Physical Vapor Transport Method Research Articles

Carbon pair defects in aluminum nitride

AlN bulk single crystals grown by the physical vapor transport method may be beneficially applied as substrates for deep ultraviolet light emitting devices or as a basic material for piezoelectric resonators operating at high temperatures. Identification of point defects which deteriorate the optical, electrical, and electromechanical properties of AlN crystals for such applications is the subject of the present work. Using Raman spectroscopy, two local vibrational modes (LVMs) were discovered at wave numbers of 1189 cm−1 and 1148 cm−1. By analyzing an AlN crystal intentionally enriched with the carbon isotope 13C, it is unambiguously shown that the two LVMs originate from two different, but in each case carbon-related defects. Furthermore, it is evidenced that the defect underlying the LVM at 1189 cm−1 contains exactly two carbon atoms. The tricarbon defect-related LVM reported earlier in an infrared absorption study is found to be Raman active at 1772 cm−1. The Raman scattering intensity of all three LVMs strongly depends on the photon energy of the exciting light what is interpreted as a resonance Raman effect. This allows linking the identified defects with their contribution to the strong, carbon-related ultraviolet absorption around 4.7 eV and proves that these defects introduce optically and electrically active deep levels in the bandgap of AlN.

Effect of Temperature Gradient on AlN Crystal Growth by Physical Vapor Transport Method

In this study, the effect of different temperature field distributions on the growth of AlN crystal via spontaneous nucleation by physical vapor transport (PVT) method was investigated through simu...

Negative Thermal Expansion of Mercurous Halides

Thermal expansions of single crystals of mercurous halides grown by the physical vapor transport method have been investigated. The unit cell lattice parameters were measured by using x-ray diffraction, whereas, the bulk thermal expansion was investigated using the dilatometer instrument as a function of temperature. It was found by both techniques that mercurous halides exhibit positive linear thermal expansion along the a-axis and negative linear thermal expansion along the c-axis. However, the volume expansion in both methods was found to be positive. The coefficient of thermal expansion along the a-axis was significantly larger than the one along the c-axis giving a net positive volume expansion.

Characterization of 60 mm AlN Single Crystal Wafers Grown by the Physical Vapor Transport Method

Physical Vapor Transport Crack-free bulk AlN single crystals up to 60 mm in diameter are grown for the first time by the physical vapor transport method. The demonstrated entire wafer exhibits excellent ultraviolet transparency with absorption coefficients of 14–21 cm−1 in the range 260–280 nm, and the Raman spectra show an E2(high) FWHM of 2.86 cm−1. More details can be found in article number 1900118 by Qikun Wang, Jason Wu, and co-workers.

Analysis of the Basal Plane Dislocation Density and Thermomechanical Stress during 100 mm PVT Growth of 4H-SiC.

Basal plane dislocations (BPDs) in 4H silicon carbide (SiC) crystals grown using the physical vapor transport (PVT) method are diminishing the performance of SiC-based power electronic devices such as pn-junction diodes or MOSFETs. Therefore, understanding the generation and movement of BPDs is crucial to grow SiC suitable for device manufacturing. In this paper, the impact of the cooldown step in PVT-growth on the defect distribution is investigated utilizing two similar SiC seeds and identical growth parameters except for a cooldown duration of 40 h and 70 h, respectively. The two resulting crystals were cut into wafers, which were characterized by birefringence imaging and KOH etching. The initial defect distribution of the seed wafer was characterized by synchrotron white beam X-ray topography (SWXRT) mapping. It was found that the BPD density increases with a prolonged cooldown time. Furthermore, small angle grain boundaries based on threading edge dislocation (TED) arrays, which are normally only inherited by the seed, were also generated in the case of the crystal cooled down in 70 h. The role of temperature gradients inside the crystal during growth and post-growth concerning the generation of shear stress is discussed and supported by numerical calculations.

Fluorescence properties of β-perylene crystals prepared by a physical vapor transport method under atmospheric pressure

We selectively prepared a metastable phase of perylene crystals (β-phase) by a physical vapor transport method under atmospheric pressure. The fluorescence lifetime (τF) was determined to be 12.3 ns and the annihilation rate constant (γss) of singlet exciton was estimated to be 3 × 10−10 cm3 s−1 from fluorescence decay profile at high excitation density. The fluorescence quantum yield (ΦF) was determined to be 0.6 and ΦF decreased dramatically with decreasing wavelength below 300 nm, suggesting that fission from a highly excited singlet state into two triplet states occurred. The radiative lifetime τR (=τF/ΦF) was 20 ns, which indicates that the fluorescence originated from the excimer with nonparallel configuration in the crystals.

Characterization of 60 mm AlN Single Crystal Wafers Grown by the Physical Vapor Transport Method

Crack‐free bulk AlN single crystals up to 60 mm in diameter are successfully grown for the first time using a series of proprietary techniques by the physical vapor transport method. The single crystals are sliced into on‐axis (±0.2°) wafers and then lapped/polished following common wafering standards. The obtained wafers are characterized by Raman spectroscopy and high‐resolution X‐ray diffraction (HRXRD). The Raman spectra show an E2(high) full width at half maximum (FWHM) of 2.85–2.87 cm−1. The symmetric and asymmetric HRXRD rocking curves show FWHMs of 172–288 and 103–242 arcsec, respectively. The optical transmission spectra reveal that the entire wafers exhibit excellent ultraviolet (UV) transparency with absorption coefficients of 14–21 cm−1 in the UV range 4.43–4.77 eV (260–280 nm). The average etch pit density (EPD) determined by preferential chemical etching is about 2.3 × 105 cm−2. The major impurities determined by evolved gas analysis and glow discharge mass spectrometry are carbon at 7.4 × 1018 cm−3 (45 ppmw), oxygen at 1.2 × 1019 cm−3 (100 ppmw), and silicon at 6.8 × 1017 cm−3 (9.7 ppmw). The usable area of the 60 mm wafers exceeds 98%.

Investigating the single crystal OFET and photo-responsive characteristics based on an anthracene linked benzo[b]benzo[4,5]thieno[2,3-d]thiophene semiconductor

Abstract Organic single crystals have great potential in optoelectronic devices owing to their grain-boundary free nature and long exciton diffusion length. In this report, an anthracene linked benzo[b]benzo[4,5]thieno[2,3-d]thiophene semiconductor, 2-An-BTBT was designed and synthesized. Belt like organic single crystals based on 2-An-BTBT were grown via physical vapor transport method. Using an ultrathin wire as the shadow mask, organic field effect transistor (OFET) with a channel length of around 15 μm was fabricated and the highest mobility of 0.9 cm2V−1s−1 and average mobility of 0.58 cm2/V were obtained. Furthermore, photo-response characteristics of the 2-An-BTBT based device were also investigated. Photo-response of as high as 7136 AW-1 was achieved via 17.7 μW/cm2 @380 nm mono wavelength light illumination which is ascribed to the long exciton diffusion length of organic single crystal and ultra-short channel length of the fabricated OFET. Stable switching behavior with rising time and decline time of several seconds is attained which indicates the existence of shallow traps at the insulator/semiconductor interface.

High thermal conductivity of pure dense SiC ceramics prepared via HTPVT

Pure dense silicon carbide (SiC) ceramics were obtained via the high-temperature physical vapor transport (HTPVT) method using graphite paper as the growth substrate. The phase composition, the evolution of microstructure, the thermal diffusivity and thermal conductivity at RT to 200∘C were investigated. The obtained samples had a relative density of higher than 98.7% and a large grain size of 1[Formula: see text]mm, the samples also had a room-temperature thermal conductivity of [Formula: see text] and with the temperature increased to 200∘C, the thermal conductivity still maintained at [Formula: see text].

Investigation of the 6-Folded Pattern in the Facetted Region of 4° Off-Axis 4H-SiC

We demonstrate 4o off-axis 4H-SiC bulk crystal growth using physical vapor transport method (PVT). A radial 6-folded pattern is discovered in the facetted region of the growth surface by differential interference contrast microscopy. SICA88 Wafer Inspection/Review System was used to further inspect such 6-folded pattern with photoluminescence channel for wafers cut from that 4H-SiC crystal. It was found that the 6-folded pattern exists in all the wafers from the bottom to the top of that crystal. Several radial 6-folded patterns are found in the bottom wafer near the seed, but there is only one in the other two wafers. Defects distribution of these wafers after etched were also researched to find out the relationship between the radial 6-folded pattern and the defects. Based on the results, the formation and evolution mechanism of the radial 6-folded pattern on 4o off-axis 4H-SiC crystal is discussed.

Optimization of total resolved shear stress in AlN single crystals homoepitaxially grown by physical vapor transport method

We investigate the distribution and evolution of the total resolved shear stress (TRSS) in AlN single crystals homoepitaxially grown by the physical vapor transport method based on an in-house three-dimensional thermal-elastic stress code. The thermal stresses of three primary slip systems in 2H-AlN crystals are modeled. The simulation results show that the magnitude of the TRSS increases tremendously after the crystal reaches its critical thickness δc at 6 mm under our specific hotzone and growth conditions. The location of the maximum TRSS transitions from the bottom surface of the crystal to the peripheral region beneath the seed crystal during the growing process. The influences of the power ratio between two resistant heaters on the TRSS and the growth rate are also investigated. The results suggest that a high growth rate could be maintained until the crystal thickness reaches δc and that the temperature gradient inside the growth chamber should be decreased during the later stages by adjusting the power ratio to reduce the overall TRSS and defect propagation. The TRSS could be reduced by 93.31% by rapidly decreasing the temperature differences in AlN crystals immediately after the growth process.

High-aspect-ratio single-crystalline AlN nanowires: Free-catalytic PVT growth and field-emission studies

Nanowires (NWs) with high aspect ratios (HARs) have great advantages for the fabrication of nanodevices. Herein, a high-efficiency and simple physical vapor transport (PVT) method is utilized to synthesize the uniform HAR aluminum nitride (AlN) NWs on a tungsten substrate without any catalysts. Synergistic effect of high surface energy of (0001), low saturated vapor pressure and large axial temperature gradient leads to the growth of HAR AlN NWs, which provides new insight for the growth of low-dimensional AlN nanostructures. The as-obtained AlN NWs with super HAR have hexagonal wurtzite structure, the diameters are about 100 nm and the lengths are over 200 μm. The AlN NWs have an intensive deep ultraviolet (DUV) absorption peak at 5.94 eV and exhibit a relatively high electrical conductivity (1.29 × 10−3 Ω−1 cm−1), low turn-on field (6.2 V μm−1) and threshold field (8.5 V μm−1). These results indicate that PVT method is efficient to fabricate HAR AlN NWs and the AlN NWs not only play an important role in DUV photoelectric devices but also have tremendous potential as a candidate for field-emission nanodevices.

The Physical Vapor Transport Method for Bulk AlN Crystal Growth.

In this report, the development of physical vapor transport (PVT) methods for bulk aluminum nitride (AlN) crystal growth is reviewed. Three modified PVT methods with different features including selected growth at a conical zone, freestanding growth on a perforated sheet, and nucleation control with an inverse temperature gradient are discussed and compared in terms of the size and quality of the bulk AlN crystals they can produce as well as the process complexity. The PVT method with an inverse temperature gradient is able to significantly reduce the nucleation rate and realize the dominant growth of only one bulk AlN single crystal, and thus grow centimeter-sized bulk AlN single crystals. X-ray rocking curve (XRC) and Raman spectroscopy measurements showed a high crystalline quality of the prepared AlN crystals. The inverse temperature gradient provides an efficient and relatively low-cost method for the preparation of large-sized and high-quality AlN seed crystals used for seeded growth, devoted to the diameter enlargement and quality improvement of bulk AlN single crystals.

Sublimation growth and property characterization of p-type 4H-SiC by Al[sbnd]B co-doping technique

Silicon carbide (SiC) is a wide bandgap semiconductor with promising application prospect in high-voltage power electronics devices. Herein, p-type 4H-SiC boules without foreign polytype inclusions were grown by physical vapor transport method using AlB co-doping technique. The concentrations of Al and B were measured to be 2.0 × 1019, 4.7 × 1017 cm−3, respectively. The small lattice distortion was verified by X-ray diffraction and quantum simulation methods. Furthermore, the polytype stabilization mechanism was clarified and electrical properties were investigated. This study shed light on growing p-type SiC for high-voltage power electronics devices.

Centimeter size BiSeI crystal grown by physical vapor transport method

On the purpose to get large sized BiSeI crystals for diverse applications based on its physical properties, such as thermoelectric, photoelectric and ferroelectric, etc. We developed a synthesis process to obtain pure BiSeI polycrystal, which was used as source material for further physical vapor transport (PVT) growth of BiSeI single crystal. Strip-shaped BiSeI single crystal with the size of ∼80 × 4 × 0.5 mm3 was obtained. Needle-like and layered microstructures with homogeneous composition were observed in BiSeI crystal by the SEM. The band-gap of as-grown BiSeI crystal was estimated to be ∼1.29 eV according to the UV–Vis diffuse reflectance spectra. Hall measurement shows that BiSeI single crystal is n-type conductivity with the anisotropic resistivity of 39.3, 3.4, and 1650.4 Ω·cm along a-, b- and c-axis, respectively.

Morphological and Optical Characteristics of Transition Metal Doped PVT Grown Zinc Selenide Single Crystal

AbstractAn experimental study on the absorption, emission, crystal quality, and morphological characteristics of the transition metal doped zinc selenide (ZnSe) crystals is performed. This study focuses on the effect of dopant and thermal convection on crystal characteristics. Two ZnSe crystals, one doped with Fe2+ and the other with Cr2+, are grown using the physical vapor transport method. The bulk crystal samples are further divided into localized zones and overall crystallinity is evaluated using optical transparency, scanning electron microscopy, and X‐ray diffraction analyses. Gross defects, such as large precipitates, inclusions and voids, are not observed. The radial segregation and its effect on morphology and optical fluorescence and emission are studied for both doped crystals using different spots along the radius for both crystals. The absorption and emission properties are investigated and the results are discussed in terms of the energy levels from which the optical transitions occur.

Electrical properties of x-ray detector based on bismuth tri-iodide single crystal with electrode configuration considering

Bismuth tri-iodide (BiI3) single crystals for x-ray detectors application were grown by using physical vapor transport method. Detectors with different electrode configuration were fabricated for electrical and x-ray detection performance measurements. Electrical anisotropy including resistivity, dark current, carrier transport and x-ray induced photoelectricity properties was obtained. Carrier scattering effect deriving from lattice structure, combination bond, dislocation and stacking fault is responsible for the anisotropy properties. Dark current of the detectors with electrical field parallel (E//c) and perpendicular (E ⊥ c) to crystal c-axis were obtained as 0.5 pA and 2.1 pA, respectively. A remarkable signal-to-noise ratio of 896.4 and the sensitivity up to 0.526 × 104 μC Gy−1 cm−2 are achieved in E ⊥ c-axis configuration under 0.02 V μm−1 DC electric field and x-ray exposure dosage rate of 489.78 μGy h−1.

Structural and vibrational properties of PVT grown BiTeCl microcrystals

High-quality BiTeCl microcrystals are grown by the physical vapor transport (PVT) method without using a foreign transport agent. The plate-like microcrystals with a developed (0001) surface are up to ∼500 μm in diameter. The grown crystal phase composition was identified by the x-ray single crystal structure analysis in space group P63mc: a = 4.2475(6) Å, c = 12.409(2) Ǻ, Z = 2 (R = 0.0343). The BiTeCl microcrystal phase purity was verified by Raman microspectrometry under excitation at λ = 632.8 and 532.1 nm.

Photoluminescence properties of N and B codoped fluorescent 4H-SiC and 6H-SiC single crystals

In this paper, N and B codoped 4H-SiC and 6H-SiC single crystals were prepared by physical vapour transport method and their photoluminescence properties were studied. The photoluminescence spectra, dopant concentrations, Raman spectra, and transmission spectra of these obtained crystals were characterized. It is observed that the fluorescent crystals radiate a warm white light that covers a wide band spectrum from 450 nm to 750 nm when they are excited by a 325 nm laser. The doped 4H-SiC single crystal presents a higher photoluminescence intensity and larger spectral band compared to that of doped 6H-SiC single crystal under the similar growth and measurement conditions. Further analyze by doping concentration and transmission spectra indicate that the photoluminescence property is strongly influenced by the N donor and B acceptor impurity level.

Preparation and characterization of free-standing BiI3 single-crystal flakes for X-ray detection application

Free-standing BiI3 single-crystal flakes with a rhombohedral appearance and 10–100 µm in thickness were prepared by using physical vapor transport method. The (00l) (l = 3, 6, 9, 12) planes of the as-grown crystal present a distribution of layered structure and approximatively atomically smooth surface with 1.3–1.7 nm step height and roughness average 0.38–0.65 nm. Both planar and coplanar electrode configuration devices for X-ray detection were fabricated with the as-grown BiI3 single crystals and the dark resistivity 5.8–6.4 × 1011 Ω cm and 1.2–1.8 × 1011 Ω cm at room temperature were obtained, respectively. A low dark current or high resistivity for planar device is on account of carrier scattering from the I–Bi–I interlayer van der Waals bonding interface. A high net photocurrent and good sensitivity 1.22–1.36 × 104 µC/Gyair/cm2 under X-ray excitation were also obtained with planar device owing to the uniform electric field distribution and high charge collection efficiency.