ETE:S Multi-quantum-well nanowire heterostructures for wavelength-controlled lasers FANG QIAN1, YAT L*, SILVIJA GRADECAK1* HONG-GYU PARK1* YAJIE DONG, YONG DING2 ZHONG LIN WANG2+ AND CHARLES M. LIEBER1, 3+ epartment of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, US sChool of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA Present address: Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, USA YL); Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA(SG ) Department of Physics, Korea University, Seoul 136-713, Korea(H.-GP) e-mail: zhong wang @mse gatech. edur; cmlecmlinis harvardedu ublished online: 17 August 2008; doi: 10.1038/nmat2253 Rational des reasingly believe that a decoupling of st complex structures can yield enhanced and/or novel electronic could offer advantages over and photonic functions. For example, Ge/Si core/shell designed laser wavelength nanowires have exhibited substantially higher performance optimization of the cavity as field-effect transistors and low-temperature quantum To address this general concept in nanolasers, we have focused devices"compared with homogeneous materials, and on group-IlI nitride multi-quantum-well(MQW)nanowire nano-roughened Si nanowires were recently shown to have heterostructures. Our MQW nanowire structure design(Fig. la) an unusually high thermoelectric figure of merit. Here, we consists of a gan nanowire core, which functions as the primary report the first multi-quantum-well (MQW) core/shell nanowire part of the optical cavity o, and epitaxial In GaN/GaN MQW heterostructures based on well-defined Ill-nitride materials shells, which serve as the composition-tunable gain medium. that enable lasing over a broad range of wavelengths at room An MQW structural motif was chosen because of its potential temperature. Transmission electron microscopy studies show as a low-threshold and tunable gain medium6.7, although such hat the triangular Gan nanowire cores enable epitaxial and complex shell structures have not been previously dislocation-free growth of highly uniform (In GaN/GaN)n nanowire materials. In comparison with planar MQW structures, quantum wells with n = 3, 13 and 26 and In Gan well nanowire heterostructure growth has the potential advantage of a thicknesses of 1-3 nm. Optical excitation of individual MQw dislocation-free and unstrained Gan nanowire 'substrate but has nanowire structures yielded lasing with In Gan quantum-well the added complexity of distinct lateral facets that could affect, for composition-dependent emission from 365 to 494 nm, and example, growth rate threshold dependent on quantum well number, n Our work MQW nanowire heterostructures with 3-26 quantum demonstrates a new level of complexity in nanowire structures, wells were prepared by metal-organic chemical vapour which potentially can yield free-standing injection nanolasers. deposition, where the gan nanowire core was elaborated in a Miniaturized multicolour lasers could be enabled with the metal-nanocluster-catalysed growth step followed immediately by levelopment of a tunable-bandgap nanoscale gain medium that MQW shell deposition without removal from the growth reactor coupled effectively into a small optical cavity. Free-standing (see the Methods section). Transmission electron nanowires have received considerable attention as nanolasers'-,(tEM) data recorded with the electron beam perpendicular to the where the semiconductor nanowires have functioned as both the axis of a typical MQW nanowire(Fig. 1b) reveals that the structure gain medium and optical cavity. So far, nanowire lasers have is single crystalline and dislocation free; measurements made on been reported for several homogeneous binary semiconductors, 50 independent samples showed similar results. High-resolution including GaSb(refs 7, 8), Zno(ref. 9), Gan (refs 10, 11), Cds TEM images and electron diffraction data(Fig. Ic)further confirm (ref. 12) and Zns (ref. 13), where the lasing wavelength in the single-crystalline structure, demonstrate that the nanowires of the respective homogeneous nanowire materials he dirges grow along the (1120) direction and show that nanowire surfaces istinct are atomically smooth. The smooth interfaces are consistent nanowire bandgaps have led to lasing over a relatively wide range with the growth of continuous quantum-well shells on the gan of discrete wavelengths but have not enabled continuous tuning in nanowire core, and are also expected to reduce optical losses at the emission colour, although engineered lasing wavelength has been nanowire-air interface, which is an important factor for reducing demonstrated in alloy nanoribb More generally, we threshold in nanowire laser naturematerialsvol7iSepTembeR2008iwww.nature.com/naturematerials @2008 Macmillan Publishers Limited. All rights reserved.LETTERS Multi-quantum-well nanowire heterostructures for wavelength-controlled lasers FANG QIAN1 , YAT LI1 *, SILVIJA GRADECAK ˇ 1 *, HONG-GYU PARK1 *, YAJIE DONG1 , YONG DING2 , ZHONG LIN WANG2† AND CHARLES M. LIEBER1,3† 1Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA 2 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA 3 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA *Present address: Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, USA (Y.L.); Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA (S.G.); Department of Physics, Korea University, Seoul 136-713, Korea (H.-G.P.) † e-mail: zhong.wang@mse.gatech.edu; cml@cmliris.harvard.edu Published online: 17 August 2008; doi:10.1038/nmat2253 Rational design and synthesis of nanowires with increasingly complex structures can yield enhanced and/or novel electronic and photonic functions1,2 . For example, Ge/Si core/shell nanowires have exhibited substantially higher performance as field-effect transistors3 and low-temperature quantum devices4,5 compared with homogeneous materials, and nano-roughened Si nanowires were recently shown to have an unusually high thermoelectric figure of merit6 . Here, we report the first multi-quantum-well (MQW) core/shell nanowire heterostructures based on well-defined III-nitride materials that enable lasing over a broad range of wavelengths at room temperature. Transmission electron microscopy studies show that the triangular GaN nanowire cores enable epitaxial and dislocation-free growth of highly uniform (InGaN/GaN)n quantum wells with n = 3, 13 and 26 and InGaN well thicknesses of 1–3 nm. Optical excitation of individual MQW nanowire structures yielded lasing with InGaN quantum-well composition-dependent emission from 365 to 494 nm, and threshold dependent on quantum well number, n. Our work demonstrates a new level of complexity in nanowire structures, which potentially can yield free-standing injection nanolasers. Miniaturized multicolour lasers could be enabled with the development of a tunable-bandgap nanoscale gain medium that is coupled effectively into a small optical cavity. Free-standing nanowires have received considerable attention as nanolasers7–13 , where the semiconductor nanowires have functioned as both the gain medium and optical cavity. So far, nanowire lasers have been reported for several homogeneous binary semiconductors, including GaSb (refs 7,8), ZnO (ref. 9), GaN (refs 10,11), CdS (ref. 12) and ZnS (ref. 13), where the lasing wavelength in these studies corresponded to the fundamental bandgap energies of the respective homogeneous nanowire materials. The distinct nanowire bandgaps have led to lasing over a relatively wide range of discrete wavelengths but have not enabled continuous tuning in emission colour, although engineered lasing wavelength has been demonstrated in alloy nanoribbon systems14,15. More generally, we believe that a decoupling of studies of the gain medium and cavity could offer advantages over homogeneous nanowire structures for designed laser wavelength output in parallel with independent optimization of the cavity. To address this general concept in nanolasers, we have focused on group-III nitride multi-quantum-well (MQW) nanowire heterostructures. Our MQW nanowire structure design (Fig. 1a) consists of a GaN nanowire core, which functions as the primary part of the optical cavity10,11, and epitaxial InGaN/GaN MQW shells, which serve as the composition-tunable gain medium. An MQW structural motif was chosen because of its potential as a low-threshold and tunable gain medium16,17, although such complex shell structures have not been previously reported for nanowire materials. In comparison with planar MQW structures, nanowire heterostructure growth has the potential advantage of a dislocation-free and unstrained GaN nanowire ‘substrate’ but has the added complexity of distinct lateral facets that could affect, for example, growth rates. MQW nanowire heterostructures with 3–26 quantum wells were prepared by metal–organic chemical vapour deposition, where the GaN nanowire core was elaborated in a metal-nanocluster-catalysed growth step followed immediately by MQW shell deposition without removal from the growth reactor (see the Methods section). Transmission electron microscopy (TEM) data recorded with the electron beam perpendicular to the axis of a typical MQW nanowire (Fig. 1b) reveals that the structure is single crystalline and dislocation free; measurements made on 50 independent samples showed similar results. High-resolution TEM images and electron diffraction data (Fig. 1c) further confirm the single-crystalline structure, demonstrate that the nanowires grow along the h1120¯ i direction and show that nanowire surfaces are atomically smooth. The smooth interfaces are consistent with the growth of continuous quantum-well shells on the GaN nanowire core, and are also expected to reduce optical losses at the nanowire–air interface, which is an important factor for reducing threshold in nanowire lasers11 . nature materials VOL 7 SEPTEMBER 2008 www.nature.com/naturematerials 701 © 2008 Macmillan Publishers Limited. All rights reserved