Blue Luminescence Band Research Articles

Luminescence from defects in GaN

We briefly review the luminescence properties of defects in GaN and focus on the most interesting defects. In particular, the blue luminescence band peaking at about 3 eV is assigned to different defects and even different types of transitions in undoped, Zn-, C-, and Mg-doped GaN. Another omnipresent luminescence band, the yellow luminescence band may have different origin in nearly dislocation-free freestanding GaN templates, undoped thin layers, and carbon-doped GaN. The Y 4 and Y 7 lines are caused by recombination at unidentified point defects captured by threading edge dislocations.

EPR and optical studies of Cr-doped PbWO4 single crystals

Abstract Electron paramagnetic resonance (EPR) studies were performed on Cr ions in PbWO 4 crystals using an X-band spectrometer. With the help of Fourier-transform infrared (FTIR) and thermoluminescence (TSL) data, it was proposed that Cr 3+ impurities enter PbWO 4 lattice in the Pb-site and result in large relaxation of crystal lattice, which was illustrated by the significant distortion of WO 4 tetrahedron and the rapid decrease of blue luminescence band. Further, for heavily doped samples some new EPR signals, which did not appear in the lightly doped sample, were detected. The new EPR lines were ascribed to the exchange-coupled effect of Cr 3+ -ion pairs. Luminescence spectra provide support to the idea about defect clusters. The presence of Cr 3+ -ion pairs or Cr 3+ -ion associations in heavily doped PWO is the reason for the luminescence quench of PWO host and Cr 3+ ion.

Synthesis of single-crystalline α-Si3N4 nanobelts by extended vapour–liquid–solid growth

A simple chemical method for the production of single-crystallineα-Si3N4 nanobelts has been developed, consisting of nitridation of a high-Si-content Fe–Si ‘catalyst’ by ammoniaat 1300 °C. The as-synthesized product was characterized by means of x-ray diffraction,electron microscopy and energy-dispersive x-ray spectroscopy. Theα-Si3N4 nanobelts have widths of 60–120 nm, thicknesses of 10–30 nm and lengths up to microns.Four intense green–blue luminescence bands at 398 nm (3.12 eV), 434 nm (2.86 eV), 492 nm(2.52 eV) and 540 nm (2.30 eV) were observed and analysed for the product, which indicatesthe potential applications in optoelectronics. The growth mechanism has also beenspeculated upon. The potential technological importance of the product, the simplicity ofthe preparation procedure, as well as the cheap commercial precursor of Fe–Sialloy particles makes this study both scientifically and technologically interesting.

Effect of potential fluctuations on photoluminescence in Mg‐doped GaN

A model is proposed to explain dependencies of the defect-related photoluminescence (PL) intensities on temperature and excitation intensity in Mg-doped p-type GaN layers. Typical PL spectrum in Mg-doped GaN is dominated by a blue luminescence (BL) band with a maximum at about 2.8–2.9 eV and an ultraviolet luminescence (UVL) band with a maximum at about 3.1–3.2 eV. The BL intensity increases linearly while the UVL band intensity increases with a square power with excitation intensity in selected samples. In the proposed model, which is based on the concept of giant long-range potential fluctuations in Mg-doped GaN, the UVL band dominates in the potential valleys where electrons bound to shallow donors tend to accumulate. In contrast, the BL band dominates in the potential hills where the equilibrium concentration of holes bound to the shallow acceptor is much higher. The effect of potential fluctuations explains also pequliarities of quenching of the UVL and BL bands with temperature. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Luminescence of crystals of divalent tungstates

Crystals of divalent tungstates are characterized by two main luminescence spectral ranges: a short-wavelength (blue) luminescence band in the range 390–420 nm and a group (often two groups) of longer wavelength (green) bands in the range 480–520 nm. For crystals of calcium, strontium, barium, cadmium, magnesium, zinc, and lead tungstates, it is shown that the wavelength corresponding to the maximum of the blue luminescence band (λmax) correlates with the melting temperature (T m) of these compounds. The position of the blue luminescence band is the same (in the range 510–530 nm) for crystals with different divalent cations. Annealing in vacuum and electron irradiation decrease the intensity of both blue and green luminescence bands but do not change the ratio of their maximum intensities. This circumstance suggests that vacancies serve as luminescence quenchers to a greater extent rather than facilitate the formation of emission centers responsible for a particular luminescence band.

Synthesis and characterization of ultra-long silica nanowires

Large-scale synthesis of amorphous silica nanowires was achieved by using simple physical evaporation of a mixture of Si and Al2O3 powders. Scanning electron microscopy observations showed that the silica nanowires have lengths of several mm and diameters of 20–100 nm. X-ray photoelectron spectroscopy revealed that these nanowires consisted of Si and O elements in an atomic ratio of approximately 1:2, consistent with the stoichiometric formula SiO2. Photoluminescence measurements showed a blue luminescence band in the wavelength range of 400–500 nm with three peaks at 420 nm, 440 nm, and 470 nm, respectively, which also differs from the previous reports of the photoluminescence for oxygen-deficient silica nanowires. This may suggest that the fully oxidized silica nanowires still have many structural defects that contribute to the photoluminescence.

Dislocation Reduction and Structural Properties of GaN layers Grown on N+-implanted AlN/Si (111) Substrates

AbstractAn alternative scheme to the growth of crack free, dislocation reduced III-Nitride layers on Silicon substrate has been previously introduced that relies on formation of an ion implanted defective layer in the substrate with implantation taking place in the presence of AlN buffer layer. Here, the effects of N+ ion implantation of AlN/Si (111) substrate on the structural and optical properties of the overgrown GaN epilayers have been investigated. Temperature dependent photoluminescence has been used to investigate the impact of the implantation conditions (energy and dose) on optical and structural quality of the GaN overgrown layers. A correlation between PL and high resolution x-ray diffraction (XRD) of the overgrown GaN layers show that the lowest FWHM of bandedge, the highest bandedge to deep defect blue luminescence band ratio, and the lowest symmetric rocking curve FWHM are achieved for the optimized implantation conditions. This correlates well with the results of etch pit density measurement showing an order of magnitude reduction in threading dislocation defect density

Effect of vacuum annealing on the edge luminescence of undoped zinc selenide

It has been established experimentally that vacuum annealing of undoped ZnSe crystals in the temperature range 700–1200 K leads to a significant increase in the intensity of the blue luminescence band and suppression of the red-orange band. A model for defect formation is proposed to explain the observed characteristics.

Synthesis and photoluminescence ofα-Al2O3 nanowires

α-Al2O3 nanowires have been synthesized in bulk quantity by using simplephysical evaporation of mixture of pure Al powders and nanometre-sizedTiO2 powdersat 1150 °C. Scanning electron microscopy and transmission electron microscopy observations show thatα-Al2O3 nanowires have diameters ranging from 20 to 60 nm and lengths up to several tens of micrometres. Thegrowth of α-Al2O3 nanowires is controlled by the vapour–liquid–solid (VLS) mechanism. Photoluminescencemeasurements revealed a blue luminescence band in the wavelength range of400–500 nm with two peaks at 415 and 440 nm, which could be attributed to theF+ centresin the α-Al2O3 nanowires.

Investigation of optical metastability in GaN using photoluminescence spectroscopy

We report photo-induced changes of optically active defects in GaN under 325nm excitation. Cooling the sample in the dark and then performing rapid measurements using a Fourier transform spectrometer allows the changes in the spectrum to be followed on time scales of minutes. At first, the characteristic blue and yellow luminescence bands are observed, with the blue band roughly four times stronger than the yellow. Upon continuous irradiation the yellow band increases rapidly at first, and then more slowly to become the dominant band after approximately 60min. During this time the blue band decreases. The temperature dependence of this fatigue behaviour is reported for both constant and intermittent illumination.

Persistent photoluminescence in high-purity GaN

High-purity GaN grown by hydride vapor-phase epitaxy was studied by time-resolved photoluminescence (PL). Two types of samples, 6–30-μm-thick GaN layers on sapphire substrate and a 200-μm-thick freestanding template, exhibited similar PL behavior. At low temperatures, the luminescence signal from the red, yellow, blue, and ultraviolet luminescence bands could be detected at delay times up to several minutes following excitation light pulse. The persistent PL in freestanding template can be attributed solely to a donor–acceptor-pair-type recombination in very pure GaN. However, a similar slow decay of radiative transitions in apparently less-pure GaN films on sapphire raises the possibility of additional mechanisms for the observed persistent PL.

Luminescence in GaN co-doped with carbon and silicon

GaN samples, containing various concentrations of carbon and doped intentionally with silicon, have been grown heteroepitaxially on sapphire using metal-organic-chemical-vapor deposition. Previous electrical and optical data, together with Density Functional calculations, have suggested that carbon is incorporated at acceptor and donor substitutional sites in this material; the relative importance of each is determined by the Fermi level position and the growth conditions. Here the luminescence behavior of these materials is examined in more detail, including spectral, temperature, and time dependences under ultraviolet light and electron beam excitation conditions. Particular attention is given to the commonly observed “yellow band” at ∼2.2 eV , a blue luminescence at ∼3 eV seen only in samples where carbon is the majority dopant, and ultraviolet bands near ∼3.3 eV . Our data suggest that the latter two bands are both donor–acceptor related with the final state being the negatively charged state of a carbon atom substituting for nitrogen. In samples where carbon is the majority dopant, extended luminescence excitation at low temperatures results in large changes in the brightness of the yellow and blue luminescence bands. These effects are similar to other recent observations of luminescence metastability in high resistivity GaN, and we suggest that carbon plays a crucial role in this phenomenon.

Blue emission band in compensated GaN:Mg codoped with Si

Codoping of p-type GaN with Mg and Si was investigated to determine its effect on deep level luminescence. By codoping with Si, the absolute intensity of the 2.8-eV blue luminescence band, previously tentatively attributed to deep donor-acceptor-pair (DAP) emission, decreased by more than an order of magnitude. The observed decrease is attributed to the reduction of the concentration of deep donor nitrogen vacancy complexes. The dependence of the emission peak position on hole concentration was investigated. A blueshift was observed with increasing carrier concentration. The shift of the blue band with carrier concentration and excitation intensity is explained semiquantitatively by a potential fluctuation model indicating its importance in DAP recombination.

Photoluminescence of Mg-doped GaN grown by metalorganic chemical vapor deposition

Two Mg-doped GaN films with different doping concentrations were grown by a metalorganic chemical vapor deposition technique. Photoluminescence (PL) experiments were carried out to investigate the optical properties of these films. For highly Mg-doped GaN, the PL spectra at 10 K are composed of a blue luminescence (BL) band at 2.857 eV and two excitonic luminescence lines at 3.342 eV and 3.282 eV, in addition to a L2 phonon replica at 3.212 eV. The intensity of the L1 line decreases monotonously with an increase in temperature. However, the intensity of the L2 line first slowly increases at first, and then decreases quickly with an increase in temperature. The two lines are attributed to bound excitonic emissions at extended defects. The BL band is most likely due to the transition from deep donor Mg–VN complex to Mg shallow acceptor. From the temperature dependence of the luminescence peak intensity of the BL band, the activation energy of acceptor Mg was found to be 290 meV.

Temperature-controlled growth of α-Al2O3nanobelts and nanosheets

α-Al2O3 Nanobelts and nanosheets with different morphologies and sizes have been successfully synthesized by a simple chemical route from H2O and Al in an argon atmosphere. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations show that the as-synthesized α-Al2O3 nanostructures consist of nanobelts and nanosheets. We find that the temperature distribution inside the alumina tube is the critical factor determining the morphologies and sizes of the α-Al2O3 nanostructures. Photoluminescence measurements showed a blue luminescence band in the wavelength range of 400–500 nm, which can be attributed to F+ centers in the α-Al2O3 nanostructures. The growth of these products is believed to be mainly determined by growth kinetics as well as thermodynamics.

Electrical and optical properties of GaN films implanted with Mn and Co

Optical transmission spectra, microcathodoluminescence spectra, capacitance–voltage and capacitance–frequency curves, temperature dependence of resistivity and deep level spectra with both electrical and optical injection were measured on n-GaN samples implanted with high doses of Mn (3×1016 and 4×1016 cm−2) and Co (4×1016 cm−2). From optical transmission it was found that Mn forms a deep acceptor near Ev+1.8 eV while the Co acceptor is about 0.1 eV deeper. In addition, Mn and Co form complexes with native defects and these complexes are deep electron traps with a level near Ec−0.5 eV. Such complexes are most likely responsible for a strong blue luminescence band with energy near 2.9 eV. Adjacent to the implanted region a defect region about 1 μm deep is formed, most likely by out-diffusion of point defects from the implanted zone during the 700 °C annealing used to partially remove the radiation damage. This region is characterized by a high density of electron traps at Ec−0.25 eV and Ec−0.7 eV and hole traps at Ev+0.2 eV, Ev+0.35 eV and Ev+0.45 eV.

Optical metastability in undoped GaN grown on Ga-rich GaN buffer layers

Investigations on defect-related optical metastability in undoped GaN epilayers grown on GaN buffer layers are presented. The III/V ratio in the buffer layer was varied over a range such that the resistivity of the GaN epilayers traversed a semiconducting to semi-insulating transition. The high-resistive and semi-insulating GaN epilayers show photo induced metastability, which is revealed through a number of features: (i) the intensity of the blue luminescence band decreases as the intensity of the yellow luminescence band increases; (ii) quenching of photocurrent; and (iii) persistent photoconductivity, under the illumination of a low-power ultraviolet laser. It has been shown that these unwanted transient effects can be eliminated in the GaN epilayers by reducing the III/V ratio in the buffer layer. A qualitative discussion of these results suggests that the metastable defects, which are associated with both yellow and blue luminescence bands, may have important consequences for our understanding of defect-related luminescence in GaN.

Influence of high Mg doping on the microstructural and optoelectronic properties of GaN

A transmission electron microscopy study of a wide range of p-type GaN samples reveals that high Mg doping has a strong influence on the polarity of GaN. The main characteristic of Mg-doped metal organic vapour phase epitaxy (MOVPE) and bulk GaN is the presence of pyramidal inversion domains (PIDs). It is shown that the appearance of PIDs is correlated with a decrease of the free hole concentration and with the appearance of the blue photoluminescence band which is characteristic of MOVPE-grown Mg-doped GaN. A tentative model based on electrostatic considerations is proposed for this blue luminescence band.

Optical and electrical properties of semi-insulating GaN:C grown by MBE

ABSTRACTSemi-insulating wurtzite GaN:C of high optical quality is obtained with CCl4or CS2 doping sources in plasma-assisted molecular-beam epitaxy in Ga-rich growth conditions. The highest resistivity (107Ω-cm) is found for [C] in the low 1018cm−3range. An increasing fraction of carbon appears to form electrically inactive pair defects for higher doping levels causing the concentration of uncompensated residual donors to be higher in films with [C] in the 1019cm−3range compared with [C] in the 1018cm−3range. Blue (2.9 eV) and yellow (2.2 eV) luminescence bands are associated with carbon-related defects, and additional support is provided for the association of the blue luminescence with the carbon-acceptor deactivating pair defect. Finally, the temperature dependence of the resistivity is described within the grain-boundary controlled transport model of Salzmanet al., Appl. Phys. Lett.76, 1431 (2000).

Exciton dynamics related with phase transitions in CsPbCl 3 single crystals

Characteristics of excitonic luminescence in CsPbCl 3 single crystals have been investigated in a wide range of temperature. A blue luminescence band observed in the temperature range 4–300 K is due to radiative decay of free excitons. In addition to the blue band, a broad red band appears below ∼180 K. Temperature dependence of the steepness of the Urbach absorption tail suggests that a relaxation pathway of excitons into a self-trapped state gets opened below ∼180 K. The change in the excitonic luminescence as well as in the steepness of the absorption tail is ascribed to a phase transition at 176 K.