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Effect of RF power and substrate temperature on the properties of boron and gallium co-doped ZnO films
Materials Science in Semiconductor Processing 53 (2016) 84–88
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Effect of RF power and substrate temperature on the properties of boron and gallium co-doped ZnO films Jin Yang, Jian Huang n, Huanhuan Ji, Ke Tang, Lei Zhang, Bing Ren, Meng Cao, Linjun Wang School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 March 2016 Received in revised form 10 June 2016 Accepted 13 June 2016 Available online 22 June 2016
Boron and gallium co-doped ZnO (BGZO) films were prepared by radio-frequency (RF) magnetron sputtering under different RF powers (50–250 W) at room temperature and 200 °C, respectively. The influence of sputtering power and substrate temperature on the structural, morphological, electrical and optical properties of BGZO films was investigated. The results indicated that all the films showed preferentially c-axis orientation and structure of hexagonal wurtzite. The grain size decreased at higher sputtering power above 150 W. The carrier concentration and optical band gap (Eg) increased with the increasing of RF sputtering power. At RF power of 150 W, the films showed higher mobility and lower resistivity. Average optical transmittance of all the BGZO films is greater than 85% in the visible wavelength and did not change obviously with the sputtering power or substrate temperature. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Boron and gallium co-doped Zno Magnetron sputtering RF power
1. Introduction Zinc oxide (ZnO) has been extensively researched because of its wide direct band gap of 3.37 eV at room temperature, high chemical stability and large exciton binding energy ( 60 meV) [1]. Doped-ZnO is considered to be one of the most promising transparent conductive oxide (TCO) materials applied in optoelectronic devices, such as flat panel displays, light emitting diodes (LEDs) and thin-film solar cells, due to its excellent electrical, optical properties, good adhesion to the substrate, low cost, non-toxicity and stability in hydrogen plasma processes [2–4]. The conductivity of the singly-doped ZnO films can be changed by several orders of magnitude by doping with Al, Ga, B, etc. Among those, Al doping is most commonly used because of its advantages such as high conductivity and thermal/chemical stability, while Ga-doped ZnO films have been widely researched due to Ga3 þ ions have the similar ionic radius with Zn2 þ ions, which would result in small ZnO lattice distortion. Meanwhile, compared with Al dopant, Ga dopant is less reactive with oxygen which means it can function as better dopant within ZnO [5–8]. B-doped ZnO, which also get extensively research because it could exhibits higher transparency, higher conductivity and improved crystalline quality, beyond that, it is reported that B-doped ZnO films have improved properties of thermal stability [9–12]. Thus, it appears that co-doping with Al, Ga and/or B is attractive because the improved performance can be expected. Co-doped ZnO films, such as n
Corresponding author. E-mail address: [email protected] (J. Huang).
http://dx.doi.org/10.1016/j.mssp.2016.06.007 1369-8001/& 2016 Elsevier Ltd. All rights reserved.
B and Al co-doped ZnO (BAZO) [13,14], Al and Ga co-doped ZnO (AGZO) [15] have already been reported in recent years and some improvements in properties have been obtained as expected. However, up to now, there have been few reports on B and Ga codoped ZnO films [16]. Therefore, we investigated the properties of sputtering B and Ga co-doped ZnO (BGZO) films, and their correlations with the sputtering process parameters (substrate temperature and sputtering power) were proposed in this paper. A variety of deposition techniques, such as magnetron sputtering, pulsed laser deposition and sol–gel have been applied for the preparation of ZnO films. Among the methods above, magnetron sputtering is one of the best choices because of its advantages: high deposition rate, low substrate temperature, strong adhesion and long-term stability [17–19]. It is well known that magnetron sputtering process parameters such as substrate temperature and RF power have much important influence on properties of the films [20]. In this paper, transparent conducting BGZO films were deposited by RF magnetron sputtering technique. The effects of RF power and substrate temperature on structural, electrical and optical properties of the BGZO films were investigated in detail.
2. Experimental BGZO films were prepared by using RF magnetron sputtering method with a BGZO ceramic target (98 wt% ZnO with 1.8 wt% Ga2O3 and 0.2 wt% B2O3, purity 99.99%). Glass sheets with area of 10 10 mm2 and thickness of 1 mm were used as the substrates for BGZO film deposition. The substrates were cleaned in an
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ultrasonic bath with acetone for 15 min before being loaded into the chamber. The vacuum chamber was evacuated until the residual gas pressure was less than 5 10 7 Torr. Sputtering was performed under an Ar atmosphere with an operating pressure of 6 mTorr. The thicknesses of all the prepared BGZO films were about 300 nm controlled by a vibrating quartz crystal. The BGZO films with various RF sputtering power (50, 100, 150, 200, 250 W) were prepared at substrate temperature of room temperature and 200 °C, respectively. The influence of RF power and substrate temperature on properties of the BGZO films is investigated. The microstructure of the BGZO films was analyzed by X-ray diffraction (XRD, 3KW D/MAX-2200 V PC, CuKα1, λ ¼0.154056 nm). The optical absorption and transmission spectra of the films were measured by UV–visible spectrophotometer (Shimadzu UV-2501PC). The electrical property of the films was investigated by Hall effect measurement system (Accent model HL5500PC). The surface morphology of the films was observed by scanning electron microscope (SEM, TESCAN MIRA II LMH).
3. Results and discussion Fig. 1 shows the deposition rate of BGZO films under different RF sputtering power at room temperature and 200 °C. It is seen that, for room temperature deposited films, the deposition rate of the films increased from about 1.2–7 nm/min with the increase of sputtering power from 50 to 250 W. The kinetic energy of the sputter species increases with the increasing of RF power is the reason for higher deposition rate at higher RF power [21]. Compared with room temperature deposited films, the deposition rate of films is growing with increasing substrate temperature to 200 °C. The higher deposition rate at higher substrate temperature is probably because the chemical absorption and the reorganization of certain chemical bonds at the substrate surface is speed up at higher substrate temperature [22]. To investigate the influence of RF sputtering power and substrate temperature on the microstructure of BGZO films, XRD measurements were performed. Fig. 2 shows the XRD patterns of BGZO films with different RF power at room temperature (Fig. 2 (a)) and 200 °C (Fig. 2(b)). It is obviously that all the films had a strong (002) peak at 2θ near 34.4° in the angle region from 20° to 70°, which indicate a well-developed hexagonal wurtzite structure of the BGZO films with preferentially (002) orientation. The results indicate that interstitial/substitute B3 þ ions or Ga3 þ ions did not change the hexagonal wurtzite structure in B and Ga co-doped ZnO films with different RF power and substrate temperature. From Fig. 2, there are no other Bragg reflections, which could
Fig. 1. The deposition rate of BGZO films under different RF power at room temperature and 200 °C.
Fig. 2. The XRD patterns of BGZO films under different RF power at room temperature (a) and 200 °C (b).
indicate the presence of any other crystallographic orientations of ZnO, or phases of impurity (such as Ga2O3 or B2O3), are not detected. This means that single phase BGZO films are grown on the glass substrate successfully. It is also found in Fig. 2 that the intensity of (002) peak of the BGZO films increased with the increasing RF power up to 150 W and then decrease with further increasing of RF power, which indicates an improvement of the crystalline quality with relatively higher RF power. The degradation of crystallinity at RF power above 150 W may be due to the higher energy and higher reaction rate of sputter species resulting in the damage of film surface. The better crystalline quality of the films obtained at RF power of 150 W reveals this power is appropriate for growth of BGZO films with high crystalline quality. The result is similar to those of literatures [23,24]. The RF power and substrate temperature dependences of surface morphology for BGZO films are revealed by the SEM, as shown in Fig. 3. For films deposited at room temperature, we can observe that with the increasing of RF power to 150 W, the crystallinity of films is improved that the crystalline size growing larger and grain boundaries became smaller, and then the grain size slightly decreased with further increasing power. Compared with films prepared at room temperature, the surface morphology of films at 200 °C shows the similar trend with the increasing of power. The observations indicate that sputtering power has an important influence on surface morphology of the films and relatively higher RF power can promotes the formation of dense and smooth film structure. On the other hand, it can be found that with the increasing substrate temperature, the grain size of the films slightly decreased under the same RF power especially above 150 W. It is considered that this tendency is influenced by the volatilization of the particles. It is worthy to noting that the shape of the grain changes from the round type at room temperature to the longish
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Fig. 3. SEM images of BGZO films under different RF power at room temperature ((a)–(e)) and 200 °C ((f)–(j)).
type like a worm at 200 °C and lower sputtering power, which may be due to incomplete thermal decomposition of the reactants [25]. Fig. 4 presents the electrical properties (resistivity (ρ), carrier concentration (n), and Hall mobility (μ)) of BGZO films under different RF power at room temperature (a) and 200 °C (b). The results confirmed the n-type nature of all the films. It can also be found that, with increasing sputtering power from 50 to 250 W,
the carrier concentration increase from 3.783 1020 to 5.761 1020 cm 3 and 2.531 1020 to 5.348 1020 cm 3 for films prepared at room temperature and 200 °C, respectively. As compared with films deposited at room temperature, the films prepared at 200 °C shows relatively lower carrier concentration. The changes of Hall mobility of the films at room temperature and 200 °C show the similar trend with varying the sputtering power. The mobility first increase from 3.88 to 7.71 cm2/V s and 6.62 to
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Fig. 4. The resistivity (ρ), carrier concentration (n), and Hall mobility (μ) of BGZO films under different RF power at room temperature (a) and 200 °C (b).
16.8 cm2/V s with the increasing of RF power from 50 to 150 W, then decreases to about 2.33 cm2/V s and 13.1 cm2/V s as the power further increase to 250 W for films deposited at room temperature and 200 °C, respectively. The carrier concentration increases with the increasing of sputtering power maybe due to the higher doping efficiency under higher sputtering power [5]. The electrical properties of BGZO films are improved up to RF power of 150 W, which is attributed to the improvement of crystallinity and reduced grain boundary scattering for charge carriers in the films as the results obtained from Figs. 2 and 3. From Fig. 4 we can also find that the films deposited at RF power of 150 W shows a relatively smaller resistivity. As is known to all, the resistivity is proportional to the reciprocal of the product of carrier concentration and mobility which can be expressed as [26]:
ρ ¼1/Nеμ
(1)
Where, ρ is resistivity, N is carrier concentration and μ is mobility, thus the resistivity is closely related to carrier concentration and mobility. From Fig. 4, it can be seen that when the sputtering power below 150 W at room temperature, the resistivity decrease due to the carrier concentration and mobility increasing. However, the sharply decreased mobility is the reason for increase of film resistivity at sputtering power above 150 W. For films deposited at 200 °C, the changed trend of resistivity below 150 W is similar with films prepared at room temperature but at sputtering power higher than 150 W, the slightly decreased mobility and increased carrier concentration resulting in a subtle difference of resistivity. The transmission and absorption spectra in the wavelength range 300–800 nm for BGZO films deposited at room temperature and 200 °C under different sputtering power were shown in Fig. 5. The average transmittance of all the BGZO films in visible region is over 85% including the substrate and the fundamental absorption edges are clearly observed for all samples. The good structural uniformity and crystallinity of the films is the reason for high
Fig. 5. The transmission and absorption spectra of BGZO films deposited under different RF power at room temperature (a) and 200 °C (b).
transparency. Both the BGZO films deposited at room temperature and at 200 °C show a blue shift in absorption edge with the increasing of sputtering power, which is due to the Burstein-Moss effect [27]. Fig. 6 shows the plot of (αhν)2 versus hν, where α is the optical absorption coefficient and hν is photon energy. The optical energy band gap (Eg) values of the films with a direct band gap were calculated using the well-known Tauc's formula [28]:
αhv ¼C(hv Eg)1/2
(2)
Where C is a constant, hν is photon energy. The extrapolation of the linear portion of plots to the energy axis α ¼0 leads to the Eg value of the films. The optical energy band gap of films deposited at room temperature increase from 3.51 eV to 3.58 eV with RF power increase from 50 W to 250 W. At deposition temperature of 200 °C, with the increasing of sputtering power, the optical energy band gap varies from 3.47 eV to 3.67 eV. Whether the films deposited at room temperature or the films at 200 °C, the optical energy band gap increase with increasing sputtering power. It is well known that the donor electrons occupy states at the bottom of the conduction band in the heavily doped semiconductors. The raising Fermi level into the conduction band will lead to energy band broadening, which can be expressed as [29]:
ΔEg =
2/3 h2 ⎛ 3 ⎞ ⎜ ⎟ n2/3 e * ⎝ ⎠ 8m π
(3)
Where ΔEg is the shift in doped semiconductor compared to undoped semiconductor, h is Planck's constant, m* is the electron effective mass in conduction band, and ne is the electron carrier concentration. From the equation, it can be found that the optical
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200 °C, especially at sputtering power above 150 W. With sputtering power increased up to 150 W, the carrier mobility increased, resistivity decreased and the film crystallinity improved. The carrier concentration increased with the increasing of sputtering power from 50 W to 250 W. All the BGZO films show high optical transmittance above 85% in the visible range and blue shift of optical band gap with the increasing of sputtering power.
Acknowledgments This work was supported by Science and Technology Commission of Shanghai (No. 15520500200). The authors also thank Instrumental Analysis and Research Center of Shanghai University for the XRD work carried out.
References
Fig. 6. The (αhν)2 vs. hν plots for BGZO films deposited under different RF power at room temperature (a) and 200 °C (b). The insets show the Eg of the films as a function of RF power.
energy band gap increase with increasing sputtering power is relates to the increase in the carrier concentration as shown in Fig. 4. It is also worth mentioning, compared with films deposited at room temperature, films prepared at 200 °C shows a slightly increase in the optical energy band when sputtering power above 150 W, which may be attributed to the decrease in the grain size [30].
4. Conclusions BGZO films were prepared by RF magnetron sputtering at room temperature and 200 °C, respectively. The effects of RF power and substrate temperature on properties of the films were investigated. All the BGZO films had hexagonal wurtzite structure with highly c-axis orientation. The grain size slightly decreased when substrate temperature increased from room temperature to
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Effect of RF power and substrate temperature on the properties of boron and gallium co-doped ZnO films Jin Yang, Jian Huang n, Huanhuan Ji, Ke Tang, Lei Zhang, Bing Ren, Meng Cao, Linjun Wang School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 March 2016 Received in revised form 10 June 2016 Accepted 13 June 2016 Available online 22 June 2016
Boron and gallium co-doped ZnO (BGZO) films were prepared by radio-frequency (RF) magnetron sputtering under different RF powers (50–250 W) at room temperature and 200 °C, respectively. The influence of sputtering power and substrate temperature on the structural, morphological, electrical and optical properties of BGZO films was investigated. The results indicated that all the films showed preferentially c-axis orientation and structure of hexagonal wurtzite. The grain size decreased at higher sputtering power above 150 W. The carrier concentration and optical band gap (Eg) increased with the increasing of RF sputtering power. At RF power of 150 W, the films showed higher mobility and lower resistivity. Average optical transmittance of all the BGZO films is greater than 85% in the visible wavelength and did not change obviously with the sputtering power or substrate temperature. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Boron and gallium co-doped Zno Magnetron sputtering RF power
1. Introduction Zinc oxide (ZnO) has been extensively researched because of its wide direct band gap of 3.37 eV at room temperature, high chemical stability and large exciton binding energy ( 60 meV) [1]. Doped-ZnO is considered to be one of the most promising transparent conductive oxide (TCO) materials applied in optoelectronic devices, such as flat panel displays, light emitting diodes (LEDs) and thin-film solar cells, due to its excellent electrical, optical properties, good adhesion to the substrate, low cost, non-toxicity and stability in hydrogen plasma processes [2–4]. The conductivity of the singly-doped ZnO films can be changed by several orders of magnitude by doping with Al, Ga, B, etc. Among those, Al doping is most commonly used because of its advantages such as high conductivity and thermal/chemical stability, while Ga-doped ZnO films have been widely researched due to Ga3 þ ions have the similar ionic radius with Zn2 þ ions, which would result in small ZnO lattice distortion. Meanwhile, compared with Al dopant, Ga dopant is less reactive with oxygen which means it can function as better dopant within ZnO [5–8]. B-doped ZnO, which also get extensively research because it could exhibits higher transparency, higher conductivity and improved crystalline quality, beyond that, it is reported that B-doped ZnO films have improved properties of thermal stability [9–12]. Thus, it appears that co-doping with Al, Ga and/or B is attractive because the improved performance can be expected. Co-doped ZnO films, such as n
Corresponding author. E-mail address: [email protected] (J. Huang).
http://dx.doi.org/10.1016/j.mssp.2016.06.007 1369-8001/& 2016 Elsevier Ltd. All rights reserved.
B and Al co-doped ZnO (BAZO) [13,14], Al and Ga co-doped ZnO (AGZO) [15] have already been reported in recent years and some improvements in properties have been obtained as expected. However, up to now, there have been few reports on B and Ga codoped ZnO films [16]. Therefore, we investigated the properties of sputtering B and Ga co-doped ZnO (BGZO) films, and their correlations with the sputtering process parameters (substrate temperature and sputtering power) were proposed in this paper. A variety of deposition techniques, such as magnetron sputtering, pulsed laser deposition and sol–gel have been applied for the preparation of ZnO films. Among the methods above, magnetron sputtering is one of the best choices because of its advantages: high deposition rate, low substrate temperature, strong adhesion and long-term stability [17–19]. It is well known that magnetron sputtering process parameters such as substrate temperature and RF power have much important influence on properties of the films [20]. In this paper, transparent conducting BGZO films were deposited by RF magnetron sputtering technique. The effects of RF power and substrate temperature on structural, electrical and optical properties of the BGZO films were investigated in detail.
2. Experimental BGZO films were prepared by using RF magnetron sputtering method with a BGZO ceramic target (98 wt% ZnO with 1.8 wt% Ga2O3 and 0.2 wt% B2O3, purity 99.99%). Glass sheets with area of 10 10 mm2 and thickness of 1 mm were used as the substrates for BGZO film deposition. The substrates were cleaned in an
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ultrasonic bath with acetone for 15 min before being loaded into the chamber. The vacuum chamber was evacuated until the residual gas pressure was less than 5 10 7 Torr. Sputtering was performed under an Ar atmosphere with an operating pressure of 6 mTorr. The thicknesses of all the prepared BGZO films were about 300 nm controlled by a vibrating quartz crystal. The BGZO films with various RF sputtering power (50, 100, 150, 200, 250 W) were prepared at substrate temperature of room temperature and 200 °C, respectively. The influence of RF power and substrate temperature on properties of the BGZO films is investigated. The microstructure of the BGZO films was analyzed by X-ray diffraction (XRD, 3KW D/MAX-2200 V PC, CuKα1, λ ¼0.154056 nm). The optical absorption and transmission spectra of the films were measured by UV–visible spectrophotometer (Shimadzu UV-2501PC). The electrical property of the films was investigated by Hall effect measurement system (Accent model HL5500PC). The surface morphology of the films was observed by scanning electron microscope (SEM, TESCAN MIRA II LMH).
3. Results and discussion Fig. 1 shows the deposition rate of BGZO films under different RF sputtering power at room temperature and 200 °C. It is seen that, for room temperature deposited films, the deposition rate of the films increased from about 1.2–7 nm/min with the increase of sputtering power from 50 to 250 W. The kinetic energy of the sputter species increases with the increasing of RF power is the reason for higher deposition rate at higher RF power [21]. Compared with room temperature deposited films, the deposition rate of films is growing with increasing substrate temperature to 200 °C. The higher deposition rate at higher substrate temperature is probably because the chemical absorption and the reorganization of certain chemical bonds at the substrate surface is speed up at higher substrate temperature [22]. To investigate the influence of RF sputtering power and substrate temperature on the microstructure of BGZO films, XRD measurements were performed. Fig. 2 shows the XRD patterns of BGZO films with different RF power at room temperature (Fig. 2 (a)) and 200 °C (Fig. 2(b)). It is obviously that all the films had a strong (002) peak at 2θ near 34.4° in the angle region from 20° to 70°, which indicate a well-developed hexagonal wurtzite structure of the BGZO films with preferentially (002) orientation. The results indicate that interstitial/substitute B3 þ ions or Ga3 þ ions did not change the hexagonal wurtzite structure in B and Ga co-doped ZnO films with different RF power and substrate temperature. From Fig. 2, there are no other Bragg reflections, which could
Fig. 1. The deposition rate of BGZO films under different RF power at room temperature and 200 °C.
Fig. 2. The XRD patterns of BGZO films under different RF power at room temperature (a) and 200 °C (b).
indicate the presence of any other crystallographic orientations of ZnO, or phases of impurity (such as Ga2O3 or B2O3), are not detected. This means that single phase BGZO films are grown on the glass substrate successfully. It is also found in Fig. 2 that the intensity of (002) peak of the BGZO films increased with the increasing RF power up to 150 W and then decrease with further increasing of RF power, which indicates an improvement of the crystalline quality with relatively higher RF power. The degradation of crystallinity at RF power above 150 W may be due to the higher energy and higher reaction rate of sputter species resulting in the damage of film surface. The better crystalline quality of the films obtained at RF power of 150 W reveals this power is appropriate for growth of BGZO films with high crystalline quality. The result is similar to those of literatures [23,24]. The RF power and substrate temperature dependences of surface morphology for BGZO films are revealed by the SEM, as shown in Fig. 3. For films deposited at room temperature, we can observe that with the increasing of RF power to 150 W, the crystallinity of films is improved that the crystalline size growing larger and grain boundaries became smaller, and then the grain size slightly decreased with further increasing power. Compared with films prepared at room temperature, the surface morphology of films at 200 °C shows the similar trend with the increasing of power. The observations indicate that sputtering power has an important influence on surface morphology of the films and relatively higher RF power can promotes the formation of dense and smooth film structure. On the other hand, it can be found that with the increasing substrate temperature, the grain size of the films slightly decreased under the same RF power especially above 150 W. It is considered that this tendency is influenced by the volatilization of the particles. It is worthy to noting that the shape of the grain changes from the round type at room temperature to the longish
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Fig. 3. SEM images of BGZO films under different RF power at room temperature ((a)–(e)) and 200 °C ((f)–(j)).
type like a worm at 200 °C and lower sputtering power, which may be due to incomplete thermal decomposition of the reactants [25]. Fig. 4 presents the electrical properties (resistivity (ρ), carrier concentration (n), and Hall mobility (μ)) of BGZO films under different RF power at room temperature (a) and 200 °C (b). The results confirmed the n-type nature of all the films. It can also be found that, with increasing sputtering power from 50 to 250 W,
the carrier concentration increase from 3.783 1020 to 5.761 1020 cm 3 and 2.531 1020 to 5.348 1020 cm 3 for films prepared at room temperature and 200 °C, respectively. As compared with films deposited at room temperature, the films prepared at 200 °C shows relatively lower carrier concentration. The changes of Hall mobility of the films at room temperature and 200 °C show the similar trend with varying the sputtering power. The mobility first increase from 3.88 to 7.71 cm2/V s and 6.62 to
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Fig. 4. The resistivity (ρ), carrier concentration (n), and Hall mobility (μ) of BGZO films under different RF power at room temperature (a) and 200 °C (b).
16.8 cm2/V s with the increasing of RF power from 50 to 150 W, then decreases to about 2.33 cm2/V s and 13.1 cm2/V s as the power further increase to 250 W for films deposited at room temperature and 200 °C, respectively. The carrier concentration increases with the increasing of sputtering power maybe due to the higher doping efficiency under higher sputtering power [5]. The electrical properties of BGZO films are improved up to RF power of 150 W, which is attributed to the improvement of crystallinity and reduced grain boundary scattering for charge carriers in the films as the results obtained from Figs. 2 and 3. From Fig. 4 we can also find that the films deposited at RF power of 150 W shows a relatively smaller resistivity. As is known to all, the resistivity is proportional to the reciprocal of the product of carrier concentration and mobility which can be expressed as [26]:
ρ ¼1/Nеμ
(1)
Where, ρ is resistivity, N is carrier concentration and μ is mobility, thus the resistivity is closely related to carrier concentration and mobility. From Fig. 4, it can be seen that when the sputtering power below 150 W at room temperature, the resistivity decrease due to the carrier concentration and mobility increasing. However, the sharply decreased mobility is the reason for increase of film resistivity at sputtering power above 150 W. For films deposited at 200 °C, the changed trend of resistivity below 150 W is similar with films prepared at room temperature but at sputtering power higher than 150 W, the slightly decreased mobility and increased carrier concentration resulting in a subtle difference of resistivity. The transmission and absorption spectra in the wavelength range 300–800 nm for BGZO films deposited at room temperature and 200 °C under different sputtering power were shown in Fig. 5. The average transmittance of all the BGZO films in visible region is over 85% including the substrate and the fundamental absorption edges are clearly observed for all samples. The good structural uniformity and crystallinity of the films is the reason for high
Fig. 5. The transmission and absorption spectra of BGZO films deposited under different RF power at room temperature (a) and 200 °C (b).
transparency. Both the BGZO films deposited at room temperature and at 200 °C show a blue shift in absorption edge with the increasing of sputtering power, which is due to the Burstein-Moss effect [27]. Fig. 6 shows the plot of (αhν)2 versus hν, where α is the optical absorption coefficient and hν is photon energy. The optical energy band gap (Eg) values of the films with a direct band gap were calculated using the well-known Tauc's formula [28]:
αhv ¼C(hv Eg)1/2
(2)
Where C is a constant, hν is photon energy. The extrapolation of the linear portion of plots to the energy axis α ¼0 leads to the Eg value of the films. The optical energy band gap of films deposited at room temperature increase from 3.51 eV to 3.58 eV with RF power increase from 50 W to 250 W. At deposition temperature of 200 °C, with the increasing of sputtering power, the optical energy band gap varies from 3.47 eV to 3.67 eV. Whether the films deposited at room temperature or the films at 200 °C, the optical energy band gap increase with increasing sputtering power. It is well known that the donor electrons occupy states at the bottom of the conduction band in the heavily doped semiconductors. The raising Fermi level into the conduction band will lead to energy band broadening, which can be expressed as [29]:
ΔEg =
2/3 h2 ⎛ 3 ⎞ ⎜ ⎟ n2/3 e * ⎝ ⎠ 8m π
(3)
Where ΔEg is the shift in doped semiconductor compared to undoped semiconductor, h is Planck's constant, m* is the electron effective mass in conduction band, and ne is the electron carrier concentration. From the equation, it can be found that the optical
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200 °C, especially at sputtering power above 150 W. With sputtering power increased up to 150 W, the carrier mobility increased, resistivity decreased and the film crystallinity improved. The carrier concentration increased with the increasing of sputtering power from 50 W to 250 W. All the BGZO films show high optical transmittance above 85% in the visible range and blue shift of optical band gap with the increasing of sputtering power.
Acknowledgments This work was supported by Science and Technology Commission of Shanghai (No. 15520500200). The authors also thank Instrumental Analysis and Research Center of Shanghai University for the XRD work carried out.
References
Fig. 6. The (αhν)2 vs. hν plots for BGZO films deposited under different RF power at room temperature (a) and 200 °C (b). The insets show the Eg of the films as a function of RF power.
energy band gap increase with increasing sputtering power is relates to the increase in the carrier concentration as shown in Fig. 4. It is also worth mentioning, compared with films deposited at room temperature, films prepared at 200 °C shows a slightly increase in the optical energy band when sputtering power above 150 W, which may be attributed to the decrease in the grain size [30].
4. Conclusions BGZO films were prepared by RF magnetron sputtering at room temperature and 200 °C, respectively. The effects of RF power and substrate temperature on properties of the films were investigated. All the BGZO films had hexagonal wurtzite structure with highly c-axis orientation. The grain size slightly decreased when substrate temperature increased from room temperature to
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