The Orientations and Applications of Synthetic Sapphire Crystals

Synthetic sapphire is a synthetic single-crystal form of aluminum oxide (Al2O3) that has excellent mechanical, optical, and chemical properties, making it suitable for demanding applications in industries like aerospace, semiconductors, and medicine. The orientation of synthetic sapphire, which refers to the angle of the sapphire crystal from its optical axis, impacts the performance of the part produced.

Sapphire is a single crystal of aluminum oxide, belonging to the trigonal system and hexagonal structure. Its crystal structure is composed of three oxygen atoms and two aluminum atoms bonded in a covalent bond form. The arrangement is very tight, with strong bonding chains and lattice energy. At the same time, there are almost no impurities or defects inside the crystal, so it has excellent electrical insulation, exceptional optical transmission, good thermal conductivity, and high hardness. 

-Learn more about sapphire windows in our related blogs:

Sapphire (Al2O3)

Exploration of Current Sapphire Window Production Techniques

Advantages and Applications of Sapphire Windows

The Optical Transmission and Properties of Sapphire Windows

What are the advantages of sapphire crystals as optical windows and substrates:

It is widely used as an optical window and high-performance substrate material. Sapphire's high hardness and thermal conductivity also make it an ideal material for high-power laser devices. It can be used as a window material for laser cavities, able to withstand high-intensity laser radiation and high-temperature environments and maintain a stable working state for a long time. In addition, sapphire can be used as a gain medium in laser amplifiers to provide efficient laser output.

However, the molecular structure of sapphire is complex and anisotropic. The processing and use of different crystal orientations have different effects on the corresponding physical properties, so there are also differences in uses. Generally speaking, sapphire substrates are available in C, R, A, and M plane orientations. This article introduces the typical orientations for sapphire and the suitable applications for each orientation.

The sapphire crystal gained using the Kyropolous method (KY method) has A plane direction.

What is crystal orientation?

A crystal, including sapphire crystals, is a repeating pattern of atoms, and the crystal orientation refers to the arrangement of atoms in a crystal lattice along a specific direction. Crystal orientation" is defined by the plane (Miller) indices of the lattice plane of a crystal. Crystal orientation is measured using techniques that determine the alignment of atomic planes in a crystal lattice. The most common methods include X-ray diffraction (XRD), Laue back-reflection, optical goniometry, and electron backscatter diffraction (EBSD).

In the case of X-ray diffraction, after passing through the molecular layer, X-rays will be refracted and reflected. When incident at a certain angle, the reflected X-rays will be parallel (as shown below), and the X-ray intensity received by the receiver is relatively large. This angle is called the crystal direction value. However, due to the different molecular layer gaps on each plane, the resulting crystal orientation values are also different.

The standard crystal direction values are as follows:
C plane: 20°50′
A Plane: 18°55′
M Plane: 34°06′
R Plane: 26°16′
N Plane: 21°43′

Typical orientations of sapphire and their alternative names:

Miller Indices	Common Name	Alternative Names	Other Names	Remarks
(0001)	C-plane	Basal plane, (0001) plane	C-cut, Z-cut, Zero-degree	Most common for optical applications with minimum birefringence
(11̅20)	A-plane	(1-120) plane	A-cut	Highest hardness and scratch resistance
(1̅102)	R-plane	(10-12) plane	R-cut	Suitable for laser and epitaxial growth applications.
(10̅10)	M-plane	(1-100) plane	M-cut	Preferred for some semiconductor applications (e.g., GaN growth).
(01̅12)	N-plane	(01-12) plane	N-cut	Less common, sometimes used in advanced optoelectronics.

• Zero Degree/C-Axis/C-Plane: The C-plane is the natural growth plane of sapphire crystal, which has lower surface energy and better thermal conductivity and is widely used as a substrate for LEDs and lasers. C-plane sapphire windows also exhibits high optical transparencies, making it ideal for optical windows, wafers, and substrates. It also has a low dielectric loss, benefiting RF and microwave applications. The direction of view is parallel to the crystal's optical axis. It is commonly used to avoid birefringence in applications such as windows and lenses. In rods, the direction lies along its length, and in windows, it is perpendicular to the face. 
• 90 Degree/A-Plane: The direction of view is perpendicular to the crystal's optical axis. The plane is perpendicular to the A-axis and contains the C-axis. A plane has a higher atomic density and is suitable for applications that require high crystal isotropy, such as optical components.
• M-Plane: The plane contains the optic axis (C) and is inclined 30 degrees to the A-axis. 
• R-Plane: A plane inclined 57.5667 degrees to the optic axis and in the same zone as the M-plane. R-plane is used in special applications, such as in certain lasers and optical window materials, and can provide excellent mechanical and optical properties.
• Random: There is no specified relationship between the part and the crystalline orientation. The part is manufactured without concern about orientation

The typical applications of different orientations of synthetic sapphire:

The crystalline orientation of synthetic sapphire significantly impacts its mechanical, optical, and electrical properties, making orientation selection critical for specific applications. Below are the typical applications for each orientation:

C-Plane (0001)
1. Optical sapphire windows and domes: High transparency in the UV, visible, and IR spectra makes it ideal for optical applications, including aerospace, defense, and industrial imaging. Ultraviolet sapphire windows are mainly used in high-power ultraviolet lasers and detectors, while infrared sapphire windows are used in infrared imaging systems and sensors to effectively transmit and detect infrared light signals.

1. Wafers for RF and microwave applications: Low dielectric loss and high insulation properties make C-plane sapphire a preferred substrate for RF and microwave circuits.

1. GaN Thin Film Growth: Gallium nitride (GaN) material is a third-generation wide bandgap semiconductor with wide direct bandgap, strong atomic bonds, high thermal conductivity, good chemical stability (almost not corroded by any acid), and strong radiation resistance. It has broad prospects in the application of optoelectronics, high-temperature high-power devices, and high-frequency microwave devices. However, due to the high melting point of GaN, it is currently difficult to obtain large-sized single-crystal materials. Therefore, the common method is to perform heteroepitaxial growth on other substrates, which have high requirements for substrate materials. GaN-based materials grown on sapphire substrates are widely used in high-power blue and ultraviolet LEDs, lasers, and high-frequency electronic devices. Due to its excellent optical transparency and thermal conductivity, sapphire substrates can effectively improve the light output efficiency and heat dissipation performance of LEDs.

A-Plane
Sapphire single crystals have excellent comprehensive properties, especially excellent transmittance, which can enhance the penetration of infrared rays and become ideal mid-infrared window materials. Among them, the A-plane sapphire is the plane in the normal direction of the polar plane (C-plane), which is a non-polar plane. Usually, the quality of sapphire crystals grown in the a-direction is better than that grown in the c-direction, with fewer dislocations, fewer mosaic structures and more complete crystal structures, so it has better light transmission performance. At the same time, due to the atomic bonding mode of Al-O-Al-O on the A-plane, the hardness and wear resistance of a-direction sapphire are significantly higher than those of c-direction, so A-direction sapphire wafers are mostly used as sapphire window materials; in addition, A-direction sapphire also has uniform dielectric constant and high insulation properties, so it can be used in hybrid microelectronics technology, and can also be used for the growth of high superconductors, such as using TlBaCaCuO (TbBaCaCuO) and Tl-2212 to grow heteroepitaxial superconducting films on sapphire cerium oxide (CeO2) composite substrates. However, due to the large bond energy of Al-O, it is quite difficult to process.

M-Plane
The M-plane is the semi-polar plane of sapphire. Due to the application prospects of sapphire in solar-blind ultraviolet detection, wide-bandgap MgZnO alloy semiconductor films have attracted more and more attention. A series of Mg x Zn 1 -x O films with different compositions were prepared on M-plane (10-10) sapphire wafers by low-pressure metal organic chemical vapor deposition (LP-MOCVD).

 

R-Plane 
The R-plane is the non-polar plane of sapphire, so the change of the R-plane position in the sapphire device gives it different mechanical, thermal, electrical and optical properties. Generally speaking, R-plane sapphire substrates are preferably used for heteroepitaxial deposition of silicon, mainly for the manufacture of semiconductors, microwaves and microelectronic integrated circuit applications. R-type substrate growth can also be applied when making indium, other superconducting components, high-resistance resistors, and gallium arsenide. Currently, with the popularity of smartphones and tablet computer systems, R-plane sapphire substrates have replaced existing compound SAW devices used in smartphones and tablet computers, providing a substrate for devices that can improve performance.

Random
Random-cut sapphire windows are often used where slight birefringence is less of a concern. It is common where ideal optical or mechanical qualities are not required because of its lower costs.

By selecting the appropriate orientation, synthetic sapphire can be optimized for a wide range of high-performance applications in optics, semiconductors, aerospace, and industrial manufacturing.

 

Comparison of each cutting orientation of sapphire:

Optical properties: The C axis has crystal light, and the other axes have negative light. (So the general substrate industry uses C-direction wafers.)

b. Hardness: The A-direction hardness is significantly higher than the C-direction, which is specifically manifested in wear resistance, scratch resistance, and high hardness. (We need to make a special grinding wheel for grinding the A-direction, or the grinding efficiency will be significantly reduced. A-direction wafers are mostly used as window materials, such as watch lenses)

c. The M surface is easy to crack during cutting: The C surface is a flat surface and is the best to cut. The A surface is a Z-shaped serrated surface, which is easier to cut. The M surface is a step-shaped serrated surface, which is difficult to cut and easy to crack.

Tags: Synthetic Sapphire Crystals

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