Sapphire is a single crystal of alumina, belongs to the tripartite crystal system, hexagonal structure, its crystal structure is composed of three oxygen atoms and two aluminum atoms in covalent bond type, arranged very closely, with strong bonding chain and lattice energy, while its crystal interior almost no impurities or defects, so it has excellent electrical insulation, transparency, good thermal conductivity and high rigidity characteristics. Widely used as optical window and high performance substrate materials. However, the molecular structure of sapphire is complex and there is anisotropy, and the impact on the corresponding physical properties is also very different for the processing and use of different crystal directions, so the use is also different. In general, sapphire substrates are available in C, R, A and M plane directions.
The application of C-plane sapphire wafer
Gallium nitride (GaN) as a wide bandgap third generation semiconductor, has wide direct band gap, strong atomic bond, high thermal conductivity, good chemical stability (almost not corroded by any acid) and strong anti-irradiation ability, and has broad prospects in the application of optoelectronics, high temperature and power devices and high frequency microwave devices. However, due to the high melting point of GaN, it is difficult to obtain large-size single crystal materials, so the common way is to carry out heteroepitaxy growth on other substrates, which has higher requirements for substrate materials.
Compared with the sapphire substrate with other crystal faces, the lattice constant mismatch rate between the C-plane (<0001> orientation) sapphire wafer and the films deposited in groups Ⅲ-Ⅴ and Ⅱ-Ⅵ (such as GaN) is relatively small, and the lattice constant mismatch rate between the two and the AlN films that can be used as buffer layer is even smaller, and it meets the requirements of high temperature resistance in the GaN crystallization process. Therefore, it is a common substrate material for GaN growth, which can be used to make white/blue/green leds, laser diodes, infrared detectors and so on.
It is worth mentioning that the GaN film grown on the C-plane sapphire substrate grows along its polar axis, that is, the direction of the C-axis, which is not only mature growth process and epitaxy process, relatively low cost, stable physical and chemical properties, but also better processing performance. The atoms of the C-oriented sapphire wafer are bonded in an O-al-al-o-al-O arrangement, while the M-oriented and A-oriented sapphire crystals are bonded in al-O-al-O. Because Al-Al has lower bonding energy and weaker bonding than Al-O, compared with the M-oriented and A-oriented sapphire crystals, The processing of C-sapphire is mainly to open the Al-Al key, which is easier to process, and can obtain higher surface quality, and then obtain better gallium nitride epitaxial quality, which can improve the quality of ultra-high brightness white/blue LED. On the other hand, the films grown along the C-axis have spontaneous and piezoelectric polarization effects, resulting in a strong internal electric field inside the films (active layer quantum Wells), which greatly reduces the luminous efficiency of GaN films.
A-plane sapphire wafer application
Because of its excellent comprehensive performance, especially excellent transmittance, sapphire single crystal can enhance the infrared penetration effect, and become an ideal mid-infrared window material, which has been widely used in military photoelectric equipment. Where A sapphire is a polar plane (C plane) in the normal direction of the face, is a non-polar surface. Generally, the quality of A-oriented sapphire crystal is better than that of C-oriented crystal, with less dislocation, less Mosaic structure and more complete crystal structure, so it has better light transmission performance. At the same time, due to the Al-O-Al-O atomic bonding mode on plane a, the hardness and wear resistance of A-oriented sapphire are significantly higher than that of C-oriented sapphire. Therefore, A-directional chips are mostly used as window materials; In addition, A sapphire also has uniform dielectric constant and high insulation properties, so it can be applied to hybrid microelectronics technology, but also for the growth of superb conductors, such as the use of TlBaCaCuO (TbBaCaCuO), Tl-2212, the growth of heterogeneous epitaxial superconducting films on cerium oxide (CeO2) sapphire composite substrate. However, also because of the large bond energy of Al-O, it is more difficult to process.
Application of R /M plane sapphire wafer
The R-plane is the non-polar surface of a sapphire, so the change in the R-plane position in a sapphire device gives it different mechanical, thermal, electrical, and optical properties. In general, R-surface sapphire substrate is preferred for heteroepitaxial deposition of silicon, mainly for semiconductor, microwave and microelectronics integrated circuit applications, in the production of lead, other superconducting components, high resistance resistors, gallium arsenide can also be used for R-type substrate growth. At present, with the popularity of smart phones and tablet computer systems, R-face sapphire substrate has replaced the existing compound SAW devices used for smart phones and tablet computers, providing a substrate for devices that can improve performance.
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Post time: Jul-16-2024