**1. Introduction**

#### **1.1. The ALD technique**

The chemical vapor deposition (CVD) technique is a chemical reaction of a volatile compound with other gases for the deposition of a solid on a substrate. This reaction takes place inside a reactor in which the reactants are inserted as gases. Temperature and pressure are the two main parameters to be controlled. Temperature affects the rate in a predictable manner (Arrhenius behavior). The pressure range, even when it has a lower impact, determines whether the deposition mechanism will be surface-reaction limited or transport limited. During a transport-limited process, the deposition speed is very high, and the growth rate is very sensitive to the temperature. On the contrary, a surface-reaction regime can reach rates as low as an atomic layer per cycle by means of an oversupply of reactants available in the vicinity of the surface. This regime is more conditioned by the boundary layer than by the main flow of reactants, being less dependent on the temperature.

and precursor, and purge gases), the process is repeated as many times as required to obtain the desired film thickness. It is worth to point out that the two gas-phase reactants are not in contact in the gas phase since the surface reactions are performed sequentially. The reaction (Eq. (1)) occurs when the second reactant reaches the surface. This sequence avoids possible reactions in the gas phase that could collapse at the surface forming undesired grains. Typical

The Atomic Layer Deposition Technique for the Fabrication of Memristive Devices: Impact of the Precursor…

ALD is a very versatile technique for many reasons. Among them, we emphasize here those that allow large-scale production: suitability to be applied in a wide temperature range, low cost, and easy scalability. Many reviews have addressed the fundamentals of ALD and its applications [1, 4]. Instead, the aim of the present study is to demonstrate how a spurious phase, produced by some inherent details of the technique, could be used to achieve devices with improved characteristics. This chapter highlights exactly such an issue in a concrete

strate. We will show how an unconsidered change, produced by the nature of the oxidant itself, has occurred at the Ti buffer during the ALD cycle, determining the growth of an extra

We will take advantage of several microscopic and macroscopic techniques in order to fully characterize the resulting oxide layer, in order to fully elucidate the effect produced by the

quite difficult to disentangle the oxidant and/or precursor effect, some signs seem to point to the former as the main responsible one of the produced changes. We also include macroscopic

metal-insulator-metal nonvolatile memory device. Besides, an additional deposition process was carried out in order to clarify whether we could be in presence of another unconsidered effect. Finally, we will introduce a phenomenological model to describe the electrical response

growth, the oxygen source is typically either H2

metal source could be for example Tetrakis(dimethylamino)hafnium (TDMAH) or TEMAH

cess—using either ozone as oxygen source and TDMAH as the metal precursor—was car-

of 5 lines by 16 columns was determined by means of optical photolithography, delimiting 80 squared-shaped structures of 200 μm lateral size covered with sputtered Pd (40 nm)/Co (35 nm) acting as the top electrode (TE). To complete these two terminal devices, the access to

size. The Ti layer (20 nm) was sputtered on top. After Ti sputtering, the ALD pro-

and microscopic transport properties that support the microscopic picture achieved.

although we will also include some comparison with samples grown using H2

The substrate employed was commercial Si (highly doped)/thermally oxidized SiO2

In brief, we report here the structural evidence for the presence of TiOx

**1.2. Details of the ALD technique and the device's fabrication**

and thus providing interesting and useful features to the final device.

precursor in its formation during the ALD process. Although it is

and TDMAH were the oxygen and metal source, respectively,

by ALD on top of a Ti-buffered Si/SiO<sup>2</sup>

layer determines the occurrence of a very interesting

below the HfO2

http://dx.doi.org/10.5772/intechopen.78937

O [4] or O3

layer. After this deposition process an array

sub-

5

layer and

[4] while the

O instead of O3

(120 nm)

.

deposition rates are in the order of 0.1 nm per cycle [2].

example: the deposition of a thin film of HfO<sup>2</sup>

oxide layer of TiOx

surface reaction.

discuss the key role of the O3

For ALD-based HfO<sup>2</sup>

of 1 cm2

[5]. In our specific case, O<sup>3</sup>

electrical evidence to support that the TiOx

ried out to obtain a uniform 20 nm-thick HfO2

the bottom contact was achieved by a scratch.

The atomic layer deposition (ALD) technique, though bearing many resemblances to CVD, excludes the gas phase reaction of the precursors. ALD is characterized by chemisorption steps while the physisorbed molecules are purged away during the necessary purge steps [1–3]. ALD relies on the activation of the surface on top of which the resultant layer is placed. This activation insures the growth to be self-limited to the minimum thickness determined by the reaction (usually an atomic layer). The substrate surface exhibits a certain density of surface sites, for example, OH groups, which serve as "anchors" for the metal precursor molecules. ALD requires four steps: I—chemisorption of the metal precursor to surface (OH) groups; II—release of the by-products during the purge; III—the reaction of the oxygen source with the remaining reactive groups at the metal ion; and IV—the purge of the by-products. After steps I–IV, the surface is again covered with (OH) groups, now on the deposited surface layer. The chemisorption of the precursor molecules to the surface, which can happen via different chemical reactions (1), stops suddenly when all surface sites are occupied. As a consequence, the ALD process exhibits extremely low deposition rates and an accurate control of the film thickness. The strict timely separation of the two precursor materials, which is achieved with the purge steps, gives the difference between ALD and CVD.

The key role played by the buffer material in the ALD technique can be identified from the described procedure. Surface preparation consists of the chamber introduction of some precursor that reacts with the former and improves the adhesion of the deposited compound. After that, deposition is performed by cycles in order to grow, layer by layer, the required material in a controlled manner. In the case of binary oxides, each cycle requires an oxygen source and a metal precursor. The oxidant and precursor chemically react, giving rise to a conformal growth layer above the buffered surface. The reaction during each cycle is summarized in Eq. (1):

$$\text{AB } \text{(precursive)} \text{+ CD } \text{(precursive)} \text{\textquotedbl{}AC \textquotedbl{}} \text{(solid formed at the surface)} + \text{BD } \text{(gas)}\tag{1}$$

Before repeating each cycle, purge gases are inserted into the chamber to avoid further reactions with possible remaining products. Once a complete run has occurred (activation, oxidant and precursor, and purge gases), the process is repeated as many times as required to obtain the desired film thickness. It is worth to point out that the two gas-phase reactants are not in contact in the gas phase since the surface reactions are performed sequentially. The reaction (Eq. (1)) occurs when the second reactant reaches the surface. This sequence avoids possible reactions in the gas phase that could collapse at the surface forming undesired grains. Typical deposition rates are in the order of 0.1 nm per cycle [2].

**1. Introduction**

**1.1. The ALD technique**

4 New Uses of Micro and Nanomaterials

The chemical vapor deposition (CVD) technique is a chemical reaction of a volatile compound with other gases for the deposition of a solid on a substrate. This reaction takes place inside a reactor in which the reactants are inserted as gases. Temperature and pressure are the two main parameters to be controlled. Temperature affects the rate in a predictable manner (Arrhenius behavior). The pressure range, even when it has a lower impact, determines whether the deposition mechanism will be surface-reaction limited or transport limited. During a transport-limited process, the deposition speed is very high, and the growth rate is very sensitive to the temperature. On the contrary, a surface-reaction regime can reach rates as low as an atomic layer per cycle by means of an oversupply of reactants available in the vicinity of the surface. This regime is more conditioned by the boundary layer than by the

The atomic layer deposition (ALD) technique, though bearing many resemblances to CVD, excludes the gas phase reaction of the precursors. ALD is characterized by chemisorption steps while the physisorbed molecules are purged away during the necessary purge steps [1–3]. ALD relies on the activation of the surface on top of which the resultant layer is placed. This activation insures the growth to be self-limited to the minimum thickness determined by the reaction (usually an atomic layer). The substrate surface exhibits a certain density of surface sites, for example, OH groups, which serve as "anchors" for the metal precursor molecules. ALD requires four steps: I—chemisorption of the metal precursor to surface (OH) groups; II—release of the by-products during the purge; III—the reaction of the oxygen source with the remaining reactive groups at the metal ion; and IV—the purge of the by-products. After steps I–IV, the surface is again covered with (OH) groups, now on the deposited surface layer. The chemisorption of the precursor molecules to the surface, which can happen via different chemical reactions (1), stops suddenly when all surface sites are occupied. As a consequence, the ALD process exhibits extremely low deposition rates and an accurate control of the film thickness. The strict timely separation of the two precursor materials, which is achieved with

The key role played by the buffer material in the ALD technique can be identified from the described procedure. Surface preparation consists of the chamber introduction of some precursor that reacts with the former and improves the adhesion of the deposited compound. After that, deposition is performed by cycles in order to grow, layer by layer, the required material in a controlled manner. In the case of binary oxides, each cycle requires an oxygen source and a metal precursor. The oxidant and precursor chemically react, giving rise to a conformal growth layer above the buffered surface. The reaction during each cycle is summarized in Eq. (1):

*AB* (*precursor*) + *CD* (*precursor*)*AC* (*solid formed at the surface*) + *BD* (*gas*) (1)

Before repeating each cycle, purge gases are inserted into the chamber to avoid further reactions with possible remaining products. Once a complete run has occurred (activation, oxidant

main flow of reactants, being less dependent on the temperature.

the purge steps, gives the difference between ALD and CVD.

ALD is a very versatile technique for many reasons. Among them, we emphasize here those that allow large-scale production: suitability to be applied in a wide temperature range, low cost, and easy scalability. Many reviews have addressed the fundamentals of ALD and its applications [1, 4]. Instead, the aim of the present study is to demonstrate how a spurious phase, produced by some inherent details of the technique, could be used to achieve devices with improved characteristics. This chapter highlights exactly such an issue in a concrete example: the deposition of a thin film of HfO<sup>2</sup> by ALD on top of a Ti-buffered Si/SiO<sup>2</sup> substrate. We will show how an unconsidered change, produced by the nature of the oxidant itself, has occurred at the Ti buffer during the ALD cycle, determining the growth of an extra oxide layer of TiOx and thus providing interesting and useful features to the final device. We will take advantage of several microscopic and macroscopic techniques in order to fully characterize the resulting oxide layer, in order to fully elucidate the effect produced by the surface reaction.

In brief, we report here the structural evidence for the presence of TiOx below the HfO2 layer and discuss the key role of the O3 precursor in its formation during the ALD process. Although it is quite difficult to disentangle the oxidant and/or precursor effect, some signs seem to point to the former as the main responsible one of the produced changes. We also include macroscopic electrical evidence to support that the TiOx layer determines the occurrence of a very interesting metal-insulator-metal nonvolatile memory device. Besides, an additional deposition process was carried out in order to clarify whether we could be in presence of another unconsidered effect. Finally, we will introduce a phenomenological model to describe the electrical response and microscopic transport properties that support the microscopic picture achieved.

## **1.2. Details of the ALD technique and the device's fabrication**

For ALD-based HfO<sup>2</sup> growth, the oxygen source is typically either H2 O [4] or O3 [4] while the metal source could be for example Tetrakis(dimethylamino)hafnium (TDMAH) or TEMAH [5]. In our specific case, O<sup>3</sup> and TDMAH were the oxygen and metal source, respectively, although we will also include some comparison with samples grown using H2 O instead of O3 . The substrate employed was commercial Si (highly doped)/thermally oxidized SiO2 (120 nm) of 1 cm2 size. The Ti layer (20 nm) was sputtered on top. After Ti sputtering, the ALD process—using either ozone as oxygen source and TDMAH as the metal precursor—was carried out to obtain a uniform 20 nm-thick HfO2 layer. After this deposition process an array of 5 lines by 16 columns was determined by means of optical photolithography, delimiting 80 squared-shaped structures of 200 μm lateral size covered with sputtered Pd (40 nm)/Co (35 nm) acting as the top electrode (TE). To complete these two terminal devices, the access to the bottom contact was achieved by a scratch.

#### **1.3. The ReRAM scenario**

As downscaling of storage devices is approaching its physical limits, new strategies based on emergent materials and non-previously explored effects are moving into the focus of intense research as FLASH memory replacement. In particular, the metal-insulator-metal (MIM) structures acting as memory cells are developing as prominent candidates for such replacement. The resistive switching (RS) is the mechanism underlying the memory behavior. Its appealing features (speed, downscaling, retention, endurance) have evolved into a nowadays mature technology, coined as resistive random access memory (ReRAM).

**2. Understanding the origin of remarkable electrical properties in** 

The MIM stack was electrically characterized in a two-terminal configuration (see inset **Figure 1a**). We used a Keithley 4200 unit hooked through coaxial wires to a room-temperature probe station. Applying voltage while recording the current flowing through the stack allowed us to identify the general properties of the devices. In particular, sweeping voltage in a pulsed

The Atomic Layer Deposition Technique for the Fabrication of Memristive Devices: Impact of the Precursor…

http://dx.doi.org/10.5772/intechopen.78937

7

**Figure 1a** shows a typical current-voltage (I-V) dependence of a fabricated stack, obtained by sweeping voltage pulses, as demonstrated in the inset in **Figure 1a**. Four branches can be defined: (a) from 0 to +15 V, (b) from +15 to 0 V, (c) from 0 to −15 V, and (d) from −15 to 0 V.After deposition (pristine state), all devices are found in HRS (branch A in **Figure 1a**). When the positive voltage applied to the top electrode is increased, the device abruptly switches to LRS in a SET operation happening at around +5 V. No additional previous "forming" is required. Upon decreasing the stimulus (branch B), the I-V curve exhibits huge hysteresis (the semi-

Further cycling with positive voltages reproduces the LRS, as shown in the second and fourth sweeps in **Figure 1b**. Thus, the programmed state is nonvolatile with respect to the time scale

When the polarity is reversed, the negative voltage cycle starts in the HRS (branch C in **Figure 1a**), even when the positive cycle finished in a LRS, that is, a rectifying response is found. This behavior is referred to as a non-crossing hysteresis in the literature [15]. From that HRS on, the description of the negative voltage cycle is completely analogous to the positive

**Figure 1.** (a) Current as a function of voltage (I-V) measured with a pulsed sweep (see lower right inset). The upper left inset sketches the stack (TE stands for top electrode while SP is the way to access to the bottom contact of the structure). (b) I-V curve displayed in semi-logarithmic scale. As can be seen from the inset, the protocol consists in two repetitions

**simple RS stacks based on ALD-deposited HfO2**

logarithmic scale in **Figure 1b** highlights the change rate).

of each polarity sweep in order to test the nonvolatile behavior.

**2.1. Electrical characterization**

of the measurement.

way is suitable for avoiding heating effects.

The ability to produce a reversible change of conductance in these technologically simple structures (the RS effect) relies on the extremely large electric field applied to the strategically engineered thin insulating layer (i.e., between the metallic electrodes) but also on the choice of the electrodes' material. Basically, the effect consists of a switching process between a high resistance state (HRS) and a low resistance state (LRS) through a soft dielectric breakdown of the insulating layer(s). The change from HRS to LRS is called the SET process and the opposite one (from LRS to HRS) is referred to as RESET. Thus, memristive cells are resistive switching units, and their unique properties are strongly dependent on the materials used and on the fabrication details. They could need an initial electroforming process or not, and the polarity could be a relevant parameter (bipolar switches) or not (unipolar switches). Comprehensive discussions on the resistive switching phenomena are found in Ref. [6].

The simplicity of the geometrical structure and the absence of transistors make the concept extremely interesting for low-power, high-density, and nonvolatile memory applications. However, a challenge to achieve a technological implementation, using the RS concept, is to be allowed to select a designated cell within a passive crossbar array without interference from neighboring cells (i.e., the sneak currents problem) [7].

A way to overcome the sneak currents problem includes the use of rectifying elements to isolate each nonvolatile memory cell. The integration of a rectifying element, to achieve bipolar operation, would solve the sneak path problem "in situ." But so far no sufficiently scalable material has been found yet [7]. Therefore, simple structures based on nanometer thick oxides are a major topic of work in scientific and industrial research. A detailed knowledge of expected behaviors allows material engineering. In that sense, rectifying metal/oxide junctions, based on TiOx , ZnO, and on TaO2−x, has been recently described [8, 9], where upon appropriate oxygen vacancy accumulation the interface is switched to a non-rectifying resistive device. In particular, structures based on HfO2 have shown excellent rectifying capabilities [10, 11]. Besides, hafnium oxide is a preferred high-k material, and therefore it is one of the most promising ReRAM materials since it has been already added to the complementary metal-oxide semiconductor (CMOS). From the industrial point of view, this fact is an enormous advantage and explains why so much effort is being done related with HfO<sup>2</sup> .

In this scenario, and responding to the actual trend on multifunctional components, there is a renewed interest on the mechanisms governing its dielectric behavior. These facts, combined with the observation of perpendicular magnetic anisotropy (which involves another promising low-power memory mechanism [12]) in Co films deposited on high-k materials [13] and FeFETs based on doped HfO<sup>2</sup> [14], put devices based on HfO2 again in the focus of the attention.
