Yoshiharu Enta

Structure, Chemical Bonding and These Thermal Stabilities of Diamond-Like Carbon (DLC) Films by RF Magnetron Sputtering

We have deposited diamond-like carbon (DLC) films using RF magnetron sputtering techniques, and investigated structure, chemical bonding of deposited films and these thermal stabilities by Raman spectroscopy and photoelectron spectroscopy. It has been found that the film deposited under typical conditions is amorphous carbon (a-C) with 62% sp2 and 38% sp3 bonds. Ordering of a-C has been observed with an increase in substrate temperature during deposition and similarly observed after postannealing, although the sp3/sp2 ratio in a film does not change even at 900°C. The absence of conversion between sp3 and sp2 bonds indicates that the DLC films have high thermal stabilities.

Real-time observation of the dry oxidation of the Si (100) surface with ambient pressure x-ray photoelectron spectroscopy

We have applied ambient-pressure x-ray photoelectron spectroscopy with Si 2p chemical shifts to study the real-time dry oxidation of Si(100), using pressures in the range of 0.01-1 Torr and temperatures of 300-530 oC, and examining the oxide thickness range from 0 to ~;;25 Angstrom. The oxidation rate is initially very high (with rates of up to ~;;225 Angstrom/h) and then, after a certain initial thickness of the oxide in the range of 6-22 Angstrom is formed, decreases to a slow state (with rates of ~;;1.5-4.0 Angstrom/h). Neither the rapid nor the slow regime is explained by the standard Deal-Grove model for Si oxidation.

Controls over Structural and Electronic Properties of Epitaxial Graphene on Silicon Using Surface Termination of 3C-SiC(111)/Si

Epitaxial graphene on Si (GOS) using a heteroepitaxy of 3C-SiC/Si has attracted recent attention owing to its capability to fuse graphene with Si-based electronics. We demonstrate that the stacking, interface structure, and hence, electronic properties of GOS can be controlled by tuning the surface termination of 3C-SiC(111)/Si, with a proper choice of Si substrate and SiC growth conditions. On the Si-terminated 3C-SiC(111)/Si(111) surface, GOS is Bernal-stacked with a band splitting, while on the C-terminated 3C-SiC(111)/Si(110) surface, GOS is turbostratically stacked without a band splitting. This work enables us to precisely control the electronic properties of GOS for forthcoming devices.

Photoemission study of the negative electron affinity surfaces of O/Cs/Si(001)2×1 and O/K/Si(001)2×1

We have found that a negative electron affinity (NEA) surface can be formed by exposing a K‐saturated Si(001)2×1 surface to oxygen gas just as in the formation of a NEA surface of O/Cs/Si(001)2×1. By x‐ray photoelectron diffraction we have found that the arrangement of O and K atoms in the NEA O/K/Si(001)2×1 surface is essentially the same as that in the NEA O/Cs/Si(001)2×1 surface in which a double layer of alkali metal is preserved. Angle resolved ultraviolet photoelectron spectroscopy (ARUPS) has been applied to the two NEA surfaces. The resulting ARUPS spectra can give useful information on bonding at the NEA surfaces if they are combined with theoretical calculations.

Real-time core-level spectroscopy of initial thermal oxide on Si(100)

A Si 2p core-level spectroscopic study has been performed in real time for initial thermal oxide on Si(100) by O2 gas. Time evolutions of the intensities of chemically shifted Si 2p peaks during oxidation have been compared with those of O 2p state, as well as with a simulation from a set of rate equations assuming a simple oxidation model. From the best fits to the data, rate constants relevant to the oxidation of the first and the second silicon layers were successfully derived as a function of the oxidation temperature. In particular, the oxidation of the first layer for temperatures of 540–620 °C was found to occur through direct oxidation of silicon atoms to stoichiometric silicon dioxide, without formation of any suboxides.

Precise control of epitaxy of graphene by microfabricating SiC substrate

Epitaxial graphene (EG) on SiC is promising owing to a capability to produce high-quality film on a wafer scale. One of the remaining issues is microscopic thickness variation of EG near surface steps, which induces variations in its electronic properties and device characteristics. We demonstrate here that the variations of layer thickness and electronic properties are minimized by using microfabricated SiC substrates which spatially confines the epitaxy. This technique will contribute to the realization of highly reliable graphene devices.

Control of epitaxy of graphene by crystallographic orientation of a Si substrate toward device applications

Graphene is a promising material in next-generation devices. Large-scale epitaxial graphene should be grown on Si substrates to transfer the accumulated technologies to integrated devices. We have for this reason developed epitaxy of graphene on Si (GOS) and device operation of the backgate field-effect transistors (FETs) using GOS has been confirmed. It is demonstrated in this paper that the GOS method enables us to tune the structural and electronic properties of graphene in terms of the crystallographic orientation of the Si substrate. Furthermore, it is shown that the uniformity of the GOS process within a sizable area enables us to reliably fabricate topgate FETs using conventional lithography techniques. GOS can be thus the key material in next-generation devices owing to the tunability of the electronic structure by the crystallographic orientation of the Si substrate.

Thin-Film Deposition of Silicon-Incorporated Diamond-Like Carbon by Plasma-Enhanced Chemical Vapor Deposition Using Monomethylsilane as a Silicon Source

We have deposited Si-incorporated diamond-like carbon (DLC) films by radio-frequency plasma-enhanced chemical vapor deposition using methane, argon, and monomethylsilane (MMS; CH3SiH3) as a silicon source, and have investigated the structural and mechanical properties of the films. The deposition rate and Si atomic fraction [Si/(Si+C)] in the DLC films increased with increasing MMS flow ratio. The Si fraction was approximately 13% at a MMS flow ratio [MMS/(MMS+CH4)] of 3%, showing that the deposition using a combination of CH4 and MMS produces films with high Si content compared with those deposited using conventional C and Si sources. The Si fraction was also found to increase with a decrease in Ar flow rate under a constant MMS flow ratio. Many particles composed mainly of Si, whose size was 0.3–1 µm in diameter, were observed on the surface when deposition was carried out at MMS flow ratios of 15 and 30%. Compressive internal stress in the films decreased with the MMS flow ratio and/or with the Ar flow rate. The decrease in internal stress is probably due to the relaxation of a three-dimensional rigid network by the formation of Si–C and Si–H bonds in the films as well as Ar+ ion bombardment.

Electronic Structures of the Single-Domain Si(001)2×1 and 2×8 Surfaces

Wide-terrace single-domain Si(001)2×1 and 2×8 surfaces have been obtained on well-oriented Si(001) wafers. Angle-resolved ultraviolet photoelectron spectra have been measured for the two surfaces at room temperature. For the 2×1 surface, two surface state bands appear that are not expected for the usual asymmetric dimer model. We infer that the c (4×2) and/or p (2×2) structures coexist with the 2×1 periodicity. For the 2×8 surface, its electronic structures are very different from the 2×1 ones. This implies that a significant difference exists between the 2×1 and 2×8 surfaces.

Structural and electrical properties and current–voltage characteristics of nitrogen-doped diamond-like carbon films on Si substrates by plasma-enhanced chemical vapor deposition

We have deposited nitrogen-doped diamond-like carbon (N-DLC) films by plasma-enhanced chemical vapor deposition using CH4, N2, and Ar, and investigated the effects of N doping on the structure and the electrical, mechanical, and optical properties of the N-DLC films. We fabricated undoped DLC/p-type Si and N-DLC/p-type Si heterojunctions and examined the current–voltage characteristics of the heterojunctions. When the N2 flow ratio was increased from 0 to 3.64%, the resistivity markedly decreased from the order of 105 Ωcm to that of 10−2 Ωcm and the internal stress also decreased. The resistivity gradually increased with increasing N2 flow ratio from 3.64 to 13.6%, and then it decreased at a N2 flow ratio of 13.6%. These behaviors can be explained in terms of the clustering of sp2 carbons and the formation of sp3C–N, sp2C=N, sp1C≡N, and C–H n bonds. The rectification ratio of the heterojunction using the N-DLC film prepared at 3.64% was 35.8 at ±0.5 V.