*2.3. KOH Wet Etching*

2.3.1. Principle

The wet etching of III–V nitrides in general involves the formation of an oxide on the surface and the subsequent dissolution of the oxide [24]. The flowing reactions occur when AlN/Al1−xScxN is subjected to the alkaline environment [29]:

$$2\text{AlN} + 3\text{H}\_2\text{O} \quad \overset{\text{KOH}}{\rightarrow} \quad \text{Al}\_2\text{O}\_3 + 2\text{NH}\_3\tag{1}$$

$$\text{AlN} + 3\text{H}\_2\text{O} \quad \overset{\text{KOH}}{\rightarrow} \quad \text{Al(OH)}\_3 + \text{NH}\_3 \tag{2}$$

$$2\text{ScN} + 3\text{H}\_2\text{O} \quad \overset{\text{KOH}}{\rightarrow} \quad \text{Sc}\_2\text{O}\_3 + 2\text{NH}\_3\tag{3}$$

$$\text{ScN} + 3\text{H}\_2\text{O} \quad \overset{\text{KOH}}{\rightarrow} \text{Sc(OH)}\_3 + \text{NH}\_3\tag{4}$$

**Figure 2.** (**a**) Al1−xScxN and SiNx hard mask deposition; (**b**) photoresist patterning; (**c**) SiNx dry etch; (**d**) resist stripping and Al1−xScxN KOH wet etch. The opening area for the etching is 30 × 200 µm2 . In this reaction, KOH acts as the catalyst that pushes the equation to the right side. Due to the origin of the reaction, N-polar AlN/Al1−xScxN are preferred to be etched as it is difficult for OH− to make contact with the Al/Sc atoms in the metal polar state because of repulsion from the negatively charged dangling nitrogen bonds [24].

*Micromachines* **2022**, *13*, x FOR PEER REVIEW 6 of 19

(**a**) (**b**)

Al0.64Sc0.36N film.

*2.2. Film Patterning*

illustrates the fabrication process.

**Figure 1.** (**a**) AFM of the Al0.64Sc0.36N Film. (**b**) Rocking curve measurement (Omega Scan) of an

To selectively etch AlN/Al1−xScxN, the film must be patterned so that only locations of interest would be exposed in the etchant. Silicon nitride (SiNx) was chosen as our hard mask, as it has an etch rate of 0.67 nm/min in 30% KOH at 80 °C [55], which is negligible compared to the etch rates of Al1−xScxN. A 200 nm SiNx film was deposited on top of the Al1−xScxN film in an Oxford Plasma Lab 100 Plasma-enhanced Chemical Vapor Deposition (PECVD) machine (Oxford Instruments Plasma Technology, Bristol, United Kingdom), and on the backside of the wafer as well to protect the Si substrate during the etch. Afterwards, photoresist MICROPOSIT® S1813 was spin-coated and exposed in a Karl Süss MA6 Mask Aligner (SÜ SS MicroTec SE, Garching, Germany) via contact lithography. The exposed film was developed in TMAH-0.26N developer, then transferred to an Oxford 80 plus Reactive-ion Etching (RIE) etcher (Oxford Instruments Plasma Technology, Bristol, United Kingdom). A 30 s O2 descum was performed first to remove the remaining resist, followed by 3 min of SiNx etch with CHF3/O2 mixture. Finally, the resist was stripped in MICROPOSIT® remover 1165 with ultrasonic bath at 60 °C and plasma cleaned. Figure 2

**Figure 2.** (**a**) Al1−xScxN and SiNx hard mask deposition; (**b**) photoresist patterning; (**c**) SiNx dry etch; (**d**) resist stripping and Al1−xScxN KOH wet etch. The opening area for the etching is 30 × 200 µm2 . **Figure 2.** (**a**) Al1−xScxN and SiNx hard mask deposition; (**b**) photoresist patterning; (**c**) SiNx dry etch; (**d**) resist stripping and Al1−xScxN KOH wet etch. The opening area for the etching is 30 × 200 µm2 . difficult for − to make contact with the Al/Sc atoms in the metal polar state because of repulsion from the negatively charged dangling nitrogen bonds [24].

(1)

(2)

(3)

(4)

#### 2.3.2. Etching Process 2.3.2. Etching Process

A water bath was established for the etching to be conducted in a stable temperature environment. A trough was filled with deionized (DI) water and placed upon an Echo Thermal HP30 hotplate (Torrey Pines Scientific, Inc., Carlsbad, California). Around 300 mL of 30% KOH/diluted KOH was poured into a beaker, which was later transferred into the trough. A Teflon plate was added in between to avoid direct heating. Finally, a thermocouple was submerged into the KOH solution and connected to the hotplate through a proportional–integral–derivative (PID) control loop to adjust the solution temperature. With all these measures, the solution temperature was able to be stabilized within ±1 ◦C during etching. The test sample was cleaved into an 8 × 10 mm2 die and clamped by a tweezer when dipped into the solution. When submerged in the KOH, samples experienced minimal agitation, and upon removal, they were rinsed under DI water and subsequently dried with N2. Finally, the sample was cleaved from the middle (Figure 3a) and a cross-section was imaged in a FEI Quanta 600 Environmental Scanning Electron Microscope (ESEM) (FEI Company, Hillsboro, OR, USA) (Figure 3b). A water bath was established for the etching to be conducted in a stable temperature environment. A trough was filled with deionized (DI) water and placed upon an Echo Thermal HP30 hotplate (Torrey Pines Scientific, Inc., Carlsbad, California). Around 300 mL of 30% KOH/diluted KOH was poured into a beaker, which was later transferred into the trough. A Teflon plate was added in between to avoid direct heating. Finally, a thermocouple was submerged into the KOH solution and connected to the hotplate through a proportional–integral–derivative (PID) control loop to adjust the solution temperature. With all these measures, the solution temperature was able to be stabilized within ±1 °C during etching. The test sample was cleaved into an 8 × 10 mm2 die and clamped by a tweezer when dipped into the solution. When submerged in the KOH, samples experienced minimal agitation, and upon removal, they were rinsed under DI water and subsequently dried with N2. Finally, the sample was cleaved from the middle (Figure 3a) and a cross-section was imaged in a FEI Quanta 600 Environmental Scanning Electron Microscope (ESEM) (FEI Company, Hillsboro, United States) (Figure 3b).

#### 2.3.3. Data Interpretation 2.3.3. Data Interpretation

Three types of data were extracted from the SEM images: vertical etch depth, lateral etch length (undercutting) and the sidewall angle. The vertical etch depth is defined as the etching depth into the AlN/Al1−xScxN film from the bottom of the SiNx hard mask. It can be somewhat ambiguous, as hexagonal-shaped hillocks are known to form after KOH etching [29], which makes it difficult to identify the end point of the etch. Since the phenomenon is presumed to be defect-related in AlN [28,55], we conclude that the 'tip' of the pyramid acts as a mask in this process and thus the etch front should be read as the basal plane of the hexagonal pyramid. Figure 4a is a demonstration of how the etch depth was measured in case of the existence of the hillocks. The lateral etch length is defined as the etch length underneath the SiNx hard mask (Figure 4b). Finally, the sidewall angle is simply the angle between the sidewall and base plane. Three types of data were extracted from the SEM images: vertical etch depth, lateral etch length (undercutting) and the sidewall angle. The vertical etch depth is defined as the etching depth into the AlN/Al1−xScxN film from the bottom of the SiNx hard mask. It can be somewhat ambiguous, as hexagonal-shaped hillocks are known to form after KOH etching [29], which makes it difficult to identify the end point of the etch. Since the phenomenon is presumed to be defect-related in AlN [28,55], we conclude that the 'tip' of the pyramid acts as a mask in this process and thus the etch front should be read as the basal plane of the hexagonal pyramid. Figure 4a is a demonstration of how the etch depth was measured in case of the existence of the hillocks. The lateral etch length is defined as the etch length underneath the SiNx hard mask (Figure 4b). Finally, the sidewall angle is simply the angle between the sidewall and base plane.

**Figure 4.** (**a**) Al0.95Sc0.05N etched for 10 s in 30% KOH at 45 °C. (**b**) Al0.85Sc0.15N etched for 2.5 min in 30% KOH at 65 °C. **Figure 4.** (**a**) Al0.95Sc0.05N etched for 10 s in 30% KOH at 45 ◦C. (**b**) Al0.85Sc0.15N etched for 2.5 min in 30% KOH at 65 ◦C.

#### **3. Results and Analysis 3. Results and Analysis**

*3.1. Etch Result with 30% KOH at 45 °C 3.1. Etch Result with 30% KOH at 45* ◦*C*

Due to the limited film thickness and the vastly different etch rates of films with varying Sc concentrations, it was not practical to use the same etch time when determining the vertical etch rate. Instead, films with lower Sc concentration were etched for a shorter amount of time. The following table (Table 4) illustrates the time used in each case: Due to the limited film thickness and the vastly different etch rates of films with varying Sc concentrations, it was not practical to use the same etch time when determining the vertical etch rate. Instead, films with lower Sc concentration were etched for a shorter amount of time. The following table (Table 4) illustrates the time used in each case:

**Table 4.** Scandium concentration vs. etch time (short etch time in 30% KOH at 45 °C). **Table 4.** Scandium concentration vs. etch time (short etch time in 30% KOH at 45 ◦C).

vertical etch rate measured due to the availability of the sample at the time this experiment was conducted. For the etched samples, cross-section images were taken from several spots to avoid local variations. Figure 5 summarizes the vertical and lateral etch rates vs. Sc concentration for etching in 30% KOH at 45 °C. It should be noted that not all samples fabricated, as mentioned in Section 2, had their vertical etch rate measured due to the availability of the sample at the time this experiment was conducted. For the etched samples, cross-section images were taken from several spots to avoid local variations. Figure 5 summarizes the vertical and lateral etch rates vs. Sc concentration for etching in 30% KOH at 45 ◦C.

It should be noted that not all samples fabricated, as mentioned in Section 2, had their

**Figure 5.** (**a**). Vertical etch rate of Al1−xScxN in 30% KOH at 45 °C. (**b**) Lateral etch rate of Al1−xScxN in 30% KOH at 45 °C (short etch time). **Figure 5.** (**a**). Vertical etch rate of Al1−xScxN in 30% KOH at 45 ◦C. (**b**) Lateral etch rate of Al1−xScxN in 30% KOH at 45 ◦C (short etch time).

Intuitively, the vertical etch rate matches our expectation: a steady decline with increasing Sc concentration. The Al0.80Sc0.20N has an average vertical etch rate of 7.58 nm/s, and the Al0.64Sc0.36N has an average vertical etch rate of 3.68 nm/s. Compared to the values of 3.59 nm/s and 2.77 nm/s obtained by K. Bespalova et al. [25] and S. Fichtner et al. [23] respectively, they are not exact matches but in the same order of magnitude. It should be noted that as per the view of A. Ababneh et al. [56], the etch rate of AlN is a strong function of sputtering conditions; thus, this study can be used to predict the trend instead of the absolute etch rate when applied to films deposited under different conditions. The lateral etch rate has several distinctive features requiring further examination. First, the standard deviation of the lateral etch rate is considerably larger than that for the vertical etch. This is due to the finite amount of lateral etching performed. As exhibited in Figure 4a, the film has a lateral etch length of only a few dozen nanometers in some cases, which makes it difficult to accurately measure. Secondly, the etch rate of Al0.72Sc0.28N does not fit in the line. Lastly, the lateral etch rate begins to increase for concentrations in excess of 20% Sc. Intuitively, the vertical etch rate matches our expectation: a steady decline with increasing Sc concentration. The Al0.80Sc0.20N has an average vertical etch rate of 7.58 nm/s, and the Al0.64Sc0.36N has an average vertical etch rate of 3.68 nm/s. Compared to the values of 3.59 nm/s and 2.77 nm/s obtained by K. Bespalova et al. [25] and S. Fichtner et al. [23] respectively, they are not exact matches but in the same order of magnitude. It should be noted that as per the view of A. Ababneh et al. [56], the etch rate of AlN is a strong function of sputtering conditions; thus, this study can be used to predict the trend instead of the absolute etch rate when applied to films deposited under different conditions. The lateral etch rate has several distinctive features requiring further examination. First, the standard deviation of the lateral etch rate is considerably larger than that for the vertical etch. This is due to the finite amount of lateral etching performed. As exhibited in Figure 4a, the film has a lateral etch length of only a few dozen nanometers in some cases, which makes it difficult to accurately measure. Secondly, the etch rate of Al0.72Sc0.28N does not fit in the line. Lastly, the lateral etch rate begins to increase for concentrations in excess of 20% Sc.

To further explore the lateral etching, a second experiment was conducted in which the lateral etch length was extracted over a longer etching period. The number of Sc concentrations in this study was also expanded for higher resolution as shown in Table 5: To further explore the lateral etching, a second experiment was conducted in which the lateral etch length was extracted over a longer etching period. The number of Sc concentrations in this study was also expanded for higher resolution as shown in Table 5:

**Table 5.** Scandium concentration vs. etch time (long etch in 30% KOH at 45 °C). **Table 5.** Scandium concentration vs. etch time (long etch in 30% KOH at 45 ◦C).

Before the long-span etching was performed, a linearity check was conducted to make sure that the etch does not change during the etching process. The film tested was Al0.64Sc0.36N and it was subjected to etches of 60 s, 80 s, 300 s, 1200 s and 2400 s, respectively. A linear regression was performed, and the lateral etching proved to be highly linear with the R2 = 0.9968. The fact that the same etch rate was retained after 40 min indicates that the etch is reaction-limited, e.g., not confined by mass transferring, and matches the description of AlN etching in KOH given by Mileham et al. [26]. Before the long-span etching was performed, a linearity check was conducted to make sure that the etch does not change during the etching process. The film tested was Al0.64Sc0.36N and it was subjected to etches of 60 s, 80 s, 300 s, 1200 s and 2400 s, respectively. A linear regression was performed, and the lateral etching proved to be highly linear with the R2 = 0.9968. The fact that the same etch rate was retained after 40 min indicates that the etch is reaction-limited, e.g., not confined by mass transferring, and matches the description of AlN etching in KOH given by Mileham et al. [26] (Figure 6).

**Figure 6.** (**a**)**.** Lateral etch length with respect to etch time of Al0.64Sc0.36N in 30% KOH at 45 °C. (**b**) Lateral etch rate of Al1−xScxN in 30% KOH at 45 °C (long etch time). **Figure 6.** (**a**). Lateral etch length with respect to etch time of Al0.64Sc0.36N in 30% KOH at 45 ◦C. (**b**) Lateral etch rate of Al1−xScxN in 30% KOH at 45 ◦C (long etch time).

The long etch time returned similar results compared with the short etch time. The etch rate experienced a transition where it reaches the lowest at x = 0.125. However, discrepancies still exist for the etch rates of x = 0.28, 0.30, 0.34 and 0.38 when compared to other films with Sc concentration x > 0.125. One explanation might be that these films were deposited in different batches as opposed to the rest of the films. As stated above, what exhibits as different etch rates here might be the result of the different chamber condition when the films were being sputtered. It was apparent that even though they do not fit in the line made from the x = 0.125, 0.15, 0.20, 0.25, 0.32 and 0.36 films, they were able to constitute their own trend line with almost has the same slope. Deeper examination revealed that the etch rate of the x = 0.28 film was lower than expected on two different films deposited in different batches, and further studies should be carried out to better understand these subtle trends in lateral etch rate. The long etch time returned similar results compared with the short etch time. The etch rate experienced a transition where it reaches the lowest at x = 0.125. However, discrepancies still exist for the etch rates of x = 0.28, 0.30, 0.34 and 0.38 when compared to other films with Sc concentration x > 0.125. One explanation might be that these films were deposited in different batches as opposed to the rest of the films. As stated above, what exhibits as different etch rates here might be the result of the different chamber condition when the films were being sputtered. It was apparent that even though they do not fit in the line made from the x = 0.125, 0.15, 0.20, 0.25, 0.32 and 0.36 films, they were able to constitute their own trend line with almost has the same slope. Deeper examination revealed that the etch rate of the x = 0.28 film was lower than expected on two different films deposited in different batches, and further studies should be carried out to better understand these subtle trends in lateral etch rate.

For most Sc concentrations, the sidewall angle of films remains invariant throughout the etching. As demonstrated in Figure 7, an extra-long submerge of the sample does not change the sidewall angle by a visible amount, and for the changes that could be measured, it can be attributed to the tilting of the sample itself during imaging. For most Sc concentrations, the sidewall angle of films remains invariant throughout the etching. As demonstrated in Figure 7, an extra-long submerge of the sample does not change the sidewall angle by a visible amount, and for the changes that could be measured, it can be attributed to the tilting of the sample itself during imaging.

The sidewall is a reflection of the crystal structure of the Al1−xScxN films. As per the findings of W. Guo et al., due to the energy difference between crystalline planes, the c-plane 0001 will be etched first prior to the deterioration of the 1011 planes [31]. Hence, during the anisotropic etching process, the exposed h1123i slip edges between the boundary of the 1011 planes forms the facets that follow the 1212 of the hexagonal crystal structure, behind which lateral etching ceases advancing (Figure 8).

**55.8 °**

**56.7 °**

**Figure 7.** (**a**) Al0.90Sc0.10N etched for 20 s; (**b**) Al0.75Sc0.25N etched for 60 s; (**c**) Al0.64Sc0.36N etched for 80 s; (**d**) Al0.90Sc0.10N etched for 10 min; (**e**) Al0.75Sc0.25N etched for 5 min; (**f**) Al0.64Sc0.36N etched for 20 min. All etching was performed in 30 wt % KOH at 45 °C. The sidewall angle is preserved after the long etch. **Figure 7.** (**a**) Al0.90Sc0.10N etched for 20 s; (**b**) Al0.75Sc0.25N etched for 60 s; (**c**) Al0.64Sc0.36N etched for 80 s; (**d**) Al0.90Sc0.10N etched for 10 min; (**e**) Al0.75Sc0.25N etched for 5 min; (**f**) Al0.64Sc0.36N etched for 20 min. All etching was performed in 30 wt% KOH at 45 ◦C. The sidewall angle is preserved after the long etch.

*Micromachines* **2022**, *13*, x FOR PEER REVIEW 11 of 19

**Figure 8.** (**a**). Exposed 〈1123〉 slip edge in an Al0.95Sc0.05N film etched in 30% KOH at 45 °C for 10 min; (**b**) the sidewall that follows the {1212} facet created by the boundary between the {1011} planes of the same etch condition for 5 min. **Figure 8.** (**a**). Exposed h1123i slip edge in an Al0.95Sc0.05N film etched in 30% KOH at 45 ◦C for 10 min; (**b**) the sidewall that follows the 1212 facet created by the boundary between the 1011 planes of the same etch condition for 5 min. **Figure 8.** (**a**). Exposed 〈1123〉 slip edge in an Al0.95Sc0.05N film etched in 30% KOH at 45 °C for 10 min; (**b**) the sidewall that follows the {1212} facet created by the boundary between the {1011} planes of the same etch condition for 5 min.

Moreover, for a Hexagonal Close-Packed (HCP) unit cell with an axis length *c*/*a*, the sidewall angle, *θ* (Figure 9), of the {1212} facets as a function of lattice length can be calculated as = arctan(/): Moreover, for a Hexagonal Close-Packed (HCP) unit cell with an axis length *c*/*a*, the sidewall angle, *θ* (Figure 9), of the 1212 facets as a function of lattice length can be calculated as *θ* = arctan(*c*/*a*): Moreover, for a Hexagonal Close-Packed (HCP) unit cell with an axis length *c*/*a*, the sidewall angle, *θ* (Figure 9), of the {1212} facets as a function of lattice length can be calculated as = arctan(/):

**Figure 9.** Crystal planes in Al1−xScxN HCP lattice. **Figure 9.** Crystal planes in Al1−xScxN HCP lattice. **Figure 9.** Crystal planes in Al1−xScxN HCP lattice.

against the experimental value (Figure 10).

Using the *c/a* data from the work presented by Ö sterlund et al. [45], and considering the isotropic etching of the {1011} plane, the theoretical sidewall angle *φ* can be plotted against the experimental value (Figure 10). Using the *c/a* data from the work presented by Ö sterlund et al. [45], and considering the isotropic etching of the {1011} plane, the theoretical sidewall angle *φ* can be plotted Using the *c*/*a* data from the work presented by Österlund et al. [45], and considering the isotropic etching of the 1011 plane, the theoretical sidewall angle *ϕ* can be plotted against the experimental value (Figure 10).

**Figure 10.** Comparison of experimental and theoretical sidewall angle vs. Sc concentration. **Figure 10.** Comparison of experimental and theoretical sidewall angle vs. Sc concentration.

The absolute value of the experimental and theoretical angles follows the same trajectory with a parabolic downtrend with increasing Sc concentration until the Sc concentration exceeds 40%. We hypothesize that the side profile is the result of both anisotropic and isotropic etching. At lower Sc concentration, the etch rate on the c-plane {0001} is significantly higher, the momentum of the etch is downwards, and thus the side wall creates a facet that follows the {1212} of the hexagonal crystal structure. At a higher Sc concentration, the low vertical etch rate slows down the descending penetration, allowing the etchant to further react with the sidewall planes already exposed. Therefore, the closer the {1011} planes are to the surface, the more they are etched away; as a result, the sidewall angle becomes lower than that predicted solely based on the anisotropic crystal etching, i.e., lower than *θ*. This can be partially verified by some of the abnormal points on the graph, most of which have a small lateral etch rate (e.g., 12.5% and 42% Sc), which prevents the etching of their corresponding {1011} planes. The absolute value of the experimental and theoretical angles follows the same trajectory with a parabolic downtrend with increasing Sc concentration until the Sc concentration exceeds 40%. We hypothesize that the side profile is the result of both anisotropic and isotropic etching. At lower Sc concentration, the etch rate on the c-plane 0001 is significantly higher, the momentum of the etch is downwards, and thus the side wall creates a facet that follows the 1212 of the hexagonal crystal structure. At a higher Sc concentration, the low vertical etch rate slows down the descending penetration, allowing the etchant to further react with the sidewall planes already exposed. Therefore, the closer the 1011 planes are to the surface, the more they are etched away; as a result, the sidewall angle becomes lower than that predicted solely based on the anisotropic crystal etching, i.e., lower than *θ*. This can be partially verified by some of the abnormal points on the graph, most of which have a small lateral etch rate (e.g., 12.5% and 42% Sc), which prevents the etching of their corresponding 1011 planes.

### *3.2. Etch Results with 30 wt% KOH at 65* ◦*C and 10 wt% KOH at 65* ◦*C*

*3.2. Etch Results with 30 wt % KOH at 65 °C and 10 wt % KOH at 65 °C* The experiment was also carried out at an elevated temperature and with lower KOH concentration to rule out any possible interference to the outcome except for the intrinsic material properties. At elevated temperature, the etching was performed with a short etching time of 150 s, except for Al0.875Sc0.125N, which was etched for 450 s. Etching with lower KOH concentration was performed for 20 min for all Sc alloying concentrations. As a result of the faster etching rate, the vertical etch rate could not be measured. The lateral The experiment was also carried out at an elevated temperature and with lower KOH concentration to rule out any possible interference to the outcome except for the intrinsic material properties. At elevated temperature, the etching was performed with a short etching time of 150 s, except for Al0.875Sc0.125N, which was etched for 450 s. Etching with lower KOH concentration was performed for 20 min for all Sc alloying concentrations. As a result of the faster etching rate, the vertical etch rate could not be measured. The lateral etch rate and sidewall angle of the experiment are presented below (Figure 11):

etch rate and sidewall angle of the experiment are presented below (Figure 11):

**Figure 11.** (**a**) Lateral etch rate at elevated temperature and lower etchant concentration. (**b**) Sidewall angle at elevated temperature and lower etchant concentration. **Figure 11.** (**a**) Lateral etch rate at elevated temperature and lower etchant concentration. (**b**) Sidewall angle at elevated temperature and lower etchant concentration.

A trend consistent with the etching studies reported above was also observed here, where the lowest etch rate was found to be at x = 0.125 and a decreasing sidewall angle was observed with increasing Sc concentration. Under the SEM, white clusters can be seen occasionally near the etch front, which we assume to be the unsoluable reaction by-product Sc(OH)3. Further research still needs to be conducted on the effect of its presence. A trend consistent with the etching studies reported above was also observed here, where the lowest etch rate was found to be at x = 0.125 and a decreasing sidewall angle was observed with increasing Sc concentration. Under the SEM, white clusters can be seen occasionally near the etch front, which we assume to be the unsoluable reaction by-product Sc(OH)3. Further research still needs to be conducted on the effect of its presence.

#### *3.3. Formation of Vertical Sidewall in Al0.875Sc0.125N 3.3. Formation of Vertical Sidewall in Al0.875Sc0.125N*

While differences can be observed across the spectrum, the lateral etch rate unanimously reaches its lowest point when the Sc concentration is at 12.5%. An inspection of the etch results have shown that during the etching, a more vertical side wall can be formed, as shown in Figure 12. While differences can be observed across the spectrum, the lateral etch rate unanimously reaches its lowest point when the Sc concentration is at 12.5%. An inspection of the etch results have shown that during the etching, a more vertical side wall can be formed, as shown in Figure 12.

**Figure 12.** Lateral etch of Al0.875Sc0.125N in (**a**) 30% KOH at 45 °C for 10 min; (**b**–**d**) 10% KOH at 65 °C for 20 min. **Figure 12.** Lateral etch of Al0.875Sc0.125N in (**a**) 30% KOH at 45 ◦C for 10 min; (**b**–**d**) 10% KOH at 65 ◦C for 20 min.

Although (a) was etched with different parameters compared to (b)(c)(d)—plus, (b)(c)(d) were etched for almost the same time—we conjecture that these images are demonstrating the transient response of the same etching dynamics. The KOH etch is slowed significantly at the Si {111} plane and the Al0.875Sc0.125N {1011} plane due to the low etch rate. Because of the energy differences between removing {1011} Al0.875Sc0.125N and {111} Si , the KOH slowly etches Si {111} until it comes into contact with the Al0.875Sc0.125N {1100} planes, which requires lower energy to react than the {1011} planes. As a result, the etchant begins to remove and simultaneously etch Si {111} and {1100} AlScN. By the time the entire {1100} planes were exposed, a vertical sidewall was formed. It is possible that the slow lateral etch rate is necessary but not sufficient for the exposure of the {1100} AlScN to occur. This has been demonstrated with the lateral etching of Al0.85Sc0.15N and Al0.72Sc0.28N, which have slightly higher lateral etch rates than Al0.875Sc0.125N. As shown below (Figure 13), the etchant may preferentially etch Si {111} instead of m-plane Al1−xScxN. Although (a) was etched with different parameters compared to (b–d)—plus, (b–d) were etched for almost the same time—we conjecture that these images are demonstrating the transient response of the same etching dynamics. The KOH etch is slowed significantly at the Si {111} plane and the Al0.875Sc0.125N 1011 plane due to the low etch rate. Because of the energy differences between removing 1011 Al0.875Sc0.125N and {111} Si, the KOH slowly etches Si {111} until it comes into contact with the Al0.875Sc0.125N 1100 planes, which requires lower energy to react than the 1011 planes. As a result, the etchant begins to remove and simultaneously etch Si {111} and 1100 AlScN. By the time the entire 1100 planes were exposed, a vertical sidewall was formed. It is possible that the slow lateral etch rate is necessary but not sufficient for the exposure of the 1100 AlScN to occur. This has been demonstrated with the lateral etching of Al0.85Sc0.15N and Al0.72Sc0.28N, which have slightly higher lateral etch rates than Al0.875Sc0.125N. As shown below (Figure 13), the etchant may preferentially etch Si {111} instead of m-plane Al1−xScxN.

**Figure 13.** (**a**) Al0.85Sc0.15N etched for 20 min in 10 wt % KOH, etch front of AlScN approaching Si {111}; (**b**) KOH preferentially etches Si {111} instead of {1212} AlScN; (**c**) Al0.72Sc0.28N etched for 20 min in 10 wt % KOH, etch front of AlScN aligned with Si {111}; (**d**) KOH preferential etching of Si {111} instead of {1212} AlScN. All etches were performed at 60 °C. **Figure 13.** (**a**) Al0.85Sc0.15N etched for 20 min in 10 wt% KOH, etch front of AlScN approaching Si {111}; (**b**) KOH preferentially etches Si {111} instead of 1212 AlScN; (**c**) Al0.72Sc0.28N etched for 20 min in 10 wt% KOH, etch front of AlScN aligned with Si {111}; (**d**) KOH preferential etching of Si {111} instead of 1212 AlScN. All etches were performed at 60 ◦C.

The {1100} plane etching has been reported before in single-crystal GaN [57] and Al1−xGaxN [58], and as per the findings of W. Chen et al. [57], the preference of its etching in GaN is a result of its smaller dangling bond density, which makes it more stable in KOH than the {1011} plane. Therefore, one explanation might be that the Al0.875Sc0.125N film has the highest activation energy in terms of the lateral etching, which was calculated to be 23.14 kcal/mol based on the data available. Nevertheless, this is one of the limited examples where this is reported in sputtered Al1-xScxN, and more research needs to be conducted to reveal the mechanism behind the vertical sidewall formation. This method, combined with BCl3/Cl2 dry etching, could potentially be applied in fabricating vertical side walls using a two-step fabrication process, which will benefit the research and production of LWR, laser mirrors, UV LEDs and a variety of MEMS devices. The 1100 plane etching has been reported before in single-crystal GaN [57] and Al1−xGaxN [58], and as per the findings of W. Chen et al. [57], the preference of its etching in GaN is a result of its smaller dangling bond density, which makes it more stable in KOH than the 1011 plane. Therefore, one explanation might be that the Al0.875Sc0.125N film has the highest activation energy in terms of the lateral etching, which was calculated to be 23.14 kcal/mol based on the data available. Nevertheless, this is one of the limited examples where this is reported in sputtered Al1−xScxN, and more research needs to be conducted to reveal the mechanism behind the vertical sidewall formation. This method, combined with BCl3/Cl2 dry etching, could potentially be applied in fabricating vertical side walls using a two-step fabrication process, which will benefit the research and production of LWR, laser mirrors, UV LEDs and a variety of MEMS devices.


**4. Performance evaluation in vivo**

The biocompatibility and separation performance of PES-based hemodialysis membranes in vivo are also discussed. Animal experiments are carried out to evaluate the PES hollow fiber membranes firstly, and goat was selected as the experimental animal. Experiments were performed to evaluate the solute clearance and the blood compatibility. The blood compatibility and performance of the PES-based high-flux hemodialysis membrane in hemodialyzation were also clinically evaluated, and compared with those of two conventional high-flux membranes, polysulfone (PSF) and polyamide (PA) membranes. The PES and PSF membranes showed similar blood compatibility and solute clearance, and the blood compatibility for PES and PSF might be better than that of the PA membrane.

Polyethersulfone Hollow Fiber Membranes for Hemodialysis 79

Table 1 also summarizes the clearance data and the reduction ratio after the dialysis for small molecules in vivo. Changes in 2-microglobulin during the dialysis for the goats are

The ultrafiltration coefficient was obtained by the hemodialysis process using the simulated solution with a value of 81ml/h.mmHg, from which we could conclude that the PES

The PES membrane was able to reduce the plasma burden of 2-microglobulin during the treatment, as shown in Figure 8. The data were analyzed by consideration of actual values and the percentage reductions achieved. The reduction ratio was about 50% after the treatment for 4 hrs; this value is comparable to that for PSF membrane and polyflux

As shown in table 1, the reduction ratio for the 2-microglobulin was smaller than that for the urea and creatinine due to the higher molecular weight (p<0.05). The alteration of 2 microglobulin in plasma levels may not simply be a result of trans-membrane transport; the adsorption to the membrane may also play a role in the observed plasma changes (Hoenich & Katopodis, 2002). For the removal of 2-microglobulin, cellulose derived membrane is impermeable to 2- microglobulin due to its dense symmetrical structure which does not permit the easy diffusion or convection of proteins through the membrane, while polyacrylonitrile (PAN), polysulfone and polymethylmethacrylate (PMMA) membrane could be used (Moachona et al., 2002). The PMMA membrane could also adsorb 2 microglobulin. To remove 2-microglobulin more efficiently from plasma, hemodialysis membranes must therefore not simply be considered as filters of low-molecular-weight metabolites but should be equally assessed for their capacity to eliminate potentially deleterious low-molecular-weight plasma proteins. For the PES membrane, 2 microglobulin adsorption is not an important mechanism of removal. The large solute removal by the membrane is mainly caused by the asymmetric structure and the higher ultra-filtration coefficient, which was presumably caused by the larger pore size and the

plotted in Figure 8. The reduction ratio was about 50% after the treatment for 4hrs.

**4.1.2 Results and discussion**

(Hoenich & Katopodis, 2002).

hydrophilicity of the membrane.

0

Fig. 8. Changes in 2-microglobulin during the dialysis. Data are expressed as the

0 50 100 150 200 250 Time (mins)

20

40

60

Normalized value (%

meansSD, n =3 (From reference, Su et al., 2008)

80

100

membrane was a high-flux hemodialysis membrane.

**<IP_ADDRESS> Solute transport**


**1. Introduction**

The smart grid is a newly evolving next-generation intelligent power grid, and many technological research works were conducted in different countries over the last decade to increase the efficiency of their power grids. The primary consideration in adopting the smart grid should be protection of the users privacy [1–3]. In other words, unlike existing security measures, the smart grid system should basically focus on the security of users rather than suppliers. More personal and specific information can be exposed in the smart grid environment by smart devices or hacking attacks when diverse types of information are combined [4–6]. The major issue of personal information protection in the distribution of smart grid technology is that it is possible to infer a user's behavioral pattern based on his/her energy consumption data by collecting and analyzing more detailed personal data, such as the characteristics of the user's energy usage or the frequency of energy production obtained, by applying the latest electric meters and other related equipment and technologies. In addition, the data read by smart meters inevitably require a certain monitoring or surveillance scheme as they are electronically collected and transmitted, rather than manually processed as in the past. The capacity of a meter capable of assessing consumer patterns or types of appliance depends on the frequency with which it collects data and the types of data being collected. Also, the user's behavior at home can make it is easier to infer his/her activity patterns in other places.

The factors associated with privacy intrusion scenarios in a smart grid environment include the following: (1) information concerning the use of a particular medical device or piece of electronic equipment which indicates their activation times and personal patterns, segmented data pertaining to the power consumption of each household appliance and its measurement location, and detailed information on the use of the appliances or equipment in use at a specific location; (2) the possibility of tracking a physical location through newly consumed energy, for instance, the charging of an electric automobile; (3) the activities in a certain house or building can be inferred from the electronic signature or use time pattern upon activation of a device or piece of equipment, which can form the basis for understanding a specific user's activities. Thus, the collection of a consumer's energy use data by a third party should be limited to the information required to serve the third party's purpose and which is authorized by the consumer.

The anonymous signature scheme comprises a function for authenticating signed messages while hiding the actual identity of the signer, which in itself is a common method in current systems that require the input signature to be authenticated. This scheme was developed by Chaum and Heyst in 1991 [7]. As for the group signature scheme, a member of a certain group is able to attach his/her signature in a message to prove that he is actually a member, and the verifier of the message will be able to confirm that person's membership only, without actually identifying the signer. However, it is possible for the opener, who authenticates the input signature, to identify the signer with the information of the signer previously stored in the system. The opener can be an organization or institution that deals with incidents associated with signatures. The group signature scheme is widely used as an anonymous signature scheme because of its reliability [8]. Despite its reliable performance, however, the security of personal information is called into question as many users consider that the opener has sufficient power to identify the signer and obtain the latter's personal information or information on anonymous activities for other purposes. To resolve this problem, Sakai et al. [9] introduced a complementary scheme by adding an "admitter" to the anonymous signature scheme. Thus, Sakai added the admitter and limited the opener's access to the signer's identification only by obtaining the consent of the admitter. In 2013, Ohara et al. [10] resolved the problem raised by Sakai (2012), which was the admitter's limited amount of token issuance.

The group signature scheme is often used when it is necessary for the authenticator to verify that the signer is a member of a particular group without revealing the actual identity. The real identity of the signer can be disclosed to the authenticator only if there are incidents or disputes that need to be solved. Nevertheless, it is quite clear that the signer will feel the burden of revealing his/her identity or anonymous activities to the authenticator without his/her consent and consider that the authority of the authenticator is too great. Thus, to limit the authority of the authenticator while maintaining the effectiveness of the group signature schemes, an anonymous signature scheme which authorizes the authenticator to identify the signer only with the token issued by the signer him/herself when generating a signature is proposed in this study.

#### **2. Related Research**

In a conventional power grid where electric power is delivered to the end users via substations (Figure 1), the power generation and distribution processes are centralized by the system, which assumes the role of mapping and visualizing the routine operations while controlling these processes to meet the power supply/demand schedule and its storage.

**Figure 1.** A typical power grid structure.

However, following the rapid development of information technology (IT), such a grid architecture transformed in a way that can provide a more efficient and effective means of power management by integrating with Internet Protocol (IP)-based technologies. The network convergence based on these technologies [11] allows the grid to interwork with an external network(s) by adopting the Transmission Control Protocol (TCP)/IP for a more efficient power management and provision of flexible but efficient service operations.

For the last decade, the development in the hardware, software, and communication technologies led to more advanced and sophisticated information and communications technology (ICT) which were the major factor of widespread mobile smart devices, software applications, or communication architectures [12,13].

The next-generation (21st century) power grid being called the smart grid (Figure 2) enables a smarter, interactive, and dynamic grid management and services based on the ubiquitous computing and advanced ICT technology to respond to the era of the fourth industrial revolution. One of the major advantages of the smart grid is that its bi-directional communication capability can not only improve the power management or operating process but also be utilized for establishing an Internet of things (IoT) system for the users' residences.

The conceptual smart grid model developed by the United States (US) National Institute of Standards and Technology (NIST) defined a smart grid as a complex infrastructure based on a set of seven chief domains [14], namely bulk generation, energy distribution, power transmission, operation and control, market, service providers, and customers and individual domains, composed of heterogeneous elements (e.g., organizations, buildings, individuals, and systems, including system resources and other entities). Also, the backhaul network is essential for achieving smooth but efficient communications between customers and utility companies when advanced power management systems such as advanced metering infrastructure (AMI) are to be embedded into the smart grid [15,16].

**Figure 2.** The architecture of a smart grid.

The problem pertaining to breach of privacy is one of the major issues when people are using a service which requires the user to be authenticated. A series of privacy protection schemes were introduced to let users remain anonymous by allowing only encrypted information or minimum user information to be disclosed to the system administrators; however, the security levels and the means of protection provided by those schemes vary and can be inadequate sometimes. The blind signature [17] or the homomorphic encryption [18] scheme is mainly used [19]. To simply describe them, for instance, the former is a scheme where the first party (Party 1) attaches his signature to the message generated by the second party (Party 2) without having any knowledge about the content of the message. Then, the third party (Party 3) can receive the message but the identity of the message sender (Party 2) will remain secure as his/her signature will not be authenticated. Meanwhile, in the latter scheme, a specific mathematical or a computational manipulation is applied to the message or the text to create a ciphertext so that only the authorized party with the right decryption key will be able to decipher the encrypted message. The smart meters usually adopt the latter scheme to encrypt and transmit their requirements to their central control system (utility company administrator) along with a specific encryption function to let the system to decrypt the contents of the requirement with an appropriate decryption key. These schemes were originally developed for the electronic voting systems to conceal the voters' information in the application layer but did not consider the possibility leaking the information from the lower layers (i.e., link layer or network layer) of the protocol stack. It is quite possible that the repeated use of the same IP address overtime may provide access to the identities of the communicating parties or a means for hackers to analyze the traffic [20].

Nonetheless, it is also true that such benefits may be provided at the cost of breaching privacy. That is, a large volume of generated data and its high granularity in which more information is contained would allow any third party with malicious intent to grasp the lifestyles of the customers. Also, there were some claims in some countries that the use of smart meters further endangered the security/privacy of the customers [21]. The balance between achieving an efficient and effective smart metering and guaranteeing the adequate level of personal information protection is always the focus of such a controversy. Using the terminology from Reference [22], the solutions that aim to protect the privacy should guarantee the customers a suitable level of anonymity together with a temporary unlinkability which disconnects them from the metering infrastructure (i.e., disabling power usage reading, etc.). However, the question here is whether the unlinkability can or should be fully achieved even when customers are required to settle their bills at some time or another. The same question can be addressed to unobservability, which refers to the condition where one's power usage cannot be observed by others. Although it is possible to keep the record of the total aggregated amount of one's power usage at the substation level, it still needs to be delivered to the main system for the smart metering system to be fully functional [22,23].

#### *2.1. Anonymous Authentication and Anonymous Signature Schemes*

The term "anonymous authentication" refers to a cryptographic technology that allows the person or entity requesting authentication to authenticate him or herself as a legitimate entity while remaining anonymous. Commonly, simple aliases designed to preserve anonymity cannot be used for this type of authentication as the user trail can be traced easily; thus, using them cannot be considered an anonymous authentication scheme. A group signature, anonymous letter of credit, and more were introduced for the purpose of anonymous authentication in a number of research works. The group signature is an electronic signature scheme which the signer can verify him/herself as a member of a particular group without having to reveal his/her identity, thus enabling the authenticator to determine that the person concerned is actually a member without being able to identify him/her. Also, the group signature scheme often involves a credible third-party organization referred to as an "opener", e.g., the police or an internet-related authority. The opener is authorized to identify a signer from the group signature and can track the identity of any user who displays inappropriate behavior (or commits illegal acts) while using anonymous services. The group signature scheme is

currently considered the most practical for real-world applications such as web application services as it offers traceable anonymity. In general, the group signature scheme offers anonymity, traceability, and linkability.

Figure 3 shows a schematic representation of the group signature scheme, whose members are normally distinguished as the group manager who sets the parameters, the opener who is authorized to trace a specific signature in a group, the signer, and the authenticator. Each signer in the same group has his/her own private signature key, whereas the authenticator can verify the signatures with an open group key. Also, information that can be used to identify a signer is encrypted in the signature value so that only the opener can trace the identity of a group signer with his open group key.

**Figure 3.** Diagram of the group signature scheme.

Figure 4 represents a group signature scheme that provides linkability, which was studied with a view of applying it to a variety of applications. Linkability is a basis for determining uniformity in a number of signatures so as to determine whether the signatures were written by the same person. Although the linker may detect uniformity in the signatures, he/she is not able to identify the signer.

**Figure 4.** Diagram of group signature scheme with linkability.

In the smart grid environment, service providers can enhance the quality of their services by performing big data analyses of users' data, such as their real-time power usage patterns, etc., and then processing them into meaningful information. Thus, the level of privacy protection can be increased by offering anonymity through group signatures, while the service providers are able to provide flexible services by linking with the data of an anonymous user (signer). Jeong-Yeon Hwang et al. introduced a group signature scheme that provides local linkability [6]; however, in this study, the linker refers to an organization or institution that has a linking key generated by the group manager so that, in general, it becomes the service provider. The linker has the authority to check the link status for all signature values.

Figure 5 shows a group signature scheme offering limited linkability. Unlike existing group signature schemes where the opener is a credible third-party organization, the linker in this scheme is the service provider itself or the organization or institution designated by the service provider, with could result in privacy violations of the service users. For example, let us assume that an anonymous user in the smart grid environment uses a power usage analysis service along with an IoT service. In this case, the service provider will be able to link the power usage information of person A (who just entered his/her signature with the group signature key) with the information about his/her IoT service use. At this time, the service provider does not know the identity of A but it is able to determine whether the user currently using these two services is one and the same, potentially leading to an undesirable breach of privacy. As such, while studies related to existing group signature schemes focused on managing the system for the designated linker so as to be able to test the linkability of all signature values, this study aims to secure a fundamental technology capable of preventing unnecessary information exposures by developing a group signature scheme that allows the designated linker to test the linkability only for those signatures desired by the signer. Thus, in the example shown above, an anonymous signer A can transfer the power usage values to the linker for the linkability test, using a group signature key while transmitting the IoT usage information separately with the same key for the same test. Thus, this scheme can provide a more secure method of preventing privacy breaches by minimizing the level of personal information exposure.

**Figure 5.** Diagram of group signature scheme with limited linkability.

#### **3. Anonymous Signature with Signer-Controlled Opening Capability**

The anonymous signature scheme allows authentication of the signer without revealing his/her identity, whereas the group signature scheme is a method of verifying that the signer is a member of a certain group, also without exposing the signer's identity. Nevertheless, it is possible for an opener to identify an anonymous signer based on the information of the signer, which is neither desirable nor favorable for the signer who wishes his/her signature to be authenticated but does not want to reveal his/her actual identity. Thus, this section discusses a solution whereby the signer obtains a (security) token upon entering his/her signature so that the opener is not able to find the information of the signer without permission.


**Author details**

Venkatesan Srinivasadesikan1 \*, Chitra Varadaraju2 , Raghunath Putikam3 and Shyi-Long Lee4

1 Division of Chemistry, Department of Sciences and Humanities, Vignan's Foundation for Science, Technology and Research, Vadlamudi, Andhra Pradesh, India

2 Department of Chemistry, St. Joseph's College for Women, Tiruppur, Tamil Nadu, India

3 Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan

4 Department of Chemistry and Biochemistry, National Chung Cheng University, Chia-Yi, Taiwan

\*Address all correspondence to<EMAIL_ADDRESS>
© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Applications of Density Functional Theory on Heavy Metal Sensor and Hydrogen Evolution… DOI: http://dx.doi.org/10.5772/intechopen.99825*


**6. References**

**7**

*Flinders University*

*Australia*

**Fully Coupled 6 Degree-of-Freedom**

Unmanned underwater vehicles (UUVs) are increasingly being used by civilian and defence operators for ever more complex and dangerous missions. This is due to the underlying characteristics of safety and cost effectiveness when compared to manned vehicles. UUVs require no human operator be subject to the conditions and dangers inherent in the underwater environment that the vehicle is exposed to, and therefore the risk to human life is greatly minimised or even removed. Cost effectiveness, in both time and financial respects, comes from a much smaller vehicle not containing the various subsystems required to sustain life whilst underwater, as well as smaller, less powerful actuators not placing the same levels of stress and strain on the vehicles as compared to a manned vehicle. This leads to a much smaller team required to undertake the regular maintenance needed to keep a vehicle operational. Taking these two main factors into account, the progression from manned vehicles to unmanned vehicles is a logical step within the oceanographic industry. Within the broad class of UUVs are the remotely operated vehicles (ROVs) and the autonomous underwater vehicles (AUVs). Both of these types of vehicles have been successfully used in industry, and their fundamental differences determine which type of vehicle is suited to a particular mission. The key difference between the two is that an ROV requires a tether of some description back to a base station, whereas an AUV does not. This tether connects the ROV to a human operator who can observe the current state of the vehicle and therefore provide the control for the vehicle while it executes its mission. This tether, depending on the configuration of the vehicle, can also provide the electricity to

power the vehicles actuators, sensors and various internal electronic systems.

AUVs have an advantage over ROVs of not requiring this tether, which leads to two main benefits. Firstly, an AUV requires little or no human interaction while the vehicle is executing its mission. The vehicle is pre-programmed with the desired mission objectives and, upon launch, attempts to complete these objectives without intervention from personnel located at the base station. This minimises the effect of human error while the vehicle is operational. The second benefit is the increased manoeuvrability that is possible without a cable continuously attached to the AUV. This tether has the potential to become caught on underwater structures, which could limit the possible working environments of an ROV, as well as cause drag on the motion of the vehicle, thus affecting its manoeuvring

**1. Introduction**

**Autonomous Underwater Vehicle**

Matthew Kokegei, Fangpo He and Karl Sammut

**Control of an Over-Actuated**


*4.1. Evaluation of Test Site near Montpellier*

In Figure 6, a subset of true color images for the reconstructed time series is shown. The subset corresponds to the eight acquisitions that are also shown in Figure 2 and illustrates qualitatively that cloudy and shadow pixels were reconstructed successfully.

**Figure 6.** Reconstructed true color images corresponding to the acquisitions in Figure 2.

Scatterplots for the spectral bands under study were then created for the same test site (Figure 7). The pixel values of the observed and the reconstructed time series are compared at acquisition time *i*clear and visualized based on the kernel density estimate. It is shown that the proposed RTSR method outperforms SG and DTS. The linear regression model indicates a better fit for RTSR, with a strong correlation (*R*2). In general, the SG- and DTSreconstructed surface reflectance values over-estimated the observed surface reflectance value. This is also illustrated for the example pixel indicated by the red square in Figure 7 (except for DTS in B8). The time series in Figure 3f is based on the same pixel. Both SGand DTS-reconstructed values for the clear observation on 23 May 2019 (first vertical line) indeed over-estimate the observation in B4 (lower graph). For this particular pixel and acquisition time, only SG over-estimated the observation in B8. The DTS-reconstructed surface reflectance in B8 was slightly lower than the clear observation, as can seen in the upper graph in Figure 3f. The RTSR reconstructed value shows little bias in any of the bands.

Furthermore, the RMSE based on the difference between the reconstructed and clear observations on 23 May 2019 was lower for RTSR than for the other two method, except for B8, where it was slightly larger the for this specific test site (0.22 with respect to 0.20 for SG and DTS). However, it will be shown that for the majority of test sites, RTSR has a smaller RMSE for all tested bands.

**Figure 7.** Scatterplot comparing the pixel values of the observed and the reconstructed time series for each spectral bands at acquisition time *i*clear. The red square corresponds to the pixel that is also indicated in Figures 2 and 6. The RMSE value, slope, intercept, and coefficient of determination (*R*2) of the linear regression are also shown.

#### *4.2. Smoothness Index for Aggregated Test Sites*

The smoothness for the three methods was evaluated by calculating TSI according to Equation (2). A cumulative frequency distribution representing all pixels aggregated for all test sites is shown in Figure 8. The results show little difference in the smoothness between the three reconstructed methods, with values for the 95 percentile that are almost identical. Most variation was found for spectral band B2, with smoothest results obtained for the non-adaptive Savitzky–Golay filter (SG) followed by the proposed RTSR and DTS. This is not surprising, as the SG-reconstructed time series represents the long-term change trend with a relatively wide half-width of the smoothing window. Both RTSR and DTS constrain the smoothing filter to capture dynamic changes of interest in vegetation.

**Figure 8.** Cumulative frequency distribution for the time-series smoothness index in the visible (B2, B3, B4), near infrared (B8) and short-wave infrared (B12) calculated for three reconstruction methods (SG, DTS, and the proposed RTSR). Small values indicate smoother time series. The 95 percentile is indicated in brackets.

### *4.3. Statistical Analysis of RMSE Metrics for Aggregated Test Sites*

As shown by the box plot in Figure 9a, the RMSE for RTSR was found to be smaller than for SG and DTS for the majority of test sites. This was the case for all spectral bands. The difference was most distinct in the visual to near infrared part of the electomagnetic spectrum. Furthermore, in the short wave infrared (band B12), the minimum as well as the 25 percentile value (Q1) and the median value of the RMSE was minimum over all test sites

for the proposed RTSR. However, for some test sites the RMSE was higher for the RTSR than for the SG without adjusting the smoothed value, as expressed by the slightly higher 75 percentile value (Q3) and upper whisker (Q3 + 1.5 × (Q3 − Q1)).

Following the addition of an extra gap in the time series, the three tested reconstruction methods performed more similarly (see Figure 9b). This can be expected given that the gap introduced exactly at acquisition time *i*clear was also used as a reference. This trustworthy reflectance value in the time series was not available to adjust the smoothed value in the RTSR-reconstructed time series. Only clear observations from nearby acquisition times were able to contribute to the reconstructed time series. When introducing three consecutive gaps (*i*clear − 1, *i*clear, *i*clear + 1), the difference between the compared methods was even more reduced, resulting in a comparable RMSE3 (see Figure 9c).

The results for the maximum-value composite over the four-month period 1 May 2019– 31 August 2019 are shown in Figure 9d. The advantage of the proposed RTSR reconstruction method with respect to the other two reference methods is more expressed than in the previous metrics. A potential explanation is that only a single observation was used for the previous metrics. Although the most clear observation was selected, varying atmospheric conditions over the test site result in reflectance values that are not homogeneous. The pixels in this reference image contain some noise, which blurs differences in the evaluation metrics. As motivated in other evaluation studies that obtained reference data from the 10-year average (2011–2020) of NDVI data [25], aggregating data over time reduces noise in the reference data.

**Figure 9.** RMSE based on the difference between the reconstructed image and the original observation for the most clear observation (RMSE0, RMSE1, and RMSE3) and for the maximum NDVI value composite over a four-month period (1 May 2019–31 August 2019).


*3.1. Participant Characteristics*

Participants were recruited from June 2020 to December 2021. Thirty-nine participants randomized to the placebo (*n* = 13), CIT (*n* = 13), and CIT+GSH (*n* = 13) completed the study (Figure 1). Compliance to the supplements were 94.9 ± 1.3%, 95.7 ± 1.1%, and 95.5 ± 1.2% for placebo, CIT, and CIT+GSH groups, respectively. No adverse effects of the supplementations were reported by participants during the study.

Participant characteristics at baseline did not differ among the groups, except for FBG (Table 1). FBG was higher in CIT+GSH compared to the placebo but not to CIT (*p* = 0.29).

Values are the mean ± SD or number (*n*) of participants. Abbreviations: BMI, body mass index; WC, waist circumference; FBG, fasting blood glucose; CIT, citrulline; GSH, glutathione; ARB, angiotensin receptor blocker; ACE, angiotensin converting enzyme; † vs. placebo.

### *3.2. Flow-Mediated Dilation and Arterial Stiffness*

Baseline FMD and PWV segments did not differ among the groups. There was a timeby-group interaction for FMD. CIT+GSH increased the FMD by 2.9% (*p* = 0.045) compared with placebo, but not with CIT (*p* = 0.18) (Figure 2). A low effect size was seen between CIT and placebo (*d* = 0.32) and, although insignificant (*p* = 0.10), there was a moderate effect size between CIT+GSH and CIT (*d* = 0.67). To compliment the significant increase in FMD, there was a high effect size between CIT+GSH and placebo (*d* = 0.91, *p* = 0.03). The change in FMD from 0–4 weeks was correlated with the baseline FMD (Figure 3). The Pearson correlation coefficient for placebo, CIT, and CIT+GSH were −0.26 (*p* = 0.39), 0.16 (*p* = 0.60), and −0.68 (*p =* 0.01), respectively. There were no significant time-by-group interactions for cfPWV, crPWV, cdPWV, or faPWV. However, CIT+GSH supplementation decreased crPWV (arm PWV) by 0.66 ± 0.93 m/s (*p* = 0.05, Table 2).

**Figure 2.** Changes (Δ) in brachial artery flow-mediated dilation (ΔFMD%) from 0 to 4 weeks. Abbreviations: CIT, citrulline; CIT+GSH, CIT+glutathione. \* *p* < 0.05 vs. placebo.

**Figure 3.** Relationship between changes (Δ) in the brachial artery flow-mediated dilation (ΔFMD%) from 0 to 4 weeks and baseline (0 week) FMD%. Abbreviations: CIT, citrulline; CIT+GSH, CIT+glutathione.

**Table 2.** Brachial artery endothelial function and arterial stiffness.

Values are the mean ± SD. Abbreviations: cfPWV, carotid-femoral PWV; crPWV, carotid-radial PWV; cdPWV, carotid-distal PWV; CIT, citrulline; CIT+GSH, CIT+glutathione; faPWV, femoral-ankle PWV. *p*-values are the time-by-group interaction from two-way repeated measures ANOVA. \* *p* < 0.05 vs. before; ‡ *p* < 0.05 vs. placebo.