Takenori Osada

Polymer-based light-emitting devices: investigations on the role of the indium—tin oxide (ITO) electrode

Abstract We have studied the cleaning procedure dependence of the chemical composition and work function for different indium—tin oxide (ITO) samples using ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). Also, the surface morphology of ITO was investigated by atomic force microscopy (AFM). Despite the pronounced differences in surface morphology and the In Sn ratio, the variation of the work function between different ITO samples was very small after each cleaning procedure. The work functions of ITO samples cleaned with organic solvents and hydrogen peroxide were 4.4–4.5 and 4.7–4.8 eV, respectively. Ne-ion sputtering preferentially removed oxygen, which resulted in a partial reduction of the surface and a lowering of the work function to 4.0–4.1 eV. Used as hole-injecting electrodes in organic and polymer light-emitting devices (LEDs), different ITOs resulted in pronounced differences in the LED performances despite their almost identical work functions after the hydrogen peroxide treatment. This reveals that the work function of the ITO is not the only factor to determine the hole-injection characteristics in polymer-based LEDs.

Strain-induced enhancement of plasma dispersion effect and free-carrier absorption in SiGe optical modulators

The plasma dispersion effect and free-carrier absorption are widely used to change refractive index and absorption coefficient in Si-based optical modulators. However, the weak free-carrier effects in Si cause low modulation efficiency, resulting in large device footprint and power consumption. Here, we theoretically and experimentally investigate the enhancement of the free-carrier effects by strain-induced mass modulation in silicon-germanium (SiGe). The application of compressive strain to SiGe reduces the conductivity effective mass of holes, resulting in the enhanced free-carrier effects. Thus, the strained SiGe-based optical modulator exhibits more than twice modulation efficiency as large as that of the Si modulator. To the best of our knowledge, this is the first demonstration of the enhanced free-carrier effects in strained SiGe at the near-infrared telecommunication wavelength. The strain-induced enhancement technology for the free-carrier effects is expected to boost modulation efficiency of the most Si-based optical modulators thanks to high complementary metal-oxide-semiconductor (CMOS) compatibility.

High Performance Tri-Gate Extremely Thin-Body InAs-On-Insulator MOSFETs With High Short Channel Effect Immunity and

We have investigated the effects of the tri-gate channel structure on electrical properties of extremely thin-body (ETB) InAs-on-insulator (-OI) MOSFETs. It was found that the tri-gate structure provides significant improvement in short channel effect (SCE) control even in ETB-OI MOSFETs by the simulation. We have fabricated and demonstrated tri-gate InAsOI MOSFETs with fin width of the top surface down to 40 nm. The tri-gate ETB InAs-OI MOSFETs shows better SCEs control with small effective mobility (μeff) reduction. Thus, we have demonstrated the operation of sub-20-nm-channel length (Lch) InAs-OI MOSFETs. The 20-nm-Lch InAs-OI MOSFETs show good electrostatic with subthreshold slope of 120 mV/decade and drain induced barrier lowering of 110 mV/V, and high transconductance (Gm) of 1.64 mS/μm. Furthermore, we have realized a wide-range threshold voltage (Vth) tunability in tri-gate InAs-OI MOSFETs through back bias voltage (VB) control.

V_{\rm th}

Using an atmospheric metal-organic chemical vapor deposition system, we passivated GaAs with AlN prior to atomic layer deposition of Al2O3. This AlN passivation incorporated nitrogen at the Al2O3/GaAs interface, improving the capacitance-voltage (C–V) characteristics of the resultant metal-oxide-semiconductor capacitors (MOSCAPs). The C–V curves of these devices showed a remarkable reduction in the frequency dispersion of the accumulation capacitance. Using the conductance method at various temperatures, we extracted the interfacial density of states (Dit). The Dit was reduced over the entire GaAs band gap. In particular, these devices exhibited Dit around the midgap of less than 4 × 1012 cm−2eV−1, showing that AlN passivation effectively reduced interfacial traps in the MOS structure.

Tunability

We have systematically analyzed the components of source/drain (S/D) resistance (RSD) in InGaAs n-MOSFETs with Ni-InGaAs metal S/D. It is found that Ni-InGaAs has a low resistivity of ~250 μΩ·cm in a thickness of Ni-InGaAs (T_Ni-InGaAs) of down to ~4 nm. Contact resistance between the contact pads and Ni-InGaAs (R_C) is found to be the most dominant component of R_SD in control InGaAs MOSFETs, because of the existence of Ni oxides. By developing a surface cleaning process using NH₄OH and H₂ plasma for Ni-InGaAs surfaces, we have reduced R_C down to 11 Ω·μm without any accompanying drawbacks. Also, the increase in the channel indium (In) content has provided further R_SD reduction. Employing these R_SD reduction technologies, we present 20-nm-channel length (L_ch) InAs-on-insulator n-MOSFETs on Si substrates with Ni-InGaAs metal S/D. The devices provide a high maximum ON-current (ION) and maximum transconductance (G_m) of 2.38 mA/μm and 1.95 mS/μm at drain voltage (V_D) of 0.5 V. This high performance is attributable to the low R_SD realized by the surface cleaning process of Ni-InGaAs surfaces before the contact pad formation as well as the increase in the In content in the channel layer. Furthermore, it is found that the interface resistance (R_interface) between Ni-InGaAs and InGaAs channels can be reduced down to 50 Ω·μm by increasing the In content in the channel layers.

Nitride passivation reduces interfacial traps in atomic-layer-deposited Al2O3/GaAs (001) metal-oxide-semiconductor capacitors using atmospheric metal-organic chemical vapor deposition

We have demonstrated the operation of high on-off current ratio (Ion/Ioff) and low subthreshold slope planar-type InGaAs tunnel field effect transistors (TFETs) with Zn-diffused source junctions. The solid-phase Zn diffusion process has been shown to form defect-less p+/n source junctions with steep profiles because of the inherent nature of Zn diffusion into InGaAs, which has significantly improved the TFET performance. The devices presented in this paper have exhibited a record low subthreshold slope of 64 mV/dec for planar-type III-V TFETs and a large Ion/Ioff ratio of more than 106 at the same time.

High-Performance InAs-On-Insulator n-MOSFETs With Ni-InGaAs S/D Realized by Contact Resistance Reduction Technology

The electronic structure of the interfacial region between tris(8-hydroxy-quinoline) aluminum (Alq) and alkali metal (Li and K) was studied by ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS). Condensed molecular solid films of Alq were doped by alkali metal deposited on the surface. The alkali metal atoms were found to distribute uniformly through out the depth to which the sample could be studied. A new feature of the density of states was observed in the band gap, 1.7eV higher than the HOMO of the pristine Alq. The binding energy of N(1s) core level of the quinoline ligand of Alq was shifted by doping to lower binding energies, of 1.6 eV and 1.8 eV from the original peak for Li and K doping, respectively. The intensity of the new shifted peak was saturated when Alq was fully doped. The full doping level, determined by chemical analysis (XPS), was Alq:metal=1:3.

High Ion/Ioff and low subthreshold slope planar-type InGaAs tunnel field effect transistors with Zn-diffused source junctions

We have demonstrated the operation of high I_on/I_off and low subthreshold slope planar-type InGaAs Tunnel FETs with Zn-diffused source junctions. It has been found that the solid-phase Zn diffusion can form steep-profile and defect-less p⁺/n source junctions because of the inherent nature of Zn diffusion into InGaAs, which has significantly improved the TFET performance. The present devices have exhibited the record small S.S. of 64 mV/dec and large I_on/I_off ratio over10⁶ as the planar-type III-V TFETs.

XPS and UPS study of charge transport material/electrode interface of light emitting diodes.

An electron cyclotron resonance (ECR) plasma post-nitridation method has been investigated for improving Al2O3/SiGe metal–oxide–semiconductor (MOS) interfaces. We evaluated the interface trap density (Dit) of Al/Al2O3/Si0.75Ge0.25/p-Si MOS capacitors by the conductance method, showing that Dit of the Al2O3/SiGe interfaces was reduced to 3×1011 cm-2 eV-1 at the minimum expense of equivalent oxide thickness (EOT). X-ray photoelectron spectroscopy (XPS) revealed that a SiGe–ON interfacial layer was formed between the Al2O3 and SiGe layers, which improved the interfacial properties through nitrogen passivation of SiGe interfaces. As a result, a superior Al2O3/SiGe MOS interface with a 0.2 nm EOT increase can be obtained by plasma post-nitridation.

High I on /I off and low subthreshold slope planar-type InGaAs tunnel FETs with Zn-diffused source junctions

We report the electrical characteristics of strained In0.53Ga0.47As-on-insulator (-OI) metal-oxide-semiconductor field-effect-transistors (MOSFETs) on Si substrates fabricated by a direct wafer bonding (DWB) technique. 1.7% highly strained In0.53Ga0.47As-OI structures are fabricated on Si substrate by DWB. Strained In0.53Ga0.47As-OI MOSFETs with Ni-InGaAs metal source/drain (S/D) have been operated with high on-current (Ion)/off-current (Ioff) ratio of ∼105 and good current saturation in output characteristics. MOSFETs with 1.7% tensile strain exhibits 1.65 × effective mobility (μeff) enhancement against In0.53Ga0.47As MOSFET without strain. We found that this μeff enhancement is attributed to the increase in mobile free electron concentration under tensile strain, which leads to the lowering in the conduction band minimum (CBM) and the increase in the energy difference between CBM and the Fermi level pinning position due to a large amount of interface states by Hall measurements.