Metal-semiconductor contacts have been the subject of intense investigation and study for several decades. With the advent of atomically-thin semiconductors new opportunities have emerged in investigating these mysterious yet, critical buried interfaces. In this talk we will show our recent advances in probing the nature of these contacts by a variety of scanning probe techniques. The primary model system will be that a bulk noble metal such as gold or silver and a 2D chalcogenides semiconductor such a molybdenum disulfide (MoS2). We will discuss impact of contact type and evaporation technique on the electrical and optical properties of the junction and suggest ways to make idealized contacts. We will then extend our analysis to looking at the same interface from an optical perspective and investigating hybrid states of excitons and plasmons observed via near-field photoluminescence micro-spectroscopy.
Efficient doping of 2D materials, including carrier type, concentration and mobility, is challenging but essential for enabling their future electronic and photonic applications. We are developing substitutional n- and p- doping of InSe semiconductor by introducing Sn and Zn, respectively, in the Bridgman bulk crystal growth. Electrical transport properties of undoped vs. n- and p- doped InSe crystals are compared by conducting Hall measurements on bulk crystals and FET transport measurements on exfoliated thin layers. Undoped InSe is intrinsically n-type in both bulk and thin-film forms, with [n]~3.5E14 cm-3 and mu values of up to 1,400 cm2 V-1 s-1 for thick layers at 300K. Carrier concentration in Sn-doped thick layers increases approximately two-fold, while the corresponding mobility reduces ~2 times at 300 K. Zn-doped InSe shows p- behavior for bulk InSe with [p]~7.9E13 cm-3 and mu~43 cm2 V-1 s-1 at 300 K, which reverts to ambipolar/n- type behavior for thin layers in FET devices.
Proc. SPIE. 11468, Enhanced Spectroscopies and Nanoimaging 2020
KEYWORDS: Semiconductors, Spectroscopy, Imaging spectroscopy, Transition metals, Near field scanning optical microscopy, Near field, Scanning probe microscopy, Molybdenum, Heterojunctions, Near field optics
In this talk we will present the use of near-field scanning probe microscopy and spectroscopy to investigate the electronic and optical quality of excitonic semiconductors. We will use two-dimensional (2D) transition metal dichalcogenides (TMDCs) of Mo and W as prototypical examples but extend our measurements to other low-dimensional excitonic systems including colloidal quantum dots, organic assemblies and layered hybrid perovskites. Via near-field photoluminescence spectroscopy we will show the nanoscale variations in quality of the contact with substrates and disorder at the interface in case of junctions or heterostructures. By placing a plasmonic metal substrate nearby and varying the distance, we will also show exciton hybridization with surface plasmons into propagating hybrid surface modes.
In this talk, we will focus on the subject of strong light-matter coupling in excitonic 2D semiconductors. We will present our recent work on the fundamental physics of light trapping in multi-layer TMDCs when coupled to plasmonic substrates. We systematically demonstrate via calculations and matching experiments that the presence of strong excitonic resonances in multilayers (< 20 nm thickness) combined with surface plasmon excitations of the nearby metals can achieve strongly coupled modes with apparent voided crossings in reflectance spectra. Further, we explore additional light confinement by patterning 1D arrays of rectangular resonators of varying widths and periods (100 nm to 500 nm) showing three mode couplings. We will further present extensions of our studies to resonators with dielectric spaces and optical superlattices in 1D.
I will present our recent works on confining visible frequency photons in heterostructures for plasmonic metals and excitonic transition metal dichalcogenides (TMDCs) of Mo and W. Evidence of strongly coupling between excitonic modes and Fabry-Perot like resonances will be presented in unpatterned case. When the TMDC layer is patterned, plasmonic and dielectric grating modes emerge which lead to further coupling with excitonic modes resulting in tunable strong coupling and light confiementment. Finally, I will extend this notion to monolayer TMDCs and show evidence of near-unity absorption in metamaterials of the same for applications ranging from optical modulators to photodetectors and photovoltaics.