Characterization of N-polar GaN/AlGaN/GaN Heterostructures Using Electron Holography

III-nitride high-electron mobility transistors (HEMTs) are in demand as commercial power amplifying devices based on their high breakdown field and high operating voltage, as well as wide band gap [1]. As opposed to the standard Ga-polar heterostructures, N-polar devices are better suited for sensors and enhancement mode transistors [2]; advantages include a strong back-barrier and low contact resistance [3]. One important feature of nitride-based HEMTs is the occurrence of a two-dimensional electron gas (2DEG) at or near the interface between adjacent layers [4]. The location of the 2DEG in nitride HEMTs has been shown to vary depending on whether the material is Ga-polar or N-polar. The 2DEG in GaN/AlGaN/GaN heterostructures grown on sapphire is located just above the upper GaN/AlGaN interface for the N-polar structure, and just below the lower AlGaN/GaN interface for the conventional Ga-polar structure [5]. In the current work, N-polar GaN/Al0.3Ga0.7N/GaN HEMT structures were grown on GaN substrates, as shown in Fig. 1, samples for TEM observation were prepared using Focused Ion Beam (FIB) standard lift-out and backside milling techniques (FEI Nova 200), and the piezoelectric fields and sheet charge were investigated using off-axis electron holography (Phillips CM200 FEG).