Optical Characterization of GaN-based Nanowires : From Nanometric Scale to Light Emitting Devices A-L. Bavencove*, E. Pougeoise, J. Garcia, P. Gilet, F. Levy, B. André CEA, LETI, MINATEC, DOPT/SIONA/LTN, Grenoble, France G. Tourbot, B. Gayral, B. Daudin CEA-INAC, SP2M/NPSC, CEA, Grenoble, France Le Si Dang Institut Néel, CNRS et Université Joseph Fourier, Grenoble, France * CL measurements on Single Nanowire Extraction of the nanowires Electron Beam Scanning (d~50nm) Si Substrate (n+) Overall CL Spectra (8K – 30 kV) Localized CL Spectra (8K – 30kV) The different regions of the nanowire exhibit different optical responses under electron beam radiation : p-type GaN : Structural Defects (Yellow Band Emission) Undoped GaN : Excitonic 357 nm InGaN QWs : No optical signature n-type GaN : DAP-Band (Interface defects?) CL Imaging Mode (8K – 30kV) GaN Band Edge DAP Band Edge 2 nd order Intensity (a.u.) Wavelength (nm) Energy (eV) 383 nm 356 nm CL and PL InGaN QWs Signals 500 nm SEM Image 500 nm CL Panchromatic Image Aimed Structure SEM Image (Top View) Si Substrate (n+) Undoped AlN Buffer Undoped GaN InGaN (3QWs) 100 nm Conclusions et Perspectives EL Measurements on Nanowire-based LEDs Optical signature identification of InGaN Quantum Wells inserted in undoped GaN nanowires Aimed Structure Wavelength (nm) Energy (eV) Intensity (a.u.) I/526 I/5758 I/9325 I/39193 Introduction Efficient integration of GaN-based nanowires into light emitting devices (LEDs) requires deep investigation of their optical properties. GaN-based nanowires, with and without InGaN multiple quantum wells were grown on n-doped silicon (111) substrate by molecular beam epitaxy (MBE). Si and Mg can be used during the growth process to obtain nanodiode structures (p-i-n junctions) which are characterized through 3 different techniques: Cathodoluminescence (CL), Photoluminescence (PL) and Electroluminescent (EL) spectroscopy. CL measurements have been conducted on both single nanowires and vertically self-assembled nanowires. A technological process has been developed in order to fabricate and characterize LEDs. For 350x350  m 2 devices, approximately 10 6 nanowires are electrically connected and operating in parallel. Thanks to this multiple-scale characterization approach, the light emission characteristics of GaN- based nanowires can be accurately investigated. Cold Plate (He Flow) (300  4K) Tungsten Filament Light Collection Optical System Monochromator Diffraction Grating Electron Beam Sample Detector (CCD Camera or PM) Cathodoluminescence Setup GaN-based Nanowires Integration Direct visualization of the luminescent regions can be achieved through CL imaging mode experiments Autoplanarized Process : 350x350  m 2 and 1x1mm 2 (~3x10 6 and 3x10 7 nanowires electrically connected) LEDs have been processed by directly depositing semitransparent electrodes and thick metal pads on the cone-shaped p-type region of the nanowires. The n-type contact is taken on the back-side of the wafer. GaN-based nanowires have been efficiently characterized : - Structural defects have been precisely localized - Active InGaN-based quantum wells have been identified These results have led to the drastic improvement of nanowire-based LEDs performances (EBL). Further electrical characterizations are in process: - n-type doping level evaluation (FET measurements) - Single vertical nanowires I-V characteristics by AFM (conductive mode) Aimed Structure n-type GaN (Si) p-type GaN (Mg) Undoped GaN 5 x InGaN QWs Si Substrate (n+) n-type AlN Buffer (Si) Schematic Cross Section of a GaN Nanowire-based LED 350  m p-type Transparent Contact Thick p-type Contact Schematic Top View of a GaN Nanowire-based LED p-type Contacts n-type Contact Si Substrate (n+) n-type AlN Buffer (Si) 2 excitation sources are used on vertically self-assembled nanowires : CL : Electron Beam - d~50nm – E = 30keV PL : Ar + Laser Beam - d~50  m – = 244nm Intensity (a.u.) Cathodoluminescence Spectra (300 et 9 K) Wavelength (nm) Energy (eV) 350 meV Photoluminescence Spectrum (7K) Intensity (a.u.) Wavelength (nm) Energy (eV) 270 meV Intensity (a.u.) Wavelength (nm) Intensity (a.u.) Wavelength (nm) With EBL Without EBL mA) Without EBL 50mA) With EBL 50mA) Room temperature EL characteristics of 1x1 mm 2 GaN nanowire-based LED. (A) EL spectra at various applied DC currents of nanowire-based LEDs with and without (INSET) an EBL. (B,C) Associated CCD Images. (D) Room Temperature Current-Voltage characteristics of GaN nanowire-based LEDs. GaN-based nanowires containing a p-type AlGaN Electron Blocking Layer (EBL) have been epitaxied and processed into LEDs according to the integration process described above. The AlGaN layer is inserted at the active region/p-type part interface and is aimed at localizing the recombinations of e - /h pairs in the InGaN-based QWs. The performances of devices with and without EBL are compared : n-type GaN (Si) p-type GaN (Mg) Undoped GaN 3 x InGaN QWs p-type AlGaN (Mg) Band Edge : RED SHIFT 1mm InGaN Signal : BLUE SHIFT (A) (B) (C) Voltage (V) Current (mA) (D) Exposition / 300 SEM image of a nanowire CL Panchromatic Image CL Monochromatic 356 nm CL Monochromatic 383 nm