Dielectric Anisotropy of the GaP=Sið001Þ Interface from First-Principles Theory Pankaj Kumar and Charles H. Patterson * School of Physics, Trinity College Dublin, Dublin 2, Ireland (Received 15 February 2017; published 9 June 2017) First-principles calculations of the dielectric anisotropy of the GaP=Sið001Þ interface are compared to the anisotropy extracted from reflectance measurements on GaP thin films on Si(001) [O. Supplie et al., Phys. Rev. B 86, 035308 (2012)]. Optical excitations from two states localized in several Si layers adjacent to the interface result in the observed anisotropy of the interface. The calculations show excellent agreement with experiment only for a gapped interface with a P layer in contact with Si and show that a combination of theory and experiment can reveal localized electronic states and the atomic structure at buried interfaces. DOI: 10.1103/PhysRevLett.118.237403 Thin layers of III-V semiconductors grown on Si(001) have been widely investigated for III-V on-silicon integra- tion [1]. Applications for GaP on Si include buffer layers in multijunction GaAsP=Si photovoltaics [2,3]. Obtaining a high quality interface and avoiding defects such as thread- ing dislocations [3] or antiphase domains [4,5] in the GaP layer is essential to the performance of III-V optoelectronic devices [3]. Techniques for observing the microscopic properties of the buried interface between a thin dielectric layer and an underlying dielectric substrate are limited. The short mean free paths of charged particles in matter, lack of interface sensitivity, and need for ultrahigh vacuum mean that electron spectroscopies are unsuitable. On the other hand, light has a great potential for directly probing buried interfaces since visible light can penetrate thin layers and reflect from the interface. An optical probe can have interface (or surface) sensi- tivity when the reflecting boundary is anisotropic and the surrounding bulk media are isotropic. The (001) interfaces between tetrahedral semiconductors are anisotropic while bulk tetrahedral semiconductors are isotropic on a meso- scopic length scale. Light induces transitions between valence and conduction band states in the bulk and at an interface, but only transitions between states localized near the interface contribute to the difference in reflectance parallel and perpendicular to rows of atoms at the interface. This is known as reflectance anisotropy (RA) and the associated difference in dielectric functions is the interface dielectric anisotropy (IDA). In this Letter we report first-principles calculations of the IDA of thin GaP layers on Si(001) and compare them to experimental measurements by Supplie et al. [5]. The agreement between theory and experiment is excellent when the last complete layer in GaP is P (as in the experiment) and when the interface is “gapped, ” i.e., when there is a gap between the interface valence and con- duction band states. A modified version of the electron counting arguments by Pashley [6] given in the Supplemental Material [7] shows that when the GaP surface is terminated by P dimers with one H atom per dimer, there is one electron more per dimer than is needed to satisfy valence requirements at the surface. This electron transfers to the GaP=Si interface and a gapped interface results when the Si layer is doped with one Ga (P) per surface dimer, when the last GaP layer is P (Ga). Electrons transferred to the interface occupy localized states at the buried interface. Below we show that it is the optical transitions between these interface localized states that are responsible for the observed IDA [5]. The states are localized within three Si bilayers of the interface. Their filling determines whether the interface is electrically conducting or semiconducting. The predicted IDA is strikingly different when the last layer is Ga or P and when the interface is electrically conducting. Measurement of the IDA at buried interfaces interpreted by first-principles calculations therefore offers a unique means of probing the atomic and electronic states of interfaces between dielectrics. The approach that is applied here was previously applied to the RA of surfaces of systems including clean Si and Ge(001) [8] and clean and transition metal covered Si(111) [9–11]. As far as the authors are aware, this is the first application of first-principles calcu- lations to determine the semiconductor buried interface anisotropic optical response. RA [5,12,13], LEED, XPS, and density functional theory (DFT) calculations [12,14] have shown that GaP grown on Si(001) by MOVPE has a mixed (2 × 2) and cð4 × 2Þ P dimer surface. The dimers are tilted and have a H atom on the down-tilted P atom. This is supported by DFT calculations of the surface energies of GaP(001) surfaces as a function of the P and H chemical potential [15], which show that the GaPð001Þ-ð2 × 2ÞH monohy- dride surface has the lowest formation energy at moderate P and H chemical potentials. Interface RA has been measured in several cases including InP=GaAsð001Þ [16], ZnSe=GaAsð001Þ and PRL 118, 237403 (2017) PHYSICAL REVIEW LETTERS week ending 9 JUNE 2017 0031-9007=17=118(23)=237403(5) 237403-1 © 2017 American Physical Society