The equations indicate that for a given case, both positive and negative values of X and B are possible. Positive and negativeX implies an inductor and capacitor respectively, while positive and negative values of B implies a capacitor and an inductor respectively. For an unmatched SAW device with ZL= 40−j120, operating atf = 200 MHz, two L-section matching solutions exists as shown in Fig. 6.9c and 6.9d respectively. Fig. 6.9e shows the variation in reflection coefficient for the unmatched case and the two possible matching solutions. The reflection coefficient becomes close to zero near the operating frequency for the matched case, indicating that the undesirable reflections are eliminated. Smith chart can also be used to verify the matching attained for load ZL. The Smith chart shows the normalized impedances, with the upper and lower half of the circle representing the normalized inductive and capacitive reactances respectively. The rightmost point indicates an open circuit with infinite impedance while the leftmost point indicates a short circuit. Fig. 6.9f shows that if the load is matched correctly, the frequency response curve passes through the center of the Smith chart where the normalized impedance is unity. A small program is written in MATLAB using equations (6.1) and (6.2) to calculate the correct values of L and C required for designing the matching circuit for a given load impedance of SAW device.
Fig. 6.10a and 6.10b show the LW resonator and delay line devices with matching circuit connected between the coaxial connector and the device. The matching is achieved by connecting a series variable capacitor and a shunt variable inductor (C ≈ 3 pF and L ≈ 80 nH). Rohde
& Schwarz ZVA24 vector network analyzer (VNA)was used to characterize the device. Firstly, the device was connected without matching circuit to calculate the load impedance ZL at the operating frequency. Next, the required values ofCandLwere calculated and then implemented in the circuit. The matching was verified by plotting the Smith chart as shown in Fig. 6.10c.
Variation in S11 (dB) and phase with frequency of the LW resonator is plotted in Fig. 6.10d, indicating the resonance occurring at 195.5 MHz. The matching circuit was connected on either side of the two port LW delay line device. A minimum insertion loss of -20.6 dB is obtained at f = 208.2 MHz as shown in Fig. 6.10e.
6.5. ZnO nanorod fabrication
(a)
15 20 25 30 35 40 45 50 55 60 65 70 0
500 1000 1500 2000 2500 3000 3500 4000
Intensity(a.u)
2 (deg)
annealed at 350 C
Non-annealed (002)
(b)
(c) (d)
Figure 6.11: (a) Small wafer pieces kept on a quartz boat for annealing the ZnO thin film in the furnace. (b) XRD plot of ZnO thin film before and after annealing at 350◦C. AFM images showing the surface morphology of ZnO film (c) before annealing and (d) after annealing.
(a) (b)
Figure 6.12: (a) ZnO nanorods grown in a sealed glass bottle containing hydrothermal solution at 90–95◦C by sticking the sample on a glass slide and keeping it upside down. As the reaction proceeds, color of the solution changes to white. (b) ZnO nanorods grown by direct method at 95◦C for 3 hours in 0.05 M solution.
(XRD) measurement was carried out to investigate the crystalline property of the ZnO thin film, and the results are shown in Fig. 6.11b. Annealing leads to preferential orientation of the film along the c-axis representing the (002) plane of wurtzite hexagonal structure of ZnO.
AFM image displaying the surface morphology of the film before and after annealing is shown in Fig. 6.11c and 6.11d respectively. The plain film has a low grain density of about 30 µm−2, but annealing increases the grain density to about 90 µm−2. The size of ZnO nano-grains is about 40–50 nm. Equimolar concentration of zinc nitrate hexahydrate (Zn(NO3)·6H2O) and hexamethylenetetramine (HMTA) (C6H12N4) are mixed, and the solution is heated to 90–95
◦C for a few hours to start the growth of ZnO nanorods on the surface. ZnO nanorods of about 100 nm diameter and different heights can be made on the device surface by varying the duration of hydrothermal growth process. Growth trials were conducted on some test samples.
Three methods namely, direct growth, PEI assisted growth, and e-beam template growth were attempted.
6.5.1 Direct growth
Annealed samples were stuck to the glass slide upside down, and the slides were placed in a sealed glass bottle containing hydrothermal solution as shown in Fig. 6.12a. The bottles were placed in an oven at 90–95 ◦C for a few hours. As the reaction proceeds, the solution turns white. The mechanism of the reaction is explained below
C6H12N4+ 6H2O→4NH3+ 6HCHO NH3+ H2O→NH+4 + OH−
Zn(NO3)·6H2O→Zn2++ 2NO−3
Zn2++ 2OH−→Zn(OH)2 →ZnO + H2O
(6.3)
HMTA decomposes to provide four NH3 molecules and six HCHO molecules, where the HCHO molecules do not take part in the nanorod growth. The OH− ions are produced via the pro- tonation of NH3. The Zn2+ ions are released by Zn(NO3)·6H2O in water which reacts with OH− to form Zn(OH)2, causing the solution to become turbid white. Zn(OH)2 is further de- composed under heat to form ZnO nanorods. FESEM image of the ZnO nanorods grown by the direct method at 95 ◦C for 3 hours in 0.05 M solution is shown in Fig. 6.12b. The di- ameter of the nanorods depend on the solution concentration, and the growth rate depends on the temperature. Directly grown nanorods have less height, tend to merge and have low aspect ratio.
6.5.2 PEI assisted growth
It has been reported that PEI has a prominent effect on controlling the aspect ratio of ZnO nanorods grown by the hydrothermal solution [194]. Without the addition of PEI, as-grown ZnO nanorods possess wider diameter and shorter length along the c-axis direction. PEI sticks to the side walls of the nanorods during growth and inhibits the lateral expansion of the rods,
6.5. ZnO nanorod fabrication
(a) (b) (c)
(d) (e) (f)
Figure 6.13: (a) Schematic of ZnO nanorod growing along thec direction with PEI sticking to the sides of the nanorod to inhibit the lateral growth. ZnO nanorods grown in 80 mL, 0.025 M hydrothermal solution for 3 h by adding different volumes of PEI (b) 0.1 mL, (c) 0.2 mL, (d) 0.5 mL, (e) 1 mL, and (f) 1.5 mL.
thereby increasing the aspect ratio as shown in Fig. 6.13a. ZnO nanorods were grown at 90 ◦C for 3 h in a sealed glass bottle containing 80 mL of 0.025 M hydrothermal solution by adding different amounts of PEI. The diameter of the rods progressively decreased from 250 nm to under 100 nm, and the rods tend to become longer with increasing PEI concentration as shown in Fig. 6.13b–6.13f. In 2–3 h, nanorod height of 1–2 µm can be attained. However, acquiring the cross-section image of nanorods and measuring the exact height in FESEM was difficult.
Moreover, the nanorods tend to branch out and with poor orientation control.
6.5.3 E-beam template growth
E-beam lithography (EBL) was performed on few test samples to make pattern comprising of holes of 100 nm size, 2 µm apart in the PMMA resist using the Raith eLine lithography equipment. The details of the process are shown in Table6.7. Firstly, the samples are dipped in IPA solution for 1 min, followed by drying in N2. Next, the sample is spin coated with e-beam resist (950 PMMA A4) at 6000 rpm for 1 min. The wafer is then baked at 180◦C for 3 min, followed by e-beam exposure with a dosage of 250µC cm−2. Square holes of size 100 nm, 2 µm apart were made on a 1 mm area of the resist. Methyl isobutyl ketone (MIBK) solvent is the key ingredient in the developer that controls the solubility and swelling of the resist, while IPA is the alcohol (non-solvent). PMMA resist is developed using MIBK:IPA (1:3) solution.
(a) (b)
(c) (d)
Figure 6.14: ZnO nanorod grown in a 0.05 M solution at 90◦C using e-beam template containing 100 nm holes for different times (a) 2.5 h, (b) 4 h, and (c) 1.5 h. (d) Hexagonal shape of nanorod.
E-beam lithography
Step Process Details
1 Cleaning IPA dip for 1 min,
drying with nitrogen 2 Spin coat resist
(950 PMMA A4) 6000 rpm for 1 min
3 Baking 180◦C for 3 min
4 E-beam exposure
Dosage : 250 µC/cm2 Beam current : 360 pA
Aperture: 30µm
5 Develop MIBK:IPA (1:3)
for 40 sec
Table 6.7: Process and parameters used in E-beam lithography.