Tarek A. Ameen

Dielectric Engineered Tunnel Field-Effect Transistor

The dielectric engineered tunnel field-effect transistor (DE-TFET) as a high-performance steep transistor is proposed. In this device, a combination of high-k and low-k dielectrics results in a high electric field at the tunnel junction. As a result, a record ON-current of ~1000 μA/μm and a subthreshold swing (SS) below 20 mV/decade are predicted for WTe2 DE-TFET. The proposed TFET works based on a homojunction channel and electrically doped contacts both of which are immune to interface states, dopant fluctuations, and dopant states in the bandgap, which typically deteriorate the OFF-state performance and SS in the conventional TFETs.

Few-layer Phosphorene: An Ideal 2D Material For Tunnel Transistors

2D transition metal dichalcogenides (TMDs) have attracted a lot of attention recently for energy-efficient tunneling-field-effect transistor (TFET) applications due to their excellent gate control resulting from their atomically thin dimensions. However, most TMDs have bandgaps (Eg) and effective masses (m*) outside the optimum range needed for high performance. It is shown here that the newly discovered 2D material, few-layer phosphorene, has several properties ideally suited for TFET applications: 1) direct Eg in the optimum range ~1.0–0.4 eV, 2) light transport m* (0.15 m0), 3) anisotropic m* which increases the density of states near the band edges, and 4) a high mobility. These properties combine to provide phosphorene TFET outstanding ION ~ 1 mA/um, ON/OFF ratio ~ 106 for a 15 nm channel and 0.5 V supply voltage, thereby significantly outperforming the best TMD-TFETs and CMOS in many aspects such as ON/OFF current ratio and energy-delay products. Furthermore, phosphorene TFETS can scale down to 6 nm channel length and 0.2 V supply voltage within acceptable range in deterioration of the performance metrics. Full-band atomistic quantum transport simulations establish phosphorene TFETs as serious candidates for energy-efficient and scalable replacements of MOSFETs.

Thickness Engineered Tunnel Field-Effect Transistors Based on Phosphorene

Thickness engineered tunneling field-effect transistors (TE-TFET) as a high-performance ultra-scaled steep transistor is proposed. This device exploits a specific property of 2-D materials: layer thickness-dependent energy bandgaps (Eg). Unlike the conventional hetero-junction TFETs, TE-TFET uses spatially varying layer thickness to form a hetero-junction. This offers advantages by avoiding the lattice mismatch problems at the interface. Furthermore, it boosts the ON-current to 1280 μA/μm with 15-nm channel length. Providing higher ON currents, phosphorene TE-TFET outperforms the homojunction phosphorene and the TMD TFETs in terms of extrinsic energy-delay product. TE-TFET also scales well to 9 nm with constant field scaling E = VDD/Lch= 33

Extremely high simulated ballistic currents in triple-heterojunction tunnel transistors

mV/nm. In this letter, the operation principles of TE-TFET and its performance sensitivity to the design parameters are investigated through full-band atomistic quantum transport simulations.

Universal Behavior of Atomistic Strain in Self-Assembled Quantum Dots

Future VLSI devices will require low CV_dd²/2 switching energy, large on-currents (I_on), and small off-currents (I_off). Low switching energy requires a low supply voltage V_dd, yet reducing V_dd typically increases /off and reduces the I_on/I_off ratio. Though tunnel FETs (TFETs) have steep subthreshold swings and can operate at a low V_dd, yet their I_on is limited by low tunneling probability. Even with a GaSb/InAs heterojunction (HJ), given a 2nm-thick-channel (001)-confined TFET, [100] transport, and assuming V_dd=0.3V and I_oFF=10^-3A/m, the peak tunneling probability is <;3 % (fig. 1 a) and I_on is only 24 A/m (fig. 1b) [1]. This low I_on will result in large CV_dd/I delay and slow logic operation. Techniques to increase /on include graded AlSb/AlGaSb source HJs [2,3] and tunneling resonant states [4]. We had previously shown that tunneling probability is increased using (11 0) confinement and channel heterojunctions [1], the latter increasing the junction built-in potential and junction field, hence reducing the tunneling distance. Here we propose a triple heterojunction TFET combining these techniques. The triple-HJ design further thins the tunnel barrier to 1.2 nm, and creates two closely aligned resonant states 57meV apart. The tunneling probability is very high, >50% over a 120meV range, and the ballistic I_on is extremely high, 800A/m at 30nm Lg and 475 A/m at 15nm Lg, both with I_off=10^-3 A/m and V_dd=0.3 V. Compared to a (001) GaSb/InAs TFET, the triple-HJ design increases the ballistic /on by 26:1 at 30nm L_g and 19:1 at 15nm L_g. The designs may, however, suffer from increased phonon-assisted tunneling.

Optimization of the anharmonic strain model to capture realistic strain distributions in quantum dots

Self-assembled quantum dots (QDs) are highly strained heterostructures. the lattice strain significantly modifies the electronic and optical properties of these devices. A universal behavior is observed in atomistic strain simulations (in terms of both strain magnitude and profile) of QDs with different shapes and materials. In this paper, this universal behavior is investigated by atomistic as well as analytic continuum models. Atomistic strain simulations are very accurate but computationally expensive. On the other hand, analytic continuum solutions are based onassumptions that significantly reduce the accuracy of the strain calculations, but are very fast. Both techniques indicate that the strain depends on the aspect ratio (AR) of the QDs, and not on the individual dimensions. Thus simple closed form equations are introduced which directly provide the atomistic strain values inside the QD as a function of the AR and the material parameters. Moreover, the conduction and valence band edges EC/V and their effective masses mC/V of the QDs are dictated by the strain and AR consequently. The universal dependence of atomistic strain on the AR is useful in many ways; Not only does it reduce the computational cost of atomistic simulations significantly, but it also provides information about the optical transitions of QDs given the knowledge of EC/V and m ∗ C/V from AR. Finally, these expressions are used to calculate optical transition wavelengths in InAs/GaAs QDs and the results agree well with experimental measurements and atomistic simulations. Keywords— Self-assembled quantum dots, Stranski-Krastanov, Atomistic strain, Analytical continuum strain, Continuum elasticity, Optical transition.Self-assembled quantum dots (QDs) are highly strained heterostructures. The lattice strain significantly modifies the electronic and optical properties of these devices. A universal behavior is observed in atomistic strain simulations (in terms of both strain magnitude and profile) of QDs with different shapes and materials. In this paper, this universal behavior is investigated by atomistic as well as analytic continuum models. Atomistic strain simulations are very accurate but computationally expensive. On the other hand, analytic continuum solutions are based on assumptions that significantly reduce the accuracy of the strain calculations, but are very fast. Both techniques indicate that the strain depends on the aspect ratio (AR) of the QDs, and not on the individual dimensions. Thus, simple closed-form equations are introduced which directly provide the atomistic strain values inside the QD as a function of the AR and the material parameters. Moreover, the conduction and valence band edges EC/V and their effective masses m*C/V of the QDs are dictated by the strain and AR consequently. The universal dependence of atomistic strain on the AR is useful in many ways. Not only does it reduce the computational cost of atomistic simulations significantly, but it also provides information about the optical transitions of QDs given the knowledge of EC/V and m*C/V from AR. Finally, these expressions are used to calculate optical transition wavelengths in InAs/GaAs QDs, and the results agree well with the experimental measurements and atomistic simulations.

Combination of Equilibrium and Nonequilibrium Carrier Statistics Into an Atomistic Quantum Transport Model for Tunneling Heterojunctions

Self-assembled quantum dots are highly strained heterostructures, and a rigorous atomistic strain model is needed to predict the behavior of these devices. An anharmonic strain model reported by Lazarenkova, et al. [1] modifies the well-known harmonic Keating model [2] to include the effect of anharmonicity in the lattice potential. The Lazarenkova strain parameters were originally optimized to deliver correct Grüneisen parameters, however this optimization does not provide strain values that compare well to the values obtained in experiments on both quantum wells and dots. Our new approach in optimizing the model parameters to obtain correct biaxial strain ratio in quantum wells has resulted in a significant improvement in the quantum dot simulations in terms of reproducing experimental optical transitions.

III-N heterostructure devices for low-power logic

Tunneling heterojunctions (THJs) have confined states close to the tunneling region, which significantly affect their transport properties. Accurate numerical modeling of THJs requires combining the nonequilibrium coherent quantum transport through the tunneling region as well as the quasi-equilibrium statistics arising from the strong scattering in the confined states. In this paper, a novel atomistic model is proposed to include both the effects: the strong scattering in the regions around THJ and the coherent tunneling. The new model matches reasonably well with experimental measurements of Nitride THJ and provides an efficient engineering tool for performance prediction and design of THJ-based devices.

Theoretical study of strain-dependent optical absorption in a doped self-assembled InAs/InGaAs/GaAs/AlGaAs quantum dot

Future generations of ultra-scaled logic may require alternative device technologies to transcend the limitations of Si CMOS; in particular, power dissipation constraints in aggressively-scaled, highly-integrated systems make device concepts capable of achieving switching slopes (SS) steeper than 60 mV/decade especially attractive. Tunneling field effect transistors (TFETs) are one such device technology alternative. While a great deal of research into TFETs based on Si, Ge, and narrow band gap III-Vs has been reported, these approaches each face significant challenges. An alternative approach based on the use of III-N wide band gap semiconductors in conjunction with polarization engineering offers potential advantages in terms of drain current density and switching slope. In this talk, the prospects for III-N based TFETs for logic will be discussed, including both simulation projections as well as experimental progress.

Multiscale transport simulation of nanoelectronic devices with NEMO5

A detailed theoretical study of the optical absorption in doped self-assembled quantum dots is presented. A rigorous atomistic strain model as well as a sophisticated 20-band tight-binding model are used to ensure accurate prediction of the single particle states in these devices. We also show that for doped quantum dots, many-particle configuration interaction is also critical to accurately capture the optical transitions of the system. The sophisticated models presented in this work reproduce the experimental results for both undoped and doped quantum dot systems. The effects of alloy mole fraction of the strain controlling layer and quantum dot dimensions are discussed. Increasing the mole fraction of the strain controlling layer leads to a lower energy gap and a larger absorption wavelength. Surprisingly, the absorption wavelength is highly sensitive to the changes in the diameter, but almost insensitive to the changes in dot height. This behavior is explained by a detailed sensitivity analysis of different factors affecting the optical transition energy.