Corresponding Author

Jochen Mannhart

Max Planck Institute for Solid State Research

References

1.
Ohtomo, A.; Hwang, H.Y.
A high-mobility electron gas at the LaAlO3/SrTiO3heterointerface
2.
Förg, B.; Richter, C.; Mannhart, J.
Field-effect Devices Utilizing LaAlO3/SrTiO3 Interfaces
3.
Jany, R.; Richter, C.; Woltmann, C.; Pfanzelt, G.; Foerg, B.; Rommel, M.; Reindl, T.; Waizmann, U.; Weis, J.; Mundy, J.A.; Muller, D.A.; Boschker, H.; Mannhart, J.
Monolithically Integrated Circuits from Functional Oxides


In collaboration with:

R. Jany (Augsburg University, Augsburg, Germany)

J.A. Mundy, D.A. Muller (Cornell University, Ithaca, USA)

Department "Solid State Quantum Electronics"

Monolithically integrated circuits from functional oxides

Authors

C. Richter, C. Woltmann, G. Pfanzelt, B. Foerg, M. Rommel, T. Reindl, U. Waizmann, J. Weis, H. Boschker, and J. Mannhart

Departments

Solid State Quantum Electronics (Jochen Mannhart)

The rich array of conventional and exotic electronic properties that can be generated by oxide heterostructures is of great potential value for device applications. However, only single transistors bare of any circuit functionality have been realized from complex oxides. Here we report on monolithically integrated NMOS logic circuits that utilize a two-dimensional electron liquid generated at an oxide interface as channel material. These results illustrate the practicability and the potential of oxide electronics.

Introduction

A particularly intriguing aspect of oxide electronics is the combination of functional oxides with Si-CMOS (complementary metal-oxide-semiconductor) technology, which has the potential to greatly extend the performance of silicon devices. Integrated circuits (ICs) built from functional oxides are needed to fully exploit the functionality of oxides. Whereas ZnO-based ICs have been demonstrated, they solve this issue only partially, because the mobile electron system of ZnO is based on s and p electrons and therefore bears the traits of standard, mean-field semiconductors such as Si and GaAs. Much richer functional electronic properties are provided by complex oxides with electron systems based on d or f electrons. Electronic correlations are found in such systems and the materials display functional properties that include magnetism of many kinds, memristive effects, multiferroicity, and sensor functions. However, it has not yet been possible to fabricate from complex oxides transistors that switch other transistors, which is a prerequisite for integrating functional oxides into ICs.

<br /><strong>Fig. 1:</strong> a) Photograph of an array of LaAlO<sub>3</sub>&ndash;SrTiO<sub>3</sub> FETs. Source (S), Drain (D), and Gate (G) of three FETs are contacted via wirebonding. b) Photograph of a LaAlO<sub>3</sub>&ndash;SrTiO<sub>3</sub> chip carrying arrays with more than 700,000 FETs with channel lengths as small as &asymp;350nm. The colors are interference colors arising from the transistor patterns. From [3]. Zoom Image

Fig. 1: a) Photograph of an array of LaAlO3–SrTiO3 FETs. Source (S), Drain (D), and Gate (G) of three FETs are contacted via wirebonding. b) Photograph of a LaAlO3–SrTiO3 chip carrying arrays with more than 700,000 FETs with channel lengths as small as ≈350nm. The colors are interference colors arising from the transistor patterns. From [3]. [less]

To fabricate oxide field-effect transistors (FETs) with a voltage gain large enough for the devices to be used as building blocks of integrated circuits, materials with conductances that show a strong response to transverse electric fields have to be found for the drain-source channels. Among others, two-dimensional (2D) electron systems induced in oxide heterostructures appear to be promising candidates. First, the electric-field response of such systems is expected to be high, because their electrostatic screening lengths usually exceed the thickness of the conducting sheet. Second, some of these 2D-systems undergo a metal-insulator transition as a function of carrier density, making them candidates for phase-transition transistors. Electron systems that are ultrathin and in many cases even two-dimensional have been induced in heterostructures of complex oxides [1]. Using the 2D electron liquid (2DEL) which forms at the LaAlO3-SrTiO3 interfaces, n-channel field effect devices have been fabricated (Fig. 1) and voltage gains larger than one have recently been obtained [2]. In such heterostructures the voltage gain is particularly large because the drain-source channels are embedded in materials with high electronic susceptibilities, the room-temperature small-field dielectric constants of LaAlO3 and SrTiO3 equaling ≈24 and ≈300, respectively.

Results

<strong>Fig. 2:</strong> a) Optical microscopy image (interference contrast) of a LaAlO<sub>3</sub>&ndash;SrTiO<sub>3</sub> ring-oscillator chip, showing parts of five oscillators. b) View of two FETs of one inverter stage. c) View of one voltage divider and one output stage. From [3]. Zoom Image
Fig. 2: a) Optical microscopy image (interference contrast) of a LaAlO3–SrTiO3 ring-oscillator chip, showing parts of five oscillators. b) View of two FETs of one inverter stage. c) View of one voltage divider and one output stage. From [3]. [less]
<p><strong>Fig. 3:</strong> Output signal of a LaAlO<sub>3</sub>-SrTiO<sub>3</sub> ring-oscillator operated at room temperature. From [3].</p> Zoom Image

Fig. 3: Output signal of a LaAlO3-SrTiO3 ring-oscillator operated at room temperature. From [3].

We employed top-gated LaAlO3-SrTiO3 FETs to build NMOS (n-type metal-oxide-semiconductor) ring-oscillators [3]. To facilitate the fabrication of the integrated circuits, FETs with gate lengths of tens or hundreds of micrometer were used. NMOS-type inverters were built by connecting switching-FETs and load-FETs in series. To obtain the different saturation currents required, we used switching-FETs and load-FETs of different channel lengths (50–100µm for the load FETs; l=100–200µm for the switching-FETs) and kept the channel width fixed at 600µm. The ring-oscillators consist of three cascaded inverter stages and an output stage. Each oscillator comprises eight FETs as well as eight LaAlO3-SrTiO3 40kΩ resistors, forming four 1:1 voltage dividers. These circuits had a minimum feature size of 40μm (Fig. 2).

As shown in Fig. 3, the oscillators work with an output voltage swing of ≈300mV, and an RC time-constant-controlled oscillation frequency of 1.4kHz, both values matching the expectation. Each chip fabricated carries six oscillators.

Conclusion

The successful operation of integrated circuits based on LaAlO3-SrTiO3 heterostructures proves that with the deposition and patterning processes available, monolithically integrated, active circuits of functional oxides can be fabricated, despite the defects present in heterostructures of complex oxides. The fabricated circuits consist of oxide FETs, oxide resistors, and oxide interconnects. Diodes and capacitors based on the same material system have already been demonstrated. While these results were obtained with one specific heterostructure system, the LaAlO3-SrTiO3 interface, they indicate the viability of oxide components and devices to complement Si CMOS technology. We expect that faster devices and CMOS circuits can be implemented once suitable materials with a higher room-temperature mobility and also hole-based oxide interface systems are implemented and optimized [3]. Here, a wide spectrum of possibilities is offered by the large variety of oxide materials that are available and by the freedom of choosing the architecture of the heterostructures. Oxide electronic circuits will be characterized by the high functionality of the oxides and by the fact that their fundamental properties, such as the possible presence of phase transitions or the existence of strong electronic correlations, differ from those of conventional semiconductors.


 
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