Data retention Measurement 24

Before this data retention test, the switching voltage formed all resistance states.

After the resistance switching process, HP4156 was used to detect the resistance of memory cell for a period of time. By means of this periodicity reading and waiting process, we can observe the data retention performance in this test. Moreover, the reading voltage of this data retention test must be small because the memory cell may be damaged by large voltages in the reading process. With this long time resistance reading process, we can obverse the data retention performance. Figure 2-15 shows a typical data retention test of 2bits/cell RRAM device. In this figure, all four-resistance states keep the same resistance value more than 10⁴ src.

Figure 2-15. The data retention test of 2 bits/cell RRAM device^[29]. 2.3.1.5 Read Disturb Measurement

In the read-disturb performance test, it is similar to the data retention test. Before the read-disturb test, the switching voltage formed all resistance states. The different part with the data retention test is the waiting time. In this test, there are only reading processes in this measurement. With these repeating resistance reading process, we can observe the read-disturb performance of memory cell. Moreover, the small reading voltage is used to avoid damaging the memory cell in the test process. Figure 4-16 shows the typical read-disturb performance test. In this figure, The “On” state is influence by the reading voltage, and the “Off” state keeps the same value in the read-disturb process.

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Figure 2-16. The typical read disturb test of RRAM device^[37].

2.3.2.1 XPS Analysis

X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopy technology to determine the elemental composition, empirical formula, and electronic state of elements. It is the use of low-energy X-ray source as the excitation source and through the analysis of samples with a characteristic energy of emitted electrons to achieve the purpose of analyzing the chemical composition; that is an ample surface analysis technology. Figure 2-17 shows the construction of XPS. XPS analysis is made into the X-ray beam; the atoms interact with the sample surface after the electronic excitation of atomic inner-shell ionization to detect the sample composition and structure. This is the characteristic X-ray excitation, and the electron here is called ionization photoelectron.

Because of the specific wavelength of the X-ray, its energy is known, and the electron binding energy can be calculated by Eq.(2-1).

)

Here, Ebinding is the binding energy of electron, Ephoton is the energy of the X-ray photon, Ekinetic is the kinetic energy of the electron as measured by the instrument, and ψis the work function of the spectrometer. The XPS spectrum can be observed by using binding energy as X-axis and relative intensity as Y-axis. With this spectrum, we can get the informant of samples with elemental composition and chemical state. XPS is the most useful for chemical analysis, and it is also call “electron spectroscopy for chemical analysis” (ESCA).

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Figure 2-17. The sketch of XPS¹⁶.

2.3.2.2 TEM Analysis

Transmission electron microscope (TEM) uses high-energy electron beam (about 100keV ~ 1MeV) through the thin samples (below 100nm), and various structure within the thin samples have different degrees of scattering. Scattering of electrons by means of different routes goes through the subsequent combination of lens aperture lens, forming the contrast images of light and dark, and the microstructure of these images is shown with the fluorescent plate. Therefore, transmission electron microscopy analysis of thin samples is acquired through transmitted electron or elastic scattering electron, or diffraction pattern microstructure, and thus resolves the structure of the thin samples and the crystal structure. Figure 2-18 shows the construction of TEM.

Moreover, the selected area electron diffraction (SAED) in TEM instrument is also can be used to check the sample structure. The SAED principle is in that thin crystal sample, the high-energy parallel ray electron beam can go through this thin sample. In this case, electron is the corresponding volatility, rather than the particle nature. As the energy of the electron wavelength is nanometers in length, and the wavelength is relatively much larger than the spacing between atoms, the atoms are arranged in this electron diffraction grating. This means that a portion of the wavelength will be scattered out of a particular point of view (different parallel surfaces) and will decide the crystal of the sample.

16 http://wiki.utep.edu/display/~vrrangel/X-ray+Photoelectron+Spectroscopy+(XPS)

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Figure 2-18. The sketch of TEM¹⁷. 2.3.2.3 PPMS Measurement

The temperature dependent electric character data was analyzed by the PPMS system PPMS. The cooling system uses the liquid helium (He) to cool the measurement system, and the minimum temperature is about 4K. The temperature range is between the room temperature and 4K. In the cooling process from room temperature to 4K, we can observe the resistance change in this cooling process.

17 http://universe-review.ca/R11-13-microscopes.htm

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Chapter 3 : Principles

In recent years, many researches study the resistive switching mechanisms of novel RRAM. According to the result of these studies, we discuss several important physics parameters which influence the performance of RRAM. These physics parameters including cell thickness^[38], cell size^[39], doping material^[40] , electrode^[41], density of oxygen vacancies, and electric field^[38,42]. Moreover, the RRAM performance can also be improved with modified operation process^[36,43,44]. By applying suitable device structure and modified operation to potential materials, the RRAM device performance shows much improvement in the last ten years.

For the electric characteristics analysis of RRAM, many researches on different materials elucidate the electron transport mechanisms of both high and low resistance states follow various conduction mechanisms, such as metallic transportation^[20], Schottky emission^[45], tunneling, space-charge-limited-current (SCLC)^[46], Frenkel-Poole emission^[47-49], Trap-assisted-Tunneling (TAT)^[50], electron hopping transportation^[20], and so on.

Moreover, for the resistive switching mechanism studies, most of the literatures indicate the conducting filament^[51-53] mechanism related to oxygen-vacancy^[54,55]. Also, various models, such as stochastic model^[56], two-variable resistor model^[46,57], compact model^[58], thermal dissolution model^[48,59], rupture ball model^[38], etc., were proposed to explain the resistive switching phenomenon in their researches.

In order to show a specific RRAM profile for readers, we introduce the basic resistance switching characteristics and nomenclature, electron transportation mechanism, resistance switching mechanism and model, key physics parameters and modified operation process in this chapter.

3.1 Resistance Switching Characteristics and Nomenclature

Before the introduction of RRAM principle, we describe the basic resistance switching characteristics including bipolar, unipolar, and nonpolar operations. Also, we discuss the basic resistance switching nomenclature, including forming process, set (programming) process, reset (erasing) process, forming voltage , set voltage (current), reset voltage, HRS (reset state), LRS (set state), resistance window (on/off ratio), dc voltage (current) sweep, and pulse switching in this section.

3.1.1 Bipolar, Unipolar, and Nonpolar Operations

For the resistance switching phenomenon in RRAM, bipolar and unipolar are the two major operational methods in the bistable resistance switching process. In earlier researches at 1970s, the most common resistance switching phenomenon of RRAM displays the bipolar operation. It means that the resistance state depends on the polarity

29

of applied bias. Biases with different polarities induce different memory states.

However, the bipolar RRAM resistor always integrates with one transistor in device applications. This one-transistor-one-resistor (1T1R) device has a larger device size and it will limit the capacity in memory applications. In recent years, the unipolar RRAM becomes an important memory application because it can integrate with a diode to form a one-diode-one-resistor (1D1R) device for memory applications. This 1D1R device has a smaller device size and it shows higher potential for high capacity memory application.

In this section, we describe the typical RRAM operation method.

Figure 3-1 shows the typical bipolar operation of resistance switching phenomenon^[60]. The resistance state depends on the polarity of applied bias. For example, the resistance state can be switched from low resistance state (LRS) to high resistance state (HRS) by applying a sweep bias, which shows from loop 1 to loop 2 in figure 3-1. Also, this HRS can be switched back to LRS by applying an opposite bias (from loop 3 to loop 4). Moreover, in order to protect the memory cell in this resistance switching process from HRS to LRS, the current compliance is necessary in this resistance switching process, and it can be observed on the dash line of figure 3-1.

Figure 3-1. The typical dc sweep bipolar operation characteristics.

The bipolar operation, which is described in the previous page, belongs to the dc sweep operation. Another bipolar operation is the pulse voltage operation, which shows fast speed in the resistance switching process. For example, in figure 3-2 the resistance state can be switched with the applied pulse shot (~80ns) and this resistance switching characteristics exhibits polarity dependence relationship of pulse shot. The opposite pulse shot induces opposite resistance state. This kind of operation belongs to the pulse voltage operation.

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Figure 3-2. The bipolar operation characteristics by applied pulse voltage.

Another polarity operation of resistance switching phenomenon is the unipolar operation method. The typical unipolar operation is shown in the upper-right region of figure 3-3. The resistance state can be switched from LRS to HRS by applying a sweep bias, which shows from loop 1 to loop 2 in figure 3-3. Also, this HRS can be switched back to LRS (from loop 3 to loop 4) by a higher applied bias. In this unipolar operation process, the current compliance is necessary to protect the memory cell in resistance switching process from HRS to LRS, and it can also be observed on the dash line in figure 3-3.

Usually, most unipolar operation RRAM materials exhibit the resistance switching characteristics by only one-way polarity voltage. Moreover, the resistance state can’t be switched by the opposite polarity voltage. However, only several materials exhibit two-way unipolar operation, which we call “nonpolar operation”. The unipolar resistance switching characteristics can be observed by both positive and negative voltages. As shown in figure 3-3, the two unipolar operation characteristics can be observed in both upper-right and lower-left region. This operation method is called

“nonpolar operation” and it shows symmetrical resistance switching behavior.

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Figure 3-3. The typical unipolar and nonpolar operation characteristics.

In the unipolar operation, the resistance state can also be switched with an applied pulse voltage. Figure 3-4 shows the unipolar resistance switching characteristics^[60] with the applied pulse voltage. In this case, we fix the applied voltage about 4V and increase the pulse width from 10ns to 1us. With the pulse width increasing, it is clear to see that the resistance state increases first and then decrease in the unipolar operation process.

Figure 3-4. The unipolar operation characteristics by applied pulse voltage.

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3.1.2 Basic Resistance Switching Nomenclature

In the researches of RRAM, different literature used different nomenclature that always confuses readers. Before the discussion of RRAM principles in this section, we introduce the nomenclature of RRAM to avoid the confusion.

First of all, we introduce the initial resistance state, which means the resistance of as-prepared. The resistance of this state is always higher than other states due to the high resistance metal oxide film. Before the resistance switching operation, most materials need a high voltage applied to work up the resistance switching characteristics.

For the example of unipolar operation^[61] RRAM in figure 3-5, this memory cell needs a high voltage about 7.5V applied to work up the memory characteristics. This voltage is called the “forming voltage” (Vform) and this process is called the “forming process”

(from arrow 1 to arrow2). As shown in figure 3-5, the resistance state is switched from the initial resistance state to the LRS in this forming process. However, several researches use current sweep method in this forming process to protect the memory cell, and the resistance switching current is also called “forming current”.

Secondly, the resistance state can also be switched from LRS to HRS with another applied bias. For example, in figure 3-5 the resistance state can be switched from LRS to HRS at about 2V. This voltage is called the “reset voltage” (Vreset) and this process is called the “reset process” (from arrow 3 to arrow 5). In other literature, this reset voltage is called the “erasing voltage” and this process is called the “erasing process”, and this HRS the “reset state” or “off state”.

Figure 3-5. The resistance switching characteristics of unipolar RRAM.

Next, this HRS can also be switched to LRS with a higher applied voltage. For example, in figure 3-5 the resistance state can be switched from HRS to LRS at about 5V. This voltage is called the “set voltage” (Vset) and this process the “set process”

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(from arrow 6 to arrow 8). In other literatures, this set voltage is called as

“programming voltage”, this process as “programming process”, and this LRS as the

“set state” or “on state”. However, several researches use current sweep method in the set process to protect the memory cell, and the resistance switching current is called “set current”.

What’s more, the on/off ratio between HRS and LRS is called the “resistance window”. Other researches also call it “On/Off ratio”. The larger On/Off ratio sample indicates that it has more potential for multiple-level-cell (MLC) application and is suitable for high density storage memory application, which is shown in figure 3-6.

Figure 3-6. The multiple-level-cell (MLC) of 2bits/cell RRAM.

3.2 Electron Transportation Mechanism

After the discussion of RRAM resistance switching characteristics and nomenclature, we describe the electron transportation mechanism of both LRS and HRS in this section. In recent years, numerous electron transportation mechanisms have been proposed for the RRAM researches. These mechanisms include the metallic transportation^[20], electron hopping transportation^[20], Schottky emission^[45], SCLC^[46], TAT^[50], Pool-Frenkel emission^[47-49], etc. The metallic transportation mechanism with linear I-V curve, which follows Ohm’s law, is usually used to explain the electrical characteristics of LRS. Also, the Poole-Frenkel emission and TAT mechanisms with non-linear I-V curve are only used to explain the behavior of HRS. Furthermore, other mechanisms with non-linear I-V curve are used to explain both LRS and HRS in several literatures^[45].

3.2.1 The Metallic Transportation Mechanism

The metallic transportation mechanism includes two parts. One is the linear I-V curve, and it follows the Ohm’s law. As shown in arrow 1 of figure 3-3, the LRS shows the linear I-V curve, and the current linearly depends on the voltage. The electrical character follows the Ohm’s law and it is also shown in Eq. (3-1).

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R

I V ……….. (3-1)

The other part is the temperature effect on the electrical characteristics^[20]. When the temperature decreases, the resistance decreases in the meantime. As shown in figure 3-7, the resistance reduces when the temperature decreases both at high and low temperatures, and this characteristic displays the metal behavior.

This metal transportation mechanism is always used to describe the electrical characteristics of the LRS due to the linear I-V curve.

Figure 3-7. The temperature dependence relationship of metal electrical characteristics.

3.2.2 The Schottky Emission

Except for the metal transportation mechanism, all of the electrical transportation mechanisms show the non-linear I-V curve. Another common transportation mechanism of RRAM is the Schottky emission^[45]. This mechanism is attributed to the metal-semiconductor contact, which induces the barrier height at the interface, and this barrier height also influences the characteristic of the electrical transportation characteristics. Figure 3-8 shows the energy band diagram of the metal-semiconductor contact and the electrical transportation follows the Schottky emission.

Figure 3-8. The energy band diagram of a metal-semiconductor contact¹⁸.

18 http://www.iue.tuwien.ac.at/phd/ayalew/node56.html

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Moreover, this transport mechanism is dependent on the barrier height and the dielectric constant, as is shown in Eq. (3-2).

4 ])

where A* is the effective Richardson constant, υB is the barrier height, ξ is the electric field, and εi is the insulator dielectric constant. In the electrical character of this Schottky emission, the value of log current increases in the same amount with the square root of voltage, and the typical Schottky emission electrical curve^[45] is shown in figure 3-9. The Schottky emission electric characteristic appears in both LRS and HRS of RRAM electrical transportation mechanism. Because several metal oxides (nickel oxide, etc.) are n-type semiconductors, the RRAM structure also forms the metal-semiconductor contact, and the electrical characteristics also shows the Schottky emission in both LRS and HRS.

Figure 3-9. The typical electrical curve of Schottky emission.

3.2.3 The Poole-Frenkel Emission

Another common electrical transportation mechanism of RRAM is the Poole-Frenkel emission^[47-49] and it also shows a non-linear I-V curve. This mechanism, which is attributed to the electrons, can move slowly through an insulator and these electrons are generally trapped in localized states. Moreover, the random thermal fluctuations or large electrical field will give the electrons enough energy to get out of its localized state and move to the conduction band. Figure 3-10 shows the energy band

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diagram of the Poole-Frenkel emission.

Moreover, the transport mechanism is dependent on the barrier height and the dielectric constant, as is shown in Eq. (3-3).

]) [

exp(  ^qE

T k E q

J _B

B

 

 ………...(3-3)

For Poole-Frenkel emission, the value of log(J/E) increases in the same amount with the square root of electrical field, and the typical Poole-Frenkel emission electrical curve^[49]

is shown in figure 3-11. The Poole-Frenkel emission characteristic appears in HRS of RRAM electrical transportation mechanism. Because the metal oxide film shows the insulator characteristic, the electrical curve of RRAM also shows the Poole-Frenkel emission behavior in HRS.

Figure 3-10. The energy band diagram of Poole-Frenkel emission¹⁹.

Figure 3-11. The typical electrical curve of Poole-Frenkel emission.

19 Sanjeev K. Gupta, A. Azam, and J. Akhtar, J. Phys., vol.74, No.2, 327, (2010)

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3.2.4 The Electron Hopping Transportation Mechanism

The electron hopping transportation^[20] is another common electrical transportation of RRAM and it shows a non-linear I-V curve. The early researches indicated that this mechanism has low temperature conduction behavior in strongly disordered systems with localized states. Several researches reported the Mott variable range hopping (VRH) characteristics of RRAM materials. Figure 3-12 shows the band diagram of VRH. These gradation localized states is formed by the strongly disordered systems.

Figure 3-12. The band diagram of variable range hopping²⁰.

The VRH^[62] transport mechanism is depicted by Eq. (3-4). Here, R is the hopping distance, N(Ef) is the density of state, k is the Boltzman’s constant, E is the electric field, α is the decay parameter of wave function, and νph is dependent on the frequency of

However, if eRF is much smaller than kT in a weak electric field, we can approximate the relation as simplification equation. This simplification Mott VRH equation as shown by Eq. (3-5).

20 http://people.math.gatech.edu/~jeanbel/TalksE/mott09.pdf

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) exp(

'  ^1/4

A BT

J …..(3-5)

Moreover, with the calculation of Origin software, we can obtain the value of (eRE/kt) with the hyperbolic sine fitting curve. Also, the hopping distance is calculated by the fitting value, and the relationship between this value and hopping distance is shown by Eq. (3-6).

kTd CV eRV kT

eRF   ……(3-6)

The typical VRH electrical curve is shown in figure 3-13. The VRH electric characteristic appears both in LRS and HRS of RRAM electrical transportation mechanism. In several researches^[20,29] of gradation oxidation systems, for example of WOx-based RRAM, the week metal oxide film shows gradation WOx system and the electron transportation characteristic follows the Mott VRH mechanism.

Figure 3-13. The electrical curve of Mott variable range hopping (VRH).

3.2.5 SCLC mechanism

Another electron transportation mechanism of RRAM is the SCLC^[46]. This mechanism occurs before the charge injection when the charges accumulate at the

Another electron transportation mechanism of RRAM is the SCLC^[46]. This mechanism occurs before the charge injection when the charges accumulate at the

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