Thermo-desorption measurements during N-doped Ge-rich Ge₂Sb₂Te₅crystallization

Area of Science:

• Materials Science
• Solid-state Chemistry
• Semiconductor Physics

Background:

• Germanium-Antimony-Tellurium (Ge2Sb2Te5 or GST) is a key material for phase change memory devices.
• Encapsulation with TiN or SiN layers and doping (N, C, O, Bi) are common strategies to stabilize GST properties and prevent oxidation.
• Controlling crystallization temperature and material composition during fabrication and operation is critical for device reliability.

Purpose of the Study:

• To investigate the desorption behavior of a nitrogen-doped GST (N-doped GGST) film encapsulated with a TiN layer during thermal annealing.
• To determine the chemical species desorbing from the N-doped GGST film and their correlation with the crystallization process.
• To assess the effectiveness of the TiN encapsulation layer in preventing the loss of dopant atoms.

Main Methods:

• Ultrahigh vacuum thermal desorption spectroscopy (TDS) was employed to analyze desorbing species.
• Isochronal annealing was performed on a N-doped GGST film covered by a 10 nm-thick TiN layer.
• In situ X-ray diffraction (XRD) measurements were used to monitor the crystallization state of the GGST film.

Main Results:

• Desorption of species including Ar, N2, H2, and H was observed.
• The primary desorption peak occurred between 653 K and 703 K, after the full crystallization of the GGST film.
• Comparison of TDS and XRD data revealed that the TiN layer did not completely prevent nitrogen atoms from leaving the GGST film during annealing.

Conclusions:

• The TiN encapsulation layer is not fully effective in preventing the out-diffusion of nitrogen from N-doped GGST films during thermal annealing.
• The observed desorption suggests a progressive change in the chemical composition of the N-doped GGST film during annealing and crystallization.
• These findings have implications for the thermal stability and long-term reliability of phase change memory devices based on N-doped GGST.