Well ordered semiconductor interfaces exhibit optical anisotropy due to the preferred bond orientations at the interface. Two types of interface systems are studies.

1. Heterovalent interfaces, such as ZnSe/GaAs interface

"Interface States of ZnSe/GaAs Interfaces", Z. Yang, I. K. Sou, Y. H. Yeung, G. K. L. Wong, Jie Wang, Cai-xia Jin, Xiao-yuan Hou, J. of Sci. and Technol. B14, 2973 (1996)

The in-plane anisotropy ( ex  ‡  ey ) and the off-plane anisotropy ( ex  ‡  ez ) of ZnSe/GaAs interfaces formed under conditions that promote the formation of either Zn-As or Ga-Se bonds are studied by reflectance difference spectroscopy (RDS). Two resonance lines, one at 2.70 eV and the other around 3.0 eV, have been observed. The in-plane anisotropy is along the [110] and the [110] principal axes. The dependence of the resonances on interface formation conditions, the results of the photoreflectance spectroscopy, and the annealing experiments all suggest that the anisotropy is not due to the electro-optic effect resulting from an interface electric field. The RDS results are consistent with the assumption that the anisotropy is the intrinsic properties of ordered ZnSe/GaAs heterovalent interface, that the resonance at 2.70 eV is associated with the interface state of Zn-As bonds and the resonance near 3.0 eV is associated with the interface state of Ga-Se bonds. The presence of a thin layer of S at the ZnSe/GaAs interfaces results in a third resonance at 3.2 eV, probably due to the Ga-S bonds. The resonance line shape changes as the top layer thickness varies. Such change can be well explained by a three-layer system with the conventional Fresnel optics, and is understood as due to the ZnSe top layer. It is our hope that our results will stimulate more interests in the theoretical study of this interface.

"Numerical Simulation of ZnSe/GaAs Interface Reflectance Difference Spectroscopy", Tak-Kun Kwok and Z. Yang, J. Appl. Phys. 80 (8), 4621 (1996)

A numerical method based on the matrices established by Yeh (14) is developed to simulate the non-normal incidence reflectance difference spectroscopy (RDS) spectra of biaxial anisotropic ( e_x  ‡  e_y  ‡  e_z ) multilayer systems. The main features of the RDS spectra obtained from the biaxial anisotropic ZnSe/GaAs interface are reproduced by the numerical method. It has demonstrated that in the cases of near normal incidence and when the anisotropy within the layer plane (in-plane anisotropy) is small (  e_x - e_y << e_x ) an RDS spectrum can be separated into two spectra, namely the in-plane anisotropic spectrum and the off-plane anisotropic spectrum. The reflectance of the s-wave and the p-wave can be calculated separately when the in-plane principal axes are at certain orientations, making it possible to obtain the anisotropic dielectric tensor directly from the measured spectra.

"Observation of ZnSe/GaAs interface states by photomodulation reflectance difference spectroscopy", Z. Yang, Y. H. Chen, and I. K. Sou, Appl. Phys. Lett. 75 (4), 528 (1999)

We show that part of the resonance feature at 2.7 eV in the reflectance difference (RD) spectra of ZnSe thin film grown on (100) GaAs substrate ¹ can be suppressed by a beam of monochromatic light below the ZnSe band gap. This light-sensitive part of the resonance contains contributions from at least two anisotropic interface electron traps near the conduction band edge of ZnSe that have distinctly different photocarrier release characteristics. Each state releases the captured electrons through more than one channel. Our results reconfirm that at least part of the RD resonance feature at 2.7 eV is due to the electronic transitions involving anisotropic interface states.

"ZnSe/GaAs interface state probed by time-resolved reflectance difference spectroscopy", K. S. Wong, H. Wang, Z. Yang, I. K. Sou and G. K. L. Wong, Appl. Phys. Lett. 74 (24), 3663 (1999)

Time-resolved reflectance difference spectroscopy (TRDS) has been applied to study the dynamics and relaxation processes of the 2.7eV ZnSe/GaAs interface state associated with Zn-As bonds. The instantaneous screening due to photoexcited carriers and ~18 ps recovery time of the 2.7eV interface state is observed in the TRDS spectra. The rapid cooling of the hot carrier in the spectral region above the ZnSe band edge is also observed.

"One- and two-photon-excited time-resolved photoluminescence investigation of bulk and surface recombination dynamics in ZnSe", H. Wang, K. S. Wong, B. A. Foreman, Z. Yang, and G. K. L. Wong, J. Appl. Phys. 83 (9), 4773 (1998)

"Anisotropic strain in (100) ZnSe epilayers grown on lattice mismatched substrates", Z. Yang, and I. K. Sou, and Y. H. Chen, J. Vac. Sci. and Technol. B18 (4), 2271 (2000)

We show that part of the reflectance difference resonance near the E0 energy of ZnSe is due to the anisotropic in-plane strain in the ZnSe thin films, as films grown on three distinctly different substrates, GaAs, GaP, and ZnS, all show the resonance at the same energy. Such anisotropic strain induced resonance is predicted and also observed near the E1/E1 + D1 energies in ZnSe grown on GaAs. The theory also predicts that there should be no resonance due to strain at the E0 + D0 energy, which is consistent with experiments. The strain anisotropy is rather independent of the ZnSe layer thickness, or whether the film is strain relaxed. For ZnSe films with large lattice mismatch with substrates, the resonance at the E1/E1 + D1 energies is absent, very likely due to the poor crystalline quality of the 20 nm or so surface layer.

2. Common-atom interfaces, such as GaAs/GaAlAs (or GaInAs) interfaces

"Quantum well anisotropic forbidden transitions induced by a common-atom interface potential", Y. H. Chen, Z. Yang, Z. G. Wang, Xu Bo, and J. B. Liang, Phys. Rev. B60 (3), 1783 - 1787 (1999)

A prominent effect of the interface potential (IP), ^{1, 2} the optical anisotropy of the forbidden transitions in quantum wells, have been observed by reflectance-difference spectroscopy. Predictions by the heavy-light hole coupling IP models are qualitatively consistent with all the observed features of the forbidden and the allowed transitions. The fact that the predicted value of the relative transition strength, which depends on neither the IP strength nor the electric field, disagrees with the observed one indicates that coupling involving X and/or L bands may also be important.

"Determination of the value of hole-mixing coefficients due to interface and electric field in GaAs/Al(Ga)As superlattices", Xiao-Lin Ye, Yong-Hai Chen, J. Z. Wang, Z. G. Wang, Z. Yang, Phys. Rev. B (in print)