Yuehui Jia

Gamma‐rays irradiation: An effective method for improving light emission stability of porous silicon

We report a study on gamma irradiated porous Si. The electron paramagnetic resonance study on porous Si irradiated by gamma rays shows that the observed signals come from an intrinsic defect, a Si dangling bond, at the interface of Si/SiOx in porous Si. The photoluminescence measurements show that the gamma irradiation not only increases the intensity of the photoluminescence but also greatly improves its stability. The spectra of the Fourier transform infrared absorption show that the gamma irradiation is an effective method for accelerating oxidation of porous Si. All experimental results can be explained by the increase of the oxidation layer thickness which decreases the nonradiative recombination probability of electron‐hole pairs.

Electron paramagnetic resonance observation of trigonally symmetric Si dangling bonds in porous silicon layers: Evidence for crystalline Si phase

In photoluminescent porous Si layers is observed a dominant intrinsic EPR signal of trigonal symmetry with g∥=2.0023±0.0003 and g⊥=2.0086±0.0003 as principal g values. This EPR signal can be identified with Si dangling bonds by its symmetry and characteristic g values. The rotation pattern of the EPR signal indicates that the axial directions of the dangling bonds are distributed in all the four 〈111〉 crystal axes of the original silicon lattice. These results can be exclusively explained by the existence of the crystalline Si phase with retention of the original crystal orientation in porous Si. The dangling bond formation is found to be closely related to the surface oxidation.

EFFECTS OF HYDROGEN ON AL/P-SI SCHOTTKY-BARRIER DIODES

Hydrogen was incorporated into B‐doped p‐type crystalline silicon in three different ways: boiling the sample in water, exposing the sample to hydrogen plasma, growing silicon in a hydrogen atmosphere. Al‐contact Schottky barrier diodes were made on both the hydrogenated and unhydrogenated samples. It was found that the Schottky barrier height (SBH) is increased due to the hydrogenation. The current‐voltage measurement showed an increase of 0.06–0.10 eV in the effective SBH and the activation energy measurement revealed an increase of 0.07–0.09 eV in the SBH under a forward bias of 0.15 V. If the silicon grown in a hydrogen atmosphere was thermal annealed at 650 °C to drive out hydrogen before Schottky metallization, then the SBH approached that of the unhydrogenated sample. It is demonstrated that the SBH in Al/p‐Si can be increased with hydrogen incorporated in silicon.

HETEROJUNCTIONS OF SOLID C70 AND CRYSTALLINE SILICON : RECTIFYING PROPERTIES AND BARRIER HEIGHTS

Heterojunctions of solid C70 and n‐ or p‐type crystalline Si have been made. Current–voltage measurements show that both C70/n‐Si and C70/p‐Si contacts are rectifying but their directions of rectification are opposite to each other. Thermal activation measurements at a fixed forward bias show an exponential dependence of current on the reciprocal of temperature, from which we determine the effective barrier height as 0.23 eV for C70/n‐Si and 0.27 eV for C70/p‐Si. Relative dielectric constant of solid C70 was determined to be 4.96 through the study of high‐frequency capacitance–voltage characteristics for Ti/C70/p‐Si structures.

Toward High Carrier Mobility and Low Contact Resistance: Laser Cleaning of PMMA Residues on Graphene Surfaces

Poly(methyl methacrylate) (PMMA) is widely used for graphene transfer and device fabrication. However, it inevitably leaves a thin layer of polymer residues after acetone rinsing and leads to dramatic degradation of device performance. How to eliminate contamination and restore clean surfaces of graphene is still highly demanded. In this paper, we present a reliable and position-controllable method to remove the polymer residues on graphene films by laser exposure. Under proper laser conditions, PMMA residues can be substantially reduced without introducing defects to the underlying graphene. Furthermore, by applying this laser cleaning technique to the channel and contacts of graphene field-effect transistors (GFETs), higher carrier mobility as well as lower contact resistance can be realized. This work opens a way for probing intrinsic properties of contaminant-free graphene and fabricating high-performance GFETs with both clean channel and intimate graphene/metal contact.Graphical Abstract

Heterojunctions of solid C60 and crystalline silicon: rectifying properties and energy-band models

Heterojunctions of undoped solid C60 and n- or p-type-doped crystalline Si have been obtained. Current-voltage measurements show that both C60/n-Si and C60/p-Si contacts are rectifying but their directions of rectification are opposite. Thermal activation measurements at a fixed forward bias show an exponential dependence of current on the reciprocal of temperature, from which we determine the effective barrier height as 0.30 eV for C60/n-Si and 0.48 eV for C60/p-Si. Using energy-band models for heterojunctions we assign values to the positions of the conduction and valence bands of the solid C60 relative to those of crystalline Si and derive the electron affinity and band gap of solid C60 film as 3.92 eV and <1.72 eV, respectively.

Characterization of stain etched porous Si with photoluminescence, electron paramagnetic resonance, and infrared absorption spectroscopy

Porous Si (PS) layers are prepared by stain etching in a HF/NaNO2 solution on both p- and n-type crystal Si substrates, and are characterized by photoluminescence (PL), electron paramagnetic resonance (EPR), and infrared absorption (IR) spectroscopy. The PL spectra under 488 nm laser excitation exhibit a strong peak at 680-720 nm for various samples of different substrate parameters and remain stable upon aging in air or gamma irradiation; as-etched (approximately 20 min in air before measurement) and aged (for up to six months) samples show no detectable EPR signal but the gamma-irradiated samples show an isotropic g = 2.006 signal of peak-to-peak linewidth of 1.1 mT supporting an amorphous Si structure; the IR spectra show both hydrogen and oxygen related IR modes in the as-etched samples and the former decreases with aging time in air while the latter increases. Comparing our results with those of anodically etched PS samples we conclude that: (1) the PL peak position of the stain PS seems to be unique and stable as compared with that of the anodic PS varying in 620-830 nm; (2) the isotropic EPR signal of the stain PS reflects no crystallinity, in contrast with the anisotropic signal of the anodic PS; and (3) obvious oxidation in the as-etched stain PS is also in contrast with the nonobservation of oxygen-related IR modes in the as-etched anodic PS. We discuss the results in terms of structural properties and PL mechanism of PS.

Low insertion loss of 200 μm-long graphite coplanar waveguide

The graphene coplanar waveguide (CPW) has recently been found to have large insertion loss (typically larger than 50 dB/100 μm), which mainly results from the large resistance of graphene. The poor radio-frequency transmission property of graphene hampers its application in interconnect, a low loss material is thus required. In this paper, low-resistance graphite CPWs with effective graphite length up to 200 μm were fabricated. A record low insertion loss of graphite CPW (2.76 dB/100 μm) is demonstrated, and the average insertion loss of our graphite CPWs is only ∼1/5 of that of our monolayer graphene CPWs. Moreover, we find the insertion loss of graphite CPW may be even smaller at higher frequencies. Our investigation shows that graphite is a possible candidate for interconnect and may even be more applicable at ultra-high frequencies.

Electron spin resonance study of RbxC60 and KxC60 powders

Abstract Rb and K doped C60 compounds are prepared by the vapor-solid reaction method. Superconductive shielding fraction is determined as 35–75% for RbxC60 and 10% for KxC60. Electron spin resonance (ESR) measurements in absorption mode are performed in the temperature range of 5–300 K. A strong ESR signal at g=2.002 for RbxC60 and g=2.003 for KxC60 has been observed. The g-value is nearly independent upon temperature for both RbxC60 and KxC60 while the linewidth behaves differently, increasing from 2.0 G at 5 K to 6.0 G at 250 K in RbxC60 and remaining constant of 2.3 G in KxC60. No drastic change of the ESR spectra is observed with the onset of superconductivity, leading to a conclusion that the observed dominant ESR signal originates from a non-superconducting phase.

Synthesis of highly uniform monolayer graphene by etching the multilayer spots for electronic devices

We demonstrate a facile method to grow highly uniform monolayer graphene films on copper foils by atmospheric pressure chemical vapor deposition (APCVD). The technique in this method includes lowering flow ratio of methane/hydrogen and extending exposure time to hydrogen. All the multilayer islands will be etched away by hydrogen during this growth process, resulting in obtaining highly uniform monolayer graphene. A mechanism for the suppression of mutilayer spots based on the etching effect of hydrogen is proposed. The electron and hole room-temperature mobilities for the back-gated graphene transistors are up to about 3800 cm2V−1s−1 and 3300 cm2V−1s−1, respectively.