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Gallium nitride (GaN) is a wide-bandgap semiconductor that is useful for optoelectronics and high speed and high power electronics. Fabrication of GaN devices requires etching for many processing steps. Gas phase thermal atomic-layer-controlled etching is desirable for damage-free isotropic etching. In this letter, the thermal atomic layer etching (ALE) of crystalline GaN was demonstrated using sequential exposures of XeF2 and BCl3. GaN ALE was achieved with an etch rate of 0.55 Å/cycle at 195 °C using XeF2 exposures for 20 s at 40 mTorr and BCl3 exposures for 0.5 s at 50 mTorr. At the same reactant exposures, GaN etch rates varied with temperature from 0.18 Å/cycle at 170 °C to 0.72 Å/cycle at 300 °C. The GaN etch rates increased slowly with increasing XeF2 exposure. In addition, the GaN etch rate was self-limiting with respect to both increasing BCl3 pressures and BCl3 exposure times. This self-limiting behavior for BCl3 is consistent with a ligand-exchange mechanism for GaN ALE. Alternative fluorination reactants were also investigated including HF, SF4, and NF3 plasma. Sequential exposures of NF3 plasma and BCl3 yielded GaN etch rates of 2.5–2.9 Å/cycle at 250 °C. In contrast, the HF and SF4 fluorination reactants could not etch crystalline GaN. [ABSTRACT FROM AUTHOR]

Codoping of gallium nitride for improved acceptor ionization has long been theorized; however, reduction to practice proves difficult via growth. Herein, implementation of codoping via ion implantation and symmetric multicycle rapid thermal annealing utilizing magnesium codoped with silicon or oxygen is demonstrated. Results show enhanced photoluminescence with both donor species but with an order of magnitude greater increase with concurrent p‐type hall for codoping with oxygen. Furthermore, the addition of nitrogen to balance stoichiometry suppresses defect photoluminescence signals. The incorporation of the donor and nitrogen demonstrates defect reduction beyond magnesium, only implants despite the additional implant dose and resultant damage with coimplantation. The enhanced hole concentrations evident with oxygen incorporation reveal important considerations for device design given unintentional doping during growth and future incorporation of ion implantation capabilities. [ABSTRACT FROM AUTHOR]

In preparing periodically-oriented gallium nitride (PO GaN) samples for optical applications, smooth surfaces are essential. Although post-growth processing of these surfaces is possible, it is challenged by significantly different reactivity of the two polar surfaces. To this end, it would be highly desirable to grow PO GaN structures with smooth surfaces. In this paper, the influence of growth parameters on the morphology of PO GaN structures is investigated. The three primary parameters investigated include the V/III ratio of the gas phase precursors, the nature of the carrier gas, and the geometry of the structure. The V/III ratio was varied from 1100 to 5500. Substitution of N2 for the standard H2 carrier gas is investigated. Finally, the effects of changing the structure periodicity from 2 to 100 mm are studied. Smoother structures (resulting from more equal growth rates of the two polarities) are achieved with increases in V/III ratio, substitution of H2 with N2, and by reducing the stripe width. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

The low thermal conductivity of β-Ga2O3 is a significant concern for maximizing the potential of this ultra-wide bandgap semiconductor as a power switching device technology. Here, we report on the use of nanocrystalline diamond (NCD) deposited via microwave plasma enhanced chemical vapor deposition (MP-CVD) as a top-side, device-level thermal management solution on a lateral β-Ga2O3 transistor. NCD was grown via MP-CVD on β-(AlxGa1−x)2O3/β-Ga2O3 heterostructures prior to the gate formation of the field-effect transistor. A reduced growth temperature of 400 °C and a SiNx barrier layer were used to protect the oxide semiconductors from etching in the MP-CVD H2 plasma environment. Raman spectroscopy showed a highly sp3-bonded NCD film was obtained at 400 °C, with grain size of about 50–100 nm imaged via atomic force microscopy. The incorporation of the NCD heat-spreading layer resulted in a β-(AlxGa1−x)2O3/β-Ga2O3 heterostructure field-effect transistor showing a decrease in the total thermal resistance at the gate by 42%. The fabrication process, including the NCD etch in the gate region, will need to be improved to minimize the impact of these processes on important device characteristics (i.e., drain current, threshold voltage, and leakage current). [ABSTRACT FROM AUTHOR]

Ion implantation of magnesium for p‐type GaN presents many opportunities; however, activation has proven difficult due to the decomposition of GaN at relevant annealing temperatures. Herein, testing the efficacy of multiple in situ and ex situ caps based on aluminum nitride and silicon nitride for GaN protection during annealing is presented. Photoluminescence shows better activation for in situ metal organic chemical vapor deposition (MOCVD)‐grown aluminum nitride caps compared with ex situ sputtered aluminum nitride and the best performance by ex situ plasma‐enhanced chemical vapor deposition (PECVD) silicon nitride. Furthermore, only samples annealed at the highest temperatures tested show preferential growth of UV luminescence to yellow‐green luminescence reinforcing the need for better capping solutions and higher temperature annealing. [ABSTRACT FROM AUTHOR]

• Deep-ultraviolet (DUV) laser source for laboratory-based photoemission microscopy. • Intense, narrowband, and tunable output from 192 nm to 210 nm (6.5 eV to 5.9 eV). • Characterize space charge effects at 80 MHz repetition rate in PEEM measurements. • High DUV flux allows high resolution studies of wide bandgap semiconductors. Photoemission electron microscopy (PEEM) is a unique and powerful tool for studying the electronic properties of materials and surfaces. However, it requires intense and well-controlled light sources with photon energies ranging from the UV to soft X-rays for achieving high spatial resolution and image contrast. Traditionally, many PEEMs were installed at synchrotron light sources to access intense and tunable soft X-rays. More recently, the maturation of solid-state lasers has opened a new avenue for laboratory-based PEEMs using laser-based UV light at lower photon energies. Here, we report on the characteristics of a laser-based UV light source that was recently integrated with a PEEM instrument. The system consists of a high repetition rate, tunable wavelength laser coupled to a harmonics generation module, which generates deep-UV radiation from 192 nm to 210 nm. We comment on the spectral characteristics and overall laser system stability, as well as on the effects of space charge within the PEEM microscope at high UV laser fluxes. Further, we show an example of imaging on gallium nitride, where the higher UV photon energy and flux of the laser provides considerably improved image quality, compared to a conventional light source. These results demonstrate the capabilities of laser-based UV light sources for advancing laboratory-based PEEMs. [ABSTRACT FROM AUTHOR]

The effect of heteroepitaxial β-(AlxGa1−x)2O3 film thickness and Al content on surface morphology was characterized to experimentally determine the critical thickness limitations of the (010) β-(AlxGa1−x)2O3/Ga2O3 heterostructure. High-resolution x-ray diffraction was used to assess the strain state of the films; reciprocal space mapping (RSM) revealed that even cracked films were still fully strained. In cracked films, diffuse scattering was observed in RSMs, indicating lattice tilting. Cracking of the films was investigated using atomic force microscopy (AFM), x-ray topography (XRT), bright-field scanning transmission electron microscopy (BF-STEM), and high-resolution transmission electron microscopy. Using both AFM and XRT, the [001] direction was observed to be the most prevalent crack direction; however, cracks were also observed in the [100] direction. In uncracked regions of the films, XRT revealed the alignment of threading dislocations along the [001] direction. Cross-sectional imaging of the crack geometry and propagation was performed using BF-STEM, and it was observed that the cracks in the [001] direction extended through the thickness of the β-(AlxGa1−x)2O3 film (∼205 nm) and a further ∼100–200 nm into the β-Ga2O3 substrate. Experimental data for critical film thickness showed good agreement with previous theoretical calculations based on the Griffith criterion for crack propagation. [ABSTRACT FROM AUTHOR]

We report the growth and characterization of III-nitride ternary thin films (Al x Ga 1 − x N, In x Al 1 − x N and In x Ga 1 − x N) at ≤ 500 °C by plasma assisted atomic layer epitaxy (PA-ALE) over a wide stoichiometric range including the range where phase separation has been an issue for films grown by molecular beam epitaxy and metal organic chemical vapor deposition. The composition of these ternaries was intentionally varied through alterations in the cycle ratios of the III-nitride binary layers (AlN, GaN, and InN). By this digital alloy growth method, we are able to grow III-nitride ternaries by PA-ALE over nearly the entire stoichiometry range including in the spinodal decomposition region (x = 15–85%). These early efforts suggest great promise of PA-ALE at low temperatures for addressing miscibility gap challenges encountered with conventional growth methods and realizing high performance optoelectronic and electronic devices involving ternary/binary heterojunctions, which are not currently possible. [ABSTRACT FROM AUTHOR]

An all-epitaxial approach was demonstrated to create coaxial plasmon laser structures composed of an aluminum plasmonic metal/SiNx dielectric/InGaN quantum well shell surrounding a p-GaN nanowire core. Strong UV luminescence was observed from as-grown vertically-aligned arrays as well as horizontally-aligned nanowires transferred to a transparent carrier wafer. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Measuring the thermal conductivity of sub-surface buried substrates is of significant practical interests. However, this remains challenging with traditional pump–probe spectroscopies due to their limited thermal penetration depths. Here, we experimentally and numerically investigate the TPD of the recently developed optical pump–probe technique steady-state thermoreflectance (SSTR) and explore its capability for measuring the thermal properties of buried substrates. The conventional definition of the TPD (i.e., the depth at which temperature drops to 1/e value of the maximum surface temperature) does not truly represent the upper limit of how far beneath the surface SSTR can probe. For estimating the uncertainty of SSTR measurements of a buried substrate a priori, sensitivity calculations provide the best means. Thus, detailed sensitivity calculations are provided to guide future measurements. Due to the steady-state nature of SSTR, it can measure the thermal conductivity of buried substrates that are traditionally challenging by transient pump–probe techniques, exemplified by measuring three control samples. We also discuss the required criteria for SSTR to isolate the thermal properties of a buried film. Our study establishes SSTR as a suitable technique for thermal characterizations of sub-surface buried substrates in typical device geometries. [ABSTRACT FROM AUTHOR]

The ultra-wide bandgap semiconductor gallium oxide (Ga2O3) offers substantial promise to significantly advance power electronic devices as a result of its high breakdown electric field and maturing substrate technology. A key remaining challenge is the ability to grow electronic-grade epitaxial layers at rates consistent with 20–40 μm thick drift regions needed for 20 kV and above technologies. This work reports on extensive characterization of epitaxial layers grown in a novel metalorganic chemical vapor deposition tool that permits growth rates of 1.0–4.0 μm h−1. Specifically, optical, structural and electrical properties of epilayers grown at ∼1 μm h−1 are reported, including employment in an operating MOSFET. The films demonstrate relatively smooth surfaces with a high degree of structural order, limited point defectivity (Nd − Na ≈ 5 × 1015 cm−3) and an optical bandgap of 4.50 eV. Further, when employed in a MOSFET test structure with an n+ doped channel, a record high mobility for a transistor structure with a doped channel of 170 cm2 V−1 s−1 was measured via the Hall technique at room temperature. This work reports for the first time a β-Ga2O3 MOSFET grown using Agnitron Technology's high growth rate MOCVD homoepitaxial process. These results clearly establish a significant improvement in epilayer quality at growth rates that can support future high voltage power device technologies. [ABSTRACT FROM AUTHOR]

The first step of a multi-cycle rapid thermal annealing process was systematically studied. The surface, structure, and optical properties of Mg implanted GaN thin films annealed at temperatures ranging from 900 to 1200 °C were investigated by Raman spectroscopy, photoluminescence, UV-visible spectroscopy, atomic force microscopy, and Nomarski microscopy. The GaN thin films are capped with two layers of in-situ metal organic chemical vapor deposition -grown AlN and annealed in 24 bar of N2 overpressure to avoid GaN decomposition. The crystal quality of the GaN improves with increasing annealing temperature as confirmed by UV-visible spectroscopy and the full widths at half maximums of the E2 and A1 (LO) Raman modes. The crystal quality of films annealed above 1100 °C exceeds the quality of the as-grown films. At 1200 °C, Mg is optically activated, which is determined by photoluminescence measurements. However, at 1200 °C, the GaN begins to decompose as evidenced by pit formation on the surface of the samples. Therefore, it was determined that the optimal temperature for the first step in a multi-cycle rapid thermal anneal process should be conducted at 1150 °C due to crystal quality and surface morphology considerations. [ABSTRACT FROM AUTHOR]

This paper reports pH-selective potentiometric responses of epitaxial nitrogen-polar (0001) GaN films on sapphire substrates. The potential of a nitrogen-polar GaN substrate increased with decreasing solution pH from 10 to 2 in a Nernstian manner, whereas it did not significantly change upon increasing the concentrations of the cationic and anionic interfering species. In particular, the negligible responses to anions marked a sharp contrast with previously reported results on gallium-polar GaN that exhibited Nernstian responses to anions. The potentiometric response to pH probably originates from the adsorption of H+ onto oxide-coated nitrogen-polar GaN substrates having polarization-induced negative surface charge. [ABSTRACT FROM AUTHOR]

The evolution of wide bandgap semiconductor materials has led to dramatic improvements for electronic applications at high powers and temperatures. However, the propensity of extended defects provides significant challenges for implementing these materials in commercial electronic and optical applications. While a range of spectroscopic and microscopic tools have been developed for identifying and characterizing these defects, such techniques typically offer either technique exclusively, and/or may be destructive. Scattering‐type scanning near‐field optical microscopy (s‐SNOM) is a nondestructive method capable of simultaneously collecting topographic and spectroscopic information with frequency‐independent nanoscale spatial precision (≈20 nm). Here, how extended defects within 4H‐SiC manifest in the infrared phonon response using s‐SNOM is investigated and the response with UV‐photoluminescence, secondary electron and electron channeling contrast imaging, and transmission electron microscopy is correlated. The s‐SNOM technique identifies evidence of step‐bunching, recombination‐induced stacking faults, and threading screw dislocations, and demonstrates interaction of surface phonon polaritons with extended defects. The results demonstrate that phonon‐enhanced infrared nanospectroscopy and spatial mapping via s‐SNOM provide a complementary, nondestructive technique offering significant insights into extended defects within emerging semiconductor materials and devices and thus serves as an important diagnostic tool to help advance material growth efforts for electronic, photonic, phononic, and quantum optical applications. [ABSTRACT FROM AUTHOR]

We report selective area n-type doping using ion implantation of Si into semi-insulating, C-doped GaN samples activated using both conventional rapid thermal annealing (RTA) and 30 atm N2 overpressure annealing. Implanted regions were tested for Si activation using Circular Transmission Line Measurements (CTLM), while linear and circular photoconductive switches (PCSS) in the unimplanted regions were used as a test vehicle to separate implanted Si dopant activation from leakage paths generated by N vacancy formation due to damage and decomposition during annealing. We observed that at an optimal temperature around 1060 °C, a low contact resistivity of $1\times 10^{-6}\,\,\Omega $ -cm2 was obtained while preserving the breakdown of the unimplanted regions. [ABSTRACT FROM AUTHOR]

The controlled and reproducible growth of hexagonal boron nitride (h-BN) by chemical vapor deposition on polycrystalline copper foil substrates remains a challenge as typical growth surfaces contain microscopic ridges (height ≈ 100 μm) arising from the foil manufacturing process. In this work, we report a method to prepare commercially cold-rolled polycrystalline copper substrates for greatly improved growth of h-BN by a combination of thermal annealing in a reducing environment and electrochemical polishing to create an excellent surface that enables control of BN nucleation sites. We report a root mean square roughness of 1.2 nm for the Cu substrate after electropolishing and a reduction of nucleation sites along with enlargement of h-BN crystals with this combined approach. We also assess the potential role of surface features that exist on the Cu surface as nucleation sites. The development of an electrochemical process to prepare two-dimensional (2D) material growth substrates and demonstration of greatly improved growth of 2D materials directly point to more pragmatic large scale processing of 2D materials since such techniques are already utilized in large scale industrial processing. [ABSTRACT FROM AUTHOR]

The activation of ion implanted p-type dopants in GaN is notoriously difficult as the extremely high temperatures required to activate implanted Mg also damage the GaN crystal. In this paper, we present refinements to our novel annealing process (symmetric multicycle rapid thermal annealing) to reduce surface damage and contamination responsible for elevated leakage currents and non-ideal diode behavior. Furthermore, we apply the technique to Mg-implanted bulk GaN substrates to enable vertical power device structures, demonstrating rectifying p-i-n junctions. In addition, the technique was applied for edge termination in both p-i-n and Schottky barrier diodes, realizing floating guard ring and junction termination extension structures. The processes demonstrated here represents a key enabling step for future GaN-based power devices. [ABSTRACT FROM AUTHOR]

Top-side integration of nanocrystalline diamond films in the fabrication sequence of AlGaN/GaN high electron mobility transistors is demonstrated. Reliable oxygen plasma etching of the diamond capping layer, required for a diamond-before-gate process, was implemented by using a sacrificial SiN 'dummy' gate. Hall characterization showed minimal (∼6%) reduction in sheet carrier density and commensurate increase in sheet resistance, while maintaining mobility and on-state drain current density. Off-state drain current and threshold voltage were increased, likely by fluorination of the AlGaN surface after removal of the sacrificial gate, even though a 20 nm thick Al2O3 layer was used as a SF6-plasma etch stop. Pulsed IDS and on-resistance were improved, indicating that a 10 nm SiN/500 nm NCD could offer improved AlGaN surface passivation compared to a more conventional 100 nm thick PECVD SiN film. [ABSTRACT FROM AUTHOR]

A dry process for selective etching of nanocrystalline diamond thin films has been developed as an alternative to plasma etching. This process relies on subjecting masked diamond films to a controlled oxygen atmosphere at temperatures of 700–800 °C to controllably etch both vertically through the film and laterally underneath the mask. SiO 2 , SiN x , and Al 2 O 3 films constitute viable mask materials for this process, provided that the underlying diamond film is fully outgassed before mask deposition and diamond etching. As expected, etching occurred more rapidly at higher temperatures. The etch rate was higher in the lateral direction than the vertical direction, which has been attributed to accelerated etching along disordered grain boundaries and the underlying nucleation layer. Similar activation energies (136–140 kJ/mol) were obtained for both lateral and vertical etching from 700 to 800 °C. Using the dry etch process developed in this research, diamond films can be removed from exposed features and undercut masked regions at a controlled rate, as indicated by microscopy and Raman spectroscopy. [ABSTRACT FROM AUTHOR]

Controlling the uniformity and morphology of graphene grown on the C-face of SiC is more difficult than on the Si-face. To improve graphene grown on the C-face, a continuous growth process was developed in a conventional tube furnace that included in situ surface preparation by annealing in H 2 followed by an Ar-mediated growth, which was done at a variety of different temperatures and pressures. Optimized H 2 etch conditions for the C-face were developed to improve the starting substrate morphology and reduce the effect of substrate defects on growth. The resulting graphene film, however, had non-uniform thickness due to intrinsic bulk defects within the SiC substrate and an interfacial oxide. Differences between substrate properties, such as polytype, are shown to have a significant effect on growth, with a 4H substrate displaying faster in-plane graphene growth than a 6H substrate. A primarily 2-domain graphene film with significant rotational disorder was found regardless of the starting substrate and growth conditions. Ultra-high vacuum desorption of the interfacial oxide caused the graphene to reorder into a single preferred rotational orientation, suggesting trace oxygen impurities in the growth chamber can play an important role in graphene growth on the C-face of SiC. [ABSTRACT FROM AUTHOR]

This article presents the use of flexible carbon substrates for the growth of III-nitride nanowire light emitters. A dense packing of gallium nitride nanowires were grown on a carbon paper substrate. The nanowires grew predominantly along the a-plane direction, normal to the local surface of the carbon paper. Strong photo- and electro-luminescence was observed from InGaN quantum well light emitting diode nanowires. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

InN thin films possessing either a novel cubic or a hexagonal phasewere grown by plasma-assisted atomic layer epitaxy on an a-plane sapphire, Si(111), and GaN/sapphire templates, simultaneously.Two ALE growth temperature windows were found between 175–185°C and 220–260 °C, in which the growth process isself-limiting. In the lower temperature ALE window, InN on an a-plane sapphire crystallized in a face-centered cubic latticewith a NaCl type structure, which has never been previously reported.InN grown on other substrates formed the more common hexagonal phase.In the higher temperature ALE window, the InN films grown on all substrateswere of hexagonal phase. The NaCl phase and the epitaxial nature ofthe InN thin films on the a-plane sapphire grownat 183 °C are confirmed independently by X-ray diffraction, transmissionelectron microscopy, and numerical simulations. These results arevery promising and demonstrate the tremendous potential for the PA-ALEin the growth of crystalline III-N materials with novel phases unachievableby other deposition techniques. [ABSTRACT FROM AUTHOR]

An undoped AlGaN/GaN nanowire (NW) demonstrated p-type conductivity solely based on the formation of hole carriers in response to the negative polarization field at the (000-1) AlGaN/GaN facet. A transistor based on this NW displayed a low-voltage transition from ohmic to space-charge-limited conduction. A numerical simulation showed that a highly asymmetric strain exists across the triangular cross section, which creates a doublet peak in the piezoelectric-induced polarization sheet charge at the (000-1) facet. Additionally, there is a strong interplay between the charge at the (000-1) AlGaN/GaN interface with depletion from the three surfaces, as well as an interaction with the opposing polarization fields at two semipolar {-110-1} facets. The charge distribution and resultant conduction regime is highly interdependent on the configuration of the multilayer structure, and it is not amenable to an analytical model. [ABSTRACT FROM AUTHOR]

Abstract: A near atmospheric pressure solution growth process was developed to produce an abundant quantity of GaN micro-wires in the c-direction with a length of tens of microns and a diameter of approximately 1–10μm. Raman analysis showed that the micro-wires were free of stress which was expected for a free-forming crystal. The lack of stress and extended defects in the GaN micro-wire provided a useful test-bed to directly compare the wet-etch behavior of the polar c-planes and non-polar m-planes in GaN. The etch behavior at the end of the micro-wire (±c) was dramatically different, with the (0001) plane found to be stable and the (000−1) plane observed to rapidly etch into nanoscale hexagonal pyramids. Additionally a dielectrophoretic method was employed to readily align the wires as part of a larger device processing sequence. [Copyright &y& Elsevier]

Abstract: The surface of a non-polar a-plane GaN light-emitting diode (LED) was intentionally damaged with a KOH wet etch to enhance the extraction of light. This roughening technique has been commonly applied to c-plane polar GaN LEDs to extract photons that would otherwise suffer from total internal refraction. We show that wet etching of the non-polar LED does create a textured surface that increases the light extraction efficiency; however, the mechanism of the etch is quite dissimilar to the etch mechanism observed for c-plane LEDs. In fact, the etch proceeds perpendicular to the a-plane surface along unstable N-face (000−1) plane with the Ga-face plane resistant to the etch. The photoluminescence intensity from a-plane non-polar LED after KOH-based wet etching was increased by 83% in our experiments. Therefore, surface roughening by KOH-based wet etch was found to be very effective to extract photons from a-plane non-polar GaN-based LEDs. [Copyright &y& Elsevier]

Abstract: Gallium nitride light emitting diodes were deposited on a sapphire substrate that was pre-patterned with an ordered two-dimensional structure. The size and arrangement of the substrate surface pattern was designed to increase the diffraction and extraction of light from the device as well as define the grain size and thus dislocation density of the GaN crystal. A close-packing of self-assembled SiO2 nanospheres was used as the sacrificial etch mask. The etch process transferred a two-dimensional pattern into the sapphire substrate with a peak-to-peak dimension of approximately 250 nm. The distance was selected to match the emission wavelength in the crystal for optimal light scattering. Additionally, the dimensions of the crystal artificially defined the grain size of the GaN in contrast to the kinetically controlled grain size in a standard GaN on sapphire growth process. [Copyright &y& Elsevier]

An approach to fabricate a set of simultaneously operating dual-UV-wavelength detectors is described. The fabrication flow relies on the confined-epitaxy growth method. The confined epitaxial Alα Ga1--α N-layer stacking approach is used to establish simultaneous multiple UV-wavelength detection. The chosen stoichiometries of specific epitaxial layers set the wavelength sensitivity at approximately 355 nm for pixel A and 320 nm for pixel B. Spectral responsivity plots of the detectors clearly show the dual-UV-color sensitivity of the pair. The detectors have signal-to-noise ratios of 15 and 17 and spectral responsivity values of 0.12 A/W and 0.05 A/W for pixel A and pixel B, respectively. [ABSTRACT FROM AUTHOR]

Abstract: A polariton-based light emitter is a extraordinary concept as an alternative to a light-emitting diode (LED) or laser diode. The physics of a polariton laser is fundamentally different from the spontaneous emission process of an LED or the inversion and stimulated emission process of a laser diode. The rapid decay and emission from this polariton–exciton state bypasses the normal irreversible spontaneous emission and associated non-radiative decay mechanisms. An AlGaN/AlN nucleation bilayer was employed on -plane sapphire to deposit non-polar GaN quantum wells embedded in an AlGaN-based cavity surrounded by top and bottom distributed Bragg reflectors (DBRs). The reflectance data show that the exciton and photon states can be tuned (by changing the angle of the sample) to the same energy. The characteristic strong coupling was observed in the reflectance data where the states of the exciton and photon do not overlap; rather they split into an upper polariton state and a lower polariton state. The photoluminescence (PL) showed a strong emission at a low stimulation level at a similar energy and angle. [Copyright &y& Elsevier]

We report on the development of a large-area few-layer graphene (FLG)-based transparent conductive electrode as a current spreading layer for GaN-based ultraviolet (UV) light-emitting diodes (LEDs). Large-area FLG was deposited on Cu using the chemical vapor deposition (CVD) method and subsequently transferred to the surface of the UV LED. UV light at a peak of 372 nm was emitted through the FLG-based transparent conductive electrode. The current spreading effects of FLG were clearly evident in both the optical images of electroluminescence (EL) and current-voltage (I-V) characteristics. Degradation of the FLG-based transparent conductive electrode could be induced by high power operation. Our results indicate that a large-area FLG-based electrode on GaN offers excellent current spreading and ultra-violet transparency properties when compared to the standard optoelectronic indium tin oxide (ITO) contact layer. [ABSTRACT FROM AUTHOR]

We report that the oxidation of graphene-based highly transparent conductive layers to AlGaN/GaN/AlGaN ultra-violet (UV) light-emitting diodes (LEDs) was suppressed by the use of SiNX passivation layers. Although graphene is considered to be an ideal candidate as the transparent conductive layer to UV-LEDs, oxidation of these layers at high operating temperatures has been an issue. The oxidation is initiated at the un-saturated carbon atoms at the edges of the graphene and reduces the UV light intensity and degrades the current-voltage (I-V) characteristics. The oxidation also can occur at defects, including vacancies. However, GaN-based UV-LEDs deposited with SiNX by plasma-enhanced chemical vapor deposition showed minimal degradation of light output intensity and I-V characteristics because the graphene-based UV transparent conductive layers were shielded from the oxygen molecules. This is a simple and effective approach for maintaining the advantages of graphene conducting layers as electrodes on UV-LEDs. [ABSTRACT FROM AUTHOR]