Silicon Face Research Articles

Photoelectron spectroscopy of atomic core levels on the silicon surface: A review

Recent studies of the atomic structure of the single-crystal silicon surface (both clean and covered by adsorbates) that are performed by high-resolution core-level photoelectron spectroscopy using synchrotron radiation are reviewed. The physical principles of the method, experimental techniques, the spectrum processing procedure, and the procedure of determining the energy shifts of the core levels in the subsurface layer are outlined. Emphasis is placed on the surface modes of silicon 2p spectra, which are observed for the main types of silicon surface reconstruction (Si(111)-7×7 and Si(100)-2×1), and on a correlation between these modes and the atomic structure of the (111) and (100) surfaces. Also, particular attention is given to the studies of the Ge/Si system, which is viewed as a promising material of nanoelectronics, as well as to those concerned with metal and gas adsorption on basic (low-index) silicon faces. These studies clearly demonstrate that core-level photoelectron spectroscopy provides extremely detailed information on the structure of adsorbed layers and on the adsorption-stimulated reconstruction of the substrate surface.

Thermal oxidation of 6H-SiC studied by oxygen isotopic tracing and narrow nuclear resonance profiling

We show that on SiC ( 0 0 0 1 ̄ ) (the fast-oxidizing carbon face), at 1100 °C and 100 mbar, the oxide exhibits an initial fast growth regime, followed by a constant growth rate confirming previous results that oxide growth is not limited by diffusion of the oxidizing species or reaction products through the oxide. At 1100 °C, in this linear regime, the growth rate also exhibits a linear dependence with oxygen pressure. The silicon face shows sub-linear pressure dependence. A simple oxidation model is ruled out since, on both SiC faces, sequential 16O 2/ 18O 2/ 16O 2 oxidations show that oxygen fixed in the oxide, near the SiC/SiO 2 interface moves during subsequent growth.

Improved ohmic contact to the n-type 4H-SiC semiconductor using cobalt silicides

Multilayer structures of cobalt and silicon have been deposited as an ohmic contact on n-type 4H-SiC substrates in order to obtain lower contact resistance and higher thermal stability. The metal structures were prepared by using electron beam evaporation on top of the silicon face of the 4H-SiC substrates, and were annealed in an atmosphere of argon with 10% hydrogen. The metal film thickness was monitored during the film deposition, and the ratio of the cobalt and silicon was fixed at 0.5 for the formation of the silicon-rich silicide structure (CoSi2). The electrical property of the ohmic contact has been significantly improved by the reduction of the oxide content in the metal contact layer. A two-step annealing process was employed to reduce oxidation problems that may occur in the heat treatment at high temperatures. The specific contact resistance of the contact structure prepared by the two-step annealing process was measured to decrease by more than one order of magnitude compared to that prepared by one-step annealing. The best result has been obtained as 1.8 × 10−6 Ω cm2 for Co/Si/Co/Si/Co metal structures after two-step annealing, at 500 °C for 600 s and 800 °C for 120 s. In the field emission scanning electron microscopy, the interface of the contact structure and SiC substrate was observed to have smooth surface morphology with CoSi2 grains.

SPACE-CHARGE LAYERS AND SURFACE STATES IN p-TYPE CRYSTALLINE SILICON

Pulse measurements on the silicon/electrolyte interface have been used to study space-charge layers and surface states on the (100) faces of p-type silicon. The techniques used enable both the creation and study of space-charge layers at the semiconductor surface, ranging from large-depletion to strong-accumulation conditions. They also permit a straightforward separation of the different components of the induced charge at the silicon/electrolyte interface, so as to yield the variation of both the free-electron density in the space-charge layer and the density of occupied surface states with barrier height. The measured space-charge characteristics are in very good agreement with theory. The data in strong-accumulation layers indicate the presence of an insulating buffer layer (such as an oxide), 3–4 monolayers thick. As to surface states, we find that for CP-4-etched silicon, a distribution of states exists ~ 0.34 eV above the valence-band edge, with total density ~ 6 × 1011 cm -2.

Basal plane slip and formation of mixed-tilt boundaries in sublimation-grown hexagonal polytype silicon carbide single crystals

Optical microscopy, synchrotron white beam x-ray topography (SWBXT), and high resolution x-ray diffraction (HRXRD) were used to study the distribution of basal plane dislocations in bulk 4H silicon carbide crystals grown by the physical vapor transport method. An etch pit array was observed on the silicon face of KOH-etched off-cut wafers. The arrays were aligned parallel to each other and perpendicular to the off-cut direction. The etch pits were oval-shaped, which is characteristic of basal plane dislocations. Corresponding array images have been observed by SWBXT. Based on the characteristic distribution, the etch pit arrays are interpreted as the slip traces of high temperature deformation during the growth process. Thermoelastic stress is proposed as the plausible cause of the deformation. In addition, basal plane dislocation pileups were found in the proximity of polygonized threading edge dislocation arrays. SWBXT and HRXRD were used to study the misorientation related to such dislocation structure. The pileups are interpreted as the origin of the basal-plane-tilt domains in SiC crystals.

Self-assembled germanium nanocrystals on SiC{0001}

The self-organized growth of Ge islands on various hexagonal SiC{0001} surfaces is discussed. The formation of two (2-D) or three-dimensional (3-D) Ge islands depend on the polarity of the SiC substrate (silicon or carbon face), the SiC surface stoichiometry and reconstruction, the growth rate and temperature. Furthermore, the nucleation conditions can be modified by an oxidation of the Si-adlayer present on Si-rich SiC surfaces. On the Si face of SiC(0001)-(√3×√3)R30°, 2-D Ge islands of lateral dimensions between 2 and 4 nm and a density of more than 10 12 cm −2 are initially formed on a 0.3 nm thick Ge wetting layer. Similar to Si substrates, a further growth leads to the formation of equilibrium 3-D Ge nanocrystals of typically more than 20 nm in size and a density of about 10 10 cm −2. A Ge deposition on the C face of SiC, as well as on the oxidized Si adlayer of a Si face of SiC results in small 3-D Ge islands of only a few nanometers in size and a number density between 10 11 and 10 12 cm −2.

MOBILITY KEY TO SILICON ETCHING

BUCKING CONVENTIONAL wisdom, researchers at the University of North Carolina, Chapel Hill, report that surface diffusion plays a key role in silicon etching. The finding contradicts current models that describe halogen etching of semiconductors as a purely kinetic process. Etching silicon with chlorine or bromine is a standard procedure for preparing the semiconductor for applications in micro-electronics. The process has been well studied, and molecular mechanisms have been developed to describe the reaction's elemental steps. Although successful in some regards, today's models fall short when it comes to providing a detailed account of certain common etching scenarios. For example, bromine etching of the (100) crystal face of silicon is known to result in two types of surface morphologies, depending on reaction temperature. Typically, scientists have invoked separate mechanisms to rationalize each type of morphology. Now, based on real-time scanning tunneling microscopy analysis of etching, chemistry pr...

Growth related distribution of secondary phase inclusions in 6H–SiC single crystals

The formation of grown in defects like misoriented regions and micropipes is correlated with second phase inclusions such as silicon droplets or carbon particles. Crystals up to 35mm diameter were grown on the silicon face by the modified Lely method. Inclusions could be identified as carbon particles by SEM and EELS. Their distribution along the growth direction and at the phase boundary were investigated by optical microscopy. It was found that their density varies in axial as well as radial direction. The dependency of the carbon particle concentration on growth parameters such as seed temperature, time, pressure and the distance between source and seed is shown. A concentration model will be discussed. Therefore any change of the vapor phase composition characterized by the loss of silicon and caused by changing of a growth parameter may result in a drastic local increase of the carbon particle concentration. If a critical value is exceeded the lattice information of the substrate is lost and defects may be formed.

Atomic hydrogen beam etching of carbon superstructures on 6H-SiC (0001) studied by reflection high-energy electron diffraction

Abstract A route for the regeneration of smooth √3×√3 R30 face on 6H-SiC(0001) by atomic hydrogen beam etching following the carbonization of the SiC surface at high temperatures had been investigated. The various stages during the segregation of carbonaceous super-structures at high temperatures as well as the layer-by-layer restructuring of the 6H-SiC(0001) surface by atomic H beam were studied by reflection high energy electron diffraction (RHEED). A smooth silicate-terminated √3×√3 R30 surface could be obtained after hydrogen-plasma beam treatment at 800°C. Annealing the √3×√3 R30 face to 900°C readily resulted in the segregation of 1×1 graphite islands on the surface, with the basis vectors of the graphite unit cell rotated 30° with respect to the bulk SiC. Further annealing to temperatures between 1000 and 1200°C resulted in the coalescence of the graphite islands to form an epitaxial layer, which adopted an incommensurate 6√3×6√3 R30-C structure with respect to the bulk. The epitaxial 6√3×6√3 R30-C layer acted as a template for the further growth of smooth single-crystalline graphite multilayers upon prolonged annealing. Atomic force microscopic (AFM) analysis revealed that the epitaxial graphite formed by this method was atomically smooth. Re-exposing the graphite-covered surface to a second hydrogen-plasma treatment readily converted the carbonized surface to a silicate-terminated √3×√3 R30 face. The layer-by-layer etching mechanism of the carbonized SiC by the atomic-H beam source constituted an effective route for the regeneration of the smooth silicon face.

Growth Related Distribution of Secondary Phase Inclusions in 6H-SiC Single Crystals

The formation of grown in defects like misoriented regions and micropipes is correlated with second phase inclusions such as silicon droplets or carbon particles. Crystals up to 35 mm diameter were grown on the silicon face by the modified Lely method. Inclusions could be identified as carbon particles by SEM and EELS. Their distribution along the growth direction and at the phase boundary were investigated by optical microscopy. It was found that their density varies in axial as well as radial direction. The dependency of the carbon particle concentration on growth parameters such as seed temperature, time, pressure and the distance between source and seed is shown. A concentration model will be discussed. Therefore any change of the vapor phase composition characterized by the loss of silicon and caused by changing of a growth parameter may result in a drastic local increase of the carbon particle concentration. If a critical value is exceeded the lattice information of the substrate is lost and defects may be formed.

Surface preparation for Schottky metal - 4H-SiC contactsformed on plasma-etched SiC

Silicon-carbide-based devices frequently require aSchottky gate to be deposited on a plasma-etched surface. Thispaper considers the effectiveness of nine differentpre-metallization surface preparation procedures in removingthe etch damage. The surfaces were assessed by x-rayphotoelectron spectroscopy and by current-voltage measurementof nickel Schottky diodes formed on both reactive-ion-etchedand non-reactive-ion-etched silicon face 4H-SiC. Thetreatments included simple UV-ozone and solvent cleans, oxygenplasma, deposited oxide and thermal oxidation. It was confirmedthat the only process which removed all traces of surfacecontamination and etch damage, producing ideal Schottky diodes,was sacrificial oxidation.

Stability-instability transitions in silicon crystal growth.

In order for a crystal to grow, source atoms must be incorporated into the underlying lattice. Typically, this process occurs on the surface in one of two modes: either through island nucleation or through step flow. However, a third, morphologically unstable growth mode has been predicted. Monitoring the surface of ultraflat substrates with an in situ scanning electron microscope, we prove that for the (111) face of silicon there is a transition from stable step flow to morphological instability and then to island nucleation.

Monte Carlo modeling of silicon crystal growth

The crystallization of pure silicon has been simulated using Monte Carlo computer modeling. These are atomic scale simulations, which permit an examination of the relationship between the configuration of atoms at the interface and the local growth conditions. It is well known that silicon growing from its melt facets on the (1 1 1) face, which implies a nucleation barrier to growth. None of the other orientations facet, and the interface for these other orientations lies very close to the melting point isotherm. The relationship between the growth rate and undercooling has been modeled for the (1 0 0) and (1 1 1) faces, and the results of the simulations agree well with experimental observations.The data indicate that the specific interfacial free energy of a step edge on the (1 1 1) face of silicon in contact with its melt is about one-tenth of the specific interfacial free energy of the (1 1 1) facet.

Kev Ion Beam Induced Surface Modification of Sic Hydrogen Sensor

AbstractSilicon carbide, a wide-bandgap semiconductor, is currently used to fabricate an efficient high temperature hydrogen sensor. When a palladium coating is applied on the exposed surface of silicon carbide, the chemical reaction between palladium and hydrogen produces a detectable change in the surface chemical potential. Rather than applying a palladium film, we have implanted palladium ions into the silicon face of 6H, n-type SiC samples. The implantation energies and fluences, as well as the results obtained by monitoring the current through the sample in the presence of hydrogen are included below. The exposure to hydrogen of this kind of sensor while monitoring the current flow with respect to time, has revealed a completely different behavior than the samples that have Pd deposited as a surface layer.

Crystalline Oxides on Silicon: The First Five Monolayers

The long-standing problem of growing a commensurate crystalline oxide interface with silicon has been solved. Alkaline earth and perovskite oxides can be grown in perfect registry on the (001) face of silicon, totally avoiding the amorphous silica phase that ordinarily forms when silicon is exposed to an oxygen containing environment. The physics of the heteroepitaxy lies in establishing a sequenced transition that uniquely addresses the thermodynamics of a layer-by-layer energy minimization at the interface. A metal-oxide-semiconductor capacitor using $\mathrm{SrTiO}{}_{3}$ as an alternative to $\mathrm{SiO}{}_{2}$ yields the extraordinary result of ${t}_{\mathrm{eq}}<10\AA{}$.

Electron focusing in backscattering from single-crystal Si(100)

Diffraction patterns produced by quasi-elastically backscattered electrons focused in a thin single-crystal Si(100)2×1 near-surface layer have been studied. The measurements performed in the 0.6–2-keV range are compared with calculations made in the single-scattering cluster approximation. This model is shown to describe adequately the experiment. An analysis is made of the relation among the diffraction patterns observed for different silicon faces, and of the effect of the primary-electron beam orientation. The relations governing the focusing of quasi-elastically backscattered electrons escaping from the crystal along the main crystallographic directions have been established. The various aspects of the effect for backscattered electrons undergoing inelastic interaction with the electron subsystem of the crystal have been investigated.

Scanning Tunneling Microscopy of Ordered Organic Monolayer Films on Si(001)

Scanning tunneling microscopy and optical spectroscopy techniques have been utilized to investigate the formation of ordered organic monolayer films on the (001) face of silicon. Cyclopentene and 1,5-cyclooctadiene both produce monolayer films that are ordered translationally and rotationally. The rotational orientations of the molecules arise from the directional interaction of the π orbitals of the starting alkene with the π orbital of the dimers comprising the reconstructed Si(001) surface, with the Si(001) surface acting as a template for determining the directionality of molecules in the subsequent organic film. Using single-domain Si(001) samples, it is shown that the molecular films also exhibit anisotropy in optical properties when measured on centimeter length scales.

Infrared Reflectance of Extremely Thin AlN Epi-Films Deposited on SiC Substrates

Silicon Carbide (SiC) is an important wide band gap semiconductor with outstanding electronic properties. With figures of merit far better than silicon, SiC is believed to replace and outcompete silicon in many applications using high frequencies, high voltage and high temperatures. With the introduction of seeded sublimation technique, a realisation of substrates with large diameter and high quality became possible. Recent progress in the bulk growth using high temperature chemical vapour deposition (HTCVD) has shown excellent results with high purity substrates with semi insulating (SI) properties. The availability of high quality SI substrates allows the fabrication of microwave devices with low rf losses such as the Metal Schottky Field Effect Transistor (MESFET). With the introduction of the hot-wall CVD technique, thick low doped n-type epitaxial layers have been grown for high power devices (> 4 kV) such as the PiN diode. The main contribution of the present work relates to the investigation of growth of MESFET structures. The goal has been to demonstrate the ability to grow MESFET structures using the hot-wall CVD technique. The challenge with abrupt interfaces and controlled doping has been investigated. A comprehensive investigation has been made on how nitrogen and aluminum dopant atoms incorporate into the SiC lattice using the hot-wall CVD technique. Fundamental research of MESFET structures has been combined with growth of device structures for both Swedish and European groups as well as industries. The research has been focused towards the understanding of dopant incorporation, characterization of doped epitaxial layers, the growth of device structures, the modelling of temperature distribution in a hot-wall susceptor and the development of growth systems for future up scaling. In paper 1 we present how the nitrogen dopant is incorporated into the SiC lattice. The influence of several different growth parameters on the nitrogen incorporation is presented. Equilibrium thermodynamical calculations have been performed to give a further insight into the incorporation mechanism. The investigation shows that the N2 molecule itself does not contribute directly to the nitrogen incorporation, however, molecules like the HCN and HNC are more likely. In paper 2 the incorporation of the aluminum dopant into the SiC lattice is investigated in a similar way as the nitrogen incorporation in paper 1. The results show that the aluminum incorporation in SiC is mainly controlled by the carbon coverage on the SiC surface. The investigation shows that it is difficult to obtain high aluminum doping on carbon face whereas the silicon face is sensitive to changes of the growth parameters. High growth rate resulted in a diffusion controlled incorporation. In Paper 3 we present the results from the growth of MESFET structures as well as characterization of the structures and final device properties. Knowledge taken from paper 1 and 2 was used to improve the abruptness of the grown structures. Paper 4 presents the results obtained by low temperature photoluminescence (LTPL) on separately grown 4H-SiC epitaxial layers. Doping calibration curves for nitrogen in the doping range from 1⋅1014 to 2⋅1019 cm-3 are presented. A discussion concerning the Mott transition is also presented. Paper 5 presents the results of the use of simulation to investigate the heating of a hot-wall CVD reactor. New susceptor and coil design are tested. The simulation has been verified with experimental heating tests which show excellent agreement. The new design has a temperature variation of less than 0.5 % over more than 70% of the total susceptor length in addition to a decreased power input of 15 %. In the final two papers, paper 6 and 7, we present work of growth of AlN on SiC. Thin films were grown and characterized with different techniques concerning crystal quality and thickness. The use of infrared reflectance and the features of the AlN reststrahl reflectance band allowed us to determine the thickness of AlN films as thin as 250 A.

Defect Characterization of Homo-Epitaxially Grown 6H-SiC on (0001) Silicon and (000-1) Carbon Faces

Defect Characterization of Homo-Epitaxially Grown 6H-SiC on (0001) Silicon and (000-1) Carbon Faces

Thin oxide growth on 6H-silicon carbide

A marked difference is observed for the initial oxidation rates of 6HSiC (silicon face) obtained with oxygen and argon as carrier gases in a wet oxidation process. A modified Deal-Grove model with surface and bulk reaction constants is presented to model the early oxidation process at the SiO 2SiC interface in the case of oxygen as the carrier gas. In the early phase of oxidation there is rapid growth with the oxygen carrier gas, in contrast with the argon carrier gas. This is a surprising result, since silicon oxidation does not demonstrate a dependence on the carrier gas. After a time of approximately 150 minutes, a linear growth rate of 1.2Ȧ/min. occurs for both carrier gases. Thermal oxides on p-type (doping=4×10 16/cm 3) 6HSiC using oxygen and argon as the carrier gases have interface trap densities (D it) of 2.5×10 11/cm 2eV and 5×10 11/cm 2eV respectively. Fixed charge (Q f ) was determined to be 4×10 11/cm 2 and 8×10 11/cm 2 for the oxygen and argon carrier gas case respectively. Electrical breakdown measurements indicated oxide breakdown field strengths on fabricated MOS capacitors to be 8.5 MV/cm.