The D-Team is focused on the search of new organic/inorganic semiconductor and dielectric materials for modern electronics applications. One of the main goals of our group is the fabrication of transparent and flexible electronic devices using inexpensive, high-throughput, and scalable processing technologies for circuit fabrication. Applications for these materials could include printed circuits, rollable newspapers, head-up displays, real-time wearable displays, spatial light modulators…
In the organic electronic field, our group has pioneered the development of n-type (electron transport) oligothiophene semiconductors, and has presented one of the highest mobilities reported to date for air-stable arylene-based semiconductors. Furthermore, a great number of oligomer and polymer materials have been and are being synthesized in our group, which are both air stable and solution processed. We continue to look for new semiconductors and small molecules for transistor applications.
Quite recently our group also realized low temperature processed, transparent, flexible TFTs using all-transparent single nanowires as the semiconductor channel, showing high optical transmission and electrical performance. These high-performance nanowire-based devices are attractive as pixel switching and driving transistors in active matrix organic light-emitting diode (AMOLED) displays. In fact, in 2008, our group demonstrated the first optical transparent AMOLED display driven exclusively by nanowire electronics.
The field of amorphous oxide semiconductors (AOSs) has grown rapidly since their initial discovery by Hosono less than ten years ago. Hydrogenate amorphous silicon, the current standard in the field of amorphous semiconductors, possesses severely degraded electron mobility with respect to its crystalline counterpart due to poor sp 3-orbital overlap, which constitute the main conduction path, in the amorphous state. In contrast, post-transition-metal oxide semiconductors’ conduction path is consisted mostly of metal ns-orbitals which are able to maintain significant overlap even in the amorphous state. The metal oxide section of the Marks Group D-Team is working to extend and improve the arsenal of amorphous oxide semiconductor materials available for electronic applications. Specifically, group members are designing materials that can be fabricated with solution processing, at low temperatures, for use with flexible substrates.
Our group is trying to develop new dielectrics for low operating voltage of TFTs (Thin-Film Transistors). Self-assembled nanodielectrics (SANDs) are representative examples. These dielectrics are compatible with organic semiconductors, single-walled carbon nanotubes(SWCNT), Single crystal Si nanoribbone, and inorganic semiconductors such as metal oxide nanowires, as well as thin films. Furthermore, the resulting SAND-based TFTs operate at extremely low voltages and exhibit greater mobility of FETs than analogous TFTs fabricated with conventional SiO2 gate dielectrics owing to the unique SAND combination of high dielectric constant, nanometer thickness, low gate leakage current, and low interface trap state densities.
Our modeling goals were to obtain a greater understanding of how the chemistry of 0-3 nanocomposite systems leads to device performance by comparing modeling and experiments, and to use this understanding to suggest new physicochemical systems for investigation. This understanding is crucial in designing new capacitors with the desired properties of high permittivity, low leakage current density, and low dielectric loss. We have obtained randomly dispersed particles, both spheres and rods, in a polyolefin matrix (see A-Team work) whose effective permittivites can be roughly predicted by effective medium theories for low volume fractions (νf).
The most common effective medium models are derived for the simple case of a spherical dielectric inclusion embedded in a sphere of host material. However, most materials do not occur naturally as spheres, and therefore effective medium models for other inclusion shapes have also been developed. Simple analytical solutions for the effective permittivity (εeff) can only be achieved for ellipsoids, whereas all other shapes require numerical solutions. The polarization of the composite is a function of the inclusion geometry and orientation with respect to the applied field. The depolarization factors (Nx, Ny, Nz) describe the extent to which the inclusion polarization is reduced according to its shape and orientation along each semi-axis of the ellip s oid. The depolarization factors are calculated from integrals, (eq. 1)
where ax, ay, and az are the semi-axes of the ellipsoid. For spheres, all three depolarization factors are equal (1/3, 1/3, 1/3), and for ellipsoids (infinite needles) the depolarization factors are, 0, 1/2, 1/2, respectively (for discs, 1, 0, 0, respectively). For the case of spherical inclusions, the effective permittivities are estimated using the Maxwell-Garnett (MG) effective medium theory (eq. 2), and for the case of randomly aligned ellipsoidal inclusions, the effective permittivities are estimated using the Polder-Van Santen (PVS) formalism (eq. 3), where εa is the relative permittivity of the inclusions, εb is the relative permittivity of the matrix, vf is the volume fraction of filler in the matrix, and the Nj’s are the depolarization factors. Figure 1 shows a substantial enhancement of εeff is predicted for ellipsoidal compared to spherical inclusions.
We have investigated the current (voltage, temperature) (I (V,T)) transport characteristics of our molecular self-assembled nanodielectrics (SANDs). Both, SAND types II 06/14/2010ong>(σ-saturated + π-conjugated layers) in Si/native SiO2/SAND/Au metal-insulator-metal (MIS) devices were looked at over the temperature range -60 to +100°C. It is found that the location of the π-conjugated layer with respect to the Si/SiO2 substrate surface in combination with a saturated alkylsilane tunneling barrier is crucial in controlling the overall leakage current through the various SAND structures. For small applied voltages, hopping transport dominates at all temperatures for the π-conjugated system (type II). However, for type III SANDs, the σ- and π- monolayers dominate the transport in two different transport regimes: hopping between +25°C and +100°C, and an apparent switch to tunneling for temperatures below 25°C. The σ-saturated alkylsilane tunneling barrier functions to reduce type III current leakage by blocking injected electrons, and by enabling bulk-dominated (Poole-Frenkel) transport vs electrode-dominated (Schottky) transport in type II SANDs. These observations provide insights for designing next-generation self-assembled gate dielectrics, since the bulk-dominated transport resulting from combining σ- and π-layers should enable realization of gate dielectrics with further enhanced performance.