Investigation into the charge distribution and barrier profile tailoring in AlGaN/GaN double heterostructures by self-consistent Poisson-Schrodinger calculations and capacitance-voltage profiling

摘要

The two-dimensional electron gas (2DEG) distribution and conduction-band profile tailoring of the Al_(x)Ga_(1-x)N/GaN/Al_(y)Ga_(1-y)N/GaN double heterostructure (DH) has been studied in detail by self-consistent Poisson-Schrodinger (SCPS) calculations. We show that a 2DEG is always created at the Al_(y)Ga_(1-y)N/GaN interface beyond the GaN quantum well but the latter will not be occupied if the lower barrier thickness or Al content exceed those of the top barrier. These findings were confirmed by capacitance-voltage profiling of a 5 nm Al_(0.3)Ga_(0.7)N/5 nm GaN/Al_(y)Ga_(1-y)N/GaN DH grown by molecular beam epitaxy on n~(+) GaN, where the lower barrier thickness was varied between 10 and 40 nm. A maximum 2DEG density of 1.0×10~(13) cm~(-2) was achieved for a 40 nm lower barrier and y=0.2. We found good agreement between the integrated carrier concentration versus depth curve and the calculated equilibrium 2DEG density. The bias required to bring about a flatband condition at the lower Al_(y)Ga_(1-y)N/GaN interface increased with the thickness of the lower barrier up to -9 V for 40 nm and y=0.2, in agreement with SCPS calculations of the subband depletion under negative bias. We discuss the potential use of an AlGaN/GaN DH with a thick GaN well for the realization of a 2DEG isolated from the buffer by strong barrier enhancement.