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Monodisperse aerosol particle deposition: Prospects for nanoelectronics
MICROELECTRONIC ENGINEERING
ELSEVIER
Microelectronic Engineering 41/42 (1998) 535-538
Monodisperse Aerosol Particle Deposition: Prospects for Nanoelectronics W.Prost", F.E.Kruis", F.Otten", K.Nielsch ~, B.Rellinghaud, U.Auer", A.Peled"", E.F.Wassermann<, H.Fissan". F.J.Tegude" "Halbleitertechnik/Halbleitertechnologie, "Prozeg- und Aerosolmegtechnik, ~Exp. Tieftemperaturphysik, "
2. Nanoparticle synthesis and deposition
The exploitation of semiconductor quantum effects enabling new devices with size-dependent working principles is very attractive for future nanoelectronics I II. In addition, a full 3-dimensional (3-D) scaling down is indispensable for both a further increase in integration density and a reductit)n of the power consumption for each device operation. On the other hand, a prerequisite for the usage of size-dependent working principles on a circuit level is an extremely precise size control. l~arge efforts are directed towards a full 3-D scaling th)wn using established high performance lithography tools, but inherently with scaling down the relative size error increases. A different route is the direct generation of 3-D nanostructures by various methods [e.g. 2-6]. For opto-electronic applications compatible to standard microelectronics a deposition of high-purity nanoparticles on planar substrates can be advantageous. This favours gasphase synthesis such as done recently by aerosol techniques [3, 4]. This approach using PbS nanoparticles is chosen in this work. The crystalline properties and the size distribution of the PbS particles arc analyzed. In addition, a lateral control ~fl particle deposition will be demonstrated and a first device application will be shown.
The design of quantum effect devices based on
PbS particles requires: 1) stoichiometric PbS, 2) crystalline particles, 3) sizes smaller than 20 nm. and 4) spherical, non-agglomerated particles. PbS is chosen becausc EI-Rahaiby and Rao [7] have shown that PbS retains its molecular form upon vaporisation. Aerosol particles are obtained here (of. Fig. I ) at atmospheric pressure by evaporating a PbS powder (Aldridge, purity > 99.9~) from a ceranlic boat into a stream tfi: nitrogen in a tube lurnace (furnace 1). The deposition rate of particles of N2r'mMFC bipolar T~C L_J charger L ~ , ', = ', ~ _ _ . ~ [ ~ ] ~ ..... q HV MFC ' ' PbS Tube furnace 1 Differential I! I I "1
Mo
Mobility
Analyzer I T=450°C i--~ Deposition I chamber [ _ ~
I'~11 I',\~
I I
I
Tube furnace 2 I
Exhaust,~ J I H V = 2 - 8 k V Figure 1. Aerosol sct-up for the synthesis, fractionation and deposition el nammacter-sized particles.
0167-9317/98/$19.00© ElsevierScienceB.V All rightsreserved. PII: S0167-9317(98)00125-7
536
W. Prost et aL /Microelectronic Engineering 41/42 (1998) 535-538
detectable sizes (>3 nm) amounts to 10" cln 2 h" when the temperature of furnace 1 is set at 680 °C, which is far underneath the melting point of PbS (1114 °C). The polydisperse aerosol is then passed through a bipolar charger containing a radioactive source (*~Kr). Charged particles smaller than 20 nm can be assumed to be singly charged [8]. In order to obtain a monodisperse aerosol, a Differential Mobility Analyzer (Nano-DMA, TSI, Minneapolis, USA) is used as size fractionator. The DMA takes out particle fractions with equal electrical mobility by applying an electric field [9]. First TEM investigations showed agglomerates consisting of partially sintered primary particles. For this reason the size-fractionated particles are sintered in a second furnace. The experimental set-up (Fig. 1) is completed by an electrostatic precipitator set at a voltage between 2 and 8 KV.
3. Nanoparticle properties The sintering and crystallisation step proved to be necessary in order to obtain quasi-spherical particles (Fig. 2). There is an optimum sintering temperature (furnace 2) for each particle size, due to the fact that above a certain temperature the standard deviation of the distribution rises sharply and the number concentration decreases. This optimum temperature decreases when going to smaller particles [11]. The standard deviation of the size distributions as determined from the TEM micrographs is typically 13 % for particles ranging in size from 5 nm to 20 nm. The degree of crystallisation of PbS particles produced by aerosol techniques is investigated
utilising specular X-Ray diffraction experiments with Co-K,, radiation at )v = 0.17902 nm. In the diffraction angle range 20 ° < 2 0 < 120 ° eight Bragg peaks related to PbS particles are detected apart from the dominating peaks from the substrate, which is copper for this experiment. Fig. 3 shows from a sample of monodisperse particles that part of the O - 2 0 scan which displays the two peaks with highest intensity. From the angular position of the Bragg peaks attributed to the PbS particles the lattice constant is determined to be a = 0.5945(_+0.0005) nm and agrees within 0.15% with the NaCI type lattice constant of bulk PbS [ 10]. The PbS related peaks have been fitted with Lorentzian curves. Assuming the particle powder to consist of cubic crystallites only, the crystallite cubic edge length L can be estimated from the full width at half maximum (FWHM) of the Lorentzians using the Scherrer equation [12]. Therefrom the mean particle size is derived to be L ........ = 21(_+2) nm. The individual L-values for the two Bragg peaks shown in Fig. 3 are given, too. Taking into account the simplifying model assumption of monodisperse cubic particles, this agrees well with the mean particle diameter as determined from TEM investigations, DTEM= 18(+3) nm [ 1 I]. We therefore conclude that the investigated PbS particles exhibit a well defined crystal structure which extends over the whole particle size.
,
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,
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p
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,
I
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,
I
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,
I
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2®(deg) Figure 2. TEM micrographs showing PbS nanoparticles. The mean particle size is 17.8 nm with a standard deviation of 13%.
Figure 3. Part of a specular O-20 X-Ray diffraction scan of PbS particles sintered at T s = 500 °C. The curve is corrected with respect to the background signal of the substrate. The Bragg peaks are labelled with their Miller indices and their individual particle size (Lorentzian fit).
W Prost et al./Microelectronic Engineering 41/42 (1998) 535-538
4. Lateral d e p o s i t i o n control Nanoparticles generated by this aerosol techniques are singly charged, which is a prerequisite for size control and effective deposition. This property may be used for a lateral control during the gas-phase deposition process by an additional electric field. On the other hand, for the first generation of structures and devices conventional photoresist patterning is a recommended technique. In this paragraph the compatibility with microelectronic patterning will be studied and first results of electrostatic control will be described. Using optical lithography a GaAs-substrate is patterned with different resist structures with a minimum feature size of 600 nm. On these structures PbS particles are deposited and the distribution of particles is analyzed using scanning electron microscopy. In Fig. 4 the result of 13.5 h deposition with 20 nm PbS particles is shown. The deposition on the resist is very low. In addition, on the substrate along the resist edge a spacer almost without particles can clearly be seen. We attribute this behaviour to the extremely low conductivity of the resist as compared to the substrate. This results in a selective charging of the resist which repels a further deposition of particles of the same charge polarity. This effect is especially strong in lateral direction at the edge of the resist resulting in the described spacer. This spacer was
537
observed in all experiments and has a thickness of about 250 nm for a particle size of 7 nm to 20 nm. After deposition a lift-off is carried out in cold acetone without agitation. This process removes the resist while almost all particles remain on tile semiconductor surface. Furthermore a second resist patterning crossing the particles is possible without a visible loss of particles in the SEM images. For the lateral control via an additional electric field, metal contacts with a spacing down to I pare are fabricated on s.i. GaAs substrates. While the precipitator voltage is set to 2 KV a battery with 1.5 V is inserted in the deposition apparatus. One contact of the battery is connected to the high voltage supply while the other one provides a voltage drop between the metal contacts. The lateral electric field between the metal contacts fi)r 2 ram spacing is approximately 7.5 KV/cm and hence three times higher than the main perpendicular field. Under our conditions a particle distribution according to Fig. 5 can be observed. An enhanced deposition on the left electrode due to the laterally applied w~ltage is observed. In addition, the deposition rate decreases towards the right electrode. But, the distribution on the wafer is inhomogenous and a strong control by the local and lateral electric field could not be observed, yet. This result is attributed to the chaotic Brownian movement which competes especially at atmospheric pressure with the lateral electric field. Although the selectivity is still quite poor, this result is encouraging having in mind that significant improvements may be possible if the deposition is carried out at lower pressure. In
500 nm Figure 4. SEM image of a resist pattern on n-GaAs suhstrate after 13.5 h deposition with PbS particles of d = 20 nm size.
Figure 5. SEM picture of metal electrodes on s.i. GaAs substrate alier 13.5 h PbS = (20 nm) particle deposilion under vertical and lateral electric field.
VKProst et al./Microelectronic Engineering 41/42 (1998) 535-538
538
addition, the lateral electric field can easily be increased due to smaller spacing and higher battery voltage. 5. Device application Based on a metal-semiconductor-metal structure detectors with fingers of 50 pm in width are fabricated, both on Si and GaAs s.i. substrates. On the Si-substrate 1000 fingers of 1 pm spacing are fabricated whereas on the GaAs-substrate 10 fingers with 2.5 pm spacing are realised. The photo current of the detectors prior to and after about 33 h deposition of 7 nm PbS particles is studied. The photo current with no bias applied is measured as a function of incident wavelength using a chopper and lock-in technique. In Fig. 6 the results on both substrates are shown. In the wavelength range of 2000 nm < ;L < 2500 nm a high photo current is observed especially after PbS particle deposition. This wavelength range corresponds to an energy of 0.5 eV < E < 0.62 eV which is higher than the bulk value for PbS ( E = 0.41 eV) but still below E = 0.75 eV which is expected from 7 nm PbS particles [5]. Moreover, the particles result in an almost wavelength-independent increase of sensitivity (GaAs 4x; Si 60x). Hence we do not attribute the higher sensitivity after deposition to a quantum effect. This was proven by experiments with larger particles which result in a similar behaviour. On the other hand, a high photo current is obtained only in the energy range close to the band gap of PbS, which implies that the material properties of the particles contribute to the /~
3000
/
-o- no
Si-Au -IF 7 nm PbS
\
GaAs-Au -o- no
[
o 11)00
0 2000
2500
3000
wavelength Mnm Figure 6. Photo current of detectors on s.i. substrates prior lo and after deposition of 7 nm thick PbS particles
substantial sensitivity improvement.
6. Conclusions Aerosol techniques are capable of providing monodisperse, crystalline particles of controllable size down to 3 rim. In addition, the particles are compatible to conventional microelectronic patterning and first results of a lateral deposition control are presented. These properties are promising for future fabrication of devices based on 3-D quantum effects.
Acknowledgement: We like to thank M. Vennemann for conducting the X-Ray diffraction measurements, and A. Osinski for technical support. Literature: [1] C. Weisbuch, B. Vinter, "Quantum Semiconductor Structures: Fundamentals and Applications" Academic Press, San Diego, 1991. [2] R. N6tzel, Semicond. Sci. Technol. 11 (1996) 1365. [3] K. Deppert, J.-O. Bovin, J.-O. Maim, L. Samuelson, J. Crystal Growth 169 (1996) 13. [4] F . E . Kruis, A. Goossens, H. Fissan, J. Aerosol Sci. 27S1 (1996) 165. [5] Y. Wang, N. Herron, J. Phys. Chem. 95 (1991) 525. [6] C.B. Murray, C.R. Kagan, M.G. Bawendi, Science 2 70 (1995) 1335. [7] S. K. EI-Rahaiby, Y. K. Rao, Metallurgical Trans. 13B (1982) 633. [8] A. Wiedensohler, J.Aerosol. Sci. 19 (1988) 387. [9] H. Fissan, D. Hummes, F. Stratmann, P. BiJscher, S. Neumann, D.Y.H. Pui, D. R. Chert, Aerosol Sci. Technol. 24 (1996) 1. [10] B.E. Warren: "X-Ray Diffraction", Dover Publications Inc., 1990, pp. 251. [ 11 ] K. Nielsch, Diploma Thesis, Gerhard-MercatorUniversity Duisburg, 1997. [12] E. Preuss, B. Krahl-Urban, R. Butz: "Laue Atlas", edited by the Jiilich Nuclear Research Center. John Wile); and Sons, New York, 1974, p. 48.
ELSEVIER
Microelectronic Engineering 41/42 (1998) 535-538
Monodisperse Aerosol Particle Deposition: Prospects for Nanoelectronics W.Prost", F.E.Kruis", F.Otten", K.Nielsch ~, B.Rellinghaud, U.Auer", A.Peled"", E.F.Wassermann<, H.Fissan". F.J.Tegude" "Halbleitertechnik/Halbleitertechnologie, "Prozeg- und Aerosolmegtechnik, ~Exp. Tieftemperaturphysik, "
2. Nanoparticle synthesis and deposition
The exploitation of semiconductor quantum effects enabling new devices with size-dependent working principles is very attractive for future nanoelectronics I II. In addition, a full 3-dimensional (3-D) scaling down is indispensable for both a further increase in integration density and a reductit)n of the power consumption for each device operation. On the other hand, a prerequisite for the usage of size-dependent working principles on a circuit level is an extremely precise size control. l~arge efforts are directed towards a full 3-D scaling th)wn using established high performance lithography tools, but inherently with scaling down the relative size error increases. A different route is the direct generation of 3-D nanostructures by various methods [e.g. 2-6]. For opto-electronic applications compatible to standard microelectronics a deposition of high-purity nanoparticles on planar substrates can be advantageous. This favours gasphase synthesis such as done recently by aerosol techniques [3, 4]. This approach using PbS nanoparticles is chosen in this work. The crystalline properties and the size distribution of the PbS particles arc analyzed. In addition, a lateral control ~fl particle deposition will be demonstrated and a first device application will be shown.
The design of quantum effect devices based on
PbS particles requires: 1) stoichiometric PbS, 2) crystalline particles, 3) sizes smaller than 20 nm. and 4) spherical, non-agglomerated particles. PbS is chosen becausc EI-Rahaiby and Rao [7] have shown that PbS retains its molecular form upon vaporisation. Aerosol particles are obtained here (of. Fig. I ) at atmospheric pressure by evaporating a PbS powder (Aldridge, purity > 99.9~) from a ceranlic boat into a stream tfi: nitrogen in a tube lurnace (furnace 1). The deposition rate of particles of N2r'mMFC bipolar T~C L_J charger L ~ , ', = ', ~ _ _ . ~ [ ~ ] ~ ..... q HV MFC ' ' PbS Tube furnace 1 Differential I! I I "1
Mo
Mobility
Analyzer I T=450°C i--~ Deposition I chamber [ _ ~
I'~11 I',\~
I I
I
Tube furnace 2 I
Exhaust,~ J I H V = 2 - 8 k V Figure 1. Aerosol sct-up for the synthesis, fractionation and deposition el nammacter-sized particles.
0167-9317/98/$19.00© ElsevierScienceB.V All rightsreserved. PII: S0167-9317(98)00125-7
536
W. Prost et aL /Microelectronic Engineering 41/42 (1998) 535-538
detectable sizes (>3 nm) amounts to 10" cln 2 h" when the temperature of furnace 1 is set at 680 °C, which is far underneath the melting point of PbS (1114 °C). The polydisperse aerosol is then passed through a bipolar charger containing a radioactive source (*~Kr). Charged particles smaller than 20 nm can be assumed to be singly charged [8]. In order to obtain a monodisperse aerosol, a Differential Mobility Analyzer (Nano-DMA, TSI, Minneapolis, USA) is used as size fractionator. The DMA takes out particle fractions with equal electrical mobility by applying an electric field [9]. First TEM investigations showed agglomerates consisting of partially sintered primary particles. For this reason the size-fractionated particles are sintered in a second furnace. The experimental set-up (Fig. 1) is completed by an electrostatic precipitator set at a voltage between 2 and 8 KV.
3. Nanoparticle properties The sintering and crystallisation step proved to be necessary in order to obtain quasi-spherical particles (Fig. 2). There is an optimum sintering temperature (furnace 2) for each particle size, due to the fact that above a certain temperature the standard deviation of the distribution rises sharply and the number concentration decreases. This optimum temperature decreases when going to smaller particles [11]. The standard deviation of the size distributions as determined from the TEM micrographs is typically 13 % for particles ranging in size from 5 nm to 20 nm. The degree of crystallisation of PbS particles produced by aerosol techniques is investigated
utilising specular X-Ray diffraction experiments with Co-K,, radiation at )v = 0.17902 nm. In the diffraction angle range 20 ° < 2 0 < 120 ° eight Bragg peaks related to PbS particles are detected apart from the dominating peaks from the substrate, which is copper for this experiment. Fig. 3 shows from a sample of monodisperse particles that part of the O - 2 0 scan which displays the two peaks with highest intensity. From the angular position of the Bragg peaks attributed to the PbS particles the lattice constant is determined to be a = 0.5945(_+0.0005) nm and agrees within 0.15% with the NaCI type lattice constant of bulk PbS [ 10]. The PbS related peaks have been fitted with Lorentzian curves. Assuming the particle powder to consist of cubic crystallites only, the crystallite cubic edge length L can be estimated from the full width at half maximum (FWHM) of the Lorentzians using the Scherrer equation [12]. Therefrom the mean particle size is derived to be L ........ = 21(_+2) nm. The individual L-values for the two Bragg peaks shown in Fig. 3 are given, too. Taking into account the simplifying model assumption of monodisperse cubic particles, this agrees well with the mean particle diameter as determined from TEM investigations, DTEM= 18(+3) nm [ 1 I]. We therefore conclude that the investigated PbS particles exhibit a well defined crystal structure which extends over the whole particle size.
,
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,
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2®(deg) Figure 2. TEM micrographs showing PbS nanoparticles. The mean particle size is 17.8 nm with a standard deviation of 13%.
Figure 3. Part of a specular O-20 X-Ray diffraction scan of PbS particles sintered at T s = 500 °C. The curve is corrected with respect to the background signal of the substrate. The Bragg peaks are labelled with their Miller indices and their individual particle size (Lorentzian fit).
W Prost et al./Microelectronic Engineering 41/42 (1998) 535-538
4. Lateral d e p o s i t i o n control Nanoparticles generated by this aerosol techniques are singly charged, which is a prerequisite for size control and effective deposition. This property may be used for a lateral control during the gas-phase deposition process by an additional electric field. On the other hand, for the first generation of structures and devices conventional photoresist patterning is a recommended technique. In this paragraph the compatibility with microelectronic patterning will be studied and first results of electrostatic control will be described. Using optical lithography a GaAs-substrate is patterned with different resist structures with a minimum feature size of 600 nm. On these structures PbS particles are deposited and the distribution of particles is analyzed using scanning electron microscopy. In Fig. 4 the result of 13.5 h deposition with 20 nm PbS particles is shown. The deposition on the resist is very low. In addition, on the substrate along the resist edge a spacer almost without particles can clearly be seen. We attribute this behaviour to the extremely low conductivity of the resist as compared to the substrate. This results in a selective charging of the resist which repels a further deposition of particles of the same charge polarity. This effect is especially strong in lateral direction at the edge of the resist resulting in the described spacer. This spacer was
537
observed in all experiments and has a thickness of about 250 nm for a particle size of 7 nm to 20 nm. After deposition a lift-off is carried out in cold acetone without agitation. This process removes the resist while almost all particles remain on tile semiconductor surface. Furthermore a second resist patterning crossing the particles is possible without a visible loss of particles in the SEM images. For the lateral control via an additional electric field, metal contacts with a spacing down to I pare are fabricated on s.i. GaAs substrates. While the precipitator voltage is set to 2 KV a battery with 1.5 V is inserted in the deposition apparatus. One contact of the battery is connected to the high voltage supply while the other one provides a voltage drop between the metal contacts. The lateral electric field between the metal contacts fi)r 2 ram spacing is approximately 7.5 KV/cm and hence three times higher than the main perpendicular field. Under our conditions a particle distribution according to Fig. 5 can be observed. An enhanced deposition on the left electrode due to the laterally applied w~ltage is observed. In addition, the deposition rate decreases towards the right electrode. But, the distribution on the wafer is inhomogenous and a strong control by the local and lateral electric field could not be observed, yet. This result is attributed to the chaotic Brownian movement which competes especially at atmospheric pressure with the lateral electric field. Although the selectivity is still quite poor, this result is encouraging having in mind that significant improvements may be possible if the deposition is carried out at lower pressure. In
500 nm Figure 4. SEM image of a resist pattern on n-GaAs suhstrate after 13.5 h deposition with PbS particles of d = 20 nm size.
Figure 5. SEM picture of metal electrodes on s.i. GaAs substrate alier 13.5 h PbS = (20 nm) particle deposilion under vertical and lateral electric field.
VKProst et al./Microelectronic Engineering 41/42 (1998) 535-538
538
addition, the lateral electric field can easily be increased due to smaller spacing and higher battery voltage. 5. Device application Based on a metal-semiconductor-metal structure detectors with fingers of 50 pm in width are fabricated, both on Si and GaAs s.i. substrates. On the Si-substrate 1000 fingers of 1 pm spacing are fabricated whereas on the GaAs-substrate 10 fingers with 2.5 pm spacing are realised. The photo current of the detectors prior to and after about 33 h deposition of 7 nm PbS particles is studied. The photo current with no bias applied is measured as a function of incident wavelength using a chopper and lock-in technique. In Fig. 6 the results on both substrates are shown. In the wavelength range of 2000 nm < ;L < 2500 nm a high photo current is observed especially after PbS particle deposition. This wavelength range corresponds to an energy of 0.5 eV < E < 0.62 eV which is higher than the bulk value for PbS ( E = 0.41 eV) but still below E = 0.75 eV which is expected from 7 nm PbS particles [5]. Moreover, the particles result in an almost wavelength-independent increase of sensitivity (GaAs 4x; Si 60x). Hence we do not attribute the higher sensitivity after deposition to a quantum effect. This was proven by experiments with larger particles which result in a similar behaviour. On the other hand, a high photo current is obtained only in the energy range close to the band gap of PbS, which implies that the material properties of the particles contribute to the /~
3000
/
-o- no
Si-Au -IF 7 nm PbS
\
GaAs-Au -o- no
[
o 11)00
0 2000
2500
3000
wavelength Mnm Figure 6. Photo current of detectors on s.i. substrates prior lo and after deposition of 7 nm thick PbS particles
substantial sensitivity improvement.
6. Conclusions Aerosol techniques are capable of providing monodisperse, crystalline particles of controllable size down to 3 rim. In addition, the particles are compatible to conventional microelectronic patterning and first results of a lateral deposition control are presented. These properties are promising for future fabrication of devices based on 3-D quantum effects.
Acknowledgement: We like to thank M. Vennemann for conducting the X-Ray diffraction measurements, and A. Osinski for technical support. Literature: [1] C. Weisbuch, B. Vinter, "Quantum Semiconductor Structures: Fundamentals and Applications" Academic Press, San Diego, 1991. [2] R. N6tzel, Semicond. Sci. Technol. 11 (1996) 1365. [3] K. Deppert, J.-O. Bovin, J.-O. Maim, L. Samuelson, J. Crystal Growth 169 (1996) 13. [4] F . E . Kruis, A. Goossens, H. Fissan, J. Aerosol Sci. 27S1 (1996) 165. [5] Y. Wang, N. Herron, J. Phys. Chem. 95 (1991) 525. [6] C.B. Murray, C.R. Kagan, M.G. Bawendi, Science 2 70 (1995) 1335. [7] S. K. EI-Rahaiby, Y. K. Rao, Metallurgical Trans. 13B (1982) 633. [8] A. Wiedensohler, J.Aerosol. Sci. 19 (1988) 387. [9] H. Fissan, D. Hummes, F. Stratmann, P. BiJscher, S. Neumann, D.Y.H. Pui, D. R. Chert, Aerosol Sci. Technol. 24 (1996) 1. [10] B.E. Warren: "X-Ray Diffraction", Dover Publications Inc., 1990, pp. 251. [ 11 ] K. Nielsch, Diploma Thesis, Gerhard-MercatorUniversity Duisburg, 1997. [12] E. Preuss, B. Krahl-Urban, R. Butz: "Laue Atlas", edited by the Jiilich Nuclear Research Center. John Wile); and Sons, New York, 1974, p. 48.