Resistive random access memory device with a solid electrolyte including a region made of a first metal oxide and doped by a second element distinct from the first metal

Abstract

A resistive random access memory device includes a first electrode made of inert material; a second electrode made of soluble material; a solid electrolyte including a region made of an oxide of a first metal element, referred to as first metal oxide doped by a second element, distinct from the first metal and able to form a second oxide, the second element being selected such that the band gap energy of the second oxide is strictly greater than the band gap energy of the first metal oxide, the atomic percentage of the second element within the region of the solid electrolyte being comprised between 5% and 20%.

Claims

1. A Resistive random access memory device comprising: a first electrode made of inert material; a second electrode made of soluble material, and a solid electrolyte, the first and second electrodes being respectively in contact with one of the faces of the solid electrolyte on either side of the solid electrolyte, the second electrode being configured to supply mobile ions circulating in the solid electrolyte to the first electrode to form a conductive filament between the first and second electrodes when a voltage is applied between the first and second electrodes, wherein the solid electrolyte comprises a region made of an oxide of a first metal element that forms a first metal oxide, and wherein the region is doped by a second element, distinct from the first metal and able to form a second oxide, the second element being selected such that a band gap energy of the second oxide is strictly greater than the band gap energy of the first metal oxide, an atomic percentage of the second element within the region of the solid electrolyte being comprised between 5% and 20%, and wherein the second element is selected such that an electrical permittivity of the material of the doped region is less than or equal to the electrical permittivity of the first metal oxide.

2. The device according to claim 1, wherein the second element is a metal distinct from the first metal and able to form a second metal oxide.

3. The device according to claim 1, wherein the atomic percentage of the second element within the region of the solid electrolyte is substantially equal to 10%.

4. The device according to claim 1, wherein the first metal oxide is gadolinium oxide and wherein the second element is aluminium.

5. The device according to claim 1, wherein the solid electrolyte comprises: a first sub-layer in contact with the first electrode made of inert material, and a second sub-layer in contact with the second electrode made of soluble material; the region of the solid electrolyte made of the first metal oxide doped by the second element being a central sub-layer comprised between the first and second sub-layers.

6. The device according to claim 1, wherein the solid electrolyte comprises: a first sub-layer in contact with the first electrode made of inert material; a second sub-layer in contact with the second electrode made of soluble material, and a central sub-layer comprised between the first and second sub-layers, the region of the solid electrolyte made of first metal oxide doped by the second element being the first sub-layer and/or the second sub-layer.

7. The device according to claim 1, wherein the solid electrolyte is entirely formed by the region made of the first metal oxide doped by the second element.

8. A Resistive random access memory device comprising: a first electrode made of inert material; a second electrode made of soluble material, and a solid electrolyte, the first and second electrodes being respectively in contact with one of the faces of the solid electrolyte on either side of the solid electrolyte, the second electrode being configured to supply mobile ions circulating in the solid electrolyte to the first electrode to form a conductive filament between the first and second electrodes when a voltage is applied between the first and second electrodes, wherein the solid electrolyte comprises a region made of an oxide of a first metal element that forms a first metal oxide, and wherein the region is doped by a second element, distinct from the first metal and able to form a second oxide, the second element being selected such that a band gap energy of the second oxide is strictly greater than the band gap energy of the first metal oxide, an atomic percentage of the second element within the region of the solid electrolyte being comprised between 5% and 20%, and wherein the second element is selected such that an electrical permittivity of the second oxide is strictly less than the electrical permittivity of the first metal oxide.

9. A Resistive random access memory device comprising: a first electrode made of inert material; a second electrode made of soluble material, and a solid electrolyte, the first and second electrodes being respectively in contact with one of the faces of the solid electrolyte on either side of the solid electrolyte, the second electrode being configured to supply mobile ions circulating in the solid electrolyte to the first electrode to form a conductive filament between the first and second electrodes when a voltage is applied between the first and second electrodes, wherein the solid electrolyte comprises a region made of an oxide of a first metal element that forms a first metal oxide, and wherein the region is doped by a second element, distinct from the first metal and able to form a second oxide, the second element being selected such that a band gap energy of the second oxide is strictly greater than the band gap energy of the first metal oxide, an atomic percentage of the second element within the region of the solid electrolyte being comprised between 5% and 20%, and wherein the second element is selected such that the first metal oxide doped by the second element has a band gap energy substantially equal to the band gap energy of the first metal oxide not doped by the second element.

10. A Resistive random access memory device comprising: a first electrode made of inert material; a second electrode made of soluble material, and a solid electrolyte, the first and second electrodes being respectively in contact with one of the faces of the solid electrolyte on either side of the solid electrolyte, the second electrode being configured to supply mobile ions circulating in the solid electrolyte to the first electrode to form a conductive filament between the first and second electrodes when a voltage is applied between the first and second electrodes, wherein the solid electrolyte comprises a region made of an oxide of a first metal element that forms a first metal oxide, and wherein the region is doped by a second element, distinct from the first metal and able to form a second oxide, the second element being selected such that a band gap energy of the second oxide is strictly greater than the band gap energy of the first metal oxide, an atomic percentage of the second element within the region of the solid electrolyte being comprised between 5% and 20%, and wherein the second element is selected such that a length of a bond between the second element and oxygen is less than the length of the bond between the first metal element and oxygen.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The figures are presented for indicative purposes and in no way limit the invention.

(2) FIG. 1 schematically illustrates the passage from an “OFF” state to an “ON” state for a CBRAM type memory device;

(3) FIG. 2 illustrates the evolution of the “OFF” resistance of a CBRAM memory cell for different atomic percentages of second doping metal within the region of the solid electrolyte;

(4) FIG. 3 schematically shows the structure of an oxide based CBRAM memory cell according to an embodiment of the invention, and

(5) FIG. 4 schematically shows the structure of an oxide based CBRAM memory cell according to a variant of the embodiment of the invention of FIG. 3.

DETAILED DESCRIPTION

(6) Unless stated otherwise, a same element appearing in the different Figures has a single reference.

(7) In the present description, “oxide based CBRAM memory cell” is taken to mean a CBRAM memory cell comprising an electrolyte based on a metal oxide.

(8) As mentioned above, an aspect of the invention beneficially makes it possible to contribute to the widening of the memory window by using a doping of the electrolyte made of metal oxide MOx (for example a gadolinium oxide GdOx such as Gd.sub.2O.sub.3) by a second element (in an embodiment a metal, for example aluminium Al) selected such that the band gap energy of the oxide DOx is strictly greater than the band gap energy of the oxide MOx. To achieve this result, the atomic percentage of the second element D within the doped region of the solid electrolyte is comprised between 5% and 20%.

(9) An example of a CBRAM memory cell 10 according to an embodiment of the invention is illustrated in FIG. 3.

(10) The memory cell 10 comprises: a first electrode 11, also called cathode or inert electrode hereafter; a solid electrolyte 12. The solid electrolyte 12 comprises a region made of oxide of a first metal, designated “first metal oxide”, the region being doped by a second metal, distinct from the first metal and able to form a second metal oxide. The second metal is selected such that the band gap energy of the second metal oxide is strictly greater than the band gap energy of the first metal oxide and the atomic percentage of the second metal within the region of the solid electrolyte 12 is comprised between 5% and 20%; a second electrode 17, also called anode or soluble electrode hereafter, and comprising a source of ions layer 13 and a metal line 14.

(11) When a voltage is applied between the inert electrode 11 and the metal line 14 of the soluble electrode 17, the source of ions layer 13 supplies mobile ions which circulate in the solid electrolyte 12 to the inert electrode 11 to form a conductive filament between the inert electrode 11 and the soluble electrode 17.

(12) According to the first embodiment illustrated in FIG. 3, the inert electrode 11 is a pad, for example made from an inert interconnection metal, such as tungsten W, titanium nitride TiN or tantalum nitride TaN.

(13) According to the first embodiment illustrated in FIG. 3, the source of ions layer 13 of the soluble electrode 17 is made from an alloy of copper Cu and of an element of the chalcogen family such as Te. The source of ions layer 13 of the soluble electrode 17 may thus be made from CuTe. More generally, the source of ions layer 13 may be made from copper Cu and alloys thereof, silver Ag and alloys thereof, zinc Zn and alloys thereof, an alloy of copper and/or zinc and/or silver, such as: AgCu, AgZn, CuZn, AgCuZn, and alloys thereof.

(14) According to a particularly beneficial embodiment illustrated in FIG. 3, the solid electrolyte 12 is made of gadolinium oxide Gd.sub.2O.sub.3 and the second doping metal is aluminium Al with an atomic percentage selected so that the first metal oxide doped by the second metal (i.e. Gd.sub.2O.sub.3:Al) has a band gap energy substantially equal to the band gap energy of the non-doped first metal oxide (i.e. Gd.sub.2O.sub.3) at some 200 meV. An atomic percentage of Al substantially equal to 10% beneficially meets this latter constraint and makes it possible to obtain the desired effect on the memory window without degrading the forming voltage and the retention.

(15) The effect of the introduction of doping metal in the electrolyte on the memory window is particularly illustrated in FIG. 2 which represents the evolution of the value of the resistance R.sub.OFF in the “OFF” state as a function of the erasing voltage (i.e. the RESET voltage). Three curves (i.e. three evolutions of resistance) are represented for three doping levels: atomic percentage of 20% of Al in the Gd.sub.2O.sub.3 electrolyte; atomic percentage of 10% of Al in the Gd.sub.2O.sub.3 electrolyte; atomic percentage of 0% of Al (i.e. non-doped reference sample) in the Gd.sub.2O.sub.3 electrolyte.

(16) FIG. 2 also schematically represents the value of the resistance in the “ON” state; it will be noted that this resistance R.sub.ON practically does not vary once the SET voltage threshold is exceeded; the atomic percentage of Al in the electrolyte also has little effect on the value of R.sub.ON which remains substantially constant at 10.sup.4 Ohms. Conversely, it is observed that the value R.sub.OFF is much more dependent on the technology used. The more the RESET voltage is increased the more the value of R.sub.OFF increases and consequently, the bigger the memory window.

(17) An aspect of the invention is based on the finding that the behaviour of the resistance R.sub.OFF as a function of the RESET voltage is not the same according to the atomic percentage of dopant in the electrolyte. Firstly it is observed that the resistance R.sub.OFF is higher when the electrolyte is effectively doped by a second metal (i.e. compared to the non-doped reference sample); this phenomenon may be explained by the fact that a doping metal element has been selected in which the associated metal oxide (here Al.sub.2O.sub.3) has a bigger window than that of the material of the electrolyte (i.e. Gd.sub.2O.sub.3). Beyond this first observation linked to the doping, the applicant has also observed that the atomic doping percentage also has an effect on the value of the memory window. Thus, a doping level of 10% makes it possible to obtain a wider memory window than a doping level of 20%.

(18) Once this particularly beneficial effect observed on the memory window, it is also advisable to ensure that the doping is not going to degrade other electrical characteristics of the memory, particularly the forming voltage and the retention. To do so, the memory according to an embodiment of the invention has an optimised atomic percentage of metal dopant comprised between 5 and 20%, it being understood that a percentage substantially equal to 10% represents a particularly beneficial embodiment (substantial improvement of the memory window without degradation of the forming voltage and the retention).

(19) As regards retention, according to an embodiment, the metal dopant may be selected so that the bond between the dopant D (for illustrative purposes Al) and oxygen (D-O bond) is Smaller than that of Gd—O: such a selection makes it possible to conserve or even improve retention. In the case in point, the Al—O bond has a length of 1.8 A whereas the Gd—O bond has a length of 2.2 A.

(20) The example of Gd.sub.2O.sub.3 doped by 10% of Al is not limiting; it will be appreciated that several variants are possible for the pair formed by the electrolyte material and the dopant, among which: a gadolinium oxide Gd.sub.2O.sub.3 doped for example by Si (here the second doping element is not metal but semiconductor), Be, B, Mg, Ca or Sr; an aluminium oxide Al.sub.2O.sub.3 doped by Si; a hafnium oxide HfO.sub.2 doped by Al or Si; a zirconium oxide ZrO.sub.2 doped by Hf, Gd, Al or Si; a titanium oxide TiO.sub.2 doped by Zr, Hf, Gd, Al or Si.

(21) The doped region of the solid electrolyte 12 may for example be made by carrying out a co-sputtering of a target of first metal oxide and a target of second metal. It is particularly possible to measure the atomic percentage of the second doping metal within the doped region of the solid electrolyte 12 by a Rutherford Backscattering Spectroscopy (RBS) technique.

(22) FIG. 4 illustrates a second variant of a memory cell 10 according to an embodiment of the invention in which the doped region of the solid electrolyte 12 is a central sub-layer 12-c of the solid electrolyte 12, the atomic percentage of aluminium Al in the central sub-layer 12-c being substantially equal to 100. The central sub-layer 12-c of the solid electrolyte 12 is comprised between first and second sub-layers 12-1 and 12-2 of the solid electrolyte 12, the first sub-layer 12-1 being in contact with the inert electrode 11, and the second sub-layer 12-2 being in contact with the source of ions layer 13.