A review of β-Ga_2O_3 single crystal defects, their effects on device performance and their formation mechanism

As a wide-bandgap semiconductor(WBG), β-Ga_2O_3 is expected to be applied to power electronics and solar blind UV photodetectors. In this review, defects in β-Ga_2O_3 single crystals were summarized, including dislocations, voids, twin, and small defects. Their effects on device performance were discussed. Dislocations and their surrounding regions can act as paths for the leakage current of SBD in single crystals. However, not all voids lead to leakage current. There's no strong evidence yet to show small defects affect the electrical properties. Doping impurity was definitely irrelated to the leakage current. Finally, the formation mechanism of the defects was analyzed. Most small defects were induced by mechanical damages. The screw dislocation originated from a subgrain boundary. The edge dislocation lying on a plane slightly tilted towards the(102) plane, the(101) being the possible slip plane. The voids defects like hollow nanopipes, PNPs, NSGs and line-shaped grooves may be caused by the condensation of excess oxygen vacancies, penetration of tiny bubbles or local meltback. The nucleation of twin lamellae occurred at the initial stage of "shoulder part" during the crystal growth. These results are helpful in controlling the occurrence of crystal defects and improving the device performance.

Raman gains of ADP and KDP crystals

In this paper, the Raman gain coefficients of ammonium dihydrogen phosphate （ADP） and potassium dihydrogen phosphate （KDP） crystals are measured. By using a pump source of a 30-ps, 532-nm laser, the gain coefficients of ADP and KDP are 1.22 cm/GW, and 0.91 cm/GW, respectively. While for a 20-ps, 355-nm pump laser, the gain coefficients of these two crystals are similar, which are 1.95 cm/GW for ADP and 1.86 for KDP. The present results indicate that for ultra-violet frequency conversion, the problem of stimulated Raman scattering for ADP crystal will not be more serious than that for KDP crystal. Considering other advantages such the larger nonlinear optical coefficient, higher laser damage threshold, and lower noncritical phase-matching temperature, it can be anticipated that ADP will be a powerful competitor to KDP in large aperture, high energy third-harmonic generation or fourth-harmonic generation applications.

Photoluminescence characteristics of ZnTe bulk crystal and ZnTe epilayer grown on GaAs substrate by MOVPE

Excitation power and temperature-dependent photoluminescence（PL） spectra of the ZnTe epilayer grown on（100）Ga As substrate and ZnTe bulk crystal are investigated. The measurement results show that both the structures are of good structural quality due to their sharp bound excitonic emissions and absence of the deep level structural defect-related emissions. Furthermore, in contrast to the ZnTe bulk crystal, although excitonic emissions for the ZnTe epilayer are somewhat weak, perhaps due to As atoms diffusing from the Ga As substrate into the ZnTe epilayer and/or because of the strain-induced degradation of the crystalline quality of the ZnTe epilayer, neither the donor–acceptor pair（DAP） nor conduction band-acceptor（e–A） emissions are observed in the ZnTe epilayer. This indicates that by further optimizing the growth process it is possible to obtain a high-crystalline quality ZnTe heteroepitaxial layer that is comparable to the ZnTe bulk crystal.

Electrical Conduction in Deuterated Ammonium Dihydrogen Phosphate Crystals with Different Degrees of Deuteration

Conductivity measurements of deuterated ammonium dihydrogen phosphate （DADP） crystals with different deuterated degrees are described. The conductivities increase with the deuterium content, and the value of the a-direction is larger than that of the e-direction. Compared with DKDP crystals, DADP crystals have larger conductivities, which is partly due to the existence of A defects. The ac conductivity over the temperature range 25-170℃has shown a knee in the curve ofln（σT） versus T-1. The conductivity activation energy calculated by the slope of the high temperature region decreases with the deuterium content. The previously reported phase transition is not seen.

Bulk gallium oxide single crystal growth

Gallium oxide, as a new type of ultra-wide bandgap semiconductor, is expected to be used in power electronics and solar blind UV photodetectors. The main cause of research and development onβ-Ga2O3 is inspired by its larger bandgap, higher breakdown field, bigger Baliga figure-of-merit (FOM), shorter absorption edge and lower cost compared to the third-generation semiconductors, such as SiC and GaN.

Intrinsic relationship between photoluminescence and electrical characteristics in modulation Fe-doped AlGaN/GaN HEMTs

The photoluminescence（PL） and electrical properties of Al GaN/GaN high electron mobility transistors（HEMTs） with different Fe doping concentrations in the GaN buffer layers were studied. It was found that, at low Fe doping concentrations,the introduction of Fe atoms can result in a downward shift of the Fermi level in the GaN buffer layer, since the Fe atoms substitute Ga and introduce an FeGa^3＋/2＋ acceptor level. This results in a decrease in the yellow luminescence（YL） emission intensity accompanied by the appearance of an infrared（IR） emission, and a decrease in the off-state buffer leakage current（BLC）. However, a further increase in the Fe doping concentration will conversely result in the upward shift of the Fermi level due to the incorporation of O donors under the large flow rate of the Fe source. This results in an increased YL emission intensity accompanied by a decrease in the IR emission intensity, and an increase in the BLC. The intrinsic relationship between the PL and BLC characteristics is expected to provide a simple and effective method to understand the variation of the electrical characteristic in the modulation Fe-doped HEMTs by optical measurements.

β-BaB_2O_4 with special cut-angle applied to single crystal cascaded third-harmonic generation

High-efficiency single crystal cascaded third-harmonic generation（THG） was realized in β-BaB_2 O_4（BBO） material with special cut-angle. By analyzing effective nonlinear optical coefficient（deff） of the cascaded THG process, which was composed by type-II frequency doubling and type-I sum-frequency, the optimum phase matching（PM） direction in BBO crystal was determined to be（θ = 32.1？, φ = 11？）. With an optimized 9-mm long sample which was processed along this direction, the highest cascaded THG conversion efficiency reached 42.3%, which is much superior to the similar components reported previously, including ADP, KDP, and Gd_x Y_（1-x） COB crystals.

Effect of raw material and growth method on optical properties of DKDP crystal

Three kinds of KH2PO4 raw material are used to grow deuterated potassium dihydrogen phosphate （DKDP） crystals by traditional and rapid growth methods, respectively. The growth habit dependence on the purity of raw material is described and analyzed. The optical properties including transmission spectra and laser-induced damage threshold of these crystals are measured. It is found that the growth method affects the optical properties of crystal more obviously than the raw material with the mass content of main metal ions below 1 ppm. Moreover, the morphology of the core in the observed damage sites indicates that an explosion process probably occurs during laser-induced breakdown.

Steady-state Raman gain in visible and near-infrared waveband of Sr WO_4 and Ba WO_4 crystals

The steady-state stimulated Raman scattering （SRS） gain with different excitation wavelengths ranging from 400 to 1100 nm of tungstate crystals, SrWO4 and BaWO4, is systematically researched. As excitation frequency is close to electronic transition frequency, molecular polarizability is not a constant, which has to be taken into account in our work. The experiment and theory agree well with each other and show that SRS gain is not only proportional to Stokes light frequency, but is also inversely proportional to biquadratic excitation frequency.

Single Crystal Fibers:Diversified Functional Crystal Material

Single crystal fibers(SCF)are considered to be a combination of bulk crystals and conventional fibers,thereby possessing the stable physical and chemical properties accompanied with excellent waveguide properties.This paper gives a detailed introduction to the development history of single crystal fibers,including the evolution of the growth technique and the optimization of the growth process.Laser-heated pedestal growth(LHPG)and Micro-pulling-down(μ-PD)methods are considered to be the most widely used growth techniques for growing single crystal fibers,and the advantages of the two methods are also introduced in detail.The second part of this paper describes the characterization of single crystal fibers,including diameter fluctuation,crystal quality and optical losses.A series of cladding approaches for SCF,such as magnetron sputtering,sol-gel,liquid phase epitaxy,co-drawing LHPG,ion implantation and micro-structure cladding will be reviewed.In addition,the research status of single crystal fiber laser and single crystal fiber sensor are also summarized in view of the current research foundation.

Phase matching characteristics of deuterated ammonium dihydrogen phosphate crystals

Refractive indices for crystals ammonium dihydrogen phosphate （ADP）, 30% deuterated ADP （DADP）, 50% DADP, and 70% DADP are measured from 253 to 1529 nm with 5 × 10^-6 accuracy. Numerical fits to modified double-pole Sellmeier equation are made. Second-harmonic generation, third-harmonic generation phase match- ing （PM） angles, and noncritical PM （NCPM） wavelengths are calculated using the Sellmeier parameters. The deuterated crystals show smaller PM angles than pure crystal. Fourth-harmonic generation process can be real- ized by DADP in smaller deuterium content than deuterated potassium dihydrogen phosphate （DKDP）. The measured NCPM wavelengths are consistent with the calculated value. PM characteristics are compared between DADP and DKDP.

Two-dimensional tellurene nanosheets as saturable absorber of passively Q-switched Nd:YAG solid-state laser

For the first time,a group-Ⅵsingle element nanomaterial was used as the optical saturable absorber(SA)to generate laser pulses.With two-dimensional(2D)tellurene as a passive Q-switch,1.06μm and 1.3μm pulse laser operations were realized from a diode-pumped Nd:YAG crystal.The shortest pulse widths were 98 ns and 178 ns,and the highest peak powers were 2.68 W and 2.45 W,respectively.Our research determines that tellurene is an excellent SA material in the near-infrared region.

Influence of excitation power on temperature-dependent photoluminescence of phase-separated InGaN quantum wells

Temperature-dependent photoluminescence （PL） of phase-separated InGaN quantum wells is investigated over a broader excitation power range. With increasing excitation power from 0.5 pW to 50 mW, the In-rich quasi-quantum dot （QD）-related PL peak disappears at about 3 mW, while temperature behavior of the InGaN matrix-related PL peak energy （linewidth） gradually evolves from a strong ＂S-shaped＂ （＂W-shaped＂） temperature dependence into a weak ＂S-shaped＂ （an approximately ＂V-shaped＂）, until becoming an inverted ＂V-shaped＂ （a monotonically increasing） temperature dependence. This indicates that, with increasing excitation power, the carrier localization effect is gradually reduced and the QD-related transition is submerged by the significantly enhanced InGaN matrix-related transition, while the carrier thermalization effect gradually increases to become predominant at high excitation powers.

Passively Q-switched self-frequency doubling Nd3+:ReCa4O（BO3）3（Re=Y,Gd） lasers with tin diselenide as a saturable absorber

With tin diselenide(SnSe2)film as a saturable absorber(SA),the passively Q-switched self-frequency doubling(SFD)lasers were realized in Nd^3+:ReCa4O(BO3)3(Re=Y,Gd)crystals.For Nd:YCa4O(BO3)3 crystal,the maximum average output power at 532 nm was 19.6 mW,and the corresponding pulse repetition frequency,pulse duration,single pulse energy,and peak power were 17.6 kHz,91.9 ns,1.1μJ,and 12.1 W,respectively.For Nd:GdCa4 O(BO3)3 crystal,these values were 14.5 mW,22.1 kHz,48.7 ns,0.66μJ,and 13.5 W.

Highly efficient Nd:(La_xGd_(1-x))_3Gd_5O_(12) laser operation at 1.33 μm

The continuous wave （CW） and passively Q-switched （PQS） performances of diode-pumped Nd： （LaxGd1-x）3Gd5O12 （Nd：LaGGG） at 1.33 μm are achieved for the first time to our knowledge. The maximum CW output power of 5.1 W is obtained with the optical-optical conversion efficiency of 25.3% and the slope efficiency of 26.6%. In the PQS operation, by using the V3＋：YAG crystal as the saturable absorber, the maxi- mum average output power, shortest pulse width, largest pulse energy, and highest peak power are measured to be 1.1 W, 27.54 ns, 75.78 μJ, and 2.44 kW, respectively.