Tuning Lasing Emission toward Long Wavelengths in GaAs-(In,Al

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Tuning lasing emission towards long wavelengths in GaAs-(In,Al)GaAs core-multishell nanowires Thomas Stettner, Andreas Thurn, Markus Döblinger, Megan O Hill, Jochen Bissinger, Paul Schmiedeke, Sonja Matich, Tobias Kostenbader, Daniel Ruhstorfer, Hubert Riedl, Michael Kaniber, Lincoln J. Lauhon, Jonathan J. Finley, and Gregor Koblmueller Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b02503 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 6, 2018

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Tuning lasing emission towards long wavelengths in GaAs(In,Al)GaAs core-multishell nanowires T. Stettner1, A. Thurn1, M. Döblinger2, M. O. Hill3, J. Bissinger1, P. Schmiedeke1, S. Matich1, T. Kostenbader1, D. Ruhstorfer1, H. Riedl1, M. Kaniber1, L. J. Lauhon3, J. J. Finley1, and G. Koblmüller1 1

Walter Schottky Institut and Physik Department, Technische Universität München, 85748

Garching, Germany 2

Department of Chemistry, Ludwig-Maximilians-Universität München, 81377 München, Germany 3

Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States

KEYWORDS. Nanowire lasers, quantum wells, monolithic III/V integration on Si, InGaAs, scanning transmission electron microscopy, photoluminescence, molecular beam epitaxy

Semiconductor nanowire (NW) lasers are attractive as integrated on-chip coherent light sources with strong potential for applications in optical communication and sensing. Realizing lasers from individual bulk-type NWs with emission tunable from the near-infrared to the telecommunications spectral region is, however, challenging and requires lowdimensional active gain regions with an adjustable band-gap and quantum confinement. Here, we demonstrate lasing from GaAs-(InGaAs/AlGaAs) core-shell NWs with multiple InGaAs quantum wells (QW) and lasing wavelengths tunable from ~0.8 µm to ~1.1 µm. Our investigation emphasizes particularly the critical interplay between QW design, growth kinetics and the control of InGaAs composition in the active region needed for effective tuning of the lasing wavelength. A low shell growth temperature and GaAs interlayers at the QW/barrier interfaces enable In-molar fractions up to ~25% without plastic strain relaxation or alloy intermixing in the QWs. Correlated scanning transmission electron microscopy, atom probe tomography, and confocal PL spectroscopy analyses illustrate the high sensitivity of the optically pumped lasing characteristics on microscopic properties, providing useful guidelines for other III-V-based NW laser systems.

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Semiconductor nanowires (NWs) offer new routes towards coherent nanoscale light sources that can be monolithically grown on low-cost Si substrates1–4 and be site-selectively integrated onto silicon photonic circuits3,5–7. Their unique geometry allows a wide range of different materials to be grown site-selectively on Si, despite the presence of considerable lattice mismatch. Moreover, they facilitate low-loss single-mode optical waveguiding, integrate an internal gain medium and host resonant optical recirculation needed for lasing. Intense research into different NW laser material systems has been ongoing for several years now, with early efforts focused mostly on group-III nitrides8–10 and II-VI materials (e.g., ZnO11,12 and CdS13,14) emitting in the ultraviolet and visible spectral range. More recent endeavors were increasingly directed towards NW-lasers in the near-infrared (NIR) using IIIV compound semiconductors such as GaAs, InP and GaAsSb15–18. Compositional tuning of the bandgap of NW-lasers allows their emission wavelength to be tuned towards the technologically relevant telecommunication band (~1.15-1.5 µm). However, to date lasing from bulk NW materials in the telecommunications band with a tunable emission wavelength has not been demonstrated for isolated NWs and a 3D (bulk) gain medium. Although bandgap tunable bulk III-V NWs, e.g. InGaAs nanopillars, can be monolithically integrated on silicon and silicon-on-insulator platform19, lasing at telecom wavelengths was only achieved in one-dimensional photonic crystal arrays.6 An alternative approach for tuning lasing wavelength to the desired spectral range is to use quantum-confined structures such as quantum wells (QW) or quantum dots (QD) as the active gain medium. Due to their modified density of states20 these low-dimensional gain media are expected to simultaneously improve lasing performance by lowering the lasing threshold while increasing modulation bandwidth, differential gain, and temperature stability.2,4,21 Furthermore, coaxial QWs in core-shell NW-lasers can host optical modes that may provide efficient in-coupling of lasing emission into proximal Si waveguides7,22,23. In this respect, relatively few near-IR NW lasers using quantum confined gain media have been reported for III-V systems: axial InGaAs QDs in GaAs-based NWs21, GaAs QWs in GaAsAlGaAs core-shell NWs22,24, InGaAs QWs in InP nanopillars (NP)25 and GaAs-GaAsSb QDsuperlattice NW structures18. In particular, InGaAs QWs arranged in coaxial NP or NW geometries appear most promising as they offer excellent luminescence yield at lasing wavelengths in the Si-transparent wavelength range25,26. While coaxial InGaAs QWnanopillar lasers can be realized in the closely lattice-matched InP system, it remains unclear whether they can be also implemented on the well-established GaAs/Si platform where


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mismatch strain comes into play. Furthermore, the links between lasing characteristics and underlying structural and compositional properties of the respective QWs mediated by QW design and growth kinetics is another open question that remains to be explored. In this work, we demonstrate the growth and explore the lasing properties of monolithically integrated GaAs-(InGaAs/AlGaAs) core-shell NWs on Si with multiple composition-tunable InGaAs quantum wells (MQW) in the active region. Using finitedifference time-domain (FDTD) simulations, the sizes of our NW-laser cavities are precisely tuned in the radial dimension to optimize the overlap of the desired optical (TE01) mode with the quantum confined gain region for any given lasing wavelength, i.e., InGaAs composition. Importantly, we explore the role of InGaAs-QW/AlGaAs-barrier design and respective growth kinetics on the In-incorporation, alloy intermixing, and structural integrity as well as the resulting lasing properties of these NW-QW lasers. Using correlated scanning transmission electron microscopy (TEM), atom probe tomography (APT) and confocal photoluminescence (PL) spectroscopy, we systematically study the growth mediated differences in the quality of the InGaAs QW based on two growth series with variable III/V ratio and temperature. We demonstrate that low-temperature shell growth conditions and GaAs interlayers surrounding the InGaAs QWs enhance In-incorporation and reduce alloy intermixing in the MQW structure, yielding tunability of the lasing wavelength over the range from ~0.8 µm to ~1.1 µm. The NW lasers were grown by solid source molecular beam epitaxy (MBE) on Si (111) substrates through random nanoscale openings in ultrathin SiO2 mask layers, which act as sites of selective nucleation27. To define the resonator length of the NW laser, the GaAs NW core was grown vertically aligned along the [111] orientation using a self-catalyzed vapor-liquid-solid (VLS) growth at a substrate temperature of 650 °C and Ga and As fluxes of 0.05 nm s-1 and 0.24 nm s-1, respectively. A total growth time of 60 min resulted in ~6-10 µm long and ~50-60 nm thick GaAs NWs, similar to our previous NW lasers16,22,28,29. After defining the core, the growth temperature was reduced and As supply increased, in order to consume the Ga droplet and to set conditions for subsequent radial growth at the six {1-10} NW sidewall facets16,30. This radial growth step defines the exact cavity width (NW laser diameter) as well as the coaxial InGaAs/AlGaAs MQW structure. First, an Al0.3Ga0.7As spacer layer was grown at a thickness chosen to maximize the overlap of the gain region with the supported mode (TE01 mode) for the respective emission wavelength. With increasing wavelength emission the entire NW laser cavity needs to be thicker, meaning that the


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AlGaAs spacer layer thickness was increased accordingly (see FDTD simulations in Supporting Information for more details). After the AlGaAs spacer layer, a total of seven 8nm thick InxGa1-xAs QWs were embedded, which are separated by 6-nm Al0.3Ga0.7As barriers. Here, an additional 2-nm thin GaAs interlayer was introduced at both upper and lower interfaces of the InGaAs QWs in order to reduce alloy intermixing of the ternary InGaAs and AlGaAs compounds. Corresponding NW lasers were also grown without GaAs interlayers and compared in terms of alloy intermixing (see Supporting Information). The QW and barrier thicknesses were adapted from former studies of GaAs/AlGaAs MQW NW systems22,24 as well as planar structures31,32 to achieve uniform carrier distributions via weakly coupled QWs to achieve low threshold lasing. In total, two distinct series of samples were grown to study the effect of varying In/Ga-flux ratio and temperature on the incorporation of In into the QW region. In the first series, the shell growth temperature was fixed at 510 °C while the nominal In-molar fraction was steadily increased up to 40% under a constant group III flux of 0.09 nm s-1 (more details can be found in the Supporting Information). In the second series, the impact of the shell growth temperature in the range of 420 °C to 510°C was studied on the In-incorporation while the nominal In-molar fraction was fixed at 30% via a constant group-III supply. All samples were finally capped with a 5-nm thick GaAs shell to prevent oxidation of the active region. All growth stages were performed under constant substrate rotation of 5 rpm (rotations per minute). Figure 1(a) shows a scanning electron microscopy (SEM) image of as-grown MQWNW lasers grown at a shell growth temperature of 510 °C and a nominal In-molar fraction of 30%. The length of the NWs on this sample varies between ~6-8 µm and the diameter, measured between two opposite {1-10} side facets, is on average ~440 nm. Overall, the lengths of NWs from all samples are in the range of 6-10 µm, irrespective of the different Inmolar fractions and shell growth temperatures. In contrast, the diameters differ between ~360 nm and ~480 nm for samples with different In-molar fraction due to the differently optimized AlGaAs spacer thickness for each desired emission wavelength (see Supporting Information). The intended inner AlGaAs spacer thickness for the present sample in Figure 1(a) was 75 nm. The NWs also exhibit a pronounced tip end that represents the crystal facets obtained from crystallization of the Ga droplet after core growth. Thin cross-sectional lamellas were prepared from NWs from the same sample as in Figure 1(a) using focused ion beam (FIB) milling to analyze quantum wells, barrier layers, and their interfaces via scanning


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transmission electron microscopy (STEM). A typical STEM-HAADF (high angle annular dark field) image of the cross-section of an individual NW is shown in Figure 1(b), clearly showing the symmetric and coaxial arrangement of alternating hexagonally-shaped QWs and barriers around the core. All layers in the shell structure exhibit thicknesses very close to the nominally expected values, i.e., an AlGaAs spacer thickness of 69 (±13) nm (nominal value of 75 nm), QW thicknesses of 7.9 (±1.1) nm, and AlGaAs/GaAs barrier thicknesses of 8.1 (±0.5) nm. The total diameter of this particular NW cross-section amounts to ~422 nm. Fluctuations in the InGaAs QW thicknesses as seen in Figure 1(b) are random and ascribed to slight asymmetries in the radial growth facets, while the nominal growth rate of all consecutive QWs was identical. To better observe the respective layers and their interfaces, Figure 1(c) depicts a high-resolution STEM image recorded in the corner region of the MQW heterostructure. From the corresponding Z-contrast we can clearly distinguish the AlGaAs barrier, the ~2-nm thin GaAs interlayer, and the InGaAs QW. For better orientation, dashed lines in the corresponding colors indicate the interfaces. The interface sharpness appears abrupt to a precision within ~2-3 monolayers, similar to recent findings in GaAs-AlGaAs core-multishell NW lasers22. In addition, we also observe dark contrast stripes along the {112} corner facets of the AlGaAs barriers, which can be attributed to Al-enrichment as noted in previous studies.30,33 A more detailed element-specific analysis of the individual layers is further shown below in Figure 4.

Figure 1: (a) SEM image of as-grown MQW-NW laser structure on Si (111) with a nominal In-molar fraction of 30% grown at a shell temperature of 510 °C. The length is typically ~6-8 µm and the diameter ~440 nm, respectively. (b) Cross-sectional STEM-HAADF image overlaid with a schematic illustration of the core-shell MQW heterostructure. (c) Highresolution STEM image of a region close to one of the six {11-2} corners, showing the high-


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quality interfaces between InGaAs QWs and AlGaAs/GaAs barriers (dashed lines indicate the interfaces). Although the as-grown NW lasers are designed to ultimately operate on a Si photonic platform, where direct coupling of coherent emission into proximal waveguides can be realized7, we mechanically transferred and dispersed NW-lasers onto sapphire templates, which provides sufficient refractive index contrast to enable lasing.16,34 This allows us to obtain a benchmark of the intrinsic lasing properties relative to recently reported NW-lasers emitting in the near-IR spectral range.16,34 The lasing characteristics were then investigated on individual NWs subject to pulsed optical excitation in low-temperature confocal photoluminescence microscopy22,28,29. Hereby, the NWs were probed by optical pulses incident along a direction perpendicular to the NW-axis at a wavelength of 780 nm (pulse length 200 fs and 82 MHz repetition rate) with a spot size of ~1.9 µm. Using the confocal measurement geometry, the emitted light scattered from the NW end facets was collected via a single-mode optical fiber. All measurements presented here were performed in a He-flow cryostat at a nominal temperature of ~10 K. Figure 2(a) shows typical pump power dependent PL spectra of a NW like that shown in Figure 1 (with nominally 30% In-molar fraction in the MQW) as a function of the emitted photon energy. The excitation pump pulse fluence ranges from 0.24 to 2.6 mJ/cm². A low pump power of 0.24 mJ/cm² (corresponding to P/Pth = 0.3) produces a broad spectrum (black) dominated by spontaneous emission (SE) around an energy of 1.36 eV. The periodic oscillations are attributed to the feedback of the Fabry-Perotlike nature of the NW cavity.34 With increasing pump power, the SE spectra increase in intensity until the lasing threshold is reached and a superlinear peak emerges at an energy near ~1.39 eV, accompanied by a clamping of the spontaneous emission. To investigate the lasing properties in more detail, a double-logarithmic light-in – light-out (L-L) curve of the integrated peak intensity as a function of the pump pulse fluence (black circles) is plotted in Figure 2(b), showing a distinct “s-shape” dependency. A clear transition from SE to amplified spontaneous emission (ASE) and ultimately to lasing is observed with increasing pump power.35 Fitting the integrated intensities with an allometric power law I ∝ Pk, with P being the pump pulse fluence and k the exponent, we find for the ASE regime a value of kASE ~ 3.45 proving the superlinear increase in intensity. The exponent in the lasing regime is close to the expected value of unity (kLasing ~ 0.94).36 Linearly plotting the input-output characteristic allows the extraction of the lasing threshold of Pth = 0.80 ± 0.06 mJ/cm². The transition from the SE to the ASE regime is also reflected by a decreasing


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full width at half maximum (FWHM) of the dominant peak from > 10 meV below threshold to < 3.5 meV above (blue triangles).

Figure 2: (a) Pump power dependent lasing spectra of the sample with a nominal In-molar fraction of 30% below threshold (black), at threshold (blue) and above threshold (red, yellow and purple), respectively. Above 0.80 ± 0.06 mJ/cm², a superlinear lasing peak arises at an energy of ~1.39 eV. (b) Corresponding L-L curve of the integrated peak intensity as a function of the pump pulse fluence on a double-logarithmic scale (black circles). Corresponding FWHM (blue triangles) as obtained from fitting the lasing peak area shows a distinct decrease of the line width upon exceeding the lasing threshold.

The lasing emission energy is tuned by varying the In-content in the MQWs. Figure 3(a) shows the normalized lasing emission spectra (each at max. pump powers) from typical NWs with nominal In-molar fractions in the MQWs between 15% and 40% when grown at a shell growth temperature of 510 °C. For comparison, a lasing spectrum of a NW with binary GaAs MQWs (blue) embedded in identical AlGaAs barriers and otherwise very similar geometry is also shown.22 While pure GaAs MQWs lase at ~1.57 eV (blue),22 increasing the nominal In-molar fraction to 15% shifts the dominant lasing peak to the lower energy of ~1.47 eV (red). Further increases in the In-molar fraction further red-shifts the lasing energy down to 1.36 eV for an In-molar fraction of nominally 40% (green). Additional statistics illustrating the typical peak energy dispersion of a larger number of NW lasers per given sample are further shown in the Supporting Information (Fig. S2(a)). Furthermore, Fig. S2(b) illustrates the corresponding lasing energies and thresholds of all investigated samples. The average lasing thresholds tend to increase from 0.4 mJ/cm² to 1.3 mJ/cm² with increasing Inmolar fraction. NW lasers from this series were also characterized at room temperature (RT), and all samples evidence clear lasing at RT (Figure 3c) except for the NWs with the highest


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nominal In-molar fraction of 40%. An exemplary LL-curve of the sample with a nominal Inmolar fraction of 15% is shown in Figure 3(d), revealing a lasing threshold of ~3.6 mJ/cm². As depicted in Figure 3(c), the lasing energies are red-shifted by ~40-90 meV compared to the energies at low-temperature which is expected from the Varshni-like37 band gap shrinkage of the gain medium38. The thresholds at RT are about 3 – 10 times higher compared to cryogenic temperatures, similar to previous observations in coaxial GaAsAlGaAs MQW-NW lasers.22,39

Figure 3: (a) Normalized lasing emission of NW lasers at 10 K with different nominal Inmolar fractions in the InGaAs MQW grown at 510 °C. For comparison, NW lasers with binary GaAs MQWs (blue) show lasing emission at an energy of ~1.57 eV.22 (b) 1D simulations of the transition energies between electron ground state and first heavy hole state showing the dependence on QW thickness and In-molar fraction without taking strain into account. The dashed line marks the nominal QW thickness of 8 nm, illustrating the approximate emission energies for the different In-molar fractions. (c) Room temperature lasing of the same NW lasers as in (a) for the different In-molar fractions in the InGaAs MQWs. The lasing energies are red-shifted by ~40-90 meV compared to the lasing energies at cryogenic temperatures. (d) Typical L-L curve of the integrated peak intensity as a function


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of the pump pulse fluence on a double-logarithmic scale of the sample with a nominal Inmolar fraction of 15%, revealing a threshold of ~3.6 mJ/cm².

To compare the observed lasing energy shifts (Figure 3(a)) with respect to the expected shifts based on a Vegard’s law relationship between bandgap and nominal In-molar fraction, we performed simulations using an effective mass model (Figure 3(b)). Hereby, numerical 1D simulations of the optical transition energies in the (Al)GaAs–InGaAs heterostructure system were conducted using the Schrödinger-Poisson solver nextnano3.40 Transition energies at 10 K were calculated based on an effective mass approximation in a Kronig-Penney type model for situations with and without strain in the InGaAs QWs. Figure 3(b) presents the resulting energies of the first electron to heavy hole (HH) transitions for nominal In-molar fractions between 5% and 60% as a function of the QW thickness for the unstrained case. For comparison, the transition energies under hydrostatic strain are shown in the Supporting Information (Fig. S3). Irrespective of the strain, we find that for the range of nominal In-molar fractions studied here and the specific QW thickness of 8 nm in our MQW structure (dashed line) the shifts in observed transition energy should be much larger (up to few hundreds of meV) than those found by the respective lasing emission. For example, an InGaAs MQW structure with an In-molar fraction of 30% should yield an emission well below 1.3 eV (compare Figs. 3(b) and Fig. S3(b)). However, the NW laser structure with a nominal In-molar fraction of 30% exhibits lasing emission at much higher energy close to ~1.4 eV (see purple spectrum in Figure 3(a)). This suggests that the actual In-molar fraction of the InGaAs MQW structure is significantly smaller than the nominal In-molar fraction, as further verified below. Furthermore, we emphasize that the present simulations of the groundstate transition energies are not expected to directly predict the lasing energy, since it reflects a complex interplay of local, steady-state carrier density in the lasing system that impacts on the effective modal refractive index, the effects of local band gap renormalization, as well as heating upon pumping under strong excitation power densities. Nevertheless, the observation of a clear red shift of the lasing emission is clearly indicative of enhanced In-incorporation in the structures investigated. To test if the In-molar fractions, estimated from the calculated transition energies, indeed deviate from their nominal values, we performed element specific analysis of the respective NW lasers. In particular, we employed two independent methods, STEM-EDXS


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(energy dispersive x-ray spectroscopy) on axial and radial NW cross-sections and atom probe tomography (APT) on individual NWs. Figure 4 summarizes the respective quantitative elemental analysis of the In-, Al- and Ga-molar distribution in the MQW region of representative NW lasers with a nominal In-molar fraction of 30% (i.e., sample as in Figure 1 and Figure 2). In Figure 4(a) we show a cross-section along the length axis of the NW, as prepared by FIB cut along the [111] growth direction. The seven InGaAs QWs can be clearly identified (layers with bright contrast), showing overall good homogeneity along and across the NW. Performing EDXS on sets of QWs allows us to identify the respective alloy compositions along the NW growth axis. The measured In-molar fraction, averaged over all seven QWs, is found to remain fairly constant in the range of ~8.5±1.5%, when measured at three different positions (dashed blue lines). This reveals two important findings: First, that the In-incorporation during growth is not diffusion limited as the In-content is approximately the same at the bottom and top of the NW.41,42 Secondly, the In-molar fraction is indeed much lower than the nominally expected value. To identify reasons for the lower than expected Inmolar fraction in the InGaAs MQWs, we analyze in more detail radial cross-sections by STEM-EDXS (see Figure 4(b)). The top STEM-HAADF image illustrates the investigated section from which subsequent EDXS analysis was performed (blue dashed line) and resulting data shown in the bottom plot. The elemental distribution as found by the EDXS linescan clearly reproduces the structure of the alternating InGaAs QW – AlGaAs/GaAsbarrier period. In addition, the 2-nm thin GaAs interlayers on each side of the InGaAs QW separating the AlGaAs barrier can be clearly resolved. The In-molar fraction is ~9±2%, which is consistent with the axial cross-sectional analysis. Also, the In-molar fraction does not change with increasing QW number, suggesting that strain effects and corresponding gradients in the In-content along the radial direction are negligible.43 For the element specific analysis, we note that the K-line of the X-ray spectra was used for all group-III elements (Ga, Al) except for In, due to spectral limitations of the detector. This leads to an underestimation of the actual Al molar fraction (more details can be found in the Supporting Information). To independently verify the In-molar fraction in the InGaAs QWs and further obtain insights into atomic intermixing effects, atom probe tomography (APT) was performed on specific reference samples. Hereby, the specimen required the total NW diameter not to exceed ~100 nm to avoid fracture of the specimen under typical APT operating conditions44. Therefore, we designed the reference sample in such a way that the NW core is only ~20 nm thick33 while only one InGaAs QW was implemented in an otherwise identical AlGaAs/GaAs


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barrier heterostructure as for the NW lasers. The sample was also grown at a growth temperature of 510 °C with a nominal In-molar fraction of 30%, leading to a total NW diameter of ~90 nm. See Supporting Information for more details about the reference samples and APT conditions. Figure 4(c) depicts the resulting APT 2D elemental maps of Ga, Al and In, respectively. Note that the full NW cross-section is not visible in the reconstruction, due to the limited field of view in APT for specimen that are not perfectly vertically oriented on the probe tip45. Nevertheless, all the relevant details can be analyzed as the GaAs core, AlGaAs/GaAs barrier and InGaAs QW region are clearly identified. A 1D composition profile, determined using proximity histogram analysis, allows quantification of the respective elemental distribution along the direction normal to an isoconcentration surface, in this case, defined by the {1-10} facets of the InGaAs quantum well (blue dashed arrows). From this we observe that an increase in the In and Al molar fraction coincides with a decrease in the Ga molar fraction as expected.

Figure 4: (a) Cross-sectional STEM image along the growth axis of a NW laser with a nominal In-molar fraction of 30% in the MQWs as prepared by FIB cutting along the [111] direction. EDXS line scans reveal a high compositional homogeneity along the NW with an average In molar fraction in the range of 8.5±1.5%, measured at three different positions (dashed blue lines). (b) STEM image of a representative section of the hexagonal NW crossACS Paragon Plus Environment

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section, illustrating the Z-contrast of the MQW region and barriers in the radial growth direction. EDXS linescans were recorded along the dashed blue line with corresponding plot on the bottom illustrating the elemental Ga, Al, and In distribution. (c) Atom probe tomography (APT) data on equivalent reference sample with a single InGaAs QW, depicting 2D compositional maps of Ga, Al and In, respectively. A 1D proxigram moving from the NW center perpendicular to one of the {1-10} side facets (blue dashed arrows) is shown in the bottom right. In the QW region, the In molar fraction is ~8 %, while additional Al with a molar fraction of ~4.5% is also present in the QW region. For instance, the Al-molar fraction in the barrier is ~30±2%, while the Ga-molar fraction is ~70±2%, respectively. In the QW, the In-molar fraction is approximately 8% which is similar to the findings by STEM-EDXS (8.5±1.5%). Due to potential trajectory overlap effects during atom probe (see Supporting Information), the In concentration should be treated as a lower bound. Further, the extent of Al and In intermixing should be treated as an upper bound. Interestingly, a residual Al-molar fraction as high as ~4.5% can be observed in the center of the QW, while the Ga-molar fraction in the QW is also higher than nominally expected. In addition, the Al molar fraction is increased along the {11-2} corner facets as already described in previous studies.30,33,45 This suggests that Al and Ga atoms diffuse from the adjacent AlGaAs/GaAs barrier into the InGaAs QW, causing alloy intermixing effects as also seen in planar structures.46–49 For comparison, we also performed APT analysis on a similar sample with nominally lower In-molar fraction in the InGaAs QW (i.e., 5% In) and without the 2-nm thick GaAs interlayers on each side of the InGaAs QW. In this case, the Inmolar fraction in the QW was ~2%, while the Al-molar fraction was increased to ~5% (see Supporting Information). This clearly shows that the use of GaAs interlayers between AlGaAs barrier and InGaAs QW effectively reduces Al diffusion into the QW, although full suppression of alloy intermixing effects requires further optimization in the design and growth parameters. The residual Al-molar fraction in the InGaAs QW impacts the lasing emission energy, causing a slight blue-shift to higher energies as compared to nominally Al-free InGaAs QW. A comparative 1D-simulation of the optical transition energy with and without a residual Almolar fraction of ~4.5% in the InGaAs QWs (cf. Figure 4 with 8% In-molar fraction) results in a ~20 meV higher transition energy for the case of intermixed QW. Although a measureable effect, it is insignificant since the dominant effect limiting the emission wavelength in the present InGaAs QWs is rather given by the overall low In incorporation. We suspect that under the given growth conditions, In incorporation is limited by competing In desorption likely due to the high growth temperature.50–52 Hence, in a second growth


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series, we explored the role of shell growth temperature by successively lowering the temperature from 510 °C to 460 °C, 420 °C and 360 °C, respectively. In this series, the InGaAs MQWs were grown with a nominal In-molar fraction of 30%, and all other parameters were kept fixed. Corresponding SEM images of all four NW laser samples show that the morphologies are pristine with well-defined sidewall facets, while for the lowest growth temperatures the facets exhibit increased roughness and disorder (see Supporting Information). We suggest that the increased roughness stems from substantial strain relaxation effects, as more In is incorporated at lower shell growth temperatures.53 Indeed, roughening of the NW sidewall surfaces in GaAs-InGaAs core-shell NWs was recently also observed for high In-content InGaAs shells, where strain relaxation and associated mound formation occurs predominantly via the {11-2} corner facets.54


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Figure 5: (a) Cross-sectional STEM-HAADF image of a NW laser with a nominal In-molar fraction of 30%, grown at a shell temperature of 420 °C. (b) EDXS linescan taken along the blue dashed line, revealing more abrupt QW-barrier interfaces and a significantly increased In -molar fraction of ~24±1% in the MQWs. (c) Normalized lasing emission for a constant nominal In-molar fraction of 30% grown at different shell growth temperatures. The lasing energies show a substantial red shift from initially 1.39 eV for Tshell = 510 °C to 1.34 eV for Tshell = 460 °C and 1.18 eV for Tshell = 420 °C, respectively.

To identify how the lower shell growth temperature influenced growth kinetics and resulting In incorporation in the MQWs, a cross-sectional lamella for STEM analysis was prepared from NW lasers grown at a shell temperature of 420 °C. As shown in the STEMHAADF image of Figure 5(a) and Supporting Information, the morphology and hexagonal


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symmetry are well preserved and appear comparable to the sample grown at 510 °C. The total diameter was measured to be ~450 nm, i.e., in excellent agreement with the nominally expected diameter (462 nm), illustrating that the QWs and barriers exhibit accurate dimensions with high homogeneity. Subsequent EDXS analysis performed along the radial InGaAs-AlGaAs/GaAs heterointerfaces (i.e., blue dashed line across the {1-10} facet) reveals two marked differences in the alloy compositional profile as compared to the NW laser structure grown at higher temperature (cf. Figure 4(b) and Figure 5(b)). First, the characteristic peaks of the Ga elemental profile in the GaAs interlayers are much sharper and more pronounced. This indicates a more abrupt heterointerface between the AlGaAs barriers and adjacent GaAs interlayers, and likely also decreases alloy intermixing at the QW-barrier interfaces as the lower growth temperature reduces atomic interdiffusion.55 Secondly, and most importantly, the In-molar fraction in the MQWs increases to about ~ 24±1%. This is consistent with the expected reduction of In desorption at the lower growth temperature and, thus, the In-molar fraction is relatively close to the nominally expected value. Additional EDXS maps (illustrated in the Supporting Information) show further that at the {11-2} corner facets the In-molar fraction is even further increased up to ~27%. Overall, the increased In-molar fraction in the InGaAs MQWs is expected to lead to a more substantial shift in the lasing emission closer to the telecommunication bands. Consequently, pump-power dependent PL measurements were performed on this growth series, and the resulting lasing spectra at cryogenic temperature are summarized in Figure 5(c). Clearly, the lasing energies are further red-shifted. i.e., from ~1.39 eV for NW lasers grown at Tshell = 510 °C to ~1.34 eV for Tshell = 460 °C and 1.18 eV for Tshell = 420 °C, respectively. The NW-laser structure grown at even lower Tshell = 360 °C did not show lasing, likely due to severe strain relaxation54,56 as suggested by the SEM images in the Supporting Information. Nevertheless, compared to NW-lasers that use pure GaAs MQWs as emitters22, the present NW-lasers realize a lasing energy shifted by > 380 meV, i.e., > 250 nm to the red with the longest lasing wavelength obtained at around 1050 nm. From the respective L-L curves the lasing thresholds were also characterized on a large number of NW-lasers. Interestingly, despite the gradual increase in In-molar fraction, and hence longer wavelength emission, the lasing threshold does not increase on average within the experimental error in the range between 1.1 – 1.4 mJ/cm² (see Supporting Information). This behavior is quite different from the first series (shell growth temperature of 510°C, variable In-molar fraction), in which the lasing threshold was found to increase with higher In-molar fraction. We believe


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that the invariant lasing threshold of the second growth series is likely a result of the suppressed alloy intermixing. Further shifting of the lasing emission to longer, Si-transparent wavelengths, will require additional optimization of growth parameters and strategies to avoid plastic relaxation. For example, one should consider strain-compensation layers (between core and active region), ultrathin NW cores33, and alternative barrier materials, e.g. InAlAs, which could be lattice-matched to InGaAs QWs. In conclusion, we investigated the lasing characteristics of individual GaAs(In,Al)GaAs core-multishell NWs under optical pumping. Specifically, we explored how NWQW lasing performance is impacted by InGaAs-QW/AlGaAs-barrier design principles and deviations from ideality as influenced by In-incorporation kinetics, alloy intermixing, and structural integrity. Using correlated scanning transmission electron microscopy (TEM), atom probe tomography (APT) and confocal photoluminescence (PL) spectroscopy, the growth mediated effects on the quality of the InGaAs QW were illustrated for two growth series with variable III/V ratio and temperature. Low-temperature shell growth conditions and GaAs interlayers surrounding the InGaAs QWs give rise to enhanced In-incorporation and reduced alloy intermixing in the MQW structure. Clear evidence of lasing (s-shaped LL-curve, SE

clamping and linewidth-narrowing) was found from the InGaAs MQW active region with lasing emission tunable over a wide spectral range. Particularly, the lasing emission was redshifted by ~400 meV to 1.18 eV (1050 nm) in GaAs-InGaAs MQW NW lasers, compared to pure GaAs-AlGaAs MQW NW lasing structures. These results pave the way for future alloptical on-chip communication platform at a favorable low-loss range in the telecommunication wavelength regime.


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ASSOCIATED CONTENT Supporting

Information.

The

following

files

are

available

free

of

charge.

Details on FDTD simulations, MBE growth parameters, detailed lasing statistics, STEMEDXS data analysis, and additional APT data. (PDF)

AUTHOR INFORMATION Corresponding Authors * [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported financially by the Deutsche Forschungsgemeinschaft (DFG) via Project Grants FI 947/4-1 and KO 4005/7-1, the Nanosystems Initiative Munich (NIM), the International Graduate School of Science and Engineering (TUM-IGSSE), and the IBM International Ph.D. Fellowship Program. M. O. H. and L. J. L. further acknowledge support by NSF DMR-1611341. M.O.H. acknowledges support of the NSF GRFP. Atom-probe tomography was performed at the Northwestern University Center for Atom-Probe Tomography (NUCAPT). The LEAP tomograph at NUCAPT was purchased and upgraded with grants from the NSF-MRI (DMR-0420532) and ONR-DURIP (N00014-0400798, N00014-0610539, N00014-0910781, N00014-1712870) programs. NUCAPT received support from the MRSEC program (NSF DMR-1720139) at the Materials Research Center, the SHyNE Resource (NSF ECCS-1542205), and the Initiative for Sustainability and Energy (ISEN) at Northwestern University.


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For Table of Contents Use Only Tuning lasing emission towards long wavelengths in GaAs(In,Al)GaAs core-multishell nanowires T. Stettner1, A. Thurn1, M. Döblinger2, M. O. Hill3, J. Bissinger1, P. Schmiedeke1, S. Matich1, T. Kostenbader1, D. Ruhstorfer1, H. Riedl1, M. Kaniber1, L. J. Lauhon3, J. J. Finley1, and G. Koblmüller1


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Nano Letters

FIG1 138x68mm (150 x 150 DPI)


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

FIG2 129x61mm (150 x 150 DPI)


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Nano Letters

FIG3 131x127mm (150 x 150 DPI)


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

FIG4 146x108mm (150 x 150 DPI)


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Nano Letters

FIG5 65x164mm (150 x 150 DPI)


Tuning Lasing Emission toward Long Wavelengths in GaAs-(In,Al

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