Structural properties of AlGaN/GaN heterostructures on Si(111) substrates suitable for high-electron mobility transistors

Abstract

Transmission electron microscopy (TEM) investigations of metal organic vapor phase deposition grown AlxGa1−xN/GaN heterostructures on Si(111) containing an AlN high-temperature buffer layer have been carried out. The structural properties at the interface and in the epilayer as well as the electronic properties suitable for a high electron mobility transistor (HEMT) were analyzed and compared with systems grown on Al2O3(0001). High resolution TEM (HRTEM) at the AlN/Si(111) interface reveals a 1.5–2.7 nm thick amorphous SiNx layer due to the high growth temperature of TAlN=1040 °C. Therefore, a grain-like GaN/AlN region extending 40–60 nm appears and it is subsequently overgrown with (0001) orientated GaN material because of geometrical selection. The residual strain at the AlN/Si(111) interface is estimated to be εr=0.3±0.6% by Fourier filtering of HRTEM images and a moiré fringe analysis. This indicates almost complete relaxation of the large mismatch f(AlN/Si)=+23.4% which seems to be supported by the SiNx layer. Weak beam imaging and plan view TEM show typical threading dislocations in the epilayer with a density of 3×109 cm−2 extending along 〈0001〉 which sometimes form grain boundaries. An AlxGa1−xN/GaN interface roughness of 3 monolayers is estimated and a small AlxGa1−xN surface roughness of 1.5 nm is obtained by HRTEM and atomic force microscopy investigations which correspond to two-dimensional growth. C–V and Hall measurements reveal two-dimensional electron gas at the Al32Ga68N/GaN interface that has a sheet carrier concentration of 4×1012 cm−2. The electron mobility of 820 cm2/Vs measured at room temperature is applicable for a HEMT grown on Si(111).