Improving the gaSb / High - k oxide Interface Using an InAs Capping layer CNF

Figure 2: n-GaSb/Al2O3 MOSCaps without (a) and with (b) InAs showing improvement in C-V characteristics with the InAs interface layer. Figure 1: TEM image showing the GaSb/InAs/Al2O3 gate stack with EDS scan to confirm the InAs interface layer. A highly reactive GaSb surface was passivated by growing a thin epitaxial InAs capping layer to improve the GaSb/high-k oxide interface. MOSCap structures were fabricated with and without InAs to study improvements in C-V characteristics and interface trap state density (Dit). Al2O3 and Al2O3/ HfO2 high-k oxides were deposited using atomic layer deposition (ALD) in the Oxford ALD FlexAL and annealed in forming gas (FG) at 350°C and 450°C. Optimized processing with (NH4)2S pre-treatment and 350°C anneal resulted in low frequency dispersion, improved stretch-out and a large reduction in Dit for both oxides. The interface with ALD oxides was thermally stable to 450°C. Summary of Research: Alternative p-channel materials are being investigated for their high hole mobility and low switching voltage potential in future CMOS circuits. III-Sb based heterostructures offer a promising replacement material that utilizes quantum confinement and bi-axial strain engineering to increase hole mobility [1]. The main challenge with GaSb is the high density of trap states formed at the III-V/high-k interface when exposed to ambient conditions. Terminated surface bonds react with oxygen and form states within the bandgap that cause Fermi level pinning. InGaAs III-V/high-k interfaces show a reduction in Dit when using chemical pretreatments and ALD of Al2O3 with trimethylaluminum (TMA) as a precursor [2]. During TMA exposure to the native III-V surface, a ligand exchange process occurs which reduces oxygen on the surface. This forms a high quality interface with a low Dit and an unpinned Fermi level. This project implemented a thin 2 nm InAs interface layer on a thick 1 μm n-GaSb (n = 5 × 1017cm-3 by Te) MOSCap structure grown on a n-GaAs substrate using MBE (Figure 1). Chemical pretreatments were used at CNF immediately prior to ALD of high-k oxide. An HCl:H2O pre-treatment was used to remove native oxides or (NH4)2S:H2O was used to remove oxides and passivate surface dangling bonds. Quality gate oxide was needed to allow for proper gate control while maintaining a low leakage current. The Oxford ALD FlexAL tool at CNF was used with the guidance of Vince Genova to deposit 10 nm oxide after surface pretreatments. Al2O3 was deposited using 100 cycles of TMA and H2O, and an alternative stack was deposited using ten cycles of Al2O3 and 100 cycles of TMAH and H2O to deposit Al2O3/HfO2. This gate oxide stack takes advantage of the initial TMA “selfcleaning” effect and the high permittivity of HfO2.