Influence of AlN Growth Temperature on the Electrical Properties of Buffer Layers for GaN HEMTs on Silicon

Despite the fact that a lower growth temperature is generally considered as a drawback for achieving high crystal quality, the necessity to reduce the nucleation temperature of AlN on Silicon has permitted Molecular Beam Epitaxy (MBE) to demonstrate high performance for GaN transistors operating at high frequency. Compared to Metal-Organic Vapor Phase Epitaxy (MOVPE), the control of the interface between the AlN nucleation layer and the substrate is easier and the reduced growth temperature allows to obtain a more electrically resistive interface while keeping good crystal quality. Moreover, it is shown that further reducing the growth temperature within the nucleation and stress mitigating layers has a noticeable impact on the lateral and vertical buffer leakage currents. At the same time the buffer of MBE grown HEMT structures exhibits low RF propagation losses (below 0.5dBmm(-1) up to 70GHz). Also, results obtained with structures regrown by MOVPE on MBE AlN-on-Si templates confirm that the thermal budget is critical for the resistivity of AlN/Si. On the other hand, the insertion of a 1.5m thick Al0.05Ga0.95N layer within a 2m HEMT structure significantly improves the vertical breakdown voltage up to 740V permitting to compare favorably with MOVPE epilayers with similar total thickness.