Analog-compatibility in metastable correlated perovskites via μm-thick wafer-scale growths

The intricate d-orbital electronic phase diagram in correlated perovskite oxides gives rise to rich quantum phenomena as primarily driven by strong electron correlation effects, providing an analog paradigm to transcend performance limitations of post-Moore semiconductor devices. Among existing correlated systems, the nickelates (RENiO₃) exhibit metal-insulator transitions (MIT) with the highest tunability, stemming from their high tolerance in distorting the NiO₆ octahedra for broad regulatory scope of Ni-3d/O-2p orbital overlapping and its potential to (un)shield electron Coulomb energy U. Beyond conventional Mott-typed MIT via switching between manifestation and shielding of U, the strength of U is more directly adjustable from the perspective of Ni-3d orbital occupations via Mottronic protonation. Harnessing the resultant extensive resistive modulations to exploit analog neuromorphic simulations for neural networks or bionic sensing systems has realized a quantum leap in computability and energy efficiency, rivaling biological brains.

Nevertheless, the analog electronic applications of correlated perovskites are constrained by their currently ineffective material growths, in terms of fidelity and scalability. Limitation in fidelity stems from the absence in feasible avenue to fabricate correlated perovskites at mm-thickness, a critical scale to enable their device resistances as compatible with efficient analog circuit requirements. The scalability in high quality film growths of correlated perovskites is struggled to source a lattice matching substrate at wafer-scale. Also, the inherent metastability of RENiO₃ also challenges their scalability in growth, as it requires MPa-high oxygen pressure (p_O2) to counteract their positive formation free energies (ΔG). To bridge the gap between the fundamental research frontier and practical electronic applications of correlated perovskites, it is urgent to develop an effective avenue that enables their scalable growth at mm-thickness.

Realizing oriented growth of RENiO₃ over large thickness is conventionally obstructed by the inevitable thermodynamic fluctuation during long periods. This is exemplified in RENiO₃ that relies on post annealing of their pre-deposited cation precursors at MPa-high p_O2 for crystallization, impeding their scalable growth. Also, the post annealing results in distinct thermodynamic landscape toward and far away from a coherent interface with the substrate. Consequently, the oriented growth is only sustained within ~10 nm thickness from the interface, while precursors located further away usually homogeneously crystallized, showing polycrystallinity and less abrupt MITs.

The researchers develop an alkali-chloride molten salt assisted low temperature liquid phase epitaxy (LPE) process, through which much more stable thermodynamic environment and constant ionic precursors are provided sustaining long period epitaxy. The molten alkali-chloride gradually dissolves the RE₂O₃ and NiO to steadily supply ionic RE³⁺ and Ni³⁺ at constant concentrations at the surface region at low temperatures. Furthermore, it is capable of descending the temperature of LPE down to exceptionally low magnitude without sacrificing the kinetic perspective. Descending temperatures provides an alternative route to reduce ΔG, apart from involving high-p_O2, shedding a light on their scalable growth at ambient pressure.

To elucidate the underneath microscopic mechanisms for extending oriented growths of RENiO₃ beyond μm-thick, the high-angle annular dark-field was performed to investigate their cross-section morphology. The stepwise formation of stacking faults mildly relaxes the lattice constants of RENiO₃ initially compliant to the substrate towards their bulk magnitudes, avoiding the previously sudden changes in thermodynamic conditions owing to more abruptly relaxed interfacial coherency. Hence, the preferential crystal orientation of RENiO₃ is sustainable over μm-thickness and across large magnitudes of lattice mismatches. Consequently, widely adjustable MIT properties were achieved for as-grown RENiO₃ via substituting RE, with a wide range in MIT temperature from 100 K to 500 K. Their μm-scaled thicknesses enable the optimum range of sheet resistance for both insulating and metallic phases catering for direct analog electronic applications, as previously unachievable.

The LPE strategy further provides an alternative avenue to manufacture miniaturized discrete devices via encapsulating the μm-thick correlated perovskites over external oxide supporters, instead of directly using their bulk pellets. The perovskite encapsulations provide an alternative avenue for their applications in discrete device, by separating the electronic and mechanical functions as provided by the encapsulation layer and supporter, respectively. This well circumvents the inherent high fragility and low resistivity of correlated perovskites, as two major obstacles impeding their practical discrete device applications.

As-made critical temperature resistor (CTR) devices based on RENiO₃ encapsulations largely extend the range of critical temperature (T_C) down to cryogenic temperature ranges, as yet uncoverable via mature CTR thermistors based on vanadium oxides or doped BaTiO₃. To build a temperature alarming module for a specific T_C, as-made CTR thermistors via RENiO₃ encapsulations were connected with a comparator circuit. Integrating such modules with various T_MIT establishes a temperature alarming array that can alarm a gradient of T_C with similar intervals. Compared to conventional cryogenic temperature sensing systems based on Pt sensors and analog-to-digital converter, the system exhibits one order higher sensitivity and two orders of magnitude lower energy consumption. It well caters for the extensive application demands for temperature alarming in comprehensive cryogenic fields.

To further cater for prospect integrated circuit analog electronic applications, RENiO₃ must be grown at wafer-size on scalable substrate wafers with potential lattice coherency at high-index crystallographic planes. They successfully grew RENiO₃ covering various rare-earth compositions on sapphire via LPE, with a preferential RENiO₃ (101) orientation compared to the Al₂O₃ (0001). To amplify the growth of RENiO₃ towards wafer scale, it is further necessary to stabilize their metastable perovskite structure in ambient oxygen or even in air, relieving their previous reliance on MPa-high p_O2. They used eutectic KCl and LiCl molten salt to grow RENiO₃ in air or flowing oxygen, in which situation abrupt MIT properties similar to the MPa-high p_O2 growth were achieved for RENiO₃ with a variety of RE. Hence, it is capable of amplifying the growth of RENiO₃ on 2-inch and 3-inch sapphire wafers, paving the way to their practical applications in the integrated circuit electronics.

In addition to nickelates, the LPE strategy was also successfully extended to other multicomponent oxides, such as Pr_0.67Sr_0.33MnO₃, SrRuO₃ and Bi₂Sr₂CaCu₂O₈. Thus, it exhibits broad applicability in the μm-thick film growth of both perovskites and non-perovskites correlated systems, covering a wealth of functionalities, such as magnetoresistance, oxide electrode, and superconductivity.