Optical Phonons in a-plane GaN under Anisotropic Strain

Optical Phonons in a-plane GaN under Anisotropic Strain 

Vanya Darakchieva, Tanya Paskova, and Mathias Schubert 

The phonon spectrum is one of the fundamental characteristics of crystals. The behavior of phonon dispersion branches reflects specific features of the crystal structure and the interatomic interactions, and therefore gives the most comprehensive and detailed information about the dynamical properties of crystals. Phonons are straightforward signatures of bond lengths and chemical species, and phonon parameters can be used to obtain information on strain, alloy composition, free arrier concentration, and mobility in individual layers and constituents of device structures. Phonon mode parameters can be also used to study compositional fluctuations, phase mixing, interface morphologies, as well as defects and impurities. To use the phonons as a gauge and tool to extract information on material properties and characteristics, it is mandatory to establish first the phonon mode behavior. 

Phonon frequencies are often employed as a tool for strain assessment in mismatched semiconductor heterostructures [1]. This involves IR and Raman measurements of phonon frequency shifts with respect to unstrained material, and their conversion into strain or stress components via the phonon deformation potentials. The knowledge of precise values of the strain-free frequencies and the phonon deformation potentials is of key importance in such studies. Phonon deformation potentials can be obtained from control experiments, in which the frequencies of the phonons under consideration are calibrated versus well-defined applied strains (or stresses) on otherwise strain-free material. Alternatively, the phonon frequencies can be caKbrated versus strain components in layers exhibiting different intrinsic strains. The latter requires determination of the intrinsic strain components. 

The application of these principles for studying phonons and strain effects in GaN and related materials turned out to be challenging. The main obstacle is the lack of native substrates in the growth process and consequent difficulties 

---

I 9 Optical Phonons in a-plane GaN under Anisotropic Strain 9.2.6.3.1 Infrared Dielectric Model... 

The anisotropic strain in the nonpolar nitride materials films heteroepitax-ially grown in nonpolar directions inevitably leads to more complex behavior of the optical phonons. The theory predicts that some of the phonons in the nitrides split under anisotropic strain. Recent reports on a-plane nonpolar GaN... 

---