Browsing by Subject "growth mechanism"

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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.