Oleg A. Kirillov

Contact-induced crystallinity for high-performance soluble acene-based transistors and circuits

The use of organic materials presents a tremendous opportunity to significantly impact the functionality and pervasiveness of large-area electronics. Commercialization of this technology requires reduction in manufacturing costs by exploiting inexpensive low-temperature deposition and patterning techniques, which typically lead to lower device performance. We report a low-cost approach to control the microstructure of solution-cast acene-based organic thin films through modification of interfacial chemistry. Chemically and selectively tailoring the source/drain contact interface is a novel route to initiating the crystallization of soluble organic semiconductors, leading to the growth on opposing contacts of crystalline films that extend into the transistor channel. This selective crystallization enables us to fabricate high-performance organic thin-film transistors and circuits, and to deterministically study the influence of the microstructure on the device characteristics. By connecting device fabrication to molecular design, we demonstrate that rapid film processing under ambient room conditions and high performance are not mutually exclusive.

Band offsets of atomic-layer-deposited Al2O3 on GaAs and the effects of surface treatment

The metal gate/high-k dielectric/III-V semiconductor band alignment is one of the most technologically important parameters. We report the band offsets of the Al/Al2O3/GaAs structure and the effect of GaAs surface treatment. The energy barrier at the Al2O3 and sulfur-passivated GaAs interface is found to be 3.0±0.1 eV whereas for the unpassivated or NH4OH-treated GaAs is 3.6 eV. At the Al/Al2O3 interface, all samples yield the same barrier height of 2.9±0.2 eV. With a band gap of 6.4±0.05 eV for Al2O3, the band alignments at both Al2O3 interfaces are established.

Band offsets of Al2O3/InxGa1−xAs (x=0.53 and 0.75) and the effects of postdeposition annealing

Band offsets at the interfaces of InxGa1−xAs/Al2O3/Al where x=0.53 and 0.75 were determined by internal photoemission and spectroscopic ellipsometry. The photoemission energy threshold at the InxGa1−xAs/Al2O3 interface was found to be insensitive to the indium composition but shifted to a lower energy after a postdeposition annealing at high temperatures. Subthreshold electron photoemission was also observed for the annealed sample and was attributed to interfacial layer formation during the annealing process.

Graphene as transparent electrode for direct observation of hole photoemission from silicon to oxide

We demonstrate the application of graphene as collector material in internal photoemission (IPE) spectroscopy, which enables direct observation of both electron and hole injections at a Si/Al2O3 interface and overcomes the long-standing difficulty of detecting holes in IPE measurements. The observed electron and hole barrier heights are 3.5 ± 0.1 eV and 4.1 ± 0.1 eV, respectively. Thus, the bandgap of Al2O3 can be deduced to be 6.5 ± 0.2 eV, in good agreement with the value obtained by ellipsometry analysis. Our modeling effort reveals that, by using graphene, the carrier injection from the emitter is significantly enhanced and the contribution from the collector electrode is minimal.

Non-volatile memory with self-assembled ferrocene charge trapping layer

A metal/oxide/molecule/oxide/Si capacitor structure containing redox-active ferrocene molecules has been fabricated for non-volatile memory application. Cyclic voltammetry and X-ray photoelectron spectroscopy were used to measure the molecules in the structure, showing that the molecules attach on SiO2/Si and the molecules are functional after device fabrication. These solid-state molecular memory devices have fast charge-storage speed and can endure more than 109 program/erase cycles. This excellent performance is derived from the intrinsic properties of the redox-active molecules and the hybrid Si-molecular device structure. These molecular devices are very attractive for future high-level non-volatile memory applications.

Tunnel field-effect transistor heterojunction band alignment by internal photoemission spectroscopy

The electron energy band alignment of a metal-oxide-semiconductor tunnel field-effect transistor heterojunction, W/Al2O3/InGaAs/InAs/InP, is determined by internal photoemission spectroscopy. At the oxide flat-band condition, the barrier height from the top of the InGaAs/InAs valence band and the top of the InP valence band to the bottom of the Al2O3 conduction band is determined to be 3.5 and 2.8 eV, respectively. The simulated energy band diagram of the heterostructure is shown to be consistent with the measured band alignments if an equivalent positive charge of 6.0 × 1012 cm−2 is present at the Al2O3/InGaAs. This interface charge is in agreement with previously reported capacitance-voltage measurements.

Discrete charge states in nanowire flash memory with multiple Ta2O5 charge-trapping stacks

In this work, multi-bit flash-like memory cell based on Si nanowire field-effect transistor and multiple Ta2O5 charge-trapping stacks have been fabricated and fully characterized. The memory cells exhibited staircase, discrete charged states at small gate voltages. Such discrete multi-bit on one memory cell is attractive for high memory density. These non-volatile memory devices exhibited fast programming/erasing speed, excellent retention, and endurance, indicating the advantages of integrating the multilayer of charge-storage stacks on the nanowire channel. Such high-performance flash-like non-volatile memory can be integrated into the microprocessor chip as the local memory which requires high density and good endurance.

A unique photoemission method to measure semiconductor heterojunction band offsets

We report a unique way to measure the energy band offset of a heterojunction by exploiting the light absorption profile in the heterojunction under visible-ultraviolet internal photoemission. This method was used to determine the band alignment of W/Al2O3/n+InAs/p+Al0.45Ga0.55Sb heterojunctions that are of interest for tunnel field-effect transistors. The barrier heights from the InAs and Al0.45Ga0.55Sb valence band maxima to the Al2O3 conduction band minimum are found to be 3.24 eV± 0.05 eV and 2.79 eV± 0.05 eV, respectively, yielding a 0.4 eV± 0.1 eV offset at the InAs/AlGaSb interface. This approach can readily be applied to characterize a wide range of other semiconductor heterojunctions.

Design and Fabrication of Ta

In this paper, tantalum pentoxide (Ta₂O₅) has been investigated as the charge-storage dielectric for discrete, multibit memory applications. Two Ta₂O₅ containing dielectric stacks, Al₂O₃/Ta₂O₅/SiO₂ and Al₂O₃/Ta₂O₅/Al₂O₃/Ta₂O₅/SiO₂, have been fabricated and measured. Both structures exhibited excellent nonvolatile memory characteristics: fast program/erase speed with significant flat-band voltage shift at 1 μs, long retention, and good endurance. Interesting multiple staircase memory characteristics observed in the devices with two layers of Ta₂O₅ exhibit multiple, discrete charge-storage states. Such an integration of multibit in one cell is very attractive for developing high-density nonvolatile memory.

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We report on electron spin transport through electrochemically precipitated copper filaments formed in TaOx memristive devices consisting of Co/TaOx/Cu/Py with crossbar-type electrode geometry. The devices show memristive behavior having a typical OFF/ON resistance ratio of 105. Magnetoresistance measurements performed by sweeping an external magnetic field clearly indicate spin transport through an electrochemically formed copper nano-filament as long as 16 nm in the memristive ON-state at 77 K. Spin transport vanishes in the OFF-state. These data are strong evidence that the fundamental switching mechanism in these metal-oxide devices is the formation of continuous metallic conduction paths.