Thermal Stability of ALD-HfO 2 /GaAs Pretreated with Trimethylaluminium

Abstract

The simultaneous self-cleaning and passivation of an as-received GaAs substrate using trimethylaluminium (TMA) pretreatment before the atomic layer deposition (ALD) of HfO 2 was systematically investigated. The change of the interfacial characteristics was probed as a function of the number of treatment cycles and the post-deposition annealing temperature and related with various electrical properties of the HfO 2 film. The TMA pretreatment removed the Ga-and As-oxides more effectively than the ALD-HfO 2 process, which reduced the amount of frequency dispersion. In addition, compared to the As case, it showed more predominant suppression of both Ga out-diffusion and Ga-O bond formation, which is believed to have delayed the thermal degradation of the capacitance equivalent thickness (CET). The optimal number of TMA treatment exhibited the best thermal stability without increasing the as-deposited CET. For the development of high-k/GaAs metal-oxide-semiconductor (MOS) capacitors in high performance transistors with superior elec-trical properties, researchers have used various deposition techniques of high-k dielectrics such as molecular beam epitaxy, 1 sputtering, 2 and atomic layer deposition (ALD). 3 Among these techniques, ALD has been acknowledged as a strong candidate for industrial applica-tion due to its fine film quality, nearly perfect uniformity, and precise thickness controllability, attributed to its surface-saturation controlled, layer-by-layer deposition kinetics. 4 The ALD processing of HfO 2 and Al 2 O 3 films − the most widely studied high-k materials due to their outstanding dielectric properties – on GaAs substrates has recently revealed the self-cleaning effect of ALD as an additional benefit: the precursor dependent, in situ removal process of Ga-and As-related native oxides. 5, 6 Despite the introduction of additional ex situ clean-ing/passivation solutions, such as NH 4 OH, and (NH 4) 2 S, 8 prior to the high-k deposition, prompt surface contamination of GaAs substrates by ambient air exposure cannot be avoided during the sample transfer to the deposition chamber. Therefore, the development of in situ self-cleaning using a precursor with a strong reduction capability can be an essential choice for the high-k/GaAs integration process. When used as a metal precursor for the Al 2 O 3 deposition, trimethylaluminium (TMA) is known to have more effective self-cleaning capability for removing the Ga-and As-oxides than tetrakis(ethylmethylamino)hafnium (TEMAHf) used for the HfO 2 deposition. 5 Furthermore, Al 2 O 3 is known to have less Fermi level pinning and better thermal stability than HfO 2 on a GaAs substrate. 7