Polarization Effects in the Photoluminescence of AlGaN and AlInGaN Based Quantum Well Structures
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Polarization Effects in the Photoluminescence of AlGaN and AlInGaN Based Quantum Well Structures Mee-Yi Ryu, E. Kuokstis, C. Q. Chen, J. P. Zhang, J. W. Yang, G. Simin, and M. Asif Khan University of South Carolina, Department of Electrical Engineering, Columbia, SC 29208, U.S. A. ABSTRACT We have analyzed photoluminescence (PL) dynamics of GaN/AlGaN and AlInGaN/AlInGaN multiple quantum wells (MQWs) with different well and barrier widths. The quantum structures were grown by conventional metalorganic chemical vapor deposition and novel pulsed atomic layer epitaxy. In both types of MQWs a blueshift followed by a redshift of the PL peak position were observed with increasing excitation power, which we attributed to the screening of built-in electric fields and band gap renormalization, respectively. In bulk AlInGaN material or in MQWs with thin well widths the blueshift was not observed. This means that the incorporation of In into AlInGaN material in the amount required to fabricate smooth layers with strong emission at 330-350 nm does not create significant concentration of band-tail states. We have also evaluated the internal field in the MQW structures by comparing the experimental PL data to the simulations based on triangular quantum well model resulting from the polarization fields. INTRODUCTION Solid-state white-light emitters require efficient deep ultraviolet (UV) pumping in the 250350 nm spectral region. Nitride-based alloys such as AlxGa1-xN [1] and quaternary AlxInyGa1-xyN layers [2,3] are among the most promising candidates for these UV devices due to their appropriate wide direct band gap. In nitride-based multiple quantum well (MQW) structures the spontaneous and piezoelectric polarization fields have a strong influence on the optical properties. In both GaN/AlGaN and AlInGaN/AlInGaN MQWs, the optical transitions change from spatially direct to indirect due to the internal polarization fields, resulting in weak emission and peak wavelength shift with the excitation power density. For practical device applications, further understanding of internal field effects is necessary to improve the output power and efficiency of nitride-based UV emission devices. In this study, we analyze the internal electric field effects in the photoluminescence (PL) dynamics of GaN/AlGaN and AlInGaN/AlInGaN MQWs with different well and barrier widths. EXPERIMENTAL DETAILS The GaN/AlxGa1-xN MQW samples were grown on sapphire substrates by a conventional low-pressure metalorganic chemical vapor deposition technique. Prior to the deposition of the GaN/AlxGa1-xN MQW layers, a Si-doped 1-µm-thick Al0.2Ga0.8N layer was also grown on the sapphire substrate with a low temperature deposited thin AlN buffer layer. The MQW structures had 8 periods of GaN wells and AlxGa1-xN barriers. The GaN well width was varied from 2.5 to 7.5 nm and the AlxGa1-xN barrier width was kept fixed at 6 nm. The Al molar fraction in the AlxGa1-xN barriers was about 8 %. I8.8.1
In order to study the internal electric field effects in quaternary AlxInyGa1-x
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