Defect-related Photoluminescence Research Articles

Radiation Defects in Heterostructures 3C-SiC/4H-SiC

The effect of 8 MeV proton irradiation on n-3C-SiC epitaxial layers grown by sublimation on semi-insulating 4H-SiC substrates has been studied. Changes in sample parameters were recorded using the Hall-effect method and judged from photoluminescence spectra. It was found that the carrier removal rate (Vd) in 3C-SiC is ~100 cm−1, which is close to Vd in 4H-SiC. Compared with 4H and 6H silicon carbide, no significant increase in the intensity of the so-called defect-related photoluminescence was observed. An assumption is made that radiation-induced compensation processes in 3C-SiC are affected by structural defects (twin boundaries), which are always present in epitaxial cubic silicon carbide layers grown on substrates of the hexagonal polytypes.

Sub-wavelength waveguide properties of 1D and surface-functionalized SnO2 nanostructures of various morphologies.

One-dimensional (1D) SnO2 sub-wavelength waveguides are a critical contribution to advanced optoelectronics. Further understanding of the surface defects and role of morphology in 1D SnO2 nanowires can help to better utilize these nanostructures more efficiently. For this purpose, three different nanowires (NWs), namely belts, cylindrical- and square-shaped structures were grown using SnO2 quantum dots as a precursor material. The growth process of these NWs is discussed. The nanobelts were observed to grow up to 3 mm in length. Morphological and structural studies of the nanostructures were also carried out. All NWs showed waveguide behavior with visible photoluminescence (PL) upon excitation with a 325 nm laser. This behavior was also demonstrated in tapered and surface-functionalized SnO2 NWs. While the tapered waveguide can allow for easy focusing of light, the simple surface chemistry offers selective light propagation by tuning the luminescence. Defect-related PL in NWs is studied using temperature-dependent measurements and a band diagram is proposed.

Enhanced room temperature exciton photoluminescence of plasma hydrogenated ZnO nanocolumns

Zinc oxide (ZnO) nanocolumns have interesting material properties such as the direct band gap, tunable electrical conductivity, high optical transparency, high thermal conductivity, and large surface to volume ratio. Here we present the latest results on the enhancement of the infrared optical absorptance and ultraviolet photoluminescence in ZnO nanocolumns after hydrogen plasma treatment. The photoluminescence in near UV region at 378 nm has been significantly enhanced by hydrogen plasma treatment at roo temperature whereas the defect related yellow photoluminescence (broad band 550–650 nm) decreased.

Structural and optical studies of gamma irradiated N-doped 4H-SiC

In this paper, the gamma irradiation effects on the structural and optical properties of N-doped 4H-SiC (n-4H-SiC) is presented up to a cumulative gamma radiation dose of 1500 kGy. The studies showed marginal and inconsistent variation in the c- and a-axis lattice constants of 4H-SiC due to the accumulation of gamma-induced defect states. The modifications in the longitudinal optical plasmon-phonon coupled (LOPC) modes, Biedermann absorption bands, Urbach energy (EU) and defect related photoluminescence (DPL) bands are discussed at different (500, 1000 and 1500 kGy) irradiation doses. Despite these effects, the overall gamma-induced disorder (1-Anorm) and variation in the free carrier concentrations (n) are found to be negligible and demonstrating the radiation resistant property of n-4H-SiC under gamma radiation environment.

Annealing Induced Oxygen Defects on Green Sonochemically Synthesized ZnO Nanoparticles for Photoelectrochemical Water Splitting

AbstractZinc oxide (ZnO) nanoparticles were prepared by a green process using a simple sonochemical method. The prepared nanoparticles were annealed at different annealing temperatures in order to determine the role of the annealing temperature on the structural, morphological and optical properties of the prepared material. The defects related photoluminescence properties of the ZnO nanoparticles are discussed in detail. X‐ray photoelectron spectroscopy provided the information regarding the presence of the defects in the material. ZnO emission related to defect emission was observed to increase with annealing temperature upto 600°C and then decreased. These defects played a crucial role in the performance of the photo‐electrochemical (PEC) activity. The PEC performance has increased with an increase in the amount of oxygen related defects present in the material due to annealing.

Fabrication and visible emissions of ZnO nanocrystal doped transparent zinc silicate glass-ceramics

ZnO nanocrystals doped transparent glass and glass-ceramics with visible emissions were successfully synthesized through melt-quenching method. Structures and defect-related photoluminescence of quenched glasses and glass-ceramics were significantly dependent on ZnO content in precursor and post heat treatment time. Nano-sized ZnO crystals could generate in quenched glass before heat treatment with a particular precursor composition (44SiO2-11Al2O3-40ZnO-10K2O, mol%), and this quenched glass emitted strong green emission. For glass-ceramics, short time (5 min) heat treatment was conducive to intensive green or yellow emission depending on the excitation energy. Further extending of heat treatment resulted in yellow emission as the dominant one, together with a decline of intensity due to reduced defects. Moreover, the emission peaks shifted towards longer wavelength, from violet, blue, to green and yellow, with an increase of heat treatment time, which can be ascribed to different defect states along with crystal growth during heat treatment. Green light corresponded to oxygen vacancies (Vo), the main defect of ZnO before heat treatment, whereas yellow light was associated with the transition from zinc interstitials (Zni) to oxygen interstitials (Oi) produced with the structure transformation during heat treatment. Yellow emission can emit with both above- and below-bandgap excitation, and preferentially excited by energy (395 nm, ∼3.14 eV) below the bandgap, near Zni level. Green emission was only efficient under the excitation larger than bandgap. Defect-based mechanisms of visible emissions with different initial states were proposed.

Two charge states of the CN acceptor in GaN: Evidence from photoluminescence

We have found a photoluminescence (PL) band with unusual properties in GaN. The blue band, termed as the ${\mathrm{BL}}_{C}$ band, has a maximum at about 2.9 eV and an extremely short lifetime (shorter than 1 ns for a free-electron concentration of about ${10}^{18}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$). The electron- and hole-capture coefficients for this defect-related band are estimated as ${10}^{\ensuremath{-}9}$ and ${10}^{\ensuremath{-}10}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{3}/\mathrm{s}$, respectively. The ${\mathrm{BL}}_{C}$ band is observed only in GaN samples with relatively high concentration of carbon impurity, where the yellow luminescence (the YL1 band) with a maximum at 2.2 eV is the dominant defect-related PL. Both the YL1 and ${\mathrm{BL}}_{C}$ bands likely originate from the ${\mathrm{C}}_{\mathrm{N}}$ defect, namely, from electron transitions via the $\ensuremath{-}/0$ and $0/+$ thermodynamic transition levels of the ${\mathrm{C}}_{\mathrm{N}}$. The ${\mathrm{BL}}_{C}$ band appears only at high excitation intensities in $n$-type GaN samples codoped with Si and C, and it can be found in a wide range of excitation intensities in semi-insulating (presumably $p$-type) GaN samples doped with C. The properties and behavior of the YL1 and ${\mathrm{BL}}_{C}$ bands can be explained using phenomenological models and first-principles calculations.

Effect of growth temperature on the structural and optical properties of few-layer hexagonal boron nitride by molecular beam epitaxy.

We have studied the epitaxy of few-layer hexagonal boron nitride (h-BN) by plasma-assisted molecular beam epitaxy (MBE) using a low growth rate and nitrogen-rich condition. It has been determined that under such conditions, the growth temperature is the factor having the most significant impact on the structural and optical quality of the material. When grown at temperatures <1000 °C, the h-BN film is polycrystalline, and defect-related photoluminescence (PL) emission dominates. Epitaxial domains of exceptional crystalline quality are obtained at elevated substrate temperatures of ~1300 °C, which exhibit strong band-edge PL emission at ~220 nm and negligible defect-related emission at room temperature. Our atomistic calculations reveal that, even though the gap of h-BN is indirect, it luminesces as strongly as direct-gap materials. Experimentally, the luminescence intensity of such a few-layer h-BN sample is measured to be two orders of magnitude stronger than that of a 4-µm thick commercially grown AlN template on sapphire, demonstrating the extraordinary potential of epitaxial h-BN for deep ultraviolet (UV) optoelectronics and quantum photonics.

Luminescence of silicon nanoparticles from oxygen implanted silicon

Oxygen with a kinetic energy of 20 keV is implanted in a silicon wafer (100) at different fluences, followed by post-implantation thermal annealing (PIA) performed at temperatures ranging from 1000 to 1200 °C, in order to form luminescent silicon nanoparticles (SiNPs) and also to reduce the damage induced by the implantation. As a result of this procedure, a surface SiOx layer (with 0 < x < 2) with embedded crystalline Si nanoparticles has been created. The samples yield similar luminescence in terms of peak wavelength, lifetime, and absorption as recorded from SiNPs obtained by the more conventional method of implanting silicon into silicon dioxide. The oxygen implantation profile is characterized by elastic recoil detection (ERD) technique to obtain the excess concentration of Si in a presumed SiO2 environment. The physical structure of the implanted Si wafer is examined by grazing incidence X-ray diffraction (GIXRD). Photoluminescence (PL) techniques, including PL spectroscopy, time-resolved PL (TRPL), and photoluminescence excitation (PLE) spectroscopy are carried out in order to identify the PL origin. The results show that luminescent SiNPs are formed in a Si sample implanted by oxygen with a fluence of 2 × 1017 atoms cm−2 and PIA at 1000 °C. These SiNPs have a broad size range of 6–24 nm, as evaluated from the GIXRD result. Samples implanted at a lower fluence and/or annealed at higher temperature show only weak defect-related PL. With further optimization of the SiNP luminescence, the method may offer a simple route for integration of luminescent Si in mainstream semiconductor fabrication.

Magnesium acceptor in gallium nitride. I. Photoluminescence from Mg-doped GaN

Defect-related photoluminescence (PL) is analyzed in detail for $n$-type, $p$-type, and semi-insulating Mg-doped GaN grown by different techniques. The ultraviolet luminescence (UVL) band is the dominant PL band in conductive $n$-type and $p$-type GaN:Mg samples grown by hydride vapor phase epitaxy (HVPE) and molecular beam epitaxy. The UVL band in undoped and Mg-doped GaN samples is attributed to the shallow $\mathrm{M}{\mathrm{g}}_{\mathrm{Ga}}$ acceptor with the ionization energy of 223 meV. In semi-insulating GaN:Mg samples, very large shifts of the UVL band (up to 0.6 eV) are observed with variation of temperature or excitation intensity. The shifts are attributed to diagonal transitions, likely due to potential fluctuations or near-surface band bending. The blue luminescence $(\mathrm{B}{\mathrm{L}}_{\mathrm{Mg}})$ band is observed only in GaN:Mg samples grown by HVPE or metalorganic chemical vapor deposition when the concentration of Mg exceeds ${10}^{19}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$. The $\mathrm{B}{\mathrm{L}}_{\mathrm{Mg}}$ band is attributed to electron transitions from an unknown deep donor to the shallow $\mathrm{M}{\mathrm{g}}_{\mathrm{Ga}}$ acceptor. Basic properties of the observed PL are explained with a phenomenological model.

Determination of donor and DX center capture characteristics by pulsed photoluminescence

In AlN, several broad defect-related photoluminescence bands in the range from 1.6 eV to 2.4 eV have been observed and were associated with donor or DX– to deep electron trap transitions. By the variation of the length of sub-bandgap laser excitation pulses, we find remarkable changes in the wavelength and intensity of the photoluminescence band at 2.4 eV. Purely based on photoluminescence experiments with variable pulsed excitation, we can determine the capture cross sections for the capture of free electrons into the initial D+ and D0 states and the photon absorption cross section for the excitation of the deep electron trap.

Angstrom-Size Defect Creation and Ionic Transport through Pores in Single-Layer MoS2.

Atomic-defect engineering in thin membranes provides opportunities for ionic and molecular filtration and analysis. While molecular-dynamics (MD) calculations have been used to model conductance through atomic vacancies, corresponding experiments are lacking. We create sub-nanometer vacancies in suspended single-layer molybdenum disulfide (MoS2) via Ga+ ion irradiation, producing membranes containing ∼300 to 1200 pores with average and maximum diameters of ∼0.5 and ∼1 nm, respectively. Vacancies exhibit missing Mo and S atoms, as shown by aberration-corrected scanning transmission electron microscopy (AC-STEM). The longitudinal acoustic band and defect-related photoluminescence were observed in Raman and photoluminescence spectroscopy, respectively. As the irradiation dose is increased, the median vacancy area remains roughly constant, while the number of vacancies (pores) increases. Ionic current versus voltage is nonlinear and conductance is comparable to that of ∼1 nm diameter single MoS2 pores, proving that the smaller pores in the distribution display negligible conductance. Consistently, MD simulations show that pores with diameters <0.6 nm are almost impermeable to ionic flow. Atomic pore structure and geometry, studied by AC-STEM, are critical in the sub-nanometer regime in which the pores are not circular and the diameter is not well-defined. This study lays the foundation for future experiments to probe transport in large distributions of angstrom-size pores.

Defect states and room temperature ferromagnetism in cerium oxide nanopowders prepared by decomposition of Ce-propionate

Four batches of cerium oxide powders (with nanocrystallite size of 6.9 nm–572 nm) were prepared from four precursor nanopowders by thermal decomposition of Ce-propionate and annealing in air between 250 °C–1200 °C for 10 min–240 min. Ceria formation reactions, structure, vibrational, luminescence and magnetic properties were investigated by differential scanning calorimetry, x-ray diffraction, electron microscopy, infrared spectroscopy, photoluminescence and SQUID. All the samples exhibit room temperature ferromagnetism, RTFM, (with coercivity, Hc, of 8 Oe - 121 Oe and saturation magnetization, Ms, of up to 6.7*10−3 emu/g) and a broad defect-related photoluminescence, PL, emission in the visible range. The samples derived from the same precursor show Ms proportional to the peak area of defect-related PL emission whereas this is not valid for the samples derived from the different precursors. An improvement of ferromagnetism and intensity of defect-related PL emission was observed when annealing the products in which nanocrystalline cerium oxide coexists with Ce - oxicarbonate traces, Ce2O2CO3. The experimental results were explained based on the following considerations: room temperature ferromagnetism was induced by the defective ceria with high concentration of oxygen vacancies generated by decomposition of Ce-propionate; oxygen vacancies of the starting precursor nanopowders could be redistributed (at the surfaces/grain boundaries, GBs) upon heating under conditions that promote an inert local environment; the decomposition of Ce2O2CO3 residues can provide an excess of oxygen vacancies at the nanoparticles surfaces or GBs, which can induce or enhance ferromagnetism; surfaces/GBs rather than bulk defects appear responsible for RTFM – this can explain the (often reported in literature) inconsistency between oxygen vacancies concentration and Ms.

Photoluminescence: Synthesis of Highly Luminescent SnO2 Nanocrystals: Analysis of their Defect-Related Photoluminescence Using Polyoxometalates as Quenchers (Adv. Funct. Mater. 4/2018)

Photoluminescence: Synthesis of Highly Luminescent SnO₂ Nanocrystals: Analysis of their Defect-Related Photoluminescence Using Polyoxometalates as Quenchers (Adv. Funct. Mater. 4/2018)

Surface modification effects on defect-related photoluminescence in colloidal CdS quantum dots.

We investigated the effects of surface modification on the defect-related photoluminescence (PL) band in colloidal CdS quantum dots (QDs). A size-selective photoetching process and a surface modification technique with a Cd(OH)2 layer enabled the preparation of size-controlled CdS QDs with high PL efficiency. The Stokes shift of the defect-related PL band before and after the surface modification was ∼1.0 eV and ∼0.63 eV, respectively. This difference in the Stokes shifts suggests that the origin of the defect-related PL band was changed by the surface modification. Analysis by X-ray photoelectron spectroscopy revealed that the surface of the CdS QDs before and after the surface modification was S rich and Cd rich, respectively. These results suggest that Cd-vacancy acceptors and S-vacancy donors affect PL processes in CdS QDs before and after the surface modification, respectively.

Synthesis of Highly Luminescent SnO2 Nanocrystals: Analysis of their Defect‐Related Photoluminescence Using Polyoxometalates as Quenchers

AbstractColloidal semiconductor nanocrystals (NCs), called quantum dots (QDs), have been intensively studied because of their excellent photoluminescence (PL) quantum yields. However, commercial QDs such as CdSe and InP contain toxic or expensive rare elements, limiting their sustainable use. This study focuses on nontoxic, stable, and cheap tin oxides, and synthesized luminescent SnO2 NCs of ≈2 nm in size by a heating‐up method. Tin precursors and diols in a high‐boiling point solvent with oleylamine as the surfactant are heated at 240 °C. SnO2 NCs show defect‐related photoluminescence at 400–460 nm by excitation at 370 nm, achieving a high quantum yield of more than 60%. The PL intensity is stable even when the NCs are stored in atmospheric air at room temperature for over 1 year. The defect‐related emissions of the SnO2 NCs are studied using polyoxometalates (POMs) as the PL quencher. POMs efficiently quench the PL emissions by extracting excited electrons from the conduction band and shallow surface defects. The results reveal that PL emissions from SnO2 NCs are associated with radiative charge recombination via shallow defect levels on the surface and in the bulk, demonstrating the effectiveness of the PL quenching technique using POMs in studying the PL emission mechanism in QDs.

Structural properties and self-activated photoluminescence emissions in hydroxyapatite with distinct particle shapes

The understanding on defect-related photoluminescence (PL) properties of hydroxyapatite (HA) particles has a fundamental importance in the technological field for the development of new non-toxic biomedical and optical devices. However, the mechanisms responsible for this intrinsic PL in HA are not completely elucidated in the literature yet. In the present paper, stoichiometric and calcium-deficient HA nano- and micro-particles were synthesized by chemical precipitation. The influence of structural and morphological features on the intrinsic PL and electronic structure of this material were investigated by varying the addition rate of the phosphate precursor (0.15, 7.00 or 600.00mL/min) and pH (4.5–5.0 or 9.5–10.0) value adopted in the precipitation. The results indicated that the structural order at long- and short-range varied with the synthesis conditions and particle shapes (rods, needles, plates, and rices). The blue and green PL emissions were attributed to defects (bulk, surface and interface) in the samples. These defects promoted the formation of additional energy levels within the band gap, as revealed by using two distinct excitation wavelengths for photoemission measurements. The energies of these wavelengths (~ 3.54 and ~ 2.98eV at 350 and 415nm, respectively) were lower than the band gap energies of HA samples (from 5.59 to 5.72eV). A general model was proposed to explain the occurrence of self-activated PL in HA structure.

Defect-related optical bandgap narrowing and visible photoluminescence of hydrothermal-derived SnO2 nanoparticles

Tin oxide (SnO2) nanoparticles are fabricated by a novel hydrothermal route using H2O2 as oxidizer without adding any corrosive acids or bases. The morphology, composition, and microstructure are characterized by transmission electron microscopy, X-ray diffraction (XRD) and Raman spectra. XRD analysis reveals the formation of single phase rutile type tetragonal structure of all samples which is further supported by Raman studies. The average crystallite size is observed to vary from 2.8 to 4.2 nm as the reacting temperature increases from 120 to 180 °C, suggesting the promotion of crystal growth with increasing temperature. Raman spectroscopy measurement presents the existence of the defect modes, of which the mode at about 565 cm−1 is identified to relate to oxygen vacancies as this mode becomes more prominent in the as-grown sample. Unusual optical band gap narrowing and the excitation wavelength dependent visible luminescence are observed, and both are supposed to be induced by the intrinsic defects. The photoluminescence (PL) spectra of all samples consist of two defect-related subbands, i.e., orange and blue luminescence bands. The two subbands are attributed to optical transitions in oxygen vacancies and some intrinsic surface states, respectively. Spectral analysis suggests that the excitation wavelength dependences of the two subbands are due to a distribution of the band gap of the nanoparticles induced by the internal strain, defect concentration as well as the quantum confinement effect. This work provides a feasible route to modulate the band gap and defect emissions and a good understanding of the PL behavior in the present SnO2 nanoparticles.

Evaluation of the concentration of point defects in GaN

Photoluminescence (PL) was used to estimate the concentration of point defects in GaN. The results are compared with data from positron annihilation spectroscopy (PAS), secondary ion mass spectrometry (SIMS), and deep level transient spectroscopy (DLTS). Defect-related PL intensity in undoped GaN grown by hydride vapor phase epitaxy increases linearly with the concentration of related defects only up to 1016 cm−3. At higher concentrations, the PL intensity associated with individual defects tends to saturate, and accordingly, does not directly correlate with the concentration of defects. For this reason, SIMS analysis, with relatively high detection limits, may not be helpful for classifying unidentified point defects in GaN. Additionally, we highlight challenges in correlating defects identified by PL with those by PAS and DLTS methods.

Defect-related luminescence in InAlAs on InP grown by molecular beam epitaxy

The photoluminescence (PL) of InAlAs grown on InP has been studied in a wide range of temperatures and excitation intensities. A novel emission ascribed to the presence of defects has been found by about 120–180 meV below the near band edge (NBE) line. The novel wide PL band is observed in the spectra only in a limited range of temperatures of 50–160 K, and is seen neither at liquid helium nor at room temperatures. The analysis of the PL behaviour with sample temperature and excitation power together with non-stationary PL kinetics allows us to conclude that both the NBE PL and the novel PL band are controlled by transitions via states of band tails formed due to alloy disorder in these films. The NBE PL is caused by recombination of carriers in the band tails, while deep levels related to defects and located in the same regions as the deepest band tail states are supposedly involved in the defect-related PL transitions. We demonstrate that no defect-related PL is found in the spectra if quasi-stoichiometric growth conditions were used during film growth, which resulted in a PL efficiency by about 1–2 orders of magnitude greater than that of samples grown under more common As-rich conditions.