Microstructural Observations on Gallium Nitride Light-Emitting Diodes

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Abstract
Microstructural observations, utilizing optical microscopy, scanning electron microscopy, and transmission electron microscopy, were made on gallium nitride metal/insulator/n‐type semiconductor (m‐i‐n) light‐emitting diodes. The 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.

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