Miyeon Cheona,
Yong Chan Chob,
Chae-Ryong Choc,
Chul Hong Park*d and
Se-Young Jeong*ef
aCrystal Bank Research Institute, Pusan National University, Miryang, 50463, Republic of Korea
bKorea Research Institute of Standard and Science, Daejeon 34113, Republic of Korea
cDepartment of Nanoenergy Engineering, Pusan National University, Busan 46241, Republic of Korea
dDepartment of Physics Education, Pusan National University, Busan, 46241, Republic of Korea. E-mail: cpark@pusan.ac.kr; Fax: +82-55-353-1314; Tel: +82-55-350-5273
eDepartment of Cogno-Mechatronics Engineering, Pusan National University, Busan, 46241, Republic of Korea. E-mail: syjeong@pusan.ac.kr
fDepartment of Optics and Mechatronics Engineering, Pusan National University, Miryang, 50643, Republic of Korea
First published on 5th October 2016
The magnetoresistance of a hydrogenated Zn0.8Co0.2O film with an inverted thin film transistor structure was measured at 7 K, in magnetic fields up to 3 T in order to verify the magnetoresistance dependency on carrier density. The gate voltage-dependent magnetoresistance was measured between −15 V and 15 V. A large positive magnetoresistance was identified at all gate biases. Changes in positive gate voltage did not result in significant changes in the magnetoresistance; however, an increase in negative gate voltage resulted in an increase in magnetoresistance and zero field resistivity. The magnetoresistance comprises a positive component, as a result of quantum conductivity correction due to the s–d exchange interaction, and a negative component induced by weak localization. The gate voltage-dependence of the positive and negative magnetoresistances was described by the density of states for ZnCoO:H.
Magnetoresistance in ZnO systems comprises both positive and negative components, with or without TM-doping.13,16–18 Negative magnetoresistance (nMR) results from suppression of weak localization, even in strongly localized systems,19,20 or 3rd order expansion of the spin-exchange Hamiltonian between localized moments and itinerant electrons.13,21 Positive magnetoresistance (pMR) in TM-doped ZnO and TiO2 has been attributed to the spin-splitting effect of the conduction band, owing to the strong s–d exchange interaction between carriers and doped TM atoms.18,22,23 Others have suggested that the pMR is developed in a two band model from the action of Lorentz force on the mobile carriers.13,24,25
Understanding the origins of MR in TM-doped oxide semiconductors is necessary when solving theoretical issues and designing spintronic devices. In this study, we examined the MR and electrical properties of Co-doped ZnO with various carrier densities.
Carrier density can be controlled through chemical doping methods, such as chemical substitution and intercalation. However, it is difficult to synthesize a variety of samples with precisely controlled doping ratios. Doped elements inherently introduce disorder and may, therefore, introduce an increase in resistivity. Carrier density can also be controlled by an electric field introduced by an applied gate voltage.26,27 This method is free from disorder. The application of a gate voltage changes the density of carriers; i.e., changes the position of the Fermi energy, and induces carrier accumulation or depletion as a function of the bias. This significant carrier accumulation has led to various achievements in condensed matter physics, such as metal–insulator transitions in ZnO.28
In this work, we report the magnetoresistance of hydrogenated Co-doped ZnO thin films of varying carrier density, by changing the bottom gate at low temperatures. We discuss the transverse magnetoresistance of ZnCoO:H, in which the magnetic field is applied perpendicular to the input current.
A schematic illustration of the Hall bar used in magnetotransport measurements is shown in Fig. 1(a). The magnetotransport properties of the sample at 7 K were measured using a Physical Properties Measurement System (PPMS-9, Quantum Design), two source meters (Keithley 2425C, 2635A), a nanovoltmeter (Keithley 2182A), and a switcher (Keithley 7001). During the magnetotransport measurements, the magnetic field was swept between −3 T and 3 T, perpendicular to both the sample surface and the direction of the current, with a fixed gate voltage. To eliminate spurious voltages, the magnetoresistance voltage Vxx(H) was calculated by Vxx(H) = ½[Vxx(H) + Vxx(−H)]. The voltage was averaged as the direction of the current was changed for each magnetic field strength. The longitudinal resistivity ρxx is defined as , where w, a, and t are the channel width, the distance between the contacts, and the film thickness, respectively. The leakage current was less than 2 nA when the gate voltage was swept, which is much less than the 1 μA that was applied during the measurements.
Fig. 1(b) shows the density of states (DOS) of the hydrogenated ZnCoO. The red and blue colors represent the occupied majority and minority spin states, respectively, and the white color represents the empty states. The localized states that originate from the hybridization of H-s and Co-t2g orbitals are in the conduction band. The electron mobility can be reduced significantly by the presence of hydrogen, and electrons can be transported by hopping between the localized states.29
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Fig. 2 (a) Resistivity of Zn0.8Co0.2O:H as a function of the magnetic field and various gate voltages. The magnetic fields were applied perpendicular to the Zn0.8Co0.2O:H film. The solid curves are the result of fitting with eqn (5). (b) Gate voltage-dependence of the resistivity at 0 T and 3 T. The resistivity increased with increase of the absolute value of the negative gate voltage. |
The magnetoresistance is defined as where ρ(H) and ρ(0) are the resistivity with and without an applied magnetic field, respectively. A large positive magnetoresistance is shown at each gate voltage, as shown in Fig. 3(a).
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Fig. 3 (a) Measured (symbol) and simulated (solid curves) transverse magnetoresistance of Zn0.8Co0.2O:H with zero gate voltage. The solid curves are the fitted results using eqn (5). The experimental data for the other gate voltages fitted well to eqn (5), which suggests that there is excellent agreement between the theoretical and experimental MR. The red and blue curves are the pMR and nMR, respectively. (b) Gate voltage-dependence of the magnetoresistance as 3 T. The positive and negative magnetoresistance were calculated using eqn (5), and the fitting parameters in Fig. 4. |
The magnetoresistance of ZnCoO:H was studied through the use of a model that considers both negative and positive components. Several groups have observed negative magnetoresistance in ZnO systems, and the mechanism was ascribed to the weak-localization effect.16,18,20 The field suppression of the constructive interference in weak localization is one possible source of the small negative MR. Owing to the thickness, the film is expected to exhibit three-dimensional (3-D) transport. The negative MR due to the field-suppressed weak localization in 3-D systems can be expressed as follows:19,20
![]() | (1) |
![]() | (2) |
An exchange interaction between the s-orbit and d-orbit has been proposed for a large pMR.16,30 In this model, the pMR arises from the quantum correction of the conductivity on the disorder-modified electron–electron interaction due to the s–d exchange interaction.
The change of conductivity in a 3-D system can be modeled as follows:
![]() | (3) |
The analytical expression
![]() | (4) |
The parameter β = [gμBH + xeffαN0SBS(T, H)]/kT represents the spin-splitting of the conduction band comprising the Zeeman energy gμBH and the s–d contribution xeffαN0SBS(T, H). Here, the g-factor is given by g = 2, and the s–d exchange energy is αN0 = 0.2 eV;32 xeff is the effective magnetic ion mole fraction, and BS(T, H) is the Brillouin function, in which S = 3/2 for Co2+.
The total magnetoresistance is the sum of the negative and positive components, as given by
![]() | (5) |
In the simulation, the fitting parameters were LTh for nMR, and Fσ and xeff for pMR.
Eqn (5) well fit the experimental data, as shown in Fig. 2, which suggests excellent agreement between the theoretical and experimental MR behaviors. The fitting parameters for the least square fit are shown in Fig. 4.
The magnetoresistance for each magnetic field with zero gate bias is shown in Fig. 3(a); the red curve is the calculated pMR, which is a product of the quantum correction of the s–d spin splitting on the disorder-modified electron–electron interaction. The blue curve is the nMR due to the suppression of weak localization. The green curve is the magnetoresistance, specifically, the sum of pMR and nMR.
The calculated pMR did not change with gate voltage, as shown in Fig. 3(b). However, the calculated nMR decreased as the gate voltage decreases to −15 V. Because pMR remains constant and nMR decreases, MR also decreases as the gate voltage decreases to −15 V. The fact that pMR stays constant indicates that the gate voltages that were applied were insufficient to change the s–d spin splitting which results pMR.
The gate voltage-dependence of MR at 3 T is shown in Fig. 3(b). The positive and negative MR were calculated from eqn (5), and the fitting parameters in Fig. 4. The magnitude of MR at 3 T increased by 22.9% from 0.310 to 0.381 as the gate voltage was lowered from 0 V to −15 V. The changes in resistivity and MR at 0 T and 3 T are summarized in Table 1. For positive gate voltages, the MR did not change significantly. The gate voltage-dependence of the resistivity and MR were similar, but the change in MR was less significant. When the magnitude of the negative gate voltage increased, nMR was reduced to zero due to electron depletion, and pMR did not change. It was found that metallic Co-doped ZnO showed a negative MR, and insulating Co-doped ZnO samples showed a positive MR. The increase in pMR with decreasing carrier density may be the result of the larger s-d spin splitting effect at lower conduction electron concentrations.17 This study shows that pMR does not depend on the carrier concentration and an increase in MR is not caused by an increase in pMR, but by a decrease in nMR.
ρ (Ω·cm)@0 T | ρ (Ω·cm)@3 T | MR@3 T | pMR@3 T | nMR@3 T | |
---|---|---|---|---|---|
−15 V | 6.383 (+37.3%) | 8.876 (+45.6%) | 0.381 (+22.9%) | 0.421 (−2.10%) | −0.04 (−66.3%) |
0 V | 4.648 | 6.096 | 0.310 | 0.429 | −0.118 |
15 V | 4.260 (−8.35%) | 5.583 (−8.42%) | 0.309 (−0.326%) | 0.425 (−0.93%) | −0.116 (−1.69%) |
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