Epitaxial growth of silicon carbide layers by sublimation “Sandwich method” (II) structural defects and growth mechanism

Structural defects of α-SiC epitaxial layers grown by sublimation “sandwich-method” in vacuum at the temperatures ranging from 1600 to 2100 °C have been investigated by X-ray topography and optical microscopy methods. It was shown, that perfect SiC layers with the homogeneous polytype structure and small dislocation density (≦ 10² cm⁻²) may be obtained on the substrates with any crystallographic orientation at the conditions close to quasi-equilibrium one. The presence of impurities and silicon deficiency in the vapour phase, lead usually to the deterioration of morphological and structural perfection of SiC layers. There are the following structural defects: uncoherent polytype inclusions (mainly β-SiC), pores, dislocations, specific stacking faults. Morphological peculiarities of the SiC epitaxial layers and possible growth mechanisms are discussed.     

Three-layer monoclinic 3M 2 polytype of muscovite revealed from oblique-texture electron diffraction patterns

The inhomogeneous (complex) polytype 3M ₂ of finely dispersed muscovite with the structure formed by layers with different parity of azimuthal orientations (561) or relative rotations $[11\bar 2]$ was identified for the first time using the oblique-texture electron diffraction patterns. The specimen had no rigorous order in layer alternation, which is reflected in the distortion of the reflection positions and intensities. It is shown that the most probable defects reduce to the replacement of the layer orientations characteristic of the 3M ₂ polytype by layer orientation characteristic of the 2M ₂ polytype. This fact indicates that, under certain conditions, the prismatic coordination of interlayer cations in micas is preferable.     

Studies on syntactically coalesced Silicon Carbide polytype structures

The structure of two new silicon carbide polytypes, found in syntactic coalescence with a basic structure 6 H, have been worked out. The structure of polytype 21 H is [(33)₂63] and that of polytype 93R is [(33)₄34]₃. The polytype 93R belongs to the structure series [(33)_n34]₃. One of the (00.1) growth face reveals the growth spiral. The splitting of diffraction spots indicates the existence of a high period polytype in the crystal. This high period polytype have been identified as 558H or 1674R. The growth mechanism have been discussed in the light of above observations.     

Stacking faults in lizardites 1T from electron diffraction data and modeling of diffraction effects

Stacking faults in lizardites 1T (polytype group A) have been studied by analyzing the oblique-texture electron diffraction patterns in combination with a numerical simulation of diffraction patterns. The simulation of diffraction profiles was performed for finite (10-layer) sequences using the statistical Markov model in the quasi-homogeneous approximation. Diffraction criteria are revealed, which indicate the presence of defects in lizardite 1T; these defects are related to the change in the layer orientation by 180° and displacements of adjacent layers by a/3. It is shown that in the case of the statistical distribution of defects, along with pairs of adjacent layers with layer packing features typical of polytype groups D and B, there are also sequences of layers 1T(A), correspondingly rotated by 180° or displaced by a/3 and 2a/3 with respect to the layers in the 1T matrix. The data obtained give deeper insight into the variations in layer packing in lizardites 1T.     

Complex (nonstandard) six-layer polytypes of lizardite revealed from oblique-texture electron diffraction patterns

Association of simple (1T and 3R) and two complex (nonstandard) orthogonal polytypes of the serpentine mineral lizardite from the Catoca kimberlite pipe (West Africa) association is revealed from oblique-texture electron diffraction patterns. A six-layer polytype with an ordered superposition of equally oriented layers (notation 3₂3₂3₄3₄3₆3₆ or ++??00) belonging to the structural group A and a three-layer (336 or I,I,II) or a six-layer (336366 or I,I,II,I,II,II) polytype with alternating oppositely oriented layers and semi-disordered structure are identified using polytype analysis.     

Ion channeling study of defects in CuInTe2 single crystals

Ion channeling spectra of as-grown CuInTe₂ single crystals are measured using a 2 MeV ⁴He⁺ analysing beam. It is found that the measured minimum yields cannot be explained within a model of randomly distributed non-interacting point defects caused by deviations from ideal stoichiometry. A model with extended defects due to vacancy clustering and dislocation formation is proposed to interprete the experimental results.     

Zn1−xCdx polytype single crystals with various degree of order

The results of X-ray investigations of the polytype structures formed in solid solutions of Zn_1–xCd_xSe are presented. For compositions of 0.22 < x < 0.35 polytypes 8H, 6H, 4H, and DS (disordered structure) are present. The Zn_1–xCd_xS single crystals of 0 ≦ x ≦ 0 were obtained by Bridgman's method under high protective argon pressure from the melt. The structures are characterised by FARKASZ -JAHNKE statistical parameters π(m,p), and by parameters P_LH of the polytype unit cell formation probability.     

Effect of γ-irradiation on mechanically activated and on non-activated LiF

By means of EPR spectroscopy lattice defects have been investigated, which had been produced in OH⁻ containing LiF by mechanical activation and subsequent irradiation (mainly γ-irradiation). The structure of irradiation-induced hydrogen centers shows characteristic differences dependent on the kind of LiF: H_s⁰ (hydrogen atoms in anion vacancies) are created in mechanically activated LiF powders, whereas H_i⁰ (hydrogen atoms in interstitial positions) in LiF single crystals. In both the cases also F centers are produced besides other paramagnetic defects. The observed defects can be used for an estimation of the degree of distortion in mechanically activated LiF.     

Structure of lanthanum prepared by quenching from a liquid state

The structure of lanthanum quenched from a liquid state at cooling rates of 2.5 × 10⁵–10⁷ K/s is investigated. It is shown that a mixture of the face-centered cubic β-La phase stable above 583 K and the metastable lanthanum modification with a seven-layer hexagonal lattice consisting of alternating atomic layers ABCABAC… is formed in rapidly cooled foils. A mechanism is proposed for the formation of the metastable polytype, according to which the polytype is formed as a result of saturation of the main β-phase with well-ordered stacking faults.     

Solvent effect on the polytype structure of CdI2 crystals

Single crystal platelets of cadmium iodide have been grown from alcohols: n-propyl, isobutyl and isoamyl and from their aqueous solutions. The structures of 880 crystals has been identified and the effect of solvent on the polytype structure of CdI₂ has been discussed regarding: the stability of the basic structures 2H and 4H, the structure of complex polytypes, the effect of the solvent on the structure of faults, the period of polytypes. Some relations indicating the role of the solvent in the formation of polytype structure of CdI₂ have been found.     

Zur Epitaxie von Galliumnitrid auf Korund im System GaCl/NH3/Ar

The vapour growth of single crystalline layers of GaN on {0001} oriented sapphire substrates in the temperature range of 500–1100°C is described for the system GaCl/NH₃/Ar. The RHEED micrographes of samples grown in the high temperature range of 1000–1050°C show KIKUCHI pattern, thus indicating a good crystalline perfection of the layers. The large free electron concentration of about 10²⁰ cm⁻³ decreases below 700°C rapidly with decreasing growth temperature. Simultaneously the crystalline perfection is remarcably lowered due to increasing misorientation of the subgrains, but no arcing of the RHEED spots due to polycrystalline behaviour is observed. The layers grown at 500°C are contaminated by chloride. Sometimes a dendritic overgrowth of the layers by the GaN sphalerite polytype takes place.     

Total pressure‐controlled PVT SiC growth for polytype stability during using 2D nucleation theory

A total pressure‐controlled physical vapor transport growth method that stabilizes SiC polytype is proposed. The supersaturation of carbon during SiC growth changed as a function of the growth time due to changes in the temperature difference between the surfaces of the source and the grown crystal. Supersaturation also varied as a function of the pressure inside the furnace. Therefore, modification of the pressure as a function of growth time allowed for constant supersaturation during growth. The supersaturation was calculated based on classical thermodynamic nucleation theory using data for heat and species of Si₂C and SiC₂ transfer in a furnace obtained from a global model. Based on this analysis, a method for polytype‐stabilized SiC growth was proposed that involves decreasing the pressure as a function of growth time. The 4H‐SiC prepared using this pressure‐controlled method was more stable than that of 4H‐SiC formed using the conventional constant‐pressure method.     

On the Structural and Electronic Characteristics of Titanium Sulphide (Tis1.7) Crystals

TiS_1.7 crystals have been grown by vapour transport technique employing a two-zone furnace with the temperatures of reaction and growth zone maintained at 1073 K and 973 K, respectively. We have measured the variation of electrical conductivity (s̀) with temperature (T) of TiS_1.7 single crystals. It has been found that the conductivity increases at temperatures T > 433 K, which provides convincing evidence that the TiS_1.7 crystal is a semiconductor. Another electronic characteristic of TiS_1.7 crystals observed in the present investigation is the occurrence of voltage controlled negative resistance (VCNR) at a field of 32.1 V cm^-1 to 35.7 V cm^-1. All the polytypes of TiS_1.7 were found to exhibit VCNR nearly at the same field which indicates that the VCNR is polytype independent property. The occurrence of VCNR has been explained on the intervally transfer of electrons in the conduction band.     

Cr doping influence in ZnS single crystals on the complex disordered polytypical structure

The results of X-ray investigation of real structures of ZnS:Cr single crystals are presented. Doped ZnS single crystals were grown from the melt by Bridgman's high pressure method. Cr dopant converts the structure of ZnS crystals from 3C with individual stacking faults (for pure ZnS) to the complex polytypical structures disordered along one dimension and including specific orderings of cp layers that form polytype cells LH (L = 2l, l = 2, 3, 4 and 5). The structures are characterised by Farkas-Jahnkes statistical parameters π(m, p) and by parameters P_LH of the polytype unit cell formation probability (Kozielski, Tomaszewicz 1986; Palosz, Przedmojski).     

Lattice disorder and optical absorption of PbTiO3 crystals

The electric conductivity and the optical absorption of PbTiO₃ crystals are explained by the existence of Pb³⁺ ions and free holes as electronic defects and of O^– vacancies and interstitials as ionic anti-Frenkel defects. The absorption band at 475 nm is caused by Pb³⁺ ions and produces the yellowish or brownish colour of the crystals. Using fluoridic fluxes instead of oxidic fluxes for the crystals growth a small part of O^– ions is replaced by F⁻ foreign ions. This substitution diminishes the Pb³⁺ ion content and causes an increased optical transmission of the PbTiO₃ crystals.     

High temperature synthesis and crystal structure of new representatives of the huntite family

Single crystals of REE, Al borates, new and known representatives of huntite family are obtained by annealing of REEBO₃ on Al₂O₃ surface at 1100°C. Phase identification for REE=Pr, Eu, Tb, Tm, Ho, Yb has been carried out using unit cell parameter determination on single crystals. Crystal structures of C 2/c (PrAl₃(BO₃)₄) and C 2 (EuAl₃(BO₃)₄) ‐ two polytypic modifications, previously determined for other REE's have been studied. Formation of one or another polytype in similar thermodynamic conditions is probably dependent on electronic structure of rare earth ion. Crystal field stabilization energy in crystal fields of different symmetry is a possible factor of polytype stability (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of composition on ionic transport in SiO2–B2O3–Na2O glasses

Ionic diffusion was investigated in the SiO₂–B₂O₃–Na₂O glass system over a wide composition range by impedance spectroscopy measurements. The Na⁺ cation transport mechanism was described by an interstitial pair migration model based on Frenkel defects in ionic crystals. The activation energy of the static electrical conductivity is shown to be correlated with the boron coordination number in these glasses. Published ¹¹B NMR results were used to calculate the activation energies of sodium cations acting as charge compensators for the [BO_4/2]⁻ tetrahedron and of sodium cations bonded to non-bridging oxygen atoms. These values are in agreement with the activation energies of the Na₂O–B₂O₃ and Na₂O–SiO₂ binary systems, respectively.     

The Temperature Dependence of Electron Spin-Lattice Relaxation Data in Trans-Polyacetylene and the Evidence for a Soliton-Phonon Interaction

Abstract

The temperature and frequency dependence of the electron spin-lattice relaxation rate Rj for the paramagnetic defects in pristine transpolyacetylene is analyzed in terms of a model in which the electron-electron dipolar interaction between spins is modulated by one-dimensional diffusion induced by phonon scattering of the spins. According to this model, at room temperatures R₁ has a T¹/² temperature dependence; at low temperatures R₁ has a T² or T⁵/² temperature dependence. R₁ is described by a ω^?1/² frequency dependence. The model shows the symmetry between moving and stationary solitons scattered by phonons. The soliton wave function obtained from experimental ENDOR data is used to obtain analytical results. We discuss the possible temperature and frequency limitations of this model.     

Influence of proton implantation on the properties of CuInSe2 single crystals (I). Ion channeling study of lattice damage

The lattice damage of p-type CuInSe₂ single crystals implanted with 10 keV protons in the fluence range from 2.5 · 10¹⁴ to 8 · 10¹⁵ cm⁻² was investigated using the Rutherford backscattering/channeling technique. At proton fluences up to about 2 · 10¹⁵ cm⁻² radiation annealing of the defects is observed which is ascribed to very high defect concentrations in the unimplanted samples. At higher fluences radiation damage occurs but the concentration of radiation induced defects ramains low. There are indications that selenium interstitials or defect complexes with selenium interstitials involved are stable defects at room temperature.     

Bulk and epitaxial growth of silicon carbide

Silicon carbide (SiC) is a wide bandgap semiconductor having high critical electric field strength, making it especially attractive for high-power and high-temperature devices. Recent development of SiC devices relies on rapid progress in bulk and epitaxial growth technology of high-quality SiC crystals. At present, the standard technique for SiC bulk growth is the seeded sublimation method. In spite of difficulties in the growth at very high temperature above 2300 °C, 150-mm-diameter SiC wafers are currently produced. Through extensive growth simulation studies and minimizing thermal stress during sublimation growth, the dislocation density of SiC wafers has been reduced to 3000–5000 cm⁻² or lower. Homoepitaxial growth of SiC by chemical vapor deposition has shown remarkable progress, with polytype replication and wide range control of doping densities (10¹⁴–10¹⁹ cm⁻³) in both n- and p-type materials, which was achieved using step-flow growth and controlling the C/Si ratio, respectively. Types and structures of major extended and point defects in SiC epitaxial layers have been investigated, and basic phenomena of defect generation and reduction during SiC epitaxy have been clarified. In this paper, the fundamental aspects and technological developments involved in SiC bulk and homoepitaxial growth are reviewed.