Persistent photocurrent effects in GaN/AlGaN multiquantum wells

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Persistent photocurrent effects in GaN/AlGaN multiquantum wells A.Bonfiglio1, G.Traetta2, M.Lomascolo3, A.Passaseo2, and R.Cingolani2 1 Unita’ INFM, Dept. Ingegneria Elettrica ed Elettronica, University of Cagliari, ITALY 2 Unita’ INFM, Dept. Ingegneria dell’Innovazione, University of Lecce, ITALY 3 IME-CNR, Via Arnesano, I-73100, Lecce, ITALY

ABSTRACT PhotoCurrent (PC) experiments on GaN/AlGaN multiple quantum wells, both spectrally and temporally resolved, have shown that a photopersistence effect exists in our samples even at room temperature. A peak in the rise and decay times of PC vs. time appears in correspondence of wavelengths typical of Yellow-Band features while this is not seen in PL. This fact seems to indicate that YB defects give rise here to carrier trapping rather than recombination. Furthermore, the observed persistence times seem to be peculiar of MultiQuantum Wells and this possibly calls into play the role of built-in fields in the defects dynamics. INTRODUCTION The transport of GaN and GaN/AlGaN heterostructures is attracting increasing interest [1-8] due to the potential application of these materials to solar blind photodetectors and high mobility transistors. However the understanding of charge transport in these heterostructures is still quite poor. Recent investigations indicate a possible correlation between the Yellow Luminescence (YL) and the occurrence of persistent photoconductivity (PPC). Some papers [1,2] suggested that both effects are caused by Gallium vacancies. In fact, in Mg-doped GaN both phenomena are reduced due to the partial compensation of Ga vacancies by the Mg ions [1]. On the other hand, other works [3,4] show that even in very good quality samples, where the YL is completely absent in PL spectra, very long transients (>1000hrs) in the rise and decay dynamics of the photo-induced current during and after illumination are observed. This issue is even more relevant in GaN/AlGaN MQWs, which have not been investigated so far. For these reasons, in this work we present a careful study of the phototransport properties of GaN/AlGaN QWs. Our data show a correlation between the persistent photoconductivity and the defects causing the yellow band, independent of the radiative efficiency of the yellow band itself. Wavelength dependent photoconductivity transients of the order of thousands of seconds are indeed observed for excitation energies corresponding to the yellow band (2.2 eV). EXPERIMENTAL DETAILS The samples investigated in this study were grown in a horizontal LP-MOCVD system (AIXTRON 200 AIX RF), equipped with a rotating substrate holder, with TMGa, TMAl and pure NH3 as source materials; palladium purified H2 was used as a carrier gas. The growth was performed on (0001) c-plane Al2O3 substrates cleaned in solvents, and then annealed in situ at 1100 °C. After the deposition of a low temperature nucleation layer, a GaN buffer layer 1 µm thick was grown at 1150 °C. Ten GaN/Al0.15Ga0.85N QWs were grown under the same growth condition, keeping the TMAl/TMGa flow ratio in o