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Cracking Generation And Avoidance During The Growth of Sapphire Domes from An Element of Shape

A failure mechanism is proposed in order to explain the generation and propagation of cracks during the growth of sapphire domes from an element of shape. According to this model, 10 μm gas bubbles expansion is induced by glide dislocations (Orowan's model), up to a critical size for which the crack is initiated. Numerical simulation of stresses during the growth explains how cracks propagate first vertically (V-type cracks) then horizontally (H-type cracks). A criterion based on plastic strain relaxation is defined in order to determine the growth parameters (pulling and rotation rates) as a function of the measured thermal gradients in the crystal. This led to the growth of crack-free sapphire hemispheres up to 50 mm in diameter that can be used for infrared dome blanks.

Growth of AlN on sapphire substrates by hydride vapor-phase epitaxy (HVPE) was investigated using AlCl3 and NH3 as source gases in the temperature range from 950 to 1100 °C. It was found that the full-width at half-maximum (FWHM) values of the X-ray diffraction (XRD) rocking curves of the and View the MathML source planes of the c-axis oriented AlN layers decreased with increasing growth temperature. An AlN layer showing a mirror-like surface could be grown at 1100 °C with a growth rate of 1.7 μm/h. Growth rate was found to increase with increasing AlCl3 input partial pressure and with decreasing distance (L) between the end of the AlCl3 injection nozzle and the sapphire substrate. The growth rate reached 122 μm/h with an AlCl3 input partial pressure of 2.0×10−3 atm and View the MathML source but the crystalline quality became poor when the growth rate rose above 10 μm/h.


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