NANO LETTERS
Photonic Crystal-Assisted Light Extraction from a Colloidal Quantum Dot/GaN Hybrid Structure
2006 Vol. 6, No. 6 1116-1120
Fre´de´ric S. Diana,*,† Aure´lien David,‡ Ines Meinel,| Rajat Sharma,‡ Claude Weisbuch,‡ Shuji Nakamura,‡ and Pierre M. Petroff†,§ Materials Department, Mitsubishi Chemical Center for AdVanced Materials, and Department of Electrical and Computer Engineering, UniVersity of California, Santa Barbara, California 93106, and Mitsubishi Chemical Research and InnoVation Center, 601 Pine AVenue, Goleta, California 93117 Received March 9, 2006; Revised Manuscript Received April 13, 2006
ABSTRACT We present a study of the light extraction from CdSe/ZnS core/shell colloidal quantum dot thin films deposited on quantum well InGaN/GaN photonic crystal structures. The two-dimensional photonic crystal defined by nanoimprint lithography is used to efficiently extract the guided light modes originating from both the quantum dot thin films and the InGaN quantum wells. Far-field photoluminescence spectra are used to measure the extraction enhancement factor of the quantum dot emission (×1.4). Microphotoluminescence measurements show that the guided mode effective extraction lengths range between 70 and 180 µm, depending on the wavelength of light.
The unique size-dependent optical properties of colloidal semiconductor quantum dots (QDs) and the ability to synthesize them through solution chemistry1 have opened a number of potential applications. Their high extinction coefficient (up to 107 cm-1‚M-1), high internal quantum efficiency (>50%), and emission stability2 are important optical properties that are relevant to new technologies. These QDs can be deposited into thin films with excellent optical quality (i.e., limited scattering and self-absorption).3 Such characteristics could make them useful as nanophosphors for light down-conversion in GaN-based light-emitting diodes (LEDs).4,5 A candidate structure could consist of a GaN layer with embedded InGaN quantum wells (QWs) grown on a sapphire substrate by metal-organic chemical vapor deposition (MOCVD) and coated with a thin film of colloidal QDs to be used as nanophosphors. The presence of the nearby high-index semiconductor layers would allow for an increase of the QDs radiative recombination rate through the Purcell effect. However, with this structure, a large fraction of the total light emitted by the QWs and QDs would be trapped in guided modes of the GaN cavity. This effect is responsible for large losses in LEDs, and much effort has been made to provide solutions for either extracting guided modes or * To whom correspondence should be addressed. E-mail: frederic@ engineering.ucsb.edu. † Materials Department & Mitsubishi Chemical Center for Advanced Materials, University of California, Santa Barbara. ‡ Materials Department, University of California, Santa Barbara. § Department of Electrical and Computer Engineering, University of California, Santa Barbara. | Mitsubishi Chemical Research and Innovation Center. 10.1021/nl060535b CCC: $33.50 Published on Web 04/27/2006
© 2006 American Chemical Society
preventing the sources from emitting into these.6,7 Recently, the use of two-dimensional (2D) photonic crystals (PhCs) have shown significant enhancement of the light extraction efficiency for monochromatic LEDs.8,9 Following this method, we report on the fabrication of a 2D PhC into the top surface of the GaN layer via nanoimprint lithography (NIL) before deposition of the QD thin film. We demonstrate that the 2D PhC allows the simultaneous extraction of the QW and QD guided modes by using far-field and microphotoluminescence measurements. Usually, guided modes are efficiently stimulated by the InGaN QWs because these are positioned inside the GaN planar cavity. It is known that roughly 12% of their emitted light escapes in air directly, ∼66% is guided in the GaN layer, and the remainder is emitted into “delocalized” modes. This designation stems from the fact that these modes have their transverse profile spread over the whole structure, including the substrate, where they are guided as well.7 Guided and delocalized modes can also be excited by external sources (i.e., sources placed outside the cavity), even though the electromagnetic field associated with these modes is evanescent in the outer media. Indeed, the field leakage of guided modes extends outside over a fraction of wavelength (∼100 nm for visible light) so that QDs (with sizes typically 2.5 are purely evanescent and do not contribute in the radiated intensity. A computation of the various contributions to the total radiated power emitted by the randomly oriented dipole was also performed as a function of its distance to the cavity interface. Figure 1C is a plot of the normalized cumulated powers carried by the different types of emitted waves, as a function of the dipole position with respect to the cavity interface. These powers are normalized by the total radiated power emitted by a Nano Lett., Vol. 6, No. 6, 2006
similar source in vacuum. For dipole-cavity distances smaller than 100 nm, the dipole emits a large portion of its total radiated power into guided modes (up to 50%). The QDs then experience a Purcell effect with a Purcell factor up to about 2 in the vicinity of the GaN interface,14 increasing their internal quantum efficiency. These calculations confirm that waveguiding of light from external sources is important and that our structure should include an element to make use of this light, which would otherwise be lost. In the following, we show that a 2D PhC defined by nanoimprint lithography (NIL)15 and etched into the top surface of the GaN cavity can be used to out-couple guided modes and therefore enhance both light extraction and light downconversion. Holographic photolithography is employed to make the Si master stamp to be used for NIL. The expanded beam of a HeCd laser emitting at 325 nm is used to expose a 100nm-thick photoresist (PR) layer on top of a 60-nm-thick SiO2 film coating a silicon substrate. Two successive exposures (with 60° sample rotation between) are carried out to produce a triangular pattern of PR nanopillars with a periodicity Λ ) 250 nm (nearest neighbors distance: a ) 2Λ/31/2 ) 290 nm), which corresponds to a second-order grating for visible light in GaN. Two consecutive inductively coupled plasma reactive ion etches (ICP-RIE) allow the transfer of the PR pattern first into the SiO2 layer and then into the Si substrate. The final depth of the induced Si nanopillars was about 250 nm, as measured by cross-section SEM (see Figure 2A). We apply NIL to produce a triangular pattern of holes in the GaN film. The unintentionally doped GaN planar cavity (about 1.75 µm in thickness), grown by MOCVD on a sapphire substrate, includes three InGaN QWs emitting at 440 nm placed 200 nm below the GaN cavity surface. The NIL process makes use of the Si master stamp to imprint a polymer layer, Nanonex NXR-1010, which is spin-coated on the GaN substrate to be etched (imprint conditions: 130 °C and 300 psi). The thin layer of polymer remaining at the bottom of the imprinted holes (see SEM image in Figure 2B) is removed by performing a RIE dry etch using only O2. After this step, the resist layer is about 250-nm-thick 1117
Figure 2. A: Master stamp with 250-nm-high Si nanopillars ordered in a triangular lattice (nearest neighbors distance is 290 nm). B: SEM image of the cross-section of the imprinted polymer layer, showing that a 150 nm layer remains on the substrate, while the imprinted layer thickness is about 250 nm. C: AFM image of GaN surface after O2 resist removal and RIE etch, showing the 2D triangular PhC, 50 nm in depth.
(as checked with AFM). A final Cl2-based RIE etch is used to transfer the resist pattern into the GaN layer, inducing a triangular PhC of 50-nm-deep holes (Figure 2C). Note that the relatively shallow depth of the PhC is chosen not to affect the PL efficiency of the InGaN QWs embedded inside the GaN cavity. Square mesas are then RIE-etched in GaN (800 × 800 µm2 and ∼700 nm in height) after a photolithographic process. Finally, CdSe/ZnS core/shell colloidal QDs (Evident Technologies) emitting around 620 nm are diluted in toluene and drop-casted on top of the patterned GaN region, giving a ∼100-nm-thick QD layer (from AFM measurement), after solvent evaporation. Figure 3A shows a schematic of the final structure of the sample with the relevant dimensions. A far-field photoluminescence (PL) detection setup is used to observe the 2D PhC-assisted extraction of guided modes originating from both InGaN QWs and colloidal QDs PL. In this experiment, the beam of a HeCd laser emitting at 325 nm incident 60° from the normal to the sample is focused 1118
using a quartz lens (spot size: 300 µm in diameter). The sample is placed on a rotation stage, for control of its azimuthal angle (φ), itself supported by a manual X-Y translation stage. A motorized rotation stage is used to collect light as a function of the zenith angle, θ, by supporting the following optical components: a linear analyzing polarizer, an aperture (1 mm in diameter), and a lens to focus the collected light into an optical fiber. The distance between the entrance end of the fiber and the sample center is 15 cm, giving an angular aperture of 0.4°. A micro-PL setup is used to measure the guided mode effective extraction lengths (noted Lext in the following). In both cases the collected light is dispersed by a spectrometer and analyzed by a Peltiercooled CCD detector array (1024 × 256 pixels). Figure 3B shows a far-field PL spectrum obtained after simultaneously pumping CdSe/ZnS QDs and InGaN QWs with the HeCd laser. The orientation of the sample (azimuthal angle φ) was set such that the PhC was in the ΓM direction (φ ) 0°) with respect to the detector rotation plane. This spectrum combines two sets of angular-resolved PL spectra, collected from θ ) -90° to θ ) +90°, 0° representing the normal to the sample surface, with the analyzing polarizer in the TE configuration (that is, with its axis parallel to the direction φ ) 90°). The first set of PL spectra contains wavelengths between 400 and 560 nm, and the second one contains wavelengths between 540 and 700 nm. The two main PL bands arising from the emission and direct extraction of the QDs, around 620 nm, and of the QWs, around 440 nm, are observed, with each band having a full width at half-maximum of about 25 nm (the GaN “yellow band” is also measured around 540 nm). In addition to these two emission bands, we observe several other narrow lines with different slopes, curvatures, and intensities, depending on wavelength and angle. All of these extra lines stem from the extraction of guided modes due to the presence of the PhC, each line being associated with a given guided mode order. We transformed the far-field PL spectrum to obtain the guided modes dispersion relationships (reduced frequency u ) Λ/λ0 vs in-plane wavevector, k|), by using the relation k| ) k0 sin θ. In addition, for each wavelength, IPL(θ) is normalized relative to its integrated intensity. The resulting normalized far-field spectrum, shown on Figure 3C, reveals the dispersion relationships of guided modes and points to details about their relative excitation and out-coupling efficiency. QD emission corresponds to 0.38 < u < 0.42, and QW emission corresponds to 0.53 < u < 0.57. The measurement covers all possible k| values in the light cone (delimited by the air light line and seen clearly in this plot because no emission occurs below the light line), for each reduced frequency. Another characteristic line arises from the presence of the frequency cutoff for any given guided mode, below which it disappears. This occurs when k| reaches the sapphire line, defined by k| ≈ 1.7k0. For any given frequency, there is a discrete number of guided modes carried by the planar cavity, with 1.7 < neff < 2.5. The lowest order mode has neff ≈ 2.5. This mode almost perfectly follows the GaN line, defined by k| ≈ 2.5k0. The number of Nano Lett., Vol. 6, No. 6, 2006
Figure 3. A: Schematic showing the sample structure and the PhC-assisted QD guided modes extraction process. B: Measured far-field PL spectrum. C: Normalized far-field PL spectrum obtained from B. D: Reciprocal lattice associated with the 2D PhC and origin of extracted guided modes. The blue and gray circles indicate, respectively, the light cone and the trace of points with identical k|. The gray hexagon is the first Brillouin zone boundary.
guided modes as measured with this sample is in accordance with the simulation shown previously (Figure 1B). To explain all of the effects caused by a 2D PhC (in particular the effects related to polarization), the field, associated with a guided mode for example, should be described as a Bloch mode: E(r) ) ΣGEG‚exp[i(k| + G)‚ r], where EG is the electric field component corresponding to harmonic G, and k| is the in-plane wavevector of the Bloch mode. With our PhC structure, the reciprocal lattice (RL) is a 2D triangular lattice rotated by 30° with respect to the direct lattice (DL) and RL vectors can be written as: G ) ha1*+ ka2*, where h and k are integers, and a1* and a2* are the two RL basis vectors (Figure 3D). Harmonics of the Bloch mode are extracted if their in-plane wavevectors are within the light cone: |k| + G| < k0. Bloch mode harmonics intensities decay like G-3 to infinity, with G the RL radial coordinate: high-order harmonics carry negligible intensity, and efficient extraction should involve low-order harmonics (associated with short RL vectors) in order to occur over reasonable lengths. The most striking feature observed in this plot is the detection of the radiative components of guided modes. The sets of lines labeled 2a and 2b are induced by the radiative harmonics of the TE-polarized guided modes Nano Lett., Vol. 6, No. 6, 2006
propagating in the ΓM direction with in-plane wavevectors k| + G10 and k| + G-10 (see Figure 3D where we show only a radiative harmonic associated with set 2b, for clarity; 2a is obtained by symmetry). The sets of lines labeled 3a and 3b are formed by the combination of two harmonics, as shown in Figure 3D (for a line associated with set 3a). These radiative harmonics are not associated with guided modes propagating in the ΓM direction but in directions about φ ) (60°. The measurement of these components constitutes direct evidence of 2D PhC-assisted light extraction.16 The polarization of these radiative harmonics is not collinear to the analyzing polarizer axis, set in the TE configuration, and therefore only a fraction of their intensity is transmitted through the analyzing polarizer, explaining why these lines appear dimmer than set 2. Their measured intensity becomes comparable to the latter when the analyzing polarizer is set in the TM configuration (axis orientation perpendicular to the direction φ ) 90°). The lines labeled 4a and 4b are identified after following a similar analysis, arising from radiative harmonics associated with higher-order RL vectors. The broad background modulations labeled 1 are due to Fabry-Perot interferences, associated with the directly extracted light. 1119
Figure 4. Integrated intensities, associated with QD emission only, as a function of θ, in the ΓM direction and TE polarization, on or off the PhC: the curves were normalized such that they have the same intensity for θ ) 0 because no guided mode extraction occurs at that angle. The black thin line shows the case of a perfect Lambertian source.
For a given wavelength, the brightest modes in Figure 3D are those that carry the most intensity and have the highest extraction efficiency. The guided modes excited by CdSe/ ZnS QDs nicely confirm the results of the computations presented above (Figure 1B) in that one can observe a gradual increase of their excitation and extraction efficiency as the mode order increases. This is another consequence of the fact that their excitation source lies outside of the planar cavity. The excitation of guided modes from such sources is greater for high-order modes because of their larger field leakage outside the GaN layer. In addition, the high-order modes are also more efficiently out-coupled because their overlap with the 2D PhC is larger than the low-order guided modes, which are more confined in the middle of the cavity.16 We estimated the enhancement of light extraction for the emission from CdSe/ZnS QDs due to the presence of the 2D PhC by comparing the QDs integrated intensity spectrum, IPL(θ), to the same emission without PhC in Figure 4. Noting that the directly extracted light is not reduced because of the presence of the PhC, and because no guided mode induced by the QDs is extracted at 0°, the backgrounds for both spectra (i.e., the directly extracted light intensity) should be equal at that angle. With this normalization, we obtained an enhancement factor of ×1.4 in the presence of the PhC, when measuring in the ΓM direction (another measurement in the ΓK direction did not produce a significant difference). This number is a rough estimate because the excitation of QDs by the QWs emission is also increased due to the presence of the PhC because guided light emitted by QWs is also extracted. Thus, this estimate is a lower limit. Finally, for a given guided mode, the extracted power decays exponentially with in-plane distance (in conjunction with a 1/r decay). Delocalized modes are extracted over much longer distances than cavity guided modes because they do not overlap as much with the PhC. A micro-PL measurement allowed us to obtain an estimation of the guided mode effective extraction lengths, Lext. The collection beam was spatially filtered by using a 1 mm aperture; therefore, few guided modes were collected simultaneously at a given 1120
wavelength. Lext ranged between 70 µm (for the blue) and 180 µm (for the yellow), with part of this difference being caused by a larger absorption of the QWs in the blue.17 These extraction lengths could be reduced by optimizing the PhC lattice parameter and filling factor, and by increasing its depth. In conclusion, we have analyzed theoretically and experimentally the optical characteristics of a planar InGaN/GaN QWs structure coated with a thin film of CdSe/ZnS colloidal QDs. We have shown, by numerical simulations, that the QDs internal quantum efficiency is enhanced through the Purcell effect and that their emission includes guided modes inside the GaN layer. Far-field photoluminescence measurements show that the introduction of a 2D PhC extracts the GaN cavity guided modes excited by the QWs and QDs. We obtained a lower limit of the light extraction enhancement factor of ×1.4 for the extracted light emission from the QDs, and we estimated the guided mode extraction lengths to range between 70 and 180 µm, depending on the wavelength, from micro-PL measurements. These results demonstrate that the use of nanophosphors in thin films for GaN LEDs combined to a 2D PhC made by NIL could be an interesting alternative to current structures, allowing for a better control of emitted light and improved extraction potential for most guided modes induced by all light sources. Acknowledgment. F.S.D. thanks Mark White and Michael Tambe for fruitful discussions. This work made use of UCSB MRL Central Facilities supported by the National Science Foundation under award no. DMR00-80034. References (1) Yin, Y.; Alivisatos, A. P. Nature 2005, 437, 664. (2) Talapin, D. V.; Koeppe, R.; Gotzinger, S.; Kornowski, A.; Lupton, J. M.; Rogach, A. L.; Benson, O.; Feldmann, J.; Weller, H. Nano Lett. 2003, 3, 12, 1677. (3) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H.-J.; Bawendi, M. G. Science 2000, 290, 314. (4) Lee, J.; Sundar, V. C.; Heine, J. R.; Bawendi, M. G.; Jensen, K. F. AdV. Mater. 2000, 12, 15, 1102. (5) Chen, H. S.; Wang, S. J. J.; Lo, C. J.; Chi, J. Y. App. Phys. Lett. 2005, 86, 131905. (6) Ichikawa, H.; Baba, T. Appl. Phys. Lett. 2004, 84, 456. (7) David, A.; Fujii, T.; Sharma, R.; McGroddy, K.; Nakamura, S.; DenBaars, S. P.; Hu, E. L.; Weisbuch, C.; Benisty, H. Appl. Phys. Lett. 2006, 88, 61124. (8) Ichikawa, H.; Baba, T. Appl. Phys. Lett. 2004, 84, 456. (9) David, A.; Fujii, T.; Sharma, R.; McGroddy, K.; Nakamura, S.; DenBaars, S. P.; Hu, E. L.; Weisbuch, C.; Benisty, H. Appl. Phys. Lett. 2006, 88, 61124. (10) Lukosz, W. Phys. ReV. B 1980, 22, 6, 3030. (11) Benisty, H.; Ge´rard, J.-M.; Houdre´, R.; Rarity, J.; Weisbuch, C. Confined Photon Systems: Fundamentals and Applications; Springer, New York, 1999. (12) Benisty, H.; Stanley, R.; Mayer, M. J. Opt. Soc. Am. A 1998, 15, 5, 1192. (13) Soller, B. J.; Stuart, H. R.; Hall, D. G. Opt. Lett. 2001, 26, 18, 1421. (14) Zhang, J.-Y.; Wang, X.-Y.; Xiao, M. Opt. Lett. 2002, 27, 14, 1253. (15) Melle, E.; Di Benedetto, F.; Persano, L.; Cingolani, R.; Pisignano, D. Nano Lett. 2005, 5, 10, 1915. (16) David, A.; Meier, C.; Sharma, R.; Diana, F. S.; DenBaars, S. P.; Hu, E.; Nakamura, S.; Weisbuch, C.; Benisty, H. Appl. Phys. Lett. 2005, 87, 10. (17) Experimental setup and results to be presented elsewhere.
NL060535B Nano Lett., Vol. 6, No. 6, 2006
