Browsing by Subject "black phosphorus"

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    Engineering Novel Transistors Based On Black Phosphorus
    (2019-02) Robbins, Matthew
    Black phosphorus (BP), a layered 2D semiconductor that can be isolated to one monolayer thicknesses re-emerged in 2014 because of its promise for use in applications such as high performance MOSFETs, optoelectronic devices, novel devices like tunneling-field-effect-transistors (TFETs), and flexible electronics. The promise of BP comes from its unique material properties such as a high mobility, crystal anisotropy, a tunable direct band gap, an anisotropic effective mass, and the ability to scale to sub-1 nm thicknesses while retaining good electronic properties. These properties make BP particularly interesting as a possible post-silicon channel material in advanced logic transistors which could enable the continuation of transistor scaling beyond the foreseeable future. However, most experimental demonstrations of BP transistors have displayed poor OFF-state performance caused by gate-induced-drain-leakage (GIDL) which limits the device's overall usefulness. In this dissertation, novel BP transistors that utilize the unique properties of BP to improve OFF-state performance are demonstrated. These novel devices include a heterostructure BP MOSFET which utilizes the thickness-tunable band gap of BP to supress GIDL current, an electrostatically doped BP MOSFET which takes advantage the thin body of BP with a novel device structure used to effectively dope the source and drain regions of the BP, to again suppress GIDL current, and BP TFETs which utilize the anisotropic effective mass in order to open a path for realizing transistors with a subthreshold slope (SS) of less than 60 mV/dec in BP.
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    Modeling of Transport Phenomena in Two-Dimensional Semiconductors
    (2016-12) Liu, Yue
    Recently, transition metal dichalcogenides and black phosphorus (BP) emerged as new 2D semiconductors due to the advantages of moderate energy band gap, high carrier mobility, ultra thin film and high anisotropy. Together with graphene, 2D materials have been utilized in the development of biomedical devices, touch screen and display technologies, and flexible applications such as wearable electronics and IoT devices. They also open up new opportunities in research fields including spintronics, optoelectronics and next generation post-silicon transistor. In this dissertation, we present theoretical modeling for several topics related to 2D materials. Starting with the fundamental tight-binding theory of graphene, we review electronic properties for graphene including massless 2x2 Dirac Hamiltonian and pseudo-spin wave function. Followed by discussion of ballistic transport, a detailed analysis on graphene diffusive transport is provided. Ionized impurity scattering and carrier screening effect is considered in the model. The momentum relaxation time and mobility for graphene is modeled. A non-linear Thomas Fermi screening is introduced to improve the simulation accuracy. Taking the real spin into account, the new Hamiltonian is a 4x4 matrix. An external field perpendicular to the graphene breaks the reflection symmetry and introduces a Rashba spin-orbit interaction, which couples pseudo-spin and real spin. The relevant charge carrier states are no longer spin eigenstates. Rashba interaction is found to be quite small compared to Coulomb impurity scattering. To characterize the spin-polarized electrons tunneling from electrodes and transport in graphene, a spin valve device modeling and magnetoresistance calculation is developed. Black phosphorus possesses excellent properties like other 2D materials for high performance nanoelectronic applications. Moreover, there is a uniquely high in-plane anisotropy in BP due to its puckered crystal structure. To model the anisotropic transport, a model based on the BTE is developed, considering the full anisotropic electronic structure. For zero temperature calculation with ionized impurity limited scattering, anisotropy ratio 3-4 can be obtained from the model. Due to the dominating effect of screening, mobility is found to decrease weakly with increasing temperature. For , a smaller anisotropy ratio of 1.8-3.5 matching experimental measurements indicates that impurity scattering is an important mechanism for black phosphorus.
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    Study Of The Birefriengence Of Black Phosphorus By Picosecond Interferometry
    (2017-05) Zheng, Wei
    This thesis presents a modified picosecond interferometry method to study the optical properties of bulk black phosphorus (BP). BP is an emerging two-dimensional material which exhibits great potential for use in future nano-photonic and nano-electronic devices. BP differentiates itself from other two-dimensional materials such as graphene in that it possesses anisotropy in in-plane direction. It has zigzag and armchair in-plane crystalline direction, which gives its unique optical and electrical properties along these two directions, and the interlayers are under Van der Waals interactions. BP has direct band gap which is tunable via controlling the number of layers, strain and the applied electric field, making it a versatile material for use in semi-conductor industry. Currently, BP has been used as few-layer materials for devices such as photodetector and field effect transistor. However, the studies on the bulk optical properties of BP are still lacking, partially due to its tendency to degrade when exposed to air. This work focused on presenting picosecond interferometry as a new method for indirectly measuring the optical properties of BP and discuss the extended application of picosecond interferometry for studying other two-dimensional materials. Picosecond interferometry is a modified pump-probe method. It observes Brillouin scattering in a crystalline system to measure the optical and acoustic properties of the crystalline system. In this study, I modified the pump-probe system in our laboratory into a polarization-sensitive picosecond interferometry setup. I studied BP’s birefringent optical properties at several different wavelengths using picosecond interferometry. The Brillouin scattering signals was modelled by exponential decaying function. The polarization-resolved optical properties of BP were extracted by fitting the exponential decaying function. The thermal backgrounds of the measurement is analyzed with computational simulation.
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    Thin Film Growth of Black Phosphorus and Black Arsenic Phosphorus
    (2020-09) Izquierdo, Nezhueyotl
    A single-step, direct silicon-substrate growth of black phosphorus (b-P) thin films is achieved by a self-contained (ampule) short-way transport method. The synthesis reactants include tin (Sn), tin tetraiodide (SnI4), and red phosphorus (r-P). A self-generated low-pressure condition of < 1.5 MPa is reached at the maximum soak temperature of 650 ℃. A well-defined phosphorus phase dependency was determined, by adjusting the SnI4 concentration, forming either violet phosphorus (v-P) or b-P. Furthermore, in situ Sn passivation for both thin film and bulk b-P is experimentally verified, enhancing the long-term stability after 4 months of exposure to ambient conditions. A b-P hero single crystal is formed with lateral dimensions of 10 × 85 μm and 115 nm thick. Electron backscatter diffraction (EBSD) measurements determined b-P thin films do not grow epitaxially with the substrate. Cross-sectional transmission electron microscopy (CS-TEM) of a b-P thin film provides valuable insight into the growth mechanism that is difficult to achieve analyzing bulk b-P. Crystalline inclusions are discovered throughout the b-P crystal with a Sn:I ratio of 1.1-1.4, and may be responsible for the dominant mechanism for seeding vertical growth. Thin film and bulk b-P recipe crystals show an equal response below Eg dominated by free carrier absorption for IR absorption measurements. Black arsenic phosphorus (b-As1-xPx) thin films can be achieved with slight modifications to the previous method. The synthesis reactants include Sn, SnI4, grey arsenic (g-As), and red r-P. An in situ Sn passivation layer was found at the surface of the b-AsP, however, at the wafer interface an amorphous layer, with Sn0.07P0.20O0.71 composition, is found. The crystal structure and elemental composition of b-P, b-AsP, v-P, v-AsP, and c-AsP thin films were characterized using the following techniques: Raman spectroscopy, x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS), cross-sectional transmission microscopy (CS-TEM) and electron backscatter diffraction (EBSD). The data provides valuable insight into the growth mechanism which motivated the proposed growth mechanism. Thin film b-P field-effect transistors (FET) devices show improved device performance compared to unpassivated b-P films of equivalent thickness with an on/off current ratio >102. Thin film b-ASP FET’s fabricated from exfoliated bulk-b-AsP grown in the same conditions as the thin film growth process show an on-off current ratio of 102, a threshold voltage of -60 V, and a peak field-effect hole mobility of 23 cm2/V·s at Vd=-0.9 and Vg=-60 V.