Growth And Optical Properties Of Sapphire Single Crystals Engineering Essay

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Sapphire (Al2O3) crystal is an important and widely used for high technology, optical and electro optical application. It is used for lasing material in solid state lasers, substrate for micro-electronics, radiations dosimeter, an insulator and so on [1]. As an optical material, sapphire has a broad transmission band the ultra violet, visible and infrared region. Sapphire also has very good mechanical and physical properties, such as tensile strength, abrasion resistance, thermal conductivity and mechanical stability [2].

Commercial methods for growing sapphire crystals are: Czochralski technique (CzT)[3,5], Horizontal Bridgman technique(HBT) [4.5], Verneuil technique (VT)[5], Floating-zone technique (FZT)[5], and Heat-exchange method (HEM)[5] Kyropulos technique [5], and temperature Gradient technique (TGT) [2,6,7]. These techniques can produce sapphire with maximum width or diameter of four inches. In the 1960s, requirements for large size sapphire were for transparent armor applications because of sapphire's high hardness and toughness properties. Therefore, emphasis was placed on adapting several crystal growth processes to produce sapphire crystals. Attempts were made to scale up the Cz, Bridgman, Verneuil, and TGT processes to grow large sapphire, but the inherent properties of sapphire made it difficult to produce large sizes using these techniques [8]. Other properties that made sapphire difficult to grow were its high melting point and low thermal conductivity compared to metal systems. We had early successfully grown high quality sapphire crystals by CzT.

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The aim of this work is to investigate the mechanism CzT growth process of sapphire single crystals with ADC system and the effect of pulling rate to high quality and optical properties of sapphire crystals.

EXPERIMENTAL

Growth of sapphire single crystals

Crystals of sapphire were grown by the radio frequency (r.f) heated with CzT using automatic diameter control (ADC) system to generate identical conditions and to produce same sizes of crystals. Computer controlled growth parameters are tabulated in Table 1. The crucible is inductively heated by cooper coils with AC current at a frequency of 25 kHz. The input power is 3200 mV with use of 20-27 percent. The experimental apparatus used this study is shown in Fig. 1. A iridium crucible, 60 mm in diameter and 60 mm in height with a thickness of 1.5 mm, was imbedded in a granular zirconia thermal insulator contained in a quartz crucible. The crucible is supported by zirconia ceramic disks. To ensure uniform temperature in the heating process and to reduce the temperature gradient, then the above crucible is constructed a insulator of insulating materials at high temperatures, 60 mm in diameter and 30 in height. An automatic power controller system controlled the process by regulating the power in the RF-coil to control and stabilizing the melt temperature. The melt surface temperature was monitored using the pyrometer.

Oxide powder of high-purity (99.999%) Al2O3 were weighed and pressed into used the raw material. The melt charge occupied about 90% volume of the iridium crucible used. Argon flows into create an inert environment for the crystal growth, preventing the melt component and iridium crucible from oxidation. The argon flow the furnace from side thought special holes, so the flow structure in the furnace is three dimensional.

Table 1: Crystal growth conditions and parameter for sapphire crystal with ADC

No.

Parameter

Value for Growth

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

Seed Diameter

Full Diameter

Solder Angle

Pull Speed

Rotation Rate

Solid Density

Liquid Density

Crucible Diameter

Neck Length

Desired length

Tail Off Angle

Tail off Min. Diameter

Tail length

Maximum Control Power

Cooling-down Time

Pulling direction

Atmosphere

5 mm

20 mm

63.44°

0.5 - 1 mmh-1

15 rpm

3.98 g/cm3

3.7 g/cm3

60 mm

3 mm

20 -25 mm

75.56°

5 mm

2 mm

3115 mV

60 h

c-axis <001>

76.7% argon

The crystals are grown from seeds attached to an iridium coupler and held by an alumina rod. It is rotating at a rate of 15 rpm to reduce same defects when being pulled by the alumina tube. The pulling rate was 0.5 to 1 mm/h. Growth is along the c-axis in both cases. The crucible was not rotated during the growth. After the growth run, the crystal boule was cooled at a rate of about 37 °C/h down to room temperature.

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Fig. 1. Experimental apparatus of Cz technique, (a) iridium crucible, (b) polycrystalline melt of Al2O3, (c) Al2O3 crystal (d) seed crystal (e) seed holder, (f) granular ZrO2 (g) quartz (h) coil (i) ZrO2 plate.

Characterization of sapphire single crystals

The absorption spectrum was carried in the range of 200-800 nm by Simadzu UV-Vis NIR spectrometer and the emission spectra was carried in the range 200-900 nm by Perkin Elmer LS 55 luminescence Spectrometer at room temperature (300K). The samples were c-cut plates with thickness of ~2 mm, both c-faces polished.

RESULTS & DISCUSSION

Crystal growth of the sapphire

The typical crystals boules of sapphire grown by Cz technique with ADC system were shown in Fig. 2. In the same figures are shown sapphire crystals with different pulling rate while the rotation rate is kept constant at 15 rpm during the growth process. The sapphire crystals were grown with the pulling rate 0.5 mmh-1, 0.75 mmh-1 and 1 mmh-1 (designated S1, S2 and S3 respectively). In this growing crystals there are three parameters which could affect the domain structure, (1) the temperature variation, (2) the off-centered crystal geometry and (3) the variation of the rotation rate [9,10]. Growing crystals by using ADC system can have been control of diameter crystal with the set diameter in the parameter growing (see Table 1). In the case of growth, dimensions of diameter crystals are larger from diameter in the parameter growth (> 20 mm). Power fluctuations create a periodical the temperature fluctuations, variation in the growth rate and thus a periodical change in the crystal diameter. Another factor affecting the diameter crystal increases as the growth is going on around the equilibrium temperature of the system. As the growth progress, the diameter of the growing crystal is controlled by adjusting the crucible temperature. Lowering the heating power will accelerate the crystallization and lead to a diameter increase, while increasing the power will act to decrease the crystal diameter [11].

S1

S3

S2

(a) (b) (c)

Fig. 2. The photograph of sapphire crystals grown by the CZ technique using ADC system with rotation rate 15 rpm and pulling rate

(a) 0.5 mmh-1 (b) 0.75 mmh-1 (c) 1 mmh-1

From the observations on the growing sapphire crystals (Fig.2), the crystals formed are deferent. As shown in the Fig. 2 (a) and Fig. 2 (c), the crystals are not clear and are have bubble, while in the Fig. 2 (b) crystal is transparent and has not bubble. In the Fig. 2 shows that the crystals produced with different pulling rate will determine the quality of the crystals formed. This indicates that these factors greatly affect the quality of the resulting crystals. The all crystals have grown are crack-free. The best result of crystal sapphire have grown (free bubble, free crack and transparent) were obtained with the pulling rate at 0.75 mmh-1 and the crystal rotation rate at 15 rpm.

Optical absorption study

The optical absorption of sapphire crystals grown by Cz technique with different pulling rate have been recorder and presented in Fig. 3. The absorption band of sapphire crystals (S1,S2 and S3) are similar. As shown in Fig.3, the intensity of optical absorption of crystal S1 in the range 200 - 300 nm was stronger than that of crystal S2 and crystal S3. The variation of the intensity of the absorption band in the range 200 - 300 nm was closely related to the concentration of the intrinsic defects, which mainly depended of the pulling rate. From the Fig. 3, there are three obvious absorption bands at 206 nm, 226 nm and 256 nm. The absorption spectrums of sapphire crystals have a prominent 206 nm absorption band and weaker bands absorbing at 226 nm and 256 nm. This bands are similar reported by Zhou, et.al [2,7]. They are reported absorption spectrum of Al2O3 crystal grown by Temperature Gradient Technique (TGT) at 204 nm and 232 nm. The band at 206 nm has attracted more attention because it scarcely appears in grown sapphire single crystals. It is a well established fact that the 206 nm band is associated with F (an oxygen-ion vacancy occupied by two electron) center. As described above, the sapphire crystals were grown by Cz method using ADC system with experimental set up (see. Fig.1), so a strongly reducing atmosphere is maintained during the growth process. As result, a high concentration of oxygen vacancies was created in the sapphire crystals. When these O2- vacancies captured two or one electrons, they formed F centers and F+ centers [12]. The F- type centers located at 206 nm band regarded as associated with atomic-displacement-type damage and it only can created by bombardment with particles (electrons, neutron and ions)[2]

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Fig. 3. The room temperature optical absorption spectrum of sapphire crystals

Photoluminescence study

Fig. 4. Shows the excitation and emission spectra of as-grown sapphire crystals by Cz technique using ADC system with different pulling rate. Its can been seen that all samples are similar results of excitation and emission peaks. The effect of pulling rate will be cause different intensity of the excitation and emission peaks. An emission band centered at 375nm is observed upon exciting with 226 nm and 256 nm also shows the same results.

Fig. 4. Excitation and emission spectrum of sapphire single crystals at room temperature

The excitation spectra of the 375 nm (emission) not consist of two bands peaks at 226 nm and 256 nm, but occurs shift band at 210 nm and 238 nm. Its can indicated that the sapphire crystals have same defects (defects intrinsic). Defect susceptible to give photon emission are oxygen vacancies trapping one or two electrons giving respectively F+ and F centres [13]. We show that the existence of these defects, in particular F+ greatly depends on the history of the crystal growth and on the thermal annealing achieved at high temperature after mechanical polishing. The 226 nm and 256 nm bands assigned to the F+ (an oxygen-ion vacancy occupied by one electron) center. The F+ center in sapphire has C2 symmetry, wich causes the 2p state to split into 1B, 2A and 2B states [2]. Fig. 5 shows the energy level scheme for absorption and emission of the F+ center. Absorption and emission observed in present study could well arise from such a transition.

Fig. 5. Schematic energy level diagram for absoption and emission of the F+ center.

CONCLUSION

Sapphire bulk crystals have been successfully grown with á001ñ oriented seed from the melt by CzT using ADC system. The best result of crystal sapphire have grown (free bubble, free crack and transparent) were obtained with the pulling rate at 0.75 mmh-1 and the crystal rotation rate at 15 rpm. The effect of pulling rate will be cause different intensity of the absorption and fluorescence spectra.

ACKNOWLEDGEMENT

The authors wish to thank the Ministry of Science, Technology and Innovation for their financial support via Science Fund number 03-01-06-SF0572. We would also thanks to UTM for the support on this project.