JP2013055209A - Resistance change type memory element in mis structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ReRAM element that stably operates on an off-state current of 10 μA or less by changing one side of an electrode from metal to p-type Si semiconductor for conversion into MIS type, in a conventional MIM type ReRAM element that cannot avoid unstable off operation because of large off-state currents.SOLUTION: With such a structure that off operations of a ReRAM element are executed using an off structure by conversion into hot electrons, activation energy of electrons is required, which causes enlargement of off-state currents. By reviewing the principle of such an off structure to change the structure into an off mechanism by depletion of a p-n junction section using voltages requiring no activation energy, a ReRAM element of extremely low power consumption can be provided.

Description

The present invention relates to an element structure of a resistance change type memory (Resistivi Random Access Memory: ReRAM).
Specifically, the ReRAM has a three-layer structure in which both sides of an insulator are sandwiched between electrodes, and is a non-volatile memory that utilizes the phenomenon that the insulator changes its resistance by voltage application. Various metal oxides are used for the insulator, and various metals such as Pt, Ni, Ti, and Al are used for the electrodes.
The present invention is characterized by a metal / insulator / semiconductor MIS structure in which one of the electrodes uses a conductive p-type Si semiconductor instead of a metal.

      The basic structure of a conventional ReRAM is a metal / insulator / metal MIM structure, and the research and development of ReRAM is mainly related to the search for metal oxide films that function as insulating films that change resistance, and electrode metals suitable for them. (For example, see Non-Patent Document 1, Non-Patent Document 2, and Non-Patent Document 3) Further, a technique for improving the switching characteristics by compositely laminating insulating films (for example, see Non-Patent Document 4) Has been done. However, in all cases, metals such as Pt were used for the electrodes, and the same was true regarding the MIM structure.

      The reason is that a Schottky barrier is inevitably formed between the metal oxide film of the MIM structure and the electrode metal, which plays an important function in the switching mechanism, and the use of a metal electrode has been considered a necessary condition. It was. (For example, refer to Patent Document 1 of the inventors)

    However, because the MIM structure of ReRAM has a large off-state current, it does not meet the conditions for operating at 50 μA or less, which is required for power-saving non-volatile memories, and it causes damage to the resistance change film (insulating film). It is poor in performance and hinders practical application.

JP 2005-183570 A

Z. Wei, T. Takagi et al. IEDM (2008) Highly Reliable TaOx ReRAM and Direct Evidence of Redox Reaction Mechanism Toru Tsuruoka et al., 70th Society of Natural Sciences (2009) 8p-H-3 Tanaka Hayato et al., 72nd Society of Natural Sciences (2011) 30a-ZK-12 Natsuki Fukuda et al., 72nd Applied Physics (2011) 31p-ZK-12

    The problem to be solved by the present invention is to significantly reduce the off-current in order to make the ReRAM a power saving type and at the same time improve the durability.

  The inventors have already developed a Schottky junction type nonvolatile memory (Patent No. 3887754) using an Al anodic oxide film. However, in order to cope with the reduction of power consumption for social needs, the off-current is reduced. I have made ingenuity to do that.

Many ReRAM developers have conducted a large number of prototype experiments with different types of metal electrodes in order to control the threshold of on / off voltage by changing the Schottky barrier of MIM type ReRAM. However, even if rare metals such as Ti, Pt, and Ru are used, a stable off-current lowering effect has not been obtained.

    Inventors of the present invention have a conventional MIM structure (metal / insulator / metal) ReRAM in which the Schottky barrier formed between the metal and the insulator is turned on during the on-state change from the high resistance state to the low resistance state. It was necessary to keep the leakage current at a low level until just before the transition to the on state, but it was found that the Schottky barrier is unnecessary during the off operation in which the low resistance state changes to the high resistance state. Based on this knowledge, one of the electrodes was made of metal as in the conventional case, and the other electrode was changed to an electrode using a semiconductor that has a rectifying property with a reverse electric field. In other words, the pn junction, which is a semiconductor technology, was incorporated into the basic structure of ReRAM, and the reduction in off current, which was a problem of ReRAM, was realized.

  The background of the discovery of the present technology by the present inventors is considered to be effective in understanding this technology, and will be described below. The present inventors verified the electronic state in the on / off state derived from the first-principles calculation results by measuring the thermally stimulated current. As a result, the electrons trapped in the insulating film are at a level 0.17 to 0.41 eV below the conduction band, and it can be seen that the activation energy that excites deep-level electrons into hot electrons and excites them in the conduction band is large. It was. The operation of the ReRAM element will be described with reference to FIG. 1 showing current MIM current-voltage (I-V) characteristics. First, a voltage is applied in a state where the current is limited to 35 μA by a current limiting diode, and when the threshold value (2.5 V) is reached, the high resistance state is changed to the low resistance state and the ON state is set. Next, the current limit is removed in the ON state, and when a voltage is applied, the maximum OFF current (18 mA) flows immediately before reaching 1 V, and the operation is turned off.

    The activation energy for exciting electrons to make hot electrons is the cause of increasing the off-current, and in order to solve this problem, electron emission that does not require hot electronization, that is, by an electric field It turns out that the method of extracting an electron directly to an electrode is effective. In order to efficiently extract electrons by an electric field, a pn junction in which the electric field acts on the electrons in a state where no current flows is considered effective. Based on these findings, we fabricated a metal / insulator / semiconductor MIS structure element in which one of the MIM structure metal electrodes was changed to a conductive p-type Si semiconductor, and applied a reverse voltage to the ON voltage. As a result of the bipolar operation for the OFF operation, as shown in FIG. 2, an IV characteristic for an OFF operation with an orderly small OFF current (10 μA) was obtained.

FIG. 3 shows a cross-sectional image of the ReRAM element in which the IV characteristics shown in FIG. 2 are obtained.
Incidentally, in a prototype experiment using n-type Si instead of p-type Si, there was no effect of reducing the off-current as theoretically expected.
Even when a metal oxide film including oxygen vacancies (Vo) other than Al anodization is used for the insulating film to be the resistance change layer, the effect of reducing the off-current by the MIS structure is the same. The reason is that this effect is in principle due to the rectification effect by the pn junction of the MIS structure. This will be described with reference to FIG. 4 which compares the conventional MIM type and the MIS type of the present invention.

  According to the Vo band model derived from the first-principles calculation, as shown in the center row (off → on) of FIG. 4, both the MIM type and the MIS type are between the metal and the metal oxide film. Electrons that tunnel through the formed Schottky barrier with an electric field are trapped at the Vo site, and the Vo electrons are spatially overlapped to form a band and become metal conduction (ON state).

  On the other hand, the off mechanism is greatly different between the MIM type and the MIS type as shown in the right column of FIG. 4 (on → off). In the MIM type, when a large current flows, some of the conduction electrons become hot electrons, and when some electrons are excited to the upper conduction band by the increased kinetic energy, the wave function of the electrons in that part Overlap for a moment and electrons on the downstream side are extracted to the electrode by the electric field. Some of the excited electrons lose energy and are captured by Vo again.However, the entire system loses and localizes the electrons captured by Vo, and the band disappears and becomes a band insulator (off Return to the state.

  On the other hand, in the MIS type off mechanism, the pn junction is depleted by the reverse voltage, and electrons on the downstream side of the pn junction are extracted to the electrode by the electric field. Instead of extracting electrons triggered by hot electronization as in the MIM type, electrons are directly extracted by the rectifying effect of the pn junction. It is considered that the off-current is greatly reduced by the basic difference of the off-mechanism, that is, the activation energy required for hot electronization is not required.

  This technology is a result obtained by elucidating the operating principle of ReRAM based on the first-principles calculation results, not only the Al anodic oxide film, but also the Al oxide film by sputtering deposition including Vo, It is a general-purpose technology that can be applied to ReRAM using transition metal oxide films containing Vo.

              According to a first aspect of the present invention, there is provided a ReRAM characterized in that an off current is reduced by adopting an MIS structure.

            The second effect of the present invention can be obtained by using p-type Si as a conductive semiconductor material used for the MIS structure. In the third and fourth aspects of the present invention, an effect is obtained by using an Al anodic oxide film and an oxygen-deficient Al oxide film as a metal oxide insulator used in the MIS structure.

      The resistance value of p-type Si used as an electrode is 10Ω or less, preferably 1 to 0.1Ω. The reason is that the resistance ratio (ratio of the resistance value between the on state and the off state) at the time of memory read increases, and current amplification is not required even at a low read voltage, resulting in significant advantages due to power saving and device circuit simplification. .

        The ReRAM with the MIS structure according to the present invention operates with an off-current that is three orders of magnitude less than the conventional ReRAM with the MIM structure, and can provide a revolutionary power-saving nonvolatile memory technology.

Typical MIM ReRAM IV characteristics. Typical MIS type ReRAM IV characteristics. Basic structure of MIS type ReRAM. Comparison of on / off mechanism between (1) MIM type and (2) MIS type. IV characteristics of MIS type ReRAM using oxygen deficient sputtered Al oxide film.

<Example 1>
(MIS type ReRAM device using Al anodized film)
Using a 50 nm thick Al film deposited on the surface of a low resistance p-Si substrate of 0.1 to 1Ω by vacuum deposition, in a 0.3 M oxalic acid solution kept at a constant temperature of 20 ° C., a voltage of 40 V is applied for 12 seconds. Then, anodization was performed to produce an Al anodized film on the surface of the p-Si substrate. A MIS type ReRAM device was fabricated by depositing Al with a thickness of 80 nm on the surface of the Al anodic oxide film that had been washed with pure water and dried under reduced pressure by vacuum deposition to form a 0.2 mmφ upper electrode and a p-Si substrate as the lower electrode.

  FIG. 2 shows the IV characteristics of the fabricated device. The bipole operation changed from the high resistance state to the low resistance state at 2V, and returned from the low resistance state to the high resistance state at -0.7V. The off current decreased to 10 μA. The on-current is controlled to 28 μA by a current limiting diode. By using the MIS type, the drive current could be reduced to 50 μA or less, which is a practical condition.

<Example 2>
(MIS-type ReRAM device using oxygen-deficient Al oxide film)
A 50 nm thick Al oxide film containing a large amount of oxygen vacancies was formed on the surface of a low resistance p-Si substrate of 0.1 to 1Ω by resistance heating in a low vacuum state of 10 ⁻³ pa, and Al was deposited on the surface by high vacuum deposition to 80 nm. A MIS-type ReRAM device was fabricated using a thick film to form a 0.2 mmφ upper electrode and a p-Si substrate as a lower electrode. FIG. 5 shows the 4-cycle IV characteristics of the fabricated device. As in the first embodiment, when the bipolar operation is performed, the off-current is reduced by two orders of magnitude or more compared to the on-current, and the off-current on the vertical axis in FIG. .

The on-current was limited to 28 μA by a current limiting diode as in Example 1.
The off-state current was further reduced from that of Example 1 to 0.2 μA or less, which was more than 5 orders of magnitude lower than the off-state current of the conventional MIM type ReRAM. According to Example 2, it was revealed that the technology of the present invention is a fundamental technology for a revolutionary power-saving nonvolatile memory that significantly reduces off-state current.

<Comparative Example 1>
(MIM type ReRAM device using Al anodized film)
Using a 0.3 mm thick Al rolled material, a voltage of 40 V was applied in 0.3 M oxalic acid solution maintained at a constant temperature of 20 ° C. as in Example 1, and an anodized Al anodized film was pure for 64 seconds. After washing with water and drying under reduced pressure, an MIM-type ReRAM device was fabricated by depositing Al with a thickness of 80 nm on the surface to form a 0.2 mmΦ upper electrode and an Al ingot as the lower electrode.

  FIG. 1 shows the IV characteristics of the fabricated device. With a current limiting diode of 35μA, the resistance state changes from high resistance to 2.5V at 2.5V, and after reaching the maximum off current of 18mA just before reaching 1V with the current limiting diode bypassed, Unipole operation to return to the high resistance state. The off-state current (18 mA) of Comparative Example 1 is three orders of magnitude greater than that of Example 1 and five orders of magnitude greater than that of Example 2, and the superiority of the MIS type ReRAM device in terms of power saving has been clarified. .

  If the MIS type ReRAM element of the present invention is used, an innovative power-saving non-volatile memory becomes possible, which can contribute to the realization of a normally-off computer that enables the ultimate power saving.

1 Limit on-current (35μA)
2 Off-state current (18mA)
3 Limit on-current (28μA)
4 Off-state current (10μA)
5 p-Si semiconductor (lower electrode)
6 Al anodic oxide film 7 Al (upper electrode)
8 Oxygen deficient Al oxide film 9 Al ingot (lower electrode)
10 Off-state current (0.2μA)

Claims (5)

A variable resistance memory (ReRAM) device with a metal / insulator / conductive p-type semiconductor structure
A ReRAM device comprising three layers.       2. The ReRAM element according to claim 1, wherein p-type Si having an electric resistance of 10 [Omega] or less is used for the conductive p-type semiconductor.   2. The ReRAM element according to claim 1, wherein p-type Si having an electric resistance of 0.1 to 1 [Omega] is used for the conductive p-type semiconductor.   The ReRAM element according to claim 1, wherein the metal oxide insulator of the ReRAM element is an Al anodic oxide film anodized in an oxalic acid solution. 2. The ReRAM device according to claim 1, wherein the metal oxide insulator of the ReRAM device is an oxygen-deficient Al oxide film formed by resistance heat treatment in a low vacuum.