Showing papers in "Journal of Applied Physics in 1984"

New material for permanent magnets on a base of Nd and Fe (invited)

Abstract: A new compound composed of Nd, Fe, and a small quantity of B (about 1 wt. %) has been found, which has a tetragonal structure with lattice constants a=0.880 nm and c=1.221 nm. This phase, which has the approximate composition, 12 at. % Nd, 6 at. % B and balance Fe, possesses remarkable magnetic properties. From the approach to saturation an anisotroy constant of about 3.5 MJ/m3 can be calculated, while saturation magnetization amounts to 1.35 T. The magnetization versus temperature curve shows a Curie temperature of 585 K, which is much higher than those of the Fe and light rare earth binary compounds. Based on the new compound, sintered permanent magnets have been developed which have a record high energy product. Permanent magnet properties and physical properties of a typical specimen which has the composition Nd15B8Fe77 are as follows: Br =1.23 T, HcB =880 kA/m, HcI =960 kA/m, (BH)max =290 kJ/m3, temperature coefficient of Br =−1260 ppm/K, density=7.4 Mg/m3, specific resistivity=1.4 μΩm, Vickers hardn...

Pr‐Fe and Nd‐Fe‐based materials: A new class of high‐performance permanent magnets (invited)

Abstract: We report the properties of a new class of high‐performance permanent magnets prepared from Nd‐Fe‐B and Pr‐Fe‐B alloys Magnetic hardening is achieved by rapid solidification Energy products of these isotropic materials can exceed 14 MGOe with intrinsic coercivities of ∼15 kOe X‐ray and microstructural analyses indicate that the alloys exhibiting optimum characteristics are comprised of roughly spherical crystallites, strongly suggesting that the coercivity mechanism is of the single‐domain particle type The crystallites are composed of an equilibrium R‐Fe‐B intermetallic phase having tetragonal symmetry, and the stability of this phase with respect to other rare earths and other metalloids has been investigated

Precise determination of lattice parameter and thermal expansion coefficient of silicon between 300 and 1500 K

Abstract: The lattice parameter of high‐purity silicon is measured as a function of temperature between 300 and 1500 K, and the linear thermal expansion coefficient is accurately determined. Precise measurements are made by the high‐temperature attachment for Bond’s x‐ray method to a few parts per million. It is found that the temperature dependence of the linear thermal expansion coefficient α(t) is empirically given by α(t)=(3.725{1−exp[−5.88×10−3{(t−124)} +5.548×10−4t)×10−6 (K−1), where t is the absolute temperature ranging from 120 to 1500 K. It is shown that the lattice parameter in the above temperature range can be calculated using α(t) and the lattice parameter at 273.2 K (0.5430741 nm). Measured values of the lattice parameter and the thermal expansion coefficient for high‐purity float‐zoned (100 kΩ cm) and Czochralski‐grown (30 Ω cm) single crystals are uniformly distributed within ±1×10−5 nm and ±2×10−7 K−1 with respect to the values obtained from the above empirical formula.

Magnetic cooling near Curie temperatures above 300 K

Abstract: Selection of materials and expected magnetocaloric effects are discussed for magnetic cooling applications at elevated temperatures (400–800 K). Various considerations result in the selection of rare earth‐transition metal compounds such as Sm2Fe17−xCox for this task. These materials offer a wide range of suitable magnetic ordering temperatures as a function of x. They also show relatively high effective magnetic moments per volume. Molecular field models are developed for analytically predicting entropy changes at and above the ordering temperature. Concomitant adiabatic cooling ΔT is accordingly computed for these compounds near the ordering temperatures. It is found that for a family of compounds ΔT values increase somewhat with increasing ordering temperatures due to the decreasing influence of the lattice heat capacity at higher temperatures. Adiabatic cooling of ΔT=−7.5 K at 70 kOe to ΔT=−9.2 K at 70 kOe is predicted for materials Y2Fe17−xCox near their Curie points of 300 and 600 K, respectively (c...

Hole traps and trivalent silicon centers in metal/oxide/silicon devices

Abstract: We report electron spin resonance (ESR) measurements of E′‐center (a ‘‘trivalent silicon’’ center in SiO2) density as well as capacitance versus voltage (C‐V) measurements on γ‐irradiated metal/oxide/silicon (MOS) structures. We also report a considerable refinement of earlier ESR measurements of the dependence of radiation‐induced Pb ‐center (a ‘‘trivalent silicon’’ center at the Si/SiO2 interface) occupation as a function of the Fermi level at the Si/SiO2 interface. These measurements indicate that the Pb centers are neutral when the Fermi level is at mid‐gap. Since the Pb centers are largely responsible for the radiation‐induced interface states, one may take ΔVmg Cox/e (where ΔVmg is the ‘‘mid‐gap’’ C‐V shift, Cox is the oxide capacitance, and e is the electronic charge) as the density of holes trapped in the oxide. We find that radiation‐induced E′ density equals ΔVmg Cox/e in oxides grown in both stream and dry oxygen. Etch‐back experiments demonstrate that the E′ centers are concentrated very near the Si/SiO2 interface (as are the trapped holes). Furthermore, we have subjected irradiated oxide structures to a sequence of isochronal anneals and find that the E′ density and ΔVmg annealing characteristics are virtually identical. We conclude that the E′ centers are largely responsible for the deep hole traps in thermal SiO2 on silicon. This observation coupled with observations regarding the Pb center indicates that two intrinsic centers, both involving silicon atoms lacking one bond to an oxygen atom, are largely responsible for the two electrically significant aspects of radiation damage in MOS devices: charge buildup in the oxide and interface‐state creation at the Si/SiO2 interface.

Theory of ferromagnetic hysteresis (invited)

Abstract: A mathematical theory of hysteresis in ferromagnetic materials is presented based on existing ideas of domain wall motion and domain rotation. Hysteresis is shown to occur as a result of impedances to changes of magnetization such as when domain walls are pinned, while the mutual interactions of the magnetic moments are shown to be of secondary importance in this respect. An equation for the anhysteretic or ideal magnetization curve is derived based on a mean field approximation and this is shown to be dependent on the mutual interactions of the moments but independent of impedances such as pinning. The introduction of a term which measures the impedance to changes in magnetization leads to a simple differential equation of state for a ferromagnet which exhibits all the features of hysteresis. Some modifications of the simple model are necessary in order to bring the solution closer to the real situation. Results are presented which show all the features of hysteresis such as initial magnetization curve, major hysteresis loops, and minor hysteresis loops in excellent agreement with experimental results.

Exchange interaction between ferromagnetic domain wall and electric current in very thin metallic films

Abstract: Since the wall thickness is at least 102 electron wavelengths, a domain wall acts, through the s‐d exchange interaction, like a very weak and smooth potential barrier of height ≂10−2 eV, which does not reflect conduction electrons appreciably. The potential energy arises from the 4s conduction electron spin making a small angle with the local s‐d exchange field. Anisotropic s‐d exchange may also contribute to the potential barrier. A solution of the electron transport equations in the wall is obtained. In order for an electron current crossing the wall to exert an appreciable drive force on it, nonzero interband electron scattering by impurities or phonons is needed. The electron density for a given band varies slightly with location inside the wall, and depends on the current. Mutual electrostatic shielding between bands is taken into account. The drive force per unit wall area is F≂2Ms μ−1i (ve −vw), where ve and vw are electron drift speed and wall speed. In other words, this drive force at ve ≠0 is g...

Thermal detectors as X-ray spectrometers

Abstract: Sensitive thermal detectors should be useful for measuring very small energy pulses, such as those produced by the absorption of X-ray photons. The measurement uncertainty can be very small, making the technique promising for high resolution nondispersive X-ray spectroscopy. The limits to the energy resolution of such thermal detectors are derived and used to find the resolution to be expected for a detector suitable for X-ray spectroscopy in the 100 eV to 10,000 eV range. If there is no noise in the thermalization of the X-ray, resolution better than 1 eV full width at half maximum is possible for detectors operating at 0.1 K. Energy loss in the conversion of the photon energy to heat is a potential problem. The loss mechanisms may include emission of photons or electrons, or the trapping of energy in long lived metastable states. Fluctuations in the phonon spectrum could also limit the resolution if phonon relaxation times are very long. Conceptual solutions are given for each of these possible problems.

Measurement of adherence of residually stressed thin films by indentation. I. Mechanics of interface delamination

Abstract: A fracture analysis of indentation‐induced delamination of thin films is presented. The analysis is based on a model system in which the section of film above the delaminating crack is treated as a rigidly clamped disc, and the crack extension force is derived from changes in strain energy of the system as the crack extends. Residual deposition stresses influence the cracking response by inducing buckling of the film above the crack and by providing an additional crack driving force once buckling occurs. A relation for the equilibrium crack length is derived in terms of the indenter load and geometry, the film thickness and mechanical properties, the residual stress level, and the fracture toughness of the interface. The analysis provides a basis for using controlled indentation cracking as a quantitative measure of interface toughness and for evaluating contact‐induced damage in thin films.

Physics of amorphous silicon based alloy field‐effect transistors

Abstract: In this paper we develop a new theory to describe the characteristics of amorphous silicon based alloy field‐effect transistors. We show that the transition from below to above threshold operation occurs when the Fermi level in the accumulation region moves from the deep to tail localized states in the energy gap. The current‐voltage and capacitance‐voltage characteristics are related to the basic material parameters such as the distribution of localized states in the energy gap, band mobility, device geometry, channel doping, and series resistances. Our analysis shows that an on current in excess of 2×10−7 A/μm gate width can be obtained with a 10‐μm gate length. We also demonstrate that even in the above threshold regime the field‐effect mobility is dependent on the gate voltage. Our theory can be used to optimize the design of amorphous silicon based alloy field‐effect transistors.

The equation of state of the gold calibration standard

Abstract: The compression of Au has been measured at room temperature to 70 GPa (700 kbar) using x‐ray diffraction through a diamond‐anvil cell and the ruby‐fluorescence pressure scale. Based on these data, the isothermal bulk modulus and its pressure derivative at zero pressure are K0T =167 (±11) GPa, and K′0T=5.5 (±0.8). These results are in excellent agreement with ultrasonic measurements of the elastic constants as well as an equation of state based on shock‐wave data. Hence, this study represents an independent experimental confirmation of both the ruby fluorescence pressure scale, and the predicted equation of state of the proposed Au pressure calibration standard. We derive a thermal equation of state for gold by inverting all equation‐of‐state data simultaneously. From this, we extend the gold pressure‐calibration standard to cover the range 0–200 GPa in pressure and 300–3000 K in temperature.

Optical properties and solar selectivity of coevaporated Co‐Al2O3 composite films

Abstract: Co‐Al2O3 composite films were produced by electron‐beam coevaporation in a system with elaborate process control. The deposits were analyzed by transmission electron microscopy, electron diffraction, Auger electron spectroscopy, secondary ion mass spectroscopy, Rutherford backscattering, field ion microscopy, mechanical stylus measurements, and electrical dc and ac measurements. A uniform separated‐grain structure with regular hcp Co particles embedded in Al2O3 was found for Co contents (fCos’) up to ∼30 vol %. The complex dielectric permeability e was evaluated in the 0.3≲λ≲40‐μm wavelength range for samples with 0.11≲ fCo ≲0.60 by carefully selected combinations of spectrophotometric transmittance and reflectance data. Numerical accuracy and internal consistency were investigated. Effective medium theories for e were derived by applying classical scattering theory to spherical random unit cells defined so as to properly represent a number of typical microgeometries. The formulations due to Maxwell Garnett, Bruggeman, and others were thus rederived in a unified way. Large‐size limits of validity and extensions to nonspherical particles were treated. It was found that the Maxwell Garnett theory could reproduce the experimental e’s in detail at low fCo provided that some Co was taken to be dispersed in the insulating matrix. At larger fCo’s we found discrepancies which are likely to be due mainly to dipole‐dipole coupling among adjacent particles. A comparison of the experimental e’s and the rigorous Bergman–Milton bounds, which hold irrespective of detailed microgeometry, gave several interesting results: the e’s varied monotonically along one of the bounds for isotropic materials as fCo was increased, at large fCo we found evidence for anisotropy, and at λ≊12.5 μm we noted certain cases of disagreement with even the most generous bounds. The empirical e’s were used to construct surfaces which combine a high solar absorptance as with a low hemispherical thermal emittance eH. From a computer optimization study we found that as =0.95 and eH =0.07 could be obtained with 0.07 μm of Co‐Al2O3 ( fCo≊0.6) antireflected with 0.07 μm of Al2O3 and laid on Ni. These results were verified by measurements on samples which approximate the ideal design.

Electronic traps and Pb centers at the Si/SiO2 interface: Band‐gap energy distribution

Abstract: Energy distribution of Pb centers (⋅Si≡Si3) and electronic traps (Dit) at the Si/SiO2 interface in metal‐oxide‐silicon (MOS) structures was examined by electric‐field‐controlled electron paramagnetic resonance (EPR) and capacitance‐voltage (C‐V) analysis on the same samples Chips of (111)‐oriented silicon were dry‐oxidized for maximum Pb and trap density, and metallized with a large MOS capacitor for EPR and adjacent small dots for C‐V measurements Analysis of C‐V data shows two Dit peaks of amplitude 2×1013 eV−1 cm−2 at Ev+026 eV and Ev+084 eV The EPR spin density reflects addition or subtraction of an electron from the singly occupied paramagnetic state and shows transitions of amplitude 15×1013 eV−1 cm−2 at Ev+031 eV and Ev+080 eV This correlation of electrical and EPR responses and their identical chemical and physical behavior are strong evidence that ⋅Si≡Si3 is a major source of interface electronic traps in the 015–095 eV region of the Si band gap in unpassivated material

Electroluminescence studies in silicon dioxide films containing tiny silicon islands

Abstract: Electroluminescence from metal‐insulator‐semiconductor structures with silicon dioxide (SiO2) layers containing varying amounts of excess silicon (Si) in the form of tiny Si precipitates have been studied in detail. Bulk insulator emission from the Si islands is shown to dominate over emission from either the SiO2 matrix material or the metallic gate material by studies of oxide or metal gate material, voltage polarity, and insulator thickness dependencies. Several distinct spectral peaks are observed in the energy range from 1.5 to 5 eV which cannot be attributed to optical interference effects. The higher‐energy peaks show a strong dependence on electric field relative to that at the lowest energy (1.7–2 eV). The entire spectral amplitude shows a strong dependence on high‐temperature annealing and excess Si content, decreasing drastically with increasing Si or decreasing annealing temperature. These results are shown to be consistent with light emission during electronic transitions between discrete energy levels associated with Si islands and/or their interface with the SiO2 host matrix material. Quantum size effects, similar to those observed in semiconductor superlattices, are proposed as one possible explanation.

Lattice match: An application to heteroepitaxy

Abstract: We define the concept of lattice match for any pair of crystal lattices in any given crystal direction, allowing for a periodic reconstruction of the interface. An algorithm for a systematic search for all possible matches is developed, and some examples of nonstandard lattice matches are given for CdTe on GaAs and sapphire to illustrate the method. For the case of CdTe on GaAs, our results agree with published results, both with respect to growth plane and orientation for CdTe(111) on GaAs(100). For CdTe on sapphire, our results agree with published results with respect to growth plane.

Slow fracture model based on strained silicate structures

Abstract: Slow crack growth data, molecular‐orbital calculations, and vibrational spectroscopy results are used to develop an atomistic model for environmentally controlled fracture of silica glass. The model is based on chemically active bond defects generated by the strain field of the crack tip. Molecular‐orbital results suggest that bond angle deformations are most effective in increasing the chemical activity of the Si–O–Si bond. Vibrational spectra identify silica polymorphs containing highly strained bonding configurations. A comparison of strained bond reactivity with crack growth results shows that strained silica polymorphs can be used to model the crack tip chemical reactions controlling slow fracture. Based on model complexes, a two‐dimensional fracture model involving kink site nucleation and motion is developed. The model shows that the stress intensity dependence of the crack growth rate is controlled by the energy required to form chemically active defects in the silica structure. The absolute rate of fracture depends on the combined rates of active bond formation and chemical attack at strain induced defects. Crack growth data suggest that for silica, strain activation occurs prior to the adsorption of environmental chemicals.

Optical constants of a hydrogenated amorphous carbon film

Abstract: The real and imaginary parts, n and k, of the complex index of refraction of a hydrogenated amorphous carbon (a‐C:H) film have been determined for photon energies between 145 and 49 eV from measurements of reflectance R and transmittance T Both n and k, and consequently e1 and e2 (the real and imaginary parts of the dielectric constant), show a considerable variation with subsequent annealing of the a‐C:H film up to Ta=750 °C, with the most rapid changes occurring for Ta=450 °C and above The optical gap Eopt is observed to decrease from 22 eV to 0 as the film is annealed, signaling the development of graphitic short range order in the film Using an effective medium approximation to model the optical properties of the film, we find that the as‐deposited film contains amorphous diamond‐like, graphitic, and polymeric components With annealing, the amorphous diamond‐like and polymeric components decrease, the amorphous graphitic component grows, and a void component appears We discuss the local order

Electrical properties of n-amorphous/p-crystalline silicon heterojunctions

Abstract: We have measured C‐V characteristics and temperature dependence of J‐V characteristics of undoped hydrogenated amorphous silicon (a‐Si:H) heterojunctions formed on p‐type crystalline silicon ( p c‐Si) substrates with different resistivities. It has been found that an abrupt heterojunction model is valid for a‐Si:H/p c‐Si heterojunctions, and the electron affinity of a‐Si:H has been estimated as 3.93±0.07 eV from C‐V characteristics. The forward current of all the junctions studied shows voltage and temperature dependence expressed as exp(−ΔEa f/kT) exp(AV), where ΔEa f and A are constants independent of voltage and temperature, being successfully explained by a multitunneling capture‐emission model. The reverse current is proportional to exp(−ΔEar/kT)(VD−V)1/2, where VD is the diffusion voltage and ΔEar is a constant. This current is probably limited by generation‐recombination process.

X‐ray rocking curve analysis of superlattices

Abstract: We present detailed analyses of x-ray double-crystal rocking curve measurements of superlattices. The technique measures depth profiles of structure factor, and profiles of perpendicular and parallel strains relative to the underlying substrate. In addition to providing a detailed picture of the state of stress, the profiles are a direct measure of the composition modulation. The thickness of the period of modulation and the average strain are determined with a precision of ~1%. The detailed structure of the period is determined to ~5%. We obtain an expression relating the structure of the rocking curve to the structure of the period. This expression allows analytic determination of the structure without Fourier transformation or computer fitting. We show the influence of small random fluctuations in layer thicknesses and strains. The technique is applied to a 15-period GaAlAs/GaAs and a ten-period AlSb/GaSb superlattice grown on GaAs and GaSb substrates, respectively. In the former, the thickness of the period was 676 A and the perpendicular strain varied between zero for the GaAs layer and 0.249% for the layer with peak (93%) Al concentration. Transition regions, ~100 A thick, with continuously varying composition, were found between the GaAs and the Ga0.07 Al0.93As layers. Fluctuations in structural properties were less than 5% of the average. The AlSb/GaSb superlattice had a period of 610 A with sharp transition regions between the layers and negligible fluctuations from period to period. The perpendicular strains were −0.03% and 1.25%, respectively, for the GaSb and AlSb layers. A uniform parallel strain of 0.03% was found throughout the superlattice. Nonzero parallel strain indicates that a small fraction of the misfit between the superlattice and the substrate is plastically accommodated by net edge dislocations lying in a narrow region (a few hundred A thick) at the interface with the substrate. The net number of edge dislocations was calculated to be ~1×10^4/cm^2. The measured perpendicular strains were in excellent agreement with the values calculated from bulk lattice parameters, elastic properties, and the parallel strain. For both superlattices, the standard deviation of random atomic displacements away from perfect crystal sites was below 0.1 A, in agreement with reported ion channeling and electron diffraction measurements of superlattices. The rocking curve method is a major tool for quantitative analysis of superlattices.

The residual streamer channel: Return strokes and secondary streamers

Abstract: The streamer and return stroke concepts in positive corona breakdown are reviewed, and examined critically for the case of short (<5 cm) gaps in ambient air. It is shown that return strokes normally should not be observed, but that return streamers may occur in longer or highly overvolted streamer channels. A new physical picture is presented of the secondary streamer, as one or several high field domains formed by Douglas‐Hamilton–Mani‐type attachment instability of the residual channel created by the passage of the primary streamer. In this way, both the erratic origin and development of the secondary streamer in air are easily explained, as well as the fact that spark breakdown follows if and when the secondary streamer fills the complete residual channel length. Further, it follows that the secondary streamer is not the cause of breakdown, but only a secondary side effect.

A mathematical model for spin coating of polymer resists

Abstract: The success of lithographic processes in microelectronics fabrication depends on the reproducible generation of desired polymer resist film thickness and profile uniformity. Numerous process variables affect the outcome of spin coating of resists on wafers. A thorough understanding of the intricate interdependence of process parameters is essential to guide future process design and improvement. A mathematical model is derived to elucidate the dominant mechanisms governing film formation. The non‐Newtonian character of the resist solution is taken into account, as well as the changes in resist viscosity and solvent diffusivity with changing polymer concentration. Results obtained from this model show that polymer film thickness is controlled by convective radial flow of the resist solution and solvent evaporation. The former process governs film thickness during the early stages of the process, while the latter becomes significant in later stages. The model accurately describes the experimentally observed dependence of film thickness on the variables affecting the spin‐coating process.

Modification of the optical and structural properties of dielectric ZrO2 films by ion‐assisted deposition

Abstract: Low‐energy bombardment by argon and oxygen ions has been used in the deposition of thin dielectric films of ZrO2. The film packing density has been improved from 0.83 to unity with a corresponding increase in the refractive index from 1.84 to 2.19. The highest stable refractive index measured was 2.23 for oxygen ion‐assisted deposition of ZrO2 on a substrate heated to 300 °C. Ion bombardment during condensation of evaporated ZrO2 on a room temperature substrate results in crystallization into the cubic phase which is consistent with previous studies of ion impact crystallization by thermal‐spike processes. At elevated substrate temperatures the monoclinic phase is also present.

Thermodynamic considerations in refractory metal-silicon-oxygen systems

Abstract: Thermodynamic considerations in thin‐film reactions involving refractory metals, refractory metal silicides, silicon, and silicon dioxide are described using ternary phase diagrams. Calculated metal‐silicon‐oxygen phase diagrams for Mo, W, Ta, and Ti are used to explain the reactivity of the metal with silicon dioxide, the effectiveness of native oxide in preventing metal‐silicon interdiffusion, and the formation of silicon dioxide in preference to metal oxides during silicide oxidation. Distinctions are drawn between experimental results which can be explained solely on thermodynamic grounds and those requiring consideration of both thermodynamic and kinetic factors.

Formation of thin films of NiSi: Metastable structure, diffusion mechanisms in intermetallic compounds

Abstract: The formation of NiSi films from the reaction of Ni2Si with (100) and (111) silicon substrates was found to be controlled by a lattice diffusion process with an activation energy of 1.70 eV. In order to correlate kinetic information obtained by Rutherford backscattering with x‐ray diffraction data, ‘‘standard’’ diffraction powder patterns for both Ni2Si and NiSi have been established. The existence of a metastable hexagonal form of NiSi has been confirmed. Observations on the formation of Ni2Si confirm previous investigations. The diffusion process at work during the formation of NiSi is discussed in terms of the crystalline anisotropy of this compound and compared to what is known about diffusion in other silicides.

Electrical and optical properties of zinc oxide thin films prepared by rf magnetron sputtering for transparent electrode applications

Abstract: Zinc oxide films were prepared on unheated glass substrates by rf magnetron sputtering under an applied external dc magnetic field in pure argon gas, and electrical and optical properties of the deposited films were investigated. Highly transparent films with resistivity as low as 10−4 Ω cm, which were weakly oriented perpendicular to the substrate surface(c‐axis orientation), could be produced with a relatively high deposition rate on the substrate suspended perpendicular to the target surface by controlling the sputtering gas pressure and the external dc magnetic field, without any postdeposition preparative treatment. The Hall mobility of the film with the highest conductivity was about 120 cm2/V sec, which was the highest yet reported for thin films on ZnO. The increase in the conductivity was related to the increase in Hall mobility which was caused by the decrease of carrier scattering from grain boundaries due to the grain growth resulting from the improvement of crystallization. The improvement of...

Hole diffusion length in n‐TiO2 single crystals and sintered electrodes: Photoelectrochemical determination and comparative analysis

Abstract: The photoelectrochemical behavior of an n‐TiO2 (rutile) single crystal under different treatments (mechanical polishing and chemical and photoelectrochemical etching) is analyzed and the conditions under which Gartner’s model can be applied are determined. The hole diffusion length Lp obtained from the photoelectrochemical data is about 10−6 cm for the etched single crystal and seems to be governed by electron‐hole recombination at centers associated to impurities introduced in the lattice during the manufacturing process. This value of the hole diffusion length determines the minimum value of the donor concentration Nd for an efficient separation of carriers within the semiconductor depletion layer. This explains the existence of a maximum of the quantum efficiency for Nd∼6×1018 cm−3, which has been found to be a value common to n‐TiO2 single crystals of different origin. Lattice deffects introduced near the crystal surface by mechanical polishing behave as recombination centers which reduce Lp to values...

Improved quartz crystal microbalance technique

Abstract: The importance of using the most pertinent mathematical description of the mass load versus frequency relation for quartz crystal thin‐film thickness monitors is reviewed. The different usable crystal load ranges of the so‐called frequency‐ and period‐measurement techniques in comparison with the Z‐Match technique are calculated for most of the commonly used deposition materials. A new thin‐film thickness monitoring procedure is described, which takes the influence of the acoustic film properties on the mass load versus frequency slope into consideration without need for the explicit knowledge of the acoustic impedance ratio z of the deposited film and the quartz crystal. It is shown how the effective z value in the composite resonator built by the quartz crystal and the deposited foreign layer can be derived from a measurement of a quasiharmonic overtone resonance frequency in addition to the commonly practiced exclusive measurement of the fundamental resonance frequency. The presently established relati...

Electrofracture mechanics of dielectric aging

Abstract: We introduce a fracture mechanics concept of aging in solid dielectrics. The key element is that growth of a damage structure is only possible if the release of electrostatic energy due to growth exceeds the formation energy of the damage structure. Both the electrostatic energy release and the formation energy are calculated for simple examples.

Early stages of oxygen segregation and precipitation in silicon

Abstract: The early stages of oxygen segregation at dislocation and precipitation in the bulk have been investigated by high‐resolution electron microscopy in Czochralski grown silicon. Two kinds of precipitates are observed: a crystalline silica phase, coesite, and an amorphous phase. Both forms coexist after 650 °C heat treatment: the so‐called rodlike defects are in fact long 〈011〉 ribbons of coesite associated with interstitial dipoles. This crystalline form is favored by a high oxygen supersaturation and a low carbon content. Above 870 °C amorphous platelets of silica are formed on the {100} planes, whereas coesite is no longer observed but interstitial dislocation loops are always present. The strain produced by such precipitates is partially relaxed by Si interstitial emission, which explains the internal formation of dislocations. It is suggested that both forms are nucleated on two different species of nuclei. At 〈011〉 dislocation cores it is shown that the coesite phase is stabilized over a wide range of oxygen content or annealing temperature.

Organic‐on‐inorganic semiconductor contact barrier diodes. I. Theory with applications to organic thin films and prototype devices

Abstract: We discuss the properties of organic‐on‐inorganic (OI) semiconductor contact barrier diodes. A model for charge transport is developed which suggests that thermionic emission over the organic/inorganic contact barrier dominates at low current densities, whereas space‐charge effects dominate transport through the organic layer at high current densities. The effects of charge trapping in the organic layer are also considered. This model is applied to OI diodes using thin films of the prototypical aromatic compound; 3,4,9,10‐perylenetetracarboxylic dianhydride, (PTCDA) vapor‐deposited onto n‐ and p‐Si substrates. Several electrical and optical properties of PTCDA are investigated to provide a basis for analyzing the OI diodes. Both ohmic and space‐charge‐limited transport are observed in the PTCDA. We discuss mobility, transient response, and photoresponse of the thin‐film organic material. Also described are the general properties of organic‐on‐inorganic contact barrier diodes which employ PTCDA and related...