Progress in the growth and research of crystals for wide-gap semiconducting materials

The feasibility of a sublimation sandwich method for controlled growth of single crystals and epitaxial layers of different SiC and GaN polytypes is demonstrated. The controlled production of pure (n _i <10¹⁶ cm⁻³) and heavily-doped crystals and epitaxial layers of these materials has made it possible to study their semiconducting parameters in detail and to identify the nature of a number of the most important impurity centers. It is shown for the example of SiC that the typically high chemical-binding energy of atoms in these compounds is the reason for the formation of stable metastable compounds, among them associations and clusters that include intrinsic defects which have a significant effect on the properties of the material. Clusters formed on the surface can serve as seeds for different polytypes during crystal growth. Fiz. Tverd. Tela (St. Petersburg) 41, 822–825 (May 1999)     

属特殊结构系列的若干碳化硅多型体的晶体结构

碳化硅(SiC)是典型的层状结构化合物。到目前为止,已发现了150种以上的多型体。作者曾发展了一种特殊的劳厄法可有效地鉴定碳化硅多型体。这种方法,我们发现了85种碳化硅新多型体。为了测定其中一些多型体的结构,拍摄了回摆和魏森堡照相作结构分析,但没有成功。因为这些照相中只有一些基本类型6H,15R和8H的衍射斑点而没有高层多型体的斑点,这是由于新多型体在晶体中含量很少且这些薄晶体多型体处于基本类型6H,8H,15R中间的缘故。本文提出了一种测定碳化硅多型体晶体结构的劳厄法。提出了计算劳厄斑点衍射强度的方法。对结构系列[(33)_m32]₃,[(33)_m34]₃,[(22)_m23]₃和[(44)_m43]₃多型体的结构因子F_hkl的计算方法作了简化。利用这种方法对9种碳化硅新多型体的晶体结构作了测定,其结构用z字形堆垛(Жданов符号)表示时为231R:[(33)₁₂32]₃,249R:[(33)₁₃32]₃,321R:[(33)₁₇32]₃, 339R:[(33)₁₈32]₃,237R:[(33)₁₂34]₃,417R:[(33)₂₂34]₃, 453R:[(33)₂₄34]₃,93R:[(44)₃43]₃,261R:[(44)₁₀43]₃。 关键词：     

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.     

Wannier-Stark resonances under strong localization conditions in natural silicon-carbide superlattices

A detailed experimental investigation is made of the electronic transport under conditions of Wannier-Stark localization of carriers in a natural superlattice of hexagonal polytypes of silicon carbide. The 4H and 6H polytypes, which possess different superlattice and miniband spectrum parameters, are employed. Direct measurements of the electronic current versus the average electric field in the active region of the sample revealed a series of regions of negative differential conductivity in fields ranging from 500 to 2100 kV/cm. Analysis of the results shows that the observed current resonances are associated with the development of the Wannier-Stark quantization process and are due to conduction mechanisms such as hopping conduction, induced between the levels of a Wannier-Stark ladder by a resonant electron-phonon interaction, and the resonant interminiband tunneling from the first into the second miniband. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 2, 105–109 (25 July 1996)     

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.     

Radiospectroscopy of wide-gap semiconductors: SiC and GaN

The present report submitted to the Anniversary Conference of the A. F. Ioffe Physicotechnical Institute, “Physics at the Turn of the 21st Century,” deals with recent EPR studies of main impurities in the wide-gap semiconductors SiC and GaN, which appear to be the most promising materials for microelectronics and quantum semiconductor electronics at the start of the 21st century. Fiz. Tverd. Tela (St. Petersburg) 41, 789–793 (May 1999)     

Pyroelectric properties of β-BaB2O4 in the temperature range from 2.5 to 90 K

Results are presented of experimental studies of how the spontaneous polarization of β-BaB₂O₄ (BBO) varies with temperature in the range from 2.5 to 90 K. Values of the pyroelectric coefficient γ _s are calculated, and effective values of the Debye temperature (Θ=112 K) and the Einstein temperature (Θ_E=28 K) are estimated. It is shown that the dominant contribution to γ _s comes from acoustic lattice modes. A mechanism is proposed to explain why the temperature dependence of the pyroelectric effect in linear pyroelectric materials is different from that in ferroelectrics. Fiz. Tverd. Tela (St. Petersburg) 39, 1631–1633 (September 1997)     

Fermionic string from Abelian Higgs model with monopoles and Θ-term

The four-dimensional Abelian Higgs model with monopoles and Θ-term is considered in the limit of large mass of the Higgs boson. We show that for Θ=2π the theory is equivalent, at large distances, to summation over all possible world-sheets of fermionic strings with Dirichlet-type boundary conditions on the string coordinates. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 2, 76–80 (25 July 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Electronic structure and local interactions on a S(100) 2×1 surface with submonolayer Ba overlayers

The electronic structure and ionization energy for the system Ba/Si(100)2×1 have been studied as functions of the submonolayer coverage. It is found that there is an energy gap in the surface states spectrum and that the Ba/Si(100)2×1 interface is semiconducting up to 1.5 monolayers of Ba. Two surface bands induced by Ba adsorption have been detected. The evolution of the spectrum with increasing degree of Ba coverage points to the existence of two nonequivalent “adsorption sites,” which differ in binding energy by 0.11 eV. The development of the Ba-induced bands is found to terminate at a coverage corresponding to the minimum ionization energy and close to one monolayer. The adsorption bond is shown to have a primarily covalent character. Zh. éksp. Teor. Fiz. 114, 2145–2152 (December 1998)     

Strong and weak modes in polytypes of SiC

The concept of an extended Brillouin zone is used to analyze the intensity of phonon modes observed in optical and Raman investigations of different polytypes of silicon carbide. It is shown that the relative intensity of these modes agrees with the magnitude of the splitting of the doublets, which are degenerate in the extended zone. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 3, 247–250 (10 February 1999)     

Photosensitivity of GaAs: NGaP: N/GaAs (GaP) heterojunctions in linearly polarized radiation

Results are presented of investigations of the photoelectric properties of nitrided layer/GaAs (GaP) heterojunctions prepared by plasma treatment of GaAs and GaP crystals in the presence of nitrogen ions. The heterojunctions exhibited broad-band photosensitivity relative to the intensity of the natural radiation. It was established that when linearly polarized radiation is obliquely incident on the surface of nitrided layers, polarization photosensitivity occurs which is controlled by the angle of incidence Θ and increases proportionately as Θ². The spectral dependences of the induced photopleochroism are attributed to the antireflecting properties of the wide-gap layers. Nitrided-layer heterojunctions can be used as broad-band photoanalyzers for linearly polarized radiation. Zh. Tekh. Fiz. 69, 138–142 (June 1999)     

Effect of thermal expansion on the elastic moduli and Debye temperature of paramagnetic lutetium

The temperature dependence of the Young’s modulus E of paramagnetic lutetium has been studied. It has been shown that an important reason for the dependence E(T) is thermal expansion of the crystal lattice, which also leads to a change in the Debye temperature Θ. The effect of this factor is also revealed in the thermodynamic properties of metals. In particular, we have shown that there is another contribution to variation of the total specific heat of lutetium, associated with the Θ(T) dependence and comparable with the electronic contribution. Fiz. Tverd. Tela (St. Petersburg) 40, 1581–1584 (September 1998)     

Calculation of the ranges of slow, heavy ions in an amorphous medium

The low-energy range corresponding to reduced energies ɛ⩽0.1 is investigated. A method is developed for calculating the ranges of heavy ions in an amorphous medium. Analytical expressions for calculating the projective range of ions and the standard deviation of the projective ranges are obtained. The computational results are in good agreement with experiment. Zh. Tekh. Fiz. 67, 16–20 (October 1997)     

Electron transport properties of MgB<Subscript>2</Subscript> in the normal state

We report measurements of the resistivity, ρ, and the Seebeck coefficient, S , of a MgB₂ sintered sample, and compare S with theoretical calculations based on precise electronic structure calculations. ρ is fitted well by a generalized Bloch-Grüneisen equation with a Debye temperature Θ _R of 1050 K. S is given by the sum of a diffusive and a phonon drag term and the behavior in the temperature region T _c < T < 0.1Θ _R follows the relationship AT+BT³. The phonon drag term indicates a strong electron-phonon interaction. The diffusive term, compared with calculations, suggests that σ bands give the main contribution to the Seebeck effect. Received 16 November 2001 and Received in final form 21 December 2001     

Surface saturation control on the formation of wurtzite polytypes in zinc blende SiC nanofilms grown on Si-（100） substrates

We investigate the formations of wurtzite （WZ） SiC nano polytypes in zinc blende （ZB） SiC nanofilms hetero-grown on Si-（100） substrates via low pressure chemical vapor deposition （LPCVD） by adjusting the Si/C ratio of the introduced precursors. Through SEM, TEM, and Raman characterizations, we find that the nanofilms consist of discrete WZ SiC nano polytypes and ZB SiC polytypes composed of WZ polytypes （WZ ＋ ZB） and disordered ZB SiC polytypes, respectively, according to Si/C ratios of 0.5, 1.5, and 3. We attribute the WZ polytype formation to being due to a kinetic mechanism based on the Si/C surface saturation control.     

A theory of the origin of silicon carbide polytypes

A new theory of the origin of SiC polytypes is presented in which the observed polytypes are stabilised as equilibrium phases at the high temperatures of growth, the structures becoming frozen in on cooling. Results of first principles pseudopotential total energy calculations show that the SiC polytypes containing only bands of width 2 and 3 are nearly degenerate in energy while all other polytypes have higher energies. We develop an inter-layer interaction model to describe the energies of the polytypes. Calculations of the phonon free energies of several SiC polytypes using the valence overlap shell model are described. These calculations show that the phonon free energy gives rise to effectively long ranged interactions between the SiC layers which can stabilise a large number of ordered polytypes as equilibrium phases.     

The case for the cosmological constant

I present a short overview of current observational results and theoretical models for a cosmological constant. The main motivation for invoking a small cosmological constant (or A-term) at the present epoch has to do with observations of high redshift Type Ia supernovae which suggest an accelerating universe. A flat accelerating universe is strongly favoured by combining supernovae observations with observations of CMB anisotropies on degree scales which give the ‘best-fit’ values Θ_A ⋍ 0.7 and Θ _m ⋍ 0.3. A time dependent cosmological A-term can be generated by scalar field models with exponential and power law potentials. Some of these models can alleviate the ‘fine tuning’ problem which faces the cosmological constant.     

Effect of impurities on thermoelectric power due to phonon drag in metals

The effect of inelastic impurity scattering of electrons on the thermoelectric power due to phonon drag in metals has been studied. It is shown that this is the main cause of the thermoelectric power suppression due to doping at low temperatures. The thermoelectric power in a metal with a quadratic electron spectrum has been calculated as a function of temperature and impurity concentration. In addition to the impurity concentration, the correction to the thermoelectric power due to inelastic scattering contains the large factor Θ_D/T. Zh. éksp. Teor. Fiz. 111, 2237–2242 (June 1997)     

Passive scalar in a large-scale velocity field

We consider advection of a passive scalar θ(t,r) by an incompressible large-scale turbulent flow. In the framework of the Kraichnan model all PDF’s (probability distribution functions) for the single-point statistics of θ and for the passive scalar difference θ(r ₁)−θ(r ₂) (for separations r ₁−r ₂ lying in the convective interval) are found. Zh. éksp. Teor. Fiz. 115, 920–939 (March 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Exchange interactions and spin dynamics in a [CuNd2(C4O4)4(H2O)16]·2H2O crystal

Crystalline [CuNd₂(C₄O₄)₄(H₂O)₁₆]·2H₂O constructed of complexes of trivalent neodymium and divalent copper, has been synthesized and studied by EPR. The square anion groups (C₄O₄) enter as bridge ligands, forming chains of neodymium ions interconnected by (C₄O₄)Cu(C₄O₄) fragments. It is found that the relaxation rate of the neodymium subsystem at room temperature significantly exceeds the exchange interaction rate between copper and neodymium ions. Under these conditions the magnetic properties of the crystal are determined by two magnetically nonequivalent chains of copper ions, which do not interact. The intrachain exchange interaction via hydrogen bonds is estimated to be ∼0.1 cm⁻¹. As one proceeds from the high-temperature (250<T<300 K) to the low-temperature region (T<40 K), a substantial change in the nature of the interaction is revealed. An unusual magnetic structure given in a crystal is observed at low temperatures, which is determined by the presence of two magnetically nonequivalent “ribbons,” formed by the interacting copper and neodymium ions: chains of copper ions are framed on two sides by chains of neodymium ions. The magnitude of the parameter of the exchange interaction between the copper and neodymium ions is estimated as J ^Cu-Nd⩾0.2 cm⁻¹. An exchange interaction between magnetically nonequivalent neodymium ions is not revealed in the EPR spectra. Fiz. Tverd. Tela (St. Petersburg) 39, 2057–2061 (November 1997)