Plasma assisted molecular beam epitaxial growth of GaN with low growth rates and their properties

A systematic investigation on PA-MBE grown GaN with low growth rates(less than 0.2μm/h)has been conducted in a wide growth temperature range,in order to guide future growth of sophisticated fine structures for quantum device applications.Similar to usual growths with higher growth rates,three growth regions have been revealed,namely,Ga droplets,slightly Ga-rich and N-rich 3D growth regions.The slightly Ga-rich region is preferred,in which GaN epilayers demonstrate optimal crystalline quality,which has been demonstrated by streaky RHEED patterns,atomic smooth surface morphology,and very low defect related yellow and blue luminescence bands.The growth temperature is a critical parameter to obtain high quality materials and the optimal growth temperature window(~700-760℃)has been identified.The growth rate shows a strong dependence on growth temperatures in the optimal temperature window,and attention must be paid when growing fine structures at a low growth rate.Mg and Si doped GaN were also studied,and both p-and n-type materials were obtained.

State of polar climate in 2023

The year 2023 has become the warmest year on global record.As the Antarctic and Arctic are sensitive regions to global warming,the climate changes in 2023 in these regions have attracted widespread attention.In this study,using observations,reanalysis and remote sensing data,we reported detailed polar climate changes in 2023,including warming,sea ice,atmospheric composition and extreme events.Antarctic exhibited large east-west regional differences and the coexistence of extreme warm and cold events.In Coats Land,Queen Maud Land and the Antarctic Peninsula,three and seven stations recorded the second and third highest autumn air temperatures in history,respectively.The Amundsen-Scott station experienced extreme warming event in July,with the temperature increasing by 40℃ in one day.Abnormal cooling was evident in the Ross Sea and neighboring regions which were predominantly winter(June-August)cold anomalies,with Marylin Station reaching the lowest winter temperature in history.The Arctic experienced the warmest summer after 1979,with an overall distribution of warm land-cold sea'on annual average.Compared with the 1991-2020 average,the annual air temperature anomalies reached more than 2℃in northern Canada and the Barents Sea-Kara Sea coast.Abnormal high summer temperature caused most severe wildfires in Canada on record and second largest daily cumulative melt area over the Greenland ice Sheet daily post-1979.Polar sea ice continued to decrease rapidly,with minimum sea ice extent in Antarctic and Arctic ranking the first and sixth lowest post-1979.For melt season,Arctic Ocean sea ice began to melt later in 2023 than the 2011-2023 average,and freeze onset was delayed due to high temperatures in summer and autumn.Additionally,the status of polar atmospheric greenhouse gases remains bleak,and major greenhouse gas concentrations continue to increase.The Antarctic ozone hole in 2023 formed approximately 10 d earlier and lasted longer than the 1979-2023 average,with a maximum daily area of 2.6×10^(7)km^(2) on 21 September.This summary of polar climate changes in 2023 will help people better understand global climate change and draw attention to polar regions.