Physical Vapor Transport Method Research Articles

Optimizing crucible geometry to improve the quality of AlN crystals by the physical vapor transport method

In the conventional crucible structure for AlN crystal growth by physical vapor transport, owing to the long molecular transport path of Al vapor and the disruption of the gas flow by the presence of a deflector, the Al vapor easily forms polycrystals in the growth domain. The result is increased internal stress in the crystals and increased difficulty in growing large-sized crystals. On this basis, with the help of finite element simulations, a novel crucible structure is designed. This crucible not only optimizes the gas transport but also increases the radial gradient of the AlN crystal surface, making the enhanced growth rate in the central region more obvious. The thermal stresses between the deflector and the crystal are also reduced. High-quality AlN crystals with an FWHM of 79 arcsec were successfully grown with this structure, verifying the accuracy of finite element simulation of the growth of AlN crystals. Our work has important guiding significance for the growth of high-quality AlN crystals.

Synthesis-in-place of V2O5 nanobelts for wide range humidity detection

Resistive type humidity sensors offer advantages in simplicity, low cost, compact size, and remote operation. Due to the benefits, they have great potential for use in a variety of environments and future applications, including the Internet of Everything (IoE). However, resistive humidity sensors have limitations in detecting extreme humidity levels below 10 % or above 90 % relative humidity (RH) and are also vulnerable to high temperatures, which hinders their use in harsh environments applications. For broader applications, humidity sensors with reliable performance across the full range of humidity levels and high stability at elevated temperatures need to be developed. Here, we report high performance resistive humidity sensors based on V2O5 nanobelts operating at high temperatures. The V2O5 nanobelts are directly deposited between Pt electrodes by O2-assisted physical vapor transport (PVT) method. The V2O5 nanobelt humidity sensor exhibits reliable humidity sensing properties over a wide range from 1 % to 100 % RH. Furthermore, long-term stability and reproducibility of V2O5 nanobelts are demonstrated through 25 consecutive humid air pulses and measurements obtained after 4 months of storage. The high performance of the V2O5 nanobelt humidity sensor is advantageous for practical use in applications ranging from IoE environments to harsh industrial conditions.

Determination of Thermal Properties of Carbon Materials above 2000 °C for Application in High Temperature Crystal Growth

AbstractThis work reports on the determination of the heat conductivity of high temperature stable carbon materials in the temperature range well above 2000 °C where classic material characterization methods fail. Dense graphite (DG) materials as well as rigid and soft felt isolation (RFI/SFI) components have been investigated which are used during crystal growth of SiC by the physical vapor transport method (PVT) in the temperature regime of 2000 and 2400 °C. The applied materials characterization methods include low temperature physical heat conductivity measurements using laser flash analysis (LFA) in the temperature range 25–1200 °C, data extrapolation to elevated temperatures up to 2400 °C, and a correlation of heating processes and computer simulation of the temperature field of different hot zone designs. Using this approach, the calculated temperatures and experimentally determined values with an error of less than ± 2% at 2400 °C can be merged.

Numerical Simulation of the Transport of Gas Species in the PVT Growth of Single‐Crystal SiC

AbstractSingle‐crystal silicon carbide (SiC) is an important semiconductor material for the fabrication of power and radio frequency (RF) devices. The major technique for growing single‐crystal SiC is the so‐called physical vapor transport (PVT) method, in which not only the thermal field but also the fluid‐flow field and the distribution of gas species can be hardly measured directly. In this study, a multi‐component flow model is proposed that includes the inside and outside of a growth chamber and a joint between the seed crystal holder and crucible which allows exchanges of the gas species. The joint is simulated as a thin porous graphite sheet. The Hertz‐Knudsen equation is used to describe the sublimation and deposition. The convection and diffusion are described by the Navier–Stokes equations and mixture‐averaged diffusion model, in which the Stefan flow is taken into account. The numerical simulations are conducted by the finite element method (FEM) with a multi‐physics coupled model, which is able to predict the fluid flow field, species distribution field, crystal growth rate, and evolution of the molar concentration of dopant gas. Using this model, the effects of several experimental conditions on the transport of gas species and the growth rate of single‐crystal SiC are analyzed.

The Numerical Simulations and Experimental Study of an 8-Inch SiC Single Crystal with Reduced BPD Density.

The basal plane dislocation (BPD) density is one of the most important defects affecting the application of SiC wafers. In this study, numerical simulations and corresponding experiments were conducted to investigate the influence of cooling processes, seed-bonding methods, and graphite crucible materials on the BPD density in an 8-inch N-type 4H-SiC single crystal grown by the physical vapor transport (PVT) method. The results showed that the BPD density could be effectively reduced by increasing the cooling rate, optimizing the seed-bonding method, and adopting a graphite crucible with a similar coefficient of thermal expansion as the SiC single crystal. The BPD density in the experiments showed that a high cooling rate reduced the BPD density from 4689 cm-2 to 2925 cm-2; optimization of the seed-bonding method decreased the BPD density to 1560 cm-2. The BPD density was further reduced to 704 cm-2 through the adoption of a graphite crucible with a smaller thermal expansion coefficient.

Growth and properties of tantalum carbide coatings on graphite by TRD technique

The tantalum carbide-coated graphite material exhibits remarkable chemical stability, corrosion resistance, and heat shock resistance, enabling it to demonstrate excellent stability during the growth of wide bandgap semiconductors. Its application significantly enhances the quality of third-generation semiconductor crystals. Nevertheless, the high cost of this material hinders its widespread utilization in the field of wide bandgap semiconductors. To address this challenge, this study employs the Thermo-reaction deposition and diffusion (TRD) technique to successfully fabricate a tantalum carbide coating on the surface of graphite. Through thermodynamic analysis, this article validate the thermodynamic feasibility of preparing tantalum carbide coatings using the TRD process. To achieve a dense and uniform TaC coating on the graphite surface, we systematically investigate the effects of reducing agent content, reaction temperature, and reaction time on the microstructure of the coating. Experimental research has found that a molar ratio of B4C to Ta2O5 of 1:0.64 optimizes the coating uniformity. Deviations from this ratio, either higher or lower, can lead to coating inhomogeneity or even failure in coating formation. Additionally, this article determine that a reaction temperature range of 1200–1400 °C, coupled with a reaction time not exceeding 6 h, represents the most suitable conditions for preparing TaC coatings on the graphite surface. Finally, to evaluate the practical performance of the coated graphite, this article simulate the corrosive environment encountered during the physical vapor transport method for growing aluminum nitride single crystals. The results demonstrate a significant improvement in the corrosion resistance of the coated graphite. This enhancement is primarily attributed to the formation of a dense TaC coating on the graphite surface, which exhibits remarkable stability at elevated temperatures.

Diameter enlargement of SiC bulk single crystals based on simulation and experiment

This study employs the physical vapor transport method for diameter enlargement of silicon carbide single crystal. Numerical simulations were utilized to investigate the impact of different crucible designs on crystal enlargement. The guiding angle and ratio of seed diameter to crucible inner diameter are thoroughly examined in this study. Simulations demonstrated that under appropriate conditions for crystal growth, there is no deposition of poly-SiC around the periphery of ingot. These findings were well validated through crystal growth experiments. Subsequently, the crucible was optimized based on the simulation results for crystal growth. The final achievement was the successful enlargement of silicon carbide single crystal from 150 mm to 200 mm, with Raman mapping analysis confirming the absence of foreign polytypes around the periphery of ingot.

Characteristics of the Discoloration Switching Phenomenon of 4H-SiC Single Crystals Grown by PVT Method Using ToF-SIMS and Micro-Raman Analysis.

The discoloration switching appearing in the initial and final growth stages of 4H-silicon carbide (4H-SiC) single crystals grown using the physical vapor transport (PVT) technique was investigated. This phenomenon was studied, investigating the correlation with linear-type micro-pipe defects on the surface of 4H-SiC single crystals. Based on the experimental results obtained using time-of-flight secondary ion mass spectrometry (ToF-SIMS) and micro-Raman analysis, it was deduced that the orientation of the 4H-SiC c-axis causes an axial change that correlates with low levels of carbon. In addition, it was confirmed that the incorporation of additional elements and the concentrations of these doped impurity elements were the main causes of discoloration and changes in growth orientation. Overall, this work provides guidelines for evaluating the discoloration switching in 4H-SiC single crystals and contributes to a greater understanding of this phenomenon.

Effects of Surface Size and Shape of Evaporation Area on SiC Single-Crystal Growth Using the PVT Method

Silicon carbide (SiC) polycrystalline powder. As the raw material for SiC single-crystal growth through the physical vapor transport (PVT) method, its surface size and shape have a great influence on growth of crystal. The surface size and shape of the evaporation area filled with polycrystalline powder were investigated by numerical simulation in this study. Firstly, the temperature distribution and deposition rate distribution for the PVT system were calculated by global numerical simulation, and the optimal ratio of polycrystalline powder surface diameter to seed crystal diameter was determined to be 1.6. Secondly, the surface of the evaporation area filled with polycrystalline powder was covered by a graphite ring and a graphite disc, respectively, to change its surface shape. The results show that adjusting the surface size and shape of the evaporation area filled with polycrystalline powder is an effective method to control the growth rate, growth stability, and growth surface shape of the single crystal. Finally, the result obtained by selecting appropriate covered structures for actual growth indicates that this process can act as a reference for improving the quality of single crystals.

Improvement of the resistivity uniformity of 8-inch 4H–SiC wafers by optimizing the thermal field

8-inch N-type 4H–SiC single crystals were grown on 4° off-axis seeds by the physical vapor transport (PVT) method. The electrical properties of 8-inch 4H–SiC wafers were assessed by contactless resistivity mapping. The resistivity of the whole wafer is inhomogeneous with an inhomogeneity of 4.8 %, much higher than that of standard 6-inch wafers with a resistivity inhomogeneity of 1.2 %. This nonuniformity phenomenon is attributed to facet formation caused by discontinuities in the thermal field as the crystal diameter increases. Due to the facet effect, the nitrogen doping concentration in the facet region is higher than that in other regions. By optimizing the thermal field through adjustment of the growth conditions, a nearly flat and slightly convex 8-inch SiC crystal growth interface was obtained. The uniformity of the resistivity of 8-inch SiC wafers is significantly improved, with an inhomogeneity of 1.6 %.

Synchrotron X-Ray Topography Studies for Defect Formation at the Early Stage of PVT-Grown 4H-SiC Crystals

Silicon Carbide (SiC), typically 4H-SiC, is replacing conventional silicon materials for high power and high frequency applications due to its excellent electrical and thermal properties [1]. However, high quality SiC wafers with low defect density are still in urgent need for automotive and energy saving applications. SiC wafers are obtained from bulk growth by physical vapor transport (PVT) method and considerable efforts have been expended to optimize growth process to lower defect densities. One major approach is the study of initial stages of crystal growth as most defects are nucleated at this stage and propagate into the boule. Ailihumaer et al [2] demonstrated the behaviors of threading edge dislocations (TEDs), threading screw dislocations (TSDs)/threading mixed dislocations (TMDs), and basal plane dislocations (BPDs) across the seed/newly grown layer interface in PVT-grown 4H-SiC. Sanchez et al [3] revealed the nucleation of TEDs and TSDs at the initial growth stage in PVT-grown SiC.However, more detailed investigations are still in demand for PVT grown 4H-SiC crystals during initial stage of growth. This study investigates 4° off-axis 4H-SiC wafers with several hundred microns of initial-stage growth PVT method. Synchrotron X-ray topography (SXRT) technique is employed and shows dramatically modified defect distribution across the seed/newly grown layer interface (Figure 1). The formation of Shockley stacking fault starting at initial stage of crystal growth is suggested as partial dislocations running along <11-20> directions on basal plane are observed to form rhombus shapes (Figure 2). Prismatic slip is observed to take place near the wafer edges and prismatic dislocations undergo cross slip to form unique dislocation configurations. A statistical analysis on defect changes across the seed/newly grown layer is being carried out and the results will be reported. These results will be complemented by observations from Nomarski optical microscopy (NOM) and Raman Spectroscopy studies to propose the formation mechanism of defects across the seed/newly grown layers.

Correlating Young's Modulus with High Thermal Conductivity in Organic Conjugated Small Molecules.

Attaining elevated thermal conductivity in organic materials stands as a coveted objective, particularly within electronic packaging, thermal interface materials, and organic matrix heat exchangers. These applications have reignited interest in researching thermally conductive organic materials. The understanding of thermal transport mechanisms in these organic materials is currently constrained. This study concentrates on N, N'-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8), an organic conjugated crystal. A correlation between elevated thermal conductivity and augmented Young's modulus is substantiated through meticulous experimentation. Achievement via employing the physical vapor transport method, capitalizing on the robust C═C covalent linkages running through the organic matrix chain, bolstered by π-π stacking and noncovalent affiliations that intertwine the chains. The coexistence of these dynamic interactions, alongside the perpendicular alignment of PTCDI-C8 molecules, is confirmed through structural analysis. PTCDI-C8 thin film exhibits an out-of-plane thermal conductivity of 3.1±0.1Wm-1K-1, as determined by time-domain thermoreflectance. This outpaces conventional organic materials by an order of magnitude. Nanoindentation tests and molecular dynamics simulations elucidate how molecular orientation and intermolecular forces within PTCDI-C8 molecules drive the film's high Young's modulus, contributing to its elevated thermal conductivity. This study's progress offers theoretical guidance for designing high thermal conductivity organic materials, expanding their applications and performance potential.

A solar-blind vacuum-ultraviolet photodetector based on free-standing lamellar aluminum nitride single crystal

High-quality aluminum nitride (AlN) crystals are the key material for the development of high-performance solid-state solar-blind vacuum-ultraviolet (VUV) photodetectors. However, the commonly used epitaxial method to grow AlN crystals would limit this development due to the existence of indispensable substrates. This study addresses this issue using free-standing lamellar AlN single crystals that are grown using the physical vapor transport method. The large lateral dimension of the crystal enables the construction of an Au-AlN-graphene van der Waals heterojunction, which can function as a vertical VUV photodetector with the graphene serving as the light window. The asymmetric junctions formed on the two sides of the crystal and the limited penetration of the VUV endow the device with a bias polarity-dependent photoresponse feature arising from different photoelectric processes. Furthermore, the device demonstrates a high responsivity of 5.77 A W−1 and a high specific detectivity of 1.71 × 1013 cm Hz1/2 W−1 under the illumination of a 193 nm laser. The high crystallinity of the AlN guarantees a high spectral selectivity of responsivity with a 193 nm/280 nm rejection ratio of 3 × 102. This work would inspire the development of wide-bandgap-semiconductor-based VUV photodetectors in terms of methodology and mechanism.

Growth of Hexagonal Boron Nitride (hBN) on Silicon Carbide Substrates by the Physical Vapor Transport Method

Growth of Hexagonal Boron Nitride (hBN) on Silicon Carbide Substrates by the Physical Vapor Transport Method

Bandgap engineering and photodetector applications in Bi(I1-xBrx)3 single crystals

Exploration of low-dimensional semiconductors with tunable band structures is of particular interest in the applications of nano-electronics and optoelectronics. In this work, Bi(I1−xBrx)3 single crystals have been synthesized by a flux-improved physical vapor transport method, where the electronic bandgaps of these single crystals are effectively modulated by the concentration of the halide elements ratios. The first-principle calculations confirm the modulation of bandgap and reveal the orbit contributions for the conduction band minimum and valence band maximum. The properties of Bi(I1-xBrx)3-based photodetectors are measured, where a competition mechanism is identified, leading to the realization of best performance sample with a Br content of 0.18. Our results provide a route to improve the performance of BiI3-based photodetectors and to achieve controllable response spectra.

Growth and Characterization of Centimeter-Scale Pentacene Crystals for Optoelectronic Devices

In this work, we present results on the growth of centimeter-scale pentacene crystals using the physical vapor transport method in a dual-temperature zone horizontal furnace. It was established that intensive crystal growth processes occurred in transition regions with sudden temperature changes, while crystal growth was practically not observed in regions with slightly varying temperatures. During crystal growth, co-precipitating golden needle-like crystals reaching lengths of more than 10 mm were obtained. Using the method of single-crystal X-ray diffraction at 85 and 293 K for dark-blue lamellar pentacene crystals, the crystal structure was refined in a triclinic system with sp.gr. P1¯. It was established that the golden needle crystals consisted of molecules of the pentacene derivative—5,14-pentacenedione, the crystal structure of which was solved for the first time in a rhombic system with sp.gr. P212121. The absorption and luminescence spectra of pentacene and 5,14-pentacenedione in toluene solutions were obtained and analyzed. The electrical properties of the prepared pentacene thin films and single crystals grown under physical vapor transport conditions were evaluated by fabricating and characterizing field-effect transistors (FETs). It was shown that the presence of impurities in the commercial pentacene material had a significant effect on the morphological quality of thin polycrystalline films and noticeably reduced the hole mobility.

Characterization of Prismatic Slip in SiC Crystals by Chemical Etching Method

In 4H-silicon carbide crystals, basal plane slip is the predominant deformation mechanism. However, prismatic slip is often observed in single crystals grown by the physical vapor transport method as the diameter expands to 6 inches or larger. Thermal modeling has shown that occurrence of prismatic slip is attributed to increased radial thermal gradients. While X-ray topography can be used to characterize the presence and extent of prismatic slip, the feasibility of using the chemical etching method to assess the extent of prismatic slip in an industrial setting is investigated. The distribution of scallop shaped etch pits oriented along the directions that correspond to prismatic dislocations, correlate well with the results of the thermal model that predicts the occurrence of prismatic slip dislocations. This capability of the etch pit method to characterize prismatic slip can be used to manage radial thermal gradients during PVT growth.

Optimization of the thermal field of 8-inch SiC crystal growth by PVT method with “3 separation heater method”

Silicon carbide (SiC) has important application prospects in power and radio frequency (RF) devices. However, preparing large-diameter, high-quality SiC crystals is challenging. The major technique for growing large-diameter SiC crystals is the physical vapor transport (PVT) method and the key point for adjusting conditions is to find an optimal thermal field. In this study, we propose a resistance heating method called the “3 separation heater method” with 3 heaters separated by the graphite foam insulations so that 3 important parts (the SiC source powder area, SiC seed area, and seed holder) of the growth cell can be heated independently through thermal radiation from each separated heater. Further, the insulation separators play an important role as a switch of the heat flux. The numerical studies of the thermal field design for this model are conducted with the help of the numerical simulation software COMSOL Multiphysics and its appropriate parameters are explored. At last, the relationship between the parameters and crystal growth conditions, such as the radial temperature difference (RTD) and edge temperature gradient (ETG), is expressed by the back-propagation (BP) neural network and the optimal parameter values for crystal growth conditions have been confirmed using the non-dominated sorting genetic algorithm (NSGA-II).

Tellurium filled carbon nanotubes cathodes for Li-Te batteries with high capacity and long-term cyclability

Li-Te batteries, compared with traditional lithium-ion batteries and Li-other group VIA elements (including O, S, Se) batteries, have shown overwhelming features of high specific volumetric capacity and superior electrical conductivity. These features make the Li-Te batteries possess great importance in modern portable electronics and electric vehicles with limited battery package size. However, the Achilles' Heel in Li-Te batteries is the huge volume change and inevitable dissolution of polytellurides during cycling. Herein, we synthesize a cathode material based on polycrystalline Te encapsulated in N-doped multiwall carbon nanotubes (Te-filled CNTs) by physical vapor transport (PVT) method. With unique spatial restriction of the CNT hosts, the electrochemically active Te content can be well preserved, and the prepared Te-filled CNTs cathodes deliver a high specific capacity of 590 mAh g−1 based on Te content. More importantly, the reaction kinetics and electrochemical reversibility of the cathodes have been significantly improved by using nanoscale cavities (4–6 nm) which stabilize the polytellurides (Li2Ten). Meanwhile, a series of in situ characterizations are carried out to straightforwardly reveal a two-step discharge/charge mechanism for Te-filled CNTs. In addition, theoretical calculations prove that the encapsulated Te ensure a lower energy barrier for lithiation compared with bare CNT. This work sheds light on the nanoconfinement effect of CNT for improving the utilization and cycling stability of Te based cathodes in Li-Te batteries.

Coexistence of logarithmic and SdH quantum oscillations in ferromagnetic Cr-doped tellurium single crystals

We report the synthesis of transition-metal-doped ferromagnetic elemental single-crystal semiconductors with quantum oscillations using the physical vapor transport method. The 7.7 atom% Cr-doped Te crystals (Cr:Te) show ferromagnetism, butterfly-like negative magnetoresistance in the low temperature (<3.8 K) and low field (<0.15 T) region, and high Hall mobility, e.g. 1320 cm2 V−1 s−1 at 30 K and 350 cm2 V−1 s−1 at 300 K, implying that Cr:Te crystals are ferromagnetic elemental semiconductors. When B // [001] // I, the maximum negative MR is ∼−27% at T = 20 K and B = 8 T. In the low temperature semiconducting region, Cr:Te crystals show strong discrete scale invariance dominated logarithmic quantum oscillations when the direction of the magnetic field B is parallel to the [100] crystallographic direction (B // [100]) and show Landau quantization dominated Shubnikov-de Haas oscillations for B // [210] direction, which suggests the broken rotation symmetry of the Fermi pockets in the Cr:Te crystals. The findings of coexistence of multiple quantum oscillations and ferromagnetism in such an elemental quantum material may inspire more study of narrow bandgap semiconductors with ferromagnetism and quantum phenomena.