Physical Vapor Transport Method Research Articles

Broadband White-Light Emission from Alumina Nitride Bulk Single Crystals

Alumina nitride bulk single crystals (AlN BSCs) were grown using a two-heater Physical Vapor Transport (th-PVT) method. The crystal contains massive lattice defects including aluminum vacancy (VAl) and oxygen substitution (ON). The photoluminescence (PL) spectrum of the crystal demonstrated a broad emission covering from 250 to 1000 nm. By study the PL spectrum, abundant midgap states in the wide band gap of AlN were nailed down. Based on the crystals, metal-AlN-metal Schottky devices were fabricated. These devices emitted high quality white light with color rendering index (CRI) over 90 under a bias of 60 V. Moreover, it was found that the white light emitting property of AlN BSCs was adjustable through changing impurity density and device structure. This research aims to pave a new way for solid-state white light source.

Crystal growth and evaluation of nitrogen and aluminum co-doped N-type 4H-SiC grown by physical vapor transport

N-type 4H-SiC crystals were grown by the physical vapor transport (PVT) method with nitrogen and aluminum (N–Al) co-doping. By using aluminum carbide powder preannealed in nitrogen gas atmosphere as an aluminum doping source, we obtained highly N–Al co-doped crystals with a nitrogen concentration higher than that in nitrogen-only-doped crystals. The dislocation densities of N-Al co-doped crystals with a high aluminum concentration (>1 × 1019 cm−3) were found to become higher than those with a low aluminum concentration (<1 × 1019 cm−3). Moreover, we investigated the expansion velocities of double Shockley-type stacking faults (DSFs) in the N–Al co-doped and the nitrogen-only-doped crystals. We found that the DSF expansion velocities in the N–Al co-doped crystals were lower than those in the nitrogen-only-doped crystals. This difference in the DSF expansion velocity is discussed with respect to the quantum well action model.

Structural Characterization of the Growth Front of 4H-SiC Boules Grown Using the Physical Vapor Transport Growth Method

The defect structure at the growth front of 4H-SiC boules grown using the physical vapor transport (PVT) method has been investigated using high resolution x-ray diffraction and x-ray topography. The crystal parameters such as the c-lattice constant exhibited characteristic variations across the growth front, which appeared to be caused by variation in surface morphology of the as-grown surface of the boules rather than the defect structure underneath the surface. X-ray topography also revealed that basal plane dislocations are hardly nucleated at the growth front during PVT growth of 4H-SiC crystals.

Epitaxial growth of γ-InSe and α, β, and γ-In2Se3 on ε-GaSe

We demonstrate that γ-InSe and the α, β and γ phases of In2Se3 can be grown epitaxially on ε-GaSe substrates using a physical vapour transport method. By exploiting the temperature gradient within the tube furnace, we can grow selectively different phases of InxSey depending on the position of the substrate within the furnace. The uniform cleaved surface of ε-GaSe enables the epitaxial growth of the InxSey layers, which are aligned over large areas. The InxSey epilayers are characterised using Raman, photoluminescence, x-ray photoelectron and electron dispersive x-ray spectroscopies. Each InxSey phase and stoichiometry exhibits distinct optical and vibrational properties, providing a tuneable photoluminescence emission range from 1.3 eV to ~2 eV suitable for exploitation in electronics and optoelectronics.

Anisotropic Three-Dimensional Thermal Stress Modeling and Simulation of Homoepitaxial AlN Single Crystal Growth by the Physical Vapor Transport Method

We develop a thermal-elastic stress model using the finite element method to predict three-dimensional anisotropic stress in AlN single crystals homoepitaxially grown by the physical vapor transport process; we also perform numerical experiments for a 1-in. AlN crystal surrounded by different cone-tube designs and grown along various orientations. The influences of the cone-tube shape and the growth orientation on the stresses inside the AlN crystal are investigated in detail. The simulation results show that the von Mises stress exceeds 1.11 GPa under all specified growth conditions, while the anisotropy is negligible. The resolved shear stresses are strongly dependent on the thermal gradient inside the growing crystal and the growth orientation. Strong anisotropy of the resolved shear stress is observed upon tilting of the growth orientation. The resolved shear stress along {0001}⟨112̅0⟩ primary slip system reveals that the c-axis growing crystal is under tensile stress along all three primary slip directions. Nevertheless, an inversion of the resolved shear stress from tensile to compressive along the −a3 slip direction is observed when changing the growth orientation. The total resolved shear stress shows 6-fold symmetry, reflection symmetry and 2-fold symmetry along [001], [10√3], and [100] growth orientations, respectively.

Mapping of Lattice Strain in 4H-SiC Crystals by Synchrotron Double-Crystal X-ray Topography

The presence of lattice strain in n-doped 4H-SiC substrate crystals grown by a physical vapor transport method can strongly influence the performance of related power devices that are fabricated on them. Information on the level and the variation of lattice strain in these wafer crystals is thus important. In this study, a non-destructive method is developed based on synchrotron double-crystal x-ray topography to map lattice strains in 4H-SiC wafers. Measurements are made on two 4H-SiC substrate crystals—one is an unprocessed commercial wafer while the other was subject to a post-growth high-temperature heat treatment. Maps of different strain components are generated from the equi-misorientation contour maps recorded using synchrotron monochromatic radiation. The technique is demonstrated to be a powerful tool in estimating strain fields in 4H-SiC crystals. Analysis of the strain maps also shows that the normal strain components vary much more significantly than do the shear/rotation components, indicating that lattice dilation/compression rather than lattice tilt is the major type of deformation caused by both the incorporation of nitrogen dopants and the nucleation of basal plane dislocations.

Nanomorphology and performance of pure and doped lead selenide for infrared detector

Lead selenide (PbSe) has been studied as a promising material for room temperature midwave infrared detection. We have investigated pure PbSe, as well as tin (Tn) and cadmium (Cd)-doped PbSe, nanocrystalline materials produced using physical vapor transport methods on glass and high-resistivity silicon substrates. The morphologies were investigated by scanning electron microscopy and energy-dispersive x-ray analysis. Pure PbSe layers consisted of nanocrystals that change into cubes and cuboids upon annealing. Cuboids generally grew in [100] orientation and ultimately developed in nanorods. Growth on silicon and glass substrates showed different morphologies of pure PbSe material. Parabolic and elongated morphologies resulted in nanowires on the top of thin layers of PbSe nanofilm, which acted as the substrate. Under low gradient annealing conditions (<20 K/cm), elongated morphologies grew into nanorods. Annealing of these samples resulted in coarse nanomorphologies with higher resistivity. In the case of Tn-doped PbSe, annealing dissolved a Tn-rich phase observed in as-grown films. Cd- and iodine-doped films produced through the addition of Cd selenide and Cd iodide, respectively, showed higher resistivity than similarly treated pure PbSe films. Annealing of as-grown materials in the presence of oxygen or iodine showed increased resistivity and significant changes in optical characteristics.

Growth and Temperature-Depending Raman Characterization of Different Nitrogen-Doped 4H-SiC Crystals

Different nitrogen-doped 4H-SiC single crystals were grown by the physical vapor transport method through mixing nitrogen gas to the argon growth atmosphere in the composition range from 0% to 10%. The electrical properties of the crystals, including resistivity and mobility were measured by Hall effect and contactless eddy current measurements. The Raman spectra of different N-doped 4H-SiC single crystals were investigated from 173 to 473 K. The temperature and doping dependence of optical modes was analyzed with anharmonic effect. The phonon lifetime was derived from the linewidths of Raman spectra via the energy-time uncertainty relation.

Reduction of Dislocation Density in Bulk Silicon Carbide Crystals Grown by PVT on Profiled Seeds

The distribution of extended defects in silicon carbide (SiC) crystals grown on profiled seeds by the sublimation (physical vapor transport) method has been studied by optical microscopy in combination with chemical etching. It is established that free lateral growth on protruding relief elements (mesas) is accompanied by a sharp decrease in the density of threading dislocations and micropipes. The decreased density of dislocations is retained after growing a thick layer that involves the overgrowth of grooves that separated individual mesas.

PVT growth of exfoliated CdZnTe polycrystalline thick films based on stress mismatch mechanism

The exfoliated CdZnTe (CZT) films were grown on quartz substrates by closed tube physical vapor transport method based on stress mismatch mechanism. The structure and morphology of the exfoliated CZT thick films were characterized by X-ray diffraction and scanning electron microscope. The as-grown CZT films are polycrystalline with (111) preferential orientation and exhibit a dense pyramid surface structure and a thickness of more than 600 μm. The coplanar and planar X-ray detectors were fabricated based on the exfoliated CZT film. The discrepant dark resistivity is 3.17 × 109 and 5.14 × 1010 Ω cm with regard to the coplanar and planar structure detector, respectively. The planar structure detector exhibits higher quantum efficiency at low doses X-ray radiation.

Growth of P-type 4H–SiC single crystals by physical vapor transport using aluminum and nitrogen co-doping

P-type 4H–silicon carbide (SiC) crystal growth has been achieved by physical vapor transport using aluminum and nitrogen co-doping. Aluminum carbide with a two-zone heating furnace was used for p-type doping, and yielded homogenous aluminum doping during SiC crystal growth by physical vapor transport. The 4H–SiC polytype with high-aluminum doping was unstable, but aluminum–nitrogen co-doping improved its stability. We grew p-type 4H–SiC bulk crystals of less than 90mΩcm by using co-doping. Secondary-ion mass spectrometry and Raman spectroscopy showed that the crystal growth of highly doped p-type SiC can be achieved by using the physical vapor transport method.

Growth of Hexagonal AlN Crystalline Microrod by Physical Vapor Transport Method

Hexagonal aluminium nitride (AIN) microrods with high crystalline quality were grown by physical vapor transport (PVT) method at low growth temperature between 1700 and 1850 degrees C. The length of as-grown microrod is around 1 cm, and the width between 200-400 mu m. The microrod exhibits typical hexagonal geometrical shape with pale yellow color under optical microscopy. Scanning electron microscope (SEM) and atomic force microscope (AFM) images show each microrod with closely arranged step waviness, of which the step interval is 2-4 mu m and the height several nanometers. Raman spectrum characterization showed characteristic peaks of high crystalline AlN. The rod-like structure is attributed to slow growth velocity at lower crystalline temperature, enabling Al and N atoms having enough time to move to the lower energy site and to form heiagonal microrod along direction. High quality hexagonal AlN microrod is an enrichment to one-dimensional semiconductor materials. Data from this study suggest that, by further study on size and impurity control, high performance miniaturized opto-electronic device is hopeful to be achieved.

Luminescence of Donor-acceptor-pair in Fluorescent 4H-SiC Doped with Nitrogen, Boron and Aluminum

Fluorescent 4H-SiC, co-doped with donor and acceptor impurities, can work as phosphor for visible light emission by recombination. Donor concentration and acceptor impurities are two important factors which influence luminescent properties of fluorescent 4H-SiC. In this study, 3 inch N-B-Al co-doped 4H-SiC crystals were prepared by physical vapor transport method. Crystalline type and N-B-Al concentration of the doped 4H-SiC were measured by Raman spectrum and secondary ion mass spectra. Influences of doping concentrations on the photoluminescence were studied by photoluminescence excitation and emission spectra, luminescence decay curves, and internal and external quantum efficiencies. It is found that p-type fluorescent 4H-SiC with low Al doping concentration shows intensive yellow-green fluorescence at room temperature. Heavily doped N and B and lightly doped Al in 4H-SiC produce enough electron hole pairs for the recombination. This sufficient recombination enhances internal quantum efficiency of fluorescent 4H-SiC. Furthermore, the recombination is also helpful to increase their photoluminescence intensity and average fluorescence lifetime.

Electrogenerated chemiluminescence logic gate operations based on molecule-responsive organic microwires.

Complex logic gate operations using organic microwires as signal transducers based on electrogenerated chemiluminescence (ECL) intensity as the optical readout signal have been developed by taking advantage of the unique ECL reaction between organic semiconductor 9,10-bis(phenylethynyl)anthracene (BPEA) microwires and small molecules. The BPEA microwires, prepared on cleaned-ITO substrate using a simple physical vapor transport (PVT) method, were subsequently used for construction of the ECL sensors. The developed sensor exhibits high ECL efficiency and excellent stability in the presence of co-reactant tripropylamine. Based on the remarkable detection performance of BPEA MWs/TPrA system, the sensors manifested high sensitive ECL response in a wide linear range with low detection limit for the detection of dopamine, proline or methylene blue, which behaves on the basis of molecule-responsive ECL properties based on different ECL reaction mechanisms. Inspired by this, these sensing systems can be utilized to design OR, XOR and INHIBIT logic gates, which would be used for the determination of dopamine, proline and ethylene blue via logic outputs. Importantly, the individual logic gates can be easily brought together through three-input operations to function as integrated logic gates.

Prismatic Slip in PVT-Grown 4H-SiC Crystals

Basal plane slip is the most frequently observed deformation mechanism in 4H-type silicon carbon (4H-SiC) single crystals grown by the physical vapor transport (PVT) method. However, it was recently reported that dislocations in such crystals can also glide in prismatic slip systems. In this study, we observed nonuniform distributions of three sets of prismatic dislocations in a commercial 4H-SiC substrate wafer. The nonuniformity is a result of the distribution of resolved shear stress on each prismatic slip system caused by radial thermal gradients in the growing crystal boule. A radial thermal model has been developed to estimate the thermal stress across the entire area of the crystal boule during PVT growth. The model results show excellent agreement with the observations, confirming that radial thermal gradients play a key role in activating prismatic slip in 4H-SiC during bulk growth.

Characterization of Basal Plane Dislocations in PVT-Grown SiC by Transmission Electron Microscopy

4H- and 6H-SiC grown by physical vapor transport method were investigated by transmission electron microscopy (TEM). From the TEM diffraction patterns observed along the [11-20] zone axis, 4H- and 6H-SiC were identified due to their additional diffraction spots, indicating atomic stacking sequences. However, identification was not possible in the [10-10] zone axis due to the absence of additional diffraction spots. Basal plane dislocations (BPDs) were investigated in the TEM specimen prepared along the [10-10] zone axis using the two-beam technique. BPDs were two Shockley partial dislocations with a stacking fault (SF) between them. Shockley partial BPDs arrayed along the [0001] growth direction were observed in the investigated 4H-SiC. This arrayed configuration of Shockley partial BPDs cannot be recognized from the plan view TEM with the [0001] zone axis. The evaluated distances between the two Shockley partial dislocations for the investigated samples were similar to the equilibrium distance, with values of several hundreds of nanometers or even values as large as over a few micrometers.

Optical and morphological characteristics of zinc selenide-zinc sulfide solid solution crystals

Experiments were performed to study the effect of point defects on the optical and morphological characteristics of zinc selenide-zinc sulfide ZnSe-ZnS (ZnSexS(1-x)) solid solution crystals grown under terrestrial (1-g) condition. We used the composition ZnSe0.91S0.09 and ZnSe0.73S0.27 for the detailed studies. Crystals of 8 mm and 12 mm diameter were grown using physical vapor transport methods. These crystals did not exhibit gross defects such as voids, bubbles or precipitates. The photoluminescence spectra indicated strong red emission for the 610–630-nm wavelength region in both crystals. This emission could be explained on the basis of high energy irradiation of Zn selenide. For the ZnSe0.73S0.27 crystal, absorption starts at a lower wavelength range (300 nm) when compared to the ZnSe0.91S0.09 crystal presumably due to the much higher bandgap of ZnS than that of ZnSe. Sharp peaks at 451 and 455 nm were observed for both samples corresponding to the band edge transitions, followed by a strong peak at 632 nm. These results were consistent with the observations based on Raman spectroscopy studies. Under 532-nm laser illumination both transverse optical (TO) and longitudinal optical (LO) phonon peaks appeared at Raman shifts of 220 and 280 Δcm−1, respectively. These peaks are similar to those observed for pure ZnSe Raman spectra for which TO and LO occur at 200 and 250 Δcm−1 for the x-axis (first order) polarization.

PVT(Physical Vapor Transport) 법으로 AlN 결정 성장에서 결정립의 성장 거동에 관한 연구

AlN 결정을 PVT(물리 기상 이동법, Physical Vapor Transport) 법으로 성장함에 있어 결정립의 성장 거동을 관찰하였다. 작은 AlN 결정립이 성장한 이후 결정립들은 이웃한 결정립과 합쳐지면서 성장하는 거동을 보였다. 이를 실체현미경을 이용하여 관찰한 결과를 보고하고자 한다. It was observed that the single grain of crystallite growth behavior in AlN (Aluminum Nitride) single crystal growth by PVT (Physical Vapor Transport) method. The single grain of AlN was grown in sequent experiments and adjacent crystallites were joined together after small grain was grown. The sequential results of those grains observed by stereoscopic microscope were reported.

Application of a niching genetic algorithm to the optimization of a SiC crystal growth system

It was demonstrated that a niching genetic algorithm (NGA) could be efficient for the optimization of a SiC crystal growth system. And several design parameters of SiC crystal growth system could be optimized at the same time, and high diversity of population was maintained to obtain global optimization solution by NGA. Firstly, the NGA and thermal models were described and applied for SiC crystal growth by physical vapor transport (PVT) method, and the combination method of NGA and thermal models were presented. Then two cases were carried out to demonstrate the automatic optimization of SiC crystal growth system by NGA. One case was a single-objective optimization problem, in which the axial position of coils was optimized to improve the growth rate of crystal. The another was a multi-objective optimization problem, in which the thickness of substrate holder and input current were optimized for uniform temperature distribution along the growth surface for reducing the thermal stresses in growing crystal. Finally, all the optimization results were analyzed.

(Invited) Study on the Role of Thermal Stress on Prismatic Slip of Dislocations in 4H-SiC Crystals Grown by PVT Method

Basal plane slip is most frequently observed deformation mechanism in 4H type silicon carbon (4H-SiC) single crystals grown by physical vapor transport (PVT) method. It has recently been reported, however, dislocations in such crystals can also glide in the prismatic slip systems. In our study, we have observed non-uniform distributions of three sets of prismatic dislocations in a commercial 4H-SiC substrate wafer. The non-uniformity is likely a result of the distribution of resolved shear stress on each prismatic slip system caused by the radial thermal gradients in the growing crystal boule. A radial thermal model during PVT growth has been developed to estimate the thermal stress across the entire area of the crystal boule. The model makes an excellent agreement with the actual observation, confirming that radial thermal gradients play a key role in activating prismatic slip in 4H-SiC during bulk growth.