Investigation of the Chaotic Dynamics of an Electron Beam with a Virtual Cathode in an External Magnetic Field

The effect of the strength of the focusing magnetic field on chaotic dynamic processes occurring inan electron beam with a virtual cathode, as well as on the processes whereby the structures form in the beamand interact with each other, is studied by means of two-dimensional numerical simulations based on solving a self-consistent set of Vlasov-Maxwell equations. It is shown that, as the focusing magnetic field is decreased,the dynamics of an electron beam with a virtual cathode becomes more complicated due to the formation andinteraction of spatio-temporal longitudinal and transverse structures in the interaction region of a vircator. The optimum efficiency of the interaction of an electron beam with the electromagnetic field of the vircator isachieved at a comparatively weak external magnetic field and is determined by the fundamentally two-dimensional nature of the motion of the beam electrons near the virtual cathode.Comment: 12 pages, 8 figure

ТЕНДЕНЦИИ РАЗВИТИЯ ЭПИТАКСИАЛЬНОЙ ТЕХНОЛОГИИ НИТРИДНЫХ СОЕДИНЕНИЙ

The main trends in the development of technology for nitride heterostructures element base of microwave−technology and power electronics, as well as light−emitting diodes have been reviewed. It has been noted that most modern technological focus is the development of nitride heterostructures on silicon substrates. The basic problems of nitride compounds on silicon substrate and the ways of their solution have been discussed. Some results of GaN/Si heterostructures technology development in «Elma−Malachit» JSC have been presented. The AlGaN/GaN/Si heterostructures have been grown by MOCVD. We show that early process stages such as Si−surface treatment and Al pre−deposition are of great importance for the growth of crack−free structures with good structural and surface quality. Meanwhile the surface curvature of the grown structures is influenced mainly by the composition of multilayered transition region between the AlN nucleation layer and the GaN layer. Transistors fabricated on AlGaN/GaN structures grown on Si substrates under optimized conditions demonstrated rather good static characteristics: Id,max = 800 mA/mm, Ubr > 120 V, gm = = 170 mS/mm. For the further technology development experimental and technological work should be arranged in close coordination with analytical prediction and calculation of properties of the grown material with mathematical modeling methods. This approach will help enhance the efficiency of technology development and deepen scientific views on the processes responsible for the formation of properties of heterostructures. Рассмотрены основные тенденции в развитии технологии гетероструктур нитридных соединений для элементной базы СВЧ−техники и силовой электроники, а также светоизлучающих диодов. Отмечено, что важнейшим современным технологическим направлением является разработка гетероструктур нитридных соединений на подложках кремния. Рассмотрены основные проблемы гетероэпитаксии нитридных соединений на подложке кремния и пути их решения. Представлены некоторые результаты разработок технологии гетероструктур нитридных соединений на подложках кремния в ЗАО «Элма−Малахит». Гетерострукуры AlGaN/GaN/Si выращены МОС−гидридным методом. Показано, что предэпитаксиальная обработка подложек кремния и начальная стадия процесса выращивания, включающая предварительное покрытие поверхности Si алюминием при подаче в реактор потока ТМА, играют большую роль в формировании гетероструктур, свободных от трещин и с хорошей морфологией. В то же время установлено, что форма поверхности гетероструктур определяется главным образом композицией переходной области между зародышевым слоем AlN и слоем GaN. Транзисторы, изготовленные на основе выращенных гетероструктур AlGaN/GaN/Si, продемонстрировали приемлемые статические характеристики: максимальная плотность тока составила 800 мА/мм, пробивное напряжение — более 120 В, крутизна — 170 мСм/мм. Показано, что для дальнейшего развития гетероструктур нитридных соединений на подложках кремния экспериментально−технологическую работу необходимо организовать в тесном взаимодействии с аналитическим прогнозированием и расчетами свойств выращиваемого материала методами математического моделирования. Такой подход поможет повысить результативность разработок технологии и углубит научные представления в отношении процессов, ответственных за формирование свойств гетероструктур.

The formation of low-dimensional structures by compressive plasma flows

The process of the deposition of low-dimensional structures on the silicon surface exposed to the compression plasma flow has been studied. Scanning electron microscopy, transmission electron microscopy and Rutherford backscattering spectroscopy have been used to analyze the morphology, microstructure and elemental composition of the near-surface layer. The deposited coating consists of a spherical metal containing particles with a size of 50–200 nm. Possible mechanism of the coating formation is discussed

Iron surface cellular structure formed by action of compression plasma flows

Polygonal cellular structure on the surface of iron samples appear as a result of processing by compression plasma flows. Typical cell size of this structures is about 400 nm. Formation of such structure takes place during the melt crystallization when the melt is concentrated and undercooled

Iron surface cellular structure formed by action of compression plasma flows

Polygonal cellular structure on the surface of iron samples appear as a result of processing by compression plasma flows. Typical cell size of this structures is about 400 nm. Formation of such structure takes place during the melt crystallization when the melt is concentrated and undercooled