Atomic Layer Deposition of Ultrathin Tunnel Barriers
This novel process uses atomic layer deposition (ALD) to grow a tunnel barrier one atomic layer at a time. Placed between two electrically conducting materials, a tunnel barrier forms a tunnel junction. Electrons pass through the barrier by quantum tunneling. This process forms more uniform, thinner, and lower defect tunnel barriers. By improving the likelihood of quantum tunneling, this process creates higher performing tunnel junction devices.
This process forms a tunnel barrier through multiple cycles of linked and self-limiting chemical reactions. Because each reaction is self-limiting, the growth surface can be overexposed to reactants which ensures uniform coverage of the growth surface while limiting the growth of the tunnel barrier to a single layer of atoms each cycle.
Tunnel junction-based devices have a broad range of application for products in both mature or rapidly maturing industries. They are critical in the design of high efficiency multijunction photovoltaic cells, magnetic RAM, hard disk read heads, Ezaki diodes, and qbits in quantum computers.
Atomic layer deposition uses self-limited surface reactions to grow films one atomic layer at a time. A Al2O3 tunnel barrier forms through a series of alternating pulses of water and trimethylaluminum. These precursors chemically react on the sample’s surface to create a fully oxidized, uniform and pinhole-free Al2O3 barrier with atomic-scale thickness control. Barrier height, a measure of tunnel barrier quality, increased by 40-50% compared to the industry standard AlOx barriers created using O2 diffusion into an Al film.
In general, over time material processing techniques move towards the manufacture of smaller feature sizes with greater processing control. This process enables a thinner, more uniform and thus higher performing tunnel barrier. Also, ALD is a low temperature process that reduces the thermal disruption of pre-exiting structures in the substrate.
This invention produces thinner, more uniform tunnel barriers. Consequently, there are fewer pinholes in the insulator for any given thickness, which improves the performance of the resulting tunnel junction device.