Papers by Keyword: Critical Thickness

Abstract: Morphology-related anomaly was found in the photoluminescence (PL) of Ge/Si type-II double heterostructures with varying Ge coverage. PL Stokes shifts of 2-D Ge wetting layers functioning as a quantum well for holes switched from negative to positive under electric field as Ge coverage crossed 3.7 monolayers, which correlates well with the onset of Stranski-Krastanow growth mode transition. The origin of such shift polarity switch is discussed in terms of the linear Stark effect in an asymmetric confinement potential developing due to 3-D Ge islands.

Abstract: The performances of heterostructural devices are often limited by misfit dislocation. In this paper, a theoretical approach for misfit dislocation reduction of wurtzite In_xGa_1-xN/GaN is presented. The linear and exponential grading techniques have been modeled for the reduction of dislocation. An energy balance model has been taken into consideration and modified for wurtzite structure to evaluate the misfit dislocation density. The value of misfit dislocation has been reduced from 7.112×10¹⁰ cm^-2 to 6.19×10⁶ cm^-2 and 7.039×10¹⁰ cm^-2 to 6.121×10⁶ cm^-2 at the plane 1/3<> {} and 1/3<>{} respectively for linear grading. In case of exponential grading the dislocation density has been reduced to 2.762×10⁵ cm^-2 for both slip systems. Because of tapered grading coefficient a tapered dislocation profile has been reported in case of exponential grading technique. Finally, a comparative study has been shown among without graded, linear and exponential grading.

Abstract: During the growth of an epitaxial overlayer on a thick substrate (GeSi on Si), an interfacial misfit dislocation becomes energetically favourable on exceeding the critical thickness. In substrates of finite thickness, the value of critical thickness is altered with respect to thick substrates. Thin substrates can bend and partially relax the coherency stresses, thus contributing to the altered value of the critical thickness. The current work aims at simulating the stress state of a growing finite epitaxial overlayer on a substrate of finite thickness, using finite element method. The numerical model is used to calculate the critical thickness for substrates with finite thickness. Eigenstrains will be imposed in selected regions in the domain towards this end. Size of the substrate for which it is not energetically favourable to accommodate a misfit dislocation is determined from the simulations (i.e. the system remains coherent for substrates below this thickness). Important effects arising in the transition regime of substrate thicknesses are also investigated.

Abstract: With the development of economical society and the stone industry, the product of granite thin slab becomes a trend, and gets the welcome of people. The grinding technology influences the quality of granite thin slab. This paper first introduced the situation of the grinding technology of granite thin slab, analyzed the mechanism of the grinding process of granite thin slab and fractured problem by observing the fractured granite thin slab. Then, based on the Hoek-Brown criterion, we researched the fracture criterion and the critical thickness of granite thin slab when grinding. Finally, we set up the fracture criterion of the granite thin slab and built the mathematical model between the thickness of the granite thin slab and grinding process parameters. According to the thickness of the granite thin slab, grinding process parameters can be formulated, in order to ensure the granite thin slab no fractured.

Abstract: This paper presents a theoretical calculation of misfit dislocation and strain relaxation in compositionally step graded In_xGa _1-x N grown on GaN using the total dislocation energy at each interface. The results also compared with uniform layer of In _0.17 Ga _0.83 N and In _0.14 Ga _0.86 N grown differently on GaN. Due to having residual strain and a step increase in indium composition a lower misfit strain in upper layers and hence larger critical thickness at each interface has been reported. These effects significantly reduced the misfit dislocations from 2.6×10⁵ cm^-1 to 9.5×10⁴ cm^-1 in step graded In _0.14 Ga _0.86 N(500nm)/In _0.09 Ga _0.91 N(100nm)/In _0.05 Ga _0.95 N(100nm)/GaN layers instead of a uniform In _0.14 Ga _0.86 N(700nm)/GaN. A small residual strain of 0.0007 after 700 nm graded layer thickness has been reported with 87.04% strain relaxation.

Abstract: Based on the broadly obtained experimental results, a particular simulant curve was firstly given to visually describe the variety of friction coefficient (μ) with the thickness of sulfide layer on hard steel substrate. The curve includes two inflexions named super-critical thickness (tC2) and sub-critical thickness (tC1) respectively. By means of “micro-peak and soft/hard plane” contact model given in the present work, physical definition and influence factors of tC1and tC2 were discussed in detailed. Theory analysis presents that a friction-reducing coating would obtain a minimum of μ at tC1，and good friction-reducing and anti-wear behaviors with its thickness range from tC1to tC2. Therefore, it is beneficial to improve tC2 or decrease tC1, especially keep a wide difference ( tC = tC2 - tC1). Herein, two optimized designation principles of the sulfide layer on a hard steel substrate were presented, which apparently apply to all lubricate situation with soft layers.

Abstract: We investigate critical thicknesses of InGaN epilayers grown on GaN substrates with the growth-plane not being the c-plane. In particular, we focus on non-polar orientations with growth planes being the m- and a-planes. We have taken into account the proper hexagonal symmetry of wurtzite GaN. We have found that there is only a small difference in the critical thickness for the cplane and the a-plane material; however, in the case of the m-plane material, we predict a quite different behaviour along the (in-plane) c-axis and the perpendicular (in-plane) a-direction.

Abstract: Epitaxial group-III nitride films, although in single crystalline form, contain still a large number of threading dislocations. These set limits to performance and lifetime of devices, notably to high power structures like lasers. The strategy in material development was and will be (at least until lattice-matched substrates become available) to reduce the dislocation densities. The present contribution elaborates on possible dislocation origination mechanisms that determine the population of dislocations in the epitaxial layers. These mechanisms can be controlled to a certain degree by proper deposition procedures. The achieved dislocation populations then determine the processes that can reduce the dislocation densities during growth of the epitaxial layers. The mutual annihilation of threading dislocations is rather efficient although affected by the glide properties of the growing epitaxial crystal and the thermal stresses during the cooling down after growth.