Photonic Crystal-Assisted Light Extraction from a Colloidal Quantum Dot/GaN Hybrid Structure Fre ´de ´ ric S. Diana,* ,² Aure ´ lien David, ‡ Ines Meinel, | Rajat Sharma, ‡ Claude Weisbuch, ‡ Shuji Nakamura, ‡ and Pierre M. Petroff ²,§ 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 chemistry 1 have opened a number of potential applications. Their high extinction coefficient (up to 10 7 cm -1 ‚M -1 ), high internal quantum efficiency (>50%), and emission stability 2 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 deposi- tion (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 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 wave- length (∼100 nm for visible light) so that QDs (with sizes typically <10 nm) placed on or near such cavities fully overlap with all possible guided modes and feed them with a large portion of their radiated power. The field produced by a randomly oriented oscillating electric dipole source, corresponding to the emission of a QD, is composed of plane- wave components propagating in all directions and of * To whom correspondence should be addressed. E-mail: frederic@ engineering.ucsb.edu. ² 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. NANO LETTERS 2006 Vol. 6, No. 6 1116-1120 10.1021/nl060535b CCC: $33.50 © 2006 American Chemical Society Published on Web 04/27/2006