The high pressure phase transformations of silicon and germanium at the nanoscale

Semiconducting materials are critical for the electronics industry with the two most important semiconductors being Si and Ge. Most Si and Ge has a diamond cubic (dc) structure. However, many other phases of Si and Ge that are accessible via the application of high pressure. Compression to 10-11 GPa leads to both Si and Ge phase transforming to a metallic (b-Sn) structure. On decompression, the b-Sn phase transforms into one of several metastable phases; bc8-Si, r8-Si, hd Si and Ge, and st12-Ge depending on a number of factors including temperature, decompression rate and shear. The difference between nanoscale and bulk behaviour is a recurring theme of materials science over the past decades. However, this effect has not been investigated with respect to high pressure phase transformations of Si and Ge. To determine the effect of size on the phase transformations of Si and Ge, nanowires (NWs) were compressed and low load nanoindentation was performed. By using these two methods, other effects such as large pressure gradients in a sample and interaction with an underlying substrate could be probed. To add further understanding, the effect on temperature and decompression rate on these small volumes of Si and Ge is also investigated. To study the effects of size SiNWs of two sizes (80-150 nm and 200-250 nm) and GeNWs (40-60 nm) in diameter were compressed using a DAC, and low load nanoindentation of Si and Ge was performed at various temperatures and decompression rates. The materials were analysed using x-ray diffraction, Raman spectroscopy, and transmission electron microscopy. At ambient temperature that both sets of SiNWs experienced a suppressed dc-Si to b-Sn-Si phase transformation, with some of the smaller diameter SiNWs observed to phase transform directly to sh-Si. On decompression b-Sn-Si was found to persist until lower pressures than in bulk-Si, and a-Si was the dominant end phase. These suppressed phase transformations were attributed to the small size of the SiNWs making nucleation of new crystalline phases difficult. The effect of temperature on the high pressure phase transformation of the SiNWs was also investigated. Temperature was found to have a significant impact on the end phases formed. At low temperatures, a-Si was the dominant end phase, at moderate temperatures bc8-Si and dc-Si were present, and at the high temperatures dc-Si was the dominant end phase. This behaviour differed to bulk Si. Nanoindentation at ambient temperature and 105C was performed, noting that the phase transformed material volume is also small. At low temperatures, a-Si was the dominant end phase. At the 105C, a-Si was the dominant phase for fast unloading, however the portion of r8/bc8-Si increased with the next lowest unloading rate. For indents in dc-Si at the slowest unloading rate, the only end phase observed was dc-Si. This phase may have formed via nucleation and growth from the underlying crystalline substrate. A similar study was performed on Ge. Like in Si, the dc-Ge to b-Sn-Ge phase transformation was suppressed on compression and b-Sn-Ge persisted until lower pressures than bulk. The end phases of GeNWs were found to be a-Ge, hd-Ge, and dc-Ge. For nanoindentation of Ge, it was found that lowering the temperature of nanoindentation promotes the formation of r8-Ge and a-Si end phases instead of defective dc-Ge at ambient temperature. These results further the understanding on how size, temperature and decompression affect the pressure induced phase transformation pathways of Si and Ge are formed. These results are of technological significance as the synthesis of near phase pure nanowires should allow for the testing of their properties

The high pressure phase transformation behavior of silicon nanowires

Si nanowires of 80–150 nm and 200–250 nm diameter are pressurized up to 22 GPa using a diamond anvil cell. Raman and x-ray diffraction data were collected during both compression and decompression. Electron microscopy images reveal that the nanowires retain a nanowire-like morphology (after high pressure treatment). On compression, dc-Si was observed to persist at pressures up to 19 GPa compared to 11 GPa for bulk-Si. On decompression, the metallic b-Sn phase was found to be more stable for Si nanowires compared with bulk-Si when lowering the pressure and was observed as low as 6 GPa. For the smallest nanowires studied (80–150 nm), predominately a-Si was obtained on decompression, whereas for larger nanowires (200–250 nm), clear evidence for the r8/bc8-Si phase was obtained. We suggest that the small volume of the individual Si nanowires compared with bulk-Si inhibits the nucleation of the r8-Si phase on decompression. This study shows that there is a size dependence in the high pressure behavior of Si nanowires during both compression and decompressionL.Q.H. acknowledges her support from an Australian Government Research Training Program Scholarship. J.E.B. would like to acknowledge funding from the ARC Future Fellowship Scheme. A.L. acknowledges financial support from the Austrian Science Fund (FWF): Project No. P28175- N27 and e-beam lithography support by Manfred Reiche from the Max Planck Institute of Microstructure Physics, Halle, German

Thermal stability of simple tetragonal and hexagonal diamond germanium

Exotic phases of germanium, that form under high pressure but persist under ambient conditions, are of technological interest due to their unique optical and electrical properties. The thermal evolution and stability of two of these exotic Ge phases, the simple tetragonal (st12) and hexagonal diamond (hd) phases, are investigated in detail. These metastable phases, formed by high pressure decompression in either a diamond anvil cell or by nanoindentation, are annealed at temperatures ranging from 280 to 320 C for st12-Ge and 200 to 550 C for hd-Ge. In both cases, the exotic phases originated from entirely pure Ge precursor materials. Raman microspectroscopy is used to monitor the phase changes ex situ following annealing. Our results show that hd-Ge synthesized via a pure form of a-Ge first undergoes a subtle change in structure and then an irreversible phase transformation to dc-Ge with an activation energy of (4.3 6 0.2) eV at higher temperatures. St12-Ge was found to transform to dc-Ge with an activation energy of (1.44 6 0.08) eV. Taken together with results from previous studies, this study allows for intriguing comparisons with silicon and suggests promising technological applications.This work was supported by the Australian Research Council under the Discovery Project Scheme. L.Q.H. is supported by an Australian Government Research Training Program Scholarship. J.E.B. acknowledges the ARC for the award of a Future Fellowship. B.H. was supported through a Weinberg Fellowship (ORNL) and the Neutron Scattering User Facilities (ORNL), supported by the U.S. Department of Energy, Office of Sciences, Basic Energy Sciences. The ORNL is funded under DOE-BES Contract No. DE-AC05-00OR22725 and the Alvin M. Weinberg Fellowship by the ORNL LDRD scheme under Project No. 7620

Suppression of ion-implantation induced porosity in germanium by a silicon dioxide capping layer

Ion implantation with high ion fluences is indispensable for successful use of germanium (Ge) in the next generation of electronic and photonic devices. However, Ge readily becomes porous after a moderate fluence implant (∼1×1015 ion cm−2) at room temperature, and for heavy ion species such as tin (Sn), holding the target at liquid nitrogen (LN2) temperature suppresses porosity formation only up to a fluence of 2×1016 ion cm−2. We show, using stylus profilometry and electron microscopy, that a nanometer scale capping layer of silicon dioxide significantly suppresses the development of the porous structure in Ge during a Sn − implant at a fluence of 4.5×1016 ion cm−2 at LN2 temperature. The significant loss of the implanted species through sputtering is also suppressed. The effectiveness of the capping layer in preventing porosity, as well as suppressing sputter removal of Ge, permits the attainment of an implanted Sn concentration in Ge of ∼15 at.%, which is about 2.5 times the maximum value previously attained. The crystallinity of the Ge-Sn layer following pulsed-laser-melting induced solidification is also greatly improved compared with that of uncapped material, thus opening up potential applications of the Ge-Sn alloy as a direct bandgap material fabricated by an ion beam synthesis technique

Trans-Regime Structural Transition of (In3+ + Nb5+) Co-Doped Anatase TiO2 Nanocrystals under High Pressure

Chemical co-doping and high pressure reactions have been broadly used to synthesize novel materials or to tune the physicochemical properties of traditional materials. Here, we take In3+ and Nb5+ ions co-doped anatase TiO2 nanocrystals as an example and report that a combination of both a chemical and a high pressure reaction route is more powerful for the preparation of metastable polymorphs. It is experimentally demonstrated that In3+ and Nb5+ co-doping significantly changes the high-pressure reaction behaviors of anatase TiO2 nanocrystals (<10 nm) and leads to their trans-regime structural transition in terms of in situ Raman analysis, from an anatase to a baddeleyite-like phase under compressive pressures and then to an α-PbO2-like structure under decompressive pressures. This abnormal phase transition is attributed to a defect-induced heterogeneous nucleation mechanism. Furthermore, the stiffness of co-doped TiO2 nanocrystals is significantly enhanced due to the synergistic effects of co-dopants. This research not only proposes a potentially effective strategy to synthesize co-doped metastable polymorphic phases but also suggests one feasible method to improve the mechanical properties of anatase TiO2 nanocrystals

Understanding the Unusual Response to High Pressure in KBe2BO3F2

Strong anisotropic compression with pressure on the remarkable non-linear optical material KBe2BO3F2 has been observed with the linear compression coefficient along the c axis found to be about 40 times larger than that along the a axis. An unusual non-monotonic pressure response was observed for the a lattice parameter. The derived bulk modulus of 31 ± 1 GPa indicates that KBe2BO3F2 is a very soft oxide material yet with stable structure up to 45 GPa. A combination of high-pressure synchrotron powder X-ray diffraction, high-pressure Raman spectroscopy, and Density Functional Theory calculations points to the mechanism for the unusual pressure response being due to the competition between the K-F bond length and K-F-K bond angle and the coupling between the stretching and twisting vibration modes.This work was supported by NSFC (50990303 and 50925205) and 973 program (2010CB833103), and the Program of Introducing Talents of Discipline to Universities in China (111 program No. b06015). The experiment was performed at the Powder Diffraction Beamline, Australian Synchrotron. DS would like to acknowledge the financial support from ANSTO during his research visit. JEB is supported by an ARC future fellowship

Multi-phase equation of state of ultrapure hafnium to 120 GPa

Hafnium (Hf) is an industrially important material due to its large neutron absorption cross-section and its high corrosion resistance. When subjected to high pressure, Hf phase transforms from its hexagonal close packed α-Hf phase to the hexagonal ω-Hf phase. Upon further compression, ω-Hf phase transforms to the body centered cubic β-Hf phase. In this study, the high pressure phase transformations of Hf are studied by compressing and decompressing a well-characterized Hf sample in diamond anvil cells up to 120 GPa while collecting x-ray diffraction data. The phase transformations of Hf were compared in both a He pressure transmitting medium (PTM) and no PTM over several experiments. It was found that the α-Hf to ω-Hf phase transition occurs at a higher pressure during compression and lower pressure during decompression with a helium (He) PTM compared to using no PTM. There was little difference in the ω-Hf to β-Hf phase transition pressure between the He PTM and no PTM. The equation of state was fit for all three phases of Hf and under both PTM and no-PTM

Cold nanoindentation of germanium

Diamond cubic Ge is subjected to high pressures via nanoindentation at temperatures between 45 C and 20 C. The residual impressions are studied using ex-situ Raman microspectroscopy and cross-sectional transmission electron microscopy. The deformation mechanism at 20 C is predominately via the generation of crystalline defects. However, when the temperature is lowered, the analysis of residual indentation impressions provides evidence for deformation by phase transformation and formation of additional phases such as r8-Ge, hd-Ge, and amorphous Ge. Furthermore, these results show that at 0 C and below, dc-Ge will reliably phase transform via nanoindentation. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4993163]We acknowledge the support of the ANFF ACT Node and Centre for Advanced Microscopy at ANU in carrying out this research. This work was supported by the Australian Research Council. J.E.B. acknowledges the ARC for a Future Fellowship

Chemical Synthesis and High-Pressure Reaction of Nb5+ Monodoped Rutile TiO2 Nanocrystals

Identifying the doping effects of extrinsic ions both during chemical reactions and in resultant products is important to deeply understand the associated material properties and to develop novel materials for practical applications. Here, we experimentally demonstrate the significant inhibitor effect of Nb5+ dopants on the formation of rutile TiO2 nanocrystals through dopant concentration-modified solvothermal reaction processes. A lower Nb5+ doping level (≤9.09 atom %) is found to be more beneficial for the nucleation and growth of rutile Ti1 2x4+Tix3+Nbx5+O2 nanocrystals while a higher one (>9.09 at.%) leads to the preferable formation of the anatase phase. At a pressure range of up to 30 GPa, the synthesized Nb5+ monodoped rutile TiO2 nanocrystals almost possess an equal slope in their respective plot of the pressure-dependent Raman frequency shifts and a similar structural transformation from rutile to baddeleyite-like (pressurization) and then to an α-PbO2-like phase (depressurization). They thus present a dopant concentration-independent high-pressure reaction behavior due to a small change in their average and local defect structures evidenced by the bond valence sum analysis and density functional theory calculations. This work not only emphasizes the key roles of dopants in material synthesis but also broadens insights into the intrinsic correlations between material properties and their specific local defect

Suppression of ion-implantation induced porosity in germanium by a silicon dioxide capping layer

Ion implantation with high ion fluences is indispensable for successful use of germanium (Ge) in the next generation of electronic and photonic devices. However, Ge readily becomes porous after a moderate fluence implant (∼1×1015 ion cm−2) at room temperature, and for heavy ion species such as tin (Sn), holding the target at liquid nitrogen (LN2) temperature suppresses porosity formation only up to a fluence of 2×1016 ion cm−2. We show, using stylus profilometry and electron microscopy, that a nanometer scale capping layer of silicon dioxide significantly suppresses the development of the porous structure in Ge during a Sn− implant at a fluence of 4.5×1016 ion cm−2 at LN2 temperature. The significant loss of the implanted species through sputtering is also suppressed. The effectiveness of the capping layer in preventing porosity, as well as suppressing sputter removal of Ge, permits the attainment of an implanted Sn concentration in Ge of ∼15 at.%, which is about 2.5 times the maximum value previously attained. The crystallinity of the Ge-Sn layer following pulsed-laser-melting induced solidification is also greatly improved compared with that of uncapped material, thus opening up potential applications of the Ge-Sn alloy as a direct bandgap material fabricated by an ion beam synthesis technique