Deterministic Placement of Quantum-Size Controlled Quantum Dots

Sep 11, 2017 - Semiconductor quantum dots (QDs) have become an integral light source in the fields of nanophotonics and quantum science.1−11 QDs pos...
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Letter pubs.acs.org/journal/apchd5

Deterministic Placement of Quantum-Size Controlled Quantum Dots for Seamless Top-Down Integration Arthur J. Fischer,† P. Duke Anderson,†,‡ Daniel D. Koleske,† and Ganapathi Subramania*,†,§ †

Sandia National Laboratories, Albuquerque, New Mexico 87185, United States Ming Hsieh Department of Electrical Engineering, University of Southern California, Los Angeles, California 90089, United States § Department of Electrical and Computer Engineering, University of New Mexico, Albuquerque, New Mexico 87131, United States ‡

ABSTRACT: We demonstrate a new route toward the integration and deterministic placement of quantum dots (QDs) within prepatterned nanostructures. Using standard electron-beam lithography (EBL) and inductively coupled plasma reactive-ion etching (ICP-RIE), we fabricate arrays of nanowires on a III-nitride platform. Next, we integrate QDs of controlled size within the prepatterned nanowires using a bandgap-selective, wet-etching technique: quantum-size-controlled photoelectrochemical (QSC-PEC) etching. Low-temperature microphotoluminescence (μ-PL) measurements of individual nanowires reveal sharp spectral signatures, indicative of QD formation. Further, internal quantum efficiency (IQE) measurements reveal a near order of magnitude improvement in emitter efficiency following QSC-PEC etching. Finally, second-order cross-correlation (g(2)(0)) measurements of individual QDs directly confirm nonclassical, antibunching behavior. Our results illustrate an exciting approach toward the top-down integration of nonclassical light sources within nanophotonic platforms. KEYWORDS: quantum dots, InGaN, photoelectrochemical etching, nanophotonics, photoluminescence, antibunched

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photonic applications.3,25−29 However, two principal challenges encountered when using either synthesis approach are (1) deterministic placement and (2) size control.3 Bottom-up growth techniques such as the Stranksi-Krastanov (SK)8 growth result in III-nitride QDs with superior surface quality due to the natural in situ passivation provided by the subsequently grown semiconductor layers. Nevertheless, the self-assembled nature of such QDs result in large uncertainty in QD location and significant fluctuations in QD size.3 As a result, integrating bottom-up growth techniques with nanophotonic designs, such as high-quality factor (Q-factor) microcavities, often involve complex alignment procedures and can face low device yields.3,30−32 Recently, a top-down wetetch approach33,34 has been demonstrated for the fabrication of III-nitride QDs using quantum-size-controlled photoelectrochemical (QSC-PEC) etching. This approach has the advantage of starting with a pregrown planar epitaxial III-nitride film and subsequently wet-etching the film to fabricate monodisperse QDs of desired size (controlled emission wavelength) in potentially desirable locations. Consequently, this approach is more amenable to subsequent photonic device integration and higher device yields. Previously, low temperature timed wetetching techniques have been used to create self-aligned quantum dots in III−V multiquantum well (MQW) photonic crystal systems.35

emiconductor quantum dots (QDs) have become an integral light source in the fields of nanophotonics and quantum science.1−11 QDs possess several advantages unique to their artificial, atomic-like construction. First, owing to their size, which is on the order of a semiconductor’s exciton Bohr radius, quantum-size effects grow prominent in QD nanostructures.12 Consequently, QDs contain discrete energy levels that determine their absorption/emission spectra. Second, QDs have demonstrated superior internal quantum efficiencies (IQEs) when compared with more traditional semiconductor emitters, such as quantum wells (QWs).13,14 The increased quantum efficiency is attributed primarily to increased carrier confinement and reduction in material strain.13,15 Recent theoretical work has also suggested QDs may be superior gain media, owing to their delta-like electronic density of states.16 Finally, and of fundamental importance, QDs enable functionality that is largely unachievable with classical light sources. In particular, QDs can exhibit single photon emission and photon antibunching.1,3,9−11,17−19 Such nonclassical photoemission processes are integral to the fields of quantum encryption, computing and metrology.18,20−22 Currently, several nanofabrication approaches exist for producing semiconductor QDs, with many of the methods relying upon solution-based synthesis techniques3,19,23,24 that are mostly suitable for incorporation on the surface of photonic structures. On the other hand, nanofabrication techniques that either involve bottom-up self-assembly or top-down synthesis enable the introduction of QDs inside the semiconductor and, thus, are more compatible with semiconductor-based nano© XXXX American Chemical Society

Received: July 17, 2017

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of light. A cathodic In electrode is bonded to the III-nitride sample while an anodic Pt electrode is suspended in the electrolyte solution. During the PEC etch, electron−hole pairs are generated using a relatively narrow line width laser (Δλfwhm ∼ 1 nm). Due to the applied electric potential, photoexcited holes are attracted to the material surface where they contribute to surface oxidation. As the size of the QW region reduces, quantum size effects emerge and begin to increase the electronic bandgap of the structure. Eventually, a QD is formed and the bandgap becomes too large for light to be absorbed, thereby self-terminating the etch process in the III-nitride sample. Thus, properly selected narrowband light can be absorbed by large nanostructures but not by smaller nanostructures. In this manner, the size of the resulting nanostructures is determined by the wavelength of the incident light. The epitaxial structures used in this work were grown on cplane GaN/Sapphire using metal−organic chemical vapor deposition (MOCVD). The wafer consists of a nominally 4.7 μm thick n-GaN buffer layer grown on an Al2O3 substrate. The thick n-GaN growth is followed with a 185 nm thick In0.04 Ga0.96 N underlayer, a single 2.7 nm thick In0.15 Ga0.85 N QW layer and, finally, a 30 nm cap of n-GaN. For an unpatterned substrate, the etchant solution can enter through micro/ nanoscale growth defects present in relatively thin GaN capping layers. The QSC-PEC etching of the QW then proceeds predominately laterally, ultimately creating a nonuniform distribution of QDs with nonpreferential positioning. In the present work, however, we focus our attention on the fabrication of QDs within predefined locations (nanowires) utilizing electron beam lithography (EBL) patterning. Precise positional accuracies of sub-100 nm for placement of functional nanoscale objects can be achieved with EBL.40 The top-down nanowire fabrication process is illustrated schematically in Figure 2. We begin by spinning on

QSC-PEC etching utilizes a light-assisted wet-etching technique whose initial step is surface oxidation by photoexcited holes.36,37 The photoexcitation depends on absorption, with absorption depending on the bandgap of the material and the bandgap depending on nanostructure size. Thus, QSC-PEC etching is a technique that self-terminates after quantum-size effects begin to suppress absorption and the subsequent photogeneration of carriers. Previous work demonstrated the feasibility of QSC-PEC etching on an unpatterned III-nitride platform, containing an uncapped InxGa1−xN QW.34 As a consequence, the QDs exhibited no preferential in-plane placement. For the first time, we demonstrate deterministic positioning of QDs by first fabricating a series of III-nitride nanowires. We integrate QDs of controlled size (and hence electronic bandgap energy) within the prepatterned nanowires using QSC-PEC etching. An illustration of one fabricated nanowire is depicted in Figure 1a. In the figure, the QSC-PEC etching technique has

Figure 1. (a) Illustration of a single QD emitting (violet) near the center of a QSC-PEC etched nanorod. (b) Schematic of the QSC-PEC etching setup. A tunable laser (blue) excites carriers in a III-nitride sample (green) that is partially submerged in an electrolyte solution (clear).

removed the bulk of an axial QW structure, leaving behind a single InxGa1−xN QD near its center. Low-temperature microphotoluminescence (μ-PL) measurements of fabricated nanowires reveal sharp spectral signatures limited by the spectrometer resolution (Δλfwhm < 0.3 nm), indicative of QD formation. Additionally, IQE measurements reveal a near order of magnitude improvement in emitter efficiency following QSC-PEC etching. Finally, second-order cross-correlation (g(2)(0)) measurements of isolated QD photoluminescence (PL) reveal photon antibunching (g(2)(0) ∼ 0.5). Our results illustrate an exciting method for the top-down integration of nonclassical light sources within prepatterned nanophotonic platforms. Further, we note that the diameters of our fabricated nanowires are comparable to those of defect regions in IIInitride L1 photonic crystal (PhC) cavities.38,39 Consequently, our results indicate real potential for the seamless, top-down incorporation of QDs within prefabricated microcavities. The precise spatial and spectral matching of QDs within high-Q microcavities promises to be of great use to the fields of quantum science and nanophotonics. III-Nitride materials are inherently difficult to wet-etch. However, light-assisted photoelectrochemical (PEC) etching has been shown to greatly improve the etching of such material systems.33,34 A typical PEC etch setup is illustrated in Figure 1b. In the setup, a III-nitride sample is immersed in an electrolyte solution. Here, we select H2SO4 as our aqueous electrolyte, allowing us to avoid material etching in the absence

Figure 2. Nanowire fabrication: (a) EBL patterning and development, (b) Ni deposition and lift-off, (c) Cl-based dry etch, and (d) QSCPEC etch. Green indicates GaN, blue indicates PMMA, silver indicates Ni, and a black line indicate a single, axial InGaN QW. Fabricated (e) nanowire array and (f) single nanowire. Black scale bars represent physical lengths of 500 nm. A white arrow indicates the position of a QSC-PEC etched QW. B

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poly(methyl methacrylate) (PMMA) resist and patterning a series of holes using EBL. The pattern consists of a square array of holes, approximately 100−200 nm in diameter with pitches of 1 and 2 μm. Following development, the openings are filled with approximately 30 nm of Ni via electron beam evaporation. Next, Ni islands are formed by ultrasonicating the sample in an acetone bath. Lift-off is followed by an inductively coupled plasma (ICP) etch utilizing a Cl2/BCl3-based chemistry, with the Ni islands functioning as a hard mask. Following the Cl2/ BCl3-based dry etch, the remaining Ni is removed in a dilute solution of H2SO4. Next, approximately half of our patterned sample is suspended in the PEC setup shown in Figure 1b. Partial sample immersion permits the comparison of unetched regions (QWs) to QSC-PEC etched regions (QDs) later in our study. During the QSC-PEC etch, our photoexcitation source consists of a frequency-doubled Ti:sapphire laser with a 82 MHz repetition rate and a 2 ps pulse width, producing an average power of 30 mW at a wavelength of 420 nm. The excitation wavelength corresponds to an energy lower than the bandgap of GaN but above that of the InGaN QW, enabling selective PEC etching of the QW. The QSC-PEC etch was performed for 90 min. At this point in the etch process, the etch current converged to a few nA, indicating that no appreciable etching was occurring and that the etch process had terminated. Figure 2e shows a scanning electron micrograph (SEM) of a fabricated nanowire array. In the figure, nanowires have nominal dimeters of 200 nm and are separated by 1 μm. Figure 2f shows a magnified SEM of a single QSC-PEC etched nanowire. A white arrow in the figure indicates the position of an etched InGaN QW. We obtained spectroscopic information on etched nanowires by performing low-temperature μ-PL measurements. The device sample was housed in a cryostation that was nominally cooled to a temperature, T = 10 K. The sample was optically excited (using the same tunable laser) at a wavelength of 375 nm and average power of 5 μW. A 50× near-UV Mitutoyo objective focused the laser to a spot size of nearly 1 μm2 on the sample. The device was imaged using the same objective and directed into a 0.3 m focal length spectrometer with an attached liquid N2 cooled charge-coupled device (CCD) camera. Emission from a QSC-PEC etched nanowire with a diameter of 200 nm is shown in Figure 3a. In the figure, three peaks are observed at wavelengths of 411.2, 413.4, and 417.9 nm, with the third peak being most prominent, exhibiting an intensity nearly 5× larger than the other two peaks (after background PL correction). The narrow line widths associated with the peaks (Δλfwhm < 0.3 nm) are indicative of QD-like emission signatures. Further, the original single QW had a PL emission wavelength of 450 nm (data not shown), corresponding to an In fraction of nearly 15%. However, following QSC-PEC etching, a significant blue shift in the emission wavelength (λ ∼ 415 nm) is observed. When moving from a QW to a QD, the emission blue shift results from an energy level increase due to increased carrier confinement. This is the fundamental principle behind QD formation using QCS-PEC etching, and is described in greater detail in ref.33 Multiple emission peaks, however, suggests the presence of multiple QDs within a single nanowire. One possible reason behind this could be due to the relatively large nanowire diameter (∼200 nm) compared to the GaN capping layer (∼30 nm). Consequently, the lateral PEC etch, which is the desired mechanism, competes with the vertical etching of

Figure 3. (a) Low-temperature μ-PL collected from a QSC-PEC etched nanowire with (a) multiple QDs and (b) a single QD. Dashed blue lines indicate the peak emission wavelengths of individual QDs.

nitride material that takes place through unavoidable threading dislocation defects in the GaN capping layer. These competing PEC etch directions may result in the formation of multiple QDs in a single nanowire. Furthermore, the spacing between neighboring nanowires (1 μm) is comparable to the spot-size diameter of our focused pump beam. Consequently, misalignment or slight defocusing may contribute to pumping more than one nanowire simultaneously. In order to address these challenges, we fabricated QSC-PEC etched nanowire arrays featuring smaller diameters (∼100 nm) and greater neighbor-to-neighbor spacing (2 μm). Figure 3b shows the low-temperature μ-PL associated with one such nanowire. In the figure, a single sharp peak with a narrow peak line width (Δλfwhm ∼ 0.3 nm) is observed at a wavelength of 414.0 nm. This peak is highly suggestive of single QD formation. Here, it should be noted that the isolated QD location has been fully controlled by the following considerations: (1) the vertical location of the QW in the original epitaxial structure (determined during growth) and (2) the cross-sectional location of the nanowire (determined during patterning). Therefore, careful selection of these parameters, in addition to the etch-assisted laser wavelength, offers a unique opportunity in spatially and spectrally coupling QSC-PEC etched QDs to prepatterned PhC cavities. For instance, L1 PhC cavities are an immediately attractive candidate, containing defect surface areas comparable with the nanowire diameters studied here. Further, our approach is top-down and highly sensitive to material absorption windows, allowing preferential etching of InGaN structures. Lastly, knowing the position of resonances preceding QSC-PEC etching provides direction in selecting the wavelength necessary for spectrally coupling the QD to a resonant cavity mode. To study the effect of quantum size control on emitter efficiency we performed IQE measurements using the same μPL setup. To measure QD IQE we considered large, unpatterned QSC-PEC etched areas (∼1 μm2) containing ensembles of QDs. Figure 4a shows the PL evolution of one such ensemble as a function of pump power (log scale) both at T = 10 and 300 K. The samples were pumped with increasing power until PL intensity saturated near an average input power of 45 mW. At low-temperature (T = 10 K) there is negligible C

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Figure 5. (a) Hanbury-Brown and Twiss (HBT) setup consisting of QD PL (blue) being split by a 50/50 beamsplitter (BS) and focused onto two, single-photon avalanche detectors (SPADs). Each signal is fed into a time-correlated single photon counting (TCSPS) module. (b) Second-order cross-correlation (g(2)(0)) of low-temperature PL collected from a single QD. The QD was pumped using a pulsed, tunable laser.

Figure 4. (a) Low-temperature and room-temperature PL collected from a QD ensemble as a function of input pump power. (b) IQE of a QD ensemble and QW as a function of input pump power.

phonon contribution and at higher pump intensities defectrelated nonrecombination is minimal. Therefore, at low temperature and high pump density, the recombination is assumed to be purely radiative and thus corresponds to perfect IQE: each absorbed pump photon leads to one emitted photon. Taking the ratio of saturated intensities at the two temperatures produces a 2.49× difference, indicating a QD IQE of nearly 40.2%. Figure 4b shows the IQE evolution as a function of pump power for the QD ensemble as well as an unetched (QW) region. The IQE of each structure again saturates near an average input power of 45 mW. When comparing the two IQEs at this pump power, the QDs exhibit a near order of magnitude improvement in IQE compared to the QW. The significant increase in IQE illustrates that a fundamental change to the original material has taken place. The IQE improvement can be attributed to the resulting strain relaxation within the emitting material in addition to increased carrier confinement in the QD compared to the QW. Another potential explanation for the increased IQE could be related to the proposed etch mechanism whereby PEC etching occurs through vertical threading dislocations propagating through the GaN cap layer. In this situation, QW material near the threading dislocation, which likely has reduced optical efficiency, will be preferentially etched away during the QD fabrication process. The motivation for creating site-controllable quantum dots is for subsequent utilization in a quantum light source. In order to study this, we investigate the photon statistics of isolated QDs using a Hanbury-Brown and Twiss (HBT) setup, illustrated in Figure 5a. The setup consists of low-temperature QD PL that is split into two paths using a 50/50 beamsplitter (BS). Each light path is optically focused onto two, single-photon avalanche detectors (SPADs). The signal from each detector is then fed into a timecorrelated single photon counting (TCSPC) module. Figure 5b shows the second-order cross-correlation (g(2)(0)) of lowtemperature QD PL collected under pulsed operation. At t = 0, there is a large reduction in photon counts, yielding a g(2)(0) ∼

0.5. The g(2)(0) result clearly indicates photon antibunching, a nonclassical behavior associated with single photon emission. However, low light-collection and potential background emission from defect GaN sites leads to large noise in the measurement. We believe incorporating QSC-PEC etched QDs into properly designed PhC defect cavities will enable simultaneously improvement of light-collection and emission rate enhancement, through the Purcell effect.41 Improving the emission signatures of QSC-PEC etched QDs using such structures could greatly improve signal-to-noise ratios and enable the development of room-temperature, single photon sources. In conclusion, we have presented a new method for the topdown integration of QDs in prepatterned nanostructures. By first fabricating nanowires from MOCVD-grown III-nitride material, we integrated QDs of controlled size and directed placement using a QSC-PEC etching technique. Low-temperature μ-PL measurements of individual nanowires revealed sharp spectral signatures, indicative of QD formation. Moreover, IQE measurements revealed a 10-fold improvement in emitter efficiency following QSC-PEC etching. Finally, secondorder cross-correlation (g(2)(0)) measurements of individual QDs directly confirmed photon antibunching and single photon emission. Our results illustrate an exciting route for the top-down integration of nonclassical light sources within nanophotonic platforms. We believe our method offers great potential for spatially and spectrally coupling QDs into future high-Q microcavities.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ganapathi Subramania: 0000-0002-6288-0344 D

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Author Contributions

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The manuscript was written through contributions of all the authors. All authors have given approval to the final version of the manuscript. A.J.F., P.D.A., and G.S. interpreted the data and drafted the manuscript. A.J.F. designed the experiments and conducted PEC etching and photoluminescence measurements. D.D.K. prepared the epitaxial films. P.D.A. and G.S. prepared samples and fabricated the nanowires. P.D.A. conducted SEM measurements. All authors 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 Laboratory Directed Research and Development Program. Portions of this work were performed at the Center for Integrated Nanotechnologies (CINT), a U.S. Department of Energy, Office of Basic Energy Sciences user facility. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under Contract DE-NA-0003525. We thank O. Spahn, X. Xiao, J. Y. Tsao, G. T. Wang, M. Crawford, M. Smith, and Anthony J. Coley for helpful discussions.

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