Biaxial strain

Although nearly all creep and stress-relaxation tests are made in uniaxial tension, it is possible to make biaxial tests in which two stresses are applied at 90° to one another, as discussed in Section VI. In a uniaxial test there is a contraction in the transverse direction, but in a biaxial test the transverse contraction is reduced or even prevented. As a result, biaxial creep is less than uniaxial creep--in cquihiaxial loading it is roughly hall as much for equivalent loading conditions. In the linear region the biaxial strain 2 in each direction is (255.256)... 

Fig. 26. Computed minimum value of the magnetic field Hm necessary to align the saturation value of magnetization along the hard axis as a function of biaxial strain component for two values of the hole concentrations in Gao.95Mno.05As. The symbol (100) - 1001) means that the easy axis is along IOO], so that Hun is applied along [001] (Dietl et al. 2001c).

For the inflation method the test piece is held with a circular clamp and pressurized through a central hole. The biaxial strain state holds near the apex and optical methods can be used to find the radius of curvature and the extension of a grid drawn on the test piece. The nominal stress is then ... 

Various biaxial straining devices have been constructed, as for example those described in references 91 - 96. References 92 and 96 cover the inflation method and reference 95 describes a device that will give unequal biaxial and sequential biaxial straining. 

For a sample in the shape of a circular disk (Fig. 9b) having a radius R, the biaxial strain rate, e b is the following ... 

The properties of embedded nanoclusters can be controlled by the properties of the matrix. For example, the matrix can exert uniaxial or biaxial strain on the embedded nanoclusters. The corresponding strain can modify the properties of nanostructure considerably. For example, Zeng et... 

Of more subtle influence is the biaxial strain in thin film samples, which is introduced upon different thermal expansion coefficients for the film and the substrate material. Furthermore, the biaxial strain can depend on the growth history. On this matter the amount of available information is not exhaustive. 

Most of the nitride samples are in the form of heteroepitaxial layers. The presence of substrates modifies the structure of such layers by a biaxial strain parallel to the surface. The strain is caused by lattice mismatch and the difference in the thermal expansion of the substrate and the layer. 

DOS at the VBM is largely reduced, compared with unstrained or biaxially strained conditions. In principle, the role of uniaxial strain in WZ structure is the same as that of biaxial strain in ZB structure. 

FIGURE 9 Schematic band structures in the kx-ky plane around the top of the valence bands of wurtzite GaN (a) without strain, (b) with biaxial strain, and (c) with uniaxial strain in the (0001) plane, (d) shows the direction of each strain. 

FIGURE 3 Valence subband structures of the strained wurtzite GaN/Al0 2Gao gN single quantum wells with 0.5% compressive biaxial strain, with the well lengths Lz being (a) 30 A, and (b) 50 A. 

FIGURE 6 Valence subband structures of the strained zincblende GaN/Alo2Gao 8N single quantum wells with (a) 0.5% compressive, and (b) 0.2% tensile biaxial strains, with the well length Lz being 40 A. 

Next, compressive and tensile biaxial strain effects are discussed, because biaxial strains are usually adopted to reduce the threshold carrier density in conventional ZB QWs. FIGURE 5 shows the maximum optical gain of biaxially strained ZB GaN/Alo.2Gao. N QWs with well width Lz = 60 A. The compressive biaxial strain strongly depresses the TM-mode optical gain and enhances the TE-mode optical gain. On the other hand, the effect of tensile biaxial strain is the reverse of that of compressive strain. These results can easily be understood from the feature of bulk ZB GaN, where compressive... 

FIGURE 5 Strain effect on optical gain of zincblende GaN/Alo 2Gao.gN quantum wells with well lengths L2 being 60 A. (a) and (b) correspond to the TE- and TM-modes, respectively. The solid, dotted and dash-dotted lines stand for the results without strain, with 0.5% compressive strain and with 0.2% tensile biaxial strain, respectively. 

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