micropipes ? makes me think of The Patch <g>
A crystal defect, a small tubular void normal to the wafer suface.
Degenerate: yes, I assume that whenever they refer to "a conductive silicon carbide substrate," in patents such as this one:
patents.ibm.com
This patent is their earliest on LEDs made from GaN on SiC, and it provides some good background. They put it in layman's terms in one of their earlier patents on SiC-only devices (US 5338944):
As used herein, the term "degenerate" has its ordinary meaning; i.e., a semiconductor material which has been extremely heavily doped with desirable impurities to give it a certain type of conductive character; i.e., a character which is more in the nature of a conductor than a semiconductor.
It's interesting to consider that a structure in which the conduction path is through the adjacent epitaxial buffer layer can result in superior device performance. SiC has advantages of a close match in lattice constant and thermal expansion coefficient, but even Cree must use transition layers. It wouldn't surprise me if other epitaxial techniques are developed to do a vertical structure from other substrates, perhaps with the lower contact being made entirely through an epi layer.
>>can't grow epi on-axis<<
U.S. #4,912,064 issued 03/27/90 to Kong et al. {Cree is a licensee}:
Homoepitaxial growth of alpha-SiC thin films and semiconductor devices fabricated thereon
patents.ibm.com
See the section of the DETAILED DESCRIPTION on Theory of "Off-Axis" Homoepitaxial Growth. This covers Alpha-SiC, 6H polytope in particular. 4H polytope is probably same concept, different angle.
WT |
