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Broad-band light absorption and high photocurrent of (In,Ga)N nanowire photoanodes resulting from a radial Stark effect Jumpei Kamimura, Peter Bogdanoff, Pierre Corfdir, Oliver Brandt, Henning Riechert, and Lutz Geelhaar ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12874 • Publication Date (Web): 30 Nov 2016 Downloaded from http://pubs.acs.org on December 1, 2016

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ACS Applied Materials & Interfaces

Broad-band light absorption and high photocurrent of (In,Ga)N nanowire photoanodes resulting from a radial Stark effect Jumpei Kamimura,*,† Peter Bogdanoff,*,‡, Pierre Corfdir,† Oliver Brandt,† Henning Riechert,† Lutz Geelhaar† †

‡

Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany

Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institute for Solar Fuels, Hahn-MeitnerPlatz 1, 14109 Berlin, Germany

Address correspondence to [email protected], [email protected] ABSTRACT: The photoelectrochemical properties of (In,Ga)N nanowire photoanodes are investigated using H2O2 as a hole scavenger in order to prevent photocorrosion. Under simulated solar illumination, In0.16Ga0.84N nanowires grown by plasma-assisted molecular beam epitaxy show a high photocurrent of 2.7 mA/cm2 at 1.2 V vs. reversible hydrogen electrode (RHE). This value is almost the theoretical maximum expected from the corresponding bandgap (2.8 eV) for homogeneous bulk material without taking into account surface effects. These nanowires exhibit a higher incident photon-to-current conversion efficiency over a broader wavelength range and a higher photocurrent than a compact layer with higher In content of 28%. These results are explained by the combination of built-in electric fields at the nanowire sidewall surfaces and compositional fluctuations in (In,Ga)N, which gives rise to a radial Stark effect. This effect enables spatially indirect transitions at energies much lower than the bandgap. The resulting broad-band light absorption leads to the high photocurrents. This benefit of the ACS Paragon Plus Environment

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radial Stark effect in (In,Ga)N nanowires for solar harvesting applications opens up the perspective to break the theoretical limit for photocurrents.

KEYWORDS: nanowire, photoelectrode, water splitting, molecular beam epitaxy, H2O2 INTRODUCTION (In,Ga)N alloys are very promising materials for solar water splitting due to their large absorption coefficient and the possibility to tune the bandgap energy across the entire solar spectrum.1 In addition, their conduction and valence band edges have been predicted to straddle the H+/H2 and O2/H2O redox potentials up to an In content of about 50%.2 However, it is difficult to grow (In,Ga)N bulk layers in high structural perfection because of the lack of lattice-matched substrates.3 In contrast, in nanowires (NWs) dislocations formed at the NW substrate interface do not propagate along the NW axis.4-6 Therefore, (In,Ga)N NWs are an attractive platform for solar energy harvesting.7-13 Another consequence of the NW geometry that is potentially highly beneficial for such applications is a radial Stark effect in (In,Ga)N NWs. Recently, we proposed this effect to explain the occurrence of two separate emission bands in the luminescence spectra of (In,Ga)N NWs.14 One of these bands is caused by direct transitions at the energy corresponding to the average bandgap of the material. The other band is strongly redshifted and results from the spatially indirect recombination of electrons localized close to the core of the NWs with holes localized near the sidewall surfaces. The pronounced redshift is due to the strong electric fields in radial direction that arise from Fermi level pinning at the sidewalls, while localization is mediated by the compositional fluctuations inherent to (In,Ga)N. Here, we demonstrate in a systematic study that this radial Stark effect enables photoelectrochemical (PEC) reactions at (In,Ga)N NWs over a much wider wavelength range than direct transitions at the band edges would allow. In order to investigate the influence of the charge carrier generation and separation on the photocurrent of (In,Ga)N NWs as a possible photoanode for water oxidation, we use H2O2 as a hole scavenger.15,16 The fast oxidation kinetics of H2O2 prevents a limitation of the ACS Paragon Plus Environment

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photocurrent by the electrochemical reaction, so that only the influence of electronic effects within the (In,Ga)N NWs becomes visible. Furthermore, by addition of H2O2 the anodic photocorrosion is suppressed which is normally a limiting factor for n-type GaN and (In,Ga)N.9,17-21 For NWs with an In content of 16% we find photocurrents that are very close to the theoretical maximum expected for the corresponding bandgap.

RESULTS AND DISCUSSION The evolution of the morphology of the (In,Ga)N samples with substrate temperature (Tsub) decreasing from 750 to 540 °C is displayed in Figures 1a-d. As reported previously in ref 22, decreasing Tsub leads to an increased lateral growth and coalescence. In particular, at the lowest growth temperature, the sample does not exhibit a NW morphology anymore but rather consists of a compact layer with a highly faceted surface (Figure 1d). Figure 1e shows ω-2θ X-ray diffraction (XRD) scans for the same samples. Assuming that the InxGa1-xN samples are entirely relaxed, the peak position of the (0002) reflection indicates that the average In concentration (x) is 0, 6, 16, and 28% for Tsub = 750, 640, 590, and 540 °C, respectively. For Tsub = 750 °C, In is actually not incorporated into the NWs due to its significant desorption rate at such a high temperature. The bandgap of InxGa1-xN can be expressed as:

E g ( x) = E gGaN (1 − x) + E gInN x − bx(1 − x) ,

(1)

where E gGaN = 3.4 eV and E gInN = 0.65 eV are the bandgaps of GaN and InN at 300 K, and b = 1.4 eV is the bowing parameter.2,23 We deduce from Equation (1) that the bandgap for the samples grown at Tsub = 750, 640, 590, and 540 °C is 3.4, 3.2, 2.8, and 2.4 eV, respectively.

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Figure 1. Top-view and cross-sectional scanning electron microscope (SEM) images of (In,Ga)N samples grown at (a) Tsub = 750 °C, (b) Tsub = 640 °C, (c) Tsub = 590 °C, and (d) Tsub = 540 °C. (e) ω-2θ XRD scans taken on the samples shown in panels a-d. The average In content deduced from these measurements is indicated in the text labels of the corresponding micrographs.

Before investigating the performance of these (In,Ga)N samples as photoanodes in 1M NaOH, we verify their photoelectrochemical stability. Previous differential electrochemical mass spectroscopy (DEMS) experiments carried out during cyclic voltammetry in the absence of scavengers revealed that for n-type (In,Ga)N NWs under illumination photocorrosion accompanied by the formation of N2 takes place instead of water oxidation.9 We note that in the same publication, p-type (In,Ga)N NWs were found to be stable. However, the growth of p-type (In,Ga)N NWs with In contents exceeding a few percent has not been achieved yet. In the present study we aim at investigating charge carrier generation and separation in (In,Ga)N NWs with higher In content, and thus we employ n-type material. Since its photocorrosion would hinder the assessment of the photoanode performance,20,21 we study first the impact of adding H2O2 as a hole scavenger on the stability of the electrode. The oxidation of H2O2 is described by the following equation: H 2O2 + 2h + → 2 H + + O2 , E 0 = +0.68 V vs. RHE. ACS Paragon Plus Environment

(2) 4

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Since this reaction is much faster than water oxidation, one expects photogenerated holes to be effectively consumed for the oxidation of H2O2 so that they are no longer available for corrosion processes. To confirm this assumption, we have analyzed the gas evolving during cyclic voltammetry using DEMS. Figure 2 shows as a representative example the current-voltage (J−V) curve measured for In0.16Ga0.84N NWs under illumination and in presence of H2O2, together with the mass signals of O2 and N2 observed by DEMS. While the signal related to O2 clearly increases with increasing voltage, the N2 mass signal remains at the background level. Since there is a clear correlation between the O2 mass signal and the photocurrent, the absence of any N2 signal indicates that the use of H2O2 as a hole scavenger indeed suppresses the photocorrosion of In0.16Ga0.84N NWs during PEC experiments and only the oxidation of H2O2 takes place. In other words, using an electrolyte containing H2O2 allows one to study the performance of (In,Ga)N NW samples as photoanodes without being masked by parasitic corrosion processes or limits in electrochemical kinetics.

Figure 2. (a) J−V curve acquired during cyclic voltammetry and (b) the mass signals simultaneously detected by DEMS under halogen light with an intensity of ≈40 mW/cm2 (λ > 410 nm) for n-type In0.16Ga0.84N NWs.


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To this end, we carried out further cyclic voltammetry experiments using a solar simulator. Figure 3a shows the J-V characteristics as a function of x. As expected for the given solar illumination, GaN NWs (x = 0%) exhibit the lowest photocurrent since their bandgap is the largest among the investigated samples. With x increasing from 0 to 16%, the photocurrent increases for the entire voltage range. In particular, for x = 16% the photocurrent reaches 2.7 mA/cm2 at 1.2 V vs. RHE. For all samples, the photocurrent obtained at this bias potential is compared in Figure 3b with the theoretical maximum photocurrent. The theoretical maximum was obtained for AM 1.5G irradiation (100 mW/cm2) assuming all light above the bandgap is absorbed and converted into current (optical absorption limit) for homogeneous bulk material without taking into account surface effects.24,25 Strikingly, the current value for our NW sample with x = 16% is close to the theoretical maximum for the bandgap 2.8 eV of In0.16Ga0.84N. In contrast, for the In0.28Ga0.72N compact layer, the measured photocurrent is much lower (0.7 mA/cm2), although the larger x in this sample should result in the absorption of light with a much longer wavelength and thus in an increase in photocurrent. In order to identify the origin of the high photocurrent found for x ≤ 16% and the decrease observed for the sample with larger x value, we measured at 1.2 V vs. RHE the incident-photon-to-current conversion efficiency (IPCE) under monochromatic light with energy varying between 1.7 and 3.8 eV. The results of these experiments are presented in Figure 3c. The vertical dotted lines indicate for each sample the bandgap expected for the average x deduced from Figure 1b, and we define the onset of absorption as the photon energy at which the IPCE is equal to 1%. For the GaN NW sample, the onset of absorption is close to the bandgap of GaN. We note that the weak absorption below the bandgap seen in Figure 3c has also been observed for planar GaN layers and is related to the Urbach absorption tail.26-29 The onset of absorption for In0.06Ga0.94N and In0.16Ga0.84N NWs is observed at 2.4 and 1.8 eV, respectively. Very remarkably, these values are about 0.8–1 eV lower than those expected from the calculated bandgaps.


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Figure 3. Investigation of the maximum performance as photoanodes for (In,Ga)N samples with x between 0 and 28%. (a) Current voltage curves acquired during cyclic voltammetry under chopped AM 1.5G illumination with 100 mW/cm2. (b) Photocurrent values at 1.2 VRHE (symbols) as a function of bandgap energy in comparison to the theoretical maximum photocurrent (dashed curve). (c) IPCE measured under monochromatic light with varying energy.


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Coming to the In0.28Ga0.72N compact layer, the onset of absorption occurs at 2.1 eV, a value smaller than the bandgap of this sample (2.4 eV), but higher than the onset measured for In0.16Ga0.84N NWs (1.8 eV), despite the larger x. In addition, not only the onset of the absorption increases when x increases from 16 to 28%, but also the IPCE value in the 2.4–3.6 eV range decreases from about 20–60% to 10– 20%. An obvious explanation for the lower IPCE of the sample with x = 28% is the fact that it is a compact layer that has formed as a consequence of massive NW coalescence. This phenomenon is associated with high densities of grain boundaries which in turn act as nonradiative recombination centers.30,31 However, a high density of nonradiative defects cannot account for the fact that the energy for the onset of absorption observed for the In0.28Ga0.72N compact layer is higher than that of the In0.16Ga0.84N NWs. In order to explain the strongly redshifted absorption found in this study, a detailed discussion is required. Local fluctuations in In content are inherent to the ternary alloy (In,Ga)N,32 and such compositional fluctuations lead to regions with lower bandgap than corresponding to the average In content detected by XRD. However, the volume fraction of regions with a bandgap much lower than the average is small and unlikely to give rise to strong absorption. Moreover, the Stokes shift that results from compositional inhomogeneities systematically increases with increasing In content.33,34 Hence, on the basis of this effect the In0.28Ga0.72N sample should exhibit an energy for the onset of absorption lower than that of the sample with x = 0.16, but the opposite is observed. Therefore, compositional fluctuations alone cannot explain our experimental results. The strong difference between the onset of absorption and the bandgap corresponding to the In content found here resembles the large discrepancy between the luminescence peak energy and the bandgap expected from the In content measured by XRD and energy dispersive X-ray spectroscopy that we reported recently for (In,Ga)N NWs.14 We explained the latter discrepancy as follows and as is sketched in Figure 4a. We assume that our unintentionally doped (In,Ga)N NWs present donor concentrations of at least 1017 cm-3.35 In each NW, the pinning of the Fermi level at the sidewalls leads to a bending of the conduction and valence bands across the diameter of the NW and results thus in ACS Paragon Plus Environment

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built-in radial electric fields. The combination of these fields with the local potential fluctuations related to compositional fluctuations gives rise to a radial Stark effect that enables spatially indirect optical transitions at much lower energy than the average bandgap. Here, we propose that this effect is also the origin of the large redshifts observed in the IPCE spectra of (In,Ga)N NWs. In the case of In0.16Ga0.84N NWs, light is absorbed in a broader spectral range than normally possible due to the spatially indirect transitions, which in turn leads to the low energy for the onset of absorption seen in Figure 3c and to the large photocurrent shown in Figures 3a and 3b. In contrast, the morphology of the sample with x = 28% resembles rather a facetted compact film than NWs (Figure 1d). As depicted in Figure 4b, in a compact layer spatially indirect transitions can take place only in a small volume fraction close to the electrolyte interface. The decrease in photocurrent for the latter sample (Figures 3a and 3b) is, in addition to defects induced by coalescence, a direct consequence of its layer morphology, since the radial Stark effect and its beneficial impact on the photocurrent are expected only for the high surface-to-volume ratio of NWs.

Figure 4. Schematic representation of the band profile (black solid lines) across (a) an In0.16Ga0.84N NW and (b) an In0.28Ga0.72N compact layer. Surfaces are indicated by the vertical grey lines. The band bending shown with dashed lines results from surface depletion, and the local potential fluctuations arise from inhomogeneities of the In content. A localized electron may recombine with a hole localized in the same or in a separate potential minimum (cyan and magenta arrows, respectively). Close to the surface, spatially indirect optical transitions are redshifted compared to the spatially direct ones as a result of the Stark effect.


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We note that localization of charge carriers is typically considered a disadvantage in electrochemistry because charge transport to the electrolyte interface may be impeded. At the same time, in our experiments we do measure considerable currents. These currents can be explained by strong radial electric fields, as the following rough estimation shows. On the one hand, the local potential minima in (In,Ga)N typically exhibit an energy depth of 30 meV and a spatial extension of 3 nm.36 Thus, charge carriers localized in such minima can be expected to escape in an electric field of 100 kV/cm. On the other hand, the band bending at the sidewall surfaces is about 0.6 eV already in equilibrium,37,38 and even higher under the biases applied here. Since the radius of the NWs is about 40 nm, the radial electric fields are hence of the same order of magnitude as needed to facilitate charge transport. Therefore, the present local potential fluctuations are beneficial for an enhanced light absorption but do not hinder the charge transfer towards the semiconductor/electrolyte interface where electrochemical reactions take place. Furthermore, the radial Stark effect resolves the seeming discrepancy between the photocurrents close to the theoretical maximum and efficiencies clearly below 100% shown in Figures 3b and 3c, respectively. The photocurrent results from illumination with simulated solar light covering a large range of wavelengths. For the determination of the theoretical maximum in Figure 3b, out of the solar spectrum only light with energy greater than the bandgap calculated from XRD is taken into account, i.e. 2.8 eV for the sample with x = 16%. However, by virtue of the radial Stark effect the material actually absorbs light also at energies lower than the average bandgap. Thus, the agreement between the experimental photocurrent and its theoretical maximum for x = 16% in Figure 3b does not imply an efficiency close to 100%. In order to verify the consistency of our data, we multiplied for this sample the spectrally resolved experimental IPCE values with the corresponding photon flux (equivalent to electric current) of the solar simulator at each wavelength and integrated them over all wavelengths. This procedure results in a current of 3.1 mA/cm2, which agrees well with the photocurrent of ≈3 mA/cm2 obtained by the cyclic voltammetry measurement. Hence, this comparison demonstrates that the


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extension of the absorption range by the radial Stark effect leads to higher photocurrents than can be expected for the same material in bulk geometry. To confirm the hypothesis that the large photocurrents and the small onset of absorption for NWs with x ≤ 16% in Figure 3 arise from the radial Stark effect, we performed in addition photoluminescence (PL) measurements. Figure 5a shows the PL spectra obtained at 150 K. The spectrum for the sample with x = 6% is broad and exhibits two separated maxima. As discussed in ref 14 and depicted schematically in Figure 4a, the high energy band arises from the recombination of electrons and holes localized at the same sites, while the low energy band is due to the recombination of holes at the surface with electrons closer to the core of the NW. Also note that the observation in the PL spectrum of two separated maxima instead of a single broad band indicates that the density of states deep enough to localize electrons close to the surface is small.14 The two-band behavior observed for the sample with x = 6% is less pronounced for the samples with x = 16 and 28%. As shown in Figure 5a, the band at high energy for these samples appears in fact as a shoulder. The position of the high-energy maximum is 3.2 eV for the sample with x = 6%. For the samples with x = 16 and 28%, the energy of the high-energy shoulder is 2.8 and 2.4 eV, respectively. As highlighted by the dashed lines in Figure 5a, this dependence is in good agreement with that expected for the bandgap of (In,Ga)N [Equation (1)], confirming that the emission at high energy arises from the recombination of electrons and holes localized at the same site. We show in Figure 5b that the PL peak energy decreases monotonically with increasing x. For the sample with x = 16%, the low-energy peak related to spatially indirect transitions is centered at 2.1 eV. This value is about 700 meV smaller than that expected for the bandgap, in qualitative agreement with the IPCE spectrum in Figure 3c. Therefore, the redshift of the absorption edge by the radial Stark effect is responsible for the low energy at which the IPCE rises for NWs with x ≤ 16%. In turn, this widening of the absorption range significantly increases the photocurrents measured for these samples close to the values predicted theoretically, although the actual IPCE values are well below 100%.


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Further observations are also consistent with the radial Stark effect, as discussed in the following. First, the two PL bands observed for the sample with x = 6% develop into one single broad band for the samples with higher x. Increasing x leads to an increase in alloy disorder39 and thus to an increase in the density of potential minima that are deep enough to localize electrons.14 Therefore, for large values of x, not only localized holes but also electrons are homogeneously distributed across the nanowire, and the nanowire PL spectrum consists of one single broad band, in agreement with what is observed in Figure 5a. Second, not only the samples with NW morphology but also the In0.28Ga0.72N compact layer shows indirect transitions. This finding is probably related to the fact that the In0.28Ga0.72N compact layer exhibits a rough and faceted surface (Figure 1d). In other words, compared to a smooth film a fairly large volume in this layer may be depleted, as depicted schematically in Figure 4b. However, as indicated by the relatively large onset of absorption for the In0.28Ga0.72N compact layer compared to the NWs with x = 16% (Figure 3c), this depletion volume is substantially smaller than that in the NWs. The beneficial impact of the radial Stark effect is thus reduced, demonstrating the superiority of NW photoanodes for PEC water splitting applications.

Figure 5. (a) PL spectra taken at 150 K on (In,Ga)N samples with x between 6 and 28%. The dashed lines show the corresponding (In,Ga)N bandgaps obtained from Equation (1). (b) PL peak energies as a function of x. The line is a guide-to-the-eye. ACS Paragon Plus Environment

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CONCLUSION We investigated the performance of (In,Ga)N NWs as photoanodes in aqueous electrolytes using H2O2 as a hole scavenger in order to prevent photocorrosion. In0.16Ga0.84N NWs show under simulated solar illumination IPCE values between 20 and 60% for photon energies between 2.4 and 3.6 eV and a photocurrent as high as 2.7 mA at 1.2 V vs. RHE. Very remarkably, the latter value is the theoretical maximum photocurrent expected for homogeneous bulk material of this semiconductor with a bandgap of 2.8 eV. For higher In content, we observe a decrease in photocurrent, despite the fact that larger In contents should give rise to an increased light absorption at longer wavelengths. Based on IPCE and PL experiments, we demonstrate that the high photocurrent in In0.16Ga0.84N NWs is a result of the radial Stark effect in these structures. The combination of compositional fluctuations typical for this alloy with the radial built-in electric fields that result from the pinning of the Fermi level at the sidewalls of the NWs gives rise to spatially indirect transitions that extend the absorption towards longer wavelengths. Therefore, the absorption range of In0.16Ga0.84N NWs is much broader than that expected for bulk, which enables the observed large photocurrent. For an In content of 28%, radial growth leads to massive coalescence and the photoanode consists rather of a facetted layer than of NWs. The beneficial impact of the radial Stark effect is thus reduced, leading to a decrease in photocurrent. To take full advantage of the radial Stark effect described in this study, it is necessary to grow (In,Ga)N photoelectrodes with higher In content without compromising the NW morphology. This is a challenging task, since higher In incorporation necessitates lower substrate temperatures, which causes lateral growth. This drawback is particularly true for p-type (In,Ga)N NWs, which are known to be stable photocathodes,9 since p-type doping with Mg induces significant coalescence even in GaN NWs grown at high temperature by plasma-assisted molecular beam epitaxy (MBE).35,40 At the same time, we emphasize that such growth efforts would be very rewarding. With further optimization of (In,Ga)N NW photoanodes, higher IPCE values above the average bandgap could be achieved. Then, the additional absorption by the radial Stark effect could allow the theoretical limit for photocurrents to be broken.


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EXPERIMENTAL SECTION The growth of (In,Ga)N NWs was carried out on n-type Si(111) substrates by MBE relying on selfassembly processes.41 To modify the incorporation of In, we varied the Tsub between 540 and 750 oC while keeping the supplies of In, Ga, and active N fixed.42,43 A more detailed description of the growth process can be found in ref 22. The NW morphology was analyzed with a SEM, and their average In content was determined by XRD in the ω-2θ geometry. Cyclic voltammetry measurements were carried out in a three-electrode PEC cell using as-grown (In,Ga)N NW samples (1 × 1 cm2 in size), a platinum wire, and a Ag/AgCl electrode as working, counter, and reference electrodes, respectively, a solution of 1 M NaOH (10 mL) and H2O2 (200 µL with a concentration of 30%) as an electrolyte, and AM 1.5 G solar illumination (intensity 100 mW/cm2) obtained from a solar simulator. Dark currents were probed by chopping the illumination at a frequency of 0.5 Hz using a scan rate of 30 mV/s. In order to test the stability of the samples and to detect gases evolving during the cyclic voltammetry experiments, DEMS was performed at a scan rate of 2 mV/s using halogen light with an intensity of about 40 mW/cm2 (wavelength λ > 410 nm).9 Potentials vs. the Ag/AgCl electrode were converted to the RHE scale by the equation VRHE = VAg/AgCl + 197 mV + (59 mV)pH, based on the fact that (In,Ga)N follows the Nernstian pH response.44,45 PL spectroscopy was carried out at 150 K using the 325-nm line of a continuous-wave HeCd laser for excitation. The laser light was focused down to a 2-µm diameter spot using a near-UV objective with a numerical aperture of 0.65. The PL signal was collected with the same objective, was sent to a 80-cm focal length monochromator for spectral dispersion (600 grooves/mm grating), and was detected using a liquid N2 cooled charge-coupled device. ACKNOWLEDGMENTS The authors thank A.-K. Bluhm for SEM support, C. Herrmann as well as H.-P. Schönherr for the maintenance of the MBE system, and C. Pfüller as well as R. van de Krol for a critical reading of the manuscript. J.K. is grateful for a JSPS Postdoctoral Fellowship for Research Abroad. P.C. acknowledges funding from the Fonds National Suisse de la Recherche Scientifique through project 161032. ACS Paragon Plus Environment

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