Lithium niobate is a multi-functional material with wide reaching applications in acoustics, optics, and electronics. Commercial applications for lithium niobate require high crystalline quality currently limited to bulk and ion sliced material. Thin film lithium niobate is an attractive option for a variety of integrated devices, but the research effort has been stagnant due to poor material quality. Both lattice matched and mismatched lithium niobate are grown by molecular beam epitaxy (MBE) and studied to understand the role of substrate and temperature on nucleation conditions and material quality. Growth on sapphire produces partially coalesced columnar grains with atomically flat plateaus and no twin planes. A symmetric rocking curve shows a narrow linewidth with a full width at halfmaximum (FWHM) of 8.6 arcsec (0.0024°) which is comparable to the 5.8 arcsec rocking curve FWHM of the substrate, while the film asymmetric rocking curve is 510 arcsec FWHM. These values indicate that the individual grains are relatively free of long-range
Metal-Nb2O5−x-metal memdiodes exhibiting rectification, hysteresis, and capacitance are demonstrated for applications in neuromorphic circuitry. These devices do not require any post-fabrication treatments such as filament creation by electroforming that would impede circuit scalability. Instead these devices operate due to Poole-Frenkel defect controlled transport where the high defect density is inherent to the Nb2O5−x deposition rather than post-fabrication treatments. Temperature dependent measurements reveal that the dominant trap energy is 0.22 eV suggesting it results from the oxygen deficiencies in the amorphous Nb2O5−x. Rectification occurs due to a transition from thermionic emission to tunneling current and is present even in thick devices (>100 nm) due to charge trapping which controls the tunneling distance. The turn-on voltage is linearly proportional to the Schottky barrier height and, in contrast to traditional metal-insulator-metal diodes, is logarithmically proportional to the device thickness. Hysteresis in the I–V curve occurs due to the current limited filling of traps.
Analog memristors that exhibit an electronic conductivity change in response to ionic motion have been simulated using the finite element method. Several physical mechanisms are considered for the redistribution of dopants within the device and all result in minimal resistance changes. The mechanisms considered that result in minimal resistance changes are initial ion concentration, hole mobility dependence on acceptor concentration, and geometry. In contrast, ion extraction results in a significant change in the simulated analog memristor resistance (many orders of magnitude). It is determined that if ions can be repeatedly cycled without damage to the crystal structure, ion extraction is the optimal analog ionic memristor operation mechanism. Given this conclusion, battery technology materials known for their robustness in spite of repeated ion extraction/replacement should be considered for reliable analog memristor applications.
The growth of seeded and self-nucleated LiNbO 2 crystals with a simplified chemistry has been demonstrated using a liquid phase electro-epitaxy method at high temperatures (900 °C). X-ray diffraction (XRD) was used to quantify the crystal quality and confirmed that both the seeded and self-nucleated crystals grown were single-orientation; the full width at halfmaximum (FWHM) for the symmetric XRD double crystal diffraction scan was 230 arcsec, while the FWHM for the XRD omega rocking curve was 19.2 arcmin and 310 arcsec for selfnucleated and seeded crystals, respectively. The LiNbO 2 crystals have undetectable amounts of contamination as determined via X-ray photoelectron spectroscopy and secondary ion mass spectrometry. Finally, as-grown LiNbO 2 crystals were shown to exhibit a memristive response and could be utilized in future neuromorphic computing architectures.
The role of stoichiometry and growth temperature in the preferential nucleation of material phases in the Li-Nb-O family are explored yielding an empirical growth phase diagram. It is shown that while single parameter variation often produces multi-phase films, combining substrate temperature control with the previously published lithium flux limited growth allows the repeatable growth of high quality single crystalline films of many different oxide phases. Higher temperatures (800-1050°C) than normally used in MBE were necessary to achieve high quality materials. At these temperatures the desorption of surface species is shown to play an important role in film composition. Using this method single phase films of NbO, NbO2, LiNbO2, Li3NbO4, LiNbO3, and LiNb3O8 have been achieved in the same growth system, all on c-plane sapphire. Finally, the future of these films in functional oxide heterostructures is briefly discussed.The lithium niobium oxide family consists of conducting, semiconducting, and insulating materials across a wide resistivity and bandgap range. Figure 1 lists the resistivity of a few materials in the system as a function of niobium valence, ranging from conducting niobium to insulating lithium niobite (LiNbO3). These materials span 22 orders of magnitude in resistivity with bandgaps from IR to UV [1][2][3][4][5][6][7][8]. Oxides in general have many desirable multifunctional properties, for example; piezoelectric, pyroelectric, and ferroelectric effects which can exist in a single material such as lithium niobate [9,10]. Lithium niobite (LiNbO2), a suboxide of the same family, is currently the focus of multiple research areas. LiNbO2 is used as a memristor for neuromorphic applications, a battery cathode material showing potential for high rate capability and long term cycle stability, and is also studied for unique optical properties [11][12][13]. NbO2 acts as a digital memristor, a device with discrete on and off resistance states. NbO2 is currently used in memory, neuristor circuitry, and relaxation oscillator circuitry [14][15][16]. The Li-Nb-O material family also includes other ceramics of various dielectric constants used in a variety of applications including battery electrodes, microwave frequency dielectrics, phosphors, photocatalysts for water reduction, and hysteretic MIM tunnel diodes or "memdiodes" (Li3NbO4, LiNb3O8, and Nb2O5) [17][18][19][20][21][22]5,23,24].This fundamental understanding will allow new devices and heterostructures to be explored including but not limited to neuromorphic computing elements, strain enhanced sensors and MEMS, high K dielectrics, tunable dielectrics, ferroelectric superlattices, and ferroelectric switching transistors with switchable enhanced channel conductance.With this motivation in mind the materials in the Li-Nb-O family are analyzed by crystal quality and surface morphology for use in future heterostructure devices. In this architecture it is shown that LiNbO2, LiNbO3, and NbO2 are the multi-functional active materials while niobium can be used...
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