Chapter 3 Experiments
3.2 Experimental equipments
The GZO thin films are deposited by APPCVD system as shown in Figure 3-2 (the sketch of diagram isn’t proportioned to the real size of equipment). Glass (25mm×25mm) is placed on hot plate. The GZO solvent is prepared by 1M Zn(NO3)2 and 0.1M Ga(NO3)2 which was mixed by atomic percentage. It is hazed by 2.45MHZ Ultrasonic and used as the precursors to deposit films. We apply N2 gas as carrier gas to deliver the precursors into the inner nozzle. The main gas also is N2 gas.
After plasma decomposed precursors, reactants diffuse and attach onto the glass with spray flow.
In the experiments, there are various parameters including Ga concentration, substrate temperature, gap distance, scan time, carrier gas flow rate and Zn(NO3)2 concentration. All the parameters are shown in Table 3-1. We will alter one of these parameters and retain the others.
Besides, the main gas flow rate, power system, nozzle speed and ultrasonic frequency are fixed. The variations of parameters are shown in the brackets underneath the “Values”.
Figure 3-2 Schematic illustration of GZO thin films prepared with APPCVD system.
Figure 3-3 APPCVD system of ITRI.
Main gas
Ultrasonic(2.45MHZ)
Gap distance
Ga Concertration
Substrate Temperature
Carrier Gas Flow Rate
Scan Times
Zn(NO3)2 Concertration
3
5
4
2 6
1
DC
Table 3-1 Parameters of GZO thin films.
Parameters Values Ga concentration (%) 8 (2~20)
Substrate Temperature (°C) 150 (100~300)
Gap Distance (mm) 5(5~20)
Ultrasonic Frequency (MHZ) 2.45
Table 3-2 Characterization analysis equipments
Company and type
SEM Hitachi S-4700I
AFM Veeco Dimension 3100
XRD PANalytical X'Pert Pro
ESCA PerkinElmer PHI1600
Four point probe Mitsubishi chemical Loresta-GP MCP-T610 Hall measurement ECOPIA HMS-3000
Spectrophotometer VASCO V-570
Haze measurement BYK-Gardner haze gard plus
Chapter 4
Results and Discussion
4.1 Ga concentration
The FE-SEM and AFM of ZnO and GZO thin films show in Fig 4-1 and Fig 4-2. The different Ga doping concentrations show similar surface morphological properties. The film is mainly made by ZnO films. Ga concentration is not effect the ZnO films seriously.
Fig 4-3 shows the GIXRD patterns of GZO films with different Ga concentration. XRD patterns show the obvious (002) peak of hexagonal (wurtzite) ZnO, indicating an oriented growth along the c-axis perpendicular to the substrate surface. The peak intensity increases with the Ga concentration up to 2at%, which then decreased for the higher doping levels. This is may be due to the fact that up to 2at% doping Ga3+
ions replace the Zn2+ ions in the ZnO lattice. However at higher Ga doping concentration, apart form replace the Zn2+ ions, Ga3+ ions may occupy the interstitial positions in the ZnO lattice. Some other additional peaks associated with the (103), (101), (102), (103), (112) directions also appear, but their intensities are weaker than the (002) signal. This means that variation in the optical and electrical properties are not caused by changes in the crystalline structure. The FWHM also increase from 0.36°
to 0.75°. The crystallite size “D” is calculated using the Scherrer’s equation.
D = 0.94λ/ Bcosθ (Eq. 4-1)
where D is the crystallite size, B is the broadening of the diffraction line
measured at half of its maximum intensity (rad) FWHM and λ is the x-ray wavelength (1.5405nm). The crystal size of GZO film is decrease from 17.79nm to 9.86nm, which means that the light scattering from the surface of ZnO was much smaller than from GZO.
The electric characterization shows in Fig 4-6. For electrical resistivity as a function of the [Ga]/ [Ga] + [Zn] ratio in the starting solution shows a decrease as the [Ga]/ [Ga] + [Zn] rate is increased, reaching a minimum value at certain [Ga]/ [Ga] + [Zn] ratio (8at%), further increase in the resistivity values is observe when the [Ga]/ [Ga] + [Zn] rate increases. For low [Ga]/ [Ga] + [Zn] the carrier concentration decreasing is due to the increase of the Ga atoms that are incorporated into the lattice ZnO in the Zn sites, supplying one electron to the conduction band for each Ga atom, until the maximum solubility of Ga into the ZnO lattice is reached (maximum value of the carrier concentration curve). For higher [Ga]/ [Ga] + [Zn] rates in solution, the Ga ions don’t occupy more Zn sites, and a segregation of Ga in an oxide form take place in the grain boundaries or interstices, causing a decrease in the carrier concentration. In heavily doped (>1018 cm-3) GZO films, the observed differences in Hall mobility values is due to a grain barrier limited mobility by Seto. [11] Hall mobility increase as the carrier concentration is increased. The resistivity of as-deposited films was from 4.04×10-3 to 8.06×10-4. The minimum value on resistivity was obtained at [Ga] /[Ga] + [Zn] = 8at%. [12]
The optical transmittance shows in Fig 4-7. The differences of these measurements are not obvious. The average transparency in the visible range was around ~75-85% and this is good for device application. The energy band gap of the films evaluated from the (αhע)2 versus hע plots vary in the range 3.7 to 3.8 eV, with changing Ga concentration which
may be attributed to the Moss-Burstein shift. The Moss-Burstein shift which occurs in heavily doped semiconductors is due to the filling of states at the bottom of the band minima as the doping level is increased.
This filling of phase space leads to transitions from the valence to conduction band occurring at higher energies and at k-vectors away from the zone centre, increasing the onset of absorption by an amount approximately equal to the fermi energy, although allowance must also be made for the non-parabolicity of the bands. [13]
Fig 4-9 shows the variation of the figure of merit and the sheet resistance. The higher Φ at the 6-10 at% doped Ga. The quality of transparent conducting films can be judged by the figure of merit (Φ), calculated from the transmittance and sheet resistance (which are inversely proportional to each other) data [14].
Φ = T10 / Rs (Eq. 4-2) where T and Rs are the transmittance at 550 nm and the sheet resistance, respectively. The higher values of the figure of merit represent the better performance of the transparent conducting film. Usually, the front contacts to the solar cell are metallic grids. While designing grid contacts, one must balance shading effects against electrical resistance losses. An alternative to metallic grid contacts is a TCO layer. The advantage of TCOs is that they are nearly invisible to incoming light, and they form a good bridge from the semiconductor material to the external electrical circuit. Therefore, both the electrical conductivity and the transmittance of TCO film should be as high as possible for application in solar cell.
Fig 4-10 shows the room temperature PL excitation and emission for all the sample were measured in the wavelength range 325-700nm. The excitation spectra showed the 324nm (3.8ev) prominent peak. The first
peak position for ZnO and GZO films were found to be 400nm, 410nm and 410nm, respectivity, and those peaks are originated from the near band edge emission. The secondary peak position for ZnO and GZO films were found to be 492nm, 463nm and 468nm, respectivity, and those peaks are originated from the deep-level emission. The deep-level emission was commonly associated with the lattice defects in the films such as oxygen vacancy, zinc vacancy, interstitial zinc, and interstitial oxygen.
4.2 Substrate temperature
The FE-SEM and AFM of GZO thin films show in Fig 4-11 to Fig 4-13. Surface morphology is not big different by the substrate temperature. It means that substrate temperature is not influence the GZO thin film seriously.
Fig 4-15 shows the GIXRD patterns of GZO films with different Ga concentration. The structural properties are not affected by the Substrate temperature seriously. Fig 4-18 shows the changes of the electrical properties of the GZO films with the different substrate temperature.
From Fig 4-18 we can see that as substrate temperature increased, resistivity also increased, and the mobility μ, carrier concentration decreased. This result is similar to anneal. The increases in resistivity observed in all the films by the substrate temperature increased can be attributed to a more appropriate stoichiometry are obtained. For films deposited at higher substrate temperatures, the following two aspects should be consider: a more stoichiometry film is formed, and a diffusion of alkaline impurities coming from the substrate to the films is present;
both facts lead to GZO films more resistive. For fig 4-19 scattering center decreases at higher substrate temperature, so transmittance increases at
higher substrate temperature.
4.3 Gap distance
Fig 4-21 shows the SEM image with different gap distance. More gap distance, more particle and roughness in the GZO thin films. This is because loner gap distance has high probability to react with air. Fig 4-22 shows the deposition rate is decrease at higher gap distance. This is attributed to the CVD process in Fig 2-2. High gap distance cause GZO precursor can’t be injected to sample completely.
Fig 4-23 shows the GIXRD patterns of GZO films with different Ga concentration. XRD patterns show that (002) peak was decrease with the increase gap distance. Longer gap distance would bring about the waste dispersion of ZnO. A short gap distance is advantageous for preventing the wasteful dispersion of source materials into the surrounding atmosphere. On the other hand, longer gap distance would bring about the waste dispersion of ZnO particles, causing a decrease in deposition rate.
In fig 4-24, contact area decreases at higher gap distance. Therefore, electrical resistivity as gap distance in the starting solution shows a decrease as the gap distance is increased. In fig 4-25, more gap distance lead to more particle, causing scatter in GZO thin films, so that the transmittances decrease at higher gap distance.
4.4 Scan time
Fig 4-26 shows the SEM images of the GZO films with the different scan times. In the Fig 4-29(a), the GZO thin film is not a continuous film.
When scan time increase, the GZO thin film stable growth. In fig 4-28, GZO thin films grow about 28nm in each scan time.
Fig 4-29 shows the changes of the electrical properties of the GZO films with the different scan times. From Fig 4-27 we can see that as scan times increased, mobility μ also increased which carrier concentration rose to a certain value and become saturated. This increase of carrier concentration along with the increase of scan time was attributed to the increase of the relative density of the GZO films. At the same time, more carriers were released as more Ga could be embedded in the host lattice of ZnO crystallites. When carrier concentration n became saturated and mobility μ rose, resistivity also dropped slowly after it had reduced to a certain value.
4.5 Carrier gas flow rate
The high carrier gas flow rate means more gas molecule will be inject to the APPCVD system, so that the film thickness is increased with the carrier gas flow rate. At the same times, some GZO solutions are not reaction completely. Therefore, Transmittance decreases with the increase thickness and particle.
4.6 Zn(NO3)2 concentration
Zn(NO3)2 concentration is similar to the Carrier gas flow rate, more gas molecule will be inject to the APPCVD system at higher Zn(NO3)2 concentration.
Figure 4-1 SEM of deposited GZO films prepared with different Ga
concentration (a) 2at% (b) 4at% (c) 6at% (d) 8at% (e) 10at% (f) 12at% (g) 20at%. (300 sccm, 10 times, 150°C, 0.2M GZO)
(a) (b)
(c) (d)
(e) (f)
(g)
Figure 4-2 AFM of deposited GZO films prepared with different Ga concentration (a) 2at% (b) 4at% (c) 6at% (d) 8at% (e) 10at% (f) 12at% (g) 20at%. (300 sccm, 10 times, 150°C, 5mm,0.2M GZO)
(a) (b)
(c) (d)
(e) (f)
(g)
Figure 4-3 GIXRD patterns of ZnO and GZO films prepared with different Ga concentration.
Figure 4-4 Variation of the (002) peak position and FWHM of GZO thin films with different Ga concentration.
Figure 4-5 Variation of crystallite size estimated along the (002) peak of GZO thin films with different Ga concentration.
Figure 4-6 Variation of the resistivity, carrier concentration and hall mobility of GZO thin films with different Ga concentration.
Figure 4-7 Optical transmission spectra of glass, ZnO and GZO thin films prepared with different Ga concentration.
Figure 4-8 Plot of (αhע)2 versus hע for GZO thin films with different Ga concentration.
Figure 4-9 Variation of sheet resistance and figure of merit with different Ga concentration for GZO thin films.
Figure 4-10 PL emission spectra of ZnO and GZO thin films with different Ga concentration at 324nm excitation.
Table 4-1 All data with different Ga concentration for GZO.
7.587 10.73 13.27 14.17 11.59 11.48 10.44
Concentration
Rms (nm) 11.552 12.229 12.886 13.204 13.315 12.445 8.691 Rs (Ω/□) 158 48 27.2 32.6 33.2 41.2 54 Haze 5.72 3.99 3.99 3.74 3.14 5.11 2.30 Transmittance
(%) (at 550nm)
78.06 78.85 79.95 80.21 82.58 78.31 81.59
Figure of merit
Figure 4-11 (a-1) SEM and (b-1) AFM of deposited GZO films at a substrate temperature of 100°C. (300sccm, 10times, 5mm, 0.2M GZO 8at%)
Figure 4-12 (a-2) SEM and (b-2) AFM of deposited GZO films at a substrate temperature of 100°C. (300sccm, 10times, 5mm, 0.2M GZO 8at%)
Figure 4-13 (a-3) SEM and (b-3) AFM of deposited GZO films at a substrate temperature of 100°C. (300sccm, 10times, 5mm, 0.2M GZO 8at%)
(a-1) (b-1)
(a-2)
(a-3)
(b-2)
(b-3)
Figure 4-14 Variation of the haze and thickness of GZO thin films with different substrate temperature.
Figure 4-15 GIXRD patterns of GZO films prepared with different substrate temperature.
Figure 4-16 Variation of the (002) peak position and FWHM of GZO thin films with different substrate temperature.
Figure 4-17 Variation of crystallite size estimated along the (002) peak of GZO thin films with different substrate temperature.
Figure 4-18 Variation of the resistivity, carrier concentration and hall mobility of GZO thin films with different substrate temperature.
Figure 4-19 Optical transmission spectra of glass and GZO thin films prepared with different substrate temperature.
Figure 4-20 PL emission spectra of GZO thin films with different substrate temperature at 324nm excitation.
Table 4-2 Element composition and percentage with different substrate temperature by XPS.
C1s N1s O1s Zn2p3 Ga2p3 O/Zn 100°C 8.88% 8.51% 45.56% 34.83% 2.23% 1.308
200°C 17.82% 7.64% 40.25% 31.80% 2.48% 1.266
300°C 13.42% 6.94% 42.34% 34.83% 2.48% 1.216
Table 4-3 All data with different substrate temperature for GZO.
100°C 200°C 300°C
Resistivity (Ωcm) 8.35X10-4 1.34X10-3 3.45X10-3 Mobility (cm2/Vs) 14.58 10.45 6.423 Concentration (cm-3) -5.14x1020 -4.47x1020 -2.84x1020
Thickness(nm) (SEM) 281 305 301
Ra (nm) 8.404 10.513 11.087
Rs (Ω/□) 33 36.4 101.2
Haze 11.2 6.42 3.03
Transmittance (%) (at 550nm)
72.59 84.86 86.96
Figure of merit (1/Ω) 1.23x10-3 5.32x10-2 2.44x10-2
Grain size (nm) 16.32 17.1 15.86
Energy gap (eV) 3.76 3.77 3.79
Figure 4-21 SEM of deposited GZO films prepared with different gap distance (a) 5mm (b) 10mm (c) 15mm (d) 20mm. (300 sccm, 10 times, 150°C, 0.2M GZO 8at%)
Figure 4-22 Variation of the haze and thickness of GZO thin films with different gap distance.
(a) (b)
(c) (d)
Figure 4-23 GIXRD patterns of GZO films prepared with different gap distance.
Figure 4-24 Variation of the resistivity, carrier concentration and hall mobility of GZO thin films with different gap distance.
Figure 4-25 Optical transmission spectra of glass and GZO thin films prepared with different gap distance.
Table 4-4 All data with different gap distance for GZO.
5mm 10mm 15mm 20mm
Resistivity (Ωcm) 7.868X10-4 1.51X10-3 1.348X10-2 7.201X10-1 Mobility (cm2/Vs) 14.28 8.12 1.386 0.03508 Concentration (cm-3) -5.554x1020-5.099x1020-3.341x1020 -3.498x1020 Thickness (nm)(SEM) 286 238 182 151 Rs (Ω/□) 26.2 51.8 560 3.6x105
Haze 4.2 6.35 8.84 7.76
Transmittance (%) (at 550nm)
79.38 80.25 80.42 78.72
Figure of merit (1/Ω) 3.79x10-2 2.14x10-2 2.02x10-4 2.54x10-7 Energy gap (eV) 3.79 3.79 3.79 3.6
Figure 4-26 SEM of deposited GZO films prepared with different scan times (a) 1time (b) 2times (c) 3times (d) 4times (e) 5times (f) 10times (g) 15times. (300 sccm, 100°C, 5mm, 0.2M GZO 8 at%)
(a) (b)
(c) (d)
(e) (f)
(g)
Figure 4-27 AFM of deposited GZO films prepared with different scan times (a) 1time (b) 2times (c) 3times (d) 4times (e) 5times (f) 10times (g) 15times. (300 sccm, 100°C, 5mm, 0.2M GZO 8 at%)
(a) (b)
(c) (d)
(e) (f)
(g)
Figure 4-28 Variation of the haze and thickness of GZO thin films with different scan times.
Figure 4-29 Variation of the resistivity, carrier concentration and hall mobility of GZO thin films with different scan times.
Figure 4-30 Optical transmission spectra of glass and GZO thin films prepared with different scan times.
Table 4-5 All data with different scan times for GZO.
1time 2times 3times 4times 5times 10times 15times Resistivity
0.626 1.051 3.584 5.016 7.275 14.16 14.11
Concentration
Figure 4-31 SEM of deposited GZO films prepared with different carrier gas flow rate (a) 150sccm (b) 300sccm (c) 450sccm (d) 600sccm. (10 times, 100°C, 5mm, 0.2M GZO 8at%)
Figure 4-32 Variation of the haze and thickness of GZO thin films with different carrier gas flow rate.
(a) (b)
(c) (d)
Figure 4-33 Optical transmission spectra of glass and GZO thin films prepared with different carrier gas flow rate.
Table 4-6 All data with different carrier gas flow rate for GZO.
150sccm 30sccm 450sccm 600sccm Thickness (nm)(SEM) 159 282 349 413
Rs (Ω/□) 240 28.4 21 17.2
Haze 0.22 5.29 17.8 15.2
Transmission (%) (at 550nm)
85.34 79.81 73.78 72.94
Figure of merit (1/Ω) 8.54 x10-4 3.69 x10-3 2.28 x10-3 2.48 x10-3
Figure 4-34 SEM of deposited GZO films prepared with different Zn(NO3)2 concentration (a) 0.1M (b) 0.2M (c) 0.3M (d) 0.5M (e) 1M.
(300sccm, 10 times, 100°C, 5mm, GZO 8at%) (d)
(e) (c)
(a) (b)
Figure 4-35 Variation of the haze and thickness of GZO thin films with different Zn(NO3)2 concentration.
Figure 4-36 Optical transmission spectra of glass and GZO thin films prepared with different Zn(NO3)2 concentration.
Table 4-7 All data with different Zn(NO3)2 concentration for GZO.
0.1M 0.2M 0.3M 0.5M 1M
Thickness (nm) (SEM)
135 282 349 421 437
Rs (Ω/□) 164.4 28.4 18.4 14.4 16.8
Haze 1.19 5.29 8.99 21.7 20
Transmittance(%) (at 550nm)
84.09 79.81 81.86 65.41 65.21
Figure of merit (1/Ω)
1.08x10-3 3.69x10-3 7.34x10-3 9.96x10-4 8.28x10-4
Chpater 5
Conclusions and Future Work
5.1 Conclusions
1. The GZO thin film deposited by atmospheric-pressure plasma jet will cost less when used for opto-electrical devices, because of low process temperatures and no vacuum chamber.
2. Ga concerntration, substrate temperature and gap distance are the major condition to dominate the quality of film.
3. Ga concerntration are mainly change the opto-electrical characterization but not effect the thickness and structure significantly.
4. The substrate temperature does not influence the GZO thin film properties significantly.
5. Smaller gap distance could get better GZO thin film properties in APPJ system.
6. [Ga] / [Ga] + [Zn] ratio = 8at%, substrate temperature = 100oC, gap distance = 5mm, scan time = 10 times, N2 flow rate = 300 sccm, Zn(NO3)2 = 0.2M, will get better performance in GZO films.
7. The best GZO thin films presented an optical transmittance of around 80 % and a resistivity value of 8x10-4Ωcm. Form Hall measurements performed an electronic mobility ranging from 0.035 to 14.58cm2V-1s-1
and a carrier concentration of 2x1020 to 6x1020cm-3 were obtained.
5.2 Future Work
1. We can use GZO films in large scale applications and in electrical devices.
2. To deposit ZnO films with the inclusion of materials such as B, Al, and In.
.
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簡歷 姓 名：陳偉強
性 別：男
出生日期：民國 72 年 12 月 31 日 出生地： 台灣省基隆市
住 址：台灣省台北市內湖路一段 285 巷 69 弄 55 號 3 樓 學 歷：西松高中
(民國 88 年 09 月～民國 91 年 06 月) 中央大學電機工程學系
(民國 91 年 09 月～民國 96 年 01 月)
交通大學電機學院微電子奈米科技產業研發碩士班 (民國 97 年 09 月～民國 99 年 08 月)
碩士論文：藉由大氣電漿在不同狀態下沉積氧化鋅摻雜鎵薄膜其 光電特性與材料分析之研究
Study on the opto-electrical characterization and material analysis of gallium doped zinc oxide film deposited by atmosphere-pressure plasma jet