Low Pressure Source for Mass-Selective, Diffusion Assisted Epitaxy

Dramatic differences in the diffusivities of the constituents of novel thin film materials and structures limit material perfection under far from equilibrium growth conditions. We will develop a new light-mass ion source, compatible with the low pressurerequirements of molecular beam epitaxy (MBE), to provide selective enhancement of the motion of surface atoms. Helium or hydrogen ions incident on a surface at approximately 100 eV will mainly transfer their energy to lower mass, surface atoms. Thislow-pressure ion source will be compared to a higher pressure Kaufman source for the MBE growth of technologically important III-nitride materials. The low-mass ions will overwhelmingly deposit their energy at the N atoms, effectively setting the growthconditions closer to equilibrium. The results will be compared to literature reports using higher mass ions. For low-mass ions incident at low angles with low energy, the energy transfer will be well below sputtering thresholds. The impinging ions willonly be able to excite local phonon modes that selectively enhance surface diffusion. If more momentum transfer is desired, higher mass ions could be used. Development of this new ion source would greatly impact a wide range of materials systems.Currently, the growth of multicomponent thin films and films with hyperabrupt interfaces challenges all growth techniques. The proposed low-mass ion source will allow control of defects, grain size, surface chemistry, and enable abrupt hetero-epitaxialcombinations not currently possible. As a proof of concept, the enhanced MBE growth of GaN and InGaN will be demonstrated. This technique would greatly benefit the development of novel semiconductor thin films and structures.