Pseudocrystalline growth technology

In epitaxial growth, when the lattice constants of the growth layer and the substrate are mismatched, there will be stress and strain energy accumulated in the growth layer, and this energy will increase as the thickness of the growth layer increases. Increase; when the film grows to a certain thickness, so-called misfit dislocations will be generated at the heterojunction interface to release the accumulated strain energy. And these misfit dislocations are extremely harmful to the electrical properties of the heterojunction (because these dislocations will cause a large number of surface states-minority carrier recombination centers), therefore, in order to grow high-quality lattice mismatched For heterojunctions, the thickness of the growth layer must be controlled to avoid misfit dislocations at the interface. The maximum thickness of the growth layer without misfit dislocations on the interface is an important process control parameter called critical thickness (when the thickness is less than this thickness, no misfit dislocations will appear, otherwise Lattice relaxation will occur-dislocations will occur). The thin film with no misfit dislocations and internal stress at the interface of this heterojunction is called a strain layer. At this time, the valence bond inside the thin film is in a state of distortion-the crystal lattice occurs. Elastic strain, there is stress (compressive stress or tensile stress). This kind of non-strictly complete and non-amorphous thin film material—strained layer is often called pseudocrystalline, and the corresponding epitaxial growth is called pseudo-form growth or coordinate growth b>.

Different pseudocrystalline films have different components and different types of substrates, and the strain energy in the epitaxial film will also be different, so the critical thickness of the pseudocrystalline film will also be different. For Si(1-x)Ge(x) films grown on Si substrates, the greater the Ge content x, the smaller the critical thickness of the film; for example, when x=0.5, Si(1-x) The critical thickness of the Ge(x) film is only about 8nm. If it is a pure Ge film (ie x=1), the critical thickness is even smaller (only about 1nm).

If the lattice constants of the substrate and the epitaxial film are a2 and a1, respectively, the mismatch rate of the lattice is D=|a1-a2|/a1. For example (room temperature), from the lattice constants of Si and Ge [a(Si)=5.431 angstroms and a(Ge)=5.658 angstroms], D≈4.17% is obtained. It can be seen that Si/Ge is a heterogeneous system with large lattice mismatch. Therefore, it is quite difficult to produce high-quality crystalline Si/Ge heterojunctions. The lattice constant a of Si(1-x)Ge(x) alloy is larger than that of Si, and the lattice constant of Si(1-x)Ge(x) thin film material almost increases with the increase of Ge content x. Increase linearly:

a(x)= 5.431+(5.658-5.431)x [Angstroms]

Then the crystal of Si(1-x)Ge(x) alloy film The lattice mismatch rate D is also approximately linearly related to the Ge content x:

D =0.0418 x

The greater the lattice mismatch rate between the grown film and the substrate , The critical thickness of the epitaxial pseudocrystalline film must be thinner. Therefore, there is a certain relationship between the critical thickness t(c) and the lattice mismatch rate D (empirical relationship):

t(c )=a1/2D. For example, when D=2%, for a1=5Ǻ epitaxial pseudocrystalline film, its critical thickness is about 10nm.

In short, through the pseudo-crystal growth technology-strained epitaxy technology, it is possible to fabricate heterojunctions with lattice mismatches that meet the requirements of a variety of applications. This is useful for expanding semiconductor materials and heterojunctions. The scope of application and the development of new devices and circuits are of great significance.