Blue Luminescence Intensity Research Articles

Vertical GaN PIN Structure with Intrinsic AlGaN Drift Layer Grown Using Metal‐Organic Chemical Vapor Deposition

In this study, it is aimed to examine the DC characteristics of a vertical PIN diode featuring AlGaN as the drift layers. In the investigation, observing the changes in DC characteristics with the variation of Al content (0%–4%) in the drift layers as well as its thickness is focused on. By applying AlGaN as the drift layer, the semi‐insulating properties are augmented and the detrimental background carbon impurity is reduced, leading to a reduction in the leakage current and an improvement in the breakdown voltage characteristics of the device. Room‐temperature and low‐temperature photoluminescence measurements confirm a reduction in the intensity of both yellow luminescence and blue luminescence, which can be attributed to the decrease in defect concentration. Furthermore, to minimize the occurrence of premature breakdown voltage, p‐type GaN is meticulously etched into a bevel shape, and subsequent confirmation through the utilization of a scanning electron microscope reveals an inclination of 13°. The outcome of the analysis on the DC characteristic indicates an improvement in reverse‐bias characteristics with the increase in the Al ratio in the drift layer for the 2 μm series. Furthermore, for the 20 μm Al0.04Ga0.96N, a breakdown voltage of 2540 V is recorded.

Electrofluorochromism based on the valence change of europium complexes in electrochemical devices with Prussian blue as the counter electrode.

The electrofluorochromism of Eu complexes based on the valence change between Eu3+ and Eu2+ is demonstrated in a two-electrode electrochemical device consisting of Prussian blue (PB) as the counter electrode. This study aims to improve the electrofluorochromic (EFC) performance of luminescence switching between Eu3+ and Eu2+ by enhancing the electrochemical reactivity of the EFC device. By introducing a PB film as a counter electrode in a two-electrode device, the redox reaction of Eu3+/2+ is promoted because of charge compensation by the counter PB film. The increase in the reaction charge enables faster changes in the photoluminescence from Eu3+ to Eu2+ and an increase in the blue luminescence intensity from the Eu2+ state. This approach achieves a lowered driving voltage, accelerates the electrochemical redox reaction of the Eu complex, and enhances the reversibility of the valence change of the Eu ion.

Quantitative analysis of carbon impurity concentrations in GaN epilayers by cathodoluminescence

In this work, a technique for quantifying carbon doping concentrations in GaN:C/AlGaN buffer structures using cathodoluminescence (CL) is presented. The method stems from the knowledge that the blue and yellow luminescence intensity in CL spectra of GaN varies with the carbon doping concentration. By calculating the blue and yellow luminescence peak intensities normalised to the peak GaN near-band-edge intensity for GaN layers of known carbon concentrations, calibration curves that show the change in normalised blue and yellow luminescence intensity with carbon concentration in the 1016 − 1019 cm−3 range were derived at both room temperature and 10 K. The utility of such calibration curves was then examined by testing against an unknown sample containing multiple carbon-doped GaN layers. The results obtained from CL using the normalised blue luminescence calibration curves are in close agreement with those from secondary-ion mass spectroscopy (SIMS). However,the method fails when applying calibration curves obtained from the normalised yellow luminescence likely due to the influence of native VGa defects acting in this luminescence region. Although this work shows that indeed CL can be used as a quantitative tool to measure carbon doping concentrations in GaN:C, it is noted that the intrinsic broadening effects innate to CL can make it difficult to differentiate between the intensity variations in thin ( < 500 nm) multilayered GaN:C structures such as the ones studied in this work.

Spectral studies of Dy3+:zincphosphate glasses for white light source emission applications: A comparative study

Recently, much efforts have been focused on Dy3+-doped glasses due to their optical and luminescent properties for several applications such as display devices, white light emitting diodes, waveguides for optical telecommunication, etc. In the present study, the Dy3+-doped zinc phosphate (Dy3+:PZKAN (P2O5+ZnO+K2O+Al2O3+Na2O+Dy2O3)) glasses have been prepared by melt quenching technique and characterized through absorption (optical band gap, Urbach energies and Judd-Ofelt (JO) theory), photoluminescence (yellow (Y) to blue (B) luminescence intensity ratios (Y/B) and CIE color coordinates), decay time and quantum yield measurements. With the help of optical absorption, transition intensities and the well proposed JO theory, three phenomenological JO fitted parameters were estimated and in turn adopted to evaluate the required radiative parameters for specific excited luminescent levels of Dy3+ ion. The luminescence transitions exhibit de-excitation (refers radiative-emission) levels in the optical region of 450-750 nm. Also while increasing Dy3+ ion content, the de-excitation intensities were quenched due to presence of higher content of Dy3+ ions. The fluorescence decay curve for all the glass samples have been recorded with 350 nm pumping and observed the de-excitation at 575 nm that belongs to the 4F9/2 → 6H13/2 level. The CIE color coordinates were estimated with the help of CIE chromaticity diagram and the quantities were in close proximity of the optimised white light (towards yellow) region for all the glasses. The lifetime (952 μs to 358 μs) and quantum yield (67 to 11) values decreases with increasing concentration (Dy3+:0.05 mol % to 3.0 mol %). The present results have been compared and discussed with those of 72 reported Dy3+:glasses besides uniform labeling of glass compositions. From these results, it is concluded that the Dy3+:PZKAN glasses could also be useful for optical display devices and yellow laser applications.

Blue Persistent Phosphor of YSiO2N:Ce3+ Developed by Codoping Sm3+ or Tm3+ Ions and Thermoluminescence Analysis of Their Trap Distributions

By estimating the trap depth from the relative position of the zigzag curve of divalent lanthanoid ions and conduction band bottom, the YSiO2N:Ce3+‐based blue persistent phosphors are developed, and their persistent luminescence and thermoluminescence properties are characterized to identify the electron trap distributions. Taking the nephelauxetic effect caused by covalent nitrogen into account for the vacuum referred binding energy diagram, it is predicted that Tm3+ and Sm3+ ions with the trap depth of 0.726 and 1.04 eV are an optimal codopant to obtain Ce3+ persistent luminescence. The prepared YSiO2N:Ce3+‐Ln3+ (Ln = Sm or Tm) samples enhance the Ce3+: 5d → 4f blue persistent luminescence intensity. In the thermoluminescence glow curves, the Ce3+ singly doped sample shows two glow peaks related to intrinsic defects; on the other hand, the Ln3+ codoped samples show an additional glow peak related to the Ln3+ traps. The thermoluminescence glow curve analyses through the initial rise method combined with the thermal cleaning and trap density methods reveal that the electron traps of Ln3+‐related defects have a distribution with ≈0.25 eV width and depths of 0.802 eV (Tm3+) and 1.10 eV (Sm3+).

Modulating the blue and green luminescence in the β-Ga2O3 films

In this work, the β-Ga2O3 films were fabricated by radio frequency magnetron sputtering on the c-plane sapphire substrates with the (2¯01) preferred orientation. The blue and green luminescence of β-Ga2O3 films have been systemically studied. By changing the concentration of intrinsic vacancies in β-Ga2O3 films through increasing oxygen flow rate during sputtering, the intensity of blue and green luminescence were modulated. The maximum value of the intensity ratio for green luminescence to blue luminescence reached to 433.37% when the oxygen flow rate arrived at 6 sccm. Combining experiments with density functional theory calculations, this work had proved the green luminescence in β-Ga2O3 mainly originated from the isolated gallium vacancies. While the isolated oxygen vacancies could not play the important roles in the blue emission independently. The blue luminescence most likely resulted from complex defects such as (VOI+VGaI)− or the optical transitions between VO and (VO, VGa). Thus, changing the concentration of intrinsic vacancies is highly effective for β-Ga2O3 to modulate the intensity of green luminescence and blue luminescence.

Cathodoluminescence of Diamond: Features of Visualization

Zonal and sectorial heterogeneities in natural diamonds provide information on the growth conditions and post-growth changes. Cathodoluminescence (CL) microscopy revealed these heterogeneities in a very detailed manner with high spatial resolution. In this study, factors affecting the CL images of two natural diamonds were analyzed and the results of cathodoluminescence studies in steady-state (SS-CL) and scanning modes were compared. SS-CL was observed using an optical microscope, and scanning mode was evaluated via SEM (SEM-CL). It was demonstrated that the relative brightness of the &lt;111&gt; and &lt;100&gt; growth sectors in diamond crystals depends on the nature of defects in them and on the method of image detection (steady-state/scanning versus color/panchromatic). The differences between SS-CL and SEM-CL images can be attributed to the kinetics of luminescence and spectral sensitivity of the detectors. It was established that the nature of lattice defects around small inclusions can be changed (e.g., the intensity of blue luminescence from nitrogen-vacancy defects (N3V) decreases due to their transformation into nitrogen–hydrogen defects (N3VH). The hydrogen disproportion between the sectors is caused by different growth mechanisms. Hydrogen atoms in the diamond matrix can affect the kinetics of transformation of the defects by transforming a part of N3V to N3VH.

Investigation of the passivation-induced VTH shift in p-GaN HEMTs with Au-free gate-first process

In this work, we observe the distinct VTH characteristics in the Au-free gate-first processing p-GaN/AlGaN/GaN HEMTs with two commonly used passivation layers, i.e., SiN and SiO2. The lower incorporated H was found in the p-GaN/AlGaN/GaN heterostructure with higher activation anneal temperature (i.e., 700 °C). Furthermore, Photoluminescence (PL) spectrum demonstrates a higher blue luminescence (BL) intensity after higher annealing treatment. The X-ray photoelectron spectroscopy (XPS) spectrum near valence band edge depicts a similar valence band maximum (VBM) characteristic, by means no impact on p-GaN surface bending by using distinct thermal treatment. The device with SiN shows a depletion-mode (D-mode) characteristic (VTH ~ -5 V) whereas the device with SiO2 passivation exhibits an enhancement-mode (E-mode) characteristic (VTH ~ +0.7 V). Moreover, Transmission Line Model (TLM) devices are fabricated to investigate the effects of the passivation on two-dimensional electron gas (2DEG) in p-GaN/AlGaN/GaN stack. The results indicate that a low Rsh is obtained while passivating device surface with SiN layer, suggesting that 2DEG is present, which is most probably due to an unfunctional p-GaN layer. The secondary ion mass spectrometry (SIMS) results indicate a high hydrogen intensity in the p-GaN/AlGaN/GaN stack with a SiN passivation layer. Thus, the p-GaN deactivation process that correlates to the formation of complex MgH after SiN passivation is proposed to explain the D-mode characteristic in the device with a SiN passivation layer.

Investigation of Thermal Effect on Reliability and Chromaticity Shift in QD-LEDs Displays

In this work, thermal effect on reliability and chromaticity shift in quantum dots (QDs)-based light emitting diodes (LEDs) are investigated by long term lifetime test and IR thermal image. The results have suggested that the heat power generation during luminescence conversion process play a key role in the compatibility between QD and encapsulation resin, which is correlated to the blue luminescence intensity generated from LED chip and the QD luminescence intensity converted by QDs. The ligands detachment of QD at luminescence conversion process results in poor reliability of QDs and influence on the thermal load ability of encapsulation resin, whereas the condition of QD should be a dominant factor. It was also found that QD with thicker shell could prevented heat power generation since higher conversion efficiency and better stability, shown the better reliability of QD-LEDs and less chromaticity shift.

Effect of temperature on up-conversion phenomena in Gd3(Al,Ga)5O12 crystals co-doped with Yb3+ and Tm3+

Single crystals of Gd3(Al,Ga)5O12 with intentional stoichiometry Gd3Ga2.5Al2.5O12 doped with 1%Tm3++x%Yb3+; (x = 1, 3, 5); were grown by the Czochralski method. Spectra and intensities of up-converted luminescence excited at 975 nm or 803 nm were measured as a function of temperature within 295 K–775 K region. It was found that for the two excitation wavelengths the growth of temperature brings about a decrease of an up-converted blue luminescence intensity. Thermally enhanced growth of nonradiative relaxation rates for excited states involved in the up-conversion process was determined based on an analysis of respective luminescence decay curves. Temperature-dependent change of fluorescence intensity ratio (FIR) for the 3F3,2 – 3H6/3H4 – 3H6 up-converted bands was observed and resulting absolute temperature sensitivity of 0.058 K-1 was found for GAGG:1%Tm,3%Yb at 725 K.

The self-compensation effect of heavily Mg doped p-GaN films studied by SIMS and photoluminescence

Abstract The electrical resistivity and compensation mechanism in heavily Mg doped p-GaN grown by metal organic chemical vapor deposition (MOCVD) are investigated. It is noted that when the Mg doping concentration is too high, the hole concentration measured by Hall test of p-GaN may decrease compared to the p-GaN samples with lower Mg doping concentration. This phenomenon suggests a possible self-compensation of Mg impurity in p-GaN. It is found that Mg concentration tested by secondary ion mass spectroscopy (SIMS) does not increase linearly when the Cp2Mg/TMGa flow rate ratio increases, and H concentration also varies with increasing Cp2Mg flow rate. The change of hole concentration may be attributed to the decrease of active acceptor (MgGa) concentration related to the self-compensation effect and the reduction of Mg incorporation rate. Actually, the integral intensity of blue luminescence (BL) caused by donor-acceptor pair (DAP) recombination in room temperature photoluminescence (PL) spectra is strengthened with increasing Cp2Mg flow rate at first, and then the BL intensity decreases when the Cp2Mg flow rate is over 100 sccm. The change of BL intensity is consistent with the conclusion that the acceptor concentration may decrease at a high Mg doping level.

Modeling and interpretation of UV and blue luminescence intensity in β-Ga2O3 by silicon and nitrogen doping

Temperature-dependent cathodoluminescence spectra of (2¯01) Si-doped β-Ga2O3 single crystals and (010) N-doped epitaxial films were comprehensively shown to investigate their electronic structure and defect states. The decrease in the self-trapped exciton (STE) emission at low temperatures in heavily Si-doped crystals implied the Debye-Hückel screening of STEs with the critical charge density larger than 2 × 1018 cm−3. The analysis based on the rate equation model suggested a significant influence of the donor-acceptor-pair recombination involving a SiGa donor on the UV luminescence band in the Si-doped crystals. The blue luminescence band was suppressed for the heavily N-doped epitaxial films with a N concentration of 1 × 1018 cm−3, implying the decrease in the oxygen vacancy (VO) concentration by N doping. The increase in the NO acceptor concentration as well as the decrease in the VO concentration were found to contribute to the compensation of residual Si donors resulting in high resistivity of N-doped epitaxial films.

Near-infrared and blue cooperative Yb3+ luminescence in Lu3Sc2Ga3O12 nano-garnets

Nanocrystalline Lu3Sc2Ga3O12 garnet powders doped with different concentrations of trivalent ytterbium (Yb3+) ions have been prepared by sol-gel method. These garnet nano-powders have been characterized for their structural and luminescence properties using X-ray diffraction, diffuse reflectance and luminescence spectra. Near-infrared and blue cooperative upconversion luminescence spectra and decay curves of Yb3+ ions in these nano-garnets have been measured as a function of the optically active ion concentration. Quenching of near-infrared and cooperative luminescence intensity beyond 5.0 mol% of Yb3+ ion concentration have been observed. Variation of blue cooperative luminescence intensity with pump power has also been discussed.

Controlling n-type conductivity of β-Ga2O3 by Nb doping

It has been confirmed that Si, Sn, and Ge are effective n-type dopants for β-Ga2O3. This letter shows that Nb doping is also a viable method for controlling the electrical resistivity and carrier density of β-Ga2O3, corresponding to the theoretical calculations about the doping of Ga2O3 with Nb [H. Peelaers and C. G. Van de Walle, Phys. Rev. B 94(19), 4 (2016)]. β-Ga2O3 single crystals with different Nb concentrations were grown by the optical floating zone method. The electrical resistivity can be varied from 3.6 × 102 Ω cm to 5.5 × 10−3 Ω cm by increasing the Nb doping concentration, and the related free carrier concentration increases from 9.55 × 1016 cm−3 to 1.8 × 1019 cm−3. The transmittance spectra and photoluminescence spectra were measured to systematically study the optical properties of Nb-doped β-Ga2O3 single crystals. The strong absorption near the IR region in the crystals is related to the increase in conductive electrons, and the decrease in blue luminescence intensity indicates a decrease in the VO concentration induced by increasing the carrier concentration.

In situ characterization of radiation sensors based on GaN LED structure by pulsed capacitance technique and luminescence spectroscopy

We report on variations of the electrical and optical characteristics of commercial GaN blue LEDs, acting as radiation sensors or dosimeters, during 1.6MeV proton irradiation. Transients of the barrier and storage capacitance have been examined simultaneously with proton-excited luminescence. The pulsed capacitance technique was shown to be a useful tool for the control of changes of the effective density of dopants in the LED structure during proton irradiation, due to introduced radiation defects. The minority carrier injection efficiency can be estimated by measuring the charging current transients of the barrier and storage capacitance. The transient current measurements imply rather fast response of the GaN LED hadron radiation sensors. Also, our results show that the intensity and spectral variations of the proton-excited blue luminescence of the LEDs can be used for the estimation of hadron flux and fluence, respectively. This implies that GaN LED-based sensors can be used for the synchronous detection of hadron irradiation by recording the optical and electrical signals.

Bright white upconversion luminescence from Er3+/Tm3+/Yb3+-doped titanate-based glasses prepared by aerodynamic levitation method

Aerodynamic levitation method was employed to prepare Er3+/Tm3+/Yb3+-doped titanate-based glasses. DTA results show that the glass performs high thermal stability with the glass transition temperature of 799 °C. The interaction among rare earth ions has been discussed by adjusting the relative concentration. Er3+ ions can quench the upconversion luminescence of Tm3+ ions. Tm3+ ions play a strong role in quenching the emissions of Er3+ and Tm3+ when the content of Tm3+ ions is greater than or equal 0.05. From the view of the ratio of red emission to green emission, Tm3+ ions can improve the red emission of Er3+ ions to some extent in contrast with the green emissions of Er3+ ions. 980 nm incident laser can be efficiently absorbed by Yb3+ ions. The relative intensity of red, green, and blue upconversion luminescence has been tuned to obtain white light. The composition with white upconversion luminescence of the color coordinate (0.291, 0.3292) has been found. Moreover, white upconversion luminescence mechanism is a two-photon process of ET, ESA, and cooperative sensitization. Rare earth ions doped titanate-based glasses with bright upconversion luminescence perform potential applications in color display, back light, et al.

Experimental optimal design on BaY2ZnO5: Tm3+/Yb3+ phosphor and its up-conversion luminescence property

To obtain a maximal blue up-conversion luminescence of Tm3+/Yb3+ co-doped BaY2ZnO5 phosphors, orthogonal experimental design combined with quadratic general rotary unitized design method is employed to optimize the Tm3+ and Yb3+ ions doping concentration. Two sets of BaY2ZnO5:Tm3+/Yb3+ phosphors are synthesized by the traditional high temperature solid reaction method. The doping concentration ranges of Tm3+ and Yb3+ are first narrowed by orthogonal experimental design, and then quadratic general rotary unitized design is performed and one regression equation is established based on the experimental results from the latter design. The theoretical maximum value of the blue up-conversion luminescence intensity and the optimal Tm3+ and Yb3+ doping concentrations are obtained by genetic algorithm. The optimal sample is synthesized and its crystal structure and up-conversion luminescence properties are investigated. It is found that the blue up-conversion luminescence originates from three photon processes under 980 nm excitation. Temperature dependent up-conversion luminescence spectra of the optimal sample show that the blue up-conversion luminescence intensity declines with increasing temperature, implying the occurrence of thermal quenching of up-conversion luminescence. The calculated excitation energy is about 0.602 eV.

Correlation of carbon doping variations with the vertical breakdown of GaN-on-Si for power electronics

In this work gallium nitride (GaN) grown on silicon substrates was investigated in order to determine critical defects responsible for differences in the vertical breakdown of HEMT structures. Cathodoluminescence studies at the SEM revealed a direct correlation between the intensity of the blue luminescence (BL) band and the carbon doping concentration. Observing this, carbon depletion zones were found around threading dislocations in the active GaN layer, as well as a deep depletion in growth columns concluded from a reduction of the BL intensity. Using the given results a model of a defect, here called the deep carbon depletion (DCD), is proposed to explain the correlation between carbon variations and the vertical breakdown.

Structural and optical investigations on Dy3+doped lithium tellurofluoroborate glasses for white light applications

Dy3+doped lithium tellurofluoroborate glasses were prepared with the chemical composition (40–x)B2O3+15TeO2+15Li2O+15LiF+15NaF+xDy2O3 (BTLNxD; where x=0.05, 0.1, 0.25, 0.5, 1.0 and 2.0 in wt%) following conventional melt quenching technique and characterized through FTIR, optical absorption, luminescence and lifetime measurements. The vibrations of the borate and tellurite network have been identified through FTIR spectral measurements. The optical properties have been explored from the absorption and luminescence spectra. The optical band gap of direct and indirect allowed transitions was determined from Tauc׳s plot and the variations in the band gap energy with structural arrangements in the prepared glasses is also discussed. Judd–Ofelt theory has been used to evaluate the intensity parameters Ω2, Ω4 and Ω6 using the experimental oscillator strengths and absorption energy levels. The radiative parameters such as transition probability (A), stimulated emission cross-section (σPE), branching ratios (βR) and radiative lifetime (τR) corresponding to the 4F9/2→6H11/2, 6H13/2 and 6H15/2 emission transitions were calculated from the luminescence spectra. The yellow-to−blue (Y/B) luminescence intensity ratios and colour chromaticity coordinates of the title glasses have also been estimated to evaluate the white light emission as a function of Dy3+ion concentration. These results have been used to identify the suitability of the prepared glasses for W-LED applications. The decay rates for the 4F9/2 level have been measured and found to deviate from exponential to non-exponential nature with the increase of Dy3+ion concentration. The non-exponential decay rates have been fitted with the Inokuti–Hirayama model for S=6 which indicate that dipole–dipole mechanism is responsible for the energy transfer processes through Dy3+–Dy3+interaction.

Correlation between surface modification and photoluminescence properties of β-Ga2O3 nanostructures

In this work three different growth methods have been used to grow β-Ga2O3 nanostructures. The nanostructures were characterized by Grazing Incident X-Ray Diffraction, Scanning Electron Microscopy, Transmission Electron Microscopy and Photoluminescence Spectroscopy. Photoluminescence spectra for all the samples of β-Ga2O3 nanostructures exhibit an UV and blue emission band. The relative intensity of UV and blue luminescence is strongly affected by the surface defects present on the nanostructures. Our study shows that Photoluminescence intensity of UV and blue luminescence can be reliably used to determine the quality of β-Ga2O3 nanostructures. Further the work opens up the possibility of using UV excitation and subsequent Photoluminescence analysis as a possible means for oxygen sensing. The Photoluminescence mechanism in β-Ga2O3 nanostructures is also discussed.