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Determination of growth mechanisms of MBE grown BaF2 on Si(100) by target angle dependence of RBS yields
Nuclear instruments
and Methods in Physics Research B 95 (1995) 319-322
Beam Interactions with Materials 6 Atoms EISEVIER
Determination
of growth mechanisms of MBE grown BaF, on Si( 100) by target angle dependence of RBS yields
Michael F. Stumborg
*,
a3 Tak K. Chu b, Noel A. Guardala b, Jack L. Price b, Francisco Santiago b
’ The Catholic University of America, Washington, D.C. 20064, USA h NaL:alSurface Warfare Center, White Oak Laboratory, Sill,er Spring, Md, 20903-5000, USA Received 4 April 1994; revised form received 31 October 1994
Abstract Rutherford backscattering spectroscopy (RBS) yields have been used to The angular dependence of 2.0 MeV 4He’+ determine the growth mechanism of epitaxial BaF, films grown on Si(100) substrates by molecular beam epitaxy (MBE). RBS yields from uniformly thick layers are characterized by a (cos 4)-l dependence, where C#Jis the angle between the incident beam and the target normal. Deviations from this relationship have been attributed to layers which are composed of islands, rather than films of uniform thickness. A series of BaF, films of increasing deposition time were examined in this way. The results of this analysis show that the BaF, at first grows in small islands which eventually coalesce into a uniform epitaxial layer.
1. Introduction RBS has been used for many years to study the properties of thin films. The thickness of a uniformly flat layer is determined by dividing the area1 density (g/cm2) by the bulk volume density (g/cm3) of the layer. The area1 density can be found from RBS by measuring the yield of backscattered ions and applying the relationship: Y= a(@)RN,N,, where Ni is the number of incident (projectile) ions, N, is the number of target atoms per cm2, 0 is the solid angle of the detector, and u(0) is the Rutherford cross section in cm’: u(e)
=
w2e2)2 (4-G2
l
sin4( 19/z) ’
where Z, and Z, are the atomic numbers of the projectile and target ions respectively, e’ = 1.44 X lo-r3 MeV cm, E, is the energy of the incident projectile ion in MeV, and # is the backscattered angle. During our investigation of barium fluoride films on silicon substrates, it was found that for 5, 10, and 4.5
* Corresponding author. Naval Surface Warfare Center, White Oak Detachment. Code 842, Silver Spring, MD 20903-50-00, USA. Tel. + 1 301 3941470. Fax + 1 301 3943179. 0168-583X/95/%09.50 0 1995 Elsevier Science B.V. All rights resewed SSDI 0168-583X(94)00545-1
minute depositions the thicknesses determined by RBS varied linearly with deposition time. The deposition rate was found to be 5.95 f 0.25 A per minute. For 1, 2, and 5 minute deposition times the thicknesses of 8, 25, and 29 A respectively, were not linear with deposition time indicating that the assumptions of uniformly flat films with bulk stoichiometry and density used in the thickness determinations were questionable. In addition, during X-ray photoelectron spectroscopy (XPS) analysis of the surface chemistry, the intensity of Ba 3d s,? photoelectron signals did not follow the self-attenuated exponential function expected of a uniform film i.e. [l - exp( - t/c)], where t is the thickness of the film, and c is the quotient of the deposition rate and the mean free path of the electrons. Because the sample thicknesses being measured are small, there is a large uncertainty in the RBS determinations. There is also some uncertainty in the constancy of the BaF, flux during deposition. The latter was not considered an important factor since the flux as monitored by ion gauge measurements did not show large fluctuations. While the nonlinear relationship between thickness and deposition time for the 1, 2, and 5 minute depositions could be attributed to large uncertainties in thickness measurements, it was decided that the alternative explanations of non-bulk BaFz density and three dimensional film morphology had to be examined. It should be pointed out that BaF, films have an extremely strong tendency to grow with a three dimensional growth mode in the (111) direction. This is characteristic of all of the group IIa fluorides because the
320
h4.F. Stumborg et al. / Nucl. Instr. and Meth. in Phys. Res. B 95 (1995) 319-322
free energy of the (1111 surface is much lower than the free energy of other surfaces [ 11. Therefore, it is likely that the BaF, actually grows in a three dimensional island mode on the silicon substrates which are (100) oriented, similar to the behavior of CaF, on silicon [l]. In order to investigate the possibility of island-like growth, angular dependent RBS was performed in the manner of Campisano, et al. [2]. In angular dependent RBS, the plane of the sample is oriented at an angle 4 to the incident beam. For a film of uniform thickness, the number of scattering centers (N,) encountered by a projectile ion will have a (cos 4)-’ dependence. If the yield at an angle 4 is normalized to the yield at 4 = 0, then it should follow this (cos 41-l dependence. If a film is not uniform in thickness and contains surface structures, a reduction in yield due to shadowing effects of the three dimensional geometry should be observed. This was demonstrated in ref. [2], where it was shown that the angular dependent RBS yield from an annealed lead film composed of islands fell below the (cos $)-’ curve. For the present study, only the barium RBS yields from the BaF, layers were used in the analysis because the barium signal is separated from that of the silicon substrate, while the fluorine yield is too small to be cleanly separated from the silicon substrate signal.
Grounded
Vacuum
Fig. 1. Schematic diagram of the scattering chamber. The sample is mounted on an electrically isolated target holder equipped with a rotatable feedthrough. The suppression cage is cylindrical in shape and rests on insulating legs on the chamber floor. The cage top is removable. The defining aperture is grounded to the chamber.
2.2. RBS experiments 2. Experiment
2.1. Film deposition BaF, films were prepared by MBE in a Vacuum Generator VG80H molecular beam deposition chamber. The single crystal Si(100) substrates were vacuum annealed at 900°C for 1 hour under a pressure of lower than 5.0 X 10 K9 mbar to rid them of their surface oxide. The absence of the oxide was confirmed by the chemical identification of surface species via XPS measurements. Results of the XPS study are still being analyzed and will be the subject of another report. The silicon substrates were then cooled and transferred in vacua to the growth chamber. During deposition the substrates were kept at 750°C. A BaF, flux was provided by a 1050°C effusion cell, 15 inches below the rotating substrate. The BaF, flux was monitored by an ion gauge before and after each deposition. Although an absolute value for the flux cannot be determined by this measurement, the ion gauge readings were consistent during each deposition, and from one deposition to the next. After deposition these films were cooled to room temperature over a 1 hour period and examined by reflective high energy electron diffraction (RHEED), and then transferred in vacua to an analysis chamber where they were examined by XPS. The films were then removed from the MBE chamber for X-ray diffraction analysis, and then examined by RBS for the purpose of the present investigation.
The experimental setup is shown schematically in Fig. 1. The 2 MeV 4He’+ projectile ions were produced in the RF Source of an NEC model 9SDH-2 Pelletron positive ion accelerator with a beam energy width of AE/E < 0.1%. The number of incident ions was determined by integrating the current of the ion beam. Suppression of secondary electrons created in the scattering process was necessary to insure accurate current integration and was accomplished by a wire mesh cage enclosing the target which was negatively biased to 1100 V [3,4]. Backscattered ions were detected by a surface barrier detector placed at a backscattered angle of 150”, and were energy analyzed by a ND6600 multichannel analyzer and computer. Current integration and electron suppression were, tested by measuring the angular dependent yield of a 500 A thick lead film vacuum evaporated on a silicon substrate. Tbe normalized yields from this film followed the (cos 41-i dependence in agreement with Ref. [2], verifying the accuracy of the current integration and secondary electron suppression. Angular dependent RBS yields of barium were then measured for the BaF, samples of 1, 2, 5, 10 and 45 minute depositions used in the standard RBS analysis: each at target angles of 0, 30, 60, 70, and 80”. Ten yield measurements were carried out at each angle for each sample. The charge collected for each of the ten yield measurement was 50 p,C (Ni = 3.1 X 1014 particles).
M.F. Stumborg et al./Nucl. Instr. and Meth. in Phys. Res. B 95 (1995) 319-322
321
3. Data analysis If the measured yields at each target angle are to be normalized to the 4 = 0 yield, one must assume that the factors on the right-hand side of Eq. (1) (other than N,) are independent of 4. Of the three factors involved, 0 is obviously independent of 4. In order for Ni to be independent of d, it must remain constant at every depth of the analyzed surface layer. This is of particular importance for angular dependent RBS since the “apparent depth” of many of the scattering centers increases as 4 increases. In our case, N, is approximately constant (and therefore independent of 4) at every depth for these film thicknesses because the number of projectiles scattered per monolayer is given by the product of (T( 0) and N,, typically 1O-‘4 cm’ X 1015 atoms/cm’/monolayer [5]. Since the thickness of these films is less than 100 monolayers, the total change in N, is N lo-’ particles, this is insignificant compared to the total N, of 3.1 X lOI particles. The Rutherford cross section however, does not remain constant in general because E, decreases as a function of depth due to the electronic stopping power S, of the (target) atoms. A corrected cross section, a(@),, can be determined by dividing the film into a large number of sublayers, calculating Eocn, at each of the n sublayers, and then calculating c~(0), = (&,/E,,,>%(0) for each sublayer. This change in a(0) leads to a correction factor, F, for the yield which is always less than unity. This analysis was undertaken using published values of the stopping power [6]. For the lead calibration experiment, this factor reached a minimum of F = 0.989 at (p = 80”. The correction factors for the BaF, films were all greater than F = 0.996, a value insignificant when compared to experimental uncertainties. We can, therefore, safely conclude that a(0) is constant for the present investigation. The normalized yields of the BaF, films are shown in Fig. 2. The results indicate that for the BaF, film of 5 minute deposition time, the (cos &I-’ dependence is followed closely. Films of deposition times of 10 and 45 minutes exhibited the same behavior, and are not shown here. For shorter times of deposition there are significant deviations from the (cos 41-l dependence. These deviations can be attributed to the shadowing effect of the three dimensional structures on the surface of the deposited films. Conceptually, deviation from the (cos I$-’ behavior must go through a maximum as the thickness-to-area ratio of the surface structure varies from zero to infinity. For the films studied here a large thickness-to-area ratio is highly unlikely because of the small rate of deposition. Hence, the mode of growth of the surface structure must have started with small islands. The growth can then proceed either by a build-up of the islands (i.e. increasing the thickness-to-area ratio), or by an island-plus-monolayer mode. In the latter mode, the islands coalesce laterally while their thickness increases. This mode is favored by
65~
. .
5.0
minute
deposition
2.0
minute
deposition
.
1 .O minute
deposition
4
3 2 1 I_
OL 0
-1-I-.1
10
20
30 Target
40
50
Angle
(degrees)
60
70
80
90
Fig. 2. Normalized RBS yields of barium from BaFz films. The solid line is the (cos Cp)Y’ line. Error bars for the 5 minute film represent one standard deviation. The 1 and 2 minute films have error bars of the same magnitude.
the results of Fig. 2, where the deviation from the (cos 41-l behavior is largest for the film of one minute deposition. A further test for this conclusion is provided for by subtracting the yield of the film of one minute deposition from that of the two minute deposition, as shown in Fig. 3. The good agreement with the (cos 41-l dependence can be explained by a growth mode where the one minute film is composed mostly of islands, and the two minute film is composed of a uniform underlayer plus islands on top. Furthermore it may be interpreted that the islands on the top of the two minute film are similar in size and structure to those of the one minute film.
6
I f
,,i’/ OL0
_A.-
10
20
30 Target
60
40
50
Angle
(degrees)
70
80
90
Fig. 3. Subtraction of the yield of the one minute film from that of the two minute film. The solid line is the (cos I#J-’ line. Error bars represent one standard deviation.
322
M.F. Stumborg et al. / Nucl. Instr. and Meth. in Phys. Rex B 95 (I 995) 319-322
4. Discussion and conclusion In conclusion, deposition of BaF, on Si(100) proceeds in a fashion whereby the BaF, at first condenses into three dimensional islands in the first minute of deposition. Subsequent addition of material is then distributed evenly over the substrate and existing islands resulting in a uniform layer of material with islands on top. The magnitude of the shadowing effect of these islands becomes smaller as the contribution to the yield from the two dimensional portion of the film increases. After approximately five minutes of deposition any reduction in yield due to shadowing effects is such a small portion of the total signal that the angular dependent RBS yields begin to follow the (cos 41-l behavior of a uniformly flat film. RHEED studies showed the films of deposition time greater than 5 minutes were epitaxial, and X-ray diffraction showed all films to be oriented in the (111) direction. Hence, it is concluded that the individual islands are aligned in a preferential direction before they coalesce to completely cover the substrate. This is a reasonable conclusion, even though an actual proof is beyond the scope of the present investigation. The analysis assumes the constancy of Ni and ~(0) for each measured yield. While these two quantities are not strictly constant, the errors were found to be insignificant for the cases studied here. Improvements in experimental uncertainties and/or examination of films with high stop-
ping powers may cause the error to become significant. For this reason, these assumptions should be tested on a case-by-case basis in any future study.
Acknowledgements The authors wish to thank Dr. David J. Land of NSWC/White Oak for the computer code used in the RBS analysis, and Mr. Pat Cady and Mr. Bill Freeman (also of NSWC/White Oak) for their help and advice in designing and constructing the experimental apparatus. This work was supported in part by the Independent Research program at NSWC/White Oak.
References [l] L.J. Schowalter and R.W. Fathauer, in: CRC Critical Reviews on Solid State and Materials Sciences, vol. 15 (4) (1989) pp. 367-421. [2] S.U. Campisano, G. Ciavola, E. Costanzo, G. Foti and E. Rimini, Nucl. Instr. and Meth. 149 (1978) 229. [3] S. Matteson and M.A. Nicolet, Nucl. Instr. and Meth. 160 (1979) 301. [4] H.M. Loebenstein, D.W. Mingay and C.S. Zaidins, Nucl. Instr. and Meth. 33 (1965) 175. [5] W.K. Chu, J.W. Meyer and M.A. Nicolet, Backscattering Spectrometry (Academic Press, New York, 1986) p. 31. [6] J.F. Ziegler, Helium: Stopping Power and Ranges in all Elements (Pergamon, New York, 1977) pp. 46 and 53.
and Methods in Physics Research B 95 (1995) 319-322
Beam Interactions with Materials 6 Atoms EISEVIER
Determination
of growth mechanisms of MBE grown BaF, on Si( 100) by target angle dependence of RBS yields
Michael F. Stumborg
*,
a3 Tak K. Chu b, Noel A. Guardala b, Jack L. Price b, Francisco Santiago b
’ The Catholic University of America, Washington, D.C. 20064, USA h NaL:alSurface Warfare Center, White Oak Laboratory, Sill,er Spring, Md, 20903-5000, USA Received 4 April 1994; revised form received 31 October 1994
Abstract Rutherford backscattering spectroscopy (RBS) yields have been used to The angular dependence of 2.0 MeV 4He’+ determine the growth mechanism of epitaxial BaF, films grown on Si(100) substrates by molecular beam epitaxy (MBE). RBS yields from uniformly thick layers are characterized by a (cos 4)-l dependence, where C#Jis the angle between the incident beam and the target normal. Deviations from this relationship have been attributed to layers which are composed of islands, rather than films of uniform thickness. A series of BaF, films of increasing deposition time were examined in this way. The results of this analysis show that the BaF, at first grows in small islands which eventually coalesce into a uniform epitaxial layer.
1. Introduction RBS has been used for many years to study the properties of thin films. The thickness of a uniformly flat layer is determined by dividing the area1 density (g/cm2) by the bulk volume density (g/cm3) of the layer. The area1 density can be found from RBS by measuring the yield of backscattered ions and applying the relationship: Y= a(@)RN,N,, where Ni is the number of incident (projectile) ions, N, is the number of target atoms per cm2, 0 is the solid angle of the detector, and u(0) is the Rutherford cross section in cm’: u(e)
=
w2e2)2 (4-G2
l
sin4( 19/z) ’
where Z, and Z, are the atomic numbers of the projectile and target ions respectively, e’ = 1.44 X lo-r3 MeV cm, E, is the energy of the incident projectile ion in MeV, and # is the backscattered angle. During our investigation of barium fluoride films on silicon substrates, it was found that for 5, 10, and 4.5
* Corresponding author. Naval Surface Warfare Center, White Oak Detachment. Code 842, Silver Spring, MD 20903-50-00, USA. Tel. + 1 301 3941470. Fax + 1 301 3943179. 0168-583X/95/%09.50 0 1995 Elsevier Science B.V. All rights resewed SSDI 0168-583X(94)00545-1
minute depositions the thicknesses determined by RBS varied linearly with deposition time. The deposition rate was found to be 5.95 f 0.25 A per minute. For 1, 2, and 5 minute deposition times the thicknesses of 8, 25, and 29 A respectively, were not linear with deposition time indicating that the assumptions of uniformly flat films with bulk stoichiometry and density used in the thickness determinations were questionable. In addition, during X-ray photoelectron spectroscopy (XPS) analysis of the surface chemistry, the intensity of Ba 3d s,? photoelectron signals did not follow the self-attenuated exponential function expected of a uniform film i.e. [l - exp( - t/c)], where t is the thickness of the film, and c is the quotient of the deposition rate and the mean free path of the electrons. Because the sample thicknesses being measured are small, there is a large uncertainty in the RBS determinations. There is also some uncertainty in the constancy of the BaF, flux during deposition. The latter was not considered an important factor since the flux as monitored by ion gauge measurements did not show large fluctuations. While the nonlinear relationship between thickness and deposition time for the 1, 2, and 5 minute depositions could be attributed to large uncertainties in thickness measurements, it was decided that the alternative explanations of non-bulk BaFz density and three dimensional film morphology had to be examined. It should be pointed out that BaF, films have an extremely strong tendency to grow with a three dimensional growth mode in the (111) direction. This is characteristic of all of the group IIa fluorides because the
320
h4.F. Stumborg et al. / Nucl. Instr. and Meth. in Phys. Res. B 95 (1995) 319-322
free energy of the (1111 surface is much lower than the free energy of other surfaces [ 11. Therefore, it is likely that the BaF, actually grows in a three dimensional island mode on the silicon substrates which are (100) oriented, similar to the behavior of CaF, on silicon [l]. In order to investigate the possibility of island-like growth, angular dependent RBS was performed in the manner of Campisano, et al. [2]. In angular dependent RBS, the plane of the sample is oriented at an angle 4 to the incident beam. For a film of uniform thickness, the number of scattering centers (N,) encountered by a projectile ion will have a (cos 4)-’ dependence. If the yield at an angle 4 is normalized to the yield at 4 = 0, then it should follow this (cos 41-l dependence. If a film is not uniform in thickness and contains surface structures, a reduction in yield due to shadowing effects of the three dimensional geometry should be observed. This was demonstrated in ref. [2], where it was shown that the angular dependent RBS yield from an annealed lead film composed of islands fell below the (cos $)-’ curve. For the present study, only the barium RBS yields from the BaF, layers were used in the analysis because the barium signal is separated from that of the silicon substrate, while the fluorine yield is too small to be cleanly separated from the silicon substrate signal.
Grounded
Vacuum
Fig. 1. Schematic diagram of the scattering chamber. The sample is mounted on an electrically isolated target holder equipped with a rotatable feedthrough. The suppression cage is cylindrical in shape and rests on insulating legs on the chamber floor. The cage top is removable. The defining aperture is grounded to the chamber.
2.2. RBS experiments 2. Experiment
2.1. Film deposition BaF, films were prepared by MBE in a Vacuum Generator VG80H molecular beam deposition chamber. The single crystal Si(100) substrates were vacuum annealed at 900°C for 1 hour under a pressure of lower than 5.0 X 10 K9 mbar to rid them of their surface oxide. The absence of the oxide was confirmed by the chemical identification of surface species via XPS measurements. Results of the XPS study are still being analyzed and will be the subject of another report. The silicon substrates were then cooled and transferred in vacua to the growth chamber. During deposition the substrates were kept at 750°C. A BaF, flux was provided by a 1050°C effusion cell, 15 inches below the rotating substrate. The BaF, flux was monitored by an ion gauge before and after each deposition. Although an absolute value for the flux cannot be determined by this measurement, the ion gauge readings were consistent during each deposition, and from one deposition to the next. After deposition these films were cooled to room temperature over a 1 hour period and examined by reflective high energy electron diffraction (RHEED), and then transferred in vacua to an analysis chamber where they were examined by XPS. The films were then removed from the MBE chamber for X-ray diffraction analysis, and then examined by RBS for the purpose of the present investigation.
The experimental setup is shown schematically in Fig. 1. The 2 MeV 4He’+ projectile ions were produced in the RF Source of an NEC model 9SDH-2 Pelletron positive ion accelerator with a beam energy width of AE/E < 0.1%. The number of incident ions was determined by integrating the current of the ion beam. Suppression of secondary electrons created in the scattering process was necessary to insure accurate current integration and was accomplished by a wire mesh cage enclosing the target which was negatively biased to 1100 V [3,4]. Backscattered ions were detected by a surface barrier detector placed at a backscattered angle of 150”, and were energy analyzed by a ND6600 multichannel analyzer and computer. Current integration and electron suppression were, tested by measuring the angular dependent yield of a 500 A thick lead film vacuum evaporated on a silicon substrate. Tbe normalized yields from this film followed the (cos 41-i dependence in agreement with Ref. [2], verifying the accuracy of the current integration and secondary electron suppression. Angular dependent RBS yields of barium were then measured for the BaF, samples of 1, 2, 5, 10 and 45 minute depositions used in the standard RBS analysis: each at target angles of 0, 30, 60, 70, and 80”. Ten yield measurements were carried out at each angle for each sample. The charge collected for each of the ten yield measurement was 50 p,C (Ni = 3.1 X 1014 particles).
M.F. Stumborg et al./Nucl. Instr. and Meth. in Phys. Res. B 95 (1995) 319-322
321
3. Data analysis If the measured yields at each target angle are to be normalized to the 4 = 0 yield, one must assume that the factors on the right-hand side of Eq. (1) (other than N,) are independent of 4. Of the three factors involved, 0 is obviously independent of 4. In order for Ni to be independent of d, it must remain constant at every depth of the analyzed surface layer. This is of particular importance for angular dependent RBS since the “apparent depth” of many of the scattering centers increases as 4 increases. In our case, N, is approximately constant (and therefore independent of 4) at every depth for these film thicknesses because the number of projectiles scattered per monolayer is given by the product of (T( 0) and N,, typically 1O-‘4 cm’ X 1015 atoms/cm’/monolayer [5]. Since the thickness of these films is less than 100 monolayers, the total change in N, is N lo-’ particles, this is insignificant compared to the total N, of 3.1 X lOI particles. The Rutherford cross section however, does not remain constant in general because E, decreases as a function of depth due to the electronic stopping power S, of the (target) atoms. A corrected cross section, a(@),, can be determined by dividing the film into a large number of sublayers, calculating Eocn, at each of the n sublayers, and then calculating c~(0), = (&,/E,,,>%(0) for each sublayer. This change in a(0) leads to a correction factor, F, for the yield which is always less than unity. This analysis was undertaken using published values of the stopping power [6]. For the lead calibration experiment, this factor reached a minimum of F = 0.989 at (p = 80”. The correction factors for the BaF, films were all greater than F = 0.996, a value insignificant when compared to experimental uncertainties. We can, therefore, safely conclude that a(0) is constant for the present investigation. The normalized yields of the BaF, films are shown in Fig. 2. The results indicate that for the BaF, film of 5 minute deposition time, the (cos &I-’ dependence is followed closely. Films of deposition times of 10 and 45 minutes exhibited the same behavior, and are not shown here. For shorter times of deposition there are significant deviations from the (cos 41-l dependence. These deviations can be attributed to the shadowing effect of the three dimensional structures on the surface of the deposited films. Conceptually, deviation from the (cos I$-’ behavior must go through a maximum as the thickness-to-area ratio of the surface structure varies from zero to infinity. For the films studied here a large thickness-to-area ratio is highly unlikely because of the small rate of deposition. Hence, the mode of growth of the surface structure must have started with small islands. The growth can then proceed either by a build-up of the islands (i.e. increasing the thickness-to-area ratio), or by an island-plus-monolayer mode. In the latter mode, the islands coalesce laterally while their thickness increases. This mode is favored by
65~
. .
5.0
minute
deposition
2.0
minute
deposition
.
1 .O minute
deposition
4
3 2 1 I_
OL 0
-1-I-.1
10
20
30 Target
40
50
Angle
(degrees)
60
70
80
90
Fig. 2. Normalized RBS yields of barium from BaFz films. The solid line is the (cos Cp)Y’ line. Error bars for the 5 minute film represent one standard deviation. The 1 and 2 minute films have error bars of the same magnitude.
the results of Fig. 2, where the deviation from the (cos 41-l behavior is largest for the film of one minute deposition. A further test for this conclusion is provided for by subtracting the yield of the film of one minute deposition from that of the two minute deposition, as shown in Fig. 3. The good agreement with the (cos 41-l dependence can be explained by a growth mode where the one minute film is composed mostly of islands, and the two minute film is composed of a uniform underlayer plus islands on top. Furthermore it may be interpreted that the islands on the top of the two minute film are similar in size and structure to those of the one minute film.
6
I f
,,i’/ OL0
_A.-
10
20
30 Target
60
40
50
Angle
(degrees)
70
80
90
Fig. 3. Subtraction of the yield of the one minute film from that of the two minute film. The solid line is the (cos I#J-’ line. Error bars represent one standard deviation.
322
M.F. Stumborg et al. / Nucl. Instr. and Meth. in Phys. Rex B 95 (I 995) 319-322
4. Discussion and conclusion In conclusion, deposition of BaF, on Si(100) proceeds in a fashion whereby the BaF, at first condenses into three dimensional islands in the first minute of deposition. Subsequent addition of material is then distributed evenly over the substrate and existing islands resulting in a uniform layer of material with islands on top. The magnitude of the shadowing effect of these islands becomes smaller as the contribution to the yield from the two dimensional portion of the film increases. After approximately five minutes of deposition any reduction in yield due to shadowing effects is such a small portion of the total signal that the angular dependent RBS yields begin to follow the (cos 41-l behavior of a uniformly flat film. RHEED studies showed the films of deposition time greater than 5 minutes were epitaxial, and X-ray diffraction showed all films to be oriented in the (111) direction. Hence, it is concluded that the individual islands are aligned in a preferential direction before they coalesce to completely cover the substrate. This is a reasonable conclusion, even though an actual proof is beyond the scope of the present investigation. The analysis assumes the constancy of Ni and ~(0) for each measured yield. While these two quantities are not strictly constant, the errors were found to be insignificant for the cases studied here. Improvements in experimental uncertainties and/or examination of films with high stop-
ping powers may cause the error to become significant. For this reason, these assumptions should be tested on a case-by-case basis in any future study.
Acknowledgements The authors wish to thank Dr. David J. Land of NSWC/White Oak for the computer code used in the RBS analysis, and Mr. Pat Cady and Mr. Bill Freeman (also of NSWC/White Oak) for their help and advice in designing and constructing the experimental apparatus. This work was supported in part by the Independent Research program at NSWC/White Oak.
References [l] L.J. Schowalter and R.W. Fathauer, in: CRC Critical Reviews on Solid State and Materials Sciences, vol. 15 (4) (1989) pp. 367-421. [2] S.U. Campisano, G. Ciavola, E. Costanzo, G. Foti and E. Rimini, Nucl. Instr. and Meth. 149 (1978) 229. [3] S. Matteson and M.A. Nicolet, Nucl. Instr. and Meth. 160 (1979) 301. [4] H.M. Loebenstein, D.W. Mingay and C.S. Zaidins, Nucl. Instr. and Meth. 33 (1965) 175. [5] W.K. Chu, J.W. Meyer and M.A. Nicolet, Backscattering Spectrometry (Academic Press, New York, 1986) p. 31. [6] J.F. Ziegler, Helium: Stopping Power and Ranges in all Elements (Pergamon, New York, 1977) pp. 46 and 53.