Well-width dependence of the emission linewidth in ZnO/MgZnO quantum wells
© Lv et al.; licensee Springer. 2012
Received: 13 September 2012
Accepted: 21 October 2012
Published: 31 October 2012
Photoluminescence (PL) spectra were measured as a function of well width (LW) and temperature in ZnO/Mg0.1Zn0.9O single quantum wells (QWs) with graded thickness. The emission linewidth (full width at half maximum) was extracted from the emission spectra, and its variation as a function of LW was studied. The inhomogeneous linewidth obtained at 5 K was found to decrease with increasing LW from 1.8 to 3.3 nm due to the reduced potential variation caused by the LW fluctuation. Above 3.3 nm, however, the linewidth became larger with increasing LW, which was explained by the effect related with defect generation due to strain relaxation and exciton expansion in the QW. For the homogenous linewidth broadening, longitudinal optical (LO) phonon scattering and impurity scattering were taken into account. The LO phonon scattering coefficient ΓLO and impurity scattering coefficient Γimp were deduced from the temperature dependence of the linewidth of the PL spectra. Evident reduction of ΓLO with decreasing LW was observed, which was ascribed to the confinement-induced enhancement of the exciton binding energy. Different from ΓLO, a monotonic increase in Γimp was observed with decreasing LW, which was attributed to the enhanced penetration of the exciton wave function into the barrier layers.
ZnO has been attracting much attention recently due to its potential applications in light-emitting devices in the ultraviolet spectral region. An important issue in enhancing the emitting efficiency of optoelectronic devices is the bandgap engineering to form a low-dimensional structure [1–4]. ZnO/MgZnO quantum well (QW) has been considered as one of the most promising structures due to its larger oscillation strength, enhanced binding energy in the excitonic region, and tunability of operating wavelength. Up to now, this structure has been demonstrated on various substrates such as ScAlMgO4, ZnO , sapphire , and silicon . The optical properties have been investigated widely, including quantum confinement effect [5–7], quantum-confined Stark effect (QCSE) [8–10], temperature dependence of excitonic emission [11–13], localized characteristics of excitons [14–16], and so on. Besides, the linewidth of absorption or photoluminescence (PL) is also crucial to understand the fundamental physics and optical properties of semiconductor microstructure. On the one hand, the structural quality of the QW can be characterized by studying the inhomogeneous broadening generally induced by the well width (LW) fluctuation and alloy disorder. On the other hand, the value of carrier-scattering parameters in semiconductors, such as longitudinal acoustic phonon, longitudinal optical (LO) phonon, and impurity scatterings, can be extracted from the homogeneous broadening [17, 18]. In addition, for optoelectronic device applications such as the laser diode, the linewidth has a direct effect on performance and, especially, is directly related to the lasing threshold. Thus, the linewidth measurement is also of critical importance in the performance of optoelectronic device based on QW. Sun et al.  investigated the homogenous linewidth broadening of the excitonic absorption peak in ZnO/MgZnO multi-QWs. Effective reduction of the exciton-LO phonon coupling with decreasing LW was observed. However, more detailed study of the dependence of emission linewidth broadening on LW was not reported due to the difficulty in sample preparation. In this paper, a special ZnO/Mg0.1Zn0.9O single QW sample, in which the LW was continuously changed from 1.4 to 7.5 nm, was used to evaluate the PL linewidth-broadening mechanisms. It was found that inhomogenous broadening, LO phonon scattering, and impurity scattering contributed to the PL linewidth, and all of them were strongly dependent on the LW. A detailed analysis of the results was conducted.
ZnO/Mg0.1Zn0.9O single QW was grown by a metalorganic chemical vapor deposition system. Al2O3 () wafers were used as substrates because of the larger critical thickness for ZnO layer-by-layer growth . The sample consists of a three-layer Mg0.1Zn0.9O/ZnO/Mg0.1Zn0.9O sandwich structure. The growth temperatures of the Mg0.1Zn0.9O barrier layer and ZnO well layer were 425°C and 475°C, respectively. By introducing a gradient in the growth rate across the sample, a graded layer thickness was obtained. The details of the growth procedure and method to determine the LW can be found elsewhere [2, 9]. In order to mark the sample position, a thin film of Au metal was deposited on the sample surface followed by an opening of hole arrays using standard photolithography and liftoff processes . The holes with a diameter of 5 μm were used for the PL measurements, whereas the area without holes was covered with the Au metal. The sample was then characterized by micro-PL spectroscopy from 5 to 300 K. A continuous He-Cd laser operating at 325 nm was used as the excitation source. A reflective objective lens was applied to focus the laser beam to a diameter of approximately 5 μm into the holes. The luminescence from the sample was collected by the same objective lens, dispersed by a spectrometer, and detected using a charge-coupled device. By conducting the laser beam to different holes, the luminescence from different layer thicknesses was obtained.
Results and discussion
The individual contributions to the FWHM from inhomogenous broadening and interactions with LO phonon and impurities are presented with colored lines in Figure 4. It is seen that except for inhomogenous broadening, below 250 K, the impurity scattering mainly contributes to the FWHM. As the temperature increases above 250 K, scattering by LO phonons becomes the main temperature-dependent contributor due to the increasing LO phonon population.
On the other hand, different from ΓLO, a monotonic increase in Γimp is observed with decreasing LW. The coefficient Γimp is thought to be a measurement for the scattering of shallow donor defects and impurities such as oxygen vacancies, zinc interstitials, hydrogen, etc. Generally, the Γimp depends on the density of defect and impurity sites. Contrary to the distribution of defects in the ZnO QW with different LWs as indicated in the above analysis, we suppose that defects, impurities, and composition fluctuation in the barrier layers are the dominant scatters. For the narrow QW geometry, the exciton wave function penetrates deeply into the adjacent barrier layers , and therefore, the scattering coming from the barrier layers is remarkable. As the well thickness increases, the extension of the exciton wave function into the barrier layers is suppressed; hence, the influence of defect and impurity scattering was sufficiently reduced, leading to a decrease of Γimp with increasing LW. In addition, in the large-LW range, the defects induced by the strain relaxation in the QW may also contribute to the scattering process, showing a slow decreasing trend in Γimp with increasing LW.
In conclusion, the broadening mechanisms of the PL excitonic linewidth were investigated in ZnO/Mg0.1Zn0.9O single QWs with graded thickness. The in homogenous broadening obtained from the 5-K LW-dependent PL spectra decreased first and then increased with increasing LW. This was mainly explained by the reduced potential fluctuation and the generated defects in the QW by strain relaxation, respectively. Furthermore, the homogenous broadening mechanisms including LO phonon scattering and impurity scattering were determined by fitting the temperature-dependent PL linewidth to a theoretical model. The LO phonon scattering coefficient ΓLO and impurity scattering coefficient Γimp showed different LW dependence. The monotonic decrease in ΓLO with decreasing LW was explained in terms of the confinement-induced enhancement of the exciton binding energy, while the continuous increase in Γimp with decreasing LW was attributed to the enhanced penetration of the exciton wave function into the barrier layers.
This work was supported by the National Natural Science Foundation of China (grant nos. 91023048, 61106044, and 61274052) and the Open Fund of the State Key Laboratory of Optoelectronic Materials and Technologies (Sun Yat-sen University, grant no. KF2010-ZD-08).
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