Microstructural Observations on Gallium Nitride Light-Emitting Diodes

Abstract

Microstructural observations, utilizing optical microscopy, scanning electron microscopy, and transmission electron microscopy, were made on gallium nitride (GaN) metal/insulator/n-type semiconductor (m-i-n) light-emitting diodes. The GaN films from which the diodes were fabricated were grown by heteroepitaxial chemical vapor deposition on sapphire substrates, and consisted of an undoped n-layer and a Mg-doped i-layer; a metal contact was subsequently placed on the surface of the i-layer forming an m-i and an n-i junction. Significant observations include the following: optical microscopy and scanning electron microscopy show a faceted surface of the layers and the existence of cell boundaries; the cell boundaries result from the coalescence of the individual crystallites formed on the substrate in the early stages of growth; x-ray Laue patterns consist of spots that are slightly diffuse, indicating that the cells are only slightly misoriented with respect to their adjoining cells; light emission occurs as a pattern of small discrete spots and this pattern of spots correlates with the faceted structure of the film; the light emanates from the cathode regions (the i-n junction with forward bias and the m-i junction with reverse bias); steep electrical potential gradients (~105 V/cm), measured using line scan techniques in the scanning electron microscope, occur at the m-i and the n-i junctions. The observations are consistent with an impact ionization model for light emission. © 1974, The Electrochemical Society, Inc. All rights reserved.