Subsurface Boron Research Articles

Effects of Subsurface Boron and Phosphorus on Surface Reactivity of Si(001): Water and Ammonia Adsorption

Using density functional slab calculations with planewave basis set and pseudopotentials, we have examined the effect of subsurface boron and phosphorus on the surface chemical properties of Si(001). We compare the adsorption behaviors of water and ammonia between B- or P-modified and clean Si(001) surfaces. On the B-modified surface, the charge polarization of two B-connected dimers is altered significantly. Our calculation shows that approximately one electron is transferred from the local surface to the subsurface boron, thereby leading both buckled-up and -down atoms of B-connected dimers to be electron-poor (electrophilic). On the other hand, subsurface P brings about no significant change in surface electronic structure. Only about one-tenth of an electron is transferred from the subsurface P to the local surface. As a result, relative to the clean surface, the adsorption properties of water and ammonia change noticeably on the B-modified surface while varying insignificantly on the P-modified surface.

Function of subsurface boron on Si(0 0 1)-2 × 1: water adsorption

Using density functional theory calculations we investigate the function of subsurface boron in determining surface properties of Si(0 0 1). To demonstrate its effect on surface reactivity we compare the behaviors of water adsorption on the clean and B-modified surfaces. We find that subsurface boron brings about a significant change in surface chemical properties by altering charge polarization of Si(0 0 1) locally. As a consequence, water adsorption on the B-modified surface shows a distinctively different feature from that on the clean surface.

Hydrogen chemisorption on the Si(1 1 1)√3 × √3R30 °-Al, -Ga, -B surfaces: An ab initio HF/DFT molecular orbital modelling using atomic clusters

A comprehensive study of atomic hydrogen chemisorption on the Si(1 1 1) √3 × √3R30 ° -Al, -Ga and -B cluster modelled surfaces is presented using Hartree-Fock/density functional theory methods. Extrapolation of the results to the extended (1 1 1) silicon surface is also discussed. It is found that the chemisorption of hydrogen on the Al and Ga terminated surfaces induces a transition from the √3 × √3 structure to a local 1 × 1: H-like reconstruction with a stable SiAl (or SiGa) sites. The subsurface boron induced √3 × √3 reconstruction is also lifted by hydrogen chemisorption but, in this case, boron adatoms are likely to be segregated on the surface, predominantly as BH or/and BH2.

Evolution of subsurface hydrogen from boron-doped Si(100)

The reactions of atomic hydrogen with boron-doped Si(100) were studied using temperature programmed desorption (TPD). In addition to adsorbing at surface sites, hydrogen penetrates into boron-doped Si(100) samples and gets trapped by forming subsurface boron–hydrogen complexes. H2-TPD spectra, taken after exposure to atomic hydrogen, showed, in addition to the well known dihydride (680 K) and monohydride (795 K) desorption features, two peaks at 600 and 630 K due to decomposition of subsurface boron–hydrogen complexes. Increasing total hydrogen uptake with increasing dosing temperature (1.7 ML at 300 K, 4.2 ML at 500 K), suggests an activation barrier for subsurface hydrogen uptake. A quantitative correlation between boron concentration and subsurface hydrogen uptake is shown.

Changes in Subsurface Drainage Water Salinity and Boron Concentrations

The electrical conductivity (salinity) and boron concentrations of subsurface agricultural drainage effluent are analyzed. The effluent is from drainage systems ranging from 0 to 40 years of age, located on irrigated saline soils in the San Joaquin Valley, California. Data collected from 1962 to 1990 verifies that the equation developed between 1959 and 1962 is valid for use in predicting the quality of effluent that will be discharged from artificial subsurface drainage systems installed in irrigated agricultural soils of the San Joaquin Valley, California. The work is useful in that it demonstrates the predictability of the quality of subsurface drainage effluent from drains installed in irrigated land. Such relationships are important in arid areas of the world where reclamation of irrigated land is required, particularly when there is growing concern about the sustainability of irrigated agriculture on saline soils and the environmental impact of saline drainage effluent.

Surface chemistry of silicon-the behaviour of dangling bonds

The role of surface dangling bonds in controlling the surface chemistry of Si(100) and Si(111) has been investigated using surface science methods. The adsorption and decomposition of acetylene on Si(100) has been shown to occur by means of a mobile precursor mechanism, leading to a di- sigma chemisorbed ethylenic species occupying Si dimer sites on Si(100). This species decomposes at elevated temperatures to produce adsorbed carbon and hydrogen (800 K) and then silicon carbide (1000 K). The activity of Si(111) for the adsorption of species such as ammonia and atomic hydrogen can be significantly reduced by doping of the surface region with boron. It has been shown that subsurface boron significantly changes the structure of Si(111)-(7*7) to a square root 3* square root 3 R30 degrees structure. In addition, the local doping of the surface reduces the chemical activity of the dangling bonds so that neither the dissociation of NH3 nor the adsorption of atomic hydrogen will occur.

Inhibition of atomic hydrogen etching of Si(111) by boron doping

Subsurface boron doping reconstructs the Si(111) surface and alters the electronic character of the surface Si atoms. The interaction of atomic hydrogen with the boron-modified Si(111)-(√3×√3)-R30° surface was studied using temperature programmed desorption (TPD), high-resolution electron energy-loss spectroscopy (HREELS), and low-energy electron diffraction. In comparison to the Si(111)-(7×7) surface, we observe a significantly reduced hydrogen saturation coverage, measured by TPD and HREELS, and the absence of silane production. The ordered (1/3 ML) subsurface boron atoms passivate the surface Si atoms and reduce their reactivity with atomic hydrogen. This leads to a surface condition causing suppression of silicon etching by atomic hydrogen, compared to the unmodified Si(111)-(7×7) surface.