Yellow Luminescence Band Research Articles

Two yellow luminescence bands in undoped GaN

Two yellow luminescence bands related to different defects have been revealed in undoped GaN grown by hydride vapor phase epitaxy (HVPE). One of them, labeled YL1, has the zero-phonon line (ZPL) at 2.57 eV and the band maximum at 2.20 eV at low temperature. This luminescence band is the ubiquitous yellow band observed in GaN grown by metalorganic chemical vapor deposition, either undoped (but containing carbon with high concentration) or doped with Si. Another yellow band, labeled YL3, has the ZPL at 2.36 eV and the band maximum at 2.09 eV. Previously, the ZPL and fine structure of this band were erroneously attributed to the red luminescence band. Both the YL1 and YL3 bands show phonon-related fine structure at the high-energy side, which is caused by strong electron-phonon coupling involving the LO and pseudo-local phonon modes. The shapes of the bands are described with a one-dimensional configuration coordinate model, and the Huang-Rhys factors are found. Possible origins of the defect-related luminescence bands are discussed.

Fabrication and UV photoresponse of GaN nanowire-film hybrid films on sapphire substrates by chemical vapor deposition method

Hybrid films composed of GaN nanowires and GaN films were prepared on sapphire substrates by chemical vapor deposition. The GaN films and GaN nanowires are all indexed to the hexagonal wurtzite structure. The photoluminescence spectra of GaN nanowire-film hybrid films are composed of a strong ultraviolet emission peak (365 nm) and a weak yellow luminescence band (∼600 nm). The UV detector based on GaN nanowire-film hybrid films shows a high responsivity of ∼2.5 A/W at 360 nm and a fast response speed of ∼22 μs.

Degradation mechanisms of optoelectric properties of GaN via highly-charged 209Bi33+ ions irradiation

N-type gallium nitride (GaN) epitaxial layers were subjected to 990-keV Bi33+ ions irradiation to various fluences. Optoelectric properties of the irradiated-GaN specimens were studied by means of Raman scattering and variable temperature photoluminescence (PL) spectroscopy. Raman spectra reveal that both the free-carrier concentration and its mobility generally decrease with a successive increase in ion fluence. Electro-optic mechanisms dominated the electrical transport to a fluence of 1.061 × 1012 Bi33+/cm2. Above this fluence, electrical properties were governed by the deformation potential. The appearance of vacancy-type defects results in an abrupt degradation in electrical transports. Varying temperature photoluminescence (PL) spectra display that all emission lines of 1.061 × 1012 Bi33+/cm2-irradiated specimen present a general remarkable thermal redshift, quenching, and broadening, including donor-bound-exciton peak, yellow luminescence band, and LO-phonon replicas. Moreover, as the temperature rises, a transformation from excitons (donor-acceptor pairs’ luminescence) to band-to-band transitions (donor-acceptor combinations) was found, and the shrinkage effect of the band gap dominated the shift of the peak position gradually, especially the temperature increases above 150 K. In contrast to the un-irradiated specimen, a sensitive temperature dependence of all photoluminescence (PL) lines’ intensity obtained from 1.061 × 1012 Bi33+/cm2-irradiated specimen was found. Mechanisms underlying were discussed.

GaN nanophosphors for white-light applications

GaN nanoparticles (NPs) were synthesized by carbothermal reduction combined with nitridation, using Ga2O3 powder and graphitic carbon nitride (g-C3N4) as precursors. Characterization of the NPs was performed by X-ray diffraction, scanning electron microscopy, and room-temperature photoluminescence measurements. X-ray photoelectron spectroscopy was also performed to detect the chemical states of the different species. A universal yellow luminescence (YL) band was observed from complexes of Ga vacancies with O anti-sites and of O anti-sites with C. Further increments in the C content were observed with continued growth and induced an additional blue luminescence (BL) band. Tuning of the YL and BL bands resulted in white-light emission under certain experimental conditions, thus offering a new way of employing GaN nanophosphors for solid-state white lighting. Calculations of the correlated color temperature and color-quality scale parameters confirmed the utility of the experimental process for different applications.

Thermal quenching of the yellow luminescence in GaN

We observed varying thermal quenching behavior of the yellow luminescence band near 2.2 eV in different GaN samples. In spite of the different behavior, the yellow band in all the samples is caused by the same defect—the YL1 center. In conductive n-type GaN, the YL1 band quenches with exponential law, and the Arrhenius plot reveals an ionization energy of ∼0.9 eV for the YL1 center. In semi-insulating GaN, an abrupt and tunable quenching of the YL1 band is observed, where the apparent activation energy in the Arrhenius plot is not related to the ionization energy of the defect. In this case, the ionization energy can be found by analyzing the shift of the characteristic temperature of PL quenching with excitation intensity. We conclude that only one defect, namely, the YL1 center, is responsible for the yellow band in undoped and doped GaN samples grown by different techniques.

Efficient temperature sensing using photoluminescence of Er/Yb implanted GaN thin films

The luminescence characteristics of GaN films implanted with Er at low doses were evaluated. The defect-related yellow luminescence (YL) and green luminescence (GL) bands observed under direct excitation with 488nm were attributed to the transitions via different charge levels of the same defect. The quenching behavior of the luminescence intensity either with the temperature or concentration variation can be attributed to nonradiative energy transfer (ET) and/or charge transfer by trapping impurities. The temperature dependence of the YL band allowed us to identify the defect responsible for this emission. The best candidate for this defect was found to be a nitrogen-vacancy. A GaN sample co-doped with Er3+ and Yb3+ ions was prepared, and its optical properties were analyzed. The incorporation of Yb3+ improved the PL emission intensity in the visible region. This feature results from the efficient ET processes between these two doping ions. The color coordinate analysis indicates that Er3+/Yb3+ co-doped GaN semiconductor emits light with color in the white-light region. To investigate the temperature sensing application of the synthesized co-doped semiconductor, the temperature-sensing performance was evaluated using the fluorescence intensity ratio technique in the temperature range 200–300K. The significant temperature sensitivity indicates its potential as a temperature sensing probe. The maximum sensitivity was 15×10−4K−1 at 200K.

High-Quality and Strain-Relaxation GaN Epilayer Grown on SiC Substrates Using AlN Buffer and AlGaN Interlayer**Supported by the National Key R&D Program of China under Grant No 2016YFB0400200.

We study the effect of the AlGaN interlayer on structural quality and strain engineering of the GaN films grown on SiC substrates with an AlN buffer layer. Improved structural quality and tensile stress releasing are realized in unintentionally doped GaN thin films grown on 6H–SiC substrates by metal organic chemical vapor deposition. Using the optimized AlGaN interlayer, we find that the full width at half maximum of x-ray diffraction peaks for GaN decreases dramatically, indicating an improved crystalline quality. Meanwhile, it is revealed that the biaxial tensile stress in the GaN film is significantly reduced from the Raman results. Photoluminescence spectra exhibit a shift of the peak position of the near-band-edge emission, as well as the integrated intensity ratio variation of the near-band-edge emission to the yellow luminescence band. Thus by optimizing the AlGaN interlayer, we could acquire the high-quality and strain-relaxation GaN epilayer with large thickness on SiC substrates.

Luminescence properties of silicate apatite phosphors M2La8Si6O26:Eu (M = Mg, Ca, Sr)

Europium ions doped M2La8Si6O26 (M = Mg, Ca, Sr) silicate apatite phosphors have been synthesized by using solid-state reaction method, post-treated in a reducing atmosphere and studied by optical spectroscopy techniques. It has been revealed that the emission spectra from all the synthesized samples contain a set of spectral lines due to the 4f – 4f transitions of Eu3+ ions, which predominantly occupy the A2 sites of Cs symmetry in the apatite structure. Efficient conversion of Eu3+ to Eu2+ has been achieved in a reducing H2/Ar atmosphere for the Mg2La8(SiO4)6O2 apatite due to the charge compensation mechanism by oxygen vacancies. This apatite possesses broadband yellow emission with the band maximum at 560nm and the luminescence decay time τ < 1 μs due to the 5d − 4f Eu2+ transitions and this yellow luminescence band can be efficiently excited by near-UV to blue light (300–450nm). It is assumed that by optimization of the europium concentration and Eu3+/Eu2+ ratio promising single-phase phosphors can be obtained for warm white LEDs having high color rendering indexes.

Electronic excitation induced structural and optical modifications in InGaN/GaN quantum well structures grown by MOCVD

The present study focuses on the electronic excitation induced structural and optical properties of InGaN/GaN quantum well (QW) structures grown by metal organic chemical vapor deposition technique. These excitations were produced using Au7+ ion irradiation with 100MeV energy. The X-ray rocking curves intensity and full width at half-maximum values corresponding to the planes of (0002) and (10−15) of the irradiated QW structures show the modifications in the screw and edge-type dislocation densities vary with the ion fluences. The structural characteristics using the reciprocal space mapping indicate the intermixing effects in InGaN/GaN QW structures. Atomic force microscopy images confirmed the presence of nanostructures and the surface modification due to heavy ion irradiation. The irradiated QW structures exhibited degraded photoluminescence intensity and a subsequent decrease in the yellow luminescence band intensity with the fluences of 1×1011 and 5×1012ions/cm2 compared to the pristine QW structures.

Yellow luminescence band in undoped GaN revealed by two-wavelength excited photoluminescence

The below-gap emission components including yellow luminescence (YL) band of an MOCVD grown undoped GaN have been studied by the two-wavelength-excited photoluminescence (TWEPL). The nature of each emission line has been investigated by using an intermittent below-gap excitation (BGE) light of 1.17 eV on an above-gap excitation (AGE) light of 3.49 eV. The intensity of DAP and the YL decreased while it increased for IOX after irradiation of the BGE. The intensity change in PL after addition of the BGE implies the presence of defect levels in the energy position corresponding to the photon energy of the BGE. Possible recombination models are listed and examined. Only the recombination model in which the YL corresponds to the transition from a shallow donor to a deep state at about 1 eV above the valence band maximum satisfies our experimental result. The possible origin of this defect state is discussed.

Photoluminescence enhancement from GaN by beryllium doping

High quality Be-doped (Be = 0.19 at.%) GaN powder has been grown by reacting high purity Ga diluted alloys (Be-Ga) with ultra high purity ammonia in a horizontal quartz tube reactor at 1200 °C. An initial low-temperature treatment to dissolve ammonia into the Ga melt produced GaN powders with 100% reaction efficiency. Doping was achieved by dissolving beryllium into the gallium metal. The powders synthesized by this method regularly consist of two particle size distributions: large hollow columns with lengths between 5 and 10 μm and small platelets in a range of diameters among 1 and 3 μm. The GaN:Be powders present a high quality polycrystalline profile with preferential growth on the [101¯1] plane, observed by means of X-ray diffraction. The three characteristics growth planes of the GaN crystalline phase were found by using high resolution TEM microscopy. The optical enhancing of the emission in the GaN powder is attributed to defects created with the beryllium doping. The room temperature photoluminescence emission spectra of GaN:Be powders, revealed the presence of beryllium on a shoulder peak at 3.39 eV and an unusual Y6 emission at 3.32eV related to surface donor-acceptor pairs. Also, a donor-acceptor-pair transition at 3.17 eV and a phonon replica transition at 3.1 eV were observed at low temperature (10 K). The well-known yellow luminescence band coming from defects was observed in both spectra at room and low temperature. Cathodoluminescence emission from GaN:Be powders presents two main peaks associated with an ultraviolet band emission and the yellow emission known from defects. To study the trapping levels related with the defects formed in the GaN:Be, thermoluminescence glow curves were obtained using UV and β radiation in the range of 50 and 150 °C.

Zero-phonon line and fine structure of the yellow luminescence band in GaN

The yellow luminescence band was studied in undoped and Si-doped GaN samples by steady-state and time-resolved photoluminescence. At low temperature (18 K), the zero-phonon line (ZPL) for the yellow band is observed at 2.57 eV and attributed to electron transitions from a shallow donor to a deep-level defect. At higher temperatures, the ZPL at 2.59 eV emerges, which is attributed to electron transitions from the conduction band to the same defect. In addition to the ZPL, a set of phonon replicas is observed, which is caused by the emission of phonons with energies of 39.5 meV and 91.5 meV. The defect is called the YL1 center. The possible identity of the YL1 center is discussed. The results indicate that the same defect is responsible for the strong YL1 band in undoped and Si-doped GaN samples.

Deep traps determining the non-radiative lifetime and defect band yellow luminescence in n-GaN

Deep traps spectra measurements were performed for a group of n-GaN films grown by metalorganic chemical vapor deposition (MOCVD) on sapphire using standard MOCVD and two versions of lateral overgrowth techniques, the epitaxial lateral overgrowth (ELOG) and pendeo epitaxy (PE). Deep levels transient spectroscopy with electrical (DLTS) or optical (ODLTS) injection showed that in all cases the dominant electron traps were the well known Ec-0.56 eV centers. DLTS spectra taken with constant illumination indicate that not only has this trap a high density and electron capture cross section, but that it also has a high hole capture cross section and thus can be an efficient recombination center. Comparison with diffusion length measurements supports this conclusion. Among the hole traps the most prominent centers are the traps near Ev+0.9 eV. However, direct estimate of the electron capture cross section by these traps give very low values close to 10−22 cm2 thus excluding the Ev+0.9 eV traps from the list of potential lifetime killers, despite the high density of these centers. Annealing at 800 °C of one of the PE samples led to an increase of the concentration of the Ev+0.9 eV traps, the increase of the centers with optical ionization energy 1.3 eV density (observed in photocapacitance), and also to increased intensity of yellow luminescence band.

Study of optical properties of bulk GaN crystals grown by HVPE

We investigated the optical properties of a series of GaN samples sliced from the same bulk crystal grown using hydride vapor phase epitaxy. The high crystalline quality of the samples was evaluated using cathodoluminescence measurements, and the dislocation density ranged from 2.4 × 106 to 2.3 × 105 cm−2. The impurity concentration was determined using secondary-ion mass spectroscopy, and photoluminescence (PL) measurements were conducted in the range of 3–300 K. We did not find a correlation between the O or C impurities and the weak yellow luminescence (YL) band. As the dislocation density decreased, the intensity of the band edge emission increased and that of the YL band decreased. A competition between the two-electron satellite lines correlated to Si and the YL band was also observed in the low-temperature PL spectra, which demonstrated that the Si impurity also plays an important role in the weak YL band of these GaN samples. These results indicate that the Si donors around the dislocations, as reasonable sources of shallow donors, will recombine with possible deep acceptors and finally respond with the YL.

Hydrogen-carbon complexes and the blue luminescence band in GaN

The blue luminescence band with a maximum at 3.0 eV and the zero-phonon line at 3.33 eV (labeled BL2) is observed in high-resistivity GaN. Under prolonged ultraviolet (UV) light exposure, the BL2 band transforms into the yellow luminescence (YL) band with a maximum at 2.2 eV. Our hybrid functional calculations suggest that the BL2 band is related to a hydrogen-carbon defect complex, either CNON-Hi or CN-Hi. The complex creates defect transition level close to the valence band, which is responsible for the BL2 band. Under UV illumination, the complex dissociates, leaving as byproduct the source of the YL band (CNON or CN) and interstitial hydrogen.

Effect of light Si doping on the properties of GaN

An obvious increase in electron mobility and yellow luminescence (YL) band intensity was found in light Si doping GaN. For a series of GaN samples with different doping concentration, the dislocation density is almost the same. It is inferred that the abrupt increase in mobility and YL intensity does not originate from the change of dislocation density. The mobility behavior is attributed to the screening of scattering by dislocation and increase of ionized impurity scattering with the increase of Si doping concentration. At lower doping level, the screening of dislocation scattering is dominant, which results in the increase in carrier mobility. At higher doping level, the increase in ionized impurity scattering leads to the decrease in carrier mobility. Higher mobility causes longer diffusion length of nonequilibrium carrier. More dislocations will participate in the recombination process which induces stronger YL intensity in light Si doping GaN.

Analysis of Point Defect Distributions in AlGaN/GaN Heterostructures via Spectroscopic Photo Current-Voltage Measurements

Raman spectroscopy, photoluminescence (PL), and spectroscopic photo current-voltage measurements were performed to characterize point defects at surface/interface and bulk in AlGaN/GaN high electron mobility transistors (HEMTs) wafer. Raman spectroscopy displayed that two samples have similar crystalline quality and stress. PL measurements revealed that samples possessed strong near band edge peak at around 3.42 eV and broad blue luminescence. No yellow luminescence band was observed. The spectroscopic photo current-voltage (I-V) measurements with sub-bandgap illumination exhibited the existence of sub-bandgap defects with different activation energies for each sample. The depth-resolved ultra-violet (UV) spectroscopic photo current-voltage (I-V) (DR-UV-SPIV) measurements revealed that the depth dependent distribution of the defects for these samples are different. It was demonstrated that even though electrically active defects for two devices on the same pieces of AlGaN/GaN HEMTs heterostructures cannot be distinguished in Raman spectroscopy and PL measurements, they can be differentiated by using the spectroscopic photo I-V with sub-bandgap illumination and DR-UV-SPIV measurements. It was concluded that DR-UV-SPIV and spectroscopic photo I-V measurements with sub-bandgap illumination can be used to assess the uniformity of in-depth and spectral distribution of sub-bandgap point defects across the AlGaN/GaN wafers, respectively.

Radiative recombination in GaN/InGaN heterojunction bipolar transistors

We report an electroluminescence (EL) study on npn GaN/InGaN heterojunction bipolar transistors (HBTs). Three radiative recombination paths are resolved in the HBTs, corresponding to the band-to-band transition (3.3 eV), conduction-band-to-acceptor-level transition (3.15 eV), and yellow luminescence (YL) with the emission peak at 2.2 eV. We further study possible light emission paths by operating the HBTs under different biasing conditions. The band-to-band and the conduction-band-to-acceptor-level transitions mostly arise from the intrinsic base region, while a defect-related YL band could likely originate from the quasi-neutral base region of a GaN/InGaN HBT. The IB-dependent EL intensities for these three recombination paths are discussed. The results also show the radiative emission under the forward-active transistor mode operation is more effective than that using a diode-based emitter due to the enhanced excess electron concentration in the base region as increasing the collector current increases.

Influence of residual carbon impurities in i-GaN layer on the performance of GaN-based p-i-n photodetectors

The influence of unintentionally doped carbon impurities of i-GaN layer on the performance of GaN-based p-i-n photodetectors is investigated. The photoluminescence spectra exhibits that the carbon impurities are strongly involved in deep trap level-related yellow luminescence band. The results of secondary ion mass spectroscopy suggest that the residual carbon impurities in the i-layer have great effect on the generation of deep trap levels, and have a strong influence on the spectral responsivity and dark current of photodetectors. Thus, the way to decrease the residual carbon impurity concentration of the i-GaN layer, such as enlarging the growth pressure, can improve the performance of p-i-n photodetectors.

A direct evidence of allocating yellow luminescence band in undoped GaN by two-wavelength excited photoluminescence

The behavior of below-gap luminescence of undoped GaN grown by MOCVD has been studied by the scheme of two-wavelength-excited photoluminescence. The emission intensity of shallow donor to valence band transition (IOX) increased while intensities of donor-acceptor pair transition and the Yellow Luminescence band (YLB) decreased after the irradiation of a below-gap excitation source of 1.17 eV. The conventional energy schemes and recombination models have been considered to explain our experimental result but only one model in which YLB is the transition of a shallow donor to a deep state placed at ∼1 eV above the valence band maximum satisfies our result. The defect related parameters that give a qualitative insight in the samples have been evaluated by systematically solving the rate equations and fitting the result with the experiment.