Prediction of Conditions for Chloride–Hydride Epitaxy of Ga1 – yInyAs1 – xPx Layers Isoperiodic with GaAs and GaAs1 – xPx

Semiconductors - For obtaining multicomponent III–V solid solutions, the vapor-phase methods such as the chloride–hydride epitaxy, the epitaxy from organometallic compounds, and the...     

Conduction in titanium dioxide films and metal–TiO<Subscript>2</Subscript>–Si structures

The effect of the annealing of titanium oxide films on the electrical properties of metal–TiO₂–n-Si structures is investigated. It is shown that, regardless of the annealing temperature, the conductivity of the structures at positive gate potentials is determined by the space-charge-limited current in the insulator with traps exponentially distributed in terms of energy. At negative gate potentials, the main contribution to the current is provided by the generation of electron–hole pairs in the space-charge region in silicon. The properties of the TiO₂/n-Si interface depend on the structure and phase state of the oxide film, which are determined by the annealing temperature.     

Specific features of the capacitance–voltage characteristics of a Cu–SiO<Subscript>2</Subscript>–<Emphasis Type="Italic">p</Emphasis>-InSb MIS structure

The capacitance–voltage and conductance–voltage characteristics of InSb-based MIS structures are measured at different probe signal frequencies with the aim of studying the influence exerted by the technological-synthesis conditions on the capacitive properties of these structures. The influence of positive charge built into the insulator on the sample characteristics is discussed. This influence manifests itself as a sharp capacitance “switch” upon changing the polarity of a low-field (E < 10⁶ V/cm) external signal.     

Mechanisms of formation of <Emphasis Type="Italic">N–S</Emphasis> transition in nonisothermal <Emphasis Type="Italic">I–V</Emphasis> characteristics of a <Emphasis Type="Italic">p–i–n</Emphasis> diode

On the basis of the self-consistent model of transport processes in the semiconductor p–i–n diode during its self-heating under conditions of limited heat sink, the mechanisms of unusual effect—the formations of N–S transition in nonisothermal I–V characteristics of the device were numerically analyzed. It is established that such an effect is caused by a pronounced temperature reduction of the mobility of carriers in the high-resistivity base and the injection-level saturation at the current densities J > 300–500 A/cm². The saturation is attained due to the Auger recombination or the leakage of carriers from plasma into heavily doped emitter layers, the integrated current of which, as a rule, exceeds the recombination integrated current in the base under these conditions. The Auger recombination in the anode emitter also starts to play an appreciable role in the injection-level restriction in the base if the impurity concentration becomes higher than 10¹⁸ cm⁻³ in there.     

Photoluminescence spectra of thin films of ZnTPP–C<Subscript>60</Subscript> and CuTPP–C<Subscript>60</Subscript> molecular complexes

The results of studies of thin composite films of zinc and copper tetraphenylporphyrins with different fractions of fullerene C₆₀ are reported. The photoluminescence spectra are recorded, and the composition and surface morphology are analyzed by means of scanning electron microscopy. The results show a difference in the structure of films with two types of metals (Zn, Cu) entering into the complex of the porphyrin macrocycle. An additional long-wavelength photoluminescence band at 1.4 eV is detected for the first time, which is evidence of the formation of ZnTPP–C₆₀ molecular complexes from a gas-dynamic vapor flow upon condensation. In CuTPP thin films, the processes of self-assembly into nanowires 20 nm in diameter and up to 50 µm in length and the formation of nanoheterojunctions upon the addition of fullerene C₆₀ are observed. Quantum-chemical calculations in the context of density-functional theory are carried out to interpret the experimental data.     

Effect of constructional features of the insulating gap of open TiN–SiO<Subscript>2</Subscript>–W and Si–SiO<Subscript>2</Subscript>–W “sandwich” structures on the process of their electroforming

It is experimentally shown that the introduction of an additional dielectrical layer several nanometers thick (more than 2.5 and 1.3 nm for TiO₂ and SiO₂, respectively) into open TiN–SiO₂–W and Si–SiO₂–W “sandwich”-structures on their anode (lower electrode, i.e., TiN and Si, respectively) in series with respect to the conducting medium formed in the insulating gap during electroformation appreciably decreases the probability of an electrical breakdown. This effect is produced by the limitation of the current by the resistance of an additional dielectrical layer at the prebreakdown level. Some experimental dependences of the probability of successfully electroforming on the thickness of an additional dielectrical layer (TiO₂) are obtained for the TiN–SiO₂–W structure.     

Conductivity of Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>–GaAs Heterojunctions

The effect of annealing in argon at temperatures of T_an = 700–900°C on the I–V characteristics of metal–Ga₂O₃–GaAs structures is investigated. Samples are prepared by the thermal deposition of Ga₂O₃ powder onto GaAs wafers with a donor concentration of N _d = 2 × 10¹⁶ cm^–3. To measure theI–V characteristics, V/Ni metal electrodes are deposited: the upper electrode (gate) is formed on the Ga₂O₃ film through masks with an area of S _k = 1.04 × 10^–2 cm² and the lower electrode in the form of a continuous metallic film is deposited onto GaAs. After annealing in argon at T_an ≥ 700°C, the Ga₂O₃-n-GaAs structures acquire the properties of isotype n-heterojunctions. It is demonstrated that the conductivity of the structures at positive gate potentials is determined by the thermionic emission from GaAs to Ga₂O₃. Under negative biases, current growth with an increase in the voltage and temperature is caused by field-assisted thermal emission in gallium arsenide. In the range of high electric fields, electron phonon-assisted tunneling through the top of the potential barrier is dominant. High-temperature annealing does not change the electron density in the oxide film, but affects the energy density of surface states at the GaAs–Ga₂O₃ interface.     

Forming-Free Reversible Bipolar Resistive Switching Behavior in Al-Doped HfO<Subscript>2</Subscript> Metal–Insulator–Metal Devices

Resistive switching (RS) characteristics are investigated in fabricated Al-doped HfO₂ metal–insulator–metal devices. It is proposed that oxygen vacancies in Al-doped HfO₂ devices play a key role as electron trap centers, leading to the forming-free reversible bipolar resistance switching behavior. The conduction mechanism can be explained by electron trapping and detrapping from such oxygen vacancy-related traps in the Al-doped HfO₂ films and is dominated by a trap-controlled space-charge-limited current (SCLC) mechanism. A large RS ratio (~10⁶) and excellent retention characteristics are also observed at room temperature as well as at 85°C. Such devices have potential for application in nonvolatile random-access memory.     

Thermal Conductivity Reduction in CoSb<Subscript>3</Subscript>–CeO<Subscript>2</Subscript> Nanocomposites

CoSb₃ + x% CeO₂ nanocomposites (x = 1, 3, 5) were synthesized by ball-milling and spark plasma sintering. Scanning electron microscopy showed that some CeO₂ nano-inclusions sit at the boundaries of CoSb₃ grains. These inclusions also reduce the sizes of the CoSb₃ grains and crystallites by inhibiting their growth during sintering. Hall-effect measurements show that the CeO₂ inclusions modify the charge-carrier concentration in CoSb₃. The variations of the electrical resistivity for the 1% and 3% CeO₂ samples can at least partially be attributed to these modifications of the carrier concentration. Nonetheless, the resistivity increase in the 5% CeO₂ sample can unambiguously be ascribed to the presence of the CeO₂ inclusions. Thermal conductivity is systematically reduced (by more than 15% at 300 K) upon CeO₂ addition. Phonon diffusion by the increased number of CoSb₃ grain boundaries is one of the mechanisms involved in this reduction.     

Electronic properties and pinning of the Fermi level in irradiated II–IV–V<Subscript>2</Subscript> semiconductors

Experimental data on the electronic properties of II–IV–V₂ semiconductors irradiated with high-energy particles (electrons, protons, and neutrons) are reported. The limiting electrical characteristics of irradiated ternary compounds are evaluated and compared with the corresponding data for the binary III–V analogues of the compounds. Particular emphasis is placed on the determination of the limiting position of the Fermi level, F _lim, in irradiated II–IV–V₂ compounds. The results of calculations of the energy position of the intrinsic charge-neutrality level are presented.     

Judd–Ofelt Analysis of Dy<Superscript>3+</Superscript>-Activated Aluminosilicate Glasses Prepared by Sol–Gel Method

Aluminosilicate (AS) glasses doped with different Dy³⁺ concentrations were synthesized via sol–gel method. Absorption, photoluminescence spectra and lifetime of this material have been studied. From analytical results of absorption spectra, the Judd–Ofelt (JO) parameters of prepared samples have been determined. These JO parameters combined with photoluminescence spectra have been used to evaluate transition probabilities (A_R), branching ratios (β) and the calculated oscillator strengths of AS:Dy³⁺ glasses. The radiative branching ratio of ⁴F_9/2 → ⁶H_13/2 transition has a minimum value at 62.2% for β_R which predicts that this transition in AS:Dy³⁺ glasses can give rise to lasing action. JO parameters show that the Ω₂ increases with the increasing of Dy³⁺ ion concentration due to the increased polarizability of the average coordination medium and decreased average symmetry.     

Amplification of terahertz radiation in a plasmon <Emphasis Type="Italic">n–i–p–i</Emphasis> graphene structure with charge-carrier injection

The absorption/amplification spectrum of terahertz radiation in inhomogeneous graphene (n–i–p–i structure) with a periodic dual metal grating is theoretically investigated. It is shown that the amplification of terahertz radiation sharply increases at the plasmon-resonance frequency, when losses due to electron scattering and emission are balanced by the plasmon gain (related to the stimulated radiative interband recombination of electron–hole pairs in the inverted region of graphene).     

Radiative <Emphasis Type="Italic">d–d</Emphasis> transitions at tungsten centers in II–VI semiconductors

The luminescence spectra of W impurity centers in II–VI semiconductors, specifically, ZnSe, CdS, and CdSe, are studied. It is found that, if the electron system of 5d (W) centers is considered instead of the electron system of 3d (Cr) centers, the spectral characteristics of the impurity radiation are substantially changed. The electron transitions are identified in accordance with Tanabe–Sugano diagrams of crystal field theory. With consideration for the specific features of the spectra, it is established that, in the crystals under study, radiative transitions at 5d W centers occur between levels with different spins in the region of a weak crystal field.     

Mechanism of the Semiconductor–Metal Phase Transition in Sm<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Gd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>S Thin Films

The influence of the Gd content on the semiconductor–metal phase transition in polycrystalline Sm_1–xGd _x S thin films produced by the flash evaporation of a powder in vacuum is studied. It is shown that the basic factor of the physical mechanism of the phase transition is compression of the film material. It is established that the films retain their semiconductor properties only up to a Gd content of x = 0.12.     

Fabrication of a <Emphasis Type="Italic">p</Emphasis>–<Emphasis Type="Italic">n</Emphasis> Heterojunction Using Topological Insulator Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>–Si and Its Annealing Response

A junction device has been fabricated by growing p-type Bi₂Te₃ topological insulator (TI) film on an n-type silicon (Si) substrate using a thermal evaporation technique. Annealing using different temperatures and durations was employed to improve the quality of the film, as confirmed by microstructural study using x-ray diffraction (XRD) analysis and atomic force microscopy (AFM). The p–n diode characteristics of the junction devices were studied, and the effect of annealing investigated. An improved diode characteristic with good rectification ratio (RR) was observed for devices annealed for longer duration. Reduction in the leakage or reverse saturation current (\( I_{\rm{R}} \)) was observed with increase in the annealing temperature. The forward-bias current (\( I_{\rm{F}} \)) dropped in devices annealed above 400°C. The best results were observed for the sample device annealed at 450°C for 3 h, showing figure of merit (FOM) of 0.621 with RR ≈ 504 and \( I_{\rm{R}} \) = 0.25 μA. In terms of ideality factor, the sample device annealed at 550°C for 2 h was found to be the best with \( n \) = 6.5, RR ≈ 52.4, \( I_{\rm{R}} \) = 0.61 μA, and FOM = 0.358. The majority-carrier density \( \left( {N_{\rm{A}} } \right) \) in the p-Bi₂Te₃ film of the heterojunction was found to be on the order of 10⁹/cm³ to 10¹¹/cm³, quite close to its intrinsic carrier concentration. These results are significant for fundamental understanding of device applications of TI materials as well as future applications in solar cells.     

Diffusion of Selenium in Liquid-Phase Epitaxy–Grown Hg<Subscript>0.78</Subscript>Cd<Subscript>0.22</Subscript>Te

We present preliminary results on Se diffusion in liquid-phase epitaxy (LPE)–grown HgCdTe epilayers. The LPE Hg_0.78Cd_0.22Te samples were implanted with Se of 2.0 × 10¹⁴/cm² at 100 keV and annealed at 350–450°C in mercury saturated vapor. Secondary ion mass spectrometry (SIMS) profiles were obtained for each sample. From a Gaussian fit, we find that the Se diffusion coefficient D _Se is about 1–2 orders of magnitude smaller than that of arsenic. The as-implanted Se distribution is taken into account in case of small diffusion length in Gaussian fitting. The D _Se was found to satisfy the Arrhenius relationship .     

Preparation and Characterization of BaTiO<Subscript>3</Subscript>–PbZrTiO<Subscript>3</Subscript> Coating for Pyroelectric Energy Harvesting

Harvesting energy from waste heat is a promising field of research as there are significant energy recovery opportunities from various waste thermal energy sources. The present study reports pyroelectric energy harvesting using thick film prepared from a (x)BaTiO₃–(1 ? x)PbZr_0.52Ti_0.48O₃ (BT–PZT) solid solution. The developed BT–PZT system is engineered to tune the ferro to paraelectric phase transition temperature of it in-between the phase transition temperature of BaTiO₃ (393 K) and PbZrTiO₃ (573 K) with higher pyroelectric figure-of-merit (FOM). The temperature-dependent dielectric behavior of the material has revealed the ferro- to paraelectric phase transition at 427 K with a maximum dielectric constant of 755. The room-temperature (298 K) pyroelectric coefficient (Pi) of the material was obtained as 738.63 μC/m²K which has yielded a significantly high FOM of 1745.8 J m^?3 K^?2. The enhancement in pyroelectric property is attributed to the morphotopic phase transition between tetragonal and rhombohedral PZT phases in the BT–PZT system. The developed BT–PZT system is capable of generating a power output of 1.3 mW/m² near the Curie temperature with a constant rate (0.11 K/s) of heating. A signal conditioning circuit has been developed to rectify the time-varying current and voltage signals obtained from the harvester during heating cycles. The output voltage generated by the pyroelectric harvester has been stored in a capacitor for powering wearable electronics.     

Influence of Isotropic Pressure on the Current–Voltage Characteristics of Surface-Barrier Diodes Sb–<Emphasis Type="Italic">p</Emphasis>-Si〈Mn〉–Au

The influence of hydrostatic pressure on the current–voltage characteristics of surface-barrier diode structures of the Sb–p-Si〈Mn〉–Au type are investigated. The potential-barrier height and the baric coefficient of its variation are found to be e?_δ = 0.75 eV and δ =–1.54 × 10^–11 eV/Pa, respectively.     

Deposition of heteroepitaxial layers of topological insulator Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> in the trimethylbismuth–isopropylselenide–hydrogen system on the (0001) Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and (100) GaAs substrates

Thin solid layers that are formed upon heating of the gaseous trimethylbismuth–isopropylselenide–hydrogen system on the (0001) Al₂O₃ and singular and vicinal (100) GaAs surfaces are studied. The conditions for deposition of metal Bi and phases of Bi₄Se₃, BiSe, and topological insulator Bi₂Se₃ using the MOCVD method are determined. Pure metastable phase BiSe is obtained for the first time. Bi₂Se₃ films with a thickness of no less than 200 nm, a relatively low volume concentration of 3 ×10¹⁸ cm^–3, and a high mobility of carriers at 300 K (1000 cm² V^–1 s^–1) are fabricated.