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Relative detection factor for quantification of Secondary Neutral Mass Spectrometry measurements of PbTe binary telluride
Vacuum 163 (2019) 99–102
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Relative detection factor for quantification of Secondary Neutral Mass Spectrometry measurements of PbTe binary telluride
T
Dmytro M. Zayachuka, Vasyl E. Slynkob, Csaba Bugac, Attila Csíkc,∗ a
Lviv Polytechnic National University, S. Bandera Str., 12, 79013, Lviv, Ukraine Institute for Problems of Material Science NASU, Chernivtsy Branch, Vilde Str., 5, 58001, Chernivtsy, Ukraine c Institute for Nuclear Research, Hungarian Academy of Sciences (ATOMKI), H-4026, Debrecen, Bem sqr. 18/c, Hungary b
A R T I C LE I N FO
A B S T R A C T
Keywords: PbTe Sputtering Relative detection factor SNMS
Knowledge of an exact value of the relative detection factor (RDF) is important for correct quantification of measurements of Secondary Neutral Mass Spectrometry (SNMS). A special crystal ingot was grown from the vapor phase to determine this coefficient for PbTe binary telluride. Sputtering of the samples was carried out by Ar+ ions with energy of 350 eV. High structural quality of the crystal grown from vapor phase allowed minimizing the density of the surface structures forming on the sputtered surface and the dimple relief of the surface. The magnitude of RDF is determined and the analytical expression is given for its energy dependence, taking into account correction caused by the impact of the transmission factors which determine the fractions of emitted Pb and Te atoms collected into the mass spectrometer.
1. Introduction As lead telluride and its solid solutions are good based materials for manufacturing of infrared photodetectors, laser structures, and thermoelectric devices, a large number of studies focused on these materials and their applications [1–3]. One of the main focuses of research is the composition analysis and the study of distribution of impurities. Complete characterization of samples requires a sensitive chemical analysis including identification of compounds and quantitative determination of elements. Ion bombardment methods, in particular SNMS, are usually used for such analysis [4–6]. In SNMS method the neutral particles are sputtered from the surface and post-ionized via interaction with energetic electrons. Since this method uses very low ion bombardment energies, while the current densities are uniform over the entire analyzed area, the ∼1 nm resolution for the depth profile analysis can be achieved [7–9]. The main advantage of the method is the separation of the emission process from the post-ionization step (ionization takes outside of the sputtered material), which results in low dependence on the nature of matrix compared to the other sputtering methods, e.g. Secondary Ion Mass Spectrometry. The neutral particles make up approximately 99% of the species removed during the target bombardment [10], therefore their flux reflects the true surface composition. The flux intensity I of a sputtered chemical element measured in the SNMS conditions depends on its absolute sensitivity factor D o , which, in turn, depends on the different characteristics of the detected ∗
element, the plasma, and the mass spectrometer and is not known in advance [6,11]. As a result, for straightforward quantification of SNMS data for a diatomic (with A and B components) solid it is necessary to know the relative detection factor (RDF) of the sputtered species A and B: DA (B ) = DAo / DBo . Knowing RDF and having determined experimentally the ratio of the intensities I of the sputtered species IA/IB, one can establish the ratio of their sputter yields YA/ YB = (IA/ IB )/ DA (B ) and further convert it into the concentrations of the sputtered species in the sputtered phase. It is assumed that RDF of any two elements is largely independent of sample composition [6] and thus can be derived from a standard sample and used for other measurements. For given plasma and sputtering conditions RDFs are expected to be independent of both the actual sample to be analyzed [12] and the bombarding ion energy if the oblique take-off technique is used [13]. However, different chemical elements have different ionization ability and this should complicate correct determination of RDF of target constituents. In our recent study [14], examining the samples of IV-VI semiconductors PbTe, SnTe, and GeTe we found that in SNMS conditions the magnitude of RDF of the intrinsic components of compounds is actually dependent on sputtering energy. It was shown that this is caused by the changes of the sputtering surface morphology and the differences in the constituents’ masses. Since RDF is one of the basic parameters for quantitative analysis of solids by SNMS, the knowledge of their correct value is crucial. In the present work we were able, by
Corresponding author. E-mail address: [email protected] (A. Csík).
https://doi.org/10.1016/j.vacuum.2019.02.008 Received 28 October 2018; Received in revised form 12 January 2019; Accepted 5 February 2019 Available online 07 February 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
Vacuum 163 (2019) 99–102
D.M. Zayachuk, et al.
preparing a special type of the PbTe sample, to eliminate the surface morphology effect and determine the value of DTe(Pb). The analytical expression for energy dependence thereof is given. 2. Experimental details In order to achieve high depth resolution of SNMS, low sputtering energy should be used [7–9]. For PbTe crystals it was recently reported that sputtering with ion energies of 50–550 eV in SNMS conditions led to the aperiodical oscillations of Pb and Te sputtering yield, and significant changes in the morphological state of sputtered crystal surface were observed [15–18]. It was shown that the lower the sputtering energy, the higher is the density of surface structures and smaller their average sizes. The morphology of the sputtered crystal surface was modified under impact of Ar+ ions, dimple reliefs were occurred and the surface was covered by an array of small formations, like hillocks, pyramids, cones and others. The change of surface morphology in such form can change the incidence angle of the Ar+ ions, which take effect on the value of RDF [14] and makes more difficult its experimental determination. A direct solution of this problem would be to sputter the surface under absence of formation of any surface structures or at least a significant decrease of the structure density. In the case of our previous works the investigated samples were manufactured from the crystals grown from melt by the Bridgman method, which is commonly used in the technology of IV-VI crystal growth. However, for these samples the surface quality was always insufficient to completely prevent the formation of the surface structures during the sputtering [15–18]. It is well known that the highest quality of IV-VI crystals can be achieved by growing them from the vapor phase [19]. Therefore, a special crystal ingot for the new sputtering experiments dedicated to the determination of DTe(Pb) value was grown in such way. Fig. 1 shows prepared crystal ingot which was used for manufacturing of the investigated samples. The crystal was grown in a free volume on the seed of (100) orientation. The flat end of the crystal had very high structural quality and the same crystallographic orientation (100), which was confirmed by X-Ray diffraction measurement (Fig. 2). At the end of the crystal one can see also small-cut surface of (111) crystallographic orientation. Sputtering experiments were carried out in an INA-X type SNMS instrument produced by SPECS GmbH, Berlin [20]. Ar+ ions were extracted from the low pressure plasma by the negatively biased sample surface (−350 V) in direct bombardment mode. The sputtered area was confined to a circle of 2 mm in diameter by a Ta mask. Post-ionized particles were directed into a quadruple mass spectrometer type Balzers QMA 410 by electrostatic lenses. The surface morphology of samples after ion sputtering was analyzed by Scanning Electron Microscopy (SEM) (Hitachi S-4300 CFE). The composition of the studied samples was verified by Energy Dispersive X-Ray Analysis (EDX) method. Ten measurements were carried out at different points on the surface of the samples, and the
Fig. 2. X-ray diffraction spectra recorded from the flat end of the crystal ingot shown in Fig. 1.
results were expressed as the mean ± standard deviation. For the sample grown from melt by the Bridgman method we found 50.7 ± 1.6 at% for Pb and 49.3 ± 1.6 at% for Te. For the sample grown from vapor phase the 50.8 ± 1.2 at% (Pb) and 49.2 ± 1.2 at% (Te) has been found. 3. Results and discussion For comparative analysis four PbTe crystals samples were used in this study for the sputtering experiments. Two samples were fabricated from the flat end of the vapor phase crystal shown in Fig. 1. Other two were fabricated from the lateral surfaces of the crystal ingot grown from melt by the Bridgman method. Duration of the sample sputtering was 50 min. The SEM pictures of the sputtered surfaces were examined and the SNMS sputtering spectra were recorded. SEM images of the sputtered surfaces are presented in Fig. 3. It is clearly seen that the crystal grown from vapor phase featured much lower density of the surface structures formed on the sputtered crystal surface. Only a few individual surface formations were observed on a sputtered surface of the Sample 1 (Fig. 3a). For the Sample 2, manufactured from the same crystal, the density of surface structures was somewhat higher (Fig. 3b), although still significantly lower than the typical for the samples grown from melt by the Bridgman method (Fig. 3c, and d). The SNMS spectra of the investigated samples are presented in Fig. 4 as dependence of the ratio of integral intensities of sputtered Te and Pb Itot(Te)/Itot(Pb) on the sputtering time. It is evident that the type of sputtering surface does not affect the dominant features of the temporal changes of sputtering spectra. At the initial stage of sputtering the ratio Itot(Te)/Itot(Pb) increases rapidly with sputtering time, after approximately 10 min of sputtering the ratio of measured integral intensities of the sputtered matrix constituents is stabilized. This is the result of preferential sputtering of Pb. As the sputter yield of Pb is higher, the sample sputtering surface is enriched by Te during the sputtering process. The surface enrichment by Te increases its sputter yield and decreases the sputter yield of Pb. This leads to the increase of the Itot(Te)/Itot(Pb) ratio over time. Under prolonged sputtering, increasing surface concentration of Te fully balances the preferential sputtering of Pb. As a result, the ratio of their sputter yields is stabilized, monotonically tending, in accordance with the law of conservation of mass, to the ratio of the bulk elements’ concentrations [21]. Two observations are of special interest here. The first is that the magnitude of the stabilized value, to which the ratio of the intensities of measured signals of the sputtered elements tends with the increase of
Fig. 1. The general view of the PbTe crystal ingot grown from vapor phase. 100
Vacuum 163 (2019) 99–102
D.M. Zayachuk, et al.
Fig. 3. SEM pictures of PbTe crystal surfaces after sputtering by Ar+ beam with energy of 350 eV during 50 min. a) Sample 1, vapor phase; b) Sample 2, vapor phase; c) Sample 3, Bridgman method; d) Sample 4, Bridgman method.
Itot(Te)/Itot(Pb) obtained above, in the relation (1). By applying the measured data we obtain magnitude of DTe(Pb) in the range of 0.87 (Sample 1) - 1.4 (Sample 4). RDF of any two elements A and B generally depends on the ratio of their neutral-to-ion conversion factor in the plasma (αA/αB) and on the ratio of the transmission factors (GA/GB) which determines the fraction of the emitted atoms collected by the mass spectrometer [12]. The neutral-to-ion conversion factor (α) in the plasma firstly depends on the energy of the atom ionization. The higher the ionization energy, the lower the probability of the ionization. The lower the probability of the atom ionization, the smaller fraction of the sputtered atoms will be taken into account in the process of their counting, and the lower the measured intensity of the sputtered element's signal. The ionization energy of Pb is 7.4167 eV; the ionization energy of Te is 9.0097 eV [23]. Consequently, in the process of SNMS analysis a smaller fraction of the sputtered Te atoms is counted than of the sputtered Pb atoms. So, the ratio αTe/αPb must lead to a lower than one DTe(Pb). The ratio of the transmission factors GTe/GPb, which determines the fraction of the emitted Pb and Te atoms collected into the mass spectrometer, depends on the sputtering surface morphology (because it changes the angular distribution of the sputtered species) and on the sputtered atom energy (because of the difference in the Pb and Te atom masses). The first factor monotonically influences the magnitude of DTe(Pb) in the entire range of the sputtering energies E, the second one contributes to it only in the range of low sputtering energies below approximately 200 eV [14]. Under impact of sputtering by Ar+ ions the PbTe sample surface morphology is modified by the dimple relief and the formed surface structures. Their formation leads to the increase of DTe(Pb) [14]. Among the investigated samples only Sample 1 has the sputtered surface practically free from the surface structures and a very slightly developed dimple relief. Taking this into account we may consider the obtained magnitude 0.87 of DTe(Pb) for Sample 1 as a magnitude close to the correct value of RDF of Pb and Te, which is determined by their ionization efficiency in the plasma. In the absence of the formation of the surface structure on the sputtering surface this value may be used for quantitative calculations and for other energies of Ar+ ions larger than at least 200 eV. For the sputtering energy lower than 200 eV the
Fig. 4. Ratio of the experimental integral intensities Itot(Te)/Itot(Pb) of the PbTe samples for sputtering energy of 350 eV vs. sputtering time: 1(black) – Sample 1; 2 (red) – Sample 2; 3 (green) – Sample 3; 4 (blue) – Sample 4.
the sputtering time, is not a constant. It varies from sample to sample. This magnitude for the Sample 4 exceeds the similar value for the Sample 1 by approximately 61.5%. This significantly exceeds the possible experimental error of measurement. The second peculiarity is that there is correlation between the magnitude of the stabilized value of the ratio Itot(Te)/Itot(Pb) and the morphology of the sputtering sample surface. The magnitude of the ratio of sputtering rates was lowest for the sample with the fewest surface structures - Sample 1. The mean of the stabilized ratio Itot(Te)/ Itot(Pb) for it is 0.87 with the standard deviation of 0.003. RDF of any A and B elements can be defined as
DA (B ) =
CB IA CA IB
(1)
where IA and IB are the sputtered ion intensities, CA and CB are the concentrations of the elements [6,22]. The composition of PbTe crystals grown from vapor phase is always very close to stoichiometry [19]. Therefore, to calculate DTe(Pb) we may put the ratio of Pb and Te concentrations CPb/CTe = 1 and the stabilized values of the ratio 101
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corrective term on the transmission factor for DTe(Pb) must be inE troduced. Using its simplest approximation 1 + C exp − Δ suggested in Ref. [14] and the values of the parameters C and Δ for PbTe obtained there, namely 24 and 34 eV respectively, we may write the followed empirical expression for RDF of Pb and Te as it dependence on sputtering energy (E) of Ar+ ions:
[2] M. Rahim, F. Felder, M. Fill, H. Zogg, Optically pumped 5 ??m IV–VI VECSEL with Al-heat spreader, Optic Lett. 33 (24) (2008) 3010–3012 https://doi.org/10.1364/ OL.33.003010. [3] E. Hazan, O. Ben-Yehuda, N. Madar, Y. Gelbstein, Functional graded Germanium–Lead chalcogenide‐based thermoelectric module for renewable energy applications, Adv. Energy Mater. 5 (2015) 1500272 https://doi.org/10.1002/aenm. 201500272. [4] P. Sigmund, Elements of sputtering theory, in: T. Som, D. Kanjilal (Eds.), Nanofabrication by Ion-Beam Sputtering, Pan Stanford Publishing, 2013, pp. 1–40 https://doi.org/10.4032/9789814303767. [5] M. Kopnarski, H. Jennet, Electron impact (EI) secondary neutral mass spectrometry (SNMS), in: Bubert H. Friedbacher (Ed.), Surface and Thin Film Analysis. A Compendium of Principles, Instrumentation, and Applications, Wiley-VCH Verlag GmbH &Co.KGaA, 2011, pp. 161–177 https://doi.org/10.1002/9783527636921. ch9. [6] H. Oechsner, Secondary Neutral Mass Spectrometry (SNMS) and its application to depth profile and interface analysis, in: H. Oechsner (Ed.), Thin Film and Depth Profile Analysis, Springer-Verlag, 1984, pp. 63–86 https://doi.org/10.1007/978-3642-46499-7. [7] E. Stumpe, H. Oechsner, H. Schoof, High-resolution sputter depth profiling with a low pressure HF plasma, Appl. Phys. 20 (1979) 55–60 https://doi.org/10.1007/ BF00901787. [8] S. Hofmann, High resolution compositional depth profiling, J. Vac. Sci. Technol. 9 (3) (1991) 1466–1476 https://doi.org/10.1116/1.577647. [9] S. Hofmann, Sputter depth profile analysis of interfaces, Rep. Prog. Phys. 61 (1998) 827–888 https://doi.org/10.1088/0034-4885/61/7/002. [10] W. Husinsky, G. Betz, Fundamental aspects of SNMS for thin film characterization: experimental studies and computer simulations, Thin Solid Films 272 (1996) 289–309 https://doi.org/10.1016/0040-6090(95)06954-2. [11] W. Bieck, H. Gnaser, H. Oechsner, Analytical performance of a secondary neutral microprobe with electron gas post ionization and magnetic sector mass spectrometer, J. Vac. Sci. Technol. 12 (1994) 2537–2543 https://doi.org/10.1116/1. 579053. [12] H. Oechsner, Secondary neutral mass spectrometry (SNMS)-recent methodical progress and applications to fundamental studies in particle/surface interaction, Int. J. Mass Spectrom. Ion Process. 143 (1995) 271–282 https://doi.org/10.1016/ 0168-1176(94)04122-N. [13] J. Jorzick, J. Losch, M. Kopnarski, H. Oechsner, Detection in the ppm range and high-resolution depth profiling with the new SNMS instrument INA-X, Appl. Phys. A 78 (2004) 655–658 https://doi.org/10.1007/s00339-003-2275-5. [14] D.M. Zayachuk, V.E. Slynko, A. Csik, Peculiar properties of preferential sputtering of PbTe, SnTe, and GeTe by Ar+ ion plasma, Mater. Sci. Semicond. Process. 88 (2018) 103–108 https://doi.org/10.1016/j.mssp.2018.07.037. [15] D.M. Zayachuk, E.I. Slynko, V.E. Slynko, A. Csik, Oscillations and huge preferences of PbTe crystal surface sputtering under Secondary Neutral Mass Spectrometry conditions, Mater. Lett. 173 (2016) 167–169 https://doi.org/10.1016/j.matlet. 2016.03.038. [16] D. Zayachuk, A. Csik, PbTe Crystal Sputtering and Re-deposition of Sputtered Species, LAP Lambert Academic Publishing, Saarbrucken, Germany, 2016. [17] D.M. Zayachuk, V.E. Slynko, A. Csik, Morphology of PbTe crystal surface sputtered by argon plasma under Secondary Neutral Mass Spectrometry conditions, Phys. Chem. Solid States 17 (2016) 336–341 https://doi.org/10.15330/pcss.17.3.336341. [18] D.M. Zayachuk, V.E. Slynko, A. Csik, Formation of the sputtered phase of PbTe crystals by Ar+ plasma and re-deposition of the sputtered Species at Secondary Neutral Mass Spectrometry conditions, Phys. Chem. Solid States 18 (2017) 21–28 https://doi.org/10.15330/pcss.18.1.21-28. [19] G. Nimtz, B. Schlicht, Narrow-Gap Semiconductors: Narrow Gap Lead Salts, Springer, Berlin, 1985, pp. 1–117 https://doi.org/10.1007/BFb0044919. [20] K. Vad, A. Csik, G.A. Langer, Secondary neutral mass spectrometry-a powerful technique for quantitative elemental and depth profiling analyses of nanostructures, Spectrosc. Eur. 21 (2009) 13–17. [21] L.C. Feldman, J.W. Mayer, Fundamentals of Surface and Thin Film Analysis, NorthHolland, Amsterdam, 1986https://doi.org/10.1016/0168-583X(87)90553-2. [22] A. Wucher, F. Novak, W. Reuter, Relative elemental sensitivity factors in secondary neutral mass spectrometry, J. Vac. Sci. Technol. 6/4 (1988) 2265–2270 https://doi. org/10.1116/1.575022. [23] A. Kramida, Yu. Ralchenko, J. Reader, NIST ASD Team, NIST Atomic Spectra Database (Ver. 5.5.6), National Institute of Standards and Technology, Gaithersburg, MD, 2018 Online https://doi.org/10.18434/T4W30F https:// physics.nist.gov/PhysRefData/ASD/ionEnergy.html.
( )
DTe (Pb)(E ) = 0.87(1 + 24 exp( −E /34))
(2)
The obtained RDF value and its energy dependence may be used for straightforward quantification of SNMS data for any PbTe samples if the sample surface remains practically flat after sputtering, regardless of whether it is a thin layer or a bulk material. Because of the low sputtering energy, as usually applied in SNMS method, the sputtering rate in range of nm/sec and as a results of this only the top few atomic layers of the sample involved in sputtering process. Therefore, the origin of the surface (thin film or bulk material) cannot take significant effect on sputtering process not like surface morphology. The change of surface morphology e.g. can change the incidence angle of the Ar+ ions, which can take effect on the value of RDF. If the sample surface is covered by surface structures in the process of its sputtering, it is necessary to introduce an adjustment to the value of RDF, which must be determined individually in each particular case. 4. Conclusions For determination of relative detection factor (DTe(Pb)) of Te and Pb the sputtering by Ar plasma beams with energy of 350 eV during 50 min of the (100) natural surface of the special PbTe crystal ingot grown from the vapor phase in a free volume on the seed of (100) orientation was carried out. The high quality and crystallographic orientation of the initial crystal surface to be sputtered was confirmed by X-ray diffraction method. The morphology of the sputtered surfaces was examined by SEM. The study has shown that the quality of the crystalline faceting surfaces of the PbTe crystal grown from vapor phase is sufficient both to prevent the formation of surface structures in the process of sputtering under the used conditions and to minimize the formation of dimple relief on the sputtered surface. It allowed determination of the correct value of DTe(Pb), 0.87. Taking into account the impact of the transmission factors determining the fraction of the emitted Pb and Te atoms collected by the mass spectrometer on DTe(Pb), the empirical expression for dependence of relative detection factor on the sputtering Ar+ ions energy for the case of a flat sputtered surface was obtained, DTe (Pb)(E ) = 0.87 (1 + 24 exp(-E/34)). Acknowledgement The work is supported by GINOP-2.3.2-15-2016-00041 project, which is co-financed by the European Union and the European Regional Development Fund. References [1] Lead chalcogenides: physics and applications, Series Editor M.O. Manasreh, in: D. Khokhlov (Ed.), Vol. 18 of Optoelectronic Properties of Semiconductors and Superlattices, Taylor & Francis Books Inc., New York and London, 2002.
102
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Relative detection factor for quantification of Secondary Neutral Mass Spectrometry measurements of PbTe binary telluride
T
Dmytro M. Zayachuka, Vasyl E. Slynkob, Csaba Bugac, Attila Csíkc,∗ a
Lviv Polytechnic National University, S. Bandera Str., 12, 79013, Lviv, Ukraine Institute for Problems of Material Science NASU, Chernivtsy Branch, Vilde Str., 5, 58001, Chernivtsy, Ukraine c Institute for Nuclear Research, Hungarian Academy of Sciences (ATOMKI), H-4026, Debrecen, Bem sqr. 18/c, Hungary b
A R T I C LE I N FO
A B S T R A C T
Keywords: PbTe Sputtering Relative detection factor SNMS
Knowledge of an exact value of the relative detection factor (RDF) is important for correct quantification of measurements of Secondary Neutral Mass Spectrometry (SNMS). A special crystal ingot was grown from the vapor phase to determine this coefficient for PbTe binary telluride. Sputtering of the samples was carried out by Ar+ ions with energy of 350 eV. High structural quality of the crystal grown from vapor phase allowed minimizing the density of the surface structures forming on the sputtered surface and the dimple relief of the surface. The magnitude of RDF is determined and the analytical expression is given for its energy dependence, taking into account correction caused by the impact of the transmission factors which determine the fractions of emitted Pb and Te atoms collected into the mass spectrometer.
1. Introduction As lead telluride and its solid solutions are good based materials for manufacturing of infrared photodetectors, laser structures, and thermoelectric devices, a large number of studies focused on these materials and their applications [1–3]. One of the main focuses of research is the composition analysis and the study of distribution of impurities. Complete characterization of samples requires a sensitive chemical analysis including identification of compounds and quantitative determination of elements. Ion bombardment methods, in particular SNMS, are usually used for such analysis [4–6]. In SNMS method the neutral particles are sputtered from the surface and post-ionized via interaction with energetic electrons. Since this method uses very low ion bombardment energies, while the current densities are uniform over the entire analyzed area, the ∼1 nm resolution for the depth profile analysis can be achieved [7–9]. The main advantage of the method is the separation of the emission process from the post-ionization step (ionization takes outside of the sputtered material), which results in low dependence on the nature of matrix compared to the other sputtering methods, e.g. Secondary Ion Mass Spectrometry. The neutral particles make up approximately 99% of the species removed during the target bombardment [10], therefore their flux reflects the true surface composition. The flux intensity I of a sputtered chemical element measured in the SNMS conditions depends on its absolute sensitivity factor D o , which, in turn, depends on the different characteristics of the detected ∗
element, the plasma, and the mass spectrometer and is not known in advance [6,11]. As a result, for straightforward quantification of SNMS data for a diatomic (with A and B components) solid it is necessary to know the relative detection factor (RDF) of the sputtered species A and B: DA (B ) = DAo / DBo . Knowing RDF and having determined experimentally the ratio of the intensities I of the sputtered species IA/IB, one can establish the ratio of their sputter yields YA/ YB = (IA/ IB )/ DA (B ) and further convert it into the concentrations of the sputtered species in the sputtered phase. It is assumed that RDF of any two elements is largely independent of sample composition [6] and thus can be derived from a standard sample and used for other measurements. For given plasma and sputtering conditions RDFs are expected to be independent of both the actual sample to be analyzed [12] and the bombarding ion energy if the oblique take-off technique is used [13]. However, different chemical elements have different ionization ability and this should complicate correct determination of RDF of target constituents. In our recent study [14], examining the samples of IV-VI semiconductors PbTe, SnTe, and GeTe we found that in SNMS conditions the magnitude of RDF of the intrinsic components of compounds is actually dependent on sputtering energy. It was shown that this is caused by the changes of the sputtering surface morphology and the differences in the constituents’ masses. Since RDF is one of the basic parameters for quantitative analysis of solids by SNMS, the knowledge of their correct value is crucial. In the present work we were able, by
Corresponding author. E-mail address: [email protected] (A. Csík).
https://doi.org/10.1016/j.vacuum.2019.02.008 Received 28 October 2018; Received in revised form 12 January 2019; Accepted 5 February 2019 Available online 07 February 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
Vacuum 163 (2019) 99–102
D.M. Zayachuk, et al.
preparing a special type of the PbTe sample, to eliminate the surface morphology effect and determine the value of DTe(Pb). The analytical expression for energy dependence thereof is given. 2. Experimental details In order to achieve high depth resolution of SNMS, low sputtering energy should be used [7–9]. For PbTe crystals it was recently reported that sputtering with ion energies of 50–550 eV in SNMS conditions led to the aperiodical oscillations of Pb and Te sputtering yield, and significant changes in the morphological state of sputtered crystal surface were observed [15–18]. It was shown that the lower the sputtering energy, the higher is the density of surface structures and smaller their average sizes. The morphology of the sputtered crystal surface was modified under impact of Ar+ ions, dimple reliefs were occurred and the surface was covered by an array of small formations, like hillocks, pyramids, cones and others. The change of surface morphology in such form can change the incidence angle of the Ar+ ions, which take effect on the value of RDF [14] and makes more difficult its experimental determination. A direct solution of this problem would be to sputter the surface under absence of formation of any surface structures or at least a significant decrease of the structure density. In the case of our previous works the investigated samples were manufactured from the crystals grown from melt by the Bridgman method, which is commonly used in the technology of IV-VI crystal growth. However, for these samples the surface quality was always insufficient to completely prevent the formation of the surface structures during the sputtering [15–18]. It is well known that the highest quality of IV-VI crystals can be achieved by growing them from the vapor phase [19]. Therefore, a special crystal ingot for the new sputtering experiments dedicated to the determination of DTe(Pb) value was grown in such way. Fig. 1 shows prepared crystal ingot which was used for manufacturing of the investigated samples. The crystal was grown in a free volume on the seed of (100) orientation. The flat end of the crystal had very high structural quality and the same crystallographic orientation (100), which was confirmed by X-Ray diffraction measurement (Fig. 2). At the end of the crystal one can see also small-cut surface of (111) crystallographic orientation. Sputtering experiments were carried out in an INA-X type SNMS instrument produced by SPECS GmbH, Berlin [20]. Ar+ ions were extracted from the low pressure plasma by the negatively biased sample surface (−350 V) in direct bombardment mode. The sputtered area was confined to a circle of 2 mm in diameter by a Ta mask. Post-ionized particles were directed into a quadruple mass spectrometer type Balzers QMA 410 by electrostatic lenses. The surface morphology of samples after ion sputtering was analyzed by Scanning Electron Microscopy (SEM) (Hitachi S-4300 CFE). The composition of the studied samples was verified by Energy Dispersive X-Ray Analysis (EDX) method. Ten measurements were carried out at different points on the surface of the samples, and the
Fig. 2. X-ray diffraction spectra recorded from the flat end of the crystal ingot shown in Fig. 1.
results were expressed as the mean ± standard deviation. For the sample grown from melt by the Bridgman method we found 50.7 ± 1.6 at% for Pb and 49.3 ± 1.6 at% for Te. For the sample grown from vapor phase the 50.8 ± 1.2 at% (Pb) and 49.2 ± 1.2 at% (Te) has been found. 3. Results and discussion For comparative analysis four PbTe crystals samples were used in this study for the sputtering experiments. Two samples were fabricated from the flat end of the vapor phase crystal shown in Fig. 1. Other two were fabricated from the lateral surfaces of the crystal ingot grown from melt by the Bridgman method. Duration of the sample sputtering was 50 min. The SEM pictures of the sputtered surfaces were examined and the SNMS sputtering spectra were recorded. SEM images of the sputtered surfaces are presented in Fig. 3. It is clearly seen that the crystal grown from vapor phase featured much lower density of the surface structures formed on the sputtered crystal surface. Only a few individual surface formations were observed on a sputtered surface of the Sample 1 (Fig. 3a). For the Sample 2, manufactured from the same crystal, the density of surface structures was somewhat higher (Fig. 3b), although still significantly lower than the typical for the samples grown from melt by the Bridgman method (Fig. 3c, and d). The SNMS spectra of the investigated samples are presented in Fig. 4 as dependence of the ratio of integral intensities of sputtered Te and Pb Itot(Te)/Itot(Pb) on the sputtering time. It is evident that the type of sputtering surface does not affect the dominant features of the temporal changes of sputtering spectra. At the initial stage of sputtering the ratio Itot(Te)/Itot(Pb) increases rapidly with sputtering time, after approximately 10 min of sputtering the ratio of measured integral intensities of the sputtered matrix constituents is stabilized. This is the result of preferential sputtering of Pb. As the sputter yield of Pb is higher, the sample sputtering surface is enriched by Te during the sputtering process. The surface enrichment by Te increases its sputter yield and decreases the sputter yield of Pb. This leads to the increase of the Itot(Te)/Itot(Pb) ratio over time. Under prolonged sputtering, increasing surface concentration of Te fully balances the preferential sputtering of Pb. As a result, the ratio of their sputter yields is stabilized, monotonically tending, in accordance with the law of conservation of mass, to the ratio of the bulk elements’ concentrations [21]. Two observations are of special interest here. The first is that the magnitude of the stabilized value, to which the ratio of the intensities of measured signals of the sputtered elements tends with the increase of
Fig. 1. The general view of the PbTe crystal ingot grown from vapor phase. 100
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Fig. 3. SEM pictures of PbTe crystal surfaces after sputtering by Ar+ beam with energy of 350 eV during 50 min. a) Sample 1, vapor phase; b) Sample 2, vapor phase; c) Sample 3, Bridgman method; d) Sample 4, Bridgman method.
Itot(Te)/Itot(Pb) obtained above, in the relation (1). By applying the measured data we obtain magnitude of DTe(Pb) in the range of 0.87 (Sample 1) - 1.4 (Sample 4). RDF of any two elements A and B generally depends on the ratio of their neutral-to-ion conversion factor in the plasma (αA/αB) and on the ratio of the transmission factors (GA/GB) which determines the fraction of the emitted atoms collected by the mass spectrometer [12]. The neutral-to-ion conversion factor (α) in the plasma firstly depends on the energy of the atom ionization. The higher the ionization energy, the lower the probability of the ionization. The lower the probability of the atom ionization, the smaller fraction of the sputtered atoms will be taken into account in the process of their counting, and the lower the measured intensity of the sputtered element's signal. The ionization energy of Pb is 7.4167 eV; the ionization energy of Te is 9.0097 eV [23]. Consequently, in the process of SNMS analysis a smaller fraction of the sputtered Te atoms is counted than of the sputtered Pb atoms. So, the ratio αTe/αPb must lead to a lower than one DTe(Pb). The ratio of the transmission factors GTe/GPb, which determines the fraction of the emitted Pb and Te atoms collected into the mass spectrometer, depends on the sputtering surface morphology (because it changes the angular distribution of the sputtered species) and on the sputtered atom energy (because of the difference in the Pb and Te atom masses). The first factor monotonically influences the magnitude of DTe(Pb) in the entire range of the sputtering energies E, the second one contributes to it only in the range of low sputtering energies below approximately 200 eV [14]. Under impact of sputtering by Ar+ ions the PbTe sample surface morphology is modified by the dimple relief and the formed surface structures. Their formation leads to the increase of DTe(Pb) [14]. Among the investigated samples only Sample 1 has the sputtered surface practically free from the surface structures and a very slightly developed dimple relief. Taking this into account we may consider the obtained magnitude 0.87 of DTe(Pb) for Sample 1 as a magnitude close to the correct value of RDF of Pb and Te, which is determined by their ionization efficiency in the plasma. In the absence of the formation of the surface structure on the sputtering surface this value may be used for quantitative calculations and for other energies of Ar+ ions larger than at least 200 eV. For the sputtering energy lower than 200 eV the
Fig. 4. Ratio of the experimental integral intensities Itot(Te)/Itot(Pb) of the PbTe samples for sputtering energy of 350 eV vs. sputtering time: 1(black) – Sample 1; 2 (red) – Sample 2; 3 (green) – Sample 3; 4 (blue) – Sample 4.
the sputtering time, is not a constant. It varies from sample to sample. This magnitude for the Sample 4 exceeds the similar value for the Sample 1 by approximately 61.5%. This significantly exceeds the possible experimental error of measurement. The second peculiarity is that there is correlation between the magnitude of the stabilized value of the ratio Itot(Te)/Itot(Pb) and the morphology of the sputtering sample surface. The magnitude of the ratio of sputtering rates was lowest for the sample with the fewest surface structures - Sample 1. The mean of the stabilized ratio Itot(Te)/ Itot(Pb) for it is 0.87 with the standard deviation of 0.003. RDF of any A and B elements can be defined as
DA (B ) =
CB IA CA IB
(1)
where IA and IB are the sputtered ion intensities, CA and CB are the concentrations of the elements [6,22]. The composition of PbTe crystals grown from vapor phase is always very close to stoichiometry [19]. Therefore, to calculate DTe(Pb) we may put the ratio of Pb and Te concentrations CPb/CTe = 1 and the stabilized values of the ratio 101
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corrective term on the transmission factor for DTe(Pb) must be inE troduced. Using its simplest approximation 1 + C exp − Δ suggested in Ref. [14] and the values of the parameters C and Δ for PbTe obtained there, namely 24 and 34 eV respectively, we may write the followed empirical expression for RDF of Pb and Te as it dependence on sputtering energy (E) of Ar+ ions:
[2] M. Rahim, F. Felder, M. Fill, H. Zogg, Optically pumped 5 ??m IV–VI VECSEL with Al-heat spreader, Optic Lett. 33 (24) (2008) 3010–3012 https://doi.org/10.1364/ OL.33.003010. [3] E. Hazan, O. Ben-Yehuda, N. Madar, Y. Gelbstein, Functional graded Germanium–Lead chalcogenide‐based thermoelectric module for renewable energy applications, Adv. Energy Mater. 5 (2015) 1500272 https://doi.org/10.1002/aenm. 201500272. [4] P. Sigmund, Elements of sputtering theory, in: T. Som, D. Kanjilal (Eds.), Nanofabrication by Ion-Beam Sputtering, Pan Stanford Publishing, 2013, pp. 1–40 https://doi.org/10.4032/9789814303767. [5] M. Kopnarski, H. Jennet, Electron impact (EI) secondary neutral mass spectrometry (SNMS), in: Bubert H. Friedbacher (Ed.), Surface and Thin Film Analysis. A Compendium of Principles, Instrumentation, and Applications, Wiley-VCH Verlag GmbH &Co.KGaA, 2011, pp. 161–177 https://doi.org/10.1002/9783527636921. ch9. [6] H. Oechsner, Secondary Neutral Mass Spectrometry (SNMS) and its application to depth profile and interface analysis, in: H. Oechsner (Ed.), Thin Film and Depth Profile Analysis, Springer-Verlag, 1984, pp. 63–86 https://doi.org/10.1007/978-3642-46499-7. [7] E. Stumpe, H. Oechsner, H. Schoof, High-resolution sputter depth profiling with a low pressure HF plasma, Appl. Phys. 20 (1979) 55–60 https://doi.org/10.1007/ BF00901787. [8] S. Hofmann, High resolution compositional depth profiling, J. Vac. Sci. Technol. 9 (3) (1991) 1466–1476 https://doi.org/10.1116/1.577647. [9] S. Hofmann, Sputter depth profile analysis of interfaces, Rep. Prog. Phys. 61 (1998) 827–888 https://doi.org/10.1088/0034-4885/61/7/002. [10] W. Husinsky, G. Betz, Fundamental aspects of SNMS for thin film characterization: experimental studies and computer simulations, Thin Solid Films 272 (1996) 289–309 https://doi.org/10.1016/0040-6090(95)06954-2. [11] W. Bieck, H. Gnaser, H. Oechsner, Analytical performance of a secondary neutral microprobe with electron gas post ionization and magnetic sector mass spectrometer, J. Vac. Sci. Technol. 12 (1994) 2537–2543 https://doi.org/10.1116/1. 579053. [12] H. Oechsner, Secondary neutral mass spectrometry (SNMS)-recent methodical progress and applications to fundamental studies in particle/surface interaction, Int. J. Mass Spectrom. Ion Process. 143 (1995) 271–282 https://doi.org/10.1016/ 0168-1176(94)04122-N. [13] J. Jorzick, J. Losch, M. Kopnarski, H. Oechsner, Detection in the ppm range and high-resolution depth profiling with the new SNMS instrument INA-X, Appl. Phys. A 78 (2004) 655–658 https://doi.org/10.1007/s00339-003-2275-5. [14] D.M. Zayachuk, V.E. Slynko, A. Csik, Peculiar properties of preferential sputtering of PbTe, SnTe, and GeTe by Ar+ ion plasma, Mater. Sci. Semicond. Process. 88 (2018) 103–108 https://doi.org/10.1016/j.mssp.2018.07.037. [15] D.M. Zayachuk, E.I. Slynko, V.E. Slynko, A. Csik, Oscillations and huge preferences of PbTe crystal surface sputtering under Secondary Neutral Mass Spectrometry conditions, Mater. Lett. 173 (2016) 167–169 https://doi.org/10.1016/j.matlet. 2016.03.038. [16] D. Zayachuk, A. Csik, PbTe Crystal Sputtering and Re-deposition of Sputtered Species, LAP Lambert Academic Publishing, Saarbrucken, Germany, 2016. [17] D.M. Zayachuk, V.E. Slynko, A. Csik, Morphology of PbTe crystal surface sputtered by argon plasma under Secondary Neutral Mass Spectrometry conditions, Phys. Chem. Solid States 17 (2016) 336–341 https://doi.org/10.15330/pcss.17.3.336341. [18] D.M. Zayachuk, V.E. Slynko, A. Csik, Formation of the sputtered phase of PbTe crystals by Ar+ plasma and re-deposition of the sputtered Species at Secondary Neutral Mass Spectrometry conditions, Phys. Chem. Solid States 18 (2017) 21–28 https://doi.org/10.15330/pcss.18.1.21-28. [19] G. Nimtz, B. Schlicht, Narrow-Gap Semiconductors: Narrow Gap Lead Salts, Springer, Berlin, 1985, pp. 1–117 https://doi.org/10.1007/BFb0044919. [20] K. Vad, A. Csik, G.A. Langer, Secondary neutral mass spectrometry-a powerful technique for quantitative elemental and depth profiling analyses of nanostructures, Spectrosc. Eur. 21 (2009) 13–17. [21] L.C. Feldman, J.W. Mayer, Fundamentals of Surface and Thin Film Analysis, NorthHolland, Amsterdam, 1986https://doi.org/10.1016/0168-583X(87)90553-2. [22] A. Wucher, F. Novak, W. Reuter, Relative elemental sensitivity factors in secondary neutral mass spectrometry, J. Vac. Sci. Technol. 6/4 (1988) 2265–2270 https://doi. org/10.1116/1.575022. [23] A. Kramida, Yu. Ralchenko, J. Reader, NIST ASD Team, NIST Atomic Spectra Database (Ver. 5.5.6), National Institute of Standards and Technology, Gaithersburg, MD, 2018 Online https://doi.org/10.18434/T4W30F https:// physics.nist.gov/PhysRefData/ASD/ionEnergy.html.
( )
DTe (Pb)(E ) = 0.87(1 + 24 exp( −E /34))
(2)
The obtained RDF value and its energy dependence may be used for straightforward quantification of SNMS data for any PbTe samples if the sample surface remains practically flat after sputtering, regardless of whether it is a thin layer or a bulk material. Because of the low sputtering energy, as usually applied in SNMS method, the sputtering rate in range of nm/sec and as a results of this only the top few atomic layers of the sample involved in sputtering process. Therefore, the origin of the surface (thin film or bulk material) cannot take significant effect on sputtering process not like surface morphology. The change of surface morphology e.g. can change the incidence angle of the Ar+ ions, which can take effect on the value of RDF. If the sample surface is covered by surface structures in the process of its sputtering, it is necessary to introduce an adjustment to the value of RDF, which must be determined individually in each particular case. 4. Conclusions For determination of relative detection factor (DTe(Pb)) of Te and Pb the sputtering by Ar plasma beams with energy of 350 eV during 50 min of the (100) natural surface of the special PbTe crystal ingot grown from the vapor phase in a free volume on the seed of (100) orientation was carried out. The high quality and crystallographic orientation of the initial crystal surface to be sputtered was confirmed by X-ray diffraction method. The morphology of the sputtered surfaces was examined by SEM. The study has shown that the quality of the crystalline faceting surfaces of the PbTe crystal grown from vapor phase is sufficient both to prevent the formation of surface structures in the process of sputtering under the used conditions and to minimize the formation of dimple relief on the sputtered surface. It allowed determination of the correct value of DTe(Pb), 0.87. Taking into account the impact of the transmission factors determining the fraction of the emitted Pb and Te atoms collected by the mass spectrometer on DTe(Pb), the empirical expression for dependence of relative detection factor on the sputtering Ar+ ions energy for the case of a flat sputtered surface was obtained, DTe (Pb)(E ) = 0.87 (1 + 24 exp(-E/34)). Acknowledgement The work is supported by GINOP-2.3.2-15-2016-00041 project, which is co-financed by the European Union and the European Regional Development Fund. References [1] Lead chalcogenides: physics and applications, Series Editor M.O. Manasreh, in: D. Khokhlov (Ed.), Vol. 18 of Optoelectronic Properties of Semiconductors and Superlattices, Taylor & Francis Books Inc., New York and London, 2002.
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