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Preparation and properties of MgO single crystals grown by chemical vapor deposition
Journal of Crystal Growth 29 (1975) 257—262
North-Holland Publishing Co.
PREPARATION AND PROPERTIES OF MgO SINGLE CRYSTALS GROWN BY CHEMICAL VAPOR DEPOSITION~ J. ROBERT BOOTH**, W. DAVID KINGERY and H. KENT BOWEN Ceramic Dirision, Depart,nent ot Materials Science and Engineering. Massachasetts institute of Technology. Cambridge, Massachusetts 02/39, U.S.A.
Received 6 October 1973; revised manuscript received 25 February 1975 Growth of MgO single crystals by dynamic chemical vapor deposition has been achieved by generating MgCI 2 vapor by the reaction: MgO (s)-1-C12 (g)+CO (g) ~ MgCI2 (g)H-C02 (g) and subsequently inducing MgO deposition by adding hydrogen to give: MgCI2 (g)+C02 (g)+H2 (g) ~ MgO (s)+2 Ha (g)
CO (g)
all at 1600 C and a total pressure of 5 torr. The influence of gas flow rate, temperature and oxidizing potential on epitaxial single crystal growth, needle growth and powder formation due to nucleation in the gas phase is reported. Epitaxial single crystals I xl xO.2 cm were grown at rates of 4.2 x l06 cm/see; 1.5 xO.3 xO.2 cm needles were grown at linear rates of 2.8 x l0~ cm/sec. Chemical 3/cm2.analysis indicated less than 150 ppm by weight, cation impurity, and dislocation densities were 5 x l0
1. Introduction
purity than the source material, and crystals grown at a low temperature relative to their melting point.
A limitation to research on MgO has been the purity, structural perfection, and optical clarity of available crystals. Most of the crystals available have been produced by arc-melting~4),and often contain entrapped gas, subgrain boundaries and a high dislocation density. Crystals formed by solution growth56) using a flux and isothermal evaporation are often opaque as a resuit of microscopic inclusions and have high impurity levels. The third technique used for MgO single crystal growth is chemical vapor deposition7~’).The majority of this work has been limited to whiskers, needles and thin films. A recent work by Vasilos, Wuensch and Gruber’°11) has produced epitaxial layers of MgO 1.5 mm thick using a closed chamber or static growth technique. In the present studychemical vapor deposition (CVD) has been developed for the preparation of large MgO crystals. This was chosen for it allows preparation of a crystal free from thermal strains and a reduction of small scale inhomogeneties which are produced by instabilities of the growing interface, a crystal of higher Supported by the Atomic Energy Commission. Technical Staffs Division, Corning Glass Works, Corning. New York 4830, U.S.A. *
**
2. Experiment Dynamic forced-flow CVD was employed. The growth system consisted of three sub-units; a gas flow system for producing and metering the reactants, a deposition furnace, and a vacuum system to regulate pressure and dispose of product gases. A schematic diagram of the system is shown in fig. I. Source gases were argon (99.995 ~), carbon monoxide (99.5 ~) carbon dioxide (99.95 ~ chlorine and hydrogen (99.95 ~. The argon was added to both the injector and sheath (the area described by the outer diameter of the injector and the inner diameter of the reactor). The reaction chamber located within the furnace is shown in fig. 2. The furnace heating element was molybdenum which required a hydrogen atmosphere within the furnace shell. The gases entered the reaction chamber by passing through alumina injectors. The center injector contained the source material, MgO prepared by firing Baker reagent grade MgO powder (99.4 ~) in MgO crucibles at 2000 °Cand then crushing (99.50/)
.
the agregate to 0.1—0.5cm diameter particles. The injector was covered with a platinum screen to hold
257
258
J. ROBERT BOOTH,
W. DAVID
KINGERY AND
H. KENT BOWEN
~
AIR INLET TO THROTTLE-PUMP
NEEDLEVALVE ~
VACUUM FURNACE
4~t~
It!
~MPOUND
VACUUM
MgO SOURCE
VACUUM GAUGE
CO
2
Ar
H2
Cl2
Fig. I.
CO Schematic of CVD of MgO. H2 + CO2 + ARGON
MOLYBDENUM HEATING ELEMENT 0
000
000
00
MgCI2 + CO2
00
000
00
“MgO
00
000
0
00
00
0
000
00
00
~l____________
PLATINUM SCREEN
SUBSTRATES
00
SUBSTRATES
+
CO + ARGON
0)0
00
ALUMINA TUBE
Fig. 2.
Injector and reactor for CVD of MgO.
particles of MgO in the injector allowing the chloride reaction to proceed. The reactant gases entered the deposition chamber from the injector and from the sheath at velocities between 300 and 1200 cm/sec (within the laminar flow region). To achieve equal velocities of all gas streams entering the reaction zone, Ar was added as a filler gas where necessary. Solid MgO was deposited in the reaction zone; the unreacted gases and
MgO were obtained from Norton Research Corporation, Niagara Falls, Ontario: Oak Ridge National Laboratory, Oak Ridge, Tennessee; Semi-Elements Inc., Saxonburg, Pennsylvania and W. C. Spicer Ltd., Winchombe, Gloucestershire, England. There were two samples analyzed from Spicer, a 99.99 MgO referred to as 4N and a 99.9 MgO referred to as 3N. Chemical analysis, dislocation count, and optical transmission
gaseous products discharged into the vacuum pumping system. The substrates which were used as seeds for the epitaxial growth were arc-melt crystals from Norton Research Corporation Ltd., Niagara Falls, Ontario, Canada. The reaction chamber was heated and cooled under a vacuum of 5 torr with argon flowing over the source material into the chamber. The argon became saturated with MgO in the source area which minimized thermal etching of the substrate. To compare the properties of the crystals grown in this study with available single crystals, samples of
measurements were conducted on all samples. The chemical analysis (table I) was spectrographic by spark gas absorption. Before analysis, all samples were placed in boiling phosphoric acid to remove a thin layer of MgO and any surface impurities. Dislocation Counts were made on freshly cleaved surfaces which were etched in a 1:1 solution of saturated animonium chloride and concentrated sulphuric acid. Dislocation etch pits were counted from photomicrographs of the etched surface. Optical transmission measurements were performed on I x I x 0.1 cm polished samples using double beam spectrophotometers.
°~
°~
PREPARATION
TABLE
AND PROPERTIES OF
MgO
259
SINGLE CRYSTALS
I
Chemical analysis of single crystals and CVD source material*
—
40—
Spicer SemiNor- Oak Spicer CVD 3-N Elements ton Ridge 4-N
CVD source
-
~2O~ Al
70
10
10
Ca Cr Cu
100
50
50
Fe Mn Si
—
—
10
5 20 10 50
10
10
50 10 50
50 5 10
—
—
—
10 5
10 5
100 >1 10
10000 50
—
—
5 10
10
10
500
50
10
40
5
10
1000
10
50
~..
B
305
180
-~---~—________ *
145 ~
—
~ ~5 0— ~
90
80
—
140
500 12095
w Ui ~ ~-
Given as ppm by weight.
—O
~
-
‘-20 ~ w Z Ui..
5
—
—
-
40
Zr Total
—2
~
.
—
.
— -
-
....— —
...--
-
—
~
— —
-60
-~
-4.E
-
b a)MgCI2+C02+H2MgO+2HCI+CO
—
6
b)MgO+C12+COMgCI2+C02
—
—8
-
~-8O
3. Results and discussion Growth of single crystals of MgO by dynamic CVD was achieved by a two step process involving chemical reactions for both vaporization and deposition. Production of metal monoxide chloride was by assumed reacting chlorine andthe carbon withachieved MgO. The reaction was: MgO (s)+Cl 2 (g)+CO (g) ~ MgCl2(g)+CO2 (g). (1)
HOC 0
—
I 500
I
I
1000 1500 TEMPERATURE 1°C)
I I I
10
2000
Fig. 3.
Standard free energy versus temperature for dynamic 1)J. growth of MgO [the logarithm of the equilibrium constant for the deposition reaction (a) is plotted on the right ordinat&
tion zone on the central axis of the furnace. As the gases flowed from the injector through the reaction
chamber, diffusive mixing took place. The flow length to achieve mixing with the sheath gases was determined by the velocity of the gases and diffusion coefficient of the mixing gases. A 5 cm length of the deposition zone MgCl2 (g)+C02 (g)+ H2 (g) ~s MgO (s) immediately following the injector was maintained at +2HCI(g)+CO(g). (2) the same temperature as the injector. Beyond that As shown in fig. 3, the vaporization reaction (I) has a point, the temperature decreased by about 25 °C/cm. large negative standard free energy which is conducive Therefore, the two gas streams flowed into the deposito nearly stoichiometric vaporization independent of tion zone and mixed together in a constant temperature minor operational variables. The deposition reaction region. The deposition reaction took place only after (2) has a less negative standard free energy at 1600 °C adequate mixing of the two streams. (fig. 3) such that control of deposition could be achieved In order to have control of the gas flow patterns in by variations of the input gases. the reaction zone of the furnace, it was essential that The depletion rate of the MgO source material was laminar flow be maintained. Any loss of laminar flow measured for injector temperatures between 1000 °C caused supersaturation in the gas phase and powder and 16003/min. °CandFor forinjector gas flow rates (Cl2 and CO)than up mize formation due to in changes the gas phase. temperatures greater the effect ofnucleation gas velocity withinTo theminisysto 50 ~C cm one mole of MgO was transported for each tern, Ar was added as an inert filler gas so that there 1325 mole of Cl 2 and CO that was introduced, and varia- was no velocity difference between the gases in the tions in the flow rate had no observable effect, sug- sheath and injector. gesting that complete conversion occurred. When laminar flow was maintained, it was possible The injector (internal generator) was placed in the to calculate the radial diffusion coefficient for the furnace as shown in fig. 2 such that its axis was aligned magnesium chloride—carbon dioxide mixture. With a with that of the outer reaction tube. Therefore, gases gas velocity of 900 cm/sec a deposit was observed on traveling through the injector exited into the deposi- the wall of the reaction tube 3 cm from the end of the Deposition occurred when hydrogen was added according to the following reaction:
260
J. ROBERT BOOTH, W. DAVID KINGERY AND Ii. KENT BOWEN
injector. By calculating the time required to travel this distance and knowing the radial distance to the wall (1 cm), a diffusion coefficient of 300 cm2/sec was estimated. In the diffusive flow region, a conversion equation is given by Schaefer’ 3), for estimating the diffusion coefficient (D) at a temperature (T) and pressure (P) from the diffusivity (D 0) at room temperature (T0) and atmospheric pressure (P0):
the molar quantity of H, at 1600 ~C. When a gas velocity of 900 cm/sec was maintained through the injector and the deposition zone, epitaxial single crystals I x I x 0.2 cm were grown. Two of these crystals are shown in fig. 4.
P0 ~ 8 _________________________________________________ . (3) P T~ For heavy molecules Schaefer reports a value of 0.1 cm2/sec for D 0. Thus, the calculated diffusion co2/sec which 1 lfl C efficient at 1600 °C and 5 torr is 330 cm compares favorably with that obtained from the cxFig. 4. CVD grown MgO single crystal. Magn 1.6. perimental estimations. Epitaxial growth occurred on the substrates held at En the early trials, a translucent interface between temperatures between 1600°Cand 1550 CC. At 1550 CC, the substrate and the epitaxial growth was observed as single crystal needles with growth rates of 2.8 x l0~ shown by the crystal on the right in hg. 4. It was cm/sec formed on the substrate and on the furnace assumed that thermal etching prior to deposition caused wall. Between 1550CC and 1300CC powder formation the translucent interface. By flowing 10 cm3/min of Ar occurred indicating nucleation in the gas phase. Below through the injector during heating a clear transparent 1300 CC there was no MgO formation and below a interface was obtained as seen with the crystal on the temperature of 600 CC there was precipitation of mag- left. nesium chloride needles and powder. These results corThe growth rates observed were approximately respond to expectations from the thermodynamic driv- 4.2 x 106 cm/sec; the leading edge and top edge of ing forces. As shown in fig. 3, the value of the equilib- the crystal had the highest growth rates. The growth rium constant for the deposition reaction, and there- rate near the bottom of the crystal adjacent to the wall fore the supersaturation increases on lowering the tern- of the reaction tube was slower by an order of magniperature from 1600 CC to 1400 CC Below 1400 CC the tude. This would be expected because of the radial driving force begins to decrease, and no deposition diffusive transport of reactant gases, since the highest occurs below 1300 CC. Below 600 CC magnesium chlo- concentration would be near the center of the gas ride becomes the stable phase and deposits first as stream. AS the gas diffused out to the wall, the concenneedles and then as powder in cooler areas of the re- tration would be lower and, therefore, result in slower action tube, growth rates. For the same gas-flow conditions, large Variations in the oxidizing potential were accom- needles I .5 x 0.3 x 0.2 cm formed at 1550 CC with a panied by using excess CO 2 supplied in the sheath with growth rate of 2.8 x l0~ cm/sec. the H2 or Ar. Increasing the oxygen partial pressure The chemical analyses of commercially available and caused high supersaturation and gas phase nucleation CVD crystals are shown in table I along with the resiat all temperatures between 1600 CC and 1300 CC Dc- duel feed material after reaction. The major source of’ creasing the oxygen partial pressure by using CO in cation impurity in the CVD crystals is aluminum which the H2 and Ar gas stream decreased the supersatura- is not unexpected since the reaction tube is constructed tion and slowed the growth rate. of this material and since the granulated source mateThe growth of large epitaxial single crystals of MgO rial contained excessive aluminum. was accomplished using equi-molar quantities of Cl2 Optical transmission of CVD crystals (fig. 5) was and CO reacted with granulated MgO at 1600 °Cto similar to the commerciallyavailable samples. However, produce MgCl2 and CO2 and then react with If times the high purity Oak Ridge and the CVD crystals had D=D0
-~r
—-,—
.---.---,---——
—
-
PREPARATION
00
AND PROPERTIES OF
I
MgO
liii
261
SINGLE CRYSTALS I
I I
..—.
80-
.-,-
•\\“.,,,
“~
\\\
‘~
~-°6O
~ 40
‘—.—‘
CVD Grown Oak Ridge Spicer 4-N
———
Semi—elements Spicer 3-N
—
-
I —
20
—
-
-
\,(l
I
-
“P1
Norton
—
0
I 0.4
0.2
I 0.6
1 3
I
4
5
6
7
8
9
10
II
12
Wavelength (microns)
Fig. 5.
Percent transmission versus wavelength for MgO single crystals.
longer wavelength IR absorption edges than the other crystals examined. Dislocation density measurements reported in table 2 show that the CVD grown crystals have a significantly lower dislocation content than crystals formed from a melt. This is probably a consequence of the isothermal,
Dislocation densities TABLE of MgO 2 single crystals Single crystals Spicer 4—9 Oak Ridge Norton Semi-Elements Spicer 3—9 CVD
Dislocations 2) (cm I.Ox 10* 1.0 x l0~’ 5.0 x I0~ 3.6 x l0~ 2.8 x Io~ 5.Ox Io~
_____
strain free growth conditions. Fig. 6 shows that there is a large decrease in dislocation density as the crystal grows away from the substrate interface, which is consistent with observations of CVD growth in other systems’4). 4.
______________________________________ ________________
______
Epitaxial Layer
_____________ ________________
Substrate
ii~~ ___________
Fig. 6. Photomicrograph of dislocation etch pits in a cleaved cross section of substrate and epitaxial growth.
cm/sec. Chemical analyses of the CVD grown crystals indicated less than ISO ppm total cation impurity with 100 ppm of that being aluminum contamination from the reaction chamber material and source material. The dislocation density of the CVD grown crystals (5 x 103/cm2) was much less than the melt grown sampIes examined (3 x l0~—lx 106/cm2). References
Conclusions
The growth of large single crystals of MgO by CVD was achieved with a dynamic growth system. Crystals I x I x 0.2 cm and needles 1.5 x 0.3 x 0.1 cm were grown. Growth rates for the crystals were 4.2x 10—6 cm/sec and the linear rate for the needles was 2.8 x JO~
I) C. T. Butler, B. J. Sturm and R. B. Quincy, Jr., J. Crystal Growth 8 (1971) 197. 2) J. Strong and R. T. Brice, J. Opt. Soc. Am. 25 (1935) 207. 4) L. J. Schupp, Electrochem. Technol. 6 (1968) 219. 3) E. G. Rochow, J. AppI. Phys. 9 (1938) 664. 5) F. W. Webster and E. A. D. White, J. Crystal Growth 5 (1969) 167. 6) H. Vora and R. R. Zupp, Mater. Res. Bull. 5 (1970) 977.
262
J. ROBERT BOOTH, W. I)AVID KINGERY AND I-I. KENT BOWEN
7) E. G. Wolff and T. D. Coskren, J. Am. Ceram. Soc. 48 (1965) 277. 8) 3. E. Mee and G. R. Pulliam, in: Crystal Growth, Ed. H. S.
Peiser (Pergamon. Oxford. 1967) pp. 333—335. 9) G. Cockayne, J. D. Filby and D. B. Gasson. J. Crystal Growth 9 (1971) 340. 10) T. Vasilos, B. J. Wuensch and P. E. Gruber, Technical Report AVSD-0468-71-RR (1971) Appendix C.
II) P. E. Gruber. J. Crystal Growth 18(1973)94. I2) 0. Kubaschewski, E. L. Evans and C. B. Alcock, tvktallurgicalThermochernistr.v. 4th ed. (Pergamon. New York. 1967) pp. 420—429. 3) H. Schaefer, Chemical Transport Reactions’ lAcademic Press, New York. 1964) p. 24. 14) H. K. Bowen, W. D. Kingery. M. Kinoshita and C. A. Goodwin. J. Crystal Growth 13/14 (1972) 402.
North-Holland Publishing Co.
PREPARATION AND PROPERTIES OF MgO SINGLE CRYSTALS GROWN BY CHEMICAL VAPOR DEPOSITION~ J. ROBERT BOOTH**, W. DAVID KINGERY and H. KENT BOWEN Ceramic Dirision, Depart,nent ot Materials Science and Engineering. Massachasetts institute of Technology. Cambridge, Massachusetts 02/39, U.S.A.
Received 6 October 1973; revised manuscript received 25 February 1975 Growth of MgO single crystals by dynamic chemical vapor deposition has been achieved by generating MgCI 2 vapor by the reaction: MgO (s)-1-C12 (g)+CO (g) ~ MgCI2 (g)H-C02 (g) and subsequently inducing MgO deposition by adding hydrogen to give: MgCI2 (g)+C02 (g)+H2 (g) ~ MgO (s)+2 Ha (g)
CO (g)
all at 1600 C and a total pressure of 5 torr. The influence of gas flow rate, temperature and oxidizing potential on epitaxial single crystal growth, needle growth and powder formation due to nucleation in the gas phase is reported. Epitaxial single crystals I xl xO.2 cm were grown at rates of 4.2 x l06 cm/see; 1.5 xO.3 xO.2 cm needles were grown at linear rates of 2.8 x l0~ cm/sec. Chemical 3/cm2.analysis indicated less than 150 ppm by weight, cation impurity, and dislocation densities were 5 x l0
1. Introduction
purity than the source material, and crystals grown at a low temperature relative to their melting point.
A limitation to research on MgO has been the purity, structural perfection, and optical clarity of available crystals. Most of the crystals available have been produced by arc-melting~4),and often contain entrapped gas, subgrain boundaries and a high dislocation density. Crystals formed by solution growth56) using a flux and isothermal evaporation are often opaque as a resuit of microscopic inclusions and have high impurity levels. The third technique used for MgO single crystal growth is chemical vapor deposition7~’).The majority of this work has been limited to whiskers, needles and thin films. A recent work by Vasilos, Wuensch and Gruber’°11) has produced epitaxial layers of MgO 1.5 mm thick using a closed chamber or static growth technique. In the present studychemical vapor deposition (CVD) has been developed for the preparation of large MgO crystals. This was chosen for it allows preparation of a crystal free from thermal strains and a reduction of small scale inhomogeneties which are produced by instabilities of the growing interface, a crystal of higher Supported by the Atomic Energy Commission. Technical Staffs Division, Corning Glass Works, Corning. New York 4830, U.S.A. *
**
2. Experiment Dynamic forced-flow CVD was employed. The growth system consisted of three sub-units; a gas flow system for producing and metering the reactants, a deposition furnace, and a vacuum system to regulate pressure and dispose of product gases. A schematic diagram of the system is shown in fig. I. Source gases were argon (99.995 ~), carbon monoxide (99.5 ~) carbon dioxide (99.95 ~ chlorine and hydrogen (99.95 ~. The argon was added to both the injector and sheath (the area described by the outer diameter of the injector and the inner diameter of the reactor). The reaction chamber located within the furnace is shown in fig. 2. The furnace heating element was molybdenum which required a hydrogen atmosphere within the furnace shell. The gases entered the reaction chamber by passing through alumina injectors. The center injector contained the source material, MgO prepared by firing Baker reagent grade MgO powder (99.4 ~) in MgO crucibles at 2000 °Cand then crushing (99.50/)
.
the agregate to 0.1—0.5cm diameter particles. The injector was covered with a platinum screen to hold
257
258
J. ROBERT BOOTH,
W. DAVID
KINGERY AND
H. KENT BOWEN
~
AIR INLET TO THROTTLE-PUMP
NEEDLEVALVE ~
VACUUM FURNACE
4~t~
It!
~MPOUND
VACUUM
MgO SOURCE
VACUUM GAUGE
CO
2
Ar
H2
Cl2
Fig. I.
CO Schematic of CVD of MgO. H2 + CO2 + ARGON
MOLYBDENUM HEATING ELEMENT 0
000
000
00
MgCI2 + CO2
00
000
00
“MgO
00
000
0
00
00
0
000
00
00
~l____________
PLATINUM SCREEN
SUBSTRATES
00
SUBSTRATES
+
CO + ARGON
0)0
00
ALUMINA TUBE
Fig. 2.
Injector and reactor for CVD of MgO.
particles of MgO in the injector allowing the chloride reaction to proceed. The reactant gases entered the deposition chamber from the injector and from the sheath at velocities between 300 and 1200 cm/sec (within the laminar flow region). To achieve equal velocities of all gas streams entering the reaction zone, Ar was added as a filler gas where necessary. Solid MgO was deposited in the reaction zone; the unreacted gases and
MgO were obtained from Norton Research Corporation, Niagara Falls, Ontario: Oak Ridge National Laboratory, Oak Ridge, Tennessee; Semi-Elements Inc., Saxonburg, Pennsylvania and W. C. Spicer Ltd., Winchombe, Gloucestershire, England. There were two samples analyzed from Spicer, a 99.99 MgO referred to as 4N and a 99.9 MgO referred to as 3N. Chemical analysis, dislocation count, and optical transmission
gaseous products discharged into the vacuum pumping system. The substrates which were used as seeds for the epitaxial growth were arc-melt crystals from Norton Research Corporation Ltd., Niagara Falls, Ontario, Canada. The reaction chamber was heated and cooled under a vacuum of 5 torr with argon flowing over the source material into the chamber. The argon became saturated with MgO in the source area which minimized thermal etching of the substrate. To compare the properties of the crystals grown in this study with available single crystals, samples of
measurements were conducted on all samples. The chemical analysis (table I) was spectrographic by spark gas absorption. Before analysis, all samples were placed in boiling phosphoric acid to remove a thin layer of MgO and any surface impurities. Dislocation Counts were made on freshly cleaved surfaces which were etched in a 1:1 solution of saturated animonium chloride and concentrated sulphuric acid. Dislocation etch pits were counted from photomicrographs of the etched surface. Optical transmission measurements were performed on I x I x 0.1 cm polished samples using double beam spectrophotometers.
°~
°~
PREPARATION
TABLE
AND PROPERTIES OF
MgO
259
SINGLE CRYSTALS
I
Chemical analysis of single crystals and CVD source material*
—
40—
Spicer SemiNor- Oak Spicer CVD 3-N Elements ton Ridge 4-N
CVD source
-
~2O~ Al
70
10
10
Ca Cr Cu
100
50
50
Fe Mn Si
—
—
10
5 20 10 50
10
10
50 10 50
50 5 10
—
—
—
10 5
10 5
100 >1 10
10000 50
—
—
5 10
10
10
500
50
10
40
5
10
1000
10
50
~..
B
305
180
-~---~—________ *
145 ~
—
~ ~5 0— ~
90
80
—
140
500 12095
w Ui ~ ~-
Given as ppm by weight.
—O
~
-
‘-20 ~ w Z Ui..
5
—
—
-
40
Zr Total
—2
~
.
—
.
— -
-
....— —
...--
-
—
~
— —
-60
-~
-4.E
-
b a)MgCI2+C02+H2MgO+2HCI+CO
—
6
b)MgO+C12+COMgCI2+C02
—
—8
-
~-8O
3. Results and discussion Growth of single crystals of MgO by dynamic CVD was achieved by a two step process involving chemical reactions for both vaporization and deposition. Production of metal monoxide chloride was by assumed reacting chlorine andthe carbon withachieved MgO. The reaction was: MgO (s)+Cl 2 (g)+CO (g) ~ MgCl2(g)+CO2 (g). (1)
HOC 0
—
I 500
I
I
1000 1500 TEMPERATURE 1°C)
I I I
10
2000
Fig. 3.
Standard free energy versus temperature for dynamic 1)J. growth of MgO [the logarithm of the equilibrium constant for the deposition reaction (a) is plotted on the right ordinat&
tion zone on the central axis of the furnace. As the gases flowed from the injector through the reaction
chamber, diffusive mixing took place. The flow length to achieve mixing with the sheath gases was determined by the velocity of the gases and diffusion coefficient of the mixing gases. A 5 cm length of the deposition zone MgCl2 (g)+C02 (g)+ H2 (g) ~s MgO (s) immediately following the injector was maintained at +2HCI(g)+CO(g). (2) the same temperature as the injector. Beyond that As shown in fig. 3, the vaporization reaction (I) has a point, the temperature decreased by about 25 °C/cm. large negative standard free energy which is conducive Therefore, the two gas streams flowed into the deposito nearly stoichiometric vaporization independent of tion zone and mixed together in a constant temperature minor operational variables. The deposition reaction region. The deposition reaction took place only after (2) has a less negative standard free energy at 1600 °C adequate mixing of the two streams. (fig. 3) such that control of deposition could be achieved In order to have control of the gas flow patterns in by variations of the input gases. the reaction zone of the furnace, it was essential that The depletion rate of the MgO source material was laminar flow be maintained. Any loss of laminar flow measured for injector temperatures between 1000 °C caused supersaturation in the gas phase and powder and 16003/min. °CandFor forinjector gas flow rates (Cl2 and CO)than up mize formation due to in changes the gas phase. temperatures greater the effect ofnucleation gas velocity withinTo theminisysto 50 ~C cm one mole of MgO was transported for each tern, Ar was added as an inert filler gas so that there 1325 mole of Cl 2 and CO that was introduced, and varia- was no velocity difference between the gases in the tions in the flow rate had no observable effect, sug- sheath and injector. gesting that complete conversion occurred. When laminar flow was maintained, it was possible The injector (internal generator) was placed in the to calculate the radial diffusion coefficient for the furnace as shown in fig. 2 such that its axis was aligned magnesium chloride—carbon dioxide mixture. With a with that of the outer reaction tube. Therefore, gases gas velocity of 900 cm/sec a deposit was observed on traveling through the injector exited into the deposi- the wall of the reaction tube 3 cm from the end of the Deposition occurred when hydrogen was added according to the following reaction:
260
J. ROBERT BOOTH, W. DAVID KINGERY AND Ii. KENT BOWEN
injector. By calculating the time required to travel this distance and knowing the radial distance to the wall (1 cm), a diffusion coefficient of 300 cm2/sec was estimated. In the diffusive flow region, a conversion equation is given by Schaefer’ 3), for estimating the diffusion coefficient (D) at a temperature (T) and pressure (P) from the diffusivity (D 0) at room temperature (T0) and atmospheric pressure (P0):
the molar quantity of H, at 1600 ~C. When a gas velocity of 900 cm/sec was maintained through the injector and the deposition zone, epitaxial single crystals I x I x 0.2 cm were grown. Two of these crystals are shown in fig. 4.
P0 ~ 8 _________________________________________________ . (3) P T~ For heavy molecules Schaefer reports a value of 0.1 cm2/sec for D 0. Thus, the calculated diffusion co2/sec which 1 lfl C efficient at 1600 °C and 5 torr is 330 cm compares favorably with that obtained from the cxFig. 4. CVD grown MgO single crystal. Magn 1.6. perimental estimations. Epitaxial growth occurred on the substrates held at En the early trials, a translucent interface between temperatures between 1600°Cand 1550 CC. At 1550 CC, the substrate and the epitaxial growth was observed as single crystal needles with growth rates of 2.8 x l0~ shown by the crystal on the right in hg. 4. It was cm/sec formed on the substrate and on the furnace assumed that thermal etching prior to deposition caused wall. Between 1550CC and 1300CC powder formation the translucent interface. By flowing 10 cm3/min of Ar occurred indicating nucleation in the gas phase. Below through the injector during heating a clear transparent 1300 CC there was no MgO formation and below a interface was obtained as seen with the crystal on the temperature of 600 CC there was precipitation of mag- left. nesium chloride needles and powder. These results corThe growth rates observed were approximately respond to expectations from the thermodynamic driv- 4.2 x 106 cm/sec; the leading edge and top edge of ing forces. As shown in fig. 3, the value of the equilib- the crystal had the highest growth rates. The growth rium constant for the deposition reaction, and there- rate near the bottom of the crystal adjacent to the wall fore the supersaturation increases on lowering the tern- of the reaction tube was slower by an order of magniperature from 1600 CC to 1400 CC Below 1400 CC the tude. This would be expected because of the radial driving force begins to decrease, and no deposition diffusive transport of reactant gases, since the highest occurs below 1300 CC. Below 600 CC magnesium chlo- concentration would be near the center of the gas ride becomes the stable phase and deposits first as stream. AS the gas diffused out to the wall, the concenneedles and then as powder in cooler areas of the re- tration would be lower and, therefore, result in slower action tube, growth rates. For the same gas-flow conditions, large Variations in the oxidizing potential were accom- needles I .5 x 0.3 x 0.2 cm formed at 1550 CC with a panied by using excess CO 2 supplied in the sheath with growth rate of 2.8 x l0~ cm/sec. the H2 or Ar. Increasing the oxygen partial pressure The chemical analyses of commercially available and caused high supersaturation and gas phase nucleation CVD crystals are shown in table I along with the resiat all temperatures between 1600 CC and 1300 CC Dc- duel feed material after reaction. The major source of’ creasing the oxygen partial pressure by using CO in cation impurity in the CVD crystals is aluminum which the H2 and Ar gas stream decreased the supersatura- is not unexpected since the reaction tube is constructed tion and slowed the growth rate. of this material and since the granulated source mateThe growth of large epitaxial single crystals of MgO rial contained excessive aluminum. was accomplished using equi-molar quantities of Cl2 Optical transmission of CVD crystals (fig. 5) was and CO reacted with granulated MgO at 1600 °Cto similar to the commerciallyavailable samples. However, produce MgCl2 and CO2 and then react with If times the high purity Oak Ridge and the CVD crystals had D=D0
-~r
—-,—
.---.---,---——
—
-
PREPARATION
00
AND PROPERTIES OF
I
MgO
liii
261
SINGLE CRYSTALS I
I I
..—.
80-
.-,-
•\\“.,,,
“~
\\\
‘~
~-°6O
~ 40
‘—.—‘
CVD Grown Oak Ridge Spicer 4-N
———
Semi—elements Spicer 3-N
—
-
I —
20
—
-
-
\,(l
I
-
“P1
Norton
—
0
I 0.4
0.2
I 0.6
1 3
I
4
5
6
7
8
9
10
II
12
Wavelength (microns)
Fig. 5.
Percent transmission versus wavelength for MgO single crystals.
longer wavelength IR absorption edges than the other crystals examined. Dislocation density measurements reported in table 2 show that the CVD grown crystals have a significantly lower dislocation content than crystals formed from a melt. This is probably a consequence of the isothermal,
Dislocation densities TABLE of MgO 2 single crystals Single crystals Spicer 4—9 Oak Ridge Norton Semi-Elements Spicer 3—9 CVD
Dislocations 2) (cm I.Ox 10* 1.0 x l0~’ 5.0 x I0~ 3.6 x l0~ 2.8 x Io~ 5.Ox Io~
_____
strain free growth conditions. Fig. 6 shows that there is a large decrease in dislocation density as the crystal grows away from the substrate interface, which is consistent with observations of CVD growth in other systems’4). 4.
______________________________________ ________________
______
Epitaxial Layer
_____________ ________________
Substrate
ii~~ ___________
Fig. 6. Photomicrograph of dislocation etch pits in a cleaved cross section of substrate and epitaxial growth.
cm/sec. Chemical analyses of the CVD grown crystals indicated less than ISO ppm total cation impurity with 100 ppm of that being aluminum contamination from the reaction chamber material and source material. The dislocation density of the CVD grown crystals (5 x 103/cm2) was much less than the melt grown sampIes examined (3 x l0~—lx 106/cm2). References
Conclusions
The growth of large single crystals of MgO by CVD was achieved with a dynamic growth system. Crystals I x I x 0.2 cm and needles 1.5 x 0.3 x 0.1 cm were grown. Growth rates for the crystals were 4.2x 10—6 cm/sec and the linear rate for the needles was 2.8 x JO~
I) C. T. Butler, B. J. Sturm and R. B. Quincy, Jr., J. Crystal Growth 8 (1971) 197. 2) J. Strong and R. T. Brice, J. Opt. Soc. Am. 25 (1935) 207. 4) L. J. Schupp, Electrochem. Technol. 6 (1968) 219. 3) E. G. Rochow, J. AppI. Phys. 9 (1938) 664. 5) F. W. Webster and E. A. D. White, J. Crystal Growth 5 (1969) 167. 6) H. Vora and R. R. Zupp, Mater. Res. Bull. 5 (1970) 977.
262
J. ROBERT BOOTH, W. I)AVID KINGERY AND I-I. KENT BOWEN
7) E. G. Wolff and T. D. Coskren, J. Am. Ceram. Soc. 48 (1965) 277. 8) 3. E. Mee and G. R. Pulliam, in: Crystal Growth, Ed. H. S.
Peiser (Pergamon. Oxford. 1967) pp. 333—335. 9) G. Cockayne, J. D. Filby and D. B. Gasson. J. Crystal Growth 9 (1971) 340. 10) T. Vasilos, B. J. Wuensch and P. E. Gruber, Technical Report AVSD-0468-71-RR (1971) Appendix C.
II) P. E. Gruber. J. Crystal Growth 18(1973)94. I2) 0. Kubaschewski, E. L. Evans and C. B. Alcock, tvktallurgicalThermochernistr.v. 4th ed. (Pergamon. New York. 1967) pp. 420—429. 3) H. Schaefer, Chemical Transport Reactions’ lAcademic Press, New York. 1964) p. 24. 14) H. K. Bowen, W. D. Kingery. M. Kinoshita and C. A. Goodwin. J. Crystal Growth 13/14 (1972) 402.