High‐Performance Phototransistors by Alumina Encapsulation of a 2D Semiconductor with Self‐Aligned Contacts

2D semiconductors are promising candidates for next generation electronics and optoelectronics. However, their exposure to air and/or resists during device fabrication can cause considerable degradation of material quality, hindering their study and exploitation. Here, field effect transistors (FETs) are designed and fabricated by encapsulation of the 2D semiconductor indium selenide (InSe) with alumina (Al ₂O ₃) and by self-aligned electrical contacts. The Al ₂O ₃-film is grown directly on InSe immediately after its exfoliation to provide a protecting capping layer during and after device fabrication. The InSe-FETs exhibit a high electron mobility of up to ≈10 ³ cm ² V ⁻¹ s ⁻¹ at room temperature for a 4-nm-thick InSe layer, a low contact resistance (down to 0.18 kΩ) and a high, fast, and broad-band photoresponsivity. The photoresponsivity depends on the InSe-layer thickness and photon wavelength, reaching a value of up to 10 ⁸ A W ⁻¹ in the visible spectral range, at least one order of magnitude larger than previously reported for similar photodetectors. The proposed fabrication is scalable and suitable for high-precision pattern definition. It could be extended to other 2D materials and multilayer structures where alumina could also provide effective screening of the electric field induced by polar molecules and/or charged impurities present near the surface of the 2D layer.