High-pressure phase transition and phase diagram of gallium arsenide

Theoretical and Experimental Advances in High-Pressure Behaviors of Nanoparticles

Using compressive mechanical forces, such as pressure, to induce crystallographic phase transitions and mesostructural changes while modulating material properties in nanoparticles (NPs) is a unique way to discover new phase behaviors, create novel nanostructures, and study emerging properties that are difficult to achieve under conventional conditions. In recent decades, NPs of a plethora of chemical compositions, sizes, shapes, surface ligands, and self-assembled mesostructures have been studied under pressure by in-situ scattering and/or spectroscopy techniques. As a result, the fundamental knowledge of pressure-structure-property relationships has been significantly improved, leading to a better understanding of the design guidelines for nanomaterial synthesis. In the present review, we discuss experimental progress in NP high-pressure research conducted primarily over roughly the past four years on semiconductor NPs, metal and metal oxide NPs, and perovskite NPs. We focus on the pressure-induced behaviors of NPs at both the atomic- and mesoscales, inorganic NP property changes upon compression, and the structural and property transitions of perovskite NPs under pressure. We further discuss in depth progress on molecular modeling, including simulations of ligand behavior, phase-change chalcogenides, layered transition metal dichalcogenides, boron nitride, and inorganic and hybrid organic-inorganic perovskites NPs. These models now provide both mechanistic explanations of experimental observations and predictive guidelines for future experimental design. We conclude with a summary and our insights on future directions for exploration of nanomaterial phase transition, coupling, growth, and nanoelectronic and photonic properties.

Probing shock-induced structural changes in GaSb

Laser-driven hypervelocity impact experiments were used to study pressure-induced long-term effects on the crystal structure of undoped GaSb. X-ray diffraction and confocal micro-Raman spectra were collected on unshocked and shock-compressed samples, with corresponding pressures ranging between 8 and 23 GPa. GaSb retained bulk crystallinity at 8 GPa but showed localized site disordering, transformed into an amorphous state at 13 GPa, and stayed in that phase until 23 GPa.

Nanocrystals with metastable high-pressure phases under ambient conditions

The ambient metastability of the rock-salt phase in well-defined model systems comprising nanospheres or nanorods of cadmium selenide, cadmium sulfide, or both was investigated as a function of composition, initial crystal phase, particle structure, shape, surface functionalization, and ordering level of their assemblies. Our experiments show that these nanocrystal systems exhibit ligand-tailorable reversibility in the rock salt-to-zinc blende solid-phase transformation. Interparticle sintering was used to engineer kinetic barriers in the phase transformation to produce ambient-pressure metastable rock-salt structures in a controllable manner. Interconnected nanocrystal networks were identified as an essential structure that hosted metastable high-energy phases at ambient conditions. These findings suggest general rules for transformation-barrier engineering that are useful in the rational design of next-generation materials.

The role of pressure and defects in the wurtzite to rock salt transition in cadmium selenide

Using molecular dynamics and path sampling techniques we investigated the effect of pressure and defects in the wurtzite to rock salt transition in cadmium selenide (CdSe). In the pressure range 2-10 GPa, rate constants of transition are in the order of 10^-23 to 10⁵ s^-1 for the transformation of a relatively small wurtzite crystal consisting of 1024 atoms with periodic boundary conditions. The transition paths predominantly evolve through an intermediate 5-coordinated structure, as reported before, though its typical lifetime within the transition paths is particularly long in the intermediate pressure range (4-6 GPa). The defects were created by removing Cd-Se pairs from an otherwise perfect crystal. The removals were either selected fully randomized or grouped in clusters (cavity creation). We find that the rate of transition due to the defects increases by several orders of magnitude even for a single pair removal. This is caused by a change in the transition mechanism that no longer proceeds via the intermediate 5-coordinated structure, when defects are present. Further, the cavity creation yields a lower rate than the fully randomized removal.

Subsurface Deformation Mechanism in Nano-cutting of Gallium Arsenide Using Molecular Dynamics Simulation

During the nano-cutting process, monocrystalline gallium arsenide is faced with various surface/subsurface deformations and damages that significantly influence the product's performance. In this paper, molecular dynamics simulations of nano-cutting on gallium arsenide are conducted to investigate the surface and subsurface deformation mechanism. Dislocations are found in the machined subsurface. Phase transformation and amorphization are studied by means of coordination numbers. Results reveal the existence of an intermediate phase with a coordination number of five during the cutting process. Models with different cutting speeds are established to investigate the effects on the dislocation. The effect of crystal anisotropy on the dislocation type and density is studied via models with different cutting orientations. In addition, the subsurface stress is also analyzed.

Strong enhancement of direct transition photoluminescence at room temperature for highly tensile-strained Ge decorated using 5 nm gold nanoparticles

Strain engineering of germanium has recently attracted tremendous research interest. The primary goal of this approach is to exploit mechanical strain to tune the electrical and optical properties of Ge to ultimately achieve an on-chip light source compatible with silicon technology. Additionally, this can result in enhanced electrical performance for high-speed optoelectronic applications. In this paper, we demonstrate the formation of highly tensile-strained Ge islands grown on a pre-patterned (110) GaAs substrate using a depth controlled nanoindentation process. Results show that a biaxial tensile strain, up to ∼2%, can be transferred from the mechanically stamped substrate to Ge islands by optimizing the parameters of the nanoindentation process. We verified our measurements by observing the islands' photoluminescence (PL) emission properties. A strong emission at room-temperature was observed around the wavelength of 1.9 µm (650 meV). This strain-induced redshift of the PL spectra is consistent with theoretical predictions, revealing a direct Ge bandgap formation. Furthermore, we demonstrate a significant 6.5x enhancement in the PL emission signal of the direct-transition when the Ge islands are decorated by 5 nm gold nanoparticles. This is attributed to a longer optical path length interaction and a plasmonic induced high-field enhancement which increases the light absorption in the Ge islands. Furthermore, results show that GNPs can significantly modulate the energy band structure and the carrier's transportation at the nanoscale metal-germanium Schottky interface. This maskless physical approach can offer a pathway towards a practical CMOS-compatible integrated laser. Additionally, it opens possibilities for designing innovative optoelectronic devices.

High-pressure characterization of multifunctional CrVO4

The structural stability and physical properties of CrVO4under compression were studied by x-ray diffraction, Raman spectroscopy, optical absorption, resistivity measurements, andab initiocalculations up to 10 GPa. High-pressure x-ray diffraction and Raman measurements show that CrVO4undergoes a phase transition from the ambient pressure orthorhombic CrVO4-type structure (Cmcm space group, phase III) to the high-pressure monoclinic CrVO4-V phase, which is proposed to be isomorphic to the wolframite structure. Such a phase transition (CrVO4-type → wolframite), driven by pressure, also was previously observed in indium vanadate. The crystal structure of both phases and the pressure dependence in unit-cell parameters, Raman-active modes, resistivity, and electronic band gap, are reported. Vanadium atoms are sixth-fold coordinated in the wolframite phase, which is related to the collapse in the volume at the phase transition. Besides, we also observed drastic changes in the phonon spectrum, a drop of the band-gap, and a sharp decrease of resistivity. All the observed phenomena are explained with the help of first-principles calculations.

Mechanical Nano-Patterning: Toward Highly-Aligned Ge Self-Assembly on Low Lattice Mismatched GaAs Substrate

Low-dimensional semiconductor structurers formed on a substrate surface at pre-defined locations and with nano-precision placement is of vital interest. The potential of tailoring their electrical and optical properties will revolutionize the next generation of optoelectronic devices. Traditionally, highly aligned self-assembly of semiconductors relies on Stranski- Krastanov growth mode. In this work, we demonstrate a pathway towards ordered configuration of Ge islands on low lattice mismatch GaAs (110) substrate patterned using depth-controlled nanoindentation. Diamond probe tips with different geometries are used to nano-mechanically stamp the surface of GaAs (110). This creates nanoscale volumes of dislocation-mediated deformation which acts to bias nucleation. Results show that nanostamped GaAs exhibits selective-nucleation of Ge at the indent sites. Ge islands formed on a surface patterned using cube corner tip have height of ~10 nm and lateral size of ~225 nm. Larger islands are formed by using Vickers and Berkovich diamond tips (~400 nm). The strain state of the patterned structures is characterized by micro-Raman spectroscopy. A strain value up to 2% for all tip geometries has been obtained. Additionally, strong room temperature photoluminescence (PL) emission is observed around 1.9 µm (650 meV). The observed strain-induced enhancement in the light-emission efficiency is attributed to direct conduction to heavy-hole (cΓ-HH) and conduction to light-hole (cΓ-LH) transitions. The inherent simplicity of the proposed method offers an attractive technique to manufacture semiconductor quantum dot structures for future electronic and photonic applications.

Carrier profiling with fast Fourier transform scanning spreading resistance microscopy: A case study for Ge, GaAs, InGaAs, and InP

Quantitative scanning spreading resistance microscopy is currently a powerful method for carrier profiling in scaled nanoelectronic devices. Faced with the further reduction of dimensions and increasing architecture complexity, a force modulation method was developed to address the challenges associated with parasitic series resistances. Called fast Fourier transform scanning spreading resistance microscopy, the method has been shown to increase dynamic range when profiling Si devices and retains the doping contrast even in the presence of a series resistance. In this work we systematically investigate the potential of fast Fourier transform scanning spreading resistance microscopy for Ge, GaAs, InP, and InGaAs, presenting a quantitative comparison with Si as well as a more in-depth understanding of the capabilities and limitations of the method. Our results show that both GaAs and InP greatly benefit, with a significantly larger dynamic range and the ability to filter undesired series resistances. Doping concentration contrast in the presence of a series resistance can also be maintained in Ge but with high noise. For InGaAs there are only minor benefits. These findings prove that fast Fourier transform scanning spreading resistance microscopy is a valuable extension to regular scanning spreading resistance microscopy for more accurate carrier profiling in Si and non-Si materials, especially in architectures where parasitic series resistances are present.

Photodegradation of Si-doped GaAs nanowire

Researching optical effects in nanowires may require a high pump intensity which under ambient conditions can degrade nanowires due to thermal oxidation. In this work we investigated the photodegradation of a single Si-doped GaAs nanowire by laser heating in air. To understand the changes that occurred on the nanowire we carried out Raman spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, and photoluminescence spectroscopy in laser damaged regions as well as in non-affected ones. From Raman Stokes and anti-Stokes measurements we estimated the local temperature that the oxidation process of the nanowire (NW) surface starts at as 661 K, resulting in two new Raman modes at 200 cm⁻¹ and 259 cm⁻¹. Scanning electron microscopy and energy dispersive X-ray spectroscopy measurements showed a significant loss of arsenic in the oxidized regions, but no erosion of the nanowire. Micro-photoluminescence measurements showed the near-band-edge emission of GaAs along the nanowire, as well as a new emission band at 755 nm corresponding to polycrystalline β-Ga₂O₃ formation. Our results also indicate that neither amorphous As nor crystalline As were deposited on the surface of the nanowire. Combining different experimental techniques, this study showed the formation of polycrystalline β-Ga₂O₃ by oxidation of the nanowire surface and the limits for performing spectroscopic investigations on individual GaAs NWs under ambient air conditions.

In order to comprehend the photodegradation of GaAs NWs, we investigated their thermal oxidation process in air induced by laser heating in a broad local temperature range.

Study of Nanoscratching Process of GaAs Using Molecular Dynamics

In this paper, molecular dynamics method was employed to investigate the nanoscratching process of gallium arsenide (GaAs) in order to gain insights into the material deformation and removal mechanisms in chemical mechanical polishing of GaAs. By analyzing the distribution of hydrostatic pressure and coordination number of GaAs atoms, it was found that phase transformation and amorphization were the dominant deformation mechanisms of GaAs in the scratching process. Furthermore, anisotropic effect in nanoscratching of GaAs was observed. The diverse deformation behaviors of GaAs with different crystal orientations were due to differences in the atomic structure of GaAs. The scratching resistance of GaAs(001) surface was the biggest, while the friction coefficient of GaAs(111) surface was the smallest. These findings shed light on the mechanical wear mechanism in chemical mechanical polishing of GaAs.

Effects of pressure on the structure and lattice dynamics of ammonium perchlorate: A combined experimental and theoretical study

Ammonium perchlorate NH₄ClO₄ (AP) was studied using synchrotron angle-dispersive X-ray powder diffraction (XRPD) and Raman spectroscopy. A diamond-anvil cell was used to compress AP up to 50 GPa at room temperature (RT). Density functional theory (DFT) calculations were performed to provide further insight and comparison to the experimental data. A high-pressure barite-type structure (Phase II) forms at ≈4 GPa and appears stable up to 40 GPa. Refined atomic coordinates for Phase II are provided, and details for the Phase I → II transition mechanics are outlined. Pressure-dependent enthalpies computed for DFT-optimized crystal structures confirm the Phase I → II transition sequence, and the interpolated transition pressure is in excellent agreement with the experiment. Evidence for additional (underlying) structural modifications include a marked decrease in the Phase II b'-axis compressibility starting at 15 GPa and an unambiguous stress relaxation in the normalized stress-strain response at 36 GPa. Above 47 GPa, XRD Bragg peaks begin to decrease in amplitude and broaden. The apparent loss of crystalline long-range order likely signals the onset of amorphization. Three isostructural modifications were discovered within Phase II via Raman spectroscopy. A revised RT isothermal phase diagram is discussed based on the findings of this study.

Structural and vibrational properties of corundum-type In2O3 nanocrystals under compression

This work reports the structural and vibrational properties of nanocrystals of corundum-type In₂O₃ (rh-In₂O₃) at high pressures by using angle-dispersive x-ray diffraction and Raman scattering measurements up to 30 GPa. The equation of state and the pressure dependence of the Raman-active modes of the corundum phase in nanocrystals are in good agreement with previous studies on bulk material and theoretical simulations on bulk rh-In₂O₃. Nanocrystalline rh-In₂O₃ showed stability under compression at least up to 20 GPa, unlike bulk rh-In₂O₃ which gradually transforms to the orthorhombic Pbca (Rh₂O₃-III-type) structure above 12-14 GPa. The different stability range found in nanocrystalline and bulk corundum-type In₂O₃ is discussed.

A thermodynamic approach of self- and hetero-diffusion in GaAs: connecting point defect parameters with bulk properties

GaAs diffusion is investigated with respect to temperature and pressure using a model that interconnects point defect with bulk properties.

Pressure induced semiconductor–metal phase transition in GaAs: experimental and theoretical approaches

GaAs undergoes a semiconductor–metal transition, which was investigated by in situ electrical measurements and first-principles calculations under a high pressure.

Enhanced photothermal conversion in vertically oriented gallium arsenide nanowire arrays

The photothermal properties of vertically etched gallium arsenide nanowire arrays are examined using Raman spectroscopy. The nanowires are arranged in square lattices with a constant pitch of 400 nm and diameters ranging from 50 to 155 nm. The arrays were illuminated using a 532 nm laser with an incident energy density of 10 W/mm(2). Nanowire temperatures were highly dependent on the nanowire diameter and were determined by measuring the spectral red-shift for both TO and LO phonons. The highest temperatures were observed for 95 nm diameter nanowires, whose top facets and sidewalls heated up to 600 and 440 K, respectively, and decreased significantly for the smaller or larger diameters studied. The diameter-dependent heating is explained by resonant coupling of the incident laser light into optical modes of the nanowires, resulting in increased absorption. Photothermal activity in a given nanowire diameter can be optimized by proper wavelength selection, as confirmed using computer simulations. This demonstrates that the photothermal properties of GaAs nanowires can be enhanced and tuned by using a photonic lattice structure and that smaller nanowire diameters are not necessarily better to achieve efficient photothermal conversion. The diameter and wavelength dependence of the optical coupling could allow for localized temperature gradients by creating arrays which consist of different diameters.

Vibrational, electronic and structural properties of wurtzite GaAs nanowires under hydrostatic pressure

The structural, vibrational, and electronic properties of GaAs nanowires have been studied in the metastable wurtzite phase via Resonant Raman spectroscopy and synchrotron X-ray diffraction measurements in diamond anvil cells under hydrostatic conditions between 0 and 23 GPa. The direct band gap E₀ and the crystal field split-off gap E₀ + Δ of wurtzite GaAs increase with pressure and their values become close to those of zinc-blende GaAs at 5 GPa, while being reported slightly larger at lower pressures. Above 21 GPa, a complete structural transition from the wurtzite to an orthorhombic phase is observed in both Raman and X-ray diffraction experiments.

Pressure tuning of the optical properties of GaAs nanowires

The tuning of the optical and electronic properties of semiconductor nanowires can be achieved by crystal phase engineering. Zinc-blende and diamond semiconductors exhibit pressure-induced structural transitions as well as a strong pressure dependence of the band gaps. When reduced to nanoscale dimensions, new phenomena may appear. We demonstrate the tuning of the optical properties of GaAs nanowires and the induction of a phase transition by applying an external pressure. The dependence of the E(0) gap on the applied pressure was measured, and a direct-to-indirect transition was found. Resonant Raman scattering was obtained by pressure tuning of the E(0) and the E(0) + Δ(SO) gaps with respect to the excitation energy. The resonances of the longitudinal optical modes LO and 2LO indicate the presence of electron-phonon Fröhlich interactions. These measurements show for the first time a variation of ionicity in GaAs when in nanowire form. Furthermore, the dependence of the lattice constant on applied pressure was estimated. Finally, we found a clear indication of a structural transition above 16 GPa.

Local modification of GaAs nanowires induced by laser heating

GaAs nanowires were heated locally under ambient air conditions by a focused laser beam which led to oxidation and formation of crystalline arsenic on the nanowire surface. Atomic force microscopy, photoluminescence and Raman spectroscopy experiments were performed on the same single GaAs nanowires in order to correlate their structural and optical properties. We show that the local changes of the nanowires act as a barrier for thermal transport which is of interest for thermoelectric applications.

The exponent 3/2 at pyramidal nanoindentations

The analysis of published loading curves reveals the exponent 3/2 to the depth for nanoindentations with sharp pyramidal or conical tips. This has geometric reasons, as it occurs independent on the bonding states and indentation mechanisms. Nevertheless, most mathematical deductions and finite element simulations of nanomechanical parameters in the literature continue using the experimentally not supported Hertzian exponent 2. Therefore, numerous published loading curves of various authors are plotted using the experimental exponent 3/2 to present unbiased proof for its generality with metals, oxides, semiconductors, biomaterials, polymers, and organics. Linearity is independent of equipment and valid for load controlled, or depth controlled, or continuous stiffness, or AFM force measurements. The linearity with exponent 3/2 often extends from the nano- into the microindentation ranges. The tip rounding and taper influence of the "geometrical similar" indenters are discussed. When kinks occur in such linear plots through the origin, these indicate change of the materials' mechanical properties under pressure by phase transition. These events are discussed for nanoindentations with respect to the known hydrostatic transformation pressures that are, of course, always higher than the necessary indentation mean pressure. Numerous Raman, as well as X-ray and electron diffraction results from the literature support the phase transitions that are now easily detected. Nanoporous materials first fill the pores upon indentation. Published loading curves exhibit more information than hitherto assumed.

Nucleation and growth in structural transformations of nanocrystals

Using transition path sampling computer simulations, we reveal the nucleation mechanism of a pressure-induced structural transformation in CdSe nanocrystals. Consistent with experiments, the thermodynamic transition pressure of the transformation increases with decreasing crystal size. Through transition state analysis, we identify the critical nuclei and characterize them by calculating activation enthalpies and volumes. Our simulations reproduce the trends with crystal size observed in experiments. This result supports the observed transformation mechanism, which consists of nucleation on the crystal surface and growth by sliding of parallel crystal planes.

Nondislocation origin of GaAs nanoindentation pop-in event

The present Letter demonstrates a pop-in event that is caused by a nanoindentation-induced phase transformation in GaAs, and not accompanied by any dislocation nucleation. Our computer simulations reveal the appearance of the new phase, documented by the structural correlation functions and visualization of the atomic positions. This challenges the orthodox view that the initial pop-in event reflects nucleation of dislocations or their movement, and has a bearing on materials where dislocation activity is not present.

Size-Dependent Phase Transformation Kinetics in Nanocrystalline ZnS

Nanocrystalline ZnS was coarsened under hydrothermal conditions to investigate the effect of particle size on phase transformation kinetics. Although bulk wurtzite is metastable relative to sphalerite below 1020 degrees C at low pressure, sphalerite transforms to wurtzite at 225 degrees C in the hydrothermal experiments. This indicates that nanocrystalline wurtzite is stable at low temperature. High-resolution transmission electron microscope data indicate there are no pure wurtzite particles in the coarsened samples and that wurtzite only grows on the surface of coarsened sphalerite particles. Crystal growth of wurtzite stops when the diameter of the sphalerite-wurtzite interface reaches approximately 22 nm. We infer that crystal growth of wurtzite is kinetically controlled by the radius of the sphalerite-wurtzite interface. A new phase transformation kinetic model based on collective movement of atoms across the interface is developed to explain the experimental results.

Multiple grains in nanocrystals: effect of initial shape and size on transformed structures under pressure

Pressure-induced structural transformations in spherical and faceted gallium arsenide nanocrystals of various shapes and sizes are investigated with a parallel molecular-dynamics approach. The results show that the pressure for zinc blende to rocksalt structural transformation depends on the nanocrystal size, and all nanocrystals undergo nonuniform deformation during the transformation. Spherical nanocrystals above a critical diameter >/=44 A transform with grain boundaries. Faceted nanocrystals of comparable size have grain boundaries in 60% of the cases, whereas the other 40% are free of grain boundaries. The structure of transformed nanocrystals shows that domain orientation and strain relative to the initial zinc blende lattice are not equivalent. These observations may have implications in interpreting the experimental x-ray line shapes from transformed nanocrystals.

Grain Boundaries in Gallium Arsenide Nanocrystals Under Pressure: A Parallel Molecular-Dynamics Study

Structural transformation in gallium arsenide nanocrystals under pressure is studied using molecular-dynamics simulations on parallel computers. It is found that the transformation from fourfold to sixfold coordination is nucleated on the nanocrystal surface and proceeds inwards with increasing pressure. Inequivalent nucleation of the high-pressure phase at different sites leads to inhomogeneous deformation of the nanocrystal. This results in the transformed nanocrystal having grains of different orientations separated by grain boundaries. A new method based on microscopic transition paths is introduced to uniquely characterize grains and deformations.

Size Dependence of Structural Metastability in Semiconductor Nanocrystals

The kinetics of a first-order, solid-solid phase transition were investigated in the prototypical nanocrystal system CdSe as a function of crystallite size. In contrast to extended solids, nanocrystals convert from one structure to another by single nucleation events, and the transformations obey simple unimolecular kinetics. Barrier heights were observed to increase with increasing nanocrystal size, although they also depend on the nature of the nanocrystal surface. These results are analogous to magnetic phase transitions in nanocrystals and suggest general rules that may be of use in the discovery of new metastable phases.