First Principles Study of Electronic and Optical Properties of Type-II InAs/GaSb Superlattices

Type-II strained-layer superlattices (T2SLs) research began in 1977 by combining two types of semiconductor materials with type-II band alignment. Type-II band alignment is a material in which the entire band is formed according to the relative positions of the conduction and valence bands of the components. In InAs/GaSb T2SLs, the conduction band of InAs is lower than the valence band of GaSb, which is known as broken-gap alignment. This feature allows that the band gap can be freely tuned and mid-wave and long-wave infrared detector can be developed by the heterostructure. In addition, the heterostructure with type-II band alignment have a distinct advantage in suppressed Auger recombination compared to bulk narrow band gap materials. However, T2SLs have a low lifetime and high level of dark current, with a generation-recombination center associated with native defects and residual impurities, thus have limitations in which practical functions cannot be properly performed.<br/>In order to understand these phenomena, it is necessary to have a microscopic understanding based on the electronic properties through the analysis of the band structures of superlattice. The empirical model methods are used to study the electronic structures of superlattices so far. These methods have the advantage of being easy to calculate, but it is difficult to realize microscopic aspects such as phenomena on various atomic combinations of heterostructure, effects of interfaces, and physical properties for various defects. Therefore, for microscopic accurate electronic structure analysis of narrow band gap T2SLs, the first-principles calculation is inevitable. However, the research on first-principles calculations for T2SLs has not been done due to computational expense and complexity.<br/>In this study, to microscopically understand the electronic and optical properties of long period T2SLs, we performed first-principles density-functional theory (DFT) calculations for InAs/GaSb superlattices using the Vienna ab initio simulation package (VASP). In DFT calculations, a kinetic energy cutoff of 400 eV for plane-wave expansion, projected augmented wave (PAW) potentials, and Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional were used. For band gap correction of well-known error in DFT calculations, LDA-1/2 method was introduced to increase calculation efficiency and accuracy.<br/>We studied several InAs/GaSb T2SLs, including (4,4), (6,6), (8,8), and (10,10) in (monolayers(ML) of InAs, monolayers of GaSb). We used the average value of lattice constants of InAs and GaSb for lattice constants of InAs/GaSb SL. The band gaps of (4,4), (6,6), (8,8) and (10,10) InAs/GaSb SL are 0.353 eV, 0.337 eV, 0.33 eV, and 0.32 eV at Γ-point, respectively. As increasing superlattice size, the band gap is slightly reduced. This trend is consistent with experimental values and previous calculation results. Moreover, we calculated complex dielectric constants as depending on wavelength of these materials. Based on the Kramers-Kronig relation, we obtained absorption spectra and extinction coefficient using complex dielectric constants. Superlattice is highly anisotropic, it gives different characteristics along the in-plane and growth-directions (z-direction). The imaginary part of dielectric function and absorption spectra shows anisotropic behavior clearly. In the growth-direction, there is remarkable broad peak between 1~2 μm compared to in-plane directions. Moreover, the absorption coefficient of SL varies with superlattice size in the mid-infrared energy range. We demonstrated that DFT can be used to predict bandgap and optical properties in heterostructure superlattices with good precision. We expect that our study contributes to the design technology for the band gap engineering for next generation of IR detectors.