Structural, magnetic and transport study of DBPLD fabricated magnetic semiconductors 3

CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION CHAPTER DEPENDENCE OF PROPERTIES OF Zn1-xCoxO THIN FILMS GROWN ON (0001) SAPPHIRE SUBSTRATES ON Co CONCENTRATION 6.1 Introduction Today, the fabrication of room temperature DMSs is still a great challenge. For the purpose of fabrication of room temperature DMSs, Zn1-xCoxO materials have widely studied, and many experimental results have been reported. However it is hard to see these results show consistency, especially discrepancies were reported by different groups for the magnetic properties. Beside ferromagnetic behaviours, paramagnetic or spin glass behaviours were all reported [1-6]. For example, Zn1-xCoxO thin films were reported to be paramagnetic for x < 0.12 in Ref. [5]; in contrast, M-H hysteresis loops were observed for Zn1-xCoxO thin films with similar Co concentrations [7]. In our view, the differences are probably due to different experimental conditions and the sensitivities of the properties for Zn1-xCoxO thin films. Hence it is necessary to prepare the films under the identical experimental conditions with a large range of Co concentrations and study the properties dependence on Co concentration x. In our study, the DBPLD was used to synthesize Zn1-xCoxO thin films, hence the films could be prepared under the identical experimental conditions with a large range of Co concentrations. Magnetic behaviors and semiconductive properties are the most National University of Singapore 97 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION important properties for DMSs. In this chapter, the magnetic and transport behaviors of the films were characterized. From our experimental results, we conclude that Zn1-xCoxO with x < 0.1 is a candidate material exhibiting both magnetic and semiconductive properties at room temperature. That is, the magnetism can be realized with Co doped into ZnO. However, the improvement was limited. The transport behaviors of the Zn1-xCoxO thin films can be explained by a hopping mechanism, whose electrical states of Co ions is suggested to be localized. In this chapter, we are only concerned with magnetic behaviors. The magnetic mechanisms will be discussed in a specific chapter (see Chapter 7). 6.2 Magnetic Properties of Zn1-xCoxO Thin Films 6.2.1 M-H Loops of Zn1-xCoxO Thin Films at Room Temperature Figure 6-1(a) illustrates the M-H curves of Zn1-xCoxO films at room temperature with Co concentration x ranging from 0.015 to 0.27 by VSM. Even in the presence of some noise due to weak signals, hysteresis loops were observed. The coercivity measured was around 100 Oe, including the curve of Fig. 6-1. Our experimental results showed that magnetism could be realized by even a very low Co concentration. From curve 1-3 of Fig. 6-1, there are not apparent differences between them. It seems that with increasing Co concentration, the improvement of magnetism was limited. Further increase in Co concentration, magnetism may increase, as shown in curve of Fig. 6-1(a), and magnetic anisotropy may even be observed [Fig. 6-1(b)]. However, the XPS Co (2p) spectrum indicates presence of Co–Co binding, as shown in the inset of National University of Singapore 98 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION Fig. 6-1(b). A peak centered at 778.9 eV was observed outside the binding energy range of Co–O (779.4– 780.2 eV) [8]. Hence, the magnetic anisotropy may be due to Magnetization (emu/cm ) the Co precipitates in the film. 40 (a) 20 Excimer Laser 10 Hz, 3J/cm -5 5x10 Torr o 650 C -20 -40 1.Co 0.015 2.Co 0.02 3.Co 0.05 4.Co 0.16 5.Co 0.27 -6000 -4000 -2000 2000 4000 6000 40 o o 30 60 90 (b) Co 0.27 20 c/s Magnetization (emu/cm ) Magnetic Field (Oe) -20 -40 22000 778.9 eV 20000 18000 16000 14000 790 785 780 775 BE (eV) -4000 4000 8000 Magnetic Field (Oe) Fig. 6-1(a) Magnetic loops of thin films with different Co concentrations measured at room temperature by VSM; (b) Magnetic loops of thin films with Co concentration of x = 0.27 at different angles by VSM. Inset shows the XPS Co 2p3/2 spectrum. National University of Singapore 99 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION Figure 6-2 depicts the saturation magnetization per Co atom dependence on Co concentration. When the Co concentration is very low (x ~ 0.01), the film reveals a relatively high effective number of Bohr magnetons, around µ B / Co , agreeing with the value in reference [9]. It is consistent with the calculated value for the effective number of Bohr magnetons under the condition of quenched orbital moments for Co2+ with the configuration of 3d7 [10]. It is worth noting that the expected value of magnetic moment per Co atom in [9] is obtained under the condition of p-type ZnO situation. It was also found that the magnetic moment per Co atom decreased with increasing Co concentration for x < 0.1. The decrease of magnetic moment per Co atom with x suggests the presence of antiferromagnetic interactions in the Zn1-xCoxO system. The details of antiferromagnetic coupling mechanism between magnetic ions will be discussed in the section 7.4.2. Here only a brief description is given as follows. It is reasonable to ascribe the antiferromagnetic coupling between magnetic ions to superexchange interaction [11]. With increase in Co concentration, the mean distance between Co atoms is reduced, and thus the antiferromagnetic interaction is reinforced. When the Co concentration is over 0.1, magnetic moment per Co stops decreasing. The reason maybe related to the Co cluster induced which is the results of Co concentration over the percolation limit. This result corresponds to those of structure studies in section 5.6, in which Zn1-xCoxO films showed good crystallinity, good lattice without obvious clusters with x < 0.1. Similar results will be discussed in the section 6.2.3, where the magnetic moment obtained at 1000 Oe was observed to decrease when the Co concentration increases from 0.02 to 0.09, as shown in Fig. 6-5(b). National University of Singapore 100 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION 3.5 Excimer Laser 10 Hz, 3J/cm -5 5x10 Torr o Growth temperature 650 C Ms/Co atom (µB/Co) 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 Co concentration x Fig. 6-2 Ms per Co atom dependence on Co concentration at room temperature. 6.2.2 Magnetic Properties of a Zn0.98Co0.02O Film A SQUID magnetometer was used to precisely characterize the magnetic properties of the Zn1-xCoxO thin films with the applied field parallel to the film plane. The M-H curve of Zn1-xCoxO film at 300 K with the Co concentration of x = 0.02 is shown in Fig. 6-3(a). A clear hysteresis loop was observed. It is obvious that the Zn1-xCoxO thin film is magnetic up to 300 K. At room temperature, the coercivity (Hc) is about 90 Oe and the saturation magnetization is about 12.5 emu/cm3, in agreement with VSM results obtained at room temperature. The coercivity of the film increased with decreasing temperature, as shown in the inset of Fig. 6-3(a). According to our experimental results, magnetic hysteresis loops were observed at least up to room temperature, and National University of Singapore 101 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION magnetism could be realized with Co doped into ZnO. However, the improvement was limited. It is experimentally shown that most of the specimens revealed small coercivity, and the maximum Hc obtained without perceptible precipitates was less than 300 Oe. Figure 6-3(b) shows a typical Hc dependence on temperature for Zn1-xCoxO thin films with Co concentration x = 0.02. We observed that Hc remains at about 100 Oe near room temperature, and it increases with decreasing temperature. When temperature goes below 100 K, it reaches about 200 Oe. National University of Singapore 102 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION (a) 10 Co 0.02 300 K M (emu/cm ) Magnetization (emu/cm ) 15 -5 -10 300 K 100 K 30 K -2 -200 -15 -4000 -3000 -2000 -1000 200 Magnetic Field (Oe) 1000 2000 3000 4000 Magnetic Field (Oe) 220 (b) Hc (Oe) 200 180 160 140 120 100 80 50 100 150 200 250 300 T (K) Fig. 6-3(a) Magnetic loop of the thin film with Co concentration of x = 0.02 measured at 300 K by SQUID. Inset: Enlarged view of the low field region to show the presence of hysteresis and remanence for the samples measured at 300, 100 and 30 K; (b) Hc dependence on temperature of the film with Co concentration of x = 0.02. National University of Singapore 103 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION 6.2.3 Magnetic Properties of Zn1-xCoxO Thin Films Dependence on Temperature In order to obtain more details of the magnetic behaviors of Zn1-xCoxO thin films, magnetization (M) dependence on temperature was measured via a SQUID. The specimen was placed in an applied field parallel to the specimen plane at a temperature ranging from to 400 K. To obtain the temperature dependence of the magnetization, zero-field-cooled (ZFC) and field-cooled methods were applied. For ZFC magnetization measurement, the sample was first cooled down to K in the absence of an applied magnetic field, and the magnetization of the sample was measured in the temperature range up to 400 K. On the other hand, for FC magnetization measurement, the sample was cooled down to K in an applied magnetic field, and measurements of magnetic moment at each intermediate temperature were carried out at constant applied fields. By Figure 6-4(a) we describe the temperature dependence of magnetization of the Zn1-xCoxO film with Co concentration x = 0.05. The film was measured by the applied field parallel to the specimen planes by means of ZFC and FC methods with the magnetic field of 100, 1000 and 2000 Oe. Curie temperature for this film could not be reached due to the operation limit of the SQUID equipment used. We estimate that the Tc is higher than 400 K. The curve obtained in a magnetic field of 100 Oe shows a nonzero magnetization up to room temperature, which is in accordance with the M-H curves. Discrepancy between the ZFC and the FC curves at a lower magnetic field was observed. The magnetization decreases slowly with decreasing temperature for National University of Singapore 104 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION temperatures up to 50 K. This is not concluded a rule in most DMS materials [12] but rather an exception. References [13-15] attributed it to the effect of randomness and disorder on the percolating FM clusters. In our view, it can be explained by spin glass behaviors. Antiferromagnetic coupling leads to the decrease in magnetism under the condition of ZFC. Comparing the curves under different magnetic fields, the point at which ZFC and FC starts to deviate tends to shift toward low temperatures for large magnetic fields. This is one of the features for spin glass [16,17]. The magnetization abruptly increases when the temperature is lower than 40K, exhibiting a low temperature tail. Using Figure 6-4(b) we can make out the difference, denoted by D, in magnetization of FC and ZFC for Zn1-xCoxO thin films with Co concentration x = 0.05 measured in 100, 1000 and 2000 Oe. It is clear that, under the three different magnetic fields, the difference D decreases with increasing temperature, and D is not zero at temperatures close to 300 K. We can observe a ferromagnetic contribution in the film up to 300 K, which is agreement with our M-H results. It is interesting to note that D increases when the field is increased from the smallest value at 100 Oe to 1000 Oe, but decreases again when the magnetic field increases beyond 2000 Oe. This is the result of competition of positive magnetic moment from the film and negative magnetic moment from the diamagnetic sapphire substrate. National University of Singapore 105 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION Excimer Laser 10 Hz, 3J/cm -5 5x10 Torr o 650 C Co 0.05 (a) 2000 Oe 1000 Oe Magnetization ( emu/cm ) Magnetization (emu/cm ) 0.30 100 Oe 0.25 100 Oe 0.20 0.15 100 200 300 Temperature (K) 400 50 100 150 200 250 300 350 400 450 Temperature (K) 0.5 100 Oe 1000 Oe 2000 Oe (b) D. M (emu/cm ) 0.4 0.3 0.2 0.1 0.0 50 100 150 200 250 300 350 Temperature (K) Fig. 6-4(a) ZFC (open) and FC (solid) curves and (b) temperature dependence of the difference magnetization between FC and ZFC magnetizations for Zn1-xCoxO thin films with Co concentration 0.05 measured in 100 Oe, 1000 Oe and 2000 Oe. National University of Singapore 106 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION (a) 0.7 0.6 (d) 0.3 100 Oe 100 Oe Co 0.02 Co 0.05 Co 0.09 D. M (emu/cm ) Magnetization (emu/cm ) 0.8 0.5 0.4 0.3 0.2 0.1 0.2 0.1 0.0 0.0 -50 50 100 150 200 250 300 350 400 450 50 100 200 250 300 350 0.8 (b) 1000 Oe 0.7 D. M (emu/cm ) -50 (e) 1000 Oe 0.6 Co 0.02 Co 0.05 Co 0.09 Magnetization (emu/cm ) 150 Temperature (K) Temperature (K) 0.5 0.4 0.3 0.2 0.1 0.0 50 100 150 200 250 300 350 400 450 50 100 Temperature (K) 150 200 250 300 350 Temperature (K) (c) 2000 Oe 0.2 c3 0.05 c4 0.09 -50 (f) 2000 Oe D. M (emu/cm ) Magnetization (emu/cm ) 0.1 0.0 50 100 150 200 250 300 350 400 450 Temperature (K) 50 100 150 200 250 300 350 Temperature (K) Fig. 6-5 ZFC/FC curves measured at (a) 100 Oe, (b) 1000 Oe and (c) 2000 Oe, respectively; and temperature dependence of the difference magnetization between FC and ZFC magnetizations measured at (e) 100 Oe, (f) 1000 Oe and (g) 2000 Oe, respectively, for Zn1-xCoxO thin films with Co concentration x = 0.02, 0.05 and 0.09, denoted by squares, circles and triangles, respectively. Here, solid symbols denote FC and open ones denote ZFC for (a-c). National University of Singapore 107 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION Figure 6-5(a-c) shows the ZFC and FC curves of Zn1-xCoxO films with different Co concentrations at respective magnetic fields of 100, 1000 and 2000 Oe. All show positive curvatures. The discrepant point at which ZFC and FC start to deviate can be observed for all the curves, and these points decreases with increasing magnetic field. From them, the discrepant points between the ZFC and the FC curves were estimated and listed in Table 5-1. It revealed the behaviour of the discrepant point shifting. The above properties are typical features for a fine particle system, such as superparamagnetic or spin glass system. To estimate the size of the clusters in a supermagnetic system, we proceed as follows. The highest blocking temperature is reached when the magnetic field is much smaller than the so-called anisotropy field [17]. Hence, when a very small magnetic field is applied, the discrepancy is caused by a large anisotropy of the system which is due to the structure formed by widely separated chains of Co atoms. The blocking temperature ( TB ) obtained at low magnetic field was used to estimate the mean size of clusters [17] using the formula V = 25 k B T B Ku , where V is the mean volume of the cluster and kB is the Boltzmann constant. From the Table 6-1, the blocking temperature for the film with Co concentration x = 0.09 is estimated to be about 380 K. The anisotropy constant ( K u ) of fine Co particle is 7×105 J/m3 [17]. Hence the cluster size is deduced to be about nm. We should observe such size of cluster under a HRTEM observation. However we did not observe clusters in the Zn1-xCoxO with x < 0.1, suggesting that it should be explained using National University of Singapore 108 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION another system, such as spin glass system. Table 6-1 The discrepant points (K) estimated from Fig. 5-5 (a-c) x H 100 Oe 1000 Oe 2000 Oe 0.02 >400 >400 0.05 >400 400 360 0.09 380 360 350 From Fig. 6-5(b), we can see that when Co concentration increases from 0.02 to 0.09, the magnetization obtained at 1000 Oe was observed to decrease, which coincides with the previous results in Fig. 6-2. Figure 6-5(d-f) show the difference between FC and ZFC of the Zn1-xCoxO films for different Co concentrations at a magnetic field of 100, 1000 and 2000 Oe, respectively. For all of the curves, D decreases with increasing temperature, and films with lower Co concentrations exhibit larger magnetization difference between FC and ZFC. National University of Singapore 109 Magnetic moment (uB/Co) CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION 1.0 (a) 0.8 1000 Oe Co 0.02 Co 0.05 Co 0.09 0.6 0.4 0.2 0.0 100 200 300 400 Temperature (K) Magnetic moment (uB/Co) 0.12 0.11 (b) 0.10 100 Oe Co 0.02 Co 0.05 Co 0.27 0.09 0.08 0.025 0.020 0.015 0.010 100 200 300 400 Temperature (K) Fig. 6-6 Magnetic moment per Co atom dependence on temperature of the Zn1-xCoxO thin films with different Co concentrations obtained at the magnetic field of (a) 1000 Oe and (b) 100 Oe. National University of Singapore 110 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION Figure 6-6 displays magnetic moment per Co atom, denoted by m, dependence on temperature T for the films with different Co concentrations at a magnetic field of 1000 Oe and 100 Oe, respectively. The shapes of m-T curves are similar to those of M-T curves, as shown in Fig. 6-6(a). To show the features of m-T curves for the Zn1-xCoxO films with small Co concentrations, here we also present a m-T curve of the Zn1-xCoxO film with x = 0.27. The m-T curve exhibits a typical ferromagnetic behaviour [10], as shown in Fig. 6-6(b). For the film with a relatively lower Co concentration, the value of magnetic moment per Co is higher. It agrees with our previous result in Fig. 6-2. 6.2.4 Overview Studies of Magnetic Properties of Zn1-xCoxO Thin Films In this section, we will show some results of samples which were obtained under different experimental conditions and different Co concentrations. All these data were obtained from M-H loops from AGM. Figure 6-7(a-c) present an overview of the coercivity Hc, magnetization M and magnetic moment per Co atom (m) as a function of the Co concentration x. The data scattered in the Fig. 6-7(a) figures indicates that there is no apparent dependence of coercivity Hc on Co concentration x. From Fig. 6-7 (b), the contribution of Co to magnetization trends to decrease with increasing Co concentration, which coincides with our previous results in Fig. 6-2. The inset of Fig. 6-7(c) shows a linear relationship between the inverse of magnetic moment per Co atom and Co concentration. Using the effective concentration ( x s ), we can explain it by the formula National University of Singapore 111 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION x s = xM s / M , where x stands for the Co concentration, which would have led to a saturation moment M s if the Co spins were fully aligned [18]. Hence, the linear dependence of m −1 on x indicates that x s decreases with increasing x, which is similar to that observed in the magnetic semiconductor (Cd, Mn)Se. This indicates that the antiferromagnetic interactions counteract some part exchange interactions in the Zn1-xCoxO system. In the inset of Fig. 5-7(c), the fitting line for the dependence of m −1 on x is m −1 = −0.2 + 41.9 x . Hence we have x ≈ 0.005 for m −1 = . This implied that basically at x around 0.005, all the Co ions have the contribution to the total magnetic moment. National University of Singapore 112 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION 140 Hc (Oe) 120 100 80 60 40 20 0.00 0.05 0.00 0.05 0.10 0.15 0.20 0.25 Co concentration (%) 0.30 Magnetization (emu/cm ) 20 15 10 0.10 0.15 0.20 0.25 0.30 Co concentration (%)) (c) 20 -1 m [µB/Co] -1 m(µΒ/Co)) Linear Fit -1 m =-0.2+41.9x 15 10 0.0 0.1 0.2 0.3 Co concentration x 0.00 0.05 0.10 0.15 0.20 0.25 0.30 Co concentration x Fig. 6-7 Coercivity, magnetization and magnetic moment per Co (m) versus Co concentration x of series of Zn1-xCoxO thin films deposited at different DBPLD processing parameters (different growth temperatures and vacuum pressures). The inset shows m-1 dependence on x. National University of Singapore 113 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION Based on our experimental results, though experimental results are a little scattered, the trends are similar. Under the optimum experimental conditions mentioned before, usually the M-H loop with coecivity around 100 Oe could be obtained. 6.3 Transport Properties of Zn1-xCoxO Thin Films The resistivity and carrier density of the Zn1-xCoxO thin film with Co concentration of x =0.015 were measured at room temperature by Hall effect measurement using a Van der Pauw configuration. The transport properties were taken using the software of the Hall effect system (Lakeshore 7500). By proper settings, such as field profile as linear sweep with field reversal, maximum field as 8000G, removing the result values near field and taking the average values, the results from the Hall effect measurement could be obtained, as shown in Table 6-2. The resistivity is between 101.9 mohm-cm to 120 mohm-cm with the average resistivity around 102 mohm-cm, and the carrier density is between -1.71×1018 cm-3 to -2.98×1018 cm-3 with the average carrier density about -2.2×1018 cm-3. It can been seen that the film is n-type semiconductor. To confirm its semiconductor behaviour, we tested its electrical properties at 77 K. It was found that the film resistivity increases while its carrier density decreases slightly as the temperature decreases to 77 K (refer to Table 6-2). This implied that the film shows n-type semiconductor properties. National University of Singapore 114 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION Table 6-2 Electrical properties obtained from the Hall measurement of the film Zn0.985Co0.015O Testing temperature Type (K) Resistivity Carrier density (mΩ-cm) (cm-3) Room temperature n 102 2.2x1018 77 n 167 1.9x1018 Figure 6-8 displays the variation in the Hall resistivity and carrier density of the thin films as a function of Co concentration. The Hall resistivity tended to increase, while carrier density tended to decrease with Co concentration until a certain Co concentration is achieved. However the Hall resistivity decreased with the further increase of the Co concentration; namely the Hall resistivity tended to decrease with carrier density especially at low Co concentration, which correspond to the general relationship of resistivity and carrier density in semiconductors [19]. With increasing Co concentration, the compensation of oxygen deficiency is enhanced, thus the carrier density decreased. Hence, the Hall resistivity increased with Co concentration. However, further increasing Co concentration did not result in the further increase of the resistivity; instead, the resistivity decreased with Co concentration. From our previous HRTEM data (refer to section 5.3 for details), it follows that when more Co are incorporated, the lattice crystal structure tends to change and more defects are induced. The presence of defects may result in the formation of precipitates. The transport behaviors related to these metal precipitates may be understood by a hopping conduction mechanism over a percolation threshold [20]. The term “percolation” was used, conceptually. The simplest problem in that theory are lattice problems, one great National University of Singapore 115 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION importance of which is the notion of a “percolation threshold”, which is defined as the upper limit of the values, which varies with concentration [20]. According to the theory, the connectivity of a pair of impurities depends on their separation. Dependence of the logarithm of the conductivity on the “mean” separation between impurities, showing that resistance between any two sites depends on their separation. With gradually increases concentration until the condition of percolation over conducting resistances reaches. This obviously occurs at percolation threshold. Thus, the resistivity of the films peak at Co concentration of x = 0.1 and decreases with increasing Co concentration beyond this concentration. At this point, the strongest localization of electrons occurred due to the highest degree of disorder of the lattice. In Fig. 6-8, except for the point at x = 0.015, Hall mobility decreases with increasing Co concentration until x = 0.1. For x > 0.1, Hall mobility increases with Co ρH(Ω cm) concentration, indicating more dopant ions scattering in this range. 0.6 resistivity 0.4 Excimer Laser 10 Hz, 3J/cm -5 5x10 Torr o 650 C 0.2 µH (cm /VS) N (1/cm ) 18 6.0x10 18 4.0x10 18 2.0x10 0.0 density 40 30 20 10 0.00 mobility 0.05 0.10 0.15 0.20 0.25 Co concentration x Fig. 6-8 Variation in Hall resistivity, carrier density and Hall mobility with Co concentration x of Zn1-xCoxO thin films. National University of Singapore 116 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION In the following, more evidences will be given. Figure 6-9 shows the variation in the Hall resistivity with Co concentration of Zn1-xCoxO thin films grown at 650 ℃ and 750 ℃. Both the films grown at 650 ℃ and 750 ℃ have the similar variety in resistivity. It can be seen that the films obtained at 650 ℃ have higher resistivitities. When grown at 750 ℃, the film trends to have larger grain size and less crystal defects, thus less electron traps. Larger carrier concentration leads to lower resistivity. Excimer Laser 10 Hz, 3J/cm -5 5x10 Torr o 650 C o 650 C o 750 C Hall Resistivity (Ω cm) 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.00 0.05 0.10 0.15 0.20 Co concentration x 0.25 Fig. 6-9 Variation in Hall resistivity with Co concentration of Zn1-xCoxO thin films grown at 650 ℃ and 750 ℃. Using Figure 6-10, we analyze the resistance dependence on temperature (T) and found that for temperature above 80 K, the resistance decreases with increasing temperature. The inset of Fig. 6-10 shows a plot of the logarithm of the resistance National University of Singapore 117 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION versus 1/T. When the temperature is above 80 K, the graph of the logarithm of the resistance versus 1/T shows a non-linear dependence. This indicates that more than one mechanism is involved in this temperature range. The kink near 160 K divide the curve into two parts which are quasi-linear. From these quasi-linear parts, we can establish the activation energy in the range of 80 K -160 K and 160 K - 300 K of the Zn1-xCoxO thin film. Refer to Table 6-3 for details. The conductance can be expressed by the simple form [21]: G = G exp[ − Ec − Ed ] kT , where E a = E c − E d stands for the activation energy and G0 is a temperature independent factor. Therefore, the thermal activation energies of conductance can be estimated from the nearly linear segments near 300 K, as listed in Table 6-3. From Table 6-3 one can see that activation energy in the range of 80 - 160 K is smaller than that obtained in the range near 300 K. From the physical point of view, when the Fermi level falls in the localized region, charge transport occurs either via thermal activation of carriers into the delocalized state region or by activated hopping between localized states [20, pp. 36]. The conductivity of an extrinsic semiconductor doped with donors depends on the electron concentration and the drift mobility. In our case, activated hopping dominates the charge transport in the range of 80 - 160 K compared with that in the range of 160 – 300 K. Transport via thermal activation is easier when temperature increases. The activation energy corresponding to hopping conduction is smaller compared to that corresponding to band conduction (thermal activation). Hence, when temperature decreased, the activation energy decreased. From Table 6-3, National University of Singapore 118 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION we can also observe that the thermal activation energy decreases with increasing Co concentration in the range of 160 – 300 K. The activation energy for hopping conduction is due to the dispersion of impurity levels. The dependence on Co concentration can be explained by the mechanism of activated hopping between localized states using the variation in the probability of a jump between two impurities [20, pp. 36]. 8.6 8.4 ln(ρ) R(T)/R(300k) 2.0 8.2 8.0 7.8 1.5 0.003 0.006 0.009 0.012 -1 T (1/K) 1.0 Co 0.09 50 100 150 200 250 300 Temperature (K) Fig. 6-10 A typical resistance dependence on temperature for Zn1-xCoxO thin film at x = 0.09. Inset shows the plot of the logarithm of the resistance versus 1/T. The straight lines are fitting lines. Table 6-3 Evaluated activation energy values (eV) of Zn1-xCoxO at different temperatures T Temperature (K) x = 0.02 x = 0.09 x = 0.2 80-160 5.52 4.31 5.04 160-300 24.4 16.6 14.71 National University of Singapore 119 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION 6.4 Summary In this chapter, we studied the magnetic and electrical properties of Zn1-xCoxO thin films and obtained the following conclusions: z Magnetism could be realized by even a very low Co concentration for Zn1-xCoxO thin films. However, the improvement of magnetism was limited. The Curie temperature is higher than 400 K. z The magnetic moment per Co atom decreases with increasing Co concentration. When the Co concentration is over 0.1, magnetic moment per Co atom stops decreasing. The magnetization decreases with decreasing temperature. Deviation between the ZFC and the FC curves at a lower magnetic field was observed and the point at which ZFC and FC start to deviate tends to shift toward low temperatures for large magnetic fields. Some spin glass behaviors can be observed. z There is no evidence to show the apparent dependence of Hc on Co concentration z The films have doped-semiconductor properties. The Hall resistivity tended to increase, while carrier density tended to decrease with Co concentration until a certain Co concentration is achieved. However the Hall resistivity decreased with the further increase of the Co concentration; namely the Hall resistivity tended to decrease with carrier density especially at low Co concentration. The transport behaviors of the films can be understood by hopping mechanisms. Activation energy in the range of 80 - 160 K is smaller than that obtained in the range near 300 K. The thermal activation energy of Zn1-xCoxO decreases with increasing Co concentration in the range of 160 – 300 K. National University of Singapore 120 [...]... 0.5 0.4 0 .3 0.2 0.1 0.0 0 50 100 150 200 250 30 0 35 0 400 450 0 50 100 Temperature (K) 150 200 250 30 0 35 0 Temperature (K) 7 (c) 2000 Oe 0.2 c3 0.05 c4 0.09 3 6 5 4 3 2 1 -50 (f) 2000 Oe D M (emu/cm ) 3 Magnetization (emu/cm ) 8 0.1 0.0 0 50 100 150 200 250 30 0 35 0 400 450 Temperature (K) 0 50 100 150 200 250 30 0 35 0 Temperature (K) Fig 6-5 ZFC/FC curves measured at (a) 100 Oe, (b) 1000 Oe and (c) 2000...CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION (a) 0.7 0.6 (d) 0 .3 100 Oe 100 Oe Co 0.02 Co 0.05 Co 0.09 3 D M (emu/cm ) 3 Magnetization (emu/cm ) 0.8 0.5 0.4 0 .3 0.2 0.1 0.2 0.1 0.0 0.0 -50 0 50 100 150 200 250 30 0 35 0 400 450 0 50 100 6 200 250 30 0 35 0 0.8 (b) 1000 Oe 5 0.7 D M (emu/cm ) 4 3 2 1 0 -50 (e) 1000 Oe 0.6 Co 0.02 Co 0.05 Co 0.09 3 3 Magnetization (emu/cm ) 150... 400 36 0 0.09 38 0 36 0 35 0 From Fig 6-5(b), we can see that when Co concentration increases from 0.02 to 0.09, the magnetization obtained at 1000 Oe was observed to decrease, which coincides with the previous results in Fig 6-2 Figure 6-5(d-f) show the difference between FC and ZFC of the Zn1-xCoxO films for different Co concentrations at a magnetic field of 100, 1000 and 2000 Oe, respectively For all of. .. 0.05 0.10 0.15 0.20 0.25 0 .30 Co concentration x Fig 6-7 Coercivity, magnetization and magnetic moment per Co (m) versus Co concentration x of series of Zn1-xCoxO thin films deposited at different DBPLD processing parameters (different growth temperatures and vacuum pressures) The inset shows m-1 dependence on x National University of Singapore 1 13 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co... be obtained 6 .3 Transport Properties of Zn1-xCoxO Thin Films The resistivity and carrier density of the Zn1-xCoxO thin film with Co concentration of x =0.015 were measured at room temperature by Hall effect measurement using a Van der Pauw configuration The transport properties were taken using the software of the Hall effect system (Lakeshore 7500) By proper settings, such as field profile as linear... CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION Figure 6-5(a-c) shows the ZFC and FC curves of Zn1-xCoxO films with different Co concentrations at respective magnetic fields of 100, 1000 and 2000 Oe All show positive curvatures The discrepant point at which ZFC and FC start to deviate can be observed for all the curves, and these points decreases with increasing magnetic field From... 0.08 0.025 0.020 0.015 0.010 0 100 200 30 0 400 Temperature (K) Fig 6-6 Magnetic moment per Co atom dependence on temperature of the Zn1-xCoxO thin films with different Co concentrations obtained at the magnetic field of (a) 1000 Oe and (b) 100 Oe National University of Singapore 110 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION Figure 6-6 displays magnetic moment per Co atom, denoted... great National University of Singapore 115 CHAPTER 6: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON Co CONCENTRATION importance of which is the notion of a “percolation threshold”, which is defined as the upper limit of the values, which varies with concentration [20] According to the theory, the connectivity of a pair of impurities depends on their separation Dependence of the logarithm of the conductivity on... range of 80 K -160 K and 160 K - 30 0 K of the Zn1-xCoxO thin film Refer to Table 6 -3 for details The conductance can be expressed by the simple form [21]: G = G 0 exp[ − Ec − Ed ] kT , where E a = E c − E d stands for the activation energy and G0 is a temperature independent factor Therefore, the thermal activation energies of conductance can be estimated from the nearly linear segments near 30 0 K,... 6 -3 From Table 6 -3 one can see that activation energy in the range of 80 - 160 K is smaller than that obtained in the range near 30 0 K From the physical point of view, when the Fermi level falls in the localized region, charge transport occurs either via thermal activation of carriers into the delocalized state region or by activated hopping between localized states [20, pp 36 ] The conductivity of . -4000 -30 00 -2000 -1000 0 1000 2000 30 00 4000 -15 -10 -5 0 5 10 15 -200 0 200 -2 0 2 30 0 K 100 K 30 K M (emu/cm 3 ) Magnetic Field (Oe) Co 0.02 30 0 K Magnetization (emu/cm 3 ) Magnetic. presence of hysteresis and remanence for the samples measured at 30 0, 100 and 30 K; (b) Hc dependence on temperature of the film with Co concentration of x = 0.02. National University of Singapore. (emu/cm 3 ) Temperature (K) 100 Oe (d) -50 0 50 100 150 200 250 30 0 35 0 400 450 0 1 2 3 4 5 6 Magnetization (emu/cm 3 ) Temperature (K) (b) 1000 Oe 0 50 100 150 200 250 30 0 35 0 0.0 0.1 0.2 0 .3 0.4 0.5 0.6 0.7 0.8