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Abstract: The barrier effect of Pt and Ni has been investigated in the Ti/Al/Pt/Au and Ti/Al/Ni/Au Ohmic contacts to n-GaN, respectively. Severe indiffusion of Pt and Au has been found in the 800°C annealed Au/Pt/Al/Ti/n-GaN samples. However, Ni does not penetrate into the n-GaN and shows better barrier effect for preventing Au penetration into Au/Ni/Al/Ti/n-GaN samples. The reaction between Ga and Pt has also been identified in the samples annealed at 600°C. At the same time, the electrical degradation was found for the Ti/Al/Pt/Au samples aged at 600°C. However, no obvious degradation was found for the aged Ti/Al/Ni/Au samples. It is suggested that the reaction between Ga and Pt may cause the electrical degradation of the Ti/Al/Pt/Au metallization system. [Copyright &y& Elsevier]

Abstract: Effect of long anneals on densities of different types of threading dislocations (TDs) in GaN films grown onto sapphire substrate by metal-organic chemical vapor deposition was investigated by high-resolution X-ray diffraction. The results showed that the densities of both types of TDs changed obviously but oppositely, and residual stress in the GaN films was relaxed by generating edge-type TDs instead of screw-type TDs. The results obtained from chemical etching experiments and grazing-incidence X-ray diffraction (GIXRD) also supported the proposed defect structure evolution. [Copyright &y& Elsevier]

Abstract: The temperature dependent diffusion and epitaxial behavior of oxidized Au/Ni/p-GaN ohmic contact were studied with Rutherford backscattering spectroscopy/channeling (RBS/C) and synchrotron X-ray diffraction (XRD). It is found that the Au diffuses to the surface of p-GaN to form an epitaxial structure on p-GaN after annealing at 450°C. At the same time, the O diffuses to the metal–semiconductor interface and forms NiO. Both of them are suggested to be responsible for the sharp decrease in the specific contact resistance (ρ c) at 450°C. At 500°C, the epitaxial structure of Au develops further and the O also diffuses deeper into the interface. As a result, the ρ c reaches the lowest value at this temperature. However, when annealing temperature reaches 600°C, part or all of the interfacial NiO is detached from the p-GaN and diffuses out, which cause the ρ c to increase greatly. [Copyright &y& Elsevier]

Abstracts: The microstructure evolution of oxidized Ni (20nm)/Au (20nm) contact to p-GaN with increasing annealing temperature was studied by synchrotron X-ray diffraction (XRD) and powder XRD. With the increase of annealing temperature, it was observed that the Au on the p-GaN gradually grew on the p-GaN with the orientation relationship of Au(111)//GaN(0002). NiO also had such a orientation relationship of NiO(111)//GaN(0002) at 500°C and gradually turned back to polycrystalline structure at higher temperature. The orientation relationship of Au, NiO and GaN implied that the epitaxial crystallization might occur during annealing. The specific contact resistance also reduced to the minimum 2.73×10−4 Ωcm2 at 500°C. Therefore, plausibly the epitaxial crystallization is an important mechanism for lowering the contact resistance between oxidized Ni/Au contacts and p-GaN. [Copyright &y& Elsevier]

In this paper, an approach to determine the average In composition in InGaN/GaN multiple quantum wells (MQWs) was suggested. The period thickness and the average lattice constants (〈aepi〉 and 〈cepi〉) of MQWs were calculated from high-resolution X-ray diffraction (HRXRD). Then by applying the Vegard''s law, the average In composition 〈x〉 was simultaneously determined by considering the relationship between the lattice constants under strain. At the same time, by using the period thickness, the average In composition was obtained from the simulation of the Rutherford backscattering spectroscopy (RBS). The results obtained from the two techniques are agreeable. So the approach of combining both HRXRD and RBS can tell reliable chemical composition in InGaN/GaN MQWs. [Copyright &y& Elsevier]

InGaN/GaN layers have been grown under low pressure by metal-organic vapor phase epitaxy on sapphire substrate. X-ray diffraction (XRD), photoluminescence, and optical absorption measurements have been performed to study the radiative recombination mechanisms in the samples. The In composition was determined by XRD measurement using Vegard''s law. With increasing In composition, a red shift of absorption edge and a broad Urbach tail in absorption spectra were observed. The InN inclusions in InGaN played a key role in long-wavelength absorption. There existed large Stokes shifts in all thick InGaN layers with different In compositions. The intensities of low-energy emissions were enhanced compared to those of high-energy ones with increasing In composition. We attributed low-energy emissions to deep levels and high-energy emissions to In-rich quantum dots. [Copyright &y& Elsevier]

Current–voltage (I–V) characteristics, contact resistance, and optical transmittance measurements are performed to investigate the effects of different annealing ambient conditions on properties of contacts of Ni/Au/p-GaN. With increasing annealing temperature in the range from room temperature to 600 °C in air, I–V characteristics become linear, and the specific contact resistance (ρc) reduces continually. ρc of Ni (20 nm)/Au (50 nm) contact to p-GaN annealed in air at 600 °C is obtained as 2.76×10−2 Ω cm2. However, in N2 ambient, ρc is higher and shows a more complex evolution with increasing annealing temperature. The optical transmittance of the specimens increases and maintains a high value with the annealing temperature in air while it increases first, and then decreases monotonically in N2 atmosphere. We suggest that formation of low-resistance and high optical transmittance p-type Ohmic contact may be attributed to the oxidized Ni layer. The mechanism of Ohmic contact formation under different ambient conditions is also discussed. [Copyright &y& Elsevier]

The evolution of wing tilt in laterally and epitaxially overgrown (LEO) GaN thick film was studied using cross-sectional transmission electron microscope (TEM) observation and high-resolution X-ray diffraction (XRD). The LEO GaN film was grown by hydride vapor epitaxy (HVPE) on Si(1 1 1) substrate. Horizontally propagating dislocations (HDs) and low density of threading dislocation (TD) in LEO wings were observed in TEM micrographs. Tilt of LEO wing to the GaN seed region was measured as 3.3° at interface of HVPE–GaN/metal organic vapor phase epitaxy (MOVPE)–GaN by selective area electron diffraction, which is much more than that in the surface region estimated as 0.4° by XRD measurements. With φ-scan XRD technique, we found that there exist many differences of tilts and twists of c-axis between the opposite wings on each mask. Asymmetry growth of the opposite wings on SiO2 mask may be due to the flow asymmetry of precursor in vertical reactor. The tilting generation and diminishing increasing with film thickness was discussed too. [Copyright &y& Elsevier]

• A tangential XICS has been upgraded and the temporal evolution of Ar density were observed by XICS on J-TEXT. • The behavior of argon was investigated under different discharge conditions. • STRAHL simulation results showed that the convection velocity changed from negative to positive as the ne increased. A tangential X-ray imaging crystal spectrometer (XICS) has been upgraded in the J-TEXT tokamak. The XICS system has been used to measure ion and electron temperature, plasma rotation velocities, selected impurity densities, and can be used for impurity transport. The XICS on J-TEXT was designed to receive Ar XVII emissions from the -0.5 a to +0.5 a region with a spatial resolution of 1.8 cm in the vertical direction, a is the minor radius. The temporal evolution of argon impurity density profiles after an argon gas puffed were observed with a time resolution of up to 2 ms. The behaviour of argon was investigated under different discharge conditions. The experimental results show that, as the density increased, the impurity confinement time obviously increased. Simulation results obtained by STRAHL showed that the convection velocity changed from negative to positive as the electron density increased. This indicated that impurities moved more favourably toward the plasma boundary. [ABSTRACT FROM AUTHOR]

Abstract: The annealing time-dependent diffusion behavior of oxidized Au/Ni/p-GaN ohmic contact was studied with Rutherford backscattering spectroscopy/Channeling (RBS/C). It was found that, at 500°C, 1min annealing caused obvious indiffusion of Au, O and outdiffusion of Ni. And the Au even has diffused to the p-GaN/metal interface. According to the annealing time-dependent RBS spectra and referring to the correspondent specific contact resistance of the samples, it is suggested that indiffusion of Au and formation of NiO at the interface are among the critical mechanisms for the formation of low resistance ohmic contact with Au/Ni/p-GaN contact structure. [Copyright &y& Elsevier]

Abstract: We report the growth and characterization of the AlGaN on low temperature (LT) AlN interlayer (IL) and effect of AlN IL on incorporation efficiency of Al composition in the AlGaN. The AlGaN layers were grown on AlN ILs by metalorganic vapor phase epitaxy (MOVPE) and characterized by X-ray diffraction (XRD) and MeV He ion Rutherford backscattering spectrometry (RBS) and cathodoluminescence (CL), the results indicate that introduction of double AlN IL resulted in an increase in Al composition incorporation efficiency. [Copyright &y& Elsevier]

• The upgraded major imaging optics is designed to work in both 16 and 24 vertical channels configurations. • The radial focus position of each array is able to move from LFS edge to HFS edge independently without affecting the image plane shapes. • Two LO optics systems are demonstrated enough coupling efficiency to drive all mixers on antenna arrays according to the laboratory test. The new quasi-optical electron cyclotron emission imaging (ECEI) diagnostic system on the J-TEXT tokamak has been developed and characterized. The quasi-optical system consists of two subsystems: the major imaging optics system for plasma imaging and the local oscillator (LO) optics system to drive the mixers in the antenna array. For the updated demands, some modifications and adaptations are accomplished on the major imaging optics. The vertical zoom ratio, focus capability, and field curvature have been measured on the J-TEXT facility, with good matching with the simulation results. For the various experimental requirements on J-TEXT, the remote control system is adopted for the major imaging optics. As for LO optics systems, spherical LO optics and hyperbolic LO optics have demonstrated high performance. Uniform power deposition has been optimized to drive all mixers and minimize insertion loss. [ABSTRACT FROM AUTHOR]

• A spectral diagnostic system for carbon transport study combined with three ionization states is presented. • The effect of the local impurity source on the carbon III diagnostics are tested. • Carbon transport asymmetry in edge is verified by modifying the relative position of plasma and limiters. A light impurity diagnostic system is developed for the J-TEXT tokamak. This diagnostic system consists of carbon V (227.09 nm), carbon III (464.7 nm) and edge rotation diagnostics. The carbon V diagnostic consists of a 12-core fiber bundle for light collection, an aberration-corrected monochromator, and a 12-channel photomultiplier tube array. The carbon III diagnostic adopts three identical 18-channel Si photodiode arrays to cover the edge region of the high-field side (HFS), low-field side (LFS) and limiter regions at different toroidal positions, with a specified front filter. The carbon VI (529.26 nm) diagnostic consists of a 10-core fiber bundle, an aberration-corrected monochromator, and an electron-multiplying CCD camera. The system coverage range is 0.2 m to 0.07 m of the HFS. A high-resolution spectroscopic diagnostic can detect carbon III (229.69 nm) and carbon V (227.09 nm) radiation intensities in the LFS. An accurate absolute calibration is carried out by an integrating sphere light source, and a relative position calibration is developed for all diagnoses. Combining the impurity emission profile which derived from inversion and plasma parameter profile, the impurity density of each ion stage and impurity transport coefficients are obtained by a one-dimensional impurity transport code. [ABSTRACT FROM AUTHOR]