A distributed Bragg reflector (DBR) is a high-quality mirror based on alternating layers with high and low refractive indices. To grow an epitaxial DBR both layers should have the same lattice parameter. Although for III-V DBRs there is a pair of almost lattice-matched materials with significantly different refractive indices: GaAs and AlAs, for II-VI semiconductors, all lattice-matched DBRs required so far growth of ternary or even quaternary compounds or complex digital alloys to fulfill the requirement of lattice matching. Here we propose a much simpler approach with only binary compounds, an almost lattice-matched DBR made of CdSe and ZnTe.
Several properties of our DBR are different from II-VI DBRs grown so far. Firstly, our DBR does not include magnesium, so it is not hygroscopic and it should be stable for years even outdoors, which can be important e.g. in solar cell applications. Secondly, the layer with a lower refractive index is also one with a lower energy gap. It is an exceptional situation because usually, semiconductors with high energy gap have a low refractive index, which is not true for considered pair of materials: ZnTe (Eg = 2.4 eV, n = 2.9) and CdSe (Eg = 1.7 eV, n = 2.6). Consequently, refractive index difference (a crucial parameter for DBRs) is increasing for long wavelengths and decreases close to the energy gap short wavelength, which is unusual for semiconductor DBRs. From a practical point of view, it is important that both materials have similar lattice constant, which is 0.608 nm for ZnTe and 0.605 nm for CdSe, and both materials can be epitaxially grown in a zinc blende structure.
We realized a new kind of DBR using molecular beam epitaxy (MBE) assisted with in-situ reflectivity. On top of GaAs or almost lattice-matched GaSb substrate we grew 20-30 pairs of CdSe and ZnTe. In reflectance spectra we obtained stop-band width of about 50 nm and reflectance over 95% if the stop-band is in the spectral range between 900 and 1700 nm (see Fig. 1). Since several optical properties are known only for wurtzite CdSe, therefore we use our relatively thick structures to determine optical parameters of zinc blende CdSe. In particular analytical formula for the refractive index of zinc blende CdSe was obtained by fit to experimental data.
Authors: K. E. Połczyńska, K.Sobczak, W.Pacuski
Superlattices and Microstructures
Volume 139, March 2020, 106422
https://doi.org/10.1016/j.spmi.2020.106422
