The growth of semipolar AlN and AlGaN epilayers on m‐plane sapphire substrates by metalorganic chemical vapor deposition (MOCVD) has been investigated for the first time. The implementation of pulsed MOCVD technique for the deposition of the AlN buffer, and the insertion of a strain relieving AlN/AlGaN short‐period superlattice structure proved instrumental in the growth of a thick, crack‐free, and single domain n‐Al0.56Ga0.44N ($11{\bar {2}}2$) films, which also exhibit a good crystal quality. To assess the suitability of this AlGaN/AlN/m‐sapphire template for practical use in deep‐ultraviolet light emitters, both polar (0001) and semipolar ($11{\bar {2}}2$) AlGaN multiple quantum wells (MQWs) were grown side‐by‐side. The room temperature photoluminescence (PL) spectra of the semipolar MQW structure peaked at 305 nm, which was approximately 10 nm longer than the PL peak originating from the reference polar MQWs. Besides, the semipolar MQWs showed a fairly excitation‐independent emission wavelength, which suggests the absence of any polarization‐induced electric fields. The intensity of the luminescence from the c‐oriented MQWs was, however, stronger than that of its semipolar counterpart. The result indicates that further reduction of the extended defects density in ($11{\bar {2}}2$) AlGaN is critical to significantly improving the optical properties of the overlying quantum heterostructures, and potentially achieving efficient optoelectronic devices.
Electron microscopy investigation has been carried for an n- ZnO /p- GaN:Mg heterojunction ultraviolet (UV) light-emitting diode device, where the n- ZnO layer was grown by atomic layer deposition on the p-type GaN:Mg /undoped GaN structure prepared on c- Al 2 O 3 substrate. Threading dislocations, formed at the interface of the GaN/Al2O3 , disappeared at the interface of n- ZnO /p- GaN during post-deposited rapid thermal annealing and accordingly the n- ZnO layer became an almost perfect single crystal including only a few surviving dislocations. An interfacial layer was found along the (0001) interface between the n- ZnO and p- GaN:Mg layers. Scanning transmission microscopy analysis revealed that the interfacial layer was composed of ZnO crystal, which connected coherently with the neighboring n- ZnO and p- GaN:Mg . This interfacial layer contained a few atomic percents of Mg , the atoms of which had been diffused from the p- GaN:Mg . The high quality crystalline n- ZnO and the interfacial layer forming a ZnO/GaN interface states are responsible for the UV electroluminescence from the ZnO .
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