Estimation of Crystallite Size, Density, and Compositional of the Ti: Al2O3 Single Crystal

Received: August 27, 2020 Accepted: October 19, 2020 Published: October 30, 2020 The purposes of this research were to estimate the crystallite size, density, and chemical composition of the ingot Ti: Al2O3 crystal grown by the Czochralski method. The crystallite size and composition of Ti: Al2O3 crystals had been determined using x-ray diffraction (XRD) and energydispersive x-ray spectroscopy (EDXS). Based on the Archimedes principle, the density of the crystals had been determined. The XRD patterns showed a single central peak with high intensity for all samples. It indicated that all samples had a single crystal. The average value of the samples' crystallite size was in the range of 20.798 nm to 34.294 nm. The ingot crystal density and Ti composition increased from the top to the bottom part because the solid solution was distributed unevenly during the growth process.


INTRODUCTION
Titanium-doped sapphire (Ti: Al2O3) is an excellent laser material (Nehari et al., 2011;Sawada et al., 2017). It is the most widely used crystal for wavelength-tunable laser (Panahi et al., 2015;Wu et al., 2020), and it is a leading material in the field of femtosecond pulse lasers (Kozlov & Samartsev, 2013). The Ti: Al2O3 single crystal with high quality and uniform dopant distribution can be produced as a highpower tunable laser with a range of wavelengths around 600nm to 1100nm (Raeder et al., 2020;Sawada et al., 2017;Zong et al., 2019). A high-quality crystal is required for a tunable solid-state laser.
Single crystal with good characteristics such as high homogeneities and large dimensions can be used on high power laser applications. Its properties affect the compactness of the laser system (Li et al., 2014;D. Zhou et al., 2015). The quality of the crystal can be improved by knowledge of the growth process's growth parameters and phenomena (Alombert-Goget et al., 2016). A single crystal of Ti: Al2O3 can be grown by several methods, i.e., Verneuil method (Alombert-Goget et al., 2016), kyropoulos (Ky) method (C. H. Chen et al., 2012;Gao et al., 2015;Nehari et al., 2011;Sen et al., 2020;C. Stelian et al., 2016;Carmen Stelian et al., 2017), temperature gradient technique (TGT) (Ren et al., 2016;G. Zhou et al., 2006), heat exchange method (HEM) (Dong & Deng, 2004;Joyce & Schmid, 2010), vertical gradient freeze (VGF) method, hydrothermal technique (Song et al., 2005;Wang et al., 2009), micro-pulling down (-PD) (Ghezal et al., 2012;Kamada et al., 2018;Zhou et al., 2015), Bridgman method (Han et al., 2020) and Czochralski (Cz) (Alombert-Goget et al., 2016;FIELITZ et al., 2008;Hur et al., 2017;Li et al., 2013Li et al., , 2014. Many researchers utilized the grown single crystal from oxide materials as optical and electronic components using the Cz method (Moulton et al., 2019;Spassky et al., 2017). It is because the Cz method can produce high-quality crystals, high homogeneity, and controllable crystal diameter. For the Cz method, a suitable temperature gradient hot zone was required. That means defect can occur during crystal growth into the cooling process when the temperature gradient is more extended (Li et al., 2013(Li et al., , 2014. Knowledge of the growth with the Cz method can produce a single crystal with high homogeneity, high quality, and large dimension. Thereby, a wellcontrolled single crystal growth process with appropriate growth parameters, including temperature gradients, pull rate, and rotating speed, must be achieved to produce a single crystal with high homogeneity and high quality (Li et al., 2014;Moulton et al., 2019). The crystal's quality is closely related to the transport phenomena and thermal gradient in the furnace (C.-H. Chen et al., 2014;Hur et al., 2017). So, high-quality and homogeneous crystals can be obtained (Li et al., 2014).
The bubbles formation and their incorporation (Ghezal et al., 2012;Li et al., 2013Li et al., , 2014 in titanium doped sapphire crystal and the influence of the pulling rate on color center (Alombert-Goget et al., 2016;Li et al., 2014;D. Zhou et al., 2015) was investigated. The influence of control power on the diameter was studied (Jainal et al., 2010) and the pulling rate on the density and compositional (Kusuma, 2015;Kusuma et al., 2018) of Ti: Al2O3 single crystal. This research evaluated the effect of the wafer cut from different positions (top, middle, and bottom) of Ti's ingots: Al2O3 with constant pull and rotation rate on the crystallite size, structure, density, and chemical composition.

METHODS
This research employed an experimental method with a quantitative approach. The crystal was grown using the Cz method with Automatic Diameter Control (ADC) System available at Advanced Optical Material Laboratory, Physics Department Universiti Teknologi Malaysia. The characterization of X-ray diffraction (XRD), density, and energy dispersive X-ray spectroscopy (EDXS) were analyzed at Ibnu Sina Institute, Universiti Teknologi Malaysia.

Crystal Growth Process
The raw materials used were polycrystalline with a purity of 99.99% of Ti (0.1 wt. %) doped with Al2O3 powders. They were purchased from the Shanghai Institute of Ceramics. The crystal was grown by the Cz method with the ADC system. In the growth process, the iridium crucible with 56.4 mm diameter was inductively heated. The diameter of the crystal can be automatically controlled due to the time derivative of the crystal weight. During the whole crystal growth process, a high vacuum was utilized to avoid oxidation or other crucible damage and crystallization. The Ti: Al2O3 crystal was grown with the direction along the c-axis, constant pull rates at 1.50 mmh -1, and rotation rate at and 15 rpm.

Characterization of Ti: Al2O3 Crystal
The Ti: Al2O3 crystals (top, middle, and bottom part of boule crystal) were analyzed using the XRD, density, and EDXS. XRD analysis was performed to determine the crystal structure and the crystallite size. The crystallite size of the crystal was determined using Debye-Scherrer Equation (1).
where t is the crystallite size of the crystal,  is the X-ray source with wavelength where  is the density of crystal, l is the density of the fluid, WA is the weight of sample in air, and Wl is the weight in fluid. The chemical compositional was analyzed using EDXS to identify the elements and concentrations.

RESULTS AND DISCUSSION
Ti crystal: Al2O3 was transparent, with no crack and no bubbles (Figure 1). Crystal as wafer was cut with a better mention here of the thickness. For the study, the titanium distribution inside the crystal was chosen three wafers from the top, middle, and bottom (see Figure 1) of the crystal, respectively. The structure and crystallite size of Ti: Al2O3 crystal can be determined by using Xray diffraction. All samples have shown a similar diffraction peak. Figure 2 shows that only a single peak was seen and observed for all the samples. It shows that the Ti: Al2O3 is a single crystal with relatively good quality. The intensity of the XRD peaks can be affected by many factors related to the crystal morphology composition . Figure 2 shows that Ti doping causes the peak to become more intense, and the increase in the relative intensity is proportional to the doping amount. Diffraction peak intensities were decreased when the titanium doped in Al2O3 increased. It indicates that the quality of the crystal from top to bottom was decreased. The XRD peaks (see Figure 2) shifted towards the higher angles when titanium doped in Al2O3 increased.
The diffraction peak resulted from the XRD pattern was then compared to the Joint Committee on Powder Diffraction Standards (JCPDS) index to determine the diffraction pattern. Therefore, based on the XRD pattern of Ti: Al2O3 crystals and JCPDS data (no. 46-1212), The crystal formation of Ti: Al2O3 was hexagonal.
Based on the XRD peak of broadening, the crystallite size for Ti: Al2O3 crystal was calculated using equation 1. The crystallite size can be estimated using the central peak position (2 angle) and FWHM of the central peak. The estimated crystallite size of Ti: Al2O3 crystals from different positions of the ingot (top, middle, and bottom) are shown in Table 1. It can be seen that the crystallite sizes of Ti: Al2O3 single crystal were similar, which suggested the excellent quality single crystal.
Furthermore, the density and chemical composition by EDX analysis can be seen in Tabel 2.  Based on Table 2, the density of the Ti: Al2O3 single crystal from different positions of the ingot (top, middle, and bottom) and the amount of titanium doped in Al2O3 had increased. During the crystal growth process, the increase was due to the distribution coefficient and the  of the solid-solution. Therefore, the foreign impurity may easily be trapped and enveloped by the solid phase when the solid-liquid surface advanced slower than mass transport (C.-H. Chen et al., 2014;C. Chen et al., 2014;C. H. Chen et al., 2012;Kamaruddin et al., 2013;Song et al., 2005). The segregation of Ti atoms was more dominant at the upper than the bottom part of the ingot crystal due to the solid-liquid interface's speed, including the Ti atom, which was faster than that entering into the solid phase. During the growth process, the interface was kept convex so that the entire titanium ions may be rejected to the bottom of the boules. The critical factor affecting the crystals' mechanical and optical properties was the transport of impurities and dopants in the growth process (Alombert-Goget et al., 2014;Hur et al., 2017). The concentration of dopant had been changing continuously during the growth process.

CONCLUSION AND SUGGESTION
The crystal of Ti: Al 2 O 3 was transparent, with no crack and no bubbles. Based on the XRD, it has a single peak, which indicated that it has good quality. The crystallite size of samples was obtained between 20.798 nm to 34.294 nm. The density of the Ti: Al2O3 single crystal from different positions of the ingot (top, middle, and bottom) and also the amount of titanium doped in Al2O3 were increased.