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 false(normalGaNfalse) metal/insulator/n‐type semiconductor (m‐i‐n) light‐emitting diodes. The normalGaN 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.

Progress in the growth of nonpolar gallium nitride

“…Unfortunately, attempts over thirty years of GaN epitaxy research repeatedly indicated that the nonpolar planes were 'unstable' [6][7][8][9][10][11][12][13]. One study after another yielded nonpolar films with surfaces too rough and/or faceted for device growth.…”

Optical Properties of GaN Light Emitting Diodes

“…The large difference between the EL and PL spectra of the blue LED at RT in the spectrum peak energy is presumed to be caused by the Zn concentration gradient near the i-n junction along the crystal growth direction. The electric potential distributions across the junction measured by Pankove (13) and Maruska et al (14) change near the i-n junction, but not abruptly. This potential distribution indicates that the Zn concentration near the junction increases along the crystal growth direction.…”

“…This potential distribution indicates that the Zn concentration near the junction increases along the crystal growth direction. The EL occurs precisely at the junction (14,15), i.e., at the interface of the n-and i-layers.…”

Extended defect structure of a-plane GaN produced by sidewall lateral epitaxial overgrowth

“…One alternative approach is the heteroepitaxial method, where NP and SP GaN are grown directly on foreign substrates. Heteroepitaxial systems have been realized for a-plane GaN on r-plane sapphire [13][14][15][16] or on a-plane SiC with AlN buffer layer [17,18], The major issue for heteroepitaxially grown nonpolar and semipolar GaN is microstructural defects, where high densities of basal plane stacking faults (BSF; typically I 1 ) and high density of threading dislocations (TDs; partial and primary) are typically observed. There are three main types of BSFs in the wurtzite structure.…”