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The competitive laser-induced local heating and recrystallization effects in ZnO nanocrystals embedded in a MgO/ZnO stack are reported via resonance Raman spectra. The dependence of the intensity, energy, and resonance effects of the longitudinal optical (LO) phonon on laser excitation condition are discussed in the context of Fröhlich interaction. Redistribution of defects, impurity-diffusion, and grain regrowth caused by thermal and photochemical effects lead to significant changes in coupling strength of electron-phonon interaction, and the resonance behaviors are strongly affected by the interplay of local heating, heat trapping, and local structural modification in such nanostructures.
©2012 Optical Society of America
1. Introduction
ZnO materials in the low dimensional regime have been of great research interest in recent years, due to their promising applications in various optoelectronic devices. From a fundamental viewpoint, it is highly interesting to probe the fundamental phenomena in low dimensional systems. Resonant Raman scattering (RRS) spectroscopy is a powerful technique to investigate the physical properties of semiconductors, including the interband electronic transitions, excitons, and electron-phonon interactions, which are of great importance for device applications [1, 2]. However, the well-known laser heating effect always causes anharmonic effects in solids and the resonance Raman scattering often shows a softening of the phonon energy with a shorter phonon decay time [2–7]. The photo-induced localized heating can give rise to changes in the electronic states distributions, local stress field, and optical transitions [8–13]. Simultaneously, under laser prolonged irradiation, structural transitions and crystalline changes could occur due to long-range ordering, as well as local or macroscopic ordering, which is typically referred to as “laser crystallization” [14, 15]. The interplay of such competing effects is vital to the stability of quantum systems or nanostructures under high-power ultraviolet laser excitation [16, 17]. In particular, they significantly alter various properties at the interface where a large difference in size or phase can modify the properties of an otherwise homogeneous structure. Thermal and photochemical effects in ZnO nanostructures and the reversible/irreversible laser crystallization in a-SiGe alloy have been reported in literatures [13, 18]. A few studies on the laser heating effect in ZnO have been reported recently [10, 13, 14]. In case of nanocrystals embedded in amorphous matrix, the laser crystallization effect would be of particular importance under ultraviolet laser irradiation. In this paper, using the ultraviolet resonant Raman spectroscopy, we report on competing behaviors of simultaneous laser heating and local recrystallization effects in ZnO nanocrystals embedded in ZnO/MgO multilayers in the context of Frohlich electron-phonon interaction and spatial correlation model. The microstructural evolution induced by photo-thermal and photochemical effects led to significant changes in the strength of electron-phonon interaction.
2. Experiments
For this study, a ZnO/MgO stack was prepared on Si (100) by electron-beam evaporation [19], consisting of alternating MgO and ZnO layers with each thickness of ~15 nm as shown in Fig. 1(b) . The X-ray diffraction (XRD) pattern in Fig. 1(a) indicates that the evaporated ZnO nanograins are of wurtzite structure. The high-resolution cross-sectional transmission electron microscopy in Fig. 1(c) shows ZnO nanograins with an average size of ~7 nm embedded within MgO sub-layers. This is well consistent with the value of 7.7nm calculated from XRD pattern of ZnO using the Debye-Scherrer formula. Corresponding to the selected area electron diffraction of Fig. 1(d), we observed that ZnO nano-grains are polycrystalline with random orientations. The resonant Raman spectra were recorded in a z(x,-)z configuration with resolution of ~0.6 cm−1. A He-Cd laser (λ = 325nm) was focused through microscope objectives onto the sample surface with an incident power of 0.5 and 0.1 mW and focusing spot size of about 1 and 10 μm in diameters, respectively. The corresponding excitation densities are about 63.7 and 0.13 kW/cm2. The spectra acquisition time were kept constant for 30 seconds.
Fig. 1 (a) 2θ-ω scan X-ray diffraction and (b) cross-sectional TEM image of the ZnO/MgO stack, and the arrows are used to point out the nanocrystals; (c) High-resolution TEM image and (d) selective area electron diffraction (SAED) of the ZnO sub-layer embedded in MgO.
3. Results and discussion
Figure 2(a) and 2(b) show the resonance Raman spectra of ZnO/MgO stack with various irradiation durations recorded under conditions of 63.7 kW/cm2 at 300K and 0.13 kW/cm2, at 80K, respectively. The Raman spectrum of ZnO (0001) single crystal grown by hydrothermal growth method is also shown for reference. Since the laser excitation energy (Ei = 3.815 eV) lies between the band gaps of MgO (7.6 eV) and ZnO (3.37 eV), the multi-phonon resonance processes in ZnO dominated the spectrum and only near band-edge photoluminescence emission of ZnO is observed. It is found that the frequency of the first-order LO (1LO) phonon is blue-shifted with respect to the dipole-allowed A1(LO) mode in bulk ZnO under the z(x,-)z configuration, which is affected by quasi-mode mixing of the polar modes (A1 and E1 phonons) when the high-symmetry axes of wurtzite crystals are randomly oriented with respect to the phonon propagation direction [8]. Alarcon-Lialdo et al found that the average wavenumber of the quasimodes for randomly oriented ZnO nanowires is slightly red-shifted, just about 0.7cm−1, with respect to the pure E1(LO) mode of bulk ZnO by taking a refractive index value of n = 2 for ZnO [12]. In particular, under the resonance condition, the transverse optical (TO) Raman scattering induced by the deformation potential mechanism is almost insensitive, strongly implying that the short-range interaction between lattice displacement and the electrons is relatively weak [1]. As a consequence, the enhanced LO modes are mainly excited through dipole-forbidden Fröhlich- interaction due to its long range nature and especially, the forbidden E1(LO) mode has a polar character, allowing strong Fröhlich interaction with charged impurities and defects and thus giving rise to the much enhanced Raman bands as compared to A1(LO) [20–23].
Fig. 2 Resonant Raman spectra of ZnO/MgO stack with various irradiation duration recorded under (a) 300 K, 63.7 kW/cm2 and (b) 80K, 0.13 kW/cm2.
Under the excitation condition of 63.7 kW/cm2 at 300 K, it is found that the LO phonons exhibit a monotonic increase in intensity when the irradiation duration increases on the same probing spot. In contrast, shown in Fig. 2(b), under laser irradiation of 0.13 kW/cm2 at 80 K, the first- and second-order quasi-mode LO phonons (~576 and 1155 cm−1) show a decrease in intensity and then grew in intensity with increasing irradiation duration on the same spot. The reduction of scattering intensity of phonons can be well understood in terms of laser heating. In general, Raman scattering efficiency decreases exponentially with increasing temperature due to the increase of optical absorption and reflectivity, as well as the weakening of the Raman susceptibility [5, 6]. However, the abnormal feature of intensity increase of phonons has not been observed and cannot be interpreted in the framework of conventional laser heating effect. Therefore, simultaneous laser recrystallization effect should be taken into account. In the MgO/ZnO stack, the heat trapping and retention are remarkable due to the lower thermal conductivity coefficient of MgO compared to ZnO [24]. In this case, thermal energy is trapped, retained, and accumulated in ZnO nanocrystals surrounded in MgO ultra-thin layer [13, 15]. Once the local temperature rises to a certain point, the softening of phonons due to local heating effect is stabilized while the laser reoxidation of nanoclusters became important, accompanied with the laser heating and photochemical effect [14], which could be responsible for the further enhancement of the dipole-forbidden LO scattering.
The electron-phonon interaction has a vital influence on the electronic and optical properties of semiconductor and its coupling strength is sensitive to the lattice temperature, impurity defects and domain size of nanomaterial. Within the Frank-Condon approximation, the coupling strength of the electron-phonon interaction can be assessed by the electronic scillator strength distribution over n modes with the relationship, in which, S is Huang-Rhys parameter [20]. As shown in Fig. 3 , the typical integrated intensity ratio (η = I2LO/I1LO) of the second- to first-order LO phonons is ranging from 0.36 to 0.5, which is well consistent with the reported values in ZnO nanoparticles with diameters below 10 nm because of the quantum confinement effect involved [20]. For η <0.5, the value of S is expected to be lower than unity, which is much smaller than that in bulk ZnO due to poor crystalline and finite phonon correlation length in these nano-scaled grains. Figure 3 shows that the integrated intensity ratio (η) exhibits an anomalous dependence on the irradiation duration, which can be interpreted in terms of dipole-forbidden Fröhlich electron-phonon interaction. The contribution of Fröhlich interaction to the forbidden LO-phonon scattering efficiency is possibly through the intrinsic (q-induced) processes and/or extrinsic (impurity-induced) enhancement [1]. First, the coupling strength of intrinsic Fröhlich interaction would be enhanced in the finite-size domain due to relaxation of the q = 0 selection rules and ~q2 proportional dependence. Quantum confinement strongly modifies the eigenfunctions in nanocrystals, and causes changes in the exciton radius and dielectric coefficient, which have profound consequences on the magnitude of the exciton-phonon coupling [21]. Alternatively, the localized electronic states bound to defect or impurity will act as an intermediate state for extrinsic Fröhlich interaction, which leads to additional enhancement of forbidden scattering efficiency independent of the wave vector q.
Fig. 3 The ratio of integrated intensity of 2LO to 1LO phonons under different resonant conditions of (i) 80 K, 0.13 kW/cm2, (ii) 150 K, 0.13 kW/cm2, (iii) 300 K, 0.13 kW/cm2, and (iv) 300 K, 63.7 kW/cm2.
Knowing the nature of Fröhlich interaction, we look at the anomalous variation of Raman scattering cross section ratio. At the initial irradiation stage, the laser heating is dominant and the temperature of nanocrystals increases quickly. Under resonance condition, the intrinsic Fröhlich interaction exhibit strong dependence on the temperature based on the third-order perturbation theory of resonance Raman scattering and can be expressed as [9]:
where M is electron-phonon matrix element, is the incident photon energy, is the 1LO phonon energy, is the Bose factor, and is the width of the electronic states. To illustrate the electron-phonon interaction, we first have to examine the temporal evolution of optical bandgap. Figure 4(a) and 4(b) exhibit the temporal near-band edge (NBE) emissions for ZnO single crystal and ZnO nanocrystals, respectively. As summarized in Fig. 4(c), for ZnO single crystal, the irradiation with a low power density (100 W/cm2) will not cause an obvious peak-shift and broadening of exciton emission at 3.296eV, while the distinct red-shift and reduced intensity of NBE emission are due to the laser heating effect under illumination with a higher power density (10 kW/cm2). As compared to ZnO single crystal, the optical bandgap of the nanocrystals determined by emission peak reduced monotonically from 3.356 to 3.312 eV as irradiation time increased from 0.5 and 8mins under excitation with the power density of 63.7 kW/cm2.Fig. 4 (a) Temporal photoluminescence of ZnO single-crystal irradiated at temperature of 300 K and power density of 10 kW/cm2 and 0.1 kW/cm2 respectively; (b) temporal photoluminescence of ZnO/MgO stack at 300 K and 63.7 kW/cm2; (c) Optical bandgap and (d) normalized PL intensity as a function of irradiation duration.
The overall larger optical bandgap of the embedded ZnO nanocrystals is caused by the distinct quantum confinement effect. It is noted that the shrinkage of optical bandgap in ZnO nanocrystals in the initial stage is larger than that of ZnO single crystal, indicative of large rise in temperature. It is expected that nonradiative transition in the embedded ZnO nanocrystals is dominant due to the presence of large number of defects, and thus more heat energy is generated. The drastically reduced thermal conductivity of the matrix also enhances heating retention, thus inducing a rapid local increase in temperature. In this case, the drop of optical bandgap induced by laser heating effect makes the denominator in Eq. (1) approaching the maximum value, and thus 2LO/1LO intensity ratio is reduced. It can also be understood that the free excitons localized in the nanograins are delocalized and dissociated, thus weakening the Fröhlich interaction via excitonic intermediate states. Under low excitation power (0.13 kW/cm2), the laser heating effect can be greatly suppressed, and the thermal energy is not enough to dissociate the localized excitons due to large binding energy in ZnO. Therefore, one cannot find an obvious decrease of 2LO/1LO ratio in case (iii). As the excitation power is constant, the retention and accumulation of thermal energy provided by laser irradiation is more difficult and need more time to reach thermal equilibrium with the environment which is at with low temperature in the case of (i) and (ii).
On the other hand, the photon energy of He-Cd laser (367 kJ/mol) is enough to break Zn-O bonds (248 kJ/mol), and the laser irradiation will cause photochemical reaction and modify the states of the surface of ZnO nanocrystals. For instance, the photon excited electron-hole pairs can modify oxygen adsorption and cause the formation of electronic defects into clusters on the surface or along the grain boundaries [14], and the laser-induced temperature rise enhances diffusion of structural defects formed by the displacement of impurity atoms in the ZnO lattice. The electronic states localized in these defects will enhance the extrinsic Fröhlich interaction and thus the forbidden E1(LO) phonon scattering will increase in intensity. The enhancement is expected to be largest for 1LO phonon [25], while such intermediate resonance is relatively suppressed for second-order scattering by LO phonons [1]. Thus, both intrinsic and extrinsic Fröhlich interactions contribute to the reduction of the 2LO/1LO Raman scattering ratio by laser heating effect of the nano-sized ZnO crystals during the initial stages.
After prolonged laser irradiation on the same spot, the laser-enhanced reoxidation or recrystallization effect have been proven to be important, in particular in cases where the photon energy (3.815eV) matches the dissociation energy (3.8eV) for adsorbed O2̄ species located at the oxide grain boundaries [14]. It is well known that evaporation-grown ZnO is always oxygen deficient. Under UV irradiation, the chemisorbed O2̄ species are dissociated and react with extra Zn ions at the grain boundaries. This re-oxidation process will be promoted as the temperature rises, thus leading to grain recrystallization and further oxide growth. The increase in grain size is evidenced by both the enhanced electron-phonon coupling strength characterized by RRS and shrinkage of optical bandgap monitored by temporal PL spectra. The ratio between second- and first-order scattering cross section was found to be almost insensitive to the heating process, but very sensitive to electron-phonon coupling strength [21]. The strength of intrinsic Fröhlich interaction will be reduced due to weak phonon confinement effect in enlarged ZnO nanodomains, resulting in an increased value of the ratio ‘η’. The weak quantum confinement effect could also be confirmed by the temporal photoluminescence characteristics in Fig. 4(c). Even when the system reaches thermal equilibrium, the optical bandgap continued to shift to lower energy side. Figure 4(d) shows that the exciton emission become narrower and more intense with increasing irradiation duration, while intensity reduction of exciton emission in ZnO single crystal is observed under high power irradiation. The contrast indicates that the recrystallization in embedded ZnO nanocrystals is the dominant mechanism, which gives rise to grain growth and improvement of crystalline quality. Therefore, the temporal reduction of line-widths in photoluminescence and Raman measurement indicates the reduced width of the electronic states Г0, thus resulting in the reduction of the denominator in Eq. (1) and enhancing the electron-phonon coupling strength.
Thus, it is concluded that the 2LO/1LO intensity ratio is contributed by the combined action of extrinsic and intrinsic Fröhlich interactions, where a competition between laser heating and local recrystallization effects occur during irradiation. Note that the valley region can be considered as the thermal equilibrium state, and the irradiation time can be considered as the equilibrium duration (τ1). It is remarkable that τ1 is shortened from 30 to 12 mins when the sample temperature increases from 80 to 300 K and irradiation power from 0.13 to 63.7 kW/cm2. This implies the increased dominance of laser recrystallization effect at higher sample temperature and with higher irradiation power density. The grain growth of ZnO nanostructures induced by laser recrystallization can be discussed in terms of increasing phonon correlation length (L). Due to translational symmetry breakdown, the spatial confinement model developed by Richter et al. [27] can be used to evaluate the phonon confinement or the average size of ZnO nanostructures [28, 29]. Assuming a spherical shape of the ZnO nanocrystals, the intensity of 1-LO phonons is given by [28]
where a is the lattice constant of ZnO, and Г is the full width at half maximum. The phonon dispersion function ω(q) can be expressed as [23]where ω (0) is the zone-center phonon frequency and Δω is the difference between the zone-center and zone-boundary frequencies in the phonon dispersion curve of bulk ZnO. The parameters used in the calculations are identical with the values in Ref [29]. The calculated confined phonon line shapes are consistent with the experimental data, as shown in Fig. 5(a) . The calculated size of the ZnO nanocrystals without irradiation is about 5.6 nm, somewhat smaller than the average size observed from the TEM study. This deviation may arise from the peak broadening by laser heating [21]. However, taking into account the thermal expansion and anharmonic coupling effects, the anharmonic coefficient for the broadening of E1(LO) is Γ1 = 2.5cm−1, as determined by temperature-dependent Raman scattering measurements. The value is much smaller than Γ0 = 18.7cm−1, which is the background contribution due to impurity and/or defect scattering and isotopic broadening. Thus, the phonon confinement plays a main role in determining changes in shift, broadening and asymmetry of first order optical phonon. By fitting the temporal 1LO phonon curves in Fig. 5(a), the nanocrystal diameters are extracted as a function of duration shown in Fig. 5(b). In terms of the effective mass model with a Coulomb interaction term [20], we, in turn, calculated the corresponding bandgap of different sized nanoparticles as a function of irradiation duration and plotted the results in Fig. 4(c). The calculated bandgap exhibits good coincidences with those derived from photoluminescence except in the initial stage, where laser heating effect cause distinct drop of the bandgap. It is conclusive that the prolonged irradiation with high power density will lead to the recrystallization of grain growth.Fig. 5 (a) 300 K Raman spectra of 1LO phonon (open circles) at 63.7 kW/cm2. The solid lines represent line shape fitting from the spatial correlation model, (b) The calculated phonon correlation length (squares) as a function of irradiation time and the solid line is the fitting of the exponential decay relationship from Eq. (4).
Upon continuous irradiation, the grain size increases gradually with exponential growth dependence on the irradiation duration.
where L0 is the correlation length without irradiation, A is a constant prefactor related to growth rate under laser irradiation. The parameter τ is estimated to be ~12.8 min from the best fitting of the experimental data (Fig. 5(b)) using Eq. (4). The value of τ is almost equal to the equilibrium irradiation time of τ1, which implies the competition between laser heating and recrystallization, and the possibility of an equilibrium state under certain irradiation conditions. To separate these two competitive effects, a supplemental experiment was adopted. The sample was irradiated with high power excitation for a fixed duration and subsequently, Raman spectrum was recorded at low power to investigate the structural transformation. It is found that the 2LO/1LO ratios excited at low power density increases with prolonged duration of high power irradiation, clearly indicating the grain growth process induced by laser recrystallization effect.In addition, as shown in Fig. 6 , the optical phonon frequency shifts exhibit a different dependence on the irradiation duration under different laser excitation conditions. At low temperature and power density, the frequency of LO phonons shows a weak red-shift due to the well-known temperature-dependent anharmonic effect because the spatial confinement within the grain boundaries is weakened as irradiation proceeds [23, 30, 31]. On the other hand, the LO phonons show a shift to higher frequencies under irradiation with high laser power density and higher sample temperature. The main contribution of alloying effect due to Mg thermal diffusion could be excluded [32], because the PL emissions exhibit a red-shift with increased irradiation duration in Fig. 4 [9, 13]. The other two possible reasons may contribute to our observation: (i) weakening of optical phonon confinement due to grain growth [20], and (ii) efficient sharing of compressive stress due to local thermal expansion and change in the surface to volume ratio in the nanocrystals [21]. The different types of LO phonon peak shifts provide further evidence of such competitive effect of laser heating and local recrystallization. It implies that for such systems, the behavior of optical phonons in the Raman spectrum should be carefully analyzed under UV laser irradiation conditions. Based on these observations, we can emphasize that the laser irradiation conditions significantly affects the vibrational properties of ZnO nanostructures than ZnO films or bulk ZnO, due to the lower thermal conductivities of nanostructures when compared to the bulk materials [10].
Fig. 6 (a) 1LO and (b) 2LO phonon frequencies as a function of irradiation duration under different UV laser excitation power density.
4. Summary
In summary, in the context of Fröhlich interaction and spatial correlation model, the competitive effect of laser heating and local recrystallization of the embedded ZnO nanocrystals under different UV irradiation conditions has been demonstrated. The microstructural evolution under irradiation was discussed in terms of photo-induced thermal and photochemical effects, as well as the spatial heat transfer dynamics in the ZnO/MgO stack. This comprehensive study would be useful for the interpretation of the phonon behaviors in UV micro-Raman scattering.
Acknowledgments
This work is supported by the Australian Research Council Discovery Project Grant (DP1096918) and by the State Key Program for Basic Research of China under Grant No. 2011CB302003, NSFC (No. 61025020, 60990312 and 11104130) and Basic Research Program of Jiangsu Province (BK2011437 and BK2011556).
References and links
1. W. Kauschke and M. Cardona, “Resonant Raman scattering in semiconductors,” Phys. Scr. T25, 201–205 (1989). [CrossRef]
2. M. R. Wagner, P. Zimmer, A. Hoffmann, and C. Thomsen, “Resonant Raman scattering at exciton intermediate states in ZnO,” Phys. Status Solidi 1(5), 169–171 (2007) (RRL). [CrossRef]
3. A. Compaan, M. C. Lee, and G. J. Trott, “Phonon populations by nanosecond-pulsed Raman scattering in Si,” Phys. Rev. B Condens. Matter 32(10), 6731–6741 (1985). [CrossRef] [PubMed]
4. S. K. Arguirova, Tz. Arguirov, D. Wolfframm, and J. Reif, “Influence of local heating on micro-Raman spectroscopy of silicon,” J. Appl. Phys. 94(8), 4946–4949 (2003).
5. K. W. Adu, H. R. Gutierrez, U. J. Kim, and P. C. Eklund, “Inhomogeneous laser heating and phonon confinement in silicon nanowires: A micro-Raman scattering study,” Phys. Rev. B 73(15), 155333 (2006). [CrossRef]
6. R. Jalilian, G. U. Sumanasekera, H. Chandrasekharan, and M. K. Sunkara, “Phonon confinement and laser heating effects in Germanium nanowires,” Phys. Rev. B 74(15), 155421 (2006). [CrossRef]
7. K. A. Alim, V. A. Fonoberov, M. Shamsa, and A. A. Balandin, “Micro-Raman investigation of optical phonons in ZnO nanocrystals,” J. Appl. Phys. 97(12), 124313 (2005). [CrossRef]
8. E. Alarcon-Liado, J. Ibanez, R. Cusco, L. Artus, J. D. Prades, S. Estrade, and J. R. Morante, “Ultraviolet Raman scattering in ZnO nanowires: quasimode mixing and temperature effects,” J. Raman Spectrosc. 42, 153–159 (2011).
9. S. Sahoo and A. K. Arora, “Laser-power-induced multiphonon resonant Raman scattering in laser-heated CdS nanocrystal,” J. Phys. Chem. B 114(12), 4199–4203 (2010). [CrossRef] [PubMed]
10. S. S. Kurbanov, K. T. Igamberdiev, and T. W. Kang, “The UV-laser induced heating effect on photoluminescence from ZnO nanocrystals deposited on different substrates,” J. Phys. D Appl. Phys. 43(11), 115401 (2010). [CrossRef]
11. M. F. Cerqueira, M. I. Vasilevskiy, F. Oliveira, A. G. Rolo, T. Viseu, J. Ayres de Campos, E. Alves, and R. Correia, “Resonant Raman scattering in ZnO:Mn and ZnO:Mn:Al thin films grown by RF sputtering,” J. Phys. Condens. Matter 23(33), 334205 (2011). [CrossRef] [PubMed]
12. E. Alarcon-Liado, R. Cusco, L. Artus, J. Jimenez, B. Wang, and M. Callahan, “Raman scattering of quasimodes in ZnO,” J. Phys. Condens. Matter 20, 445211 (2008).
13. L. Bergman, X. B. Chen, J. L. Morrison, J. Huso, and A. P. Purdy, “Photoluminescence dynamics in ensembles of wide-band-gap nanocrystallites and powders,” J. Appl. Phys. 96(1), 675–682 (2004). [CrossRef]
14. D. Bäuerle, Laser Processing and Chemistry, Fourth Edition (Springer, 2011).
15. X. B. Chen, J. L. Morrison, J. Huso, L. Bergman, and A. P. Purdy, “Ultraviolet Raman scattering of GaN nanocrystallites: Intrinsic versus collective phenomena,” J. Appl. Phys. 97(2), 024302 (2005). [CrossRef]
16. L. Bergman, X. B. Chen, J. Huso, J. L. Morrison, and H. Hoeck, “Raman scattering of polar modes of ZnO crystallites,” J. Appl. Phys. 98(9), 093507 (2005). [CrossRef]
17. A. Kaschner, M. Strassburg, A. Hoffmann, C. Thomsen, M. Bartels, K. Lischka, and D. Schikora, “Temporal evolution of resonant Raman-scattering in ZnCdSe quantum dots,” Appl. Phys. Lett. 76(19), 2662–2664 (2000). [CrossRef]
18. P. Martin, A. Torres, J. Jimenez, A. Rodriguez, J. Sangrador, and T. Rodriguez, “Reversible crystallization of a-Si1−xGex alloys under the combined effect of light and temperature,” J. Appl. Phys. 96(1), 155–163 (2004). [CrossRef]
19. J. D. Ye, K. W. Teoh, X. W. Sun, G. Q. Lo, D. L. Kwong, H. Zhao, S. L. Gu, R. Zhang, Y. D. Zheng, S. A. Oh, X. H. Zhang, and S. Tripathy, “Effects of alloying and localized electronic states on the resonant Raman spectra of Zn1−xMgxO nanocrystals,” Appl. Phys. Lett. 91(9), 091901 (2007). [CrossRef]
20. H. M. Cheng, K. F. Lin, H. C. Hsu, and W. F. Hsieh, “Size dependence of photoluminescence and resonant Raman scattering from ZnO quantum dots,” Appl. Phys. Lett. 88(26), 261909 (2006). [CrossRef]
21. R. P. Wang, G. Xu, and P. Jin, “Size dependence of electron-phonon coupling in ZnO nanowires,” Phys. Rev. B 69(11), 113303 (2004). [CrossRef]
22. J. Menéndez and M. Cardona, “Interference effects: A key to understanding forbidden Raman scattering by LO phonons in GaAs,” Phys. Rev. B Condens. Matter 31(6), 3696–3704 (1985). [CrossRef] [PubMed]
23. R. Cusco, E. A. Llado, J. Ibanez, L. Artus, J. Jimenez, B. G. Wang, and M. J. Callahan, “Temperature dependence of Raman scattering in ZnO,” Phys. Rev. B 75(16), 165202 (2007). [CrossRef]
24. T. Tsubota, M. Ohtaki, K. Eguchi, and H. Arai, “Transport properties and thermoelectric performance of (Zn1–yMgy)1–xAlxO,” J. Mater. Chem. 8(2), 409–412 (1998). [CrossRef]
25. R. M. Martin and T. C. Damen, “Breakdown of Selection Rules in Resonance Raman Scattering,” Phys. Rev. Lett. 26(2), 86–88 (1971). [CrossRef]
26. I. Ozerov, M. Arab, V. I. Safarov, W. Marine, S. Giorgio, M. Sentis, and L. Nanai, “Enhancement of exciton emission from ZnO nanocrystalline films by pulsed laser annealing,” Appl. Surf. Sci. 226(1-3), 242–248 (2004). [CrossRef]
27. H. Richter, Z. P. Wang, and L. Ley, “The one phonon Raman spectrum in microcrystalline silicon,” Solid State Commun. 39(5), 625–629 (1981). [CrossRef]
28. I. H. Campbell and P. M. Fauchet, “The effects of microcrystal size and shape on the one phonon Raman spectra of crystalline semiconductors,” Solid State Commun. 58(10), 739–741 (1986). [CrossRef]
29. M. Rajalakshmi, A. K. Arora, B. S. Bendre, and S. Mahamuni, “Optical phonon confinement in zinc oxide nanoparticles,” J. Appl. Phys. 87(5), 2445–2448 (2000). [CrossRef]
30. K. Samanta, P. Bhattacharya, and R. S. Katiyar, “Temperature dependent E2 Raman modes in the ZnCoO ternary alloy,” Phys. Rev. B 75(3), 035208 (2007). [CrossRef]
31. V. A. Coleman, M. Buda, H. H. Tan, C. Jagadish, M. R. Phillips, K. Koike, S. Sasa, M. Inoue, and M. Yano, “Observation of blue shifts in ZnO/ZnMgO multiple quantum well structures by ion-implantation induced intermixing,” Semicond. Sci. Technol. 21(3), L25–L28 (2006). [CrossRef]
32. C. Bundesmann, A. Rahm, M. Lorenz, M. Grundmann, and M. Schubert, “Infrared optical properties of MgxZn1−xO thin films (0 < x < 1): Long-wavelength optical phonons and dielectric constants,” J. Appl. Phys. 99(11), 113504 (2006). [CrossRef]
