Radiative recombination in In 0.15 Ga 0.85 N/GaN multiple quantum well structures
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Internet Journal Nitride Semiconductor Research
Radiative recombination in In0.15Ga0.85N/GaN multiple quantum well structures B. Monemar1, J. P. Bergman1, J. Dalfors1, G. Pozina1, B.E. Sernelius1, P.O. Holtz1, H. Amano23 and I. Akasaki23 1Department
of Physics and Measurement Technology, Linköping University, of Electrical and Electronic Engineering, Meijo University, 3High-Tech Research Center, Meijo University, 2Department
(Received Thursday, September 9, 1999; accepted Saturday, December 4, 1999)
We present a study of the radiative recombination in In0.15Ga0.85N/GaN multiple quantum well samples, where the conditions of growth of the InGaN quantum layers were varied in terms of growth temperature (< 800 °C) and donor doping. The photoluminescence peak position varies strongly (over a range as large as 0.3 eV) with delay time after pulsed excitation, but also with donor doping and with excitation intensity. The peak position is mainly determined by the Stark effect induced by the piezoelectric field. In addition potential fluctuations, originating from segregation effects in the InGaN material, from interface roughness, and the strain fluctuations related to these phenomena, play an important role, and largely determine the width of the emission. These potential fluctuations may be as large as 0.2 eV in the present samples, and appear to be important for all studied growth temperatures for the InGaN layers. Screening effects from donor electrons and excited electron-hole pairs are important, and account for a large part of the spectral shift with donor doping (an upward shift of the photoluminescence peak up to 0.2 eV is observed for a Si donor density of 2 x 1018 cm-3 in the well), with excitation intensity and with delay time after pulsed excitation (also shifts up to 0.2 eV). We suggest a two-dimensional model for electron- and donor screening in this case, which is in reasonable agreement with the observed data, if rather strong localization potentials of short range (of the order 100 Å) are present. The possibility that excitons as well as shallow donors are impact ionized by electrons in the rather strong lateral potential fluctuations present at this In composition is discussed.
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Introduction
InxGa1-xN/GaN multiple quantum well (MQW) structures provide an important active medium for the violet III-Nitride lasers [1]. The recombination processes in these structures therefore are of considerable interest. It has recently been claimed that the broad spontaneous photoluminescence (PL) emission spectra in these structures are due to an inhomogeneously broadened envelope of excitonic emissions from "quantum dots" (QDs) [2] [3]. These QDs are supposed to consist of regions of a size 3-5 nm having a bandgap much lower than the alloy matrix, due to segregation of In during growth [2]. Other recent work points towards the importance of the piezoelectric field as the dominant mechanism governing the electronic energy levels and the recombination processes in these QW structures [4] [5]. In the previ-
ous stu
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