In Situ Observation of Two-Dimensional Electron Gas Creation at the Interface of an Atomic Layer-Deposited Al₂O₃/TiO₂ Thin-Film Heterostructure

Abstract

A two-dimensional electron gas (2DEG) was formed at the interface of an ultrathin Al 2 O 3 /TiO 2 heterostructure that was fabricated using atomic layer deposition (ALD) at a low temperature (<300 °C) on a thermally oxidized SiO 2 /Si substrate. A high electron density (∼10 14 cm −2 ) and mobility (∼4 cm 2 V −1 s −1 ) were achieved, which are comparable to those of the epitaxial LaAlO 3 /SrTiO 3 heterostructure. An in situ resistance measurement directly demonstrated that the resistance of the heterostructure interface dropped significantly with the injection of trimethylaluminum (TMA), indicating that oxygen vacancies were formed on the TiO 2 surface during the TMA pulse in the ALD of Al 2 O 3 films, such that they provide electron donor states to generate free electrons at the interface of the ultrathin Al 2 O 3 /TiO 2 heterostructure. The activation energy of the electron donor states to move to the Ti 3d conduction band plays an essential role in the electrical characteristics of the 2DEG. Interestingly, the donor state level can be tailored by the control of TiO 2 crystallinity, which eventually adjusts the electron density. The activation energy was decreased to less than 20 meV to generate ultrashallow donor states while improving the TiO 2 crystallinity, such that the 2D electrons become readily delocalized, even at room temperature, to create a 2DEG.

Harnessing Conductive Oxide Interfaces for Resistive Random-Access Memories

“…We consider the progress made with the All-ALD 2DEGs [25,33] to be a defining point. The All-ALD 2DEGs have liberated 2DEGs from small and expensive single-crystal substrates and from costly high-temperature fabrication processes.…”

“…2DEGs are now being fabricated by ALD on many substrates while keeping a low thermal budget and using low-cost, highly scalable, mature, and microelectronics-compatible techniques. The use of anatase TiO 2 provides another advantage of a potentially more conductive 2DEG compared to the LaAlO 3 /SrTiO 3 interface at room temperature [64] and offers tunability of the conductivity [32,33]. Higher conductivity of the 2DEG is desirable in RRAM applications because the resistance of the bottom electrode is in series with the resistive switching layer, making lower resistance beneficial for stabilizing the resistive switching behavior, and for reducing the operating power.…”

“…Later work has demonstrated that 2DEGs can be formed at more simple interfaces, between amorphous and single-crystalline oxides [30,31] with the benefit of room temperature preparation. Recently 2DEGs were shown to form even at amorphous/polycrystalline oxide interfaces [17,21,32,33] (Figure 1C). Furthermore, the oxide deposition temperatures were reduced from 650-900 °C to 25-300 °C, and the deposition techniques have been extended from PLD to the more scalable atomic layer deposition (ALD), which is widely used by the microelectronics industry.…”

“…The use of amorphous oxide on single crystal oxide substrates [16,24,30,31,[64][65][66][67][68] (Figure 1B) lowered the oxide deposition temperatures to <300 °C and expanded the deposition methods to ALD. The use of amorphous oxides deposited on polycrystalline oxide [17,21,25,32,33,69], which we term "All-ALD 2DEGs", provides a great advantage towards scalability and integration with existing and future technologies (Figure 1C). We note that significant progress has been demonstrated in MBE-based integration of epitaxial oxide 2DEGs systems with silicon [70][71][72] and other semiconductors [73,74].…”

Constructing Electron Levers in Perovskite Nanocrystals to Regulate the Local Electron Density for Intensive Chemodynamic Therapy

“…Chemical reactions are known to take place at the electrons in the outershell of reaction center atoms,w hose local electron density is the key to determine the reaction characteristics.A ny changes in electron density distribution following functionalization can change the chemical potential of reactive electrons,i nducing far-reaching impact on the activation energy of reactions. [1] Especially for transition metal atoms with reactive d-band electrons,such as Pt, Pd, Au and Fe,w hose catalytic reactivity are strongly dependent on their local electron density. [2] Increasing electron density with exogenous electron injection has been studied to improve the reactivity,b ut the side effects,i ncluding unexpected redox reactions and uncontrollable electrons accumulation, are inevitable.…”