Equilibrium Solid Solubility Limit Research Articles

Crack‐Free Tungsten Fabricated via Laser Powder Bed Fusion Additive Manufacturing

AbstractAdditive manufacturing of tungsten (W) is challenging due to its high melting point, high thermal conductivity, oxidation tendency, and brittleness from grain boundary (GB) oxides. In this study, the processing of W through laser powder bed fusion is investigated. Parts are fabricated under argon (Ar) and nitrogen (N2) atmospheres using the same processing parameters. The part produced in Ar has cracks with oxide precipitates decorating the fractured GBs. On the other hand, crack‐free W samples are produced under N2 atmosphere without any additional process modification. In both cases, the oxygen (O) content in the LPBF samples is similar to the starting powder. Interestingly, the analysis of the samples fabricated in nitrogen suggests that nitrogen is retained beyond the equilibrium solid solubility limit, while high‐resolution electron micrographs of fractured surfaces reveal reduced levels of oxides at GBs. Increased hardness for samples processed under N2 atmosphere is observed. Density Functional Theory (DFT) calculations performed to study the influence of interstitial nitrogen on oxygen diffusion in W indicated a hindrance to O diffusion from the presence of dissolved N.

Non-equilibrium solid-phase growth of amorphous GeSn layer on Ge-on-insulator wafer induced by flash lamp annealing

The effect of flash lamp annealing on the crystal growth of amorphous germanium-tin (GeSn) layer deposited on Ge-on-insulator wafer was investigated. It was found that the presence of Sn in the amorphous Ge significantly promoted solid-phase growth (SPG). The diffusion of Sn during the SPG of GeSn was completely suppressed owing to milli-second thermal processing, providing a high-quality GeSn layer with Sn content of 13%, which far exceeds the equilibrium solid solubility limit (∼1%). The fabricated GeSn/Ge-on-insulator wafer exhibited improved infrared absorption beyond 2000 nm, which would be a suitable platform for near-infrared image sensors based on group-IV materials.

Surface segregated Ga, In, and Al activation in high Ge content SiGe during UV melt laser induced non-equilibrium solidification

The impact of solidification front velocity (SFV) on dopant segregation and activation surpassing the equilibrium solid solubility limit was investigated in high Ge content SiGe using UV nanosecond melt laser annealing (MLA). We first simulated the SFV evolution in the MLA-induced solidification of a SiGe binary system. It was then combined with the near-surface atomic and electrically active dopant concentrations measured in the Ga, In, and Al-implanted SiGe after MLA. Solute trapping phenomenon and self-compensation effect of the dopants were discussed to capture a part of their segregation and activation dynamics in the presented SiGeX ternary systems.

Nanoscale n++-p junction formation in GeOI probed by tip-enhanced Raman spectroscopy and conductive atomic force microscopy

Ge-on-Si and Ge-on-insulator (GeOI) are the most promising materials for the next-generation nanoelectronics that can be fully integrated with silicon technology. To this day, the fabrication of Ge-based transistors with a n-type channel doping above 5 × 1019 cm−3 remains challenging. Here, we report on n-type doping of Ge beyond the equilibrium solubility limit (ne ≈ 6 × 1020 cm−3) together with a nanoscale technique to inspect the dopant distribution in n++-p junctions in GeOI. The n++ layer in Ge is realized by P+ ion implantation followed by millisecond-flashlamp annealing. The electron concentration is found to be three times higher than the equilibrium solid solubility limit of P in Ge determined at 800 °C. The millisecond-flashlamp annealing process is used for the electrical activation of the implanted P dopant and to fully suppress its diffusion. The study of the P activation and distribution in implanted GeOI relies on the combination of Raman spectroscopy, conductive atomic force microscopy, and secondary ion mass spectrometry. The linear dependence between the Fano asymmetry parameter q and the active carrier concentration makes Raman spectroscopy a powerful tool to study the electrical properties of semiconductors. We also demonstrate the high electrical activation efficiency together with the formation of ohmic contacts through Ni germanidation via a single-step flashlamp annealing process.

Segregation and activation of Ga in high Ge content SiGe by UV melt laser anneal

The feasibility of dopant activation surpassing the equilibrium solid solubility limit by using an out of equilibrium melt laser annealing (MLA) process was investigated. To that end, we used an UV excimer nanosecond laser annealing and studied the segregation and activation of dopants in a Ga-implanted SiGe 50% epilayer. Dopant segregation is of great interest for future nodes to further improve contact resistivity in transistors. However, there is a lack of in-depth study about their activation. In this paper, we first reported very high Ga activation well above the equilibrium solid solubility limit when the partial Si0.5Ge0.5:Ga melt regime was assessed. The dopant segregation phenomenon, together with the surface morphology change of the Si0.5Ge0.5:Ga epilayer, was then induced by MLA. A very clear honeycomblike surface pattern was observed in the full Si0.5Ge0.5:Ga melt regime, while it was less pronounced in the partial melt regime. This honeycomblike pattern would be the result of dopant precipitation at the liquid–solid interface during solidification. Our simulation results highlighted that solidification velocity could play a key role in the substitutional incorporation of Ga atoms in a SiGe lattice.

Metallic conduction behavior in selenium-hyperdoped silicon

Hall measurements at 80–300K are performed on crystalline silicon doped with selenium exceeding the equilibrium solid solubility limit using ion implantation combined with furnace annealing. The temperature dependence of free carrier density and sheet conductivity of the Se doping layer changes with implantation dose. Metallic conduction behavior is well observed in the sample doped with selenium to be 7.4×1020/cm3. The overlapping between Se impurity states below Si conduction band might give a microscopic explanation.

Structural and optoelectronic properties of selenium‐doped silicon formed using picosecond pulsed laser mixing

AbstractMicrostructured silicon doped with selenium beyond the equilibrium solid solubility limit was formed by picosecond pulsed laser mixing of a 20‐nm Se film with the underlying p‐Si wafer. The effects of pulse fluence varying from 0.40 to 0.72 J cm−2 on the structural and electronic properties of this material were investigated. It was found that the surface roughness in the laser‐irradiated regions increases with increasing pulse fluence, and a phase transition from the crystalline to the amorphous occurs at a pulse fluence of 0.72 J cm−2. Silicon microstructures were doped with selenium of the order of 1018 cm−3 within a depth of 100 nm below the surface, and n+p junctions were formed between the Se‐doped layers and the silicon substrates. The current–voltage as well as spectral response characteristics of the photodiodes fabricated from microstructured silicon were also analyzed and discussed. The leakage currents and responsivities of these photodiodes strongly depend on the pulse fluence, presumably due to the effects of picosecond laser modification on the crystal structure and electronic structure of the surface Se‐doped layers.

Kinetics of large B clusters in crystalline and preamorphized silicon

We present an extended model for B clustering in crystalline or in preamorphized Si and with validity under conditions below and above the equilibrium solid solubility limit of B in Si. This model includes boron-interstitial clusters (BICs) with BnIm configurations—complexes with n B atoms and m Si interstitials—larger (n > 4), and eventually more stable, than those included in previous models. In crystalline Si, the formation and dissolution pathways into large BICs configurations require high B concentration and depend on the flux of Si interstitials. In the presence of high Si interstitial flux, large BICs with a relatively large number of interstitials (m ≥ n) are formed, dissolving under relatively low thermal budgets. On the contrary, for low Si interstitial flux large BICs with few interstitials (m ≪ n) can form, which are more stable than small BICs, and whose complete dissolution requires very intense thermal budgets. We have also investigated the kinetics of large BICs in preamorphized Si, both experimentally and theoretically. B was implanted at a high-dose into preamorphized Si, and the B precipitation was studied by transmission electron microscopy and by sheet resistance and Hall measurement techniques. A simplified model for B clustering and redistribution in amorphous Si is proposed, including the experimental value for the B diffusivity in amorphous Si and the energetics of BICs. Our model suggests that B2, B3I, B4I and B4I2 clusters are the most energetically favored configurations, with relative abundance depending on B concentration. After recrystallization, thermal anneals up to 1100 °C evidence that BICs evolve under very low flux of Si interstitials under the particular experimental conditions considered. Simulations indicate that for very high B concentrations and low Si interstitial flux a significant fraction of the initial small BICs evolves into larger and very stable BIC configurations that survive even after intense thermal budgets, as confirmed by energy filtered transmission electron microscopy analyses. The correlation between simulations and Hall measurements on these samples suggest that hole mobility is significantly degraded by the presence of a high concentration of BICs.

Heavily Ga-doped germanium layers produced by ion implantation and flash lamp annealing: Structure and electrical activation

Heavily p-type doped Ge layers were fabricated by 100 keV Ga implantation and subsequent flash lamp annealing for 3 ms in the temperature range between 700 and 900 °C. For comparison, some samples were annealed in a rapid thermal processor for 60 s. Ga fluences of 2×1015, 6×1015, and 2×1016 cm−2 were chosen in order to achieve Ga peak concentrations ranging from values slightly below the equilibrium solid solubility limit of 4.9×1020 cm−3 up to 3.5×1021 cm−3 which corresponds to a maximum Ga content of about 8 at. %. The structure of the doped layer and the Ga distribution were investigated by Rutherford backscattering spectrometry in combination with ion channeling, cross-sectional electron microscopy, and secondary ion mass spectrometry. Temperature dependent Hall effect measurements were carried out in order to determine the electrical properties of the Ga-doped Ge layers. It is shown that by flash lamp annealing Ga diffusion into the bulk can be completely avoided and the Ga loss by outdiffusion from the surface is reduced. The lowest sheet resistance of 36 Ω/sq. was achieved for the medium Ga concentration annealed at 900 °C. The best Ga activation values are 73%, 60%, and 24% for the three Ga fluences under investigation. The Ga activation is correlated with the layer regrowth. Incomplete epitaxial regrowth as observed in some samples leads to lower activation.

High Activation of Ga at Low Temperatures

We found that ion-implanted Ga was activated at a concentration of around 4×1019 cm-3, independent of annealing temperature. This active concentration is higher than the solid solubility at 500°C by about three orders. The activated Ga was deactivated during subsequent long-term annealing and approached its thermal equilibrium solid-solubility limit. Ga in the surface high-concentration region moved towards the surface and showed significant out-diffusion. We further evaluated the solid solubility limit of Ga, which we found to be much lower than the reported value.

Reactive sintering of boron-doped Ni76Al24 intermetallic

A preliminary investigation into the formation of boron-doped nickel-rich Ni3Al with boron additions up to 2 wt% (i.e. to levels above the equilibrium solid solubility limit of boron in Ni3Al) from elemental powders by reaction synthesis was carried out. The application of reaction synthesis was seen as a low-energy alternative to the production of Ni3Al/boride composite suitable for wear applications. X-ray diffraction, Neutron diffraction, SEM/EDS,WDS, Image analysis, Archimedes principle and Rockwell hardness measurements; were used to study the effect of boron addition on the final microstructure, average grain size, bulk density and hardness of as-prepared Ni76Al24. Up to 0.3 wt% boron content, the microstructure consisted of single-phase Ni3Al, however, at a boron content of 0.5 wt% an apparent transition from a single phase microstructure to a two-phase intermetallic/boride composite microstructure was observed, which dominated when the boron content increased, up to 2 wt%. The two-phase microstructure was identified as Ni3Al (particles) within an Ni41Al5B12 boride matrix, with no remaining un-reacted boron. The boron addition was found to increase the Rockwell hardness of Ni3Al via two mechanisms. Below the solubility limit, the increase in hardness was due to solution hardening. Above 0.5 wt%B, solution hardening in addition to the formation of the harder boride phase, were found to amount to up to 50% increase in the hardness compared with boron free Ni3Al. The extrusion of semi-molten beads at the surface of the compact at high B-content may be a limiting factor, in the formation of Ni3Al/boride composites via this route.

Zn solubility limit in GaAs: growth versus equilibrium

We have found that the equilibrium Zn solubility limit in GaAs, as set by formation of the second condensed phase, Zn 3As 2, exhibits a retrograde behavior over the temperature range 575 ≤ T ≤ 700°C, at constant arsenic partial pressure. The results show that the maximum Zn concentration, N Zn, which can be grown into GaAs by organometallic vapor phase epitaxy is limited by the equilibrium solid solubility limit. Below the solid solubility limit, we observe N Zn ∼ P Zn for rapid growth rates, r g > 1.6 μm/h. We demonstrate a clear transition to the expected equilibrium relationship, N Zn ∼ P 1 2 Zn , at lower growth rates. Both results are shown to be consistent with Fermi energy pinning at the surface at high temperatures. The equilibrium constant and the enthalpy of the formation of Zn 3As 2 have been estimated from the growth of Zn 3As 2 at several temperatures and compared to the results of others.

Amorphous/Crystalline Structure And Phase Transformations In Metastable Semiconducting Ge1−xSnx

ABSTRACTThe semiconducting crystalline alloys, Ge1−xSnx, are of interest due to theoretical predictions about their electronic band structures which make them useful in infrared photodetectors. However the composition region where these alloys have the desired properties is greater than the equilibrium solid solubility limit of Sn in Ge (x<0.01). We have circumvented the solubility limits and produced thin (2000Å) and thick (4–8Μm) films of Gei.xSnx (x<0.31) by rf sputtering. Differential scanning calorimetry (DSC) Measurements were performed to study grain growth and crystallization processes in these highly metastable semiconductors. X-ray and electron diffraction measurements indicated the materials were amorphous, but the fact that some of the films were fine grained polycrystalline samples only became apparent in their DSC spectra. We present models that describe quantitatively the transformation behavior in both sets of films.

I n s i t u doping by As ion implantation of silicon grown by molecular-beam epitaxy

The incorporation and electrical activation of As, implanted in situ during molecular-beam epitaxial growth of epilayers on Si(100), is reported. Parameters varied included growth temperature (460–700 °C), implantation energy (500–1000 eV), and concentration (1017→>1020/cm3 ). In general, the material was excellent with 100% activation and bulk mobilities for concentrations up to the equilibrium solid solubility limit and carrier densities in excess of five times this limit in highly doped samples.

Metastable (GaSb) (1−x)(Sn 2) x alloys: Crystal growth and phase stability of single crystal and polycrystalline layers

Single-phase metastable (GaSb) (1−x)(Sn 2) x alloys with x ⩽ 0.23 far exceeding the equilibrium solid solubility limit of x ⪅ 0.02 for Sn in GaSb, have been grown by rf sputter deposition. The key feature allowing the growth of single phase alloys was the use of low energy ion-bombardment-induced collisional mixing during deposition. Films grown at elevated temperatures T s on GaAs(100) substrates were found to be single crystals while polycrystalline films with very strong (220) perferred orientation and grain sizes of 20–40 nm were obtained on amorphous glass substrates. The maximum film growth temperatures T m at which single phase films could be obtained was a function of the negative substrate potential V s during deposition and the composition x. For V s = 75 V, T m ranged from 375 °C at x = 0.06 to 125 °C at x = 0.23. Crystal growth and post-annealing studies both indicated that the transformation from the single-phase metastable state to the equilibrium (GaSb + βSn) state occurs through a continuous set of metastable GaSb-rich phases and that Sn precipitates out first in the α-diamond structure and then transforms to the β-tetragonal structure. In single crystal films, the GaSb-rich phase precipitates out coherently with the alloy matrix. The activation energy for phase separation ranged from ∼ 1.4 eV for alloys with x = 0.06 to 0.9 eV at x = 0.23. A phase map for crystal growth, plotted as a function of T s and x, and an annealing transformation diagram for (GaSb) 0.86(Sn 2) 0.14 was determined.

Substrate and doping effects upon laser-induced epitaxy of amorphous silicon

UHV-deposited amorphous silicon was recrystallized on (100) single-crystal substrates using pulsed Nd : YAG laser irradiation. Prior to deposition, substrates were prepared with either atomically clean surfaces, residual argon ion sputter damage, or from one to three monolayers of residual oxide. Epitaxial regrowth of bulk crystalline quality occurred regardless of substrate preparation. Annealing energy densities were significantly higher, however, for substrates retaining residual impurities. Certain layers were doped with Ga. At moderate doping levels recrystallization behavior was unaffected. At a 2% doping level, the annealing energy threshold dropped by one-half and Ga atoms were displaced toward the epitaxial surface. It is shown that the decrease in energy threshold is due to an impurity-induced increase in light absorption within the amorphous layer. Layers were crystallized with substitutional Ga concentrations of up to 10 times the equilibrium solid solubility limit.