Letter pubs.acs.org/journal/apchd5
Improving the Radiative Efficiency of InGaN Quantum Dots via an Open Top Cavity Brandon Demory,† Adam Katcher,‡ Tyler Hill,‡ Chu-Hsiang Teng,† Cheng Zhang,†,§ L. Jay Guo,† Hui Deng,‡ and P. C. Ku*,† †
Department of Electrical Engineering and Computer Science, The University of Michigan, Ann Arbor, Michigan 48109, United States ‡ Department of Physics, The University of Michigan, Ann Arbor, Michigan 48109, United States § Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland 20878, United States ABSTRACT: A self-aligned, highly tolerant “open top” cavity design is proposed and shown to enhance the spontaneous emission of InGaN quantum dots. Compared to the “closed top” counterpart, removing the metal cap, creating an “open top” cavity, enhances both the emission intensity and recombination rate. The localized surface plasmon resonance of the Ag “open top” cavity was matched to the InGaN quantum dots’ emission wavelength, resulting in an observed final emitter lifetime of 132 ps. The proposed cavity structure exhibits high cavity antenna quantum efficiency, can be self-aligned to the quantum dot, and has good tolerance to quantum dot emission wavelength, polarization, and processing variations. KEYWORDS: plasmonic coupling, metallic cavity, quantum dots, III-nitride, quantum efficiency, site control
S
η = η0
emiconductor single-photon emitters are a critical resource for quantum science and technology. Group III-nitride quantum dot (QD) based single-photon emitters have attracted much attention, as they can operate at room temperature1,2 and be designed and processed using mature semiconductor technologies, such as with p−n junctions for electrical injection. Single photons of deterministic linear polarizations have been recently demonstrated in this material system.3 However, due to the wurtzite crystal symmetry, strained GaN heterostructures exhibit a large piezoelectric field,4 which separates the electron and hole wave functions and slows down the rate at which ondemand single photons can be generated.5,6 For InGaN/GaN QDs with a moderate (15−20%) indium composition, the rate is only tens of MHz as compared to GHz typically seen in IIIarsenic QDs. For high-speed applications, it is necessary to increase the spontaneous emission rate of III-nitride QDs into the GHz range. The radiative lifetime of QDs can be reduced by coupling to optical cavities.7−14 Metallic cavities exhibit nanoscale dimensions and are more tolerant to QD emission wavelengths, owing to their broad resonances.13,15,16 Using an Ag coating on top of an InGaN QD emitter, namely, a “closed top” design, we have previously shown enhancements of both the radiative recombination and photon generation rates on the order of 10×.16 In this work, we propose an improved “open top” cavity with good tolerance to quantum dot emission wavelength, polarization, and processing variations, to further enhance the spontaneous emission. The radiative quantum efficiency (η) of the QD-cavity system is given by © XXXX American Chemical Society
1 1 − η0 Fp
+
η0 ηa
(1)
where η0 is the quantum efficiency of the bare QD without the cavity, Fp is the Purcell factor, and ηa is the antenna quantum efficiency (AQE) of the cavity.17,18 In the previous “closed top” experiment, η0 and η were estimated to be around 17% and 46%, respectively, with an average Fp of 46.16 A further increase of η can be achieved by an increased Fp, ηa, or both. The Purcell factor of the “closed top” design, shown in the inset of Figure 1a, is limited by the metal loss. The metal layer on top of the GaN nanopillar increases the absorption loss but contributes little in reducing the optical mode volume. Moreover, the presence of the metal cap may potentially interfere with a p−n junction and top metal contact structure for electrical injection. As a result, the removal of the metal cap (shown in the inset of Figure 1b) while keeping only the metal layer on the sidewall of the nanopillar is expected to increase the Purcell factor and improve the compatibility for electrical injection. Without the metal cap, the structure resembles a zero-mode waveguide with a small optical mode volume.19 This hypothesis was confirmed by the finitedifference-time-domain (FDTD) calculations,20 comparing the Purcell factor and AQE of the QD-cavity system with and without the metal cap. As shown in Figure 1, comparing the longer wavelength resonance peaks, there is about a 6-fold Received: February 14, 2017 Published: March 28, 2017 A
DOI: 10.1021/acsphotonics.7b00140 ACS Photonics XXXX, XXX, XXX−XXX
ACS Photonics
Letter
Figure 1. (a) Calculated Purcell factor (solid black line) and cavity AQE (blue dashed line) for the “closed top” (a) and “open top” (b) cavity designs. Both cavities have a 30 nm thick Ag layer (blue) and a 2 nm thick Al2O3 layer (yellow) between the Ag and the GaN nanopillar (red).
increase in the Purcell factor. The resonance wavelength shift is due to the change in the Ag film geometry. In addition, there is a 5−10% increase in the cavity AQE around this resonance, due to the removal of the metal cap layer. The fabrication procedure of the “open top” QD-cavity structure is illustrated in Figure 2. We used a site-controlled
Figure 3. SEM images of the “open top” cavity fabrication process. The scale bar is 300 nm. (a) Bare 48 nm diameter GaN/InGaN vertical nanopillars after wet etching. (b) After Al2O3 and Ag deposition. (c) After SiO2 planarization and etchback. (d) After removal of SiO2 and Ag top. The arrows indicate the exposed GaN tops.
Figure 2. Fabrication steps of the proposed “open top” cavity: (a) fabrication of InGaN/GaN QD nanopillars by lithography and etching; (b) coating of Al2O3 by ALD; (c) deposition of Al-doped Ag by sputtering; (d) planarization of the sample using SiO2; (e) etchback to expose Ag; (f) removal of Ag and Al2O3 top.
measurement was performed to record the “baseline” luminescence properties before Ag was coated. After the initial PL measurements, the sample was divided into two sections: an experimental section to receive the Ag cavity and a control section to remain unchanged throughout the remainder of the experiment. The control section was protected by a 3 μm thick photoresist layer for the remaining fabrication. For Ag deposition, sputtering was used to help with vertical sidewall coverage. It is known that it is hard to form high-quality Ag thin films due to its 3D growth model.25 This creates fabrication challenges in coating Ag films on the sidewall of QDs, which also may have a certain roughness from the previous lithography and etching processes. Due to the sensitive dependence of the localized surface-plasmon resonance (LSPR) on the Ag film thickness, the Ag film coverage is critical. To solve the above issue, Ag was co-sputtered with Al at a 6% ratio to reduce the grain size and improve the film coverage and uniformity over the pillar surface, whose details can be found in previous publications.26−28 The deposition resulted in 30 nm of Ag on the vertical sidewalls after 66 s of sputtering at a rate of 11.7 Å/s, which was confirmed using SEM and surface profilometry. Figure 3b shows a bird’s-eye view of the Ag-coated nanopillars. A second PL measurement
InGaN/GaN QD emitter made by lithographically defining and etching a single InGaN/GaN quantum well (QW) active region. The details of the QD fabrication can be found in previous references.5,21 The strain-induced quantum confinement allows the quantization effect to be possible, even with a QD dimension 10 times the exciton Bohr radius (∼3 nm in bulk GaN), as well as keeping the excitons away from the surface of the nanopillar structure.22 To minimize the cavity mode volume, we etched the tapered nanopillars for 35 min after dry etching using 2% buffered KOH solution (AZ-400 K).23,24 This wet etching process is anisotropic, resulting in a vertical nanopillar sidewall shown in Figure 2b. Figure 3a shows a tilted scanning electron microscope (SEM) image of the resulting 48 nm diameter QD nanopillar structure after etching. The nanopillar is 135 nm tall. After QD fabrication, the nanopillar was conformally coated with a 2 nm thick Al2O3 layer using atomic-layer deposition (ALD) at 250 °C with a rate of 1.1 Å/pulse. The refractive index of the Al2O3 was measured to be 1.65. At this point, a photoluminescence (PL) B
DOI: 10.1021/acsphotonics.7b00140 ACS Photonics XXXX, XXX, XXX−XXX
ACS Photonics
Letter
excitation-dependent measurement also ensured the luminescence property was analyzed when the QD was populated all the time, to simulate a CW measurement, as well as to rule out any contribution from the enhanced absorption of the pump laser. Figure 4 shows that the “closed top” Ag cavity enhances the QD emission intensity with respect to the control, at the saturation intensity of 2.12 kW/cm2, by a factor of 1.71. The “open top” cavity has an enhancement factor of 2.72. The corresponding emission spectra taken at saturation are shown in Figure 5. It is observed that while the emission wavelengths
was taken after Ag deposition to capture the effect of the closed Ag film on the luminescence properties versus the control section and to have a comparison for the “open top” Ag cavity later. Removal of the top section of Ag to create the “open top” cavity was done using planarization and etch-back. The structure was first planarized by evaporating a 550 nm thick SiO2 layer at an angle. After planarization, reactive ion etching (RIE) was used to etch back the SiO2 until the surface of the Ag film was exposed (Figure 3c). The final step was to remove the Ag and Al2O3 at the top of the nanopillar, using RIE with CF4 and Ar+ at 9 mTorr and 500 W. The CF4 both etched through SiO2 and created a byproduct with Ag.29 The Ar+ subsequently removed the Ag byproduct. The high-power and low-pressure RIE conditions are important to help maintain the vertical etching profile, since the structure requires removal of the top Ag surface without damaging the Ag on the sidewall or the substrate. Figure 3d shows the resulting “open top” structure. The white arrows highlight the GaN nanopillars, where it is easy to see that the Ag layer has been broken, as indicated by the image contrast under SEM. The oxide barrier was mostly etched away during this process as well since both RIE gas species attacked the oxide, leaving behind small SiO2 islands on the surface in between the nanopillars. The third PL measurement was taken at this stage. All PL and time-resolved PL (TRPL) measurements performed in this work were based on a confocal setup.5 All measurements were taken at a 10 K temperature. The QD nanopillars were optically excited from the front (metal side), at a 55° angle, using a femtosecond laser at a 390 nm wavelength. The PL signal was collected from the front, at the surface normal direction, using a 0.6 NA objective lens. On the basis of the laser beam size, we estimated the number of QD nanopillars being covered by the excitation laser beam was ∼10 000. Figure 4 shows the integrated PL intensity versus the excitation laser intensity measured at the three different fabrication stages described above. The measurements were repeated on both experimental and control sections at each stage. The data taken on the control section were used to verify whether the data from the experimental section, taken at different stages and different days, can be compared. The
Figure 5. Integrated PL intensity of the 48 nm diameter QD array at the saturation intensity (2.12 kW/cm2) measured at the three stages of the experiment. The “open top” Ag intensity is the largest and broadest, showing the improved quantum efficiency from 5% to 23%. The simulated Purcell factor for the experimental Al-doped silver cavity is shown by the dotted line.
from the control and the “closed top” cavity are nearly identical, the “open top” cavity exhibits a blue shift in its emission wavelength. This blue shift is attributed to the cavity pulling effect, as confirmed by the calculated Purcell factor using the measured dielectric constant of the Al-doped Ag shown by the dashed line in Figure 5.27 This Purcell factor calculation uses the experimental geometry, including the wet etched (vertical) pillar and the experimentally determined n and k parameters for the Al-doped Ag film, which are different from the calculation using the tapered pillar and empirical Ag data30 in Figure 1b. The emission wavelength approaches the local maximum in the enhancement. Because the QD array is inhomogeneously broadened, the QDs with resonances close to the peak (∼467 nm) of the Purcell factor are preferentially enhanced by a factor of 3.46 compared to the control at the same wavelength. Figure 6 shows the results from TRPL measurements using a streak camera. The analysis for QD lifetimes was performed on the initial part of the PL decay. Because the QDs were pumped at saturation, any change of the lifetime due to the screening of the piezoelectric field, as a result of the enhanced absorption of the pump laser, is expected to be the same for bare QDs and “closed top” and “open top” cavities in the initial part of the PL decay when the QDs are still mostly populated. If the QDs are not pumped at saturation, more piezoelectric field can be screened if the pump absorption is enhanced due to the Purcell effect, which will make the comparison of data difficult and inaccurate. In Figure 6, the lifetime was calculated by summing the PL counts, within the full width at half-maximum wavelength range of each trace in Figure 5, at each time delay and fitting the resulting trace with a biexponential function:
Figure 4. Measured PL intensity versus the laser excitation intensity for the 48 nm diameter QD array at the three stages of measurements: control (after Al2O3 deposition), “closed top” Ag (after Ag deposition), and “open top” Ag (after removal of Ag and Al2O3 top). The dotted black line corresponds to the laser intensity used for subsequent PL and TRPL measurements. The consistency of the PL intensity values was verified between measurement days using a control array.
a1e−t / τ1 + a 2e−t / τ2 C
(2) DOI: 10.1021/acsphotonics.7b00140 ACS Photonics XXXX, XXX, XXX−XXX
ACS Photonics
■
Letter
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +1(734)-764-7134. ORCID
Brandon Demory: 0000-0002-9674-958X Funding
This work was supported by National Science Foundation MRSEC Center for Photonic and Multiscale Nanomaterials (DMR 1120923). Notes
The authors declare no competing financial interest.
■
Figure 6. TRPL measured at the saturation intensity (2.12 kW/cm2) for the 48 nm diameter QD array. The integrated PL intensity versus time for the three measurement stages, control (blue), “closed top” (black), and “open top” (red), shows lifetime reduction at each measurement stage. The lifetimes are fit with a biexponential fitting. The lifetimes and exponential weights are (0.85) 108 ps and (0.17) 1480 ps for the control, (0.76) 105 ps and (0.24) 680 ps for the “closed top” cavity, and (0.77) 46 ps and (0.23) 420 ps for the “open top” cavity.
REFERENCES
(1) Holmes, M. J.; Choi, K.; Kako, S.; Arita, M.; Arakawa, Y. RoomTemperature Triggered Single Photon Emission from a III-Nitride Site-Controlled Nanowire Quantum Dot. Nano Lett. 2014, 14, 982− 986. (2) Deshpande, S.; Frost, T.; Hazari, A.; Bhattacharya, P. Electrically pumped single-photon emission at room temperature from a single InGaN/GaN quantum dot. Appl. Phys. Lett. 2014, 105, 141109. (3) Teng, C.-H.; Zhang, L.; Hill, T. A.; Demory, B.; Deng, H.; Ku, P.C. Elliptical quantum dots as on-demand single photons sources with deterministic polarization states. Appl. Phys. Lett. 2015, 107, 191105. (4) Takeuchi, T.; Sota, S.; Katsuragawa, M.; Komori, M.; Takeuchi, H.; Amano, H.; Akasaki, I. Quantum-confined Stark effect due to piezoelectric fields in GaInN strained quantum wells. Jpn. J. Appl. Phys. 1997, 36, L382. (5) Zhang, L.; Teng, C. H.; Hill, T. A.; Lee, L. K.; Ku, P. C.; Deng, H. Single photon emission from site-controlled InGaN/GaN quantum dots. Appl. Phys. Lett. 2013, 103, 5. (6) Zhang, L.; Lee, L. K.; Teng, C. H.; Hill, T. A.; Ku, P. C.; Deng, H. How much better are InGaN/GaN nanodisks than quantum wellsOscillator strength enhancement and changes in optical properties. Appl. Phys. Lett. 2014, 104, 5. (7) Purcell, E. M. Spontaneous Emission Probabilities at Radio Frequencies. Phys. Rev. 1946, 69, 681−681. (8) Rao, V.; Hughes, S. Single quantum dot spontaneous emission in a finite-size photonic crystal waveguide: Proposal for an efficient ″On chip″ single photon gun. Phys. Rev. Lett. 2007, 99, 193901. (9) Hennessy, K.; Badolato, A.; Winger, M.; Gerace, D.; Atature, M.; Gulde, S.; Falt, S.; Hu, E. L.; Imamoglu, A. Quantum nature of a strongly coupled single quantum dot-cavity system. Nature 2007, 445, 896−899. (10) Tandaechanurat, A.; Ishida, S.; Guimard, D.; Nomura, M.; Iwamoto, S.; Arakawa, Y. Lasing oscillation in a three-dimensional photonic crystal nanocavity with a complete bandgap. Nat. Photonics 2011, 5, 91−94. (11) Akimov, A. V.; Mukherjee, A.; Yu, C. L.; Chang, D. E.; Zibrov, A. S.; Hemmer, P. R.; Park, H.; Lukin, M. D. Generation of single optical plasmons in metallic nanowires coupled to quantum dots. Nature 2007, 450, 402−406. (12) Russell, K. J.; Liu, T. L.; Cui, S. Y.; Hu, E. L. Large spontaneous emission enhancement in plasmonic nanocavities. Nat. Photonics 2012, 6, 459−462. (13) Hofmann, C. E.; de Abajo, F. J. G.; Atwater, H. A. Enhancing the Radiative Rate in III-V Semiconductor Plasmonic Core-Shell Nanowire Resonators. Nano Lett. 2011, 11, 372−376. (14) Ellis, D. J. P.; Bennett, A. J.; Dewhurst, S. J.; Nicoll, C. A.; Ritchie, D. A.; Shields, A. J. Cavity-enhanced radiative emission rate in a single-photon-emitting diode operating at 0.5 GHz. New J. Phys. 2008, 10, 043035. (15) Prodan, E.; Radloff, C.; Halas, N. J.; Nordlander, P. A hybridization model for the plasmon response of complex nanostructures. Science 2003, 302, 419−422. (16) Demory, B.; Hill, T. A.; Teng, C.-H.; Zhang, L.; Deng, H.; Ku, P.-C. Plasmonic Enhancement of Single Photon Emission from a SiteControlled Quantum Dot. ACS Photonics 2015, 2, 1065−1070.
Compared to a stretched exponential fitting, the biexponential function gives a better fit. The deviation from the ideal monoexponential decay previously observed in a single InGaN QD nanopillar,5 of a similar structure and materials, was attributed to the QD inhomogeneous broadening present in this experiment. Using the biexponential fitting results, we assigned the QD lifetime as the weighted average of the two biexponential lifetimes: τeff = a1τ1 + a2τ2. The lifetimes for the array of bare QDs, “closed top” cavity QDs, and “open top” cavity QDs were found to be 343, 243, and 132 ps, respectively. The respective quantum efficiencies were estimated to be 5%, 9%, and 23%. The Purcell factors for the “closed top” and “open top” cavities were found to be 2.5 and 6.2, respectively. Hence our optical measurements have confirmed the improvements from a “closed top” to an “open top” cavity. Currently, the measured Purcell factor is still much lower than the design (Figure 1), which is attributed to the higher intrinsic loss in Aldoped Ag than that in pure Ag. The Al-doped Ag was chosen in this work in favor of Ag, as it enabled a smooth coverage over the nanopillar profile. To further increase the Purcell factor, it is essential to develop a process for smooth Ag deposition with an even lower optical loss. In addition, there may also be scattering losses due to the remaining SiO2 on the sample surface, which can be seen to be very rough. The light scattering off of the random structures could reduce the collectable emission within our numerical aperture at the “open top” cavity stage, which would produce a lower PL enhancement ratio and reduce the Purcell factor. In summary, we proposed and experimentally demonstrated an “open top” cavity design to enhance the spontaneous emission of InGaN/GaN QDs. The design was motivated by removing the metal cap from a previous “closed top” cavity to reduce the metal absorption loss. The experiment showed that the “open top” cavity exhibited a Purcell factor 2.5 times that of the “closed top” cavity, as evidenced by both a higher emission intensity and shorter QD lifetime. An effective lifetime of 132 ps was observed, which was comparable to that of III-arsenic QDs.31 This makes III-N QDs viable for high-speed applications. D
DOI: 10.1021/acsphotonics.7b00140 ACS Photonics XXXX, XXX, XXX−XXX
ACS Photonics
Letter
(17) Lu, G. W.; Zhang, T. Y.; Li, W. Q.; Hou, L.; Liu, J.; Gong, Q. H. Single-Molecule Spontaneous Emission in the Vicinity of an Individual Gold Nanorod. J. Phys. Chem. C 2011, 115, 15822−15828. (18) Bharadwaj, P.; Novotny, L. Spectral dependence of single molecule fluorescence enhancement. Opt. Express 2007, 15, 14266− 14274. (19) Levene, M. J.; Korlach, J.; Turner, S. W.; Foquet, M.; Craighead, H. G.; Webb, W. W. Zero-mode waveguides for single-molecule analysis at high concentrations. Science 2003, 299, 682−686. (20) Lumerical Solutions, Inc. http://www.lumerical.com/tcadproducts/fdtd/. (21) Lee, L. K.; Ku, P. C. Fabrication of site-controlled InGaN quantum dots using reactive-ion etching. Phys. Status Solidi C 2012, 9, 609−612. (22) Zhang, L.; Hill, T. A.; Teng, C.-H.; Demory, B.; Ku, P.-C.; Deng, H. Carrier dynamics in site- and structure-controlled InGaN/ GaN quantum dots. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 90, 245311. (23) Stocker, D. A.; Schubert, E. F.; Redwing, J. M. Crystallographic wet chemical etching of GaN. Appl. Phys. Lett. 1998, 73, 2654−2656. (24) Li, Q. M.; Westlake, K. R.; Crawford, M. H.; Lee, S. R.; Koleske, D. D.; Figiel, J. J.; Cross, K. C.; Fathololoumi, S.; Mi, Z. T.; Wang, G. T. Optical performance of top-down fabricated InGaN/GaN nanorod light emitting diode arrays. Opt. Express 2011, 19, 25528−25534. (25) Sennett, R. S.; Scott, G. D. The Structure of Evaporated Metal Films and Their Optical Properties. J. Opt. Soc. Am. 1950, 40, 203− 211. (26) Gu, D.; Zhang, C.; Wu, Y.-K.; Guo, L. J. Ultrasmooth and thermally stable silver-based thin films with subnanometer roughness by aluminum doping. ACS Nano 2014, 8, 10343−10351. (27) Zhang, C.; Zhao, D.; Gu, D.; Kim, H.; Ling, T.; Wu, Y. K. R.; Guo, L. J. An Ultrathin, Smooth, and Low-Loss Al-Doped Ag Film and Its Application as a Transparent Electrode in Organic Photovoltaics. Adv. Mater. 2014, 26, 5696−5701. (28) Zhang, C.; Kinsey, N.; Chen, L.; Ji, C.; Xu, M.; Ferrera, M.; Pan, X.; Boltasseva, A.; Shalaev, V.; Guo, L. J. High-performance Doped Silver Films: Overcoming Fundamental Material Limits for Nanophotonic Applications. Adv. Mater. 2017, 1605177. (29) Park, S. D.; Lee, Y. J.; Kim, S. G.; Choe, H. H.; Hong, M. P.; Yeom, G. Y. Etch characteristics of silver by inductively coupled fluorine-based plasmas. Thin Solid Films 2003, 445, 138−143. (30) Johnson, P. B.; Christy, R. W. Optical Constants of Noble Metals. Phys. Rev. B 1972, 6, 4370−4379. (31) Stock, E.; Unrau, W.; Lochmann, A.; Tofflinger, J. A.; Ozturk, M.; Toropov, A. I.; Bakarov, A. K.; Haisler, V. A.; Bimberg, D. Highspeed single-photon source based on self-organized quantum dots. Semicond. Sci. Technol. 2011, 26, 4.
E
DOI: 10.1021/acsphotonics.7b00140 ACS Photonics XXXX, XXX, XXX−XXX
