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Crystal structure, Raman spectra, electrical properties, and photoluminescence of unintentionally doped (UID) and W-doped β-Ga2O3 (W:β-Ga2O3) crystals grown using the optical floating zone technique were investigated. Based on the experimental data, W6+ ions substitute Ga3+ ions mainly in the octahedral lattice site, as revealed by the Raman spectroscopic assessment of W:β-Ga2O3 crystals. The carrier concentration of 0.10 mol. % W:β-Ga2O3 (3.92 × 1018 cm3) is more than forty times that of UID crystal (9.55 × 1016 cm3). In addition, the resistivity and mobility of 0.10 mol. % W: β-Ga2O3 decreased from 0.603 to 0.032 Ω cm and 153.1 to 126 cm−2 V−1 s−1, respectively. The transmittance of W:β-Ga2O3 crystals decreases with increasing W content (0.01, 0.05, and 0.10 mol. %) but remains high in the visible wavelength range. Three distinct emissions are observed in the photoluminescence spectra: two blue emissions and a UV band emission. These bands are owing to the −1 charge states of Ga(I) vacancies (VGa1−) at the octahedral site, the −1 charge states of gallium and oxygen vacancy pairs (VGa + VO)1−, as well as the recombination of self-trapped holes (STHs) are confined between two O(II)-s sites, respectively. [ABSTRACT FROM AUTHOR]

Electrical properties, electronic defects, and photoluminescence (PL) of Nb-doped β-Ga2O3 crystals grown by the floating zone method have been studied in the temperature range from 10 to 350 K. The activation energies of shallow and deep traps were obtained and compared with their counterparts. The 0.25-Nb-doped Ga2O3 crystal has a metallic behavior at room temperature and a semiconducting behavior at low temperatures. This behavior was interpreted by the quantum corrections to the electrical resistivity resulting from the weak localization and Coulomb interaction. In addition, the 0.25-Nb-doped Ga2O3 crystal has a shallow donor ionization energy of 0.03 eV, indicating that it requires less energy to enable n-type conductivity. Hence, Nb is a good choice for improving the electrical conductivity of the β-Ga2O3 crystal. Three defect trap states were found at EC-0.81 eV, EC-0.49 eV, and EC-0.12 eV below the conduction band edge. The PL spectra of the undoped sample show two characteristic bands in the UV and blue regions under the excitation wavelength of 256 nm. On the other hand, in the Nb-doped β-Ga2O3 sample, a novel UV emission band with a shorter wavelength was observed. [ABSTRACT FROM AUTHOR]

To increase the n-type conductivity of single-crystal substrates, the Sn element was purposefully inserted into β-Ga2O3 crystals as n-type dopants. However, it was found that the color of the β-Ga2O3 crystal changed significantly after high-temperature remelting in the edge-defined film-fed growth (EFG) method. According to Hall and optical transmission spectroscopy, the carrier concentration in the colorless region is significantly lower than that in the blue region, resulting in a decrease in electrical conductivity after high-temperature remelting of Sn-doped β-Ga2O3. This is a challenge for the application of β-Ga2O3 as an electronic power device. Therefore, we focused on the impact of the two color boundary areas on the structure, electronic, optical properties, and (100) surface defects after high-temperature remelting, and discussed the reason for this color change. Inductive coupled plasma emission spectroscopy (ICP) was used to detect the content of Sn. According to the results of high-resolution X-ray diffraction and atomic force microscopy, compared with the low crystal in the colorless area, the single crystal in the blue area showed higher crystal quality. The PL results indicate that the colorless area has more defects caused by oxygen vacancies than the blue area. [ABSTRACT FROM AUTHOR]

Twins are common defects in β-Ga2O3 crystals that affect the development of large single crystal substrates. To investigate the generation and development of twins, we grew a (100) oriented β-Ga2O3 crystal using edge-defined film-fed growth (EFG). We characterized the (010) sample containing twins by polarized light stress meter, chemical etching, laser scanning confocal microscopy (LSCM) and transmission electron microscopy (TEM). The polarized light stress meter clearly distinguishes areas containing twins from those without. After H3PO4 etching at 150 °C for 1 h, LSCM observations revealed etch pits with mirror symmetry across the (100) twin boundary, with an angle of approximately 77° between the array of etch pits and the boundary. The twins originate at the boundary (102¯) along the [2¯01¯] direction, which is nearly perpendicular to the (100) plane, and then grow in steps on the side near the interior of the crystal. The twin boundary on the side near the edge of the crystal first undergoes stepwise extension and then gradually bends and grows, eventually forming a (100) twin boundary on both sides. TEM observations reveal a different arrangement of atomic density on either side of the twin boundary possessing the [2¯01¯] orientation component. Furthermore, both (102¯) and (100) are slip planes for the β-Ga2O3 crystal, indicating that the generation of twins is closely related to the slip plane. [ABSTRACT FROM AUTHOR]

The long-range crystallographic order and anisotropy in β-(AlxGa1−x)2O3 (x = 0.0, 0.06, 0.11, 0.17, 0.26) crystals, prepared by optical floating zone method with different Al composition, is systematically studied by spatial correlation model and using an angle-resolved polarized Raman spectroscopy. Alloying with aluminum is seen as causing Raman peaks to blue shift while their full widths at half maxima broadened. As x increased, the correlation length (CL) of the Raman modes decreased. By changing x, the CL is more strongly affected for low-frequency phonons than the modes in the high-frequency region. For each Raman mode, the CL is decreased by increasing temperature. The results of angle-resolved polarized Raman spectroscopy have revealed that the intensities of β-(AlxGa1−x)2O3 peaks are highly polarization dependent, with significant effects on the anisotropy with alloying. As the Al composition increased, the anisotropy of Raman tensor elements was enhanced for the two strongest phonon modes in the low-frequency range, while the anisotropy of the sharpest Raman phonon modes in the high-frequency region decreased. Our comprehensive study has provided meaningful results for comprehending the long-range orderliness and anisotropy in technologically important β-(AlxGa1−x)2O3 crystals. [ABSTRACT FROM AUTHOR]

The introduction of different elements into semiconductors to achieve conductivity control of n-type doping has considerable scientific significance and practical application prospects, which can further promote their extensive application in the field of electronic devices. Herein, a series of V-doped beta-gallium oxide (β-Ga2O3) single crystals with different concentrations were cultivated using the optical floating zone method. The influence of V doping on the structural, electrical, and optical properties of β-Ga2O3 single crystals was systematically investigated. The characterization results revealed that V-doped β-Ga2O3 single crystals exhibited superior crystalline quality; moreover, the free carrier concentration increased from 6.9 × 1016 cm−3 to 6.4 × 1018 cm−3 and then decreased to 3.1 × 1016 cm−3 with an increase in the V concentration. The optical transmission of the crystals initially decreased in the infrared region and then increased, which is related to the change in the conductive electrons. The intensities of the two absorption peaks located at approximately 400 nm and 610 nm increased with increasing V doping concentration. From the Raman scattering spectra, the inhibition of V on the [GaIIO6] octahedral peak intensity became evident with increasing V content. These valuable findings may contribute to the research on β-Ga2O3-based electronic and optical devices. [ABSTRACT FROM AUTHOR]

In this paper, the effects of oxygen vacancy and gallium vacancy on the optical and scintillation properties of undoped β-Ga2O3 crystal and 2.5 mol % Al doped gallium oxide were investigated. For the undoped β-Ga2O3, the transmittance is improved after annealing in oxygen or nitrogen atmosphere. After the introduction of Al element, the absorption cutoff appears slightly blue shift, and the band gap increases. For the undoped as-grown β-Ga2O3 single crystals, the decay time consists of a fast component (τ1) of the order of nanoseconds, and two slow components (τ2, τ3) of tens to hundreds of nanoseconds. The contribution of the fast decay time component in the decay times is 2.78%. While for Al-doped β-Ga2O3, the faster (τ1) time is 2.33 ns for the as-grown one, and the contribution is 68.02%. However, the pulse height spectrum shows that the introduction of 2.5 mol % Al will reduce the light yield of the β-Ga2O3 crystal. [ABSTRACT FROM AUTHOR]

A crack extending downward along the seed of a beta-gallium oxide (β-Ga2O3) crystal grown using the edge-defined film-fed growth (EFG) method severely affects the efficiency of crystal slicing. In this study, the influence of the crack on the structural, electronic, and optical properties, as well as the defects in the β-Ga2O3 crystal, was investigated. The causes of the crack were also discussed in this study. Compared with the low crystalline crystal on the right side of the crack, the single crystal on the left side exhibits superior crystalline quality, according to the high-resolution X-ray diffraction and Raman scattering spectra. The Hall data and optical transmission spectra reveal that the crack has a negligible effect on the carrier concentration and optical bandgap. The PL results indicate that the right crystal has more defects caused by oxygen vacancies than the left crystal. Rugby-ball-shaped dislocation etch pits were discovered using a chemical etching approach. The average dislocation densities on the left and right sides of the crack were 9 × 103 cm−2 and 3 × 104 cm−2, respectively. [ABSTRACT FROM AUTHOR]

Room temperature sub-gap optical absorption spectra measured by photothermal deflection spectroscopy were investigated for hetero- and homo-epitaxial β-Ga2O3 layers grown by plasma-assisted molecular beam epitaxy as well as for a bulk crystal. The absorption spectra show a pronounced exponential Urbach tail with slope parameters of 120–150 meV in the spectral region between 4.5 and 5 eV, indicating an unusually large self-trapping energy of excitons. In addition, an absorption band related to deep defects is observed in the spectral region from 2.5 to 4.5 eV. The steepness of the Urbach tail as well as the strength of the defect-related absorption can be influenced and optimized by annealing at 900–1000 °C in an oxygen atmosphere. Similar features were also observed for bulk β-Ga2O3 crystals and for homoepitaxial β-Ga2O3 layers. The present results for β-Ga2O3 are compared and discussed in the context of similar measurements for other wide-bandgap semiconductors of current interest in electronics and photocatalysis: GaN, ZnO, TiO2, and BiVO4. [ABSTRACT FROM AUTHOR]

X-ray photoelectron spectroscopy (XPS), photoluminescence spectroscopy and Raman spectroscopy were applied to study β-Ga2O3 single crystals before and after annealing. By systematic analysis, the mechanism of the carrier concentration decrease after annealing was explained using electronic trap of GaI3+. XPS measurements showed that the O content increases while the Ga3+ content decreases after annealing, which is related to the carrier concentration decrease. The shift of blue emission band center which related to donors was attributed to the electron capture by Ga3+ nearby conduction band. The Raman spectra confirmed this capture, as based on the Raman peak intensity changes, and the electrons were judged to be mainly captured by GaI of β-Ga2O3. [ABSTRACT FROM AUTHOR]

Combining a blue LED chip with phosphor, white light can be obtained. In traditional white LEDs, the phosphor was dispersed in resin in a form of particles. Particle-resin composite has complex packaging and poor thermal stability. For high-power white LEDs, this combination cannot provide long-term stable work because of photo-degradation of the organic epoxy resin under high irradiation and high temperatures. Here we develop resin-free phosphor (Tb1-nCen)3Al5O12-Al2O3 eutectic mixed crystals (Ce:TAG-Al2O3) to eliminate the photo-degradation issue of the organic material. They were grown by optical floating zone method. X-ray diffraction and scanning electron microscopy show that these materials have typical eutectic mixed crystal structure of sapphire and garnet phases, in which lamellar spacing is in an order of tens of microns. The doped Ce3+ ions do not influence basic structure of TAG crystal. Ce3+ ion replaces Tb3+ ion rather than Al3+ ion. Photoluminescence emission and excitation properties of the eutectic mixed crystals were investigated. They get to maximal value with 2.0% Ce3+, and have wide excitation band at 470 nm. Luminous efficiency of eutectic mixed crystal-packaged LED is higher than that of particle-resin composite packaged LED. These results show that Ce:TAG-Al2O3 eutectic mixed crystals are promising phosphor for white LEDs. [ABSTRACT FROM AUTHOR]

In this paper, Ba2SiO4:Eu2+green phosphor was synthesized by sol-gel method. TGA-DTA, XRD, SEM and PL spectra were used to characterize the as-prepared phosphors. The phosphors are composed of nanoparticles with 60 nm grain size and exhibit green light with a broad peak around 500 nm. The relationship between crystal growth, morphology and luminescent properties was studied. The structure and luminescence properties of phosphors synthesized in different conditions were also discussed. [ABSTRACT FROM AUTHOR]

Nd:Lu3ScAl4O12 single crystal has been grown by the Czochralski method. The segregation coefficient of Nd3+ ions in LuSAG crystal is 0.22. X-ray powder diffraction (XRD) experiments show that the Nd:LuSAG crystal crystallizes in cubic structure with space group Ia3d and its cell parameters are a = 1.1953 nm. The absorption and fluorescence spectra of Nd:LuSAG crystal were investigated at room temperature. The spectral parameters were calculated based on Judd–Ofelt (J–O) theory, and the intensity parameters Ω2, Ω4 and Ω6 were obtained to be 0.14, 3.38, and 4.88 × 10−20 cm2, respectively. The calculated spontaneous radiative probabilities, branching ratios, radiative lifetime and quantum efficiency have been evaluated for the 4F3/2 excited state using the calculated intensity parameters. The results indicate that this crystal can be considered as a promising material for solid-state laser application. [ABSTRACT FROM AUTHOR]

Highlights: [•] Ce doped Al2O3−YAG eutectic has been successfully grown by the Czochralski method. [•] The eutectic has a higher segregation coefficient of Ce3+ than YAG single crystal. [•] Photoluminescence properties of Ce-doped Al2O3–YAG eutectic were investigated. [•] Variouswhitecolourscouldbeobtainedasrequired,byvaryingtheeutecticphosphorthickness. [•] The distinctive structure of eutectic made it have a better performance in LED than particle-dispersed system. [ABSTRACT FROM AUTHOR]

Abstract: The Mn-, Cr-doped and Mn, Cr-co-doped MgAl2O4 powders have been synthesized via a gel–solid reaction method. Energy transfer from Mn2+ to Cr3+ has been observed for the first time in the co-doped MgAl2O4 phosphors. When excited with blue light with a wavelength of 450nm at room temperature, both green emission from Mn2+ around 520nm and red emission from Cr3+ around 675and 693nm were generated. Moreover, the color of the emission can be modified by controlling the doping concentrations of Mn2+ and Cr3+. Therefore, MgAl2O4: Mn2+, Cr3+ could be used as a single-phased phosphor for white LED with a blue LED chip. The energy transfer in terms of Mn2+ to Cr3+ is determined by means of radiation and reabsorption. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Systemic investigations are performed to comprehend the structural, optical, and electrical characteristics of four niobium (Nb) doped β-Ga2O3 crystals (β-Ga2O3:Nb) grown by the optical floating zone (OFZ) method. All of the β-Ga2O3:Nb crystals revealed monoclinic phases and good crystalline qualities. While the different Nb doped (i.e., 0.0001 mol%, 0.01 mol%, 0.1 mol% and 0.5 mol%) samples exhibited slightly changed bandgap energies Eg (≡ 4.72 eV, 4.73 eV, 4.81 eV, 4.68 eV)—the luminescence features indicated distinctive defect levels—affecting the electronic energy structure significantly. By increasing the Nb doping level from 0.0001 mol% to 0.1 mol%, the Fermi level (EF) moves closer to the bottom of the conduction band. For the sample with Nb doping 0.5 mol%—no further improvement is noticed in the electronic properties. Finally, the energy band diagrams of four samples are given. [ABSTRACT FROM AUTHOR]

In the traditional white LED, the phosphor is combined with the blue LED chip after mixed with the resin composite material. This combination affects the final light efficiency and color temperature, and cannot provide long-term stable work because of high emission and high temperature. Based on Tb 3 Al 5 O 12 -Al 2 O 3 :Ce (TAG-Al 2 O 3 :Ce) eutectic, we developed the eutectic Tb 1.5 Gd 1.5 Al 5 O 12 -Al 2 O 3 :Ce ((TG) AG-Al 2 O 3 :Ce) to overcome the shortcomings of traditional phosphors. The eutectic was grown by an optical floating zone method. X-ray diffraction shows that the structure of (TG) AG-Al 2 O 3 do not have significant changes compared to TAG-Al 2 O 3 , and the doped Ce 3+ ions do not influence the basic structure of eutectic. Scanning electron microscopy shows that these materials have typical eutectic structure of sapphire and garnet phases, which can increase the length of the optical path to reduce the reflection loss and cause a higher luminous efficiency compared to the resin phosphor. Photoluminescence emission and excitation properties of (TG) AG-Al 2 O 3 eutectic were investigated. The main excitation is in the luminous range (450 nm to 480 nm) of the blue LED chip for white LED. Compared with TAG-Al 2 O 3 :Ce, the emission peak is slightly shifted to the red, which cause the color temperature of (TG) AG-Al 2 O 3 :Ce eutectic LED is closer to natural light. The luminous efficiency of (TG) AG-Al 2 O 3 :Ce eutectic white LED is much higher than that of TAG-Al 2 O 3 :Ce eutectic. These results show that (TG) AG-Al 2 O 3 : Ce eutectic is promising phosphor for white LEDs. [ABSTRACT FROM AUTHOR]

The growth, XRD patterns, spectral properties, and fluorescence decays of Yb:Ca 0.28 Ba 0.72 Nb 2 O 6 (Yb:CBN) with doping concentration of 1 at.% and 5 at.% were studied. The peak absorption cross-section and the emission cross-section were calculated. Larger Stark splitting of Yb:CBN offers the prospect of the quasi-four level laser operation. The fluorescence lifetimes are 0.64 and 0.73 ms for 1 at.% and 5 at.%, respectively. The FWHM of absorption and fluorescence spectra of 5 at.% Yb:CBN are 62 and 68 nm, which are broader than other ultrashort laser media. The results prove that Yb:CBN crystal is a good candidate as an ultrashort laser medium. [ABSTRACT FROM AUTHOR]

Highlights: [•] Spectroscopic properties and crystal growth of Tm:LYSO single crystal were studied for the first time. [•] The large absorption and emission cross-section and broad FWHM make Tm:LYSO a promising media for tunable and ultrashort pulses. [•] The J–O parameters and gain spectra reveal that the excellent properties for laser applications. [Copyright &y& Elsevier]

Abstract: Pr3+, Yb3+ co-doped CaYAlO4 (Pr, Yb: CYA) single crystal was grown using the Czochralski (CZ) method. For near-infrared quantum-cutting system, a quantum efficiency of 145% for Pr3+, Yb3+-codoped CYA crystal has been obtained by ignoring the nonradiative losses. Under excitation at 940nm, high intensities of 502nm, 555nm, 628nm, 661nm and 729nm emissions turn up. Fluorescence decay time of 3P0→3F2 emission of Pr3+ ions and 2F5/2→2F7/2 transition of Yb3+ ions were also studied as evidences for energy transfer. [Copyright &y& Elsevier]

Abstract: Nd,Co:LaMgAl11O19 (abbreviated as Co,Nd:LMA) was grown using the Czochralski method. The structure, polarized absorption spectrum, fluorescence spectrum, and fluorescence decay time were analyzed. The as-grown crystal had very wide absorption bands at 794nm, which can be pumped by GaAs laser diode without temperature stabilization. Two strong emission bands were present at 1056nm and 1082nm with full-width at half-maximum (FWHM) of 6 and 8nm, respectively. The large FWHM is due to the inhomogeneity of the Nd ion sites. The lifetimes of the 4F3/2 manifold of Co,Nd:LMA at room temperature monitored at 905nm, 1056nm, and 1344nm were 292, 288, and 350μs, respectively, which was caused by the different contribution of the three different sites with D3h and C2v symmetry. The absorption band of Co is from 1.3μm to 1.6μm, and Co,Nd:LMA still has a strong emission around the 1.38μm, indicating that the Co,Nd:LMA can be applied as a potential self-Q-switched material operating at 1.3μm. [Copyright &y& Elsevier]

The long-range crystallographic order and anisotropy in β-(Al x Ga 1-x ) 2 O 3 (x = 0.0, 0.06, 0.11, 0.17, 0.26) crystals, prepared by optical floating zone method with different Al composition, is systematically studied by spatial correlation model and using an angle-resolved polarized Raman spectroscopy. Alloying with aluminum is seen as causing Raman peaks to blue shift while their full widths at half maxima broadened. As x increased, the correlation length (CL) of the Raman modes decreased. By changing x, the CL is more strongly affected for low-frequency phonons than the modes in the high-frequency region. For each Raman mode, the CL is decreased by increasing temperature. The results of angle-resolved polarized Raman spectroscopy have revealed that the intensities of β-(Al x Ga 1-x ) 2 O 3 peaks are highly polarization dependent, with significant effects on the anisotropy with alloying. As the Al composition increased, the anisotropy of Raman tensor elements was enhanced for the two strongest phonon modes in the low-frequency range, while the anisotropy of the sharpest Raman phonon modes in the high-frequency region decreased. Our comprehensive study has provided meaningful results for comprehending the long-range orderliness and anisotropy in technologically important β-(Al x Ga 1-x ) 2 O 3 crystals.