| dc.description.abstract | Because of the breakthrough of the epitaxial growth technique in the past decade, the semiconductor solid-state lighting becomes the focus of new lighting source. GaN-based LED (light emitting diode) emitting in blue to green region is one of the topics that have been expected to realize the solid-state lighting. The fabrication technique of GaN-based LED device not only depends on GaN crystal quality but also the light extraction efficiency. The different LED structures have been developed to improve the light extraction efficiency. Vertical LED structures attract a lot of attention because it has the excellent light extraction efficiency and heat dispersion ability. The efficiency improvement of the vertical LED relies on three key process, which are thin-film transfer, heat dissipation and light extraction. We provide and apply some new idea and concept to improve the efficiency of vertical LED in this study.
In the view point of thin-film transfer, it includes the wafer bonding of new substrate and stripping the sapphire substrate. We use the Au/Si wafer bonding, which is generally used in the package of IC (integrated circuit) chips. When the Au/Si bonding is applied in large area thin-film transfer, the bonding uniformity and voids formation at the bonding interface will be a serious problem. We add an amorphous Si layer at the bonding interface and the loose amorphous Si layer react with Au layer to form an uniform liquid alloy firstly. It has been proved that an uniform and void-free interface can be obtained. We also study the temperature effect on the laser lift-off process on sapphire substrate. The elevated temperature was employed to reduce the laser damage that from the abruptly stress change and adjust the excess laser energy in LLO process.
The most important advantage of the vertical LED is the excellent heat dissipation ability that caused by the high conductive transferring substrate. But, the heat spot that caused by high current injection is still a big problem in the vertical LED. Therefore, we propose a concept of the heat spreading layer to promote the heat dissipation ability. A high thermal conductive diamond-like carbon layer was employed to spread the heat in the horizontal direction rapidly. The experiment verifies the heat spreading layer can solve the problem of heat spot and enhance the heat dissipation ability of new substrate and the enhancement effect will be more obvious in the high current injection condition.
The light extraction of the vertical LED can be separated to the top side and the bottom side. The top side is the texturing of N-polar surface that appear after LLO process. It has been reported that the uniform hexagonal cone can be obtained by the wet etching process and the light output enhancement can reach 260%. It is a mature process and has been generally used technique. The reflector in the bottom side is used to reflect the light from MQW, which is the other key point to improve the light extraction of vertical LED. The reflector in LED device has to satisfy both requirements of the high reflectivity and low contact resistance between metal and GaN semiconductor. Yet, due to the low work functions of Ag and Al, they have been shown to have poor specific contact resistance to p-GaN. Among the high work function metals, Pt can achieve a good ohmic contact to p-GaN due to its high work function (5.36 eV). But, the low reflectivity of Pt layer in the visible region will limit Pt layer used in the high performance LED. In this study, we observed a hexagonal nano meshed Pt thin-film, which self-formed on the GaN surface. Combining with a metal reflector (Ag), meshed Pt/Ag can provide a low contact resistance to p-GaN and the mesh can provide a directly reflection path to Ag layer.
This is the first report about the self-assembled Pt hexagonal nano-meshed Pt layer. We believe that the hexagonal mesh formation is related to the regular arrangement of Pt in the GaN surface. A meta-stable state between Pt thin film on GaN leads de-wetting of Pt layer on the GaN surface form a specific pattern. In meshed Pt/Ag metal contact scheme to p-GaN, the meshed Pt thin-film plays the role of the current paths and the meshes allow the light get reflected by the Ag layer directly.
An energy favorite stacking configuration is proposed to explain the mechanism of the hexagonal structure formation. Also, the origin of meshes on the meshed Pt layer is explained by the defect theory. As for optical property of the meshed Pt layer, we conclude a relation between the reflectivity (transmittance) of the meshed Pt layer and the size of meshes on the meshed Pt layer.
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