Pressure-induced amorphization and an amorphous–amorphous transition in densified porous silicon

Abstract

Crystalline and amorphous forms of silicon are the principal materials used for solid-state electronics and photovoltaics technologies. Silicon is therefore a well-studied material, although new structures and properties are still being discovered. Compression of bulk silicon, which is tetrahedrally coordinated at atmospheric pressure, results in a transition to octahedrally coordinated metallic phases. In compressed nanocrystalline Si particles, the initial diamond structure persists to higher pressure than for bulk material, before transforming to high-density crystals. Here we report compression experiments on films of porous Si, which contains nanometre-sized domains of diamond-structured material. At pressures larger than 10 GPa we observed pressure-induced amorphization. Furthermore, we find from Raman spectroscopy measurements that the high-density amorphous form obtained by this process transforms to low-density amorphous silicon upon decompression. This amorphous-amorphous transition is remarkably similar to that reported previously for water, which suggests an underlying transition between a high-density and a low-density liquid phase in supercooled Si (refs 10, 14, 15). The Si melting temperature decreases with increasing pressure, and the crystalline semiconductor melts to a metallic liquid with average coordination approximately 5 (ref. 16).

High-pressure studies with x-rays using diamond anvil cells

“…Interesting phenomena have been observed, including coordination number change [60,61], polyamorphism [197,198], and long-range topological order [199]. In DAC experiments, the amorphous XRD method is often utilized with a high energy monochromatic beam, in order to have a large coverage in momentum transfer Q=4sin [61,179,180] for sufficient real space resolution.…”

“…HP XRS allows direct probing of the chemical bonding changes of light elements in noncrystalline materials [267,382]. Relatively speaking, HP XRD remains a primary technique for studying the structural evolution of non-crystalline materials [97,120,198,204,262,264,[383][384][385][386][387][388][389], pressure-induced amorphization or melting [346,[390][391][392][393][394][395][396][397][398][399], and pressure-induced crystallization [58,346,393,397,[400][401][402].…”

Polyamorphic transitions in Ce-based metallic glasses by synchrotron radiation

“…Such transitions have been observed, for example, in liquid phosphorous 2 and sulfur, 3 by X-ray diffraction and neutron diffraction techniques respectively, regarding for static signatures. Other examples of this phenomena include Si, [4][5][6] binary liquids such as Y 2 O 3 -Al 2 O 3 , 7,8 vitreous SiO 2 9,10 and GeAsS chalcogenide glasses. 11 More recently, metallic glasses (MGs) have attracted interest as a new class of amorphous materials due to their potential for structural applications.…”

The model developed for stress-induced structural phase transformations of micro-crystalline silicon films

“…The indentation stress-strain curve exhibits a series of yielding events, attributed to the nucleation and movement of dislocations [18]. In porous silicon [19], a transition from the diamond phase to a high density amorphous (HDA) phase has been reported. Nanocrystalline silicon [20] undergoes a direct transition from diamond to a simple hexagonal structure.…”

“…Here, we examine the atomistic response to nanoindentation by a sharp indenter with various centerlineto-face angles α. From the stress response at incipient plasticity developed by Pan et al [13], the contact stress V c and indentation strain H c in Fig Stress-induced phase transitions in microcrystalline silicon (μc-Si) have been extensively studied [19][20][21][22][23][24]. In order to study the model of crystallization, the volume-dependent Gibbs free energy is evaluated.…”