- Email: [email protected]
Ion beam deposition of β-SiC layers onto α-SiC substrates
Vacuum/volume 39/numbers Printed in Great Britain
11 I1 2lpages
1065 to 1068/1989
Pergamon Press plc
Ion beam deposition substrates S P Withrow,
K L More,
0042-207X/89$3.00+.00
of P_SiC layers onto a-Sic
R A Zuhr and T E Haynes,
Oak Ridge National
Laboratory,
Oak Ridge,
TN 37831
USA
Thin films of B-Sic have been grown epitaxically on 6H u-Sic substrates by direct deposition of alternating layers of 13C+ and 3oSi+. The carbon and silicon ions were obtained from an ion implanter by decelerating mass-analyzed ion beams down to 40 eV. The substrate was held at 700°C during growth. Rutherford backscattering and high resolution transmission electron microscopy techniques have been used to determine the composition and structure of the resulting P-Sic layer and the nature of the interface. This is the first reported use of the ion beam deposition technique to grow P-Sic films and demonstrates the capability of using alternating beam deposition to prepare compound structures.
Introduction In recent years, the use of SIC for applications in high-temperature, high frequency, and high power electronic devices has emerged as a promising technology. There is particular interest in the application of B-Sic, the only cubic form of Sic, because its band gap, 2.39 eV, is the lowest of all the common Sic polytypes. Thin films of B-Sic grown using both chemical vapor deposition (CVD)’ and molecular beam epitaxy (MBE)’ techniques have been reported, and Sic devices have been successfully fabricated from these films3v4. However, both of these techniques require relatively high growth temperatures. Another problem has been the choice of an optimum substrate material for the growth of B-Sic. The most common substrate material has been (100) silicon. Unfortunately, as a result of the -8% difference in thermal expansion coefficients and an -20% difference in lattice parameters, the films contain a high density of defects, the most common of which are microtwins, stacking faults,5 and antiphase domain boundaries6. A number of candidate substrates for the deposition of improved B-Sic layers have been suggested by prior work. AlphaSIC, when used as a substrate material for the epitaxical growth of B-Sic, should result in the best film/substrate matching. In fact, the growth of B-Sic on Cc-Sic has been previously reported. Muench and Pfaffeneder’ described the deposition of epitaxical layers of /?-Sic on Cc-Sic substrates in the temperature range 1700-185O”C, and reported that the /3-Sic layers were generally polycrystalline. Beta-Sic thin films were grown on a-Sic substrates via CVD by Kong et al* in the temperature range 13601410°C. It was determined that the major defects observed in these films were double positioning boundariesg. Different microstructures were observed depending on the polarity of the (0001) growth face”. Alpha-Sic has also been used as a substrate to grow a-Sic layers’*’ ‘-I 3. These CVD and MBE growth experiments have been carried out at temperatures well over 1000°C. Recently, however, the heteroepitaxical growth of D-Sic on Si substrates held below 800°C using a reactive ion beam deposition technique has been reported 14.
An alternative method for growing thin layers is ion beam deposition (IBD) Is. Using this technique, a mass-analyzed, lowenergy ion beam of a desired species is directed onto a substrate. The energy is chosen low enough that the incident ions do not penetrate deeply into the substrate but rather deposit onto the surface. By alternating the ion species to maintain stoichiometry, it is possible to grow compound materials. Ion beam deposition of epitaxical GaAs on both silicon and germanium has been achieved in this manner’ 6*I’. The present work was initiated to explore the possibilities of growing the compound semiconductor SIC on 6H a-Sic substrates using the IBD technique. The results represent the lowest reported temperature for forming epitaxical B-Sic on any substrate material. Rutherford backscattering (RBS) and high resolution electron microscopy (HREM) techniques were used to characterize the resulting films. Experimental procedure The substrates were grown in an Acheson furnace used to make SIC grit for industrial applications. These crystals generally terminate with only one flat surface, either a silicon (0001) or carbon (OOOi) face. For this work, crystals with terminating silicon (0001) faces were utilized since previous results all indicate that /?-Sic crystals grown on this face are generally of a higher quality than on carbon-terminating substrates’s’2. To prepare a clean, smooth surface for the deposition of alternating carbon and silicon layers, the substrates were first oxidized at 1200°C in a flowing dry oxygen atmosphere for -90 min. The oxide was then removed with a 1: 1 HF-HNO, solution before mounting in the IBD chamber. The samples were further cleaned in situ by bombarding with 40-eV H+ ions, with the sample held at > 500°C. Base pressure in the IBD chamber was 5 x lo- lo T. The IBD apparatus and process have been detailed previously”. In the present work, “C and “Si ions were produced simultaneously in an accelerator ion source using a mixture of 13C-enriched CO2 and SiF, as source gases. The ions were then extracted at 35 keV, electromagnetically mass-analyzed, transported to the IBD chamber, and finally decelerated to
S P Withrow
et al:
Ion
beam
deposition
of P-Sic
layers
onto
?-Sic
substrates
40
eV before impinging on the substrate. Previous work on other systems suggests that energies in this range minimize damage in the substrate. The ion beam was switched under computer control between the carbon and silicon beams, providing alternating deposition of approximately one monolayer of each. Since the substrate terminated with a silicon (0001) face. the initial layer deposited was “C. Beam currents of -2 /tA of “C’ and - I /IA of 3oSi+ were used, providing an overlayer growth rate of approximately 0.1 nm/min. The sample temperature was held at 700°C during deposition by resistively heating the substrate holder. The temperature was periodically monitored using an infrared pyrometer. The deposited films were characterized using RBS and ion channeling of 2.0 MeV helium. The scattering angle was set at 160”. In these spectra, the scattering yield from the overlayer was easily separated from that of the substrate since heavy isotopes (“C and “Si) were used for the deposition. Transmission electron microscopy (TEM) was also used to determine the structure of the deposited layer. Cross-section TEM specimens were prepared by creating a “sandwich” with the IBD layers epoxied together face-to-face. Slices -300 ,um thick were then cut perpendicular to the interface with a low speed diamond saw. Each slice was then mechanically thinned to - 75 /lrn, dimpled across the center of the interface to - 20 pm, and finally ion milled using 6 keV Ar+ ions on a stage cooled with liquid nitrogen. The crosssection specimens were initially examined in a JEOL 2000FX transmission electron microscope operated at 200 kV. High-resolution crystal structure images were recorded in a JEOL 4000EX transmission electron microscope operated at 400 kV. All photographs were taken near Scherzer defocus (- - 50 nm) and a convergence angle of - 1 mr. These conditions were used for the image simulation calculations obtained using the MacTempas program, which were compared with the structure images obtained. Results and discussion Figure 1 shows backscattering spectra obtained from a sample which was grown by alternating 50 layers each of 13C and ‘“Si. The total dose for each atomic layer, integrated at the target, was
1
Virgin a-Sic, IBD 30Si+“C
0
axis 1
”
0.5
0.6
0.7
Figure 1. Rutherford backscattering deposited on an z-Sic substrate. 1066
0.8
0.9
ENERGY
(MeV)
spectra
1 .o
1.1
1.2
from /J-Sic layer ion beam
chosen to be 1.2x IO ” atoms/cm’, the atomic density for a (I I 1) /j-Sic layer. Part of the sample was masked during IBD so that spectra from a region without a deposited overlayer could be obtained. For the channeled spectra, the incident beam was aligned along the (0001) axis of this undeposited 6H r-Sic substrate Aligned spectra are shown for both an undeposited, virgin region as well as a region with an overlayer. In addition. a random spectrum is shown which was taken with the He+ beam incident on the sample at the same location as the channeled spectrum for the overlayer. The sample was continuously rotated while the random spectrum was bcin, m obtained. and the yield in this spectrum represents the yield that would be obtained if the sample were amorphous. The energies at which He backscatters from surface atoms of mass 12, 13,28, and 30 are marked in the figure. The “Si surface peak in the aligned, undepositcd spectrum can be seen at 1.14 MeV. Peaks corresponding to other isotopes of Si as well as to ’ 'C are not easily resolved for the statistics of the aligned spectra. The ratio of the yield in the undeposited spectra to that of the random spectrum at an energy just below the Si surface peak is 0.03. which indicates that the substrate has exccllent crystallinity. Backscattering from C and Si atoms in the deposited layer are easily resolved as peaks at energies corresponding to scattering from atoms with mass 13 and 30, respectively. The total 13C and “Si yields are essentially the same in both the aligned and random spectra, indicating that either the overlayer is not an undamaged single crystal or the incident beam is not aligned with a crystalline axis. Scattering from Si in the substrate at the interface occurs at a lower energy than the Si surface peak in the virgin spectrum due to energy loss of the He beam in the overlayer. The small increase in yield seen in the channeled spectrum between approximately I .05 and 1.15MeV, which has no corresponding yield in the virgin spectrum, indicates that the Si lattice has been slightly damaged during the IBD process. This damage extends approx. 80 nm into the substrate, as can be determined by making an energy-to-depth calibration using standard Hc stopping powers. The integrated scattering yields in the “Si and 13C peaks were quantified by comparison to a spectrum obtained from a virgin Si sample. The overlayer represents I .03 f 0.1 x 10 ’ ’ Si cm ’ and 1.38+0.15x IO” C cm-‘. This result shows the C:Si ratio in the deposited layer to be on the order of 1.3 : 1. For a beta overlayer, the Si amount calculated above would correspond to an overlayer thickness of 215 A. It should be noted, however, that backscattering spectra obtained from different spots on the sample indicated the D-Sic film was not uniform in thickness, varying by as much as 50%. Transmission electron microscopy analysis of the sample also showed a thickness non-uniformity. For the channeled spectra in Figure 1 the incident beam was aligned along the substrate (0001) axis. Additional spectra were obtained by tilting the sample up to 2’ away from the (0001) in an attempt to find a crystal axis in the overlayer. While small reductions (up to -5%) in the yield in the ‘“Si peak were observed, no well-defined channel was found in the overlayer. A low magnification TEM image of the resulting IBD layer is shown in Figure 2. The thickness of the layer was not uniform across the substrate surface ; the maximum thickness was found to bc - 20 nm. The RBS results in Figure I indicate the presence of a damage region down to - 80 nm below the substrate surface : however, as shown in Figure 2, there is no significant damage detected by TEM (e.g. no dislocations or dislocation loops) indicating that the damage is limited to point defects generated by the IBD process.
S P Withrow
et a/; Ion beam deposition
of P-Sic
layers onto cr-Sic substrates IBD Sic +
alpha-SiC substrate
220 l
111 0
002 l
0’ twin refleetlons
iii l
Figure 2. Low magnification TEM micrograph of IBD layer on j%SiC. a-SiC substrate was oriented with (1120) parallel to the electron beam.
b)
0 /
0
0 l l
l
IBD beta-Sic film
The deposited layer was too thin to permit use of standard electron diffraction techniques to determine the structure of the overlayer and its orientation with respect to the substrate. Thus, high-resolution electron microscopy (HREM) was employed. Figure 3 in a cross-section high-resolution structure image of the IBD layer on the Cc-Sic substrate. The structure of the overlayer and the orientation relationship between the overlayer and substrate were determined from HREM micrographs using optical diffraction techniques on a laser optical diffractometer. Properly oriented optical diffraction patterns from the substrate and overlayer are shown in Figures 4(a) and 4(b), respectively. The substrate was oriented such that (1 l?O), was perpendicular to the plane of the image. The optical diffraction pattern obtained from the thin film is consistent with a cubic /J-Sic (110) pattern, indicating that the film grew with (1 lO),II(l IZO),. Thus, a film grew such that (1 lO),ll( 1 l?O), and the atoms were deposited such that { 11 l}pll {OOOI},. The P-Sic overlayer was completely epitaxical with respect to the a-Sic.
Figure 4. Properly oriented substrate
oriented and (b)
optical diffraction
patterns
from (a) (1 lzO),
(I lo), oriented IBD film.
A variety of defects were observed in the B-Sic film. The most common defects, identified by arrows in Figure 3, were small areas or boundaries which appeared amorphous in nature. The atomic stacking was disrupted in these regions but the lattice was continuous on either side of the boundary and throughout the film. As discussed earlier, the stoichiometry of the film was not 1Si : lC, but had an excess of -30% carbon. It is possible that the excess carbon causes the disruptions in the lattice. This is currently being examined in more detail: Other defects observed in the film include dislocations and twins. In fact, the optical diffraction pattern from the P-Sic in Figure 4(b) shows extra reflections due to a twin on (111). Conclusions
A /?-Sic overlayer has been deposited on an Cc-Sic substrate by bombardment with alternating low energy ion beams of C and Si. The substrate temperature during bombardment was 7OO”C, which represents the lowest reported temperature for the formation of P-Sic. While TEM micrographs and diffraction patterns conclusively showed the overlayer was /&Sic, ion beam channeling in the overlayer was not possible. Further work is under way to understand this result. Acknowledgements
Figure 3. High resolution structure arrows identify small amorphous-like
image of IBD layer on B-Sic. The boundaries in the D-Sic film.
The authors thank RF Davis and J Bumgamer at North Carolina State University for supplying the 6H Cc-Sic substrate crystals and L F Allard at Oak Ridge National Laboratory for help with the image simulations. This research was sponsored by the Division of Materials Sciences, U.S. Department of Energy and also by the Assistant Secretary for Conservation and Renewable Energy, Office of Transportation Systems, as part of the HighTemperature Materials Laboratory Program under contract DEAC05840R21400 with Martin Marietta Energy Systems, Inc. 1067
S P Withrow et a/: Ion beam deposition
of B-Sic layers onto n-Sic
substrates
References
’ H P Liaw and R F Davis, J Elec(rochrm Sue 132, 642 (1985). ‘Shigeo Kaneda, Yoshiki Sakamoto, Tadashi Mihara and Takao Tanaka, J Crysfal Gronrh 81, 536 (1987). ‘J W Palmour, H S Kong and R F Davis, J Appl Phys 64,2168 (1988). ‘J A Edmond, K Das and R F Davis. J Appl Phys 63,922 (1988). ‘C H Carter Jr, R F Davis and S R Nutt, J Muf Res 1, 811 (1986). ‘P Pirouz. C M Chorey and J A Powell, Appl Phys Leff 50,221 (1987). ’ W V Muench and I Pfaffeneder, Thin So/id Films 31, 39 (1976). ‘H S Kong J T Glass and R F Davis, Appl Phys Lerf 49, 1074 (1986). ‘H S Kong, B L Jiang. J T Glass, G A Rozgonyi and K L More, J Appl Phys 63, 2645 (1985). ‘“K L More, H S Kong, J T Glass and R F Davis, submitted to J Anwr Cur sot. ” H S Kong, J T Glass and R F Davis, J Appl Phys 64,2672 (1988). “B Wessels, H C Gatos and A F Witt, in Silicon Carbide-1973 (Edited
1068
by R C Marshall. J W Foust Jr and C E Ryan) (University of South Carolina Press, Columbia. SC, 1974) 144. “S Nishino, H Matsunami and T Tanaka, J Cry.trtr/ Growth 45, 144 (1978). I4 Hiroshi Yamada, J Appl Phys 65,2084 (1989). ” R A Zuhr, G D Alton, B R Appleton, N Herbots. T S Noggle and S J Pennycook, p 243 in Materials Modification und Growfh Using Ion Beams (Edited-by M J Gibson, A E White and P P Pronko). Materials Research Societv. Pittsbureh. PA. 1987; B R Appleton, S J Pennycook. R A Zuhr, N Herbots and ? S Noggle, Nuclear I~,~trumMethods B-19/20, 975 (1987). 16T E Haynes, R A Zuhr, S J Pennycook and B R Appleton, Appl P/IY.Y Lerr 54, 1439 (1989). “T E Haynes, R A Zuhr and S J Pennycook, in Advmws in Muferuis, Processina. and Devices in III-V Compound Semiconductors (Edited by D K Sad&a, L Eastman and D Dupuis), Materials Research Society. Pittsburgh, PA, 1989 (in press).
11 I1 2lpages
1065 to 1068/1989
Pergamon Press plc
Ion beam deposition substrates S P Withrow,
K L More,
0042-207X/89$3.00+.00
of P_SiC layers onto a-Sic
R A Zuhr and T E Haynes,
Oak Ridge National
Laboratory,
Oak Ridge,
TN 37831
USA
Thin films of B-Sic have been grown epitaxically on 6H u-Sic substrates by direct deposition of alternating layers of 13C+ and 3oSi+. The carbon and silicon ions were obtained from an ion implanter by decelerating mass-analyzed ion beams down to 40 eV. The substrate was held at 700°C during growth. Rutherford backscattering and high resolution transmission electron microscopy techniques have been used to determine the composition and structure of the resulting P-Sic layer and the nature of the interface. This is the first reported use of the ion beam deposition technique to grow P-Sic films and demonstrates the capability of using alternating beam deposition to prepare compound structures.
Introduction In recent years, the use of SIC for applications in high-temperature, high frequency, and high power electronic devices has emerged as a promising technology. There is particular interest in the application of B-Sic, the only cubic form of Sic, because its band gap, 2.39 eV, is the lowest of all the common Sic polytypes. Thin films of B-Sic grown using both chemical vapor deposition (CVD)’ and molecular beam epitaxy (MBE)’ techniques have been reported, and Sic devices have been successfully fabricated from these films3v4. However, both of these techniques require relatively high growth temperatures. Another problem has been the choice of an optimum substrate material for the growth of B-Sic. The most common substrate material has been (100) silicon. Unfortunately, as a result of the -8% difference in thermal expansion coefficients and an -20% difference in lattice parameters, the films contain a high density of defects, the most common of which are microtwins, stacking faults,5 and antiphase domain boundaries6. A number of candidate substrates for the deposition of improved B-Sic layers have been suggested by prior work. AlphaSIC, when used as a substrate material for the epitaxical growth of B-Sic, should result in the best film/substrate matching. In fact, the growth of B-Sic on Cc-Sic has been previously reported. Muench and Pfaffeneder’ described the deposition of epitaxical layers of /?-Sic on Cc-Sic substrates in the temperature range 1700-185O”C, and reported that the /3-Sic layers were generally polycrystalline. Beta-Sic thin films were grown on a-Sic substrates via CVD by Kong et al* in the temperature range 13601410°C. It was determined that the major defects observed in these films were double positioning boundariesg. Different microstructures were observed depending on the polarity of the (0001) growth face”. Alpha-Sic has also been used as a substrate to grow a-Sic layers’*’ ‘-I 3. These CVD and MBE growth experiments have been carried out at temperatures well over 1000°C. Recently, however, the heteroepitaxical growth of D-Sic on Si substrates held below 800°C using a reactive ion beam deposition technique has been reported 14.
An alternative method for growing thin layers is ion beam deposition (IBD) Is. Using this technique, a mass-analyzed, lowenergy ion beam of a desired species is directed onto a substrate. The energy is chosen low enough that the incident ions do not penetrate deeply into the substrate but rather deposit onto the surface. By alternating the ion species to maintain stoichiometry, it is possible to grow compound materials. Ion beam deposition of epitaxical GaAs on both silicon and germanium has been achieved in this manner’ 6*I’. The present work was initiated to explore the possibilities of growing the compound semiconductor SIC on 6H a-Sic substrates using the IBD technique. The results represent the lowest reported temperature for forming epitaxical B-Sic on any substrate material. Rutherford backscattering (RBS) and high resolution electron microscopy (HREM) techniques were used to characterize the resulting films. Experimental procedure The substrates were grown in an Acheson furnace used to make SIC grit for industrial applications. These crystals generally terminate with only one flat surface, either a silicon (0001) or carbon (OOOi) face. For this work, crystals with terminating silicon (0001) faces were utilized since previous results all indicate that /?-Sic crystals grown on this face are generally of a higher quality than on carbon-terminating substrates’s’2. To prepare a clean, smooth surface for the deposition of alternating carbon and silicon layers, the substrates were first oxidized at 1200°C in a flowing dry oxygen atmosphere for -90 min. The oxide was then removed with a 1: 1 HF-HNO, solution before mounting in the IBD chamber. The samples were further cleaned in situ by bombarding with 40-eV H+ ions, with the sample held at > 500°C. Base pressure in the IBD chamber was 5 x lo- lo T. The IBD apparatus and process have been detailed previously”. In the present work, “C and “Si ions were produced simultaneously in an accelerator ion source using a mixture of 13C-enriched CO2 and SiF, as source gases. The ions were then extracted at 35 keV, electromagnetically mass-analyzed, transported to the IBD chamber, and finally decelerated to
S P Withrow
et al:
Ion
beam
deposition
of P-Sic
layers
onto
?-Sic
substrates
40
eV before impinging on the substrate. Previous work on other systems suggests that energies in this range minimize damage in the substrate. The ion beam was switched under computer control between the carbon and silicon beams, providing alternating deposition of approximately one monolayer of each. Since the substrate terminated with a silicon (0001) face. the initial layer deposited was “C. Beam currents of -2 /tA of “C’ and - I /IA of 3oSi+ were used, providing an overlayer growth rate of approximately 0.1 nm/min. The sample temperature was held at 700°C during deposition by resistively heating the substrate holder. The temperature was periodically monitored using an infrared pyrometer. The deposited films were characterized using RBS and ion channeling of 2.0 MeV helium. The scattering angle was set at 160”. In these spectra, the scattering yield from the overlayer was easily separated from that of the substrate since heavy isotopes (“C and “Si) were used for the deposition. Transmission electron microscopy (TEM) was also used to determine the structure of the deposited layer. Cross-section TEM specimens were prepared by creating a “sandwich” with the IBD layers epoxied together face-to-face. Slices -300 ,um thick were then cut perpendicular to the interface with a low speed diamond saw. Each slice was then mechanically thinned to - 75 /lrn, dimpled across the center of the interface to - 20 pm, and finally ion milled using 6 keV Ar+ ions on a stage cooled with liquid nitrogen. The crosssection specimens were initially examined in a JEOL 2000FX transmission electron microscope operated at 200 kV. High-resolution crystal structure images were recorded in a JEOL 4000EX transmission electron microscope operated at 400 kV. All photographs were taken near Scherzer defocus (- - 50 nm) and a convergence angle of - 1 mr. These conditions were used for the image simulation calculations obtained using the MacTempas program, which were compared with the structure images obtained. Results and discussion Figure 1 shows backscattering spectra obtained from a sample which was grown by alternating 50 layers each of 13C and ‘“Si. The total dose for each atomic layer, integrated at the target, was
1
Virgin a-Sic, IBD 30Si+“C
0
axis 1
”
0.5
0.6
0.7
Figure 1. Rutherford backscattering deposited on an z-Sic substrate. 1066
0.8
0.9
ENERGY
(MeV)
spectra
1 .o
1.1
1.2
from /J-Sic layer ion beam
chosen to be 1.2x IO ” atoms/cm’, the atomic density for a (I I 1) /j-Sic layer. Part of the sample was masked during IBD so that spectra from a region without a deposited overlayer could be obtained. For the channeled spectra, the incident beam was aligned along the (0001) axis of this undeposited 6H r-Sic substrate Aligned spectra are shown for both an undeposited, virgin region as well as a region with an overlayer. In addition. a random spectrum is shown which was taken with the He+ beam incident on the sample at the same location as the channeled spectrum for the overlayer. The sample was continuously rotated while the random spectrum was bcin, m obtained. and the yield in this spectrum represents the yield that would be obtained if the sample were amorphous. The energies at which He backscatters from surface atoms of mass 12, 13,28, and 30 are marked in the figure. The “Si surface peak in the aligned, undepositcd spectrum can be seen at 1.14 MeV. Peaks corresponding to other isotopes of Si as well as to ’ 'C are not easily resolved for the statistics of the aligned spectra. The ratio of the yield in the undeposited spectra to that of the random spectrum at an energy just below the Si surface peak is 0.03. which indicates that the substrate has exccllent crystallinity. Backscattering from C and Si atoms in the deposited layer are easily resolved as peaks at energies corresponding to scattering from atoms with mass 13 and 30, respectively. The total 13C and “Si yields are essentially the same in both the aligned and random spectra, indicating that either the overlayer is not an undamaged single crystal or the incident beam is not aligned with a crystalline axis. Scattering from Si in the substrate at the interface occurs at a lower energy than the Si surface peak in the virgin spectrum due to energy loss of the He beam in the overlayer. The small increase in yield seen in the channeled spectrum between approximately I .05 and 1.15MeV, which has no corresponding yield in the virgin spectrum, indicates that the Si lattice has been slightly damaged during the IBD process. This damage extends approx. 80 nm into the substrate, as can be determined by making an energy-to-depth calibration using standard Hc stopping powers. The integrated scattering yields in the “Si and 13C peaks were quantified by comparison to a spectrum obtained from a virgin Si sample. The overlayer represents I .03 f 0.1 x 10 ’ ’ Si cm ’ and 1.38+0.15x IO” C cm-‘. This result shows the C:Si ratio in the deposited layer to be on the order of 1.3 : 1. For a beta overlayer, the Si amount calculated above would correspond to an overlayer thickness of 215 A. It should be noted, however, that backscattering spectra obtained from different spots on the sample indicated the D-Sic film was not uniform in thickness, varying by as much as 50%. Transmission electron microscopy analysis of the sample also showed a thickness non-uniformity. For the channeled spectra in Figure 1 the incident beam was aligned along the substrate (0001) axis. Additional spectra were obtained by tilting the sample up to 2’ away from the (0001) in an attempt to find a crystal axis in the overlayer. While small reductions (up to -5%) in the yield in the ‘“Si peak were observed, no well-defined channel was found in the overlayer. A low magnification TEM image of the resulting IBD layer is shown in Figure 2. The thickness of the layer was not uniform across the substrate surface ; the maximum thickness was found to bc - 20 nm. The RBS results in Figure I indicate the presence of a damage region down to - 80 nm below the substrate surface : however, as shown in Figure 2, there is no significant damage detected by TEM (e.g. no dislocations or dislocation loops) indicating that the damage is limited to point defects generated by the IBD process.
S P Withrow
et a/; Ion beam deposition
of P-Sic
layers onto cr-Sic substrates IBD Sic +
alpha-SiC substrate
220 l
111 0
002 l
0’ twin refleetlons
iii l
Figure 2. Low magnification TEM micrograph of IBD layer on j%SiC. a-SiC substrate was oriented with (1120) parallel to the electron beam.
b)
0 /
0
0 l l
l
IBD beta-Sic film
The deposited layer was too thin to permit use of standard electron diffraction techniques to determine the structure of the overlayer and its orientation with respect to the substrate. Thus, high-resolution electron microscopy (HREM) was employed. Figure 3 in a cross-section high-resolution structure image of the IBD layer on the Cc-Sic substrate. The structure of the overlayer and the orientation relationship between the overlayer and substrate were determined from HREM micrographs using optical diffraction techniques on a laser optical diffractometer. Properly oriented optical diffraction patterns from the substrate and overlayer are shown in Figures 4(a) and 4(b), respectively. The substrate was oriented such that (1 l?O), was perpendicular to the plane of the image. The optical diffraction pattern obtained from the thin film is consistent with a cubic /J-Sic (110) pattern, indicating that the film grew with (1 lO),II(l IZO),. Thus, a film grew such that (1 lO),ll( 1 l?O), and the atoms were deposited such that { 11 l}pll {OOOI},. The P-Sic overlayer was completely epitaxical with respect to the a-Sic.
Figure 4. Properly oriented substrate
oriented and (b)
optical diffraction
patterns
from (a) (1 lzO),
(I lo), oriented IBD film.
A variety of defects were observed in the B-Sic film. The most common defects, identified by arrows in Figure 3, were small areas or boundaries which appeared amorphous in nature. The atomic stacking was disrupted in these regions but the lattice was continuous on either side of the boundary and throughout the film. As discussed earlier, the stoichiometry of the film was not 1Si : lC, but had an excess of -30% carbon. It is possible that the excess carbon causes the disruptions in the lattice. This is currently being examined in more detail: Other defects observed in the film include dislocations and twins. In fact, the optical diffraction pattern from the P-Sic in Figure 4(b) shows extra reflections due to a twin on (111). Conclusions
A /?-Sic overlayer has been deposited on an Cc-Sic substrate by bombardment with alternating low energy ion beams of C and Si. The substrate temperature during bombardment was 7OO”C, which represents the lowest reported temperature for the formation of P-Sic. While TEM micrographs and diffraction patterns conclusively showed the overlayer was /&Sic, ion beam channeling in the overlayer was not possible. Further work is under way to understand this result. Acknowledgements
Figure 3. High resolution structure arrows identify small amorphous-like
image of IBD layer on B-Sic. The boundaries in the D-Sic film.
The authors thank RF Davis and J Bumgamer at North Carolina State University for supplying the 6H Cc-Sic substrate crystals and L F Allard at Oak Ridge National Laboratory for help with the image simulations. This research was sponsored by the Division of Materials Sciences, U.S. Department of Energy and also by the Assistant Secretary for Conservation and Renewable Energy, Office of Transportation Systems, as part of the HighTemperature Materials Laboratory Program under contract DEAC05840R21400 with Martin Marietta Energy Systems, Inc. 1067
S P Withrow et a/: Ion beam deposition
of B-Sic layers onto n-Sic
substrates
References
’ H P Liaw and R F Davis, J Elec(rochrm Sue 132, 642 (1985). ‘Shigeo Kaneda, Yoshiki Sakamoto, Tadashi Mihara and Takao Tanaka, J Crysfal Gronrh 81, 536 (1987). ‘J W Palmour, H S Kong and R F Davis, J Appl Phys 64,2168 (1988). ‘J A Edmond, K Das and R F Davis. J Appl Phys 63,922 (1988). ‘C H Carter Jr, R F Davis and S R Nutt, J Muf Res 1, 811 (1986). ‘P Pirouz. C M Chorey and J A Powell, Appl Phys Leff 50,221 (1987). ’ W V Muench and I Pfaffeneder, Thin So/id Films 31, 39 (1976). ‘H S Kong J T Glass and R F Davis, Appl Phys Lerf 49, 1074 (1986). ‘H S Kong, B L Jiang. J T Glass, G A Rozgonyi and K L More, J Appl Phys 63, 2645 (1985). ‘“K L More, H S Kong, J T Glass and R F Davis, submitted to J Anwr Cur sot. ” H S Kong, J T Glass and R F Davis, J Appl Phys 64,2672 (1988). “B Wessels, H C Gatos and A F Witt, in Silicon Carbide-1973 (Edited
1068
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