Vertically Stacked Color Tunable Light-Emitting Diodes Fabricated Using Wafer Bonding and Transfer Printing Jaeyi Chun, † Kwang Jae Lee, ‡ Young-Chul Leem, ‡ Won-Mo Kang, ‡ Tak Jeong, § Jong Hyeob Baek, § Hyung Joo Lee, # Bong-Joong Kim, ‡ and Seong-Ju Park* ,†,‡ † Department of Nanobio Materials and Electronics and ‡ School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea § LED Device Research Center, Korea Photonics Technology Institute, Gwangju 500-779, Republic of Korea # CF Technology Division, AUK Corporation, Iksan 570-210, Republic of Korea * S Supporting Information ABSTRACT: We report on the vertically stacked color tunable light-emitting diodes (LEDs) fabricated using wafer bonding with an indium tin oxide (ITO) layer and transfer printing by the laser lift-off process. Employing optically transparent and electrically conductive ITO as an adhesion layer enables to bond the GaN-based blue and AlGaInP-based yellow LEDs. We find out that the interdiffusion of In, O, and Ga at the interface between ITO and GaP allows the strong bonding of the heterogeneous optoelectronic materials and the integration of two different color LEDs on a single substrate. The efficacy of this method is demonstrated by showing the successful control of color coordinate from the vertically stacked LEDs by modulating the individual intensity of blue and yellow emissions. KEYWORDS: color tunable light-emitting diodes, vertical integration, ITO adhesion layer, wafer bonding, transfer printing I norganic light-emitting diode (LED) has been focused as a key component in solid-state lighting source because of its intrinsic properties such as high internal quantum efficiency (IQE), external quantum efficiency, low power consumption, and long-term stability. 1-3 Numerous efforts in past decade have significantly developed the LED technology to satisfy the diverse requirements for solid-state lighting systems including general lighting and outdoor digital signage. 4,5 Charge separation in a GaN-based active region which reduces the IQE has been improved using staggered quantum well structures 6,7 and nonpolar or semipolar GaN LEDs. 8,9 Epitaxial lateral overgrowth (ELOG) and surface plasmon have been also managed to increase the IQE, 10,11 and surface texturing or photonic crystal have been suggested for high light extraction efficiency. 12,13 In addition, previous studies have introduced methods to fabricate passive or active matrix monochromatic microdisplays using GaN-based blue or green LEDs. 14-17 These addressing techniques show the feasibility for use in next-generation visual systems that are high resolution, near-to-eye, or for projections. However, different structural aspects between GaN-based LEDs covering from blue to green and AlGaInP-based LEDs emitting in the range from yellow to red make it difficult to grow and integrate both materials on a same substrate. 18 Current skills to generate multicolor LEDs through the mechanical packaging of different color LED chips in a lateral configuration impose a limit on the resolution due to chip placement accuracy which is a few hundred microns. 17 Therefore, the problematic issue of integrating red, green, and blue primary color LEDs on a single wafer should be challenged to expand its applications to high- resolution full-color compact displays, biomedical, and optogenetic applications which require multiwavelengths for in vivo or in vitro illumination of biological units located on tiny area. 19 Previously, several studies have suggested approaches to fabricate single wafer-based color tunable LEDs using multifaceted GaN nanorods, 20 high In-content InGaN- based LEDs, 21 and inkjet printing of organic color converters on ultraviolet LEDs, 22 but the external bias-dependent color change and the degradation of emission efficiency because of color converters still require breakthroughs to achieve full range color tunability by controlling the emission intensity of the primary colors. Here, we report color tunable dual wavelength LEDs by vertically stacking GaN-based blue LEDs onto AlGaInP-based yellow LEDs using wafer bonding and transfer printing processes. This scheme not only enables the construction of heterogeneous materials on a single substrate for compact pixel design using a simple fabrication process, but also provides a wide range of color control by modulating the individual intensity of highly pure blue and yellow emission. The bonding Received: August 12, 2014 Accepted: November 3, 2014 Published: November 3, 2014 Letter www.acsami.org © 2014 American Chemical Society 19482 dx.doi.org/10.1021/am505415q | ACS Appl. Mater. Interfaces 2014, 6, 19482-19487