Nitrogen-doped Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> films for nonvolatile memory

Nitrogen-doped Ge₂Sb₂Te₅ (GST) films for nonvolatile memories were prepared by reactive sputtering with a GST alloy target. Doped nitrogen content was determined by using x-ray photoelectron spectroscopy (XPS). The crystallization behavior of the films was investigated by analyzing x-ray diffraction (XRD) and differential scanning calorimetry (DSC). Results show that nitrogen doping increases crystallization temperature, crystallization-activation energy, and phase transformation temperature from fcc to hexagonal (hex) structure. Doped nitrogen probably exists in the grain vacancies or grain boundaries and suppresses grain growth. The electrical properties of the films were studied by analyzing the optical band gap and the dependence of the resistivity on the annealing temperature. The optical band gap of the nitrogen-doped GST film is slightly larger than that of the pure GST film. Energy band theory is used to analyze the effect of doped nitrogen on electrical properties of GST films. Studies reveal that nitrogen doping increases resistivity and produces three relatively stable resistivity states in the plot of resistivity versus annealing temperature, which makes GST-based multilevel storage possible. Current-voltage (I-V) characteristics of the devices show that nitrogen doping increases the memory’s dynamic resistance, which reduces writing current from milliampere to microampere.     

Effect of Rapid Thermal Annealing on Sputtered CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> Thin Films

Calcium copper titanium oxide (CaCu₃Ti₄O₁₂, abbreviated to CCTO) films were deposited on Pt/Ti/SiO₂/Si substrates at room temperature (RT) by radiofrequency magnetron sputtering. As-deposited CCTO films were treated by rapid thermal annealing (RTA) at various temperatures and in various atmospheres. X-ray diffraction patterns and scanning electron microscope (SEM) images demonstrated that the crystalline structures and surface morphologies of CCTO thin films were sensitive to the annealing temperature and ambient atmosphere. Polycrystalline CCTO films could be obtained when the annealing temperature was 700°C in air, and the grain size increased signifi- cantly with annealing in O₂. The 0.8-μm CCTO thin film that was deposited at RT for 2 h and then annealed at 700°C in O₂ exhibited a high dielectric constant (ε′) of 410, a dielectric loss (tan δ) of 0.17 (at 10 kHz), and a leakage current density (J) of 1.28 × 10⁻⁵ A/cm² (at 25 kV/cm).     

Band structure of Mg<Subscript>2</Subscript>Si and Mg<Subscript>2</Subscript>Ge semiconducting compounds with a strained crystal lattice

The effect of isotropic and uniaxial deformation of a lattice on the electronic band structure of the indirect band gap Mg₂Si and Mg₂Ge semiconductors was investigated using the method of linear augmented plane waves. The reduction of the lattice constant down to 95% results in a linear increase in the energy gap for the direct transition in magnesium silicide by 48%. In contrast, the indirect band gap decreases and tends to overlap with the valence band, which is typical of the zero-gar semiconductors. The stresses arising under uniaxial deformation not only shift the bands but also split the degenerate states. The changes in the interband transitions under the uniaxial deformation are found to be nonlinear.     

Effect of erbium fluoride doping on the photoluminescence of SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> films

The photoluminescence of SiO _x films deposited on c-Si wafers by the thermal evaporation of SiO in a vacuum and, for the first time, doped with ErF₃ by coevaporation is studied. It is shown that, like undoped SiO _x films, the unannealed SiO _x :ErF₃ films passivate the surface of the Si wafers and, thus, increase their edge photoluminescence intensity almost fivefold. A similar increase is observed after annealing of the doped films in air at 750°C. Doping with ErF₃ suppresses the photoluminescence of Si nanoclusters, if the films have been subjected to high-temperature annealing (at 750°C). In this case, the PL intensity of the band with a peak at ∼890 nm decreases as well. The ∼890 nm band is observed for the first time and, due to its features, is attributed to transitions in SiO _x matrix defects. The experimentally observed effect of ErF₃ doping on SiO _x film photoluminescence is interpreted. An intense photoluminescence signal from Er³⁺ ions in the nearinfrared spectral region (the ⁴ I _11/2 → ⁴ I _15/2 and ⁴ I _13/2 → ⁴ I _15/2 transitions) is observed in the SiO _x :ErF₃ films annealed in air at 750°C. This finding shows that 1.54 μm luminescent emitters, which are currently in popular demand, can be produced by a simple low-cost method.     

Schottky barriers based on <Emphasis Type="Italic">n</Emphasis>-In<Subscript>2</Subscript>S<Subscript>3</Subscript> films obtained by laser-induced evaporation

The method of evaporation of starting targets with subsequent deposition onto glass substrates at temperatures of 480–720 K is used to grow homogeneous thin (0.6–1.5 μm) n-In₂S₃ films on which the In/n-In₂S₃ Schottky barriers were formed for the first time; evaporation was induced by a pulsed laser. The temperature dependence of resistivity of the films with the n-type conductivity was studied and the activation energy of donor centers in these films was determined. Spectral dependences of the photoconversion quantum efficiency η(?ω) for the barriers obtained were studied. An analysis of the spectral dependences η(?ω) made it possible to identify the type of band-to-band transitions and estimate the band gap in the In₂S₃ films. It is concluded that the thin In₂S₃ films can be used in broadband photoconverters of optical radiation.     

The effect of O<Subscript>2</Subscript> ambient annealing on the microstructure of Cu(Mg) in the form of a Cu(Mg)/SiO<Subscript>2</Subscript>/Si multilayer

The effect of annealing in an O₂ ambient on Cu(Mg)/SiO₂/Si multilayer films was investigated. As-deposited Cu(Mg)/SiO₂/Si multilayer samples with film thicknesses in the 1,000–3,000 ? range were annealed for 30 min in oxygen ambients at pressures ranging from vacuum to 100 mtorr. The results showed that annealing in an 8-mtorr O₂ ambient significantly decreased the electrical resistivity of a 1,000 ? sample from 10.5 μΩ-cm to 3.7 μΩ-cm. Annealing in the O₂ ambient enhanced Mg diffusion to the surface in comparison to vacuum annealing. Furthermore, O₂ ambient annealing leads to excessive grain growth. However, the effect of O₂ ambient annealing on resistivity is less when the thickness of the film increases.     

Microstructure and Characteristics of Thin-Film La<Subscript>0.8</Subscript>Sr<Subscript>0.2</Subscript>Co<Subscript>0.5</Subscript>Ni<Subscript>0.5</Subscript>O<Subscript>3</Subscript>/ZnO:Al Heterocontact CO Sensors Prepared by RF Magnetron Sputtering

Highly sensitive CO gas sensors based on heterocontacts of ZnO:Al on La_0.8Sr_0.2Co_0.5Ni_0.5O₃ (LSCNO) were fabricated successfully. La_0.8Sr_0.2Co_0.5Ni_0.5O₃ thin films were coated on (100) silicon wafers by a sol-gel method including the Pechini process followed by a spin-coating procedure. Then, ZnO:Al films prepared by radiofrequency (RF) magnetron sputtering at various oxygen partial pressures and deposited on as-deposited La_0.8Sr_0.2Co_0.5Ni_0.5O₃ films were investigated. The results revealed that the CO sensing ability of the film prepared with the ratio of O₂/Ar = 5/5 (ratio of volume flow rate) was the worst, owing to the highest (002) plane orientation in the ZnO:Al film. In contrast, the ZnO:Al film prepared with O₂/Ar = 3/7 exhibited better CO sensitivity. Furthermore, all two-layer samples showed higher CO sensitivities than single-layer samples. The CO sensitivity of ZnO:Al/La_0.8Sr_0.2Co_0.5Ni_0.5O₃ thin film was 45% for 500 ppm CO at a sensing temperature of 200°C.     

Structural and luminescent properties of ZnO:P films produced by annealing ZnP<Subscript>2</Subscript> wafers in atomic oxygen

The epitaxial ZnO:P films are fabricated by annealing the ZnP₂ wafers in atomic oxygen (oxygen radicals). The properties of the films are studied by X-ray diffraction analysis, atomic force microscopy, Auger spectroscopy, and photoluminescence measurements. In the X-ray diffraction spectra of the samples, the (002) peak is observed, suggesting that the ZnO:P films are oriented along the c axis. The Auger spectroscopy data show that the ZnO films contain phosphorus. The low-temperature photoluminescence spectrum observed for the ZnO: P films exhibits a 3.356 eV peak presumably corresponding to excitons bound at neutral acceptors and a peak at 3.306 eV (free electron)-acceptor transitions associated with the P_O level. The phosphorus related acceptor level is localized at 128 meV above the top of the valence band. The origin of the 3.312 eV band related to recombination at donor-acceptor pairs is discussed.     

Specific Features and Nature of the 890 nm Photoluminescence Band Detected in SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Films after Low-Temperature Annealing

A band with a peak at 890 nm is detected in the photoluminescence spectra of SiO _x (x ≈ 1.3) films deposited by thermal evaporation of SiO and annealed in air at 650–1150°C. The 890-nm band appears after low-temperature (∼650°C) annealing and exhibits a number of features: (i) as the annealing temperature is elevated to 1150°C, the position of the band peak remains unchanged, whereas the intensity increases by two orders of magnitude; (ii) the effects of the annealing atmosphere (air, vacuum) and the excitation wavelength and power density on the intensity of the 890-nm band differ from the corresponding effects on the well-known bands observable in the ranges 600–650 and 700–800 nm; and (iii) the photoluminescence decay is first fast and then much slower, with corresponding lifetimes of ∼9 and ∼70 μs. The observed features are inconsistent with the interpretation of photoluminescence observed in SiO _x so far. Specifically, the earlier observed photoluminescence was attributed to transitions between the band and defect states in the matrix and between the states of band tails, transitions inside Si nanoclusters, and intraion transitions in rare-earth impurity ions. Therefore, we consider here the possibility of attributing the 890-nm band to transitions in local centers formed by silicon ions twofold- and/or threefold-coordinated with oxygen; i.e., we attempt to interpret the 890-nm band in the same manner as was done for luminescence in SiO₂ glasses and films slightly deficient in oxygen.     

Free-Standing PEDOT-PSS/Ca<Subscript>3</Subscript>Co<Subscript>4</Subscript>O<Subscript>9</Subscript> Composite Films as Novel Thermoelectric Materials

Free-standing poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT- PSS)/Ca₃Co₄O₉ composite films have been successfully prepared by mechanically blending Ca₃Co₄O₉ powder and PEDOT-PSS solution (Baytron P) and casting the mixed solution on polypropylene (PP) film substrates. X-ray diffraction (XRD) and scanning electron microscopy (SEM) characterization indicated that the Ca₃Co₄O₉ particles were in the shape of sheets and composited well together with PEDOT-PSS. Thermoelectric (TE) measurements revealed that the Seebeck coefficient can be improved by increasing the Ca₃Co₄O₉ content in the composite films, with the largest enhancement being 24.8% compared with a free-standing PEDOT-PSS film. However, it is also shown that the power factor of the composite films decreases with increasing Ca₃Co₄O₉ content, mainly due to the decline of electrical conductivity and the limited improvement of the Seebeck coefficient.     

Charge transport in thin layers of ferroelectric Hf<Subscript>0.5</Subscript>Zr<Subscript>0.5</Subscript>O<Subscript>2</Subscript>

The mechanism responsible for the charge transport in thin ferroelectric Hf_0.5Zr_0.5O₂ films has been studied. It is shown that in these films the transport mechanism is phonon-assisted tunneling between the traps. The optimal thickness of dielectric film for TiN/Hf_0.5Zr_0.5O₂/Pt structures is determined. As a result of comparing the experimental current–voltage (I–V) characteristics of TiN/Hf_0.5Zr_0.5O₂/Pt structures with the calculated ones, the thermal and optical energies of the traps are determined and the concentration of the traps is estimated. A comparison between the transport properties of ferroelectric and amorphous Hf_0.5Zr_0.5O₂ films is carried out. It is shown that the charge transport mechanism in this dielectric does not depend on its crystalline phase. A method for decreasing leakage currents in Hf_0.5Zr_0.5O₂ is proposed. A study of the resource of repolarization cycles for TiN/Hf_0.5Zr_0.5O₂/TiN metal-dielectric-metal (MDM) structures fully grown by atomic layer deposition (ALD) has been carried out.     

Photosensitivity of ZnO/CdS/Cu(In,Ga)Se<Subscript>2</Subscript>/Mo thin-film solar cells fabricated on various substrates

The results of measuring the first spectra of relative quantum efficiency for photoconversion in thin-film ZnO/CdS/Cu(In,Ga)Se₂/Mo solar cells fabricated on rigid (glass) and flexible (polyimide) substrates are reported. The character of interband transitions has been studied and the values of the band gap for direct and indirect transitions in thin Cu(In,Ga)Se₂ films are determined. It is found that a shift of the maximal photosensitivity for the obtained solar cells to shorter wavelengths is observed as rigid substrates are replaced by flexible ones. It is concluded that thin-film Cu(In,Ga)Se₂ structures can be used as broad-band photoconverters of solar radiation.     

The electrical properties and stability of the hafnium silicate/Si<Subscript>0.8</Subscript>Ge<Subscript>0.2</Subscript>(100) interface

The effect of post-oxidation N₂ annealing and post-metallization forming-gas annealing on the electrical properties of Pt/Hf-silicate (3 nm)/Si_0.8Ge_0.2(100)/n-type Si(100) metal-oxide semiconductor (MOS) capacitors is reported. Capacitance-voltage (C-V) and current density-voltage (J-V) measurements of asgrown, 3-nm-thick, hafnium-silicate films containing ∼12at.%Hf indicate a large number of bulk and interface traps with a current density of ∼10⁻² A/cm² at V_FB+1 V. Post-ultraviolet (UV)/O₃ oxidation annealing in N₂ at 350°C for 30 min leads to a significant improvement in the electrical characteristics of the film. A post-metallization anneal (PMA) at 450°C for 30 min in forming gas (90% N₂:10% H₂), however, degraded the electrical properties of the films. X-ray photoelectron spectroscopy (XPS) analyses of the forming-gas-annealed films indicate that a possible cause for the degradation in electrical properties is the hydrogen-induced reduction of GeO₂ in the interfacial Si_xGe_1−xO₂ oxide layer to elemental germanium. Implications for the introduction of hafnium silicate as a viable gate dielectric for SiGe-based devices are discussed.     

Electronic states on silicon surface after deposition and annealing of SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> films

The spectrum of the photoconductivity induced by the polarization field of charges at surface states and traps in the film bulk has been analyzed to determine the energy band diagram at the c-Si-SiO _x interface and the changes in the electronic states after the film annealing. It is found that the energy bands are bent at the Si-SiO _x interface and the Si surface is enriched in electrons. In equilibrium the photocurrent peak at 1.1 eV is due to the band-to-band transitions in the silicon part of the interface. Annealing shifts the peak to higher energies; this shift increases with an increase in the annealing temperature from 650 to 1000°C. This effect is accompanied by a decrease in the photocurrent at ≤1.1 eV and weakening of the band-edge photoluminescence near the Si surface. The changes revealed are explained by the formation of an oxide layer with Si nanoclusters at the Si-SiO _x interface upon annealing. This process is caused by oxygen diffusion from the SiO _x film, which occurs mainly via defects on the Si wafer surface. The photoconductivity spectrum of the samples charged by short-term application of a negative potential to silicon exhibits electronic transitions in the SiO _x film, both from the matrix electronic states and from the states of the defects and Si nanoclusters in the film.     

Electrical properties of In<Subscript>2</Subscript>Se<Subscript>3</Subscript> single crystals and photosensitivity of Al/In<Subscript>2</Subscript>Se<Subscript>3</Subscript> Schottky barriers

In₂Se₃ single crystals ∼40 mm long and 14 mm in diameter were grown by the Bridgman method. The composition of grown single crystals and their crystal structure were determined. The conductivity (σ) and Hall constant (R) of grown single crystals were measured and the first Schottky barriers Al/n-In₂Se₃ were fabricated. Rectification and photovoltaic effect were detected in the new structures. Based on the study of the photosensitivity spectra of Al/n-In₂Se₃ structures, the nature of the interband transitions and band gap of In₂Se₃ crystals were determined. It was concluded that the new structures can be applied to develop broadband photoconverters of optical radiation.     

Effect of Annealing Temperature on Flowerlike Cu<Subscript>3</Subscript>BiS<Subscript>3</Subscript> Thin Films Grown by Chemical Bath Deposition

For widespread application of thin-film photovoltaic solar cells, synthesis of inexpensive absorber material is essential. In this work, deposition of ternary Cu₃BiS₃ absorber material, which contains abundant and environmentally benign elements, was carried out on glass substrate. Flowerlike Cu₃BiS₃ thin films with nanoflakes as building block were formed on glass substrate by chemical bath deposition. These films were annealed at 573 K and 673 K in sulfur ambient for structural improvement. Their structure was characterized using Raman spectroscopy, as well as their surface morphological and optical properties. The x-ray diffraction profile of as-deposited Cu₃BiS₃ thin film revealed amorphous structure, which transformed to orthorhombic phase after annealing. The Raman spectrum exhibited a characteristic peak at 290 cm^?1. Scanning electron microscopy of as-deposited Cu₃BiS₃ film confirmed formation of nanoflowers with diameter of around 1052 nm. Wettability testing of as-deposited Cu₃BiS₃ thin film demonstrated hydrophobic nature, which became hydrophilic after annealing. The measured ultraviolet–visible (UV–Vis) absorption spectra of the Cu₃BiS₃ thin films gave an absorption coefficient of 10⁵ cm^?1 and direct optical bandgap of about 1.42 eV after annealing treatment. Based on all these results, such Cu₃BiS₃ material may have potential applications in the photovoltaic field as an absorber layer.     

Electrical and Structural Real-Time Changes in Thin Thermoelectric (Bi<Subscript>0.15</Subscript>Sb<Subscript>0.85</Subscript>)<Subscript>2</Subscript>Te<Subscript>3</Subscript> Films by Dynamic Thermal Treatment

A recent trend in thermoelectrics is miniaturization of generators or Peltier coolers using the broad spectrum of thin-film and nanotechnologies. Power supplies for energy self-sufficient micro and sensor systems are a wide application field for such generators. It is well known that thermal treatment of as-deposited p-type (Bi_0.15Sb_0.85)₂Te₃ films leads to enhancement of their power factors. Whereas up to now only the start (as-deposited) and the end (after annealing) film stages were investigated, herein for the first time, the dynamical changes of sputter-deposited film properties have been observed by real-time measurements. The electrical conductivity shows a distinct, irreversible increase during a thermal cycle of heating to about 320°C followed by cooling to room temperature. The interpretation of the Seebeck and Hall coefficients points to an enhancement in Hall mobility after annealing. In situ x-ray diffractometry shows the generation of an additional Te phase depending on temperature. This is also confirmed by energy-dispersive x-ray microanalysis and the corresponding mapping by scanning electron microscopy. It is presumed that the Te enrichment in a separate, locally well-defined phase is the reason for the improvement in the integral film transport properties.     

Radiative recombination in Ge<Superscript>+</Superscript>-implanted SiO<Subscript>2</Subscript> films annealed under hydrostatic pressure

Photoluminescence (PL) spectra and PL excitation spectra were recorded at room temperature from SiO₂ films implanted with Ge⁺ ions and annealed at temperature T _a =450–1100°C under hydrostatic pressure P=12 kbar. The emergence of features in the violet and green bands of the PL and PL excitation spectra correlates with the formation of hydrostatically strained Ge nanocrystals. The shift of the PL bands to higher energies, which occurs as the annealing temperature is raised to T _a ≥800°C, can be attributed to a shift of the energy levels related to the radiative recombination centers, which is caused by the increasing deformation potential. The observed PL is accounted for by the enhanced probability of direct radiative transitions in Ge nanocrystals with an X-like conduction band.     

Transmittance spectra of the CuGa<Subscript>3</Subscript>Se<Subscript>5</Subscript> ternary compound near the fundamental absorption edge

The CuGa₃Se₅ ternary compound films are produced by laser deposition at the substrate temperatures 480 and 580 K. The composition and structure of the films are studied. It is shown that, similarly to the corresponding crystals, the CuGa₃Se₅ films crystallize into the imperfect chalcopyrite structure. The transmittance spectra near the fundamental absorption edge are used to establish the energies and nature of optical transitions. The energies of crystal-field splitting (Δ_cr) and spin-orbit splitting (Δ_SO of the valence band of the CuGa₃Se₅ ternary compound are calculated in the context of the Hopfield quasi-cubic model.     

Thermoelectric power of Bi<Subscript>92</Subscript>Sb<Subscript>8</Subscript> and Bi<Subscript>85</Subscript>Sb<Subscript>15</Subscript> thin films

The results of studying the galvanomagnetic and thermoelectric properties of thin block Bi₉₂Sb₈ and Bi₈₅Sb₁₅ films on mica and polyimide substrates are presented. The method used for measuring the thermoelectric power allowed us to study the temperature dependence the thermoelectric power, without introducing additional deformations into the substrate–film system. A significant difference in the temperature dependences of the galvanomagnetic and thermoelectric properties of films on mica and polyimide is found. The free charge-carrier concentrations and mobilities in the films on mica and polyimide and levels of the chemical potential for electrons and holes are calculated within the two-band approximation. The difference in the charge-carrier parameters for films on mica and polyimide is associated with strains in the film–substrate system.