Lateral Homogeneity Research Articles

ITO/PVK/Alq/metal LEDs: influence of PVK doping with DCM and of passivation with sputtered Si 3N 4

This paper reports on the fabrication and on the characterization of LEDs consisting of two layers: a spinned-on hole transport layer, PVK layer and an evaporated tris(8-hydroxy) quinoline aluminum (Alq) emitter and an electron injection layer. Dye-doping of PVK with DCM induced a red shift of the spectrum due to concomitant emission from Alq and DCM. Some devices were passivated with Si 3N 4 deposited by RF sputtering. In that latter case, the threshold voltage of the LED slightly increased, but the emitted power was kept constant. The LED surface was imaged with a lateral resolution as low as 30 μm using Scanning ElectroLuminescence Imaging Microscopy (SELIM), a technique derived from the well-known Scanning Photo-Luminescence (SPL) widely used for III–V and II–VI semiconductors. This technique was successfully used to assess the lateral homogeneity of the emitting surface, and the aging of the diodes. At increasing working time, the devices did not show dark spots formation but a uniform decrease of the luminescence.

Lateral inhomogeneities in engineered Schottky barriers

We have measured the lateral homogeneity in Al/GaAs(0 0 1) Schottky barriers engineered for low barrier height through fabrication of a Si layer at the interface under As flux. We used a spectroscopic photo emission and low-energy electron microscope (SPELEEM) which combines a low-energy electron microscope with an imaging band pass filter to allow spatially resolved synchrotron radiation photoemission experiments. As-grown samples with a Si interlayer thickness of 0.5 and 3 monolayers appeared homogeneous within the spatial resolution of the SPELEEM (22 nm). Annealing an Al/Si(As)/GaAs(0 0 1) heterostructure for 10 min at 500°C, however, caused inhomogeneous As out-diffusion which is correlated with a local As 3d core level shift of 0.3 eV. We suggest that the reported degradation of such engineered Schottky barriers might be correlated with laterally inhomogeneous As out-diffusion upon annealing.

Growth of CdS/ZnS strained layer superlattices on GaAs(0 0 1) by molecular-beam epitaxy with special reference to their structural properties and lattice dynamics

Strained layer CdS/ZnS superlattices have been grown on GaAs(0 0 1) by molecular-beam epitaxy using CdS and ZnS compound sources. The samples were investigated with special reference to their structural properties and lattice dynamics by means of X-ray diffraction and Raman spectroscopy. The results of four superlattices with different period length are discussed in detail. X-ray diffraction profiles show superlattice satellite peaks up to the fourth order indicating a high degree of periodicity. The lateral and in-depth homogeneity of the period length is also confirmed by Raman investigations. Folded longitudinal acoustic phonons in CdS/ZnS superlattices were observed for the first time. The experimental values agree very well with theoretical calculations based on the Rytov model and show the expected dependency on the superlattice period. The behaviour of the optical phonons is mainly determined by strain induced shifts caused by the high lattice mismatch (−7%) for this system. A good agreement between theoretical predictions and detected frequencies is obtained.

Correlation between Structural Properties and Optical Amplification in InGaN/GaN Heterostructures Grown by Molecular Beam Epitaxy

AbstractWe comprehensively studied InGaN/GaN heterostructures grown by molecular beam epitaxy (MBE) using a variety of methods of optical spectroscopy, such as cathodoluminescence microscopy (CL), time-integrated and time-resolved photoluminescence. To correlate the fluctuations in emission wavelength with values for the optical amplification we performed gain measurements in edge-stripe geometry. The lateral homogeneity can be drastically improved using a template of GaN grown on the sapphire substrate by metal-organic vapor phase epitaxy (MOVPE). Gain values up to 62 cm-1 were found in samples with low indium fluctuations, which is comparable to values for high-quality InGaN/GaN heterostructures grown by MOVPE.

Very high compositional homogeneity of 1.55 μm strain-compensated InGaAsP MQW structures by MOVPE under N 2 atmosphere

In order to improve the compositional homogeneity in metalorganic vapor phase epitaxy (MOVPE), growth under N 2 used as carrier gas has been applied for the first time to strain-compensated InGaAsP multi-quantum-well (MQW) structures on InP. Various InGaAs(P) layers and MQW structures have been grown by low-pressure MOVPE in a horizontal reactor at total gas flows Q tot=3.0–7.6 slm with substrate rotation using standard precursors. By replacing H 2 by N 2 the lateral homogeneities of the gap wavelength, lattice mismatch and layer thickness were considerably improved. The role of the carrier gas is discussed. The standard deviation σ(λ) of the wavelength over 40 mm of a 50 mm wafer of strain-compensated λ=1.55 μm laser structures with 10 QWs was reduced from 6.5 nm (H 2) to 0.55 nm (N 2), which is a factor of about 10! This is one of the best homogeneity values reported so far for InGaAsP grown in any type of MOVPE reactor with any precursors. Details of the growth parameters and results are presented along with the characteristics of distributed feedback (DFB) lasers and laser arrays produced from these structures.

Si-doping of MOVPE grown InP and GaAs by using the liquid Si source ditertiarybutyl silane

The liquid Si-compound ditertiarybutyl silane (DTBSi) has been investigated as a doping source for the metal organic vapour-phase epitaxy (MOVPE) of InP and GaAs using TBP or DTBP and TBAs as less hazardous group-V-sources. For both material systems the measured carrier concentration is directly proportional to the DTBSi/group-III-partial pressure ratio with a slope of unity. Uncompensated n-type InP as well as GaAs layers are achieved up to the 10 18 cm −3 doping range. Additional calibrated SIMS-studies of InP layers show a coincidence of the Si-concentration in the InP layers with the measured net n-type carrier concentration within the experimental errors. With a reduction of the growth temperature from 610°C to 570°C a significant reduction of the Si-doping efficiency is observed. However, for temperature in excess of 610°C up to 700°C an almost constant doping efficiency for GaAs is obtained. For GaAs a slight reduction in Si-incorporation efficiency with increasing V/III-ratio is detected. The successful doping characteristics for GaAs have been applied to realize n-GaAs : Si Hall sensor device structures. The lateral homogeneity of the sheet resistance is evaluated as a function of the V/III-ratio and the substrate material. Under optimum conditions the normalized variance of the sheet resistance can be as low as 1%.

Surface- and microanalytical characterization of silicon-carbonitride thin films prepared by means of radio-frequency magnetron co-sputtering

Thin Si-C-N films were produced by means of reactive radio-frequency (RF) magnetron co-sputtering using a special geometry for the sputter target consisting of silicon and carbon. The stoichiometry and the vertical and lateral homogeneity of these layers as well as the reproducibility have been investigated by means of X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), non-Rutherford backscattering spectroscopy (n-RBS) and nuclear reaction analysis (NRA). After calibration of the surface analytical techniques using absolute data derived from the nuclear methods, AES sensitivity factors for the ESCALAB-5 spectrometer (SSi=1.65, SC=0.57, SO=1.10, SN=0.50) as well as the sputtering yield for ternary Si-C-N compounds (YSi-C-N=1.00) have been estimated. The corresponding XPS binding energies were found for Si 2p=102.8 eV (Si-N-C), C 1s=285.9 eV (C(sp2)-N) and C 1s = 288.2 eV (Si-N-C), N 1s = 398.9 eV (Si-N-C) and N 1s = 400.6 eV (N-C(sp2)).

Analysis compositional homogeneity for MOVPE grown ternary alloys in the horizontal reactor with a rotating susceptor

Using the rotation of the susceptor, the lateral homogeneity of the epilayers can be improved in a horizontal MOVPE-reactor, but the uniformity of the composition along the growth direction has been neither theoretically nor experimentally studied. In this paper the model developed by Xu et al. [J. Crystal Growth 187 (1998) 203] has been applied to discuss both the lateral and vertical homogeneity of ternary alloys grown on the rotating susceptor in a horizontal MOVPE-reactor. The model calculation has revealed that the rotation results in a lateral uniformity, but at the same time it also causes a periodical variation of the composition in the growth direction.

Calculation of lateral distribution of lattice constant for horizontal MOVPE grown quaternary alloys

Lateral compositional inhomogeneity for ternary and quaternary alloys is a general problem in horizontal MOVPE system. According to the model developed by Xu et al. in previous work, a calculation method for analysing lateral homogeneity of lattice constant in horizontal MOVPE grown quaternary alloys was presented. Based on the conclusions of this model several experiments were performed. The experimental results demonstrated that this model has properly described the compositional uniformity of MOVPE grown ternary and quaternary alloys.

Propagation factor errors due to the assumption of lateral homogeneity

Propagation conditions are frequently assessed by making a single measurement of the vertical refractivity and injecting this profile into a propagation model from which the two‐dimensional propagation factor field is calculated. Such an assessment tacitly assumes lateral homogeneity of the refractivity environment. That is, the refractive index is assumed to vary only as a function of height above the Earth's surface. Hitherto, the authors, using only case studies, have determined errors in propagation assessment employing the above assumptions. The objective of this paper is to characterize the errors in the propagation factor field using a statistical database of multiple arrays of refractivity profiles covering many days. In this effort, errors in the propagation factor field at 1, 3, and 10 GHz are derived for the case in which single refractive index profile measurements are used to calculate the two‐dimensional propagation factor field. This “error” field is subsequently compared with the “benchmark” or “truth” propagation factor field calculated using an array of refractivity profiles. The errors are statistically deduced by combining measurements from 30 days over defined two‐dimensional areas. The methodology for which the above type of propagation assessment is addressed may also be used to establish temporal and lateral spacing requirements employing similar statistical databases.

Formation of Nickel, Cobalt, Copper, and Iron Aluminates fromα- andγ-Alumina-Supported Oxides: A Comparative Study

The rates of metal aluminate formation inMeOx/Al2O3systems (Me=Ni, Co, Cu, Fe) at 500–1000°C in O2or N2atmospheres were compared, using bothα-Al2O3andγ-Al2O3substrates. Interfacial reaction toMeAl2O4was assessed using Rutherford backscattering spectrometry and X-ray diffraction. The lateral homogeneity of the samples was investigated using scanning electron microscopy. The spinel formation rate follows the sequence FeAl2O4<NiAl2O4<CoAl2O4<CuAl2O4. This is explained in terms of thermodynamic limitations (FeAl2O4) and site-preference energies (CoAl2O4, NiAl2O4). The fast reaction to copper aluminate was accompanied by a severe rupture of the top layer (∼1 μm) of the substrate. Penetration of transition metal ions intoγ-Al2O3substrates is notable at considerably lower temperatures than withα-Al2O3substrates, mainly due to grain boundary diffusion. Annealing conditions that favor the formation of transition metal cations of a low valency (Co2+, Cu+) are beneficial for reactions to aluminates.

Lateral implantation homogeneity of wedge-shaped samples treated by plasma immersion ion implantation

Plasma immersion ion implantation (PIII) is a rapidly developing technique for ion implantation in metallurgical and semiconductor applications. In contrast to conventional beam-line ion implantation, the treatment of three-dimensional samples does not require complicated workpiece and ion beam manipulation. However, recent results have shown that, in addition to the shape of the workpiece, treatment homogeneity is strongly dependent on process parameters such as ion density and gas pressure. Knowledge of homogeneity is of great importance for the further development of PIII. In the present paper, homogeneity measurements of a model system are discussed. Wedge-shaped silicon specimens with different angles (30 ° to 90 °) were treated by plasma immersion ion implantation with argon ions with 45 kV high-voltage pulses. The samples were analyzed for lateral implantation concentration by Rutherford backscattering measurements. The implanted concentration, the projected range and their gradient are a strong function of the edge angle. The results show that for acute angles (30 °), gradients in implantation concentration up to a factor five develop.

Zircon typology and geochemistry in the palaeogeographic reconstruction of the Late Ordovician of Sardinia (Italy)

The present study concerns the Upper Ordovician sedimentary succession which crops out in the Sulcis-Iglesiente area autochthon and in the Sarrabus-Arburese allochthonous units, with the aim of contributing to a better understanding of the pre-Hercynian palaeogeography. This study is based on the typology of zircon and trace-element geochemistry of zircon and titaniferous minerals. The Upper Ordovician rocks cropping out in the Sarrabus-Arburese area (allochthon belt) mainly result from the erosion of pre-Caradocian volcanics (the so-called ‘Porfidi grigi’), which were emplaced in a magmatic arc setting. The lateral and vertical homogeneity of the terrigenous supply during the Caradoc and early Ashgill, and the argillaceous-carbonate sediments with shallow shelf faunas during Ashgillian times, suggest that stable geodynamic conditions persisted throughout this time span. In the Sulcis-Iglesiente area (autochthon belt), the nature of the terrigenous influx and the source-rocks remained the same from the Late Cambrian to the early Ashgill. The constant nature of the detrital zircon grains rules out any possibility that the ‘Porfidi grigi’ might have contributed to the sedimentary supply; the source-rocks were probably located within an internal cratonic area, distinct from the magmatic arc of the Sarrabus area. In the early Himantian, new source-rocks contributed to the terrigenous supply. They were volcanics with a strong mantle component. This new material was later progressively diluted by terrigenous supply of cratonic origin. The significant differences in the composition and origin of the terrigenous material deposited in the autochthonous and the allochthonous units may reflect distinct source-areas with different geodynamic histories. It is therefore possible that the two depositional areas were geographically separated. Location of the Sulcis-Iglesiente area within the North Gondwanan shelf is a. plausible scenario. But the occurrence of Baltic taxa in the Sarrabus area suggests that this area was originally located in an intermediate position between North Gondwana and Baltica.

Comparison of PdGeTiPt and NiGeAu ohmic contacts to n-GaAs and PdGeTiPt and TiPd contacts to p+-GaAs

NiGeAu and PdGeTiPt ohmic contacts to n-GaAs and TiPd and PdGeTiPt ohmic contacts to p+-GaAs are examined by comparing their contact resistances, chemical intermixing as determined by Auger electron microscopy, interface structure as determined by transmission electron microscopy, and surface roughness as determined by surface profiling all measured as a function of annealing time and temperature. The n-PdGeTiPt contact annealed for short times, ⩽15 s, and at low temperatures, ⩽395 °C, was superior to the NiGeAu contact because it had a comparable contact resistance, less interface mixing, better lateral homogeneity, and a smoother surface. However, its contact resistance increased substantially with the annealing time and temperature, whereas the NiGeAu contact was relatively unaffected. For all annealing times and temperatures except the one at 550 °C, the TiPd contact to p+ GaAs was superior as it had a lower contact resistance and a comparable amount of interface intermixing, lateral homogeneity, and surface roughness. However, it had a complete chemical breakdown at 550 °C, whereas the PdGeTiPt contact resistance remained relatively stable.

An analytical approach to migration velocity analysis

Prestack depth migration provides a powerful tool for velocity analysis in complex media. Both prominent approaches to velocity analysis—depth‐focusing analysis and residual‐curvature analysis, rely on approximate formulas to update velocity. Generally, these formulas are derived under the assumptions of horizontal reflector, lateral velocity homogeneity, or small offset. Therefore, the conventional methods for updating velocity lack accuracy and computational efficiency when velocity has large, lateral variations. Here, based on ray theory, I find the analytic representation for the derivative of imaged depths with respect to migration velocity. This derivative function characterizes a general relationship between residual moveout and residual velocity. Using the derivative function and the perturbation method, I derive a new formula to update velocity from residual moveout. In the derivation, I impose no limitation on offset, dip, or velocity distribution. Consequently, I revise the residual‐curvature‐analysis method for velocity estimation in the postmigrated domain. Furthermore, my formula provides sensitivity and error estimation for migration‐based velocity analysis, which is helpful in quantifying the reliability of the estimated velocity. The theory and methodology in this paper have been tested on synthetic data (including the Marmousi data).

Upper ocean thermohaline fields near 2°S, 156°E, during the Tropical Ocean‐Global Atmosphere‐Coupled Ocean‐Atmosphere Response Experiment, November 1992 to February 1993

Zonal and meridional Seasoar sections centered at 1°50′S, 156°06′E were repeated &gt;30 times in three 20‐day periods between November 13, 1992, and February 15, 1993. Both sections were 130 km long, and sampling depth was 0–280 m, with a vertical resolution of ∼2 dbar (2×104 Pa) and a horizontal resolution of ∼3 km. The observed fields are complex and variable and are summarized graphically in several forms. Characteristics of the near‐surface layer varied with the local winds which were variable and weak (&lt;6 m s−1) during the first 20‐day period, strong and westerly (&gt;10 m s−1) during much of the second, and moderate and westerly (4–10 m s−1) during most of the third. Near‐surface temperatures were warmest (up to 30°C) during the first period and coldest (often &lt;29°C) during the second. Thermal stratification in the near‐surface layer was strong under weak winds and weak under strong and moderate winds. Except during and after heavy rainfall, salinity stratification in the near‐surface layer was generally weak. Surface salinity generally decreased toward the north. The depth of the surface isopycnal layer was often but not always limited by salinity stratification; the surface isohaline layer was shallower than the top of the thermocline throughout. Strong lateral temperature and salinity gradients occurred on a few occasions. Increased wind stress was associated with lateral homogenization as well as vertical mixing. Structure and water properties of the thermocline also varied between cruises and within each cruise. The upper thermocline was shallowest in late January after prolonged easterly winds. Isotherms in the upper and midthermocline (20°–25°C) sloped generally upward to the north, while those in the lower thermocline (12°–14°C) sloped down to envelop the core of the Equatorial Undercurrent, which shoaled (from 225 to 160 m) and warmed (from 15°to 20°C) between the first and last survey periods. Mesoscale and fine‐scale water mass features were usually recognizable in sections less than a few days apart and migrated eastward at about 0.3 m s−1. There is a remarkably high degree of nonstationarity in these thermohaline fields from the Warm Pool of the western Pacific Ocean.

Bulk sound travel times and implications for mantle composition and outer core heterogeneity

We present a new methodology for constraining mantle composition directly from observed seismic travel times. By measuring travel times of PcP, PcS, ScS, and PKiKP phases generated by the Chinese nuclear explosion of 21 May 1992 and recorded at epicentral distances of ∼10°, we directly determine bulk sound travel times (TΦ' for the mantle and outer core. This eliminates the need to combine independent VP and VS profiles, characterizing different paths and frequencies, to obtain a reference VΦ profile. Our observed PKiKP‐PcP differential travel time indicates that the outer core model of AK135 is slow by 2.1 s (that of IASP91 by 2.3 s' while that of PREM is slow by only 0.7 s and suggests, through comparison with other data, relative lateral homogeneity of the outer core. Furthermore, we find that T0Φ for the crust and mantle in this region exceeds the AK135 prediction by 3.5 s (IASP91 by 3.9 s' ±1.0 s. Since the lower mantle contribution to this delay depends upon how much arises in the upper mantle, we present two end‐member models for such a slower mantle VΦ, in which the delay resides entirely above or below 660 km depth. The latter yields best‐fitting lower mantle compositions which are less enriched in Si than those fit to T0Φ for AK135 (or IASP91' for the former, and neither supports Feenrichment of the lower mantle. Applicability of such localized observations to the larger mantle is suggested by tomographic results indicating that lateral heterogeneity in VΦ is very small relative to that in VS. Our results illustrate the potential utility of T0Φ in constraining mantle composition.

Ex-situ spectroscopic ellipsometry studies of micron thick CVD diamond films

Micron thick diamond films have been studied by spectroscopic ellipsometry (SE). The films were grown, on previously prepared Si(100) substrates, by the plasma enhanced chemical vapor deposition (PECVD) technique. Ex situ SE measurements were carried out on samples grown under different conditions, such as substrate temperature and methane fraction in the gas mixture. An optical model consisting of five layers was constructed in order to explain the SE spectra and to provide the optical and structural parameters of the films. This model was deduced from results of various measurements performed by other characterization techniques (Raman spectroscopy, scanning electron microscopy, atomic force microscopy and positron annihilation spectroscopy) which have revealed the optical and structural parameters of the samples. Its sensitivity to the surface and interface roughness as well as to the absorption of the nondiamond phase of the film is demonstrated. Several values of the percentage of the nondiamond phase can be obtained, with the same fit quality, however, depending on the amorphous carbon reference used in the model. These references were obtained by performing SE measurements on various amorphous carbon films. Finally, our SE analysis has allowed us to monitor the lateral homogeneity of the thickness, surface and interface roughness and nondiamond phase concentration over the diamond film.

Investigation of ultrathin iron and iron/nickel layers prepared from Langmuir-Blodgett films

Oxidic and metallic iron and iron/nickel multilayers, formed after thermal and chemical treatment of Langmuir-Blodgett films, were investigated. Reduced iron-stearate multilayers were compared with evaporated films concerning their phase composition and their lateral homogeneity. The sequence of metallic and oxidic iron in the films after reduction was determined. Oxidic iron/nickel mixed layers can be prepared from Langmuir-Blodgett films, in which a magnetically non-ordered Fe3+ phase exhibiting a distribution of the quadrupole splitting is observed. After reduction of such layers, two metallic iron phases appear dependent on the nickel concentration. The hyperfine field distribution of the magnetically ordered metallic phase can be evaluated using a method proposed by Rancourt.

MOVPE for InP-based optoelectronic device application

The rapid development of fibre optical communications requires highly sophisticated devices for the 1.3 μm and 1.5 μm wavelength regions. The basis for the development of devices like lasers, detectors, modulators, wavelength converters and optical switches as single devices or in integrated form is the availability of high quality epitaxial layers. The requirements are the exact and reproducible control of the composition (lattice matched or strained layers), the lateral and vertical homogeneity of composition, doping level and layer thickness as well as the atomically sharp heterointerface, the very low (crystalline and surface) defect density and thermal stability of a layer structure. The epilayers should have a low background doping level, i.e. a minimum concentration of unintentional dopants.