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Unintentional incorporation of contaminants during chemical vapour deposition of silicon carbide
MATERIALS SCIENCE & ENGINEERING
g
ELSEVIER
Materials Science and Engineering B29 (1995) 134-137
Unintentional incorporation of contaminants during chemical vapour deposition of silicon carbide Stephan Karmann ~, L6a Di Cioccio, Bruno Blanchard, Thierry Ouisse 2, Denise Muyard, Claude Jaussaud LET1 (CEA- Technologies A vancdes), DMEL-CENG, 17 rue des Martyrs, 38054 Grenoble Cedex, France
Abstract 6H SiC layers were grown by chemical vapour deposition. The contents of nitrogen, aluminum and boron contaminants were determined by secondary ion mass spectroscopy. The incorporation of these elements leads to unintentionially p-type layers when growth is performed on uncoated graphite susceptors. The use of coated susceptor materials yields n-type layers. The lowest achieved doping levels are determined by capacitance-voltage measurements to be N a - N D = 1 × 1014 cm-3 and N D - N A= 4 x 1()~ cm ~. An attempt is made to explain the observed doping mechanism using a recently proposed model of"site competition" during epitaxial growth.
Keywords: Silicon carbide; Doping effects; Chemical vapour deposition
1. Introduction
2. Experimental procedure
Chemical vapour deposition (CVD) is a technique frequently used to grow single-crystal layers of siicon carbide [ 1-4]. n-Type doped layers can be obtained by adding a nitrogen-containing gas such as N 2 or NH 3 to the reactor system during growth, while the analogous addition of aluminum- or boron-containing compounds is used for p-type doping. This kind of in situ doping is one of the basic steps in the fabrication of desired high temperature, high power and high speed SiC devices in planar technology [5]. Nevertheless, nitrogen, aluminum and boron might also appear as residual, and therefore possibly uncontrolled, background elements in the environment of the growth chamber, resulting in unintentional incorporation of these elements in the films. The influence on the electrical characteristics might render the layer useless for device applications.
2.1. Growth experiments
~Permanent address: Friedrich-Schiller-Universit~it Jena, Institut ffir Festk6rperphysik, Max-Wien-Platz 1, D07743 Jena, Germany. 2Laboratoire de Physique des Composants h Semiconducteurs (URA-CNRS 840) E N S E R G , 23 rue des Martyrs, 38016 Grenoble Cedex, France.
All examined layers were grown in an atmospheric pressure CVD system consisting of a horizontal watercooled quartz reactor tube of 13.5 cm inner diameter with stainless steel flanges. The substrates rest on top of a rectangular graphite susceptor (12 x 12 x 4 cm 3) which is supported by a quartz-boat (catamaran) and inductively heated (8 kHz, maximum power 50 kW). The gas flow is guided by a quartz-ramp, to give a homogeneous flow profile over the surface of the substrates. The substrates are silicon wafers up to 4 in in diameter, on which cubic SiC is successfully grown. Further details of the reactor system and previous experiments performed on silicon substrates are described elsewhere [6-8]. In the present article we report the growth of single-crystal 6H SiC layers on commercially available 6H SiC substrate material (Cree Research, off-axis, p- and n-type, classified as ~'research grade"). The standardized growth parameters were a temperature of 1390°C and 30 standard 1 min 1 hydrogen carrier gas, in which silane (3 standard cm 3 rain -1) and propane (2.5 standard cm 3 rain 1) were diluted as sources of silicon and carbon atoms; the growth rate was 0.7 ~m h
0921-5107/95/$9.50 © 1995 - Elsevier Science S.A. All rights reserved SSDI 0921-5107(94)04021 -U
S. Karmann et al.
/
Materials Science and Engineering B29 (1995) 134-137
Prior to layer deposition all substrates were subjected to an in situ hydrogen etch (10 min in 15 standard 1 min -~ at 1390°C). After deposition the system cooled down in hydrogen atmosphere. Between single growth runs the reaction chamber was evacuated to about 0.02 Torr, and the susceptors brought into air via a transfer chamber to allow unloading of the substrates. T h e non-adherent SiC covering of the susceptor which is formed during the growth process was carefully removed after every run by light blowing with argon gas.
2.2. Secondary ion mass spectroscopy Secondary ion mass spectroscopy (SIMS) measurements were taken of every deposited layer to study the incorporation of different chemical elements during C V D growth of the SiC layers. In a modified Cameca IMS 300 system with enhanced vacuum conditions, 6 keV Xe primary ions were accelerated at an incident angle of 60 ° towards the SiC layers. T h e counting rates were calibrated using samples implanted with N, AI or B. Owing to the weak emission of N + ions at mass 14 in our SIMS apparatus, quantitative analysis of nitrogen was performed by the detection of Si2N + ions (mass 70). T h e two species that can form acceptors in SiC are easily detected by the signals at mass 27 (AI + signal) and mass 11 (B + signal) with detection limits of 1015 atoms cm -3 for A1 and 1014 atoms cm 3 for B. All given atomic concentrations are average values obtained over the film thickness; error bars indicate the fluctuations of the count rates.
3. Experimental results T h e use of pure graphite susceptors is reported in the literature as well as the use of graphite susceptors covered with a silicon carbide coating. On both types of susceptors we observe a certain spread of the nonintentional doping level. To examine in more detail the role of the susceptor, we performed series of growth runs with the same standard growth parameters as listed above. Each series started with a "new" susceptor as delivered by the manufacturer without any cleaning or heating to "out-gas" the susceptor. We report in detail the results obtained for a pure graphite susceptor, guaranteed to contain not more than 20 ppm (metallic) contamination, and for a second susceptor covered with a 100 # m thick silicon carbide coating by the manufacturer.
3.1. Graphite susceptors In Fig. 1 we show the results of SIMS measurements of a series of layers grown using the pure graphite susceptor. It is evident that the aluminum content in the layers increases steadily from 1 x 101~' to 8 x 10 ~" atoms cm 3 during the first four growth experiments. T h e boron content remains almost constant in the lower 10 ~5 range while the nitrogen content seems to decrease slightly, at least for the first three experiments from 2 x 10 TM atoms cm 3 to the higher 1() ~s range (see Table 1 ). N o n e of the SIMS signals shows a systematic
1E+20~ °Eo 1E+19~
2.3. Electrical characterization
135
~
~
{,
:;
70 Si2N
._ 1E+18~
Capacitance-voltage ( C V ) measurements were performed on M O S capacitors as basic electrical characterization of the deposited layers. Details of the devices, the measurement and evaluation techniques as well as typical results on commercially available 6 H SiC layers by Cree have already been published [9,10]. T h e characteristic data, such as flatband voltage or interface state density, show no significant differences for capacitors made on our own epilayers compared with the Cree material. From the 1/(72 curves the net carrier concentrations of the layers are determined.
2 AI
1E+17
,~
0
~-
1E+16
~
-
T,
~
:11B
~"
._o
T
Z
oE 1E+15 1E+14
i 2 3 4 ' Experiments on noncoated graphite
Fig. 1. Atomic concentrations of nitrogen, aluminum and b o r o n as incorporated unintentionally during growth on an uncoated graphite susceptor.
Table 1 C o m p a r i s o n of SIMS results for 6H SiC layers grown on different susceptor materials Susceptor
70 SizN
27 AI
11 B
SiC coated Graphite of high purity (20 ppm) Less pure graphite (40 ppm)
8 x 101s-3 x 10 TM 8 x 101s-2 x 10 ~9 1 x lOiS-2 x 10 TM
6 x 1()1~-6 x 101~ (1-8) × 10 ~' 6 × 101~-6 × 1 0 "
= 1014 (1.5-3) × 1() ~5 1 × 1()15-1 × 1 0 "
A', Karmann el al.
136
/
Mawrials Science and Engineering B29 (1995) 134-137
increase or decrease with depth, but they are all more or less constant throughout the analysis. Unintentional p-type doping was determined for several dozens of 6H epilayers grown using uncoated graphite susceptors in our CVD apparatus (with only one exception). CV measurements by mercury probe (to the as-grown surfaces) and on MOS capacitors (as described above) revealed corresponding values. N A- ND rises from 1014 cm s for layer 1 to the high l() 1(~ range for layer 4, in analogy with increasing aluminum incorporation. In general, however, we are not able to correlate the CV and SIMS results of layers grown on pure graphite susceptors. The use of another uncoated graphite susceptor of less purity (40 ppm) resulted in p-type layers for all experiments performed under a variety of growth conditions with N a - N D ranging between 1015 and some 1()~7 cm s. The SIMS results are listed in Table 1, to be compared with the other experiments. 3.2. Coated susceptors
All layers grown on SiC-coated susceptors under different conditions (temperature, time, gas composition) show n-type conductivity without exception. Again we performed a series of deposition experiments starting with a new susceptor. The nitrogen content of these layers increased slightly but steadily from 8 x l0 Is to 3 x 101~) atoms cm ~ (Fig. 2). The picture for aluminum incorporation is totally indifferent, since the aluminum content varies by a full order from 6 x 10 ~~ to 6 x 1()~(~ atoms cm -~ without showing any distinct trend. The boron content, finally, of all layers grown on SiC-coated susceptors ranged around the detection limit of 1014 atoms cm s. This is at least one order below the usual boron content reached on uncoated graphite in our experiments. The very first layer (sample 5) grown on a fairly new SiC-coated susceptor was doped to N D - NA = 1 x 10 ~s 1E+20z o
I.
g 1E+181 ~E 1E+171 r c
o
!
70 Si2N
1E+19
27 AI i[ 11B
1E+16
oE 1 E + 1 5 ,< 1E+14 ] Experiments on SiC-coated graphite
Fig. 2. Atomic concentrations of nitrogen, aluminum and boron as incorporated unintentionally during growth on an SiC-coated susceptor.
cm 3. In the following experiments the unintentional doping increased from N D - NA= 2 × 1() I¢~ cm s for sample 6 to the low 1()~7 range for sample 8, in analogy with increasing nitrogen incorporation. A few layers grown on a susceptor with a 120 /zm thick coating of pyrolytic graphite all showed n-type conductivity; the lowest obtained N D - N A value was 4 x 10IS cm -~. SIMS results for these layers are not yet available. Unfortunately, the pyrolytic graphite coating was found to be very unstable in the reactive ambient of the growth chamber, making further experiments impossible. Aluminum nitride coating was found to cause serious problems owing to unsuitable grime of the reaction chamber.
4. D i s c u s s i o n and summary
Under the described experimental conditions n-type layers can be grown using coated susceptors, while the operation with uncoated graphite material introduces p-type contamination in the layers. To explain this effect, we use a recently proposed model [11] that describes the incorporation of contaminants during the CVD growth of SiC. Since the nitrogen donor rests on a carbon site of the SiC lattice, a high carbon to silicon ratio [C]/[Si] in the gas phase decreases the probability of nitrogen being incorporated (so-called "site competition" between C and N atoms). However, the incorporation of aluminum on the desired silicon site is enhanced by a high [C]/[Si] ratio. Further, it is known that an uncoated graphite susceptor is attacked in the reactive ambient of the growth chamber. This introduces additional carbon-containing species to the gas phase, emerging from the susceptor. From simple thermodynamic considerations one can even show [12] that for a sufficient high temperature and susceptor surface the [C]/[Si] ratio in the gas phase is totally determined by carbon from the graphite and is independent of the maintained propane flow. Attack of the susceptor is least for newly manufactured surfaces and increases from experiment to experiment leading to increasing porosity of the susceptor surface. So the uncoated susceptor contributes more and more carbon to the gas phase. The assumed site competition during epitaxial growth explains the decrease in nitrogen content and the simultaneous increase in aluminium content of layers 1 to 4. The small boron atom, however, probably occupies interstitial sites, so its incorporation is not affected by the competition mechanism and remains constant for all layers. Using an SiC-coated susceptor, one can assume no additional carbon contribution to the gas phase. On the contrary, silicon is more likely to be evaporated from an SiC surface during heating [13]. This effect reduces
S. Karmann et al.
/
Materials Science and Engineering B29 (199.5) 134-137
the [C]/[Si] ratio and might explain the increasing nitrogen content of samples 5 to 8, since the "site competition" favours nitrogen in this case. The large variations in the aluminum content of the layers cannot be explained as easily. Finally, we try to identify the sources of the introduced contaminants nitrogen, aluminum and boron. No leakage of the reactor system was found during repeated and careful testing, so we conclude the nitrogen source to be located "inside" the reactor itself. It might be the working gases, since the purity of the propane (99.95%) was unsatisfactory [14]. Contamination of the susceptor material by nitrogen from ambient air during loading of the substrates might also occur, but is not very likely to lead to the observed constant nitrogen level during the entire growth of one layer. The boron source might be the used susceptor, as indicated by the reduction in boron content when an SiC-coated susceptor is used and an increase on using the less pure uncoated graphite. Aluminum might out diffuse from the susceptors too. In addition, even the quartz catamaran could act as an aluminum source when it becomes hot owing to direct contact with the heated susceptor [15]. Even if we are not able at the moment to explain all the reported observations in complete detail, our experiments might help other groups to understand results obtained using the common CVD technique to produce SiC layers. This could start a search for possible improvements in the reactor set-up and the deposition process.
Acknowledgments We are indebted to our former colleagues N. B6court and J.-L. Ponthenier, who in their work on the experimental set-up of the CVD reactor as well as on
137
the basic process parameters gave us a very helpful starting platform for the actual study. We thank T. Billon for his assistance concerning the device technology. J.F. Michaud for the ion implantation, J. Collet for his part of the electrical measurements and all those who contributed by discussions and suggestions. The work is partly funded by the DRET; S.K. was additionally supported by the SFB 169, Jena.
References [1] J.A. Powell, D.J. Larkin, L.G. Matus, W.J. Choyke, J.L. Bradshaw, L. Henderson, M. Yoganathan, J. Yang and R Pirouz, Appl. Phys. Lett., 56 (1990) 1442. [2] Y.C. Wang and R.F. Davis, J. Electron. Mater., 20 (1991) 869. [3] S. Karmann, W. Suttrop, A. Sch6ner, M. Schadt, C. Haberstroh, F. Engelbrecht, R. Helbig, G. Pensl, R.A. Stein and S. Leibenzeder, J. Appl. Phys., 72 (1992) 5437. [4] T. Kimoto, H. Nishino, W.S. Yoo and H. Matsunami, J. Appl. Phys., 73 (1993) 726. [5] 5th ICSCR, Washington, DC, November 1-3, 1993, Proc. 2nd Hitec, June 5-10, 1994, Charlotte. [6] N. Bdcourt, PhD Thesis, Montpellier, 1993. [7] N. B6court, J.L. Ponthenier, A.M. Papon and C. Jaussaud, Physica B, 185 (1993) 79. [8] N. B6court, B. Cros, J.L. Ponthenier, R. Berjoan, A.M. Papon and C. Jaussaud, Appl. Surf. Sci., 68 (1993) 461. [9] T. Ouisse, N. Bdcourt, C. Jaussaud and F. Templier, J. Appl. Physl., 75 (1994) 604. [10] T. Ouisse, N. Bdcourt, F. Templier, J. Vuillod, S. Cristoloveanu, T. Billon, J.L. Ponthenier, C. Jaussaud and F. Mondon, presented at 5th 1CSCRM, Washington, DC, 1993. [111 D.J. Larkin, EG. Neudeck, J.A. Powell and L.G. Matus, presented at 5th ICSCRM, Washington, DC, 1993. [12] K. Rottner and R. Helbig, presented at the MRS Spring Meet. San Francisco, 1994, J. Crvst. Growth, submitted for publication. [13] L. Muelhoff, W.J. Choyke, M.J. Bozack and J.T. Yates, J. Appl. Phys., 00 (1986)2842. [14] M. Anikin, personal communication, 1994. [ 15] W.J. Choyke, personal communication, 1994.
g
ELSEVIER
Materials Science and Engineering B29 (1995) 134-137
Unintentional incorporation of contaminants during chemical vapour deposition of silicon carbide Stephan Karmann ~, L6a Di Cioccio, Bruno Blanchard, Thierry Ouisse 2, Denise Muyard, Claude Jaussaud LET1 (CEA- Technologies A vancdes), DMEL-CENG, 17 rue des Martyrs, 38054 Grenoble Cedex, France
Abstract 6H SiC layers were grown by chemical vapour deposition. The contents of nitrogen, aluminum and boron contaminants were determined by secondary ion mass spectroscopy. The incorporation of these elements leads to unintentionially p-type layers when growth is performed on uncoated graphite susceptors. The use of coated susceptor materials yields n-type layers. The lowest achieved doping levels are determined by capacitance-voltage measurements to be N a - N D = 1 × 1014 cm-3 and N D - N A= 4 x 1()~ cm ~. An attempt is made to explain the observed doping mechanism using a recently proposed model of"site competition" during epitaxial growth.
Keywords: Silicon carbide; Doping effects; Chemical vapour deposition
1. Introduction
2. Experimental procedure
Chemical vapour deposition (CVD) is a technique frequently used to grow single-crystal layers of siicon carbide [ 1-4]. n-Type doped layers can be obtained by adding a nitrogen-containing gas such as N 2 or NH 3 to the reactor system during growth, while the analogous addition of aluminum- or boron-containing compounds is used for p-type doping. This kind of in situ doping is one of the basic steps in the fabrication of desired high temperature, high power and high speed SiC devices in planar technology [5]. Nevertheless, nitrogen, aluminum and boron might also appear as residual, and therefore possibly uncontrolled, background elements in the environment of the growth chamber, resulting in unintentional incorporation of these elements in the films. The influence on the electrical characteristics might render the layer useless for device applications.
2.1. Growth experiments
~Permanent address: Friedrich-Schiller-Universit~it Jena, Institut ffir Festk6rperphysik, Max-Wien-Platz 1, D07743 Jena, Germany. 2Laboratoire de Physique des Composants h Semiconducteurs (URA-CNRS 840) E N S E R G , 23 rue des Martyrs, 38016 Grenoble Cedex, France.
All examined layers were grown in an atmospheric pressure CVD system consisting of a horizontal watercooled quartz reactor tube of 13.5 cm inner diameter with stainless steel flanges. The substrates rest on top of a rectangular graphite susceptor (12 x 12 x 4 cm 3) which is supported by a quartz-boat (catamaran) and inductively heated (8 kHz, maximum power 50 kW). The gas flow is guided by a quartz-ramp, to give a homogeneous flow profile over the surface of the substrates. The substrates are silicon wafers up to 4 in in diameter, on which cubic SiC is successfully grown. Further details of the reactor system and previous experiments performed on silicon substrates are described elsewhere [6-8]. In the present article we report the growth of single-crystal 6H SiC layers on commercially available 6H SiC substrate material (Cree Research, off-axis, p- and n-type, classified as ~'research grade"). The standardized growth parameters were a temperature of 1390°C and 30 standard 1 min 1 hydrogen carrier gas, in which silane (3 standard cm 3 rain -1) and propane (2.5 standard cm 3 rain 1) were diluted as sources of silicon and carbon atoms; the growth rate was 0.7 ~m h
0921-5107/95/$9.50 © 1995 - Elsevier Science S.A. All rights reserved SSDI 0921-5107(94)04021 -U
S. Karmann et al.
/
Materials Science and Engineering B29 (1995) 134-137
Prior to layer deposition all substrates were subjected to an in situ hydrogen etch (10 min in 15 standard 1 min -~ at 1390°C). After deposition the system cooled down in hydrogen atmosphere. Between single growth runs the reaction chamber was evacuated to about 0.02 Torr, and the susceptors brought into air via a transfer chamber to allow unloading of the substrates. T h e non-adherent SiC covering of the susceptor which is formed during the growth process was carefully removed after every run by light blowing with argon gas.
2.2. Secondary ion mass spectroscopy Secondary ion mass spectroscopy (SIMS) measurements were taken of every deposited layer to study the incorporation of different chemical elements during C V D growth of the SiC layers. In a modified Cameca IMS 300 system with enhanced vacuum conditions, 6 keV Xe primary ions were accelerated at an incident angle of 60 ° towards the SiC layers. T h e counting rates were calibrated using samples implanted with N, AI or B. Owing to the weak emission of N + ions at mass 14 in our SIMS apparatus, quantitative analysis of nitrogen was performed by the detection of Si2N + ions (mass 70). T h e two species that can form acceptors in SiC are easily detected by the signals at mass 27 (AI + signal) and mass 11 (B + signal) with detection limits of 1015 atoms cm -3 for A1 and 1014 atoms cm 3 for B. All given atomic concentrations are average values obtained over the film thickness; error bars indicate the fluctuations of the count rates.
3. Experimental results T h e use of pure graphite susceptors is reported in the literature as well as the use of graphite susceptors covered with a silicon carbide coating. On both types of susceptors we observe a certain spread of the nonintentional doping level. To examine in more detail the role of the susceptor, we performed series of growth runs with the same standard growth parameters as listed above. Each series started with a "new" susceptor as delivered by the manufacturer without any cleaning or heating to "out-gas" the susceptor. We report in detail the results obtained for a pure graphite susceptor, guaranteed to contain not more than 20 ppm (metallic) contamination, and for a second susceptor covered with a 100 # m thick silicon carbide coating by the manufacturer.
3.1. Graphite susceptors In Fig. 1 we show the results of SIMS measurements of a series of layers grown using the pure graphite susceptor. It is evident that the aluminum content in the layers increases steadily from 1 x 101~' to 8 x 10 ~" atoms cm 3 during the first four growth experiments. T h e boron content remains almost constant in the lower 10 ~5 range while the nitrogen content seems to decrease slightly, at least for the first three experiments from 2 x 10 TM atoms cm 3 to the higher 1() ~s range (see Table 1 ). N o n e of the SIMS signals shows a systematic
1E+20~ °Eo 1E+19~
2.3. Electrical characterization
135
~
~
{,
:;
70 Si2N
._ 1E+18~
Capacitance-voltage ( C V ) measurements were performed on M O S capacitors as basic electrical characterization of the deposited layers. Details of the devices, the measurement and evaluation techniques as well as typical results on commercially available 6 H SiC layers by Cree have already been published [9,10]. T h e characteristic data, such as flatband voltage or interface state density, show no significant differences for capacitors made on our own epilayers compared with the Cree material. From the 1/(72 curves the net carrier concentrations of the layers are determined.
2 AI
1E+17
,~
0
~-
1E+16
~
-
T,
~
:11B
~"
._o
T
Z
oE 1E+15 1E+14
i 2 3 4 ' Experiments on noncoated graphite
Fig. 1. Atomic concentrations of nitrogen, aluminum and b o r o n as incorporated unintentionally during growth on an uncoated graphite susceptor.
Table 1 C o m p a r i s o n of SIMS results for 6H SiC layers grown on different susceptor materials Susceptor
70 SizN
27 AI
11 B
SiC coated Graphite of high purity (20 ppm) Less pure graphite (40 ppm)
8 x 101s-3 x 10 TM 8 x 101s-2 x 10 ~9 1 x lOiS-2 x 10 TM
6 x 1()1~-6 x 101~ (1-8) × 10 ~' 6 × 101~-6 × 1 0 "
= 1014 (1.5-3) × 1() ~5 1 × 1()15-1 × 1 0 "
A', Karmann el al.
136
/
Mawrials Science and Engineering B29 (1995) 134-137
increase or decrease with depth, but they are all more or less constant throughout the analysis. Unintentional p-type doping was determined for several dozens of 6H epilayers grown using uncoated graphite susceptors in our CVD apparatus (with only one exception). CV measurements by mercury probe (to the as-grown surfaces) and on MOS capacitors (as described above) revealed corresponding values. N A- ND rises from 1014 cm s for layer 1 to the high l() 1(~ range for layer 4, in analogy with increasing aluminum incorporation. In general, however, we are not able to correlate the CV and SIMS results of layers grown on pure graphite susceptors. The use of another uncoated graphite susceptor of less purity (40 ppm) resulted in p-type layers for all experiments performed under a variety of growth conditions with N a - N D ranging between 1015 and some 1()~7 cm s. The SIMS results are listed in Table 1, to be compared with the other experiments. 3.2. Coated susceptors
All layers grown on SiC-coated susceptors under different conditions (temperature, time, gas composition) show n-type conductivity without exception. Again we performed a series of deposition experiments starting with a new susceptor. The nitrogen content of these layers increased slightly but steadily from 8 x l0 Is to 3 x 101~) atoms cm ~ (Fig. 2). The picture for aluminum incorporation is totally indifferent, since the aluminum content varies by a full order from 6 x 10 ~~ to 6 x 1()~(~ atoms cm -~ without showing any distinct trend. The boron content, finally, of all layers grown on SiC-coated susceptors ranged around the detection limit of 1014 atoms cm s. This is at least one order below the usual boron content reached on uncoated graphite in our experiments. The very first layer (sample 5) grown on a fairly new SiC-coated susceptor was doped to N D - NA = 1 x 10 ~s 1E+20z o
I.
g 1E+181 ~E 1E+171 r c
o
!
70 Si2N
1E+19
27 AI i[ 11B
1E+16
oE 1 E + 1 5 ,< 1E+14 ] Experiments on SiC-coated graphite
Fig. 2. Atomic concentrations of nitrogen, aluminum and boron as incorporated unintentionally during growth on an SiC-coated susceptor.
cm 3. In the following experiments the unintentional doping increased from N D - NA= 2 × 1() I¢~ cm s for sample 6 to the low 1()~7 range for sample 8, in analogy with increasing nitrogen incorporation. A few layers grown on a susceptor with a 120 /zm thick coating of pyrolytic graphite all showed n-type conductivity; the lowest obtained N D - N A value was 4 x 10IS cm -~. SIMS results for these layers are not yet available. Unfortunately, the pyrolytic graphite coating was found to be very unstable in the reactive ambient of the growth chamber, making further experiments impossible. Aluminum nitride coating was found to cause serious problems owing to unsuitable grime of the reaction chamber.
4. D i s c u s s i o n and summary
Under the described experimental conditions n-type layers can be grown using coated susceptors, while the operation with uncoated graphite material introduces p-type contamination in the layers. To explain this effect, we use a recently proposed model [11] that describes the incorporation of contaminants during the CVD growth of SiC. Since the nitrogen donor rests on a carbon site of the SiC lattice, a high carbon to silicon ratio [C]/[Si] in the gas phase decreases the probability of nitrogen being incorporated (so-called "site competition" between C and N atoms). However, the incorporation of aluminum on the desired silicon site is enhanced by a high [C]/[Si] ratio. Further, it is known that an uncoated graphite susceptor is attacked in the reactive ambient of the growth chamber. This introduces additional carbon-containing species to the gas phase, emerging from the susceptor. From simple thermodynamic considerations one can even show [12] that for a sufficient high temperature and susceptor surface the [C]/[Si] ratio in the gas phase is totally determined by carbon from the graphite and is independent of the maintained propane flow. Attack of the susceptor is least for newly manufactured surfaces and increases from experiment to experiment leading to increasing porosity of the susceptor surface. So the uncoated susceptor contributes more and more carbon to the gas phase. The assumed site competition during epitaxial growth explains the decrease in nitrogen content and the simultaneous increase in aluminium content of layers 1 to 4. The small boron atom, however, probably occupies interstitial sites, so its incorporation is not affected by the competition mechanism and remains constant for all layers. Using an SiC-coated susceptor, one can assume no additional carbon contribution to the gas phase. On the contrary, silicon is more likely to be evaporated from an SiC surface during heating [13]. This effect reduces
S. Karmann et al.
/
Materials Science and Engineering B29 (199.5) 134-137
the [C]/[Si] ratio and might explain the increasing nitrogen content of samples 5 to 8, since the "site competition" favours nitrogen in this case. The large variations in the aluminum content of the layers cannot be explained as easily. Finally, we try to identify the sources of the introduced contaminants nitrogen, aluminum and boron. No leakage of the reactor system was found during repeated and careful testing, so we conclude the nitrogen source to be located "inside" the reactor itself. It might be the working gases, since the purity of the propane (99.95%) was unsatisfactory [14]. Contamination of the susceptor material by nitrogen from ambient air during loading of the substrates might also occur, but is not very likely to lead to the observed constant nitrogen level during the entire growth of one layer. The boron source might be the used susceptor, as indicated by the reduction in boron content when an SiC-coated susceptor is used and an increase on using the less pure uncoated graphite. Aluminum might out diffuse from the susceptors too. In addition, even the quartz catamaran could act as an aluminum source when it becomes hot owing to direct contact with the heated susceptor [15]. Even if we are not able at the moment to explain all the reported observations in complete detail, our experiments might help other groups to understand results obtained using the common CVD technique to produce SiC layers. This could start a search for possible improvements in the reactor set-up and the deposition process.
Acknowledgments We are indebted to our former colleagues N. B6court and J.-L. Ponthenier, who in their work on the experimental set-up of the CVD reactor as well as on
137
the basic process parameters gave us a very helpful starting platform for the actual study. We thank T. Billon for his assistance concerning the device technology. J.F. Michaud for the ion implantation, J. Collet for his part of the electrical measurements and all those who contributed by discussions and suggestions. The work is partly funded by the DRET; S.K. was additionally supported by the SFB 169, Jena.
References [1] J.A. Powell, D.J. Larkin, L.G. Matus, W.J. Choyke, J.L. Bradshaw, L. Henderson, M. Yoganathan, J. Yang and R Pirouz, Appl. Phys. Lett., 56 (1990) 1442. [2] Y.C. Wang and R.F. Davis, J. Electron. Mater., 20 (1991) 869. [3] S. Karmann, W. Suttrop, A. Sch6ner, M. Schadt, C. Haberstroh, F. Engelbrecht, R. Helbig, G. Pensl, R.A. Stein and S. Leibenzeder, J. Appl. Phys., 72 (1992) 5437. [4] T. Kimoto, H. Nishino, W.S. Yoo and H. Matsunami, J. Appl. Phys., 73 (1993) 726. [5] 5th ICSCR, Washington, DC, November 1-3, 1993, Proc. 2nd Hitec, June 5-10, 1994, Charlotte. [6] N. Bdcourt, PhD Thesis, Montpellier, 1993. [7] N. B6court, J.L. Ponthenier, A.M. Papon and C. Jaussaud, Physica B, 185 (1993) 79. [8] N. B6court, B. Cros, J.L. Ponthenier, R. Berjoan, A.M. Papon and C. Jaussaud, Appl. Surf. Sci., 68 (1993) 461. [9] T. Ouisse, N. Bdcourt, C. Jaussaud and F. Templier, J. Appl. Physl., 75 (1994) 604. [10] T. Ouisse, N. Bdcourt, F. Templier, J. Vuillod, S. Cristoloveanu, T. Billon, J.L. Ponthenier, C. Jaussaud and F. Mondon, presented at 5th 1CSCRM, Washington, DC, 1993. [111 D.J. Larkin, EG. Neudeck, J.A. Powell and L.G. Matus, presented at 5th ICSCRM, Washington, DC, 1993. [12] K. Rottner and R. Helbig, presented at the MRS Spring Meet. San Francisco, 1994, J. Crvst. Growth, submitted for publication. [13] L. Muelhoff, W.J. Choyke, M.J. Bozack and J.T. Yates, J. Appl. Phys., 00 (1986)2842. [14] M. Anikin, personal communication, 1994. [ 15] W.J. Choyke, personal communication, 1994.