Chemical Segregation of Titanium in Sapphire Single Crystals Grown by Micro-Pulling-Down Technique: Analytical Model and Experiments

Abstract

International audienceThe micro-pulling-down (mu-PD) process consists in pulling a crystal under a capillary channel placed at the bottom of a crucible. Despite it being limited to rather small liquid volumes, it is used to grow single crystal fibers and shaped crystals of various cross sections, mainly applied industrially for optical applications, such as lasers, optics, or scintillators. Consequently, those crystals should be doped with active elements to fit the target application. Unfortunately, whatever the growth parameters and the dopant type, quite often segregation problems are observed. It is generally believed that chemical partition in mu-PD technique is restricted to the first grown millimeters, but some experiments show that it is not always the case. An analytical one-dimensional model is presented, aiming to predict the longitudinal segregation along the growth direction. It is shown that it depends in practice on growth parameters such as capillary length, meniscus height, capillary section, and pulling rate. The characteristic numbers controlling the segregation profile are derived and a parametric study is performed in the case of Ti-doped sapphire single crystal fibers. Ti3+:Al2O3 single crystal fibers oriented along c-axis have been grown under stationary stable growth regime using different pulling rates and the longitudinal chemical segregation has been characterized by photoluminescence. Results are in agreement with the model predictions

Identification of hydrogen defects inα−Al2O3by first-principles local vibration mode calculations

“…For example, it can be used as a substrate for epitaxial growth [15], a surface passivation layer of Si solar cells [16], and a hydrogen permeation barrier [17,18]. In addition, α-Al 2 O 3 can be used in optical devices when it is doped with Cr or Ti [19,20]. Generally, the presence of unintentional defects in α-Al 2 O 3 , such as native point defects and hydrogen, can either degrade or enhance its electrical, mechanical, and optical properties [12,21].…”

Lasing of Nd3+ in sapphire

“…Thicker films could enable power scaling of Nd:sapphire in thin disk configuration or by direct end pumping of planar waveguides with diode bars . As laser action of rare‐earth‐doped sapphire is now proven it is worthwhile to investigate other crystal growth techniques apart from thermal equilibrium such as micro‐pulling‐down at high growth rates .…”

On the way to modelling dopant valence in ionic crystal growth: the Ti:Al2O3 case

“…Many simulations and experimental results show that crystal growth rate could affect solute distribution in grown crystal and the crystal quality. [32][33][34] Therefore, growths at rates of 1 mm h −1 , 3 mm h −1 and 10 mm h −1 are simulated, under the temperature gradient of 2244 K m −1 , so that the boundary conditions are not changed. In axial direction (Fig.…”