Magneto-transport properties of MOVPE-grown Al_xGa(1-x)N/AlN/GaN heterostructures with high-mobility two-dimensional electron gas

摘要

We study Al_xGa_(1-x)N/AlN/GaN heterostructures with a two-dimensional-electron-gas (2DEG) grown on different GaN templates using low-temperature magneto-transport measurements. Heterostructures with different Al compositions are grown by metal-organic vapor phase epitaxy (MOVPE) on three different templates; conventional undoped GaN (u-GaN), epitaxial lateral overgrown GaN (ELO-GaN), and in situ ELO-GaN using a Si_xN_y nanomask layer (SiN-GaN). Field-dependent magneto-resistance and Hall measurements indicated that in addition to 2DEG, the overgrown heterostructures had a parallel conducting layer. The contact resistance for the parallel channels was large so that it introduced errors in the quantitative mobility spectrum analysis (QMSA) of the data. Notwithstanding complexities introduced by parallel conducting channels in mobility analysis in SiN-GaN and ELO-GaN samples, we were able to observe Shubnikov-de Haas (SdH) oscillations in all samples, which confirmed the existence of 2DEGs. To characterize the parallel channel, we repeated the transport measurements after the removal of the 2DEG by etching the heterostructure. The 2DEG carrier density values were extracted from the SdH data, whereas the zero-field 2DEG conductivity was determined by subtracting the parallel channel conductivity from the total conductivity. The resulting 2DEG mobility was significantly higher (about a factor of 2) in the ELO-GaN and SiN-GaN samples as compared to the standard control sample. The mobility enhancement is attributed to the threading dislocation reduction by both ELO techniques.