eV Photoluminescence Band Research Articles

Evolution of Au nanoparticles in c-plane GaN under the heavy ion implantation and their optical properties

The modification of GaN with Au nanoparticles is a promising way for manipulating optical properties in optoelectronics and enhancing the sensitivity of GaN-based substrates used in Surface Enhanced Raman Spectroscopy. Ion implantation is an attractive method for the preparation of high-purity metal nanoparticles on the surface or within the bulk of solids, although this process in crystals is not yet fully understood. Here we study the specific stages of Au nanoparticle formation in c-plane GaN crystals implanted with 1.85 MeV Au ions to the fluence range of 1.5×1016 - 7×1016 cm−2. The implanted samples were annealed at 800 °C in an ammonia atmosphere for 20 minutes to support the nucleation of Au nanoparticles and prevent surface decomposition. The structural and optical properties of the samples were investigated by a combination of Rutherford backscattering spectroscopy in channeling mode (RBS-C), Transmission Electron Microscopy (TEM), Raman spectroscopy, photoluminescence (PL) and diffuse-reflectance spectroscopy (DRS). The two damaged regions, namely at the depth and at the surface, were identified. The depth region exhibits saturation of damage, unlike the surface region reaching an amorphous state for the highest implantation fluence. RBS-C revealed multimodal Au depth profiles for higher ion fluences, which were subsequently redistributed after thermal annealing due to the formation of Au nanoparticles, as confirmed by TEM. Au nanoparticles with fcc structure have been successfully synthesized with the Au-ion implantation fluence of 5×1016 cm−2 with the sizes mostly of 4 – 20 nm resulting in increased optical scattering and absorption within the spectral range of 500 – 700 nm associated with surface plasmon resonance (SPR) and the emergence of blue (2.8 eV) and red (2 eV) photoluminescence bands linked to the combined effect of SPR and implantation-induced defects.

Identification of the Nitrogen Interstitial as Origin of the 3.1 eV Photoluminescence Band in Hexagonal Boron Nitride

Nitrogen interstitials () have the lowest formation energy among intrinsic defects of hexagonal boron nitride (hBN) under n‐type and N‐rich conditions. Using an optimized hybrid functional, which reproduces the gap and satisfies the generalized Koopman's condition, an configuration is found, which is lower in energy than the ones reported so far. The (0/–) charge transition level is also much deeper, so acts as a very efficient compensating center in n‐type samples. Its calculated photoluminescence (PL) at 3.0 eV agrees well with the position of an N‐sensitive band measured at 3.1 eV. It is also found that the nitrogen vacancy () cannot be the origin of the three‐boron‐center (TBC) electron paramagnetic resonance (EPR) center and in thermal equilibrium it is even unlikely to exist in n‐type samples.

Shallow and Deep States of Beryllium Acceptor in GaN: Why Photoluminescence Experiments Do Not Reveal Small Polarons for Defects in Semiconductors.

Currently, only one shallow acceptor (Mg) has been discovered in GaN. Here, using photoluminescence (PL) measurements combined with hybrid density functional theory, we demonstrate that a shallow effective-mass state also exists for the Be_{Ga} acceptor. A PL band with a maximum at 3.38eV reveals a shallow Be_{Ga} acceptor level at 113±5 meV above the valence band, which is the lowest value among any dopants in GaN reported to date. Calculations suggest that the Be_{Ga} is a dual-nature acceptor with the "bright" shallow state responsible for the 3.38eV PL band, and the "dark," strongly localized small polaronic state with a significantly lower hole capture efficiency.

The 1.1, 0.8 and 0.55–0.60 eV deep bands in detector-grade CdMnTe studied by photoluminescence spectroscopy

We present low-temperature photoluminescence (PL) spectra of Cd0·95Mn0·05Te grown by the Bridgman method. This is a promising material for nuclear detectors. We focus on three deep bands: 1.1, 0.8 and 0.55–0.60 eV. In order to achieve high resistivity in our crystals, essential in detector applications, we confirm the role of a deep intrinsic donor level at 0.8 eV, related to TeCd2+. It can pin the Fermi level in the mid-gap and increase the resistivity value. We suggest two interpretations of the 0.55–0.60 eV PL band origin. It can be connected with the deionization of a deep acceptor, such as Cd2−, or with simultaneous transitions of an electron (1.1 eV) and a hole (0.55–0.60 eV) followed by their recombination. These two energies together give the band gap value for Cd0·95Mn0·05Te at 5 K. The 1.1 eV luminescence in our crystals is related to Te secondary phases, the presence of which we confirm by infra-red microscopy. The correlation between the chemical composition of the crystals (changed by Cd- or Te-excess used in growth process and by post-growth annealing), the resistivity value and the PL results is shown.

Donor-acceptor nature of orange photoluminescence in AlN

Recombination dynamics, photoluminescence and photoluminescence excitation spectra have been investigated for 1.9 eV photoluminescence band in AlN in the temperature range of 5–650 K. The recombination dynamics for the 1.9 eV photoluminescence band has been described by a model of donor-acceptor recombination with taking into account a broadening due to electron coupling with local lattice vibrations of a deep level defect. The experimental results have been compared with density functional theory calculations of luminescence peak energies and line shapes of band to defect and donor-acceptor transitions, and possible origin of the orange photoluminescence band in AlN has been discussed.

Emission and structure variations at aging ZnO:Ag nanocrystals prepared by spray pyrolysis

The emission and structure of ZnO:Ag nanocrystal (NC) films obtained by ultrasonic spray pyrolysis have been studied in the as-grown state and after aging. The scanning electronic microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL) methods have been applied. The films are characterized by the wurtzite crystal structure with the NC sizes about of 8.0–10.0 nm in the as-grown state and 6.0–8.0 nm after aging in ambient air. The Ag content was varied within the range of 1–4 at.% in the ZnO films. PL spectra of as-grown ZnO:Ag films are complex and can be presented as a set of PL bands at 10 K: the near band edge emission with the peak at 3.18 eV, two Ag doping—related PL bands centered at 2.95 and 2.68 eV, and native defect-related green (2.40 eV) and orange (1.90 eV) PL bands. The high PL intensities of Ag doping-related PL bands were detected for the Ag concentrations of 2–3 at.%. PL intensities of high energy PL bands (3.18, 2.95 and 2.68 eV) fall down at aging together with intensity stimulating the orange and green PL bands. The joint analysis of XRD and PL results permit to confirm that the 2.95 eV PL band is connected with the optical transition via the substitutional AgZn defects. It was shown as well that aging in ambient air is owing to the diffusion of oxygen interstitials in the ZnO:Ag NCs that leads to intensity increasing the orange PL band and modification of the AgZn radiative defects.

Radiative Defect Varying in Aging ZnO:Ag Nanocrystals

ZnO nanocrystal (NC) films prepared by ultrasonic spray pyrolysis and doped with Ag have been studied in as-grown and aged states. Photoluminescence (PL) and x-ray photo electronic spectroscopy (XPS) have been used in this study. PL spectra of as-grown ZnO:Ag NCs include a number of PL bands peaking at: 3.18 eV, 2.85 eV, 2.70 eV, 2.50 eV and 2.10 eV. It is revealed that the 2.70 eV and 2.85 eV PL bands are connected with Ag-doping ZnO NCs. PL intensities of the first three PL bands decline at aging, but the PL intensity of the orange PL band increases. This last fact indicates that aging at 300 K is connected with increasing the concentration of oxygen interstitials in ZnO NC films. In XPS research, the variation of substitutional AgZn defects and the formation of Ag-O bounds at aging have been detected. The nature of optical transitions and radiative defects varying at aging have been analyzed and discussed.

Optical properties of LiGaSe2 noncentrosymmetric crystal

Large single crystals of LiGaSe2 were grown and their spectroscopic properties were studied. As-grown LiGaSe2 crystals are transparent in the 0.5–14 μm spectral region as well as at λ > 200 μm in the THz domain. A large blue shift of the fundamental absorption edge was established after high-temperature annealing in the presence of Li. Intense low-temperature photoluminescence (PL) in the 1.77 eV band is excited in the 3.0–3.6 eV absorption band near the fundamental absorption edge; PL intensity decreases by two orders of magnitude as temperature grows to 300 K. The X-ray irradiation results in crystal darkening, whereas the original state is restored after heating to 500 K or by visible light illumination. This effect is due to six traps that capture charge carriers: these traps were revealed using the thermoluminesence technique and their parameters were calculated. Pyroelectric luminescence in the 100–270 K range confirms the noncentrosymmetric structure of this crystal. DFT calculations were carried out to simulate the phonon density of states, optical absorption and Raman spectra. Calculations show that the intense 3.0–3.6 eV absorption band and corresponding 1.77 eV PL band may be associated with the cation antisite defects (GaLi), i.e. gallium in the Li site. The 2.15 and 2.4 eV emissions are likely due to the recombination of self-trapped excitons.

Origins of a damage-induced green photoluminescence band in fused silica revealed by time-resolved photoluminescence spectroscopy

With combined lifetime and intensity spectra in time-resolved photoluminescence (PL) spectroscopy, the origins of a green PL band consisting of a 2.39eV-band with lifetime ~10μs and a 2.25eV-band with lifetime ~50μs, of high-purity fused silica under ArF excimer laser (6.4eV) excitation were revealed. The 2.39eV PL defects from the surface were annealed due to the deoxidation of the dioxasilyrane group ( = SiO2). The 2.25eV PL defects from bulk showed a UV induced growth. Mechanical and laser damage induced growth of the 2.25eV PL band confirmed that it was due to physical disorder from bending of Si-O-Si bonds. Theoretical calculations further assigned the 2.25eV PL band to silanone groups ( = Si = O), which can be created by relaxation process of strained Si-O bonds in SiO2 network.

Influence of electronic and optical properties of GaN nanoparticles as potential electrocatalyst in hydrogen production

Due the high electron density of the anion lattice found in nitrides, these compounds have been used for such applications as high power devices, photonics and, more recently, hydrogen production. Among nitride compounds, GaN is well known for its thermal stability, while the different physical techniques used for its synthesis have been reported to obtain different physicochemical properties. However, these techniques are not designed for the mass production of GaN. For these reasons, two different methods for both the chemical synthesis of GaN and its physical chemical characterization are reported in this study, with both synthesis methods presenting wurtzite phase crystallization. Furthermore, both samples presented agglomerates formed by similar sized nanoparticles (~20nm). Despite the similarity in the particle size, higher pore volume and surface area as well as carbon traces are promoted by the solvothermal route. In contrast, structural and photoluminescence (PL) characterization revealed that Ga vacancies and interstitial N formed in those samples synthesized by the nitridation method, promoting higher intensity in ~2.8eV PL band. Finally, based on cathodic linear sweep voltammetry, the samples prepared in this study can be considered good candidates for use in hydrogen production.

Willemite photoluminescence in Zn‐implanted silica glasses

AbstractThe photoluminescence properties of α‐Zn2SiO4 nanoparticles formed in the SiO2 by means of ion implantation and subsequent 900 °C, 1 hour annealing in air atmosphere. Both X‐ray diffraction, optical and photoluminescence (PL) spectra confirm the formation of willemite phase Zn2SiO4 in SiO2 matrices. The characteristic 2.4 eV PL band observed resembles the properties of classical Zn2SiO4:Mn phosphor, though no manganese were introduced into the samples. PL excitation spectra demonstrate the contribution of willemite and silica band states, thus suggesting interphase energy transfer mechanism. Thermal quenching and lifetimes of the luminescence observed provide the clues to figure out the nature of emission centres. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural and light emitting properties of silicon-rich silicon nitride films grown by plasma enhanced-chemical vapor deposition

Si-rich Silicon nitride films fabricated by plasma enhanced chemical vapor deposition (PECVD) on p-type silicon substrates were investigated by means of photoluminescence (PL) and X-ray diffraction (XRD) methods. The film stoichiometry was controlled via varying the NH3/SiH4 ratio (R) in the range R=0.56–1.0. Thermal annealing at 1100°C for 30min in the nitrogen flow was applied to form the Si nanocrystals (NCs) embedded in the films. XRD experiment has revealed also the formation of hexagonal silicon nitride NCs with the sizes of 26–30nm in the films grown with R=1. When R decreasing, the sizes of silicon nitride NCs decreases down to 2.8–3.0nm (R=0.63). In the films grown with R=0.56 the amorphous silicon nitride, amorphous Si and crystallized Si phases have been detected. PL study showed the non-monotonic behavior of PL intensity with the R decreasing. The highest PL intensity was detected for the films grown with R=0.63. PL spectra of all films were found to be complex containing a set of PL bands peaked at about 2.8–3.0, 2.5–2.7, 2.1–2.2 and 1.8–2.0eV. Temperature behavior of PL spectra was investigated in the range 20–300K that allowed analyzing the nature of each PL component. It was shown that the peak position of all PL bands varies with the SiNx stoichiometry and Si NC sizes, demonstrating a “red” shift with R decreasing. It was revealed that the former three PL bands did not change their peak positions versus temperature of measurement that permits to assign them to the carrier recombination via radiative defects in the silicon nitride matrix. The PL band at 1.8–2.0eV has demonstrated a “blue” spectral shift with cooling similar to the Si band gap shrinkage. Based on this behavior, the 1.8–2.0eV PL band has been attributed to exciton emission inside of Si NCs. The role of silicon nitride NCs in photoluminescence and its excitation is discussed.

Time‐resolved photoluminescence characterization of 2 eV band in AlN

AbstractWe report time‐resolved and temperature‐dependent photoluminescence investigations of 2 eV photoluminescence band in AlN with below bandgap excitation. Series of the samples grown by molecular beam epitaxy on sapphire substrates with varying growth conditions have been studied. Intensity of the 2 eV photoluminescence band has been found to increase with increasing III/V flux ratio. The 2 eV photoluminescence band has been described in one‐dimensional configuration coordinate model. A possible origin of this orange emission has been discussed. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Properties of the main Mg-related acceptors in GaN from optical and structural studies

The luminescent properties of Mg-doped GaN have recently received particular attention, e.g., in the light of new theoretical calculations, where the deep 2.9 eV luminescence band was suggested to be the main optical signature of the substitutional MgGa acceptor, thus, having a rather large binding energy and a strong phonon coupling in optical transitions. We present new experimental data on homoepitaxial Mg-doped layers, which together with the previous collection of data give an improved experimental picture of the various luminescence features in Mg-doped GaN. In n-type GaN with moderate Mg doping (<1018 cm−3), the 3.466 eV ABE1 acceptor bound exciton and the associated 3.27 eV donor-acceptor pair (DAP) band are the only strong photoluminescence (PL) signals at 2 K, and are identified as related to the substitutional Mg acceptor with a binding energy of 0.225 ± 0.005 eV, and with a moderate phonon coupling strength. Interaction between basal plane stacking faults (BSFs) and Mg acceptors is suggested to give rise to a second deeper Mg acceptor species, with optical signatures ABE2 at 3.455 eV and a corresponding weak and broad DAP peak at about 3.15 eV. The 2.9 eV PL band has been ascribed to many different processes in the literature. It might be correlated with another deep level having a low concentration, only prominent at high Mg doping in material grown by the Metal Organic Chemical Vapor Deposition technique. The origin of the low temperature metastability of the Mg-related luminescence observed by many authors is here reinterpreted and explained as related to a separate non-radiative metastable deep level defect, i.e., not the MgGa acceptor.

Hydrogen effect on nanostructural features of nanocrystalline silicon thin films deposited at 200 °C by PECVD

In this work, we reported hydrogen effect on nanostructural features of nanocrystalline silicon (nc-Si) thin films by using a plasma-enhanced chemical vapor deposition (PECVD) method at low deposition temperatures. Structural and optical properties have been investigated by means of X-ray diffraction, Raman scattering, photoluminescence (PL), and oscillator strength measurements. In addition, mechanical property was also investigated by stress measurements. It is shown that with an increase of the hydrogen content, R=[H2]/[SiH4], the crystalline volume fraction, the average crystallite size, <δ>, and the intensity of the 1.75–1.85eV PL band were increased, while the optical band-gap, Egopt, the peak energy and the intensity of the 2.15–2.25eV PL band were decreased. The oscillator strength and Egopt were increased when <δ> decreased. The remarkable result obtained from PL measurements showed that amorphous silicon (a-Si) exhibits a PL peak at 1.65eV and also possesses a wider band-gap than that of bulk Si. The stress in the nc-Si films is slightly compressive, which is favored for application of nc-Si to device technology. Moreover, addition of hydrogen-related molecules to the feed gases under low deposition temperature plays an important role, such as etching, cleaning, and coverage on the growing surface, in the crystallization of nc-Si.

On the origin of deep oxygen defects in hydrogenated nanocrystalline silicon thin films used in photovoltaic applications

To effectively mitigate oxygen induced degradation of electronic properties in hydrogenated nanocrystalline silicon (nc-Si:H) the microscopic origins of the associated defects must be determined. This work elucidates the origin of a 0.7eV photoluminescence (PL) band observed in thermally annealed, oxygen contaminated nc-Si:H. Raman and PL spectroscopy data demonstrate the deep defects responsible for the 0.7eV PL reside in the crystalline phase. The electronic levels enabling the emission exist 0.4eV below the conduction band edge, or 0.4eV above the valence band edge, or both, with transitions into or out of extended states as opposed to band tail recombination. The defect formation energy is 0.6eV, which suggests that the formation involves a local reconfiguration of oxygen and hydrogen as opposed to bulk diffusion. Hydrogen plays multiple roles in the microstructural evolution and electronic activity of these centers. TEM micrographs show dislocations in the nanocrystalline regions, which suggests that oxygen gettering occurs. It seems in the absence of hydrogen dislocations in Si nanocrystals scavenge oxygen impurities from grain boundaries to become optically active, enabling emission at 0.7eV.

Below band-gap optical absorption and photoluminescence excitation spectroscopy at room temperature in low-defect-density bulk GaN:Fe

We present a detailed study of the below band-gap optical absorption at room temperature in bulk semi-insulating GaN:Fe versus the Fe-doping. It was established that the 1.24 eV photoluminescence band at 300 K consists of only vibrational replicas of the Fe3+ 4T1(G)→6A1(S) internal transition. We also studied the below band-gap photoluminescence excitation of the 1.24 eV band. The identical exponential rise of the photoluminescence excitation and the optical absorption coefficient identify the Fe3+/2+ charge-transfer as the main contributor to the 300 K optical absorption in the range 400-500 nm. Practical implications of these results for Fe-doping determination are discussed.

Temperature dependent photoluminescence of PECVD a-SiO x:H ( x

The temperature dependent visible photoluminescence (PL) property of a-SiO x :H ( x<2) samples prepared in a PECVD system by using SiH 4+CO 2 gas mixture is investigated at a temperature range of 20 K–400 K. One of the two explicitly distinguished PL bands, with varying peak photon energies between 1.70 and 2.05 eV, can be detected at only low temperatures below 200 K, which is attributed to tail-to-tail radiative recombination. Thermal quenching parameter ( T L ) of the tail-to-tail PL band is calculated as varying between 120 and 280 K as the atomic oxygen concentration ([O]at.%) of the samples increases. Stokes shift (Δ E Stokes ) of the tail-to-tail PL band is found to change from 85 meV to 420 meV due to band tail widening. The other PL band emerges at 2.1 eV and can be detected at higher temperatures with thermal activation behavior. The activation energies calculated about room temperature vary in the range of 8 meV–50 meV with oxygen concentration. Thermal activation of the 2.1 eV PL band is attributed to the behavior of thermally activated incoherent hopping migration of electrons. These electrons combine with self trapped holes (STHs) to form self trapped excitons (STEs). STEs are localized at intrinsic defects of SiO 2 structure such as oxygen vacancies ( E′ centers) and non-bridging oxygen hole centers (NBOHC).

Excitation power dependent photolu‐ minescence of PECVD a‐SiOx:H (x&lt;2) thin films

AbstractThe excitation power dependent visible photoluminescence (PL) property of an a‐SiOx:H (x &lt; 2) sample prepared in a PECVD system is studied within a temperature range of 12 K and 400 K. An explicit PL band was observed at the photon energy of 2.1 eV. It was observed that the PL intensity increased with a power of the excitation intensity (IPL∝Lγ) changed between 1.3 mWcm‐2 and 25 mWcm‐2. γ was calculated to change between 0.8‐1.2 depending on the measurement temperature. The 2.1 eV PL band was found to show weak thermal activation energy around room temperature and calculated as about 1meV. The temperature dependence of PL for each excitation power was fitted to a function proposed due to the thermal activated radiative recombination and the defect related non‐radiative recombination mechanism. As a result of the model the ratio of the average distance between a carrier and the non‐radiative defect center to the decay length of the wave function of the defect was calculated to be varying between 21 and 33 with increased generation rate range of 1.75x1016 cm‐3/s‐3.5x1017 cm‐3/s. The results were discussed in terms of the separation of phases and the behavior of thermally activated incoherent hopping migration of electrons to combine with self trapped holes (STH) to form self trapped excitons (STE). (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Conformational heterogeneity of the point defects in silica: The lifetime of the phosphorescence band at 2.7 eV

We have measured the excitation and emission energy dependence of the lifetimes of the 2.7eV photoluminescence band associated to oxygen deficient centers in silica glasses. The non-exponential behavior of this time decay is consistent with intrinsic conformational heterogeneity of these point defects in the amorphous matrix. Accordingly, we have analyzed the data in terms of a radiative rates distribution. Moreover, both surface and bulk typologies of these point defects have been studied. The mean value of the lifetime distribution of the surface defects increases from 12 to 15ms varying the excitation energy from 4.6 to 5.2eV, and it increases from 14 to 15ms in the emission energy interval between 2.6 and 3.0eV. As well similar variations of the lifetime are observed for interior defects, when measured at different excitation and emission energies. We can also estimate the width of the lifetime distribution of this ensemble of point defects in silica glass.