WO2007111478A1

WO2007111478A1 - Materials of continuous metal-insulator transition and device using the same

Info

Publication number: WO2007111478A1
Application number: PCT/KR2007/001518
Authority: WO
Inventors: Jungwook Lim; Sun-Jin Yun; Hyun-Tak Kim
Original assignee: Electronics and Telecommunications Research Institute ETRI
Current assignee: Electronics and Telecommunications Research Institute ETRI
Priority date: 2006-03-28
Filing date: 2007-03-28
Publication date: 2007-10-04
Anticipated expiration: 2008-09-28

Abstract

Provided is continuous metal-insulator transition (MIT) material including a transition region in which resistance varies continuously from an insulator or a semiconductor to a metal by energy change between electrons.

Description

Description Materials of continuous metal-insulator transition and device using the same

Technical Field

[1] The present invention relates to a transition material whichcan change from an insulator to a metal, and a device using the transition material, and more particularly, to a transition material whichcan change from an insulator to a metaland undergo a gradual variation in resistanceaccording to a voltage, and a device using the transition material. Background Art

[2] Recently, much research has focused on an insulator having a resistance variation when a voltage is supplied. One promising example is a metal-insulator transition (MIT) insulator, which abruptly changes from an insulator to a metal. Hyun-Tak Kim et al. have presented in New Journal of Physics, Volume 6, p 52, that although it is known in general that an MIT insulator is accompanied by a structure change, the MIT insulator undergoes aMIT without a structure change when an electric field is changed to VO . The MIT insulator is currently the subject of active research, especially a Mott transition, which includes discontinuous and abrupt current variation with respect to an applied electric field. The discontinuous metal-insulator transition can be applied to various devices. For example, U.S. Patent No. 6,624,463 discloses a field effect transistor using the MIT of Hyun-Tak Kim et al.

[3] Meanwhile, when resistance is connected to an insulator which changes discon- tinuously, the current is varied continuously. A material causing a continuous transition current variation is defined as a continuous transition material, distinguished from a Mott transition in which abrupt discontinuous current variation occurs. Continuous transition materials usually change from an insulator to a metal due to a temperature change. However, there is great demand for a material that can change continuously from an insulator to a metal according to an electric field, and a device using the material. Disclosure of Invention Technical Problem

[4] The present invention provides a continuous transition material from an insulator to a metal with respect to electric field.

[5] The present invention also provides a device using the material.

Technical Solution

[6] According to an aspect of the present invention, there is provided a continuous metal- insulator transition (MIT) material comprising a transition region in which resistance varies continuously from an insulator or a semiconductor to a metal by energy change between electrons.

[7] The MIT material is at least one selected from the group consisting of an inorganic semiconductor, an inorganic insulator, an organic semiconductor and organic insulator, each of which has a low density of holes. The MIT material of claim 1, wherein the MIT material includes at least one selected from the group consisting of oxygen, carbon, Si, Ge, semiconductor compound in group III- V and II- VI, a transition metal element, a rare-earth element or a lanthanum element. The MIT material of claim 1, wherein the MIT material is at least one selected from Si Ti O, Al Ti O, Zn Ti O, Zr Ti O, Ta Ti O, V Ti O, La Ti O, Ba Ti O, Sr Ti O, Al O^*, VO , ZrO \ ZnO₅ HfO , Ta oj Ni0,^*Mg0, GaAs, GaS^*b, InP, InAs" GST(GeSbTe), Si and Ge.

[8] The transition region may comprise an inflection point, and the transition region comprising the inflection point may be narrower than the transition region not comprising an inflection point.

[9] According to another aspect of the present invention, there is provided a device using continuous MIT, comprising: an MIT material layer comprising a transition region in which resistance changes continuously from an insulator or a semiconductor to a metal by energy variation between electrons; and electrodes, one end of which is connected to the MIT material layer and which supply power to the MIT material. Description of Drawings

[10] The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:

[11] FIG. 1 is a graph of voltage V versus current I of a continuous transition material according to an embodiment of the present invention;

[12] FIG. 2 is a graph of voltage V versus current I of a continuous transition material according to another embodiment of the present invention;

[13] FIG. 3 is a schematic view for explaining a voltage control system according to an application example of the present invention; and

[14] FIG. 4 is a graph of input voltage V versus output voltage Vo when series resistance

R is kept constant, in the voltage control system according to the present invention.

Best Mode

[15] The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in many different forms, and should not be construed as being limited to the embodiments set forth herein; rather, these em- bodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art. Like reference numerals denote like elements in the drawings, and thus their description will not be repeated.

[16] A continuous transition material, which undergoes a continuoustransition from one state to another, according to an embodiment of the present invention, and a device using the continuous transition material, will now be described. The continuous transition material changes from an insulator to a metal, during which its resistance varies continuously. The continuous transition material will be described first, followed by a voltage control system which is an example using the continuous transition material. The insulator of the continuous transition material includes a semiconductor state.

[17] (continuous transition material)

[18] FIG. 1 is a graph of voltage V versus current I of a continuous transition material according to an embodiment of the present invention.

[19] Referring to FIG. 1, the electrical states of the continuous transition material include an insulator state, or semiconductor state (a), a first transition region (b), and a metal state (c), according to the applied voltage. In the first transition region (b), the electrical characteristics change between the insulator or semiconductor state (a) and the metal state (c) according to voltages. The metal state (c) has the properties of a metal, where voltage (V) and current (I) are proportional to each other.

[20] The continuous transition material has different voltage-current characteristics to a typical semiconductor. In a typical semiconductor, the current increases exponentially with voltage or shows a similar curve to (a) of FIG. 1 at a low voltage but a non-linear, abrupt increase at a high voltage. However, in the continuous transition material of the current embodiment, the current increases linearly with respect to the voltage as in a metal. Accordingly, an increase in voltage indicates that the continuous transition material according to the current embodiment of the present invention has changed to a metal.

[21] The continuous transition material differs from an abrupt metal- insulator transition insulator (a-MIT insulator), which changes abruptly from an insulator or a semiconductor (a) to a metal (c), in having the first transition region (b). In other words, the a-MIT insulator changes abruptly from an insulator to a metal at a critical voltage, and thus shows a discontinuous jump in current at the critical voltage. On the other hand, the continuous transition material according to the current embodiment of the present inventionhas no critical voltage, but instead has the first transition region (b), which is a voltage region in which the insulator or semiconductor (a) changes to a metal (c).

[22] The continuous transition material having the first transition region (b) is at least one selected from the group consisting of an inorganic semiconductor, an inorganic insulator, an organic semiconductor and organic insulator, each of which has a low density of holes. The continuous transition material may include at least one selected from the group consisting of oxygen, carbon, Si, Ge, semiconductor compound in group III- V and II- VI, a transition metal element, a rare-earth element or a lanthanum element. The continuous transition material may be at least one material selected from an oxide layer comprising Ti, Al 2 O3 , VO2 , ZrO2 , ZnO, HfO 2 , Ta2 O5 , NiO, MgO, GaAs,

GaSb, InP, InAs, GST (GeSbTe), Si, and Ge. The oxide layer comprising Ti may be at least one selected from the group consisting of Si Ti O, Al Ti O, Zn Ti O, Zr Ti O, Ta x y ^χ y ^χ y ^χ y ^χ

Ti y O, V x Tiy O, La x Tiy O, Ba x Tiy O, Sr x Tiy O.

[23] FIG. 2 is a graph of voltage V versus current I of a continuous transition material according to another embodiment of the present invention.

[24] Referring to FIG. 2, the continuous transition material has a second transition region

(d) having a continuous but steep inclination. Since the second transition region (d) does not show a discontinuous jump which is shown by the a-MIT insulator, the second transition region (d) may be seen as a part of a continuous transition process. However, the voltage region in which the transition from an insulator or semiconductor (a) to a metal (c) occurs in the second transition region (d) is narrower than the first transition region (b). The second transition region (d) includes an inflection point where the curvature of the current- voltage curve changes. In the metal state (c), the current is proportional to the voltage.

[25] The continuous transition material showing the second transition region (d) may be the same material as the continuous transition material having the first transition region (b). However, the first transition region (b) or the second transition region (d) occurs selectively according to the voltage applied to the continuous transition material.

[26] (Examples of application of the continuous transition material)

[27] The continuous transition material according to the current embodiment of the present invention can be used in various types of devices. For example, the continuous transition material can be used in a device which shows high current and low current states at high voltages and low voltages. If a resistor is connected to the continuous transition material, the voltage can be kept uniform with respect to a predetermined electric field. The device whichmaintainsa uniform voltage is called a voltage control system.

[28] FIG. 3 is a schematic view for explaining a voltage control system 100 according to an application example of the present invention. Here, a device including the continuous transition material is called a continuous transition material device (continuous transition device) 20.

[29] As illustrated in FIG. 3, the voltage control system 100 includes an input power source 10, a continuous transition device 20, and a series resistance R connected in series. The voltage control system 100 may further include an electrical system R that is connected to the voltage control system 100 parallel. Voltages which are controlled by the voltage control system 100 are applied to the electrical system R . [30] The width of the voltage controlling section maintaining a uniform output voltage

(also referred to as a device voltage) according to the series resistance R is changed in the continuous transition device 20, which will be also described in FIG. 4. The continuous transition device 20 includes a continuous transition material layer 24, a first electrode 22 on one side of the continuous transition material layer 24, and a second electrode 26 on the other side of the continuous transition material layer 24 and connected to the series resistance R in series.

[31] The first electrode 22 and the second electrode may be formed of a material selected from Al, Cu, Ni, W, Mo, Cr, Zn, Mg, Fe, Co, Sn, Pb, Au, Ag, Pt, Ti, Ta, TaN, TaW, WN, TiN, TiW, poly-Si, IrO, RuO, ITO, and ZnO. When the continuous transition device 20 changes to a metal, the current flows perpendicular to the continuous transition material layer 24. Though not described in detail, the present invention may also be applied to a device in which the direction of the current is parallel to the continuous transition material layer 24.

[32] Methods of forming the layers of the continuous transition device 20 are not limited, and examples include sputtering, molecular beam epitaxy (MBE), E-beam evaporation, thermal evaporation, atomic layer epitaxy (ALE), pulsed laser deposition (PLD), chemical vapor deposition (CVD), Sol-Gel method, and atomic layer deposition (ALD).

[33] The resistance of the continuous transition material layer 24 according to the present invention may change according to the type of the continuous transition material and the structure of the continuous transition device. The voltage control characteristics can be observed when the continuous transition device 20 is connected to a series resistance R . The value of the series resistance R can vary from several Ω to K Ω , and the voltage control capability of the continuous transition device 20 varies depending on the value of the series resistance R .

[34] Also, since the continuous transition applied to the voltage control system according to the present invention usually occurs in an insulator and a semiconductor, the voltage control characteristics can be realized on any substrate by depositing a continuous transition material, provided that there is no stress problem. The process temperature can be within a wide range from room temperature to 900 ⁰C . In addition, the voltage control system can be easily manufactured using the continuous transition material as one layer.

[35] FIG. 4 is a graph of an input voltage V versus an output voltage Vo when series resistance R is kept constant, in the voltage control system according to the present invention. The MIT insulator is an Al x Ti y O thin layer formed using a plasma type ALD deposition method. The value of the series resistance R is greater than the resistance of the Al x Ti y O thin layer in a metal state.

[36] As illustrated in FIG. 4, the output voltage V increases according to the input voltage, passing a transient output voltage V (t) and staying at a uniform saturation output voltage V (s) . Also, the output voltage V increases in proportion to the input voltage V until the input voltage V reaches the voltage controlling section. The output voltage V is a voltage included in the continuous transition device 20 of FIG. 3.

The output voltage V stays at the saturation output voltage V (s), which is constant even when the input voltage V increases over the saturation input voltage V (s) . That is, when the input voltage becomes greater than the saturation input voltage, the voltage across the continuous transition device 20 is constant, and the rest of the input voltage is across the series resistance R . Accordingly, the voltage control system according to the present invention can maintain a predetermined voltage using the continuous transition device 20 despite the increase of the input voltage V .

[37] The continuous transition material and a device using the continuous transition material according to the present invention have voltage controlling characteristics with a large variation of current according to the voltage. Thus a device can be realized using these characteristics.

[38] While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.

Claims

[1] A continuous metal-insulator transition (MIT) material comprising a transition region in which resistance varies continuously from an insulator or a semiconductor to a metal by energy change between electrons.

[2] The MIT material of claim 1, wherein the MIT material is at least one selected from the group consisting of an inorganic semiconductor, an inorganic insulator, an organic semiconductor and organic insulator, each of which has a low density of holes.

[3] The MIT material of claim 1, wherein the MIT material includes at least one selected from the group consisting of oxygen, carbon, Si, Ge, semiconductor compound in group III- V and II- VI, a transition metal element, a rare-earth element or a lanthanum element.

[4] The MIT material of claim 1, wherein the MIT material is at least one selected from Si Ti O, Al Ti O, Zn Ti O, Zr Ti O, Ta Ti O, V Ti O, La Ti O, Ba Ti O, Sr Ji O, Al₂O₃, VO₂, ZrO₂, ZnO, HfO^*, Ta₃O₅, NiO, MgO^GaAs^* GaSb, InP, InAs, GST(GeSbTe), Si and Ge.

[5] The MIT material of claim 1, wherein the transition region comprises an inflection point.

[6] The MIT material of claim 5, wherein the transition region comprising the inflection point is narrower than the transition region not comprising an inflection point.

[7] A device using continuous MIT, comprising: an MIT material layer comprising a transition region in which resistance changes continuously from an insulator or a semiconductor to a metal by energy variation between electrons; and electrodes, one end of which is connected to the MIT material layer and which supply power to the MIT material.

[8] The device using continuous MIT of claim 7, wherein the MIT material is at least one selected from the group consisting of an inorganic semiconductor, an inorganic insulator, an organic semiconductor and organic insulator, each of which has a low density of holes.

[9] The device using continuous MIT of claim 7, wherein the MIT material includes at least one selected from the group consisting of oxygen, carbon, Si, Ge, semiconductor compound in group III-V and II- VI, a transition metal element, a rare- earth element or a lanthanum element.

[10] The device using continuous MIT of claim 7, wherein the MIT material is at least one selected from Si Ti O, Al Ti O, Zn Ti O, Zr Ti O, Ta Ti O, V Ti O, La Ti x y ^χ y ^χ y ^χ y ^χ y ^χ y ^χ y O, Ba x Tiy O, Sr x Tiy O, Al 2 O3 , VO2 , ZrO 2 , ZnO, HfO 2 , Ta205 , NiO, MgO, GaAs,

GaSb, InP, InAs, GST(GeSbTe), Si and Ge.

[11] The device using continuous MIT of claim 7, wherein the transition region comprises an inflection point.

[12] The device using MIT of claim 7, wherein the electrode is formed of at least one selected from the group consisting of Al, Cu, Ni, W, Mo, Cr, Zn, Mg, Fe, Co, Sn,

Pb, Au, Ag, Pt, Ti, Ta, TaN, TaW, WN, TiN, TiW, poly-Si, IrO, RuO, ITO, and

ZnO.

[13] The device using continuous MIT of claim 7, wherein the electrodes are formed on the MIT material layer, facing one another and spaced apart.

[14] The device using continuous MIT of claim 7, wherein the electrodes are formed on both sides of the MIT material layer, having the MIT material layer therebetween.

[15] The device using continuous MIT of claim 7, wherein the MIT material layer is connected in series with a resistance R and a voltage control capability varies depending on the value of the series resistance R .

[16] The device using continuous MIT of claim 15, wherein the Rc is connected in series with an input voltage.