Metal-insulator multistacks for absorption and photodetection
Dereshgi, Sina Abedini
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Metal-insulator (MI) stacks are one of the most studied nanoscale devices of the recent decade. These structures have opened a new door to endless photonic applications ranging from solar cells to waveguides and polarizers. The main attribute of metal-insulator stacks is possibility of scaling down device dimensions with them that is the main trend in photonic and electronic technology nowadays. The conventional photonic structures require very high thicknesses where novel photonic devices can show many arti cial properties by tailoring speci cally designed metal-insulator cells also known as metamaterials. In this thesis, we will investigate some metal-insulator absorber stacks with capability of highly con ning light speci cally for photodetection. The near-infrared part of the electromagnetic spectrum is problematic in photocurrent generation due to the fact that conventional narrow band gap PN photodiodes fail to function in room temperature. Adding to this predicament is their large dimensions. Some of these problems are addressed in this thesis. First a plasmonic MIM structure is studied with random nanoparticles obtained by dewetting in the top layer which con nes the incident light in the plasmonic MIM cavity and gives rise to high absorption through surface plasmon polariton excitation in the bottom lossy metal. Several materials are investigated in order to engineer best absorbers with the focus on absorption in the bottom metal which is critical for photodetection. Our simulations and experimental results demonstrate over 90 percent absorption for most of the visible and near-infrared region. The absorption in the bottom metal in a structure comprised of chromium-aluminum oxide-silver nanoparticles (bottom to top) reaches 82 percent at 850 nm. After obtaining appropriate NIR absorption, an MIMIM photodetector is designed and fabricated where another insulator-metal layer is added to the bottom of the previous absorber. The formerly reported plasmonic photodetectors put the burden of absorption and photocurrent path on the same MIM structure putting restrictions on device design. In our proposed structure, however, tunneling MIM photocurrent junction is used which shares only its top metal with the top absorbing MIM. The main advantage of this structure is that it separates the absorption and photocurrent parts of the photodetector, making separate optimization of each MIM possible. The best structure which is silver-hafnium oxide-chromium-aluminum oxide-silver nanoparticles (top to bottom) demonstrates a peak photoresponsivity (from nonradiative decay of surface plasmon polaritons) of 0.962 mA/W at 1000 nm and a dark current of only 7 nA in a bias of 50 mV. Our results demonstrate approximately two orders of magnitude enhancement in photoresponsivity compared to previously reported MIMIM photodetectors. In another attempt to obtain perfect absorbers for visible and near-infrared regions, we put forth an MIMI absorber. In this work, the contribution of metal layers is studied in detail and material choice is discussed. Our optimization process suggests a versatile method for designing perfect absorbers. Transfer matrix method as well as FDTD simulations are used to optimize thicknesses. Furthermore, in order to shed light on material selection, impedance matching of the waves in the multilayer media to free space is proposed for the extraction of ideal metal permittivity values and comparing them to existing metals. Our experimental result of a tungsten-aluminum oxide-titanium-aluminum oxide (bottom to top) structure illustrates over 90 percent absorption for wavelength range of 400 nm to 1642 nm which is the highest perfect absorption bandwidth reported in similar MIMI structures to the best of our knowledge.
Broadband perfect absorption