The Common Defects of Sapphire Crystal Caused by Internal Stress

The composition of Sapphire is Aluminum Oxide (Al₂O₃), and its crystal structure is a hexahedral lattice structure. C-plane is the most commonly used sapphire substrate. Due to its wide optical penetration band , it has good light transmittance from near-ultraviolet to mid-infrared, so it is widely used in optical components, infrared devices, high-intensity laser lens materials and mask materials.

At present, the quality of ultra-high brightness white and blue LED depends on the material quality of GaN, and the quality of GaN is closely related to the surface processing quality of the sapphire substrate used.

Recently, the market has shifted from 2-inch sapphire single crystal to 4-inch, 6-inch and 8-inch. The rapid development of the LED market requires the growth of sapphire crystals of large size, high quality and stable performance, which puts forward higher requirements for sapphire growth technology. However, in the growth process of sapphire single crystal, there are often some defects that significantly affect the performance of sapphire, such as crystal cracks, dislocations, impurities and color centers, air bubbles, etc.

Below we focus on two types of sapphire crystal defects.

1. Crystal cracks

During the growth process, the generation of various stresses inside the crystal will cause strain. When the strain is greater than the elastic limit of the crystal itself, the crystal will crack. The stress in the crystal mainly includes the following three types:

(1) Thermal stress: Thermal stress is a kind of internal stress caused by the uneven heating of the crystal and the temperature difference, resulting in inconsistent expansion or contraction deformation of the crystal, and mutual restraint between the various parts of the crystal. So as long as there is a temperature gradient within the crystal, there will be thermal stress.

(2) Chemical stress: caused by the uneven distribution of various components in the crystal.

(3) Mechanical stress: caused by mechanical vibration during crystal growth.

During the growth of sapphire single crystal, thermal stress is the most important form of all stress. The main reasons for excessive thermal stress in the crystal include the following aspects:

a. The growth rate is too fast.

b. The temperature field is unreasonable and the temperature gradient is too large.

c. The cooling rate is too fast.

d. Crystal orientation.

e. Crystal size.

slight stress

medium stress

2. Dislocation

Dislocation is a lattice defect with a special structure. The actual crystal is subjected to the action of the external environment or various internal stresses during crystallization, and the arrangement of the particles inside the crystal is deformed and is no longer ordered into an ideal lattice state , resulting in a linear defect in the crystal called a dislocation.

The causes of dislocations in sapphire crystals mainly include the following three aspects:

a. Primary dislocations: If there are dislocations in the selected seed crystal, they can be extended into new crystals through growth. Dislocations in the seed crystal include dislocations inherent in the seed crystal, dislocations generated by excessive stress during processing, and dislocations generated by thermal shock during seeding.

b. Dislocations are generated during crystal growth. Its main sources are:

(1) The axial and radial temperature gradients of the crystal near the interface generate thermal stress, both of which exceed the critical value.

(2) Changes in lattice constants caused by component segregation: due to the presence of impurity atoms in the melt, the crystals will solidify successively during the solidification process, resulting in differences in composition and possible formation of dislocations.

(3) Point defects (vacancies and interstitials) cause local stress concentration.

(4) The influence of mechanical vibration causes the crystal to deflect or bend, and a phase difference occurs between adjacent crystal blocks, thereby forming dislocations.

Stress concentration is prone to occur in the vicinity of interfaces such as twins and grain boundaries inside the crystal and near micro-cracks. If the stress exceeds the slip stress, when the crystal slips in this area, dislocations will occur in this area.