A. Di Cicco1,3, A. Congeduti2, F. Coppari1, J. C. Chervin1, F. Baudelet2 and A. Polian1
1IMPMC-CNRS, Université Pierre et Marie Curie, 140 rue de Lourmel, 75015 Paris, France
2Synchrotron SOLEIL, L’Orme des Merisiers Saint-Aubin, 91192 Gif-sur-Yvette, France
3CNISM, CNR-INFM Soft, Dipartimento di Fisica, Università di Camerino, Via Madonna delle Carceri, I-62032 Camerino (MC), Italy
A coupled x-ray-absorption and Raman scattering experiment carried out within a collaboration between the ODE absorption beam line and IMPMC laboratory has shown a spectacular evidence of metallization of amorphous Ge films under high pressure. This transformation is identified as a low-density amorphous LDA to high-density amorphous HDA transition, relevant to a large class of systems including amorphous ice, semiconductors, and oxides. We have discovered that this transition initiates at the surface of the LDA sample, characterized intrinsically by a thickness dependent density of voids. Contrary to the case of transitions involving stable crystalline solid phases, our observations show that pressure-induced phase transitions in inhomogeneous amorphous samples are morphology driven and are favored for lower defect void densities. The metal disordered phase is observed first at 8 GPa for a lower density of voids, transforming to a metastable ordered phase upon depressurization. The local HDA structure is characterized by an increase in the first-neighbor coordination number, average distance, and variance.
FIG. 1. (Color online) Upper figures: images of the surface of the a-Ge film (thickness of about 3μm) collected using a metallographic optical microscope. The morphologies of the film removed from the evaporation substrate (glass) are different on the two sides. The substrate side S (toward the glass) contains a larger number of voids than the V (vacuum) side. The estimated surface density fraction of voids, evaluated on a set of micrographs, changes from 5.8% to about 1% from the substrate to the vacuum side. Lower figure:
section of the sample evaporated on the glass substrate showing the
void density change and the S and V sides of the films.
FIG. 2. (Color online)
Panel (a): Raman-scattering patterns for HDA (7.9 GPa) and LDA (0 and 4.8 GPa) phases shown on an extended range and on absolute intensity scale. The HDA sample shows a stronger diffuse scattering and loss of Raman signals.
Panel (b): a-Ge Raman patterns for increasing pressures. The a-Ge LDA Raman pattern shows a dominant contribution around 280 cm-1, which shifts to higher frequencies at higher pressures, as a consequence of the bond shortening. The a-Ge films undergo a LDA-HDA transition at about 8 GPa which involves the V (vacuum) side of the films. The substrate side (S) is found to be still LDA up to about 10 GPa. Raman measurements at 7.9 and 8.8 GPa are shown for both phases on the two sides of the a-Ge films.
Panel (c): images of the a-Ge sample through the circular gasket hole (initial aperture, 250 μm) for increasing pressure. Two samples have been loaded showing both sides V and S for Raman measurements (through diamond, first column). The rightmost images were collected on the opposite side through the NaCI medium and they show clearly a complementary trend. Most Raman measurements have been performed on the V sample indicated in the figure. The transition to a new phase is clearly visible at 7.9 GPa and is completed at 8.8 GPa. The transition of side S is observed at about 10 GPa (image shown for both sides at 12.5 GPa).
FIG. 3. (Color online)
Left panel: shift of the Ge K edge structure at high pressures (all data normalized to 1). The shift to lower energies around 8 GPa is associated with the disappearance of the gap in a metallic state.
Right panel: micrographs of the a-Ge sample through the diamonds. The transition to a highly reflective metallic state is clearly visible.
FIG. 4. (Color online)
Panel (a): measured XAS kχ(k) data during a compression-decompression cycle shown from the bottom to the top. The data were smoothed for clarity of the presentation. The dramatic intensity and phase change of the signal associated with the LDA-HDA transition is clearly visible in the data above 7.9 GPa.
Panel (b): the FT of the experimental data shows a clear trend of the intensity (shown in the inset) and position of the first peak.
Inset (c): average first-neighbor distance R and variance σ2 obtained by XAS data analysis shown as a function of pressure. The structural changes associated with the transition are clearly visible looking at the sudden increases in R and σ2 (intensity decrease of the first FT peak).
Ref.: PHYSICAL REVIEW B 78, 033309 (2008).
