Role of polarization in the photoluminescence of C- and M-plane oriented GaN/AlGaN multiple quantum wells
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L5.3.1
Role of polarization in the photoluminescence of C- and M-plane oriented GaN/AlGaN multiple quantum wells
E. Kuokstis, C. Q. Chen, M. E. Gaevski, W. H. Sun, J. W. Yang , G. Simin, M. Asif Khan, H. P. Maruska,1 D. W. Hill,1 M. M. C. Chou,1 J. J. Gallagher,1 and B. H. Chai1 Department of Electrical Engineering, University of South Carolina, Columbia, SC 29208, U.S.A. 1 Crystal Photonics, Inc., Sanford, FL 32773, U.S.A. ABSTRACT The comparative study of photoluminescence (PL) dynamics of wurtzite-type GaN/AlGaN multiple quantum wells (MQWs) fabricated using low-pressure metalorganic chemical vapor deposition technique over GaN coated [0001]-sapphire (Cplane) and single crystalline [1 1 00] -oriented freestanding GaN (M-plane) substrates is presented. The MQWs on C-plane sapphire at low excitation exhibited much lower (~30 times) PL intensity in comparison with M-plane samples. The C-plane MQWs showed a strong excitation intensity-induced PL spectrum line blueshift (up to 140 meV). Meanwhile identical MQW structures on M-plane substrates demonstrated no PL peak shifts indicating an absence of polarization fields. At higher excitation (>50 kW/cm2) the PL intensity and spectra peak positions for both the C- and the M-plane MQWs become nearly the same and do not differ with subsequent increase of pumping. Theoretical analysis and comparison with PL experimental data revealed strong (up to ~1.2-1.3 MV/cm) built-in electrostatic fields in the C-plane structures whereas M-plane structures are almost non-polar. INTRODUCTION Thin films of N-based wurtzite (WZ) compounds grown along the c-axis ([0001] direction) usually have strong built-in electrostatic field as a result of spontaneous and piezo-elelectric polarization, due to the noncentrosymmetric nature of WZ C-plane oriented layers[1]. In quantum wells, these electrostatic fields tend to bend the energy bands, and electrons and holes typically become situated in nearly triangular potential wells. The modulation of the conduction and valence band edges due to large internal electrostatic fields leads to the Quantum Confined Stark Effect (QCSE) which is undesirable in light-emitting devices because the fields across the individual quantum wells give rise to a spatial separation of electrons and holes [2]. This results in a reduced oscillator strength and lower quantum efficiency for radiative transitions. Furthermore, in multiple quantum well (MQW) structures designed for deep UV emission, QCSE causes an undesirable red shift in the emission spectra [3]. A useful approach for reducing the deleterious effects of built-in fields is to fabricate GaN-based structures along non-polar directions, e.g., to grow [1 1 00] -oriented M-plane films since these surfaces contain equal number of Ga and N atoms and therefore are non-polar [4].
L5.3.2
In the present study, we demonstrate fabrication of non-polar N-based WZ layers and analyze the dynamics of photoluminescence (PL) in highly excited GaN/AlGaN MQWs with C- and M-plane surfaces, seeking to gain control over the built-
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