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.

Growth and fundamentals of bulk β-Ga_2O_3 single crystals

The rapid development of bulk β-Ga_2O_3 crystals has attracted much attention to their use as ultra-wide bandgap materials for next-generation power devices owing to its large bandgap(～ 4.9 eV) and large breakdown electric field of about8 MV/cm. Low cost and high quality of large β-Ga_2O_3 single-crystal substrates can be attained by melting growth techniques widely used in the industry. In this paper, we first present an overview of the properties of β-Ga_2O_3 crystals in bulk form. We then describe the various methods for producing bulk β-Ga_2O_3 crystals and their applications. Finally, we will present a future perspective of the research in the area in the area of single crystal growth.

Effect of temperature on photoresponse properties of solar-blind Schottky barrier diode photodetector based on single crystal Ga_2O_3

A solar-blind photodetector is fabricated on single crystal Ga_2O_3 based on vertical structure Schottky barrier diode. A Cu Schottky contact electrode is prepared in a honeycomb porous structure to increase the ultraviolet(UV) transmittance.The quantum efficiency is about 400% at 42 V. The Ga_2O_3 photodetector shows a sharp cutoff wavelength at 259 nm with high solar-blind/visible(= 3213) and solar-blind/UV(= 834) rejection ratio. Time-resolved photoresponse of the photodetector is investigated at 253-nm illumination from room temperature(RT) to 85.8℃. The photodetector maintains a high reversibility and response speed, even at high temperatures.

Growth and physical characterization of high resistivityFe:β-Ga2O3 crystals

High quality 0.02 mol%,0.05 mol%,and 0.08 mol%Fe:β-Ga2O3 single crystals were grown by the floating zone method.The crystal structure,optical,electrical,and thermal properties were measured and discussed.Fe:β-Ga2O3 single crystals showed transmittance of higher than 80%in the near infrared region.With the increase of the Fe doping concentration,the optical bandgaps reduced and room temperature resistivity increased.The resistivity of 0.08 mol%Fe:β-Ga2O3 crystal reached to 3.63×1011Ω·cm.The high resistivity Fe:β-Ga2O3 single crystals could be applied as the substrate for the high-power field effect transistors(FETs).

Single crystal β-Ga_2O_3: Cr grown by floating zone technique and its optical properties

β-Ga2O3: Cr single crystals were grown by floating zone technique. Absorption spectra and fluorescence spectra were measured at room temperature. The values of field splitting parameter Dq and Racah parameter B were obtained by the peak values of absorption spectra. The value 10Dq/B=23.14 manifests that in β-Ga2O3 crystals Cr3+ ions are influenced by low energy crystal field. After high temperature annealing in air, the Cr3+ intrinsic emission was enhanced and the green lumines-cence disappeared. The strong and broad 691 nm emission was obtained at 420 nm excitation due to the electron transition occurred from 4T2 to 4A2. The studies manifest that the β-Ga2O3 crystals have the potential application for tunable laser.

Facile synthesis of coordination polymer nanocubes and their conversion into mesoporous single crystal-like Y_2O_3 nanocubes

Mesoporous single crystal-like Y2O3 nanocubes have been prepared through a coordination-based self- assembly process. Firstly, a uniform nanocube-like Y-lysine precursor was simply prepared with hydrothermal treatment. After the simple thermal treatment process, nanocube-shaped yttrium oxides with the morphology inherited from the Y-lysine precursor were successfully prepared. The phase, morphology, size and crystalline structure were well characterized by XRD, SEM and TEM. N2 adsorption-desorption demonstrates the mesoporous characteristics of the Y2O3 nanocubes, showing a relatively high surface area of 60 m^2/g.

Elaboration and characterization of a KCl single crystal doped with nanocrystals of a Sb_2O_3 semiconductor

Abstract： Undoped and doped KC1 single crystals have been successfully elaborated via the Czochralski （Cz） method. The effects of dopant Sb2O3 nanocrystals on structural and optical properties were investigated by a number of techniques, including X-ray diffraction （XRD）, scanning electron microscopy （SEM）, energy disper- sive X-ray （EDAX） analysis, UV-visible and photoluminescence （PL） spectrophotometers. An XRD pattern of KCI：Sb2O3 reveals that the Sb2O3 nanocrystals are in the well-crystalline orthorhombic phase. The broadening of diffraction peaks indicated the presence of a Sb2O3 semiconductor in the nanometer size regime. The shift of ab- sorption and PL peaks is observed near 334 nm and 360 nm respectively due to the quantum confinement effect in Sb2O3 nanocrystals. Particle sizes calculated from XRD studies agree fairly well with those estimated from optical studies. An SEM image of the surface KCI：Sb2O3 single crystal shows large quasi-spherical of Sb2O3 crystallites scattered on the surface. The elemental analysis from EDAX demonstrates that the KCI：Sb2O3 single crystal is slightly rich in oxygen and a source of excessive quantities of oxygen is discussed.

Carbon agent chemical vapor transport growth of Ga2O3 crystal

Beta-type gallium oxide（β-Ga2O3） is a new attractive material for optoelectronic devices. Different methods had been tried to grow high quality β-Ga2O3 crystals. In this work, crystal growth of Ga2O3 has been carried out by chemical vapor transport（CVT） method in a closed quartz tube using C as transport agent and sapphire wafer as seed. The CVT mass flux has been analyzed by theoretical calculations based on equilibrium thermodynamics and 1D diffusional mass transport. The crystal growth experimental results are in agreement with the theoretical predictions. Influence factors of Ga2O3 crystal growth, such as temperature distribution, amount of C as transport agent used, have also been discussed. Structural（XRD） and optical（Raman spectroscopy, photoluminescence spectrum） properties of the CVT-Ga2O3 crystal are presented.

Pulling growth technique towards rare earth single crystals

Pulling growth technique serves as a popular method to grow congruent melting single crystals with multiscale sizes ranging from micrometers to centimeters.In order to obtain high quality single crystals,the crystal constituents would be arranged at the lattice sites by precisely controlling the crystal growth process.Growing interface is the position where the phase transition of crystal constituents occurs during pulling growth process.The precise control of energy at the growing interface becomes a key technique in pulling growth.In this work,we review some recent advances of pulling technique towards rare earth single crystal growth.In Czochralski pulling growth,the optimized growth parameters were designed for rare earth ions doped Y_3Al_5O_(12)and Ce:(Lu_(1-x)Y_x)_2Si O_5on the basis of anisotropic chemical bonding and isotropic mass transfer calculations at the growing interface.The fast growth of high quality rare earth single crystals is realized by controlling crystallization thermodynamics and kinetics in different size zones.On the other hand,the micro pulling down technique can be used for high throughput screening novel rare earth optical crystals.The growth interface control is realized by improving the crucible bottom and temperature field,which favors the growth of rare earth crystal fibers.The rare earth laser crystal fiber can serve as another kind of laser gain medium between conventional bulk single crystal and glass fiber.The future work on pulling technique might focus on the mass production of rare earth single crystals with extreme size and with the size near that of devices.

光学浮区法生长掺锗氧化镓单晶及其性质研究

采用光学浮区法生长了尺寸8 mm×40 mm的Ge∶β-Ga_2O_3单晶。XRD物相分析表明Ge∶β-Ga_2O_3单晶仍属于单斜晶系。为对其内部缺陷进行表征,进行了腐蚀实验,在光学显微镜下观察到缺陷密度为6×10~4/cm^2。光学测试表明,与纯单晶相比,Ge∶β-Ga_2O_3单晶在红外波段存在明显吸收,只有位于蓝光区域的两个荧光峰,抑制了紫外与绿光波段的发光。电学性能测试得出,Ge∶β-Ga_2O_3单晶的电导率在10-3量级,说明掺杂Ge4+对β-Ga_2O_3单晶的电学性能的确有改善。