Large area SiC substrates and epitaxial layers for high power semiconductor devices — An industrial perspective

We review the progress in the industrial production of SiC substrates and epitaxial layers for high power semiconductor devices. Optimization of SiC bulk growth by the sublimation method has resulted in the commercial release of 100 mm n-type 4H-SiC wafers and the demonstration of micropipe densities as low as 0.7 cm⁻² over a full 100 mm diameter. Modelling results link the formation of basal plane dislocations in SiC crystals to thermoelastic stress during growth. A warm-wall planetary SiC-VPE reactor has been optimized up to a 8×100 mm configuration for the growth of uniform 0.01–80-micron thick, specular, device-quality SiC epitaxial layers with low background doping concentrations of <1×10¹⁴ cm⁻³, and intentional p- and n-type doping from 1×10¹⁵ to >1×10¹⁹ cm⁻³. We address the observed degradation of the forward characteristics of bipolar SiC PiN diodes [H. Lendenmann, F. Dahlquist, J.P. Bergmann, H. Bleichner, C. Hallin, Mater. Sci. Forum 389–393 (2002) 1259], and discuss the underlying mechanism due to stacking fault formation in the epitaxial layers. A process for the growth of the epitaxial layers with a basal plane dislocation density <10 cm⁻² is demonstrated to eliminate the formation of these stacking faults during device operation [J.J. Sumakeris, M. Das, H.McD. Hobgood, S.G. Müller, M.J. Paisley, S. Ha, M. Skowronski, J.W. Palmour, C.H. Carter Jr., Mater. Sci. Forum 457–460 (2004) 1113].     

Differences in the atomic environment of nonequivalent sites in SiC-polytype structures

A general algorithm is proposed for calculating the Θ series of SiC polytypes. The obtained Θ series of the main SiC polytypes can be useful in calculating lattice sums, in particular when using the Mellin transform of the Θ series. By expanding the Θ series in the Jacobi parameter, one obtains sequences of coordination numbers for crystallographically nonequivalent atomic sites in the main SiC polytypes. A nontrivial interrelationship is demonstrated between these numerical sequences and the local symmetry of the nonequivalent sites. Fiz. Tverd. Tela (St. Petersburg) 41, 183–186 (February 1999)     

Concentration or modulation layering of perfect single crystals Bi2Sr2?xLaxCuO6+δ and Bi2?yPbySr2?xLaxCuO6

Precision structural studies of layering of perfect crystals Bi ₂ Sr _2−x La _x CuO _6+δ (BSLCO) ∼ 10 μm thick, grown by free growth within crystallized melt cavities, detect a macrolayer structure with each layer up to 0.1 μm thick. In the lanthanum concentration range x = 0.6−0.8, only modulation layering is observed. In the concentration range x = 0.3−0.5, either layers with two different lanthanum concentrations, but with the same modulated superlattice type, or layers with the same lanthanum concentration, but with two different modulated superlattice types are observed. At low lanthanum concentrations (0 < x < 0.26), layering into two or even three layer types with different lanthanum concentrations are almost always observed. Modulation suppression when lead is added to a mixture leads to the same layered structure of samples, but with appreciable variations in lattice parameters in the ab plane of individual layers and a and b axis rotation by several degrees with respect to each other in these layers. Thus, the superlattice in BSLCO single crystals stabilizes their composition in the ab plane, and inevitable variations in growth conditions lead to the layered structure of such crystals. Original Russian Text ? V.P. Martovitsky, A. Krapf, 2008, published in Kratkie Soobshcheniya po Fizike, 2008, Vol. 35, No. 3, pp. 29–38.     

Transport characteristics of a single-layer graphene field-effect transistor grown on 4H-silicon carbide

We have investigated transport characteristics of epitaxial graphene grown on semi-insulating silicon-face 4H-silicon carbide (SiC) substrate by thermal decomposition method in relatively high N₂ pressure atmosphere. We have succeeded in forming 1–2 layers of graphene on SiC in controlled manner. The surface morphology of formed graphene was analyzed by atomic force microscopy (AFM), low-energy electron diffraction (LEED) and low-energy electron microscope (LEEM). We have confirmed single-layer graphene growth in average by this method. Top-gated, single-layer graphene field-effect transistors (FETs) were fabricated on epitaxial graphene grown on 4H-SiC. Increased on/off ratio of nearly 100 at low temperature and extremely small minimum conductance (0.018–0.3 in 4 e²/h) in gated Hall-bar samples suggest possible band-gap opening of single-layer epitaxial graphene grown on Si-face SiC.     

SiC polytypes process affected by Ge predeposition on Si(111) substrates

Structural and optical measurements were performed on silicon carbide (SiC) samples containing several polytypes. The SiC samples investigated were grown on (111) Si substrates by solid source molecular beam epitaxy (SSMBE). Several quantities of Ge were predeposited before the growth procedure. The influence of Ge on the SiC polytypes formation was studied by X-Ray, FTIR and μ-Raman characterizations methods. The spectra of the samples with less than one Ge monolayer exhibit a mixture of 2H, 15R and 3C–SiC polytypes. This mixture is due to the mismatch between the heterostructure layers. We propose that the Ge predeposition in the heterostructure can be used to stabilize and unify the polytypes formation.     

Physical properties of bulk single-crystal wafers of gallium nitride

A study has been performed of the crystalline structure and optical characteristics of single crystals of gallium nitride (GaN). The crystals were grown from a gallium-based flux. X-ray structural analysis showed that the crystals have wurzite structure 2H-GaN. From their luminescence characteristics and optical absorption spectra the crystals are similar to 2H-GaN epitaxial layers described in the literature. Fiz. Tverd. Tela (St. Petersburg) 39, 858–860 (May 1977)     

Spin dynamics in (( BEDO-TTF )6[ M (CN)6](H3O,CH3CN)2 ( M = Fe, Cr) crystals

Layered single crystals of the (BEDO-TTF)₆[M(CN)₆](H₃O,CH₃CN)₂ (M = Fe, Cr) compounds with alternating conducting layers of BEDO-TTF and [M(CN)₆](H₃O,CH₃CN)₂ are studied. The contributions to the magnetic susceptibility from charge carriers in BEDO-TTF layers and from the subsystem of localized magnetic moments of iron (or chromium) transition metal complexes are separated for both compounds under investigation. It is revealed that the crystals with [Fe(CN))₆]³⁻ anions at a temperature of ∼80 K and the crystals with [Cr(CN))₆]³⁻ anions at ∼30 K undergo magnetic transitions which are accompanied by drastic changes in the parameters of the EPR lines associated with the BEDO-TTF layers and the subsystem of localized spins of transition metal complexes. It is established that the presence of the BEDO-TTF layers in the structure affects the magnetic properties of iron and chromium hexacyanide complexes. Original Russian Text ? R.B. Morgunov, E.V. Kurganova, T.G. Prokhorova, E.B. Yagubskiĭ, S.V. Simonov, R.P. Shibaeva, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 4, pp. 657–663.     

Epitaxial combination of NdBa2Cu3O7−δ /SrTiO3: growth characteristics, structure, and parameters

We have studied the growth characteristics, structure, and parameters of the epitaxial heterostructures (001)NdBa₂Cu₃O_7−δ /(100)SrTiO₃/(001)NdBa₂Cu₃O_7−δ grown by laser ablation on a (100)LaAlO₃ substrate with a thin (∼2 nm) YBa₂Cu₃O_7−δ intermediate layer. The use of an YBa₂Cu₃O_7−δ intermediate layer promotes layered growth of the (200 nm) NdBa₂Cu₃O_7−δ layer, whose free-surface roughness is 4–5 nm. The resistance of the NdBa₂Cu₃O_7−δ layers began to fall off abruptly at T=92 K, and at T≈87 K it vanished completely. The critical current density in the NdBa₂Cu₃O_7−δ layers at T=76 K exceeded 10⁶ cm² A/cm². The dielectric constant of the (400 nm) SrTiO₃ layer sandwiched between the NdBa₂Cu₃O_7−δ epitaxial layers grew by roughly threefold as the temperature was lowered in the interval 300–4.2 K. When a bias voltage of ±2.5V was applied to the NdBa₂Cu₃O_7−δ electrodes, the relative dielectric constant of the (400 nm) SrTiO₃ intermediate layer fell from 1150 to 400 (T=32 K, f=100 kHz). The conductivity of the SrTiO₃ intermediate layer in the direction perpendicular to the substrate plane increased with temperature and the electric field strength. Fiz. Tverd. Tela (St. Petersburg) 41, 395–403 (March 1999)     

Study of YBa2Cu3O7−x films at various stages of their growth by medium-energy ion scattering

The initial stages in the formation and growth of yttrium-barium cuprate films have been studied in the course of magnetron sputtering of a ceramic target by combining medium-energy-ion scattering (MEIS) and scanning-electron microscopy. The growth mechanisms of YBa₂Cu₃O_7−x films on MgO and substrates having the perovskite structure, SrTiO₃ and LaAlO₃, at deposition temperatures of 700–780 °C and (Ar+O₂) pressure of ∼70 Pa were found to be essentially different. Simulation of MEIS spectra (H⁺ or He⁺ ions with initial energies of 150–250 keV) and comparison of these results with experimental data revealed that in the first case a film forms from pyramid-shaped islands and, in the second, it grows by a two-dimensional layer-by-layer process starting practically with the first monolayer. For the island mechanism, MEIS permitted determination of the substrate surface coverage and showed the growth of the YBa₂Cu₃O_7−x phase to be paralleled by formation of epitaxial nuclei of a Cu₂O phase. After the first, initial stage in the film formation, the second stage, regular growth within reduced thicknesses of 7–15 nm, sets in. This stage is characterized by a practically complete coverage of the substrate and a stable composition. The third stage, regular growth, of an apparently helical nature, was observed to set in at thicknesses above 100 nm. In this stage, the quality of film structure in the bulk and on the surface was found to be somewhat inferior to that of YBa₂Cu₃O_7−x single crystals and did not depend on substrate type. Fiz. Tverd. Tela (St. Petersburg) 41, 588–595 (April 1999)     

On new two-dimensional structures produced on the Si (111) and Si (100) surface upon molecular-beam epitaxy of cobalt and silicon

Highly perfect epitaxial heterostructure CoSi₂ films have been grown on the surface of Si (111) and Si (100) single crystals by the method of molecular-beam epitaxy. The optimal regimes of the film growth with different thicknesses have been determined. It has been shown that short-term annealing of epitaxial films at T = 900–950 K leads to the formation of new CoSi₂/Si(111)-2 × 2 and CoSi₂/Si(100)−2 × 4 superstructures.     

Formation of multilayer strained-layer heterostructures by liquid epitaxy. II. Simulation of the fabrication of heterostructures based on indium-arsenic-antimony-bismuth solid solutions

The formation of InAs_1−x−y Sb_xBi_y/InSb and InAs_1−x−y Sb_xBi_y/InSb_yBi_y strained-layer heterostructures by “capillary” LPE is simulated. The laws governing the dependence of the gap width E _g and the thickness d of the epilayers on the conditions of the process are revealed. It is shown that because of the sharp increase in the rate of epitaxial deposition as the LPE temperature is raised, the successful growth of epilayers of subcritical thickness is possible only up to T<550 K. The influence of the rate of laminar flow of the liquid in the growth channel in a relaxation regime and in a continuous pumping regime on the uniformity of the distribution of E _g and d in the epitaxial heterostructures is analyzed. Effective combinations of parameters for carrying out the process, which ensure the achievement of E _g ≈0.1 eV (77 K) in the active layers with variable-band-gap layers of minimal thickness, are established. Zh. Tekh. Fiz. 67, 50–56 (July 1997)     

Nonlinear-optical and structural properties of nanocrystalline silicon carbide films

The aim of this study is to investigate the nonlinearity of refraction in nanostructured silicon carbide films depending on their structural features (synthesis conditions for such films, substrate temperature during their deposition, concentration of the crystalline phase in the film, Si/C ratio of atomic concentrations in the film, and size of SiC nanocrystals formed in the film). The corresponding dependences are obtained, as well as the values of nonlinear-optical third-order susceptibility χ⁽³⁾(ω; ω, −ω, ω) for various silicon polytypes (3C, 21R, and 27R) which exceed the value of χ⁽³⁾ in bulk silicon carbide single crystals by four orders of magnitude.     

Transmission spectra of epitaxial layers of Pb1 ? xEuxTe (0 ≤ x ≤ 0.37) solid solutions in the frequency range 7–4000 cm?1

The spectra of epitaxial Pb_{1 − x} Eu _x Te (0 ≤ x ≤ 0.37) solid solution layers grown on BaF₂ and Si substrates have been investigated over a wide frequency range 7–4000 cm⁻¹ at temperatures of 5–300 K. Apart from the phonon and impurity absorption lines, the absorption in a local mode in PbEuTe layers of substrates and buffer layers has been observed in the frequency range 110–114 cm⁻¹. As the temperature decreases from 300 to 5 K, the transverse phonon mode softens from 33.0 to 19.5 cm⁻¹.     

Diffusion interaction between Al and Mg controlled by a high magnetic field

The effects of an 11.5 T high magnetic field on the growth behavior of compounds layers during reactive diffusion in solid Al/solid Mg diffusion couples have been investigated. After annealing with and without a magnetic field, the interfacial zone was still composed of two layers of stable compounds β (Mg₂Al₃) and γ (Mg₁₇Al₁₂), but the thicknesses of the layers were increased by the magnetic field. Data analysis of the thicknesses of compound layers showed that the growth of the γ layer was controlled by grain boundary diffusion as well as volume diffusion at B=0 T, but only controlled by volume diffusion when an 11.5 T high magnetic field was applied. The interdiffusion coefficients for the samples annealed at 643–693 K were calculated from the parabolic relationship between the migration of the interface and the annealing time. The results show that the diffusion constant increased due to the high magnetic field, increasing the chemical potential gradient, and in turn it caused the interdiffusion coefficient to increase.     

Evolution of dislocation structure during deformation of γ-irradiated LiF crystals

The effect of γ irradiation on the mechanical characteristics and dislocation structure of slip bands in LiF crystals is studied at doses D⩽7.3×10⁸ R. Irradiation causes a substantial increase (up to a factor of 30) in the yield stress τ _y of the crystals, with τ _y ∝ D ^0.4 in the first approximation. The deformation shear increases in the slip bands of irradiated crystals, as do the densities of the screw and edge dislocation components, while the dislocation mean free paths decrease. Irradiation also raises the probability of twinning cross slip for screw dislocations. The observed effects are assumed to be related to the formation of a different kind of defects in the irradiated crystals, primarily clusters of implanted atoms. Fiz. Tverd. Tela (St. Petersburg) 39, 1072–1075 (June 1997)     

Physical properties of n-CdGeAs2 single crystals prepared by low-temperature crystallization

The first results obtained in studies of the temperature dependences of electrical conductivity and Hall constant of n-CdGeAs₂ single crystals prepared by low-temperature crystallization are reported. It has been established that the method developed permits growing single crystals with a free-electron concentration ⋍(1−2)×10¹⁸ cm⁻³ and a Hall mobility ⋍10000 cm²/(Vs) at T=300 K. It is shown that the temperature dependence of Hall mobility exhibits a behavior characteristic of electron scattering by lattice vibrations, whereas below 150 K a deviation from this law is observed to occur evidencing an increasing contribution of static lattice defects to scattering. The Hall mobility in the crystals prepared was found to reach ⋍36000 cm²/(Vs) at 77 K. Photosensitive heterojunctions based on n-CdGeAs₂ single crystals were prepared. The spectral response of the photosensitivity of these structures is analyzed. It is concluded that this method is promising for preparation of perfect CdGeAs₂ crystals. Fiz. Tverd. Tela (St. Petersburg) 41, 1190–1193 (July 1999)     

Long SiC nanowires synthesized from off-gases of the polycarbosilane-derived SiC preparation

In this communication, we report on the attempt to make full use of the off-gases from polycarbosilane-derived SiC preparation, and we successfully synthesized long SiC nanowires in large areas with Fe(NO₃)₃ as catalyst. The nanowires have diameters of about 80–300 nm and lengths of millimeters, and they are identified as single crystals β-SiC along the 〈111〉 direction. The VLS mechanism was employed to interpret the nanowire growth.     

Growth of periodic Hg1−x MnxTe structures by liquid-phase epitaxy

A procedure for the calculation of phase equilibria in the tellurium corner of the phase diagram of the ternary system Mn-Hg-Te is described within the framework of the model of regular associated solutions. The initial concentrations of the nonstoichiometric fluxed melts and the temperature regimes of their cooling for the controlled liquid-phase epitaxial growth of inhomogeneous Hg_1−x Mn_xTe structures with prescribed modulation of the composition along their thickness are determined by numerical solution of a nonlinear inverse Stefan problem. Zh. Tekh. Fiz. 67, 64–67 (September 1997)     

Photoluminescence and multiphonon resonant Raman scattering in Ni-and Co-doped Zn1−x MnxTe crystals

Low-temperature photoluminescence, exciton reflection, and multiphonon resonant Raman scattering spectra of Ni-and Co-doped Zn_1−x Mn_xTe crystals were investigated. Intense emission occurs in a broad spectral region (1100–17 000 cm⁻¹) in the crystals containing Ni atoms. It is caused by intracenter transitions involving Mn²⁺ ions and transitions between the conduction band and a level of the doubly charged acceptor. The features of the exciton photoluminescence and multiphonon resonant Raman scattering involving longitudinal-optical (LO) phonons at various temperatures are investigated. The insignificant efficiency of the localization of excitons on potential fluctuations in the Zn_1−x Mn_xTe:Co crystals is established. A temperature-induced increase in the intensity of the 5LO multiphonon resonant Raman scattering line due to the approach of the conditions for resonance between this line and the ground exciton state is observed in these crystals. Fiz. Tverd. Tela (St. Petersburg) 40, 616–621 (April 1998)     

Electron paramagnetic resonance of scandium in silicon carbide

The EPR spectra of scandium acceptors and Sc²⁺(3d) ions are observed in 6H-SiC crystals containing a scandium impurity. The EPR spectra of scandium acceptors are characterized by comparatively small hyperfine interaction constants, whose values are consistent with the constants for other group III elements in SiC: boron, aluminum, and gallium acceptors. The EPR spectra of scandium acceptors undergo major changes in the temperature interval 20–30 K. In the low-temperature phase the EPR spectra are characterized by orthorhombic symmetry, whereas the high-temperature phase has higher axial symmetry. The EPR spectra observed at temperatures above 35 K and ascribed by the authors to Sc²⁺(3d) ions, or to the A ²⁻ state of scandium, have significantly larger hyperfine structure constants and narrower lines in comparison with the EPR spectra of scandium acceptors. The parameters of these EPR spectra are close to those of Sc²⁺(3d) in ionic crystals and ZnS, whereas the parameters of the EPR spectra of scandium acceptors correspond more closely to the parameters of holes localized at group III atoms, in particular, at scandium atoms in GeO₂. It is concluded that in all centers the scandium atoms occupy silicon sites. Fiz. Tverd. Tela (St. Petersburg) 39, 52–57 (January 1997)