J. W. Keister

STRUCTURE OF ULTRATHIN SIO2/SI(111) INTERFACES STUDIED BY PHOTOELECTRON SPECTROSCOPY

Device-grade ultrathin (9–22 A) films of silicon dioxide, prepared from crystalline silicon by remote-plasma oxidation, are studied by soft x-ray photoelectron spectroscopy (SXPS). The 2p core-level spectra for silicon show evidence of five distinct states of Si, attributable to the five oxidation states of silicon between Si0 (the Si substrate) and Si4+ (the thin SiO2 film). The relative binding energy shifts for peaks Si1+ through Si4+ (with respect to Si0) are in agreement with earlier work. The relatively weaker signals found for the three intermediate states (I1, I2, and I3) are attributed to silicon atoms at the abrupt interface between the thin SiO2 film and substrate. Estimates of the interface state density from these interface signals agree with the values reported earlier of ∼2 monolayers (ML). The position and intensity of the five peaks are measured as a function of post-growth annealing temperature, crystal orientation, and exposure to He/N2 plasma. We find that annealing produces more abrup...

Band offsets for ultrathin SiO2 and Si3N4 films on Si(111) and Si(100) from photoemission spectroscopy

High resolution soft x-ray photoelectron spectroscopy with synchrotron radiation is used to study the interfaces of SiO2/Si(111), SiO2/Si(100), Si(111)/Si3N4, and SiO2/Si3N4 for device-quality ultrathin gate oxides and nitrides. The thin oxides and nitrides were grown by remote plasma deposition at a temperature of 300 °C. Aftergrowth samples were further processed by rapid thermal annealing for 30 s at various temperatures from 700 to 950 °C. The Si(111)/Si3N4 samples were air exposed and formed a thin ∼6 A SiO2 layer with a Si(2p) core-level shift of 3.9 eV, thus allowing us to study both the Si(111)/Si3N4 and SiO2/Si3N4 interfaces with a single type of sample. We obtain band offsets of 4.54±0.06 eV for SiO2/Si(111) and 4.35±0.06 eV for SiO2/Si(100) with film thicknesses in the range 8–12 A. The Si(111)/Si3N4 nitrides show 1.78±0.09 eV valence-band offset for 15–21 A films. This value agrees using the additivity relationship with our independent photoemission measurements of the nitride–oxide valence-ba...

The effects of interfacial sub-oxide transition regions and monolayer level nitridation on tunneling currents in silicon devices

Direct tunneling (D-T) in Si metal-oxide-semiconductor (MOS) devices having 1.8 to 3 nm thick gate oxides is reduced approximately tenfold by monolayer Si-dielectric interface nitridation with respect to devices with nonnitrided interfaces. The reduction is independent of gate oxide-equivalent thickness, and gate or substrate injection, and extends into the Fowler-Nordheim tunneling (F-N-T) regime for thicker oxides as well. A barrier layer model, including sub-oxide transition regions, has been developed for the interface electronic structure for tunneling calculations using X-ray photoelectron spectroscopy data. These calculations provide a quantitative explanation for the observed tunneling current reductions.

Intrinsic limitations on device performance and reliability from bond-constraint induced transition regions at interfaces of stacked dielectrics

The substitution of deposited alternative gate dielectrics for thermally grown SiO2 in aggressively scaled complementary metal–oxide–semiconductor devices requires separate and independent processing steps for (i) the oxidation of the Si substrate to form the Si-dielectric interface and (ii) the deposition of thin film dielectric. Ultrathin plasma-oxidized Si–SiO2 interface layers which contribute approximately 0.3–0.4 nm to the overall electrical oxide thickness have been integrated into devices with Si nitride, Si oxynitride, and Ta2O5 alternative dielectrics. This article proposes an analogy between (i) microscopically inhomogeneous bulk glass alloys such as GeSex with 1<x<2, and (ii) interfaces included in these composite gate dielectric-semiconductor structures including, for examples, the Si–SiO2 and internal dielectric SiO2–Si3N4 interfaces. Scaling relationships for bond defect states applied initially to microscopically inhomogeneous glasses and thin films are applied here to interfaces in stacke...

Separate and independent reductions in direct tunneling in oxide/nitride stacks with monolayer interface nitridation associated with the (i) interface nitridation and (ii) increased physical thickness

Direct tunneling limits aggressive scaling of thermally grown oxides to about 1.6 nm, a thickness at which the tunneling current density Jg at 1 V is ∼1 A/cm2. This article demonstrates that stacked gate dielectrics prepared by remote plasma processing and including (i) ultrathin nitrided SiO2 interfacial layers and (ii) either silicon nitride or oxynitride bulk dielectrics can extend the equivalent oxide thickness to 1.1–1.0 nm before Jg exceeds 1 A/cm2. Significant reductions in direct tunneling are derived from (i) interface nitridation at the monolayer level and (ii) the increased physical thickness of the nitride or oxynitride alloy layers. The “portability” of the interface contribution is demonstrated by combining the nitrided SiO2 interface layers with transition-metal oxides, e.g., Ta2O5, in stacked gate dielectric structures and obtaining essentially the same reductions in tunneling current on n- and p-type substrates with respect to non-nitrided plasma-grown interface layers.

Ultrathin metals films on W(221): Structure, electronic properties and reactivity

Nanoscale pyramidal facets with (211) faces are formed when W(111) surface is covered by monolayer film of certain metals (including Pt, Pd and Au) and annealed to T ≥ 750 K. In the present work, we focus on the structure, electronic properties and reactivity of planar W(211) covered by ultrathin films of platinum and palladium. The measurements include soft X-ray photoelectron spectroscopy using synchrotron radiation, Auger electron spectroscopy, low energy electron diffraction (LEED) and thermal desorption spectroscopy. The metal film growth and evolution during annealing has been investigated for coverages ranging from 0 to 8 monolayers. The films grow initially in a layer-by-layer mode at 300 K. LEED, Auger, and Surface Core Level Shift (SCLS) measurements reveal that for coverages of one monolayer, the films are stable up to temperatures at which desorption occurs. In contrast, at higher coverages, SCLS data indicate that surface alloys are formed upon annealing films of Pt and Pd; surface alloy formation is not seen for Au overlayers. These findings are discussed in terms of structural and electronic properties of these bimetallic systems. Relevance to catalytic properties for acetylene cyclization over PdW(211) is also discussed.

Photoemission spectroscopy of platinum overlayers on silicon dioxide films

Soft x-ray photoelectron spectroscopy (SXPS) has been used to study ultrathin Pt films on silicon dioxide as model supported-catalyst materials. Using monochromatic synchrotron radiation. Pt 4f and Si 2p core level photoelectron peaks were measured as a function of platinum coverage in the range 0–10 ML. The bulk silicon and silicon dioxide film Si 2p peaks each show a binding energy decrease within the first ML of dosing. However, the effect is stronger for the silicon dioxide Si 2p peak, indicating an increased screening of the 2p electrons by the metal overlayer. We also observe a monotonic increase of the work function of the ultrathin film Pt/SiO2 system with coverage from 4.52 initially to 5.58 eV at ∼10 ML. The Pt 4f7/2 core level binding energy decreased from ∼72.2 to ∼70.9 eV between 0 and ∼10 ML coverage. This binding energy shift at low dose (⩽1 ML), the coverage dependence of the Pt line shape and intensity, and the large saturation coverage for the work function are each consistent with two-d...

The effects of interfacial suboxide transition regions on direct tunneling in oxide and stacked oxide-nitride gate dielectrics

Abstract This paper builds on previous work that has demonstrated the effects of interfacial suboxide transition regions at SiSiO 2 interfaces on tunneling oscillations in the Fowler-Nordheim regime. This paper extends thes effects to the direct tunneling regime and focuses on differences in interfacial transition regions between SiSiO 2 interfaces with, and without monolayer level interface nitridation. Tunneling currents in devices with the same oxide-equivalent thickness are reduced by monolayer level interfacial nitrogen with respect to devices without interface nitridation for i) substrate and gate injection and ii) in both the direct and Fowler-Nordheim tunneling regimes.

Comparison of ultrathin SiO2∕Si(100) and SiO2∕Si(111) interfaces from soft x-ray photoelectron spectroscopy

The limitations of soft x-ray photoelectron spectroscopy (SXPS) for determining structural information of the SiO2∕Si interface for device-grade ultrathin (∼6–22A) films of SiO2 prepared from crystalline silicon by remote plasma assisted oxidation are explored. The main focus of this article is the limitation of data analysis and sensitivity to structural parameters. In particular, annealing data shows a significant decrease in the integrated density of suboxide bonding arrangements as determined from analysis of SXPS data. These decreases and changes are interpreted as evidence for reorganization of specific interface bonding arrangements due to the annealing process. Moreover, these results suggest that sample preparation and processing history are both critical for defining the nature of the SiO2∕Si interface, and therefore its electrical properties. Quantitative estimates of the interface state densities are derived from SXPS data revealing ∼2 monolayers (ML) of suboxide as prepared and ∼1.5 ML of sub...