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Flow visualization studies for optimization of OMVPE reactor design
136
Journal of Crystal Growth 77 (1986) 136—143 North-Holland, Amsterdam
FLOW VISUALIZATION STUDIES FOR OPTIMIZATION OF OMVPE REACTOR DESIGN C.A. WANG, S.H. GROVES and S.C. PALMATEER Lincoln Laboratory, Massachusetts Institute of Technology. Lexington, Massachusetts 021 73-0073, USA
D.W. WEYBURNE Rome Air Development Center, Hanscom Air Force Base, Massachusetts 01731, USA
and R.A. BROWN Department of Chemical Engineering; Massachusetts Insitute of Technology, Cambridge, Massachusetts 02139, USA
Gas flow visualization studies have been performed in a vertical reactor tube incorporating a rotating disk susceptor. Smoke particles of Ti0 2 transported in a stream of He gas are illuminated by a sheet of laser light. This technique permits detailed observation of gas flow patterns. To study the influence of the method of gas injection, the gas was injected through (1) a vertical pipe inlet coaxial with the tube, (2) four inlets tangential to the tube and (3) a radial inlet above a porous plug. The method of injection was found to have a critical effect on vortex fonnation, which is minimized by using the porous plug. Disk rotation can be effective in creating a uniform boundary layer at the disk surface. When the susceptor is heated, the flow is strongly perturbed by thermally induced convection, which can be reduced by lowering the operating pressure.
1. Introduction Reproducible growth of laterally uniform epilayers that have abrupt interfaces and can be made thin enough for quantum well and superlattice structures is necessary for many electronic and optoelectronic applications. Deposition of epilayers by the OMVPE technique is usually limited by mass transport of reactants to the substrate, Consequently, epilayer quality is determined largely by gas dynamics in the reactor. In this investigation, gas flow visualization achieved by the scattering of a sheet of laser light from TiO2 particles has been utilized in optimizing the design and operation of OMVPE reactors. By limiting the source of light scattering to a cross-sectional plane, this visualization technique permits detailed observation of gas flow patterns [1]. Previously, flow visualization using incoherent illumination of TiO2 particles has been used to interpret growth results and confirm thermal models in vapor phase epi-
0022-0248/86/$03.50
taxial reactors [2—7].Interference holography has also been employed for mapping gas densities in such reactors [8]. For optimum reactor design, two criteria should be met: (1) for epilayer uniformity, the thickness of the boundary layer at the substrate should be uniform, so that deposition occurs at the same rate for all points on the substrate; (2) for interface abruptness, the flow field in the reactor should be free of any laminar vortices in order to minimize gas residence times. The reactor tube used in this investigation is vertical and has a rotating susceptor disk. Rotation of the disk can be used to establish a self-similar velocity profile along the substrate, which produces a uniform boundary layer [9]. The effects of gas injection and mass flow rate, disk rotation and heating, and reactor pressure on flow patterns have been studied. The method of gas injection is found to b~crucial to the control of vortex formation.
© Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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/ Flow visualization studies for OMVPE reactor design
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2. Experimental procedure
Gas flow patterns are made visible by illuminating Ti02 particles averaging about 1 ~smin
The experimental apparatus is shown schematically in fig. 1. The vertical quartz tube, 10 cm ID, incorporates a 6.7 cm diameter RF-heated rotating graphite susceptor disk. The distance between the disk and the gas inlet is adjustable from 5 to 25 cm; a spacing of 15 cm was used in the experiments reported here. An optical pyrometer is used to measure the disk temperature. The system is equipped with a vacuum pump and control valve for low pressure operation. Gas injection was studied by the use of: (a) a vertical 6-mm pipe inlet coaxial with the tube, (b) four equally spaced inlets tangential to the tube, and (c) a radial inlet above a porous plug, either 3.8 or 7.6 cm in diameter, formed by metal screens. The first was selected for study because of its common use and simplicity, the second and third to give good mixing and permit loading a substrate through a load lock at the top of a working reactor tube.
diameter [10] generated by the reaction of TiCl4 and H 20. Separate streams of He carrier gas, whose properties are similar to those of H2, are passed through two bubblers, one containing H 20 and the other a mixture of TiCl4 and Cd4. Addition of CC14 minimizes formation of solid deposits and clogging of gas lines [11]. In order to keep their concentration small enough to avoid influencing the flow patterns, the TiO2 particles are generated at low flow rates (-‘- 1 LPM), and two additional gas lines are provided to obtain the desired total flow of He. All flows are monitored by rotameters. Another feature of the system is a fast-switching pressure-balanced manifold that has been used for quantitative studies of gas residence times, employing SF6 as a tracer gas, which will be reported elsewhere. Vertical and horizontal cross sections through the tube are illuminated with a sheet of light formed by directing the beam of a 5 mW He—Ne laser through a cylindrical lens. Images are recorded on video tape and sheet film. Since the intensity of scattered light is proportional to the local concentration of Ti02 particles, gas mixing in the tube can be observed by tracing the paths of these particles following their introduction. Gas residence times are estimated by measuring the
tangential coaxial
porous plug _______
I
,
switchg
.
.
injectors
.
.
~ cylindrical lens H20
pump
V V V V
4—He Fig. 1. Schematic diagram of apparatus for gas flow visualiza-
tion.
.
. . . injection on the flow field was studied for isother-
mal gas flow, with and without disk rotation. Changes in flow patterns resulting when the disk was heated to 600°Cwere studied only for the
,
:coil TiCl4 Cd4
time required for particles to clear the tube after smoke generation is stopped by bypassing the TiCl4 bubbler. In both instances, the total mass flow rate through the tube is maintained constant. Three-dimensional flows are observed by projecting white light down the axis of the tube. In this study the effect of the method of gas
porous plug inlet, using disk rotation rates between 0 and 1800 RPM. Gas flow rates were typical of those used in growth and varied from 2 LPM for coaxial injection to 10 LPM for tangential and porous plug injection. The reactor pressure was controlled at values between 0.1 and 1
atm.
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3. Results 3.1. Gas injection method 3.1.1. Vertical pipe injection The most common method of gas injection in OMVPE reactors is introduction through a pipe inlet that is coaxial with the reactor tube. Fig. 2 shows cross-sectional photographs of gas flow patterns at room temperature obtained when this method was used with a total flow of 2 LPM and a tube pressure of 1 atm. (The disk was not rotated during any of the experiments described in section 3.1). One second after the initial introduction of smoke (fig. 2a), a jet is observed that extends from the inlet to the disk, flows outward radially across the disk and strikes the side of the tube. By 20 s (fig. 2b), vortices extending from the top of the disk are observed around the jet. At 20 s the concentration of smoke is higher in the jet than in the vortices, but the concentration in the vortices increases with time, as some of the smoke being added becomes trapped there. After the smoke is turned off, it clears rapidly from the jet
and across the disk (fig. 2c), but persists in the recirculating cell for several minutes. The presence of the inertial jet shows that the gas flow from the pipe inlet is not fully developed when it reaches the disk. Intense gas recirculation occurs throughout the reactor as a result of the impinging jet. An increase in the gas flow rate increases both the velocity of the jet and the gas recirculation velocities. Gas trapped in vortices is isolated from the main flow, and its composition can adjust only by diffusion across the separating streamline. This diffusive control leads to long gas residence times for dopant and alloying components. A high degree of radial non-uniformity and marked grading between epilayers is therefore expected from coaxial injection. 3.1.2. Tangential injection Injection through tangential inlets introduces swirl to the gas, increasing its path length and dissipating the momentum of the inlet stream, thereby producing a fully developed flow at the disk surface. Steady-state patterns recorded at room temperature and 1 atm for a flow rate of 10
Fig. 2. Vertical cross sections showing flow patterns obtained for gas injection through a coaxial vertical pipe: (a) I s after initial introduction of smoke; (b) 20 s after initial introduction of smoke; (c) 1 mm after smoke was turned off.
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I. Fig. 3. Steady-state flow patterns for gas injection through tangential inlets: (a) ~hite light photograph: (b) horizontal cross section; (c) vertical cross section.
LPM are shown in fig. 3. A white light photograph (fig. 3a) shows a complex. spiraling motion of gas around the circumference of the tube with
penetration to the centerline. The spiral pattern is obvious in a horizontal cross section (fig. 3b). By observing the pattern for a vertical cross section
II Fig. 4. Vertical cross sections showing steady-state flow patterns obtained for gas injection through a porous plug: (a) 3.8 cm diameter: (b) 7.6 cm diameter.
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(fig. 3c) as a function of time, it was found that smoke particles follow a path around the lobes near the outer edges of the tube, repeatedly traversing down, around, then up, while swirling around the tube, until the steady-state pattern develops approximately 5 s after initial introduction of smoke. The oscillatory particle path arises from centrifugal instabilities that develop from angular momentum introduced by the tangential inlets. The flow has the form of Goertler vortices [12] superimposed on the downward motion of the gas. Although the flow is fully developed when it reaches the disk, variations in smoke concentration in the vicinity of the disk indicate that the gas is not well mixed. The gas swirl results in the formation of a vortex at the top of the tube. After the smoke is turned off, clearing occurs first from the edges of the lobes. In 10—15 s it becomes complete except within the central vortex where smoke particles continue to recirculate for several minutes.
3.1.3. Porous plug injection The use of a radial inlet followed by a porous plug results in uniform injection over a large cross section of the tube. The steady-state flow pattern obtained at room temperature and 1 atm for a flow rate of 10 LPM through a 3.8 cm diameter plug is shown in fig. 4a. Gas enters over a diameter equal to that of the plug with a velocity nearly independent of radial distance, characteristics typical of plug flow. The uniform intensity of smoke particles indicates that the gas is uniformly mixed (the vertical streaks in the photograph are reflections from the walls of the tube). However, flow separation causes formation of vortices. Increasing the diameter of the plug to 7.6 cm (fig. 4b) results in a uniform flow that sweeps out all but an extremely small volume at the top of the tube. The time elapsed before laminar flow reaches the disk is approximately 4—5 s, while 5—6 s are required for the tube to be cleared after the smoke is turned off. These times are comparable to those
Fig. 5. Vertical cross sections showing steady-state flow patterns for gas injection through tangential inlets with disk rotation: (a) 40 RPM; (b) 300 RPM; (c) 1000 RPM.
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calculated by dividing the distance between the inlet and the disk by the average volocity of the gas. Reducing the reactor pressure to 0.1 atm without changing the mass flow rate increases the gas velocity by a factor of 10 and reduces the initial and final transient times by the same factor. 3.2. Disk rotation Disk rotation forces gas radially outward and vertically downward all across the disk, acting like a pump. This effect is illustrated in fig. 5, which shows steady-state flow patterns obtained by using tangential injection at 10 LPM and 1 atm. Comparison of the patterns for rotation at 40 RPM (fig. 5a) and no rotation (fig. 3c) shows that streamlines in the center of the tube are pulled toward the disk. At higher rates, the flow above the disk appears laminar and the gas at the disk surface is uniformly mixed. Increasing the rotation rate to 300 RPM (fig. Sb) results in the formation of an eddy near the disk at the outer edge of the tube. As the rotation rate is increased further, this side eddy enlarges, as seen in fig. Sc for 1000 RPM. Rotation affects all the gas above the disk. The central vortex at the top of the tube is pulled downward, and can be pulled all the way to the disk at high rotation rates (fig. Sc). When the smoke is turned off, it lingers in the central vortex and in the side eddy for several minutes.
.
Fig. 6. Vertical cross section showing flow pattern for gas injection through 7.6 cm diameter porous plug with disk temperature of 600°C.
hydrodynamic boundary layer cannot be identified with the thickness of this smoke-free region. Due to thermal convective flows, the residence times at 1 atm were on the order of 15 s. At 0.1 atm, the residence times were reduced to a fraction of a second.
3.3. Disk heating The effect of heating the susceptor disk was studied for gas injection at 10 LPM through the 7.6 cm diameter porous plug. A flow pattern observed at 1 atm for a disk temperature of 600°Cis shown in fig. 6. The flow, which is dominated by thermal convection, is very complicated. Rotation of the disk at rates up to 1800 RPM failed to eliminate thermal convection effects. However, reducing the pressure below 0.3 atm (without rotation) did eliminate these effects, and laminar flows like those observed without heating (fig. 4b) were obtained. Smoke was absent from a region 5—10 mm thick above the disk because of the thermophoretic migration of TiO2 particles away from the heated surface. The thickness of the
4. Discussion Optimizing the experimental conditions in an OMVPE reactor to achieve epilayer uniformity and permit abrupt changes in composition requires control of the gas flow adjacent to the substrate as well as throughout the reactor tube. For epilayer uniformity, the boundary layer at the substrate must be of uniform thickness. For a vertical-flow, rotating-disk reactor, two mechanisms are available for forming such a uniform layer. One utilizes the stagnation flow that occurs when a fluid impinges normally on a flat surface. If the velocity of the impinging fluid is uniform across the surface, this flow produces a uniform
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boundary layer in which fluid moves radially outward. Such a layer should be obtained for injection through a porous plug, since the flow visualization studies show that the velocity obtained by this method is nearly uniform. The thickness 6 of the boundary layer produced by1”2, stagnation is where flow ~j and proportional to the ratio (ii/pV) p are the fluid viscosity and density, respectively, and V is the velocity of the impinging gas. Assuming the gas is ideal, p is proportional to the reactor pressure P. Since i~ is independent of F, for a constant mass flow rate (proportional to the volumetric flow rate at 1 atm), 6 is independent of P. Thus injection through a porous plug should be as effective in producing a uniform boundary layer at reduced pressures as at atmospheric pressure. Rotating the disk provides a second mechanism for obtaining a boundary layer of uniform thickness. The isothermal scaling analysis introduced by von Karman [9] for an infinitely large disk implies that the vertical velocity supplying the centrifugal pump is radially uniform. In this case, 6 is proportional to (~/pw)1’~2, where w is the angular rotation rate. Since p is proportional to F, 6 (caP)”2. Consequently, as P is reduced below 1 atm, w must be increased in order to maintain the same 6. Heating the disk can result in a flow dominated by thermal convection. Such a flow is not uniform, and the gas residence time is increased. If the fluid is treated as incompressible, the dependence of buoyancy-driven convection on pressure is characterized by the Grashof number, — —
2 gp 2r~3o~’r, ~‘ p~.i’ / ~
which is the ratio of the buoyancy to viscous forces in a flow driven by a temperature difference i.~ T (gm gravitational acceleration and /3mthermal expansion coefficient). Since $ is independent of pressure, Gr P 2 Scaling analysis for simple flows with both forces and thermal convection suggests that the ratio of these effects varies as Re/Gr1”2 [12], where Re is the Reynolds number (= VLp/i~, with L a characteristic length), which characterizes forced convection. Consequently, the contnbution of thermal convection is expected to decrease with decreasing pressure, as observed in the experiments reported in section 3.3. ,
.
.
5. Conclusion Visualization of gas flows by using a sheet of laser light to illuminate TiO2 particles is an effective technique for use in optimizing reactor designs forpipe OMVPE growth.jet Gas injection coaxial inlet caused flow to the through disk anda large recirculation flows. Tangential injection resuited in fully developed flow that could be made laminar by disk rotation. Gas injection through a porous plug produced the most uniform flow to the disk. In addition, gas residence times were minimized when this injection geometry was combined with low pressure. Thermal convective effects from disk heating were eliminated by reducing the pressure. These observations suggest that low-pressure operation of an OMVPE growth reactor with gas injection through a porous plug is the preferred approach for growing uniform epilayers with abrupt compositional changes. Disk rotation at sufficiently high rates can be utilized to influence the gas flow and thereby improve uniformity.
Acknowledgements The authors would like to thank A.J. Strauss for helpful discussions and critical reading of the manuscript, J.H. Mahoney for photography, J.W. Caunt for assistance in system design, and D.M. Tracy for technical assistance. This work was sponsored by the Defense Advanced Research Projects Agency and the Air Force Office of Scientific Research.
References [1] W. Merzkirch, York 1974). Flow Visualization (Academic Press, New [2] F.C. Eversteyn, P.J.W. Severin, C.H.J. van den Brekel and H.L. Peek, J. Electrochem. Soc. 117 (1970) 925. [3] R. Takahashi, Y. Koga, and K. Sugawara, J. Electrochem. Soc. 119 (1972) 1406. [4] V.S. Ban and S.L. Gilbert, J. Crystal Growth 31(1975) 284 [5] G. Wahl, Thin Solid Films 40 (1977) 13. [6] T. Suzuki, Y. Inoue, T. Aoyama and M. Maid, J. Electrochem. Soc. 132 (1985) 1480.
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/ Flow visualization studies for OMVPE reactor design
[7] J.I. Davies, G. Fan and J.O. Williams, J. Chem. Soc. Faraday Trans. I, 81(1985) 2711. [8] L.J. Gilling, J. Electrochem. Soc. 129 (1982) 634. [9] T. von Karman, Z. Angew. Math. Mech. 1 (1921) 233. [10] P. Freymuth, W. Bank and M. Palmer, in: Proc. 3rd. Intern. Symp. on Flow Visualization, Ann Arbor, MI, 1983, p. 99.
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[11] R.L. Maitby and R.F.A. Keating, AGARDograph No. 70 (1962) 87. [12] H. Schlichting, Boundary Layer Theory (McGraw-Hill, New York, 1972).
Journal of Crystal Growth 77 (1986) 136—143 North-Holland, Amsterdam
FLOW VISUALIZATION STUDIES FOR OPTIMIZATION OF OMVPE REACTOR DESIGN C.A. WANG, S.H. GROVES and S.C. PALMATEER Lincoln Laboratory, Massachusetts Institute of Technology. Lexington, Massachusetts 021 73-0073, USA
D.W. WEYBURNE Rome Air Development Center, Hanscom Air Force Base, Massachusetts 01731, USA
and R.A. BROWN Department of Chemical Engineering; Massachusetts Insitute of Technology, Cambridge, Massachusetts 02139, USA
Gas flow visualization studies have been performed in a vertical reactor tube incorporating a rotating disk susceptor. Smoke particles of Ti0 2 transported in a stream of He gas are illuminated by a sheet of laser light. This technique permits detailed observation of gas flow patterns. To study the influence of the method of gas injection, the gas was injected through (1) a vertical pipe inlet coaxial with the tube, (2) four inlets tangential to the tube and (3) a radial inlet above a porous plug. The method of injection was found to have a critical effect on vortex fonnation, which is minimized by using the porous plug. Disk rotation can be effective in creating a uniform boundary layer at the disk surface. When the susceptor is heated, the flow is strongly perturbed by thermally induced convection, which can be reduced by lowering the operating pressure.
1. Introduction Reproducible growth of laterally uniform epilayers that have abrupt interfaces and can be made thin enough for quantum well and superlattice structures is necessary for many electronic and optoelectronic applications. Deposition of epilayers by the OMVPE technique is usually limited by mass transport of reactants to the substrate, Consequently, epilayer quality is determined largely by gas dynamics in the reactor. In this investigation, gas flow visualization achieved by the scattering of a sheet of laser light from TiO2 particles has been utilized in optimizing the design and operation of OMVPE reactors. By limiting the source of light scattering to a cross-sectional plane, this visualization technique permits detailed observation of gas flow patterns [1]. Previously, flow visualization using incoherent illumination of TiO2 particles has been used to interpret growth results and confirm thermal models in vapor phase epi-
0022-0248/86/$03.50
taxial reactors [2—7].Interference holography has also been employed for mapping gas densities in such reactors [8]. For optimum reactor design, two criteria should be met: (1) for epilayer uniformity, the thickness of the boundary layer at the substrate should be uniform, so that deposition occurs at the same rate for all points on the substrate; (2) for interface abruptness, the flow field in the reactor should be free of any laminar vortices in order to minimize gas residence times. The reactor tube used in this investigation is vertical and has a rotating susceptor disk. Rotation of the disk can be used to establish a self-similar velocity profile along the substrate, which produces a uniform boundary layer [9]. The effects of gas injection and mass flow rate, disk rotation and heating, and reactor pressure on flow patterns have been studied. The method of gas injection is found to b~crucial to the control of vortex formation.
© Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
C.A. Wang et al.
/ Flow visualization studies for OMVPE reactor design
137
2. Experimental procedure
Gas flow patterns are made visible by illuminating Ti02 particles averaging about 1 ~smin
The experimental apparatus is shown schematically in fig. 1. The vertical quartz tube, 10 cm ID, incorporates a 6.7 cm diameter RF-heated rotating graphite susceptor disk. The distance between the disk and the gas inlet is adjustable from 5 to 25 cm; a spacing of 15 cm was used in the experiments reported here. An optical pyrometer is used to measure the disk temperature. The system is equipped with a vacuum pump and control valve for low pressure operation. Gas injection was studied by the use of: (a) a vertical 6-mm pipe inlet coaxial with the tube, (b) four equally spaced inlets tangential to the tube, and (c) a radial inlet above a porous plug, either 3.8 or 7.6 cm in diameter, formed by metal screens. The first was selected for study because of its common use and simplicity, the second and third to give good mixing and permit loading a substrate through a load lock at the top of a working reactor tube.
diameter [10] generated by the reaction of TiCl4 and H 20. Separate streams of He carrier gas, whose properties are similar to those of H2, are passed through two bubblers, one containing H 20 and the other a mixture of TiCl4 and Cd4. Addition of CC14 minimizes formation of solid deposits and clogging of gas lines [11]. In order to keep their concentration small enough to avoid influencing the flow patterns, the TiO2 particles are generated at low flow rates (-‘- 1 LPM), and two additional gas lines are provided to obtain the desired total flow of He. All flows are monitored by rotameters. Another feature of the system is a fast-switching pressure-balanced manifold that has been used for quantitative studies of gas residence times, employing SF6 as a tracer gas, which will be reported elsewhere. Vertical and horizontal cross sections through the tube are illuminated with a sheet of light formed by directing the beam of a 5 mW He—Ne laser through a cylindrical lens. Images are recorded on video tape and sheet film. Since the intensity of scattered light is proportional to the local concentration of Ti02 particles, gas mixing in the tube can be observed by tracing the paths of these particles following their introduction. Gas residence times are estimated by measuring the
tangential coaxial
porous plug _______
I
,
switchg
.
.
injectors
.
.
~ cylindrical lens H20
pump
V V V V
4—He Fig. 1. Schematic diagram of apparatus for gas flow visualiza-
tion.
.
. . . injection on the flow field was studied for isother-
mal gas flow, with and without disk rotation. Changes in flow patterns resulting when the disk was heated to 600°Cwere studied only for the
,
:coil TiCl4 Cd4
time required for particles to clear the tube after smoke generation is stopped by bypassing the TiCl4 bubbler. In both instances, the total mass flow rate through the tube is maintained constant. Three-dimensional flows are observed by projecting white light down the axis of the tube. In this study the effect of the method of gas
porous plug inlet, using disk rotation rates between 0 and 1800 RPM. Gas flow rates were typical of those used in growth and varied from 2 LPM for coaxial injection to 10 LPM for tangential and porous plug injection. The reactor pressure was controlled at values between 0.1 and 1
atm.
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3. Results 3.1. Gas injection method 3.1.1. Vertical pipe injection The most common method of gas injection in OMVPE reactors is introduction through a pipe inlet that is coaxial with the reactor tube. Fig. 2 shows cross-sectional photographs of gas flow patterns at room temperature obtained when this method was used with a total flow of 2 LPM and a tube pressure of 1 atm. (The disk was not rotated during any of the experiments described in section 3.1). One second after the initial introduction of smoke (fig. 2a), a jet is observed that extends from the inlet to the disk, flows outward radially across the disk and strikes the side of the tube. By 20 s (fig. 2b), vortices extending from the top of the disk are observed around the jet. At 20 s the concentration of smoke is higher in the jet than in the vortices, but the concentration in the vortices increases with time, as some of the smoke being added becomes trapped there. After the smoke is turned off, it clears rapidly from the jet
and across the disk (fig. 2c), but persists in the recirculating cell for several minutes. The presence of the inertial jet shows that the gas flow from the pipe inlet is not fully developed when it reaches the disk. Intense gas recirculation occurs throughout the reactor as a result of the impinging jet. An increase in the gas flow rate increases both the velocity of the jet and the gas recirculation velocities. Gas trapped in vortices is isolated from the main flow, and its composition can adjust only by diffusion across the separating streamline. This diffusive control leads to long gas residence times for dopant and alloying components. A high degree of radial non-uniformity and marked grading between epilayers is therefore expected from coaxial injection. 3.1.2. Tangential injection Injection through tangential inlets introduces swirl to the gas, increasing its path length and dissipating the momentum of the inlet stream, thereby producing a fully developed flow at the disk surface. Steady-state patterns recorded at room temperature and 1 atm for a flow rate of 10
Fig. 2. Vertical cross sections showing flow patterns obtained for gas injection through a coaxial vertical pipe: (a) I s after initial introduction of smoke; (b) 20 s after initial introduction of smoke; (c) 1 mm after smoke was turned off.
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I. Fig. 3. Steady-state flow patterns for gas injection through tangential inlets: (a) ~hite light photograph: (b) horizontal cross section; (c) vertical cross section.
LPM are shown in fig. 3. A white light photograph (fig. 3a) shows a complex. spiraling motion of gas around the circumference of the tube with
penetration to the centerline. The spiral pattern is obvious in a horizontal cross section (fig. 3b). By observing the pattern for a vertical cross section
II Fig. 4. Vertical cross sections showing steady-state flow patterns obtained for gas injection through a porous plug: (a) 3.8 cm diameter: (b) 7.6 cm diameter.
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(fig. 3c) as a function of time, it was found that smoke particles follow a path around the lobes near the outer edges of the tube, repeatedly traversing down, around, then up, while swirling around the tube, until the steady-state pattern develops approximately 5 s after initial introduction of smoke. The oscillatory particle path arises from centrifugal instabilities that develop from angular momentum introduced by the tangential inlets. The flow has the form of Goertler vortices [12] superimposed on the downward motion of the gas. Although the flow is fully developed when it reaches the disk, variations in smoke concentration in the vicinity of the disk indicate that the gas is not well mixed. The gas swirl results in the formation of a vortex at the top of the tube. After the smoke is turned off, clearing occurs first from the edges of the lobes. In 10—15 s it becomes complete except within the central vortex where smoke particles continue to recirculate for several minutes.
3.1.3. Porous plug injection The use of a radial inlet followed by a porous plug results in uniform injection over a large cross section of the tube. The steady-state flow pattern obtained at room temperature and 1 atm for a flow rate of 10 LPM through a 3.8 cm diameter plug is shown in fig. 4a. Gas enters over a diameter equal to that of the plug with a velocity nearly independent of radial distance, characteristics typical of plug flow. The uniform intensity of smoke particles indicates that the gas is uniformly mixed (the vertical streaks in the photograph are reflections from the walls of the tube). However, flow separation causes formation of vortices. Increasing the diameter of the plug to 7.6 cm (fig. 4b) results in a uniform flow that sweeps out all but an extremely small volume at the top of the tube. The time elapsed before laminar flow reaches the disk is approximately 4—5 s, while 5—6 s are required for the tube to be cleared after the smoke is turned off. These times are comparable to those
Fig. 5. Vertical cross sections showing steady-state flow patterns for gas injection through tangential inlets with disk rotation: (a) 40 RPM; (b) 300 RPM; (c) 1000 RPM.
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calculated by dividing the distance between the inlet and the disk by the average volocity of the gas. Reducing the reactor pressure to 0.1 atm without changing the mass flow rate increases the gas velocity by a factor of 10 and reduces the initial and final transient times by the same factor. 3.2. Disk rotation Disk rotation forces gas radially outward and vertically downward all across the disk, acting like a pump. This effect is illustrated in fig. 5, which shows steady-state flow patterns obtained by using tangential injection at 10 LPM and 1 atm. Comparison of the patterns for rotation at 40 RPM (fig. 5a) and no rotation (fig. 3c) shows that streamlines in the center of the tube are pulled toward the disk. At higher rates, the flow above the disk appears laminar and the gas at the disk surface is uniformly mixed. Increasing the rotation rate to 300 RPM (fig. Sb) results in the formation of an eddy near the disk at the outer edge of the tube. As the rotation rate is increased further, this side eddy enlarges, as seen in fig. Sc for 1000 RPM. Rotation affects all the gas above the disk. The central vortex at the top of the tube is pulled downward, and can be pulled all the way to the disk at high rotation rates (fig. Sc). When the smoke is turned off, it lingers in the central vortex and in the side eddy for several minutes.
.
Fig. 6. Vertical cross section showing flow pattern for gas injection through 7.6 cm diameter porous plug with disk temperature of 600°C.
hydrodynamic boundary layer cannot be identified with the thickness of this smoke-free region. Due to thermal convective flows, the residence times at 1 atm were on the order of 15 s. At 0.1 atm, the residence times were reduced to a fraction of a second.
3.3. Disk heating The effect of heating the susceptor disk was studied for gas injection at 10 LPM through the 7.6 cm diameter porous plug. A flow pattern observed at 1 atm for a disk temperature of 600°Cis shown in fig. 6. The flow, which is dominated by thermal convection, is very complicated. Rotation of the disk at rates up to 1800 RPM failed to eliminate thermal convection effects. However, reducing the pressure below 0.3 atm (without rotation) did eliminate these effects, and laminar flows like those observed without heating (fig. 4b) were obtained. Smoke was absent from a region 5—10 mm thick above the disk because of the thermophoretic migration of TiO2 particles away from the heated surface. The thickness of the
4. Discussion Optimizing the experimental conditions in an OMVPE reactor to achieve epilayer uniformity and permit abrupt changes in composition requires control of the gas flow adjacent to the substrate as well as throughout the reactor tube. For epilayer uniformity, the boundary layer at the substrate must be of uniform thickness. For a vertical-flow, rotating-disk reactor, two mechanisms are available for forming such a uniform layer. One utilizes the stagnation flow that occurs when a fluid impinges normally on a flat surface. If the velocity of the impinging fluid is uniform across the surface, this flow produces a uniform
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boundary layer in which fluid moves radially outward. Such a layer should be obtained for injection through a porous plug, since the flow visualization studies show that the velocity obtained by this method is nearly uniform. The thickness 6 of the boundary layer produced by1”2, stagnation is where flow ~j and proportional to the ratio (ii/pV) p are the fluid viscosity and density, respectively, and V is the velocity of the impinging gas. Assuming the gas is ideal, p is proportional to the reactor pressure P. Since i~ is independent of F, for a constant mass flow rate (proportional to the volumetric flow rate at 1 atm), 6 is independent of P. Thus injection through a porous plug should be as effective in producing a uniform boundary layer at reduced pressures as at atmospheric pressure. Rotating the disk provides a second mechanism for obtaining a boundary layer of uniform thickness. The isothermal scaling analysis introduced by von Karman [9] for an infinitely large disk implies that the vertical velocity supplying the centrifugal pump is radially uniform. In this case, 6 is proportional to (~/pw)1’~2, where w is the angular rotation rate. Since p is proportional to F, 6 (caP)”2. Consequently, as P is reduced below 1 atm, w must be increased in order to maintain the same 6. Heating the disk can result in a flow dominated by thermal convection. Such a flow is not uniform, and the gas residence time is increased. If the fluid is treated as incompressible, the dependence of buoyancy-driven convection on pressure is characterized by the Grashof number, — —
2 gp 2r~3o~’r, ~‘ p~.i’ / ~
which is the ratio of the buoyancy to viscous forces in a flow driven by a temperature difference i.~ T (gm gravitational acceleration and /3mthermal expansion coefficient). Since $ is independent of pressure, Gr P 2 Scaling analysis for simple flows with both forces and thermal convection suggests that the ratio of these effects varies as Re/Gr1”2 [12], where Re is the Reynolds number (= VLp/i~, with L a characteristic length), which characterizes forced convection. Consequently, the contnbution of thermal convection is expected to decrease with decreasing pressure, as observed in the experiments reported in section 3.3. ,
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5. Conclusion Visualization of gas flows by using a sheet of laser light to illuminate TiO2 particles is an effective technique for use in optimizing reactor designs forpipe OMVPE growth.jet Gas injection coaxial inlet caused flow to the through disk anda large recirculation flows. Tangential injection resuited in fully developed flow that could be made laminar by disk rotation. Gas injection through a porous plug produced the most uniform flow to the disk. In addition, gas residence times were minimized when this injection geometry was combined with low pressure. Thermal convective effects from disk heating were eliminated by reducing the pressure. These observations suggest that low-pressure operation of an OMVPE growth reactor with gas injection through a porous plug is the preferred approach for growing uniform epilayers with abrupt compositional changes. Disk rotation at sufficiently high rates can be utilized to influence the gas flow and thereby improve uniformity.
Acknowledgements The authors would like to thank A.J. Strauss for helpful discussions and critical reading of the manuscript, J.H. Mahoney for photography, J.W. Caunt for assistance in system design, and D.M. Tracy for technical assistance. This work was sponsored by the Defense Advanced Research Projects Agency and the Air Force Office of Scientific Research.
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[7] J.I. Davies, G. Fan and J.O. Williams, J. Chem. Soc. Faraday Trans. I, 81(1985) 2711. [8] L.J. Gilling, J. Electrochem. Soc. 129 (1982) 634. [9] T. von Karman, Z. Angew. Math. Mech. 1 (1921) 233. [10] P. Freymuth, W. Bank and M. Palmer, in: Proc. 3rd. Intern. Symp. on Flow Visualization, Ann Arbor, MI, 1983, p. 99.
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