Gallium nitride (GaN) epitaxial layers were deposited by metalorganic chemical vapor deposition (MOCVD) on (0001) sapphire with the goal to reduce threading dislocation density through etching, agglomeration and plugging. An in-situ SiH4 etching method was adopted as an essential step to delineate pits on dislocations and to cluster pits in the GaN layer. After the etching, the GaN layer at low temperature was grown to increase the size of etch pits and to agglomerate them. An amorphous material like silicon dioxide (SiO2) or silicon nitride (Si 3N4) was deposited either ex-situ or in-situ inside optimally sized pits and agglomerated pits in the GaN. This new method is termed Etch pit, pit agglomeration, and plug-in approach; subsequently, epitaxial lateral overgrowth (ELOG) was carried out and was expected to provide lower threading dislocation density (TDD). In this work, two methods were studied to reduce the threading dislocations in GaN: ex-situ and in-situprocess. Amorphous plugs on etch-pits using ex-situ process can prevent the replication of dislocations and may lead to the loss of the crystal information associated with TDs. However, it was found that TDs were replicated into epilayer. Half of the dislocations propagated along the side facets of pits and were reconstituted above the pits during growth due to a memory of crystal information. So, an all-in-one-reactor growth process using an in-situ method for depositing mask material was developed. The enlargement and agglomeration of pits was very important because it led to selective in-situ deposition of a Si3N4. The low temperature GaN growth and SiH 4 etching were crucial factors to achieve opening of the pits and to agglomerate them. It also produced a larger inner free surface during the ELOG. Most of the dislocations were terminated at the surface of voids to minimize the free energy. Some bending of TDs also is observed above the SiN x layer due to strong image force. These voids and SiNx are the main factors for reduction of dislocations. The presence of these voids did not deteriorate the upper material quality and the flatness of the resulting film. So, this in-situ process can be used as a high quality crystal growth of GaN layers with ultra low TDD. TDD can be reduced by three orders of magnitude as compared to the standard two-step growth.
|School:||Arizona State University|
|School Location:||United States -- Arizona|
|Source:||DAI-B 71/01, Dissertation Abstracts International|
|Keywords:||Epitaxial growth, Gallium nitride, Threading dislocation|
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