Shift In Optical Band Gap Research Articles

Analysis of thickness-dependent optical parameters of a-Si:H/nc-Si:H multilayer thin films

Multilayer thin films with alternate hydrogenated amorphous (a-Si:H) and nanocrystalline silicon (nc-Si:H) layers are deposited in hot wire chemical vapor deposition (HWCVD) chamber using hydrogen (H2) dilution of silane (SiH4). Two sets (two samples for each set) have different nc-Si:H layer thickness with the same a-Si:H thickness. The transmittance spectra, refractive index and absorption coefficient are obtained using UV–Vis spectroscopy. An analysis of Tauc plot suggests thickness-dependent optical band gap shift. The films with thick nc-Si:H layers show a decrease in band gap from 1.78 to 1.68 eV, whereas the films with lower nc-Si:H layer thickness have an increased band gap from 1.64 to 1.72 eV. Further, the slope of the Tauc plot (B) and disorder indicator (Urbach energy, Eu) show that the films with thick nc-Si:H layers are poorly ordered as hydrogen dilution increases. These results illustrate that band gap shift can be attributed to the crystal size effect and film disorder. This shifting of optical band gap with the nc-Si:H layer thickness is useful in tuning of the absorber layer band gap which finds an application in photovoltaics.

Quantum confinement effect and size-dependent photoluminescence in laser ablated ultra-thin GZO films

The structural, morphological, and optical characteristics of GZO ultra-thin films were investigated using XRD (X-ray diffraction), FE-SEM (field emission electron microscopy), in situ EDS (Energy Dispersive X-ray) spectroscopy, UV–VIS-IR spectroscopy and photoluminescence spectroscopy (PL) respectively. Morphological analysis reveals the noodle, seed and particle like structure of GZO for GaN, sapphire and Si substrates respectively with average grain size ranging from 5 to 20nm. An effective mass model (EM-Model) for particle in a cylindrical wave function of e–h pair was correlated with experimental results. The reduction in FWHM value (from 31nm to 13nm) of NBE (near-band-edge) emission peak and enhanced NBE intensity have been achieved with small grain size. Blue shift in optical band gap is explained in term of grain radius by EM-model. Improved optical and structural properties were found in relation with quantum confinement effect. The current study states that grain size plays vital role in order to tailor optical properties of GZO thin films.

Synthesis of ZnO nanoporous structure materials by two-step thermal oxidation of Zn film

We describe the synthesis of ZnO nanostructures materials on quartz substrates by a simple two-step thermal oxidation process. Firstly, a thin layer of metallic Zn films are oxidized in nitrogen atmosphere at 350℃, and then the samples annealed at high temperature (from 600 to 800℃) to improve the crystal quality. The properties of the materials are characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy and UV–Visible spectroscopy at room temperature. The results show that nano porous structure materials have been formed after the samples annealed at high temperature. Since the small grain size and grain boundary induced effect of conduction band bending, the optical band gap shifts to low energy.

Structural, optical and electrical properties of ZnO/ZnAl2O4 nanocomposites prepared via thermal reduction approach

ZnO/ZnAl2O4 nanocomposites (Z r A-MMO) with homogeneously distributed ZnAl2O4 nanoparticles in the interpenetrating network of ZnO were successfully prepared via two-step solution process, namely co-precipitation of Zn/Al–NO3 layered double hydroxide (Z r A-LDH) precursor and thermal treatment methods. The effect of cation molar ratio (r) between Zn2+ and Al3+ on the morphological, structural, thermal, optical as well as electrical properties of ZnO-based MMOs was investigated in detail. A mechanism study related to the surface modification, optical band edge alteration as well as improvement of electrical conductivity after phase transformation from LDH to MMO was also elucidated. The optimum Z5A-MMO sample presents the highest electrical conductivity of 8.23 × 10−3 S/cm as compared to 7.26 × 10−7 S/cm for the standard ZnO nanocrystal. It is believed that the bathochromic shift in optical band gap of Z5A-MMO plays a pivotal role in facilitating the photoemission process and leads to the increase in number of carrier concentration in the sample. This method could provide alternative pathways for fabricating a high electrical conductivity MMO-based material and benefit in future optoelectronic devices.

Optical band gap shift in thin LiNbO3 films grown by radio-frequency magnetron sputtering

Abstract Thin polycrystalline LiNbO3 films were deposited by the radio-frequency magnetron sputtering (RFMS) method and ion-beam sputtering (IBS) method under different conditions. Study of the adsorption band edge of fabricated films reveals direct and indirect optical transition. Depending on the particular technological sputtering RFMS regime, the direct energy gap varies from 3.8 to 4.4 eV. Band tails induced by the defects formation due to the reactive plasma effect on the film structure are responsible for indirect optical transitions in the studied films. Thermal annealing has a prominent effect on trap concentration and strain in as-grown films leading to rise in direct band energy up to 4.4 eV which is close to the value for bulk LiNbO3.

Synthesis of SnSe2 thin films by thermally induced phase transition in SnSe

The synthesis of SnSe2 thin films, for phase change memory applications, generally require selenization and higher temperature. However, there are many critical problems associated with selenization, such as use of highly toxic H2Se/Se gas at high temperature, slow reaction rate and non-uniform composition of thin films. To overcome these problems, a novel and relatively low temperature synthesis of SnSe2 thin films has been reported here. Selenium rich Sn30Se70 (prepared by melt quenching technique) has been thermally evaporated in presence of Ar followed by vacuum annealing at 200 °C. An attempt has been made to elucidate the structural, optical and electrical properties of these thin films. X-ray diffraction and UV–Vis studies confirmed a phase change from orthorhombic SnSe to hexagonal SnSe2 followed by improvement in crystallinity (from ≈5 nm to 15 nm) and a red shift in optical band gap on annealing. Electrical studies revealed that SnSe phase have single thermal and photo activation energies in temperature range from −20 °C to 100 °C. A large contrast (four orders) in dark-conductivity [(3.19 ± 0.02) × 10−5Ω−1cm−1 to (5.16 ± 0.02) × 10−1Ω−1cm−1] between two phases of Sn30Se70, suggests its use in phase change memory devices. The SnSe2 phase is observed to be highly stable in the presence of visible light as compared to SnSe.

Structure, optical and electronic properties of solid solution Zn(O,S) thin films and the effect of annealing

Structure, optical and electronic properties of solid solution thin films of Zn(O,S) developed by spray pyrolysis are investigated, and the effect of annealing is investigated for a possible improvement in its electrical properties. A variation in the initial sulphur (S) precursor concentration with fixed Zn-precursor concentration during the spray deposition produces a solid solution of ZnO and ZnS. Both the lower ( 80%) concentrations of the S-precursor cause isovalent substitution in the system, retaining hexagonal ZnO and cubic ZnS structures, respectively, whereas other concentrations lead to the formation of films in a phase-separated solid solution of ZnO and ZnS. A strain-induced shift in optical band gap in the range of 3.3–3.6 eV is unique in the system with a strong bowing effect. A post-annealing at 370 °C in air is found to improve electron mobility in the film, whereas the annealing in argon ambient improves its electrical conductivity.

Tunability of Band Gap and Photoluminescence in CH3NH3PbI3 Films by Anodized Aluminum Oxide Templates

Hybrid organic-inorganic halide CH3NH3PbI3 perovskite films are deposited on anodized aluminum oxide templates with the different pore diameters via one-step spin coating method. The obvious 0.082 eV blue shift of optical band gap is observed in films with decreasing the diameters of pores from 400 to 30 nm. And numerical simulations based on finite element modeling are carried out to represent the absorption edge and consistent with the experiment results. It is interesting that the films show the intense photoluminescence with the excitation intensity of less than 1 μW. Moreover, the photoluminescence intensity is increased with increasing pore diameters, which is attributed to the radiative recombination rate of photogenerated electrons and holes. These results pave a way for the further understanding of tunable photophysical properties of perovskite films.

Growth and characterization of TbAs films

We report on the molecular beam epitaxy growth and characterization of TbAs films. In situ reflection high energy electron diffraction and ex situ high resolution X-ray diffraction, reciprocal space mapping, and both scanning and transmission electron microscopy are used to confirm the complete film growth and study the films' morphology. Spectrophotometry measurements provide the energy of optical transitions, revealing a red shift in optical band gap with increasing thickness. The Hall effect measurements show temperature insensitive carrier concentrations, resistivities, and mobilities. The carrier concentration decreases and resistivity increases with increasing film thickness; mobility appears thickness independent. The films' reflectivity, obtained via Fourier transform infrared spectroscopy, shows a possible Drude edge that differs from the trend of other lanthanide monopnictides. These measurements show that TbAs is a degenerately doped semiconductor with a combination of electronic and optical properties that is dissimilar to other lanthanide monopnictides.

Influence of (Zn + F) double doping on the structural, morphological, photoluminescence, optoelectrical properties and antibacterial activity of CdS thin films

The effects of (Zn + F) double doping on the structural, morphological, optical and electrical properties of CdS thin films is reported in this paper. Polycrystalline nature is observed for all the films. Zn-doped and (Zn + F) doubly doped CdS films exhibit a strong (0 0 2) preferential orientation similar to that of the undoped film. The (0 0 2) plane of the Zn-doped and (Zn + F) doubly doped films shift towards higher Bragg angles favoring a contraction in their lattice parameter values. Increased transparency and blue shift in optical band gap is observed for the doubly doped films. The electrical resistivity values of the undoped, Zn-doped, (Zn + F) doubly doped CdS thin films are found to be in the order of 10−1 Ω-cm. From the obtained results it is found that the physical properties of Zn-doped CdS films got enhanced when co-doped with fluorine, and the (Zn + F) doubly doped CdS thin films seem to be a potential candidate for future optoelectronic device applications. Antibacterial activity of the as deposited films were carried against E. coli gram negative bacteria and from the zone of inhibition it is confirmed that the (Zn + F) doubly doped CdS thin films can be used as a good antimicrobial agent against pathogenic microorganisms.

Influence of electron-hole recombination on optical properties of boro-silicate glasses containing CdS quantum dots

Semiconductor (CdS) quantum dots were grown in a glass matrix (SiO2-B2O3-ZnO-Na2O-K2O) by controlled heat treatment. Growth of CdS quantum dots were confirmed by optical absorption (UV–vis), photoluminescence (PL), micro-Raman spectroscopy, X-ray diffraction (XRD) and High resolution transmission electron microscopy (HRTEM). The structural modification occur in glass matrix were studied with Fourier transform infrared spectroscopy (FTIR). Clusters formation due to heat treatment was studied by field emission gun scanning electron microscopy (FEG-SEM). The observed experimental data shows that the CdS QDs size is very well controlled by single stage heat treatment. Size dependent blue shift in optical energy band gap and blue energy shift in PL intensity was observed with annealing time. The overall study of these glasses suggests these glasses are useful as an optical filters.

Effects of Rapid Thermal Annealing on the Structural, Electrical, and Optical Properties of Zr-Doped ZnO Thin Films Grown by Atomic Layer Deposition.

The 4 at. % zirconium-doped zinc oxide (ZnO:Zr) films grown by atomic layer deposition (ALD) were annealed at various temperatures ranging from 350 to 950 °C. The structural, electrical, and optical properties of rapid thermal annealing (RTA) treated ZnO:Zr films have been evaluated to find out the stability limit. It was found that the grain size increased at 350 °C and decreased between 350 and 850 °C, while creeping up again at 850 °C. UV–vis characterization shows that the optical band gap shifts towards larger wavelengths. The Hall measurement shows that the resistivity almost keeps constant at low annealing temperatures, and increases rapidly after treatment at 750 °C due to the effect of both the carrier concentration and the Hall mobility. The best annealing temperature is found in the range of 350–550 °C. The ZnO:Zr film-coated glass substrates show good optical and electrical performance up to 550 °C during superstrate thin film solar cell deposition.

Properties of CdS films doped with magnesium and fluorine

Nanostructured CdS thin films co-doped with Mg and F were prepared on glass substrates maintained at 400°C by spray technique using perfume atomiser. Structural, morphological, optical and electrical properties of the films were investigated in order to study the effect of Mg, F doping concentration. XRD patterns confirm that both undoped and co-doped CdS films exhibit hexagonal crystal structure with preferential orientation along the (0 0 2) plane. Crystallite size decreased from 33.19 to 24.51 nm with increase in Mg, F concentration. Increased transparency and blue shift in optical band gap from 2.48 to 2.75 eV is observed with increase in Mg, F concentration. All the films have resistivity in the order of 10−2 Ω cm which decreased with co-doping. Improved optical transparency, widened band gap and decreased resistivity were observed for the CdS films simultaneously doped with Mg and F.

Ferroelectric photovoltaic response to structural transformations in doped BiFeO3 derivative thin films

We report an investigation on crystallographic texturing, optical bandgap modulation and ferroelectric photovoltaic (FE-PV) response of semitransparent Au/BiFeO3 derivatives/Sn:In2O3 (ITO) thin film capacitors. Optimized A/B-site doping of respective Ce3+ and Mn2+ drives large-amplitude structural changes in BiFeO3 (BFO) films with partial tetragonal transformation and upsurge in extent of texture of (110) preferred orientation. Spin coated BFO, doped BiFe0.9Mn0.1O3, Bi0.88Ce0.12FeO3 and codoped Bi0.88Ce0.12Fe0.9Mn0.1O3 films of 350nm thickness on ITO coated corning glass exhibited optical transmittance of 65–80% in visible light range and revealed blue shift in optical band gap from 2.53 to 2.81eV due to splitting of t2g and eg orbital. Influence of single and co-doping on FE-PV response demonstrated crystallographic orientation and depolarizing field dependent behavior of short circuit current (JSC) and open circuit voltage (VOC). Current-voltage (J-V) characteristics of semitransparent Au/Bi0.88Ce0.12Fe0.9Mn0.1O3/ITO capacitors possessed large JON/JOFF ratio of approx. 104 at zero bias. Temporal dependence of photocurrent showed highly repeatable and stable response for consecutive ON/OFF cycles of illumination under solid state violet laser (405nm) of 160mW/cm2 intensity, a forward step towards non-destructive memory combining electronic and optical functionalities.

Influence of Lithium doping on the correlated ferromagnetic ordering and red shift of band gap in weakly Mn-doped ZnO nanoparticles

Manganese and Lithium co-doped ZnO nanoparticles were synthesized by a simple wet chemical method. In this study we report the influence of lithium on room temperature ferromagnetic ordering of Mn-doped ZnO nanoparticles. The influence of defect chemistry on the exchange interaction between localized magnetic moments of Mn2+ ions and free charge carriers responsible for room temperature ferromagnetism has been studied in detail. The study also reveals a strong correlation between enhanced ferromagnetic ordering and red shift of optical band gap in (Li, Mn) co-doped ZnO nanoparticles.

Effect of Sputtering Pressure on the Properties of Boron and Gallium Co-doped ZnO Thin Films

B and Ga co-doped ZnO films were fabricated by RF magnetron sputtering method. The effects of sputtering pressure on the electrical, optical, structural and morphological properties of the films (BGZO) were investigated. As sputtering pressure increased up to 6mTorr, the film crystallinity was improved. At the sputtering pressure of 6mTorr, the films showed smooth and dense of film surface, lower resistivity and higher Hall mobility. It was also observed that all films showed high transparency in the visible range. The film showed blue shift of optical band gap with increaseing of sputtering pressure.

Effects of Al-2N doping on the photoelectric properties of ZnO

In a similar range of Al-2N doping amount to that in the present paper, the absorption spectra of ZnO doped system and two kinds of experimental results have been reported in the literature. However, there is no reasonable explanation for the absorption spectra of ZnO doped system. In order to solve the problem, all calculations in the present paper are carried out by the CASTEP tool in the Materials Studio software based on the first principal ultrasoft pseudopotential of the density functional theory, and the geometric structures of ZnO, Zn0.98148Al0.01852O0.96296N0.03704 and Zn0.96875Al0.03125O0.9375N0.0625 systems are constructed by first-principal. All the models are based on the optimization of the geometry structure. And the distribution of the band structure, the density of states and the absorption spectra of the doping system are calculated by the method of GGA+U. The results indicate that in the range of the doping content restricted in the present paper, the bigger the doping amount of Al-2N, the smaller the volume of doped system is; the higher the total energy, the more the stability decreases; the higher the formation energy, the harder the doping becomes and the narrower the optical band gap of doped system. Meanwhile, the higher the Al-2N doping content, the narrower the optical bandgap of the doping system becomes, which suggests that the more significant the red shift of absorption spectrum of Al-2N doped ZnO system is. Therefore, the doped system is controlled within the doping content in experiment to obtain the narrow optical band gap and red shift in absorption spectrum in Al-2N doped ZnO, in addition to the control of lower nanoscale of Al-2N doped in ZnO. At the same time, all doping systems are p-type degenerated semiconductors. Then, the higher the Al-2N doping content, the smaller the relative concentration of free holes of doped system is; the smaller the hole effective mass, the lower the mobility is; the lower the hole conductivity, the worse the conductive property of doping system is. The calculated results are in agreement with the experimental results. The research shows that Al-2N co-doped ZnO can be a new type of semiconductor material, a functional material which is used at low temperature end of thermoelectric power generation.

New Insights on the Burstein-Moss Shift and Band Gap Narrowing in Indium-Doped Zinc Oxide Thin Films.

The Burstein-Moss shift and band gap narrowing of sputtered indium-doped zinc oxide (IZO) thin films are investigated as a function of carrier concentrations. The optical band gap shifts below the carrier concentration of 5.61 × 1019 cm-3 are well-described by the Burstein-Moss model. For carrier concentrations higher than 8.71 × 1019 cm-3 the shift decreases, indicating that band gap narrowing mechanisms are increasingly significant and are competing with the Burstein-Moss effect. The incorporation of In causes the resistivity to decrease three orders of magnitude. As the mean-free path of carriers is less than the crystallite size, the resistivity is probably affected by ionized impurities as well as defect scattering mechanisms, but not grain boundary scattering. The c lattice constant as well as film stress is observed to increase in stages with increasing carrier concentration. The asymmetric XPS Zn 2p3/2 peak in the film with the highest carrier concentration of 7.02 × 1020 cm-3 suggests the presence of stacking defects in the ZnO lattice. The Raman peak at 274 cm-1 is attributed to lattice defects introduced by In dopants.

Consequence of cobalt on structural, optical and dielectric properties in ZnO nanostructures

The critical role of cobalt dopant in ZnO nanostructures with different cobalt concentrations has been explored on the basis of structural, optical and dielectric mechanisms. X-ray diffraction (XRD) analysis shows that the Co+2 ions replace Zn+2 ions in the ZnO matrix, producing lattice strain. Diffused Reflectance Spectroscopy (DRS) shows a red shift in optical energy band gap with increase in cobalt content, along with the presence of transitions in high spin states due to tetrahedrally coordinated cobalt ions. The dielectric characterization explains the disparity in dynamic dielectric parameters like capacitance, dielectric constant, tangent loss, AC conductivity and impedance as a function of frequency. Capacitance and both static and dynamic dielectric constants found to be decreasing with cobalt addition. The anomaly in these pronounced parameters can address the key problems of the material at higher frequencies device operation.

Optical band-gap shift in (InAs)GaAs/AlGaAs HEMTs structures studied by photoluminescence spectroscopy

The optical band-gap has been investigated in detail in (InAs)GaAs/AlGaAs HEMTs structures using a photoluminescence spectroscopy. A power dependent PL study allows highlighting the different causes of the shift band-gap. We can explain the shift by more effects: quantum confined Stark effect (QCSE), band-gap renormalisation (BGR) and Burstein–Moss (BM) effects. We are interested in these effects based on the carriers’ behaviour inside GaAs and InAs/GaAs channels. These effects were affected by carrier concentration depending on Si-δ-doping density and AlGaAs spacer layer thickness as well as the strain relaxation effect. The photoluminescence characterisations have been studied in the 1–10Wcm−2 power range.