Pressure dependence of the Sn-P phase diagram and investigations in the ternary system Ni-P-Sn

In der Elektronikindustrie wird Nickel oft zur Beschichtung von Metalloberflächen, z.B. Kupfer, verwendet um diese gegen Korrosion zu schützen, zu härten oder die Lötfähigkeit zu erhöhen. Diese Beschichtung wird üblicherweise mittels electroless plating unter Verwendung von phosphorhältigen Substanzen aufgetragen. Dabei werden Ni-P-Schichten gebildet, die einen P-Gehalt von bis zu 15 at% aufweise können. Während des Lötvorgangs tritt diese Ni(P) Schicht mit zinnreichen Lötlegierungen in Kontakt. Die Qualität dieser Lötverbindungen ist ein wesentliches Thema in der Werkstoff- und Elektrotechnik, deshalb ist es von essentieller Bedeutung, die Bildung intermetallischer Verbindungen (IMC) an den Lötstellen zu verstehen. Um die auftretenden Reaktionen und entstehenden Verbindungen erklären zu können, ist es nötig die dazugehörigen Phasendiagramme zu erforschen. In der vorliegenden Arbeit wurden die Systeme Sn-P und Ni-P-Sn untersucht. Das binäre System zeigt eine starke Abhängigkeit vom Dampfdruck des Phosphors. Diesbezüglich wurden isopiestische Versuche mit unterschiedlichen P-Dampfdrücken durchgeführt. Für neun verschiedene Drücke im Bereich zwischen 0,006 und 0,69 bar konnten erfolgreich Phasendiagrammeversionen bis zu einem maximalen P-Gehalt von 70 at% erstellt werden. Mit den erhaltenen Daten wurden die Gleichgewichtstemperaturen der Bildungsreaktionen für die drei existierenden binären Verbindungen (Sn4P3, Sn3P4, SnP3) für unterschiedliche Dampfdrücke verglichen und auf 1 bar extrapoliert. Weiters wurde die ebenfalls druckabhängige maximale Löslichkeit von Phosphor in der zinnreichen Flüssigkeit mit Hilfe der experimentellen Ergebnisse abgeschätzt. Abschließend wurden partielle thermodynamische Eigenschaften wie partielle Mischungsenthalpien und Phosphoraktivitäten sowie integrale Gibbs Bildungsenergien der binären Verbindungen abgeleitet und mit Literaturdaten verglichen. Im ternären System wurden Proben im Sn-reichen Teil des Phasendiagramms hergestellt, und mit Hilfe von Röntgenbeugung (XRD) und Elektronenmikroskopie (SEM) Daten sollte ein isothermer Schnitt bei 300 °C erstellt werden. Die Probenherstellung sowie die Interpretation der Ergebnisse wurden jedoch maßgeblich durch den hohen Dampfdruck von Phosphor erschwert. Proben ab einem Phosphorgehalt von mehr als 60 at% konnten nicht für die Auswertung herangezogen werden. Einige Quarzrohre explodierten während der Probenherstellung, in anderen Proben kondensierte P während des Abschreckens an der Quarzglaswand. Weiter waren die meisten Proben ab einem Phosphorgehalt von 15 at% auch nach langem Tempern nicht im thermodynamischen Gleichgewicht. Dies ist vermutlich auf die entstehende phosphorhaltige Gasphase in den Proben und die niedrige Gleichgewichtsglühtemperatur zurückzuführen. Aufgrund dieser experimentellen Schwierigkeiten konnte kein konsistenter isothermer Schnitt des Ni-P-Sn Phasendiagramms erstellt werden.In the electronic industry nickel is often used for protective coatings of metal surfaces, e.g. Cu, to protect them against corrosion, to increase the hardness or the solderability. These protective layers are usually applied by means of electroless plating using phosphorus-containing solutions. The Ni-P layers that are formed in this process can have a P content of up to 15 at%. During the soldering process there is an interaction of these Ni(P)-layers with the tin-rich solder alloy. This causes the formation of so called intermetallic compounds (IMC) in the solder joint. In order to explain the occurring reactions and the reaction products that are formed during the soldering process, it is necessary to investigate the corresponding phase diagrams. In the present thesis, the systems Sn-P and Ni-Sn-P have been investigated. The binary system shows a strong dependence on the vapor pressure of phosphorus. Hence, isopiestic experiments with different vapor pressures of P were performed. Successful measurements were carried out at nine different pressures in a range from 0.006 to 0.69 bar and for these pressures phase diagrams could be created up to a P content of 70 at%. With the obtained data the change of the equilibrium temperatures of the formation reactions for the three binary compounds (Sn4P3, Sn3P4, SnP3) with increasing pressure could be analyzed and extrapolated to 1 bar. Further, the pressure-dependent maximum solubility of phosphorus in the tin-rich liquidus was investigated according to the obtained results. Finally, partial thermodynamic properties of phosphorus, such as partial enthalpies of mixing and activities, as well as integral Gibbs energies of formation were derived and compared with literature data. In the ternary system samples in the Sn-rich part of the phase diagram were prepared and then investigated by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). With these data an isothermal section at 300 ° C should have been created. However, phosphorus and its vapor pressure caused significant problems during sample preparation and interpretation of the results. Samples with a phosphorus content of more than 60 at% could not be used for evaluation. Some quartz tubes burst during the sample preparation, in other samples P condensed during quenching on the quartz wall. Further, most samples with a P-content of more than 15 at% were not in thermodynamic equilibrium. These non-equilibrium samples were most likely obtained due to the influence of the gas phase and the low annealing temperature. As a result of these experimental difficulties, no reliable and consistent isothermal section of the Ni-Sn-P phase diagram could be created

Synthese und strukturelle Charakterisierung der Halbleiterverbindung Kesterit

The here-presented thesis provides detailed investigations on the structural characteristics of the chalcogenide Cu2ZnSnS4 (CZTS). This semiconductor material is a promising candidate for absorber layers in solar cells due to its desirable properties for thin film photovoltaic applications. Yet, compared to current used chalcopyrite-based devices, efficiencies are significantly lower. This could be attributed to structural effects. Fundamental understanding of the structural characteristics of potential thin film absorber compounds is crucial for proper materials design in the field of solar technology. Therefore, intensive research is necessary to obtain knowledge on so far unexplored structural features or to certify and extend current literature data. The main objective of the here-presented work was to deepen the understanding of the quaternary sulfide Cu2ZnSnS4. Primary, this implied the full characterization of structural properties of the compound using various diffraction techniques. As the synthesis of phase-pure kesterite powder is a challenging problem, one of the central points of this work was the development of a novel rapid and facile synthesis process leading to single phase material. A mechanochemical approach was successfully introduced, including the reaction of the corresponding binary sulfides in a planetary ball mill followed by an annealing procedure in H2S atmosphere, leading to highly crystalline powder samples. The crystallization of the as-milled powder during annealing was tracked by high temperature X-ray diffraction measurements. With this synthesis method it is nicely possible to control the composition of the synthesized powder. Therefore, it was feasible to prepare single phase stoichiometric as well as off-stoichiometric samples with desired compositions. Phase purity and composition were determined by means of electron microprobe analysis and X-ray absorption spectroscopy. First, structural analysis of a stoichiometric sample was performed using X-ray powder diffraction methods including Rietveld refinement. As copper and zinc are not distinguishable using conventional X-ray diffraction methods, only the Sn/(Cu/Zn) distribution could be disclosed, whereas Cu/Zn order remained unknown. For full cation distribution analysis, neutron diffraction measurements were performed, as copper and zinc show a significant difference in the neutron scattering length. It could be shown that the powder sample adopts the kesterite-type structure with a partial disorder of copper and zinc on the two Wyckoff positions 2c and 2d. Another key issue of the present thesis was the investigation of the order-disorder transition in Cu2ZnSnS4. For this purpose, a series of stoichiometric CZTS samples were synthesized according to the mechanochemical synthesis process and afterwards annealed at different temperatures in a range of 473–623K. Again, neutron diffraction techniques were used to investigate the samples. Detailed structural analysis revealed a Landau-type second order transition from an ordered to a disordered kesterite-type structure at a critical temperature of 552 ± 2 K. Additionally, a fully ordered Cu2ZnSnS4 powder sample (within the standard deviation) was successfully synthesized at 473 K. In a final step, special focus was put on the study of intrinsic point defects in off-stoichiometric kesterite. For this purpose, B- and C-type off-stoichiometric samples were prepared and analyzed by means of X-ray and neutron diffraction measurements. It could be shown that it is possible to synthesize phase-pure kesterite samples with a composition far off the stoichiometric point.Die vorliegende Arbeit liefert eine detaillierte Untersuchung der strukturellen Eigenschaften des Chalkogenides Cu2ZnSnS4 (CZTS). Die Halbleiterverbindung gilt als ein vielversprechender Kandidat für Absorberschichten in Solarzellen aufgrund ihrer geeigneten Eigenschaften für Dünnschicht-Photovoltaik-Anwendungen. Im Vergleich zu aktuellen verwendeten Chalkopyrit-basierten Solarzellen sind die Effizienzen jedoch deutlich geringer. Dies könnte auf strukturelle Effekte zurückzuführen sein. Ein fundamentales Verständnis der strukturellen Merkmale von potentiellen Dünnschicht-Absorber-Verbindungen ist für die Materialentwicklung auf dem Gebiet der Solartechnik von entscheidender Bedeutung. Die detaillierte Erforschung der Kristallstruktur ist notwendig um deren Beeinflussung der physikalischen und chemischen Eigenschaften des Materials festzustellen. Das Hauptziel der hier präsentierten Arbeit war es, das Verständnis des quaternären Sulfids Cu2ZnSnS4 zu vertiefen. Primär impliziert dies die vollständige Charakterisierung der strukturellen Eigenschaften der Verbindung mit Hilfe verschiedener Beugungstechniken. Da die Synthese von phasenreinem Kesteritpulver eine anspruchsvolle Aufgabe ist, war die Entwicklung eines neuartigen, schnellen und einfachen Syntheseverfahrens zu Herstellung einphasigen CZTS-Materials ein zentraler Punkt dieser Arbeit. Ein mechanochemischer Prozess wurde erfolgreich entwickelt. Dieser umfasst die Umsetzung der entsprechenden binären Sulfide in einer Planetenkugelmühle und ein anschließendes Temperverfahren in H2S Atmosphäre, wodurch ein hochkristallines Pulver erhalten wird. Mittels Hochtemperatur-Röntgenbeugungsmessungen konnte die Kristallisation des gemahlenen Pulvers in situ verfolgt werden. Mit dem neu entwickelten Verfahren ist es möglich, die Zusammensetzung des synthetisierten Pulvers zu steuern. Daher war die Herstellung einphasiger stöchiometrischer sowie nicht-stöchiometrischer Proben mit gewünschten Zusammensetzungen realisierbar. Die Phasenreinheit und die exakte Zusammensetzung wurden mittels Elektronenstrahlmikroanalyse und Röntgenabsorptionsspektroskopie bestimmt. Erste strukturelle Untersuchungen einer stöchiometrischen Probe wurden mittels Pulver-Röntgenbeugung und anschließender Rietveld-Verfeinerung durchgeführt. Kupfer und Zink sind mit herkömmlichen Röntgenbeugungsmethoden nicht unterscheidbar, dadurch ließ sich nur die Sn/(Cu/Zn) Verteilung bestimmen, während die Cu/Zn-Ordnung unklar blieb. Zur vollständigen Kationenverteilungsanalyse wurden Neutronenbeugungsmessungen durchgeführt, da Kupfer und Zink einen signifikanten Unterschied in der Neutronenstreulänge aufweisen. Es konnte gezeigt werden, dass das Pulver im Kesterit Strukturtyp kristallisiert und eine partielle Unordnung von Kupfer und Zink auf den beiden Wyckoff-Positionen 2c und 2d aufweist. Ein weiterer Schwerpunkt der vorliegenden Arbeit war die Untersuchung des Ordnungs-Unordnungs-Übergangs in Cu2ZnSnS4. Hierfür wurde eine Reihe stöchiometrischer CZTS Proben nach dem mechanochemischen Syntheseverfahren hergestellt und anschließend bei verschiedenen Temperaturen in einem Bereich von 473 – 623 K getempert. Zur detaillierten Strukturanalyse wurden Neutronenbeugungsmessungen durchgeführt und die Kristallstrukturen wurden mit Hilfe der Rietveld-Methode verfeinert. Diese Untersuchungen ergaben einen Landau–Übergang zweiter Ordnung. Die kritische Temperatur des Übergangs von der geordneten zur ungeordneten Kesterit-Struktur konnte mit 552±2K bestimmt werden. Zusätzlich konnte bei 473K eine vollständig geordnete Cu2ZnSnS4 Probe (innerhalb der Standardabweichung) erfolgreich synthetisiert werden. Abschließend wurden Untersuchungen zu intrinsischen Punktdefekten nicht-stöchiometrischer Kesterit-Proben durchgeführt. Zu diesem Zweck wurden Pulverproben des nicht-stöchiometrischen B- und C-Typs gemäß der mechanochemischen Route hergestellt und mittels Röntgen- und Neutronenbeugungsmessungen untersucht. Es konnte gezeigt werden, dass es möglich ist, phasenreine Kesterit-Proben mit einer Zusammensetzung weit vom stöchiometrischen Punkt zu synthetisieren

Synthesis and Structural Characterization of Semiconductors based on Kesterites

The here presented thesis provides detailed investigations on the structural characteristics of the chalcogenide Cu2ZnSnS4 CZTS . This semiconductor material is a promising candidate for absorber layers in solar cells due to its desirable properties for thin film photovoltaic applications. Yet, compared to current used chalcopyrite based devices, efficiencies are significantly lower. This could be attributed to structural effects. Fundamental understanding of the structural characteristics of potential thin film absorber compounds is crucial for proper materials design in the field of solar technology. Therefore, intensive research is necessary to obtain knowledge on so far unexplored structural features or to certify and extend current literature data. The main objective of the here presented work was to deepen the understanding of the quaternary sulfide Cu2ZnSnS4. Primary, this implied the full characterization of structural properties of the compound using various diffraction techniques. As the synthesis of phase pure kesterite powder is a challenging problem, one of the central points of this work was the development of a novel rapid and facile synthesis process leading to single phase material. A mechanochemical approach was successfully introduced, including the reaction of the corresponding binary sulfides in a planetary ball mill followed by an annealing procedure in H2S atmosphere, leading to highly crystalline powder samples. The crystallization of the as milled powder during annealing was tracked by high temperature X ray diffraction measurements. With this synthesis method it is nicely possible to control the composition of the synthesized powder. Therefore, it was feasible to prepare single phase stoichiometric as well as off stoichiometric samples with desired compositions. Phase purity and composition were determined by means of electron microprobe analysis and X ray absorption spectroscopy. First, structural analysis of a stoichiometric sample was performed using X ray powder diffraction methods including Rietveld refinement. As copper and zinc are not distinguishable using conventional X ray diffraction methods, only the Sn Cu Zn istribution could be disclosed, whereas Cu Zn order remained unknown. For full cation distribution analysis, neutron diffraction measurements were performed, as copper and zinc show a significant difference in the neutron scattering length. It could be shown that the powder sample adopts the kesterite type structure with a partial disorder of copper and zinc on the two Wyckoff positions 2c and 2d. Another key issue of the present thesis was the investigation of the order disorder transition in Cu2ZnSnS4. For this purpose, a series of stoichiometric CZTS samples were synthesized according to the mechanochemical synthesis process and afterwards annealed at different temperatures in a range of 473 623 K. Again, neutron diffraction techniques were used to investigate the samples. Detailed structural analysis revealed a Landau type second order transition from an ordered to a disordered kesterite type structure at a critical temperature of 552 2 K. Additionally, a fully ordered Cu2ZnSnS4 powder sample within the standard deviation was successfully synthesized at 473 K. In a final step, special focus was put on the study of intrinsic point defects in offstoichiometric kesterite. For this purpose, B and C type off stoichiometric samples were prepared and analyzed by means of X ray and neutron diffraction measurements. It could be shown that it is possible to synthesize phase pure kesterite samples with a composition far off the stoichiometric poin

Pressure dependence of the tin–phosphorus phase diagram

The pressure dependence of the binary Sn–P phase diagram was investigated using an isopiestic equilibration method. Successful experiments were carried out at nine different pressures in the range from 0.006 to 0.69 bar, and the corresponding phase diagrams for these pressures were created up to a content of x P = 70 %. The change of the equilibrium temperatures of the formation reactions for the three binary compounds (Sn4P3, Sn3P4, SnP3) with increasing pressure was analyzed and extrapolated to 1 bar. The pressure-dependent maximum solubility of phosphorus in the tin-rich liquid was investigated and partial thermodynamic properties of P were derived for the liquid phase. Standard molar Gibbs energies of formation were derived for all three stable compounds

High-pressure behavior of disodered kesterite-type Cu2_2ZnSnS4_4

We have investigated the high-pressure structural and vibrational behavior of the disordered kesterite-type Cu$_2$ZnSnS$_4$ compound at ambient temperature. Our experimental and theoretical investigations have revealed a clear structural transition to a GeSb-type phase close to 15 GPa, a tetragonally distorted variant of the NaCl-type phase. The latter transformation is accompanied by a cationic coordination increase from fourfold to sixfold with respect to the sulfur anions. In addition, a change in the compressibility rate was detected at about 8 GPa within the pressure stability range of the disordered kesterite-type phase. Upon decompression, a disordered zinc blende/sphalerite structure is recovered. We discuss our findings in close conjunction with our recent high-pressure work on the ordered Cu$_2$ZnSnS$_4$ modification

High-pressure behavior of disodered kesterite-type Cu2ZnSnS4

We have investigated the high-pressure structural and vibrational behavior of the disordered kesterite-type Cu2ZnSnS4 compound at ambient temperature. Our experimental and theoretical investigations have revealed a clear structural transition to a GeSb-type phase close to 15 GPa, a tetragonally distorted variant of the NaCl-type phase. The latter transformation is accompanied by a cationic coordination increase from fourfold to sixfold with respect to the sulfur anions. In addition, a change in the compressibility rate was detected at about 8 GPa within the pressure stability range of the disordered kesterite-type phase. Upon decompression, a disordered zinc blende/sphalerite structure is recovered. We discuss our findings in close conjunction with our recent high-pressure work on the ordered Cu2ZnSnS4 modification

Pressure-induced structural and electronic transitions in kesterite-type Cu2ZnSnS4\mathrm{Cu_{2}ZnSnS_{4}}

We have performed structural investigations of ordered kesterite-type Cu$_2$ZnSnS$_4$ up to 30 GPa. Our current X-ray diffraction results clearly excluded the presence of a kesterite → disordered kesterite transition reported previously between 7 and 9 GPa. Nevertheless, specific anomalies connected with the Cu-S bond length of the starting kesterite-type phase are evidenced close to 6 GPa, indicating subtle structural effects at play in this system. Moreover, we have indexed the high-pressure modification of Cu$_2$ZnSnS$_4$ adopted above 16 GPa to a disordered GeSb-type structure, a tetragonally distorted rocksalt-type modification. Full decompression leads to the adoption of a disordered sphalerite/zincblende-type structure. Our complementary density functional theory calculations reproduce accurately the experimental observations and indicate the possibility of a metallic high-pressure GeSb-type phase, unlike the starting semiconducting kesterite-type Cu$_2$ZnSnS$_4$structure

Pressure-induced structural and electronic transitions in kesterite-type Cu2ZnSnS4

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Journal of Applied Physics 124, 085905 (2018) and may be found at https://doi.org/10.1063/1.5047842.We have performed structural investigations of ordered kesterite-type Cu2ZnSnS4 up to 30 GPa. Our current X-ray diffraction results clearly excluded the presence of a kesterite → disordered kesterite transition reported previously between 7 and 9 GPa. Nevertheless, specific anomalies connected with the Cu-S bond length of the starting kesterite-type phase are evidenced close to 6 GPa, indicating subtle structural effects at play in this system. Moreover, we have indexed the high-pressure modification of Cu2ZnSnS4 adopted above 16 GPa to a disordered GeSb-type structure, a tetragonally distorted rocksalt-type modification. Full decompression leads to the adoption of a disordered sphalerite/zincblende-type structure. Our complementary density functional theory calculations reproduce accurately the experimental observations and indicate the possibility of a metallic high-pressure GeSb-type phase, unlike the starting semiconducting kesterite-type Cu2ZnSnS4 structure