strainedlayer

A strained layer is a thin film grown on a substrate whose lattice constant differs from the film’s natural lattice constant. Because the film must fit the substrate in the plane of the interface, it adopts the substrate’s in-plane lattice spacing and becomes elastically strained. If the film remains coherent with the substrate, the strain is called pseudomorphic and is typically biaxial, either compressive or tensile depending on whether the film’s natural lattice is larger or smaller than the substrate’s.

The concept hinges on lattice mismatch and its consequences. Strain energy accumulates as the film grows, raising the possibility that, beyond a certain thickness, the layer relaxes by forming defects such as misfit dislocations. This critical thickness separates fully strained (coherent) layers from partially or fully relaxed ones. Strained layers are deliberately engineered to modify electronic and optical properties, a practice known as strain engineering.

Applications and materials systems commonly involve semiconductor heterostructures, such as GaAs/AlGaAs, InGaAs on GaAs, or silicon with a silicon-germanium layer. Strain can alter band structure, increase carrier mobility, and tailor bandgaps, enabling devices such as high-electron-mobility transistors, laser diodes, LEDs, infrared detectors, and advanced photovoltaics. Strained silicon (via SiGe) is widely exploited to boost transistor performance by enhancing carrier transport.

Growth methods for strained layers typically involve epitaxy, including molecular beam epitaxy, metal-organic chemical vapor deposition, and chemical vapor deposition. Precise control of layer thickness, composition, and interface abruptness is essential to maintain the desired strain and prevent relaxation.

In summary, a strained layer is a coherent, lattice-mismatched thin film whose induced strain tunes material properties, with practical importance in modern semiconductor devices and optoelectronics.