A review of the properties of hydrogen in crystalline semiconductors is presented. The equilibrium lattice positions of the various states of hydrogen are detailed, together with the reactions of atomic hydrogen with shallow and deep level impurities that passivate their electrical activity. Evidence for several charge states of mobile hydrogen provides a consistent picture for both the temperature dependence of its diffusivity and the chemical reactions with shallow level dopants. The electrical and optical characteristics of hydrogen-related defects in both elemental and compound semiconductors are discussed, along with the surface damage caused by hydrogen bombardment. The bonding configurations of hydrogen on semiconductor surfaces and the prevalence of its incorporation during many benign processing steps are reviewed. We conclude by identifying the most important areas for future effort.

Hydrogen in crystalline semiconductors

Although it has been qualitatively demonstrated that surface roughness can reduce the thermal conductivity of crystalline Si nanowires (SiNWs), the underlying reasons remain unknown and warrant quantitative studies and analysis. In this work, vapor–liquid–solid (VLS) grown SiNWs were controllably roughened and then thoroughly characterized with transmission electron microscopy to obtain detailed surface profiles. Once the roughness information (root-mean-square, σ, correlation length, L, and power spectra) was extracted from the surface profile of a specific SiNW, the thermal conductivity of the same SiNW was measured. The thermal conductivity correlated well with the power spectra of surface roughness, which varies as a power law in the 1–100 nm length scale range. These results suggest a new realm of phonon scattering from rough interfaces, which restricts phonon transport below the Casimir limit. Insights gained from this study can help develop a more concrete theoretical understanding of phonon–surfac...

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Quantifying surface roughness effects on phonon transport in silicon nanowires.

A comprehensive study on the inversion layer mobility improvement of n-channel MOSFETs with nanometer-range ultra-thin (reoxidized) nitrided oxides is presented. The performance improvement is described from both the device physics modeling and device/circuit design points of view by using the various kinds of current drivability and mobility data over wide ranges of gate drive and E/sub eff/. The nitridation- and reoxidation-condition dependencies of the mobility values are extensively reported. A mechanism for improvement is discussed in conjunction with the observed results of Auger electron spectroscopy, capacitance-voltage measurements, hole mobility measurements of p-channel MOSFETs, and transmission electron microscopy. The (reoxidized) nitrided oxides were fabricated fully by rapid thermal processing instead of conventional quartz-tube furnace annealing, whose major drawback is a considerable redistribution of dopant impurities. >

Inversion layer mobility under high normal field in nitrided-oxide MOSFETs

Silicon dominates the semiconductor industry for good reasons. One factor is the stable, easily formed, insulating oxide, which aids high performance and allows practical processing. How well can these virtues survive as new demands are made on integrity, on smallness of feature sizes and other dimensions, and on constraints on processing and manufacturing methods? These demands make it critical to identify, quantify and predict the key controlling growth and defect processes on an atomic scale. The combination of theory and novel experiments (isotope methods, electronic noise, spin resonance, pulsed laser atom probes and other desorption methods, and especially scanning tunnelling or atomic force microscopies) provide tools whose impact on models is just being appreciated. We discuss the current unified model for silicon oxidation, which goes beyond the traditional descriptions of kinetic and ellipsometric data by explicitly addressing the issues raised in isotope experiments. The framework is still the Deal-Grove model, which provides a phenomenology to describe the major regimes of behaviour, and gives a base from which the substantial deviations can be characterized. In this model, growth is limited by diffusion and interfacial reactions operating in series. The deviations from Deal-Grove are most significant for just those first tens of atomic layers of oxide which are critical for the ultra-thin oxide layers now demanded. Several features emerge as important. First is the role of stress and stress relaxation. Second is the nature of the oxide closest to the Si, both its defects and its differences from the amorphous stoichiometric oxide further out, whether in composition, in network topology, or otherwise. Thirdly, we must consider the charge states of both fixed and mobile species. In thin films with very different dielectric constants, image terms can be important; these terms affect interpretation of spectroscopies, the injection of oxidant species and relative defect stabilities. This has added importance now that Pb concentrations have been correlated with interfacial stress. This raises further issues about the perfection of the oxide random network and the incorporation of interstitial species like molecular oxygen. Finally, the roles of contamination, particles, metals, hydrocarbons etc. are important, as is interface roughness. These features depend on pre-gate oxide cleaning and define the Si surface that is to be oxidized which may have an influence on the features listed above.

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Oxidation of silicon: the VLSI gate dielectric

An overview of the growth and characterization of epitaxial insulators on semiconductors is presented. The potential applications include semiconductor-on-insulator structures, three-dimensional and/or high-density integrated circuits, optoelectronic applications such as integrated waveguides and improved gate insulators. The growth and physical characterization of epitaxial fluorides on semiconductors are discussed. Consideration is also given to the epitaxial overgrowth of epitaxial metal layers with CaF2 as well as to the formation of a novel superlattice of Ca particles in epitaxial CaF2.

Growth and characterization of single crystal insulators on silicon

Scattering of inversion layer electrons at the oxide semiconductor interface is a major source of mobility degradation in MOSFETs. We have studied, on the same sample, high resolution TEM cross sections of the Si–SiO2 interface and detailed temperature dependent electronic and magnetic transport measurements. The expeirmental mobility data are compared to a model containing terms for surface roughness and impurity scattering including carrier screening. The statistical properties of the interface fluctuations appear as parameters which are adjusted to fit the transport data. While the TEM pictures indicated an interfacial boundary which deviated by only one atomic layer, an assumed peak‐to‐peak deviation of approximately five atomic layers is required to fit the experimental mobility. Thus, while the overall comparison of the surface parameters suggests that roughness scattering is a plausible mechanism, close agreement between the theory and experiment is not obtained.

Surface roughness scattering at the Si–SiO2 interface

Ohmic contacts to GaAs have been studied using a high resolution TEM, an SEM and a STEM with an energy dispersive x-ray attachment. Two different deposition sequences of the constituent Au-Ge-Ni metals yielded specific contact resistivities that varied by one order of magnitude. Cross-section images of the interface between the GaAs and the metal Au-Ge-Ni layers following alloying showed protrusions at the interface. Contacts with low specific resistivities showed deeper protrusion and also significantly more Ge and Ni in the GaAs.

X-ray Microanalysis and High Resolution Imaging of Ge-Au-Ni Metal Layers on Gallium Arsenide

Silicon wafers have been implanted with H+ (90 keV) to doses of 5.0E15/ cm2 and 2.OE16/cm2. The wafers were annealed in nitrogen at temperatures between 450 and 700°C for times between 10 and 60 min. The electrically active carrier profiles were measured by capacitance voltage and spreading resistance techniques. The residual damage was measured by TEM and RBS. The electrical measurements were essentially the same in both FZ and CZ silicon implying that oxygen is not playing a role in the donor formation which was observed. The donor concentration peaks near the projected range of the hydrogen after annealing at temperatures between 450–500°C. As reported previously 1000 H+ ions generate 1 donor in the implant peak. In addition, the donor concentration between the surface and Rp has increased more than a factor of 10 above the background concentration after a 450°C 10 min anneal. Anneals of 550°C for 30 min or more annihilates essentially all of the donors. The RBS results show small amounts of damage for the 5.0E15/cm2 implant dose but considerable crystal damage with a dose of 2.0E16/cm2, even after a 500°C, 30 min anneal. Cross-sectional TEM analysis of 500°C annealed samples showed a large number of small loops at depths corresonding to the depth of the peak electrical carrier concentration. The donors are directly correlated to the implant damage and resultant defects. SIMS data shows little diffusion for anneals of 500°C or less but after 550°C, 30 min the peak H concentration decreases by approximately a factor of 10.

D. Fathy

Papers

Surface roughness scattering at the Si–SiO2 interface

X-ray Microanalysis and High Resolution Imaging of Ge-Au-Ni Metal Layers on Gallium Arsenide

Characterization of N-Type Layers Formed in Si by Ion Implantation of Hydrogen