ARTICLE pubs.acs.org/JPCC
Lattice Dynamics and Crystalline Properties of Wurtzite Zn1xMgxO Powders under High Pressure Y. C. Lin,* C. L. Tseng, and W. C. Chou* Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan
C. H. Chia and T. C. Han Department of Applied Physics, National University of Kaohsiung, Kaohsiung 81148, Taiwan
J. L. Shen Department of Physics, Chung Yuan Christian University, Jhongli 32023, Taiwan ABSTRACT: This investigation systematically studies the lattice dynamics and crystalline properties in Zn1xMgxO using highpressure Raman spectroscopy. The incorporation of Mg and the application of external pressure cause distinct phonon vibrational behaviors in ZnO. Accordingly, the 202.7, 332.7, and 511.5 cm1 phonons, which have been controversially assigned, can be conclusively identified. Detailed Raman spectra reveal that the metallic phase transition of ZnO is complete by around 13.2 GPa, which pressure is found to decrease as the Mg content increases. Upon pressure release, an unusual hysteresis effect (>10.0 GPa) in Zn1xMgxO is observed. The degree of crystal ionicity and anisotropy importantly affects the phase transition pressure of Zn1xMgxO. Under ambient conditions, ZnO becomes more ionic upon the incorporation of Mg and becomes more covalent under higher pressure. These results are caused by the interplay between the pressure dependence of the high-frequency dielectric constant and Born’s transverse dynamical effective charge. The E1A1 splitting of the longitudinal and transverse optical phonons is analyzed, yielding insight into the pressure-dependent crystal anisotropy of Zn1xMgxO.
’ INTRODUCTION An urgent and global need for renewable energy sources has aroused widespread research interest in potential photovoltaic materials. Such research efforts may pave the way to the gradual phasing out of conventional and nuclear power. Oxides are promising owing to their natural availability, environmental stability, and ecologically friendly characteristics.1 Moreover, because of its large electronegativity, oxygen forms chemical bonds with almost all elements to give the corresponding oxides. Zinc oxide (ZnO) is a wide-bandgap (∼3.4 eV at 300 K) semiconductor and crystallizes preferentially in the hexagonal wurtzite structure. By comparison with ZnSe and GaN, the relatively strong polar binding, deep excitonic level (∼60 meV), and biocompatibility with organic systems of ZnO make it highly attractive for use in functional optoelectronic devices.27 Substitution of an increasing fraction of the Zn atoms by Mg can shift the band gap of Zn1xMgxO into the deep ultraviolet region.6,8 Additionally, the ionic radius of Mg2+ (0.57 Å) closely matches to that of Zn2+ (0.60 Å), making this ternary alloy a suitable barrier material in ZnO-based heterostructures and solar-blind devices.26 The technological importance of Zn1xMgxO has motivated detailed and fundamental studies as well as application-oriented research. Although the optical properties of Zn1xMgxO have been extensively investigated, the effects of both Mg and pressure r 2011 American Chemical Society
on the lattice dynamics and crystalline properties of Zn1xMgxO remain unexplored. Previous Raman studies of ZnO at ambient and high pressures have yielded somewhat unclear and contradictory results. (i) The assignments of the vibrational modes at 202.7 and 332.7 cm1 are contentious.9,10 Furthermore, origin of the phonon mode at 511.5 cm1, which appears only after intentional doping, is controversial.1113 (ii) Evidence of the pressure-dependent LO-TO splitting and transverse effective charge in ZnO is contradictory. Decremps et al. and Manjon et al. found that the LO-TO splitting and the transverse effective charge of the E1 mode in ZnO increase upon compression.14,15 However, Reparaz et al. demonstrated that the LO-TO splitting and the transverse effective charge decrease with increasing pressure in both A1 and E1 modes.16 (iii) Previous high-pressure Raman measurements have shown that the wurtzite-to-rocksalt phase transition of ZnO completes at 8.39.0 GPa.14,15,17 These values are far below the experimental values (above 12.8 GPa) that were obtained using high-resolution angular dispersive X-ray diffraction (XRD).18 Such a difference is probably caused by a weak Raman signal, large pressure interval, and an undetected Received: August 10, 2011 Revised: September 7, 2011 Published: September 08, 2011 19962
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Figure 2. Raman spectra of Zn1xMgxO (x = 0, 3.0, 7.0, and 10.0%) under ambient pressure. Figure 1. SEM images of ZnO (inset) and Zn0.90Mg0.10O powders.
low-frequency ( 0) are also found. The origins of these phonons are controversial and will be discussed later. The observed Raman phonon frequencies of ZnO correlate well with previous works.9,10,14,16 In this study, the optical phonons are 19963
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Table I. Zone-Center Optical Phonon Frequencies and LO-TO Splittings for Zn1-xMgxO phonon frequency ωi (cm1) phonon mode (i)
x = 0%
observable at both the Stokes and the anti-Stokes sides of each Raman spectrum. Moreover, the phonon frequencies are independent of excitation wavelength (457.9, 488.0, and 514.5 nm). Unlike the higher oriented c axes of bulks and films on specific substrates, the crystal axes of powder are randomly tilted relative to the laser excitation polarization. Therefore, the peculiar crystalline geometries of Zn1xMgxO powders lead to our observation of all Raman-active phonon modes. To gain insight into the lattice dynamics of Zn1xMgxO, parts ac of Figure 4 show the highlighted zone-center Raman spectra. Table I presents the phonon frequency and the LO-TO splitting of each phonon. Clearly, as Mg is substituted into ZnO, the A1, E1, and Elow [Figure 4b] phonons together with the acoustic 2 phonons shift to higher frequencies. However, the Ehigh [Figure 4c] 2 and 332.7 cm1 phonons behave oppositely. These phonon shifts are accompanied by intensity quenching and line width broadening, which could be attributed to Mg-induced translational crystal asymmetry and alloy fluctuations. Additionally, as indicated in Table I, the LO-TO splittings of both A1 and E1 modes increase with Mg content. The results imply that incorporating Mg makes ZnO more ionic. This behavior is consistent with the fact that the ionicity (fi; Phillips’ ionic scale) of MgO (0.841) exceeds that of ZnO (0.616).23 Generally, the substitution of Mg for Zn atoms decreases the reduced mass of the oscillator, shifting the Raman phonons to and 332.7 cm1 phonons higher frequencies. However, the Ehigh 2 phonon correshift to lower frequencies. This is because the Ehigh 2 sponds mainly to the vibrations of the oxygen (lighter) atoms,24 so the difference between atomic masses, mMg(24) < mZn(65), cannot phonon frequency. As be responsible for the decrease of the Ehigh 2 previously demonstrated, the lattice constant a of Zn1xMgxO increases monotonically with x.25 Therefore, the lattice expansion in the ab plane accounts for the Ehigh 2 phonon softening (Figure 3). A similar result is obtained for the 332.7 cm1 phonon, which has been previously attributed to either the transverse acoustic overtone at the 9,10 low As x K-M-∑ point or the difference between Ehigh 2 and E2 in ZnO. is increased, the phonon frequency correlates closely with the difand Elow ference between Ehigh 2 2 . Moreover, the phonon shift agrees and Ehigh well with the opposite vibrational behaviors of Elow 2 2 high phonons. These experimental results indicate that the E2 - Elow 2 complex phonon is most likely the origin of the 332.7 cm1 mode.
x = 7.0%
x = 10.0%
E2(low)
98.8
99.4
100.4
101.2
E2(high) E2(low)
332.7
331.6
331.0
330.5
A1(TO) E1(TO)
380.6 411.1
381.8 412.1
382.7 413.2
383.5 414.0
E2(high)
438.3
437.5
D
Figure 4. Highlighted zone-center Raman spectra of (a) A1, E1, D, and 332.7 cm1 phonons, including Lorentzian fits for LO modes, and (b) and (c) Ehigh phonons of Zn1xMgxO under ambient pressure. Elow 2 2 Dashed lines and solid circles are guides for the eyes.
x = 3.0%
438.0
437.7
511.5
511.5
511.5 585.2
A1(LO)
574.4
577.3
582.8
E1(LO)
584.0
589.4
597.5
601.3 1
phonon frequency Δω (cm )
phonon splitting A1(LO) A1(TO)
193.8
195.5
200.1
201.7
E1(LO) E1(TO)
172.9
177.3
184.3
187.3
Such an assignment will be further verified by making measurements under high pressure. In addition to the host Raman phonons of ZnO, a vibrational mode at 511.5 cm1 (D hereafter) is clearly observed when Mg is intentionally incorporated. Interestingly, increasing the Mg content slightly increases the intensity of the D mode but its frequency is unaffected. The origin of this phonon is still under debate. Kaschner et al. observed this additional phonon in N-doped ZnO films and interpreted the occurrence as an N-related local vibrational mode.11 However, Bundesmann et al. detected the phonon in ZnO films that were doped with Fe, Sb, and Al, and intentionally grown without N incorporation.12 Because of the large variation in the masses of these dopants, they suggested that the intrinsic host lattice defects were responsible for this phonon. Manjon et al. attributed this phonon in N-doped ZnO films to the disorder-activated 2Blow 1 mode due to the relaxation of Raman selection rules that is induced by the breakdown of the crystal symmetry.13 In the case considered herein, the D mode cannot be ascribed to the N-related phonon because the Zn1xMgxO powders were all grown without additional N-doping. Also, the D mode cannot be an activated silent Blow 1 mode because (i) B1-related phonons have not been observed in Zn1xMgxO even under high-pressure conditions (high crystal asymmetry) and (ii) the D mode behaves entirely differently from Blow 1 , which should shift toward higher frequencies as x increases (Figure 3). On the basis of these observations, a structure or a complex defect with a crystal symmetry that differs from that of the host lattice accounts for the D mode in Zn1xMgxO (x > 0). Therefore, results of this study suggest that the complex defects of Zn and O interstitials (ZnI-OI), which are induced by incorporating Mg, are the origin of the D mode. Zinc (oxygen) vacancies and antisites can be ruled out since they degrade the entire crystal isotropy without providing related phonon modes.26 Further evidence for the ZnI-OI complexes under high pressure will be discussed later. B. Under High Pressure. Figure 5a shows the pressuredependent Raman spectra of ZnO recorded with increasing pressure up to 20.0 GPa. Figure 5b plots the pressure dependence of the phonon frequencies, to which straight lines can be fitted (Table II). As external pressure is applied, the reduction in lattice constants should shift all Raman phonons toward higher frequencies. However, two remarkable exceptions—the Elow 2 and 19964
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Figure 5. (a) Upstroke pressure-dependent Raman spectra of ZnO. Asterisks indicate the 202.7 cm1 phonon. (b) Pressure dependence of observed optical phonon frequencies. Solid lines are linear least-square fits to experimental points.
the 202.7 cm1 phonons—are observed in Figure 5a. The softening of the Elow 2 phonons with increasing pressure is observed in all of the Zn1xMgxO samples discussed herein. A similar experimental result was obtained for wurtzite GaN.27 The negative pressure coefficient of the Elow 2 phonon in ZnO and GaN can be attributed to the soft pressure-dependent C66 elastic constant; however, the hard C66 elastic constants in wurtzite AlN and SiC 28 result in the positive pressure coefficients of the Elow 2 phonons. The phonon at 202.7 cm1 in ZnO has been previously assigned 9,10 In this study, although incorporating Mg and to 2Elow 2 or 2TA. applying external pressure cause opposite phonon behavior of the phonon, the frequency shift of the 202.7 cm1 phonon Elow 2 phonon. These results closely corresponds to that of the Elow 2 suggest that the Elow 2 phonon should contribute significantly to the 202.7 cm1 phonon and, therefore, the 202.7 cm1 phonon can be assigned to the 2Elow 2 . The assignment will be further confirmed under high-pressure measurements. In Figure 5a, the pressure-induced phonon shifts are accompanied by significant falls in phonon intensities. As the pressure is increased to 9.6 GPa, three additional phonons with similar intensities simultaneously appear at around 150, 550, and 590 cm1, and the latter two peaks overlap as a broad vibrational band in the range between 500 and 650 cm1. Above 9.6 GPa, all A1 and E1 phonons gradually vanish, and the Raman spectra are dominated by the three additional phonons. As the pressure further increases and Ehigh phonons in the to 13.2 GPa, the two intense Elow 2 2 wurtzite structure completely disappear, and the sample darkens. These phenomena are strong evidence of a pressure-induced metallic phase transition.21,29,30 In fact, the first-order Raman phonon modes are forbidden by the selection rules in rocksalt structures. Thus, the metallic phase transition should be accompanied by the wurtzite-to-rocksalt phase change as previously described.14,15,17 The wurtzite-to-rocksalt phase transition of ZnO is complete by around 13.2 GPa, as determined using detailed high-pressure Raman measurements, which finding agrees closely with the high-pressure XRD results obtained by Mao’s group.18 A close inspection of the three additional modes in Figure 5a indicates that these phonons emerge at around 9.6 GPa and become more intense with increasing pressure until the wurtzite-to-rocksalt
phase transition is complete. Obviously, these phonons are, in contrast with the first-order Raman phonons in the wurtzite phase, insensitive to pressure. In the rocksalt phase, these additional phonons are still observable and slightly shift to higher frequencies with increasing pressure. On the basis of these findings, these additional Raman phonons that appeared at high pressures are vibrational modes of rocksalt ZnO. The complicated Raman signals in the pressure range between 9.6 and 13.2 GPa reflect the coexistence of wurtzite and the rocksalt phase, revealing that the ZnO undergoes a gradual phase transformation from wurtzite to rocksalt. Figure 6a show the upstroke pressure-dependent Raman spectra of Zn0.90Mg0.10O. Several interesting conclusions can be drawn from a comparison with those of ZnO. (i) In addition to the A1, E1, and E2 phonons, the D mode, whose frequency is unaffected by the Mg concentration at ambient pressure shifts to higher frequencies with increasing pressure. (ii) The additional rocksalt modes of Zn0.90Mg0.10O appear at around 8.1 GPa. This result indicates that the onset of the wurtzite-to-rocksalt phase transition of Zn0.90Mg0.10O is at lower pressure than that of ZnO (∼9.6 GPa). (iii) The first-order Raman phonon modes in the high modes vanish wurtzite phase including the intense Elow 2 and E2 at 10.4 GPa. Also, the sample in the DAC suddenly changes from bright to dark, as shown in parts b and c of Figure 6, respectively. Restated, the metallic phase transition of the Zn0.90Mg0.10O is complete at around 10.4 GPa, which value is lower than that of ZnO. Before the effect of Mg on the phase transitions can be discussed, the downstroke pressure-dependent Raman spectra of ZnO and Zn0.90Mg0.10O, shown in parts a and b of Figure 7, respectively, must be considered. Clearly, upon decompression, the rocksalt phase in both samples is maintained beyond the upstroke phase-transition pressure, revealing substantial phase hysteresis. The ZnO and Zn0.90Mg0.10O revert to the wurtzite high phonons first reappear, at about phase, in which the Elow 2 and E2 2.4 and 1.2 GPa, respectively. Notably, the degree of phase hysteresis (the pressure difference between the end of the upward transition and the onset of downward transition) declines as Mg content increases. As the pressure is further released, the rest of the wurtzite-phase phonons, including the D mode appear. Once 19965
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Table II. Linear Pressure Coefficients (dωi/dp), Mode Gr€uneisen Parameters (γi), and Phase Transition Pressures for Zn1xMgxO Mg content (x) 0%
3.0%
7.0%
10.0%
phonon mode (i)
dωi/dp (cm1/GPa)
Gr€uneisen parameter (γi)
0.76
1.04
E2(high) E2(low) A1(TO)
5.67 4.73
2.31 1.68
E1(TO)
4.98
1.64
E2(high)
4.86
1.50
A1(LO)
4.14
0.98
E2(low)
E1(LO)
4.32
1.00
E2(low)
0.71
0.97
E2(high) E2(low)
5.52
2.25
A1(TO) E1(TO)
4.67 4.95
1.65 1.63
E2(high)
4.77
1.47
D
4.61
1.22
A1(LO)
4.26
1.00
E1(LO)
4.49
1.03
E2(low)
0.68
0.92
E2(high) E2(low)
5.41
2.21
A1(TO) E1(TO)
4.62 4.97
1.63 1.63
E2(high)
4.70
1.45
D
4.64
1.23
A1(LO)
4.32
1.00
E1(LO)
4.60
1.04
E2(low)
0.66
0.88
E2(high) E2(low)
5.33
2.18
A1(TO) E1(TO)
4.60 4.96
1.62 1.62
E2(high)
4.62
1.43
D
4.62
1.22
A1(LO)
4.38
1.01
E1(LO)
4.71
1.06
phase transition pressure (GPa) 9.6 ( 0.2a, 13.2 ( 0.2b, 2.6 ( 0.2c
9.2 ( 0.2a, 12.6 ( 0.2b, 2.4 ( 0.2c
8.5 ( 0.2a, 11.8 ( 0.2b, 1.8 ( 0.2c
8.1 ( 0.2a, 10.4 ( 0.2b, 1.4 ( 0.2c
The onset of wurtzite-to-rocksalt phase transition. b The finish of wurtzite-to-rocksalt phase transition. c The onset of rocksalt-to-wurtzite phase transition. a
the applied pressure is completely released, the samples behave in a time-independent manner, even after two months. Such an unusual hysteresis effect (>10 GPa) has also been observed in AlN, GaN, InN, and zincblende (3C-type) SiC.31,32 However, materials such as ZnSe, CdSe, and CdTe, exhibit quite small phase hysteresis (
