Edge-defined Film-fed Growth Research Articles

Schottky barrier diode based on β-Ga2O3 (100) single crystal substrate and its temperature-dependent electrical characteristics

The Pt/β-Ga2O3 Schottky barrier diode and its temperature-dependent current-voltage characteristics were investigated for power device application. The edge-defined film-fed growth (EFG) technique was utilized to grow the (100)-oriented β-Ga2O3 single crystal substrate that shows good crystal quality characterized by X-ray diffraction and high resolution transmission electron microscope. Ohmic and Schottky electrodes were fabricated by depositing Ti and Pt metals on the two surfaces, respectively. Through the current-voltage (I-V) measurement under different temperature and the thermionic emission modeling, the fabricated Pt/β-Ga2O3 Schottky diode was found to show good performances at room temperature, including rectification ratio of 1010, ideality factor (n) of 1.1, Schottky barrier height (ΦB) of 1.39 eV, threshold voltage (Vbi) of 1.07 V, ON-resistance (RON) of 12.5 mΩ·cm2, forward current density at 2 V (J@2V) of 56 A/cm2, and saturation current density (J0) of 2 × 10−16 A/cm2. The effective donor concentration Nd − Na was calculated to be about 2.3 × 1014 cm3. Good temperature dependent performance was also found in the device. The Schottky barrier height was estimated to be about 1.3 eV–1.39 eV at temperatures ranging from room temperature to 150 °C. With increasing temperature, parameters such as RON and J@2V become better, proving that the diode can work well at high temperature. The EFG grown β-Ga2O3 single crystal is a promising material to be used in the power devices.

Residual stress analysis and bow simulation of crystalline silicon solar cells

The pressure to reduce solar energy costs encourages efforts to reduce the thickness of silicon wafers. Thus, the cell bowing problem associated with the use of thin wafers has become increasingly important, as it can lead to the cracking of cells and thus to high yield losses. In this paper, a systematic approach for simulating the cell bowing induced by the firing process is presented. This approach consists of three processes: (1) the material properties are determined using a nanoidentation test; (2) the thicknesses of aluminum (Al) paste and silver (Ag) busbars and fingers are measured using scanning electron microscopy; (3) non-linear finite element analysis (FEA) is used for simulating the cell bowing induced by the firing process. As a result, the bowing obtained using FEA simulation agrees better with the experimental data than that using the bowing calculations suggested in literature. In addition, the total in-plane residual stress state in the wafer/cell due to the firing process can be determined using the FEA simulation. A detailed analysis of the firing-induced stress state in single crystalline silicon (sc-Si), cast, and edge-defined film-fed growth (EFG) multi-crystalline silicon wafers of different thicknesses is presented. Based on this analysis, a simple residual stress calculation is developed to estimate the maximum in-plane principal stress in the wafers. It is also proposed that the metallization pattern, Ag busbars and fingers screen printed on the front of a solar cell, can be designed using this approach. A practical case of a 3-busbar Si solar cell is presented.

One-step exfoliation of ultra-smooth β-Ga2O3 wafers from bulk crystal for photodetectors

High-quality bulk β-Ga2O3 single crystals have been grown by optimized edge-defined film-fed growth (EFG) method.

Communication—Electrical Characterization of β-Ga2O3 Single Crystal Substrates

We have electrically investigated deep-level defects in unintentionally-doped β-Ga2O3 single crystal substrates fabricated by an edge-defined film-fed growth method, employing capacitance-voltage (C-V) and steady-state photo-capacitance spectroscopy (SSPC) techniques. From C-V measurements, n-type residual carriers with their concentration of ∼8.7 × 1016 cm−3 were almost uniformly distributed over the measured depth region of 188 – 336 nm from the β-Ga2O3 surface. SSPC measurements revealed four dominant deep-level defects located at ∼0.8, ∼2.04, ∼2.71, and ∼3.87 eV below the conduction band, in addition to near-band-edge emissions at ∼4.49 eV. These deep-level defects tended to be more densely present with shallowing the probing depth range.

Concentration and composition of gas inclusions in some oxide crystals

A method of concentration of gas impurities contained in a melt into sealed cavities in a crystal has been proposed for the first time. This makes it possible to determine the amount of gases dissolved in the melt during crystallization by the Edge-defined Film-fed Growth (EGF) technique and the gas pressure in cavities inside the crystals. We also measure the composition of gas inclusions in crystallized melts of Al2O3, Y3Al5O12 and Bi4Ge3O12 and discuss it in connection with crystal growth procedure and quality of crystals.

Structural evaluation of defects in β-Ga2O3 single crystals grown by edge-defined film-fed growth process

We have structurally evaluated β-Ga2O3 crystals grown by edge-defined film-fed growth process using etch pitting, focused ion beam scanning ion microscopy, transmission electron microscopy, and related techniques. We found three types of defects: arrays of edge dislocations corresponding to etch pit arrays on -oriented wafers, platelike nanopipes corresponding to etch pits revealed on the (010)-oriented wafers, and twins including twin lamellae.

High speed growth of SrI2 scintillator crystals by the EFG process

Strontium iodide (SrI2), an important new scintillator crystal having a high light yield and excellent energy resolution, was grown for the first time by the edge-defined film-fed (EFG) growth method. Using high purity starting materials and floating dies made of graphite, fused quartz or AlN, large cylindrical, planar or square cross-section single crystals (12–15mm across and >7cm long) were produced at growth rates up to 15mm/h, significantly faster than the current Bridgman growth technology. Details on the equipment used to grow this deliquescent material and on its growth behavior are given along with some discussion of crystalline quality.

Terahertz Photonic Crystal Waveguides Based on Sapphire Shaped Crystals

In this paper, an ability for highly efficient terahertz (THz) waveguiding in multichannel sapphire shaped crystals is demonstrated. The edge-defined film-fed growth (EFG) technique (or Stepanov technique) of shaped crystal growth has been implemented to manufacture the THz photonic crystalline (PC) waveguide. The PC waveguide has been characterized using both numerical simulations and experimental study. It allows guiding the THz waves in multimode regime with the minimal dispersion in frequency range of $1.0$ – $1.55$ THz and the minimal power extinction coefficient of $0.02$ dB $/$ cm at $1.45$ THz. The mode interference phenomenon has been observed in this waveguide highlighting the prospectives of its use for intrawaveguide interferometry. These results demonstrate the capabilities of combining the EFG/Stepanov technique advantages with unique properties of sapphire, such as relatively low THz-wave absorption, high mechanical, thermal, chemical, and radiation strength, for manufacturing the THz waveguides characterized with low loss and dispersion and suitable for use in wide range of THz technology applications in biomedical and material sciences, including sensing in aggressive environment.

Modeling high speed growth of large rods of cesium iodide crystals by edge-defined film-fed growth (EFG)

A thermocapillary model of edge-defined film-fed growth (EFG) is developed to analyze an experimental system for high speed growth of cesium iodide as a model system for halide scintillator production. The model simulates heat transfer and fluid dynamics in the die, melt, and crystal under conditions of steady growth. Appropriate mass, force, and energy balances are used to compute self-consistent shapes of the growth interface and melt–vapor meniscus. The model is applied to study the effects of growth rate, die geometry, and furnace heat transfer on the limits of system operability. An inverse problem formulation is used to seek operable states at high growth rates by adjusting the overall temperature level and thermal gradient in the furnace. The model predicts that steady growth is feasible at rates greater than 20mm/h for crystals up to 18mm in diameter under reasonable furnace gradients.

Conduction mechanism in highly doped β-Ga2O3 single crystals grown by edge-defined film-fed growth method and their Schottky barrier diodes

Edge-defined fed-grown β-Ga2O3 single crystals with high electron concentration of 3.9 × 1018 cm−3 at 300 K were characterized by Hall effect measurement, and Schottky barrier diodes have been demonstrated. Electron mobility was as high as 74 cm2/(V·s) at 300 K regardless of the high doping concentration. The electron concentration did not change substantially in the low temperature below 160 K. This properties can be explained by the two-band model due to the inter-band conduction. On the Schottky barrier diodes, the rectification characteristics were clearly observed, and the current density of 96.8 A/cm2 at the forward voltage of 1.6 V was obtained.

Deep level defects throughout the bandgap of (010) β-Ga2O3 detected by optically and thermally stimulated defect spectroscopy

Deep level optical spectroscopy (DLOS) and deep level transient spectroscopy (DLTS) measurements performed on Ni/β-Ga2O3 Schottky diodes fabricated on unintentionally doped (010) substrates prepared by edge-defined film-fed growth revealed a rich spectrum of defect states throughout the 4.84 eV bandgap of β-Ga2O3. Five distinct defect states were detected at EC − 0.62 eV, 0.82 eV, 1.00 eV, 2.16 eV, and 4.40 eV. The EC − 0.82 eV and 4.40 eV levels are dominant, with concentrations on the order of 1016 cm−3. The three DLTS-detected traps at EC − 0.62 eV, 0.82 eV, and 1.00 eV are similar to traps reported in Czochralski-grown β-Ga2O3, [K. Irmscher et al., J. Appl. Phys. 110, 063720 (2011)], suggesting possibly common sources. The DLOS-detected states at EC − 2.16 eV and 4.40 eV exhibit significant lattice relaxation effects in their optical transitions associated with strongly bound defects. As a consequence of this study, the Ni/β-Ga2O3 (010) Schottky barrier height was determined to be 1.55 eV, with good consistency achieved between different characterization techniques.

Growth of sapphire and oxide eutectic fibers by the EFG technique

Single-crystal sapphire and oxide binary (Al2O3-Y3Al5O12, Al2O3-Er3Al5O12, Al2O3- GdAlO3) and ternary (Al2O3-ZrO2(Y2O3)) eutectic fibers of 150-300 µm in diameters were grown by the edge-defined film-fed growth (EFG) technique. Microstructure and some properties of fibers were investigated.

Dopant activation in Sn-doped Ga2O3 investigated by X-ray absorption spectroscopy

Doping activity in both beta-phase (β-) and amorphous (a-) Sn-doped gallium oxide (Ga2O3:Sn) is investigated by X-ray absorption spectroscopy (XAS). A single crystal of β-Ga2O3:Sn grown using edge-defined film-fed growth at 1725 °C is compared with amorphous Ga2O3:Sn films deposited at low temperature (<300 °C). Our XAS analyses indicate that activated Sn dopant atoms in conductive single crystal β-Ga2O3:Sn are present as Sn4+, preferentially substituting for Ga at the octahedral site, as predicted by theoretical calculations. In contrast, inactive Sn atoms in resistive a-Ga2O3:Sn are present in either +2 or +4 charge states depending on growth conditions. These observations suggest the importance of growing Ga2O3:Sn at high temperature to obtain a crystalline phase and controlling the oxidation state of Sn during growth to achieve dopant activation.

Investigations of thermoluminescence properties of multicrystalline LiF: Mg, Cu, Si phosphor prepared by edge defined film fed growth technique

Thermoluminescence (TL) properties of LiF: Mg, Cu, Si phosphor prepared in multicrystalline form using edge defined film fed growth (EFG) technique has been investigated. The effect of preparation route on TL properties and thermal stability has been studied. To improve the TL dosimetry properties, phosphor is subjected to different annealing temperatures ranging from 250 °C to 450 °C. The shape of the glow curve structure and peak temperature remains similar at different annealing temperatures, however peak intensities vary. The consistency in the glow curve structure with annealing temperature elucidate that TL trapping states are stable in nature. Thermal annealing at 300 °C for 10 min gives maximum TL intensity with main dosimetry peak at 209 °C. The TL intensity of the main dosimetry peak is increased by a factor of five as compared to as-grown crystal. The thermal stability of LiF: Mg, Cu, Si is found to be better than LiF: Mg, Cu, P. Trapping parameters are calculated to have an insight study of defect states. A simple glow curve structure, tissue equivalency, thermal stability, low residual signal, linear response and reusability makes LiF: Mg, Cu, Si a suitable phosphor for radiation therapy, radio diagnostics and personnel dosimetry applications.

Investigation of liquid oxide interactions with refractory substrates via sessile drop method

The wetting of refractory substrates by oxide alloys has been investigated using a high-temperature sessile drop bench. The aim of this study was to select the best crucible material for directional solidification processes such as the Bridgman, edge-defined film-fed growth or micro-pulling down methods. Contact angles and surface tension were determined via high-resolution imaging. The O2 partial pressure in the furnace atmosphere was also measured. Alumina and a eutectic ceramic called A/YAG/Z (Al2O3/Y3Al5O12/ZrO2) were studied by investigating the following systems: alumina/molybdenum, alumina/tungsten and alumina/iridium; A/YAG/Z–molybdenum, A/YAG/Z–tungsten and A/YAG/Z–iridium. The results show intermediate wetting for both oxides (30° < θ <50°). This is an original result for A/YAG/Z. Several previous studies reported good wetting (7° < θ <20°) for alumina. Accurate surface tension values could not be obtained by image processing. Therefore, the Wilhelmy method was used, giving γ = 0.63 ± 0.03 N m−1 for alumina, which is comparable to the value of 0.67 ± 0.03 N m−1 reported in the literature. The surface tension obtained for A/YAG/Z was 0.71 N m−1, but this value was influenced by the approximate density used. A metal-like layer was observed on the apex of the drops after solidification. According to an SEM–EDS analysis, most of these layers were composed of molybdenum, tungsten and oxygen. Based on thermodynamic calculations, a mechanism which takes into account the oxidation of Mo parts and the dissolution of oxides in the drop is proposed.

Influence of relative positions between RF coil and crucible on sapphire crystals by edge-defined film-fed growth (EFG) technique

To obtain the stable temperature field required for growing sapphire crystals, the influence of relative positions between RF coil and crucible on the performances of sapphires produced by edge-defined film-fed growth (EFG) technique was investigated. For comparison, the crucible was located at the top (case A) and the middle (case B) of the RF coil, respectively. Furthermore, the lattice integrities were studied by the double-crystal X-ray diffraction, and the dislocations were observed under the optical microscope and atomic force microscope after corroding in molten KOH at 390 °C. The crystals in case B exhibit better lattice integrity with smaller full width at half maximum of 29.13 rad·s, while the value in case A is 45.17 rad·s. The morphologies of dislocation etch pits in both cases show typical triangular symmetry with smooth surfaces. However, the dislocation density of 2.8×104 cm-2 in case B is only half of that in case A, and the distribution is more uniform, compared to the U-shaper in case A.

High-mobility β-Ga2O3() single crystals grown by edge-defined film-fed growth method and their Schottky barrier diodes with Ni contact

High-Hall-electron-mobility and high-performance Schottky barrier diodes for edge-defined fed-grown () β-Ga2O3 single crystals have been demonstrated. A high electron mobility of 886 cm2/(V·s) at 85 K was obtained. By theoretical specific scattering mechanisms, it was found that the electron mobility for >200 K is limited by optical phonon scattering and that for <100 K by ionized impurity scattering. On Schottky barrier diodes with Ni contacts, the current density for the forward voltage was 70.3 A/cm2 at 2.0 V, and a nearly ideal ideality factor of 1.01 was obtained.

Enhanced performance of EFG silicon solar cells by using vapor texturing process

Surface texturing of silicon (Si) wafer is an effective and more lasting technique for improving the conversion efficiency of solar cells by improving light trapping effects compared to hydrogenated silicon nitride (SiNx:H) antireflection (AR) coatings. In this paper, we present the method of vapor texturing for achieving a very low reflectance of edge-defined film-fed growth (EFG) Si surface. Prior to vapor texturing, silicon wafers were textured by alkaline (NaOH) and mixed acidic (HF, HNO3 and DI water) solutions to investigate a uniformly textured surface. Also, we carried out vapor texturing for achieving the minimum reflectance by uniform and homogeneous surface morphology following surface texturing optimized by alkaline or acidic solutions. Using the optimum process conditions, EFG Si solar cell showed conversion efficiency, fill factor, short circuit current density and open circuit voltages as high as 14.1%, 74.7%, 32 mA/cm2 and 588 mV, respectively.

Growth of SrTiO3bulk single crystals using edge-defined film-fed growth and the Czochralski methods

Cylindrical SrTiO3 bulk single crystals with diameters of about 15 mm and lengths between 13 and 50 mm were grown from stoichiometric melts by the edge-defined film fed growth (EFG) method. Typical average etch pit density values are in the range of 1.7–2.0 × 105 cm−2. Rocking curve measurements revealed FWHM values between 28′′ and 41′′. Suitable conditions for the growth of puck-shaped single crystals were also found for the Czochralski (Cz) method. However, the structural quality was lower as compared to crystals grown by the EFG method. So far pulling SrTiO3 crystals from the melt using the Czochralski method suffered from growth instabilities like diameter fluctuation, foot formation and subsequent spiraling immediately after the seeding stage. We achieved crystals with diameters up to 42 mm and lengths of 15 mm.

Fabrication of Al&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;/YAG eutectic ceramics utilizing a fast combustion reaction heating method

Al2O3/Y3Al5O12(YAG) eutectic ceramics are considered to be candidate materials for use in high temperature and oxidizing environments owing to their remarkable thermodynamic stabilities. Many techniques have been used to prepare eutectic ceramics, including the Bridgeman, edge-defined film-fed growth, micro-pulling-down, floating zone, and laser zone re-melting methods. However, these methods use complex high temperature equipment. In this work, simple equipment was used to achieve combustion reaction heating with the advantages of a high heating rate (2000℃/min), high temperature, and high cooling rate. Using this set-up we could melt and fabricate eutectic ceramics. The microstructure and physical properties of the obtained Al2O3/YAG eutectic ceramics were studied. The eutectic ceramics consisted of Al2O3 and YAG phases. The two phases exhibited coupled growth with well-matched and clean phase boundaries. Several special morphologies were obtained besides the typical Chinese script microstructure, and their formation mechanisms are discussed. The Vickers hardness of the eutectic ceramics was 20.52 GPa, which is higher than that previously reported for any binary eutectic ceramic system. The fracture toughness was 2.64 MPa m1/2, which is slightly higher than about 2 MPa m1/2 previously reported. The crack propagated in a straight line from the indentation corner and did not deflect at the interface between the Al2O3 and YAG domains, compared with the intergranular fracture in the hot-pressed sample. This weak interaction of the crack path with the microstructure was induced by the absence of residual stress and the excellent bonding between the eutectic phases. Samples treated at 1500℃ for 20 h exhibited no weight loss or obvious change in microstructure, indicating the high temperature stability of these eutectic ceramics.