Electrostatic Mechanism of Emission Enhancement in Hybrid Metal-semiconductor Light-emitting Heterostructures Page: 8
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These compounds are generally not used in bulk structures, but are instead used
in GaN heterostructures to provide either barriers for GaN quantum well in the case of
AIGaN, or as quantum wells confined by GaN barriers in the case of InGaN. Progress
has also been made on using them in quantum wire or quantum dot structures .
2.3 Growth Issues and Defects
Initial attempts at growing GaN and its alloys were limited to heteroepitaxial
growth. This is due to the difficulty in producing high-quality GaN crystals for use as
growth substrates . Instead the initial substrate of choice was sapphire (A1203).
Although progress has been made on free-standing HPVE GaN substrates , most
growth continues on sapphire or silicon carbide substrates.
Growth of the heterostructures can be done via molecular beam epitaxy (MBE) or
metal-organic chemical vapor deposition (MOCVD). Until recently MOCVD resulted in
higher quality samples and required less growth time than MBE . This was part of
the reason for the quick adoption of the nitrides for light emission, as MOCVD is fast
enough to be used for commercial growth.
Due to the lattice mismatch between sapphire and GaN, a high number of
defects occur at the interface between the two. The most common defect arising from
this mismatch is the threading dislocation (TD), which can have dislocation densities
between 106-1010 cm-2. Threading dislocations arise at GaN/sapphire interface at the
boundaries of separate regions of GaN which are rotated with respect to each other
oriented along the c-axis. These dislocations propagate to the surface from the
substrate boundary, with some being annihilated or terminating before reaching the
surface . The efficiency of GaN and AIGaN-based LDs and LEDs turns out to be
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Llopis, Antonio. Electrostatic Mechanism of Emission Enhancement in Hybrid Metal-semiconductor Light-emitting Heterostructures, dissertation, May 2012; Denton, Texas. (digital.library.unt.edu/ark:/67531/metadc115113/m1/18/: accessed March 23, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; .