Electrophysical properties of lead zirconate titanate films doped with lanthanum

The influence of lanthanum doping of lead zirconate titanate films (PZTL) on their polarization properties and leakage currents is studied. The films of Pb_{1 ? x} La _x (Zr_0.48Ti_0.52)_{1 ? x/4}O₃ with x = 0, 0.02, 0.05, 0.08, and 0.1 and a thickness of 240 nm were produced using the sol-gel method at an annealing temperature of 650°C on the substrates with a Si-SiO₂-TiO₂-Pt structure. It is found that the residual polarization and the coercive field are weakened after doping with La. The films with x = 0.02 exhibit satisfactory ferroelectric characteristics (7 μC/cm², 25 kV/cm) that make them suitable for use in ferroelectric memory. The current-voltage curves of PZTL films have two characteristic regions. In weak fields (up to 80–90 kV/cm), the leakage current is attributable primarily to the high resistance of the depletion region of the reverse-biased Schottky barrier at the electrode-ferroelectric interface, and the leakage current is thus practically independent of the La content. When the threshold voltage is exceeded, the Schottky barrier breakdown occurs, and the leakage current in strong fields is defined by the density of free carriers in the bulk of the film. This density depends on the La doping level. The leakage current at x = 0.02 is reduced by about two orders of magnitude compared to undoped PZT film. Further increases in the La content are accompanied by an increase in the leakage current in strong fields. This effect is attributed to a change in the conduction type and an increase in the density of n-type carriers.     

Energy parameters of two-electron tin centers in PbSe

Relations that make it possible to use an experimentally measured temperature dependence of carrier concentration to determine the Hubbard energy U and temperature dependence of the Fermi level F for two-electron tin centers in lead selenide are derived. A study of Pb_1?x?ySn_xNa_ySe solid solutions shows that their Fermi level in the temperature region 100–600 K lies below the valence band top E _v and that their F(T) dependences are linear, with extrapolation to T = 0 yielding E _V ?F = 210±10 meV. The Hubbard energy of the two-electron tin centers in PbSe is found to be U = ?80±20 meV.     

Improved Hydrogen Capping Effect in n-Type Crystalline Silicon Solar Cells by SiN(Si-Rich)/SiN(N-Rich) Stacked Passivation

The effect of hydrogen capping of SiN(Si-rich)/SiN(N-rich) stacks for n-type c-Si solar cells was investigated. Use of a passivation layer consisting of Si-rich SiN with a refractive index (n) of 2.7 and N-rich SiN with a refractive index of 2.1 improved the thermal stability. A single SiN passivation layer with a refractive index of 2.05 resulted in an initial lifetime of 200 μs whereas the layer with a refractive index of 2.7 resulted in a high initial lifetime of 2 ms, but the layer degraded rapidly after firing. A stacked passivation layer with refractive indices of 2.1 and 2.7 had a stable lifetime of 1.5 ms with an implied open-circuit voltage (iV _oc) of 720 mV after firing. The thermally stable passivation mechanism with changing amounts of Si–N and Si–H bonding was analyzed by Fourier-transform infrared (FTIR) spectroscopy. Incorporation of the SiN _x stack layer (2.7 + 2.1) into the passivated rear of n-type Cz silicon screen-printed solar cells resulted in energy conversion efficiency of 19.69%. Improved internal quantum efficiency in the long-wavelength range above 900 nm, with V _oc of 630 mV, is mainly because of superior passivation of the rear surface compared with conventional solar cells.     

Electrical Characteristics of Al/Poly(methyl methacrylate)/p-Si Schottky Device

Al/Poly(methyl methacrylate)(PMMA)/p-Si organic Schottky devices were fabricated on a p-Si semiconductor wafer by spin coating of PMMA solution. The capacitance–voltage (C–V) and conductance–voltage (G–V) characteristics of Al/PMMA/p-Si structures have been investigated in the frequency range of 1 kHz–10 MHz at room temperature. The diode parameters such as ideality factor, series resistance and barrier height were calculated from the forward bias current–voltage (I–V) characteristics. In order to explain the electrical characteristics of metal–polymer–semiconductor (MPS) with a PMMA interface, the investigation of interface states density and series resistance from C–V and G–V characteristics in the MPS structures with thin interfacial insulator layer have been reported. The measurements of capacitance (C) and conductance (G) were found to be strongly dependent on bias voltage and frequency for Al/PMMA/p-Si structures. The values of interface state density (D _it) were calculated. These values of D _it and series resistance (R _s) were responsible for the non-ideal behavior of I–V and C–V characteristics.     

Characterization of Double-Junction GaAsP Two-Color LED Structure

The structural, optical, electrical and electrical–optical properties of a double-junction GaAsP light-emitting diode (LED) structure grown on a GaP (100) substrate by using a molecular beam epitaxy technique were investigated. The p–n junction layers of GaAs_1?xP_x and GaAs_1?yP_y, which form the double-junction LED structure, were grown with two different P/As ratios. High-resolution x-ray diffraction (HRXRD), photoluminescence (PL), and current–voltage (I–V) measurements were used to investigate the structural, optical and electrical properties of the sample. Alloy composition values (x, y) and some crystal structure parameters were determined using HRXRD measurements. The phosphorus compositions of the first and second junctions were found to be 63.120% and 82.040%, respectively. Using PL emission peak positions at room temperature, the band gap energies (E_g) of the first and second junctions were found to be 1.867 eV and 2.098 eV, respectively. In addition, the alloy compositions were calculated by Vegard’s law using PL measurements. The turn-on voltage (V_on) and series resistance (R_s) of the device were obtained from the I–V measurements to be 4.548 V and 119 Ω, respectively. It was observed that the LED device emitted in the red (664.020 nm) and yellow (591.325 nm) color regions.     

Growth and characterization of superconducting V3Si on Si and Al2O3 by molecular beam epitaxial techniques

A study of the growth parameters governing the nucleation of metastable superconducting A15 V₃Si on Si and A1₂O₃ is presented. Nominally, 500Å films of V_1-xSi_x were produced through codeposition of V and Si onto heated (111) Si and (1102) A1₂O₃ substrates. Samples were prepared in a custom-built ultrahigh vacuum (UHV) chamber containing dual e-beam evaporation sources and a high temperature substrate heater. V and Si fluxes were adjusted to result in the desired average film composition. V_0.75Si_0.25 films prepared at temperatures in excess of 550° C on Si show significant reaction with the substrate and are nonsuperconducting while similar films grown on A1₂O₃ exhibit superconducting transition temperatures(@#@ T_c @#@) approaching bulk values for V₃Si (16.6-17.1 K). Codeposition at temperatures between 350 and 550° C results in superconducting films on Si substrates while growth at lower temperatures results in nonsuperconducting films. Lowering the growth temperature to 400° C has been shown throughex situ transmission electron microscopy (TEM) and Auger compositional profiling to minimize the reaction with the Si substrate while still activating the surface migration processes needed to nucleate A15 V₃Si. Variation of film composition aboutx = 0.25 is shown to result in nonsuperconducting films for highx and superconducting films with T_c approaching the bulk V value (5.4 K) for lowx. Finally, lowering the V_0.75Si_0.25 deposition rate is shown to raise T_c.     

Charge transport in Si-SiO2 and Si-TiO2 nanocomposite structures

The experimental data on dispersed Si-SiO₂ and Si-TiO₂ nanocomposite structures different in terms of physical, chemical, and insulator properties of oxide components are reported. The parameters of the nanocomposite structures are studied by FTIR spectroscopy and impedance spectroscopy. It is shown that, in such structures, the mechanisms of charge-carrier transport are defined by the properties of Si nanocrystallites and the corresponding oxide as well as by interaction processes at interfaces between grains.     

Origin of an absorption band peaked at 5560 cm−1 and related to divacancies in Si1−x Gex

It is found that two types of centers are formed in Si_1?x Ge_x single crystals as a result of irradiation with fast electrons: divacancies (V ₂) characteristic of silicon and the V ₂ ^* centers; the latter are complexes of divacancies V ₂ with germanium atoms (V ₂Ge). It is shown that an absorption band peaked at about 5560 cm^?1 is a superposition of two absorption bands that correspond to the above centers. The V ₂ divacancies diffuse during isochronous heat treatment and interact with germanium atoms, thus giving rise to additional V ₂ ^* centers. The latter have a higher thermal stability than the V ₂ centers do, and their annealing temperature increases with increasing content of germanium.     

Paradoxical Enhancement of the Power Factor of Polycrystalline Silicon as a Result of the Formation of Nanovoids

Hole-containing silicon has been regarded as a viable candidate thermoelectric material because of its low thermal conductivity. However, because voids are efficient scattering centers not just for phonons but also for charge carriers, achievable power factors (PFs) are normally too low for its most common form, i.e. porous silicon, to be of practical interest. In this communication we report that high PFs can, indeed, be achieved with nanoporous structures obtained from highly doped silicon. High PFs, up to a huge 22 mW K^?2 m^?1 (more than six times higher than values for the bulk material), were observed for heavily boron-doped nanocrystalline silicon films in which nanovoids (NVs) were generated by He⁺ ion implantation. In contrast with single-crystalline silicon in which He⁺ implantation leads to large voids, in polycrystalline films implantation followed by annealing at 1000°C results in homogeneous distribution of NVs with final diameters of approximately 2 nm and densities of the order of 10¹⁹ cm^?3 with average spacing of 10 nm. Study of its morphology revealed silicon nanograins 50 nm in diameter coated with 5-nm precipitates of SiB _x . We recently reported that PFs up to 15 mW K^?2 m^?1 could be achieved for silicon–boron nanocomposites (without NVs) because of a simultaneous increase of electrical conductivity and Seebeck coefficient. In that case, the high Seebeck coefficient was achieved as a result of potential barriers on the grain boundaries, and high electrical conductivity was achieved as a result of extremely high levels of doping. The additional increase in the PF observed in the presence of NVs (which also include SiB _x precipitates) might have several possible explanations; these are currently under investigation. Experimental results are reported which might clarify the reason for this paradoxical effect of NVs on silicon PF.     

Defects of the structure of semiconductor superlattices grown on the basis of II–VI alloys

The specific features of defects of crystal lattices in multilayer device structures containing small-period (T ? 20 nm) superlattices of type I ZnSe/Cd _x Zn_{1 ? x} Se/ZnSe/.../ZnSe/(001)GaAs and type II ZnS/ZnSe_{1 ? x} S _x /ZnS/.../ZnS/(001)GaAs are studied by the methods of X-ray diffractometry and diffraction rocking pseudocurves, and partially by the X-ray topography. According to the data of quantitative analysis of the X-ray diffraction spectra, the periods of superlattices are in the range T _I = 11.3–16.1 nm (for the compositions Cd _x Zn_{1 ? x} Se with x ₁ = 0.047 and x ₂ = 0.107) for type I superlattices and T _II = 15.6–17.2 nm for type II superlattices (for the compositions ZnSe_{1 ? x} S _x with x ₁ = 0.20 and x ₂ = 0.10). The widths of diffraction peaks from both the ZnSe layers and small-period superlattices in the diffraction rocking pseudocurves considerably exceed their widths in the X-ray diffraction spectra. This fact proves that a pronounced plastic strain with the formation of series of rectilinear dislocations in the crossing slip systems took place in the studied device structures. In order to exclude the generation of dislocations in the growth processes, it is necessary to decrease the concentration of the solid solution to the values x < 0.047 for the first type of superlattices and to the values x ? 0.062 for the second type of superlattices, and to decrease the thickness of the ZnSe and ZnS layers.     

Use of photocurrent-voltage characteristics of MOS structures to determine insulator bulk trapped charge densities and centroids

A new technique is described for determining the density and centroid of trapped space charge in the oxide layer of metal-oxide-semiconductor (MOS) structures. Photo-current-voltage (photo I-V) characteristics for both the metal-oxide and Si-oxide interfaces are used to determine the internal fields due to bulk trapped charge, and hence its density and centroid. Experimental examples for locating both positive and negative charge are presented. For negative charge, an Al-Si0₂-W-SiO₂-Si structure (MOWOS) with a layer of approximately 10¹⁴ W atoms/cm² deposited 80 Å from the Si-SiO₂ interface and charged by electronic internal photoemission is investigated. For positive charge, the location of trapped holes generated by 16.85 eV photons or x-rays in the SiO₂ layer of an MOS structure under a voltage bias is discussed. The photo I-V technique is compared to others in terms of its direct, rapid, minimally perturbing, low current, and low field characteristics.     

Electrically-detected electron paramagnetic resonance of point centers in 6H-SiC nanostructures

The results of investigation of electrically-detected electron paramagnetic resonance (EDEPR) and classical electron paramagnetic resonance (EPR) (X-band) for the identification of shallow and deep boron centers, NV _Si defects, and isolated silicon vacancies (V _Si), which are formed directly during the preparation of planar nanostructures under conditions of silicon-vacancy injection at the SiO₂/n-6H-SiC interface without any subsequent irradiation, are presented. The prepared sandwich nanostructures are an ultra-narrow p-type quantum well, confined by δ barriers heavily doped with boron on an n-6H-SiC surface, which are self-ordered during pyrolytic-oxide deposition and subsequent short-time boron diffusion. The EDEPR data of point centers in sandwich nanostructures, prepared within the framework of Hall geometry, are recorded by measuring the field dependences of the magnetoresistance without an external cavity, microwave source and detector, due to the presence of microcavities embedded in the quantum-well plane and microwave generation under conditions of a stabilized source-drain current from δ barriers containing dipole boron centers. The obtained EDEPR spectra of the shallow and deep boron centers are analyzed using the data of EPR studies in 6H-SiC bulk crystals [10]. The EDEPR spectrum of the isolated silicon vacancy reveals both the negatively charged state V _Si ^? (S = 3/2) and the neutral state in hexagonal (V _Si(h)) and quasicubic (V _{Si(k1, k2)}) states (S = 1). In turn, NV _Si defects are detected not only by the EDEPR method at 77 K, but also through the use of a Bruker ELEXSYS E580 EPP spectrometer at 9.7 GHz, in a temperature range of 5–40 K. The EDEPR and EPR spectra recorded on the same sandwich nanostructure are virtually identical and correspond to the center in the triplet state with spin S = 1. The EPR spectrum, which is a lowintensity line doublet with a splitting value equal to ΔB = 237.6 mT, is observed in the background of the EPR spectrum from donors of nitrogen, the concentration of which in the n-6H-SiC initial sample was 5 × 10¹⁸ cm^?3, whereas nitrogen donor centers are not revealed in the EDEPR spectrum because of total occupation by silicon vacancies inside the 6H-SiC sandwich nanostructure.     

Ohmic contacts to InP-based materials induced by means of rapid thermal low pressure (metallorganic) chemical vapor deposition technique

A rapid-thermal-low-pressure-metallorganic-chemical-vapor-deposition (RT-LPMOCVD) technique was executed in order to deposit non-semiconductor thin layer materials, necessary for producing metal contact to InP-based microelectronic devices. Silicon dioxide (SiO₂) films were deposited onto InP substrates in rapid thermal cycles, using O₂ and 2% diluted SiH₄ in Ar, with very fast growth kinetics and low activation energy. The SiO₂ film exhibited excellent properties, such as refractivity index, density, internal stress, and wetp-etch rates. The SiO₂ films were dry etched in a given pattern to allow for the formation of a small metal contact to the InP-based material, onto which the SiO₂ layer was deposited. Subsequently, titanium-nitride (TiN _x ) thin films were deposited onto the InP substrate through rapid thermal deposition cycles, using a tetrakis (dimethylamido) titanium (DMATi) metallorganic liquid source as the precursor for the process, with fast kinetics. The deposited TiN _x films had a stoichiometric structure and contained nitrogen and titanium in a ratio close to unity, but incorporated a large amount of carbon and oxygen. The film properties, such as resistivity (40–80 μΩ·mm) and stress (compressive; ?0.5 to ?2.0×10⁹ dyne·cm^?2), were studied in addition to an intensive investigation of its microstructure and morphology, and their performance as an ohmic contacts while deposited ontop?In_0.53Ga_0.47As material (Zn doped 1.2×10¹⁸ cm^?3).     

Planar light-emitting microcavities based on hydrogenated amorphous silicon carbide

The plasma-enhanced chemical vapor deposition (PECVD) method is used to fabricate planar Fabry-Perot microcavities (MCs) with an active region emitting light at the boundary between the visible and infrared (IR) spectral ranges. The MCs comprise an α-Si_{1 ? x} C _x :H active layer with an increased carbon content and distributed Bragg reflectors (DBRs) constituted by alternating nonemitting α-Si_{1 ? x} C _x :H/α-SiO₂ layers. The active layer and the DBRs are grown in a single technological cycle. Owing to the high optical contrast and low absorption of the layers constituting the DBRs, a high Q factor of the microcavities (Q = 316) and high emission directivity from the MCs for three pairs of layers in the DBRs are achieved. The intensity of the room-temperature photoluminescence exceeds by two orders of magnitude the emission intensity of an identical α-Si_{1 ? x} C _x :H layer without DBRs. Comparison of the experimental transmittance spectra and those calculated by the transfer-matrix method with consideration for dispersion of the real and imaginary parts of the refractive index of α-Si_{1 ? x} C _x :H is used to estimate the degree of systematic deviation of the layer thicknesses in the DBRs and to determine the upper limit of the absorption coefficient in α-Si_{1 ? x} C _x :H layers.     

Effect of Rare Earth Elements on Electromagnetic and Microwave Absorption Properties of Fe-Based Alloys

The RE₂Fe₁₇ (RE = Ce, Pr, Nd, Sm) and La_xPr_2?xFe₁₇ (x = 0.0, 0.1, 0.2, 0.3, 0.4) alloys were synthesized by arc smelting and high-energy ball milling methods. The phase structure, morphology, magnetic properties and electromagnetic parameters of the powders were characterized by x-ray diffraction, scanning electron microscopy, vibrating sample magnetometer and vector network analyzer, respectively. The results reveal that the lattice parameters a and c and unit-cell volume V of the La_xPr_2?xFe₁₇ alloys increase linearly upon the La content. The minimum absorption peak frequency shifts towards a lower-frequency region upon the La content. And the minimum reflection loss and saturation magnetization of the La_xPr_2?xFe₁₇ alloys decrease upon the La content, while the minimum reflection loss of Pr₂Fe₁₇ and La_0.4Pr_1.6Fe₁₇ alloy of the 2.0 mm coating thickness reaches about ?13.65 dB and ?7.15 dB at 5.92 GHz and 3.6 GHz, respectively.     

Effects of Partial Replacement of Iron with Tungsten on Microstructure,Electrical, Magnetic and Humidity Properties of Copper-Zinc Ferrite Material

This paper presents the analysis of the influence of partial replacement of iron with tungsten on the properties of copper-zinc spinel ferrite material. The samples of Cu_0.5Zn_0.5W _x Fe_2?x O₄ spinel powder ferrites were prepared by using a sol–gel self-combustion technology. The ferrite samples were treated for 30 min at 1000°C. The x-ray diffraction was used in order to establish the differences between the phase compositions of ferrites with different tungsten content. Scanning electron microscopy was used to highlight the influence of the tungsten content on the crystallites. All the samples of Cu_0.5Zn_0.5W _x Fe_2?x O₄ were subject to investigation of the influence of the substitution of tungsten upon their electrical and magnetic properties. The measurements of the electrical properties were performed in different humidity conditions, in order to highlight the effect of moisture conditions on the electrical properties of the material and to analyze the applicability of Cu_0.5Zn_0.5W _x Fe_2?x O₄ ferrite for resistive or capacitive humidity sensors.     

Optically active centers in Si/Si<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>:Er heterostructures containing Er<Superscript>3+</Superscript> ions

The basic types of optically active erbium centers that make the major contribution to the photo-luminescence signal from the Si/Si_{1 ? x} Ge _x :Er heterostructures with the Ge content from 10 to 30% are analyzed in detail. It is shown that the origin of the optically active centers containing Er³⁺ ions correlates with the molar composition of the Si_{1 ? x} Ge _x :Er layer and the content of oxygen impurity in the layer. The major contribution to the photoluminescence signal from the Si/Si_{1 ? x} Ge _x :Er heterostructures with the Ge content below 25% is made by the well-known centers containing Er³⁺ ions and oxygen. An increase in the Ge content in the Si_{1 ? x} Ge _x :Er layer (x ≥ 25%) yields the formation of a new type of centers, specifically, the Gecontaining optically active erbium centers unobserved in the Si-based structures previously.     

Growth of Ge1 − x Sn x solid solution films and study of their structural properties and some of their photoelectric properties

From the charge state and closeness of the covalent radii of molecules of the solution-forming components, the possibility of the formation of the solutions Si_{1 ? x} Ge _x , Si_{1 ? x} Sn _x , (Si₂)_{1 ? x} (SnC) _x , Ge_{1 ? x} Sn _x , (Ge₂)_{1 ? x} (SiSn) _x , (SiC)_{1 ? x} (GeC) _x , (GeC)_{1 ? x} (SnC) _x , and (SiGe)_{1 ? x} (SnC) _x based on chemical elements of Group IV has been predicted. Single-crystal films of the substitutional solid solution Ge_{1 ? x} Sn _x (0 ≤ x ≤ 0.03) have been grown on Ge substrates by liquid-phase epitaxy. X-ray diffraction patterns, spectral photosensitivity, and the I–V characteristics of the obtained n-Ge-p-Ge_{1 ? x} Sn _x heterostructures have been investigated. The lattice parameters of the epitaxial film and the substrate a _f = 5.6812 Å and a _s = 5.6561 Å have been determined. The spectral photosensitivity of the n-Ge-p-Ge_{1 ? x} Sn _x heterostructures encompasses the photon energy range from 0.4 to 1.4 eV. It is shown that the forward portion of the I–V characteristics of the investigated structures at low voltages (up to 0.5 V) is described by the exponential dependence I = I ₀exp(qV/ckT) and at high voltages (V > 0.5 V), by the power dependence I ∝ V ^α with the values α = 2 at V = (0.5–0.9) V, α = 1.3 at V = (0.9–1.4) V, and α = 2 at V > 1.4 V. The experimental data are explained within the double injection model for the n-p-p structure using the drift mechanism of current transport in the ohmic relaxation mode taking into account the inertia of the electron exchange inside a recombination complex.     

Ion-beam synthesis of InSb nanocrystals in the buried SiO2 layer of a silicon-on-insulator structure

The ion-beam synthesis of InSb nanocrystals in the buried SiO₂ layer of a silicon-on-insulator structure is investigated. The distributions of In and Sb atoms after annealing at a temperature of T _a = 500–1100°C are studied. It is established that the redistribution of implanted atoms is unsteadily dependent on the annealing temperature. The formation of InSb nanocrystals occurs at Ta ≥ 800°C near the Si/SiO₂ interface and at a depth corresponding to the mean paths R _p . Analysis of the profiles of implanted atoms and of the structure and depth distribution of nanocrystals formed allows an inference regarding the two-stage character of formation of the InSb phase. In the initial stage, antimony precipitates are formed; further the precipitates serve as nuclei for indium and antimony to flow to them.     

Growth,structure, and properties of GaAs-based (GaAs)1–x–y (Ge2) x (ZnSe) y epitaxial films

The possibility of growing the (GaAs)_1–x–y (Ge₂) _x (ZnSe) _y alloy on GaAs substrates by the method of liquid-phase epitaxy from a tin solution–melt is shown. X-ray diffraction shows that the grown film is single-crystal with the (100) orientation and has the sphalerite structure. The crystal-lattice parameter of the film is a _f = 0.56697 nm. The features of the spectral dependence of the photosensitivity are caused by the formation of various complexes of charged components. It is established that the I–V characteristic of such structures is described by the exponential dependence I = I ₀exp(qV/ckT) at low voltages (no higher than 0.4 V) and by the power dependence J ~ V ^α, where the exponent α varies with increasing voltage at high voltages (V > 0.5 V). The results are treated within the framework of the theory of the drift mechanism of current transfer taking into account the possibility of the exchange of free carriers within the recombination complex.