Edge-defined Film-fed Growth Research Articles

Large-area total-thickness imaging and Burgers vector analysis of dislocations in β -Ga2O3 using bright-field x-ray topography based on anomalous transmission

Using bright-field x-ray topography based on anomalous transmission (AT), we have demonstrated the first large-area total-thickness imaging of dislocations in β-Ga2O3 at the substrate scale. The dislocation images were acquired from the entire 10 mm × 15 mm × 680 μm (001)-oriented substrate prepared by edge-defined film-fed growth (EFG) by stitching together hundreds of topographic images, each recorded with the forward-diffracted beam in the Laue geometry for g = 020, 0–20, 022, and 400, under the conditions in which AT occurred. Dislocations distributed over the entire crystal volume were imaged as long as their Burgers vectors (b) were not orthogonal to the g-vectors. The results of the g·b analysis of the dislocation contrasts clearly revealed three major dislocation types that were numerically dominant in the EFG crystal: (i) b-axis screw-type dislocations with b∥ξ∥[010] (ξ is the unit vector of line direction), (ii) b-axis edge-type dislocations with b∥[001] and ξ∥[010], and (iii) curved mixed-type dislocations lying on the (001) planes with b∥[010]. Based on their b- and ξ-vectors, types (i) and (ii) were attributed to dislocations that propagated during EFG pulling up along the [010] direction, while type (iii) was attributed to dislocations generated through glide in the [010](001) slip system under stress. The extent to which AT can manifest itself is explained by using the effective absorption coefficient calculated for the above g-vectors based on dynamical x-ray diffraction theory.

Optical and electronic effects of rapid thermal annealing at Ir–Ga2O3 interfaces

Ir is a common impurity in the edge-defined film-fed growth method used to produce high-power Ga2O3 device structures and growth substrates; yet, the effects of this deep level defect on carrier transport and recombination, particularly at high device processing and operating temperatures and at metal interfaces, have yet to be explored. The optical and electronic nature of Ir in Ga2O3 and at Ir/ Ga2O3 interfaces change on a near-nanometer to micrometer scale after nitrogen rapid thermal annealing; the understanding and control of which could lower defect-assisted tunneling, improve diode rectification, and preserve critical electric field strength. Depth-resolved cathodoluminescence spectroscopy, interface electronic current, and capacitance–voltage measurements reveal how this dominant impurity in bulk, edge-fed crystal growth diffuses, evolves with temperature, and impacts defect formation and Schottky barrier formation.

High crystal quality of vertical Bridgman and edge-defined film-fed growth β-Ga2O3 bulk crystals investigated using high-resolution X-ray diffraction and synchrotron X-ray topography

High crystal quality of vertical Bridgman (VB)-grown (001) β-Ga2O3 single-crystal was confirmed in comparison with edge-defined film-fed growth (EFG)-grown (001) β-Ga2O3 single-crystal. The FWHMs of VB were lower than FWHMs of EFG from X-ray diffraction rocking curve analysis. VB and EFG crystal curvatures were measured as 232.24 m and 45.83 m, respectively. By using transmission and reflection geometry in synchrotron X-ray topography, we unambiguously determined the Burgers vector ( b ) = [010] and [001]. For the VB crystal, screw dislocations with b = [010] and dislocation direction ( t ) = [010] were observed. For the EFG crystal, edge dislocations with b = [001] and t = [010] were observed. In addition, wandering dislocations on the (001) plane with b = [010] for EFG and b = [001] for both EFG and VB were observed. These phenomena indicate that the wandering dislocations were formed by a slip motion on the (001) slip plane.

Crystal structure analysis of stacking faults through scanning transmission electron microscopy of β-Ga2O3 (001) layer grown via halide vapor phase epitaxy

A crystal structure analysis of stacking faults (SFs) in a β-Ga2O3 (001) layer grown by halide vapor phase epitaxy (HVPE) was performed using scanning transmission electron microscopy. Etch pits were formed on the surface using alkaline molten salt etching and were used as a fiduciary mark during the analysis. The results revealed three types of defects. These defects were generated in the HVPE-grown layer (propagation from the defects of the β-Ga2O3 substrate grown using the edge-defined film-fed growth method played no role in the generation of these defects). The first type of defect involved SFs on the (−310) plane and resulted from the vacancy of gallium (Ga) atom sites of the GaO4 tetrahedral unit and the GaO6 octahedral unit. The second type involved SFs on the (1−11) plane and resulted from displacement of the GaO6 octahedral unit. The third type was presumed as a ribbon-folded defect with repeated SFs on the (111) and (7−27) planes or the (1−11) and (727) planes.

Effect of substrate orientation on homoepitaxial growth of β -Ga2O3 by halide vapor phase epitaxy

The influence of substrate orientation on homoepitaxial growth of beta-gallium oxide by halide vapor phase epitaxy was investigated. Substrates were cut at various angles Δb from the (001) plane (Δb = 0°) to the (010) plane (Δb = 90°) of bulk crystals grown by the edge-defined film-fed growth method. The growth rate increased with increasing absolute value of Δb near the (001). However, from the (001) to the (010), as Δb increased, the growth rate decreased sharply, and streaky grooves observed in the grown layer on the (001) substrate became triangular pits. The length of the pits decreased with increasing Δb, and a pit-free homoepitaxial layer grew at Δb ≈ 60°. The valley line of the pits was parallel to the [010] direction; therefore, the length of the pits decreased with increasing Δb. In addition, transmission electron microscopy observations of the deepest part of a pit revealed that the pits originate from dislocations propagating in the substrate at an angle of 60° with respect to the (001) plane. Therefore, pits are not formed on the grown layer surface when the Δb of the substrate is ∼60°, because its surface is substantially parallel to the dislocations. The homoepitaxial growth of a pit-free layer on the (011) substrate (Δb = 61.7°) was demonstrated, and void defects and dislocations in the substrate were confirmed by the etch-pit method to not be inherited by the homoepitaxial layer.

Defect energy levels in monoclinic β-Ga2O3

The defect levels in semiconductor are crucial to the performance of related devices. However, the positions of the defect levels in monoclinic β-Ga2O3, such as that for oxygen vacancy (VO), Ga–O divacancy, and self-trapped hole (STH), are still controversial. In this paper, we systematically investigated the photoluminescence and photoluminescence excitation spectra of undoped, Si-doped, and Mg-doped β-Ga2O3 single crystals grown by the edge-defined film-fed growth method. It is found that the oxygen vacancies are deep donors and their levels are related to the coordinations of the vacancies. For each crystal, the recombination between VO and Ga–O divacancy levels is the origin of the blue emission, and the red emission originates from the transition from VO to STH levels. The location of STH level is also related to the geometrical position of oxygen. For the Si-doped crystal, no red emission is observed due to the annihilation of STHs. While a Mg impurity level is produced at ∼1 eV above the valence band maximum, which causes an extra red emission in the spectra of Mg-doped crystal. Our results strongly support the recent theoretical results obtained via first-principle calculations [Q. D. Ho et al., Phys. Rev. B 97 (2018) 115163] and could be benefit for designing β-Ga2O3 related devices.

Anisotropies of angle-resolved polarized Raman response identifying in low miller index β-Ga2O3 single crystal

Beta-phase gallium oxide (β-Ga2O3) with a monoclinic lattice structure is a research hotspot in the field of ultra-wide bandgap semiconductors in the world and displays intriguing anisotropic properties. Here, we synthesize β-Ga2O3 single-crystal (SC) using the edge-defined film-fed growth method and systematically investigate the anisotropic structural and vibrational properties. The anisotropic nature of β-Ga2O3 SC is revealed by angle-resolved polarized Raman spectroscopy under linearly polarized excitations, in which different vibration modes exhibit pronounced periodic variations in intensity. In the detection of different excitation wavelengths, not only the Raman intensity show significant change, but also the strongest main peak of the Raman modes also shows significant difference. In addition, the angular dependence of the peak intensities of the Ag(4), Ag(6), Ag(7), Ag(10) and Bg modes will also be affected by different low Miller index. All the experimental results above are beneficial to the understanding of inelastic light-scattering process of β-Ga2O3 SC.

Proof of concept for the sapphire scalpel combining tissue dissection and optical diagnosis.

The development of compact diagnostic probes and instruments with an ability to direct access to organs and tissues and integration of these instruments into surgical workflows is an important task of modern physics and medicine. The need for such tools is essential for surgical oncology, where intraoperative visualization and demarcation of tumor margins define further prognosis and survival of patients. In this paper, the possible solution for this intraoperative imaging problem is proposed and its feasibility to detect tumorous tissue is studied experimentally. For this aim, the sapphire scalpel was developed and fabricated using the edge-defined film-fed growth technique aided by mechanical grinding, polishing, and chemical sharpening of the cutting edge. It possesses optical transparency, mechanical strength, chemical inertness, and thermal resistance alongsidethe presence of the as-grown hollow capillary channels in its volume for accommodating optical fibers. The rounding of the cutting edge exceeds the same for metal scalpels and can be as small as 110 nm. Thanks to these features, sapphire scalpel combines tissue dissection with light delivering and optical diagnosis. The feasibility for the tumor margin detection was studied, including both gelatin-based tissue phantoms and ex vivo freshly excised specimens of the basal cell carcinomafrom humans and the glioma model 101.8 from rats. These tumors are commonly diagnosed either non-invasively or intraoperatively using different modalities of fluorescence spectroscopy and imaging, which makes them ideal candidates for our feasibility test. For this purpose, fiber-based spectroscopic measurements of the backscattered laser radiation and the fluorescence signals were carried out in the visible range. Experimental studies show the feasibility of the proposed sapphire scalpel to provide a 2-mm-resolution of the tumor margins' detection, along with an ability to distinguish the tumor invasion region, which results from analysis of the backscattered optical fields and the endogenous or exogenous fluorescence data. Our findings justified a strong potential of the sapphire scalpel for surgical oncology. However, further research and engineering efforts are required to optimize the sapphire scalpel geometry and the optical diagnosis protocols to meet the requirements of oncosurgery, including diagnosis and resection of neoplasms with different localizations and nosologies.

Crystal growth and design of Sn-doped β-Ga2O3: Morphology, defect and property studies of cylindrical crystal by EFG

The cylindrical Sn: β-Ga2O3 crystal with high crystalline quality was successfully designed and grown by the innovative edge-defined film-fed growth (EFG) method equipped with a cylindrical Iridium die. The challenges for the growth of Sn: β-Ga2O3 crystals were overcome by optimizing the design of an afterheater. The growth morphology of cylindrical β-Ga2O3 crystal was studied using a theoretical model and the results from experimental crystal growth. The order of importance of growth conditions affecting β-Ga2O3 crystal growth morphology was examined, based on the morphological features of cylindrical β-Ga2O3 crystals obtained by the EFG and Czochralski methods. The iridium inclusions with three shapes were observed in bulk β-Ga2O3 crystal, and the formation mechanism was carefully discussed. The optical bandgap and valence band maximum (VBM) of Sn: β-Ga2O3 crystal were calculated to be 4.74 eV and 3.49 eV by absorption spectra and X-ray photoelectron spectroscopy (XPS), respectively. The corresponding surface barrier height (Φsurf) was 1.25 eV. The carrier concentration of 5.95 × 1018 cm−3 was characterized by capacitance-voltage (C-V) measurement. By the Hall measurement, the carrier mobility and resistivity were estimated to be around 51 cm2 V−1 s−1 and 3.55 × 10–2 Ω cm, respectively.

Control methodology for EFG sapphire crystals

A control methodology is presented for the growth of sapphire crystals by the Edge-defined, Film-fed Growth (EFG) process. This technique is particularly useful for crystals with large cross sections and lengths. To prevent the crystal from sticking, i.e., freezing, to the die, a unique algorithm for calculating the growth rate is described.

Growth of large sapphire crystals: Lessons learned

The article reviews current state of the art in growing large sapphire crystals used for epitaxial substrates of GaN light-emitting diodes and optical windows. Results of crystal growth with the Czochralski, Kyropoulos, heat-exchanger methods, horizontal directional crystallization, and edge-defined film-fed growth are demonstrated and discussed, along with recommendations of useful tools to increase crystal size.

Visualization of the curving of crystal planes in β-Ga2O3 by X-ray topography

The curving of crystal planes (CCP) in semiconductor wafers poses significant challenges for subsequent epitaxial growth and device processing, and must, therefore, be mitigated. This paper demonstrates a method of using the shift of the apparent Bragg angles measured by X-ray topography (XRT) to reconstruct and visualize the three-dimensional shape of curved β-Ga2O3 crystal planes. General geometrical considerations regarding the CCP are presented first, followed by simulations of the CCP using some simple models, including the cylindrical, spherical, ellipsoidal, and saddle-shaped models. Based on the results obtained from the simulations, the method of experimentally deducing the vector field of the surface normal by means of ω-rocking XRT measurements is described, based on which, the shape of the (002) plane in the β-Ga2O3 substrate grown by edge-defined film-fed growth is visualized. If a synchrotron source is unavailable, the method demonstrated here can also be performed using conventional laboratory X-ray diffraction equipment. The visualization of CCP provides crucial feedback for the optimization of crystal growth and wafer fabrication processes.

Investigation on β-Ga2O3 (101) plane with high-density surface dangling bonds

This work studied the structural, thermal, optical and electrical properties of β-Ga2O3 (101) plane with high-density surface dangling bonds for the first time. The high quality (101)-oriented substrate with a full width at half maximum (FWHM) of 90″ in X-ray rocking curve was grown by the edge-defined film-fed growth (EFG) method. The atomic arrangement of (101) surface was terminated either by Ga or O. The dangling bond densities of Ga- and O-terminated surfaces were calculated to be 2.230 × 1015 cm−2 and 2.376 × 1015 cm−2, respectively. The thermal property of the (101) plane was characterized. The Raman active modes of the (101) plane were allowed for selective observation with the variation of polarizing angle. The surface and Schottky barrier height of the (101) plane were estimated to be as low as 1.14 eV and 1.07 eV by optical and electrical measurements, respectively. The activation energy for wet chemical etching was determined to be 0.451 eV for (101) plane and the high etching rate was obtained. By comprehensive comparisons with other common planes, the potential applications of β-Ga2O3 (101) plane should be comparable to the widely used 2̅01 plane.

Edge-defined film-fed growth of incongruent-melting Ce,Ga:GIG crystal with high magneto-optical performance

Ce-doped Gd3Fe5O12 (GIG) crystals, which are a kind of incongruent melting compound, were successfully grown into centimeter size and good quality by an improved edge-defined film-fed growth (EFG) method. Annealing has obviously affected crystal quality, optical transmittance and saturation magnetization. Doping Ce3+ ions greatly enhances the magnetic-optical properties of GIG crystals. For Ce:GIG crystal, a suitable concentration of Ga3+ ion replacing Fe3+ is helpful to improve its magneto-optical properties. The specific Faraday rotation angle of (Ce0.35Gd2.65)(Ga1.04Fe3.95V0.01)O12 crystal is great and reaches 896 Deg. cm−1 that meets commercial requirements.

Young's modulus and corresponding orientation in β-Ga2O3 thin films resolved by nanomechanical resonators

We report on the nondestructive measurement of Young's modulus of thin-film single crystal beta gallium oxide (β-Ga2O3) out of its nanoscale mechanical structures by measuring their fundamental mode resonance frequencies. From the measurements, we extract the Young's modulus in the (100) plane, EY,(100) = 261.4 ± 20.6 GPa, for β-Ga2O3 nanoflakes synthesized by low-pressure chemical vapor deposition (LPCVD), and in the [010] direction, EY,[010] = 245.8 ± 9.2 GPa, for β-Ga2O3 nanobelts mechanically cleaved from bulk β-Ga2O3 crystal grown by the edge-defined film-fed growth (EFG) method. The Young's moduli extracted directly on nanomechanical resonant device platforms are comparable to theoretical values from first-principle calculations and experimentally extracted values from bulk crystal. This study yields important quantitative nanomechanical properties of β-Ga2O3 crystals and helps pave the way for further engineering of β-Ga2O3 micro/nanoelectromechanical systems (M/NEMS) and transducers.

Temperature-dependent Raman and photoluminescence of β-Ga2O3 doped with shallow donors and deep acceptors impurities

Ion doping technology with precise control of doping concentration and configuration can optimize the performance of β-phase gallium oxide (β-Ga2O3) high-power electronic and optoelectronic devices. In this work, shallow-donor Si and deep-acceptor Mg impurities are doped in β-Ga2O3 separately using edge-defined film-fed growth (EFG) method. Laser scanning confocal microscopy and X-ray diffraction analyses show that the as-grown un/Si/Mg-doped β-Ga2O3 substrates have superior qualities such as smooth surface, homogenous phase, and high crystallinity. Raman spectroscopy analysis indicate that the Raman shift and full width at half-maximum (FWHM) has a linear relationship with temperature in the range of 77–297 K. The temperature-variable photoluminescence (PL) spectroscopy suggests that the Si doping introduces less damage to the β-Ga2O3 lattice structure and the Si-doped sample has higher stability at different temperatures, however, the local atomic configurations of Mg impurities can be more easily/significantly affected by temperature. The work can offer an insightful reference to applications of ion-doped β-Ga2O3 optoelectronic devices.

Crystal growth, spectral properties and Judd-Ofelt analysis of Ho:GdScO3 crystal

Ho3+-doped GdScO3 crystal was successfully grown by the edge-defined film-fed growth (EFG) method. The spectroscopic properties of Ho:GdScO3 crystal were investigated at room temperature. The peak absorption cross-sections were 0.33 × 10−20 cm2 and 1.24 × 10−20 cm2 at 1160 nm and 653 nm with full widths at half maximum of 171.7 nm and 11.5 nm. The peak emission cross-sections of 5I7→5I8 and 5I6→5I7 transitions for the crystal were 0.49 × 10−20 cm2 and 1.24 × 10−20 cm2, respectively. The fluorescence lifetimes of the 5I6 and 5I7 multiplets were measured to be 1.12 ms and 6.13 ms, respectively. The results show that Ho:GdScO3 crystal is a very promising material for 3 μm laser operation.

Growth Rate Calculations for Edge‐Defined, Film‐Fed Growth of Sapphire Crystals

AbstractA new model for the calculation of the growth rate of edge‐defined, film‐fed growth (EFG) crystals is proposed based on surface tension proportional to the area of the crystal/die interface. A comparison to the classical model, which includes a surface tension factor proportional to the length of the crystal's peripheral edge, is shown. Equations are derived that accurately predict the growth rate derived from weight versus time calculations. Examples are shown for various geometries of crystal shape.

New near-infrared optical modulator of Co2+:β-Ga2O3 single crystal

As a binary oxide, β-Ga2O3 is a promising host material with high thermal conductivity compared with the traditional laser materials. In this work, Co2+-doped β-Ga2O3 single crystal was successfully grown by the edge-defined film-fed growth (EFG) method and firstly designed as a near-infrared (NIR) optical modulator by doping with cobalt. The doping concentration of Co2+ ions was determined to be 0.029 at.% by X-ray fluorescence. The thermal conductivity was determined to be 13.0 W·m-1·K-1 along a* direction. There were two typical broadband absorption peaks around 1173 nm and 1588 nm related to the transitions of transitions of 4A2(4F)→4T1(4F). By means of Z-scan technique, the third-order nonlinear refractive index of Co2+:β-Ga2O3 was measured to be 9.14×10−13 (esu), which was much larger than that of Cr4+:YAG crystal. It was further successfully employed as a saturable absorber for passively Q-switched lasers at 1342 nm. In the Q-switched regime, the maximum average output power of 35 mW was obtained with the shortest pulse width of 280 ns and repetition rate of 181 kHz. The results indicated that Co2+:β-Ga2O3 crystals have important potential to be used as optical modulators for passive Q-switching laser generation at the NIR band.

Terahertz transmission-mode scanning-probe near-field optical microscopy based on a flexible step-index sapphire fiber

Terahertz scanning-probe near-field optical microscope (THz-SNOM), operating in transmission mode and relying on a flexible sapphire fiber, was developed. For this aim, a 300-μm-diameter step-index sapphire optical fiber with flat ends, high surface, and volume quality was fabricated using the edge-defined film-fed growth technique. For the lowest-order guided mode of such fiber, numerical analysis revealed small-to-moderate THz-wave propagation loss, as well as strongly subwavelength guided mode confinement in a fiber core thanks to a high refractive index of sapphire in the THz range. Experimental setup realizing the THz-SNOM principles was assembled based on thus fabricated fiber, as a probe, a backward-wave oscillator, as a continuous-wave emitter with the output wavelength of λ = 1.2 mm, and a Golay cell, as a THz beam power detector. By combining numerical analysis and experimental study, a subwavelength resolution of our THz-SNOM setup was demonstrated. The resolution depends on orientation of the guided mode polarization toward the image plane, but always remains strongly subwavelengths and overcomes the Abbe diffraction limit. For certain orientations, it can be as low as 0.23λ. In our opinion, thanks to the subwavelength resolution, high-energy efficiency, and flexibility of design, the developed sapphire-fiber-based THz-SNOM holds strong potential in different branches of THz science and technology.