Comparison of electrical, optical and structural properties of epitaxially grown HEMT's type AlGaN/AlN/GaN heterostructures on Al2O3, Si and SiC substrates

In this work we present the results of experimental studies on HEMT's (High Electron Mobility Transistor) type AlGaN/GaN heterostructures deposited by MOVPE (Metal Organic Vapor Phase Epitaxy) on two inch diameter Al2O3, Si and SiC substrates. The layer structure as well as detailed grown conditions of buffer layers of the investigated structures are described. The main attention is paid to comparison of the properties of nominally the same, AlGaN/GaN heterostructures deposited on the different substrates. SEM investigations show significant differences in size of hexagonal shaped mosaic observed on the surface of AlGaN/AlN/GaN heterostructures. In this paper an effort is made to explain the mechanism of grooves and micro-pits formation in the heterostructures deposited on Al2O3, Si and SiC substrates. The AlGaN/AlN/GaN heterostructures electrical parameters, investigated by impedance spectroscopy, optical properties measured by room temperature photoluminescence and surface morphology examined by scanning electron microscopy are presented and discussed. In addition, XRD measurments results are presented. All samples, regardless of the used substrate, exhibit good electrical performance with sheet carrier concentration and mobility of 2DEG (Two Dimensional Electron Gas) above 0.95 × 1013 cm−2 and 1450 cm2 V−1 s−1, respectively. Depending on the substrate type, distinct positions of AlGaN and GaN related photoluminescence peaks are observed. It is shown, that structures grown at the same conditions on different substrates, exhibit different electrical and optical behavior as a result of the different material quality and a surface microstructure.