Electrostatic Mechanism of Emission Enhancement in Hybrid Metal-semiconductor Light-emitting Heterostructures Page: 54
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"wing" regions. These wing regions experience strain relaxation during growth and so
have a lower overall strain than the GaN in the seed areas. The growth process is
further detailed in 2.3.1. Figure 4.6 depicts the seed and wing regions as imaged using
an optical microscope as well as AFM. Having been studied before , this controlled
wide-area strain variation provides an excellent baseline for testing our strain mapping
method. In order to extract S empirically, near-field photoluminscence (PL)
measurements were conducted using a near-field scanning optical microspectrometer
(NSOM) over a 45x45 pm region of the sample. At each measurement point, the
spectrum was fitted using the method described in 3.7.
This fitting allows for the extraction of various parameters at each measurement
point including the phonon replica intensities, which are then used to calculate S
empirically according to the previously given equation S = (n+1) In,/In. Furthermore, I
was able to simultaneously map the topology of the system to allow me to associate the
emission with the seed and wing regions. Figure 4.7 depicts the extracted parameters,
including the max intensity and S, along with the near-field topology. As can be seen in
panel d of Figure 4.7, there are regions near the center of each wing region which are
relaxed compared to the seed regions. It is interesting to note that the majority of
relaxation appears to occur at the boundary where the two halves of the wing region
meet. It is unclear as to whether this is a result of the way growth proceeds when the
two facets meet, or whether it is a direct result of the gradual relaxation that should
occur as they grow away from the seed regions.
While the strain map in Figure 4.7d agrees well qualitatively with the expected
variations, the values for strain produced from the theoretical model are around -0.2%
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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/m1/64/: accessed April 24, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; .