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Deep drilling into the ice sheet at Vostok station, Antarctica, was started by specialists of the Leningrad Mining Institute (since 1991, St Petersburg State Mining Institute) in 1970. Five deep holes were cored: hole No. 1 to 952 m; hole No. 2 to 450.4 m; hole No. 3G (3G-1, 3G-2) to 2201.7 m; hole No. 4G (4G-1, 4G-2) to 2546.4 m; and hole No. 5G (5G-1) to 3650.2 m depth. Drilling of hole 5G-1 is not yet complete. The deep drilling at Vostok station has had successes and problems. All the deep holes at Vostok have undergone at least one offset drilling operation because of problems with lost drills. These deviations were made successfully using a thermal drilling technique. Several drilling records have been achieved at Vostok station. The deepest dry hole, No. 1 (952 m), was made during Soviet Antarctic Expedition (SAE) 17 in 1972. The deepest fluid-filled hole, No. 5G-1, made by a thermal drill (TBZS-132), reached 2755 m during SAE 38 in 1993. The deepest fluid-filled hole in ice, No. 5G-1, was drilled with a KEMS-132 electromechanical drill and was stopped above Vostok Subglacial Lake at 3650.2 m depth during Russian Antarctic Expedition (RAE) 51 in 2006.
This paper documents the drilling of the 5G deep hole at the Russian Vostok station, Antarctica. The hole construction is described and the specifications of the drill and surface drilling equipment are given. The peculiarities of drilling at various depths are considered. Based on the extensive experimental data collected at Vostok station, the processes occurring at the hole bottom are investigated: ice breaking and cutting, bottom cleaning and chip transport, and accumulation of the chips in the screen. The main factors affecting ice-drilling efficiency are the coarseness of ice crystals and the ice temperature. When ice crystal size exceeds 10 mm the cutting efficiency sharply decreases, and when ice temperature exceeds –5°C the chip transport from the hole bottom and the chip density in the screen are reduced. The drill advance then becomes irregular, slows down and may even be terminated in some cases. Optimal values of the basic drilling parameters (e.g. cutter head rotation rate and drilling fluid flow rate in the circulation system) have been deduced empirically. The designs of the cutter head and circulation system including screens have been significantly modified, allowing the drilling process to proceed at a normal rate even in the vicinity of the subglacial lake surface.
The VF3-type compound GaF3 has been studied by high-pressure angle-dispersive X-ray diffraction in the pressure range from 0.0001 to 10 GPa. The compression mechanism was found to be highly anisotropic. The c-axis shows little pressure dependence (≈0.4%), but exhibits negative linear compressibility up to ≈3 GPa where it achieves its maximum length. In contrast, the length of the a-axis is reduced by ≈8.8% at the highest measured pressure and an anomalous reduction in the linear compressibility is observed at 4 GPa. The zero pressure bulk modulus B0 was determined to B0 = 28(1) GPa. The compression mechanism of GaF3 is discussed in terms of deformation of an 8/3/c2 sphere-packing model. The volume reduction of GaF3 is mainly achieved through coupled rotations of the GaF6 octahedra within the entire measured pressure range, which reduces the volume of the cubooctahedral voids. In addition, the volume of the GaF6 octahedra also decreases for p ≲ 4.0 GPa, but remains constant above this pressure. The volume reduction of the GaF6 octahedra is accompanied by an increasing octahedral strain. Isosurfaces of the procrystal electron density are used for visualization of the cubooctahedral voids at different pressures.
On Powers, an antiquarian treatise with a political agenda by John Lydus, an erudite and imperial official in the late fifth and early sixth century, has been interpreted as either a panegyric to the emperor or as a work that was critical not only of Justinian’s rule but of monarchy in general. By reexamining it within the broader context of Byzantine political philosophy, this article proposes to see On Powers as a display of a traditional meritorious approach to the emperor, who was judged by measuring his personal and political qualities against the ideal image of the ruler.
Following the idea first expressed by Heinrich Swoboda, there is a general perception that the meaning of ἀτιμία in Athens eventually evolved from the original ‘outlawry’, when an ἄτιμος was liable to being deprived of his property and slayed with impunity if he returned to the land from which he had been banished, into a certain limitation on civic status, which has often been rendered as a ‘disfranchisement’. Specific outcomes of this later form of ἀτιμία varied depending on the dating and circumstances of individual cases, thereby giving rise to theories of a so-called full – or ‘total’ – and partial ἀτιμία. Still, whether it was viewed as ‘full’ or ‘partial’, this ἀτιμία did not inflict the death penalty. The precise dating of the transformation of ἀτιμία has also been debated, with opinions ranging from pre-Solonian times (L'Homme-Wéry) to the late sixth (Swoboda, Hansen, Manville) or the late fifth century (Scafuro). While the exact dating is unknown, this transformation was definitely over in the fifth century, when inscriptions and literary texts mentioned punishment by ἀτιμία alongside the death penalty and the confiscation of property. Thus, according to Raphael Sealey, ἀτιμία evolved ‘from a more severe to a milder sense’, and Alick R.W. Harrison pointed to the evidence that, by the fourth century, any willing Athenian could seize an ἄτιμος who happened to be in Athenian territory and surrender him to the θεσμοθέται, instead of killing him.
Free standing GaN platelets were fabricated by hydride vapor phase epitaxy (HVPE). The platelets having a current maximum size of 7×6×0.1 mm3 were obtained by HVPE growth of ~100 μm thick GaN layers on SiC substrates and subsequent removal of the substrates by reactive ion etching (RIE). Surface of the GaN platelets was characterized by reflectance high energy election diffraction (RHEED), and Auger electron spectroscopy (AES). Crystal structure and optical properties of the platelets were studied by x-ray diffraction and photoluminescence (PL), respectively. Raman spectroscopy was also applied for material characterization. Residual strain was detected in the crystals. The stress was eliminated by high temperature anneal.
We describe the growth of high quality AlN and GaN on Si(111) by gas source molecular beam epitaxy (GSMBE) with ammonia (NH3). The initial nucleation (at 1130−1190K) of an AlN monolayer with full substrate coverage resulted in a very rapid transition to two-dimensional (2D) growth mode of AlN. The rapid transition to the 2D growth mode of AlN is essential for the subsequent growth of high quality GaN, and complete elimination of cracking in thick ( > 2 μm) GaN layers. We show, using Raman scattering (RS) and photoluminescence (PL) measurements, that the tensile stress in the GaN is due to thermal expansion mismatch, is below the ultimate strength of breaking of GaN, and produces a sizable shift in the bandgap. We show that the GSMBE AlN and GaN layers grown on Si can be used as a substrate for subsequent deposition of thick AlN and GaN layers by hydride vapor phase epitaxy (HVPE).
In this paper we report on the first GaN p-n diodes fabricated by Mg ion implantation doping of n-type GaN epitaxial layers. Ion implantation was performed at room temperature. Implantation dose ranged from 1013 to 2×1016 cm−2. After implantation samples were annealed for 10-15 s at a wide temperature interval from 600°C to 1200°C in flowing N2 to form p-type layers. Secondary ion mass spectroscopy, scanning electron microscopy with electron beam induced current and back scattered electron modes as well as current-voltage and capacitance-voltage measurements were used to study structural and electrical characteristics of the Mg implanted p-n structures.
We report on AlN wafers fabricated by hydride vapor phase epitaxy (HVPE). AlN thick layers were grown on Si substrates by HVPE. Growth rate was up to 60 microns per hour. After the growth of AlN layers, initial substrates were removed resulting in free-standing AlN wafers. The maximum thickness of AlN layer was about 1 mm. AlN free-standing single crystal wafers with a thickness ranging from 0.05 to 0.8 mm were studied by x-ray diffraction, transmission electron microscopy, optical absorption, and cathodoluminescence.
Deep levels studies on a set of n-GaN films grown by MOCVD and HVPE reveal the presence of electron traps with levels near Ec−0.25 eV, Ec−0.55 eV, Ec−0.8 eV, Ec−1 eV, hole traps with levels near Ev+0.9 eV and a band of relatively shallow states in the lower half of the bandgap. The total density of these latter states was estimated to be some 1016 cm−3 and they were tentatively associated with dislocations in GaN based on their high concentration and band-like character. None of the electron or hole traps could be unambiguously related with strong changes of diffusion lengths of minority carriers in various samples. It is proposed that such changes occur due to different surface recombination velocities. An important role of Ec−0.55 eV traps in persistent photoconductivity phenomena in n-GaN has been demonstrated.
Gallium nitride films were successfully grown by HVPE technique on p-type 6H-SiC substrate. The layers exhibit high crystal quality as was determined by X-ray diffraction. Photoluminescence (PL) of these films was measured. The PL spectra were dominated by band edge emission. Concentration Nd-Na in undoped epitaxial layers ranged from 2×1017 to 1×1019cm−3. Mesa-structures formed by reactive ion etching showed good rectifying current-voltage characteristics for GaN/SiC pn heterojunctions.
Wide band gap nitrides(InN, GaN, AlN) have been considered promising optoelectronics materials for many years . Recently two main technological problems in the nitrides were overcome: (1)high quality layers has been grown on both sapphire and SiC substrates and(2) p-type GaN and AlGaN material has been obtained. These achievements resulted in the fabrication of bright light emitters in the violet, blue and green spectral regions .First injection laser has been demonstrated . This paper reviews results obtained over the last few years on nitride p-n junctions, particularly on GaN based p-n junctions grown on SiC substrates. We will consider GaN p-n junctions, AlGaN p-n junctions, GaN and AlGaN p-i-n structures, and, finally, GaN/SiC p-n structures.
The luminescent properties of AlGaN epitaxial layers with AlN mole fractions up to 30% and various types of AlGaN/GaN-based heterostructures have been studied. The structures were grown on 6H-SiC substrates by MOCVD. The structures' cathodoluminescence and electroluminescence were measured. A “blue” shift of the edge luminescent peak position for AlGaN alloys was measured to be a non-linear function on the AlN mole fraction. For p-AlGaN/n-GaN double heterostructures (DH), the edge peak position was detected at 365 nm (300K). For a p-Al0.05Ga0.95N/n-Al0.03Ga0.97N heterostructure, the electroluminescent edge peak was observed at 355 nm (300K). The effects of temperature and forward current on the edge electroluminescence of theAlGaN/GaN DH's were investigated.
We report on nickel based technology for the fabrication of GaN mesa structures. Ti/Ni ohmic contacts for n-doped GaN with contact resistivity Rc ~2×10−5 Ω×cm2 and Ni ohmic contacts for p-doped GaN with Rc ~ 4×10−2 Ω×cm2 were formed. Both types of contacts were used as masks for GaN reactive ion etching (RIE) in a CCl2F2/Ar gas mixture. Maximum etch rates of ~ 40 nm/min were obtained. Mesa structures up to 3 μm in height were formed.
The radiative recombination rates have been calculated for the first time in the wide band gap wurtzite semiconductors GaN, InN and AlN and their solid solutions GaxAl1−xN and InxAl1−xN on the base of existing data on the energy band structure and optical absorption in these materials. We calculated the interband matrix elements for the direct optical transitions between the conductivity band and the valence one using the experimental photon energy dependence of the absorption coefficient near the band edge. In our calculations we assumed that the material parameters of the solid solutions (the interband matrix element, carrier effective masses and so on) could be obtained by a linear interpolation between their values in the alloy components. The temperature dependence of the energy gap was taken in the form proposed by Varshni. The calculations of the radiative recombination rates were performed in the wide range of temperature and alloy compositions.
We have studied epitaxial GaN layers grown by hydride vapour phase epitaxy (HVPE) on porous GaN sublayers formed on SiC substrates. It was shown that these layers can be grown with good surface morphology and high crystalline quality. X-ray, Raman and photoluminescent (PL) measurements showed that the stress in the layers grown on porous GaN was reduced to 0.1-0.2 GPa, while the stress in the layers grown directly on 6H-SiC substrates remains at its usual level of about 1 GPa. Thus, we have shown that growth on porous GaN sublayer is a promising method for fabrication of high quality epitaxial layers of GaN with low strain values.
In this paper, we describe the seeded growth of ∼20 mm diameter 15 mm long GaN ingots from the melt-solution. This is the first successful attempt to conduct growth of GaN boule-crystals. GaN ingots were grown from Ga-based melt in the temperature range of 800-1000°C at less than 2 atm ambient pressure. Growth was performed at ∼2 mm/hr growth rate. X-ray diffraction revealed polycrystalline structure of the ingots. Homoepitaxial GaN layers were deposited by HVPE technique on the substrates, which were fabricated from the grown GaN ingots.
Heavy-ion bombardment of a glass surface is a conventional laboratory
technique for producing damage of interest for radioactive waste
encapsulation. At energy of order 100 keV such a bombardment simulates the
damage produced by α-recoil nuclei and fission fragments resulting from the
nuclear decay. The damage region is 100–500 nm depending on conditions of
In the present work some results of EPR study of point defects formed in
silicate, borate, borosilicate, phosphate and other oxide glasses irradiated
with different charge particles (C, N, O, Ar. Mn, Cu, Pb) at energy E=150
keV and large total fluence of ions (up to 1017 cm-2)
are reported. Electron paramagnetic resonance (EPR) is a very sensitive
technique which gives an information on the structure of point defects and
their content. It is shown that in some cases (for example, in borate
glasses) the oxygen hole centers similar to ones observed in γ-irradiated
glasses are formed after ion bombardment. However, in the majority of cases
new defects which are not typical of γ-irradiated oxide glasses were found
They were large molecular oxygen ions (O2-O3-O4-) located in the cavities formed under ion bombardment in the
near surface layer of glass. It should be noted that the relative content of
these defects is of the order of several tens per 1000 incident ions. This
content decreases with increasing fluence and atomic mass of incident ions.
It indicates indirectly that point defects are clustered when the damage of
the near surface layer becomes strong. The formation of gaseous oxygen is
possible in cavities of the damage surface layer.
It was found that some elements (for example C, N and transition metals)
form chemical compounds with oxygen. The migration of alkali ions promotes
the formation of such compounds since the chemical compounds were detected
by means EPR in glasses rich in alkali oxides.