Charge is observed at the regrowth interface for heterostructure field-effect transistors (HFETs) grown on semi-insulating (SI) bulk GaN substrates, even with Fe doping in the regrown buffer layer for reduction of the interface charge. Ultraviolet photoenhanced chemical (PEC) etching is used to treat the surface of SI bulk GaN substrates. Employing optimized etching conditions, a very smooth surface is achieved for the bulk GaN substrate after the etching. The charge at the regrowth interface is eliminated for HFETs grown on etched SI GaN substrates. Secondary ion mass spectrometry measurements show that the Si impurity concentration at the regrowth interface for HFETs grown on etched SI GaN substrates is much lower than that for HFETs grown on unetched SI GaN substrates, which suggests that the charge-containing layer on the SI substrate is removed by PEC etching and that the effects of the reduced charge layer near the regrowth interface can be eliminated by Fe doping for HFETs grown on etched SI substrates.
We describe the effect of carbon incorporation into Si on dopant surface segregation during molecular-beam epitaxial growth. Low concentration of carbon can significantly reduce the surface segregation of boron and phosphorus. Combining the surface diffusion model with a two-state exchange process, we are able to model the experimental results over the whole temperature range between 350 and 800 °C. Each exchange process alone is not sufficient to describe surface segregation at all investigated temperatures. Our results show that the presence of carbon lowers the energy difference for boron in subsurface and surface states. The energy barriers for surface diffusion as well as for the two-state exchange process are not affected by carbon.
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