蓝宝石衬底上GaN/AlxGa1-xN超晶格插入层对AlxGa1-xN外延薄膜应变及缺陷密度的影响

室温300K下,由于Al_xGa_1-xN的带隙宽度可以从GaN的3.42eV到AlN的6.2eV之间变化,所以Al_xGa_1-xN是紫外光探测器和深紫外LED所必需的外延材料.高质量高铝组分Al_xGa_1-xN材料生长的一大困难就是Al_xGa_1-xN与常用的蓝宝石衬底之间大的晶格失配和热失配.因而采用MOCVD在GaN/蓝宝石上生长的Al_xGa_1-xN薄膜由于受张应力作用非常容易发生龟裂.GaN/Al_xGa_1-xN超晶格插入层技术是释放应力和减少Al_xGa_1-xN薄膜中缺陷的有效方法.研究了GaN/Al_xGa_1-xN超晶格插入层对GaN/蓝宝石上Al_xGa_1-xN外延薄膜应变状态和缺陷密度的影响.通过拉曼散射探测声子频率从而得到材料中的残余应力是一种简便常用的方法,Al_xGa_1-xN外延薄膜的应变状态可通过拉曼光谱测量得到.Al_xGa_1-xN外延薄膜的缺陷密度通过测量X射线衍射得到.对于具有相同阱垒厚度的超晶格,例如4nm/4nm,5nm/5nm,8nm/8nm的GaN/Al_0.3Ga_0.7N超晶格,研究发现随着超晶格周期厚度的增加Al_xGa_1-xN外延薄膜缺陷密度降低,Al_xGa_1-xN外延薄膜处于张应变状态,且5nm/5nmGaN/Al_0.3Ga_0.7N超晶格插入层Al_xGa_1-xN外延薄膜的张应变最小.在保持5nm阱宽不变的情况下,将垒宽增大到8nm,即十个周期的5nm/8nmGaN/Al_0.3Ga_0.7N超晶格插入层使Al_xGa_1-xN外延层应变状态由张应变变为压应变.由X射线衍射结果计算了Al_xGa_1-xN外延薄膜的刃型位错和螺型位错密度,结果表明超晶格插入层对螺型位错和刃型位错都有一定的抑制效果.透射电镜图像表明超晶格插入层使位错发生合并、转向或是使位错终止,且5nm/8nmGaN/Al_0.3Ga_0.7N超晶格插入层导致Al_xGa_1-xN外延薄膜中的刃型位错倾斜30°左右,释放一部分压应变.     

Evidence for a transition in deformation mechanism in nanocrystalline pure titanium processed by high-pressure torsion

Nanocrystalline titanium with an average grain size of about 60–70 nm was prepared by high-pressure torsion. The results of hardness and structural evolutions indicate that a strain-induced hardening–softening–hardening–softening behaviour occurs. For coarse-grained titanium, 〈a〉-type dislocation multiplication, twinning and a high pressure-induced α-to-ω phase transformation play major roles to accommodate deformation, leading to a significant strain hardening. As deformation proceeds, dynamic recrystallisation leads to a decrease in dislocation density, especially for 〈a〉-type dislocations, leading to a slight strain softening. The 〈c〉-component dislocation multiplication dominates the deformation when the grain size decreases to 100 nm and 〈c〉-component dislocation multiplication, grain refinement and the α-to-ω phase transformation contribute to the second strain hardening. The following strain softening is attributed to dynamic recovery.     

Climb via vacancy diffusion of edge dislocations in 2D dislocation microstructures

The prevention of strength degradation of components is one of the great challenges in solid mechanics. In particular, at high temperatures material may deform even at low stresses, a deformation mode known as deformation creep. One of the microstructural mechanisms that governs deformation creep is dislocation motion due to the absorption or emission of vacancies, which results in motion perpendicular to the glide plane, called dislocation climb. However, the importance of the dislocation network for the deformation creep remains far from being understood. In this study, a climb model that accounts for the dislocation network is developed, by solving the diffusion equation for vacancies in a region with a general dislocation distribution. The definition of the sink strength is extended, to account for the contributions of neighbouring dislocations to the climb rate. The model is then applied to dislocation dipoles and dislocation pile-ups, which are dense dislocation structures and it is found that the sink strength of dislocations in a pile-up is reduced since the vacancy field is distributed between the dislocations. Finally, the importance of the results for modelling deformation creep is discussed.     

How to Avoid Plastic Deformation in GaAs Wafers during Molecular Beam Epitaxial Growth

Plastic deformation in two‐inch diameter GaAs wafers resulted from standard thermal treatments which accompanied epitaxial growth in molecular beam epitaxy (MBE) machines of three different makes. Synchrotron based X‐ray transmission topography was used to distinguish between thermal treatment induced dislocation bundles and misfit dislocations. Eradication of the wafer slip related dislocation bundles has been achieved by modifications to the sample holder of a user built MBE machine. These modifications are discussed, the extent of the problem is briefly outlined, and an extrapolation of the susceptibility of GaAs wafers of higher diameters to this type of plastic deformation is given.     

A possible role of amorphous regions at grain-boundary vertex points as linear defects on flux pinning in MgB2 films with columnar grain structure

We measured the transport properties of MgB₂ films having columnar grain structure with their axis normal to the substrate. When an external magnetic field was applied parallel to the grain axis, an enhanced critical current density has been observed, and this result has been ascribed to flux pinning induced by grain boundaries. The shape of the angular dependence of critical current density and its magnetic field dependence showed a quite similar resemblance to those of YBa₂Cu₃O_x films containing columnar defects, implying a possible existence of linear defects in MgB₂ films of columnar structure. We propose that the amorphous regions at the vertex points of three or more grain boundaries observed in microstructural studies correspond to the linear defects and these linear defects anchor the end points of the flux line dislocations of Frank-Read sources, by which the shear in the flux line lattice is actuated. This assumed mechanism is found to reasonably explain the magnetic field dependence of the flux pinning force density of MgB₂ films with columnar grain structure.     

Buckling-induced dislocation emission in thin films on substrates

Atomistic simulations of the evolution of a strained thin film on a substrate has been reported and the formation of dislocations has been observed in the film/substrate interface after the film has buckled. In the framework of the linear elasticity theory, an analytical model has been developed to explain the buckle effect on the formation of the dislocations. A stability diagram with respect to the buckling and dislocation emission phenomena is finally presented for the film as a function of the uniaxial strain and the Burgers vector.     

On the reverse leakage current of Schottky contacts on free-standing GaN at high reverse biases

In this work, a dislocation-related tunneling leakage current model is developed to explain the temperature-dependent reverse current–voltage(I–V –T) characteristics of a Schottky barrier diode fabricated on free-standing GaN substrate for reverse-bias voltages up to-150 V. The model suggests that the reverse leakage current is dominated by the direct tunneling of electrons from Schottky contact metal into a continuum of states associated with conductive dislocations in GaN epilayer.A reverse leakage current ideality factor, which originates from the scattering effect at metal/GaN interface, is introduced into the model. Good agreement between the experimental data and the simulated I–V curves is obtained.     

Continuum representation of systems of dislocation lines: A general method for deriving closed-form evolution equations

Plasticity is governed by the evolution of, in general anisotropic, systems of dislocations. We seek to faithfully represent this evolution in terms of density-like variables which average over the discrete dislocation microstructure. Starting from T. Hochrainer's continuum theory of dislocations (CDD) (Hochrainer, 2015), we introduce a methodology based on the ‘Maximum Information Entropy Principle’ (MIEP) for deriving closed-form evolution equations for dislocation density measures of different order. These equations provide an optimum representation of the kinematic properties of systems of curved and connected dislocation lines with the information contained in a given set of density measures. The performance of the derived equations is benchmarked against other models proposed in the literature, using discrete dislocation dynamics simulations as a reference. As a benchmark problem we study dislocations moving in a highly heterogeneous, persistent-slip-band like geometry. We demonstrate that excellent agreement with discrete simulations can be obtained in terms of a very small number of averaged dislocation fields containing information about the edge and screw components of the total and excess (geometrically necessary) dislocation densities. From these the full dislocation orientation distribution which emerges as dislocations move through a channel-wall structure can be faithfully reconstructed.