JP7170273B2 - Ferroelectric thin film, its manufacturing method and device - Google Patents

Description

The present invention relates to a ferroelectric thin film using zinc oxide, a method for manufacturing the same, and a device.

Zinc oxide (ZnO) has a large energy gap of 3.37 eV, so it transmits visible light, and since it has oxygen with a large electronegativity, oxygen deficiency is likely to occur, and it tends to be an n-type semiconductor (Non-Patent Document 1 reference). For this reason, zinc oxide thin films have been used as transparent n-type semiconductors. Recently, it has become possible to introduce (dope) a p-type impurity into a zinc oxide thin film (see Non-Patent Document 2), and pn junctions using zinc oxide thin films have been actively studied. Zinc oxide also has a piezo-electric effect. In particular, it is known that the piezoelectric effect of zinc oxide is enhanced by making it thinner than the bulk material (see Non-Patent Document 3). Since zinc oxide has a particularly large electromechanical coupling factor, it can be expected to effectively obtain a large piezoelectric effect (see Non-Patent Document 4).

In electronic device applications, particularly as field effect transistors (FETs), the gate insulating layer is becoming thinner as device dimensions are reduced. As a result, leakage current from the gate electrode cannot be ignored. In order to prevent such drawbacks, it is advantageous to use a material with a high dielectric constant for the gate insulating layer, which can store a large amount of charge even if it is thin. For this reason, hafnium oxide (HfO ₂ ) is widely used as a material for gate insulating layers in silicon-based semiconductor circuits.

More recently, it has been found that doping hafnium with silicon oxide (SiO ₂ ) exhibits ferroelectric properties (see Non-Patent Document 5). The development of ferroelectric properties is thought to be caused by removing the symmetry of crystals, and for this reason, various materials can be used as dopants to develop ferroelectric properties. For example, a mixture of zirconia oxide (ZrO ₂ ) and hafnium oxide has been shown to have ferroelectric properties (see Non-Patent Document 6), and various applications are expanding. Similarly, zinc oxide has been shown to exhibit ferroelectric properties by doping with Li (see Non-Patent Document 7).

A. F. Kohan et al. "First-principles study of native point defects in ZnO", Physical Review B, vol. 61, no. 22, pp. 15019-15027, 2000. M. Joseph et al., "p-Type Electrical Conduction in ZnO Thin Films by Ga and N Codoping", Japanese Journal of Applied Physics, vol. 38, Pt. 2, no. 11A, pp. L1205-L1207, 1999. M. Zhao et al., "Piezoelectric Characterization of Individual Zinc Oxide Nanobelt Probed by Piezoresponse Force Microscope", Nano Letters, vol. 4, no. 4, pp. 587-590, 2004. T. Yamamoto et al., "Characterization of ZnO piezoelectric films prepared by rf planar-magnetron sputtering", Journal of Applied Physics, vol. 51, no. 6, pp. 3133-3120, 1980. T. S. Boscke et al., "Ferroelectricity in hafnium oxide thin films", Applied Physics Letters, vol. 99, no. 10, 102903, 2011. J. Muller et al., "Ferroelectricity in Simple Binary ZrO2 and HfO2", Nano Letters, vol. 12, pp. 4318-4323, 2012. A. Onodera et al., "Dielectric Activity and Ferroelectricity in Piezoelectric Semiconductor Li-Doped ZnO", Japanese Journal of Applied Physics, vol. 35, Pt. 1, no. 9B, pp. 5160-5162, 1996. R. L. Puurunen, "Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process", Journal of Applied Physics, vol. 97, no. 12, 121301, 2005. M. Matsuoka, "Nonohmic Properties of Zinc Oxide Ceramics", Japanese Journal of Applied Physics, vol. 10, no. 6, pp. 736-746, 1971. H. Kohlstedt et al., "Theoretical current-voltage characteristics of ferroelectric tunnel junctions", Physical Review B, vol. 72, no. 12, 125341, 2005. I. B. Misirlioglu and K. Sendur, "Ferroelectric/Semiconductor/Tunnel-Junction Stacks for Nondestructive and Low-Power Read-Out Memory", IEEE Transactions on Electron Devices, vol. 63, no. 6, pp. 2374-2379, 2016. V. Garcia et al., "Giant tunnel electroresistance for non-destructive readout of ferroelectric states", Nature, vol. 460, pp. 81-84, 2009. S. Salahuddin and S. Datta, "Use of Negative Capacitance to Provide Voltage Amplification for Low Power Nanoscale Devices", Nano Letters, vol. 8, no. 2, pp. 405-410, 2008.

As described above, research and development of zinc oxide thin films exhibiting ferroelectric properties have been actively carried out, and zinc oxide thin films with new structures exhibiting ferroelectric properties are desired.

SUMMARY OF THE INVENTION An object of the present invention is to provide a thin film having a new structure using zinc oxide and exhibiting ferroelectric properties.

The ferroelectric thin film according to the present invention is composed of a zinc oxide layer in which 3 to 5 molecular layers composed of Zn and O are laminated, and an aluminum oxide layer composed of one molecular layer composed of Al and O. The zinc oxide layer and the aluminum oxide layer are stacked, and the zinc oxide layer and the aluminum oxide layer adjacent to each other in the stacking direction are in contact with each other.

In one structural example of the ferroelectric thin film, a plurality of zinc oxide layers and aluminum oxide layers are alternately laminated.

The method for producing a ferroelectric thin film according to the present invention is an atomic layer deposition method using a zinc raw material and an oxidizing agent to form a zinc oxide layer in which 3 to 5 molecular layers composed of Zn and O are laminated. and a second step of forming, on and in contact with the zinc oxide layer, an aluminum oxide layer composed of one molecular layer composed of Al and O by an atomic layer deposition method using an aluminum raw material and an oxidizing agent. and a step.

In one structural example of the method for producing a ferroelectric thin film, the first step and the second step are repeated to alternately laminate a plurality of zinc oxide layers and aluminum oxide layers.

In one structural example of the method for producing a ferroelectric thin film, the first step includes a step of supplying a zinc raw material onto a substrate to form a single atomic layer of Zn; a step of oxidizing the atomic layer to form a molecular layer composed of Zn and O; the second step includes a step of supplying an aluminum raw material onto the substrate to form one atomic layer of Al; A step of oxidizing the one atomic layer of Al formed thereon to form a molecular layer composed of Al and O is included.

A device according to the present invention comprises a first layer made of zinc oxide, a tunnel barrier layer formed on the first layer as a tunnel barrier, and the ferroelectric thin film formed on the tunnel barrier layer. and a second layer of zinc oxide formed on the ferroelectric thin film.

A device according to the present invention comprises a channel layer formed on a substrate, the ferroelectric thin film formed on the channel layer, and a source and a drain formed on the substrate with the channel layer interposed therebetween. and a gate electrode made of zinc oxide formed on the ferroelectric thin film with a gate insulating layer interposed therebetween.

As described above, according to the present invention, the zinc oxide layer is composed of three or more molecular layers composed of Zn and O, and the aluminum oxide layer is composed of one molecular layer composed of Al and O. Therefore, it is possible to provide a zinc oxide thin film having a novel structure that exhibits ferroelectric properties.

FIG. 1 is a cross-sectional view showing the structure of a ferroelectric thin film according to an embodiment of the invention. FIG. 2 is a configuration diagram showing the configuration of a manufacturing apparatus for carrying out the method of manufacturing a ferroelectric thin film. FIG. 3 is a timing chart for explaining the method of manufacturing a ferroelectric thin film. FIG. 4 is a characteristic diagram showing measurement results of current-voltage characteristics of the fabricated ferroelectric thin film. FIG. 5 is a cross-sectional view showing the configuration of a device according to an embodiment of the invention. FIG. 6 is a cross-sectional view showing the configuration of another device according to the embodiment of the invention.

A ferroelectric thin film according to an embodiment of the present invention will be described below with reference to FIG. This ferroelectric thin film is composed of a zinc oxide layer 102 and an aluminum oxide layer 103 formed on a substrate 101 . In the zinc oxide layer 102, three or more molecular layers 121 made of Zn and O are laminated. The aluminum oxide layer 103 is composed of one molecular layer made of Al and O. As shown in FIG. The zinc oxide layer 102 and the aluminum oxide layer 103 are stacked, and the zinc oxide layer 102 and the aluminum oxide layer 103 adjacent in the stacking direction are in contact with each other. In this example, a plurality of zinc oxide layers 102 and aluminum oxide layers 103 are alternately laminated. The number of laminations can be set to 10, for example.

Next, a method for manufacturing a ferroelectric thin film according to an embodiment of the present invention will be described. This manufacturing method is an atomic layer deposition method using a zinc raw material and an oxidizing agent, and includes a first step of forming a zinc oxide layer 102, and an atomic layer deposition method using an aluminum raw material and an oxidizing agent, to form an aluminum oxide layer. and a second step of forming 103 . In the second step, an aluminum oxide layer 103 is formed on and in contact with the zinc oxide layer 102 . By repeating the first step and the second step, a plurality of zinc oxide layers 102 and aluminum oxide layers 103 can be alternately laminated.

Here, the first step includes a step of supplying a zinc raw material on the substrate 101 to form a monoatomic layer of Zn, and a step of oxidizing the monoatomic layer of Zn formed on the substrate 101 to form Zn and O. and forming a molecular layer 121 consisting of: The second step includes a step of supplying an aluminum raw material on the substrate 101 to form a monoatomic layer of Al, and a step of oxidizing the monoatomic layer of Al formed on the substrate 101 to form Al and O. forming a molecular layer consisting of

The manufacturing method will be described in more detail below. A ferroelectric thin film can be manufactured by the atomic layer deposition method described above, for example, by using the apparatus shown in FIG. This apparatus includes a reaction vessel 201 for processing, a dry pump 202 for exhausting the inside of the reaction vessel 201, a raw material supply unit 203 containing a zinc raw material, a raw material supply unit 204 containing an aluminum raw material, an oxidation and an oxidizing agent supply unit 205 containing an oxidizing agent.

The zinc raw material is, for example, diethyl zinc [DEZ: Zn _{(C2H5)2 ] .} The aluminum raw material is, for example, trimethylaluminum [TMA: Al(CH ₃ ) ₃ ]. The oxidant is, for example, water ( _H2O ). In atomic layer deposition, diethylzinc and trimethylaluminum are organometallic compounds called precursors.

The zinc raw material contained in the raw material supply unit 203 is introduced into the pipe 221 by opening the pulsing valve 206 and transported to the reaction vessel 201 by the carrier gas 209 . Also, the aluminum raw material contained in the raw material supply unit 204 is introduced into the pipe 222 by opening the pulsing valve 207 and transported to the reaction vessel 201 by the carrier gas 209 . Also, the oxidant contained in the oxidant supply unit 205 is introduced into the pipe 223 by opening the pulsing valve 208 and transported to the reaction vessel 201 by the carrier gas 209 . A carrier gas 209 is also supplied to the reaction vessel 201 through a pipe 224 .

Further, the dry pump 202 discharges (purges) surplus raw materials and reaction gas in the reaction vessel 201 to the outside of the reaction vessel 201 . In this discharge, carrier gas 209 supplied to reaction vessel 201 through pipe 224 is used as a purge gas.

For example, the substrate 211 is loaded and placed inside the reaction container 201, and the reaction container 201 is sealed. Next, by opening the pulsing valve 206 , the zinc raw material contained in the raw material supply unit 203 is introduced into the reaction vessel 201 through the pipe 221 and supplied onto the substrate 211 . The zinc raw material supplied onto the substrate 211 adsorbs only one atomic layer on the surface of the substrate 211 due to the self-limiting mechanism of the zinc raw material, which is the precursor, and does not become thicker than this.

Next, the supply of the zinc raw material is stopped, the carrier gas 209 is supplied to the reaction vessel 201 , and the dry pump 202 purges excess raw material gas and reaction gas in the reaction vessel 201 .

Next, by opening the pulsing valve 208 , the oxidant contained in the oxidant supply unit 205 is introduced into the reaction vessel 201 through the pipe 223 and supplied onto the substrate 211 . Only one atomic layer of the zinc raw material is adsorbed on the substrate 211 , and is oxidized by the supply of the oxidizing agent to form a (ZnO) molecular layer composed of Zn and O on the substrate 211 . Become.

Next, the supply of the oxidant is stopped, the carrier gas 209 is supplied to the reaction vessel 201 , and the dry pump 202 purges excess oxidant gas and the like in the reaction vessel 201 .

Next, by opening the pulsing valve 207 , the aluminum raw material contained in the raw material supply unit 204 is introduced into the reaction vessel 201 through the pipe 223 and supplied onto the substrate 211 . The aluminum raw material supplied onto the substrate 211 adsorbs only one atomic layer on the surface of the ZnO molecular layer already formed on the substrate 211 due to the self-limiting mechanism of the precursor aluminum raw material. It should not be thicker than

Next, the supply of the aluminum raw material is stopped, the carrier gas 209 is supplied to the reaction vessel 201 , and the dry pump 202 purges excess raw material gas and reaction gas in the reaction vessel 201 .

Next, by opening the pulsing valve 208 , the oxidant contained in the oxidant supply unit 205 is introduced into the reaction vessel 201 through the pipe 223 and supplied onto the substrate 211 . Only one atomic layer of the aluminum raw material is adsorbed on the surface of the ZnO molecular layer already formed on the substrate 211, and is oxidized by the supply of the oxidizing agent to form (aluminum oxide) molecules composed of Al and O. A layer is now formed on the substrate 211 .

The Atomic Layer Deposition (ALD) method described above is a film formation method that digitally grows atomic layers one by one using an organometallic compound called a precursor and its oxidizing agent (mainly water). be. It is possible to grow one atomic layer at a time by using the self-limiting mechanism of the precursor, that is, the property that if only one atomic layer is adsorbed on the surface, no further thickness is obtained (see Non-Patent Document 8).

The atomic layer deposition method is a kind of chemical vapor deposition (CVD) method, but differs from the usual CVD method in that the atomic layer is grown one atomic layer at a time by a self-limiting mechanism due to surface adsorption. In the chemical vapor deposition method, a source gas and an oxidizing agent are simultaneously supplied onto the substrate, and the material reacted in the vapor phase on the substrate is deposited and grown on the substrate. In the atomic layer deposition method, on the other hand, the major difference is that, firstly, the atomic layer grows by surface chemical reaction on the substrate surface, and secondly, the atomic layer grows layer by layer. For this reason, it has the advantage of being able to grow at a low temperature compared to the chemical vapor deposition method. In the case of growing one atomic layer at a time and growing at a low temperature, the formed film is a dense amorphous film. In the atomic layer deposition method, since atomic layers are grown one by one, the film formation speed is slower than other chemical vapor deposition methods.

Next, the result of fabricating the ferroelectric thin film according to the embodiment by the atomic layer deposition method will be described.

First, an ALD film forming apparatus "Sunale R-200" manufactured by Picosan Co., Ltd. was used as an atomic layer deposition apparatus. Diethyl zinc [DEZ] was used as the zinc raw material. Trimethylaluminum (TMA) was used as the aluminum raw material. Also, H ₂ O was used as an oxidizing agent. Also, silicon was used as the substrate material. Moreover, the internal temperature (processing temperature) of the reaction vessel was set to around 150°C.

First, the A cycle for forming a monomolecular layer of zinc oxide was set to "supply DEZ for 0.1 seconds, purge for 4 seconds, supply H ₂ O for 0.1 seconds, and purge for 4 seconds" (Fig. 3). It is assumed that a monolayer of ZnO is formed in one A cycle. In addition, the B cycle for forming a monomolecular layer of aluminum oxide was defined as "supplying TMA for 0.1 seconds, purging for 4 seconds, supplying H ₂ O for 0.1 seconds, and purging for 4 seconds" (Fig. 3). It is assumed that one monolayer of Al ₂ O ₃ is formed in one B cycle.

Further, the A cycle described above was repeated three times in succession, followed by one B cycle, and this cycle was repeated 10 times to form a ferroelectric thin film (ZnO; Al ₂ O ₃ ). In the process of repeating the A cycle three times, when the carrier gas supply rate is 50 to 80 sccm, both the DEZ supply process and the H ₂ O supply process are performed for 0.1 second or longer. and the duration of the purge step is 3.2-4.0 seconds.

The ferroelectric thin film produced as described above had a thickness of about 7.6 nm. This ferroelectric thin film was sandwiched between _two zinc oxide films each having a thickness of ₃₀ nm to form a "ZnO/ZnO; It was measured by the 4-probe method. The measurement results are shown in FIG. The dashed line shows the sweep results of the first measurement and the solid line shows the sweep results of the fifth measurement.

As shown in FIG. 4, the hysteresis curve changes as the number of sweeps increases due to charge accumulation in the “ZnO/ZnO; Al ₂ O ₃ /ZnO” structure. Here, non-linearity is seen in the current-voltage characteristics, which is considered to be due to the varistor characteristics of zinc oxide (see Non-Patent Document 9). It is known that the varistor characteristics also change greatly depending on the growth cycle of the atomic layer deposition method, and various optimizations can be considered for this as well.

By the way, ZnO;Al ₂ O ₃ formed by performing the above-described A cycle twice and the B cycle once did not exhibit ferroelectric properties. Therefore, it is considered that the ZnO:Al ₂ O ₃ film formed by performing the A cycle 3 times per B cycle 1 time exhibits dielectric properties. In fact, ZnO;Al ₂ O ₃ formed with 4 A cycles and 1 B cycle, and ZnO;Al ₂ O ₃ formed with 5 A cycles and 1 B cycle showed strong Dielectric properties were confirmed.

In other words, a film in which a zinc oxide layer in which three or more molecular layers of Zn and O are laminated and an aluminum oxide layer in which one molecular layer of Al and O are laminated alternately. It is thought that ferroelectric properties can be obtained by

Next, a device using the ferroelectric thin film according to the embodiment described above will be described with reference to FIG. This device comprises a first layer 301 made of zinc oxide, a tunnel barrier layer 302 formed on the first layer 301 as a tunnel barrier, and a ferroelectric thin film formed on the tunnel barrier layer 302. 303 and a second layer 304 of zinc oxide formed on the ferroelectric thin film 303 . The tunnel barrier layer ₃₀₂ is composed of _Al2O3 . The tunnel barrier layer 302 has a thickness of about 2-4 nm. Each layer can be successively formed in the same apparatus by the atomic layer deposition method described above.

A ferroelectric tunnel junction can be constructed by combining a ferroelectric thin film and a tunnel barrier (see Non-Patent Document 10). Ferroelectric tunnel junctions are being studied for application to memory devices (Non-Patent Document 11). So far, barium titanate (BaTiO ₃ ) and lead titanate (PbTiO ₃ ) have been used as ferroelectric materials applicable to memory elements (Non-Patent Documents 10 and 12). Further, recently, studies using HfO ₂ are also progressing.

According to the device described above, the tunnel junction is formed by the tunnel barrier layer 302 and the ferroelectric thin film 303 . Also, the first layer 301 and the second layer 304 can each be an electrode. As described above, these layers are excellent in reliability and controllability of characteristics because they can be continuously formed in the same apparatus by the atomic layer deposition method.

Next, another device using the ferroelectric thin film according to the embodiment described above will be described with reference to FIG. This device has a channel layer 402 formed on a substrate 401 , a source 403 and a drain 404 formed on the substrate 401 with the channel layer 402 interposed therebetween, and a ferroelectric layer 404 formed on the channel layer 402 . A thin film 405 and a gate electrode 407 formed on the ferroelectric thin film 405 with a gate insulating layer 406 interposed therebetween.

The substrate 401 is made of Si or GaAs, for example. Channel layer 402 is composed of, for example, Si or GaAs, which may be n-type or p-type. Also, the gate insulating layer 406 is composed of Al ₂ O ₃ . Also, the gate electrode 407 can be made of, for example, zinc oxide. Each layer of the ferroelectric thin film 405, the gate insulating layer 406, and the gate electrode 407 can be continuously formed in the same apparatus by the atomic layer deposition method described above. This device is a field effect transistor.

At present, with the miniaturization of circuits, it is necessary to make the gate insulating layer thinner in field effect transistors, and gate leakage has become a serious problem. In order to solve this problem, it has been proposed to form the gate insulating layer from a material with a high dielectric constant (high-k). For example, HfO ₂ is widely used as the gate insulating material. However, in order to lower the operating voltage and reduce power consumption, it is important to change the current significantly at a subthresholdvoltage near the threshold voltage. As one technique for realizing this, the use of ferroelectric negative capacitance has been proposed (see Non-Patent Document 13).

In the device according to the embodiment described above, the ferroelectric thin film 405 according to the embodiment is formed on the channel layer 402 _, and the gate insulating layer 406 made of _Al2O3 is formed thereon. , and a gate electrode 407 made of zinc oxide is laminated thereon to realize a negative capacitance field effect transistor. According to this structure, since the gate electrode 407 is made of ZnO, which is a transparent electrode material, it can operate as an optical switching field effect transistor. Further, according to this structure, by thinning the gate insulating layer 406 to form a tunnel junction structure, it is possible to operate it as a memory transistor.

As described above, according to the present invention, a zinc oxide layer in which three or more molecular layers made of Zn and O are laminated and an aluminum oxide layer made of one molecular layer made of Al and O are combined. Since it is constructed, it is possible to provide a thin film having a new structure using zinc oxide and exhibiting ferroelectric properties.

It should be noted that the present invention is not limited to the embodiments described above, and many modifications and combinations can be implemented by those skilled in the art within the technical concept of the present invention. It is clear.

DESCRIPTION OF SYMBOLS 101... Substrate, 102... Zinc oxide layer, 103... Aluminum oxide layer, 121... Molecular layer.

Claims (7)

a zinc oxide layer in which 3 to 5 molecular layers composed of Zn and O are laminated;
An aluminum oxide layer consisting of one molecular layer made of Al and O,
The zinc oxide layer and the aluminum oxide layer are laminated,
A ferroelectric thin film, wherein the zinc oxide layer and the aluminum oxide layer adjacent to each other in the stacking direction are in contact with each other. In the ferroelectric thin film according to claim 1,
A ferroelectric thin film, wherein a plurality of said zinc oxide layers and said aluminum oxide layers are alternately laminated. a first step of forming a zinc oxide layer in which 3 to 5 molecular layers composed of Zn and O are laminated by an atomic layer deposition method using a zinc raw material and an oxidizing agent;
a second step of forming, on and in contact with the zinc oxide layer, an aluminum oxide layer composed of one molecular layer composed of Al and O by an atomic layer deposition method using an aluminum raw material and an oxidizing agent; A method for manufacturing a dielectric thin film. In the method for manufacturing a ferroelectric thin film according to claim 3,
Repeating the first step and the second step,
A method of manufacturing a ferroelectric thin film, comprising alternately laminating a plurality of the zinc oxide layers and the aluminum oxide layers. In the method for manufacturing a ferroelectric thin film according to claim 3 or 4,
The first step includes supplying a zinc raw material onto a substrate to form a monoatomic layer of Zn, and oxidizing the monoatomic layer of Zn formed on the substrate to form Zn and O. forming a molecular layer;
The second step includes a step of supplying an aluminum raw material onto the substrate to form a monoatomic layer of Al; A method for producing a ferroelectric thin film, comprising the step of forming a molecular layer consisting of: a first layer made of zinc oxide;
a tunnel barrier layer formed on the first layer and serving as a tunnel barrier;
formed over the tunnel barrier layerstronga dielectric thin film;
a second layer of zinc oxide formed on the ferroelectric thin film;
equipped with,
The ferroelectric thin film is
a zinc oxide layer in which three or more molecular layers composed of Zn and O are laminated;
An aluminum oxide layer consisting of one molecular layer made of Al and O,
The zinc oxide layer and the aluminum oxide layer are laminated,
The zinc oxide layer and the aluminum oxide layer adjacent to each other in the stacking direction are in contact with each other.
characterized by device. a channel layer formed over a substrate;
formed on the channel layerstronga dielectric thin film;
a source and a drain formed on the substrate with the channel layer interposed therebetween;
a gate electrode formed on the ferroelectric thin film via a gate insulating layer and made of zinc oxide;
equipped with,
The ferroelectric thin film is
a zinc oxide layer in which three or more molecular layers composed of Zn and O are laminated;
An aluminum oxide layer consisting of one molecular layer made of Al and O,
The zinc oxide layer and the aluminum oxide layer are laminated,
The zinc oxide layer and the aluminum oxide layer adjacent to each other in the stacking direction are in contact with each other.
characterized by device.

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