A comparative study on Zn0.8Cd0.2O films deposited on different substrates

Highly (0 0 2)-oriented Zn_0.8Cd_0.2O crystal films were prepared on different substrates, namely, glass, Si(1 1 1) and α-Al₂O₃(0 0 1) wafers by the dc reactive magnetron sputtering technique. The Zn_0.8Cd_0.2O/α-Al₂O₃ film has the best crystal quality with a FWHM of (0 0 2) peak of 0.3700°, an average grain size of about 200 nm and a root-mean-square surface roughness of about 70 nm; yet the Zn_0.8Cd_0.2O/glass holds the worst crystal quality with a much larger FWHM of 0.6281°. SIMS depth profile shows that the Zn and O compositions change little along the film depth direction; the Cd incorporation also almost holds the line towards the top surface other than an accumulation at the interface between the film and the substrate. The Cd content in the film is nearly consistent with that in target.     

Ultraviolet electroluminescence from ZnO-based light-emitting diode with p-ZnO:N/n-GaN:Si heterojunction structure

A p-ZnO:N/n-GaN:Si structure heterojunction light-emitting diode (LED) is fabricated on c-plane sapphire by full metal organic chemical vapor deposition (MOCVD) technique. The p-type layer with hole concentration of 8.94×10¹⁶ cm⁻³ is composed of nitrogen-doped ZnO using NH₃ as the doping source with subsequent annealing in N₂O plasma ambient. Silicon-doped GaN film with electron concentration of 1.15×10¹⁸ cm⁻³ is used as the n-type layer. Desirable rectifying behavior is observed from the current-voltage (I-V) curve of the device. The forward turn on voltage is about 4 V and the reverse breakdown voltage is more than 7 V. A distinct ultraviolet (UV) electroluminescence (EL) with a dominant emission peak centered at 390 nm is detected at room temperature from the heterojunction structure under forward bias conditions. The origins of the EL emissions are discussed in comparison with the photoluminescence (PL) spectra.     

Effect of NaF on optical and structural properties of CdxZn1−xS nano crystalline films

This work investigates the effect of NaF on optical and structural properties of nano crystalline Cd_xZn_1?xS films. The Cd_xZn_1?xS films are prepared through chemical bath deposition (CBD) technique in aqueous alkaline bath and their subsequent condensation on substrates. The as-obtained samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV–VIS absorption spectroscopy. Micro structural features, obtained from XRD analysis confirm the formation of cubic phase of undoped as well as NaF doped Cd_xZn_1?xS nano particles while SEM observations depict non-uniform distribution of grains. These results show the average grain size of pure as well as NaF doped samples to range from 50 to 90 nm. Tauc's plots, extracted from absorption spectra exhibit absorption to be dominating mainly in blue-green region of visible spectrum. The room-temperature photoluminescence (PL) spectra of Cd_xZn_1?xS samples show a peak around 425 nm, which gets blue shifted for doped sample indicating improvement in PL properties on its addition.     

Fabrication of coaxial p-Cu2O/n-ZnO nanowire photodiodes

The authors report the deposition of Cu₂O onto vertically well aligned ZnO nanowires by DC sputtering. The average length, average diameter and density of these VLS-synthesized ZnO nanowires were 1 μm, 100 nm and 23 wires/μm², respectively. With proper sputtering parameters, the deposited Cu₂O could fill the gaps between the ZnO nanowires with good step coverage to form coaxial p-Cu₂O/n-ZnO nanowires with a rectifying current–voltage characteristic. Furthermore, the fabricated coaxial p-Cu₂O/n-ZnO nanowire photodiodes exhibit reasonably large photocurrent-to-dark-current contrast ratio and the fast responses.     

Fabrication and characterization of n-ZnO on p-SiC heterojunction diodes on 4H-SiC substrates

We report on the growth and characterization of n-ZnO/p-4H-SiC heterojunction diodes. Our n-ZnO layers were grown with radical-source molecular beam epitaxy (RS-MBE) on p-4H-SiC epilayers, which was previously prepared in a horizontal hot-wall reactor by chemical vapour deposition (CVD) on the n-type 4H-SiC wafers. Details on the n-ZnO growth on 8-off 4H-SiC wafers, the quality of the layers and the nature of realized p–n structures are discussed. Mesa diode structures were fabricated. Al was sputtered through a circle mask with diameter 1 mm and annealed to form Ohmic contacts to p-SiC. Ohmic contacts to the n-ZnO were formed by 30 nm/300 nm Ti/Au sputtered by electron beam evaporation. Electrical properties of the structures obtained have been studied with Hall measurements, and current–voltage measurements (I–V). I–V measurements of the device showed good rectifying behavior, from which a turn-on voltage of about 2 V was obtained.     

Electro-Optic Coefficient Measurements for Zn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript> Te Single Crystals at 1550 nm Wavelength

Nonlinear optical materials of the type Zn _x Cd_1-x Te single crystals, where x = 0.0, 0.2, 0.4, 0.6, 0.7, 0.8, and 1.0, have been grown by the Bridgman method, using a vertical furnace. We have investigated the electro-optic (E-O) coefficient and refractive index of Zn _x Cd_1-x Te single crystals at optical communication wavelength (1550 nm). In the case of CdTe crystal, the E-O coefficient was 15.5 × 10^-12 m/V, which is the biggest among the E-O coefficients of Zn _x Cd_1-x Te crystals. The E-O modulation signal was very big in low frequency range (DC-100kHz), but the signal amplitude became smaller as we increased frequency above 100 kHz. We also found the acousto-optic modulation at CdTe single crystals.     

Embedded-Ge source and drain in InGaAs/GaAs dual channel MESFET

We report the first demonstration of n-type III–V metal-semiconductor field-effect transistors (nMESFETs) with IV group material hetero-junction source and drain (S/D) technology. A selective epitaxial growth of germanium (Ge) in the recessed gallium arsenide (GaAs) S/D regions is successfully developed using ultra-high vacuum chemical vapor deposition (UHVCVD) system. The dual channel structure includes an additional 10-nm higher mobility n-In_0.2Ga_0.8As layer on n-GaAs channel and is introduced to further improve the device performance. The n-MESFET, combining embedded-Ge S/D with In_0.2Ga_0.8As/GaAs channel, exhibits good transfer properties with a drain current on/off ratio of approximately 10³. Due to the small barrier height of Ti/In_0.2Ga_0.8As Schottky contact, a lattice-matched wide bandgap In_0.49Ga_0.51P dielectric layer is also integrated into the device architecture to build a higher electron Schottky barrier height (SBH) for gate leakage current reduction. The Ti/In_0.49Ga_0.51P/n-In_0.2Ga_0.8As Schottky diode shows a comparable leakage level to Ti/n-GaAs with 2 × 10^?2 A/cm² at a gate voltage of ?2.0 V.     

Ternary alloys CdyZn1 ? yO and MgxZn1 ? xO as materials for optoelectronics

Thin films of Cd _y Zn_{1 − y} O and Mg _x Zn_{1 − x} O (y = 0−0.35, x = 0−0.45) ternary alloys have been grown by pulsed laser deposition onto sapphire substrates. The record solubility limits of Cd (y = 0.3) and Mg (x = 0.35) have been achieved in hexagonal zinc oxide. The mismatch of the lattice parameters a of Cd_0.2Zn_0.8O and Mg_0.35Zn_0.65O does not exceed 1%; in this case, the band gap discontinuity is 1.3 eV. The surface roughness of the films does not exceed 2.5 nm at x = 0−0.27 and y = 0−0.20.     

Optical and transport properties of Cd0.8Zn0.2S:Cu compounds prepared by modified chemical co-precipitation method

Cd_0.8Zn_0.2S bulk compound doped with different concentrations of copper have been prepared by chemical co-precipitation (CCP-I) and modified CCP method (CCP-II). The structural, optical and electrical properties of Cd_0.8Zn_0.2S:Cu compounds grown by these methods have been investigated by using XRD, SEM, EDAX, UV spectroscope and DC electrical conductivity techniques. X-ray diffraction studies on Cd_0.8Zn_0.2S compounds doped with various mole% of copper revealed that they possessed polycrystalline nature with hexagonal structure. SEM micrographs taken on samples grown by CCP-I showed that the crystallites have needle like shape and no such regular shape was observed on CCP-II grown samples and in both of them the crystallite size varied differently with the variation in the concentration of copper. The energy gap of a sample grown by CCP-II, evaluated from optical absorption studies, are more than that of the sample grown by CCP-I. This may be attributed to the variation observed in the crystallite sizes of the respective sample grown by two methods. Further, the electrical conductivity variations in CCP-I and II samples were explained based on E_g value, crystallite size and grain boundary conduction in the samples. The activation energies of both CCP-I and II samples were explained on the basis of the differences in their preparation methods. Finally, it was concluded that the CCP-II method is more suitable to grow Cd_0.8Zn_0.2S:Cu compounds with uniform average crystallite size and energy gap for low doping concentration of Cu (∼4 mole%).     

Electro-optical properties of all-oxide p-CuAlO<Subscript>2</Subscript>/n-ZnO: Al transparent heterojunction thin film diode fabricated on glass substrate

We report on the fabrication of all transparent heterojunction thin film diodes of the form glass/n-ZnO: Al/p-CuAlO₂ produced by a combinatorial chemical and physical technique and on a study of their electro-optical properties. The n-ZnO: Al layer was deposited by a sol-gel-dip-coating process whereas the p-CuAlO₂ layer was deposited by direct current sputtering techniques. The diode structure, with a total thickness of 1100 nm showed around 60% transmittance in the visible region. The current-voltage characteristics of the device showed a rectifying nature, with a low turn-on voltage around 0.8 V, having a rectification ratio > 50 at ± 2 V. The low turn-on voltage and moderate visible transmittance of the transparent diode indicate its potential application in the field of “Transparent” or “Invisible Electronics”.      

I–V characteristics of ZnO/Cu2O thin film n–i–p heterojunction

ZnO/Cu₂O thin film n–i–p heterojunctions were fabricated by magnetron sputtering. The microstructure, optical, and electrical properties of n-type (n‐) ZnO, insulating (i‐) ZnO, and p-type (p‐) Cu₂O films deposited on glass substrates were characterized by X-Ray diffraction (XRD), spectrophotometer, and the van der Pauw method, respectively. XRD results show that the mean grain size of i-ZnO film is much larger than that of n-ZnO film. The optical band gap energies of n-ZnO, i-ZnO, and p-Cu₂O film are 3.27, 3.47, and 2.00 eV, respectively. The carrier concentration of n-ZnO film is two orders of magnitude larger than that of p-Cu₂O film. The current–voltage (I–V) characteristics of ZnO/Cu₂O thin film n–i–p heterojunctions with different i-ZnO film thicknesses were investigated. Results show that ZnO/Cu₂O n–i–p heterojunctions have well-defined rectifying behavior. All ideality factors of these n–i–p heterojunctions are larger than 2.0. The forward bias threshold voltage and ideality factor increase when i-ZnO layer thickness increases from 100 to 200 nm. An energy band diagram was proposed to analyze the I–V characteristics of these n–i–p heterojunctions.     

Optical,structural and magnetic properties of Zn0.9Cd0.1S:yCo nanoparticles

Zn_0.9Cd_0.1S:yCo nanoparticles were prepared by a co-precipitation method at low temperature. The obtained products were identified to be of cubic structure without any impurity phase. Cobalt incorporation leads to an increase in the local strain value and a decrease in the lattice constants as measured from XRD. Magnetic measurements showed that cobalt was incorporated in the Zn_0.9Cd_0.1S lattice as Co²⁺ and substituted for the Zn site as there was no evidence of the presence of metallic cobalt. Transmission electron microscopy suggests the crystalline nature of nanoparticles, with average particle size of ∼3.5 nm. UV-Vis measurements showed a red shift with respect to undoped nanoparticles in energy band gap with increasing cobalt concentration. Photoluminescence spectra reveal the defect-related emissions. The decay time constant is found to be in the nanosecond regime and is attributed to the spatial confinement of photo generated electron–hole pairs.     

Growth, structural and optical properties of CdxZn1−xS thin films deposited using spray pyrolysis technique

Cd_xZn_(1−x)S (x = 0, 0.2, 0.4, 0.6, 0.8, and 1) thin films were deposited by the chemical spray pyrolysis technique using a less used combination of chemicals. Depositions were done at 573 K on cleaned glass substrates. The composition, surface morphology and structural properties of deposited films were studied using EDAX, SEM and X-ray diffraction technique. XRD studies reveal that all the films are crystalline with hexagonal (wurtzite) structure and inclusion of Cd into the structure of ZnS improved the crystallinity of the films. The value of lattice constant ‘a’ and ‘c’ have been observed to vary with composition from 0.382 to 0.415 nm and 0.625 to 0.675 nm, respectively. The band gap of the thin films varied from 3.32 to 2.41 eV as composition varied from x = 0.0–1.0. It was observed that presence of small amount of cadmium results in marked changes in the optical band gap of ZnS.     

Effects of Cd concentration on structure and optical properties of the ternary Zn1−xCdxO nanopowder prepared by sol–gel method

Zn_1−xCd_xO nanocrystalline powder with different Cd contents (0≤x≤1) has been prepared by new facile sol–gel route. The crystal structure and optical properties were investigated by X-ray diffraction patterns, Transmission electron microscope, X-ray photoelectron spectroscopy, Photoluminescence. As x varied from x=0 to 0.25, the Zn_1−xCd_xO nanopowder exhibits a hexagonal wurtzite structure of pure ZnO without any significant formation of a separated CdO phase. For the samples with 0.5≤x≤0.85, the Zn_1−xCd_xO nanopowder exhibits the coexistence of hexagonal ZnO and cubic CdO phase, meanwhile, the content of ZnO phase decreases while that of CdO increases with increasing the Cd content x. The ultra-violet near-band-edge emission of the Zn_1−xCd_xO nanopowder was monotonously red-shifted from 389 nm (x=0) to 406 nm (x=0.25) due to the direct modulation of band gap caused by Cd substitution.     

Structural, electrical, optical and magnetic properties of Co0.2AlxZn0.8−xO films

Co_0.2Al_xZn_0.8−xO films prepared with different molar ratio of aluminum nitrate to zinc acetate were deposited on substrates by the sol-gel technique. X-ray diffraction, photoluminescence and ferromagnetism measurements were used to characterize the Co_0.2Al_xZn_0.8−xO diluted magnetic semiconductors. The authors found that the intensity of the acceptor-related photoluminescence increased with increasing aluminum concentration and an increase in the number of the acceptor-like defects (zinc vacancies especially) in the Co_0.2Al_xZn_0.8−xO film might lead to the enhancement of the magnetic properties. This implies that controls of the aluminum concentration and the number of the acceptor-like defects are important factors to produce strong ferromagnetism Co_0.2Al_xZn_0.8−xO films prepared by the sol-gel method.     

On the origin of the ferromagnetism in Zn0.8Mn0.2O having a higher Curie temperature than Zn0.8Co0.2O

Zn_0.8Co_0.2O and Zn_0.8Mn_0.2O films were deposited on substrates by a sol–gel technique. X-ray diffraction, field-emission scanning electron microscopy, photoluminescence, and ferromagnetism measurements were used to characterize these dilute magnetic semiconductors. It is shown that the ferromagnetic properties might be related to the formation of acceptor-like defects in the Zn_0.8Co_0.2O and Zn_0.8Mn_0.2O films. It is found that ferromagnetic Zn_0.8Mn_0.2O has a higher Curie temperature than Zn_0.8Co_0.2O. In addition, the higher ratio of grain-boundary area to grain volume of Zn_0.8Mn_0.2O than Zn_0.8Co_0.2O indicates that grain boundaries and related acceptors are the intrinsic origin for ferromagnetism.     

Microwave-assisted synthesis of Zn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>S–MWCNT heterostructures and their photocatalytic properties

The multi-walled carbon nanotubes (MWCNTs) wrapped with hexagonal wurtzite Zn _x Cd_1−x S nanoparticles with a uniform and small diameter have been prepared to form Zn _x Cd_1−x S–MWCNT heterostructures by microwave-assisted route using Zn(Ac)₂, Cd(NO₃)₂, and thioacetamide as the reactants_. The heterostructures have been characterized by X-ray powder diffraction, scanning and transmission electron microscopy, high-resolution transmission electron microscopy, photoluminescence (PL) and PL excited lifetime. Despite the analogous size and configuration, the Zn _x Cd_1−x S–MWCNT (x = 0, 0.2, 0.5, 0.8, 1) with different Zn concentration exhibit composition-dependent absorption properties in the visible zone. The PL peak positions of Zn _x Cd_1−x S–MWCNT change gradually from ZnS–MWCNT to CdS–MWCNT. The Zn _x Cd_1−x S–MWCNT shows different photocatalytic activity towards the photodegradation of fuchsin acid under visible light illumination, photocatalytic activity of the Zn _x Cd_1−x S–MWCNT decreases gradually with the increase in the Zn concentration, the Zn_0.2Cd_0.8S–MWCNT possessed the best photocatalytic activity. After recycling thrice, the photocatalysts still have about 85% efficiency.     

Long-wavelength optical phonons in the ZnTe/Zn<Subscript>0.8</Subscript>Cd<Subscript>0.2</Subscript>Te superlattice

The lattice IR reflection spectra of a ZnTe/Zn_0.8Cd_0.2Te superlattice measured at temperatures of 300 and 10 K are analyzed. The ZnTe/Zn_0.8Cd_0.2Te superlattice is grown by molecular-beam epitaxy on a GaAs substrate with a ZnTe buffer layer. It is found that the lattice IR reflection spectra of the studied structure exhibit only one reflection band. Dispersion analysis of the experimental spectrum has revealed the presence of one lattice TO mode close in frequency to the mode of pure ZnTe. This result is explained by a shift in the frequency of the lattice modes of the ZnTe and Zn_0.8Cd_0.2Te layers of the superlattice toward each other. In turn, this shift is caused by internal elastic stresses in the superlattice due to a mismatch between the lattice parameters of the materials of these layers.     

Investigation of Li-induced structural disorder and phase transition in ZnO by Raman spectroscopy

Zn_0.8Li_0.2O ceramics with wurtzite structure have been fabricated by a solid reaction of ZnO and Li₂CO₃. The effects of substitutional Li atoms on the crystal structure and structural phase transition of ZnO are studied by Raman spectroscopy. The enhancement of E₁(LO) mode in Zn_0.8Li_0.2O ceramics reveals the occurrence of Li-induced structural disorder. Temperature dependent Raman spectra strongly indicate that a structural phase transition occurs at about 448 K in Zn_0.8Li_0.2O ceramics.     

Structure and optical properties of Cd-substituted ZnO (Zn1−x Cd x O) thin films synthesized by the dc reactive magnetron sputtering

The ternary Zn_1?x Cd _x O (x = 0, 0.2) thin films with wurtzite structure and highly (002)-preferred orientations were deposited on glass substrates by the direct current (dc) reactive magnetron sputtering method. The X-ray diffraction, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), optical absorption spectra and photoluminescence (PL) were employed to investigate the structural and the optical properties in detail. The results indicated that as x varied from x = 0–0.2, the diffraction angle of the (002) peaks decreased from ~34.36° to ~33.38° and the lattice spacing increased from 0.260 to 0.268 nm. Moreover, the optical band-gap of the Zn_1?x Cd _x O thin films with the wurtzite structure decreased from 3.20 eV at x = 0–2.70 eV at x = 0.2. Correspondingly, the near-band-edge PL was tuned in a wide visible region from ~393 to 467 nm. The chemical bonding states of Cd in Zn_1?x Cd _x O alloy thin films were examined by XPS analysis.