(440c) Silicon-Based Infrared Photodetectors Enabled By Hot Electrons

Conventionally, photonic infrared (IR) detectors employ low band gap materials such as InGaAs, InSb, or HgCdTe. However, these materials include elements that are rare, expensive, or toxic. Past research indicates that crystalline Si (c-Si), which is a much cheaper and more abundant element, could be used for IR detection when metal electrodes are cleverly nanostructured. In this type of photodetection systems, the IR light with energies below the c-Si band gap is strongly absorbed by the metal structures, rather than by c-Si. The photoexcited electrons in the metal can then be injected into the conduction band of c-Si before being thermalized and electric current can be generated. These non-thermalized electrons, called hot electrons, enable the detection of IR light with energies below the c-Si band gap. For efficient transport of electrons in the metal before thermalization, the metal layer should be as thin as approximately the electron mean free path. Accordingly, the metal layer thickness should be only a few tens of nanometers. To induce strong optical absorption in such a thin metal layer, surface plasmon polaritons (SPPs) can be excited at the metal surface. Previous studies on hot electron photodetection utilized small-scale metamaterials or deep trench resonators to have strong resonant absorption of SPPs in thin metal films on c-Si at the desired frequencies. However, these structures should be fabricated with a high precision because the metal structure sizes determine resonances. Accordingly, in many cases, expensive techniques such as electron beam lithography have been commonly used to fabricate the structures. However, for mass production, it is important to obtain metallic structures that do not require expensive techniques and tolerate practical fabrication errors. In this study, we use metal metasurfaces that can be fabricated by scalable, inexpensive techniques and achieve a broad-band IR absorption of over 95% in 15-nm-thick metal films. This unprecedented strong absorption, in terms of both the absorptance magnitude and the band width, is enabled by a new scheme where the light takes multiple passes within the c-Si substrate. During the passage, light is preferentially absorbed by the thin metal layer that is on one side of the substrate. Absorption on the other side is efficiently eliminated by using a dielectric layer. In our effort, the surface of the c-Si substrate where thin meal film is deposited is structured by a simple optical lithography. The structured surface admits the incident light into the substrate and prevents the light from leaking out of the substrate. In our scheme of multiple light passes, extremely strong resonances are not necessary and fabrication errors would not destroy the optical properties appreciably. In this talk, we will discuss the details of the optical absorption in our scheme. We will also present our experimental results on the electronic characteristics of our hot electron devices.