Nonlinear Photonic Diode Behavior in Energy-Graded Core–Shell

Letter pubs.acs.org/NanoLett

Nonlinear Photonic Diode Behavior in Energy-Graded Core−Shell Quantum Well Semiconductor Rod Suk-Min Ko, Su-Hyun Gong, and Yong-Hoon Cho* Department of Physics and KI for the NanoCentury, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea S Supporting Information *

ABSTRACT: Future technologies require faster data transfer and processing with lower loss. A photonic diode could be an attractive alternative to the present Si-based electronic diode for rapid optical signal processing and communication. Here, we report highly asymmetric photonic diode behavior with low scattering loss, from tapered core−shell quantum well semiconductor rods that were fabricated to have a large gradient in their bandgap energy along their growth direction. Local laser illumination of the core−shell quantum well rods yielded a huge contrast in light output intensities from opposite ends of the rod. KEYWORDS: Photonic diode, asymmetric waveguide, nonlinear, GaN/InGaN core−shell quantum well

T

grown by the vapor transport technique, CdSSe rods are hard to align vertically and consist of toxic elements such as Cd, Se, and S. Moreover, the surfaces of the CdSSe rods flex and are easily oxidized in air to CdO, resulting in numerous scattering centers which reduce the waveguide and/or emission efficiency of the photonic diode. From a materials’ viewpoint, nontoxic, chemically stable, straight GaN/InGaN core−shell QW rods would be more appropriate for highly efficient photonic diodes than the current CdSSe rods, due to the issues noted above. Moreover, the QW structure in the semiconductor rods permits a large and controllable Eg gradient, which is achieved by simultaneously varying the composition and the quantum confinement effect. This also results in highly asymmetric light propagation, due to the remarkable optical absorption along the reverse direction by excitonic states bound in QWs, even though the effective volume of the QW structure is much smaller than that of the bulk.7 In addition, when grown along the c-axis, Eg-graded GaN/InGaN core−shell rods have an active region (InGaN QWs) on the nonpolar sidewall (mplane) with negligible piezoelectric internal field, in contrast to conventional QWs grown on the polar (c-) plane.8 The Eg-graded GaN/InGaN core−shell QW rods in this study were synthesized on Si (100) substrates by metal− organic chemical vapor deposition. First, to obtain the tapered GaN core rods, we employed the growth conditions of a low V/III molar ratio (66.7) and a high reactor pressure (500 Torr), rather than those typically used for a conventional two-

here is growing interest in nano/microscale optoelectronic devices that can provide faster information transfer and optical signal processing and communication. Such performance would be a welcome alternative to the inherent limitations of conventional Si-based electronic integrated circuits. Recently, compound semiconductor rods have emerged as promising optical components for highly efficient, compact optoelectronic devices because of their tunable wide direct bandgap and efficient waveguiding by unique onedimensional structure. In particular, wavelength tuned compound semiconductor rods with a spatially graded bandgap energy (Eg) along the growth axis can be used not only as conventional waveguides but also as nonlinear photonic diodes. Such diodes allow light propagation in the forward direction but restrict it in the opposite (reverse) direction, analogous to an electronic diode. To date, other time-reversal symmetry broken asymmetric optical configurations have been reported, such as anisotropic planar chiral structures,1 or nonlinear, magnetic, and spatial reversal antisymmetric photonic crystal structures.2−5 However, the semiconductor rods are more promising for highly integrated circuits than any other approach, due to their relatively small dimensions. Here, we present a highly efficient nonlinear photonic diode composed of a tapered GaN/InGaN core−shell quantum well (QW) rod with a large gradient in bandgap energy. The Eggraded GaN/InxGa1−xN core−shell QW rods were formed naturally by gradually varying both composition and well thickness and are good candidates for application as highly efficient photonic diodes with superior asymmetric light propagation. CdSySe1−y rods with graded composition have recently been utilized for asymmetric light propagation.6 However, when © 2014 American Chemical Society

Received: March 1, 2014 Revised: April 17, 2014 Published: April 21, 2014 4937

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Figure 1. (a) Growth procedure of GaN/InGaN core−shell quantum well semiconductor rod on Si (100) substrate. (b−c) Cross-sectional and top view SEM images of GaN core rods. (d) CL spectra taken at various positions (P1−P12) along the growth direction. Inset figure shows the crosssectional SEM image of an individual GaN rod on the Si (100) substrate with the white scale bar of 1 μm.

dimensional thin film. Details concerning these growth conditions can be found in a previously published work.9 Next, as a shell layer, an InGaN/GaN multiple QWs (MQWs) structure was deposited on the tapered GaN core rods. A high V/III molar ratio (>1875) and large indium mole fraction were used, as well as reduced growth temperature (680 °C), to obtain high-quality InGaN/GaN MQWs. Finally, to prevent degradation of the crystal quality of the InGaN/GaN MQWs and to make a smooth surface on the sidewall, the GaN/InGaN core−shell rods were covered by a GaN capping layer ∼50 nm thick. Figure 1a illustrates the procedure for growing of the tapered GaN/InGaN QW rods on the Si (100) substrates. The rod structure, grown in sequence, consists of a tapered GaN core rod (which acts as an optical waveguide), five-period InGaN/GaN MQWs (which act as a tunable active medium), and GaN cap. Since the microstructure and crystal quality of the GaN core rods directly influence the properties of the GaN/InGaN core− shell QW rods, these were investigated first, prior to examining the properties of the GaN/InGaN core−shell QW rods. Figure 1b and c shows cross-sectional and top-view scanning electron microscopy (SEM) images, respectively, of the GaN core rod ensemble grown on Si (100) substrate. The as-grown wurtzite GaN core rods are well-aligned and predominantly tilted in the four cubic [111] directions of the Si (100), with an angle of about 55° to the surface of the Si substrate because of the crystal symmetric tilt. Moreover, the shape of the GaN core rods look slightly tapered toward their top. This is due to a reduced supply of Ga to the upper part of the rod relative to its lower part, which occurs when the length of the GaN core rod is much longer (>few μm) than the surface diffusion length of the Ga adatom (submicrometer).10,11 Figure 1d shows the cathodoluminescence (CL) spectra of an individual GaN core rod, taken at various positions along the growth directions. A strong near-band-edge (NBE) emission peak is located at 364 nm, together with weak defect-related yellow luminescence at ∼560 nm. As shown in Figure 1d, the GaN core rod has a higher ratio of NBE emission to yellow luminescence for most

of the positions except the lower side (P1−P2) of the rod. In this region, lattice disorders (e.g., grain boundaries) have been induced by the four different crystal symmetric tilts, with corresponding stacking faults near the interface between the rod and the Si substrate. The results indicate that the upper side of the GaN core rods is highly crystalline without obvious defects or disorder. Furthermore, the generation of defects at the interface almost relaxes the biaxial misfit strain. Therefore, the NBE peak is not significantly shifted by the biaxial strain on the upper part of the GaN core rod, as compared with that of strain-free GaN (∼3.4 eV), as shown in Figure 1d. Figure 2a shows the SEM image of an individual GaN/ InGaN core−shell QW rod with five period MQWs. This

Figure 2. (a) SEM image of a single GaN/InGaN core−shell rod. (b) Monochromatic CL images of the single rod. (c) Spatially resolved CL spectra taken at different locations (P1−P5). 4938

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Figure 3. STEM HAADF images of a GaN/InGaN core−shell QW rod: (a−e) Dark field images of a QW taken at various positions from a single rod. (f) Overall image of a GaN/InGaN core−shell rod. (g) Variation of QW thickness and core diameter as a function of the growth direction. (h) Schematics of a kinetic Wulff’s plot (right) and the corresponding equilibrium shapes of GaN (left). The magnitude of the Wulff’s plot along a certain direction reflects the surface energy of the plane normal to that direction. (i) Comparison of the grown volumes of the upper and lower QW with a similar relative volume growth rate. (j) Variation of the indium content inside a QW along the distance from bottom of the rod

GaN/InGaN core−shell QW rod, as well as the GaN core rod, is slightly tapered and has a length and diameter of ∼10 μm and ∼1 μm. We checked the distribution in length and diameter of the dispersed 136 rods. The average length, the mean diameter, and the tapered angle of the rods are about 11.3 ± 4.5 μm, 2.1 ± 0.7 μm, and 0.77 ± 0.49°, respectively (Supporting Information, Figure S2). Figure 2b presents the monochromatic CL images of the same GaN/InGaN core−shell QW rod shown in Figure 2a. The peak wavelength of the rod gradually changes from 405 nm (bottom) to 525 nm (top). There are two likely explanations for the Eg-gradient of the GaN/InGaN core−shell QW rod along the growth axis. The first possibility has to do with the variation in the thickness of the InGaN QWs along their length, which would produce peak shifts due to changes in the quantum confinement effect.12 Another possibility is the higher rate of indium incorporation into the QWs in the upper part of the rod (referred to as upper QWs) than in the lower part of the rod (referred to as lower QWs). This may be due to the longer surface migration length of the In adatom compared to the Ga adatom13,14 and the lower dissociation rate of indium caused by the slightly lower surface temperature on the upper QW versus the lower QW. These two effects can contribute to the Eg-gradient of the GaN/ InGaN core−shell QW rods along the growth direction. To compare the QW peak energy and intensity at various positions (P1−P5) along the growth axis, spatially resolved CL spectra were taken, as shown in Figure 2c. We found that the QW peak intensity slightly increased along the growth axis (i.e., as the thickness of QWs increases) since the QW active volume becomes larger while the quantum confined Stark effect (QCSE) inside QWs grown on nonpolar (m-) plane of the sidewalls is negligible. This is in contrast to the case of InGaN QWs grown on the c-plane, where the QW peak intensity

degrades with increasing the thickness of QWs due to the strong QCSE.8 To precisely evaluate the variation of QW thickness and indium content inside the GaN/InGaN core−shell rod along the growth direction, scanning transmission electron microscopy high-angle annular dark field (STEM HAADF) images were taken along the rod, as shown in Figure 3a−f. The InGaN/GaN MQWs shell was formed on the {101x}̅ plane (0 < x ≪ 2) of the GaN core rod with a tapered angle of ∼0.64° from the growth axis. As shown in Figure 3g, the thickness of the QW increases by a factor of 2.3 from 1.1 nm (at the bottom) to 2.5 nm (at the top) along the growth direction of [0001]. This is in contrast to the GaN core rod whose diameter gradually decreases along the length. There are two reasons for the increase of InGaN QW thickness along the length of the GaN/InGaN core−shell rod. Generally, it is well-known that the surface energy of the {1010} plane will be smaller than that of the {101x}̅ plane (0 < x ≪ 2) when InGaN/GaN MQWs (shell) are grown under conditions of lower pressure (∼300 Torr) and a higher V/III ratio (∼1875) which are similar to the equilibrium growth conditions of GaN, as shown in the kinetic Wulff plot (Figure 3h).15 The {1010} plane is preferred over the {101x}̅ plane for growth of the InGaN QW (shell), since the {1010} plane has much smaller total free energy. Thus, the tapered angle (∼0.64°) of the GaN core rod finally becomes zero after sufficient InGaN/GaN MQWs shell growth. As a result, the upper QWs grow faster than the lower QWs. The other reason is that the tapered shape of the GaN core rod results in a similar relative volume growth rate (Supporting Information, Table S1), when the volume growth rate is defined as dV = 2√3(dcore + tQW)tQWdh. Here dV is the volume growth rate of the surface of the tapered GaN core rod with an infinitesimal length (dh) at a certain height of the rod, and dcore and tQW are the diameter of the tapered GaN core rod and QW 4939

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Figure 4. (a) Schematic illustration of the unidirectional (asymmetric) light propagation along the Eg-graded rod with local excitation by a 405 nm cw laser. (b−c) Panchromatic and monochromatic micro-PL image taken by CCD pixel array at the image plane for reverse and forward guides. Dotted lines in the panchromatic images of b, c indicate the morphology of the corresponding single core−shell QW rod. To see the peak shift clearly, the white spectra in monochromatic PL mapping in b, c are normalized.

QWs. Therefore, the upper QWs can have a higher indium content than the lower QWs. Consequently, the large Eggradient in our tapered GaN/InGaN core−shell QW rods is derived from variations in QW thickness and indium content along the growth direction. Because of the Eg-gradient of the QWs along the growth axis, inside the rods the light propagates asymmetrically depending on the direction. This is in contrast to conventional Eg-homogeneous GaN/InGaN core−shell QW rods with bidirectional emission, which lack aymmetric light absorption along their length. Figure 4a illustrates the principle of this asymmetric light propagation in the Eg-graded GaN/InGaN core−shell QW rods using a locally focused 405 nm continuous wave (cw) laser spot. When the focused laser is flashed at the QWs located at bottom of the rod (with the largest Eg), the intensity of the blue light emitted from the bottom QWs penetrating upward attenuates rapidly, and the peak position of the light simultaneously shifts to a lower energy due to energy transfer loss, such as reabsorption and emission (reverse guide). On the other hand, very little absorption takes place during the downward propagation along the rod since the green light emitted from the top QWs has a smaller photon energy than the Eg of the bottom QWs (forward guide). Figure 4b and c contains the panchromatic (left) and monochromatic (right)

thickness, respectively. In other words, the InGaN QW grows faster on the smaller diameter surface of the GaN core rod (dcore = 660 nm at the top of the GaN core rod) than on the larger core diameter (dcore = 840 nm at the bottom of the GaN core rod) as indicated in Figure 3i. In the same vein, the outer QW (QW5) is thinner than the inner QW (QW1) for all positions. The indium content inside the QWs along the growth direction was investigated using the STEM HAADF images.16 The indium content inside the QWs was extracted by the following equation and calibrated via energy dispersive spectroscopy (Supporting Information, Figure S7): IInGaN/IGaN =

ε xZ Inε + (1 − x)ZGa + Z Nε ε ZGa + Z Nε

where I indicates the STEM HAADF intensity, Z represents the atomic number of the materials, and ε is a factor depending on the collection angle of the STEM detector. As shown in Figure 3j, the indium content increases along the growth direction due to the higher rate of indium incorporation, since it has a much longer surface diffusion length than that of the Ga adatom.13,14 Moreover, since the dissociation rate of indium is proportional to the growth temperature, the upper QWs, which have a slightly lower surface temperature, can contain more indium than the lower 4940

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Figure 5. (a) Panchromatic PL image of Eg-graded rod with various local excitations of a 405 nm cw laser. (b) Integrated PL intensity taken from both ends of the Eg-graded GaN/InGaN core−shell QW rod as a function of the propagation length. (c) Intensity contrast of the integrated PL intensity between both ends of the rod (Iforward/Ireverse) with/without the gradient in emission intensity along the length as a function of the propagation length of the emission.

propagation length (L) between the locally excited spot and the two opposite ends of the rod. By fitting the propagationdependent integrated PL intensity spectra (I = I0e−αL), the values of the absorption coefficient (α) for the forward and reverse guides were estimated to be 0.015 and 0.161. This is caused by the much higher absorption which occurs during the upward-waveguiding (reverse guide) compared to the downward-waveguiding (forward guide). Moreover, since the PL intensity of the QWs of the rod becomes larger along the growth direction, it seems that the Eg-graded GaN/InGaN core−shell QW rod has a much smaller absorption coefficient (α = 0.015) than would be expected just from scattering loss (α = 0.045) during the forward guiding. This is due to compensation by the rising emission intensity of the upper QW. On the other hand, as the propagation length of the reverse guide increases, the PL output intensity decreases further due to the smaller intensity at the lower QWs. Thus, in the reverse guide case, the Eg-graded rod has a larger absorption loss (α = 0.161) than would result from just energy transfer losses (α = 0.142). Based on the results above (Figure 5b), we assume that a core−shell QW rod with a gradient in emission intensity as well as the Eg-gradient has a more asymmetric light absorption behavior along both ends of the rod than would be the case without the gradient in emission intensity (i.e., intensity-homogeneous). Figure 5c shows a comparison of the integrated PL intensity between both ends of the rods (Iforward/ Ireverse) with/without the gradient in emission intensity, as a function of the propagation length of the emission. Under the same excitation conditions, the GaN/InGaN core−shell QW rod with the intensity gradient shows a higher intensity contrast (∼2.70) than the intensity-homogeneous core−shell QW rod (∼2.18), after light propagation of ∼8 μm. This result obviously

photoluminescence (PL) images of the GaN/InGaN core− shell rod, whose bottom and top QWs, respectively, were locally excited. These images were taken at the image plane by a charge-coupled device (CCD) array. In the case of the reverse guide, the intensity of the blue emitted light declines by 67.7% or more (Figure 4b, left), and the peak of the blue emitted light remarkably shifts to a longer wavelength (from 442 to 481 nm) (Figure 4b, right) due to the band-to-band reabsorption and emission process during wave-guiding. In contrast, there is little intensity attenuation (Figure 4c, left) in the forward guide case, and only a small peak red-shift (from 496 to 501 nm) of the green light emitted from the top QWs (Figure 4c, right). This is because only the high energy part of the green emitted light has been slightly absorbed by the Urbach tail state of the QWs located lower than the excited spot, which have a higher Eg than the photon energy of the emitted light. Therefore, we can obtain asymmetric light output with different PL intensities from two opposite ends of our Eg-graded GaN/InGaN core− shell QW rods. Figure 5a shows the panchromatic PL images taken by CCD array following various local excitations with a 405 nm cw laser. The intensities of the light outputs from the bottom end in the forward guide mode (from F7 to F1) seem to be almost constant, whereas the intensities of the outputs from the top end in the reverse guide mode (from R1 to R7) diminish drastically as the propagating length of the emitted light increases. The PL intensities of the spots excited by the focused 405 nm laser vary depending on the locations of the excitation spot. These results agree fairly well with the position-dependent CL intensity variation (Figure 2c). Figure 5b indicates the integrated PL intensity (I) taken from both ends of the Eggraded GaN/InGaN core−shell QW rod, as a function of the 4941

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indicates that the graded-intensity core−shell rod is a more efficient structure than the homogeneous-intensity core−shell rod for producing more highly asymmetric light propagation. It is worth noting that the integrated PL intensity contrast between the two opposite ends of the rods can be further enhanced for real device application (e.g., photonic diode). This can be accomplished by adjusting the tapered angle of the GaN core rod to obtain a larger Eg-gradient, growing larger numbers of QWs to increase the absorption cross-section along the reverse guide, reducing the Urbach tail state in the QWs to reduce the scattering along the forward guide, and elongating the length of the rod. Importantly, our study of the Eg-graded GaN/InGaN core−shell QW rod with the intensity gradient represents significant progress toward the development of a highly efficient photonic diode and also offers a path toward application of optoelectronic devices to fast data processing and communication. Methods. The microstructure of the GaN/InGaN core− shell rods was observed by SEM (FEI Philips XL-30 SFEG) and STEM HAADF (JEOL JEM-ARM200F). The optical properties of an indiviual GaN/InGaN core−shell rod were investigated by CL (Gatan monoCL 4) experiments and micro-PL with a 405 nm cw laser as an excitation source. In the micro-PL experiment, the GaN/InGaN core−shell QW rods were dispersed onto a transparent glass substrate (Supporting Information, Figure S1). A microscope objective lens with a long working distance (Mitutoyo, ×100, N.A. = 0.5) was used to excite an area of approximately 1 μm2 of individual core− shell QW rods and to simultaneously collect the PL from the samples in the normal direction. A cw 405 nm semiconductor laser diode was used to selectively excite the InGaN layer. Next, the axis of the core−shell QW rod was completely aligned with the direction of the monochromator entrance slit. The PL spectra along the length of the rod were measured using a monochromator (Acton, SP2500) in conjunction with a twodimensional CCD detector. Motorized stages were employed to change the excitation position of the single core−shell QW rod (Supporting Information, Figure S4).

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NOTE ADDED AFTER ASAP PUBLICATION The caption of Figure 4 has been updated. The revised version was re-posted on April 30, 2014.

ASSOCIATED CONTENT

S Supporting Information *

Schematic illustrations for the sample preparation and the experimental setup for micro-PL measurement as well as descriptions of more detailed structural properties. 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]. Tel.: +82-42-350-2549. Fax: +82-42350-5549. Author Contributions

All authors contributed to all aspects of this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation (NRF-2013R1A2A1A01016914, NRF-2013R1A1A2011750) of the Ministry of Education, the Industrial Strategic Technology Development Program (10041878) of the Ministry of Knowledge Economy, and KAIST EEWS Initiative. 4942

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