고휘도 발광다이오드를 위한 수소기상증착법을 이용한 질화갈륨 기판 성장과 가공 연구

Abstract

Gallium nitride substrates are being applied to light emitting devices based on their excellent optical and electrical properties, and are also being applied to blue and ultraviolet light emitting diodes having a short wavelength using a gallium nitride band gap(3.4eV). As blue light emitting diodes have been developed, all three primary colors of light can be realized, and light emitting diodes for illumination are developed and applied, and high efficiency light emitting diodes for indoor and outdoor lighting have been developed starting from the lighting fields of smart phones and TVs. Gallium nitride substrates can be obtained by epitaxial growth on different substrates. Threading dislocation and bow are inevitable due to differences in material properties between mother board and gallium nitride. Particularly, the gallium nitride obtained by the epitaxial growth method using the sapphire substrate has a dislocation density of about 1×109/cm2 due to a difference of about 16% between the lattice constants of the two materials. In addition, the gallium nitride epitaxial growth using a sapphire substrate causes an inevitable wafer bowing after growing the thermal expansion coefficient to about 25%. In order to solve this problem, a buffer layer for stress release is formed on the sapphire substrate during gallium nitride epitaxial growth. In this case, the ratio of growth gas is a very important issue. In this study, the growth of gallium nitride on a sapphire substrate is controlled by growing a buffer layer of 10 layers or more in a single layer. The thermal stress of gallium nitride is controlled and the threading dislocation density is 1×106/cm2. In addition, the gallium nitride grown on the sapphire wafer was separated from the sapphire substrate by using a laser. The stress relaxation effect of the buffer layer was verified based on the bow of the gallium nitride wafer after the separation. In addition, the damaged layer generated on the surface of the gallium nitride wafer by the size of the diamond particles in the freestanding gallium nitride wafer polishing process was confirmed by TEM analysis, and the influence of the tensile stress generated by the damaged layer on the gallium nitride wafer bow Respectively. Also, it was found that the bowing of the gallium nitride substrate decreases as the thickness of the gallium nitride substrate decreases with the thickness of the gallium nitride substrate including the buffer layer during the polishing process using the diamond particles, as the thermal stress of the gallium nitride substrate decreases. Therefore, it was confirmed that the thickness ratio in the polishing process of the gallium nitride wafer including the buffer layer is very important. However, when the bow of a gallium nitride wafer becomes zero, the thickness of the gallium nitride wafer including the buffer layer is as thin as about 185 um, and a thickness of about 400 to 450 um or more should be grown for a freestanding gallium nitride wafer with a thickness of 300 um or more. Polishing of the gallium nitride wafer using diamond particles resulted in a surface with no roughness and scratches of about 0.3 nm and no damaged layer. During the growth of gallium nitride wafer using sapphire substrate, the sapphire substrate is separated at high temperature by using laser. Therefore, we have developed an automatic separation technology that spontaneously separates by using the difference of thermal expansion coefficient generated after the growth of gallium nitride, while skipping the separation process using laser. Hydrogen ions were implanted into the gallium nitride substrate using an ion implanter to create a separate interface for automatic isolation. The concentration of implanted hydrogen ions was 2, 3, 4 x 1017 and the injection energy was 75 keV. It was confirmed that the hydrogen ions were clustered and defected in the gallium nitride substrate through the heat treatment after the hydrogen ion implantation to form a blister layer of the hydrogen ion. As the hydrogen ion implantation concentration increased, the blister was observed to burst. After the hydrogen ion implantation, the blister layer was formed in the gallium nitride substrate through the heat treatment and the gallium nitride was regrowthed to confirm the automatic separation. As a result, the ion implantation concentration was automatically separated at 2.0 × 1017 and at the higher concentration, it was confirmed that the gallium nitride substrate broke without automatic separation.