Optical studies of exciton-phonon coupling in nonpolar plane GaN

Abstract

Optical studies of nonpolar plane GaN substrates with emphasis on exciton-phonon coupling were carried carefully. These substrates were grown via HVPE method and had a good quality which enable us to distinguish diverse exciton lines and therefor have a profounder understanding of the exciton-phonon coupling process. It is widely believed that nonpolar GaN is one of the ultimate solutions for producing LEDs and HEMTs. Its excellent optical and electric properties take them an invincible lead in industries. Here, in this thesis, we use PL spectroscopy and Raman spectroscopy to study these samples. Temperature dependent PL spectra were carried with two different polarities of laser. The zero phonon lines of m-, a- and c-plane GaN were well resolved at low temperatures. For all samples, the dominated near band edge emission comprised free exciton transitions, donor bound electron transitions and an abnormal line. The binding energy of A-type exciton is estimated to 22 meV which is in a good agreement with theoretical one. B-type exciton transition is also resolved and found a strong polarity dependence on FXB. There are two kinds of donor bound excitons observed in PL spectra, which are A- and B-type free excitons bounded to neutral donors (Si on Ga site) independently. An abnormal line Ix is observed at the lower energy side of D0X whose origin is still under debate. TES transitions can be used to calculate the binding energy of bound exciton, in which radiative recombination leaves an excited electron on donor. By taking PL spectra measurement in various temperature, we know that Ix rapidly quenches, and there is an alteration of bound excitons to free excitons. Then, the attention was directed on the exciton-phonon coupling process. Detailed luminescence spectra were further investigated for corresponding LO-phonon replicas. The line shape and intensity of phonon-assisted emission is full of information. We use several theoretical models to give a reasonable explanation on the exciton-phonon coupling. We started with Huang-Rhys factor which characterizes the coupling strength of excitons and phonons. We can calculate the S factor of different transitions. Especially, B-type exciton shows a weak coupling strength, which can be theoretical attributed to its light effective mass. From the model proposed by Segall and Mahan, we can see the line shape of emission lines with phonon participation will reflect the Maxwell-Boltzmann distribution of the free exciton kinetic energy. A formula to describe this process was proposed and used to check experimental result immediately. It turns out that the Segall Mahan model runs very well for free exciton. Another model proposed by Hopfield was used to check donor bound exciton transition, moreover, a MBO model which propose a new phonon bath mode is also given. These two models somehow play a complementary role in investigation of exciton-phonon coupling. A well-defined oscillatory feature located in Fano resonance part was found in the low-temperature PL spectra of nonpolar plane GaN only.