Progress in Crystal Growth and Characterization of Materials最新文献

Concentrations of elements in single hair samples were evaluated by X-ray fluorescence by scanning with a narrow beam in the growth direction. Zn binds to the hair protein molecules, and is distributed uniformly from hair tip to root bulb by steady-state growth. To avoid the effect of thickness variation for the bulb, the hair elements were evaluated as the amount per protein molecule using the hair [Zn], resulting in the fault-bounded [S] change typical for a solid–liquid interface; the papilla is in a liquid state and the segregation of elements occurs so as to maintain the amount of shaft element equal to the element inflow into the papilla from the blood, leading to the relationship between hair and blood concentrations. The diffusion boundary layer of S segregation in the bulb gives the diffusion coefficient of D∼1 × 10⁻⁸ cm²/s. The liquid papilla during hair growth solidifies with temperature decrease with the formation of the hair specimen, and the results for solidified papilla are different from the state during growth. It is proposed that the serum protein supplied into dermal papilla changes into precursor keratin molecules, and then into insolvable keratin in the hair matrix cells, i.e., hair makes “protein-melt growth.” The pulsed or stepwise variations of [Ca] and [Sr] occur due to the ion channel gating of matrix cells; such variations can never be expected for the cell division growth as deduced from the solidified papilla. The hair growth reflects the status of ion channels and pumping only possible because of the solid–liquid growth interface driven by the gradient in chemical potential nearly perpendicular to the skin surface. Thus, a hair root is a solid–liquid system for hair formation from serum protein.

Thermoelectric devices convert thermal energy, i.e. heat, into electric energy. With no moving parts, the thermoelectric generator has demonstrated its advantage of long-duration operational reliability. The IV–VI compound semiconductor PbTe-based materials have been widely adopted for the thermoelectric applications in the medium temperature range of 350–650°C. In most of the reports, thermoelectric materials were manufactured by a hot pressing or quench and annealing method. The recent advancements in the converting efficiency of thermoelectrics, including PbTe-based materials, have been attributed to the modification on material inhomogeneity of microstructures by hot pressing or simply cooling the melt to reduce the thermal conductivity. On the other hand, due to its time-consuming preparation/processing and unnecessary good crystalline quality (for thermoelectric applications), the processing of thermoelectric materials by crystal growth resulted in very few investigations. In this report, the design and growth of the PbTe-based materials solidified from the melt for thermoelectric applications as well as the results of their thermoelectric characterizations will be reviewed. It shows that, besides its Figure of Merit comparable to other processing methods, the melt grown PbTe material has several additional capabilities, including the reproducibility, thermal stability and the functional gradient characteristics from the variation of properties along the growth length.

High-efficiency semiconductor lasers and light-emitting diodes operating in the 3–5 μm mid-infrared (mid-IR) spectral range are currently of great demand for a wide variety of applications, in particular, gas sensing, noninvasive medical tests, IR spectroscopy etc. III-V compounds with a lattice constant of about 6.1 Å are traditionally used for this spectral range. The attractive idea to fabricate such emitters on GaAs substrates by using In(Ga,Al)As compounds is restricted by either the minimum operating wavelength of ∼8 μm in case of pseudomorphic AlGaAs-based quantum cascade lasers or requires utilization of thick metamorphic In_xAl_1-xAs buffer layers (MBLs) playing a key role in reducing the density of threading dislocations (TDs) in an active region, which otherwise result in a strong decay of the quantum efficiency of such mid-IR emitters. In this review we present the results of careful investigations of employing the convex-graded In_xAl_1-xAs MBLs for fabrication by molecular beam epitaxy on GaAs (001) substrates of In(Ga,Al)As heterostructures with a combined type-II/type-I InSb/InAs/InGaAs quantum well (QW) for efficient mid-IR emitters (3–3.6 μm). The issues of strain relaxation, elastic stress balance, efficiency of radiative and non-radiative recombination at T = 10–300 K are discussed in relation to molecular beam epitaxy (MBE) growth conditions and designs of the structures. A wide complex of techniques including in-situ reflection high-energy electron diffraction, atomic force microscopy (AFM), scanning and transmission electron microscopies, X-ray diffractometry, reciprocal space mapping, selective area electron diffraction, as well as photoluminescence (PL) and Fourier-transformed infrared spectroscopy was used to study in detail structural and optical properties of the metamorphic QW structures. Optimization of the growth conditions (the substrate temperature, the As₄/III ratio) and elastic strain profiles governed by variation of an inverse step in the In content profile between the MBL and the InAlAs virtual substrate results in decrease in the TD density (down to 3 × 10⁷ cm⁻²), increase of the thickness of the low-TD-density near-surface MBL region to 250–300 nm, the extremely low surface roughness with the RMS value of 1.6–2.4 nm, measured by AFM, as well as rather high 3.5 μm-PL intensity at temperatures up to 300 K in such structures. The obtained results indicate that the metamorphic InSb/In(Ga,Al)As QW heterostructures of proper design, grown under the optimum MBE conditions, are very promising for fabricating the efficient mid-IR emitters on a GaAs platform.

Secondary ion mass spectrometry (SIMS) is a well-known technique for 3D chemical mapping at the nanoscale, with detection sensitivity in the range of ppm or even ppb. Energy dispersive X-ray spectroscopy (EDS) is the standard chemical analysis and imaging technique in modern scanning electron microscopes (SEM), and related dual-beam focussed ion beam (FIBSEM) instruments. Contrary to the use of an electron beam, in the past the ion beam in FIBSEMs has predominantly been used for local milling or deposition of material. Here, we review the emerging FIBSIMS technique which exploits the focused ion beam as an analytical probe, providing the capability to perform secondary ion mass spectrometry measurements on FIBSEM instruments: secondary ions, sputtered by the FIB, are collected and selected according to their mass by a mass spectrometer. In this way a complete 3D chemical analysis with high lateral resolution < 50 nm and a depth resolution < 10 nm is attainable.

We first report on the historical developments of both SIMS and FIB techniques and review recent developments in both instruments. We then review the physics of interaction for incident particles using Monte Carlo simulations. Next, the components of modern FIBSIMS instruments, from the primary ion generation in the liquid metal source in the FIB column, the focussing optics, the sputtered ion extraction optics, to the different mass spectrometer types are all detailed. The advantages and disadvantages of parallel and serial mass selection in terms of data acquisition and interpretation are highlighted, while the effects of pressure in the FIBSEM, acceleration voltage, ion take-off angles and charge compensation techniques on the analysis results are then discussed. The capabilities of FIBSIMS in terms of sensitivity, lateral and depth resolution and mass resolution are reviewed. Different data acquisition strategies related to dwell time, binning and beam control strategies as well as roughness and edge effects are discussed. Data analysis routines for mass identification based on isotope ratios and molecular fragments are outlined. Application examples are then presented for the fields of thin films, polycrystalline metals, batteries, cultural heritage materials, isotope labelling, and geological materials. Finally, FIBSIMS is compared to EDS, and the potential of the technique for correlative microscopy with other FIBSEM based imaging techniques is discussed.

It has been known that, in growing silicon from melts, vacancies (Vs) predominantly exist in crystals obtained by high-rate growth, while interstitial atoms (Is) predominantly exist in crystals obtained by low-rate growth. To reveal the cause, the temperature distributions in growing crystal surfaces were measured. From this result, it was presumed that the high-rate growth causes a small temperature gradient between the growth interface and the interior of the crystal; in contrast, the low-rate growth causes a large temperature gradient between the growth interface and the interior of the crystal. However, this presumption is opposite to the commonly-accepted notion in melt growth. In order to experimentally demonstrate that the low-rate growth increases the temperature gradient and consequently generates Is, crystals were filled with vacancies by the high-rate growth, and then the pulling was stopped as the extreme condition of the low-rate growth. Nevertheless, the crystals continued to grow spontaneously after the pulling was stopped. Hence, simultaneously with the pulling-stop, the temperature of the melts was increased to melt the spontaneously grown portions, so that the diameters were restored to sizes at the moment of pulling-stop. Then, the crystals were cooled as the cooling time elapsed, and the temperature gradient in the crystals was increased. By using X-ray topographs before and after oxygen precipitation in combination with a minority carrier lifetime distribution, a time-dependent change in the defect type distribution was successfully observed in a three-dimensional manner from the growth interface to the low-temperature portion where the cooling progressed. This result revealed that Vs are uniformly introduced in a grown crystal regardless of the pulling rate as long as the growth continues, and the Vs agglomerate as a void and remain in the crystal, unless recombined with Is. On the other hand, Is are generated only in a region where the temperature gradient is large by low-rate growth. In particular, the generation starts near the peripheral portion in the vicinity of the solid–liquid interface. First, the generated Is are recombined with Vs introduced into the growth interface, so that a recombination region is always formed which is regarded as substantially defect free. Excessively generated Is after the recombination agglomerate and form a dislocation loop region. Unlike conventional Voronkov's diffusion model, Is hardly diffuse over a long distance. Is are generated by re-heating after growth.

[In a steady state, the crystal growth rate is synonymous with the pulling rate. Meanwhile, when an atypical operation is performed, the pulling rate is specifically used.]

This review on point defects formation intends to contribute further silicon crystals development, because electronic devices are aimed to have finer structures, a