**4. Solid‐state molecular nonvolatile memory**

#### **4.1. Introduction**

1:3.5 were selected. The CyV curves of the mixed molecules are shown in **Figure 5a**. The results from the nonmixed pure molecules were also illustrated for comparison. One pair of Fc‐BzOH peaks and two pairs of Por‐BzOH peaks were clearly observed for the three mixed SAMs. With decreasing Fc‐BzOH molar percentage, the corresponding peak height decreased substantially due to the changing surface coverage density. The coverage of Fc‐BzOH was higher than that of Por‐BzOH in the pure SAMs and even mixed SAMs with 1:1 molar ratio, because of the smaller size and the faster attachment of smaller molecule of Fc‐BzOH. The redox peaks exhibited a scan rate‐dependent peak separation, as shown in **Figure 5b**. Such phenomena could arise from either an increasing resistive drop in the electrolyte or limitations imposed by the intrinsic electron‐transfer rate of redox centers on the Si substrate [51]. **Figure 5c**–**e** shows the *C*‐*V* and *G*‐*V* characteristics of the SAMs at 100 Hz. Each of the mixed SAMs shows three pairs of peaks corresponding to the charging and discharging behaviors of the Fc‐BzOH and Por‐BzOH mole‐ cules. As the molar ratio decreases, the capacitance and conductance peaks of Por‐BzOH increase while those of Fc‐BzOH decrease. This is also in agreement with the CyV measurement results. Such a mixed SAM approach is very attractive owing to its simpler synthesis and more dis‐ tinct potential of the redox states, as compared with a single molecule with multiple redox

**Figure 5.** (a) CyV of SAMs of pure and mixed Fc‐BzOH and Por‐BzOH with different molar ratios. The scan rate was 5 V/s. (b) CyV at increasing scan rates of 10, 50, and 100 V/s. *C*‐*V* and *G*‐*V* characteristics with mixed SAMs with molar

ratios of (c) 1:0.35, (d) 1:1.4, and (e) 1:3.5 [51].

66 Redox - Principles and Advanced Applications

CMOS and semiconductor nonvolatile memory scaling have create huge demands for alternative memory technologies with higher scalability and better performance. The well‐studied charge‐ trapping memory relies on hot electron injection from the channel into the charge‐trapping medium through a tunneling layer. By attaching redox‐active molecules onto silicon structures as the charge‐storage medium can further enhance the memory density and enable further cell scaling [30, 53–55]. The previous section has introduced the implementation of redox molecules for nonvolatile memory applications. However, the liquid electrolyte‐involved structure lacks effective protection for the molecules leading to deteriorated memory performance. By integrat‐ ing redox molecules in a solid‐state molecular memory cell with a solid‐state insulating barrier deposited on both sides of the molecules, the possibility of orbital hybridization from the gate can be lessened. The structure of the solid‐state molecular memory can be engineered with the material and layer thickness as well as the molecules, whose linker also works as the tunnel bar‐ rier and can be optimized by variation in structure and connectivity to obtain the desired tun‐ neling probability, redox potential, and retention time. Such an integration of redox molecules in a solid‐state memory provides an excellent platform to study the electrical behavior of the molecules and the devices with universal microelectronic characterization metrologies.

In this section, we first review a solid‐state capacitor structure incorporating embedded redox molecules as the charge‐trapping medium for high‐endurance memory applications. Then, a Si nanowire FET‐based molecular flash‐like memory with faster operation speed, lower operation voltage, better reliability, and multibit charge storage will be introduced.

#### **4.2. Metal/dielectric/molecule/oxide/Si capacitors for memory application**

#### *4.2.1. Device fabrication*

**Figure 6a** shows the structure of the ferrocene molecule and the schematic of a metal/ Al<sup>2</sup> O3 /ferrocene/SiO2 /Si (MAFOS) solid‐state capacitor structure [30]. The most important fabrication process steps are the molecule attachment on SiO<sup>2</sup> and the formation of Al<sup>2</sup> O3 encapsulating the molecules. After the 100 μm × 100 μm active areas were defined by photo‐ lithography and wet etching, a ∼1.5 nm SiO<sup>2</sup> was grown in the active area by rapid thermal oxidation at 850°C for 2 min. The SAM was formed by placing the substrate in a solution of 3 mM ferrocene in dichloromethane at 100°C for 20 min. After the attachment, dichloro‐ methane was used again to rinse the substrate to remove any physisorbed residuals on the surface.

**Figure 6.** (a) Molecule structure of α‐ferrocenylethanol (ferrocene) and the schematic of the MAFOS capacitor memory cell. (b) CyV curves of ferrocene‐attached capacitor at different scan rates. Inset: CyV curves of the reference sample with ferrocene attached on the Si surface. (c) XPS spectra of the samples with ferrocene attached on Si and SiO<sup>2</sup> surfaces. (d) The XPS spectrum of the ferrocene‐attached SiO<sup>2</sup> substrate with 5 nm Al<sup>2</sup> O3 covered on the top [30].

CyV was performed after the attachment of molecule. Other substrates were immediately loaded into the atomic layer deposition (ALD) vacuum chamber to deposit 20 nm Al<sup>2</sup> O3 at 100°C. Trimethyl aluminum (TMA) and H<sup>2</sup> O were used as precursors. In the final step, a layer of 80 nm Pd was deposited and patterned on Al<sup>2</sup> O3 as the top gate. Three reference samples were fabricated for comparison: metal/Al<sup>2</sup> O3 /ferrocene/Si (MAFS), metal/Al<sup>2</sup> O3 / SiO2 /Si (MAOS), and metal/Al<sup>2</sup> O3 /Si (MAS). For the ferrocene attachment on the Si surface in MAFS device, a hydrogen‐terminated Si surface was obtained by dipping the substrate into 2% hydrofluoric acid for 30 s prior to the attachment [30].

#### *4.2.2. Electrical characterization*

**Figure 6b** shows the CyV curves of the electrolyte/ferrocene/SiO<sup>2</sup> /Si structure at various scan rates. Oxidation and reduction peaks were observed at negative and positive gate voltage (*V*G), respectively, through losing and restoring electrons from the SAM. By comparing the CyV curves with the results from the electrolyte/ferrocene/Si structure (inset in **Figure 6b**), the clear peak separation is due to the tunneling barrier (∼1.5 nm SiO<sup>2</sup> ) and the molecular linker. The ferrocene surface coverage can be calculated from the oxidation peak, and the coverage was found to be 5.23 × 10<sup>13</sup> and 3.14 × 1013 cm−2 for ferrocene attachment on Si and SiO2 surfaces, respectively. X‐ray photoelectron spectroscopy (XPS) was carried out before and after the ALD to examine the viability of ferrocene during the deposition. From the XPS spectra shown in **Figure 6c** and **d**, the ferrocene SAMs were well attached on both Si and SiO<sup>2</sup> surfaces, and the SAM survives the deposition of Al<sup>2</sup> O3 .

The memory behavior was characterized by measuring the *C*‐*V* hysteresis at 1 MHz. As shown in **Figure 7a**, large memory window was observed with the MAFOS device, and the counterclockwise hysteresis loop indicates the charge storage in ferrocene SAM. **Figure 7b** shows the flat‐band voltage shift (Δ*V*FB) of the MAFOS and three control samples as a func‐ tion of applied programming/erasing (P/E) operations. The much smaller Δ*V*FB observed from MAOS and MAS devices indicates the negligible charge storage at the dielectric interface traps. The asymmetric behavior observed from the MAFS device suggests the difficulty of

CyV was performed after the attachment of molecule. Other substrates were immediately loaded into the atomic layer deposition (ALD) vacuum chamber to deposit 20 nm Al<sup>2</sup>

substrate with 5 nm Al<sup>2</sup>

**Figure 6.** (a) Molecule structure of α‐ferrocenylethanol (ferrocene) and the schematic of the MAFOS capacitor memory cell. (b) CyV curves of ferrocene‐attached capacitor at different scan rates. Inset: CyV curves of the reference sample with

ferrocene attached on the Si surface. (c) XPS spectra of the samples with ferrocene attached on Si and SiO<sup>2</sup>

MAFS device, a hydrogen‐terminated Si surface was obtained by dipping the substrate into

rates. Oxidation and reduction peaks were observed at negative and positive gate voltage (*V*G), respectively, through losing and restoring electrons from the SAM. By comparing the CyV curves with the results from the electrolyte/ferrocene/Si structure (inset in **Figure 6b**),

100°C. Trimethyl aluminum (TMA) and H<sup>2</sup>

/Si (MAOS), and metal/Al<sup>2</sup>

The XPS spectrum of the ferrocene‐attached SiO<sup>2</sup>

68 Redox - Principles and Advanced Applications

*4.2.2. Electrical characterization*

SiO2

layer of 80 nm Pd was deposited and patterned on Al<sup>2</sup>

2% hydrofluoric acid for 30 s prior to the attachment [30].

O3

**Figure 6b** shows the CyV curves of the electrolyte/ferrocene/SiO<sup>2</sup>

samples were fabricated for comparison: metal/Al<sup>2</sup>

O3 at

surfaces. (d)

O3 /

O were used as precursors. In the final step, a

covered on the top [30].

as the top gate. Three reference

/Si structure at various scan

/ferrocene/Si (MAFS), metal/Al<sup>2</sup>

O3

O3

/Si (MAS). For the ferrocene attachment on the Si surface in

O3

**Figure 7.** (a) *C*‐*V* hysteresis curves of the MAFOS device at 1 MHz with different gate voltage scan ranges. Hysteresis of MAFOS and three control samples with ±5 V scan range were compared in the inset. (b) Δ*V*FB of MAFOS and three control samples versus P/E voltage with 500 μs pulse width. (c) Retention properties of MAFOS and MAFS at room temperature. (d) Endurance properties of MAFOS with ±10 V P/E voltage but different pulse width [30].

maintaining molecules positively charged without the SiO<sup>2</sup> tunnel barrier. The stable and symmetric staircase Δ*V*FB of MAFOS device originated from the reliable charging/discharging behavior of ferrocene and the effective charge separation by the SiO<sup>2</sup> tunnel barrier [30]. The charge density of the ferrocene SAM was calculated by using the following equation [30]:

charge density of the ferromagneticSAM calculated by using the following equation [30]: 
$$\Delta V\_{\text{FB}} = \frac{e \cdot n}{C} = e \cdot n \cdot \left(\frac{T\_{\text{Al}\_i\text{O}\_i}}{\varepsilon\_0 \varepsilon\_{\text{Al}\_i\text{O}\_i}}\right) \tag{1}$$

where *e* is the elementary charge, *n* is the number of stored electrons, *T*Al2O3 and *ε*Al2O3 are the thickness and relative dielectric constant of Al<sup>2</sup> O3 , respectively. The charge density was calculated to be 4.82 × 1012 cm−2, which is only a small portion (∼15%) of the coverage density obtained from the CyV results. Nevertheless, effective memory can still be realized and the charge density of the solid‐state molecular memory is sufficiently high [30].

The room temperature retention of MAFOS and MAFS devices is shown in **Figure 7c** [30]. Note that ∼60% charge loss was observed after 10<sup>6</sup> s retention time for MAFOS. Such fair retention is due to the relative thin SiO<sup>2</sup> , and the oxide quality by rapid thermal oxidation is not satisfactory. The endurance property of the MAFOS device was measured with ±10 V P/E voltages and different pulse width. The memory device continues to behave well after 10<sup>9</sup> P/E cycle with the same memory window and a slightly upshift which is due to the accumulation of electron in deep traps [30]. Such excellent endurance characteristics are naturally derived from the intrinsic redox properties of the redox molecules, which have been well protected by the device structure design and fabrication.

#### **4.3. Nanowire/nanotube‐based flash‐like molecular memory**

Semiconductor nanowires and nanotube MOSFETs have been regarded as the building blocks for nanoelectronics and circuits [56, 57]. The quasi‐one‐dimensional nanowires have a larger surface‐to‐volume ratio as compared with the bulk materials. Therefore, less stored charges are needed to induce a same memory window. In addition, the nanowire or nanotube can enable a gate‐surrounding structure, allowing excellent electrostatic gate control. The integra‐ tion of redox molecules in semiconductor nanowire FET for solid‐state flash‐like memory can be expected to significantly enhance the memory performance by taking advantages of the inherent properties of redox molecules [24, 58–60].

#### *4.3.1. Device fabrication*

**Figure 8** illustrates the fabrication process of a molecular flash memory based on a so‐called "self‐aligned" silicon nanowire FET [24, 61]. A thin film of Au catalyst (2–3 nm) was first sputtered at predefined locations by photolithography on a 300‐nm SiO<sup>2</sup> /Si substrate. The Si nanowires were then grown from the catalyst following the vapor‐liquid‐solid (VLS) mecha‐ nism in a low‐pressure chemical vapor deposition (LPCVD) furnace at 440°C for 2 h with an ambient SiH<sup>4</sup> stream under a pressure of 500 mTorr. Immediately after the growth, the nanow‐ ires were oxidized at 750°C for 30 min by dry oxidation to form a 3‐nm SiO<sup>2</sup> on which the SAM will be attached [24]. As compared with the SiO<sup>2</sup> grown by rapid thermal oxidation, the SiO<sup>2</sup> formed here is of much better quality by using the dry oxidation technique [30]. Therefore, enhanced memory retention can be expected. The next step is to pattern the source/drain

**Figure 8.** Schematic of the fabrication process of a molecular flash memory. (a) Au patterning on SiO<sup>2</sup> , synthesis of Si nanowires and nanowire oxidation. (b) Formation of S/D electrodes and SAM attachment. (c) Deposition of Al<sup>2</sup> O3 by ALD and fabrication of top gate electrode. (d) Schematic structure of a completed molecular flash memory [24].

(S/D) electrodes with photolithography. A 2% HF wet etch was applied to remove the oxide from the nanowire at the patterned S/D area before 5/100 (unit:nm) Ti/Pt was deposited and lift‐off to form S/D electrodes.

The SAM of ferrocene and Ru<sup>2</sup> redox molecules (**Figure 1**) were then formed on the SiO<sup>2</sup> / Si nanowire channel by placing droplets of a solution of dichloromethane with 3‐mM fer‐ rocene and 2‐mM Ru<sup>2</sup> on active areas separately [24]. Each drop was in place for 3–4 min and the substrates were held at 100°C in an N<sup>2</sup> environment during attachment. Saturated SAMs were formed after ∼30 min. Dichloromethane solvent was used to rinse the substrates to remove the residual molecules. The samples were then immediately loaded into the ALD chamber for a layer of 25 nm Al<sup>2</sup> O3 deposition at 100°C with TMA and H<sup>2</sup> O as precursors. The final step is the fabrication of 100‐nm Pd top gate with the same photolithography and lift‐off process as S/D electrodes. A reference sample without redox molecules was fabricated for comparison [24].

#### *4.3.2. Electrical characterization*

maintaining molecules positively charged without the SiO<sup>2</sup>

*<sup>Δ</sup> <sup>V</sup>*FB <sup>=</sup> \_\_\_\_ *<sup>e</sup>* <sup>⋅</sup> *<sup>n</sup>*

70 Redox - Principles and Advanced Applications

the thickness and relative dielectric constant of Al<sup>2</sup>

Note that ∼60% charge loss was observed after 10<sup>6</sup>

**4.3. Nanowire/nanotube‐based flash‐like molecular memory**

retention is due to the relative thin SiO<sup>2</sup>

the device structure design and fabrication.

inherent properties of redox molecules [24, 58–60].

will be attached [24]. As compared with the SiO<sup>2</sup>

*4.3.1. Device fabrication*

ambient SiH<sup>4</sup>

behavior of ferrocene and the effective charge separation by the SiO<sup>2</sup>

charge density of the solid‐state molecular memory is sufficiently high [30].

symmetric staircase Δ*V*FB of MAFOS device originated from the reliable charging/discharging

*<sup>C</sup>* = *e* ⋅ *n* ⋅

where *e* is the elementary charge, *n* is the number of stored electrons, *T*Al2O3 and *ε*Al2O3 are

calculated to be 4.82 × 1012 cm−2, which is only a small portion (∼15%) of the coverage density obtained from the CyV results. Nevertheless, effective memory can still be realized and the

The room temperature retention of MAFOS and MAFS devices is shown in **Figure 7c** [30].

not satisfactory. The endurance property of the MAFOS device was measured with ±10 V P/E voltages and different pulse width. The memory device continues to behave well after 10<sup>9</sup>

cycle with the same memory window and a slightly upshift which is due to the accumulation of electron in deep traps [30]. Such excellent endurance characteristics are naturally derived from the intrinsic redox properties of the redox molecules, which have been well protected by

Semiconductor nanowires and nanotube MOSFETs have been regarded as the building blocks for nanoelectronics and circuits [56, 57]. The quasi‐one‐dimensional nanowires have a larger surface‐to‐volume ratio as compared with the bulk materials. Therefore, less stored charges are needed to induce a same memory window. In addition, the nanowire or nanotube can enable a gate‐surrounding structure, allowing excellent electrostatic gate control. The integra‐ tion of redox molecules in semiconductor nanowire FET for solid‐state flash‐like memory can be expected to significantly enhance the memory performance by taking advantages of the

**Figure 8** illustrates the fabrication process of a molecular flash memory based on a so‐called "self‐aligned" silicon nanowire FET [24, 61]. A thin film of Au catalyst (2–3 nm) was first

nanowires were then grown from the catalyst following the vapor‐liquid‐solid (VLS) mecha‐ nism in a low‐pressure chemical vapor deposition (LPCVD) furnace at 440°C for 2 h with an

formed here is of much better quality by using the dry oxidation technique [30]. Therefore, enhanced memory retention can be expected. The next step is to pattern the source/drain

stream under a pressure of 500 mTorr. Immediately after the growth, the nanow‐

sputtered at predefined locations by photolithography on a 300‐nm SiO<sup>2</sup>

ires were oxidized at 750°C for 30 min by dry oxidation to form a 3‐nm SiO<sup>2</sup>

(

O3

*T*Al \_2 O3 *ε*<sup>0</sup> *ε*Al<sup>2</sup>

charge density of the ferrocene SAM was calculated by using the following equation [30]:

tunnel barrier. The stable and

O3) (1)

, respectively. The charge density was

s retention time for MAFOS. Such fair

, and the oxide quality by rapid thermal oxidation is

tunnel barrier [30]. The

P/E

/Si substrate. The Si

on which the SAM

grown by rapid thermal oxidation, the SiO<sup>2</sup>

**Figure 9** shows the transmission electron microscopy (TEM) image of the cross‐section of a ferrocene‐attached molecular flash memory device [24]. Clear gate‐surrounding structure has been achieved, with a ∼6‐nm "intermixed" region observed.

**Figure 10a** and **b** shows the out characteristics, drain current (*I* DS) versus drain voltage (*V*DS) of the ferrocene molecular flash memory in linear and log‐scale, respectively. Smooth and well‐saturated *I* DS‐*V*DS curves have been observed with negligible contact resistance. The leak‐ age‐affected and the weak, moderate, and strong inversion operation regions are shown in **Figure 10b**. From the transfer characteristics shown in **Figure 10c** and **d**, counterclockwise hysteresis loops have been observed at different gate voltage (*V*GS) sweep ranges for both ferrocene and Ru<sup>2</sup> memory devices, suggesting the charge‐trapping mechanism. From the log‐scale *I*DS‐VGS curves shown in the insets of **Figure 10c** and **d**, clear off states were achieved, and the on/off current ratio was as high as ∼107 [24].

The P/E speed characteristics of the molecular memory (ferrocene) are shown in **Figure 11a** and **b**, demonstrated by the threshold voltage shift (Δ*V*TH) under different P/E gate voltage pulses [24]. The P/E operations were performed by applying top gate voltage pulses while the substrate, S/D electrodes were all grounded. As shown in **Figure 11a**, with accumulative +10 V programming pulses, the threshold voltage showed a clear shift toward positive direction, indicating that the electrons were injected from the nanowire channel through the tunnel bar‐ rier and stored in the redox centers of the molecules. Erasing operations with −10 V gate volt‐ age pulses back shifted the threshold voltage toward the negative direction, suggesting hole injection from the nanowire channel during erasing operations. The P/E speed characteristics of ferrocene and Ru<sup>2</sup> memory are summarized in **Figure 11c** and **d**, respectively. Both devices showed fast P/E speed, which arises from the intrinsic fast speed of the charging behavior of the redox molecules. The slightly faster erasing speed over programming speed was attrib‐ uted to the more favorable hole injection in the Si/SiO<sup>2</sup> /molecule/Al<sup>2</sup> O3 /gate interface states, though the amount charge is very small even with ±10 V, 1 s stressing [24].

**Figure 12a** shows the Δ*V*TH of the ferrocene molecular memory and the reference sample as a function of P/E voltages at a fixed pulse width of 500 μs. Negligible Δ*V*TH was observed for

**Figure 9.** TEM image of the cross‐section of a ferrocene‐attached molecular flash memory device. The red‐dashed line indicates the ferrocene‐embedded region. Inset: cross‐section of the nanowire channel, with the SiO<sup>2</sup> layer indicated by the dashed line [24].

Redox-Active Molecules for Novel Nonvolatile Memory Applications http://dx.doi.org/10.5772/intechopen.68726 73

**Figure 10a** and **b** shows the out characteristics, drain current (*I*

and the on/off current ratio was as high as ∼107

uted to the more favorable hole injection in the Si/SiO<sup>2</sup>

though the amount charge is very small even with ±10 V, 1 s stressing [24].

well‐saturated *I*

72 Redox - Principles and Advanced Applications

ferrocene and Ru<sup>2</sup>

of ferrocene and Ru<sup>2</sup>

the dashed line [24].

of the ferrocene molecular flash memory in linear and log‐scale, respectively. Smooth and

age‐affected and the weak, moderate, and strong inversion operation regions are shown in **Figure 10b**. From the transfer characteristics shown in **Figure 10c** and **d**, counterclockwise hysteresis loops have been observed at different gate voltage (*V*GS) sweep ranges for both

log‐scale *I*DS‐VGS curves shown in the insets of **Figure 10c** and **d**, clear off states were achieved,

The P/E speed characteristics of the molecular memory (ferrocene) are shown in **Figure 11a** and **b**, demonstrated by the threshold voltage shift (Δ*V*TH) under different P/E gate voltage pulses [24]. The P/E operations were performed by applying top gate voltage pulses while the substrate, S/D electrodes were all grounded. As shown in **Figure 11a**, with accumulative +10 V programming pulses, the threshold voltage showed a clear shift toward positive direction, indicating that the electrons were injected from the nanowire channel through the tunnel bar‐ rier and stored in the redox centers of the molecules. Erasing operations with −10 V gate volt‐ age pulses back shifted the threshold voltage toward the negative direction, suggesting hole injection from the nanowire channel during erasing operations. The P/E speed characteristics

showed fast P/E speed, which arises from the intrinsic fast speed of the charging behavior of the redox molecules. The slightly faster erasing speed over programming speed was attrib‐

**Figure 12a** shows the Δ*V*TH of the ferrocene molecular memory and the reference sample as a function of P/E voltages at a fixed pulse width of 500 μs. Negligible Δ*V*TH was observed for

**Figure 9.** TEM image of the cross‐section of a ferrocene‐attached molecular flash memory device. The red‐dashed line

indicates the ferrocene‐embedded region. Inset: cross‐section of the nanowire channel, with the SiO<sup>2</sup>

[24].

DS‐*V*DS curves have been observed with negligible contact resistance. The leak‐

memory devices, suggesting the charge‐trapping mechanism. From the

memory are summarized in **Figure 11c** and **d**, respectively. Both devices

/molecule/Al<sup>2</sup>

O3

/gate interface states,

layer indicated by

DS) versus drain voltage (*V*DS)

**Figure 10.** Output characteristics of ferrocene molecular memory in (a) linear and (b) log‐scale. Transfer characteristics of (c) ferrocene and (d) Ru<sup>2</sup> molecular memory with different gate voltage sweep ranges. *V*DS was set to −50 mV [24].

the reference sample, indicating the fact that the dominant charge‐storage location lies in the molecules, rather than the solely traps within the gate dielectric interface or a dielectric interface. Clear staircase behavior was observed for the ferrocene memory, demonstrating discrete energy levels corresponding to various molecular orbitals. A saturated Δ*V*TH was observed beyond ±26 V gate voltage, suggesting that all the available redox centers in the SAM have been occupied by injected charges. The charging density in the SAM can be thus calculated by

Calculated by 
$$\Delta V\_{\rm TH} = \frac{qN}{\overline{C}\_{\rm{Rok}}} \not\equiv \frac{qN}{\overline{C}\_{\rm{AO}}} = qN \frac{\ln\left(\frac{t\_{\rm{AO}\to\alpha}}{t\_{\rm{AO}\to\alpha}}\right)}{2\pi\,\varepsilon\_0\,\varepsilon\_{\rm{AO}}L} \tag{2}$$

where *q* is the elementary charge, *N* is the total charge stored in the redox centers, *C*Redox is the total capacitance arising between the redox centers and the metal gate, *C*AlO and *ε;*AlO are the capacitance and relative dielectric constant of the Al<sup>2</sup> O3 layer, respectively, *L* is the channel length, *t*AlO‐out and *t*AlO‐in are the distances from the center of nanowire channel to the outside and inside surfaces of the Al<sup>2</sup> O3 layer. The charging density was found to be 6.96 × 1012 cm−2, which is about 22% as compared with the coverage den‐ sity from the CyV results [24]. This indicates that effective memory can be realized even with a partial (i.e., non‐continuous) ferrocene SAM. Two‐step charge‐storage behavior was observed in the Ru<sup>2</sup> molecular memory due to the two redox centers in the Ru<sup>2</sup> molecule which can exhibit stable and distinct charged states at different voltage level. As shown in **Figure 12b**, with increasing programming gate voltage, the first step charged state was found at 10 V, with 0.8 V Δ*V*TH. This means that the Ru<sup>2</sup> redox cen‐ ters with lower voltage level have been occupied by electrons. Further increasing the programming voltage led to a second charged state, which was observed beyond 14 V *V*GS, with 1.95 V Δ*V*TH. Up to now, all the redox centers in Ru<sup>2</sup> SAM have been filled with injected electrons. Similarly, the overall charging density of Ru<sup>2</sup> SAM was calculated, and was found to be 1.12 × 1013 cm−2, which is about 44% of the freshly attached SAM before Al<sup>2</sup> O3 deposition [24]. Such discrete charging behavior in Ru<sup>2</sup> molecules with multiple redox centers is very attractive for discrete multibit memory applications.

**Figure 12c** and **d** shows the room temperature retention properties of the ferrocene and Ru<sup>2</sup> molecular memory devices, respectively [24]. The devices were initially programmed/ erased by ±10 V gate voltage with 500 and 100 μs pulse width. Only ∼20% charge loss was observed with a projected 10‐year memory window. As compared with the previous capacitor structure molecular memory, the improved retention shown here is attributed

**Figure 11.** (a) Programming and (b) erasing operations of the ferrocene molecular memory under ±10 V P/E gate voltage pulses with accumulative time. P/E speed characterization of (c) ferrocene and (d) Ru<sup>2</sup> molecular memory devices, respectively [24].

with a partial (i.e., non‐continuous) ferrocene SAM. Two‐step charge‐storage behavior

molecule which can exhibit stable and distinct charged states at different voltage level. As shown in **Figure 12b**, with increasing programming gate voltage, the first step

ters with lower voltage level have been occupied by electrons. Further increasing the programming voltage led to a second charged state, which was observed beyond 14 V

was found to be 1.12 × 1013 cm−2, which is about 44% of the freshly attached SAM before

**Figure 12c** and **d** shows the room temperature retention properties of the ferrocene and

**Figure 11.** (a) Programming and (b) erasing operations of the ferrocene molecular memory under ±10 V P/E gate voltage

pulses with accumulative time. P/E speed characterization of (c) ferrocene and (d) Ru<sup>2</sup>

 molecular memory devices, respectively [24]. The devices were initially programmed/ erased by ±10 V gate voltage with 500 and 100 μs pulse width. Only ∼20% charge loss was observed with a projected 10‐year memory window. As compared with the previous capacitor structure molecular memory, the improved retention shown here is attributed

charged state was found at 10 V, with 0.8 V Δ*V*TH. This means that the Ru<sup>2</sup>

*V*GS, with 1.95 V Δ*V*TH. Up to now, all the redox centers in Ru<sup>2</sup>

injected electrons. Similarly, the overall charging density of Ru<sup>2</sup>

deposition [24]. Such discrete charging behavior in Ru<sup>2</sup>

redox centers is very attractive for discrete multibit memory applications.

molecular memory due to the two redox centers in the Ru<sup>2</sup>

redox cen‐

SAM have been filled with

SAM was calculated, and

molecules with multiple

molecular memory devices,

was observed in the Ru<sup>2</sup>

74 Redox - Principles and Advanced Applications

Al<sup>2</sup> O3

Ru<sup>2</sup>

respectively [24].

**Figure 12.** Δ*V*TH of (a) ferrocene and reference sample and (b) Ru<sup>2</sup> molecular memory as a function of P/E gate voltage with 500 μs pulse width and increasing pulse height. Room temperature retention properties of (c) ferrocene and (d) Ru<sup>2</sup> molecular memory. The initial P/E pulses were ±10 V gate voltage with 500 and 100 μs pulse width, respectively. Endurance properties of (e) ferrocene and (f) Ru<sup>2</sup> molecular memory. Note that ±10 V gate voltage with 500 and 100 μs pulse width was applied [24].

to the high‐quality tunnel oxide with clean solid/molecule and dielectric interfaces during the fabrication process. This further supports the mechanism of dominant charge stor‐ age located in the redox centers instead of interface states, since the recovery process of the interface states is quite fast leading to a poor retention. The endurance properties of the molecular memory devices are shown in **Figure 12e** and **f** [24]. Both devices exhib‐ ited excellent endurance characteristics with ±10 V P/E gate voltages and 500 and 100 μs pulse width. Negligible memory window degradation was observed after 10<sup>8</sup> P/E cycles by applying 500 μs P/E voltages. With shorter P/E voltages (100 μs), both devices still func‐ tioned perfectly after 109 P/E operation cycles. Such excellent endurance is 10,000 times better than that of the conventional floating‐gate memory, and is resulted from the excel‐ lent reliability of the redox properties of the ferrocene and Ru<sup>2</sup> molecules [24]. However, less than 50% of the molecules in the SAM were effectively involved in the redox pro‐ cess, though the portion is slightly higher than that of the capacitor structure memory cell. Further research efforts are needed to increase the redox efficiency so as to lower the operation voltage and improve the operation speed. The memory density can be fur‐ ther increased with more carefully engineered molecules, and the demonstrated multibit memory concept is more reasonable and feasible than by just modulating the voltage level, as precise controlling of the charged states can be clearly defined and monitored with the physically discrete redox states.
