Showing papers in "Applied Physics A in 1984"

Dynamic RHEED observations of the MBE growth of GaAs

Abstract: Detailed measurements have been made of the specular beam intensity in RHEED patterns from static and growing GaAs surfaces. The basic parameters investigated were substrate temperature and electron beam azimuth. The results have provided further understanding of growth dynamics and surface disorder, respectively. There is a significant trend away from two-dimensional growth at the higher temperatures, which also correspond to more Ga-rich surface structures. Conversely, surface disorder is apparently greater during growth at the lower temperatures, where the structure is As-rich. The static As-stable 2×4 surface is, however, the most ordered and the most closely two-dimensional. It has also been shown that ordered, two-dimensional growth can be initiated from excess Ga adatom populations.

Shape of a liquid metal ion source

Abstract: A model of the liquid-metal ion-source shape consisting of a jet-like protrusion on the end of a Taylor cone shape is shown to be consistent with a field evaporation mechanism of ion formation, fluid dynamic considerations, space charge effects and recent TEM observations. The diameter of the ion emitting area is found to be only a few tens of A. Self-consistent numerical calculations of electric potential and particle trajectories predict emission characteristics which compare favorably with experimental results.

Kinetic limitations to surface segregation during MBE growth of III–V compounds: Sn in GaAs

Abstract: The incorporation of Sn as a dopant in GaAs has been studied in the temperature range of 500°–650° C, over a wide range of Ga and As fluxes, the latter being incident as either As4 or As2 molecules. The results are explained in terms of a surface segregation model in which the behaviour at high growth temperatures (above ∼600 °C) approaches thermal equilibrium, but growth at lower temperatures involves a kinetic limitation to the segregation process.

Diffusion of gold in silicon studied by means of neutron-activation analysis and spreading-resistance measurements

Abstract: In- and out-diffusion of gold in silicon were investigated with the aid of a neutronactivation analysis in combination with mechanical sectioning or by the spreadingresistance technique. In-diffusion profiles in the range 1371–1073 K show that Au diffuses in Si mainly via the so-called kick-out mechanism. From the Au diffusion and solubility measurements the interstitialcy contributionDISD to the Si self-diffusion coefficient was determined, which shows that the self-diffusion occurs to a considerable extent via selfinterstitials. Out-diffusion profiles at 1173 K were measured on wafers homogeneously supersaturated with Au. The observed decrease of the electrical activity of Au in the bulk indicates that during the out-diffusion anneal the majority of Au atoms originally dissolved substitutionally changes its configuration.

Infrared absorption enhancement of monolayer species on thin evaporated Ag films by use of a Kretschmann configuration: evidence for two types of enhanced surface electric fields

Abstract: Infrared absorption enhancement of m- and p-nitrobenzoic acid deposited on thin-evaporated silver films has been investigated using the Kretschmann's ATR coupling method. The absorption spectra provide direct evidence that enhancement is prominent only for vibrations of the first monolayer adsorbed on the Ag surface. It is shown that all of the vibrational modes observed obey the normal dipole selection rule. Moreover, it was found that there exist two types of absorption enhancement; the first is enhanced (∼300) by both p- and s-polarized radiation, and the second is enhanced (∼60) by p-polarized radiation alone. The Ag film thickness optimum is about 50 A in the former case and about 200 A in the latter. The enhancement insensitive to the polarization state of radiation can well be explained by the excitation of the transverse collective electron resonance of the Ag islands, whereas that obtained by p-polarized radiation may be due to the excitation of delocalized surface plasmons modified by surface roughness. The image-dipole effect may also be significant in the mechanisms.

Measurement of surface defects by low-energy electron diffraction

Abstract: Low-energy electron diffraction is capable of detection of non-periodic arrangements by spot profile analysis (SPA-LEED). All kinds of defects may be identified by a qualitative and nevertheless systematic evaluation of the diffraction pattern with respect to spot shape, background and energy dependance. It is shown that the kinematical approximation provides far reaching results. The importance and achievements of the newly developed high-resolution instrumentation is demonstrated. The quantitative evaluation provides data on defect density and distribution, which are not available otherwise, as demonstrated with examples. A comparison of imaging and diffraction techniques shows the special advandages of diffraction with respect to quantitative analysis.

Temperature-dependent sputtering of metals and insulators

Abstract: The temperature dependence of the sputter yield and the energy spectrum of sputtered atoms have been investigated on the basis of a standard model for thermal spikes. A high-temperature and a low-temperature regime have been identified in the temperature spectrum making up the evaporation yield. The high-temperature component of the yield as well as the associated energy spectrum are only very weakly dependent on ambient target temperature. The relative variation is the less pronounced the higher the spike temperature. The low-temperature component is associated with the long-time behavior of the spike, and measurable evaporation takes place over time intervals where spikes overlap. The importance of time constants for macroscopic heat transport is pointed out. The results are shown to provide a framework within which experimental results on the temperature dependence of the sputter yield of metals can be explained. The results are also consistent with measured temperature dependences in the sputter yield of insulators.

Kinetics of CoSi2 from evaporated silicon

Abstract: 2 MeV4He+ backscattering spectrometry and CuK α x-ray diffraction were used to study CoSi2 formed by annealing at temperatures between 405° and 500 °C from CoSi with evaporated Si films. A laterally uniform layer of CoSi2 forms, in contrast to the laterally nonuniform CoSi2 layer that is obtained on single crystal Si substrates. The thickness of the CoSi2 film formed is proportional to the square root of time at a fixed temperature. The activation energy of this reaction is about 2.3 eV.

Influence of the optical activity on hologram formation in photorefractive crystals

Abstract: It is shown that dynamical hologram formation in photorefractive, optically active crystals is accompanied by polarization and energy exchange. These effects may be used like new effective holographic methods for determinations of crystal parameters and optical signal processing in real time. In self-diffraction a polarization rotation the order of degrees/cm for Bi12TiO20 and Bi12SiO20 is predicted.

Selectively dopedn-AlxGa1−xAs/GaAs heterostructures with high-mobility two-dimensional electron gas for field effect transistors

Abstract: In selectively dopedn-AlxGa1−xAs/GaAs heterostructures with high-mobility two-dimensional electron gas (2 DEG) at the heterointerface a second conductive channel exists, if the AlxGa1−xAs layer is not totally depleted from free carries. The occurrence of parallel conductance has a deleterious effect on the performance of high-electron mobility transistors (HEMTs) fabricated from this material. Although in principle computable, parallel conductance depends on a large number of design parameters to be chosen for the heterostructure, which are additionally affected by the presence of deep electron traps inn-AlxGa1−xAs of composition 0.25

Subpicosecond carrier trapping in high-defect-density amorphous Si and GaAs

Abstract: The relaxation of photoexcited carriers in a-Si and a-GaAs has been studied with subpicosecond time resolution by excite-and-probe experiments. The measured time dependence of the induced reflectivity and absorption reveals that the initial trapping of mobile carriers in high-defect-density materials occurs on a subpicosecond time scale.

The dynamical behavior of periodic surface profiles on metals under the influence of anisotropic surface energy

Abstract: The kinetics of shaping of periodic surface profiles under the action of surface selfdiffusion were calculated for the special case of anisotropic surface energy. The orientation-dependent surface energy was allowed to contain a local minimum of very high curvature (cusp). The orientation of this cusp was equal to the macroscopic orientation of the crystal surface. The periodic surface profiles exhibited in all cases extended, nearly flat regions centered at the cusp orientation. This type of faceting which causes the profile to be trapezoidal in shape agrees well with experimentally observed profiles on (111) and (100) Ni single crystal surfaces. A procedure is outlined by which the orientation-dependent surface energy along a certain azimuth and the surface self-diffusion coefficient can be extracted from the decay kinetics of faceted periodic surface profiles. This procedure demands the accurate experimental assessment of profile shapes and amplitudes.

Coexistence problem of magnetism and superconductivity

Abstract: The recent development of the problem of coexistence of magnetism and superconductivity will be reviewed. The possible coexistence of superconductivity with various types of magnetism carried by localized magnetic moments and itinerant electrons is critically discussed in connection with several existing materials such as rare earth ternary compounds [(RE)Mo6S8, (RE)Mo6Se8 and (RE)Rh4B4], newly synthesized organic superconductors [(TMTSF)2X] and Y9Co7. In order to obtain a coherent picture for the understanding the rich phenomena associated with the interplay between two mutually exclusive types of long-range order, a unified point of view based on a key concept of the partial gapping model proposed by the author is introduced as a working hypothesis.

Trap spectrum of the ``new oxygen donor'' in silicon

Abstract: Electronic properties of thenew oxygen donor generated in phosphorus-doped Czochralski-silicon at 650‡C are investigated by deep level transient spectroscopy. A continuous distribution of trap states (1014–1016 cm−3 eV−1) is detected in the upper half of the band gap with increasing values towards the conduction band. The magnitude of the state density observed increases with the oxygen content, the heat duration, and a preanneal at temperatures lower than 650‡C. The continuous trap spectrum of thenew donor is explained by interface states occuring at the surface of SiOx precipitates.

Positron transmission and effective mass absorption coefficient in nickel

Abstract: The penetration of positrons from a22Na source into Ni foils sandwiched between different backscattering materials (Kapton, NaCl, Ni, and Mo) has been studied for the geometry commonly used in PAT experiments. Transmitted intensities and effective mass absorption coefficients have been found and the dependence on the backscattering material is discussed. General formulas for the effective mass absorption coefficients are derived. The large geometrical effects are stressed. Suggestions for source corrections are supplied.

Donor states in tellurium-doped silicon

Abstract: Hall effect and conductivity measurements are performed on Te-doped silicon in the temperature range 30K≦T≦800K. A Hall equipment suited for high temperatures up to 800 K has been constructed. The temperature dependence of the free electron concentration is analyzed for Te-doped silicon including one double-donor and several monovalent donor species. A deep level with an electrical activation energy of 200 meV is determined from the saturation of the free electron concentration at temperatures above 400 K. This level represents the first ionization stage of the Te double-donor. The second ionization stage is estimated to have an activation energy of 440 meV. The maximum electrically active Te concentration obtained is 5×1016cm−3. Three different shallow donor states are resolved in the low-temperature range. The concentrations of these shallow donors are partially sensitive to a subsequent heat-treatment.

The morphology and electrical properties of heteroepitaxial InAs prepared by MBE

Abstract: An investigation of the growth of heteroepitaxial InAs by MBE is reported. The surface morphology and electrical properties are shown to be critically dependent on growth parameters and the conditions necessary to obtain good material quality are deduced. Analysis of the thickness dependence of the electrical properties of undoped, and Si- or Te-doped InAs shows that interfacial effects contribute to the measured properties. Material remote from the interfacial region compares favourably with VPE- and bulkgrown InAs.

Ion mixing in Al, Si, and their oxides

Abstract: The redistribution of thin metallic markers due to ion irradiation was studied by backscattering spectrometry in Al, Al2O3, Si, and SiO2. Marker species were selected for their similar masses and different chemical reactivities with the host media and included Ti, Fe, W, Pt, and Au. It was found that the marker signals are Gaussian and that the varianceσ2 of the marker atom distributions increases linearly with the dose of the irradiation, is insensitive to the temperature of irradiation in the range of 80–∼300 K, and depends linearly on the nuclear stopping power of the incident ions. The absolute values ofσ2 for Ti, Fe, W, Pt, and Au markers in Al and Al2O3, W, and Pt in SiO2 and W in Si is, within±50 %, of 6.5×103A2 for 300 keV, 8×1015 Xe ions/cm2. These observations suggest that collisional cascade mixing is a dominant mechanism in this type of impurity-matrix combinations. Only Au and Pt in Si mix at a larger rate:σ2 for Pt is about 3 and for Au about 5 times larger thanσ2 for all other markers. Lower threshold displacement energies and/or the contribution of processes other than cascade mixing are possible considered reasons. In polycrystalline Al, a rapid migration of Au and Pt atoms throughout the Al layer, similar to grain boundary diffusion, is observed.

Lateral growth rates in laser CVD of microstructures

Abstract: Lateral growth rates of Ni spots deposited on absorbing substrates by decomposition of Ni(CO)4 with visible Kr+ laser light have been measured. The experimental data are consistent with the calculated temperature distributions. The mechanism of decomposition is thermal with an apparent chemical activation energy of 22±3 kcal/mole for the temperature range 350 K≦T≦500 K.

Direct high-resolution excimer laser photoetching

Abstract: Direct high-resolution photoetching of various resists and polymers has been demonstrated using an ArF excimer laser at 193 nm. Features as small as 0.3 μm have been produced in 1 μm thick films with no subsequent wet processing steps necessary.

Intense source of slow positrons from pulsed electron accelerators

Abstract: A pulsed LINAC is used for pair production in a tantalum target of 2.5 radiation lengths in an energy range from 80 to 260 MeV. Several well-annealed tungsten vanes are placed immediately behind the target and thermalize a small fraction of the fast positrons. The slow positrons are extracted from the target region and magnetically guided over a distance of 17 m to the detector at the end of an S-shaped solenoid. Two Nal detectors with well-known detection efficiency are used to register the 511 keV annihilationγ-rays. To reduce pile-up effects 50 mm of Pb were placed in front of the detectors. At an average electron current of 1 μA we could detect about 107 slow positrons per second. The positron yield is proportional to the electron current, and shows an increase with the electron energy for our target. The positron energy distribution has a FWHM of ∼ 1.8 eV.

A note on evaporation from heated spikes

Abstract: We have investigated the effect of heat loss through evaporation on the surface temperature profile and the evaporation yield of an ion-induced spike. We derive a three-dimensional extension of a nonlinear integral equation first found by Mann and Wolf to describe the temperature profile in a semiinfinite medium in the presence of heat loss through the surface. The equation has been solved by perturbation expansion in powers of the evaporation rate. For heavy-ion induced, cylindrical elastic-collision spikes, noticeable but moderate corrections are found to evaporation yields estimated previously by neglecting heat loss due to evaporation. These results are relevant mainly to sputtering of metals by heavy atomic and molecular ion bombardment. Comments are also made on sputting of insulators both by heavy keV ions and by ionizing particles. Expressions for an effective sputter time and sputter area are derived for cylindrical geometry; both quantities turn out independent of the initial spike temperature. The sputter radius is normally greater than the depth of the crater formed; we conclude that the influence of crater formation on the evaporation yield is normally negligible.

Measurement of the Refractive Index and Optical Absorption Spectra of Epitaxial Bismuth Substituted Yttrium Iron Garnet Films at uv to Near-ir Wavelengths

Abstract: Refractive-index and optical-absorption spectra of Bi-substituted yttrium iron garnet films, epitaxially grown by liquid-phase epitaxy, have been measured in the spectral regime 0.26 μm≦λ≦1.9 μm by thin-film interference for λ ≳0.52 μm and by ellipsometry forλ≲0.52 μm. The Y3−x−yBixPbyFe5−zPtzO12 films contain bismuth in the range O≦x ≦1.42, lead in the range 0.01 ≦y≦0.08 and platinum in the range 0.005<=z≦0.03. There is satisfactory coincidence between the results from ellipsometry and thin-film interference in the overlapping wavelength region. The materials investigated are the same as reported earlier from this laboratory in ter mof their magnetic and magnetooptic properties.

Ion implantation in IV–VI semiconductors

Abstract: In this review, ion-implantation doping and ion-implantation damage in IV–VI semiconductors are discussed. Doping can be achieved by implanting impurity atoms as well as the constituent elements themselves since the electrical properties of these compounds are determined not only by impurities but also by deviations from stoichiometry. Results obtained with implantation doping are presented and the applications of ion implantation to the fabrication of infrared photodetectors and laser diodes are summarized. The main feature of the implantation damage is a saturation of the carrier concentration as a function of the dose, independent of the ion species and implantation conditions. This unusual behaviour has been explained recently by a defect-level model, which is discussed in detail. The similarity of the damage caused by ion implantation and by electron irradiation is pointed out.

Temperature dependence of positron annihilation characteristics on the surfaces of graphite powders

Abstract: Positron lifetime measurements have been made on graphite powders, grafoils, and pyrolytic graphite crystals with different surface areas in the temperature range between 25° and 600 °C. Three positron lifetimes were found in these systems: a short-lived component (∼0.2 ns) due to positrons in the bulk; a component (∼0.45 ns) due to surface-trapped positrons; and a long-lived component (∼2 ns) ofo-Ps in the voids or the interfacial spaces of powders. Both bulk and surface positron lifetimes increase as a function of temperature. Correlations between the intensity of surface-trapped positrons and the surface area and between Ps formation and the surface area of graphite are found. The Ps formation probability increases as a function of temperature. A thermal desorption model interprets the emission process of Ps atoms from the surface of graphite to the vacuum and gives an activation energy of 0.23±0.02 eV.

Formation of palladium silicide by rapid thermal annealing

Abstract: The formation of palladium silicide Pd2Si by rapid thermal annealing of Pd layers on silicon has been studied as a function of annealing time (1–60s) in the temperature range 350–500 °C. It is shown that the results found for conventional furnace annealing (long duration, low temperature) can be extrapolated for rapid thermal annealing (shorter time, higher temperature) when taking into account the exact time dependence of the short temperature cycle. The growth rate is essentially diffusion limited and the activation energy is close to 1.1±0.1 eV. Silicide resistivity of about 30–40 Ω cm was obtained for 200–400 nm thick Pd2Si layers formed at 400 °C for a few seconds.

Vanadium oxidation as a result of cw CO2 laser irradiation in atmospheric air

Abstract: New results are reported concerning the vanadium oxidation by cw CO2 laser irradiation in air at atmospheric pressure. Particular emphasis is paid both to the initial stage and the development of the oxidation process under the action of the laser radiation. Some aspects are finally discussed concerning the quantitative theoretical interpretation of the experimentally recorded data.

Dependence of properties of hydrogenated microcrystalline and amorphous silicon films prepared by planar magnetron sputtering in inert gas

Abstract: Microcrystalline and amorphous hydrogenated silicon films were preparaed by rf planar magnetron sputtering in the four kinds of inert gas, i.e., He, Ne, Ar, and Kr. The dependence of such properties as x-ray diffraction, ir spectra, absorption coefficient, hydrogen content, dark conductivity and photoconductivity on the kind of inert gas was investigated. Such deposition conditions as hydrogen partial pressure, sputtering pressure and rf power were also studied mainly in relation to the microcrystallization of the films. Microcrystalline films with noticeably high deposition rate could be obtained in the case of Kr and Ar, compared to the case of He and Ne. Hydrogen concentration was found to correlate to the photoconductivity and activation energy of dark conductivity except for the case of He. Photosensitivity was appreciably larger for amorphous film than for microcrystalline one. Especially in the case of Kr, it was considerably larger than in other cases.

Photoluminescence of AlxGa1−xAs/GaAs quantum well heterostructures grown by molecular beam epitaxy. II: Intrinsic free-exciton nature of quantum well luminescence

Abstract: Low-temperature photoluminescence measurements on nominally undoped AlxGa1−xAs/GaAs quantum well heterostructures (QWHs) grown by molecular beam epitaxy (MBE) exemplified the exclusivelyintrinsic free-exciton nature of the luminescence under moderate excitation conditions. Neither any spectroscopic evidence for alloy clustering in the AlxGa1−xAs barriers nor any extrinsic luminescence due to recombination with residual acceptors has been detected in single and double QWHs when grown at 670 °C under optimized MBE growth conditions. Carrier confinement in AlxGa1−xAs/GaAs QWHs starts at a well width ofLz≌30 nm when x≌0.25. The minor average well thickness fluctuation ofΔLz=4×10−2nm as determined from the excitonic halfwidth allowed the realization of well widths as low asLz=1 nm and thus a shift of the free-exciton line as high as 2.01 eV which is close to the conduction band edge of the employed Al0.43Ga0.57As confinement layer. The measurements further revealed a strongly enhanced luminescence efficiency of the quantum wells as compared to bulk material which is caused by the modified exciton transition probabilities due to carrier localization.

AES and ELS Study of Titanium Oxidation in High Vacuum

Abstract: In the present work, results of the interaction of O2 with polycrystalline titanium using AES, and ELS techniques, are presented. Changes in the shapes of Ti(LMV) and Ti(LMM) transitions and in the Ti(LM)O(V)/Ti(LMV) and Ti(LMV) Ti(LMM) amplitude ratios as well as a shift of the 34 eV loss peak [Ti(3p level], are studied as function of the oxygen exposure. At O2 pressures equal or less than 10−6 Pa and exposure up to 2000 L(O2) (weak oxidation), the Ti(LM)O(V)/Ti(LMV) and Ti(LMV)/Ti(LMM) ratios show three clearly distinct regions: (i) up to 20L there is a chemisorbed phase with the Ti(LMV)/Ti(LMM) ratio remaining constant and a fast linear increase in the Ti(LM)O(V)/Ti(LMM) ratio. (ii) At exposures higher than 20L the oxidation begins being characterized by a linear variation of the Ti(LM)O(V)/Ti(LMV) and Ti(LMV)/Ti(LMV) ratios, with positive and negative slopes, respectively. (iii) At 125L there is a change in both slopes but no stable value is achieved, despite the O(KLL)/Ti(LMM) ratio remains constant. Hence, it can be concluded that TiO is the final oxide obtained after that treatment, from features like the shape of the transitions and a maximum shift of 3eV of the Ti(3p) level, complemented with ESD experiments. On the other hand, experiments at pressures higher than 10−4Pa with or without the surfaces at 900K (strong oxidation) indicate that the most likely final oxide is TiO2.