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...

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

A molecule containing a nitroamine redox center (2'-amino-4-ethynylphenyl-4'-ethynylphenyl-5'-nitro-1-benzenethiol) was used in the active self-assembled monolayer in an electronic device. Current-voltage measurements of the device exhibited negative differential resistance and an on-off peak-to-valley ratio in excess of 1000:1.

https://science.sciencemag.org/content/sci/286/5444/1550.full.pdf

Large On-Off Ratios and Negative Differential Resistance in a Molecular Electronic Device.

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...

Hafnium and zirconium silicates for advanced gate dielectrics

Electron tunneling through self-assembled monolayers (SAM's) of alkanethiols is investigated using nanometer-scale devices. Temperature-dependent current-voltage measurements are performed on alkanethiol SAM's to distinguish between different conduction mechanisms. Temperature-independent electron transport is observed, proving that tunneling is the dominant conduction mechanism of alkanethiols, as well as exhibiting an exponential dependence of tunneling current on the molecule length with a decay coefficient \ensuremath{\beta}. From the bias dependence of \ensuremath{\beta}, a barrier height ${\ensuremath{\Phi}}_{B}$ of $1.39\ifmmode\pm\else\textpm\fi{}0.01\mathrm{eV}$ and a zero-field decay coefficient ${\ensuremath{\beta}}_{0}$ of $0.79\ifmmode\pm\else\textpm\fi{}0.01{\AA{}}^{\ensuremath{-}1}$ are determined for alkanethiols.

Mechanism of electron conduction in self-assembled alkanethiol monolayer devices

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.

Electrical properties of hafnium silicate gate dielectrics deposited directly on silicon

Electron transport in ultrathin (tox<40 A) Al/SiO2/n−Si structures is dominated by direct tunneling of electrons across the SiO2 barrier. By analyzing the tunneling currents as a function of the SiO2 layer thickness for a comprehensive set of otherwise identical samples, we are able to extract an effective mass for the tunneling electron in the SiO2 layer. Oxide films 16–35 A thick were thermally grown in situ in a dry oxygen ambient. The oxide thicknesses were determined by capacitance–voltage measurements and by spectroscopic ellipsometry. The tunneling effective mass was extracted from the thickness dependence of the direct tunneling current between an applied voltage of 0 and 2 V, a bias range that has not been previously explored. Employing both a parabolic and a nonparabolic assumption of the E−κ relationship in the oxide forbidden gap, we found the SiO2 electron mass to be mP*=0.30±0.02me, and mNP*=0.41±0.01me, respectively, independent of bias. Because this method is based on a large sample set, t...

Direct extraction of the electron tunneling effective mass in ultrathin SiO2

The combination of resonant-tunneling diodes and heterostructure field-effect transistors provides a versatile technology for implementing microwave digital and mixed-signal applications. Here we demonstrate and characterize the first monolithic flash analog-to-digital converter (ADC) in this technology. The first-pass ADC achieved 2.7 effective bits at 2 gigasamples per second (Gsps) for a 220-MHz input signal. The one-bit quantizer achieved a single-tone spurious free dynamic range greater than 40 dB at 2 Gsps for a 220-MHz single-tone input with dithering.

A monolithic 4-bit 2-Gsps resonant tunneling analog-to-digital converter

Indium phosphide heterojunction bipolar transistors (HBTs) find applications in very wide-band digital and mixed-signal integrated circuits (ICs). Devices fabricated in high-yield process flows at 500 nm feature size obtain 450 GHz cutoff frequencies and 5 V breakdown and enable high yield fabrication of integrated circuits having more than 3000 transistors. Laboratory devices at 250 nm feature size obtain 755 GHz . We describe device and circuit bandwidth limits associated with HBTs, develop scaling roadmaps for HBTs having lithographic minimum feature sizes between 512 and 64 nm, and identify key technological challenges in realizing 480-GHz digital ICs and 1000-GHz amplifiers. Key features of manufacturable self-aligned dielectric sidewall processes are described in detail.

InP Bipolar ICs: Scaling Roadmaps, Frequency Limits, Manufacturable Technologies

A large-scale integration (LSI) InP-based technology is described for high-speed mixed-signal circuits. The monolithic 75-mm wafer process uses molecular beam epitaxy, InP etch stop layers, an electron-beam-defined gate, non-alloyed ohmic contacts, and 10 mask levels to provide resonant tunneling diodes (RTD's), 0.25- or 0.5-μm gate-length high electron mobility transistors (HEMT's), Schottky diodes, resistors, capacitors and two and a half levels of interconnect. Resonant tunneling circuits described here for the first time include a 2.5-GHz, ten stage, tapped shift register, a 6.5-GHz clock generator and a multivalued-to-binary converter.

Resonant-tunneling mixed-signal circuit technology

The combination of resonant tunneling diodes (RTDs) and heterostructure field-effect transistors (HFETs) provides a versatile technology for implementing microwave digital and mixed-signal applications. Here we demonstrate the first monolithic flash RTD/HFET analog-to-digital converter (ADC). The first pass ADC achieved 2.7 effective bits at 2 GSps. The one bit quantizer achieved a single tone spurious free dynamic range (SFDR) of greater than 40 dB at 2 GSps for a 220 MHz single tone input with dithering.

Bobby Brar

Papers

Direct extraction of the electron tunneling effective mass in ultrathin SiO2

A monolithic 4-bit 2-Gsps resonant tunneling analog-to-digital converter

InP Bipolar ICs: Scaling Roadmaps, Frequency Limits, Manufacturable Technologies

Resonant-tunneling mixed-signal circuit technology

A monolithic 4 bit 2 GSps resonant tunneling analog-to-digital converter