Nitride Based Metal Insulator Semiconductor Heterostructure Material and Device Design and Characterization
Al2O3/AlGaN/GaN metal-insulator-semiconductor-heterostructures (MISH) were designed, fabricated and characterized. The effects of different dielectric deposition techniques, surface treatments and post deposition treatments were investigated by comprehensive material and electrical characterization to understand the Al2O3 dielectric and Al2O3/AlGaN interfacial properties.
Thermal ALD and PEALD Al2O3 thin films were successfully deposited on MBE grown AlGaN/GaN layers. An XPS study reveals the band offset of Al2O3/AlGaN interface. In addition, pre-deposition treatments show a reduction of Ga-O bonds at the interface after ALD growth.
The fabrication of Al2O3/AlGaN/GaN MISH diodes were achieved with deposition of Ti/Al/Ni/Au ohmic contacts and Ni/Au gate contacts. C-V characterization of MISH diodes was applied to evaluate Al2O3/AlGaN interface states. Traps with different energy levels were differentiated by C-V hysteresis curves and multi-frequency C-V. C-V analysis suggests that PEALD provides better film quality with lower defect densities than thermal ALD. The implementation of NH3 and N2 pre-deposition surface plasma treatment and N2 post-deposition annealing can also improve interfacial properties.
Al2O3 dielectric thin film leakage current and conduction mechanisms were also studied by I-V characterization. PEALD Al2O3 thin films exhibit better leakage current suppression compared to thermal ALD films. Temperature dependent I-V characterization shows that Poole-Frenkel emission dominates in dielectric current transport at medium electric fields, while at high electric fields, Fowler-Nordheim tunneling and trap-assisted tunneling dominate at low and high temperatures, respectively. Various dielectric reliability tests were employed on Al2O3 thin films. The results of time dependent dielectric breakdown (TDDB) test can be fit into 1/E field dependent model and a Weibull slope of 2.87 is extracted for PEALD Al2O3 thin films. The dielectric breakdown field distribution statistics show that PEALD Al2O3 films have a larger average dielectric breakdown field than thermal ALD films, and the plasma N2O post deposition annealing improves the average breakdown field. The improvements from integration of pre-deposition and post deposition treatments may offer a better device performance and reliability in MIS-HEMTs, and enable further progress and development of nitride based power electronics.