Farhan Rana

A silicon nanocrystals based memory

A new memory structure using threshold shifting from charge stored in nanocrystals of silicon (≊5nm in size) is described. The devices utilize direct tunneling and storage of electrons in the nanocrystals. The limited size and capacitance of the nanocrystals limit the numbers of stored electrons. Coulomb blockade effects may be important in these structures but are not necessary for their operation. The threshold shifts of 0.2–0.4 V with read and write times less than 100’s of a nanosecond at operating voltages below 2.5 V have been obtained experimentally. The retention times are measured in days and weeks, and the structures have been operated in an excess of 109 cycles without degradation in performance. This nanomemory exhibits characteristics necessary for high density and low power.

Measurement of ultrafast carrier dynamics in epitaxial graphene

Using ultrafast optical pump-probe spectroscopy, we have measured carrier relaxation times in epitaxial graphene layers grown on SiC wafers. We find two distinct time scales associated with the relaxation of nonequilibrium photogenerated carriers. An initial fast relaxation transient in the 70–120fs range is followed by a slower relaxation process in the 0.4–1.7ps range. The slower relaxation time is found to be inversely proportional to the degree of crystalline disorder in the graphene layers as measured by Raman spectroscopy. We relate the measured fast and slow time constants to carrier-carrier and carrier-phonon intraband and interband scattering processes in graphene.

Ultrafast Optical-Pump Terahertz-Probe Spectroscopy of the Carrier Relaxation and Recombination Dynamics in Epitaxial Graphene

The ultrafast relaxation and recombination dynamics of photogenerated electrons and holes in epitaxial graphene are studied using optical-pump terahertz-probe spectroscopy. The conductivity in graphene at terahertz frequencies depends on the carrier concentration as well as the carrier distribution in energy. Time-resolved studies of the conductivity can therefore be used to probe the dynamics associated with carrier intraband relaxation and interband recombination. We report the electron-hole recombination times in epitaxial graphene for the first time. Our results show that carrier cooling occurs on subpicosecond time scales and that interband recombination times are carrier density dependent.

Graphene Terahertz Plasmon Oscillators

In this paper we propose and discuss coherent terahertz sources based on charge density wave (plasmon) amplification in two-dimensional graphene. The coupling of the plasmons to interband electron-hole transitions in population inverted graphene layers can lead to plasmon amplification through stimulated emission. Plasmon gain values in graphene can be very large due to the small group velocity of the plasmons and the strong confinement of the plasmon field in the vicinity of the graphene layer. We present a transmission line model for plasmon propagation in graphene that includes plasmon dissipation and plasmon interband gain due to stimulated emission. Using this model, we discuss design for terahertz plasmon oscillators and derive the threshold condition for oscillation taking into account internal losses and also losses due to external coupling. The threshold condition is shown to depend on the ratio of the external impedance and the characteristic impedance of the plasmon transmission line. The large gain values available at terahertz frequencies in graphene can lead to integrated oscillators that have dimensions in the 1-10 mum range.

Single charge and confinement effects in nano‐crystal memories

Use of nano‐crystals of silicon in close proximity (1.5–4.5 nm) of a transistor channel lead to structures with pronounced memory where effects due to discrete number of electrons, confinement‐induced subbands in inversion layers and discrete energy states in quantum dots, random charge distribution in quantum dots, and transmission through a strong barrier are very important. Experimental results show plateaus in threshold voltage at low temperatures, spaced nearly equally apart, and indicative of single electron effects. Varying the oxide thickness shows strong influence on speed and charge retention. We confirm the strength of confinement effects and discuss the underlying considerations in the operation of the memory that are related to the reduced volume, strength of the barrier, and random distribution of the trapped charge in nano‐crystals.

Measurement of the optical absorption spectra of epitaxial graphene from terahertz to visible

We present experimental results on the optical absorption spectra of epitaxial graphene from the visible to the terahertz frequency range. In the terahertz range, the absorption is dominated by intraband processes with a frequency dependence similar to the Drude model. In the near-IR range, the absorption is due to interband processes and the measured optical conductivity is close to the theoretical value of e2/4ℏ. We extract values for the carrier densities, the number of carbon atom layers, and the intraband scattering times from the measurements.

Volatile and non-volatile memories in silicon with nano-crystal storage

A single transistor memory structure, with changes in threshold voltage exceeding /spl ap/0.25 V corresponding to single electron storage in individual nano-crystals, operating in the sub-3 V range, and exhibiting long term to non-volatile charge storage is reported. As a consequence of Coulombic effects, operation at 77 K shows a saturation in threshold voltage in a range of gate voltages with steps in the threshold voltage corresponding to single and multiple electron storage. The plateauing of threshold shift, operation at ultra-low power, low voltages, and single element implementation utilizing current sensing makes this an alternative memory at speeds lower than those of DRAMs and higher than those of E/sup 2/PROMs, but with potential for significantly higher density, lower power, and faster read.

Self‐consistent modeling of accumulation layers and tunneling currents through very thin oxides

Poisson and Schrodinger equations are solved self‐consistently for accumulated layers in metal‐oxide‐semiconductor devices and applied to the calculation of tunneling currents at 300 K and 77 K and extraction of parameters for very thin oxides. Calculations at 300 K show strong agreement with measured tunneling currents and also point out the sources of inaccuracies in extracting thicknesses of oxides by electrical methods such as through measurement of capacitance. Direct tunneling current in thin oxides (1.5–2.0 nm) are shown to achieve larger than 1  A /cm2 current density for applied voltages smaller than 3 V, pointing to possibilities of achieving high endurance injection across thin oxides. Comparison of calculations using a classical approach and self‐consistent approach shows fortuitous agreements in tunneling currents despite large differences in the physical models. Appropriate methods for calculating tunneling currents from bound and extended quantum states are also described.

Ultrafast relaxation dynamics of hot optical phonons in graphene

Using ultrafast optical pump-probe spectroscopy, we study the relaxation dynamics of hot optical phonons in few-layer and multilayer graphene films grown by epitaxy on silicon carbide substrates and by chemical vapor deposition on nickel substrates. In the first few hundred femtoseconds after photoexcitation, the hot carriers lose most of their energy to the generation of hot optical phonons which then present the main bottleneck to subsequent cooling. Optical phonon cooling on short time scales is found to be independent of the graphene growth technique, the number of layers, and the type of the substrate. We find average phonon lifetimes in the 2.5–2.55 ps range. We model the relaxation dynamics of the coupled carrier-phonon system with rate equations and find a good agreement between the experimental data and the theory. The extracted optical phonon lifetimes agree very well with the theory based on anharmonic phonon interactions.

Ultrafast Dynamics of Defect-Assisted Electron–Hole Recombination in Monolayer MoS2

In this Letter, we present nondegenerate ultrafast optical pump-probe studies of the carrier recombination dynamics in MoS2 monolayers. By tuning the probe to wavelengths much longer than the exciton line, we make the probe transmission sensitive to the total population of photoexcited electrons and holes. Our measurement reveals two distinct time scales over which the photoexcited electrons and holes recombine; a fast time scale that lasts ∼ 2 ps and a slow time scale that lasts longer than ∼ 100 ps. The temperature and the pump fluence dependence of the observed carrier dynamics are consistent with defect-assisted recombination as being the dominant mechanism for electron-hole recombination in which the electrons and holes are captured by defects via Auger processes. Strong Coulomb interactions in two-dimensional atomic materials, together with strong electron and hole correlations in two-dimensional metal dichalcogenides, make Auger processes particularly effective for carrier capture by defects. We present a model for carrier recombination dynamics that quantitatively explains all features of our data for different temperatures and pump fluences. The theoretical estimates for the rate constants for Auger carrier capture are in good agreement with the experimentally determined values. Our results underscore the important role played by Auger processes in two-dimensional atomic materials.