Ultrafast Optical Studies of Phonons and Phase Transitions in Ge2Sb2Te5 thin films
Thesis or dissertation
University of Exeter
This dissertation reports the results of optical studies of epitaxial (e), polycrystalline (p) and amorphous (a) Ge2Sb2Te5 (GST) thin films. The dynamic properties of GST films in all three (e/p/a) phases were investigated by a time-resolved optical pump-probe technique in which a femtosecond pump pulse of 55 fs duration was used to excite the sample. The intensity and polarization of the reflected probe beam respectively provide information about the transient reflectance (R) and anisotropic reflectance (AR) induced in the sample, that in turn provide the information about the crystal structure, phonon spectrum, and ultimately phase transitions within the sample. The study of an epitaxial sample provides an opportunity to explore the character of the modes within the phonon spectrum. The epitaxial GST film was grown upon a homoepitaxial layer of GaSb grown upon a GaSb wafer. We observed a 6.7 THz coherent optical phonon (COP) in GaSb(001). The dependence of the signal strength upon the pump and probe polarization was explained in terms of a model that considered both Transient stimulated Raman Scattering (TSRS) and the action of a Surface Space-Charge (SSC) field. The presence of the 6.7 THz transverse COP in the AR channel and its four fold dependence on pump and probe polarization suggests a three-dimensional T2 character. The COP amplitude was maximum when the probe was polarized parallel to the cube edge (GaSb) and the pump polarization was set parallel to a face diagonal (GaSb). The results were fully understood using a microscopic model of selective bond breaking. The AR response of e-GST/GaSb(001) reveals the presence of a 3D 3.4 THz transverse optical phonon. The mode amplitude was independent of pump polarization indicating that the mode is excited by a SSC field. This SSC field could exist within the GST, if the distorted rock-salt structure of GST lacks inversion symmetry, or GaSb, which has the non-centrosymmetric zincblende structure, leading to impulsive excitation of phonons at the GST/GaSb interface. The mode in GST was inferred to be T2-like. The observation of a T2-like phonon mode confirms that GST is cubic in structure and challenges previous studies where 1D or 2D character was assigned to the 3.4 THz mode. While pump-probe measurements displayed the presence of a 3D 3.4 THz mode in the AR response of e-GST/GaSb(001), a 4.5 THz mode was observed in both R and AR channels for p-GST(37 nm)/Si(001) and a-GST(57 nm)/Si(001). The mode character was identified to be either of A or E type by comparing the frequency with frequencies reported in the literature. Additional Raman microscope measurements confirmed the presence of the modes observed in the pump-probe measurements and also revealed additional frequencies. The differences in the frequencies observed from the different samples are quite small suggesting the presence of similar bonds that are modified to some extent by the different structural environment found within each sample. After exposure to high pump fluence the original modes disappeared and were replaced by new modes with frequencies at 4.2 THz and 3.1 THz in e-GaSb, 4.2 THz in e-GST, 3.5 THz in p-GST and 3.6 THz in a-GST. The difference in the final frequencies observed for p and a-GST sample may result from the difference in stack structure affecting the time-dependent temperature profile in each sample. The dependence of the temperature profile on the sample stack was understood from an experimental study of the phase transition between the amorphous and crystalline states induced by exposure to a series of amplified laser pulses. The dependence of the crystalline area and its reflectivity upon the number of pulses and fluence was described using a simple algebraic model. The results justify the assumption of one-dimensional heat flow. The growth velocity of the crystalline region was calculated to be 7-9 m/s. Apparatus and methods were developed to extend the time-resolved optical studies described previously. Firstly, an apparatus was constructed for the measurement of the wavelength dependent sample reflectance with a white-light pulse. A reference arm was employed to allow normalization and hence removal of the intensity noise arising in the laser regenerative amplifier system. Secondly an electrical measurement apparatus was constructed to allow combined electro-optical measurements in future. Switching of GST vertical memory cells was successfully demonstrated. The cells were fabricated on a borosilicate substrate with TiW top and bottom electrodes. A DC voltage of 4.5 to 6 V was required to induce switching, while in pulsed measurements, the device demonstrated switching in response to a pulse with minimum duration of 100 ns.
PhD in Physics