3.6.1 Basic principle of PL spectroscopy
PL spectroscopy is a non-contact, non-destructive method of probing the electronic structure of materials. PL is the spontaneous photon emission from a material under optical excitation. Typical luminescences process in semiconductor can be simplified as shown in
figure 3.15. When photons with energy higher than the bandgap of the material, Eg are
incident on the sample, the sample is excited from the ground state. This creates electron- hole pairs due to the transfer of electrons from the valence band into the conduction band. The non-equilibrium electron and hole distribution relaxes via interband relaxation back to
3. Experimental techniques
Figure 3.15: Schematic of the basic processes involved in a typical luminescence experiment in optically excited semiconductors representing the process of a) excitation, b) interband relaxation and c) recombination.
With different variants of PL spectroscopy techniques such as temperature- dependent PL and excitation power dependent PL, one can determine the bandgap of the material, the crystal quality, detect impurity level and defects and study the recombination mechanisms . The bandgap of a semiconductor material depends on temperature due to the expansion of the crystal lattice. Temperature dependence of the bandgap can be fitted using the empirical Varshni equation given by the following:
𝐸! 𝑇 = 𝐸! 0 −!!
where Eg (0) is the bandgap value at 0 K, α and β are constants corresponding to different
Other than the shift of bandgap due to lattice expansion, an increase in temperature can also cause a change of carrier population of the conduction and valence bands. Free carriers at higher temperature perturb the band-edge and induce a tail of band states into the energy gap. At low temperature when the thermal energy is lower than the exciton binding energy, exciton related emission could be observed . Hence the transition or the changes observed with temperature-dependence PL can help determine the bandgap, recombination mechanisms, elemental composition, and whether there are defects related states.
Excitation power dependent PL can identify some underlying recombination
mechanisms. The intensity of the excitation light can control the density of photoexcited electrons and holes. Low carrier density means that the measurement is dominated by
defects and impurities at the interfaces. The increase in intensity with the excitation power should provide information whether this is the case. The rate of recombination at these sites
is proportional to the dominant carrier density, n. Discrete states can be filled as the
excitation power increases and bulk radiative recombination plays a greater role. The
radiative rate is proportional to n2 with the intensity.
3.6.2 Components of PL setup
The main components of a typical PL system consist of a laser, lenses to align the laser beam, detectors and spectrometer. Additional components include a microscope with magnification lenses for micro-PL and cryostat for temperature-dependent measurements. Figure 3.16 shows the typical setup for photoluminescence spectroscopy.
Figure 3.16: Schematic of a typical PL setup.
Two different experimental PL are carried out in this thesis, micro-PL for single nanowire measurements and macro-PL for measurements of nanowire ensembles. For single nanowire measurements of bandgap below 1600 nm, HORIBA Jobin Yvon T64000 Raman/PL system located at The Australian National University is used. The setup consists of a 632 nm He-Ne laser, Si CCD detector (detection wavelength: 350 – 1100 nm) and InGaAs array detector (800 – 1700 nm) and 50X magnifications near-infrared (NIR) objective lenses.
3. Experimental techniques
For narrow bandgap (above 1600 nm) samples such as high In concentration InGaAs nanowires discussed in chapter 3, macro-PL measurements on ensembles of nanowires are carried out with a continuous-wave, frequency doubled Nd:YAg laser (532 nm), liquid nitrogen cooled mercury cadmium telluride (MCT) detector connected to a lock-in-amplifier by collaborators at University of Oxford.
In summary, the experimental techniques for the growth of nanowires and their characterization used for this thesis have been presented in this chapter. Principles of MOVPE, SEM, TEM, EDX and PL were briefly discussed. Each of these techniques on its own is highly complex, but relevant details to the work in the thesis are covered. Further details can be found in the cited references provided.
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