Deep UV Photons Research Articles

Plasmonic Silver Nanoparticles Facilitate Electron Emission from Diamond upon Sun‐Like Excitation

AbstractThe development of a stable, non‐toxic material that emits electrons following absorption of visible light may have a major impact on the solar photocatalysis of difficult reactions such as CO2 and N2 reduction, as well as for targeted chemical transformations in general. Diamond is a good candidate, however it is a wide bandgap material requiring deep UV photons ( <227 nm) to promote electrons from the valence band into the conduction band. Embedding silver nanoparticles under the diamond surface allows the photoconductivity of the diamond in the spectral region of the surface plasmon resonance to be increased, while also leading to an enhancement of visible light photoemission. Considering the low intensity of the light sources used in this work and the spectral properties of the enhanced photoconductivity and photoemission a mechanism based on plasmonically enhanced photoconductivity which in turn allows surface states emptied by photoemission to be recharged thus leading to enhanced photoemission in the visible range is proposed.

Polarization-induced 2D electron gases in N-polar AlGaN/AlN heterostructures on single-crystal AlN substrates

Polarization-induced carriers play an important role in achieving high electrical conductivity in ultrawide bandgap semiconductor AlGaN, which is essential for various applications ranging from radio frequency and power electronics to deep UV photonics. Despite significant scientific and technological interest, studies on polarization-induced carriers in N-polar AlGaN are rare. We report the observation and properties of polarization-induced two-dimensional electron gases (2DEGs) in N-polar AlGaN/AlN heterostructures on single-crystal AlN substrates by systematically varying the Al content in the 8 nm top layers from x = 0 to x = 0.6, spanning energy bandgaps from 3.56 to 4.77 eV. The 2DEG density drops monotonically with increasing Al content, from 3.8 × 1013/cm2 in the GaN channel, down to no measurable conductivity for x = 0.6. Alloy scattering limits the 2DEG mobility to below 50 cm2/V s for x = 0.49. These results provide valuable insights for designing N-polar AlGaN channel high electron mobility transistors on AlN for extreme electronics at high voltages and high temperatures, and for UV photonic devices.

Boosted Spontaneous Formation of High-Aspect Ratio Nanopeaks on Ultrafast Laser-Irradiated Ni Surface.

The capacity to synthesize and design highly intricated nanoscale objects of different sizes, surfaces, and shapes dramatically conditions the development of multifunctional nanomaterials. Ultrafast laser technology holds great promise as a contactless process able to rationally and rapidly manufacture complex nanostructures bringing innovative surface functions. The most critical challenge in controlling the growth of laser‐induced structures below the light diffraction limit is the absence of external order associated to the inherent local interaction due to the self‐organizing nature of the phenomenon. Here high aspect‐ratio nanopatterns driven by near‐field surface coupling and architectured by timely‐controlled polarization pulse shaping are reported. Electromagnetic coupled with hydrodynamic simulations reveal why this unique optical manipulation allows peaks generation by inhomogeneous local absorption sustained by nanoscale convection. The obtained high aspect‐ratio surface nanotopography is expected to prevent bacterial proliferation, and have great potential for catalysis, vacuum to deep UV photonics and sensing.

High-efficiency AlGaN/GaN/AlGaN tunnel junction ultraviolet light-emitting diodes

AlGaN is the material of choice for high-efficiency deep UV light sources, which is the only alternative technology to replace mercury lamps for water purification and disinfection. At present, however, AlGaN-based mid- and deep UV LEDs exhibit very low efficiency. Here, we report a detailed investigation of the epitaxy and characterization of LEDs utilizing an AlGaN/GaN/AlGaN tunnel junction structure, operating at ∼ 265 nm , which have the potential to break the efficiency bottleneck of deep UV photonics. A thin GaN layer was incorporated between p + and n + -AlGaN to reduce the tunneling barrier. By optimizing the thickness of the GaN layer and thickness of the top n -AlGaN contact layer, we demonstrate AlGaN deep UV LEDs with a maximum external quantum efficiency of 11% and wall-plug efficiency of 7.6% for direct on-wafer measurement. It is also observed that the devices exhibit severe efficiency droop under low current densities, which is explained by the low hole mobility, due to the hole hopping conduction in the Mg impurity band and the resulting electron overflow.

Universal direct patterning of colloidal quantum dots by (extreme) ultraviolet and electron beam lithography.

Colloidal quantum dots have found many applications and patterning them on micro- and nanoscale would open a new dimension of tunability for the creation of smaller scale (flexible) electronics or nanophotonic structures. Here we present a simple, general, one-step top-down patterning technique for colloidal quantum dots by means of direct optical or electron beam lithography. We find that both photons and electrons can induce a solubility switch of both PbS and CdSe quantum dot films. The solubility switch can be ascribed to cross-linking of the organic ligands, which we observe from exposure with deep-UV photons (5.5 eV) to extreme-UV photons (91.9 eV), and low-energy (3-70 eV) as well as highly energetic electrons (50 keV). The required doses for patterning are relatively low and feature sizes can be as small as tens of nanometers. The luminescence properties as well as carrier lifetimes remain similar after patterning.

Direct Wavelength-Selective Optical and Electron-Beam Lithography of Functional Inorganic Nanomaterials.

Direct optical lithography of functional inorganic nanomaterials (DOLFIN) is a photoresist-free method for high-resolution patterning of inorganic nanocrystals (NCs) that has been demonstrated using deep UV (DUV, 254 nm) photons. Here, we expand the versatility of DOLFIN by designing a series of photochemically active NC surface ligands for direct patterning using various photon energies including DUV, near-UV (i-line, 365 nm), blue (h-line, 405 nm), and visible (450 nm) light. We show that the exposure dose for DOLFIN can be ∼30 mJ/cm2, which is small compared to most commercial photopolymer resists. Patterned nanomaterials can serve as highly robust optical diffraction gratings. We also introduce a general approach for resist-free direct electron-beam lithography of functional inorganic nanomaterials (DELFIN) which enables all-inorganic NC patterns with feature size down to 30 nm, while preserving the optical and electronic properties of patterned NCs. The designed ligand chemistries and patterning techniques offer a versatile platform for nano- and micron-scale additive manufacturing, complementing the existing toolbox for device fabrication.

Effective decrease of photoelectric emission threshold from gold plated surfaces.

Many applications require charge neutralization of isolated test bodies, and this has been successfully done using photoelectric emission from surfaces which are electrically benign (gold) or superconducting (niobium). Gold surfaces nominally have a high work function (∼5.1 eV) which should require deep UV photons for photoemission. In practice, it has been found that it can be achieved with somewhat lower energy photons with indicative work functions of (4.1-4.3 eV). A detailed working understanding of the process is lacking, and this work reports on a study of the photoelectric emission properties of 4.6 × 4.6 cm2 gold plated surfaces, representative of those used in typical satellite applications with a film thickness of 800 nm, and measured surface roughnesses between 7 and 340 nm. Various UV sources with photon energies from 4.8 to 6.2 eV and power outputs from 1 nW to 1000 nW illuminated ∼0.3 cm2 of the central surface region at angles of incidence from 0° to 60°. Final extrinsic quantum yields in the range 10 ppm-44 ppm were reliably obtained during 8 campaigns, covering a period of ∼3 years but with intermediate long-term variations lasting several weeks and, in some cases, bake-out procedures at up to 200 °C. Experimental results were obtained in a vacuum system with a baseline pressure of ∼10-7 mbar at room temperature. A working model, designed to allow accurate simulation of any experimental configuration, is proposed.

Growth of hexagonal boron nitride on (111) Si for deep UV photonics and thermal neutron detection

Hexagonal boron nitride (hBN) growth was carried out on (111) Si substrates at a temperature of 1350 °C using a cold wall chemical vapor deposition system. The hBN phase of the deposited films was identified by the characteristic Raman peak at 1370 cm−1 with a full width at half maximum of 25 cm−1, corresponding to the in-plane stretch of B and N atoms. Chemical bonding states and composition of the hBN films were analyzed by X-ray photoelectron spectroscopy; the extracted B/N ratio was 1.03:1, which is 1:1 within the experimental error. The fabricated metal-hBN-metal devices demonstrate a strong deep UV (DUV) response. Further, the hBN growth on the vertical (111) surfaces of parallel trenches fabricated in (110) Si was explored to achieve a thermal neutron detector. These results demonstrate that hBN-based detectors represent a promising approach towards the development of DUV photodetectors and efficient solid-state thermal neutron detectors.

Luminescence of unfused 95%SiO2–5%GeO2 amorphous films with fluorine additive: No evidence for presence of GeODC(I) defects found

Photoluminescence (PL) of unfused amorphous germanosilicate films with fluorine additive is studied in 2–8.5eV spectral range. Experiments are based on films deposited on silica substrates by means of the surface-plasma chemical vapor deposition (SPCVD). Films of about 100μm in thickness with “high F” (~4.2wt.%) and “low F” (~0.5wt.%) fluorine content have been fabricated for the experiments. KrF (248nm), ArF (193nm) and F2 (157nm) excimer lasers are used to pump PL. It is found that absorption and luminescence associated with germanium oxygen deficient centers (GeODCs) in “high F” and “low F” films differ. In the “high F” unfused film absorption coefficient of the band at 5eV as well as intensity of the blue PL band at 3.1eV are significantly greater. This film proves features of the so called GeODCs(II), which symbolize twofold coordinated germanium defects in silica network. In the “low F” unfused film absorption band at 5eV is feebly marked. Poorly resolved PL intrinsic to GeODCs(II) can be detected in this film under the KrF laser pump. The most significant PL features are revealed under deeper UV pump by ArF and F2 lasers. Spectral positions of PL bands excited by these lasers correspond to GeODCs(II). However PL decay kinetics dramatically differs from that one intrinsic to GeODCs(II). Noticeable growth of PL intensity caused by permanent (half an hour and more) exposure to ArF and/or F2 lasers takes place, indicating GeODC(II) formation. It is found that considerable body of fluorine additive has the same effect as profusion for “low F” and/or fluorine free germanosilicate amorphous material synthesized by SPCVD. In the “high F” film yield of GeODC(II) PL pumped by the F2 laser remains high. This speaks for the suppression of the competitive 7.6eV absorption band associated with SiODCs(I) by fluorine additive indicating a decrease in the content of this type of defects in the material. High yield of GeODC(II) luminescence pumped by deep UV photons as well as hypothetical similarity of SiODC(I) and GeODC(I) permit one to conclude that GeODCs(II) are the only defects dominating in the materials under study.

The bright side of defects: Chemistry and physics of persistent and storage phosphors

Ceramic specimens of singly doped Lu2O3:Tb, and doubly activated Lu2O3:Tb,Ca, Lu2O3:Tb,Hf and Lu2O3:Pr,Hf were prepared and their thermoluminescent properties were investigated. After irradiation with X-rays or deep-UV photons the singly doped Lu2O3:Tb was found to produce a glow curve with a few thermoluminescent bands spread across the temperature range of 50–450°C. (Tb,Ca) co-doped material produced basically one strong thermoluminescent band peaking at around 120°C and thus was able to produce persistent luminescence lasting about 10h, as visible to the human eye. In the (Tb,Hf) co-doped ceramics the thermoluminescence at around 100°C was seen as a vestige only and was responsible for persistent emission lasting merely a few minutes. Instead, a strong thermoluminescence glow band at around 250°C appeared. Its intensity faded by about 10% after 6hours in the dark. Lu2O3:Pr,Hf co-doped ceramics also showed intense thermoluminescence with the main components at around 200°C and 270°C. The latter was found not to fade in time, while intensity of the former dropped by about 7–8% after 6h. All these effects were observed in ceramics processed at temperatures as high as 1700°C. It is proposed that high temperature of processing is necessary as only then clustering of individual defects occurs and the energy released upon heating can diffuse towards the nearby located emitting center efficiently.

Deep ultraviolet photoluminescence studies of AlN photonic crystals

Two-dimensional AlN photonic crystals (PCs) with varying periodicity/diameter down to 150∕75nm were fabricated. Deep ultraviolet photoluminescence spectroscopy has been employed to study the optical properties of AlN PCs. With PC formation, a 20-fold enhancement in the band edge emission intensity at 208nm over unpatterned AlN epilayer has been observed. The emission intensity increases with decreasing lattice constant of AlN PCs. However, the spectral peak energy decreases with decreasing lattice constant, indicating a possible release of compressive stresses as a result of PC formation. Successful fabrication of AlN PCs opens up new opportunities for exploring novel physical phenomena in the artificially structured photonic band gap material system and their applications, particularly in the area of deep UV photonics.

DUV photons to probe the photoconductive properties of diamond films in the surface proximity

We suggest the use of deep UV photons as a probe to investigate the photoelectrical properties of different depths in the diamond films. The penetration depth of UV photons decreases rapidly towards shorter wavelengths in the region of and just below the band gap energy, achieving ∼2 nm at wavelengths approximately 100 nm. Therefore, they are suitable for probing the bulk properties and the surface proximity of diamond layers. We have irradiated different CVD diamond samples with UV laser pulses at 193, 213 and 266 nm as well as continuous radiation emitted by a UV source in the 140–250 nm wavelength range. We have analysed the photoconductivity at different depths as a function of material thickness, electric field value and polarity, radiation intensity, time response to laser pulses and to illumination transients. We have observed different behaviours above and below the band gap energy that could be due to polarization effects and a photo-response quenching above the band gap that is enhanced by reversing the electric field.

Leakage currents induced in ultrathin oxides on (100)Si by deep-UV photons

Degradation of ultrathin (dox=3–6 nm) oxides on (100)Si under irradiation with deep-UV (hν=10 eV) photons was studied in order to evaluate the possible oxide damage by deep-UV lithography. The irradiation was found to lead to a significant enhancement in electrical conduction of the ultrathin oxides. The leakage current observed after irradiation correlates with the concentration of radiolytic atomic hydrogen in the oxide and includes two components. A first transient one is related to thermally unstable defects similar to the H-related donor interface states observed after irradiation of thicker oxides. The second, stable component correlates with the enhanced density of Si dangling bond defects in the oxide (E′ center) which points to the H-assisted rupture of SiO bonds. It is also found that the leakage currents observed after electrical stress and after deep-UV irradiation exhibit similar annealing behavior. The two revealed sources of the degradation-induced leakage current may account for the enhanced electron transport across ultrathin Si oxides under various circumstances.