Physical Vapor Transport Research Articles

High-Performance Organic Field-Effect Transistors Based on Two-Dimensional Vat Orange 3 Crystals

The exploration and research of low-cost, environmentally friendly, and sustainable organic semiconductor materials are of immense significance in various fields, including electronics, optoelectronics, and energy conversion. Unfortunately, these semiconductors have almost poor charge transport properties, which range from ∼ 10−4 cm2⋅V−1⋅s−1 to ∼ 10−2 cm2⋅V−1⋅s−1. Vat orange 3, as one of these organic semiconductors, has great potential due to its highly conjugated structure. We obtain high-quality multilayered Vat orange 3 crystals with two-dimensional (2D) growth on h-BN surfaces with thickness of 10–100 nm using physical vapor transport. Raman’s results confirm the stability of the chemical structure of Vat orange 3 during growth. Furthermore, by leveraging the structural advantages of 2D materials, an organic field-effect transistor with a 2D vdW vertical heterostructure is further realized with h-BN encapsulation and multilayered graphene contact electrodes, resulting in an excellent transistor performance with On/Off ratio of 104 and high field-effect mobility of 0.14 cm2⋅V−1⋅s−1. Our results show the great potential of Vat orange 3 with 2D structures in future nano-electronic applications. Furthermore, we showcase an approach that integrates organic semiconductors with 2D materials, aiming to offer new insights into the study of organic semiconductors.

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 %.

Effect of subsurface damages in seed crystals on the crystal quality of 4H-SiC single crystals grown by the PVT technology

This study focuses on the generation and transformation of defects associated with subsurface damages (SSDs) in seed crystals during the physical vapor transport (PVT) growth of 4H-SiC crystals.

Fabrication of a freestanding AlN substrate via HVPE homoepitaxy on a PVT-AlN substrate

Hydride vapor phase epitaxy (HVPE) is employed for the homoepitaxial development of AlN thick films on AlN substrates grown via physical vapor transport (PVT). A freestanding AlN substrate with a 200 μm thickness is then obtained by mechanically grinding away the PVT-AlN substrate. The as-grown HVPE AlN layer has a smooth surface with long parallel atomic steps. The freestanding HVPE-AlN substrate is crack-free and stress-free. In comparison to PVT-AlN substrate, HVPE-AlN substrate not only has better crystal quality but also substantially lower C, O, and Si impurity concentrations. The deep ultraviolet (DUV) transmittance of the 200 μm thick freestanding AlN substrate is as high as 66% at 265 nm. This performance aligns perfectly with the demands of AlGaN-based DUV optoelectronic devices.

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.

Investigating the Distribution Pattern of Threading Edge Dislocation Low Angle Grain Boundaries in 4H-SiC Wafers Using Synchrotron X-Ray Topography

Silicon carbide (SiC) has emerged as a significant wide bandgap semiconducting material, playing a pivotal role in the fabrication of high-frequency, high-power electronic devices in modern times. Its exceptional properties, such as high thermal conductivity, high saturation velocity, and high breakdown field, have positioned it as a valuable constituent in various application fields, including transportation, communication, and bioelectronics. However, the efficacy and long-term reliability of SiC-based devices are critically dependent on the quality of both the SiC substrate and epitaxial material employed. The presence of deleterious defects within these materials can induce device degradation and failure, posing a significant challenge in realizing the full potential of the device. Therefore, it is imperative to address the quality while maintaining the availability of large-scale single crystals of SiC to further expand the development of SiC-based device technology. Among various kinds of crystallographic defects in bulk crystal, low angle grain boundary (LAGB) is a commonly observed planar defect that prevents the implementation of large size (≥ 1 cm2) SiC devices [1]. In both 6H and 4H-SiC, LAGBs have been observed to replicate during the epitaxial growth [2], which can lead to profound impact on the physical and electronic properties of the crystal that subsequently triggers device degradation. Literature reveals that the presence of LAGBs in commercial wafers can lead to increased leakage currents during reverse-bias operation of devices [3]. Besides, the existence of prismatic tilt boundaries is usually accompanied with aggregation of edge type basal plane dislocations (BPDs) [4], which cause severe degradation in SiC bipolar devices [5] as BPD segments can dissociate into partials and form stacking faults in between [6].This study investigated a unique distribution pattern of LAGBs observed on physical vapor transport (PVT) grown off-axis 4H-SiC wafers. Synchrotron X-ray topographs in both transmission and grazing geometry reveal that three sets of LAGBs are presented on one side of the wafer, away from the facet position, each set contains numerous LAGB arrays. Figure 1 (a) shows a schematic diagram where each oval outlines the approximate location of each LAGB set. As the grazing-incidence topographs shown in Figure 1 (b), the LAGB sets distributed at the upper and lower region of the wafer consist of white contrast threading edge dislocations (TEDs) extending from the inner area towards the edge approximately along 60º and 120º direction, respectively. The LAGB set positioned at the middle region of the wafer consists of dark contrast TEDs extending from the inner area towards the edge approximately along 90º direction. All individual TED arrays in each LAGB set extends along the <1-100> directions, with identical Burgers vectors perpendicular to the array direction. The white and dark contrast of TEDs corresponds to tilt in opposite directions. White contrast TEDs are with Burgers vectors of 1/3[2-1-10] and 1/3[1-210], dark contrast TEDs are with Burgers vectors of 1/3[-2110] and 1/3[-12-10]. Such TED LAGBs are identified as pure tilt boundaries that accommodate the rotational misorientation between two adjacent regions. Therefore, a flat wafer is expected to exhibit zero net misorientation due to these TED arrays. However, any non-zero net misorientation resulting from these arrays is likely to promote instability, ultimately leading to the curvature of the plane. A systematic analysis is being carried out across several wafers and this relationship between the distribution of TED arrays and lattice curvature will be discussed.Reference:[1] N. Ohtani et al. Journal of Crystal Growth 237–239, 1180–1186, (2002).[2] C. Hallin et al, J. Cryst. Growth 181, 241 (1997).[3] R. Singh and M. Pecht, IEEE Industrial Electronics Magazine, 2, 19-31, (2008).[4] Y Chen et al, Materials Science Forum 556, 231-234 (2007).[5] T. Kimoto, et al., Proc. IEEE Int. Rel. Phys. Symp., 2A-1.1–2A-1.7, (2017).[6] M. Dudley, et al, Materials Science Forum, 600-603, 261-266 (2008). Figure 1

Investigation of Growth Sectors in Gallium Nitride Substrate Wafers from Ammonothermal and Patterned Hvpe Growth Methods

Gallium Nitride (GaN) is a wide band gap semiconductor material that has the potential to replace silicon (Si) in power electronics. However, the quality and availability of bulk GaN substrates is currently limiting its use in high-performance power electronics. Several methods have been developed to grow bulk GaN crystals, including physical vapor transport (PVT) [1, 2], Na-flux [3], ammonothermal [4, 5], and hydride vapor phase epitaxy (HVPE) [6]. Additionally, patterned HVPE growth methods have been developed to produce GaN substrates with reduced dislocation densities [7]. Ammonothermal and patterned HVPE growth methods have shown promise in producing high-quality GaN crystals for use in power devices [8-10]. Ammonothermal-grown GaN has an overall low dislocation density of 1000 cm-2, while patterned growth can concentrate dislocations into small areas, leaving the rest of the crystal relatively dislocation-free (< 102 cm-2). Besides dislocations, the overall homogeneity of these wafers is also important for devices. Recent synchrotron X-ray topography studies revealed domain features in both ammonothermal and patterned HVPE GaN wafers. Using synchrotron X-Ray Plane-Wave Topography (SXPWT, previously known as synchrotron X-Ray Rocking Curve Topography), these domain features that have enclosed diamond or quadrilateral shapes are found to have relative compressive strain values (~ 10-5) (Figure 1b). These features are visible on SEM micrographs using a Robinson detector (RBSD) (Figure 1c). The origin of these strains is likely the different impurity incorporation rates in different growth sectors.In the patterned HVPE growth method, growth sectors have been reported to exist due to growth along different directions [11]. It is likely that a similar situation exists for ammonothermal substrates, although no studies have been reported on this. Understanding the nature of these domain areas is important for device fabrication as the inhomogeneities will likely be replicated into active regions. SIMS measurements are currently being conducted to characterize the differences in impurity concentrations in these domains. These results will be used to confirm the origin of the strain.Figure 1. (a) Synchrotron monochromatic beam X-ray topograph of a selected area on a patterned HVPE grown GaN substrate; (b) Cropped strain map of the same area obtained by SXPWT; (c) SEM image of the same area with RBSD contrast. All three images show contrast from the domains defined by the growth sectors.[1] H. Wu, J. Spinelli, P. Konkapaka, M. Spencer, MRS Online Proceedings Library Archive 892 (2005).[2] D. Siche, D. Gogova, S. Lehmann, T. Fizia, R. Fornari, M. Andrasch, A. Pipa, J. Ehlbeck, Journal of crystal growth 318 (2011) 406-410.[3] M. Aoki, H. Yamane, M. Shimada, S. Sarayama, F.J. DiSalvo, Journal of crystal growth 242 (2002) 70-76.[4] R. Dwiliński, R. Doradziński, J. Garczyński, L. Sierzputowski, A. Puchalski, Y. Kanbara, K. Yagi, H. Minakuchi, H. Hayashi, Journal of Crystal Growth 310 (2008) 3911-3916.[5] T. Hashimoto, F. Wu, J.S. Speck, S. Nakamura, Journal of Crystal Growth 310 (2008) 3907-3910.[6] H.P. Maruska, J. Tietjen, Applied Physics Letters 15 (1969) 327-329.[7] T. Nakamura, K. Motoki, GaN substrate technologies for optical devices, IEEE, vol 101, 2013, p. 2221.[8] B. Raghothamachar, Y. Liu, H. Peng, T. Ailihumaer, M. Dudley, F.S. Shahedipour-Sandvik, K.A. Jones, A. Armstrong, A.A. Allerman, J. Han, H. Fu, K. Fu, Y. Zhao, Journal of Crystal Growth 544 (2020) 125709.[9] Y. Liu, B. Raghothamachar, H. Peng, T. Ailihumaer, M. Dudley, R. Collazo, J. Tweedie, Z. Sitar, F.S. Shahedipour-Sandvik, K.A. Jones, Journal of Crystal Growth 551 (2020) 125903.[10] Y. Liu, H. Peng, T. Ailihumaer, B. Raghothamachar, M. Dudley, Journal of Electronic Materials 50 (2021) 2981-2989.[11] K. Motoki, T. Okahisa, S. Nakahata, N. Matsumoto, H. Kimura, H. Kasai, K. Takemoto, K. Uematsu, M. Ueno, Y. Kumagai, Journal of Crystal Growth 237 (2002) 912-921. Figure 1

(Invited) Application of 3D in-Situ X-Ray Visualization to Track the Formation of Dislocation Clusters during PVT Growth of SiC

SiC has become the key player among wide bandgap semiconductors for power electronic applications. Since the first description of the physical vapour transport (PVT) growth process of SiC by Tairov and Tsvetkov (J. Crystal Growth, 43, 209(1978)), there has been steady progress in SiC-based crystal growth, epitaxy and device processing. The success of SiC compared to Si is related to its superior material properties such as extremely high electrical breakdown field and high thermal conductivity compared to the standard silicon counterpart. In addition, SiC device processing utilises much of the standard Si processing equipment. A major reason for the success of SiC in power electronic applications compared to other wide bandgap counterparts such as GaN, Ga2O3 and diamond is related to the availability of large diameter SiC wafer materials (150mm = standard, 200mm = developped). Bulk SiC growth is now a very well developed process with comparatively high yields. The extraordinary physical properties also include obstacles related to the strong chemical bonding and complex phase diagram of the material, which pose challenges to the growth process. Therefore, there are still a number of open questions related to the nucleation, progression and termination of the bulk growth process that require fundamental research in materials science and technology.The aim of this presentation is (i) to give an overview of the state-of-the-art PVT growth process and (ii) to discuss a current research topic dealing with the early stage of the growth process and the defect formation that can occur during the initial nucleation of SiC. We have applied 3D in-situ visualisation of the growth process using X-ray computed tomography to visualise island formation on the large seeding area. These data are related to growth process instabilities such as temperature variations during the seeding process and axial doping level changes from the seed to the newly grown crystal. Both process instabilities induce mechanical stress on the SiC lattice and act as sources for dislocation generation and multiplication. We will show a series of growth processes with varying growth parameters that shed light on the initial growth stage of SiC.As the crystal diameter of SiC increases from 150 mm to 200 mm, the results of this study become increasingly important.

Mapping Analysis of Crystalline Perfection and UV‐C Transparency of 2‐in. Aluminum Nitride Substrates Grown by Physical Vapor Transport

Large‐diameter, single‐crystalline aluminum nitride (AlN) substrates are sought for growth of AlGaN‐based devices. A combination of high crystalline perfection and optical transparency is desired for UV‐C emitter applications. A mapping analysis of 2‐in. AlN substrates grown by physical vapor transport is performed to study the within‐wafer uniformity of structural and optical properties. Substrates with spatially uniform crystalline perfection and UV‐C absorption coefficients, as determined by typical X‐ray rocking curve widths below 15 arcsec and 4.68 eV absorption coefficients below 20 cm−1, are shown. These findings demonstrate a robust mass production process for high‐quality, 2 in. AlN substrates possessing uniform structural and UV optical properties, making them highly suited for growth of next‐generation, AlGaN‐based devices.

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.

Study of effects of varying parameters on the dislocation density in 200 mm SiC bulk growth

Physical vapor transport (PVT) is the dominant method for growing 200 mm SiC crystals, and the crystals produced by this method still have dislocations, which affect the performance of the device. In this study, the finite element analysis of 200 mm SiC crystal growth has been conducted to investigate the influencing parameters on the dislocation density. The calculations are based on the model of multiple resistance heating. The transient heat transfer for the crystal growth is first calculated. A dynamic mesh technique is then employed to consider the shape evolution of the crystal during the growth. Finally, the distributions of the internal stress and dislocation density have been calculated based on the Alexander-Haasen model. The comparison among different parameters provides guidance for reducing the thermal stress and dislocation density in the 200 mm SiC crystal growth.

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.

Morphology evolution of homoepitaxial growth of aluminum nitride by hydride vapor phase epitaxy

Free-standing aluminum nitride (AlN) crystals prepared by physical vapor transport (PVT) have been utilized as substrates for the homoepitaxial growth of AlN thick layers by using hydride vapor phase epitaxy (HVPE). The morphology evolution has been investigated in details under various growth conditions. The surface conditions from step flow to step bunching growth mode are governed by growth rates and V/III ratios, which is attributed to the vapor supersaturation. The presence of microcrystal inclusions, formation of spiral hillocks and occurrence of cracks could cause severe surface degradation. Additionally, the substrate misorientation also contributes to the morphology variation to some extent. By optimizing key growth parameters, a 300-μm thick homoepitaxial AlN layer with a smooth surface and high deep UV transparency has been successfully prepared by HVPE method.

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

Structural Design Optimization of Physical Vapor Transport Furnace for Aluminum Nitride Crystal Growth via Modeling and Simulation

Structural Design Optimization of Physical Vapor Transport Furnace for Aluminum Nitride Crystal Growth via Modeling and Simulation

The Thermal Field Design of a 150 mm Sic Crystal Growth System by Numerical Simulation

AbstractNumerical simulation analysis, enabled by the physical vapor transport (PVT) technique, has been widely adopted to study the growth process of silicon carbide (SiC). In this study, a 2D axisymmetric model employed the finite volume method (FVM) is adopted to simulate the thermal fields and mass transfer of SiC powders with different particle sizes, thicknesses of the top thermal insulation, side wall thicknesses of the lid, and distances between the lid center and the top thermal insulation in a SiC crystal growth system. The simulation results revealed that the reduction in particle size increased the radial and axial temperature gradients in the powder region. Meanwhile, when the thickness of the top thermal insulation decreased, the axial temperature gradient inside the crucible increased significantly. Furthermore, the change in the side wall thickness of the lid significantly affected the thermal field distribution on the crystal surface and consequently affected the crystal growth shape. Finally, the variation in the distance between the lid center and the top thermal insulation has a significant influence on the axial and radial temperature gradients. Overall, these results indicate that the quality of SiC crystal growth may be improved by adjusting particle size, thermal insulation and crucible design.

Comparison of temperature-dependent resistivity of heavily Al- and N-codoped 4H-SiC grown by physical vapor transport and heavily Al-doped 4H-SiC grown by chemical vapor deposition

The temperature-dependent resistivity of heavily Al- and N-codoped 4H-SiC grown by physical vapor transport (PVT) with Al concentrations (C Al) higher than 1019 cm−3 is investigated to obtain high-growth-rate and low-cost p+-type substrates suitable for the collectors of n-channel insulated-gate bipolar transistors. The resistivity is compared with that of heavily Al-doped 4H-SiC grown by CVD. In the band conduction region, the hole mobility of the PVT-grown codoped samples is slightly lower than that of the CVD-grown sample at the same C Al. At C Al values of around 2 × 1020 cm−3, the temperature range in the variable-range-hopping conduction region for the PVT-grown codoped samples is much wider than that for the CVD-grown samples.

Micropipe-Like Defects in the Expanded Diameter Region of 8 in. SiC Grown by Physical Vapor Transport

Micropipe-Like Defects in the Expanded Diameter Region of 8 in. SiC Grown by Physical Vapor Transport

Experimental and simulation studies of surface segregation-limited nitrogen incorporation at the growth front of physical vapor transport-grown 4H-SiC crystals

Experimental and simulation studies were conducted for surface segregation-limited kinetics of nitrogen incorporation into a 4H-SiC crystal during physical vapor transport (PVT) crystal growth. It was revealed that the nitrogen incorporation is kinetically limited by the step-flow velocity on the growing crystal surface of a 4H-SiC crystal; in this study, the surface step-flow velocity at the growth front was deduced from the local inclination angle of the growth front measured from the (0001¯) plane, assuming a uniform growth rate along the c-axis (crystal growth direction) across the growth front, and the nitrogen concentration across the growth front was measured using Raman scattering microscopy. The step-flow velocity dependence of nitrogen incorporation was theoretically analyzed using a two-site-exchange model, and the simulated dependence using the model was in good agreement with the experimental data. On the basis of these experimental and simulation results, kinematical and energetical aspects of nitrogen incorporation at the growth front of a 4H-SiC crystal during PVT growth are discussed.

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.