Optical Floating-zone Research Articles

Study of optical float-zone grown gallium oxide Schottky barrier diode

β−Ga2O3 based lateral Schottky barrier diodes are fabricated using Ni/Au as Schottky contact and Ti/Au as Ohmic contact. The Sn-doped β−Ga2O3 sample is grown by the optical float-zone technique. The effect of trenches, which are used for mesa isolation, on breakdown voltage is investigated. The device shows near-ideal characteristics in terms of built-in potential. The parasitic series and shunt resistances are extracted, and their dependency on temperature is established. Modeling of temperature-dependent reverse leakage current is demonstrated using the thermionic emission model, Poole–Frenkel emission model, and Fowler-Nordheim tunneling mechanism. The procedure to extract relevant parameters is explained in terms of bias and temperature dependency. The combined model shows excellent agreement with experimental data over a wide range of bias and temperature. The maximum electric field of 2.3 MV cm−1 is achieved.

Optical properties of pure and Sn-doped β-Ga2O3 single crystals grown by optical float zone technique

High-quality single crystals of both pure and Sn:β-Ga2O3 single crystals weregrown using the optical floating zone technique and a comprehensive study of its luminescence and scintillation properties was carried out. The pure and Sn:β-Ga2O3 grown crystals were oriented along (100) by Laue diffraction pattern and its monoclinic structure was confirmed using a single crystal XRD. X-ray induced luminescence spectrum shows a maximum emission peak at 365 nm. The cut off wavelength was observed around 258 nm and the optical band gap was calculated to be 4.64 eV from UV–Vis-NIR transmission spectroscopy. The room temperature Raman spectra were recorded and the various vibration modes were investigated. Low temperature (10 to 300 K) and high temperature (325 to 675 K) TL measurements were carried out on X-ray irradiated crystals and its TL kinetic parameters such as activation energy (eV) and frequency factor (S) were calculated. Scintillation decay time profiles were measured for both pure and Sn-doped crystals under 137Cs γ-ray excitation. The calculated scintillation light output is 2300 Ph/5.5 Mev for Sn:β-Ga2O3 crystal under α-particle excitation from 241Am radiation source.

Magnetic Characterization of Zone-Refined Rare-Earth Orthoferrite Crystals Grown by the Optical Floating Zone Method

Magnetic Characterization of Zone-Refined Rare-Earth Orthoferrite Crystals Grown by the Optical Floating Zone Method

Frustrated Magnet Mn3Al2Ge3O12 Garnet: Crystal Growth by the Optical Floating Zone Method

Mn3Al2Ge3O12 is a member of the garnet family of compounds, A3B2(CO4)3, whose magnetic properties are affected by a high degree of geometrical frustration. The magnetic frustration is at the origin of the intriguing magnetic properties that these materials exhibit, such as a long range hidden order derived from multipoles formed from 10-spin loops in the gadolinium gallium garnet, Gd3Ga5O12. Mn3Al2Ge3O12 garnet is isostructural to the thoroughly investigated Gd garnets, Gd3Ga5O12 and Gd3Al5O12. Moreover, in Mn3Al2Ge3O12, the Heisenberg-like Mn2+ magnetic ions (L= 0) are also arranged in corner sharing triangles that form a hyperkagomé structure. The identical crystallographic structures and similar Heisenberg-like behaviour of the magnetic ions make manganese aluminium germanium garnet the closest compound to the gadolinium garnets in its magnetic properties. Here, we report, for the first time, the growth of a large, high quality single crystal of the Mn3Al2Ge3O12 garnet by the floating zone method. X-ray diffraction techniques were used to characterise and confirm the high crystalline quality of the Mn3Al2Ge3O12 crystal boule. Temperature-dependent magnetic susceptibility measurements reveal an antiferromagnetic ordering of the Mn2+ ions below TN= 6.5 K. The high quality of the single crystal obtained makes it ideal for detailed investigations of the magnetic properties of the system, especially using neutron scattering techniques.

Eu3+-Doped (Gd, La)AlO3 Perovskite Single Crystals: Growth and Red-Emitting Luminescence.

Eu3+-doped GdAlO3 (Eu:GAP) and Gd0.5La0.5AlO3 (Eu:GLAP) perovskite single crystals were successfully grown using the optical floating zone (OFZ) method. The microstructure, optical, photoluminescence (PL) and radioluminescence (under X-ray excitation, XEL) were investigated. Under the PL excitation of 275 nm, obvious emission bands peaking at 556 nm, 592 nm, 617 nm, 625 nm, 655 nm, and 706 nm were demonstrated, which correspond to the 5D0 → 7Fj (j = 0-4) transitions of Eu3+. The grown Eu:GAP single crystal showed a stronger PL intensity compared with that of Eu:GLAP in the red light region. After annealing at 1000 °C for 4 h in weak reductive atmosphere (Ar + 5% H2), a slight redshift and dramatic enhancement of PL and XEL intensity occurred. In addition, Eu:GLAP show a more intense XEL emission than that of Eu:GAP. Considering their different densities, these two kinds of red luminescence phosphors are proposed to be promising in a wide field of X-ray imaging, warm white, or plant lighting, respectively.

Optical and electrical properties of Sb-doped β-Ga2O3 crystals grown by OFZ method

Sb-doped β-Ga2O3 crystals were grown using the optical floating zone (OFZ) method. X-ray diffraction data and X-ray rocking curves were obtained, and the results revealed that the Sb-doped single crystals were of high quality. Raman spectra revealed that Sb substituted Ga mainly in the octahedral lattice. The carrier concentration of the Sb-doped single crystals increased from 9.55×1016 to 8.10×1018 cm-3, the electronic mobility depicted a decreasing trend from 153.1 to 108.7 cm2 ·V-1 ·s-1, and the electrical resistivity varied from 0.603 to 0.017 Ω·cm with the increasing Sb doping concentration. The un-doped and Sb-doped β-Ga2O3 crystals exhibited good light transmittance in the visible region; however, the evident decrease in the infrared region was caused by increase in the carrier concentration. The Sb-doped β-Ga2O3 single crystals had high transmittance in the UV region as well, and the cutoff edge appeared at 258 nm.

A homogeneity study on (Ce,Gd)3Ga2Al3O12 crystal scintillators grown by an optical floating zone method and a traveling solvent floating zone method

1 at% Ce3+-doped Gd3Ga2Al3O12 (GGAG) single crystals were grown by an optical floating zone (OFZ) method and a traveling solvent floating zone (TSFZ) method.

Ce3+, Pr3+ Co-Doped Lu3Al5O12 Single Crystals and Ceramics: A Comparative Study.

Ce3+, Pr3+ co-doped Lu3Al5O12 (Ce, Pr:LuAG) single crystals and ceramics were prepared using the optical floating zone (OFZ) and reactive vacuum sintering methods, respectively. The microstructure, photo- (λex = 450 nm), and radio-luminescence (under X-ray excitation) performance, as well as scintillation light yield (LY, under γ-ray, 137Cs source) of both materials, were investigated and compared. Ce, Pr:LuAG ceramics had an in-line transmittance of approximately 20% in the visible light range, while the analogous crystals were more transparent (~65%). The X-ray excited luminescent (XEL) spectra showed the characteristic Ce 3+ and Pr3+ emissions located at 310 nm, 380 nm, and 510 nm. The highest LY of the Ce, Pr:LuAG ceramics reached 34,112 pho/MeV at 2 μs time gate, which is higher than that of a single crystal. The ratio of LY values (LY2/LY0.75) between shaping times of 0.75 μs and 2 μs indicated a faster scintillation decay of ceramics regarding single crystals. It was ascribed to the lower effective concentration of luminescent activators in single crystals because of the coefficient segregation effect.

Radiation hard gallium oxide scintillators for high count rate radiation detection

Despite decades of materials research, the availability of appropriate crystalline radiation-hard scintillators with ultrafast decay times (∼10ns) and high light yields (>2000 ph/MeV) are still limited. In this study, we demonstrate the potential of gallium oxide (β-Ga2O3) based scintillators for high count rate applications. The low-cost rugged β-Ga2O3 scintillators were grown using the optical float zone (FZ) technique. Several dopants and growth atmospheres were used to demonstrate the balance between ultrafast primary decay time and the light yield of the scintillators. Light yields as high as 6446 ± 716 ph/MeV were obtained with 18.3 ns decay time for 662 keV gamma rays. Decay times as fast as 12ns were obtained with a 3212 ± 357 ph/MeV light yield. An excellent 662 keV gamma energy resolution of 7% was obtained using the β-Ga2O3:Ce crystals. Even with these excellent scintillation properties, the gamma radiation hardness of β-Ga2O3 crystals was better than the leading radiation-hard lead tungstate (PbWO4) crystals. Twenty-six scintillators fabricated from the grown β-Ga2O3 crystals were tested for radiation hardness with a 60Co gamma irradiation rate of 14 krad/h with a total radiation dose of 2 Mrad. The scintillation properties of these crystals remained unaffected. To date, no other scintillator has demonstrated such high radiation resistance. These FZ-grown low-cost, non-hygroscopic, and high-density β-Ga2O3 scintillators can replace the existing scintillators in many applications ranging from nuclear and high energy physics experiments to nuclear security.

Advances in Floating Zone Crystal Growth

Optical floating zone (OFZ) method has been applied to grow various materials for semiconducting industry applications as well as basic research on quantum materials. This article describes the OFZ method in detail and briefly introduces two recent advances in floating zone method by incorporating new techniques, namely, laser diode as optical source and high pressure environments. These developments have made it possible to grow materials that were previously challenging in conventional OFZ method and greatly expanded the range of accessible systems to search for exotic quantum phenomena.

Tensile creep behavior of HfNbTaTiZr refractory high entropy alloy at elevated temperatures

Tensile creep, which is one of the most important deformation modes for high temperature applications, is rarely reported for refractory high entropy alloys (RHEAs). In the present study, the optical floating zone (OFZ) technique was used to fabricate HfNbTaTiZr with grain size larger than 1 mm on average; tensile creep tests under vacuum at 1100-1250°C and stepwise loading of 5–30 MPa were conducted. The stress exponents and creep activation energies were determined to be 2.5–2.8 and 273 ± 15 kJ mol–1, respectively. The stress exponents determined have suggested solute drag creep behavior, and deformation was governed by a/2<111> type dislocations. To elucidate the effect of alloying constituents on solute drag creep, intrinsic diffusion coefficients of all elements were determined by simulation, and theoretical minimum creep strain rates were compared with those of experimental values. Analysis suggests that creep rate of HfNbTaTiZr appears to be controlled by Ta, which possesses the lowest intrinsic diffusivity and contributes the most to drag dislocations. To our knowledge, this work is the first to report tensile creep deformation mechanism of HfNbTaTiZr, especially up to 1250°C.

Determination of the phase diagram Tb2O3–SiO2 and crystal growth of the rare earth pyrosilicate Tb2Si2O7 by the optical floating-zone (OFZ) technique

By differential thermal analysis (DTA) is was shown that the pseudo-binary system Tb2O3–SiO2 contains intermediate compounds Tb2SiO5, Tb4Si3O12 and Tb2Si2O7. All three compounds melt congruently, a phase diagram was proposed, and from the latter compound single crystals could be grown for the first time. This was possible from stoichiometric feed rods by the optical floating-zone (OFZ) technique. X-ray powder analysis proved the Pna21 phase of the crystals. Melting temperature and heat of fusion were measured by DTA to be 1769◦C and ≈150 kJ/mol, respectively. The specific heat capacity of Tb2Si2O7 was measured by dynamic DSC to be cp ​= ​149.4982 ​+ ​0.2242 ⋅ T − 7.3913 ​× ​10−5 ⋅ T2 + 558449.3624 ⋅ T−2.

Modeling optical floating zone crystal growth in a high-pressure, single-lamp furnace

An optical floating zone (OFZ) model is developed, validated, and applied to obtain initial results for crystal growth in a novel high-pressure, single-lamp furnace. Computation with a realistic irradiation profile for zone heating results in dramatically different growth behavior than predicted using idealized Gaussian heating, though the discontinuities of the realistic profile prove challenging from a computational perspective. System pressure significantly affects buoyant flows in the surrounding atmosphere. Both driving force and flow strength increase nonlinearly with pressure, with the Grashof number growing with the cube of pressure and the Reynolds number scaling with pressure to the 3/2 power. For pressures of 100 bar and greater, flows of the surrounding phase strongly cool the growth sample, leading to significantly shorter melt zones, more deflected melt-solid interfaces, and weaker flows in the melt zone. Such effects reduce the likelihood of achieving stable growth conditions under very high pressure in this OFZ system.

Effective Suppression of MIS Interface Defects Using Boron Nitride toward High-Performance Ta-Doped-β-Ga2O3 MISFETs.

β-Ga2O3 is considered an attractive candidate for next-generation high-power electronics due to its large band gap of 4.9 eV and high breakdown electrical field of 8 MV/cm. However, the relatively low carrier concentration and low electron mobility in the β-Ga2O3-based device limit its application. Herein, the high-quality β-Ga2O3 single crystal with high doping concentration of ∼3.2 × 1019 cm-3 was realized using an optical float-zone method through Ta doping. In contrast to the SiO2/β-Ga2O3 gate stack structure, we used hexagonal boron nitride as the gate insulator, which is sufficient to suppress the metal-insulator-semiconductor (MIS) interface defects of the β-Ga2O3-based MIS field-effect transistors (FETs), exhibiting outstanding performances with a low specific on-resistance of ∼6.3 mΩ·cm2, a high current on/off ratio of ∼108, and a high mobility of ∼91.0 cm2/(V s). Our findings offer a unique perspective to fabricate high-performance β-Ga2O3 FETs for next-generation high-power nanoelectronic applications.

Microstructure and mechanical properties of twinning induced plasticity steel prepared by optical floating zone method

The three twinning induced plasticity (TWIP) steels were directionally solidified (DS) using the optical floating zone (OFZ) method at the withdrawal rates of 7 (DS7), 15 (DS15), and 30 μm s-1 (DS30) for the first time. The results showed that the relatively low withdrawal rate of 7 μm s-1 was unfavorable to the growth of DS, and the hybrid morphology was observed, which included equiaxed grains, columnar grains, amorphous coarse grains, and a few stray grains. With the increase of withdrawal rate, the volume fraction of columnar grain and dendrite grain increases, and the spacing decreased correspondingly, increasing microhardness and tensile strength. The samples exhibited outstanding plasticity, especially the DS15 had the highest elongation of 96%. This is attributed to the strong TWIP effect with a higher work hardening rate and the strongest ability of uniform plastic deformation at medium to large strains. Compared with the other two samples, the twin structure formed in the DS15 is more homogeneous and prevalent. The interaction between high-density dislocations and increased grain boundaries from the refined columnar grains and dendrites coupled with activated deformation twins, which contributes to the highest work hardening rate in the whole plastic deformation process of DS30.

Wide Concentration Range of Tb3+ Doping Influence on Scintillation Properties of (Ce, Tb, Gd)3Ga2Al3O12 Crystals Grown by the Optical Floating Zone Method.

To obtain a deeper understand of the energy transfer mechanism between Ce3+ and Tb3+ ions in the aluminum garnet hosts, (Ce, Tb, Gd)3Ga2Al3O12 (GGAG:Ce, Tb) single crystals grown by the optical floating zone (OFZ) method were investigated systematically in a wide range of Tb3+ doping concentration (1–66 at.%). Among those, crystal with 7 at.% Tb reached a single garnet phase while the crystals with other Tb3+ concentrations are mixed phases of garnet and perovskite. Obvious Ce and Ga loss can be observed by an energy dispersive X-ray spectroscope (EDS) technology. The absorption bands belonging to both Ce3+ and Tb3+ ions can be observed in all crystals. Photoluminescence (PL) spectra show the presence of an efficient energy transfer from the Tb3+ to Ce3+ and the gradually quenching effect with increasing of Tb3+ concentration. GGAG: 1% Ce3+, 7% Tb3+ crystal was found to possess the highest PL intensity under excitation of 450 nm. The maximum light yield (LY) reaches 18,941 pho/MeV. The improved luminescent and scintillation characteristics indicate that the cation engineering of Tb3+ can optimize the photoconversion performance of GGAG:Ce.

Concentration and temperature dependence behaviors of photoluminescence and scintillation properties of Lu2O3:Pr single crystals

Praseodymium doped lutecia (Lu1−xPrx)2O3 (x = 0.003, 0.006 and 0.01) single crystals have been prepared by optical floating zone (OFZ) method. The concentration and temperature dependence of photoluminescence and scintillation properties, including photoluminescence excitation (PLE) and photoluminescence (PL), PL decay, X-ray excited luminescence (XEL) and afterglow curves were systematically investigated. Both onward (Pr(S6)→Pr(C2)) and inverse (Pr(C2)→Pr(S6)) energy transfers are observed in Lu2O3:Pr. It is found that with the increasing of Pr3+ concentration, the Pr(S6)→Pr(C2) energy transfers efficiency increases. And the PL decay time becomes faster due to the increasing cross relaxation transition. Meanwhile, as the temperature increases (78–500 K), the onward and inverse energy transfer efficiencies between the two kinds of Pr3+ are enhanced simultaneously. The highest scintillation efficiency is Lu2O3:0.6%Pr and afterglow levels of obtained single crystals were comparable.

Micro-defects and luminescence of thulium-doped yttrium aluminum garnet single crystals

Thulium-doped yttrium aluminum garnet (YAG) single crystals with composition (TmxY(1-x))3Al5O12 (x = 0.001–0.02) were prepared by the optical floating zone method. The crystals were of high quality with no cracks, bubbles, or inclusions, and with >80% visible light transmittance. The lowest value for the mean positron lifetime was observed with crystals which had x = 0.01, which demonstrates that this composition had the fewest number of defects. This sample also produced the highest emission intensities in the PL spectra, and had narrower lines than those previously reported for Tm3+-doped yttria-stabilized zirconia. However, with higher Tm3+ concentrations, interactions between neighboring Tm3+ ions resulted in partial quenching of the luminescence. Overall, Tm-doped YAG single crystals synthesized by the optical floating zone (OFZ) method contained few defect centers and exhibited narrow luminescence linewidths, which demonstrate potential uses in high resolution applications.

β-Ga2O3 Used as a Saturable Sbsorber to Realize Passively Q-Switched Laser Output

β-Ga2O3 crystals have attracted great attention in the fields of photonics and photoelectronics because of their ultrawide band gap and high thermal conductivity. Here, a pure β-Ga2O3 crystal was successfully grown by the optical floating zone (OFZ) method, and was used as a saturable absorber to realize a passively Q-switched all-solid-state 1 μm laser for the first time. By placing the as-grown β-Ga2O3 crystal into the resonator of the Nd:GYAP solid-state laser, Q-switched pulses at the center wavelength of 1080.4 nm are generated under a output coupling of 10%. The maximum output power is 191.5 mW, while the shortest pulse width is 606.54 ns, and the maximum repetition frequency is 344.06 kHz. The maximum pulse energy and peak power are 0.567 μJ and 0.93 W, respectively. Our experimental results show that the β-Ga2O3 crystal has great potential in the development of an all-solid-state 1 μm pulsed laser.

Fast (Ce,Gd)3Ga2Al3O12 Scintillators Grown by the Optical Floating Zone Method

A series of (CexGd1–x)3Ga2Al3O12 (x = 0–0.8%) (Ce/GGAG) crystals were grown by the optical floating zone (OFZ) method. Ce/GGAG single crystals with good crystallinity were successfully obtained by ...