Si Surface Region Research Articles

Atom layer deposited TiO2 electron transport layer for silicon heterojunction solar cells to achieve high performance

Silicon/compound heterojunction (SCH) solar cells based on p-type and n-type c-Si wafers using atom layer deposition-TiO2 and low work function metal as the electron transport layer and rear electrode are fabricated. Efficiencies of 20.6 % and 21.6 % are achieved on p-type and n-type silicon solar cells, respectively. High Voc exceeding 700 mV have been realized on both kinds of Si wafers. Special attention has been paid to the SCH solar cells based on p-type c-Si wafers in which the TiO2 electron transport layer serves as the emitter. Carrier transport mechanisms and junction characteristics of the SCH solar cells are analyzed based on dark J-V measurement. The formation of a strong inversion layer at the Si surface region is determined, which implies the high Voc potential of the SCH solar cells based on p-type Si. An optical loss analysis is performed along with a possible solution to further improve the Jsc. The feasibility of TiO2 applied as an efficient electron transport layer (emitter) in p-type SCH solar cells with high performance has been showed.

Deep UV transparent conductive Si-doped Ga2O3 thin films grown on Al2O3 substrates

β-Ga2O3 is attracting considerable attention for applications in power electronics and deep ultraviolet (DUV) optoelectronics owing to the ultra-wide bandgap of 4.85 eV and amendable n-type conductivity. In this work, we report the achievement of Si-doped β-Ga2O3 (Si:β-Ga2O3) thin films grown on vicinal α-Al2O3 (0001) substrates with high electrical conductivity and DUV transparency of promising potential as transparent electrodes. The use of Al2O3 substrates with miscut angles promotes step-flow growth mode, leading to substantial improvement of crystalline quality and electrical properties of the Si:β-Ga2O3 films. A high conductivity of 37 S·cm−1 and average DUV transparency of 85% have been achieved for 0.5% Si-doped film grown on a 6° miscut substrate. High-resolution x-ray and ultraviolet photoemission spectroscopy were further used to elucidate the surface electronic properties of the grown Si:β-Ga2O3 films. An upward surface band bending was found at the surface region of Si:β-Ga2O3 films. Interestingly, all the Si:β-Ga2O3 films have a very low work function of approximately 3.3 eV, which makes Si:β-Ga2O3 suitable materials for efficient electron injection. The present Si:β-Ga2O3 films with high conductivity, DUV transparency, and low work function would be useful as the DUV transparent electrode to develop advanced DUV optoelectronic devices.

On the Band-Gap Width of NiSi2 Nanocrystals Created in the Surface Region of Si Using Ion Implantation

On the Band-Gap Width of NiSi2 Nanocrystals Created in the Surface Region of Si Using Ion Implantation

Origin of carrier lifetime degradation in floating-zone silicon during a high-temperature process for insulated gate bipolar transistor

We report on the relationship between carrier lifetime degradation of floating-zone silicon (FZ Si) and the guard ring formation process for silicon integrated gate bipolar transistor (Si-IGBT). A clear carrier lifetime degradation was observed through the guard ring oxidation and annealing processes for Si-IGBT and showed interstitial oxygen (Oi) concentration dependence, which was obtained by Fourier transform infrared absorption spectroscopy. Based on the carrier lifetime measurements through the step etching of the FZ Si substrate, it has been suggested that the carrier lifetime degradation is not only caused by the Oi itself near the FZ Si surface region but also the other defects induced by the Oi injection. Diffused interstitial Si atoms kicked-out by the Oi into the FZ Si substrate, which has a longer diffusion length than Oi, can be considered to be the origin of the carrier lifetime degradation.

Optical second harmonic generation from silicon (100) crystals with process tailored surface and embedded silver nanostructures for silicon nonlinear nanophotonics

The development of on-chip nonlinear optical devices in silicon is of great importance to silicon photonics and silicon chip based quantum information processing technologies. With the aim for a viable solution to overcome the lack of second harmonic generation (SHG) in Si, which is fundamentally limited by its centrosymmetric lattice structure, our work investigates SHG behaviors from Si (100) crystals with silver nanostructures formed following deposition of an ultrathin silver film and subsequent annealing. This study is aided by additional techniques, including x-ray photoelectron spectroscopy for measuring surface band bending, secondary electron microscopy for monitoring surface morphology, and Raman scattering for assessing crystal stress. The resultant Ag nanostructures are found to strongly impact the second order nonlinear polarizations in the Si surface regions rather than the bulk. The SHG intensities are increased following the Ag deposition but reduced below the Si control levels after annealing at 600 and 700 °C, which may be due to charge transfer from Ag to SiO2/Si and/or passivation of interfacial defects. Interestingly, annealing at higher temperatures (800 and 900 °C) leads to the formation of Ag nano-shell structures embedded below the SiO2/Si interface, different from the as-deposited and low-temperature annealing cases with Ag nano-spheroid structures appearing on the surface, and concomitantly, the SHG intensities are recovered and even exceed the level for the as-deposited sample in the p-Si case. The enhanced SHG following high-temperature annealing, particularly at 800 °C, is attributed to a redshift of the localized plasmon resonance of these embedded Ag nano-shells relative to the surface-covered nano-spheroids that leads to localized surface plasmon resonance around the fundamental light wavelength. Other factors including electric-field-induced SHG and the effects of stress or oxide thickness can be ruled out. This work suggests a novel approach to enhance second-order optical nonlinearity in Si through the formation of embedded metal nanostructures.

Low-temperature grown single-crystal Si on epi Ge(001)-2 × 1 and its oxidation: electronic structure study via synchrotron radiation photoemission

The capped Si expels completely the Ge dimers on top of the epi Ge(001)-2 × 1 and exhibits a multiphase electronic structure consisting of strained surface atoms bonded with Ge in the film. The deposition of molecular oxygen at room temperature removes a portion of the segregated Ge atoms and oxidizes some of the rest. The surface Si atoms exhibit 1+ to 4+ charge states. Heat eliminates the embedded Ge in the capped Si film and transfers the oxygen bonded with the surface Ge to Si. The oxidation reactions occur primarily on the capped Si surface region without involving the Si/Ge interface.

Structure and electronic properties of nanoscale phases and nanofilms of metal silicides produced by ion implantation in combination with annealing

It is established that ion implantation in combination with annealing makes it possible to produce regularly arranged nanocrystalline phases and continuous films of metal silicides in the surface region of Si. It is shown that silicide nanocrystals and nanofilms crystallize in the cubic lattice. A model of a Si surface with silicide nanocrystals is developed.

Evolution of surface stress during oxygen exposure of clean Si(111), Si(100), and amorphous Si surfaces

The evolutions of the surface stress of Si(111)-7 × 7, Si(100)-2 × 1, and a-Si surfaces upon oxygen exposure at pO2 = 1 × 10−4 Pa and room temperature have been investigated in a comparative manner using a specimen-curvature based technique. To this end, a generally applicable, dedicated set of experiments has been devised and performed to deduce and correct for the surface stress change owing to oxygen reaction(s) at the (poorly-defined) back face of the specimen only. On this basis, it could be demonstrated that exposure of clean Si(111)-7 × 7, Si(100)-2 × 1 and a-Si surfaces to pure oxygen gas results in compressive surface stress changes for all three surfaces due to the incorporation of oxygen into Si backbonds. The measured surface stress change decreases with decreasing atomic packing density at the clean Si surfaces, which complies well with the less-densily packed Si surface regions containing more free volume for the accommodation of adsorbed O atoms.

Annealing of Si surface region modified by plasma immersion implantation of nitrogen

In the present work, the formation of a nano-scale Si surface layer is studied after high-temperature annealing of Si modified by shallow plasma immersion implantation of nitrogen with fluences of 1016 - 1018 cm−2. The implanted profiles of the atomic (N+) and molecular nitrogen (N2+) are modeled by SRIM for different annealing durations taking into account the diffusion process. The presence of Si-O and Si-N bonds is established by Fourier (FTIR) spectral analysis and spectroscopic ellipsometry (SE). The refractive index value measured at 632.8 nm varies between 1.46 and 1.59, corresponding to a low y/x ratio. The models using VIS and IR ellipsometric data reveal formation of nanostructured SiOxNy layer with Si inclusions.

Dependence of Si + and Si 2+ sputtering yields on residual oxygen impurity

The Si + and Si 2+ sputtering yields have been measured by time-of-flight (TOF) method under the bombardment of a pulsed 11 keV Ar 0 beam on a nominally clean Si(1 1 1) surface. Although the residual oxygen impurity is below the detection limit of Auger electron analysis (∼0.01 monolayer), the SiO + ions are clearly observed and its yield can be regarded as a measure of the residual O impurity on the Si surface region. The Si + TOF spectrum changes its shape and the Si + yield increases linearly with the SiO + yield, while the shape of the Si 2+ TOF spectrum and the Si 2+ yield remain almost unchanged. The change in the Si + TOF spectrum is attributed to the increasing SiH + yield, which may be caused by the H 2O adhesion to the Si surface during the TOF measurement. The increase in the adsorbing H 2O may also lead to the enhancement of SiO + and Si + yields; Si + ions may be created through the charge exchange process between O and Si due to difference in their electron affinity. The insensitivity of the Si 2+ yield to the residual O impurity is consistent with the Si 2+ formation by the Auger ionization process of the excited Si + having the 2p hole, which is produced in the collision cascades.

Defects in SiO2/Si Structures Formed by Dry Thermal Oxidation of RF Hydrogen Plasma Cleaned Si

The present paper reports characterization of the defects in ultrathin (∼10 nm) oxides grown by low-temperature (850°C) thermal oxidation of hydrogen plasma hydrogenated (100)Si and (111)Si substrates. Electrically active defects, studied by analyzing the frequency dispersion of the capacitance-voltage (C-V) and conductance-voltage (G-V) characteristics are dominated by distinct defects, related to interface traps with different localized energy levels in the Si bandgap, border traps and bulk Si traps. Precursors of the trapping centers are defects in a thin, less dense Si surface region containing voids, which is formed during hydrogenation and is incorporated into the growing oxide layer. Oxidation-induced stress level, evaluated from ellipsometric and electroreflectance data analysis, is in the order of 108 N/m2.

Study of Si and C adatoms and SiC clusters on the silicon surface by the molecular dynamics method

Individual Si and C adatoms, as well as SiC clusters, on a Si surface are simulated by the molecular dynamics method in the course of investigation of the initial stages of formation of a SiC layer on silicon with the help of molecular beam epitaxy. The potential energy surfaces for Si and C adatoms on the (2 × 1) reconstructed Si(001) surface and on the nonreconstructed Si(111) surface, as well as on the Si(111) surface with a SiC cluster, are calculated and analyzed. The values of migration barriers for adatoms on these surfaces are calculated. The effect of the SiC cluster on deformation of the surface region of Si(111) and on the migration of adatoms is investigated. The deep minima observed on the potential energy surfaces immediately above a cluster and at its boundaries can trap diffusing adatoms. The distributions of stresses and strains in the silicon lattice under a cluster on the surface are studied and described.

Spectroscopic second harmonic generation as a diagnostic tool in silicon materials processing

Spectroscopic second harmonic generation (SHG) has been applied to study thin layers of amorphous silicon in the second harmonic photon energy range of 2.7 - 3.5 eV. The layers were synthesized by hot-wire CVD of hydrogenated amorphous silicon (a-Si:H) and by Ar + ion bombardment of crystalline silicon (c-Si). For a-Si:H a broad feature has been observed in the SHG spectrum. It is discussed that the SHG signal originates from strained Si-Si bonds in the surface or interface region of the a-Si:H film. For H-terminated Si(100) both in spectroscopic and real-time experiments the SHG signal increases by an order of magnitude upon bombardment with 70 eV Ar + ions. We argue that the SHG signal from the amorphized Si layer is generated mainly at the buried interface with c-Si, while an additional contribution seems to originate from the amorphous Si surface region. From the combination of spectroscopic and real-time SHG studies insight into the role of strained bonds in the growth and etching processes of silicon can be gained.

An Investigation of Enhanced Secondary Ion Emission Under Au n+ (n = 1–7) Bombardment

We investigate the mechanism of the nonlinear secondary ion yield enhancement using Au(n)+ (n = 1, 2, 3, 5, 7) primary ions bombarding thin films of Irganox 1010, DL-phenylalanine and polystyrene on Si, Al, and Ag substrates. The largest differences in secondary ion yields are found using Au+, Au2+, and Au3+ primary ion beams. A smaller increase in secondary ion yield is observed using Au5+ and Au7+ primary ions. The yield enhancement is found to be larger on Si than on Al, while the ion yield is smaller using an Au+ beam on Si than on Al. Using Au(n)+ ion structures obtained from Density Functional Theory, we demonstrate that the secondary yield enhancement is not simply due to an increase in energy per area deposited into the surface (energy deposition density). Instead, based on simple mechanical arguments and molecular dynamics results from Medvedeva et al, we suggest a mechanism for nonlinear secondary ion yield enhancement wherein the action of multiple concerted Au impacts leads to efficient energy transfer to substrate atoms in the near surface region and an increase in the number of secondary ions ejected from the surface. Such concerted impacts involve one, two, or three Au atoms, which explains well the large nonlinear yield enhancements observed going from Au+ to Au2+ to Au3+ primary ions. This model is also able to explain the observed substrate effect. For an Au+ ion passing through the more open Si surface, it contacts fewer substrate atoms than in the more dense Al surface. Less energy is deposited in the Si surface region by the Au+ primary ion and the secondary ion yield will be lower for adsorbates on Si than on Al. In the case of Au(n)+ the greater density of Al leads to earlier break-up of the primary ion and a consequent reduction in energy transfer to the near-surface region when compared with Si. This results in higher secondary ion yields and yield enhancements on silicon than aluminum substrates.

Kinetics of the growth of an amorphous layer at the surface of silicon bombarded with fast heavy ions at low temperatures

The accumulation of defects in the surface region of Si bombarded with 0.5-MeV Bi ions at a temperature of −196°C is considered. It is shown that the buildup of disorder in the surface region as the ion dose increases manifests itself in the planar growth of an amorphous layer starting from the Si-SiO2 interface, and this growth sets in after a threshold implantation dose is attained. The results obtained can be described adequately in the context of a model based on the migration of mobile point defects generated by fast ions to the surface, subsequent processes of segregation of these defects at the interphase boundary, and the presence of saturable sinks in the initial samples.

Response function during oxygen sputter profiling for deconvolution of boron spatial distribution

We have studied the spatial distribution of a surface deposited boron layer by secondary ion mass spectrometry (SIMS) using oxygen beams of various incident energies. A boron layer around 0.4 nm thick was deposited by using electron-gun evaporation combined with liquid nitrogen cooling of the target. The SIMS boron profiles can be approximated closely by exponential-like distributions with decay lengths that are linear functions of the incident energy of the oxygen beam. Furthermore, our studies show that corrections to the depth and yield scales due to transient sputtering near the Si surface region must be considered for a plausible deconvolution of the original boron distribution.

Response function during oxygen sputter profiling and its application to deconvolution of ultrashallow B depth profiles in Si

The secondary ion mass spectrometry (SIMS) response function to a B “δ surface layer” has been investigated. Using electron-gun evaporation combined with liquid nitrogen cooling of target, we are able to deposit an ultrathin B layer without detectable island formation. The B spatial distribution obtained from SIMS is exponentially decaying with a decay length approximately a linear function of the incident energy of the oxygen during the SIMS analysis. Deconvolution with the response function has been applied to reconstruct the spatial distribution of ultra-low-energy B implants. A correction to depth and yield scales due to transient sputtering near the Si surface region was also applied. Transient erosion shifts the profile shallower, but beam mixing shifts it deeper. These mutually compensating effects make the adjusted distribution almost the same as original data. The one significant difference is a buried B peak observed near the surface region.

Defect formation inSi(111)7×7surfaces due to 200 eVAr+ion bombardment

Surface x-ray diffraction measurements have been made to study defect formation at the atomic level in the near surface region of Si(111)7×7 surface during low energy (200 eV) Ar + ion bombardment. We have observed the two characteristics of the defects: missing atoms at different layers in the surface region as well as outward strain. We provide calculations that demonstrate how crystal truncation rod (CTR) measurements are sensitive to these modifications in the surface regions. We find that the measurement is sensitive to lattice strain and site occupancy of individual atomic layers near to the surface. We have thus determined occupancy profiles that represent the density of missing atoms with depth after irradiation. Both the strain and the atom deficit are found to increase with dose while the two distributions extend to about the same depth. These results are consistent with simulations of the ion-solid interaction mechanism using SRIM2000 with appropriate choice of surface and bulk displacement energies.

Computer simulation of ion channeling in Si containing structurally relaxed point defects

Atomistic simulation of ion channeling in Si shows that the lattice deformation around point defects, calculated by empirical potentials, gives a significant contribution to the backscattering yield of He ions channeled along major axial or planar directions. This effect, whose amount depends on defect type, beam-alignment condition and model potential used to relax the system, is expected to have consequences on the interpretation of Rutherford backscattering–channeling analysis. A model of structurally relaxed point defects, including split- 〈1 1 0〉 interstitials and vacancies, has been applied to the interpretation of multiaxial channeling analysis in the slightly damaged surface region of Si implanted with MeV ions. Despite the simple microscopic structure of damage assumed in the model, the method leads to a significantly better agreement with experiments then the standard models which treat defects as individually displaced atoms in an unperturbed lattice.

Electron transfer at n-silicon∣methanol interfaces: effects of ferricenium pretreatment and silicon dioxide overlayers

Previous studies have shown that electron transfer rate constants for a bimolecular reaction between conduction band electrons and solution acceptors can be obtained at the n -Si ∣ methanol interface [A.M. Fajardo, N.S. Lewis, Science 274 (1996) 969]. These rate constants (derived from current–potential curves) and band edge positions (derived from interfacial capacitance measurements) are shown to depend on the length of exposure to solutions containing either ferricenium cations or cyclopentadienide anions. Surprisingly, the effects of these pretreatments are reversed when the electrodes are removed from solution. Catalyzed binary reaction sequence chemistry was used to fabricate n -Si electrodes with silicon dioxide overlayers with thicknesses ranging from 13 to 49 Å. Initiation of the growth process by a water plasma resulted in changes to the n -Si surface region producing highly non-ideal electrochemical behavior, which prevented the measurement of electron transfer rate constants at the oxide covered electrodes. Electrodeposition of silver on these electrodes demonstrated that the oxide contained pinholes that accommodated Faradaic current flow.