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Development and Characterization of Piezoelectric Alscn-Based Alloys for Electroacoustic Applications

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Development and Characterization of Piezoelectric Alscn-Based Alloys for Electroacoustic Applications Development and characterization of piezoelectric AlScN-based alloys for electroacoustic applications Yuan Lu Dissertation zur Erlangung des Doktorgrades der Technischen Fakultät der Albert-Ludwigs-Universität Freiburg im Breisgau 2019 Dekanin Prof. Dr. Hannah Bast Referenten Prof. Dr. Oliver Ambacher Prof. Dr. Michael Fiederle Datum der Promotion 21.05.2019 Erklärung Ich erkläre, dass ich die vorliegende Arbeit ohne unzulässige Hilfe Dritter und ohne Benutzung anderer als der angegebenen Hilfsmittel angefertigt habe. Die aus anderen Quellen direkt oder indirekt übernommenen Daten und Konzepte sind unter Angabe der Quelle gekennzeichnet. Insbesondere habe ich hierfür nicht die entgeltliche Hilfe von Vermittlungs- oder Beratungsdiensten (Promotionsberaterinnen oder Promotionsberater oder anderer Personen) in Anspruch genommen. Niemand hat von mir unmittelbar oder mittelbar geldwerte Leistungen für Arbeiten erhalten, die im Zusammenhang mit dem Inhalt der vorgelegten Dissertation stehen. Die Arbeit wurde bisher weder im In- noch im Ausland in gleicher oder ähnlicher Form einer anderen Prüfungsbehörde vorgelegt. Ich erkläre hiermit, dass ich mich noch nie an einer in- oder ausländischen wissenschaftlichen Hochschule um die Promotion beworben habe oder gleichzeitig bewerbe. Yuan Lu Freiburg im Breisgau, den 30. Januar 2019 Abstract The mobile communication standard of the 5th generation (5G) is nowadays a great technological in- novation for both industry and consumers. To meet the requirement of higher data rates in 5G, a higher electromechanical coupling of electro-acoustic devices is needed. One of the most successful piezoelectric materials in electro-acoustic applications is aluminum nitride (AlN). However, AlN-based devices have a rather low piezoelectric coefficient d33 = 5.5 pC/N, and the electromechanical coupling of AlN is limited to 7%. These parameters can be significantly enhanced by incorporating scandium (Sc) in the wurtzite structure AlN, making AlScN a promising material for future mobile communica- tion applications. This work aims at the development of wurtzite AlScN thin films with homogenous microstructure, high crystalline quality and high Sc-concentration (x > 0.4) to achieve a large piezoelectric coefficient and electromechanical coupling in electro-acoustic resonators. Therefore, Al1-xScxN thin films on Si(001) substrates were grown by reactive DC-pulsed magnetron co-sputtering, which is capable of adjusting the Sc-concentration. By tuning process pressure and tar- get-to-substrate distance, AlN thin film with straight columnar microstructure along substrate normal was achieved, which served as a reference for sputtering AlScN films. Replacing Al by Sc degraded the crystal quality and resulted in misoriented grains and localized non-piezoelectric areas. Optimized gas ratio eliminated misoriented grains and enabled the growth of a homogenous piezoelectric phase, max- imizing the piezoelectric coefficient at Sc concentration x = 0.13. Later on, the Sc concentration was further increased without phase separation. Finally, a state-of-the-art Al0.54Sc0.46N/Si thin film was synthesized with columnar microstructure, excellent crystalline quality (FWHM < 2°) and high pie- zoelectric coefficient of d33 = 44 pC/N. Furthermore, epitaxial Al1-xScxN (0 ≤ x ≤ 0.46) thin films were also synthesized on Al2O3(0001) substrates. The mechanical and optical properties of AlScN/Al2O3 samples were thoroughly analyzed. The elastic modulus E and the coefficient of thermal expansion α were experimentally determined by thermal- cycling up to 400°C. The sample with a composition of Al0.59Sc0.41N showed E = 270 GPa and α = 4.29 × 10-6 K-1, respectively. The evolution of the band gap in higher Sc concentration was deter- mined by spectroscope ellipsometry, indicating the band gap can be tuned from 5.8 eV in AlN to 4.4 eV in Al0.59Sc0.41N, which makes AlScN also promising material in the optoelectronics. Finally, AlScN-based surface acoustic wave (SAW) resonators were fabricated and an effective elec- tromechanical coupling of 2.2% at around 2 GHz could be obtained for Al0.68Sc0.32N-based resonators, more than 4 times higher compared to the 0.5% of conventional AlN-resonators. The enhanced pie- zoelectric coefficient and electromechanical coupling underlined the outstanding quality of AlScN and its huge potential in electro-acoustic applications. Kurzfassung Der Mobilfunkstandard der 5. Generation (5G) ist in der heutigen Zeit sowohl für die Industrie wie auch die Verbraucher eine wichtige technologische Innovation. Um die Voraussetzungen der hohen Datenraten von 5G zu erfüllen, wird eine höhere elektronisch-mechanischer Kopplung der elektroakustischen Bauteile benötigt. Eines der erfolgreichsten piezoelektrischen Materialien in elektroakustischen Anwendungen ist Aluminiumnitrid (AlN). Bauteile, welche auf AlN basieren, besitzen jedoch eine relativ niedrige piezoelektrischen Koeffizienten d33 = 5,5 pC/N und die elektromechanische Kopplung ist auf 7% beschränkt. Durch den Einbau von Scandium (Sc) in die wurtzitische Phase des AlN lassen sich diese Parameter signifikant erhöhen und machen AlScN zu einem vielversprechenden Material in zukünftigen Anwendungen der mobilen Kommunikation. Diese Arbeit zielt auf die Entwicklung von wurtzitischem AlScN Dünnschichten ab, welche mit einer homogenen Mikrostruktur, hoher kristalliner Qualität und Sc-Konzentration (x > 0,4) einen großen piezoelektrischen Koeffizienten und elektromechanische Kopplung in elektroakustischen Bauteilen erreichen sollen. Hierfür wurden Al1-xScxN Dünnschichten auf Si(001) Substrate mittels reaktivem DC-gepulstem Magnetron-Co-Sputtern gewachsen, welches ein Einstellen der Sc-Konzentration erlaubt. Durch eine Anpassung des Prozessdrucks sowie der Abstand zwischen Target und Substrat, konnten AlN Dünnschichten mit einer senkrecht zur Substratnormalen ausgebildeten kolumnaren Mikrostruktur gezüchtet werden, die als hervorragende Referenz für das AlScN-Wachstum dienten. Der Austausch von Al durch Sc führt jedoch zu einer Degradation der Kristallqualität und im Weiteren zu fehlorientierten Körnern und lokalen Bereichen ohne piezoelektrische Eigenschaften. Optimierte Verhältnisse zwischen den Prozessgasen eliminierte die Fehlorientierung der Körner und ermöglichte das Wachstum einer homogenen piezoelektrischen Phase, welche bei einer Zusammensetzung von x = 0,13 einen maximalen piezoelektrischen Koeffizienten d33,clamp = 12,3 pC/N aufwies. Im weiteren Verlauf der Arbeit konnte die Sc-Konzentration ohne Phasenseparation weiter erhöht werden. Dies resultierte in einer state-of-the-art Al0.54Sc0.46N Dünnschicht auf Silizium, welche eine kolumnare Mikrostruktur, exzellente Kristallqualität (FWHM < 2°) und einen hohen piezoelektrischen Koeffizienten mit d33 = 44 pC/N aufwies. Zusätzlich wurden epitaktische Dünnschichten mit Al1-xScxN (0 ≤ x ≤ 0.46) auf Al2O3(0001) aufgewachsen. Die mechanischen und optischen Eigenschaften von AlScN/Al2O3 Proben wurden eingehend analysiert. Das Elastizitätsmodul E und der thermische Ausdehnungskoeffizient α wurden experimentell durch thermische zyklische Belastung bis 400 °C bestimmt, welche bei einer Zusammensetzung von Al0.59Sc0.41N in E = 270 GPa und α = 4,29 × 10-6 K-1 resultierte. Die Abhängigkeit der Bandlücke von der Sc-Konzentration wurde über spektroskopische Ellipsometrie bestimmt. Diese zeigte eine Einstellbarkeit der Bandlücke von 5,8 eV für AlN zu 4,4 eV in Al0.59Sc0.41N, welches AlScN auch für optoelektronische Anwendungen interessant macht. Zuletzt wurden AlScN-basierte elektronische Bauteile hergestellt, sogenannte surface acoustic wave (SAW) resonators, die eine effektive elektromechanische Kopplung von 2.2% bei etwa 2 GHz und einer Zusammensetzung von Al0.68Sc0.32N besaßen. Dies entspricht einer Erhöhung von mehr als dem 4- fachen im Vergleich zu den 0,5% in konventionellen AlN-Bauteilen. Der verbesserte piezoelektrische Koeffizient und die elektromechanische Kopplung unterstreichen die herausragende Qualität von AlScN und sein immenses Potential für den Einsatz in elektroakustischen Anwendungen. Publications Some ideas, results, and figures have been published previously in the following publications: 1. Y. Lu, M. Reusch, N. Kurz, A. Ding, T. Christoph, L. Kirste, V. Lebedev, and A. Žukauskaitė, Surface morphology and microstructure of pulsed DC magnetron sputtered piezoelectric AlN and AlScN thin films, Physica Status Solidi (A). 215, 1700559 (2018). 2. Y. Lu, M. Reusch, N. Kurz, A. Ding, T. Christoph, M. Prescher, L. Kirste, O. Ambacher, and A. Žukauskaitė, Elastic modulus and coefficient of thermal expansion of piezoelectric Al1-xScxN (up to x = 0.41) thin films, APL Mater. 6, 076105 (2018). 3. A. Ding, M. Reusch, Y. Lu, N. Kurz, R. Lozar, T. Christoph, R. Driad, O. Ambacher, and A. Žukauskaitė, Investigation of Temperature Characteristics and Substrate Influence on AlScN- Based SAW Resonators, in 2018 IEEE Int. Ultrason. Symp. (IEEE, Kobe, 2018), pp. 1–9. Table of contents 1 Introduction ......................................................................................................... 1 1.1 Motivation...................................................................................................................................................... 1 1.2 The aim of this work .................................................................................................................................... 3 1.3 Outline ...........................................................................................................................................................
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