Vapour phase growth of epitaxial silicon carbide layers

After a brief overview of different epitaxial layer growth techniques, the homoepitaxial chemical vapour deposition (CVD) of SiC with a focus on hot-wall CVD is reviewed. Step-controlled epitaxy and site competition epitaxy have been utilized to grow polytype stable layers more than 50 μm in thickness and of high purity and crystalline perfection for power devices. The influence of growth parameters including gas flow, C/Si ratio, growth temperature and pressure on growth rate and layer uniformity in thickness and doping are discussed. Background doping levels as low as 10¹⁴ cm⁻³ have been achieved as well as layers doped over a wide n-type (nitrogen) and p-type (aluminium) range.

Furthermore the status of numerical process simulation is mentioned and SiC substrate preparation is described. In order to get flat and damage free epi-ready surfaces, they are prepared by different methods and characterised by atomic force microscopy and by scanning electron microscope using channelling patterns. For the investigation of defects in SiC high purity CVD layers are grown. The improvement of the quality of bulk crystal substrates by micropipe healing and so-called dislocation stop layers can further decrease the defect density and thus increase the yield and performance of devices. Due to its high growth rate functionality and scope for the use of multi-wafer equipment hot-wall CVD has become a well-established method in SiC-technology and has therefore great industrial potential.     

Atomistic examinations of the solid-phase epitaxial growth of silicon

The low-temperature vapor deposition of silicon thin films and the ion implantation of silicon can result in the formation of amorphous silicon layers on a crystalline silicon substrate. These amorphous layers can be crystallized by a thermally activated solid-phase epitaxial (SPE) growth process. The transformations are rapid and initiate at the buried amorphous to crystalline interface within the film. The initial stages of the transformation are investigated here using a molecular dynamics simulation approach based upon a recently proposed bond order potential for silicon. The method is used first to predict an amorphous structure for a rapidly cooled silicon melt. The radial distribution function of this structure is shown to be similar to that observed experimentally. Molecular dynamics simulations of its subsequent crystallization indicate that the early stage, rate limiting mechanism appears to be removal of tetrahedrally coordinated interstitial defects in the nominally crystalline region just behind the advancing amorphous to crystalline transition front. The activation barriers for this interstitials migration within the bulk crystal lattice are calculated and are found to be comparable to the activation energy of the overall solid-phase epitaxial growth process simulated here.     

Study of temperature distribution in high-temperature furnaces for silicon carbide monocrystal and epitaxial layer growing

A method to determine temperature gradient and distribution in high-temperature furnaces for silicon carbide monocrystal and epitaxial layer growing is suggested on the basis of the results obtained from the experiments on silicon carbide epitaxial layer growing kinetics in gas using 2 „sandwiches”︁.     

Investigation of the formation and growth processes of silicon carbide monocrystals

The formation and growth processes of SiC monocrystals from vapour phase by sublimation method have been investigated. It is shown that the peaks of the evaporation and growth whiskers are the centres of SiC monocrystals formation. One of the mechanisms of development of the whisker into SiC platelet is established. It is shown that on the naturally mirror like crystal face either the edge of the crystal or the screw dislocation, situated on the crystal edge, is the source of the steps. On the stepped crystal face the root of the crystal is the source of the steps.     

Predicted nitrogen doping concentrations in silicon carbide epitaxial layers grown by hot-wall chemical vapor deposition

A simple quantitative model for the surface adsorption of nitrogen has been developed to simulate the doping incorporation in intentionally doped 4H–SiC samples during epitaxial growth. Different reaction schemes are necessary for the two faces of SiC. The differences are discussed, and implications to the necessary model adjustments are stressed. The simulations are validated by experimental values for a large number of different process parameters with good agreement.     

Investigation of the thermal conditions during silicon carbide crystal growth

In the present work an analysis of the thermal conditions during silicon carbide crystal growth from the vapour phase by the sublimation method is carried out. On the basis of the obtained results from the calculation of the temperature distribution along the length of the growing crystal it was drawn a conclusion that in order to decrease the density of dislocations in the growing crystals it is necessary to decrease the temperature gradients in the crucible for growing.     

CBr4 as precursor for VPE growth of cubic silicon carbide

This work presents a study of carbon tetrabromide (CBr₄) as precursor to deposit 3C‐SiC on (001) and (111) Si by VPE technique at temperatures ranging between 1000 °C and 1250 °C. TEM, AFM and SEM results indicate that the epitaxy proceeds as a 3D growth of uncoalesced islands at low temperature, whereas a continuous crystalline layer with hillocks on top is obtained above 1200 °C. The hillocks observed at high temperature appear well faceted and their shape and orientation are analyzed in detail by AFM, showing a {311} preferred orientation. 3D island growth was suppressed by adding C₃H₈ to the precursor gases. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thin silicon epitaxial layers on preannealed silicon substrates

Multistep-temperature pretreatment is used to create defect denuded zones at surfaces in conjunction with bulk defects in oxygen rich substrate wafers. Using structural etching, RF–CV measurements, EBIC investigations, and gamma spectroscopy it is shown that bulk defects act as sinks for metallic impurities being fixed at an early stage of bipolar technology, already Etchable defects induced due to subsequent processing steps – e.g., emerging from bulk regions and entering denuded zones – contain less metallic impurities. 2 μm epitaxial layers deposited on such substrates with bulk defects and denuded zones reveal excellent structural and electrical properties.     

Effect of growth conditions on formation of grain boundaries in epitaxial silicon layers

The formation of grain boundaries of the general type, along with small-and large-angle symmetric grain boundaries with the 〈 〉 axis in the epitaxial layers grown onto bicrystal substrates by the method of thermal migration has been studied. The solvent was aluminum. It is shown that if the grain boundaries in the epitaxial layer are tilted to the crystallization front or if there is a temperature gradient tangential to this front, their orientation differs from their orientation in the substrate. The large-angle symmetric boundaries are more stable than the boundaries of the general type. The grain-boundary energy and rotation moment of large-angle symmetric boundaries are evaluated.     

Crystal structure of the growth surface of silicon carbide obtained by sublimation

Structural study of polycrystalline silicon carbide obtained by sublimation performed via X-ray luminescence and X-ray diffraction analysis. It is shown that chemical vapor deposition of silicon carbide results in the formation of grains with the (00.1), (01.1), and (12.3) crystallographic planes parallel to the growth surface. The grains with the (00.1) growth planes are characterized by perfect structure and by red luminescence. Domains with yellow luminescence have a mosaic structure with the (01.1) and (12.3) growth planes.     

Numerical investigation of carbon and silicon carbide contamination during the melting process of the Czochralski silicon crystal growth

Carbon contamination in single crystalline silicon is detrimental to the minority carrier lifetime, one of the critical parameters for electronic wafers. In order to study the generation and accumulation of carbon contamination, transient global modeling of heat and mass transport was performed for the melting process of the Czochralski silicon crystal growth. Carbon contamination, caused by the presence of carbon monoxide in argon gas and silicon carbide in the silicon feedstock, was predicted by the fully coupled chemical model; the model included six reactions taking place in the chamber. A simplified model for silicon carbide generation by the reaction between carbon monoxide and solid silicon was proposed using the closest packing assumption for the blocky silicon feedstock. The accumulation of carbon in the melted silicon feedstock during the melting and stabilization stages was predicted. Owing to this initial carbon content in the melt, controlling carbon contamination before the growth stage becomes crucial for reducing the carbon incorporation in a growing crystal.     

Study of the evaporation mechanism in silicon carbide crystals growth from vapour phase

Investigation of the thermal etching (evaporation) of the source material in the process of silicon carbide crystal growth from the vapour phase by the sublimation method has been carried out. It has been established that silicon carbide etching in the temperature interval (2200–2600) °C takes place with the formation of whiskers of evaporation. The mechanism of their formation has been examined and the character of the whisker distribution in the charge of the source silicon carbide has been studied.     

Anodic oxide films on silicon carbide

Anodic oxide films were grown on SiC using various electrolytes. The obtained oxide films were compared and some of their electrophysical properties were investigated. Anodic oxidation of SiC was shown to be useful for precise removal of layers as well as for identification of the polar faces of SiC crystals. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Blowing of silica microforms on silicon carbide

Silicon carbide ceramics have great potential for use in harsh environment applications, however many technical challenges still need to be addressed, including high temperature stability. The oxidation of SiC in air up to temperatures of 2053 K was conducted and resulted in the formation of a thermally grown silica scale that does not prove to be protective for very high temperature applications because of its rapid degradation. Examination of the oxide scale using scanning electron microscopy revealed the presence of striking features formed in a manner analogous to conventional glass blowing techniques. These features occurred for oxidation temperatures of at least 1773 K. The interfacial reaction between SiC and the oxide scale is responsible for the production of gases that must somehow escape. If the viscosity of the silica scale is low enough then it can be deformed freely by this pressure buildup.     

On the adsorption- and equilibrium-referred interpretation of the doping element incorporation during the growth of CVD epitaxial silicon

For investigating the partial steps of CVD-silicon doping: gas transport, adsorption-desorption, incorporation, a solution reflecting the kinetic influence of the layer growth in a satisfactory way, is obtained only if specific forward and backward reactions of the incorporation step are taken into account. Regardless of assuming a thermodynamical decomposition equilibrium for the doping source material and the incorporation equilibria for each of the individual components of the decomposition equilibrium, these components are not interchangeable in representing the total incorporation flow of dopants. The incorporation process is dominated by that component which exhibits the largest relative equilibrium partial pressure.     

Optically monitoring and controlling epitaxial growth

We provide a perspective on current capabilities for optically monitoring and controlling epitaxial growth, and discuss examples taken from recent work at Bellcore.     

Investigation of nitrogen solubility process in silicon carbide

On the basis of the epitaxial layers of SiC (N) grown from vapour phase by the sublimation method in a temperature interval (1900–2200) °C and at nitrogen partial pressures from 4 to 760 Torr has been determined the dependence of the nitrogen content in the epitaxial layers of SiC(N) on the nitrogen partial pressure and the temperature of growing. A thermodynamics analysis of the nitrogen solubility process in silicon carbide has been developed taking account of the electron-hole interaction. The experimental and calculation results, we have obtained, allowed us to determine the partial mol solubility heat of nitrogen in SiC.     

Synthesis and tribological properties of hexagonal titanium silicon carbide crystals

Hexagonal titanium silicon carbide (Ti₃SiC₂) crystals were prepared by vacuum sintering of Ti, Si, and C powders at 1300 °C. The microstructure and grain deformations of Ti₃SiC₂ were examined by scanning electron microscopy and transmission electron microscopy. The tribological properties of hexagonal Ti₃SiC₂ crystals as lubrication additive in HVI500 base oil were investigated by a UMT‐2 ball‐on‐plate friction and wear tester. The Ti₃SiC₂ additives exhibited good friction reduction. Under determinate conditions, the friction coefficient of base oil containing Ti₃SiC₂ crystals is lower than that of pure base oil. The base oil with 3.0 wt.% hexagonal Ti₃SiC₂ crystals presented good anti‐wear capability. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)