The ALD Films of Al2O3, SiNx, and SiON as Passivation Coatings in AlGaN/GaN HEMT

In the field-effect transistors based on the wide-band-gap nitride heterostructures, the dielectric layers are in widespread use as one of the main elements in the active regions of the devices and the passivation layers. Stringent requirements are imposed on the dielectrics in terms of the high dielectric capacitance, large band-gap energy, and the coating integrity. Furthermore, the films must withstand high electric fields and have a low surface state density in the dielectric/semiconductor interface. For these purposes, the low temperature films grown by plasma-enhanced chemical vapor deposition, atomic layer deposition (ALD), and plasma-enhanced deposition are usually used as effective coatings. The ALD films of Al 2 О 3 , SiN х (Si 3 N 4 ), SiON, and ALD AlN are used most often, and offer the greatest promise for the AlGaN/GaN heterostructures. The influence of the passivation of the ALD Al 2 O 3 , SiN x , and SiON coatings of various thicknesses on the change of the charge and the density of the states of the AlGaN/GaN heterostructures is investigated. The physical parameters of the structures are estimated by the C – V characteristics measured on various frequencies and the I – V characteristics. It is demonstrated, according to the examined energy band diagrams of the structures at various control voltages, and estimation of the elemental composition of the films by the method of Auger electron spectroscopy, that the reason for the generation of a high positive charge at the deposition of the ALD Al 2 O 3 and SiN x films is the occurrence of an additional piezoelectric charge in the buffer layer of AlGaN. It is demonstrated that use of the SiON films with the oxygen concentration in them higher than 3% does not result in the generation of an additional positive charge but can initiate current fluctuations during the measurement of the I – V characteristics. A possible mechanism of carrier transport in the space charge region resulting in such fluctuations is discussed.