Atomic layer deposition of ultrathin tunnel barriers
Methods for forming tunnel barrier layers are provided, including a method comprising exposing a surface of a material, the surface free of oxygen, to an initial water pulse for a pulse time and at a pulse temperature, the pulse time and pulse temperature selected to maximize hydroxylation of the surface; and exposing the hydroxylated surface to alternating, separated pulses of precursors under conditions to induce reactions between the hydroxylated surface and the precursors to form a tunnel barrier layer on the surface of the material via atomic layer deposition (ALD), the tunnel barrier layer having an average thickness of no more than 1 nm and being formed without an intervening interfacial layer between the tunnel barrier layer and the surface of the material.
1. A method for forming a tunnel barrier layer for a multilayer structure, the method comprising:
(a) exposing a surface of a material, the surface free of oxygen, to an initial water pulse for a pulse time and at a pulse temperature, the pulse time and pulse temperature selected to maximize hydroxylation of the surface; and
(b) exposing the hydroxylated surface to alternating, separated pulses of precursors under conditions to induce reactions between the hydroxylated surface and the precursors to form a tunnel barrier layer on the surface of the material via atomic layer deposition (ALD), the tunnel barrier layer having an average thickness of no more than 1 nm and being formed without an intervening interfacial layer between the tunnel barrier layer and the surface of the material.
2. The method of claim 1 , wherein the material is a wetting layer.
3. The method of claim 2 , wherein the wetting layer has an average thickness of at least 5 nm.
4. The method of claim 2 , wherein the wetting layer is formed directly on a layer of a metal, a layer of a ferromagnetic material, or a layer of a semiconductor and the method further comprises forming another layer of the metal, another layer of the ferromagnetic material, or another layer of the semiconductor directly on the tunnel barrier layer.
5. The method of claim 1 , wherein the material is a ferromagnetic material and the method further comprises forming another layer of the ferromagnetic material directly on the tunnel barrier layer.
6. The method of claim 5 , wherein the ferromagnetic material is Fe having a single-crystalline Fe(100) surface, a single-crystalline Fe(110) surface, or a polycrystalline Fe(100)/Fe(110) surface.
7. The method of claim 1 , wherein the pulse time and the pulse temperature are selected to provide an ALD coverage of at least about 85% after 1 ALD cycle.
8. The method of claim 1 , wherein the pulse time is selected to provide no more than a monolayer of adsorbed water on the surface.
9. The method of claim 8 , wherein the pulse time is in the range of from about 1 second to about 3 seconds.
10. The method of claim 1 , wherein the pulse temperature is in the range of from about 150° C. to about 190° C.
11. The method of claim 1 , wherein the tunnel barrier layer has an average thickness corresponding to one ALD cycle.
12. The method of claim 1 , wherein steps (a) and (b) are carried out in an ALD chamber which has been preheated prior to carrying out steps (a) and (b).
13. The method of claim 1 , further comprising, prior to step (a), dynamically heating the surface at a heating rate and for a heating time to achieve the selected pulse temperature.
14. The method of claim 13 , wherein the heating time is no greater than about 30 minutes.
15. The method of claim 1 , wherein the tunnel barrier layer is composed of Al 2 O 3 or MgO and is characterized by an average tunnel barrier height E b of at least 1.2 eV.
16. The method of claim 1 , wherein the tunnel barrier layer is characterized by an average tunnel barrier height E b which is the same over an average thickness range of from 0.12 to 1 nm.
17. The method of claim 1 , wherein the pulse time is in the range of from about 1 second to about 3 seconds and the pulse temperature is in the range of from about 150° C. to about 190° C.
18. The method of claim 17 , further comprising, prior to step (a), dynamically heating the surface at a heating rate and for a heating time of no greater than about 30 minutes to achieve the selected pulse temperature.