Chapter 6 Reducing exciton–longitudinal optical phonon coupling with
6.2 Binding energy
Figure 6-4 and 6-5 show the peak intensity of the emission as a function of reciprocal temperature for the undoped and Mg-doped samples, respectively. The data can be described by the equation [20]
=1 constants, and k is the Boltzmann constant. The curve fitting gives rise to two activation energies of about 55 and 11 meV for ZnO powders (Fig. 6-4) and about 73
0.01 temperature-dependent capture cross sections of the carriers at the recombination centers, and not from a genuine thermal activation energy. Furthermore, Ea1 for ZnO is comparable with EXB, 60 meV, for ZnO bulk, while that for Mg0.05Zn0.95O is larger than 60 meV. Table 6-1 clearly shows that the binding energy of increases markedly in samples of higher x. Therefore, it was implied that exciton localization takes place and that the degree of localization increases with increasing x. The essential origin of the localization is thought to be the spatial fluctuation of the local composition of Mg in the alloys, which results in the spatial fluctuation of the potential energy for the excitons.
=1 n
FXA
Fig. 6-4 Normalized integrated intensity of ZnO sample as a function of temperature.
Fig. 6-5Normalized integrated intensity of Mg Zn O sample as a function of temperature.
6.3 Photoluminescence spectra analysis
It is well known the increase of EXB may result from the decrease of aEB or effective reduced mass. Due to the electron is more delocalized than the hole in the bulk exciton, the polar lattice experience a net negative charge in the outer regions of the exciton, while it experiences a net positive one in the inner regions. This charge inhomogeneity couples to the polar lattice via the Fröhlich mechanism that leads to reduce aEB and to reduce the exciton-LO-phonon coupling.[21] Thus, we further deduced the coupling strength of free exciton A with LO phonon from temperature-dependent energy shift of as increasing Mg concentration according to the Bose-Einstein expression [22]:
=1 meV is the LO phonon energy, and λ is a proportional coefficient which reflects a change in the exciton-LO-phonon interaction. The physical mechanism about exciton-phonon interaction was shown in Fig. 6-6.
Fig. 6-6 Diagram of exciton-phonon interaction in the unit cell.
0 50 100 150 200 250
Fig. 6-7 Dependence of PL peak energy positions on temperature for the Mg Znx 1−xO alloys (x = 5%, 3%, and zero).
Figure 6-7 depicts the PL-peak energy shift as a function of the temperature with different Mg concentrations. Notice that we had set the PL-peak energy position to zero for Mg0.05Zn0.95O powders at 15 K. By fitting the experimental data for all the MgxZn1−xO and ZnO powders from T = 15 to 260 K with Eq. (6.2), the exciton-LO-phonon coupling of each MgxZn1−xO sample are compared with ZnO powder by taking the average ratios, λMgxZn1−xO/λZnO, which are listed in Table 6-1 clearly show decreasing as more Mg incorporation. Even though aEB and λ are not directly proportional, there is a relation: If aEB reduces λ will also reduce.
Assuming that the Mg-doped ZnO powders have almost the same effective electron and hole masses as the undoped ZnO powders due to a small amount of Mg2+
substitution for Zn2+, we calculated aEB from EXB. The correlation between λ and aEB
normalized to ZnO powders are shown in Fig. 6-8 with open triangles and squares, respectively. Besides, the solid dots are obtained by the modified effective masses, which were corrected from MgZnO electronic band structure using semi-empirical
tight-binding approach sp3 model [23,24] and virtual-crystal approximation method[25]. Therefore, a contraction of aEB will make it less polar thereby reducing the coupling to LO phonons. The results show that relaxation by means of LO phonon becomes the less important as the more Mg incorporation. Consequently, we attribute this reducing coupling effect to increase in EXB with raising Mg mole fraction up to 5% of MgxZn1−xO powders. The reduction of λEx-LO has to take into account for the MgZnO-based excitonic device performance in which carrier relaxation to the exciton ground state is a crucial parameter.
0 1 2 3 4 5
Fig. 6-8 The coupling strength of the exciton-LO-phonon given as the average ratio λratio = λMgxZn1−
xO/λZnO. For comparison, the diminution of aEB for experiment and correction is also given as aEB ratio = aEB MgxZn1−xO/aEB ZnO.
Table 6-1 Summary of the results of the temperature-dependent PL characterization.
x = 0 x = 3% x = 5%
Samples: MgxZn O 1-x
Calculated Mg content (%) 0 2.9 4.5
Exciton binding energy (meV) 55 ± 4.7 70 ± 6.7 73 ± 8.4 Exciton-LO-phonon coupling strength
1 0.95 0.91
λ(MgxZn1-xO)/λ(ZnO)
6.4 Summary
The temperature-dependent NBE PL spectra of MgxZn1−xO powders within the range 0 ≤ x ≤ 0.05 were measured from 15 K to RT. The RTexcitonic transition energy showed being tuned by ~ 74 meV towards the UV range upon more Mg substitution. We deduced experimentally EXB showing elevation in powders up to 5% Mg substitution. It is suggests that the localization of excitons, because of the compositional fluctuation, takes place in MgxZn1-xO alloys and that the degree of the localization increases with increasing x. The reduction of λEx-LO may originate from a diminution in aEB making the exciton less polar, which could be explained by the dopant-induced increase of EXB.
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