Deep Ultraviolet Emission Research Articles

Low absorption loss p-AlGaN superlattice cladding layer for current-injection deep ultraviolet laser diodes

Current injection into AlGaN-based laser diode structures with high aluminum mole fractions for deep ultraviolet emission is investigated. The electrical characteristics of laser diode structures with different p-AlGaN short period superlattice (SPSL) cladding layers with various aluminum mole fractions are compared. The heterostructures contain all elements that are needed for a current-injection laser diode including cladding and waveguide layers as well as an AlGaN quantum well active region emitting near 270 nm. We found that with increasing aluminum content in the p-AlGaN cladding, the diode turn-on voltage increases, while the series resistance slightly decreases. By introducing an SPSL instead of bulk layers, the operating voltage is significantly reduced. A gain guided broad area laser diode structure with transparent p-Al0.70Ga0.30N waveguide layers and a transparent p-cladding with an average aluminum content of 81% was designed for strong confinement of the transverse optical mode and low optical losses. Using an optimized SPSL, this diode could sustain current densities of more than 4.5 kA/cm2.

Large Optical Gain AlInN-Delta-GaN Quantum Well for Deep Ultraviolet Emitters.

The optical gain and spontaneous emission characteristics of low In-content AlInN-delta-GaN quantum wells (QWs) are analyzed for deep ultraviolet (UV) light emitting diodes (LEDs) and lasers. Our analysis shows a large increase in the dominant transverse electric (TE) polarized spontaneous emission rate and optical gain. The remarkable enhancements in TE-polarized optical gain and spontaneous emission characteristics are attributed to the dominant conduction (C)-heavy hole (HH) transitions achieved by the AlInN-delta-GaN QW structure, which could lead to its potential application as the active region material for high performance deep UV emitters. In addition, our findings show that further optimizations of the delta-GaN layer in the active region are required to realize the high performance AlInN-based LEDs and lasers with the desired emission wavelength. This work illuminates the high potential of the low In-content AlInN-delta-GaN QW structure to achieve large dominant TE-polarized spontaneous emission rates and optical gains for high performance AlN-based UV devices.

Vertically conducting deep-ultraviolet light-emitting diodes with interband tunneling junction grown on 6H-SiC substrate

Vertically conducting deep-ultraviolet (DUV) light-emitting diodes (LEDs) with a polarization-induced backward-tunneling junction (PIBTJ) were grown by metal–organic chemical vapor deposition (MOCVD) on 6H-SiC substrates. A self-consistent solution of Poisson–Schrödinger equations combined with polarization-induced theory was applied to simulate the PIBTJ structure, energy band diagrams, and free-carrier concentration distribution. AlN and graded AlxGa1−xN interlayers were introduced between the PIBTJ and multiple quantum well layers to avoid cracking of the n-Al0.5Ga0.5N top layer. At a driving current of 20 mA, an intense DUV emission at ∼288 nm and a weak shoulder at ∼386 nm were observed from the AlGaN top layer side. This demonstrates that the PIBTJ can be used to fabricate vertically conducting DUV LED on SiC substrates.

Hexagonal boron nitride is an indirect bandgap semiconductor

Hexagonal boron nitride is a wide bandgap semiconductor with very high thermal and chemical stability that is used in devices operating under extreme conditions. The growth of high-purity crystals has recently revealed the potential of this material for deep ultraviolet emission, with intense emission around 215 nm. In the last few years, hexagonal boron nitride has been attracting even more attention with the emergence of two-dimensional atomic crystals and van der Waals heterostructures, initiated with the discovery of graphene. Despite this growing interest and a seemingly simple structure, the basic questions of the bandgap nature and value are still controversial. Here, we resolve this long-debated issue by demonstrating evidence for an indirect bandgap at 5.955 eV by means of optical spectroscopy. We demonstrate the existence of phonon-assisted optical transitions and we measure an exciton binding energy of about 130 meV by two-photon spectroscopy. Scientists resolve the long-debated issue of the nature and value of the bandgap in hexagonal boron nitride by providing evidence for an indirect bandgap at 5.955 eV and an exciton binding energy of about 130 meV by means of optical spectroscopy.

Intrinsic n- and p-Type MgZnO Nanorods for Deep-UV Detection and Room-Temperature Gas Sensing

MgZnO (MZO) family, like other ternary compounds, offering tunable energy bandgap covering from blue to deep UV emission has attracted attentions for new optoelectronics, where both n- and p-types are essential. Nevertheless, only limited success is reported for p-type only by extrinsic doing. In the work, intrinsic n- and p-type single-crystalline MZO nanorods (NRs) were synthesized using a low-cost multistep process via the solid-state diffusion of Mg into intrinsic n- and p-type ZnO NRs prepared using chemical bath deposition or chemical vapor deposition. Cathodoluminescence spectra demonstrate substantial blue-shifted ultraviolet (UV) emission from 377 to 357 nm for MZO NRs with the highest Mg content. Photosensing measurements of the MZO NR devices show high sensitivity to UVC compared to UVA with bias-dependent sensitivity. Room-temperature gas sensing using MZO NRs exhibits enhanced sensitivity compared to that for ZnO, with opposite responses in current variation, supporting different carrier type...

Localized surface plasmon-enhanced light emission using platinum nanorings in deep ultraviolet-emitting AlGaN quantum wells.

We report the enhancement of deep ultraviolet emissions from AlGaN-based quantum wells (QWs) using energy-matched localized surface plasmons (LSPs) in platinum (Pt) nanoring arrays. The peak resonances of the extinction spectra were shifted to the red spectral region as the nanoring diameters increased, and the Pt nanorings with a diameter of 325nm exhibited strong photoluminescence (PL) resonance at 279nm. The emission enhancement ratio was calculated to be 304% in peak PL intensity when compared to that of the bare AlGaN QWs; this is attributed to the strong coupling of QWs with LSPs from the Pt nanorings.

Effect of quantum well shape and width on deep ultraviolet emission in AlGaN nanowire LEDs

The effect of quantum well shape and width on the emission properties of nanowire ultraviolet LEDs is experimentally investigated. Nanowire LEDs are grown by plasma-assisted molecular beam epitaxy and their electroluminescence is compared. A two- to three-fold increase in optical output is observed from 5 nm wide parabolic quantum wells compared with square quantum wells. This increased output is attributed to the both increased electron-hole overlap in agreement with band edge modeling, and supports previous theoretical predictions of decreased Auger recombination due to softening of the potential well in parabolic quantum wells. It is experimentally observed that when the quantum well width is decreased to 2 nm, the parabolic quantum wells exhibit a twofold output improvement over the square wells at low injection. However, at high current injection, the parabolic and square quantum wells perform similarly. Peak emission wavelengths for the nanowire LEDs is pushed into the deep ultraviolet by decreasing the AlGaN quantum well width. The emission wavelength shifts from 315 to 290 nm when the width is decreased from nominally 5 to 0.5 nm.

Deep ultraviolet emission photoluminescence and high luminescece efficiency of ferric passivated graphene quantum dots: Strong negative inductive effect of Fe

It is necessary to broaden the finite band gap of the graphene quantum dots (GQDs) and prepare deep ultraviolet emission GQDs with high luminescence efficiency. In this study, the deep ultraviolet emission ferric passivated GQDs (Fe-GQDs) are successfully prepared. Due to the strong negative inductive effect of Fe, the band gap of the Fe-GQDs was broadened, and the photoluminescence emission wavelength is 275nm for an excitation at 224nm with a very high fluorescent quantum yield (φ=0.63). This is the shortest emission wavelength reported for solution-based inorganic and organic quantum dots. The high efficient ultraviolet emission, as well as the good stability and low in vitro cytotoxicity, may facilitate their applications to carbon-based light-emitting devices and lasers.

Recent progress and future prospects of AlGaN-based high-efficiency deep-ultraviolet light-emitting diodes

In this paper, recent advances in AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) are demonstrated. 220–350-nm-band DUV LEDs have been realized by developing crystal growth techniques for wide-bandgap AlN and AlGaN semiconductors. Significant increases in internal quantum efficiency (IQE) have been achieved for AlGaN DUV emissions by developing low-threading-dislocation-density (TDD) AlN buffer layers grown on sapphire substrates. The electron injection efficiency (EIE) of the LEDs was also significantly increased by introducing a multiquantum barrier (MQB). We also discuss light extraction efficiency (LEE), which is the most important parameter for achieving high-efficiency DUV LEDs. We succeeded in improving LEE by developing a transparent p-AlGaN contact layer. The maximum external quantum efficiency (EQE) obtained was 7% for a 279 nm DUV LED. EQE could be increased by up to several tens of percent through the improvement of LEE by utilizing transparent contact layers and photonic nanostructures in the near future.

Plasma induced deep ultraviolet emissions from MgZnO films

Cubic rocksalt structured Mg0.54Zn0.46O films have been prepared, and the films show a wide bandgap of 5.47eV. Obvious emission at around 276nm has been detected when the films are placed in the ambient of plasma, and the output power of the plasma induced deep ultraviolet emission can reach around 56μW, which is the highest value ever reported for ZnO-based DUV light-emitters. The deep ultraviolet emission is believed to come from the Mg0.54Zn0.46O films excited by the kinetic radicals of the plasma.

High quality semipolar (11¯02) AlGaN/AlN quantum wells with remarkably enhanced optical transition probabilities

Adjusting the growth conditions from those for c-plane growth realizes high-quality semipolar (11¯02) AlGaN/AlN quantum wells (QWs) with atomically smooth surfaces and abrupt interfaces on AlN substrates. Upon comparing the optical properties to those of c-plane QWs using time-integrated and time-resolved photoluminescence spectroscopy, the estimated internal electric field is much smaller in (11¯02) AlGaN/AlN QWs than in c-plane QWs. Thus, (11¯02) AlGaN/AlN QWs have narrower emission line widths and remarkably faster radiative recombination lifetimes, realizing highly efficient deep ultraviolet emissions.

Hexagonal boron nitride for deep ultraviolet photonic devices

This paper provides a brief overview on recent advances in tackling the doping and optical polarization issues involved in the development of high performance deep ultraviolet (DUV) light emitting devices. In particular, recent developments in the exploitation of a novel DUV emitter layer structure based on a hexagonal boron nitride (hBN) and AlGaN p–n junction and doping engineering to potentially overcome the intrinsic problem of low p-type conductivity (or low free hole concentration) in Al-rich AlGaN are summarized. By implementing the wide bandgap and highly conductive hBN p-type layer strategy in nitride DUV emitters, p-type conductivities and DUV transparency of the electron blocking layer and p-type contact layer will be dramatically increased. This will significantly improve the free hole injection and quantum efficiency, reduce the operating voltage and heat generation, and increase the device operating lifetime. The growth of undoped and Mg-doped p-type hBN via a metal organic chemical vapor deposition technique has been studied. Furthermore, p-hBN/n-AlGaN p–n junctions have been fabricated and characterized to demonstrate the feasibility and potential of p-hBN/n-AlGaN p–n heterostructure based DUV light emitting devices. Further improvements in material quality, p-type conductivity control and device processing procedures would enhance the properties of these p–n structures, which could ultimately pave the way towards the realization of high efficiency nitride DUV photonic devices.

Structure and optical investigation of faceted hexagonal aluminum nitride nanotube arrays

Arrays of single-crystalline aluminum nitride nanotubes (AlNNTs) were successfully synthesized at 780 °C by a facile CVD method without templates or catalysts. Faceted hexagonal AlNNTs with diameters from tens to hundreds of nanometers were observed. 264 nm deep-ultraviolet emission associated with Al vacancies was detected by photoluminescence. Raman characterization indicates that the lattice of the AlNNTs is notably defective, and disorder-activated silent B1 (low) and B1 (high) were detected at 583.0 and 730.0 cm−1 respectively, which were allowed by the breakdown of the translational crystal symmetry. The redshift of E2 (high) Raman modes indicates that biaxial tensile stress exists in the samples.

Seven-photon ultraviolet upconversion emission of Er3+ induced by 1,540-nm laser excitation

The multi-photon ultraviolet upconversion emission properties and synergistic effect are investigated in BaSr2Y6O12:Er3+ phosphor. The deep-ultraviolet emissions centered at 274, 297 and 324-nm are observed under the 1,540-nm excitation, which results from a seven-, six- and six-photon upconversion process, respectively. A synergistic effect is found, which shows that the red emission intensity under 351- and 1,540-nm dual excitation is 4.7 % time stronger than the sum of red emission intensities under the 351 and 1,540-nm single excitation. This phenomenon is attributed to the 4I13/2 and 4I11/2 levels of Er3+ from non-radiative transition process under the 351-nm excitation are excited again to 4F9/2 level by absorbing 1,540-nm photon in the 351- and 1,540-nm dual-excitation process.

Ultraviolet–green photoluminescence of β-Ga2O3 films deposited on MgAl6O10 (1 0 0) substrate

β-Ga2O3 films were grown on double-side polished MgAl6O10 (100) substrate by metal organic chemical vapor deposition (MOCVD) at 600, 650 and 700°C. The refractive index dispersive behaviors of Ga2O3 films have the typical shape of the normal dispersion curve. Photoluminescence (PL) spectra measured at room temperature revealed that all the films exhibited intense ultraviolet (UV)–green emission from 300 to 650nm. A minor deep UV emission around 275nm (∼4.51eV) was observed for the sample prepared at 700°C. The intensity of the emission increased markedly when measured at low temperature. The corresponding PL mechanisms were discussed in detail and a schematic diagram was proposed.

Deep Ultra-Violet Emission from GaN/AlN matrix grown by Plasma-Assisted Molecular Beam Epitaxy

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

A 2-inch, large-size deep ultraviolet light-emitting device using dynamically controlled micro-plasma-excited AlGaN

A dynamically controlled micro-plasma-excited (MIPE) aluminum gallium nitride deep ultraviolet (DUV) light-emitting device is demonstrated. This device provides high-power DUV emission at any desired wavelength and allows enlargement of emission areas like plasma display panels for easy, low-cost fabrication. Neither p-n junctions nor electrode contacts are required for device fabrication. We fabricated 2-in. diameter wafer-size MIPE emitters of DUV light at specific wavelengths from AlGaN quantum wells with 50 mW average output power. We can also fabricate 6-in. diameter DUV emitters using sapphire wafers and 1 m × 5 m panel-type DUV emitters.

Automated method for analysis of tryptophan and tyrosine metabolites using capillary electrophoresis with native fluorescence detection

Capillary electrophoresis (CE) with laser-induced native fluorescence (LINF) detection offers the ability to characterize low levels of selected analyte classes, depending on the excitation and emission wavelengths used. Here a new automated CE-LINF system that provides deep ultraviolet (DUV) excitation (224 nm) and variable emission wavelength detection was evaluated for the analysis of small molecule tryptophan- and tyrosine-related metabolites. The optimized instrument design includes several features that increase throughput, lower instrument cost and maintenance, and decrease complexity when compared with earlier systems using DUV excitation. Sensitivity is enhanced by using an ellipsoid detection cell to increase the fluorescence collection efficiency. The limits of detection ranged from 4 to 30 nmol/L for serotonin and tyrosine, respectively. The system demonstrated excellent linearity over several orders of magnitude of concentration and intraday precision from 1-11 % relative standard deviation (RSD). The instrument's performance was validated via tryptophan and serotonin characterization using tissue extracts from the mammalian brain stem, with RSDs of less than 10 % for both metabolites. The flexibility and sensitivity offered by DUV laser excitation and tunable emission enables a broad range of small-volume measurements.

A facile chemical exfoliation method to obtain large size boron nitride nanosheets

Owning many peculiar properties, hexagonal boron nitride nanosheets (BNNSs) have lots of potential applications, such as electronic devices and deep ultraviolet emitters. In this article, a chemical exfoliation method to prepare few-layer and large size BNNSs is reported. Through related instrument characterizations, we demonstrated that this preparation method can allow the exfoliation of BNNSs from bulk BN powder successfully. From CL spectra, the as-prepared BNNSs were proved to show stronger CL emission ability than BN powder. Based on the experiment results analysis, we proposed an exfoliation mechanism and verified it through in situ SEM detection.

Handheld deep ultraviolet emission device based on aluminum nitride quantum wells and graphene nanoneedle field emitters

We report the successful fabrication of a compact deep ultraviolet emission device via a marriage of AlGaN quantum wells and graphene nanoneedle field electron emitters. The device demonstrated a 20-mW deep ultraviolet output power and an approximately 4% power efficiency. The performance of this device may lead toward the realization of an environmentally friendly, convenient and practical deep ultraviolet light source.