Showing papers in "Applied Physics A in 1983"

Transition metals in silicon

Abstract: A review is given on the diffusion, solubility and electrical activity of 3d transition metals in silicon. Transition elements (especially, Cr, Mn, Fe, Co, Ni, and Cu) diffuse interstitially and stay in the interstitial site in thermal equilibrium at the diffusion temperature. The parameters of the liquidus curves are identical for the Si:Ti — Si:Ni melts, indicating comparable silicon-metal interaction for all these elements. Only Cr, Mn, and Fe could be identified in undisturbed interstitial sites after quenching, the others precipitated or formed complexes. The 3d elements can be divided into two groups according to the respective enthalpy of formation of the solid solution. The distinction can arise from different charge states of these impurities at the diffusion temperature. For the interstitial 3d atoms remaining after quenching, reliable energy levels are established from the literature and compared with recent calculations.

Dynamics of film growth of GaAs by MBE from Rheed observations

Abstract: Detailed observations have been made of the intensity oscillations in the specularly reflected and various diffracted beams in the RHEED pattern during MBE growth of GaAs, Ga x Al1−x As and Ge. The results indicate that growth occurs predominantly in a two-dimensional layer-by-layer mode, but there is some roughening, which is enhanced by deviations from stoichiometry and the presence of impurities. In the case of the GaAs (001) −2×4 reconstructed surface a combination of dynamic and static RHEED measurements has provided firm evidence for the presence of one-dimensional disorder features as well as surface steps.

Photothermal displacement spectroscopy: An optical probe for solids and surfaces

Abstract: We present a sensitive technique for determining the optical and thermal properties of solids, surfaces and thin films. This technique, photothermal displacement Spectroscopy, is based on the detection of the thermal expansion of a sample upon absorption of electromagnetic radiation. The technique is well suited for in situ ultrahigh vacuum studies and for experiments where wide temperature ranges are required. We show that surface and bulk optical absorption can be distinguished and that surface absorptions of αL=10−6/W of incident power can be measured. The theoretical basis of the signal generation is given, and excellent experimental and theoretical agreement is demonstrated. The implications of our findings to imaging and microscopy are discussed.

Monte-Carlo calculations of keV electron and positron slowing down in solids

Abstract: A Monte-Carlo simulation technique based on the screened Rutherford differential cross section for the elastic scattering and Gryzinski's semiempirical expression for the inelastic core and valence electron excitation is used to describe electrons and positrons slowing down in solids. The theoretical results are compared with the experimental backscattering, absorption and transmission results for aluminum, silicon, copper, and gold thin film and semi-infinite targets and good agreement is observed. The simulated stopping profiles are fitted with a simple analytic expression. The profiles are Laplace-transformed to give a useful data base for analyzing phenomena associated with slow positron re-emission from solids.

The activation of FeTi for hydrogen absorption

Abstract: FeTi is an interesting hydrogen storage material which has to be activated at ≈670 K for the absorption of hydrogen. We review critically the great number of previously published results and models on this activation process and emphasize the controversial points. To eliminate the controversy we analysed the variation of the surface composition of FeTi upon activation in the high-pressure cell of a photoelectron spectrometer. The initially passivating surface oxide is shown to be converted into a mixture of TiO2 and Fe by surface segregation and chemical reduction. No evidence for the formation of Fe2Ti4O x , FeTiO x , and TiH x is found. H2/D2 exchange reactions show that H2 dissociates rapidly on Fe and FeTi, but not on TiO2. The surface of FeTi is activated easily at 670 K. Difficulties encountered with the initial hydrogen absorption by virgin high purity FeTi are probably related to bulk (H diffusion, fracture toughness) rather than surface properties.

Chromium and Chromium-Boron Pairs in Silicon

Abstract: Four different measurement techniques: EPR, NAA, DLTS, and luminescence were applied to characterize the properties of chromium in silicon. The solubility of chromium in silicon was determined and its pairing reaction with boron was studied. The energy level atEc−0.23 eV was attributed to interstitial chromium and a second level atEv+0.27 eV was correlated to chromium-boron pairs. The luminescence band of the chromium-boron pairs was clearly identified. The properties of chromium are compared with those of other transition metal impurities in silicon crystals.

The characteristics and properties of optimised amorphous silicon field effect transistors

Abstract: A series of experiments aimed at improving the performance of amorphous silicon field effect transistors has been carried out. The dc and dynamic characteristics of the optimised devices are described. Stable devices capable of ON-currents of the order of 100 μA with OFF-currents ≃10−11 A can be fabricated which could, in principle, be used to address more than 1000 lines of a liquid crystal display. The properties of the highly conducting ON-state channel have also been studied. The field effect mobility, 0.3 cm2 V−1 s−1 at room temperature, has an activation energy of 0.1 eV at the higher gate voltages. The possible reasons for the improvement in performance over earlier devices are discussed.

Silicon doping of MBE-grown GaAs films

Abstract: Two concentration ranges of silicon doping in MBE-grown GaAs films have been investigated in some detail. In lightly doped films, with a free-electron concentration of ≈1016cm−3, low-temperature photoluminescence spectra have been analysed to develop a model to account for spectral features previously attributed to Ge and Si acceptor levels. In heavily doped films, a maximum free-electron concentration of ≈7×1018 cm−3 has been obtained, which is only rather weakly dependent on growth conditions and the nature of the arsenic species (As2 or As4). Transmission electron microscopy has shown that no significant precipitation effects occur when higher Si fluxes are used but there is evidence for autocompensation. The maximum PL intensity (300 K) is found at a lower free electron concentration then with Sn-doped films, and is more sharply peaked, but there is no evidence for an anomalous Moss-Burstein shift.

Improved slow-positron yield using a single crystal tungsten moderator

Abstract: A well-annealed W(110) single crystal was used as a fast-to-slow positron moderator. The measured moderator efficiency at room temperature using a58Co positron source in the backscattering geometry isɛ =(3.2±0.4)×10−3, roughly a factor of three better thanɛ for the best previously reported Cu(111)+S moderator. We find a stable positron moderation efficiency over a period of several weeks when maintained at pressures around 10−9 Torr and an energy spreadΔE = 0.7 eV of the emitted slow positrons. An initial attempt was made to fabricate a hybrid Cu on W(110) moderator, which yieldedɛ of about 1.2×10−3 after annealing.

Mechanism of ion beam induced mixing of layered solids

Abstract: A brief review of recent experiments on interface mixing by high-energy heavy-ion bombardment indicates that observed low-fluence mixing rates through interfaces are substantially larger than what can be accounted for by collisional mixing at low and high recoil energies. Diffusion and subsequent trapping of point defects within individual damaged zones is shown to lead to gaussian relocation profiles even at moderate bombarding-ion fluences. Estimated half widths are consistent with measured mixing rates. The depth and energy dependences of these half widths differ markedly from those predicted for low-energy cascade mixing.

Laser induced chemical vapor deposition of Ni by decomposition of Ni(CO)4

Abstract: Polycrystalline Ni has been grown by decomposition of Ni(CO)4 using different wavelengths of the visible radiation of a Kr+ laser. The influence of laser irradiance, substrate material and scanning velocity on deposition rate and widths of patterns has been investigated. The deposition rates achieved are typically several μm/s, and the lateral dimensions of the deposits can be as small as 1 μm.

The lanthanide β″ aluminas

Abstract: The entire sodium ion content of sodiumβ″ alumina (Na1.67Mg0.67Al10.33O17) can be replaced with a variety of lanthanide ions by simple diffusion reactions at moderate temperatures (500–700°C). Lanthanideβ″ alumina crystals are hard, clear, chemically stable, and have well-defined crystal structures. The fluorescence spectrum of Nd3+ inβ″ alumina is similar to that in YAG. The lifetime of the4 F 3/2 state of Nd3+ in completely-exchangedβ″ alumina (350μs at 1021 Nd3+ cm−3) is about 45% longer than in YAG (240μs at 1020Nd3+ cm−3). The lanthanideβ″ aluminas may be of considerable interest as new phosphor and laser host materials.

Shape effects in the interaction between an edge dislocation and an elliptical inhomogeneity

Abstract: The plane elasticity problem of the interaction between an edge dislocation and an elliptical inhomogeneity is solved, and the image glide-force on dislocation is computed. Contour plots of the force exterted by either an elliptic hole (crack) or a rigid elliptical inhomogeneity show that force is stronger for more elongated shapes, and that in some cases dislocation trapping effects undergo drastic changes even for slight shape variations. The general case is investigated by means of angular plots of force. They show increasing oscillatory angular depence on increasing both elongation and shear moduli difference.

Investigation of impurity levels in n-type indium selenide by means of Hall effect and deep level transient spectroscopy

Abstract: We report the results of Hall effect and deep-level-transient-spectroscopy measurements onn-type indium selenide samples cleaved from non-intentionally doped material and from chlorine, tin, gallium sulphide or oxygen doped ingots. Some unidentified shallow and deep levels are found. Impurity levels attributed to Cl (310 meV) and to Sn (44 and 120 meV) are also reported. Anomalous degeneracy of electrons at low temperature is discussed.

Observations of higher-order laser-induced surface ripples on 〈111〉 germanium

Abstract: We report the observation and analysis of higher-order optical diffraction patterns arising from linear combinations of the primary laser-induced gratings or ripples on germanium surfaces. These higher-order surface structures presumably arise from nonlinear interactions between superimposed primary gratings with different grating wave vectors. For gratings produced by normally incident laser beams on Ge 〈111〉 surfaces, the diffraction patterns exhibit a strongly hexagonal symmetry.

On the nature of point defects and the effect of oxidation on substitutional dopant diffusion in silicon

Abstract: An extensive analysis of the substitutional dopant diffusion phenomena in silicon during oxidation is presented. The analysis covers qualitative as well as quantitative aspects of the oxidation-enhanced and -retarded diffusion (OED and ORD) phenomena, and examines three different possible assumptions that can be made on the nature of the silicon thermal equilibrium point defect species: silicon self-interstitials (I) only, vacancies (V) only, coexistence of I and V. The only consistent way to interpret all properly documented OED/ORD data is to assume that I and V coexist under oxidation as well as under thermal equilibrium conditions at high temperatures.

Laser-induced optical recording in thin films. Modelling of the hole opening and experiments with organic sublayers

Abstract: A model is presented to describe the hole opening in ablative optical recording. The model accounts for changes in geometry of the sensitive film, for adhesion of the film to the substrate, for surface tension gradients in the sensitive film and for evaporation or sublimation at the film-substrate interface. The changes in geometry during hole opening are shown to be responsible for an energy barrierFact=10−16 to 10−15 J, corresponding to 1.5–6J/cm3 which is much too large to make thermal activation of hole opening possible.Fact is reduced by surface tension gradients in the liquid film and/or evaporation at the filmsubstrate interface.

Proton conductivity in the layer compound H 3 OUO 2 AsO 4 ·3H 2 O (HU As)

Abstract: The proton conductivity in H3OUO2AsO4·3H2O (HUAs) has been measured below the transition temperature at 299 K. A consistent picture of the elementary process taking place is developed from separate electrochemical, spectroscopic and calorimetric measurements. The conductivity is proposed to occur by the “vehicle mechanism” of proton transport, i.e. the cooperative motion of H3O+ and H2O. This mechanism is compared with the Grotthuss and the simple ion hopping mechanism.

Laser grown single crystals of silicon

Abstract: Single crystal rods of silicon have been grown by laser-induced chemical vapor deposition (LCVD) using visible light. We believe these to be the first reported single crystals of any material grown by LCVD.

Energy- and spin-analysis of polarized photoelectrons from NEA GaAsP

Abstract: Polarized photoelectrons from NEA GaAs0.6P0.4 are for the first time analysed simultaneously with respect to spin and energy. Parameters of photon energy and temperature were chosen suitable for a polarized electron source cathode. Various contributions to the intensity- and polarization-distribution in connection with several depolarization mechanisms are discussed. Electron-hole exchange scattering together with multiple reflections in the space charge region qualitatively explain the observed distributions. Consequences for a polarized electron source are pointed out.

A search for a thermal spike effect in sputtering

Abstract: The temperature dependence of the sputtering yield was measured for lOOkeV per atom bombardment of silver with molecular and atomic antimony ions. No exponential increase of the yield was found in the temperature range from 25° to 775 °C, although a pronounced nonlinear effect is observed when the yield of these projectiles is compared,Y(Sb 2 + )≈ 1.5×[2Y(Sb+)]. We conclude that there is no influence of the lattice temperature on collision spikes.

Investigation of persistent photoconductivity in Si-dopedn-Al x Ga1−x as grown by molecular beam epitaxy

Abstract: The persistent photoconductivity effect in Si-dopedn-AlxGa1−xAs layers grown by molecular beam epitaxy on (100)GaAs substrates has been investigated by detailed Halleffect and capacitance measurements at 10–300 K. In the alloy composition range 0.25

Refractive index profiles of LiNbO3: Ti waveguides

Abstract: Eight LiNbO3: Ti waveguides have been fabricated under differing conditions The effective refractive indices of guided modes have been measured and successfully fitted to Gaussian index profiles The change of refractive index and titanium concentration are related by a power law, the exponents being different for the extraordinary and the ordinary beam

Gaussmeter application of an organic conductor

Abstract: We present several examples for the application of organic conductors like (Fluoranthene)2PF6 as magnetic field probes. Due to their frequency independent, extremely narrow electron spin resonance line, magnetic flux densities ranging from the earth's field until thousands of Gauss can easily be evaluated with an accuracy better than 1.5 mG. The small sample size required — eg. 2 mm3 for a flux density of about 5 G (S/N ratio 10∶1) — allows to analyse strong magnetic field gradients. Examples for the adaptation of field or frequency-modulation and pulse techniques are given. We point to the requirement of single crystalline samples for high field applications.

A study of built-in potential in a-Si solar cells by means of back-surface reflected electroabsorption

Abstract: The electroreflectance (ER) signal has been studied for the purpose of identifying the built-in field in practical amorphous silicon (a-Si∶H) solar cells. Through both theoretical and experimental considerations, it has been confirmed that the ER signal essentially comes from the light which is reflected at the back surface and hence experiences the internal electric field within thea-Si∶H layer. By analyzing the ER signal, which is really the back-surface reflected electroabsorption signal, the built-in potentialV bcan be evaluated. This method has been applied to various types ofp-i-n junctiona-Si solar cells.V bof a usual homojunction solar cell was about 0.85 V. Increases ofV bby 50≈130mV have been found in heterojunction solar cells constructed withp-type amorphous silicon carbide (a-SiC∶H) and/orn-type microcrystalline silicon (μc-Si) as compared with homojunctionp-i-n solar cells. Moreover, a clear dependence ofV bon the substrate materials has been observed. These experimental results are described in connection with cell performances.

The effect ofγ-irradiation on amorphous silicon field effect transistors

Abstract: The paper deals with the effect ofγ-radiation from a Co60 source on the electronic properties of amorphous silicon field effect transistors. These thin film devices, deposited by the glow discharge technique, are being developed for addressable liquid crystal displays, logic circuits and other applications. 1 Mrad (Si) and 5 Mrad (Si) doses were used and the transistors were held at gate voltages between −8V and +8V during irradiation. Measurements on irradiated specimens showed shifts in threshold voltage of less than 3 V and a change in transconductance below 10%, both of which could be removed by annealing above 130 °C. These results are compared with presently available “radiation hardened” crystalline silicon device structures and it is concluded that in spite of the thicker gate insulation layer (0.3 μm of silicon nitride) of the amorphous devices, the latter are remarkably radiation tolerant, with little degradation in performance. Measurements on irradiatedα-Si films deposited on glass show pronounced conductivity changes, not observed in the transistors. It is suggested that these effects arise at the Si/glass interface, and are prevented by the presence of the silicon nitride film in the devices.

The superconductivity and structure of equiatomic ternary transition metal pnictides

Abstract: Recent superconductivity and crystallographic data for the MM′X ternary compounds with M = Ti, Zr, or Hf, M′ = Ru or Os, and X = P or As are presented. Moderate to high superconducting transition temperatures (T c 's)are exhibited by the ZrRuSi-type hexagonal phase which is found to be metastable at low temperatures. The low-temperature phase of ZrRuP has the TiNiSi-type orthorhombic structure and exhibits superconductivity at 3.9K. The low-temperature phase of HfRuAs has the TiFeSi-type orthorhombic superstructure and remains normal at 1.0K. TiRuAs exists only with the superstructure and remains normal at 0.35K. The details of these various structural modifications point to the importance of undistorted zig-zag chains of metal atoms for the occurrence of superconductivity in these compounds. The lowT c for TiRuP and the absence of superconductivity for TiOsP above 0.35K are not accounted for by these structural modifications.

Raman diagnostics of CVD systems: Determination of local temperatures

Abstract: The suitability of local temperature measurements by cw Raman spectroscopy for the CH4/H2 CVD system has been established. The temperature profiles in a model reactor were derived from H2 pure rotational lines and from hot bands of thev1 vibrational band of CH4. Experimental results are presented for substrate temperatures of 773 K and of 1473 K. High accuracy of measurement and excellent agreement with theoretical solutions for the temperature field within the reactor were found.

Computer simulation of liquid metal ion source optics

Abstract: A numerical calculation of the electric field and current density distribution for a liquid metal ion (LMI) source has been carried out. If a field evaporation mechanism for ion formation is assumed an elongated Taylor cone shape emitter is required to account for the observed total currents. Trajectory calculations including the effect of uniform space charge have been carried out as a function of total current and particle mass. The predicted emission characteristics compare favorably with experimental results for Ga, however the homogeneous space charge model is unable to account for all of the experimentally observed increase in beam divergence with increasing mass and current.

Surface electric fields of tourmaline

Abstract: The N2, O2, H2O, and CO2 molecules that have condensed on the surface of a pyroelectric tourmaline crystal were degassed successively by means of electron bombardment. The temperature dependence of the electrostatic field strength on the specimen surface was observed by electron diffraction; it decreased as the degassing advanced. The tourmaline surface behaved as a gas Chromatographic adsorbent.