Defect-related Transitions Research Articles

Electronic states near surfaces and interfaces of β-Ga2O3 and κ-Ga2O3 epilayers investigated by surface photovoltage spectroscopy, photoconductivity and optical absorption

Transient surface photovoltage (SPV) spectroscopy, optical absorption, and photoconductivity (PC) were applied to study electronic transitions in (-201) β-Ga2O3 and (001) κ-Ga2O3 epitaxial layers on c-plane sapphire substrates. SPV signals were distinguished for charge separation near the surface and near the layer/substrate interface. The bandgaps of β-Ga2O3 and κ-Ga2O3 epitaxial layers were found to be, respectively, about 4.75 and 4.79 eV from optical absorption measurements, about 4.8 and 4.9 eV from PC, and 4.65 and 4.9 (near the surface) from SPV measurements. Near the κ-Ga2O3 layer/substrate interface SPV instead gives a value of 4.75 eV, possibly related to the presence of an interlayer. Defect-related transitions with onset energies at about 4.5, 4.1, and 3.5 eV were observed along the whole β-Ga2O3 layer cross-section. In contrast, defects related to different transitions were differently distributed in κ-Ga2O3 epitaxial layers: transitions setting on at about 4.3 eV originate from defects uniformly distributed across the layer, while transitions starting at 2.4 eV and 1.7 eV were respectively connected with defects located near the surface or near the layer/substrate interface. The PC measurements at photo-excitation energy above the bandgap indicated that defect densities and recombination losses were much larger in κ-Ga2O3 than in β-Ga2O3.

Investigation of highly efficient self-powered photo-detector performance of all inorganic lead-free halide perovskites (CsSnCl3) nanocrystals (NCs) decorated Er: ZnO nanowires (NWs)/Si heterojunctions

In this study, a zero-biased or self-powered heterojunction photodetector is fabricated by using the combination of lead-free halide-based inorganic perovskite (CsSnCl3) and erbium (Er)-doped ZnO nanowires (NWs) on Si platform. The main advantages of such types of heterojunctions are cost-effective fabrication procedure, highly stable and responsive photoresponse, and, such devices exhibit low dark current and high photo-to-dark current ratio of >102. The HRTEM images confirm that CsSnCl3 perovskites nanocrystals (NCs) are randomly decorated over the ZnO NWs. The UV–visible spectroscopy shows the absorption edges and bandgaps of perovskites NCs, ZnO NWs, and combined heterojunctions. The structural, chemical compositional, and defect-related optical transitions are studied in detail by employing XRD, XPS, and PL results. The highest zero-biased photo-response at 374 nm is achieved for the 1%Er: ZnO–CsSnCl3 NWs/Si heterojunction and corresponding calculated responsivity and detectivity values are obtained to be 0.196 A/W and 2.48 × 1011 Jones, respectively. Along with it, such selective Er-doped NWs perovskites heterojunction exhibits relatively superior linear dynamic range (LDR) (51.76 dB) and rapid response speed (rise time: fall time = 83 ms:63 ms) as compared to reported values. Hence, current work demonstrates that Er-doped oxide-perovskite heterojunctions can be a promising candidate for developing high-performance UV/Visible photodetector under self-powered mode.

Luminescence study of ZnS/PVK nanocomposites

Sulphide-based luminescence materials have attracted more curiosity for their extensive applications in electro and photoluminescence devices. One of the sulphides, such as zinc sulphide (ZnS), is a favourable material for phosphor growth in various bands for visible emission. The composition of the polymer matrix with ZnS demonstrates its special functions and stabilizes the nanoparticles. We have used chemical methods for the preparation of ZnS/PVK nanocomposites and reported their synthesis, photoluminescence (PL), and electroluminescence (EL) studies. The samples were analyzed through SEM (Scanning Electron Microscope), AFM (Atomic Force Microscope), and XRD (X-ray Diffraction). The XRD study shows the formation of ZnS nanocrystals with a cubic crystal structure, having a size between 3 and 12 nm and the broadening of peaks shows the formation of an amorphous compound. The AFM and SEM show clusters of particles in the range of a few tens to a few hundred nm. The results from all three studies show an increase in the size of particles with the ZnS loading in PVK (poly N-vinylcarbazole). The PL of a PVK sample excited at 400 nm shows a single PL peak at 451 nm and covers the spectral range from 400 to 600 nm. ZnS/PVK samples excited by 370 nm, give a PL peak due to PVK near 450 nm and when excited by 425 nm, the PL peak is found near 470 nm due to defect-related transitions in ZnS. In both cases, PL intensity increases with increasing ZnS loading in PVK. This may be because of interfacial charge transfer and radiative recombination. The EL studies show that corresponding to the applied voltage, the brightness is increased. The EL emission starts at a lower threshold voltage with a higher loading of ZnS in PVK and at a higher threshold voltage for higher frequencies. EL intensity of the composites increases with increasing ZnS loading in PVK. The voltage-current curve shows the linear relationship between voltage and current that is ohmic in nature. The frequency of applied voltage increases as impedance decreases.

Visible and infrared photoluminescence of Er-doped AlN films: Excitation of Er ions via efficient nonradiative energy transfer from the host level

Aluminum nitride (AlN) is a multifunctional semiconductor. AlN doped with rare earth ions has broad application prospects in monochromatic lighting, color display, laser medium, medical treatment, etc. In this paper, an Er3+-doped AlN film has been prepared by radio frequency magnetron sputtering and characterized by surface morphology, chemical composition and photoluminescence (PL), focusing on the emission mechanism of Er3+ in the AlN host. The films show PL in a wide range of the visible (540, 560 and 668 nm) and near-infrared (816, 869, 985 and 1534 nm) ranges. It has been found a certain energy transfer bridge between Er3+ and AlN defect-related optical transitions. At 532 nm excitation, VAl-ON vacancy-oxygen complex in the AlN host efficiently absorbs photons, then, nonradiative energy transfer (NRET) occurs from the defect levels to the levels of Er3+ since they are in resonance. Since there is a resonance of a donor emission level and an acceptor absorption level, Förster resonance energy transfer (FRET) occurs with the transfer efficiency about 51%. This results in a very strong enhancement of excitation of the 4f related PL. On the contrary, optical excitation at other energies results in a low intensity of the Er3+ PL. As a matter of facts, a strong absorption of the AlN host and the FRET-enabling resonance of donor-acceptor levels allow for a great enhancement of the PL efficiency of Er3+ ions. The results show that the impurity defect state of AlN has an important influence on the Er3+ luminescence efficiency.

Morphological and optical characterizations of different ZnO nanostructures grown by mist-CVD

In this study, the effects of growth temperatures, post-growth treatment, and substrate cleaning recipes on structural, morphological, luminescence, and growth kinetics of ZnO nanostructures grown by mist Chemical Vapor Deposition method (mist-CVD) on SiO2/Si substrates are investigated. The grown ZnO nanostructures have polycrystalline wurtzite (wz) hexagonal crystal structure according to X-ray Diffraction (XRD) measurements. However, the lower growth temperature is found to lead to the formation of ZnO nanostructures oriented in the [0002] direction. From Atomic Force Microscope (AFM) measurements, it is seen that the Root Mean Square (RMS) values decrease with decreasing growth temperature. In addition, it is shown that the surface covering is tightly dependent on decreasing growth temperature of the ZnO nanostructures. The hexagonal and triangular pyramid-like ZnO nanostructures are grown on the surface according to the Scanning Electron Microscope (SEM) measurements. The wurtzite type of crystallization is also demonstrated by confocal Raman measurements. The characteristic vibration modes related to Zn and O motion in the lattice are found to be ∼435 cm−1 and ∼100 cm−1, respectively. The luminescence properties are studied by low-temperature (10 K) Photoluminescence (PL) measurements. The spectra are dominated by the transition related to the donor-bound exciton (DBE) with a peak position typically observed at ∼369 nm (3.362 eV) in hexagonal ZnO structures. Defect-related transitions are observed to be strongly correlated with growth temperature and post-growth treatment.

Tuning the optical properties of molybdenum oxide using electric field: A theoretical and experimental study

Molybdenum Oxide (MoO3) with van der Waals structure can lead to the exotic properties under the action of external physical perturbation. Here, a combined density functional theory and an experimental study were performed to analyze the response of MoO3 under electric field. The electric field with the strength varying from 0.05 V/Å to 0.23 V/Å was applied across the MoO3 cell which induced the splitting in the valence and conduction band and decreases the bandgap. A critical electric field of 0.23 V/Å resulted a closing of the bandgap of MoO3 and led semiconductor to metal transition. The results of DFT regarding decrease in the bandgap of MoO3 were further supported experimentally. For this, MoO3 films were fabricated using thermal evaporation and their chemical and optical properties were analyzed using X-ray photoelectron spectroscopy and Electroreflectance (ER). To analyze the optical response, an ER analysis with external voltage varying from 10 to 50 volts was performed on Aluminum/MoO3/Aluminum heterostructure. The resultant ER spectra revealed three distinct critical points that correspond to the fundamental as well as defect related optical transitions involved in MoO3. The Third derivative functional form model applied on the ER spectra depicted a monotonical decrease in the critical points of MoO3 at lower voltage. At high voltage of about 80 volts, the existence of high built-in electric field results the delocalization in the electrons and hole wave function and gives rise to the Franz-Keldysh oscillations.

Effect of Microindentation on Electroluminescence of SiC P-I-N Junctions.

The influence of microindentation on the electroluminescence of silicon carbide was studied in forward-biased 4H SiC p-i-n junctions. Four spectral regions at approximately 390, 420, 445 and 500 nm initially observed on virgin samples strongly depend, in regard to magnitude, on the condition of the starting die. These spectral regions may be interpreted as arising from either phonon-assisted band-to-band transitions or from defect-related transitions. The same SiC die were then subjected to mechanical damage brought about by a series of closely spaced microindentations directed approximately perpendicular to the c-axis. The spectra taken after a first set and subsequently a second set of microindentations are distinct from the initial spectra in all cases, and differences are interpreted as being due to the modification of existing defects or additional defects being generated mechanically. The influence of microindentation on the ideality factor is measured and discussed. Measured light flux with respect to a standard light source is also shown at each microindentation stage.

Direct Fabrication of CsPbxMn1-x(Br,Cl)3 Thin Film by a Facile Solution Spraying Approach.

Nowadays, Mn-doping is considered as a promising dissolution for the heavy usage of toxic lead in CsPbX3 perovskite material. Interestingly, Mn-doping also introduces an additional photoluminescence band, which is favorable to enrich the emission gamut of this cesium lead halide. Here, a solution spraying strategy was employed for the direct preparation of CsPbxMn1−x(Br,Cl)3 film through MnCl2 doping in host CsPbBr3 material. The possible fabrication mechanism of the provided approach and the dependences of material properties on Mn-doping were investigated in detail. As the results shown, Pb was partially substituted by Mn as expected. With the ratio of PbBr2:MnCl2 increasing from 3:0 to 1:1, the obtained film separately featured green, cyan, orange-red and pink-red emission, which was caused by the energy transferring process. Moreover, the combining energy of Cs, Pb, and Mn gradually red-shifted resulted from the formation of Cs-Cl, Pb-Cl and Mn-Br coordination bonding as MnCl2 doping increased. In addition, the weight of short decay lifetime of prepared samples increased with the doping rising, which indicated a better exciton emission and less defect-related transition. The aiming of current work is to provide a new possibility for the facile preparation of Mn-doping CsPbX3 film material.

Activation energy distribution in thermal quenching of exciton and defect-related photoluminescence of InP/ZnS quantum dots

Thermal quenching is one of the essential factors in reducing the efficiency of radiative processes in luminophores of various nature. The emission activity of low-dimensional structures is influenced also by multiplicity of parameters that are related to synthesis processes, treatment regimes, etc. In the present work, we have investigated the temperature dependence of photoluminescence caused by exciton and defect-related transitions in ensembles of biocompatible InP/ZnS core/shell nanocrystals with an average size of 2.1 and 2.3 nm. The spread in the positions of energy levels is shown to be due to size distribution of quantum dots in the ensembles under study. For a quantitative analysis of the experimental data, we have proposed a band model accounting for the Gaussian distribution of the thermally activated barriers in the photoluminescence quenching processes. The model offers the thermal escape of an electrons from the core into the shell as the main mechanism for non-radiative decay of excitons. In turn, the quenching of defect-related emission is predominantly brought about through the emptying of the hole capture centers based on dangling phosphorus bonds. We have revealed the correlation between size distributions of quantum dots and scatter of the activation energy of exciton luminescence quenching. The developed approach will give further the possibility to optimize technological regimes and methods for band engineering of indium phosphide-based type-I quantum dots.

Ab initio calculations for electronic and optical properties of ErW defects in single-layer tungsten disulfide

Ab initio calculations for the electronic and optical properties of single-layer (SL) tungsten disulfide (SL WS2) in the presence of substitutional Erbium defects (ErW) are presented, where the W atom is replaced by an Er atom. Although Er is much larger than W, we show that Er:SL WS2 is stable using density functional theory. In order to understand defect related optical transitions, odd states, which are usually neglected for pristine cases, need to be considered in addition to even states. We use group theory to derive strict selection rules for the optical transitions, which are in excellent agreement with the absorption spectrum calculated by means of the Kubo–Greenwood formula using the Kohn–Sham orbitals. Defects usually play an important role in tailoring electronic and optical properties of semiconductors. We show that neutral and negatively charged ErW defects lead to localized defect states in the band structure due to the f-orbital states of Er, which in turn give rise to sharp optical transitions in in-plane and out-of-plane components of the susceptibility tensor Imχ∥ and Imχ⊥, respectively. We identify the optical transitions at 5.3 μm, 1.5 μm, 1.2 μm, 920 nm, 780 nm, 660 nm, and 550 nm to originate from ErW defect states. Our results for the optical spectra are in good agreement with experimental data.

Modulation on Radiative Recombination Rate of CdS Nanobelts by Selective Rare Earth Ions

AbstractRare earth ions Er3+, La3+, and Y3+ have been introduced into the lattice of CdS nanobelts (NBs) to modulate its radiative recombination rate (RRR) during photoluminescence (PL) process. It is found that these ions have different quenching effects on the RRRs of direct interband transition and defect related transition. Only the former is greatly reduced through Er3+ doping, however, both of them are suppressed to a certain extent by La3+ and Y3+ doping. It is valuable that specific solid color luminescence can be realized through well‐chosen rare earth ions.

Defect-related photoluminescence and photoluminescence excitation as a method to study the excitonic bandgap of AlN epitaxial layers: Experimental and ab initio analysis

We report defect-related photoluminescence (PL) and its vacuum ultraviolet photoluminescence excitation (PLE) spectra of aluminum nitride layers with various layer thicknesses and dislocation densities grown on two different substrates: sapphire and silicon. The defect-related transitions have been distinguished and examined in the emission and excitation spectra investigated under synchrotron radiation. The broad PL bands of two defect levels in the AlN were detected at around 3 eV and 4 eV. In the PLE spectra of these bands, a sharp excitonic peak originating most probably from the A-exciton of AlN was clearly visible. Taking into account the exciton binding energy, the measurements allow determination of the bandgaps of the investigated AlN samples and their temperature dependencies. Next, they are compared with the literature data obtained by other experimental techniques for bulk AlN crystals and layers grown on different substrates. The obtained results revealed that the AlN bandgap depends on the substrate. The theoretical analysis using density functional theory calculations showed that the effect is induced by the tetragonal strain related to the lattice mismatch between the substrate and the AlN layer, which has a strong influence on the spectral positions of the intrinsic excitons, and consequently on the bandgap of AlN layers.

Density functional studies of defects and defect-related luminescence in Mg3N2

$\mathrm{M}{\mathrm{g}}_{3}{\mathrm{N}}_{2}$ is a promising material for light-emitting-diode applications. However, the microscopic origin of the broad yellow photoluminescence in $\mathrm{M}{\mathrm{g}}_{3}{\mathrm{N}}_{2}$ remains unknown. Here, defect properties and defect-related optical transitions are investigated based on the hybrid functional calculation. Our results show that the nitrogen vacancy introduces multiple localized defect states within the band gap, which play a dominant role in luminescent properties of $\mathrm{M}{\mathrm{g}}_{3}{\mathrm{N}}_{2}$. Common impurities like hydrogen, oxygen, and carbon and their complexes with native defects are also studied. Compared with isolated ${\mathrm{V}}_{\mathrm{Mg}}, {\mathrm{V}}_{\mathrm{Mg}}$-H and ${\mathrm{V}}_{\mathrm{Mg}}\ensuremath{-}{\mathrm{O}}_{\mathrm{N}}$ complexes have shallower transition levels. Our calculated optical excitation and emission peaks associated with isolated ${\mathrm{V}}_{\mathrm{N}}$ are in good agreement with those observed in experiments. In addition, the impurity ${\mathrm{C}}_{\mathrm{N}}$ is also a potential source for the observed yellow emission in $\mathrm{M}{\mathrm{g}}_{3}{\mathrm{N}}_{2}$.

Non-radiative relaxation processes in luminescence of InP/ZnS quantum dots

Photoluminescence (PL) thermal quenching of InP/ZnS core/shell nanocrystals of 4.2 (QD-1) and 4.6 (QD-2) nm in diameter is investigated in the temperature range of 6.5-296 K. Observed PL spectra are found to originate from exciton and defect-related transitions. Temperature behavior of the emission bands indicates effects of inhomogeneous broadening due to distributions of quantum dots parameters in the ensembles. PL thermal quenching analysis unveils two non-radiative relaxation channels for each band in both samples. Activation energies of 130 and 119 meV in QD-1 and QD-2, correspondingly, for the exciton band can be assigned to the bright-bright splitting of the exciton fine structure.

Strain-free ultrathin AlN epilayers grown directly on sapphire by high-temperature molecular beam epitaxy

High-quality AlN ultrathin films on sapphire substrate were grown by molecular beam epitaxy using an in situ high-temperature annealing approach. From transmission electron microscopy studies, it was found that the AlN epilayers are strain relaxed within the first nm, thus growing nearly strain free. Many of the dislocations generated at the AlN/sapphire interface are reduced within the first 50 nm of growth. Epitaxial films grown directly on sapphire, which are ∼100 nm thick, show X-ray diffraction (002) and (102) rocking curve peaks with full widths at half maximum of less than 150 and 1400 arc sec, respectively, which are the narrowest linewidths reported for AlN of this thickness. Detailed photoluminescence studies further showed that such AlN epilayers exhibit relatively high luminescence efficiency and strong near-band edge emission without defect-related transitions.

First-principles calculations of Stark shifts of electronic transitions for defects in semiconductors: the Si vacancy in 4H-SiC

Point defects in solids are promising single-photon sources with application in quantum sensing, computing and communication. Herein, we describe a theoretical framework for studying electric field effects on defect-related electronic transitions, based on density functional theory calculations with periodic boundary conditions. Sawtooth-shaped electric fields are applied perpendicular to the surface of a two-dimensional defective slab, with induced charge singularities being placed in the vacuum layer. The silicon vacancy (V Si) in 4H-SiC is employed as a benchmark system, having three zero-phonon lines in the near-infrared (V1, V1′ and V2) and exhibiting Stark tunability via fabrication of Schottky barrier or p-i-n diodes. In agreement with experimental observations, we find an approximately linear field response for the zero-phonon transitions of V Si involving the decay from the first excited state (named V1 and V2). However, the magnitude of the Stark shifts are overestimated by nearly a factor of 10 when comparing to experimental findings. We discuss several theoretical and experimental aspects which could affect the agreement.

Understanding the excitation wavelength dependent spectral shift and large exciton binding energy of tungsten disulfide quantum dots and its interaction with single-walled carbon nanotubes

Herein, we investigate the origin of excitation wavelength dependent spectral features and high fluorescence quantum yield in fluorescent 2D tungsten disulfide (WS2) quantum dots (QDs) of average size 2.4 nm. The as-prepared WS2 QDs possess high optical bandgap and reasonably high fluorescence quantum yield ~15.4% in the green region without any functionalization. The broad photoluminescence (PL) spectrum consists of multiple peaks owing to emissions from excitonic transitions and surface defect-related transitions. The excitation wavelength-dependent spectral redshift and narrowing of line shape in the PL peak are analyzed carefully, and it is attributed to the selective excitation/recombination of carriers from different energy levels. The temperature-dependent PL analysis yields an exciton binding energy of ~301 meV in the QDs. Furthermore, we study the interaction between fluorescent WS2 QDs and single-walled carbon nanotubes (SWCNTs) and explore the mechanism of systematic quenching of PL of QDs by SWCNTs. The nature of the Stern–Volmer plot is found to be linear, and the time-resolved fluorescence measurements reveal that the quenching follows primarily the static behavior. Our study further reveals that defect sites in SWCNTs primarily act as the binding sites for WS2 QDs and form non-fluorescent complexes for effective quenching of the PL. The strong interaction between the WS2 QDs and the SWCNTs is evidenced from the spectral shift in the X-ray photoelectron spectroscopy and Raman peaks. Our study reveals the origin of excitation wavelength dependent PL emission from WS2 QDs and the nature of the interaction between WS2 QDs and SWCNTs, which are important for their applications in biomedical imaging and sensing, such as surface-enhanced Raman scattering, etc.

Excitation-intensity dependence of shallow and deep-level photoluminescence transitions in semiconductors

Photoluminescence characterization of semiconductors is a powerful tool for studying shallow and deep defects. Excitation-intensity-dependent measurements at low temperatures are typically analyzed to distinguish between exciton and defect related transitions. We have extended existing models based on rate equations to include the contribution of deep defects. Generally, it is observed that the photoluminescence intensity IPL follows a power law IPL∝ϕk with the excitation intensity ϕ. We show that the exponent k takes on values of multiples of 1/2. The values depend on the availability of additional recombination channels. Defect levels can saturate at high enough excitation intensities, leading to one or several crossover points from one power law behavior to another. Power law exponents different from n/2 can result from the transition region between two limiting cases of linear power laws. Model functions for the analytical description of these transitional excitation dependencies are derived and the analysis is applied to chalcopyrite thin films and to numerical data. The saturation effects of defects by excess carriers as well as the influence of deep recombination centers can be extracted with the help of the presented model, which extends existing theories.

Formation Mechanism and Properties of Foam-Structured ZnO Films Prepared Using Catalyst-Free Carbothermal Method

Foam-structured ZnO films were formed on c-axis Al2O3 substrates by using a catalyst-free carbothermal method. The morphological properties of the films strongly depended on their substrate position in the reactor, implying that the quantity of Zn vapor transported played a crucial role in determining the film structure. The formation mechanism of the films is proposed to be as follows: with increased transport of Zn vapor, vaporized Zn elements transformed into liquid Zn nanoparticles, and the liquid Zn elements on the rods promoted the new nucleation of ZnO nanorods. The repeated growth of ZnO rods at the nucleation sites led to the foam-like structure. The as-formed foam-structured ZnO films exhibited the wurtzite crystal structure, and the atomic composition ratio of Zn to O was 1 : 0.85. The main photoluminescence emissions were centered at 3.3 and 2.5 eV, and they were attributed to band-to-band and defect-related transitions, respectively.

A study of structural parameters and photoluminescence of Tb doped ZnO nanoparticles

Systematic structural studies on Tb3+ ion doped ZnO nanoparticles were carried out using X-ray diffraction, X-ray photoelectron spectroscopy and photoluminescence studies. Our XRD study shows a contraction effect as the dopant ion concentration is increased. The dislocation densities and the surface area calculation also show an increase up to a certain concentration of Tb, which reveals the increase in the surface defects when Tb is incorporated in ZnO host lattice. PL study confirms the presence of band edge as well as defect related transition.