Robert M. Wallace

High-κ gate dielectrics: Current status and materials properties considerations

Many materials systems are currently under consideration as potential replacements for SiO2 as the gate dielectric material for sub-0.1 μm complementary metal–oxide–semiconductor (CMOS) technology. A systematic consideration of the required properties of gate dielectrics indicates that the key guidelines for selecting an alternative gate dielectric are (a) permittivity, band gap, and band alignment to silicon, (b) thermodynamic stability, (c) film morphology, (d) interface quality, (e) compatibility with the current or expected materials to be used in processing for CMOS devices, (f) process compatibility, and (g) reliability. Many dielectrics appear favorable in some of these areas, but very few materials are promising with respect to all of these guidelines. A review of current work and literature in the area of alternate gate dielectrics is given. Based on reported results and fundamental considerations, the pseudobinary materials systems offer large flexibility and show the most promise toward success...

Carbon-Based Supercapacitors Produced by Activation of Graphene

Activated microwave-exfoliated graphite oxide combined with an ionic liquid can be used to make an enhanced capacitor. Supercapacitors, also called ultracapacitors or electrochemical capacitors, store electrical charge on high-surface-area conducting materials. Their widespread use is limited by their low energy storage density and relatively high effective series resistance. Using chemical activation of exfoliated graphite oxide, we synthesized a porous carbon with a Brunauer-Emmett-Teller surface area of up to 3100 square meters per gram, a high electrical conductivity, and a low oxygen and hydrogen content. This sp2-bonded carbon has a continuous three-dimensional network of highly curved, atom-thick walls that form primarily 0.6- to 5-nanometer-width pores. Two-electrode supercapacitor cells constructed with this carbon yielded high values of gravimetric capacitance and energy density with organic and ionic liquid electrolytes. The processes used to make this carbon are readily scalable to industrial levels.

Hafnium and zirconium silicates for advanced gate dielectrics

Hafnium and zirconium silicate (HfSixOy and ZrSixOy, respectively) gate dielectric films with metal contents ranging from ∼3 to 30 at. % Hf, or 2 to 27 at. % Zr (±1 at. % for Hf and Zr, respectively, within a given film), have been investigated, and films with ∼2–8 at. % Hf or Zr exhibit excellent electrical properties and high thermal stability in direct contact with Si. Capacitance–voltage measurements show an equivalent oxide thickness tox of about 18 A (21 A) for a 50 A HfSixOy (50 A ZrSixOy) film deposited directly on a Si substrate. Current–voltage measurements show for the same films a leakage current of less than 2×10−6 A/cm2 at 1.0 V bias. Hysteresis in these films is measured to be less than 10 mV, the breakdown field is measured to be EBD∼10 MV/cm, and the midgap interface state density is estimated to be Dit∼1–5×1011 cm−2 eV−1. Au electrodes produce excellent electrical properties, while Al electrodes produce very good electrical results, but also react with the silicates, creating a lower e l...

The effect of chemical residues on the physical and electrical properties of chemical vapor deposited graphene transferred to SiO2

The effects of residues introduced during the transfer of chemical vapor deposited graphene from a Cu substrate to an insulating (SiO2) substrate on the physical and electrical of the transferred graphene are studied. X-ray photoelectron spectroscopy and atomic force microscopy show that this residue can be substantially reduced by annealing in vacuum. The impact of the removal of poly(methyl methacrylate) residue on the electrical properties of graphene field effect devices is demonstrated, including a nearly 2 × increase in average mobility from 1400 to 2700 cm2/Vs. The electrical results are compared with graphene doping measurements by Raman spectroscopy.

Electrical properties of hafnium silicate gate dielectrics deposited directly on silicon

Hafnium silicate (HfSixOy) gate dielectric films with ∼6 at. % Hf exhibit significantly improved leakage properties over SiO2 in the ultrathin regime while remaining thermally stable in direct contact with Si. Capacitance–voltage measurements show an equivalent oxide thickness (tox) of less than 18 A for a 50 A HfSixOy film deposited directly on a Si substrate, with no significant dispersion of the capacitance for frequencies ranging from 10 kHz to 1 MHz. Current–voltage measurements show for the same film a leakage current of 1.2×10−6 A/cm2 at 1 V bias. Hysteresis in these films is measured to be less than 20 mV, the breakdown field is measured to be EBD∼10 MV/cm, and the midgap interface state density is Dit∼1011 cm−2 eV−1. Cross-sectional transmission electron microscopy shows no signs of reaction or crystallization in HfSixOy films on Si after being annealed at 800 °C for 30 min.

Stable zirconium silicate gate dielectrics deposited directly on silicon

Zirconium silicate (ZrSixOy) gate dielectric films with ∼3–5 at. % Zr exhibit excellent electrical properties and high thermal stability in direct contact with Si. We demonstrate an equivalent oxide thickness of about 21 A for a 50 A ZrSixOy film sputter-deposited directly on a Si substrate, as measured by capacitance–voltage techniques, with a hysteresis shift less than 10 mV. Leakage currents for these films are very low, approximately 1×10−6 A/cm2 at 1.0 V bias in accumulation. Films ramped to hard breakdown exhibit breakdown fields Ebd ∼10 MV/cm. Excellent electrical properties are obtained with Au electrodes, in particular.Zirconium silicate (ZrSixOy) gate dielectric films with ∼3–5 at. % Zr exhibit excellent electrical properties and high thermal stability in direct contact with Si. We demonstrate an equivalent oxide thickness of about 21 A for a 50 A ZrSixOy film sputter-deposited directly on a Si substrate, as measured by capacitance–voltage techniques, with a hysteresis shift less than 10 mV. Leakage currents for these films are very low, approximately 1×10−6 A/cm2 at 1.0 V bias in accumulation. Films ramped to hard breakdown exhibit breakdown fields Ebd ∼10 MV/cm. Excellent electrical properties are obtained with Au electrodes, in particular.

GaAs interfacial self-cleaning by atomic layer deposition

The reduction and removal of surface oxides from GaAs substrates by atomic layer deposition (ALD) of Al2O3 and HfO2 are studied using in situ monochromatic x-ray photoelectron spectroscopy. Using the combination of in situ deposition and analysis techniques, the interfacial “self-cleaning” is shown to be oxidation state dependent as well as metal organic precursor dependent. Thermodynamics, charge balance, and oxygen coordination drive the removal of certain species of surface oxides while allowing others to remain. These factors suggest proper selection of surface treatments and ALD precursors can result in selective interfacial bonding arrangements.

Defect-Dominated Doping and Contact Resistance in MoS2

Achieving low resistance contacts is vital for the realization of nanoelectronic devices based on transition metal dichalcogenides. We find that intrinsic defects in MoS2 dominate the metal/MoS2 contact resistance and provide a low Schottky barrier independent of metal contact work function. Furthermore, we show that MoS2 can exhibit both n-type and p-type conduction at different points on a same sample. We identify these regions independently by complementary characterization techniques and show how the Fermi level can shift by 1 eV over tens of nanometers in spatial resolution. We find that these variations in doping are defect-chemistry-related and are independent of contact metal. This raises questions on previous reports of metal-induced doping of MoS2 since the same metal in contact with MoS2 can exhibit both n- and p-type behavior. These results may provide a potential route for achieving low electron and hole Schottky barrier contacts with a single metal deposition.

Near-unity photoluminescence quantum yield in MoS2

Brighter molybdenum layers The confined layers of molybdenum disulphide (MoS2) exhibit photoluminescence that is attractive for optolectronic applications. In practice, efficiencies are low, presumably because defects trap excitons before they can recombine and radiate light. Amani et al. show that treatment of monolayer MoS2 with a nonoxidizing organic superacid, bis(trifluoromethane) sulfonimide, increased luminescence efficiency in excess of 95%. The enhancement mechanism may be related to the shielding of defects, such as sulfur vacancies. Science, this issue p. 1065 Superacid treatment enhances the luminescence efficiency of monolayer molybdenum disulfide from 1% to >95%. Two-dimensional (2D) transition metal dichalcogenides have emerged as a promising material system for optoelectronic applications, but their primary figure of merit, the room-temperature photoluminescence quantum yield (QY), is extremely low. The prototypical 2D material molybdenum disulfide (MoS2) is reported to have a maximum QY of 0.6%, which indicates a considerable defect density. Here we report on an air-stable, solution-based chemical treatment by an organic superacid, which uniformly enhances the photoluminescence and minority carrier lifetime of MoS2 monolayers by more than two orders of magnitude. The treatment eliminates defect-mediated nonradiative recombination, thus resulting in a final QY of more than 95%, with a longest-observed lifetime of 10.8 ± 0.6 nanoseconds. Our ability to obtain optoelectronic monolayers with near-perfect properties opens the door for the development of highly efficient light-emitting diodes, lasers, and solar cells based on 2D materials.

Band alignment of two-dimensional transition metal dichalcogenides: Application in tunnel field effect transistors

Tunnel field effect transistors (TFETs) based on vertical stacking of two dimensional materials are of interest for low-power logic devices. The monolayer transition metal dichalcogenides (TMDs) with sizable band gaps show promise in building p-n junctions (couples) for TFET applications. Band alignment information is essential for realizing broken gap junctions with excellent electron tunneling efficiencies. Promising couples composed of monolayer TMDs are suggested to be VIB-MeX2 (Me = W, Mo; X = Te, Se) as the n-type source and IVB-MeX2 (Me = Zr, Hf; X = S, Se) as the p-type drain by density functional theory calculations.