Bulk Crystals Research Articles

Investigation of Doping Processes to Achieve Highly Doped Czochralski Germanium Ingots

Highly doped germanium (HD-Ge) is a promising material for mid-infrared detectors, bio-sensors, and other devices. Bulk crystals with a doping concentration higher than 1018 cm−3 would be desirable for such device fabrication technologies. Hence, an effective method needs to be developed to dope germanium (Ge) ingots in the Czochralski (Cz) growth process. In this study, a total of 5 ingots were grown by the Cz technique: two undoped Ge ingots as a reference and three doped ingots with 1018, 1019 , and 1020 atoms/cm3 respectively. To obtain a uniform p-type doping concentration along the crystal, co-doping of boron-gallium (B-Ga) via the Ge feed material was also attempted. Both B and Ga are p-type dopants, but with a large difference in their segregation behavior (contrary segregation profile) in Ge, and hence it is expected that the incorporation of dopants in the crystal would be uniform along the crystal length. The distribution of the dopants followed the Scheil-predicted profile. The etch pit density maps of the grown crystals showed an average dislocation density in the order of 105 cm−2. No increase in the overall etch pit count was observed with increasing dopant concentration in the crystal. The grown highly doped Ge crystals have a good structural quality as confirmed by x-ray diffraction rocking curve measurements.

Dissolved-Cl2 triggered redox reaction enables high-performance perovskite solar cells

Constructing 2D/3D perovskite heterojunctions is effective for the surface passivation of perovskite solar cells (PSCs). However, previous reports that studying perovskite post-treatment only physically deposits 2D perovskite on the 3D perovskite, and the bulk 3D perovskite remains defective. Herein, we propose Cl2-dissolved chloroform as a multifunctional solvent for concurrently constructing 2D/3D perovskite heterojunction and inducing the secondary growth of the bulk grains. The mechanism of how Cl2 affects the performance of PSCs is clarified. Specifically, the dissolved Cl2 reacts with the 3D perovskite, leading to Cl/I ionic exchange and Ostwald ripening of the bulk grains. The generated Cl− further diffuses to passivate the bulk crystal and buried interface of PSCs. Hexylammonium bromide dissolved in the solvent reacts with the residual PbI2 to form 2D/3D heterojunctions on the surface. As a result, we achieved high-performance PSCs with a champion efficiency of 24.21% and substantially improved thermal, ambient, and operational stability.

Polymer co-crystallization from LLE: Crystallization kinetics of POCB hydrate from two-phase mixtures of POCB and water

Polyoxacyclobutane (POCB, –[CH2CH2CH2O−]n) has the rare ability to co-crystallize with water to form a hydrate. Above the melting point of the hydrate (∼37 °C), mixtures exist in liquid-liquid equilibrium (LLE) between a POCB-rich and a water-rich liquid phase over a wide range of POCB:water ratios. Such phase behavior, which combines liquid-liquid equilibrium (LLE) and co-crystallization of a polymer with a small molecule, appears to be unique to POCB-water mixtures. Here, we examine the kinetics of isothermal hydrate co-crystallization when mixtures that are initially in liquid-liquid equilibrium (LLE) are cooled to below the hydrate melting point. Hydrate co-crystallization requires both POCB and water, and hence, it is coupled with transport of species between the liquid phases. Optical microscopy shows that hydrate crystallization occurs within the POCB-rich domains, and hydrate spherulites grow at constant speed. The temperature-dependence of the speed of hydrate growth front is consistent with the Hoffman Lauritzen model for homopolymer crystallization. Dilatometric measurements of bulk co-crystallization kinetics are conducted with constant stirring to avoid gravitational separation of the liquid phases. The temperature-dependence of bulk crystallization kinetics of these stirred mixtures in LLE is found to be far weaker than that of spherulitic growth kinetics, or of the quiescent bulk crystallization kinetics of mixtures starting from homogeneous conditions.

Effect of crystal growth conditions on carrier lifetime and lattice defects in the solar absorber material ZnSnP2

We investigated the minority carrier lifetime and behavior of lattice defects in ZnSnP2 bulk crystals through experiments on carrier recombination and defect properties. Advanced deep level transient spectroscopy (DLTS) revealed that an electron trap with a short time constant at 0.2 eV below the conduction band minimum edge may contribute to the short minority carrier lifetime evaluated by time-resolved photoluminescence (TRPL). The temperature dependence of steady-state photoluminescence suggested that the carrier recombination through the electron trap was nonradiative around room temperature, which supports the fact of the short carrier lifetime and lower current density in ZnSnP2 solar cells. Previously reported theoretical calculation suggests that such a trap comes from the antisite defect of Sn from the viewpoint of the thermodynamic transition level. We, thus, prepared ZnSnP2 crystals by the solution growth method under conditions with a higher chemical potential of Zn, and we achieved the enhancement of the carrier lifetime compared to that under other growth conditions. In this case, the evaluation of the liquidus temperature and chemical potentials by a thermodynamic model indicated that the formation of Sn antisite was effectively suppressed by a lower precipitation temperature in addition to the effect of chemical potentials. Finally, we demonstrated the improvement of current density in ZnSnP2 solar cells using crystals with a longer lifetime, especially in the longer wavelength range.

Interface birefringence in asymmetric CdTe/CdZnTe quantum wells

Photoluminescence and polarized reflection spectra of quantum well structures with symmetric ${\mathrm{Cd}}_{0.9}{\mathrm{Zn}}_{0.1}\mathrm{Te}/\mathrm{CdTe}/{\mathrm{Cd}}_{0.9}{\mathrm{Zn}}_{0.1}\mathrm{Te}$ and asymmetric ${\mathrm{Cd}}_{0.9}{\mathrm{Zn}}_{0.1}\mathrm{Te}/\mathrm{CdTe}/{\mathrm{Cd}}_{0.4}{\mathrm{Mg}}_{0.6}\mathrm{Te}$ barriers were studied. The Stokes parameters of the reflected light from these structures were measured. In structures with asymmetric barriers, in the region of exciton resonances, the phenomenon of light birefringence was detected, caused by a lower symmetry of the interfaces compared with the symmetry of bulk crystals. A discussion of this phenomenon is given.

Full Temporal Characterization of Ultrabroadband Few-Cycle Laser Pulses Using Atomically Thin WS2

Nowadays, the accurate and full temporal characterization of ultrabroadband few-cycle laser pulses with pulse durations below 7 fs is of great importance in fields of science that investigate ultrafast dynamic processes. There are several indirect methods that use nonlinear optical signals to retrieve the complex electric field of femtosecond lasers. However, the precise characterization of few-cycle femtosecond laser sources with an ultrabroadband spectrum presents additional difficulties, such as reabsorption of nonlinear signals, partial phase matching, and spatiotemporal mismatches. In this work, we combine the dispersion scan (d-scan) method with atomically thin WS2 flakes to overcome these difficulties and fully characterize ultrabroadband laser pulses with a pulse duration of 6.9 fs and a spectrum that ranges from 650 to 1050 nm. Two-dimensional WS2 acts as a remarkably efficient nonlinear medium that offers a broad transparency range and allows for achieving relaxed phase-matching conditions due to its atomic thickness. Using mono- and trilayers of WS2, we acquire d-scan traces by measuring the second-harmonic generation (SHG) signal, originated via laser–WS2 interaction, as a function of optical dispersion (i.e., glass thickness) and wavelength. Our retrieval algorithm extracts a pulse duration at full-width half-maximum of 6.9 fs and the same spectral phase function irrespective of the number of layers. We benchmark and validate our results obtained using WS2 by comparing them with those obtained using a 10-μm-thick BBO crystal. Our findings show that atomically thin media can be an interesting alternative to micrometer-thick bulk crystals to characterize ultrabroadband femtosecond laser pulses using SHG-d-scan with an error below 100 as (attoseconds).

Measurement of nonlinear refractive indices of bulk and liquid crystals by nonlinear chirped interferometry.

The nonlinear refractive indices (n2) of a selection of bulk (LiB3O5, KTiOAsO4, MgO:LiNbO3, LiGaS2, ZnSe) and liquid (E7, MLC2132) crystals are measured at 1030 nm in the sub-picosecond regime (200 fs) by nonlinear chirped interferometry. The reported values provide key parameters for the design of near- to mid-infrared parametric sources, as well as all-optical delay lines.

Electronic transport mechanisms in a thin crystal of the Kitaev candidate α -RuCl3 probed through guarded high impedance measurements

α-RuCl3 is considered to be the top candidate material for the experimental realization of the celebrated Kitaev model, where ground states are quantum spin liquids with interesting fractionalized excitations. It is, however, known that additional interactions beyond the Kitaev model trigger in α-RuCl3 a long-range zigzag antiferromagnetic ground state. In this work, we investigate a nanoflake of α-RuCl3 through guarded high impedance measurements aimed at reaching the regime where the system turns into a zigzag antiferromagnet. We investigated a variety of temperatures (1.45–175 K) and out-of-plane magnetic fields (up to 11 T), finding a clear signature of a structural phase transition at ≈160 K as reported for thin crystals of α-RuCl3, as well as a thermally activated behavior at temperatures above ≈30 K, with a characteristic activation energy significantly smaller than the energy gap that we observe for α-RuCl3 bulk crystals through our angle resolved photoemission spectroscopy (ARPES) experiments. Additionally, we found that below ≈30 K, transport is ruled by Efros–Shklovskii variable range hopping (VRH). Most importantly, our data show that below the magnetic ordering transition known for bulk α-RuCl3 in the frame of the Kitaev–Heisenberg model (≈7 K), there is a clear deviation from VRH or thermal activation transport mechanisms. Our work demonstrates the possibility of reaching, through specialized high impedance measurements, the thrilling ground states predicted for α-RuCl3 at low temperatures in the frame of the Kitaev–Heisenberg model and informs about the transport mechanisms in this material in a wide temperature range.

Origin of chromitites in the Norilsk-1 intrusion (Siberian LIP) triggered by assimilation of argillaceous rocks by Cr-rich basic magma

Сhromite-rich rocks and monomineralic chromitites occur in mafic-ultramafic layered intrusions worldwide, and are economically important as they host major resources of chromium and platinum group elements. One of the acknowledged genetic concepts for such mineralization advocates bulk chromite crystallization in a response to hybridization of Cr-rich basic magma with felsic melts, derived from partial melting of the wall rocks. However, there has been lack of direct insights into this process and its details have been remaining largely obscured. In this study, we report data on chromite-rich assemblages of the Noril'sk-1 intrusion (Siberian LIP), with a particular focus on chromite-hosted multiphase inclusions. Composition of the latter is different from the rocks of the Noril'sk-1 intrusion and inherits geochemical fingerprints of the wall rock argillites and, therefore, represent snapshots of the heavily-contaminated medium of the magma-wall rock reaction front. Mineral relationships in chromite-rich breccias with fragments of the wall rocks also provide valuable insight into chromite mineralization mechanisms. Our results, along with geological evidence and data on other Noril'sk-type intrusions, indicate that bulk crystallization of chromite, aided by partial suppression of silicate crystallization, occurred in the hybrid medium during digestion of the wall rocks by ascending and emplacing basic magma. Continuous flow of the magma around outshoots of the wall rocks or through a “magmatic karst” in the wall rocks, allowed for a continuous precipitation of chromite. Concentration of chromite grains to form dense mineralization apparently occurred due to formation of chromite-rich blobs around the wall rock fragments, and selective collection and transport of chromite by fluid bubbles, which formed via degassing of wall rocks and then carried chromite and relics of the wall rocks (xenoliths) to the upper parts of the pluton. We propose that assimilation-driven formation of chromite-rich lithologies in intrusions requires: (1) sufficient Cr contents in magma, which allow for its oversaturation in Cr-spinel only by a sudden cooling and addition of SiO2, K2O or/and H2O; (2) efficient disintegration and digestion of the host rocks, releasing considerable amounts of these components to the magma, and (3) a mechanism ensuring accumulation of excess chromite within a small volume (e.g. continuous reaction of Cr-rich magma with the products of the host rocks' assimilation or/and mechanical concentration of chromite).

Tutorial: Metalorganic chemical vapor deposition of β-Ga2O3 thin films, alloys, and heterostructures

β-phase gallium oxide (Ga2O3) is an emerging ultrawide bandgap (UWBG) semiconductor with a bandgap energy of ∼ 4.8 eV and a predicted high critical electric field strength of ∼8 MV/cm, enabling promising applications in next generation high power electronics and deep ultraviolet optoelectronics. The advantages of Ga2O3 also stem from its availability of single crystal bulk native substrates synthesized from melt, and its well-controllable n-type doping from both bulk growth and thin film epitaxy. Among several thin film growth methods, metalorganic chemical vapor deposition (MOCVD) has been demonstrated as an enabling technology for developing high-quality epitaxy of Ga2O3 thin films, (AlxGa1−x)2O3 alloys, and heterostructures along various crystal orientations and with different phases. This tutorial summarizes the recent progresses in the epitaxial growth of β-Ga2O3 thin films via different growth methods, with a focus on the growth of Ga2O3 and its compositional alloys by MOCVD. The challenges for the epitaxial development of β-Ga2O3 are discussed, along with the opportunities of future works to enhance the state-of-the-art device performance based on this emerging UWBG semiconductor material system.

Cross-plane thermal conductivity of h-BN thin films grown by pulsed laser deposition

The distinguished properties of hexagonal boron nitride (h-BN), specifically its atomically smooth surface, large critical electric field, and large electronic bandgap, make it ideal for thin film microelectronics and as an ultrawide bandgap semiconductor. Owing to weak van der Waals interactions between layers, h-BN exhibits a significant degree of anisotropic thermal conductivity. The in-plane thermal conductivity of h-BN has extensively been studied, yet the only measured data of cross-plane thermal conductivity (k⊥) are for exfoliated h-BN films. Exfoliation from bulk crystals is not a sustainable method for scalable production of h-BN due to its low repeatability, low yield, poor control of sample thickness, and limitation to small areas. Thus, it is necessary to investigate the thickness-dependence of k⊥ for thin films grown by a practical growth method, such as pulsed laser deposition (PLD), which enables the production of reliable and large-area h-BN films with a control of film thickness. We grew h-BN using PLD at 750 °C and observed a decreasing trend of k⊥ as thickness increases from 30 to 300 nm, varying from ∼1.5 to ∼0.2 W/(m K). We observed a relatively high k⊥ value for h-BN at a thickness of 30 nm, providing insight into the k⊥ of PLD-grown films suitable for electronics applications.

Glass and glass-ceramics from red mud tailings: Understanding the evolution mechanism

Large quantities of red mud tailings cause serious negative impacts to both the environmental sustainability and public health. Eco-friendly utilization of tailings can address this issue and achieve its commercial value. We reported an approach to preparing transparent basic glass and opaque glass-ceramics utilizing red mud tailings as the starting material. Crystallization kinetics, phase transition, microstructure evolution, and performances of glass-ceramics were studied. The crystallization activation energy was calculated to be only 125.5 kJ/mol, which made the transformation from red mud tailings to glass-ceramics much easier. The Avrami coefficient of crystallization was calculated to be around 2, revealing that one-dimensional bulk crystallization occurred in glass-ceramics. Fluorapatite crystals having a one-dimensional morphology were found to generate in glass-ceramics, which endowed the glass-ceramics with superior performances, such as the density of 2.7 g/cm3, the crack resistance of 2.43 MPa m1/2, and the Vickers hardness of 643.67 HV. These results and findings demonstrate that hazardous red mud tailings could become eco-friendly glass and glass-ceramics for construction applications.

Shift-Current Photovoltaics Based on a Non-Centrosymmetric Phase in In-Plane Ferroelectric SnS.

The shift-current photovoltaics of group-IV monochalcogenides has been predicted to be comparable to those of state-of-the-art Si-based solar cells. However, its exploration has been prevented from the centrosymmetric layer stacking in the thermodynamically stable bulk crystal. Herein, the non-centrosymmetric layer stacking of tin sulfide (SnS) is stabilized in the bottom regions of SnS crystals grown on a van der Waals substrate by physical vapor deposition and the shift current of SnS, by combining the polarization angle dependence and circular photogalvanic effect, is demonstrated. Furthermore, 180° ferroelectric domains in SnS are verified through both piezoresponse force microscopy and shift-current mapping techniques. Based on these results, an atomic model of the ferroelectric domain boundary is proposed. The direct observation of shift current and ferroelectric domains reported herein paves a new path for future studies on shift-current photovoltaics.

THERMAL STABILITY OF CAST CONDUCTOR MICROALLOYED ALUMINUM ALLOYS

The nucleation of the Al3X (X = Zr, Yb, Er, Hf) particles in the cast conductor Al alloys including the alloys additionally doped with Mg and Si was studied. The alloys were made by induction casting. To investigate the particle nucleation kinetics, the specific electrical resistivity (SER) and microhardness measurements were applied. It was shown that the investigated alloys can be subdivided into three groups. Group I includes the alloys, which the decrease in the SER with increasing annealing temperature takes place in due to the particle nucleation. Group II includes the alloys, which the particle nucleation takes place in during the bulk crystallization. The SER magnitude of such alloy was close to the SER of pure Al. The SER of the alloys of Group III almost doesn’t change during annealing and is 3.0-3.4 ·cm that evidences a high alloy solid solution stability. Using Jonhnson-Mehl-Avrami-Kolmogorov equation, the particle nucleation kinetics in the Group I alloys was analyzed. The activation energy of the particle nucleation in the Group I alloys was found to be close to the activation energy of volume diffusion, but the values of the decomposition intensity coefficient (n = 0.5-0.8) in Johnson-Mehl-Avrami-Kolmogorov equation appeared to be smaller that the theoretical value n = 1.5 typical for the particle nucleation inside the bulk crystal lattice. This contradiction was related to the presence of large primary or eutectic Al3X particles in the alloy structure. The Al-0.25%Zr-0.25%Er-0.15%Si alloy was shown to have the optimal set of properties: the characteristics of this alloy after annealing match the requirements to the alloys being developed: SER less than 2.95 ·cm, microhardness Hv ~ 550 MPa.

2D Hemiporphyrazine: A new nanoporous material

Crystalline microporous materials are solids formed by interconnected pores of less than 2 nm in size. Typically, they possess large surface areas desirable for versatile applications such as catalysis, gas adsorption, and energy storage. In the present work, we propose a new porphyrin-based 2D nanoporous crystal, named 2D Porphyrazine (2DP), which is formed by topological assembling H$_{5}$C$_{13}$N$_{4}$ porphyrins. We have considered its monolayer, bi-layer, and molecular crystal (bulk) arrangements. We carried out DFT calculations to investigate 2DP structural and electronic properties. Results show that 2DP is a very stable structure with a direct bandgap of 0.65 eV and significant optical absorption in the visible range. 2DP exhibited satisfactory affinity to lithium atoms. Simulations also showed the existence of proton transfer between nitrogen atoms. It is the first report on the site-specific hydrogen exchange process in 2D crystals.

Al0.64Ga0.36N channel MOSHFET on single crystal bulk AlN substrate

We report MOCVD-grown Al0.87Ga0.13N/Al0.64Ga0.36N metal-oxide-semiconductor-heterojunction-field-effect-transistors on single crystal bulk AlN substrate. As compared to control devices on AlN template, thermal impedance for devices on single crystal AlN decreased to 1/3 from 31 to 10 K mm W−1, comparable to SiC and copper heat-sinks. This represents a significant thermo-electric co-design advantage over other semiconductors. As a result, the peak drain saturation current increased from 410 to 610 mAmm−1. A 3-terminal breakdown field of 3.7 MV cm−1 was measured, which to date represents state-of-the-art performance for devices with similar Al x Ga1−x N-channel composition. This translates to a measured Baliga figure of merit of 460 MWcm−2.

Improvement of the Conformational Stability of 150 mm 4H SiC Wafers

A method for mitigating loss of conformational stability in 150 mm n-type 4H SiC wafers was investigated. Modifications to the physical vapor transport (PVT) process used to grow the parent bulk crystals, combined with post-growth thermal treatment, were examined as means of reducing the internal stresses hypothesized to promote instability. The magnitude of the stresses was analyzed by mechanically thinning sets of wafers produced from each process to determine the critical thickness of stability loss. The average critical thickness was found to be reduced by 13% via growth cell modification, at a reduced level of thermal treatment relative to a control process, with all wafers becoming unstable greater than 30 μm below the minimum recorded production thickness. Assessment of the spatial uniformity of dislocations indicated that lower conformational stability corresponded to elevated densities of basal plane dislocations (BPDs) and threading edge dislocations (TEDs) at the wafer edge relative to the center.

Dislocation arrangements in 4H-SiC and their influence on the local crystal lattice properties.

Two wafers of one 4H-silicon carbide (4H-SiC) bulk crystal, one cut from a longitudinal position close to the crystal's seed and the other close to the cap, were characterized with synchrotron white-beam X-ray topography (SWXRT) in back-reflection and transmission geometry to investigate the dislocation formation and propagation during growth. For the first time, full wafer mappings were recorded in 00012 back-reflection geometry with a CCD camera system, providing an overview of the dislocation arrangement in terms of dislocation type, density and homogeneous distribution. Furthermore, by having similar resolution to conventional SWXRT photographic film, the method enables identification of individual dislocations, even single threading screw dislocations, which appear as white spots with a diameter in the range of 10 to 30 µm. Both investigated wafers showed a similar dislocation arrangement, suggesting a constant propagation of dislocations during crystal growth. A systematic investigation of crystal lattice strain and tilt at selected wafer areas with different dislocation arrangements was achieved with high-resolution X-ray diffractometry reciprocal-space map (RSM) measurements in the symmetric 0004 reflection. It was shown that the diffracted intensity distribution of the RSM for different dislocation arrangements depends on the locally predominant dislocation type and density. Moreover, the orientation of specific dislocation types along the RSM scanning direction has a strong influence on the local crystal lattice properties.

Self-organization of ferroelectric domains induced by water and reinforced via ultrasonic vibration

Pattern formation caused by self-organization is a fascinating phenomenon that appears in biological, chemical, and physical systems. In ferroelectrics, although a variety of domain patterns have been reported at different scales and dimensions, the self-organization process of ferroelectric domains was rarely investigated. Here, in 0.72Pb(Mg1/3Nb2/3)O3−0.28PbTiO3 bulk crystals exposed to water, the self-organized formation process of domain structures is observed and reinforced by ultrasonic vibration. By combining experimental observations and theoretical analysis, we find that adsorbed H+/OH− ions on the sample surface act as screening charges to induce the coarsening of the ferroelectric domains. Meanwhile, interactions among dipoles determine the ordering of the domain configuration, while ultrasonic vibration reduces the barrier height for polarization switching. The process of domain evolution deviates from that of the non-conservative dynamic system, and instead fits a percolation model with a clear transition point. This work demonstrates the self-organization of ferroelectric domains induced by water, which is of value for understanding domain dynamics and for the development of high-performance ferroelectric materials.

Emergence of a Non-Van der Waals Magnetic Phase in a Van der Waals Ferromagnet.

Manipulation of long-range order in 2D van der Waals (vdW) magnetic materials (e.g., CrI3 , CrSiTe3 ,etc.), exfoliated in few-atomic layer, can be achieved via application of electric field, mechanical-constraint, interface engineering, or even by chemical substitution/doping. Usually, active surface oxidation due to the exposure in the ambient condition and hydrolysis in the presence of water/moisture causes degradation in magnetic nanosheets that, in turn, affects the nanoelectronic /spintronic device performance. Counterintuitively, the current study reveals that exposure to the air at ambient atmosphere results in advent of a stable nonlayered secondary ferromagnetic phase in the form of Cr2 Te3 (TC2 ≈160K) in the parent vdW magnetic semiconductor Cr2 Ge2 Te6 (TC1 ≈69K). The coexistence of the two ferromagnetic phases in the time elapsed bulk crystal is confirmed through systematic investigation of crystal structure along with detailed dc/ac magnetic susceptibility, specific heat, and magneto-transport measurement. To capture the concurrence of the two ferromagnetic phases in a single material, Ginzburg-Landau theory with two independent order parameters (as magnetization) with a coupling term can be introduced. In contrast to the rather common poor environmental stability of the vdW magnets, the results open possibilities of finding air-stable novel materials having multiple magnetic phases.