Transformations Revealed by Dynamic TEM

Photo-Emission TEM for Nanosecond Scale Materials Science

  • Fig. 1: False color image of laser crystallization of a PCM. Crystalline regions (yellow) grow into amorphous GeTe (blue) at the center of a 3.2 μJ laser shot. Each frame is taken with a 20-ns electron pulse and the time signatures are relative to the peak intensity of the 12-ns laser shot on the specimen.Fig. 1: False color image of laser crystallization of a PCM. Crystalline regions (yellow) grow into amorphous GeTe (blue) at the center of a 3.2 μJ laser shot. Each frame is taken with a 20-ns electron pulse and the time signatures are relative to the peak intensity of the 12-ns laser shot on the specimen.
  • Fig. 1: False color image of laser crystallization of a PCM. Crystalline regions (yellow) grow into amorphous GeTe (blue) at the center of a 3.2 μJ laser shot. Each frame is taken with a 20-ns electron pulse and the time signatures are relative to the peak intensity of the 12-ns laser shot on the specimen.
  • Fig. 2: Schematic of the DTEM at LLNL. Reactions may be initiated with the specimen laser. The cathode laser pulses enter the TEM column and are directed onto the cathode with a mirror. The photo-emitted electron pulses used to image the reacting specimen are directed onto different regions of the CCD camera with an electrostatic deflector.
  • Fig. 3: Crystallization of amorphous GeTe as in figure 1, but the longer inter-frame time and lower magnification give an overview of the crystallization process. The nucleation rate is low near the center of the laser shot and large grains grow. The outer edge of the crystallizing region is cooler and grows slowly outward, promoted by the heat released during crystallization.
  • Fig. 4: Laser crystallization of amorphous Ge. The 95-ns inter-frame time allows the crystallization front, moving at 10 m/s, to be imaged. The region imaged in the 20-ns exposures (left) is circled in the conventional TEM image (right).

Dynamic Transmission Electron Microscopy, a photo-emission TEM technique, is emerging as a tool for studying rapid transformations in materials. The technique enables electron imaging and diffraction on the nanosecond and microsecond scale, allowing kinetic characterization of processes that cannot be resolved with other microscopic techniques. Examples are shown for laser crystallization of amorphous phase change materials and semiconductors. Application to metals, alloys, and other materials are described.

Introduction

Transmission electron microscopy (TEM) is a powerful tool for studying materials' microstructure and crystal structure with high spatial resolution. It is also used to characterize dynamic processes, yet many processes, such as dislocation motion and phase transformations, propagate at rates far exceeding the temporal resolution of thermionic- or field emission-based TEMs. Photo-emission-based TEM techniques that use lasers to extract electrons from the cathode have enabled the study of processes on time scales many orders of magnitude faster than what may be captured by conventional TEM. This work focuses on dynamic TEM (DTEM) and its application to problems in materials science. DTEM is distinguished from other photo-emission TEM techniques in that each electron pulse contains enough electrons (~1010) to form an image or diffraction pattern [1], making it well-suited for studying the irreversible transformations that are core to understanding materials processing. The related technique of stroboscopic photo-emission TEM, known as ultrafast electron microscopy (UEM) uses many pulses each containing ~1 electron to form images. UEM can attain sub-picosecond time resolution, is especially good for studying highly reversible reactions, and is described elsewhere [2].

The DTEM at Lawrence Livermore National Laboratory (LLNL) (fig. 2), originally built to capture single images of reacting specimens, is capable of generating multiple nanosecond-scale electron pulses spaced over several microseconds in its current stage of development. A laser system delivers a series of laser pulses to the TEM cathode with precise temporal shaping and intensity resulting in multiple photo-emitted electron pulses, which probe a specimen multiple times during a reaction, forming a movie of a unique irreversible process.

An electrostatic deflector placed in the TEM column below the imaging system deflects each image to a different part of the CCD camera, overcoming the ~ms limit on the refresh rate of the camera.

Phase Transformations in Phase Change Materials for Memory Applications

Chalcogenide-based phase change materials (PCMs) such as Ge2Sb2Te5 and GeTe are important for phase-change-based memory devices. For memory applications, it must be possible to switch between the amorphous and crystalline phases in nanoseconds with rapid laser or electrical pulses. Crystallization is the data-rate-limiting process and the kinetics of crystallization are important as they directly impact device speed. Laser crystallization has been tracked in Ge2Sb2Te5 in diffraction mode using single shot DTEM [3], but imaging mode has proved especially powerful as it can distinguish the individual contributions of nucleation and growth to the overall crystallization process. Multi-frame DTEM has yielded direct measurement of crystal growth rates at the high temperatures and short times especially relevant to the function of PCM memory devices. DTEM imaging of laser crystallization of GeTe as shown in figure 1, has yielded crystal growth rate measurements exceeding 3 m/s, more than 6 orders of magnitude faster than what has been measured with other microscopic imaging techniques, and allowing kinetic models to be fit with experimental measurements in technological relevant time and temperature regimes [4].

Changing the inter-frame spacing and the magnification gives an overview of nucleation and growth during laser crystallization of GeTe (fig. 3). Crystallization begins in an annulus where the nucleation rate is highest. The density of nuclei is low in the center of the laser spot where the temperature is highest. Further from the center, the outer edge of the crystallizing region is cooler and grows slowly outward, promoted by the heat released during crystallization. Single shot DTEM was initially used to study nucleation in GeTe [5], but current efforts using multi-frame DTEM will yield far more accurate measures of nucleation rates in PCMs.

Laser Crystallization of Semiconductors

Laser crystallization of amorphous semiconductor thin films is an important processing path for electronic devices. Laser heating enables extremely high heating rates to highly localized area of a device. It is possible to achieve a variety of grain sizes and textures, including very large grain sizes, which are important for the processing of thin film transistors. Laser crystallization accesses metastable and unstable states that are inaccessible with other processing methods, but as with PCMs, the transformation occurs far too fast to image with traditional microscopy techniques. Microstructural development of amorphous semiconductors during laser heating is also an incredibly complex process that involves several distinct modes of crystal nucleation and growth and the development of intricate microstructural patterns that begins within nanosecond of laser heating and continues for many microseconds. One of the earliest applications of photoemission TEM imaging was to crystallization of amorphous Ge in the late 1980's [6]. More recently DTEM has revealed the complex crystallization dynamics with 10 nm spatial and 15 ns temporal resolution [1,7]. Super lateral growth of crystallizing Ge results in grains many microns long and the DTEM can image the propagation and roughening of the crystallization front as it sweeps along at speeds exceeding 10 m/s (fig. 4). This growth is followed by a transition to a ledge-like growth mechanism that produces a layered microstructure over several microseconds.

Applications to Metals, Metal Alloys, and Other Materials Systems

DTEM has also revealed important information on a variety of transformations in metal systems including alloy formation in Ni-Al multilayer reactive foils [8], crystal-growth modes during rapid solidification of Al and Al-Cu alloys [1,9], and crystallization kinetics of amorphous NiTi alloys [1]. DTEM observations of morphological changes during dewetting of ultra-thin Ni films have revealed the physical mechanism driving the liquid-phase self-assembly of metal nanoparticles [10].

Outlook
The application of photoemission TEM to materials science problems is currently a small but growing field. Potential applications of DTEM go far beyond the those described here, including processes occurring in fluid and biological systems [1] and phase transformations induced by electrical biasing.

Acknowledgements
This work performed under the auspices of the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under FWP SCW0974 by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. MKS thanks Simone Raoux for preparation of GeTe and Ge films.

References
[1] T. LaGrange et al.: Micron 43, 1108-1120 (2012)
[2] A. H. Zewail: Science 328, 187-193 (2010)
[3] M. K. Santala et al.: J. Appl. Phys. 111, 024309 (2012)
[4] M. K. Santala et al.: Appl. Phys. Lett. 102, 174105 (2013)
[5] M. K. Santala et al.: Physica Status Solidi B 249, 1907-1913 (2012)
[6] O. Bostanjoglo et al.: Ultramicroscopy 21, 367-372 (1987)
[7] L. Nikolova et al.: Phys. Rev. B 87, 064105 (2013)
[8] J. S. Kim et al.: Science 321, 1472-1475 (2008)
[9] J. T. McKeown et al.: Acta Materialia 65, 56-68 (2014)
[10] J. T. McKeown et al.: Langmuir 28, 17168-17175 (2012)

Author
Dr. Melissa Santala

Lawrence Livermore National Laboratory
Condensed Matter and Materials Division

Livermore, CA, USA

Contact

Lawrence Livermore National Laboratory
7000 East Ave.
Livermore, CA 94550
USA
Phone: +1 510 422 1100
Telefax: +1 510 422 09 98

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