What materials can grow on sapphire substrates?
Sapphire is an oxide crystal, also known as corundum. Its main chemical composition is alumina (Al2O3), and a pure alumina crystal is colorless and transparent.
The substrate mentioned herein is actually a single-crystal wafer utilized for the epitaxial growth of crystal films, also referred to as the chip. It can directly enter the wafer manufacturing process to produce semiconductor devices in the semiconductor field. Additionally, it can also undergo meticulous processing after cutting, grinding, polishing, etc., under certain conditions, serving as the basis for epitaxial growth, enabling the same or another single crystal to grow directionally on its surface.
The substrate itself may possess specific functions and applications, or it may serve merely as a substrate for new materials with no additional distinctive properties. Previously, the materials used as film substrates were glass, ceramics and metal sheets. However, the more prevalent materials in current use are sapphire, Si and SiC, with a particularly notable quantity of sapphire.
The popularity of sapphire as substrates can be attributed to a number of factors:
- The production technology of sapphire substrates is well established and the quality of the devices produced is of a high standard.
- Sapphire demonstrates excellent thermal stability, rendering it suitable for utilization in high-temperature growth processes.
- Sapphire exhibitshigh mechanical strength and is easy to handle and clean.
Materials that can grow on sapphire substrates
- Photoelectric devices:
In this field, the representative material that can be grown on sapphire substrates is gallium nitride (GaN), a highly renowned material. The structure of sapphire crystal is identical to that of gallium nitride (GaN), which is hexagonal. Sapphire displays a wide band gap, a low refractive index (1.7), good chemical stability, and does not react with hydrogen even at a high temperature up to 1000°C. These properties make it suitable for high-temperature growth, with excellent light transmission and mature technology.
Currently, sapphire is a suitable substrate material for high-temperature growth processes due to its mature production technology, excellent device quality and high stability. Furthermore, it exhibits high mechanical strength, ease of handling and cleaning, rendering it a favourable material for the LED industry. Presently, the utilization of sapphire in LED substrates represents 80% of all downstream consumption. Driven by pertinent policies implemented in recent years, the demand for sapphire has exhibited a gradual increase, and the market size has continued to demonstrate a growth trajectory.
- High-temperature superconducting film
The development of modern electronic technology, particularly in the field of microwave electronic passive devices, has been significantly influenced by the advancement of thin film technology. Consequently, the research focus on high temperature superconducting thin films has assumed great importance in the context of the utilization of modern microwave electronic devices. Presently, high-temperature superconducting devices, particularly microwave passive devices, are extensively utilized in the domain of electronic communication, with high-temperature superconducting thin films serving as the fundamental carriers of superconducting microwave devices.
Among the numerous substrate materials used in the preparation of Tl high-temperature superconducting thin films, sapphire and MgO single-crystal substrates exhibit a moderate dielectric constant and low microwave loss, rendering them particularly well-suited for applications in the microwave field. Of particular significance is the relatively low price of large-area sapphire single crystal substrates, which is of great importance for the application and popularization of high temperature superconducting thin films in the microwave field.
- Infrared optical material
The mid-infrared band, spanning 3-5 μm, is a commonly utilized window for infrared detectors. However, the use of high-energy laser radiation during operation may potentially damage the detectors. Therefore, it is essential to prepare infrared anti-reflection films with laser protection on the surface of the infrared window to safeguard against such damage.
Due to its favourable transmittance properties across the ultraviolet, visible and infrared band, sapphire crystal is frequently used as a window material for high-intensity lasers. However, when used as a window material in the middle infrared wavelength of 3-5 μm, the transmittance of the sapphire substrate is inadequate and is highly susceptible to damage from high-energy laser systems. Therefore, the design and preparation of an anti-reflection film on the sapphire substrate surface is essential, to enhance the transmittance and anti-laser damage ability, thereby compensating for the shortcomings of the sapphire substrate.
