Emission Peak Energy Research Articles

Intrinsic lithium indium diselenide: Scintillation properties and defect states

A scintillating lithium indium diselenide (LISe) single crystal has recently been found to exhibit an eight-fold increase in its apparent light yield within 80 µm across the surface when excited by cold neutrons. The cause of this observation is currently unknown. In this paper, we report on our investigations to explain the previous observations of nonuniformity in scintillating LISe. Using a two-photon interband excitation instrument, we found that the lower and higher light yield regions “scintillate” at peak emission energies of 1.98 eV and 2.4 eV, respectively, and no region investigated exhibited both. Volumetric evaluation found a relatively uniform scintillation response in each region, verifying that the relatively flat spatial resolution of LISe substrates of varying thickness applied to neutron imaging is not from only near-surface scintillation mechanisms. Raman investigations indicate that the vibrational modes of the scintillating LISe substrate is somewhat different than that reported in literature, but no apparent difference between the two regions was observed. Using ToF-SIMS in each region, we found that the 1.98 eV emission region contains additional lithium, which is consistent with neutron absorption results. We suspect that the sharp transition in peak scintillation emission is due to a change in the charge state of Li-In antisites, but that the observed scintillation light yield and emission energy change is not directly correlated to crystal color. Qualitative comparison between the observed noise in the two-photon “scintillation” spectra indicates that the light yield between each region is not due to the quantum efficiency of the CCD used during cold neutron interrogation experiments, but an actual change in the light yield.

Control growth orientation of semipolar GaN layers grown on 3C-SiC/(001) Si

Heteroepitaxial growth of GaN buffer layers on 3C-SiC/(001) Si substrates (4°-miscut towards [110]) by metalorganic vapour phase epitaxy has been investigated. High-temperature grown AlxGa1-xN/AlN interlayers were employed to control GaN surface orientations. Semipolar GaN layers with (101¯1), (202¯3) and (101¯2) surface orientations were achieved, as confirmed by X-ray diffraction. Due to the substrate miscut, the growth of (101¯1) layers was twinned along [11¯0]3C-SiC/Si and [1¯10]3C-SiC/Si while the growth of (202¯3) and (101¯2) layers was only along [110]3C-SiC/Si. The (101¯1) layers have rough surface morphology while the (202¯3) and (101¯2) layers have mirror-like smooth surface. For all samples with various surface orientations, different photoluminescence peak emission energies were observed at ∼3.45 eV, 3.78 eV and 3.27 eV at 10 K. These emissions are attributed to the near-band edge of hexagonal GaN, basal-plane stacking faults and partial dislocations, respectively. The dominant luminescence intensity of stacking faults indicates their high density in the GaN layers.

Self-trapped holes in BaTiO3

The behavior of holes in the valence band of BaTiO3 is investigated using hybrid density-functional calculations. We find that holes tend to self-trap, localizing on individual O atoms and causing local lattice distortions, forming small hole-polarons. This takes place even in the absence of intrinsic defects or impurities. The self-trapped hole (STH) is more energetically favorable than the delocalized hole in the valence band. The calculated emission peak energy corresponding to the recombination of a conduction band electron with a STH can explain the observed photoluminescence at low temperatures. The stability of the STH, its migration barrier, and the related emission peak are then compared to those of SrTiO3.

Luminescence properties of layered mixed-anion compounds Sr2ScCuSeO3 and Sr3Sc2Cu2Se2O5

We report the synthesis and luminescence properties of new layered mixed-anion compounds, Sr2ScCuSeO3 and Sr3Sc2Cu2Se2O5. Sr2ScCuSeO3 and Sr3Sc2Cu2Se2O5 were synthesized by solid-state reaction and were investigated using x-ray diffraction, diffuse reflectance, and photoluminescence spectroscopies. Both compounds have very similar properties except for their optical band gaps and emission peak energies. At room temperature, Sr2ScCuSeO3 and Sr3Sc2Cu2Se2O5 exhibit optical band gaps energies of 2.9–3.0 eV. The excitonic luminescence with very fast emissions of ∼30 ps were observed in both compounds even at room temperature. This feature of the compounds makes them suitable for fast scintillator applications. Our results suggest that the luminescence properties such as emission peak energies and lifetimes of these layered mixed-anion compounds can be controlled by changing their structure and composition.

Microscopic View of Defect Evolution in Thermal Treated AlGaInAs Quantum Well Revealed by Spatially Resolved Cathodoluminescence

An aluminum gallium indium arsenic (AlGaInAs) material system is indispensable as the active layer of diode lasers emitting at 1310 or 1550 nm, which are used in optical fiber communications. However, the course of the high-temperature instability of a quantum well structure, which is closely related to the diffusion of indium atoms, is still not clear due to the system’s complexity. The diffusion process of indium atoms was simulated by thermal treatment, and the changes in the optical and structural properties of an AlGaInAs quantum well are investigated in this paper. Compressive strained Al0.07Ga0.22In0.71As quantum wells were treated at 170 °C with different heat durations. A significant decrement of photoluminescence decay time was observed on the quantum well of a sample that was annealed after 4 h. The microscopic cathodoluminescent (CL) spectra of these quantum wells were measured by scanning electron microscope-cathodoluminescence (SEM-CL). The thermal treatment effect on quantum wells was characterized via CL emission peak wavelength and energy density distribution, which were obtained by spatially resolved cathodoluminescence. The defect area was clearly observed in the Al0.07Ga0.22In0.71As quantum wells layer after thermal treatment. CL emissions from the defect core have higher emission energy than those from the defect-free regions. The defect core distribution, which was associated with indium segregation gradient distribution, showed asymmetric character.

Photoluminescence Spectra From The Direct Energy Gap of a-SiQDs

A theoretical model for radiative recombination in amorphous silicon quantum dots (a-SiQDs) was developed. In this model, for the first time, the coexistence of both spatial and quantum confinements were considered. Also, it is found that the photoluminescence exhibits significant size dependence in the range (1-4) nm of the quantum dots. a-SiQDs show visible light emission peak energies and high radiative quantum efficiency at room temperature,in contrast to bulk a-Si structures. The quantum efficiency is sensitive to any change in defect density (the volume nonradiative centers density and/or the surface nonradiative centers density) but, with small dots sizes, the quantum efficiency is insensitive to such defects. Our analysis shows that the photoluminescence intensity increases or decreases by the effect of radiative quantum efficiency. By controlling the size of a-SiQDs, we note that the energy of emission can be tuned. The blue shift is attributed to quantum confinement effect. Meanwhile, the spatial confinement effect is clearly observed in red shift in emission spectra. we found a good agreement with the experimental published data. Therefore, we assert that a-SiQDs material is a promising candidate for visible, tunable, and high performance devices of light emitting.

Preparation of Mg1+yAl2-xO4:xMn4+, yMg2+ deep red phosphor and their optical properties

Exploration of efficient deep red phosphor based on non-rare-earth ion activated oxide is of great practical value in the field of phosphors converted white light-emitting diode lighting. A spinel Mg1+yAl2-xO4:xMn4+, yMg2+ phosphor with deep red emission is synthesized by a solid-state reaction route. The crystal structure and morphology are characterized by powder X-ray diffraction and scanning electron microscopy. The luminescent performance is characterized by fluorescence spectrophotometer and fluorescence decay curves. The results demonstrate that the synthesized phosphor shows that two excited spectrum bands centered at 290 nm and 438 nm cover a broad spectral region from 220 nm to 500 nm due to the Mn4+-O2- charge transfer band and the 4A2-4T1 and 4T2 transitions of Mn4+ ions. Upon excitation at 300 nm, a strong, narrow red emission band is observed between 600 and 700 nm peaked at 652 nm as a result of the spin-forbidden 2Eg-4A2g electron transition of Mn4+. The corresponding chromaticity coordinate is (0.7256, 0.2854). Additionally, the concentration quenching of Mn4+ in the MgAl2O4 host is evaluated in detail, which indicates that the optimum doping concentration of Mn4+ is experimentally determined to be 0.14 mol%. The critical distance is calculated to be 52.15 according to the Blasse equation, which elucidates that the concentration quenching mechanism is consequently very likely to be induced by the multipole-multipole interaction. The crystal field strength (Dq) and the Racah parameters (B and C) are estimated to evaluate the nephelauxetic effect of Mn4+ suffered in MgAl2O4:Mn4+ host lattice. Luminous mechanism is explained by Tanabe-Sugano energy level diagram of Mn4+ ion. The ratio of Dq/B equals 1.74, indicating that Mn4+ ions experience a weak crystal field in the MgAl2O4 host and emission peak energy of 2Eg-4A2g transition is dependent on the nephelauxetic effect. The red emission intensity of Mg1+yAl2-xO4:xMn4+, yMg2+ increases on account of excess Mg2+ which would compensate for the local charge balance surrounding Mn4+ ions, furthermore, lead the Mn4+-Mn4+ pairs connected with interstitial O2- to transform into isolated Mn4+ ions, and thus eliminating energy transfer and enhancing the luminescence efficiency effectively. The decay times of two time-dependent curves of Mg1+yAl2-xO4:xMn4+,yMg2+ are 0.672 ms and 0.604 ms, and each entire decay curve could be well-fitted to single-exponential, confirming that there is only a single Mn4+ ion luminescence center. The decay time of Mn4+ luminescence is prolonged with the increase of Mg2+ content, indicating that excitation energy transfer and non-radiative relaxation between Mn4+-Mn4+ pairs decrease, the reason is that photoexcitation energy can be temporarily stored in the trapping centers induced by excess positive charges. These results imply that Mn4+ doped Mg1+yAl2 -xO4:xMn4+, yMg2+ is a promising candidate of deep-red phosphors for near-UV and blue light emitting diodes. These findings in the paper would be beneficial not only to developing a low-cost and safe strategy to produce high-efficient Mn4+ activated luminescent materials for white light emitting diodes, but also to providing a new insight into improving the photoluminescence properties of Mn4+.

Size-Dependent Photoluminescence Efficiency of Silicon Nanocrystal Quantum Dots

Photoluminescence (PL) spectra were measured for dodecene-capped Si nanocrystals with a wide range of average diameters, from 1.8 to 9.1 nm. Nanocrystals larger than 3 nm exhibited relatively high PL quantum yields of 30%–45%. Smaller nanocrystals exhibited lower quantum yields that decreased significantly with reduced size. Because smaller nanocrystals also have lower optical absorption there is a significant biasing of the PL spectra by the larger nanocrystals. We show that with proper accounting of polydispersity and size-dependent quantum yields and optical absorption the effective mass approximation (EMA) accurately estimates the average diameter of silicon (Si) nanocrystals from experimentally determined PL emission peak energies. A finite confinement model is presented that explains the decreased PL quantum yields of the smaller diameter nanocrystals.

Non uniform distribution of indium in the InxGa1−xN/GaN quantum well is more advantageous for quantum well light emitting diodes

We investigate the impact of non uniform distribution of indium (In) in the InxGa1-xN/GaN quantum well (QW) light emitting diodes as compared to the uniform distribution of In. Computations are based on the self-consistence solutions of Schrödinger and Poisson's equations. The transition probabilities have been studied for various In distributions in the QW. A very interesting and striking information emerges through the studies, that the overlap of electron and hole wave functions increase in the non uniformly distributed In leading to higher luminosity. With the consideration of multistep staggered QWs, the overlap of electron and hole wave functions increased more significantly. The results of the overlap and peak emission energies for the uniform and non uniform distribution of In in the QW are presented with suitable discussions.Results show that the QW with uniform In distribution is not the best for InxGa1-xN/GaN QW LEDs.

Evidence of quantum dot size uniformity in strain-coupled multilayered In(Ga)As/GaAs QDs grown with constant overgrowth percentage

Strain coupled multi-layer quantum dot (QD) structures are limited due to their non-uniform dot size distribution, as-grown defects and dislocations. Even with these limitations, they are subject of interest because of high optical and device efficiency. In this work, we propose a modified growth strategy in which monolayer (ML) deposition of QDs is varied based on reflection high energy electron diffraction (RHEED) pattern to achieve defect free structure with uniform dot size distribution. The overgrowth percentage, i.e. monolayer deposition above critical thickness (minimum thickness required for transition from two-dimensional (2D) to three-dimensional (3D) confined structures) was kept constant in modified strategy. To verify this claim, we have compared it with conventional strain coupled multilayer structure and demonstrated improved optical performance and device characteristics. Photoluminescence spectroscopy exhibited monomodal QD distribution in strain-coupled multi-stacked heterostructure grown by the proposed strategy. Power dependent photoluminescence (PL) measurement confirms existence of single ground state peak. The proposed strategy yielded a defect free structure and better carrier capture rate compared to conventional multilayer QDs. The spectral response observed from proposed quantum dot infrared photodetector (QDIP) exhibited multiple peaks in mid-IR region (3.92µm and 4.54µm), which is one of the essential requirements for fabricating infrared detectors or focal plane arrays. Post-growth thermal annealing treatment at various temperatures (650–800°C) were performed to verify the change in activation energy, ground-state emission peak, and thermal stability. Negligible change in the ground-state PL emission peak (< 2%) and thermal activation energy (< 2%) were observed up to 750°C. The full width at half maximum (FWHM) values of the ground-state peak of the annealed samples exhibited a similar trend. High-temperature thermal stability was achieved probably due to high strain field hindering material interdiffusion and improved quantum confinement realized from uniform QD sizes. The modified growth strategy used in this paper is helpful not only for improved optical characteristics, but also for useful device performances, such as a night vision camera in defense areas, cancer detector in medical applications and fabrication of thermal imagers; focal plane arrays.

Tunable white light emitting Sr2V2O7:Bi3 + phosphors: Role of bismuth ion

Phase purity, optical properties and defects in new near white light emitting rare earth free Sr2V2O7:Bi3+phosphors were investigated. Emission spectrum of Sr2V2O7 displayed two peaks at all excitation wavelength approximately around 450 (P1) and 600nm (P2). With increase in the excitation wavelength; emission intensity increases and there is a red and blue shift in Peak P1 and P2, respectively. On bismuth doping, the two Peaks P1 and P2 slowly starts merging and become a single broad peak at 5% doping. Bismuth doping also reduces the emission peak energy. This is attributed to substitution of Bi3+ at Sr2+ site which would distort VO4 tetrahedra, thereby reducing the energy difference between the 3T and 1A levels of VO43− distorted tetrahedral. This dual role of bismuth doping and excitation energy leads to interesting tunability in white emission from cool to warm white. This opens a new arena in phosphor research because of great role of white LEDs (both warm and cool) in outdoor and indoor lighting.

Effect of small flow hydrogen treatment at the upper well/barrier interface on the properties of InGaN/GaN multiple quantum wells

To enhance the quality of InGaN/GaN multiple quantum wells (MQWs), a small hydrogen flow is introduced to treat the upper well/barrier interface. High-resolution X-ray diffraction results indicate that hydrogen treatment improves the interface quality of MQWs. Room temperature photoluminescence (PL) tests show that integrated PL intensity is enhanced by 57.6% and line width is narrowed, while emission peak energy is almost unchanged. On the basis of temperature-dependent PL characteristics analysis, it is concluded that hydrogen treatment decreases non-radiative recombination centers in active region, yet has little impact on carrier localization. Moreover, surface roughness and V-pit density are significantly reduced after hydrogen treatment as revealed by atomic force microscopy. Because the emission energy is quite stable after the hydrogen treatment, this method can also be promoted to improve the quality of green and yellow-green emission MQWs.

Substrate-Mediated Cooperative Adsorption of Sodium Cholate on (6,5) Single-Wall Carbon Nanotubes.

The interaction of sodium cholate (NaC) with (6,5) single-wall carbon nanotubes (SWNTs) is investigated using photoluminescence spectroscopy. Dilution of SWNT-NaC suspensions is accompanied by changes in the exciton PL quantum yield and peak emission energy. An abrupt change of the exciton emission peak energy at NaC concentrations between 10 and 14 mM indicates strongly cooperative formation of a micellar phase on (6,5) SWNT surfaces with a Hill coefficient of nH = 65 ± 6. This is in contrast to the formation of free NaC micelles with aggregation numbers of only about 4 and suggests that the cooperativity of NaC micelle formation on nanotube surfaces is strongly substrate-enhanced. The temperature dependence of this previously unobserved transition is used for a determination of ΔmicG(⊖)/(1 + β) = -(11.4 ± 0.2) kJ·mol(-1) which, for typical Na(+) counterion binding with β ≈ 0.2, yields a free SWNT-NaC micellization enthalpy ΔmicG(⊖) of -13.7 kJ·mol(-1).

Size dependent photoluminescence properties of CdZnS nanostructures

Photoluminescence response of cadmium zinc sulfide (CdZnS) nanostructures has been studied through low-temperature and time-resolved photoluminescence measurements. The as synthesize nanostructures are single crystalline having growth along [102] direction. The CdZnS nanostructures exhibited near band edge and deep level emission bands at 2.485eV and 2.09eV, displaying bi-exponential excitonic decay with an average lifetime of 2.56ns and 4.11ns, respectively. Furthermore, single CdZnS nanostructures have been investigated for their size dependent photoluminescence properties. For isolated nanostructures, emission band systematically shifted towards the higher-energy, and defect-related low energy emission peak disappeared with a decrease in nanostructure width. Results revealed that emission originated from the radiative recombination process of the donor and acceptor bound excitons. Detrimental effect of low temperature Photoluminescence measurements have been observed in individual nanostructures.

Time-resolved analysis of the white photoluminescence from chemically synthesized SiCxOy thin films and nanowires

The study reported herein presents results on the room-temperature photoluminescence (PL) dynamics of chemically synthesized SiCxOy≤1.6 (0.19 &amp;lt; x &amp;lt; 0.6) thin films and corresponding nanowire (NW) arrays. The PL decay transients of the SiCxOy films/NWs are characterized by fast luminescence decay lifetimes that span in the range of 350–950 ps, as determined from their deconvoluted PL decay spectra and their stretched-exponential recombination behavior. Complementary steady-state PL emission peak position studies for SiCxOy thin films with varying C content showed similar characteristics pertaining to the variation of their emission peak position with respect to the excitation photon energy. A nearly monotonic increase in the PL energy emission peak, before reaching an energy plateau, was observed with increasing excitation energy. This behavior suggests that band-tail states, related to C-Si/Si-O-C bonding, play a prominent role in the recombination of photo-generated carriers in SiCxOy. Furthermore, the PL lifetime behavior of the SiCxOy thin films and their NWs was analyzed with respect to their luminescence emission energy. An emission-energy-dependent lifetime was observed, as a result of the modulation of their band-tail states statistics with varying C content and with the reduced dimensionality of the NWs.

ZnO thin films prepared by atomic layer deposition at various temperatures from 100 to 180 °C with three-pulsed precursors in every growth cycle

Zinc oxide (ZnO) thin films are grown at various temperatures from 100 to 180 °C by atomic layer deposition (ALD) with three-pulsed precursors per ALD cycle on sapphire (0001) substrates. In the texture analyses of x-ray diffraction (XRD), hexagonal (002) and (101) crystalline orientations are shown. The dominant diffraction is (002) above 140 °C and (101) at and below 140 °C. Hall effect measurements demonstrate remarkable decrease of free electron concentration below 140 °C and the lowest value is 2.4 × 1016 cm−3 at 100 °C. Photoluminescence (PL) spectra of samples display no defect-related luminescence band. The emission peak energy of PL spectra show blueshift with the increase of temperature from 100 to 140 °C, which is originated from band-filling (Burstein–Moss) effect. However, redshift occurred at temperature above 140 °C owing to bandgap renormalization (shrinkage) with the increase of free electron concentration. Enhancement of PL peak intensity and reduction of free electron concentration below 140 °C result from the large decline of intrinsic donor-like defects. 140 °C is a critical temperature for thermal activation of donors-like defects and alteration of dominant crystalline orientation as well as the electrical and optical behaviors in ZnO. The growth method also constitutes smooth surface of ZnO when it comes to the analysis of surface roughness of samples from topographic image forming by atomic force microscope (AFM).

Electrical Characterization of High Energy Electron Irradiated Ni/4H-SiC Schottky Barrier Diodes

The effect of high energy electron irradiation on Ni/4H-SiC Schottky barrier diodes was evaluated by current–voltage (I–V) and capacitance–voltage (C–V) measurements at room temperature. Electron irradiation was achieved by using a radioactive strontium source with peak emission energy of 2.3 MeV. Irradiation was performed in fluence steps of 4.9 × 1013 cm−2 until a total fluence of 5.4 × 1014 cm−2 was reached. The Schottky barrier height determined from I–V measurements was not significantly changed by irradiation while that obtained from C–V measurements increased with irradiation. The ideality factor was obtained before irradiation as 1.05 and this value did not significantly change as a result of irradiation. The series resistance increased from 47 Ω before irradiation to 74 Ω after a total electron fluence of 5.4 × 1014 cm−2. The net donor concentration decreased with increasing irradiation fluence from 4.6 × 1014 cm−3 to 3.0 × 1014 cm−3 from which the carrier removal rate was calculated to be 0.37 cm−1.

Room temperature PL efficiency of InGaN/GaN quantum well structures with prelayers as a function of number of quantum wells

AbstractWe report on the effects of varying the number of quantum wells (QWs) in an InGaN/GaN multiple QW (MQW) structure containing a 23 nm thick In0.05Ga0.95N prelayer doped with Si. The calculated conduction and valence bands for the structures show an increasing total electric field across the QWs with increasing number of QWs. This is due to the reduced strength of the surface polarisation field, which opposes the built‐in field across the QWs, as its range is increased over thicker samples. Low temperature photoluminescence (PL) measurements show a red shifted QW emission peak energy, which is attributed to the enhanced quantum confined Stark effect with increasing total field strength across the QWs. Low temperature PL time decay measurements and room temperature internal quantum efficiency (IQE) measurements show decreasing radiative recombination rates and decreasing IQE, respectively, with increasing number of QWs. These are attributed to the increased spatial separation of the electron and hole wavefunctions, consistent with the calculated band profiles. It is also shown that, for samples with fewer QWs, the reduction of the total field across the QWs makes the radiative recombination rate sufficiently fast that it is competitive with the efficiency losses associated with the thermal escape of carriers. (© 2016 The Authors. Phys. Status Solidi C published by WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Evaluation of field emission properties from multiple-stacked Si quantum dots

Multiple-stacked Si quantum dots (QDs) with ultrathin SiO2 interlayers were formed on ultrathin SiO2 layers by repeating a process sequence consisting of the formation of Si-QDs by low pressure chemical vapor deposition using a SiH4 gas and the surface oxidation and subsequent surface modification by remote hydrogen and oxygen plasmas, respectively. To clarify the electron emission mechanism from multiple-stacked Si-QDs covered with an ultrathin Au top electrode, the energy distribution of the emitted electrons and its electric field dependence was measured using a hemispherical electron energy analyzer in an X-ray photoelectron spectroscopy system under DC bias application to the multiple-stacked Si-QD structure. At −6V and over, the energy distributions reached a peak at ~2.5eV with a tail toward the higher energy side. While the electron emission intensity was increased exponentially with an increase in the applied DC bias, there was no significant increase in the emission peak energy. The observed emission characteristics can be interpreted in terms of field emissions from the second and/or third topmost Si-QDs resulting from the electric concentration there.

Spontaneous lateral phase separation of AlInP during thin film growth and its effect on luminescence

The occurrence of spontaneous lateral phase separation during thin film growth of AlxIn1−xP by metal-organic chemical vapor deposition was investigated using a combination of transmission electron microscopy and atom probe tomography to obtain a quantitative view of this phenomenon. An anisotropic and coherent composition modulation was observed in the nearly lattice-matched films deposited below 750 °C with a quasi-linear amplification with thickness that was inversely proportional to the growth temperature. The periodicity of the modulation increased exponentially with the growth temperature. A comparison of photoluminescence from phase separated and homogenous direct band gap AlxIn1−xP deposited on metamorphic InyGa1−yAs graded buffers showed a lowering of peak-emission energy in accordance with the atom probe compositional characterization without any degradation in luminous intensity. Additionally, indications of carrier trapping in the low band gap regions were observed even at room-temperature. While some of these results are in qualitative agreement with theoretical models of kinetic instability in unstrained alloy growth in the literature, significant discrepancies remain.