) with 15 nm IrO2 layer and probed at 800 nm, and (d) with 15 nm IrO2 layer and probed at 400 nm.

transient reflectance is larger by at least an order of magnitude than that induced by photogenerated strain waves in GaAs alone, as seen in other studies.14,21,22 Simply photoexciting the TMO without the underlying p-GaAs/TMO interface does not generate coherent oscillations.23 Since the oscillations are only present with the TMO layer and with a surface sensitive probe, they should originate from the photoexcited carrier distribution at the p-GaAs/TMO interface. The timing of reflected strain waves that propagate across the length of the device, shown in Figure 3, confirms this hypothesis. These are measured with the surface sensitive 400 nm probe after excitation from the back. Strain waves propagate across the n-p GaAs region by reflecting at the back (InGaP/ air) interface and then returning to the probed region at the front (Figure 3b). We refer to these recurring oscillations in the transient reflectance as echoes. Notably, there are two echo types; minor echoes of smaller amplitude first appear at 215 ps, and major echoes of larger amplitude begin at 430 ps. Both echo types recur with a period of 430 ps. The propagation intervals are described by 923

DOI: 10.1021/acs.jpclett.6b02835 J. Phys. Chem. Lett. 2017, 8, 922−928

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The Journal of Physical Chemistry Letters

Figure 4. Left: Transient reflectivity with TMO layer type denoted in the figure (pump back, λprobe = 400 nm). Dashed line is the fit described in Figure S6. Right: corresponding fast Fourier transform spectra for pump back (black) and pump front (red) geometries, adjusted for reflection losses (Table S1 in the Supporting Information).

Figure 3. (a) Transient reflectivity with 25 nm Co3O4 layer (solid) or 15 nm IrO2 layer (dotted) in pump back (black) or pump front (red) geometries. Pump back spectra show major echoes in Co3O4 (fine resolution) and IrO2 (course resolution) layers. Minor echoes are only visible for the Co3O4 layer. Spectra are offset and IrO2 spectrum is multiplied by −1 for ease of comparison. A close-up view of each major and minor echo for the Co3O4 layer can be found in the Supporting Information Figure S7. (b) Schematic of acoustic echo formation described in the text.

T=

2L νs

ing Figure S6 in the Supporting Information). The characteristic oscillations of each oxide are most prominent with back excitation. For the 15 nm IrO2 layer, the FFT shows a major component at 0.07 THz and a weaker, second harmonic at 0.145 THz. For the 80 nm Co3O4 layer, the major component is again 0.07 THz, with weaker components at 0.13 THz and at 0.18 THz. On the other hand, the 25 nm Co3O4 layer only contains one peak in the FFT at 0.12 THz. With front excitation, the 0.07 THz frequency occurs without a distinct second harmonic for the IrO2 layer and the 0.12 THz occurs with reduced amplitude for the 25 nm Co3O4 layer. The magnitudes of the FFT spectra, whether exciting from the front or the back, have been scaled to represent a 2.5 mJ cm−2 absorption along the GaAs cell (Table S1 in the Supporting Information describes how the reflectivity and absorption of the TMO layer were taken into account). The two reccurring frequencies in the FFT spectra of 0.07 THz (ν1) and 0.12−0.13 THz (ν2) correspond to that expected for a coherent logitudinal acoustic phonon (CLAP) in TMO materials and GaAs respectively when probed by λprobe = 400 nm. A CLAP induces a change in the index of refraction that travels with the sound velocity of the material, as depicted in the inset of Figure 1. An oscillation appears in the transient reflectivity as a result of the interference between the reflected probe beam and this refractive index change. The period of a CLAP oscillation is determined by

(1)

where L is the length of propogation and νs is the sound velocity. Using the InGaP/n-GaAs/p-GaAs length (L) of 1010 nm and the measured period of 430 ps, νs is calculated to be 4.7× 105 cm s−1. This is consistent with previous studies of the sound velocity in GaAs.24 In order for two distinct echo types to appear in the probed region and phase delayed by half the round trip of the device, they have to be initially created at the two separate ends. The minor echo arrives in the probed region at a 215 ps phase delay from the initial excitation and therefore originates from the back of the device. The major echo occurs within the 7 ps time scale of hole propagation, and its first occurrence coincides with the oscillations seen in Figure 2d; therefore it originates from the front p-GaAs/TMO interface. The major echoes occur for both IrO2 and Co3O4 TMO layers. When exciting from the front, the excited volume is backed by the unetched GaAs substrate; the only visible echo is thus the first occurrence of the minor echo at 215 ps, which reflects off the front interface and is transmitted into the substrate. Given that the electric field across the n-p GaAs junction is strongest in the n-GaAs region (Figure S5 in the Supporting Information), photoelectrons injected into the n-GaAs induce the minor strain echo, in accordance with previous studies.13 The generation mechanism of the major echo at the p-GaAs/ TMO interface will be considered below. Figure 4 shows the oscillations (λprobe = 400 nm) specific to three different TMO layers along with corresponding fast Fourier transforms for each trace after subtraction of the exponentially decaying background (see discussion accompany-

T=

λprobe 2νsn

(2)

where νs and n are the sound velocity and refractive index at λprobe.16,18,25 For Co3O4, where n400nm = 2.23 and νs = 7 × 105 cm s−1,26−28 the CLAP period is 13 ps (ν1). For IrO2, where n400nm = 2.35 and νs = 6× 105 cm s−1,29,30 the CLAP period is 14 ps (ν1). On the other hand, in GaAs, where n400nm = 4.46 and νs = 5× 105 cm s−1,19,20,24 the CLAP period is 9 ps (ν2). Figure 5a displays the change we observe in the oscillation period with probe wavelength, along with the predicted 924

DOI: 10.1021/acs.jpclett.6b02835 J. Phys. Chem. Lett. 2017, 8, 922−928

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Figure 5. (a) Variation of observed oscillation period with probe wavelength for 15 nm IrO2 (red circles) and 25 nm Co3O4 (blue diamonds) layers (front excitation). The red and black lines are the predicted oscillation periods for IrO2 and GaAs, respectively, calculated as T = λ , where νs is the 2νn s

sound velocity and n is the wavelength-dependent refractive index of the material. (b) Representative transient reflectivity traces for samples with the IrO2 layer, with probe wavelength noted in the figure. Oscillation periods shown in (a) are obtained from fits (black dotted lines) to the data using a function described in eq S14 in the Supporting Information. Error bars in the left figure are the standard deviations of the frequency fitting parameter. Complete transient reflectivity traces are available in Figures S8 and S9 in the Supporting Information.

stemming from the TMO layer to become more prominent. In the limit of an absoprtion depth that is shorter than the thickness of the TMO layer, the CLAP frequency of the TMO is exclusively observed. The absorption depth of reflected probe light that only penetrates the thin oxide layer can be estimated from the thin film approximation of 1/α = λ/4πn.32,33 For IrO2, this absorption depth equals the thickness of the layer, 15 nm, at λprobe = 425 nm. Finally, theoretical calculations of the transient reflectence induced by a CLAP initiated at the bare GaAs surface (without the TMO layer) and propogating into the GaAs follow the oscillation closely when the GaAs is primarily probed (λprobe = 550−700 nm), while they deviate from the data when IrO2 is primarily probed (λprobe < 425 nm) (see Figure S8 in the Supporting Information). While the 25 nm Co3O4 layer does not display a CLAP associated with the TMO (Figure 5a and Figure 4), the thicker 80 nm Co3O4 has its major FFT frequency at the expected value in the TMO for λprobe = 400 nm (Figure 4). The dependence on thickness further confirms that the probe penetration depth highlights one or the other CLAP oscillation. Beyond the absorption depth through the thickness of the oxide, detecting the TMO CLAP will depend on the amplitude of the CLAP induced in the TMO, the sensitivity of the TMO refractive index to the CLAP amplitude, and the reflection coefficient for the sound wave at the GaAs/TMO interface. Any of these considerations could explain the lack of an observable CLAP in the 25 nm Co3O4 layer. The question arises as to why the presence of the GaAs/ TMO interface leads to the generation of the large amplitude CLAP that then propagates into the GaAs and the TMO layer. Insight is obtained if one takes into account the differences in carrier densities between the highly p+-doped TMO (carrier densities of about 1020 cm−3 reported by Waegele et al.23) and the lightly doped p-type GaAs (carrier densities of about 3 × 1015 cm−3). Charge transfer at the interface leads to extremely

oscillation periods for CLAPs in IrO2 and in GaAs, using literature values19,20,24,30,31 for n and vs. The oscillation period for the sample with the 25 nm Co3O4 layer is seen to closely follow the predicted oscillation period for GaAs at all probe wavelengths. The oscillation period for the sample with the IrO2 layer, on the other hand, displays three regions: from 700 to 550 nm, the period follows that predicted for GaAs, then from 550 to 425 nm, the period crosses over to that predicted for IrO2, which it follows closely below 425 nm. The crossover is accompanied by a change in sign in the transient reflectivity (Figure 5b). Below 390 nm, the oscillatory component was a small fraction of the overall transient reflectance and a frequency could not be reliably determined. Oscillation periods displayed in Figure 5a were obtained from the results of a leastsquares fit using a custom fitting function (see eq S14 in the Supporting Information) incorporating a sum of exponential decays and one exponentially decaying sinusoidal oscillation; representative fits are displayed as the black dotted lines in Figure 5b. In the middle of the crossover region, at λprobe = 453 nm, the data (Figure 5b) exhibit several spikes not captured by the single frequency fit, indicative of more than one relevant frequency. Observing the GaAs CLAP at longer wavelengths and the IrO2 CLAP at shorter ones can be explained by considering the absorption depth (1/α) of the reflected probe light. The probe light penetrates well into the GaAs at longer wavelengths (1/α = 150−450 nm for λprobe = 550−700 nm).19 Further, we can predict the depth of the probed region by the time it takes for the CLAP to move out of it, given by the exponential decay time of the sinusoidal oscillation and νs. The probed region is thus suggested to be 150−300 nm, which is in reasonable agreement with 1/α. As the absorption depth becomes shallower at shorter wavelengths, less of the GaAs region of the device is probed (1/α = 20−150 nm for the crossover region, λprobe = 425−550 nm), causing the CLAP frequency 925

DOI: 10.1021/acs.jpclett.6b02835 J. Phys. Chem. Lett. 2017, 8, 922−928

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The Journal of Physical Chemistry Letters large built-in electric fields that can be 100 kV/cm or greater (Figure S3 in the Supporting Information). There is also a smaller field of about 30 kV/cm on the back side of the sample at the interface between the n-doped GaAs layer and the pdoped GaAs layer. The large electric field at the TMO/GaAs interface is key to understanding the generation of the large CLAP. There are three mechanisms that conventionally can give rise to the coherent acoustic phonons. They include: (1) thermal stress in the TMO, (2) piezo-electric coupling of the photoexcited carriers to the phonons, and (3) deformation potential coupling of the photoexcited carriers to the phonons. We consider each mechanism below. Thermal stress occurs when the overlayer heats up from the photoexcitation of hot carriers, which then transfer their energy to the lattice, changing the temperature and the lattice constant. This typically occurs when one has a metallic overlayer on top of a semiconductor substrate. However, coherent acoustic phonons were not observed when a gold overlayer was placed on top of the GaAs samples and thermal stress from the gold film should be stronger than from the TMO (see Figure S10 in the Supporting Information). Piezo-electric coupling between the photoexcited electrons/ holes and the phonons can occur when there is a large internal electric field that can result in the photoexcited electrons and holes being spatially separated as would occur at the TMO/ GaAs interface (and the p-GaAs/n-GaAs and GaAs/InGaP to a lesser extent). However, normally piezo-electric coupling does not occur in GaAs for growth in the 100 direction. Instead, investigations into piezo-electric coupling have involved growth in the 111 direction.13,14,34 It is possible that roughness/surface charge at the interface could lead to piezo-coupling, or perhaps the piezo-electric coupling occurs from the charge carriers generated in the TMO near the GaAs interface. On the other hand, IrO2 and Co3O4 have crystal structures that are rutile and spinel, respectively, which would allow for a piezo-electric coupling to the longitudinal acoustic modes in the TMO. Another consideration for the piezo-electric effect is that for a large absorbed fluence range (up to 10 mJ/cm2), the amplitude of the oscillation does not saturate (Figure S11) and a saturation would be expected for an effect dependent on a finite internal electric field. A more likely explanation for the generation of the CLAP at the TMO/GaAs interface is related to deformation potential coupling of the photoexcited carriers with the phonons. In a uniform sample, the penetration depth in GaAs at 800 nm is quite long (890 nm). The strain pulse generated for deformation potential coupling is related to the derivative of the photoexcited carrier density, which is small for a uniform sample with a long absorption length. The presence of the TMO/GaAs interface with large electric fields leads to a nonuniform absorption on a very short length scale, the length scale of the depletion fields at the interface, which is 10 nm or less at the TMO/GaAs interface (Figure S3 in the Supporting Information). The source term for generating the coherent phonons is then35 S(z , t ) =

∂ρ (z , t ) ⎞ 1 ⎛ ∂ρe (z , t ) ⎟ ⎜ac − av h ρo ⎝ ∂z ∂z ⎠

in this case: the short length scale of the nonuniform absorption (leads to a large derivative of the photoexcited carrier density) and the fact that the electron and hole densities are spatially separated (so the deformation potentials do not cancel out). Most likely, the third mechanism is the dominant mechanism for both CLAPs, though the piezoelectric effect could contribute in the TMO. More detailed modeling should be undertaken to understand the internal electric fields. In fact, these experiments lay the groundwork for determining the equilibrium and excited state electronic properties of the buried semiconductor/TMO interface, including the built-in electric field, the internal Fermi level, and the photoexcited carrier distribution.36,37 Details of the experimental data, such as the appearance of the second harmonic of the CLAP in the IrO2 layer, the prominence of either the GaAs or TMO CLAP with different TMO layers, and differences between back and front excitation, should aid in constraining parameters. Future work will also investigate CLAPs in more complex multilayered semiconductor/TMO architectures designed such that one buried interface is truly selectively excited and the response at another is probed.

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EXPERIMENTAL METHODS Transient reflectivity measurements were performed with a pump−probe geometry using ∼120 fs laser pulses from a Ti:sapphire laser with a fundamental wavelength of 800 nm. Probe wavelengths of 400 nm were generated by reflecting a portion of the fundamental with a beam splitter and directing the beam onto a beta barium borate (BBO) crystal. Probe wavelengths between 475 and 700 nm were generated by focusing the fundamental beam through a sapphire plate. Probe wavelengths between 400 and 475 nm and below 400 nm were generated with an OPerA Solo optical parametric amplifier (OPA) from Coherent Inc. (Santa Clara, CA). The 800 nm pump beam was focused on the sample from either the back (n−p junction) side or the front (TMO) side, depending on the particular experiment, while the probe beam was focused on the sample from the front, as shown in Figure 1. The reflected 400 nm beam was collected on a Si photodiode and recorded with a lock-in amplifier. Probe wavelengths between 400 and 475 nm and below 400 nm were collected with a CCD from Ultrafast Systems (Sarasota, FL). Changes in reflectivity (ΔR/ R) were monitored by delaying the probe pulse in time with respect to the pump pulse using an optical delay stage. The pump beam was mechanically chopped at 300 Hz. Samples were scored and removed from a customized n-p GaAs epitaxial junction purchased from Intelligent Epitaxy Technology, Inc. (Richardson, TX 75081). The substrate was a 4′′ semi-insulating (SI) GaAs(100) with 100 nm InGaP layer followed by 50 nm Si-doped (5× 1017 cm−3) and 860 nm Bedoped (3× 1015 cm−3) epitaxial GaAs layers. 80 nm Co3O4 and 15 nm IrO2 were grown on top of the n-p GaAs device by RF reactive sputtering using a 3 in. Co and Ir target, respectively. Sputtered Co3O4 deposition used a power of 200 W, a 9:1 ratio of Ar:O2 at a total pressure of 5 mTorr, and a substrate temperature of 500 °C. Sputtered IrO2 deposition used a power of 75 W, a 3:2 ratio of Ar:O2 at a total working pressure of 5 mTorr, and a substrate temperature of 300 °C. Twenty-five nanometer Co3O4 was grown on top of the n-p GaAs device by atomic layer deposition (ALD). ALDdeposited Co3O4 was deposited in an Oxford FLexAl system at 100 °C from a CoCp2 precursor with oxygen plasma as the

(3)

where av and ac are the deformation potential constants for electron, ρe and hole, ρh, densities, respectively; ρo is the mass density. Two effects enhance the generation of the strain pulse 926

DOI: 10.1021/acs.jpclett.6b02835 J. Phys. Chem. Lett. 2017, 8, 922−928

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The Journal of Physical Chemistry Letters oxidant.38−40 Sputtered samples were sonicated in acetone and isopropanol prior to deposition, and ALD samples were subject to a 2 min buffered HF etch prior to deposition to remove the native oxide of the material. Stoichiometry of sputtered films was verified via X-ray diffraction. Film thicknesses were estimated using step contact profilometry on thick depositions, then using a linear calibration to extrapolate the deposited thickness for thinner films. In order to directly excite the n-p GaAs junction, the semiinsulating GaAs substrate was selectively removed down to the InGaP layer by wet etching with a 4.2 M solution of citric acid mixed with 30% hydrogen peroxide in a 5:1 volume ratio. The n−p junction was protected from the etch solution by adhering a silicon wafer with Crystalbond 509 adhesive, and an etch mask was formed with Kapton tape. The etch proceeded at a rate of ∼10 μm per hour at room temperature.

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(6) Sun, K.; Saadi, F. H.; Lichterman, M. F.; Hale, W. G.; Wang, H.P.; Zhou, X.; Plymale, N. T.; Omelchenko, S. T.; He, J.-H.; Papadantonakis, K. M.; et al. Stable solar-driven oxidation of water by semiconducting photoanodes protected by transparent catalytic nickel oxide films. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 3612−3617. (7) Zhong, D. K.; Cornuz, M.; Sivula, K.; Grätzel, M.; Gamelin, D. R. Photo-assisted electrodeposition of cobalt−phosphate (Co−Pi) catalyst on hematite photoanodes for solar water oxidation. Energy Environ. Sci. 2011, 4, 1759−1764. (8) Tilley, S. D.; Cornuz, M.; Sivula, K.; Grätzel, M. Light-Induced Water Splitting with Hematite: Improved Nanostructure and Iridium Oxide Catalysis. Angew. Chem. 2010, 122, 6549−6552. (9) Scheuermann, A. G.; Lawrence, J. P.; Kemp, K. W.; Ito, T.; Walsh, A.; Chidsey, C. E.; Hurley, P. K.; McIntyre, P. C. Design principles for maximizing photovoltage in metal-oxide-protected water-splitting photoanodes. Nat. Mater. 2015, 15, 99. (10) Joshya, R.; Ptak, A.; France, R.; Mascarenhas, A.; Kini, R. Coherent acoustic phonon generation in GaAs1−xBix. Appl. Phys. Lett. 2014, 104, 091903−4. (11) Lim, D.; Averitt, R. D.; Demsar, J.; Taylor, A. J.; Hur, N.; Cheong, S. W. Coherent acoustic phonons in hexagonal Manganite LuMnO3. Appl. Phys. Lett. 2003, 83, 4800−4802. (12) Liu, R.; Sanders, G.; Stanton, C.; Kim, C.; Yahng, J.; Jho, Y.; Yee, K.; Oh, E.; Kim, D. Femtosecond pump-probe spectroscopy of propagating coherent acoustic phonons in InxGa1−xN/GaN heterostructures. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72, 195335. (13) Babilotte, P.; Ruello, P.; Pezeril, T.; Vaudel, G.; Mounier, D.; Breteau, J.-M.; Gusev, V. Transition from piezoelectric to deformation potential mechanism via photogeneration in n-doped GaAs semiconductors. J. Appl. Phys. 2011, 109, 064909. (14) Babilotte, P.; Ruello, P.; Vaudel, G.; Pezeril, T.; Mounier, D.; Breteau, J.-M.; Gusev, V. Picosecond acoustics in p-doped piezoelectric semiconductors. Appl. Phys. Lett. 2010, 97, 174103. (15) Babilotte, P.; Mounier, D.; Pezeril, T.; Vaudel, G.; Edely, M.; Breteau, J.-M.; Gusev, V.; Blary, K. Femtosecond laser generation and detection of high-frequency acoustic phonons in GaAs semiconductors. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 245207. (16) Bozovic, I.; Schneider, M.; Xu, Y.; Sobolewski, R.; Ren, Y.; Lupke, G.; Demsar, J.; Taylor, A.; Onellion, M. Long-lived coherent acoustic waves generated by femtosecond light pulses. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 69, 132503. (17) Grahn, H. T.; Maris, H. J.; Tauc, J. Picosecond ultrasonics. IEEE J. Quantum Electron. 1989, 25, 2562−2569. (18) Xu, Y.; Qi, J.; Miller, J.; Cho, Y.-J.; Liu, X.; Furdyna, J.; Shahbazyan, T.; Tolk, N. Pump-probe studies of traveling coherent longitudinal acoustic phonon oscillations in GaAs. Phys. Status Solidi C 2008, 5, 2632−2636. (19) Aspnes, D. E.; Kelso, S. M.; Logan, R. A.; Bhat, R. Optical properties of AlxGa1−xAs. J. Appl. Phys. 1986, 60, 754−767. (20) Jellison, G. Optical functions of GaAs, GaP, and Ge determined by two-channel polarization modulation ellipsometry. Opt. Mater. 1992, 1, 151−160. (21) Ruello, P.; Gusev, V. Physical mechanisms of coherent acoustic phonons generation by ultrafast laser action. Ultrasonics 2015, 56, 21− 35. (22) Wright, O. B.; Perrin, B.; Matsuda, O.; Gusev, V. E. Ultrafast carrier diffusion in gallium arsenide probed with picosecond acoustic pulses. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 64, 081202. (23) Waegele, M.; Doan, H.; Cuk, T. Long-lived photoexcited carrier dynamics of d-d excitations in spinel ordered Co3O4. J. Phys. Chem. C 2014, 118, 3426−3432. (24) McSkimin, H. J.; Jayaraman, A.; Andreatch, P. Elastic Moduli of GaAs at Moderate Pressures and the Evaluation of Compression to 250 kbar. J. Appl. Phys. 1967, 38, 2362−2364. (25) Thomsen, C.; Grahn, H.; Maris, H.; Tauc, J. Surface generation and detection of phonons by picosecond light pulses. Phys. Rev. B: Condens. Matter Mater. Phys. 1986, 34, 4129−4138.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b02835. Theoretical calculations of internal electric fields of the device shown in Figure 1 of the main text; fitting and FFT procedure for extracting the osillation frequencies; zoom in of data shown in Figure 3 of the main text; extended data set of that shown in Figure 5 of the main text, including theory curves; transient reflectivity with Au as the overlayer; transient reflectivity dependence on fluence (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tanja Cuk: 0000-0002-1635-2946 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Thanks to Dr. Jinhui Yang for help with ALD deposition. This material is based upon work supported by the Air Force Office of Scientific Research under AFOSR Award No. FA9550-12-10337 (K.L.P., H.Q.D.) and through AFOSR Award No. FA9550-14-1-0376 (C.J.S., A.R.).

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REFERENCES

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