Surface preparation of AB semiconductors (Cd3As2, Zn3As2, Cd3P2, Zn3P2) under ultra-high vacuum conditions

The UHV surface preparation of AB materials (crystals and thin films) has been monitored with XPS and AES. Clean and stoichiometric surfaces of AB crystals were prepared by means of low energy ion bombardment and subsequent low temperature annealing. Stoichiometric Cd₃As₂ and Zn₃P₂ thin films with very low amounts of C and O were deposited by the evaporation of bulk material in the UHV. The quality of prepared AB crystal and thin film surfaces was sufficient to carry out density of states investigations (UPS, RELS) with success.     

Thermal expansion anisotropy and individual bond expansion coefficients in ternary chalcopyrite compounds

The individual bond expansion coefficients of the A^IIB^IVC and A^IB^IIIC₂^VI chalcopyrite compounds are calculated from the principal linear thermal expansion coefficients of the lattice parameters using the regular B C tetrahedron model and a model with a temperature independent free parameter of the lattice. It is shown that the bond expansion coefficients derived from the latter model are in better agreement with the trends found for the interatomic forces in the chalcopyrite compounds and observed for the thermal expansion coefficients in the binary AC, A^IIC and BC compounds.     

Dependence of the Electric Properties and the Positron Lifetimes on the Dopant Content in La-doped SrTiO3

The resistivity, the dielectric constant, and the positron lifetimes in La-doped SrTiO₃ have been measured for La content x of 0—10 at.%. It was found that with increasing x, the variations of the resistivity and the dielectric constant and the positron lifetime parameters are nonmonotonic. The positron experiments have shown that the La-doping induces mainly formation of Sr vacancies and variation of Sr vacancy configuration; the most probable configuration is the isolated Sr vacancies (V) as x < 0.5 at.%, the associated defects (La V) for 0.5 < x < 1 at.%, and the associated defects (2 La V) above x = 1 at.%. The results suggest that the variation of the resistivity can be regarded as variation of electron density, and the variation of the dielectric constant results mainly from variations of the space-charge polarizaion and Sr-vacancy concentration and configuration.     

The influence of the direct quench and indirect quench to Ta > Th on the decomposition behaviour of an Al-2 at.% Zn-1 at.% Mg alloy

The decomposition behaviour of an Al-2.0 at.% Zn-1.0 at.% Mg alloy after direct quench (DQ) and indirect quench (IQ:T = 23 °C, t) to the ageing temperature T = 160 °C (above the temperature of the rapid homogeneous nucleation, T_h) was investigated by means of TEM and isothermal resistivity measurements. The precipitation density of the η-particles yields a maximum value at pre-ageing times about 10 min at T = 23 °C. To clear up the reason for that phenomenon a loop-formation and clustering model was introduced.     

On the mechanism of chemical polishing of GaAs crystals

In order to establish reproducible conditions for chemical lap polishing of GaAs with NaOCl solutions, containing also OH⁻- and CO -ions, the mechanism of dissolution was investigated using the rotating disk arrangement. In solutions with excess OCl⁻ concentrations likewise diffusion and a reaction of the first order with respect to OH⁻ and CO, complexing agents for Ga-ions, control the dissolution rate, and the surfaces of the wafers are polished. OH⁻ and CO₃^2– concentrations in excess with respect to OCl⁻ lead to rate determining OCl⁻ diffusion and to irregularly polished or film covered surfaces. Diffusion constants for OH⁻, CO and OCl⁻ are given.     

Absorption and ionic conductivity of oxygen-containing SrF2 crystals

An absorption band at 3644 cm⁻¹ is caused by isolated OH⁻ ions. O⁻ ions cause an absorption band at 213 nm the oscillator strength of which is 0.020. Charge-compensation of O⁻ ions is effected by F⁻ ion vacancies (F). As for CaF₂ crystals there occur monomers and dimers of [O⁻ – F] complexes. The mass action constants of association of F with oxygen centres are K_AD = ⅓ exp (4.28 – 0.82 eV/kT) for [O⁻ – F], K_AT = 4 exp (17.4 – 1.25 eV/kT) for [2 O⁻ – F] and K_AQ = exp (4.2 – 0.89 eV/kT) for [2 O⁻ – 2 F].     

Interface Kinetical Limitations in Closed-Tube Chemical Vapour Transport (I)

A semiempirical method is proposed, by which the role of kinetical constraints to the growth in closed tube chemical vapour transport can be evidenced. The method is based on the evaluation of the ratio (Φ) between the quantity D which is related to the semiempirically-determined average diffusivity of the gaseous species during the growth process, and the quantity D, which is related to the maximum average diffusivity of the gaseous species as is predicted, through the choice of a suitable diffusional model, by the equilibrium thermodynamics and diffusion properties of the vapour phase.     

Some correlations in liquid phase growth (I)

In this work we consider some circumstances of the growth process of an n-component solid phase from an n-component liquid phase and provide some general relationships, which must be observed in this growth process. In more detail we treat the two extreme cases: growth when (∂C/∂t = 0) — the stationary case; growth when (∂C/∂x = 0) — no concentration gradients in the liquid phase. In our opinion these theoretical considerations will simplify the investigations of multicomponent systems.     

Trends in the microhardness of the CuBIIIC chalcopyrite compounds

On the basis of a microhardness scaling law found for the A^IIB^IVC compounds and of the dependence of the microhardness on conductivity type and composition observed experimentally in CuInSe₂ a model is proposed to describe the trends in the microhardness of the CuB^IIIC compounds. The results are compared with experimental data and other microhardness estimates. It is found that most of the existing microhardness measurements can be interpreted within the model.     

Hyperfine structure of Cr5+ EPR spectra in X-irradiated potassium halide crystals doped with CrO and Ca2+

The hyperfine structure has been investigated in the EPR spectra of X-irradiated KCl: CrO, Ca²⁺ and KBr: CrO, Ca²⁺ crystals. Preliminarily spectra were simplified by means of heating the crystals up to 400 K for KCl and 440 K for KBr, that destroys the less stable CrO · V centers. It is ascertained, that principal directions of g- and A-tensors do not coincide, and principal A-values are determined. Calculations, making use of the experimental meanings of g- and A-tensor components, showed, that the degree of distortion of oxygen tetrahedra in CrO · Ca²⁺ · V centers is almost the same in both KCl and KBr crystals; besides, these calculations suggest the existence of a strong covalent bonding between the central Cr⁵⁺ ion and four oxygen ligands in CrO ions.     

Stability Determinant of Anisotropic Phases

The thermodynamic stability of crystal determinant D) and nonstability D_{n st}⁽¹⁴) of the fourteenth order, their adiabatic and corresponding isodynamical coefficients are discussed.     

On the structure of the rhombohedral α′R-phase in Al Zn alloys

The lattice parameters α and α of precipitates of the rhombohedral α′_R-phase grown in two Al Zn alloys with 10 and 12 at.% Zn, respectively, at elevated temperatures were determined by transmission electron microscopy (TEM). Two approximative methods were derived to evaluate the Moiré fringes occuring in the TEM micrographs. The results obtained by these two methods agree within the range of experimental error with those presented in the literature (mainly gained from X-ray methods).     

Density and mobility of anti-frenkel defects in SrF2 crystals

By fitting the theoretically calculated temperature-dependent conductivity σ to the measured dependencies log σT ÷ 1/T the following parameters have been determined: free formation enthalpy of anti-Frenkel defects g_AF = 2.05 eV – 6.35 kT; mobilities of F⁻ion vacancies F and interstitials F: v_νT = 600 exp (-0.70 eV/kT) cm² K/Vs, v_iT = 1.1 · 10⁴ exp (-0.93 eV/kT) cm² K/Vs. — The free association enthalpies of complexes consisting of single foreign ions (Sc³⁺, Y³⁺, La³⁺, Sm³⁺, Li⁺, K⁺, Na⁺, O⁻) and the charge-compensating defect were obtained. The vibration frequency of F⁻ ions in the neighbourhood of F and F is changed by a factor of 2.6 and 0.6, respectively.     

Incorporation concentration dependence of phosphorus in the deposition of silicon epitaxial layers from silane-phosphine-hydrogen mixtures

The empirical incorporation characteristic N with the phosphine input pressure (P) as the controlling variable and the phosphorus incorporation concentration ((14 < lg N<18)) as the controlled variable is theoretically explained by comparing the thermodynamic equilibrium pressures of the various phosphorous hydrides and the various molecular species of the phosphorus vapour in the Si P H system in their respective incorporation equilibria with regard to the phosphorus incorporation concentration. In the lower incorporation concentration range (lgN > 18) there is an almost total agreement of the values of the phosphine equilibrium pressure (PPH_j; j = 2; 3) and the phosphine input pressure. In the upper concentration range (lg N > 18) there is a close approximation of the equilibrium partial pressure of the dimeric phosphorous vapour (PP₂) to the phosphine input pressure. There is no marked effect of the conduction electron density in silicon at the deposition temperatures considered on the incorporation characteristic.     

Sputter depth profiling of GaAlAs double heterostructures using auger electron spectroscopy

Sputter depth profiles of GaAlAs double heterostructures had been taken by the help of Auger electron spectroscopy (AES). The sputtering ion beam consisted of either N⁺ — or Ar⁺ ions. In the case of using N⁺ ions an essentially better depth resolution was found comparing with Ar⁺ ions, especially in a sputter depth of some μm. Investigations of the sputter—crater using Scanning Electron Microscopy (SEM) and Electron Probe Micro Analysis (EPMA) showed, that the improved depth resolution during N ion sputtering is a result of the essential lowered sputter induced surface roughness. This should be caused by the stronger amorphisation of the target surface using N ions which is a consequence of the chemical reactions taking place at the surface with the sputtering ions.     

Experimental investigations of buoyancy-driven convection in vertical (BixSb1−x)2Te3 molten zones

Experimental results for various states of buoyancy driven flow in vertical (Bi_0.23Sb_0.75)₂Te₃ molten zones with covered surface are presented. Critical thermal wall Rayleigh numbers Ra for the onset of time-dependent convection have been determined by means of temperature measurements. The stability diagram obtained for the existing buoyancy driven convection shows the increase of Ra with increasing aspect ratio. This relation is also known from other crystal growth configurations and is due to the damping influence of container walls. At the beginning in the oscillatory region of convection extremely long periods of oscillation (maximum 850 s) were observed, which are caused by another mechanism than periods (25 … 37 s) registered at increasing melt heights. Furthermore, Bi_0.5Sb_1.5Te₃ crystals were grown by using the vertical zone melting technique. The microscopic striations observed in the grown crystals correlate exactly with the temperature signals caused by time dependent convection. However, the fluctuations of the tellurium distribution in axial direction measured by scanning the Seebeck coefficient are presumably generated by unsteady solutal convection during growth.     

Temperature dependence of doping element incorporation with the chemical vapour deposition of epitaxial silicon (IV). Incorporation of phosphorus in the Si-H-Cl-lP system

The lower temperature dependence of the phosphorus incorporation with the use of phosphine together with chlorosilanes (ΔH = −13 to −15 kcal/ mole) instead of silane (ΔH = −43 kcal/mole) is explained by introducing a special incorporation equilibrium of phosphine bound to the silicon surface. The source materials phosphorus trichloride and phosphorus pentachloride may be incorporated with this equilibrium.     

Strukturelle Umklapp-Vorgänge in metastabilen β1-Kupfer-Zink-Mischkristallen bei tiefen und höheren Temperaturen (I). Gitterdynamische Skizze1)

The quantity of the shear-modulus G′ = (c₁₁ – c₁₂)/2 is a measure for the probability of structural Umklapp-processes to occur in metastable β₁-Cu-Zn-solid solutions. Such processes take place, if G′ = G′(T, x) would fall below a critical limit, G′_crit, e.g. by lowering of temperature T or/and Zn-concentration x, because the Fermi-contribution to G′will sink in the 1^st and with him additional the Coulomb-contribution in te 2^nd case. Both ones are the authoritative stabilizing factors for β₁ and therefore specific fo its lattice-dynamical behaviour, especially in the longwavy range of thermoacoustic lattice-vibrations, by which the cooperative Umklapp-motion will escape. Umklapping can also be initiated by favourably oriented dislocations, is G′ would approach to G′_crit in their neighbourhood: G′_critG′(xM8), with xM8  Umklapp-concentration. This is possible by variation of x during isothermal reactions, in the course of whicht he β₁-matrix will heterogenize itself into Zn-poorer and -richer districts β and β (pre-diffusion by means of quenchy vacancies). Both kinds of districts, among which the latter ones will enhance their degree of stability, are joint together coherently. They build up a so called β /β -parquet. The parquet-bricks can reach a critical size, which is necessary that sufficient large atom-collectives can simultaneously be caused to an Umklapp-motion and to occupate new equilibrium positions by thermo-acoustic shear-waves (comparison with a sin-wave beeing changed to a zigzag-line). Only at higher temperatures the bricks come up to the critical size. Umklapping comes about only in the β -bricks, which turn by it to a transition lattice (β₂) with a structure similary to that of the lowtemperature-martensite β′′. After that β₂ changes to α-phase. The way β ⇒ α is marked by the following steps: prediffusion, Umklapping + dislocation, leading to β₂, and a further dislocation dissoziation, leading the atoms to the equilibrium positions in the α-lattice. The so stepped mechanism acts an nucleationmechanism of the α-phase. After the nucleation the α-nuclei grow at the cost of too much formed β (postdiffusion). By isothermal reactions at too low temperatures a mini-herterogenized state of the β₁-matrix will be caused comparable with coldhardening states of other alloys (Guinier-Preston-zones). A β₁-matrix in such a state is unable to isothermal Umklapping, so that α-crystals can be formed – provided that the mini-heterogenities are resolved by increasing of the reaction temperature.     

The Structure of Metastable Phases in Al?Zn Alloys (II). HREM Investigations on α-Precipitates

Precipitates of the metastable -phase grown in an Al-(6.8 at.%)Zn alloy are investigated by means of a technique of high resolution electron microscopy, namely by two-beam lattice fringe imaging with tilted illumination. It is demonstrated by this direct method that the -precipitates are in fact semicoherent. The transition of fully coherent Guinier-Preston zones into semicoherent precipitates of the -phase takes place by formation of additional (III)^rh and { III }^rh planes inside the zones.     

Chrial Smectic Side-chain Copolymers and their Properties

Copolymers have been synthesized in which chiral monomer units are combined with those of a tilted smetic homopolymer. The copolymers with the proportion of chiral units not exceeding 37% form a S phase and have therefore ferroelectric properties. Temperature dependences of the pyroelectric coefficient, spontaneous polarization and relaxation time have been mesured for a series of ferroelectric LC copolymers.