<p>Nitride semiconductors are promising for applications in
opto-electronic devices due to their wide band gap that is adjustable by
appropriate choice of alloy composition. To date, many III-nitride devices have
been demonstrated, such as light-emitting diodes, lasers, etc. Most
opto-electronic devices make use of the optical transition from conduction band
to valence band. Moreover, the large conduction band offset achieved by
III-nitrides makes it possible to take advantage of transitions inside the
conduction band or valence band, which provide much more freedom for band
engineering. Although many III-nitrides based opto-electronic devices have been
invented and implemented in commercial use, there is still a need for more compact,
rugged, higher efficiency devices with lower cost. Many challenges of
III-nitride semiconductors are related to material defects, lattice mismatch
and internal polarization fields. Photoluminescence is a convenient technique
to characterize sample quality and optical properties. It does not destroy the
samples or need any electrical contacts. Therefore, it is commonly used in
qualitative analysis of III-nitrides. This thesis focuses on non-polar m-plane
III-nitrides structures, because this crystal orientation eliminates internal
polarization fields in heterostructures. We first performed a photoluminescence
study of a series of m-plane InGaN thin films with In compositions up to 24.5%.
Evidence of large In composition fluctuations was observed. This inhomogeneity of
In composition contributes to the non-monotonic temperature dependence of
photoluminescence peak energy and linewidth. A large drop of internal quantum
efficiency when temperature increases to room temperature was observed, which
indicates the presence of a large number of non-radiative recombination
centers. This is due to low temperature growth of InGaN by plasma assisted
molecular beam epitaxy. The InGaN film with 11% has a linewidth close to
theoretical calculations for InGaN with random In distribution, and much smaller
than many reported polar c-plane InGaN films with comparable In compositions, which
suggests improved material quality. This In composition was selected for the
design of InGaN/AlGaN superlattices.</p>
<p>In order to avoid the disadvantage of strain buildup, we designed
nearly strain-balanced non-polar m-plane InGaN/AlGaN structures with In
composition of about 9%. Steady-state photoluminescence and time-resolved
photoluminescence were performed on these structures. A significant discrepancy
between measured and calculated PL peak positions was observed. This is likely
due to the In composition fluctuations and quantum confinement in quantum wells.
The broadening mechanism of the PL in the superlattices was investigated. The
low-temperature linewidth of undoped superlattices is comparable to many
previously reported values for m-plane InGaN/GaN quantum wells. Similar to
InGaN films, the internal quantum efficiency drops dramatically when
temperature reaches room temperature. Regions with high In compositions act as
localization centers for excitons. An average localization potential depth of
21 meV was estimated for undoped superlattices. This small potential depth does
not reduce the degree of polarization of emitted light, and contributes to the narrow
linewidth. A fast decay time of 0.3 ns at 2 K was observed for both doped and undoped
superlattices. This value is much smaller than that for polar c-plane InGaN/GaN
superlattices. The localization of excitons was found to be strong and not
affected by magnetic field at low temperatures. Compared with undoped
superlattices, the doping sheets reduce decay pathways of excitons in doped
superlattices.</p>
<p> </p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/17300222 |
| Date | 03 January 2022 |
| Creators | Yang Cao (11858636) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/PHOTOLUMINESCENCE_STUDY_OF_NON-POLAR_III-NITRIDE_SEMICONDUCTORS/17300222 |
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