Bidirectional light-emitting diode as a visible light source driven by alternating current.

Gallium nitride-based light-emitting diodes have revolutionized the lighting market by becoming the most energy-efficient light sources. However, the power grid, in example electricity delivery system, is built based on alternating current, which raises problems for directly driving light emitting diodes that require direct current to operate effectively. In this paper, we demonstrate a proof-of-concept device that addresses this fundamental issue - a gallium nitride-based bidirectional light-emitting diode. Its structure is symmetrical with respect to the active region, which, depending on the positive or negative bias, allows for the injection of either electrons or holes from each side. It is composed of two tunnel junctions that surround the active region. In this work, the optical and electrical properties of bidirectional light emitting diodes are investigated under direct and alternating current conditions. We find that the light is emitted in both directions of the supplied current, contrary to conventional light emitting diodes; hence, bidirectional light-emitting diodes can be considered a semiconductor light source powered directly with alternating current. In addition, we show that bidirectional light-emitting diodes can be stacked vertically to multiply the optical power achieved from a single device.

Role of Metallic Adlayer in Limiting Ge Incorporation into GaN.

Atomically thin metal adlayers are used as surfactants in semiconductor crystal growth. The role of the adlayer in the incorporation of dopants in GaN is completely unexplored, probably because n-type doping of GaN with Si is relatively straightforward and can be scaled up with available Si atomic flux in a wide range of dopant concentrations. However, a surprisingly different behavior of the Ge dopant is observed, and the presence of atomically thin gallium or an indium layer dramatically affects Ge incorporation, hindering the fabrication of GaN:Ge structures with abrupt doping profiles. Here, we show an experimental study presenting a striking improvement in sharpness of the Ge doping profile obtained for indium as compared to the gallium surfactant layer during GaN-plasma-assisted molecular beam epitaxy. We show that the atomically thin indium surfactant layer promotes the incorporation of Ge in contrast to the gallium surfactant layer, which promotes segregation of Ge to the surface and Ge crystallite formation. Understanding the role of the surfactant is essential to control GaN doping and to obtain extremely high n-type doped III-nitride layers using Ge, because doping levels >1020 cm−3 are not easily available with Si.

Mitigating Ammonia and Greenhouse Gaseous Emission From Arable Land by Co-application of Zeolite and Biochar.

The intensive use of chemical fertilizers in arable farming dramatically increased environmental pollution through anthropogenic ammonia (NH3) and greenhouse gaseous emissions. Therefore, there is a need to develop improved fertilizer management practices that can reduce these losses. An experiment was conducted to assess the mitigating effects of sole or combined application of zeolite with biochar on gaseous emissions from arable land. For this purpose, zeolite (clinoptilolite) was mixed with different doses of biochar (produced from Dalbergia Sissoo wood chips) and applied along with the recommended dose of chemical fertilizer (NPK @ 150, 100, and 60 kg ha-1, respectively) on arable land in years 2013-14 and 2014-15. Immediately after application, these were incorporated into the top 10 cm of the soil layer and wheat was sown. Treatments were as follows: C = control, Z = zeolite @ 5 t ha-1, B1Z = biochar @ 3 t ha-1 + zeolite @ 5 t ha-1, B2Z = biochar @ 6 t ha-1 + zeolite @ 5 t ha-1, and B3Z = biochar @ 9 t ha-1 + zeolite @ 5 t ha-1. The experiment was laid out in a randomized complete block design (RCBD) with three replicates. The experimental plot size was 6 m × 4 m. Randomly, ten soil samples from each plot were taken at a depth of 0-15 cm and mixed to get a composite sample. All the samples were immediately stored in a freezer at -18°C until gaseous analysis in order to prevent N transformations. Each soil sample was analyzed for emission of NH3, CO2, and CH4 by using a selected-ion flow-tube mass spectrometer (SIFT-MS). Co-application of zeolite and biochar reduced NH3 and CH4 emissions by an average of 87 and 58% compared to the control, respectively. However, CO2 emission was increased by 104% relative to the control. The NH3 emission was decreased by an average of 61, 78, 90, and 92% by Z, B1Z, B2Z, and B3Z treatments compared to the control. Similarly, the decrement in CH4 emission was 47, 54, 55, and 65%. In contrast, the increment in CO2 emission was 42, 110, and 160% for B1Z, B2Z, and B3Z, respectively, while interestingly, a reduction of 12% was observed in Z treatment. Besides, co-application of zeolite and biochar at the highest dose (B3Z) improved soil chemical properties such as soil EC, OM, total N, as well as available P and K relative to zeolite alone. It is concluded that the combined application of zeolite and biochar can mitigate NH3 and greenhouse emissions and improve soil chemical characteristics, thus enhancing the environmental worth of arable farming.

Electrically pumped blue laser diodes with nanoporous bottom cladding.

We demonstrate electrically pumped III-nitride edge-emitting laser diodes (LDs) with nanoporous bottom cladding grown by plasma-assisted molecular beam epitaxy on c-plane (0001) GaN. After the epitaxy of the LD structure, highly doped 350 nm thick GaN:Si cladding layer with Si concentration of 6·1019 cm-3 was electrochemically etched to obtain porosity of 15 ± 3% with pore size of 20 ± 9 nm. The devices with nanoporous bottom cladding are compared to the reference structures. The pulse mode operation was obtained at 448.7 nm with a slope efficiency (SE) of 0.2 W/A while the reference device without etched cladding layer was lasing at 457 nm with SE of 0.56 W/A. The design of the LDs with porous bottom cladding was modelled theoretically. Performed calculations allowed to choose the optimum porosity and thickness of the cladding needed for the desired optical mode confinement and reduced the risk of light leakage to the substrate and to the top-metal contact. This demonstration opens new possibilities for the fabrication of III-nitride LDs.

Dependence of InGaN Quantum Well Thickness on the Nature of Optical Transitions in LEDs.

The design of the active region is one of the most crucial problems to address in light emitting devices (LEDs) based on III-nitride, due to the spatial separation of carriers by the built-in polarization. Here, we studied radiative transitions in InGaN-based LEDs with various quantum well (QW) thicknesses-2.6, 6.5, 7.8, 12, and 15 nm. In the case of the thinnest QW, we observed a typical effect of screening of the built-in field manifested with a blue shift of the electroluminescence spectrum at high current densities, whereas the LEDs with 6.5 and 7.8 nm QWs exhibited extremely high blue shift at low current densities accompanied by complex spectrum with multiple optical transitions. On the other hand, LEDs with the thickest QWs showed a stable, single-peak emission throughout the whole current density range. In order to obtain insight into the physical mechanisms behind this complex behavior, we performed self-consistent Schrodinger-Poisson simulations. We show that variation in the emission spectra between the samples is related to changes in the carrier density and differences in the magnitude of screening of the built-in field inside QWs. Moreover, we show that the excited states play a major role in carrier recombination for all QWs, apart from the thinnest one.

Revealing inhomogeneous Si incorporation into GaN at the nanometer scale by electrochemical etching.

Typical methods of doping quantification are based on spectroscopy or conductivity measurements. The spatial dopant distribution assessment with nanometer-scale precision is limited usually to one or two dimensions. Here we demonstrate an approach to detect three-dimensional dopant homogeneity in GaN:Si layers using electrochemical etching (ECE). GaN:Si layers are grown by plasma-assisted molecular beam epitaxy. Dopant incorporation is uniform when the growth front morphology is atomically flat. Non-uniform Si incorporation into GaN is observed when step-bunches are present on the surface during epitaxy. In this study we show that local Si concentration in the area of step-bunch is about three times higher than in the area between step-bunches. ECE spatial resolution in our experiment is estimated to be about 50 nm. This makes ECE a simple and quantitative probing tool for local three-dimensional conductivity homogeneity assessment. Our study proves that ECE could be important both for fundamental studies of crystal growth physics and impurity incorporation and for ion-implanted structures and post-processing device control.