Relaxation of layers

A method of forming a layer of elastically unstrained crystalline material, which method comprises:

• providing first and second crystalline layers adjacent each other, wherein one layer is elastically strained under tension and the other layer is elastically strained in compression;
• providing a step of diffusion between the layers so that the differences between the respective compositions of the two layers reduces progressively until they are substantially the same, the two layers then forming just a single final layer of crystalline material having, in aggregate, a uniform composition, and wherein the respective compositions, thicknesses, and degrees of strain of the two layers are initially selected so that, after diffusion, the material then constituting the final layer no longer, in the aggregate, exhibits elastic strain, wherein the first layer is formed from Si1-xGex, the second layer is formed from Si1-yGey and the final layer is formed from Si1-zGez, with x and y respectively being in the range of 0 to 1, and x being substantially different from y, and with z satisfying the formula:
• z=(x*th1+y*th2)/(th1+th2)
• wherein th1 is the thickness of the first layer;
• th2 is the thickness of the second layer;
• and the respective lattice parameters of the first and second strained layers are respectively equal to the nominal lattice parameter of Si1-zGez.