# Sub-nanometre resolution of atomic motion during electronic excitation in
phase-change materials

Kirill V. Mitrofanov  Paul Fons<EMAIL_ADDRESS>Kotaro Makino
Nanoelectronics Research Institute, AIST, Tsukuba Central 4, Higashi
1-1-1,Tsukuba 305-8562, Japan  Ryo Terashima  Division of Applied Physics,
Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai,
Tsukuba 305-8573, Japan  Toru Shimada  Department of Science, Faculty of
Education, Hirosaki University, 1 Bunkyo-cho, Hirosaki, Aomori 036-8560, Japan
Alexander V. Kolobov  Junji Tominaga  Nanoelectronics Research Institute,
AIST, Tsukuba Central 4, Higashi 1-1-1,Tsukuba 305-8562, Japan  Valeria
Bragaglia  Alessandro Giussani  Raffaella Calarco  Paul-Drude-Institut fur
Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany  Henning
Riechert  Takahiro Sato  RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo-cho, Hyogo
679-5148, Japan  Tetsuo Katayama  Kanade Ogawa  Tadashi Togashi  Japan
Synchrotron Radiation Research Institute, 1-1-1 Kouto, Sayo-cho, Hyogo
679-5198, Japan  Makina Yabashi  RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo-cho,
Hyogo 679-5148, Japan  Simon Wall  ICFO - Institut de Ciències Fotòniques, The
Barcelona Institute of Science and Technology, 08860, Castelldefels,
Barcelona, Spain  Dale Brewe  X-ray Science Division, Argonne National
Laboratory, 9700 S. Cass Ave, Lemont, IL 60439, USA  Muneaki Hase
<EMAIL_ADDRESS>Division of Applied Physics, Faculty of Pure and
Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8573,
Japan

###### Abstract

Phase-change materials based on Ge-Sb-Te alloys are widely used in industrial
applications such as nonvolatile memories, but reaction pathways for
crystalline-to-amorphous phase-change on picosecond timescales remain unknown.
Femtosecond laser excitation and an ultrashort x-ray probe is used to show the
temporal separation of electronic and thermal effects in a long-lived ($>$100
ps) transient metastable state of Ge2Sb2Te5 with muted interatomic interaction
induced by a weakening of resonant bonding. Due to a specific electronic
state, the lattice undergoes a reversible nondestructive modification over a
nanoscale region, remaining cold for 4 ps. An independent time-resolved x-ray
absorption fine structure experiment confirms the existence of an intermediate
state with disordered bonds. This newly unveiled effect allows the utilization
of non-thermal ultra-fast pathways enabling artificial manipulation of the
switching process, ultimately leading to a redefined speed limit, and improved
energy efficiency and reliability of phase-change memory technologies.

phase-change materials, time-resolved x-ray diffraction, ultra-fast phenomena

## Introduction

The class of chalcogenide phase-change materials, such as Ge-Sb-Te and Ag-In-
Sb-Te, has been found to be the most appropriate candidate for optical data
storage media in the forms of rewritable CDs and DVDs, as well as Blu-ray
discs. Ge2Sb2Te5 (GST) has also been demonstrated to be a compound well suited
for non-volatile memory applications, owing to a fast phase-switching process
between amorphous and crystalline phases (10 $\sim$ 100 ns), excellent thermal
and chemical stability of the end phases and good reliability allowing more
than 109 write-erase cyclesCheng2013 . GST superlattice films are now being
applied in a new generation of non-volatile electrical memory, interfacial
phase-change memorySimpson2011nn , surpassing current FLASH technology both in
cyclability and speedRaouxPCM2009 . The process of rapid phase change involved
in the writing and erasing of data in conventional optical recording is
presently induced by a purely thermal process using nanosecond laser pulses:
heating of the material leads to the formation of a molten phase and
subsequently the crystalline (SET) or amorphous (RESET) state, depending on
the cooling speed. Thus in phase-change memory devices the speed of write
cycles is limited to tens of nanosecondsWuttig2007nmat . Therefore in order to
overcome this limitation, an alternative method of switching between SET and
RESET states of GST is necessary. Avoidance of thermally based processes
implies the necessity of using electronic effects, and thus the nature of the
electronic structure of GST needs to be taken into account. While its
amorphous phase is characterized by its covalent bonding nature, the distorted
rock-salt crystalline phase is locally rhombohedral with three long and three
short bonds and is usually described in terms of resonant bondsshportko2008nm
; Kolobov2012pss ; krbal2012prb . Recently, the possibility of an ultrafast
phase transition triggered by an electronic excitation due to the breaking of
resonant bonds has been proposedkolobov2014jpc ; Li2011prl . Based upon the
experimental results from a time-resolved x-ray absorption fine structure
measurement, the possible presence of non-thermal contributions to the
amorphization of GST alloy on sub-nanosecond time scales was reported
Fons2010prb . The existence of a solid-solid amorphization process induced via
electronic excitation and subsequent lattice relaxation was further argued for
by density functional calculations Kolobov2011nc  and time-resolved electron
diffraction studiesHada2015 ; Hu2015 , in which, however, due to the
polycrystalline samples used and the transmission mode applied, the direct
demonstration of the existence of electronically driven effects without the
significant influence of thermal processes is challenging. This reason, in
addition to the fact that electronic excitation is in general immediately
followed by electron-phonon coupling-induced heating of the lattice, makes the
dynamics induced by an increase in lattice temperature difficult to separate
from purely electronic effects.

Very recently, Waldecker et al. reported on the decoupling of the electronic
and lattice degrees of freedom on a several picosecond time scale after
optical excitation. They interpreted the temporal separation of optical
properties from the structural transition in GST alloy films to be a result of
the depopulation of resonant bonds before electron-phonon energy exchange
occurredWaldecker2015 . These observations on a several picosecond time scale
will open a new route to control phase change in resonantly bonded materials.
However, the observation of the decoupling was limited to a narrow time
window, preventing from evaluating the lifetime and exact structure, both of
which are required for real device applications. Here we provide new insight
into a possible solid-solid transformation process in an epitaxially-grown
single-crystal film of GST based on reversible conditions through time-
resolved x-ray techniques with sub-nanometre resolution allowing the temporal
ranges of the lattice evolution to be discerned. The use of a much wider time
window up to $>$ 1 ns reveals the long lifetime of the transient state and
associated bond angles disordering. The x-ray diffraction intensity decreases
as a function of time delay without a thermal shift in peak position for
several picoseconds after photo-excitation, followed by the sudden onset of a
thermal-expansion-induced shift. These features coincide well with previous
resultsWaldecker2015 , but, being obtained from the direct observation of
atomic motion as opposed to indirect information derived from the evolution of
the optically excited dielectric function, strongly and unambiguously support
the existence of the separation of non-thermal (electronic) effects from
thermal (lattice) effects. These observations were also made possible by the
use of an epitaxial sample in the rocking curve mode measurements, providing
clear evidence of the movement of atoms independent from the optically-induced
heating of the sample. Our x-ray absorption fine structure (XAFS) data further
prove that the transient state is an intermediate state between the original
crystalline and amorphous states, but different from the liquid phase that
would be expected to exist for the case of switching based on a melting
process.

Figure 1: Layout of the experiment and schematic representation of the GST
structural change. The sub-nanometre dynamical atomic configuration processes
in GST are schematically shown at the bottom with three different time frames.
(I) Before the excitation ($\tau=0$) the atomic configuration is resonantly
bonded (resonant bonds are shown in red color thin interconnections between
atoms). (II) At $0<\tau<4$ ps the resonant bonds are disrupted and local
lattice distortions appear, while no thermal energy from the electrons was
transferred to the lattice, which corresponds to a new metastable state. (III)
At $\tau>4$ ps the optical phonons start to emit (heating of the lattice),
giving rise to bigger oscillations amplitudes of the atoms, which is
represented by thermal ellipsoids around them.

## Results

We used a time-resolved x-ray diffraction (XRD) techniqueSiders1999 ;
Sokolowski-Tinten2003nat ; Ichikawa2011  (Figure 1) to directly probe the
ultrafast structural dynamics in a 35-nm-thick single crystalline GST film
grown on a Si (111) substrate photo-excited by 1.55 eV (800 nm) photons. The
laser-induced dynamics of the GST lattice were studied by taking time-resolved
XRD rocking curves of the GST (222) Bragg diffraction peak (Figure 2a,b). This
peak served as an optimal peak for observations of the lattice dynamics due to
the boundary conditions imposed by the pump laser; the corresponding
integrated intensities and peak shift were measured as a function of the time
delay (Figure 2c,d). The diffraction intensity of the (222) peak decreases
immediately after excitation, and at $\sim$4 ps the intensity level reaches
$\approx$ 80% of its initial value. In contrast, the (222) diffraction peak
position starts to shift towards lower $Q_{z}$ values only after a $\sim$4 ps
time delay with a concomitant further decrease in intensity (Figure 2d). The
integrated intensity taken at a fixed incident angle at the (222) peak
position with high time resolution ($Q_{z}$=const) shows that the diffracted
signal intensity has an inflection point at $\approx$ 4 ps, which, together
with a combination of intensity and position of the diffraction peak,
demonstrates the existence of two-step dynamics. The (222) peak reaches a
maximum deflection $\sim$ 20 ps after the excitation. The shift of the
position of the Bragg peak towards smaller $Q_{z}$ values after excitation
implies that the maximum thermal expansion of the GST film is ${\Delta}$d/d
$\approx$ 1.4%Fons2014 , where $\Delta d$ is the fractional change in $d$
spacing. Subsequently, the peak position reverts to higher $Q_{z}$ values,
corresponding to contraction of the lattice.

The unshifted position of the (222) diffraction peak, observed with better
time resolution than in previous workFons2014 , implies that thermal effects
leading to expansion of the lattice do not occur during the first $\sim$4 ps,
since a diffraction peak shift toward lower $Q_{z}$ values would be expected
if the lattice temperature had risen due to electron-phonon
interactionsLahme2014 . Such a delay in the lattice heating onset may be due
to the presence of charge screening leading to a decrease of electron-phonon
coupling and/or Auger recombination in the early stages of the carrier
relaxation processCavalleri2000 ; Downer1986 ; Chin1999 . These processes make
possible the conservation of electron energy, leaving the electrons excited
for a time much longer than the characteristic time of optical phonon
emission. Such a long-lived excited state in GST results in a specific non-
thermal lattice response in which nanoscale local order changes due to the
high concentration of electrons remaining in an excited state. This premise is
supported by a combination of the diffraction main peak intensity and position
dynamics: while the intensity of the diffraction signal has a contribution
from thermal effects, including the Debye-Waller factor (DWF), the diffraction
peak position reflects solely the lattice strain (expansion or contraction)
along the direction of the scattering vector. The correlation between the
increase in the lattice temperature and the shift of the diffraction intensity
peak is based on the fact that the excess energy transferred to the lattice
from the optically excited electrons leads to an increase in the stress, which
has gradients at the film interfaces. This produces a strain wave propagating
into the sample at the speed of sound and an increase in the portion of the
film with modified lattice spacing as the wave propagates. This results in a
new Bragg condition and changes in the corresponding dynamics of the
diffraction rocking curve profile, in particular, a shift of the main
diffraction peak position.

Figure 2: Time-resolved x-ray diffraction results. (a) X-ray diffraction
rocking curves as a function of time delay for the GST (222) reflection
centered at 3.603 Å-1, observed for a 13.9 mJ cm-2 pump fluence and a $\geq$10
ps time delay step, over a wide $Q_{z}$ range, where $Q$ represents the
momentum transfer of the elastic scattering process and $z$ is the direction
of the scattering vector. The white dashed line traces the center of the
diffraction peak. (b) The same as (a) but with a 1 ps time delay step, over a
narrow $Q_{z}$ range. The green dashed lines trace the diffraction intensity
level. The black dashed lines indicate time zero and the time delay
corresponding to the beginning of the diffraction peak shift. (c) Normalized
integrated diffraction intensity (inverted triangles) and corresponding peak
position shift along $Q_{z}$ (squares), together with normalized integrated
diffraction intensity (diamonds) for a fixed x-ray beam angle of incidence as
a function of time up to 1.8 ns. (d) The same as (c) but for a finer time
delay step up to 10 ps. The arrow indicates the different dynamics between the
diffraction integrated intensity and the peak position. The data evolution is
divided into three stages using background color of the plot, corresponding to
the atomic configuration process in Figure 1: ground state - (I), disrupted
crystalline state with a cold lattice - (II) and a state with thermal effects
started to take place - (III).

## Discussion

Moderate decreases in diffraction intensity can be described by the Debye-
Waller factor, $e^{-\frac{1}{2}Q^{2}\langle u^{2}\rangle}$, where $\langle
u^{2}\rangle$ denotes the mean-square displacement of collective atoms along
the scattering vector $Q$AlsNielsenXrayPhys2011 , but the absence of a
thermally-induced diffraction peak position shift in the experimental data is
an evidence of a different scenario. While one might assume that the observed
changes are result of a response from a complete disordering of the part of
the crystal lattice, this explanation is not valid: dramatic decreases in the
diffraction intensity from crystalline samples are usually associated with
structural disorder, which in turn is connected with randomization or
fluctuations in atomic positions, but in tetrahedral covalently bonded
semiconductors non-thermal melting has been reported to occur within several
hundred femtosecondslindenberg2005sci , a time scale much faster than seen in
the current GST film. In fact, the observed recovery of the Bragg peak
intensity on the nanosecond time scale demonstrates that the initial
crystalline state remains. Re-crystallization from the molten state also
cannot explain the observed dynamics as optically induced crystallization
processes in GST require much longer times - typically more than 10
nsSiegel2004 . Therefore, the observed decrease in diffraction intensity for
times $<$ 4 ps cannot be interpreted to be the result of non-thermal melting
in the general sense, i.e. a complete loss of long-range order. The absence of
a thermal-shift during the first 4 ps, however, indicates the presence of non-
thermal effects in photo-excited GST, arising due to the effects of intense
optical excitation on resonant bonding. It should be noted that the term
”resonant bonding” with respect to phase-change materials is used differently
by different authors: while some authors consider all bonds to be resonant,
others refer to only longer bonds as being so. In this manuscript, we have
adopted the latter approach. The experimentally observed dynamics indicate
that the diffraction peak intensity does not fully recover even 1.8 ns after
excitation (Figure 2c) due to residual thermal effects, which completely
disappear after 3 $\sim$ 5 ns depending on the excitation fluence. The
corresponding return of the diffraction peak to its initial intensity
demonstrates that the GST film was not amorphized, but transiently transformed
to an intermediate state. The drop in the diffraction signal intensity cannot
be explained soley by the DWF for the following two major reasons. First, the
absence of the diffraction peak position shift until 4 ps after the
excitation, which is more clearly represented by an inflection point on the
time-dependent diffraction intensity curve, indicates the lack of thermal
effects immediately after laser exposure. The inflection point marks the start
of the shift of the diffraction signal peak position due to the expansion of
the GST film, i.e. the onset of thermal effects. Second, if one assumes that
the diffraction peak shift is the result of the strain in the sample due to
thermal effects, then the acoustic phonon-induced contribution of the strain,
corresponding to the shift at 128 ps after excitation, should be highly
damped. This assumption leads to a calculated lattice temperature using the
thermal expansion coefficient of 1.74x10-5 K-1 (ref. Park2008tsf ) of
$\approx$ 930 K, a temperature which is higher than the melting point, which
is inconsistent with observation and thus indicates the presence of long-lived
non-thermal effectsshu2013apl .

Figure 3: Simulation results for the lattice dynamics of an optically excited
GST sample. (a) Time dependence of the crystal lattice strain for the case of
the current experiment conditions except the literature value of the sound
velocity (3.19 nm ps-1)(see ref. Fons2014 ). (b) Corresponding diffraction
data evolution with time for the (222) reflection, the white dashed line
traces the diffraction peak position, the black dashed lines indicate the
value of the diffraction maximum peak shift along $Q_{z}$ (vertical line) and
time delays corresponding to the beginning of the peak shift and the maximum
shift (horizontal lines). (c) The same as (b), but for the case of the sound
velocity value obtained from the current experiment (2.19 nm ps-1).

In addition to thermal expansion, the laser-induced thermal stress in the film
gives rise to the generation of acoustic phonons originating from the
interfaces of the film with the capping layer and the substrate (Figure
3)Thomsen1986 . The half-period of the acoustic phonon (T/2 is equal to the
ratio of the sample thickness (l) and the sound velocity (v): T/2=l/v. The
half-period of the experimentally observed strain waves originating from the
interfaces was estimated to be $\approx$ 16 ps (Figure 3b). This allows the
calculation of the sound velocity in the metastable state to be $\approx$ 2.19
nm ps-1, a value which is $\approx$ 2/3 of the literature value for
crystalline GSTFons2014 ; lyeo2006apl  and close to the value for the
amorphous caselyeo2006apl . To account for the observed strain dynamics, the
propagation of strain waves in laser-excited GST as well as the resulting
transient diffraction intensity change was simulated using a one-dimensional
linear chain model as implemented in the package UDKM1DSIMFons2014 ;
Schick2013cpc  for two cases: using the literature value of the sound velocity
for crystalline GST and the experimentally estimated sound velocity as an
input parameter. In the first case the obtained time delay for the maximum
deflection of the diffraction peak was about 2/3 of the half-period of the
experimentally observed strain wave (Figure 3b). On the other hand, for the
case of the experimentally estimated speed of sound value, the simulation
results are in good agreement with the experimental data (Figure 3c), which
indicates a significant change in the interatomic potentials from the ground
state, a result which can be related to the local reorientation of atoms
ensembles, leading to increased bonds lengths and a wider angular distribution
of bonds. The results of the simulated transient diffraction pattern obtained
using the UDKM1DSIM package are in a good agreement with the experimental data
for the case of the experimentally estimated speed of sound in the GST film,
but it should be noted that an earlier work, in which much slower optical
excitation was used, the simulation data matched the experimental results for
the literature value of the sound velocitySiegel2004 . This disparity is a
consequence of the differences in excitation conditions (in the current work
the pump pulse is much faster: 30 fs instead of the 700 fs (presented in ref.
Fons2014 ), giving rise to the markedly different dynamics observed in the
current work. In addition it should be pointed out that the speed of the
diffraction response in the earlier work was also masked by a $\approx$100 ps
convolution in time, due to the probe pulse width, making small changes in
sound velocity difficult to distinguish.

We argue that the ultrafast excitation of electrons from bonding to anti-
bonding states leads to the breaking of longer (weaker) Ge/Sb-Te
bondskolobov2014jpc ; Waldecker2015 , i.e., the non-thermal melting of
resonant bonds, leaving stronger covalent bonds intact. Based on the results
presented in refs. Lee2011prl  and Akola2007spt , we schematically represent
the excited structure as non-connected cubes (Figure 1). The process of the
formation of such building blocks is expected to occur over very short times,
less than the characteristic recombination time of non-equilibrium charge
carriers. The breaking of the longer Ge/Sb-Te bonds results in local
structural relaxation, while the average long range order in GST crystal
persists. The corresponding state can be characterized as a ”disrupted
crystalline” state. This transient disordering in GST is schematically
visualized in Figure 1. In the ground state, building blocks are connected to
each other by resonant bonds; in contrast, upon electronic excitation the
building blocks locally relax due to the loss of resonant bonding, leading to
the tilting and shifting of the blocks from their initial
positionsKolobov2011nc ; kolobov2014jpc .

The decrease in the (222) Bragg diffraction peak intensity continues until the
excited system gradually reverts to its initial state with resonant bond
interaction over the course of at least 100 ps (Figure 2c). This implies that
despite in the initial stage (first 4 ps) the excited state is purely
electronic, it persists for much longer times, accompanied and thus hidden by
thermal effects, which does not prevent its usage for ultrafast structural
modification. Upon photo-excitation, local distortions can lead to a change in
some Ge atomic configurations from an ”octahedral” to a tetrahedral geometry
with sp3-hybridizationAkola2007spt ; caravati2007cta ; Micoulaut2014 . Local
structural relaxation back to the ground state is suppressed by a decrease in
the probability of the restoration of resonant bonding due to a transient
increase in mean-square atomic displacements and fluctuations in bond angles.
These effects extend the long relaxation period of the excited electrons and
serve as a kinetic barrier leading to the unusually long duration of
structural recovery from the excited state in GST.

The existence of a metastable (intermediate) state is also supported by an
independent time-resolved XAFS experiment. A unique XAFS $k^{2}\chi(k)$ signal
was collected in which an excited state structure of GST (Figure 4a),
different from both the liquid and amorphous states (Figure 4b), was
transiently observed upon ultrafast optical excitation with the sample quickly
reverting to its crystalline state (within $\approx$1 ns).

Figure 4: XAFS experimental and simulations results. (a) Ge K-edge XAFS
oscillations of GST for the crystalline cubic state before, during and after
the excitation, the red arrow points to the ”beat” in the ground state and the
state after the excitation at $\sim$3.7 Å-1. (b) Ge K-edge XAFS oscillations
of GST for amorphous, liquid and excited state. (c) MD simulation results of
Ge K-edge XAFS signal for 3 $<k<$ 4.5 Å-1 (see explanations in Methods
section) for GeTe in the ground and excited states, the arrow points out the
”beat” in the ground state at $\sim$3.8 Å-1. The light-orange area in the plot
traces the damping of the function, corresponding to the excited state.

A similar change could be reproduced based upon a molecular dynamics (MD)
simulation using the plane wave density functional program VASP (see
Experimental Section for details), in which 4% of the valence electrons were
placed into the conduction band (a value equal to the theoretically estimated
concentration of the excited electrons for the experimental conditions used);
the MD simulations were carried out using a Nosé thermostat, which maintained
the lattice temperature at approximately 300 K (the standard deviation of the
temperature fluctuations was approximately 20 degrees implying that
temperature fluctuations do not lead to significant changes in the XAFS
spectra and can be neglected), i.e. the observed change is a consequence of a
purely electronic effect. Figure 4c shows the results of the simulated time
averaged XAFS signal for both the ground and excited states. In the MD
simulations, we have used GeTe as an approximant for GST to avoid the
configurational complexity of the vacancy distribution associated with GST.
While for the ground state spectra, obtained from the experiment, there is a
clear ”beat” at approximately 3.7 Å-1 that arises from quantum mechanical
interference of the outgoing photoelectron wave function with itself due to
backscattering from a well-defined second coordination shell, for the excited
state, the interference is strongly diminished, and the spectra itself closely
approximates a damped sine function, a shape characteristic of backscattering
from a single coordination shell (Figure 4a). The simulations of the 300 K
systems reproduce the experimental trends well with the difference between the
two simulated spectra being solely due to the change in the electronic
conditions, strongly supporting the non-thermal nature of the lattice
perturbations. This is interpreted to be a consequence of ultrafast strong
optical excitation leading to a transient increase in lattice local disorder;
the same disorder is speculated to lead to the experimentally observed
transient dramatic change in sound velocity and the extraordinary behavior of
the diffraction signal peak position/intensity, measured in the XFEL pump-
probe experiment.

The existence of a disrupted crystalline state in GST can be used in phase-
change memory to transiently lower the potential barrier between the SET and
RESET states allowing energetically preferable switching with anticipated
improvements over the current speed limit to rates in the GHz region (which
corresponds to a few picoseconds for the structural relaxation to occur). The
long lived excited state can be utilized in a different types of applications,
like optical and electrical switches and sensors, possibly working even in the
THz region, since the observed excited state appears in the first tens of
femtoseconds (just after the laser excitation).

The process of breaking the resonant bonds, leading to significant lattice
local distortions, can serve as a precursor of non-thermal high-speed phase-
switching. Moreover, it represents a new metastable state, characterized by
both electronic and crystal lattice conditions, that results in optical and
electric properties, different from the ground state. This difference can be
utilized for the introduction of fundamentally new concepts of device
operation for the recording and processing of data, including the possibility
for applications in multi-level memory devices. The understanding of the
transformation dynamics on ultrafast time scales over a few picoseconds
provides new insight into the ultimate speed limit of phase change
materialsLoke2012  and suggests that electronic effects may play a heretofore
unrecognized important role in device switching, possibly leading to
significant reductions in the energy requirements. Ultimately, resonant
bonding state manipulation may allow the alteration of the crystal structure
solely by the selective modification of the electronic state. In particular,
the recent development of interfacial phase-change memory with spatially
separated GeTe and Sb2Te3 layers, in which switching between the SET and RESET
state is achieved without meltingSimpson2011nn , may be assisted by the
presence of excited states induced by the large electric fields applied during
switching.

## Methods

### Sample preparation:

Crystalline Ge2Sb2Te5 (GST) thin films were fabricated by molecular beam
epitaxy (MBE) on the Si(111)-($\surd$3x$\surd$3)R30∘-Sb reconstructed surface
as described in detail elsewhereBoschker2014nano . According to x-ray
reflectivity measurements (not shown) the film thickness of the sample was 35
nm. The GST films were investigated by x-ray diffraction (XRD) and showed a
cubic phase characterized by a high degree of texture and structural order in
both the out-of-plane and in-plane directions Boschker2014nano ; Katmis2011 ;
Rodenbach2012pss . The lattice constant $a$ was 6.04 Å, which is consistent
with the literature value of 6.01 Å $\sim$ 6.04 ÅRodenbach2012pss . Neglecting
the slight rhombohedral distortion, GST is hence lattice-mismatched by
$\approx$11% to Si (a = 5.431 Å). The samples were capped with 30 nm of
sputtered amorphous Si3N4 to prevent oxidation.

### Pump-probe x-ray diffraction:

Ultrafast time-resolved XRD measurements were conducted at the Japanese XFEL
facility SACLATono2013  using the output of the x-ray FEL in spontaneous
emission mode. A monochromatic beam was generated from the XFEL beam using two
channel-cut Si(111) crystals in a non-dispersive arrangement corresponding to
a wavelength of 1.24 Å with a FWHM pulse of $\approx$ 10 fs. This beam was
directed onto the sample as a probe pulse and the diffraction reflections
obtained in the Bragg condition were detected by a 512 x 1024 channel multi-
port CCD camera (Figure 1). Each measurement was repeated up to 250 times to
improve statistics. A high-speed chopper was used to isolate individual x-ray
pulses at 30 Hz. The p-polarized optical laser pulses for the pump
($\approx$30 fs-duration at a center wavelength of 800 nm) were synchronized
with the XFEL; the angle between the laser and x-ray beams was $\approx$5∘.
The maximum pump power was 3 mW (or 55 $\mu$J pulse-1) and the laser was
focused into a 700 $\mu$m diameter spot, a size significantly larger than the
diameter of x-ray beam spot (250 $\mu$m). The results presented here were
obtained with pump fluences up to 13.9 mJ cm-2 at the sample surface, which is
below the threshold value for the irreversible changes in GSTWaldecker2015
after taking into account the reflectivity and the ellipsoidal broadening of
the laser beam spot for the used angle of incidence. The jitter in the time
delay between x-ray and laser pulses was $\leq$ 0.2 ps, supporting an
effective time resolution better than 200 fs. The time delay ($\tau$) between
the pump laser and the x-ray pulse was adjusted by an optical delay line in
time increments of 60 fs to 100 ps. An effective time resolution of 60 fs was
obtained by time sorting x-ray pulses using a timing monitor for the
diffraction intensity plot shown in Figure 2d.

### Time-resolved XAFS measurement:

XAFS measurements were taken at sector 20 of the advanced photon source (APS).
In the experiment, a 40 nm thick polycrystalline layer of GST encapsulated by
20 nm thick transparent Si3N4 layers with a ZnS-SiO2 protective cap was
irradiated by a 190 fs, 800 nm pump laser pulse and the resulting lattice
dynamics were probed via measurement of Ge K-edge XAFS as a function of laser
delay. The fluence used was 9 mJ cm-2. The spot size of the laser was
approximately 55 $\mu$m and the $\sim$ 5 $\mu$m Kirkpatrick-Baez focused x-ray
probe beam was well contained within the pump beam spot. A quartz disk served
as the substrate and was continuously rotated using an ultra-low wobble
spindle motor to eliminate heat build up and reduce radiation damage. The
laser system consisted of a Coherent Millenia Ti-Sapphire oscillator phase-
locked to the RF signal of the storage ring and a Coherent RegA regenerative
amplifier system which could provide power up to 4 $\mu$J pulse-1. The disk
was rotated assuring that several microseconds passed before a given area on
the sample was subsequently irradiated. The Ge fluorescence signal was
detected via a 100,000 element gated Pilatus detector. By varying the laser
trigger signal delay, the relative position of the laser pump to the x-ray
probe pulse could be varied in $\sim$ 18 ps intervals. The x-ray probe pulse
had a duration of approximately 100 ps resulting in a similar convolution in
time of the experimental data. The fact that the unique signature of the
excited state is visible despite the convolution resulting from the duration
of the x-ray pulse, substantiates the long lifetime of the excited state.

### VASP simulations:

The simulated XAFS spectra were generated in two steps. In the first, ab-inito
molecular dynamics (AIMD) were used to follow the trajectory of a 128 atom
GeTe cluster for 30 picoseconds using density functional theory and the plane
wave code VASP 5.4.1VASP . The effects of optical excitation were simulated by
constraining the band occupancies such that 4% of the valence electrons were
promoted to the conduction band. TThe generated trajectories were then used as
input to the real-space multiple scattering code FEFF 9.64feff8  to generate
XAFS spectra, which were then averaged to generate the final simulated XAFS
spectra. A cutoff energy of 175 eV was used for the plane waves in VASP and
k-point sampling was carried out using the gamma point. A convergence study
showed that use of the gamma point for integration of the Brillioun zone was
sufficient to reflect the the general trends of the XAFS data. The energy
difference was found to be less than 10 meV/atom. Projector augmented waves
were used to include the effects of the core electrons. In Figure 4c the data
is presented for k $>$ 3 Å-1, due to limitations of the extended x-ray
absorption fine structure approximation used.

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## Acknowledgements

This work was supported by X-ray Free Electron Laser Priority Strategy
Program, entitled Lattice dynamics of phase change materials by time-resolved
X-ray diffraction (NO. 12013011 and 12013023), from the Ministry of Education,
Science, Sports, and Culture of Japan.

The XFEL experiments were performed at the BL3(EH2) of SACLA with the approval
of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No.
2012B8041, 2013A8051, 2013B8056, 2014A8039 and 2014B8061).

Sector 20 facilities at the Advanced Photon Source, and research at these
facilities, are supported by the US Department of Energy - Basic Energy
Sciences, the Canadian Light Source and its funding partners, the University
of Washington, and the Advanced Photon Source. Use of the Advanced Photon
Source, an Office of Science User Facility operated for the U.S. Department of
Energy (DOE) Office of Science by Argonne National Laboratory, was supported
by the U.S. DOE under Contract No. DE-AC02-06CH11357.

## Author contributions statement

K.V.M., P.F., K.M., R.T., T.Shimada, A.V.K., A.G., T.Sato, T.K., K.O., T.T.,
S.W., D.B. and M.H. carried out the experiments. K.V.M., P.F. and M.H.
processed the data and performed the simulations. V.B., A.G. and R.C. prepared
the samples. K.V.M., P.F., A.V.K. and M.H. wrote the manuscript. All of the
co-authors contributed to the discussions of the results and manuscript.

## Additional information

Competing financial interests: The authors declare no competing financial
interests.