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Abstract 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. 

Abstract Infrared spectroscopy currently requires the use of bulky, expensive, and/or fragile spectrometers. For gas sensing, environmental monitoring, or other applications, an inexpensive, compact, robust on‐chip spectrometer is needed. One way to achieve this is through gradient permittivity materials, in which the material permittivity changes as a function of position in the plane. Here, synthesis of infrared gradient permittivity materials is demonstrated using shadow mask molecular beam epitaxy. The permittivity of the material changes as a function of position in the lateral direction, confining varying wavelengths of infrared light at varying horizontal locations. An electric field enhancement corresponding to wavenumbers ranging from ≈650 to 900 cm⁻¹over an in‐plane width of ≈13 µm on the flat mesa of the sample is shown. An electric field enhancement corresponding to wavenumbers ranging from ≈900 to 1250 cm⁻¹over an in‐plane width of ≈13 µm on the slope of the sample is also shown. These two different regions of electric field enhancement develop on two opposite sides of the material. This demonstration of a scalable method of creating in‐plane gradient permittivity material can be leveraged for the creation of a variety of miniature infrared devices, such as an ultracompact spectrometer. 

GaAs(111)B are commercially available substrates widely used for the growth of van der Waals chalcogenide films. Wafer-scale, high-quality crystalline films can be deposited on GaAs(111)B substrates using molecular beam epitaxy. However, two obstacles persist in the use of GaAs(111)B: first, the surface dangling bonds make it challenging for the growth of van der Waals materials; second, the As-terminated surface is prone to aging in air. This study investigated a thermal treatment method for deoxidizing GaAs(111)B substrates while simultaneously passivating the surface dangling bonds with Se. By optimizing the treatment parameters, we obtained a flat and completely deoxidized platform for subsequent film growth, with highly reproducible operations. Furthermore, through first-principle calculations, we find that the most energetically favorable surface of GaAs(111)B after Se passivation consists of 25% As atoms and 75% Se atoms. Finally, we discovered that the common storage method using food-grade vacuum packaging cannot completely prevent substrate aging, and even after thermal treatment, aging still affects subsequent growth. Therefore, we recommend using N2-purged containers for better preservation. 

Abstract Understanding surface stability becomes critical as 2D materials like SnSe are developed for piezoelectric and optical applications. SnSe thin films deposited by molecular beam epitaxy showed no structural changes after a two-year exposure to atmosphere, as confirmed by X-ray diffraction and Raman spectroscopy. X-ray photoelectron spectroscopy and reflectivity show a stable 3.5 nm surface oxide layer, indicating a self-arresting oxidative process. Resistivity measurements show an electrical response dominated by SnSe post-exposure. This work shows that SnSe films can be used in ambient conditions with minimal risk of long-term degradation, which is critical for the development of piezoelectric or photovoltaic devices. Graphical Abstract