Search

The Karoo region of South Africa is an ideal laboratory to use ambient seismic signals to map the shallow subsurface, as it is a quiet and pristine environment with a geology that is relatively well known. Ambient seismic signals were continuously recorded for a ten week period between August and October 2015. The ambient seismic noise network consisted of two groups of 17 temporary, stand-alone seismic stations each. These were installed in the southeastern Cape Karoo region, near the town of Jansenville. Here we present data on the retrieval and coherency of Rayleigh surface waves extracted from the vertical component recordings. We reconstruct and show, for the first time in the southeastern Cape Karoo, estimates of Green's function from cross-correlating ambient noise data between stations pairs, which can be successfully used to image the subsurface. The stacked cross-correlations between all station pairs show clear arrivals of the Rayleigh surface waves. The group velocities of the Rayleigh waves in the 3 to 7 seconds period range were picked and inverted to compute the 2-D group velocity maps. The resulting 2-D group velocity maps at different periods resulted in a group velocity model from approximately 2 to 7 km depth, which shows a high velocity anomaly in the north of the study area, most likely imaging the denser, thick sedimentary basin of the Karoo (Carboniferous-Permian). To the south, the low velocity anomaly could correspond to the overlying Jurassic-Cretaceous sedimentary sequences of the younger Algoa Basin (Uitenhage Group).

We report on the characterization of an epitaxial layer of InAs0.94Sb0.06 grown on semi insulating GaAs by metal organic vapour phase epitaxy (MOVPE). Transmission electron microscopy, x-ray diffraction, photoluminescence and photoconductivity spectroscopy were used to characterize the layers in terms of its structural and luminescent properties, with emphasis on residual stress/strain, extended defects like dislocations, band gap energies and impurity states. Temperature dependant Hall measurements were also performed. The composition of the layer as well as strain relaxation was confirmed by High resolution X-ray diffraction. Photoluminescence produced a broad emission peak which was shown to be a convolution of extrinsic recombination and band-to-band transitions. A comparison between photoluminescence and photoconductivity results confirmed this analysis. The absorption edges, determined from photocurrent spectroscopy measurements at different temperatures, were fitted with an Einstein single oscillator model, yielding a 0 K band gap energy of 355 meV. Hall mobility results showed a peak mobility of 9.5 × 103 cm2 /V.s at 115 K; results were fitted with a two-layer Hall model yielding a bulk peak mobility of 22.4 × 103 cm2 /V.s at 88 K, with a majority donor carrier concentration of roughly 5 × 10-15 cm-3. These results continue to show that InAs1-xSbx has great potential for use within mid-wave infrared detectors, however further improvement in deposition growth is still needed for final device manufacture.

The growth of atmospheric pressure metal-organic vapour phase epitaxial (AP-MOVPE) GaSb and Ga 1− x In x Sb layers with different nominal V/III ratios (1.0–2.0) at 550 °C, using trimethylantimony (TMSb), trimethylindium (TMIn) and triethylgallium (TEGa), is being reported. Ga 1− x In x Sb layers were grown with a constant indium mole fraction in the vapour phase ( x v ) in order to study the dependence of the indium mole fraction in the solid ( x ) on the V/III ratio. The surface morphology, crystallinity and optical properties of these layers are compared to those of GaSb layers with the aim of understanding their level of sensitivity to changes in the V/III ratio. It has been observed that the indium incorporation into GaSb increases with an increase in the V/III ratio. The shift of the free exciton/band-to-band transition in Ga 0.96 In 0.04 Sb with temperature follows the Varshni relation: E ( T ) = 0.777 – 7.559 × 10 − 4 T 2 / ( 720 + T ) .

The triethylgallium/trimethylantimony (TEGa/TMSb) precursor combination was used for the metal-organic vapour phase epitaxial growth of GaSb at a growth temperature of 520 °C at atmospheric pressure. Trimethylindium was added in the case of Ga1−xInxSb growth. The effects of group V flux to group III flux ratio (V/III ratio) on the crystallinity and optical properties of GaSb layers are reported. It has been observed from the crystalline quality and optical properties that nominal V/III ratios of values greater than unity are required for GaSb epitaxial layers grown at this temperature. It has also been shown that Ga1−xInxSb can be grown using TEGa as a source of gallium species at atmospheric pressure. The relationship between Ga1−xInxSb vapour composition and solid composition has been studied at a V/III ratio of 0.78.

Excess carrier lifetimes (77 K) have been measured as function of the absorbed flux density in undoped p-type gallium antimonide films (GaSb/GaAs) using steady state photoconductivity measurements with the illumination wavelength of 1.1 μm. Using the results from Hall effect measurements along with the relations describing the lifetimes of the excess minority carriers in the bulk of the films and at the surface, the theoretical values of the effective excess carrier lifetime in the materials were also calculated. Discrepancies between the experimental and theoretical results were described using a two-layer model, by considering the variation in the charge distribution within the layer due to the presence of surface states, as well as the band offset between the layer and the substrate. Theoretical modeling of the experimental result yields values of different parameters such as band bending at the surface, minimum value of Shockley–Read–Hall lifetime and maximum value of the surface recombination velocity.

The free carrier concentration and compensation ratio of unintentionally doped InAs were reliably determined by evaluating the conduction characteristics of as-grown and hydrogenated epilayers. The free carrier concentration has also been obtained using steady-state capacitance measurements performed on p–i–n diodes fabricated using various growth conditions. The influence the growth temperature has on the electrical characteristics of unintentionally doped InAs thin films is presented.

This paper compares the galvanometric and thermoelectric evaluation of the electrical characteristics of narrow gap semiconductors. In particular, the influence of a surface inversion layer is incorporated into the analysis of the temperature-dependent Hall and thermoelectric measurements of p-type InAs. The temperature at which the Seebeck coefficient of p-type material changes sign is shown to be unaffected by the presence of degenerate conduction paths. This finding consequently facilitated the direct determination of the acceptor density of lightly doped thin film InAs.

During August and September 2015, a seismic network consisting of 17 stations was installed in the south-eastern Cape-Karoo region, near the town of Jansenville. Ambient seismic signals were continuously recorded for a three week period. In this chapter, we reconstruct estimates of the seismic Green’s functions between sensor pairs by cross-correlating the ambient seismic signals recorded in the vertical component of each station. The resulting Green’s functions contain clear direct Rayleigh wave arrivals. The measured group velocity dispersion curves of the Rayleigh waves were averaged in the period range from 2.5 to 5.5 s (approximately 2–5 km depth). The arrival times of the Rayleigh waves are picked at different periods and then inverted to compute 2-D group velocity maps. This resulted in a velocity model up to depths of 5 km. The results reveal two different velocity regions, broadly corresponding to the Cape Fold Belt and the flanking Karoo Basin. The higher group velocity anomalies (3–5 km in thickness) most likely represent the Carboniferous-Permian sequences of the Karoo Basin (Dwyka-Ecca-Beaufort Groups). A lowermost velocity region in the south-eastern study area could correspond to the Jurassic-Cretaceous sequences of the Algoa Basin that directly overlie the Cape Fold Belt.

This paper reports on the optical and structural properties of strained type-I Ga1−xInxSb quantum wells embedded in GaSb from a metal-organic vapour phase epitaxial growth perspective. Photoluminescence measurements and transmission electron microscopy were used to evaluate the effect of the growth temperature on the quality of Ga1−xInxSb strained layers with varied alloy compositions and thicknesses. Although the various factors contributing to the overall quality of the strained layers are difficult to separate, the quantum well characteristics are significantly altered by the growth temperature. Despite the high growth rates (∼2 nm/s), quantum wells grown at 607 °C display photoluminescence emissions with full-width at half-maximum of 3.5–5.0 meV for an indium solid content (x) up to 0.15.

Parts of the gas-phase spectrum of the linear cumulene-type molecule tricarbon oxide sulfide, OCCCS, were recorded in the region of its lowest-lying CCC bending vibration ν 7 around 2.4 THz. The Doppler-limited spectrum was obtained at low temperatures using a tunable far-infrared spectrometer. In addition, the rotational spectrum was recorded between 78 and 118 GHz with microwave techniques at room temperature. The analysis of 412 rotational transition frequencies obtained during this and previous work allowed us to assign 71 resolved rovibrational transitions of the ν 7 band system in the far-infrared region. They are distributed among 16 subbands of the fundamental band and the first three hotbands. The center of the fundamental band was found to be at 2 325 894.20(30) MHz. Forty-five spectroscopic constants describing the vibrational states ν 7 = 0, . . . , 4 were obtained by fitting all data simultaneously to an effective Hamiltonian for polyatomic linear molecules. The anharmonicity coefficient x 0 77 of the bending vibration was found to be only −0.2% of the harmonic frequency ω 0 7 , which was determined to be ω 0 7 = 2 320 995(13) MHz.

The r Q K a branches of carbodiimide, HNCNH, have been recorded with a tunable far-infrared spectrometer. The r Q K a branches with K a ∈ { 2, 3, 4 }, which occur at approximately 1.8, 2.5, and 3.2 THz, respectively, were measured rotationally resolved. The torsional doublet splitting observed in the r Q 0 branch and several r R branches by Birk et al. ( J. Mol. Spectrosc. 136, 402-445 (1989)) was also observed in all the Q branches reported here. In addition, anomalous K a -type doubling was detected within the r Q 2 branch for rotational quantum numbers J > 18. Molecular constants were obtained by fitting the line positions, including older data, with Watson′s Hamiltonian in S -reduction. The anomalous K a -type doubling could be described as a centrifugal distortion interaction with Δ K a = 4. The splitting is called anomalous because it inverts the K a = 2 energy levels of an accidentally nearly prolate symmetric top molecule (Ray′s asymmetry parameter for HNCNH κ = −0.999995343(32)).

Background Pancreatic resections can be performed with great safety. However, the morbidity rate is reported to be 40–60 per cent with a high prevalence of pancreatic complications. The aim of this study was to analyse complications after pancreatic head resection, with particular attention to morbidity and pancreatic fistula. Methods From November 1993 to May 1999, perioperative and postoperative data from 331 consecutive patients undergoing pancreatic head resection were recorded prospectively. Data were analysed and grouped according to the procedure performed: classic Whipple resection, pylorus-preserving pancreatoduodenectomy (PPPD) or duodenum-preserving pancreatic head resection (DPPHR). Results Pancreatic head resection had a mortality rate of 2·1 per cent; the difference in mortality rate between the three groups (0·9–3·0 per cent) was not significant. Total and local morbidity rates were 38·4 and 28 per cent respectively. DPPHR had a lower morbidity, both local and systemic, than pancreatoduodenectomy. The prevalence of pancreatic fistula was 2·1 per cent in 331 patients, and was not dependent on the procedure or the aetiology of the disease. Reoperations were performed in 3·9 per cent of patients, predominantly for bleeding and non-pancreatic fistula. None of the patients with pancreatic fistula required reoperation or died in the postoperative course. Conclusion A standardized technique and a continuing effort to improve perioperative management may be responsible for low mortality and surgical morbidity rates after pancreatic head resection. Pancreatic complications occur with Whipple, PPPD and DPPHR procedures with a similar prevalence. Pancreatic fistula no longer seems to be a major problem after pancreatic head resection and rarely necessitates surgical treatment.

Three prominent electron traps, 0.167 eV, 0.243 eV, and 0.295 eV below the conduction band minimum were detected in Te doped MOCVD grown n-GaSb using an Au Schottky barrier diode. The free carrier concentration of the ∼3 μm epilayer grown on n+ (>1018 cm−3) substrate, confirmed by Hall and capacitance-voltage measurements, was 5–7 × 1016 cm−3. The low doping concentration of the epitaxial layers was achieved using diethyl tellurium as the dopant source. Defect concentration profiles suggest that Ec-0.167 eV and Ec-0.243 are predominantly confined to the surface of the epilayer and that the concentration, thereof, approximates the free carrier concentration of the material close to the metal-semiconductor interface.

The injection dependence of the excess carrier lifetime has been investigated for undoped and zinc doped p-type gallium antimonide thin films using steady-state photoconductivity measurements. Discrepancies between the observed lifetimes and the theoretical values are resolved using a two-layer generation/recombination model. Simulation of the room temperature experimental results yields values for the surface band bending, an upper limit to the surface recombination velocity and a lower limit to the Shockley-Read-Hall carrier lifetime.

The use of dimethylcadmium as a potential p-type dopant source during the growth of InAs by metalorganic vapor phase epitaxy has been investigated. Thermoelectric measurements were used to accurately determine the doping density of the epilayers. A linear doping efficiency was attained for low DMCd mole fractions within the 550 °C to 650 °C growth temperature range with the doping density spanning from low 1016 to a maximum of 6 × 1017 cm−3. The surfactant action of a small mole fraction of Sb increased the maximum p-type doping density by a factor of two.

Thermoelectric measurements were performed on p-type InAs thin films grown by metalorganic vapor phase epitaxy. The measured Seebeck coefficient displayed an anomalous temperature dependence due to the existence of a highly conductive surface inversion layer. The effect of a degenerate conduction layer has been incorporated into our analysis and revealed that the sign-reversal temperature of the Seebeck coefficient remained unaffected by the surface layer. This finding consequently facilitated the direct determination of the acceptor density of lightly doped thin film InAs.