Multiple quantum well (MQW) InGaN light emitting diodes with and without electron blocking layers, with relatively small and large barriers, with and without p-type doping in the MQW region emitting at ∼420nm were used to determine the genesis of efficiency droop observed at injection levels of approximately ⩾50A∕cm2. Pulsed electroluminescence measurements, to avoid heating effects, revealed that the efficiency peak occurs at ∼900A∕cm2 current density for the Mg-doped barrier, near 550A∕cm2 for the lightly doped n-GaN injection layer, meant to bring the electron injection level closer to that of holes, and below 220A∕cm2 for the undoped InGaN barrier cases. For samples with GaN barriers (larger band discontinuity) or without p-AlGaN electron blocking layers the droop occurred at much lower current densities (⩽110A∕cm2). In contrast, photoluminescence measurements revealed no efficiency droop for optical carrier generation rates corresponding to the maximum current density employed in pulsed injection measurements. All the data are consistent with heavy effective mass of holes, low hole injection efficiency (due to relatively lower p-doping) leading to severe electron leakage being responsible for efficiency droop.

Light emitting diodes ͑LEDs͒ based on InGaN suffer from efficiency droop at current injection levels as low as 50 A cm −2. We investigated multiple quantum well InGaN LEDs with varying InGaN barrier thicknesses ͑3-12 nm͒ emitting at ϳ400-410 nm to investigate the effect of hole mass and also to find out possible solutions to prevent the efficiency droop. In LEDs with electron blocking layers, when we reduced the InGaN barriers from 12 to 3 nm, the current density for the peak or saturation of external quantum efficiency increased from 200 to 1100 A cm −2 under pulsed injection conditions, which eliminates the heating effects to a large extent. Our calculations show that such reduction in the barrier thickness makes the hole distribution more uniform among the wells. These results suggest that the inferior low hole transport through the barriers exacerbated by large hole effective mass and low hole injection due to relatively low hole concentration and the consequent electron leakage are responsible for the efficiency droop at high current injection levels.

Areas of dark ice have appeared on the Greenland ice sheet every summer in recent years. These are likely to have a great impact on the mass balance of the ice sheet because of their low albedo. We report annual and geographical variations in the bare ice and dark ice areas that appeared on the Greenland Ice Sheet from 2000 to 2014 by using MODIS satellite images. The July monthly mean of the extent of bare ice showed a positive trend over these 15 years, and large annual variability ranging from 89,975 to 279,075 km 2 , 5 and 16% of the entire ice sheet, respectively. The extent of dark ice also showed a positive trend and varied annually, ranging from 3575 to 26,975 km 2 , 4 and 10% of the bare ice extent. These areas are geographically varied, and their expansion is the greatest on the western side, particularly the southwestern side of the ice sheet. The bare ice extent correlates strongly with the monthly mean air temperature in July, suggesting that the extent was determined by snow melt. The dark ice extent also correlates with the air temperature; however, the correlation is weaker. The dark ice extent further correlates negatively with solar radiation. This suggests that the extent of dark ice is not only controlled by snow melt on the ice, but also by changes in the surface structures of the bare ice surface, such as cryoconite holes, which are associated with impurities appearing on the ice surface.

Cryoconite holes are water-filled cylindrical holes formed on ablation ice surfaces and commonly observed on glaciers worldwide. Temporal changes of cryoconite holes characteristically <5 cm in diameter were monitored with a time-lapse interval camera over 15 d during the melting season on Qaanaaq Glacier in northwest Greenland. The holes drastically changed their dimensions and synchronously collapsed twice during the study period. When the holes collapsed, the coverage of cryoconite on the ice surface increased from 1.0 to 3.5% in the field of view of the camera, and then decreased again to 0.4% after the holes reformed. Comparison with meteorological data showed that the collapses occurred in cloudy and rainy or windy weather conditions, corresponding to low shortwave solar radiation (68–126 W m−2, 40–55% of the incoming flux). In contrast, holes developed in sunny conditions correspond to high solar radiation (186–278 W m−2, 63–88%). Results suggest that the dimensions of holes drastically changed depending on the weather conditions and that frequent cloudy, warm and windy conditions would cause a decay of holes and weathering crust, inducing an increase in the cryoconite coverage on the ice, consequently darkening the glacier surface.

) on snow surface at the study sites. On Qaanaaq Glacier (an outlet glacier of Qaanaaq Ice Cap) impurity abundance was greatest at mid-elevations, with fewer impurities at upper and lower sites. Surface reflectivity was lowest in the mid-elevation area, suggesting that impurities substantially reduce ice surface albedo at mid-elevations on glacier surfaces. Organic matter content in the impurities ranged from 1.4 to 12.0% (mean: 5.4%) on the ice and from 3.2 to 10.6% (mean: 6.7%) on the snow surface. Microscopy revealed that impurities in the ice areas mainly consisted of cryoconite granules, which are aggregations of mineral particles, filamentous cyanobacteria and other microbes and organic matter, while those in snow areas consisted of mineral particles and snow algae. Results suggest that the spatial variation in the abundance of impurities is caused by supply of mineral particles both from air and ice, and microbial production of organic matter on the glacier surface.