Semiconductor materials are being used in an increasingly diverse array of applications, with new device concepts being proposed each year for solar cells, flat-panel displays, sensors, memory, and spin transport. This rapid progress of invention outpaces the development of new semiconductor materials with the required properties and performance. In many applications, high carrier mobility at room temperature is required in addition to specific functional properties critical to the device concept. We review recent developments on high mobility stannate perovskite oxide materials and devices.

Many electronic device architectures require multiple functionalities from the constituent materials. For example, transparent conducting oxides for display applications must combine high conductivity with low carrier concentration and absence of interband transitions to reduce optical absorption; high on/off ratios for transistors can only be achieved with a highly conducting on state combined with a highly insulating off state. For widespread use, all devices must be designed to operate at or above room temperature.

Perovskite oxides, with the chemical formula ABO3, represent an important, useful, and broad class of materials which includes metals, semiconductors, and insulators displaying a rich variety of functional properties such as magnetism, piezoelectricity, ferroelectricity, or superconductivity depending on their chemical composition.1–12 The edge-sharing octahedral oxygen network in perovskites can support a wide variety of cations on the A and B sites,1 and combined with advances in layer-by-layer growth, a dizzying variety of heterostructures and artificially layered materials with tunable physical properties can be realized. With major improvements in synthesis in recent years, it has been possible to achieve ultrahigh mobilities in doped perovskite semiconductors at low temperatures.13,14 However, mobilities are strongly temperature dependent and become small near room temperature: to date, this low mobility at room temperature has been a major obstacle to perovskite-oxide electronics. The discovery and development of perovskite materials with high carrier mobility at room temperature are thus of significant interest.

Recently, a critical breakthrough in room temperature mobility of perovskite oxides has been reported in alkaline earth stannates RSnO3 (R = Ba, Sr, and Ca), with most attention focused on R = Ba (Fig. 1). BaSnO3 is a cubic insulating perovskite with lattice parameter of 4.12 Å15 and a wide band gap, at least 3.3 eV while certain high quality crystals show band gaps up to 4.05 eV.16,17 It has excellent thermal stability up to 1000 °C,18 making it useful as a material for thermally stable capacitors, humidity sensors, gas sensors, and similar high temperature applications.19–21 

FIG. 1.

Four single crystals of BaSnO3 (BSO) placed on a 1 cm2 SrTiO3 substrate which itself rests on silicon integrated circuits. Reddish crystals of BaSnO3 are grown with PbO-based flux while the white ones are grown with Cu2O-based flux. The former shows a mobility of ∼100 and the latter shows a mobility of ∼300 (both in units of cm2V −1s−1).

FIG. 1.

Four single crystals of BaSnO3 (BSO) placed on a 1 cm2 SrTiO3 substrate which itself rests on silicon integrated circuits. Reddish crystals of BaSnO3 are grown with PbO-based flux while the white ones are grown with Cu2O-based flux. The former shows a mobility of ∼100 and the latter shows a mobility of ∼300 (both in units of cm2V −1s−1).

Close modal

BaSnO3 (BSO) can be doped with a variety of dopants both on the A site and the B site, including Sb, Cr, Ni, Mn, Fe, Pb, and La, to induce magnetism, conductivity, or photocatalytic activity.22–27 Remarkably, La doped single-crystals of LaxBa1−xSnO3, where x ranges from 0 to 0.07, have shown room temperature mobilities of ∼100 cm2 V−1 s−1 (Ref. 16) and up to ∼300 cm2 V−1 s−1.17 For comparison, at room temperature n-doped ZnO has a mobility of ∼200 cm2 V−1 s−1 (Ref. 28) and up to >400 cm2 V−1 s−1 for undoped 2DEGs.29 Doped titanate perovskites have much lower values of ∼2 for BaTiO3,30 ∼8 for CaTiO3,31 or ∼11 for SrTiO3,32,13 (all in units of cm2 V−1 s−1). The high electron mobilities in La-doped BaSnO3 (LBSO) are associated with the cubic structure and the nature of the Sn 5s dominated conduction bands: the cubic structure results in straight O–Sn–O connectivity (180° bond angles) which, combined with the relatively large size of the Sn 5s orbital, results in a dispersive conduction band with computed effective masses of m* ≤ 0.4me (me is the bare electron mass).33,17,34,35 The room temperature mobilities of single crystals are understood to be limited by acoustic phonon-electron scattering.17 

High room-temperature mobility, ∼70 cm2 V−1 s−1, has also been achieved in thin film samples of LBSO grown on SrTiO3.17 The significant drop in mobility compared to bulk samples is explained by extrinsic scattering mechanisms, primarily due to grain boundaries and threading dislocations;17 in thin films, such defects can dramatically suppress the mobility. The lattice mismatch with SrTiO3 is above 4%, and thus, an abundance of such defects is expected to be present in the thin films. Similar reductions in mobility due to dislocations are observed for GaN thin films,36 and one can expect mobilities for thin film BaSnO3 to further improve once better lattice matched substrates are utilized.

These advances open up many possibilities for all-oxide electronic devices. Here, we highlight the demonstration of an all epitaxial field-effect transistor based on LBSO by Kim et al.37 A lightly La doped channel of LBSO is epitaxially grown on a single crystal SrTiO3 substrate. Epitaxial LaInO3 is used as an insulating gate oxide, and highly doped LBSO is used for the source and drain. This all-epitaxial device achieves a large on-off ratio of ∼107 along with a room temperature mobility—derived from the transistor characteristics and an electrostatic estimate of carrier concentration—that is as high as 90 cm2 V−1 s−1 (see Figure 2). Misfit dislocations due to the large BSO-SrTiO3 lattice mismatch likely limit the mobility (Fig. 2): the use of single crystal substrates of BSO promises to improve the mobility for such a conventional FET device.

FIG. 2.

(a) Basic device performance of an all perovskite FET based on LBSO. Left axis shows source-drain current modulation by ∼107 and right axis presents mobility, all as a function of source-gate voltage. From Ref. 37. Reproduced with permission from APL Mater. 3, 036101 (2015). Copyright 2015 Author(s), licensed under a Creative Commons Attribution. (b) Cross sectional transmission electron micrograph of a LBSO film grown on a SrTiO3 substrate. Misfit dislocation arrays in the LBSO film (see inset) with a period of 8 nm are found.45 

FIG. 2.

(a) Basic device performance of an all perovskite FET based on LBSO. Left axis shows source-drain current modulation by ∼107 and right axis presents mobility, all as a function of source-gate voltage. From Ref. 37. Reproduced with permission from APL Mater. 3, 036101 (2015). Copyright 2015 Author(s), licensed under a Creative Commons Attribution. (b) Cross sectional transmission electron micrograph of a LBSO film grown on a SrTiO3 substrate. Misfit dislocation arrays in the LBSO film (see inset) with a period of 8 nm are found.45 

Close modal

Polarization doping of pure BSO may produce even higher mobilities.38,39 The most direct approach is to begin with a high quality crystal and to grow an epitaxial film on it that electron dopes the interfacial region. Like the polar mechanism invoked for the SrTiO3/LaAlO3 system,14,40–42 a thin film of an A+3B+3(O−2)3 insulator with relatively good lattice match, such as LaScO3 and LaInO3, on the SnO2-terminated (001) surface of a BSO single crystal could lead to the formation of a two-dimensional electron gas. Doping due to a polar discontinuity is particularly appealing, as it is a form of “remote doping” without the direct introduction of dopant atoms or chemical disorder. Further, interface-induced local distortions may impact the mobility, and careful experimental and theoretical studies are needed to understand and control this effect by characterizing the interface structure and connecting structural changes to modifications in the electronic states and transport.43 

Another example of the opportunities for using a high mobility channel with a perovskite structure is the development of all-epitaxial heterostructures that incorporate perovskites with large lattice polarizations44 to achieve changes in sheet carrier concentrations exceeding 1013 cm−2, or non-volatile switching using perovskite ferroelectrics. For a ferroelectric non-volatile switch on a perovskite semiconductor, achieving a working device requires the right combination of materials properties such as channel doping concentration, ferroelectric coercive field, ferroelectric polarization and domain structure, interface trap concentration, and chemical compatibility. These challenges are the same as those faced for integration of ferroelectrics with conventional semiconductors, but by using perovskite oxides, the choices of materials are increased dramatically.

In addition to the switchable conductivity through modification of carrier density, ferroelectric/oxide thin film heterostructures provide additional control due to the switching-induced changes in local structural distortions at the interface. This is accomplished by changing the bonding configuration of the interfacial atoms (i.e., bond lengths and bond angles), thereby modifying the band structure parameters that control the interfacial mobility.43 With the appropriate choice of ferroelectric, these changes could amplify the effect of switching on the conductivity.

Further theoretical study of the transport of doped BSO is needed to establish mechanisms for high mobility. In particular, it is of great interest to separate the impact of the nature of the conduction band from the effects of geometry on the band structure. A quantitative model could yield design principles to be used in searching for other high-mobility doped perovskites, with those that have additional functional properties being of particular interest. The high room-temperature mobility of BaSnO3 may be the harbinger of a new generation of perovskite oxide-based devices.

Primary support was provided by NSF MRSEC DMR-1119826. Additional support was provided by ONR through Grant No. N00014-11-1-0666 as well as by the ONR MURI EXtreme Electron DEvices (EXEDE) program via Grant No. N00014-12-1-0976.

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