Electrical Transport Properties of SnO under High Pressure - The

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Electrical Transport Properties of SnO under High Pressure Junkai Zhang,† Yonghao Han,† Cailong Liu,† Wanbin Ren,† Yan Li,† Qinglin Wang,† Ningning Su,† Yuqiang Li,† Boheng Ma,‡ Yanzhang Ma,§ and Chunxiao Gao*,† †

State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People's Republic of China A James Clark School of Engineering, University of Maryland, College Park, Maryland 20742, United States § Department of Mechanical Engineering, Texas Tech University, Lubbock, Texas 79409, United States ‡

ABSTRACT: We carried out the accurate in situ Hall-effect measurements, the temperature dependence of electrical resistivity measurements and the first-principles calculations in SnO under high pressure. The results of Hall-effect measurements display the carrier transport behavior of SnO under pressure, which indicates that SnO undergoes a carrier-type inversion around 1.3 GPa and an underlying phase transition at 2
3 GPa. In addition, the temperature dependence of electrical resistivity shows that SnO undergoes a semiconductor-to-metal transition around 5 GPa. The calculated band structures based on the first-principles method illustrate that the indirect band gap of SnO vanishes around 4 GPa. In particular, the results of total and partial density of states indicate that the closure of the indirect fundamental gap is mostly attributed to Sn-5s and 5p states hybridized with O-2p states at the Fermi level.

1. INTRODUCTION Tin monoxide (SnO) has attracted much attention due to its applications in many fields. SnO could be used as an active layer in gas sensors,1,2 as anode materials for lithium rechargeable batteries,3,4 and as a precursor for the production of SnO2.5 In particular, previous researches have indicated that SnO could be a p-type semiconductor with a more effective hole transport path and higher hole mobility.6,7 These particular properties label SnO as a promising p-type oxide semiconductor utilized for novel optoelectronic and electrical devices. For example, SnO could produce perfect p-type oxide thin-film transistors (TFTs) exhibiting field-effect mobilities of 1.3 cm2V
1s
1, on/off current ratios of ∼102, and threshold voltages of 4.8 V.7,8 At ambient pressure, SnO crystallizes in a tetragonal crystal structure. As shown in Figure 1, the tetragonal structure of SnO is layered with an AA . . . stacking sequence of slabs, and each Sn atom is at the apex of a square pyramid whose base is formed by four oxygen atoms. All Sn-nearest oxygen neighbor distances are equal to 2.22 Å.9,10 Relatively weak van der Waals forces exist among the Sn
O
Sn slabs along the c-axis.11 Under high pressure, early X-ray investigations indicated that SnO underwent a phase transition from the tetragonal phase (α-SnO) to the lower-symmetry orthorhombic phase (γ-SnO) in the pressure range of 1.5
2.5 GPa.12
14 Li et al.15 studied the pressureinduced phase transition by evaluating the enthalpy difference of the two crystal structures and suggested the existence of γ-SnO in SnO around 1.1 GPa. However, Wang et al.10 revealed no evidence of the pressure-induced phase transition up to 20 GPa by angle-dispersive X-ray diffraction and high-pressure Raman measurements. The most recent experiments by Giefers et al.16 r 2011 American Chemical Society

Figure 1. Tetragonal crystal structure of SnO at ambient pressure.

also showed no high-pressure phase of SnO up to 51 GPa except for a orthorhombic distorted phase under nonhydrostatic conditions by energy-dispersive X-ray diffraction and edge X-ray absorption fine structure measurements. As stated above, the phase transition of SnO from α-SnO to γ-SnO under high pressure is still questionable. The electrical resistivity and other electrical transport parameters may imply a phase transition or provide some clues of the changes in the electrical configuration. Thus, whether or not the electrical transport properties of SnO reflect a phase transition becomes one of our research purposes. In addition, whether or not the metal oxide semiconductor SnO becomes metallized under high pressure is another meaningful subject. Infrared reflectivity measurements by Wang et al.10 predicted that a semiconductor-to-metal transition in SnO might Received: July 1, 2011 Revised: September 16, 2011 Published: September 20, 2011 20710

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Figure 3. Pressure dependence of electrical resistivity of SnO at room temperature. The vertical dashed line indicates the phase transition.

Figure 2. (a) XPS survey spectra of the sample. (b) Configuration of a microcircuit on a diamond anvil. (c) Cross section of the designed DAC. Here A is Mo electrode, B is alumina layer, C is ruby, D is the gasket, and 1, 2, 3, 4 are the lead wires.

occur around 5 GPa. Later calculations also revealed that the indirect minimal gap could close at a pressure of 5 GPa.17 In fact, we still require more direct and effective measurements to prove the metallization of SnO at certain pressures. In the present study, we carried out the in situ Hall-effect measurements under high pressure to verify whether a phase transition existed in SnO and investigated the temperature dependence of electrical resistivity to verify whether SnO underwent a semiconductor-to-metal transition at certain pressures. Meanwhile, we also discussed many electrical transport properties of SnO under given conditions in order to find its new characteristics. In addition, we tried to observe the changes in band structures and density of states under high pressure by the calculations based on the first-principles method.

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS High-pressure experiments were carried out using a nonmagnetic diamond anvil cell (DAC) with an anvil culet of 300 μm in diameter. A nonmagnetic rhenium flake used as a gasket was indented to a thickness of 50 μm. A hole with 100 μm in diameter was drilled in the center of the indentation by an electric discharge machine and served as a sample chamber, and alumina film with thickness of 2 μm was sputtered on the gasket for insulation. Polycrystalline SnO powder brought from Alfa Aesar Co. with a stated purity of 99.9% was used as the sample. A quantitative analysis of the X-ray photoelectron spectrum has confirmed that the sample is stoichiometric with the [O]/[Sn] atomic ratio equal to 1.06, as shown in Figure 2(a). The sample thickness under pressure was determined by a micrometer with a precision of 0.5 μm, and the deformation of diamond anvils was taken into account.18 No pressure medium was loaded in order to avoid the introduction of impurities for measurement of electrical parameters and to ensure good electrical contact. A ruby fluorescence method was used for pressure calibration. Van der Pauw electrodes were integrated on one diamond anvil for the high-pressure experiments. Fabrication of the detecting microcircuit on a diamond anvil has been reported previously.19,20

Figure 2(b),(c) shows the microcircuit and the sectional view of DAC to illustrate the sample loading method. In situ Hall-effect measurements under high pressure were conducted using the current reversal method to avoid thermoelectric offsets where current, typically 100 mA, was sourced by a Keithley 2400 SourceMeter and the voltage was measured by a Keithley 2700 multimeter. All instruments were connected to a computer via a Keithley Kusb-488 interface adapter and a general purpose interface bus. The measurement process was automatically performed according to the Van der Pauw method. The Hall-effect measurements were made in magnetic fields of 1.2T. The temperature dependence of electrical resistivity measurements were conducted by placing the DAC into a tropical drying cabinet for more than 10 min to achieve thermal equilibrium. The electrical current was also provided by a KEITHLET 2400 Source Meter, and the voltage drop was measured by a KEITHLEY 2700 Multimeter. The first-principles calculations were performed on the basis of the density functional theory (DFT) and pseudopotential method.21 All electrical structure calculations were implemented using the CASTEP code.22 The electron-ion interaction was described by Vanderbilt-type ultrasoft pseudopotentials.23 The exchange and correlation terms were described with generalized gradient approximations (GGA) in the scheme of Perdew
Burke
Ernzerh (PBE) parametrization.24 The geometric optimization of the unit cell was carried out with the BFGS minimization algorithm.22 The configurations of Sn and O were 5s25p2 and 2s22p4, respectively. Convergence of the total energy was checked with respect to both the plane wave cutoff energy and k-point density, which were obtained using the MonkhorstPack method.25 The one-electron valence state was expanded on the basis of plane wave with the cutoff energy of 500 and 480 eV, respectively. Integration in the Brillouin zone was performed using special k points generated with 9  9  7 and 9  8  9 mesh parameter grids for α-SnO and γ-SnO. The space group of α-SnO was P4/nmm (no.129) at 0 GPa with the calculated lattice parameters a = b = 3.80 Å and c = 4.83 Å,26 and the space group of γ-SnO was Pm21n (no. 31) at 2.6 GPa with the calculated lattice parameters a = 3.82 Å, b = 3.81 Å, and c = 4.62 Å.15

3. RESULTS AND DISCUSSION 3.1. In Situ Hall-Effect Measurements under High Pressure. The pressure dependence of electrical resistivity at room

temperature is shown in Figure 3. At ambient pressure, the electrical resistivity of sample is 29.6 Ω 3 cm. During the compression, the electrical resistivity of SnO decreases with pressure increasing. However, a significant inflection point of electrical 20711

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Figure 5. Temperature dependence of electrical resistivity of SnO plotted in Arrhenius format at several pressures.

Figure 4. Pressure dependence of Hall coefficient, carrier concentration, and mobility of SnO at room temperature. The vertical dashed line indicates the carrier-type inversion.

resistivity appears around 2 GPa. After this inflection point, the electrical resistivity decreases relatively slowly with pressure increasing. During the decompression, the abnormal inflection point of electrical resistivity reoccurs around 4 GPa and the electrical resistivity indeed returns to its original magnitude when the pressure returns to ambient. Figure 4 shows the results of Hall-effect measurements in magnetic fields of 1.2 T. At ambient pressure, the Hall coefficient (RH), carrier concentration (n), and carrier mobility (μ) are 0.86 cm3C1
, 5.09  1020 cm
3 and 0.56 cm2V
1s
1, respectively. During the compression, these three electrical transport parameters also show discontinuous changes at 2
3 GPa. From ambient pressure to 2 GPa, RH decreases with pressure increasing. Specifically, RH changes its sign from positive to negative around 1.3 GPa, indicating that SnO undergoes a carrier-type inversion from p- to n-type. This reveals that p-type thin-film transistors based on SnO are inapplicable for the highpressure environments. From 2 to 7 GPa, RH turns to increase with pressure increasing. Apparently, RH shows the existence of a significant inflection point around 2 GPa and an abnormal leap point around 3 GPa. Above 7 GPa, RH remains nearly constant up to 12 GPa. From ambient pressure to 2 GPa, n first increases with pressure increasing up to 1.3 GPa, and then begins to decrease. Subsequently, n increases rapidly with pressure increasing up to 7 GPa. In this pressure interval, the increase of n is the dominant effect that produces a decrease in electrical resistivity. Above 7 GPa, the variation of n becomes gentle, which is mostly caused that the excited carriers tend to be saturated. However, from ambient pressure to 2 GPa, μ decreases with pressure increasing

up to 1.3 GPa, and turns to increase abruptly, which is likely to indicate that the electrons with higher mobility27 start to prevail under pressure, corresponding to the Hall coefficient RH tends to inversion herein, it is further explicit that SnO undergoes the transition from hole- to electron-dominant conduction at 1.3 GPa. Sequentially, μ continues to increase with pressure increasing up to 3 GPa. From 3 to 7 GPa, μ decreases slowly with pressure increasing. Above 7 GPa, μ returns to increase with pressure increasing, which leads to a decrease in electrical resistivity in this pressure interval. Because the electrical transport behavior is related to the crystal structure, the significant changes in these electrical transport parameters (the electrical resistivity, Hall coefficient, carrier concentration and mobility) during the compression could imply the phase transition of SnO at 2
3 GPa, which is consistent with other studies.12
15 In addition, the changes in these electrical transport parameters are similar in two processes of the compression and the decompression. They also return to their original state when the pressure returns to ambient. Specifically, the Hall coefficient recovers after a pressure-circulation. These results demonstrate the reversibility of the phase transition and the carrier-type inversion. 3.2. Temperature Dependence of Electrical Resistivity Measurements. In order to determine whether or not SnO becomes metallized under high pressure, we carried out the temperature dependence of electrical resistivity measurements at several pressures. The results are plotted in the Arrhenius format and shown in Figure 5. Below 5.1 GPa, the electrical resistivity of SnO decreases with temperature increasing, indicating that SnO is a semiconductor. Above 5.1 GPa, the electrical resistivity shows a positive relationship with temperature, indicating that SnO becomes metallized at this pressure. Thus, the transition reflected by the abnormal electrical resistivity change is a typical semiconductor-to-metal transition. To obtain more information about the electrical transport properties, we studied the activation energy of SnO at several pressures. The relationship between the electrical resistivity and the temperature of semiconductor could be represented by the following equation: F ¼ F0 expðEi =2kB TÞ where F0 is a constant that depends partially upon the electron and hole effective masses; Ei is the activation energy; kB is Boltzmann constant; and T is the temperature. Further, Ei could be obtained by linearly fitting the plots of lnF vs 1000/T. When trying to obtain the activation energy of SnO, an ignorable phenomenon was found. As shown in Figure 5, we observed two different changes in electrical resistivity with temperature before the metallization of SnO. This phenomenon was further shown in an 20712

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Figure 6. Pressure dependence of activation energy of SnO in two different temperature regions: between 200 K and room temperature (solid circle), between 70 and 200 K (open circle). The inset shows the plots of lnF vs 1000/T at 1.2, 2.1, 2.8, and 4.3 GPa.

inset of Figure 6. The plots of electrical resistivity vs temperature at several pressures can be fitted by two lines. Between 70 and 200 K, the electrical resistivity decreases slowly with temperature increasing. However, between 200 K and room temperature, the electrical resistivity decreases relatively fast with temperature increasing. To gain a better understanding, we obtained two different activation energies by linearly fitting the plots of lnF vs 1000/T in these two different temperature regions, as shown in Figure 6. As well-known, commercially obtained SnO is still nonintrinsic semiconductor accompanied with the impurity levels existing in the band gap. Between 70 and 200 K, the activation energy is relatively low, as shown in Figure 6, so that the carriers are easily excited to a lower impurity level. Moreover, the excited carriers become saturated with temperature increasing. Between 200 K and room temperature, the carriers are not easily further excited to a higher impurity level because of the increase in activation energy. Thus, the slope of electrical resistivity vs temperature is steep. In addition, we find out that the initial inflection point (200 K) shifts slightly to the lower temperature with pressure increasing. It is proposed that this relates to the decrease in the energy separation between these two impurity levels with increasing pressure. The energy separations at several pressures are also labeled in Figure 6. In addition, the pressure dependence of activation energy in these two temperature regions expresses more messages. As shown in Figure 6, between 200 K and room temperature, Ei decreases with pressure increasing at an initial rate of ∼
8.41 meV/GPa, decreases more slowly above 2 GPa, and then extrapolates to zero around 5 GPa. Meanwhile, between 70 and 200 K, Ei has a synchronous variation. The decrease in these two activation energy with pressure increasing can both be explained by the fundamental properties of semiconductor. The slopes of Ei vs pressure express the effect of pressure on the energy barriers to the charge transport. Furthermore, the negative slopes indicate that the carrier transport could become easier with pressure increasing. Once Ei f 0, the typical behavior of a semiconductor would vanish, accompanied by the disappearance of energy barriers. In particular, the same inflection point (2 GPa) existing in these two plots of activation energy vs pressure also indicates that the phase transition of SnO occurred herein, which correlates significantly with our other experimental results. 3.3. Band Structure Calculation. Owing to the particular layered crystal structure and weak van der Waals interactions among the Sn
O
Sn layers, SnO is highly compressible in the direction perpendicular to the layers under high pressure. Some calculations10,28,29 revealed a significant shortening of the interlayer

Figure 7. (a) Calculated band structure of α-SnO along high-symmetry directions at ambient pressure. (b) Calculated total and partial density of states at ambient pressure.

Sn
Sn distances and suggested a large decrease of the indirect gap with decreasing interlayer spacing. Further, we deduced that the covalent binding with atoms becomes more remarkable, directly affecting the band edge crossing under pressure. To gain further insight into the effect of covalent binding on the semiconductor-to-metal transition of SnO, we performed a first-principles ultrasoft pseudopotential band structure calculations. The calculated band structure at ambient pressure along selected high-symmetry lines is shown in Figure 7(a). The zeropressure structural SnO is clearly a semiconductor with an indirect band gap of 0.3 eV, which is in agreement with other theoretical studies.30 The valence-band maximum (VBM) is found to be located at the Γ point, and the conduction-band minimum (CBM) is found to be located at the M point. From the PDOS at ambient pressure plotted in Figure 7(b), where the vertical dotted line is the Fermi level, we can see that in the lower VB in the range between
20 and
10 eV below the Fermi level, the O-2s states of SnO have a predominate contribution, showing the strong characteristic of lone electron pairs. The upper VB in the range between 0 and
10 eV is mainly divided into three regions, which are the low-energy, the moderate-energy and the high-energy regions. In the low-energy region, the Sn-5s states strongly hybridize with O-2p states. The moderate-energy region is mainly caused by the hybridizing Sn-5p states with O-2p states. The high-energy region is also predominately made up of antibonding Sn-5s and O-2p combinations. In addition, Sn-derived 5pz character is mixed into the occupied states near the top of the high-energy region. Above the Fermi level, the conduction band of SnO is dominated by Sn-5p states. However, there is still a considerable mixture with Sn-5s states (and, to a lesser extent, O-2p states). 20713

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of SnO reveal that the closure of the indirect fundamental gap results in the semiconductor-to-metal transition under high pressure, and the results of total and partial density of states indicate that the closure of the indirect fundamental gap is mostly caused by the interaction of Sn-5s and 5p states with O-2p states at the Fermi level.

’ AUTHOR INFORMATION Corresponding Author

*Phone: +86-431-85168878-601; Fax: +86-431-85168878-602; E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant Nos. 10874053, 11074094, 50802033, and 91014004), by the National Basic Research Program of China (Grant No. 2011CB808204). ’ REFERENCES

Figure 8. (a) Calculated band structure of γ-SnO along high-symmetry directions at 4 GPa. (b) Calculated total and partial density of states at 4 GPa.

With increasing pressure, the conduction band of SnO intrudes downward into the upper valence band. As shown in Figure 8(b), owing to the effect of external force, the Brillouin zone broadens with shrinking atomic distances, and the distribution of total and partial density of states trends to become scattered. Thus, the upper valence band broadens significantly toward the two opposite directions. Meanwhile, the conductor band broadens slightly toward the upper valence band, accompanied with the separation of each peak in the conductor band. Ultimately, the overlap of the three subpeaks of Sn-5s, Sn-5p, and O-2p states at the Fermi level directly results in the closure of the indirect fundamental gap, which indicates the transition from semiconductor to metal in SnO.

4. CONCLUSIONS In the present study, we have performed the in situ Hall-effect measurements, the temperature dependence of electrical resistivity measurements and the first-principles calculations in SnO under high pressure. A reversible carrier-type inversion of SnO around 1.3 GPa has been obtained by the changes in Hall coefficient. Subsequently, a reversible pressure-induced phase transition of SnO at 2
3 GPa has also been observed by the results of the pressure dependence of electrical resistivity, Hall coefficient, carrier concentration, and mobility. Furthermore, a semiconductor-to-metal transition of SnO around 5 GPa has been observed by the measurements of the temperature dependence of electrical resistivity at several pressures. And then two significantly different activation energies of SnO have been obtained and discussed, providing evidence for the phase transition at 2
3 GPa. In addition, the calculations on band structures

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