The influence of resistivity for n-AlGaN is investigated by changing Si concentration, growth temperature and growth rate. It is found that the resistivity strongly depended on the growth conditions and it reaches to a small value of 5 x 10-3 omega cm at the growth temperature of 1040 degrees C. In addition, it is found that there is a strong correlation between the resistivity and the PL emission intensity of group-III-vacancy-Si (VIII-Si) complexes. It indicates that defect compensation can play a key role in high resistivity of n-AlGaN films.

A spin-triplet superconductor can harbor Majorana bound states that can be used in topological quantum computing. Recently, K2Cr3As3 and its variants with critical temperature Tc as high as 8 kelvin have emerged as a new class of superconductors with ferromagnetic spin fluctuations. Here, we report a discovery in K2Cr3As3 single crystal that the spin susceptibility measured by 75As Knight shift below Tc is unchanged with the magnetic field H0 applied in the ab plane but vanishes toward zero temperature when H0 is along the c axis, which unambiguously establishes this compound as a spin-triplet superconductor described by a vector order parameter d→ parallel to the c axis. Combining with point nodal gap, we show that K2Cr3As3 is a new platform for the study of topological superconductivity and its possible technical application.

The mechanism for the formation of V-pits in InGaN/GaN multi-quantum well (MQW) growth and its effect on the performance of GaN based laser diodes (LDs) are investigated in detail. It is observed that the V-pits in InGaN/GaN MQWs begin at the GaN barrier layer rather than InGaN well layer due to the low atomic migration ability of Ga atoms at low growth temperature. In addition, it is found that the formation of V-pits can be suppressed remarkably when a small amount of In atoms is introduced during GaN barrier layer growth. This may be attributed to the decrease of the potential barrier for atom migration around the dislocation region. In this way, the thermal stability of InGaN/(In)GaN MQWs is enhanced. Therefore, the threshold current decreases abruptly when using InGaN/InGaN MQWs instead of InGaN/GaN MQWs.

Performance of InGaN-based laser diodes (LDs) with different Mg concentrations of electron blocking layer (EBL) is investigated by simulation and experimental methods. It is found from the simulation results that the threshold current decreases and slope efficiency increases, when the Mg concentration of EBL increases from 2 × 10 18 to 6 × 10 19 cm −3 ; it is attributed to the suppression of the leakage of electrons and the enhancement of the injection of holes due to the variation of potential barrier for them as the increase of Mg concentration of EBL. These simulation results agree well with the experimental ones, when the Mg concentration of EBL is lower than 7.5 × 10 18 cm −3 . However, it deteriorates when the Mg concentration increases to 1.2 × 10 19 cm −3 . It may be due to the increase of the absorption loss of LDs.

The link between the Milton pallasite and the South Byron Trio irons is examined through metallography and metallogaphic cooling rates; major, minor, and trace element compositions of metal; inclusion mineralogy and mineral compositions; and oxygen isotopic compositions. The metallic hosts of these Ni-rich meteorites (18.2-20.3 wt% Ni) are dominated by plessite with spindles of kamacite and schreibersite. The presence of similar to 50 nm wide tetrataenite and absence of high-Ni particles in the cloudy zone in Milton suggest cooling of similar to 2000 K/Myr or >10,000 K/Myr. Compositionally, the metallic host in all four meteorites exhibits modest (1-2 orders of magnitude compared to CI chondrites) depletions of volatile elements relative to refractory elements, and marked depletions in the redox sensitive elements W, Mo, Fe, and P. Oxygen isotopic compositions (Delta O-17) are, within uncertainty, the same for the Milton and the South Byron Trio and for IVB irons. Similarities in metallography, metal composition, inclusion mineralogy, and oxygen (Delta O-17), molybdenum and ruthenium isotopic composition suggest that the Milton pallasite and South Byron Trio irons could have originated on a common parent body as chemically distinct melt, or on separate parent bodies that experience similar cosmochemical and geochemical processes. The Milton pallasite and South Byron Trio irons share a number of properties with IVB irons, including metallography, enrichment in highly siderophile elements and nickel, inclusion mineralogy and oxygen isotopic composition, suggesting they formed in a similar nebular region through common processes, although Milton and the South Byron Trio did not experience the dramatic volatile loss of the IVB irons. Depletions in W, Mo, Fe, and P relative to elements of similar volatility likely result from oxidation, either in the nebula prior to accretion or on the parent body during melting. Oxidation of similar to 73 wt% Fe is indicated, with a correspondingly FeO-rich mantle and smaller core. If Milton and the South Byron Trio sample a common core, Milton formed near the surface of the core after stripping of the silicate shell and may have experienced rapid solidification and contamination by an impactor. The molten core, from which the South Byron Trio irons crystallized, solidified from the outside in. (C) 2019 Elsevier Ltd. All rights reserved.

Although superconductivity in the vicinity of antiferromagnetic (AFM) instability has been extensively explored in the last three decades or so, superconductivity in compounds with a background of ferromagnetic (FM) spin fluctuations is still rare. We report 75As nuclear quadrupole resonance measurements on the A2Cr3As3 family, which is the first group of Cr-based superconductors at ambient pressure, with A being alkali elements. From the temperature dependence of the spin-lattice relaxation rate (1/T1), we find that by changing A in the order of A=Na, Na0.75K0.25, K, and Rb, the system is tuned to approach a FM quantum critical point (QCP). This may be ascribed to the Cr2-As2-Cr2 bond angle that decreases towards 90 degrees, which enhances the FM interaction via the Cr2-As2-Cr2 path. Upon moving away from the QCP, the superconducting transition temperature Tsc increases progressively up to 8.0 K in Na2Cr3As3, which is in sharp contrast to the AFM case where Tsc usually shows a maximum around a QCP. The 1/T1 decreases rapidly below Tsc with no Hebel-Slichter peak, and ubiquitously follows a T5 variation below a characteristic temperature T*=0.6 Tsc, which indicates the existence of point nodes in the superconducting gap function commonly in the family. These results suggest that the A2Cr3As3 family is a possible solid-state analog of superfluid 3He.

The Spectrometer Ring (SRing) is an essential part of the High Intensity heavy-ion Accelerator Facility project (HIAF) in China. It is designed as a multi-functional experimental storage ring, which will be able to operate in three ion optical operation modes. The SRing will be used as a time-of-flight mass spectrometer for short-lived, especially neutron-rich nuclei. It will also be used to collect and cool Rare Isotope Beams (RIBs) or highly-charged stable ion beams for nuclear and atomic physics experiments. The design magnetic rigidity is in the range 1.5 to 15 Tm. The beam cooling system consists of stochastic cooling and electron cooling devices. With a help of an electron cooler, stored ions will be decelerated to a minimum energy of 30 MeV/u by RF cavities. The extraction system of the SRing will allow cooled ion beams to be extracted to an external target for further ion manipulations or reaction experiments. The general ion optics design and technical requirements of SRing subsystems are presented and discussed in this paper.

Three InGaN/GaN multi-quantum well (MQW) samples are grown on c-plane sapphire substrate and free standing GaN substrate, respectively. The emission and structural sproperties of these samples are studied in detail. It is found that the thermal stability and emission homogeneity of InGaN/GaN MQWs grown on GaN substrate is much better than that grown on sapphire substrate. This may be attributed to the lower density of threading dislocations in InGaN/GaN MQW regions grown on GaN substrate. Threading dislocations often end with V-pits at InGaN surface, and often contain In-rich clusters around them. These In-rich regions are easy to segregate during high temperature p-type layer growth, leading to the degradation of InGaN/GaN MQWs ultimately.

InGaN samples are grown using metalorganic chemical vapor deposition (MOCVD) and the dependences of structural and luminescence properties of InGaN layers on growth temperature are studied. It is found that the luminescence properties of InGaN layer are improved by increasing growth temperature properly. However, when the growth temperature of InGaN layer is too higher (740 degrees C in our work), a large amount of unintentionally incorporated gallium atoms enter into InGaN, and a spiral growth mode dominates in this case. It results in an inferior crystalline and interface quality, and ultimately degrades the luminescence of InGaN. (C) 2017 Elsevier Ltd. All rights reserved.

An optical memory effect is found in a 20 nm InGaN film. With increasing illumination time, photoluminescence (PL) intensity of InGaN rises at first and then falls. We present that this effect is caused by carriers capture in deep levels near interfaces between GaN and InGaN. Firstly, carriers captured by deep levels near the interfaces reduces the band inclination in InGaN. This cause the rise of PL intensity. Secondly, more and more captured carriers may form anti-shielding, which enhances band inclination and results in the decrease of PL intensity. Carriers captured in previous illumination can remain for a long time after illumination is blocked, which make InGaN show an optical memory effect.

Transportation of carriers in c-plane InGaN-based blue laser diodes (LDs) has been investigated by both experimental and simulation methods. It is found that excepting the leakage of electrons, holes may overflow from InGaN/GaN multiple quantum wells (MQWs) and enter into InGaN lower waveguide (LWG) layer. It leads to the increase of recombination rate in LWG layer and results in the waste of carriers. Ultimately degrades the performance of LDs. In addition, we also found that reducing the thickness of GaN first barrier (FB) layer can suppress the leakage of holes due to the decrease of the effective depth of potential well in InGaN LWG layer, a lower effective depth of potential well in InGaN LWG layer is helpful the holes coming back to well layer and therefore the threshold current decreases to 80% when the FB layer thickness decreases from 20 nm to 1 nm.

Series of green laser diodes (LDs) with different (In)GaN barrier layers are investigated. It is found that the optical confinement factor of multi-quantum well (MQW) always increases with increasing indium content of InGaN barrier layer, which results in a decrease of threshold current when indium content of InGaN barrier layer increases from 0 to 5%. However, when a high In content InGaN barrier is used (> 5%), both threshold current and slop efficiency of LDs deteriorate. It may be attributed to the waste of carriers in the potential well at the interface between the last barrier (LB) and the upper waveguide (UWG) layers, which is induced by the piezoelectric polarization effect in high In content InGaN LB layer. Therefore, a new LD structure using a thin thickness of the LB layer to reduce the effect of polarization shows a low threshold current and a high output power even when the In content of barrier layers is as large as 7%.

The optical power emitting from the cavity facet of blue InGaN-based laser diodes (LDs) is measured to investigate the efficiency droop. The efficiency droop behavior of blue InGaN-based LDs near the threshold is confirmed in our experiments. From measurements of optical power at different wavelengths, it is analyzed that the droop behavior of LDs can be ascribed to the efficiency reduction of longer wavelengths. The efficiency of longer wavelengths is subject to the carrier occupation process in quantum levels. In addition, it is found that the droop behavior may be largely affected by the relatively large threshold current of InGaN-based LDs and the screening effect of polarization, and it can be suppressed by stimulated emission.

In this work, the Cp2Mg flux and growth pressure influence to Mg doping concentration and depth profiles is studied. From the SIMS measurement we found that a transition layer exists at the bottom region of the layer in which the Mg doping concentration changes gradually. The thickness of transition layer decreases with the increases of Mg doping concentration. Through analysis, we found that this is caused by Ga memory effect which the Ga atoms stay residual in MOCVD system will react with Mg source, leading a transition layer formation and improve the growth rate. And the Ga memory effect can be well suppressed by increasing Mg doping concentration and growth pressure and thus get a steep Mg doping at the bottom region of p type layer.

The hole distribution and electroluminescence property improvement by adjusting the relative position between quantum wells and p-doped region in InGaN/GaN multiple-quantum-well structures are experimentally and theoretically investigated. Five designed samples with different barrier layer parameters of multiple-quantum-well structure are grown by MOCVD and then fabricated into devices. The electroluminescence properties of these samples are measured and compared. It is found that the output electroluminescence intensity of samples is enhanced if the position of quantum wells shifts towards p-side, while the output power is reduced if their position is shifted towards the n-side. The theoretical calculation of characteristics of these devices using the simulation program APSYS agrees well with the experimental data, illustrating that the effect of relative position between p-doped region and quantum wells on the improvement of hole distribution and electroluminescence performance is significant, especially for InGaN/GaN multiple-quantum-well devices operated under high injection condition.

Three series of samples with different NH 3 flow rate are grown and the optical and structural properties are investigated. It is found that apart from a positive effect on keeping a high partial pressure of nitrogen to enhance indium incorporation, NH 3 may also play a negative effect on indium incorporation during InGaN growth. Especially, when temperature is relatively high, the hydrogen generated from the dissociation of NH 3 may suppress the chemical reaction which produces InN, leading to a reduced indium incorporation efficiency during the InGaN layer growth.

In this work, epitaxial GaN with different Mg doping concentration annealed at different temperature is investigated. Through Hall and PL spectra measurement we found that when Mg doping concentration is different, different annealing temperature is needed for obtaining the best p-type conduction of GaN, and this difference comes from the different influence of annealing on compensated donors. For ultra-heavily Mg doped sample, the process of Mg related donors transferring to non-radiative recombination centers is dominated, so the performance of P-GaN deteriorates with temperature increase. But for low Mg doped sample, the process of Mg related donors transfer to non-raditive recombination is weak compare to the Mg acceptor activation, so along the annealing temperature increase the performance GaN gets better.

The field-emission (FE) characteristics of 200 nm-thick Si-doped AlN films with different concentration of Si-dopant is investigated. It is found that the AlN film with lower concentration of Si-dopant has a smaller turn-on voltage, larger maximum current density and more stable FE current. Accompanying with atomic force surface micro-images, it is suggested that the stable current is attributed to a uniform surface, which is related a uniform local field enhancement factors and thus a stable FE current without discontinuous fluctuations. Furthermore, the analysis of scanning electron microscopy indicates that a smaller turn-on voltage and larger maximum current density may result from the electron transport channel of V-defects.

Three series of InGaN samples with different growth pressures are grown in a vertical metal organic chemical vapor deposition (MOCVD) system and the indium incorporation efficiency during InGaN layer growth is investigated. It is found that the indium content in InGaN layer decreases when the NH3 flow rate increases at a higher growth pressure and it increases with the NH3 flow rate at a lower growth pressure, This may be attributed to the higher dissociation rate of NH3 into N2 and H2 at a higher growth pressure, leading to a higher H2 concentration in reactor during InGaN growth. Therefore, changing growth conditions to suppress the dissociation of NH3 into N2 and H2 can increase the indium incorporation efficiency during InGaN film growth.

Photoelectron spectroscopy has been employed to analyze the content and chemical states of the elements on the surface of AlN films with different thickness, which are synthesized by metalorganic chemical vapor deposition on the n-type SiC substrates under low pressure. It is found that, besides the carbon and gallium on the AlN surface, the atom percentage of surface oxygen increases from 4.9 to 8.4, and the electron affinity also increases from 0.36 to 0.97 eV, when the thickness of AlN films increase from 50 to 400 nm. Furthermore, accompanying with the high-resolution XPS spectra of the O 1s, it is speculated that surface oxygen may be the major influence on the electron affinity, where the surface oxygen changes the surface chemical states through replacing N to form Al–O bond and Ga–O bond, although there are also a few of Ga and C contaminations in the chemical sate of Ga–O and C–C, respectively.