Growth And Optical Properties Of Sapphire Single Crystals Engineering Essay

Growth And Optical Properties Of Sapphire Single Crystals Engineering Essay

Sapphire ( Al2O3 ) crystal is an of import and widely used for high engineering, optical and electro optical application. It is used for lasing stuff in solid province optical masers, substrate for micro-electronics, radiations dosemeter, an dielectric and so on [ 1 ] . As an optical stuff, sapphire has a wide transmittal band the extremist violet, seeable and infrared part. Sapphire besides has really good mechanical and physical belongingss, such as tensile strength, scratch opposition, thermic conduction and mechanical stableness [ 2 ] .

Commercial methods for turning 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 bring forth sapphire with maximal breadth or diameter of four inches. In the sixtiess, demands for big size sapphire were for crystalline armour applications because of sapphire ‘s high hardness and stamina belongingss. Therefore, accent was placed on accommodating several crystal growing processes to bring forth sapphire crystals. Attempts were made to scale up the Cz, Bridgman, Verneuil, and TGT processes to turn big sapphire, but the built-in belongingss of sapphire made it hard to bring forth big sizes utilizing these techniques [ 8 ] . Other belongingss that made sapphire hard to turn were its high thaw point and low thermic conduction compared to metal systems. We had early successfully grown high quality sapphire crystals by CzT.

The purpose of this work is to look into the mechanism CzT growing procedure of sapphire individual crystals with ADC system and the consequence of drawing rate to high quality and optical belongingss of sapphire crystals.

EXPERIMENTAL

Growth of sapphire individual crystals

Crystals of sapphire were grown by the wireless frequence ( r.f ) heated with CzT utilizing automatic diameter control ( ADC ) system to bring forth indistinguishable conditions and to bring forth same sizes of crystals. Computer controlled growing parametric quantities are tabulated in Table 1. The crucible is inductively heated by cooper spirals with AC current at a frequence of 25 kilohertz. The input power is 3200 millivolt with usage of 20-27 per centum. The experimental setup used this survey is shown in Fig. 1. A Ir melting pot, 60 millimeter in diameter and 60 millimeter in tallness with a thickness of 1.5 millimeter, was imbedded in a farinaceous zirconium oxide thermic dielectric contained in a vitreous silica melting pot. The crucible is supported by zirconia ceramic discs. To guarantee unvarying temperature in the warming procedure and to cut down the temperature gradient, so the above melting pot is constructed a dielectric of insulating stuffs at high temperatures, 60 millimeter in diameter and 30 in tallness. An automatic power accountant system controlled the procedure by modulating the power in the RF-coil to command and stabilising the thaw temperature. The thaw surface temperature was monitored utilizing the pyrometer.

Oxide pulverization of high-purity ( 99.999 % ) Al2O3 were weighed and pressed into used the natural stuff. The melt charge occupied about 90 % volume of the Ir melting pot used. Argon flows into create an inert environment for the crystal growing, forestalling the thaw constituent and Ir melting pot from oxidization. The Ar flow the furnace from side thought particular holes, so the flow construction in the furnace is three dimensional.

Table 1: Crystal growing conditions and parametric quantity 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 way

Atmosphere

5 millimeter

20 millimeter

63.44A°

0.5 – 1 mmh-1

15 revolutions per minute

3.98 g/cm3

3.7 g/cm3

60 millimeter

3 millimeter

20 -25 millimeter

75.56A°

5 millimeter

2 millimeter

3115 millivolt

60 H

c-axis & lt ; 001 & gt ;

76.7 % Ar

The crystals are grown from seeds attached to an Ir coupling and held by an aluminum oxide rod. It is revolving at a rate of 15 revolutions per minute to cut down same defects when being pulled by the aluminum oxide tubing. The drawing rate was 0.5 to 1 mm/h. Growth is along the c-axis in both instances. The melting pot was non rotated during the growing. After the growing tally, the crystal boulle was cooled at a rate of about 37 A°C/h down to room temperature.

Fig. 1. Experimental setup of Cz technique, ( a ) Ir crucible, ( B ) polycrystalline thaw of Al2O3, ( degree Celsius ) Al2O3 crystal ( vitamin D ) seed crystal ( vitamin E ) seed holder, ( degree Fahrenheit ) granular ZrO2 ( g ) vitreous silica ( H ) spiral ( I ) ZrO2 home base.

Word picture of sapphire individual crystals

The soaking up spectrum was carried in the scope of 200-800 nanometer by Simadzu UV-Vis NIR spectrometer and the emanation spectra was carried in the scope 200-900 nanometer by Perkin Elmer LS 55 luminescence Spectrometer at room temperature ( 300K ) . The samples were c-cut home bases with thickness of ~2 millimeter, both c-faces polished.

RESULTS & A ; DISCUSSION

Crystal growing of the sapphire

The typical crystals boulles of sapphire grown by Cz technique with ADC system were shown in Fig. 2. In the same figures are shown sapphire crystals with different drawing rate while the rotary motion rate is kept changeless at 15 revolutions per minute during the growing procedure. The sapphire crystals were grown with the drawing rate 0.5 mmh-1, 0.75 mmh-1 and 1 mmh-1 ( designated S1, S2 and S3 severally ) . In this turning crystals there are three parametric quantities which could impact the sphere construction, ( 1 ) the temperature fluctuation, ( 2 ) the off-centered crystal geometry and ( 3 ) the fluctuation of the rotary motion rate [ 9,10 ] . Turning crystals by utilizing ADC system can hold been control of diameter crystal with the set diameter in the parametric quantity growth ( see Table 1 ) . In the instance of growing, dimensions of diameter crystals are larger from diameter in the parametric quantity growing ( & gt ; 20 millimeter ) . Power fluctuations create a periodical the temperature fluctuations, fluctuation in the growing rate and therefore a periodical alteration in the crystal diameter. Another factor impacting the diameter crystal additions as the growing is traveling on around the equilibrium temperature of the system. As the growing advancement, the diameter of the turning crystal is controlled by seting the crucible temperature. Lowering the heating power will speed up the crystallisation and lead to a diameter addition, while increasing the power will move to diminish the crystal diameter [ 11 ] .

S1

S3

S2

( a ) ( B ) ( degree Celsius )

Fig. 2. The exposure of sapphire crystals grown by the CZ technique utilizing ADC system with rotary motion rate 15 revolutions per minute and drawing rate

( a ) 0.5 mmh-1 ( B ) 0.75 mmh-1 ( degree Celsius ) 1 mmh-1

From the observations on the turning sapphire crystals ( Fig.2 ) , the crystals formed are regardful. As shown in the Fig. 2 ( a ) and Fig. 2 ( degree Celsius ) , the crystals are non clear and are have bubble, while in the Fig. 2 ( B ) crystal is crystalline and has non bubble. In the Fig. 2 shows that the crystals produced with different drawing rate will find 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 consequence of crystal sapphire have grown ( free bubble, free cleft and transparent ) were obtained with the drawing rate at 0.75 mmh-1 and the crystal rotary motion rate at 15 revolutions per minute.

Optical soaking up survey

The optical soaking up of sapphire crystals grown by Cz technique with different drawing rate have been recorder and presented in Fig. 3. The soaking up set of sapphire crystals ( S1, S2 and S3 ) are similar. As shown in Fig.3, the strength of optical soaking up of crystal S1 in the scope 200 – 300 nanometer was stronger than that of crystal S2 and crystal S3. The fluctuation of the strength of the soaking up set in the scope 200 – 300 nanometer was closely related to the concentration of the intrinsic defects, which chiefly depended of the drawing rate. From the Fig. 3, there are three obvious soaking up sets at 206 nanometers, 226 nanometer and 256 nanometer. The soaking up spectrums of sapphire crystals have a outstanding 206 nm soaking up set and weaker sets absorbing at 226 nanometers and 256 nanometer. This sets 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 nanometers and 232 nanometer. The set at 206 nanometer has attracted more attending because it barely appears in adult sapphire individual crystals. It is a good established fact that the 206 nm set is associated with F ( an oxygen-ion vacancy occupied by two negatron ) centre. As described above, the sapphire crystals were grown by Cz method utilizing ADC system with experimental set up ( see. Fig.1 ) , so a strongly cut downing ambiance is maintained during the growing procedure. As consequence, a high concentration of O vacancies was created in the sapphire crystals. When these O2- vacancies captured two or one negatrons, they formed F centres and F+ centres [ 12 ] . The F- type centres located at 206 nm set regarded as associated with atomic-displacement-type harm and it merely can created by barrage with atoms ( negatrons, neutron and ions ) [ 2 ]

Fig. 3. The room temperature optical soaking up spectrum of sapphire crystals

Photoluminescence survey

Fig. 4. Shows the excitement and emanation spectra of as-grown sapphire crystals by Cz technique utilizing ADC system with different drawing rate. Its can been seen that all samples are similar consequences of excitement and emanation extremums. The consequence of drawing rate will be cause different strength of the excitement and emanation extremums. An emanation set centered at 375nm is observed upon exciting with 226 nanometers and 256 nanometer besides shows the same consequences.

Fig. 4. Excitement and emanation spectrum of sapphire individual crystals at room temperature

The excitement spectra of the 375 nanometer ( emanation ) non dwell of two sets extremums at 226 nanometers and 256 nanometer, but occurs displacement set at 210 nanometers and 238 nanometer. Its can bespeak that the sapphire crystals have same defects ( defects intrinsic ) . Defect susceptible to give photon emanation are oxygen vacancies pin downing one or two negatrons giving severally F+ and F centres [ 13 ] . We show that the being of these defects, in peculiar F+ greatly depends on the history of the crystal growing and on the thermic tempering achieved at high temperature after mechanical shining. The 226 nanometer and 256 nm sets assigned to the F+ ( an oxygen-ion vacancy occupied by one negatron ) centre. The F+ centre in sapphire has C2 symmetricalness, wich causes the 2p province to divide into 1B, 2A and 2B provinces [ 2 ] . Fig. 5 shows the energy degree strategy for soaking up and emanation of the F+ centre. Absorption and emanation observed in present survey could good originate from such a passage.

Fig. 5. Conventional energy degree diagram for absoption and emanation of the F+ centre.

Decision

Sapphire majority crystals have been successfully grown with a001n oriented seed from the thaw by CzT utilizing ADC system. The best consequence of crystal sapphire have grown ( free bubble, free cleft and transparent ) were obtained with the drawing rate at 0.75 mmh-1 and the crystal rotary motion rate at 15 revolutions per minute. The consequence of drawing rate will be cause different strength of the soaking up and fluorescence spectra.

Recognition

The writers wish to thank the Ministry of Science, Technology and Innovation for their fiscal support via Science Fund figure 03-01-06-SF0572. We would besides thanks to UTM for the support on this undertaking.