Results of characterization of AlGaN/GaN high electron mobility transistor (HEMT) structures grown by plasma-assisted molecular beam epitaxy (PAMBE) are reported. High resolution X-ray diffraction (HRXRD) and X-ray reflectivity (XRR) were applied to show that structural properties of the AlGaN/GaN layers strongly depend on the substrate used for growth. It has been found that an additional 10 μm thick HVPE GaN layer grown on a commercial GaN/sapphire substrate significantly improves structural quality of AlGaN layer. However, the best structural parameters have been obtained for the HEMT sample grown on free-standing HVPE bulk GaN substrate.
The novel method of preparation of epi-ready ZnO substrates is demonstrated. The substrates were made of unique ZnO crystals grown by chemical vapor transport method using hydrogen as the transport agent. The effect of low-level doping (Mn, Co, Cu, and V) on the structural quality of the crystals was investigated. Atomic layer deposition was used to verify usability of the substrates for homoepitaxy. The thermal annealing prior to the atomic layer deposition process and effect of thermal annealing of the epitaxial layers was studied. The X-ray diffraction and atomic force microscopy methods were applied to study the structural quality of the ZnO layers. Detection of the dopants in the substrates by secondary ion mass spectroscopy made possible the measurement of the thickness of the layers. The obtained root mean square roughness for both the substrates and layers ranged between 0.2 nm and 5 nm, and was dependent on the sample crystallographic orientation and sequence of polishing and annealing procedures. The optimal recipe for the epi-ready substrate preparation was formulated.
A single crystal of (Pb,Cd)Te solid solution with Cd content equal to 5% was grown by self-selecting vapour growth technique and characterized by powder X-ray diffraction using the X'Pert PANalytical diffractometer and Cu K_{α₁} radiation. The X-ray diffraction pattern refinement demonstrated the fcc structure of the rock-salt type of investigated sample, no precipitates or other crystal phases were detected. The sample chemical composition was determined on the basis of measured lattice parameter value. Next, the Young modulus and microhardness were determined by the nanoindentation for carefully prepared, (001), (011) and (111)-oriented single crystal plates. The slight anisotropy of two parameters mentioned above has been found and compared with available literature data.
We show that post growth annealing of GaMnAs under As capping at temperatures in the range of 180-210ºC leads to significant surface modifications. Depending on GaMnAs layer thickness and composition, we obtain either a smooth continuous reacted (MnAs) surface layer or 3D islands (quantum dots). The surface modifications are due to a solid phase epitaxial process, in which Mn interstitials diffusing to the GaMnAs surface are bound with the As.
Structures containing magnetic metallic layers attract a lot of attention because of their possible applications in the area of spintronics. The hybrid structures compatible with the Si crystal lattice parameter are of special interest. In this work the short-period Fe/Si multilayers were grown by the sputtering onto (001)-oriented Si substrate and investigated by various techniques. After the deposition, all multilayers were characterized by atomic force microscopy. The goal of the present paper was to determine the chemical composition of thin layer created at the interface in Fe/Si multilayers due to the Fe diffusion into Si, as well as to analyze the phenomena, which take place in this area. The results of the optical characterization by the Raman scattering were correlated with the magnetic properties of investigated structures (determined by means of the Kerr rotation).
Recently we demonstrated growth of monocrystalline ZnO films by atomic layer epitaxy in the gas flow variant using inorganic precursors. In this study, we discuss properties of ZnO films grown with organic precursors. Successful Mn doping of the ZnO films during the growth was achieved using the Mn-thd complex. Secondary ion mass spectroscopy and X-ray investigations reveal the contents of Mn up to about 20% of the cationic component.
The photoluminescence studies in CdTe/CdMnTe quantum wells are reported in the temperature range 10-300 K. The MnTe concentration in the barriers is x = 0.3, 0.5, 0.63 and 0.68. Thus the potential wells in our samples are very deep, of the order of ≈ 800 meV in the conduction band and ≈ 200 meV in the valence band in the case of the x = 0.68 sample. In spite of the large lattice mismatch (related to high x value) between the wells and the barriers the observed line widths are as narrow as 2 meV in the case of 100 Å. Clear manifestations of internal strain are observed. In particular, the temperature coefficient of the luminescence energies shows strong dependence on the width of wells.
We report on fabrication of hybrid inorganic-on-organic thin film structures with polycrystalline zinc oxide films grown by atomic layer deposition technique. ZnO films were deposited on two kinds of thin organic films, i.e. pentacene and poly(dimethylosiloxane) elastomer with a carbon nanotube content (PDMS:CNT). Surface morphology as well as electrical measurements of the films and devices were analyzed. The current density versus voltage (I-V) characteristics of ITO/pentacene/ZnO/Au structure show a low-voltage switching phenomenon typical of organic memory elements. The I-V studies of ITO/PDMS:CNT/ZnO/Au structure indicate some charging effects in the system under applied voltages.
The paper quotes the results of investigations concerning planar optical waveguides with a high value of the refractive index, achieved basing on a broad-band gap semiconductor ZnO, deposited on glass or quartz substrates. The investigations were focused on the properties of the waveguides, determining the modal characteristics, the attenuation coefficient and the structure of the surface.
We report on an extensive structural and electrical characterization of undergate dielectric oxide insulators Al_2O_3 and HfO_2 grown by atomic layer deposition. We elaborate the atomic layer deposition growth window for these oxides, finding that the 40-100 nm thick layers of both oxides exhibit fine surface flatness and required amorphous structure. These layers constitute a base for further metallic gate evaporation to complete the metal-insulator-semiconductor structure. Our best devices survive energizing up to ≈ 3 MV/cm at 77 K with the leakage current staying below the state-of-the-art level of 1 nA. At these conditions the displaced charge corresponds to a change of the sheet carrier density of 3 × 10^{13} cm^{-2}, which promises an effective modulation of the micromagnetic properties in diluted ferromagnetic semiconductors.
We report on results of magneto-transport measurements performed on four-arm nanostructure fabricated from p-type ferromagnetic Ga_{0.92}Mn_{0.08}As layer. The results reveal hysteresis-like behaviors of low field magnetoresistance. We interpret the magnetoresistance in terms of domain walls, which are expected to be trapped inside the nanostructure at some particular positions and which contribute to the total resistance.
MnAs layer has been grown by means of MBE on the GaN(000_1)-(1 x 1) surface. Spontaneous formation of MnAs grains with a diameter of 30-60 nm (as observed by atomic force microscopy) occurred for the layer thickness bigger than 7 ML. Ferromagnetic properties of the layer with Curie temperature higher than 330 K were detected by SQUID measurements. Electronic structure of the system was investigated in situ by resonant photoemission spectroscopy for MnAs layer thickness of 1, 2, and 8 ML. Density of the valence band states of MnAs and its changes due to the increase in the layer thickness were revealed.
ZnO thin films were grown by atomic layer deposition method at extremely low temperature using a reactive diethylzinc as a zinc precursor. Optical properties, electrical properties and surface morphology were examined by photoluminescence, Hall effect and atomic force microscope. The study shows correlation between optical, electrical properties and surface morphology in a series of samples of different thickness.
We report on the MBE growth and magnetooptical studies of (120)-oriented CdTe/CdMnTe quantum well structures. The quality of structures, as evaluated by the photoluminescence line width, was as good as that of the best structures grown in ⟨100⟩ direction. No spin splitting enhancement, expected theoretically, due to the reduction of the antiferromagnetic interaction between Mn ions in CdTe/CdMnTe digital alloy quantum wells grown along ⟨120⟩ direction was observed.
The 3D-architecture is a prospective way in miniaturization of electronic devices. However, this approach can be realized only if metal paths are placed not only at the top, but also beneath the electronic parts, which imposes drastic temperature limitations for the electronic device processing. Therefore last years a lot of investigations are focused on materials which can be grown at low temperature with electrical parameters appropriate for electronic applications. Zinc oxide grown by the atomic layer deposition method is one of the materials of choice. We obtained ZnO-ALD films at growth temperature range between 100°C and 200°C, and with controllable electrical parameters. Free carrier concentration was found to scale with deposition temperature, so it is possible to grow ZnO films with desired conductivity without any intentional doping. We used correlation of electrical and optical parameters to optimize the deposition process. Zinc oxide layers obtained in that way have free carrier concentration as low as 10^{16} cm^{-3} and high mobility (10-50 cm^{2}/(Vs)), which satisfies requirements for a material used in three-dimensional memories.
The process of growth of single crystals of Cd_{1-x}Zn_{x}Te (x ≤ 0.25) and ZnTe by physical vapour transport has been optimized and the twin-free single crystals with a very good crystal structure and low density of dislocations are grown as substrates for MBE and other techniques of epitaxy. Characterization of the crystals is described.
Monocrystalline thin layers of (Eu,Gd)Te, n-type ferromagnetic semiconductor, were grown by molecular beam epitaxy technique on BaF_2 (111) substrates. Reflection high-energy electron diffraction, X-ray diffraction, and atomic force microscopy characterization proved epitaxial mode of growth and high crystal quality of the layers. Magnetic susceptibility and magnetic resonance measurements showed that in (Eu,Gd)Te layers ferromagnetic transition takes place at about 13 K. Electrical characterization carried out by the Hall effect and resistivity measurements revealed very high electron concentration of 10^{20}~cm^{-3} and sharp maximum of resistivity at transition temperature.
MBE grown Ge_{1-x-y}Mn_xSn_yTe layers with Mn content ranging from 10 to 30% and Sn content ranging from 2 to 5% have been characterized with X-ray diffraction, energy-dispersive X-Ray spectroscopy, atomic force microscopy, SQUID magnetometry, and ferromagnetic resonance. All layers (except the one with the highest Mn and Sn content) were found to be single phase rhombohedral, with the distortion axis perpendicular to the layer surface, and ferromagnetic. Ferromagnetic resonance studies have shown that co-doping with a few percent of tin makes the lattice more rigid and changes considerably the magnetocrystalline anisotropy, from purely uniaxial in GeMnTe to distorted cubic in Ge_{1-x-y}Mn_xSn_yTe at the same Mn content.
Magnetic, structural, and optical properties of ZnMnO films grown with atomic layer epitaxy are discussed. Atomic layer epitaxy films were grown at low temperature using organic zinc and manganese precursors. From magnetometry and electron spin resonance investigations we conclude that lowering of a growth temperature significantly limits formation of Mn precipitates and inclusions of different foreign phases of manganese oxides to ZnMnO host.
Bulk monocrystals of Pb_{1-x}Cd_{x}Te, with the Cd content x up to 0.11, were grown by physical vapour transport method. The structural, electrical and optical properties of these ternary crystals were studied experimentally and theoretically. All investigated samples exhibit rock-salt structure and high crystal quality, which was confirmed by X-ray rocking curve width parameter of about 100 arcsec. The decrease of the lattice parameter with increasing Cd content x was found experimentally, in agreement with ab initio calculations. The band structures of Pb_{1-x}Cd_{x}Te mixed crystals for x values up to 0.2 were calculated using tight binding approach. The calculated band gap in the L-point increases with the Cd content in qualitative agreement with photoluminescence measurements in the infrared. For all studied Pb_{1-x}Cd_{x}Te samples, the Hall effect and electrical conductivity measurements, performed in the temperature range from 4 to 300 K, revealed p-type conductivity.
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