Photoluminescence of Excitons in n-Type In 0.11 Ga 0.89 N/In 0.01 Ga 0.99 N Multiple Quantum Wells
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Photoluminescence of Excitons in n-Type In0.11Ga0.89N/In0.01Ga0.99N Multiple Quantum Wells B. Monemar, P.P. Paskov, J.P. Bergman, G. Pozina, S. Kamiyama1, M. Iwaya1, H. Amano1and I. Akasaki1 Department of Physics and Measurement Technology, Linköping University, S-581 83 Linköping, Sweden 1 Department of Electrical Engineering and Electronics and High-Tech Research Center, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468, Japan
ABSTRACT In0.11Ga0.89N/In0.01Ga0.99N multiple quantum wells (MQWs) with heavily Si-doped barriers are shown to be very sensitive to a near surface depletion field. For a sample with 3 QWs, similar to what is often used in LEDs, only the QW most distant from the surface is observed in photoluminescence (PL). The appearance of a second lower energy PL peak below the ordinary QW exciton peak is a proof of a substantial band bending across the MQW structure. A similar effect seems to occur in pn-junctions having an MQW in the depletion region of a highly doped n-side. The strong depletion field is suggested to explain these results. The apparent absence of PL from the QWs closer to the surface (pn-junction) is tentatively ascribed to a loss of hole confinement in the strong depletion field.
INTRODUCTION Photoluminescence (PL) is a very common technique used to characterize the electronic properties of multiple quantum well (MQW) systems. The InxGa1-xN/InyGa1-yN materials system employing quantum wells of a thickness about 3 nm is of very high interest for applications in LEDs and lasers for the visible and near UV spectral regions [1]. In the literature it is generally assumed that PL measurements on MQW samples provide an average spectral information of the MQW region, so that all QWs contribute to the observed PL spectrum. We will demonstrate that this is often not true for this materials system, in the common cases demonstrated here it seems like only one QW contributes to the measured PL signal. A special property of the III-nitride materials is the presence of strong polarization charges at surfaces and interfaces in all device structures [2]. Both spontaneous (sp) and piezoelectric (pz) polarization are in general important, but for the InGaN/GaN MQW system the pz part is expected to dominate by far the properties of the QWs, due to the strong coherent strain built into these systems [3,4]. At an outer surface such polarization charges are expected to have a strong influence on the pinning of the Fermi level. In the common case of Ga(In) face surfaces a pinning at the valence band edge, or possibly in the lower part of the bandgap in the case of a large number of surface states, is expected [5]. Since the Fermi level further inside the grown structure is typically close to the conduction band edge, a depletion region is present I7.8.1
close to the surface [4-6]. For the common case of an n-doped MQW structure ending at the surface the QWs will experience a strong depletion field, which is also expected to vary strongly between the different QWs in the MQW region. The electr
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