Oxidation of the Si(001) surface: lateral growth and formation of P(b0) centers

Bridging the gap between surface physics and photonics

Use and performance criteria of photonic devices increase in various application areas such as information and communication, lighting, and photovoltaics. In many current and future photonic devices, surfaces of a semiconductor crystal are a weak part causing significant photo-electric losses and malfunctions in applications. These surface challenges, many of which arise from material defects at semiconductor surfaces, include signal attenuation in waveguides, light absorption in light emitting diodes, non-radiative recombination of carriers in solar cells, leakage (dark) current of photodiodes, and light reflection at solar cell interfaces for instance. To reduce harmful surface effects, the optical and electrical passivation of devices has been developed for several decades, especially with the methods of semiconductor technology. Because atomic scale control and knowledge of surface-related phenomena have become relevant to increase the performance of different devices, it might be useful to enhance the bridging of surface physics to photonics. Toward that target, we review some evolving research subjects with open questions and possible solutions, which hopefully provide example connecting points between photonic device passivation and surface physics. One question is related to the properties of the wet chemically cleaned semiconductor surfaces which are typically utilized in device manufacturing processes, but which appear to be different from crystalline surfaces studied in ultrahigh vacuum by physicists. In devices, a defective semiconductor surface often lies at an embedded interface formed by a thin metal or insulator film grown on the semiconductor crystal, which makes the measurements of its atomic and electronic structures difficult. To understand these interface properties, it is essential to combine quantum mechanical simulation methods. This review also covers metal-semiconductor interfaces which are included in most photonic devices to transmit electric carriers to the semiconductor structure. Low-resistive and passivated contacts with an ultrathin tunneling barrier are an emergent solution to control electrical losses in photonic devices.

Strain-driven diffusion process during silicon oxidation investigated by coupling density functional theory and activation relaxation technique

The reaction of oxygen molecules on an oxidized silicon model-substrate is investigated using an efficient potential energy hypersurface exploration that provides a rich picture of the associated energy landscape, energy barriers, and insertion mechanisms. Oxygen molecules are brought in, one by one, onto an oxidized silicon substrate, and accurate pathways for sublayer oxidation are identified through the coupling of density functional theory to the activation relaxation technique nouveau, an open-ended unbiased reaction pathway searching method, allowing full exploration of potential energy surface. We show that strain energy increases with O coverage, driving the kinetics of diffusion at the Si/SiO₂ interface in the interfacial layer and deeper into the bulk: at low coverage, interface reconstruction dominates while at high coverage, oxygen diffusion at the interface or even deeper into the bottom layers is favored. A changing trend in energetics is observed that favors atomic diffusions to occur at high coverage while they appear to be unlikely at low coverage. Upon increasing coverage, strain is accumulated at the interface, allowing the oxygen atom to diffuse as the strain becomes large enough. The observed atomic diffusion at the interface releases the accumulated strain, which is consistent with a layer-by-layer oxidation growth.

Stress inversion from initial tensile to compressive side during ultrathin oxide growth of the Si(100) surface

We report the real-time observation of the stress change during sub-nanometer oxide growth on the Si(100) surface. Oxidation initially induced a rapid buildup of tensile stress up to -1.9 × 10(8) N m(-2) with an oxide thickness of 0.25 nm, followed by gradual compensation by a compressive stress. The compressive stress saturated at 5 × 10(7) N m(-2) for an oxide thickness of 1.2 nm. The analysis, assisted by theoretical study, indicates that the observed initial tensile stress is caused by oxygen bridge-bonding between the Si dimers. Atomistic model calculations considering mutually orthogonal orientations of the Si(100) surface structure reproduce the stress inversion from the tensile to the compressive side.

First-principles molecular-dynamics study of native oxide growth on Si(001)

Through first-principles molecular dynamics we study the low-temperature oxidation of the Si(001) surface from the initial adsorption of an O2 molecule to the formation of a native oxide layer. Peculiar features of the oxidation process are the early, spontaneous formation of Si4+ species, and the enhanced reactivity of the surface while the reactions proceed, until saturation is reached at a coverage of 1.5 ML. The channels for barrierless oxidation are found to be widened in the presence of both boron and phosphorous impurities.

Initial Stage of Si(001) Surface Oxidation from First-Principles Calculations

A comprehensive density-functional theory (DFT) study of the atomic structure, electronic properties, and optical response of the Si(001) surface at the initial stages of oxidation is presented. The most favored adsorption position of a single O atom on top of the (4 x 2)-reconstructed Si(001) surface is found at the back-bond of the "down" Si dimer atom. There is no energy barrier for oxygen insertion into this bond. The ionization energy of the surface reaches a maximum when the oxidation of the second Si monolayer starts. Oxidation leads to an increase of the energy gap between occupied and empty surface states. The calculated reflectance anisotropy spectroscopy (RAS) data in comparison with experiment suggest a considerable amount of surface disorder already after oxidation of the first monolayer.

Reaction of the oxygen molecule at the Si(100)-SiO2 interface during silicon oxidation

Using constrained ab initio molecular dynamics, we investigate the reaction of the O2 molecule at the Si(100)-SiO2 interface during Si oxidation. The reaction proceeds sequentially through the incorporation of the O2 molecule in a Si-Si bond and the dissociation of the resulting network O2 species. The oxidation reaction occurs nearly spontaneously and is exothermic, irrespective of the O2 spin state or of the amount of excess negative charge available at the interface. The reaction evolves through the generation of network coordination defects associated with charge transfers. Our investigation suggests that the Si oxidation process is fully governed by diffusion.