Marissa A. Caldwell

Crystallization times of Ge–Te phase change materials as a function of composition

The crystallization times of Ge–Te phase change materials with variable Ge concentrations (29.5–72.4 at. %) were studied. A very strong dependence of the crystallization time on the composition for as-deposited, amorphous films was confirmed, with a minimum for the stoichiometric composition GeTe. The dependence is weaker for melt-quenched, amorphous material and crystallization times are between one to almost four orders of magnitude shorter than for as-deposited materials. This is promising for applications because recrystallization from the melt-quenched phase is the relevant process for optical and solid state memory, and fast crystallization and weak dependence on compositional variations are desirable.

Nanoscale phase change memory materials

Phase change memory materials store information through their reversible transitions between crystalline and amorphous states. For typical metal chalcogenide compounds, their phase transition properties directly impact critical memory characteristics and the manipulation of these is a major focus in the field. Here, we discuss recent work that explores the tuning of such properties by scaling the materials to nanoscale dimensions, including fabrication and synthetic strategies used to produce nanoscale phase change memory materials. The trends that emerge are relevant to understanding how such memory technologies will function as they scale to ever smaller dimensions and also suggest new approaches to designing materials for phase change applications. Finally, the challenges and opportunities raised by integrating nanoscale phase change materials into switching devices are discussed.

Electronic and optical switching of solution-phase deposited SnSe2 phase change memory material

We report the use of chalcogenidometallate clusters as a solution-processable precursor to SnSe2 for phase change memory applications. This precursor is spin-coated onto substrates and then thermally decomposed into a crystalline SnSe2 film. Laser testing of our SnSe2 films indicate very fast recrystallization times of 20 ns. We also fabricate simple planar SnSe2 electronic switching devices that demonstrate switching between ON and OFF resistance states with resistance ratios varying from 7−76. The simple cell design resulted in poor cycling endurance. To demonstrate the precursor’s applicability to advanced via-geometry memory devices, we use the precursor to create void-free SnSe2 structures inside nanowells of ∼25 nm in diameter and ∼40 nm in depth.

Phase change nanodots patterning using a self-assembled polymer lithography and crystallization analysis

Crystallization behavior of scalable phase change materials can be studied on nanoscale structures. In this paper, high density ordered phase change nanodot arrays were fabricated using the lift-off technique on a self-assembled diblock copolymer template, polystyrene-poly(methyl-methacrylate). The size of the nanodots was less than 15 nm in diameter with 40 nm spacing. This method is quite flexible regarding the patterned materials and can be used on different substrates. The crystallization behavior of small scale phase change nanodot arrays was studied using time-resolved x-ray diffraction, which showed the phase transition for different materials such as Ge15Sb85, Ge2Sb2Te5, and Ag and In doped Sb2Te. The transition temperatures of these nanodot samples were also compared with their corresponding blanket thin films, and it was found that the nanodots had higher crystallization temperatures and crystallized over a broader temperature range.

Synthesis and Size-Dependent Crystallization of Colloidal Germanium Telluride

Colloidal nanocrystals have long been used to study the dependence of phase stability and transitions on size. Both structural phase stability and phase transitions change dramatically in the nanometre size regime where the surface plays a significant role in determining the overall energetics of the system. We investigate the solid-solid phase transformation of crystallization in amorphous GeTenanoparticles. We report a colloidal synthetic route to amorphous GeTenanoparticles. Using in situ X-ray diffraction while heating, we observe the crystallization of the nanoparticles and find a dramatic increase of the crystallization temperature of over 150 C above the bulk value. Using size-selected nanoparticle films, we show that the crystallization temperature depends strongly on the particle size. In addition, we measure the electrical resistance of nanoparticle films and observe over 5 orders of magnitude lower resistance for the crystalline film compared to the amorphous film. Finally, we discuss the implications of the size-dependence of crystallization in the context of both understanding the behavior of phase stability in the nanosize regime and applications to phase change memory devices.

Recent progress of phase change memory (PCM) and resistive switching random access memory (RRAM)

Conventional memories such as SRAM, DRAM, and FLASH have set a very high cost/performance standard. Yet, recent advances in new materials, device technologies and circuits have made many emerging memories attractive candidates for a new generation of memories. This paper gives an overview of our recent research work on phase change memory (PCM) and metal oxide resistive switching memory (RRAM).

Phase Change Memory: Scaling and applications

Phase Change Memory (PCM) technology is a promising candidate for the future non-volatile memory applications. Scaling of PCM into the sub-10 nm regime has been demonstrated using novel applications of nanofabrication techniques. PCM devices using solution-processed GeTe nanoparticles of diameter range 1.8-3.4nm has been demonstrated. Highly scaled (<;2nm) PCM cross-point device using carbon nanotube as the electrode is fabricated proving the scalability of PCM to ultra small dimensions. The use of PCM as a nanoelectronic synapse for neuromorphic computation is also demonstrated as an illustration of PCM application beyond digital memory.

First demonstration of phase change memory device using solution processed GeTe nanoparticles

We present the first demonstration of a functional Phase Change Memory (PCM) device fabricated using solution processed GeTe phase change nanoparticle. The device shows the characteristic memory behavior of crystallization and threshold switching. The cycling endurance of the device is up to 100 cycles. The cells are currently the best performing solution processed phase change material based memory devices reported so far.