Showing papers in "Applied Physics A in 1991"

Quantum crystallites and nonlinear optics

Abstract: This is a review and analysis of the optical properties of quantum crystallites, with principal emphasis on the electro-optic Stark effect and all optical third order nonlinearity. There are also introductory discussions on physical size regimes, crystallite synthesis, quantum confinement theory, and linear optical properties. The experiments describe CdSe crystallites, exhibiting strong confinement of electrons and holes, and CuCl crystallites, exhibiting weak confinement of the exciton center of mass. In the CdSe system, neither the Stark effect nor the third order nonlinearity is well understood. The Stark shifts appear to be smaller than calculated, and field inducted broadening also occurs. The third order nonlinearity is only modestly stronger than in bulk material, despite theoretical prediction. Unexpectedly large homogeneous widths, due to surface carrier trapping, in the nominally discrete crystallite excited states appear to be involved. The CuCl system shows far narrower spectroscopic homogeneous widths, and corresponds more closely to an ideal quantum dot in the weak confinement limit. CuCl also exhibits exciton superradiance at low temperature. Surface chemistry and crystallite encapsulation are critical in achieving the predicted large Stark and third order optical effects in II-VI and III-V crystallites.

Transport properties of silicon

Abstract: Electrical conductivity, thermal conductivity, and thermoelectric power of single-crystalline silicon are investigated at temperatures between 2 and 300 K. From the measured data we calculate the mean free path of electrons and phonons and separate diffusion part and phonon-drag part of the thermoelectric power. Using a new method, we evaluate the mean free path of those phonons which are responsible for the phonon drag effect.

Beta barium borate (BBO)

Abstract: The paper contains a review of crystallographic, optical and nonlinear optical properties of beta barium borate (β-BaB2O4 or BBO) crystal and presents a description of its typical applications in nonlinear optics and quantum electronics.

Properties of silicon-electrolyte junctions and their application to silicon characterization

Abstract: A number of interesting and still not fully understood phenomena occur if silicon is used as an electrode in an electrochemical cell. Effects include porous silicon layer (PSL) formation with features on a nanometer scale, surface roughening on a micrometer scale, quantum efficiencies for light generated currents much larger than 1, preferential etching of defects, electropolishing, and voltage or current oscillations. It is shown that despite the complexities of chemical reactions involved, a basic understanding of the electrode behavior is possible from a semiconductor physics point of view and that it can be advantageous to use the silicon — electrolyte junction for analytical purposes. Topics such as defect characterization, measurements of minority carrier diffusion length, or surface recombination velocities can be addressed in unique ways by taking advantage of particular properties of the silicon — hydrofluoric acid system. Based on the general description of the Si — electrolyte junction given in this paper, strengths and limitations of some electrochemical methods are discussed in some detail and illustrated by examples.

Investigation of self-organizing thiol films by optical second harmonic generation and X-ray photoelectron spectroscopy

Abstract: The adsorption of self-organizing thiol films on polycrystalline gold substrates was investigated in situ by optical second harmonic generation. Growth of the films could be detected at submonolayer coverage. For a comparison with classical surface analytical methods the thiol coverage was measured ex situ by X-ray photoelectron spectroscopy. We find that formation of ordered thiol films up to monolayer coverage can be described by Langmuir adsorption kinetics.

Solar energy materials. Overview and some examples

Abstract: This paper introduces materials for energy efficiency and solar energy utilization and discusses some current trends for basic research and development. Most of the materials involve thin surface coatings. Overviews are given for solar absorber surfaces, transparent infrared reflectors and transparent conductors, large-area chromogenics for transmittance control in “smart windows”, and transparent convection-suppressing materials, whereas solar cell materials are not included. The paper also treats a few examples of specific coatings that are presently being investigated; data are given for angular-selective transmittance through porous Cr films with oblique columnar microstructure, transparent and conducting non-stoichiometric SnO2 films, and chromogenic effects in Li-intercalated VO2 films.

Liquid phase epitaxy

Abstract: This paper presents a comprehensive review of the method of liquid phase epitaxy (LPE) of semiconductors. In Sect. 1 the physical principles including diffusion-limited growth and solid-liquid phase diagrams are treated in detail. In Sect. 2 technological aspects and various kind of growth systems including industrial versions are described. Section 3 summarizes the relevant properties of LPE grown layers. Section 4 contains the application of LPE to the material system InP/InGaAs/InGaAsP as a model system. In Sect. 5 the advantages and weaknesses of LPE with respect to device applications in comparison with competing methods are discussed, and finally we attempt to predict the future direction of LPE.

Total reflection X-ray fluorescence spectrometry for quantitative surface and layer analysis

Abstract: Measurements of X-ray fluorescence spectra versus grazing incident angles provide information on elemental composition as well as density and thickness of near surface layers. Calculations of fluorescence intensities are presented, which are used for the evaluation of data obtained by total reflection X-ray fluorescence (TXRF) spectrometry. The calculation is based on a matrix formalism to account for standing wave phenomena due to transmission and reflection in layered material. For the determination of concentrations the model makes additional use of the fundamental parameter technique in order to include absorption and enhancement effects of the fluorescence radiation. On the basis of experimental data some capabilities of this nondestructive and contactless probing technique are presented.

Analysis of reflectance data using the Kramers-Kronig Relations

Abstract: Kramers-Kronig Relations (KKR) are a well-known tool to interpret reflectance spectra by reconstructing the reflected wave's phase from its modulus with the help of a dispersion relation. However, a unscrupulous application is only possible in the case of semiinfinite media at perpendicular incidence.

New phenomena in homoepitaxial growth of metals

Abstract: The growth of Pt(111) by Pt vapour deposition is studied by He diffraction as a function of substrate temperature and deposition rate. At a deposition rate of about 2.5×10−2 monolayers/second several growth modes are observed: layer-by-layer (2D-) growth at 450 K≲Ts≲800 K, multilayer (3D-) growth at 340 K≲Ts≲450 K and reentrant layer-by-layer (2D-) growth at Ts≲340 K. The observed growth modes and in particular the reentrant 2D-growth are shown to be characteristic of growing Pt(111) under clean conditions, i.e. not influenced by contaminants. The influence of the intra- and interlayer mass transport on the growth mode is discussed in the light of experimental and simulation results. The 3D-growth mode is attributed to the existence of an activation barrier which suppresses the descent of adatoms from the top of the growing adatom islands onto the lower terraces. The barrier can be overcome by thermal adatoms at Ts≳450 K enabling interlayer mass transport which leads to 2D-growth. The reentrant 2D-growth occurs due to a break down of this barrier for small, irregularly shaped islands.

Two distinct transitions in ultrafast solid-liquid phase transformations of GaAs

Abstract: Femtosecond-laser-induced changes of the optical reflectivity and the reflected second harmonic are measured over a wide range of times and laser fluences. Changes of the linear and nonlinear optical properties suggestive of a transition to a metallic state have been observed. For relatively low fluences these changes take tens of picoseconds to develop. For higher fluences the transition takes place in just a few hundred femtoseconds. Our data suggest that, depending on the laser excitation conditions, two distinctly different types of phase transformations are observed.

Optical emission diagnostics of laser-induced plasma for diamond-like film deposition

Abstract: Optical emission from a laser-induced plasma plume is recorded during KrF excimer laser ablation of graphite in a gas mixture of Ar and H2 (3%) for deposition of diamond-like thin films. At sub-GW/cm2 laser intensities the spectrum is dominated by the bands of C2 and CN. From the band intensities, the vibrational temperatures of both radicals are calculated to be 12–15×103 K, and their concentrations are estimated to be 5×1014 cm−3 and 2×1014 cm−3, respectively.

Structure formation in UV-laser ablated poly-ethylene-terephthalate (PET)

Abstract: Structure formation observed in UV-laser ablated poly-ethylene-terephthalate (PET) foils can uniquely be assigned to mechanical and thermal pretreatments.

Microstructure of annealed low-temperature-grown GaAs layers

Abstract: The crystal structure and orientation of As precipitates in annealed low-temperature GaAs (LT-GaAs) layers have been investigated by transmission electron microscopy. Three types of As precipitates were identified in layers grown by molecular beam epitaxy at substrate temperatures from 180° to 210° C. In the monocrystalline LT-GaAs layers small “pseudocubic” As precipitates (2–3 nm diameter) coherent with the GaAs lattice were observed. These precipitates lose their coherency when a certain critical size is exceeded. Precipitates of similar sizes are occasionally found for which a TEM lattice image cannot be obtained. These precipitates are believed to be amorphous. Larger As precipitates with a hexagonal structure (>4 nm diameter) were also found in the layers. These hexagonal As precipitates were observed to be largest near structural defects. The effect of these precipitates on the structure and on the electronic properties of the host GaAs is discussed.

STM investigation of the adsorption and temperature dependent reactions of ethylene on Pt(111)

Abstract: The adsorption and reactions of ethylene adsorbed in UHV on Pt(111) have been studied as a function of temperature by STM. The STM images taken at 160K show an ordered structure of adsorbed ethylene. Annealing to 300 K produces ethylidyne (C-CH3) irreversibly, as has been demonstrated by a wide variety of surface science techniques. The ethylidyne on Pt(111) is not visible to the STM at room temperature. Cooling the sample allows direct observation of the ethylidyne ordered structure by STM. Annealing above 430 K results in further dehydrogenation, eventually leaving only carbon on the surface. The decomposition products appear as small clusters which are localized and uniformly distributed over the surface. Further annealing to temperatures >800 K results in the growth of graphite islands on the Pt(111) surface. The annealed graphite islands exhibit several supersturctures with lattice parameters of up to 22 A, which are thought to result from the higher order commensurability with the Pt(111) substrate at different relative rotations.

Detection of surface acoustic waves by scanning tunneling microscopy

Abstract: A new method for the investigation of ultrasonic waves on surfaces of solids based on scanning tunneling microscopy is presented. A sinusoidal high frequency signal is added to the tip voltage. Hence the tunneling current contains a component whose frequency is the difference of the frequencies of the acoustic wave field and the ac tip voltage. Amplitude and phase of this component carry the full information about the wave field.

Blow-off of Aluminium Films

Abstract: The blow-off of Al targets from a transparent support by single pulses of intense laser irradiation through the support is investigated for target thicknesses up to some μm. The blow-off process is theoretically described by calculations of the phase front dynamics in the target, adopted from Harrach's analytical solutions for laser heating and burnthrough of opaque solid slabs. Experimentally two laser beam sources were used: a Nd:YAG laser with a Gaussian intensity profile and a Nd:Glass-laser system with a homogenized flat-top profile. Two types of removal mechanism could be identified. In the first mode (low laser intensities and/or thin targets) the target is completely molten and can easily be blown-off by the vapor originating at the target-support interface. In the second mode (high laser intensities and/or thick targets) superheating of the liquid Al or a burst of the remaining solid layer of the target occurs. In both cases in the second mode the blow-off process is characterized by higher vapor pressures.

Relaxation semiconductors: In theory and in practice

Abstract: Relaxation semiconductors are materials dominated by free carrier transport and defined by the condition that the dielectric relaxation time τD is longer than the free carrier lifetime τ0. Novel transport behavior has been demonstrated, both theoretically and experimentally, to be associated with this regime of semiconductor behavior. This review surveys the history of the field, emphasizes recent experimental and modeling work and summarizes our current understanding of relaxation behavior in crystalline semiconductors.

Photoconductivity and light-induced absorption in knbo3:fe

Abstract: Measurements of photoconductivity and light-induced absorption in KNbO3: Fe are performed at different light intensities and crystal temperatures. The results are interpreted in terms of a two-center charge transport model. Different model parameters may be evaluated from the experimental data. A complete set of parameters is suggested explaining the dependences of photoconductivity and light-induced absorption on light intensity and temperature for the KNbO3: Fe crystal investigated.

Generation of three-dimensional free-standing metal micro-objects by Laser Chemical processing

Abstract: Three-dimensional free-standing metal micro-objects were created using Laser Chemical Vapor Deposition (LCVD) of aluminum from aluminum-trihydride-trimethylamine. A dense grid of thin lines (about 10 µm diameter) was deposited on a pre-formed polycarbonate substrate. The substrate was removed by dissolving it in a suitable solvent after ablating an unsoluble by-product by UV excimer laser pulses. The result is a free-standing three-dimensional aluminum grid micro-structure.

Porphyrin-sensitized laser swelling and ablation of polymer films

Abstract: Laser-induced morphological changes of poly(methyl methacrylate), poly(N-vinylcarbazole), and gelatin films doped with porphyrins have been studied by etch depth measurement and scanning electron microscopy. An irreversible swelling of the irradiated surface was observed for all films in the case of low laser fluence. The swelling was replaced by ablation when the fluence was increased. The etch depth depends on the irradiation fluence and the dye concentration in the polymer. The observation of the irradiated surfaces suggests that the thermal effect is predominant both for swelling and ablation. The surface temperature at which swelling or ablation is initiated was estimated, assuming that these morphological changes take place at a certain temperature for any dye concentration in each polymer film.

Individual, attractive defect centers in the SiO2-Si interface of μm-sized MOSFETs

Abstract: In a micron-sized p-channel MOSFET, the alternate capture and emission of holes into a Coulomb-attractive defect center is analyzed by the random telegraph signal in the source-drain current. Anomalous switching is observed with a high channel conductance when the defect center is occupied, and a low channel conductance after re-emission. The rate constants show an inverse symmetry for capture and emission. The measured results are interpreted by a tunneling transfer of a hole bound in the channel at the attractive center to the defect center 2.4 nm deep in the oxide and vice versa. The energy offset of the two stable configurations can be linearly varied by the gate voltage. An excited state 40 meV above the ground state is observed for the defect level. The anomalous switching is caused by a mobility change rather than by a change in mobile charge carrier density. The tunneling transfer for Coulomb-attractive centers differs from the transfer observed for Coulomb-repulsive centers where activated emission is reported.

Surface and bulk electrical properties of the hematite phase Fe2O3

Abstract: Electrical properties of Fe2O3 were studied by using several electrical methods such as electrical conductivity, thermopower (Seebeck effect) and work function. The studies were performed at elevated temperatures (1053–1153 K) and under controlled oxygen activity (102∓105 Pa). Samples of different thickness varying between 103 nm and 1 mm were taken for the measurements of both electrical conductivity and thermopower. It has been found that the exponent of the po2 dependence resulting from the work function measurements (1/nφ) is about 1/2. Both thermopower and electrical conductivity data are well consistent with work function data for the thin film (1000 nm) of Fe2O3. The charge transport in Fe2O3 has been interpreted in terms of small polaron mechanism. Analysis of measured electrical parameters, regarding the thickness of studied specimens, indicates that the near-surface layer of Fe2O3 exhibits much higher deviation from stoichiometry than the bulk phase and resulting strong interaction between charge carriers. This effect has been interpreted in terms of segregation of intrinsic lattice defects to the surface, and presumably also to grain boundaries, of Fe2O3.

Mapping the fermi surface of graphite with a display-type photoelectron spectrometer

Abstract: A display spectrometer is used to image the momentum distribution of photoelectrons from the Fermi level in graphite. The Fermi “surface” consists of six points at the corners K of the hexagonal, two-dimensional Brillouin zone, in agreement with band theory. The method is also applied to other equal energy surfaces below the Fermi level, thereby giving the band dispersion of the π-bands.

Growth and melting behaviour of thin in films on Ge(100)

Abstract: Growth and melting behaviour of thin indium films on Ge(100) have been investigated by Auger-electron spectroscopy (AES), atomic force microscopy (AFM) and perturbed γγ angular correlation (PAC) spectroscopy, respectively. At room temperature inidium is found to grow in three-dimensional islands even at submonolayer coverages. A very rough film surface is observed for thicknesses up to 230 ML. The melting behaviour of such films has been studied by PAC. A reduction of the melting temperature T m as well as a strong supercooling of the films is observed. The electric field gradient for 111In(111Cd) in the indium islands is determined as a function of temperature and is used to monitor the local crystalline order of the films up to temperatures just below the melting point.

ESCA studies of Si-Fe alloys

Abstract: ESCA examination of films formed on Si-Fe alloys after immersion in 0.1 M NaCl for 24 h has hown that the thickness of passive films decreased with an increase in silicon content. A thick passive film containing oxidized silicon and oxidized iron was formed on Fe-20 wt% Si and the oxidized iron was about three times higher than the oxidized silicon in the passive film. However, an obvious reduction in the oxidized iron in the film on Fe-30 wt % Si was observed. Oxidized iron was detected up to a depth of 1.0 nm and at a depth greater than 1.0 nm from the surface, the film was exclusively in oxidized silicon. The film was exclusively silicon oxide when the silicon content was increased to 50 wt %. Electrochemical techniques according to ASTM G59 and ASTM G5 were used for the determination of the relative corrosion rate. Fe-50 wt % Si was found to have a corrosion rate smaller than those lower silicon alloys. This relates to the surface film composition and structure as determined by ESCA.

Matching factors for various light-source-photodetector combinations

Abstract: The matching between the spectral distribution of the light emitted from a light source and the spectral distribution of the sensitivity of a light detector is characterized by the “matching factor”. In the paper we present a table of 500 matching factors for the combination of various phosphors with the most common photodetectors.

Phonons on surfaces: The importance of structure and adsorbates

Abstract: Recent progress in detecting thermal energy vibrations at solid surfaces will be reviewed. Phonons on clean, well-ordered metals will be discussed as well as vibrations within ordered and disordered adlayers of adsorbed atoms and molecules. Simple lattice dynamical models will be used to demonstrate the interconnection between vibrations, structure, and force fields at a surface.

Pulsed laser processing of Ge/Se thin film structures

Abstract: The comparison between results of a systematic experimental study on Q-switched ruby laser processing of supported thin Ge/Se bilayer structures and model calculations permits us to unfold the rather complicated events into elementary steps. Depending on the sequence of the elemental layers and the absorbed fluence, compound synthesis, total or partial ablation of one of the constituents or the compound formed, and simultaneous transfer of the ablated material onto a separate substrate in close proximity is possible with a single laser pulse. The striking agreement between experimental findings and model calculations indicates the validity of the simple thermal model applied. The results establish a novel, inherently simple, single-step technique for local deposition of compound films from stacked elemental layers as a source onto any substrate.

Ultraviolet photoablation of a plasma-synthesized fluorocarbon polymer

Abstract: Plasma polymerized tetrafluoroethylene (PPTFE) is shown to undergo efficient 248 nm excimer laser ablation. The principle difference between this material and the analogous polytetrafluoroethylene (PTFE), which results in only poor quality ablation, is PPTFE's much greater absorption coefficient (7×104 vs. ∼102 cm−1). A plot of the ablation depth per pulse versus incident fluence indicates that the threshold for significant ablation occurs near 50 mJ/cm2, and that approximately 0.7 μm/pulse can be removed at 800 mJ/cm2. Near threshold, the ablation rate curve can be fit by a single Arrhenius-type exponential. This suggests that the removal process is at least partially governed by a photothermal process, similar to well-known laser induced thermal desorption experiments. In the very low fluence regime between 10 and 30 mJ/cm2, small removal rates are measured in a process likely dominated by non-thermal ablation. The paper concludes with a discussion of the high quality, micron-size features that can be directly patterned into PPTFE surfaces.