Interdiffusion in two-layer Pd/Ag films II. “Cold” homogenization mechanisms

Interdiffusion processes in thin epitaxial polycrystalline Pd/Ag films in the temperature range 20–500°C are studied by transmission electron microscopy, electron diffraction and electrical resistance methods. Homogenization is investigated both during condensation and under conditions of postcondensation annealing.The basic processes of homogenization associated with GB diffusion along migrating boundaries. It is found that in real polycrystal films with wide spectrum of grain sizes few mechanisms can occur simultaneously or subsequently: recrystallization induced diffusion, diffusion induced grain boundary migration, activation of bulk diffusion in fine grain clusters, bulk diffusion through interphase boundary. The conditions for prevailing one of them can be provided by changing condensation and postcondensation annealing temperatures or by choosing certain grain size.     

The Effect of Point Defects on the Activation Processes in Semiinsulating GaAs Irradiated by Si28 Ions

The layered concentration, concentration profile, and mobility of electrons in the Si²⁸ ion-implanted layers (IIL) of semiinsulating GaAs are investigated. The specific resistance of the latter is also studied upon radiation annealing (RA) in the temperature range 590–800 °C using electron energies higher than the threshold energy of defect formation. The IIL are shown to form during RA at much lower temperatures. The layers exhibit high electrical activation of Si²⁸ ions, with the electron-concentration profile corresponding to the calculated one, and a low concentration of residual defects limiting the electron mobility. The radiation annealing increases the resistance of semiinsulating GaAs. The calculations show that these effects are due to the Frenkel pairs (FP) generated by radiation. A high degree of ionization of GaAs atoms significantly reduces energies of potential barriers of diffusion, FP recombination, and electrical impurity activation.     

EPMA-EDS surface measurements of interdiffusion coefficients between miscible metals in thin films

A new technique is developed to study interdiffusion between two miscible metals. The technique is applied to the Ni-Pd system. It consists in measuring the change of apparent surface composition of a Pd substrate coated with an 800 nm Ni thin film during annealing at a given temperature. The measurement is carried out in-situ inside the chamber of a SEM (scanning electron microscope) by EPMA-EDS (electron probe microanalysis-energy dispersive X-ray spectroscopy). The experimental data are processed using a model that mixes the Fick's diffusion equations and the electron probe microanalysis equation. This process allows the determination of the mean interdiffusion coefficient at a given annealing temperature. The main advantages of the technique are the possible determination of interdiffusion coefficients in thin films and at very low temperature (down to 430 °C, i.e. ∼0.4 T_m), which is not achievable with other techniques conventionally used for the study of interdiffusion. The Ni-Pd mean interdiffusion coefficient is shown to follow an Arrhenius law between 430 °C and 900 °C, in relatively good agreement with previous interdiffusion measurements made on the Ni-Pd system at higher temperature.     

Thermal annealing effects on a compositionally graded SiGe layer fabricated by oxidizing a strained SiGe layer

Thermal annealing effects on a thin compositionally graded SiGe buffer layer on silicon substrate fabricated by oxidizing a strained SiGe layer are investigated with X-ray diffraction, ultraviolet Raman spectra and atomic force microscopy. Interestingly, we found that the surface roughness and the threading dislocation densities are kept low during the whole annealing processes, while the Ge concentration at the oxidizing interface decreases exponentially with annealing time and the strain in the layer is only relaxed about 66% even at 1000 °C for 180 min. We realized that the strain relaxation of such a compositionally graded SiGe buffer layer is dominated by Si-Ge intermixing, rather than generation and propagation of misfit dislocations or surface undulation.     

Two groups of misfit dislocations in GaAs on Si

High-resolution cross-sectional and conventional plan-view transmission electron microscope observations have been carried out for molecular beam epitaxially grown GaAs films on vicinal Si (001) before and after annealing as a function of film thicknesses and observation directions between two orthogonal 110 directions. Two groups of misfit dislocations are characterized at the interface regions between GaAs and Si by analyzing whether their extra half planes exist in the film or the substrate side. Group I misfit dislocations due to stress caused by a lattice misfit between GaAs and Si consist of partial dislocations and 60° and 90° complete dislocations in an as-grown state. With an increase in the film thickness, partial dislocations decrease and complete dislocations increase. After annealing, partial dislocations almost completely disappear and 90° perfect dislocations are predominantly observed. Group II misfit dislocations due to thermal-expansion misfit-induced stress are all 60°-type complete dislocations regardless of film thicknesses and annealing treatment.On leave from Central Research Laboratory, Hitachi, Ltd., Tokyo 185, Japan     

XPS study of annealing induced effects on surface and interface electronic properties of Si/Ge nanostructures

The present study is focused on the influence of vacuum thermal treatment on surface/interface electronic properties of Si/Ge multilayer structures (MLS) characterized using X-ray photoelectron spectroscopy (XPS) technique. Desired [Si(5 nm)/Ge(5 nm)]_×10 MLS were prepared using electron beam evaporation technique under ultra high vacuum (UHV) conditions. The core-level XPS spectra of as-deposited as well as multilayer samples annealed at different temperatures such as 100 °C, 150 °C and 200 °C for 1 h show substantial reduction in Ge 2p peak integrated intensity, whereas peak intensity of Si 2p remains almost constant. The complete interdiffusion took place after annealing the sample at 200 °C for 5 h as confirmed from depth profiling of annealed MLS. The asymmetric behaviour in intensity patterns of Si and Ge with annealing was attributed to faster interdiffusion of Si into Ge layer. However, another set of experiments on these MLS annealed at 500 °C suggests that interdiffusion can also be studied by annealing the system at higher temperature for relatively shorter time duration.     

Threading dislocations in GaAs on Si grown with ∼1 nm thick Si interlayers

Threading dislocation morphologies and characteristics have been investigated in 3 m thick GaAs films with ultrathin (1 nm) Si interlayers grown by molecular beam epitaxy on tilted (2° toward [110]) Si (001) substrates using cross-sectional transmission electron microscopy. Four sample structures are studied in which different numbers of ultrathin Si layers are grown at different positions in the GaAs film, and the results are compared for samples observed before and after ex situ annealing. In all of the sample structures, some mixed-type dislocations are clearly blocked by the Si interlayers, although some of them pass through these layers, resulting in propagation of their threading dislocations to the sample surface. After annealing at 900 °C for 10 s or 800 °C for 30 min, the interactions of the dislocations with the interlayers are enhanced, and there is an increase in the number of dislocations which are bent along the 110 directions at the positions of the Si barrier layers. However, a considerable number of dislocations still escape from the Si barriers by gliding on {111} slip planes during annealing. Moreover, ex situ annealing generates new edge-type dislocations through interactions between moving threading dislocations. The nature of the dislocations that cross the Si barriers is especially discussed.     

Electrical activity of dislocations: Prospects for practical utilization

The electrical activity of interfacial misfit dislocations in silicon has been examined using the electron beam induced current technique (EBIC) in a scanning electron microscope. Clean misfit dislocations, i.e. no EBIC contrast, formed during high-temperature Si(Ge) chemical vapor epitaxy were studied. These defects were subsequently decorated with known metallic impurities (Au and Ni) by diffusion at 400° C to 1130° C from a back-side evaporated layer. Qualitative analysis of the electrical activity in relation to the energy levels anticipated for the clean or decorated dislocations is presented. Of particular interest is the case of defect-induced conductivity type inversion which occurred both at the top surface and at the buried dislocated interfaces of the multilayer. The prospects for using dislocations in a beneficial manner as active elements in electronic devices are discussed.     

Mechanism for coarsening of P-mediated Ge quantum dots during in-situ annealing

The coarsening of phosphorus-mediated Ge quantum dots (QDs) on Si(0 0 1) during in-situ annealing at 550 °C is studied. In-situ annealing makes the as-grown sample morphology be remarkably changed: the larger dots are formed and the dot density is greatly reduced. The results of chemical etching and Raman spectra reveal that the incorporation of Ge atoms which originate from the diminishing dots, rather than substrate Si atom incorporation is responsible for the dot coarsening at the incipient stage of in-situ annealing. Besides, Raman spectra suggest that the larger dots formed during in-situ annealing are dislocated, which was confirmed by cross-sectional high-resolution electron microscopy observation. Through the generation of dislocations, the strain in the dots is relaxed by about 50%.     

Effects of processing on the electrical and structural properties of spray deposited CdS:In thin films

Polycrystalline CdS:In thin films were prepared by the Spray pyrolysis technique (SP) at a substrate temperature T_s=490 °C. The effects of annealing in nitrogen atmosphere at 400 °C and HCl-etching on the electrical and structural properties of the films were investigated. The electrical properties were studied through the analysis of the I-V curves, while the structural properties were studied through the analysis of the X-ray diffraction (XRD) patterns and the scanning electron microscope (SEM) images. An increase in the films’ resistivity was occurred after annealing and/or HCl-etching, which was accompanied by changes in the XRD patterns and SEM images. These changes were related to a phase change from the mixed (cubic and hexagonal) phase to the hexagonal phase which was expected to occur during the aforementioned processes. The X-ray diffraction (XRD) patterns and the scanning electron microscope images confirm this expectation.     

Electrical resistance,compressional strength,and extension of Ni-Mo and Ni-W alloys

An experimental study has been made of the temperature dependences of the compressional strength of Ni alloys with 5 and 10 at.% Mo and 5 at.% W, of the kinetic curves for the electrical resistance during annealing at 400 °C, and of the nature of the tensile deformation at various temperatures. The mechanism primarily responsible for the anomalous behavior of the temperature-rate dependence of the strength of these alloys may be the formation of microscopic carbide inclusions and Cottrell carbon atmospheres at dislocations during the deformation. Presence of the K state amplifies the jumps on the flow curves.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 1, pp. 89–93, January, 1970.In conclusion we note that this strengthening is displayed to some extent in other systems characterized by a K state.     

Metal contacts to n-GaN

Al, Au, Ti/Al and Ti/Au contacts were prepared on n-GaN and annealed up to 900 °C. The structure, phase and morphology were studied by cross-sectional transmission and scanning electron microscopy as well as by X-ray diffraction (XRD), the electrical behaviour by current-voltage measurements. It was obtained that annealing resulted in interdiffusion, lateral diffusion along the surface, alloying and bowling up of the metal layers. The current-voltage characteristics of as-deposited Al and Ti/Al contacts were linear, while the Au and Ti/Au contacts exhibited rectifying behaviour. Except the Ti/Au contact which became linear, the contacts degraded during heat treatment at 900 °C. The surface of Au and Ti/Au contacts annealed at 900 °C have shown fractal-like structures revealed by scanning electron microscopy. Transmission electron microscopy and XRD investigations of the Ti/Au contact revealed that Au diffused into the n-GaN layer at 900 °C. X-ray diffraction examinations showed, that new Ti₂N, Au₂Ga and Ga₃Ti₂ interface phases formed in Ti/Au contact at 900 °C, new Ti₂N phase formed in Ti/Al contact at 700 and 900 °C, as well as new AlN interface phase developed in Ti/Al contact at 900 °C.     

Physical properties of amorphous Si: The role of annealing

The influence of annealing (up to 900 °C) on the absorption edge and the temperature dependence of the electrical conductivity of amorphous silicon (a-Si) thin layers prepared by cathode sputtering is reported. Up to 400 °C the changes by annealing on the above mentioned properties are explained by the decrease of the density of states within the mobility gap. At higher temperatures, two processes start to play role: the crystalization of the layer and the probable contamination, especially by oxygen.We thank Dr. M.Rozsíval for testing our samples by the electron diffraction.     

Electrical, structural and surface morphological properties of thermally stable low-resistance W/Ti/Au multilayer ohmic contacts to n-type GaN

A W/Ti/Au multilayer scheme has been fabricated for achieving thermally stable low-resistance ohmic contact to n-type GaN (4.0 × 10¹⁸ cm⁻³). It is shown that the as-deposited W/Ti/Au contact exhibits near linear I-V behaviour. However, annealing at temperature below 800 °C the contacts exhibit non-linear behaviour. After annealing at a temperature in excess of 850 °C, the W/Ti/Au contact showed ohmic behaviour. The W/Ti/Au contact produced specific contact resistance as low as 6.7 × 10⁻⁶ Ω cm² after annealing at 900 °C for 1 min in a N₂ ambient. It is noted that the specific contact resistance decreases with increase in annealing temperature. It is also noted that annealing the contacts at 900 °C for 30 min causes insignificant degradation of the electrical and thermal properties. It is further shown that the overall surface morphology of the W/Ti/Au stayed fairly smooth even after annealing at 900 °C. The W/Ti/Au ohmic contact showed good edge sharpness after annealing at 900 °C for 30 min. Based on the Auger electron spectroscopy and glancing angle X-ray diffraction results, possible explanation for the annealing dependence of the specific contact resistance of the W/Ti/Au contacts are described and discussed.     

Crystallization and electrochemical performance of La0.6Sr0.4Co0.2Fe0.8O3 − δ-Ce0.8Gd0.2O1.9 thin film cathodes processed by single solution spray pyrolysis

La_0.6Sr_0.4Co_0.2Fe_0.8O_3 − δ-Ce_0.8Gd_0.2O_1.9 (LSCF-CGO) thin films obtained by spray pyrolysis of a single precursor solution were investigated by XRD, TEM and impedance spectroscopy at annealing temperatures ranging from 500 to 900 °C. Films annealed at 600 °C contained a mixture of amorphous regions and crystalline regions composed of fine crystallites (< 5 nm). Annealing above 600 °C increased the ratio of crystalline to amorphous material, led to the segregation of the films into distinct LSCF and CGO phases, and promoted grain growth. The electrical behavior of the films depended on annealing temperature. At testing temperatures of 400 °C and below, the polarization resistance of films with lower annealing temperatures was larger than the polarization resistance of films with higher annealing temperatures. However, at testing temperatures of 500 °C and above the polarization resistance of films with lower annealing temperatures was equal to or lower than the polarization resistance of films with higher annealing temperatures. This was reflected by the activation energy that decreased with increasing annealing temperature. The varying electrical behavior may be related to microstructural changes that caused bulk diffusion to be the rate-limiting step in films with lower annealing temperatures and oxygen dissociation to be the rate-limiting step in films with higher annealing temperatures.     

Influence of vacuum annealing on the asymmetry of the electrical conductivity of tantalum pentoxide films

In this work we demonstrate the influence of interphase interaction processes, taking place at the layer boundaries during vacuum annealing of Ta-Ta₂O₅ sandwiches, on the asymmetry of the electrical conductivity of Ta₂O₅ films. These processes are demonstrated to be local in nature.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 46–50, January, 1991.     

Annealing effects on electrical and optical properties of ZnO thin films synthesized by the electrochemical method

Microstructural and electrical properties of potentiostatically electrodeposited ZnO thin films from an aqueous bath were investigated after annealing at different temperatures in Ar and 5% H₂/Ar atmospheres. It is confirmed that the bandgap energy of ZnO thin films decreased with annealing from 3.42 to 3.27-3.29 eV by calculating the wavelength of the absorption region. The annealing at temperatures as low as 200 °C decreased the sheet resistance of ZnO thin films because of the extinction of Zn(OH)₂ in the atmosphere. In addition, the sheet resistance of ZnO thin films decreased by annealing in a 5% H₂ atmosphere, which caused an increase of carrier concentration by hydrogen reduction.     

Formation of thermally stable low-resistance Ti/W/Au ohmic contacts on n-type GaN

A Ti(12 nm)/W(20 nm)/Au(50 nm) metallization scheme has been investigated for obtaining thermally stable low-resistance ohmic contacts to n-type GaN (4.0×10¹⁸ cm^-3). It is shown that the current–voltage (I–V) characteristics of the samples are abnormally dependent on the annealing temperature. For example, the samples that were annealed at temperatures below 750 °C for 1 min in a N₂ ambient show rectifying behavior. However, annealing the samples at temperatures in excess of 850 °C results in linear I–V characteristics. The contact produces a specific contact resistance as low as 8.4×10^-6 Ω cm² when annealed at 900 °C. It is further shown that the contacts are fairly thermally stable even after annealing at 900 °C; annealing the samples at 900 °C for 30 min causes insignificant degradation of the electrical and structural properties. Based on glancing angle X-ray diffraction and Auger electron microscopy results, the abnormal temperature dependence of the ohmic behavior is described and discussed. PACS 72.80.Ey; 73.40.Cg; 73.20.At; 79.60Bm; 73.40.Gk     

Enhanced/suppressed interdiffusion of lattice-matched and pseudomorphic III–V heterostructures by controlling Ga vacancies

Data are presented showing that the interdiffusion of Ga and Al (AlGaAs-GaAs) and of Ga and In (pseudomorphic InGaAs-GaAs) at a heterojunction can be significantly enhanced or suppressed by controlling group III vacancies and interstitials. High resistivity low-temperature GaAs which contains excess As is shown to accelerate the interdiffusion process. The emission wavelength from monolayer-thick quantum wells can remain relatively unchanged even after annealing for 200 h at 850° C.     

Theory of electrical resistance of multivalley semiconductors with screw dislocations

The diffusion theory of electrical resistance in multivalley semiconductors of Si-type with screw dislocations is developed. Intervalley electron transitions are taken into account. The effect of the inhomogeneous dislocation distribution on the electrical resistance of the crystal is considered.It is shown that randomly distributed dislocations having the mean density 10¹⁰ cm–2 contribute some percents to the electrical resistance at room temperature. Intervalley transitions make this contribution much lower (by one order or more). The inhomogeneity of dislocation distribution enhances the electrical resistance several orders as compared with randomly distributed dislocations.