Electrical measurement of internal quantum efficiency and extraction efficiency of III-N light-emitting diodes

We propose a direct electrical measurement method for determining the extraction efficiency (EXE) and internal quantum efficiency(IQE) of III-Nitride light-emitting diodes(LEDs). The method is based on measuring the optical output power as a function of injection current at current densities near the external quantum efficiency (EQE) maximum and extracting IQE and EXE from the measurement data. In contrast to conventional methods, our method requires no low temperaturemeasurements or prior knowledge of the device structure. The method is far more convenient than commonly used methods because it enables measuring the EXE and IQE of different LED structures at room temperature directly in a repeatable and consistent way. This enables convenient comparison of LED structures. We apply the method to determine the IQE and EXE of one commercial LED and selected self-grown planar LED chips to compare the effects of different LED structure designs. Our results are in line with published experimental results and also give more insight to our earlier findings regarding the effects of growth parameters on the quantum efficiency. In addition, our measurement method allows estimating the Shockley-Read-Hall and radiative recombination parameters if the Auger parameter is known.Peer reviewe

Effect of business opportunity recognition on entrepreneurs’ market perception

To be able to act efficiently and productively on a certain market, an entrepreneur must have a perception and understanding of the driving forces and critical factors of the market. Prior research has found two business opportunity recognition manners, which are opportunity discovery and opportunity creation. Discovered opportunities are based on existing markets with an existing demand or supply. Created opportunities on the other hand are based on entrepreneurs’ own interests and the entrepreneur has to create both supply and demand for these innovations. This study aims to add understanding on how the business opportunity recognition manner affects the entrepreneurs’ market perception. Market perceptions are studied through market elements, which describe the market from a market actor’s perspective. As the aim of this study is to better understand a certain phenomena, a qualitative research method is used. The empirical research is conducted as a case study and the data is collected through semi-constructed interviews. There were in total eight entrepreneurs acting on an eHealth market, who were interviewed for this research. According to the empirical results of this study, the business opportunity recognition manner does have an effect on the entrepreneurs’ market perception. For both discovery and creation entrepreneurs, the demand and supply factors are in the core of their market perception, which both entrepreneurs can identify well. In general, the discovery entrepreneurs have a better understanding on market and its driving forces compared to creation entrepreneurs, who tend to struggle with finding their place on the market. This study helps entrepreneurs to understand how their business opportunity recognition manner affects their business. Entrepreneurs who have created their business opportunities need to accept that since they have not started their business based on an existing demand, they have to work harder to understand who their customers are and how they can serve them compared to discovery entrepreneurs. The results of this study should not be generalized unconditionally to other markets, but this study brought interesting insights to the opportunity recognition discussion

Seasonal cycle, size dependencies, and source analyses of aerosol optical properties at the SMEAR II measurement station in Hyytiälä, Finland

Scattering and absorption were measured at the Station for Measuring Ecosystem–Atmosphere Relations (SMEAR II) station in Hyytiälä, Finland, from October 2006 to May 2009. The average scattering coefficient σ<sub>SP</sub> (λ = 550 nm) 18 Mm<sup>−1</sup> was about twice as much as at the Pallas Global Atmosphere Watch (GAW) station in Finnish Lapland. The average absorption coefficient σ<sub>AP</sub> (λ = 550 nm) was 2.1 Mm<sup>−1</sup>. The seasonal cycles were analyzed from hourly-averaged data classified according to the measurement month. The ratio of the highest to the lowest average σ<sub>SP</sub> and σ<sub>AP</sub> was ~1.8 and ~2.8, respectively. The average single-scattering albedo (ω<sub>0</sub>) was 0.86 in winter and 0.91 in summer. σ<sub>SP</sub> was highly correlated with the volume concentrations calculated from number size distributions in the size range 0.003–10 μm. Assuming that the particle density was 1.5 g cm<sup>−3</sup>, the PM<sub>10</sub> mass scattering efficiency was 3.1 ± 0.9 g m<sup>−2</sup> at λ = 550 nm. Scattering coefficients were also calculated from the number size distributions by using a Mie code and the refractive index of ammonium sulfate. The linear regression yielded σ<sub>SP</sub>(modelled) = 1.046 × σ<sub>SP</sub>(measured) for the data with the low nephelometer sample volume relative humidity (RH<sub>NEPH</sub> = 30 ± 9 %) and σ<sub>SP</sub>(modelled) = 0.985 × σ<sub>SP</sub>(measured) when RH<sub>NEPH</sub> = 55 ± 4 %. The effective complex refractive index was obtained by an iterative approach, by matching the measured and the modelled σ<sub>SP</sub>and σ<sub>AP</sub>. The average effective complex refractive index was (1.517 ± 0.057) + (0.019 ± 0.015)<i>i</i> at λ = 550 nm. The iterated imaginary part had a strong seasonal cycle, with smallest values in summer and highest in winter. The contribution of submicron particles to scattering was ~90 %. The Ångström exponent of scattering, σ<sub>SP</sub>, was compared with the following weighted mean diameters: count mean diameter (CMD), surface mean diameter (SMD), scattering mean diameter (ScMD), condensation sink mean diameter (CsMD), and volume mean diameter (VMD). If α<sub>SP</sub> is to be used for estimating some measure of the size of particles, the best choice would be ScMD, then SMD, and then VMD. In all of these the qualitative relationship is similar: the larger the Ångström exponent, the smaller the weighted mean diameter. Contrary to these, CMD increased with increasing α<sub>SP</sub> and CsMD did not have any clear relationship with α<sub>SP</sub>. Source regions were estimated with backtrajectories and trajectory statistics. The geometric mean σ<sub>SP</sub> and σ<sub>AP</sub> associated with the grid cells in Eastern Europe were in the range 20–40 Mm<sup>−1</sup> and 4–6 Mm<sup>−1</sup>, respectively. The respective geometric means of σ<sub>SP</sub> and σ<sub>AP</sub> in the grid cells over Norwegian Sea were in the range 5–10 Mm<sup>−1</sup> and <1 Mm<sup>−1</sup>. The source areas associated with high α<sub>SP</sub> values were norther than those for σ<sub>SP</sub> and σ<sub>AP</sub>. The trajectory statistical approach and a simple wind sector classification agreed well

Plasmon phonon modes and optical resonances in n-GaN

We study theoretically and experimentally transverse and longitudinal electromagnetic waves in n-GaN epitaxial layers. The studies are carried out on the epitaxial layers with various doping levels. Simulation of the reflectivity and absorptivity spectra is performed in a wide frequency range. Additionally, the radiation emission spectra are simulated for various temperatures of the GaN epitaxial layer. It is shown that the resonance peculiarities of the optical spectra are located closely to the frequencies of the coupled plasmon phonon modes. Experimental studies of the reflectivity spectra have been performed in the spectral range 2-20 THz. The experimental spectra are well fitted by the simulated ones and can be used for contactless determination of the electron concentration and mobility in GaN epitaxial layers. The experimental and theoretical results of the present work provide insights for the development of GaN-based devices aimed to absorb/emit terahertz or mid infrared radiation selectively.Peer reviewe

Interaction of surface plasmon polaritons in heavily doped GaN microstructures with terahertz radiation

We present the results of experimental and theoretical studies of the surface plasmon polariton excitations in heavily doped GaN epitaxial layers. Reflection and emission of radiation in the frequency range of 2-20 THz including the Reststrahlen band were investigated for samples with grating etched on the sample surface, as well as for samples with flat surface. The reflectivity spectrum for p-polarized radiation measured for the sample with the surface-relief grating demonstrates a set of resonances associated with excitations of different surface plasmon polariton modes. Spectral peculiarities due to the diffraction effect have been also revealed. The characteristic features of the reflectivity spectrum, namely, frequencies, amplitudes, and widths of the resonance dips, are well described theoretically by a modified technique of rigorous coupled-wave analysis of Maxwell equations. The emissivity spectra of the samples were measured under epilayer temperature modulation by pulsed electric field. The emissivity spectrum of the sample with surface-relief grating shows emission peaks in the frequency ranges corresponding to the decay of the surface plasmon polariton modes. Theoretical analysis based on the blackbody-like radiation theory well describes the main peculiarities of the observed THz emission.Peer reviewe

Observational evidence for aerosols increasing upper tropospheric humidity

Aerosol–cloud interactions are the largest source of uncertainty in the radiative forcing of the global climate. A phenomenon not included in the estimates of the total net forcing is the potential increase in upper tropospheric humidity (UTH) by anthropogenic aerosols via changes in the microphysics of deep convection. Using remote sensing data over the ocean east of China in summer, we show that increased aerosol loads are associated with an UTH increase of 2.2 ± 1.5 in units of relative humidity. We show that humidification of aerosols or other meteorological covariation is very unlikely to be the cause of this result, indicating relevance for the global climate. In tropical moist air such an UTH increase leads to a regional radiative effect of 0.5 ± 0.4 W m−2. We conclude that the effect of aerosols on UTH should be included in future studies of anthropogenic climate change and climate sensitivity