Heterojunction performance of dip coated n-Cd0.5Zn0.5S thin films on different metal sulfide substrates

In this work, n-type cadmium zinc sulfide (n-Cd_0.5Zn_0.5S) films were grown by a dip coating technique on different p-type metal sulfide substrates. The morphology, structure, and composition of the yielded materials have been con?rmed by scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray analysis, respectively. Using the absorption measurements, the direct allowed band gap energies for Cd_0.5Zn_0.5S, PbS, CuS, CuInS₂, were found to be 2.93, 1.83, 1.98 and 2.5 eV at room temperature, respectively. The substrate dependence of the current density–voltage (J–V) characteristics of n-Cd_0.5Zn_0.5S/p-PbS, n-Cd_0.5Zn_0.5S/p-CuS, n-Cd_0.5Zn_0.5S/p-Cu_0.8In_1.2S₂, n-Cd_0.5Zn_0.5S/p-Cu_0.9In_1.1S₂, n-Cd_0.5Zn_0.5S/p-CuInS₂, n-Cd_0.5Zn_0.5S/p-Cu_1.1In_0.9S₂ and n-Cd_0.5Zn_0.5S/p-Cu_1.2In_0.8S₂ heterojunctions were measured at room temperature (～300 K). These characteristics showed a rectifying behavior consistent with a potential barrier formed at the interface for all the studied devices. The forward current density–voltage characteristics under low voltage bias were explained on the basis of thermionic emission mechanism. Heterojunction parameters such as ideality factor, n, series resistance, R_S and barrier height, Φ_b were obtained from J–V measurements using Cheung's method. The heterojunctions show non-ideal J–V behavior with an ideality factor greater than unity. Analysis of the experimental data under reverse voltage bias suggests that Schottky effect is the dominant mechanism. The dark capacitance–voltage characteristics of the heterojunctions were studied at 1 MHz. High value of built-in potential of 0.58 V was obtained for n-Cd_0.5Zn_0.5S/p-Cu_0.9In_1.1S₂ heterojunction as compared to the other studied heterojunctions. The photovoltaic characteristics were analyzed for the heterojunctions under illumination of 100 mW/cm².     

Enhanced photocatalytic activity for hydrogen evolution from water by Zn0.5Cd0.5S/WS2 heterostructure

Zn_0.5Cd_0.5S/WS₂ nanocomposites with different amount of Zn_0.5Cd_0.5S solid solution deposited onto the surface of WS₂ were prepared by a one-step hydrothermal method with thioacetamide as sulfur source. Intimate heterojunctions between Zn_0.5Cd_0.5S and WS₂ were identified by TEM and HRTEM technologies in these materials. The chemical composition and valence of the elements were characterized by XPS experiments. The band energies were characterized by UV–vis diffusive reflectance spectroscopy (DRS). The photocatalytic properties of Zn_0.5Cd_0.5S/WS₂ nanocomposites for H₂ evolution from water in the presence of sacrificial reagents were tested. The highest rate of H₂ evolution achieved for these hybrid materials under visible-light irradiation (λ≥420 nm) is about 6 times higher than that of pristine Zn_0.5Cd_0.5S. The promoted catalytic activity of this hybrid material can be ascribed to the formation of the heterojunctions between Zn_0.5Cd_0.5S and WS₂, which enhanced the separation of the photogenerated hole-electron pairs.     

Silicon nanowire array architecture for heterojunction electronics

Photosensitive nanostructured heterojunctions n-TiN/p-Si were fabricated by means of titanium nitride thin films deposition (n-type conductivity) by the DC reactive magnetron sputtering onto nano structured single crystal substrates of p-type Si (100). The temperature dependencies of the height of the potential barrier and series resistance of the n-TiN/p-Si heterojunctions were investigated. The dominant current transport mechanisms through the heterojunctions under investigation were determined at forward and reverse bias. The heterojunctions under investigation generate open-circuit voltage V _oc = 0.8 V, short-circuit current I _sc = 3.72 mA/cm² and fill factor FF = 0.5 under illumination of 100 mW/cm².

     

Ultra-violet photo-response characteristics of p-Si/i-SiO2/n-ZnO heterojunctions based on hydrothermal ZnO nanorods

Hydrothermal zinc oxide (ZnO) nanorod (NR)-based p-Si/n-ZnO and p-Si/i-SiO₂/n-ZnO heterojunctions were fabricated, and the effects of interfacial native SiO₂ (~4 nm) on the I-V characteristics of heterojunctions under dark and ultra-violet illumination conditions were investigated. First, the structural and optical properties of ZnO seed crystals grown by sol-gel method and hydrothermal ZnO NRs on two different substrates of p-Si and p-Si/i-SiO₂ were examined, and more improved optical and crystalline quality was obtained as revealed by photoluminescence and X-ray diffraction. The p-i-n heterojunctions showed ~3 times greater forward-bias currents and enhanced rectifying property than those of p-n junctions, which is attributed to the role of native SiO₂ in carrier confinement by promoting the electron-hole recombination current through the deep level states of ZnO crystal. The measured ratios of photocurrent to dark current of the p-i-n structure were also greater under reverse bias (92–260) and forward bias (2.3–7.1) conditions than those (28–225 for reverse bias, 1.6–6.8 for forward bias) of p-n structure, and the improved photosensitivity of the p-i-n structure under reverse bias is due to lower density of recombination centers in the ZnO NR crystals. Fabricated ZnO NR heterojunction showed repeatable and fast photo-response transients under forward bias condition of which response and recovery times were 7.2 and 3.5 s for p-i-n and 4.3 and 1.7 s for p-n structures, respectively.     

Charge transport mechanisms in anisotype n-TiO2/p-Si heterostructures

Anisotype n-TiO₂/p-Si heterojunctions are fabricated by the deposition of a TiO₂ film on a polished poly-Si substrate using magnetron sputtering. The electrical properties of the heterojunctions are investigated and the dominant charge transport mechanisms are established; these are multi-step tunneling recombination via surface states at the metallurgical TiO₂/Si interface at low forward biases V and tunneling at V > 0.6 V. The reverse current through the heterojunctions under study is analyzed within the tunneling mechanism.     

<Emphasis Type="Italic">p</Emphasis><Superscript>+</Superscript>-Si-<Emphasis Type="Italic">n</Emphasis>-CdF<Subscript>2</Subscript> heterojunctions

Boron diffusion and the vapor-phase deposition of silicon layers are used to prepare ultrashallow p⁺-n junctions and p⁺-Si-n-CdF₂ heterostructures on an n-CdF₂ crystal surface. Forward portions of the I–V characteristics of the p⁺-n junctions and p⁺-Si-n-CdF₂ heterojunctions reveal the CdF₂ band gap (7.8 eV), as well as allow the identification of the valence-band structure of cadmium fluoride crystals. Under conditions in which forward bias is applied to the p⁺-Si-n-CdF₂ heterojunctions, electroluminescence spectra are measured for the first time in the visible spectral region.     

Photoelectric properties of isotype and anisotype Si/GaN:O heterojunctions

Results of a comprehensive study of electrical and photoelectric properties of isotype (p-Si/p-GaN:O) and anisotype n-Si/p-GaN:O) heterojunctions are reported. The structures were fabricated by chemical vapor deposition of thin films of GaN:O solid solutions on silicon substrates by pyrolytic decomposition of gallium monoammine chloride in the presence of water vapor. Total and spectral photosensitivity and photoresponse kinetics in the current mode under forward and reverse biases, current-voltage characteristics, and saturation open-circuit photovoltage were studied. It was found that in both kinds of heterojunctions the charge distribution near the contacts is mainly governed by carrier capture into interface states (with density estimated to be ～10¹⁴–10¹⁵ cm^?2) with the formation of depletion layers on both sides of the interface. Photosensitivity mechanisms are analyzed for anisotype and isotype heterojunctions. It is shown that the differential nature of the photoresponse kinetics is related to the recharging of interface states, and the strong rise in the photoresponse signal of a biased anisotype heterojunction is attributed to the phototransistor effect. The proposed energy band models of the heterojunctions consistently describe the observed effects.     

Electrical and photoelectric properties of anisotype n-TiN/p-Si heterojunctions

Photosensitive n-TiN/p-Si heterojunctions are fabricated by the reactive magnetron sputtering of a thin titanium-nitride film with n-type conductivity onto polished polycrystalline p-Si wafers. The I–V characteristics of the heterostructures are measured at different temperatures. The temperature dependences of the potential-barrier height and series resistance of the n-TiN/p-Si heterojunction are studied. The dominant mechanisms of current transport through the heterojunction in the cases of forward and reverse bias are established. The heterostructures generate the open-circuit voltage V _oc = 0.4 V and the short-circuit current I _sc = 1.36 mA/cm² under illumination with a power density of 80 mW/cm².     

Temperature dependent current-voltage characteristics of n-MgxZn1−xO/p-GaN junction diodes

This study investigates the temperature dependence of the current-voltage (I-V) characteristics of n-Mg_xZn_1−xO/p-GaN junction diodes. The n-Mg_xZn_1−xO films were deposited on p-GaN using a radio-frequency (rf) magnetron sputtering system followed by annealing at 500, 600, 700, and 800 °C in nitrogen ambient for 60 s, respectively. The n-Mg_xZn_1−xO/p-GaN diode at a substrate temperature of 25 °C had the lowest leakage current in reverse bias. However, the leakage current of the diodes increased with an increase in annealing temperatures. The temperature sensitivity coefficients of the I-V characterizations were obtained at different substrate temperatures (25, 50, 75 100, and 125 °C) providing extracted values of 26.4, 27.2, 17.9, and 0.0 mV/°C in forward bias and 168.8, 143.4, 84.6, and 6.4 mV/°C in reverse bias, respectively. The n-Mg_xZn_1−xO/p-GaN junction diode fabricated with Mg_xZn_1−xO annealed at 800 °C demonstrated the lowest temperature dependence. Based on these findings, the n-Mg_xZn_1−xO/p-GaN junction diode is feasible for GaN-based heterojunction bipolar transistors (HBTs).     

Electrical and Photoelectric Properties of the TiN/<Emphasis Type="Italic">p</Emphasis>-InSe Heterojunction

The conditions for fabricating photosensitive TiN/p-InSe heterojunctions by the reactive-magnetron sputtering of thin titanium-nitride films onto freshly cleaved p-InSe single-crystal substrates is investigated. The presence of a tunnel-transparent high-resistivity In₂Se₃ layer at the heterojunction is revealed from analysis of the I–V characteristics, and the effect of this layer on the electrical properties and photosensitivity spectra of the heterostructures is analyzed. The dominant current transport mechanisms through the TiN/p-InSe energy barrier under forward and reverse bias are determined.     

Structure and magnetic properties evolution of cobalt–zinc ferrite with lithium substitution

Li_xCo_0.5Zn_0.5−xFe₂O₄ (0.0≤x≤0.3) is obtained by calcining precursor oxalates at 900 °C in air. The precursor and its calcined products are characterized by thermogravimetry and differential scanning calorimetry, X-ray powder diffraction, scanning electron microscopy, and vibrating sample magnetometer. A high-crystallized Li_xCo_0.5Zn_0.5−xFe₂O₄ with a cubic structure is obtained when the precursor is calcined at 900 °C in air for 3 h. Lattice parameters decrease with the increase of Li⁺ addition amount. The magnetic properties of Li_xCo_0.5Zn_0.5−xFe₂O₄ depend on Li⁺ doped amount and calcination temperature. Li_0.3Co_0.5Zn_0.2Fe₂O₄ obtained at 900 °C has the highest specific saturation magnetization value, 70.24 emu/g. However, Li_0.3Co_0.5Zn_0.2Fe₂O₄ obtained at 800 °C has the highest remanence (8.29 emu/g) and coercivity value (97.8 Oe).     

Characterization of AZO/p-Si heterojunction prepared by DC magnetron sputtering

Al-doped ZnO (AZO) film was deposited by direct-current (DC) magnetron sputtering on p-Si (1 0 0) wafer to fabricate Al-doped n-ZnO/p-Si heterojunctions. The microstructural, optical and electrical properties of the AZO film were characterized by XRD, SEM; UV–vis spectrophotometer; four-point probe and Hall effect measurement, respectively. Results show that the AZO film is of good quality. The electrical junction properties were investigated by I–V measurement, which reveals that the heterojunction shows rectifying behavior under a dark condition. The ideality factor and the saturation current of this diode are 20.1 and 1.19×10⁻⁴ A, respectively. The value of I_F/I_R (I_F and I_R stand for forward and reverse current, respectively) at 5 V is found to be as high as 19.7. It shows fairly good rectifying behavior, indicating formation of a diode between AZO and p-Si. High photocurrent is obtained under a reverse bias when the crystalline quality of AZO film is good enough to transmit light into p-Si.     

Electrical properties of MOS diodes In/TiO2/p-CdTe

In/TiO₂/p-CdTe MOS diodes, which have a rectification coefficient of K = 6 × 10³ at an external bias of 2 V, are fabricated for the first time by means of the inexpensive spray-pyrolysis method. It is established that tunnel-recombination processes in the MOS structures under investigation for forward and reverse voltages with the participation of levels at an energy depth of 0.25 eV are the dominant current-flow mechanism. The features of the voltage-capacitance characteristics of In/TiO₂/p-CdTe MOS diodes testify to a sharp decrease in the resistance of the TiO₂ high-resistance layer at forward bias, which is caused by the relation between the energy parameters of components of the MOS structure under investigation.     

Characterization of ternary MgxZn1−xO thin films deposited by electron beam evaporation

Mg_xZn_1−xO (0≤x≤1) thin films were deposited on glass and quartz substrates by electron beam evaporation and effect of the Mg content of the film on its structural, optical and electrical properties were investigated. The structure, surface morphology, optical transmittance, band gap, refractive index and electrical resistivity were found to depend on the Mg content of the film. XRD data revealed that films were polycrystalline in nature. The structure of the films having Mg content in the range of 1–0.74 was cubic, mixed cubic-hexagonal phases for x=0.47 and hexagonal phase for x=0. The composition analysis showed that Mg content in Mg_xZn_1−xO film is high as compared to the corresponding target alloy. It was observed that the optical band gap increases from 3.3 to 6.09 eV, refractive index at 550 nm decreases from 1.99 to 1.75, transmittance increases from about 70% to 90% and electrical resistivity increases from 0.5 to 1.48×10⁶ Ω cm with the increase of Mg concentration in the film from 0 to 1. The results reported in this work are useful for window layer of solar cells and other optoelectronic devices.     

Study of magnetic impurity as defects in ZnO grown by pulsed laser deposition

The structural, electrical and magnetic properties of Zn_0.95Mn_0.05O films grown by a pulsed laser deposition system were studied. An X-ray diffraction was tried to verify a crystal structure of the sample. A capacitance-voltage measurement showed that the Zn_0.95Mn_0.05O has electrical properties of an n-type semiconductor, and its carrier concentration appears 5×10¹⁸ cm⁻³. From a deep level transient spectroscopy measurement, an oxygen vacancy and a Mn-related electron trap in the Zn_0.95Mn_0.05O films were appeared as E_c−0.62 eV and E_c−0.13 eV, respectively. A magnetic hysteresis of ferromagnetic was measured in the Zn_0.95Mn_0.05O at temperature of 15 K. The hydrogen plasma-annealed sample had larger magnetization than non-annealed sample because of interstitially located hydrogen atoms-mediated double exchange interaction.     

Density functional perturbation theory calculations of vibrational and thermodynamic properties of Zn1−xBexO alloys

The elastic, phonon and thermodynamic properties of Zn_1−xBe_xO alloy are investigated by performing density functional theory (DFT) and density functional perturbation theory (DFPT) calculations. The calculated lattice parameters decreases with the increase of Be content that is in good agreement with the available theoretical and experimental data. The effect of Be composition on elastic constants was investigated for Zn_1−xBe_xO alloys. Phonon dispersion curves show that Zn_1−xBe_xO are dynamically stable. Thermodynamic properties, including Helmholtz free energy, enthalpy, entropy and heat capacity, were evaluated under quasi-harmonic approximation using the calculated phonon density of states. Finally, the results show that Zn_1−xBe_xO alloys with lower Be content are more thermodynamically stable. The agreement between the present results and the known data that are available only for ZnO and BeO is generally satisfactory.     

Type II heterojunctions in an InGaAsSb/GaSb system: Magnetotransport properties

Magnetotransport properties of the narrow-gap In_xGa_1?x As_ySb_1?y /GaSb heterojunctions grown by liquid-phase epitaxy with various In content in the solid solution (x=0.85–0.95 and E _g ≤0.4 eV) were studied. It is shown that, depending on the In content in these heterostructures, type II staggered-lineup (x=0.85) or broken-gap heterojunctions (x=0.95) with high mobility in the electron channel at the interface (μ?20000 cm²/(V s)) can be realized. For x=0.92, depending on temperature, both types of heterojunctions were observed. Obtained results are in good agreement with the band energy diagram of the type II InGaAsSb/GaSb heterostructures under study.     

Isotype surface-barrier n-TiN/n-Si heterostructure

n-TiN/n-Si heterostructures are prepared by reactive magnetron sputtering. The current-voltage characteristics of the heterostructures are measured at different temperatures. The temperature dependences of the potential-barrier height and the series resistance of the heterojunction are analyzed. The energy-band diagram for the heterojunctions under study is constructed. The concentration of heterojunction surface states is estimated to be 2.67 × 10¹³ cm^?2. It is established that the dominant mechanisms of current transport through forward- and reverse-biased n-TiN/n-Si heterojunctions are described well within the tunnel and emission models.     

Properties of Zn1 − x Co x O films produced by pulsed laser deposition with fast particle separation

The study is concerned with the structural, optical, magnetic, and transport properties of Zn_{1 ? x} Co _x O (x = 0.05–0.45) films produced on Al₂O₃ (0001) substrates at a temperature of T _s = 500°C by pulsed laser deposition with fast particle separation. The film thickness is d = 60–300 nm. It is found that the Zn_{1 ? x} Co _x O ternary alloy retains its wurtzite-type crystal structure up to x = 0.35, if the films are produced at low buffer-oxygen pressures (～10^?6 Torr). It is established that, in these conditions, the electron concentration is higher than 10²⁰ cm^?3 because of the high density of oxygen donor vacancies. In this case, the films start to exhibit ferromagnetism in the magnetization and the anomalous Hall effect at temperatures above 100 K. The sign of the anomalous Hall effect is found to be positive and opposite to the sign of the normal Hall effect, as occurs in Co metal layers. This is indicative of the cluster nature of ferromagnetism of the Zn_{1 ? x} Co _x O films. For thin Zn_{1 ? x} Co _x O layers (d = 60 nm, x = 0.2) in a transverse magnetic field, profound hysteresis of the magnetoresistance is observed, which is indicative of the out-of-plain easy axis of magnetization of the films. The magnetic anisotropy is attributed to the structuring of the layers (elongation of magnetic clusters along the growth axis of the films). The structuring can lead to noticeable strengthening of the layer ferromagnetism.     

A simplified model for graded-gap heterojunctions

A simplified method for calculating the energy band profiles of graded-gap heterojunctions, based on the generalized model of Oldham and Milnes, is presented. The profiles are derived by superposing an energy band grading function and the electrostatic potential in the heterojunction. The latter is obtained by using the depletion layer approximation as for conventional p-n homojunctions. The energy band profiles of hypothetical p(GaAs)-n(Al_0·4Ga_0·6As) heterojunctions are calculated using the simplified method. For small grading layer widths, the results are in good agreement with the generalized model. The barrier lowering factor η as a function of the graded layer width l is calculated for such heterojunctions. It is found, for acceptor and donor densities of 10¹⁸ and 10¹⁶ cm^?3 respectively, that the barrier height is reduced from 0·47 eV to zero as l increases from zero (abrupt case) to ≈300 Å. The applications of these analyses to practical heterojunctions are discussed.