Electrode interface for high-dielectric-constant materials

Abstract

A preferred embodiment of this invention comprises a first thin dielectric buffer layer of a first leakage-current-density material (e.g. strontium titanate 32) with a first moderate-dielectric-constant, a high-dielectric-constant layer of a second leakage-current-density material (e.g. barium strontium titanate 34) overlaying the first thin dielectric buffer layer, and a second thin dielectric buffer layer of a third leakage-current-density material (e.g. strontium titanate 36) with a second moderate-dielectric-constant overlaying the high-dielectric-constant layer, wherein the first and third leakage-current-density materials have substantially lower leakage-current-densities than the second leakage-current-density material. The first and second thin moderate-dielectric-constant buffer layers (e.g. strontium titanate 32, 36) substantially limit the leakage-current-density of the structure, with only modest degradation of the dielectric constant of the structure. The possibly lower dielectric constant of the structure is generally compensated for by the reduced leakage current of the structure. The additional layers generally require only minor modifications of existing processes, since the same processes that are used for the high-dielectric-constant oxide can generally be used for the low leakage-current-density dielectric. These structures may also be used for multilayer capacitors and other thin-film ferroelectric devices such as pyroelectric materials, non-volatile memories, thin-film piezoelectric and thin-film electro-optic oxides.

Description

FIELD OF THE INVENTION

This invention generally relates to improving the electrode interface for materials with high-dielectric-constants, such as in the construction of capacitors.

BACKGROUND OF THE INVENTION

Without limiting the scope of the invention, its background is described in connection with current methods of forming electrical connections to high-dielectric-constant materials, as an example.

The increasing density of integrated circuits (e.g. DRAMs) is increasing the need for materials with high-dielectric-constants to be used in electrical devices such as capacitors. The current method generally utilized to achieve higher capacitance per unit area is to increase the surface area/unit area by increasing the topography, such as in trench and stack capacitors using SiO.sub.2 or SiO.sub.2 /Si.sub.3 N.sub.4 as the dielectric. This approach becomes very difficult in terms of manufacturability for devices such as the 256 Mbit and 1 Gbit DRAMs.

An alternative approach is to use a high permittivity dielectric material. Many perovskite, ferroelectric, or high-dielectric-constant (hereafter abbreviated HDC) oxides such as (Ba,Sr)TiO.sub.3 (BST) usually have much larger capacitance densities than standard SiO.sub.2 --Si.sub.3 N.sub.4 --SiO.sub.2 capacitors. Many different materials, such as RuO.sub.2 and Pt, have been proposed for the electrodes for HDC materials. To be useful in electronic devices, the electrode/HDC material interface should generally form a connection with good overall electrical characteristics.

SUMMARY OF THE INVENTION

As used herein the term moderate-dielectric-constant (hereafter abbreviated MDC) means having a dielectric constant greater than about 150. HDC materials (i.e. having a dielectric constant of greater than about 300) are useful for the fabrication of many electrical devices, such as capacitors. Many thin-film (generally less than 5 um) applications require a small leakage-current-density in addition to a large capacitance per unit area, so it is important to find methods which reduce the leakage current but do not substantially degrade the capacitance. The leakage current is sensitive to many variables such as thickness, microstructure, electrodes, electrode geometry and composition. In particular, it appears that the leakage current is controlled by Schottky barriers. For example, the leakage current of lead zirconium titanate (PZT) using RuO.sub.2 electrodes is several orders of magnitude larger than the leakage current of PZT using Pt electrodes. The smaller leakage current with Pt electrodes appears to be due to the larger work function.

One method of controlling the leakage current is to control the materials at the electrode/dielectric interface. MDC materials (e.g. SrTiO.sub.3 (ST)) generally have lower leakage-current-density (and lower permittivity) than HDC materials (e.g. BST) with the same thickness and electrodes. Surrounding the relatively higher permittivity HDC material with a thin layer of an MDC material generally results in a structure with lower leakage current yet relatively little dielectric constant degradation. Thin layers of the MDC material at the electrode interface(s) would have essentially the same effect.

The disclosed structures generally provide the high permittivity of HDC materials, with relatively smaller leakage currents when compared to structures in which the dielectric is all HDC material. One embodiment of this invention comprises a first thin dielectric buffer layer of a first leakage-current-density material with a first moderate-dielectric-constant, a high-dielectric-constant layer of a second leakage-current-density material overlaying the first thin dielectric buffer layer, and a second thin dielectric buffer layer of a third leakage-current-density material with a second moderate-dielectric-constant overlaying the high-dielectric-constant layer, wherein the first and third leakage-current-density materials have substantially lower leakage-current-densities than the second leakage-current-density material. The first and second thin dielectric buffer layers substantially limit the leakage-current-density of the structure, with only modest degradation of the dielectric constant of the structure. A method of forming an embodiment of this invention comprises the steps of forming a first thin dielectric buffer layer of a first leakage-current-density material with a first moderate-dielectric-constant, forming a high-dielectric-constant layer of a second leakage-current-density material on the first thin dielectric buffer layer, and forming a second thin dielectric buffer layer of a third leakage-current-density material with a second moderate-dielectric-constant on the high-dielectric-constant layer, wherein the first and third leakage-current-density materials have substantially lower leakage-current-densities than the second leakage-current-density material.

These are apparently the first thin-film structures wherein a thin dielectric barrier layer of a low leakage-current-density MDC material is used to reduce the leakage current of a thin-film ferroelectric or high-dielectric-constant oxide structure. It is generally desirable to minimize the ratio of the thickness of the low leakage-current-density MDC material to the thickness of the HDC material in order to keep the dielectric constant of the structure within 50% (preferably within 90% and more preferably within 95%) of the dielectric constant of the HDC material when used alone. As used herein, the term "thin", when used in conjunction with the dielectric barrier layer, means less than 1/10 (preferably less than 1/20 and more preferably less than 1/50) the thickness of the high-dielectric-constant material. As used herein, the terms "low" and "lower", when used in reference to leakage-current-density of the dielectric buffer layer, mean substantially smaller (and preferably less than 1/5 and more preferably less than 1/10) than the leakage-current-density of a layer of HDC material with the same thickness, electrodes, etc. The possibly lower dielectric constant of the structure is generally compensated for by the reduced leakage current of the structure. The additional layers generally require only minor modifications of existing processes, since the same processes that are used for the high-dielectric-constant oxide can generally be used for the moderate-dielectric-constant material. These structures may also be used for multilayer capacitors and other thin-film ferroelectric devices such as pyroelectric materials, non-volatile memories, thin-film piezoelectric and thin-film electro-optic oxides.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as other features and advantages thereof, will be best understood by reference to the detailed description which follows, read in conjunction with the accompanying drawings, wherein:

FIGS. 1-3 are cross-sectional views of a method for constructing a relatively low leakage current interface for a high-dielectric-constant material;

FIG. 4 is a cross-sectional view of an HDC material surrounded by an MDC material; and

FIG. 5 is a cross-sectional view of a relatively low leakage current HDC capacitor formed on the surface of a semiconductor substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIGS. 1-3, there is shown a method of forming a preferred embodiment of this invention, a high-dielectric-constant material sandwiched between two low leakage-current-density MDC materials. FIG. 1 illustrates a thin strontium titanate layer 32 deposited on the surface of a platinum electrode 30. FIG. 2 illustrates a barium strontium titanate layer 34 deposited on the strontium titanate layer 32. FIG. 3 illustrates a second thin layer of strontium titanate 36 deposited on the barium strontium titanate layer 34. ST thin-films generally have lower leakage currents than BST films of the same thickness and electrodes, so sandwiching the BST layer 34 between the thin ST layers 32 and 34 should result in a structure with lower leakage current yet relatively little dielectric constant degradation. As an example, the leakage current of a ST film 53 nm thick at 2V is 10.sup.-8 A/cm.sup.2, while the leakage current of a BST film 70 nm at 2V is 10.sup.-7 A/cm.sup.2. Table 1, below, illustrates various examples of the effective dielectric constant of the structure for different thicknesses of the ST 32 and 36, and for different values of the dielectric constant of the BST 34. The percent degradation in dielectric constant of the structure to that of a structure in which the dielectric is all HDC material is also shown. Table 1 assumes a total thickness of 100 nm, thicknesses of 2, 5, and 10 nm each for the ST layers 32 and 36, a dielectric constant of 230 for the ST 32 and 36, and dielectric constants of 300, 500 and 700 for the BST 36. Standard equations for capacitance were used to derive the values in Table 1.

TABLE 1______________________________________ (K.sub.BST = 300) (K.sub.BST = 500) (K.sub.BST = 700)ST Layer K.sub.effective, (% K.sub.effective, (% K.sub.effective, (%Thickness degradation) degradation) degradation)______________________________________2 nm 296 (1.3%) 478 (4.4%) 647 (7.6%)5 nm 291 (3.0%) 447 (10.6%) 581 (17%)10 nm 283 (5.7%) 405 (19%) 497 (29%)______________________________________

As expected, there is significant degradation in effective dielectric constant for thicker layers of ST 32 and 36, and for larger BST 34 dielectric constants. The highest known achieved dielectric constant for BST films 100 nm thick is currently about 460. Due to thin-film effects, the dielectric constant of a material in a thin-film is generally substantially less (typically more than an order of magnitude less) than the dielectric constant of the material in bulk. Future research will probably produce even higher dielectric constants for BST films, and PZT films are generally known to have even higher dielectric constants. Table 1, above, shows that a 2 nm thick ST layer at each interface with a BST layer with a dielectric constant of 700 results in only about a 7.6% degradation in the effective dielectric constant or equivalently the capacitance per unit area. The dielectric constant is only modestly lower but the leakage current is substantially lower.

The additional ST layers 32 and 36 require very little additional processing. The deposition of the ST layers 32 and 36, and the BST layer 34 may be performed using substantially the same processes. In the case of CVD, the deposition of the ST 32 and 36 is easier than the BST 34 and does not require any more metalorganic sources. In the case of sputtered deposition, the ST 32 and 36, and BST 34 deposition may be performed in the same chamber with one additional sputtered target. Since the ST 32 and 36, and BST 34 are substantially similar chemically, it is generally possible to etch the ST 32 and 36 using the essentially the same process as to etch the BST 34.

In an alternate embodiment, FIG. 4 shows a layer of BST 34 surrounded by a thin-film of strontium titanate 38. This serves to illustrate that the ST 38 may formed in such a way that it surrounds the BST 34, as opposed to just being formed at the electrode interfaces as in FIGS. 1-3.

In another alternate embodiment, FIG. 5 illustrates a microelectronic capacitor formed on a silicon substrate 40. The Pt lower electrode 30, the thin ST layers 32 and 36, and the BST layer 34 are essentially the same as described in FIG. 3. A TiN upper electrode 46 is shown overlaying the upper ST layer 36, and a TiN conductive plug 44 is shown making an electrical connection to the Pt lower electrode 30 through the SiO.sub.2 insulating layer 42.

Table 2, below, provides an overview of some embodiments and the drawings.

TABLE 2______________________________________ Preferred orDrawing Generic Specific Other AlternateElement Term Examples Examples______________________________________30 Lower Platinum Other noble metals electrode (e.g. palladium, rhodium, gold, iridium, silver) Conductive metal compounds (e.g. oxides: ruthenium dioxide, tin oxide, titanium monoxide; nitrides: ruthenium nitride, tin nitride, titanium nitride, zirconium nitride)32 Lower Strontium Other relatively low dielectric titanate leakage-current-density buffer layer perovskite, ferroelectric, or36 Upper high-dielectric-constant dielectric oxides buffer layer (e.g. (Sr, Ca, Mg)(Ti, Zr, Hf)O.sub.3)38 Surrounding Relatively low leakage- dielectric current-density acceptor buffer layer (e.g. Na, Al, Mn, Ca, K, Cr, Mn, Co, Ni, Cu, Zn, Li, Mg) and/or donor (e.g. F, Cl, V, Nb, Mo, La, Ce, Pr, Nd, Sm, Eu, Gd, Th, Dy, Ho, Er, Ta, W) doped perovskite, ferroelectric, or high-dielectric-constant oxides (e.g. (Sr, Ca, Mg)(Ti, Zr, Hf)O.sub.3)34 High- Barium Other perovskite, dielectric- strontium ferroelectric or high- constant titanate dielectric-constant oxides material (e.g. (Ba, Sr, Pb, La)(Ti, Zr)O.sub.3, bismuth titanate, potassium tantalate, lead niobate, lead zinc niobate, potassium niobate, lead magnesium niobate) Acceptor (e.g. Na, Al, Mn, Ca, K, Cr, Mn, Co, Ni, Cu, Zn, Li, Mg) and/or donor (e.g. F, Cl, V, Nb, Mo, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Ta, W) doped perovskite, ferroelectric, or high- dielectric-constant oxides (e.g. (Ba, Sr, Pb, La)(Ti, Zr)O.sub.3, bismuth titanate, potassium tantalate, lead niobate, lead zinc niobate, potassium niobate, lead magnesium niobate)40 Semiconductor Silicon Other single component substrate semiconductors (e.g. germanium, diamond) Compound semiconductors (e.g. GaAs, InP, Si/Ge, SiC)42 Insulator Silicon Other insulators dioxide (e.g. silicon nitride)44 Conductive Titanium Reactive metals plug nitride (e.g. tantalum, tungsten, titanium, molybdenum) Other reactive metal compounds (e.g. nitrides: zirconium nitride; silicides: titanium silicide, tantalum silicide, tungsten silicide, molybdenum silicide, nickel silicide; carbides: tantalum carbide; borides: titanium boride) Conductive carbides and borides (e.g. boron carbide) Single component semiconductors (e.g. single crystalline and polycrystalline silicon, germanium, diamond) Compound semiconductors (e.g. GaAs, InP, Si/Ge, SiC)46 Upper Titanium Other conductive metal electrode nitride compounds (e.g. nitrides: ruthenium nitride, tin nitride, zirconium nitride; oxides: ruthenium dioxide, tin oxide, titanium monoxide) Noble metals (e.g. platinum, palladium, rhodium, gold, iridium, silver) Other common seniiconductor electrodes (e.g. silicides, Al, Cu)______________________________________

A few preferred embodiments have been described in detail hereinabove. It is to be understood that the scope of the invention also comprehends embodiments different from those described, yet within the scope of the claims. With reference to the structures described, electrical connections to such structures can be ohmic, rectifying, capacitive, direct or indirect, via intervening circuits or otherwise. Implementation is contemplated in discrete components or fully integrated circuits in silicon, germanium, gallium arsenide, or other electronic materials families. In general the preferred or specific examples are preferred over the other alternate examples.

While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.

Claims

1. A microelectronic structure on a semiconductor substrate comprising:

a first thin dielectric buffer layer of a first leakage-current-density material with a first dielectric-constant;

a high-dielectric-constant layer of a second leakage-current-density material overlying said first thin dielectric buffer layer; and

a second thin dielectric buffer layer of a third leakage-current-density material with a second dielectric-constant overlying said high-dielectric-constant layer, wherein said first and third leakage-current-density materials have substantially lower leakage-current-densities than said second leakage-current-density material, and said first and said second thin dielectric buffer layers are thin to the extent that they are less than one-tenth the thickness of said high-dielectric-constant layer, whereby said first and second thin dielectric buffer layers substantially limit the leakage-current-density of the structure, with only modest degradation of the dielectric constant of the structure.

2. The structure of claim 1, wherein said first and second thin dielectric buffer layers have substantially similar compositions.

3. The structure of claim 1, wherein said first and second thin dielectric buffer layers completely surround said high-dielectric-constant material layer.

4. The structure according to claim 1, further comprising an electrically conductive layer on said first thin dielectric buffer layer.

5. The structure according to claim 4 wherein said electrically conductive layer is selected from the group consisting of: platinum, palladium, rhodium, gold, iridium, silver, titanium nitride, tin nitride, ruthenium nitride, zirconium nitride, ruthenium dioxide, tin oxide, titanium monoxide, and combinations thereof.

6. The structure according to claim 1, further comprising forming an electrically conductive layer on said second thin dielectric buffer layer.

7. The structure according to claim 6, wherein said electrically conductive layer is selected from the group consisting of: platinum, palladium, rhodium, gold, iridium, silver, titanium nitride, tin nitride, ruthenium nitride, zirconium nitride, ruthenium dioxide, tin oxide, titanium monoxide, titanium silicide, aluminum, and combinations thereof.

8. The structure according to claim 1, wherein said first thin dielectric buffer layer is selected from the group consisting of: perovskites, ferroelectrics, high-dielectric-constant oxides, acceptor doped perovskites, acceptor doped ferroelectrics, acceptor doped high-dielectric-constant oxides, donor doped perovskites, donor doped ferroelectrics, donor doped high-dielectric-constant oxides, and combinations thereof.

9. The structure according to claim 1, wherein said first thin dielectric buffer layer is selected from the group consisting of: (Na,Al,Mn,Ca) doped (Sr,Ca,Mg)(Ti,Zr,Hf)O.sub.3, (La,Nb,F) doped (Sr,CA,Mg)(Ti,Zr,Hf)O.sub.3, and combinations thereof.

10. The structure according to claim 1, wherein said first thin dielectric buffer layer is selected from the group consisting of: (Sr,Ca,Mg)(Ti,Zr,Hf)O.sub.3, (K,Cr,Mn,Co,Ni,Cu,Zn,Li,Mg) doped (Sr,Ca,Mg)(Ti,Zr,Hf)O.sub.3, (Cl,V,Mo,Ce,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er,Ta,W) doped (Sr,Ca,Mg)(Ti,Zr,Hf)O.sub.3, and combinations thereof.

11. The structure according to claim 1, wherein said second thin dielectric buffer layer is selected from the group consisting of: perovskites, ferroelectrics, high-dielectric-constant oxides, acceptor doped perovskites, acceptor doped ferroelectrics, acceptor doped high-dielectric-constant oxides, donor doped perovskites, donor doped ferroelectrics, donor doped high-dielectric-constant oxides, and combinations thereof.

12. The structure according to claim 1, wherein said second thin dielectric buffer layer is selected from the group consisting of: (Sr,Ca,Mg)(Ti,Zr,Hf)O.sub.3, (Na,Al,Mn,Ca) doped (Sr,Ca,Mg)(Ti,Zr,Hf)O.sub.3, (La,Nb,F) doped (Sr,Ca,Mg)(Ti,Zr,Hf)O.sub.3, and combination as thereof.

13. The structure according to claim 1, wherein said second thin dielectric buffer layer is selected from the group consisting of: (Sr,Ca,Mg)(Ti,Zr,Hf)O.sub.3, (K,Cr,Mn,Co,Ni,Cu,Zn,Li,Mg) doped (Sr,Ca,Mg)(Ti,Zr,Hf)O.sub.3, (Cl,V,Mo,Ce,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er,Ta,W) doped (Sr,Ca,Mg) (Ti,Zr,Hf)O.sub.3, and combinations thereof.

14. The structure according to claim 1, wherein said high-dielectric-constant layer is selected from the group consisting of: perovskites, ferroelectrics, high-dielectric-constant oxides, acceptor doped perovskites, acceptor doped ferroelectrics, acceptor doped high-dielectric-constant oxides, donor doped perovskites, donor doped ferroelectrics, donor doped high-dielectric-constant oxides, and combinations thereof.

15. The structure according to claim 1, wherein said high-dielectric-constant layer is selected from the group consisting of: (Ba,Sr,Pb,La)(Ti,Zr)O.sub.3, bismuth titanate, potassium tantalate, lead niobate, lead zinc niobate, potassium niobate, lead magnesium niobate, and combinations thereof.

16. The structure according to claim 1, wherein said high-dielectric-constant layer is doped with a material selected from the group consisting of: Na, Al, Mn, Ca, La, Nb, F, K, Cr, Mn, Co, Ni, Cu, Zn, Li, Mg, Cl, V, Mo, Ce, Pr, Nd, Sm, Eu, Gd, Th, Dy, Ho, Er, Ta, W, and combinations thereof.

17. A microelectronic structure on a semiconductor substrate comprising:

a first thin dielectric buffer layer of a first leakage-current-density material with a first dielectric-constant;

a high-dielectric-constant layer of a second leakage-current-density material overlying said first thin dielectric buffer layer; and

a second thin dielectric buffer layer of a third leakage-current-density material with a second dielectric-constant overlying said high-dielectric-constant layer, wherein said first and third leakage-current-density materials have substantially lower leakage-current-densities than said second leakage-current-density material, and said first and said second thin dielectric buffer layers are thin to the extent that they are less than 1/20 the thickness of said high-dielectric-constant layer, wherein said first thin dielectric buffer layer is selected from the group consisting of: (Na,Al,Mn,Ca) doped (Sr,Ca,Mg)(Ti,Zr,Hf)O.sub.3, (La,Nb,F) doped (Sr,Ca,Mg)(Ti,Zr,Hf)O.sub.3, (Sr,Ca,Mg)(Ti,Zr,Hf)O.sub.3, (K,Cr,Mn,Co,Ni,Cu,Zn,Li,Mg) doped (Sr,Ca,Mg)(Ti,Zr,Hf)O.sub.3, (Cl,V,Mo,Ce,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er,Ta,W) doped (Sr,Ca,Mg)(Ti,Zr,Hf)O.sub.3, and combinations thereof; whereby said first and second thin dielectric buffer layers substantially limit the leakage-current-density of the structure, with only modest degradation of the dielectric constant of the structure.

18. The structure according to claim 17, wherein said high-dielectric-constant layer is selected from the group consisting of: (Ba,Sr,Pb,La)(Ti,Zr)O.sub.3, bismuth titanate, potassium tantalate, lead niobate, lead zinc niobate, potassium niobate, lead magnesium niobate, and combinations thereof; and said high-dielectric-constant layer is doped with a material selected from the group consisting of: Na, Al, Mn, Ca, La, Nb, F, K, Cr, Mn, Co, Ni, Cu, Zn, Li, Mg, Cl, V, Mo, Ce, Pr, Nd, Sm, Eu, Gd, Th, Dy, Ho, Er, Ta, W, and combinations thereof.

19. A microelectronic structure on a semiconductor substrate comprising:

a first thin dielectric buffer layer of a first leakage-current-density material with a first dielectric-constant;

a high-dielectric-constant layer of a second leakage-current-density material overlying said first thin dielectric buffer layer; and

a second thin dielectric buffer layer of a third leakage-current-density material with a second dielectric-constant overlying said high-dielectric-constant layer, wherein said first and third leakage-current-density materials have substantially lower leakage-current-densities than said second leakage-current-density material, and said first and said second thin dielectric buffer layers are thin to the extent that they are less than 1/50 thickness of said high-dielectric-constant layer, wherein said first thin dielectric buffer layer is selected from the group consisting of: (Na,Al,Mn,Ca) doped (Sr,Ca,Mg)(Ti,Zr,Hf)O.sub.3, (La,Nb,F) doped (Sr,Ca,Mg)(Ti,Zr,Hf)O.sub.3, (Sr,Ca,Mg)(Ti,Zr,Hf)O.sub.3, (K,Cr,Mn,Co,Ni,Cu,Zn,Li,Mg) doped (Sr,Ca,Mg)(Ti,Zr,Hf)O.sub.3, (Cl,V,Mo,Ce,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er,Ta,W) doped (Sr,Ca,Mg)(Ti,Zr,Hf)O.sub.3, and combinations thereof; whereby said first and second thin dielectric buffer layers substantially limit the leakage-current-density of the structure, with only modest degradation of the dielectric constant of the structure.

20. The structure according to claim 19, wherein said high-dielectric-constant layer is selected from the group consisting of: (Ba,Sr,Pb,La)(Ti,Zr)O.sub.3, bismuth titanate, potassium tantalate, lead niobate, lead zinc niobate, potassium niobate, lead magnesium niobate, and combinations thereof; and said high-dielectric-constant layer is doped with a material selected from the group consisting of: Na, Al, Mn, Ca, La, Nb, F, K, Cr, Mn, Co, Ni, Cu, Zn, Li, Mg, Cl, V, Mo, Ce, Pr, Nd, Sm, Eu, Gd, Th, Dy, Ho, Er, Ta, W, and combinations thereof.