Tailoring in-Plane Permittivity Gradients by Shadow Mask Molecular Beam Epitaxy

Infrared (IR) gradient permittivity materials are the potential building blocks of miniature IR-devices such as an on-chip spectrometer. The manufacture of materials with permittivities that vary in the horizontal plane is demonstrated using shadow mask molecular beam epitaxy in Si:InAs films. However, to be useful, the permittivity gradient needs to be of high crystalline quality and its properties need to be tunable. In this paper, it is shown that it can control the permittivity gradient length and steepness by varying the shadow mask thickness. Samples grown with similar growth parameters and with 200 and 500 µm mask thicknesses show permittivity gradient widths of 18 and 39 µm on the flat mesa on one side and 11 and 23 µm on the film slope on the other side, respectively. The gradient steepnesses are 23.3 and 11.3 cm⁻¹/µm on the flat mesa and 21.8 and 9.1 cm⁻¹/µm on the film slope, for samples made with the 200 and 500 µm masks, respectively. This work clearly shows the ability to control the in-plane permittivity gradient in Si:InAs films, setting the stage for the creation of miniature IR devices.