Chalcogenide superlattices : growth, structure and applications
- Chalkogenid-Übergitter : Wachstum, Struktur und Anwendungen
Lange, Felix Rolf Lutz; Wuttig, Matthias (Thesis advisor); Kooi, Bart J. (Thesis advisor)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2016
Phase-change materials (PCMs) are one of the most promising candidates for emerging storage class memory (SCM) technology. SCMs aim at bridging the cost-performance gap between nowadays established memory solutions (fast, but volatile, e.g. DRAM) and data storage technologies (slow, but non-volatile, e.g. FLASH). The application potential of PCMs rests on a high endurance, high bit density (multi-level cell capability) and programming times of the order of nanoseconds. In conventional PCM technology, the binary information is encoded by two different solid states: a crystalline (SET state) and an amorphous phase (RESET state) which differ by orders of magnitude in their electrical resistance. The phase transition is induced thermally allowing for crystallization (SET operation) or melt-quenching (RESET operation) and thereby amorphization.Only recently, superior switching properties have been reported for device-integrated GeTe/Sb2Te3 superlattice (SL) PCMs that outperform GeTe-Sb2Te3 alloys in terms of programming time, power consumption and endurance. In difference to conventional PCM technology, the contrast between SET and RESET is argued to stem from two crystalline states that differ in their local atomic arrangement at the GeTe/Sb2Te3 interface. The switching between these two atomic configurations is argued to be electric-field-driven, rather than thermally induced as in conventional PCMs. This novel switching mechanism has been ascribed for the improved device performance. At present, the superior switching behavior is confirmed in numerous publications, yet the underlying switching mechanism is under strong debate.This thesis provides insights into the structure, growth, thermal stability and thermal conductivity of GeTe/Sb2Te3 superlattices (chalcogenide superlattice: CSL). Highly textured CSLs were grown by direct-current magnetron sputter deposition. Their structural quality is mirrored in prominent SL X-ray diffraction features such as SL Bragg peaks and equally spaced satellites. CSLs have been prepared with bilayer thicknesses Λ as small as Λ=34Å.Yet, intermixing was observed at such small values of Λ giving rise to local GeTe-Sb2Te3 alloying at the interface. This tendency for intermixing was also found in case of post-deposition annealing of the CSLs. The SL stacking is unaffected by any thermal treatment at temperatures up to 300°C. At higher temperatures, the artificial stacking dissolves into a stable GeTe-Sb2Te3 compound.At the beginning of this thesis, no data on the thermal conductivity κ was available for GeTe/Sb2Te3 SL. Yet, κ is a parameter that crucially determines the device performance in conventional PCM technology. Therefore, the thermal conductivity of CSLs with different values of Λ was measured. It was found that κ is significantly reduced compared to GeTe and Sb2Te3, respectively. Moreover, κ develops a minimum in dependence of Λ which approaches values of amorphous PCMs. Such a minimum is a fingerprint of SL structures with high interface quality.The present switching models consider neither the local intermixing at the GeTe/Sb2Te3 interface, nor the strikingly small thermal conductivity κ of CSLs. This thesis hence provides valuable results that may help to unravel the cause for the improved device performance of CSLs.
- Chair of Experimental Physics I A and I. Institute of Physics 
- Department of Physics