Non-polar GaN epilayers and heterostructures for photonic applications

This PhD thesis describes the experimental study of wurtzite III-nitride semiconductors grown on non-polar crystal orientations, namely (1120) a- and (1010) m-planes. Hindered by poor material quality, they were not as extensively investigated as the polar basal (0001) c-plane. Nevertheless, the renewed interest in this topic has been gaining momentum since 2000, thanks to the improvement in material quality. The motivation is to overcome the quantum-confined Stark effect (QCSE) in heterostructures grown on c-plane, which is induced by the intrinsic spontaneous and piezoelectric polarization field. QCSE weakens the performance of c-plane optoelectronic devices such as light emitting diodes (LEDs) and laser diodes and it can be avoided by growing these devices on non-polar planes. In addition, non-polar LEDs are also featured by the spontaneously polarized light emission and thus have a large potential for liquid crystal display backlighting application. This thesis includes two aspects: (1) Since previous study in our laboratory was lacking before this work, the first target was to establish the heteroepitaxial growth procedures for a- and m-plane GaN layers on r- and m-plane sapphire substrates, respectively, by hydride vapor phase epitaxy (HVPE). Then, smooth planar templates have been obtained. Particularly, m-plane GaN has been demonstrated on m-plane sapphire. Further analysis revealed that actually three orientations (two semi-polar and one non-polar) can be grown on m-plane sapphire through the modification of sapphire surface chemistry and/or growth procedure. Nevertheless, the material quality of these non-polar templates is inferior to that of c-plane as they are characterized by the presence of high-density stacking faults and threading dislocations. To reduce the defect density, epitaxial lateral overgrowth (ELO) technique has been applied. As a result, material quality close to that of planar c-plane was achieved in GaN layers overgrown on the mask areas. Overall, the experimental results revealed peculiar characteristics of non-polar orientation, induced by the intrinsic anisotropy of wurtzite structure parallel and perpendicular to the basal plane. It also lays the foundation for further improvement in material quality. (2) Based on these aforementioned results, the next stage was to investigate the heterostructures grown on those HVPE ELO templates. GaN/AlGaN single quantum wells (QWs) and AlN/GaN distributed Bragg reflectors (DBR) have been grown by NH3-source molecular beam epitaxy (MBE). The experiments confirm that QWs are free from QCSE and their optical properties are comparable to those grown on planar c-plane templates. Furthermore, nitride-based microcavities (MCs) have been demonstrated on a-plane ELO templates. Their optical properties are encouraging and open the way for more sophisticated structure aimed at polariton physics. In conclusion, non-polar a- and m-plane GaN planar and ELO templates have been obtained through epitaxy on r- and m-plane sapphire substrates, respectively, by HVPE. Nanostructures such as QWs, DBRs, and MCs have been grown on non-polar HVPE GaN ELO templates by MBE.

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