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).

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to ferroelectric devices. 
       BACKGROUND 
       [0002]    When an electric field is applied to an object, the resulting charge separation in the object is referred to as electric polarization. Although most materials exhibit electric polarization only when an electric field is applied to the material, some materials exhibit electric polarization without an applied electric field (i.e., a spontaneous polarization). Such materials are often referred to as pyroelectric materials. For some pyroelectric materials, the direction of the spontaneous electric polarization can be switched using an applied electric field. Such materials are referred to as ferroelectric materials, and the switching of electric polarization in ferroelectric materials is analogous to the switching of magnetic moments in ferromagnetic materials. In particular, the polarization state of a ferroelectric is a non-volatile parameter that retains its value even if no power is supplied to a device including the ferroelectric. 
         [0003]    Thus, applications of ferroelectrics to electronic device technology have been under investigation for some time. Representative examples of the state of the art include U.S. Pat. No. 6,093,242, U.S. Pat. No. 6,023,082, U.S. Pat. No. 6,080,235, and U.S. Pat. No. 6,602,720. However, it remains difficult to provide long-term stability of ferroelectric switching in connection with microelectronic devices. 
       SUMMARY 
       [0004]    As described in greater detail below, we have found that interface effects, such as pinning of the electric polarization at an interface, can markedly affect whether or not a layer of material disposed on a substrate can actually have its polarization switched by an applied field. For example, the theoretically predicted ferroelectricity of epitaxial SrTiO 3  on Si (001) appears to be prevented by polarization pinning at the Si/SrTiO 3  interface. More importantly, from a practical perspective, we have also identified situations where interface effects promote ferroelectric switching rather than hinder it. 
         [0005]    In particular, a layer of material that is not ferroelectric in bulk can exhibit ferroelectric switching when disposed at an interface. Such layers can advantageously be inserted into device structures to promote ferroelectric switching. For example, a layer of ZrO 2  (which is not ferroelectric in bulk) between a Si substrate with a (001) orientation and an SrTiO 3  layer provides a structure that is ferroelectrically switchable (i.e., both the ZrO 2  and SrTiO 3  layers have two stable polarization states, and switching between these states can be accomplished with an electric field). 
         [0006]      FIG. 1   a  shows a simple example of such a structure. Here, a ferroelectric layer  104  is disposed on a semiconductor substrate  102 .  FIG. 1   b  shows an example similar to the example of  FIG. 1   a , except that an insulator  106  is disposed on the side of layer  104  that is opposite to substrate  102  (i.e., layer  104  is sandwiched between substrate  102  and layer  106 ). Layer  104  has at least two stable configurations (schematically shown as  103   b  and  105   b  on  FIG. 1   a ) having different electrical polarizations ( 103   a  and  105   a  on  FIG. 1   a ) and can be switched between these stable configurations with an applied electric field. Thus, layer  104  is ferroelectric. However, the material(s) used for layer  104  is/are not ferroelectric in bulk. We have unexpectedly found that layers of materials which are not ferroelectric in bulk can be ferroelectric in sufficiently thin layers, and that such ferroelectric layers can be advantageous for reducing/eliminating undesirable effects, such as interface pinning of electric polarization. 
         [0007]    One class of materials that has been found to be suitable for layer  104  has composition MX 2 , where M can be any of Zr, Hf, Ce, Ca, Pt, Pd, Rh, Ir, Ti, Fe, Ni, Co and V, and X can be any of O, F, S, As and P. Suitable materials also include transition metal oxides, chalcogenides, fluorides, or pnictides with a layered bulk crystal structure. Zirconium oxide (ZrO 2 ), hafnium oxide (HfO 2 ), calcium fluoride (CaF 2 ), cerium oxide (CeO 2 ) and mixtures thereof are some preferred compositions for layer  104 . Layer  104  is a thin layer (thickness &lt;20 nm) and is preferably a monolayer (i.e., a single layer of lattice unit cells). 
         [0008]    Practice of the invention does not depend critically on the material of substrate  102 , so any semiconductor can be employed for the substrate, including but not limited to Si and Ge. Practice of the invention also does not depend critically on the composition of layer  106  (if present). Layer  106  can be either ferroelectric or non-ferroelectric. If layer  106  is ferroelectric, the material of layer  106  can be either ferroelectric or non-ferroelectric in bulk. Preferably, layer  106  is ferroelectric. An important advantage of the present approach is that layer  106  can be a layer that would not be ferroelectric if disposed directly on substrate  102 , but is ferroelectric when disposed on layer  104 . For example, the SrTiO 3  in a SrTiO 3 /ZrO 2 /Si structure can be ferroelectric, while the SrTiO 3  in a SrTiO 3 /Si structure would not be ferroelectric. Suitable materials for layer  106  include, but are not limited to: lead titanate, barium titanate, strontium titanate, and alloys or mixtures of these. Layer  106  can be either amorphous or crystalline, and preferably has a thickness sufficient to provide a barrier to the flow of electric current when a voltage is applied between a top electrode  206  (e.g., as shown on  FIG. 2 ) and substrate  102 . Preferably, layer  106  is substantially matched in size and symmetry to substrate  102  when this layer is crystalline. 
         [0009]    Practice of the invention does not depend critically on the geometrical location of the ferroelectric layer  104  in a stack of multiple layers. For example,  FIG. 1   c  shows a multi-layer stack  110  disposed on substrate  102 . The stack of this example includes four layers  112 ,  114 ,  116 , and  118 . Any of these layers can have the properties of layer  104  as described above (i.e., not ferroelectric in bulk, ferroelectric in a thin layer as disposed in the stack). More generally, one or more layers such as layer  104  above can be included in a multi-layer stack having any number of layers. 
         [0010]      FIG. 2  shows an application of the present approach. In this example, a transistor having source and drain terminals  202  and  204  is controlled by a gate electrode  206 . An insulating stack  110  including a ferroelectric layer is disposed under gate electrode  206 . Such a device can provide a non-volatile memory cell, based on the effect of the switchable electric polarization in stack  110  on current flow between source and drain. Other applications of such ferroelectric layers include transistors and volatile memory cells. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1   a  shows a first embodiment of the invention. 
           [0012]      FIG. 1   b  shows a second embodiment of the invention. 
           [0013]      FIG. 1   c  is shows a third embodiment of the invention. 
           [0014]      FIG. 2  shows a transistor including a ferroelectric layer according to principles of the invention. 
           [0015]      FIG. 3  shows experimental measurements of displacement in a SrTiO 3  film. 
           [0016]      FIGS. 4   a - b  show two stable configurations for a Si/ZrO 2 /SrTiO 3  structure. 
           [0017]      FIG. 5  shows plots of the electrostatic potential in a Si/ZrO 2 /SrTiO 3  structure for the two stable polarization states. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    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 3 /Si system is considered. In the second sub-section, ferroelectric layer approaches are described. 
       A) Pinning of SrTiO 3  Polarization on Si 
     A1) Introduction 
       [0019]    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 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 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. 
         [0020]    While bulk SrTiO 3  is not ferroelectric, epitaxial SrTiO 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 3  films fully relax to the unstrained, non-ferroelectric state when thicker than −10 unit cells, limiting the possibility for ferroelectric SrTiO 3  on silicon to the regime of very thin SrTiO 3  films. At this scale, it is known that depolarizing fields can significantly reduce or eliminate polarization, even for films on a metallic substrate. 
         [0021]    Despite these considerations, experimental studies of ultra-thin SrTiO 3  films on silicon have found signatures of ferroelectricity: X-ray adsorption near-edge spectroscopy (XANES) studies show that 2 nm SrTiO 3  films on silicon are polar, and recent piezoelectric force microscopy (PFM) results show that 2-4 nm SrTiO 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 phenomena—both of which can significantly affect the behavior of ferroelectric thin films—play an important role in the Si/SrTiO 3  system. While a number of studies have focused on determining the atomic structure and electronic properties of the Si/SrTiO 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. 
       A2) Interface Geometry 
       [0022]    We begin by discussing the behavior of the model Si/SrTiO 3  system, first via our theoretical work, and then through our experimental results. To understand the effects of the Si/SrTiO 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 3  films. In addition, we used this data set to determine the thermodynamic phase diagram of the Si/SrTiO 3  interface. Under the constraint that neither SiO 2  nor TiSi 2  forms at the interface, we predicted the structure that should be observed experimentally. 
         [0023]    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 (1×1), ii) there are no Si dimers, iii) a full monolayer of Sr atoms resides immediately above the silicon, and iv) the Ti—O 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. 
       A3) Theoretical Results 
       [0024]    The most striking result of our DFT calculations is that the Si/SrTiO 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 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 int , is greater than or comparable to P bulk , the polarization in bulk strained SrTiO 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 3  interface. 
         [0025]    Another key feature of the Si/SrTiO 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 int  is a fixed property determined only by the interface. The fixed nature of P 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 3  system, such a perturbation results in a small net negative polarization; however, the interface polarization remains unchanged. In other words, P 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 3  interface prevents ferroelectric switching between monodomain polarization states. 
         [0026]    The largely general features described above suggest that the interface phenomena observed in the Si/SrTiO 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 3  in our calculations with PbTiO 3  or BaTiO 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. 
       A4) Experimental Confirmation 
       [0027]    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 3  film on silicon using temperature-dependent anomalous XRD measurements. Landau-Ginzberg-Devonshire theory predicts a transition temperature of Tc ˜280 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 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 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 3  thin films on silicon. 
       B) Ferroelectric Layers 
       [0028]    Thus far we have extracted the key features governing ferroelectric behavior in the epitaxial Si/SrTiO 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. 
         [0029]    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 2  that crystallize in the CdI 2 , pyrite, fluorite, or related layered structures. One can imagine removing a single monolayer of MX 2  from a bulk crystal. When MX 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.) 
         [0030]    Performing DFT calculations, we find a number of bistable MX 2  interfaces; for example, Si/MX 2 /SrTiO 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 2  and ZrO 2 , which in bulk crystallize in the CdI 2  and fluorite structures, respectively, also exhibit bistable behavior, as does a monolayer of fluorite itself (CaF 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 2  have been used as a buffer layer in the growth of SrTiO 3  films on silicon. 
         [0031]    As an example of this new class of interface structures,  FIGS. 4   a - b  shows the atomic structure of the two stable Si/ZrO 2 /SrTiO 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 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 2 /SrTiO 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. 
         [0032]    The class of bistable MX 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 2 /SrTiO 3  structures is required to avoid direct bonding between the Si and SrTiO 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 2 /SrTiO 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.