Ferroelectric devices including a layer having two or more stable configurations

Abstract

Ferroelectric semiconductor devices are provided by including a ferroelectric layer in the device that is made of a material that is not ferroelectric in bulk. Such layers can be disposed at interfaces to promote ferroelectric switching in a semiconductor device. Switching of conduction in the semiconductor is effected by the polarization of a mechanically bi-stable material. This material is not ferroelectric in bulk but can be considered to be when the thickness is sufficiently reduced down to a few atomic layers. Devices including such ferroelectric layers are suitable for various applications, such as transistors and memory cells (both volatile and non-volatile).

Claims

1. A method of ferroelectric switching, the method comprising: providing a semiconductor substrate; providing a ferroelectric layer of a material disposed on or above said substrate; wherein said ferroelectric layer has at least two stable configurations having different electrical polarizations; switching said ferroelectric layer between said stable configurations with an applied electric field; wherein said material does not exhibit bulk ferroelectricity; and wherein said ferroelectric layer is a single layer of lattice unit cells.

2. The method of claim 1, wherein said substrate comprises silicon or germanium.

3. The method of claim 1, wherein said material has a composition MX.sub.2.

4. The method of claim 3, wherein said composition MX.sub.2 has M selected from the group consisting of Zr, Hf, Ce, Ca, Pt, Pd, Rh, Ir, Ti, Fe, Ni, Co and V, and has X selected from the group consisting of O, F, S, As and P.

5. The method of claim 4, wherein said ferroelectric layer comprises a material selected from the group consisting of: zirconium oxide, hafnium oxide, calcium fluoride, cerium oxide and mixtures thereof.

6. The method of claim 1, wherein said ferroelectric layer is disposed on said substrate and wherein an insulator is disposed on a surface of said ferroelectric layer opposite said substrate.

7. The method of claim 6, wherein said insulator comprises a material selected from the group consisting of lead titanate, barium titanate, strontium titanate, and alloys or mixtures thereof.

8. The method of claim 7, wherein said insulator comprises strontium titanate.

9. The method of claim 6, wherein said insulator comprises a material that does not exhibit bulk ferroelectricity, but is ferroelectric when substantially matched in size and symmetry to said substrate.

10. The method of claim 6, wherein said insulator comprises a material that is not ferroelectric.

11. The method of claim 6, wherein said insulator is amorphous.

12. The method of claim 1, wherein an insulating layer stack is disposed on said substrate, wherein said ferroelectric layer is one of the layers of said insulating layer stack.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1a shows a first embodiment of the invention.

(2) FIG. 1b shows a second embodiment of the invention.

(3) FIG. 1c shows a third embodiment of the invention.

(4) FIG. 2 shows a transistor including a ferroelectric layer according to principles of the invention.

(5) FIG. 3 shows experimental measurements of displacement in a SrTiO.sub.3 film.

(6) FIGS. 4a-b show two stable configurations for a Si/ZrO.sub.2/SrTiO.sub.3 structure.

(7) FIG. 5 shows plots of the electrostatic potential in a Si/ZrO.sub.2/SrTiO.sub.3 structure for the two stable polarization states.

DETAILED DESCRIPTION

(8) This section provides theoretical and experimental results relating to the above-described concepts. Two sub-sections follow. In the first sub-section, evidence of pinning in the SrTiO.sub.3/Si system is considered. In the second sub-section, ferroelectric layer approaches are described.

(9) A) Pinning of SrTiO.sub.3 Polarization on Si

(10) A1) Introduction

(11) The development of methods to incorporate functional properties such as ferroelectricity and ferromagnetism into existing silicon devices is a quintessential goal of materials research. A promising route towards achieving this goal is the epitaxial growth of perovskite oxides on silicon. By incorporating a Sr-based atomic buffer layer at the oxide-semiconductor interface, high quality SrTiO.sub.3 (001) thin films can be grown directly on silicon substrates. As this method has so far been unsuccessful for most other perovskites, the SrTiO.sub.3 on silicon system has received a great deal of attention as the most promising candidate for ferroelectric field effect transistors, as well as other applications.

(12) While bulk SrTiO.sub.3 is not ferroelectric, epitaxial SrTiO.sub.3 on silicon is predicted, in the limit of thick films, to have an out-of-plane polarization and a room-temperature paraelectric-ferroelectric phase transition due to ferroelectric-strain coupling. However, epitaxial SrTiO.sub.3 films fully relax to the unstrained, non-ferroelectric state when thicker than 10 unit cells, limiting the possibility for ferroelectric SrTiO.sub.3 on silicon to the regime of very thin SrTiO.sub.3 films. At this scale, it is known that depolarizing fields can significantly reduce or eliminate polarization, even for films on a metallic substrate.

(13) Despite these considerations, experimental studies of ultra-thin SrTiO.sub.3 films on silicon have found signatures of ferroelectricity: X-ray adsorption near-edge spectroscopy (XANES) studies show that 2 nm SrTiO.sub.3 films on silicon are polar, and recent piezoelectric force microscopy (PFM) results show that 2-4 nm SrTiO.sub.3 films on silicon exhibit a stable, reversible PFM contrast in response to an applied tip voltage. These observations suggest that interface and/or surface phenomenaboth of which can significantly affect the behavior of ferroelectric thin filmsplay an important role in the Si/SrTiO.sub.3 system. While a number of studies have focused on determining the atomic structure and electronic properties of the Si/SrTiO.sub.3 interface, the relationship between the interface and the potential ferroelectric properties in this system has not been examined. Furthermore, the effects of interface structure and chemistry on the properties of complex oxide films on semiconductors in general are unknown.

(14) A2) Interface Geometry

(15) We begin by discussing the behavior of the model Si/SrTiO.sub.3 system, first via our theoretical work, and then through our experimental results. To understand the effects of the Si/SrTiO.sub.3 interface structure, we performed DFT (density functional theory) calculations to determine the minimum energy atomic structures for 56 different interface compositions with varying amounts of Sr, O, Ti, and Si in the interface region. For each interface composition, we searched for metastable states corresponding to positively poled, negatively poled, and paraelectric SrTiO.sub.3 films. In addition, we used this data set to determine the thermodynamic phase diagram of the Si/SrTiO.sub.3 interface. Under the constraint that neither SiO.sub.2 nor TiSi.sub.2 forms at the interface, we predicted the structure that should be observed experimentally.

(16) The predicted interface structure is in excellent agreement with our experimental STEM (scanning transmission electron microscope) data. In fact, this interface is the only one of the 56 studied that exhibits all four structural characteristics identified in the STEM image: i) the symmetry is (11), ii) there are no Si dimers, iii) a full monolayer of Sr atoms resides immediately above the silicon, and iv) the TiO column in the oxide is aligned with the top-most layer of silicon atoms. Furthermore, it matches well with our synchrotron XRD measurements, and with the structure proposed based on previous high resolution STEM images. In the following, we use this predicted interface structure as the primary example, ensuring that the theoretical principles presented here can be directly applied to the experimentally studied system.

(17) A3) Theoretical Results

(18) The most striking result of our DFT calculations is that the Si/SrTiO.sub.3 heterostructure has one and only one metastable state in each of the 56 interface compositions studied. In every case, this state is characterized by a net positive polarization in the SrTiO.sub.3 film. (We use the convention that positive polarization is directed away from the silicon substrate.) Furthermore, we observe that the polarization in the first oxide layer at the interface, P.sub.int, is greater than or comparable to P.sub.bulk, the polarization in bulk strained SrTiO.sub.3. Away from the interface, the polarization decreases, asymptoting to a finite positive value if the polarization charge is screened by a top metal electrode or decaying to zero if it is not. Both the positively poled ground state and the lack of a metastable negatively poled state arise directly from the fundamental chemical interactions that characterize the Si/SrTiO.sub.3 interface.

(19) Another key feature of the Si/SrTiO.sub.3 interface is that while the average film polarization depends on many parameters of the structure, such as the presence or absence of a top electrode, P.sub.int is a fixed property determined only by the interface. The fixed nature of P.sub.int is demonstrated most dramatically by replacing the top electrode by a full monolayer of surface oxygen vacancies, a perturbation that induces a large, monodomain negative polarization in other thin film ferroelectric systems. In the Si/SrTiO.sub.3 system, such a perturbation results in a small net negative polarization; however, the interface polarization remains unchanged. In other words, P.sub.int is an intrinsic structural property of the interface. This has profound consequences for the potential ferroelectric behavior of the system: By imposing a pinned structural boundary condition on the polarization, the Si/SrTiO.sub.3 interface prevents ferroelectric switching between monodomain polarization states.

(20) The largely general features described above suggest that the interface phenomena observed in the Si/SrTiO.sub.3 system are not unique to this interface but will be observed at any non-polar semiconductor/complex oxide interface. Indeed, we find that substituting the SrTiO.sub.3 in our calculations with PbTiO.sub.3 or BaTiO.sub.3 results in a similar interfacial electron rearrangement and a large interface polarization, providing evidence that our results also describe the behavior of other systems. This generality has important consequences for the design of ferroelectric field effect transistors and other devices, as we expect that the fundamental physical properties of these interfaces will hinder ferroelectric switching.

(21) A4) Experimental Confirmation

(22) As a corollary to the lack of ferroelectric switching, we also expect that no paraelectric-ferroelectric phase transition will be observed in these systems. We now show that the experiments are indeed consistent with this theoretical model. We determine the displacement of the Ti from centrosymmetry as a fraction of the unit cell for a 5-unit-cell-thick SrTiO.sub.3 film on silicon using temperature-dependent anomalous XRD measurements. Landau-Ginzberg-Devonshire theory predicts a transition temperature of Tc280 K for the limit of thick films with perfect electrodes, and it is well known that both imperfect screening and decreasing film thickness suppress Tc. Therefore, one would expect to see evidence of a phase transition (i.e., a relatively sharp decrease of the Ti displacement to zero) below this temperature if Si/SrTiO.sub.3 is ferroelectric at 0 K. However, as FIG. 3 shows, the experimental data demonstrate a temperature independent polarization, having a 2.5% displacement of the Ti in the SrTiO.sub.3 unit cell, directed away from the Si substrate. In addition to confirming the theoretical prediction, both the direction and magnitude of the measured displacement are in good agreement with our DFT results, as indicated by the dashed lines in the figure. While the experimental data in FIG. 3 cannot rule out a phase transition above 380 K, the observation of a constant Ti displacement in the experimentally measured range, combined with our theoretical understanding of the interface properties, strongly suggests that no ferroelectric phase transition occurs in these epitaxial SrTiO.sub.3 thin films on silicon.

(23) B) Ferroelectric Layers

(24) Thus far we have extracted the key features governing ferroelectric behavior in the epitaxial Si/SrTiO.sub.3 system. While these features lead to the inhibition of ferroelectric switching, they also suggest means by which to overcome this inhibition. In particular, a switchable ferroelectric oxide on silicon should have a mechanically bistable interface structure with oppositely directed interface dipoles.

(25) One route towards achieving these conditions is to use cation and/or anion substitutions to create a mechanically bistable interface. Compounds that form layered materials, composed of alternating planes of cations and anions along one crystallographic direction, turn out to be a useful starting point. For example, there are numerous transition metal chalcogenides, pnictides, fluorides, and even some oxides, with the formula MX.sub.2 that crystallize in the CdI.sub.2, pyrite, fluorite, or related layered structures. One can imagine removing a single monolayer of MX.sub.2 from a bulk crystal. When MX.sub.2 is a layered material, one expects the potential energy surface of the monolayer to be a double well: the cation can reside on either side of the plane of anions but must overcome a significant barrier to go from one side to the other, with the maximum energy corresponding to the fully planar configuration. (Depending on the bulk crystal structure, the potential energy surface may also include a third minimum between these two, corresponding to an X-M-X orientation.)

(26) Performing DFT calculations, we find a number of bistable MX.sub.2 interfaces; for example, Si/MX.sub.2/SrTiO.sub.3 films with M=Ti, Fe, Ni, and V and X=S, As, and P have bistable interface structures and two stable film polarization states. Monolayers of the oxides PtO.sub.2 and ZrO.sub.2, which in bulk crystallize in the CdI.sub.2 and fluorite structures, respectively, also exhibit bistable behavior, as does a monolayer of fluorite itself (CaF.sub.2). Perhaps the most promising interface structures are the latter two, as epitaxial thin films of each can be grown successfully on silicon, suggesting that growth of the proposed heterostructures will be experimentally feasible; in fact, thin (5-15 nm) layers of epitaxial CaF.sub.2 have been used as a buffer layer in the growth of SrTiO.sub.3 films on silicon.

(27) As an example of this new class of interface structures, FIGS. 4a-b shows the atomic structure of the two stable Si/ZrO.sub.2/SrTiO.sub.3 interfaces. Continuous switching between the two minima in this system is possible, with computed barrier heights of 0.42 and 0.95 eV from the negatively and positively poled states, respectively. Similar values are found for the CaF.sub.2-based heterostructure. In addition to stabilizing two polarization states in the oxide film, the presence of the bistable interface layer also couples the ferroelectric polarization to the electronic structure of the silicon substrate. As FIG. 5 illustrates, the electrostatic potential in the bulk silicon region differs by 0.45 eV for the two polarization states in the Si/ZrO.sub.2/SrTiO.sub.3 system. This large change of silicon potential (close to half the Si band gap) at the interface directly translates into substantial changes of carrier density in the silicon substrate, satisfying a key requirement for designing a ferroelectric field effect device.

(28) The class of bistable MX.sub.2 interfaces is a novel type of ferroelectric, which, in contrast to traditional ferroelectrics, is only stable in very thin films. The complexity of these Si/MX.sub.2/SrTiO.sub.3 structures is required to avoid direct bonding between the Si and SrTiO.sub.3, which our calculations show to never lead to bistable polarization behavior. If these structure are grown epitaxially on Si, the interfaces exhibit the key properties identified above as necessary for a ferroelectric on silicon device. Furthermore, the reversible interface polarization of the Si/MX.sub.2/SrTiO.sub.3 structures can affect charge carriers in the silicon, directly coupling the ferroelectric polarization to the silicon substrate. In addition to the examples mentioned above, we expect that atomic layers composed of other transition metal chalcogenides, pnictides, and fluorides that have layered bulk crystal structures will also exhibit similar properties, providing a large phase space within which to tailor the behavior of the system. Consideration of this class of interface structures thus opens a number of possibilities for engineering the properties of silicon/functional oxide systems.