Letter pubs.acs.org/NanoLett
Quantum-Size-Controlled Photoelectrochemical Fabrication of Epitaxial InGaN Quantum Dots Xiaoyin Xiao,† Arthur J. Fischer,† George T. Wang,† Ping Lu,† Daniel D. Koleske,† Michael E. Coltrin,† Jeremy B. Wright,†,‡ Sheng Liu,†,‡ Igal Brener,† Ganapathi S. Subramania,† and Jeffrey Y. Tsao*,† †
Solid-State Lighting Science Energy Frontier Research Center and ‡Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, New Mexico 87185, United States S Supporting Information *
ABSTRACT: We demonstrate a new route to the precision fabrication of epitaxial semiconductor nanostructures in the sub-10 nm size regime: quantum-size-controlled photoelectrochemical (QSC-PEC) etching. We show that quantum dots (QDs) can be QSC-PEC-etched from epitaxial InGaN thin films using narrowband laser photoexcitation, and that the QD sizes (and hence bandgaps and photoluminescence wavelengths) are determined by the photoexcitation wavelength. Lowtemperature photoluminescence from ensembles of such QDs have peak wavelengths that can be tunably blue shifted by 35 nm (from 440 to 405 nm) and have line widths that narrow by 3 times (from 19 to 6 nm). KEYWORDS: Quantum dots, InGaN, photoelectrochemical etching, quantum-size effects
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corresponding to the calculated11 0 K absorption edge of an In0.13Ga0.87N QD as a function of its diameter. In the wavelength range 420−430 nm, a change in photoexcitation wavelength of 1 nm corresponds to a change in QD diameter of approximately 0.1 nm. In other words, though the nanofabrication process uses light, its resolution is not determined, as is usually the case in lithography, by the spatial imaging resolution (hence by the shortness of the optical wavelength) of the optical source. Instead, it is determined to first order by the spectral line width of the optical source (which can be quite narrow) convolved with the homogeneous absorption line widths of the QDs themselves (assuming the PEC etch rate of each QD depends only on its own absorption and thus terminates when it itself stops absorbing, independent of the etch progress of the other QDs). Note, though, that homogeneous absorption line widths of QDs, particularly in InGaN materials, are complicated by many factors, including point/line/surface defects, size/shape/strain dependent polarization fields, and Urbach tails. In the experiments we report here, QSC-PEC etching was used to realize InGaN QDs of controlled size starting from epitaxial InGaN thin films. The wide-bandgap III-nitrides are of broad interest in electronics and optoelectronics,12−14 many of whose device functionalities would benefit enormously from nanostructures in the quantum-size regime.15,16 For example, single QDs resonantly coupled to high-Q microcavities would enable high-performance single-photon sources for quantum communications,1 while monodisperse ensemble QD gain media would enable lower threshold and higher efficiency
he past two decades have seen an explosion of interest in semiconductor nanophotonics and nanoelectronics,1−3 a large fraction enabled by controlled fabrication of epitaxial semiconductor nanostructures. At the current frontier are nanostructures in the sub-10 nm size regime; however, precise control in that size regime is extremely difficult. Coincidentally, the sub-10 nm size regime is comparable to the exciton Bohr radius in most semiconductors and thus is also the regime in which semiconductor nanostructures exhibit quantum-size effects.4 If some aspect of a nanofabrication process were sensitive to such quantum-size effects, that process might be used to control nanostructure distribution in size much more precisely than current processes can. In fact, in pioneering work in the 1990s Yoneyama5,6 and others7,8 showed that quantumsize effects could be used to size-selectively photoetch “nonepitaxial” (colloidal) quantum dots (QDs) in solution. Here we show for the first time that quantum-size effects can be used to control the fabrication of epitaxial nanostructures with significant implications for the realization of a broad range of future nanoelectronic and nanophotonic devices. The process we demonstrate is quantum-size-controlled photoelectrochemical (QSC-PEC) etching, whose initial step is semiconductor surface oxidization by photoexcited holes.9,10 As illustrated in Figure 1a, photoexcitation depends on light absorption; light absorption depends on bandgap; and in the quantum-size regime bandgap depends on nanostructure size. Thus, properly selected narrowband light will be absorbed by large but not small nanostructures, and PEC etching can be self-terminated at a size determined by the wavelength of that narrowband light. The sensitivity of nanostructure size to that wavelength and, therefore, the precision that can be achieved is illustrated in Figure 1b. There, we show the photoexcitation wavelength © XXXX American Chemical Society
Received: June 9, 2014 Revised: August 15, 2014
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Figure 1. Quantum-size-controlled (QSC) photoelectrochemical (PEC) etching scheme. (a) How quantum-size effects can be used to control photoelectrochemical (PEC) etching. (b) QD diameter dependence of the photoexcitation wavelengths that correspond to the calculated 0 K absorption edge (exciton formation energy) of idealized spherical In0.13Ga0.87N QDs embedded in GaN.
First, the initial rate is low due to the known resistance of the Ga-terminated (0001) top surface against etching.22 We exclude the possibility that intermediate products are formed that then catalyze dark reactions, as the dark etch rate in the absence of photoexcitation (the “off” portions of the etch-rate curve in Figure 2) is negligible throughout the life cycle. Second, the rate increases steadily as etching proceeds nonuniformly and more surface area is exposed (some which is non-(0001) oriented and etches at a faster rate). Third, the etch rate ultimately peaks then decays due to a reduction in the volume of absorbing InGaN, a decrease in the remaining InGaN surface area, and a decrease in light absorption due to the increase in bandgap of the InGaN nanostructures. For the conditions of Figure 2, the cumulative charge collected asymptotes to that corresponding to etching of ∼91% of the InGaN thin film (assuming the number of electrons transferred per unit In0.13Ga0.87N is 3, consistent with the halfreaction 3h+ + GaN → Ga3+ + (1/2)N2 in which the oxidation state of N is increased from −3 to 0 at the anode), indicating that some unetched InGaN remains. To assess this unetched InGaN, we performed atomic force microscopy (AFM) measurements at the end of two PEC etching life cycles, one for a photoexcitation wavelength of 445 nm and one for a higher-energy photoexcitation wavelength of 420 nm (the conditions of Figure 2). That remaining InGaN, as seen in the AFM images in Figure 3a,b, is typically composed of two superimposed distributions of nanodots. The first distribution is sparse (∼15−30 μm−2) and composed of larger (∼4−10 nm high) dots. These large nanodots are In0.13Ga0.87N, as confirmed by scanning transmission electron microscope (STEM) energy-dispersive X-ray spectroscopy (EDS) mapping, and persist even for laser excitation at wavelengths so short (405 nm) that the thick GaN underlayer itself begins to PEC etch. We speculate that these large nanodots coincide with surface-intersecting dislocations or other defects that either repel carriers or at which carrier recombination is extremely fast23 and PEC etching relatively slower. The second distribution is dense (∼1100 μm−2) and composed of smaller QDs located randomly at both terraces and step edges. These small QDs are also In0.13Ga0.87N and are quite sensitive to PEC etching conditions. As illustrated in Figure 3c, at a laser excitation wavelength of 445 nm (red curve) the best-fit Gaussian QD height distribution is centered at 3.3 nm with a fwhm of 2.4 nm, while at a laser excitation wavelength of 420 nm (blue curve), the best-fit Gaussian QD height distribution is centered at 1.0 nm with a fwhm of 1.1 nm. Note that the shift in the height distributions, larger QDs for
visible and UV lasers of interest for displays, optical storage, and ultraefficient and smart solid-state lighting.17 We emphasize, though, that the principle of QSC-PEC fabrication of epitaxial nanostructures should be widely applicable to other semiconductors10 and not limited to the InGaN example discussed here. Our initial set of samples consisted of thin (3−20 nm) epitaxial In0.13Ga0.87N films grown on c-plane GaN/sapphire under conditions18 similar to those used for state-of-the-art InGaN light-emitting diodes. The samples had metallic In electrodes applied to the underlying GaN and were then suspended in a PEC cell for etching. For the PEC etching, we mostly follow other work19−21 except in two respects. First, for the working electrolyte we used an H2SO4 aqueous solution rather than the more common KOH aqueous solution,19,21 which enabled us to eliminate etching in the absence of light. Second, for photoexcitation we used a tunable, relatively narrow-band (∼1 nm line width) laser source, following recent reports that such a source enables PEC etch sensitivity to fine absorption features.20 To elucidate the electrochemistry during PEC etching, we measured the current and charge collection dynamics over a PEC etching life cycle at a photoexcitation wavelength of 420 nm. That life cycle is shown in Figure 2 and follows a characteristic pattern: first, an ∼100 s incubation time; second, a steady increase culminating in a sharp peak; and third, a decay, initially quick but with a long tail. The mechanisms at play during this life cycle, inferred from atomic force microscopy snapshots in time, appear to include the following.
Figure 2. PEC etching of InGaN/GaN structures. Current-density versus time (j−t) and charge-density versus time (σ−t) dynamics over a PEC etching life cycle (potential fixed at 0.9 V) of an epitaxial In0.13Ga0.87N thin film using λ = 420 nm photoexcitation. The current density goes to zero when the laser photoexcitation is turned off at 100 and 600 s, indicating a zero dark etch rate. B
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Figure 3. AFM images of QDs etched using photoexcitation at (a) λ = 445 nm and (b) λ = 420 nm. (c) Probability densities over QD heights inferred from those AFM images: solid circles are AFM measurements and dashed lines are best-fit Gaussian distributions.
Figure 4. STEM of QSC-PEC-etched InGaN/GaN structures. (a) STEM-HAADF images of InGaN QDs etched using λ = 445 nm photoexcitation. The red box shows the region where (b) STEM-EDS mapping was done (red has been scaled to In0.13Ga0.87N, green has been scaled to pure GaN). (c,d) Higher-resolution STEM-HAADF images of QDs etched using photoexcitation at (c) λ = 445 nm and (d) λ = 420 nm, showing the epitaxial nature of the InGaN QDs.
In0.13Ga0.87N QDs with bottom and top surfaces passivated by In0.02Ga0.98N and GaN, respectively. Except for an additional incubation period, PEC etching to form these buried QDs appears to be similar to PEC etching to form the surface QDs of Figures 2, 3, and 4. STEM and STEM-EDS images of one such buried QD are shown in Figures 5a and 5b, showing that it is indeed epitaxial with the underlying In0.02Ga0.98N and overlying GaN. Though not fully passivated, these samples do exhibit much (∼100x) higher PL intensities (Figures 5c and d) than the samples shown in Figure 4. Figure 5c shows low-temperature micro-PL from a ∼1 μm2 region containing an ensemble of the order of 103 QDs. The PL spectra blue shift (from 440 to 405 nm) with decreasing PEC photoexcitation wavelength (from 440 to 410 nm), consistent with a controlled, tunable decrease in effective QD diameter (from ∼9 to ∼2.5 nm), and too much to be associated with other possible explanations such as elastic strain relaxation. The PL spectra also narrow (from Δλfwhm ∼19 to 6 nm), consistent with a narrowing of the QD size distribution. Note that our narrowest observed PL line width (Δλfwhm ∼ 6 nm) and implied distribution of QD sizes is much narrower than both those typical for conventional Stranski-Krastanov grown epitaxial InGaN QD ensembles (Δλfwhm ∼ 30−40 nm)15,26 and those for size-selective photoetching of nonepitaxial (colloidal) II−VI QDs (Δλfwhm ∼ 23.5 nm).27 However, they are not narrower than those observed for tightly lithographically controlled selective-area-epitaxial InGaAs QDs.28 We emphasize, though, that our results are a first rather than an optimized demonstration, are for the less mature InGaN materials system, and do not require such tight lithographic control. Figure 5d shows low-temperature micro-PL from a much smaller ensemble of the order of 10’s of QDs. This ensemble was created by prepatterning (e-beam lithography and reactive ion etching) the sample into 150 nm diameter nanoposts before PEC etching (at 420 nm) and thus is composed of a
laser excitation at 445 nm and smaller QDs for laser excitation at 420 nm, is qualitatively consistent with the quantum-sizecontrolled bandedge shift shown in Figure 1 for In0.13Ga0.87N with bulk bandedge at 450 nm. However, it differs quantitatively, likely because these QDs are not spherical but are flattened laterally (as seen below in Figure 4). To assess at a higher level of detail the second distribution of small QDs, we imaged cross sections of these QDs using highangle annular dark-field aberration-corrected cross-sectional scanning transmission electron microscopy (STEM-HAADF). An ensemble of eight similarly sized QDs is shown in Figure 4a. An energy-dispersive X-ray spectroscopy (STEM-EDS) map of a smaller ensemble of four QDs is shown in Figure 4b, verifying that the QDs are indeed In0.13Ga0.87N. Atomically resolved STEM-HAADF images of QDs PEC etched at photoexcitation wavelengths of 445 and 420 nm are shown in Figure 4c,d, respectively. These atomically resolved images show that the In0.13Ga0.87N QDs are epitaxial to the underlying GaN. They appear to be unfaceted, unlike InGaN QDs formed by growth, but it is possible that crystallographic dependencies24 would be observed under closer examination or for other PEC etch conditions. Most importantly, they decrease in size with decreasing photoexcitation wavelength, consistent with the quantum-size-controlled nature of the PEC etch. Photoluminescence (PL) intensities measured from these QDs were relatively weak, however, perhaps due to the unpassivated surfaces and the lack of a low-In-content InGaN underlayer whose presence is well-known to improve radiative efficiency.25 To improve the PL intensities, another set of samples was prepared, similar to the set just discussed except with a low-In-content In0.02Ga0.98N epitaxial underlayer beneath, and a thin (10 nm) epitaxial GaN cap on top of the now-buried In0.13Ga0.87N layer. The thin GaN cap is somewhat permeable to the PEC etch, etching in localized spots and exposing the underlying In0.13Ga0.87N. The In0.13Ga0.87N is then laterally etched underneath the GaN to create buried C
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transport, subsequent passivation of exposed surfaces, and on the semiconductor material itself. However, we believe this work has the potential to open up a very large application space that can make use of self-limiting subtractive processes to control nanostructure distributions precisely both in size and space.
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ASSOCIATED CONTENT
* Supporting Information S
Description of sample preparation by metal−organic vaporphase epitaxy (MOVPE), PEC etching conditions, and AFM, STEM, and PL measurements. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: 505-844-7092. Fax: 505844-4045. Author Contributions
Figure 5. QSC-PEC etching of GaN/InGaN/GaN structures. (a) STEM-HAADF image and (b) STEM-EDS map of QDs etched using 420 nm laser light (red = In0.13Ga0.87N, yellow = In0.02Ga0.98N, green = GaN). The black regions in (a) and (b) are the absence of material. (c) Micro-PL spectra at 5 K from ∼1 μm2 areas fabricated using laser photoexcitation at the indicated wavelengths, as well as from an unetched sample. The PL fwhm spectral widths from these large ensembles of QDs were (6, 8, 11, 19 nm) for photoexcitation wavelengths (410, 420, 430, 440 nm), respectively, and 25 nm for the unetched sample. The Fabry−Perot fringes in the PL spectra are due to optical thin-film interference effects caused by reflection at the underlying GaN/sapphire interface. (d) Micro-PL spectra at 5 K from smaller ensembles of QDs fabricated using a laser photoexcitation wavelength 420 nm on small (150 nm diameter) prepatterned nanoposts. The spectrum shows both discrete lines indicative of single QDs as well as a broader background indicative of an ensemble of QDs.
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. X.X., A.J.F., G.T.W., and J.Y.T. designed the experiments, interpreted the data, and drafted the manuscript. X.X. conducted PEC etching. X.X. and G.T.W. did the AFM analyses of the QDs. A.J.F. conducted photoluminescence measurements. D.D.K. prepared the epitaxial films. P.L. conducted TEM measurements. X.X., A.J.F., I.B., G.T.W., and J.Y.T. helped conceive the idea and coordinate measurements. J.B.W and S.L. helped with PL measurements. G.S. designed and prepared samples. M.E.C. performed QD energy calculations. All contributed to editing the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was primarily supported by Sandia’s Solid-State Lighting Science Energy Frontier Research Center, funded by the U.S. Department of Energy, Office of Basic Energy Sciences. The portion of the work which led to single QD emission was funded by Sandia’s Laboratory Directed Research and Development program. Portions of this work were also performed at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. We thank S.A. Casalnuovo, W. Pan, R. Polsky, G. Montano, F. Leonard, J.E. Martin, J.J. Wierer and R.P. Schneider (Sandia Labs) for helpful discussions. We thank C. Weisbuch (UC Santa Barbara) for encouragement, and especially thank Professor T. Torimoto (Nagoya University) for helpful insights.
dramatically reduced number of QDs on top and near the center of the nanoposts. Micro-PL measurements using a 1 μm diameter pump spot centered on one post show emission from single QDs. Figure 5d shows these as sharp, resolution-limited (Δλfwhm < 0.3 nm) spikes at 5 K, spikes that disappear as the temperature is increased to ∼50 K. Though these spikes appear on a broader background from a smaller ensemble of QDs on one nanopost, it should ultimately be possible with QSC-PEC etching to fabricate both single or arrayed QDs within single or arrayed nanoposts or nanowires, thereby potentially achieving the holy grail of simultaneous precise spatial and spectral matching of QDs to optical cavities within a nanophotonic structure.29 In summary, we have demonstrated quantum-size-controlled photoelectrochemical (QSC-PEC) etching, a novel new class of nanofabrication process in which quantum-size effects are used in a self-consistent and self-limited manner to fabricate epitaxial semiconductor nanostructures on surfaces and in thin films. InGaN QDs with dimensions
