Optimization of Wurtzite Sc-doped AlN Thin Films for Next-Generation Electronics

Master’s Project Proposal

1. Introduction and Motivation

Ferroelectric materials are central to emerging electronic, electromechanical, and memory technologies because of their switchable spontaneous polarization. Recently, wurtzite-structured III-nitride ferroelectrics, particularly scandium-doped aluminum nitride (Sc_xAl_1-xN), have attracted strong interest due to their high remanent polarization, high Curie temperature, CMOS compatibility, and robustness under harsh thermal and mechanical conditions^[1].
A key challenge in Sc_xAl_1-xN is achieving high crystalline quality and stable wurtzite phase at elevated Sc concentrations, where the material approaches the wurtzite–zinc blend phase boundary. High Sc content is desirable to reduce the bond stiffness and hence, coercive field. Yet, it often leads to phase instability, defect formation, and increased leakage. Recent studies have demonstrated that carefully engineered low-temperature growth techniques, particularly variants of pulsed laser deposition (PLD), can stabilize high-Sc wurtzite ScAlN films with excellent piezoelectric and ferroelectric performance^[2].
In this project, we propose to grow high-quality wurtzite AlN and ScAlN thin films using a newly developed CO₂-laser heating–assisted ultra-high-vacuum pulsed laser deposition (UHV-PLD) system. The project will systematically explore growth conditions to optimize minimum defect denisty on the wurtzite phase and evaluate ferroelectric properties, with the aim of establishing structure-defect-ferroelectric property relationship.

2. Objectives of the Project

The main objectives of this Master’s project are:

Optimize the growth of epitaxial nitride thin films (parent and Sc-doped).
Structure–property correlation: To correlate structural, morphological, and compositional properties with piezoelectric and ferroelectric behavior.
Ferroelectric model system demonstration: To fabricate and characterize simple metal/ScAlN/metal capacitor structures and demonstrate ferroelectric switching.

3. Experimental Approach and Methodology

3.1 Growth Technique: CO₂-Laser–Heated UHV-PLD

Thin films will be deposited using a UHV pulsed laser deposition system equipped with a CO₂ laser–based substrate heating stage and high-resolution reflection high-energy electron diffraction (RHEED). Unlike conventional resistive or lamp heating, CO₂-laser heating provides:

Fast- and localized heating of the substrate
Precise control over substrate temperature
Growth at elevated temperatures (300 <T <1500°C)
Fast cooling

Ceramic AlN and ScAlN targets with varying Sc compositions (e.g., 0–40% Sc) will be ablated using a high-energy pulsed laser. Film growth will be performed under ultra-high vacuum with controlled nitrogen background pressure to ensure stoichiometric nitride formation.

3.2 Substrates and Film Stack Design

Candidate substrates include sapphire (Al₂O₃), silicon with buffer layers, or conductive nitride electrodes such as TiN. A typical heterostructure will consist of:

Substrate (e.g., Al₂O₃ (0001) or Si (111))
Bottom electrode (TiN or similar)
AlN and Sc_xAl_1-xN ferroelectric layer
Top metal electrode (e.g., Pt or Au)

This architecture enables direct electrical characterization of ferroelectric properties.

3.3 Structural and Materials Characterization

The deposited films will be characterized using standard thin-film techniques:

X-ray diffraction (XRD): Phase identification, orientation, and crystalline quality
Reflection high-energy electron diffraction (RHEED): surface morphology
Atomic force microscopy (AFM): Surface morphology and roughness.
Scanning probe microscope (SPM): Ferroelectric reading and writing
X-ray photoelectron spectroscopy (XPS): Compositional analysis and verification of Sc/Al atomic% ratio.

These measurements will establish the growth–structure relationships critical for ferroelectric performance.

4. Ferroelectric Model System and Electrical Testing

To test ferroelectric behavior, simple capacitor structures will be fabricated using standard lithography or shadow masking techniques. The key electrical measurements will include:

Polarization–electric field (P–E) hysteresis loops to confirm switchable polarization
Leakage current measurements to assess film quality
Endurance and retention tests (time permitting) to evaluate stability of polarization states

The results will be compared across AlN (non-ferroelectric reference) and ScAlN films with varying Sc concentrations to clearly identify the onset and evolution of ferroelectricity.

Figure 1 A. Cross-section of the ferroelectric capacitor and B. Switchable ferroelectric polarization of wurtzite Sc-doped AlN.

This schematic highlights the integration of CO₂-laser heating with UHV-PLD growth and subsequent ferroelectric device characterization.

5. Expected Outcomes and Significance

By the end of this Master’s project, the student is expected to:

Demonstrate the growth of high-quality wurtzite AlN and ScxAl1-xN thin films.
Understand the role of Sc concentration and growth conditions on phase stability and ferroelectric behavior
Gain hands-on experience in thin-film growth, vacuum systems, and functional electrical characterization

The project will provide a strong experimental foundation in nitride ferroelectrics and contribute to the broader effort of integrating ferroelectric nitrides into next-generation electronic and MEMS devices.

6. Timeline

Month 1: Literature review, system training, and calibration of CO₂-laser heating
Month 2: Growth of AlN and ScAlN films
Month 3: Structural, morphological and composition analysis
Month 4: Optimization of ScAlN composition and thickness
Month 5: Ferroelectric measurements
Month 6: Data analysis, Thesis writing and presentation
Month 7: Manuscript preparation for publication

Reference:
[1] Kim, K. D., et al., Adv. Electron. Mater. 2023, 9, 2201142.
[2] Li, Chao., et al., Nano Lett. 2025, 25, 46, 16356-16365.

This Master’s project combines advanced thin-film growth with functional ferroelectric (& piezoelectric) testing and is well suited for training in experimental materials science and applied solid-state chemistry/physics.

For more information contact:
Dr. Arindom Chatterjee, daily supervisor (a.chatterjee-1@utwente.nl)
Prof. dr. Gertjan Koster, supervisor, (g.koster@utwente.nl)