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Structure of the reconstructed domains on a high density stepped W(001) surface
Surface Science 122 (1982) L635-L641 North-Holland Publishing Company
L635
SURFACE SCIENCE LETTERS STRUCTURE OF THE RECONSTRUCTED DOMAINS ON A HIGH DEI~SITY STEPPED W(001) SURFACE * G.-C. WANG Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, USA
and T.-M. LU ** Department of Metallurgical and Mineral Engineering and Materials Science Center, University of Wisconsin, Madison, Wisconsin 53706, USA Received 13 July 1982
We have observed, using low-energy electron diffraction, the reconstructed (vf2 X V~-)R45° structure on a high density stepped W(001) surface ( - 28 A average terrace width) cooled below room temperature. The reconstructed domains are preferentially oriented with the zig-zag displacements of the surface atoms perpendicular to the step edge which is parallel to the [110] direction. The step edges are found to inhibit the reconstruction for a considerably smaller range than reported from previous LEED observations.
A clean W(001) crystal surface reconstructs to a (v~-× v~)R45 ° structure [1-4]. Detailed low-energy electron diffraction (LEED) studies by Debe and King [5] (DK) have shown that reconstruction within two types of rotationally equivalent domains, each possessing a p2mg space group symmetry, occurs during the reconstruction. Based on this symmetry, DK proposed [5] a zig-zag model in which the displacements of the surface atoms are parallel to the (110) directions. Also, they suggested a 20 .~ long range inhibition of the reconstruction due to each step edge (total of - 40 A unreconstructed region on a terrace). In this Letter we report the first LEED observation of the reconstruction on a high density stepped W(001) surface ( - 2 8 ,A average terrace width) prepared so that the step edges are parallel to the [110] direction. The (h/2, h/2) beam intensity is observed to be much higher than that of the (h/2, h/2) * Research sponsored b y the Division of Materials Sciences, US Department of Energy under contract W-7405-eng-26 with Union Carbide Corporation. ** Supported by National Science Foundation grant NSFDNR78-25754.
0039-6028/82/0000-0000/$02.75 © 1982 North-Holland
L636
G.-C. Wang, T.- M. Lu / Structure of reconstructed domains on W(O01)
beam. This implies that the reconstructed domains are preferentially oriented. This observation is consistent with the lateral zig-zag model [5] proposed by DK. Furthermore, we determined that the preferentially nucleated domains are those in which the zig-zag displacements of the surface atoms are perpendicular to the step edges. From the measurements of the two-dimensional angular distribution of the half-order beam intensities, we have deduced the size and shape of both types of rotationally equivalent domains. It is shown quantitatively that the reconstruction occurs within a distance much closer to the step edges than suggested by D K [5]. The W(001) sample was cut 3.25 ° + 0.25 ° off the (001) orientation with the step edges parallel to the [110] direction. The sample could be cooled to 170 K within 3 min after flashing to 2500 K. The diffraction beam intensity was collected using a Faraday cup with a 0.5 mm aperture. The angular distribution of an intensity profile was obtained by scanning the Faraday cup through a diffracted beam at a fixed azimuthal angle and the intensity versus polar angle was plotted on an X - Y recorder. Thus, by measuring the intensity profile of a diffracted beam at different azimuthal angles, we obtained the two-dimensional intensity distribution of a beam. All LEED measurements were made using conditions of normal incidence to the (001). Normal incidence was determined within 0.25 ° by adjusting the crystal orientation until the four (20) beams showed almost identical intensity versus energy ( 1 - V ) profiles between 90 and 120 eV where they exhibited no apparent beam splitting. The upper part of fig. 1 is the schematic of a LEED pattern obtained from the crystal cooled to a temperature of 170 K. In addition to the characteristic splitting of the (11} integral order beams, distinct half-order diffracted beams, which decay rapidly above room temperature, were observed from this surface with a high density of steps. The I - V profiles of the (½ ½) beams are identical to that obtained by D K [5]. This superlattice formation is interpreted as due to the clean surface reconstruction. Results of recent field-ion microscopy expoeriments also indicated that reconstruction can occur on a tungsten tip < 35 A in diameter [4]. In contrast, no reconstruction was observed for terraces < 40 wide for cleaved semiconductor surfaces such as Si(111)-(2 x 1) and Ge(111)-(2 x 1) structures [6,7]. The lower part of fig. 1 shows the 1-V profiles for the (-~ ½) and (½ ½) beams. The intensity of the ({ ½) beam is approximately 4.5 times_greater than_ _ that of the (½ ½) beam. Similar results were obtained for (½ 1) and (½ ½) beams. According to DK's model, the ( h / 2 , h/2) and (h/2, h/2) beams result from diffraction from two rotationally equivalent domains with the zig-zag movements of the surface atoms perpendicular (type A, see fig. 2a) and parallel (type B, see fig. 2b) to the step edge respectively. Since the intensity of the (½ ½) beam is much higher than that of the (½ ½) beam, we conclude that type A domains have been preferentially nucleated. This observation is in contrast to the case of hydrogen-induced c(2 x 2) reconstruction on a stepped W(001)
G.-C Wang, T.-M. l_,u / Structure of reconstructed domains on W(O01)
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G.-C. Wang, T..M. Lu / Structure of reconstructed domains on W(O01)
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Fig. 2. The top-layer zig-zag model for the two (V/2 x v~-)R45 ° rotational domains having p2mg symmetry on the W(001) surface. The dots represent the bulk lattice and the open circles represent the displaced atomic positions.
where the displacements of the W atoms along the step edge is more favorable [8]. (In that case the step edges and the atomic displacements are all in the [010].) The origin of the difference between the two cases is not clear at the present time. A scan of diffracted intensity versus polar angle along the (00)-* (11)
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G.-C. Wang, T.- M. Lu / Structure of reconstructed domains on W(O01)
L639
diagonal direction (refers to the A" in fig. 4a) for an incident beam energy of 110 eV is shown in fig. 3a. The splitting of the integra| order beam is very sharp and the angular separation of 2.8 + 0.2 ° implies an average terrace width of 28 + 2 A. The half-order beams do not split [5] but show uniform broadening for all energies. Fig. 3b is a summary of the measured angular profiles plotted in reciprocal space. This is a two-dimensional cut along the (00) -0 (I 1) direction of the diffracted intensity distribution in the three-dimensional reciprocal space. The width of the (½ ½) rod in fig. 3b represents the full width at half maximum (FWHM) of the angular profile after removing the instrumental broadening effect. (We have made a best estimate of the amount of broadening of the (½ ½) beam due to the instrumental effect.) The absence of splitting can be interpreted as due to the existence of a random antiphase relationship among the domains nucleated at random positions on different terraces [9]. As a result, different domains (even if they have the same type of orientation) give negligible interference effect and the angular distribution of the half-order beam intensity can be regarded as the sum of the intensity scattered from the individual domains [9]. The width of the half-order beams is then a reflection of the size of the domains in the direction under consideration. From the width of the (-i2 ½) beam, we have estimated the average domain size for type A domains measured in the [110] direction to be between 22 + 2 and 24 + 2 ,~ [10] (determined by the lower and upper limits of the measured instrument response). As mentioned before, the splitting of the integral order beam is very sharp and the individual split profiles give no measurable broadening other than the instrumental effect. If one assumes that the spread of the terrace width distribution is negligibly small, then from the above information on the estimates of the reconstructed domain size, the unreconstructed area on the terrace would be less than 8 A wide [11]. In order to study in more detail the shape of the reconstructed domains for bo~h orientations, the two-dimensional angular distribution of the (~ ½) and (½ ½) beam intensities were measured. Fig. 4a shows schematically the diffracted beam shapes at E = 68 eV. The FWHM's of the (½ ½) and (½ ½) beams measured in the X direction are approximately the same and the sizes of both types of domains in the direction perpendicular to the step edge agree with the results obtained at E--- 114 eV for the (½ ½) beam. The F W H M of the (~ ½) beam measured in the ~r direction is narrower than that measured in the X direction, and the size of type A domains parallel to the [110] step edge is estimated to be twice as large as the size perpendicular to the step edge. On the other hand, the (½ ½) beam has approximately the same size in both ~" and ~" directions and appeared to be rounded, an indication that type B domains are more or less rounded. A possible arrangement of the two domains is shown in fig, 4b. The arrows indicate the direction o f the zig-zag movements of the surface atoms to form the domains.
G.-C. Wang, T.- M. Lu / Structure of reconstructed domains on W(O01)
L640
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Fig. 4. (a) A schematic Of the measured two-dimensional angular distribution of intensity in a LEED pattern at E = 68 eV. (b) A schematic of a possible arrangement of the reconstructed domains. A and B denote type A (fig. 2a) and type B (fig. 2b) domains. Arrows indicate the directions of atomic displacements from bulk lattice positions.
In conclusion, our observation of the preferential nucleation of one type of orientational domain on a stepped W(001) surface, with edges parallel to the [110], is consistent with DK's zig-zag model of the reconstruction, and these domains are those in which the zig-zag displacemen.t of the surface atoms is perpendicular to the step edge. The total unreconstructed area on a terrace is
G.-C. Wang, T.-M. Lu / Structure of reconstructed domains on W(O01)
L641
shown to be - 8 ~,, which is considerably smaller than the value ( - 40 A) suggested by DK. We acknowledge E. Bauer, D.A. King, P.J. Estrup, A.J. Melmed, L.D. Roelofs, and the Surface Physics Group at ORNL for invaluable discussions. We also thank J.F. Barhorst and G.W. Ownby for crystal preparation.
References [1] [2] [3] [4] [5] [6] [7] [8]
[9] [10]
[11]
Y. Yonehara and L.D. Schmidt, Surface Sci. 25 (1971) 238. M.K. Debe and D.A. King, J. Phys. CI0 (1977) L303. T.E. Felter, R.A. Barker and P.J. Estrup, Phys. Rev. Letters 38 (1977) 1138. A.J. Melmed, R.T. Tung, W.R. Graham and G.D.W. Smith, Phys. Rev. Letters 43 (1979) 1521. M.K. Debe and D.A. King, Phys. Rev. Letters 39 (1977) 708; Surface Sci. 81 (1979) 193; J.A. Walker, M.K. Debe and D.A. King, Surface Sci. 104 (1981) 405. J.E. Rowe, S.B. Christman and H. Ibach, Phys. Rev. Letters 34 (1975) 874. M. Henzler, Surface Sci. 19 (1970) 159. R.A. Barker and P.J. Estrup, J. Chem. Phys. 74 (1981) 1442. T.-M. Lu and G.-C. Wang, Surface Sci. 107 (1981) 139; T.-M. Lu, L.-H. Zhao, M.G. Lagally, G.-C. Wang and J.E. Houston, to be published. Due to the large (2.9+0.2 °) domain size broadening, the uncertainty of the instrument response used does not affect the domain size determination very much. For instance, at E = I I 4 eV, the width of the measured profile is 2.9+0.2 °. If one takes the instrument response width of the (11) beam (which is 1.5 ° at E = 114 eV) as the upper limit of the response width of the (~ ½~ beam at that energy, then the upper limit of the domain size can be calculated to be 2 4 + 2 A. On the other hand, if one takes hte lower limit of the instrument response width of the (-i2 ½) beam to be 1° (which presumably underestimates its value), then the lower limit of the domain size can be estimated to be 22 + 2 A. Rigorously speaking, one has to consider the possibility of the existence of a finite spread in the terrace width distribution (J.E. Houston and R.L. Park, Surface Sci. 26 (1971) 269). We have estimated that the spread should not be larger than + 15 A around the average value of 28 A. A spread larger than + 15 A would result in broadening of the individual split beams that would become observable with.the resolving capability of our instrument. If the terrace width does spread to within + 15 A and the reconstruction occurs only on the few larger terraces, then the unreconstructed region in each terrace may be twice as large as the estimated value of 8 A. (If the same analysis is applied, to refs. [6] and [7], then the reconstruction may be inhibited on terraces as large as 65 A in those semiconductor cases.) However, due to the observed strong half-order beam intensity, it seems inconceivable that only the larger terraces are involved in the reconstruction. The ratio of our (~ ½) beam intensity compared to the (I1) beam is as large as that reported by D K [5].
L635
SURFACE SCIENCE LETTERS STRUCTURE OF THE RECONSTRUCTED DOMAINS ON A HIGH DEI~SITY STEPPED W(001) SURFACE * G.-C. WANG Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, USA
and T.-M. LU ** Department of Metallurgical and Mineral Engineering and Materials Science Center, University of Wisconsin, Madison, Wisconsin 53706, USA Received 13 July 1982
We have observed, using low-energy electron diffraction, the reconstructed (vf2 X V~-)R45° structure on a high density stepped W(001) surface ( - 28 A average terrace width) cooled below room temperature. The reconstructed domains are preferentially oriented with the zig-zag displacements of the surface atoms perpendicular to the step edge which is parallel to the [110] direction. The step edges are found to inhibit the reconstruction for a considerably smaller range than reported from previous LEED observations.
A clean W(001) crystal surface reconstructs to a (v~-× v~)R45 ° structure [1-4]. Detailed low-energy electron diffraction (LEED) studies by Debe and King [5] (DK) have shown that reconstruction within two types of rotationally equivalent domains, each possessing a p2mg space group symmetry, occurs during the reconstruction. Based on this symmetry, DK proposed [5] a zig-zag model in which the displacements of the surface atoms are parallel to the (110) directions. Also, they suggested a 20 .~ long range inhibition of the reconstruction due to each step edge (total of - 40 A unreconstructed region on a terrace). In this Letter we report the first LEED observation of the reconstruction on a high density stepped W(001) surface ( - 2 8 ,A average terrace width) prepared so that the step edges are parallel to the [110] direction. The (h/2, h/2) beam intensity is observed to be much higher than that of the (h/2, h/2) * Research sponsored b y the Division of Materials Sciences, US Department of Energy under contract W-7405-eng-26 with Union Carbide Corporation. ** Supported by National Science Foundation grant NSFDNR78-25754.
0039-6028/82/0000-0000/$02.75 © 1982 North-Holland
L636
G.-C. Wang, T.- M. Lu / Structure of reconstructed domains on W(O01)
beam. This implies that the reconstructed domains are preferentially oriented. This observation is consistent with the lateral zig-zag model [5] proposed by DK. Furthermore, we determined that the preferentially nucleated domains are those in which the zig-zag displacements of the surface atoms are perpendicular to the step edges. From the measurements of the two-dimensional angular distribution of the half-order beam intensities, we have deduced the size and shape of both types of rotationally equivalent domains. It is shown quantitatively that the reconstruction occurs within a distance much closer to the step edges than suggested by D K [5]. The W(001) sample was cut 3.25 ° + 0.25 ° off the (001) orientation with the step edges parallel to the [110] direction. The sample could be cooled to 170 K within 3 min after flashing to 2500 K. The diffraction beam intensity was collected using a Faraday cup with a 0.5 mm aperture. The angular distribution of an intensity profile was obtained by scanning the Faraday cup through a diffracted beam at a fixed azimuthal angle and the intensity versus polar angle was plotted on an X - Y recorder. Thus, by measuring the intensity profile of a diffracted beam at different azimuthal angles, we obtained the two-dimensional intensity distribution of a beam. All LEED measurements were made using conditions of normal incidence to the (001). Normal incidence was determined within 0.25 ° by adjusting the crystal orientation until the four (20) beams showed almost identical intensity versus energy ( 1 - V ) profiles between 90 and 120 eV where they exhibited no apparent beam splitting. The upper part of fig. 1 is the schematic of a LEED pattern obtained from the crystal cooled to a temperature of 170 K. In addition to the characteristic splitting of the (11} integral order beams, distinct half-order diffracted beams, which decay rapidly above room temperature, were observed from this surface with a high density of steps. The I - V profiles of the (½ ½) beams are identical to that obtained by D K [5]. This superlattice formation is interpreted as due to the clean surface reconstruction. Results of recent field-ion microscopy expoeriments also indicated that reconstruction can occur on a tungsten tip < 35 A in diameter [4]. In contrast, no reconstruction was observed for terraces < 40 wide for cleaved semiconductor surfaces such as Si(111)-(2 x 1) and Ge(111)-(2 x 1) structures [6,7]. The lower part of fig. 1 shows the 1-V profiles for the (-~ ½) and (½ ½) beams. The intensity of the ({ ½) beam is approximately 4.5 times_greater than_ _ that of the (½ ½) beam. Similar results were obtained for (½ 1) and (½ ½) beams. According to DK's model, the ( h / 2 , h/2) and (h/2, h/2) beams result from diffraction from two rotationally equivalent domains with the zig-zag movements of the surface atoms perpendicular (type A, see fig. 2a) and parallel (type B, see fig. 2b) to the step edge respectively. Since the intensity of the (½ ½) beam is much higher than that of the (½ ½) beam, we conclude that type A domains have been preferentially nucleated. This observation is in contrast to the case of hydrogen-induced c(2 x 2) reconstruction on a stepped W(001)
G.-C Wang, T.-M. l_,u / Structure of reconstructed domains on W(O01)
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G.-C. Wang, T..M. Lu / Structure of reconstructed domains on W(O01)
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Fig. 2. The top-layer zig-zag model for the two (V/2 x v~-)R45 ° rotational domains having p2mg symmetry on the W(001) surface. The dots represent the bulk lattice and the open circles represent the displaced atomic positions.
where the displacements of the W atoms along the step edge is more favorable [8]. (In that case the step edges and the atomic displacements are all in the [010].) The origin of the difference between the two cases is not clear at the present time. A scan of diffracted intensity versus polar angle along the (00)-* (11)
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G.-C. Wang, T.- M. Lu / Structure of reconstructed domains on W(O01)
L639
diagonal direction (refers to the A" in fig. 4a) for an incident beam energy of 110 eV is shown in fig. 3a. The splitting of the integra| order beam is very sharp and the angular separation of 2.8 + 0.2 ° implies an average terrace width of 28 + 2 A. The half-order beams do not split [5] but show uniform broadening for all energies. Fig. 3b is a summary of the measured angular profiles plotted in reciprocal space. This is a two-dimensional cut along the (00) -0 (I 1) direction of the diffracted intensity distribution in the three-dimensional reciprocal space. The width of the (½ ½) rod in fig. 3b represents the full width at half maximum (FWHM) of the angular profile after removing the instrumental broadening effect. (We have made a best estimate of the amount of broadening of the (½ ½) beam due to the instrumental effect.) The absence of splitting can be interpreted as due to the existence of a random antiphase relationship among the domains nucleated at random positions on different terraces [9]. As a result, different domains (even if they have the same type of orientation) give negligible interference effect and the angular distribution of the half-order beam intensity can be regarded as the sum of the intensity scattered from the individual domains [9]. The width of the half-order beams is then a reflection of the size of the domains in the direction under consideration. From the width of the (-i2 ½) beam, we have estimated the average domain size for type A domains measured in the [110] direction to be between 22 + 2 and 24 + 2 ,~ [10] (determined by the lower and upper limits of the measured instrument response). As mentioned before, the splitting of the integral order beam is very sharp and the individual split profiles give no measurable broadening other than the instrumental effect. If one assumes that the spread of the terrace width distribution is negligibly small, then from the above information on the estimates of the reconstructed domain size, the unreconstructed area on the terrace would be less than 8 A wide [11]. In order to study in more detail the shape of the reconstructed domains for bo~h orientations, the two-dimensional angular distribution of the (~ ½) and (½ ½) beam intensities were measured. Fig. 4a shows schematically the diffracted beam shapes at E = 68 eV. The FWHM's of the (½ ½) and (½ ½) beams measured in the X direction are approximately the same and the sizes of both types of domains in the direction perpendicular to the step edge agree with the results obtained at E--- 114 eV for the (½ ½) beam. The F W H M of the (~ ½) beam measured in the ~r direction is narrower than that measured in the X direction, and the size of type A domains parallel to the [110] step edge is estimated to be twice as large as the size perpendicular to the step edge. On the other hand, the (½ ½) beam has approximately the same size in both ~" and ~" directions and appeared to be rounded, an indication that type B domains are more or less rounded. A possible arrangement of the two domains is shown in fig, 4b. The arrows indicate the direction o f the zig-zag movements of the surface atoms to form the domains.
G.-C. Wang, T.- M. Lu / Structure of reconstructed domains on W(O01)
L640
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Fig. 4. (a) A schematic Of the measured two-dimensional angular distribution of intensity in a LEED pattern at E = 68 eV. (b) A schematic of a possible arrangement of the reconstructed domains. A and B denote type A (fig. 2a) and type B (fig. 2b) domains. Arrows indicate the directions of atomic displacements from bulk lattice positions.
In conclusion, our observation of the preferential nucleation of one type of orientational domain on a stepped W(001) surface, with edges parallel to the [110], is consistent with DK's zig-zag model of the reconstruction, and these domains are those in which the zig-zag displacemen.t of the surface atoms is perpendicular to the step edge. The total unreconstructed area on a terrace is
G.-C. Wang, T.-M. Lu / Structure of reconstructed domains on W(O01)
L641
shown to be - 8 ~,, which is considerably smaller than the value ( - 40 A) suggested by DK. We acknowledge E. Bauer, D.A. King, P.J. Estrup, A.J. Melmed, L.D. Roelofs, and the Surface Physics Group at ORNL for invaluable discussions. We also thank J.F. Barhorst and G.W. Ownby for crystal preparation.
References [1] [2] [3] [4] [5] [6] [7] [8]
[9] [10]
[11]
Y. Yonehara and L.D. Schmidt, Surface Sci. 25 (1971) 238. M.K. Debe and D.A. King, J. Phys. CI0 (1977) L303. T.E. Felter, R.A. Barker and P.J. Estrup, Phys. Rev. Letters 38 (1977) 1138. A.J. Melmed, R.T. Tung, W.R. Graham and G.D.W. Smith, Phys. Rev. Letters 43 (1979) 1521. M.K. Debe and D.A. King, Phys. Rev. Letters 39 (1977) 708; Surface Sci. 81 (1979) 193; J.A. Walker, M.K. Debe and D.A. King, Surface Sci. 104 (1981) 405. J.E. Rowe, S.B. Christman and H. Ibach, Phys. Rev. Letters 34 (1975) 874. M. Henzler, Surface Sci. 19 (1970) 159. R.A. Barker and P.J. Estrup, J. Chem. Phys. 74 (1981) 1442. T.-M. Lu and G.-C. Wang, Surface Sci. 107 (1981) 139; T.-M. Lu, L.-H. Zhao, M.G. Lagally, G.-C. Wang and J.E. Houston, to be published. Due to the large (2.9+0.2 °) domain size broadening, the uncertainty of the instrument response used does not affect the domain size determination very much. For instance, at E = I I 4 eV, the width of the measured profile is 2.9+0.2 °. If one takes the instrument response width of the (11) beam (which is 1.5 ° at E = 114 eV) as the upper limit of the response width of the (~ ½~ beam at that energy, then the upper limit of the domain size can be calculated to be 2 4 + 2 A. On the other hand, if one takes hte lower limit of the instrument response width of the (-i2 ½) beam to be 1° (which presumably underestimates its value), then the lower limit of the domain size can be estimated to be 22 + 2 A. Rigorously speaking, one has to consider the possibility of the existence of a finite spread in the terrace width distribution (J.E. Houston and R.L. Park, Surface Sci. 26 (1971) 269). We have estimated that the spread should not be larger than + 15 A around the average value of 28 A. A spread larger than + 15 A would result in broadening of the individual split beams that would become observable with.the resolving capability of our instrument. If the terrace width does spread to within + 15 A and the reconstruction occurs only on the few larger terraces, then the unreconstructed region in each terrace may be twice as large as the estimated value of 8 A. (If the same analysis is applied, to refs. [6] and [7], then the reconstruction may be inhibited on terraces as large as 65 A in those semiconductor cases.) However, due to the observed strong half-order beam intensity, it seems inconceivable that only the larger terraces are involved in the reconstruction. The ratio of our (~ ½) beam intensity compared to the (I1) beam is as large as that reported by D K [5].