Christoph Buchal

Christoph Buchal
Forschungszentrum Jülich · Peter Grünberg Institute (PGI)

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45
Publications
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662
Citations

Publications

Publications (45)
Article
The SQUID magnetic measurements were performed on the GaN films prepared by metal-organic vapour phase epitaxy and implanted by Tb3+, Tm3+, Sm3+, and Ho3+ ions. The sapphire substrate was checked by the electron paramagnetic resonance method which showed a content of Cr3+ and Fe3+ impurities. The samples 5 × 5 mm2 were positioned in the classical s...
Data
Full-text available
. We present a method for the selective two- and three-dimensional patterning of sapphire using light ion-beam implantation to generate severe lattice damage to depths exceeding 1 μm and subsequent selective wet chemical etching of the damaged regions by hot H3PO4. C-cut sapphire crystals were implanted through contact masks using ion fluences of 1...
Article
We present a complex study of rare earth elements implanted GaN layers grown by low pressure metal-organic vapor phase epitaxy on c-plane sapphire substrates. Gd, Dy, La and Lu ions were implanted with energies of 200 keV and doses ranging from 5 × 1013 to 4 × 1017 atoms.cm− 2. The chemical composition and concentration profiles of ion-implanted la...
Article
The wurzite type gallium nitride doped by gadolinium, Ga <sub>1-x</sub> Gd <sub>x</sub> N (x∼0.01–0.07) , was prepared by Gd ion implantation of the parent GaN thin films deposited on sapphire substrates. The material obtained exhibits a weak ferromagnetism (FM) persisting up to 700 K . At higher Gd concentrations, the minute FM component coexists...
Article
We report about properties of Gallium Nitride layers doped by Erbium and Erbium/Ytterbium ions. The GaN layers were fabricated by Metal Organic Chemical Vapor Deposition on sapphire substrate, and Er3+ and Yb3+ ions were incorporated into the deposited layers by using ion implantation. After the implantation the samples were annealed in nitrogen at...
Conference Paper
Full-text available
We report about properties of Gallium Nitride thin films containing erbium and ytterbium ions. The galliumnitride was fabricated by Metal Organic Vapor Phase Epitaxy and Erbium and Ytterbium ions were incorporated intogallium nitride by using ion implantation. After the implantation the samples were annealed in nitrogen atmosphereat 800 °C. The pro...
Article
The nonlinear absorption of nanocomposite layers based on ZnO implanted with Cu+ ions with an energy of 160 keV in implantation doses of 10(16) and 10(17) ions/cm2 was investigated. The values of the nonlinear absorption coefficient were measured by the Z-scan method at a wavelength of 532 nm by use of nanosecond and picosecond laser pulses. Possib...
Article
Crack-free polycrystalline BaTiO3 films of up to 1-mum thickness were grown on silicon and on silicon-on-sapphire (SoS) substrates by pulsed laser deposition. The quality of films of different thickness was studied, particularly the onset of the formation of cracks. The crystallinity of the films was confirmed by x-ray diffraction measurements. The...
Article
Full-text available
We present a method for the selective two- and three-dimensional patterning of sapphire using light ion-beam implantation to generate severe lattice damage to depths exceeding 1 μm and subsequent selective wet chemical etching of the damaged regions by hot H3PO4. C-cut sapphire crystals were implanted through contact masks using ion fluences of 1×1...
Article
Ti:sapphire is an attractive material for applications as a tunable or short-pulse laser and as a broadband light source in low-coherence interferometry. We investigated several methods to fabricate rib structures in sapphire that can induce channel waveguiding in Ti:sapphire planar waveguides. These methods include direct laser ablation, reactive...
Article
Preferentially oriented perovskite-structured Na0.5K0.5NbO3 (NKN) thin films have been deposited on hexagonal Al2O3(011_2) substrates using rf magnetron sputtering of a stoichiometric, high-density, ceramic target. Structural and film surface properties were measured using x-ray diffraction and atomic force microscopy, respectively. Optical and wav...
Article
Presently the installation of optical fibers for high bandwidth communication services experiences an explosive growth on a world wide scale. As a consequence, a growing demand for more complex integrated optical devices is forseeable. At present, the technology for elec-trooptic integrated devices and components has been mostly satisfied by bulk L...
Article
For the development of optical devices both defect engineering and dopant introduction are the most important current uses of ion implantation. Defect engineering is a well-established concept for changing the optical index. Especially in well-ordered crystalline materials, as the optical ferroelectrics, the nuclear recoils from ion implantation ge...
Article
Presently commercial optical fiber links experience an explosive growth. On the other hand, the integration of optical functions on a single substrate (“Integrated Optical Device”) is still mostly limited to the R & D-laboratories. Only Mach-Zehnder modulators, made from ferroelectric crystals, have found commercial applications so far. The develop...
Article
Ridged channel waveguides in KNbO3 were produced using He+ ion implantation, photolithographic masking, and subsequent Ar+ ion sputtering. We investigated the linear and nonlinear optical characteristics of the waveguides. The effective mode indices are derived from the refractive index profiles using the effective index method. The losses are inve...
Article
Ridged channel waveguides in KNbO3 were fabricated by a technique using He+ ion implantation, photolithographic masking, and subsequent Ar+ ion sputtering. A continuous-wave second-harmonic output power of 14 mW at 438 nm was obtained with an in-coupled fundamental power of 340 mW in a 0.73 cm long waveguide. Phase matching was provided by material...
Article
The losses of ion-implanted potassium niobate (KNbO3) waveguides are evaluated theoretically and experimentally in dependence on wavelength, irradiation dose, waveguide thickness, and waveguide width. Irradiation-induced absorption and tunneling are identified as the main sources of loss. The contributions from surface scattering and intrinsic mate...
Article
We report on the epitaxial growth of CoSi2 on silicon using ion beam synthesis and molecular beam allotaxy. The latter process uses molecular beam epitaxy to grow a silicide precipitate distribution embedded in single crystalline silicon and thermal annealing to form the epitaxial layer. Both, ion beam synthesis and molecular beam allotaxy, are cap...
Conference Paper
Ridged channel waveguides in KNbO 3 were fabricated using a new technique involving He ⁺ ion implantation and Ar ⁺ ion sputtering. A continuous-wave second-harmonic power of 14 mW at 438 nm was obtained with an in-coupled fundamental power of 340 mW.
Article
We report on photorefractive two-beam coupling in proton-implanted Fe-doped KNbO3 waveguides at wavelengths from 632.8 to 1550 nm. Exponential gain coefficients of 11 and 0.9 cm -1 were measured at wavelengths of 632.8 nm and 1550 nm, respectively. Photorefractive response times as low as 120 mu s are reported at 632.8 nm for a pump power of 4 mW....
Article
Results on annealing experiments of He<sup>+</sup> ion‐implanted KNbO 3 planar and channel waveguides are reported. Annealing at 150 °C for several hours leads to a reduction of the waveguide attenuation constant by more than 5 dB cm<sup>-1</sup> at a wavelength of 457 nm without significant change of the profile of the mode confining barrier. A mi...
Article
We report on design, fabrication, and characterization of an ultrafast vertical metal–semiconductor–metal photodetector. A CoSi2 layer in silicon acts both as a bottom Schottky contact and a buried light reflector. A semi‐transparent metallization on top of a photosensitive silicon mesa serves as top Schottky contact. Time‐domain studies of the pul...
Article
For our studies of laseractive rare earth ions in dielectric optical waveguides we have implanted Nd and Er ions into x and zcut LiNbO3 single crystals. We show the recrystallization of the host and the rare earth diffusion during annealing. Nd and Er have different solubilities and different diffusion constants in LiNbO3. The solubility is strongl...
Article
LiNbO3 is the best substrate for modulators and switches for integrated optics. Efficient low-loss waveguides are fabricated by introducing Ti into the crystal. The maximum titanium concentration and the corresponding Δn as well as the minimum waveguide dimensions are normally limited by the diffusion characteristics. Ion implantation offers an int...
Article
Low temperature (0.5–20 K) high field (0–10T) heat capacity and low field (0–2T) magnetic susceptibility (60 μK−300K) measurements were used to study and characterize the quenching of spin fluctuations (paramagnons) in the highly exchange enhanced RCo2 phases, where R = Sc, Y and Lu. These RCo2 compounds are characterized by large many-body mass en...
Article
Measurements of the heat capacity of dilute solutions of ³He in ⁴He give useful information about the interactions of the ³He quasiparticles; however, they should also be a reliable indicator for any phase transition which occurs in the solutions. The most recently published calculation predicts a transition of the quasiparticles into a superfl...
Article
Investigations of the superconducting transition temperatures and of critical magnetic fields of Au1−c In alloys with c ⩽ 0.1 from 0.038 to 80 mK give, as an extrapolation, Tc ≃ 100 μK for pure Au. The pair-breaking influence of magnetic impurities in the low ppm range is studied. Even their sub-ppm concentration may be responsible for the failure...
Article
Investigations of superconductivity of Au-In alloys display evidence for pair breaking by magnetic impurities. A "pure" Au sample (rrr > 10000, 0.6ppm 3d-impurities) remained normal conducting down to 42 μK. The upper limit of Tc for a perfectly clean sample is estimated to be 100 μK.
Article
An analysis of the nuclear specific heat of PrNi5 yields its magnetization and susceptibility. The results are compared with the directly measured quantities. From the susceptibility we obtain a Weiss temperature (0.42±0.03) mK, consistent with the ferromagnetic Curie temperature (0.40±0.02) mK. For the Korringa constant we find κ < 10 μK sec, the...
Article
Pd1−xAgx alloys with 0.065 ≤ × ≤ 0.55 were examined for superconductivity down to temperatures of 0.1 mK. The theoretically predicted superconducting transition between 10 mK and 500 mK for 0.2 ≤ × ≤ 0.4 could not be detected.
Article
The construction and performance of a double-stage nuclear demagnetization refrigerator are reported. The two nuclear stages are precooled to about 25 mK by a He3−He4 dilution refrigerator. They are thermally connected to each other and to the dilution refrigerator by superconducting aluminium heat switches. The first demagnetization stage of the a...
Article
We describe a double stage nuclear demagnetization refrigerator (4.2 moles of PrNi5 in 6 T and 10 moles of Cu in 8 T) which has cooled about 2 kg Cu to a measured electronic temperature of 50 μK. Temperatures are determined by NMR measurements of nuclear magnetization and from spin-lattice relaxation times of Pt and Cu.
Article
We report on the construction and behavior of a platinum wire NMR thermometer. The thermometer has been calibrated down to 14.6 mK, and used down to 0.55 mK; the latter temperature has been achieved by nuclear demagnetization of PrNi(5).
Article
We have measured the thermal conductivity of a 0.1-mm-thick Al foil in the normal and superconducting state down to 58 mK. At this temperature, our data give a ratio for the conductivities of k(n)/k(s)=1600 T(-2). They show that Al is a better material for a heat switch than the usually used superconductors because of its large k(n) and large Debye...
Article
The attenuation, the velocity u, and the dispersion D=u()-u(0) of first sound have been determined in He3-He4 mixtures (molar He3 concentrations X3=0, 0.070, 0.194, 0.377, and 0.517) at frequencies 2.32627 kHz, and in the temperature range 1 K |T-T| 10 mK. From the measured velocities we calculate the thermodynamic velocity u(0), as well as (SP) an...
Article
Attenuation alpha and dispersion D=u(omega) - u(0) of first sound have been measured in a 19.4% 3He-4He mixture for frequencies 2.25 kHz
Article
Sapphire is an attractive material for micro- and opto-electronic systems applications because of its excellent mechanical and chemical properties. However, because of its hardness, sapphire is difficult to machine. Titanium-doped sapphire is a well-known broadly tunable and short-pulse laser material and a promising broadband light source for appl...
Article
The year 2007 was a particularly important year for Forschungszentrum Jülich and Prof. Peter Grünberg. With this book, we not only want to honour Grünberg but also to thank him for his discoveries, which he made here in Jülich. What was to become known as the giant magnetoresistance effect (GMR effect) was discovered by Grünberg in 1988 within the...

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