Microscopy and Microanalysis

Materials Applications

Insight into the Compositional and Structural Nano Features of AlN/GaN DBRs by EELS-HAADF

Alberto Eljarrata1 c1, Lluís López-Conesaa1, César Magéna2a3, Žarko Gačevića4, Sergio Fernández-Garridoa4 p1, Enrique Callejaa4, Sónia Estradéa1a5 and Francesca Peiróa1

a1 Laboratory of Electron NanoScopies, LENS-MIND-IN2UB, Dept. Electrónica, Universitat de Barcelona, Marti i Franqués 1, 08028 Barcelona, Spain

a2 Laboratorio de Microscopías Avanzadas (LMA), Instituto de Nanociencia de Aragón (INA), Departamento de Física de la Materia Condensada, Universidad de Zaragoza, 50018 Zaragoza, Spain

a3 Fundación ARAID, 50018 Zaragoza, Spain

a4 Instituto de sistemas optoelectrónicos y Microtecnología (ISOM), Universidad Politécnica de Madrid, Ciudad Universitaria s/n, 28040 Madrid, Spain

a5 TEM-MAT, (CCiT), Universitat de Barcelona, Lluís Solé i Sabarís 1-3, 08028 Barcelona, Spain

Abstract

III-V nitride (AlGa)N distributed Bragg reflector devices are characterized by combined high-angle annular dark-field (HAADF) and electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope. Besides the complete structural characterization of the AlN and GaN layers, the formation of AlGaN transient layers is revealed using Vegard law on profiles of the position of the bulk plasmon peak maximum. This result is confirmed by comparison of experimental and simulated HAADF intensities. In addition, we present an advantageous method for the characterization of nano-feature structures using low-loss EELS spectrum image (EEL-SI) analysis. Information from the materials in the sample is extracted from these EEL-SI at high spatial resolution.The log-ratio formula is used to calculate the relative thickness, related to the electron inelastic mean free path. Fitting of the bulk plasmon is performed using a damped plasmon model (DPM) equation. The maximum of this peak is related to the chemical composition variation using the previous Vegard law analysis. In addition, within the context of the DPM, information regarding the structural properties of the material can be obtained from the lifetime of the oscillation. Three anomalous segregation regions are characterized, revealing formation of metallic Al islands.

(Received November 23 2012)

(Accepted February 11 2013)

Key words

  • EELS;
  • HAADF;
  • III-V nitrides;
  • DBR;
  • STEM

Correspondence

c1 Corresponding author. aeljarrat@el.ub.edu

p1 Current address: Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 57, 10117 Berlin, Germany