Epitaxy And Optical Properties Of AlGaN Quantum Structure Materials

With the enhancing awareness of global environment-protection,mercury lamps will gradually disappear in our daily life after 2020.Thus,it is imminent to develop high-power AlGaN deep-ultraviolet light-emitting diodes(deep-UV LEDs)to completely replace the conventional mercury-containing UV light source.Despite the tremendous efforts have been made in preparation technology and application performance,some instinct challenges still serve as an obstacle on the way.With the increase of A1 component in AlGaN multiple quantum wells(MQWs),challenges are gaining in material growth and process preparation,i.e.,difficulty in epitaxy of high crystal quality of high A1 content AlGaN,poor p-type conductivity,and lower light extraction efficiency due to the unique optical polarization property.Those challenges are getting seriously with the emission wavelength shorter than 300 nm.Thus the external-quantum efficiency as well as the output power are still insufficiently low,which hinder the utilization of AlGaN-based DUV LEDs in massive application areas.Understanding and improving the optical properties of AlGaN quantum structures is the one of the most important objects for improving the quantum efficiency of DUV LEDs.In this thesis,we studied the emission mechanism of AlGaN MQWs firstly,then a novel MQWs structure with ultrathin AIN insertion layers to modify the strain status as well as optical polarization properties.The major results are as follow:Firstly,the emission mechanism of AlGaN MQWs were studied.The measurement results of cathodo luminescence of the traditional Alo.4Gao.6N/AlonsGao.sN MQWs structure show that the emission wavelength of QWs redshifts in the edge regions of the"hexagonal" structure on the surface.On the basis of analysis of Raman spectra and the first principle of simulations,such a phenomenon is mainly caused by the following reasons:During the growth of AlGaN,sub-grains slightly misorient with respect to each other due to the lattice-mismatch strain,leading to a large tensile stress accumulates at the grain boundary.As a result,Ga atoms segregate at the grain boundary due to that formation energy of Ga atoms is lower than A1 atoms under the tensile strain.Secondly,we propose an approach to modify the strain field in MQWs with introducing one or two ultrathin AIN layers(UALs)at the interfaces between well and barrier.The cross-sectional transmission electron microscopy image verifies that well-defined UALs have been inserted at the interfaces between the well and barrier layers.The analysis of Raman shifts confirms that stress variations of about-1.24 GPa and-1.46 GPa are achieved by introducing one and two ultrathin layers at the interfaces of MQWs,respectively.As a result,the degree of polarization is increased to 18.5%and 22.3%,respectively,from that of 17.8%in traditional MQWs.Moreover,the quantum well emission is found to become symmetric and narrower due to the suppression of compositional fluctuation.These results provide a simple technique to modify the strain field of MQWs so as to improve transverse-electric(TE)polarized emission for DUV LEDs.Thirdly,AlGaN micro-rod array were grown by Metal-organic vapor phase epitaxy system.Hexagonal GaN micro-rod arrays were firstly grown as the template by using self-Metal catalytic method.Then AlGaN micro-rod array were grown with ultra-short-period AlN/GaN superlattice in the pulse growth mode.The scanning electron microscope measurement results show that AlGaN micro-rod array with a diameter of 1.5?3 ?m and a density of about 3.9X1O6/cm2 has been achieved.In addition,some of the AlGaN micro-rods exhibit truncated-hexagonal-pyramid(THP)structures.Cathodoluminescence shows that the emission wavelength of the semipolar facets on the THP structure is about 260 nm,which is significantly blue shifted by about 30 nm from that of(0001)polar facets.This phenomenon may be caused by the difference growth rates of AIN and GaN on different polar surfaces.