Semiconductor Atoms Research Articles

Incorporation of dopant atoms and defects in semiconductors: a microscopic view

We demonstrate that scanning tunneling microscopy is an ideal tool for the investigation of individual bulk defects and dopant atoms in semiconductors. This technique allows not only an atomically resolved imaging of individual defects and dopant atoms, but rather a detailed determination of their nanoscale electronic as well as structural properties and their concentrations. The capacities of scanning tunneling microscopy for defect investigations will be presented with help of selected examples.

Photoreflectance study of changes in the QW profile of 1.55-micrometer laser structure induced by SiO 2 cap layers

Quantum well (QW) band gap modification has been carried out in 1.55 μm InGaAsP-based laser structures using a silicon dioxide cap (SiO 2) and rapid thermal annealing (RTA). Migration of the semiconductor atoms across the QW interface changes the profile of QW from a square well to a rounded well. The modification of the profile causes the shift of energy levels in the QW. In consequence, a blue shift of the emission peak is observed in photoluminescence (PL). In this paper, the contactless electromodulation technique called photoreflectance (PR), which is an absorption-like experiment, has been used. This method allows the detection of excited states transitions, as well as the fundamental one, in low-dimensional structures. In PR experiment, different energy shifts are observed for different confined state transitions reflecting the character of the changes in the well profile. The role of the microwave power of ECR-PECVD process is analysed and its impact on the blue shift of both ground and excited states is discussed in this paper. It is shown that the value of the blue shift can be controlled by the microwave power.

Imaging defects and dopants

The electrical properties of semiconductor materials are, to a large degree, governed by defects and dopant atoms incorporated during growth and production processes. Unfortunately, direct experimental access to such bulk point defects is very difficult. In recent years, however, scanning tunneling microscopy has developed into an ideal tool for the investigation of individual bulk defects and dopant atoms in semiconductors. This technique allows not only atomically resolved imaging of individual defects and dopant atoms, but also detailed determination of nanoscale electronic as well as structural properties and their concentrations. This provides new access to the physical mechanisms involved in the incorporation and interaction of defects and dopant atoms in semiconductors, leading eventually to an atomic level understanding of materials’ properties.With the invention of the transistor, a revolution in the development of semiconductor-based electronic devices began. However, even in the very early stages, the importance of defects and dopant atoms became obvious. In fact, if one incorporates the right defects and dopant atoms into semiconductor materials, one can tune their electrical properties such that optimal device characteristics are achieved. Unfortunately, counteractive defects are often also formed unintentionally during semiconductor<@> <@$p>processing, leading to unfavorable electronic properties. Considerable research efforts have, therefore, focused on understanding the nanoscale physics that governs the formation of point defects, the incorporation behavior of impurities, and their respective electronic properties.

Aberration-Corrected Scanning Transmission Electron Microscopy: The Potential for Nano- and Interface Science

Abstract The sub-Angstrom probe of an aberration-corrected scanning transmission electron microscope will enable imaging and analysis of nanostructures and interfaces with unprecedented resolution and sensitivity. In conjunction with first-principles theory, new insights are anticipated into the atomistic processes of growth and the subtle link between structure and functionality. We present initial results from the aberration-corrected microscopes at Oak Ridge National Laboratory that indicate the kinds of studies that will become feasible in the near future. Examples include (1) the three-dimensional location and identification of individual dopant and impurity atoms in semiconductor interfaces, and their effect on local electronic structure; (2) the accurate reconstruction of surface atomic and electronic structure on nanocrystals, and the effect on optical properties; and (3) the ability to distinguish which configurations of catalyst atoms are active, and why.

Intermetallic bonds and midgap interface states at epitaxial Al/GaAs(001) junctions

Using first-principles pseudopotential calculations, we have investigated the nature of the electronic states with energies within the semiconductor band gap of abrupt, defect-free As-terminated Al/GaAs(001) junctions. While bondinglike/antibondinglike semiconductor evanescent states occur near the valenceband/conduction-band edges, the semiconductor midgap region is characterized by a different type of electronic states, not accounted for by commonly accepted models. These states, which correspond to intermetallic bonds between the outermost Ga cations of the semiconductor and Al atoms of the metal occur near the Fermi energy. They are localized at the interface and are located around the $\mathbf{J}$ point of the Brillouin zone. These new interface states derive from an interaction between localized states of the Al(001) surface and bulk GaAs conduction-band states, mediated by localized states of the unreconstructed As-terminated GaAs(001) surface.

Dopant mapping for the nanotechnology age.

The push for smaller microelectronics poses many challenges, such as locating dopant atoms in semiconductors with ever-increasing precision. The ideal technique must be able to detect single dopants with atomic resolution, and identify their electronic state. Neither is an easy task.

Lattice relaxation around impurity atoms in semiconductors – arsenic in silicon – a comparison between experiment and theory

We have measured the lattice relaxation around As in Si at a homogeneous As concentration of 4×10 18 cm −3 by EXAFS spectroscopy. From the absorption spectra, distances up to the 4th shell could be extracted. A sizeable misfit due to an increased distance is only observed for the 1st shell. Complementing our experimental work we have performed ab initio calculations based on the density functional theory with the WIEN97 package which uses the linearised augmented plane wave method and with the FHI96md program which uses first-principles pseudo-potentials and a plane wave basis set to investigate the size dependence of the super-cells constructed around one substitutional As atom. The calculations yielded good agreement with our EXAFS experiment so that the determined relaxations can be used as a solid basis for further interpretations of derived parameters such as hyperfine interaction parameters in defect complexes.

Investigation of the Non-Square InGaAsP/InP Quantum Wells in the Electric Field by Photoreflectance

Non-square quantum wells in electric field have been investigated by photoreflectance and photoluminescence spectroscopies. The structures have been obtained by a post-growth modification (rapid thermal annealing) of standard 1.55 μm InGaAsP-based laser structures that were grown by gas source molecular beam epitaxy. During rapid thermal annealing a migration of semiconductor atoms across quantum well interfaces changes the quantum well profile from a square well to a rounded well. The modification of the profile changes energy levels in the quantum well and in consequence a blue shift of the quantum well emission peak is observed in photoluminescence. In this paper the blue shift of the ground state transition of post-growth modified quantum well structures has been investigated by both photoluminescence and photoreflectance techniques. Also a blue shift of excited state transitions has been observed in photoreflectance spectra. Generally, a stronger blue shift for the ground state transition than for excited state transitions has been observed. Additionally, oscillator strengths for all quantum well transitions have been determined from photoreflectance spectra. It has been found that the oscillator strength is constant for all quantum well transitions despite of modification of the quantum well profile.

GaAs surface modifications under light irradiation in vacuum

The interaction between metal atoms and near-surface semiconductor atoms from GaAs results in metal and Ga and As atoms intermixing, or solid solutions and new compounds formed during the metal deposition on the GaAs surface. Near-surface GaAs layer changes induced by non-loaded evaporator light emission in situ in vacuum were studied in this work. The GaAs near-surface layer state changes were followed by measurements of resistivity as a function of light intensity and duration of exposure, and of SAW amplitude and phase velocity variations. These measurements have been correlated with the structure and composition changes of the semiconductor native oxide layer. The GaAs surface changes were additionally studied with HEED and microscopy methods. They can be explained by diffusion of Ga and As atoms from the crystalline GaAs into the oxide layer, and back, caused by the plastic deformation, as well as the interaction between the Ga and the As atoms and residual gas in the vacuum chamber resulting in changing the thickness and composition of the oxide layer.

Intermixing of InGaAsP/InGaAsP quantum-well structures using dielectric films

The enhancement of quantum-well intermixing by SiOxNy and commercial spin-on glass dielectric films deposited on InP-based practical laser structures with all-quaternary active regions has been studied. Migration of semiconductor atoms into the dielectric films was observed using secondary ion mass spectrometry, and this migration behaviour has been correlated with the amount of band-gap change. Based on the results presented, it is suggested that the enhanced intermixing effect is due to the injection of group V interstitials into the underlying laser structure. These are formed at the dielectric/semiconductor interface as Ga and In atoms migrate into the dielectric layer. However, some interfacial process appears to influence the release of the interstitials from the interface region depending on the composition of the SiOxNy film.

Mössbauer-effect study of off-center atoms in IV-VI semiconductors

PbTeSnSe and GeSnTe compound semiconductors were studied by 119Sn Mossbauer spectroscopy in the temperature range from 5 to 240 K. Analysis of temperature dependences of the quadrupole splitting of Mossbauer spectra measured during a cooling-heating cycle confirmed the presence of off-center atoms in these materials.

Influence of Heat on Impurity States in an Artificial Semiconductor Atom

semiconductor as the potential barrier material of the artificial atom. Different heat dependent effective masses and different heat dependent dielectric constants are used for the two semiconductors constitute the ASA. The lowest energy and the binding energy of the ground state are calculated. The calculations have shown that the lowest electron energy decreases by increasing the ASA radius. For very large radii the lowest energies corresponding to different aluminum contents approach a certain value which is the bulk limit. At a constant ASA radius the lowest energy increases as a function of temperature. A pronounced deviation is obtained when the calculations of the present work are compared with those of Montenegro and Merchancano [18] in which the effects of heat and mismatches for both effective masses and dielectric constants were neglected. It is found that decreasing the temperature and shrinking the radius of the ASA lead to more binding of the donor electron. Increasing the aluminum concentration also enhances the donor electron binding energy. Therefore the electron ground state energy and its associated binding energy are both mainly functions of temperature, ASA radius, and aluminum content.

Direct coulomb and exchange interaction in artificial atoms

We determine contributions from the direct Coulomb and exchange interactions to the total interaction in artificial semiconductor atoms. We tune the relative strengths of the two interactions and measure them as a function of the number of confined electrons. The electrons tend to have parallel spins when they occupy nearly degenerate single-particle states. We use a magnetic field to adjust the single-particle-state degeneracy, and find that the spin configurations in an arbitrary magnetic field are well explained in terms of two-electron singlet and triplet states.

Advances in electron emission channeling measurements in semiconductors

This paper reports on recent advances in the application of electron emission channeling to studies of the lattice location of radioactive probe atoms in semiconductors. The introduction of position-sensitive electron detection has resulted in an improvement in the experimental efficiency of this method by two orders of magnitude. Electron emission channeling now is able to carry out detailed lattice location studies, and is especially interesting for cases where conventional ion beam lattice location techniques cannot be applied due to a lack of sensitivity. Characteristic features of electron emission channeling with position-sensitive detection are discussed and illustrated by results on the lattice location of Er in Si and GaAs, and on the lattice sites and stability of implanted transition metals in Si.

Electronic and Positronic Distribution in the Ternary Alloy Hg1-xCdxTe

The electron and positron charge densities are calculated as a function of position in the unit cell for the Hg1–xCdxTe alloy system using the empirical pseudopotential method combined with the virtual-crystal approximation. The results show that the positron has a strong affinity to one sort of atoms in binary semiconductors. The results of the positron charge density should provide valuable insight into the effect of annihilation. The electron charge density and the ionicity character with respect to the variation of the mole fraction are discussed.

Effect of temperature and surface coverage on the samarium interaction with Si(111)

The interaction of Sm atoms with Si(111) has been studied. The study was carried out by low-energy electron diffraction, Auger-electron spectroscopy, and contact-potential difference method over a broad range of temperatures (from room temperature to 1140 K) at which samarium was deposited on the silicon sample surface. The surface coverage varied from zero to 55 monolayers. It was shown that the shape of the low-energy Auger spectrum of samarium depends on coverage, and that its variation correlates with that of the Sm atom valence state. It was established that no ordered structures form when samarium is deposited on silicon at room temperature, and that partial mixing of the metal and semiconductor atoms takes place in the initial stages of this process. If samarium is deposited on heated silicon (900 and 1140 K), an adsorbed film (transition layer), whose structure is determined by the coverage and temperature, is the first to form. After that, three-dimensional silicide crystallites begin to grow on this transition layer. Their shape depends on the substrate temperature. This dependence accounts for the relation between temperature and the coverage at which the crystallites coalesce.

Local structures around Er atoms doped in InP revealed by fluorescence EXAFS

We have investigated OMVPE-grown InP doped with Er by fluorescence EXAFS (extended X-ray absorption fine structure) in order to study the local structures around Er atoms, so as to understand the luminescence property of Er atoms in semiconductors. The EXAFS analysis revealed that the Er atoms substituted into the In-site of InP, in the samples grown below 550°C, which exhibited high efficiency of luminescence, while the Er atoms constructed the NaCl-structure ErP, in the samples grown above 580°C, which exhibited low efficiency of luminescence.

XPS INVESTIGATION OF INTERFACE CHARACTERISTICS IN Au/GaP CONTACT SYSTEM

The variation of interface structure and composition with heat treatment temperature in Au-GaP contact system are investigated by X-ray photo-emission spectroscopy. The experimental results show that the diffusion and migration between the metal and semiconductor have taken place even if at room temperature. Annealing of Au-GaP contact leads to GaP decomposition in the interface accompanied by rapid atomic interdiffusion. During the increase of temperature the interreaction of Au-Ga atoms is enhanced and AuGa multiphase composition compounds are formed. The interfaces of Au-GaP contact system are an alloying regrowth layer containing metal and semiconductor atoms. The temperature dependence of interface characteristics are discussed from the point of view of Metallurgy.

Behavior of fluorine atoms in compound semiconductors

Behavior of fluorine atoms in compound semiconductors

Influence of electrostatic forces on the investigation of dopant atoms in layered semiconductors by scanning tunneling microscopy/spectroscopy and atomic force microscopy

Investigation of the atomic scale topography and electronic structure of dopant sites in semiconductor materials is a promising application of scanning probe microscopies. Dopants have been imaged with scanning tunneling microscopy (STM) on and near the surface of conventional semiconductor materials as well as on layered compounds. On both kinds of materials, dopants are detected as either protrusions or depressions in the STM image. The comparison of the measured heights between the materials shows that the values on layered materials are considerably larger than those on the conventional (three-dimensional) semiconductors. We interpret this as the influence of dopant induced electrostatic forces between the tip and sample leading to a structural deformation of the surface around dopant atoms. In order to investigate the influence of electrostatic forces, we performed STM measurements on p-type MoS2 at different bias voltages. The bias dependence of the images indicates the presence of electrostatic forces and demonstrates the influence of screening due to the surrounding electron density. Additional measurements with current imaging tunneling spectroscopy show that changes in the density of states at dopant sites plays only a minor role and cannot account for the large protrusions observed. Atomic force microscopy measurements, with an applied dc voltage between cantilever and sample, also confirm the role of electrostatic forces since voltage dependent changes in the topograpy were observed.