- Email: [email protected]
Influence of the reactor inlet configuration on the AlGaN growth efficiency
ARTICLE IN PRESS
Journal of Crystal Growth 298 (2007) 413–417 www.elsevier.com/locate/jcrysgro
Influence of the reactor inlet configuration on the AlGaN growth efficiency E.V. Yakovleva,, R.A. Talalaevb, N. Kaluzac, H. Hardtdegenc, H.L. Bayc a
Soft-Impact Ltd., P.O. Box 83, 194156, St.Petersburg, Russia Semiconductor Technology Research GmbH, 91002 Erlangen, Germany c Institute of Thin Films and Interfaces (ISG-1), Center of Nanoelectronic Systems for Information Technology, Research Center Juelich, 52425 Juelich, Germany b
Available online 22 November 2006
Abstract This paper discusses the results of a combined modeling and experimental analysis of AlGaN deposition in the horizontal two-flow AIX 200/4 RF-S reactor. The purpose of this study is to examine conventional and inverted supply of the precursors into the reactor with respect to the growth reproducibility and efficiency of the aluminum (Al) incorporation. It has been found that the use of the inverted inlet improves the reproducibility of the growth process and provides a good control of AlGaN deposition. At the same time, the Al content appears to be somewhat lower for the inverted inlet configuration. A good agreement between the experimental data and model predictions allows us to use the modeling results for interpretation of the experimental findings. r 2006 Elsevier B.V. All rights reserved. PACS: 81.05.Ea; 81.15.Gh; 82.20.Wt Keywords: A1. AlN nanoparticles; A1. Computer simulation; A3. Metalorganic vapor phase epitaxy; B1. Nitrides
1. Introduction Despite significant progress in AlGaN deposition by metalorganic vapor phase epitaxy (MOVPE) that has been achieved in recent years, the effective growth of AlGaN layers with high Al content still remains a crucial issue. Difficulties with aluminum (Al) incorporation are related to parasitic gas-phase reactions and formation of nano-size particles in the volume of a deposition reactor. The presence of particles has been detected experimentally, using laser light scattering, in a vertical inverted flow reactor [1]. The reduction of particle formation intensity and enhancement of the Al incorporation can be achieved via lowering of the reactor pressure [2–3], decrease of the gas residence time (higher inflow velocity) [1,3], reduction of the TMAl flow rate [3], and using lower V/III ratios [4–5]. Another factor that may affect the AlGaN growth efficiency is the reactor design. The intensity of parasitic chemical processes and the range of the optimal operating Corresponding author. Tel.: +7 812 703 15 22; fax: +7 812 326 61 94.
E-mail address: [email protected] (E.V. Yakovlev). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.10.047
conditions are dependent on the type of the reactor and schematics of the precursor supply. Within this particular study, AlGaN layers have been grown in a horizontal twoflow reactor at relatively low pressure of 50 mbar for a wide range of the layer composition and two different configurations of the reactor hardware. Recently, it has been demonstrated [6] that inversion of the precursor supply (metalorganics are supplied through the lower channel, ammonia is supplied through the upper one) in the horizontal AIX 200/4 RF-S reactor reduces effectively parasitic deposition during GaN growth. In addition, the inverted inlet configuration leads to a higher reproducibility of the growth process and longer uptimes of the reactor without maintenance. In this work, the ability to grow AlGaN with high Al contents has been studied for both conventional and inverted supply of the precursors. 2. Experimental procedure Experiments have been performed in a standard AIX 200/4 RF-S horizontal reactor (AIXTRON) equipped with
ARTICLE IN PRESS E.V. Yakovlev et al. / Journal of Crystal Growth 298 (2007) 413–417
a straight separation plate in the gas inlet, that allows a separate injection of the group III sources and the ammonia. A general view of the central cross-section of the reactor liner and the scheme of the precursor supply are shown in Fig. 1. The reactor is additionally equipped with valves, so that both conventional (MO species are supplied through the upper channel and ammonia comes through the lower one) and inverted (vice versa) inlet configurations can be used. AlGaN layers with the thickness above 1 mm were grown on 1–2 mm thick GaN buffer layers at a pressure of 50 mbar and a nominal temperature of 1100 1C. The total flow (7.5 slm) and ammonia flow rate (5.8 10 2 mole/min) were kept constant and the same for both inlet configurations, whereas the TMAl and TMGa flow rates were varied to perform the growth at different Al contents in the gas phase. The AlGaN growth rate was determined from growth transients, using in situ reflectometry, and the layer composition was measured ex situ using X-ray diffraction (XRD) and Rutherford back scattering (RBS) techniques.
inlet improves the reproducibility of the growth process and provides a good control of the AlGaN deposition. Also the AlGaN growth efficiency appears to be fairly high for both inlet configurations and AlGaN layers with the Al
a 90 Al content in the solid phase, %
414
experiment, RBS experiment, XRD calculations ideal calc. (const. flow)
80 70 60 50 40 30 20 10
conventional inlet
0
3. Modeling approach
0
10
20
30
40
50
60
70
80
90
Al content in the gas phase, %
4. Results and discussion The AlGaN growth experiments have demonstrated that, similarly to GaN deposition, the use of the inverted
b 90 Al content in the solid phase, %
The modeling approach consists in a 2D/3D consideration of flow, heat transfer and multi-component mass transport in the reactor, gas-phase and surface chemistry and nucleation, growth and transport of particles produced in the gas phase and serving as a source of material losses. The computations have been performed using the CVDSim software (http://www.semitech.us). We consider that the gas-phase reaction mechanism represents a multi-step pathway [7]. Reaction between TMAl and ammonia gives rise to TMAl:NH3 adduct that produces DMAl:NH2 via methane elimination reaction and interactions with ammonia. Subsequently formed (DMAl:NH2)2 and (DMAl:NH2)3 species may produce AlN in the gas phase, initiating AlN particle nucleation. Further growth of the solid particles takes place at the expense of interactions between the AlN nuclei and Alcontaining species such as AlN, DMAlNH2, and [DMAlNH2]2. Another way of Al losses is the formation of oligomers ((DMAl:NH2)n, nX3) that do not contribute to the layer deposition.
experiment, RBS experiment, XRD calculations ideal calc. (const. flow)
80 70 60 50 40 30 20 10
inverted inlet
0 0
10
20
30
40
50
60
70
80
90
Al content in the gas phase, % Fig. 2. AlGaN solid-vapor relationship for the conventional (a) and inverted (b) inlet configurations. Symbols indicate experimental results, solid lines are for the modeling predictions. Dash-dotted lines correspond to the results of computations performed for the constant total flow of 7.5 slm when the Al gas-phase composition is raised from 55% to 79.5%. Dashed line shows for reference the ideal (no aluminum losses) solid-vapor relationship.
Fig. 1. Schematic view of the AIX 200/4 RF-S reactor.
ARTICLE IN PRESS E.V. Yakovlev et al. / Journal of Crystal Growth 298 (2007) 413–417
content up to about 60%. At the same time, the AlGaN growth rate appears to be somewhat lower for the inverted inlet configuration. Note that while the GaN growth rate varies linearly with the TMGa flow rate, the AlN growth rate increases sub-linearly and even saturates at high TMAl flow rates, indicating some losses of Al. This behavior is characteristic for both inlet configurations. The AlGaN solid-vapor relationship is shown in Fig. 2 for both conventional and inverted inlets as a comparison of the experimental data and modeling predictions. Here, enhancement of the Al gas-phase composition (the ratio QTMAl/(QTMAl+QTMGa)) from 9% to 55% is made via a gradual increase of the TMAl flow rate and simultaneous decrease of the TMGa flow. The last step (when the Al gasphase content grows from 55 to 79.5%) corresponds to an increase of the TMAl flow rate only, keeping the TMGa flow rate constant. Fig. 2 shows that the dependence of the
415
Al content in the solid phase on the Al content in the gas phase is reproduced well by the modeling. Both inlet configurations allow the growth of AlGaN with Al content up to about 70% and exhibit a non-linear dependence of the Al incorporation into the solid on the Al gas-phase composition: at higher TMAl flow rates a deviation from linearity of the relationship between the Al content in the solid and Al content in the gas phase is observed. The higher is the TMAl flow the larger is this deviation. Nevertheless, the Al incorporation efficiency is fairly high up to the Al content of about 60%. However, further increase of Al incorporation is hindered due to enhancement of the Al losses. So, to get the Al composition of about 70%, the total flow in the reactor was increased up to 10 slm. It is interesting to consider which Al content could be achieved if the TMAl flow was raised without increasing
a conventional susceptor
inverted susceptor
b
susceptor
susceptor
c
susceptor
susceptor Fig. 3. Distributions of the DMAl:NH2 molar fraction (a), AlN molar fraction (b) and particle density (c) for conventional and inverted inlet configurations.
ARTICLE IN PRESS E.V. Yakovlev et al. / Journal of Crystal Growth 298 (2007) 413–417
416
total flow in the reactor. For this purpose, a series of computations has been done, keeping both TMGa and total flow rate constant and varying the TMAl flow rate only. The results of such computations are presented in Fig. 2 as the dash-dotted lines. One can see that, at the constant total flow of 7.5 slm, increase of the Al gas-phase composition from 55 to 79.5% does not provide a significant enhancement of the Al incorporation efficiency. The Al content in the growing layer saturates at some TMAl flow rate and even decreases further to some extent. This behavior is related to intensification of particle formation and strong depletion of the gaseous mixture with Al in flow direction. The effect is somewhat stronger for the inverted supply of the precursors: the Al percentage corresponding to the highest Al gas-phase composition of 79.5% is even lower than that at the Al content in the gas phase of 55%. Regarding differences between the two inlet configurations, we should emphasize that the inverted supply
xAl(solid)=79.5%
Al content in the solid phase, %
80 conventional inverted 70
60
50
40 xAl(gas)=79.5% 30 8
9
10
11
Total flow, slm Fig. 4. AlGaN layer composition as a function of the total flow. Al content in the gas phase is 79.5%.
provides generally lower Al contents, which is especially pronounced at high TMAl flow rates. This effect can be understood form the analysis of the modeling results. We consider that in comparison to the conventional inlet configuration reduced Al incorporation for the inverted inlet is due to specificities of gas-phase chemistry and particle formation. To illustrate these differences, Fig. 3 represents the distributions of the DMAl:NH2 and AlN concentrations and the particle density patterns. These components have been chosen for illustration from the analysis of the species fluxes onto the deposition surface, which has revealed that the main Al-containing species contributing to the layer growth are DMAl:NH2 and AlN vapor. The contributions of these two species are almost equal for the inverted inlet, whereas the AlN flux prevails for the conventional one. These conclusions are supported by the distributions presented in Fig. 3. When the conventional supply is used, ammonia diffuses rather intensively into the upper MO channel, initiating the formation of the adduct and subsequent gas-phase reactions. As a result, DMAl:NH2 is formed away from the deposition surface with the maximal concentration near the separation plate edge (Fig. 3(a)). In case of the inverted supply, the majority of the gas-phase reactions proceed somewhat more downstream and closer to the susceptor. In particular, the concentration of DMAl:NH2 is higher and its the relative contribution to the layer deposition becomes greater. The distributions of the AlN vapor molar fraction (Fig. 3(b)) also differ significantly between the considered inlet configurations. For the conventional supply of the precursors, the AlN concentration has a pronounced maximum between the reactor ceiling and susceptor. Inversion of the precursor supply changes the AlN concentration pattern so that it has some elongated form. Eventually, the particles form a layer immediately above the susceptor in case of the inverted precursor supply, while the particle density is maximal in the central part of the reactor and closer to its upper wall for the conventional supply. In addition, the particle density is higher above the susceptor in the former case (see Fig. 3(c)). Taking into
a
susceptor
b
Fig. 5. Particle density distributions for the inverted inlet and total flow rate of 7.5 slm (a) and 11 slm (b).
ARTICLE IN PRESS E.V. Yakovlev et al. / Journal of Crystal Growth 298 (2007) 413–417
account that both DMAl:NH2 and AlN not only contribute to the layer deposition but can also be consumed by particle growth, we can conclude that the losses of Al are greater in case of the inverted supply, when both species are of the same importance with respect to the Al incorporation into the growing layer. A promising way to get even higher Al contents up to about 70% is to increase the total flow in the reactor in order to suppress partly the formation of particles by lowering of the Al-containing species partial pressures and the gas residence time. To study this effect in more detail, we have performed computations for the highest Al gasphase composition of 79.5% and varied total flow rate. The computed dependencies, shown in Fig. 4, demonstrate a strong effect of the total flow on the AlGaN layer composition. At the total flow of 11 slm, the Al content in the solid is practically the same as the Al gas-phase composition for the conventional inlet, but still remains somewhat lower for the inverted one. For both inlet configurations, the Al incorporation efficiency increases significantly as the total flow is raised due to gradual suppression of particle formation, as one can see from Fig. 5 presenting the particle density distributions for the inverted supply and two different total flow rates. The reduction of particle formation is due to the decrease of the Al-containing species partial pressures (via dilution) and lowering of the gas residence time (via higher inlet velocities). 5. Summary A combined modeling and experimental study of AlGaN deposition in the AIX 200/4 RF-S reactor has been made with the aim to reveal the effect of the reactor
417
inlet configuration on the Al incorporation efficiency. It has been found that the use of the inverted inlet configuration improves the reproducibility of the growth process, allowing a good control of AlGaN deposition. For both configurations of the reactor inlet, an Al content of about 70% has been achieved, but the inverted precursor supply results in generally lower Al incorporation. This effect has been understood from the modeling results: inverted inlet configuration results in more intensive formation of particles in the gas phase that consume the main Al-containing species contributing to the layer deposition. Additionally it was demonstrated that a reduction of particle formation and thereby an increase of the Al content can be achieved for both configurations by increasing the total flow in the reactor.
References [1] J.R. Creighton, G.T. Wang, W.G. Breiland, M.E. Coltrin, J. Crystal Growth. 261 (2004) 204. [2] A.A. Allerman, M.H. Crawford, A.J. Fischer, K.H.A. Bogart, S.R. Lee, D.M. Follstaedt, P.P. Provencio, D.D. Koleske, J. Crystal Growth 272 (2004) 227. [3] A.V. Kondratyev, R.A. Talalaev, W.V. Lundin, A.V. Sakharov, A.F. Tsatsul’nikov, E.E. Zavarin, A.V. Fomin, D.S. Sizov, J. Crystal Growth 272 (2004) 420. [4] S.C. Choi, J.-H. Kim, J.Y. Choi, K.J. Lee, K.Y. Lim, G.M. Yang, J. of Appl.Phys. 87 (2002) 172. [5] A.V. Lobanova, K.M. Mazaev, R.A. Talalaev, M. Leys, S. Boeykens, K. Cheng, S. Degroote, J. Crystal Growth 287 (2006) 601. [6] H. Hardtdegen, N. Kaluza, R. Steins, R. Schmidt, K. Wirtz, E.V. Yakovlev, R.A. Talalaev, Yu.N. Makarov, J. Crystal Growth 272 (2004) 407. [7] E.V. Yakovlev, R.A. Talalaev, A.N. Vorob’ev, Yu.N. Makarov, Electrochem. Soc. Proc. 2003-08 (2003) 258.
Journal of Crystal Growth 298 (2007) 413–417 www.elsevier.com/locate/jcrysgro
Influence of the reactor inlet configuration on the AlGaN growth efficiency E.V. Yakovleva,, R.A. Talalaevb, N. Kaluzac, H. Hardtdegenc, H.L. Bayc a
Soft-Impact Ltd., P.O. Box 83, 194156, St.Petersburg, Russia Semiconductor Technology Research GmbH, 91002 Erlangen, Germany c Institute of Thin Films and Interfaces (ISG-1), Center of Nanoelectronic Systems for Information Technology, Research Center Juelich, 52425 Juelich, Germany b
Available online 22 November 2006
Abstract This paper discusses the results of a combined modeling and experimental analysis of AlGaN deposition in the horizontal two-flow AIX 200/4 RF-S reactor. The purpose of this study is to examine conventional and inverted supply of the precursors into the reactor with respect to the growth reproducibility and efficiency of the aluminum (Al) incorporation. It has been found that the use of the inverted inlet improves the reproducibility of the growth process and provides a good control of AlGaN deposition. At the same time, the Al content appears to be somewhat lower for the inverted inlet configuration. A good agreement between the experimental data and model predictions allows us to use the modeling results for interpretation of the experimental findings. r 2006 Elsevier B.V. All rights reserved. PACS: 81.05.Ea; 81.15.Gh; 82.20.Wt Keywords: A1. AlN nanoparticles; A1. Computer simulation; A3. Metalorganic vapor phase epitaxy; B1. Nitrides
1. Introduction Despite significant progress in AlGaN deposition by metalorganic vapor phase epitaxy (MOVPE) that has been achieved in recent years, the effective growth of AlGaN layers with high Al content still remains a crucial issue. Difficulties with aluminum (Al) incorporation are related to parasitic gas-phase reactions and formation of nano-size particles in the volume of a deposition reactor. The presence of particles has been detected experimentally, using laser light scattering, in a vertical inverted flow reactor [1]. The reduction of particle formation intensity and enhancement of the Al incorporation can be achieved via lowering of the reactor pressure [2–3], decrease of the gas residence time (higher inflow velocity) [1,3], reduction of the TMAl flow rate [3], and using lower V/III ratios [4–5]. Another factor that may affect the AlGaN growth efficiency is the reactor design. The intensity of parasitic chemical processes and the range of the optimal operating Corresponding author. Tel.: +7 812 703 15 22; fax: +7 812 326 61 94.
E-mail address: [email protected] (E.V. Yakovlev). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.10.047
conditions are dependent on the type of the reactor and schematics of the precursor supply. Within this particular study, AlGaN layers have been grown in a horizontal twoflow reactor at relatively low pressure of 50 mbar for a wide range of the layer composition and two different configurations of the reactor hardware. Recently, it has been demonstrated [6] that inversion of the precursor supply (metalorganics are supplied through the lower channel, ammonia is supplied through the upper one) in the horizontal AIX 200/4 RF-S reactor reduces effectively parasitic deposition during GaN growth. In addition, the inverted inlet configuration leads to a higher reproducibility of the growth process and longer uptimes of the reactor without maintenance. In this work, the ability to grow AlGaN with high Al contents has been studied for both conventional and inverted supply of the precursors. 2. Experimental procedure Experiments have been performed in a standard AIX 200/4 RF-S horizontal reactor (AIXTRON) equipped with
ARTICLE IN PRESS E.V. Yakovlev et al. / Journal of Crystal Growth 298 (2007) 413–417
a straight separation plate in the gas inlet, that allows a separate injection of the group III sources and the ammonia. A general view of the central cross-section of the reactor liner and the scheme of the precursor supply are shown in Fig. 1. The reactor is additionally equipped with valves, so that both conventional (MO species are supplied through the upper channel and ammonia comes through the lower one) and inverted (vice versa) inlet configurations can be used. AlGaN layers with the thickness above 1 mm were grown on 1–2 mm thick GaN buffer layers at a pressure of 50 mbar and a nominal temperature of 1100 1C. The total flow (7.5 slm) and ammonia flow rate (5.8 10 2 mole/min) were kept constant and the same for both inlet configurations, whereas the TMAl and TMGa flow rates were varied to perform the growth at different Al contents in the gas phase. The AlGaN growth rate was determined from growth transients, using in situ reflectometry, and the layer composition was measured ex situ using X-ray diffraction (XRD) and Rutherford back scattering (RBS) techniques.
inlet improves the reproducibility of the growth process and provides a good control of the AlGaN deposition. Also the AlGaN growth efficiency appears to be fairly high for both inlet configurations and AlGaN layers with the Al
a 90 Al content in the solid phase, %
414
experiment, RBS experiment, XRD calculations ideal calc. (const. flow)
80 70 60 50 40 30 20 10
conventional inlet
0
3. Modeling approach
0
10
20
30
40
50
60
70
80
90
Al content in the gas phase, %
4. Results and discussion The AlGaN growth experiments have demonstrated that, similarly to GaN deposition, the use of the inverted
b 90 Al content in the solid phase, %
The modeling approach consists in a 2D/3D consideration of flow, heat transfer and multi-component mass transport in the reactor, gas-phase and surface chemistry and nucleation, growth and transport of particles produced in the gas phase and serving as a source of material losses. The computations have been performed using the CVDSim software (http://www.semitech.us). We consider that the gas-phase reaction mechanism represents a multi-step pathway [7]. Reaction between TMAl and ammonia gives rise to TMAl:NH3 adduct that produces DMAl:NH2 via methane elimination reaction and interactions with ammonia. Subsequently formed (DMAl:NH2)2 and (DMAl:NH2)3 species may produce AlN in the gas phase, initiating AlN particle nucleation. Further growth of the solid particles takes place at the expense of interactions between the AlN nuclei and Alcontaining species such as AlN, DMAlNH2, and [DMAlNH2]2. Another way of Al losses is the formation of oligomers ((DMAl:NH2)n, nX3) that do not contribute to the layer deposition.
experiment, RBS experiment, XRD calculations ideal calc. (const. flow)
80 70 60 50 40 30 20 10
inverted inlet
0 0
10
20
30
40
50
60
70
80
90
Al content in the gas phase, % Fig. 2. AlGaN solid-vapor relationship for the conventional (a) and inverted (b) inlet configurations. Symbols indicate experimental results, solid lines are for the modeling predictions. Dash-dotted lines correspond to the results of computations performed for the constant total flow of 7.5 slm when the Al gas-phase composition is raised from 55% to 79.5%. Dashed line shows for reference the ideal (no aluminum losses) solid-vapor relationship.
Fig. 1. Schematic view of the AIX 200/4 RF-S reactor.
ARTICLE IN PRESS E.V. Yakovlev et al. / Journal of Crystal Growth 298 (2007) 413–417
content up to about 60%. At the same time, the AlGaN growth rate appears to be somewhat lower for the inverted inlet configuration. Note that while the GaN growth rate varies linearly with the TMGa flow rate, the AlN growth rate increases sub-linearly and even saturates at high TMAl flow rates, indicating some losses of Al. This behavior is characteristic for both inlet configurations. The AlGaN solid-vapor relationship is shown in Fig. 2 for both conventional and inverted inlets as a comparison of the experimental data and modeling predictions. Here, enhancement of the Al gas-phase composition (the ratio QTMAl/(QTMAl+QTMGa)) from 9% to 55% is made via a gradual increase of the TMAl flow rate and simultaneous decrease of the TMGa flow. The last step (when the Al gasphase content grows from 55 to 79.5%) corresponds to an increase of the TMAl flow rate only, keeping the TMGa flow rate constant. Fig. 2 shows that the dependence of the
415
Al content in the solid phase on the Al content in the gas phase is reproduced well by the modeling. Both inlet configurations allow the growth of AlGaN with Al content up to about 70% and exhibit a non-linear dependence of the Al incorporation into the solid on the Al gas-phase composition: at higher TMAl flow rates a deviation from linearity of the relationship between the Al content in the solid and Al content in the gas phase is observed. The higher is the TMAl flow the larger is this deviation. Nevertheless, the Al incorporation efficiency is fairly high up to the Al content of about 60%. However, further increase of Al incorporation is hindered due to enhancement of the Al losses. So, to get the Al composition of about 70%, the total flow in the reactor was increased up to 10 slm. It is interesting to consider which Al content could be achieved if the TMAl flow was raised without increasing
a conventional susceptor
inverted susceptor
b
susceptor
susceptor
c
susceptor
susceptor Fig. 3. Distributions of the DMAl:NH2 molar fraction (a), AlN molar fraction (b) and particle density (c) for conventional and inverted inlet configurations.
ARTICLE IN PRESS E.V. Yakovlev et al. / Journal of Crystal Growth 298 (2007) 413–417
416
total flow in the reactor. For this purpose, a series of computations has been done, keeping both TMGa and total flow rate constant and varying the TMAl flow rate only. The results of such computations are presented in Fig. 2 as the dash-dotted lines. One can see that, at the constant total flow of 7.5 slm, increase of the Al gas-phase composition from 55 to 79.5% does not provide a significant enhancement of the Al incorporation efficiency. The Al content in the growing layer saturates at some TMAl flow rate and even decreases further to some extent. This behavior is related to intensification of particle formation and strong depletion of the gaseous mixture with Al in flow direction. The effect is somewhat stronger for the inverted supply of the precursors: the Al percentage corresponding to the highest Al gas-phase composition of 79.5% is even lower than that at the Al content in the gas phase of 55%. Regarding differences between the two inlet configurations, we should emphasize that the inverted supply
xAl(solid)=79.5%
Al content in the solid phase, %
80 conventional inverted 70
60
50
40 xAl(gas)=79.5% 30 8
9
10
11
Total flow, slm Fig. 4. AlGaN layer composition as a function of the total flow. Al content in the gas phase is 79.5%.
provides generally lower Al contents, which is especially pronounced at high TMAl flow rates. This effect can be understood form the analysis of the modeling results. We consider that in comparison to the conventional inlet configuration reduced Al incorporation for the inverted inlet is due to specificities of gas-phase chemistry and particle formation. To illustrate these differences, Fig. 3 represents the distributions of the DMAl:NH2 and AlN concentrations and the particle density patterns. These components have been chosen for illustration from the analysis of the species fluxes onto the deposition surface, which has revealed that the main Al-containing species contributing to the layer growth are DMAl:NH2 and AlN vapor. The contributions of these two species are almost equal for the inverted inlet, whereas the AlN flux prevails for the conventional one. These conclusions are supported by the distributions presented in Fig. 3. When the conventional supply is used, ammonia diffuses rather intensively into the upper MO channel, initiating the formation of the adduct and subsequent gas-phase reactions. As a result, DMAl:NH2 is formed away from the deposition surface with the maximal concentration near the separation plate edge (Fig. 3(a)). In case of the inverted supply, the majority of the gas-phase reactions proceed somewhat more downstream and closer to the susceptor. In particular, the concentration of DMAl:NH2 is higher and its the relative contribution to the layer deposition becomes greater. The distributions of the AlN vapor molar fraction (Fig. 3(b)) also differ significantly between the considered inlet configurations. For the conventional supply of the precursors, the AlN concentration has a pronounced maximum between the reactor ceiling and susceptor. Inversion of the precursor supply changes the AlN concentration pattern so that it has some elongated form. Eventually, the particles form a layer immediately above the susceptor in case of the inverted precursor supply, while the particle density is maximal in the central part of the reactor and closer to its upper wall for the conventional supply. In addition, the particle density is higher above the susceptor in the former case (see Fig. 3(c)). Taking into
a
susceptor
b
Fig. 5. Particle density distributions for the inverted inlet and total flow rate of 7.5 slm (a) and 11 slm (b).
ARTICLE IN PRESS E.V. Yakovlev et al. / Journal of Crystal Growth 298 (2007) 413–417
account that both DMAl:NH2 and AlN not only contribute to the layer deposition but can also be consumed by particle growth, we can conclude that the losses of Al are greater in case of the inverted supply, when both species are of the same importance with respect to the Al incorporation into the growing layer. A promising way to get even higher Al contents up to about 70% is to increase the total flow in the reactor in order to suppress partly the formation of particles by lowering of the Al-containing species partial pressures and the gas residence time. To study this effect in more detail, we have performed computations for the highest Al gasphase composition of 79.5% and varied total flow rate. The computed dependencies, shown in Fig. 4, demonstrate a strong effect of the total flow on the AlGaN layer composition. At the total flow of 11 slm, the Al content in the solid is practically the same as the Al gas-phase composition for the conventional inlet, but still remains somewhat lower for the inverted one. For both inlet configurations, the Al incorporation efficiency increases significantly as the total flow is raised due to gradual suppression of particle formation, as one can see from Fig. 5 presenting the particle density distributions for the inverted supply and two different total flow rates. The reduction of particle formation is due to the decrease of the Al-containing species partial pressures (via dilution) and lowering of the gas residence time (via higher inlet velocities). 5. Summary A combined modeling and experimental study of AlGaN deposition in the AIX 200/4 RF-S reactor has been made with the aim to reveal the effect of the reactor
417
inlet configuration on the Al incorporation efficiency. It has been found that the use of the inverted inlet configuration improves the reproducibility of the growth process, allowing a good control of AlGaN deposition. For both configurations of the reactor inlet, an Al content of about 70% has been achieved, but the inverted precursor supply results in generally lower Al incorporation. This effect has been understood from the modeling results: inverted inlet configuration results in more intensive formation of particles in the gas phase that consume the main Al-containing species contributing to the layer deposition. Additionally it was demonstrated that a reduction of particle formation and thereby an increase of the Al content can be achieved for both configurations by increasing the total flow in the reactor.
References [1] J.R. Creighton, G.T. Wang, W.G. Breiland, M.E. Coltrin, J. Crystal Growth. 261 (2004) 204. [2] A.A. Allerman, M.H. Crawford, A.J. Fischer, K.H.A. Bogart, S.R. Lee, D.M. Follstaedt, P.P. Provencio, D.D. Koleske, J. Crystal Growth 272 (2004) 227. [3] A.V. Kondratyev, R.A. Talalaev, W.V. Lundin, A.V. Sakharov, A.F. Tsatsul’nikov, E.E. Zavarin, A.V. Fomin, D.S. Sizov, J. Crystal Growth 272 (2004) 420. [4] S.C. Choi, J.-H. Kim, J.Y. Choi, K.J. Lee, K.Y. Lim, G.M. Yang, J. of Appl.Phys. 87 (2002) 172. [5] A.V. Lobanova, K.M. Mazaev, R.A. Talalaev, M. Leys, S. Boeykens, K. Cheng, S. Degroote, J. Crystal Growth 287 (2006) 601. [6] H. Hardtdegen, N. Kaluza, R. Steins, R. Schmidt, K. Wirtz, E.V. Yakovlev, R.A. Talalaev, Yu.N. Makarov, J. Crystal Growth 272 (2004) 407. [7] E.V. Yakovlev, R.A. Talalaev, A.N. Vorob’ev, Yu.N. Makarov, Electrochem. Soc. Proc. 2003-08 (2003) 258.