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Stoichiometry and impurity concentrations in II–VI compounds measured by elastic recoil detection analysis (ERDA)
Journal of Crystal Growth 197 (1999) 571—575
Stoichiometry and impurity concentrations in II—VI compounds measured by elastic recoil detection analysis (ERDA) M. Birkholz*, W. Bohne, J. Ro¨hrich, A. Ja¨ger-Waldau, M.C. Lux-Steiner Hahn-Meitner-Institut, Bereich Festko( rperphysik, Glienicker Str. 100, D-14109 Berlin, Germany
Abstract Polycrystalline ZnSe will be used in a chemical vapour deposition process for thin-film solar-cell emitter layers. The precursor bulk material was characterised with respect to stoichiometry and impurity concentrations by the ERDA method, for which an experimental setup is installed at the Berlin Ion Beam Facility (ISL). The sample material was irradiated with high-energy Kr and Xe projectile ions of 120 and 250 MeV, respectively. The energy and time-of-flight (TOF) of the released sample atoms were detected. In contrast to the Rutherford backscattering technique the measurement of both parameters enables the separation of different masses not only in thin-films but also in bulk material. ERDA allows the simultaneous depth profiling of heavy and light elements including hydrogen. We found ZnSe bulk samples to exhibit an oxygen surface contamination with a Zn/O ratio of 1 : 2 and thickness of 27;10 at/cm. An accuracy in stoichiometry, i.e., Zn/Se ratio of better than 1% could be achieved and impurity concentrations could be detected down to the 100 ppm range. Detection limits of the ERDA technique and its capacity for concentration profiling in II—VI materials will be discussed. 1999 Elsevier Science B.V. All rights reserved. PACS: 61.18.Bn; 61.50.Nw; 85.60.Dw Keywords: Solar-cell materials; Stoichiometry; Doping; Elastic recoil detection
Among other applications, II—VI compounds are currently under investigation as buffer layers in thin film solar cells utilising chalcopyrite absorber layers. High efficiencies of almost 18% have been achieved for n-CdS/p-Cu(Ga,In)Se devices under * Corresponding author. Fax: #49 30 6705 3333; e-mail: [email protected].
AM 1.5 radiation conditions [1]. The aim of our work is to develop a n-ZnSe/p-CuGaSe hetero diode for which the preparation of CuGaSe source material and thin films has already been succeeded [2,3]. Quantitative knowledge on stoichiometry and contamination is an important prerequisite for the optimisation of solar-cell materials. Therefore, we investigated the elastic recoil detection analysis
0022-0248/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 7 5 5 - 6
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M. Birkholz et al. / Journal of Crystal Growth 197 (1999) 571–575
(ERDA) [4] to determine these values for bulk material and thin films. Like the well-known Rutherford backscattering (RBS) technique, ERDA is another ion beam method, for which the sample is irradiated by projectile ions of kinetic energies in the MeV range. But whereas for RBS only one single energy spectrum of the projectile ions scattered at all the different constituents of the sample is obtained, ERDA facilitates to extract separate energy spectra for each element. Therefore, ERDA does not only measure the energy but in coincidence also the mass or atomic number of each recoil that is scattered out of the sample. The amount of ejected atoms, however, maximally attains the ppb range, causing only a negligible loss of sample mass. The number of ejected atoms depends on the amount of a certain element contained in the sample, on the number of projectile ions impinging the sample and on the corresponding exactly known Rutherford cross section, i.e., ERDA is an absolute method which does not need calibration standards. An outstanding feature of ERDA is the almost equal sensitivity for the detection of all elements (except for hydrogen for which the sensitivity is even enhanced by a factor of four). As for RBS the maximum energy for each element spectrum is obtained from the corresponding scattering at the surface of the sample, while scatter processes from deeper parts are found at smaller energies, since the particles are slowed down along their trajectories through the sample. Therefore, the ERDA method delivers depth-resolved concentrations. The investigations were carried out with the ERDA setup installed at the ion beam laboratory (ISL) of Hahn-Meitner-Institut Berlin, a detailed description of which is given in Ref. [5]. Typical ions used were Kr and Xe with energies of 120 and 250 MeV, respectively. Because the experiments were performed in reflection geometry with a scattering angle of 40° the energies allow to analyse samples up to depths of a few lm. For the element identification and separation the time-of-flight method (TOF) is used. The spectrometer consists of two TOF-energy telescopes. One, the “long telescope”, uses start and stop channel plate detectors with a long flight path of 123 cm in between and a large (6;7.2 cm) Si-energy detector placed behind the stop detector. With the achieved time
resolution of approximately 135 ps and typical TOF values of 30—100 ns a sufficient mass resolution is possible. Because the detection efficiency of this system for hydrogen and helium is less than 100%, an additional “short telescope” is installed with a flight path of only 46 cm and without separate time detectors. In this case the time-of-flight is measured with the fast signal of the Si-energy detector relative to the RF signal of the cyclotron which delivers a pulsed beam with a pulse width of about 400 ps. Although the time resolution is reduced compared to the long TOF telescope it is still good enough to resolve light masses with the advantage of a detection efficiency of unity for all elements. The coincident energy E and time t signals (due to electronic reasons start and stop signals are reversed by an internal delay t ) are recorded event by event resulting in a three-dimensional intensity histogram N(E, t) which is called scatter plot. According to MJEt separated curves for each recoil mass M are obtained, broadened due to the energy and time resolution. From these branches energy spectra for all outscattered elements of the sample are extracted. Similar to RBS, the depth and concentration profile of the corresponding element can be obtained by simulating with a numerical program for which we made use of SIMNRA [6]. Example 1. Fig. 1a depicts the ERDA scatter plot of a polycrystalline bulk ZnSe sample that was commercially available (II—VI Inc.). The sample had the typical feature of IR window material, i.e., it was about 5 mm thick and exhibited parallel and smooth surfaces. The ERDA experiment was carried out with 250 MeV Xe ions. The sample was irradiated with a total dose of 5.3;10 particles. The mass branches of zinc and selenium are clearly separated. Both curves are significantly broadened not only due to the detectors resolution but mainly due to the fact that natural zinc and selenium occur with five and six stable isotopes, respectively. In the high-energy range the curves are split up due to isotope separation demonstrating the high-quality mass separation of the setup. In terms of phase diagrams, zinc selenide may be considered as a line compound since stoichiometry deviations are known to occur only in the 10\ range [7]. The
M. Birkholz et al. / Journal of Crystal Growth 197 (1999) 571–575
573
Fig. 1. (a) Scatter plot and (b) elemental energy projections of a bulk ZnSe sample. The solid line in (b) represents the simulation with the Zn/Se concentration ratio fixed to the best fitting value 1.01(1) : 0.99(1).
ERDA measurement of ZnSe therefore was used as a test to determine the precision obtainable in stoichiometry measurements. From the events of Zn and Se branches in the scatter plot separate energy spectra were extracted, which are shown in Fig. 1b. The simulation of the spectra were performed assuming a depth-independent stoichiometry. Simulated spectra are shown as solid lines in the figure. The optimum fit to the data was obtained with atomic concentrations c "1.01(1) 8 and c "0.99(1), where the estimated standard de1 viations are given in parentheses. The ideal atomic ratio Zn/Se"1 : 1 is seen to fall within the confidence interval of the measurement. It is concluded that a precision of about 1% can be obtained for the stoichiometry measurement so far. If one is interested in even more precise measurements, the limitations have to be considered. First of all, the precision is limited by statistics, but this factor can be improved by a higher dose. The next point is that for samples thicker than the detectable range the stopping power influences the integral of the calculated spectra. That is not the case for thin films where the spectra have a finite energy width for each element detected. Stopping powers in the high-energy range are not yet known with the necessary precision. Another limiting factor is the energy straggling and the multiple scattering the probability of which increase with increasing depth.
Finally, the analysis makes use of Bragg’s rule which is a superposition principle for stopping powers and states that the stopping of ions in compounds is the sum of the stopping power of its constituents. However, according to previous investigations, this rule is fulfilled for the compounds considered here, because the samples consist of heavy elements and because the ion energy is larger than 250 keV/amu [8]. Example 2. One of the investigated ZnSe samples exhibited an oxygen surface contamination. The measurement was done with an 120 MeV Kr ion beam using a total radiation dose of 3.1;10. The energy projections of zinc and oxygen are shown in Fig. 2. An energy projection of selenium could not be obtained due to a large overlap of the selenium branch with the backscattered Kr projectile ions in the scatter plot. Elemental spectra were simulated by assuming the bulk material to be covered by a thin oxide layer. The best fit to the experimental data was obtained by setting the Zn/O ratio to 1 : 2 and the layer thickness to 27;10 at/cm. Assuming the density of the layer to be equal to the bulk density of ZnSe a thickness of 62 A> is calculated from the areal density value. This investigation demonstrates the capability of the ERDA technique with respect to the detection of surface contamination of compound semiconductors.
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M. Birkholz et al. / Journal of Crystal Growth 197 (1999) 571–575
in order to obtain separate cadmium and sulphur energy spectra and to investigate their relative concentrations. No deviations from the ideal atomic ratio of Cd/S"1 : 1 could be detected within the present limits of precision of our setup and analysis routines.
Fig. 2. Zinc and oxygen energy projections of ERDA measurement of a bulk ZnSe sample. The solid line accounts for the simulation of a 62 A> thick oxide surface layer with a Zn/O ratio of 1 : 2 that was assumed to cover the bulk sample.
Fig. 3. Scatter plot of a CdS bulk sample measured with the short TOF telescope.
Example 3. In Fig. 3 we present the scatter plot of a measured CdS single crystal (2;10 mm, (1 0 0) Crystec GmbH) that was obtained with the short TOF configuration (250 MeV Xe, 5 min, total dose 2.75;10 particles). A contamination of the sample surface by hydrogen and carbon was observed. The surface layer was simulated with an areal density of 44;10 at/cm and a composition of C/H"3 : 1. The sample was also investigated with the long TOF configuration (not shown here)
Example 4. Finally, in Fig. 4, we present the ERDA scatter plots of two ZnSe thin films grown by MOCVD on GaAs. The production of both samples occurred within the same run, they only differ with respect to substrate orientation which were either (1 0 0) or (1 1 1) wafers. Accordingly, the growth mode of the ZnSe layer was by stacking of either cubic or hexagonal layers. Doping of ZnSe was done by supplying chlorine in small amount during the growth. For both samples no chlorine could be measured by virtue of EDX. It can be seen, however, from the scatter plot that a Cl doping is detectable by ERDA. Surprisingly, a measurable chlorine content only occurs for the (1 1 1) oriented sample that could be simulated under the assumption of a homogeneous Cl distribution of 4400 ppm in the ZnSe layer. The N(E, t) scatter plot region of the (1 0 0) sample, where Cl events arise in the (1 1 1) sample, displays a few events which may account for background or a very weak Cl doping. Assuming the latter hypothesis to be valid, a maximum doping of 1200 ppm would be inferred from the count rate. However, this value can only be considered as an upper limit, since the background density in the neighboring (E, t) regions is of comparable strength. For a quantitative analysis the background was determined and subtracted from the count number. A chlorine concentration of less than 600 ppm could then be inferred for the (1 0 0)grown sample. Although this example is a thin film example, the conclusions concerning detection limits of contamination are also valid for bulk material. In the meantime a reduction of the background was achieved by a change of detector parameters resulting in an improved impurity detection limit of about 100 ppm. The experiment illustrates the capability of the ERDA method to reveal interesting details of the dependency of the doping mechanism from the growth mode of crystal layers. In conclusion, we have achieved a precision in stoichiometry measurements of 1% and in impurity
M. Birkholz et al. / Journal of Crystal Growth 197 (1999) 571–575
575
Fig. 4. Scatter plots of ZnSe thin films deposited by MOCVD on (1 0 0) and (1 1 1)GaAs wafers. Chlorine is seen to be built in detectable amounts only for deposition on (1 1 1) oriented GaAs wafer. The Cl concentration was analysed to amount 4400 ppm.
concentrations of approximately 100 ppm for bulk II—VI materials. We thank T. Kampschulte and U. Blieske for supplying thin MOCVD ZnSe films. The work was partially supported by BMBF under contract number 0329740. References [1] L.L. Kazmerski, Ren. Sust. Energ. Rev. 1 (1997) 71. [2] T. Weiss, M. Birkholz, Y. Tomm, A. Ja¨ger-Waldau, M.C. Lux-Steiner, J. Mater. Sci. (1998), submitted.
[3] A. Ja¨ger-Waldau, N. Meyer, T. Weiss, S. Fiechter, M.C. Lux-Steiner, K. Tempelhoff, W. Richter, Jpn. J. Appl. Phys. 37 (1998) 1617. [4] J.R. Tesmer, M. Nastasi (Eds.), Handbook of Modern Ion Beam Materials Analysis, Pittsburg, 1995, p. 83. [5] W. Bohne, J. Ro¨hrich, G. Ro¨schert, Nucl. Instr. and Meth. B 136—138 (1998) 633. [6] M. Mayer, SIMNRA User’s Guide, Max-Planck-Institut fu¨r Plasmaphysik, 1996. [7] H. Hartmann, R. Mach, B. Selle, in: E. Kaldis (Ed.), Current Topics in Materials Science, vol. 9, North-Holland, Amsterdam, 1982, p. 1. [8] J.F. Ziegler, J.M. Manoyan, Nucl. Instr. and Meth. B 35 (1988) 215.
Stoichiometry and impurity concentrations in II—VI compounds measured by elastic recoil detection analysis (ERDA) M. Birkholz*, W. Bohne, J. Ro¨hrich, A. Ja¨ger-Waldau, M.C. Lux-Steiner Hahn-Meitner-Institut, Bereich Festko( rperphysik, Glienicker Str. 100, D-14109 Berlin, Germany
Abstract Polycrystalline ZnSe will be used in a chemical vapour deposition process for thin-film solar-cell emitter layers. The precursor bulk material was characterised with respect to stoichiometry and impurity concentrations by the ERDA method, for which an experimental setup is installed at the Berlin Ion Beam Facility (ISL). The sample material was irradiated with high-energy Kr and Xe projectile ions of 120 and 250 MeV, respectively. The energy and time-of-flight (TOF) of the released sample atoms were detected. In contrast to the Rutherford backscattering technique the measurement of both parameters enables the separation of different masses not only in thin-films but also in bulk material. ERDA allows the simultaneous depth profiling of heavy and light elements including hydrogen. We found ZnSe bulk samples to exhibit an oxygen surface contamination with a Zn/O ratio of 1 : 2 and thickness of 27;10 at/cm. An accuracy in stoichiometry, i.e., Zn/Se ratio of better than 1% could be achieved and impurity concentrations could be detected down to the 100 ppm range. Detection limits of the ERDA technique and its capacity for concentration profiling in II—VI materials will be discussed. 1999 Elsevier Science B.V. All rights reserved. PACS: 61.18.Bn; 61.50.Nw; 85.60.Dw Keywords: Solar-cell materials; Stoichiometry; Doping; Elastic recoil detection
Among other applications, II—VI compounds are currently under investigation as buffer layers in thin film solar cells utilising chalcopyrite absorber layers. High efficiencies of almost 18% have been achieved for n-CdS/p-Cu(Ga,In)Se devices under * Corresponding author. Fax: #49 30 6705 3333; e-mail: [email protected].
AM 1.5 radiation conditions [1]. The aim of our work is to develop a n-ZnSe/p-CuGaSe hetero diode for which the preparation of CuGaSe source material and thin films has already been succeeded [2,3]. Quantitative knowledge on stoichiometry and contamination is an important prerequisite for the optimisation of solar-cell materials. Therefore, we investigated the elastic recoil detection analysis
0022-0248/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 7 5 5 - 6
572
M. Birkholz et al. / Journal of Crystal Growth 197 (1999) 571–575
(ERDA) [4] to determine these values for bulk material and thin films. Like the well-known Rutherford backscattering (RBS) technique, ERDA is another ion beam method, for which the sample is irradiated by projectile ions of kinetic energies in the MeV range. But whereas for RBS only one single energy spectrum of the projectile ions scattered at all the different constituents of the sample is obtained, ERDA facilitates to extract separate energy spectra for each element. Therefore, ERDA does not only measure the energy but in coincidence also the mass or atomic number of each recoil that is scattered out of the sample. The amount of ejected atoms, however, maximally attains the ppb range, causing only a negligible loss of sample mass. The number of ejected atoms depends on the amount of a certain element contained in the sample, on the number of projectile ions impinging the sample and on the corresponding exactly known Rutherford cross section, i.e., ERDA is an absolute method which does not need calibration standards. An outstanding feature of ERDA is the almost equal sensitivity for the detection of all elements (except for hydrogen for which the sensitivity is even enhanced by a factor of four). As for RBS the maximum energy for each element spectrum is obtained from the corresponding scattering at the surface of the sample, while scatter processes from deeper parts are found at smaller energies, since the particles are slowed down along their trajectories through the sample. Therefore, the ERDA method delivers depth-resolved concentrations. The investigations were carried out with the ERDA setup installed at the ion beam laboratory (ISL) of Hahn-Meitner-Institut Berlin, a detailed description of which is given in Ref. [5]. Typical ions used were Kr and Xe with energies of 120 and 250 MeV, respectively. Because the experiments were performed in reflection geometry with a scattering angle of 40° the energies allow to analyse samples up to depths of a few lm. For the element identification and separation the time-of-flight method (TOF) is used. The spectrometer consists of two TOF-energy telescopes. One, the “long telescope”, uses start and stop channel plate detectors with a long flight path of 123 cm in between and a large (6;7.2 cm) Si-energy detector placed behind the stop detector. With the achieved time
resolution of approximately 135 ps and typical TOF values of 30—100 ns a sufficient mass resolution is possible. Because the detection efficiency of this system for hydrogen and helium is less than 100%, an additional “short telescope” is installed with a flight path of only 46 cm and without separate time detectors. In this case the time-of-flight is measured with the fast signal of the Si-energy detector relative to the RF signal of the cyclotron which delivers a pulsed beam with a pulse width of about 400 ps. Although the time resolution is reduced compared to the long TOF telescope it is still good enough to resolve light masses with the advantage of a detection efficiency of unity for all elements. The coincident energy E and time t signals (due to electronic reasons start and stop signals are reversed by an internal delay t ) are recorded event by event resulting in a three-dimensional intensity histogram N(E, t) which is called scatter plot. According to MJEt separated curves for each recoil mass M are obtained, broadened due to the energy and time resolution. From these branches energy spectra for all outscattered elements of the sample are extracted. Similar to RBS, the depth and concentration profile of the corresponding element can be obtained by simulating with a numerical program for which we made use of SIMNRA [6]. Example 1. Fig. 1a depicts the ERDA scatter plot of a polycrystalline bulk ZnSe sample that was commercially available (II—VI Inc.). The sample had the typical feature of IR window material, i.e., it was about 5 mm thick and exhibited parallel and smooth surfaces. The ERDA experiment was carried out with 250 MeV Xe ions. The sample was irradiated with a total dose of 5.3;10 particles. The mass branches of zinc and selenium are clearly separated. Both curves are significantly broadened not only due to the detectors resolution but mainly due to the fact that natural zinc and selenium occur with five and six stable isotopes, respectively. In the high-energy range the curves are split up due to isotope separation demonstrating the high-quality mass separation of the setup. In terms of phase diagrams, zinc selenide may be considered as a line compound since stoichiometry deviations are known to occur only in the 10\ range [7]. The
M. Birkholz et al. / Journal of Crystal Growth 197 (1999) 571–575
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Fig. 1. (a) Scatter plot and (b) elemental energy projections of a bulk ZnSe sample. The solid line in (b) represents the simulation with the Zn/Se concentration ratio fixed to the best fitting value 1.01(1) : 0.99(1).
ERDA measurement of ZnSe therefore was used as a test to determine the precision obtainable in stoichiometry measurements. From the events of Zn and Se branches in the scatter plot separate energy spectra were extracted, which are shown in Fig. 1b. The simulation of the spectra were performed assuming a depth-independent stoichiometry. Simulated spectra are shown as solid lines in the figure. The optimum fit to the data was obtained with atomic concentrations c "1.01(1) 8 and c "0.99(1), where the estimated standard de1 viations are given in parentheses. The ideal atomic ratio Zn/Se"1 : 1 is seen to fall within the confidence interval of the measurement. It is concluded that a precision of about 1% can be obtained for the stoichiometry measurement so far. If one is interested in even more precise measurements, the limitations have to be considered. First of all, the precision is limited by statistics, but this factor can be improved by a higher dose. The next point is that for samples thicker than the detectable range the stopping power influences the integral of the calculated spectra. That is not the case for thin films where the spectra have a finite energy width for each element detected. Stopping powers in the high-energy range are not yet known with the necessary precision. Another limiting factor is the energy straggling and the multiple scattering the probability of which increase with increasing depth.
Finally, the analysis makes use of Bragg’s rule which is a superposition principle for stopping powers and states that the stopping of ions in compounds is the sum of the stopping power of its constituents. However, according to previous investigations, this rule is fulfilled for the compounds considered here, because the samples consist of heavy elements and because the ion energy is larger than 250 keV/amu [8]. Example 2. One of the investigated ZnSe samples exhibited an oxygen surface contamination. The measurement was done with an 120 MeV Kr ion beam using a total radiation dose of 3.1;10. The energy projections of zinc and oxygen are shown in Fig. 2. An energy projection of selenium could not be obtained due to a large overlap of the selenium branch with the backscattered Kr projectile ions in the scatter plot. Elemental spectra were simulated by assuming the bulk material to be covered by a thin oxide layer. The best fit to the experimental data was obtained by setting the Zn/O ratio to 1 : 2 and the layer thickness to 27;10 at/cm. Assuming the density of the layer to be equal to the bulk density of ZnSe a thickness of 62 A> is calculated from the areal density value. This investigation demonstrates the capability of the ERDA technique with respect to the detection of surface contamination of compound semiconductors.
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M. Birkholz et al. / Journal of Crystal Growth 197 (1999) 571–575
in order to obtain separate cadmium and sulphur energy spectra and to investigate their relative concentrations. No deviations from the ideal atomic ratio of Cd/S"1 : 1 could be detected within the present limits of precision of our setup and analysis routines.
Fig. 2. Zinc and oxygen energy projections of ERDA measurement of a bulk ZnSe sample. The solid line accounts for the simulation of a 62 A> thick oxide surface layer with a Zn/O ratio of 1 : 2 that was assumed to cover the bulk sample.
Fig. 3. Scatter plot of a CdS bulk sample measured with the short TOF telescope.
Example 3. In Fig. 3 we present the scatter plot of a measured CdS single crystal (2;10 mm, (1 0 0) Crystec GmbH) that was obtained with the short TOF configuration (250 MeV Xe, 5 min, total dose 2.75;10 particles). A contamination of the sample surface by hydrogen and carbon was observed. The surface layer was simulated with an areal density of 44;10 at/cm and a composition of C/H"3 : 1. The sample was also investigated with the long TOF configuration (not shown here)
Example 4. Finally, in Fig. 4, we present the ERDA scatter plots of two ZnSe thin films grown by MOCVD on GaAs. The production of both samples occurred within the same run, they only differ with respect to substrate orientation which were either (1 0 0) or (1 1 1) wafers. Accordingly, the growth mode of the ZnSe layer was by stacking of either cubic or hexagonal layers. Doping of ZnSe was done by supplying chlorine in small amount during the growth. For both samples no chlorine could be measured by virtue of EDX. It can be seen, however, from the scatter plot that a Cl doping is detectable by ERDA. Surprisingly, a measurable chlorine content only occurs for the (1 1 1) oriented sample that could be simulated under the assumption of a homogeneous Cl distribution of 4400 ppm in the ZnSe layer. The N(E, t) scatter plot region of the (1 0 0) sample, where Cl events arise in the (1 1 1) sample, displays a few events which may account for background or a very weak Cl doping. Assuming the latter hypothesis to be valid, a maximum doping of 1200 ppm would be inferred from the count rate. However, this value can only be considered as an upper limit, since the background density in the neighboring (E, t) regions is of comparable strength. For a quantitative analysis the background was determined and subtracted from the count number. A chlorine concentration of less than 600 ppm could then be inferred for the (1 0 0)grown sample. Although this example is a thin film example, the conclusions concerning detection limits of contamination are also valid for bulk material. In the meantime a reduction of the background was achieved by a change of detector parameters resulting in an improved impurity detection limit of about 100 ppm. The experiment illustrates the capability of the ERDA method to reveal interesting details of the dependency of the doping mechanism from the growth mode of crystal layers. In conclusion, we have achieved a precision in stoichiometry measurements of 1% and in impurity
M. Birkholz et al. / Journal of Crystal Growth 197 (1999) 571–575
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Fig. 4. Scatter plots of ZnSe thin films deposited by MOCVD on (1 0 0) and (1 1 1)GaAs wafers. Chlorine is seen to be built in detectable amounts only for deposition on (1 1 1) oriented GaAs wafer. The Cl concentration was analysed to amount 4400 ppm.
concentrations of approximately 100 ppm for bulk II—VI materials. We thank T. Kampschulte and U. Blieske for supplying thin MOCVD ZnSe films. The work was partially supported by BMBF under contract number 0329740. References [1] L.L. Kazmerski, Ren. Sust. Energ. Rev. 1 (1997) 71. [2] T. Weiss, M. Birkholz, Y. Tomm, A. Ja¨ger-Waldau, M.C. Lux-Steiner, J. Mater. Sci. (1998), submitted.
[3] A. Ja¨ger-Waldau, N. Meyer, T. Weiss, S. Fiechter, M.C. Lux-Steiner, K. Tempelhoff, W. Richter, Jpn. J. Appl. Phys. 37 (1998) 1617. [4] J.R. Tesmer, M. Nastasi (Eds.), Handbook of Modern Ion Beam Materials Analysis, Pittsburg, 1995, p. 83. [5] W. Bohne, J. Ro¨hrich, G. Ro¨schert, Nucl. Instr. and Meth. B 136—138 (1998) 633. [6] M. Mayer, SIMNRA User’s Guide, Max-Planck-Institut fu¨r Plasmaphysik, 1996. [7] H. Hartmann, R. Mach, B. Selle, in: E. Kaldis (Ed.), Current Topics in Materials Science, vol. 9, North-Holland, Amsterdam, 1982, p. 1. [8] J.F. Ziegler, J.M. Manoyan, Nucl. Instr. and Meth. B 35 (1988) 215.