Carbon doped GaN buffer layer using propane for high electron mobility transistor applications: Growth and device results

Abstract

The creation of a semi insulating (SI) buffer layer in AlGaN/GaN High Electron Mobility Transistor (HEMT) devices is crucial for preventing a current path beneath the two-dimensional electron gas (2DEG). In this investigation, we evaluate the use of a gaseous carbon gas precursor, propane, for creating a SI GaN buffer layer in a HEMT structure. The carbon doped profile, using propane gas, is a two stepped profile with a high carbon doping (1.5 × 1018 cm−3) epitaxial layer closest to the substrate and a lower doped layer (3 × 1016 cm−3) closest to the 2DEG channel. Secondary Ion Mass Spectrometry measurement shows a uniform incorporation versus depth, and no memory effect from carbon doping can be seen. The high carbon doping (1.5 × 1018 cm−3) does not influence the surface morphology, and a roughness root-mean-square value of 0.43 nm is obtained from Atomic Force Microscopy. High resolution X-ray diffraction measurements show very sharp peaks and no structural degradation can be seen related to the heavy carbon doped layer. HEMTs are fabricated and show an extremely low drain induced barrier lowering value of 0.1 mV/V, demonstrating an excellent buffer isolation. The carbon doped GaN buffer layer using propane gas is compared to samples using carbon from the trimethylgallium molecule, showing equally low leakage currents, demonstrating the capability of growing highly resistive buffer layers using a gaseous carbon source.

Study of GaN doping with carbon from propane in a wide range of MOVPE conditions

“…For the data presented in [5], this gives approximately 100 μmole/min of TMGa and 3 slm of NH 3 flows. These estimations match well data presented by the same group in [6]. It means that very low ammonia concentration was used in [5] for growing GaN:C epitaxial layers.…”

“…Later the same team demonstrated HEMTs with GaN:C buffer intentionally doped from propane [6,7] and quite recently similar results were published by another group [8]. These works are a significant step in the development of controllable carbon doping of gallium nitride.…”

“…It should be noted also that both our experiments and those described in [5][6][7][8] were carried out in horizontal flow or planetary reactors which means rather long residence time of precursors in the hot zone. This results in efficient propane pyrolysis which is a very important stage of the studied process.…”

“…Most interesting is the evaluation of the effect of growth rate on carbon concentration. In work [5] (and likely in [6,7]), the growth rate of GaN:C epilayers was only 0.65 μm/h. This value is too low for production.…”

“…These works are a significant step in the development of controllable carbon doping of gallium nitride. However, in the recent works [6][7][8], the authors have focused on properties of device structures while the main objective of paper [5] was a comparison of carbon incorporation efficiency for a number of carbon precursors and growth temperature was the only parameter studied. An observed tendency -increase of carbon concentration with temperature reduction -was the same as for background carbon doping.…”