Low-dimensional electronic states at silicon surfaces

Self-assembled atomic chains can be triggered at stepped Si(111) surfaces by adding sub-monolayer amounts of metals, such as gold, silver, platinum, alkali metals, alkaline earths, and rare earths. A common feature of all these structures is the honeycomb chain, a graphitic strip of Si atoms at the step edge that is lattice matched in the direction parallel to the edge but completely mismatched perpendicular to it. This honeycomb chain drives one-dimensional surface reconstructions even on the flat Si(111) surface, breaking its three-fold symmetry. Particularly interesting are metallic chain structures, such as those induced by gold. The Au atoms are locked rigidly to the Si substrate but the electrons near the Fermi level completely decouple from the substrate because they lie in the band gap of silicon. The electronic structure of one-dimensional electrons is predicted to be qualitatively different from that of higher dimensions, since electrons cannot avoid each other when moving along the same line. The single-electron picture has to be abandoned, making way for collective excitations, such as spinons and holons, where the spins and charges of electrons become separated. Although such excitations have yet to be confirmed definitively, the band structure seen in angle-resoled photoemission exhibits a variety of unusual features, such as a fractional electron count and a doublet of nearly half-filled bands. Chains of tunable spins can be created with rare earths. The dimensionality can be controlled by adjusting the step spacing with intra- and inter-chain coupling ratios from 10:1 to >70:1. Thus, metal-induced chain structures on stepped silicon provide a versatile class of low-dimensional materials for approaching the one-dimensional limit and exploring the exotic electronic states predicted for one dimension. PACS 73.20.At; 71.10.Pm; 79.60.Jv; 81.07.Vb; 73.21.Hb     

Surface-state conduction through dangling-bond states

The surface-state conduction through dangling-bond states on the (1 1 1) ideal surfaces of group-IV semiconductors is studied theoretically. Localization strength of wave functions is an important factor determining the surface-state conduction. Approximate expressions for the decay constants of the dangling-bond states are presented using a tight-binding model. The origin of the difference in the decay constants of diamond, Si, and Ge is clarified in terms of linear combination of the states at the top of valence bands and the bottom of conduction bands. The ballistic conductance from a probe to the surfaces is calculated using the Landauer formalism. The surface-state conductance is much larger than the bulk-state one, which is due to small band dispersions of the dangling-bond states.     

Electronic properties of cleaved CdTe(110) surfaces

Photoemission yield spectroscopy measurements were performed on a set of n- and p-doped CdTe single crystals. The surfaces were obtained by cleavage in ultrahigh vacuum and characterized by low energy electron diffraction and Auger electron spectroscopy. On clean and properly cleaved surfaces, no band bending was found, neither on n- nor on p-type samples, showing the absence of intrinsic surface states in the gap. The ionization energy is found at 5.80±0.05 eV. Oxygen adsorption removes defect-induced surface states on the valence band side of the gap and develops a band bending on n-type samples which indicates the presence of acceptor surface states in the gap down to 0.70 eV below the conduction band edge. The ionization energy remains constant.     

Surface states at the clean surfaces of cleaved Si(111) and GaAs(110)

A combination of ellipsometric data on the electronic transitions from occupied to unoccupied surface states and published photoemission data on the energy distribution of the occupied surface states has been used to construct models of the surface states densities at the cleaved Si (111) and GaAs (110) surfaces.     

Non-equilibrium surface state properties at clean cleaved silicon surface as measured by surface photovoltage

Surface photovoltage transients were measured at clean cleaved silicon (111) faces in ultrahigh vacuum. The temperature and doping of the samples, the intensity of the stimulating light pulses (energy less than band gap), and the surface coverage (clean and adsorbed water vapor) were varied systematically. The results yield information on the charge transfer at the surface and on surface recombination. The calculation of the surface photovoltage (using only the generation rates into and out of the surface states and data of thermal equilibrium) shows, that only one bulk band (conduction band for n-doped samples and valence band for p-doped samples) controls completely signal height and its relaxation via charge transfer to the surface states. The determined surface state parameters are: relaxation time constants, capture cross-sections for photons and transition probabilities. On the basis of the model all decay curves can be reproduced quantitatively.     

Absence of Fermi-level pinning at cleaved nonpolar InN surfaces

Prior experimental work had found that the Fermi level at InN growth surfaces is pinned well above the conduction band edge, leading to strong surface band bending and electron accumulation. Using cross-sectional scanning photoelectron microscopy and spectroscopy, we show definitive evidence of unpinned Fermi level for in situ cleaved a-plane InN surfaces. To confirm the presence or absence of band bending, the surface Fermi level relative to the valence band edge was precisely measured by using both the Fermi edge of Au reference sample and the core level of ultrathin Au overlayer. It is confirmed that flat surface bands only occur at cleaved nonpolar surfaces, consistent with the recent theoretical predictions.     

Electronic and structural properties of steps on cleaved semiconductor surfaces

The work function of clean, cleaved p-Si(111) surfaces was measured in dependence of the density of cleavage steps. The contact potential difference CPD was observed first to increase then to decrease linearly with increasing step density. This dependence is caused by an increase in band bending, which saturates at a density of approximately 1 × 10⁶ steps per cm, and a decrease of the surface dipole which is proportional to the density of steps. Since the 2 × 1 reconstruction of the cleaved Si surface is ionic and buckled it increases the surface dipole moment compared with the 1 × 1 structure. Thus, a decrease of the surface dipole may be caused by removal of the 2 × 1 reconstruction on part of the step terraces. The experimental data give a width of about 28 Å for the unreconstructed stripe along the step edge.     

Optical properties of porous silicon

We have measured the absorption spectra and the dispersion of refractive index for porous silicon samples with different porosities in the energy range 1.5–3.5 eV at room temperatures. The experimental data are compared with the dependences calculated by using Bruggeman’s theory for the dielectric constant of a multicomponent system composed of crystal silicon, SiO₂, amorphous silicon, and voids (pores). The best agreement between the experimental and theoretical dependences is achieved for a significant percentage of SiO₂ in the porous silicon samples.     

Theoretical search for atomic-like states of silicon at the surface of thermally grown SiO2 films on silicon surfaces

The existence of atomic-silicon cryptates in siloxanic networks has been studied theoretically via standard density functional theory calculations. Mimicking with model molecules the candidate sites to host atomic silicon, we found that stationary states are impossible in highly reticulated siloxanic networks; metastable adducts can only be formed at their external surfaces or in regions where the siloxanic network is subjected to weak steric constraints. This analysis suggests that the atomic silicon injected into the oxide during thermal oxidation of silicon by O₂ may be trapped as a metastable adduct at the oxide surface. PACS 31.10.+z; 31.15.Ar; 31.70.-f; 31.70.Dk; 31.90.+s     

Strain relaxation at cleaved surfaces studied by atomic force microscopy

Atomic force microscopy (AFM) in air is used to study the (110) cleaved surface of strained (100) In_xGa_1-xAs/ InP heterostructures for different compositions and thicknesses of the ternary compound layers. We find that the elastic strain relaxation induces a surface undulation of a few ? amplitude, even for very small misfits, provided the layers are thick enough. Using finite-element calculations of the strain relaxation near the cleaved edge, we reproduce quantitatively the AFM observations for compressive- as well as for tensile-strained layers with an accuracy better than 0.1 nm. This demonstrates the ability of AFM to quantify strain distributions by making use of surface profile measurements. Received: 9 November 1998 / Accepted: 11 March 1999 / Published online: 7 July 1999     

Surface photovoltage spectroscopy of electronic surface states on cleaved germanium(111) surfaces

Surface photovoltage (SPV) measurements on UHV cleaved Ge(111) surfaces at 100 K are reported for photon energies 0.4 < ?ω < 1 eV. The SPV spectra are sensitive to surface treatment. Upon annealing to temperatures above 200°C, which is accompanied by a reconstruction change from the (2 × 1) to an (8) superstructure, the SPV spectrum shows 2 shoulders below band gap energy with threshold energies near 0.4 and 0.45 eV. These structures are interpreted in terms of electronic transitions from the valence band into empty surface state levels which are related to the (8) superstructure. Adsorbed oxygen and water vapor both cause new similar transitions from the valence band into empty surface states at 0.08 eV below the bottom of the conduction band.     

Optical properties of silicon nanocrystal LEDs

In this work, we describe how to fabricate good quality 3 nm nc-Si with low size distribution in thermal SiO₂ oxides. Photoluminescence, excited photoluminescence, and photocurrent measurements are discussed on the basis of theoretical calculations of the quantified levels in nc-Si. The impact of shape and size in quantum dots on transition energies has been highlighted, thanks to 2D symmetrical self-consistent Poisson–Schrödinger simulations. Both direct and indirect gaps in silicon have been considered in order to carry out a better comparison between simulations and optical measurements. A good agreement is found between simulations and experimental data for the indirect gap of 3 nm dots which show a threshold energy around 2 eV. However, the optical recombinations seems to be related to lower energy states probably due to interfacial radiative defects around 1.58 eV. On the basis of highly luminescent nc-Si, we have fabricated an optimized light emitting device (LED) with a calculated design in order to favour both electron and hole injection. Stable red electroluminescence has been obtained at room temperature and the I–V measurements confirm that the current is related to a pure tunnelling process. A modelling of I–V curves confirms a Hopping mechanism with an average trap distance between 1.4 and 1.9 nm. The Fowler–Nordheim process is not observed during light emission for electric fields below 5 MV/cm. Finally, we have not hot carrier injection and thus it seems possible to develop Si-based LEDs with a good reliability.     

Optical rectification at metal surfaces

The emission of freely propagating terahertz (THz) radiation coming from optical rectification at metallic surfaces has been detected and characterized for the first time to the authors' knowledge. The observed THz transients are induced through nonlinear electronic processes at gold and silver surfaces on intense pulsed optical photoexcitation and exhibit a peak electric field of as much as 200 V/cm. This finding opens a qualitatively new way to investigate nonlinear phenomena at metal surfaces and also can be exploited for the development of new THz emitters.