Present-day computing devices are approaching performance limits due to excess heat and parasitic capacitance degrading performance. One proposed solution, termed “neuromorphic computing”, is to perform computations similarly to the brain.

A key enabling device for neuromorphic computing is the lithium niobite (LiNbO2) memristor[1]. Like synapses in the brain, LiNbO2 memristors can exhibit excitatory or inhibitory behavior. This is due to the ability to produce both n and p-type LiNbO2 memristors, a unique property for an oxide. As shown in Figure 1, when a voltage is applied across an n-type memristor (left), resistance increases over time, whereas a p-type memristor exhibits decreasing resistance (right). N-type memristors are grown with excess oxygen vacancies while p-type memristors are grown with excess lithium vacancies. N- and p- type films are very conductive, exhibiting resistances of 4.3x10-4 Ω-cm(p-type) and 3.9x10-4 Ω-cm(n-type) with carrier concentrations higher than 1021 cm-3 and Hall mobilities greater than 8 cm2/V∙s.

The epitaxial growth of LiNbO2 is the enabling technology for the memristors described above. Growths are conducted using an oxy-chloride MBE system in which metal-halide sources interact with lithium and oxygen [2]. Lithium acts as a getterer for the chlorine from the metal-halide, and the resulting LiCl is desorbed from the heated growth surface. The bare metal then oxidizes under the application of a oxygen plasma and combines with lithium. This growth chemistry has been used to produce dielectric/ferroelectric lithium niobate (LiNbO3), semiconducting lithium niobite (LiNbO2), and lithium cobalt oxide (LiCoO2).

Insulating LiNbO3 (Figure 2) and semiconducting LiNbO2 (Figure 3) are grown using NbCl5, and the phase of the material is controlled by the niobium to oxygen ratio delivered to the growth surface. Both single crystal LiNbO3 and LiNbO2 have been epitaxially grown on Al2O3 and SiC. LiNbO3 is a promising lattice matched, ferroelectric transistor gate high-k oxide for its semiconducting sub-oxide, LiNbO2. LiNbO2 has a layered, lithium-intercalated structure [3] (Figure 4). When lithium is removed, holes are introduced which increases the conductivity of p-type material and decreases the conductivity of n-type material, thus producing the memristance effect described herein.

LiCoO2 is grown on Al2O3 using CoCl2 as the precursor (Figure 5). Like LiNbO2, LiCoO2 has a layered structure that facilitates lithium movement. The lattice spacing of LiCoO2 is closely matched to LiNbO2 (2.81 Å and 2.91 Å respectively), thus making it an attractive material for heterostructures of lithium-bearing semiconductor materials.