Interface Sulfur Passivation for Advanced MOSFETs with High Mobility Channel Materials

Abstract

Approaching with speed limit for nano-electronic device built on Si substrates, III-V compound semiconductor/Ge with a high mobility are gaining great interests to replace the Si substrate. However, the growth of interfacial layer (IL) between the gate insulator and III-V compound semiconductor/Ge substrate, which deteriorates the interfacial property, is still a critical problem. Passivating the interface of gate insulator (GI)/III-V and Ge substrates using sulfur (S) has been known to effectively suppress the interfacial layer growth and to improve the electrical properties of interface. Among the various methods for sulfur passivation, (NH4)2S solution treatment was commonly adopted because of simplicity of the process for laboratory experiments. However, the S passivation using (NH4)2S solution may not be appropriate for industrial mass-production since it is a wet chemical approach which can cause non-uniform S distribution on the substrate, difficulty in controlling S concentration, and surface residues of contaminants. Pre-deposition annealing of the substrate under H2S atmosphere is believed to be an appropriate method for replacing the S passivation using (NH4)2S solution wet process. In this work, the feasibility to replace the wet-process using (NH4)2S solution with the dry-process by annealing under H2S atmosphere was examined for the interface S passivation in metal-insulator-semiconductor capacitor (MISCAP) devices fabricated on Ge substrate. Atomic-layer-deposited (ALD) HfO2 film was grown on Ge substrate after surface S passivation. The H2S annealing provided uniform distribution over Ge surface and solid S passivation with the strong resistance against oxidation during ALD process. The electrical thickness of the gate insulator by S passivation decreased and interface state density near the valence band edge suppressed, as the annealing temperature increases, because thermal energy during the annealing resulted in stronger S bonding and Ge surface reconstruction. Moreover, the hysteresis was lower for the device with H2S annealing at 400 oC. Surface Sulfur(S) passivation on InP substrate was performed using a dry process - rapid thermal annealing under H2S atmosphere for III-V compound-semiconductor-based devices. In order to minimize thermal degradation of the InP compound semiconductor surface, rapid thermal annealing (RTA) was a good process under the H2S environment. The electrical properties of metal-oxide-semiconductor capacitor fabricated with atomic-layer-deposited HfO2 film as a gate insulator were examined, and were compared with the similar devices with S passivation using a wet process-(NH4)2S solution treatment. The H2S annealing provided solid S passivation with the strong resistance against oxidation compared with the (NH4)2S solution treatment, although S profiles at the interface of HfO2/InP were similar. The decrease in electrical thickness of the gate insulator by S passivation was similar for both methods. However, the H2S annealing was more effective to suppress interface state density near the valence band edge, because thermal energy during the annealing resulted in stronger S bonding and InP surface reconstruction. Moreover, the flatband voltage shift by constant voltage stress was lower for the device with H2S annealing. Atomic-layer-deposited Al2O3 films were grown on ultrathin-body In0.53Ga0.47As substrates for III-V compound-semiconductor-based devices. Interface sulfur (S) passivation was performed with wet processing using ammonium sulfide ((NH4)2S) solution, and dry processing using post-deposition annealing (PDA) under a H2S atmosphere. The PDA under the H2S atmosphere resulted in a lower S concentration at the interface and a thicker interfacial layer than the case with (NH4)2S wet-treatment. The electrical properties of the device, including the interface property estimated through frequency dispersion in capacitance, were better for (NH4)2S wet-treatment than the PDA under a H2S atmosphere. They might be improved, however, by optimizing the process conditions of PDA. The PDA under a H2S atmosphere following (NH4)2S wet-treatment resulted in an increased S concentration at the interface, which improved the electrical properties of the devices.