Optical investigations of Zn,Hg and Li doped GaN

The use of infrared quenching of photoluminescence to study the spectral dependence of the photoionization cross-section in GaN doped with Zn, Hg, and Li is reported. It is shown that these impurities produce deep level centres 0.48 eV, 0.41 eV, and 0.75 eV, respectively, above the valence band. In addition, excitation spectra are investigated for Zn and Li doped GaN, giving values of 3.17 eV and 2.86 eV at 78 K for the energy distance of these levels from the conduction band. Finally, from the temperature dependence of the excitation spectra, it is concluded that the levels are probably pinned to the valence band.     

Hydrogen depassivation of the magnesium acceptor by beryllium in p-type GaN

Under nitrogen-rich growth conditions, the present ab initio study predicts that hydrogen passivation is more effective on the acceptor Be instead of Mg in a co-doped p-type GaN. The formation energy is 0.24 eV for (H-Be_Ga) complex, and 0.46 eV for (H-Mg_Ga) complex. Congruently, the binding energy is 1.40 eV for (H-Be_Ga), and 0.60 eV for (H-Mg_Ga). Owing to the lower binding energy, (H-Mg_Ga) is not thermally stable. As Be is incorporated in Mg-doped GaN, a (H-Mg_Ga) may release a H⁺ cation at relatively elevated temperatures. Consequently, the H⁺ diffuses swiftly away from a Mg⁻_Ga, across a barrier of 1.17 eV, towards a Be⁻_Ga and forms a stable (H-Be_Ga) with it. The activation of Mg acceptors can be thus facilitated. In this view, the process of hydrogen depassivation of the Mg acceptor by Be can convert the as-grown high-resistivity Mg-doped GaN into a p-conducting material, as observed in the experiments.     

Growth studies of m-GaN layers on LiAlO2 by MOCVD

This paper reports that the m-plane GaN layer is grown on （200）-plane LiAlO2 substrate by metal-organic chemical wpour deposition （MOCVD） method. Tetragonal-shaped crystallites appear at the smooth surface. Raman measurement illuminates the compressive stress in the layer which is released with increasing the layer＇s thickness. The high transmittance （80%）, sharp band edge and excitonic absorption peak show that the GaN layer has good optical quality. The donor acceptor pair emission peak located at -3.41 eV with full-width at half maximum of 120 meV and no yellow peaks in the photoluminescence spectra partially show that no Li incorporated into GaN layer from the LiAlO2 substrate.     

掺Er/Pr的GaN薄膜深能级的研究

利用深能级瞬态谱(DLTS)、傅里叶变换红外光谱(FT-IR)对GaN以及GaN掺Er/Pr的样品进行了 电学和光学特性分析.研究发现未掺杂的GaN样品只在导带下0.270eV处有一个深能级；GaN注 入Er经900℃,30min退火后的样品出现了四个深能级，能级位置位于导带下0.300 eV,0.188 eV,0.600 eV 和0.410 eV；GaN注入Pr经1050℃,30min退火后的样品同样出现了四个深能级 ，能级位置位于导带下0.280 eV,0.190 eV,0.610 eV 和0.390 eV；对每一个深能级的来源 进行了讨论.光谱研究表明，掺Er的GaN样品经900℃,30min退火后，可以观察到Er的1538nm 处的发光，而且对能量输运和发光过程进行了讨论. 关键词： GaN Er Pr 深能级     

The fate of Li and Be in stars and in the laboratory

We connect the observed under-abundances of Li and Be in dwarfs, with recent results on nuclear cross sections at low energies: for collisions of protons with atomic or molecular targets, the measured cross sections seem too high with respect to extrapolations for bare nuclei. Phenomenologically, these anomalous nuclear interactions can be described in terms of an effective screening potentialU _lab in the range of few hundred eV: in the presence of the electron cloud, nuclei become more transparent to each other as if the effective collision energy is aumented byU _lab. This implies that fusion cross sections are enlarged and at the same time elastic cross sections are lowered. If something similar occurs in stellar plasma, the nuclear burning temperatures are lowered, whereas diffusion processes are enhanced. We find that the observed Li and Be abundances in the Hyades and in the Sun can be reproduced for effective screening potentials of the plasma in the range of 600–700 eV, close to that found by experiments in the laboratory.     

Primordial beryllium as a big bang calorimeter

Many models of new physics including variants of supersymmetry predict metastable long-lived particles that can decay during or after primordial nucleosynthesis, releasing significant amounts of nonthermal energy. The hadronic energy injection in these decays leads to the formation of ?Be via the chain of nonequilibrium transformations: Energy(h)→T, 3He→?He, ?Li→?Be. We calculate the efficiency of this transformation and show that if the injection happens at cosmic times of a few hours the release of O(10 MeV) per baryon can be sufficient for obtaining a sizable ?Be abundance. The absence of a plateau structure in the ?Be/H abundance down to a O(10?1?) level allows one to use beryllium as a robust constraint on new physics models with decaying or annihilating particles.     

GaN深中心Zn的低温瞬态研究

本文研究了GaN:Zn的低温瞬态过程,同时测量了衰减中的时间分辨光谱,从两者测量的结果得出:2.89eV的光致发光寿命为300ns(2K).实验证实,Zn作为发光中心的同时也引起一些非辐射陷阱.2.89eV的瞬态曲线可分为两部分,在短时范围(1μs)内基本上是指数形式,可归结为导带中光激发的电子和束缚在ZnGa受主上空穴的复合.在长时范围(t>>1μs)内瞬态曲线则偏离了指数规律,相当好地符合Becqureal经验公式.     

Proprietes optiques de couches de Be evaporees et etudiees sous ultra-vide dans L'u.v. De 2,5–25 eV

Optical constants of thin films of Be are obtained from reflectance measurements between 2 and 25 eV. These films are obtained by evaporation in ultra-high vacuum and studied in situ. The optical conductivity σ and real part ?₁ of the dielectric constant are compared with previous theoretical and experimental results. The agreement is good, particularly for the important structure of σ at 4.8 eV. The volume plasmon is situated at 19.2 ± 0.4 eV and the surface plasmon at 13.6 eV.     

Fe掺杂GaN光电特性的第一性原理研究

基于密度泛函理论体系,计算了本征GaN材料和12.5%的Fe掺杂GaN体系的光电特性,分析了晶体结构、能带结构和电子态分布、介电函数、吸收系数、折射率、反射率、能量损失谱和消光系数,从理论上讨论了掺杂对体系光电特性的影响.计算所得理想GaN的禁带宽度为3.41 eV,Fe的重掺杂体系明显变窄,为3.06 eV,但仍为直接带隙半导体.本征GaN材料与Fe掺杂GaN体系的静态介电常数为5.74和6.20,折射率为2.39和2.48,能量损失最大值在20.02 eV和18.96 eV,最大吸收系数能量均在13.80 eV左右.计算结果为Fe掺杂GaN高压光电导开关材料及器件的进一步研究提供了有力的理论依据和实验支持.     

Effect of high temperature annealing on strain and band gap of GaN nanoparticles

Black-coloured GaN nanoparticles with an average grain size of 50 nm have been obtained by annealing GaN nanoparticles under flowing nitrogen at 1200 oC for 30 min. XRD measurement result indicates an increase in the lattice parameter of the GaN nanoparticles annealed at 1200 oC, and HRTEM image shows that the increase cannot be ascribed to other ions in the interstitial positions. If the as-synthesised GaN nanoparticles at 950 oC are regarded as standard, the thermal expansion changes nonlinearly with temperature and is anisotropic; the expansion below 1000oC is smaller than that above 1000 oC. This study provides an experimental demonstration for selecting the proper annealing temperature of GaN. In addition, a large blueshift in optical bandgap of the annealed GaN nanoparticles at 1200 oC is observed, which can be ascribed to the dominant transitions from the C(Γ₇) with the peak energy at 3.532 eV.     

Band transitions in wurtzite GaN and InN determined by valence electron energy loss spectroscopy

Valence electron energy loss spectroscopy (VEELS) was applied to determine band transitions in wurtzite InN, deposited by molecular beam epitaxy on (0001) sapphire substrates or GaN buffer layers. The GaN buffer layer was used as VEELS reference. At room temperature a band transition for wurtzite InN was found at (1.7±0.2 eV) and for wurtzite GaN at (3.3±0.2 eV) that are ascribed to the fundamental bandgap. Additional band transitions could be identified at higher and lower energy losses. The latter may be related to transitions involving defect bands. In InN, neither oxygen related crystal phases nor indium metal clusters were observed in the areas of the epilayers investigated by VEELS. Consequently, the obtained results mainly describe the properties of the InN host crystal.     

Near-Field Evaluation of a Quantum Size Effect in Self-Aligned GaN Whiskers Fabricated Using Photochemical Etching

We fabricated nanoscale GaN whiskers using photochemical etching. The fabricated GaN whiskers were conjugated and aligned perpendicular to the incident light polarization used for photochemical etching in a self-assembling manner. Their far-field photoluminescence spectra exhibited a blue-shifted photoluminescence peak at 3.60eV. Near-field photoluminescence spectra of individual GaN whiskers were obtained, for the first time. The evaluation of the near-field spectra identified several peaks from individual whiskers, corresponding to a diameter range of 5–10 nm, and revealed a stepwise change in the diameter along the axis of individual whisker.     

Measurement of the ~2H(~7Be,~6Li)~3He reaction rate and its contribution to the primordial lithium abundance

In the standard Big Bang nucleosynthesis(SBBN) model, the lithium puzzle has attracted intense interest over the past few decades, but still has not been solved. Conventionally, the approach is to include more reactions flowing into or out of lithium, and study the potential effects of those reactions which were not previously considered.~7Be(d,~3He)~6Li is a reaction that not only produces~6Li but also destroys~7Be, which decays to~7Li, thereby affecting~7Li indirectly. Therefore, this reaction could alleviate the lithium discrepancy if its reaction rate is sufficiently high.However, there is not much information available about the~7Be(d,~3He)~6Li reaction rate. In this work, the angular distributions of the~7Be(d,~3He)~6Li reaction are measured at the center of mass energies Ecm = 4.0 Me V and 6.~7Me V with secondary~7Be beams for the first time. The excitation function of the~7Be(d,~3He)~6Li reaction is first calculated with the computer code TALYS and then normalized to the experimental data, then its reaction rate is deduced. A SBBN network calculation is performed to investigate its influence on the~6Li and~7Li abundances. The results show that the~7Be(d,~3He)~6Li reaction has a minimal effect on~6Li and~7Li because of its small reaction rate. Therefore,the~7Be(d,~3He)~6Li reaction is ruled out by this experiment as a means of alleviating the lithium discrepancy.     

Work function dependence of the Li(2p) formation in slow collisions of Li atoms with partially cesiated tungsten surfaces

The reflection of Li atoms from partially cesiated polycrystalline tungsten is studied at impact energies between 100 and 1000 eV at near-grazing incidence angles. The work function dependence of the yield for Li(2p) formation is studied between φ = 1.27 and 4.8 eV together with the probability for Li⁺ formation. While Li⁺ formation is only important for φ > 3.5 eV, the probability for Li(2p) excitation has its maximum of several percents near (at φ = 1.7±0.1 eV), but not exactly at the minimum of the work function. The dependence of the absolute photon yield is given as a function of the impact energy. The polarization properties of the Li(2p → 2s) photons are also studied. The angular distribution of the excited Li(2p) atoms for various φ values is obtained by applying photon-scattered atom coincidence techniques. An effort is made to understand the Li(2p) formation on cesiated metal surfaces on the basis of the nonadiabatic coupling between different Li states induced by the motion of the Li atom relative to the surface.     

Comparison of 7Li , 7Be + 9Be elastic scattering in the coupled-reaction-channels approach

The data for the ⁷Li + ⁹Be and ⁷Be + ⁹Be elastic scattering at the energies E _lab(⁷Li) = 15.75 , 24, 30, 34, 63 and 130MeV and E _lab(⁷Be) = 17 , 19 and 21MeV were analyzed within the optical model and coupled-reaction-channels method. The elastic and inelastic scattering, reorientation of ⁷Li , ⁷Be and ⁹Be as well as most important transfer reactions were included in the coupled-channels scheme. The resulting ⁷Li and ⁷Be potentials are very similar and have the same energy dependence. The real potential for recently derived ⁸Be + ⁹Be scattering potentials is very similar to that for ⁷Li , ⁷Be but the imaginary part of the ⁸Be one has a much greater strength at longer range.     

Search for solar-neutrino related M1 transitions in 6Li using 180° electron scattering

The ⁶Li analog of the 11.5 MeV, 0⁺ state in ⁶Be proposed to explain the low flux of high-energy solar neutrinos has been searched for and not observed. An upper limit of 3 eV is placed on the M1 ground-state width of any such narrow 0⁺ state.     

A new simulated neutron workplace field at PIAF

The range of neutron energies encountered at workplaces extends from 10^?2 eV to 10⁷ eV or even higher neutron energies. The monoenergetic neutron calibration fields cover the neutron energy range from 10⁴ eV to 10⁷ eV. Hence calibrations in so-called realistic fields with a broad spectral distribution similar to those at workplaces are still essential for radiation protection equipment.This is the reason why PTB has developed a simulated workplace field. The field is produced using a proton beam on a thick Li or Be target installed in a moderating sphere which produces the intermediate and thermal part of the spectrum. Different target materials and compositions and different constructions of the target were investigated with special focus on the long-term stability of the neutron yield, as well as the stability of the spectral neutron fluence of the primary neutron spectrum. The spectral distribution outside the moderator sphere was measured using the PTB Bonner sphere spectrometer NEMUS and calculated using MCNPX.     

Measurement of astrophysical S-factor for 9Be(d, α0)7Li and 9Be(d, α1)7Li* reactions at low energies

The thick-target yield of the ⁹Be(d,α0)7Li and ⁹Be(d,α1)7Li*reactions has been first directly measured over deuteron energies from 66 to 94 keV.The obtained S(Ei)ofα0 andα1 have similar trends calculated by the thin-target yield,consistent with Yan’s report within the errors.Furthermore,the parametric expression of S(E)was obtained to calculate the theoretical thick target yield,and it roughly agrees with the experimental thick target yield.     

Segregation of dissolved Li to the surface of Si: A new activation process

Photoemission measurements have been performed on Li covered Si samples. The Li layer is formed: (i) by adsorption, (ii) by segregation of dissolved Li to the Si surface.These two methods of activation give comparable results: the photoemissive quantum yield is about 10 per cent at 5 eV and the threshold is 2.5 eV for p type Si. Moreover, the surface layer obtained by segregation seems highly pure and uniform. The segregation kinetics are analyzed using a simple theoretic model.     

Atomic layer deposition coating of ZnO shell for GaN-ZnO core-sheath heteronanowires

We have synthesized GaN-core/ZnO-shell nanowires and investigated effects of the ZnO coating. The X-ray diffraction pattern showed that as-synthesized samples are composed of GaN and ZnO. Transmission electron microscopy indicated that the deposited ZnO shell layer is poly-crystalline. The photoluminescence (PL) spectrum of GaN has been changed by the ZnO coating, where emission bands centered at roughly 1.9 eV, 2.5 eV, and 3.3 eV were newly added to the emissions from core GaN nanowires. We found that overall PL intensity has been significantly increased by coating the ZnO shell layers.